Electrochemical Reduction of Nitrates to Ammonia on Oxide Derived Cobalt

NH₃ is manufactured in an industrial scale by Haber-Bosch process which is highly energy intensive as it requires high temperatures (500 to 600 °C) and high pressures (150 to 200 atm), and it leaves a massive carbon footprint. Electrochemical route is an alternate process to synthesize NH₃ by using renewable electricity at ambient conditions. Direct electrochemical reduction of dinitrogen would be ideal, but the NH₃ Faradaic efficiency and yield is low in this case due to high mass transfer resistances and the competing hydrogen evolution reaction (HER). Electrochemical reduction of NO₃^- to NH₃ is less challenging when compared to direct N₂ reduction to NH₃ due to lower dissociation energy of N = O bond (204 kJ/mol) than N ≡ N (941 kJ/mol). A catalyst that can selectively reduce nitrates to ammonia is required. DFT calculations were performed on late transition metals to identify the most active metal for the electrochemical nitrate reduction to NH₃. Ni and Co were found to be most active among the late transition metals. Experimental investigations corroborated with the theoretical findings and Co was found to give the highest NH₃ Faradaic efficiency. Oxide-derived Cobalt was used for the electrochemical nitrate reduction as it had low onset potential compared to other forms of Co. We obtained a maximum NH₃ Faradaic efficiency of 68.13 % at -0.6 V vs RHE (pH = 14), and a high NH₃ current density of 245.34 mA/cm² at -0.8 V vs RHE (pH = 14). The oxide derived Co was found to be stable during the test period of 24 h. NMR Control studies were performed using N-15 nitrates to confirm that the source of NH₃ is nitrates.