(141e) Ultra-Thin Polymer-Graphene Oxide Membranes for H2/CH4 and H2/CO2 Separation

A novel strategy for the development of novel membranes for gas separation is based on the use of 2D materials, in view of their peculiar gas transport properties as well as extremely high aspect ratios. Graphene oxide (GO), among the others, is characterized by basically monoatomic flakes with very large lateral size and the presence of oxidized species provides the possibility to suspend the material in water dispersions, and to modify such groups to the different needs.

The layer-by-layer (LbL) technique may thus be employed to fabricate thin coatings on polymeric substrates, using GO and polyelectrolytes, deposited in an alternated fashion, driven by the electrostatic interactions of negatively charged GO with polycations. The assembly mechanism controls the coating assembly, leading to well-ordered structures, organized at the nanoscale, able to discriminate gaseous penetrant based on their molecular size, and the molecular sieving effect can be conveniently applied to the case of hydrogen purification processes, such as for pre-combustion carbon capture (H₂/CO₂) or for hydrogen production (H₂/CH₄).

The active layer fabrication is carried out using a suitable polycation (poly diallyldimethylammonium chloride) and graphene oxide, on either dense polyimide substrate film (about 30 μm thick), or on porous supports with a convenient gutter layer. The deposition is performed by alternated dip coating deposition in diluted aqueous polyelectrolyte solutions and GO dispersions, with intermediate water rinsing steps.

Membrane performances are conveniently tailored varying the number of bilayers deposited, controlling penetrant transport through the separation layer, and thus tuning the permeability and selectivity for the different gas couples of interest. The gas perm-selectivity performances of such membranes are also significantly boosted with the partial reduction of the GO sheets (by direct thermal treatment of the coating up to 200°C). The resulting arrangement of the coating and the GO structure leads to an improved sieving ability of the membrane.

Membrane performances are evaluated by direct gas permeation measurements (manometric method), revealing a pronounced sieving ability of the coating, as the gas permeability decreases dramatically as the penetrant kinetic size increases. Obtained selectivities of the GO composite layer are as high as 200 for H₂/CO₂ separation or even larger than 1000 for the H₂/CH₄. Therefore, such membranes are able to discriminate the penetrant molecules on size basis, and the very large selectivity makes them very interesting for hydrogen purification purposes.

The results obtained are then compared to the material and morphological characterization of the membranes by means of SEM microscopy, XRD and AFM analysis, as well as by ζ-potential testing of each layer surface.

Therefore, the present approach proved to be a simple and scalable method to fabricate highly selective membranes for H₂ purification exploiting their outstanding sieving ability, able to overcome technological limits of state-of-the-art membranes for H₂/CO₂ or H₂/CO₂ separations. Indeed, the very large selectivities obtained may allow for improved hydrogen purities in the separation process, thus broadening the use of membranes for gas separation purposes.