부산대학교 도서관

Currently, the supercapacitors based on electrochemical system has emerged as promising energy storage system because of its pulse power supply, long cyclic life (more than 100,000 cycles), simple operational mechanism, and high dynamics of charge propagation. In this thesis, the excellent and effective electro-active materials were explored to fabricate the high performance electrochemical double-layer capacitor (EDLC) and pseudo-supercapacitors which were compared by the reported literatures. In the first part, the mesoporous carbon nanofibers (MCFs) materials were prepared from dextrose as carbon source via thermal carbonization process using SBA-15 silica as template under N2 gas flow for effective electro-active electrode in EDLC. The prepared MCFs exhibited the high surface to volume ratio, large pore volume and high mesoporous nature. A relation of capacitive performance, specific surface area, accessible average pore size and current density was manifested to evaluate the impact of capacitance parameters. The capacitive and electrochemical properties of fabricated EDLC based on MCFs electrode were investigated by analyzing the cyclic voltammetry, electrochemical impedance and galvanostatic charge-discharge measurements. A reasonably high specific capacitance of 201 F/g at current density of 0.6 A/g was achieved by the fabricated EDLC based on MCFs electrode. The fabricated EDLC based on MCFs electrode attained the ultra-high energy density of 5.47 W h/kg and reasonable power density of 0.842 kW/kg at the current density of 0.6 A/g. The fabricated EDLC importantly showed an excellent stability by keeping about 88% of initial capacity after 1000 cycles. In order to improve the properties of commercial activated carbon (AC), a highly controlled steam pretreatment was adopted and the steam pretreated AC extensively applied as electro-active materials for EDLC. The steam pretreatment at 700oC followed by H2 gasification was found to be optimal temperature for achieving the high surface area, pore size and pore volume of AC. FTIR and XPS analysis revealed the significant reduction in hydroxyl groups after H2 gasification during the steam pretreatment on AC. The electrochemical characterizations aroused the considerable improvement in the electrocatalytic and capacitive properties of AC electrode after the steam pretreatment at 700oC. The supercapacitor based on bare AC electrode exhibited low capacitance of 155.2 F/g whereas supercapacitor based steam-700oC/AC electrode showed the appreciably increment in the capacitance of 187.2 F/g which might be due to improve surface area, and pore volume. The steam-700oC/AC electrode presented the excellent stability by recording 93.8% of initial capacity after 1000 cycles.In the second part, various transition metal oxides such as Co3O4 nanoparticles, MnO2 nanofibers and Mn2O3 nanowalls were synthesized by simple solution process, electrodeposition and hydrothermal process to fabricate the high performance pseudo-supercapacitors. In this regards, highly uniform Co3O4 nanoparticles (NPs) were synthesized by a low temperature solution process at 70oC using cobalt nitrate (Co(NO3)2·6H2O) as precursor and utilized as electro-active electrode by depositing on nickel (Ni) foam for the fabrication of pseudo-supercapacitors. The synthesized Co3O4 NPs possessed well-crystalline semispherical morphology and were extensively characterized in terms of structural, crystalline and electrochemical properties. The synthesized Co3O4 NPs based electrode exhibited a reasonable specific capacitance of ~304 F/g in 1M KOH aqueous solution as electrolyte. It was found that the capacitance was drastically increased to ~480 F/g when the electrode was prepared by the mixing Co3O4 NPs and super-P carbon (conducting agent). Interestingly, Co3O4 NPs based electrodes attained the excellent cycling stability with the retention ratio of ~88.6% after 1000 cycles. On the other hand, well ordered manganese trioxide (Mn2O3) nanowalls and manganese dioxide (MnO2) nanofiber arrays were deposited directly on Ni foam by a simple hydrothermal and electrodeposition techniques, respectively. The prepared Mn2O3 nanowalls and MnO2 nanofiber arrays were comprehensively characterized by X-ray diffraction, field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), UV-Vis, FTIR, Raman and electrochemical techniques etc. The fabricated pseudo-supercapacitor with Mn2O3 nanowalls showed high specific capacitance of 480 F/g in 6 M KOH solution, indicating the excellent electrocatalytic ability of Mn2O3 nanowalls. However, the pseudo-supercapacitor fabricated with MnO2 nanofibers arrays exhibited reasonable specific capacitance of 266 F/g at the scan rate of 10 mV/s in 0.5M Na2SO4 aqueous solution with the potential window of -0.8 ~ -0.2V. The excellent electrochemical property of Mn2O3 nanowalls based electrode might be attributed to its large active surface area, high redox current density, and a short facile diffusion path for electrolyte ions.