Redox flow batteries have emerged as one of the most promising technologies for grid-scale energy storage options. Different chemistries have been developed, including the all-vanadium redox flow battery¹. Among the different components of the all-vanadium redox flow batteries, the ion-exchange membrane plays a significant role via directly affecting the voltage and coulombic efficiencies. The ideal membrane must minimize resistance for proton flux and at the same time limit unwanted crossover of active species.

The species transport mechanism through the ion-exchange membrane has been the focus of several studies ^{2, 3}. Also, different mathematical models have been developed to predict the transport through the membrane ^4-8. However, experimental studies for the species transport through the ion-exchange membrane is rare ^{9, 10}. In this talk, we will report on results of direct species crossover measurement through the ion-exchange membrane for the all-vanadium redox flow batteries. A unique test facility utilizing several test cells and UV-Vis spectrometry has been designed and verified that enables measurement of the species crossover with and without electric fields, so that individual driving forces of transport can be deconvoluted. The experimental data reveals the different crossover rates for individual vanadium oxide species as a function of concentration and electric field effect. 

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