Porous poly(benzimidazole) membrane for all vanadium redox flow battery

• A porous poly(benzimidazole) membrane (p-PBI) is prepared by phase inversion.
• The p-PBI shows 35 times lower VO2+ permeability than the Nafion 112.
• 98% coulombic efficiency is achieved in a battery with the p-PBI.
• The battery with the p-PBI shows 10% higher energy efficiency than the Nafion 112.
• The poly(benzimidazole) shows good chemical stability in VO2+ electrolyte.

Porous poly(benzimidazole) (PBI) membranes of low vanadium ions permeability are described for an all vanadium redox flow battery (VRFB). The PBI membrane was prepared by a water vapour induced phase inversion process of a PBI polymer solution. The membrane has a symmetrical cross-sectional morphology. A low water permeability of 16.5 L (m2 h bar)−1 indicates the high hydraulic resistance stemming from a closed cell morphology with nanoporous characteristics. The PBI membrane doped with 2.5 M H2SO4 shows a proton conductivity of 16.6 mS cm−1 and VO2+ permeability as low as 4.5 × 10−8 cm2 min−1. The stability test of dense PBI membrane in VO2+ solution indicates good chemical stability. An all vanadium redox flow battery (VRFB) operated with the porous PBI membrane shows 98% coulombic efficiency and more than 10% higher energy efficiency compared to VRFB operated with Nafion 112 at applied current densities of 20–40 mA cm−2. High in situ stability of the porous PBI membrane was confirmed by about 50 cycles of continuous charge and discharge operation of the battery.