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are mitigated when first subjected to the electrochemical purification step, we also hypothesize that the impurity concentrations are relatively low and thus difficult to detect, but non-negligible, as discharge capacity decay rate is dependent on the amount of purified electrolyte volume as a function of electrode volume (Figure V-3e). If these impurities act as HER catalysts, only a small amount is required to have a significant impact as shown in the CVs in Figure V-2. Further studies leveraging more precise in situ or ex situ spectroscopy will prove valuable to ascertain the chemical identities of the deposited impurities. 4. Conclusions Hydrogen-evolution mitigation strategies in Fe-Cr RFB systems have largely focused on materials and reactor design innovation. These techniques can add cost and complexity, particularly approaches that utilizes expensive metals (e.g., Bi, In) as catalysts on the electrode or additives in the electrolyte. Here, we demonstrate an alternative, potentially low-cost approach to mitigate HER: electrochemical electrolyte purification. We demonstrate that this strategy leads to significant reduction in capacity fade at appreciable current density over extended cycling experiments and produces results that are on-par with literature that teach materials-centric strategies. We hypothesize that the purification process reduced and filtered out metal impurities that can catalyze deleterious hydrogen generation on the negative electrode lowering accessible capacity over time. Importantly, this purification process does not appear to induce a significant charge imbalance that would, in itself, reduce the accessible capacity. We show that a clear association between discharge capacity decay rate and coulombic efficiency exists based on durational cycling data obtained from literature, and that our electrochemically purified data falls within that trend, while the cell with unpurified electrolyte demonstrates abnormally high decay rate with moderate coulombic efficiency. The connection between coulombic efficiency and decay rate semi-quantitatively elucidates the importance of attenuating HER attempted through numerous strategies for more resilient Fe-Cr RFBs. Future work should focus on exploring the universality of the strategy across materials sets (i.e., electrolytes of varying purity levels, electrodes of different formats), operating conditions (i.e., temperatures, flow rates), and related electrochemical purification approaches (i.e., potential holds, different potential cutoffs), in tandem with precise methods for impurity detection. Finally, despite 105PDF Image | Bringing Redox Flow Batteries to the Grid
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