Redox flow batteries (RFBs) remain one of the most promising technologies that are actively explored for grid-scale energy storage from discontinuous power sources such as wind and solar.[1] Proliferation of RFB technology has resulted in a need for continued development and understanding of new redox couples and improved separator membranes for RFB operation, while reducing cost. Membranes employed in RFB technology are largely comprised of the perfluorosulfonic acid (PFSA) family (e.g. Nafion, Aquivion, and 3M PFSA) which own their functionality to the phase separated structure composed of ionically conductive hydrophilic domains distributed within an inert and mechanically stable hydrophobic matrix.[2] PFSA membranes are also susceptible to high crossover of redox active species, leading to parasitic losses in capacity that can only be solved by the development of highly selective separator membranes.

In this work, we report our progress in developing membranes that exhibit high selectivity by employing heteropoly acid (HPA) ion conductors that have been covalently tethered to the backbone of a polymer support. HPA-loaded membranes are promising due to the highly mobile protons associated with the HPA molecule[3] alongside the hydrophobic polymer that provides mechanical strength that is capable of preventing crossover. PFSA membranes were used as a baseline to access the quality of the newly developed HPA-loaded membranes by measuring the selectivity which involves measurement of the membrane conductivity and permeability. The HPA-loading was varied on a mass base in order to evaluate the trade-offs between increased membrane conductivity, which tends to occur with increased permeability of redox active species. Selected membranes were subjected to cell testing in a RFB based on the iron-hydrogen to gauge cell performance.

References:

1. Perry, M.L. and A.Z. Weber, Advanced Redox-Flow Batteries: A Perspective. Journal of the Electrochemical Society, 2016. 163(1): p. A5064-A5067.
2. Kusoglu, A. and A.Z. Weber, New Insights into Perfluorinated Sulfonic-Acid Ionomers. Chemical Reviews, 2017. 117(3): p. 987-1104.
3. Motz, A.R., et al., Heteropoly acid functionalized fluoroelastomer with outstanding chemical durability and performance for vehicular fuel cells. Energy & Environmental Science, 2018. 11(6): p. 1499-1509.

Acknowledgements:

This study was funded with support of the U.S. Department of Energy under contract number DE-AC02-05CH11231 as sub-contract under Colorado School of Mines and United Technologies Research Center.