Probing Anion Effect on Hydrogen Evolution and C–C Coupling on Copper in Acidic CO ₂ Reduction

Electrochemical CO2 reduction reaction (CO2RR) in acid offers a promising solution to overcome the limitation of carbon loss that happens in neutral and alkaline electrolytes. However, the CO2RR in strong acid suffers from serious competition with the hydrogen evolution reaction (HER) and the high kinetic barrier of C–C coupling. While the effect of cations on acidic CO2RR has been extensively studied, the influence of anions remains poorly understood. Herein, we systematically explored the impact of typical inorganic anions (SO42–, Cl–, Br–, and I–) on the CO2RR and HER behaviors on a Cu2O catalyst in acidic environments. Electrochemical results show that among the four anions, Cl– plays the most significant contribution in inhibiting HER and promoting C–C coupling in pH = 2 electrolytes. We revealed that anions do not affect the morphology, structure, composition, and surface hydrophobicity of the Cu2O-derived Cu catalyst. Combining in situ Raman spectroscopy and constant-potential DFT calculations underscores that Cl– near the catalyst-electrolyte interface significantly enhances *CO adsorption and coverage on the Cu (111) surface while weakening *H adsorption compared to other anions, thereby lowering the kinetic barrier of *CO–COH coupling and suppressing HER. This work establishes a fundamental platform toward the selection of suitable anions in achieving efficient CO2-to-C2+ transformation in strong acidic electrolytes.