(381aw) Innovative Graphene Based Membranes for Gas Separation with Tunable Selective Properties

Atomic-thick 2D materials attract a great interest for the development of membrane materials due to their peculiar gas transport properties, combined to extremely high aspect ratios. In particular, graphene and its derivatives are very promising in this research area due to their very low permeability to atoms and molecules coupled with interesting selective properties towards smallest species, thus opening to new viable strategies for mass transport-related applications.

In this work, the layer-by-layer (LbL) technique is employed to fabricate thin coatings on top of polymeric substrates, using graphene oxide (GO) sheets as 2-D building blocks, piled up together with polyelectrolyte species in well-ordered structures. Electrostatic interactions drive the self-assembly of GO and polyelectrolytes, controlling both the growth and the coatings arrangement at the nanoscale.

The nanostructured architecture is achieved by alternated dip coating in highly diluted (typically 0.1 wt. %) aqueous polyelectrolyte solutions and GO dispersions, with a water rinsing step performed immediately after each dipping, in order to avoid inhomogeneity on sample surface and promote the required structural order. The whole process is repeated for a prescribed number of cycles to create long selective paths and, after being validated on several different polymeric substrates, both dense and microporous, is carried out on commercial Matrimid polyimide films, a benchmark membrane material for gas separation applications.

The obtained nanostructured coatings revealed an effective and homogeneous deposition of alternated layers, as observed by SEM analysis and by advanced scanning ellipsometry micromapping that allowed also the quantitative determination of the average thickness of each layer.

Gas permeation measurements showed a significant sieving ability of the coatings, which are able to discriminate the penetrant molecules on size basis, making these membranes very interesting for hydrogen purification (e.g. for H₂/CO₂ or H₂/CH₄ separation). Furthermore, due to their small thicknesses, the GO based layers provide a very limited effect on the gas transport of small penetrants such as He or H₂, eventually allowing to overcome the Robeson’s technological limit for polymeric membranes.

Multiple strategies are explored in order to tune the permselectivities of the GO based membranes, considering different positive polyelectrolytes with varied steric properties, GO of different later sizes (as obtained by ultrasonication), and varying the ionic strength of the solutions during the LbL assembly (controlling the pH or using salts). The obtained materials are tested for He (used as safer H₂ surrogate) and CO₂, in order to evaluate the sieving ability of the membrane, and the results are compared with the state of the art data.

The best results are obtained considering a 10 bilayers coating (around 40 nm), whose values of permeability and selectivity (as high as 200) for the He/CO₂ gas pair lies well above the upper bound. Interestingly, the addition of small amount of NaCl is able to boost both selectivity and permeability, while the use of smaller GO flakes allow a more ordered packing of the layers thus leading to a decrease in the gas transport properties.

Therefore, this novel concept of membranes design demonstrated that by means of a simple and scalable method it is possible to fabricate innovative membrane able to overcome technological limits on this class of separation processes, by using innovative 2-D nanomaterial as graphene oxide.