A redox flow battery system comprising: a redox flow battery including a redox flow cell, and a supply/storage system external of the redox flow cell, the supply/storage system including first and second electrolytes (22, 26) for circulation through the redox flow cell, the first and second electrolytes (22, 26) being aqueous liquid electrolytes having electrochemically active species (24, 28) with multiple, reversible oxidation states; a gas vent passage (52; 152; 252; 352a) connected with the redox flow battery to receive water byproduct that evolves from side reaction of the first electrolyte; a bypass passage (54; 154; 354a) connected with the supply/storage system to receive the second electrolyte; and an electrochemical recovery cell including a first half-cell connected to the gas vent passage (52; 152; 252; 352a) to receive as a reactant the water byproduct and a second half-cell connected to the bypass passage (54; 154; 354a) to receive as a reactant the second electrolyte; characterised in that the first half-cell includes a first electrode that has a metal catalyst or a phthalocyanine catalyst and the second half-cell includes a second electrode that is carbon paper and excludes any metal catalyst; and in that the metal catalyst is selected from the group consisting of platinum group metals, nickel, iron, cobalt, and combinations thereof, or includes rhodium sulfide, RhxSy or the phthalocyanine catalyst is selected from the group consisting of silver phthalocyanine, iron phthalocyanine, and combinations thereof. a redox flow battery including a redox flow cell, and a supply/storage system external of the redox flow cell, the supply/storage system including first and second electrolytes (22, 26) for circulation through the redox flow cell, the first and second electrolytes (22, 26) being aqueous liquid electrolytes having electrochemically active species (24, 28) with multiple, reversible oxidation states; a redox flow cell, and a supply/storage system external of the redox flow cell, the supply/storage system including first and second electrolytes (22, 26) for circulation through the redox flow cell, the first and second electrolytes (22, 26) being aqueous liquid electrolytes having electrochemically active species (24, 28) with multiple, reversible oxidation states; a gas vent passage (52; 152; 252; 352a) connected with the redox flow battery to receive water byproduct that evolves from side reaction of the first electrolyte; a bypass passage (54; 154; 354a) connected with the supply/storage system to receive the second electrolyte; and an electrochemical recovery cell including a first half-cell connected to the gas vent passage (52; 152; 252; 352a) to receive as a reactant the water byproduct and a second half-cell connected to the bypass passage (54; 154; 354a) to receive as a reactant the second electrolyte; characterised in that the first half-cell includes a first electrode that has a metal catalyst or a phthalocyanine catalyst and the second half-cell includes a second electrode that is carbon paper and excludes any metal catalyst; and in that the metal catalyst is selected from the group consisting of platinum group metals, nickel, iron, cobalt, and combinations thereof, or includes rhodium sulfide, RhxSy or the phthalocyanine catalyst is selected from the group consisting of silver phthalocyanine, iron phthalocyanine, and combinations thereof.