Operating A Redox Flow Battery With A Negative Electrolyte Imbalance

Abstract

Loss of flow battery electrode catalyst layers during self-discharge or charge reversal may be prevented by establishing and maintaining a negative electrolyte imbalance during at least parts of a flow battery's operation. Negative imbalance may be established and/or maintained actively, passively or both. Actively establishing a negative imbalance may involve detecting an imbalance that is less negative than a desired threshold, and processing one or both electrolytes until the imbalance reaches a desired negative level. Negative imbalance may be effectively established and maintained passively within a cell by constructing a cell with a negative electrode chamber that is larger than the cell's positive electrode chamber, thereby providing a larger quantity of negative electrolyte for reaction with positive electrolyte.

1 - 21 . (canceled) 
     
     
         22 . A method of operating a flow battery, comprising:
 flowing a positive liquid electrolyte through a positive-polarity half-cell of a flow-through reaction cell and flowing a negative liquid electrolyte through a negative-polarity half-cell of the flow-through reaction cell, wherein the negative electrolyte in the negative half-cell of the flow-through reaction cell contains a first quantity of a negative reactant in an oxidized ionic state and a second quantity of the negative reactant in a reduced ionic state, and the positive liquid electrolyte in the positive half-cell of the flow-through reaction cell contains a first quantity of a positive reactant in a reduced ionic state and a second quantity of the positive reactant in an oxidized ionic state;   charging the flow battery by applying an electric current to the flow-through reaction cell, thereby reducing a portion of the first quantity of the negative reactant from the oxidized ionic state to the reduced ionic state and oxidizing a portion of the first quantity of the positive reactant from the reduced ionic state to the oxidized ionic state;   detecting a negative imbalance in which the second quantity of the negative reactant in the reduced state in the negative liquid electrolyte in the negative half-cell of the flow-through reaction cell is greater than the second quantity of the positive reactant in the oxidized state in the positive liquid electrolyte in the positive half-cell of the flow-through reaction cell, wherein the negative imbalance is a difference obtained by subtracting the second quantity of the negative reactant in the reduced state from the second quantity of the positive reactant in the oxidized state;   determining that the negative imbalance is less negative than a threshold negative imbalance; and   processing at least one of the positive liquid electrolyte and the negative liquid electrolyte to cause the negative imbalance to become more negative.   
     
     
         23 . The method of  claim 22 , further comprising maintaining the negative imbalance during at least a charging mode, a discharging mode, and a power-off mode in which electrolytes do not flow and are neither charged nor discharged. 
     
     
         24 . The method of  claim 22 , wherein processing at least one of the positive electrolyte and the negative electrolyte comprises decreasing the second quantity of the positive reactant in the oxidized ionic state in the positive electrolyte. 
     
     
         25 . The method of  claim 22 , wherein processing at least one of the positive electrolyte and the negative electrolyte comprises increasing the second quantity of the negative reactant in the reduced ionic state in the negative electrolyte. 
     
     
         26 . The method of  claim 22 , wherein processing the processing at least one of the positive liquid electrolyte and the negative liquid electrolyte to cause the negative imbalance to become more negative comprises processing the at least one of the positive liquid electrolyte and the negative liquid electrolyte until a target negative imbalance is reached, wherein the positive liquid electrolyte and the negative liquid electrolyte each contain a greater quantity of negative reactant than positive reactant, and wherein a ratio of negative reactant to positive reactant is equal to a maximum target negative imbalance. 
     
     
         27 . The method of  claim 22 , wherein processing at least one of the positive electrolyte and the negative electrolyte comprises decreasing the second quantity of the positive reactant in the oxidized ionic state in the positive electrolyte and increasing the second quantity of the negative reactant in the reduced ionic state in the negative electrolyte. 
     
     
         28 . The method of  claim 22 , wherein the second quantity of the negative reactant in the reduced ionic state in the negative liquid electrolyte, the second quantity of the positive reactant in the oxidized ionic state in the positive liquid electrolyte, and the negative imbalance are molar concentrations, and wherein the threshold negative imbalance is between −0.01M and −0.05M. 
     
     
         29 . The method of  claim 22 , wherein the second quantity of the negative reactant in the reduced ionic state in the negative liquid electrolyte, the second quantity of the positive reactant in the oxidized ionic state in the positive liquid electrolyte, and the negative imbalance are molar concentrations, and wherein processing at least one of the positive electrolyte and the negative electrolyte to cause the negative imbalance to become more negative proceeds until the negative imbalance is between −0.05M and −0.20M. 
     
     
         30 . The method of  claim 22 , wherein the second quantity of the negative reactant in the reduced ionic state in the negative electrolyte is Cr 2+  and the second quantity of the positive reactant in the oxidized ionic state in the positive electrolyte is Fe 3+ . 
     
     
         31 . The method of  claim 22 , further comprising maintaining a concentration of the negative reactant in the reduced ionic state in an entire volume of the negative liquid electrolyte at least 0.1M greater than a concentration of the positive reactant in the oxidized ionic state in an entire volume of the positive liquid electrolyte. 
     
     
         32 . The method of  claim 31 , further comprising providing a greater quantity of total negative reactant in the negative electrolyte than positive reactant in the positive electrolyte, wherein the total negative reactant is the sum of the first quantity of the negative reactant in the reduced ionic state and the second quantity of the negative reactant in the oxidized ionic state and the total positive reactant is the sum of the first quantity of the positive reactant in the reduced ionic state and the second quantity of the positive reactant in the oxidized ionic state. 
     
