The pH of a positive electrode electrolyte and a negative electrode electrolyte that are used in a redox flow battery is within the range 2-8. An anion-exchange membrane is used for the membrane between the positive electrode electrolyte and the negative electrode electrolyte. The anion-exchange membrane is obtained by reacting a tertiary amine and a graft polymer comprising a halogenated methylstyrene graft-polymerized on to a non-porous polypropylene base material. The graft polymer and tertiary amine reaction introduces an anion exchange group to a graft chain in the graft polymer.
