MOF-based Technologies as an Alternative Fine-purification Step for High-grade Hydrogen: Case Study of the Karlsruhe Refueling Station

The hydrogen produced for fuel cell vehicles must respect the purity requirements described by the ISO standards. Here the case of the hydrogen refueling station located in Karlsruhe is analyzed. The solid oxide electrolysis system installed at the station produces hydrogen on-site with nitrogen and oxygen concentrations higher than the ISO limits. A fine-purification of the product hydrogen is thus required, but all the technologies currently implemented for this purpose are energy-intensive and not suitable for the case. As a consequence, an alternative process for hydrogen purification through nitrogen removal is demanded. The highly porous MOF materials combined with membrane- or column-based technologies are an interesting candidate for gas separation applications, as they are fine-tunable and can be tailored in terms of pore size and distribution. Two MOFs, ZIF-8 and UiO-66–NH2, were chosen due to their permanent porosity, uniform pore size, and thermal and chemical stability. Both MOF membranes and powders were produced and later characterized by XRD, FTIR, SEM and defect-free membrane tests to assure their overall quality. ZIF-8 and UiO-66–NH2 performance was finally tested in gas separation experiments with four different concentrations of hydrogen and nitrogen. A selectivity for hydrogen over nitrogen was shown, but the separation factors (between 2.64 and 3.31) were not as high as required for industrial applications. Significant hydrogen permeances up to 197·10−7 mol⋅s−1⋅m−2⋅Pa−1 have been also measured.