Deciphering and Suppressing Over‐Oxidized Nitrogen in Nickel‐Catalyzed Urea Electrolysis

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Abstract
Urea electrolysis is a prospective technology for simultaneous H2 production and nitrogen suppression in the process of water being used for energy production. Its sustainability is currently founded on innocuous N2 products; however, we discovered that prevalent nickel‐based catalysts could generally over‐oxidize urea into NO2− products with ≈80 % Faradaic efficiencies, posing potential secondary hazards to the environment. Trace amounts of over‐oxidized NO3− and N2O were also detected. Using 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 via intra‐ and intermolecular coupling. DFT calculations confirmed that C−N cleavage is energetically more favorable. Inspired by the mechanism, a polyaniline‐coating strategy was developed to locally enrich urea for increasing N2 production by a factor of two. These findings provide complementary insights into the nitrogen fate in water–energy nexus systems.
Funding Information
  • National Key Research and Development Program of China (2019YFC1906700, 2019YFC1604602)
  • National Natural Science Foundation of China (22006039, 21876049, 21972023, 21773032, 22022301)
  • China Postdoctoral Science Foundation (2019M661412)

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