Accelerated degradation of quaternary ammonium functionalized anion exchange membrane in catholyte of vanadium redox flow battery

•A quaternary ammonium functionalized anion exchange membrane was degraded ex-situ.•1H NMR revealed changes in both aromatic backbone and aliphatic side chains.•FT-IR, XPS, and SEM confirmed the happening of degradation caused by VO2+.•A degradation mechanism was proposed based on the analytical results.•Four strategies were suggested to alleviate the oxidative degradation of membrane. Anion exchange membranes (AEMs) are a type of important separators for vanadium redox flow batteries (VRFBs) with unique advantage in mitigating the permeability of vanadium ions by Donnan repulsion. So far, the degradation mechanism of quaternary ammonium functionalized AEMs in acidic and oxidative VRFBs catholyte is still unclear. Herein, we report the accelerated degradation of a homemade quaternary ammonium functionalized AEM (QA-OMPFEK-20) in the catholyte of VRFBs. The experimental results show that the VO2+ species in the catholyte are reduced to VO2+ gradually with the extension of immersion time. The ion exchange capacity (IEC) of the QA-OMPFEK-20 was decreased to 88.7% of its original value after 30 days of immersion. 1H NMR, Fourier-transformed infrared spectroscopy (FT-IR), and X-ray photoelectron spectroscopy (XPS) were used to probe the changes in chemical environment of QA-OMPFEK-20. The degradation mechanism was then proposed as the critical attack of VO2+ ions on the ether bond of backbone and the C-N bond of quaternary ammonium groups, leading to the decrease of both molecular weight and IEC.