Highly selective separations of multivalent and monovalent cations in electrodialysis through Nafion membranes coated with polyelectrolyte multilayers

Abstract

Abstract Nafion membranes coated with 5.5-bilayer poly(4-styrenesulfonate) (PSS)/protonated poly(allylamine) (PAH) films show Li+/Co2+ and K+/La3+ selectivities >1000 in electrodialysis (ED). The corresponding selectivities of bare Nafion are 23. However, the resistance to monovalent-ion passage does not decrease significantly with fewer polyelectrolyte layers. At overlimiting currents, water splitting near the polyelectrolyte/Nafion interface reduces current efficiency because protons and hydroxide ions carry current in the membrane. Moreover, hydroxides from water splitting form insoluble metal hydroxides to foul the membrane. With 0.1 M source-phase salt concentrations, transference numbers for monovalent cations approach unity and selectivities are >5000 because the diffusion-limited K+ or Li+ currents exceed the applied current. However, ED selectivities gradually decline with time. Thus, future research should aim to increase membrane stability and limiting currents to fully exploit the remarkable selectivity of these membranes.