ACTIVITY AND STABILITY OF ORDERED AND DISORDERED CO-PT ALLOYS FOR PHOSPHORIC-ACID FUEL-CELLS

Co-Pt alloys were studied in detail by a corrosion test under phosphor ic acid fuel cell (PAFC) conditions on the well-defined crystallograph ic structures for a typical combination of the alloy catalysts used in PAFCs, examining the long-life stabilities of the structures and the catalytic activities for O2 electroreduction. The ordered (O) and diso rdered (D) alloys at the same particle sizes can be obtained by heat-t reating the mother alloy in different temperature sequences. The O-all oy exhibits a specific activity, an electrocatalytic activity based on the catalyst surface area, 1.35 times higher than the D-alloy before the corrosion test, but shows less activity (0.73 times) after the cor rosion test, due to a higher degradation (47%) in the O-alloy activity as compared with that of the D-alloy (1%). It was found that the Co a toms on particle surfaces of both alloys dissolve easily in the acid. This is followed by a second slow dissolution from inside the alloy pa rticles probably due to the protective action by a monolayer thickness of Pt remaining on the alloy surfaces, but the loss of Co in the seco nd stage dissolution for the O-alloy is higher by several percentage p oints compared to that of the D-alloy. It was also found that the Pt c ontent does not change on the catalyst support even after 50 h of corr osion test, but the pure Pt phase is formed in the corrosion product, where the phase for the O-alloy grows faster than that for the D-alloy with corrosion time. Based on these results, obtained by chemical, x- ray diffraction, and transmission electron microscopy with energy disp ersive spectroscopy analyses, the corrosion for Pt alloy catalysts is clearly explained, i.e., after the dissolution of Co atoms in the firs t surface layer of alloys, both Co and Pt dissolve out simultaneously from small-size alloy particles and the Pt redeposits on the surfaces of large-size alloy particles (Ostwald ripening). It is concluded that the D-alloy is preferable to the O-alloy from the viewpoint of the st abilities in the structure and the electrocatalytic activity, differin g from previous claims.