Due to their ability to selectively transport ionic species, ion-exchange membranes (IEMs) play a key role in various membrane-based technologies for water purification and energy storage/generation applications. Notable examples of such technologies include electrodialysis, redox flow batteries, and fuel cells, among others. Although IEMs have been the subject of industrial and academic interest for a long period of time, the fundamental understanding of how polymer structure influences transport properties of charges species remains incomplete. In particular, the development of a simple, predictive model for ion partitioning and diffusion in IEMs has long eluded researchers despite sustained efforts. A key challenge in this area has been the inability to accurately quantify non-idealities in IEMs arising from interactions between the polymers and ions. This presentation will discuss the use of Manning’s counter-ion condensation theory, which was originally developed for dilute polyelectrolyte solutions, to quantify such non-ideal effects in charged membranes. Despite its relative simplicity, Manning’s model adequately captured non-ideal effects in a series of homogeneous IEMs and, combined with the classical Donnan theory, enabled the prediction of equilibrium ion partitioning in the membranes with no adjustable parameters. Manning’s counter-ion condensation theory and the Mackie/Meares model were used to construct a simple, unifying model for ion diffusion in IEMs. The model predicted concentration-gradient driven transport with no adjustable parameters and electric-field driven transport using a single adjustable parameter. The experimental and modeling results elucidate key membrane structural parameters that influence ion transport in IEMs and provide guidance on how to rationally design high performance materials.

Ion partitioning and transport in ion exchange membranes: Importance of counter-ion condensation

2019 Fall National Meeting

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