Cell for electrochemically determining active species concentrations in redox flow batteries

Abstract

A redox flow battery system with a redox flow battery includes a redox flow cell, and a supply/storage system external of the redox flow cell. The supply/storage system includes first and second electrolytes for circulation through the redox flow cell. At least the first electrolyte is a liquid electrolyte that has electrochemically active species with multiple, reversible oxidation states. A secondary cell is fluidly connected with the first electrolyte and is operable to monitor concentration of one or more of the electrochemically active species. The secondary cell includes a counter electrode, a working microelectrode, and an ionically conductive path formed by the first electrolyte between the counter electrode and the working microelectrode.

What is claimed is: 
     
         1 . A redox flow battery system comprising:
 a redox flow battery including
 a redox flow cell, and 
 a supply/storage system external of the redox flow cell, the supply/storage system including first and second electrolytes for circulation through the redox flow cell, at least the first electrolyte being a liquid electrolyte having electrochemically active species with multiple, reversible oxidation states; and 
   a secondary cell fluidly connected with the first electrolyte and operable to monitor concentration of one or more of the electrochemically active species, the secondary cell including a counter electrode, a working microelectrode, and an ionically conductive path formed by the first electrolyte between the counter electrode and the working microelectrode.   
     
     
         2 . The system as recited in  claim 1 , wherein the working microelectrode has an area A1 of less than 0.8 mm 2 . 
     
     
         3 . The system as recited in  claim 2 , wherein the counter electrode has an area A2 that is equal to or greater than 0.8 mm 2 . 
     
     
         4 . The system as recited in  claim 3 , wherein the area A2 is greater than the area A1 by a factor of at least 5. 
     
     
         5 . A method for monitoring state of charge in a redox flow battery system, the method comprising:
 providing a redox flow battery system that includes:
 a redox flow battery including
 a redox flow cell, and 
 a supply/storage system external of the redox flow cell, the supply/storage system including first and second electrolytes for circulation through the redox flow cell, at least the first electrolyte being a liquid electrolyte having electrochemically active species with multiple, reversible oxidation states; and 
 
 a secondary cell receiving the first or second electrolyte and operable to monitor concentration of one or more of the electrochemically active species, the secondary cell including a counter electrode, at least one working electrode, and a separator between the counter electrode and the at least one working electrode; 
   applying a bias voltage to the working electrode relative to the counter electrode;   measuring electric current in an electrical circuit connecting the working electrode and the counter electrode to determine an electrochemical response from redox reactions of the electrochemically active species at the working electrode;   determining a concentration of at least one of the electrochemically active species based on the electrochemical response; and   determining a state of charge of the electrochemically active species based on the concentrations measured.   
     
     
         6 . The method of  claim 5 , wherein the bias voltage is a constant voltage. 
     
     
         7 . The method of  claim 5 , wherein the bias voltage varies dynamically as a function of an open cell voltage of the redox flow cell. 
     
     
         8 . The method of  claim 5 , wherein the bias voltage varies.