Calcium-hydroxyl group complex for potential hydrogen storage media: A density functional theory study

Using density functional theory electronic structure calculations, we investigate the calcium-hydroxyl group complex for potential applications to the hydrogen storage at near ambient temperature and pressure. The Ca atom is bound to the hydroxyl group with a binding energy comparable to the cohesive energy of bulk Ca, and we find that each Ca atom binds up to seven H₂ molecules in the molecular form. Binding of an unexpectedly large number of H₂ molecules is attributed to the fact that d orbitals of Ca positive ions are downshifted and partially occupied, thereby validating the empirical 18-electron rule as in the transition metal atoms. The binding energy turns out to be ∼0.1 eV/H₂, somewhat smaller than the requirement (≥0.2 eV/H₂) of the room-temperature application. We also show that an important bonding mechanism of H₂ molecules on Ca is the polarization, namely, the electric dipole moment of H₂ induced by the partially ionized Ca. Based on this result, the possibility of the organic material functionalized with hydroxyl groups for a hydrogen storage medium is discussed.