Boosting the Areal Capacity of Titanium-Manganese Single Flow Battery by Fe²⁺ /Fe³⁺ Redox Mediator

Aqueous manganese-based flow batteries (AMFBs) have attracted great attention due to the advantages of low cost and environmental friendliness. Extending the cycle life of AMFBs has long been a challenging theme. The titanium-manganese single-flow batteries (TMSFB) are promising due to their special structure and electrolyte composition. However, TMSFB with high areal capacity faces capacity decay for unknown reasons. In this work, the capacity decay mechanism (accumulation and growth of MnO₂ ) is clarified by a homemade in situ microscope system. Given that, a redox mediator of Fe²⁺ /Fe³⁺ is specially designed to boost the areal capacity of TMSFB without side reaction. The directional promoting principle of the Fe²⁺ /Fe³⁺ is elaborated in detail. Fe²⁺ chemically reacts with the residual MnO₂ to form Fe³⁺ , which is reduced to Fe²⁺ by the electrochemical reaction. And then Fe²⁺ continues reacting with MnO₂ until MnO₂ is consumed completely. As a result, the TMSFB with the areal capacity of ≈55 mA h cm^-2 can stably operate at a current density of 40 mA cm^-2 , which is the highest areal capacity reported in aqueous manganese-based batteries. This work provides a new strategy for boosting the capacity of manganese-based batteries, shedding light on the improvement of other deposition-type batteries.