A NEW SYNTHETIC ROUTE yields inexpensive iron-based catalysts for fuel cells that are nearly as active catalytically as the expensive platinum catalysts normally used in those electrochemical devices, scientists in Canada report in Science (2009, 324, 71). The study advances efforts to come up with low-cost substitutes for precious-metal catalysts and may help lower the high price of fuel cells for automotive applications.
A key obstacle to widely commercializing hydrogen-fueled electric automobiles is the cost of the fuel cells that convert hydrogen into electric power. Polymer-electrolyte-membrane (PEM) fuel cells, the type widely studied for powering cars, generally include carbon-supported platinum (Pt/C) catalysts to mediate reactions at the electrodes. For years, researchers have worked on lowering overall costs by replacing platinum with less expensive substitutes such as iron, which is generally considered a leading candidate. But until now, iron-based catalysts have remained too sluggish, especially for driving the oxygen-reduction reaction, which converts oxygen to water at the cathode.
Pt/C catalysts can mediate that reaction at a "turnover frequency" of roughly 25 reactions per active catalyst site per second. In contrast, iron-based catalysts have tended to exhibit turnover frequencies closer to 0.4 per second.
Researchers at the National Institute for Scientific Research, in Quebec, now describe a synthesis that yields iron-based catalysts for the cathode of PEM fuel cells that are comparable in activity, initially, with ones made from platinum.
Rather than using common wet-impregnation methods to load iron into carbon supports, Michel Lefèvre, Eric Proietti, Jean-Pol Dodelet, and coworkers used an intense dry-mixing method to react carbon, ferrous acetate, and phenanthroline, and then they subjected the product to heat treatments and subsequent reaction with ammonia. That procedure yields catalytically active iron cations coordinated to pyridinic groups within the micropores of the carbon support, the group proposes. The team notes that after extended use in a fuel cell, the catalyst's high initial activity decreases significantly.
In an accompanying commentary in Science, MIT's Hubert A. Gasteiger and enad M. Marković of Argonne National Laboratory remark that "despite remaining challenges, these recent successes bring us closer to completing our quest to put PEM fuel-cell technology on the road."