Electrocatalyst-induced kinetic modulation of anion-based redox mediators in aqueous iron-chromium redox flow batteries

Abstract

Aqueous iron-chromium redox flow batteries (ICRFBs) are promising candidates for large-scale, long-duration energy storage owing to their resource abundance, safety, and operational flexibility. However, their practical deployment is hindered by the sluggish kinetics of the CrIII/CrII redox couple, arising from the reorganization energy associated with the strong Jahn-Teller distortion of high-spin CrII, as well as the competing hydrogen evolution reaction (HER) at the negolyte. Here, we demonstrate dual catalytic effects that simultaneously accelerate the kinetics of the [Cr(CN)6]3-/4- redox reaction and suppress the HER through the use of electrodeposited bismuth (Bi), tin (Sn), and indium (In). Comprehensive electrochemical benchmarking on carbon substrates reveals that Bi deposition increases the standard rate constant by an order of magnitude (k0 = 3.04 × 10-3 cm s-1) and markedly reduces charge transfer resistance for the low-spin CrIII/II couple, while Sn and In effectively mitigate HER. Stability tests show robust cycling over 1,000 cycles, significantly outperforming conventional CrCl3–based systems. In full-cell configurations, Bi-deposited graphite felt electrodes sustain more than 100 cycles with an average energy efficiency of ~75 % and excellent capacity retention under alkaline conditions. Kinetic analyses indicate that Bi deposition optimizes both catalytic and anticatalytic effects, establishing selective kinetic modulation as a practical strategy to enhance the performance and durability of ICRFBs. This work highlights a pathway toward realizing the full potential of hexacyanometallate-based flow batteries.