On the nature of the adsorbed hydrogen phase in microporous metal-organic frameworks at supercritical temperatures.

Hydrogen adsorption measurements on different metal-organic frameworks (MOFs) over the 0-60 bar range at 50 and 77 K are presented. The results are discussed with respect to the materials' surface area and thermodynamic properties of the adsorbed phase. A nearly linear correlation between the maximum hydrogen excess amount adsorbed and the Brunauer-Emmett-Teller (BET) surface area was evidenced at both temperatures. Such a trend suggests that the adsorbed phase on the different materials is similar in nature. This interpretation is supported by measurements of the adsorbed hydrogen phase properties near saturation at 50 K. In particular it was found that the adsorbed hydrogen consistently exhibits liquid state properties despite significant structural and chemical differences between the tested adsorbents. This behavior is viewed as a consequence of molecular confinement in nanoscale pores. The variability in the trend relating the surface area and the amount of hydrogen adsorbed could be explained by differences in the adsorbed phase densities. Importantly, the latter were found to lie in the known range of bulk liquid hydrogen densities. The chemical composition and structure (e.g., pore size) were found to influence mainly how adsorption isotherms increase as a function of pressure. Finally, the absolute isotherms were calculated on the basis of measured adsorbed phase volumes, allowing for an estimation of the total amounts of hydrogen that can be stored in the microporous volumes at 50 K. These amounts were found to reach values up to 25% higher than their excess counterparts, and to correlate with the BET surface areas. The measurements and analysis in this study provide new insights on supercritical adsorption, as well as on possible limitations and optimization paths for MOFs as hydrogen storage materials.