A sustainable ultramicroporous MOF for hydrogen storage (Highlighted oral presentation)

Background Hydrogen storage remains a critical challenge in the transition toward sustainable energy systems. Metal-organic frameworks (MOFs) are renowned for their tunable porosity and chemical versatility, yet research has primarily focused on large-pore MOFs and frameworks with open metal sites. Ultramicroporous MOFs, by contrast, are underexplored despite their potential for enhanced hydrogen confinement effects. This study introduces magnesium gallate as a sustainable and lightweight ultramicroporous MOF for hydrogen storage. Methods The MOF was synthesized using green hydrothermal and dry mechanochemical approaches. Thermal activation was studied through in situ synchrotron X-ray diffraction. Hydrogen storage performance and adsorption enthalpies were assessed via physisorption experiments, while in situ neutron diffraction under cryogenic deuterium pressure conditions identified the hydrogen adsorption sites. Results Hydrogen storage experiments demonstrated exceptionally strong interactions, with full site occupancy achieved at pressures of a few bar at 77 K. The isosteric heat of adsorption was as high as 9.5 kJ/mol. Both physisorption and neutron diffraction confirmed the adsorption of 4 hydrogen molecules per 3 metal sites. These strong interactions were attributed to confinement effects within the ultramicropores rather than Kubas-type interactions with metal centers. Conclusion Ultramicroporous MOFs like magnesium gallate offer a promising alternative for efficient hydrogen storage. They can achieve high heats of adsorption without the need for open metal sites, emphasizing the potential of confinement effects within ultramicropores as a complementary strategy to current approaches centered on large-pore systems and open metal sites.