Stretchable Supercapacitors for the Next Stage of Wearables
A new discovery by researchers from Duke University and Michigan State University could lead to an excellent, stretchable source of power for wearable electronics.
Imagine a new type of supercapacitor that is capable of being stretched to eight times its original size while still retaining full functionality, and only after 10,000 charge cycles does it begin to lose just a modicum of its energy performance.
Well, you can stop imagining: Researchers from Duke University and Michigan State University (MSU) have done just that. In their study which was published last Thursday in the journal Matter, the research team described their novel supercapacitor as potentially forming part of a stretchable, flexible, power-independent system that could be used in wearable devices.
Carbon nanotube forests placed on an elastomer substrate pre-stretched in two directions, which creates a maze of spaghetti, improving a stretchable supercapacitor's performance. Image used courtesy of Duke University
Developing a Truly Flexible Power Source for Wearable Devices
By developing this novel supercapacitor, the team has achieved what it originally set out to do—develop a flexible power source for wearable devices.
"Our goal is to develop innovative devices that can survive mechanical deformations like stretching, twisting or bending without losing performance,” said Yunteng Cao, 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 like a battery. But, unlike a battery, it stores energy through charge separation and is not able to create its own. To charge, it needs an outside source. In contrast to batteries, however, they release energy in short but large bursts and are also capable of charging much more quickly. These properties make them ideal for short, high-powered applications in electronic systems.
A major flaw of the supercapacitor is that they are hard and rigid, just like batteries. This puts a stop to them being used in many potentially useful applications.
A Stamp-Sized Supercapacitor Made from Carbon Nanotubes
For years, Jeff Glass and Yunteng Cao have been working on an alternative. In their new paper, the duo describes how they were able to fabricate a stamp-sized supercapacitor that is capable of carrying more than two volts. When four are connected, the supercapacitors can power a two-volt Casio watch for an hour and a half.
Rows of carbon nanotubes created on an elastomer substrate and pre-stretched in one direction and allowed to contract. The process creates stretchable supercapacitors that hold more charge in less space. Image used courtesy of Duke University
To create their stretchable supercapacitors, Glass and his research team first grew a carbon nanotube forest on top of a silicon wafer. The nanotube forest consists of millions of nanotubes measuring just 15 nanometers in diameter and 20—30 nanometers in height. On top of this forest, a thin layer of gold nanofilm was applied. This layer acts as an electric collector, dropping the resistance of the device below previous versions, thus allowing the device to go through charge cycles much more quickly.
At this point, Glass hands the engineering process to Cao and his team which employs a method to crumple the nanotube forest. As Glass explained it: “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.” All the carbon nanotubes are then filled with a gel electrolyte that can trap electrons on the surface of the nanotubes.
The result is a highly stretchable supercapacitor that can be used to power wearable devices and more on their own. They could also be combined with other components to overcome engineering challenges. For example, supercapacitors that can be charged in mere seconds could be used to then slowly recharge a battery which acts as a device’s primary energy source.
“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.”