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

Electrochemical CO 2 reduction reaction (CO 2 RR) in acid offers a promising solution to overcome the limitation of carbon loss that happens in neutral and alkaline electrolytes. However, the CO 2 RR 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 CO 2 RR has been extensively studied, the influence of anions remains poorly understood. Herein, we systematically explored the impact of typical inorganic anions (SO 4 2–, Cl –, Br –, and I – ) on the CO 2 RR and HER behaviors on a Cu 2 O 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 Cu 2 O-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 CO 2 -to-C 2+ transformation in strong acidic electrolytes.