A Triphasic Membrane-Free Redox Flow Battery in a Total Aqueous System

The membrane-free redox flow battery (RFB) represents an innovative design philosophy that encompasses reduced costs, flexible design schemes, and enhanced overall performance. However, despite these advantages, membrane-free RFBs encounter several challenges including low Coulombic efficiency (CE), limited cycling stability, and elevated toxicity from organic solvents. Aiming at these issues, we constructed a triphasic membrane-free RFB using a total aqueous system. The electrolytes for both the anolyte and catholyte are extracted from the tetrabutylammonium chloride-Na₂SO₄–H₂O salting-out system, while the other phase serves as the separator. A novel organic anolyte material, N,N′-di(ethyl butyrate)-4,4′-bipyridinium dichloride, significantly boosts battery performance when paired with a 2,2,6,6-tetramethylpiperidine-N-oxyl-4-sulfate potassium catholyte. Our static battery delivers an open-circuit voltage of 1.24 V, demonstrating a stable energy efficiency with a capacity loss of 0.064% per hour and an average CE of 98.6% over 493 h (345 cycles). Furthermore, we have conducted preliminary construction of a flow battery that exhibited stable energy efficiency, with a capacity decay of 0.035% per cycle and an average CE of 98.7%. These results provide promising evidence supporting the feasibility of this triphasic all-aqueous membrane-free RFB. We also identify primary intermediate species following extended cycling and elucidate two potential degradation pathways for the anolyte material. We anticipate that this novel anolyte material, combined with our innovative design scheme and comprehensive mechanistic analysis, will expand the research scope of RFBs.