Molecularly Tuned PHFP- g -MA Anion-Exchange Membranes for High-Performance Vanadium Redox Flow Batteries

Abstract

Developing membranes with high ion selectivity and long-term chemical stability is essential for advancing vanadium redox flow battery (VRFB) technologies. In this work, uncross-linked and cross-linked anion-exchange membranes (AEMs) based on PHFP grafted with 2-(dimethylamino)ethyl methacrylate were synthesized and quaternized using mono- and dibromohexane. Spectroscopic and microscopic analyses confirmed successful grafting, quaternization, and the formation of continuous hydrophilic ion-transport pathways. Cross-linking with DBH produced a highly compact and densely interconnected network that reduced water uptake and swelling while improving mechanical strength, oxidative stability, and Donnan exclusion against vanadium ions. The optimized PHFP-g-MA-DBH membrane exhibited a VO2+ permeability of 7.35 × 10–7 cm2 min–1, ionic selectivity of 0.912 × 105 S min cm–3, and ionic conductivity of 6.71 × 10–2 S cm–1. Single-cell VRFB performance of PHFP-g-MA-DBH membrane showed a 2.03 times longer self discharge time and 1.34 times higher peak power density than Nafion-212. Moreover, PHFP-g-MA-DBH membrane exhibited 3.75 times higher capacity retention over 300 charge–discharge cycles compared to the Nafion-212 membrane. Long-term cycling at 100 mA cm–2 demonstrated robust durability, with a capacity decay rate of ∼0.13% per cycle and retained efficiencies of CE = 97.88%, VE = 78.53%, and EE = 76.87% after 500 cycles. These results position PHFP-g-MA-DBH as a cost-effective and high-performance alternative to perfluorinated membranes for durable, high-efficiency VRFB systems.