  Using first-principles calculations, we perform a search for high-capacity
hydrogen storage media based on individually dispersed calcium atoms on doped
or defective carbon nanotubes. We find that up to six H2 molecules can bind to
a Ca atom each with a desirable binding energy of ~0.2 eV/H2. The hybridization
of the empty Ca 3d states with the H2 sigma states contributes to the H2
binding, and Ca clustering is suppressed by preferential binding of Ca atoms to
doped boron and defect sites dispersed on carbon nanotubes. We also show that
individual Ca-decorated B-doped CNTs with a concentration of ~6 at. % B doping
can reach the gravimetric capacity of ~5 wt % hydrogen.
