Aromatic Ester-Functionalized Ionic Liquid for Highly Efficient CO2 Electrochemical Reduction to Oxalic Acid

Graphical CO 2 electroreduction : An aprotic aromatic ester-functionalized ionic liquid (IL), [TEA][4-MF-PhO], is designed for CO 2 electroreduction to oxalic acid. The multiple active sites exhibit excellent catalytic performance with high current density and faradaic efficiency by interacting with CO 2 . This study provides a promising prospect for using IL as electrolyte to develop efficient electrocatalysts. 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.