Researchers from Tohoku University collaborated with the supercapacitor production company TOC Capacitor Co. to develop a new material that demonstrates outstanding stability under conditions of high voltage and high temperature. The published their findings in the journal Energy and Environmental Science. (See image above of developed graphene mesosponge sheet and its supercapacitor connected to two LEDs -- copyright Hirotomo Nishihara).
"Scientists have long been looking for high-performance materials for supercapacitors that can meet the requirements for energy-intensive applications such as cars. "It is very challenging to find materials which can both operate at high-voltage and remain stable under harsh conditions," says Hirotomo Nishihara, a materials scientist at Tohoku University and co-author of the paper.
Traditionally, the electrodes of capacitors use activated carbons, but the use of activated carbons electrodes is limited by low voltage in single cells, the building blocks that make up capacitors. The single cell limitation means that a large number of cells must be stacked together to achieve the required voltage. Crucially, the new material offers a higher single-cell voltage, reducing the stacking number and allowing more compact devices.
A continuous three-dimensional framework of graphene mesosponge, a carbon-based material with nanoscale pores, was made into a sheet. One primary feature of the material is that it is seamless, and it contains a very small amount of carbon edges where corrosion reactions originate, making it extremely stable.
Electron microscopy and a variety of physical tests were used to examine the physical properties of their new material. Other testing included X-ray diffraction and vibrational spectroscopy techniques.
As a benchmark for comparison, they also tested commercial graphene-based materials, including single-walled carbon nanotubes, reduced graphene oxides, and 3D graphene.
They demonstrated that in a conventional organic electrolyte, the new graphene mesosponge material had excellent stability at high temperatures of 60°C and high voltage of 3.5V.
It also showed ultra-high stability at 25°C and 4.4V, about 2.7 times higher than the maximum voltage of the stable operation for conventional activated carbons and other graphene-based materials.
"This is a world record for voltage stability of carbon materials in a symmetric supercapacitor," says Nishihara.
The researchers speculate that the new material could lead to the development of highly durable, high-voltage supercapacitors for many applications including motor vehicles.
Reference Publication
Nomura, K., Nishihara, H., Kobayashi, N., Asada, T., and Kyotani, T. 4.4 V supercapacitors based on super-stable mesoporous carbon sheets made of edge-free graphene walls. Energy & Environmental Science. DOI: 10.1039/C8EE03184C
