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 analyzed 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 correlations using the modelled total hydrogen capacities of several MOFs. These 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 capacities 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.</p

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

Mechanical characterisation of polymer of intrinsic microporosity PIM-1 for hydrogen storage applications

Polymers of intrinsic microporosity (PIMs) are currently attracting interest due to their unusual combination of high surface areas and capability to be processed into free-standing films. However, there has been little published work with regards to their physical and mechanical properties. In this paper, detailed characterisation of PIM-1 was performed by considering its chemical, gas adsorption and mechanical properties. The polymer was cast into films, and characterised in terms of their hydrogen adsorption at −196 °C up to much higher pressures (17 MPa) than previously reported (2 MPa), demonstrating the maximum excess adsorbed capacity of the material and its uptake behaviour in higher pressure regimes. The measured tensile strength of the polymer film was 31 MPa with a Young’s modulus of 1.26 GPa, whereas the average storage modulus exceeded 960 MPa. The failure strain of the material was 4.4%. It was found that the film is thermally stable at low temperatures, down to −150 °C, and decomposition of the material occurs at 350 °C. These results suggest that PIM-1 has sufficient elasticity to withstand the elastic deformations occurring within state-of-the-art high-pressure hydrogen storage tanks and sufficient thermal stability to be applied at the range of temperatures necessary for gas storage applications