Equilibrium and Dynamic Absorption of Electrolyte Species in Cation/Anion Exchange Membranes of Vanadium Redox Flow Batteries

Vanadium redox flow batteries (VRFBs) rely on ion exchange membranes (IEMs) to separate the positive and negative compartments while maintaining electrical neutrality of the cell, by allowing the transport of ionic charge carriers. Cation exchange membranes (CEMs) and anion exchange membranes (AEMs), the two principal types of IEM, have both been employed in VRFBs. The performance of these IEMs can be influenced by the absorption of species from the electrolyte. In this study, a typical commercial CEM (Nafion 117) and AEM (FAP 450), were examined with respect to vanadium uptake, after exposure to electrolyte at different states of charge. The two types of membrane were found to behave very differently, with the AEM showing very high selectivity for V ^V , which resulted in a significant increase in area-specific resistivity. In contrast, the CEM absorbed V ^II more strongly than vanadium in other oxidation states. These findings are essential for the development of an effective membrane for VRFB applications.