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Gas-Diffusion Electrodes for Carbon Dioxide Reduction: A New Paradigm

Cite this: ACS Energy Lett. 2019, 4, 1, 317–324
Publication Date (Web):December 14, 2018
https://doi.org/10.1021/acsenergylett.8b02035
Copyright © 2018 American Chemical Society
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Abstract

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Significant advances have been made in recent years discovering new electrocatalysts and developing a fundamental understanding of electrochemical CO2 reduction processes. This field has progressed to the point that efforts can now focus on translating this knowledge toward the development of practical CO2 electrolyzers, which have the potential to replace conventional petrochemical processes as a sustainable route to produce fuels and chemicals. In this Perspective, we take a critical look at the progress in incorporating electrochemical CO2 reduction catalysts into practical device architectures that operate using vapor-phase CO2 reactants, thereby overcoming intrinsic limitations of aqueous-based systems. Performance comparison is made between state-of-the-art CO2 electrolyzers and commercial H2O electrolyzers—a well-established technology that provides realistic performance targets. Beyond just higher rates, vapor-fed reactors represent new paradigms for unprecedented control of local reaction conditions, and we provide a perspective on the challenges and opportunities for generating fundamental knowledge and achieving technological progress toward the development of practical CO2 electrolyzers.

The development of new technologies that reduce greenhouse gas emissions while producing fuels and commodity chemicals has the potential to mitigate the devastating impacts of climate change by transforming the petrochemical sector toward sustainability. Electrochemical CO2 reduction (CO2R) coupled with renewably generated electricity (wind, solar, hydro) provides an attractive approach for the carbon-neutral production of valuable hydrocarbon, alcohol, and carbonyl products that find widespread use in the energy and chemical sectors. For this artificial photosynthesis process to be implemented at scale, highly active and selective CO2R catalysts must be developed and ultimately integrated into devices that can achieve high conversion rates and energy-conversion efficiencies to the desired product(s). Vapor-fed CO2 devices represent a promising platform for such a technology.

On a fundamental level, there has been much progress understanding electrochemical CO2R in the liquid phase, where CO2 molecules are solubilized in an aqueous electrolyte and reduced on the surface of a catalyst (Figure 1a). The unprecedented level of synergy between theoretical and experimental research toward aqueous-phase CO2R has led to improved understanding regarding the impact of electrolyte ions,(1−3) pH,(4) mass transport,(5−7) temperature,(8) and pressure(8,9) on activity and selectivity. As a result, activity descriptors(10,11) and mechanistic insight into reaction pathways(12,13) have guided catalyst design efforts, leading to the discovery of new compositions(14−16) and morphologies that are more active and selective to desired CO2R products. A succinct overview of these advances has been provided in a recently published perspective piece.(17) It is furthermore expected these efforts will be accelerated with the implementation of machine learning processes for catalyst discovery.(18,19)

Figure 1

Figure 1. Different electrochemical CO2R reactor schemes. (a) Aqueous-phase CO2R, where CO2 is first solubilized in an aqueous electrolyte and then reduced at a catalyst surface. Vapor-fed CO2R employing an (b) aqueous or (c) polymer electrolyte.

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Moving toward practical reactor designs that operate using CO2 delivered to the cathode in the vapor phase can help to overcome these performance and solubility challenges.

While the above referenced studies have been critical in establishing a deeper understanding of CO2R, they have traditionally relied upon aqueous-phase CO2R reactors designed for fundamental investigations (Figure 1a). From an applied standpoint, however, these test reactors have many practical limitations that must be addressed. Most notably, the poor solubility (ca. 34 mM) of CO2 in aqueous electrolytes, along with acid/base buffer (CO2/HCO3–/CO3–2) equilibria lead to intrinsic challenges toward achieving high conversion rates and energy efficiencies.(20) Moving toward practical reactor designs that operate using CO2 delivered to the cathode in the vapor phase (Figure 1b,c) can help to overcome these performance and solubility challenges. Such gas-diffusion electrodes (GDEs) can achieve this by employing a porous catalyst layer along with diffusion media to facilitate reactant transport and distribution. GDEs have been used in other electrochemical energy-conversion devices such as fuel cells and electrolyzers, where the architecture has been optimized for high current density and low transport losses. However, the adaptation to CO2R will require further advancement, as different operating strategies and understandings are needed to address product selectivity considerations, which is important to avoid the need for costly downstream separations.(21) Furthermore, the actual electrolyte can either be aqueous to form a catalyst/liquid electrolyte interface (Figure 1b), or ideally an ion-conducting polymer that can transport charged species (e.g., H+ or OH–) and form a catalyst/polymer-electrolyte interface (Figure 1c).

A recently published article(22) provides a critical overview of various electrolyzer designs that can be considered, along with a review of the technological achievements made in recent years on electrochemical CO2R reactor designs. In this Perspective, we discuss the challenges and opportunities facing GDE development for electrochemical CO2R. We provide context in terms of CO2R electrocatalysis, and include a discussion of the intrinsic advantages and unexpected opportunities of GDEs in an effort to motivate researchers to translate current understanding toward new GDE designs. The purpose of this Perspective is not to provide a comprehensive review on the topic of electrochemical CO2R or GDE development. Instead, the goal is to provide a forward-looking perspective to inspire and provide direction for these fields of research, using the technology maturation process of commercial water electrolyzers as realistic performance targets. We identify areas deemed important for developing a fundamental understanding of the underlying chemistry, processes, and phenomena occurring in GDEs. This insight is essential for advancing the state of electrochemical CO2R technologies toward commercial viability.

State-of-the-Art. In comparison to electrodes studied in aqueous-phase electrochemical reactors, various types of vapor-fed CO2R electrodes have been successful in improving the partial current densities and energy efficiencies for CO2R.(23) This has been achieved by taking the most selective catalysts identified through fundamental aqueous-phase reactor investigations and integrating them into vapor-fed device designs. This research translation trend is depicted in Figure 2a, which summarizes state-of-the-art faradaic efficiencies versus partial current densities achieved for CO, formate, ethylene, and ethanol production. Performance obtained from vapor-fed GDEs(24−36) (solid symbols) are shown in comparison to similar catalyst compositions tested in aqueous-phase reactors(1,14,37−47) (hollow symbols). While different reactor designs and catalyst configurations were used throughout these studies, this comparison shows the general trend of vapor-fed GDE research successfully improving partial current densities beyond those achievable in aqueous-phase investigations, while retaining similar selectivity. Among these major products shown, the highest faradaic efficiencies and partial current densities are generally reported for CO and formate, as there are a number of different catalyst types that are intrinsically selective to these 2e– reduction products.(36,48−52) On the other hand, data for the further reduced (>2e–) products, ethylene and ethanol, demonstrates that selectivity is still a major challenge. This selectivity challenge is largely because ethylene and ethanol are competitively coproduced on Cu-based catalysts through very similar mechanistic reaction pathways. However, improvements in ethylene selectivity have been observed by implementing Cu-based catalysts in vapor-fed GDEs for electrochemical CO2R,(24,26) along with similar results demonstrated for electrochemical carbon-monoxide reduction.(53,54) This observation suggests that vapor-fed conditions are a promising avenue for tuning the local environment and reaction conditions that control CO2R selectivity (vide infra), while simultaneously achieving higher partial current densities. However, altering the local CO2 environment is largely underexplored for GDEs and presents an opportunity for increased understanding compared to solely aqueous-phase reactor investigations.

Figure 2

Figure 2. State-of-the-art performance of vapor-fed CO2 devices. (a) Faradaic efficiencies versus partial current densities to ethylene, ethanol, carbon monoxide, and formate. (b) Energy efficiencies versus partial current densities to ethylene, carbon monoxide, formate, and hydrogen. Performances obtained for vapor-fed CO2R electrodes are shown in solid symbols, while performance for electrodes in aqueous-phase CO2R reactors are shown in hollow symbols. All energy efficiencies were calculated as voltage efficiencies using the formula:, where Eanode0 and Ecathode0 are the reversible potentials, FE is the faradaic efficiency for the CO2R product, and Vcell is the uncompensated cell voltage.

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On a system level, Figure 2b shows a summary of state-of-the-art energy efficiencies versus partial current densities, which takes into account transport resistances (ionic and electronic), along with kinetic losses from both the anode and cathode.(55) A comparison is also provided to the performance of representative alkaline(55) and proton-exchange membrane(56) (PEM) H2O electrolyzers. It is interesting to note that all of the vapor-fed CO2R cells incorporate an aqueous electrolyte (Figure 1b), in part because alkaline electrolytes can improve energy efficiencies by reducing cathodic overpotentials. Thus, a traditional commercial H2O electrolyzer with an aqueous alkaline electrolyte(55) likely provides the most appropriate comparison. While the representative alkaline H2O electrolyzer performance is superior to that of the vapor-fed CO2 cells shown in Figure 2b, the performance of CO2R to CO cells has been recently closing the gap. Comparing CO2R cell data to the representative PEM H2O electrolyzer shows the intrinsic advantages of the PEM configuration (Figure 1c) for high current-density applications (Figure 2b) due to their more efficient reactant management, high reaction area, and minimal distances for ion transport. Clearly, there are opportunities to develop vapor-fed CO2 devices in this configuration as there is currently a dearth of such studies. Moving forward, it is necessary to understand and optimize transport properties and reaction kinetics in vapor-fed CO2R devices to advance the performance toward practical viability. While we have focused on partial current density, Faradaic efficiency, and energy efficiency as immediately important performance figures of merit, we note that other parameters such as CO2 utilization and component stability will also become increasingly important.

The challenges and opportunities facing vapor-fed CO2R electrode development relate to understanding and optimizing the multitude of processes occurring in three-dimensional GDEs. These processes span different length and time scales, with the complex interplay between phenomena ultimately having a governing effect on the CO2 reaction selectivity and the energy-conversion efficiencies and rates.

Challenges and Opportunities. A crucial first step in the development of vapor-fed CO2R devices relates to engineering the GDE structures. Despite decades of studies, GDEs continue to be an active area of research in the fuel-cell sector, and performance improvements are still being realized through GDE optimization strategies that aim to address the many open questions that remain. GDEs in fuel cells may represent a simplified case in comparison to those in CO2R cells, as reaction selectivity and different product phases (vapor versus liquid) are not as crucial considerations for fuel cells. The challenges and opportunities facing vapor-fed CO2R electrode development relate to understanding and optimizing the multitude of processes occurring in three-dimensional GDEs. These processes span different length and time scales (Figure 3), with the complex interplay between phenomena ultimately having a governing effect on the CO2 reaction selectivity and the energy-conversion efficiencies and rates. As these research efforts are accelerated, it will be necessary to translate fundamental knowledge from aqueous-phase CO2R studies to vapor-fed systems and identify gaps and emergent phenomena. The vapor-fed systems are inherently more complex, due to the presence of a myriad of heterogeneous interfaces on the micro- and nanometer scales. Future research and scientific challenges must be addressed by closely coupled experimental and theoretical investigations. Areas deemed important for knowledge generation and technological process are outlined herein.

Figure 3

Figure 3. Schematic of a three-dimensional GDE depicting the multiple length scales where phenomena are occurring during electrochemical CO2R.

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Transport of Reactants. In vapor-phase CO2R electrodes, the delivery of relevant reaction species (CO2, electrons, and H+) can be readily optimized to achieve improved conversion rates. Most notably, vapor-fed cells overcome the intrinsic solubility challenges of CO2 in aqueous electrolytes (ca. 34 mM). At these low concentrations, mass-transport limitations significantly hinder CO2 conversion rates in aqueous-phase devices when current densities exceed ca. 10 mA/cm2.(20) The type of catalyst and GDE fabrication process must be carefully selected to maximize the catalytically active surface area available, and micro- and nanoscale electrode architectures must be designed to optimize CO2, ion, and product transport simultaneously.(57) If present, the properties of the diffusion media, including porosity, pore structure, hydrophilicity, and thickness also play significant roles in governing electrode performance. These parameters have been explored and optimized in the case of fuel cells,(58) whereby H2/O2 fuel cells are able to reliably achieve current densities in excess of 1 A/cm2. This provides a good basis for comparison, yet very limited understanding exists toward the design and development of high current density CO2R electrodes, which must be established through concerted experimental and theoretical efforts.

The relative humidity and concentration of water in vapor-fed CO2R reactors can be carefully controlled to overcome the intrinsic challenges associated with aqueous-phase CO2R, where the concentration of water at the catalyst surface is ca. 55 M, whereas in a typical ion-exchange membrane, water concentrations on the order of 1–25 M or so are obtainable via humidity control although there is trade-off in ionic conductivity at low water contents.(59−61) Water can be a proton source for CO2R as well as for the undesirable HER. As the reversible potentials for most electrochemical CO2 reactions lie within 200 mV of the HER,(37) the HER provides competition to CO2R by occupying electrocatalytically active sites and consuming electrons as well as the proton source, resulting in reduced CO2 conversion rates and energy efficiencies toward the desired product(s). By delivering CO2 to the cathode in the vapor-phase, the local partial pressure of CO2 can be decoupled from the concentration of water (provided an ionic transport pathway remains), enabling strategies to steer selectivity by controlling reactant transport to tune the coverage of intermediates on the catalyst surface. The impact of CO2 partial pressure on vapor-fed device performance is, however, not well understood and should be the focus of future studies. Parametric investigations on well-characterized GDEs should be conducted and closely coupled to the development of continuum mathematical models to understand transport processes throughout these 3-dimensional porous electrodes and identify their impact on performance.

Polymer Electrolyte and Ionomer: Charge Carrier Transport and Catalyst/Electrolyte Interfaces. As previously mentioned, a key challenge of aqueous-phase CO2R is the CO2/carbonate/bicarbonate buffering equilibria that limits the range of operational pH values for CO2R, and convolutes an accurate depiction of the boundary-layer properties at the catalyst surface.(20) This leads to inflexibility in tuning the chemical properties of the catalyst/electrolyte interface, despite the importance of these chemical properties in dictating surface reaction kinetics, mechanisms, and charge-transport processes. For example, electrolyte pH is known significantly impact CO2R activity and selectivity. In particular, increased activity toward valuable C–C coupled products are favored at high pH values,(4,62) which cannot be reliably achieved for aqueous-phase CO2R due to the above-mentioned equilibria. This presents a valuable opportunity to develop and utilize polymer electrolytes that can operate in different pH regimes and may exhibit very different ion concentrations due to their thinness as well as background charge. Furthermore, advances in polymer electrolytes must be translated to the development of ionomers for incorporation throughout the three-dimensional structure of a GDE to create an interconnected thin-film network needed for ionic species transport. Despite similar structures, the behavior of ionomer thin films in an electrode can vary quite significantly from the bulk polymer,(60) and advances in their development and understanding are needed.

Solid-state polymer electrolytes (Figure 1c) pose many intrinsic advantages over liquid-phase electrolytes (Figure 1b). Particularly, simplified device designs requiring fewer auxiliary components for electrolyte circulation and replenishment, and the elimination of any mobile counterions other than protons and hydroxyls are ideal from a sustainability and CO2 utilization standpoint. Vapor-fed GDE based devices employing polymer electrolytes also provide additional transport advantages versus aqueous electrolytes as they enable shorter distances between the anode and cathode,(63) thereby minimizing ohmic resistances through a “zero-gap” complete solid-state configuration. Avoiding the use of corrosive liquid electrolytes also poses several safety advantages, including avoiding the risk of leaking or heat-induced pressure buildup. Polymer electrolytes furthermore enable operation at higher pressures and potentially allow for differential pressures to be used between the two electrodes, as reactant crossover can be suppressed.(63) Finally, they provide an opportunity for separation of volatile liquid-phase CO2R products directly at the site of generation. For example, when targeting alcohol products, in comparison to aqueous-phase CO2R, vapor-fed devices will avoid the formation of azeotropic alcohol/water mixtures that would require energy intensive downstream separation processes.(21) Clearly there is an immense opportunity for the development of solid polymer electrolytes and their integration with vapor-fed CO2R GDEs. Key challenges include designing and integrating new polymer electrolytes that simultaneously satisfy the requirements of low cost, high ionic conductivity and selectivity, resistance to reactant/product crossover, CO2 tolerance, and long-term chemical and mechanical stability under operating conditions.

On the electrode level, the ionomer properties, including type (i.e., anionic, cationic), structure and catalyst/ionomer interactions strongly influence CO2R activity and selectivity, where the tethering of the ionic groups hinder movement of their counterions as well as influence the reactivity of the ionic group themselves relative to their behavior in liquid electrolytes. Ionic species (e.g., H+, OH–, HCO3–) transport in the ionomer phase is a crucial consideration, in addition to the distribution of the ionomer phase throughout the three-dimensional GDE structure. Particularly, optimized ionomer distributions can enable good charge species transport and active site utilization, while nonideal distributions can adversely affect performance through catalytic or transport resistances.(64) There also exist enticing opportunities to modify ionomer structures to accommodate functional or ionic species that can provide promotional CO2R effects, such as increasing the local CO2 concentration, decreasing selective site poisoning through blocky architectures,(15) or impacting reaction mechanisms and routes through chemical modification(65) and field effects, where the local ion concentrations and distances can be more precisely controlled.(2)

While recent advances have enabled understanding of how different parameters (i.e., pH, electrolyte concentrations, catalyst functionalization) fundamentally affect aqueous-phase CO2R catalysis, it is an opportune time to translate and validate this current state of understanding to highly porous vapor-fed GDEs. For example, polymer electrolytes exhibit different acid/base equilibria time constants than aqueous electrolytes due to the existence of the polymer backbone.(66) Furthermore, while one may obtain the desired high pH in aqueous electrolytes using high flow rates, this provides challenges from a practicality standpoint;(24) a similar effect may perhaps be obtained with polymer electrolytes since their thinness and possibility for high current-density operation result in large hydroxide fluxes and amounts in the electrode ionomer. Targeted approaches to understand polymer electrolyte effects, ionomer distributions, ionomer/catalyst interactions and charge-carrier transport properties must be carried out on model and/or prototype vapor-fed CO2R systems, where the use of new polymer electrolytes and ionomers provide an increased ability to control and manipulate the local reaction environment at the catalyst surface. It is suggested that researchers leverage previous efforts on these topics reported in the fuel cell or electrolyzer literature, especially as anion-exchange membranes and ionomers become more prevalent and understood.

Opportunities for Fundamental Understanding. With the seemingly overwhelming number of factors that govern the multiscale processes and performance of a GDE, a detailed understanding of these phenomena will require experimental approaches closely coupled with multiscale theoretical modeling and prediction. Comprehensive models do not currently exist that simultaneously capture and bridge quantum- and molecular-level dynamics with continuum models of reactant and product transport. The difficulty lies in the disparate length- and time-scales between these processes that require the combination of nonlinear partial differential equations with complex boundary conditions. Robust numerical techniques that can accomplish this are needed, which will enable the necessary multiprocess understanding and optimization that will be essential for guiding and understanding GDE approaches.

In terms of experimental approaches, the increasing complexity of vapor-fed devices necessitates the development and utilization of operando, in situ, and ex situ probes that probe interfacial phenomena in highly porous electrodes. For this, simplified vapor-fed cells can potentially be designed to deconvolute the influence of common experimental parameters.(67) This could serve to enable facile characterization and CO2R evaluation of catalyst and electrode structures, which will accelerate the implementation of new GDE formulations in high-performance devices. Additionally, vapor-fed GDEs offer a promising platform for experimentally characterizing the multiscale properties of devices and processes occurring during operation. By minimizing the use of liquids, challenges associated with beam attenuation and refraction are avoided, enabling mechanistic probing of electrode processes using X-ray scattering, absorption, or photoelectron techniques. For example, the electronic or chemical structure of catalytically active surface sites in GDEs under reaction conditions can be probed by in situ X-ray absorption spectroscopy(68−70) or in situ X-ray photoelectron spectroscopy,(71,72) respectively; meanwhile the effects of electrode pore sizes, structures, and surface properties on microscale transport processes can be interrogated by X-ray computed tomography coupled with performance evaluation.(73−77) Developing an improved understanding of the effects of operating conditions and GDE configurations on performance will provide opportunity to engineer devices to provide multivariable optimization for achieving unprecedented knowledge and performance.

Beyond GDE designs to optimize the multiscale processes underlying their operation, electrode integration into vapor-fed reactors provides an ideal opportunity for advanced understanding. The impact of operational parameters such as relative humidity, reactant flow rates, temperature, and device electrical potential on CO2 conversion rates and efficiency remains largely unexplored, yet provide additional levers to tune performance and selectivity. The type of polymer electrolyte (proton exchange, anion exchange, bipolar) and anode design and materials are essential considerations for incorporating GDEs into working devices,(78,79) and GDE compatibility with electrolytes and anodes must be understood. The stability of GDEs under operating conditions is also an important topic that has not been addressed in detail here or in the literature, because vapor-fed CO2R electrode design is a relatively early stage field of research. Stable, long-term operation will be essential for achieving practicality of these devices. As these devices will ideally be coupled with renewable sources of power, the question of variability and how it relates to GDE performance and stability must also be understood and addressed. Furthermore, engineering vapor-fed GDEs to be capable of accommodating low-grade or dilute CO2 feed sources (e.g., atmospheric CO2) improves the practicality of these devices to different applications and elucidation of these effects is important.

Particularly, there is an immense scientific opportunity to develop fundamental understanding of the multi-scale processes occurring in three-dimensional GDEs, and to optimize GDE performance through rational engineering approaches.

Outlook. Recent efforts have demonstrated the potential of translating scientific advances made in electrochemical CO2 reduction research toward the development of practical CO2 electrolyzers. Key challenges and opportunities that remain involve the understanding and development of three-dimensional vapor-fed CO2 reduction electrodes that can achieve high conversion rates and energy efficiencies toward the desired products. Particularly, there is an immense scientific opportunity to develop fundamental understanding of the multiscale processes occurring in three-dimensional GDEs, and to optimize GDE performance through rational engineering approaches. Closely integrated experimental and theoretic investigations are required to progress upon our current state of understanding and perpetuate the advancement of CO2 electrolyzers toward practical relevance. The knowledge generated and progress made in catalyst integration, electrode engineering, and electrochemical device design will also be directly applicable to other electrochemical conversion devices that could be of technological importance in the near future to replace gigatonne-scale, carbon-intensive industrial processes. These include sustainable electrochemical technologies for the production of fuels and chemicals from carbon-based feedstocks, or the synthesis of ammonia-based fertilizers from ambient N2.

⊥Author Contributions

D.H. and C.H.: Equal author contributions.

The authors declare no competing financial interest.

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  • Corresponding Authors
    • Thomas F. Jaramillo - Department of Chemical Engineering, Stanford University, 443 Via Ortega Way, Stanford, California 94305, United States; SUNCAT Center for Interface Science and Catalysis, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, United States; Orcidhttp://orcid.org/0000-0001-9900-0622; Email: [email protected]
    • Adam Z. Weber - Energy Conversion Group, Energy Technologies Area, Lawrence Berkeley National Laboratory, 1 Cyclotron Road, MS70-108B, Berkeley, California 94720, United States; Orcidhttp://orcid.org/0000-0002-7749-1624; Email: [email protected]
  • Authors
    • Drew Higgins - Department of Chemical Engineering, Stanford University, 443 Via Ortega Way, Stanford, California 94305, United States; SUNCAT Center for Interface Science and Catalysis, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, United States; Orcidhttp://orcid.org/0000-0002-0585-2670
    • Christopher Hahn - Department of Chemical Engineering, Stanford University, 443 Via Ortega Way, Stanford, California 94305, United States; SUNCAT Center for Interface Science and Catalysis, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, United States; Orcidhttp://orcid.org/0000-0002-2772-6341
    • Chengxiang Xiang - Division of Chemistry and Chemical Engineering, California Institute of Technology, 210 Noyes Laboratory, 127-72, Pasadena, California 91125, United States; Orcidhttp://orcid.org/0000-0002-1698-6754
  • Author Contributions

    D.H. and C.H.: Equal author contributions.

  • Notes

    The authors declare no competing financial interest.

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Drew Higgins

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Drew Higgins has been at Stanford University and SLAC National Laboratory since 2015, first as a Banting Postdoctoral Fellow and then Associate Staff Scientist working on electrochemical catalyst development, understanding and device integration. In January 2019, he starts a Faculty position at McMaster University in the Department of Chemical Engineering. https://www.higginslab.com/.

Christopher Hahn

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Christopher Hahn began his current position at SLAC National Accelerator Laboratory in 2015, where he is conducting research with the Joint Center for Artificial Photosynthesis on catalyst discovery and understanding reaction mechanisms for electrochemical CO2 reduction. https://suncat.stanford.edu/people/christopher-hahn.

Chengxiang Xiang

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Chengxiang (“CX”) Xiang and his team are working on development of testbed prototypes for photoelectrochemical CO2 reduction and water-splitting. http://sunlight.caltech.edu/cx/.

Thomas F. Jaramillo

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Thomas Jaramillo is an Associate Professor at Stanford and SLAC National Accelerator Laboratory and is a Thrust Coordinator in JCAP overseeing electrocatalysis research. His laboratory focuses on fundamental catalytic processes occurring on solid-state surfaces in both the production and consumption of energy. http://jaramillogroup.stanford.edu/.

Adam Z. Weber

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Adam Weber is currently a Staff Scientist at LBNL where he leads the Energy Conversion Group and is a Thrust Coordinator in JCAP overseeing continuum modeling, multicomponent integration, and test-bed construction and evaluation. He is a Fellow of the Electrochemical Society for his research on understanding electrochemical technologies. https://weberlab.lbl.gov/.

Acknowledgments

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This work was supported by the Joint Center for Artificial Photosynthesis, a DOE Energy Innovation Hub, supported through the Office of Science of the U.S. Department of Energy under Award Number DE-SC0004993.

