Alkali metal cations act as homogeneous cocatalysts for the oxygen reduction reaction in aqueous electrolytes

Abstract

Alkali metal cations (AM+) exhibit high solubility and ionic conductivity, making them optimal components in aqueous electrolytes. Despite the conventional belief that AM+ are chemically inert spectators, the strong dependence of electrocatalysis on AM+ has recently provoked debates about their unforeseen catalytic role. However, conclusive evidence is still lacking. Here we demonstrate that AM+ can couple with reaction intermediates and determine kinetics as homogeneous cocatalysts in aqueous conditions, for the alkaline oxygen reduction reaction on a carbon catalyst. In situ X-ray absorption spectroscopy reveals a change in the electronic structure of Na+ from its hydrated state on a charged electrode. In situ Raman spectroscopy further identifies that this change is due to the formation of water-unstable NaO2 as a key intermediate in OOH- production. Together with theoretical calculations, this finding enunciates the counterintuitive cocatalytic role of AM+ in aqueous environments, highlighting the exigency of refined interface design principles for better electrocatalysis.