Platinum-iron phosphate electrocatalysts for oxygen reduction in PEMFCs

Proton exchange membrane fuel cells (PEMFCs) depend on platinum at the cathode to catalyze the oxygen reduction reaction (ORR) and maintain high performance. This report shows that the electrocatalytic activity of Pt is enhanced when it is dispersed in a matrix of hydrous iron phosphate (FePO). The Pt-FePO has 2 nm micropores with Pt dispersed as ions in Pt2+ and Pt4+ oxidation states. Increased ORR performance is demonstrated for the Pt-FePO + Vulcan carbon (VC) materials compared to a standard 20 wt % Pt-VC catalyst on rotating disk electrodes with Pt-loadings of 0.1 mg(Pt) cm(-2). The improvement in the ORR is attributed to the adsorption/storage of oxygen on the FePO, presumably as iron-hydroperoxides. The ORR activity of the Pt-FePO in air is close to that in oxygen at low current density, and therefore this catalyst has a distinctly unique behavior from Pt-VC. Contrary to Pt-VC, the Pt-FePO catalyst shows activity towards hydrogen and CO oxidation, but does not exhibit their characteristic adsorption peaks, suggesting that Pt ions in the iron phosphate structure are less sensitive to poisoning than metallic Pt. The results present opportunities for new low-Pt catalysts that extend beyond the current capabilities of Pt-VC. (C) 2004 The Electrochemical Society.