Coalescing Cation Selectivity Approaches in Ionomers

Inconsistent selectivity measures used in the literature to quantify an ionomer's preference for one ion over another make it challenging to compare uptake of different ions across studies. Preferential ion-partitioning, which leads to partially exchanged membranes, needs to be studied to gain a fundamental understanding of the nature of ion/membrane interactions as well as to begin to unravel how varying ion fractions, whether or not intentional, may affect the membrane's properties and thus durability and performance. In this focus review, we use Nafion as a prototypical example to explore partial exchange studies and codify the various literature experiments and theory into a comprehensive whole, enabling formation of ionomer databases. A few representative approaches to selectivity are dissected to identify the points of divergence between them as well as to re-examine the embedded assumptions therein. Lastly, modeling strategies for ion selectivity of the membrane are reviewed to arrive at and correlate fundamental ion properties to ionomer selectivity.

Automated summary

The inconsistent use of selectivity measures in research makes it difficult to compare the uptake of different ions. Studying preferential ion-partitioning and partially exchanged membranes is important for understanding the interactions between ions and membranes, and how varying ion fractions may impact membrane properties and performance. This review focuses on Nafion as a case study to explore partial exchange studies and consolidate literature experiments and theory into a comprehensive overview, forming ionomer databases. Various approaches to selectivity are analyzed to identify differences and re-evaluate assumptions. Lastly, modeling strategies for ion selectivity are reviewed to establish correlations with fundamental ion properties.