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 H 2 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 À1 (13 g L À1 at 298 K) compared to 29 g L À1 for the powder (6 g L À1 at 298 K) using a conventional method that takes into account the packing density of the adsorbents, or 43 g L À1 (compared to 35 g L À1 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). † Electronic supplementary information (ESI) available: Six additional gures (consisting of SEM images, TGA analysis, comparative nitrogen isotherms and pore size distribution curves, and adsorbed H 2 volume fractions), and two tables (consisting of comparative textural properties, packing density, and H 2 uptake). See

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High-pressure (700 MPa or~100 000 psi) compaction of dehydroxylated and hydroxylated UiO-66 for H2 storage applications is reported. The dehydroxylation reaction was found to occur between 150 -300 o C. The H2 uptake capacity of powdered hydroxylated UiO-66 reaches 4.6 wt% at 77 K and 100 bar, which is 21% higher than that of dehydroxylated . On compaction the H2 uptake capacity of dehydroxylated UiO-66 pellets reduces by 66% from 3.8 wt% to 1.3 wt%, while for hydroxylated UiO-66 the pellets show only a 9% reduction in capacity from 4.6 wt% to 4.2 wt%. This implies the H2 uptake capacity of compacted hydroxylated UiO-66 is at least three times higher than that of dehydroxylated UiO-66, and therefore hydroxylated UiO-66 is more promising for hydrogen storage applications. The H2 uptake capacity is closely related to compaction induced changes in the porosity of UiO-66. The effect of compaction is greatest in partially dehydroxylated UiO-66 samples that are thermally treated at 200 and 290 o C. These compacted samples exhibit XRD patterns indicative of an amorphous material, low porosity (surface area reduces from between 700 and 1300 m 2 /g to ca. 200 m 2 /g and pore volume from between 0.4 and 0.6 cm 3 /g to 0.1 and 0.15 cm 3 /g) and very low hydrogen uptake (0.7 -0.9 wt% at 77 K and 100 bar). The observed activation temperature-induced dynamic behaviour of UiO-66 is unusual for MOFs and has previously only been reported in computational studies. After compaction at 700 MPa, the structural properties and H2 uptake of hydroxylated UiO-66 remain relatively unchanged, but are extremely compromised upon compaction of dehydroxylated UiO-66. Therefore, UiO-66 responds in a dynamic manner to changes in activation temperature within the range in which it has hitherto been considered stable.

Spindle-shaped CeO 2 on bio-char carbon synthesized by in-situ hydrothermal method. Naturally occurring Spirulina platensis microalgae used as the carbon source. N, P -doped carbon and O-vacancy of Ce 3þ in CeO 2 /BC700 promote the ORR kinetics. CeO 2 /BC700 has good ORR activity and remarkable stability after 5000 cycles.