References

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    OpenURL UNIV OF NORTH TEXAS
  4. 4
    Wang, L.; Nitopi, S. A.; Bertheussen, E.; Orazov, M.; Morales-Guio, C. G.; Liu, X.; Higgins, D. C.; Chan, K.; Nørskov, J. K.; Hahn, C. Electrochemical Carbon Monoxide Reduction on Polycrystalline Copper: Effects of Potential, Pressure, and PH on Selectivity toward Multicarbon and Oxygenated Products. ACS Catal. 2018, 8, 7445– 7454,  DOI: 10.1021/acscatal.8b01200
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    4
    Electrochemical Carbon Monoxide Reduction on Polycrystalline Copper: Effects of Potential, Pressure, and pH on Selectivity toward Multicarbon and Oxygenated Products
    Wang, Lei; Nitopi, Stephanie A.; Bertheussen, Erlend; Orazov, Marat; Morales-Guio, Carlos G.; Liu, Xinyan; Higgins, Drew C.; Chan, Karen; Noerskov, Jens K.; Hahn, Christopher; Jaramillo, Thomas F.
    ACS Catalysis (2018), 8 (8), 7445-7454CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
    Understanding the surface reactivity of CO, which is a key intermediate during electrochem. CO2 redn., is crucial for the development of catalysts that selectively target desired products for the conversion of CO2 to fuels and chems. In this study, a custom-designed electrochem. cell is utilized to investigate planar polycryst. copper as an electrocatalyst for CO redn. under alk. conditions. Seven major CO redn. products have been obsd. including various hydrocarbons and oxygenates which are also common CO2 redn. products, strongly indicating that CO is a key reaction intermediate for these further-reduced products. A comparison of CO and CO2 redn. demonstrates that there is a large decrease in the overpotential for C-C coupled products under CO redn. conditions. The effects of CO partial pressure and electrolyte pH are investigated; it is concluded that the aforementioned large potential shift is primarily a pH effect. Thus, alk. conditions can be used to increase the energy efficiency of CO and CO2 redn. to C-C coupled products, when these cathode reactions are coupled to the oxygen evolution reaction at the anode. Further anal. of the reaction products reveals common trends in selectivity that indicate both the prodn. of oxygenates and C-C coupled products are favored at lower overpotentials. These selectivity trends are generalized by comparing the results on planar Cu to current state-of-the-art high-surface-area Cu catalysts, which are able to achieve high oxygenate selectivity by operating at the same geometric c.d. at lower overpotentials. Combined, these findings outline key principles for designing CO and CO2 electrolyzers that are able to produce valuable C-C coupled products with high energy efficiency.
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    OpenURL UNIV OF NORTH TEXAS
  5. 5
    Singh, M. R.; Goodpaster, J. D.; Weber, A. Z.; Head-Gordon, M.; Bell, A. T. Mechanistic Insights into Electrochemical Reduction of CO2 over Ag Using Density Functional Theory and Transport Models. Proc. Natl. Acad. Sci. U. S. A. 2017, 114 (42), E8812,  DOI: 10.1073/pnas.1713164114
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    5
    Mechanistic insights into electrochemical reduction of CO2 over Ag using density functional theory and transport models
    Singh, Meenesh R.; Goodpaster, Jason D.; Weber, Adam Z.; Head-Gordon, Martin; Bell, Alexis T.
    Proceedings of the National Academy of Sciences of the United States of America (2017), 114 (42), E8812-E8821CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
    Electrochem. redn. of CO2 using renewable sources of elec. energy holds promise for converting CO2 to fuels and chems. Since this process is complex and involves a large no. of species and phys. phenomena, a comprehensive understanding of the factors controlling product distribution is required. While the most plausible reaction pathway is usually identified from quantum-chem. calcn. of the lowest free-energy pathway, this approach can be misleading when coverages of adsorbed species detd. for alternative mechanism differ significantly, since elementary reaction rates depend on the product of the rate coeff. and the coverage of species involved in the reaction. Also, cathode polarization can influence the kinetics of CO2 redn. Here, the authors present a multiscale framework for ab initio simulation of the electrochem. redn. of CO2 over an Ag(110) surface. A continuum model for species transport is combined with a microkinetic model for the cathode reaction dynamics. Free energies of activation for all elementary reactions are detd. from d. functional theory calcns. Using this approach, three alternative mechanisms for CO2 redn. were examd. The rate-limiting step in each mechanism is **COOH formation at higher neg. potentials. However, only via the multiscale simulation was it possible to identify the mechanism that leads to a dependence of the rate of CO formation on the partial pressure of CO2 that is consistent with expts. Simulations based on this mechanism also describe the dependence of the H2 and CO current densities on cathode voltage that are in strikingly good agreement with exptl. observation.
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    OpenURL UNIV OF NORTH TEXAS
  6. 6
    Lobaccaro, P.; Singh, M. R.; Clark, E. L.; Kwon, Y.; Bell, A. T.; Ager, J. W. Effects of Temperature and Gas-Liquid Mass Transfer on the Operation of Small Electrochemical Cells for the Quantitative Evaluation of CO2 Reduction Electrocatalysts. Phys. Chem. Chem. Phys. 2016, 18 (38), 26777– 26785,  DOI: 10.1039/C6CP05287H
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    6
    Effects of temperature and gas-liquid mass transfer on the operation of small electrochemical cells for the quantitative evaluation of CO2 reduction electrocatalysts
    Lobaccaro, Peter; Singh, Meenesh R.; Clark, Ezra Lee; Kwon, Youngkook; Bell, Alexis T.; Ager, Joel W.
    Physical Chemistry Chemical Physics (2016), 18 (38), 26777-26785CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    In the last few years, there has been increased interest in electrochem. CO2 redn. (CO2R). Many exptl. studies employ a membrane sepd., electrochem. cell with a mini H-cell geometry to characterize CO2R catalysts in aq. soln. This type of electrochem. cell is a mini-chem. reactor and it is important to monitor the reaction conditions within the reactor to ensure that they are const. throughout the study. We show that operating cells with high catalyst surface area to electrolyte vol. ratios (S/V) at high current densities can have subtle consequences due to the complexity of the phys. phenomena taking place on electrode surfaces during CO2R, particularly as they relate to the cell temp. and bulk electrolyte CO2 concn. Both effects were evaluated quant. in high S/V cells using Cu electrodes and a bicarbonate buffer electrolyte. Electrolyte temp. is a function of the current/total voltage passed through the cell and the cell geometry. Even at a very high c.d., 20 mA cm-2, the temp. increase was less than 4 °C and a decrease of <10% in the dissolved CO2 concn. is predicted. In contrast, limits on the CO2 gas-liq. mass transfer into the cells produce much larger effects. By using the pH in the cell to measure the CO2 concn., significant undersatn. of CO2 is obsd. in the bulk electrolyte, even at more modest current densities of 10 mA cm-2. Undersatn. of CO2 produces large changes in the faradaic efficiency obsd. on Cu electrodes, with H2 prodn. becoming increasingly favored. We show that the size of the CO2 bubbles being introduced into the cell is crit. for maintaining the equil. CO2 concn. in the electrolyte, and we have designed a high S/V cell that is able to maintain the near-equil. CO2 concn. at current densities up to 15 mA cm-2.
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    OpenURL UNIV OF NORTH TEXAS
  7. 7
    Hashiba, H.; Weng, L.-C.; Chen, Y.; Sato, H. K.; Yotsuhashi, S.; Xiang, C.; Weber, A. Z. Effects of Electrolyte Buffer Capacity on Surface Reactant Species and the Reaction Rate of CO2 in Electrochemical Co2 Reduction. J. Phys. Chem. C 2018, 122 (7), 3719– 3726,  DOI: 10.1021/acs.jpcc.7b11316
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    7
    Effects of Electrolyte Buffer Capacity on Surface Reactant Species and the Reaction Rate of CO2 in Electrochemical CO2 Reduction
    Hashiba, Hiroshi; Weng, Lien-Chun; Chen, Yikai; Sato, Hiroki K.; Yotsuhashi, Satoshi; Xiang, Chengxiang; Weber, Adam Z.
    Journal of Physical Chemistry C (2018), 122 (7), 3719-3726CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
    In the aq. electrochem. redn. of CO2, the choice of electrolyte is responsible for the catalytic activity and selectivity, although there remains a need for more in-depth understanding of electrolyte effects and mechanisms. In this study, using both exptl. and simulation approaches, how the buffer capacity of the electrolytes affects the kinetics and equil. of surface reactant species and the resulting reaction rate of CO2 with varying partial CO2 pressure are reported. Electrolytes investigated include KCl (non-buffered), KHCO3 (buffered by bicarbonate), and phosphate-buffered electrolytes. Assuming 100% methane prodn., the simulation successfully explains the exptl. trends in max. CO2 flux in KCl and KHCO3 and also highlights the difference between KHCO3 and phosphate in terms of pKa as well as the impact of the buffer capacity. To examine the electrolyte impact on selectivity, the model is run with a const. total c.d. Using this model, several factors are elucidated, including the importance of local pH, which is not in acid/base equil., the impact of buffer identity and kinetics, and the mass-transport boundary-layer thickness. The gained understanding can help to optimize CO2 redn. in aq. environments.
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    OpenURL UNIV OF NORTH TEXAS
  8. 8
    Hashiba, H.; Yotsuhashi, S.; Deguchi, M.; Yamada, Y. Systematic Analysis of Electrochemical CO2 Reduction with Various Reaction Parameters Using Combinatorial Reactors. ACS Comb. Sci. 2016, 18 (4), 203– 208,  DOI: 10.1021/acscombsci.6b00021
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    8
    Systematic Analysis of Electrochemical CO2 Reduction with Various Reaction Parameters using Combinatorial Reactors
    Hashiba, Hiroshi; Yotsuhashi, Satoshi; Deguchi, Masahiro; Yamada, Yuka
    ACS Combinatorial Science (2016), 18 (4), 203-208CODEN: ACSCCC; ISSN:2156-8944. (American Chemical Society)
    Applying combinatorial technol. to electrochem. CO2 redn. offers a broad range of possibilities for optimizing the reaction conditions. In this work, the CO2 pressure, stirring speed, and reaction temp. were varied to investigate the effect on the rate of CO2 supply to copper electrode and the assocd. effects on reaction products, including CH4. Expts. were performed in a 0.5 M KCl soln. using a combinatorial screening reactor system consisting of eight identical, automatically controlled reactors. Increasing the CO2 pressure and stirring speed, or decreasing the temp., steadily suppressed H2 prodn. and increased the prodn. of other reaction products including CH4 across a broad range of current densities. Our anal. shows that the CO2 pressure, stirring speed, and reaction temp. independently contributed to the limiting rate of CO2 supply to the electrode (Jlim). At a const. temp., the limiting c.d. of CH4 increased proportionally with Jlim, illustrating that the prodn. rate of CH4 was proportional to CO2 supply. Varying the CO2 pressure and stirring speed hardly affected the max. Faradaic efficiency of CH4 prodn. However, changes to the reaction temp. showed a significant contribution to CH4 selectivity. This study highlights the importance of quant. anal. of CO2 supply in clarifying the role of various reaction parameters and understanding more comprehensively the selectivity and reaction rate of electrochem. CO2 redn.
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  9. 9
    Ahn, S. T.; Abu-Baker, I.; Palmore, G. T. R. Electroreduction of CO2 on Polycrystalline Copper: Effect of Temperature on Product Selectivity. Catal. Today 2017, 288, 24– 29,  DOI: 10.1016/j.cattod.2016.09.028
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    9
    Electroreduction of CO2 on polycrystalline copper: Effect of temperature on product selectivity
    Ahn, Steven T.; Abu-Baker, Ismael; Palmore, G. Tayhas R.
    Catalysis Today (2017), 288 (), 24-29CODEN: CATTEA; ISSN:0920-5861. (Elsevier B.V.)
    The authors study the effect of temp. on product selectivity for the electrochem. redn. of CO2 on polycryst. Cu. The temp. of an electrolyte soln. can influence several reaction parameters in the CO2 redn. reaction (CO2RR), including pH (both local and bulk), concn. of dissolved CO2, soln. resistance, the rate of diffusion of reactants to the electrode surface, and adsorbed intermediates. The working potential was -1.60 V vs. Ag/AgCl to allow direct comparison with literature values. Under optimal reaction conditions at 2°, the faradaic efficiency (FE) for converting CO2 to methane (CH4) on a Cu electrocatalyst increases to ∼50% while ethylene (C2H4) decreases to ∼10%. Above room temp., the prodn. of H gas (H2) dominates the electrochem. reaction, reaching >50% FE. The major products (i.e., >5% FE) obsd. at all temps. studied were H2, CH4, C2H4, CO, and formate, with product selectivity driven by changes in [CO2] rather than changes in pH. The authors' initial goal was to confirm pH dependence of CO2RR on Cu at different temps. Importantly, pH varies minimally over the temp. range studied (2° to 42°), in contrast to changes in [CO2] and corresponding changes in product selectivity.
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    OpenURL UNIV OF NORTH TEXAS
  10. 10
    Liu, X.; Xiao, J.; Peng, H.; Hong, X.; Chan, K.; Nørskov, J. K. Understanding Trends in Electrochemical Carbon Dioxide Reduction Rates. Nat. Commun. 2017, 8, 15438,  DOI: 10.1038/ncomms15438
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    10
    Understanding trends in electrochemical carbon dioxide reduction rates
    Liu, Xinyan; Xiao, Jianping; Peng, Hongjie; Hong, Xin; Chan, Karen; Noerskov, Jens K.
    Nature Communications (2017), 8 (), 15438CODEN: NCAOBW; ISSN:2041-1723. (Nature Publishing Group)
    Electrochem. carbon dioxide redn. to fuels presents one of the great challenges in chem. Herein we present an understanding of trends in electrocatalytic activity for carbon dioxide redn. over different metal catalysts that rationalize a no. of exptl. observations including the selectivity with respect to the competing hydrogen evolution reaction. We also identify two design criteria for more active catalysts. The understanding is based on d. functional theory calcns. of activation energies for electrochem. carbon monoxide redn. as a basis for an electrochem. kinetic model of the process. We develop scaling relations relating transition state energies to the carbon monoxide adsorption energy and det. the optimal value of this descriptor to be very close to that of copper.
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    OpenURL UNIV OF NORTH TEXAS
  11. 11
    Rendón-Calle, A.; Builes, S.; Calle-Vallejo, F. A Brief Review of the Computational Modeling of CO2 Electroreduction on Cu Electrodes. Curr. Opin. Electrochem. 2018, 9, 158,  DOI: 10.1016/j.coelec.2018.03.012
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    11
    A brief review of the computational modeling of CO2 electroreduction on Cu electrodes
    Rendon-Calle, Alejandra; Builes, Santiago; Calle-Vallejo, Federico
    Current Opinion in Electrochemistry (2018), 9 (), 158-165CODEN: COEUCY; ISSN:2451-9111. (Elsevier B.V.)
    Electrochem. redn. of CO2 (CO2RR) to hydrocarbons and alcs. is a promising yet intricate catalytic process with large assocd. overpotentials. On Cu, the most active metal, factors such as pH, solvation, anions and cations in soln., in addn. to the catalysts' structure, modify the reaction mechanism and play an important role on the catalytic activity and selectivity. Such an extraordinary complexity calls for an in-depth understanding of CO2RR that eventually leads to its optimization. In this brief review, we illustrate how computational methods have aided in recent years to gain insight on CO2RR. We show the achievements and limitations of well-established methods based on Gibbs energy diagrams, calcns. in vacuum, the computational hydrogen electrode and scaling-relation-based volcano plots. Besides, we review advances on kinetics of electrochem. steps, structure-sensitive screening, ion effects, and machine learning.
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  12. 12
    Garza, A. J.; Bell, A. T.; Head-Gordon, M. Mechanism of CO2 Reduction at Copper Surfaces: Pathways to C2 Products. ACS Catal. 2018, 8 (2), 1490– 1499,  DOI: 10.1021/acscatal.7b03477
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    12
    Mechanism of CO2 Reduction at Copper Surfaces: Pathways to C2 Products
    Garza, Alejandro J.; Bell, Alexis T.; Head-Gordon, Martin
    ACS Catalysis (2018), 8 (2), 1490-1499CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
    From constraints from reported exptl. observations and d. functional theory simulations, the authors propose a mechanism for the redn. of CO2 to C2 products on Cu electrodes. To model the effects of an applied potential bias on the reactions, calcns. are carried out with a variable, fractional no. of electrons on the unit cell, which is optimized so that the Fermi level matches the actual chem. potential of electrons (i.e., the applied bias); an implicit electrolyte model allows for compensation of the surface charge so that neutrality is maintained in the overall simulation cell. The authors' mechanism explains the presence of the seven C2 species that were detected in the reaction, as well as other notable exptl. observations. Also, the authors' results shed light on the difference in activities toward C2 products between the (100) and (111) facets of Cu. The authors compare the authors' methodologies and findings with those in other recent mechanistic studies of the Cu-catalyzed CO2 redn. reaction.
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    OpenURL UNIV OF NORTH TEXAS
  13. 13
    Kortlever, R.; Shen, J.; Schouten, K. J. P.; Calle-Vallejo, F.; Koper, M. T. M. Catalysts and Reaction Pathways for the Electrochemical Reduction of Carbon Dioxide. J. Phys. Chem. Lett. 2015, 6 (20), 4073– 4082,  DOI: 10.1021/acs.jpclett.5b01559
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    13
    Catalysts and Reaction Pathways for the Electrochemical Reduction of Carbon Dioxide
    Kortlever, Ruud; Shen, Jing; Schouten, Klaas Jan P.; Calle-Vallejo, Federico; Koper, Marc T. M.
    Journal of Physical Chemistry Letters (2015), 6 (20), 4073-4082CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
    A review. The electrochem. redn. of CO2 has gained significant interest recently as it has the potential to trigger a sustainable solar-fuel-based economy. In this Perspective, we highlight several heterogeneous and mol. electrocatalysts for the redn. of CO2 and discuss the reaction pathways through which they form various products. Among those, copper is a unique catalyst as it yields hydrocarbon products, mostly methane, ethylene, and ethanol, with acceptable efficiencies. As a result, substantial effort has been invested to det. the special catalytic properties of copper and to elucidate the mechanism through which hydrocarbons are formed. These mechanistic insights, together with mechanistic insights of CO2 redn. on other metals and mol. complexes, can provide crucial guidelines for the design of future catalyst materials able to efficiently and selectively reduce CO2 to useful products.
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    OpenURL UNIV OF NORTH TEXAS
  14. 14
    Clark, E. L.; Hahn, C.; Jaramillo, T. F.; Bell, A. T. Electrochemical CO2 Reduction over Compressively Strained Cuag Surface Alloys with Enhanced Multi-Carbon Oxygenate Selectivity. J. Am. Chem. Soc. 2017, 139 (44), 15848– 15857,  DOI: 10.1021/jacs.7b08607
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    14
    Electrochemical CO2 Reduction over Compressively Strained CuAg Surface Alloys with Enhanced Multi-Carbon Oxygenate Selectivity
    Clark, Ezra L.; Hahn, Christopher; Jaramillo, Thomas F.; Bell, Alexis T.
    Journal of the American Chemical Society (2017), 139 (44), 15848-15857CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    The electrochem. redn. of carbon dioxide using renewably generated electricity offers a potential means for producing fuels and chems. in a sustainable manner. To date, copper has been found to be the most effective catalyst for electrochem. reducing carbon dioxide to products such as methane, ethene, and ethanol. Unfortunately, the current efficiency of the process is limited by competition with the relatively facile hydrogen evolution reaction. Since multi-carbon products are more valuable precursors to chems. and fuels than methane, there is considerable interest in modifying copper to enhance the multi-carbon product selectivity. Here, we report our investigations of electrochem. carbon dioxide redn. over CuAg bimetallic electrodes and surface alloys, which we find to be more selective for the formation of multi-carbon products than pure copper. This selectivity enhancement is a result of the selective suppression of hydrogen evolution, which occurs due to compressive strain induced by the formation of a CuAg surface alloy. Furthermore, we report that these bimetallic electrocatalysts exhibit an unusually high selectivity for the formation of multi-carbon carbonyl-contg. products, which we hypothesize to be the consequence of a reduced coverage of adsorbed hydrogen and the reduced oxophilicity of the compressively strained copper. Thus, we show that promoting copper surface with small amts. of Ag is a promising means for improving the multi-carbon oxygenated product selectivity of copper during electrochem. CO2 redn.
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  15. 15
    Han, Z.; Kortlever, R.; Chen, H.-Y.; Peters, J. C.; Agapie, T. CO2 Reduction Selective for C ≥ 2 Products on Polycrystalline Copper with N-Substituted Pyridinium Additives. ACS Cent. Sci. 2017, 3 (8), 853– 859,  DOI: 10.1021/acscentsci.7b00180
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    15
    CO2 Reduction Selective for C≥2 Products on Polycrystalline Copper with N-Substituted Pyridinium Additives
    Han, Zhiji; Kortlever, Ruud; Chen, Hsiang-Yun; Peters, Jonas C.; Agapie, Theodor
    ACS Central Science (2017), 3 (8), 853-859CODEN: ACSCII; ISSN:2374-7951. (American Chemical Society)
    Electrocatalytic CO2 redn. to generate multicarbon products is of interest for applications in artificial photosynthetic schemes. This is a particularly attractive goal for CO2 redn. by Cu electrodes, where a broad range of hydrocarbon products can be generated but where selectivity for C-C coupled products relative to CH4 and H2 remains an impediment. Herein the authors report a simple yet highly selective catalytic system for CO2 redn. to C≥2 hydrocarbons on a polycryst. Cu electrode in bicarbonate aq. soln. that uses N-substituted pyridinium additives. Selectivities of 70-80% for C2 and C3 products with a hydrocarbon ratio of C≥2/CH4 significantly >100 were obsd. with several additives. 13C-labeling studies verify CO2 to be the sole C source in the C≥2 hydrocarbons produced. Upon electroredn., the N-substituted pyridinium additives lead to film deposition on the Cu electrode, identified in one case as the reductive coupling product of N-arylpyridinium. Product selectivity can also be tuned from C≥2 species to H2 (∼90%) while suppressing methane with certain N-heterocyclic additives.
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    OpenURL UNIV OF NORTH TEXAS
  16. 16
    Higgins, D.; Landers, A. T.; Ji, Y.; Nitopi, S.; Morales-Guio, C. G.; Wang, L.; Chan, K.; Hahn, C.; Jaramillo, T. F. Guiding Electrochemical Carbon Dioxide Reduction toward Carbonyls Using Copper Silver Thin Films with Interphase Miscibility. ACS Energy Lett. 2018, 3, 2947– 2955,  DOI: 10.1021/acsenergylett.8b01736
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    16
    Guiding Electrochemical Carbon Dioxide Reduction toward Carbonyls Using Copper Silver Thin Films with Interphase Miscibility
    Higgins, Drew; Landers, Alan T.; Ji, Yongfei; Nitopi, Stephanie; Morales-Guio, Carlos G.; Wang, Lei; Chan, Karen; Hahn, Christopher; Jaramillo, Thomas F.
    ACS Energy Letters (2018), 3 (12), 2947-2955CODEN: AELCCP; ISSN:2380-8195. (American Chemical Society)
    Steering the selectivity of Cu-based electrochem. CO2 redn. (CO2R) catalysts toward targeted products will serve to improve the technoeconomic outlook of technologies based on this process. Using phys. vapor deposition as a tool to overcome thermodn. miscibility limitations, CuAg thin films with nonequil. Cu/Ag alloying were prepd. for CO2R performance evaluation. In comparison to pure Cu, the CuAg thin films showed significantly higher activity and selectivity toward liq. carbonyl products, including acetaldehyde and acetate. Suppressed activity and selectivity toward hydrocarbons and the competing H evolution were also demonstrated on CuAg thin films, with a greater degree of suppression obsd. at increasing nominal Ag compns. Compositional-dependent CO2R trends coupled with phys. characterization and d. functional theory suggest that significant miscibility of Ag into the Cu-rich phase of the catalyst underpinned the obsd. CO2R trends through tuning of adsorbate and reaction intermediate binding energies on the surface.
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  17. 17
    Raciti, D.; Wang, C. Recent Advances in CO2 Reduction Electrocatalysis on Copper. ACS Energy Lett. 2018, 3 (7), 1545– 1556,  DOI: 10.1021/acsenergylett.8b00553
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    17
    Recent Advances in CO2 Reduction Electrocatalysis on Copper
    Raciti, David; Wang, Chao
    ACS Energy Letters (2018), 3 (7), 1545-1556CODEN: AELCCP; ISSN:2380-8195. (American Chemical Society)
    Electroredn. of CO2 represents a promising approach toward artificial C recycling for addressing global challenges in energy and sustainability. The foreground of this approach is dependent on the development of efficient electrocatalysts capable of selectively reducing CO2 to valuable (oxygenated) hydrocarbon products at low overpotentials. Here, we present an overview of the recent developments of Cu electrocatalysts for CO2 redn. The focus is placed on elucidation of the structure-property relations of monometallic Cu electrocatalysts, which is believed to be the foundation for understanding alloys and other more complex catalytic systems. Reported mechanisms are discussed in terms of grain boundaries, open facets, residual oxides, subsurface O, local pH effect, etc. After this discussion, remaining questions are raised for further development of advanced electrocatalysts for energy and chem. efficient CO2 redn.
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    OpenURL UNIV OF NORTH TEXAS
  18. 18
    De Luna, P.; Wei, J.; Bengio, Y.; Aspuru-Guzik, A.; Sargent, E. Use Machine Learning to Find Energy Materials. Nature 2017, 552 (7683), 23,  DOI: 10.1038/d41586-017-07820-6
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    18
    Use machine learning to find energy materials
    De Luna, Phil; Wei, Jennifer; Bengio, Yoshua; Aspuru-Guzik, Alan; Sargent, Edward
    Nature (London, United Kingdom) (2017), 552 (7683), 23-27CODEN: NATUAS; ISSN:0028-0836. (Nature Research)
    Artificial intelligence can speed up research into new photovoltaic, battery and carbon-capture materials, argue Edward Sargent, Ala´n Aspuru-Guzikand colleagues.
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    OpenURL UNIV OF NORTH TEXAS
  19. 19
    Ulissi, Z. W.; Tang, M. T.; Xiao, J.; Liu, X.; Torelli, D. A.; Karamad, M.; Cummins, K.; Hahn, C.; Lewis, N. S.; Jaramillo, T. F. Machine-Learning Methods Enable Exhaustive Searches for Active Bimetallic Facets and Reveal Active Site Motifs for CO2 Reduction. ACS Catal. 2017, 7 (10), 6600– 6608,  DOI: 10.1021/acscatal.7b01648
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    19
    Machine-Learning Methods Enable Exhaustive Searches for Active Bimetallic Facets and Reveal Active Site Motifs for CO2 Reduction
    Ulissi, Zachary W.; Tang, Michael T.; Xiao, Jianping; Liu, Xinyan; Torelli, Daniel A.; Karamad, Mohammadreza; Cummins, Kyle; Hahn, Christopher; Lewis, Nathan S.; Jaramillo, Thomas F.; Chan, Karen; Noerskov, Jens K.
    ACS Catalysis (2017), 7 (10), 6600-6608CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
    Bimetallic catalysts are promising for the most difficult thermal and electrochem. reactions but modeling the many diverse active sites on polycryst. samples is an open challenge. The authors present a general framework for addressing this complexity in a systematic and predictive fashion. Active sites for every stable low-index facet of a bimetallic crystal are enumerated and cataloged yielding hundreds of possible active sites. The activity of these sites is explored in parallel using a neural-network based surrogate model to share information between the many D. Functional Theory (DFT) relaxations, resulting in activity ests. with an order of magnitude fewer explicit DFT calcns. Sites with interesting activity were found and provide targets for follow-up calcns. This process was applied to the electrochem. redn. of CO2 on Ni Ga bimetallics and indicated that most facets had similar activity to Ni surfaces, but a few exposed Ni sites with a very favorable on-top CO configuration. This motif emerged naturally from the predictive modeling and represents a class of intermetallic CO2 redn. catalysts. These sites rationalize recent exptl. reports of Ni Ga activity and why previous materials screens missed this exciting material. Most importantly these methods suggest that bimetallic catalysts will be discovered by studying facet reactivity and diversity of active sites more systematically.
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    OpenURL UNIV OF NORTH TEXAS
  20. 20
    Singh, M. R.; Clark, E. L.; Bell, A. T. Effects of Electrolyte, Catalyst, and Membrane Composition and Operating Conditions on the Performance of Solar-Driven Electrochemical Reduction of Carbon Dioxide. Phys. Chem. Chem. Phys. 2015, 17 (29), 18924– 18936,  DOI: 10.1039/C5CP03283K
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    20
    Effects of electrolyte, catalyst, and membrane composition and operating conditions on the performance of solar-driven electrochemical reduction of carbon dioxide
    Singh, Meenesh R.; Clark, Ezra L.; Bell, Alexis T.
    Physical Chemistry Chemical Physics (2015), 17 (29), 18924-18936CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    Solar-driven electrochem. cells can be used to convert carbon dioxide, water, and sunlight into transportation fuels or into precursors to such fuels. The voltage efficiency of such devices depends on the (i) phys. properties of its components (catalysts, electrolyte, and membrane); (ii) operating conditions (carbon dioxide flowrate and pressure, c.d.); and (iii) phys. dimensions of the cell. The sources of energy loss in a carbon dioxide redn. (CO2R) cell are the anode and cathode overpotentials, the difference in pH between the anode and cathode, the difference in the partial pressure of carbon dioxide between the bulk electrolyte and the cathode, the ohmic loss across the electrolyte and the diffusional resistances across the boundary layers near the electrodes. In this study, we analyze the effects of these losses and propose optimal device configurations for the efficient operation of a CO2R electrochem. cell operating at a c.d. of 10 mA cm-2. Cell operation at near-neutral bulk pH offers not only lower polarization losses but also better selectivity to CO2R vs. hydrogen evolution. Addn. of supporting electrolyte to increase its cond. has a neg. impact on cell performance because it reduces the elec. field and the soly. of CO2. Addn. of a pH buffer reduces the polarization losses but may affect catalyst selectivity. The carbon dioxide flowrate and partial pressure can have severe effects on the cell efficiency if the carbon dioxide supply rate falls below the consumption rate. The overall potential losses can be reduced by use of an anion, rather than a cation, exchange membrane. We also show that the max. polarization losses occur for the electrochem. synthesis of CO and that such losses are lower for the synthesis of products requiring a larger no. of electrons per mol., assuming a fixed c.d. We also find that the reported electrocatalytic activity of copper below -1 V vs. RHE is strongly influenced by excessive polarization of the cathode and, hence, does not represent its true activity at bulk conditions. This article provides useful guidelines for minimizing polarization losses in solar-driven CO2R electrochem. cells and a method for predicting polarization losses and obtaining kinetic overpotentials from measured partial current densities.
