Continuous-flow electrosynthesis of urea and oxalic acid by CO2-nitrate reduction and glycerol oxidation

Abstract

Urea and oxalic acid are critical component in various chemical manufacturing industries. However, achieving simultaneous generation of urea and oxalic acid in a continuous-flow electrolyzer is a challenge. Herein, we report a continuous-flow electrolyzer equipped with 9-square centimeter-effective area gas diffusion electrodes (GDE) which can simultaneously catalyze the glycerol oxidation reaction in the anode region and the reduction reaction of CO₂ and nitrate in the cathode region, producing oxalic acid and urea at both the anode and cathode, respectively. The current density at low cell voltage (0.9 V) remained above 18.7 mA cm^–2 for 10 consecutive electrolysis cycles (120 h in total), and the Faraday efficiency of oxalic acid (67.1%) and urea (70.9%) did not decay. Experimental and theoretical studies show that in terms of the formation of C–N bond at the cathode, Pd-sites can provide protons for the hydrogenation process of CO₂ and NO₃^–, Cu-sites can promote the generation of *COOH and Bi-sites can stabilize *COOH. In addition, in terms of glycerol oxidation, the introduction of Cu and Bi into Pd metallene promotes the oxidation of hydroxyl groups and the cleavage of C–C bond in glycerol molecules, respectively.