Anion Effect on CO₂ Electrochemical Reduction in Acidic Media

The CO₂ electroreduction reaction (CO₂RR) 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 CO₂RR, the role of anions remains underexplored. Here, we systematically investigate the effect of three common electrolyte anions—Cl⁻, SO₄²⁻, and H₂PO₄⁻—on acidic CO₂RR. Theoretical studies suggest that Cl⁻ induces the most negative surface charge on Cu relative to SO₄²⁻ and H₂PO₄⁻, which enhances C–C coupling and thereby improves selectivity toward C₂₊ products. Electrochemical evaluation shows that Cu catalysts in Cl⁻-containing electrolytes deliver the highest C₂₊ selectivity, surpassing those in SO₄²⁻ and H₂PO₄⁻ environments. Specifically, we achieve a peak C₂₊ Faradaic efficiency of 82% at 300 mA cm⁻². The promotional role of Cl⁻ is further observed on Ag catalysts in acidic CO₂RR. In situ Raman spectroscopy reveals that Cl⁻ enhances the adsorption of *CO intermediates on Cu surfaces, thereby promoting C–C coupling and C₂₊ product generation. This work reports an effective anion electrolyte engineering strategy to enhance CO₂RR performance under acidic conditions. © 2026 Wiley-VCH GmbH.