Flow battery that includes redox mediator

A flow battery comprising:

• a liquid including a redox mediator;
• an electrode;
• an active material;
• and
• a circulator that circulates the liquid between the electrode and the active material,
• wherein the redox mediator includes a tetrathiafulvalene derivative, and
• the tetrathiafulvalene derivative is a compound represented by the following general formula (1):
• where R1 to R4 each independently denote at least one selected from the group consisting of a hydrogen atom, a chain saturated hydrocarbon group, a chain unsaturated hydrocarbon group, a cyclic saturated hydrocarbon group, a cyclic unsaturated hydrocarbon group, and a halogen atom,
• the chain saturated hydrocarbon group, the chain unsaturated hydrocarbon group, the cyclic saturated hydrocarbon group, and the cyclic unsaturated hydrocarbon group may each independently include at least one atom selected from the group consisting of boron, nitrogen, oxygen, fluorine, silicon, phosphorus, sulfur, chlorine, bromine, and iodine, and
• R1 and R2 may constitute a ring, and R3 and R4 may constitute a ring.