Abstract
A solar-driven CO2 reduction (CO2R) cell was constructed, consisting of a tandem GaAs/InGaP/TiO2/Ni photoanode in 1.0 M KOH(aq) (pH = 13.7) to facilitate the oxygen-evolution reaction (OER), a Pd/C nanoparticle-coated Ti mesh cathode in 2.8 M KHCO3(aq) (pH = 8.0) to perform the CO2R reaction, and a bipolar membrane to allow for steady-state operation of the catholyte and anolyte at different bulk pH values. At the operational current density of 8.5 mA cm–2, in 2.8 M KHCO3(aq), the cathode exhibited <100 mV overpotential and >94% Faradaic efficiency for the reduction of 1 atm of CO2(g) to formate. The anode exhibited a 320 ± 7 mV overpotential for the OER in 1.0 M KOH(aq), and the bipolar membrane exhibited ∼480 mV voltage loss with minimal product crossovers and >90 and >95% selectivity for protons and hydroxide ions, respectively. The bipolar membrane facilitated coupling between two electrodes and electrolytes, one for the CO2R reaction and one for the OER, that typically operate at mutually different pH values and produced a lower total cell overvoltage than known single-electrolyte CO2R systems while exhibiting ∼10% solar-to-fuels energy-conversion efficiency.
The sustainable electrochemical reduction of CO2 requires utilization of CO2 from the atmosphere as well as use of the electrons and protons produced by the oxidation of water to O2(g).(1, 2) However, CO2 reduction (CO2R) involves very different optimal electrolyte conditions than oxidation of water. For the cathodic CO2R reaction in alkaline conditions (e.g., pH > 10), the low concentration of dissolved CO2 imposes severe mass-transport limitations on the electroactive reagent,(3-5) whereas in acidic conditions (e.g., pH < 1), the high proton concentration favors the competing hydrogen-evolution reaction (HER). Hence, the development of catalysts for CO2R has generally focused on electrolytes having near-neutral pH values.(6, 7) At present, in near-neutral pH electrolytes, only electrochemical processes that involve the two-electron/two-proton reduction of CO2, to produce either CO or formate, can be performed efficiently and selectively at an operating current density of 101 mA cm–2.(6, 8-11) In contrast, for the oxygen-evolution reaction (OER), mixed-metal oxides have been extensively studied in strongly alkaline conditions (1.0 M KOH(aq)), with state-of-the-art catalysts exhibiting ∼250–300 mV overpotentials at 10 mA cm–2 of anodic current density.(12, 13) Electrocatalysts for the OER in near-neutral electrolytes exhibit substantially larger overpotentials than OER electrocatalysts in alkaline electrolytes(13-15) because the negatively charged hydroxide ion is more readily oxidized than a neutral water molecule, and because hydroxide is present in high concentration in alkaline solutions.(16)
Most laboratory demonstrations of solar-driven CO2R devices have used a single electrolyte at near-neutral pH values but consequently suffer substantial overpotential losses for the OER.(9, 17-19) For example, a 6.5% solar-to-fuel conversion efficiency, ηSTF, was reported using a Au catalyst for CO generation in 0.5 M NaHCO3(aq) electrolyte.(9) A value of ηSTF = 4.6% was obtained using a polymeric Ru complex for formate generation in 0.1 M aqueous phosphate buffer.(17) In addition to requiring effective ionic coupling between the catholyte and anolyte, a full solar-driven CO2R system also requires a robust means to separate the products as well as facile collection of the reduced fuels. We demonstrate herein the performance of a photovoltaic-assisted electrosynthetic cell in which the photoanode is operated in 1.0 M KOH(aq) (pH = 13.7) to perform the OER while the cathode performs the CO2R reaction in 2.8 M KHCO3(aq) (pH = 8.0) under 1 atm of CO2(g). The pH difference between the cathode chamber and the anode chamber was sustained at steady state with no accompanying chemical bias voltage by use of a bipolar membrane (fumasep FBM).(20-23) Hence, two electrolytes having mutually different pH values, with each electrolyte individually optimized for the CO2R reaction or the OER, were effectively coupled together to produce a modest combined cell overvoltage at the desired operational current density.
The photoanode (GaAs/InGaP/TiO2/Ni) consisted of a tandem-junction III–V photoabsorber, an amorphous hole-conductive TiO2 protection layer, and a thin catalyst layer to facilitate the OER.(24, 25) Details of the fabrication of the photoanode are described in the Supporting Information (SI). The water-oxidation behavior of the photoanode in 1.0 M KOH(aq) under simulated 1 sun illumination has been characterized previously,(24) and the photoanode exhibited a light-limited photocurrent density of ∼8.5 mA cm–2 and an equivalent open-circuit voltage of ∼2.4 V, (Figure S1), in accord with prior results.(24, 25)
Figure 1A shows the cyclic voltammetry, at a scan rate of 10 mV s–1 without correction for uncompensated resistance (see Figure S2A for an illustration of the three-electrode electrochemical measurement setup), of a Pd/C nanoparticle-coated Ti mesh cathode with a Pd mass loading of 250 μg cm–2 (red) and a Pd/C-coated Ti foil with a Pd mass loading of 50 μg cm–2 (black) in 2.8 M KHCO3(aq) (pH = 8) that was saturated with a stream of 1 atm of CO2(g). The Pd/C cathode was fabricated by drop-casting a solution containing 2 mg mL–1 Pd/C nanoparticles and ∼0.15 wt % Nafion in isopropanol on a Ti mesh or a Ti foil. The Pd/C-coated Ti foil (black curve in Figure 1A) exhibited very similar electrocatalytic activity to that reported previously.(8) The Pd/C-coated stacked Ti mesh electrode exhibited improved performance because of the increased mass loading and larger electrochemically accessible surface area for CO2R reduction. The forward scan indicated that the onset potential of the cathodic current was close to the equilibrium potential for CO2 reduction to formate (E°′(CO2/HCOO–) = −0.687 V versus the Ag/AgCl reference electrode).(26) An overpotential of −57 ± 8 mV was required to drive CO2R at a cathodic geometric current density of 10 mA cm–2. Figure 1B shows the Faradaic efficiency for the production of formate using the Pd/C nanoparticle-coated Ti mesh cathode in CO2-saturated 2.8 M KHCO3(aq) as a function of time, at four different overpotentials. At all overpotentials, near-unity Faradaic efficiency was observed for the first 60 min of operation. The Faradaic efficiency then decreased slowly for overpotentials between −45 and −120 mV, but still exceeded ≥94% after 3 h of electrolysis (Figure 1B). In contrast, when the electrode was held at −170 mV vs E°′(CO2/HCOO–), the Faradaic efficiency decayed quickly after 90 min and decreased to ∼80% after 3 h of continuous operation (see the SI for discussion about the time dependence of the Faradaic efficiency). The decrease of the Faradaic efficiency for formate production is consistent with the accumulation of CO at the surface of the Pd nanoparticles.(8) To characterize in detail the performance of the protection layer and electrocatalytic components of the anode, Figure 1C shows the current density vs potential (J–E) behavior of a p+-Si/TiO2/Ni dark anode affecting the OER in 1.0 M KOH(aq) (black) and in 2.8 M KHCO3(aq) (red) without any correction for uncompensated resistance. The J–E behavior of the p+-Si/TiO2/Ni dark electrode was used to provide a measure of the overpotentials of the OER catalyst in 1.0 M KOH(aq) and in 2.8 M KHCO3(aq) (see the SI for discussion of the nickel catalyst). As shown in Figure 1C, an overpotential of 330 ± 10 mV was required to produce a current density of 10 mA cm–2 in 1.0 M KOH(aq), consistent with previous results.(27) In contrast, an overpotential of 793 ± 26 mV was required in 2.8 M KHCO3(aq) to produce 10 mA cm–2 of current density.
Figure 1. (A) Cyclic voltammetry of the Pd/C nanoparticle-coated stacked Ti mesh electrode with a Pd mass loading of 250 μg cm–2 (red) and the Pd/C nanoparticle-coated Ti foil with a Pd mass loading of 50 μg cm–2 (black) in CO2-saturated 2.8 M KHCO3(aq) at pH = 8.0. The dotted line indicates the equilibrium potential for CO2 reduction to formate at pH = 8.0. The three-electrode cell configuration is shown in Figure S2A. (B) Faradaic efficiency of formate production as a function of time, for four different overpotentials, using the Pd/C nanoparticle-coated Ti mesh in CO2-saturated 2.8 M KHCO3(aq). (C) Current–potential behavior of a p+-Si/TiO2/Ni electrode for the OER in 1.0 M KOH(aq) (red) and in 2.8 M KHCO3(aq) (black).
With the anolyte at pH = 13.7 (1.0 M KOH(aq)) and the catholyte at pH = 8.0 (2.8 M KHCO3(aq)), Figure 2A shows the membrane voltage loss (left axis), as well as the measured total membrane voltage (right axis) as a function of the current density normalized to the bipolar membrane (BPM) area (see Figure S2C for an illustration of the four-point measurement configuration).(25) Two Luggin capillaries with Ag/AgCl reference electrodes were used to measure the electric potential drop across the BPM. The equilibrium potential, Vmembrane,equilibrium was calculated to be 0.336 V in the anolyte/catholyte system. At a current density of 10 mA cm–2, the measured membrane total voltage was 0.843 ± 0.038 V. Hence, to drive the reduction of CO2 to formate at steady state, the voltage loss in the BPM, Vmembrane,loss = Vmembrane,total – Vmembrane,equilibrium = 0.843 V – [0.059 V × (13.7 – 8.0)], was 0.507 V. The voltage loss primarily resulted from the resistance loss of the BPM as well as from the overvoltage required for water dissociation at the transition region in the BPM. The observed membrane voltage losses in the 1.0 M KOH(aq) (pH = 13.7)/2.8 M KHCO3(aq) (pH = 8.0) system were substantially smaller than those previously reported in 0.5 M KH2BO3(aq) (pH = 9.3)/1.0 M H2SO4(aq) (pH = 0)(25) because the flexible Luggin capillaries used in the present study were placed very close to the membrane and minimized the resistive losses due to the solution. To evaluate the ionic transport properties of the membrane, a cell with Pt mesh electrodes as the cathode and anode was operated continuously for 100 h at a current density of ∼8.5 mA cm–2 normalized to the BPM area, with a resulting change by ∼0.01 unit in the pH of the anolyte. If 100% of the charge passed had been used for electrodialysis of the electrolytes, the pH of the anolyte would have changed by >1 unit. After continuous operation of this cell for 6 h at 8.5 mA cm–2, the BPM voltage changed by <0.5% (Figure S3). Alternatively, for operation of CO2R and OER in the same electrolyte, a cation-exchange membrane, for example, Nafion, could be used to separate the cathode chamber from the anode chamber. Figure 2B shows the measured total membrane voltage as a function of the current density normalized to the Nafion area, when both the anolyte and catholyte were 2.8 M KHCO3(aq), but Nafion was used instead of a BPM. The total Nafion membrane voltage was equal to the Nafion membrane voltage loss, which largely arose due to the membrane resistance for transport of K+ ions. At a current density of 10 mA cm–2, the voltage loss across the Nafion membrane was 214 ± 15 mV.
