A\nTriphasic Membrane-Free Redox Flow Battery in a\nTotal Aqueous System

The\nmembrane-free redox flow battery (RFB) represents an innovative\ndesign philosophy that encompasses reduced costs, flexible design\nschemes, and enhanced overall performance. However, despite these\nadvantages, membrane-free RFBs encounter several challenges including\nlow Coulombic efficiency (CE), limited cycling stability, and elevated\ntoxicity from organic solvents. Aiming at these issues, we constructed\na triphasic membrane-free RFB using a total aqueous system. The electrolytes\nfor both the anolyte and catholyte are extracted from the tetrabutylammonium\nchloride-Na<sub>2</sub>SO<sub>4</sub>–H<sub>2</sub>O salting-out\nsystem, while the other phase serves as the separator. A novel organic\nanolyte material, <i>N</i>,<i>N</i>′-di(ethyl\nbutyrate)-4,4′-bipyridinium dichloride, significantly boosts\nbattery performance when paired with a 2,2,6,6-tetramethylpiperidine-<i>N</i>-oxyl-4-sulfate potassium catholyte. Our static battery\ndelivers an open-circuit voltage of 1.24 V, demonstrating a stable\nenergy efficiency with a capacity loss of 0.064% per hour and an average\nCE of 98.6% over 493 h (345 cycles). Furthermore, we have conducted\npreliminary construction of a flow battery that exhibited stable energy\nefficiency, with a capacity decay of 0.035% per cycle and an average\nCE of 98.7%. These results provide promising evidence supporting the\nfeasibility of this triphasic all-aqueous membrane-free RFB. We also\nidentify primary intermediate species following extended cycling and\nelucidate two potential degradation pathways for the anolyte material.\nWe anticipate that this novel anolyte material, combined with our\ninnovative design scheme and comprehensive mechanistic analysis, will\nexpand the research scope of RFBs.