Hydrogen storage of calcium atoms adsorbed on graphene: First-principles plane wave calculations

Abstract

Based on first-principles plane wave calculations, we showed that Ca adsorbed on graphene can serve as a high-capacity hydrogen storage medium, which can be recycled by operations at room temperature. Ca is chemisorbed by donating part of its $4s$ charge to the empty ${\ensuremath{\pi}}^{\ensuremath{\ast}}$ band of graphene. At the end the adsorbed Ca atom becomes positively charged and the semimetallic graphene changes into a metallic state. While each of the adsorbed Ca atoms forming the $(4\ifmmode\times\else\texttimes\fi{}4)$ pattern on the graphene can absorb up to five ${\text{H}}_{2}$ molecules, hydrogen storage capacity can be increased to $8.4\text{ }\text{wt}\text{ }%$ by adsorbing Ca to both sides of graphene and by increasing the coverage to form the $(2\ifmmode\times\else\texttimes\fi{}2)$ pattern. Clustering of Ca atoms is hindered by the repulsive Coulomb interaction between charged Ca atoms.