Tuning Polybenzimidazole-Derived Crosslinked Interpenetrating Network Membranes for Vanadium Redox Flow Batteries

Non-fluorinated ion exchange membranes with high proton selectivity and conductivity are sought as separators for vanadium redox flow batteries (VRFB) to substitute the typically used perfluorosulfonic acid (PFSA) polymer membranes. Polybenzimidazole based membranes offer a promising non-fluorinated alternative due to their excellent thermomechanical properties, low vanadium crossover, and ionic conductivity in acidic media. In this work, a series of polybenzimidazole-polyvinylchloride polymer blends were cast and decorated with different quaternary ammonium functionalities. The polymer blends were systematically studied with respect to the blend composition and chemical structure of the quaternary ammonium groups. Assessment of relevant membrane properties for use in an aqueous acidic flow battery was conducted through a combination of VRFB single cell testing, permeability measurements, water/electrolyte uptake, and infrared spectroscopy. The blend with a polybenzimidazole content of 90 % decorated with DABCO (1,4-diazabicyclo[2.2.2]octane) was found to combine low polarization resistance with low swelling and high stability. The performance was found to be similar to that of a benchmark polybenzimidazole membrane despite being three times thicker. This led to the conclusion that quaternary ammonium functionalized polybenzimidazole-polyvinylbenzylchloride systems is an excellent candidate for further modification or fabrication of thinner membranes to further reduce the membrane resistance without compromising on vanadium blocking properties.