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

Abstract

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 $\ensuremath{\sim}0.2\text{ }\text{eV}/{\text{H}}_{2}$. The hybridization of the empty $\text{Ca}\text{ }3d$ orbitals with the ${\text{H}}_{2}$ $\ensuremath{\sigma}$ 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 $\ensuremath{\sim}6\text{ }\text{at.}\text{ }%$ $B$ doping can reach the gravimetric capacity of $\ensuremath{\sim}5$ wt. % hydrogen.