| Summary: | 碩士 === 國立交通大學 === 電子研究所 === 106 === Supercapacitors have attracted more and more attention due to their excellent characteristics such as rapid charge and discharge rate, high power density, high reliability and long cycle life. Especially, supercapacitors have not only high power density of traditional capacitors but also high energy density of batteries; therefore, they are considered as the most promising energy storage device in the next generation. As the electrodes of supercapacitors, carbon nanotubes have high specific surface area, high porosity, excellent electrical conductivity, and unique tubular structure so that the charges and ions can be well distributed to the electrode/electrolyte interface and charge completely in very short time. In this thesis, the ultrasonic spraying system was utilized to build uniform, and nano-scale 3D porous carbon nanotubes thin films as the conductive electrodes of supercapacitors. In order to further increase the specific capacitance (Csp) performance of the CNTFs without chemical solvent and metal pollution under low temperature, the surface modification of KrF excimer laser treatment and oxygen plasma treatment were carried out, respectively.
First, the influence of the thickness of CNTFs on the Csp was discussed. The uniformity and thickness controllability of the thin film with scanning ultrasonic spraying system were verified by scanning electron microscopy (SEM). By comparing different CNTFs thicknesses and corresponding CV results, the relation between thickness and Csp was proposed. In order to save materials and obtain practical Csp, 40 layers of CNTFs were selected for the following discussion.
Secondly, in order to verify the effect of the excimer laser treatment on CNTFs, a series of material analyses including scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy were used to analyze the surface morphology and crystallinity of CNTs. By comparing the results of the material analyses with the Csp characteristics under different laser conditions for the supercapacitors, the effect of the excimer laser treatment on the CNTFs was proposed. With increasing laser energy, the crystallinity of the CNTFs would be improved and hence the better conductivity was achieved. Moreover, part of CNTs would be unzipped as graphene sheets with keeping the 3D structure and the graphenes could provide more surface area for charge storage after excimer laser irradiation with ISLED treatment. Under 400 mJ/cm2 excimer laser ISLED irradiation, the Csp would be promoted from 23.7 F/g to 39.5 F/g. The improvement ratio of Csp was 1.7x. However, too high energy densities, above 400 mJ/cm2, would cause the ablation of the CNTFs and result in the degradation of the Csp.
In the end, to verify the effect of oxygen plasma treatment on CNTFs, a series of material analyses including SEM, XPS, and Raman were again used to analyze the surface morphology and functional groups of CNTs. By comparing the results of the material analyses with the Csp characteristics under different plasma conditions for the supercapacitors, the effect of the oxygen plasma treatment on CNTFs was proposed. With oxygen plasma treatment, the oxygen-containing functional groups would be effectively decorated on the surface of CNTFs. These oxygen-containing functional groups were verified to play a role in the redox reactions to produce extra capacitance during charging/discharging so that the better Csp could be achieved. By adjusting the plasma processing time, the Csp of supercapacitors could raise to 49.3 F/g with respect to 23.7 F/g for the as-sprayed CNTFs. The improvement ratio of Csp was 2.1x. Hence, this study exhibited that the CNTFs as the electrodes of supercapacitors are promising in the future developments for the energy-storage devices.
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