Defect-induced confinement in zirconium metal-organic frameworks for enhanced hydrogen adsorption

Hydrogen is an essential gas to multiple industrial processes, and due to its high gravimetric density, it promises a large potential as a clean energy source. The risks of hydrogen at low pressures, however, have deterred substantial progress in improving hydrogen storage technology. Metal-organic frameworks (MOFs) are crystalline porous materials that have emerged as excellent gas adsorbents, and their gas storage properties can be tuned using a variety of synthetic methods. Herein, we leveraged the tunability and porosity of MOFs to introduce defect engineering as a method to improve hydrogen storage technology at low pressure regimes in two zirconium-based MOFs (Zr-MOFs), UiO-66 and NU-403, which feature ideal pore apertures (7 Å) for studying the confinement effect and ease of defect engineering. By reducing the number of defects in these Zr-MOFs, and thereby decreasing the quantity of larger pores, we can induce a structural confinement effect that increases the selectivity of hydrogen adsorption. A combination of thermogravimetric analysis (TGA) and nuclear magnetic resonance (NMR) spectroscopic analysis enabled quantification of the defect levels, confirming that each sample exhibits a distinct level of defectiveness. Gas adsorption measurements revealed that the adsorption of hydrogen is greatly enhanced in samples with fewer defects, while the calculated isosteric heats of enthalpy (Qst) indicate that there are no open metal sites in these MOFs that could be the cause. Overall, we can conclude that defect engineering for pore tailoring is a viable strategy to enhance hydrogen adsorption at low-pressure regimes.