Interfacial Cation Arrangement Governs Electrocatalytic Kinetics in CO2 Reduction

The identity of electrolyte cations is known to strongly influence electrocatalytic activity, but the relationship between their interfacial arrangement and catalytic performance remains poorly understood. Organic cations, with their molecular tunability, provide a powerful platform for systematically probing these effects. Here, we leverage phosphonium-based geminal di-cations to control interfacial cation arrangement and identify the variables that most strongly influence catalytic rates. As a case study, we examine CO2 reduction (CO2R) to CO over polycrystalline silver electrodes in dry aprotic acetonitrile. Through a combination of rotating disk electrode measurements, electrochemical impedance spectroscopy, and molecular dynamics simulations, we decouple the effects of cation–electrode distance and interfacial cation density on catalytic rates. We find that smaller, more densely packed cations induce stronger interfacial electric fields, which lower the activation barrier for CO2 adsorption and increase reaction rates. Using geminal phosphonium di-cations, we demonstrate that both the vertical and lateral positioning of organic cations within the electrical double layer independently influence reactivity. These results demonstrate that electrolyte cation identity primarily influences electrocatalytic kinetics by determining how efficiently charge can be arranged at the electrocatalytic interface. These findings support an electrostatic view of cation effects in catalysis and provide principles for designing next-generation electrolytes.