A Non-Precious Electrocatalyst for Oxygen Reduction Based on Simple Heat-Treated Precursors

Although polymer electrolyte fuel cells (PEFCs) have long been recognized as a potential power source for zero-emission vehicles, they remain economically uncompetitive with internal combustion engines. As a result of a high and continuously increasing price of platinum in recent years ($2175/oz on May 26, 20082), up to 80 % of the projected cost1 of the automotive fuel cell stack is likely to originate from the use of platinum-based catalysts. The oxygen reduction reaction (ORR) at the cathode occurs at a lower rate than hydrogen oxidation at the anode, thus requiring a much higher catalyst loading. Reduction or elimination of Pt at the cathode is therefore necessary for achieving any significant cost reduction. To address the catalyst cost issue, the potential use of non-precious materials in the PEFC cathode has attracted much attention in recent years. Organic macrocycles, such as phthalocyanines or porphyrins, bound with transition metals, have been used extensively, both with and without heat treatment.3 Metal chalcogenides (e.g., RuSe, CoS) have also shown promising results,3 but stable PEFC performance of chalcogenide catalysts, non-precious in particular, is yet to be demonstrated. The effect of the surface chemistry of the carbon support itself has also been studied. For example, carbon oxidation through acid-treatment4 and nitrogen doping via heat treatment5 have been reported to increase the activity for oxygen reduction reaction (ORR). Respectable performance has been observed for many catalysts in aqueous electrolytes, but stability in fuel cells at elevated temperatures (e.g., 80°C) remains the critical issue. Bashyam et al. reported stable performance of a Co-polypyrrole-carbon composite, notably without any heat treatment.6 The activity, however, was below that of the initial performance observed for pyrolyzed porphyrins. Thus, the focus of the current effort is the development of non-precious ORR catalysts that would be both more active than the composite and sufficiently stable. Potential vs. RHE (V) 0.0 0.2 0.4 0.6 0.8 1.