Analysis of crossover-induced capacity fade in redox flow batteries with non-selective separators

Redox flow batteries (RFBs) are candidates for grid-scale energy storage. For RFBs 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. Irrespective of whether redox couples use a common charge-balancing counterion (rocking chair configuration) or not (salt splitting configuration) the molar ratio of inert salt to redox species must exceed 50% to cycle with substantial capacity. Using Damköhler numbers (characteristic scales of reaction rates to transport rates) for both inert salt Da_salt and redox-active species Da_redox we classify three RFB operating regimes: redox shuttle limited, ohmic polarized, and sufficient supporting electrolyte. In the sufficient supporting electrolyte regime capacity fade is found to scale inversely with Da_redox, resulting in capacity fade per cycle less than 0.01% for Da_redox larger than 10⁵ and capacity utilization of approximately 80% for Da_saltsmaller than 12.