Computational optimization of room temperature usable capacity for hydrogen storage in MFU-4 metal-organic frameworks via pairwise metal substitutions

The efficient storage of hydrogen is a critical challenge in the quest for sustainable energy solutions. Current adsorbent-based methods achieve satisfactory storage densities predominantly under cryogenic temperatures and/or high pressures, which imposes problems with cost-efficient implementation of this technology. Development of materials able to bind hydrogen gas reversibly at ambient temperatures and moderate pressures could play a pivotal role in enabling hydrogen-powered technologies. In this study, we use reliable computational modelling to investigate two synthetically feasible paths for tuning the enthalpy of H₂ binding in MFU-4-type metal-organic frameworks (MOFs), aiming to maximize usable capacity. This study examines M(I)M(II)₄Cl₃(bta)₆ (bta = benzotriazolate) Kuratowski-type clusters as a model for strong binding sites in MFU-4 frameworks where M(I) is a peripheral open metal site. We systematically evaluate the impact of separately tuning the central M(II) metal ion (which plays a structural role) and peripheral M(I) metal ion (which binds the substrate) on the energetics of H₂ binding. Our computational findings reveal that there exist several combinations of M(I) and M(II) ions that can provide optimal binding enthalpies for H₂ storage. As a result, three of these new cluster compositions are able to achieve high fractional usable capacity of the total H₂ adsorbed under a pressure swing from 5 to 100 bar at room temperature. Additionally, we examine the nature of the binding interaction between the peripheral metal atom and the hydrogen molecule. While charge transfer predominantly induces this interaction, for several atom combinations changes in the polarization (associated with variations in the ionic radius of the binding atom) are essential for adjusting the strength of the interaction.