Effects of active species transport on current density distribution and performance in redox flow battery

Abstract

The vanadium redox flow battery (VRFB) is expected as a potential candidate for the next generation secondary batteries with large capacity because of its characteristics: flexible design of charging and discharging capacities, superior responsiveness and safety, and other advantageous characteristics. This study investigated the effects of active species transport in the electrolyte on the current density distribution and the cell performance of a VRFB experimentally and analytically. A method for measurements of the current density distributions along the electrolyte flow direction was developed using five segmented current collectors in flow through type electrodes. In this method, the segmented current collectors were kept equipotential by adjustable resistances. The experimental results showed that the current density during discharging decreases along the flow, and the difference between at the up and down streams becomes larger with higher current density, lower flow rate, and lower state of charge. The effects of electrolyte flow conditions at the negative electrode are larger than those at the positive electrode. An analytical model for evaluations of the active species transport and its contribution to the various types of overpotentials was also developed based on the experimental results. It was shown that the model can simulate the measured cell overpotentials and current density distributions quantitatively. In the model results, under the low current density conditions in this study, the current density distribution is caused mainly by the concentration overpotential due to decrease in active species concentration in the electrolyte flow, while the concentration overpotential at the electrode surface is kept relatively uniform and has a larger effect on an increase in total cell overpotential. As the current density increases, the distribution of the concentration overpotential at the electrode surface also becomes more uneven, and the largest overpotential at the downstream induces further deterioration of the cell performance.