The Complex Inhibiting Role of Surface Oxide in the Oxygen Reduction Reaction

Advances in Pt-based oxygen reduction reaction (ORR) fuel cell catalysts will continue to lead to remarkably more efficient fuel cells. However, more efficient Pt-based cathode catalysts will be required to deliver higher currents at lower overpotential, which will result in them becoming oxidized over periods of prolonged use. In this manuscript, a comprehensive analysis of the ORR on Pt-oxide films of varying thickness is presented. Oxides were grown for various times with various oxidizing potentials in pH 1 and pH 13 media, and the ORR process was probed on these surfaces during a negative-going voltammetry sweep. The oxide layer was found to be stronger bound in alkaline media, but acidic conditions seem to favor growth of thicker oxides. Product selectivity is also altered by the oxide layer to favor undesirable peroxide formation. A novel analysis shows how chemisorbed oxide led to alterations in Tafel slope that could be mistaken for a changing rate-determining step. This study provides strong evidence that the total quantity of oxide deposited on the Pt electrode alters the mechanism of the ORR during oxide reduction. This paper presents an investigation of ORR reactivity on oxide-covered surfaces that will become very valuable as high activity ORR catalysts begin to become feasible.