• Journal of the Korean Society of Manufacturing Technology Engineers
• /
• v.25 no.4
• /
• pp.307-311
• /
• 2016

Several types of supercapacitors have been developed for energy storage systems. Among them, the hybrid type has advantages such as a large capacitance per weight compared with the electric double-layer capacitator type. In this study, constant current charging and discharging tests were conducted for recently developed hybrid-type supercapacitors. Based on the experimental results, the capacitance and equivalent series resistance were obtained. The capacitance was larger than the designed capacitance at a low current but became small at a high current. In addition, the capacitance depended on the cell voltage. These results can be used to design an energy storage system.

• Journal of the Korean Electrochemical Society
• /
• v.6 no.4
• /
• pp.255-260
• /
• 2003

The complex capacitance analysis was performed in order to examine the potential-dependent EDLC characteristics of porous carbon electrodes. The imaginary capacitance profiles $(C_{im}\;vs.\;log\lf)$ were theoretically derived for a cylindrical pore and further extended to multiple pore systems. Two important electrochemical parameters in EDLC can be estimated from the peak-shaped imaginary capacitance plots: total capacitance from the peak area and $\alpha_0$ from the peak position. Using this method, the variation of capacitance and ion conductivity in pores can be traced as a function of electric potential. The electrochemical impedance spectroscopy was recorded on the mesoporous carbon electrode as a function of electric potential and analyzed by complex capacitance method. The capacitance values obtained from the peak area showed a maximum at 0.3V (vs. SCE), which was in accordance with cyclic voltammetry result. The ionic conductivity in pores calculated from the peak position showed a maximum at 0.2 V (vs. SCE), then decreased with an increase in potential. This behavior seems due to the enhanced electrostatic interaction between ion and surface charge that becomes enriched at more positive potentials.

• Proceedings of the KIEE Conference
• /
• 2005.07c
• /
• pp.2022-2024
• /
• 2005

Electrical double-layer capacitor (EDLC) is an electrochemical energy storage device in which electric charges only accumulated by a pure electrostatic attraction force are stored on the electrolyte-electrode interface in a form of double layer and separated by the electrolyte. The composite was prepared by mixing nanosize $TiO_2$ and activated carbon through a means of ultrasonic vibration in ethanol solution for 30 min in various mass ratios of $AC:TiO_2$ to form activated carbone-semiconducting oxide composites. Either 1.0 M $LiClO_4/EC-DEC$ or $Et_4NBF_4$/EC-DEC was used as the electrolyte. It was found that with modification of $TiO_2$, the specific capacitance of activated carbon measured at $1mA/cm^2$ was increased from 40 to 50 F/g. This method is unique in comparison the conventional method because it uses semiconducting TiO2 other than electrochemically active materials such as $RuO_2$. The increase in specific capacitance could be attributed to the decrease in electric polarization, caused by the introduction of $RuO_2$.

• Carbon letters
• /
• v.6 no.2
• /
• pp.86-88
• /
• 2005

Mesoporous activated carbon (AC) was prepared from aged petroleum coke through chemical activation. The AC has a specific surface area of 1733 $m^2/g$ and a mean pore diameter of 2.37 nm. The volume fraction of 2 to 4nm pores is 56.74%. At a current density of 10 mA/$cm^2$, a specific capacitance of 240 F/g is achieved representing the use factor of the surface area of 69.2%. And the electrical double layer capacitor (EDLC) based on the AC shows an excellent power performance. This result suggests that the presence of high fraction of mesopores can effectively increase the adsorption efficiency of the specific surface area of the AC and enhance the power performance of EDLC based on the efficient surface area of the AC.

• Bulletin of the Korean Chemical Society
• /
• v.33 no.5
• /
• pp.1523-1526
• /
• 2012

In this work, we prepared activated multi-walled carbon nanotubes/polyacrylonitrile (A-MWCNTs/C) composites by film casting and activation method. Electrochemical properties of the composites were investigated in terms of serving as MWCNTs-based electrode materials for electric double layer capacitors (EDLCs). As a result, the A-MWCNTs/C composites had much higher BET specific surface area, and pore volume, and lower volume ratio of micropores than those of pristine MWCNTs/PAN ones. Furthermore, some functional groups were added on the surface of the A-MWCNTs/C composites. The specific capacitance of the A-MWCNTs/C composites was more than 4.5 times that of the pristine ones at 0.1 V discharging voltage owing to the changes of the structure and surface characteristics of the MWCNTs by activation process.

• 한국신재생에너지학회:학술대회논문집
• /
• 2009.11a
• /
• pp.627-627
• /
• 2009

In this work, the activated carbons (ACs) with high porosity were synthesized from pitch by KOH chemical activation. The structure and surface properties of ACs were characterized by means of elemental analysis, X-ray diffraction (XRD), and X-ray photoelectron spectroscopy(XPS). And, the influence of the KOH-to-pitch ratio on the porosity of the ACs was investigated using the nitrogen adsorption isotherms at 77 K and a scanning electron microscopy (SEM). As a result, pitch could be successfully converted into ACs with well-developed micro and mesopores. The specific surface areas and pore volumes were increased with an increase of the KOH-to-pitch ratio. Furthermore, it was found that the addition of KOH led to the transformation of the micropores to the meso- and macropores. In the application to electric double layer capacitors (EDLC), the pitch-based ACs showed a higher capacitance per weight and per volume, and an excellent electrochemical stability in the high voltage region.