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    OpenURL UNIV OF NORTH TEXAS
  21. 21
    Greenblatt, J. B.; Miller, D. J.; Ager, J. W.; Houle, F. A.; Sharp, I. D. The Technical and Energetic Challenges of Separating (Photo)Electrochemical Carbon Dioxide Reduction Products. Joule 2018, 2 (3), 381– 420,  DOI: 10.1016/j.joule.2018.01.014
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    21
    The Technical and Energetic Challenges of Separating (Photo)Electrochemical Carbon Dioxide Reduction Products
    Greenblatt, Jeffery B.; Miller, Daniel J.; Ager, Joel W.; Houle, Frances A.; Sharp, Ian D.
    Joule (2018), 2 (3), 381-420CODEN: JOULBR; ISSN:2542-4351. (Cell Press)
    Known catalysts for (photo)electrochem. carbon dioxide (CO2) redn. typically generate multiple products, including hydrogen, carbon monoxide, hydrocarbons, and oxygenates, making product sepn. a ubiquitous, yet often overlooked, challenge. Here, we review CO2 redn. products using available catalysts and discuss approaches for product sepn. along with ests. of sepn. energy requirements. We illustrate potential complexities and discuss opportunities to minimize sepns. by utilizing product mixts. We also examine potential CO2 sources, their energy requirements, and net CO2 emissions. Finally, we discuss use of waste energy sources and integrate this information into an overall energy balance assessment. Using a common sustainability metric, energy return on energy investment (EROEI), we find that an EROEI of ∼2.0 may be possible, before including sepn. and CO2 prodn. energy. For EROEI to remain above one (the break-even point), these addnl. energy requirements, including embodied energy of equipment, must be no greater than half of the product energy.
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  22. 22
    Weekes, D. M.; Salvatore, D. A.; Reyes, A.; Huang, A.; Berlinguette, C. P. Electrolytic CO2 Reduction in a Flow Cell. Acc. Chem. Res. 2018, 51 (4), 910– 918,  DOI: 10.1021/acs.accounts.8b00010
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    22
    Electrolytic CO2 Reduction in a Flow Cell
    Weekes, David M.; Salvatore, Danielle A.; Reyes, Angelica; Huang, Aoxue; Berlinguette, Curtis P.
    Accounts of Chemical Research (2018), 51 (4), 910-918CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)
    A review. Electrocatalytic CO2 conversion at near ambient temps. and pressures offers a potential means of converting waste greenhouse gases into fuels or commodity chems. (e.g., CO, formic acid, methanol, ethylene, alkanes, and alcs.). This process is particularly compelling when driven by excess renewable electricity because the consequent prodn. of solar fuels would lead to a closing of the carbon cycle. However, such a technol. is not currently com. available. While CO2 electrolysis in H-cells is widely used for screening electrocatalysts, these expts. generally do not effectively report on how CO2 electrocatalysts behave in flow reactors that are more relevant to a scalable CO2 electrolyzer system. Flow reactors also offer more control over reagent delivery, which includes enabling the use of a gaseous CO2 feed to the cathode of the cell. This setup provides a platform for generating much higher current densities (J) by reducing the mass transport issues inherent to the H-cells.In this Account, we examine some of the systems-level strategies that have been applied in an effort to tailor flow reactor components to improve electrocatalytic redn. Flow reactors that have been utilized in CO2 electrolysis schemes can be categorized into two primary architectures: Membrane-based flow cells and microfluidic reactors. Each invoke different dynamic mechanisms for the delivery of gaseous CO2 to electrocatalytic sites, and both have been demonstrated to achieve high current densities (J > 200 mA cm-2) for CO2 redn. One strategy common to both reactor architectures for improving J is the delivery of CO2 to the cathode in the gas phase rather than dissolved in a liq. electrolyte. This phys. facet also presents a no. of challenges that go beyond the nature of the electrocatalyst, and we scrutinize how the judicious selection and modification of certain components in microfluidic and/or membrane-based reactors can have a profound effect on electrocatalytic performance. In membrane-based flow cells, for example, the choice of either a cation exchange membrane (CEM), anion exchange membrane (AEM), or a bipolar membrane (BPM) affects the kinetics of ion transport pathways and the range of applicable electrolyte conditions. In microfluidic cells, extensive studies have been performed upon the properties of porous carbon gas diffusion layers, materials that are equally relevant to membrane reactors. A theme that is pervasive throughout our analyses is the challenges assocd. with precise and controlled water management in gas phase CO2 electrolyzers, and we highlight studies that demonstrate the importance of maintaining adequate flow cell hydration to achieve sustained electrolysis.
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  23. 23
    Jhong, H.-R. M.; Ma, S.; Kenis, P. J. A. Electrochemical Conversion of CO2 to Useful Chemicals: Current Status, Remaining Challenges, and Future Opportunities. Curr. Opin. Chem. Eng. 2013, 2 (2), 191– 199,  DOI: 10.1016/j.coche.2013.03.005
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  24. 24
    Dinh, C.-T.; Burdyny, T.; Kibria, M. G.; Seifitokaldani, A.; Gabardo, C. M.; García de Arquer, F. P.; Kiani, A.; Edwards, J. P.; De Luna, P.; Bushuyev, O. S. CO2 Electroreduction to Ethylene Via Hydroxide-Mediated Copper Catalysis at an Abrupt Interface. Science 2018, 360 (6390), 783– 787,  DOI: 10.1126/science.aas9100
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    24
    Carbon dioxide electroreduction to ethylene via hydroxide-mediated copper catalysis at an abrupt interface
    Dinh, Cao-Thang; Burdyny, Thomas; Kibria, Md Golam; Seifitokaldani, Ali; Gabardo, Christine M.; Garcia de Arquer, F. Pelayo; Kiani, Amirreza; Edwards, Jonathan P.; De Luna, Phil; Bushuyev, Oleksandr S.; Zou, Chengqin; Quintero-Bermudez, Rafael; Pang, Yuanjie; Sinton, David; Sargent, Edward H.
    Science (Washington, DC, United States) (2018), 360 (6390), 783-787CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    Carbon dioxide (CO2) electroredn. could provide a useful source of ethylene, but low conversion efficiency, low prodn. rates, and low catalyst stability limit current systems. Here we report that a copper electrocatalyst at an abrupt reaction interface in an alk. electrolyte reduces CO2 to ethylene with 70% faradaic efficiency at a potential of -0.55 V vs. a reversible hydrogen electrode (RHE). Hydroxide ions on or near the copper surface lower the CO2 redn. and carbon monoxide (CO)-CO coupling activation energy barriers; as a result, onset of ethylene evolution at -0.165 V vs. an RHE in 10 M potassium hydroxide occurs almost simultaneously with CO prodn. Operational stability was enhanced via the introduction of a polymer-based gas diffusion layer that sandwiches the reaction interface between sep. hydrophobic and conductive supports, providing const. ethylene selectivity for an initial 150 operating hours.
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  25. 25
    Ma, S.; Sadakiyo, M.; Luo, R.; Heima, M.; Yamauchi, M.; Kenis, P. J. A. One-Step Electrosynthesis of Ethylene and Ethanol from CO2 in an Alkaline Electrolyzer. J. Power Sources 2016, 301, 219– 228,  DOI: 10.1016/j.jpowsour.2015.09.124
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    25
    One-step electrosynthesis of ethylene and ethanol from CO2 in an alkaline electrolyzer
    Ma, Sichao; Sadakiyo, Masaaki; Luo, Raymond; Heima, Minako; Yamauchi, Miho; Kenis, Paul J. A.
    Journal of Power Sources (2016), 301 (), 219-228CODEN: JPSODZ; ISSN:0378-7753. (Elsevier B.V.)
    Electroredn. of CO2 has potential for storing otherwise wasted intermittent renewable energy, while reducing emission of CO2 into the atm. Identifying robust and efficient electrocatalysts and assocd. optimum operating conditions to produce hydrocarbons at high energetic efficiency (low overpotential) remains a challenge. In this study, four Cu nanoparticle catalysts of different morphol. and compn. (amt. of surface oxide) are synthesized and their activities towards CO2 redn. are characterized in an alk. electrolyzer. Use of catalysts with large surface roughness results in a combined Faradaic efficiency (46%) for the electroredn. of CO2 to ethylene and ethanol in combination with current densities of ∼200 mA cm-2, a 10-fold increase in performance achieved at much lower overpotential (only < 0.7 V) compared to prior work. Compared to prior work, the high prodn. levels of ethylene and ethanol can be attributed mainly to the use of alk. electrolyte to improve kinetics and the suppressed evolution of H2, as well as the application of gas diffusion electrodes covered with active and rough Cu nanoparticles in the electrolyzer. These high performance levels and the gained fundamental understanding on Cu-based catalysts bring electrochem. redn. processes such as presented here closer to practical application.
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  26. 26
    Hoang, T. T. H.; Verma, S.; Ma, S.; Fister, T. T.; Timoshenko, J.; Frenkel, A. I.; Kenis, P. J. A.; Gewirth, A. A. Nanoporous Copper–Silver Alloys by Additive-Controlled Electrodeposition for the Selective Electroreduction of CO2 to Ethylene and Ethanol. J. Am. Chem. Soc. 2018, 140 (17), 5791– 5797,  DOI: 10.1021/jacs.8b01868
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    26
    Nanoporous Copper-Silver Alloys by Additive-Controlled Electrodeposition for the Selective Electroreduction of CO2 to Ethylene and Ethanol
    Hoang, Thao T. H.; Verma, Sumit; Ma, Sichao; Fister, Tim T.; Timoshenko, Janis; Frenkel, Anatoly I.; Kenis, Paul J. A.; Gewirth, Andrew A.
    Journal of the American Chemical Society (2018), 140 (17), 5791-5797CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Electrodeposition of CuAg alloy films from plating baths contg. 3,5-diamino-1,2,4-triazole (DAT) as an inhibitor yields high surface area catalysts for the active and selective electroredn. of CO2 to multicarbon hydrocarbons and oxygenates. EXAFS shows the co-deposited alloy film to be homogeneously mixed. The alloy film contg. 6% Ag exhibits the best CO2 electroredn. performance, with the faradaic efficiency for C2H4 and EtOH prodn. reaching nearly 60 and 25%, resp., at a cathode potential of just -0.7 V vs. RHE and a total c.d. of approx. - 300 mA/cm2. Such high levels of selectivity at high activity and low applied potential are the highest reported to date. In situ Raman and electroanal. studies suggest the origin of the high selectivity toward C2 products to be a combined effect of the enhanced stabilization of the Cu2O overlayer and the optimal availability of the CO intermediate due to the Ag incorporated in the alloy.
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  27. 27
    Zhuang, T.-T.; Liang, Z.-Q.; Seifitokaldani, A.; Li, Y.; De Luna, P.; Burdyny, T.; Che, F.; Meng, F.; Min, Y.; Quintero-Bermudez, R. Steering Post-C–C Coupling Selectivity Enables High Efficiency Electroreduction of Carbon Dioxide to Multi-Carbon Alcohols. Nature Catalysis 2018, 1 (6), 421– 428,  DOI: 10.1038/s41929-018-0084-7
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    OpenURL UNIV OF NORTH TEXAS
  28. 28
    Lv, J.-J.; Jouny, M.; Luc, W.; Zhu, W.; Zhu, J.-J.; Jiao, F. A Highly Porous Copper Electrocatalyst for Carbon Dioxide Reduction. Adv. Mater. 2018, 30 (49), 1803111,  DOI: 10.1002/adma.201803111
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    OpenURL UNIV OF NORTH TEXAS
  29. 29
    Verma, S.; Hamasaki, Y.; Kim, C.; Huang, W.; Lu, S.; Jhong, H.-R. M.; Gewirth, A. A.; Fujigaya, T.; Nakashima, N.; Kenis, P. J. A. Insights into the Low Overpotential Electroreduction of CO2 to CO on a Supported Gold Catalyst in an Alkaline Flow Electrolyzer. ACS Energy Lett. 2018, 3 (1), 193– 198,  DOI: 10.1021/acsenergylett.7b01096
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    29
    Insights into the Low Overpotential Electroreduction of CO2 to CO on a Supported Gold Catalyst in an Alkaline Flow Electrolyzer
    Verma, Sumit; Hamasaki, Yuki; Kim, Chaerin; Huang, Wenxin; Lu, Shawn; Jhong, Huei-Ru Molly; Gewirth, Andrew A.; Fujigaya, Tsuyohiko; Nakashima, Naotoshi; Kenis, Paul J. A.
    ACS Energy Letters (2018), 3 (1), 193-198CODEN: AELCCP; ISSN:2380-8195. (American Chemical Society)
    Cost competitive electroredn. of CO2 to CO requires electrochem. systems that exhibit partial c.d. (jCO) exceeding 150 mAcm-2 at cell overpotentials (|ηcell|) <1 V. However, achieving such benchmarks remains difficult. Here, the authors report the electroredn. of CO2 on a supported Au catalyst in an alk. flow electrolyzer with performance levels close to the economic viability criteria. Onset of CO prodn. occurred at cell and cathode overpotentials of just -0.25 and -0.02 V, resp. High jCO (∼99, 158 mAcm-2) was obtained at low |ηcell| (∼0.70, 0.94 V) and high CO energetic efficiency (∼63.8, 49.4%). The performance was stable for at least 8 h. Addnl., the onset cathode potentials, kinetic isotope effect, and Tafel slopes indicate the low overpotential prodn. of CO in alk. media to be the result of a pH-independent rate-detg. step (i.e., electron transfer) in contrast to a pH-dependent overall process.
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    OpenURL UNIV OF NORTH TEXAS
  30. 30
    Ma, S.; Luo, R.; Gold, J. I.; Yu, A. Z.; Kim, B.; Kenis, P. J. A. Carbon Nanotube Containing Ag Catalyst Layers for Efficient and Selective Reduction of Carbon Dioxide. J. Mater. Chem. A 2016, 4 (22), 8573– 8578,  DOI: 10.1039/C6TA00427J
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    30
    Carbon nanotube containing Ag catalyst layers for efficient and selective reduction of carbon dioxide
    Ma, Sichao; Luo, Raymond; Gold, Jake I.; Yu, Aaron Z.; Kim, Byoungsu; Kenis, Paul J. A.
    Journal of Materials Chemistry A: Materials for Energy and Sustainability (2016), 4 (22), 8573-8578CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
    Over the last few decades significant progress was made in the development of catalysts for efficient and selective electroredn. of CO2. These improvements in catalyst performance were of the extent that identifying electrodes of optimum structure and compn. has become key to further improve throughput levels in the electrolysis of CO2 to CO. Here the authors report on a simple 1-step method to incorporate multi-walled C nanotubes (MWCNT) in the catalyst layer to form gas diffusion electrodes with different structures: (i) a mixed catalyst layer in which the Ag nanoparticle catalyst and MWCNTs are homogeneously distributed; and (ii) a layered catalyst layer comprised of a layer of MWCNTs covered with a layer of Ag catalyst. Both approaches improve performance in the electroredn. of CO2 compared to electrodes that lack MWCNTs. The mixed layer performed best: an electrolyzer operated at a cell potential of -3 V using 1 M KOH as the electrolyte yielded unprecedented high levels of CO prodn. of up to 350 mA cm-2 at high faradaic efficiency (>95% selective for CO) and an energy efficiency of 45% under the same condition. Electrochem. impedance spectroscopy measurements indicate that the obsd. differences in electrode performance can be attributed to a lower charge transfer resistance in the mixed catalyst layer. A simple optimization of electrode structure and compn., i.e. incorporation of MWCNTs in the catalyst layer of a GDE, has a profound beneficial effect on their performance in electrocatalytic conversion of CO2, while allowing for a lower precious metal catalyst loading with improved performance.
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  31. 31
    Dufek, E. J.; Lister, T. E.; Stone, S. G.; McIlwain, M. E. Operation of a Pressurized System for Continuous Reduction of CO2. J. Electrochem. Soc. 2012, 159 (9), F514– F517,  DOI: 10.1149/2.011209jes
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    31
    Operation of a pressurized system for continuous reduction of CO2
    Dufek, Eric J.; Lister, Tedd E.; Stone, Simon G.; McIlwain, Michael E.
    Journal of the Electrochemical Society (2012), 159 (9), F514-F517CODEN: JESOAN; ISSN:0013-4651. (Electrochemical Society)
    A pressurized electrochem. system equipped for continuous redn. of CO2 is presented. At elevated pressures, using a Ag-based cathode, the quantity of CO which can be generated is 5 times that obsd. at ambient pressure with faradaic efficiencies ≤92% obsd. at 350 mA cm-2. For operation at 225 mA cm-2 and 60° the cell voltage at 18.5 atm was 0.4 V below that obsd. at ambient pressure. Increasing the temp. further to 90° led to a cell voltage <3 V (18.5 atm and 90 °C), which equates to an elec. efficiency of 50%.
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  32. 32
    Haas, T.; Krause, R.; Weber, R.; Demler, M.; Schmid, G. Technical Photosynthesis Involving CO2 Electrolysis and Fermentation. Nature Catalysis 2018, 1 (1), 32– 39,  DOI: 10.1038/s41929-017-0005-1
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    Whipple, D. T.; Finke, E. C.; Kenis, P. J. A. Microfluidic Reactor for the Electrochemical Reduction of Carbon Dioxide: The Effect of Ph. Electrochem. Solid-State Lett. 2010, 13 (9), B109– B111,  DOI: 10.1149/1.3456590
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    Microfluidic reactor for the electrochemical reduction of carbon dioxide: The effect of pH
    Whipple, Devin T.; Finke, Eryn C.; Kenis, Paul J. A.
    Electrochemical and Solid-State Letters (2010), 13 (9), B109-B111CODEN: ESLEF6; ISSN:1099-0062. (Electrochemical Society)
    This article reports the development and characterization of a microfluidic reactor for the electrochem. redn. of CO2. The use of gas diffusion electrodes enables better control of the three-phase interface where the reactions take place. Furthermore, the versatility of the microfluidic reactor enables rapid evaluation of catalysts under different operating conditions. Several catalysts as well as the effects of electrolyte pH on reactor efficiency for redn. of CO2 to formic acid were tested. Operating at acidic pH resulted in a significant increase in performance: Faradaic and energetic efficiencies of 89 and 45%, resp., and c.d. of ≈100 mA/cm2.
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  34. 34
    Lu, X.; Leung, D. Y. C.; Wang, H.; Xuan, J. A High Performance Dual Electrolyte Microfluidic Reactor for the Utilization of CO2. Appl. Energy 2017, 194, 549– 559,  DOI: 10.1016/j.apenergy.2016.05.091
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    34
    A high performance dual electrolyte microfluidic reactor for the utilization of CO2
    Lu, Xu; Leung, Dennis Y. C.; Wang, Huizhi; Xuan, Jin
    Applied Energy (2017), 194 (), 549-559CODEN: APENDX; ISSN:0306-2619. (Elsevier Ltd.)
    The pH-differential membraneless architecture could enhance the thermodn. property and raise the electrochem. performance of a dual electrolyte microfluidic reactor (DEMR) for electrochem. conversion of CO2. Freed from hindrances of membrane structure and thermodn. limitation, DEMR demonstrates the possibility of altering anolyte and catholyte pHs to achieve higher reactivity rates and efficiencies. Different operation condition parameters of a microfluidic network would affect the reactor performance to a certain extents, constraining further improvement. Therefore, we conducted exptl. anal. to study the mechanisms and intrinsic correlations of catalyst to Nafion ratio, microchannel thickness, electrolyte flow rate and CO2 supply for an optimized outcome. A comprehensive investigation on the cell durability was also carried out in the way of repetitiveness and long period operation, regarding both reactivity and efficiency. It was found that the catalyst to Nafion ratio affects the performance in a parabolic relation and there exists optimal values of electrolyte flow rate and microfluidic channel thickness for maximized cell performance. The influence of the reactant CO2 supply rate is not significant above a certain level where kinetics limitation is not dominant. The parametric study provides an operational point of view on the dual electrolyte microfluidic reactor and serves as a tool for DEMR optimization design.
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    OpenURL UNIV OF NORTH TEXAS
  35. 35
    Li, H.; Oloman, C. Development of a Continuous Reactor for the Electro-Reduction of Carbon Dioxide to Formate – Part 2: Scale-Up. J. Appl. Electrochem. 2007, 37 (10), 1107– 1117,  DOI: 10.1007/s10800-007-9371-8
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    35
    Development of a continuous reactor for the electro-reduction of carbon dioxide to formate
    Li, Hui; Oloman, Colin
    Journal of Applied Electrochemistry (2007), 37 (10), 1107-1117CODEN: JAELBJ; ISSN:0021-891X. (Springer)
    This paper reports exptl. and modeling work for the lab. scale-up of continuous "trickle-bed" reactors for the electro-redn. of CO2 to potassium formate. Two reactors (A and B) were employed, with particulate tin 3D cathodes of superficial areas, resp., 45 × 10-4 (2-14 A) and 320 × 10-4 m2 (20-100 A). Expts. in Reactor A using granulated tin cathodes (99.9 wt% Sn) and a feed gas of 100% CO2 showed slightly better performance than that of the tinned-copper mesh cathodes of our previous communications, while giving substantially improved temporal stability (200 vs. 20 min). The seven-fold scaled-up Reactor B used a feed gas of 100% CO2 with the aq. catholyte and anolyte, resp. [0.5 M KHCO3 + 2 M KCl] and 2 M KOH, at inlet pressure from 350 to 600 kPa(abs) and outlet temp. 295 to 325 K. For a superficial c.d. of 0.6-3.1 kA m-2 Reactor B achieved corresponding formate current efficiencies of 91-63%, with the same range of reactor voltage as that in Reactor A (2.7-4.3 V), which reflects the success of the scale-up in this work. Up to 1 M formate was obtained in the catholyte product from a single pass in Reactor B, but when the catholyte feed was spiked with 2-3 M potassium formate there was a large drop in current efficiency due to formate cross-over through the Nafion 117 membrane. An extended reactor (cathode) model that used four fitted kinetic parameters and assumed zero formate cross-over was able to mirror the reactor performance with reasonable fidelity over a wide range of conditions (max. error in formate CE = ±20%), including formate product concns. up to 1 M.
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    OpenURL UNIV OF NORTH TEXAS
  36. 36
    Yang, H.; Kaczur, J. J.; Sajjad, S. D.; Masel, R. I. Electrochemical Conversion of Co2 to Formic Acid Utilizing Sustainion Membranes. J. CO2 Utilization 2017, 20, 208– 217,  DOI: 10.1016/j.jcou.2017.04.011
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    Electrochemical conversion of CO2 to formic acid utilizing Sustainion membranes
    Yang, Hongzhou; Kaczur, Jerry J.; Sajjad, Syed Dawar; Masel, Richard I.
    Journal of CO2 Utilization (2017), 20 (), 208-217CODEN: JCUOAJ; ISSN:2212-9839. (Elsevier Ltd.)
    Formic acid generated from CO2 has been proposed both as a key intermediate renewable chem. feedstock as well as a potential chem.-based energy storage media for hydrogen. In this paper, we describe a novel three-compartment electrochem. cell configuration with the capability of directly producing a pure formic acid product in the concn. range of 5-20 wt% at high current densities and Faradaic yields. The electrochem. cell employs a Dioxide Materials Sustainion anion exchange membrane and a nanoparticle Sn GDE cathode contg. an imidazole ionomer, allowing for improved CO2 electrochem. redn. performance. Stable electrochem. cell performance for more than 500 h was exptl. demonstrated at a c.d. of 140 mA cm-2 at a cell voltage of only 3.5 V. Future work will include cell scale-up and increasing cell Faradaic performance using selected electrocatalysts and membranes.
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    OpenURL UNIV OF NORTH TEXAS
  37. 37
    Kuhl, K. P.; Cave, E. R.; Abram, D. N.; Jaramillo, T. F. New Insights into the Electrochemical Reduction of Carbon Dioxide on Metallic Copper Surfaces. Energy Environ. Sci. 2012, 5 (5), 7050– 7059,  DOI: 10.1039/c2ee21234j
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    New insights into the electrochemical reduction of carbon dioxide on metallic copper surfaces
    Kuhl, Kendra P.; Cave, Etosha R.; Abram, David N.; Jaramillo, Thomas F.
    Energy & Environmental Science (2012), 5 (5), 7050-7059CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)
    We report new insights into the electrochem. redn. of CO2 on a metallic copper surface, enabled by the development of an exptl. methodol. with unprecedented sensitivity for the identification and quantification of CO2 electroredn. products. This involves a custom electrochem. cell designed to maximize product concns. coupled to gas chromatog. and NMR for the identification and quantification of gas and liq. products, resp. We studied copper across a range of potentials and obsd. a total of 16 different CO2 redn. products, five of which are reported here for the first time, thus providing the most complete view of the reaction chem. reported to date. Taking into account the chem. identities of the wide range of C1-C3 products generated and the potential-dependence of their turnover frequencies, mechanistic information is deduced. We discuss a scheme for the formation of multi-carbon products involving enol-like surface intermediates as a possible pathway, accounting for the obsd. selectivity for eleven distinct C2+ oxygenated products including aldehydes, ketones, alcs., and carboxylic acids.
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    OpenURL UNIV OF NORTH TEXAS
  38. 38
    Jiang, K.; Sandberg, R. B.; Akey, A. J.; Liu, X.; Bell, D. C.; Nørskov, J. K.; Chan, K.; Wang, H. Metal Ion Cycling of Cu Foil for Selective C–C Coupling in Electrochemical CO2 Reduction. Nature Catalysis 2018, 1 (2), 111– 119,  DOI: 10.1038/s41929-017-0009-x
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    OpenURL UNIV OF NORTH TEXAS
  39. 39
    Kim, D.; Kley, C. S.; Li, Y.; Yang, P. Copper Nanoparticle Ensembles for Selective Electroreduction of CO2 to C2–C3 Products. Proc. Natl. Acad. Sci. U. S. A. 2017, 114 (40), 10560– 10565,  DOI: 10.1073/pnas.1711493114
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    Copper nanoparticle ensembles for selective electroreduction of CO2 to C2-C3 products
    Kim, Dohyung; Kley, Christopher S.; Li, Yifan; Yang, Peidong
    Proceedings of the National Academy of Sciences of the United States of America (2017), 114 (40), 10560-10565CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
    Direct conversion of carbon dioxide to multicarbon products remains as a grand challenge in electrochem. CO2 redn. Various forms of oxidized copper have been demonstrated as electrocatalysts that still require large overpotentials. Here, we show that an ensemble of Cu nanoparticles (NPs) enables selective formation of C2-C3 products at low overpotentials. Densely packed Cu NP ensembles underwent structural transformation during electrolysis into electrocatalytically active cube-like particles intermixed with smaller nanoparticles. Ethylene, ethanol, and n-propanol are the major C2-C3 products with onset potential at -0.53 V (vs. reversible hydrogen electrode, RHE) and C2-C3 faradaic efficiency (FE) reaching 50% at only -0.75 V. Thus, the catalyst exhibits selective generation of C2-C3 hydrocarbons and oxygenates at considerably lowered overpotentials in neutral pH aq. media. In addn., this approach suggests new opportunities in realizing multicarbon product formation from CO2, where the majority of efforts has been to use oxidized copper-based materials. Robust catalytic performance is demonstrated by 10 h of stable operation with C2-C3 c.d. 10 mA/cm2 (at -0.75 V), rendering it attractive for solar-to-fuel applications. Tafel anal. suggests reductive CO coupling as a rate detg. step for C2 products, while n-propanol (C3) prodn. seems to have a discrete pathway.
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    OpenURL UNIV OF NORTH TEXAS
  40. 40
    Ren, D.; Ang, B. S.-H.; Yeo, B. S. Tuning the Selectivity of Carbon Dioxide Electroreduction toward Ethanol on Oxide-Derived Cuxzn Catalysts. ACS Catal. 2016, 6 (12), 8239– 8247,  DOI: 10.1021/acscatal.6b02162
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    40
    Tuning the Selectivity of Carbon Dioxide Electroreduction toward Ethanol on Oxide-Derived CuxZn Catalysts
    Ren, Dan; Ang, Bridget Su-Hui; Yeo, Boon Siang
    ACS Catalysis (2016), 6 (12), 8239-8247CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
    The electrochem. redn. of carbon dioxide (CO2) to ethanol (C2H5OH) and ethylene (C2H4) using renewable electricity is a viable method for the prodn. of these com. vital chems. Copper (Cu) and its oxides are by far the most effective electrocatalysts for this purpose. However, the formation of ethanol using these catalysts is generally less favored in comparison to that of ethylene. In this work, we demonstrate that the selectivity of CO2 redn. toward ethanol could be tuned by introducing a cocatalyst to generate an in situ source of mobile CO reactant. Cu-based oxides with different amts. of Zn dopants (Cu, Cu10Zn, Cu4Zn, and Cu2Zn) were prepd. and used as catalysts under ambient pressure in aq. 0.1 M KHCO3 electrolyte. By varying the amt. of Zn in the bimetallic catalysts, we found that the selectivity of ethanol vs. ethylene prodn., defined by the ratio of their Faradaic efficiencies (FEethanol/FEethylene), could be tuned by a factor of up to ∼12.5. Ethanol formation was maximized on Cu4Zn at -1.05 V vs RHE, with a remarkable Faradaic efficiency and c.d. of 29.1% and -8.2 mA/cm2, resp. The Cu4Zn catalyst was also catalytically stable for the prodn. of ethanol for at least 5 h. The importance of Zn as a CO-producing site was demonstrated by performing CO2 redn. on Cu-Ni and Cu-Ag bimetallic catalysts. Operando Raman spectroscopy revealed that the as-deposited Cu-based oxide films were reduced to the metallic state during CO2 redn., after which only signals belonging to CO adsorbed on Cu sites were recorded. This showed that the redn. of CO2 probably occurred on metallic sites rather than on metal oxides. A two-site mechanism to rationalize the selective redn. of CO2 to ethanol is proposed and discussed.
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    OpenURL UNIV OF NORTH TEXAS
  41. 41
    Cave, E. R.; Montoya, J. H.; Kuhl, K. P.; Abram, D. N.; Hatsukade, T.; Shi, C.; Hahn, C.; Nørskov, J. K.; Jaramillo, T. F. Electrochemical CO2 Reduction on Au Surfaces: Mechanistic Aspects Regarding the Formation of Major and Minor Products. Phys. Chem. Chem. Phys. 2017, 19 (24), 15856– 15863,  DOI: 10.1039/C7CP02855E
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    41
    Electrochemical CO2 reduction on Au surfaces: mechanistic aspects regarding the formation of major and minor products
    Cave, Etosha R.; Montoya, Joseph H.; Kuhl, Kendra P.; Abram, David N.; Hatsukade, Toru; Shi, Chuan; Hahn, Christopher; Noerskov, Jens K.; Jaramillo, Thomas F.
