To this point we have discussed the interaction of H2 with a single Ti atom bonded to a nanotube, but clearly one can imagine attaching more Ti to a nanotube, thereby increasing the hydrogen storage capacity. In order to show the feasibility of this approach, we present two simple cases where Ti covers ¼ (a) and ½ (b) of the hexagons. The optimized bond-lengths and other parameters of the structures shown in figure are very similar to those in the single-Ti case, indicating that the system has the capacity to have many Ti and hydrogen.

In fact, these configurations, which have the chemical formulas, C 8TiH8 and C 4TiH8, store approximately 5-wt% and 8-wt% hydrogen, respectively. Due to the charge transfer to the tube, it may also be possible to attract hydrogen molecules inside the tube, further increasing the storage capacity.