Cyclable membraneless redox flow batteries based on immiscible liquid electrolytes: Demonstration with all-iron redox chemistry

Membraneless redox flow batteries reported to date are microscale designs that have shown poor capacity retention and cyclability due to reactant crossover. Here, we present a new design of macroscale membraneless redox flow battery capable of recharging and recirculation of the same electrolyte streams for multiple cycles and maintains the advantages of the decoupled power and energy densities. The battery is based on immiscible aqueous anolyte and organic catholyte liquids, which exhibits high capacity retention and columbic efficiency during cycling. The aqueous anolyte consists of iron(II) sulfate (FeSO₄) as active species, which is immiscible with the catholyte, having iron(III) acetylacetonate (Fe(acac)₃) as active species dissolved in water-immiscible ethyl acetate, supported by ionic liquid (1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide). Over 60% of initial discharge capacity was retained and over 80% of columbic efficiency was sustained after 25 cycles in a test flow cell, which is among the highest reported thus far.