Nonlinear Field-Reorganization Coupling Dictates Cation-Steered Proton Transfer at Electrified Interfaces

Electrolyte cations in the electric double layer strongly influence electrocatalytic CO2 reduction to multicarbon products, yet the microscopic origin of this cation effect remains elusive. Using metadynamics-biased ab initio molecular dynamics simulations, we identify a cation-driven electrostatic traction mechanism, where the cation-induced local electric field promotes the interfacial water activation and directly steers nascent protons toward adsorbed *CO2 via an Eley–Rideal path, effectively bypassing the Volmer H adsorption intermediate as the dominant channel under these interfacial conditions. At the electrified interface, strong electronic coupling in this inner-sphere proton-coupled electron transfer process renormalizes the classical solvent reorganization energy to effective quantum reorganization energy, flattening the reaction potential energy surface. We further establish an adiabatic Marcus–Stark framework by nonlinearly coupling the interfacial electric field with the effective reorganization energy, which quantitatively accounts for a substantial part of the experimentally observed exponential acceleration of C2 product formation with K+ over Li+ and identifies interfacial proton supply as an important rate-modulating factor within the broader C2 reaction network. This finding partially decouples reaction kinetics from intermediate binding energy scaling relations, providing a physical framework for manipulating electrochemical selectivity through interfacial microenvironment engineering.