Investigation of Anionic Metal–Organic Frameworks with Extra-Framework Cations for Room Temperature Hydrogen Storage

Hydrogen (H2) is a clean alternative to fossil fuels and can be produced by using renewable energy sources. However, due to its low volumetric energy density, H2 storage requires low temperatures and/or high pressures for practical applications. H2 storage using metal–organic frameworks (MOFs) has gained traction due to the large surface areas and highly tunable nature of MOFs, but suitable materials for room-temperature storage remain elusive. In this study, we performed ab initio calculations to study H2 storage at room temperature in two anionic MOFs (NOTT-200 and SU-102) and examined the effects of different exchanged metal cations (Li+, Na+, K+, Mg2+, and Ca2+) on hydrogen adsorption. Our results show that the H2 adsorption enthalpy on Mg2+ cations is the highest among the cations studied, with average adsorption enthalpies of −18.7 and −23.3 kJ/mol at three H2 molecules per Mg2+ in NOTT-200 and SU-102, respectively. These adsorption enthalpies are in the desired range for room temperature H2 storage. We further estimated the volumetric H2 uptake for 2,058 anionic MOFs from the Cambridge Structural Database using assumptions based on our modeling and found that 459 MOFs could potentially exhibit a H2 volumetric uptake of more than 15.0 g/L. We synthesized NOTT-200-Mg, a magnesium-exchanged version of NOTT-200, and measured the H2 heat of adsorption experimentally. The measured H2 heat of adsorption was significantly lower than the results from density functional theory (DFT). Further DFT calculations show that water molecules can bind strongly to the Mg2+ cation and thus reduce the H2 heat of adsorption.