Chemisorption of molecular hydrogen on carbon nanotubes: A route to effective hydrogen storage?

The energetics of the chemisorption of molecular hydrogen on small-diameter armchair carbon nanotubes has been investigated using first-principles density functional theory (DFT). The adsorption of hydrogen was examined at a range of coverages, from low to full monolayer coverage. Several pathways for hydrogenation were investigated, and those that could lead to energetically favorable, stable structures of fully saturated nanotubes were identified. For these routes, the calculations indicate that the addition of hydrogen, apart from at the very onset, is exothermic and also becomes increasingly more favorable with increasing degree of coverage. Carbon nanotubes of sufficiently small diameter are shown to have the capacity to store a full monolayer of hydrogen effectively via chemisorption. In addition, kinetic barriers for the dissociative chemisorption of H-2 and thermal equilibration of the system were considered. These were found to be quite large for admolecules on an otherwise-clean nanotube, but to drop substantially in the vicinity of preadsorbed hydrogen; that is, the adsorbed hydrogen acts as an autocatalyst for further hydrogenation. On the basis of these findings, the chemical reaction of hydrogen with carbon nanotubes is expected to become increasingly exothermic and also to proceed more rapidly at higher coverages.