Investigation of Kinetic and Mass Transport Behaviors of V²⁺ and V³⁺ Ions Using Symmetric Vanadium Redox Flow Battery Configuration

Vanadium redox flow batteries (VRFBs) have attracted attention for their long lifespan, high safety, and flexible design. In the overall cell reaction, the V 2+ /V 3+ redox couple on the negative side is considered performance-limiting due to its high overpotential, underscoring the need to enhance negative electrode performance. Heat treatment, an effective electrode modification method, improves the structural and interfacial properties of carbon electrodes, significantly impacting the reaction kinetics and mass transport of V 2+ and V 3+ . This study employed a symmetric cell configuration with or without heat-treated electrodes to investigate the kinetic and mass transport processes of V 2+ and V 3+ . Electrochemical impedance spectroscopy and polarization curve analysis separated impedance contributions and calculated overpotentials. The results indicated that V 2+ has a higher reaction rate constant, while V 3+ exhibits a larger diffusion coefficient, reflecting distinct kinetic and mass transport properties despite belonging to the same redox couple. Heat treatment enhanced porosity, wettability, and reaction kinetics, particularly improving V 3+ reduction. However, single-side heat treatment caused electrode performance mismatches, limiting the overall enhancement. These findings emphasize the importance of considering the distinct behaviors of V 2+ and V 3+ rather than treating them as a single redox couple, providing valuable insights for improving VRFB performance.