Crumpled ‘graphene paper’ has potential to store energy

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Layers of graphene can be bonded and then deformed to form a material with properties that could be useful for creating stretchable supercapacitors 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 claimed.

‘Many people are exploring graphene paper: It’s a good candidate for making supercapacitors, because of its large surface area per mass,’ Zhao said in a statement.

He said the development of flexible electronic devices, such as wearable or implantable biomedical sensors or monitoring devices, will require flexible power-storage systems.

Like batteries, supercapacitors can store electrical energy, but they primarily do so electrostatically, rather than chemically.

Now Zhao and his team have demonstrated that by crumpling a sheet of graphene paper into a mass of folds, they can make a supercapacitor that can easily be bent, folded, or stretched to as much as 800 per cent 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, the team has demonstrated, without a significant loss of performance.

‘The graphene paper is pretty robust and we can achieve very large deformations over multiple cycles,’ Zhao said. ‘

Graphene, a structure of pure carbon just one atom thick with its carbon atoms arranged in a hexagonal array, is one of the strongest materials known.

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 rumpled surface. When stretched, the material’s folds smooth themselves out.

Forming a capacitor requires two conductive layers – namely 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, Zhao said. 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.

The research team included Jianfeng Zang at Huazhong University of Science and Technology and Changyang Cao, Yaying Feng, and Jie Liu at Duke University. The work was supported by the US Office of Naval Research, the US National Science Foundation, and the National 1000 Talents Program of China.