  Because of their unique structure, it has been proposed that carbon nanotube
ropes may well provide an ideal container for the storage of molecular
hydrogen. Indeed, there has been some experimental evidence of enhanced
hydrogen uptake in doped Li and other alkali metal systems [Chen et al, Science
285, 91 (1999)]. We have therefore addressed this issue of hydrogen storage in
Li-doped graphite and carbon nanotube systems theoretically with ab initio
simulations. Our results find no evidence for such enhanced storage, based on
the induced structural changes. In addition, we have also investigated the
diffusion barriers for hydrogen to enter into nanotube interiors, both in the
presence and absence of topological defects. Even if nanotube interiors are
made accessible, the hydrogen uptake remains modest, i.e., less than 3.5 wt%.
Mechanically or chemically processing nanotubes is therefore not likely to lead
to greatly increased hydrogen storage.
