Ionic Strength Impacts Charge Capacity in a Redox-Matched Flow Battery: From Single-Particle Interrogation to Battery Cycling

The redox-matched flow battery (RMFB), which reversibly exchanges charge between a flowable redox mediator and stationary redox-active polymeric beads, has emerged as a viable technology for energy storage. However, RMFBs suffer from an underutilized charge capacity. In this work, we show that lower ionic strength solutions lead to significant increases in the charge capacity of ferrocene-functionalized beads in RMFBs. Single-particle experiments using scanning electrochemical cell microscopy (SECCM) showed that voltammetric peaks associated with the ferrocene redox dramatically increased in intensity (∼7-fold) as the ionic strength was decreased from 1000 to 10 mM of tetrabutylammonium hexafluorophosphate (TBAPF6) in propylene carbonate. This change was accompanied by an increase in the particle size. Furthermore, higher performance (∼92% theoretical capacity utilization) was observed in RMFB cycling at 10 mM TBAPF6 compared to 57% at 1000 mM TBAPF6. Our results highlight the critical role of supporting electrolyte concentration in polymer-bead-based redox-matched flow batteries.