Deciphering and Suppressing the Over-oxidized Nitrogen in Nickel-catalyzed Urea Electrolysis.

Urea electrolysis is a prospective technology for simultaneous H2 production and nitrogen abatement toward water-energy nexus. Its sustainability is currently founded on the innocuous N2 products; however, herein, we discovered the prevalent nickel-based catalysts could generally over-oxidize urea into NO2- products with ~80% Faradaic efficiencies, posing potential secondary hazard to the environment. Trace amounts of over-oxidized NO3- and N2O were also detected. By 15N isotopes and urea analogues, we derived a nitrogen fate network, involving a NO2- formation pathway via OH--assisted C-N cleavage and two N2 formation pathways of intra- and inter-molecular coupling. Density functional theory (DFT) calculation confirmed that C-N cleavage is energetically more favorable. Inspired by the mechanism, a polyaniline coating strategy was developed to locally enrich urea for encouraging N2 production by 2-folds. These findings provide complementary insights into the nitrogen fate in water-energy nexus systems.