A triphasic membrane-less battery based on salting-out effect employing metal-free redox materials

The utilization of membrane-less redox flow batteries (RFBs) offers a promising avenue to mitigate the dependence on ion exchange membranes (IEMs). However, there is a dearth of membrane-less RFBs developed that simultaneously exhibit high cyclability and efficiency, leveraging naturally abundant elements. Here we firstly demonstrated a triphasic membrane-less battery based on salting-out effect utilizing metal-free redox materials. With 2,2,6,6-tetramethylpiperidoxyl (TEMPO) and methyl viologen (MV2+) in the salting-out propylene carbonate (PC) solution as redox materials in catholyte and anolyte respectively, after being separated by the immiscible salting-out electrolytes, this stable triphasic membrane-less battery can deliver an open circuit voltage (OCV) of 1.11 V and achieve 98.1% theoretical capacity. Furthermore, the prolonged galvanostatic cycling maintained high Coulombic efficiencies (>99.5 %) and negligible capacity decay over about 660 cycles (552 h) under static condition. We also employed a hydrogel to immobilize the above immiscible electrolyte which functions as a separator, successfully demonstrating a flow cell with enhanced mechanical stability and good compatibility using the conventional battery stack. This particular triphasic battery not only mitigate the issues of poor mechanical stability and compatibility in membrane-less batteries, but also provides the conceptual validation of membrane-less RFB for next-generation energy storage devices.