Tungsten Oxide Modified Nickel-cobalt-based Catalyst For Urea Electrooxidation

Hydrogen（H₂）with the advantage of high energy density and renewable is considered an ideal alternative to fossil fuels.Electrochemical water splitting is one of the mature technologies for producing high-purity hydrogen.However,it requires high overpotential due to anodic oxygen evolution reaction（OER）relates multi-step proton-coupled electron transfer process.Utilizing urea as the anode reducing agent to replace OER can realize energy-saving hydrogen production and purify urea-containing wastewater at the anode.However,the problem of poor activity,high cost and unclear catalytic mechanism restricts its large-scale application.Therefore,the effect of hetero-interface,morphology control,carbon coating and other strategies on electrocatalytic performance have been further studied to prepare efficient and stable non-precious metal catalysts in this paper.The main contents are as follows:（1）In order to achieve energy-saving hydrogen production by replacing OER with thermodynamically more advantageous urea oxidation（UOR）.Herein,a carbon encapsulated Co_5.47N-WO₂ nanoparticles catalyst self-supported on nickel foam（NF）with high performance is prepared.The synergy between Co_5.47N and WO₂ can improve intrinsic activity,and the carbon layer can avoid direct contact between active material and electrolyte,thereby enhancing the stability.(Co_5.47N-WO₂)@C/NF exhibits remarkable UOR and HER activities,it only requires a potential of 1.38 V and-154 m V to reach±100 m A cm^-2.Moreover,it continuous operation at±100 m A cm^-2 for 40 h with no significant decrease of catalytic activity,which proves that(Co_5.47N-WO₂)@C/NF has excellent stability.However,the side reactions（OER）will occur as the current density increases due to its limited activity at large current density.Therefore,it is necessary to develop more efficient catalytic materials that can suppress the occurrence of side reactions.（2）In order to improve the limited activity of catalyst at large current density in the previous chapter.The self-supported carbon-coated Ni-WO₂ nanoparticles is prepared via hydrothermal method and high-temperature annealing in this chapter.The electrochemical tests show that（Ni-WO₂）@C/NF has excellent catalytic performance.When（Ni-WO₂）@C/NF is used as the anode and cathode to assemble the urea electrolyzer,the urea overall oxidation only needs 1.54 V to achieve a current density of 100 m A cm^-2,which is significantly lower than overall water splitting（1.79 V）.It proves that urea electrolysis has obvious thermodynamic and kinetic preponderance over water electrolysis.Moreover,its current density will not decrease with the increase of potential compared to(Co_5.47N-WO₂)@C/NF,and there are no side reactions in the catalytic reaction process.The main reason is that the tungsten oxide would affect the electronic structure of its neighboring metals,making Ni easier to be transformed into Ni³⁺,which is the effective active material of UOR,and further enhancing the catalytic activity of the catalyst at large current density.It provides a basis for the subsequent design and preparation of efficient and stable catalysts.（3）In this chapter,thioacetamide and nickel foam are used as sulfur sources and nickel sources,respectively.And the Ni₃S₂-WO₃ nanowire catalyst self-supported nickel foam is prepared by the one-step hydrothermal method.The WO₃ decorated Ni₃S₂ structure could promote the conversion of Ni²⁺to the UOR actual active component of Ni³⁺,thereby improving the UOR activity.Compared with the nanoparticles,the self-supported nanowire structure can promote mass transfer and enhance the stability at large current density.The electrochemical test results show that Ni₃S₂-WO₃/NF only needs a potential of1.430 V to reach the current density of 800 m A cm^-2.The above results show its broad commercial application prospects.