Researchers at Michigan State University in East Lansing and Duke University in North Carolina have engineered a new type of supercapacitor that remains fully functional even when it is stretched to eight times its original size, according to ScienceDaily.
The researchers envision it being part of a power-independent, stretchable, flexible electronic system for wearable electronics or medical devices.
The supercapacitor does not exhibit wear and tear from being stretched repeatedly and only loses a few percentage points of energy performance after 10,000 cycles of charging and discharging.
“Our goal is to develop innovative devices that can survive mechanical deformations like stretching, twisting, or bending without losing performance,” says Changyong Cao, senior author of the study and assistant professor of packaging, mechanical engineering, and electrical and computer engineering and director of the Laboratory for Soft Machines and Electronics at MSU. “But if the power source of a stretchable electronic device isn’t stretchable, then the entire device system will be constrained to be non-stretchable.”
A supercapacitor stores energy through a charge separation, unlike batteries, which store energy chemically and generate charges through chemical reactions. Supercapacitors cannot create their own energy and 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 flows quickly between them.
Supercapacitors are also able to discharge their energy in short but massive bursts, unlike batteries which discharge their energy through a long, slow trickle. They can also charge and discharge must faster than a battery and tolerate many more charge-discharge cycles. They are perfect for short, high-power applications such as setting off the flash in a camera or the amplifiers in a stereo.
Most supercapacitors are just as hard and brittle as any other component on a circuit board. The researchers created a stretchable stamp-sized supercapacitor that can carry more than two volts.
To make the stretchable supercapacitors, the researchers grow a carbon nanotube forest – a patch of millions of nanotubes on a silicon wafer. The patch and wafer are about the width of the smallest bacteria and height of the animal cell it infects. The researchers coat a thin layer of gold nanofilm on top, which acts as an electric collector, dropping the resistance of the device, which allows the device to charge and discharge much faster.
The nanotube forest is then transferred to a pre-stretched elastomer substrate gold-side down. The electrode is then relaxed to allow the pre-strain to release, causing it to shrink to a quarter of its original size. The process crumples up the thin layer of gold and smashes together the trees in the nanotube forest.
“The crumpling greatly increases the amount of surface area available in a small amount of space, which increases the amount of charge it can hold,” says Jeff Glass, senior author of the research and professor of electrical and computer engineering at Duke. “If we had all the room in the world to work with, a flat surface would work fine. But if we want a supercapacitor that can be used in real devices, we need to make it as small as possible.”
The 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 says. “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.”
The supercapacitors could power their own devices or 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. In Japan, they use them to power buses for urban commuting, completing a full recharge at each stop in the time it takes to load and unload passengers.
“A lot of people want to couple supercapacitors and batteries together,” Glass says. “A supercapacitor can charge rapidly and survive thousands or even millions of charging cycles, while batteries can store more charge so they can last a long time. Putting them together gives you the best of both worlds. They fill two different functions within the same electrical system.”
The results of the study appear in Matter and are available here.