Researchers at EPFL and Kyoto University have created the first hydride-based deep eutectic solvent-a stable hydrogen-rich liquid formed by mixing two simple chemicals. This breakthrough could make hydrogen storage easier, safer, and more efficient at room temperature.
Hydrogen can be the clean fuel of the future, but getting it from the lab to everyday life isn't simple. Most hydrogen-rich materials are solids at room temperature, or they only become liquids under extreme conditions like high pressure or freezing temperatures.
Even materials such as ammonia borane, a solid, hydrogen-rich compound that can store a lot of hydrogen, are difficult because they release hydrogen only when heated, often producing unwanted byproducts.
Making a hydrogen-rich liquid that stays stable at normal temperatures could make hydrogen storage and transport much easier. In fact, there have been efforts to improve hydrogen storage by changing the chemical makeup of current storage materials or by adding substances that help hydrogen release more easily.
One promising area is deep eutectic solvents (DESs), which are mixtures that melt at lower temperatures than their ingredients. This is important for hydrogen storage because DESs can turn solid hydrogen-rich materials into easy-to-handle liquids at much lower temperatures. Until now, though, none of these DESs had used hydride components, which are especially rich in hydrogen and could open up new ways to store more hydrogen in liquid form.
Scientists from the groups of Professors Andreas Züttel at EPFL and Satoshi Horike at Kyoto University have developed the first example of a hydride-based DES: a transparent, stable hydrogen-rich liquid that stays liquid at room temperature. The new DES can contain up to 6.9% hydrogen by weight, exceeding several technical targets for hydrogen storage including those set for 2025 by the US Department of Energy.
To make the new DES, the researchers physically mixed ammonia borane and tetrabutylammonium borohydride in different amounts to determine which combination(s) would stay liquid at room temperature. The right ratio (between 50% and 80% ammonia borane) produced a stable liquid that stayed amorphous, meaning it didn't form crystals again even at cold temperatures.
Using spectroscopy, the researchers confirmed that the molecules formed strong hydrogen bonds, breaking up their usual solid structure and keeping the mixture liquid down to minus 50°C. Tests showed the new liquid could release hydrogen when heated to just 60°C, much lower than most hydrogen-rich solids. This means hydrogen can be accessed more easily and efficiently, making storage and use much more practical for real-world applications.
Mixing ammonia borane with tetrabutylammonium borohydride creates a new, hydrogen-rich liquid that doesn't crystallize under normal conditions. The glass transition, which refers to when the liquid becomes glassy, happens at -50°C, far lower than our everyday conditions.
The mixture stays stable for weeks if kept dry, and its density is among the lowest reported for similar liquids. When heated, it releases pure hydrogen gas at relatively low temperatures without producing many impurities. Only the ammonia borane part breaks down first, which means parts of the mixture could be reused.
This new DES could make hydrogen storage and transport much simpler and safer. Instead of relying on high-pressure tanks or super-cold liquids, industries could use stable, easy-to-handle hydrogen carriers at room temperature.
Beyond hydrogen storage, these results could lead to new custom liquids for other uses, such as chemical production or green energy. The discovery opens up new directions for both hydrogen research and practical energy technology.
Other contributors
- Swiss Federal Laboratories for Materials Science and Technology (Empa)
- Vidyasirimedhi Institute of Science and Technology (Thailand)