Figure 2. (A) Membrane voltage loss (left axis) and measured total membrane voltage (right axis) as a function of the current density normalized to the 0.03 cm2 BPM area. The cell configuration was KHCO3(aq) (pH = 8.0)/BPM/KOH (aq, pH = 13.7) (Figure S2C). (B) Measured total membrane voltage (or membrane voltage loss) as a function of the current density normalized to the Nafion area. The cell configuration was KHCO3(aq) (pH = 8.0)/Nafion/KHCO3(aq) (pH = 8.0) (Figure S2C). (C) Selectivity of the BPM for protons as a function of time, when operated at two different current densities. (D) Selectivity of the BPM for hydroxide ions as a function of time, for two different current densities. Pt mesh electrodes were used as the cathode and anode in (C) and (D).
The ion-crossover fluxes in the BPM system were characterized using inductively coupled plasma mass spectrometry (ICPMS) in conjunction with a total inorganic carbon (TIC) analyzer to measure the ion concentrations in the catholyte and anolyte after charge was passed through the BPM at different current densities. Figure 2C−D shows the time dependence of the selectivity of the BPM for protons and hydroxide ions at two different operational current densities. Two major crossover pathways, cation crossover from the anolyte to the catholyte and anion crossover from the catholyte to the anolyte, were present under the electric field due to the imperfect permselectivity of the cation-exchange membrane and anion-exchange membrane portions of the BPM. To determine the cation crossover, the KHCO3(aq) catholyte was replaced by CsHCO3(aq), so that small increases in the K+ concentration could be detected. The measured K+ leak rate in the CsHCO3(aq)/KOH(aq) configuration also presented an upper bound for the behavior of the KHCO3(aq)/KOH(aq) configuration due to the absence of the diffusional driving force for K+ transport from the anolyte to the catholyte in the all-K+-containing system. The membrane selectivities, fH+ (fOH–), were defined as the ratio of proton-carried (hydroxide-carried) charge passed relative to the total charge passed through the membrane. At an operational current density of 3 mA cm–2, the potassium leak current and the bicarbonate leak current constituted 10–25% and 20–35%, respectively, of the total current passed through the BPM. When the membrane current density was increased to 8 mA cm–2, the membrane selectivity for protons increased to >90% and the membrane selectivity for hydroxide ions increased to >95%. The crossover of the formate product was low (Figure S4). In the three-electrode electrochemical measurement, the cathode compartment was separated from the Pt counter electrode by a BPM; therefore, the high Faradaic efficiency (>94%) measured in the cathode compartment at low overpotentials (from −45 to −120 mV, Figure 1B) also provides evidence for a low rate of formate crossover through the BPM. The product crossovers were minimal because the negatively charged formate ion was effectively blocked by the negatively charged cation-exchange membrane in the BPM system.
Figure 3A shows a schematic illustration of the two-electrode electrochemical setup. CO2 at 1 atm (ALPHAGAZ 1) was bubbled continuously into the 2.8 M KHCO3(aq) catholyte. The blue tubes shown in the figure were connected to a peristaltic pumping system that facilitated removal of CO2 bubbles and thus minimized associated fluctuations in the cell voltage and current. The geometric areas of the GaAs/InGaP/TiO2/Ni photoanode, BPM, Nafion membrane, and Pd/C/Ti cathode were mutually similar, at 0.030, 0.030, 0.030, and 0.040 cm2, respectively. The relatively small active device area was due to the behavior of the photoanode in 1.0 M KOH(aq). The electrocatalytic performance, stability, and Faradaic efficiency for product formation at the cathode, the J–E properties of the photoanode, and the current–voltage characteristics of the BPM were independent of the geometric areas of these cell components (Figure S5), but the stability of the photoanode was dependent on the electrode area due to pinholes and other defects at large electrode areas, providing a source for active dissolution and thus instability of the III–V semiconductors in 1.0 M KOH(aq) (see the SI). Small-area photoanodes have exhibited stable operation for >100 h with near-unity Faradaic efficiency for the OER.(24, 28) Figure 3B shows the current density for unassisted CO2R as a function of the time under 100 mW cm–2 of simulated AM1.5 illumination, when the GaAs/InGaP/TiO2/Ni photoanode was directly wired to the Pd/C nanoparticle-coated Ti mesh cathode without application of any external bias. The photocurrent density was 8.7 ± 0.5 mA cm–2. The overpotential for the Pd/C nanoparticle-coated Ti mesh cathode was recorded during the 3 h stability test, as shown in Figure S6A. During the stability test, the overpotential was between −40 and −100 mV; therefore, as shown in Figure 1B, the Faradaic efficiency of CO2 reduction to formate was ∼100, 98, 95, and 94% after 30 min, 1 h, 2 h, and 3 h, respectively. The corresponding solar-to formate conversion efficiency at these times was thus 10.5, 10.3, 10.0, and 9.9%, respectively (the see SI for details on calculation of the solar-to-formate conversion efficiency).(9, 29) The photocurrent density vs voltage behavior of the two-electrode system (Figure S6B) exhibited mutually similar onset potentials and light-limited current densities before and after a 3 h stability test (Figure 3B), indicating minimal corrosion of the photoanode over this time period.
Figure 3. (A) Schematic illustration of a two-electrode electrochemical setup. The blue tubes were connected to a peristaltic pumping system, which facilitated the removal of CO2 bubbles and prevented voltage loss caused by bubbles. (B) The unassisted CO2R current density as a function of operational time using a GaAs/InGaP/TiO2/Ni photoanode and a Pd/C-coated Ti mesh cathode in a two-electrode electrochemical configuration (panel A) under 100 mW cm–2 of simulated AM1.5 illumination. (C) The overall polarization characteristics for the CO2R reaction and the OER using a p+-Si/TiO2/Ni anode and a Pd/C-coated Ti mesh cathode in the two-electrode BMP configuration (KHCO3/Nafion/KOH) (black) as well as in the two-electrode Nafion membrane configuration (KHCO3/Nafion/KHCO3) (blue). The measured (red) and calculated (black) two-electrode current–voltage behavior of the GaAs/InGaP/TiO2/Ni photoanode wired to a Pd/C-coated Ti mesh cathode was measured under 100 mW cm–2 of simulated AM1.5 illumination. The calculated current density–voltage characteristic of the solid-state tandem cell (orange).(24, 25)
Figure 3C shows the measured (red) and calculated (dotted black) two-electrode current density vs voltage behavior of the GaAs/InGaP/TiO2/Ni photoanode wired to a Pd/C-coated Ti mesh cathode under 100 mW cm–2 of simulated Air Mass (AM)1.5 illumination. The calculated two-electrode current density vs voltage behavior (dotted black) was obtained by using the current–voltage behavior of the tandem solid-state photoabsorber (dotted orange) in conjunction with the overall polarization characteristic of a p+-Si/TiO2/Ni anode and a Pd/C-coated Ti mesh cathode in the two-electrode BMP configuration (KHCO3/BPM/KOH) (black). The calculated two-electrode current density vs voltage behavior was in good agreement with the experimental measurements. The electrosynthetic cell component required 2.04 V to operate at a current density of 8.5 mA cm–2 and was thus well matched to the maximum power point of the photovoltaic tandem junction component of the photoanode. The electrosynthetic cell thus operated with an electrical-to-fuel conversion efficiency, ηelectrolyzer, of 1.21 V/2.04 V = 59.3% at 8.5 mA cm–2 current density.(16, 29) Figure 3C also shows the overall polarization characteristics of the two-electrode Nafion membrane configuration (KHCO3/Nafion/KHCO3) (blue) using the same electrode materials. Due to the large overpotential for the Ni catalyst to affect the OER at the near-neutral pH (Figure 1C), obtaining an operational current density of ∼8.5 mA cm–2 to drive the overall CO2R reaction in conjunction with the OER required an additional ∼180 mV of voltage in the Nafion-containing cell relative to the voltage required to operate the BPM-containing cell (Tables S1 and S2). For comparison, a Nafion-containing cell for the electrochemical reduction of 1 atm of CO2 to CO while performing the OER at the anode in 0.4 M buffered KH2PO4(aq) (pH = 7.3)/Nafion/0.5 M KHCO3(aq) (pH = 7.3) required 2.5 V to produce a current density of 1 mA cm–2.(30) Transport of K+ between the anolyte and catholyte during steady-state operation would also electrodialyze the electrolytes in the Nafion-containing cell. Circulation or recirculation might potentially minimize the steady-state K+ ion concentration polarization of the system,(31) but would entail significant challenges in separation of the low concentration of the liquid product, formate, in the catholyte. In contrast, the robust product separation afforded by the BPM would allow for production of a high concentration of formate, which would be advantageous in a downstream separation process. Additionally, the Ni catalyst is not stable for OER at near-neutral pH.(32) Use of the BPM thus relaxed the electrolyte constraints and allowed the incorporation of this active OER catalyst(33) in the device.
