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.　H-1　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.　(C)　2022　Elsevier　Ltd.　All　rights　reserved.