Carbon Dioxide Reduction in Room-Temperature Ionic Liquids: The Effect of the Choice of Electrode Material, Cation, and Anion

The electroreduction of carbon dioxide is systematically investigated in Room Temperature Ionic Liquids (RTILs) to establish the effect of the electrode material, the cation, and the anion on the mechanism of reduction. Silver, gold, platinum and glassy carbon macroelectrodes are investigated using cyclic voltammetry in 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (BmimNTf2). On silver electrodes, a diffusion-limited reductive peak current is observed, whilst for gold, the peak current density is six times smaller. No appreciable reduction is observable on either platinum or glassy carbon. Moreover, on silver the overpotential required for reduction is reduced by ca. 1.5 V compared with gold. Next a cation effect and an anion effect are demonstrated on a silver electrode, with the use of the following RTILs: 1-butyl-1-methylpyrrolodinium bis(trifluoromethylsulfonyl)-imide (BmpyrrNTf2), 1-ethyl-3-methylimidazolium bis-(trifluoromethylsulfonyl)imide (EmimNTf2), 1-propyl-3-me-thylimidazolium bis(trifluoromethylsulfonyl)imide (Pmim-NTf2), and 1-butyl-3-met-hylimidazolium tris(pentafluoroethyl)trifluorophosphate (BmimFAP), and 1-butyl-3-methylimidazolium tetrafluoroborate (BmimBF4). The study of the anion and cation effect is enabled by the use of IL-based Ag/Ag+ reference electrodes, with the solvent in each reference electrode composed of the RTIL used in the primary solution. To allow direct comparison of CO2 reduction potentials between different solvents, the cobaltocenium/cobaltocene couple is used as a standard, allowing corrected relative potentials to be reported. The peak current on silver was observed in BmimNTf2 at -1.1 V vs cobaltocenium/cobaltocene, in BmpyrrNTf2 at -1.20 V, and in EmimNTf2 and PmimNTf2 at -1.50 V. Of the anions, the peak current observed in BmimBF4 occurs at a comparable potential to BmimNTf2 (-1.10 V vs cobaltocenium/cobaltocene), whilst the reduction of CO2 in BmimFAP was observed at -1.60 V. This work provides experimental evidence that silver facilitates the inner sphere reduction of carbon dioxide, and a possible mechanism is proposed.