| Supercapacitors as a new kind of energy storage system have aroused broad interest due to its faster charge and discharge performance, better cycle stability, wider working temperature range compared to the widely used energy storage devices. Among the various transition metal oxides, manganese oxide(Mn O2) has turned out to be an alternative electrode material for supercapacitors due to its high theoretical specific capacitance in aqueous electrolyte, natural abundance and low cost. Despite all of the superiority, the poor electrical conductivity of bulk Mn O2 has severely confined its capacitive performance far away from the theoretical value. Considering that Mn O2 contributes to the capacitance through a fast surface Faradic redox reaction and the rate performance of Mn O2 electrode strongly depends on how fast the electron and ion can be transported in the interior of the electrode. Therefore, much effort was made to improve the electrical conductivity and specific area. We report a facile liquid phase precipitation method and hydrothermal method to synthesize Mn O2 electrode materials. Furthermore, various high conductivity carbon material were dispersed into the Mn O2 materials to achieve high power and high energy density. Some valuable results were obtained and the main points of this thesis are summarized as follows:1. Mn O2/C composites were prepared by liquid phase precipitation method. The influence of preparation conditions on the structure and electrochemical capacitive performances of the materials has been investigated by XRD, FESEM and electrochemical properties tests. The results show the optimum reaction conditions were as follows: the manganese source is Mn(Ac)2, the molar ratio of K2S2O8 to Mn(Ac)2 is 3.0:1.0, and the reaction time is 6 h. The obtained Mn O2 is α-Mn O2 with spherical which was composed of fibrous reunion under the optimum reaction conditions, and the average diameter and the length of fibrous reunion is about 50 nm and 80-150 nm, respectively. Electrochemical properties measurements showed that under 100 m A/g, the discharge specific capacitances of the prepared sample is 268 F/g. When the current density is up to 300 m A/g, the discharge specific capacitances of the sample is 257 F/g and the capacity retention rate is 96%, showing good electrochemical performance. In addition, the capacity retention rate of the sample under 100 m A/g after 2000 cycles is more than 50% and the coulombic efficiency is close to 100%.2. Mn O2 electrode materials for supercapacitors were prepared by hydrothermal method. Single-factor fermentation experiments were employed to inspect the influences of the dosage of HCl, reaction temperature and reaction time factors on the performances of Mn O2. The optimized conditions are obtained by XRD, FESEM, N2 adsorption-desorption measurement, galvanostatic charge-discharge, and CV analyse. The results show the optimum reaction conditions were as follows: the dosage of HCl is 10 m L, the reaction temperature is 130 °C and the reaction time is 12 h. The obtained Mn O2 is α-Mn O2 with the uniform particle size. Under the optimum reaction conditions, the average diameter of the sample is around 1.0-2.0 μm. Electrochemical properties measurements showed that under the conditions, a specific capacitance of 294, 271.5, 261, 255 and 251 F/g was achieved at a current density of 100, 150, 200, 250 and 300 m A/g, respectively. In addition, the capacity attenuation rate of the sample is only 92% after 2000 cycles at 100 m A/g and yielded a specific capacitance of 114 F/g. The coulombic efficiency is above 95% in the whole cycles.3. Mn O2 materials were prepared by hydrothermal method. Based on the optimized process route, the effects of doping contents of carbon nanotubes(CNT), graphene(GR) and carbon fiber(CF) on the phase structure, surface morphology and electrochemical properties of the composite were investigated. The results show that the doping of CNT and GR have a contribution to improve the specific capacitances of Mn O2 electrode materials and the optimal doped amount is 15 wt.% CNT and 10 wt.% GR. The specific capacitances of Mn O2/CNT15 wt.% and Mn O2/GR10 wt.% composites were 339 F/g and 349 F/g, respectively, which is higer than the pure Mn O2. But the doping of CF did not improve the specific capacitances of Mn O2. The adding content of 4 wt.% CF was defined conclusively as the optimum preparative conditions. And the discharge specific capacitances of Mn O2/CF4 wt.%, composites are 273, 258, 250, 243 and 237 F/g at 100, 150, 200, 250 and 300 m A/g, respectively. |
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