Kinetic barrier networks reveal rate limitations in ion selective membranes

While polymer membranes are used to remove salts from environmental and industrial electrolytes, it remains a significant challenge to engineer them to isolate a single dissolved species from complex mixtures, which is important for lithium mining, battery and magnet recycling, and microelectronics. Underpinning this challenge has been a lack of understanding of rate-limiting mechanisms in selective ion transport. Here, we show that hydrated ions exhibit higher free energies of activation when crossing solution–membrane interfaces (i.e., partitioning) than when diffusing through polymers, which challenges historical assumptions embedded in widely used models of membrane performance. We further articulate a framework benchmarked with quantitative capabilities for predicting how functionality within polymer membranes or at their surfaces affects the selectivity towards individual dissolved species.