Electric double-layer capacitor performance of a new mesoporous carbon

A new mesoporous carbon (NMC) was prepared, and its performance in an electric double-layer capacitor (EDLC) was compared to that of a conventional carbon (a molecular-sieving carbon, MSC25). The effect of pore size and pore connection pattern on EDLC performance was demonstrated. To prepare NMC, phenol resin was synthesized inside the pores of an inorganic template, Mobile Composite Material 48 (MCM48), and the resulting resin-template composite was carbonized at 700 degrees C under Ar atmosphere. A coke-like carbonaceous material was obtained after removing the inorganic template by HF treatment. The surface area of NMC was 1257 m(2) g(-1) which is smaller than that of MSC25 (1970 m(2) g(-1)). NMC had three-dimensionally interconnected mesopores (2.3 nm average diam), but randomly connected cage-like micropores (<2.0 nm) were dominant in MSC25. The difference in the pore size and pore connection pattern between the two carbons gave rise to a remarkable difference in their EDLC performances. NMC exhibited a smaller specific capacitance (about 120 F g(-1)) than MSC25 as a result of its smaller surface area, but it showed a higher critical scan rate than the MSC25 electrode due to a smaller resistance-capacitance (RC) time constant. The specific charging capacity of the NMC electrode was about 20 mAh g(-1) and was largely invariant vs. the charge-discharge rate. This was contrasted by MSC25 which showed a steadily decreasing capacity with an increase in rate. As a result, the NMC electrode outperformed the MSC25 based on rate capability. The smaller RC time constant and better rate capability of the NMC electrode apparently arises from the lower electrolyte resistance in pores, which in turn stems from the faster ionic motion in larger pores. (C) 2000 The Electrochemical Society. S0013-4651(00)01-080-6. All rights reserved.This work was supported by the Brain Korea 21 Project.

Preparation of Nanotube TiO2-Carbon Composite and Its Anode Performance in Lithium-Ion Batteries

A nanocomposite between carbon and nanotube TiO2 (CNTT) was prepared by addition of activated carbon during hydrothermal treatment of TiO2 and following high-temperature calcinations. From morphological analysis using a scanning electron microscope, transmission electron microscope, and N2 sorption profiles, it was revealed that nanotube TiO2 was homogeneously dispersed with carbon in nanoscale for CNTT materials. When applied into the anode in a lithium-ion battery, CNTT electrodes displayed higher cyclability and better rate capability. From ac-impedance measurement, the total resistance was smaller in the CNTT electrode due to a homogeneously dispersed carbon in nanoscale and a more porous structure.This research was supported by a grant from the Fundamental Research and Development Program for Core Technology of Materials funded by the Ministry of Knowledge Economy, Republic of Korea

Synthesis of a new mesoporous carbon and its application to electrochemical double-layer capacitors

A mesoporous carbon with regular three-dimensionally interconnected 2 nm pore arrays using AlMCM-48 as a template has been synthesised; the mesoporous carbon exhibited excellent performance as an electrochemical double layer capacitor.We are grateful to the Korea Science and Engineering Foundation (Basic Research Program #98-05-02-03-01-3) for financial support.

Complex Capacitance Analysis on Leakage Current Appearing in Electric Double-layer Capacitor Carbon Electrode

imaginary capacitance profiles(Cim vs. log f) were theoretically derived for a cylindrical pore and multiple pore systems of nonuniform pore geometry. The parallel RC circuit was assumed for the interfacial impedance, where R is the charge-transfer resistance for leakage current and C the double-layer capacitance. The theoretical derivation illustrated that the resistive tail relevant to the leakage current appears in addition to the capacitive peak, which was in accordance with the experimental data taken on the porous carbon electrode. The electric double-layer capacitor (EDLC) parameters such as the extent of leakage current, total capacitance, and rate capability were visually estimated from the imaginary capacitance profiles. The more quantitative EDLC parameters were obtained by a nonlinear fitting to the experimental data.This work was supported by KOSEF through the Research Center for Energy Conversion and Storage and also by the Division of Advanced Batteries in NGE Program (project no. 10016446)

Surface modification of graphite by coke coating for reduction of initial irreversible capacity in lithium secondary batteries

