(233f) Nanoporous Atomically Thin Graphene Membranes for Selective Rare Earth Separations

Ion separation technologies play a vital role in resource recovery. The extraction of rare earth elements and lithium from acidic solutions represents a critical process for clean energy infrastructure development. Conventional solvent extraction methods consume significant amounts of energy, chemicals, and solvents. Nanoporous graphene membranes offer the potential to concentrate ionic solutions, reduce chemical use, and separate different ions. While their atomic thinness enables high permeance, creating selective pores remains challenging. This research explores pore modification using atomic layer deposition (ALD) and polyelectrolyte assembly (PEA) to improve ionic selectivity.

The study tests nanoporous graphene membranes on polyimide supports using standardized solutions containing numerous metal ions with varying valences. Ion transport is evaluated in both diffusion and pressure-driven modes. The membrane pores are modified by depositing hafnium oxide through ALD and applying positive and negative polyelectrolytes to opposite sides of the membrane.

Results show that ion transport through nanoporous graphene correlates with the hydrated radii of ions. ALD effectively seals graphene defects, enabling selective transport with an alkali-rare earth selectivity of 7. Post-pore creation ALD treatment narrows the pore size distribution, increasing alkali-rare earth selectivity of about 50.

Polyelectrolytes of different molecular weights and different pKa are tested. These include polyanions such as polyacrylic acid, polyvinylsulfonic acid, polyvinylphosphonic acid. The addition of PEA dramatically enhances alkali-rare earth selectivity. A highest alkali-rare earth selectivity of 100 is achieved with hundred times the permeances as compared to literature. Selectivity between same-valence ions is also achieved, including a K⁺/Li⁺ selectivity of 10. However, these polyelectrolyte systems show significant pH dependence with the effect being stronger with use of stronger polyelectrolytes.

The study expands the design possibilities for tuning ionic transport through atomically thin membranes through pore modification approach using ALD and PEA.