The Fabrication Of Co₃O₄ Polyhedra Composite Electrodes For Supercapacitors

Currently,it is urgent to upgrade the power sources and industry with the continuous process of the industrialization in China.The supercapacitors are novel energy storage devices which have a higher energy density than electrostatic capacitor and a higher power density than battery.Electrode materials decide the energy storage performance of supercapacitors.Compared with the carbon materials storing energy by electrostatic adsorption,the transition metal oxides store energy through not only the electrochemical double electric layer,but also the rapid reversible redox process in their surface and bulk phase,resulting in a higher mass capacitance and energy density.Therefore the transition metal oxides are considered to be promising electrode materials for supercapacitors.In addition,the studies have shown that the structure of electrode materials play an important role in their electrochemical performance.Metal organic frameworks(MOFs)are materials composed of organic ligands and metal ions/cluster with intramolecular space.Porous metal oxides with a large surface area can be obtained after high temperature calcination of MOFs and have been applied in the catalysis,energy storage,sensors,drug delivery and other fields.Among the various transition metal oxides,Co3O4 has attracted much attention by researchers because of its high theoretical specificitic capacitance,low cost and environmental friendliness.However,the conductivity of Co3O4 is poor,which affects its electrochemical performance and cycle life.Doping with conductive carbon materials is one of the effective ways to improve its electrochemical performance.In this paper,the zeolite imidazole framework(ZIF-67)was synthesized by using 2-methylimidazole as ligand and cobalt nitrate as metal source.The porous Co3O4 polyhedra(PCP),the PCP and reduced graphene oxide composites(PCP/RGO),and the PCP and carbon nanotube composites(PCP/CNTs)were prepared by subsequent calcination of the precursors.The influence of the experimental conditions on the morphology,composition and properties of the product was studied.A series of composite materials with excellent electrochemical properties were prepared by optimizing the synthesis conditions.The main work is summarized as follows:1.ZIF-67 precursor was synthesised by liquid-phase deposition and then pyrolysized in air atmosphere to obtain PCP.The effect of calcination temperature on its morphology and electrochemical performance was studied.PCP maintains the morphology of the ZIF-67 precursor at 300 ? and 350 ?,but displays a heavy collapse at 400 ?.The specific capacitances of PCP calcined at diferent temperatures were tested at 1 Ag-1,and a maximum capacitance of 383 F g-1 with cycling stability can be achieved for the PCP calcined at 350 ?.2.ZIF-67/graphene oxide(GO)precursor was synthesised by one-step liquid-phase deposition and doping with GO,and then the PCP doped with different RGO contents was prepared by subsequent high temperature reduction and thermal decomposition of ZIF-67/GO under N2 and air atmosphere.At the same time,the effects of different RGO doping contents on the electrochemical performances of the composites were studied.When the doping amount of GO is 2%,a "point-to-face" 3D structure was formed,where PCP particles were uniformly adhered onto the surface of RGO.A specific capacity of 539 F g-1 at the current density of 1 A g-1 was obtained,which is much higher than that of the pure Co3O4 electrode.3.ZIF-67/CNTs precursor was synthesised by one-step liquid-phase deposition and doping with CNTs,and then the PCP doped with different CNTs contents was prepared by subsequent heat treatment of ZIF-67/CNTs.Meanwhile,the effects of different CNTs doping contents on the electrochemical performances of the composites were studied.It is found that an interpenetrating structure,similar to"beads",of well-distributed CNTs and PCP was formed when the CNTs doping content was 2%.And this PCP/CNTs electrode exhibits a high specific capacity of 592 F g-1,higher than those of pure Co3O4 electrode and PCP/RGO composite.