Methods and systems for determining average oxidation state of redox flow battery systems using the open circuit voltage
A method for determining a storage capacity or average oxidation state (AOS) of a redox flow battery system including an anolyte and a catholyte includes discharging a portion of the anolyte and catholyte of the redox flow battery system at a discharge rate that is within 10% of a preselected discharge rate; after discharging the redox flow battery system, determining an end OCV; and determining the storage capacity or AOS from the end OCV. Other methods can be used to determine the storage capacity or AOS using a measured OCV.
1. A method for determining a storage capacity or an average oxidation state of a redox flow battery system comprising an anolyte and a catholyte, the method comprising
discharging a portion of the anolyte and the catholyte of the redox flow battery system at a discharge rate that is within 10% of a preselected discharge rate;
after discharging the portion of the anolyte and the catholyte of the redox flow battery system, determining an end open circuit voltage; and
determining the storage capacity or the average oxidation state from the end open circuit voltage.
2. The method of claim 1 , wherein determining the storage capacity or the average oxidation state comprises determining the storage capacity or the average oxidation state from a pre-determined open circuit voltage curve, look-up table, calibration table, or mathematical relationship relating the end open circuit voltage to the storage capacity or the average oxidation state under the preselected discharge rate.
3. The method of claim 1 , wherein the redox flow battery system comprises at least two half-cells and at least two electrodes with each of the electrodes associated with one of the half-cells.
4. The method of claim 3 , wherein the discharging comprises discharging the anolyte and the catholyte in at least two of the half-cells.
5. The method of claim 4 , further comprising halting flow of at least one of the anolyte or the catholyte in the redox flow battery system prior to, and during, the discharging.
6. The method of claim 4 , wherein the at least two of the half-cells are part of a battery stack of the redox flow battery system.
7. The method of claim 4 , wherein the at least two of the half-cells are not part of a battery stack of the redox flow battery system.
8. The method of claim 1 , wherein the discharging comprises discharging the portion of the anolyte and the catholyte of the redox flow battery system until there is no more than 10% Cr 2+ ions in the anolyte.
9. The method of claim 1 , wherein the discharging comprises self-discharging the redox flow battery system.
10. The method of claim 1 , wherein the anolyte comprises chromium and the catholyte comprises iron.
11. The method of claim 10 , further comprising rebalancing the average oxidation state in response to the determination.
12. The method of claim 11 , wherein rebalancing the average oxidation state comprises oxidizing vanadium ions in a balancing electrolyte to dioxovanadium ions to produce hydrogen ions, wherein the anolyte or the catholyte of the redox flow battery system form a balancing arrangement with the balancing electrolyte using at least two half-cells.
13. The method of claim 12 , further comprising regenerating the vanadium ions by reducing the dioxovanadium ions using a reductant.
14. The method of claim 10 , further comprising measuring one iron ionic species or measuring one chromium ionic species.
15. The method of claim 14 , wherein measuring the one iron ionic species or measuring the one chromium ionic species comprises reducing Fe 3+ or Cr 3+ , respectively, and oxidizing vanadium ions.
16. The method of claim 15 , further comprising regenerating the vanadium ions by reducing dioxovanadium ions using a reductant.
17. The method of claim 16 , wherein the reductant comprises an organic compound or hydrogen gas.
18. The method of claim 16 , wherein the reductant comprises a sugar, carboxylic acid, aldehyde, or alcohol.
19. The method of claim 1 , wherein discharging the portion of the anolyte and the catholyte of the redox flow battery system comprises discharging the portion of the anolyte and the catholyte of the redox flow battery system at a discharge rate that is within 5% of the preselected discharge rate.
20. The method of claim 1 , wherein discharging the portion of the anolyte and the catholyte of the redox flow battery system comprises discharging the portion of the anolyte and the catholyte of the redox flow battery system at a discharge rate that is within 1% of the preselected discharge rate.