Preparation Of MoS_x-based Materials And Study On Their Electrocatalytic Performance For Hydrogen Evolution

The rapid development of human society consumes more and more fossil energy,and the frequent occurrence of extreme climate caused by excessive CO₂ emission has seriously hindered the sustainable development of human society.Using clean,cheap and renewable energy to replace traditional fossil energy is an effective way to solve energy shortage,climate deterioration and realize the"dual carbon"goal.As an ideal energy carrier,hydrogen energy has the advantages of high energy density and zero carbon emission,which is a green energy alternative to traditional fossil fuels.The best way to solve the problem of hydrogen source is to use renewable energy such as solar energy and wind energy to generate electricity that is then used to produce hydrogen through electrolysis of water.However,at present,the commercial catalysts for hydrogen production by the electrolysis of water are mostly noble metals such as Pt etc.,which leads to a relatively high price of hydrogen.Therefore,how to obtain hydrogen cheaply is an urgent problem for the industry to solve.It is one of the feasible way to use non-noble metal instead of precious metal as catalyst for hydrogen evolution.Recent studies have shown that amorphous MoS_x has high catalytic activity for hydrogen evolution and is considered as one of the alternative materials for Pt-based precious metals.However,the amorphous structure of MoS_x makes its morphology difficult to be regulated,and the morphology just has great influence on the performance of catalyst to a large extent.In order to obtain amorphous MoS_x and give it certain geometric structure and morphology,so as to improve its specific surface area and catalytic active sites,and to improve its catalytic performance,carbon cloth（CC）,nickel foam（NF）,Ni₃S₂/NF,Ni Co₂S₄/NF,etc.,were used as substrates in this paper.Electrodes such as a-MoS_x/CC,a-MoS_x/FN,MoS_x/Ni₃S₂/NF and MoS_x/Ni Co₂S₄/NF were synthesized and prepared by potentiostatic electrodeposition,hydrothermal and other methods,and their catalytic performance was studied in detail.Meanwhile,their structure-activity relationship and catalytic reaction mechanism were revealed by experiments combining with theoretical calculation.The research results are as follows:1.Using carbon cloth as substrate,a MoS₂ electrode with 2H structure 2H-MoS₂/CC was prepared by the in situ growth of MoS₂ crystals in the hydrothermal process.Another amorphous a-MoS_xelectrode a-MoS_x/CC was obtained via deposition of MoS_x on carbon cloth by using electrochemical deposition method.The electrocatalytic hydrogen evolution performance of the both in acidic and alkaline systems was studied,respectively.The experiment results show that under acidic condition and 10 m A cm^-2current density,the hydrogen evolution overpotential of 2H-MoS₂/CC and a-MoS_x/CC is 185 and 165 m V,respectively.The Tafel slope is 80.8 and 41.7 m V dec^-1,respectively.Whereas under alkaline condition and 10 m A cm^-2 current density,the hydrogen evolution overpotential of both 2H-MoS₂/CC and a-MoS_x/CC is 256 m V,and the Tafel slope is 135.6 and 113.5 m V dec^-1,respectively.Obviously,the catalytic activity of hydrogen evolution of a-MoS_x/CC is higher than that of 2H-MoS₂/CC under both acidic and alkaline conditions.The main reason is the existence of bridging S₂^2-and hexavalent molybdenum center Mo（VI）=O in amorphous MoS_x.The results of XPS characterization and electrochemical testing indicate that the HER activity of a-MoS_x/CC increases with the increase of Mo（VI）=O content.Therefore,we conclude that Mo（VI）=O is the catalytic active site of MoS_x.2.Because MoS_x is amorphous structure,it is difficult to improve its performance by changing its morphology,but it can be deposited on the substrate with a certain morphology to give it a certain morphology indirectly.Therefore,we prepared a-MoS_x/NF electrode via deposition of MoS_x on nickel foam with certain geometric morphology and good conductivity.The experimental results show that the hydrogen evolution overpotential of this electrode is152 m V and the Tafel slope is 82.4 m V dec^-1 under alkaline condition and the current density of 10 m A cm^-2.The activity of HER is significantly higher than that of a-MoS_x/CC.It is proved by XPS characterization that there are more Mo（VI）=O active sites in a-MoS_x/NF than a-MoS_x/CC.3.In order to further improve HER activity of MoS_x,Ni₃S₂ with nanosheet morphology was first deposited on nickel foam substrate using hydrothermal method,and then MoS_x was deposited using electrodeposition method to prepare MoS_x/Ni₃S₂/NF electrode,thus giving MoS_x with nanosheet second-order morphology.The experimental results show that the hydrogen evolution overpotential of MoS_x/Ni₃S₂/NF electrode is 122 m V and the Tafel slope is74.9 m V dec^-1 under alkaline condition and 10 m A cm^-2 current density.Compared with a-MoS_x/NF,HER activity is further improved.XPS characterization proved that it is also attributed to the increase of Mo（VI）=O active sites.4.In order to improve HER activity of MoS_x one more,we used Ni Co₂S₄ that has better electrochemical tolerance and is easier to control morphology in addition to good conductivity instead of Ni₃S₂ to prepare MoS_x/Ni Co₂S₄/NF electrode using the same method.The experimental results show that the hydrogen evolution overpotential of this electrode is 108 m V and the Tafel slope is 79.4 m V dec^-1 under alkaline condition and 10 m A cm^-2current density,and the structure and activity of the electrode are almost unchanged after suffering from continuous 24 h electrolysis for hydrogen evolution,and the performance of HER is further improved.XPS characterization proved that compared with MoS_x/Ni₃S₂/NF,the content of Mo（VI）=O active site in MoS_x/Ni Co₂S₄/NF electrode is higher.5.In order to understand the HER electrocatalytic mechanism of MoS_x,the possible adsorption posture of H₂O molecules at the active site was calculated theoretically.The calculation results show that the posture in which oxygen in H₂O adsorbs on Mo（VI）in Mo（VI）=O active site,and hydrogen interacts with terminal sulfur via hydrogen bonding has the lowest energy and the longest H-O bond length,thus is the most favorable and possible adsorption state.Combined with experimental data and theoretical calculation,a novel electrocatalytic mechanism for hydrogen evolution of MoS_x is proposed.