Revisiting Electrolyte Effects in Oxygen Evolution Reaction on NiOOH: pH‐Controlled Activity Depends on Catalyst Mesostructure

The role of alkali cations and pH in governing the oxygen evolution reaction (OER) activity, catalyst structure, and intercalation behavior of nickel oxyhydroxide (NiOOH) remains debated. Here, we decouple pH and cation concentration effects by independently varying hydroxide and alkali metal concentrations and correlating electrochemical activity with simultaneous electrochemical quartz crystal microbalance (eQCM) mass measurements. We find that OER activity increases with alkalinity and depends on cation identity, while remaining independent of bulk cation concentration. Instead, increasing cation concentration solely improves the reversibility of the Ni(OH) 2 /NiOOH redox couple by enhancing ion transport in the catalyst mesopores. The apparent pH dependence is strongest at low concentrations and in thick, heterogeneous films where long diffusion pathways create transport limitations. In contrast, thin and homogeneous NiOOH layers exhibit a largely potential‐independent and strongly diminished pH effect. These results demonstrate that the widely reported yet inconsistent pH effect is highly sensitive to morphology‐related transport and conductivity limitations. Overall, pH governs cation intercalation and activity, while catalyst (meso‐) structure determines the extent to which transport and internal pH gradient effects manifest.