Split Biphasic Electrochemical Cells: Toward Membrane-Less Redox Flow Batteries

Ion-selective membranes are an essential, yet expensive fixture in redox flow batteries, preventing charge carrier crossover between the two half-cells. This work demonstrates the viability of replacing these membranes with an electrolyte solution that is mutually immiscible with the two half-cell solutions, eliminating the direct anolyte–catholyte interface that leads to self-discharge in existing biphasic cells. We developed a simple dichloromethane (DCM)/water model system consisting of charge carriers in the organic phase connected by an immiscible aqueous electrolyte sharing a common anion (PF6–) with the DCM phase. This “split biphasic” model cell maintained high Coulombic efficiencies (>99%) and capacity retention (∼95%) over a period of 24 h for fully charged cells. Lastly, we demonstrated the performance of this model system at scale in high-surface-area cells while retaining rapid charge–discharge at a rate of 1C.