Fluorinated poly(fluorenyl ether)s with linear multi-cationic side chains for vanadium redox flow batteries

Abstract

The cationic group distribution along the polymeric backbones of anion exchange membranes (AEMs) has significant influence on their microscopic morphology and anion conductivity. To develop high-performance AEMs for vanadium redox flow batteries (VRFBs), a series of poly (fluorenyl ether) samples bearing di- and tri-quaternary ammonium side chains with similar ion exchange capacities (IECs) were synthesized by grafting cationic alkyl chains with tertiary amine-containing poly(fluorenyl ether) precursors. The experimental results indicate that the introduction of the multi-cationic side chains facilitates the formation of micro-phase-separated morphologies and enhances anion conductivity. Moreover, the number of spacer atoms between the quaternary ammonium groups on the side chains affects the water uptake of the membranes, thus complicating the relationship between the density of cationic group distribution and anion conductivity. The poly(fluorenyl ether)s with dicationic side chains and six spacing atoms (DQA-PFE-C6) showed the highest anion conductivity. A VRFB assembled with DQA-PFE-C6 exhibited a maximum power density of 239.80 mW cm−2 at 250 mA cm−2, which is significantly higher than a VRFB assembled with Nafion 212. Therefore, side chain engineering is an effective chemical approach to enhance the properties of AEMs for VRFB applications.