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Redox flow batteries (RFBs) are potential solutions for grid-scale energystorage, and deeper understanding of the effect of flow rate on RFB performanceis needed to develop efficient, low-cost designs. In this study we highlightthe importance of modeling tanks, which can limit the charge/discharge capacityof redox-active polymer (RAP) based RFBs. The losses due to tank mixingdominate over the polarization-induced capacity losses that arise due toresistive processes in the reactor. A porous electrode model is used toseparate these effects by predicting the time variation of active speciesconcentration in electrodes and tanks. A simple transient model based onspecies conservation laws developed in this study reveals that chargeutilization and polarization are affected by two dimensionless numbersquantifying (1) flow rate relative to stoichiometric flow and (2) size of flowbattery tanks relative to the reactor. The RFB's utilization is shown toincrease monotonically with flow rate, reaching 90% of the theoretical valueonly when flow rate exceeds twenty-fold of the stoichiometric value. We alsoidentify polarization due to irreversibilities inherent to RFB architecture asa result of tank mixing and current distribution internal to the reactor, andthis polarization dominates over that resulting from ohmic resistancesparticularly when cycling RFBs at low flow rates and currents. These findingsare summarized in a map of utilization and polarization that can be used toselect adequate flow rate for a given tank size.

Uncovering the role of flow rate in redox-active polymer flow batteries: simulation of reaction distributions with simultaneous mixing in tanks