Preparation And Electrochemical Properties Of Porcelain / Manganese Dioxide Composites

Graphene is a unique carbon material with potential for electrochemical energy storage device applications due to its high thermal and chemical stabilities, large theoretic surface area, and high electrical conductivity. It can be used to improve the electrical conductivity of the electrode materials by improving the electron transfer from the graphene nanosheets. However, the capacitive behavior of pure RGO is much lower than its theoretic value because of the agglomeration during preparation. In order to improve the low power output of the graphene electrodes, graphene nanosheets with holey structure have been attracted, and the holey structure provides a high density of porosity to facilitate charge and ion transport. Manganese dioxides are one of the most promising pseudocapacitance electrode materials due to their high energy density, ease of preparation, low cost and abundant resources. a-MnO2has a hollandite-type structure, its (2x2) tunnels can be comprised by double chains of [MnO6] octahedra. However, the bulk manganese oxides have a small specific surface area and low electrical conductivity, which largely limit their application as the electrode materials of electrochemical capacitors. Therefore, graphene/manganese oxide hybrid electrode materials with high specific capacitance and high electrical conductivity are expected to be obtained.In this thesis, the graphene with the holey structure and α—MnO2with needle morphology are selected as the assembeled units, the graphene/a-MnO2hybrid nanocomposite with holey structure is prepared by the preparation conditions which are optimized. Also the electrochemistry property of the HRGO/α-Mn02hybrid nanocomposite has been investigated. This thesis mainly consists of five chapters. Introduction part (Chapter1) reviews the structure, property and the preparation method of graphene with holey structure, manganese dioxide, and graphene/manganese dioxide hybrid nanocomposites. Experimental parts (Chapters2and3) discuss the preparation and characterization of both graphene/manganese dioxide hybrid nanocomposite and graphene/manganese dioxide hybrid nanocomposite with holey structure on the basis of the preparation of graphene with holey structure. In chapter4, the electrochemical properties of the obtained material electrodes have been investigated. The research conclusion is finally presented in chapter5. Graphite oxide (GO) is firstly fabricated from crude flake graphite by a modified Hummer’s method. The as-prepared GO is then treated by ultrasonication treatment in a water, and GO homogeneous dispersions with differential amounts are respectively obtained. The holey graphite oxide (HGO) are prepared by a wet chemical method combined ultrasonic vibration in a mixed solution of HNO3and H2SO4, and then it is reduced into graphene with holey structure (HRGO). Graphene/manganese dioxide hybrid nanocomposite with holey structure (HRGO/α-MnO2) is prepared by a hydrothermal treatment technology. The optimized preparation conditions of HRGO(1.5)/α-MnO2are by hydrothermally treating an isopropyl alcohol suspension of HRGO, MnCl2·4H2O and KMnO4at90℃. The α-MnO2particles with needle-like morphology are uniformly dispersed on the surface of graphene with flexible holey structure, which prevent the restacking of the graphene nanosheets and allow graphene nanosheets to exist in a complete exfoliation state in HRGO(1.5)/α-MnO2hybrid material. The self-assembly between HRGO nanosheets and α-MnO2particles can increase the specific surface area of the prepared material. The specific surface area of HRGO(1.5)/α-MnO2is about196m2g-1, which is much higher than HRGO (127m2g-1) and α-MnO2(115m2g-1).The electrochemical properties of HRGO/α-MnO2hybrid electrode have been investigated by cyclic voltammetry (CV), galvanostatic charge-discharge test, and the electrochemical impedance spectroscopy (EIS). And they are performed at a potential window of-0.2-0.8V in1mol LT1Na2SO4electrolyte. The prepared HRGO(1.5)/α-MnO2electrode exhibits a high specific capacitance of348F g-1at5mV s-1with a capacitance retention of86%after1000cycles at a constant current density of10A g-1. These results show that a holey treatment and self-assembly between HRGO nanosheets and α-MnO2particles not only increase the specific surface area of the prepared materials, but also improve the transfer of the ions and electrons. The HRGO nanosheets in the hybrid electrode increase the utilization efficiency of α-MnO2and the holey structure provides a high density of porosity to facilitate charge and ion transport.