Preparation And Electrochemical Properties Of Metal-organic Framework Derived Nanocomposites

As a king of energy storage devices,supercapacitors have attracted wide attention due to their advantages such as high power density,good cycle life and fast charge and discharge rate.The electrode material is the key to determine the storage capacity and conversion efficiency of a supercapacitor.Metal-organic frameworks（MOFs）are a new class of porous crystalline materials,which are formed by metal center and organic ligands by strong covalent bonds.Due to high specific surface area,controllable morphology,adjustable pore structure,MOFs have become one of the most promising electrode materials for supercapacitors.In this thesis,several Ni-and Co-based MOFs derived/composite materials with special morphology were obtained by using Ni-and Co-based MOFs as templates and structure guide agents and influence of the morphology,structure,the charge storage mechanism on the performance of the resulted supercapacitors were discussed.Thesis included three parts as follows:1.Considering the high electrical conductivity of carbon dots（CDs）and good oxidation-reduction property of polyoxometalates（POMs）,a novel CDs coated POMs/metal organic frameworks（POMOFs）composition structure,CDs@PMo₁₂/Ni-MOF,was prepared by hydrothermal reaction.PMo₁₂ was encapsulated in the Ni-MOF framework to form the POMOF composite material,which effectively prevented erosion of water-soluble[PMo₁₂O₄₀]^3-(PMo₁₂)in the electrolyte.By experimental condition optimalization,it was found that 6CDs@PMo₁₂/Ni-MOF possessed optimal electrochemical performance with the biggest specific capacitance of 1479.6 F·g^-1 at 1A·g^-1 and high rate of 75.9%at 10 A·g^-1.Furthermore,the hybrid supercapacitor device was assembled using 6CDs@PMo₁₂/Ni-MOF as positive electrode and activated carbon（AC）as negative electrode,which showed an high energy density of41.6 Wh·kg^-1under 800 W·kg^-1 and excellent cyclic stability(only 1.6%specific capacitance loss after 20000 charge-discharge cycles at 5 A·g^-1).2.Hollow structure of transition metal sulfides with particular structural defect can expose abundant active sites and shorten electron/ion diffusion paths,improving the conductivity of the resulted materials.In this study,an effective strategy was proposed for preparing r-NiCo₂S₄ electrode materials with sulfur vacancies and hollow structures by employing ligand exchange followed wet chemical reduction.Due to the unique structure,the prepared r-NiCo₂S₄ electrode exhibited excellent specific capacitance of 1406.4 F·g^-1 at 1 A·g^-1 and good rate capacity with a specific capacitance retention of 90%when the current density increased 10 times.Additionally,a hybrid supercapacitor r-NiCo₂S₄//AC HSC was assembled using r-NiCo₂S₄ as the positive electrode and AC as the negative electrode.It presented an excellent energy density of 43.8 Wh·kg^-1 under 800 W·kg^-1 and cycling performance with 93.9%capacity retention after 10000 cycles at 5 A·g^-1.3.A metal porous carbon（Ni@NC）supported nickel/cobalt layered double hydroxide（NiCo-LDH）（Ni@NC@NiCo-LDH）with a cauliflower morphology was synthesized by a successive double template method.At first a nickel metal-organic framework（Ni-MOF）was prepared and used as a sacrifice template to produce metal porous carbon（Ni@NC）.Then the prepared porous carbon material was exploited as a substrate for in-situ growing of ZIF-67 on its surface.Lastly the obtained Ni@NC@ZIF-67 was further used as a sacrifice template to prepare the final electrode material Ni@NC@NiCo-LDH by Ni²⁺etching and co-precipitation.The cooperation of Ni@NC with excellent conductivity and NiCo-LDH with superior pseudocapacitive property yielded a synergistic effect,which effectively improved the electrochemical performance of the resulted electrode material.It is just the special flower morphology that exposed more redox active sites and provided proper charge transport path for enhanced electrochemical performance.The prepared Ni@NC@NiCo-LDH exhibited the excellent specific capacitance of 1761.8 F·g^-1 at the current density of 1 A·g^-1.The assembled Ni@NC@NiCo-LDH//AC hybrid supercapacitor also displayed an acceptable energy density(39.3 Wh·kg^-1 at the power density of 757 W·kg^-1)and ultrahigh cycling stability(94.7%capacitance retention after 25000 cycles at 10 A·g^-1).