Fabrication Of The Porous Nanocomposite Electrode Materials Derived From Metal-organic Frameworks And Their Supercapacitive Properties

In order to solve the urgent global problem in the wide use of fossil fuels and consequent lack of fuel and environmental pollution,the development of various energy storage and conversion systems has been the subject of recent attention.The advantages of high power density,high efficiency and long life expectancy have made electrochemical supercapacitors one of the major emerging devices for energy and power supply.Nanoporous composites are widely used as electrode materials for supercapacitors,owing to the huge specific surface area,good conductivity,and high chemical stability.The preparation method mainly includes sol-gel,nano casting and template method.Compared with traditional methods,Metal-organic framework(MOFs)and theirs derivative,emerging as a new class of porous crystalline materials,which was assembled by two main components of inorganic vertices and organic linkers have been used as electrode materials or precursors of metal oxide or oxide materials/carbon electrode materials for supercapacitors.As obtained hierarchical MOFs-based hybird materials can be used as supercapacitor electrodes to favor fast ion and electron transportation.The investigation will be emphasized on the relationship between their structure and electric capacity properties.Tuning the porosity and studying the effect of the pore size on capacitance could potentially improve performance.The main research content is as follows:A hierarchically porous carbon microsphere material was prepared by direct carbonization of metal phenolic precursor,which was supramolecular self-assembled by zinc(Zn)ions coordinated to ellagic acid(EA).The microstructure and electrochemical properties of as-obtained carbon microspheres were studied by TG,FTIR,Raman,XRD,SEM,TEM and electrochemical test.The results show that carbonization temperature plays an important role in determining the structure,specific surface area and electrochemical performance of the carbon microsphere electrode materials.Carbonization at 1000 ?under nitrogen produced a micro-mesoporous carbonaceous microsphere material exhibiting lamellar graphene-like structure and high specific surface area of 1238 m2 g-1,and achieved a specific capacitance of 216 F g-1 at a scan rate of 5 mV s-1 in 6 M KOH electrolyte.When the scan rate increased from 5 mV s-1 to 100 mV s-1,the specific capacitor remained 85%.Moreover,the porous carbon showed less than 3%decay in specific capacitance values over 5000 cycles at a current density of 1 A g-1.The porous carbon microsphere material has excellent electrochemical performance as an electrode material for supercapacitor.Hollow Co3O4 nanododecahedral N-doped carbons(H-Co3O4@NC)were prepared by applying metal-organic frameworks(MOFs)as precusors.Zeolitic imidazolate framework(ZIF-67)dodecahedrons were reduced to produce Co/NC composite,following by oxidized to produce H-Co3O4@NC composite,in which the hollow dodecahedrons were uniformly covered with a NC layer.The Co-MOFs transformed into hollow dodecahedrons during oxidation via the Kirkendall diffusion process.The unique hollow composite exhibited excellent electrochemical performances when used as electrode material in supercapacitors.The highest specific capacitance of 666.63 F g-1 for H-Co3O4@NC-500/250-10 composite electrode material is achieved at the scan rate of 5 mV s-1 in the 6 M KOH electrolyte.The capacitance retention rate retained 99.53%even after 2000 cycles at a current density of 2 A g-1.An asymmetric capacitor(ASC)incorporating the H-Co3O4@NC as the positive electrode and the activated carbon(AC)as the negative electrode is fabricated.A maximum energy density of 41.30 Wh kg-1 is obtained with a power density of 500 W kg-1 at a current density of 1 A g-1 in a potential window of 1.4 V.The device maintained a capacitance retention rate of 78.41%after 500 cycles at 1 A g-1 current density.The UiO-66/rGO composites with hierarchical pore structure are prepared by one-step solvothermal method using zirconium tetrachloride as the zirconium source,terephthalic acid as the organic ligand and graphene oxide as the precursor.The effects of different mass fractions of graphene oxide and reaction temperature on the phase structure,morphology and electrochemical properties of the composites are investigated.The result showed that the optimized UiO-66/rGO-8(100?)composite material delivered the highest specific capacitance of 506 F g-1,5 mV s-1 at 6 M KOH electrolyte when mass fraction of graphene oxide is 8%,the reaction temperature is 100?.An asymmetric capacitor(ASC)incorporating the UiO-66(Zr)/rGO-8(100?)as the positive electrode and the activated carbon(AC)as the negative electrode is fabricated.A maximum energy density of 27.22 Wh kg-1 is obtained with a power density of 350 W kg-1 at a current density of 0.5 A g-1,and the ASC also retained respective high specific energies of 15.75 Wh kg-1 at high specific powers of 3500 W kg-1.The device maintained a capacitance retention rate of 96.73%after 1500 cycles at 1 A g-1 current density.