Ligand content and driving force effects on ion-ion permselectivity in ligand-functionalized membranes

Ion-selective membranes could enable sustainable critical material separations processes because of their scalability, low energy consumption, and low chemical input. The effects of membrane water content and incorporation of ion-coordinating ligands have been studied via computation and experiment to develop structure-performance relationships. However, few studies systematically investigate the effects of membrane composition beyond monomer chemical identity or the balance of driving forces such as diffusion and electromigration. Here we synthesized a library of poly(ethylene glycol) acrylate membranes with varying percentages of ion-coordinating monomers (acrylic acid, 4-vinylpyridine) to investigate the influence of ligand content on ion permeabilities and permselectivities. Trends in membrane performance under electrodialysis and diffusion were compared to elucidate the relative effects of separation driving forces and to inform electrochemical operation. We observed order-of-magnitude permeability reductions with ligand content for ions capable of multidentate ligand complexation, especially for nickel in the pyridine-containing membranes. As a result, lithium/nickel permselectivity gradually increased by a factor of 1.65× from 10 to 50 mol% pyridine membranes. We further demonstrated simultaneous improvements in lithium/nickel separation productivity (1.75×) and selectivity (2.99×) with increasing electric potential driving force. Ultimately, results from this study provide design insights for ligand-functionalized membranes in electrified ion-ion separations processes.