Assessment of high‐pressure hydrogen storage performance of Basolite metal‐organic frameworks

Abstract

The hydrogen storage performance of a series of Basolite metal-organic frameworks (MOFs) has been estimated within a pressure range up to 700 bar at temperatures from 77 to 293 K. Crystal structure and texture properties of the MOFs were determined using X-ray diffraction and nitrogen cryoadsorption, respectively. Hydrogen excess adsorption isotherms were measured and then converted to gravimetric and volumetric total adsorption. To calculate the total volumetric capacity, the crystallographic density of materials for the theoretically maximum capacity was used. At 700 bar, the highest volumetric total adsorption was about 65 mg/cm3 shown by Z1200 and Z377. The heat of adsorption at 298 K vs pressure was evaluated by the Clausius-Clapeyron equation. Peculiar adsorption behavior of C300 in the high-pressure region manifested in specific forms of isotherms and baric dependence of the heat of adsorption has been revealed. The relation of this phenomenon with a possible transition from Cu2+ to Cu+ in the C300 framework under the reducing effect of hydrogen has been discussed. The upper-pressure limit of the MOFs' effectiveness in hydrogen storage systems compared to adsorbent-free compressed gas has been estimated and did not exceed 127 bar at 298 K and 82 bar at 77 K due to the large amount of hydrogen that remains adsorbed at release pressure 5 bar. It has been concluded that conventional testing methods based only on excess adsorption isotherm measurements give initial data on the surface interaction and do not reflect the actual amount of stored and deliverable hydrogen. Besides the excess adsorption, the density of the adsorbent and the low-pressure capacity are the key factors defining the applicability of materials in high-pressure storage systems.