• Amorphous Bi nanoparticles anchored on N-doped graphite felts significantly enhance the electrochemical activity of Cr3+/Cr2+ and Fe2+/Fe3+ while suppressing the hydrogen evolution reaction.
• The optimized Bi/N ratio of 2 yields a high coulombic efficiency of 97.7% and an energy efficiency of 85.8% at 60 mA cm−2, outperforming many reported materials.
• The modified electrode achieves a capacity of 862.7 mAh L−1 after 100 cycles, about 5.3 times that of bare graphite felt, demonstrating excellent stability.
• The combined self-polymerization and wet-chemistry reduction strategy provides a scalable route for fabricating high-performance negative electrodes for iron-chromium redox flow batteries.