Kinetics of oxygen evolution and dissolution on platinum electrodes

Abstract

Different pre-treatment of Pt electrodes, provided they are well anodized, does not affect the mechanism of the oxygen-electrode reaction. Due to the low exchange current density for the reaction, measurements at low (< 10 −7 A/cm 2) current densities are possible only if the solution is highly purified. Reproducible cathodic V/ln i curves can be obtained by fast measurements on pre-anodized electrodes. The instability with time of V/ln i curves at higher cathodic η is due to gradual reduction of the surface oxides. The reaction mechanism in acid solution includes the primary water discharge as the rate-controlling step. In alkaline solution, at low anodic η, a rate-controlling chemical step follows a fast OH − discharge. The same step is rate-controlling in O 2 dissolution. At high anodic η, the mechanism changes and, for the same path, the primary discharge of OH − becomes the rate-controlling step. Phase oxides cover the electrode surface during O 2 evolution. In dissolution, oxides already present reduce (at potentials < V (he)). Total oxygen coverage (and hence the average thickness of the oxides) increases linearly with applied potential both in acid and in alkaline solution. The same type of oxides forms in acids and in alkaline solution.