    Physical Chemistry Chemical Physics (2017), 19 (24), 15856-15863CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    In the future, industrial CO2 electro-redn. using renewable energy sources could be a sustainable means to convert CO2 and water to commodity chems. at room temp. and atm. pressure. This work focused on the electrocatalytic redn. of CO2 over polycryst. Au surfaces which have high activity and selectivity for CO evolution. The catalytic behavior of polycryst. Au surfaces were examd. by coupling potentiostatic CO2 electrolysis expts. in an aq. HCO3- soln. with high sensitivity product detection methods. Methanol prodn. of methanol was obsd., in addn. to detecting known products of CO2 electroredn. over Au: CO, H2, and formate. The authors suggested a mechanism to explain methanol evolution from Au, specifically, the Au surface does not favor C-O scission, thus is more selective toward methanol than CH4. These insights could aid in designing electrocatalysts selective for CO2 electroredn. to oxygenates over hydrocarbons.
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    OpenURL UNIV OF NORTH TEXAS
  42. 42
    Hatsukade, T.; Kuhl, K. P.; Cave, E. R.; Abram, D. N.; Jaramillo, T. F. Insights into the Electrocatalytic Reduction of CO2 on Metallic Silver Surfaces. Phys. Chem. Chem. Phys. 2014, 16 (27), 13814– 13819,  DOI: 10.1039/C4CP00692E
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    42
    Insights into the electrocatalytic reduction of CO2 on metallic silver surfaces
    Hatsukade, Toru; Kuhl, Kendra P.; Cave, Etosha R.; Abram, David N.; Jaramillo, Thomas F.
    Physical Chemistry Chemical Physics (2014), 16 (27), 13814-13819CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    The electrochem. redn. of CO2 could allow for a sustainable process by which renewable energy from wind and solar are used directly in the prodn. of fuels and chems. In this work we investigated the potential dependent activity and selectivity of the electrochem. redn. of CO2 on metallic silver surfaces under ambient conditions. Our results deepen our understanding of the surface chem. and provide insight into the factors important to designing better catalysts for the reaction. The high sensitivity of our exptl. methods for identifying and quantifying products of reaction allowed for the observation of six redn. products including CO and hydrogen as major products and formate, methane, methanol, and ethanol as minor products. By quantifying the potential-dependent behavior of all products, we provide insights into kinetics and mechanisms at play, in particular involving the prodn. of hydrocarbons and alcs. on catalysts with weak CO binding energy as well as the formation of a C-C bond required to produce ethanol.
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  43. 43
    Lu, Q.; Rosen, J.; Zhou, Y.; Hutchings, G. S.; Kimmel, Y. C.; Chen, J. G.; Jiao, F. A Selective and Efficient Electrocatalyst for Carbon Dioxide Reduction. Nat. Commun. 2014, 5, 3242,  DOI: 10.1038/ncomms4242
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    43
    A selective and efficient electrocatalyst for carbon dioxide reduction
    Lu Qi; Rosen Jonathan; Zhou Yang; Hutchings Gregory S; Kimmel Yannick C; Jiao Feng; Chen Jingguang G
    Nature communications (2014), 5 (), 3242 ISSN:.
    Converting carbon dioxide to useful chemicals in a selective and efficient manner remains a major challenge in renewable and sustainable energy research. Silver is an interesting electrocatalyst owing to its capability of converting carbon dioxide to carbon monoxide selectively at room temperature; however, the traditional polycrystalline silver electrocatalyst requires a large overpotential. Here we report a nanoporous silver electrocatalyst that is able to electrochemically reduce carbon dioxide to carbon monoxide with approximately 92% selectivity at a rate (that is, current) over 3,000 times higher than its polycrystalline counterpart under moderate overpotentials of <0.50 V. The high activity is a result of a large electrochemical surface area (approximately 150 times larger) and intrinsically high activity (approximately 20 times higher) compared with polycrystalline silver. The intrinsically higher activity may be due to the greater stabilization of CO2 (-) intermediates on the highly curved surface, resulting in smaller overpotentials needed to overcome the thermodynamic barrier.
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    OpenURL UNIV OF NORTH TEXAS
  44. 44
    Chen, Y.; Li, C. W.; Kanan, M. W. Aqueous CO2 Reduction at Very Low Overpotential on Oxide-Derived Au Nanoparticles. J. Am. Chem. Soc. 2012, 134 (49), 19969– 19972,  DOI: 10.1021/ja309317u
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    44
    Aqueous CO2 Reduction at Very Low Overpotential on Oxide-Derived Au Nanoparticles
    Chen, Yihong; Li, Christina W.; Kanan, Matthew W.
    Journal of the American Chemical Society (2012), 134 (49), 19969-19972CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Carbon dioxide redn. is an essential component of many prospective technologies for the renewable synthesis of carbon-contg. fuels. Known catalysts for this reaction generally suffer from low energetic efficiency, poor product selectivity, and rapid deactivation. It is shown that the redn. of thick Au oxide films results in the formation of Au nanoparticles (oxide-derived Au) that exhibit highly selective CO2 redn. to CO in water at overpotentials as low as 140 mV and retain their activity for at least 8 h. Under identical conditions, polycryst. Au electrodes and several other nanostructured Au electrodes prepd. via alternative methods require at least 200 mV of addnl. overpotential to attain comparable CO2 redn. activity and rapidly lose their activity. Electrokinetic studies indicate that the improved catalysis is linked to dramatically increased stabilization of the CO2•- intermediate on the surfaces of the oxide-derived Au electrodes.
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    OpenURL UNIV OF NORTH TEXAS
  45. 45
    Zheng, X.; De Luna, P.; García de Arquer, F. P.; Zhang, B.; Becknell, N.; Ross, M. B.; Li, Y.; Banis, M. N.; Li, Y.; Liu, M. Sulfur-Modulated Tin Sites Enable Highly Selective Electrochemical Reduction of CO2 to Formate. Joule 2017, 1 (4), 794– 805,  DOI: 10.1016/j.joule.2017.09.014
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    45
    Sulfur-Modulated Tin Sites Enable Highly Selective Electrochemical Reduction of CO2 to Formate
    Zheng, Xueli; De Luna, Phil; Garcia de Arquer, F. Pelayo; Zhang, Bo; Becknell, Nigel; Ross, Michael B.; Li, Yifan; Banis, Mohammad Norouzi; Li, Yuzhang; Liu, Min; Voznyy, Oleksandr; Dinh, Cao Thang; Zhuang, Taotao; Stadler, Philipp; Cui, Yi; Du, Xiwen; Yang, Peidong; Sargent, Edward H.
    Joule (2017), 1 (4), 794-805CODEN: JOULBR; ISSN:2542-4351. (Cell Press)
    Electrochem. redn. of carbon dioxide (CO2RR) to formate provides an avenue to the synthesis of value-added carbon-based fuels and feedstocks powered using renewable electricity. Here, we hypothesized that the presence of sulfur atoms in the catalyst surface could promote undercoordinated sites, and thereby improve the electrochem. redn. of CO2 to formate. We explored, using d. functional theory, how the incorporation of sulfur into tin may favor formate generation. We used at. layer deposition of SnSx followed by a redn. process to synthesize sulfur-modulated tin (Sn(S)) catalysts. X-ray absorption near-edge structure (XANES) studies reveal higher oxidn. states in Sn(S) compared with that of tin in Sn nanoparticles. Sn(S)/Au accelerates CO2RR at geometric current densities of 55 mA cm-2 at -0.75 V vs. reversible hydrogen electrode with a Faradaic efficiency of 93%. Furthermore, Sn(S) catalysts show excellent stability without deactivation (<2% productivity change) following more than 40 h of operation.
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  46. 46
    Feaster, J. T.; Shi, C.; Cave, E. R.; Hatsukade, T.; Abram, D. N.; Kuhl, K. P.; Hahn, C.; Nørskov, J. K.; Jaramillo, T. F. Understanding Selectivity for the Electrochemical Reduction of Carbon Dioxide to Formic Acid and Carbon Monoxide on Metal Electrodes. ACS Catal. 2017, 7 (7), 4822– 4827,  DOI: 10.1021/acscatal.7b00687
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    46
    Understanding Selectivity for the Electrochemical Reduction of Carbon Dioxide to Formic Acid and Carbon Monoxide on Metal Electrodes
    Feaster, Jeremy T.; Shi, Chuan; Cave, Etosha R.; Hatsukade, Toru; Abram, David N.; Kuhl, Kendra P.; Hahn, Christopher; Noerskov, Jens K.; Jaramillo, Thomas F.
    ACS Catalysis (2017), 7 (7), 4822-4827CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
    Increases in energy demand and in chem. prodn., together with the rise in CO2 levels in the atm., motivate the development of renewable energy sources. Electrochem. CO2 redn. to fuels and chems. is an appealing alternative to traditional pathways to fuels and chems. due to its intrinsic ability to couple to solar and wind energy sources. Formate (HCOO-) is a key chem. for many industries; however, greater understanding is needed regarding the mechanism and key intermediates for HCOO- prodn. This work reports a joint exptl. and theor. investigation of the electrochem. redn. of CO2 to HCOO- on polycryst. Sn surfaces, which have been identified as promising catalysts for selectively producing HCOO-. Our results show that Sn electrodes produce HCOO-, carbon monoxide (CO), and hydrogen (H2) across a range of potentials and that HCOO- prodn. becomes favored at potentials more neg. than -0.8 V vs RHE, reaching a max. Faradaic efficiency of 70% at -0.9 V vs RHE. Scaling relations for Sn and other transition metals are examd. using exptl. current densities and d. functional theory (DFT) binding energies. While *COOH was detd. to be the key intermediate for CO prodn. on metal surfaces, we suggest that it is unlikely to be the primary intermediate for HCOO- prodn. Instead, *OCHO is suggested to be the key intermediate for the CO2RR to HCOO- transformation, and Sn's optimal *OCHO binding energy supports its high selectivity for HCOO-. These results suggest that oxygen-bound intermediates are crit. to understand the mechanism of CO2 redn. to HCOO- on metal surfaces.
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    OpenURL UNIV OF NORTH TEXAS
  47. 47
    Gao, S.; Lin, Y.; Jiao, X.; Sun, Y.; Luo, Q.; Zhang, W.; Li, D.; Yang, J.; Xie, Y. Partially Oxidized Atomic Cobalt Layers for Carbon Dioxide Electroreduction to Liquid Fuel. Nature 2016, 529, 68,  DOI: 10.1038/nature16455
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    47
    Partially oxidized atomic cobalt layers for carbon dioxide electroreduction to liquid fuel
    Gao, Shan; Lin, Yue; Jiao, Xingchen; Sun, Yongfu; Luo, Qiquan; Zhang, Wenhua; Li, Dianqi; Yang, Jinlong; Xie, Yi
    Nature (London, United Kingdom) (2016), 529 (7584), 68-71CODEN: NATUAS; ISSN:0028-0836. (Nature Publishing Group)
    Electroredn. of CO2 into useful fuels, esp. if driven by renewable energy, represents a potentially 'clean' strategy for replacing fossil feedstocks and dealing with increasing CO2 emissions and their adverse effects on climate. The crit. bottleneck lies in activating CO2 into the CO2•- radical anion or other intermediates that can be converted further, as the activation usually requires impractically high overpotentials. Recently, electrocatalysts based on oxide-derived metal nanostructures have been shown to enable CO2 redn. at low overpotentials. However, it remains unclear how the electrocatalytic activity of these metals is influenced by their native oxides, mainly because microstructural features such as interfaces and defects influence CO2 redn. activity yet are difficult to control. To evaluate the role of the two different catalytic sites, here we fabricate two kinds of four-atom-thick layers: pure cobalt metal, and co-existing domains of cobalt metal and cobalt oxide. Cobalt mainly produces formate (HCOO-) during CO2 electroredn.; we find that surface cobalt atoms of the atomically thin layers have higher intrinsic activity and selectivity towards formate prodn., at lower overpotentials, than do surface cobalt atoms on bulk samples. Partial oxidn. of the at. layers further increases their intrinsic activity, allowing us to realize stable current densities of about 10 mA per square centimeter over 40 h, with approx. 90 per cent formate selectivity at an overpotential of only 0.24 V, which outperforms previously reported metal or metal oxide electrodes evaluated under comparable conditions. The correct morphol. and oxidn. state can thus transform a material from one considered nearly non-catalytic for the CO2 electroredn. reaction into an active catalyst. These findings point to new opportunities for manipulating and improving the CO2 electroredn. properties of metal systems, esp. once the influence of both the at.-scale structure and the presence of oxide are mechanistically better understood.
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  48. 48
    Zhao, S.; Jin, R.; Jin, R. Opportunities and Challenges in CO2 Reduction by Gold- and Silver-Based Electrocatalysts: From Bulk Metals to Nanoparticles and Atomically Precise Nanoclusters. ACS Energy Lett. 2018, 3 (2), 452– 462,  DOI: 10.1021/acsenergylett.7b01104
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    48
    Opportunities and Challenges in CO2 Reduction by Gold- and Silver-Based Electrocatalysts: From Bulk Metals to Nanoparticles and Atomically Precise Nanoclusters
    Zhao, Shuo; Jin, Renxi; Jin, Rongchao
    ACS Energy Letters (2018), 3 (2), 452-462CODEN: AELCCP; ISSN:2380-8195. (American Chemical Society)
    To tackle the excessive emission of greenhouse gas CO2, electrocatalytic redn. has been recognized as a promising way. Given the multielectron, multiproduct nature of the CO2 redn. process, an ideal catalyst should be capable of converting CO2 with high rates as well as high selectivity to either gas-phase (e.g., CO, CH4) or liq.-phase products (e.g., HCOOH, CH3OH, etc.). Gold- and silver-based materials have been extensively investigated as CO2 redn. catalysts for the formation of CO. This Perspective focuses on the advances of gold- and silver-based electrocatalysts for CO2 redn. in terms of catalyst design as well as some insights from theor. investigations. In particular, a special emphasis is placed on the newly emerging, atomically precise metal nanoclusters for CO2 electroredn. The strong quantum confinement effect and mol. purity as well as the crystallog. solved at. structures of nanoclusters make this new class of catalysts quite promising in fundamental studies, and valuable mechanistic insights for CO2 electroredn. at the at. scale can be obtained. We hope that this Perspective highlights the opportunities and challenges in the exploration of emerging nanomaterials.
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    OpenURL UNIV OF NORTH TEXAS
  49. 49
    Cai, Z.; Wu, Y.; Wu, Z.; Yin, L.; Weng, Z.; Zhong, Y.; Xu, W.; Sun, X.; Wang, H. Unlocking Bifunctional Electrocatalytic Activity for CO2 Reduction Reaction by Win-Win Metal–Oxide Cooperation. ACS Energy Lett. 2018, 3 (11), 2816– 2822,  DOI: 10.1021/acsenergylett.8b01767
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    Unlocking Bifunctional Electrocatalytic Activity for CO2 Reduction Reaction by Win-Win Metal-Oxide Cooperation
    Cai, Zhao; Wu, Yueshen; Wu, Zishan; Yin, Lichang; Weng, Zhe; Zhong, Yiren; Xu, Wenwen; Sun, Xiaoming; Wang, Hailiang
    ACS Energy Letters (2018), 3 (11), 2816-2822CODEN: AELCCP; ISSN:2380-8195. (American Chemical Society)
    Understanding how remarkable properties of materials emerge from complex interactions of their constituents and designing advanced material structures to render desired properties are grand challenges. Metal-oxide interactions are frequently utilized to improve catalytic properties but are often limited to situations where only one component is facilitated by the other. In this work, highly cooperative win-win metal-oxide interactions are demonstrated that enable unprecedented catalytic functionalities for electrochem. CO2 redn. reactions. In a single SnOx/Ag catalyst, the oxide promotes the metal in the CO prodn. mode, and meanwhile the metal promotes the oxide in the HCOOH prodn. mode, achieving potential-dependent bifunctional CO2 conversion to fuels and chems. with H2 evolution suppressed in the entire potential window. Spectroscopic studies and computational simulations reveal that electron transfer from Ag to SnOx and dual-site cooperative binding for reaction intermediates at the SnOx/Ag interface are responsible for stabilizing the key intermediate in the CO pathway, changing the potential-limiting step in the HCOOH pathway, and increasing the kinetic barrier in the H2 evolution pathway, together leading to highly synergistic CO2 electroredn.
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  50. 50
    Lu, X.; Wu, Y.; Yuan, X.; Huang, L.; Wu, Z.; Xuan, J.; Wang, Y.; Wang, H. High-Performance Electrochemical CO2 Reduction Cells Based on Non-Noble Metal Catalysts. ACS Energy Lett. 2018, 3 (10), 2527– 2532,  DOI: 10.1021/acsenergylett.8b01681
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    High-Performance Electrochemical CO2 Reduction Cells Based on Non-noble Metal Catalysts
    Lu, Xu; Wu, Yueshen; Yuan, Xiaolei; Huang, Ling; Wu, Zishan; Xuan, Jin; Wang, Yifei; Wang, Hailiang
    ACS Energy Letters (2018), 3 (10), 2527-2532CODEN: AELCCP; ISSN:2380-8195. (American Chemical Society)
    The promise and challenge of electrochem. mitigation of CO2 calls for innovations on both catalyst and reactor levels. Here, enabled by our high-performance and earth-abundant CO2 electroredn. catalyst materials, we developed alk. microflow electrolytic cells for energy-efficient, selective, fast, and durable CO2 conversion to CO and HCOO-. With a cobalt phthalocyanine-based cathode catalyst, the CO-selective cell starts to operate at a 0.26 V overpotential and reaches a Faradaic efficiency of 94% and a partial c.d. of 31 mA/cm2 at a 0.56 V overpotential. With a SnO2-based cathode catalyst, the HCOO--selective cell starts to operate at a 0.76 V overpotential and reaches a Faradaic efficiency of 82% and a partial c.d. of 113 mA/cm2 at a 1.36 V overpotential. In contrast to previous studies, we found that the overpotential redn. from using the alk. electrolyte is mostly contributed by a pH gradient near the cathode surface.
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    Kaczur, J. J.; Yang, H.; Liu, Z.; Sajjad, S. D.; Masel, R. I. Carbon Dioxide and Water Electrolysis Using New Alkaline Stable Anion Membranes. Front. Chem. 2018, 6, 263,  DOI: 10.3389/fchem.2018.00263
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    Carbon dioxide and water electrolysis using new alkaline stable anion membranes
    Kaczur, Jerry J.; Yang, Hongzhou; Liu, Zengcai; Sajjad, Syed D.; Masel, Richard I.
    Frontiers in Chemistry (Lausanne, Switzerland) (2018), 6 (), 263/1-263/16CODEN: FCLSAA; ISSN:2296-2646. (Frontiers Media S.A.)
    The recent development and market introduction of a new type of alk. stable imidazole-based anion exchange membrane and related ionomers by Dioxide Materials is enabling the advancement of new and improved electrochem. processes which can operate at com. viable operating voltages, current efficiencies, and current densities. These processes include the electrochem. conversion of CO2 to formic acid (HCOOH), CO2 to carbon monoxide (CO), and alk. water electrolysis, generating hydrogen at high current densities at low voltages without the need for any preciousmetal electrocatalysts. The first process is the direct electrochem. generation of pure formic acid in a three-compartment cell configuration using the alk. stable anion exchange membrane and a cation exchange membrane. The cell operates at a c.d. of 140 mA/cm2 at a cell voltage of 3.5 V. The power consumption for prodn. of formic acid (FA) is about 4.3-4.7 kWh/kg of FA. The second process is the electrochem. conversion of CO2 to CO, a key focus product in the generation of renewable fuels and chems. The CO2 cell consists of a two-compartment design utilizing the alk. stable anion exchange membrane to sep. the anode and cathode compartments. A nanoparticle IrO2 catalyst on a GDE structure is used as the anode and a GDE utilizing a nanoparticle Ag/imidazolium-based ionomer catalyst combination is used as a cathode. The CO2 cell has been operated at current densities of 200 to 600 mA/cm2 at voltages of 3.0 to 3.2 resp. with CO2 to CO conversion selectivities of 95-99%. The third process is an alk. water electrolysis cell process, where the alk. stable anion exchange membrane allows stable cell operation in 1M KOH electrolyte solns. at current densities of 1 A/cm2 at about 1.90 V. The cell has demonstrated operation for thousands of hours, showing a voltage increase in time of only 5 μV/h. The alk. electrolysis technol. does not require any precious metal catalysts as compared to polymer electrolytemembrane (PEM) design water electrolyzers. In this paper, we discuss the detailed tech. aspects of these three technologies utilizing this unique anion exchange membrane.
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  52. 52
    Salvatore, D. A.; Weekes, D. M.; He, J.; Dettelbach, K. E.; Li, Y. C.; Mallouk, T. E.; Berlinguette, C. P. Electrolysis of Gaseous CO2 to CO in a Flow Cell with a Bipolar Membrane. ACS Energy Lett. 2018, 3 (1), 149– 154,  DOI: 10.1021/acsenergylett.7b01017
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    Electrolysis of Gaseous CO2 to CO in a Flow Cell with a Bipolar Membrane
    Salvatore, Danielle A.; Weekes, David M.; He, Jingfu; Dettelbach, Kevan E.; Li, Yuguang C.; Mallouk, Thomas E.; Berlinguette, Curtis P.
    ACS Energy Letters (2018), 3 (1), 149-154CODEN: AELCCP; ISSN:2380-8195. (American Chemical Society)
    The conversion of CO2 to CO is demonstrated in an electrolyzer flow cell contg. a bipolar membrane at current densities of 200 mA/cm2 with a faradaic efficiency of 50%. Electrolysis was carried out by delivering gaseous CO2 at the cathode with a Ag catalyst integrated in a C-based gas diffusion layer. Nonprecious Ni foam in a strongly alk. electrolyte (1 M NaOH) was used to mediate the anode reaction. While a configuration where the anode and cathode were sepd. by only a bipolar membrane is unfavorable for robust CO2 redn., a modified configuration with a solid-supported aq. layer inserted between the Ag-based catalyst layer and the bipolar membrane enhanced the cathode selectivity for CO2 redn. to CO. The authors report higher current densities (200 mA/cm2) than previously reported for gas-phase CO2 to CO electrolysis and demonstrate the dependence of long-term stability on adequate hydration of the CO2 inlet stream.
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    Ripatti, D. S.; Veltman, T. R.; Kanan, M. W. Carbon Monoxide Gas Diffusion Electrolysis That Produces Concentrated C2 Products with High Single-Pass Conversion. Joule 2018,  DOI: 10.1016/j.joule.2018.10.007
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    Jouny, M.; Luc, W.; Jiao, F. High-Rate Electroreduction of Carbon Monoxide to Multi-Carbon Products. Nature Catal. 2018, 1 (10), 748– 755,  DOI: 10.1038/s41929-018-0133-2
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    Zeng, K.; Zhang, D. Recent Progress in Alkaline Water Electrolysis for Hydrogen Production and Applications. Prog. Energy Combust. Sci. 2010, 36 (3), 307– 326,  DOI: 10.1016/j.pecs.2009.11.002
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    Recent progress in alkaline water electrolysis for hydrogen production and applications
    Zeng, Kai; Zhang, Dongke
    Progress in Energy and Combustion Science (2010), 36 (3), 307-326CODEN: PECSDO; ISSN:0360-1285. (Elsevier Ltd.)
    A review. Alk. water electrolysis is one of the easiest methods for hydrogen prodn., offering the advantage of simplicity. The challenges for widespread use of water electrolysis are to reduce energy consumption, cost and maintenance and to increase reliability, durability and safety. This literature review examines the current state of knowledge and technol. of hydrogen prodn. by water electrolysis and identifies areas where R&D effort is needed in order to improve this technol. Following an overview of the fundamentals of alk. water electrolysis, an elec. circuit analogy of resistances in the electrolysis system is introduced. The resistances are classified into three categories, namely the elec. resistances, the reaction resistances and the transport resistances. This is followed by a thorough anal. of each of the resistances, by means of thermodn. and kinetics, to provide a scientific guidance to minimising the resistance in order to achieve a greater efficiency of alk. water electrolysis. The thermodn. anal. defines various electrolysis efficiencies based on theor. energy input and cell voltage, resp. These efficiencies are then employed to compare different electrolysis cell designs and to identify the means to overcome the key resistances for efficiency improvement. The kinetic anal. reveals the dependence of reaction resistances on the alk. concn., ion transfer, and reaction sites on the electrode surface, the latter is detd. by the electrode materials. A quant. relationship between the cell voltage components and c.d. is established, which links all the resistances and manifests the importance of reaction resistances and bubble resistances. The important effect of gas bubbles formed on the electrode surface and the need to minimise the ion transport resistance are highlighted. The historical development and continuous improvement in the alk. water electrolysis technol. are examd. and different water electrolysis technologies are systematically compared using a set of the practical parameters derived from the thermodn. and kinetic analyses. In addn. to the efficiency improvements, the needs for redn. in equipment and maintenance costs, and improvement in reliability and durability are also established. The future research needs are also discussed from the aspects of electrode materials, electrolyte additives and bubble management, serving as a comprehensive guide for continuous development of the water electrolysis technol.
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    Carmo, M.; Fritz, D. L.; Mergel, J.; Stolten, D. A Comprehensive Review on Pem Water Electrolysis. Int. J. Hydrogen Energy 2013, 38 (12), 4901– 4934,  DOI: 10.1016/j.ijhydene.2013.01.151
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    A comprehensive review on PEM water electrolysis
    Carmo, Marcelo; Fritz, David L.; Mergel, Juergen; Stolten, Detlef
    International Journal of Hydrogen Energy (2013), 38 (12), 4901-4934CODEN: IJHEDX; ISSN:0360-3199. (Elsevier Ltd.)
    A review. Hydrogen is often considered the best means by which to store energy coming from renewable and intermittent power sources. With the growing capacity of localized renewable energy sources surpassing the gigawatt range, a storage system of equal magnitude is required. PEM electrolysis provides a sustainable soln. for the prodn. of hydrogen, and is well suited to couple with energy sources such as wind and solar. However, due to low demand in electrolytic hydrogen in the last century, little research has been done on PEM electrolysis with many challenges still unexplored. The ever increasing desire for green energy has rekindled the interest on PEM electrolysis, thus the compilation and recovery of past research and developments is important and necessary. In this review, PEM water electrolysis is comprehensively highlighted and discussed. The challenges new and old related to electrocatalysts, solid electrolyte, current collectors, separator plates and modeling efforts will also be addressed. The main message is to clearly set the state-of-the-art for the PEM electrolysis technol., be insightful of the research that is already done and the challenges that still exist. This information will provide several future research directions and a road map in order to aid scientists in establishing PEM electrolysis as a com. viable hydrogen prodn. soln.
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  57. 57
    Weng, L. C.; Bell, A. T.; Weber, A. Z. Modeling Gas-Diffusion Electrodes for CO2 Reduction. Phys. Chem. Chem. Phys. 2018, 20 (25), 16973– 16984,  DOI: 10.1039/C8CP01319E
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    Modeling gas-diffusion electrodes for CO2 reduction
    Weng, Lien-Chun; Bell, Alexis T.; Weber, Adam Z.
    Physical Chemistry Chemical Physics (2018), 20 (25), 16973-16984CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    CO2 redn. conducted in electrochem. cells with planar electrodes immersed in an aq. electrolyte is severely limited by mass transport across the hydrodynamic boundary layer. This limitation can be minimized by use of vapor-fed, gas-diffusion electrodes (GDEs), enabling current densities that are almost two orders of magnitude greater at the same applied cathode overpotential than what is achievable with planar electrodes in an aq. electrolyte. The addn. of porous cathode layers, however, introduces a no. of parameters that need to be tuned in order to optimize the performance of the GDE cell. In this work, we develop a multiphysics model for gas diffusion electrodes for CO2 redn. and used it to investigate the interplay between species transport and electrochem. reaction kinetics. The model demonstrates how the local environment near the catalyst layer, which is a function of the operating conditions, affects cell performance. We also examine the effects of catalyst layer hydrophobicity, loading, porosity, and electrolyte flowrate to help guide exptl. design of vapor-fed CO2 redn. cells.
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  58. 58
    Weber, A. Z.; Borup, R. L.; Darling, R. M.; Das, P. K.; Dursch, T. J.; Gu, W. B.; Harvey, D.; Kusoglu, A.; Litster, S.; Mench, M. M. A Critical Review of Modeling Transport Phenomena in Polymer-Electrolyte Fuel Cells. J. Electrochem. Soc. 2014, 161 (12), F1254– F1299,  DOI: 10.1149/2.0751412jes
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    A critical review of modeling transport phenomena in polymer-electrolyte fuel cells
    Weber, Adam Z.; Borup, Rodney L.; Darling, Robert M.; Das, Prodip K.; Dursch, Thomas J.; Gu, Wenbin; Harvey, David; Kusoglu, Ahmet; Litster, Shawn; Mench, Matthew M.; Mukundan, Rangachary; Owejan, Jon P.; Pharoah, Jon G.; Secanell, Marc; Zenyuk, Iryna V.
    Journal of the Electrochemical Society (2014), 161 (12), F1254-F1299, 46 pp.CODEN: JESOAN; ISSN:0013-4651. (Electrochemical Society)
    A review. Polymer-electrolyte fuel cells are a promising energy-conversion technol. Over the last several decades significant progress has been made in increasing their performance and durability, of which continuum-level modeling of the transport processes has played an integral part. In this review, we examine the state-of-the-art modeling approaches, with a goal of elucidating the knowledge gaps and needs going forward in the field. In particular, the focus is on multiphase flow, esp. in terms of understanding interactions at interfaces, and catalyst layers with a focus on the impacts of ionomer thin-films and multiscale phenomena. Overall, we highlight where there is consensus in terms of modeling approaches as well as opportunities for further improvement and clarification, including identification of several crit. areas for future research.
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  59. 59
    Varcoe, J. R.; Atanassov, P.; Dekel, D. R.; Herring, A. M.; Hickner, M. A.; Kohl, P. A.; Kucernak, A. R.; Mustain, W. E.; Nijmeijer, K.; Scott, K. Anion-Exchange Membranes in Electrochemical Energy Systems. Energy Environ. Sci. 2014, 7 (10), 3135– 3191,  DOI: 10.1039/C4EE01303D
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    Anion-exchange membranes in electrochemical energy systems
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    Energy & Environmental Science (2014), 7 (10), 3135-3191CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)
