Modulating Ionic Transport Through Nanoporous Graphene Membranes for Rare-Earth Recovery

Rare earth elements are critical to the clean energy infrastructure. Current ways of extraction are limited to solvent-based extraction. Membranes provide an opportunity to conduct these separations in an energy- and material-efficient way. Nanoporous graphene, is an ideal candidate as being atomically thin it allows for high permeances, and is chemically compatible with the acidic environments from which these ions can be recovered. However, creation of selective pores is a challenge in these membranes. In this work, ionic transport through graphene pores is modulated using Atomic Layer Deposition (ALD) and Polyelectrolyte Assembly (PEA).

Graphene membranes are transferred on polyimide track-etched supports. Defects in these membranes are then sealed with ALD of hafnium oxide, pores are created using oxygen plasma and then decorated with self-assembling polyelectrolytes. Multiple polyanions, like Polyacrylic Acid (PAA) and Polyestyrenesulfonate (PSS) are assembled with Polyallylamine hydrochloride (PAH) as the polycation. These membranes are then tested in a diffusion-cell with acidic solutions containing 10+ metal ions of different valences. The effect of molecular weight of the polyelectrolyte in modulating the ionic transport and the stability of this assembly in solutions of different pH is tested.

Ionic transport in nanoporous graphene membranes shows hydrated-diameter dependence. ALD helps seal the defects and increases the selective transport in these membranes, however transport is still hydration-diameter governed. Assembling smaller molecular weight, weak-polyelectrolytes, PAA-PAH dramatically increases the monovalent-multivalent selectivity to values above 150 in a solution of pH 3. Even though the transport still points to hydration-size dependent trend, however, some affinity-based effects are also observed. Selectivity between ions of the same valence, for example a K⁺/Li⁺ selectivity of 4 is shown. Assembly of PAH with a stronger polycation PSS is tested with solutions of different pH and it is seen that at pH > pKa, the polyanion interacts strongly with the ions of interest in the solution and increases their rate of permeation, and reduces any observed selectivity. However, at pH ~ pKa, the assembly works as desired and enhances the selective permeation of the smaller monovalent ions as seen in the case of PAA.

Transport through graphene nanopores is altered through atomic layer deposition and polyelectrolyte assembly. Molecular weight and pKa of the polyelectrolytes are demonstrated as levers to modulate this transport. Alkali / rare-earth selectivity of over 150 is demonstrated with high permeance of the alkali ions.