Anions-mediated interfacial-water regulation boosts C2+ selectivity for CO2 electroreduction over Cu-based catalysts

The electrochemical CO₂ reduction reaction (CO₂RR) offers a promising strategy to produce high-value-added C₂₊ products. Cu-based catalysts enable C–C coupling but suffer from low C₂₊ selectivity. Although electronic/structural engineering and cation effects have been widely studied, the role of anions in tuning interfacial water structure and CO₂RR pathways remains insufficiently understood. Herein, a series of low-concentration anions were introduced into 1 M KOH to regulate the interfacial water environment over reconstructed CuO. By combining in-situ attenuated total reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS) and electrochemical impedance spectroscopy (EIS), it is revealed that low-concentration anions in 1 M KOH increase the proportion of strong interfacial-water structure in the order of F⁻ > Cl⁻ > Br⁻ > NO₃⁻. Moreover, the moderate strong interfacial-water structure can accelerate ∗H transfer and promote ∗CO hydrogenation to ∗COH, thus favoring asymmetric C–C coupling to produce C₂₊ products. However, excessively stable interfacial water structure in F⁻-containing electrolyte induces enhanced ∗H coverage and dominant H₂ evolution. A volcano-type relationship is thus observed between C₂₊ selectivity and the proportion of strongly hydrogen-bonded water. This work emphasizes anion-mediated interfacial water regulation as an effective strategy to enhance C₂₊ selectivity for CO₂RR.