Tuning Crystal Morphology in MOFs for Improved Hydrogen Storage

Achieving net-zero carbon emissions by 2050 is imperative to mitigating the global climate crisis, necessitating a transition to sustainable energy sources. Hydrogen, recognized for its high energy density, is a promising sustainable fuel; however, its storage and transportation are challenging, necessitating the development of more effective storage materials. Metal–organic frameworks (MOFs) possess exceptional structural integrity and diversity, making them highly valuable in applications such as catalysis, separations, and energy storage. Despite their significant potential as viable materials for H 2 storage, the poor packing of powdered MOFs limits their volumetric storage capacity. Improvement in packing density can be achieved by modifying the crystal shape and structure to reduce the void volume. In this paper, we embarked on synthesizing different nonideal crystal shapes and orientations by varying the ligand-to-metal ratio, which changes the overall crystal growth direction. We report the engineering of MIL-53-Al, focusing on tuning its crystal size and shape distribution to enhance its packing density and volumetric H 2 storage capacity. This approach allows the synthesis of MOF nanomaterials with a high surface area (up to 1900 m 2 /g) and increased packing density (0.38 g/cm 3 ), while preventing crystal damage when subjected to pressure compaction, thereby enhancing the volumetric storage capacity. After tuning, the volumetric storage capacity of the two best-performing samples, (MIL-53-Al) 1:1 and (MIL-53-Al) 0.52:1, exhibited a monoclinic crystal shape, which enhanced not only the overall packing density but also the working volumetric capacity of the system. Specifically, (MIL-53-Al) 1:1 achieved an enhancement of 25% in working capacities compared to pristine MIL-53-Al. Moreover, its volumetric working capacity was estimated to be 37 g/L under the pressure swing (PS) conditions between 77 K/70 bar and 77 K/5 bar, surpassing the 30 g/L volumetric capacity of the compressed 700-bar pressure storage systems.