Effect of Alkyl Chain Length of Imidazolium Cation on the Electroreduction of CO2 to CO on Ag Electrode in Acetonitrile

The effect of imidazolium-based ionic liquid on the electroreduction of CO2 to CO over a Ag electrode in acetonitrile catholyte was investigated. The voltage–current profiles clearly indicate that the electroreduction of CO2 is sensitive to the alkyl chain length at the N1-position in imidazolium cation (MIM+). Density functional theory computation suggests that the onset potential of CO2 reduction is related to the association degree between MIM+ and CO2•– species. More importantly, preparative scale electrolysis shows that the selectivity and output rate for the target product CO are also significantly affected by MIM+. With the elongation of the alkyl group in MIM+ from ethyl to octyl, the Faradaic efficiency for CO remarkably increases from 87 ± 4 % to 97 ± 2 % and then remains almost unchanged. However, the curve of the current density with respect to the chain length of alkyl group shows a convex style. These results indicate the dependence of CO2 reduction efficiency on the MIM+ adsorbed on the Ag electrode surface.