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

Hydrogen (H 2 ) is a clean alternative to fossil fuels and can be produced by using renewable energy sources. However, due to its low volumetric energy density, H 2 storage requires low temperatures and/or high pressures for practical applications. H 2 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 H 2 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 +, Mg 2+, and Ca 2+ ) on hydrogen adsorption. Our results show that the H 2 adsorption enthalpy on Mg 2+ cations is the highest among the cations studied, with average adsorption enthalpies of −18.7 and −23.3 kJ/mol at three H 2 molecules per Mg 2+ in NOTT-200 and SU-102, respectively. These adsorption enthalpies are in the desired range for room temperature H 2 storage. We further estimated the volumetric H 2 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 H 2 volumetric uptake of more than 15.0 g/L. We synthesized NOTT-200-Mg, a magnesium-exchanged version of NOTT-200, and measured the H 2 heat of adsorption experimentally. The measured H 2 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 Mg 2+ cation and thus reduce the H 2 heat of adsorption.