A Highly Reversible Redox Mediator Enables Long‐Term Stability Alkaline Zinc‐Iron Flow Batteries with Complete Anolyte Utilization

Abstract Alkaline zinc‐iron flow batteries (AZIFBs) represent a promising candidate for large‐scale, long‐duration energy storage applications. However, the formation and accumulation of inactive zinc (“dead Zn”) during cycling significantly compromise the reaction reversibility and cycling stability of AZIFBs. In this study, 2‐amino‐3‐hydroxyphenazine is introduced as a redox mediator (RM) to spontaneously and chemically dissolve the dead Zn, where the RM exhibits high structural stability and redox kinetics. It eliminates continuous Zn dendrite growth and dead Zn accumulation in the electrolyte, thereby significantly enhancing cycling stability. In addition, the RM is revealed to effectively suppress the parasitic hydrogen evolution reaction (HER) from water electrolysis by stabilizing the concentration of Zn(OH) 4 2− anions, even at a high anolyte utilization ratio. Therefore, the AZIFBs deliver exceptional cycling stability at 100% anolyte utilization ratio for over 2300 h at an areal capacity of 20 mAh cm −2 and a current density of 20 mA cm −2 .