Insights Into the Oxygen Reduction Reaction Activity of Pt/C and PtCu/C Catalysts

A mechanistic electrochemical study of the oxygen reduction reaction (ORR) on a carbon-supported PtCu catalyst is presented. The catalyst was prepared by galvanic displacement of nanoporous copper with Pt. The electrochemistry of the catalyst was explored at pH 1 and pH 13. Hydrogen peroxide reduction and rotating ring–disk electrode (RRDE) studies showed that the PtCu/C catalyst facilitates a 4e– direct or series reduction to water in alkaline electrolyte. A Tafel study suggests that the ORR rate-limiting step for PtCu/C does not change when the catalyst is subject to pH extremes. The ORR activity of the PtCu/C catalyst was found to be 2–3 times higher than the ORR activity of commercially available Pt/C (Johnson Matthey). Adsorption of OH– was quantified for PtCu/C and Pt/C, and PtCu/C was observed to have a reduced affinity toward OHads in both acid and alkaline electrolyte, which was found to promote the rate of the ORR relative to Pt/C. On the basis of this study, we propose that (1) each Pt-based catalyst has a unique rate of change of OHads coverage which is correlated to its ORR activity and (2) all Pt-based catalysts have the same rate of change of OHads coverage during irreversible oxide formation.