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 CO ₂. 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 CO ₂ reduction.