Emulsion-based strategy for size-exclusion redox flow batteries using compartmentalized redox-active electrolytes

While promising for grid-scale energy storage, redox flow batteries (RFBs) are hindered by ion-exchange membranes that are costly, impose conductivity-selectivity trade-offs, and undergo long-term degradation. Accordingly, replacing ion-exchange membranes with low-cost alternatives has become a critical challenge. Here, we introduce an emulsion-based size-exclusion RFB (ESRFB), representing the first demonstration of an RFB architecture that uses compartmentalized liquid droplets to suppress redox species crossover through inexpensive porous separators. Hydrophobic redox mediators, ferrocene and menadione, are confined within oil-in-water droplets whose average size (ca. 200 nm) exceeds the separator pore size (ca. 45 nm), enabling size-exclusive retention while the continuous aqueous phase maintains ionic conduction. Chronoamperometry at a carbon ultramicroelectrode verified electrochemical reactions of droplet-confined mediators upon droplet-electrode collision. Flow cell testing demonstrated stable capacity and voltage efficiency over 90 cycles, with 63% energy efficiency. These results establish the feasibility of cost-effective size-exclusion RFBs based on compartmentalized electrolytes.