Asao Oya

Influence of pore structure and surface chemistry on electric double layer capacitance in non-aqueous electrolyte

The performance as electric double layer capacitors (EDLC) in non-aqueous electrolyte of a series of alkaline agent-activated carbons with high surface area is presented in this work. The results obtained show that, in general, capacitance increases with surface area. However, the results obtained in this study confirm that capacitance not only depends on surface area, but also on two other parameters: pore size distribution and surface chemistry. It has been shown that capacitance is higher for a sample with wider micropore size distribution than for a sample with higher surface area but too narrow micropore size distribution. In addition, it has been observed that the sample with a very high amount of surface groups presents very high capacitance values. In the present study, a KOH-activated carbon with a capacitance as high as 220 F/g was prepared. Finally, the results obtained with a mesoporous sample have shown that the presence of mesopores in activated carbons with very high surface area (e.g. >2000 m2/g), do not seem to be effective for double layer capacitors.

Novel preparation method for the production of mesoporous carbon fiber from a polymer blend

In this paper we describe a novel way to produce mesoporous carbon fibers without an activation process or activation catalysts starting from a novolak/poly(vinyl butyral) blend. The carbon fibers thus made essentially do not contain metals.

Formation of mesopores in phenolic resin-derived carbon fiber by catalytic activation using cobalt

Abstract Preparation of an activated carbon fiber containing mesopores was attempted by catalytic activation using cobalt. Phenolic resin and cobalt-acetylacetonate were mixed intimately, spun, cured, carbonized at 900 C in nitrogen and finally activated at 750 900 C in steam. The carbon fibers with the cobalt contents of 38 ppm and 100 ppm were prepared together with a fiber without cobalt as a reference. The cobalt deposited a characteristic turbostratic carbon by catalytic graphitization at carbonization process. Simultaneously the cobalt accelerated activation of the fiber in steam catalytically to form mesopores preferentially. The maximum surface area. 170 m2g 1. for mesopores of several 10s of nm in radius was obtained in the fiber containing 38 ppm cobalt after 38% burn-off. This fiber also contained a relatively large amount of micropores. No damage on the liber surface after activation was observed by scanning electron microscopy, but the fiber became fragile. The formation mechanism of mesopores is discussed briefly.

Factors controlling mechanical properties of clay mineral/polypropylene nanocomposites

Effects of two factors on the mechanical properties of clay mineral/polypropylene nanocomposites were examined. The first factor is the presence of a small amount of poly(diacetone acrylamide) formed between clay mineral layers. This material was expected to separate effectively the stacked layer structure of the clay mineral on mixing with polypropylene. The stacked layer structure, however, was not separated sufficiently in spite of expansion of the interlayer distance of clay mineral, leading to poor improvement of the mechanical properties of the nanocomposite. Another factor is the kind of clay mineral. Montmorillonite and mica resulted in less favorable separation of the stacked structure than hectorite, but the resulting nanocomposites gave outstanding improvement of the properties. The stiffness of clay mineral layer was considered to influence the properties of the nanocomposite strongly.

Electric double layer capacitance of highly pure single-walled carbon nanotubes (HiPco Buckytubes) in propylene carbonate electrolytes

The double layer capacitance of highly pure single-walled carbon nanotubes (SWCNTs) prepared by the HiPco™ process was measured in 1.0moldm−3LiClO4/propylene carbonate solution. The unpurified SWCNT electrode was mainly composed of a bundle structure of SWCNTs with around 1.0 nm tube diameter, small amount of amorphous carbons, and Fe catalyst particles. The Fe catalysts in the surface of the SWCNT were removed by immersion in HClaq. The αs-SPE analysis of the N2 adsorption isotherms revealed that both the SWCNTs before and after the immersion in HClaq had relatively high specific surface areas of ∼500m2g−1 without microporosity although the tube ends were closed. The SWCNTs showed a gravimetric capacitance of around 45Fg−1. Thus, the specific capacitance per unit surface area was estimated to be around 10μFcm−2, which was higher than that of conventional activated carbon fibers. Furthermore, the capacitance of the SWCNTs did not decrease even at high current density. This good rate property of the SWCNTs is related to the large area of the external surface (∼400m2g−1) on which ion adsorption/desorption can proceed fast because of no ion sieving. On the other hand, most of the Fe catalyst in the SWCNT could be removed by thermal oxidation followed by immersion in HClaq. However, the gravimetric capacitance of this purified SWCNT was not as great as that expected by the correlation of 10μFcm−2. This is related to the formation of amorphous carbons caused by the thermal oxidation.

