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 (4x4) 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 (2x2) pattern. Clustering of Ca atoms is hindered by the repulsive Coulomb interaction between charged Ca atoms.