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

Electrochemical CO₂ methanation is crucial for sustainable carbon recycling, transforming a greenhouse gas into a valuable fuel. However, achieving high selectivity toward CH₄, particularly at high current densities, remains a significant challenge. In this study, a remarkable Faradaic efficiency for CH₄ (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 CO₂ electroreduction to CH₄. By employing other Cu-based catalysts, high CH₄ 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 CH₄.