NOVELTY - A redox flow battery comprises: a half-cell electrode chamber coupled to an electrode, where the half-cell electrode chamber comprises a first redox-active mediator and a second redox-active mediator, where the first redox-active mediator and the second redox-active mediator are circulated through the half-cell electrode chamber into a container comprising an active charge-transfer material; and a separator coupled to the half-cell electrode chamber.    USE - A redox flow battery.    ADVANTAGE - The redox flow battery utilizes mediator molecules that are designed on the molecular level to possess redox potentials for obtaining high round-trip energy efficiency and high energy density. In mediated flow batteries, mediator molecules, or charge carriers, participate in the electrochemical electrode reactions at the cell or stack while the energy is stored in immobile charge-transfer compounds (also known as charge-storage compounds) located in containers (e.g. reactors, tanks, or vessels) separate from the cell or stack. The charge-transfer compounds do not react directly on the electrodes but react with one or more mediators, which in turn react in the electrode chamber in the cell. Thus, solid materials with high charge-storage capacity per unit mass or volume can be used in the mediated flow battery system without having to flow through the electrode chamber in the cell. Also, the redox flow battery possesses the main advantages of both flow batteries (the ability to scale energy content and power independently) and solid phase charge store, e.g. lithium batteries (high energy density).    DETAILED DESCRIPTION - A redox flow battery comprises: a half-cell electrode chamber coupled to an electrode, where the half-cell electrode chamber comprises a first redox-active mediator and a second redox-active mediator, where the first redox-active mediator and the second redox-active mediator are circulated through the half-cell electrode chamber into a container comprising an active charge-transfer material, where the active charge-transfer material comprises a redox potential between a redox potential of the first redox-active mediator and a redox potential of the second redox-active mediator, and where the active charge-transfer material is a polyoxometalate or derivative; and a separator coupled to the half-cell electrode chamber. INDEPENDENT CLAIMS are included for:    (1) a method for storing, releasing, or storing and releasing electrical energy by mediating electrochemical reactions in at least one of a first or second cycle, which involves: in the first cycle: circulating a first redox-active mediator and a second redox-active mediator through a half-cell electrode chamber into a container comprising an active charge-transfer material; reducing the first redox-active mediator in the container and oxidizing the active charge-transfer material; circulating the reduced first redox-active mediator and the second redox-active mediator through the container comprising the active charge-transfer material to the half-cell electrode chamber; and oxidizing the reduced first redox-active mediator and reducing an electrode surface; and in the second cycle: circulating the first redox-active mediator and the second redox-active mediator through the half-cell electrode chamber into the container comprising the active charge-transfer material; oxidizing the second redox-active mediator in the container and reducing the active charge-transfer material; circulating the first redox-active mediator and the oxidized second redox-active mediator through the container comprising the active charge-transfer material to the half-cell electrode chamber; and reducing the oxidized second redox-active mediator and oxidizing the electrode surface in the electrode chamber, where the active charge-transfer material comprises a redox potential between a redox potential of the first redox-active mediator and a redox potential of the second redox-active mediator, and where the active charge-transfer material is a polyoxometalate or derivative; and    (2) a method of making the redox flow battery, which involves: pre-determining a redox potential of a battery half-cell, the voltage is in a range of 0-5 volts versus lithium (Li)/Li+; based on the predetermined redox potential of the battery half-cell, selecting an active charge-transfer material that comprises a polyoxometalate; selecting a first redox-active mediator and a second redox-active mediator, the redox potential of the active charge-transfer material being between a redox potential of the first redox-active mediator and a redox potential of the second redox-active material; and assembling the battery half-cell comprising: a half-cell electrode chamber coupled to an electrode; a separator membrane coupled to the half-cell electrode chamber; a container coupled to the half-cell electrode chamber, the container comprising the active charge-transfer material; and the half-cell electrode chamber comprising the first redox-active mediator and the second redox-active mediator.