Highly stable phenazine derivatives for aqueous redox flow batteries

1. An aqueous electrolyte, comprising:

water; and

a phenazine derivative according to formula I or a salt thereof, where formula I is

wherein each R 1 independently is —H, —OCH 3 , —R a SO 3 H, —R a COOH, —R a SO 3 M, —R a COOM, —R a NR 3 3 X, —R a NR 3 2 , —R a PO(OH) 2 , —R a SH, —R a PS(OH) 2 , —R a —O—PO(OH) 2 , —R a —O—PS(OH) 2 , —R a —S—PS(OH) 2 , or —(OCH 2 CH 2 ) n OR 3 ,

each R 2 independently is —H or —R a OH, wherein at least one R 2 is other than hydrogen,

each R 3 independently is H or C 1 -C 5 alkyl,

each R a independently is absent or C 1 -C 5 alkyl,

M is a cation,

X is an anion,

n is an integer ≥1, and

y+z=1, 2, 3, 4, 5, 6, 7, or 8, where y is 0, 1, 2, 3, or 4 and z is 1,2, 3, or 4,

wherein the aqueous electrolyte does not include any electrochemically active component other than the phenazine derivative.

2. The aqueous electrolyte of claim 1 , wherein:

y is 0, 1, or 2;

z is 1 or 2; and

y+z=2, 3, or 4.

3. The aqueous electrolyte of claim 1 , wherein:

each R 1 independently is —SO 3 H, —COOH, or —OCH 3 ; and

each R 2 is —OH.

4. The aqueous electrolyte of claim 1 , further comprising a base.

5. The aqueous electrolyte of claim 1 , further comprising an acid.

6. The aqueous electrolyte of claim 1 , wherein the electrolyte has an average pH within a range of 6.5 to 7.5, the electrolyte further comprising at least one aqueous-soluble neutral salt other than a phenazine derivative salt.

7. The aqueous electrolyte of claim 1 , wherein the phenazine derivative has a structure according to formula II

wherein each R 1 independently is —H, —OCH 3 , —R a SO 3 H, —R a COOH, —R a SO 3 M, —R a COOM, —R a NR 3 3 X, —R a NR 3 2 , —R a PO(OH) 2 , —R a SH, —R a PS(OH) 2 , —R a —O—PO(OH) 2 , —R a —O—PS(OH) 2 , —R a —S—PS(OH) 2 , or —(OCH 2 CH 2 ) n OR 3 ,

each R 2 independently is —H or —R a OH, wherein at least one R 2 is other than hydrogen;

each R 3 independently is H or C 1 -C 5 alkyl;

each R a independently is absent or C 1 -C 5 alkyl;

M is a cation;

X is an anion; and

n is an integer ≥1.

8. The aqueous electrolyte of claim 7 , wherein:

each R 1 independently is —H, —SO 3 H, —COOH, or —OCH 3 ; and

each R 2 independently is —H or —OH, wherein at least one R 2 is —OH.

9. The aqueous electrolyte of claim 1 , wherein the phenazine derivative is:

or any combination thereof.

10. The aqueous electrolyte of claim 1 , wherein the aqueous electrolyte further comprises an alkali metal hydroxide.

11. The aqueous electrolyte of claim 1 , wherein the phenazine derivative is

12. The aqueous electrolyte of claim 1 , consisting essentially of:

a base;

the phenazine derivative; and

water.

13. The aqueous electrolyte of claim 1 , where the phenazine derivative is present in the electrolyte at a concentration within a range of from 0.01 M to 8 M.

14. An aqueous electrolyte system for use in a redox flow battery system, comprising:

an aqueous anolyte comprising the aqueous electrolyte according to claim 1 ; and

an aqueous catholyte comprising an electrochemically active material.

15. The aqueous electrolyte system of claim 14 , wherein the aqueous catholyte further comprises a base or an acid.

16. The aqueous electrolyte system of claim 14 , wherein the electrochemically active material of the aqueous catholyte comprises K 4 Fe(CN) 6 , K 3 Fe(CN) 6 , or a combination thereof.

17. The aqueous electrolyte system of claim 14 , wherein:

the aqueous anolyte comprises an alkali metal base, and

or any combination thereof; and

the aqueous catholyte comprises an alkali metal base and K 4 Fe(CN) 6 , K 3 Fe(CN) 6 , or a combination thereof.

18. The aqueous electrolyte system of claim 14 , wherein:

the aqueous anolyte comprises an alkali metal base and

 and

the aqueous catholyte comprises the alkali metal base and K 4 Fe(CN) 6 , K 3 Fe(CN) 6 , or a combination thereof.

19. A redox flow battery system, comprising:

the aqueous electrolyte system of claim 14 ; and

a separator.

20. The redox flow battery system of claim 19 , further comprising a carbon-based anode and a carbon-based cathode.