(478a) Investigation of Room Temperature Hydrogen Storage Using Anionic Metal-Organic Frameworks with Extra-Framework Cations

Hydrogen is a promising, clean alternative to fossil fuels and can be synthesized using renewable energy sources. However, due to its low volumetric energy density, H₂ utilization requires low temperatures and/or high pressures for practical applications. H₂ storage using metal-organic frameworks (MOFs) has gained traction due to the sorbents’ large surface area and highly tunable nature, but suitable materials for room-temperature storage remain elusive. In this study, we performed ab initio calculations to explore room temperature H₂ storage in two anionic MOFs and examined the effects of different exchanged metal cations (Li⁺, Na⁺, K⁺, Mg²⁺ and Ca²⁺) on hydrogen adsorption. Our results show that the hydrogen binds weakly to the monovalent cations (Li⁺, Na⁺, K⁺) with an adsorption enthalpy between -4 and -12 kJ/mol at 298K for the first H₂ molecule and is thus not attractive for room temperature applications. Hydrogen exhibits stronger interaction with the divalent cations, resulting in adsorption enthalpies between -22 and -31 kJ/mol for Mg²⁺ and between -13 and -19 kJ/mol for Ca²⁺. More importantly, each Mg²⁺ cation can accommodate up to 3 H₂ per site, with average adsorption enthalpies of –18.7 kJ/mol and –23.3 kJ/mol in the two materials studied. These energies are in the range that is considered optimal for room-temperature hydrogen storage.