Published by the American Chemical Society

We model fullerene nanocages filled with hydrogen, of the general formula Hn@Ck, 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 C60 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. Among the intriguing properties of carbon nanostructures, the possibility to synthesize endohedral fullerenes or nanotubes containing various atoms and molecules is interesting for various possible applications.1 In such composites a fullerene or nanotube plays the role of nanocage whose inner cavity is filled with a foreign compound.2,3 In particular, existence of endohedral fullerenes with a hydrogen molecule inside, predicted theoretically in 1991,4 makes the idea of storing hydrogen inside a fullerene cage especially attractive. Although the interaction between the hydrogen and the storage media is weaker in this case than for a conventional chemisorption, a high potential barrier of breaking the cage stabilizes the hydrogen inside it. We note that the potential of giant hollow molecules to entrap “hundreds of times their own weight” of some small molecules was considered by Daedalus as early as in 1966.5 Endohedral C60 fullerene containing one H2 molecule can be produced with high yield7 by a so-called “molecular surgery” approach.6,8 Synthesis under high hydrogen pressure using laser excitation is also demonstrated.9 Insertion of hydrogen into fluorinated carbon cages is considered as another possibility in refs 10 and 11. For the recent progress in hydrogen storage inside carbon nanotubes, see the reviews in refs 3, 12, and 13 and references therein. The latest idea is an elegant “fill-and-lock” mechanism suggested in ref 14. The task of producing an endohedral fullerene with a higher hydrogen content, although experimentally challenging, can be approached theoretically. Modeling hydrogen storage in fullerene nanocages should answer three basic questions. First, how is hydrogen put inside the cage? Second, how can the release of hydrogen from the cage be controlled? And third, what are the properties of endohedral fullerenes filled with hydrogen? The present study addresses the latter question using C60 fullerene as an example. This problem concerns mainly the following issues: maximum sustainable number of encapsulated hydrogen atoms, formation energy of such endohedral complexes, and pressure produced by hydrogen inside the