  We report a first-principles study of hydrogen storage media consisting of
calcium atoms and graphene-based nanostructures. We find that Ca atoms prefer
to be individually adsorbed on the zigzag edge of graphene with a Ca-Ca
distance of 10 A without clustering of the Ca atoms, and up to six H2 molecules
can bind to a Ca atom with a binding energy of ~0.2 eV/H2. A Ca-decorated
zigzag graphene nanoribbon (ZGNR) can reach the gravimetric capacity of ~5 wt %
hydrogen. We also consider various edge geometries of the graphene for Ca
dispersion.
