"Most power sources, such as phone batteries, are not stretchable. They are very rigid," says Dr Xiaodong Chen. "My team has made stretchable electrodes, and we have integrated them into a supercapacitor, which is an energy storage device that powers electronic gadgets."
Supercapacitors have a higher power density and longer life cycle than standard capacitors or batteries but less storage time, so Chen and his team sought to develop a micro-supercapacitor from graphene.
"Graphene can be flexible and foldable, but it cannot be stretched," he said. To fix that, Chen's team took a cue from the wave-like microstructure of skin. "We started to think of how we could make graphene more like a wave," he said.
The researchers' first step was to make graphene micro-ribbons using a chemical process. "We have more control over the graphene's structure and thickness that way," he said. "It's very difficult to control that with the physical approach. Thickness can really affect the conductivity of the electrodes and how much energy the supercapacitor overall can hold."
The researchers created a stretchable polymer chip with a series of pyramidal ridges, placing graphene micro-ribbons across the ridges in a wave-like structure. The design allowed the material to stretch without the graphene electrodes of the superconductor detaching, cracking or deforming. In addition, the team developed kirigami structures, which are variations of origami folds, to make the supercapacitors 500 percent more flexible without decaying their electrochemical performance. As a final test, an LCD from a calculator was powered with the micro-supercapacitor. These stretchy supercapacitors can also be used in pressure or chemical sensors.
In future experiments, the researchers hope to increase the electrode's surface area so it can hold even more energy. The current version only stores enough energy to power LCD devices for a minute, says Chen.
The work is being presented this week at the American Chemical Society www.acs.org