The Structure and Activity of Pt‐Co Alloys as Oxygen Reduction Electrocatalysts

Carbon‐supported Pt‐Co (3:1 atom ratio) catalysts were prepared in both acid and alkaline aqueous media, followed by heat‐treatments to promote alloy formation. Both preparations began with a commercial 10% Pt‐on‐carbon catalyst with Pt particle sizes of 15–30Å. Significantly greater alloying was observed in the catalyst prepared in the acid medium. X‐ray diffraction studies of the acid‐prepared catalyst demonstrated lattice parameters tending away from Pt (3.927Å) and toward that for (3.831Å), greatly increased particle sizes, and significant ordering evidenced by the presence of superlattice reflections. In all cases, catalysts prepared in the alkaline medium were alloyed to a lesser extent, were of moderately increased particle size, and gave no indication of alloy ordering. Activity testing under phosphoric acid fuel cell conditions demonstrated that the most highly alloyed catalysts were not significantly more active than pure Pt catalyst of comparable crystallite size. Loss of cobalt in the phosphoric acid environment was the lowest in catalysts which were the most alloyed, and where the ordered phase was present.