Synergistic Effect of Mixed Cations in the Electrochemical Double Layer Enables Highly Efficient Electrochemical CO 2 Methanation

Abstract Electrochemical CO 2 methanation is crucial for sustainable carbon recycling, transforming a greenhouse gas into a valuable fuel. However, achieving high selectivity toward CH 4 , particularly at high current densities, remains a significant challenge. In this study, a remarkable Faradaic efficiency for CH 4 (FE CH4 ) of 88.7% with a current density of 461.0 mA cm −2 was achieved at −1.3 V vs. RHE over CuCeSiO x electrode in a ternary electrolyte of 1 M tetrapropylammonium hydroxide (TPAOH) + 5 mM KCl, setting a new record for CO 2 electroreduction to CH 4 . By employing other Cu‐based catalysts, high CH 4 selectivity can also be obtained in this ternary electrolyte. Mechanistic studies reveal the synergistic effect of mixed K + and TPA + in the electrochemical double layer plays a critical role in controlling the reaction pathway. Specifically, K + modulates *CO adsorption and promotes water dissociation, generating abundant H • radicals, while TPA + aggregates stabilize these radicals, accelerating the reaction kinetics, and facilitating the deep hydrogenation from *CO to CH 4 .