Small-pore hydridic frameworks store densely packed hydrogen

Nanoporous materials have attracted great attention for gas storage, but achieving high volumetric storage capacity remains a challenge. Here, by using neutron powder diffraction, volumetric gas adsorption, inelastic neutron scattering and first-principles calculations, we investigate a magnesium borohydride framework that has small pores and a partially negatively charged non-flat interior for hydrogen and nitrogen uptake. Hydrogen and nitrogen occupy distinctly different adsorption sites in the pores, with very different limiting capacities of 2.33 H$_2$ and 0.66 N$_2$ per Mg(BH$_4$)$_2$. Molecular hydrogen is packed extremely densely, with about twice the density of liquid hydrogen (144 g H$_2$ per litre of pore volume). We found a penta-dihydrogen cluster where H$_2$ molecules in one position have rotational freedom, whereas H$_2$ molecules in another position have a well-defined orientation and a directional interaction with the framework. This study reveals that densely packed hydrogen can be stabilized in small-pore materials at ambient pressures.