CHAPEL HILL - Experiments with carbon nanotubes, a new form of carbon discovered about a decade ago, suggest for the first time that it should be possible to store more energy in batteries using the tiny tubes than with conventional graphite electrodes.
The experiments, conducted at the University of North Carolina at Chapel Hill, show carbon nanotubes can contain roughly twice the energy density of graphite. One possibility, researchers say, is longer-lasting batteries.
A report on the findings appears in the Jan. 7 issue of Physical Review Letters, a top physics journal. Authors are Dr. Hideo Shimoda, postdoctoral fellow; Bo Gao, graduate student; Drs. Xiao-Ping Tang and Alfred Kleinhammes, both research assistant professors; graduate student Leslie Fleming; and Drs. Yue Wu and Otto Z. Zhou, both associate professors.
All are with UNC's department of physics and astronomy or curriculum in applied and materials sciences.
"Scientists and others, including the popular press, have shown a lot of interest in carbon nanotubes because of numerous potential applications," Zhou said. "They are very strong tubular structures formed from a single layer of carbon atoms and are only about a billionth of a meter in diameter."
Future uses may include flat panel display and telecommunications devices, fuel cells, high-strength composite materials and novel molecular electronics, he said. People have raised the possibility of using them to improve batteries, but no one has demonstrated that it might work better than the conventional materials.
"In our experiments, we used both electrochemistry and solid state nuclear magnetic resonance measurements, which show similar results," Zhou said. "With graphite, we can store, reversibly, one charged lithium ion for every six carbon atoms in graphite, but we found that with nanotubes, we can store one charged lithium ion for every three carbons, also reversibly."
Most rechargeable batteries in portable electronics today are lithium-ion batteries, which use graphite or carbonaceous materials as one of the electrodes. Redox reactions occurring at the electrodes create a flow of electrons that generate and store energy.
The UNC scientists created single-wall carbon nanotubes by subjecting a carbon target to intense laser beams. By chemical processing, the researchers were able to open the closed ends of the nanotubes and reduce their lengths.
"This allows the diffusion of lithium ions into the interior space of the nanotubes and reduces the diffusion time," Zhou said. "We believe this is the reason for the enhanced storage capacity."
The promise is great for better batteries because there's no question that the team found a significantly higher energy density, he said.
"We have shown this for the first time experimentally," Zhou said. "Now, we'll have to work on and overcome other practical issues before we can make real devices, but we are very optimistic."
The UNC scientists have received an U.S. patent on using single-wall carbon nanotubes to store electrical charges. Support for the experiments came from the Office of Naval Research and the National Science Foundation.
Zhou directs the N.C. Center for Nanoscale Materials at UNC, http://www.
Note: Zhou can be reached at 919-962-3297 or Zhou@email.unc.edu. Contact: David Williamson, 919-962-8596