Specific Adsorption of Alkaline Cations Enhances CO-CO Coupling in CO2 Electroreduction.

Electrolyte alkaline cations can significantly modulate the reaction selectivity of electrochemical CO₂ reduction (eCO₂R), enhancing the yield of the valuable multicarbon (C₂₊) chemical feedstocks. However, the mechanism underlying this cation effect on the C-C coupling remains unclear. Herein, by performing constant-potential AIMD simulations, we studied the dynamic behavior of interfacial K⁺ ions over Cu surfaces during C-C coupling and the origin of the cation effect. We showed that the specific adsorption of K⁺ readily occurs at the surface sites adjacent to the *CO intermediates on the Cu surfaces. Furthermore, this specific adsorption of K⁺ during *CO-*CO coupling is more important than quasi-specific adsorption for enhancing coupling kinetics, reducing the coupling barriers by approximately 0.20 eV. Electronic structure analysis revealed that charge redistribution occurs between the specifically adsorbed K⁺, *CO, and Cu sites, and this can account for the reduced barriers. In addition, we identified excellent *CO-*CO coupling selectivity on Cu(100) with K⁺ ions. Experimental results show that suppressing surface K⁺-specific adsorption using the surfactant cetyltrimethylammonium bromide (CTAB) significantly decreases the Faradaic efficiency for C₂ products from 41.1% to 4.3%, consistent with our computational findings. This study provides crucial insights for improving the selectivity toward C₂₊ products by rationally tuning interfacial cation adsorption during eCO₂R. Specifically, C-C coupling can be enhanced by promoting K⁺-specific adsorption, for example, by confining K⁺ within a coated layer or using pulsed negative potentials.