Optimizing the performance of meta-polybenzimidazole membranes in vanadium redox flow batteries by adding an alkaline pre-swelling step

Polybenzimidazole (PBI) is a promising material for vanadium redox flow battery (VRFB) membranes. It shows a low permeability for vanadium ions, a conductivity in the range of 5 mS cm⁻¹ in contact with 2 M sulfuric acid, and resists degradation by VO₂⁺. Recent literature showed that the conductivity of PBI can be increased to 18 mS cm⁻¹ by pre-swelling the membrane with phosphoric acid (PA), and up to 590 mS cm⁻¹ by casting a sulfonated para-PBI membrane from polyphosphoric acid before immersion in sulfuric acid. However, these membranes show an increased permeability towards VO²⁺ ions, and thus reduced coulomb efficiency in the VRFB. Here we investigate pre-swelling in 4 M alkaline solution. It increases the conductivity in 2 M sulfuric acid to 56 (potassium hydroxide) and 12 mS cm⁻¹ (sodium hydroxide). In 3 M sulfuric acid, the NaOH swollen membrane (4N3S) shows 14 mS cm⁻¹, corresponding to an area resistance of 69 mΩ cm² for a 10 µm thick membrane, lower than that of Nafion 115 (192 mΩ cm²) and even Nafion 212 (89 mΩ cm²). The selectivity (conductivity/permeability) is 9·10¹⁴ S s m⁻³, 7, 30 and 1000 times higher than for standard PBI, PA and polyphosphoric acid pre-swollen membranes, respectively. A VRFB with a 5 µm thick 4N3S membrane showed energy efficiencies of 91.3% at 80 mA cm⁻² and 95.4% at 40 mA cm⁻².