• Amorphous Bi nanoparticles anchored on N-doped graphite felts significantly inhibit the hydrogen evolution reaction and enhance the electrochemical activity of Cr3+/Cr2+ and Fe2+/Fe3+.
• The optimal 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.
• The modified electrode achieves a capacity of 862.7 mAh L−1 after 100 cycles, about 5.3 times that of bare graphite felt.
• The combined effect of Bi and N dopants reduces charge transfer resistance and improves mass transfer, leading to superior battery performance.