Stretchy power sources

Traditional batteries are thick and rigid – not ideal properties for materials used in tiny malleable devices. However, researchers are working on development of stretchy, micro-supercapacitors.

“Most power sources, such as phone batteries, are not stretchable. They are very rigid,” says Xiaodong Chen, Ph.D. “My team has made stretchable electrodes, and we have integrated them into a supercapacitor, which is an energy storage device that powers electronic gadgets.”

Developed in the 1950s, supercapacitors have a higher power density and longer life cycle than standard capacitors or batteries. And as devices have shrunk, so have supercapacitors, bringing a generation of two-dimensional micro-supercapacitors that are integrated into medical devices, cell phones, computers, and more. However, these supercapacitors have remained rigid.

In a study, Chen and a team from Nanyang Technological University in Singapore, sought to develop a micro-supercapacitor from graphene – a carbon sheet known for its thinness, strength, and conductivity. Graphene can be flexible and foldable, but it cannot be stretched. To fix that, Chen’s team took a cue from skin, which has a wave-like microstructure. From this, the team started to think of how they could make graphene more like a wave.

The researchers’ first step was to make graphene micro-ribbons. Most graphene is produced with physical methods – like shaving the tip of a pencil – but the research team uses chemistry to build this material.

“We have more control over the graphene’s structure and thickness that way,” Chen explains. “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 next step was to create the stretchable polymer chip with a series of pyramidal ridges. The researchers placed the graphene ribbons across the ridges, creating the wave-like structure. The design allows the material to stretch without the graphene electrodes of the superconductor detaching, cracking, or deforming. In addition, the team developed kirigami structures – variations of origami folds, to make the supercapacitors 500% more flexible without decaying their electrochemical performance.

As a final test, Chen powered an LCD from a calculator with the stretchy graphene-based micro-supercapacitor. Similarly, such stretchy supercapacitors can be used in pressure or chemical sensors.

In future experiments, researchers hope to increase the electrode’s surface area so it can hold more energy, since the current version only stores enough energy to power LCD devices for a minute.

Chen and the teams’ work was presented at the 252nd National Meeting & Exposition of the American Chemical Society (ACS) in August.

Nanyang Technological University

www.ntu.edu.sg