Materials for use with aqueous redox flow batteries and related methods and systems

1. A redox flow battery comprising:

a first aqueous electrolyte comprising a first type of redox active material;

a second aqueous electrolyte comprising a second type of redox active material;

wherein the first type of redox active material comprises a quinoxaline moiety, a salt of the quinoxaline moiety, or a mixture thereof.

2. The battery of claim 1 , further comprising a first electrode in contact with the first aqueous electrolyte and a second electrode in contact with the second aqueous electrolyte.

3. A method of storing energy, comprising applying a potential difference across the first electrode and second electrode of a flow battery of claim 2 , wherein the first type of redox active material is reduced.

4. The method of claim 3 , wherein a quinoxaline moiety in the first type of redox active material is converted from a compound comprising Formula (I):

or salt thereof, to a compound comprising Formula (I-B):

5. A method of releasing energy, comprising applying a potential difference across the first electrode and second electrode of a flow battery of claim 2 , wherein the first type of redox active material is oxidized.

6. The method of claim 5 , wherein a quinoxaline in the first type of redox active material is converted from a compound comprising Formula (I-B):

or salt thereof, to a compound comprising Formula (I):

7. The battery of claim 1 , further comprising a separator between the first aqueous electrolyte and second aqueous electrolyte.

8. The battery of claim 1 , wherein the first aqueous electrolyte has a pH between about 7 and about 10, or between about 10 and about 12, or between about 12 and about 14.

9. The battery of claim 1 , wherein the first type of redox active material is present in the first aqueous electrolyte in a concentration of at least about 0.5 M, at least about 1 M, at least about 2 M, at least about 4 M, or at least about 6 M.

10. The battery of claim 1 , wherein the first type of redox active material is present in the first aqueous electrolyte in a concentration of between about 0.5 and about 2 M.

11. The battery of claim 1 , wherein the quinoxaline moiety comprises a compound comprising Formula (I):

or a salt thereof, wherein:

each R 1 , R 2 , R 3 , R 4 , R 5 , and R 6 is independently hydrogen, halogen, optionally substituted alkyl, optionally substituted carbocyclyl, optionally substituted heterocyclyl, optionally substituted aryl, optionally substituted heteroaryl, —CN, —NO 2 , —OR a , —N(R a ) 2 , —C(═O)R a , —C(═O)OR a , —S(═O) 2 R a , or —S(═O) 2 OR a ;

each R a is independently hydrogen, optionally substituted alkyl, optionally substituted carbocyclyl, optionally substituted heterocyclyl, optionally substituted aryl, optionally substituted heteroaryl, an oxygen protecting group, or a nitrogen protection group; and

optionally, any two adjacent groups selected from R 1 , R 2 , R 3 , R 4 , R 5 , and R 6 are joined to form an optionally substituted aryl or optionally substituted heteroaryl ring.

12. The battery of claim 11 , wherein:

each of R 1 , R 2 , R 3 , R 4 , R 5 , and R 6 is independently hydrogen, halogen, optionally substituted alkyl, optionally substituted haloalkyl, —CN, —NO 2 , —OR a , —N(R a ) 2 , —C(═O)OR a , or —S(═O) 2 OR a ; and

each R a is independently hydrogen or optionally substituted alkyl.

13. The battery of claim 11 , wherein none of, any two of, any three of, any four of, or any five of R 1 , R 2 , R 3 , R 4 , R 5 , and R 6 are hydrogen.

14. The battery of claim 11 , wherein the compound of Formula (I) comprises the structure:

or a salt thereof.

15. The battery of claim 1 , wherein the first aqueous electrolyte comprises between 0.5 and 5 M LiCl, between 0.5 M and 5 M NaCl, or between 0.5 M and 5 M KCl, or a mixture thereof such that the combined concentration is between 0.5 M and 5 M.

16. The battery of claim 1 , wherein the quinoxaline moiety comprises Formula (IV):

or a salt thereof, wherein:

each R 1 , R 2 , R 3 , R 4 , R 5 , and R 6 is independently hydrogen, halogen, optionally substituted alkyl, optionally substituted carbocyclyl, optionally substituted heterocyclyl, optionally substituted aryl, optionally substituted heteroaryl, —CN, —NO 2 , —OR a , —N(R a ) 2 , —C(═O)R a , —C(═O)OR a , —S(═O) 2 R a , or —S(═O) 2 OR a ;

each R a is independently hydrogen, optionally substituted alkyl, optionally substituted carbocyclyl, optionally substituted heterocyclyl, optionally substituted aryl, optionally substituted heteroaryl, an oxygen protecting group, or a nitrogen protection group;

provided at least one of the nitrogen labeled a, the nitrogen labeled b, R 1 , R 2 , R 3 , R 4 , R 5 , and R 6 is associated with a metal center, and

wherein the metal center is optionally associated with one or more auxiliary ligands.

17. The battery of claim 1 , wherein the second type of redox active material comprises a hexacyanoiron complex, a quinone, a hydroquinone, or an organic dye, or a salt or mixture thereof.

18. The battery of claim 17 , wherein the second type of redox active material comprises ferrocyanide, ferricyanide, 1,2-dihydroxy-3,5-benzenedisulfonate, 1,2,4-trihydroxy-3,5-benzenedisulfonate, or indigo carmine, a salt of any one of these compounds, or a mixture thereof.

19. The battery of claim 1 , further comprising:

a first circulation loop comprising a storage tank containing the first aqueous electrolyte, piping for transporting the first aqueous electrolyte, a chamber in which a first electrode is in contact with the first aqueous electrolyte, and a pump to circulate the first aqueous electrolyte through the first circulation loop;

optionally a second circulation loop comprising a storage tank containing the second aqueous electrolyte, piping for transporting the second aqueous electrolyte, a chamber in which a second electrode is in contact with the second aqueous electrolyte, and a pump to circulate the second aqueous electrolyte through the second circulation loop; and

control hardware and software.

20. The battery of claim 1 , wherein the first type of redox active material is present in the first aqueous electrolyte in a concentration of between about 2 M and about 4 M.

21. The battery of claim 1 , wherein the first type of redox active material is present in the first aqueous electrolyte in a concentration of between about 4 M and about 6 M.

22. The battery of claim 1 , wherein the first type of redox active material is present in the first aqueous electrolyte in a concentration of between about 6 M and about 10 M.