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.