Understanding the Interplay between Electrolyte Velocity Distribution and Current Distribution in Vanadium Flow Battery Electrode

Abstract

Vanadium redox flow batteries (VRFBs) are considered a promising candidate among grid-scale energy storage technologies due to their flexibility and scalability, high coulombic efficiency, and long cycle life. However, system costs and relatively low energy and power density compared to other electrochemical energy storage devices are major challenges to commercialization. At the cell level, mass transport losses are one of the major contributors to performance losses; this mechanism is associated with inadequate delivery of active species to electrode surfaces. Improving flow field design and electrode structure or simply increasing flow rate can mitigate mass transport losses. However, due to parasitic pumping losses, optimization is not straightforward. As yet unpublished research in our lab has shown that increased convection improves mass transport of the active species through the porous electrode and it is possible to achieve better electrochemical performance with lower pressure drop in convection-dominated VRFB cells. In this study, the impact of convection on electrochemical performance is investigated using computational fluid dynamics (CFD) model and in-situ localized current distribution diagnostics. Polarization curve analysis and pressure drop measurements are also used to support the conclusions. Experiments and simulations are conducted on strip cell architecture with varying channel depths (0.25, 0.50, 1.00, 2.50 mm) and flow rates (10-50 ml min-1). The simulation domain includes both channel and porous electrodes seen in Figure 1. Conservation of mass and momentum are solved to obtain velocity and pressure distributions. It is found that there is a strong relationship between electrolyte velocity in the electrode and electrochemical performance. Similar electrolyte velocity distribution (in the electrode) predicted for two configurations: 1 mm channel depth at 40 mL min-1 flow rate and 0.50 mm channel depth at 10 mL min-1 have similar polarization curves and current distributions down the channel. However, the model predicts very different velocity distributions for these configurations in channel. Higher velocity distribution predicted for 1 mm configuration has also higher pressure drop than 0.5 mm configuration. The results of the computational and experimental study as related to the trade-off between pumping power requirements and electrochemical performance in VRFBs will be discussed in this talk. Figure 1