Improved EDLC Characteristics of the CNTs Grown on the Nanoporous Alumina Templates
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Improved EDLC Characteristics of the CNTs Grown on the Nanoporous Alumina Templates Sun Wen, Mi Jung, Oh-Shim Joo1, Sun-il Mho* Department of Molecular Science and Technology, Ajou University, Suwon 443-749, Korea 1 Korea Institute of Science and Technology, P.O.Box 131, Seoul 130-650, South Korea
ABSTRACT The specific capacitance of the carbon nanotube (CNT) electrode can be increased by using nanoporous alumina templates with the high pore density and the small and uniform pore diameter. The surface area of the CNTs was controlled and increased by preparing them with uniform diameters. The well-ordered nanoporous alumina templates were fabricated by a twostep anodization method. The cylindrical pore diameter, length, and density of the template utilized for the CNT growth was 53 ± 1 nm, 2 µm, and 3.1×1010 cm-2, respectively. The CNTs with uniform diameter of 44 ± 2 nm were grown on the porous alumina template as electrode materials for the electric double-layer capacitor (EDLC). The EDLC characteristic of the CNT electrodes was examined by measuring the capacitances from the cyclic voltammograms. The specific capacitance of the CNT electrodes can be increased to the value of 121 ± 5 F/g.
INTRODUCTION Various carbon materials (activated carbons, aerogels, xerogels, CNTs) have been extensively used as electrode materials in a variety of electrochemical systems, such as batteries and electric double-layer capacitors (EDLCs) [1-6]. The EDLC stores electric charges in the double-layer formed at the electrode/electrolyte interface, where a pure electrostatic attraction between ions and the charged surface of an electrode takes place. The double layer at the interface forms quickly, which results in high power rates of the EDLCs compared to the conventional batteries. The EDLC electrode materials should have a large surface area and an appropriate pore structure for the electrolyte wetting and rapid ionic motion. Among different carbon materials, activated carbons were used extensively because of its economical merit, however, the specific capacitances of activated carbons are dependent on the pore volume and size as well as the surface area. The pore structure and the surface morphology are strongly dependent on the activation process. Carbon nanotubes (CNTs) can be used as a novel electrode material for the EDLCs. CNTs have a narrow distribution of size, highly accessible area, low resistivity, and high stability. However, the specific capacitances of the electrode made of CNTs are smaller than that of the best activated carbons. The specific capacitance of the CNT electrode has to be improved. Several attempts have been made to increase the capacitance of the CNTs for the EDLC electrode material [4-7]. The electrochemical capacitance of the CNTs of the diameter from 15 to 30 nm, was increased from 70 F/g to 120 F/g through an additional treatment of the CNTs with nitric acid. The capacitances of CNTs was also enhanced by functionalization of the CNTs to 130 F/g, and by modifying the CNTs with PPy to 170 F/g.
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