Preparation Of MOFs-Based Transition Metal Phosphorus(Selenium) Compounds And Their Electrochemical Properties

With the development of society,the depletion of fossil fuels and environmental pollution problems are becoming more and more serious,prompting researchers to search for new energy storage devices that are safe,efficient,green and promising.Due to the ultra-high power density,fast charging and discharging rates,environmental friendliness and excellent cycling stability,supercapacitors have become a highly efficient energy storage device that has attracted a lot of attention from researchers.Commercially available carbon-based materials have lower specific capacitance and energy density,limiting their widespread application.The key to the performance of supercapacitors lies in the electrode active materials,and therefore novel pseudocapacitive materials need to be developed to improve their electrochemical performance.Metal-organic frameworks（MOFs）formed by the self-assembly of metal ions and organic ligands,are promising crystalline porous materials.With the advantages of adjustable pore size,high porosity,diverse topology and large specific surface area,MOFs are widely used in the field of supercapacitors.However,when MOFs are directly used as electrode materials for supercapacitors,disadvantages such as poor electrical conductivity and structural instability can lead to low specific capacity and poor cycling stability.MOFs derivatives and composites can be prepared by structural design and component modulation of MOFs to substantially improve electrochemical performance.In this thesis,MOFs were used as self-sacrificing templates to tune the structure and optimize the chemical composition to construct polymetallic phosphide/selenide,which leads to high-performance supercapacitor electrode materials.The main studies are as follows:（1）Firstly,zinc/cobalt metal organic framework（Zn/Co-MOF）was synthesized by precipitation method,and then nickel ions were introduced by hydrothermal reaction,followed by alkali etching and calcined phosphorization to prepare zinc-cobalt-nickel phosphide composites（ZnCoNiP-D）.The polymetallic centers provide abundant active sites for the materials,and the synergistic effect between Zn,Co and Ni improves the electrochemical properties of the materials.The nanosheet structure provides enough space for the electrolyte and ensures that more active sites are in full contact with the electrolyte ions.In addition,phosphating treatment effectively improves the electrical conductivity and electrochemical activity of the material.Thus,the obtained ZnCoNiP-D electrode showed a high capacity of770.4 C g^-1 at 1 A g^-1,good rate performance(466.7 C g^-1 at 20 A g^-1)and capacity retention of81.8%after 5000 cycles.The assembled HPC//ZnCoNiP-D asymmetric supercapacitor exhibited an energy density of 40.6 Wh kg^-1at 266.6 W kg^-1 and maintained a specific capacitance of 85.0%after 5000 cycles.（2）Metal-organic framework（Co-MOF）was used as self-sacrificing template for the preparation of phosphorus-doped cobalt-nickel-selenide(CoNi_1.5Se-P)with hollow structures by proton etching and two-step calcination.The hollow porous structure of the material facilitates ion diffusion and charge transfer,which can alleviate the volume change during the charging and discharging process.Due to the synergistic effect between phosphorus and other elements,the electrochemical performance of phosphorus-doped cobalt-nickel-selenide is superior to that of cobalt-nickel-selenide and cobalt-nickel-phosphide.The optimum electrode material CoNi_1.5Se-P was obtained by tuning the cobalt-nickel ratio.Thus,the obtained CoNi_1.5Se-P electrode shows a high capacity of 838 C g^-1 at 1 A g^-1(685 C g^-1 at 20 A g^-1)and exhibits an excellent capacity retention of 99.6%after 10,000 cycles.Asymmetric supercapacitors were assembled using CoNi_1.5Se-P and porous carbon NPC as the positive and negative electrode materials,respectively,with an energy density of 42.44 Wh kg^-1.