Improved Hydrogen Storage and Thermal Conductivity in High-Density MOF-5 Composites

Abstract

Porous adsorbents such as MOF-5 have low thermal conductivities which can limit the performance of adsorption-based hydrogen storage systems. To improve the thermal properties of these materials, we have prepared a series of high-density MOF-5 composites containing 0–10 wt % expanded natural graphite (ENG), which serves as a thermal conduction enhancer. The addition of 10 wt % ENG to MOF-5 and compaction to 0.5 g/cm3 was previously found to increase the thermal conductivity relative to neat MOF-5 of the same density by a factor of 5. In this study, detailed measurements of the hydrogen storage behavior of MOF-5/ENG composites between 77 and 295 K are reported. We find that MOF-5 pellets with 0 wt % ENG and a density of 0.5 g/cm3 have a total volumetric hydrogen storage density at 77 K and 100 bar that is 23% larger than powder MOF-5 and 41% larger than cryo-compressed hydrogen. The addition of 10% ENG to 0.5 g/cm3 MOF-5 pellets produces only a small decrease (6%) in the total volumetric hydrogen storage compared to neat MOF-5 pellets of equal density. The excess, absolute, total, and deliverable hydrogen storage amounts by the MOF-5 composites are compared for ENG additions of 0–10 wt % and pellet densities of 0.3–0.7 g/cm3. Three adsorption models (Unilan, Tóth, Dubinin–Astakhov) are compared for their effectiveness in describing hydrogen adsorption isotherms of MOF-5 and MOF-5/ENG composites. The Unilan model provides the most accurate description of the experimental data, requiring only five temperature-invariant parameters to accurately fit the data across a wide temperature range.