Influence of the Supporting Electrolyte on the Structure-Transport Properties of PFSA Membranes for Redox Flow Batteries

Abstract

Redox flow batteries (RFBs) are poised at becoming one of the most promising technologies that are 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 new redox couples and their supporting electrolyte counterparts, as well as improving 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 exhibit different material properties depending on the surrounding environment, such as transport properties.[2] The structure-property relationships of PFSAs are well-known and studied,[3] providing the initial framework for our understanding; however, much of this work has been performed in either vapor or liquid water. Thus, it is of interest to elucidate how the structure and transport properties of PFSAs change in various electrolytes used in RFBs. In this work, we discuss our progress in linking the electrolyte concentration-dependent nanostructure to the transport of ions in PFSA membranes. Sulfuric acid and hydrochloric acid represent the majority of supporting electrolytes used in RFB applications and are therefore the primary focus of this study. The polymer’s nanostructure in electrolytes is analyzed using small angle X-ray scattering (SAXS) in-situ, in order to understand the influence of the type and concentration of the supporting electrolytes on the characteristic morphological features of the membrane. Then, nanostructural information is linked to the ion transport through the membrane, via conductivity. Linking the conductivity and polymer morphology provides a better understanding of the structure-transport relationships of PFSAs in electrolytes, and their implications for membrane performance in a RFB cell. Perry, M.L. and A.Z. Weber, Advanced Redox-Flow Batteries: A Perspective. Journal of the Electrochemical Society, 2016. 163(1): p. A5064-A5067.Tang, Z., et al., Composition and Conductivity of Membranes Equilibrated with Solutions of Sulfuric Acid and Vanadyl Sulfate. Journal of the Electrochemical Society, 2013. 160(9): p. F1040-F1047.Kusoglu, A. and A.Z. Weber, New Insights into Perfluorinated Sulfonic-Acid Ionomers. Chem Rev, 2017. 117(3): p. 987-1104.