Boosting the Activity and Stability of 3‐Hydroxyphenothiazine Derivatives for Aqueous Organic Flow Batteries

Abstract Aqueous organic flow batteries (AOFBs) represent one of the most promising technologies for stationary energy storage due to their features of abundant resources and high tunability. Phenothiazines have stable conjugated structures and are considered one of the most potential catholytes for AOFBs. However, the highly conjugated structure of phenothiazines is always hydrophobic and reduces the molecular polarity, which makes it challenging to achieve high capacity and energy efficiency. Herein, a new class of 3‐hydroxyphenothiazine derivatives with a high solubility of 1.8  m and fast redox kinetics by introducing the hydrophilic tertiary ammonium groups. The designed 7‐bromo‐2,4‐dimethylaminemethylene‐3‐hydroxyphenothiazine (BDAHP) based cell not only exhibited an ultra‐stable cycling capacity (over 10 000 cycles with a capacity fade rate of 0.00048% per cycle for the symmetric cell) but also achieved a high energy efficiency of 82.3% (80 mA cm −2 at 0.5  m ). Furthermore, the cell also displayed a highly reversible catholyte capacity of 82 Ah L −1 at a high concentration of 1.7  m and wide temperature adaptability (−15–60 °C). Combining the high volumetric capacity, fast redox reaction, and stability, the hydroxyl‐substituted PTZ demonstrates great potential for large‐scale energy storage.