Electrocatalysis of aqueous dioxygen reduction

Abstract

In the reduction of dioxygen to water, proton transfer must be involved. Therefore, dioxygen reduction may be regarded as a modified type of proton transfer reaction. To understand it, the underlying phenomenon of proton transfer must first be understood. This requires an evaluation of the kinetic models for the proton transfer reaction. After deciding which model applies best, the effects of adsorption on proton transfer in the hydrogen evolution reaction (HER) are compared with those which have been observed experimentally in dioxygen reduction. Both reactions in acid media are characterized by volcano relationships. However, the volcano in the HER results from reversible absorption effects and that in dioxygen reduction results from irreversible absorption. The effect of this is to reduce the pre-exponential value in the rate expression from a figure close to the theoretical values predicted from the model to a value several orders of magnitude lower. The models of the separate oxygen electrode steps are shown to give excellent agreement with experiment in both acid and alkaline solution over a wide range of materials. The consequences for practical dioxygen reduction catalysts are discussed.