Preparation And Electrochemical Properties Of MOFs-derived Multi-core-shell Selenide/sulfide Electrode Materials For Supercapacitors

As a promising new energy device,supercapacitors boast of long service life,environmental friendliness and low preparation cost,but their development is limited by energy storage density.Theoretically,Metal-organic frameworks（MOFs）are highly potential electrode materials for supercapacitor due to their intrinsic porosity,large specific surface area,and highly adjustable composition/structure.However,when pure MOFs materials are used directly in making supercapacitors,their low conductivity and unstable metal-ligand coordination bonds in electrolytes hinder their application.The derivatives and composites of MOFs can be obtained successfully through fine regulation of components,ingenious design of its structure and reasonable transformation of preparation methods,which are seen as effective strategies to overcome the barriers of MOFs itself and improve the electrochemical properties.This paper aims to give full play to the maternal advantages of MOFs to produce MOFs-derived multi-core-shell selenides or sulfides with excellent electrochemical properties and highly cyclic stability.The main research contents and results are as follows:1.The core-shell structure ZIF8@ZIF67 was prepared,and the multi-core-shell structure was obtained by introducing appropriate amount of Ni²⁺based on cation exchange.Among them,the rich and uniform metal center provides a wide range of dispersed active sites for the material.When the amount ratio of Co Ni substance is 8:2,the electrochemical properties of the materials are greatly improved because the optimal synergistic effect occurs between metals.Next,Zn Se@Co Se/Ni Se-400,a kind of composite from the selenide and carbide,can be obtained by calcining and selenizing MOFs materials under the protection of inert gas.After characterization and electrochemical tests,the material showed a pseudo-capacitive energy storage mechanism with a specific capacity of 2208 F g^-1at 1 A g^-1,which still maintain its initial specific capacity of 88.3%after 10,000 cycles.the material could still maintain its original specific capacity of 88.3%.The prepared asymmetric supercapacitor exhibits an energy density of 46.55 Wh kg^-1at a power density of 700 W kg^-1.2.Using ZIF8@ZIF67 as template,the layered bimetallic hydroxide Co Ni-LDH is grown on the surface of core-shell structure by using the co-precipitation method.The introduction of LDH not only increases the specific surface area of the material,but also brings more redox active centers.Furthermore,the presence of layered structures can shorten the distance of mass transfer and accelerate the transport of ions or electrons.It is well known that the diffusion rate of metals differs from each other,and the higher the temperature,the more violent of core structure’s shrinking until it was completely consumed.So taking materials above as precursors for calcination and vulcanization,the sulfide and carbide composites with core-shell structure,yolk-shell structure and hollow structure are obtained at 400℃,450℃and 500℃respectively,among which the calcined carbon skeleton plays an important role in improving the stability of the material.The tests results show that the yolk-shell structure ZnS@CoS/NiS-450 has the best electrochemical properties,and its large specific surface area and mesoporous structure of the material can improve the transport rate of electrolyte ions.The material still retains a specific capacity of 91.11%after 3000 cycles at 10 A g^-1,and its specific capacity is 1500 F g^-1at 1A g^-1.The prepared asymmetric supercapacitor has a maximum energy density of50.9 Wh kg^-1and a power density of 699.9 W kg^-1.