Role of Alkaline Earth Metal Cations in Improving the Hydrogen-Storage Capacity of Polyhydroxy Adamantane: A DFT Study

The hydrogen-storage (H-storage) capacities of different polyhydroxy adamantanes or adamantanols coordinated (dressed or doped) with alkaline earth metal cations (Mg2+ and Ca2+) have been studied using the density functional theory (DFT) method. The complex coordinated with one Mg2+ cation adsorbs a four H2 molecules with a binding energy (BE) of 19.98 kcal/mol, whereas the complex coordinated with one Ca2+ cation adsorbs a maximum of six H2 molecules with a BE of 21.59 kcal/mol. The maximum number of metal cations that can be anchored to polyhydroxy adamantane is four. The complexes dressed with four Mg2+ and Ca2+ cations adsorb maxima of 16 and 24 H2 molecules, respectively. The gravimetric densities of the corresponding complexes are 8.4 and 10.4 wt %, respectively. The results reinforce the idea that the creation of a number of open metal sites on adamantanols enhances their hydrogen-storage capacities and also their binding energies.