Preparation, Modification And Electrochemical Capacitance Of Carbon Based Electrode Materials

At present, the shortage of traditional fossil energy and the environmental pollution coming from the use of fossil energy becomes more and more serious, to develop green, sustainable and efficient clean energy is our urgent task. Supercapacitor combines the advantages of the conventional capacitors and batteries, exhibiting high power density, long cycle life, good stability, and environmental friendliness. Electrode material is the most important part in supercapacitor which mainly determine the performance of supercapacitior. On the basis of summarizing the research on supercapacitor electrode materials, we mainly engaged in the promising carbon materials. In this paper, we focused on the preparation, modification and improvement on their electrochemical performance. The main research contents of this paper are as follows:（1） Thermal exfoliation is a popular approach to prepare thin-layered graphene in large scale. Generally believed that the microstructure and surface chemistry of the products could be tailored by the variation of ambient gas pressure, temperature, acid and organic substances during thermal exfoliation of graphite oxide（GO）. However,the role of physical pre-treatment process and the negative effect of volatile acid were not been taken into account. In this work, we used graphite oxide powder as the precursor to investigate the effect of different physical pre-treatment process and acid assisting on the texture, surface state and electrochemical performance of the resulting thermal exfoliated graphene, and found that the different pre-treatment process had great influence on the microstructure, specific surface area and electrochemical property of graphene. The physical pre-treatment process could cut GO into fragments, peel off them to thin sheets and form active radicals to further oxidize GO, which was beneficial to the thermal exfoliation of GO. Furthermore, the sonication was superior to stirring process. The addition of volatile acid in the sonication process could decrease the specific surface area of graphene, resulting from the strong self-assembly and coagulation of GO. Acid would decompose and react with carbon atoms nearby to produce more pore in the process of thermal expansion which was favour to the fast diffusion of the electrolyte. Not only the specific surface area but also the pore structure of electrode material had an influence on electrochemical performance. The specific surface area of graphene with acid assisting decreased, but as a result of the improvement of pore structure, electrochemical performance was better, the capacitance as much as 238 F g-1. The important role of physical pre-treatment process and acid assisting process in expansion-exfoliation of GO was discussed in detail in the third chapter.（2） Based on the experience of our preparation thermal expansion graphene, in this work, we explored a new method to preparation activated carbons（ACs）. We selected biomass as raw material, using H₂O2/HAc to activation-carbonization method. Lotus shell was activated by hydrothermal reaction using H₂O2 and HAc, followed by carbonized at 700 °C to get ACs（C-H₂O2/HAc） that had excellent electrochemical performance. The specific surface area of C- H₂O2/HAc was 1056 m2 g–1,as well as the specific capacitance at 0.5 A g-1 was 330 F g-1. To analyze the influence of different preparing process on the microstructure and electrochemical of ACs, we also compared the effect on ACs when a single component was used. The analysis results showed that in the hydrothermal process, H₂O2 would corrupt and oxidize lotus shell which was beneficial to forming more holes in the process of carbonization; at the same time, lotus shell took advantages to getting thin layer in carbonization process with HAc assisting. Furthermore, this method for preparing ACs has a guiding significance for the preparation of ACs from biomass.（3） OMC is a supporting material applied in supercapacitor, because it has suitable pore diameter and channel for electric double layer capacitance, superior conductivity and large specific surface area. But OMC has simple storage mode, and the energy density is too low to apply in production. PANI is an electrode material which has highly reversible redox reaction, as well as high specific capacitance. However, the structure of PANI is collapsed in circulation. There were many literatures reported on the OMC/PANI composite applied in supercapacitor. In this paper, not only the PANI was effectively immobilized on the surface of OMC, but also the controlled synthesis of PANI/OMC composite with different PANI content by using Mn2O3 as sacrificial template was realized. The microstructure and morphology observed indicated that a thin layer of PANI was coated on OMC uniformly. Furthermore, PANI/OMC composites with different content of PANI were gained by adjusting the content of Mn2O3 on OMC and their properties were characterized. As the supercapacitor electrode material, the discharge capacity of the optimized PANI/OMC could reach to 467 F g-1, which was far higher than that of OMC, PANI and Mn2O3/OMC. The influence of different content of PANI on the electrochemical properties of PANI/OMC composite was discussed in detail in the fifth chapter.