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

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 π∗ 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 (4×4) pattern on the graphene can absorb up to five H2 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 (2×2) pattern. Clustering of Ca atoms is hindered by the repulsive Coulomb interaction between charged Ca atoms.