Multifunctional role of bismuth: From catalytic etching of carbon felt to catalyzing VO2+/VO2+ and V3+/V2+ redox reactions for high-performance all-vanadium redox flow battery

To enhance performance of all-vanadium redox flow batteries (VRFBs) at higher current densities, this study aimed at synthesizing multifunctional electrode material to address poor electrochemical kinetics of VO²⁺/VO₂⁺ and V³⁺/V²⁺ redox reactions. In this context, carbon felt was solvothermally decorated with bismuth nanoparticles followed by thermal treatment in the air atmosphere. This unveiled role of bismuth in the catalytic etching of carbon fibers where the most favorable results were obtained at 400 °C and 1.5 h (Bi400-CF). Cyclic voltammetry and electrochemical impedance spectroscopy elucidated better redox kinetics of Bi400-CF for both positive and negative vanadium redox couples. In VRFB single cell performance, the Bi400-CF VRFB outperformed pristine CF by achieving 249, 34.7, and 34.9 % higher discharge capacity, energy efficiency, and voltage efficiency, respectively and a remarkable 3.5-fold increase in electrolyte utilization, at a current density of 150 mA cm⁻². Moreover, Bi400-CF exhibited commendable capacity retention of 86 % over 100 cycles. Therefore, the catalytic and surface etching effects of Bi₂O₃ on carbon felt demonstrate notable improvements in battery performance, suggesting strong potential for future practical implementation in VRFB systems.