Study on Mechanism and Kinetic of Air Oxidation of V(II) in Electrolyte Reservoir of a Vanadium Redox Flow Battery

Abstract

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.

Improved Vanadium Flow Battery Performance through a Pulsating Electrolyte Flow Regime

“…Argon gas (1) was employed to purge the electrolytes (2) before the experiments and to remove oxygen from the system. Oxygen spontaneously reacts with V(II), which is a strong reducing agent [Equation (4)], resulting in discharge of the negative electrolyte and loss of Coulombic efficiency [49,50] . The electrolyte reservoirs were connected through a valve (3), which was closed during the measurements but was opened to mix and rebalance the electrolytes.…”

“…Oxygen spontaneously reacts with V(II), which is a strong reducing agent [Equation ( 4)], resulting in discharge of the negative electrolyte and loss of Coulombic efficiency. [49,50] The electrolyte reservoirs were connected through a valve (3), which was closed during the measurements but was opened to mix and rebalance the electrolytes. When necessary, the rebalancing of the electrolyte started by flushing the flow system with argon, then the rebalancing took place under atmospheric pressure while the valve was opened.…”

Redox Flow Batteries: Recent Development in Main Components, Emerging Technologies, Diagnostic Techniques, Large-Scale Applications, and Challenges and Barriers

“…Oxygen gas converts V 2+ into V 3+ , which causes the battery to drain quickly. However, it may be readily avoided with the right design [190]. For the commercialization of RFBs, electrolyte cost minimization is critically essential.…”