NOVELTY - A flow battery comprises first electrolyte comprising an aqueous solution comprising a first active material (30) containing at least one mobile ion; second electrolyte comprising an aqueous solution comprising a second active material (40) and at least one mobile ion; a first electrode (10) in contact with the first aqueous electrolyte; a second electrode in contact with the second aqueous electrolyte; and a separator (20); where the flow battery is capable of operating with a current efficiency of at least 85% with a current density of at least 100 mA/cm2.    USE - As flow battery (claimed).    ADVANTAGE - The flow battery provides low-cost energy storage using aqueous, benign electrolytes and redox-active moieties that exhibit reversible electrochemistry and tunable redox potentials. This is achieved by a choice of electrolyte, separator thickness, separator composition, active material, and electrode composition to yield efficient cells that operate at high voltages. The flow battery is capable of operating with a current efficiency of at least 90%, a round trip voltage efficiency of at least 60% with a current density of at least 100 mA/cm2.    DETAILED DESCRIPTION - A flow battery comprises first electrolyte comprising an aqueous solution comprising a first active material (30) containing at least one mobile ion; second electrolyte comprising an aqueous solution comprising a second active material (40) and at least one mobile ion; a first electrode (10) in contact with the first aqueous electrolyte; a second electrode in contact with the second aqueous electrolyte; and a separator (20); where the flow battery is capable of operating with a current efficiency of at least 85% with a current density of at least 100 mA/cm2 and where the separator has a thickness of less than or equal to 100 microns. The separator comprises a polymer, where the polymer is a cationic exchange membrane comprised of anionic functional groups. The membrane further comprises a reinforcement material.    DESCRIPTION OF DRAWING(S) - The figure shows schematic of flow battery.    Electrode (10)    Separator (20)    First active material (30)    Second active material (40)    Tank (50)    Pump (60)    Power source (70)