Preparation And Capacitive Properties Of Graphene / Tetraxide Cobalt Nano Electrode Materials

Electrochemical capacitor has been considered to be a promising energy storage device due to its attractive properties such as high power density, high energy density, low cost and so on. Research results indicate that the electrode material is one of the most important factors determining the capacitive property of the electrochemical capacitor. Therefore, it is of great importance to develop new electrode materials with excellent electrochemical properties. As a kind of electrode material with the theoretically high specific capacitance, CO3O4has attracted the wide attention. However, CO3O4not only has low electronic conductivity, but also has the large volume expansion/contraction during the charge-discharge process, which makes CO3O4electrode suffer from a poor electrochemical stability and largely limits its practical application in the electrochemical capacitor. To overcome these disadvantages, CO3O4is usually combined with graphene which has the good electrical conductivity, and the synergistic electrodes with high capacitance and good cycle stability are prepared. In the present work, graphene/Co3O4electrode materials have been prepared by a nanosheet assembled reaction between the exfoliated Co(OH)2nanosheets and graphene oxide nanosheets and the subsequent thermal treatment of graphene oxide/Co(OH)2nanocomposites. The structure and morphology of the obtained materials have been thoroughly characterized by a series of testing technologies, and the electrochemical properties of the obtained materials have also been systematically investigated in a three-electrode test system.The thesis mainly includes three sections, introduction, experiment and conclusion. Chapter1reviews the research progress and existing problems of electrode material. The structure, preparation methods and the appplications of graphene, CO3O4and graphene-based nanocomposite materials are also reviewed. In the experimental part (chapter2to chapter4), α-Co(OH)2with layered structure is firstly prepared by a solvothermal treatment, and its ion intercalation reaction is investigated. On the basis of the intercalation reaction, α-Co(OH)2with layered structure is exfoliated into Co(OH)2nanosheets in the chapter2. In the chapters3and4, graphene/Co3O4electrode materials have been prepared by a flocculation technique between graphene oxide nanosheets and CO3O4nanosheets and the subsequent thermal treatment, and the electrochemical properties of RGO/CO3O4electrode materials have been investigated. Finally, the research results are conclused in chapter5.The main research works are as follows:α-Co(OH)2with high crystallinity and well-defined morphology is firstly prepared by a solvothermal treatment at130℃for3h. On the basis of the investigation of the reaction temperature and time for the ion intercalation and phase transform, ClO4--a-Co(OH)2is obtained by an ion exchange reaction. The decarbonated ClO4--α-Co(OH)2is obtained by carefully degassed deionized water washed and dried at N2atmosphere. Finally, the decarbonated is dispersed into formamide with N2purging, followed by ultrasonic treatment, the exfoliated Co(OH)2nanosheets are obtained. The GO/Co(OH)2nanocomposites with different mass ratios can be prepared by an assembled reaction between the exfoliated Co(OH)2nanosheets and RGO nanosheets. By calcinating GO/Co(OH)2layered nanocomposites with different mass ratios in a muffle furnace at suitable temperature for2h and followed by thermal annealing in tubular furnace with Ar atmosphere, the RGO/CO3O4nanocomposites with different mass ratios are obtained. The obtained materials at different stages are characterized by XRD, FESEM, TEM, FT-IR, AFM and XPS analyses. The research results indicate that RGO/CO3O4(94.3%) electrode material not only shows general multilayered morphology, but also exhibits a high specific capacitance and good cycle stability. The uniform distribution of CO3O4on RGO nanosheets can greatly increase the active site for faradic redox reaction, while the existence of CO3O4nanosheets effectively prevents the restacking of RGO nanosheets, and RGO nanosheets existe as an amorphous state in the composite.The electrochemical properties of RGO/CO3O4electrode materials with different mass ratios are systematically investigated by using a three electrode cell which contains the working electrode, counter electrode (Pt foil) and reference electrode (saturated calomel electrode) in6mol L-1KOH electrolyte. For comparison purpose, the electrochemical properties of pure RGO and CO3O4electrode are also investigated, respectively. On the basis of the experimental tests of the cycle voltammetry, galvanostatic charge-discharge and electrochemical impedance spectroscopy, the research results show that at a current density of5A g-1, the RGO/CO3O4electrode with94.3%CO3O4exhibits an overall improved specific capacitance of331F g-1, as compared with RGO (91F g-1) and CO3O4(186F g-1) electrode. The electrochemical impedance and cycle stability tests show that RGO/CO3O4(94.3%) electrode has a small internal resistance and diffusion resistance, and its capacitance retention is still122%after5000continuous charge-discharge cycles.