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 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 orbitals with the H₂ σ orbitals 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.