| Electrochemical capacitors have attracted considerable attentions to be a major concern of energy storage devices duo to their long cycle life, quickly charge and discharge, and environment-friendly. Electrode material is one of the most important factors affecting for property of the electrochemical capacitors. Graphene is considered to be the most promising electrode material for electrochemical capacitors due to its unique physical and chemical properties. However, the serious agglomeration behavior during the preparation process limits its play actual capacitance in compare with its theoretical specific capacitance. In order to prevent the aggregation bevior of graphene and improve the cpacitaive property of graphene, graphen-based electrodes with hollow structure have been widely attention. Manganese dioxide is a promising candidate for pseudocapacitive electrode material due to its low cost, relatively environmentally benign properties, and high theoretical specific capacitance. However, the bulk manganese oxides have a small specific surface area and low electrical conductivity because of their semiconducting character, which seriously limit their application as the electrode materials of electrochemical capacitors. In addition, In order to reduce the aggregation reaction of the graphene nanosheets and obtain the graphene with good electrical conductivity and high individual dispersion, some nanoparticals such as semiconductors or polymer have been intercalated into the interlayer of the graphene nanosheets, and the obtained graphene-metal oxide/or polymer nanocomposites are expected to show promising application in supercapacitor. Therefore, in this thesis, manganese dioxide/graphene and polypyrrole/graphene electrode materials with hollow nanosphere morphologies have been respectively prepared by redox-oxidation and in situ polymerization methods, and also their electrochemical properties have been investigated.The thesis includes four sections:Chapter 1 (introduction part) reviews energy storage principle, the classification of electrode materials, application and advantage-disadvantages of the electrochemical capacitors. Also the structure, property and the preparation method of manganese dioxide, graphene and polypyrrole are reviewed. In Chapter 2, MnO2-wrapped graphene hollow nanosphere (MnO2/HGR) electrodes with different mass ratios of MnO2 to HGR are prepared by a cooperative template wrapping method, and their electrochemical properties are investigated. In Chapter 3, polypyrrole/graphene hybrid electrodes with hollow nanosphere morphology are prepared by the in situ polymerization method, and their structure and morphology are characterized by using a variety of characterization methods. Moreover, the electrochemical properties of the polypyrrole/graphene hybrid electrodes with hollow nanosphere morphology are studied. The conclusion of the thesis is presented in Chapter 4.MnO2-wrapped graphene hollow nanosphere (MnO2/HGR) electrodes with different mass ratios of MnO2 to HGR are prepared by a cooperative template wrapping method. Graphene with hollow nanospheres (HGR) is firstly obtained by carbonizing phenolic resin and followed by etching of SiO2 template, then the MnO2 ultrathin nanoplates are coated on the surfaces of the HGR hollow nanospheres through a redox reaction between KMnO4 and HGR nanospheres under vigorous stirring for 3 h at 60℃. Manganese dioxide is coated on the surface of HGR as δ-MnO2 crystal. The as-prepared MnO2/HGR hollow nanospheres possess porous structure and large specific surface area (about 230 m2 g-1). The specific capacitances of MnO2/HGR nanosphere electrodes with different mass ratios of MnO2 to HGR are about 340-360 F g-1 at a scan rate of 5 mV s-1 in Na2SO4 solution in a voltage of-0.2 to 0.8 V by using the cyclic voltammetry (CV), galvanostatic charge-discharge and electrochemical impedance spectroscopy (EIS) tests, they are mainly contributed from a faradic pseudocapacitance. After 1000 cycles at a scan rate of 20 mV s-1, the capacitance retention of MnO/HGR nanosphere electrode is 86%. The good specific capacitance is ascribed to the novel hollow nanosphere structure, which possesses high surface-to-volume ratio and can be easy for mass diffusion of electrolyte and transmission of ions and electrons, and maintains the mechanical integrality and high electrical conductivity. This cooperative template wrapping method can be used for the preparation of HGR and other metal oxide hollow electrodes with good capacitance.Graphene with hollow nanosphere suspension is firstly obtained by suspensing the as-prepared HGR in deionlized water, then FeCl3-6H2O and pyrrole monomers are respectively added into this dispersion and the mixed suspension is vigorously stirred for 4 h at 5℃, PPy/HGR hollow nanosphere is obtained by in situ polymerization method. The N2 adsorption-desorption test results indicate that PPy/HGR hollow nanosphere has a high specific surface area of 441 m2 g-1.The electrochemical performance of PPy/HGR hollow nanosphere is investigated by using cyclic voltammetry, galvanostatic charge-discharge, and electrochemical impedance spectroscopy techniques. The experimental results show that the specific capacitance of PPy/HGR hollow nanosphere is 332 F g-1 at a scan rate of 5 mV s-1 in 1 mol L-1 H2SO4 electrolyte in a voltage of -0.1 to 0.6 V, and its capacitance retention is 77% after 1000 cycles at a scan rate of 20 mV s-1. The chain scission problem of polypyrrole is improved by hybriding with HGR with hollow nanospheres. Moreover, the exhibits excellent electrochemical property of PPy/HGR hybrid electrode with hollow nanosphere is ascribed to the hollow mesoporous structure, which is favorable for the diffusion and transmission of electrolyte ions. |
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