Structure property relationships in metal organic frameworks for hydrogen storage
Experimental hydrogen isotherms on several metal organic frameworks (IRMOF 1, IRMOF 3, IRMOF 9, ZIF 7, ZIF 8, ZIF 9, ZIF 11, ZIF 12, ZIF CoNIm, MIL 101 (Cr) NH2 MIL 101 (Cr) NH2 MIL 101 (Al) UiO 66, UiO 67 and HKUST 1) synthesized in house and measured at 77 K and pressures up to 18 MPa are presented, along with N2 adsorption characterization. The experimental isotherms together with literature high pressure hydrogen data were analysed in order to search for relationships between structural properties of the materials and their hydrogen uptakes. The total hydrogen capacity of the materials was calculated from the excess adsorption assuming a constant density for the adsorbed hydrogen. The surface area, pore volumes and pore sizes of the materials were related to their maximum hydrogen excess and total hydrogen capacities. Results also show that ZIF 7 and ZIF 9 (SOD topology) have unusual hydrogen isotherm shapes at relatively low pressures, which is indicative of "breathing" a phase transition in which the pore space increases due to adsorption. This work presents novel and more useful correlations using the modelled total hydrogen capacities of several MOFs. These total hydrogen capacities are more practically relevant for energy storage applications than the measured excess hydrogen capacities. Thus, these structural correlations will be advantageous for the prediction of the properties a MOF will need in order to meet the US Department of Energy targets for the mass and volume of on board storage systems. Such design tools will allow hydrogen to be used as an energy vector for sustainable mobile applications such as transport, or for providing supplementary power to the grid in times of high demand.