Fundamentals of Ion Transport in Ion Containing Membranes

Ion transport in polymer electrolytes is of significant interest in various applications (e.g., water purification, electrochemical cells, and batteries). However, these communities have evolved independently, leading to a lack of design rules applicable under broad process conditions. To connect the various, disparate communities interested in polymer electrolytes, this presentation focuses on measurements and modeling of ion transport in ion containing polymers. Ion pairing is shown to greatly enhance salt sorption in hydrated ion exchange polymers due to reduced Donnan exclusion. Although ion pairing is discussed extensively in dry polymers, this phenomenon is rarely considered in hydrated polymers. Thus, predictions made using established theories for hydrated polymers deviate from the data by over an order of magnitude. Remarkably, by reformulating these models to account for ion pairing, predictions of salt sorption are significantly improved, and, in some cases, good quantitative agreement is observed between theory and experiment using no adjustable parameters. Finally, the applicability of various models for ionic conductivity in polymer electrolytes is determined as a function of water activity, demonstrating the importance of hydration on the physics of ion transport.