Preparation And Capacitive Properties Of Graphite - Based Nanocomposite Thin Film Electrode Materials

Graphene (RGO) is ideal electrode material for electrochemical capacitor due to its good electrical conductivity, high specific surface area, and high mechanical strength. However, the lower specific capacitance and the serious agglomeration during the charge-discharge process limit its wide applications in many fields. Therefore, it is very important significance to research and develop RGO-based electrode materials with higher specific capacity and long cycle life. In addition, with the wide application of micro-electromechanical systems, the wireless sensor networks, portable electronic devices, electronic skin, and flexible displays, energy storage device have been widely required. For energy storage devices, they not only show high energy power density, high power density, and long cycle life, but also meet the safe, ultrathin, light weight, low-cost, and miniaturization. Therefore, the flexible electrode materials with good capacitance are expected to be designed and prepared.In this thesis, graphene-based hybrid film electrodes are prepared by a filtration-directed self-assembly method. Full thesis includes four chapters. Chapter 1 (Introduction) reviews the research progress and challenge of electrochemical capacitor, especially discuss the structure, property, application, and improvement methods of the graphene, manganese dioxide and polypyrrole based electrode materials. Chapters 2 and 3 (Experiment part) are respectively prepared the graphene-MnO2 hybrid film and graphene-MnO2-PPy ternary hybrid film electrodes by a filtration-directed self-assembly method, and the structure, morphology and capacitance property of the prepared hybrid film electrodes are investigated. The research conclusion is finally presented in chapter 4.The main research works are as follows:(1) [RGO/MnO2 NS]10 and [RGO/MnO2 NA]10 hybrid film electrodes are prepared by a filtration-directed self-assembly method, the film structures and its electrode performance are systematically investigated. RGO nanosheets and MnO2 NS are uniformly embedded into the RGO matrix to form interactive laminated nanostructure, which offers ion pathways for fast electrolyte diffusion. By using a three electrode system, the capacitance property of [RGO/MnO2 NS]10 and [RGO/MnO2 NA]10 hybrid film electrodes is characterized at a potential window of -0.2-0.8 Vin 1 M Na2SO4 electrolyte. The [RGO/MnO2 NS]10 film electrode obtained with a mass ratio of RGO/MnO2 NS= 2 in each layer shows not only high specific capacitance of 446 F g-1 at 5 mV s-1, but also good cycling performance. After 1000 consecutive cycles with a scan rate of 20 mV s-1, its specific capacitance retains about 96% of its original capacitance. Suggesting that spherical morphology of MnO2 are uniformly embedded into the RGO layers, the middle to form multilevel channel structure promotes the diffusion of electrolyte ions, thus improve the capacitance property of the hybrid thin film electrode materials.(2) On the basis of the preparation of [RGO/MnO2 NS]10 hybrid film electrode in Chapter 2, [RGO/MnO2 NS+PPy]10 ternary film electrode with high capacitance and good flexibility is prepared by improving filtration-directed self-assembly technique by using the good conductivity and forming-film property of PPy (Polypyrrole) and RGO and high capacitance of MnO2 and PPy. The ternary films are characterized in terms of structure, morphology and electrochemical properties. PPy and MnO2 NS are uniformly embedded in RGO matrix structured by RGO nanosheets, decreasing the agglomeration of PPy and MnO2 NS and the reassembling of RGO nanosheets. The introduce of RGO and PPy into the binder-free electrode improves its conductivity, ion diffusion rate and charge-transfer resistance, thus making the [RGO/MnO2 NS+PPy]10 ternary film electrode show high capacitance and good flexibility. The [RGO/MnO2 NS+PPy]10 ternary film electrode with a mass ratio of RGO/MnO2/PPy=2:1:2 delivers a highest specific capacitance of 682 F g-1 at a scan rate of 5 mV s-1 and good cycling stability with about 86% retention of its original capacitance after 1000 consecutive cycles. The superior performance of [RGO/MnO2 NS+PPy]10 ternary film electrode is probably ascribed to the novel film structure, the synergistic effect among PPy, RGO and MnO2 NS and the high packing density of PPy and MnO2 NS, which improves the maximum utilization of each component in ternary film and increases its specific capacitance and film-forming ability.From above experimental results, it can be seen that graphene-based hybrid film electrodes with different composition, structure and binder-free can be easily preparaed by improving filtration-directed self-assembly technique. The prepared hybrid film electrodes not only show good capacitance, but also have better forming-film property. These superior performances are ascribed to their novel film structure, the synergistic effect among each component and their packing density, they can improve the maximum utilization of each component in hybrid films and increase their specific capacitance and film-forming ability. This filtration-directed self-assembly method can also be used for the preparation of RGO and other metal oxide or conducting polymers hybrid film electrodes with good capacitance.