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We evaluate the electrochemical performance of the electrochemically purified and untreated electrolyte in a single-cell Fe-Cr RFB via galvanostatic cycling at 50 mA cm–2 at 50 °C. An electrolyte composition of 1.0 M FeCl2 and 1.0 M CrCl3 in 3.0 M HCl was selected to align with the composition used in the CV studies. Figure V-3a shows cycles 1, 10, and 30 for cell with untreated electrolyte and Figure V-3b shows the coulombic, voltaic, and energy efficiencies as a function of cycle number. In comparison, Figure V-3c shows cycles 1, 10, and 30 for the cell with purified electrolyte and Figure V-3d shows the evolution of the efficiencies over 200 cycles. Both purified and pristine electrolyte exhibit an initial discharge of ca. 0.74 Ah, demonstrating that negligible capacity is lost to charge imbalances induced by the initial electrochemical purification (i.e., a negligible amount of Fe2+ in the positive electrolyte is oxidized against any of the following counter reactions in/at the negative electrolyte/electrode: metal impurity reduction, HER, or Fe cation reduction). For an electrolyte volume of 50 mL, the maximum capacity can be calculated as 1.34 Ah, indicating a 55% electrolyte utilization efficiency. While this is a relatively low accessed capacity, it aligns with prior reports (see Table V-1); as such, understanding and expanding the limits of the Fe-Cr accessed capacity should be the focus of future work. A possible cause is the use of a relatively low upper voltage limit (e.g., 1.2 V) in order to minimize HER. This low initial utilization could also be due in part to only some of the Cr being electrochemically active (i.e., in the correct Cr-speciation) [204,205,228], which is presumably why cells run at lower temperatures have even lower utilizations (see first row in Table V-1). In both conditions, the coulombic, voltaic, and energy efficiencies are comparable. For the purified electrolyte, an average coulombic efficiency of 96.9%, average voltaic efficiency of 86.5%, and an average energy efficiency of 83.9% is achieved, with stable metrics for 200 cycles. For the unpurified electrolyte, an average coulombic efficiency of 96.9%, an average voltaic efficiency of 85.1%, and an average energy efficiency of 82.3% is achieved, though only for 30 cycles. The most notable difference in the cell cycling data across the two electrolytes is the reduced capacity fade rate for the purified electrolyte compared to the untreated electrolyte, as evinced by the slower decay in the discharge capacity as a function of cycle number (Figure V-3e). We posit that the discrepancy in capacity fade despite nearly identical coulombic, voltaic, and energy efficiencies is due to losses from HER during charging from impurities in the unpurified electrolyte, as these efficiencies are relative measures of losses for individual cycles, and are unable to capture behavior across cycles. To quantify the decay rate, we determine the number of cycles at which 50% of the maximum 97PDF Image | Bringing Redox Flow Batteries to the Grid
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