Big Energy Storage in Thin Films  

Technology Review has an article on research into combining ultracapacitors with thin film solar - Big Energy Storage in Thin Films.

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.

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.