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the number of separations and other process steps needed to convert the precursor to electrochemical grade electrolyte (since it is ultimately employed as a mixed electrolyte) [143,204]. As inorganic species, neither Fe nor Cr decompose, and crossover is remediable via the spectator strategy electrolyte configuration where the two electrolyte tanks contain both active species in equal concentrations, making it “pseudo-symmetric.” This general operating approach is only employable if both active species are stable in the (electro)chemical environment of the opposing half-cell but, if such conditions are met and in the absence of other forms of electrolyte degradation, it enables utilization of the same or substantially similar methods pioneered for remediating crossover losses in vanadium RFB (VRFB) systems. Not only does the spectator strategy facilitate crossover remediation, but it actually lowers crossover rates: by having all active species (Fe2+/3+ and Cr3+/2+) present in nearly-equal concentrations on either side of the membrane, the diffusional driving force for crossover is diminished, which, in turn, significantly reduces the net crossover rate [143]. This approach is particularly important for the Fe-Cr system, as the Fen+ and Crn+ are ~20× more permeable than vanadium cations in NafionTM membranes, the current state-of-the-art ion-exchange membranes for RFBs [144]. Further, the ability to utilize rebalancing can enable economically viable replacement of these more expensive membranes (e.g., NafionTM) with lower-cost but less-selective options (e.g., size-exclusion membranes) [25,60]. Although the chemical configuration of the spectator strategy essentially doubles the amount of active materials required and sacrifices energy density (as the solubility of true active materials is reduced due to presence of spectators) and thus increases the electrolyte cost, with sufficiently low-cost charge- storage compound this tradeoff may not be prohibitive. For the Fe-Cr system, utilizing the calculations by Rodby et al. (and adjusting the depth-of-discharge to reflect the data in the Fe-Cr RFB literature – 60%, shown in Table V-1 (vide infra)– as opposed to the 80% used in the original work), the total electrolyte cost is only ~ 31 $/kWh [120]. Thus, this RFB chemistry may represent a viable alternative to the VRFB, at least from an electrolyte cost perspective [25,120]. Further, the system shows promise from a practical standpoint; it has been successfully demonstrated with the spectator strategy in the past [143], though several technical hurdles remain that challenge the economic viability of long-term operation. Generally considered to be the first modern RFB, the Fe-Cr system was first advanced by the National Aeronautics and Space Administration (NASA) in the 1970’s and 1980’s as a potential energy storage solution for deep-space missions [146]. The system uses the following two half 87PDF Image | Bringing Redox Flow Batteries to the Grid
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