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 ${\text{H}}_2$ molecules can bind to a Ca atom each with a desirable binding energy of $\sim 0.2\text{eV}/{\text{H}}_2$. The hybridization of the empty $\text{Ca}3d$ orbitals with the ${\text{H}}_2$ $\sigma$ orbitals contributes to the ${\text{H}}_2$ 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 $\sim 6\text{at.}\%$ $B$ doping can reach the gravimetric capacity of $\sim 5$ wt. % hydrogen.

• Received 23 July 2009

DOI:https://doi.org/10.1103/PhysRevB.80.115412