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Supercapacitor could be the key to wearables

by on20 March 2020

Works at a stretch

Boffins at Duke University and Michigan State University have engineered a novel type of supercapacitor that remains fully functional even when stretched to eight times its original size.

According to Matter science journal, which we get for its spot the proton competition, the supercapacitors do not exhibit any wear and tear from being stretched repeatedly and lose only a few percentage points of energy performance after 10,000 cycles of charging and discharging. It could be the key to creating wearable electronics or biomedical devices.

The boffins included Changyong Cao, assistant professor of packaging, mechanical engineering and electrical and computer engineering at Michigan State University (MSU), and senior author Jeff Glass, professor of electrical and computer engineering at Duke. Their co-authors are doctoral students Yihao Zhou and Qiwei Han and research scientist Charles Parker from Duke, as well as Ph.D. student Yunteng Cao from the Massachusetts Institutes of Technology.

A supercapacitor stores energy like a battery, but 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.

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.

The downside of supercapacitors is that they are hard as any other component on a circuit board. That's why Cao and Glass have spent years working on a stretchable version.

In their new paper, the researchers demonstrate fabricating a stamp-sized supercapacitor that can carry more than two volts. When connecting four together, as many devices require AA or AAA batteries, the supercapacitors could power a two-volt Casio watch for an hour and a half.

Glass and his research team first grow 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. That's about the width of the smallest bacteria and height of the animal cell it infects.

The researchers then coat 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.

Cao, who transfers the carbon nanotube forest 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 super dense 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.

"We still have some work to do for building a complete stretchable electronics system", Cao said. "The supercapacitor demonstrated in this paper doesn't go as far as we want it to yet. But with this foundation of a robust stretchable supercapacitor, we will be able to integrate it into a system that consists of stretchable wires, sensors and detectors to create entirely stretchable devices."


Last modified on 20 March 2020
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