Phenoxazine Radical as a Positive Material for Neutral pH Aqueous Flow Batteries

Understanding electron transfer reactions in phenoxazine aqueous-soluble electroactive materials is crucial for developing flow battery (FB) electrolytes, especially for the positive side. Here, we prepared a water-soluble phenoxazine methyl celestine blue compound (<b>mCB</b>) to demonstrate its relatively high redox potential and study its reversible redox chemistry in aqueous KCl solutions. This flow battery (FB) electrolyte exhibited full capacity retention when tested in a symmetrical cell operated at 86% capacity during 55 charge-discharge cycles. The stability of the radical species formed during the one-electron reduction process of <b>mCB</b> (to obtain the positive electrolyte of the FB cell) was also characterized by cyclic voltammetry and electron paramagnetic resonance (EPR) spectroscopy. This electrolyte was also tested against a viologen-based <i>negolyte</i>, and the detected capacity loss (after 55 cycles) was related to a degradation mechanism of the <b>mCB</b> compound undergoing proton oxidation reactions. The experimental results suggest that a more exhaustive characterization by cyclic voltammetry be considered when analyzing FB electrolytes, in order to also take into account the possible effect of inner-sphere electron transfer reactions on the reaction mechanism and its electrochemical parameters.