TiO2-protected tandem III–V photoanodes have been used in a variety of cell configurations to construct high (>10%) efficiency solar-driven water-splitting cells.(24, 25) The J–E performance of the cathodes in such systems(34) are comparable to the J–E performance of the Pd/C nanoparticle-coated CO2R cathodes used herein, providing a basis for comparison of the efficiency losses in each of the cell types in the limit of having a low overpotential, selective cathode in each system. Although the 10% efficiency for CO2R reported herein is comparable in magnitude to the 10% efficiency reported for photoelectrosynthetic solar-driven water-splitting cells that use either anion exchange or BPMs,(24, 25, 35) the efficiencies of these different electrosynthetic cells cannot be directly compared at the system level. Solar-driven water-splitting cells in alkaline electrolytes produce physically separate streams of H2(g) and O2(g) (either at 1 atm or under higher pressures due to electrochemical compression) from an abundant reagent (including liquid or vapor H2O from humidified ambient air),(36, 37) have minimal (<100 mV) voltage losses associated with the anion-exchange membrane and electrolyte, utilize relatively low (<300 mV) overpotential electrocatalysts for the OER, and thus will provide the highest ηSTF.(24, 38) Use of a BPM allows for operation of the photoanode in electrolytes at near-neutral pH values, which are less corrosive to photoelectrodes, especially for III–V compound semiconductors, relative to the alkaline electrolyte (e.g., 1.0 M KOH), and facilitates the use of relatively large area (∼1 cm2) photoelectrodes due to stabilization by TiO2 protective coatings.(25) However, BPMs introduce additional membrane-derived voltage losses.(20-23, 25) The near-neutral pH operation also entails increased overpotential losses due to the reduced activity of available OER catalysts operating under such conditions.(13, 32, 39, 40) The available OER catalysts also obscure light and are semisoluble and unstable on the electrode surface during operation at near-neutral pH.(32) In addition, the finite ion crossovers in such cells would eventually lead to electrodialysis of the electrolyte during passive, long-term cell operation. The CO2R cells evaluated herein share the same ηSTF limitations as solar-driven water-splitting systems that utilize a BPM having one electrolyte at near-neutral pH and the other electrolyte under either strongly alkaline or acidic conditions (Tables S1 and S2). The CO2R cells moreover require that concentrated, purified (to eliminate highly electroactive, ambient O2) CO2 feeds are distributed over large electrode areas due to mass-transport limitations on the atmospheric CO2 flux into aqueous electrolytes,(5) and they also entail a loss of selectivity in reduction of CO2 relative to H2 production at atmospheric (400 ppmv) concentrations of CO2 in the cathode feed. The formate would also have to be separated from the aqueous solution, as would any other water-soluble CO2R product such as methanol, ethanol, or isopropanol, requiring an energy-intensive separation step. Use of flue gas as the (unsustainable) CO2 source would require removal of the electroactive O2(g) as well as purification of the flue-gas stream to remove NOx, SOx, Hg, and other trace flue-gas components that are either electroactive or that can poison the cathode. The gas stream would then need to be humidified and cooled to near-ambient temperatures, and the resulting gas feedstock distributed over the ∼107 m2 area would be required to collect the incident solar photon flux necessary to reduce the CO2 that is emitted from a 100 MW (electric) coal-fired power plant (see the SI for calculation details). Large storage reservoirs for the CO2 would also be required to compensate for the 20% capacity factor of the solar-driven photoelectrosynthetic cell relative to the nearly continuous CO2 emissions stream emanating from the power plant.
In summary, a solar-driven CO2 reduction photovoltaic-assisted electrosynthetic cell was demonstrated at a solar-to-fuel energy-conversion efficiency of 10% using a tandem GaAs/InGaP/TiO2/NiOx photoanode in 1.0 M KOH(aq), a Pd/C nanoparticle-coated Ti mesh cathode in 2.8 M KHCO3(aq), and a BPM reducing a purified feed stream of 1 atm of CO2(g). At the operational current density of 8.5 mA cm–2, the cathode exhibited <100 mV overpotential and >94% Faradaic efficiency for CO2 reduction to formate in 2.8 M KHCO3(aq) (pH = 8.0), the anode exhibited a 320 ± 7 mV overpotential for OER in 1.0 M KOH (aq) (pH = 13.7), and the BPM exhibited a ∼480 mV voltage loss with minimal product crossovers and >90 and >95% selectivity for proton and hydroxide ions, respectively. The BPM effectively coupled together two electrolytes that were separately effective for the CO2R reaction and for the OER and produced lower total overpotentials and higher efficiency than could at present be obtained in a single-electrolyte CO2 reduction cell. The photoelectrosynthetic cells also allowed a comparative evaluation of the operational constraints associated with sustainable solar-driven CO2 reduction systems relative to sustainable solar-driven water-splitting systems.
Supporting Information
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.6b00317.
Experimental details; calculation details; bipolar membrane and two-electrode configuration (Pd/C-coated Ti mesh/BPM/GaAs/InGaP/TiO2/Ni) stability data; voltage loss comparison for three cell configurations; low formate crossover data; Faradaic efficiency data for large-area Pd/C-coated Ti mesh cathode; and large-area bipolar membrane voltage loss data (PDF)
The authors declare no competing financial interest.
Acknowledgment
This material is based on work performed 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. S.A.F. acknowledges the Resnick Sustainability Institute at Caltech for a Postdoctoral Fellowship. The authors also thank N. Dalleska (Caltech) for his assistance with measurements and analysis of the ICPMS and TIC data.
References
This article references 40 other publications.
- 1.Lewis
, N. S.; Crabtree, G.; Nozik, A. J.; Wasielewski, M. R.; Alivisatos, P.; Kung, H.; Tsao, J.; Chandler, E.; Walukiewicz, W.; Spitler, M.; Basic Research Needs for Solar Energy Utilization. Report of the Basic Energy Sciences Workshop on Solar Energy Utilization. Office of Science, U. S. Department of Energy: Washington, DC, 2005. - 3.Gupta
, N.; Gattrell, M.; MacDougall, B.Calculation for the cathode surface concentrations in the electrochemical reduction of CO2 in KHCO3 solutions J. Appl. Electrochem. 2006, 36, 161– 172, DOI: 10.1007/s10800-005-9058-y[CrossRef], [CAS]3.https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD28XjsFKitA%253D%253D&md5=21d25aecf50b121ed3f269586680f658Gupta, N.; Gattrell, M.; MacDougall, B.Journal of Applied Electrochemistry (2006),This article presents a math. model that predicts the chem. conditions at the electrode surface during the electrochem. redn. of CO2. Such electrochem. redn. of CO2 to valuable products is an area of interest for the purpose of reducing green house gas emissions. In the reactions involved, CO2 acts as both a reactant and a buffer, consequently the estn. of local concns. at the electrode surface is not trivial and a numerical approach is required. The necessary partial differential equations (PDEs) were set-up and solved using MATLAB. The results show the local concns. at the electrode surface to be significantly different from the bulk concns. under typical reported exptl. conditions. The importance of buffer strength and a careful quantification of the degree of mixing produced in the exptl. app. is demonstrated. The model also was used to reexamine previously published data, showing that the Tafel slopes in CO2 redn. are consistent with those reported for the simpler CO redn. system. Further, the effect of pulsed electroredn. was also modeled, showing that pulsing causes corresponding swings in local pH and CO2 concns. - 4.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, 18924– 18936, DOI: 10.1039/C5CP03283K[CrossRef], [PubMed], [CAS]4.https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2MXhtVSqtrjL&md5=c959235a8ac5356c4552c963f9ad7501Singh, Meenesh R.; Clark, Ezra L.; Bell, Alexis T.Physical Chemistry Chemical Physics (2015),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. - 5.Chen
, Y.; Lewis, N. S.; Xiang, C.Operational constraints and strategies for systems to effect the sustainable, solar-driven reduction of atmospheric CO2 Energy Environ. Sci. 2015, 8, 3663– 3674, DOI: 10.1039/C5EE02908B[CrossRef], [CAS]5.https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2MXhs1amsr%252FJ&md5=13eb2c0a10f0fad475a83f011ca05e92Chen, Yikai; Lewis, Nathan S.; Xiang, ChengxiangEnergy & Environmental Science (2015),The operational constraints for a 6-electron/6-proton CO2 redn. system that operates at the concn. of CO2 in the current atm. (pCO2 = 400 ppm) have been evaluated on a variety of scale lengths that span from lab. scale to global scale. Due to the low concn. of CO2 in the atm., limitations due to mass transport of CO2 from the tropopause have been evaluated through five different regions, each with different characteristic length scales: the troposphere; the atm. boundary layer (ABL); the canopy layer; a membrane layer; and an aq. electrolyte layer. The resulting CO2 conductances, and assocd. phys. transport limitations, will set the ultimate limit on the efficiency and areal requirements of a sustainable solar-driven CO2 redn. system regardless of the activity or selectivity of catalysts for redn. of CO2 at the mol. level. At the electrolyte/electrode interface, the steady-state limiting c.d. and the concomitant voltage loss assocd. with the CO2 concn. overpotential in a one-dimensional solar-driven CO2 redn. cell have been assessed quant. using a math. model that accounts for diffusion, migration and convective transport, as well as for bulk electrochem. reactions in the electrolyte. At pCO2 = 400 ppm, the low diffusion coeff. combined with the low soly. of CO2 in aq. solns. constrains the steady-state limiting c.d. to <0.1 mA cm-2 in a typical electrochem. cell with natural convection and employing electrolytes with a range of pH values. Hence, in such a system, the CO2 capture area must be 100- to 1000-fold larger than the solar photon collection area