(664g) Selective Electrochemical Reduction of NO3- and CO2 to Urea on Silver Gas Diffusion Electrode

Electrochemical synthesis of urea from the concentrated stream of waste NO₃^- and CO₂ from flue gas is attractive, as renewable energy can be used to make green urea. Electrochemical co-reduction of NO₃^- and CO₂ can give a variety of products such as NH₃, NO₂^-, H₂, CO₂RR products (CO, C₂H₄, HCOOH, etc.), formamide, methylamine, and urea. Electrochemical C-N coupling with selective synthesis of urea is challenging and a thorough catalyst screening to selectively produce urea is required. The above-mentioned areas are not explored well in the literature. In this work, we screened a wide range of metallic catalyst materials for urea synthesis. The catalysts that reduce CO₂ to CO and HCOOH are active for urea synthesis. Ag is the best catalyst for selective synthesis of urea with near 100 % selectivity. The system can be operated dynamically over a potential range of -0.5 to -1.5 V vs. RHE without compromising the selectivity of urea. The concentration of NO₃^- and CO₂ play a major role in determining the selectivity of urea. Ag GDE is used to overcome the mass transfer limitations and we report a high urea current density of ~-100 mA/cm² with a urea Faradaic Efficiency of ~100 % at -1.2 V vs. RHE which is the highest so far reported in the literature. DFT calculations are performed to get insights on the electrochemical C-N bond formation. A combination of facile first and second C-N bond formation steps and an endergonic formamide formation step from *CONH₂ results in improved kinetics and selectivity towards urea synthesis.