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

Based on the 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 $\pi^*$-band of graphene. At the end adsorbed Ca atom becomes positively charged and the semi-metallic graphene change into a metallic state. While each of adsorbed Ca atoms forming the (4$\times$4) pattern on the graphene can absorb up to five H$_2$ molecules, hydrogen storage capacity can be increased to 8.4 wt \% by adsorbing Ca to both sides of graphene and by increasing the coverage to form the (2$\times$2) pattern. Clustering of Ca atoms is hindered by the repulsive Coulomb interaction between charged Ca atoms.