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Big Energy Storage in Thin Films

New ultracapacitor material could be fabricated directly on chips and solar cells.

Energy storage devices called ultracapacitors can be recharged many more times than batteries, but the total amount of energy they can store is limited. This means that the devices are useful for providing intense bursts of power to supplement batteries but less so for applications that require steady power over a long period, such as running a laptop or an engine.

Micro ultracapacitor: This thin-film carbon ultracapacitor electrode, shown in a microscope image, is about 50 micrometers on each side. The zigzagging, porous regions are the active part of the device.

Now researchers at Drexel University in Philadelphia have demonstrated that it’s possible to use techniques borrowed from the chip-making industry to make thin-film carbon ultracapacitors that store three times as much energy by volume as conventional ultracapacitor materials. While that is not as much as batteries, the thin-film ultracapacitors could operate without ever being replaced.

These charge-storage films could be fabricated directly onto RFID chips and the chips used in digital watches, where they would take up less space than a conventional battery. They could also be fabricated on the backside of solar cells in both portable devices and rooftop installations, to store power generated during the day for use after sundown. The materials have been licensed by Pennsylvania startup Y-Carbon.

An ultracapacitor is “an electrical energy source that has virtually unlimited lifetime,” says Yury Gogotsi, professor of materials science and engineering at Drexel University in Philadelphia, who led the development of the thin-film ultracapacitors. “It will live longer than any electronic device and never needs to be replaced.” While batteries store and release energy in the form of chemical reactions, which cause them to degrade over time, ultracapacitors work by transferring surface charges. This means they can charge and discharge rapidly, and because the electrode materials aren’t involved in any chemical reactions, they can be cycled hundreds of thousands of times. Researchers have begun developing thin-film ultracapacitor materials but have had difficulty getting high enough total energy storage using practical fabrication methods, says Gogotsi.

Gogotsi’s group uses a high-vacuum method called chemical vapor deposition to create thin films of metal carbides such as titanium carbide on the surface of a silicon wafer. The films are then chlorinated to remove the titanium, leaving behind a porous film of carbon. In each place where a titanium atom was, a small pore is left behind. “The film is like a molecular sponge, where the size of each pore is equal to the size of a single ion,” says Gogotsi. This matching means that when used as the charge-storage material in an ultracapacitor, the carbon films can accumulate a large amount of total surface charge. The Drexel researchers complete the device by adding metal electrodes to either surface to carry current into and out of the device and adding a liquid electrolyte to carry the charges. They found that the performance of the device is best when the carbon material is about 50 micrometers thick, about the same as the width of a human hair.

The Drexel researchers first developed this ultracapacitor material a few years ago; today in the journal Science they report the first demonstration of thin films made from it. Conventional ultracapacitors are made from powdered activated carbon. These powders can’t be used to make large, thin films because they won’t stick to the surface. Other groups have developed printable thin-film ultracapacitors based on carbon nanotubes; Gogotsi says his devices can store more charge.

Gogotsi says there is, in theory, no limit to the size of the films that could be made using these methods, which are used by the solar industry and display industries to make panels as large as nine square meters. Because the carbon films are thin and can be made at temperatures as low as 200 ºC, it might be possible to integrate them with flexible electronics.

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