| In the 21st century, one of the great challenges is undoubtedly energy storage. The ever worsening energy depletion and global warming issues call for not only urgent development of clean alternative energies and emission control of global warming gases, but also more advanced energy storage and management devices. Supercapacitors, offering transient but extremely high powers, are probably the most important next generation energy storage device. To develop an advanced supercapacitor device, an active electrode material with high capacity performance is indispensable. In this dissertation, MnO2 with different microstructure and morphology, carbon/MnO2 and carbon/polyaniline (PANI) composites have been synthesized by different methods. The morphology and microstructure of the samples were examined by scanning electron microscopy (SEM), transition electron microscopy (TEM), X-ray diffraction (XRD), Raman spectra and X-ray photoelectron spectroscopy (XPS). Electrochemical properties were characterized by cyclic voltammetry (CV), galvanostatic charge/discharge and electrochemical impedance spectroscopy (EIS).In this dissertation, rod-like and lamellar MnO2 were synthesized by microwave and hydrothermal method, respectively, and the effects of the microstructure on the electrochemical performances were also investigated. The results indicate that the electrochemical performances of the latter are better than those of the former. The maximum specific capacitance (based on MnO2) of the lamellar MnO2 is 242 F·g-1, and the maximum power density of 10.4 kW-kg-1 is obtained. Additionally, the specific capacitance increased by about 8.1% of initial capacitance after 200 cycles at a current density of 100 mA-cm-2.In order to improve the poor electrical conductivity of MnO2, graphite nanoplatelet (GNP)/MnO2 composites were prepared by hydrothermal synthesis. In addition, carbon nanotube (CNT)/MnO2 and graphene/MnO2 composites were also synthesized by microwave method, and the effects of the different carbon supports on the electrochemical performances of the composites were systematacially studied. MnO2 in GNP/MnO2 composites is composed of the mixture phases ofα-MnO2 andγ-MnO2. The specific capacitance of GNP/MnO2 (based on MnO2) increases monotonously with increasing GNP content, and the maximum specific capacitance is 276.3 F·g-1. The specific capacitance based on the composite could reach 158 F-g"1 by addition of only 10 wt%GNPs. For CNT/MnO2 composites, the MnO2 can be indexed to birnessite-type MnO2, and the content of MnO2 in the composites increases from 15% to 57%with the increasing amount of KMnO4. The specific capacitance based on MnO2 of the CNT-15%MnO2 composite is 944 F·g-1 (85%of the theoretical capacitance) at 1 mV·s-1. When the scan rates are increased to 500 mV·s-1, the specific capacitance could remain 522 F·g-1. Amaximum specific capacitance based on composite of 239.1 F·g-1 with a measured power density of 45.4 kW·kg-1 at an energy density of 25.2 Wh·kg-1 in 1 mol·L-1 Na2SO4 solution has been obtained for CNT-57%MnO2 composite. For graphene/MnO2 composites, the MnO2 corresponds to the monoclinic lamellar structure of birnessite-type MnO2. Nanoscale MnO2 particles (5-10 nm) disperse on the surfaces of graphene, and the growth of MnO2 is preferred near the edges of graphene layer. The specific capacitance of graphene-78%MnO2 hybrid is 310 F·g-1 at 2 mV·s-1 (228 F·g-1 at 500 mV·s-1), to the best of our knowledge, this value is the highest reported for carbon/MnO2 composites. Meanwhile, the capacitance is highly kept over a wide range of scan rates (the capacitance retention ratio is 88%and 74%at 100 and 500 mV·s-1, respectively). After 500 cycles, the specific capacitance of this electrode decreases only 1.5%of the initial capacitance.Moreover, we managed to synthesize graphene/PANI composites using graphene as deposition supports by in situ polymerization. PANI particles homogeneously coat on the surfaces of graphene with size of-2 nm. The maximum specific capacitance is 1046 F-g'1 (based on composite) at 1 mV·s-1 compared to 115 F·g-1 for pure PANI and the energy density can reach 39 Wh-kg"1 at a power density of 70 kW·kg-1. However, the capacitance rention ratio is significantly dreased with the increase of scan rates and the cycle stability is relatively poor.Finally,1 wt.%CNTs were introduced into the graphene/PANI composites to improve their long-term cycle stability. PANI particles preferentially grow on the surfaces of graphene due to its high chemical activity and large surface area. The maximium specific capacitance (based on composites) of 1035 F·g-1 at a scan rate of 1 mV·s-1 in 6 mol·L-1 KOH solution has been obtained. In addition, the electrode exhibits excellent long cycle life along with-94%specific capacitance retained after 1000 cycle tests, compared to 48%and 33%for the graphene/PANI and CNT/PANI composites, respectively. |
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