Efficient Electrocatalytic Reduction of Carbon Dioxide in 1-Ethyl-3-methylimidazolium Trifluoromethanesulfonate and Water Mixtures

The catalytic activity of room-temperature ionic liquid 1-ethyl-3-methylimidazolium trifluoromethanesulfonate ([EMIM][OTf]) for carbon dioxide (CO2) reduction is reported on a polycrystalline silver electrode as a function of the water content and solution pH. The optimal aqueous dilution was determined to be ca. 10% (v/v), resulting in a reduction of the overpotential required to reduce CO2 in [EMIM][OTf] by 400 mV. The reduction process and products are largely insensitive to pH. CO2 reduction in [EMIM][OTf] with 10% 0.1 M NaHCO3 is achieved with high selectivity for CO with 93.0 +/- 4.6% Faradaic efficiency. Hydrogen evolution reaction (HER) is suppressed in the range of neat [EMIM][OTE] (500 ppm water content) to a [EMIM][OTf]/H2O mixture of 10% water content. Conductivity and viscosity of the [EMIM][OTf]/H2O mixture suggest that the ionic liquid ion pair fully dissociates, analogous to dilute KCl solution, where each ion is completely hydrated by water molecules.