MIT 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 extremely stretchable supercapacitors to store energy for flexible electronic devices.
“Many people are exploring graphene paper: It’s a good candidate for making supercapacitors, because of its large surface area per mass,” says MIT’s Xuanhe Zhao.
Like batteries, supercapacitors can store electrical energy, but they primarily do so electrostatically, rather than chemically — meaning they can deliver their energy faster than batteries. Now, Zhao and his team have demonstrated that by crumpling a sheet of graphene paper into a chaotic mass of folds, they can make a supercapacitor that can easily be bent, folded, or stretched to as much as 800 percent of its original size. The team has made a simple supercapacitor using this method as a proof of principle.
The material can be crumpled and flattened up to 1,000 times without a significant loss of performance. “The graphene paper is pretty robust,” Zhao says, “and we can achieve very large deformations over multiple cycles.”
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 simply 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, which in this demonstration was made from a hydrogel material. Like the crumpled graphene, the hydrogel is highly deformable and stretchable, so the three layers remain in contact even while being flexed and pulled.
Though this initial demonstration was specifically to make a supercapacitor, the same crumpling technique could be applied to other uses, such as the creation of one electrode in a flexible battery, or a stretchable sensor for specific chemical or biological molecules.
Key to illustration above:
To form the crumpled graphene, a sheet of polymer material is stretched in both dimensions, then graphene paper is bonded to it. When the polymer is released in one direction, the graphene forms pleats, as shown in the bottom images, taken with a scanning electron microscope (SEM). Then, when released in the other direction, it forms a chaotic crumpled pattern (top images). The right pair of SEM images shows the material at higher magnification than the left-hand images.