Polyelectrolyte Assembly to Tune Ionic Selectivity in Nanoporous Graphene Membranes

The extraction of rare earth elements (REEs) is vital for modern technologies, including renewable energy, advanced electronics, and defense applications. However, they are present in extremely dilute concentrations and need to be recovered through chemically intensive and costly solvent extraction methods. Membrane-based separation can provide a less energy-intensive option for pre-concentration of extremely dilute streams. A potential candidate is nanoporous graphene membranes, single-atom-thick sheets of sp² carbon with nanopores created by removing parts of the hexagonal lattice structure. These membranes are capable of high permeances, and allow for creation of tunable and selective pores. In this work, we self-assmeble polyelectrolytes on nanoporous graphene membranes as a strategy to regulate ionic selectivity and enable more efficient REE separations. Polyelectrolytes of opposing charges were introduced on both sides of the membrane interface, allowing for electrostatic interactions. Assembly was implemented using four distinct polyanions: poly(acrylic acid) (PAA, 30K and 5K), poly(vinylsulfonate) (PVS, 5K), and poly(vinylphosphonic acid) (PVP, 24K), paired with poly(allylamine hydrochloride) (PAH, 5K) as the polycation. We studied simultaneous ionic diffusion across these membranes for 13 ions in a solution of supporting electrolyte. The results demonstrate diffusive selectivity between monovalent-multivalent ions (even as high as 90 for K⁺ and Dy³⁺) as well as selectivity between ions of the same valence (selectivity of 3-5 between REEs). It was observed that lighter and weaker polyelectrolytes often exhibited higher selectivity for alkali metals and REEs compared to heavier and stronger options. The observed behavior could be attributed to different mechanisms, including defect-sealing, electrostatic repulsions, and specific binding, that need to be further investigated. This study demonstrates the ability to use self-assembly of polyelectrolytes to control ionic transport across 2D membranes and enhance ion-ion selectivity.