Reduced anodic energy depletion in electrolysis by urea and water co-oxidization on NiFe-LDH : Activity origin and plasma functionalized strategy

As an alternative anodic reaction to water oxidization (oxygen evolution reaction, OER), the urea oxidization reaction (UOR) is promising due to its favorable thermodynamics. However, reliance of the proper Ni oxidization state and passivation at high potentials render UOR by Ni-based catalysts inefficient and impractical. Herein, the anodic characteristics of the conventional NiFe-layered double hydroxide (NiFe-LDH) catalyst are investigated in a urea-containing alkaline electrolyte. Fe³⁺ and derivative Ni^2+δ cations are identified as the UOR active sites before and after Ni oxidization and both UOR and OER are observed simultaneously at high potentials. The number of active sites increases at high potentials and internal atoms are activated as well. For further improvement, plasma processing is employed to produce layer reconstruction on the NiFe-LDH slabs in a controllable fashion, leading to left-shifting of the Ni oxidization threshold and enhanced UOR activity. The assembled urea-assisted hydrogen evolution device is demonstrated to have better energy-saving and passivation-avoiding characteristics.