Isolation of Mass Transport and Current Distribution in Vanadium Flow Batteries Via Segmented Strip Cell
Authors: Ertugrul Tugrul Yavuz , Clement Jason T. , Ashraf Gandomi Yasser , Aaron Douglas , Mench Matthew M. 
Published: February 2020

Vanadium redox flow batteries (VRFBs) have drawn considerable attention due to advantageous characteristics including flexible design, high coulombic efficiency, and long cycle life. But a number of challenges must be overcome for commercial implementation [1]. At the cell level, mass transport limitations can contribute to performance losses, limiting VRFB performance and depth of discharge, especially for cells operating at high power densities [2–4]. Mass transport losses are associated with inadequate delivery of active species to electrode surfaces and are controlled by multiple physicochemical phenomena. Therefore, understanding mass transport mechanisms in the porous electrode is crucial to understanding pathways to achieve optimal performance and a higher depth of discharge. In this study, bulk motion and concentration-driven mass transport mechanisms are qualitatively investigated utilizing a simplified test bed with a segmented strip cell architecture, having only one straight channel and a total of 1 cm2 active area as it is shown in Figure. The simple strip cell design has no gap between cell components [5], allowing the control and isolation of mass transport mechanisms by suppressing higher-dimensional behaviors, and measurement of localized current along the channel path. Thus, the impact of individual transport mechanisms on current distribution can be evaluated with precision. The diffusion-dominated and convection-dominated mass transport regimes are achieved by employing various channel depths (0.25 mm, 0.5 mm, 1 mm, 2.5 mm). Fully segmented, printed circuit board (PCB)-based, localized current distribution measurement is implemented on all strip cell configurations to obtain current distribution along the channel. Major parameters affecting current distribution considered in this work include constant voltage operation, channel depth, and flow rate. It is shown that charging is not a mass transport-limited process for VRFBs. However, once the mass transport-limited region is reached during discharge, non-uniform current distributions are observed. The highest current deviation down the channel is obtained for the deepest channel (2.5 mm) cell configuration. This type of current distribution behavior is attributed to concentration gradient-driven mass transport dominating in the electrode. As channel depth decreases, current distribution is flattened from inlet to outlet as a result of improved bulk motion of the active species in the electrode. It is also observed that the flow rate contribution for improving bulk motion of the active species in the electrode is more distinct in convection-dominated cells (0.25 mm, 0.5 mm) than diffusion-dominated cells (2.5 mm, 1 mm). This work reveals that it is possible to achieve enhanced electrochemical performance with reduced pressure drop in convection-dominated cells.

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Figure 1