NOVELTY - A redox flow battery consists of electrochemical cell(s) in fluid contact with an electrochemical balancing cell having electrodes (I,II), a membrane arranged between the electrodes, and a provision for contacting hydrogen with electrode (I). Electrode (I) is maintained at a potential more positive than the thermodynamic potential for hydrogen evolution. Electrode (II) is maintained at a potential sufficiently negative to reduce the negative electrolyte. The membrane is suitable to allow hydrogen cations to flow from the membrane to the negative electrolyte.    USE - Redox flow battery (claimed).    ADVANTAGE - The redox flow battery has no precious metal corrosion, as potentials on the metal carbon electrode are maintained at potentials where corrosion is minimal. Hence, crossover of corroded precious metals to the electrode in contact with the negative active material will not occur. Hence, excellent hydrogen oxidation capabilities of precious metal electrodes can be employed without concern for exacerbation of parasitic hydrogen evolution reactions. State of charge imbalance resulting from oxidation of negative electrolyte by oxygen or other deleterious reactions is mitigated. The membrane reduces inefficiencies incurred by ion conduction and hence can serve to increase voltage efficiency of the energy storage device.    DETAILED DESCRIPTION - A redox flow battery consists of electrochemical cell(s) in fluid contact with an electrochemical balancing cell. The balancing cell consists of electrodes (I,II), a membrane arranged between the electrodes, and a provision for contacting hydrogen with electrode (I). Electrode (I) comprises a gas-diffusion electrode and a hydrogen oxidation catalyst, and is maintained at a potential more positive than the thermodynamic potential for hydrogen evolution. Electrode (II) contacts negative electrolyte, and is maintained at a potential sufficiently negative to reduce the negative electrolyte. The membrane is suitable to allow hydrogen cations to flow from the membrane to the negative electrolyte. An INDEPENDENT CLAIM is included for balancing the state of charge of flow battery.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of an electrochemical cell for balancing electrolyte state of charge in a flow battery cell.