Energy Storage for Flexible Electronic DevicesOctober 03, 2014
Researchers have found that crumpling a piece of graphene "paper" — a material formed by bonding together layers of the two-dimensional form of carbon — can yield new properties that could be useful for creating stretchable supercapacitors to store energy for flexible electronic devices.
The finding is reported in the journal Scientific Reports by MIT's Xuanhe Zhao, an assistant professor of mechanical engineering and civil and environmental engineering, and four other authors. The new flexible superconductors should be easy and inexpensive to fabricate, the team says.
"Many people are exploring graphene paper: It's a good candidate for making supercapacitors, because of its large surface area per mass," Zhao says.
Like batteries, supercapacitors can store electrical energy, but they primarily do so electrostatically, rather than chemically. That means they can deliver their energy faster than batteries can. Zhao and his team demonstrated that by crumpling a sheet of graphene paper into a chaotic mass of folds, they can make a supercapacitor that can be bent, folded, or stretched to as much as 800% percent of its original size. The material can be crumpled and flattened up to 1,000 times without a significant loss of performance.
To make the crumpled graphene paper, a sheet of the material was placed in a mechanical device that first compressed it in one direction, creating a series of parallel folds or pleats, and then in the other direction, leading to a chaotic, rumpled surface. When stretched, the material's folds smooth themselves out.
Forming a capacitor requires two conductive layers — in this case, two sheets of crumpled graphene paper — with an insulating layer in between. In the demonstration, the insulating layer was made from a hydrogel material. Like the crumpled graphene, the hydrogel is deformable and stretchable, so the three layers remain in contact even while being flexed and pulled.
The crumpling technique could be applied to other uses, Zhao says. For example, the crumpled graphene material might be used as one electrode in a flexible battery, or could be used to make a stretchable sensor for specific chemical or biological molecules.