| Exploring new electrode materials is a critical task in building high-performance pseudocapacitors in order to satisfy the ever-growing demand for portable electronics devices and electric/hybrid vehicles. Normally, electrode materials of pseudocapacitors including transition metal oxides (MnO2, RuO2, NiO, Co3O4, CoO, CuO, etc) or hydroxides (Ni(OH)2, Co(OH)2, etc.), as well as electrically conductive polymers (polyaniline, polythiophenes and polypyrroles) generally employ a reversible faradaic redox reaction at the electrode surface leading to a high capacitance. Therefore, the performance of pseudocapacitors mainly depends on the electrochemical activity and kinetics of the electric materials. It is important to enhance the kinetics of the ion and electron transports in the electrodes and at the electrode/electrolyte interface. Among transition metal oxides, CoO is considered to be ideal due to its excellent chemical and physical properties, such as its high theoretical capacitance, environmental friendliness, and good easy processing. Currently, CoO in one-dimensional (1D) array architecture, which is directly grown on the collectors, has been widely investigated. This architecture allows a high porosity associated with the open nanostructures, as well as favourable orientations, which benefit for high efficiency of ion and electron transport in the electrodes and across the electrode/electrolyte interface, and therefore potentially boosts the pesudocapacitive performance. However, in many cases, the observed specific capacitance values are often far lower than the theoretical value. So, fabricating a CoO-based supercapacitor with high specific capacitances is still a challenge. As far as we understand, there are many advantages of metal core-oxide shell supercapacitors electrode, such as high rate capability and cycling stability, high power and energy density, there have been few reports on integrating metal core with CoO. In addition, hexagonal honeycomb graphene with sp2 hybrid composition film is a single layer of carbon atoms, it is a two-dimensional plane structural material of the thickness of a carbon atom. Graphene has superior stability and mechanical properties, high thermal and electrical conductivity. In recent years, graphene has attracted much attention in the research of the supercapacitor with its excellent electrical properties and unique structure. In this thesis, we design and successfully prepared porous Co/CoO core-shell composites and three-dimensional Co/CoO/graphene composites, and explored their supercapacitor performance. The main research work and innovations are as follows:(1) Interconnected continuous nanoporous Co/CoO core-shell composites on nickel foam were successfully obtained by hydrothermal method, followed by annealing the precursors in H2 atmosphere. We used XRD, SEM, TEM, HRTEM, HAADF-STEM-EDS, XPS and electrochemical method to investigate its crystal structure, morphology and electrochemical performance. The designed Co/CoO core-shell nanostructure possess following advantages:(â…°) Compared with other studies, Co/CoO core-shell nanostructure with a metal core enhanced connection and electrical contact between the electrode material and the current collector, facilitating the transport of electrons. (â…±) Structure for 5 nm thin CoO covered as shell, thinner than 20 nm-30 nm shell of other studies, resulting in a small depth of the reaction and the active material was fully reacted at a high current density. (â…²) Compared with the complex hydrothermal and electrochemical growth methods, the fabricate process of core-shell structure is simple. The desired Co/CoO core-shell structure was obtained only by hydrogen reduction of Co and placed in the air for oxidation. The Co/CoO core-shell bring low resistance, high specific capacitance of 5.632 F/cm2 and good cycle stability (81.5% capacitance retention after 6000 cycles). An asymmetric supercapacitor was assembled using the optimized Co/CoO core-shell nanostructure as positive electrode and activated carbon (AC) film on nickel foam as negative electrode. This electrode offers excellent energy density of 0.00452 W/cm3 at power density of 0.00823 W/cm3, After 10000 charge/discharge cycles, the total capacitance of the asymmetric super capacitor is still maintained at a rate of 73.94%. Such outstanding behaviors of the Co/CoO core-shell nanostructure is much higher than that of pure CoO nanowire material.(2) Graphene has excellent properties such as high specific surface area and good electrical conductivity. Based on the results of part (1), a layer of graphene was coated on the surface of Co/CoO core-shell nano-structure to form Co/CoO/graphene. The designed Co/CoO/graphene nanostructure material possess following advantages:(â…°) The method of in situ electrodeposition of graphene on the Co/CoO electrode strengthened the connection and electrical contact between graphene and Co/CoO. (â…±) Compared with the previous methods, the method of Hummer and electrochemical in-situ deposition of graphene nano-sheet is simple, low cost and controllability. (â…²) The conductive properties of the Co/CoO electrode materials were strengthened by the addition of graphene. Hummer method was used to prepared graphene oxide, and then in situ electrochemical method was used to deposit graphene nanosheet on Co/CoO. The composition, morphology and electrochemical properties of the obtained Co/CoO/graphene samples were characterized by Raman, XRD, SEM TEM, and electrochemical method. The effect of electrodeposition graphene cycles on the electrochemical properties of Co/CoO/graphene composite were investigated. The results show that the coating of graphene on Co/CoO further enhances the conductivity of the material, and the specific capacitance is 6.439 F/cm2 at the current density of 1 mA/cm2 is achieved. The Co/CoO/graphene and AC were assembled into an asymmetric supercapacitor. After 10000 charge/discharge cycles, the total capacitance of the asymmetric supercapacitor is still maintained at a rate of 79.94%. The coating of graphene not only increases the conductivity of the material, but also makes the structure more stable. The Co/CoO/graphene composite material has excellent electrochemical properties and is an ideal material for the supercapacitor. |
PDF Full Text Request