Beyond and Inside the Electrical Double Layer: Molecular-Level Cation Effects in Aqueous Electrocatalysis.

Electrocatalytic reactions occur at a dynamic, ion-regulated electrochemical interface, rather than at a static electrode boundary. In electrocatalytic processes such as hydrogen evolution/oxidation and CO2 reduction, which operate at potentials below the electrode's potential of zero charge, electrolyte cations are not mere spectators but active participants that shape reaction behavior. Their size, charge density, solvation structure, and spatial organization within the electric double layer (EDL) govern interfacial energetics, transition states, and reaction activity and selectivity, giving rise to pronounced, system-dependent cation effects. Furthermore, cation accumulation dynamically modifies the interfacial electric field, reorganizes water structure, and alters local pH, thereby coupling electrostatics, solvation, and mass transport in a highly nonlinear manner. These intertwined effects complicate mechanistic interpretation and have contributed to ongoing debates and seemingly contradictory conclusions across experimental and theoretical studies. Here, we synthesize the current understanding of cation effects at electrochemical interfaces, highlight emerging interfacial molecular structures revealed by in situ and operando studies that challenge classical EDL models, identify key unresolved debates, and outline pathways toward a quantitative, molecular-level description of cation-interface-reactivity relationships.