N‑Sulfonated Poly(arylene-oxindole) for Vanadium Redox Flow Battery Applications

We describe the synthesis of N-sulfonated poly­(arylene-oxindole) by metal-free superacid, denoted IDB, IBP, and IFP by polycondensation of isatin with 1,4-diphenoxybenzene, biphenyl, and fluorene, monomer, respectively. All three polymers provide flexible transparent films with high decomposition temperatures (400 to 500 °C) as well as good mechanical and oxidative stability. The physiochemical and electrochemical characteristics of the IDB membrane were better, in comparison to IBP and IFP. The presence of an ether linkage in IDB facilitates the formation of a highly negative charge amide ion, resulting in a high degree of N-sulfonation. To evaluate performance in VRFB, relevant physical properties and stability in a highly oxidative environment (2.1 M H₂SO₄ and 1.6 M VO₂⁺) were evaluated. In a single cell VRFB test, the peak power density for IDB, IBP, and IFP was 275 mW cm^–2, 240 mW cm^–2, and 225 mW cm^–2 respectively, at 300 mA cm^–2 current density. The IDB membrane showed 97% Coulombic, 68% energy, and 66% voltage efficiencies at 100 mA cm^–2 over 200 charge/discharge cycles. In comparison to Nafion 117 in identical experimental settings, the OCV studies validated the low rate of self-discharge, showing the IDB membrane is better suited for VRFB applications.