Elucidating conductivity-permselectivity tradeoffs in electrodialysis and reverse electrodialysis by structure-property analysis of ion-exchange membranes

Ion-exchange membranes (IEMs) are used in environmental and energy technologies of electrodialysis (ED) desalination and reverse electrodialysis (RED) power generation, respectively. Recent studies reported empirical evidence that the conductivity and permselectivity of IEMs are bound by a tradeoff relationship, where an increase in ionic conductivity is accompanied by a decrease in counterion selectivity over co-ion. A fundamental understanding of this conductivity-permselectivity tradeoff is principal to inform membrane development. This study presents an IEM transport model to analytically relate conductivity and permselectivity to intrinsic membrane chemical and structural properties. The model employs the Nernst-Planck transport framework and incorporates counterion condensation theory to simulate the performance of IEMs in a range of ED and RED operations. The analysis revealed the mechanism for the tradeoff induced by bulk solution concentration: a higher salinity suppresses IEM charge-exclusion, thus lowering permselectivity, but elevates mobile ion concentration within the membrane matrix to improve conductivity. As such, IEM applications are practically confined to sub-seawater salinities, i.e., RED using hypersaline streams will not be efficient. In another tradeoff driven by IEM water sorption, increasing membrane swelling enhances effective ion diffusivity to raise conductivity, but diminishes permselectivity due to dilution of fixed charges. The transport model indicates that increasing membrane ion-exchange capacity and reducing thickness can yield highly selective and conductive IEMs.