Membrane resistance of different separator materials in a vanadium redox flow battery

The membrane in vanadium redox flow batteries has various influences on the performance during operation due to their ohmic losses and crossover phenomena. In this study, the area specific membrane resistances of different materials (cation exchange membranes, anion exchange membranes and a porous separator) were measured under conditions relevant for practical vanadium redox flow battery operation by means of electrochemical impedance spectroscopy. The transient behavior of the membrane resistance upon current density changes was found to be strongly dependent on the thickness and the internal structure of the tested materials. In most cases, the membrane resistance decreased with increasing current density, while only the FAP-375-PP membrane showed an increasing resistance during discharge. For thin membranes and especially the Celgard® 3401 separator, a higher resistance was determined than expected from thickness and electrolyte conductivity. Regarding the influence of the state of charge on the resistance, only the separator and the FAP-375-PP membrane revealed the behavior expected from the electrolyte conductivity. Overall, our results clearly show that the processes in the membranes contributing to their resistance are highly complex and require further investigations.