to enable a >10% efficient solar-driven CO2 redn. system (based on the solar collection area). This flux limitation is consistent with ests. of oceanic CO2 uptake fluxes that have been developed in conjunction with carbon-cycle analyses for use in coupled atm./ocean general circulation models. Two strategies to improve the feasibility of obtaining efficient and sustainable CO2 transport to a cathode surface at pCO2 = 400 ppm are described and modeled quant. The first strategy employs yet unknown catalysts, analogous to carbonic anhydrases, that dramatically accelerate the chem. enhanced CO2 transport in the aq. electrolyte layer by enhancing the acid-base reactions in a bicarbonate buffer system. The rapid interconversion from bicarbonate to CO2 in the presence of such catalysts near the cathode surface would in principle yield significant increases in the steady-state limiting c.d. and allow for >10% solar-fuel operation at the cell level. The second strategy employs a thin-layer cell architecture to improve the diffusive transport of CO2 by use of an ultrathin polymeric membrane electrolyte. Rapid equilibration of CO2 at the gas/electrolyte interface, and significantly enhanced diffusive fluxes of CO2 in electrolytes, are required to increase the steady-state limiting c.d. of such a system. This latter approach however only is feasible for gaseous products, because liq. products would coat the electrode and therefore thicken the hydrodynamic boundary layer and accordingly reduce the diffusive CO2 flux to the electrode surface. Regardless of whether the limitations due to mass transport to the electrode surface are overcome on the lab. scale, at global scales the ultimate CO2 flux limitations will be dictated by mass transport considerations related to transport of atm. CO2 to the boundary plane of the solar-driven reactor system. The transport of CO2 across the troposphere/ABL interface, the ABL/canopy layer interface, and the canopy layer/electrolyte interface have therefore been assessed in this work, to provide upper bounds on the ultimate limits for the solar-to-fuel (STF) conversion efficiency for systems that are intended to effect the redn. of atm. CO2 in a sustainable fashion at global scale. - 6.Kumar
, B.; Llorente, M.; Froehlich, J.; Dang, T.; Sathrum, A.; Kubiak, C. P.Photochemical and Photoelectrochemical Reduction of CO2 Annu. Rev. Phys. Chem. 2012, 63, 541– 569, DOI: 10.1146/annurev-physchem-032511-143759[CrossRef], [PubMed], [CAS]6.https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC38Xnt1Gks7w%253D&md5=1e645496c15f5c4686db774ef297e060Kumar, Bhupendra; Llorente, Mark; Froehlich, Jesse; Dang, Tram; Sathrum, Aaron; Kubiak, Clifford P.Annual Review of Physical Chemistry (2012),A review. The recent literature on photochem. and photoelectrochem. redns. of CO2 is reviewed. The different methods of achieving light absorption, electron-hole sepn., and electrochem. redn. of CO2 are considered. Energy gap matching for redn. of CO2 to different products, including CO, formic acid, and methanol, is used to identify the most promising systems. Different approaches to lowering overpotentials and achieving high chem. selectivities by employing catalysts are described and compared. - 7.Hori
, Y.; Wakebe, H.; Tsukamoto, T.; Koga, O.Electrocatalytic Process of CO Selectivity in Electrochemical Reduction of CO2 at Metal-Electrodes in Aqueous-Media Electrochim. Acta 1994, 39, 1833– 1839, DOI: 10.1016/0013-4686(94)85172-7[CrossRef], [CAS]7.https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADyaK2cXls1GrtLs%253D&md5=f9795260a8e604a14f966ca1c8cc7998Hori, Yoshi; Wakebe, Hidetoshi; Tsukamoto, Toshio; Koga, OsamuElectrochimica Acta (1994),Electrochem. redn. of CO2 at metal electrodes yields CO, HCOO-, CH4, C2H4, and alcs. in aq. media. Metal electrodes are roughly divided into two groups, CO formation metals (Cu, Au, Ag, Zn, Pd, Ga, Ni, and Pt) and HCOO- ones (Pb, Hg, In, Sn, Tl). Foreign atom modified electrode (the coverage is virtually unity) showed variable product selectivity between CO and HCOO-, which depends upon the combination of modifier atom and substrate electrode. Crit. investigation of foreign atom modified electrodes derived a series of CO selectively, as Au > Ag > Cu > Zn » Cd > Sn > In > Pb > Tl > Hg. The electrode potentials of CO2 redn. are well correlated with the heat of fusion of metals and the potential of H2 evolution. The order of CO selectivity agrees roughly with that of the electrode potential of CO2 redn., and is rationalized in terms of stabilization of intermediate species CO2·- at the electrode surface. CO is produced from stably adsorbed CO2·-, and HCOO- is formed from free or weakly adsorbed CO2·-. - 8.Min
, X.; Kanan, M. W.Pd-Catalyzed Electrohydrogenation of Carbon Dioxide to Formate: High Mass Activity at Low Overpotential and Identification of the Deactivation Pathway J. Am. Chem. Soc. 2015, 137, 4701– 4708, DOI: 10.1021/ja511890h[ACS Full Text
], [CAS]8.https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2MXlt1WltLg%253D&md5=643928588e5d62433b98364b47f5aa3bMin, Xiaoquan; Kanan, Matthew W.Journal of the American Chemical Society (2015),Electrochem. redn. of CO2 to formate (HCO2-) powered by renewable electricity is a possible C-neg. alternative to synthesizing formate from fossil fuels. This process is energetically inefficient because >1 V of overpotential is required for CO2 redn. to HCO2- on the metals currently used as cathodic catalysts. Pd reduces CO2 to HCO2- with no overpotential, but this activity was previously limited to low synthesis rates and plagued by an unidentified deactivation pathway. Pd nanoparticles dispersed on a C support reach high mass activities (50-80 mA HCO2- synthesis per mg Pd) when driven by <200 mV of overpotential in aq. bicarbonate solns. Electrokinetic measurements are consistent with a mechanism in which the rate-detg. step is the addn. of electrochem. generated surface adsorbed H to CO2 (i.e., electrohydrogenation). The electrodes deactivate over several hours because of a minor pathway that forms CO. Activity is recovered, however, by removing CO with brief air exposure. - 9.Schreier
, M.; Curvat, L.; Giordano, F.; Steier, L.; Abate, A.; Zakeeruddin, S. M.; Luo, J.; Mayer, M. T.; Gratzel, M.Efficient photosynthesis of carbon monoxide from CO2 using perovskite photovoltaics Nat. Commun. 2015, 6, 7326, DOI: 10.1038/ncomms8326[CrossRef], [PubMed], [CAS]9.https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2MXhtF2ksrrJ&md5=adfc6c030ce870eb0c61bedafb2c0550Schreier, Marcel; Curvat, Laura; Giordano, Fabrizio; Steier, Ludmilla; Abate, Antonio; Zakeeruddin, Shaik M.; Luo, Jingshan; Mayer, Matthew T.; Gratzel, MichaelNature Communications (2015),Artificial photosynthesis, mimicking nature in its efforts to store solar energy, has received considerable attention from the research community. Most of these attempts target the prodn. of H2 as a fuel and our group recently demonstrated solar-to-hydrogen conversion at 12.3% efficiency. Here, in an effort to take this approach closer to real photosynthesis, which is based on the conversion of CO2, we demonstrate the efficient redn. of CO2 to carbon monoxide driven solely by simulated sunlight using water as the electron source. Employing series-connected perovskite photovoltaics and high-performance catalyst electrodes, we reach a solar-to-CO efficiency exceeding 6.5%, which represents a new benchmark in sunlight-driven CO2 conversion. Considering hydrogen as a secondary product, an efficiency exceeding 7% is obsd. Furthermore, this study represents one of the first demonstrations of extended, stable operation of perovskite photovoltaics, whose large open-circuit voltage is shown to be particularly suited for this process. - 10.Asadi
, M.; Kumar, B.; Behranginia, A.; Rosen, B. A.; Baskin, A.; Repnin, N.; Pisasale, D.; Phillips, P.; Zhu, W.; Haasch, R.Robust carbon dioxide reduction on molybdenum disulphide edges Nat. Commun. 2014, 5, 4470, DOI: 10.1038/ncomms5470[CrossRef], [PubMed], [CAS]10.https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A280%3ADC%252BC2cbnt1Ontg%253D%253D&md5=365954965806ac37b96d6745241a7900Asadi Mohammad; Kumar Bijandra; Behranginia Amirhossein; Pisasale Davide; Abiade Jeremiah; Salehi-Khojin Amin; Rosen Brian A; Baskin Artem; Repnin Nikita; Phillips Patrick; Klie Robert F; Zhu Wei; Haasch Richard; Kral PetrNature communications (2014),Electrochemical reduction of carbon dioxide has been recognized as an efficient way to convert carbon dioxide to energy-rich products. Noble metals (for example, gold and silver) have been demonstrated to reduce carbon dioxide at moderate rates and low overpotentials. Nevertheless, the development of inexpensive systems with an efficient carbon dioxide reduction capability remains a challenge. Here we identify molybdenum disulphide as a promising cost-effective substitute for noble metal catalysts. We uncover that molybdenum disulphide shows superior carbon dioxide reduction performance compared with the noble metals with a high current density and low overpotential (54 mV) in an ionic liquid. Scanning transmission electron microscopy analysis and first principle modelling reveal that the molybdenum-terminated edges of molybdenum disulphide are mainly responsible for its catalytic performance due to their metallic character and a high d-electron density. This is further experimentally supported by the carbon dioxide reduction performance of vertically aligned molybdenum disulphide. - 11.Asadi
, M.; Kim, K.; Liu, C.; Addepalli, A. V.; Abbasi, P.; Yasaei, P.; Phillips, P.; Behranginia, A.; Cerrato, J. M.; Haasch, R.Nanostructured transition metal dichalcogenide electrocatalysts for CO2 reduction in ionic liquid Science 2016, 353, 467– 470, DOI: 10.1126/science.aaf4767[CrossRef], [PubMed], [CAS]11.https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28Xht1ehtL7M&md5=b2a2c08639c77e7ca8afd6c4c1a08bd4Asadi, Mohammad; Kim, Kibum; Liu, Cong; Addepalli, Aditya Venkata; Abbasi, Pedram; Yasaei, Poya; Phillips, Patrick; Behranginia, Amirhossein; Cerrato, Jose M.; Haasch, Richard; Zapol, Peter; Kumar, Bijandra; Klie, Robert F.; Abiade, Jeremiah; Curtiss, Larry A.; Salehi-Khojin, AminScience (Washington, DC, United States) (2016),Conversion of carbon dioxide (CO2) into fuels is an attractive soln. to many energy and environmental challenges. However, the chem. inertness of CO2 renders many electrochem. and photochem. conversion processes inefficient. A transition metal dichalcogenide nanoarchitecture is reported for catalytic electrochem. CO2 conversion to carbon monoxide (CO) in an ionic liq. It is found that tungsten diselenide nanoflakes show a c.d. of 18.95 mA per square centimeter, CO faradaic efficiency of 24%, and CO formation turnover frequency of 0.28 per s at a low overpotential of 54 mV. The catalyst is also applied in a light-harvesting artificial leaf platform that concurrently oxidized water in the absence of any external potential. - 12.Zhang
