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. Alkali cations in electrolytes are commonly considered chemically inert species, but their role has recently been called into question. Now, using in situ spectroscopy and molecular dynamics simulations, it is shown that alkali cations couple with intermediates in the oxygen reduction reaction, acting as cocatalysts.