(644d) An Engineered Immiscible Poly (vinylidene fluoride-co-hexafluoropropuylene)-Blended Anion-Exchange Membrane for Redox Flow Batteries

Redox flow batteries (RFB) hold great potential for large-scale stationary energy storage to be integrated with solar and wind power plants. Designing more efficient membranes with high ionic conductivity, low permeability of active species, and high chemical stability will boost the RFB performance and durability.

Commercial cation-exchange membranes (CEMs, i.e., Nafion®) possess excellent ionic conductivity, chemical stability, and mechanical properties. However, they have difficulties blocking cationic active species (common in RFBs). Anion-exchange membranes (AEMs) block the active species efficiently; However, commercial AEMs are not acid-stable and as conductive as commercial CEMs. Polytetrafluoroethylene (PTFE)-reinforced quaternary ammonium cardo poly(ether)ketone (QPEK-C)-based membranes with Al₂O₃ additives (PQPAM), the AEM developed in our lab¹, possess excellent ionic conductivity with low permeability of active species and good mechanical properties and have been shown to possess better acid stability than commercial AEMs; However, their acid stability needs improvement as they degrade after 250 hours of in-situ RFB cycling due to the exposure of QPEK-C-rich side. A strategy to improve the chemical stability of the QPEK-C-based membranes is to inhibit the exposure of QPEK-C to oxidation agents by blending with a hydrophobic engineering thermoplastic like poly (vinylidene fluoride-co-hexafluoropropylene) (PVDF-co-HFP)².

Initial results of chemical stability tests agree with the mentioned hypothesis and blending PVDF-co-HFP with QPEK-C shows enhanced stability. In an ex-situ chemical stability test over 40 days, the PVDF-co-HFP/QPEK-C blended membranes nearly preserved their initial ionic conductivity and ion-exchange capacity as opposed to the diminished properties of PQPAM. Despite this positive result, the fabricated PVDF-co-HFP/QPEK-C blended membranes are not as conductive as the PQPAM. Additionally, PVDF-co-HFP and QPEK-C form immiscible blends and their morphology and phase composition varies with blend composition, casting temperatures, and preparation methods. Consequently, the macroscopic properties of the membranes (e.g., ionic conductivity, mechanical properties, cation permeability) also vary drastically; This study is focused on understanding and controlling the composition-structure property relationships in blended membranes for RFBs.

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).