, B.; Zheng, X. L.; Voznyy, O.; Comin, R.; Bajdich, M.; Garcia-Melchor, M.; Han, L. L.; Xu, J. X.; Liu, M.; Zheng, L. R.Homogeneously dispersed multimetal oxygen-evolving catalysts Science 2016, 352, 333– 337, DOI: 10.1126/science.aaf1525[CrossRef], [PubMed], [CAS]12.https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28Xlsl2isLk%253D&md5=d2b80a4091e08859b95ca5900015c186Zhang, Bo; Zheng, Xueli; Voznyy, Oleksandr; Comin, Riccardo; Bajdich, Michal; Garcia-Melchor, Max; Han, Lili; Xu, Jixian; Liu, Min; Zheng, Lirong; Garcia de Arquer, F. Pelayo; Dinh, Cao Thang; Fan, Fengjia; Yuan, Mingjian; Yassitepe, Emre; Chen, Ning; Regier, Tom; Liu, Pengfei; Li, Yuhang; De Luna, Phil; Janmohamed, Alyf; Xin, Huolin L.; Yang, Huagui; Vojvodic, Aleksandra; Sargent, Edward H.Science (Washington, DC, United States) (2016),Earth-abundant first-row (3d) transition metal-based catalysts were developed for the oxygen-evolution reaction (OER); however, they operate at overpotentials substantially above thermodn. requirements. D. functional theory suggested that non-3d high-valency metals such as tungsten can modulate 3d metal oxides, providing near-optimal adsorption energies for OER intermediates. A room-temp. synthesis is developed to produce gelled oxyhydroxides materials with an atomically homogeneous metal distribution. These gelled FeCoW oxyhydroxides exhibit the lowest overpotential (191 mV) reported at 10 mA per square centimeter in alk. electrolyte. The catalyst shows no evidence of degrdn. after more than 500 h of operation. X-ray absorption and computational studies reveal a synergistic interplay between tungsten, iron, and cobalt in producing a favorable local coordination environment and electronic structure that enhance the energetics for OER. - 13.McCrory
, C. C. L.; Jung, S. H.; Peters, J. C.; Jaramillo, T. F.Benchmarking Heterogeneous Electrocatalysts for the Oxygen Evolution Reaction J. Am. Chem. Soc. 2013, 135, 16977– 16987, DOI: 10.1021/ja407115p[ACS Full Text], [CAS]13.https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3sXhslSqtLnM&md5=105318a53866c4b147ae62288137536dMcCrory, Charles C. L.; Jung, Suho; Peters, Jonas C.; Jaramillo, Thomas F.Journal of the American Chemical Society (2013),Objective evaluation of the activity of electrocatalysts for water oxidn. is of fundamental importance for the development of promising energy conversion technologies including integrated solar water-splitting devices, water electrolyzers, and Li-air batteries. However, current methods employed to evaluate oxygen-evolving catalysts are not standardized, making it difficult to compare the activity and stability of these materials. We report a protocol for evaluating the activity, stability, and Faradaic efficiency of electrodeposited oxygen-evolving electrocatalysts. In particular, we focus on methods for detg. electrochem. active surface area and measuring electrocatalytic activity and stability under conditions relevant to an integrated solar water-splitting device. Our primary figure of merit is the overpotential required to achieve a c.d. of 10 mA cm-2 per geometric area, approx. the c.d. expected for a 10% efficient solar-to-fuels conversion device. Utilizing the aforementioned surface area measurements, one can det. electrocatalyst turnover frequencies. The reported protocol was used to examine the oxygen-evolution activity of the following systems in acidic and alk. solns.: CoOx, CoPi, CoFeOx, NiOx, NiCeOx, NiCoOx, NiCuOx, NiFeOx, and NiLaOx. The oxygen-evolving activity of an electrodeposited IrOx catalyst was also investigated for comparison. Two general observations are made from comparing the catalytic performance of the OER catalysts investigated: (1) in alk. soln., every non-noble metal system achieved 10 mA cm-2 current densities at similar operating overpotentials between 0.35 and 0.43 V, and (2) every system but IrOx was unstable under oxidative conditions in acidic solns. - 14.McCrory
, C. C. L.; Jung, S.; Ferrer, I. M.; Chatman, S. M.; Peters, J. C.; Jaramillo, T. F.Benchmarking Hydrogen Evolving Reaction and Oxygen Evolving Reaction Electrocatalysts for Solar Water Splitting Devices J. Am. Chem. Soc. 2015, 137, 4347– 4357, DOI: 10.1021/ja510442p[ACS Full Text], [CAS]14.https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2MXisFyqtrw%253D&md5=7a45e7400dde5988037aab80ed076b75McCrory, Charles C. L.; Jung, Suho; Ferrer, Ivonne M.; Chatman, Shawn M.; Peters, Jonas C.; Jaramillo, Thomas F.Journal of the American Chemical Society (2015),Objective comparisons of electrocatalyst activity and stability using std. methods under identical conditions are necessary to evaluate the viability of existing electrocatalysts for integration into solar-fuel devices as well as to help inform the development of new catalytic systems. Herein, the authors use a std. protocol as a primary screen for evaluating the activity, short-term (2 h) stability, and electrochem. active surface area (ECSA) of 18 electrocatalysts for the H evolution reaction (HER) and 26 electrocatalysts for the O evolution reaction (OER) under conditions relevant to an integrated solar H2O-splitting device in aq. acidic or alk. soln. The primary figure of merit is the overpotential necessary to achieve a magnitude c.d. of 10 mA cm-2 per geometric area, the approx. c.d. expected for a 10% efficient solar-to-fuels conversion device under 1 sun illumination. The specific activity per ECSA of each material is also reported. Among HER catalysts, several could operate at 10 mA cm-2 with overpotentials <0.1 V in acidic and/or alk. solns. Among OER catalysts in acidic soln., no nonnoble metal based materials showed promising activity and stability, whereas in alk. soln. many OER catalysts performed with similar activity achieving 10 mA cm-2 current densities at overpotentials of ∼0.33-0.5 V. Most OER catalysts showed comparable or better specific activity per ECSA when compared to Ir and Ru catalysts in alk. solns., while most HER catalysts showed much lower specific activity than Pt in both acidic and alk. solns. For select catalysts, addnl. secondary screening measurements were conducted including faradaic efficiency and extended stability measurements. - 15.Bockris
, J. O. M.; Reddy, A. K.; Gamboa-Aldeco, M. E. Modern Electrochemistry; Springer: New York, 2000.There is no corresponding record for this reference. - 16.Xiang
, C.; Papadantonakis, K. M.; Lewis, N. S.Principles and implementations of electrolysis systems for water splitting Mater. Horiz. 2016, 3, 169– 173, DOI: 10.1039/C6MH00016A[CrossRef], [CAS]16.https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28XisVegu7o%253D&md5=caad4ed84b88532d135eeb41c995dcd4Xiang, Chengxiang; Papadantonakis, Kimberly M.; Lewis, Nathan S.Materials Horizons (2016),Efforts to develop renewable sources of carbon-neutral fuels have brought a renewed focus to research and development of sunlight-driven water-splitting systems. Electrolysis of water to produce H2 and O2 gases is the foundation of such systems, is conceptually and practically simple, and has been practiced both in the lab. and industrially for many decades. In this Focus article, we present the fundamentals of water splitting and describe practices which distinguish com. water-electrolysis systems from simple lab.-scale demonstrations. - 17.Arai
, T.; Sato, S.; Morikawa, T.A monolithic device for CO2 photoreduction to generate liquid organic substances in a single-compartment reactor Energy Environ. Sci. 2015, 8, 1998– 2002, DOI: 10.1039/C5EE01314C[CrossRef], [CAS]17.https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2MXpt1Sjtbw%253D&md5=e1e8a6049245dc9519e48ca84465e18dArai, Takeo; Sato, Shunsuke; Morikawa, TakeshiEnergy & Environmental Science (2015),A solar to chem. energy conversion efficiency of 4.6% was demonstrated for CO2 photoredn. to formate utilizing water as an electron donor under simulated solar light irradn. to a monolithic tablet-shaped device. The simple CO2 photoredn. system was realized by exploiting the effect of the carbon substrate on selective CO2 redn. in the presence of oxygen and selective H2O oxidn. over IrOx catalysts in the presence of formate. - 18.Jeon
, H. S.; Koh, J. H.; Park, S. J.; Jee, M. S.; Ko, D. H.; Hwang, Y. J.; Min, B. K.A monolithic and standalone solar-fuel device having comparable efficiency to photosynthesis in nature J. Mater. Chem. A 2015, 3, 5835– 5842, DOI: 10.1039/C4TA06495J[CrossRef], [CAS]18.https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXitV2mtbnE&md5=8345b5a40bbb797ab3daf26e74e28daeJeon, Hyo Sang; Koh, Jai Hyun; Park, Se Jin; Jee, Michael Shincheon; Ko, Doo-Hyun; Hwang, Yun Jeong; Min, Byoung KounJournal of Materials Chemistry A: Materials for Energy and Sustainability (2015),The need for developing sustainable energy sources has generated academic and industrial attention in artificial photosynthesis, inspired by the natural process. In this study, a highly efficient solar energy to fuel conversion device is demonstrated using CO2 and water as feedstock. A thin film photovoltaic technol. is developed for the light absorbing component using a low cost, soln. based Cu(InxGa1-x)(SySe1-y)2 (CIGS) module fabrication method to provide sufficient potential for the conversion reactions. The solar-fuel device uses cobalt oxide (Co3O4) nanoparticle thin film deposited with a low temp. coating method as the water oxidn. catalyst and nanostructured gold film as the CO2 redn. to CO generation catalyst. It is demonstrated that the integrated monolithic device operated by energy only from sunlight, in an absence of any external energy input. The individual components showed the following abilities, solar-to-power conversion efficiency of 8.58% for the CIGS photovoltaic module photoelectrode, overpotential redn. of water oxidn. with the Co3O4 catalyst film by ∼360 mV at 5 mA cm-2, and Faradaic efficiency of over 90% by the nanostructured Au catalyst for CO2 redn. to CO. Remarkably, this is the first demonstration of a monolithic and standalone solar-fuel device whose solar-to-fuel conversion efficiency from CO2 and H2O is 4.23%, which is comparable with that of photosynthesis in nature. - 19.Sugano
