Critical aspects of membrane-free aqueous battery based on two immiscible neutral electrolytes

Redox Flow Batteries (RFB) stand out as a promising energy storage technology to mitigate the irregular energy generation from renewable sources. However, some hurdles limit their massive implementation including high cost of vanadium and the poor-performance of ion-selective membranes. Recently, we presented a revolutionary Membrane-Free Battery based on organic aqueous/nonaqueous immiscible electrolytes that eludes both separators and vanadium compounds. Here, we demonstrate the feasible application of this archetype in Aqueous Biphasic Systems (ABS) acting as an unprecedented Total Aqueous Membrane-Free Battery. After evaluating several organic molecules, methylviologen (MV) and 2,2,6,6-Tetramethyl-1-piperidinyloxy (TEMPO) were selected as active species due to their optimum electrochemical behavior and selective partitioning between the phases. When connected electrically, this redox-active ABS becomes a Membrane-Free Battery with an open circuit voltage (OCV) of 1.23 V, high peak power density (23 mWcm 2 ) and excellent long-cycling performance (99.99% capacity retention over 550 cycles). Moreover, essential aspects of this technology such as the crossover, controlled here by partition coefficients, and the inherent self-discharge phenomena were addressed for the first time. These results point out the potential of this pioneering Total Aqueous Membrane-Free Battery as a new energy storage technology.