(509y) Electrochemical Co-Reduction of CO2 and Nitrates into Urea on Cu-Co Bimetallic Gas Diffusion Electrodes (GDE)

Urea is one of the major fertilizers and it is produced in an industrial scale by Bosch-Meiser process using CO₂ and NH₃ at a high temperature of 160 °C and a high pressure of 110 atm. NH₃ which is a feedstock for this process is manufactured by Haber-Bosch process which requires a high temperature of 400 to 500 °C and a high pressure of 150 to 200 atm. This overall process to manufacture Urea leaves a larger Carbon footprint as H₂, the feedstock to produce NH₃ comes from steam reforming process. In this study, we synthesize urea by electrochemical co-reduction of nitrates and CO₂ at ambient conditions. Electrochemical water treatment generally involves conversion of nitrates into N₂. Reducing nitrates into more useful products such as NH₃ and urea are preferred. Gas Diffusion Electrodes (GDE) are used as they reduce the mass transfer resistances associated with the CO₂ solubility in aqueous medium. Cu is a well-established catalyst for electrochemical CO₂ reduction, and it has been thoroughly studied in the literature. Co was found to active for electrochemical reduction of nitrates to NH₃ and we achieved a high NH₃ Faradaic efficiency of 65 % and a high NH₃ current density of 250 mA/cm² at -0.8 V vs RHE. In this study we used Cu-Co bimetallic catalyst which is sputter coated on a porous carbon electrode that acts as a GDE to electrochemically co-reduce CO₂ and nitrates into urea. We achieved a high urea Faradaic efficiency of 42 % at -0.5 V vs RHE. The catalyst remained stable during the study period of 24 h. The effect of operating parameters such as pH, flow rate, cations, and the concentration of electrolyte were studied and optimized. DFT studies were performed to identify the reaction mechanism for urea formation on Cu-Co bimetallic catalyst.