Experiment-supported survey of inefficient electrolyte mixing and capacity loss in vanadium flow battery tanks

Abstract

This study investigates the impact of electrolyte mixing inside the tanks of Vanadium Flow Battery (VFB) on capacity degradation. Heterogeneous mixing inside the tanks may lead to a severe capacity drop due to the existence of stagnant electrolyte regions and asymmetries in the State of Charge (SoC) of the positive and negative electrolytes reaching the cell. The study is based on a preliminary order-of-magnitude analysis involving the dimensionless Richardson and Reynolds numbers. A subsequent experimental campaign is carried out in which cell voltage and current response are screened for different tank designs and operating conditions to identify different fluid-dynamics events in the tanks. The results highlight the two main competing phenomena affecting the electrolyte fluid dynamics: buoyancy effects induced by density variations, and the inertia of the inlet submerged jet. The interplay between these two forces determines the flow field and therefore the SoC of the electrolyte feed to the cell, which affects the usable capacity and battery performance. Inertia dominated flows induce homogeneous electrolyte mixing, leading to a higher usable capacity, whereas buoyancy dominated flows result in a lower one. Furthermore, various inner structures were tested within the tanks to optimize mixing. These structures were created using additive manufacturing techniques. The implementation of helicoidal geometries within the tanks, promoting longer electrolyte paths and ample cross-sectional areas for convective mixing, markedly enhanced battery capacity compared to traditional empty tanks. These findings call for further investigation into optimizing the geometry of large-scale (VFB) storage tanks.