Overpotentials and Faraday Efficiencies in CO2 Electrocatalysis-the Impact of 1-Ethyl-3-Methylimidazolium Trifluoromethanesulfonate

The mixtures of room temperature ionic liquid 1-ethyl-3-methylimidazolium trifluoromethanesulfonate ([EMIM]TFO) and water as electrolytes for reduction of CO2 to CO are reported. Linear sweep voltammetry shows overpotentials for CO2 reduction and the competing hydrogen evolution reaction (HER), both of which vary as a function of [EMIM] TFO concentration in the range from 4 x 10(-3) M (0.006 mol%) to 4869 x 10(-3) M (50 mol%). A steady lowering of overpotentials up to an optimum for 334 x 10-3 M is identified. At 20 mol% and more of [EMIM] TFO, a significant CO2 reduction plateau and inhibition of HER, which is limited by H2O diffusion, is noted. Such a plateau in CO2 reduction correlates to high CO Faraday efficiencies. In case of 50 mol% [EMIM] TFO, a broad plateau spanning over a potential range of 0.58 V evolves. At the same time, the overpotential for HER is increased by 1.20 V when compared to 334 x 10(-3) M and, in turn, HER is largely inhibited. The Faraday efficiencies for CO and H-2 formation feature 95.6% +/- 6.8% and 0.5% +/- 0.3%, respectively, over a period of 3 h in a separator divided cell. Cathodic as well as anodic electrochemical stability of the electrolyte throughout this time period is corroborated in H-1 NMR spectroscopic measurements.