Method for iron preformation in redox flow batteries

Abstract

Methods and systems are provided for iron preformation in a redox flow battery. In one example, a method may include, in a first condition, discharging and then charging the redox flow battery, and in a second condition, charging the redox flow battery including preforming iron metal at a negative electrode of the redox flow battery, and thereafter entering an idle mode of the redox flow battery including adjusting one or more electrolyte conditions. In some examples, each of preforming the iron metal and adjusting the one or more electrolyte conditions may increase a battery charge capacity to greater than a threshold battery charge capacity.

1 . A redox flow battery system, comprising:
 a redox electrode;   a plating electrode;   an electrolyte subsystem supplying an electrolyte to each of the redox electrode and the plating electrode;   one or more sensors; and   a controller operable to receive signals from the one or more sensors and storing executable instructions in non-transitory memory, the instructions executable to:
 determine one or more system conditioning entry conditions based on the signals received from the one or more sensors; and 
 responsive to the one or more system conditioning entry conditions being met:
 command a charging mode to preform an amount of iron metal at the plating electrode, 
 command an idle mode to balance one or more conditions of the electrolyte subsystem, and thereafter 
 further responsive to the amount of iron metal being greater than a threshold amount, enter battery cycling. 
 
   
     
     
         2 . The redox flow battery system of  claim 1 , wherein the idle mode includes operating the redox flow battery system at less than an idle threshold current. 
     
     
         3 . The redox flow battery system of  claim 1 , wherein the charging mode includes supplying a DC current to the redox electrode to charge the redox flow battery system by oxidizing ferrous iron at the redox electrode and reducing ferrous iron to plate the amount of iron metal at the plating electrode. 
     
     
         4 . The redox flow battery system of  claim 1 , wherein the one or more system conditioning entry conditions comprise the redox flow battery system reaching a threshold degradation level. 
     
     
         5 . The redox flow battery system of  claim 4 , wherein determining the one or more system conditioning entry conditions based on the signals received from the one or more sensors includes:
 determining a battery charge capacity based on the signals received from the one or more sensors; and   responsive to the battery charge capacity being greater than a threshold battery charge capacity, indicating that the redox flow battery system has reached the threshold degradation level.   
     
     
         6 . The redox flow battery system of  claim 1 , wherein the one or more conditions of the electrolyte subsystem comprise a ferric iron concentration, a positive electrolyte pH, a negative electrolyte pH, and an electrolyte state of charge imbalance. 
     
     
         7 . The redox flow battery system of  claim 6 , wherein the electrolyte subsystem comprises one or more electrolyte balancing reactors,
 wherein balancing the one or more conditions of the electrolyte subsystem includes operating the one or more electrolyte balancing reactors to perform catalytic hydrogen reduction of ferric iron to decrease the ferric iron concentration, and   
       wherein hydrogen gas used to perform the catalytic hydrogen reduction of ferric iron is directed from gas head spaces of an electrolyte storage tank fluidically coupled to the one or more electrolyte balancing reactors. 
     
     
         8 . A method, comprising, during system conditioning of a redox flow battery cell:
 charging the redox flow battery cell at a set point to increase an electrolyte storage tank pressure;   responsive to the electrolyte storage tank pressure being greater than a first threshold pressure, conducting electrolyte rebalancing; and thereafter   determining an iron plating amount at a negative electrode of the redox flow battery cell; and   responsive to the iron plating amount being less than a threshold amount, continuing charging the redox flow battery cell at the set point.   
     
     
         9 . The method of  claim 8 , wherein the threshold amount is selected to increase a battery charge capacity above a threshold battery charge capacity, and wherein determining the iron plating amount includes inferring the iron plating amount based on an electrolyte concentration of ferric iron. 
     
     
         10 . The method of  claim 8 , wherein the first threshold pressure is selected to prevent rupture or damage of the redox flow battery cell, and wherein the electrolyte storage tank pressure is a pressure due to storage of hydrogen gas in one or more headspaces of the electrolyte storage tank, the hydrogen gas formed at the negative electrode. 
     
     
         11 . The method of  claim 8 , wherein conducting electrolyte rebalancing includes:
 initiating electrolyte rebalancing to decrease the electrolyte storage tank pressure; and   responsive to the electrolyte storage tank pressure being less than a second threshold pressure, ending electrolyte rebalancing,   wherein the second threshold pressure is selected to indicate that a threshold amount of hydrogen gas has reacted during electrolyte rebalancing.   
     
     
         12 . The method of  claim 11 , wherein initiating electrolyte rebalancing includes:
 draining a DC current from the redox flow battery cell to end charging; and   operating one or more electrolyte rebalancing reactors,   
       wherein continuing charging the redox flow battery cell includes reentering charging after ending charging. 
     
     
         13 . The method of  claim 12 , wherein continuing charging the redox flow battery cell includes reentering charging after ending charging. 
     
     
         14 . The redox flow battery system of  claim 5 , wherein the threshold battery charge capacity is lower than a battery cycling charging threshold. 
     
     
         15 . The redox flow battery system of  claim 1 , wherein the instructions further include to determine a tank pressure, and in response to the tank pressure greater than a first threshold pressure exit the charging mode and enter an idle mode, and in response to the tank pressure less than or equal to the first threshold pressure maintain the charging mode. 
     
     
         16 . The method of  claim 8 , wherein charging the redox flow battery cell includes charging for iron preformation. 
     
     
         17 . The method of  claim 8 , wherein charging the redox flow battery cell occurs when a battery charge capacity is lower than a battery cycling charging threshold. 
     
     
         18 . The method of  claim 8 , wherein conducting electrolyte rebalancing includes employing a catalytic electrolyte rebalancing subsystem to lower imbalanced Fe 3+  by supplying hydrogen gas as a reductant to reduce excess Fe 3+  to Fe 2+ .