Electrolyte and Solvent Effects on CO Poisoning Process During CO2 Electrochemical Reduction on Silver Electrodes

• Ag electrodes undergo CO poisoning during CO₂ electroreduction in dry acetonitrile. • The presence of water effectively suppresses CO-induced poisoning of Ag during CO₂RR. • BF₄⁻ anions inhibit CO poisoning of Ag electrodes even under anhydrous acetonitrile conditions. • Specific adsorption of BMIM⁺ cations on Ag surfaces significantly enhances CO₂RR kinetics. In this study, the influence of the supporting electrolyte and water content on CO poisoning during the CO₂ reduction reaction (CO₂RR) on silver electrodes was systematically investigated in acetonitrile-based electrolytes. Sodium perchlorate (NaClO₄), tetrabutylammonium hexafluorophosphate ([TBA][PF₆]), 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF₄]), and 1-butyl-3-methylimidazolium trifluoromethanesulfonate ([BMIM][OTf]) were evaluated over a range of acetonitrile–water compositions. The results show that the nature of the electrolyte, the water content of the solvent, and the presence of CO₂ strongly influence both the kinetics and thermodynamics of the underpotential formation of silver oxides. Under strictly anhydrous conditions, the Ag electrode undergoes rapid and irreversible CO poisoning within only a few potential cycles, occurring in the potential region where CO₂RR is thermodynamically accessible. At very low water content, strong and irreversible CO adsorption on Ag produces a well-defined Gaussian peak in the voltammograms. The peak potential is strongly dependent on the supporting electrolyte, appearing at −1.40, −1.52, −1.65, and −1.75 V in [BMIM][BF₄], NaClO₄, [TBA][PF₆], and [BMIM][OTf], respectively. In contrast, the presence of water effectively suppresses CO adsorption, preventing surface deactivation and enabling CO₂RR to proceed with significantly enhanced kinetics. Notably, the BF₄⁻ anion inhibits CO-induced electrode poisoning even in dry acetonitrile. Furthermore, the results indicate that BMIM⁺ cations undergo strong specific adsorption on the Ag surface, and this interfacial interaction plays a key role in promoting CO₂RR. In the presence of BMIM⁺, the onset potential of the faradaic admittance occurs at approximately −1.4 V for X H 2 O = 0.12 , whereas in the presence of Na⁺ and TBA⁺ the onset is observed near −1.6 V vs. Ag/AgCl.