Electrocatalytic CO2 Reduction with a Homogeneous Catalyst in Ionic Liquid: High Catalytic Activity at Low Overpotential

We describe a new strategy for enhancing the efficiency of electrocatalytic CO2 reduction with a homogeneous catalyst, using a room-temperature ionic liquid as both the solvent and electrolyte. The electrochemical behavior of fac-ReCl­(2,2′-bipyridine)­(CO)3 in neat 1-ethyl-3-methylimidazolium tetracyanoborate ([emim]­[TCB]) was compared with that in acetonitrile containing 0.1 M [Bu4N]­[PF6]. Two separate one-electron reductions occur in acetonitrile (−1.74 and −2.11 V vs Fc+/0), with a modest catalytic current appearing at the second reduction wave under CO2. However, in [emim]­[TCB], a two-electron reduction wave appears at −1.66 V, resulting in a ∼0.45 V lower overpotential for catalytic reduction of CO2 to CO. Furthermore, the apparent CO2 reduction rate constant, k app, in [emim]­[TCB] exceeds that in acetonitrile by over one order of magnitude (k app = 4000 vs 100 M–1 s–1) at 25 ± 3 °C. Supported by time-resolved infrared measurements, a mechanism is proposed in which an interaction between [emim]+ and the two-electron reduced catalyst results in rapid dissociation of chloride and a decrease in the activation energy for CO2 reduction.