Polybenzimidazole membrane based aqueous redox flow batteries using anthraquinone-2,7-disulfonic acid and vanadium as redox couple

New cost-effective aqueous redox flow batteries (ARFBs) using anthraquinone-2,7-disulfonic acid (2,7-AQDS) and vanadium oxide sulfate (VOSO₄) as anolyte and catholyte are suggested with polybenzimidazole (PBI) membranes replacing the conventionally used Nafion membranes. To evaluate the conduction mechanism of PBI and Nafion membranes, the permeability of protons and hydrogen sulfate ions is measured. Permeability for charge balancing ions is 2100 (proton, Nafion), 0.78 (hydrogen sulfate, Nafion), 180 (proton, PBI) and 2 (hydrogen sulfate, PBI) 10⁻¹⁴ m² s⁻¹, indicating that PBI has much lower cation/anion selectivity than Nafion. The low permeability of 2,7-AQDS and VOSO₄ in PBI membrane ensures the stable discharge capacity of ARFBs. ARFBs using 15, 30 and 45 μm thick PBI membranes show the capacity fade rate of less than 3% even after 100 cycles. Conclusively, 30 μm thick PBI is optimal because this exhibits the balanced ohmic resistance between membrane thickness and has few voids between electrodes and membrane. When ARFB using 30 μm thick PBI membrane is operated, this shows excellent performances, such as energy efficiency of 70.1% and discharge capacity of 10.6 Ah L⁻¹ during 100 cycles.