Anion Effect on CO ₂ Electrochemical Reduction in Acidic Media

The CO 2 electroreduction reaction (CO 2 RR) in acidic media offers a promising route to convert renewable electricity into value‐added chemicals with high carbon utilization, yet its selectivity is severely limited by competing hydrogen evolution reaction. Despite extensive investigations into cation effects in acidic CO 2 RR, the role of anions remains underexplored. Here, we systematically investigate the effect of three common electrolyte anions—Cl − , SO 4 2− , and H 2 PO 4 − —on acidic CO 2 RR. Theoretical studies suggest that Cl − induces the most negative surface charge on Cu relative to SO 4 2− and H 2 PO 4 − , which enhances C–C coupling and thereby improves selectivity toward C 2+ products. Electrochemical evaluation shows that Cu catalysts in Cl − ‐containing electrolytes deliver the highest C 2+ selectivity, surpassing those in SO 4 2− and H 2 PO 4 − environments. Specifically, we achieve a peak C 2+ Faradaic efficiency of 82% at 300 mA cm −2 . The promotional role of Cl − is further observed on Ag catalysts in acidic CO 2 RR. In situ Raman spectroscopy reveals that Cl − enhances the adsorption of *CO intermediates on Cu surfaces, thereby promoting C–C coupling and C 2+ product generation. This work reports an effective anion electrolyte engineering strategy to enhance CO 2 RR performance under acidic conditions.