In a recent scientific paper published in Nature Chemistry, an international research team led by Aarhus University, Université Catholique de Louvain, Belgium, and the Max Planck Institute, Stuttgart, presents a significant development in energy storage. Using a nanoporous material based on magnesium borohydride, γ-Mg(BH₄)₂, the researchers have achieved a denser packing of hydrogen molecules than previously seen. This may inspire new ways to store and transport hydrogen, a key component in the transition to a 'greener' energy system.

Using advanced neutron scattering experiments, it has been possible to clarify the exact atomic structure and see exactly how the hydrogen molecules are organised in the nanoporous material. It is very surprising that hydrogen molecules are adsorbed in different positions than nitrogen and that there is room for 3.5 times more hydrogen than nitrogen. This is because the hydrogen molecules can form penta-dihydrogen clusters that have not been observed before.

In penta-dihydrogen clusters, the hydrogen molecules bind in two different ways. At the position that fills first, they bind with weak non-directional van der Waals interactions that allow the molecules to rotate freely around their centre of mass.

As more hydrogen is adsorbed, the next position is filled, where hydrogen binds to the nanoporous material. The research team has discovered that this is a new type of directional electrostatic bonding. The hydrogen molecules at this position cannot rotate freely, but are fixed by four other hydrogen atoms in the lattice structure. Macroscopic volumetric measurement of gas adsorption has confirmed that there are two different hydrogen positions in the solid and that one position is filled before the next. The nanoporous material can absorb an extreme amount of hydrogen corresponding to the composition, γ-Mg(BH₄)₂×2.33H₂. This means that hydrogen is twice as densely packed in the porous material as in pure liquid hydrogen.

Hydrogen has been the focus of renewable energy storage for decades because it has by far the highest energy content by weight. On the other hand, it has a low volumetric energy content. Therefore, there has previously been a lot of focus on adsorbing hydrogen in nanoporous materials. However, it has not previously been possible to get as much hydrogen into porous materials as in this case. Nor has it been possible to determine exactly how the adsorbed molecules are organised and bound to the porous material to be packed so extremely tightly.

These new discoveries provide a fundamental scientific understanding of how hydrogen can be bound in porous matter and provide inspiration for the development of new materials for hydrogen storage in a solid.