Effect of atomic ordering on the catalytic activity of carbon supported PtM (M = Fe, Co, Ni, and Cu) alloys for oxygen reduction in PEMFCs

Carbon-supported PtM (M = Fe, Co, Ni and Cu) catalysts have been synthesized by precipitating the Mn+ ions as hydroxides in presence of a commercial carbon-supported Pt catalyst followed by heat treatment in a 90% Ar-10% H-2 mixture at 900 degrees C for 1 h. X-ray diffraction indicates the PtFe and PtCo alloys thus obtained to have an ordered structure while the PtNi and PtCu alloys have a disordered FCC structure. Structural analysis of PtFe and PtCo after annealing at various temperatures 500 degrees C <= T <= 900 degrees C for 1 h indicates the ordering to be maximized for an annealing temperature of around 600 degrees C. Evaluation of the PtM alloy catalysts for oxygen reduction in proton exchange membrane fuel cells (PEMFCs) indicates that the alloys with the ordered structures have higher catalytic activity with lower polarization losses than Pt and the disordered PtM alloys. With the ordered phases, the activity increases with the extent of ordering. The enhanced catalytic activity is explained based on an optimal structural and electronic features consisting of optimum number of Pt and M nearest neighbors, d-electron density in Pt, atomic configuration on the surface, and Pt-Pt distance. (c) 2005 The Electrochemical Society. All rights reserved.