Enhancing the Stability of Aqueous Membrane-Free Flow Batteries: Insights into Interphase Processes

Membrane-free flow batteries using immiscible electrolytes aim to overcome limitations of conventional redox flow batteries by eliminating expensive ion-selective membranes. However, they face challenges including low power density due to the transport constraints in immiscible electrolytes, the need for high partitioned stable compatible active species, and the overlooked self-discharge interphase phenomena that reduce coulombic efficiency. We present a novel aqueous biphasic system based on two salts improving electrolyte ionic conductivity and viscosity. Potassium ferrocyanide (K₄[Fe(CN)₆]) and a sulfonated viologen ((SPr₂)V) species were examined computationally and experimentally, demonstrating effective redox pair separation in all oxidation states, achieving a tenfold higher concentration in their electrolyte. The mutual compatibility and stability of these species enabled unprecedented scanning electrochemical microscopy (SECM) analysis of the liquid-liquid interphase, revealing insights like species concentration gradients and crossover. The enhanced electrolyte properties expanded the open-circuit voltage to 1.1 V and improved mass transport, enabling power densities that are 3.5 times higher than previous examples. The battery achieved 80.2% energy efficiency at a C/2 rate, and under flowing conditions, it maintained stable performance over a month (400 cycles) at high states of charge. This work presents an innovative aqueous membrane-free flow battery that avoids parasitic reactions, enabling detailed interphase studies and advancing this technology.