NOVELTY - A flow battery comprises: a first aqueous electrolyte comprising a first redox active material (30); a second aqueous electrolyte comprising a second redox active material (40); 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) disposed between the electrolytes. Both of the first and second redox active materials remain soluble in both the charged and discharged states.    USE - As flow battery used in a system connected to an electrical grid configured to provide renewables integration, peak load shifting, grid firming, baseload power generation/consumption, energy arbitrage, transmission and distribution asset deferral, weak grid support, and/or frequency regulation; and for providing stable power for remote camps, forward operating bases, off-grid telecommunications, or remote sensors (all claimed).    ADVANTAGE - The flow battery has an open circuit potential of at least 1.4 V and is capable of operating or are operating at a current density of at least 50 mA/cm2. The flow battery is capable of operating with a voltage efficiency of at least 70%. The aqueous flow batteries comprise active materials comprised of metal ligand coordination compounds for energy storage. As such, these flow batteries configurations have the promise of lowering the cost of large-scale energy storage. It allows low-cost energy storage using aqueous, benign electrolytes and redox-active moieties that exhibit reversible electrochemistry and tunable redox potentials; and also provides efficient cells that operate at high voltages. The high open circuit voltages result in increased system performance at lower costs through a number of mechanisms including increased energy density of the flow battery active materials, higher electrochemical cell/stack voltage efficiency, and decreased electrochemical cell/stack size. Increased active material energy density results in a reduced system volume, thus reducing costs associated with, storage tanks, electrolyte pumps, and general system siting. Higher voltage efficiencies result in higher round-trip energy conversion efficiencies, while decreased cell/stack size results in increased modularity at lower costs.    DETAILED DESCRIPTION - A flow battery comprises: a first aqueous electrolyte comprising a first redox active material (30); second aqueous electrolyte comprising second redox active material (40); first electrode (10) in contact with first aqueous electrolyte; second electrode in contact with second aqueous electrolyte; and a separator (20) disposed between the first and second aqueous electrolytes. Both of the first and second redox active materials remain soluble in both the charged and discharged states. At least one of the first and second electrodes is a carbon electrode. At least one of the redox active materials is an organic compound devoid of metal, preferably an aromatic compound, or is a metal ligand coordination compound. The separator comprises an ionomer. INDEPENDENT CLAIMS are included for the following:    (1) method of operating the flow battery, involving charging the battery by the input of electrical energy or discharging the battery by the removal of electrical energy;    (2) charging the flow battery, with an associated flow of electrons, involving applying a potential difference across the negative and positive electrodes, so as to: a) reduce the first redox active material; or b) oxidize the second redox active material; or c) both (a) and (b);    (3) discharging the flow battery with an associated flow of electrons, involving applying an electrical load across the negative and positive electrode, so as to: a) oxidize the first redox active material; or b) reduce the second redox active material; or c) both (a) and (b); and    (4) a system comprising the flow battery, and further comprising: a first chamber containing the first aqueous electrolyte and a second chamber containing the second aqueous electrolyte; at least one electrolyte circulation loop in fluidic communication with each electrolyte chamber, where the electrolyte circulation loop comprises storage tanks and piping for containing and transporting the electrolytes; control hardware and software; and a power conditioning unit.    DESCRIPTION OF DRAWING(S) - The figure shows a view of an exemplary flow battery.    Electrode (10)    Separator (20)    First redox active material (30)    Second redox active material (40)    Tank containing first redox material (50)    Power source or load (70)