NOVELTY - A flow battery (20) consists of one or more cells (36), an external supply/storage system (30) of cells, and primary fluid electrolyte (22) and secondary fluid electrolyte (26) in the supply/storage system. The cells consist of a primary electrode (42), a secondary electrode (44), and an electrolyte separator layer. The electrolyte separator layer consists of a hydrated ion-exchange membrane (46) of polymer comprising carbon backbone chain and side chains. The side chains form clusters of water domains. The clusters have average maximum cluster size of less than 4 nm.    USE - Flow battery is used in wind-powered system for converting electrical energy to chemical energy.    ADVANTAGE - The flow battery has excellent conductivity and selectivity due to maximum water domain cluster size.    DETAILED DESCRIPTION - A flow battery consists of one or more cells, an external supply/storage system of cells, and primary fluid electrolyte and secondary fluid electrolyte in the supply/storage system. The cells consist of a primary electrode, a secondary electrode spaced apart from the primary electrode, and an electrolyte separator layer arranged between the primary electrode and the secondary electrode. The supply/storage system has primary vessel (32) and secondary vessel (34) connected fluidly with the cells. The electrolyte separator layer consists of a hydrated ion-exchange membrane of polymer comprising carbon backbone chain and side chains extending from carbon backbone chain. The side chains contain hydrophilic chemical groups having water molecules attached by secondary bonding, and form clusters of water domains. The clusters have average maximum cluster size of less than 4 nm. The average number of water molecules per hydrophilic chemical group is more than 0. The average maximum cluster size and average number of water molecules per hydrophilic chemical group limits the migration of vanadium or iron ions across the hydrated ion-exchange membrane.    DESCRIPTION OF DRAWING(S) - The drawing shows the schematic view of flow battery.    Flow battery (20)    Primary fluid electrolyte (22)    Secondary fluid electrolyte (26)    Supply/storage system (30)    Primary vessel (32)    Secondary vessel (34)    Cell (36)    Primary electrode (42)    Secondary electrode (44)    Hydrated ion exchange membrane (46)