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Air oxidation of vanadium in the reservoir of a negative electrolyte is considered a major side reaction in a vanadium redox flow battery (VRB) and would lead to electrolyte imbalance and loss of energy storage capacity in a long-term operation. In general, it can be protected by purging an inert gas (e.g., nitrogen) in the negative-electrolyte reservoir; however, this approach results in a complicated VRB system. Alternatively, the appropriate design of an electrolyte storage system could be a potential method to avoid this difficulty. In this study, the negative electrolyte solution of 80% state of charge (SOC) is employed to investigate the mechanism and kinetic of the air oxidation of V(II) reactions in the negative-electrolyte reservoir. The results show that there are two mechanisms in the reservoir consisting of the air oxidation reaction of V(II) and the diffusion of V(II) and V(III). In addition, it is found that one mole of oxygen in the air completely reacts with two moles of V(II) ion. Effects of vanadium concentration in the electrolyte solution and ratio of electrolyte volume to air/electrolyte solution interface are studied. The correlation of the electrolyte volume-to-air/electrolyte solution interface ratio and the specific reaction rate constants are presented. The obtained information will be beneficial to design of the electrolyte reservoir in the VRB system.

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