Nickel-Catalyzed Urea Electrolysis: From Nitrite and Cyanate as Major Products to Nitrogen Evolution

The electrochemical urea oxidation reaction (UOR) to N₂ represents an efficient route to simultaneous nitrogen removal from N-enriched waste and production of renewable fuels at the cathode. However, the overoxidation of urea to NO_x ^- usually dominates over its oxidation to N₂ at Ni(OH)₂ -based anodes. Furthermore, detailed reaction mechanisms of UOR remain unclear, hindering the rational catalyst design. We found that UOR to NO_x ^- on Ni(OH)₂ is accompanied by the formation of near stoichiometric amount of cyanate (NCO^- ), which enabled the elucidation of UOR mechanisms. Based on our experimental and computational findings, we show that the formation of NO_x ^- and N₂ follows two distinct vacancy-dependent pathways. We also demonstrate that the reaction selectivity can be steered towards N₂ formation by altering the composition of the catalyst, e.g., doping the catalyst with copper (Ni_0.8 Cu_0.2 (OH)₂ ) increases the faradaic efficiency of N₂ from 30 % to 55 %.