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 approximate to 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.