V⁵⁺ degradation of sulfonated Radel membranes for vanadium redox flow batteries

Insight into the degradation mechanisms of aromatic proton conducting membrane separators for vanadium redox flow batteries (VRFBs) is urgently needed for the development of long lifetime VRFBs. Other than in-cell observations of performance degradation, there is little fundamental evidence on the specific degradation pathways of aromatic ion exchange membranes for VRFBs. Herein we investigated a sulfonated Radel® membrane (S-Radel) as the degradation target to study the degradation mechanism of aromatic polymers by V(v) (or generally V⁵⁺) oxidation. It was found that the ductile S-Radel membrane, which has a similar aromatic backbone structure to the most-studied polyaromatic VRFB membranes that have shown high performance, became brittle and discolored after 3 days of immersion in 1.7 M V(v) + 3.3 M H₂SO₄ solution at 40 °C. The membrane’s intrinsic viscosity was reduced to about half of its original value after this exposure to V(v) while the ion exchange capacity did not change. In addition to chain scission, it was found that -OH groups were introduced to the backbone of S-Radel as the major degradation product. Quinone groups were also observed at 1677 cm^-1 in FTIR measurements. While the V(v) species in VRFBs is usually denoted as VO₂⁺, V(v)=O in VOCl₃ was found to not have degradation activity for S-Radel. Therefore, we hypothesized that there were other reactive forms of V(v) species that first attacked the S-Radel by incorporating hydroxyl groups into the polymer’s aromatic backbone, followed by the oxidation of these hydroxyl groups to quinone functionalities through a redox mechanism.