| The electrochemical water splitting is considered as a potential large-scale hydrogen production technology benefited from its pollution-free,efficient hydrogen production and sustainability.Due to sluggish kinetic process of hydrogen evolution and oxygen evolution in alkaline medium,the actual voltage of water splitting is much higher than the theoretical voltage.Until now,Pt and RuO2/IrO2 are the most efficient electrocatalysts for water electrolysis.However,the scarcity and high cost limit their wide application.With the advantages of the low-cost,variety and high conductivity,transition metal sulfides are expected to be the potential catalysts to replace precious metal materials in water splitting technology.Electrode material is the key factor for efficiency of electrocatalytic water.Developing self-supporting catalysts is important in recent years to improve the performance of electrodes in the practical application for water splitting.Herein,based on Ni3S2,this work focuses on the heterogeneous components design,electrode structure optimization and water splitting process mechanism of transition metal sulfide self-supported catalysts,and systematically discusses the structural characteristics and catalytic performance of the catalysts,revealing the structural changes of surface catalysts by in-situ Raman characterization method.The main research results are as follows:（1）In the view of the problems of uneven distribution and agglomeration of catalysts synthesized via traditional static hydrothermal method,the self-supporting catalyst of Ni3S2 in situ grown on nickel foam substrate（Ni3S2/NF）was prepared by one-step dynamic hydrothermal method in a homogeneous reactor.Comparsion of the electrolytic water performance for the catalysts synthesized via two hydrothermal methods was investigated.As a result,when Ni3S2/NF is directly used as a two-electrode material for overall water splitting,the required cell voltage is only 1.45V at the current density of 10 m A cm-2 with the stable operation for 24 h.The dynamic hydrothermal method overcomes the agglomeration phenomenon of Ni3S2,leading to the uniform dispersion on the surface of nickel foam substrate,which enhances the exposure of effective active area and improves the hydrogen evolution and oxygen evolution properties of Ni3S2.（2）In the view of the dissatisfied oxygen evolution activity and limited active sites of Ni3S2/NF,with the introducing of heterogeneous metals Co and Mo,a multi-metal Co-Mo-Ni sulfide self-supporting catalyst（CoMo-S-32/NF）was constructed by one-step hydrothermal method.By adjusting the molar ratio of Co and Mo,the effect of different metals ratios on the morphology,phase structure and oxygen evolution properties was investigated.As a result,heterogeneous metals can induce morphological change and promote the exposure of vertical edge sites.With the Co/Mo ratio of 3:2,a vertically grown nanoneedle array with heterostructure of MoS2 and CoNi2S4 is formed on the CoMo-S-32/NF surface,exhibiting significantly enhanced alkaline oxygen evolution activity.The required overpotential is only106 m V at the current density of 20 m A cm-2 with the Tafel slope of 53.1 m V dec-1.The unique nanoneedle structure of CoMo-S-32/NF provides sufficient space for electrolyte diffusion and gas release.The synergistic interaction between MoS2 and CoNi2S4 and the presence of defect structure promote the water oxidation process.（3）In the view of the problems of active species shedding and insufficient stability for catalysts in the long-term electrochemical process,monolithic electrodes based on porous nanofibers（Ni3S2@CNFs and FeS2@CNFs）were prepared by electrospinning and thermal treatment processes.The formation mechanism and morphology of monolithic electrodes as well as the activity and stability of electrocatalytic hydrogen evolution were systemically analyzed.As a result,the interwoven carbon fibers in situ constructe the self-supporting structure of catalyst,and the carbon matrix composite can be directly used as an electrode material for hydrogen production.Ni3S2@CNFs and FeS2@CNFs exhibit the similar HER activities with the required overpotential of 176 and 168 m V at 10 m A cm-2,respectively.FeS2@CNFs shows better catalytic stability for nearly 7 days operation with no destruction of fiber structure.In addition,the monolithic electrode FeS2@CNFs also shows good hydrogen evolution activity under simulated industrial condition（60℃,6 M KOH）with the stable operation for 45 h.During the pyrolysis process,the presence of metals promotes the formation of graphitic carbon and porous structure,constructing carbon encapsulation structure that effectively inhibits the loss of FeS2 and Ni3S2,which enhances the stability of catalysts.（4）The species of transition metal sulfides are significantly affected by the preparation conditions,the variety and amount of sulfur sources.Based on this,taking nickel sulfide as the research object,Ni and S sources were mixed and in situ electrospun.The different sulfur-phase NiSx were synthesized through the regulation of thermal-treatment conditions（annealing temperature,thiourea content）.The reaction mechanisms of different NiSx for alkaline hydrogen evolution process were preliminatively explored.As a result,with the increase of annealing temperature,the exposure of Ni9S8 crystal planes and the degree of graphitization of carbon fiber are enhanced.With the increase of thiourea content,the catalyst phase changes from sulfur-deficient phase Ni9S8 into sulfur-rich phase NiS.The hydrogen evolution results show that NiS-800@CNFs exhibits the best HER activity with an overpotential of 119 m V at10 m A cm-2.Ni O is beneficial for water dissociation,and Ni9S8 provides hydrogen adsorption sites,which synergically promoting the alkaline HER process.（5）In view of the problems of metal dissolution and poor stability of catalyst at high oxygen evolution potentials,based on the design of monolithic electrode,FeS2-Ni3S2-FeNi3heterogeneous catalyst was synthesized by in-situ electrospinning with the mixture of Ni and Fe sources.The oxygen evolution performance of catalyst was optimized by adjusting the Ni/Fe ratio.The results show that with the Ni/Fe ratio of 1:3,NiFe-S-13@CNFs electrode presents the excellent oxygen evolution activity and stability with the overpotential of 270 m V at 10 m A cm-2.The electrode operates stably for 50 and 55 h at the current density of 10 and 100 m A cm-2,respectively,and no destruction of fiber structure occurs.In situ Raman analysis reveals the evolution of metal sulfides at high potential and the reconfiguration into Fe OOH and Ni（OH）2active phases.With the assist of conductive FeNi3 and S,N co-doped carbon,Ni and Fe two sites synergistically catalyze oxygen evolution process.Fe inhibits the further oxidative degradation of Ni,and the carbon encapsulation structure prevents the loss of Fe OOH and Ni（OH）2,thus effectively improving the stability of catalyst. |
