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

Abstract

Electrochemical reduction of CO₂ into valuable chemicals is a significant route to utilize CO₂ resources. Among various electroreduction products, oxalic acid (H₂ C₂ O₄ ) 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 CO₂ 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 CO₂ 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.

Keywords: CO2 reduction; electrocatalyst; electrolysis; ionic liquid; oxalic acid.