Redox Targeting-based Neutral Aqueous Flow Battery with High Energy Density and Low Cost

Neutral aqueous flow batteries with common traits of the redox flow batteries, such as the independence of energy and power, scalability and operational flexibility, and additional merits of outstanding safety and low corrosivity show great promise for storing massive electrical energy from solar and wind energy. Particularly, the ferricyanide/ferrocyanide ([Fe(CN)₆ ]^3-/4- ) couple has been intensively employed as redox mediator to store energy in the catholyte ascribed to its abundance, low corrosivity, remarkable redox reversibility and stability. However, the low energy density arising from poor solubility of [Fe(CN)₆ ]^3-/4- restricts their commercial applications for energy storage systems. In this study, the practical energy density of a [Fe(CN)₆ ]^3-/4- -based catholyte is significantly boosted from 10.5 to 92.8 Wh L^-1 by combining the counter-ion effect and the single-molecule redox-targeting (SMRT) reactions between [Fe(CN)₆ ]^3-/4- and Prussian blue (Fe₄ [Fe(CN)₆ ]₃ , PB)/Prussian white (PW). Paired with concentrated K₂ S anolyte, we demonstrate a neutral aqueous SMRT-based PB-Fe/S flow battery with ultra-long lifespan over 7000 cycles (4500 h) and ultra-low chemical cost of electrolytes in the cell as 19.26 $ kWh^-1 . Remarkably, under the influences of SMRT reactions in the presence of PB granules in the catholyte, the capacity after 7000 cycles of the PB-Fe/S flow battery is 181.8 % of the initial capacity without PB.