Fabrication And Electrocatalytic Properties Of Molybdenum/Tungsten Carbide Nanocomposites

Hydrogen energy is a clean and efficient renewable energy.It is considered as an ideal alternative to fossil energy because of its advantages of wide sources,high energy density,clean and pollution-free.At present,hydrogen production by electrolysis of water is regard as one of the most promising methods for industrial application.Water electrolysis process includes oxygen evolution reaction（OER）and hydrogen evolution reaction（HER）.OER involves multi-electron transfer process,and has the characteristics of slow kinetics.HER usually requires high overpotential.At present,the precious metal Pt/Ru/Ir based catalyst has the best HER and OER performance,but their high price limits its wide application.Therefore,the development of new non-noble metal based catalysts is one of the key problems in the field of electrocatalytic water splitting.Molybdenum/tungsten carbide is a substitute material for Pt-based catalyst due to its similar electronic structure with Pt.But it is difficult to effectively control the morphology and structure due to the agglomeration problem during high-temperature carbonization process.In view of the above problems,the uniform dispersion and embedment of molybdenum carbide nanoparticles were realized by using two-dimensional porous nitrogen-doped carbon as template to fully expose the active sites;The uniform carbon tube arrays encapsulating Ni/MoC heterojunction nanoparticles was fabricated to prevent agglomeration of MoC particles and regulate the electronic structure of MoC;The H3PMo12O40 and H3PW12O40 clusters are homogeneously limited in the ZIF-67 nanocages through confinement strategy.During high temperature carbonization,Mo6Co6C and WC nanoparticles were homogeneously anchored on the surface of carbon matrix.The confinement effect of ZIF-67 can effectively avoid the agglomeration of the above carbide nanoparticles.The Pt-Mo6Co6C/NC and Pt-WC/NC heterojunctions were also fabricated by replacement effect of H2Pt Cl6 to regulate its electronic structure.We further explore the mechanism between HER performance and above heterojunctions.The research content of this paper is as follows:（1）The composite of two-dimensional porous nitrogen-doped carbon and molybdenum carbide material was prepared by staged calcination using ammonium molybdate,melamine and ammonium chloride as raw materials.The composite of C3N4nanosheet and Mo O2 nanoparticles was prepared during the first stage of calcination at500^oC.C3N4 was produced by melamine during calcination process and was stripped into nanosheet by gases decomposed from NH4Cl.The ammonium molybdate is converted into Mo O2 nanoparticles during this carbonization process.When the temperature rising to 800^oC,C3N4 nanosheet is transformed into two-dimensional porous nitrogen-doped carbon materials.The Mo O2 is converted intoβ-Mo2C through carbonization process.Moreover,β-Mo2C nanoparticles are uniformly anchored on the surface of two-dimensional nitrogen-doped carbon materials without agglomeration.The MoC-800 has an overpotential of 116 m V and Tafel slope of 48 m V dec^-1 in basic solution,and has an overpotential of 187 m V and Tafel slope of 59 m V dec^-1 in acid electrolyte.The MoC-800 has outstanding stability in both alkaline and acidic conditions.The excellent HER performance of MoC-800 is due to the fact that it fully expose the active sites,also present higher HER intrinsic activity and specific surface area.（2）The precursor of NiMoO nanosheet arrays fabricated on carbon cloth was calcinated with melamine at 800^oC.Uniform carbon nanotube arrays encapsulating Ni/MoC heterojunction nanoparticles was constructed during this process.The carbon tube arrays（Ni-MoC@NCNTs/CC）present excellent HER,OER and total water splitting properties.The HER and OER overpotential of Ni-MoC@NCNTs/CC was 122 m V and232 m V,respectively.The total water splitting voltage was 1.54 V.The Ni-MoC@NCNTs/CC also displayed favorable stability.The excellent performance of Ni-MoC@NCNTs/CC is mainly attributed to:the uniform carbon tube array has good conductivity and large specific surface area,which is conducive to rapid charge and mass transfer process.The confinement effect of carbon nanotubes can effectively inhibit the agglomeration of Ni and MoC nanoparticles and fully expose the active sites.In addition,the carbon layer with high stability can enhance the oxidation and corrosion resistance of Ni and MoC nanoparticles,and improve their stability during reactions.In carbon nanotubes,heterojunctions were formed by Ni and MoC nanoparticles,which can optimize the electronic structure and promote HER and OER performance.（3）Adopting MOFs confinement strategy,H3PMo12O40 cluster was successfully confined into ZIF-67 cage structure.The Co-Mo6Co6C/NC catalyst is prepared through carbonization of ZIF-67 precursor encapsulated with H3PMo12O40 cluster.Pt-Mo6Co6C heterojunction composited N-doped carbon（Pt-PMo/ZIF-67-800）was fabricated through H2Pt Cl6 replacement and acid pickling.Hence,Pt-PMo/ZIF-67-800 displays excellent HER performance than commercial Pt/C and favorable stability.At current density of 10m A cm^-2,Pt-PMo/ZIF-67-800 displays overpotential of 26 m V,22 m V and 42 m V in acid,basic and PBS solutions,respectively.This outstanding performance is ascribed to the reasons as below:Firstly,the homogeneously confinement of H3PMo12O40 cluster into ZIF-67 can effectively avoid agglomeration of Mo6Co6C nanoparticles during pyrolysis process.Secondly,the Pt-Mo6Co6C heterojunction can achieve synergistic effect through regulating electronic structure at interface.Moreover,the porous carbon substrate derived from ZIF-67 is available for mass and electron transportation.The DFT calculation reveals that the d-band center of Pt-Mo6Co6C heterojunction was lower than Pt and Mo6Co6C model,and theΔGH*of Pt-Mo6Co6C heterojunction was closer to 0 e V,demonstrating its outstanding HER performance.（4）In order to explore universality of MOFs encapsulation strategy to prepare carbides,phosphotungstic acid clusters（1.2 nm）was successfully confined into ZIF-67cages which both has similar size.The Co-WC/NC material was prepared by calcinating ZIF-67 precursor confined with phosphotungstic acid clusters at 800^oC.Meanwhile,Pt-WC heterojunction material（Pt-PW/ZIF-67-800）was achieved through H2Pt Cl6replacement and acid pickling process.The Pt-WC heterojunction presents superior HER performance than commercial Pt/C under all p H condition,and outstanding stability.At current density of 10 m A cm^-2,Pt-PW/ZIF-67-800 presents overpotential of 25 m V,20m V and 36 m V in acid,basic and PBS solutions,respectively.The excellent HER activity was attributed to the reasons as follows:First,the homogeneously confinement of H3PMo12O40 cluster into ZIF-67 can effectively avoid agglomeration of Mo6Co6C nanoparticles during pyrolysis process.Then,the Pt-Mo6Co6C heterojunction can achieve synergistic effect through regulating electronic structure at interface.Finally,the porous carbon substrate derived from ZIF-67 is available for mass and electron transportation.Theoretical calculation shows that Pt-WC heterojunction can adjust the electronic structure and optimizeΔGH*closer to 0 e V,indicating its excellent HER activity.