Osamu Tanaike

Preparation and pore control of highly mesoporous carbon from defluorinated PTFE

Abstract Porous carbon having more than 2000 m2/g of BET specific surface area was synthesized by defluorination of polytetrafluoroethylene (PTFE) at 473 K using sodium metal. The porous carbon as-prepared had a large amount of narrow mesopores 2–3 nm in pore width, together with micropores. Control of the pore structure was attempted by simple heat-treatment of the carbon in nitrogen, and change of the porous structures was characterized by nitrogen adsorption techniques. As a result, it was found that the ratio between micro- and mesopores was easily varied. Electric double layer capacitance was measured as one of the applications for the mesoporous carbon with specific porosity, and the effect of pore control on capacitance was investigated.

Electric double layer capacitance performance of porous carbons prepared by defluorination of polytetrafluoroethylene with potassium

Porous carbons (Brunauer-Emmett-Teller surface area 999 m 2 /g) prepared by defluorination of polytetrafluoroethylene (PTFE) powder with potassium metal showed a very high capacitance of the electrical double layer (229 F/g). Such a high value is considered to be due not only to the contribution of microporosity, but also to the development of mesoporosity. When the original PTFE was subjected to γ-ray irradiation to reduce the polymerization degree, the carbon obtained in the same procedure showed only a slightly higher value (237 F/g). The relationship between the capacitance and pore size was discussed by taking into consideration the results for conventional activated carbon fibers.

Mesoporous carbon from poly(tetrafluoroethylene) defluorinated by sodium metal

[4] Shethna HK, Bhatia SK. Interpretation of adsorption iso[6] Dubinin MM, Astakhov VA. Development of the concepts of therms at above-critical temperature using a modified microvolume filling of micropores in the adsorption of gases and pore filling model. Langmuir 1994;10:870–6. vapors by microporous adsorbents. Izv Akad Nauk SSSR Ser [5] Liang Z, Kaixi L, Licheng L. The preparation of superKhim 1971;1:5–11. activated carbons and its properties for hydrogen storage. Chin J New Carbon Mater 2001; in press.

Capacitor Properties and Pore Structure of Single- and Double-Walled Carbon Nanotubes

The correlation between capacitance and the structure of carbon nanotubes (CNTs) was revealed. The electrochemical behaviors of single- and double-walled CNTs were characteristic due to the electrochemical doping mechanism of semiconducting nanotubes, which are amphoteric and can be doped by both donors and acceptors. The capacitance and the sheet conductance of CNTs, as capacitor electrodes were dependent on their porous and electronic structure. High specific capacitance was obtained for uncapped single-walled CNTs having inner micropores with a diameter of < 1 nm.

Supercapacitors using Pure Single-walled Carbon Nanotubes

The excellent and characteristic capacitor performance of pure single-walled carbon nanotubes (SWNTs), which differ from conventional activated carbon electrodes, is reported. SWNTs with little bundling showed higher specific capacitance than activated carbons. High operating voltage can be expected for pure SWNTs without metal contamination and graphene edge structure.

Preparation and characterization of porous carbons by defluorination of PTFE with alkali metals: Effect of alkali metals on the porous structure

Porous carbons were prepared by defluorination of poly-tetrafluoroethylene(PTFE) with various alkali metals and their porous structures were compared. Using sodium for defluorination resulted in a characteristic mesoporous carbon having a large mesopore volume with the narrow pore-size distribution of 2-3 nm diameter, in contrast to using potassium and rubidium which produced also mesoporous carbons but their pore-size distributions were wide spread. This difference seemed to be caused by the dispersion state of the alkali metal fluoride particles formed during defluorination.

Electrochemical Behavior of Halogen-Doped Carbon Materials as Capacitor Electrodes

Electric double-layered capacitor (EDLC) usually uses high-surface area carbon materials as the electrodes, and the authors have reported that single-walled carbon nanotube (SWCNT) is a promising material for the electrode of EDLC [1-3]. For the further improvement of capacitance, modification of the electrode carbon materials is important. Doping is one of the methods to modify the carbons, and lithium doping is a recent hot topic to modify the negative carbon electrode which is the core technique of so-called lithium-ion capacitor. On the other hand, some of halogen is known to react with carbon strongly with p-doping. In particular, p-doping of SWCNT by bromine and iodine vapor has been investigated widely from the viewpoints of increase the carrier of SWCNT. We focus the p-doping by halogens to challenge the modification of positive carbon electrodes for capacitor system. In the present work, bromine and iodine vapors were reacted with various carbon materials such as SWCNT, graphite, and activated carbon to modify their electronic states, and their electrochemical behaviors in 1M Et4NBF4/PC were investigated, paying attentions to their capacitance, initial potential, and cyclic reversibility.

High-performance bioelectrocatalysts created by immobilization of an enzyme into carbon-coated composite membranes with nano-tailored structures

A large (40 mm ϕ) composite membrane with mesoporous silica nanotubes (F127MST) was coated with a thin carbon layer (1–2 graphene sheets) by carrying out chemical vapor deposition (CVD) using acetylene, and the obtained carbon-coated F127MST (C/F127MST) was used directly as an electrode. After evaluating the electrical conductivity inside the continuous mesopore network, the enzyme bilirubin oxidase (BOD) was loaded into the mesopores of C/F127MST to form BOD–C/F127MST. Indeed, this loading procedure was effective and achieved direct electron transfer between the enzymes and electrodes. The loading also was found to enhance the stability of stored BOD (it was stable for 15 days) and could be used to control the enzymatic reaction by varying the electric potential. We therefore consider BOD–C/F127MST to be a promising candidate as an effective bioelectrode in various fields.

Application of Carbon Materials Derived from Fluorocarbons in an Electrochemical Capacitor

Various carbon materials can be prepared by defluorination of fluorocarbon materials, such as polytetrafluoroethylene (PTFE), with alkali metals or alkali cation–naphthalene anion radical complexes. The pore structure of the PTFE-based porous carbons prepared by defluorination without any traditional activation processes is mesoporous/microporous depending on the defluorination conditions. The PTFE-based porous carbons show a high capacitance and rate performance as an electric double layer capacitor (EDLC) electrode compared to conventional activated carbons due to the presence of mesopores that cancel the ion-sieving effect and accelerate the electrolyte ion transfer in the pores. A nitrogen-containing fluorocarbon or graphite fluoride can be defluorinated as well as PTFE to provide nitrogen-doped porous carbons or a lamellar carbon material with high capacitance, respectively. Thus, the defluorination technique is promising for realizing a high-performance carbon electrode for the EDLC.