Research and optimization of slit issues in the kW-scale redox flow batteries stack

Abstract

The assembly of the frame and bipolar plates in redox flow batteries (RFBs) often results in assembly gaps, forming ‘slit.’ Due to differing coefficients of thermal expansion between the plate frame and bipolar plates, thermal expansion and contraction occur under the influence of assembly environment temperatures and operational temperatures of RFBs, exacerbating the ‘slit’ issue. The presence of ‘slits’ can reduce the utilization rate of the electrolyte and decrease mass transfer efficiency during the battery cycling process. However, the flow channel design for kilowatt (kW)-scale stacks often overlooks the impact of these slits. Therefore, this paper examines the influence of ‘slit’ structures on the flow characteristics of electrolytes within carbon cloth using kW-scale stacks with interdigitated flow channel structures as an example, and optimizes the interdigitated flow field with slits. The study analyzes the flow characteristics of electrolytes on carbon cloth in flow field structures with no slits, slits at both ends and slits on three sides. It further investigates the effects of slit width and the length of interdigitated flow channels on the flow patterns of electrolytes on carbon cloth when slits are present. Through numerical simulation, velocity and pressure distribution maps at the center of the carbon cloth, as well as velocity distributions at various positions, are obtained. The results show that slits affect the position and extent of the low-speed zone on the carbon cloth. As slit width increases, the resistance of the slit decreases, gradually increasing the flow velocity of the electrolyte towards the carbon cloth. However, when the slit width exceeds 0.5 mm, the impact of the slit on flow over the carbon cloth remains largely unchanged. With a fixed slit width of 0.5 mm, increasing the length of the interdigitated flow channels reduces the flow velocity and pressure within the carbon cloth but significantly improves the uniformity of the carbon cloth. Extending the interdigitated flow channel length by 15 mm demonstrates the best overall effect.