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| Schematic diagram of the synthesis of PANI-M-C catalysts. (A) Mixing of high–surface area carbon with aniline oligomers and transition-metal precursor (M: Fe and/or Co). (B) Oxidative polymerization of aniline by addition of APS. (C) First heat treatment in N2 atmosphere. (D) Acid leaching. The second heat treatment after acid leach is not shown. Source: Wu et al. Click to enlarge. |
In a paper published in Science, Los Alamos researchers Gang Wu, Christina Johnston, and Piotr Zelenay, joined by researcher Karren More of Oak Ridge National Laboratory, describe a family of non–precious metal catalysts that approach the performance of platinum-based systems at a cost sustainable for high-power fuel cell applications, possibly including automotive power.
Their approach uses polyaniline (PANI) as a precursor to a carbon-nitrogen template for high-temperature synthesis of catalysts incorporating iron and cobalt. The most active materials in the PANI-M-C (M: Fe and/or Co) catalyst group catalyze the cathodic oxygen reduction reaction (ORR) at potentials within ~60 millivolts of that delivered by state-of-the-art carbon-supported platinum, combining their high activity with remarkable performance stability for non–precious metal catalysts (700 hours at a fuel cell voltage of 0.4 volts).
| H2-O2 fuel cell polarization plots recorded with various PANI-derived cathode catalysts at a loading of ~4 mg cm-2: 1, PANI-C; 2, PANI-Co-C; 3, PANI-FeCo-C(1); 4, PANI-FeCo-C(2) (SDs from three independent measurements marked for all data points); 5, PANI-Fe-C. Performance of an H2-air fuel cell with a Pt cathode (0.2 mgPt cm-2) is shown for comparison (dashed line). Source: Wu et al. Click to enlarge. |
The prohibitive cost of platinum for catalyzing the ORR has hampered the widespread use of polymer electrolyte fuel cells. Eliminating platinum would solve a significant economic challenge that has hampered large-scale commercialization of hydrogen fuel cell systems.
The carbon-iron-cobalt catalyst fuel cells also effectively completed the conversion of hydrogen and oxygen into water, rather than producing large amounts of undesirable hydrogen peroxide (hydrogen peroxide yield <1.0%). Inefficient conversion of the fuels, which generates hydrogen peroxide, can reduce power output by up to 50%, and also has the potential to destroy fuel cell membranes.
The encouraging point is that we have found a catalyst with a good durability and life cycle relative to platinum-based catalysts. For all intents and purposes, this is a zero-cost catalyst in comparison to platinum, so it directly addresses one of the main barriers to hydrogen fuel cells.
—Piotr Zelenay, corresponding author for the paper
The next step in the team’s research will be to better understand the mechanism underlying the carbon-iron-cobalt catalyst. Micrographic images of portions of the catalyst by researcher More have provided some insight into how it functions, but further work must be done to confirm theories by the research team. Such an understanding could lead to improvements in non-precious-metal catalysts, further increasing their efficiency and lifespan.
Project funding for the Los Alamos research came from the US Department of Energy’s (DOE) Energy Efficiency and Renewable Energy (EERE) Office as well as from Los Alamos National Laboratory’s Laboratory-Directed Research and Development program. Microscopy research was done at Oak Ridge National Laboratory’s SHaRE user facility with support from the DOE’s Office of Basic Energy Sciences.
Resources
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Gang Wu, Karren L. More, Christina M. Johnston, and Piotr Zelenay (2011) High-Performance Electrocatalysts for Oxygen Reduction Derived from Polyaniline, Iron, and Cobalt. Science Vol. 332 no. 6028 pp. 443-447 doi: 10.1126/science.1200832