Para-Substituted Triphenylamine as a Catholyte for Zinc–Organic Aqueous Redox Flow Batteries

Aqueous organic redox flow batteries (RFBs) have emerged as one the most promising technologies for grid-scale energy storage because of their sustainability, safety, and high power densities. Herein, by the molecular engineering of aqueous irreversible triphenylamine (TPA) to form 4,4′,4″-trihydroxytriphenylamine (P3), a reversible two-electron transfer system based on an enol/keto redox couple was obtained, with a high kinetic rate constant of 1.1 × 10–2 cm2 s–1. P3 was used as the catholyte and paired with a zinc anolyte in an aqueous RFB, which yields a high working voltage of ∼1.4 V with good cycling behavior. A capacity of 22.5 Ah L–1 can be obtained using a mixed solvent (DMF/H2O = 1:5, volume ratio) and the battery shows a minor self-discharge rate at full state of charge. This work demonstrates that irreversible or instable redox-active organic molecules can be converted into reversibly reactive analogues by molecular engineering, thereby broadening the range of redox-active materials for organic aqueous RFBs.