(571c) Polymer-Graphene Oxide Self-Assembled Membranes for H2 Purification with Enhanced Selectivity for Carbon Capture Applications

2D materials are very attractive for the fabrication and the development of membranes for gas separation due to their peculiar gas transport properties, combined to extremely high aspect ratios. Among the others, graphene and its derivatives are very promising due to their very low permeability to atoms and molecules coupled and the possibility to finely tune its size and assembly. Such strategy can be pursued to create size-selective membranes, e.g. for hydrogen purification, in which the transport of larger molecules such as CO₂ is significantly hindered by the large diffusive paths around the piled 2D materials.

This work is focused on the use of layer-by-layer (LbL) self-assembly technique to fabricate thin coatings on top of polymeric substrates, using graphene oxide (GO) combined to polyelectrolyte species, deposited in an alternated fashion, thus leading to well-ordered structures organized at the nanoscale. The assembly mechanism, indeed, exploits electrostatic interactions of weakly negatively charged GO and polycations, controlling both the growth of the coating and the resulting arrangement of the alternated structures.

Membrane fabrication includes the preparation of a Matrimid polyimide substrate (about 30-40 μm), considered as benchmark material for H₂ separation, then the alternated dip coating deposition of highly diluted aqueous polyelectrolyte solutions and GO dispersions. Intermediate water rinsing steps are performed after each deposition, in order remove the excess of GO or PE attached on the surface and to avoid inhomogeneity on sample, ultimately guarantying the required structural order. The whole process is repeated 10 times (10 bilayers – BLs) to ensure a sufficiently homogeneous structure, in which large molecules are force to follow long selective paths.

The characterization of the obtained membranes by SEM microscopy revealed the effective and homogeneous deposition of the coating, while XRD analysis identified the alternated layer arrangement and provided a quantitative determination of the average thickness of each layer.

Membrane performances are evaluated by direct gas permeation measurements, which show a pronounced sieving ability of the coating, as the gas permeability decreases dramatically as the penetrant kinetic size increases. 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 (e.g. for H₂/CO₂ or H₂/CH₄ separation).

The gas perm-selectivity performances of such membranes may be conveniently tuned by partially reducing the GO sheets, by direct thermal treatment of the coating (T up to 200°C) causing thus the removal of some oxidized species on the graphenic layers, and leading modified arrangement of the coating (inspected by XRD) and ultimately to different permeabilities. Relevantly, the permeability of small molecules such as H₂ is enhanced almost 30 times, accompanied by one order of magnitude increase in the H₂/CO₂ selectivity, which reaches values as high as 200, overcoming the Robeson’s upper bound (Figure 1)

Therefore, a simple and scalable method is proposed for the fabrication of novel H₂ purification membranes with outstanding sieving ability, using a 2D material and a self-assembly technique, able to overcome technological limits on this class of separation processes.