NiMo-based Heterojunction Micro-nano Array Catalyst For Water Electrolysis At Large Current Density

To achieve the ambitious goals of“peak carbon dioxide emission”by 2030and“carbon neutrality”by 2060,it is imperative to develop high-energy-density,carbon-free and renewable hydrogen energy to replace the traditional fossil energy.With the advantages of simplicity and no pollution,water electrolysis is regarded as an ideal and sustainable hydrogen production technology.However,it needs highly efficient,stable and low-cost catalysts to accelerate the slow kinetic process of cathode/anode（hydrogen evolution reaction:HER,oxygen evolution reaction:OER）at large current density(>500 m A cm^-2)for reducing the energy consumption.Therefore,exploiting catalysts with ultra-high activity/stability at large current density is of great significance for promoting the development of the H₂ production industry and reducing the cost.In this thesis,the advantages of carbon-encapsulated,metal coupled with metal oxide heterostructures and micro-nano array are utilized,then taking the controllable preparation of catalysts and theoretical calculations into account to carry out the study of Ni Mo-based heterostructure catalysts for water electrolysis at large current density.The principle of metal segregation is applied to fabricate the metal/metal oxide heterogeneous crystal interface.At the same time,the organic carbon transforms into a carbon-encapsulating structure by the catalysis of segregated Fe/Co/Ni atoms.The content and main conclusions of this thesis are as follows:Firstly,the microstructure and material compositions of the heterojunction micro-nano array catalysts are regulated by controlling the time/temperature of solvothermal process,molar ratio of reactants,calcination temperature,etc.The optimal solvothermal time,temperature,and calcination temperature of the catalysts are 12 h,160 ^oC,and 450 ^oC,respectively.Besides,the optimal Ni/Mo molar ratio of carbon-encapsulated Ni modified Mo O₂ nanosheets（Ni@C-Mo O₂/NF）and carbon-encapsulated nano-needle Ni/Mo O₂ three-phase heterojunction（Ni/Mo O₂@CN）is 1:7.The optimal Ni/Co molar ratio of mesoporous Ni Co Mo O nanosheet coupled with carbon-encapsulated Ni Co alloy（Ni Co@C-Ni Co Mo O）is 1:1,and the optimal concentration of iron nitrate nonahydrate for the preparation of carbon-encapsulated Fe Ni alloy modified rod-shaped Fe₂Mo₃O₈（Fe Ni₃@CN/Fe₂Mo₃O₈）is 0.012 mmol m L^-1.Secondly,the morphology,carbon-encapsulating structure,heterogeneous crystal interface,material composition,chemical state of elements,electronic interaction and carbon defect,etc.are obtained by the field emission scanning electron microscope（SEM）,X-ray diffractometer（XRD）,transmission electron microscope（TEM）,Raman spectrometer（Raman）and X-ray photoelectron spectroscopy（XPS）.SEM,TEM,XRD and Raman results of the above characterization indicate that Ni@C-Mo O₂/NF,Ni Co@C-Ni Co Mo O,Ni/Mo O₂@CN and Fe Ni₃@CN/Fe₂Mo₃O₈ all have carbon-encapsulated structure,metal-metal oxide heterostructure and micro-nano array structure.XPS results display that there exist strong electronic interactions between nitrogen-doped carbon（CN）and metal,as well as between metal and metal oxide,which optimizes the adsorption/desorption energy of the reaction intermediate on the active site of catalysts for enhancing the HER/OER intrinsic activity.Then,the activity/stability of the catalysts is analyzed by the HER/OER polarization curves and chronopotentiometry（CP）method.Meanwhile,the catalysts are evaluated in the simulated industrial water electrolysis conditions（6.0 M KOH+60 ^oC）.Benefitting from the carbon-encapsulated structure and the metal/metal oxide heterostructure,the electrons are redistributed at the nanointerface,which is beneficial to optimize the adsorption/desorption energy of H/O-containing intermediates,thus boosting the intrinsic activity of HER/OER.Ni@C-Mo O₂/NF(HER/OER:η_±10=25/240 m V,η_±1,000=250/400 m V),Ni Co@C-Ni Co Mo O(HER/OER:η_±10=39/260 m V,η_±1,000=266/390 m V),Ni/Mo O₂@CN(HER/OER:η_±10=33/250 m V,η_±1,000=267/420 m V)and Fe Ni₃@CN/Fe₂Mo₃O₈(HER/OER:η_±10=17/216 m V,η_±1,000=170/318 m V)all show good HER/OER activity.Besides,the CN can avoid the direct contact between metal particles and the electrolyte for improving electrochemical stability of catalysts at large current density.The redistribution of interface electrons can accelerate the charge transfer at large current density.Meanwhile,the micro-nano array structure not only has a large specific surface area to expose more active sites,but also is conducive to gas-liquid mass transfer at large current density for improving the mechanical stability.Therefore,Ni@C-Mo O₂/NF can steadily work for 196 h at 1,000 m A cm^-2 in 1.0 M KOH+30 ^oC;In the simulated industrial water electrolysis environment,Ni Co@C-Ni Co Mo O,Ni/Mo O₂@CN,and Fe Ni₃@CN/Fe₂Mo₃O₈ can keep stable at the current density of 1,000 m A cm^-2 for 43,330,and 500 h,respectively,showing the potential for industrial application.Finally,the density functional theory（DFT）simulation is used to research the Gibbs free energy（ΔG）of HER/OER and the electronic structure of the catalysts.Then,the relationship/law between the physical/chemical properties of the catalysts and the electrocatalytic performance is revealed by combining the physical characterization with electrochemical analysis to explain the enhancement mechanism of activity/stability.DFT calculations show that the ortho-carbon of pyridine-N for Ni@C-Mo O₂/NF and Ni Co@C-Ni Co Mo O have the optimal H adsorption Gibbs free energy(ΔG_H*),thus displaying good HER catalytic activity.Meanwhile,electrons are all transferred from the metal to CN.The three-phase heterojunction interface in Ni/Mo O₂@CN can effectively optimize the electronic structure of CN,Ni and Mo O₂ for adjusting the adsorption energy of H/O intermediates to obtain the bestΔG_H*for HER and reduce theΔG value of OER rate-determining step,thereby enhancing the HER/OER intrinsic activity.The carbon-encapsulated structure and metal-metal oxide heterostructure of Fe Ni₃@CN/Fe₂Mo₃O₈ can simultaneously optimize the electronic structure of carbon to boost the activity of water electrolysis.To sum up,in this thesis,Ni Mo-based heterojunction micro-nano array catalysts are successfully prepared by solvothermal and high-temperature calcination methods through the principle of metal segregation and the formation of graphitic carbon on Fe/Co/Ni,which exhibits good electrocatalytic activity/stability at large current density.This thesis provides a new idea for designing and preparing the high-efficiency and low-cost non-noble metal catalytic materials for water electrolysis at large current density.