Hydrogen density in nanostructured carbon, metals and complex materials

The challenge in the research on hydrogen storage materials is to pack hydrogen atoms or molecules as close as possible. Hydrogen absorbed in metals can reach a density of more than 150 kgm^-3 (e.g. Mg ₂FeH₆ or Al(BH₄)₃) at atmospheric pressure. For metallic hydrides, however, due to the large atomic mass of the transition metals the gravimetric hydrogen density is limited to less than 5 mass%. Nanostructured carbon materials, e.g. carbon nanotubes or high surface area graphite absorb hydrogen at liquid nitrogen proportional to the specific surface area 1.5 mass%/1000 m²g^-1. Light weight group three metals, e.g. Al, B, are able to bind four hydrogen atoms and form together with an alkali metal an ionic or at least partially covalent compound. The complex hydrides can only be cycled in as nanostructured materials.