Potential-Driven Dual Effects of Cations Regulate CO2 Reduction: The Interplay between Kinetic Promotion and Steric Effects

Abstract Electrode–electrolyte interfacial microenvironments (EEIMs) critically govern the performance of electrochemical CO2 reduction (ECR). However, the potential-driven dynamic distribution of interfacial cations and their reshaping effects on the interfacial water structure and proton-coupled electron transfer kinetics remain inadequately understood, thereby hindering the highly selective conversion of CO2. Herein, we unravel the dynamic evolution of the Ag/H2O interface at the atomic scale and establish a spatially dependent dual-role mechanism for cations. Specifically, interfacial water and alkaline cations act as highly potential-sensitive dynamic participants. Specifically adsorbed cations, in synergy with the highly ordered H-down interfacial water structure, stabilize the polar charged intermediate *CO2δ– and lower the energy barrier for its subsequent hydrogenation. Concurrently, moderately enriched cations in the diffuse layer construct a physical gap zone that hinders proton transfer, thereby suppressing the competitive hydrogen evolution reaction (HER). However, at high overpotentials, the densely packed cation layer induces a severe steric effect that drastically elevates the CO2 migration barrier. This, coupled with the accelerated HER caused by the shrinkage of the gap zone, results in a volcano-shaped trend for the Faradaic efficiency of the target product. This study highlights the fundamental significance of elucidating the evolution of the EEIM in exploring electrocatalytic mechanisms, thereby providing rational design principles for tailoring the microenvironment to achieve high-performance ECR.