Tuning Crystal Morphology\nin MOFs for Improved Hydrogen\nStorage

Achieving net-zero carbon emissions by 2050 is imperative\nto mitigating\nthe global climate crisis, necessitating a transition to sustainable\nenergy sources. Hydrogen, recognized for its high energy density,\nis a promising sustainable fuel; however, its storage and transportation\nare challenging, necessitating the development of more effective storage\nmaterials. Metal–organic frameworks (MOFs) possess exceptional\nstructural integrity and diversity, making them highly valuable in\napplications such as catalysis, separations, and energy storage. Despite\ntheir significant potential as viable materials for H<sub>2</sub> storage,\nthe poor packing of powdered MOFs limits their volumetric storage\ncapacity. Improvement in packing density can be achieved by modifying\nthe crystal shape and structure to reduce the void volume. In this\npaper, we embarked on synthesizing different nonideal crystal shapes\nand orientations by varying the ligand-to-metal ratio, which changes\nthe overall crystal growth direction. We report the engineering of\nMIL-53-Al, focusing on tuning its crystal size and shape distribution\nto enhance its packing density and volumetric H<sub>2</sub> storage\ncapacity. This approach allows the synthesis of MOF nanomaterials\nwith a high surface area (up to 1900 m<sup>2</sup>/g) and increased\npacking density (0.38 g/cm<sup>3</sup>), while preventing crystal\ndamage when subjected to pressure compaction, thereby enhancing the\nvolumetric storage capacity. After tuning, the volumetric storage\ncapacity of the two best-performing samples, (MIL-53-Al)<sub>1:1</sub> and (MIL-53-Al)<sub>0.52:1</sub>, exhibited a monoclinic crystal\nshape, which enhanced not only the overall packing density but also\nthe working volumetric capacity of the system. Specifically, (MIL-53-Al)<sub>1:1</sub> achieved an enhancement of 25% in working capacities compared\nto pristine MIL-53-Al. Moreover, its volumetric working capacity was\nestimated to be 37 g/L under the pressure swing (PS) conditions between\n77 K/70 bar and 77 K/5 bar, surpassing the 30 g/L volumetric capacity\nof the compressed 700-bar pressure storage systems.