Impact of Flow Rate and Flow Field Design on Redox Flow Battery Performance

As the demand for electricity increases, a desire exists to improve grid efficiency and stability. Energy storage research can mitigate this need by providing steady power distribution to be used for peak shaving, power quality assurance, and more. Additionally, the intermittent forms of electrical production, solar and wind, can also be improved by providing a steady stream of power. One device that is a strong candidate for grid-level storage is a redox flow battery (RFB). RFBs have independent scaling of power rating and energy capacities, as well as having the active material dissolved in liquids active material. Most RFB literature focuses on identifying new active materials and electrolyte formulations, leaving cell engineering often overlooked. However, for RFBs to be viable, the energy conversion must be quick and efficient, enabled by low area specific resistance (ASR). Few systematic studies have been performed on these enhancements methods. This study investigates the performance variations by quantifying the effects of flow rates and flow field design on the mass transfer contribution to the total ASR. Single electrolyte flow cells, employing an iron (II/III) chloride electrolyte, enables this study. An equimolar solution of iron (II/III) chloride electrolyte is pumped to the cathode side of the flow battery and is reduced. The effluent is then transferred to the anode side and re-oxidized before returning to the electrolyte reservoir. We examine the interdigitated, flow through, parallel, and serpentine flow fields design with various electrolyte flow rates and active species concentrations. Potentiostatic and impedance measurements are taken to generate iR-corrected polarization curves, which are used to determine ASR. We find that ASR decreases as a function of flow rate for all flow field designs according to a nonlinear relationship. Our further studies have investigated modeling the experimental polarization curves. Furthermore, the performance of the interdigitated and flow through designs are more sensitive to changes in flow rates than serpentine or parallel designs. We performed an additional experiment in a 25 cm² single electrolyte cell to demonstrate how these performance trends scaling with increasing active area of the flow cell. With this study, an upper limit flow rate can be determined, as well as the best flow field design to enable the best overall flow cell performance.