Supercapacitors clean up with new ionic liquid electrolyte

An international team of scientists, including a professor of chemistry from the University of Bristol in the UK, has found a way to improve energy storage devices called supercapacitors, by designing a new class of ionic liquids. The scientists report their work in a paper in Nature Materials.

Currently, aqueous and organic electrolytes are used in supercapacitors, but researchers and manufacturers have recently been testing ionic liquids as a way to boost performance. Ionic liquids are salts that are liquid at room temperature, and have numerous advantages over conventional electrolytes because they are stable, non-flammable and often much more environmentally friendly.

To explore the exciting potential offered by ionic liquids for emerging electrochemical technologies, the authors designed a new set of highly efficient, detergent-like ionic liquid electrolytes and studied how they work at electrode surfaces. This understanding could help with the design of even more efficient devices for storing electrical energy.

Typically, for electrolytes in contact with a charged electrode, the distribution of ions is dominated by electrostatic Coulombic interactions. This distribution can be controlled, however, by making the ionic liquids soap-like, or amphiphilic, so that the molecules now have separate polar and non-polar domains, exactly like common detergents.

These domains cause the ionic liquids to spontaneously form bilayer structures on the electrode surfaces, leading to much improved energy storage capabilities. The scientists found that temperature and applied voltage also affect the energy storage performance.

"We engineered a new class of ionic liquids that can store energy more efficiently," explained co-author Xianwen Mao from the Massachusetts Institute of Technology (MIT). "These detergent-like ionic liquids can self-assemble into sandwich-like bilayer structures on electrode surfaces. And that is the very reason why they give better energy storage performance."

"To make this discovery required a team of scientists with a very diverse skill set, spanning chemical synthesis, advanced structural microscopy and electrical techniques as well as computational methods," said Julian Eastoe, a professor in the University of Bristol's School of Chemistry and a co-author of the paper. "This work demonstrates the power of scientific research 'without borders', the groups from different nations contributed their own expertise to make 'the whole greater than the sum of parts'."

This new class of electrolyte may be suitable for challenging operations such as oil drilling and space exploration, but it may also pave the way for new and improved supercapacitors in hybrid cars. Supercapacitors are essential components in modern hybrid cars and can outperform batteries in terms of higher power and better efficiency.

This is particularly the case during regenerative braking, where mechanical work is turned into electrical energy that is stored quickly, ready to be released. Regenerative braking reduces energy consumption and is much more environmentally friendly. Furthermore, with the new ionic liquids developed in this study, future supercapacitors may be able to store more energy than batteries, potentially replacing them in applications such as electrical vehicles, personal electronics and grid-level energy storage facilities.

This story is adapted from material from the University of Bristol, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.