Planar orientation of hydrophilic channels by biaxial deformation of perfluorinated sulfonic acid membranes for vanadium redox flow batteries

Abstract Commercially available perfluorosulfonic acid (PFSA) membranes are used as cation exchange membranes for vanadium redox flow batteries (VRFBs) due to their outstanding chemical stability and proton conductivity. However, high vanadium crossover, and expensive production cost of PFSA membranes are considered as limitations for the further commercialization of VRFBs. Here, in order to overcome the issues, PFSA membranes are biaxially stretched for planar orientation of hydrophilic channels with the expanded membrane area over 800%. The area stretching ratio (γ) of PFSA membranes is controlled by water bath temperature, and by the degree of biaxial stretching. As γ increases, vanadium permeability and proton conductivity, known as thickness independent material properties, decrease, and then reach constant values over γ ~1.25. Thus, both of the hydrophilic channel orientation and the membrane thickness matter for the VRFB cell performance. With thick PFSA membranes, a VRFB cell is not operated at high current density, but with the biaxially 827% area expanded Nafion 117, a VRFB cell shows stable efficiencies even at high current density and better capacity retention (82.9%) than a thin Nafion 211 (57.8%) during the long-term cycling. This planarly oriented morphology by biaxial stretching offers an efficient way to utilize the expansive PFSA membranes.