Capacitors are able to charge and discharge more quickly than batteries, and can do so hundreds of thousands of times. Batteries, on the other hand, are able to store more energy than capacitors. There are also electric double-layer capacitors (EDLCs), otherwise known as supercapacitors, that can hold battery-like amounts of energy while retaining the charge/discharge speed of regular capacitors. EDLCs incorporate liquid or gel-like electrolytes, however, which can break down under hot or cold conditions. Now, a new solid-state supercapacitor developed at Houston's Rice University is using nanotechnology to get around that limitation.

The Rice researchers started out by growing an array of 15-20 nanometer bundles of single-walled carbon nanotubes, each up to 50 microns in length. This "nanotube forest" served to maximize the surface area available to electrons.

That array was subsequently transferred to a copper electrode, that included thin layers of gold and titanium to help with electrical stability and adhesion. In an atomic layer deposition process, the bundles (which served as the primary electrodes) were next doped with sulfuric acid to boost their conductivity. They were then covered with aluminum oxide, which served as a dielectric layer, and aluminum-doped zinc oxide, which acted as the counterelectrode. Finally, the circuit was completed with a top electrode of silver paint.

The Rice supercapacitor is reportedly stable and scalable, holds a charge under high-frequency cycling, and isn't adversely effected by harsh temperatures. It could also be incorporated into other materials, allowing for electric car bodies that double as batteries, or microrobots that serve as their own power supply.

"All solid-state solutions to energy storage will be intimately integrated into many future devices, including flexible displays, bio-implants, many types of sensors and all electronic applications that benefit from fast charge and discharge rates," said Cary Pint, who co-led the research.

Technology that combines the attributes of capacitors and batteries is also being developed at the University of Illinois, where scientists are creating nanostructured lithium-ion batteries that charge and discharge 10 to 100 times faster than regular li-ions.