Aprotic phosphonium‐based ionic liquid as electrolyte for high CO2 electroreduction to oxalate

Abstract In this study, a new CO2 electroreduction electrolyte system consisting of tetrabutylphosphonium 4‐(methoxycarbonyl) phenol ([P4444][4‐MF‐PhO]) ionic liquid (IL) and acetonitrile (AcN) was designed to produce oxalate, and the electroreduction mechanism was studied. The results show that using the new IL‐based electrolyte, the electroreduction system exhibits 93.8% Faradaic efficiency and 12.6 mA cm−2 partial current density of oxalate at −2.6 V. The formation rate of oxalate is 234.4 μmol cm−2 h−1, which is better than those reported in the literature. The mechanism study using density functional theory (DFT) calculations reveals that [P4444][4‐MF‐PhO] can effectively activate CO2 molecule through ester and phenoxy double active sites. In addition, in the phosphonium‐based ionic environment, the potential barriers of the key intermediates *CO2− and *C2O42− are reduced by the induced electric field, which greatly facilitates the activation and conversion of CO2 molecule to oxalate.