Vanadium Modified Nickel/cobalt Nanocomposite Catalysts For Urea Oxidation Assisted Hydrogen Production

Water electrolysis is one of the most hopeful methods to product hydrogen at present.It consists of hydrogen evolution reaction（HER）at cathode and oxygen evolution reaction（OER）at anode,but OER kinetics is slow with high theoretical potential,which affects the overall water electrolysis efficiency.In recent years,urea oxidation-assisted hydrogen production has attracted great attention.The theoretical potential is only 0.37 V for urea oxidation reaction（UOR）,which can replace OER（1.23 V）to increase the energy conversion efficiency.Urea oxidation-assisted hydrogen production can not only improve the efficiency of hydrogen production,but also degrade urea to alleviate the pollution from urea-rich wastewater.Designing efficient and stable UOR catalysts can further improve the efficiency of urea oxidation-assisted hydrogen production.This thesis mainly focuses on developing vanadium modified nickel/cobalt catalysts by adopting strategies such as heterointerface construction,carbon-encapsulated structure,alloying and heteroatom doping.The major contents and basic conclusions are listed as follows:（1）Carbon-encapsulated Co₃V alloy modified Co₂VO₄ nanosheet heterostructure catalyst is synthesized by combining hydrothermal and calcination method（Co₃V@C/Co₂VO₄/NF）.The heterointerface between Co₃V and Co₂VO₄ can provide abundant active sites to induce electron redistribution,thus improving catalytic activity.Carbon-encapsulated structure protects the catalyst from corrosion in strong alkaline solution,thus enhancing stability.It demonstrates good UOR and HER activity,which the potential required at a current density of±10 m A cm^-2 is-51 m V and 1.31 V,respectively.When Co₃V@C/Co₂VO₄/NF is used as both cathode and anode for urea oxidation assisted hydrogen production device,only 1.37 V is required to drive 10 m A cm^-2,which is apparently less than the potential of overall water splitting（1.56V）,demonstrating the advantage of UOR instead of OER for assisting hydrogen production.Meanwhile,it can maintain 100 h at 500 m A cm^-2,which shows excellent stability.（2）Ni modified Co₂VO₄ mesoporous nanosheet heterostructure catalyst is prepared by combining the hydrothermal and calcination methods（Ni-Co₂VO₄/NF）.It only needs-50 m V for HER and 1.28 V for UOR at±10m A cm^-2.For overall urea oxidation,only 1.36 V is needed to reach 10 m A cm^-2.Meanwhile,it still maintains good activity after stability test at 500 m A cm^-2 for 140 h.The good performance can be owed to the coupling interface between Ni and Co₂VO₄,which is beneficial to induces charge transfer to promote the formation of electron poor/rich structures,thus boosting the absorption of urea and water;and the mesoporous nanosheet morphology endows Ni-Co₂VO₄ a high specific surface area to expose abundant active sites,which promoting the close contact with reactants and accelerating gas-liquid transmission and bubble desorption.（3）N-doped carbon-encapsulated V-doped Ni Co alloy nanoparticle catalyst is synthesized by a simple two-step reaction method（V-Ni Co@NC/NF）.Doping of metallic vanadium optimizes electronic structure to enhance the electrical conductivity and the charge transfer of Ni Co alloys,thereby enhancing the UOR/HER catalytic activity;N-doped carbon effectively optimizes the electron distribution on the catalyst surface,thereby boosting the UOR/HER activity and preventing the dissolution of Ni Co alloys in strong alkaline solution.V-Ni Co@NC/NF shows good UOR/HER（1.27 V（14）-43 m V）activity at±10 m A cm^-2,which can maintain 100 h stability at±1,000 m A cm^-2,respectively.Compared with overall water splitting（1.61 V）,it only needs 1.36V at 10 m A cm^-2 for overall urea oxidation,which fully shows the advantage of urea oxidation-assisted hydrogen production.