(226f) Co-Ion Specific Effect on Li Salt Sorption in Cation Exchange Membranes (CEM)

Understanding the permeation of lithium salt in CEMs has significant implications for both energy and resource extraction applications. In both scenarios, Li+ is transported through a CEM under the influence of an electrochemical potential gradient.

The sorption of salt in CEMs can be described by the Donnan-Manning model, which equates the electrochemical potential of ion species in the membrane with that in the solution phase. The selectivity of counter-ion over co-ion sorption is often explained by Donnan-exclusion: the electrostatic repulsion between the fixed charged group and the co-ion. Recent studies have indicated that the sorption of salts in CEMs may also be influenced by the attraction between counter and co-ions. For instance, in Li2SO4, SO4^(-2), unlike Cl^(-1), can form ion pairs and be absorbed into the membrane as a reduced-charge species, LiSO4^(-1). The ion-pairing effect diminishes the Donnan-exclusion effect on the co-ion.

For salts with the same cation, the ability to form ion pairs increases with the anion charge density. In this study, we investigated Li salt sorption in a standard CEM (CR 61) by systematically varying the co-ion charge density. The co-ions, ranging approximately from low to high charge density, include bistriflimide (TFSI^(-1)), perchlorate (ClO4^(-1)), nitrate (NO3^(-1)), chloride (Cl^(-1)), sulfate (SO4^(-2)), and oxalate (C2O4^(-2)). These ions are of interest in a wide range of energy, water, and resource recovery applications.

We found that the sorption of lithium salt in CEMs is dependent on the co-ion charge density, but this dependence decreases with increasing counter-ion charge density (from Li to Na). We further outlined the implications of our findings for the selective separation of Li salts.