Preparation Of Conductive Metal Organic Framework Nanocomposites And Their Electrical Energy Storage Properties

The depletion of fossil fuels and the environmental impact of greenhouse gas emissions have led to an increasing global demand for sustainable energy supplies worldwide.In various energy systems,supercapacitors have attracted worldwide attention due to their advantages of high-power density,fast charging and discharging,and long cycle stability,which can be applied in fields such as electric power transportation,portable electronic devices,and biomedical devices.To improve the electrochemical performance of supercapacitors,electrode materials are usually required to have a large specific surface area,large pore volume,layered porous structure,and highly exposed redox active sites.In this case,conductive metal organic frameworks(cMOFs)and their composites become ideal candidates for manufacturing highperformance electrodes.However,most of the cMOFs that have been studied have lower specific capacitance and are expensive,which greatly limits their practical application.In this thesis,a branch of cMOFs,M-HHTP(M is a metal ion,HHTP=2,3,6,7,10,11hexahydroxybenzene),is studied based on the characteristics of cMOFs as supercapacitor materials.A series of cMOFs composites were constructed by means of morphology control,structural design,introduction of conductive substrates,modification with multiple metals,and solvent adjustment.The morphology,structure,composition,and electrochemical performance of the materials were characterized,and significant research results were obtained.The specific content is as follows:(1)Transition metal hydroxides are ideal materials for obtaining stable composite materials during electrochemical testing in alkaline electrolytes.Ni(OH)2 nanosheets were synthesized by hydrothermal method.Then,using HHTP as an organic ligand and Ni2+ as a metal center,a controllable morphology of Ni(OH)2 was prepared Ni(OH)2@Ni-HHTP compound material.The effects of reactant dosage,reaction time,and different morphologies of Ni(OH)2 substrates on the morphology and electrochemical properties of Ni(OH)2@Ni-HHTP were investigated.The results showed that only Ni(OH)2 nanosheets and cMOFs can be compounded.Due to the exposure of specific crystal surfaces,Ni-HHTP nanowires are grown axially on both sides of the Ni(OH)2 nanosheet.Ni(OH)2@Ni-HHTP achieves high conductivity and ion transport efficiency.At a current density of 1.0 A g-1,the specific capacitance of Ni(OH)2@Ni-HHTP was 247.5 F g-1,which was higher than the specific capacitance of Ni-HHTP(85.5 F g-1).NiHHTP@Ni(OH)2 was assembled as a positive electrode material in an aqueous asymmetric supercapacitor,98%of the initial capacitance was maintained after 5000 cycles at a high current density of 3 A g-1.(2)MXene,as a conductive material,has a 2D layered structure,high electrochemical active surface,metal like conductivity.MXene multilayers were prepared using a molten salt method,and Ni-HHTP nanoarrays were grown on MXene using a simple hydrothermal method.The growth of cMOFs reduces the atomic exposure of MXene and expands the interlayer distance,which is conducive to inhibiting the oxidation of MXene,improving the stability of the complex and expanding the interlayer distance,resulting in rapid ion transport.Compared to Ni(OH)2 nanosheets in the previous chapter,MXene was beneficial for the growth of more cMOFs,thereby improving the specific capacitance of the composite material.In three electrode system,the specific capacitance of the MXene@Ni-HHTP was 332.4 F g-1.In asymmetric supercapacitor devices,the composite material achieves a high energy density of 53.7 Wh kg-1 at a power density of 1599 W kg-1,indicating its excellent magnification performance.(3)Due to the large surface area of atomic exposure and the multiple oxidation states of the reversible Faraday reactions,LDH has excellent redox activity and flexible ion exchange ability,especially NiCo-LDH with large interlayer spacing,which exhibits excellent electrochemical performance.NiCo-LDH nanosheets were prepared by simple solvothermal method.Subsequently,a series of composite materials were synthesized by introducing MHHTP and controlling the reaction solvent.The growth of cMOFs on NiCo-LDH not only reduces the possibility of aggregation of NiCo-LDH,but also increases the active sites.At current density of 1 A g-1,the specific capacitance of NiCo-LDH@Co-HHTP-EtOH reached 830 F g-1 compared to the previous two chapters.