Electrochemical performance of permanganate as an active material for catholyte in aqueous alkaline flow batteries

Iron (Fe)-based aqueous flow batteries (FBs) have become increasingly popular as large-scale and long-duration energy storage devices due to their advantages in safety and cost. Particularly, Fe 2,2-bis(hydroxymethyl)-2,2′,2′-nitrilotriethanol complex (Fe(BIS-TRIS)) belonging to Fe-atrane complexes is recognized as an appropriate active material for the anolyte in alkaline FBs. In contrast to the active material for anolytes, ferrocyanide ([Fe(CN)₆]⁴⁻) is still widely used for catholytes despite its drawbacks in solubility, stability, and cell voltage. This study therefore explores the feasibility of permanganate ([MnO₄]⁻) as an alternative to [Fe(CN)₆]⁴⁻, and the performance of Fe-Mn FBs using Fe(BIS-TRIS) and [MnO₄]⁻-based electrolytes is evaluated. Electrochemical investigations show that [MnO₄]⁻ exhibits better redox potential and reaction kinetics than [Fe(CN)₆]⁴⁻. Fe-Mn FB was tested using 0.5 M electrolytes, and the voltage efficiency was 92.5% at 40 mA cm⁻². Furthermore, compared to FBs using Fe(BIS-TRIS) and [Fe(CN)₆]⁴⁻-based electrolytes, the cell voltage was improved by 11% and energy efficiency was increased by 14% (61.0% to 69.6%) at 200 mA cm⁻², with a 113% increase in the maximum discharge power density (275.4 to 590.1 mW cm⁻²). This proves that [MnO₄]⁻ is a proper alternative to [Fe(CN)₆]⁴⁻ in alkaline FBs. Eventually, this study also suggests future research goals of Fe-Mn FBs, such as enhancement in membrane performance and optimization in electrolytes.