Portland, Ore. - Using conventional materials-processing technology as a foundation, researchers at the University of California-Davis have developed nanotube-based thin films to build capacitors with a power density of 30 kilowatts per kilogram, a tenfold increase over previous devices. The "supercapacitors," with a scan rate of 1,000 millivolts/second, "can be manufactured very efficiently," according to their inventor, professor Ning Pan.
Pan hopes to fabricate supercapacitors that can replace conventional batteries in handheld electronic devices. Two capacitor manufacturers are said to be competing to license the technology from UC Davis' Technology Transfer Center.
"Supercapacitors built using electrodes made from our nanotube thin films exhibit very high power density, very high scan rates and nearly ideal cyclic voltammograms," said Pan, a professor of biological and agricultural engineering. "Also, our nanotube-based electrode material is simple and fast to prepare, since no binder is required."
Researchers at the university's Nanomaterials in the Environment, Agriculture and Technology center, and consultant Jeff Yeh of Mytitek Inc. (Davis, Calif.), aided in the project. The company co-sponsored the research with the U.S. Missile Defense Agency.
Conventional Faraday capacitors store electric charge between parallel charged plates that are separated by an insulating dielectric material. Instead of flat parallel plates, capacitors that come in tubes use two metallic foils separated by an electrolyte-impregnated paper in a "sandwich" that is rolled up into the tube. For these devices, nanotube thin films can increase the surface area of the conducting foil due to the nanotubes' very small size, orderly alignment and high conductivity. "Nanotubes provide a huge surface area on which to store and release energy-that is what makes the difference," Pan said.
Supercapacitors with low leakage currents could power mobile devices, Pan said. "There is not that much difference between a battery and a capacitor. We think our technique gives us the best chance at succeeding. Our next research goal will be very durable supercapacitors with a very long life so that they can replace batteries in PDAs, cell phones and laptop computers," he said.
Today, supercapacitors are used to deliver a jolt of energy to hybrid electric/fuel-cell automobiles, which can require as much as tenfold more power to start up than their batteries can supply. For these and other applications requiring a quick jolt of current, making use of existing supercapacitor technologies can provide designers with as much as 4 kW/kg.
Other labs have reported 20-kW/kg supercapacitors that make use of nanotubes to increase power density. But, Pan said, none of these alternatives use manufacturing methods that are as easy to integrate with conventional materials processing as his approach. Pan said it will be easy to integrate his approach with existing materials-processing steps to deliver power density of 30 kW/kg.
"Our multiwalled carbon nanotubes were densely packed and aligned in a colloidal suspension with no binders, making it well-suited to efficient manufacturing methods," he said.
Pan's lab fabricated his supercapacitors by applying a thin film of nanotubes to two metal foils, separated by an insulator, before rolling them up into tubes. By reducing the resistance of the capacitors' electrodes, and their connection to the surge-delivery system, accounted for the improvement in power density.
Pan said there was no limit to how big such supercapacitors could be. He also said that catalytic methods of making the nanotubes, instead of CVD, could also be used in high-volume manufacturing settings.