Detecting and repairing micro defects in perfluorinated ion exchange membranes for redox flow batteries

Abstract

Ion exchange membranes play a vital role in redox flow batteries. However, polymer membranes with a microscopic thickness of approximately 20–50 μm are susceptible to micro defects, which substantially reduces the battery's energy efficiency and cycling stability. Hence, there is a need for an effective strategy to identify and resolve membrane imperfections, which is currently missing in the literature. In this work, a pressure-retention setup and hot-pressing method are proposed and show that defective membranes can be effectively identified and resolved. For instance, a membrane with around 100-μm pinholes exhibits a low coulombic efficiency of 77.5 % at the current density of 100 mA cm⁻². However, the coulombic efficiency can be raised to 96.3 % by removing the defects, thus attaining the level of the undamaged pristine membrane (96.4 %). The capacity retention rate of the vanadium redox flow batteries with the repaired membrane is 71.1 % over 100 cycles at the current density of 200 mA cm⁻², close to that of the pristine membrane (72.2 %). In addition, the repaired membrane exhibits quite similar physicochemical properties to the pristine membrane from various characterizations. The proposed method represents a convenient, economical, and non-destructive membrane detecting and repairing strategy, demonstrating great potential for redox flow batteries.