Impact of solution composition on the resistance of ion exchange membranes

Resistance to ion transport in ion exchange membranes (IEMs) is detrimental to the performance of IEM-based processes. In this work we measured the resistance of representative IEMs, i.e. one cation (CEM) and one anion (AEM) exchange membrane, in 15 single-salt solutions using electrochemical impedance spectroscopy. Resistance was sensitive to solute identity only in the case of the CEM for which it depended on the counter-ion identity; the resistance of the CEM was mostly insensitive to the co-ion identity, and the resistance of the AEM was mostly insensitive to both the counter-ion and co-ion identity. For all solutes, membrane resistance decreased sharply with increasing solution concentration below 0.1 M, and remained approximately constant above 0.1 M. An empirical mathematical model comprising a concentration-dependent term and a concentration-independent term successfully described membrane resistance as a function of solution concentration. The model builds on that previously proposed by Galama et al. (JMS 467 (2014), 279-291). We found that for both membranes, the concentration-dependent and concentration-independent terms of the resistance increased with increasing counter-ion hydration free energy. This was rationalized as the energy barrier to counter-ions having to shed/reorient water molecules of hydration, due to steric effects, when permeating the membranes. Also for both membranes, the concentration-dependent term of the resistance generally had a non-linear relationship with salt concentration. This result suggests that the concentration-dependent term is not attributable to bulk solution, and that there is a degree of randomness to the interconnectedness between the different membrane regions that contribute to ionic resistance. Our findings improve the understanding of the relationships between electrolyte properties and IEM resistance, and provide tools for assessing IEM resistance. This improved understanding is critical to establishing a complete IEM resistance theory and to evaluating new applications for IEMs.