Compaction of a zirconium metal-organic framework (UiO-66) for high density hydrogen storage applications

We report a rare case whereby a metal-organic framework (MOF), namely UiO-66, is compacted at high pressure (∼700 MPa or 100 000 psi) resulting in densification and improved total volumetric hydrogen storage capacity, but crucially, without compromising the total gravimetric uptake attained in the powdered form of the MOF. The applied compaction pressure is also unprecedented for MOFs as most studies have shown the MOF structure to collapse when compacted at very high pressure. The UiO-66 prepared in this study retained ∼98% of the original surface area and microporosity after compaction at ∼700 MPa, and the densified pellets achieved a total H₂ uptake of 5.1 wt% at 100 bar and 77 K compared to 5.0 wt% for the UiO-66 powder. Depending on the method used to compute the volumetric uptake, the densified UiO-66 attained unprecedented volumetric capacity at 77 K and 100 bar of up to 74 g L⁻¹ (13 g L⁻¹ at 298 K) compared to 29 g L⁻¹ for the powder (6 g L⁻¹ at 298 K) using a conventional method that takes into account the packing density of the adsorbents, or 43 g L⁻¹ (compared to 35 g L⁻¹ for the powder at 77 K and 100 bar) based on a method that uses both the single crystal and skeletal densities of MOFs. However, regardless of the difference in the calculated values according to the two methods, the concept of UiO-66 compaction for improving volumetric capacity without compromising gravimetric uptake is clearly proven in this study and shows promise for the achievement of hydrogen storage targets for a single material as set by the United States Department of Energy (DOE).