Supercapacitor and a method for expanding the voltage range of an aqueous electrolyte suprcapacitor

1. A supercapacitor comprising an electrolyte, said electrolyte comprising a mixture of water, one or more non-aqueous solvent and a salt forming a mixture of an aqueous solution and a non-aqueous solution; wherein

said non-aqueous solution has a larger electrochemical stability window than said aqueous solution;

said salt has a solubility above 0.1M in water and/or in said one or more non-aqueous solvent between −25 and 100° C.;

said mixture has a buffer capacity □ lower than 10 −5 ;

hydron (H + ) and hydroxide ion (OH − ) solubility is lower in said non-aqueous solution than in said aqueous solution;

wherein the ratio vol/vol between said aqueous solution and said non-aqueous solution is in the range between 100:1 and 1:1;

said supercapacitor further comprising two electrodes and wherein said electrolyte is located between said two electrodes, wherein said two electrodes have a higher affinity towards said non-aqueous solution than towards said aqueous solution.

2. A supercapacitor according to claim 1 , wherein said electrolyte is or comprises a gel electrolyte.

3. A supercapacitor according to claim 1 , wherein said one or more non-aqueous solvent comprises a polar organic solvent.

4. A supercapacitor according to claim 1 , wherein said salt comprises cations selected from the group comprising alkali metal or alkaline metal earth, such as Li + , Na + , K + , Be 2+ , Mg 2+ , Ca 2+ , Sr + , or NH 4 + and anions selected from the group comprising F − , Cl − , Br − , I − , NO 3 − , ClO4 − , HSO 4 + , PF 6 + , BF 4 − and SO 4 2− .

5. A supercapacitor according to claim 1 wherein said two electrodes are carbon based electrodes.

6. A supercapacitor according to claim 5 , wherein said carbon based electrodes comprise carbon based materials selected from carbon particles, nanotubes, aerogel or fibres, graphene, carbon black, activated carbon or carbides.

7. A supercapacitor according to claim 1 , wherein said two electrodes are in contact with said non-aqueous solution.

8. A supercapacitor according to claim 7 , wherein said two electrodes in contact with said non-aqueous solution are separated by said aqueous solution.

9. A method of increasing the voltage range of an aqueous electrolyte for use in a supercapacitor according to claim 1 , said method comprising:

producing an electrolyte by mixing water, one or more non-aqueous solvent and a salt forming a mixture of an aqueous solution and a non-aqueous solution.

10. A method according to claim 9 , wherein

said non-aqueous solution has a larger electrochemical stability window than said aqueous solution;

said salt has a solubility above 0.1M in water and/or in said one or more non-aqueous solvent between −25 and 100° C.;

said mixture has a buffer capacity □ lower than 10 −5 ;

hydron (H + ) and hydroxide ion (OH − ) solubility is lower in said non-aqueous solution than in said aqueous solution.

11. A carbon based electrode for use in a supercapacitor according to claim 1 , wherein said carbon electrode is characterized by having a higher affinity towards said non-aqueous solution than towards said aqueous solution.

12. A method of producing said carbon based electrode according to claim 11 , the method comprising:

providing a carbon precursor, such as a structured source of cellulose or lignin;

mechanically processing said structured source of cellulose or lignin;

thermally processing said structured source of cellulose or lignin by exposing said structured source of cellulose or lignin to heat up to 300° C. for a desired period of time in inert atmosphere, thereby forming char;

mechanically and chemically treating said char;

activating said char by exposing said char to potassium hydroxide in presence of to heat up to 900° C. for a desired period of time in inert atmosphere, thereby forming activated carbon;

cooling said activated carbon to room temperature;

treating said activated carbon with water; and

drying said activated carbon.

13. A Polymer Electrolyte Membrane (PEM) fuel cell stack comprising a supercapacitor according to claim 1 .

14. A supercapacitor according to claim 3 , wherein the polar organic solvent comprises propylene carbonate.

15. A supercapacitor according to claim 7 , wherein said two electrodes are surrounded by said non-aqueous solution.