Redox flow batteries (RFBs) are among one of the most promising technologies being explored for grid-scale energy storage from discontinuous power sources such as wind and solar.¹ This has resulted in a need for continued development and understanding of electrolytes and membranes employed in RFBs as a means of improving performance while reducing cost. New approaches in membrane design rely on size-exclusion to reduce parasitic losses resulting from water and electrolyte crossover that presently plagues ionomer membranes within the perfluorosulfonic acid (PFSA) family (e.g. Nafion, Aquivion, and 3M PFSA).² The structure-property relationships of PFSAs are well-known and studied,³ providing the initial framework for utilization of ligands to understand transport through next-generation membranes.

In this work, we discuss our efforts in developing a systematic approach towards probing the effective size-exclusion properties of the polymeric membranes by employing iron-ligand complexes. A series of aminopolycarboxylic acid ligands, shown in Figure 1, that strongly complex with iron were selected to explore the effect of molecule size on crossover in PFSAs with varying domain sizes, controlled via side chain length. We show decreases in permeability with increased size of the iron-ligand complex, providing information regarding the hydrophilic domain network of these ionomer membranes. By measuring the crossover of these complexes using diffusion cells, we explore the size-exclusion properties of PFSAs using relevant molecules for future iron RFB applications.

Acknowledgements

This work was funded as a sub-contract under the Integration and Optimization of Novel Ion-Conducting Solids program (IONICS), by the Advanced Research Projects Agency-Energy (ARPA-E), of the US Department of Energy under award number DE-AR0000773.

1. Perry, M. L.; Weber, A. Z., Advanced Redox-Flow Batteries: A Perspective. J Electrochem Soc 2016, 163 (1), A5064-A5067.
2. Darling, R.; Gallagher, K.; Xie, W.; Su, L.; Brushett, F., Transport Property Requirements for Flow Battery Separators. J Electrochem Soc 2015, 163 (1), A5029-A5040.
3. Kusoglu, A.; Weber, A. Z., New Insights into Perfluorinated Sulfonic-Acid Ionomers. Chem Rev 2017, 117 (3), 987-1104.