Fullerene nanocage capacity for hydrogen storage

We model fullerene nanocages filled with hydrogen, of the general formula H-n@C-k, and study the capacity of such endohedral fullerenes to store hydrogen. It is shown using density functional theory that for large numbers of encapsulated hydrogen atoms, some of them become chemisorbed on the inner surface of the cage. A maximum of 58 hydrogen atoms inside a C-60 cage is found to still remain a metastable structure, and the mechanism of its breaking is studied by ab initio molecular dynamics simulations. Hydrogen pressure inside the fullerene nanocage is estimated for the first time and is shown to reach the values only a few times smaller than the pressure of hydrogen metallization. We provide a general relation between the hydrogen pressure and resulting C - C bond elongation for fullerene nanocages of arbitrary radii. This opens a way to assess possible hydrogen content inside larger carbon nanocages, such as giant fullerenes, where significant capacity can be reached at reasonable conditions.