Stability of acid-excess acid-base blend membranes in all-vanadium redox-flow batteries

In this contribution the performance of two acid-base blend membranes in an all-vanadium redox-flow battery (VRFB) is studied. The first membrane is a nonfluorinated acid-base blend membrane composed of a sulfonated poly(arylene ether sulfone) and polybenzimidazole PBIOO, the second, partially fluorinated, membrane is composed of a sulfonated polymer from decafluorobiphenyl and bisphenol AF and the polybenzimidazole F6PBI. It turns out from gel permeation chromatography experiments that the molecular weight of both membranes degrades in VRFB. However it is found that the partially fluorinated membrane (S1B1) is more stable in VRFB, which can be seen in the number of charge/discharge cycles, while the nonfluorinated membrane S2B2 fails after 137 cycles, the partially fluorinated membrane S1B1 survives 200 cycles. Moreover, the percentage of residual molecular weight of the nonfluorinated membrane after failure (after 137 cycles) is 34.0%, and of the partially fluorinated membrane (after 200 cycles) is 58.8%, respectively. Both membranes show better peak power densities than a Nafion®117 membrane operated in VRFB under the same conditions. In contrast to Nafion®117 and S2B2, the S1B1 membrane shows stable voltage and energy efficiency within the first 60 charge/discharge cycles. Moreover, the Coulomb efficiency of the S1B1 membrane was higher than that of S2B2 and Nafion®117, respectively, being nearly 100%.