Quantitation of vanadium ion crossover through membranes provides essential information for optimizing membrane properties in all-vanadium redox flow batteries (VRFBs) and evaluating the efficiency of VRFB cells. However, a VRFB cell composition of highly concentrated vanadium solutions on both sides of the membrane makes it difficult to directly monitor vanadium ion crossover through the membrane. In this work, the permeation behavior of VO2+ and V3+ through Nafion 117 subjected to two different pretreatments is investigated using 48V radiotracer under conditions corresponding to a conventional permeability test employing a MgSO4/sulfuric acid solution and to a fully discharged VRFB. In situ and ex situ radiotracer permeation measurements demonstrate that use of Mg2+ on the acceptor side of the membrane is valid for quantitating concentration-gradient-induced VO2+ crossover. However, V3+ permeation through the membrane in the fully discharged VRFB does not exhibit linear behavior in contrast to a conventional permeability test. Thus, substitution of Mg2+ for VO2+ does not accurately replicate V3+ permeation behavior under these conditions. Our results indicate that radiotracer permeation has considerable potential for the direct observation of vanadium ion crossover in VRFB cells.
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