Hydrogen storage in MIL-88 series

To be selected as sorbents for gas storage, metal-organic frameworks (MOFs) must be stable to avoid collapsed in humid media. MIL-88 series (abbreviated as MIL-88s) including MIL-88A, B, C, D satisfies high flexibility and stability; it, therefore, may become a suitable candidate for hydrogen storage based on the adsorption. In this work, the grand canonical Monte Carlo simulations for the pressures below 100bar showed that in MIL-88 series MIL-88D exhibits the highest absolute and excess gravimetric H-2 capacities of 5.15wt% and 4.03wt% at 77K, and 0.69wt% and 0.23wt% at 298K, respectively. Meanwhile, MIL-88A has the highest absolute and excess volumetric H-2 uptakes of 50.69g/L and 44.32g/L at 77K, and 6.97g/L and 2.49g/L at 298K. These results are comparable to the best MOFs for hydrogen storage to date. It was shown that the hydrogen uptakes depend on the special surface area and the pore volume of the MIL-88s, apart from depending on the type of the ligand. By utilizing the van der Waals dispersion-corrected density functional theory (DFT) calculations, we elucidated the interaction between the H-2 molecule and the MIL-88 series. The adsorption energy, as well as the isosteric heat of adsorption, revealed that the H2MIL-88C interaction is strongest despite its lowest storage capacity. This observation implies an implicit role of electronic structure on the H-2 adsorption capacities at the considered conditions. However, at the low temperature, the DFT calculations could elucidate the preferred adsorption sites of hydrogen molecule on the surface of MIL-88s. Besides, we also found that the interaction is dominated by the bonding state of the H-2 molecule and the p orbitals of the O and C atoms of the MIL-88s. The most substantial overlap between the electronic density of states (DOS) of the MIL-88C and the DOS of the H-2 molecule leads to the most robust interaction between the H-2 molecule and the MIL-88C.