Preparation And Electrochemical Performance Of Co₃O₄/Graphene And Co（OH）₂/Graphene Composites

In recent years, supercapacitors have attracted growing interest as energy storage system, which features higher power density than batteries, higher energy density than traditional capacitors, fast charge and discharge rate and excellent long-term cyclability. In supercapacitors, active materials are the most crucial factor governing the electrochemical performance. Generally, the electrode materials of supercapacitors are of three types, carbon materials, conducting polymers and transition metal oxide/hydroxide. Each of them has its own advantages and disadvantages. Carbon materials have outstanding electrical properties, but small double layer capacitance. Transition metal oxide/hydroxide materials have high specific capacitance but poor electronic conductivity and cycle stability. Therefore, binary composites for use in supercapacitors have been extensively synthesized and investigated with the goal of obtaining materials with a combined and balanced merit of this two kinds of materials.This thesis demonstrates a simple hydrothermal route to prepare the composites of Co(OH)2/reduced graphene oxide(Co(OH)2/RGO) and Co3O4/reduced graphene oxide nanosheets(Co304/rGONS). The composites of Co3O4/reduced graphene oxide nanosheets (Co3O4/rGONS) have been prepared via a facile hydrothermal route followed by calcination. The effect of the amount of CoCl2·6H2O, hydrothermal temperature and time on electrochemical properties of the Co(OH)2/graphene composites is studied. Under the condition of optimized hydrothermal synthesis, in which the amount of CoCl2·6H2O is3mmol, the hydrothermal time is8h and the hydrothermal temperature is95℃, the prepared Co3O4/rGONS composites show high specific capacity and great redox performance in comparison with pure CO3O4. The measured values of specific capacitance of Co3O4/rGONS are all more than400F/g when the current densities range from0.5to2A/g. Its specific capacitance reaches up to444F/g at the current density of0.5A/g. Furthermore, the composite also shows good cyclic performance and coulomb efficiency. The specific capacitance of Co3O4/rGONS still retains56%after1500consecutive charge-discharge cycles and the coulomb efficiency is found to be more than90%. The enhanced properties of the material are mainly attributed to its special mesoporous structure and the synergistic advantage between Co3O4and reduced graphene oxide nanosheets. For the preparation of Co(OH)2/RGO, ascorbic acid is used as reducing agent to deoxidize graphene oxide. Factors such as the amount of CoCl2·6H2O, hydrothermal time and temperature were studied, and the comparison was made between the Co(OH)2/RGO composites and pure Co(OH)2. The electrochemical results demonstrate that the composite prepared under the condition of optimized hydrothermal synthesis exhibits the best electrochemical activities. The specific capacitance of the Co(OH)2/RGO electrode decreased from591to476F/g when the current density increased from0.5to2A/g. In contrast, the specific capacitance of pure Co(OH)2is only433F/g at the current density of0.5A/g. This suggests that the introduction of graphene significantly enhances the specific capacitance. The specific capacitance still keeps270F/g after1200cycles at the current density of2A/g, suggesting excellent stability of the Co(OH)2/RGO composite.