, Y.; Ono, A.; Kitagawa, R.; Tamura, J.; Yamagiwa, M.; Kudo, Y.; Tsutsumi, E.; Mikoshiba, S.Crucial role of sustainable liquid junction potential for solar-to-carbon monoxide conversion by a photovoltaic photoelectrochemical system RSC Adv. 2015, 5, 54246– 54252, DOI: 10.1039/C5RA07179H[CrossRef], [CAS]19.https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2MXhtVSgtr7O&md5=79f1e9f6c5ecb22e07d564d5042a337aSugano, Yoshitsune; Ono, Akihiko; Kitagawa, Ryota; Tamura, Jun; Yamagiwa, Masakazu; Kudo, Yuki; Tsutsumi, Eishi; Mikoshiba, SatoshiRSC Advances (2015),Solar energy conversion to carbon monoxide (CO) is carried out using a wired photovoltaic photoelectrochem. (PV PEC) system under simulated solar light irradn. The PV PEC system promotes CO generation from carbon dioxide and water with approx. 2.0% solar-to-CO conversion efficiency (ηCO) for 2 h. This is achieved via contributions from electrolyte conditions, which generate a sustainable liq. junction potential, in addn. to the combination of efficient visible light absorption by a triple-junction amorphous silicon PV cell with high electrode activities with low overpotentials. Estns. of energy conversion efficiency based on the electrochem. properties of the PV cell and electrodes exhibit that the liq. junction potential makes a huge contribution to ηCO. Moreover, the liq. junction potential created by bubbling two kinds of carrier gases produces sustainable chem. bias. This system may contribute to new strategies for the development of sustainable artificial photosynthesis. - 20.Vargas-Barbosa
, N. M.; Geise, G. M.; Hickner, M. A.; Mallouk, T. E.Assessing the Utility of Bipolar Membranes for use in Photoelectrochemical Water-Splitting Cells ChemSusChem 2014, 7, 3017– 3020, DOI: 10.1002/cssc.201402535[CrossRef], [PubMed], [CAS]20.https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXhvVeksb7J&md5=f0e908abe0f46782bf8a1670d576e8d6Vargas-Barbosa, Nella M.; Geise, Geoffrey M.; Hickner, Michael A.; Mallouk, Thomas E.ChemSusChem (2014),Membranes are important in water-splitting solar cells because they prevent crossover of hydrogen and oxygen. Here, bipolar membranes (BPMs) were tested as separators in water electrolysis cells. Steady-state membrane and soln. resistances, electrode overpotentials, and pH gradients were measured at current densities relevant to solar photoelectrolysis. Under forward bias conditions, electrodialysis of phosphate buffer ions creates a pH gradient across a BPM. Under reverse bias, the BPM can maintain a const. buffer pH on both sides of the cell, but a large membrane potential develops. Thus, the BPM does not present a viable soln. for electrolysis in buffered electrolytes. However, the membrane potential is minimized when the anode and cathode compartments of the cell contain strongly basic and acidic electrolytes, resp. - 21.McDonald
, M. B.; Ardo, S.; Lewis, N. S.; Freund, M. S.Use of Bipolar Membranes for Maintaining Steady-State pH Gradients in Membrane-Supported, Solar-Driven Water Splitting ChemSusChem 2014, 7, 3021– 3027, DOI: 10.1002/cssc.201402288[CrossRef], [PubMed], [CAS]21.https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXhvVeksb7P&md5=49da44270ba4fe23bda8a3f4e2ecc951McDonald, Michael B.; Ardo, Shane; Lewis, Nathan S.; Freund, Michael S.ChemSusChem (2014),A bipolar membrane can maintain a steady-state pH difference between the sites of oxidn. and redn. in membrane-supported, solar-driven water-splitting systems without changing the overall thermodn. required to split water. A com. available bipolar membrane that can serve this purpose has been identified, its performance has been evaluated quant., and is demonstrated to meet the requirements for this application. For effective utilization in integrated solar-driven water-splitting systems, such bipolar membranes must, however, be modified to simultaneously optimize their phys. properties such as optical transparency, electronic cond. and kinetics of water dissocn. - 22.Ünlü
, M.; Zhou, J.; Kohl, P. A.Hybrid Anion and Proton Exchange Membrane Fuel Cells J. Phys. Chem. C 2009, 113, 11416– 11423, DOI: 10.1021/jp903252u[ACS Full Text], [CAS]22.https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD1MXmvVeksLc%253D&md5=222a0994a5ed1046c751fcfbcb014350Unlu, Murat; Zhou, Junfeng; Kohl, Paul A.Journal of Physical Chemistry C (2009),A novel hybrid fuel cell is described using at least one electrode operating at high pH in an effort to use the high cond. of Nafion and exploit the electrochem. advantages of high-pH operation. The electrochem. behavior of a hybrid anion exchange membrane (AEM)/proton exchange membrane (PEM) junction and corresponding fuel cell are presented. Two AEM/PEM junctions and fuel cells have been evaluated: AEM anode/PEM cathode and PEM anode/AEM cathode. The AEM cathode/PEM membrane configuration causes proton-hydroxide recombination at the membrane junction, resulting in self-hydration of the membrane. The fuel cell performance improves at lower relative humidity. At 65° and 0% relative humidity, the hybrid cell operates at steady state whereas the performance of a conventional PEM cell decreases with time due to dry-out. - 23.Vermaas
, D. A.; Sassenburg, M.; Smith, W. A.Photo-assisted water splitting with bipolar membrane induced pH gradients for practical solar fuel devices J. Mater. Chem. A 2015, 3, 19556– 19562, DOI: 10.1039/C5TA06315A[CrossRef], [CAS]23.https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2MXhtlyjtLbN&md5=8422c57f3ae615ee695124857aa52107Vermaas, David A.; Sassenburg, Mark; Smith, Wilson A.Journal of Materials Chemistry A: Materials for Energy and Sustainability (2015),Different pH requirements for a cathode and an anode result in a non-optimal performance for practical solar fuel systems. We present for the first time a photo-assisted water splitting device using a bipolar membrane, which allows a cathode to operate in an acidic electrolyte while the photoanode is in alk. conditions. The bipolar membrane dissocs. water into H+ and OH-, which is consumed for hydrogen evolution at the cathode and oxygen evolution at the anode, resp. The introduction of such a bipolar membrane for solar fuel systems provides ultimate freedom for combining different (photo)cathodes and -anodes. This paper shows that photo-assisted water splitting at both extreme pH gradients (0-14) as well as mild pH gradients (0-7) yields current densities of 2-2.5 mA cm-2 using a BiVO4 photoanode and a bipolar membrane. The membrane potentials are within 30 mV of the theor. electrochem. potential for low current densities. The pH gradient is maintained for 4 days of continuous operation and electrolyte anal. shows that salt cross-over is minimal. The stable operation of the bipolar membrane in extreme and mild pH gradients, at negligible loss, contributes to a sustainable and practically feasible solar fuel device with existing photoactive electrodes operating at different pH. - 24.Verlage
, E.; Hu, S.; Liu, R.; Jones, R. J. R.; Sun, K.; Xiang, C.; Lewis, N. S.; Atwater, H. A.A monolithically integrated, intrinsically safe, 10% efficient, solar-driven water-splitting system based on active, stable earth-abundant electrocatalysts in conjunction with tandem III-V light absorbers protected by amorphous TiO2 films Energy Environ. Sci. 2015, 8, 3166– 3172, DOI: 10.1039/C5EE01786F[CrossRef], [CAS]24.https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2MXhtlKnsLfE&md5=e548bf4d84d606a21052c9eef1da43efVerlage, Erik; Hu, Shu; Liu, Rui; Jones, Ryan J. R.; Sun, Ke; Xiang, Chengxiang; Lewis, Nathan S.; Atwater, Harry A.Energy & Environmental Science (2015),A monolithically integrated device consisting of a tandem-junction GaAs/InGaP photoanode coated by an amorphous TiO2 stabilization layer, in conjunction with Ni-based, earth-abundant active electrocatalysts for the hydrogen-evolution and oxygen-evolution reactions, was used to effect unassisted, solar-driven water splitting in 1.0 M KOH(aq). When connected to a Ni-Mo-coated counterelectrode in a two-electrode cell configuration, the TiO2-protected III-V tandem device exhibited a solar-to-hydrogen conversion efficiency, ηSTH, of 10.5% under 1 sun illumination, with stable performance for >40 h of continuous operation at an efficiency of ηSTH > 10%. The protected tandem device also formed the basis for a monolithically integrated, intrinsically safe solar-hydrogen prototype system (1 cm2) driven by a NiMo/GaAs/InGaP/TiO2/Ni structure. The intrinsically safe system exhibited a hydrogen prodn. rate of 0.81 μL s-1 and a solar-to-hydrogen conversion efficiency of 8.6% under 1 sun illumination in 1.0 M KOH(aq), with minimal product gas crossover while allowing for beneficial collection of sep. streams of H2(g) and O2(g). - 25.Sun
, K.; Liu, R.; Chen, Y.; Verlage, E.; Lewis, N. S.; Xiang, C.A Stabilized, Intrinsically Safe, 10% Efficient, Solar-Driven Water-Splitting Cell Incorporating Earth-Abundant Electrocatalysts with Steady-State pH Gradients and Product Separation Enabled by a Bipolar Membrane Adv. Energy Mater. 2016, 6, 1600379, DOI: 10.1002/aenm.201600379 - 26.Kortlever
, R.; Peters, I.; Koper, S.; Koper, M. T. M.Electrochemical CO2 Reduction to Formic Acid at Low Overpotential and with High Faradaic Efficiency on Carbon-Supported Bimetallic Pd–Pt Nanoparticles ACS Catal. 2015, 5, 3916– 3923, DOI: 10.1021/acscatal.5b00602[ACS Full Text], [CAS]26.https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2MXosVCgs7o%253D&md5=6e2b733f6b366910025954a88300f1c6Kortlever, Ruud; Peters, Ines; Koper, Sander; Koper, Marc T. M.ACS Catalysis (2015),The electrochem. redn. of CO2 has attracted significant interest recently, as it is a possible reaction for the storage of renewable energy. Here, the authors report on the synthesis of PdxPt(100-x)/C nanoparticles and their electrocatalytic properties for the redn. of CO2 to formic acid, compared with their activity for the reverse oxidn. of formic acid to CO2. PdxPt(100-x)/C nanoparticles have a very low onset potential for the redn. of CO2 to formic acid of ∼0 V vs. RHE, which approaches the theor. equil. potential of 0.02 V vs. RHE for this reaction. Also, the Pd70Pt30/C catalyst shows a faradaic efficiency of 88% toward formic acid after 1 h of electrolysis at -0.4 V vs. RHE with an av. c.d. of ∼5 mA/cm2. Therefore, this catalyst shows a competing or even better faradaic efficiency toward formic acid compared to recently reported catalysts, at a substantially lower overpotential, while avoiding a strong deactivation that was obsd. with previously reported Pd-based catalysts. - 27.Sun
