Elucidating the role of electrolyte velocity homogeneity for scaling up flow fields in vanadium redox flow batteries

The distribution of the electrolyte over the porous electrode is one of the main critical issues that limit the power density of vanadium redox flow batteries. The flow field serves to distribute the electrolyte across the electrode active area, aiming at reducing mass transport overpotentials and pressure drops. This becomes more critical when increasing cell active area. In this work, the performance of a 25 cm2 interdigitated geometry and two 100 cm2 interdigitated flow fields characterized by different channels length were firstly compared with polarization curves adopting positive electrolyte at 50 % state of charge. The 100 cm2 configuration with longer channels exhibited the best performance, with a current density value of 439 mA cm−2 at −0.5 V. Using a 3D computational fluid dynamics model, the performance difference among the three configurations was identified in the mean under-the-rib fluxes and their homogeneity across the electrode area: the Long channels flow field exhibited a heterogeneity index of 19.0 %. Moreover, the analysis of the local pressure drops inside the cell provided further insights into the understanding of the distribution of under-the-rib fluxes. This permitted to design a 100 cm2 flow field characterized by increased under-the-rib fluxes, evenly distributed throughout the electrode area, characterized by a heterogeneity index of 3.2 %. The developed distributor outperformed the previous ones, reaching 654 mA cm−2 at −0.5 V with an electrolyte flow rate of 1.6 ml min−1 cm−2. In addition, the new flow field presented a limited pressure drop increase and it demonstrated superior performances even in full-cell configuration.