The Influence Of Electronic Structure Regulation Of Transition Metal Carbides On Their Electrocatalytic Hydrogen Evolution Reaction

The hydrogen evolution reaction(HER)from water splitting plays a vital role for industrial hydrogen production in that it is an efficient approach to solve the challenges of climate change and meanwhile to achieve carbon neutrality.Among numerous HER non-noble catalysts,transition metal carbides(TMCs)have been renowned for their "platinum-like behavior".The d-orbitals of metal hybridize with s-and p-orbitals of C lead to a broadened dorbitals of metal in TMCs,endowed with the characteristic resembling Pt.TMCs are also highlighted by high electronic conductivity as well as the wide pH applicability,featuring in the outstanding HER performance with longterm stability.However,the strong interaction between TMCs and hydrogen intermediate(H*)as well as the scarce active centers impede their further applications.Therefore,for further improving the HER activity,it is necessary to optimize the microstructure and electronic structure of TMCs.In this work,a series of TMCs-based HER catalysts with optimized microstructure and electronic structure are developed to increase the active center and intrinsic activity of the catalysts.Beyond that,the electronic structure optimization mechanism and its influence on hydrogen evolution are systematically studied by means of characterization methods and theoretical calculation.Moreover,these strategies are extended as universal methods applicable to all TMCs.The main research contents and results are as follows:(1)The strong H*adsorption on TMCs is the key factor limiting its HER catalytic activity.Based on this,a series of hierarchical branched ZnMoC/Mo2C catalysts with promoted HER kinetics are constructed,where the induced structural transition from Mo2C to MoC can be controlled by Zndoping.The different configuration of Mo2C and MoC resulting in the electrons transfer from Mo2C to MoC through the Zn-MoC/Mo2C interface,which reduces the electron density around Mo atom and then weakens the adsorption strength of H*on Mo.After the optimization of the electronic structure,Mo and Zn sites in Zn-MoC/Mo2C catalyst are activated,and thus dramatically increase the number of active centers and enhance the intrinsic catalytic activity.With an optimal feeding Zn/Mo atomic ratio of 0.2,the obtained Zn-MoC/Mo2C-0.2 catalyst presents an extremely low overpotential of 139 and 179 mV at 10 mA cm-2 current density in 1 M KOH and 0.5 M H2SO4,respectively.Nevertheless,the introduction of Zn2+ into WC lattice could not induce its structural transformation,indicating that Zn2+ doping is not a universal method to regulate electrons by inducing TMCs structural transformation.(2)In the previous chapter,the universal route for regulating the electronic structure of TMCs was not fully realized.In consideration of this,Ni activation of TMCs(VC,Mo2C,Cr2C3,and Fe3C)is proposed by the means of introducing adsorbed Ni clusters on the TMCs surface(Ni-GF/TMCs).Ni clusters adsorbed on the surface of TMCs can provide active sites for the HER process of Ni-GF/TMCs catalysts,resulting in a significantly increased number of active sites and enhanced the intrinsic catalytic activity of TMCs,which ultimately reduced the dissociation energy barrier of H2O and H2 and improved HER performance.The overpotentials and Tafel slopes of the NiGF/TMCs decreased obviously in both alkaline and acidic electrolyte.Significantly,the Ni-GF/VC catalyst features the ultra-low overpotentials of 128 mV in 1 M KOH,111 in 0.5 M H2SO4 at-10 mA cm-2,respectively.As for Ni-GF/Fe3C catalyst,its overpotentials are 93 and 112 mV in 1 M KOH and 0.5 M H2SO4,respectively.When a current density of-150 mA cm-2 is applied,their overpotentials are 270 and 291 mV,even comparable to Pt.(3)Based on the volcano plot of HER catalyst,a universal "balance effect"strategy is proposed,where nitrogen-doped graphene(NG)with weak H*adsorption is introduced to weaken the interactions of TMCs(M=Mo,W,Ti,and V)with H*.Hydrogen binding energies show that the TMCs coupled with NG appear to be thermo-neutral.Stemming from different work functions of TMCs and NG,partial electrons transfer from TMCs to the NG surface,resulting in optimized electronic structures of these electrocatalysts.In other words,the decrease of electron density around transition metal atoms leads to the weakening of TMCs adsorption on H*,and the increase of NG electron density results in the enhancement of the interaction between NG and H*.These optimized electronic structures can not only balance the H*adsorption and desorption on TMCs@NG,but also TMCs and NG,leading to synergistic ally-enhanced HER kinetics.After "balance effect" optimization,the number of active sites of TMCs@NG was significantly increased and the intrinsic activity was obviously enhanced,which ultimately reduced the dissociation energy barrier of H2O and H2 and improved HER performance.The overpotentials and Tafel slopes of the HER on these TMCs@NG electrocatalysts are thus pronouncedly reduced in both acidic and alkaline solutions.The as-prepared Mo2C@NG/CNT catalyst exhibits extremely low overpotential of 135 and 160 mV at a current density of 10 mA cm-2 in 0.5 M H2SO4 and 1 M KOH,respectively.Applying the "balance effect" to Ni and Fe3O4 can make the free energy of hydrogen adsorption value of Ni/Fe3O4 closer to 0 and its HER performance significantly improved.Therefore,the"balance effect" is universal and suitable to all HER catalysts.