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 OH_ads 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 OH_ads coverage which is correlated to its ORR activity and (2) all Pt-based catalysts have the same rate of change of OH_ads coverage during irreversible oxide formation. (Graph Presented).