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.