Invariance with pH of enthalpies of activation for O 2 reduction at Pt electrodes in acid solutions

Abstract

The apparent enthalpies of activation, Δ H * a, for O 2 reduction at Pt electrodes are determined in acid solutions of different pH. In the high current density region, characterized by a Tafel slope close to − 120 mV, Δ H * a,h is lower than Δ H * a,l in the low current density region characterized by a Tafel slope close to − 60 mV. Although Δ H * a,l > Δ H * a,h it is concluded that at every pH the enthalpies of activation at the zero Galvani potential difference are the same in both current density regions. Therefore, irrespective of the different Tafel slopes, the same mechanism is operative in both regions. In all solutions, Δ H * a for either current density region are independent of pH when they are determined at constant potentials vs rhe. This invariance with pH is unexpected in view of the suggested variance of the Galvani potential difference with pH. Alternative causes for the invariance are discussed.