(651e) Impact of Electrolyte Association on Ion Transport in Ion Containing Polymers

Selective ion transport across polymer membranes is essential in water purification and electrochemical applications. For example, hydrated ion exchange membranes (IEMs) are frequently applied in water treatment because the fixed ions tethered to the polymer backbone enable selective permeation of ions based on charge. Despite an expansive literature on hydrated IEMs, the influence of electrolyte association (i.e., ion pairing) on ion selectivity is relatively unexplored and the models used in this community assume complete dissociation. Conversely, ion pairing is discussed extensively in dry polymer electrolytes for battery applications, but the extent to which water affects dissociation in such systems is poorly understood. This presentation focuses on experimental and theoretical results of ion transport in ion containing polymers to address these knowledge gaps. Macroscopic measurements of Na₂SO₄ and MgSO₄ partitioning in hydrated IEMs were performed and compared with model predictions based on counter-ion condensation theory. This model underpredicted sulfate sorption by several orders of magnitude due to significant concentrations of ion pairs in sulfate salts that are not accounted for in this framework. A new model that captures these physics for hydrated IEMs was developed and showed markedly improved agreement with the experimental data. The role of hydration on ion dissociation in polymers was further probed by exposing model, LiTFSI doped polyether systems to humid air. Ion dissociation was determined as function of polymer water content using ionic conductivity and pulsed field gradient NMR measurements. Dissociation of LiTFSI was found to increase strongly with increasing humidity due to the strong solvation of ions by water imbibed within the polymer.