Systems and Methods for Rebalancing Redox Flow Battery Electrolytes

Abstract

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 rebalancing system configured to adjust a charge balance between a positive electrolyte and a negative electrolyte in a redox flow battery system, the rebalancing system comprising:
 a first electrochemical reaction cell comprising a cathode chamber and an anode chamber separated from the cathode chamber by a separator membrane, the separator membrane having a coating containing a catalyst on a surface thereof;   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, the negative electrolyte containing Cr 3+  ions and Cr 2+  ions;   a power supply having a first terminal and a second terminal, the first terminal connected to a first conductive element in the cathode chamber and the second terminal connected to a second conductive element in the anode chamber of the first electrochemical reaction cell; and   a controller configured to control a first flow of the negative electrolyte into the cathode chamber through the negative electrolyte inlet at a first rate, and to control a second flow of hydrogen gas into the anode chamber through the hydrogen gas inlet at a second rate, and to control the application of an electric current from the power supply to the first and second terminals, wherein the first rate, the second rate and the applied electric current are controlled so as to cause Cr 3+  ions to be reduced to Cr 2+  ions at one of: a controlled rate for a controlled period of time; or until a quantity of Cr 2+  ions exceeds a threshold quantity.   
     
     
         2 . The rebalancing system of  claim 1 , wherein the negative electrolyte does not contain Fe 3+  ions. 
     
     
         3 . The rebalancing system of  claim 1 , wherein the second flow of hydrogen gas into the anode chamber through the hydrogen gas inlet is controlled at the second rate, the second flow of hydrogen gas interacting with the catalyst coating, so as to cause H +  ions to be generated at a third rate. 
     
     
         4 . The rebalancing system of  claim 3 , wherein the first rate, the second rate, the third rate and the applied electric current are controlled so at to cause the Cr 3+  ions to be reduced to Cr 2+  ions at one of: the controlled rate for a controlled period of time; or until a quantity of Cr 2+  ions exceeds a threshold quantity by reaction with H +  ions entering the cathode chamber through the separator from the anode chamber at the third rate. 
     
     
         5 . The rebalancing system of  claim 1 , wherein the catalyst coating includes platinum. 
     
     
         6 . The rebalancing system of  claim 1 , wherein the catalyst coating includes at least one member of the group consisting of tungsten carbide, palladium, platinum alloy, and rhodium. 
     
     
         7 . The rebalancing system of  claim 1 , wherein the separator membrane includes a cation exchange membrane. 
     
     
         8 . The rebalancing system of  claim 1 , wherein the redox flow battery negative electrolyte inlet is coupled to a source of negative electrolyte 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. 
     
     
         9 . The rebalancing system of  claim 8 , wherein the low state-of-charge negative electrolyte is at a state of oxidation of less than about 40%. 
     
     
         10 . The rebalancing system of  claim 1 , further comprising at least a second electrochemical reaction cell, wherein the first electrochemical reaction cell and the second electrochemical reaction cell are configured in a bipolar stack arrangement. 
     
     
         11 . The rebalancing system of  claim 1 , wherein the hydrogen gas inlet is coupled to a source of hydrogen gas captured from a redox flow battery. 
     
     
         12 . The rebalancing system of  claim 1 , wherein the hydrogen gas inlet is coupled to a tank of hydrogen gas. 
     
     
         13 . The rebalancing system of  claim 1 , wherein the anode chamber is further configured with no outlet for hydrogen gas and is further configured to accumulate excess hydrogen gas therein. 
     
     
         14 . The rebalancing system of  claim 1 , wherein the anode chamber further comprises an outlet that vents excess hydrogen gas. 
     
     
         15 . The rebalancing system of  claim 1 , wherein the anode chamber further comprises an outlet coupled to a source of hydrogen gas, and wherein the hydrogen gas continuously recirculates between the source of hydrogen gas and the anode chamber. 
     
     
         16 - 24 . (canceled) 
     
     
         25 . The rebalancing system of  claim 1 , wherein an outlet of the cathode chamber of the first electrochemical reaction cell is in fluid communication with a negative electrolyte inlet of a redox flow battery charging/discharging stack such that the negative electrolyte solution is directed from the cathode chamber directly to the redox flow battery charging/discharging stack. 
     
     
         26 . The rebalancing system of  claim 1 , wherein an outlet of the cathode chamber of the first electrochemical reaction cell is in fluid communication with a negative electrolyte reservoir, such that the negative electrolyte solution is directed from the cathode chamber directly to the negative electrolyte reservoir.