Research teams from Lahore University Pakistan, Harbin Engineering University in China, University of West Bohemia in the Czech Republic and Saudi Arabia research centers have computed the properties of two innovative materials for solid state hydrogen storage. Their focus is on complex hydrides based on magnesium and iron with the addition of calcium (CaMg₂FeH₈) or beryllium (BeMg₂FeH₈). The latter was found to have the highest storage capacity: hydrogen accounts for up to 6.63% of its mass, and its volume storage density can be up to 129.3 g/l.
Hydrogen is characterized by very high energy density – around 120 MJ/kg, approximately 2.8 times that of gasoline. However, under normal conditions the gas has very low density and can take up a large volume. This means that it is usually either compressed and stored in specially designed high pressure tanks at 350-700 bar, or otherwise cooled to around −253°C to liquefy. Both options depend on sophisticated equipment and imply extra energy costs. There may be an alternative approach with metal hydrides – solid substances whose crystal grid can hold hydrogen by chemical bonds and release it when heated.
The researchers investigated properties of calcium magnesium iron hydride (CaMg₂FeH₈) and the similar compound with beryllium (BeMg₂FeH₈). Neither of those materials has yet been obtained in laboratory setting: their properties were estimated by way of computer modeling at the level of single atoms and electrons. The authors identified how the respective elements are positioned in the crystal grid, how tightly hydrogen is held there, and how the compounds behave when heated. The models were run under normal pressure, and also under 35 and 70 hPa. The latter being 700 times the atmospheric pressure, those high values were used to test the integrity of the material structure, and not as proposed operational parameters.
The estimates revealed that hydrogen takes up 5.28% of the total mass of the calcium compound (CaMg₂FeH₈) and 6.63%. of the beryllium compound (BeMg₂FeH₈). The volume density of storage was, respectively, 127.1 and 129.3 g of hydrogen per liter of the holding material. This is above the storage capacity of the well-known magnesium hydride which can hold about 110 g of hydrogen per liter. The new compounds also outperform a number of previously studied complex hydrides in terms of hydrogen content: for example, hydrogen accounts for 3.84% of magnesium iron hydride (MgFe₃H₈) mass, compared to 3.65% in the case of calcium manganese hydride (CaMn₃H₈).
To release the accumulated hydrogen, the material must be heated. The estimated temperature of gas release was found to be around 188°C for the calcium compound and 203°C for the beryllium compound. Heating weakens the bonds holding hydrogen inside the crystal grid, causing it to escape as molecular hydrogen. (H₂). Therefore, the beryllium compound can hold more hydrogen, while the calcium compound releases gas at a somewhat lower temperature.
Computer modeling also demonstrated that both hydrides maintain structure stability as pressure goes up to 70 hPa. Electron structure calculations suggest that part of the electronic charge is transferred from calcium/beryllium and magnesium to iron and hydrogen. Hydrogen is thus bound within the crystal grid which enables its storage in the solid material.
In fact, the research is so far purely theoretical. The proposed compounds are yet to be synthesized and subjected to experiment to verity their actual storage capacity, hydrogen absorption and release rates, and survivability through multiple absorb-release cycles.
