Design,Preparation And Performance Research Of Hydrogen Evolution Electrocatalyst Based On Polyoxometalates

With the rapid development of society,the exhaustion of traditional energy sources and the increasing demand for energy have become a problem that plagues people's continuous progress.According to the requirements of the national“14th Five-Year Plan”and people's urgent demand for renewable energy,the embryonic form of“efficient,low-cost and environmentally friendly energy conversion and storage systems”has become a steady way to solve this problem.Hydrogen production from water electrolysis has become the most potential renewable energy solution because of its clean and efficient characteristics,and has broad application prospects,in which the catalyst is the core of the electrocatalytic hydrogen evolution reaction(HER).At present,platinum group materials are recognized as the state-of-the-art catalysts for the HER.However,noble metal catalysts often suffer from a high cost and limited abundance,which inevitably hinders their large-scale production and application.Furthermore,the water-splitting devices based on proton exchange membrane technology and microbial electrolysis cells often operate in acidic and neutral media,respectively.Besides,water electrolysis in industry is usually performed in strongly basic media.Therefore,the development of efficient and durable non-precious metal p H-universal HER electrocatalysts is of great significance.Polyoxometalates(POMs)are a class of nanoscale polynucleated molecular clusters based on transition metal elements,which have structural diversity,water solubility and compatibility with almost all other materials.However,POMs have poor conductivity and dispersion,and poor stability in the electrolytes.By hybridizing POMs and graphene oxide(GO)with abundant oxygen-containing functional groups and high specific surfaces area,transition metal composites with high stability,controllable morphology and diverse active components could efficaciously design and regulate the classification and numbers of metals at molecular or even atomic levels.Accordingly,in this thesis,the electronic structure controllable nanocomposites were prepared by hydrothermal self-assembly and chemical vapor deposition(CVD)technology using POMs and GO as precursors.The performance of electrocatalytic hydrogen evolution was tested,and the internal mechanism of performance improvement was studied by theoretical calculations.The main contents of the study are as follows:(1)Dual-atom catalyst consisting of O-coordinated W-Mo heterodimers embedded in N-doped graphene(W₁Mo₁-NG DAC)was precisely constructed via a controlled self-assembly and CVD process using hexaammonium molybdate and sodium tungstate as metal precursors and GO as support.In W₁Mo₁-NG DAC,the O-bridged W-Mo atoms are anchored in N-doped graphene vacancies through oxygen atoms(W–O–Mo–O–C),which ensures the structural stability of the catalyst.In acidic and alkaline solutions,the overpotentials of W₁Mo₁-NG DAC at a current density of 10 m A cm^–2(?₁₀)are 24 m V and 67 m V,respectively,and the Tafel slopes are 30 m V dec^–1and 45 m V dec^–1,respectively,which are superior to almost all W and Mo-based HER catalysts,and even surpass commercial Pt/C catalyst.In addition,the catalyst exhibits excellent stability in the temperature range of 5?90 ^oC in the whole p H range solution.The electron delocalization of W–O–Mo–O–C configuration provides optimized H adsorption behavior and enhanced HER kinetics,thereby significantly promoting the intrinsic activity of W₁Mo₁-NG DAC.(2)Heterostructured Nb₄N_5-xO_x-Mo S₂(0<x<1)supported on N-doped graphene(Nb₄N_5-xO_x-Mo S₂/NG)is synthesized through hydrothermal reaction and controllable nitridation process using hexaammonium molybdate,niobium(V)chloride and GO as precursors.During CVD process,Mo S₂nanosheets are etched into small pieces and covalently connected with Nb₄N_5-xO_xto form fine Nb₄N_5-xO_x-Mo S₂heterostructures with abundant interface and edge active sites.Nb₄N_5-xO_x-Mo S₂/NG shows excellent HER performance in both acidic(?₁₀=39 m V,Tafel slope of 30 m V dec^–1)and alkaline(?₁₀=67 m V,Tafel slope of 44 m V dec^–1)media.The enhanced HER activity of Nb₄N_5-xO_x-Mo S₂/NG is attributed to the appropriate electron density at the covalently connected Nb–(N,S)–Mo interface,which optimizes the free energy of H adsorption and enhances the water adsorption capacity.Meantime,the chemical coupling of covalently connected Nb₄N_5-xO_x-Mo S₂heterostructures and N-doped graphene(NG)ensures the stability of Nb₄N_5-xO_x-Mo S₂/NG catalyst and the fast electron transfer in HER process,further cooperating the H₂generation.(3)Mo₂C nanosheets with abundant carbon vacancies supported on NG(H-Mo₂C/NG)were constructed in situ by hydrothermal reaction and water-assisted CVD process using hexaammonium molybdate and GO as precursors.During the CVD process,the host-guest interaction between Mo O₃and water obtains a unique Mo–O configuration,which makes Mo₂C more inclined to combine with six-membered C-ring of graphene during carbonization reduction,thereby forming ultra-thin Mo₂C nanosheets.Meanwhile,water reacts with carbon at high temperature,which leads to insufficient carbon atoms around Mo O₃that interact with water to form Mo₂C,thus producing carbon vacancies(Cv).In acidic and alkaline solutions,the H-Mo₂C/NG catalyst exhibits excellent HER activity,with?₁₀of 10 m V and 63 m V,respectively,and Tafel slopes of 38 m V dec^–1and 59 m V dec^–1,respectively.Theoretical calculations show that electron enrichment at Cv in Mo₂C nanosheets optimizes the binding strength of H and reduces the dissociation energy of water,thereby significantly improving the HER activity of H-Mo₂C/NG in the whole p H range.