Investigation of Kinetic and Mass Transport Behaviors of VO²⁺ and VO₂ ⁺ Ions by Using Symmetric Vanadium Redox Flow Battery Configuration

Improving electrode performance through heat treatment is an effective way to lower overpotentials of vanadium redox flow batteries (VRFBs). However, the effect of heat treatment on VO 2+ /VO 2 + still needs to be further clarified. This study systematically investigated how electrode heat treatment affects the kinetic and mass transport behaviors of the VO 2+ /VO 2 + couple in a symmetric VRFB. Total overpotential was separated into ohmic, activation, and mass transport contributions using electrochemical impedance spectroscopy, with additional analysis of precipitation-induced polarization. Results showed that heat treatment significantly increased the electrochemically active surface area (ECSA), improving wettability and electrolyte transport, thus markedly reducing activation and mass transport overpotentials despite a moderate decrease in the reaction rate constant. Notably, the electrode without heat treatment resulted in V 2 O 5 precipitation at moderate current densities (100-125 mA/cm 2 ) for the reduction reaction, substantially increasing both activation and mass transport overpotentials due to pore blockage and ECSA loss. These findings clarify the effects of heat treatment in the VO 2+ /VO 2 + reaction, emphasizing the importance of heat treating the VO 2 + reduction electrode to prevent precipitation-induced performance deterioration. This study provides case-specific insights for SGL 39AA under 500°C air treatment, which may inform electrode design under comparable conditions.