, K.; Saadi, F. H.; Lichterman, M. F.; Hale, W. G.; Wang, H.-P.; Zhou, X.; Plymale, N. T.; Omelchenko, S. T.; He, J.-H.; Papadantonakis, K. M.Stable solar-driven oxidation of water by semiconducting photoanodes protected by transparent catalytic nickel oxide films Proc. Natl. Acad. Sci. U. S. A. 2015, 112, 3612– 3617, DOI: 10.1073/pnas.1423034112[CrossRef], [PubMed], [CAS]27.https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2MXktFentLk%253D&md5=30b72ba4d0204711ee853587e5cf7c0fSun, Ke; Saadi, Fadl H.; Lichterman, Michael F.; Hale, William G.; Wang, Hsin-Ping; Zhou, Xinghao; Plymale, Noah T.; Omelchenko, Stefan T.; He, Jr-Hau; Papadantonakis, Kimberly M.; Brunschwig, Bruce S.; Lewis, Nathan S.Proceedings of the National Academy of Sciences of the United States of America (2015),Reactively sputtered nickel oxide (NiOx) films provide transparent, antireflective, elec. conductive, chem. stable coatings that also are highly active electrocatalysts for the oxidn. of water to O2(g). These NiOx coatings provide protective layers on a variety of technol. important semiconducting photoanodes, including textured cryst. Si passivated by amorphous silicon, cryst. n-type cadmium telluride, and hydrogenated amorphous silicon. Under anodic operation in 1.0 M aq. potassium hydroxide (pH 14) in the presence of simulated sunlight, the NiOx films stabilized all of these self-passivating, high-efficiency semiconducting photoelectrodes for >100 h of sustained, quant. solar-driven oxidn. of water to O2(g). - 28.Hu
, S.; Shaner, M. R.; Beardslee, J. A.; Lichterman, M.; Brunschwig, B. S.; Lewis, N. S.Amorphous TiO2 coatings stabilize Si, GaAs, and GaP photoanodes for efficient water oxidation Science 2014, 344, 1005– 1009, DOI: 10.1126/science.1251428[CrossRef], [PubMed], [CAS]28.https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXosFSmuro%253D&md5=9de5f3032f2f33d9db97a9759ed0307bHu, Shu; Shaner, Matthew R.; Beardslee, Joseph A.; Lichterman, Michael; Brunschwig, Bruce S.; Lewis, Nathan S.Science (Washington, DC, United States) (2014),Although semiconductors such as silicon (Si), gallium arsenide (GaAs), and gallium phosphide (GaP) have band gaps that make them efficient photoanodes for solar fuel prodn., these materials are unstable in aq. media. TiO2 coatings (4 to 143 nm thick) grown by at. layer deposition prevent corrosion, have electronic defects that promote hole conduction, and are sufficiently transparent to reach the light-limited performance of protected semiconductors. In conjunction with a thin layer or islands of Ni oxide electrocatalysts, Si photoanodes exhibited continuous oxidn. of 1.0M aq. KOH to O2 for >100 h at photocurrent densities of >30 mA per square centimeter and ~100% faradaic efficiency. TiO2-coated GaAs and GaP photoelectrodes exhibited photovoltages of 0.81 and 0.59 V and light-limiting photocurrent densities of 14.3 and 3.4 mA per square centimeter, resp., for water oxidn. - 29.Coridan
, R. H.; Nielander, A. C.; Francis, S. A.; McDowell, M. T.; Dix, V.; Chatman, S. M.; Lewis, N. S.Methods for comparing the performance of energy-conversion systems for use in solar fuels and solar electricity generation Energy Environ. Sci. 2015, 8, 2886– 2901, DOI: 10.1039/C5EE00777A[CrossRef], [CAS]29.https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2MXmsFejtr4%253D&md5=7a29a32e733b42d21ada6f6e0e3d49b4Coridan, Robert H.; Nielander, Adam C.; Francis, Sonja A.; McDowell, Matthew T.; Dix, Victoria; Chatman, Shawn M.; Lewis, Nathan S.Energy & Environmental Science (2015),The energy-conversion efficiency is a key metric that facilitates comparison of the performance of various approaches to solar energy conversion. However, a suite of disparate methodologies has been proposed and used historically to evaluate the efficiency of systems that produce fuels, either directly or indirectly, with sunlight and/or elec. power as the system inputs. A general expression for the system efficiency is given as the ratio of the total output power (elec. plus chem.) divided by the total input power (elec. plus solar). The solar-to-hydrogen (STH) efficiency follows from this globally applicable system efficiency but only is applicable in the special case for systems in which the only input power is sunlight and the only output power is in the form of hydrogen fuel derived from solar-driven water splitting. Herein, system-level efficiencies, beyond the STH efficiency, as well as component-level figures of merit are defined and discussed to describe the relative energy-conversion performance of key photoactive components of complete systems. These figures of merit facilitate the comparison of electrode materials and interfaces without conflating their fundamental properties with the engineering of the cell setup. The resulting information about the components can then be used in conjunction with a graphical circuit anal. formalism to obtain "optimal" system efficiencies that can be compared between various approaches. The approach provides a consistent method for comparison of the performance at the system and component levels of various technologies that produce fuels and/or electricity from sunlight. - 30.Tatin
, A.; Comminges, C.; Kokoh, B.; Costentin, C.; Robert, M.; Savéant, J.-M.Efficient electrolyzer for CO2 splitting in neutral water using earth-abundant materials Proc. Natl. Acad. Sci. U. S. A. 2016, 113, 5526– 5529, DOI: 10.1073/pnas.1604628113[CrossRef], [PubMed], [CAS]30.https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28XntVChu7c%253D&md5=5f3669caac6aed1d0969cf8e9c8bf150Tatin, Arnaud; Comminges, Clement; Kokoh, Boniface; Costentin, Cyrille; Robert, Marc; Saveant, Jean-MichelProceedings of the National Academy of Sciences of the United States of America (2016),Low-cost, efficient CO2-to-CO+O2 electrochem. splitting is a key step for liq.-fuel prodn. for renewable energy storage and use of CO2 as a feedstock for chems. Heterogeneous catalysts for cathodic CO2-to-CO assocd. with an O2-evolving anodic reaction in high-energy-efficiency cells are not yet available. An Fe porphyrin immobilized into a conductive Nafion/C powder layer is a stable cathode producing CO in pH neutral H2O with 90% faradaic efficiency. It is coupled with a H2O oxidn. phosphate Co oxide anode in a home-made electrolyzer by a Nafion membrane. Current densities of ∼1 mA/cm2 over 30-h electrolysis are achieved at a 2.5-V cell voltage, splitting CO2 and H2O into CO and O2 with a 50% energy efficiency. Remarkably, CO2 redn. outweighs the concurrent H2O redn. The setup does not prevent high-efficiency p transport through the Nafion membrane separator: The ohmic drop loss is only 0.1 V and the pH remains stable. These results demonstrate the possibility to set up an efficient, low-voltage, electrochem. cell that converts CO2 into CO and O2 by assocg. a cathodic-supported mol. catalyst based on an abundant transition metal with a cheap, easy-to-prep. anodic catalyst oxidizing H2O into O2. - 31.Modestino
, M. A.; Walczak, K. A.; Berger, A.; Evans, C. M.; Haussener, S.; Koval, C.; Newman, J. S.; Ager, J. W.; Segalman, R. A.Robust production of purified H2 in a stable, self-regulating, and continuously operating solar fuel generator Energy Environ. Sci. 2014, 7, 297– 301, DOI: 10.1039/C3EE43214A[CrossRef], [CAS]31.https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3sXhvFKhtrjP&md5=f7f72de9c591b35ea2c9a54c8781363bModestino, Miguel A.; Walczak, Karl A.; Berger, Alan; Evans, Christopher M.; Haussener, Sophia; Koval, Carl; Newman, John S.; Ager, Joel W.; Segalman, Rachel A.Energy & Environmental Science (2014),The development of practical solar-driven electrochem. fuel generators requires the integration of light absorbing and electrochem. components into an architecture that must also provide easy sepn. of the product fuels. Unfortunately, many of these components are not stable under the extreme pH conditions necessary to facilitate ionic transport between redox reaction sites. By using a controlled recirculating stream across reaction sites, this work demonstrates a stable, self-regulating and continuous purified solar-hydrogen generation from near neutral pH electrolytes that yield continuous nearly pure H2 streams with solar-fuel efficiencies above 6.2%. - 32.Minguzzi
, A.; Fan, F.-R. F.; Vertova, A.; Rondinini, S.; Bard, A. J.Dynamic potential-pH diagrams application to electrocatalysts for water oxidation Chem. Sci. 2012, 3, 217– 229, DOI: 10.1039/C1SC00516B[CrossRef], [CAS]32.https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3MXhsFeju7fE&md5=6ab6082546859c01942aa98d60fe1ff4Minguzzi, Alessandro; Fan, Fu-Ren F.; Vertova, Alberto; Rondinini, Sandra; Bard, Allen J.Chemical Science (2012),The construction and use of "dynamic potential-pH diagrams" (DPPDs), that are intended to extend the usefulness of thermodn. Pourbaix diagrams to include kinetic considerations is described. As an example, DPPDs are presented for the comparison of electrocatalysts for water oxidn., i.e., the oxygen evolution reaction (OER), an important electrochem. reaction because of its key role in energy conversion devices and biol. systems (water electrolyzes, photoelectrochem. water splitting, plant photosynthesis). The criteria for obtaining kinetic data are discussed and a 3-D diagram, which shows the heterogeneous electron transfer kinetics of an electrochem. system as a function of pH and applied potential is presented. DPPDs are given for four catalysts: IrO2, Co3O4, Co3O4 electrodeposited in a phosphate medium (Co-Pi) and Pt, allowing a direct comparison of the activity of different electrode materials over a broad range of exptl. conditions (pH, potential, c.d.). In addn., the exptl. setup and the factors affecting the accurate collection and presentation of data (e.g., ref. electrode system, correction of ohmic drops, bubble formation) are discussed. - 33.Trotochaud
