Study reveals how bromide ions cause ripples in semiclathrate hydrates
Osaka, Japan: Understanding and maximising the characteristics of proton-conducting materials depends greatly on how water molecules behave in these materials. To detect changes in the speed of the water, it is necessary to be able to look at extremely quick pictures. Semiclathrate hydrate crystals have been closely examined by Osaka University researchers utilising quasi-elastic neutron scattering (QENS).
The findings of the study were published in Applied Physics Letters.
Semiclathrate hydrates have water molecule frameworks that house other molecules or ions as ‘guests’ in their structures. The overall properties of the framework can therefore be controlled and tailored to particular requirements by introducing different guests.
However, some of the best proton conductors are highly acidic solutions and are difficult to be handled. Solid electrolyte alternatives are therefore needed. Tetra-n-butylammonium bromide (TBAB) semiclathrate hydrate is known to be a promising solid electrolyte, but the mechanism behind its performance has been unclear.
The researchers took a close look at the water molecule dynamics in TBAB semiclathrate hydrate using QENS. This allowed motions of the water molecules to be captured over much shorter periods than have been achieved with other techniques, providing a clearer picture of what is happening.
“The transfer of protons in the semiclathrate hydrate is suspended by the water molecules,” explains study lead author Jin Shimada. “The way the water molecules then reorient—their reorientation motion—then tells us about what might be affecting the conduction.”
QENS showed that water molecules in the crystal reorientate themselves very rapidly in much shorter times than have previously been measured. In addition, the energy needed to prompt the change is consistent with that needed to break a hydrogen bond, the type of interaction that occurs between the guest ions and the water molecules.
It is believed that the large bromide ion that forms part of TBAB activates the water to behave as it would around bromide in aqueous solution.
“The insight we have gained into TBAB semiclathrate hydrate provides an excellent grounding for future innovation,” says senior author Takeshi Sugahara. “We believe the findings will contribute to the development of batteries and thermal storage materials.”
