Watching ionic liquids on the move

As the world moves away from fossil fuels, scientists are working to develop powerful new batteries that can help electrify applications from transport to services. Key to this technology is finding the right electrolyte, the liquid that allows charged ions to circulate and so pass current.

Scientists at Khalifa University of Science and Technology have now mapped in unprecedented detail the flow behaviour of one popular green-energy electrolyte. The results could help researchers design and operate more effective energy storage devices, from batteries to supercapacitors.

Working with colleagues from the National Center for Scientific Research Demokritos in Greece, the KU scientists, led by Jamal Hassan from the Department of Physics, studied the flow of room temperature ionic liquids (RTIL). These are molecules made of oppositely charged ions, which have low melting points and so are liquid at room temperature. Also called molten salts, ionic liquids are useful solvents, and their state makes them versatile electrolytes.

“In recent years, they have attracted much attention due to their unique and tunable properties, which render them promising alternatives for a broad range of applications,” says Hassan. “However, the mechanism underlying the molecular arrangement and motion of RTILs under confinement was still obscure.”

Knowing how the molecules in ionic liquids move and squeeze through small spaces is important because it will affect their performance, especially as devices, and their batteries become more compact.

In the new study, the researchers used a technique called time-resolved diffusion NMR to track how an ionic liquid flowed through tiny silica pores just a few nanometres across. That’s about a thousand times thinner than a human hair, which forces the bulky molecules to shuffle into a specific pattern so they can pass through.

This is the first time a study has identified this pattern in an RTIL. The results showed that a ring of the ionic liquid molecules arranged themselves around the inside wall of the pore. That created a central channel through which other RTIL ions could travel more freely, moving forwards, one after the other, in a pattern called single-file diffusion.

This type of diffusion has been seen in other liquids in confined spaces, including water flowing through narrow plastic tubes. Experts thought RTIL could behave in the same way, and these new results offer the first experimental support for that idea.

“The experimental evidence for this extraordinary effect provides key information regarding the use of RTIL-based composites in energy storage applications,” says Hassan.