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Disproportionation redox flow batteries (DRFBs) employ active species whose oxidation states in both electrolytes are identical in the fully discharged condition, as seen in Figure 1. This feature allows for the use of simple porous materials for separators instead of more expensive, less robust ion-exchange membranes. DRFBs with porous separators tolerate crossover, because recombination of reduced and oxidized active species through the separator only affects round-trip efficiency, and does not permanently degrade the battery’s capacity. This advantage presents challenges when attempting to characterize DRFB performance, however, because crossover is always present, and substantial voltage relaxation occurs when performing open-circuit holds. A new characterization method is used to estimate the relationship between system state-of-charge and open-circuit potential (Nernst-equation parameterization, Figure 2) simultaneously with the instantaneous rate of crossover inside the reactor. This method, based on a simplified reactor model for DRFBs, consists of an adaptive observer which uses transient current and voltage data from the battery to quantify these interrelated phenomena. The online characterization method was used to perform a parameter study probing the effects of liquid flowrates, active-species concentration, separator thickness/porosity, and solvent choice on the performance of porous separators in DRFBs by evaluating crossover rates, Nernst curves, and efficiencies. The DRFB used in this study consisted of vanadium acetylacetonate in various organic solvents, with tetraethylammonium tetrafluoroborate as a supporting salt. Various polypropylene separators (originally designed for lithium ion batteries) were used in the DRFBs. This adaptive observer method is shown to be straightforward to implement and useful for characterizing the performance of different porous separators in DRFBs. Additionally, the overall parameter study allows for a greater understanding of the tradeoffs involved in operating DRFBs, independent of separator. Figure 1 |
