Unveiling the Cation–Anion Synergistic Effects of a Hydrophobic Ionic Liquid on Copper-Catalyzed CO ₂ Electroreduction

Ionic liquids have been proven to be promising molecules for adjusting the selectivity of copper catalysts in CO2 electroreduction, but the combined effects of cations and anions are still ambiguous. Herein, we performed in situ attenuated total reflection surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS), complemented with density functional theory (DFT) calculations and molecular dynamics (MD) simulations, to investigate the mechanism of electroreduction of CO2 to C2+ on a Cu catalyst modified by a typical hydrophobic ionic liquid, [BMIm]PF6. The ATR-SEIRAS results indicates that the key step in the conversion of CO2 to C2+ products is *CO + *COH → *OCCOH. The DFT calculations reveal [BMIm]+ can stabilize *CO and reduce the energy barrier for *OCCOH generation. MD simulations prove that [BMIm]+ can strengthen the adsorption of CO2, while PF6– can suppress water accumulation on the copper surface, thereby inhibiting hydrogen generation from water splitting. Due to these synergistic effects of [BMIm]+ and PF6–, the Faradaic efficiency of C2+ products increased from 58.2% to 74.4% at 500 mA·cm–2 Cu(0). This study clarifies the different roles of cations and anions in hydrophobic ionic liquids and their synergistic effects during CO2 reduction, which may inspire the design of more functional ionic liquids to overcome the limits of C2+ selectivity on copper-based catalysts.