Synthesis Of M(Fe/Ni)-Mo-S(Se)-Based Nanomaterials And Their Electrocatalytic Performances In Water Splitting

The ever-increasing depletion of fossil energy and environmental pollution has influenced the sustainable development of human society.Therefore,developing clean,efficient and renewable energy to reduce the dependence of social production and life on traditional fossil fuels is a matter of utmost urgency.Hydrogen is considered to be one of the most promising clean energy carriers due to its high energy density,carbon-free nature,and practicality in transportation and distribution.It can replace,at least in part,the limited fossil fuels and ease environmental pollution and global warming.Water electrolysis is considered to be a promising method for the production of hydrogen because it is eco-friendly and the only reactant is abundant and renewable water.Compared to steam methane conversion and coal gasification,H2 produced from water electrolysis is highly pure and the reaction conditions are mild.Electrocatalytic water splitting involves the cathodic hydrogen evolution reaction(HER),and the anodic oxygen evolution reaction(OER).Both HER and OER require suitable catalysts to lower the high energy barrier for the reaction.Even today,precious metals are still recognized as the most beneficial catalysts for the HER and OER,but their high cost,scarcity and instability restrict their large-scale applications.Therefore,scientists are highly engaged in the development of less expensive and highly efficient non-noble metal-based electrocatalysts,such as transition-metal chalcogenides.Specifically,Mo-S(Se)-based materials with distinctive structural features,rich active sites,and adjustable electronic properties and components have attracted widespread research attention.Although there have witnessed great accomplishments in this area,more efforts are still made to satisfy the requirements of relatively low overpotentials and long-term stability through optimizing synthesis methods,morphology and structure.In view of this,this doctoral thesis focused on the design and synthesis of M-Mo-S(Se)-based(M=metal)functional nanomaterials.The high catalytic performances of M-Mo-S(Se)-based nanomaterials were achieved through doping,morphology control,structure optimization,and construction of heterostructures.The related reaction mechanism and structure-activity relationship were explored by X-ray photoelectron spectroscopy(XPS)and density functional theory(DFT)calculations,etc.The main research results are described as follows:1.A series of Fe-doped MoS2 nanomaterials were prepared by simple one-pot solvothermal reactions of(NH4)2MoS4 with FeCl3·6H2O and the morphology evolution processes were examined.The optimized working electrode of Fe-MoS2-5 showed high HER activity in 0.5 M H2SO4.It only needs a relatively small overpotential of 173 mV to drive the current density of 10 mA cm-2,together with no significant loss in catalytic performance after 1000 cyclic voltammetry(CV)cycles.Besides,in 1.0 M KOH electrolyte,Fe-MoS2 supported on nickel foam(denoted as Fe-MoS2/NF)displayed good HER(153 mV at 10 mA cm-2)and OER(230 mV at 20 mA cm-2)catalytic performances.Under these conditions,its catalytic performance remained unchanged for more than 140 h.The two-electrode system with Fe-MoS2/NF as anode and cathode electrodes exhibited excellent electrocatalytic activity toward overall water splitting with a low voltage of 1.52 V to achieve a current density of 10 mA cm-2 in 1.0 M KOH.The remarkable electrocatalytic performance of Fe-MoS2 can be due to the successful doping of Fe and the effective construction of Fe-MoS2 and conductive NF.2.A multi-component hierarchically-assembled Fe-MoS2/Ni3S2/NF catalyst was prepared by a facile solvothermal method,in which spine-like Fe-doped MoS2 nanorods were decorated over Ni3S2 nanoplates grown on NF.Benefit from the special morphology and the synergistic interactions between Fe-MoS2,Ni3S2 and NF,the optimized electrocatalyst(denoted as Fe-MoS2/Ni3S2/NF-2)exhibited excellent catalytic activity and stability for catalyzing HER and OER in 1.0 M KOH,with low overpotentials of 130.6 mV and 256 mV(vs.RHE)at a current density of 10 mA cm-2 respectively.Besides,there is no evident loss in catalytic performance toward OER even after 180 h,which showed outstanding durability.The two-electrode system with Fe-MoS2/Ni3S2/NF-2 showed promising activity toward water splitting with current density of 10 mA cm-2 at a voltage of 1.61 V.It also held an outstanding long-term stability for over 20 h,exhibiting its great potential for application in water splitting.3.Two rare MoSe2/NiSe heterostructures were synthesized by varying the selenylation conditions of NiMoO4 nanorods.MoSe2/NiSe-1 is a triphase heterojunction with tetragonal phase MoSe2(1T-MoSe2),hexagonal phase MoSe2(2H-MoSe2)and hexagonal phase NiSe(H-NiSe)while MoSe2/NiSe-2 is a tetraphase heterojunction with 1T-MoSe2,2H-MoSe2,H-NiSe and rhombohedral phase NiSe(R-NiSe).In contrast to MoSe2/NiSe-2,MoSe2/NiSe-1 exhibited remarkably enhanced HER activity with an overpotential of 30 mV at 10 mA cm-2,and negligible voltage change even when operated at current density of 10 mA cm-2 for 40 h.The strong electronic synergistic interaction between different interfaces of mixed MoSe2/NiSe phases greatly enhanced the HER activity.DFT calculations rationalized the fact that the combination of three phases(H-NiSe and 1T/2H-MoSe2)was more active toward HER than that of four phases,which remarkably increased the interface electron concentration,facilitated electron transfer,strengthened the electron orbital coupling in the heterojunction structure and decreased the free energy ΔGH2O and AGH*.This work provideed a rational strategy to design and assemble stable and high-performance multiphasic heterojunctioned HER electrocatalysts through interface engineering.4.Ni-doped 1T-MoS2 catalysts were fabricated via one-step solvothermal strategy,with a preformed cluster[P(C6H5)4]2Ni(MoS4)2 served as the precursor.Through optimizing the reaction solvents,three kinds of Ni-doped MoS2 with different phase components were obtained:Ni-M-MoS2-1(40%1T phase),Ni-M-MoS2-2(67%1T phase)and Ni-1T-MoS2(100%1T phase).As the conductivity of 1T-MoS2 is better than 2H-MoS2,the HER performance of the three catalysts got improved with the increase of the proportion of 1T-MoS2 in the catalyst.Among them,the Ni-doped pure 1T-MoS2 nanosheets exhibited the highest catalytic activity and durability for HER,achieving a current density of 10 mA cm-2 with only 107.6 mV,a relatively low Tafel slope of 87 mV dec-1 and maintaining excellent stability for at least 20 h.This work not only provides a new route for directly preparing the metal-doped metastable catalysts with promising catalytic performance,but also opens the gate for the engagement of Mo(W)-M-S clusters as the good precursors in energy conversion and storage fields.