Surface modification of graphite to reduce the irreversible capacity loss during the first charging period of graphite anodes is described. For the surface modification, artificial graphite (Lonza KS44) is dispersed in a tetrahydrofuran/acetone solution which contains coal tar pitch. The solvent is then evaporated. The loaded pitch component is converted to coke by a heat treatment at 1000°C in argon atmosphere. The resulting coke-coated graphite has a smaller surface area than that of the pristine one. The reduction of surface area, which is due to the coverage of pores of <10 nm by the coke component, causes a decrease in the irreversible capacity on the first cycle. The extent of electrolyte decomposition, gas evolution and surface film growth is also less with the coke-coated graphite electrodeThis work was supported by the Korea Science and Engineering Foundation (98-2-03-01-01-2)

Facile synthesis of a mesostructured TiO2-graphitized carbon (TiO2-gC) composite through the hydrothermal process and its application as the anode of lithium ion batteries

A mesostructured TiO2-graphitic carbon (TiO2-gC) composite was synthesized through a simple and scalable hydrothermal method to be employed as an anode material in Li-ion batteries. In a wide voltage range (0.0-2.5 V), the TiO2-gC composite anode possesses a high initial lithiation capacity (598 mA h g(-1)) at 0.1 C (1 C: 150 mA g(-1)), and it still retains 369 mA h g(-1) after 50 cycles at 0.5 C. Furthermore, under a high current density of 2 C, the TiO2-gC anode exhibits stable capacity (252 mA h g(-1)) retention for up to 200 cycles. This excellent electrochemical performance could be ascribed to a synergistic effect of well-developed mesoporosity with a high surface area (345.4 m(2) g(-1)), the conductive graphitic carbon wall, and uniformly dispersed TiO2 nanoparticles, resulting in improved Li+ penetration, fast electron transport and high structural stability during cycling.116Nsciescopu

Rate Capability of Electric Double-Layer Capacitor (EDLC) Electrodes According to Pore Length in Spherical Porous Carbons

A series of spherical porous carbons were prepared via resorcinol-formaldehyde (RF) sol-gel polymerization in the presence of cationic surfactant (CTAB, cetyltrimethylammonium bromide), wherein the carbon sphere size was controlled by varying the CTAB introduction time after a pre-determined period of addition reaction (termed as "pre-curing"). The sphere size gradually decreases with an increase in the pre-curing time within the range of 30-150 nm. The carbons possess two types of pores; one inside carbon spheres (intra-particle pores) and the other at the interstitial sites made by carbon spheres (inter-particle pores). Of the two, the surface exposed on the former was dominant to determine the electric double-layer capacitor (EDLC) performance of porous carbons. As the intra-particle pores were generated inside RF gel spheres by gasification, the pore diameter was similar for all these carbons, thereby the pore length turned out to be a decisive factor controlling the EDLC performance. The charge-discharge voltage profiles and complex capacitance analysis consistently illustrate that the smaller-sized RF carbons deliver a better rate capability, which must be the direct result of facilitated ion penetration into shorter pores.via Research Center for Energy Conversion and Storage, and by the Division of Advanced Batteries in NGE Program(Project No. 10016439)

Complex capacitance analysis on rate capability of electric-double layer capacitor (EDLC) electrodes of different thickness

The complex capacitance analysis is utilized to examine the thickness-dependent rate capability of electric double-layer capacitor (EDLC) electrodes. Based on the transmission line model, the theoretical imaginary capacitance is derived for porous carbon electrodes, where the resistance relevant to ion transport in pores of carbon particles (intra-particle pores) and within electrode layer (inter-particle pores) is assumed to be the major component for equivalent series resistance (ESR). The use of hexagonal mesoporous carbon (HMC) as the EDLC electrode material, which has a well-defined pore structure, allows us to estimate the number of intra-particle pores in the composite electrodes such that the two resistance components are separately analyzed as a function of electrode thickness. As the theoretical derivation suggests, the time constant for intra-particle pores is invariant against the electrode thickness, whereas the time constant for inter-particle pores becomes larger for thicker electrodes. The poorer rate capability observed in the thicker electrodes is thus ascribed to a larger time constant for inter-particle pores.This work was supported by KOSEF through the Research Center for Energy Conversion and Storage. We thank them for their financial support