Zwitterionic poly(terphenylene piperidinium) membranes for vanadium redox flow batteries

Over recent years, non-fluorinated ion exchange membranes based on poly(terphenylene) backbones carrying different functional groups have shown potential application for vanadium redox flow batteries (VRFBs). Generally, the ion exchange membrane in VRFBs is a critical component in terms of the output power, long-term stability and cost. Yet, the shortcomings of commercial membranes (e.g., Nafion) have become a substantial barrier to further commercializing VRFBs. After successfully fabricating and testing poly(terphenylene)-based membranes carrying piperidinium and sulfonic acid groups, respectively, for VRFBs, we have in the present work combined both these ionic groups in a single zwitterionic membrane. A series of poly(terphenylene)-based membranes containing zwitterionic (sulfoalkylated piperidinium) and cationic (piperidinium) groups in different ratios (40–60%) were synthesized and investigated. The VRFB using the zwitterionic membranes showed competitive performance compared to Nafion 212 regarding ionic conductivity, capacity retention, and chemical stability. In addition, it was shown that the VRFB performance was improved by increasing the content of zwitterionic groups within the membrane. A self-discharge time of more than 800 h and 78.7% average capacity retention for 500 VRFB cycles were achieved using a membrane with an optimized ratio (60% zwitterionic and 40% piperidinium groups). Furthermore, the chemical stability was promising, as there was no change in the chemical structure after 500 cycles. Our results represent a critical step for developing novel and competitive ion exchange membranes as an excellent alternative to the Nafion benchmark.