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

Abstract

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 reduces 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 sulfonated viologen ((SPr2)V) species were examined computationally and experimentally demonstrating effec­tive species 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 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 being 3.5 times higher than previous examples. The battery achieved 80.2% energy efficiency at 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.