Aqueous pyridinium cation-based redox flow batteries

1. An aqueous redox flow battery comprising a negative electrode immersed in an aqueous liquid negative electrolyte, a positive electrode immersed in an aqueous liquid positive electrolyte, and a cation-permeable separator between the negative and positive electrolytes;

the negative electrode being positioned within a negative electrolyte chamber (“NE chamber”) defined by a first housing and containing the negative electrolyte, the NE chamber connecting with a first negative electrolyte reservoir (“NE reservoir”) and a second NE reservoir such that the first NE reservoir, the NE chamber, and the second NE reservoir can be placed in fluid-flow communication and collectively define a negative electrolyte circulation pathway; a first pump being operably positioned within the negative electrolyte circulation pathway to circulate the negative electrolyte back and forth between the first NE reservoir and the second NE reservoir over the negative electrode;

the positive electrode being positioned within a positive electrolyte chamber (“PE chamber”) defined by a second housing and containing the positive electrolyte, the PE chamber connecting with a first positive electrolyte reservoir (“PE reservoir”) and a second PE reservoir such that the first PE reservoir, the PE chamber, and the second PE reservoir can be placed in fluid-flow communication and collectively define a positive electrolyte circulation pathway; a second pump being positioned within the positive electrolyte circulation pathway to circulate the positive electrolyte back and forth between the first PE reservoir and the second PE reservoir over the positive electrode;

the negative and positive electrolytes each independently comprising an electrolyte salt, a redox reactant, and an aqueous solvent; and

the NE chamber and the PE chamber being separated from one another by the cation-permeable separator, such that cations from the electrolyte salt can flow back and forth between the NE chamber and the PE chamber to balance charges resulting from oxidation and reduction of the redox reactants during charging and discharging of the battery;

wherein the redox reactant of the positive electrolyte has a higher redox potential than the redox reactant of the negative electrolyte; and wherein the redox reactant of the negative electrolyte comprises a cation of Formula (IV):

wherein:

R 1 is selected from alkyl, substituted-alkyl, cycloalkyl, substituted-cycloalkyl, heterocycloalkyl, substituted-heterocycloalkyl, aryl, substituted-aryl, arylalkyl, substituted-arylalkyl, heteroaryl, and substituted-heteroaryl;

R 2 is selected from H, —C(═Y)-E, alkyl, substituted-alkyl, cycloalkyl, substituted-cycloalkyl, heterocycloalkyl, substituted-heterocycloalkyl, aryl, substituted-aryl, arylalkyl, substituted-arylalkyl, heteroaryl, and substituted-heteroaryl;

each Y independently is O or S;

E is selected from alkyl, substituted-alkyl, cycloalkyl, substituted-cycloalkyl, heterocycloalkyl, substituted-heterocycloalkyl, aryl, substituted-aryl, heteroaryl, and substituted-heteroaryl; and

Ar 1 is an aryl group or substituted aryl group.

2. The aqueous redox flow battery of claim 1 , wherein the redox reactant of the positive electrolyte comprises 2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO); 4-hydroxy-2,2,6,6-tetramethylpiperidin-1-yl)oxyl(4-hydroxy-TEMPO); or 4-methoxy-2,2,6,6-tetramethylpiperidin-1-yl)oxyl(4-methoxy-TEMPO).

3. The aqueous redox flow battery of claim 1 , wherein the cations of the electrolyte salts are selected from Li + and Na + .

4. The aqueous redox flow battery of claim 3 , wherein the anions of the electrolyte salts are selected from the group consisting of BF 4 − , PF 6 − , ClO 4 − , AsF 6 − , CF 3 SO 3 − , N(SO 2 CF 3 ) 2 − , N(SO 2 CF 2 CF 3 ) 2− , B(C 2 O 4 ) 2− , and B 12 X n H (12−n) 2− , wherein X=halogen.

5. The aqueous redox flow battery of claim 1 , wherein one or more of the substituted-cycloalkyl, substituted-heterocycloalkyl, substituted-aryl, substituted-arylalkyl, and substituted-heteroaryl groups of R 1 , R 2 or both R 1 and R 2 bears one or more substituent selected from alkyl, halogen, nitro, amino, alkoxy, aryloxy, amido, hydroxyl, sulfonyl, carboxylic acid, carboxyalkyl, and carbonylalkyl.

6. The aqueous redox flow battery of claim 1 , wherein the substituted-alkyl group of R 1 , R 2 or both R 1 and R 2 bears one or more substituent selected from halogen, nitro, amino, alkoxy, aryloxy, amido, hydroxyl, sulfonyl, carboxylic acid, carboxyalkyl, and carbonylalkyl.