PIM-MOF Composites for Use in Hybrid Hydrogen Storage Tanks

It is well understood that fossil fuels (coal, oil and natural gas) are non-renewable and their combustion is a major driver of anthropogenic climate change, and therefore replacement energy sources and energy vectors must be found. Hydrogen has long been touted as an alternative energy vector, due to its very high gravimetric energy density, and that its full combustion releases only water as a side product. However, hydrogen is a very sparse gas, and as a result its volumetric energy density is very low, making hydrogen storage a difficult technical challenge. The current industrial standard for hydrogen storage in vehicles is compression, in which hydrogen gas is compressed to 70 MPa. This technique has a high energy penalty, and safety concerns due to the high pressure. The tank must also be made out of low weight, high strength carbon fibre composite, which incurs a large economic cost. One alternative solution is adsorption, in which high surface area microporous use the physical bonding between hydrogen molecules and the solid surface area to achieve high hydrogen storage densities at reasonable pressures and temperatures.One potential class of materials for use in such a class are polymers of intrinsic microporosity (PIMs), which are polymeric materials featuring spiro-centres in their molecular chains that cause kinking and thus inefficient packing of the material, which leads to an inherent microporosity. These materials are attractive for a hybrid adsorption-compression tank due to their flexibility and processability. The central material being investigated in this study, PIM-1, is soluble in polar-aprotic solvents such as chloroform and tetrahydrofuran, and forms robust, flexible films upon solvent casting [1]. However, the BET surface area of these films is relatively low (~ 600 m2 g-1) [1], and therefore hydrogen storage capacity must be added for this material to achieve the U.S. Department of Energy targets for hydrogen storage systems [2]. This project aims to achieve this by incorporating crystals of MOF-5, a well understood metal-organic framework (MOF) that has been receiving industrial attention due to its high BET surface area and hydrogen storage capacity [3]. This work aims to synthesise and characterise PIM-1 and MOF-5 independendetly, before combining into a composite-type film material. Characterisation work on these materials is focussed on adsorption isotherms between 0 – 20 MPa at 77 K, using both nitrogen and hydrogen as sorbents. This work is supported through the use of helium pycnometry and thermogravimetric analysis. This characterisation data is compared between the materials, and the relationship between the characteristic properties of the composite and the ratio of materials in its composition is discussed. References:[1] P. M. Budd, E. S. Elabas, B. S. Ghanem, S. Makhseed, N. B. McKeown, K. J. Msayib, C. E. Tattershall & D. Wang, Adv. Mater. 16 (2004) 456-459[2] United States Department of Energy (2009) http://energy.gov/sites/prod/files/2015/01/f19/fcto_myrdd_table_onboard_h2_storage_systems_doe_targets_ldv.pdf [Accessed 13/04/2015][3] M. Veenstra, J. Yang, C. Xu, M. Gaab, L. Arnold, U. Müller, D. Siegel & Y. Ming (2014) http://www.hydrogen.energy.gov/pdfs/review14/st010_veenstra_2014_o.pdf [Accessed 13/04/2015]