NOVELTY - A rebalancing system comprises an electrochemical reaction cell (320) having a cathode chamber (226) and an anode chamber (228) separated from cathode chamber by a separator membrane (202) having a catalyst-containing coating (204); a hydrogen gas inlet in communication with anode chamber; a redox flow battery negative electrolyte inlet in communication with cathode chamber to allow a negative electrolyte to be flowed through the cathode chamber; a power supply; and a controller. The negative electrolyte contains trivalent chromium ions (Cr3+) and divalent chromium ions (Cr2+).    USE - Rebalancing system used for adjusting a charge balance between a positive electrolyte and a negative electrolyte in an iron/chromium redox flow battery system (claimed).    ADVANTAGE - The system is capable of minimizing the risk of dissolved catalyst to negatively affect flow battery performance while minimizing the release of harmful contaminants. The system reduces the chance of catalyst contamination of RFB electrolytes by employing a mediator solution to eliminate direct contact between the catalyzed membrane and the RFB electrolyte. Further, the system maintains the catalyzed electrode at a potential low enough to prevent the catalyst from dissolving.    DETAILED DESCRIPTION - A rebalancing system comprises an electrochemical reaction cell (320) comprising a cathode chamber (226) and an anode chamber (228) separated from the cathode chamber by a separator membrane (202) having a coating (204) containing a catalyst on surface; a hydrogen gas inlet in communication with the anode chamber; a redox flow battery negative electrolyte inlet in communication with the cathode chamber so as to allow a negative electrolyte to be flowed through the cathode chamber; a power supply having a first terminal and a second terminal; and a controller. The negative electrolyte contains trivalent chromium ions (Cr3+) and divalent chromium ions (Cr2+). The first terminal is connected to a first conductive element in the cathode chamber. The second terminal is connected to a second conductive element in the anode chamber of the electrochemical reaction cell. The controller controls a first flow of the negative electrolyte into the cathode chamber through the negative electrolyte inlet at a first rate; controls a second flow of hydrogen gas into the anode chamber through a hydrogen gas inlet at a second rate; and controls the application of an electric current from the power supply to the first and second terminals. The first rate, the second rate and the applied electric current are controlled so as to cause Cr3+ ions to be reduced to Cr2+ ions at one of a controlled rate for a controlled period of time, or until a quantity of Cr2+ ions exceeds a threshold quantity. An INDEPENDENT CLAIM is included for rebalancing iron/chromium (Fe/Cr) redox flow battery (RFB) electrolytes, comprising:    (A) directing a negative electrolyte solution containing Cr3+ ions and Cr2+ ions through a cathode chamber of a rebalance cell (310);    (B) directing a quantity of hydrogen gas through an anode chamber of the rebalance cell;    (C) applying an electric current to the rebalance cell; and    (D) controlling the quantity of hydrogen gas and the current applied to the rebalance cell so as to reduce at least some of the Cr3+ ions to Cr2+ ions according to a controlled reaction rate for a controlled period of time to rebalance the negative electrolyte solution.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic diagram illustrating a two-cell electrochemical system for electrolyte rebalancing.    Separator membrane (202)    Coating (204)    Cathode chamber (226)    Anode chamber (228)    Rebalance cell (310)    Electrochemical reaction cell (320)