Methods and systems for determining average oxidation state of redox flow battery systems
A method for determining an average oxidation state (AOS) of a redox flow battery system includes measuring a charge capacity for a low potential charging period starting from a discharged state of the redox flow battery system to a turning point of a charge voltage; and determining the AOS using the measured charge capacity and volumes of anolyte and catholyte of the redox flow battery system. Other methods can be used to determine the AOS for a redox flow battery system or use discharge voltage instead of charging voltage.
1. A method for operating a redox flow battery system comprising a catholyte and an anolyte that both comprise iron and chromium ions, the method comprising
discharging the redox flow battery system;
measuring a discharge rate during the discharge of the redox flow battery system;
determining a turning point of the discharge on a discharge curve and measuring an open circuit voltage (OCV) after the turning point;
estimating a concentration of Fe 3+ using the measured OCV; and
determining an average oxidation state (AOS) using the estimated concentration of Fe 3+ and volumes of anolyte and catholyte of the redox flow battery system.
2. The method of claim 1 , wherein a molar ratio of chromium in the anolyte to iron in the catholyte is at least 1.25.
3. The method of claim 1 , further comprising rebalancing the AOS in response to the determination.
4. The method of claim 3 , wherein rebalancing the AOS comprises oxidizing vanadium ions in a balancing electrolyte to dioxovanadium ions to produce hydrogen ions, wherein the anolyte or catholyte of the redox flow battery system form a balancing arrangement with the balancing electrolyte using at least two half-cells.
5. The method of claim 4 , further comprising regenerating the vanadium ions by reducing the dioxovanadium ions using a reductant.
6. The method of claim 5 , wherein the reductant comprises an organic compound or hydrogen gas.
7. The method of claim 5 , wherein the reductant comprises a sugar, carboxylic acid, aldehyde, or alcohol.
8. The method of claim 5 , wherein the reductant comprises fructose, glucose, or sucrose.
9. The method of claim 5 , wherein the reductant comprises hydrogen gas.
10. The method of claim 5 , wherein regenerating the vanadium ions comprises adding the reductant intermittently or periodically to the balance electrolyte.
11. The method of claim 5 , wherein regenerating the vanadium ions comprises
removing at least a portion of the balance electrolyte from the balance arrangement;
introducing the reductant to the removed portion of the balance electrolyte; and
returning the removed portion of the balance electrolyte to the balance arrangement.
12. The method of claim 1 , wherein the redox flow battery system comprises a battery stack comprising a first half-cell and a second half-cell and the method is performed in situ using the first and second half-cells of the battery stack.
13. The method of claim 1 , wherein the estimating comprises estimating the concentration of Fe 3+ based on a discharge rate difference between discharge prior to the turning point and discharge after the turning point.
14. The method of claim 1 , wherein the redox flow battery system comprises
a first half-cell comprising a first electrode in contact with the anolyte,
a second half-cell comprising a second electrode in contact with the catholyte, and
a first separator separating the first half-cell from the second half-cell.
15. The method of claim 14 , wherein the redox flow battery system further comprises
a balance arrangement comprising
a balance electrolyte comprising vanadium ions in solution,
a third half-cell comprising a third electrode in contact with the anolyte or the catholyte,
a fourth half-cell comprising a fourth electrode in contact with the balance electrolyte, and
a reductant in the balance electrolyte or introducible to the balance electrolyte for reducing dioxovanadium ions.
16. The method of claim 15 , wherein the redox flow battery system further comprises
an intermediate electrolyte comprising any combination of either a) V 3+ and V 2+ ions or b) Fe 3+ and Fe 3+ ions;
a fifth half-cell comprising a fifth electrode in contact with intermediate electrolyte,
a second separator between the third half-cell and the fifth half-cell,
a sixth half-cell comprising a sixth electrode in contact with the intermediate electrolyte, and
a third separator between the fourth half-cell and the sixth half-cell.
17. The method of claim 1 , further comprising estimating an amount of hydrogen generated by the redox flow battery system from the determined AOS.
18. The method of claim 1 , wherein the discharging comprises self-discharging the redox flow battery system.
19. The method of claim 1 , wherein the turning point represents a change from discharge utilizing both iron and chromium ions to discharge utilizing primarily only iron ions.
20. The method of claim 1 , wherein the turning point represents a substantial change in the discharge rate to a lower discharge rate.