Preparation and Properties of polyaniline/quantum dots nanocomposites and their applications on supercapacitor

This thesis utillized the chemical oxidation method to synthesize the polyaniline (PANI) nanowires in array on graphite foil employed for supercapacitor electrodes. First, after polymerized for 24 h with different concentration of aniline, the surface morphology was investigated by SEM, mass weight on graphite foil was estimated by TGA, and surface functional groups were characterized by FTIR-ATR. Electrochemical measurements including Galvanostatic charge discharge, cyclic voltammetry and electrochemical impedance were employed to characterize their specific capacitance. When the concentration of aniline was 0.01 M, the PANI nanowire array showed the highest surface area with the optimized specific capacitance reaching 740.54 F/g at 1 A/g, and 286.22/g at 20 A/g. The optimized electrodes were then used to adsorb the quantum dot materials including graphene quantum dots (GQDs) and ruthenium oxide(RuO2) for further studies. The modified Hummers method was used to fabricate the GQDs-1, GQDs-2, GQDs-3 by varying the hydrothermal reaction temperature at 100°C, 120°C and 140°C, respectively for 12 h. Their sizes of 7, 4, and 3.5 nm measured by TEM were similar to those measured by DLS (7.7 nm for GQDs-1 , 4.7 nm for GQDs-2. and 3.3 nm for GQDs-3 ). The PL spectra showed that with increasing the excitation wavelength, the emission peak shifted to longer wavelengths. AFM observations revealed that GQDs-1 and GQDs-3 consisted of 1~3 layers in contrast to 1~5 layers for GQDs-2. According to the XPS results, the percentage of C=O bonds decreased with higher reaction temperature, which is consistant with the FTIR observation. Furthermore, the ATR-FTIR also revealed that the peak intensity at 3256 cm-1 contributed by primary amine increased as hydrogen bonds formed between GQDs and PANI, leading to the disappearance of C-N bonds peak. Next, the GQDs/PANI electrodes were prepared by immersing PANI electrodes in diluted GQDs suspension. By dipping PANI electrode in 1 mg/ml GQDs, the best performance of GQDs-3/PANI electrode was obtained with the specific capacitance reaching 907.38 F/g at 1 A/g, and 315.32 F/g at 20A/g, which are also supported by the CV data. EIS measurements confirmed that the electron charge transfer resistances for PANI electrode were decreased significantly as the PANI electrode was incorporated with GQDs-3. Hydrous RuO2·xH2O quantum dots were fabricated by hydrothermal process. The average particle size was about 4 nm as estimated by TEM, similar to the DLS measurement. As they were adsorbed by PANI, the FTIR spectrum showed that the peak at 3485cm-1 became broader, indicating the formation of hydrogen bonds between RuO2·xH2O and PANI. As to the performance of RuO2/PANI electrodes, the best specific capacitance of RuO2/PANI electrode reached to 1098.14 F/g at 1 A/g and 504 F/g at 20A/g, which are better than that of the GQDs-3/PANI electrodes. The energy density of RuO2/PANI electrode reached to 74.72 Wh/kg with 350W/kg power density at 1 A/g compared to 61.73Wh/kg for GQDs-3/PANI electrodes. Two-electrode confuguration supercapapcitor were also fabricated by using GO/EMITFSI ionic liquid composite electrolyte system. When 4 wt% of GO was incorporated with EMITFSI, the electrolyte system became immovable. RuO2/PANI and GQDs-3/PANI electrodes showed the best specific capacitance of 231.21 and 192.79 F/g, respectively at 1 A/g. After charge-discharge test for 1000 cycles, two-electrode configuration with RuO2/PANI electrode can retain 58.2% specific capacitance, better than that with three-electrode configuration. From the Ragone plot, the energy density reached to 15.72 Wh/Kg for RuO2/PANI electrode compared to 13.12 Wh/Kg for GQDs-3/PANI electrodes in two-electrode system.