Electron transfer through thin anodic films in oxygen evolution at Pt electrodes in alkaline solutions

Abstract

Oxygen evolution reaction at Pt electrodes in alkaline solutions is characterized by two linear E-log i relationships. At low current densities (cd), the Tafel slope is close to 60 mV, whereas at high cd, it is close to 120 mV. The rates at high cd are strongly affected by the presence of an anodic oxide film over electrodes. They decrease exponentially with the film thickness. In contrast, the rates at low cd are unaffected by the film thickness. The reaction order with respect to OH − is 3/2 at high, and 2 at low cd. The mechanism of the reaction involves electron tunneling through the oxide film. The first electron transfer step is rate determining (rd) at high cd. The major factor affecting the rates is the probability for successful transfer of electrons through the potential energy barrier. The dependence of the rates on electrode potential is, however, controlled principally by the distribution of the occupied electron energy levels in solution. At low cd, a chemical step following the first electron transfer step is rd. The observed pH dependences cannot be accounted for in terms of any customary approach to electrode kinetics. They are accounted for in terms of the quasi-equilibrium that exists across the outer Helmholtz layer.