Imbalance of active ions through segmented electrodes: Mass transfer characteristics of vanadium redox flow batteries

The mass transfer that occurs at the interface between the porous electrode and electrolyte is important because it is closely related to the electrochemical performance of batteries. In general, as the materials used for electrodes have irregularly connected microscale pores, various methods exist to predict the permeation path of the electrolyte between the inlet and outlet. In this study, we investigate the use of segmented cells to experimentally visualize the mass transfer properties that occur during electrochemical reactions in porous electrodes. In this work, the measured voltages are quantitatively compared with the voltage deviation for each electrode position for a segment cell that is divided into 25 active areas, each measuring 25 cm². As a result, the electrochemical reaction for each charge/discharge process caused by the physical and electrochemical properties of the VRFB electrolyte showed a relatively uniform reactivity during the charging process. The strategy of enhancing the supply of active ions by increasing the flow rate does not contribute to reducing the voltage difference inside the active region. When the current density is increased by 100%, the rate of increase of the voltage deviation by the electrode position is 70% and 140% in the charging and discharging process, respectively. Furthermore, in the DoD 100% discharge stage based on the cell voltage, the voltage difference is approximately 6.3 times higher than the average voltage deviation in the SoC 0–100% and DoD 0–90% stages. This paper presents a strategy to ensure stable operation during the system charge/discharge process by measuring the voltage at each position of the porous electrode used as an electrode.