Stability Testing and X-ray
Characterization of Novel Cathode Electrocatalysts
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Deborah Myers demonstrates the fuel cell fixture created to allow in
situ X-ray study of transition metals. |
Researchers at Argonne are engaged in a multi-lab project, led by Los Alamos
National Laboratory, on improved, durable, low-cost cathode electrocatalysts for
polymer electrolyte fuel cells (PEFCs). This multi-lab project focuses on
developing non-platinum (or ultra-low Pt loading) PEFC cathode electrocatalysts
using three approaches and classes of materials:
- Nonprecious metal-heteroatomic polymer nanocomposites (e.g.,
Co-polypyrrole-carbon, at Los Alamos National Laboratory),
- Core-shell base metal-precious metal nanoparticles (e.g., Pt monolayers
on palladium-iron alloy (Pd3Fe) nanoparticles, at Brookhaven National
Laboratory), and
- Nanoparticle chalcogenides (e.g., ruthenium-selenium skin on iron-group
metal or zirconia nanoparticles, at the University of Illinois at
Urbana-Champaign).
One of Argonne's roles in this collaborative project is to study--at the
atomic level--the properties of these three classes of catalysts by using ex
situ and in-situ X-ray absorption spectroscopies at Argonne's Advanced
Photon Source. The properties studied include particle size, oxidation state,
identity of the reactive site, and local environment of the reactive site.
The oxidation state of the catalyst is important in determining oxygen
reduction activity; it determines the availability of catalyst sites for
adsorption and dissociation of the oxygen molecule. Characterization of these
properties, especially while the catalyst is in the reactive environment, allows
a correlation of these properties with activity, identification of the active
catalyst site, and an understanding of degradation mechanisms. Identification of
the active site, for example, would enable study of synthetic routes that
maximize site density, leading to higher catalyst specific activities.
Ultimately, the characterization findings will lead to ways to improve catalytic
activity and the durability of the cathode electrocatalyst.
A cell fixture was designed and fabricated to allow the in situ X-ray study
of low-atomic-number transition metals, such as cobalt and iron, at low
concentrations in the fuel cell environment. Thus far, the project has focused
on in situ characterization of the cobalt and cobalt-iron catalysts developed by
Los Alamos National Laboratory. Argonne researchers have found that the
catalyst's atomic structure changes with cathode conditions, such as the water
content of the oxygen stream and the fuel cell voltage. The atomic structure is
altered from a cobalt oxide-like structure at high voltages or high water
content to a structure containing hydroxy and water linkages between cobalt
atoms at low voltages and low water content.
This work is funded by the Hydrogen, Fuel Cells and Infrastructure
Technologies Program of the DOE Office of Energy Efficiency and Renewable
Energy.
Non-Platinum Bimetallic Cathode Electrocatalysts for Polymer Electrolyte
Fuel Cells
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Xiaoping Wang measures the stability of a platinum cathode
electrocatalyst. |
One of the major barriers to the commercialization of polymer electrolyte
fuel cell (PEFC) power systems, especially for the automotive application, is
cost. The high cost is largely due to the use of platinum as the fuel cell's
electrocatalysts. In the past decade, there have been significant advances in
reducing the platinum (Pt) loading of both the anode and the cathode through the
development of highly dispersed Pt nanoparticles on a carbon support, thin-film
Pt electrodes, and Pt alloy nanoparticles supported on carbon. However, further
significant cost reductions will require replacing platinum, especially in the
cathode layer, with less expensive but active and stable electrocatalytic
materials.
Argonne investigators are at work on an alternative to Pt-containing
electrocatalysts: bimetallic base metal-noble metal nanoparticles. In these
nanoparticles, the bulk of each particle is an inexpensive base metal or alloy,
the surface of which is coated by a noble metal. The base metals are chosen for
their ability to modify the electronic properties of the noble metal and thus
the bonding characteristics of the noble metal with the cathode reactant,
oxygen. The noble metal protects the base metals against dissolution caused by
the acidic environment of the fuel cell.
Thus far, Argonne investigators have studied copper, iron, cobalt, and nickel
base metals alloyed with the noble metal palladium. Oxygen reduction activities
that are 75% of those of commercial platinum catalysts (per gram of
platinum-group metal) have been achieved; these results could lead to a 60%
reduction in the cost of the cathode electrocatalyst. Argonne scientists are
currently determining the durability of these materials in an operating PEFC.
This work is funded by the Hydrogen, Fuel Cells and Infrastructure
Technologies Program of the DOE Office of Energy Efficiency and Renewable
Energy.
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