Thursday, February 18, 2010

AIST Nanotube Capacitor Development Project Uses Super Growth Method to Create Forest of Carbon Nanotubes for Capacitors

Advanced Industrial Science and Technology: 1 g single wall carbon nanotube (SWCNT) material with a high specific surface area (left: 500-yen coin), and TEM image

Image credit: AIST

Japan's National Institute of Advanced Industrial Science and Technology (AIST) Nanotube Research Center Carbon is coordinating the Nanotube Capacitor Development Project. That project is developing a mass synthesis technology for carbon nanotube (CNT) forests, as well as their the use of CNT forest in a high energy density capacitors.  The project is being highlighted at nano tech 2010 in Japan.  

Kenji Hata (Leader), Super Growth CNT Team, the Nanotube Research Center (Director: Sumio Iijima), Hiroaki Hatori (Senior Research Scientist), Energy Storage Materials Group, the Energy Technology Research Institute (Director: Hiroo Hasegawa), and others of the National Institute of Advanced Industrial Science and Technology (AIST) (President: Tamotsu Nomakuchi) have developed a fibrous material with a specific surface area of 2240 m2/g by using single-walled carbon nanotubes (SWCNTs).

Materials with large specific surface areas are used for energy storage as electricity storage devices including capacitors.  They are also used for storage, purification and separation of substances.  Many existing materials are brittle and it was difficult to make them into easy-to-handle solids while maintaining their specific surface areas.

In this study, the tips and walls of the aligned SWCNTs synthesized using “Super Growth method” were holed by oxidation.  A fibrous material with a specific surface area of 2240 m2/g was produced by this opening process.  The specific surface area of the new material is larger than that of porous silica and activated carbon which are exiting materials with high specific surface areas.  A prototype capacitor that was fabricated by using this material as its electrodes was found to have a high energy density of 24.7 Wh/kg and a high power density of 98.9 kW/kg.

The performance of the prototype was better than that of conventional capacitors. Furthermore, substances that are included in the SWCNTs can be selected by adjusting the hole diameter, which is controlled by varying the temperature in the opening process. It is expected that this novel electrode material will help realize a compact, light, high-performance capacitor. Further, the material is expected to have a wide variety of applications, ranging from energy storage to substance storage.

SWCNTs attracted attention because their specific surface areas did not become small even when they aggregated; hence, they did not suffer from the disadvantages of conventional materials with large specific surface areas.  However, a SWCNT material with a specific surface area close to the theoretical value (2630 m2/g) has not yet been obtained.  Only small values (240-1250 m2/g) have been reported so far.

AIST developed the Super Growth method—a synthesis method for SWCNTs—and an oriented high-purity SWCNT structure in which SWCNTs are vertically aligned (This structure is named “CNT Forest.”).

 AIST has recently developed a technique for densifying this aligned structure and has been conducting research on utilizing this structure as electrode materials for capacitors. The Nanotube Research Center and the Energy Technology Research Institute of AIST jointly pursued researches on the use of SWCNTs in capacitor electrodes including a research on a processing method for increasing the specific surface area.  Consequently, we developed the new materials.

This development is a result of the "Nanotechnology Program: Carbon Nanotube Capacitor Development Program," commissioned by the New Energy and Industrial Technology Development Organization (NEDO).

The CNT Forest synthesized by the Super Growth method consists of SWCNTs (99.5%) and the SWCNTs are aligned in a single direction.  The SWCNTs in CNT Forest have larger diameters and are of higher purity than other SWCNTs.  Focusing on these characteristics, we examined opening process by oxidation, through which the tips and walls of SWCNTs were holed to increase their specific surface areas.  The CNT Forest was heated to temperatures of 350 to 600 °C at a rate of 1 °C/min in dry air and oxidized by oxygen in air in order to hole the structure.

In order to encourage institutions and companies to develop applications of the SWCNT materials with high specific surface areas, AIST will supply the gram-order samples of the materials.  Furthermore, AIST will collaborate with Zeon Corporation to develop a technique for the industrial mass production of CNT Forest, which is used to obtain a material with a high specific surface area, and a high-performance capacitor will be developed in collaboration with Nippon Chemi-Con Corporation; these will be carried out under the "Nanotechnology Program: Carbon Nanotube Capacitor Development Program," commissioned by NEDO.

Details of the results will be published in a German scientific journal “Advanced Functional Materials.”

Carbon Nanotube Capacitor Development Project
National Institute of Advanced Industrial Science and Technology Nanotube Research Center
AIST Tsukuba Central5, 1-1-1 Umezono, Tsukuba, Ibaraki, Japan, 305-8565    TEL : +81-29-861-4551  FAX : +81-29-851-5425 Email : m.yumura@aist.go.jp
URL : www.aist.go.jp/ 

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