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 core-shell 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.  Tafel data 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 to 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 towards OH_ads in both acid and alkaline electrolyte, which was found to promote the rate of the ORR relative to Pt/C.  Based on this study, we propose that:  1) Pt-based catalyst has a unique rate of change of OH_ads coverage which is correlated to their ORR activity and 2) All Pt-based catalysts have the same rate of change of OH_ads coverage during irreversible oxide formation.