Rational Synthesis Of Heterostructured Transition Metal Sulfide Catalysts For Efficient Water Electrolysis

At present,sustainable energy system has inspired tremendeous research interest worldwide.As a new type of clean energy,molecular hydrogen can provide a feasible solution for sustainable energy transformation.As an emerging energy storage technology,hydrogen production by water electrolysis can convert electrical energy into chemical energy（hydrogen energy）to realize electrical energy storage.Through devices such as hydrogen-oxygen fuel cells,the chemical energy can be converted back into electricity,providing a new cycle of electricity-chemical energy conversion for smart grid,and strongly promoting the utilization of sustainable electric energy such as wind power and photovoltaic power generation.During the electrolysis process,water molecules are oxidized to oxygen on the anode via oxygen evolution reaction,（OER）,and reduced to hydrogen on the cathode via hydrogen evolution reaction,（HER）.In this process,the sluggish kinetics and high energy barrier lead to high overpotential and low energy conversion efficiency.In order to achieve the effieient electric energy conversion and sustainable conversion economy,it is urgent to develop new non-noble catalysts with high catalytic activity and stability.The construction of heterostructures composed of multiple components is considered as a feasible strategy for improving the electrocatalytic activity of catalysts.These composite heterostructure catalysts have advantages such as controllable composition and tunable structure,and can enhance the electrocatalytic performance through electronic interaction,stress effect,synergistic catalysis and other effects.Therefore,in this paper,three metal sulfide heterostructure catalysts were prepared through structure regulation for high-efficiency water electrolysis and the mechanism of activity enhancement induced by structure regulation was studied.The main researches is as follows:1.Metal-organic frameworks（MOFs）with high specific surface area and tunable ordered porous structure have shown remarkable application prospects as OER catalysts.The first work promotes OER activity by introducing materials with high conductivity into MOFs to form heterojunctions.Ni₃S₂was composited with ultrathin Ni/Fe MIL-53nanosheets on a nickel foam（NF）substrate to form a heterostructure（S-MOF/NF）,which is used for efficient OER electrocatalysis to test OER electrocatalytic performance in alkaline medium.Growth of MIL-53 nanosheets with an average thickness of as low as 2.5 nm on conductive substrate NF can fundamentally improve electron exchange,and uniform growth of nanosheets can improve mass transfer characteristics through the formation of large pores.Ni₃S₂nanoclusters embedded in nanosheets improve the conductivity of MIL-53,thereby promoting the efficient conduct of OER,significantly improving electrocatalytic performance.This study provides a new strategy for constructing heterogeneous electrocatalysts with enhanced electrochemical performance,and helps deepen our understanding of MOFs-based OER catalysts.2.Developing alkaline HER catalysts in alkaline is necessary to improve the energy conversion efficiency of water electrolysis.Tungsten trioxide（WO₃）exhibits good catalytic stability for HER due to its physicochemical properties.The second work of this thesis is designed to solves the scarcity of electron transfer ability of WO₃by introducing high-conductivity components.and the heterostructure interface can promote the migration of H_adsfrom one component to the surface of the adjacent one to improve the HER performance of the catalyst.Specifically,the Ni₃S₂component was introduced into WO₃to construct a Ni₃S₂/WO₃heterointerface（S-WO₃/NF）.The charge synergy effect adjusted the electron distribution on the catalyst surface,improved WO₃conductivity and optimized the kinetics during the reaction process.Ni₃S₂and WO₃composite nanosheets form nanosphere exhibit excellent HER performance and stability in alkaline media,and the overpotential at the same current density is much lower than that of single-component Ni₃S₂and WO₃.After 80-hour stability test,the change in its hydrogen evolution overpotential of it is negligible.Characterization proves that the excellent electrochemical performance stems from its nanosphere structure and charge synergy effect.The introduction of Ni₃S₂adjusted the electronic structure of WO₃,the H_adsproduced by the water dissociation on the WO₃sites can be transported to the plane of Ni₃S₂through the hydrogen spillover effect,and the kinetics during the electrode reaction process are optimized.3.Industrial electrolysis of water for hydrogen production requires an ampere-level current density.However,when the catalyst works at a high current density and H_adsgenerated in the water dissociation step cannot be transferred or detached from the surface active sites in time,it will hinder the subsequent water dissociation.Constructing a heterojunction can adjust the distribution of charges,and the resulted interface effect of a rationally design can make the H_adsoverflow to the another component for desorption,which can effectively solve the above problems.The second work of this paper is to optimize the electron distribution state of the active site by constructing a Ni₃S₂/Cr₂S₃heterostructure（S-NiCr/NF）,and there is a hydrogen spillover process during the reaction.The above two effects jointly promote the high-current HER reaction.S-NiCr/NF exhibits excellent HER performance in alkaline media,only requiring low overpotentials of 230 and 333 m V to achieve high currents of0.5 and 1 A cm^-2in alkaline media,respectively,The catalytic activity of S-NiCr/NF remained almost unchanged after 50-hour stability tests at high current densities of 500m A cm^-2.This excellent HER performance results from its unique synergy effect and hydrogen spillover on Ni₃S₂/Cr₂S₃heterointerface,which tunes the electronic structure and optimizes the Volmer/Heyrovsky process,so that the catalyst can achieve a stable operation of ampere-level current density with a lower overpotential in alkaline media.In conclusion,this dissertation focuses on the construction of heterointerface electrocatalysts.Three heterogeneous structured catalysts were designed and synthesized,and their catalytic activity was carefully evaluated,and their structure-activity relationship and activity improvement mechanism were analyzed.These works provide a feasible method for the design and synthesis of high-efficiency electrolytic water catalysts,and also provide a useful reference for exploring the structure-activity relationship of the heterojunction interface of non-noble metal catalysts.