Porous polymeric composite separators for redox flow batteries

Currently, the most commonly used membranes in redox flow batteries (RFB) are ion-exchange membranes. In particular, in all vanadium flow battery systems (VRB), perfluorinated polymers such as Nafion® are widely used, owing to their high proton conductivity and chemical stability; however, the extremely high cost of currently available membranes has limited the commercialization of VRB technology. Recently, low-cost porous polymeric composite separators (e.g., polytetrafluoroethylene [PTFE]/silica), as an alternative to traditional ion-exchange membranes, have attracted a great deal of interest because of their significantly lower cost. Porous separators prepared from various polymer materials and inorganic fillers have demonstrated comparable electrochemical performances to that of Nafion® in flow battery tests with different redox chemistries. This paper provides a review of porous separators for flow battery applications. In addition to discussions of separator material selection and preparation methods, we also emphasize the electrochemical performance of various flow battery systems, especially the capacity fade mechanism that is closely related to ion-transport across porous separator.