Passive Mitigation of Capacity Decay in Vanadium Redox Flow Batteries through Asymmetric Cell Configuration
Authors: Ashraf Gandomi Yasser , Houser Jacob , Aaron Douglas , Mench Matthew M. 
Published: February 2020

The performance of all-vanadium redox flow batteries (VRFBs) can be significantly improved through improved cell design and material selection. 1 However, there are certain issues yet to be addressed including demonstration of long-term, robust, and stable performance at higher power and efficiency. During the cycling of VRFBs, the capacity of the battery decreases monotonically, and the causes for this loss has yet to be fully studied and understood. The primary contributions to capacity loss are undesired vanadium and water transport through the ion-exchange membrane, degradation of cell components (predominantly electrodes and membrane), and gas-generating side reactions. Appropriate material selection and operating voltage range can greatly reduce degradation and side reactions. However, capacity decay due to vanadium and water transport across the membrane is inevitable. 2-5

Use of thicker membranes and electrolyte rebalancing are partial solutions, but each introduces additional performance losses on the system. Development of new ion exchange membranes with reduced crossover is an active area or exploration with a goal of reduced crossover while maintaining conductivity. However, a little-explored, passive method for decreasing vanadium and water crossover is design of different features that do not necessarily result in any performance loss while reducing the unwanted crossover significantly. In a recent publication from our lab, we studied the effect of the electric field on vanadium crossover and deduced interaction coefficients for quantifying vanadium crossover as a function of state of charge (SoC). As a result, the transport parameters for the solute (vanadium ions) and solvent (water) with and without the effect of electric field and as a function of SoC are now known. 6 In this talk we will report experimental data resulting from investigations into asymmetric cell features designed to passively mitigate the rate of crossover during the cell operation. A unique facility has been designed and built using multiple electrochemical and flow cells to enable real-time quantification of crossover rate. The set-up utilizes UV/Vis spectroscopy in order to assess the vanadium crossover under different operating conditions. The results of this study should provide more in-depth insight to optimize VRFBs with enhanced performance and reduced ion-crossover and pressure drop.

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Figure 1