Oxygen reduction limited by surface diffusion at a rotating ring-disk electrode

Abstract

The oxygen reduction reaction (orr) limited by surface diffusion is studied with an analytical model. The model considers the electrosorption of dissolved oxygen followed by surface diffusion of the adsorbate towards a catalyst centre for reaction. It includes the possibility of partial reduction to hydrogen peroxide. The model is solved for a catalyst dispersion embedded in the disk of a rotating ring-disk electrode (rrde). Simplified Eqs. are provided for the disk and ring currents, as well as Tafel slopes and Koutecký-Levich relations, as a function of surface diffusivity, catalyst dispersion, and reaction parameters. The disk current may show a peak at moderate overpotentials due to a maximum in surface diffusivity. Disk and ring currents are calculated for various orr mechanisms differing in the surface diffusion intermediate. The mechanism limited by OH* surface diffusion appears most favourable, however it requires partial charge transfer and O-O bond breaking reactions on the supporting surface. The model shows that orr electrocatalysis assisted by the surface transport of intermediates improves the performance of air cathodes of low temperature fuel cells and metal-air batteries.