Preparation Of W₂C/C Nanomaterials And There Electrocatalytic Performance Of Hydrogen Evolution

In recent years,transition metal carbides have the advantages of high natural reserves,low price,and high catalytic activity,and are easy to realize industrial production.Therefore,the transition metal carbide has the ability to replace the precious metal platinum as an electrocatalyst.Among them,tungsten carbide(W₂C)is used as a catalyst for the electrocatalytic hydrogen evolution reaction(HER),due to its widening of the d orbital of the metal when C is inserted in the W lattice,showing a d-band center similar to Pt.Make W₂C show physicochemical properties similar to precious metals.In fact,carbonization of W or WO₃ in hydrocarbons and in a reducing atmosphere will firstly lead to inevitable aggregation and uncontrollable particle size.Secondly,excessive gaseous carbon sources may cover the active sites,seriously affecting the catalytic effect of W₂C electrocatalysts.This thesis takes it as the research object,by designing morphology and structure to expose more active sites or by tunable components to improve the conductivity or intrinsic activity of W₂C,the main research results are as follows:(1)Firstly,WO₃ nanowires with uniform diameter were prepared by a simple hydrothermal method,and then we pyrolyzed urea and WO₃ NWs at a mass ratio of 2:1 at 700? without a catalyst to obtain a single phase wrapped in a thin carbon layer W₂C nanoparticles.On the basis of this,the influence of different calcination temperatures(600,800?)on its catalytic performance was explored.Then by adjusting the mass ratio of urea,it is found that it has adjusted the phase composition,such as W₂N(x<2),W₂C(2?x<3)or WC(x>3),where x is the mass ratio of urea and WO₃ NWs.The single-phase W₂C particles as a hydrogen evolution catalyst show good HER activity in acidic solutions.When the overpotential is 138 mV,it has reached a current density of 10 mA/cm2,the Tafel slope is as low as 72.08 mV/dec,and the resistance is 13.8 ?.And long-term stability is good.These experimental test results show that W₂C nanoparticles do have excellent hydrogen evolution catalytic performance in HER compared with other tungsten compounds.We believe that its good catalytic performance is mostly attributable to the intrinsic activity of the W₂C particles themselves,the highly conductive amorphous carbon layer,and the synergistic effect of the two to make it have a faster charge transfer rate.(2)Through electrospinning and carbon thermal hydrogen reduction technology,pre-oxidized fibers were calcined at 700? under Ar/H2 atmosphere to obtain W₂C/C nanofibers.On this basis,the phase structure and carbon content of the product were changed by controlling the hydrogen concentration.During the electrocatalytic hydrogen evolution test,W₂C/C-20 NFs showed excellent HER activity in the entire pH range:when the current density is 10 mA/cm2 in 1.0 M KOH,1.0 M PBS and 0.5 M H2SO4 the overpotential(?10)is 81,138 and 92 mV,respectively.Moreover,it shows better HER catalytic activity in alkaline media than acid and neutral media:for example,when the overpotential is 81,137,and 157 mV,the current density reaches 10,100,and 200 mA/cm2.The excellent catalytic performance of W₂C/C-20 NFs is that the constructed one-dimensional nanostructure prevents the agglomeration of W₂C and exposes more active sites.Secondly,the strong coupling of W₂C in the carbon matrix provides a faster electron transmission path,accelerates the electron transfer rate,and improves the conductivity.The synergy between W₂C and the carbon matrix regulates the electronic structure of the active site and the adsorption capacity of the catalyst to the intermediate,thereby promoting the significant enhancement of HER kinetics.(3)In this chapter,W₂C nanoparticles embedded in F,N-doped porous flexible carbon nanofibers are used as self-supporting HER electrocatalysts,which show good HER activity in alkaline media.In the process of electrospinning,a PTFE aqueous solution was added to the precursor spinning solution,followed by reduction and carbonization in an Ar/H2 atmosphere to prepare porous W₂C/C NFs flexible materials.Subsequently,the influence of PTFE on performance was explored by controlling the amount of PTFE aqueous solution.In the HER test,the self-supporting porous W₂C/C-0.8 NFs can reach a current density of 10 mA/cm2 at an overpotential of 50 mV in a 1.0 M KOH solution,and its Tafel slope is 70.9 mV/dec.At the same time,it maintained good stability during the 10 h catalytic hydrogen evolution process.The enhancement of HER activity may be attributed to the self-supporting structure and porous structure of the material,as well as the synergistic effect between F,N and W₂C.This allows W₂C/C NFs to expose more active sites,higher conductivity,and optimize the interaction with intermediates during the HER process.