NOVELTY - A flow battery comprises a first aqueous electrolyte comprising a first redox active material; a second aqueous electrolyte comprising a second redox active material; a first electrode in contact with the first aqueous electrolyte; a second electrode in contact with the second aqueous electrolyte; and a separator disposed between the first aqueous electrolyte and the second aqueous electrolyte; where the first and/or second redox active materials comprises at least 0.75M a metal-ligand coordination compound that exhibits reversible electrochemical kinetics.    USE - As flow battery used in electronic system (claimed) such as solar- and wind-based systems.    ADVANTAGE - The flow battery is capable of operating with a current density of at least 100 mA/cm2 and a round trip voltage efficiency of at least 70%; and has significant improved performance and cost parameters than previous known in the art. The cell exhibits a round trip voltage efficiency of at least 70%, when measured at 200 mA/cm2, and retains at least 70% round trip voltage efficiency when subjected to 10 charge/discharge cycles.    DETAILED DESCRIPTION - A flow battery comprises a first aqueous electrolyte comprising a first redox active material; a second aqueous electrolyte comprising a second redox active material; a first electrode in contact with the first aqueous electrolyte; a second electrode in contact with the second aqueous electrolyte; and a separator disposed between the first aqueous electrolyte and the second aqueous electrolyte; where the first and/or second redox active materials comprises at least 0.75M a metal-ligand coordination compound that exhibits reversible electrochemical kinetics; the separator has a thickness of less than or equal to 100 micron; and the energy density of the electrolytes is at least 30 Wh/L. INDEPENDENT CLAIMS are included for the following:    (1) 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 each electrolyte chamber, where the electrolyte circulation loop comprises storage tanks and piping for containing and transporting the electrolytes; control hardware and software; and an optional power conditioning unit;    (2) operating the flow battery involving charging the battery by the input of electrical energy or discharging the battery by the removal of electrical energy;    (3) charging a flow battery or system with an associated flow of electrons, involving applying a potential difference across the first and second electrode to reduce the first redox active metal-ligand coordination compound and/or oxidize the second redox active metal-ligand coordination compound; and    (4) discharging the flow battery or system with an associated flow of electrons, involving applying a potential difference across the first and second electrode to oxidize the first redox active metal-ligand coordination compound and/or reduce the second redox active metal-ligand coordination compound.