Hydrogen storage capacity of alkali and alkaline earth metal ions doped carbon based materials: A DFT study

The hydrogen storage (H-storage) capacity of alkali (Li+, Na+ and K+) and alkaline earth metal ion (Mg2+ and Ca2+) doped cubane, cyclohexane and adamantane has been investigated using Density Functional Theory (DFT) based M05-2X functional employing 6-31+G∗∗ basis set. The adsorption of number of H2 molecules on the metal ion doped complexes depends on ionic radii and charge of the metal ions. Among the 15 complexes investigated in this study, Mg2+ ion doped cubane, cyclohexane and adamantane complexes have higher H-storage capacity when compared to other complexes. The calculated binding energy (BE) of 5H2@Cub-Mg2+ complex is 46.85 kcal/mol with binding energy per H2 molecule (BE/nH2) of 9.37 kcal/mol. The corresponding gravimetric density of the complexes is 7.3 wt%. In the case of 4H2@Cyc-Mg2+ complex, the BE is 32.19 kcal/mol (BE/nH2 is 8.05 kcal/mol with 6.9 wt% in gravimetric density). The Adm-Mg2+ complexes adsorb 4H2 molecules with BE of 33.33 kcal/mol, the BE of per H2 molecule is 8.33 kcal/mol. The corresponding gravimetric density of the complex is around 4.8 wt%, respectively. A new linker modified MOP-9 has been constructed based on the results and their H-storage capacity has also estimated.