Preparation Of Multistage Heterostructure MoSe₂ Based Composites By Freeze-Drying Method For Overall Water Splitting

Hydrogen production from electrolytic water has the advantages of zero pollution and wide source of raw materials,and is one of the most promising ways to produce hydrogen.However,the catalysts of this method are mainly noble metals such as platinum（Pt）,iridium（Ir）and ruthenium（Ru）,which have high costs and seriously limit its commercial application.Therefore,it is of great significance and research value to study low-cost non-precious metal catalysts.In recent years,molybdenum-based catalysts have received a lot of attention from researchers because of their high catalytic activity,good structural stability and environmental friendliness.However,these materials suffer from poor intrinsic conductivity and weak electrochemical reversibility,and their catalytic performance is less satisfactory.In this thesis,MoSe₂/NC composites with honeycomb nitrogen-doped carbon structure and multilevel heterostructured MoSe₂/CoSe₂/NC composites were synthesized by freeze-drying method combined with in situ selenization strategy to greatly improve the catalytic performance of MoSe₂-based materials and to investigate the mechanism of catalytic performance enhancement.The details of the study are as follows:（1）Controlled preparation of MoSe₂/NC and its overall water splitting performance studyPolymer（PVP）was used as the carbon source Freeze-drying combined with in situ selenization was used to synthesize honeycomb nitrogen-doped carbon-encapsulated MoSe₂-based composites（MoSe₂/NC）.The results showed that the unique honeycomb nitrogen-doped carbon network structure enhanced the electrical conductivity and structural stability of MoSe₂/NC,promoted the catalytic reaction,and improved the all-electrolytic water performance of the composites.In the 1 M KOH solution,a hydrogen precipitation overpotential of 153 m V at 10 m A cm^-2 and an oxygen precipitation overpotential of 180 m V at 10 m A cm^-2 were demonstrated.When used for total electrolysis of water,the bifunctional MoSe₂/NC requires only 1.5 V to produce a current density of 10 m A cm^-2 and exhibits excellent electrochemical stability（2）Controlled preparation of multi-level heterostructured CoSe₂/MoSe₂/NC and its overall water splitting performance studyMoSe₂/CoSe₂/NC composites were prepared in situ with the help of freeze-drying method and high-temperature selenization using cobalt acetate and molybdate as the cobalt and molybdenum sources,respectively.The results show that this strategy can stabilize the synthesis of MoSe₂ and CoSe₂ multiphase heterostructures and simultaneously construct three-dimensional porous carbon network structures.MoSe₂/CoSe_2/NC exhibited good catalytic performance when used as HER and OER catalysts.The HER and OER overpotentials of the composites were 109 m V and 250 m V at10 m A cm^-2 respectively.After assembling as a fully electrolytic water bath,MoSe₂/CoSe₂/NC reached a current density of 10 m A cm^-2 at only 1.63 V.The impedance and post-cycle SEM image test results demonstrated the good electronic conductivity and structural stability of the composites,which in turn improved their electrocatalytic activity.This may be attributed to the three-dimensional porous carbon structure provides abundant active sites and facilitates ion diffusion and bubble release inside the material during the catalytic process.