Efficient Utilization of the Electrodes in a Redox Flow Battery by Modifying Flow Field and Electrode Morphology
We conducted both experiments and numerical simulations for higher utilization of porous carbon electrodes in vanadium redox flow batteries. We first compared an interdigitated flow field with a serpentine flow field. By the interdigitated flow field applied to the cell, reduction in concentration overpotential was observed, resulting in better cell performance, which was attributed to higher utilization of the electrodes by convective transport of reaction species. We then investigated effects of electrode thickness on cell polarization. As a result, large-thickness electrodes showed higher cell performance. This can be explained by an increase in surface area of the electrodes. We also examined electrodes modified by heat treatment under 600°C air environment. The heat treatment intensively improved cell performance reaching at 1.1A/cm2. Scanning electron microscopy observations indicated that the original carbon fibers were exposed by the heat treatment, resulting in open pore structure and enhancement of surface area of the electrode. We then performed two-dimensional numerical simulations with mass, momentum, ionic species and charge transport with electrochemical reaction in VFRBs taken into account. We observed inhomogeneous local reaction current density and reaction species distribution in the electrodes in a flow-through geometry simulating an interdigitated flow field. The numerical simulation indicated a great potential on surface modification of the electrode. Cell performance was considerably enhanced by introducing an electrode with 10times larger surface area. The presented observations give future directions on cell performance improvement with efficient utilization of the electrodes for high current density operation.