Effect of the activating gas on tensile strength and pore structure of pitch-based carbon fibres

Abstract Petroleum pitch-based activated carbon fibres have been prepared by CO2 and steam activation. The effects of the activating gas on porosity and mechanical properties of the activated carbon fibres have been analyzed. The original carbon fibre contains a narrow porosity that is well developed upon activation in both CO2 and steam (an apparent surface area of about 1700 m 2 g is obtained after a 50% burn-off). The activated carbon fibres obtained exhibit different porous texture evolution with burn-off depending on the activating gas used. CO2 essentially develops microporosity and causes a steady decrease in the tensile strength with burn-off, while the fibre diameter does not change significantly. Contrarily, steam produces a wider porous texture and, after the initial stages of the activation process, the tensile strength remains nearly constant and the fibre diameter decreases. The results have been interpreted considering the different behaviour of the two molecules (CO2 and steam) involved in the reaction in the narrow microporosity. In this sense, CO2 seems to react to a larger extent than steam inside the narrow microporosity due to its larger coefficient diffusion.

Preparation and properties of an antibacterial activated carbon fiber containing mesopores

Preparation of an antibacterial activated carbon fiber with mesopores was attempted. Phenolic resin containing cobalt as an activation catalyst and silver as an antibacterial agent was spun, stabilized, carbonized and activated in steam. The number of the metal particles increased with as activation proceeded and reached to 100–200 nm in diameter at the largest. The activated carbon fiber with 72 m2 g−1 of mesopore surface area was obtained after 51 wt% burn-off of the carbon fiber containing 0.22 wt% of Ag and 52 ppm of Co. The silver somewhat disturbed formations of both micro- and mesopores through catalytic activation by cobalt. A silver content of 0.22 wt% in the activated carbon fiber was rapidly decreased to 0.0006 wt% after immersion in flowing tap water for 20 days, but the resulting fiber exhibited antibacterial activity against Escherichia coli and Staphylococcus aureus. The above behaviors can be reasonably explained by the formation of an alloy of cobalt and silver.

Electric Double-Layer Capacitance of Meso/Macroporous Activated Carbon Fibers Prepared by the Blending Method I. Nickel-Loaded Activated Carbon Fibers in Propylene Carbonate Solution Containing LiClO 4 Salt

Meso/macroporous activated carbon fibers (ACFs), containing mesopores and macropores in addition to micropores, were prepared from carbonization and steam-activation of the phenolic resin fibers blended with a small amount (0.1 wt %) of an organic nickel complex. Conventional (microporous) ACF as reference sample, mainly composed of micropores, was also prepared from pure phenolic resin fibers without any agent. The electric double-layer capacitance of these ACFs was measured in propylene carbonate containing 1.0 mol dm -3 LiClO 4 (1.0 M LiClO 4 /PC) The correlation between the capacitance and the BET (Brunauer-Emmett-Teller) specific surface area of the microporous ACFs showed the nonlinearity due to strong ion-sieving of micropores. The double-layer capacitance of the mes/macroporous ACFs was higher than the microporous ACF, because the ion-sieving effect of micropores was relaxed by the presence of meso/macropores. The relaxation by the meso/macropores was enhanced for the cation adsorption or the high current density measurement. These results confirm that the presence of many meso/macropores promotes the formation of an effective double layer or fast transfer of ions in the microporous structure.

Preparation of general purpose carbon fibers from coal tar pitches with low softening point

This paper describes the different procedures applied for the preparation of general purpose carbon fibers from four coal tar pitches. The four raw materials have a low softening point (of about 373 K) for this application and, hence, must be subjected to a treatment before spinning to increase this temperature. The treatments applied are the following: (i) heating in N2, (ii) heating in air, (iii) consecutive heating in N2 and air, and (iv) blending of the coal tar pitch with a petroleum one and further treatment in air. The changes in chemical composition, softening point and yield of the treatments used have been followed by different techniques. Only treatments (iii) and (iv) produce materials with an adequate viscosity for spinning and a sufficiently high softening point to be transformed in carbon fibers. The carbon fibers obtained have similar mechanical properties to those prepared in a previous work from a suitable petroleum pitch with the same experimental system. However, the mechanical properties of these fibers are inferior to those of a commercial carbon fiber, the differences being due to the lower diameter of the commercial fibers.

Antibacterial activated carbon fiber derived from phenolic resin containing silver nitrate

A novolactype phenolic resin including 1 wt% of AgNO3 was spun, hardened, carbonized at 900°C under nitrogen and finally activated at 900°C for 10 and 60 min under steam. The resulting fibers showed specific N2-BET surface areas of 280 and 1,940 m2/g, respectively. The latter contained 2.2 wt% of the silver particles of which the largest size was several 100 nm and showed antibacterial activity against Staphylococcus aureus and Escherichia coli. After soaking in the flowing tap water for 20 days, this fiber lost one third of silver but kept the antibacterial activity.