Sulfonated Polybenzimidazole Membranes: How Sulfonation Affects Properties, Stability, and Performance in Vanadium Redox Flow Batteries

Poly(4,4′-diphenylether-5,5′-bibenzimidazole), OPBI, is sulfonated at different conditions to reach different degrees of sulfonation (DOS of 35, 54, 102, and 133%). The membrane with a DOS of 102% shows the most balanced properties. The tensile strength is 55 MPa and the elongation at break is 130%. Conductivity in 3 m sulfuric acid exceeds that of Nafion 212 (58 mS cm⁻¹) and reaches 70 mS cm⁻¹. The resistance of a 23 µm thick membrane is 33 mΩ cm², and energy efficiencies of 91.8% at 80 mA cm⁻² and 90.4% at 100 mA cm⁻² are achieved. OPBI showed the highest stability in contact with VO₂⁺, because the ions cannot enter the membrane. Sulfonated OPBI swells more in sulfuric acid, and therefore has a lower stability. However, it is found that the stability increases with the DOS. This indicates that attack by VO₂⁺ occurs preferentially at positions that are activated for electrophilic aromatic substitution reactions.