Phenothiazylpropylsulfonate: A High-Potential Posolyte for Redox-Flow Batteries: Study of the Instability in the Charged State

The development of aqueous organic redox-flow batteries (AORFB) requires finding new posolyte alternatives to the ferrocyanide salts used presently. Among the potential families, phenothiazine has been reported to be of interest if its solubility in aqueous medium is successfully increased. In this article, the phenothiazine propyl sulfonate (PTZPS) is evaluated as a promising high-potential posolyte for neutral-medium aqueous redox-flow battery. It is demonstrated that electrochemical reversibility is highly dependent on pH, with namely a fast capacity loss in neutral medium. Through the preparation of the oxidized form of this molecule, a kinetic study is performed, confirming the crucial role of the electron transfer step between two molecules at this redox state. Even if density functional theory (DFT) calculations of the electron transfer are not successful due to the significant multireference character of the oxidized form's dyad, using MD simulations, the behavior of the oxidized form in various media is qualitatively predicted, including the effect of addition of chaotropic additives.