Calcium-decorated carbon nanotubes for high-capacity hydrogen storage: First-principles calculations

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 H-2 molecules can bind to a Ca atom each with a desirable binding energy of similar to 0.2 eV/H-2. The hybridization of the empty Ca 3d orbitals with the H-2 sigma orbitals contributes to the H-2 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 similar to 6 at. % B doping can reach the gravimetric capacity of similar to 5 wt. % hydrogen.