Polymer-Blended and Reinforced Anion-Exchange Membranes for Methanesulfonic Acid-Based Titanium-Cerium Redox Flow Batteries

Redox flow batteries (RFBs) are a promising technology for stationary energy storage, offering decoupled power and energy units, cost-effectiveness, and flexibility. Among various types of RFBs, titanium-cerium (Ti–Ce) systems stand out due to their abundant redox species and high nominal voltage, which minimize side reactions such as hydrogen and oxygen evolution. A key challenge in Ti–Ce RFBs is the development of membranes that balance ionic conductivity, permselectivity, and chemical stability while minimizing cation crossover.This work introduces a reinforced polymer-blend membrane composed of quaternized ammonium cardo poly(ether) ketone (QPEK-C) and polybenzimidazole (PBI), designed for methanesulfonic acid (MSA)--based Ti–Ce RFBs that operate under highly acidic and oxidative conditions. The hydrophobicity of PBI contributes to chemical resistance, and the imidazole groups in PBI enhance the Donnan exclusion effect upon protonation. Reinforcement with a 10 μm expanded PTFE (e-PTFE) sheet and 7 wt/wt% aluminum oxide nanoparticles further improves the membrane's mechanical and chemical stability, resulting in a discharge capacity retention (∼94 % over 150 h) and high energy efficiency (∼70 %). Our reinforced PBI/QPEK-C blend outperforms Fumasep's FAP-330, a commercial anion-exchange membrane (AEM), demonstrating superior chemical stability, mechanical strength, and ionic conductivity. These findings underscore the potential of PBI/QPEK-C blends as durable solutions for large-scale energy storage, representing a significant advancement in addressing membrane-related challenges in Ti–Ce RFBs.