The Electrochemical Kinetics of the Evolution of Isotopically Enriched Gases and the Mechanism of the Oxygen Evolution Reaction

Abstract

This work describes a method which follows, by mass spectrometry, the kinetics of gas evolution from an isotopically enriched electrode surface. The mechanisms of both 18O16O and 4D ₂ evolution were investigated by this technique. 34O ₂ evolution was studied on platinum, NiCo₂O₄ and lithiated CO₃O₄ electrodes in alkaline electrolyte, and additionally on platinum electrodes in acid electrolyte. The kinetics were found to be consistent with a mechanism of oxygen evolution which involved successive oxidations on a single site or reaction via a bridge structure. It was found, in all cases, that the total number of sites responsible for oxygen evolution were less than the total number of sites at the surface. Additionally, on NiCo₂O₄ electrodes the number of sites which showed enrichment decreased with aging of the electrode, in agreement with evidence from its cyclic voltammogram. Tafel slopes and cyclic voltammograms are reported for each electrode. On each substrate the amount of enrichment detected was less than would have been expected from the degree of enrichment of the electrolyte. This is most readily explained on consideration of the proposed mechanism of oxygen evolution, where the first step involving oxide formation is rate determining on platinum and NiCo₂O₄ (high overpotential). The intermediate oxide species formed being rapidly removed as oxygen was evolved. On materials in which the metal was initially in a trivalent state, a mechanism involving a bridge structure between this trivalent site and a second site is postulated; its formation being rate determining at low overpotential on NiCo₂O₄.

This method was extended to follow the mechanism of hydrogen evolution on both platinum and platinised hydrogen tungsten bronzeelectrodes. The kinetics of the removal of an adsorbed deuteron were consistent with a second order recombination and found to be fast (k = 0.02 +/- 0.02 s-1 at 25°C on platinum). Experiments on platinised hydrogen tungsten bronze electrodes confirmed the existence of a synergistic effect in this system.