Investigation of Polarization in Vanadium Redox Flow Batteries with a Fluctuating Supply of Electrolyte Solution

Abstract

The rapid development of renewable energy has led to increasing demand for energy storage equipment. The vanadium redox flow battery (VRFB) has attracted more attention due to its advantages such as good safety, long service life, and flexible design [1].In the VRFB, the electrolyte flow characteristics are important for the concentration overpotential as it significantly affects the supply of vanadium ions [2]. Therefore, many studies are focusing on the influence of flow rates and electrolyte distribution on VRFB performance [3-4]. However, few studies have focused on the effects of fluctuations in electrolyte flow rates on battery polarization and overpotential, which often occurs in practical situations and affects the mass transfer of ions. In this study, we focused on the effect of fluctuations in electrolyte supply induced by the peristaltic pump on the polarization behavior of the VRFB.The electrodes of the experimental VRFB are Sigracet® GDL 38AA, the membrane is Nafion® 211, and the vanadium ion concentration of the electrolyte is 1.7 mol/L. Before the formal experiment, the cell was charged to 50% state of charge. To determine the fluctuation effect on battery polarization, a peristaltic pump was used on one electrode side and a smooth pump on the other.Fig. 1 (a) and (b) show the charging and discharging voltage variation with a current of 200 mA/cm2 at different flow rates. The average voltages during charging decrease with increasing flow rates, while the average voltages during discharging increase with increasing flow rates, and all voltages fluctuate up and down.It can be known that the voltage variation is attributed to the fluctuating supply of electrolyte affecting the mass transfer. At higher flow rates, the vanadium ion transport speed is faster, and the concentration gradient of the reaction surface is smaller, resulting in lower overpotential and polarization. Under fluctuating conditions, the changing electrolyte flow rates also lead to changes in vanadium ion supply speed, resulting in the voltage fluctuation periodically.AcknowledgementThis work was supported by JSPS KAKENHI Grant Number 21H04540. The first author was supported by the China Scholarship Council.