  Based on the first-principles plane wave calculations, we showed that Ca
adsorbed on graphene can serve as a high-capacity hydrogen storage medium,
which can be recycled by operations at room temperature. Ca is chemisorbed by
donating part of its 4s-charge to the empty $\pi^*$-band of graphene. At the
end adsorbed Ca atom becomes positively charged and the semi-metallic graphene
change into a metallic state. While each of adsorbed Ca atoms forming the (4x4)
pattern on the graphene can absorb up to five H_2 molecules, hydrogen storage
capacity can be increased to 8.4 wt % by adsorbing Ca to both sides of graphene
and by increasing the coverage to form the (2x2) pattern. Clustering of Ca
atoms is hindered by the repulsive Coulomb interaction between charged Ca
atoms.
