Ca-intercalated graphite as a hydrogen storage material: Stability against decomposition into CaH2 and graphite

We have used calculations based on density functional theory to investigate the energetics of hydrogen absorption in calcium-intercalated graphites. We focus particularly on the absorption energy and the stability of the hydrogenated material with respect to decomposition into graphite and calcium hydride, which is essential if this material is to be used for practical H2 storage. The calculations are performed with two commonly used approximations for the exchange-correlation energies. Our calculations confirm earlier predictions that the absorption energy is approximately -0.2 to -0.4 eV, which is favourable for practical use of Ca-intercalated graphite as a hydrogen storage medium. However, we find that the hydrogenated material is strongly unstable against decomposition. Our results therefore explain recent experiments which show that H2 does not remain stable in CaC6 but instead forms a hydride plus graphite. - Graphical abstract: The hydrogenation of Ca-graphite (left) results in its decomposition into pure graphite (middle) and CaH2 (right). Highlights: → We investigate the stability of hydrogenated Ca-intercalated graphite with DFT. → Dissociated H absorption in CaC6 is most favourable, with reasonable binding energies. → Molecular H2 absorption is most favourable in CaC8 and CaC14. → We find all scenarios are unstable against decomposition into CaH2 and graphite. → The decomposition will be strongly exothermic in agreement with experiments.