Systems and methods for rebalancing redox flow battery electrolytes
Various methods of rebalancing electrolytes in a redox flow battery system include various systems using a catalyzed hydrogen rebalance cell configured to minimize the risk of dissolved catalyst negatively affecting flow battery performance. Some systems described herein reduce 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. Other methods use a rebalance cell chemistry that maintains the catalyzed electrode at a potential low enough to prevent the catalyst from dissolving.
1. A method of rebalancing Fe/Cr redox flow battery electrolytes, the method comprising:
directing a negative electrolyte solution containing Cr 3+ ions and Cr 2+ ions through a cathode chamber of a first rebalance cell, the cathode chamber having a first electrode;
directing a quantity of hydrogen gas through an anode chamber of the first rebalance cell, the anode chamber having a second electrode and being separated from the cathode chamber by a separator membrane having a catalyst-containing coating on a surface facing the anode chamber;
applying an electric current to the first rebalance cell; and
controlling the quantity of hydrogen gas and the electric current applied to the first rebalance cell so as to reduce at least some of the Cr 3+ ions to Cr 2+ ions according to a controlled reaction rate for a controlled period of time to rebalance the negative electrolyte solution.
2. The method of claim 1 , wherein the negative electrolyte does not contain Fe 3+ .
3. The method of claim 1 , wherein the catalyst-containing coating includes platinum.
4. The method of claim 1 , wherein the catalyst-containing coating includes a member of the group consisting of tungsten carbide, palladium, platinum alloy, and rhodium.
5. The method of claim 1 , wherein the separator membrane includes a cation exchange membrane.
6. The method of claim 1 , wherein the negative electrolyte solution directed into the cathode chamber is at a low state of charge, in which a first quantity of the Cr 2+ ions is less than a second quantity of the Cr 3+ ions in the negative electrolyte solution.
7. The method of claim 6 , wherein the low state-of-charge negative electrolyte solution is at a state of oxidation of less than 50% prior to being directed into the cathode chamber.
8. The method of claim 1 , further comprising directing the negative electrolyte solution out of the cathode chamber and into a redox flow battery charging stack.
9. The method of claim 1 , further comprising directing negative electrolyte and hydrogen gas through respective cathode and anode chambers of at least a second rebalance cell that is combined with the first rebalance cell in a bipolar stack arrangement.