Distributed Diagnostic Techniques to Quantify Spatial and Temporal Voltage Gradients in Energy Storage Systems Utilizing Flowable Electrodes
Authors: Dennison Christopher R , Beidaghi Majid , Hatzell Kelsey B. , Gogotsi Yury , Kumbur Emin Caglan 
Published: February 2020

Recently, several electrochemical concepts utilizing flowable slurry electrodes have been proposed for various applications including electric vehicles [1-4], grid-scale energy storage [5-8], and water desalination [9, 10]. The flowable electrodes utilized in these systems consist of an electrically conductive solid phase (often carbon) and an ionically conductive liquid phase. As the electrode flows through the cell, the solid particles form a dynamic, mobile network through which electrons can percolate. It has previously been shown that the conductivity of this network strongly depends on the flow conditions present within the cell [6]. Changes in the bulk conductivity of the flowable electrode can have a significant impact on the potential and current distribution within the flow cell, potentially resulting in poor utilization of the active material and low efficiency.
In this study, we have developed a distributed diagnostics approach which enables us to resolve the localized voltage within the cell under various conditions. To accomplish this, special electrode arrays were manufactured which allowed us to precisely locate voltage probes within the cell. These electrode arrays were installed in a working cell, and real-time data was collected during potentiostatic charge/discharge operation under a variety of flow conditions. For this study, we applied this distributed diagnostic technique to an electrochemical flow capacitor (EFC) system, but the technique can be readily adapted to other flowable electrode systems as well.
From the spatially-resolved voltage profiles, we observed a significant voltage drop across the current-collector|electrode interface under static (no-flow) conditions. During flowing operation, the total voltage drop across the current-collector|electrode interface is slightly reduced, while the voltage drop within the bulk of the electrode increases significantly. As a result, the material furthest from the current-collectors is not well utilized, giving rise to reduced system capacity and efficiency. These results indicate a strong coupling between flow conditions and charge percolation within the electrodes, and provides guidance for the optimization of flowable electrodes and the design of flow cells.

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