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 conventionalredox flow batteries by eliminating expensive ion-selective membranes. However, they face challenges includinglow power density due to the transport constraints in immiscible electrolytes, the need for high partitioned stablecompatible 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 (K 4 [Fe(CN) 6 ]) and a sulfonated viologen ((SPr 2 )V) species were examined computationallyand experimentally, demonstrating effective redox pair separation in all oxidation states, achieving a tenfold higherconcentration in their electrolyte. The mutual compatibility and stability of these species enabled unprecedented scanningelectrochemical microscopy (SECM) analysis of the liquid-liquid interphase, revealing insights like species concentrationgradients and crossover. The enhanced electrolyte properties expanded the open-circuit voltage to 1.1 V and improvedmass 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) athigh 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.