In situ bismuth electrodeposition accelerates V3+/V2+ kinetics in mixed-acid VRFB electrolytes

H 2 SO 4 -HCl mixed-acid vanadium electrolytes broaden the operating window and allow higher vanadium concentrations, yet the negative V 3+ /V 2+ reaction can remain kinetically sluggish and impose substantial polarization. Here, a small amount of BiCl 3 is introduced directly into the mixed-acid negative electrolyte so that metallic bismuth forms and deposits in situ on graphite felt during cell operation, without electrode pretreatment. UV–vis spectra show that adding BiCl 3 does not measurably change the characteristic V 3+ absorption features, indicating that the bulk vanadium speciation remains unchanged. After charging, plate-like Bi 0 deposits are observed on the carbon fibers, and the V 3+ /V 2+ redox reaction is clearly accelerated. Electrochemical measurements show that 0.02 mol·L −1 Bi 3+ is most effective. The redox peak separation decreases from 535.19 mV to 212.35 mV, and the charge-transfer resistance is substantially reduced. The exchange current density increases from 0.195 mA·cm −2 to 0.324 mA·cm −2 , and the standard rate constant increases from 2.021 × 10 −9 cm·s −1 to 3.358 × 10 −9 cm·s −1 . By comparison, the double-layer capacitance increases only from 0.572 mF·cm −2 to 0.641 mF·cm −2 , suggesting that the gain mainly comes from faster interfacial charge transfer rather than a simple increase in electrochemically accessible area. In single-cell tests from 40 to 160 mA·cm −2 , the Bi-containing electrolyte shows lower polarization and improved voltage and energy efficiencies.