High-performance materials for supercapacitors

Materials World magazine
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13 Feb 2019

High-performance materials for supercapacitors, with vitality density 2.7 times greater than standard supplies, have been developed by a research team led by Tohoku University, Japan. 

An alternative to batteries, supercapacitors are rechargeable energy storage devices with a broad range of applications, from machinery to smart meters. Offering faster charging and longer lifespans, these supercapacitors are preferred over conventional batteries, but lack the capacity to store large quantities of energy. 

Carbon materials, including porous carbon, graphene, carbon fibre, carbon nanotubes, and carbon network, are most commonly used for the electrodes in supercapacitors for their high specific surface area, high electronic conductivity, high chemical stability and low cost. 

However, these are limited by low voltage in single cells, the building blocks that make up capacitors. This means that a large number of cells must be stacked together to achieve the required voltage.

The new material – a sheet made from a continuous three-dimensional framework of graphene mesosponge and a carbon-based material containing nanoscale pores – has higher single-cell voltage, reducing the stacking number and allows for more compact devices.  

Recently published in the journal Energy and Environmental Science, the researchers investigated these high-performance materials, which have the potential to meet the requirements for energy-intensive applications, such as cars. 

The team collaborated with TOC Capacitor Co – the Japan-based production company that manufactures electric double-layer capacitors. These combined efforts developed a new material that displayed exceptionally high stability under conditions of high voltage and high temperature. 

‘It is very challenging to find materials that can both operate at high-voltage and remain stable under harsh conditions,’ said Tohoku University Materials Scientist and co-author of the paper, Hirotomo Nishihara.

Investigating the physical properties of their new material, the researchers used electron microscopy and a range of physical tests, including X-ray diffraction and vibrational spectroscopy techniques. They also tested commercial graphene-based materials, including single-walled carbon nanotubes, reduced graphene oxides, and 3D graphene, using activated carbons as a benchmark for comparison.

At high temperatures of 60°C and high voltage of 3.5V, the materials exhibited excellent stability in a conventional organic electrolyte. The materials stability was ultra-high at 25°C and 4.4V – 2.7 times higher than conventional activated carbons and other graphene-based materials. 

‘This is a world record for voltage stability of carbon materials in a symmetric supercapacitor,’ said Nishihara.

In addition to its capacity for higher voltage and better stability, one of the main elements of the new materials is that it is seamless. It contains a very small number of carbon edges, only 4% of the number present in conventional activated carbons. Thesesites are where corrosion reactions originate, and this makes it extremely stable.

For the future, the research into the new materials show there is opportunity for it to pave the way for development of high durable, high-voltage supercapacitors. These could be used for a wide range of applications, including motor vehicles.