Overpotentials and Faraday Efficiencies in CO 2 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 CO 2 to CO are reported. Linear sweep voltammetry shows overpotentials for CO 2 reduction and the competing hydrogen evolution reaction (HER), both of which vary as a function of [EMIM]TFO concentration in the range from 4 × 10 −3 m (0.006 mol%) to 4869 × 10 −3 m (50 mol%). A steady lowering of overpotentials up to an optimum for 334 × 10− 3 m is identified. At 20 mol% and more of [EMIM]TFO, a significant CO 2 reduction plateau and inhibition of HER, which is limited by H 2 O diffusion, is noted. Such a plateau in CO 2 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 × 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 1 H NMR spectroscopic measurements.