Sang-k Woo

Preparation of three dimensionally ordered macroporous carbon with mesoporous walls for electric double-layer capacitors

Three dimensionally ordered macroporous (3DOM) carbons with mesoporous walls were prepared by a colloidal crystal templating method. A three dimensionally ordered composite consisting of monodisperse polystyrene (PS) latex (100–450 nm) and colloidal silica (5–50 nm) was prepared by an evaporation process of suspensions containing PS latex and colloidal silica in water. In the course of the heat treatment of this composite membrane at 573 K under an inert atmosphere, the PS was melted and penetrated into the spaces between the colloidal silica. The penetrated PS was carbonized during further heat treatment to provide a very thin carbon layer on the colloidal silica, and the macropore corresponding to the PS particle size was formed simultaneously. After this procedure, the 3DOM carbon with mesoporous walls was obtained by removing the silica particles. From the results of scanning electron microscope observations and nitrogen adsorption-desorption measurements, it was confirmed that the prepared carbon had a bimodal porous structure, and the sizes of macropores and mesopores of prepared carbon were in good agreement with the sizes of the PS and silica particles used as templates, respectively. The bimodal porous carbon, which had a specific surface area of 1500 m2 g−1 and 5 nm mesopores, showed highest capacitance of 120 F g−1 in propylene carbonate solution containing 1 mol dm−3 (C2H5)4NBF4. The mesopore size rather than macropore size gave significant effects on the rate capability of carbon electrode during charge and discharge. The bimodal porous carbon having 5 nm mesopores showed an excellent rate capability and its capacitance at a high current density of 4 A g−1 was 109 F g−1.

Improved capacitance characteristics of electrospun ACFs by pore size control and vanadium catalyst

Nano-sized carbon fibers were prepared by using electrospinning, and their electrochemical properties were investigated as a possible electrode material for use as an electric double-layer capacitor (EDLC). To improve the electrode capacitance of EDLC, we implemented a three-step optimization. First, metal catalyst was introduced into the carbon fibers due to the excellent conductivity of metal. Vanadium pentoxide was used because it could be converted to vanadium for improved conductivity as the pore structure develops during the carbonization step. Vanadium catalyst was well dispersed in the carbon fibers, improving the capacitance of the electrode. Second, pore-size development was manipulated to obtain small mesopore sizes ranging from 2 to 5 nm. Through chemical activation, carbon fibers with controlled pore sizes were prepared with a high specific surface and pore volume, and their pore structure was investigated by using a BET apparatus. Finally, polyacrylonitrile was used as a carbon precursor to enrich for nitrogen content in the final product because nitrogen is known to improve electrode capacitance. Ultimately, the electrospun activated carbon fibers containing vanadium show improved functionality in charge/discharge, cyclic voltammetry, and specific capacitance compared with other samples because of an optimal combination of vanadium, nitrogen, and fixed pore structures.

NANOCOMPOSITE ELECTRODES CONSISTING OF 3DOM CARBON WITH BIMODAL POROUS STRUCTURE AND CONDUCTING POLYMERS FOR ELECTROCHEMICAL CAPACITORS

Three-dimensionally ordered macroporous (3DOM) carbon with mesoporous walls was prepared by the colloidal crystal templating method using polystyrene (PS) and silica particles. Colloidal crystals consisting of monodispersed PS sphere (204 nm diameter) and silica particles (4–6 nm) were carbonized in Ar, and the silica was removed by etching in HF, and then 3DOM carbon with bimodal pore structure was obtained. Conducting polymers of polyaniline (PAn) and polypyrrole (PPy) were electrochemically deposited on the internal surface of macropores in 3DOM carbons. The mesopores were not closed after the deposition of the polymers. The composites were utilized as electrodes for electrochemical capacitors. The specific capacitances of prepared carbon–PAn and carbon–PPy composites were found to be 237 and 152 F g-1, respectively. The incorporation of the conducting polymers dramatically increased the charge storage capacities of the 3DOM carbon electrodes.

Preparation of Porous Carbon with Bimodal Porous Structure and as a Electrode Material for Electric Double-Layer Capacitor

The electric double layer capacitors (EDLCs) as rechargeable energy storage device have drawn considerable attention because they provide high power density compared with conventional rechargeable battery. The EDLCs are based on the double layer capacitance (DLC) at solid/solution interface. In particular, DLC in the system of electrode / electrolyte depends not only on the specific surface area (SSA), but also on the pore size distribution (PSD) for ion mobility. For this reason, some research groups have made efforts to prepare mesoand macroporous carbon with the reasonable PSD by means of chemical activation and catalytic graphitization by metal, and template method etc. In this study, we prepared porous carbons with 3D ordered meso / macroporous structures using colloidal imprinting method. Prepared carbons using colloidal crystal have bimodal porous structures of meso / macroscale. The dependence of DLC on the meso / macroporous structures will be discussed. The monodisperse PS latex (204 and 112 nm) and the silica particles (10∼20 nm) were uniformly mixed in deionized water by an ultrasonic treatment. The mixed suspension was simply evaporated in Petri dish at 60 °C for 24 hours, and the PS-silica composite was accumulated on the bottom of the Petri dish. During the evaporation, the monodisperse PS spheres self assemble into an ordered lattice where the silica particles are forced to pack closely at the interstices between the PS spheres. The volume ratio of PS and silica in the composite was controlled to be 74 : 26, which allowed the PS spheres to form close-packed lattice in the composite. The obtained PS-silica composite was treated at high temperatures in a tubular furnace with dry argon (30 mL min flow rate). The temperature was increased from room temperature to 1000 °C at a heating rate of 5 °C min and cooled to room temperature. After the carbonization, the silica particles were removed with 20 % aqueous hydrofluoric acid, and the resulting porous carbons were washed with ultra pure water and dried in a vacuum oven at 110 °C. The obtained porous carbon samples were referred to PE200 and PE100 according to the size of utilized PS particle (204 and 112 nm), respectively. A composite electrode was prepared from porous carbon, acetylene black (AB), and PTFE binder. The ratio of porous carbon, AB and PTFE in a composite electrode was 80:10:10 wt %, respectively. Titanium mesh was utilized as the current collector for the composite electrode. A standard three electrode electrochemical cell is adopted to measure the DLC of a single porous electrode. The counter electrode was activated carbon fiber (BET-SSA: 2000 m g). Lithium metal foil connected nickel wire was used as the reference electrode. The electrochemical measurement of the DLC was performed under galvanostatic conditions in propylene carbonate (PC) containing 1.0 mol dm LiClO4 and LiBF4. Cyclic voltammetry measurements were carried out at scan rate of 5 mV sec, and galvanostatic discharge / charge measurements were performed at various rates (100 mA g corresponded to 1.0 mA cm of current density). All electrochemical measurements were carried out in argon-filled grove box at room temperature. Fig. 1 shows SEM image of prepared porous carbon (PE200). Prepared carbon has macroporous structure, and size of macropore resemble to the size of utilized PS sphere. In addition, prepared porous carbon has a narrow PSD centered at 15 nm as calculated from the adsorption branch of isotherm by Barret-JoynerHalenda method. The size of mesopore calculated from adsorption isotherm is corresponds to the size of utilized silica. Consequently, it is considered that prepared carbon has bimodal pore structure of meso / macroscale. Fig. 2 shows cyclic voltammograms of prepared carbons in 1M LiClO4 / PC electrolyte at scan rate of 5mV sec. Specific capacitances of PE100 and PE200 in 1M LiClO4 / PC electrolyte (current density: 10mA g) were as high as 120 F g and 107 F g, respectively.