, L.; Young, S. L.; Ranney, J. K.; Boettcher, S. W.Nickel–Iron Oxyhydroxide Oxygen-Evolution Electrocatalysts: The Role of Intentional and Incidental Iron Incorporation J. Am. Chem. Soc. 2014, 136, 6744– 6753, DOI: 10.1021/ja502379c[ACS Full Text], [CAS]33.https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXmtlygtLc%253D&md5=6a26a961800255dfe91df6d95388f64dTrotochaud, Lena; Young, Samantha L.; Ranney, James K.; Boettcher, Shannon W.Journal of the American Chemical Society (2014),Fe plays a crit., but not yet understood, role in enhancing the activity of the Ni-based O evolution reaction (OER) electrocatalysts. The authors report electrochem., in situ elec., photoelectron spectroscopy, and x-ray diffraction measurements on Ni1-xFex(OH)2/Ni1-xFexOOH thin films to study the changes in electronic properties, OER activity, and structure as a result of Fe inclusion. The authors developed a simple method for purifn. of KOH electrolyte that uses pptd. bulk Ni(OH)2 to absorb Fe impurities. Cyclic voltammetry on rigorously Fe-free Ni(OH)2/NiOOH reveals new Ni redox features and no significant OER current until >400 mV overpotential, different from previous reports which were likely affected by Fe impurities. The authors show through controlled crystn. that β-NiOOH is less active for OER than the disordered γ-NiOOH starting material and that previous reports of increased activity for β-NiOOH are due to incorporation of Fe-impurities during the crystn. process. Through-film in situ cond. measurements show a >30-fold increase in film cond. with Fe addn., but this change in cond. is not sufficient to explain the obsd. changes in activity. Measurements of activity as a function of film thickness on Au and glassy C substrates are consistent with the hypothesis that Fe exerts a partial-charge-transfer activation effect on Ni, similar to that obsd. for noble-metal electrode surfaces. These results have significant implications for the design and study of Ni1-xFexOOH OER electrocatalysts, which are the fastest measured OER catalysts under basic conditions. - 34.Saadi
, F. H.; Carim, A. I.; Verlage, E.; Hemminger, J. C.; Lewis, N. S.; Soriaga, M. P.CoP as an Acid-Stable Active Electrocatalyst for the Hydrogen-Evolution Reaction: Electrochemical Synthesis, Interfacial Characterization and Performance Evaluation J. Phys. Chem. C 2014, 118, 29294– 29300, DOI: 10.1021/jp5054452[ACS Full Text], [CAS]34.https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC2cXhs1CitLbI&md5=f43a90164a7a3a77065454c5ecdd32f4Saadi, Fadl H.; Carim, Azhar I.; Verlage, Erik; Hemminger, John C.; Lewis, Nathan S.; Soriaga, Manuel P.Journal of Physical Chemistry C (2014),Films of CoP have been electrochem. synthesized, characterized, and evaluated for performance as a catalyst for the hydrogen-evolution reaction (HER). The film was synthesized by cathodic deposition from a boric acid soln. of Co2+ and H2PO2- on copper substrates followed by operando remediation of exogenous contaminants. The films were characterized structurally and compositionally by scanning-electron microscopy, energy-dispersive X-ray spectroscopy, XPS, and Raman spectrophotometry. The catalytic activity was evaluated by cyclic voltammetry and chronopotentiometry. Surface characterization prior to electrocatalysis indicated that the film consisted of micrometer-sized spherical clusters located randomly and loosely on a slightly roughened surface. The compn. of both the clusters and surface consisted of cobalt in the metallic, phosphide, and amorphous-oxide forms (CoO·Co2O3) and of phosphorus as phosphide and orthophosphate. The orthophosphate species, produced by air-oxidn., were eliminated upon HER electrocatalysis in sulfuric acid. The operando film purifn. yielded a functional electrocatalyst with a Co:P stoichiometric ratio of 1:1. After the HER, the surface was densely packed with micrometer-sized, mesa-like particles whose tops were flat and smooth. The CoP eletrodeposit exhibited an 85 mV overvoltage (η) for the HER at a c.d. of 10 mA cm-2 and was stable under operation in highly acidic soln., with an increase in η of 18 mV after 24 h of continuous operation. The comparative HER catalytic performance of CoP, film or nanoparticles, is as follows: ηPt < ηCoP film = ηCoP NP, ηNi2P < ηCoSe2 < ηMoS2 < ηMoSe2. - 36.Kumari
, S.; Turner White, R.; Kumar, B.; Spurgeon, J. M.Solar hydrogen production from seawater vapor electrolysis Energy Environ. Sci. 2016, 9, 1725– 1733, DOI: 10.1039/C5EE03568F[CrossRef], [CAS]36.https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC28XktFGktr4%253D&md5=c21a5f06fc323f6bcc2b69a65b702f8eKumari, Sudesh; Turner White, R.; Kumar, Bijandra; Spurgeon, Joshua M.Energy & Environmental Science (2016),Solar photovoltaic utilities require large land areas to produce power equiv. to a conventional fossil fuel utility, and also must be coupled to cost-effective energy storage to overcome the intermittency of sunlight and provide reliable, continuous energy generation. To target both of these disadvantages, a method was demonstrated to produce hydrogen fuel from solar energy by splitting seawater vapor from ambient humidity at near-surface ocean conditions. Using a proton exchange membrane electrolyzer with seawater-humidified air at 80% relative humidity at the anode and dry N2 at the cathode, sufficient electrolysis c.d. was maintained to operate near the max. energy-conversion point when driven by a triple-junction amorphous Si solar cell of equiv. area. Fluctuations in atm. water content were demonstrated to have minimal effect on the solar-to-hydrogen efficiency, remaining ∼6% with relative humidity ≥30%. Direct solar-driven H2 prodn. from seawater vapor was maintained at >4.5 mA cm-2 for >90 h. The calcd. solar-to-hydrogen conversion efficiency before and after 50 h of operation changed from 6.0% to 6.3% using an ambient seawater humidity feedstock as compared to a drop from 6.6% to 0.5% using liq. seawater feedstock. - 37.Spurgeon
, J. M.; Lewis, N. S.Proton exchange membrane electrolysis sustained by water vapor Energy Environ. Sci. 2011, 4, 2993– 2998, DOI: 10.1039/c1ee01203g[CrossRef], [CAS]37.https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3MXhtFSrs7bM&md5=399ddaebeaf2f5cfa0af303d6ccff3f0Spurgeon, Joshua M.; Lewis, Nathan S.Energy & Environmental Science (2011),The current-voltage characteristics of a proton exchange membrane (PEM) electrolyzer constructed with an IrRuOx water oxidn. catalyst and a Pt black water redn. catalyst, under operation with water vapor from a humidified carrier gas, have been investigated as a function of the gas flow rate, the relative humidity, and the presence of oxygen. The performance of the system with water vapor was also compared to the performance when the device was immersed in liq. water. With a humidified Ar(g) input stream at 20 °C, an electrolysis c.d. of 10 mA cm-2 was sustained at an applied voltage of ∼1.6 V, with a c.d. of 20 mA cm-2 obsd. at ∼1.7 V. In the system evaluated, at current densities >40 mA cm-2 the electrolysis of water vapor was limited by the mass flux of water to the PEM. At <40 mA cm-2, the electrolysis of water vapor supported a given c.d. at a lower applied bias than did the electrolysis of liq. water. The relative humidity of the input carrier gas strongly affected the current-voltage behavior, with lower electrolysis c.d. attributed to dehydration of the PEM at reduced humidity values. The results provide a proof-of-concept that, with sufficiently active catalysts, an efficient solar photoelectrolyzer could be operated only with water vapor as the feedstock, even at the low operating temps. that may result in the absence of active heating. This approach therefore offers a route to avoid the light attenuation and mass transport limitations that are assocd. with bubble formation in these systems. - 38.Chen
, Y.; Hu, S.; Xiang, C.; Lewis, N. S.A sensitivity analysis to assess the relative importance of improvements in electrocatalysts, light absorbers, and system geometry on the efficiency of solar-fuels generators Energy Environ. Sci. 2015, 8, 876– 886, DOI: 10.1039/C4EE02314E - 39.Surendranath
, Y.; Dincǎ, M.; Nocera, D. G.Electrolyte-Dependent Electrosynthesis and Activity of Cobalt-Based Water Oxidation Catalysts J. Am. Chem. Soc. 2009, 131, 2615– 2620, DOI: 10.1021/ja807769r[ACS Full Text], [CAS]39.https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD1MXhtlehtLg%253D&md5=bc3f00dcac98249d354fa70bedf6a8b6Surendranath, Yogesh; Dinca, Mircea; Nocera, Daniel G.Journal of the American Chemical Society (2009),Electrolysis of Co2+ in phosphate, methylphosphonate, and borate electrolytes effects the electrodeposition of an amorphous highly active H2O oxidn. catalyst as a thin film on an inert anode. Electrodeposition of a catalytically competent species immediately follows oxidn. of Co2+ to Co3+ in soln. Methylphosphonate and borate electrolytes support catalyst activity comparable to that obsd. for phosphate. Catalytic activity for O2 generation in aq. solns. contg. 0.5M NaCl is retained for catalysts grown from phosphate electrolyte. - 40.Kanan
, M. W.; Nocera, D. G.In Situ Formation of an Oxygen-Evolving Catalyst in Neutral Water Containing Phosphate and Co2+ Science 2008, 321, 1072– 1075, DOI: 10.1126/science.1162018[CrossRef], [PubMed], [CAS]40.https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BD1cXhtVSitrbP&md5=dbeea0b08710b7d24acf4046a56424d8Kanan, Matthew W.; Nocera, Daniel G.Science (Washington, DC, United States) (2008),The utilization of solar energy on a large scale requires its storage. In natural photosynthesis, energy from sunlight is used to rearrange the bonds of water to oxygen and hydrogen equiv. The realization of artificial systems that perform "water splitting" requires catalysts that produce oxygen from water without the need for excessive driving potentials. Here we report such a catalyst that forms upon the oxidative polarization of an inert indium tin oxide electrode in phosphate-buffered water contg. cobalt (II) ions. A variety of anal. techniques indicates the presence of phosphate in an approx. 1:2 ratio with cobalt in this material. The pH dependence of the catalytic activity also implicates the hydrogen phosphate ion as the proton acceptor in the oxygen-producing reaction. This catalyst not only forms in situ from earth-abundant materials but also operates in neutral water under ambient conditions.
