Preparation Of Metal Organic Frameworks Based Nanocomposite Materials And Their Application In Electrochemical Energy Storagre

Due to high power density and quick charging-discharging property,supercapacitor?SC?has been becoming an active investigation subject.However,relatively low energy density severely limits its practical application.High energy density can be obtained mainly by increasing specific capacitance and enhancing the potential range.In general,a larger specific capacitance can be achieved usually by improving the electrochemical performance of the positive electrode material,while a larger potential window can be obtained usually by assembling an asymmetric supercatacitor.In recent years,metal organic frameworks?MOF?have been extensively studied due to its designable structure,adjustable pore size,and large specific surface area.The electrolyte ions can quickly pass through the pores during the process of charge and discharge due to the controlled pore size and suitable pores of the MOF material.The metal cations?cluster?of MOF materials can provide redox active sites for electrode reactions,so MOF materials have been widely used as electrode materials.However,poor electrical conductivity induced by the organic ligands and unstable structure reduce their application performances.Thus,it is necessary to improve the structural stability and electrical conductivity.In this thesis,three novel MOF based nanocomposites were designed and synthesized,and their electrochemical performancs were investigated.Some asymmetric supercapacitors were assembled with the MOF based composites as positive electrode,and their practicability was further investigated.This work was divided into three parts as follows:?1?A Ni-MOF was prepared by one-step hydrothermal method using 3,5-pyrazoledicarboxylic acid as organic ligand and nitrogen source,and then Co²⁺was doped to form Ni/Co-MOF.Finally,nitrogen-sulfur co-doped bimetallic nanocomposites?NS-BNs?were prepared by vulcanizating and annealing the precursor Ni/Co-MOF.The designed and prepared NS-BNs material exhibits a more stable structure and richer active sites.As a result,it shows very excellent electrochemical performance.The specific capacitance could reach 1529 F·g^-1 at a current density of 1A·g^-1,and the superor specific capacitance can still reach 1069 F·g^-1 even after the current density is expanded ten times.Furthermore,the assembled asymmetric supercapacitor with the prepare material as positive electrode displayed a high energy density of 41.04 Wh·Kg^-1 at power density of 750 W·Kg^-1,and excellent capacitance retention rate of 89.29%after 10000 cycles.?2?Mo-based metal clusters were encapsulated into the holes of the Ni-MOF frame structure by a self-assembly strategy to prepare Mo-doped Ni-MOF nanosheets?M-NMN?,which improves the conductivity and stability of Ni-MOF.The results showed that the obtained M-NMN-1?Mo/Ni molar ratio is 1:1?materials exhibited excellent electrochemical performance.The nanosheets structure of the M-NMN-1 materials acts as a“superhigh way”for charge transport,accelerates charge transfer rate and enhances the conductivity of the elecrode materials.As-prepared M-NMN-1 electrode material exhibited high specific capacitances of 1604 F·g^-11 at 1 A·g^-1 and cycle retention of 96%after 20000 cycles.Furthermore,the assembled asymmetry supercapacitor device exhibits an excellent energy density of 59 Wh·kg^-11 at a power density of 802 W·kg^-1,and superior cycle retention of 93%after 20000 cycles.?3?Co-MOF was prepared by a hydrothermal method,and then Ni-MOF was epitaxially grown with Co-MOF as seed crystal to form the Ni-MOF@Co-MOF.Finally,MOF-derived double metal hydroxide?NC-MDH?was prepared by an alkali treatment stripping strategy using the composite MOF as a precursor.The prepared NC-MDH exhibited a nanoflower structure,which facilitated the penetration of electrolyte and the transport of electrolyte ions.The nanoflower structure exposed more redox active sites to facilitate the electrode reaction.As a result,the material utilization rate and the electrochemical performance are improved.The kinetic analysis indicated that the material was an electrode material of battery properties.The specific capacitance reached 1693 F·g^-11 at current density of 2 A·g^-1,capacitance retention reached 94%after 12000 cycles.The material was used as a positive electrode material to assemble asymmetry supercapacitor.The electrochemical test exhibited a superior power density of 825 W·Kg^-1 and the energy density of 66.5 Wh·Kg^-1.After charging and discharging cycles of 18000 cycles,the capacitance retention rate and coulomb efficiencies were95%and 86%,respectively.