Nontrivial Tuning of the Hydrogen-Binding Energy to Fullerenes with Endohedral Metal Dopants

We report a first-principle study of the tunable hydrogenation of endohedral metallofullerenes M@C60 and M2@C60, where M ) Li, Be, Mg, Ca, Al, and Sc. The interaction between the encapsulated metal atoms and the C60 cage leads to a tuning of the hydrogen binding in a desired manner as the hydrogenation proceeds. At lower H densities, when H atoms are too strongly bound to pure C60, the endohedral dopants weaken the binding. The dopants also enhance the hydrogen-binding energy at higher coverages and enable the degree of hydrogenation to be substantially increased relative to that available with empty-cage C60. Overall, the encapsulated metals increase the capacity and improve the energy efficiency for hydrogen storage. We identify materials capable of storing 6.1 wt % hydrogen and elucidate a subtle interplay between reactivity and structure which will be important for designing the next-generation hydrogen-storage materials.