A flow battery includes a cell that has a first electrode, a second electrode spaced apart from the first electrode and an electrolyte separator layer arranged between the first electrode and the second electrode. A supply/storage system is external of the at least one cell and includes first and second vessels that are fluidly connected with the at least one cell. First and second fluid electrolytes are located in the supply/storage system. The electrolyte separator layer includes a hydrated ion-exchange membrane of a polymer that has a carbon backbone chain and side chains extending from the carbon backbone chain. The side chains include hydrophilic chemical groups with water molecules attached by secondary bonding to form clusters of water domains. The clusters have an average maximum cluster size no greater than 4 nanometers, with an average number of water molecules per hydrophilic chemical group, λ (lambda), being greater than zero.