The Influence of Additives on the Polybenzimidazole Membranes in Vanadium Redox Flow Batteries

Abstract

In recent years, membrane development has been a focus to replace Nafion as the only viable choice for Vanadium redox flow batteries (VRFBs). Efforts were made to develop polymeric membranes with a lower cost and especially with lower vanadium ions crossover to minimize the costs and hassles associated with frequent electrolyte rebalancing. Among researched VRFB membranes, polybenzimidazole (PBI) membrane stands out as one of the most promising candidates thanks to its incredibly low vanadium permeability and lower cost than Nafion. However, its lower ionic conductivity when compared to Nafion which results in low voltage and energy efficiency – is a main barrier for its widespread acceptance in VRFBs.In order to improve the conductivity of PBI membranes, efforts have been made, such as chemical alteration of PBI backbone, and addition of ionic and non-ionic side groups. However, these approaches add complexity in the synthetic procedure resulting in a higher membrane cost. Therefore, simple approaches such as blending PBI polymer with additives (e.g. Nafion, polypentafluorostyrene, etc.) provide composite membranes with suitable conductivities for VRFBs. However, these additives leach into the vanadium electrolyte solution depending on their solubility. They can affect the kinetics of the anode and cathode reactions which is largely omitted in the literature. Therefore, in this study, we have investigated the effects of different additives, used for the fabrication of composite PBI membranes, on the electrode and VRFB performance. The kinetic performance loss is studied in-situ and ex-situ by electrochemical impedance spectroscopy. We have also established the Structure–Performance Relationship of additives with VRFB performance.