Research On The Preparation Of MOFs And The Performance Of Supercapacitors

With the gradual improvement of people’s life in the world,the speed of economic and scientific and technological development is also gradually accelerated,the consumption of energy is faster and faster,and many energy sources are facing exhaustion.Therefore,the research and development of renewable clean energy and electrochemical energy storage devices have become the primary problems faced by all countries in the world.Supercapacitor is a promising energy storage device due to its fast charging and discharging speed,long cycle life and high power density.Its electrode material determines its electrochemical performance.Among the electrode materials of supercapacitors,transition metal oxides have excellent electrochemical performance and are suitable to be used as pseudo capacitor electrode materials.Among them,nickel oxide and cobalt oxide based materials are considered to be very promising electrode materials because of their excellent electrochemical stability and high theoretical specific capacitance,as well as their advantages of low toxicity,low price and wide range of sources.Metal-organic Frameworks,also known as MOFs,are porous materials composed of Metal ions and Organic ligands.They have the characteristics of adjustable pore structure and high specific surface area,so they are the materials that people pay close attention to.MOFs can be converted into corresponding metals or metal oxides and carbon materials by means of pyrolysis,solvothermal and other methods.At the same time,introducing heteroatoms into the ligands for in-situ doping is an effective strategy to improve the electrochemical performance.Introducing ligands containing N or P into original MOFs is considered as a direct method to synthesize carbon-based materials containing N or P or metal oxides/carbon composites.In this thesis,Co-MOF and Ni-MOF were synthesized by solvothermal reaction with metal salts.A variety of heteroatomic co-doped composites were prepared by one-step pyrolysis.At the same time,the electrochemical performance of the composite material applied to the electrode material of supercapacitor was systematically investigated.The main research results are as follows:1.The ligand CTP-COOH was synthesized with hexachlorocyclotriphosphazene and ethylparaben as raw materials in the condition of anhydrous and anoxic conditions.CoMOF was synthesized with cobalt nitrate hexahydrate through simple solvothermal method,and the Co/C composites co-doped with N,P and O were obtained by next pyrolysis at different temperatures.After the morphology and other characteristics of the composite material,it was applied to the electrode of the supercapacitor.The best performance was found to be the Co /C-650 material with a specific capacitance of 739.6F/g at 1A /g current density.At the same time,80.6% capacity was maintained after 5000 cycles.The Co/C-650 electrode and activated carbon electrode were assembled into an asymmetric supercapacitor(ASC),and the energy density was found to be as high as 30.4Wh/kg.In the cycle performance test,the ASC still had a capacity retention rate of nearly71.3% after 10000 cycles at A current density of 5 A/g.The experimental results show that MOFs prepared by CTP-COOH can be calcined simply by one-step method to obtain high capacity N,P,O co-doped materials,and improve the utilization efficiency of Co active components.2.The same ligand CTP-COOH and nickel nitrate hexahydrate were mixed with GO to synthesize Ni-MOF@r GO by a simple solvothermal method,and then the heteroatom co-doped Ni/C/r GO composites were prepared by pyrolysis at 550℃.Three kinds of composites,Ni/C/r GO-2,Ni/C/r GO-4 and Ni/C/r GO-8,were obtained by changing the amount of GO added.Through electrochemical test,it is found that the composite materials have better performance than single Ni/C,among which,Ni/C/r GO-4 has the best electrochemical performance: 1258.7 F/g high specific capacitance at 8 A/g current density,and 110% capacity retention after 50000 cycles.Ni/C/r GO-4 //AC asymmetric supercapacitor was assembled,and the maximum energy density was found to reach79.7Wh /kg.In the cycle performance test,when the current density was 2A/g,the capacity retention rate of ASC was nearly 83.7% after 8000 cycles,indicating its excellent stability.