Regeneration of flow battery electrode

1. A method for regenerating an electrode of a flow battery, the method comprising:

during a transition of the flow battery from an active charge/discharge mode to an inactive, shut-down mode in which neither a negative electrolyte nor a positive electrolyte are circulated through at least one cell of the flow battery, driving voltage of the least one cell of the flow battery toward zero by converting, in-situ, the negative electrolyte in the at least one cell to a higher oxidation potential, the negative electrolyte in contact with an electrode of the at least one cell; and

using the higher potential negative electrolyte to regenerate, in-situ, the electrode of the at least one cell with respect to a catalytically active surface of the electrode, followed by transitioning the flow batter into the shut-down mode.

2. The method as recited in claim 1 , wherein the catalytically active surface is carbon that has oxygen-containing groups.

3. The method as recited in claim 1 , wherein the driving of the voltage of at least one cell of the flow battery toward zero includes ceasing flow of the negative electrolyte through the at least one cell to the electrode and flowing the positive electrolyte through the at least one cell.

4. The method as recited in claim 3 , wherein the ceasing of the flow of the negative electrolyte through the at least one cell to the electrode includes deactivating at least one pump which, when active, circulates the negative electrolyte between the at least one cell and an external storage vessel.

5. The method as recited in claim 3 , wherein the flowing of the positive electrolyte through the at least one cell is periodic.

6. The method as recited in claim 3 , wherein the flowing of the positive electrolyte through the at least one cell is preceded by at least partially emptying the at least one cell of the positive electrolyte.

7. The method as recited in claim 1 , further comprising voltage cycling the at least one cell that has the higher oxidation potential negative electrolyte to further convert the negative electrolyte in the at least one cell to the higher oxidation potential.

8. The method as recited in claim 7 , wherein the voltage cycling is within a range of −400 millivolts to +400 millivolts.

9. The method as recited in claim 8 , wherein the voltage cycling has a cycle frequency of 10 minutes or less.

10. The method as recited in claim 1 , further comprising applying an external discharge current to the at least one cell that has the higher oxidation potential negative electrolyte to further convert the negative electrolyte in the at least one cell to the higher oxidation potential.

11. A flow battery comprising:

at least one cell including a first electrode, a second electrode spaced apart from the first electrode, and an electrolyte separator layer arranged between the first electrode and the second electrode;

a supply/storage system external of the at least one cell and including:

a first vessel fluidly connected in a first loop with the first electrode,

a second vessel fluidly connected in a second loop with the second electrode, the first loop and the second loop being isolated from each other with respect to open fluid flow there between; and

a controller that is configured to regenerate catalytically active surfaces of one of the first electrode or the second electrode during a transition of the flow battery from an active charge/discharge mode to an inactive, shut-down mode in which neither a negative electrolyte nor a positive electrolyte are circulated through the at least one cell, by driving voltage of the least one cell toward zero by converting, in-situ, the negative electrolyte in the at least one cell to a higher oxidation state, the negative electrolyte in contact with one of the first electrode or the second electrode, and using the higher oxidation state negative electrolyte to regenerate, in-situ, the catalytically active surfaces of the electrode, followed by transitioning the flow batter into the shut-down mode.

12. The flow battery as recited in claim 11 , wherein the driving of the voltage of at least one cell of the flow battery toward zero includes ceasing flow of the negative electrolyte through the at least one cell to the electrode and flowing the positive electrolyte through the at least one cell.

13. The flow battery as recited in claim 12 , wherein the ceasing of the flow of the negative electrolyte through the at least one cell to the electrode includes deactivating at least one pump which, when active, circulates the negative electrolyte in either the first loop or the second loop.

14. The flow battery as recited in claim 12 , wherein the flowing of the positive electrolyte through the at least one cell is periodic.

15. The flow battery as recited in claim 12 , wherein the flowing of the positive electrolyte through the at least one cell is preceded by at least partially emptying the at least one cell of the positive electrolyte.

16. The flow battery as recited in claim 11 , wherein the controller is configured to voltage cycle the at least one cell that has the higher oxidation state negative electrolyte to further convert the negative electrolyte in the at least one cell to the higher state of charge.

17. The flow battery as recited in claim 16 , wherein the voltage cycling is within a range of −400 millivolts to +400 millivolts.

18. The flow battery as recited in claim 17 , wherein the voltage cycling has a cycle frequency of 10 minutes or less.

19. The flow battery as recited in claim 11 , wherein the controller is configured to apply an external discharge current to the at least one cell that has the higher oxidation state negative electrolyte to further convert the negative electrolyte in the at least one cell to the higher state of charge.

20. The method as recited in claim 1 , wherein the transitioning of the flow battery to the shut-down mode after the regeneration of the negative electrolyte includes, without an intervening charge/discharge cycle of the flow battery, deactivating pumps which when active circulate the negative electrolyte and the positive electrolyte between the at least one cell and external storage vessels.

21. The method as recited in claim 20 , wherein the driving of the voltage of at least one cell of the flow battery toward zero includes ceasing flow of the negative electrolyte through the at least one cell to the electrode and flowing the positive electrolyte through the at least one cell.

22. The method as recited in claim 21 , wherein the catalytically active surface of the electrode is carbon that has oxygen-containing groups, and the higher potential negative electrolyte regenerates the electrode by oxidizing the carbon and increase the number of oxygen-containing groups.

23. The method as recited in claim 22 , wherein the positive electrolyte and the negative electrolyte are based on vanadium.

24. The method as recited in 23 , wherein the flowing of the positive electrolyte through the at least one cell is periodic.

25. The method as recited in claim 23 , wherein the driving of the voltage of at least one cell of the flow battery toward zero further includes at least partially emptying the at least one cell of the positive electrolyte prior to the flowing of the positive electrolyte through the at least one cell, without an intervening charge/discharge cycle of the flow battery.

26. The method as recited in claim 23 , wherein the driving of the voltage of at least one cell of the flow battery toward zero further includes emptying the at least one cell of the positive electrolyte prior to the flowing of the positive electrolyte through the at least one cell, without an intervening charge/discharge cycle of the flow battery.