We've seen flexible supercapacitor designs before, but how about one that's stretchable? A new discovery by researchers at Duke University and Michigan State University could lead to an excellent stretchable power source for wearable electronics.
Supercapacitors, of course, are known for their excellent power density, charging and discharging a lot of energy quickly and having a longer lifespan than chemical batteries, which usually (but not always) have the advantage of storing significantly more energy.
This research team set out trying to develop a truly flexible power source for the wearables they were working on. "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.”
The team struck pay dirt with a design that starts out with a small "carbon nanotube forest" on a silicon wafer – millions of nanotubes about 20-30 micrometers tall in a patch about 15 nanometers in diameter. The ends are covered in a gold nanofilm layer, which drops the resistance of the final device and allows it to transfer charge much faster than previous designs.
This is laid, gold side down, onto an elastic substrate that's stretched out to four times its normal length under tension. Once it's on, the tension is released, and the whole thing crumples up, smashing the nanotube "trees" in the "forest" together in even higher density. Finally, the nanotube "forests" are filled up with a gel electrolyte capable of trapping electrons on the surfaces of the nanotubes. At this point, sandwiching two of these electrodes together and applying a voltage sends all the electrons from one side to the other, where they can be stored and later released as power.
At this stage, building a patch of this stuff the size of a postage stamp gets you a two-volt supercapacitor. If you connect four together, say the researchers, you could power a two-volt Casio watch for an hour and a half. “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.”
Like just about everyone we've spoken to who's working on next-gen supercapacitors, this team says they're likely to work best in a hybrid energy system with some chemical battery cells, which can provide the energy density to go with the supercapacitors' power density, each playing to its strengths. “A supercapacitor can charge rapidly and survive thousands or even millions of charging cycles," said Jeff Glass, professor of electrical and computer engineering at Duke. "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.”
A paper on the research was published in the SSRN Electronic Journal.
Source: Duke University