On the Kinetics and Mechanism of O 2 Reduction at Oxide Film Covered Pt Electrodes: I . Effect of Oxide Film Thickness on Kinetics

Oxygen reduction reaction is examined at oxide‐covered Pt electrodes in acid solutions. In the high current density region characterized by the Tafel slope of −120 mV, the reaction is affected by the thickness of the oxide film. As the film thickness increases, current at the same potential decreases. In the low current density region characterized by the Tafel slope of −60 mV, the rate of reduction is unaffected by film thickness. In both current density regions, the potential at a given current decreases 60 mV as p H increases one unit. This leads to the fractional reaction order of with respect to in the high current density region. In the low current density region, the order is 1. Possible mechanisms compatible with the observed kinetic parameters are discussed.