Critical to the development of low-cost redox flow batteries (RFBs) is utilization of the intrinsic charge-storage capacity of redox-active molecules over many cycles. Here, we report on our recent efforts to predict RFB utilization as a function of operating conditions, reactor/tank design, and electrolyte materials. Specifically, mechanistic understanding of redox-active species crossover is needed to optimize electrolyte composition (both of inert salt ions and redox-active species) especially when low-cost separators are used instead of ion-selective membranes. We simulate these effects using a multi-component porous electrode model with Nernst-Planck fluxes and Marcus-Hush-Chidsey kinetics to predict capacity utilization and fade. The molar ratio of inert salt to redox species and the ratio of their diffusivities are used to parameterize different electrolytes in RFBs with non-selective separators. Using Damköhler numbers (characteristic scales of reaction rates to transport rates) for both inert salt Da<sub>salt</sub> and redox-active species Da<sub>redox</sub> we classify three RFB operating regimes: redox shuttle limited, ohmic polarized, and sufficient supporting electrolyte. Simulations of RFB cycling as a function of flow rate are also conducted revealing that utilization increases monotonically with increasing flow rate – charage capacity reaches 90% of the theoretical value only when flow rate exceeds twenty-fold of the stoichiometric value. At low flow rates the capacity losses due to tank mixing dominate over the polarization-induced capacity losses that arise due to resistive processes in the reactor. Concomitant with these capacity losses, we identify sources of polarization that are due to irreversibilities arising from electrolyte mixing within tanks and current distribution within the reactor.

Continuum modeling of redox flow batteries to inform cell and material development

2019 Spring National Meeting

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