A high redox potential phenothiazine-based catholyte for aqueous organic redox flow batteries

We report the design and synthesis of a highly water-soluble sulfonic acid anion-functionalized phenothiazine (PTZS), which exhibits a solubility of 1.95 M in mixed acids (2.7 M H₂SO₄ and 1.8 M AA) and 1.7 M in water, resulting in a theoretical capacity of 52.26 Ah L⁻¹. Due to the strong electron-withdrawing effect of the sulfur atom in the skeleton, PTZS achieves a high potential of 0.5 V (vs. Ag/AgCl). When paired with an anthraquinone anolyte, the assembled aqueous organic redox flow battery (AORFB) demonstrates a capacity retention of 62 % over 1000 cycles at a current density of 30 mA cm⁻² with a capacity decay of as low as 0.038 % per cycle. We also investigated the mechanism of capacity decay by conducting a series of measurements, including cyclic voltammetry (CV), nuclear magnetic resonance spectroscopy (NMR), and high-resolution mass spectrometry (HR-MS). Results indicate that although PTZS does generate certain sulfoxide byproducts, the capacity decay of the flow battery is primarily caused by the crossover of PTZS.