    A review. This article provides an up-to-date perspective on the use of anion-exchange membranes in fuel cells, electrolyzers, redox flow batteries, reverse electrodialysis cells, and bioelectrochem. systems (e.g. microbial fuel cells). The aim is to highlight key concepts, misconceptions, the current state-of-the-art, technol. and scientific limitations, and the future challenges (research priorities) related to the use of anion-exchange membranes in these energy technologies. All the refs. that the authors deemed relevant, and were available on the web by the manuscript submission date (30th Apr. 2014), are included.
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    Kusoglu, A.; Weber, A. Z. New Insights into Perfluorinated Sulfonic-Acid Ionomers. Chem. Rev. 2017, 117 (3), 987– 1104,  DOI: 10.1021/acs.chemrev.6b00159
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    Kusoglu, Ahmet; Weber, Adam Z.
    Chemical Reviews (Washington, DC, United States) (2017), 117 (3), 987-1104CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
    In this comprehensive review, recent progress and developments on perfluorinated sulfonic-acid (PFSA) membranes have been summarized on many key topics. Although quite well investigated for decades, PFSA ionomers' complex behavior, along with their key role in many emerging technologies, have presented significant scientific challenges but also helped create a unique cross-disciplinary research field to overcome such challenges. Research and progress on PFSAs, esp. when considered with their applications, are at the forefront of bridging electrochem. and polymer (physics), which have also opened up development of state-of-the-art in situ characterization techniques as well as multiphysics computation models. Topics reviewed stem from correlating the various phys. (e.g., mech.) and transport properties with morphol. and structure across time and length scales. In addn., topics of recent interest such as structure/transport correlations and modeling, composite PFSA membranes, degrdn. phenomena, and PFSA thin films are presented. Throughout, the impact of PFSA chem. and side-chain is also discussed to present a broader perspective.
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    Dekel, D. R. Review of Cell Performance in Anion Exchange Membrane Fuel Cells. J. Power Sources 2018, 375, 158– 169,  DOI: 10.1016/j.jpowsour.2017.07.117
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    Review of performance of anion exchange membrane fuel cells
    Dekel, Dario R.
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    A review of the performance and performance stability of AEMFCs through the years since the 1st reports in the early 2000s. Anion exchange membrane fuel cells (AEMFCs) have recently received increasing attention since in principle they allow for the use of non-precious metal catalysts, which dramatically reduces the cost per kW of power in fuel cell devices. Until not long ago, the main barrier in the development of AEMFCs was the availability of highly conductive anion exchange membranes (AEMs); however, improvements on this front in the past decade show that newly developed AEMs have already reached high levels of cond., leading to satisfactory cell performance. In recent years, a growing no. of research studies have reported AEMFC performance results. In the last 3 years, new records in performance were achieved. Most of the literature reporting cell performance is based on H-AEMFCs, although an increasing no. of studies have also reported the use of fuels others than H - such as alcs., nonalc. C-based fuels, as well as N-based fuels.
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    The electrochem. redn. of CO in aq. soln. using phosphate, carbonate, and hydroxide electrolytes is reported. The highest total current efficiency was obtained at a current of 2.5 mA cm-2 with the KHCO3 electrolyte. Under these conditions the individual product-current efficiencies were CH4 16.3%, CH2:CH2 21.2%, EtOH 10.9%, PrOH 1.5%, and HCHO 0.1%.
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    Paidar, M.; Fateev, V.; Bouzek, K. Membrane Electrolysis—History, Current Status and Perspective. Electrochim. Acta 2016, 209, 737– 756,  DOI: 10.1016/j.electacta.2016.05.209
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    Membrane electrolysis - History, current status and perspective
    Paidar, M.; Fateev, V.; Bouzek, K.
    Electrochimica Acta (2016), 209 (), 737-756CODEN: ELCAAV; ISSN:0013-4686. (Elsevier Ltd.)
    A review. This review is devoted to membrane electrolysis, in particular using ion-selective membranes, as an important part of both existing and emerging industrial electrochem. processes. It aims to provide fundamental information on the history and development, current status and future perspectives of membrane electrolysis. An overview of the history of electro-membrane processes is given with the focus on brine electrolysis since it is the predominant electrochem. industrial technol. using ion-selective membranes. This is followed by a summary of the wide range of H-based energy conversion processes with different degrees of maturity, i.e. H2O electrolysis and fuel cells, which promise to become the next generation of major electro-membrane processes. The overview of the state-of-the-art is rounded off by a no. of smaller-scale processes using ionically conducting solid electrolytes and ion-selective membranes that are already com. available. The article concludes by considering potential future developments in this exciting field of electrochem.
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    Weber, A. Z.; Kusoglu, A. Unexplained Transport Resistances for Low-Loaded Fuel-Cell Catalyst Layers. J. Mater. Chem. A 2014, 2 (41), 17207– 17211,  DOI: 10.1039/C4TA02952F
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    Unexplained transport resistances for low-loaded fuel-cell catalyst layers
    Weber, Adam Z.; Kusoglu, Ahmet
    Journal of Materials Chemistry A: Materials for Energy and Sustainability (2014), 2 (41), 17207-17211CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
    For next-generation polymer-electrolyte fuel cells, material solns. are being sought to decrease the cost of the cell components and, in particular, the amt. of catalyst without sacrificing performance and lifetime. However, as recently shown, this cannot be achieved in practice due most likely to limitations caused by the ionomer thin-film surrounding the catalyst sites, where confinement and substrate interactions dominate and result in increased mass-transport limitations. Mitigation of this issue is paramount to the future com. viability of polymer-electrolyte fuel cells.
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    OpenURL UNIV OF NORTH TEXAS
  65. 65
    Tamura, J.; Ono, A.; Sugano, Y.; Huang, C.; Nishizawa, H.; Mikoshiba, S. Electrochemical Reduction of CO2 to Ethylene Glycol on Imidazolium Ion-Terminated Self-Assembly Monolayer-Modified Au Electrodes in an Aqueous Solution. Phys. Chem. Chem. Phys. 2015, 17 (39), 26072– 26078,  DOI: 10.1039/C5CP03028E
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    Electrochemical reduction of CO2 to ethylene glycol on imidazolium ion-terminated self-assembly monolayer-modified Au electrodes in an aqueous solution
    Tamura, Jun; Ono, Akihiko; Sugano, Yoshitsune; Huang, Chingchun; Nishizawa, Hideyuki; Mikoshiba, Satoshi
    Physical Chemistry Chemical Physics (2015), 17 (39), 26072-26078CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    Imidazolium ion-terminated self-assembled monolayer (SAM)-modified electrodes achieve CO2 conversion while suppressing H evolution. Immobile imidazolium ion on Au electrodes reduce CO2 at low overpotential. The distance between electrode and imidazolium ion sepd. by alkane thiol affects CO2 redn. activity. CO2 redn. current depends on the tunnel current rate. Although the product of CO2 redn. at the bare Au electrode is CO, SAM-modified electrodes produce ethylene glycol in aq. electrolyte soln. without CO evolution. The faradaic efficiency reached a max. of 87%. CO2 redn. at SAM-modified electrodes is unaffected by redn. activity of Au electrode. This phenomenon shows that the reaction field of CO2 redn. is not the electrode surface but the imidazolium ion monolayer.
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    OpenURL UNIV OF NORTH TEXAS
  66. 66
    Divekar, A. G.; Park, A. M.; Owczarczyk, Z. R.; Seifert, S.; Pivovar, B. S.; Herring, A. M. A Study of Carbonate Formation Kinetics and Morphological Effects Observed on Oh- Form of Pfaem When Exposed to Air Containing CO2. ECS Trans. 2017, 80 (8), 1005– 1011,  DOI: 10.1149/08008.1005ecst
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    A study of carbonate formation kinetics and morphological effects observed on OH- form of pfaem when exposed to air containing CO2
    Divekar, A. G.; Park, A. M.; Owczarczyk, Z. R.; Seifert, S.; Pivovar, B. S.; Herring, A. M.
    ECS Transactions (2017), 80 (8, Polymer Electrolyte Fuel Cells 17 (PEFC 17)), 1005-1011CODEN: ECSTF8; ISSN:1938-5862. (Electrochemical Society)
    For AEMFC, OH- form of membrane reacts with CO2 when exposed to air leading to loss in cond. Very few attempts have been made to understand the reaction kinetics, morphol. properties and equil. concns. at different environmental conditions. We have attempted to study the CO2 kinetics and its effect on water-uptake (or lambda) and morphol.(SAXS) when the OH- form of membrane is exposed to air which has approx. 400 ppm of CO2. The kinetics was studied by exposing the membrane to controlled environment and titrating it using Warder and Winkler titrn. methods. From transient SAXS anal. we observe the intensity of ionomer feature of membrane spectrum dropping over time. Also the d-spacing at equil. is lower than the initial value. Ultimately we want to understand the effect of CO2 on membrane from every aspect and possibly help us think about strategies to mitigate the problem.
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    OpenURL UNIV OF NORTH TEXAS
  67. 67
    Inaba, M.; Jensen, A. W.; Sievers, G. W.; Escudero-Escribano, M.; Zana, A.; Arenz, M. Benchmarking High Surface Area Electrocatalysts in a Gas Diffusion Electrode: Measurement of Oxygen Reduction Activities under Realistic Conditions. Energy Environ. Sci. 2018, 11, 988,  DOI: 10.1039/C8EE00019K
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    Benchmarking high surface area electrocatalysts in a gas diffusion electrode: measurement of oxygen reduction activities under realistic conditions
    Inaba, Masanori; Jensen, Anders Westergaard; Sievers, Gustav Wilhelm; Escudero-Escribano, Maria; Zana, Alessandro; Arenz, Matthias
    Energy & Environmental Science (2018), 11 (4), 988-994CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)
    In this work, we introduce the application of gas diffusion electrodes (GDE) for benchmarking the electrocatalytic performance of high surface area fuel cell catalysts. It is demonstrated that GDEs offer several inherent advantages over the state-of-the-art technique, i.e. thin film rotating disk electrode (TF-RDE) measurements for fast fuel cell catalyst evaluation. The most crit. advantage is reactant mass transport. While in RDE measurements the reactant mass transport is severely limited by the gas soly. of the reactant in the electrolyte, GDEs enable reactant transport rates similar to tech. fuel cell devices. Hence, in contrast to TF-RDE measurements, performance data obtained from GDE measurements can be directly compared to membrane electrode assembly (MEA) tests. Therefore, the application of GDEs for the testing of fuel cell catalysts closes the gap between catalyst research in academia and real applications.
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    OpenURL UNIV OF NORTH TEXAS
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    Wiltshire, R. J. K.; King, C. R.; Rose, A.; Wells, P. P.; Hogarth, M. P.; Thompsett, D.; Russell, A. E. A Pem Fuel Cell for in Situ Xas Studies. Electrochim. Acta 2005, 50 (25), 5208– 5217,  DOI: 10.1016/j.electacta.2005.05.038
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    A PEM fuel cell for in situ XAS studies
    Wiltshire, Richard J. K.; King, Colin R.; Rose, Abigail; Wells, Peter P.; Hogarth, Martin P.; Thompsett, David; Russell, Andrea E.
    Electrochimica Acta (2005), 50 (25-26), 5208-5217CODEN: ELCAAV; ISSN:0013-4686. (Elsevier B.V.)
    A miniature p exchange membrane (PEM) fuel cell enabled in situ XAS studies of the anode catalyst through fluorescence measurements. The development of the cell is described as well as the modifications required for elevated temp. operation and humidification of the feed gasses. The impact of the operating conditions increased the catalyst use, which is evident in the EXAFS collected at the Pt LIII and Ru K edges for a PtRu/C catalyst. The Pt component of the catalyst is readily reduced by H in the fuel, while the Ru was only fully reduced under conditions of good gas flow and electrochem. contact. Under such conditions no evidence of O neighbors were found at the Ru x-ray absorption edge. The results are interpreted in relation to the lack of surface sensitivity of the EXAFS method and indicate that the equil. coverage of O species on the Ru surface sites is too low to be obsd. using EXAFS.
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    OpenURL UNIV OF NORTH TEXAS
  69. 69
    Ramaker, D. E.; Korovina, A.; Croze, V.; Melke, J.; Roth, C. Following Orr Intermediates Adsorbed on a Pt Cathode Catalyst During Break-in of a Pem Fuel Cell by in Operando X-Ray Absorption Spectroscopy. Phys. Chem. Chem. Phys. 2014, 16 (27), 13645– 13653,  DOI: 10.1039/C4CP00192C
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    Following ORR intermediates adsorbed on a Pt cathode catalyst during break-in of a PEM fuel cell by in operando X-ray absorption spectroscopy
    Ramaker, D. E.; Korovina, A.; Croze, V.; Melke, J.; Roth, C.
    Physical Chemistry Chemical Physics (2014), 16 (27), 13645-13653CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    In operando X-ray absorption spectroscopy data using the Δμ X-ray Absorption Near Edge Spectroscopy (XANES) anal. procedure is used to follow the ORR intermediate adsorbate coverage on a working catalyst in a PEMFC during initial activation and break-in. The adsorbate coverage and log i (Tafel) curves reveal a strong correlation, i.e., an increase in adsorbate intermediate coverage poisons Pt sites thereby decreasing the current. A decrease in Pt-O bond strength commensurate with decrease in potential causes a sequence of different dominant adsorbate volcano curves to exist, namely first O, then OH, and then OOH exactly as predicted by the different ORR kinetics mechanisms. During break-in, the incipient O coverage coming from exposure to air during storage and MEA prepn. is rather quickly removed, compared to the slower and more subtle nanoparticle morphol. changes, such as the rounding of the Pt nanoparticle edges/corners and smoothing of the planar surfaces, driven by the nanoparticle's tendency to lower its surface energy. These morphol. changes increase the Pt-Pt av. coordination no., decrease the av. Pt-O bond strength, and thereby decrease the coverage of ORR intermediates, allowing increase in the current.
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    OpenURL UNIV OF NORTH TEXAS
  70. 70
    Ishiguro, N.; Saida, T.; Uruga, T.; Nagamatsu, S.-i.; Sekizawa, O.; Nitta, K.; Yamamoto, T.; Ohkoshi, S.-i.; Iwasawa, Y.; Yokoyama, T. Operando Time-Resolved X-Ray Absorption Fine Structure Study for Surface Events on a Pt3Co/C Cathode Catalyst in a Polymer Electrolyte Fuel Cell During Voltage-Operating Processes. ACS Catal. 2012, 2 (7), 1319– 1330,  DOI: 10.1021/cs300228p
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    Operando Time-Resolved X-ray Absorption Fine Structure Study for Surface Events on a Pt3Co/C Cathode Catalyst in a Polymer Electrolyte Fuel Cell during Voltage-Operating Processes
    Ishiguro, Nozomu; Saida, Takahiro; Uruga, Tomoya; Nagamatsu, Shin-ichi; Sekizawa, Oki; Nitta, Kiyofumi; Yamamoto, Takashi; Ohkoshi, Shin-ichi; Iwasawa, Yasuhiro; Yokoyama, Toshihiko; Tada, Mizuki
    ACS Catalysis (2012), 2 (7), 1319-1330CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
    The structural kinetics of surface events on a Pt3Co/C cathode catalyst in a polymer electrolyte fuel cell was investigated by operando time-resolved x-ray absorption fine structure with a time resoln. of 500 ms. The rate consts. of electrochem. reactions, the changes in charge d. on Pt, and the changes in the local coordination structures of the Pt3Co alloy catalyst in the polymer electrolyte fuel cell were successfully evaluated during fuel-cell voltage-operating processes. Significant time lags were obsd. between the electrochem. reactions and the structural changes in the Pt3Co alloy catalyst. The rate consts. of all the surface events on the Pt3Co/C catalyst were significantly higher than those on the Pt/C catalyst, suggesting the advantageous behaviors (cell performance and catalyst durability) on the Pt3Co alloy cathode catalyst.
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    OpenURL UNIV OF NORTH TEXAS
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    Casalongue, H. S.; Kaya, S.; Viswanathan, V.; Miller, D. J.; Friebel, D.; Hansen, H. A.; Nørskov, J. K.; Nilsson, A.; Ogasawara, H. Direct Observation of the Oxygenated Species During Oxygen Reduction on a Platinum Fuel Cell Cathode. Nat. Commun. 2013, 4, 2817,  DOI: 10.1038/ncomms3817
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    Sanchez Casalongue, H. G.; Ng, M. L.; Kaya, S.; Friebel, D.; Ogasawara, H.; Nilsson, A. In Situ Observation of Surface Species on Iridium Oxide Nanoparticles During the Oxygen Evolution Reaction. Angew. Chem., Int. Ed. 2014, 53 (28), 7169– 7172,  DOI: 10.1002/anie.201402311
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    In Situ Observation of Surface Species on Iridium Oxide Nanoparticles during the Oxygen Evolution Reaction
    Sanchez Casalongue, Hernan G.; Ng, May Ling; Kaya, Sarp; Friebel, Daniel; Ogasawara, Hirohito; Nilsson, Anders
    Angewandte Chemie, International Edition (2014), 53 (28), 7169-7172CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    An Ir oxide nanoparticle electrocatalyst under O evolution reaction conditions was probed in situ by ambient-pressure XPS. Under OER conditions, Ir undergoes a change in oxidn. state from IrIV to IrV that takes place predominantly at the surface of the catalyst. The chem. change in Ir is coupled to a decrease in surface hydroxide, providing exptl. evidence which strongly suggests that the O evolution reaction on Ir oxide occurs through an OOH-mediated deprotonation mechanism.
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    OpenURL UNIV OF NORTH TEXAS
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    Zenyuk, I. V.; Parkinson, D. Y.; Hwang, G.; Weber, A. Z. Probing Water Distribution in Compressed Fuel-Cell Gas-Diffusion Layers Using X-Ray Computed Tomography. Electrochem. Commun. 2015, 53, 24– 28,  DOI: 10.1016/j.elecom.2015.02.005
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    Probing water distribution in compressed fuel-cell gas-diffusion layers using X-ray computed tomography
    Zenyuk, Iryna V.; Parkinson, Dilworth Y.; Hwang, Gisuk; Weber, Adam Z.
    Electrochemistry Communications (2015), 53 (), 24-28CODEN: ECCMF9; ISSN:1388-2481. (Elsevier B.V.)
    X-ray computed tomog. was used to investigate geometrical land and channel effects on spatial liq.-water distribution in gas-diffusion layers (GDLs) of polymer-electrolyte fuel cells under different levels of compression. At low compression, a uniform liq.-water front was obsd. due to water redistribution and uniform porosity; however, at high compression, the water predominantly advanced at locations under the channel for higher liq. pressures. At low compression, no apparent correlation between the spatial liq. water and porosity distributions was obsd., whereas at high compression, a strong correlation was shown, indicating a potential for smart GDL architecture design with modulated porosity.
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    OpenURL UNIV OF NORTH TEXAS
  74. 74
    Medici, E. F.; Zenyuk, I. V.; Parkinson, D. Y.; Weber, A. Z.; Allen, J. S. Understanding Water Transport in Polymer Electrolyte Fuel Cells Using Coupled Continuum and Pore-Network Models. Fuel Cells 2016, 16 (6), 725– 733,  DOI: 10.1002/fuce.201500213
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    Understanding Water Transport in Polymer Electrolyte Fuel Cells Using Coupled Continuum and Pore-Network Models
    Medici, E. F.; Zenyuk, I. V.; Parkinson, D. Y.; Weber, A. Z.; Allen, J. S.
    Fuel Cells (Weinheim, Germany) (2016), 16 (6), 725-733CODEN: FUCEFK; ISSN:1615-6846. (Wiley-Blackwell)
    Water management remains a crit. issue for polymer electrolyte fuel cell performance and durability, esp. at lower temps. and with ultrathin electrodes. To understand and explain exptl. observations better, water transport in gas diffusion layers (GDLs) with macroscopically heterogeneous morphologies was simulated using a novel coupling of continuum and pore-network models. X-ray computed tomog. was used to ext. GDL material parameters for use in the pore-network model. The simulations were conducted to explain exptl. observations assocd. with stacking of anode GDLs, where stacking of the anode GDLs increased the limiting c.d. Through imaging, it is shown that the stacked anode GDL exhibited an interfacial region of high porosity. The coupled model shows that this morphol. allowed more efficient water movement through the anode and higher temps. at the cathode compared to the single GDL case. As a result, the cathode exhibited less flooding and hence better low temp. performance with the stacked anode GDL.
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    OpenURL UNIV OF NORTH TEXAS
  75. 75
    Cetinbas, F. C.; Wang, X. H.; Ahluwalia, R. K.; Kariuki, N. N.; Winarski, R. P.; Yang, Z. W.; Sharman, J.; Myers, D. J. Microstructural Analysis and Transport Resistances of Low-Platinum-Loaded Pefc Electrodes. J. Electrochem. Soc. 2017, 164 (14), F1596– F1607,  DOI: 10.1149/2.1111714jes
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    Microstructural Analysis and Transport Resistances of Low-Platinum-Loaded PEFC Electrodes
    Cetinbas, Firat C.; Wang, Xiaohua; Ahluwalia, Rajesh K.; Kariuki, Nancy N.; Winarski, Robert P.; Yang, Zhiwei; Sharman, Jonathan; Myers, Deborah J.
    Journal of the Electrochemical Society (2017), 164 (14), F1596-F1607CODEN: JESOAN; ISSN:0013-4651. (Electrochemical Society)
    We present microstructural characterization for polymer electrolyte fuel cell (PEFC) cathodes with low platinum group metal (PGM) loadings together with polarization data anal. Three-dimensional pore morphol. and ionomer distribution are resolved using nano-scale x-ray computed tomog. (nano-CT). Electrode structural properties are reported along with the effective ion and reactant transport properties. The characterization results are incorporated with 2-dimensional multi-physics model that accounts for energy, charge, and mass transport along with the effect of liq. water flooding. Defining total mass transport resistance for the whole polarization curve, contributions of transport mechanisms are identified. Anal. of the exptl. polarization curves at different operating pressures and temps. indicates that the mass transport resistance in the cathode is dominated by the transport processes in the electrode. It is shown that flooding in the electrode is a major contributor to transport losses esp. at elevated operating pressures while the pressure-independent resistance at the catalyst surface due to transport through the ionomer film plays a significant role, esp. at low temps. and low catalyst loading. By performing a parametric study for varying catalyst loadings, the importance of electrode roughness (i.e, electrochem.-active surface area/geometric electrode area) in detg. the mass transport losses is highlighted.
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    OpenURL UNIV OF NORTH TEXAS
  76. 76
    Komini Babu, S.; Chung, H. T.; Zelenay, P.; Litster, S. Resolving Electrode Morphology’s Impact on Platinum Group Metal-Free Cathode Performance Using Nano-Ct of 3d Hierarchical Pore and Ionomer Distribution. ACS Appl. Mater. Interfaces 2016, 8 (48), 32764– 32777,  DOI: 10.1021/acsami.6b08844
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    76
    Resolving Electrode Morphology's Impact on Platinum Group Metal-Free Cathode Performance Using Nano-CT of 3D Hierarchical Pore and Ionomer Distribution
    Komini Babu, Siddharth; Chung, Hoon T.; Zelenay, Piotr; Litster, Shawn
    ACS Applied Materials & Interfaces (2016), 8 (48), 32764-32777CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
    This article reports on the characterization of polymer electrolyte fuel cell (PEFC) cathodes featuring a platinum group metal-free (PGM-free) catalyst using nanoscale resoln. X-ray computed tomog. (nano-CT) and morphol. anal. PGM-free PEFC cathodes have gained significant interest in the past decade since they have the potential to dramatically reduce PEFC costs by eliminating the large platinum (Pt) raw material cost. However, several challenges remain before they are com. viable. Since these catalysts have lower volumetric activity, the PGM-free cathodes are thicker and subject to increased gas and proton transport resistances that reduce the performance. To better understand the efficacy of the catalyst and improve electrode performance, a detailed understanding the correlation between electrode fabrication, morphol., and performance is crucial. In this work, the pore/solid structure and the ionomer distribution was resolved in three dimensions (3D) using nano-CT for three PGM-free electrodes of varying Nafion loading. The assocd. transport properties were evaluated from pore/particle-scale simulations within the nano-CT-imaged structure. These characterizations are then used to elucidate the microstructural origins of the dramatic changes in fuel cell performance with varying Nafion ionomer loading. We show that this is primarily a result of distinct changes in ionomer's spatial distribution. The significant impact of electrode morphol. on performance highlights the importance of PGM-free electrode development in concert with efforts to improve catalyst activity and durability.
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    OpenURL UNIV OF NORTH TEXAS
  77. 77
    Cetinbas, F. C.; Ahluwalia, R. K.; Kariuki, N.; De Andrade, V.; Fongalland, D.; Smith, L.; Sharman, J.; Ferreira, P.; Rasouli, S.; Myers, D. J. Hybrid Approach Combining Multiple Characterization Techniques and Simulations for Microstructural Analysis of Proton Exchange Membrane Fuel Cell Electrodes. J. Power Sources 2017, 344, 62– 73,  DOI: 10.1016/j.jpowsour.2017.01.104
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    Hybrid approach combining multiple characterization techniques and simulations for microstructural analysis of proton exchange membrane fuel cell electrodes
    Cetinbas, Firat C.; Ahluwalia, Rajesh K.; Kariuki, Nancy; De Andrade, Vincent; Fongalland, Dash; Smith, Linda; Sharman, Jonathan; Ferreira, Paulo; Rasouli, Somaye; Myers, Deborah J.
    Journal of Power Sources (2017), 344 (), 62-73CODEN: JPSODZ; ISSN:0378-7753. (Elsevier B.V.)
    A review. The cost and performance of proton exchange membrane fuel cells strongly depend on the cathode electrode due to usage of expensive platinum (Pt) group metal catalyst and sluggish reaction kinetics. Development of low Pt content high performance cathodes requires comprehensive understanding of the electrode microstructure. In this study, a new approach is presented to characterize the detailed cathode electrode microstructure from nm to μm length scales by combining information from different exptl. techniques. In this context, nano-scale X-ray computed tomog. (nano-CT) is performed to ext. the secondary pore space of the electrode. Transmission electron microscopy (TEM) is employed to det. primary C particle and Pt particle size distributions. X-ray scattering, with its ability to provide size distributions of orders of magnitude more particles than TEM, is used to confirm the TEM-detd. size distributions. The no. of primary pores that cannot be resolved by nano-CT is approximated using mercury intrusion porosimetry. An algorithm is developed to incorporate all these exptl. data in one geometric representation. Upon validation of pore size distribution against gas adsorption and mercury intrusion porosimetry data, reconstructed ionomer size distribution is reported. In addn., transport related characteristics and effective properties are computed by performing simulations on the hybrid microstructure.
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    Vermaas, D. A.; Smith, W. A. Synergistic Electrochemical CO2 Reduction and Water Oxidation with a Bipolar Membrane. ACS Energy Lett. 2016, 1 (6), 1143– 1148,  DOI: 10.1021/acsenergylett.6b00557
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    78
    Synergistic Electrochemical CO2 Reduction and Water Oxidation with a Bipolar Membrane
    Vermaas, David A.; Smith, Wilson A.
    ACS Energy Letters (2016), 1 (6), 1143-1148CODEN: AELCCP; ISSN:2380-8195. (American Chemical Society)
    The electrochem. conversion of CO2 and water to value-added products still suffers from low efficiency, high costs, and high sensitivity to electrolyte, pH, and contaminants. Here, we present a strategy for this reaction using a silver catalyst for CO2 redn. in a neutral catholyte, sepd. by a bipolar membrane from a nickel iron hydroxide oxygen evolution catalyst in a basic anolyte. This combination of electrolytes provides a favorable environment for both catalysts and shows the effective use of bicarbonate and KOH to obtain low cell voltages. This architecture brings down the total cell voltage by more than 1 V compared to that with conventional use of a Pt counter electrode and monopolar membranes, and at the same time, it reduces contamination and improves stability at the cathode.
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    https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28XhvVSjurzI&md5=7d3fde0617927db0ab1419e8c1f621d5
    OpenURL UNIV OF NORTH TEXAS
  79. 79
    Li, Y. C.; Zhou, D.; Yan, Z.; Gonçalves, R. H.; Salvatore, D. A.; Berlinguette, C. P.; Mallouk, T. E. Electrolysis of CO2 to Syngas in Bipolar Membrane-Based Electrochemical Cells. ACS Energy Lett. 2016, 1 (6), 1149– 1153,  DOI: 10.1021/acsenergylett.6b00475
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    79
    Electrolysis of CO2 to Syngas in Bipolar Membrane-Based Electrochemical Cells
    Li, Yuguang C.; Zhou, Dekai; Yan, Zhifei; Goncalves, Ricardo H.; Salvatore, Danielle A.; Berlinguette, Curtis P.; Mallouk, Thomas E.
    ACS Energy Letters (2016), 1 (6), 1149-1153CODEN: AELCCP; ISSN:2380-8195. (American Chemical Society)
    The electrolysis of CO2 to syngas (CO + H2) using non-precious metal electrocatalysts was studied in bipolar membrane-based electrochem. cells. Electrolysis was carried out using aq. bicarbonate and humidified gaseous CO2 on the cathode side of the cell, with Ag or Bi/ionic liq. cathode electrocatalysts. In both cases, stable currents were obsd. over a period of hours with an aq. alk. electrolyte and NiFeOx electrocatalyst on the anode side of the cell. But the performance of the cells degraded rapidly when conventional anion- and cation-exchange membranes were used in place of the bipolar membrane. In agreement with earlier reports, the faradaic efficiency for CO2 redn. to CO was high at low overpotential. In the liq.-phase bipolar membrane cell, the faradaic efficiency was stable at ∼50% at 30 mA/cm2 c.d. In the gas-phase cell, current densities up to 200 mA/cm2 could be obtained, albeit at lower faradaic efficiency for CO prodn. At low overpotentials in the gas-phase cathode cell, the Faradaic efficiency for CO prodn. was initially high but dropped within 1 h, most likely because of dewetting of the ionic liq. from the Bi catalyst surface. The effective management of protons in bipolar membrane cells enables stable operation and the possibility of practical CO2 electrolysis at high current densities.
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  • Abstract

