PORTLAND, Ore. — Ultracapacitors, also called supercapacitors, serve as temporary energy storage that can quickly charge and discharge for everything from regenerative brakes in electric vehicles to cordless power tools that recharge in 90 seconds to stabilizing computer power supplies. Now researchers at George Washington University's Micro-Propulsion and Nanotechnology Laboratory report that superior ultracapacitors can be constructed from an inexpensive hybrid composite of graphene flakes mixed with single-walled carbon nanotubes.
In an American Institute of Physics (AIP) report the new style ultracapacitor is described in detail by professor Michael Keidar at George Washington University, in Washington, D.C., and doctoral candidate Jian Li, now graduated and working as a senior process engineer at Global Foundries Inc. They write:
The great advantage of these hybrid structures' design is that graphene can make the contribution for good conductivity in plane of nanostructures and high surface area for the composite ultracapacitor. However, the important role of single walled carbon nanotubes in the composite is to connect all of the structures into a uniform network, and thus enhance the performance of composite ultracapacitors.
Scanning electron microscope reveals the ultracapacitor's hybrid film contains graphene flakes and single-walled carbon nanotubes.
(Source: George Washington University)
Other research groups have experimented with using graphene in supercapacitors
as well as using carbon nanotubes in supercapacitors
, but Keidar and Li claim to lead the first research group that combined the best qualities of both forms of carbon into a superior ultracapacitor that is inexpensive to manufacture.
Both carbon nanotubes and graphene have superior electron mobility, making them perfect for meeting the ultracapacitor's need to quickly deliver power and just as quickly recharge, thus combining the energy storage capacity of a battery with the quick energy delivery and recharging capabilities of a capacitor. The materials are also 100 times stronger than steel yet conduct thermal energy even better than diamond.
The discovery of the hybrid composite material was by accident when the research team found that both could be easily synthesized by vaporizing a hollow graphite rod filled with a metallic catalyst powder with an electric arc. By thoroughly mixing the two materials to form an ink, the team was able to roll the film onto paper -- a common separator for high-capacity commercial capacitors.
When they measured the hybrid material's specific capacitance, they were surprised to find that its was three-times higher than the specific capacitance for carbon nanotubes alone. Although its high specific capacitance of 100 F/g has also been achieved by rival techniques, the researchers claim that their material is much easier to manufacture, potentially resulting in drastically cheaper ultracapacitors.
Funding for this project was supplied, in part, by the National Science Foundation and the Department of Energy Partnership in Plasma Science and Technology.
— R. Colin Johnson, Advanced Technology Editor, EE Times