Aromatic Ester‐Functionalized Ionic Liquid for Highly Efficient CO 2 Electrochemical Reduction to Oxalic Acid

Electrochemical reduction of CO 2 into valuable chemicals is a significant route to utilize CO 2 resources. Among various electroreduction products, oxalic acid (H 2 C 2 O 4 ) is an important chemical for pharmaceuticals, rare earth extraction, and metal processing. Here, an aprotic aromatic ester‐functionalized ionic liquid (IL), 4‐(methoxycarbonyl) phenol tetraethylammonium ([TEA][4‐MF‐PhO]), was designed as an electrolyte for CO 2 electroreduction into oxalic acid. It exhibited a large oxalic acid partial current density of 9.03 mA cm −2 with a faradaic efficiency (FE) of 86 % at −2.6 V (vs. Ag/Ag + ), and the oxalic acid formation rate was as high as 168.4 μmol cm −2  h −1 , which is the highest reported value to date. Moreover, the results of density functional theory calculations demonstrated that CO 2 was efficiently activated to a −COOH intermediate by bis‐active sites of the aromatic ester anion via the formation of a [4‐MF‐PhO‐COOH] − adduct, which finally dimerized into oxalic acid.