Inexpensive metal-free organic redox flow battery (ORBAT) for grid-scale storage

1. A flow battery comprising:

a positive electrode;

a positive electrode electrolyte including water and a first redox couple that includes a first organic compound, the positive electrode electrolyte flowing over and contacting the positive electrode, the positive electrode electrolyte having a pH from 0.5 to 6.5;

a negative electrode;

a negative electrode electrolyte including water and a second redox couple, the negative electrode electrolyte flowing over and contacting the negative electrode, the negative electrode electrolyte having a pH from 0.5 to 6.5, the second redox couple including a second organic compound having formula 7, a reduction product of the second organic compound being oxidized to the second organic compound during discharge:

wherein:

R 8 -R 15 are each independently selected from the group consisting of H, —SO 3 H, and electron donating groups with the proviso that at least one of R 8 -R 15 is hydroxyl, NH 2 , —NHR 2 , N(R 2 ) 2 , pyridinyl, imidazoyl, or pyrroyl, where R 2 is H or C 1-10 alkyl; and

a polymer electrolyte membrane interposed between the positive electrode and the negative electrode.

2. The flow battery of claim 1 further comprising:

a positive electrode reservoir in fluid communication with the positive electrode, the positive electrode electrolyte being stored in the positive electrode reservoir to charge and discharge the flow battery; and

a negative electrode reservoir in fluid communication with the negative electrode, the negative electrode electrolyte being stored in the negative electrode reservoir to charge and discharge the flow battery.

3. The flow battery of claim 1 wherein the first organic compound has a standard electrode potential that is at least 0.3 volts higher than a standard electrode potential for the second organic compound.

4. The flow battery of claim 1 wherein the first organic compound has a standard electrode potential greater than 0.6 volts relative to a standard hydrogen electrode and the organic second compound has a standard electrode potential less than 0.3 volts relative to a standard hydrogen electrode.

5. The flow battery of claim 1 wherein R 4 , R 5 , R 6 , R 7 are each independently H, —NO 2 , —CF 3 , or —SO 3 H.

6. The flow battery of claim 1 wherein the first organic compound has formula 5 or 6:

wherein R 4 , R 5 , R 6 , R 7 are each independently selected from the group consisting of hydrogen and electron withdrawing groups.

7. The flow battery of claim 1 wherein the first organic compound includes a component selected from the group consisting of:

and salts thereof.

8. The flow battery of claim 1 wherein at least one of R 8 -R 15 is C 1-10 alkyl, NH 2 , —NHR 2 , —N(R 2 ) 2 , —O-M + , —NHCOR 2 , —OR 2 , —CH 3 , —C 2 H 5 , or phenyl where R 2 is H or C 1-10 alkyl and M + is a positively charged counter ion.

9. The flow battery of claim 1 wherein at least one of R 8 -R 15 is —SO 3 H, —PO 3 H 2 , —COOH, —OH, —O − M + , —SO 3 − M + , —PO 3 2− M + 2 , or −COO − M + , where M + is a positively charged counter ion.

10. The flow battery of claim 1 wherein the second organic compound includes a component selected from the group consisting of:

and salts thereof.

11. The flow battery of claim 1 wherein the first organic compound has formula 6:

wherein R 4 , R 5 , R 6 , R 7 are each independently selected from the group consisting of hydrogen and electron withdrawing groups.

12. The flow battery of claim 1 wherein the first organic compound includes a component selected from the group consisting of:

and salts thereof.

13. The flow battery of claim 1 wherein at least one of R 8 -R 15 is hydroxyl.

14. The flow battery of claim 1 wherein at least one of R 8 -R 15 is NH 2 , —NHR 2 , or N(R 2 ) 2 , where R 2 is H or C 1-10 alkyl.

15. The flow battery of claim 1 wherein at least one of R 8 -R 15 is pyridinyl, imidazoyl, or pyrroyl.