The all-vanadium redox flow battery (VRFB) is one of the most promising energy storage systems to be associated with the grid. The system has been developed for almost 30 years. A key component for VRFBs is the membrane separator, which separates the positive and negative half-cells and prevents the cross-mixing of vanadium ions, while providing required ionic conductivity. In general, research is to solve a multi-variable problem which requires optimization in both physical characteristics and electrochemical performance of the 10 membrane. Nafion and its derivatives are still important materials thanks to their high chemical stability and ionic conductivity. However, weaknesses of these materials, such as high vanadium ion crossover and high cost, stimulate new approaches in materials design for VRFBs. New achievements in material sciences and polymer chemistry allow further development of other types of polymeric materials and composites as separators in VRFBs. This includes new cation exchange membranes, anion exchange membranes, amphoteric 15 ion-exchange membranes, and non-ionic porous materials. Each type of material exhibits its advantages, accompanying with its weaknesses. Recent articles in polymer-containing membranes for use as separators in VRFBs are reviewed.Journal Name, [year], [vol], 00-00 | 5 vanadium crossover in s-Radel in comparison with that in Nafion.

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