Iron-based catholytes for aqueous redox-flow batteries

Redox-flow batteries (RFBs) are promising electrochemical energy storage devices to load-level intermittent power from renewable energy. In particular, aqueous RFBs using aqueous electrolytes possess several advantages over nonaqueous ones, such as low fabrication cost, nontoxicity, safety, and environmental benignity. Therefore, developing high-performance, abundant, less-expensive iron-based catholytes for aqueous RFBs is essential toward their wide deployment in a power grid. In this Perspective, we summarize the recent progress of iron-based catholytes for aqueous RFBs. We emphasize that iron-based catholytes possess widely ranged redox potentials (−1.0 to 1.5 V vs standard hydrogen electrodes) and solubility in water (0.2–4.0 mol L−1), thereby providing a wide range of cell performance. The molecular design, such as ligand functionalization, counter ion mixing, and asymmetrization, allows for rationally improving solubility, redox potential, and energy density. Furthermore, we demonstrate a simple evaluation method of the redox potential of iron-based catholytes using the calculated energy levels of the lowest unoccupied molecular orbital of ligand molecules. Finally, we rationalize the design strategy of iron-based catholytes for advanced aqueous RFBs.