Membrane-Free Redox Flow Batteries

While membrane-free batteries have been successfully demonstrated in static batteries, membrane-free batteries in authentic flow modes with high energy capacity and high cyclability are rarely reported. Here, we present a biphasic flow battery with high capacity employing organic compound in organic phase and zinc in aqueous phase. Under ambient flow testing conditions, a capacity retention of 94.5% is obtained over 190 charging/discharging cycles with a Coulombic efficiency of &gt; 99% at a current density of 8.54 mA cm^-2. Self-discharge at full state-of-charge of the membrane-free RFB is negligible (potential drop = 0.78 mV h^-1). DFT calculation and <i>operando</i> UV-visible and FT-IR spectroscopies are performed to probe minor side reactions during cycling and monitor the water content in the organic electrolyte in real time. The successful demonstration of the prototypical membrane-free battery under flow conditions, together with the developed <i>operando</i> spectroscopic techniques, will open a new avenue towards detailed mechanistic studies and practical applications of redox flow batteries for cost-effective energy storage.