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    Figure 1

    Figure 1. Different electrochemical CO2R reactor schemes. (a) Aqueous-phase CO2R, where CO2 is first solubilized in an aqueous electrolyte and then reduced at a catalyst surface. Vapor-fed CO2R employing an (b) aqueous or (c) polymer electrolyte.

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    Figure 2

    Figure 2. State-of-the-art performance of vapor-fed CO2 devices. (a) Faradaic efficiencies versus partial current densities to ethylene, ethanol, carbon monoxide, and formate. (b) Energy efficiencies versus partial current densities to ethylene, carbon monoxide, formate, and hydrogen. Performances obtained for vapor-fed CO2R electrodes are shown in solid symbols, while performance for electrodes in aqueous-phase CO2R reactors are shown in hollow symbols. All energy efficiencies were calculated as voltage efficiencies using the formula:, where Eanode0 and Ecathode0 are the reversible potentials, FE is the faradaic efficiency for the CO2R product, and Vcell is the uncompensated cell voltage.

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    Figure 3

    Figure 3. Schematic of a three-dimensional GDE depicting the multiple length scales where phenomena are occurring during electrochemical CO2R.

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  • References

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    This article references 79 other publications.

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      Singh, M. R.; Goodpaster, J. D.; Weber, A. Z.; Head-Gordon, M.; Bell, A. T. Mechanistic Insights into Electrochemical Reduction of CO2 over Ag Using Density Functional Theory and Transport Models. Proc. Natl. Acad. Sci. U. S. A. 2017, 114 (42), E8812,  DOI: 10.1073/pnas.1713164114
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      Electrochem. redn. of CO2 using renewable sources of elec. energy holds promise for converting CO2 to fuels and chems. Since this process is complex and involves a large no. of species and phys. phenomena, a comprehensive understanding of the factors controlling product distribution is required. While the most plausible reaction pathway is usually identified from quantum-chem. calcn. of the lowest free-energy pathway, this approach can be misleading when coverages of adsorbed species detd. for alternative mechanism differ significantly, since elementary reaction rates depend on the product of the rate coeff. and the coverage of species involved in the reaction. Also, cathode polarization can influence the kinetics of CO2 redn. Here, the authors present a multiscale framework for ab initio simulation of the electrochem. redn. of CO2 over an Ag(110) surface. A continuum model for species transport is combined with a microkinetic model for the cathode reaction dynamics. Free energies of activation for all elementary reactions are detd. from d. functional theory calcns. Using this approach, three alternative mechanisms for CO2 redn. were examd. The rate-limiting step in each mechanism is **COOH formation at higher neg. potentials. However, only via the multiscale simulation was it possible to identify the mechanism that leads to a dependence of the rate of CO formation on the partial pressure of CO2 that is consistent with expts. Simulations based on this mechanism also describe the dependence of the H2 and CO current densities on cathode voltage that are in strikingly good agreement with exptl. observation.
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      In the last few years, there has been increased interest in electrochem. CO2 redn. (CO2R). Many exptl. studies employ a membrane sepd., electrochem. cell with a mini H-cell geometry to characterize CO2R catalysts in aq. soln. This type of electrochem. cell is a mini-chem. reactor and it is important to monitor the reaction conditions within the reactor to ensure that they are const. throughout the study. We show that operating cells with high catalyst surface area to electrolyte vol. ratios (S/V) at high current densities can have subtle consequences due to the complexity of the phys. phenomena taking place on electrode surfaces during CO2R, particularly as they relate to the cell temp. and bulk electrolyte CO2 concn. Both effects were evaluated quant. in high S/V cells using Cu electrodes and a bicarbonate buffer electrolyte. Electrolyte temp. is a function of the current/total voltage passed through the cell and the cell geometry. Even at a very high c.d., 20 mA cm-2, the temp. increase was less than 4 °C and a decrease of <10% in the dissolved CO2 concn. is predicted. In contrast, limits on the CO2 gas-liq. mass transfer into the cells produce much larger effects. By using the pH in the cell to measure the CO2 concn., significant undersatn. of CO2 is obsd. in the bulk electrolyte, even at more modest current densities of 10 mA cm-2. Undersatn. of CO2 produces large changes in the faradaic efficiency obsd. on Cu electrodes, with H2 prodn. becoming increasingly favored. We show that the size of the CO2 bubbles being introduced into the cell is crit. for maintaining the equil. CO2 concn. in the electrolyte, and we have designed a high S/V cell that is able to maintain the near-equil. CO2 concn. at current densities up to 15 mA cm-2.
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      OpenURL UNIV OF NORTH TEXAS
    7. 7
      Hashiba, H.; Weng, L.-C.; Chen, Y.; Sato, H. K.; Yotsuhashi, S.; Xiang, C.; Weber, A. Z. Effects of Electrolyte Buffer Capacity on Surface Reactant Species and the Reaction Rate of CO2 in Electrochemical Co2 Reduction. J. Phys. Chem. C 2018, 122 (7), 3719– 3726,  DOI: 10.1021/acs.jpcc.7b11316
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      7
      Effects of Electrolyte Buffer Capacity on Surface Reactant Species and the Reaction Rate of CO2 in Electrochemical CO2 Reduction
      Hashiba, Hiroshi; Weng, Lien-Chun; Chen, Yikai; Sato, Hiroki K.; Yotsuhashi, Satoshi; Xiang, Chengxiang; Weber, Adam Z.
      Journal of Physical Chemistry C (2018), 122 (7), 3719-3726CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
      In the aq. electrochem. redn. of CO2, the choice of electrolyte is responsible for the catalytic activity and selectivity, although there remains a need for more in-depth understanding of electrolyte effects and mechanisms. In this study, using both exptl. and simulation approaches, how the buffer capacity of the electrolytes affects the kinetics and equil. of surface reactant species and the resulting reaction rate of CO2 with varying partial CO2 pressure are reported. Electrolytes investigated include KCl (non-buffered), KHCO3 (buffered by bicarbonate), and phosphate-buffered electrolytes. Assuming 100% methane prodn., the simulation successfully explains the exptl. trends in max. CO2 flux in KCl and KHCO3 and also highlights the difference between KHCO3 and phosphate in terms of pKa as well as the impact of the buffer capacity. To examine the electrolyte impact on selectivity, the model is run with a const. total c.d. Using this model, several factors are elucidated, including the importance of local pH, which is not in acid/base equil., the impact of buffer identity and kinetics, and the mass-transport boundary-layer thickness. The gained understanding can help to optimize CO2 redn. in aq. environments.
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      OpenURL UNIV OF NORTH TEXAS
    8. 8
      Hashiba, H.; Yotsuhashi, S.; Deguchi, M.; Yamada, Y. Systematic Analysis of Electrochemical CO2 Reduction with Various Reaction Parameters Using Combinatorial Reactors. ACS Comb. Sci. 2016, 18 (4), 203– 208,  DOI: 10.1021/acscombsci.6b00021
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      8
      Systematic Analysis of Electrochemical CO2 Reduction with Various Reaction Parameters using Combinatorial Reactors
      Hashiba, Hiroshi; Yotsuhashi, Satoshi; Deguchi, Masahiro; Yamada, Yuka
      ACS Combinatorial Science (2016), 18 (4), 203-208CODEN: ACSCCC; ISSN:2156-8944. (American Chemical Society)
      Applying combinatorial technol. to electrochem. CO2 redn. offers a broad range of possibilities for optimizing the reaction conditions. In this work, the CO2 pressure, stirring speed, and reaction temp. were varied to investigate the effect on the rate of CO2 supply to copper electrode and the assocd. effects on reaction products, including CH4. Expts. were performed in a 0.5 M KCl soln. using a combinatorial screening reactor system consisting of eight identical, automatically controlled reactors. Increasing the CO2 pressure and stirring speed, or decreasing the temp., steadily suppressed H2 prodn. and increased the prodn. of other reaction products including CH4 across a broad range of current densities. Our anal. shows that the CO2 pressure, stirring speed, and reaction temp. independently contributed to the limiting rate of CO2 supply to the electrode (Jlim). At a const. temp., the limiting c.d. of CH4 increased proportionally with Jlim, illustrating that the prodn. rate of CH4 was proportional to CO2 supply. Varying the CO2 pressure and stirring speed hardly affected the max. Faradaic efficiency of CH4 prodn. However, changes to the reaction temp. showed a significant contribution to CH4 selectivity. This study highlights the importance of quant. anal. of CO2 supply in clarifying the role of various reaction parameters and understanding more comprehensively the selectivity and reaction rate of electrochem. CO2 redn.
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      OpenURL UNIV OF NORTH TEXAS
    9. 9
      Ahn, S. T.; Abu-Baker, I.; Palmore, G. T. R. Electroreduction of CO2 on Polycrystalline Copper: Effect of Temperature on Product Selectivity. Catal. Today 2017, 288, 24– 29,  DOI: 10.1016/j.cattod.2016.09.028
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      9
      Electroreduction of CO2 on polycrystalline copper: Effect of temperature on product selectivity
      Ahn, Steven T.; Abu-Baker, Ismael; Palmore, G. Tayhas R.
      Catalysis Today (2017), 288 (), 24-29CODEN: CATTEA; ISSN:0920-5861. (Elsevier B.V.)
      The authors study the effect of temp. on product selectivity for the electrochem. redn. of CO2 on polycryst. Cu. The temp. of an electrolyte soln. can influence several reaction parameters in the CO2 redn. reaction (CO2RR), including pH (both local and bulk), concn. of dissolved CO2, soln. resistance, the rate of diffusion of reactants to the electrode surface, and adsorbed intermediates. The working potential was -1.60 V vs. Ag/AgCl to allow direct comparison with literature values. Under optimal reaction conditions at 2°, the faradaic efficiency (FE) for converting CO2 to methane (CH4) on a Cu electrocatalyst increases to ∼50% while ethylene (C2H4) decreases to ∼10%. Above room temp., the prodn. of H gas (H2) dominates the electrochem. reaction, reaching >50% FE. The major products (i.e., >5% FE) obsd. at all temps. studied were H2, CH4, C2H4, CO, and formate, with product selectivity driven by changes in [CO2] rather than changes in pH. The authors' initial goal was to confirm pH dependence of CO2RR on Cu at different temps. Importantly, pH varies minimally over the temp. range studied (2° to 42°), in contrast to changes in [CO2] and corresponding changes in product selectivity.
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      OpenURL UNIV OF NORTH TEXAS
    10. 10
      Liu, X.; Xiao, J.; Peng, H.; Hong, X.; Chan, K.; Nørskov, J. K. Understanding Trends in Electrochemical Carbon Dioxide Reduction Rates. Nat. Commun. 2017, 8, 15438,  DOI: 10.1038/ncomms15438
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      10
      Understanding trends in electrochemical carbon dioxide reduction rates
      Liu, Xinyan; Xiao, Jianping; Peng, Hongjie; Hong, Xin; Chan, Karen; Noerskov, Jens K.
      Nature Communications (2017), 8 (), 15438CODEN: NCAOBW; ISSN:2041-1723. (Nature Publishing Group)
      Electrochem. carbon dioxide redn. to fuels presents one of the great challenges in chem. Herein we present an understanding of trends in electrocatalytic activity for carbon dioxide redn. over different metal catalysts that rationalize a no. of exptl. observations including the selectivity with respect to the competing hydrogen evolution reaction. We also identify two design criteria for more active catalysts. The understanding is based on d. functional theory calcns. of activation energies for electrochem. carbon monoxide redn. as a basis for an electrochem. kinetic model of the process. We develop scaling relations relating transition state energies to the carbon monoxide adsorption energy and det. the optimal value of this descriptor to be very close to that of copper.
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      OpenURL UNIV OF NORTH TEXAS
    11. 11
      Rendón-Calle, A.; Builes, S.; Calle-Vallejo, F. A Brief Review of the Computational Modeling of CO2 Electroreduction on Cu Electrodes. Curr. Opin. Electrochem. 2018, 9, 158,  DOI: 10.1016/j.coelec.2018.03.012
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      11
      A brief review of the computational modeling of CO2 electroreduction on Cu electrodes
      Rendon-Calle, Alejandra; Builes, Santiago; Calle-Vallejo, Federico
      Current Opinion in Electrochemistry (2018), 9 (), 158-165CODEN: COEUCY; ISSN:2451-9111. (Elsevier B.V.)
      Electrochem. redn. of CO2 (CO2RR) to hydrocarbons and alcs. is a promising yet intricate catalytic process with large assocd. overpotentials. On Cu, the most active metal, factors such as pH, solvation, anions and cations in soln., in addn. to the catalysts' structure, modify the reaction mechanism and play an important role on the catalytic activity and selectivity. Such an extraordinary complexity calls for an in-depth understanding of CO2RR that eventually leads to its optimization. In this brief review, we illustrate how computational methods have aided in recent years to gain insight on CO2RR. We show the achievements and limitations of well-established methods based on Gibbs energy diagrams, calcns. in vacuum, the computational hydrogen electrode and scaling-relation-based volcano plots. Besides, we review advances on kinetics of electrochem. steps, structure-sensitive screening, ion effects, and machine learning.
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      OpenURL UNIV OF NORTH TEXAS
    12. 12
      Garza, A. J.; Bell, A. T.; Head-Gordon, M. Mechanism of CO2 Reduction at Copper Surfaces: Pathways to C2 Products. ACS Catal. 2018, 8 (2), 1490– 1499,  DOI: 10.1021/acscatal.7b03477
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      12
      Mechanism of CO2 Reduction at Copper Surfaces: Pathways to C2 Products
      Garza, Alejandro J.; Bell, Alexis T.; Head-Gordon, Martin
      ACS Catalysis (2018), 8 (2), 1490-1499CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
      From constraints from reported exptl. observations and d. functional theory simulations, the authors propose a mechanism for the redn. of CO2 to C2 products on Cu electrodes. To model the effects of an applied potential bias on the reactions, calcns. are carried out with a variable, fractional no. of electrons on the unit cell, which is optimized so that the Fermi level matches the actual chem. potential of electrons (i.e., the applied bias); an implicit electrolyte model allows for compensation of the surface charge so that neutrality is maintained in the overall simulation cell. The authors' mechanism explains the presence of the seven C2 species that were detected in the reaction, as well as other notable exptl. observations. Also, the authors' results shed light on the difference in activities toward C2 products between the (100) and (111) facets of Cu. The authors compare the authors' methodologies and findings with those in other recent mechanistic studies of the Cu-catalyzed CO2 redn. reaction.
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      OpenURL UNIV OF NORTH TEXAS
    13. 13
      Kortlever, R.; Shen, J.; Schouten, K. J. P.; Calle-Vallejo, F.; Koper, M. T. M. Catalysts and Reaction Pathways for the Electrochemical Reduction of Carbon Dioxide. J. Phys. Chem. Lett. 2015, 6 (20), 4073– 4082,  DOI: 10.1021/acs.jpclett.5b01559
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      13
      Catalysts and Reaction Pathways for the Electrochemical Reduction of Carbon Dioxide
      Kortlever, Ruud; Shen, Jing; Schouten, Klaas Jan P.; Calle-Vallejo, Federico; Koper, Marc T. M.
      Journal of Physical Chemistry Letters (2015), 6 (20), 4073-4082CODEN: JPCLCD; ISSN:1948-7185. (American Chemical Society)
      A review. The electrochem. redn. of CO2 has gained significant interest recently as it has the potential to trigger a sustainable solar-fuel-based economy. In this Perspective, we highlight several heterogeneous and mol. electrocatalysts for the redn. of CO2 and discuss the reaction pathways through which they form various products. Among those, copper is a unique catalyst as it yields hydrocarbon products, mostly methane, ethylene, and ethanol, with acceptable efficiencies. As a result, substantial effort has been invested to det. the special catalytic properties of copper and to elucidate the mechanism through which hydrocarbons are formed. These mechanistic insights, together with mechanistic insights of CO2 redn. on other metals and mol. complexes, can provide crucial guidelines for the design of future catalyst materials able to efficiently and selectively reduce CO2 to useful products.
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      OpenURL UNIV OF NORTH TEXAS
    14. 14
      Clark, E. L.; Hahn, C.; Jaramillo, T. F.; Bell, A. T. Electrochemical CO2 Reduction over Compressively Strained Cuag Surface Alloys with Enhanced Multi-Carbon Oxygenate Selectivity. J. Am. Chem. Soc. 2017, 139 (44), 15848– 15857,  DOI: 10.1021/jacs.7b08607
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      14
      Electrochemical CO2 Reduction over Compressively Strained CuAg Surface Alloys with Enhanced Multi-Carbon Oxygenate Selectivity
      Clark, Ezra L.; Hahn, Christopher; Jaramillo, Thomas F.; Bell, Alexis T.
      Journal of the American Chemical Society (2017), 139 (44), 15848-15857CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      The electrochem. redn. of carbon dioxide using renewably generated electricity offers a potential means for producing fuels and chems. in a sustainable manner. To date, copper has been found to be the most effective catalyst for electrochem. reducing carbon dioxide to products such as methane, ethene, and ethanol. Unfortunately, the current efficiency of the process is limited by competition with the relatively facile hydrogen evolution reaction. Since multi-carbon products are more valuable precursors to chems. and fuels than methane, there is considerable interest in modifying copper to enhance the multi-carbon product selectivity. Here, we report our investigations of electrochem. carbon dioxide redn. over CuAg bimetallic electrodes and surface alloys, which we find to be more selective for the formation of multi-carbon products than pure copper. This selectivity enhancement is a result of the selective suppression of hydrogen evolution, which occurs due to compressive strain induced by the formation of a CuAg surface alloy. Furthermore, we report that these bimetallic electrocatalysts exhibit an unusually high selectivity for the formation of multi-carbon carbonyl-contg. products, which we hypothesize to be the consequence of a reduced coverage of adsorbed hydrogen and the reduced oxophilicity of the compressively strained copper. Thus, we show that promoting copper surface with small amts. of Ag is a promising means for improving the multi-carbon oxygenated product selectivity of copper during electrochem. CO2 redn.
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      OpenURL UNIV OF NORTH TEXAS
    15. 15
      Han, Z.; Kortlever, R.; Chen, H.-Y.; Peters, J. C.; Agapie, T. CO2 Reduction Selective for C ≥ 2 Products on Polycrystalline Copper with N-Substituted Pyridinium Additives. ACS Cent. Sci. 2017, 3 (8), 853– 859,  DOI: 10.1021/acscentsci.7b00180
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      15
      CO2 Reduction Selective for C≥2 Products on Polycrystalline Copper with N-Substituted Pyridinium Additives
      Han, Zhiji; Kortlever, Ruud; Chen, Hsiang-Yun; Peters, Jonas C.; Agapie, Theodor
      ACS Central Science (2017), 3 (8), 853-859CODEN: ACSCII; ISSN:2374-7951. (American Chemical Society)
      Electrocatalytic CO2 redn. to generate multicarbon products is of interest for applications in artificial photosynthetic schemes. This is a particularly attractive goal for CO2 redn. by Cu electrodes, where a broad range of hydrocarbon products can be generated but where selectivity for C-C coupled products relative to CH4 and H2 remains an impediment. Herein the authors report a simple yet highly selective catalytic system for CO2 redn. to C≥2 hydrocarbons on a polycryst. Cu electrode in bicarbonate aq. soln. that uses N-substituted pyridinium additives. Selectivities of 70-80% for C2 and C3 products with a hydrocarbon ratio of C≥2/CH4 significantly >100 were obsd. with several additives. 13C-labeling studies verify CO2 to be the sole C source in the C≥2 hydrocarbons produced. Upon electroredn., the N-substituted pyridinium additives lead to film deposition on the Cu electrode, identified in one case as the reductive coupling product of N-arylpyridinium. Product selectivity can also be tuned from C≥2 species to H2 (∼90%) while suppressing methane with certain N-heterocyclic additives.
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      OpenURL UNIV OF NORTH TEXAS
    16. 16
      Higgins, D.; Landers, A. T.; Ji, Y.; Nitopi, S.; Morales-Guio, C. G.; Wang, L.; Chan, K.; Hahn, C.; Jaramillo, T. F. Guiding Electrochemical Carbon Dioxide Reduction toward Carbonyls Using Copper Silver Thin Films with Interphase Miscibility. ACS Energy Lett. 2018, 3, 2947– 2955,  DOI: 10.1021/acsenergylett.8b01736
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      16
      Guiding Electrochemical Carbon Dioxide Reduction toward Carbonyls Using Copper Silver Thin Films with Interphase Miscibility
      Higgins, Drew; Landers, Alan T.; Ji, Yongfei; Nitopi, Stephanie; Morales-Guio, Carlos G.; Wang, Lei; Chan, Karen; Hahn, Christopher; Jaramillo, Thomas F.
      ACS Energy Letters (2018), 3 (12), 2947-2955CODEN: AELCCP; ISSN:2380-8195. (American Chemical Society)
      Steering the selectivity of Cu-based electrochem. CO2 redn. (CO2R) catalysts toward targeted products will serve to improve the technoeconomic outlook of technologies based on this process. Using phys. vapor deposition as a tool to overcome thermodn. miscibility limitations, CuAg thin films with nonequil. Cu/Ag alloying were prepd. for CO2R performance evaluation. In comparison to pure Cu, the CuAg thin films showed significantly higher activity and selectivity toward liq. carbonyl products, including acetaldehyde and acetate. Suppressed activity and selectivity toward hydrocarbons and the competing H evolution were also demonstrated on CuAg thin films, with a greater degree of suppression obsd. at increasing nominal Ag compns. Compositional-dependent CO2R trends coupled with phys. characterization and d. functional theory suggest that significant miscibility of Ag into the Cu-rich phase of the catalyst underpinned the obsd. CO2R trends through tuning of adsorbate and reaction intermediate binding energies on the surface.
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      OpenURL UNIV OF NORTH TEXAS
    17. 17
      Raciti, D.; Wang, C. Recent Advances in CO2 Reduction Electrocatalysis on Copper. ACS Energy Lett. 2018, 3 (7), 1545– 1556,  DOI: 10.1021/acsenergylett.8b00553
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      17
      Recent Advances in CO2 Reduction Electrocatalysis on Copper
      Raciti, David; Wang, Chao
      ACS Energy Letters (2018), 3 (7), 1545-1556CODEN: AELCCP; ISSN:2380-8195. (American Chemical Society)
      Electroredn. of CO2 represents a promising approach toward artificial C recycling for addressing global challenges in energy and sustainability. The foreground of this approach is dependent on the development of efficient electrocatalysts capable of selectively reducing CO2 to valuable (oxygenated) hydrocarbon products at low overpotentials. Here, we present an overview of the recent developments of Cu electrocatalysts for CO2 redn. The focus is placed on elucidation of the structure-property relations of monometallic Cu electrocatalysts, which is believed to be the foundation for understanding alloys and other more complex catalytic systems. Reported mechanisms are discussed in terms of grain boundaries, open facets, residual oxides, subsurface O, local pH effect, etc. After this discussion, remaining questions are raised for further development of advanced electrocatalysts for energy and chem. efficient CO2 redn.
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      OpenURL UNIV OF NORTH TEXAS
    18. 18
      De Luna, P.; Wei, J.; Bengio, Y.; Aspuru-Guzik, A.; Sargent, E. Use Machine Learning to Find Energy Materials. Nature 2017, 552 (7683), 23,  DOI: 10.1038/d41586-017-07820-6
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      18
      Use machine learning to find energy materials
      De Luna, Phil; Wei, Jennifer; Bengio, Yoshua; Aspuru-Guzik, Alan; Sargent, Edward
      Nature (London, United Kingdom) (2017), 552 (7683), 23-27CODEN: NATUAS; ISSN:0028-0836. (Nature Research)
      Artificial intelligence can speed up research into new photovoltaic, battery and carbon-capture materials, argue Edward Sargent, Ala´n Aspuru-Guzikand colleagues.
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      OpenURL UNIV OF NORTH TEXAS
    19. 19
      Ulissi, Z. W.; Tang, M. T.; Xiao, J.; Liu, X.; Torelli, D. A.; Karamad, M.; Cummins, K.; Hahn, C.; Lewis, N. S.; Jaramillo, T. F. Machine-Learning Methods Enable Exhaustive Searches for Active Bimetallic Facets and Reveal Active Site Motifs for CO2 Reduction. ACS Catal. 2017, 7 (10), 6600– 6608,  DOI: 10.1021/acscatal.7b01648
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      19
      Machine-Learning Methods Enable Exhaustive Searches for Active Bimetallic Facets and Reveal Active Site Motifs for CO2 Reduction
      Ulissi, Zachary W.; Tang, Michael T.; Xiao, Jianping; Liu, Xinyan; Torelli, Daniel A.; Karamad, Mohammadreza; Cummins, Kyle; Hahn, Christopher; Lewis, Nathan S.; Jaramillo, Thomas F.; Chan, Karen; Noerskov, Jens K.
      ACS Catalysis (2017), 7 (10), 6600-6608CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
      Bimetallic catalysts are promising for the most difficult thermal and electrochem. reactions but modeling the many diverse active sites on polycryst. samples is an open challenge. The authors present a general framework for addressing this complexity in a systematic and predictive fashion. Active sites for every stable low-index facet of a bimetallic crystal are enumerated and cataloged yielding hundreds of possible active sites. The activity of these sites is explored in parallel using a neural-network based surrogate model to share information between the many D. Functional Theory (DFT) relaxations, resulting in activity ests. with an order of magnitude fewer explicit DFT calcns. Sites with interesting activity were found and provide targets for follow-up calcns. This process was applied to the electrochem. redn. of CO2 on Ni Ga bimetallics and indicated that most facets had similar activity to Ni surfaces, but a few exposed Ni sites with a very favorable on-top CO configuration. This motif emerged naturally from the predictive modeling and represents a class of intermetallic CO2 redn. catalysts. These sites rationalize recent exptl. reports of Ni Ga activity and why previous materials screens missed this exciting material. Most importantly these methods suggest that bimetallic catalysts will be discovered by studying facet reactivity and diversity of active sites more systematically.
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      OpenURL UNIV OF NORTH TEXAS
    20. 20
      Singh, M. R.; Clark, E. L.; Bell, A. T. Effects of Electrolyte, Catalyst, and Membrane Composition and Operating Conditions on the Performance of Solar-Driven Electrochemical Reduction of Carbon Dioxide. Phys. Chem. Chem. Phys. 2015, 17 (29), 18924– 18936,  DOI: 10.1039/C5CP03283K
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      20
      Effects of electrolyte, catalyst, and membrane composition and operating conditions on the performance of solar-driven electrochemical reduction of carbon dioxide
      Singh, Meenesh R.; Clark, Ezra L.; Bell, Alexis T.
      Physical Chemistry Chemical Physics (2015), 17 (29), 18924-18936CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
      Solar-driven electrochem. cells can be used to convert carbon dioxide, water, and sunlight into transportation fuels or into precursors to such fuels. The voltage efficiency of such devices depends on the (i) phys. properties of its components (catalysts, electrolyte, and membrane); (ii) operating conditions (carbon dioxide flowrate and pressure, c.d.); and (iii) phys. dimensions of the cell. The sources of energy loss in a carbon dioxide redn. (CO2R) cell are the anode and cathode overpotentials, the difference in pH between the anode and cathode, the difference in the partial pressure of carbon dioxide between the bulk electrolyte and the cathode, the ohmic loss across the electrolyte and the diffusional resistances across the boundary layers near the electrodes. In this study, we analyze the effects of these losses and propose optimal device configurations for the efficient operation of a CO2R electrochem. cell operating at a c.d. of 10 mA cm-2. Cell operation at near-neutral bulk pH offers not only lower polarization losses but also better selectivity to CO2R vs. hydrogen evolution. Addn. of supporting electrolyte to increase its cond. has a neg. impact on cell performance because it reduces the elec. field and the soly. of CO2. Addn. of a pH buffer reduces the polarization losses but may affect catalyst selectivity. The carbon dioxide flowrate and partial pressure can have severe effects on the cell efficiency if the carbon dioxide supply rate falls below the consumption rate. The overall potential losses can be reduced by use of an anion, rather than a cation, exchange membrane. We also show that the max. polarization losses occur for the electrochem. synthesis of CO and that such losses are lower for the synthesis of products requiring a larger no. of electrons per mol., assuming a fixed c.d. We also find that the reported electrocatalytic activity of copper below -1 V vs. RHE is strongly influenced by excessive polarization of the cathode and, hence, does not represent its true activity at bulk conditions. This article provides useful guidelines for minimizing polarization losses in solar-driven CO2R electrochem. cells and a method for predicting polarization losses and obtaining kinetic overpotentials from measured partial current densities.
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      OpenURL UNIV OF NORTH TEXAS
    21. 21
      Greenblatt, J. B.; Miller, D. J.; Ager, J. W.; Houle, F. A.; Sharp, I. D. The Technical and Energetic Challenges of Separating (Photo)Electrochemical Carbon Dioxide Reduction Products. Joule 2018, 2 (3), 381– 420,  DOI: 10.1016/j.joule.2018.01.014
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      21
      The Technical and Energetic Challenges of Separating (Photo)Electrochemical Carbon Dioxide Reduction Products
      Greenblatt, Jeffery B.; Miller, Daniel J.; Ager, Joel W.; Houle, Frances A.; Sharp, Ian D.
      Joule (2018), 2 (3), 381-420CODEN: JOULBR; ISSN:2542-4351. (Cell Press)
      Known catalysts for (photo)electrochem. carbon dioxide (CO2) redn. typically generate multiple products, including hydrogen, carbon monoxide, hydrocarbons, and oxygenates, making product sepn. a ubiquitous, yet often overlooked, challenge. Here, we review CO2 redn. products using available catalysts and discuss approaches for product sepn. along with ests. of sepn. energy requirements. We illustrate potential complexities and discuss opportunities to minimize sepns. by utilizing product mixts. We also examine potential CO2 sources, their energy requirements, and net CO2 emissions. Finally, we discuss use of waste energy sources and integrate this information into an overall energy balance assessment. Using a common sustainability metric, energy return on energy investment (EROEI), we find that an EROEI of ∼2.0 may be possible, before including sepn. and CO2 prodn. energy. For EROEI to remain above one (the break-even point), these addnl. energy requirements, including embodied energy of equipment, must be no greater than half of the product energy.
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      OpenURL UNIV OF NORTH TEXAS
    22. 22
      Weekes, D. M.; Salvatore, D. A.; Reyes, A.; Huang, A.; Berlinguette, C. P. Electrolytic CO2 Reduction in a Flow Cell. Acc. Chem. Res. 2018, 51 (4), 910– 918,  DOI: 10.1021/acs.accounts.8b00010
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      22
      Electrolytic CO2 Reduction in a Flow Cell
      Weekes, David M.; Salvatore, Danielle A.; Reyes, Angelica; Huang, Aoxue; Berlinguette, Curtis P.
      Accounts of Chemical Research (2018), 51 (4), 910-918CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)
      A review. Electrocatalytic CO2 conversion at near ambient temps. and pressures offers a potential means of converting waste greenhouse gases into fuels or commodity chems. (e.g., CO, formic acid, methanol, ethylene, alkanes, and alcs.). This process is particularly compelling when driven by excess renewable electricity because the consequent prodn. of solar fuels would lead to a closing of the carbon cycle. However, such a technol. is not currently com. available. While CO2 electrolysis in H-cells is widely used for screening electrocatalysts, these expts. generally do not effectively report on how CO2 electrocatalysts behave in flow reactors that are more relevant to a scalable CO2 electrolyzer system. Flow reactors also offer more control over reagent delivery, which includes enabling the use of a gaseous CO2 feed to the cathode of the cell. This setup provides a platform for generating much higher current densities (J) by reducing the mass transport issues inherent to the H-cells.In this Account, we examine some of the systems-level strategies that have been applied in an effort to tailor flow reactor components to improve electrocatalytic redn. Flow reactors that have been utilized in CO2 electrolysis schemes can be categorized into two primary architectures: Membrane-based flow cells and microfluidic reactors. Each invoke different dynamic mechanisms for the delivery of gaseous CO2 to electrocatalytic sites, and both have been demonstrated to achieve high current densities (J > 200 mA cm-2) for CO2 redn. One strategy common to both reactor architectures for improving J is the delivery of CO2 to the cathode in the gas phase rather than dissolved in a liq. electrolyte. This phys. facet also presents a no. of challenges that go beyond the nature of the electrocatalyst, and we scrutinize how the judicious selection and modification of certain components in microfluidic and/or membrane-based reactors can have a profound effect on electrocatalytic performance. In membrane-based flow cells, for example, the choice of either a cation exchange membrane (CEM), anion exchange membrane (AEM), or a bipolar membrane (BPM) affects the kinetics of ion transport pathways and the range of applicable electrolyte conditions. In microfluidic cells, extensive studies have been performed upon the properties of porous carbon gas diffusion layers, materials that are equally relevant to membrane reactors. A theme that is pervasive throughout our analyses is the challenges assocd. with precise and controlled water management in gas phase CO2 electrolyzers, and we highlight studies that demonstrate the importance of maintaining adequate flow cell hydration to achieve sustained electrolysis.
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      OpenURL UNIV OF NORTH TEXAS
    23. 23
      Jhong, H.-R. M.; Ma, S.; Kenis, P. J. A. Electrochemical Conversion of CO2 to Useful Chemicals: Current Status, Remaining Challenges, and Future Opportunities. Curr. Opin. Chem. Eng. 2013, 2 (2), 191– 199,  DOI: 10.1016/j.coche.2013.03.005
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      OpenURL UNIV OF NORTH TEXAS
    24. 24
      Dinh, C.-T.; Burdyny, T.; Kibria, M. G.; Seifitokaldani, A.; Gabardo, C. M.; García de Arquer, F. P.; Kiani, A.; Edwards, J. P.; De Luna, P.; Bushuyev, O. S. CO2 Electroreduction to Ethylene Via Hydroxide-Mediated Copper Catalysis at an Abrupt Interface. Science 2018, 360 (6390), 783– 787,  DOI: 10.1126/science.aas9100
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      24
      Carbon dioxide electroreduction to ethylene via hydroxide-mediated copper catalysis at an abrupt interface
      Dinh, Cao-Thang; Burdyny, Thomas; Kibria, Md Golam; Seifitokaldani, Ali; Gabardo, Christine M.; Garcia de Arquer, F. Pelayo; Kiani, Amirreza; Edwards, Jonathan P.; De Luna, Phil; Bushuyev, Oleksandr S.; Zou, Chengqin; Quintero-Bermudez, Rafael; Pang, Yuanjie; Sinton, David; Sargent, Edward H.
      Science (Washington, DC, United States) (2018), 360 (6390), 783-787CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
      Carbon dioxide (CO2) electroredn. could provide a useful source of ethylene, but low conversion efficiency, low prodn. rates, and low catalyst stability limit current systems. Here we report that a copper electrocatalyst at an abrupt reaction interface in an alk. electrolyte reduces CO2 to ethylene with 70% faradaic efficiency at a potential of -0.55 V vs. a reversible hydrogen electrode (RHE). Hydroxide ions on or near the copper surface lower the CO2 redn. and carbon monoxide (CO)-CO coupling activation energy barriers; as a result, onset of ethylene evolution at -0.165 V vs. an RHE in 10 M potassium hydroxide occurs almost simultaneously with CO prodn. Operational stability was enhanced via the introduction of a polymer-based gas diffusion layer that sandwiches the reaction interface between sep. hydrophobic and conductive supports, providing const. ethylene selectivity for an initial 150 operating hours.
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      OpenURL UNIV OF NORTH TEXAS
    25. 25
      Ma, S.; Sadakiyo, M.; Luo, R.; Heima, M.; Yamauchi, M.; Kenis, P. J. A. One-Step Electrosynthesis of Ethylene and Ethanol from CO2 in an Alkaline Electrolyzer. J. Power Sources 2016, 301, 219– 228,  DOI: 10.1016/j.jpowsour.2015.09.124
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      25
      One-step electrosynthesis of ethylene and ethanol from CO2 in an alkaline electrolyzer
      Ma, Sichao; Sadakiyo, Masaaki; Luo, Raymond; Heima, Minako; Yamauchi, Miho; Kenis, Paul J. A.
      Journal of Power Sources (2016), 301 (), 219-228CODEN: JPSODZ; ISSN:0378-7753. (Elsevier B.V.)
      Electroredn. of CO2 has potential for storing otherwise wasted intermittent renewable energy, while reducing emission of CO2 into the atm. Identifying robust and efficient electrocatalysts and assocd. optimum operating conditions to produce hydrocarbons at high energetic efficiency (low overpotential) remains a challenge. In this study, four Cu nanoparticle catalysts of different morphol. and compn. (amt. of surface oxide) are synthesized and their activities towards CO2 redn. are characterized in an alk. electrolyzer. Use of catalysts with large surface roughness results in a combined Faradaic efficiency (46%) for the electroredn. of CO2 to ethylene and ethanol in combination with current densities of ∼200 mA cm-2, a 10-fold increase in performance achieved at much lower overpotential (only < 0.7 V) compared to prior work. Compared to prior work, the high prodn. levels of ethylene and ethanol can be attributed mainly to the use of alk. electrolyte to improve kinetics and the suppressed evolution of H2, as well as the application of gas diffusion electrodes covered with active and rough Cu nanoparticles in the electrolyzer. These high performance levels and the gained fundamental understanding on Cu-based catalysts bring electrochem. redn. processes such as presented here closer to practical application.
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      OpenURL UNIV OF NORTH TEXAS
    26. 26
      Hoang, T. T. H.; Verma, S.; Ma, S.; Fister, T. T.; Timoshenko, J.; Frenkel, A. I.; Kenis, P. J. A.; Gewirth, A. A. Nanoporous Copper–Silver Alloys by Additive-Controlled Electrodeposition for the Selective Electroreduction of CO2 to Ethylene and Ethanol. J. Am. Chem. Soc. 2018, 140 (17), 5791– 5797,  DOI: 10.1021/jacs.8b01868
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      26
      Nanoporous Copper-Silver Alloys by Additive-Controlled Electrodeposition for the Selective Electroreduction of CO2 to Ethylene and Ethanol
      Hoang, Thao T. H.; Verma, Sumit; Ma, Sichao; Fister, Tim T.; Timoshenko, Janis; Frenkel, Anatoly I.; Kenis, Paul J. A.; Gewirth, Andrew A.
      Journal of the American Chemical Society (2018), 140 (17), 5791-5797CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Electrodeposition of CuAg alloy films from plating baths contg. 3,5-diamino-1,2,4-triazole (DAT) as an inhibitor yields high surface area catalysts for the active and selective electroredn. of CO2 to multicarbon hydrocarbons and oxygenates. EXAFS shows the co-deposited alloy film to be homogeneously mixed. The alloy film contg. 6% Ag exhibits the best CO2 electroredn. performance, with the faradaic efficiency for C2H4 and EtOH prodn. reaching nearly 60 and 25%, resp., at a cathode potential of just -0.7 V vs. RHE and a total c.d. of approx. - 300 mA/cm2. Such high levels of selectivity at high activity and low applied potential are the highest reported to date. In situ Raman and electroanal. studies suggest the origin of the high selectivity toward C2 products to be a combined effect of the enhanced stabilization of the Cu2O overlayer and the optimal availability of the CO intermediate due to the Ag incorporated in the alloy.
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      OpenURL UNIV OF NORTH TEXAS
    27. 27
      Zhuang, T.-T.; Liang, Z.-Q.; Seifitokaldani, A.; Li, Y.; De Luna, P.; Burdyny, T.; Che, F.; Meng, F.; Min, Y.; Quintero-Bermudez, R. Steering Post-C–C Coupling Selectivity Enables High Efficiency Electroreduction of Carbon Dioxide to Multi-Carbon Alcohols. Nature Catalysis 2018, 1 (6), 421– 428,  DOI: 10.1038/s41929-018-0084-7
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      OpenURL UNIV OF NORTH TEXAS
    28. 28
      Lv, J.-J.; Jouny, M.; Luc, W.; Zhu, W.; Zhu, J.-J.; Jiao, F. A Highly Porous Copper Electrocatalyst for Carbon Dioxide Reduction. Adv. Mater. 2018, 30 (49), 1803111,  DOI: 10.1002/adma.201803111
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      OpenURL UNIV OF NORTH TEXAS
    29. 29
