Theoretical Study of Hydrogen Storage in Ca-Coated Fullerenes

First principles calculations based on gradient corrected density functional theory and molecular dynamics simulations of Ca decorated fullerene yield some novel results: (1) C-60 fullerene decorated with 32 Ca atoms on each of its 20 hexagonal and 12 pentagonal faces is extremely stable. Unlike transition metal atoms that tend to cluster on a fullerene surface, Ca atoms remain isolated even at high temperatures. (2) C60Ca32 can absorb up to 62 H-2 molecules in two layers. The first 30 H-2 molecules dissociate and bind atomically on the 60 triangular faces of the fullerene with an average binding energy of 0.45 eV/H, while the remaining 32 H-2 molecules bind on the second layer quasi-molecularly with an average binding energy of 0.11 eV/H-2. These binding energies are ideal for Ca coated C-60 to operate as a hydrogen storage material at near ambient temperatures with fast kinetics. (3) The gravimetric density of this hydrogen storage material can reach 6.2 wt %. Simple model calculations show that this density is the limiting value for higher fullerenes.