Researchers have engineered a novel type of supercapacitor that remains fully functional even when stretched to eight times its original size. It does not exhibit any wear and tear from being stretched repeatedly and loses only a few percentage points of energy performance after 10,000 cycles of charging and discharging. The supercapacitor could be part of a power-independent, stretchable, flexible electronic system for applications such as wearable electronics or biomedical devices.
A supercapacitor (also sometimes referred to as an ultracapacitor) stores energy like a battery but with some important differences. Unlike batteries, which store energy chemically and generate charges through chemical reactions, an electrostatic double-layer supercapacitor (EDLSC) stores energy through charge separation and cannot create its own electricity. It must be charged from an outside source. During charging, electrons are built up on one part of the device and removed from the other, so that when the two sides are connected, electricity quickly flows between them.
Also unlike batteries, supercapacitors are able to discharge their energy in short but massive bursts, rather than through a long, slow trickle. They can also charge and discharge much faster than a battery and tolerate many more charge-discharge cycles than a rechargeable battery. This makes them perfect for short, high-power applications such as setting off the flash in a camera or the amplifiers in a stereo. But most supercapacitors are just as hard and brittle as any other component on a circuit board.
The new stamp-sized supercapacitor can carry more than two volts. When connecting four together, as many devices require for AA or AAA batteries, the supercapacitors could power a two-volt watch for an hour and a half.
To make the stretchable supercapacitors, the team first grew a carbon nanotube forest —- a patch of millions of nanotubes just 15 nanometers in diameter and 20-30 micrometers tall — on top of a silicon wafer. They then coated a thin layer of gold nanofilm on top of the carbon nanotube forest. The gold layer acts as a sort of electric collector, dropping the resistance of the device an order of magnitude below previous versions, which allows the device to charge and discharge much faster.
The carbon nanotube forest is then transferred to a pre-stretched elastomer substrate with the base gold-side-down. The gel-filled electrode is then relaxed to allow the pre-strain to release, causing it to shrink to a quarter of its original size. This process crumples up the thin layer of gold and smashes together the “trees” in the carbon nanotube forest. The superdense forest is then filled with a gel electrolyte that can trap electrons on the surface of the nanotubes. When two of these final electrodes are sandwiched close together, an applied voltage loads one side with electrons while the other is drained, creating a charged super-stretchable supercapacitor.
Stretchable supercapacitors could power some devices on their own or they could be combined with other components to overcome engineering challenges; for example, supercapacitors can be charged in a matter of seconds and then slowly recharge a battery that acts as the primary source of energy for a device. This approach has been used for regenerative breaking in hybrid cars, where energy is generated faster than it can be stored. Supercapacitors increase the efficiency of the whole system.
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