A Stable Polymer-Blended Anion-Exchange Membrane for Enhanced Performance in Redox Flow Batteries

Redox flow batteries (RFBs) operate with a membrane, responsible for the separation of cationic active species and allowing the passage of counter-ions, to preserve electroneutrality on each side of RFBs. Commercial cation-exchange membranes (CEMs), such as Nafion®, possess excellent ionic conductivity and chemical stability. However, they face challenges in blocking cationic active species common in RFBs, leading to significant capacity loss over time. Anion-exchange membranes (AEMs) efficiently block active species with the assistance of the Donnan effect. Nevertheless, commercial AEMs lack acid stability and conductivity compared to their cationic counterparts. Polytetrafluoroethylene (PTFE)-reinforced quaternary ammonium cardo poly(ether)ketone (QPEK-C)-based membranes with Al 2 O 3 additives (PQPAM), a heterogeneous AEM developed in our lab 1 , possess excellent ionic conductivity with low permeability of active species and good mechanical properties. PQPAM has demonstrated better acid stability than commercial AEMs. However, its acidic and oxidative stability require improvement, as evidenced by degradation after 250 hours of in-situ harsh RFB cycling due to the heterogeneous nature of PQPAM and the exposure of the susceptible QPEK-C-rich side to the electrolytes 2 . To address this issue, we propose a strategy to enhance the chemical stability of QPEK-C-based membranes by inhibiting the exposure of QPEK-C to oxidation agents through blending with a hydrophobic engineering thermoplastic like polybenzimidazole (PBI) 3,4 . Furthermore, PBI's known ability to protonate in acidic media will reinforce ionic conductivity and the separation of active species, thus improving the Donnan effect, in addition to enhancing chemical and mechanical stability. Elemental analyses and microscopy images confirm a homogeneous blending of PBI and QPEK-C, promising stability improvement. The results of ex-situ tests on ionic conductivity, cation permeability, and chemical stability align with the mentioned hypotheses, showcasing that blend of PBI/QPEK-C outperforms pure PBI and QPEK-C AEMs. Additionally, the mechanical properties of the PBI/QPEK-C blended AEMs are improved compared to pure QPEK-C AEMs and PBI/QPEK-C blended AEMs outperform pure PBI and QPEK-C AEMs in in-situ cycling tests with our patented titanium-cerium RFBs 5 . Moreover, this work will explore other strategies to improve the blended AEM properties, such as utilizing inorganic fillers or PTFE reinforcement. References: 1- Sankarasubramanian, S., Ramani, V. K., Zhang, Y., Parrondo, J. & Wang, Zhongyang. Doped anion exchange membranes (AEMs) for highly selective separators in electrochemical devices. US Patent 2021/0299650 A1 (2021). 2- Hao, X., Chen, N., Chen, Y. & Chen, D. Accelerated degradation of quaternary ammonium functionalized anion exchange membrane in catholyte of vanadium redox flow battery. Polymer Degradation and Stability 197 , 109864 (2022). 3- Jung, M. et al. Blending polybenzimidazole with an anion exchange polymer increases the efficiency of vanadium redox flow batteries. Journal of Membrane Science 580 , 110–116 (2019). 4- Xu, J. et al. Facile strategy for preparing a novel reinforced blend membrane with high cycling stability for vanadium redox flow batteries. Chemical Engineering Journal 433 , 133197 (2022). 5- Sankarasubramanian, S. & Ramani, V. Redox flow battery. US Patent 2019/0280323 A1 (2019). Figure 1