     
         33 . The method of  claim 22 , wherein the negative half-cell includes a negative electrode having a catalyst plated on a surface thereof, the method further comprising maintaining the negative half-cell of the flow-through reaction cell at a negative electrochemical potential to inhibit deplating of the catalyst when the flow-through reaction cell is discharged to zero volts. 
     
     
         34 . The method of  claim 33 , wherein the catalyst comprises bismuth, the negative reactant in the reduced ionic state in the negative electrolyte is Cr 2+  and the positive reactant in the oxidized ionic state in the positive electrolyte is Fe 3+ . 
     
     
         35 . The method of  claim 22 , wherein processing at least one of the positive electrolyte and the negative electrolyte to cause the negative imbalance to become more negative is performed by a rebalance sub-system of the flow battery. 
     
     
         36 . A method of shutting down a flow battery while charging the flow battery, the method comprising:
 flowing a first flow of a positive liquid electrolyte through a positive-polarity half-cell of a flow-through reaction cell and flowing a second flow of a negative liquid electrolyte through a negative-polarity half-cell of the flow-through reaction cell   charging the flow battery by applying an electric current to the flow-through reaction cell;   stopping the first flow of the negative liquid electrolyte through the negative half-cell of the flow-through reaction cell while continuing the flowing the second flow of the positive liquid electrolyte through the positive half-cell of the cell of the flow-through reaction cell;   applying the charging current to the flow-through reaction cell while the first flow is stopped and the second flow is flowing; and   after applying the charging current for a period of time, shutting down the flow battery by stopping the second flow of the positive electrolyte and stopping the applying the charging current to the flow-through reaction cell.   
     
     
         37 . The method of  claim 36 , further comprising after the flow battery has been shut down for a period of time, re-starting one of: a charging process, a discharging process, or a rebalancing process, before a state of zero imbalance between the positive liquid electrolyte and the negative liquid electrolyte is reached due to spontaneous reactions. 
     
     
         38 . A method of shutting down a flow battery while charging the flow battery, the method comprising:
 flowing a first flow of a positive liquid electrolyte through a positive-polarity half-cell of a flow-through reaction cell and flowing a second flow of a negative liquid electrolyte through a negative-polarity half-cell of the flow-through reaction cell;   charging the flow battery by applying an electric current to the flow-through reaction cell;   stopping a second flow of the negative liquid electrolyte through the negative half-cell of the flow-through reaction cell and stopping the first flow the positive liquid electrolyte through the positive half-cell of the cell of the flow-through reaction cell;   applying the charging current to the flow-through reaction cell while the first flow and the second flow are stopped; and   after applying the charging current for a period of time, shutting down the flow battery by stopping the charging current to the flow-through reaction cell.   
     
     
         40 . A method of shutting down a flow battery while discharging the flow battery, the method comprising:
 flowing a first flow of a positive liquid electrolyte through a positive-polarity half-cell of a flow-through reaction cell and flowing a second flow of a negative liquid electrolyte through a negative-polarity half-cell of the flow-through reaction cell;   dis-charging the flow battery by applying an electric discharging current produced by the flow-through reaction cell to a load;   stopping the first flow of the positive liquid electrolyte through the positive half-cell of the flow-through reaction cell while continuing to flow the second flow of the negative electrolyte through the negative half-cell of the flow-through reaction cell;   continuing to apply the electric discharging current from the flow-through reaction cell to the load while the first flow is stopped and the second flow is flowing for a period of time; and   shutting down the flow battery by stopping the flowing of the second flow of the negative electrolyte and stopping the applying the electric discharging current from the flow-through reaction cell.   
     
     
         41 . The method of  claim 40 , further comprising, after shutting down the flow battery, initiating one of: a charging process, a discharging process, or a rebalancing process before a state of zero imbalance between the positive liquid electrolyte and the negative liquid electrolyte is reached due to spontaneous reactions. 
     
     
         42 . A flow battery system comprising:
 a first reservoir containing a negative liquid electrolyte;   a second reservoir containing a positive liquid electrolyte;   a charge/discharge stack;   at least one pump for circulating the electrolytes between the reservoirs and the stack;   an imbalance monitoring sub-system configured to provide a controller with an analog or digital signal indicative of a sign and quantity of an electrolyte charge imbalance;   a controller configured with controller-executable instructions to cause the controller to perform operations comprising determining that a negative imbalance is less negative than a threshold negative imbalance; and processing at least one of the positive liquid electrolyte and the negative liquid electrolyte to cause the negative imbalance to become more negative.   
     
     
         43 . A flow battery system comprising:
 a first reservoir containing a negative liquid electrolyte;   a second reservoir containing a positive liquid electrolyte;   a charge/discharge stack having a plurality of individual flow-through cells; and   at least one pump configured to circulate the electrolytes between the reservoirs and the stack,   wherein each of the plurality of flow-through cells has a negative electrode chamber and a positive electrode chamber, and wherein a negative electrode chamber volume of the negative electrode chamber is larger than a positive electrode chamber volume of the positive electrode chamber.   
     
     
         44 . The system of  claim 43 , wherein each of the plurality of flow-through cells has a ratio of negative electrode chamber volume to positive electrode chamber volume configured to maintain a negative imbalance within the cell to prevent de-plating of a catalyst plated onto an electrode structure within the negative electrode chamber. 
     
     
         45 . The system of  claim 43 , wherein the stack comprises a cascade stack comprising a plurality of stages arranged in fluidic series along a flow path in which ones of the plurality of flow-through cells at a first end of the flow path operate at a lower state-of-charge than ones of the plurality of flow-through cells at a second end of the flow path, and wherein ones of the plurality of flow-through cells at the first end of the flow path have higher ratios of negative electrode chamber volume to positive electrode chamber volume than ones of the plurality of flow-through cells at the second end of the flow path.