Controllable Carbon Felt Etching by Binary Nickel Bismuth Cluster for Vanadium–Manganese Redox Flow Batteries

Various redox couples have been reported to increase the energy density and reduce the price of redox flow batteries (RFBs). Among them, the vanadium electrolyte is mainly used due to its high solubility, but electrode modification is still necessary due to its low reversibility and sluggish kinetics. Also, an incompatible ion exchange membrane with redox-active species leads to self-discharge referred to as crossover. Here, we report a V/Mn RFB using an anion exchange membrane (AEM) for crossover mitigation and etched carbon felt by nickel-bismuth (NB-ECF) for the vanadium anolyte. The NB-ECF significantly enhances the reversibility and kinetics of the V2+/V3+ redox reaction, attributed to inhibited irreversible hydrogen evolution by the Bi catalyst and increased carboxyl groups by nickel (etching and NiO catalyst). Notably, the V/Mn cell employed in the NB-ECF maintains a high energy efficiency of 85.7% during 50 cycles without capacity degradation at a current density of 20 mA cm-2, which is attributed to a synergistic effect of crossover mitigation and facilitated V2+/V3+ redox reaction. This study demonstrates the novel electrocatalyst design of carbon felt using two metal species.