Morphology Control of Immiscible Polymer-Blended Anion-Exchange Membranes

Membranes are an integral component of electrochemical flow reactors (EFRs), allowing charge carrier transport between electrodes while blocking active species. Many EFR technologies utilize cation-exchange membranes (CEMs) extensively due to their high conductivity and chemical stability; however, CEMs do not obstruct cationic active species and cause unwanted electrolyte cross-contamination in EFR technologies that use cations as active species, e.g., redox flow batteries (RFBs) and electrodialysis. Anion-exchange membranes (AEMs) innately block cations; however, they are not sufficiently durable. Developing strategies to enhance their durability is necessary to expand their application in RFBs and similar EFRs. This work investigates the morphology control of immiscible polymer-blended AEMs via tuning the blend composition and casting temperature. Marangoni-Bénard effect was identified as the dominant mechanism behind the perturbations in the casting solution film. Marangoni cells appear at AEMs cast at >40 °C, leading to desirable lateral phase separation of polymers. The ionic conductivity of lateral phase-separated AEMs improved with lowering the casting temperature, indicating that excessively heightened perturbations in the casting films damage the conductivity routes. Adding 20% PVDF-co-HFP to the AEM reduced the water uptake (30 vs 58%), hence increasing the durability of the AEM (conductivity loss of 21 vs 63% over a week) while sacrificing minimal ionic conductivity (11 vs 13 mS·cm^-1). Hence, polymer blending is a useful strategy to improve the durability of membranes for RFBs and similar electrochemical systems.