      Verma, S.; Hamasaki, Y.; Kim, C.; Huang, W.; Lu, S.; Jhong, H.-R. M.; Gewirth, A. A.; Fujigaya, T.; Nakashima, N.; Kenis, P. J. A. Insights into the Low Overpotential Electroreduction of CO2 to CO on a Supported Gold Catalyst in an Alkaline Flow Electrolyzer. ACS Energy Lett. 2018, 3 (1), 193– 198,  DOI: 10.1021/acsenergylett.7b01096
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      29
      Insights into the Low Overpotential Electroreduction of CO2 to CO on a Supported Gold Catalyst in an Alkaline Flow Electrolyzer
      Verma, Sumit; Hamasaki, Yuki; Kim, Chaerin; Huang, Wenxin; Lu, Shawn; Jhong, Huei-Ru Molly; Gewirth, Andrew A.; Fujigaya, Tsuyohiko; Nakashima, Naotoshi; Kenis, Paul J. A.
      ACS Energy Letters (2018), 3 (1), 193-198CODEN: AELCCP; ISSN:2380-8195. (American Chemical Society)
      Cost competitive electroredn. of CO2 to CO requires electrochem. systems that exhibit partial c.d. (jCO) exceeding 150 mAcm-2 at cell overpotentials (|ηcell|) <1 V. However, achieving such benchmarks remains difficult. Here, the authors report the electroredn. of CO2 on a supported Au catalyst in an alk. flow electrolyzer with performance levels close to the economic viability criteria. Onset of CO prodn. occurred at cell and cathode overpotentials of just -0.25 and -0.02 V, resp. High jCO (∼99, 158 mAcm-2) was obtained at low |ηcell| (∼0.70, 0.94 V) and high CO energetic efficiency (∼63.8, 49.4%). The performance was stable for at least 8 h. Addnl., the onset cathode potentials, kinetic isotope effect, and Tafel slopes indicate the low overpotential prodn. of CO in alk. media to be the result of a pH-independent rate-detg. step (i.e., electron transfer) in contrast to a pH-dependent overall process.
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      OpenURL UNIV OF NORTH TEXAS
    30. 30
      Ma, S.; Luo, R.; Gold, J. I.; Yu, A. Z.; Kim, B.; Kenis, P. J. A. Carbon Nanotube Containing Ag Catalyst Layers for Efficient and Selective Reduction of Carbon Dioxide. J. Mater. Chem. A 2016, 4 (22), 8573– 8578,  DOI: 10.1039/C6TA00427J
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      30
      Carbon nanotube containing Ag catalyst layers for efficient and selective reduction of carbon dioxide
      Ma, Sichao; Luo, Raymond; Gold, Jake I.; Yu, Aaron Z.; Kim, Byoungsu; Kenis, Paul J. A.
      Journal of Materials Chemistry A: Materials for Energy and Sustainability (2016), 4 (22), 8573-8578CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
      Over the last few decades significant progress was made in the development of catalysts for efficient and selective electroredn. of CO2. These improvements in catalyst performance were of the extent that identifying electrodes of optimum structure and compn. has become key to further improve throughput levels in the electrolysis of CO2 to CO. Here the authors report on a simple 1-step method to incorporate multi-walled C nanotubes (MWCNT) in the catalyst layer to form gas diffusion electrodes with different structures: (i) a mixed catalyst layer in which the Ag nanoparticle catalyst and MWCNTs are homogeneously distributed; and (ii) a layered catalyst layer comprised of a layer of MWCNTs covered with a layer of Ag catalyst. Both approaches improve performance in the electroredn. of CO2 compared to electrodes that lack MWCNTs. The mixed layer performed best: an electrolyzer operated at a cell potential of -3 V using 1 M KOH as the electrolyte yielded unprecedented high levels of CO prodn. of up to 350 mA cm-2 at high faradaic efficiency (>95% selective for CO) and an energy efficiency of 45% under the same condition. Electrochem. impedance spectroscopy measurements indicate that the obsd. differences in electrode performance can be attributed to a lower charge transfer resistance in the mixed catalyst layer. A simple optimization of electrode structure and compn., i.e. incorporation of MWCNTs in the catalyst layer of a GDE, has a profound beneficial effect on their performance in electrocatalytic conversion of CO2, while allowing for a lower precious metal catalyst loading with improved performance.
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      OpenURL UNIV OF NORTH TEXAS
    31. 31
      Dufek, E. J.; Lister, T. E.; Stone, S. G.; McIlwain, M. E. Operation of a Pressurized System for Continuous Reduction of CO2. J. Electrochem. Soc. 2012, 159 (9), F514– F517,  DOI: 10.1149/2.011209jes
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      31
      Operation of a pressurized system for continuous reduction of CO2
      Dufek, Eric J.; Lister, Tedd E.; Stone, Simon G.; McIlwain, Michael E.
      Journal of the Electrochemical Society (2012), 159 (9), F514-F517CODEN: JESOAN; ISSN:0013-4651. (Electrochemical Society)
      A pressurized electrochem. system equipped for continuous redn. of CO2 is presented. At elevated pressures, using a Ag-based cathode, the quantity of CO which can be generated is 5 times that obsd. at ambient pressure with faradaic efficiencies ≤92% obsd. at 350 mA cm-2. For operation at 225 mA cm-2 and 60° the cell voltage at 18.5 atm was 0.4 V below that obsd. at ambient pressure. Increasing the temp. further to 90° led to a cell voltage <3 V (18.5 atm and 90 °C), which equates to an elec. efficiency of 50%.
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      OpenURL UNIV OF NORTH TEXAS
    32. 32
      Haas, T.; Krause, R.; Weber, R.; Demler, M.; Schmid, G. Technical Photosynthesis Involving CO2 Electrolysis and Fermentation. Nature Catalysis 2018, 1 (1), 32– 39,  DOI: 10.1038/s41929-017-0005-1
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      OpenURL UNIV OF NORTH TEXAS
    33. 33
      Whipple, D. T.; Finke, E. C.; Kenis, P. J. A. Microfluidic Reactor for the Electrochemical Reduction of Carbon Dioxide: The Effect of Ph. Electrochem. Solid-State Lett. 2010, 13 (9), B109– B111,  DOI: 10.1149/1.3456590
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      Microfluidic reactor for the electrochemical reduction of carbon dioxide: The effect of pH
      Whipple, Devin T.; Finke, Eryn C.; Kenis, Paul J. A.
      Electrochemical and Solid-State Letters (2010), 13 (9), B109-B111CODEN: ESLEF6; ISSN:1099-0062. (Electrochemical Society)
      This article reports the development and characterization of a microfluidic reactor for the electrochem. redn. of CO2. The use of gas diffusion electrodes enables better control of the three-phase interface where the reactions take place. Furthermore, the versatility of the microfluidic reactor enables rapid evaluation of catalysts under different operating conditions. Several catalysts as well as the effects of electrolyte pH on reactor efficiency for redn. of CO2 to formic acid were tested. Operating at acidic pH resulted in a significant increase in performance: Faradaic and energetic efficiencies of 89 and 45%, resp., and c.d. of ≈100 mA/cm2.
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    34. 34
      Lu, X.; Leung, D. Y. C.; Wang, H.; Xuan, J. A High Performance Dual Electrolyte Microfluidic Reactor for the Utilization of CO2. Appl. Energy 2017, 194, 549– 559,  DOI: 10.1016/j.apenergy.2016.05.091
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      34
      A high performance dual electrolyte microfluidic reactor for the utilization of CO2
      Lu, Xu; Leung, Dennis Y. C.; Wang, Huizhi; Xuan, Jin
      Applied Energy (2017), 194 (), 549-559CODEN: APENDX; ISSN:0306-2619. (Elsevier Ltd.)
      The pH-differential membraneless architecture could enhance the thermodn. property and raise the electrochem. performance of a dual electrolyte microfluidic reactor (DEMR) for electrochem. conversion of CO2. Freed from hindrances of membrane structure and thermodn. limitation, DEMR demonstrates the possibility of altering anolyte and catholyte pHs to achieve higher reactivity rates and efficiencies. Different operation condition parameters of a microfluidic network would affect the reactor performance to a certain extents, constraining further improvement. Therefore, we conducted exptl. anal. to study the mechanisms and intrinsic correlations of catalyst to Nafion ratio, microchannel thickness, electrolyte flow rate and CO2 supply for an optimized outcome. A comprehensive investigation on the cell durability was also carried out in the way of repetitiveness and long period operation, regarding both reactivity and efficiency. It was found that the catalyst to Nafion ratio affects the performance in a parabolic relation and there exists optimal values of electrolyte flow rate and microfluidic channel thickness for maximized cell performance. The influence of the reactant CO2 supply rate is not significant above a certain level where kinetics limitation is not dominant. The parametric study provides an operational point of view on the dual electrolyte microfluidic reactor and serves as a tool for DEMR optimization design.
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    35. 35
      Li, H.; Oloman, C. Development of a Continuous Reactor for the Electro-Reduction of Carbon Dioxide to Formate – Part 2: Scale-Up. J. Appl. Electrochem. 2007, 37 (10), 1107– 1117,  DOI: 10.1007/s10800-007-9371-8
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      35
      Development of a continuous reactor for the electro-reduction of carbon dioxide to formate
      Li, Hui; Oloman, Colin
      Journal of Applied Electrochemistry (2007), 37 (10), 1107-1117CODEN: JAELBJ; ISSN:0021-891X. (Springer)
      This paper reports exptl. and modeling work for the lab. scale-up of continuous "trickle-bed" reactors for the electro-redn. of CO2 to potassium formate. Two reactors (A and B) were employed, with particulate tin 3D cathodes of superficial areas, resp., 45 × 10-4 (2-14 A) and 320 × 10-4 m2 (20-100 A). Expts. in Reactor A using granulated tin cathodes (99.9 wt% Sn) and a feed gas of 100% CO2 showed slightly better performance than that of the tinned-copper mesh cathodes of our previous communications, while giving substantially improved temporal stability (200 vs. 20 min). The seven-fold scaled-up Reactor B used a feed gas of 100% CO2 with the aq. catholyte and anolyte, resp. [0.5 M KHCO3 + 2 M KCl] and 2 M KOH, at inlet pressure from 350 to 600 kPa(abs) and outlet temp. 295 to 325 K. For a superficial c.d. of 0.6-3.1 kA m-2 Reactor B achieved corresponding formate current efficiencies of 91-63%, with the same range of reactor voltage as that in Reactor A (2.7-4.3 V), which reflects the success of the scale-up in this work. Up to 1 M formate was obtained in the catholyte product from a single pass in Reactor B, but when the catholyte feed was spiked with 2-3 M potassium formate there was a large drop in current efficiency due to formate cross-over through the Nafion 117 membrane. An extended reactor (cathode) model that used four fitted kinetic parameters and assumed zero formate cross-over was able to mirror the reactor performance with reasonable fidelity over a wide range of conditions (max. error in formate CE = ±20%), including formate product concns. up to 1 M.
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    36. 36
      Yang, H.; Kaczur, J. J.; Sajjad, S. D.; Masel, R. I. Electrochemical Conversion of Co2 to Formic Acid Utilizing Sustainion Membranes. J. CO2 Utilization 2017, 20, 208– 217,  DOI: 10.1016/j.jcou.2017.04.011
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      36
      Electrochemical conversion of CO2 to formic acid utilizing Sustainion membranes
      Yang, Hongzhou; Kaczur, Jerry J.; Sajjad, Syed Dawar; Masel, Richard I.
      Journal of CO2 Utilization (2017), 20 (), 208-217CODEN: JCUOAJ; ISSN:2212-9839. (Elsevier Ltd.)
      Formic acid generated from CO2 has been proposed both as a key intermediate renewable chem. feedstock as well as a potential chem.-based energy storage media for hydrogen. In this paper, we describe a novel three-compartment electrochem. cell configuration with the capability of directly producing a pure formic acid product in the concn. range of 5-20 wt% at high current densities and Faradaic yields. The electrochem. cell employs a Dioxide Materials Sustainion anion exchange membrane and a nanoparticle Sn GDE cathode contg. an imidazole ionomer, allowing for improved CO2 electrochem. redn. performance. Stable electrochem. cell performance for more than 500 h was exptl. demonstrated at a c.d. of 140 mA cm-2 at a cell voltage of only 3.5 V. Future work will include cell scale-up and increasing cell Faradaic performance using selected electrocatalysts and membranes.
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    37. 37
      Kuhl, K. P.; Cave, E. R.; Abram, D. N.; Jaramillo, T. F. New Insights into the Electrochemical Reduction of Carbon Dioxide on Metallic Copper Surfaces. Energy Environ. Sci. 2012, 5 (5), 7050– 7059,  DOI: 10.1039/c2ee21234j
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      37
      New insights into the electrochemical reduction of carbon dioxide on metallic copper surfaces
      Kuhl, Kendra P.; Cave, Etosha R.; Abram, David N.; Jaramillo, Thomas F.
      Energy & Environmental Science (2012), 5 (5), 7050-7059CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)
      We report new insights into the electrochem. redn. of CO2 on a metallic copper surface, enabled by the development of an exptl. methodol. with unprecedented sensitivity for the identification and quantification of CO2 electroredn. products. This involves a custom electrochem. cell designed to maximize product concns. coupled to gas chromatog. and NMR for the identification and quantification of gas and liq. products, resp. We studied copper across a range of potentials and obsd. a total of 16 different CO2 redn. products, five of which are reported here for the first time, thus providing the most complete view of the reaction chem. reported to date. Taking into account the chem. identities of the wide range of C1-C3 products generated and the potential-dependence of their turnover frequencies, mechanistic information is deduced. We discuss a scheme for the formation of multi-carbon products involving enol-like surface intermediates as a possible pathway, accounting for the obsd. selectivity for eleven distinct C2+ oxygenated products including aldehydes, ketones, alcs., and carboxylic acids.
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    38. 38
      Jiang, K.; Sandberg, R. B.; Akey, A. J.; Liu, X.; Bell, D. C.; Nørskov, J. K.; Chan, K.; Wang, H. Metal Ion Cycling of Cu Foil for Selective C–C Coupling in Electrochemical CO2 Reduction. Nature Catalysis 2018, 1 (2), 111– 119,  DOI: 10.1038/s41929-017-0009-x
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      There is no corresponding record for this reference.
      OpenURL UNIV OF NORTH TEXAS
    39. 39
      Kim, D.; Kley, C. S.; Li, Y.; Yang, P. Copper Nanoparticle Ensembles for Selective Electroreduction of CO2 to C2–C3 Products. Proc. Natl. Acad. Sci. U. S. A. 2017, 114 (40), 10560– 10565,  DOI: 10.1073/pnas.1711493114
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      39
      Copper nanoparticle ensembles for selective electroreduction of CO2 to C2-C3 products
      Kim, Dohyung; Kley, Christopher S.; Li, Yifan; Yang, Peidong
      Proceedings of the National Academy of Sciences of the United States of America (2017), 114 (40), 10560-10565CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
      Direct conversion of carbon dioxide to multicarbon products remains as a grand challenge in electrochem. CO2 redn. Various forms of oxidized copper have been demonstrated as electrocatalysts that still require large overpotentials. Here, we show that an ensemble of Cu nanoparticles (NPs) enables selective formation of C2-C3 products at low overpotentials. Densely packed Cu NP ensembles underwent structural transformation during electrolysis into electrocatalytically active cube-like particles intermixed with smaller nanoparticles. Ethylene, ethanol, and n-propanol are the major C2-C3 products with onset potential at -0.53 V (vs. reversible hydrogen electrode, RHE) and C2-C3 faradaic efficiency (FE) reaching 50% at only -0.75 V. Thus, the catalyst exhibits selective generation of C2-C3 hydrocarbons and oxygenates at considerably lowered overpotentials in neutral pH aq. media. In addn., this approach suggests new opportunities in realizing multicarbon product formation from CO2, where the majority of efforts has been to use oxidized copper-based materials. Robust catalytic performance is demonstrated by 10 h of stable operation with C2-C3 c.d. 10 mA/cm2 (at -0.75 V), rendering it attractive for solar-to-fuel applications. Tafel anal. suggests reductive CO coupling as a rate detg. step for C2 products, while n-propanol (C3) prodn. seems to have a discrete pathway.
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      OpenURL UNIV OF NORTH TEXAS
    40. 40
      Ren, D.; Ang, B. S.-H.; Yeo, B. S. Tuning the Selectivity of Carbon Dioxide Electroreduction toward Ethanol on Oxide-Derived Cuxzn Catalysts. ACS Catal. 2016, 6 (12), 8239– 8247,  DOI: 10.1021/acscatal.6b02162
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      40
      Tuning the Selectivity of Carbon Dioxide Electroreduction toward Ethanol on Oxide-Derived CuxZn Catalysts
      Ren, Dan; Ang, Bridget Su-Hui; Yeo, Boon Siang
      ACS Catalysis (2016), 6 (12), 8239-8247CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
      The electrochem. redn. of carbon dioxide (CO2) to ethanol (C2H5OH) and ethylene (C2H4) using renewable electricity is a viable method for the prodn. of these com. vital chems. Copper (Cu) and its oxides are by far the most effective electrocatalysts for this purpose. However, the formation of ethanol using these catalysts is generally less favored in comparison to that of ethylene. In this work, we demonstrate that the selectivity of CO2 redn. toward ethanol could be tuned by introducing a cocatalyst to generate an in situ source of mobile CO reactant. Cu-based oxides with different amts. of Zn dopants (Cu, Cu10Zn, Cu4Zn, and Cu2Zn) were prepd. and used as catalysts under ambient pressure in aq. 0.1 M KHCO3 electrolyte. By varying the amt. of Zn in the bimetallic catalysts, we found that the selectivity of ethanol vs. ethylene prodn., defined by the ratio of their Faradaic efficiencies (FEethanol/FEethylene), could be tuned by a factor of up to ∼12.5. Ethanol formation was maximized on Cu4Zn at -1.05 V vs RHE, with a remarkable Faradaic efficiency and c.d. of 29.1% and -8.2 mA/cm2, resp. The Cu4Zn catalyst was also catalytically stable for the prodn. of ethanol for at least 5 h. The importance of Zn as a CO-producing site was demonstrated by performing CO2 redn. on Cu-Ni and Cu-Ag bimetallic catalysts. Operando Raman spectroscopy revealed that the as-deposited Cu-based oxide films were reduced to the metallic state during CO2 redn., after which only signals belonging to CO adsorbed on Cu sites were recorded. This showed that the redn. of CO2 probably occurred on metallic sites rather than on metal oxides. A two-site mechanism to rationalize the selective redn. of CO2 to ethanol is proposed and discussed.
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      OpenURL UNIV OF NORTH TEXAS
    41. 41
      Cave, E. R.; Montoya, J. H.; Kuhl, K. P.; Abram, D. N.; Hatsukade, T.; Shi, C.; Hahn, C.; Nørskov, J. K.; Jaramillo, T. F. Electrochemical CO2 Reduction on Au Surfaces: Mechanistic Aspects Regarding the Formation of Major and Minor Products. Phys. Chem. Chem. Phys. 2017, 19 (24), 15856– 15863,  DOI: 10.1039/C7CP02855E
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      41
      Electrochemical CO2 reduction on Au surfaces: mechanistic aspects regarding the formation of major and minor products
      Cave, Etosha R.; Montoya, Joseph H.; Kuhl, Kendra P.; Abram, David N.; Hatsukade, Toru; Shi, Chuan; Hahn, Christopher; Noerskov, Jens K.; Jaramillo, Thomas F.
      Physical Chemistry Chemical Physics (2017), 19 (24), 15856-15863CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
      In the future, industrial CO2 electro-redn. using renewable energy sources could be a sustainable means to convert CO2 and water to commodity chems. at room temp. and atm. pressure. This work focused on the electrocatalytic redn. of CO2 over polycryst. Au surfaces which have high activity and selectivity for CO evolution. The catalytic behavior of polycryst. Au surfaces were examd. by coupling potentiostatic CO2 electrolysis expts. in an aq. HCO3- soln. with high sensitivity product detection methods. Methanol prodn. of methanol was obsd., in addn. to detecting known products of CO2 electroredn. over Au: CO, H2, and formate. The authors suggested a mechanism to explain methanol evolution from Au, specifically, the Au surface does not favor C-O scission, thus is more selective toward methanol than CH4. These insights could aid in designing electrocatalysts selective for CO2 electroredn. to oxygenates over hydrocarbons.
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      OpenURL UNIV OF NORTH TEXAS
    42. 42
      Hatsukade, T.; Kuhl, K. P.; Cave, E. R.; Abram, D. N.; Jaramillo, T. F. Insights into the Electrocatalytic Reduction of CO2 on Metallic Silver Surfaces. Phys. Chem. Chem. Phys. 2014, 16 (27), 13814– 13819,  DOI: 10.1039/C4CP00692E
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      42
      Insights into the electrocatalytic reduction of CO2 on metallic silver surfaces
      Hatsukade, Toru; Kuhl, Kendra P.; Cave, Etosha R.; Abram, David N.; Jaramillo, Thomas F.
      Physical Chemistry Chemical Physics (2014), 16 (27), 13814-13819CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
      The electrochem. redn. of CO2 could allow for a sustainable process by which renewable energy from wind and solar are used directly in the prodn. of fuels and chems. In this work we investigated the potential dependent activity and selectivity of the electrochem. redn. of CO2 on metallic silver surfaces under ambient conditions. Our results deepen our understanding of the surface chem. and provide insight into the factors important to designing better catalysts for the reaction. The high sensitivity of our exptl. methods for identifying and quantifying products of reaction allowed for the observation of six redn. products including CO and hydrogen as major products and formate, methane, methanol, and ethanol as minor products. By quantifying the potential-dependent behavior of all products, we provide insights into kinetics and mechanisms at play, in particular involving the prodn. of hydrocarbons and alcs. on catalysts with weak CO binding energy as well as the formation of a C-C bond required to produce ethanol.
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      OpenURL UNIV OF NORTH TEXAS
    43. 43
      Lu, Q.; Rosen, J.; Zhou, Y.; Hutchings, G. S.; Kimmel, Y. C.; Chen, J. G.; Jiao, F. A Selective and Efficient Electrocatalyst for Carbon Dioxide Reduction. Nat. Commun. 2014, 5, 3242,  DOI: 10.1038/ncomms4242
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      43
      A selective and efficient electrocatalyst for carbon dioxide reduction
      Lu Qi; Rosen Jonathan; Zhou Yang; Hutchings Gregory S; Kimmel Yannick C; Jiao Feng; Chen Jingguang G
      Nature communications (2014), 5 (), 3242 ISSN:.
      Converting carbon dioxide to useful chemicals in a selective and efficient manner remains a major challenge in renewable and sustainable energy research. Silver is an interesting electrocatalyst owing to its capability of converting carbon dioxide to carbon monoxide selectively at room temperature; however, the traditional polycrystalline silver electrocatalyst requires a large overpotential. Here we report a nanoporous silver electrocatalyst that is able to electrochemically reduce carbon dioxide to carbon monoxide with approximately 92% selectivity at a rate (that is, current) over 3,000 times higher than its polycrystalline counterpart under moderate overpotentials of <0.50 V. The high activity is a result of a large electrochemical surface area (approximately 150 times larger) and intrinsically high activity (approximately 20 times higher) compared with polycrystalline silver. The intrinsically higher activity may be due to the greater stabilization of CO2 (-) intermediates on the highly curved surface, resulting in smaller overpotentials needed to overcome the thermodynamic barrier.
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      OpenURL UNIV OF NORTH TEXAS
    44. 44
      Chen, Y.; Li, C. W.; Kanan, M. W. Aqueous CO2 Reduction at Very Low Overpotential on Oxide-Derived Au Nanoparticles. J. Am. Chem. Soc. 2012, 134 (49), 19969– 19972,  DOI: 10.1021/ja309317u
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      44
      Aqueous CO2 Reduction at Very Low Overpotential on Oxide-Derived Au Nanoparticles
      Chen, Yihong; Li, Christina W.; Kanan, Matthew W.
      Journal of the American Chemical Society (2012), 134 (49), 19969-19972CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Carbon dioxide redn. is an essential component of many prospective technologies for the renewable synthesis of carbon-contg. fuels. Known catalysts for this reaction generally suffer from low energetic efficiency, poor product selectivity, and rapid deactivation. It is shown that the redn. of thick Au oxide films results in the formation of Au nanoparticles (oxide-derived Au) that exhibit highly selective CO2 redn. to CO in water at overpotentials as low as 140 mV and retain their activity for at least 8 h. Under identical conditions, polycryst. Au electrodes and several other nanostructured Au electrodes prepd. via alternative methods require at least 200 mV of addnl. overpotential to attain comparable CO2 redn. activity and rapidly lose their activity. Electrokinetic studies indicate that the improved catalysis is linked to dramatically increased stabilization of the CO2•- intermediate on the surfaces of the oxide-derived Au electrodes.
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      OpenURL UNIV OF NORTH TEXAS
    45. 45
      Zheng, X.; De Luna, P.; García de Arquer, F. P.; Zhang, B.; Becknell, N.; Ross, M. B.; Li, Y.; Banis, M. N.; Li, Y.; Liu, M. Sulfur-Modulated Tin Sites Enable Highly Selective Electrochemical Reduction of CO2 to Formate. Joule 2017, 1 (4), 794– 805,  DOI: 10.1016/j.joule.2017.09.014
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      45
      Sulfur-Modulated Tin Sites Enable Highly Selective Electrochemical Reduction of CO2 to Formate
      Zheng, Xueli; De Luna, Phil; Garcia de Arquer, F. Pelayo; Zhang, Bo; Becknell, Nigel; Ross, Michael B.; Li, Yifan; Banis, Mohammad Norouzi; Li, Yuzhang; Liu, Min; Voznyy, Oleksandr; Dinh, Cao Thang; Zhuang, Taotao; Stadler, Philipp; Cui, Yi; Du, Xiwen; Yang, Peidong; Sargent, Edward H.
      Joule (2017), 1 (4), 794-805CODEN: JOULBR; ISSN:2542-4351. (Cell Press)
      Electrochem. redn. of carbon dioxide (CO2RR) to formate provides an avenue to the synthesis of value-added carbon-based fuels and feedstocks powered using renewable electricity. Here, we hypothesized that the presence of sulfur atoms in the catalyst surface could promote undercoordinated sites, and thereby improve the electrochem. redn. of CO2 to formate. We explored, using d. functional theory, how the incorporation of sulfur into tin may favor formate generation. We used at. layer deposition of SnSx followed by a redn. process to synthesize sulfur-modulated tin (Sn(S)) catalysts. X-ray absorption near-edge structure (XANES) studies reveal higher oxidn. states in Sn(S) compared with that of tin in Sn nanoparticles. Sn(S)/Au accelerates CO2RR at geometric current densities of 55 mA cm-2 at -0.75 V vs. reversible hydrogen electrode with a Faradaic efficiency of 93%. Furthermore, Sn(S) catalysts show excellent stability without deactivation (<2% productivity change) following more than 40 h of operation.
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      OpenURL UNIV OF NORTH TEXAS
    46. 46
      Feaster, J. T.; Shi, C.; Cave, E. R.; Hatsukade, T.; Abram, D. N.; Kuhl, K. P.; Hahn, C.; Nørskov, J. K.; Jaramillo, T. F. Understanding Selectivity for the Electrochemical Reduction of Carbon Dioxide to Formic Acid and Carbon Monoxide on Metal Electrodes. ACS Catal. 2017, 7 (7), 4822– 4827,  DOI: 10.1021/acscatal.7b00687
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      46
      Understanding Selectivity for the Electrochemical Reduction of Carbon Dioxide to Formic Acid and Carbon Monoxide on Metal Electrodes
      Feaster, Jeremy T.; Shi, Chuan; Cave, Etosha R.; Hatsukade, Toru; Abram, David N.; Kuhl, Kendra P.; Hahn, Christopher; Noerskov, Jens K.; Jaramillo, Thomas F.
      ACS Catalysis (2017), 7 (7), 4822-4827CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
      Increases in energy demand and in chem. prodn., together with the rise in CO2 levels in the atm., motivate the development of renewable energy sources. Electrochem. CO2 redn. to fuels and chems. is an appealing alternative to traditional pathways to fuels and chems. due to its intrinsic ability to couple to solar and wind energy sources. Formate (HCOO-) is a key chem. for many industries; however, greater understanding is needed regarding the mechanism and key intermediates for HCOO- prodn. This work reports a joint exptl. and theor. investigation of the electrochem. redn. of CO2 to HCOO- on polycryst. Sn surfaces, which have been identified as promising catalysts for selectively producing HCOO-. Our results show that Sn electrodes produce HCOO-, carbon monoxide (CO), and hydrogen (H2) across a range of potentials and that HCOO- prodn. becomes favored at potentials more neg. than -0.8 V vs RHE, reaching a max. Faradaic efficiency of 70% at -0.9 V vs RHE. Scaling relations for Sn and other transition metals are examd. using exptl. current densities and d. functional theory (DFT) binding energies. While *COOH was detd. to be the key intermediate for CO prodn. on metal surfaces, we suggest that it is unlikely to be the primary intermediate for HCOO- prodn. Instead, *OCHO is suggested to be the key intermediate for the CO2RR to HCOO- transformation, and Sn's optimal *OCHO binding energy supports its high selectivity for HCOO-. These results suggest that oxygen-bound intermediates are crit. to understand the mechanism of CO2 redn. to HCOO- on metal surfaces.
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      OpenURL UNIV OF NORTH TEXAS
    47. 47
      Gao, S.; Lin, Y.; Jiao, X.; Sun, Y.; Luo, Q.; Zhang, W.; Li, D.; Yang, J.; Xie, Y. Partially Oxidized Atomic Cobalt Layers for Carbon Dioxide Electroreduction to Liquid Fuel. Nature 2016, 529, 68,  DOI: 10.1038/nature16455
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      47
      Partially oxidized atomic cobalt layers for carbon dioxide electroreduction to liquid fuel
      Gao, Shan; Lin, Yue; Jiao, Xingchen; Sun, Yongfu; Luo, Qiquan; Zhang, Wenhua; Li, Dianqi; Yang, Jinlong; Xie, Yi
      Nature (London, United Kingdom) (2016), 529 (7584), 68-71CODEN: NATUAS; ISSN:0028-0836. (Nature Publishing Group)
      Electroredn. of CO2 into useful fuels, esp. if driven by renewable energy, represents a potentially 'clean' strategy for replacing fossil feedstocks and dealing with increasing CO2 emissions and their adverse effects on climate. The crit. bottleneck lies in activating CO2 into the CO2•- radical anion or other intermediates that can be converted further, as the activation usually requires impractically high overpotentials. Recently, electrocatalysts based on oxide-derived metal nanostructures have been shown to enable CO2 redn. at low overpotentials. However, it remains unclear how the electrocatalytic activity of these metals is influenced by their native oxides, mainly because microstructural features such as interfaces and defects influence CO2 redn. activity yet are difficult to control. To evaluate the role of the two different catalytic sites, here we fabricate two kinds of four-atom-thick layers: pure cobalt metal, and co-existing domains of cobalt metal and cobalt oxide. Cobalt mainly produces formate (HCOO-) during CO2 electroredn.; we find that surface cobalt atoms of the atomically thin layers have higher intrinsic activity and selectivity towards formate prodn., at lower overpotentials, than do surface cobalt atoms on bulk samples. Partial oxidn. of the at. layers further increases their intrinsic activity, allowing us to realize stable current densities of about 10 mA per square centimeter over 40 h, with approx. 90 per cent formate selectivity at an overpotential of only 0.24 V, which outperforms previously reported metal or metal oxide electrodes evaluated under comparable conditions. The correct morphol. and oxidn. state can thus transform a material from one considered nearly non-catalytic for the CO2 electroredn. reaction into an active catalyst. These findings point to new opportunities for manipulating and improving the CO2 electroredn. properties of metal systems, esp. once the influence of both the at.-scale structure and the presence of oxide are mechanistically better understood.
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      OpenURL UNIV OF NORTH TEXAS
    48. 48
      Zhao, S.; Jin, R.; Jin, R. Opportunities and Challenges in CO2 Reduction by Gold- and Silver-Based Electrocatalysts: From Bulk Metals to Nanoparticles and Atomically Precise Nanoclusters. ACS Energy Lett. 2018, 3 (2), 452– 462,  DOI: 10.1021/acsenergylett.7b01104
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      48
      Opportunities and Challenges in CO2 Reduction by Gold- and Silver-Based Electrocatalysts: From Bulk Metals to Nanoparticles and Atomically Precise Nanoclusters
      Zhao, Shuo; Jin, Renxi; Jin, Rongchao
      ACS Energy Letters (2018), 3 (2), 452-462CODEN: AELCCP; ISSN:2380-8195. (American Chemical Society)
      To tackle the excessive emission of greenhouse gas CO2, electrocatalytic redn. has been recognized as a promising way. Given the multielectron, multiproduct nature of the CO2 redn. process, an ideal catalyst should be capable of converting CO2 with high rates as well as high selectivity to either gas-phase (e.g., CO, CH4) or liq.-phase products (e.g., HCOOH, CH3OH, etc.). Gold- and silver-based materials have been extensively investigated as CO2 redn. catalysts for the formation of CO. This Perspective focuses on the advances of gold- and silver-based electrocatalysts for CO2 redn. in terms of catalyst design as well as some insights from theor. investigations. In particular, a special emphasis is placed on the newly emerging, atomically precise metal nanoclusters for CO2 electroredn. The strong quantum confinement effect and mol. purity as well as the crystallog. solved at. structures of nanoclusters make this new class of catalysts quite promising in fundamental studies, and valuable mechanistic insights for CO2 electroredn. at the at. scale can be obtained. We hope that this Perspective highlights the opportunities and challenges in the exploration of emerging nanomaterials.
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      OpenURL UNIV OF NORTH TEXAS
    49. 49
      Cai, Z.; Wu, Y.; Wu, Z.; Yin, L.; Weng, Z.; Zhong, Y.; Xu, W.; Sun, X.; Wang, H. Unlocking Bifunctional Electrocatalytic Activity for CO2 Reduction Reaction by Win-Win Metal–Oxide Cooperation. ACS Energy Lett. 2018, 3 (11), 2816– 2822,  DOI: 10.1021/acsenergylett.8b01767
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      49
      Unlocking Bifunctional Electrocatalytic Activity for CO2 Reduction Reaction by Win-Win Metal-Oxide Cooperation
      Cai, Zhao; Wu, Yueshen; Wu, Zishan; Yin, Lichang; Weng, Zhe; Zhong, Yiren; Xu, Wenwen; Sun, Xiaoming; Wang, Hailiang
      ACS Energy Letters (2018), 3 (11), 2816-2822CODEN: AELCCP; ISSN:2380-8195. (American Chemical Society)
      Understanding how remarkable properties of materials emerge from complex interactions of their constituents and designing advanced material structures to render desired properties are grand challenges. Metal-oxide interactions are frequently utilized to improve catalytic properties but are often limited to situations where only one component is facilitated by the other. In this work, highly cooperative win-win metal-oxide interactions are demonstrated that enable unprecedented catalytic functionalities for electrochem. CO2 redn. reactions. In a single SnOx/Ag catalyst, the oxide promotes the metal in the CO prodn. mode, and meanwhile the metal promotes the oxide in the HCOOH prodn. mode, achieving potential-dependent bifunctional CO2 conversion to fuels and chems. with H2 evolution suppressed in the entire potential window. Spectroscopic studies and computational simulations reveal that electron transfer from Ag to SnOx and dual-site cooperative binding for reaction intermediates at the SnOx/Ag interface are responsible for stabilizing the key intermediate in the CO pathway, changing the potential-limiting step in the HCOOH pathway, and increasing the kinetic barrier in the H2 evolution pathway, together leading to highly synergistic CO2 electroredn.
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      OpenURL UNIV OF NORTH TEXAS
    50. 50
      Lu, X.; Wu, Y.; Yuan, X.; Huang, L.; Wu, Z.; Xuan, J.; Wang, Y.; Wang, H. High-Performance Electrochemical CO2 Reduction Cells Based on Non-Noble Metal Catalysts. ACS Energy Lett. 2018, 3 (10), 2527– 2532,  DOI: 10.1021/acsenergylett.8b01681
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      50
      High-Performance Electrochemical CO2 Reduction Cells Based on Non-noble Metal Catalysts
      Lu, Xu; Wu, Yueshen; Yuan, Xiaolei; Huang, Ling; Wu, Zishan; Xuan, Jin; Wang, Yifei; Wang, Hailiang
      ACS Energy Letters (2018), 3 (10), 2527-2532CODEN: AELCCP; ISSN:2380-8195. (American Chemical Society)
      The promise and challenge of electrochem. mitigation of CO2 calls for innovations on both catalyst and reactor levels. Here, enabled by our high-performance and earth-abundant CO2 electroredn. catalyst materials, we developed alk. microflow electrolytic cells for energy-efficient, selective, fast, and durable CO2 conversion to CO and HCOO-. With a cobalt phthalocyanine-based cathode catalyst, the CO-selective cell starts to operate at a 0.26 V overpotential and reaches a Faradaic efficiency of 94% and a partial c.d. of 31 mA/cm2 at a 0.56 V overpotential. With a SnO2-based cathode catalyst, the HCOO--selective cell starts to operate at a 0.76 V overpotential and reaches a Faradaic efficiency of 82% and a partial c.d. of 113 mA/cm2 at a 1.36 V overpotential. In contrast to previous studies, we found that the overpotential redn. from using the alk. electrolyte is mostly contributed by a pH gradient near the cathode surface.
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      OpenURL UNIV OF NORTH TEXAS
    51. 51
      Kaczur, J. J.; Yang, H.; Liu, Z.; Sajjad, S. D.; Masel, R. I. Carbon Dioxide and Water Electrolysis Using New Alkaline Stable Anion Membranes. Front. Chem. 2018, 6, 263,  DOI: 10.3389/fchem.2018.00263
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      51
      Carbon dioxide and water electrolysis using new alkaline stable anion membranes
      Kaczur, Jerry J.; Yang, Hongzhou; Liu, Zengcai; Sajjad, Syed D.; Masel, Richard I.
      Frontiers in Chemistry (Lausanne, Switzerland) (2018), 6 (), 263/1-263/16CODEN: FCLSAA; ISSN:2296-2646. (Frontiers Media S.A.)
      The recent development and market introduction of a new type of alk. stable imidazole-based anion exchange membrane and related ionomers by Dioxide Materials is enabling the advancement of new and improved electrochem. processes which can operate at com. viable operating voltages, current efficiencies, and current densities. These processes include the electrochem. conversion of CO2 to formic acid (HCOOH), CO2 to carbon monoxide (CO), and alk. water electrolysis, generating hydrogen at high current densities at low voltages without the need for any preciousmetal electrocatalysts. The first process is the direct electrochem. generation of pure formic acid in a three-compartment cell configuration using the alk. stable anion exchange membrane and a cation exchange membrane. The cell operates at a c.d. of 140 mA/cm2 at a cell voltage of 3.5 V. The power consumption for prodn. of formic acid (FA) is about 4.3-4.7 kWh/kg of FA. The second process is the electrochem. conversion of CO2 to CO, a key focus product in the generation of renewable fuels and chems. The CO2 cell consists of a two-compartment design utilizing the alk. stable anion exchange membrane to sep. the anode and cathode compartments. A nanoparticle IrO2 catalyst on a GDE structure is used as the anode and a GDE utilizing a nanoparticle Ag/imidazolium-based ionomer catalyst combination is used as a cathode. The CO2 cell has been operated at current densities of 200 to 600 mA/cm2 at voltages of 3.0 to 3.2 resp. with CO2 to CO conversion selectivities of 95-99%. The third process is an alk. water electrolysis cell process, where the alk. stable anion exchange membrane allows stable cell operation in 1M KOH electrolyte solns. at current densities of 1 A/cm2 at about 1.90 V. The cell has demonstrated operation for thousands of hours, showing a voltage increase in time of only 5 μV/h. The alk. electrolysis technol. does not require any precious metal catalysts as compared to polymer electrolytemembrane (PEM) design water electrolyzers. In this paper, we discuss the detailed tech. aspects of these three technologies utilizing this unique anion exchange membrane.
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      OpenURL UNIV OF NORTH TEXAS
    52. 52
      Salvatore, D. A.; Weekes, D. M.; He, J.; Dettelbach, K. E.; Li, Y. C.; Mallouk, T. E.; Berlinguette, C. P. Electrolysis of Gaseous CO2 to CO in a Flow Cell with a Bipolar Membrane. ACS Energy Lett. 2018, 3 (1), 149– 154,  DOI: 10.1021/acsenergylett.7b01017
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      Electrolysis of Gaseous CO2 to CO in a Flow Cell with a Bipolar Membrane
      Salvatore, Danielle A.; Weekes, David M.; He, Jingfu; Dettelbach, Kevan E.; Li, Yuguang C.; Mallouk, Thomas E.; Berlinguette, Curtis P.
      ACS Energy Letters (2018), 3 (1), 149-154CODEN: AELCCP; ISSN:2380-8195. (American Chemical Society)
      The conversion of CO2 to CO is demonstrated in an electrolyzer flow cell contg. a bipolar membrane at current densities of 200 mA/cm2 with a faradaic efficiency of 50%. Electrolysis was carried out by delivering gaseous CO2 at the cathode with a Ag catalyst integrated in a C-based gas diffusion layer. Nonprecious Ni foam in a strongly alk. electrolyte (1 M NaOH) was used to mediate the anode reaction. While a configuration where the anode and cathode were sepd. by only a bipolar membrane is unfavorable for robust CO2 redn., a modified configuration with a solid-supported aq. layer inserted between the Ag-based catalyst layer and the bipolar membrane enhanced the cathode selectivity for CO2 redn. to CO. The authors report higher current densities (200 mA/cm2) than previously reported for gas-phase CO2 to CO electrolysis and demonstrate the dependence of long-term stability on adequate hydration of the CO2 inlet stream.
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      OpenURL UNIV OF NORTH TEXAS
