Synergistic Effects of the Electric Field Induced by Imidazolium Rotation and Hydrogen Bonding in Electrocatalysis of CO2

The roles of the ionic liquid (IL), 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM][BF₄]), and water in controlling the mechanism, energetics, and electrocatalytic activity of CO₂ reduction to CO on silver in nonaqueous electrolytes were investigated. The first electron transfer occurs to CO₂ at reduced overpotentials when it is trapped between the planes of the [EMIM]⁺ ring and the electrode surface due to cation reorientation as determined from voltammetry, in situ surface-enhanced Raman spectroscopy, and density functional theory calculations. Within this interface, water up to 0.5 M does not induce significant Faradaic activity, opposing the notion of it being a free proton source. Instead, water acts as a hydrogen bond donor, and the proton is sourced from [EMIM]⁺. Furthermore, this study demonstrates that alcohols with varying acidities tune the hydrogen bonding network in the interfacial microenvironment to lower the energetics required for CO₂ reduction. The hydrogen bonding suppresses the formation of inactive carboxylate species, thus preserving the catalytic activity of [EMIM]⁺. The ability to tune the hydrogen bonding network opens new avenues for advancing IL-mediated electrocatalytic reactions in nonaqueous electrolytes.