• Applied Chemistry for Engineering
• /
• v.19 no.4
• /
• pp.407-412
• /
• 2008

As an activation procedure, in this study, the oxidation treatment of needle cokes with a dilute nitric acid and sodium chlorate $(NaClO_3)$, combined with heat treatment, was attempted. The structures of acid-treated and pyrolyzed coke were examined with XRD, FESEM, elemental analyzer, BET, and Raman spectroscopy. The behavior of double layer capacitance was investigated with the analysis of charge and discharge. The structure of needle coke treated with acid was revealed to a single phase of (001) diffraction peak after 24 h. On the other hand, thecoke oxidized by heat treatment was reduced to a graphite structure of (002) at $300^{\circ}C$. The distorted graphene layer structure, derived from the process of oxidation and reduction of the inter-layer, makes the pores by the electric field activation at the first charge, and generates the double layer capacitance from the second charge. The cell using pyrolyzed coke with 24 h acid treatment and $300^{\circ}C$ heat treatment exhibited the maximum capacitance per weight and volume of 33 F/g and 30 F/mL at the two-electrode system in the potential range of 0~2.5 V.

• Journal of Electrochemical Science and Technology
• /
• v.9 no.1
• /
• pp.20-27
• /
• 2018

We describe a redox-enhanced electric double-layer capacitor (EDLC) that turns the electrolyte in a conventional EDLC into an integral, active component for charge storage-charge is stored both through faradaic reactions with soluble redox-active molecules in the electrolyte, and through the double-layer capacitance in a porous carbon electrode. The mixed-redox electrolyte, composed of vanadium and iodides, was employed to achieve high power density. The electrochemical reaction in a supercapacitor with vanadium and iodide was studied to estimate the charge capacity and energy density of the redox supercapacitor. A redox supercapacitor with a mixed electrolyte composed of 0.75 M NaI and 0.5 M $VOSO_4$ was fabricated and studied. When charged to a potential of 1 V, faradaic charging processes were observed, in addition to the capacitive processes that increased the energy storage capabilities of the supercapacitor. The redox supercapacitor achieved a specific capacity of 13.44 mAh/g and an energy density of 3.81 Wh/kg in a simple Swagelok cell. A control EDLC with 1 M $H_2SO_4$ yielded 7.43 mAh/g and 2.85 Wh/kg. However, the relatively fast self-discharge in the redox-EDLC may be due to the shuttling of the redox couple between the polarized carbon electrodes.

• Journal of the Korean Electrochemical Society
• /
• v.12 no.1
• /
• pp.34-39
• /
• 2009

In this study, a needle coke was oxidized in a mixture of dilute nitric acid and sodium chlorate ($NaClO_3$) solutions and followed by heat treatment. The samples were analyzed with using XRD, FESEM, elemental analyzer, BET, and Raman spectroscopy. Double layer capacitance was measured with the charge and discharge measurements. The consisting layers of the needle coke were expanded to single phase showing only (001) diffraction peak by the acid treatment for 24 hours. The oxidized coke returned to a graphite structure appearing (002) peak after heat treatment above $200^{\circ}C$. The structure returned could be more easily accessible to the ions by the first charge, and improve the double layer capacitance at the second charge. The two-electorde cell from acid treated coke and $300^{\circ}C$ heat treatment exhibited the maximum capacitances of 32.1 F/g and 29.5 F/ml at the potential of $0{\sim}2.5\;V$.

• Journal of the Korean Chemical Society
• /
• v.54 no.1
• /
• pp.93-98
• /
• 2010

In this work, the electrochemical properties of surface treated multi-walled carbon nanotubes (MWNTs) were studied. Nitrogen and oxygen functional groups of the MWNTs were introduced by urea and acidic treatment, respectively. The surface functional groups of the MWNTs were confirmed by X-ray photoelectron spectroscopy (XPS) measurements and zeta-potential method. The characteristics of $N_2$ adsorption isotherm at 77 K, specific surface area, and total pore volumes were investigated by BET eqaution, BJH method and t-plot method. Electrochemical properties of the functionalized MWNTs were accumulated by cyclic voltammetry at the scan rates of 50 $mVs^{-1}$ and 100 $mVs^{-1}$ in 1M $H_2SO_4$ as electrolytes. As a result, the functionalized MWNTs led to an increase of capacitance as compared with pristine MWNTs. It was found that the increase of capacitance for urea treated MWNTs was attributed to the increase in density of surface functional groups, resulting in improving the wettability between electrode materials and charge species.