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      Ripatti, D. S.; Veltman, T. R.; Kanan, M. W. Carbon Monoxide Gas Diffusion Electrolysis That Produces Concentrated C2 Products with High Single-Pass Conversion. Joule 2018,  DOI: 10.1016/j.joule.2018.10.007
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      Zeng, K.; Zhang, D. Recent Progress in Alkaline Water Electrolysis for Hydrogen Production and Applications. Prog. Energy Combust. Sci. 2010, 36 (3), 307– 326,  DOI: 10.1016/j.pecs.2009.11.002
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      A review. Alk. water electrolysis is one of the easiest methods for hydrogen prodn., offering the advantage of simplicity. The challenges for widespread use of water electrolysis are to reduce energy consumption, cost and maintenance and to increase reliability, durability and safety. This literature review examines the current state of knowledge and technol. of hydrogen prodn. by water electrolysis and identifies areas where R&D effort is needed in order to improve this technol. Following an overview of the fundamentals of alk. water electrolysis, an elec. circuit analogy of resistances in the electrolysis system is introduced. The resistances are classified into three categories, namely the elec. resistances, the reaction resistances and the transport resistances. This is followed by a thorough anal. of each of the resistances, by means of thermodn. and kinetics, to provide a scientific guidance to minimising the resistance in order to achieve a greater efficiency of alk. water electrolysis. The thermodn. anal. defines various electrolysis efficiencies based on theor. energy input and cell voltage, resp. These efficiencies are then employed to compare different electrolysis cell designs and to identify the means to overcome the key resistances for efficiency improvement. The kinetic anal. reveals the dependence of reaction resistances on the alk. concn., ion transfer, and reaction sites on the electrode surface, the latter is detd. by the electrode materials. A quant. relationship between the cell voltage components and c.d. is established, which links all the resistances and manifests the importance of reaction resistances and bubble resistances. The important effect of gas bubbles formed on the electrode surface and the need to minimise the ion transport resistance are highlighted. The historical development and continuous improvement in the alk. water electrolysis technol. are examd. and different water electrolysis technologies are systematically compared using a set of the practical parameters derived from the thermodn. and kinetic analyses. In addn. to the efficiency improvements, the needs for redn. in equipment and maintenance costs, and improvement in reliability and durability are also established. The future research needs are also discussed from the aspects of electrode materials, electrolyte additives and bubble management, serving as a comprehensive guide for continuous development of the water electrolysis technol.
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      OpenURL UNIV OF NORTH TEXAS
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      Carmo, M.; Fritz, D. L.; Mergel, J.; Stolten, D. A Comprehensive Review on Pem Water Electrolysis. Int. J. Hydrogen Energy 2013, 38 (12), 4901– 4934,  DOI: 10.1016/j.ijhydene.2013.01.151
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      A comprehensive review on PEM water electrolysis
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      A review. Hydrogen is often considered the best means by which to store energy coming from renewable and intermittent power sources. With the growing capacity of localized renewable energy sources surpassing the gigawatt range, a storage system of equal magnitude is required. PEM electrolysis provides a sustainable soln. for the prodn. of hydrogen, and is well suited to couple with energy sources such as wind and solar. However, due to low demand in electrolytic hydrogen in the last century, little research has been done on PEM electrolysis with many challenges still unexplored. The ever increasing desire for green energy has rekindled the interest on PEM electrolysis, thus the compilation and recovery of past research and developments is important and necessary. In this review, PEM water electrolysis is comprehensively highlighted and discussed. The challenges new and old related to electrocatalysts, solid electrolyte, current collectors, separator plates and modeling efforts will also be addressed. The main message is to clearly set the state-of-the-art for the PEM electrolysis technol., be insightful of the research that is already done and the challenges that still exist. This information will provide several future research directions and a road map in order to aid scientists in establishing PEM electrolysis as a com. viable hydrogen prodn. soln.
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      Weng, L. C.; Bell, A. T.; Weber, A. Z. Modeling Gas-Diffusion Electrodes for CO2 Reduction. Phys. Chem. Chem. Phys. 2018, 20 (25), 16973– 16984,  DOI: 10.1039/C8CP01319E
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      Modeling gas-diffusion electrodes for CO2 reduction
      Weng, Lien-Chun; Bell, Alexis T.; Weber, Adam Z.
      Physical Chemistry Chemical Physics (2018), 20 (25), 16973-16984CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
      CO2 redn. conducted in electrochem. cells with planar electrodes immersed in an aq. electrolyte is severely limited by mass transport across the hydrodynamic boundary layer. This limitation can be minimized by use of vapor-fed, gas-diffusion electrodes (GDEs), enabling current densities that are almost two orders of magnitude greater at the same applied cathode overpotential than what is achievable with planar electrodes in an aq. electrolyte. The addn. of porous cathode layers, however, introduces a no. of parameters that need to be tuned in order to optimize the performance of the GDE cell. In this work, we develop a multiphysics model for gas diffusion electrodes for CO2 redn. and used it to investigate the interplay between species transport and electrochem. reaction kinetics. The model demonstrates how the local environment near the catalyst layer, which is a function of the operating conditions, affects cell performance. We also examine the effects of catalyst layer hydrophobicity, loading, porosity, and electrolyte flowrate to help guide exptl. design of vapor-fed CO2 redn. cells.
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      Weber, A. Z.; Borup, R. L.; Darling, R. M.; Das, P. K.; Dursch, T. J.; Gu, W. B.; Harvey, D.; Kusoglu, A.; Litster, S.; Mench, M. M. A Critical Review of Modeling Transport Phenomena in Polymer-Electrolyte Fuel Cells. J. Electrochem. Soc. 2014, 161 (12), F1254– F1299,  DOI: 10.1149/2.0751412jes
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      A critical review of modeling transport phenomena in polymer-electrolyte fuel cells
      Weber, Adam Z.; Borup, Rodney L.; Darling, Robert M.; Das, Prodip K.; Dursch, Thomas J.; Gu, Wenbin; Harvey, David; Kusoglu, Ahmet; Litster, Shawn; Mench, Matthew M.; Mukundan, Rangachary; Owejan, Jon P.; Pharoah, Jon G.; Secanell, Marc; Zenyuk, Iryna V.
      Journal of the Electrochemical Society (2014), 161 (12), F1254-F1299, 46 pp.CODEN: JESOAN; ISSN:0013-4651. (Electrochemical Society)
      A review. Polymer-electrolyte fuel cells are a promising energy-conversion technol. Over the last several decades significant progress has been made in increasing their performance and durability, of which continuum-level modeling of the transport processes has played an integral part. In this review, we examine the state-of-the-art modeling approaches, with a goal of elucidating the knowledge gaps and needs going forward in the field. In particular, the focus is on multiphase flow, esp. in terms of understanding interactions at interfaces, and catalyst layers with a focus on the impacts of ionomer thin-films and multiscale phenomena. Overall, we highlight where there is consensus in terms of modeling approaches as well as opportunities for further improvement and clarification, including identification of several crit. areas for future research.
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      Varcoe, J. R.; Atanassov, P.; Dekel, D. R.; Herring, A. M.; Hickner, M. A.; Kohl, P. A.; Kucernak, A. R.; Mustain, W. E.; Nijmeijer, K.; Scott, K. Anion-Exchange Membranes in Electrochemical Energy Systems. Energy Environ. Sci. 2014, 7 (10), 3135– 3191,  DOI: 10.1039/C4EE01303D
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      Anion-exchange membranes in electrochemical energy systems
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      A review. This article provides an up-to-date perspective on the use of anion-exchange membranes in fuel cells, electrolyzers, redox flow batteries, reverse electrodialysis cells, and bioelectrochem. systems (e.g. microbial fuel cells). The aim is to highlight key concepts, misconceptions, the current state-of-the-art, technol. and scientific limitations, and the future challenges (research priorities) related to the use of anion-exchange membranes in these energy technologies. All the refs. that the authors deemed relevant, and were available on the web by the manuscript submission date (30th Apr. 2014), are included.
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      Kusoglu, A.; Weber, A. Z. New Insights into Perfluorinated Sulfonic-Acid Ionomers. Chem. Rev. 2017, 117 (3), 987– 1104,  DOI: 10.1021/acs.chemrev.6b00159
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      Kusoglu, Ahmet; Weber, Adam Z.
      Chemical Reviews (Washington, DC, United States) (2017), 117 (3), 987-1104CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
      In this comprehensive review, recent progress and developments on perfluorinated sulfonic-acid (PFSA) membranes have been summarized on many key topics. Although quite well investigated for decades, PFSA ionomers' complex behavior, along with their key role in many emerging technologies, have presented significant scientific challenges but also helped create a unique cross-disciplinary research field to overcome such challenges. Research and progress on PFSAs, esp. when considered with their applications, are at the forefront of bridging electrochem. and polymer (physics), which have also opened up development of state-of-the-art in situ characterization techniques as well as multiphysics computation models. Topics reviewed stem from correlating the various phys. (e.g., mech.) and transport properties with morphol. and structure across time and length scales. In addn., topics of recent interest such as structure/transport correlations and modeling, composite PFSA membranes, degrdn. phenomena, and PFSA thin films are presented. Throughout, the impact of PFSA chem. and side-chain is also discussed to present a broader perspective.
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      OpenURL UNIV OF NORTH TEXAS
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      Dekel, D. R. Review of Cell Performance in Anion Exchange Membrane Fuel Cells. J. Power Sources 2018, 375, 158– 169,  DOI: 10.1016/j.jpowsour.2017.07.117
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      A review of the performance and performance stability of AEMFCs through the years since the 1st reports in the early 2000s. Anion exchange membrane fuel cells (AEMFCs) have recently received increasing attention since in principle they allow for the use of non-precious metal catalysts, which dramatically reduces the cost per kW of power in fuel cell devices. Until not long ago, the main barrier in the development of AEMFCs was the availability of highly conductive anion exchange membranes (AEMs); however, improvements on this front in the past decade show that newly developed AEMs have already reached high levels of cond., leading to satisfactory cell performance. In recent years, a growing no. of research studies have reported AEMFC performance results. In the last 3 years, new records in performance were achieved. Most of the literature reporting cell performance is based on H-AEMFCs, although an increasing no. of studies have also reported the use of fuels others than H - such as alcs., nonalc. C-based fuels, as well as N-based fuels.
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      Hori, Y.; Murata, A.; Takahashi, R.; Suzuki, S. Electroreduction of Carbon Monoxide to Methane and Ethylene at a Copper Electrode in Aqueous Solutions at Ambient Temperature and Pressure. J. Am. Chem. Soc. 1987, 109 (16), 5022– 5023,  DOI: 10.1021/ja00250a044
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      Electroreduction of carbon monoxide to methane and ethylene at a copper electrode in aqueous solutions at ambient temperature and pressure
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      Journal of the American Chemical Society (1987), 109 (16), 5022-3CODEN: JACSAT; ISSN:0002-7863.
      The electrochem. redn. of CO in aq. soln. using phosphate, carbonate, and hydroxide electrolytes is reported. The highest total current efficiency was obtained at a current of 2.5 mA cm-2 with the KHCO3 electrolyte. Under these conditions the individual product-current efficiencies were CH4 16.3%, CH2:CH2 21.2%, EtOH 10.9%, PrOH 1.5%, and HCHO 0.1%.
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      Paidar, M.; Fateev, V.; Bouzek, K. Membrane Electrolysis—History, Current Status and Perspective. Electrochim. Acta 2016, 209, 737– 756,  DOI: 10.1016/j.electacta.2016.05.209
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      Membrane electrolysis - History, current status and perspective
      Paidar, M.; Fateev, V.; Bouzek, K.
      Electrochimica Acta (2016), 209 (), 737-756CODEN: ELCAAV; ISSN:0013-4686. (Elsevier Ltd.)
      A review. This review is devoted to membrane electrolysis, in particular using ion-selective membranes, as an important part of both existing and emerging industrial electrochem. processes. It aims to provide fundamental information on the history and development, current status and future perspectives of membrane electrolysis. An overview of the history of electro-membrane processes is given with the focus on brine electrolysis since it is the predominant electrochem. industrial technol. using ion-selective membranes. This is followed by a summary of the wide range of H-based energy conversion processes with different degrees of maturity, i.e. H2O electrolysis and fuel cells, which promise to become the next generation of major electro-membrane processes. The overview of the state-of-the-art is rounded off by a no. of smaller-scale processes using ionically conducting solid electrolytes and ion-selective membranes that are already com. available. The article concludes by considering potential future developments in this exciting field of electrochem.
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      OpenURL UNIV OF NORTH TEXAS
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      Weber, A. Z.; Kusoglu, A. Unexplained Transport Resistances for Low-Loaded Fuel-Cell Catalyst Layers. J. Mater. Chem. A 2014, 2 (41), 17207– 17211,  DOI: 10.1039/C4TA02952F
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      Weber, Adam Z.; Kusoglu, Ahmet
      Journal of Materials Chemistry A: Materials for Energy and Sustainability (2014), 2 (41), 17207-17211CODEN: JMCAET; ISSN:2050-7496. (Royal Society of Chemistry)
      For next-generation polymer-electrolyte fuel cells, material solns. are being sought to decrease the cost of the cell components and, in particular, the amt. of catalyst without sacrificing performance and lifetime. However, as recently shown, this cannot be achieved in practice due most likely to limitations caused by the ionomer thin-film surrounding the catalyst sites, where confinement and substrate interactions dominate and result in increased mass-transport limitations. Mitigation of this issue is paramount to the future com. viability of polymer-electrolyte fuel cells.
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      OpenURL UNIV OF NORTH TEXAS
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      Tamura, J.; Ono, A.; Sugano, Y.; Huang, C.; Nishizawa, H.; Mikoshiba, S. Electrochemical Reduction of CO2 to Ethylene Glycol on Imidazolium Ion-Terminated Self-Assembly Monolayer-Modified Au Electrodes in an Aqueous Solution. Phys. Chem. Chem. Phys. 2015, 17 (39), 26072– 26078,  DOI: 10.1039/C5CP03028E
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      Electrochemical reduction of CO2 to ethylene glycol on imidazolium ion-terminated self-assembly monolayer-modified Au electrodes in an aqueous solution
      Tamura, Jun; Ono, Akihiko; Sugano, Yoshitsune; Huang, Chingchun; Nishizawa, Hideyuki; Mikoshiba, Satoshi
      Physical Chemistry Chemical Physics (2015), 17 (39), 26072-26078CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
      Imidazolium ion-terminated self-assembled monolayer (SAM)-modified electrodes achieve CO2 conversion while suppressing H evolution. Immobile imidazolium ion on Au electrodes reduce CO2 at low overpotential. The distance between electrode and imidazolium ion sepd. by alkane thiol affects CO2 redn. activity. CO2 redn. current depends on the tunnel current rate. Although the product of CO2 redn. at the bare Au electrode is CO, SAM-modified electrodes produce ethylene glycol in aq. electrolyte soln. without CO evolution. The faradaic efficiency reached a max. of 87%. CO2 redn. at SAM-modified electrodes is unaffected by redn. activity of Au electrode. This phenomenon shows that the reaction field of CO2 redn. is not the electrode surface but the imidazolium ion monolayer.
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      Divekar, A. G.; Park, A. M.; Owczarczyk, Z. R.; Seifert, S.; Pivovar, B. S.; Herring, A. M. A Study of Carbonate Formation Kinetics and Morphological Effects Observed on Oh- Form of Pfaem When Exposed to Air Containing CO2. ECS Trans. 2017, 80 (8), 1005– 1011,  DOI: 10.1149/08008.1005ecst
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      A study of carbonate formation kinetics and morphological effects observed on OH- form of pfaem when exposed to air containing CO2
      Divekar, A. G.; Park, A. M.; Owczarczyk, Z. R.; Seifert, S.; Pivovar, B. S.; Herring, A. M.
      ECS Transactions (2017), 80 (8, Polymer Electrolyte Fuel Cells 17 (PEFC 17)), 1005-1011CODEN: ECSTF8; ISSN:1938-5862. (Electrochemical Society)
      For AEMFC, OH- form of membrane reacts with CO2 when exposed to air leading to loss in cond. Very few attempts have been made to understand the reaction kinetics, morphol. properties and equil. concns. at different environmental conditions. We have attempted to study the CO2 kinetics and its effect on water-uptake (or lambda) and morphol.(SAXS) when the OH- form of membrane is exposed to air which has approx. 400 ppm of CO2. The kinetics was studied by exposing the membrane to controlled environment and titrating it using Warder and Winkler titrn. methods. From transient SAXS anal. we observe the intensity of ionomer feature of membrane spectrum dropping over time. Also the d-spacing at equil. is lower than the initial value. Ultimately we want to understand the effect of CO2 on membrane from every aspect and possibly help us think about strategies to mitigate the problem.
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    67. 67
      Inaba, M.; Jensen, A. W.; Sievers, G. W.; Escudero-Escribano, M.; Zana, A.; Arenz, M. Benchmarking High Surface Area Electrocatalysts in a Gas Diffusion Electrode: Measurement of Oxygen Reduction Activities under Realistic Conditions. Energy Environ. Sci. 2018, 11, 988,  DOI: 10.1039/C8EE00019K
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      Benchmarking high surface area electrocatalysts in a gas diffusion electrode: measurement of oxygen reduction activities under realistic conditions
      Inaba, Masanori; Jensen, Anders Westergaard; Sievers, Gustav Wilhelm; Escudero-Escribano, Maria; Zana, Alessandro; Arenz, Matthias
      Energy & Environmental Science (2018), 11 (4), 988-994CODEN: EESNBY; ISSN:1754-5706. (Royal Society of Chemistry)
      In this work, we introduce the application of gas diffusion electrodes (GDE) for benchmarking the electrocatalytic performance of high surface area fuel cell catalysts. It is demonstrated that GDEs offer several inherent advantages over the state-of-the-art technique, i.e. thin film rotating disk electrode (TF-RDE) measurements for fast fuel cell catalyst evaluation. The most crit. advantage is reactant mass transport. While in RDE measurements the reactant mass transport is severely limited by the gas soly. of the reactant in the electrolyte, GDEs enable reactant transport rates similar to tech. fuel cell devices. Hence, in contrast to TF-RDE measurements, performance data obtained from GDE measurements can be directly compared to membrane electrode assembly (MEA) tests. Therefore, the application of GDEs for the testing of fuel cell catalysts closes the gap between catalyst research in academia and real applications.
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      Wiltshire, R. J. K.; King, C. R.; Rose, A.; Wells, P. P.; Hogarth, M. P.; Thompsett, D.; Russell, A. E. A Pem Fuel Cell for in Situ Xas Studies. Electrochim. Acta 2005, 50 (25), 5208– 5217,  DOI: 10.1016/j.electacta.2005.05.038
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      A PEM fuel cell for in situ XAS studies
      Wiltshire, Richard J. K.; King, Colin R.; Rose, Abigail; Wells, Peter P.; Hogarth, Martin P.; Thompsett, David; Russell, Andrea E.
      Electrochimica Acta (2005), 50 (25-26), 5208-5217CODEN: ELCAAV; ISSN:0013-4686. (Elsevier B.V.)
      A miniature p exchange membrane (PEM) fuel cell enabled in situ XAS studies of the anode catalyst through fluorescence measurements. The development of the cell is described as well as the modifications required for elevated temp. operation and humidification of the feed gasses. The impact of the operating conditions increased the catalyst use, which is evident in the EXAFS collected at the Pt LIII and Ru K edges for a PtRu/C catalyst. The Pt component of the catalyst is readily reduced by H in the fuel, while the Ru was only fully reduced under conditions of good gas flow and electrochem. contact. Under such conditions no evidence of O neighbors were found at the Ru x-ray absorption edge. The results are interpreted in relation to the lack of surface sensitivity of the EXAFS method and indicate that the equil. coverage of O species on the Ru surface sites is too low to be obsd. using EXAFS.
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      OpenURL UNIV OF NORTH TEXAS
    69. 69
      Ramaker, D. E.; Korovina, A.; Croze, V.; Melke, J.; Roth, C. Following Orr Intermediates Adsorbed on a Pt Cathode Catalyst During Break-in of a Pem Fuel Cell by in Operando X-Ray Absorption Spectroscopy. Phys. Chem. Chem. Phys. 2014, 16 (27), 13645– 13653,  DOI: 10.1039/C4CP00192C
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      Following ORR intermediates adsorbed on a Pt cathode catalyst during break-in of a PEM fuel cell by in operando X-ray absorption spectroscopy
      Ramaker, D. E.; Korovina, A.; Croze, V.; Melke, J.; Roth, C.
      Physical Chemistry Chemical Physics (2014), 16 (27), 13645-13653CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
      In operando X-ray absorption spectroscopy data using the Δμ X-ray Absorption Near Edge Spectroscopy (XANES) anal. procedure is used to follow the ORR intermediate adsorbate coverage on a working catalyst in a PEMFC during initial activation and break-in. The adsorbate coverage and log i (Tafel) curves reveal a strong correlation, i.e., an increase in adsorbate intermediate coverage poisons Pt sites thereby decreasing the current. A decrease in Pt-O bond strength commensurate with decrease in potential causes a sequence of different dominant adsorbate volcano curves to exist, namely first O, then OH, and then OOH exactly as predicted by the different ORR kinetics mechanisms. During break-in, the incipient O coverage coming from exposure to air during storage and MEA prepn. is rather quickly removed, compared to the slower and more subtle nanoparticle morphol. changes, such as the rounding of the Pt nanoparticle edges/corners and smoothing of the planar surfaces, driven by the nanoparticle's tendency to lower its surface energy. These morphol. changes increase the Pt-Pt av. coordination no., decrease the av. Pt-O bond strength, and thereby decrease the coverage of ORR intermediates, allowing increase in the current.
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      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXhtVantr%252FF&md5=ab50ad8d37ddcb6fe0f29a92bc61646c
      OpenURL UNIV OF NORTH TEXAS
    70. 70
      Ishiguro, N.; Saida, T.; Uruga, T.; Nagamatsu, S.-i.; Sekizawa, O.; Nitta, K.; Yamamoto, T.; Ohkoshi, S.-i.; Iwasawa, Y.; Yokoyama, T. Operando Time-Resolved X-Ray Absorption Fine Structure Study for Surface Events on a Pt3Co/C Cathode Catalyst in a Polymer Electrolyte Fuel Cell During Voltage-Operating Processes. ACS Catal. 2012, 2 (7), 1319– 1330,  DOI: 10.1021/cs300228p
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      70
      Operando Time-Resolved X-ray Absorption Fine Structure Study for Surface Events on a Pt3Co/C Cathode Catalyst in a Polymer Electrolyte Fuel Cell during Voltage-Operating Processes
      Ishiguro, Nozomu; Saida, Takahiro; Uruga, Tomoya; Nagamatsu, Shin-ichi; Sekizawa, Oki; Nitta, Kiyofumi; Yamamoto, Takashi; Ohkoshi, Shin-ichi; Iwasawa, Yasuhiro; Yokoyama, Toshihiko; Tada, Mizuki
      ACS Catalysis (2012), 2 (7), 1319-1330CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
      The structural kinetics of surface events on a Pt3Co/C cathode catalyst in a polymer electrolyte fuel cell was investigated by operando time-resolved x-ray absorption fine structure with a time resoln. of 500 ms. The rate consts. of electrochem. reactions, the changes in charge d. on Pt, and the changes in the local coordination structures of the Pt3Co alloy catalyst in the polymer electrolyte fuel cell were successfully evaluated during fuel-cell voltage-operating processes. Significant time lags were obsd. between the electrochem. reactions and the structural changes in the Pt3Co alloy catalyst. The rate consts. of all the surface events on the Pt3Co/C catalyst were significantly higher than those on the Pt/C catalyst, suggesting the advantageous behaviors (cell performance and catalyst durability) on the Pt3Co alloy cathode catalyst.
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      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38XntVOksbg%253D&md5=7f8db9f771675754df088f0f76b143a7
      OpenURL UNIV OF NORTH TEXAS
    71. 71
      Casalongue, H. S.; Kaya, S.; Viswanathan, V.; Miller, D. J.; Friebel, D.; Hansen, H. A.; Nørskov, J. K.; Nilsson, A.; Ogasawara, H. Direct Observation of the Oxygenated Species During Oxygen Reduction on a Platinum Fuel Cell Cathode. Nat. Commun. 2013, 4, 2817,  DOI: 10.1038/ncomms3817
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      OpenURL UNIV OF NORTH TEXAS
    72. 72
      Sanchez Casalongue, H. G.; Ng, M. L.; Kaya, S.; Friebel, D.; Ogasawara, H.; Nilsson, A. In Situ Observation of Surface Species on Iridium Oxide Nanoparticles During the Oxygen Evolution Reaction. Angew. Chem., Int. Ed. 2014, 53 (28), 7169– 7172,  DOI: 10.1002/anie.201402311
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      72
      In Situ Observation of Surface Species on Iridium Oxide Nanoparticles during the Oxygen Evolution Reaction
      Sanchez Casalongue, Hernan G.; Ng, May Ling; Kaya, Sarp; Friebel, Daniel; Ogasawara, Hirohito; Nilsson, Anders
      Angewandte Chemie, International Edition (2014), 53 (28), 7169-7172CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      An Ir oxide nanoparticle electrocatalyst under O evolution reaction conditions was probed in situ by ambient-pressure XPS. Under OER conditions, Ir undergoes a change in oxidn. state from IrIV to IrV that takes place predominantly at the surface of the catalyst. The chem. change in Ir is coupled to a decrease in surface hydroxide, providing exptl. evidence which strongly suggests that the O evolution reaction on Ir oxide occurs through an OOH-mediated deprotonation mechanism.
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      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXos1CjsLs%253D&md5=114dd5f9d7fc27964b33d8e1d1a3885c
      OpenURL UNIV OF NORTH TEXAS
    73. 73
      Zenyuk, I. V.; Parkinson, D. Y.; Hwang, G.; Weber, A. Z. Probing Water Distribution in Compressed Fuel-Cell Gas-Diffusion Layers Using X-Ray Computed Tomography. Electrochem. Commun. 2015, 53, 24– 28,  DOI: 10.1016/j.elecom.2015.02.005
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      73
      Probing water distribution in compressed fuel-cell gas-diffusion layers using X-ray computed tomography
      Zenyuk, Iryna V.; Parkinson, Dilworth Y.; Hwang, Gisuk; Weber, Adam Z.
      Electrochemistry Communications (2015), 53 (), 24-28CODEN: ECCMF9; ISSN:1388-2481. (Elsevier B.V.)
      X-ray computed tomog. was used to investigate geometrical land and channel effects on spatial liq.-water distribution in gas-diffusion layers (GDLs) of polymer-electrolyte fuel cells under different levels of compression. At low compression, a uniform liq.-water front was obsd. due to water redistribution and uniform porosity; however, at high compression, the water predominantly advanced at locations under the channel for higher liq. pressures. At low compression, no apparent correlation between the spatial liq. water and porosity distributions was obsd., whereas at high compression, a strong correlation was shown, indicating a potential for smart GDL architecture design with modulated porosity.
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      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2MXivVCjs74%253D&md5=8a857638a8d6c37b2d4a409bc341c7c1
      OpenURL UNIV OF NORTH TEXAS
    74. 74
      Medici, E. F.; Zenyuk, I. V.; Parkinson, D. Y.; Weber, A. Z.; Allen, J. S. Understanding Water Transport in Polymer Electrolyte Fuel Cells Using Coupled Continuum and Pore-Network Models. Fuel Cells 2016, 16 (6), 725– 733,  DOI: 10.1002/fuce.201500213
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      74
      Understanding Water Transport in Polymer Electrolyte Fuel Cells Using Coupled Continuum and Pore-Network Models
      Medici, E. F.; Zenyuk, I. V.; Parkinson, D. Y.; Weber, A. Z.; Allen, J. S.
      Fuel Cells (Weinheim, Germany) (2016), 16 (6), 725-733CODEN: FUCEFK; ISSN:1615-6846. (Wiley-Blackwell)
      Water management remains a crit. issue for polymer electrolyte fuel cell performance and durability, esp. at lower temps. and with ultrathin electrodes. To understand and explain exptl. observations better, water transport in gas diffusion layers (GDLs) with macroscopically heterogeneous morphologies was simulated using a novel coupling of continuum and pore-network models. X-ray computed tomog. was used to ext. GDL material parameters for use in the pore-network model. The simulations were conducted to explain exptl. observations assocd. with stacking of anode GDLs, where stacking of the anode GDLs increased the limiting c.d. Through imaging, it is shown that the stacked anode GDL exhibited an interfacial region of high porosity. The coupled model shows that this morphol. allowed more efficient water movement through the anode and higher temps. at the cathode compared to the single GDL case. As a result, the cathode exhibited less flooding and hence better low temp. performance with the stacked anode GDL.
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      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28Xls1Git7k%253D&md5=53b483df6122ab6202345eed47d27914
      OpenURL UNIV OF NORTH TEXAS
    75. 75
      Cetinbas, F. C.; Wang, X. H.; Ahluwalia, R. K.; Kariuki, N. N.; Winarski, R. P.; Yang, Z. W.; Sharman, J.; Myers, D. J. Microstructural Analysis and Transport Resistances of Low-Platinum-Loaded Pefc Electrodes. J. Electrochem. Soc. 2017, 164 (14), F1596– F1607,  DOI: 10.1149/2.1111714jes
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      75
      Microstructural Analysis and Transport Resistances of Low-Platinum-Loaded PEFC Electrodes
      Cetinbas, Firat C.; Wang, Xiaohua; Ahluwalia, Rajesh K.; Kariuki, Nancy N.; Winarski, Robert P.; Yang, Zhiwei; Sharman, Jonathan; Myers, Deborah J.
      Journal of the Electrochemical Society (2017), 164 (14), F1596-F1607CODEN: JESOAN; ISSN:0013-4651. (Electrochemical Society)
      We present microstructural characterization for polymer electrolyte fuel cell (PEFC) cathodes with low platinum group metal (PGM) loadings together with polarization data anal. Three-dimensional pore morphol. and ionomer distribution are resolved using nano-scale x-ray computed tomog. (nano-CT). Electrode structural properties are reported along with the effective ion and reactant transport properties. The characterization results are incorporated with 2-dimensional multi-physics model that accounts for energy, charge, and mass transport along with the effect of liq. water flooding. Defining total mass transport resistance for the whole polarization curve, contributions of transport mechanisms are identified. Anal. of the exptl. polarization curves at different operating pressures and temps. indicates that the mass transport resistance in the cathode is dominated by the transport processes in the electrode. It is shown that flooding in the electrode is a major contributor to transport losses esp. at elevated operating pressures while the pressure-independent resistance at the catalyst surface due to transport through the ionomer film plays a significant role, esp. at low temps. and low catalyst loading. By performing a parametric study for varying catalyst loadings, the importance of electrode roughness (i.e, electrochem.-active surface area/geometric electrode area) in detg. the mass transport losses is highlighted.
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      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1cXkslajsA%253D%253D&md5=3f2297809e1599fb13f523a302f7c8cb
      OpenURL UNIV OF NORTH TEXAS
    76. 76
      Komini Babu, S.; Chung, H. T.; Zelenay, P.; Litster, S. Resolving Electrode Morphology’s Impact on Platinum Group Metal-Free Cathode Performance Using Nano-Ct of 3d Hierarchical Pore and Ionomer Distribution. ACS Appl. Mater. Interfaces 2016, 8 (48), 32764– 32777,  DOI: 10.1021/acsami.6b08844
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      76
      Resolving Electrode Morphology's Impact on Platinum Group Metal-Free Cathode Performance Using Nano-CT of 3D Hierarchical Pore and Ionomer Distribution
      Komini Babu, Siddharth; Chung, Hoon T.; Zelenay, Piotr; Litster, Shawn
      ACS Applied Materials & Interfaces (2016), 8 (48), 32764-32777CODEN: AAMICK; ISSN:1944-8244. (American Chemical Society)
      This article reports on the characterization of polymer electrolyte fuel cell (PEFC) cathodes featuring a platinum group metal-free (PGM-free) catalyst using nanoscale resoln. X-ray computed tomog. (nano-CT) and morphol. anal. PGM-free PEFC cathodes have gained significant interest in the past decade since they have the potential to dramatically reduce PEFC costs by eliminating the large platinum (Pt) raw material cost. However, several challenges remain before they are com. viable. Since these catalysts have lower volumetric activity, the PGM-free cathodes are thicker and subject to increased gas and proton transport resistances that reduce the performance. To better understand the efficacy of the catalyst and improve electrode performance, a detailed understanding the correlation between electrode fabrication, morphol., and performance is crucial. In this work, the pore/solid structure and the ionomer distribution was resolved in three dimensions (3D) using nano-CT for three PGM-free electrodes of varying Nafion loading. The assocd. transport properties were evaluated from pore/particle-scale simulations within the nano-CT-imaged structure. These characterizations are then used to elucidate the microstructural origins of the dramatic changes in fuel cell performance with varying Nafion ionomer loading. We show that this is primarily a result of distinct changes in ionomer's spatial distribution. The significant impact of electrode morphol. on performance highlights the importance of PGM-free electrode development in concert with efforts to improve catalyst activity and durability.
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      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28XhslOhsrnM&md5=7a929b66ec4d8d27cfc34d0cc67e076e
      OpenURL UNIV OF NORTH TEXAS
    77. 77
      Cetinbas, F. C.; Ahluwalia, R. K.; Kariuki, N.; De Andrade, V.; Fongalland, D.; Smith, L.; Sharman, J.; Ferreira, P.; Rasouli, S.; Myers, D. J. Hybrid Approach Combining Multiple Characterization Techniques and Simulations for Microstructural Analysis of Proton Exchange Membrane Fuel Cell Electrodes. J. Power Sources 2017, 344, 62– 73,  DOI: 10.1016/j.jpowsour.2017.01.104
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      77
      Hybrid approach combining multiple characterization techniques and simulations for microstructural analysis of proton exchange membrane fuel cell electrodes
      Cetinbas, Firat C.; Ahluwalia, Rajesh K.; Kariuki, Nancy; De Andrade, Vincent; Fongalland, Dash; Smith, Linda; Sharman, Jonathan; Ferreira, Paulo; Rasouli, Somaye; Myers, Deborah J.
      Journal of Power Sources (2017), 344 (), 62-73CODEN: JPSODZ; ISSN:0378-7753. (Elsevier B.V.)
      A review. The cost and performance of proton exchange membrane fuel cells strongly depend on the cathode electrode due to usage of expensive platinum (Pt) group metal catalyst and sluggish reaction kinetics. Development of low Pt content high performance cathodes requires comprehensive understanding of the electrode microstructure. In this study, a new approach is presented to characterize the detailed cathode electrode microstructure from nm to μm length scales by combining information from different exptl. techniques. In this context, nano-scale X-ray computed tomog. (nano-CT) is performed to ext. the secondary pore space of the electrode. Transmission electron microscopy (TEM) is employed to det. primary C particle and Pt particle size distributions. X-ray scattering, with its ability to provide size distributions of orders of magnitude more particles than TEM, is used to confirm the TEM-detd. size distributions. The no. of primary pores that cannot be resolved by nano-CT is approximated using mercury intrusion porosimetry. An algorithm is developed to incorporate all these exptl. data in one geometric representation. Upon validation of pore size distribution against gas adsorption and mercury intrusion porosimetry data, reconstructed ionomer size distribution is reported. In addn., transport related characteristics and effective properties are computed by performing simulations on the hybrid microstructure.
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      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2sXitVWitLo%253D&md5=072e92b97e253390fc095dffc4559235
      OpenURL UNIV OF NORTH TEXAS
    78. 78
      Vermaas, D. A.; Smith, W. A. Synergistic Electrochemical CO2 Reduction and Water Oxidation with a Bipolar Membrane. ACS Energy Lett. 2016, 1 (6), 1143– 1148,  DOI: 10.1021/acsenergylett.6b00557
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      78
      Synergistic Electrochemical CO2 Reduction and Water Oxidation with a Bipolar Membrane
      Vermaas, David A.; Smith, Wilson A.
      ACS Energy Letters (2016), 1 (6), 1143-1148CODEN: AELCCP; ISSN:2380-8195. (American Chemical Society)
      The electrochem. conversion of CO2 and water to value-added products still suffers from low efficiency, high costs, and high sensitivity to electrolyte, pH, and contaminants. Here, we present a strategy for this reaction using a silver catalyst for CO2 redn. in a neutral catholyte, sepd. by a bipolar membrane from a nickel iron hydroxide oxygen evolution catalyst in a basic anolyte. This combination of electrolytes provides a favorable environment for both catalysts and shows the effective use of bicarbonate and KOH to obtain low cell voltages. This architecture brings down the total cell voltage by more than 1 V compared to that with conventional use of a Pt counter electrode and monopolar membranes, and at the same time, it reduces contamination and improves stability at the cathode.
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      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28XhvVSjurzI&md5=7d3fde0617927db0ab1419e8c1f621d5
      OpenURL UNIV OF NORTH TEXAS
    79. 79
      Li, Y. C.; Zhou, D.; Yan, Z.; Gonçalves, R. H.; Salvatore, D. A.; Berlinguette, C. P.; Mallouk, T. E. Electrolysis of CO2 to Syngas in Bipolar Membrane-Based Electrochemical Cells. ACS Energy Lett. 2016, 1 (6), 1149– 1153,  DOI: 10.1021/acsenergylett.6b00475
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      79
      Electrolysis of CO2 to Syngas in Bipolar Membrane-Based Electrochemical Cells
      Li, Yuguang C.; Zhou, Dekai; Yan, Zhifei; Goncalves, Ricardo H.; Salvatore, Danielle A.; Berlinguette, Curtis P.; Mallouk, Thomas E.
      ACS Energy Letters (2016), 1 (6), 1149-1153CODEN: AELCCP; ISSN:2380-8195. (American Chemical Society)
      The electrolysis of CO2 to syngas (CO + H2) using non-precious metal electrocatalysts was studied in bipolar membrane-based electrochem. cells. Electrolysis was carried out using aq. bicarbonate and humidified gaseous CO2 on the cathode side of the cell, with Ag or Bi/ionic liq. cathode electrocatalysts. In both cases, stable currents were obsd. over a period of hours with an aq. alk. electrolyte and NiFeOx electrocatalyst on the anode side of the cell. But the performance of the cells degraded rapidly when conventional anion- and cation-exchange membranes were used in place of the bipolar membrane. In agreement with earlier reports, the faradaic efficiency for CO2 redn. to CO was high at low overpotential. In the liq.-phase bipolar membrane cell, the faradaic efficiency was stable at ∼50% at 30 mA/cm2 c.d. In the gas-phase cell, current densities up to 200 mA/cm2 could be obtained, albeit at lower faradaic efficiency for CO prodn. At low overpotentials in the gas-phase cathode cell, the Faradaic efficiency for CO prodn. was initially high but dropped within 1 h, most likely because of dewetting of the ionic liq. from the Bi catalyst surface. The effective management of protons in bipolar membrane cells enables stable operation and the possibility of practical CO2 electrolysis at high current densities.
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      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28XhvVWisbbN&md5=b997568ba8a192f378bd1dca952036e6
      OpenURL UNIV OF NORTH TEXAS
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