Proton and hydroxide crossover rates and ion conductivities of ion-exchange membranes for redox flow batteries
University of Delaware
A high selectivity and a low internal resistance are key performance properties for ion-exchange membranes in redox flow batteries. Here, the basic zinc-acidic iron double ion-exchange membrane redox flow battery is described. In order to find the best combination of an anion-exchange membrane and a cation-exchange membrane for this design, the H+/OH- permeabilities and the Cl-/Na+ conductivities of different anion-exchange membranes and cation-exchange membranes were characterized and compared. For the investigated anion-exchange membranes, the H+ permeabilities were found to be in the order of Fumapem ® FAA-3 > PTFE-QNPPO > QNPPO > Fumasep ® FAB-PK-130, but the same order (from highest to lowest) is observed for the Cl- conductivities. For the investigated cation-exchange membranes, the OH- permeabilities were found to be in the order of Nafion ® NR-212 > Fumasep® FKS-50 > Fumasep ® FKE-50, while the same order (from highest to lowest) is observed for the Na+ conductivities. Hence, the choice for a particular ion-exchange membrane in the redox flow battery design should be made based on a trade-off between a high selectivity (low crossover) and a high ion conductivity (low internal resistance).