Flexible energy storage device with redox-active polymer hydrogel electrolyte

1. A flexible energy storage device, comprising:

a pair of electrodes separated by a redox-active polymer hydrogel electrolyte, the redox-active polymer hydrogel electrolyte comprising:

a polymer hydrogel,

redox-active transition metal cations which are at least one selected from the group consisting of vanadium, chromium, manganese, cobalt, and copper, and

charge balancing anions;

wherein the flexible energy storage device retains greater than 75% of an unbent energy storage capacity when bent at an angle of 10 to 170°,

wherein the flexible energy storage device is in the form of a supercapacitor having a specific capacitance of 300 to 380 F/g.

2. The flexible energy storage device of claim 1 , wherein the polymer hydrogel is a polyacrylic acid hydrogel.

3. The flexible energy storage device of claim 2 , wherein polyacrylic acid is present in the polyacrylic acid hydrogel in an amount of 2.5 to 50 wt. %, based on a total weight of polyacrylic acid hydrogel.

4. The flexible energy storage device of claim 1 , wherein the redox-active transition metal cations are cobalt (II) ions.

5. The flexible energy storage device of claim 1 , wherein the redox-active transition metal ions are present in an amount of 0.1 to 15 wt. %, based on a total weight of the redox-active polymer hydrogel electrolyte.

6. The flexible energy storage device of claim 1 , wherein the redox-active polymer hydrogel electrolyte is substantially free of iron.

7. The flexible energy storage device of claim 1 , wherein the charge balancing anions are selected from the group consisting of hydroxide anions, halide anions, sulfate anions, nitrate anions, perchlorate anions, tetrafluoroborate anions, difluoro(oxalato)borate anions, hexafluorophosphate anions, and bis(trifluoromethanesulfonyl)/imide anions.

8. The flexible energy storage device of claim 1 , wherein the charge balancing anions are sulfate anions.

9. The flexible energy storage device of claim 1 , wherein the charge balancing anions are present in an amount of 0.1 to 15 wt. %, based on a total weight of redox-active polymer hydrogel electrolyte.

10. The flexible energy storage device of claim 1 , wherein the redox-active polymer hydrogel electrolyte has an ionic conductivity of 1.00×10 −5 to 1.00×10 −3 Scm −1 .

11. The flexible energy storage device of claim 1 , wherein one or both of the electrodes are carbon electrodes.

12. A flexible energy storage device, comprising:

a pair of electrodes separated b a redox-active polymer hydrogel electrolyte, the redox-active polymer hydrogel electrolyte comprising:

a polymer hydrogel,

redox-active transition metal cations which are at least one selected from the group consisting of vanadium, chromium, manganese, cobalt, and copper, and

charge balancing anions;

wherein the flexible energy storage device retains greater than 75% of an unbent energy storage capacity when bent at an angle of 10 to 170°, and

wherein the flexible energy storage device is in the form of a supercapacitor having an energy density of 17 to 25 Wh kg −1 at a power density of 100 to 125 W kg −1 .

13. A method of forming a flexible energy storage device, the method comprising:

soaking a polymer in a solution comprising redox-active transition metal cations, charge balancing anions, and water for 1 to 8 hours to form a redox-active polymer hydrogel electrolyte;

disposing the redox-active polymer hydrogel electrolyte on a first electrode of a pair of electrodes such that the redox-active polymer hydrogel electrolyte forms a uniform film covering an entirety of a top surface of the first electrode; and

placing onto the redox-active polymer hydrogel electrolyte a second electrode of the pair of electrodes such that the second electrode is separated from the first electrode by the redox-active polymer hydrogel electrolyte to form the flexible energy storage device,

wherein the flexible energy storage device comprises:

the pair of electrodes separated by the redox-active polymer hydrogel electrolyte, the redox-active polymer hydrogel electrolyte comprising:

the polymer hydrogel,

the redox-active transition metal cations which are at least one selected from the group consisting of vanadium, chromium, manganese, cobalt, and copper, and

the charge balancing anions;

wherein the flexible energy storage device retains greater than 75% of an unbent energy storage capacity when bent at an angle of 10 to 170°.

14. The method of claim 13 , wherein the first electrode, the second electrode, or both, are carbon electrodes prepared by uniformly depositing on a metal support a film comprising activated carbon, conductive carbon, and a binder such that an entirety of a top surface of the metal support is individually covered by the film to form a carbon electrode.

15. A redox-active polymer hydrogel electrolyte, comprising:

70 to 99.8 wt. % a polymer hydrogel;

1 to 15 wt. % redox-active transition r petal ions which are at least one selected from the group consisting of vanadium, chromium, manganese, cobalt, and copper; and

1 to 15 wt. % charge-balancing anions which are at least one selected from the group consisting of hydroxide anions, halide anions, sulfate anions, nitrate anions, perchlorate anions, tetrafluoroborate anions, difluoro(oxalato)borate anions, hexafluorophosphate anions, and bis(trifluoromethanesulfonyl)imide anions.

16. The redox-active polymer hydrogel electrolyte of claim 15 , wherein the polymer hydrogel is a polyacrylic acid hydrogel comprising 2.5 to 50 wt. % polyacrylic acid, based on a total weight of polymer hydrogel.

17. The redox-active polymer hydrogel electrolyte of claim 15 , wherein the redox-active transition metal ions are cobalt (II) ions and the charge-balancing anions are sulfate anions.

18. The redox-active polymer hydrogel electrolyte of claim 15 , which is substantially free of iron.