Water splitting has been regarded as a sustainable and environmentally-friendly technique to realize green hydrogen generation, while more energy is consumed due to the high overpotentials required for the anode oxygen evolution reaction. Urea electrooxidation, an ideal substitute, is thus received increasing attention in assisting water-splitting reactions. Note that highly efficient catalysts are still required to drive urea oxidation, and the facile generation of high valence state species is significant in the reaction based on the electrochemical-chemical mechanisms. The high cost and rareness make the noble metal catalysts impossible for further consideration in large-scale application. Ni-based catalysts are very promising due to their cheap price, facile structure tuning, good compatibility, and easy active phase formation. In the light of the significant advances made recently, herein, we reviewed the recent advances of Ni-based powder catalysts for urea oxidation in assisting water-splitting reaction. The fundamental of urea oxidation is firstly presented to clarify the mechanism of urea-assisted water splitting, and then the prevailing evaluation indicators are briefly expressed based on the electrochemical measurements. The catalyst design principle including synergistic effect, electronic effect, defect construction and surface reconstruction as well as the main fabrication approaches are presented and the advances of various Ni-based powder catalysts for urea assisted water splitting are summarized and discussed. The problems and challenges are also concluded for the Ni-based powder catalysts fabrication, the performance evaluation, and their application. Considering the key influencing factors for catalytic process and their application, attention should be given to structure−property relationship deciphering, novel Ni-based powder catalysts development and their construction in the real device; specifically, the effort should be directed to the Ni-based powder catalyst with multi-functions to simultaneously promote the fundamental steps and high anti-corrosion ability by revealing the local structure reconstruction as well as the integration in the practical application. We believe the current summarization will be instructive and helpful for the Ni-based powder catalysts development and understanding their catalytic action for urea-assisted hydrogen generation via water splitting technique. Advances and challenges of Ni based powder catalyst were reviewed for urea oxidation in assisting water-splitting reaction. 

A review of Ni based powder catalyst for urea oxidation in assisting water splitting reaction

In situ Raman spectroscopic study towards the growth and excellent HER catalysis of Ni/Ni(OH)2 heterostructure

Ultrathin Ti2NTx MXene-wrapped MOF-derived CoP frameworks towards hydrogen evolution and water oxidation

Honeycomb-like Ni-Mo-S on Ni foam as superior bifunctional electrocatalyst for hydrogen evolution and urea oxidation

Electrodeposition NiMoSe ternary nanoshperes on nickel foam as bifunctional electrocatalyst for urea electrolysis and hydrogen evolution reaction

Hairy sphere-like Ni9S8/CuS/Cu2O composites grown on nickel foam as bifunctional electrocatalysts for hydrogen evolution and urea electrooxidation

Improving the electrocatalytic activity of NiFe bimetal-organic framework toward oxygen evolution reaction by Zr doping

NiCo bimetal organic frames derived well-matched electrocatalyst pair for highly efficient overall urea solution electrolysis

Ni-doped VOOH as an efficient electrocatalyst for urea oxidation

Ammonia is important, both as a fertilizer and as a carrier of clean energy, mainly produced by the Haber-Bosch process, which consumes hydrogen and emits large amounts of carbon dioxide. The ENRR (Electronchemical Nitrogen Reduction Reaction) is considered a promising method for nitrogen fixation owing to their low energy consumption, green and mild. However, the ammonia yield and Faraday efficiency of the ENRR catalysts are low due to the competitive reaction between HER and NRR, the weak adsorption of N2 andthe strong N≡N triple bond. Oxygen vacancy engineering is the most important method to improve NRR performance, not only for fast electron transport but also for effective breaking of the N≡N bond by capturing metastable electrons in the antibonding orbitals of nitrogen molecules. In this review, the recent progress of OVs (oxygen vacancies) in ENRR has been summarized. First, the mechanism of NRR is briefly introduced, and then the generation methods of OVs and their applicationin NRR are discussed, including vacuum annealing, hydrothermal method, hydrogen reduction, wet chemical reduction, plasma treatment and heterogeneous ion doping. Finally, the development and challenges of OVs in the field of electrochemical nitrogen fixation are presented. This review shows the important areas of development of catalysts to achieve industrially viable NRR.

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Wujian Tang

Papers

Hairy sphere-like Ni9S8/CuS/Cu2O composites grown on nickel foam as bifunctional electrocatalysts for hydrogen evolution and urea electrooxidation

Improving the electrocatalytic activity of NiFe bimetal-organic framework toward oxygen evolution reaction by Zr doping

NiCo bimetal organic frames derived well-matched electrocatalyst pair for highly efficient overall urea solution electrolysis

Ni-doped VOOH as an efficient electrocatalyst for urea oxidation

Oxygen vacancies engineering in electrocatalysts nitrogen reduction reaction