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 (Q_st) 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.