Operando quantitatively analyses of polarizations in all-vanadium flow batteries

All-vanadium flow batteries (VFBs) are one of the most promising large-scale energy storage technologies. Conducting an operando quantitative analysis of the polarizations in VFBs under different conditions is essential for developing high power density batteries. Here, we employ an operando decoupling method to quantitatively analyze the polarizations in each electrochemical and chemical reaction of VFBs under different catalytic conditions. Results show that the reduction reaction of V³⁺ presents the largest activation polarization, while the reduction reaction of VO₂⁺ primarily contributes to concentration polarizations due to the formation of the intermediate product V₂O₃³⁺. Additionally, it is found that the widely used electrode catalytic methods, incorporating oxygen functional groups and electrodepositing Bi, not only enhance the reaction kinetics but also exacerbate concentration polarizations simultaneously, especially during the discharge process. Specifically, in the battery with the high oxygen-containing electrodes, the negative side still accounts for the majority of activation loss (75.3%) at 200 mA cm⁻², but it comes down to 36.9% after catalyzing the negative reactions with bismuth. This work provides an effective way to probe the limiting steps in flow batteries under various working conditions and offers insights for effectively enhancing battery performance for future developments.