Despite new methods of hydrogen production, the complexity of transportation remains an obstacle to its industrial introduction, as it directly relates to its physical properties – lightness (14 times lighter than air), chemical activity (high reaction rate with external substances) and explosiveness. Hydrogen can be transported either by compression and liquefaction or being transformed into a solid state, i.e. into a crystal of H2 molecules. However, such manipulations are rather energy-consuming: compression and cooling require from 20% to 40% of the energy that can be obtained from the fuel itself.
At the same time, even in a compacted form, hydrogen contains about half as much energy per unit volume as natural gas, which reduces its usage efficiency in transportation. Finally, due to its small size, hydrogen molecules can easily leak out of containers and even penetrate metal walls, making them brittle and causing cracks. This makes it difficult to transport H2 in tank cars and cryogenic tanks.
An alternative is chemical storage: for example, alloys of magnesium and nickel or zirconium and vanadium are able to hold hydrogen in the voids between metals atoms forming a crystal lattice. Such accumulators can pack hydrogen for storage, and then release it by heating. However, such alloys can hold not more than three hydrogen atoms per metal atom.
The scientists of Skoltech, the Institute of Crystallography of the RAS and the research centres in China, Japan and Italy have managed to circumvent this limitation by synthesising compounds, in which one metal atom has seven to nine hydrogen atoms. We are talking about cesium heptahydride (CsH7) and rubidium nonahydride (RbH9), which, according to the scientists, will remain stable at atmospheric pressure. “The proportion of hydrogen atoms in these substances is higher than in any known hydride existing at normal pressures – twice as high as in methane CH4,” Skoltech quotes Dmitry Semenyuk, a graduate student in the program Materials Science graduate.
The experiment, during which cesium- and rubidium-based compounds were synthesised, consisted of several stages. “The hydrogen-rich solid borazan (ammonia borane NH3BH3) reacts with cesium or rubidium. The resulting salt is cesium or rubidium amidoborane. When heated, the salt decomposes into cesium or rubidium monohydride and a large amount of hydrogen. Since the experiment takes place in a cell with diamond anvils providing the pressure of 100,000 atmospheres, the released hydrogen is squeezed into the voids of the crystal lattice of lower hydrides with formation of polyhydrides: cesium heptahydride and two versions of rubidium nonahydride with different topology of the crystal structure“, Skoltech quotes Artem Aganov, Head of Research, Head of the Laboratory of Material Design.
In the future, the authors plan to scale the experiment using a hydraulic press to obtain cesium and rubidium polyhydrides in larger quantities and at lower pressure (10,000 atmospheres).
