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

Sulfonation of OPBI increases the electrolyte uptake and thus increases the conductivity. Simultaneously, the more open structure increases access to vanadium ions. However, it appears that sulfonation deactivates the sites which are attacked by VO₂ ⁺. This allows stable and highly efficient vanadium redox flow batteries.

Abstract

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.