U.S. researchers say they've used carbon structures just one atom thick to develop high-performance, low-cost energy-storing ultracapacitors.
Combining two different carbon nanostructures possessing complementary properties -- single-walled nanotubes and graphene flakes -- yielded capacitors capable of containing large quantities of energy that can be quickly released to provide a power surge, the researchers at George Washington University say.
Such ultracapacitors may provide increased performance in handheld electronics like smartphones and tablets, in electric vehicles, audio systems and other applications, they say.
Excellent electronic, mechanical and thermal properties make both graphene and single-walled nanotubes attractive candidates for ultracapacitors, lead researcher Jian Li says; the trick was in bringing them together.
Although both materials had been studied singly, few efforts had been made to combine them, Li's colleague Michael Keidar says.
"In our lab we developed an approach by which we can obtain both single-walled carbon nanotubes and graphene, so we came up with the idea to take advantage of the two promising carbon nanomaterials together," he says.
Writing in the Journal of Applied Physics, the researchers describe how they combine the two different nanostructures to create ink capable of being rolled onto paper, a normal separator used in current capacitor designs.
The resulting specific capacitance -- defined as the energy performance of the capacitor compared with its weight -- was as much as three times that of a capacitor made from the carbon nanotubes by themselves, they reported.
While the graphene flakes contribute good conductivity and a large surface area, the nanotubes hold the structure tougher in a uniformly spaced network, Li explained. The carbon nanotubes act as reinforcing bars, making the single-atom graphene flakes much easier work with.
The resulting capacitor is inexpensive, since the desirable mix of nanostructures is simple to create in large amounts. The researchers said they were able to synthesize the graphene flakes and nanotubes simultaneously by vaporizing a hollow graphite rod filled with metallic catalyst powder with an electric arc.
In addition to being cheap to produce, because the ultracapacitor is light and small it will lead to ever smaller and more efficient electronic devices, they researchers said.
The research was conducted in the Micro-propulsion and Nanotechnology Laboratory at George Washington University.
