Hydrogen storage in C₆O₆M₆ (M = Li, Na): A DFT study

A density functional study of the electronic structures of C₆O₆M₆ (M = Li, Na) clusters, as well as their potentials for dihydrogen storage, have been performed by two methods. Molecular dynamic simulations confirm that both clusters are thermodynamically stable. Calculations reveal that C₆O₆Li₆/C₆O₆Na₆ can adsorb up to 36/38H₂ molecules with a gravimetric uptake capacity of 26.10/19.90 wt%, exceeding the goal (5.5 wt%) set by the US Department of Energy (DOE). The adsorption energies of H₂ on C₆O₆Li₆ and C₆O₆Na₆ are in the range of 0.093–0.119 eV and 0.112–0.228 eV, respectively. The interactions of C₆O₆M₆ (M = Li, Na) with H₂ molecules are analyzed by a variety of electronic structure methods. Thermo-chemistry calculations indicate two H₂ in C₆O₆Li₆(H₂)₃₆ and fourteen H₂ in C₆O₆Na₆(H₂)₃₈ can be readily adsorbed at 77 K and desorbed at 298.15 K under atmospheric pressure, corresponding to the maximal reversible hydrogen storage densities are 2.11 wt% and 8.38 wt%, respectively. Higher pressure can improve the maximal reversible hydrogen storage abilities. Atom density matrix propagation (ADMP) molecular dynamic simulations indicate that H₂ molecules are substantially and strongly bound at 77 K and get efficiently released at elevated temperature (300 K). The (C₆O₆M₆)₂ (M = Li, Na) dimers can also efficiently adsorb multiple H₂ molecules with high gravimetric density and reversible average adsorption energy.