Method for preventing outgassing

Abstract

A method of reducing outgassing in a supercapacitor comprised of carbon-containing electrodes and at least one ionic liquid is characterised by the steps of (a) contacting the carbon-containing electrodes with a tetrafluoroborate salt; (b) applying a potential difference across the carbon-containing electrodes whilst in contact with the salt in a cycle during which electrical charge is stored on and discharged from the electrodes; and (c) continuing further cycles of step (b) until such time as substantially no further outgassing from the system occurs.

Claims

1. A method of reducing outgassing in a supercapacitor comprised of carbon-containing electrodes and at least one ionic liquid that is an organic salt which is molten below 100 C., the method characterised by the steps of (a) treating the carbon-containing electrodes with a tetrafluoroborate salt; (b) applying a potential difference across the carbon-containing electrodes whilst in the presence of the salt in a cycle during which electrical charge is stored on and discharged from the electrodes; and (c) continuing cycles of step (b) until such time as substantially no further outgassing from the system occurs.

2. A method as claimed in claim 1 characterised in that the carbon-containing electrodes include graphene, carbon nanotubes or a mixture thereof.

3. A method as claimed in claim 1 characterised in that the ionic liquid is also a tetrafluoroborate salt.

4. A method as claimed in claim 1 characterised in that the ionic liquid is at least one C.sub.1 to C.sub.4 alkyl substituted imidazolium, piperidinium or pyrrolidinium salt.

5. A method as claimed in claim 1 characterised in that the tetrafluoroborate salt is EMIM tetrafluoroborate salt.

6. A method as claimed in claim 1 characterised in that supercapacitor comprises a plastic pouch containing the carbon-containing electrodes and the ionic liquid(s).

7. A method of reducing outgassing in a supercapacitor comprised of carbon-containing electrodes and at least one ionic liquid that is an organic salt which is molten below 100 C., the method characterised by the steps of (a) treating the carbon-containing electrodes with an ionic liquid tetrafluoroborate salt that is a quaternary salt of an alkyl or substituted-alkyl pyridinium, pyridazinium, pyrimidinium, pyrazinium, imidazolium, piperidinium, pyrrolidinium, pyrazolium, thiazolium, oxazolium, triazolium or azepanium cation and a tetrafluoroborate anion; (b) applying a potential difference across the carbon-containing electrodes whilst in the presence of the salt in a cycle during which electrical charge is stored on and discharged from the electrodes, wherein the charging and discharging parts of the cycle are performed at different temperatures; and (c) continuing cycles of step (b) at least 10 to 60 times until such time as substantially no further outgassing from the system occurs.

8. A method as claimed in claim 7 characterised in that the carbon-containing electrodes include graphene, carbon nanotubes or a mixture thereof.

9. A method as claimed in claim 7 characterised in that the ionic liquid is also a tetrafluoroborate salt.

10. A method as claimed in claim 7 characterised in that the ionic liquid is at least one C1 to C4 alkyl substituted imidazolium, piperidinium or pyrrolidinium salt.

11. A method as claimed in claim 7 characterised in that the ionic liquid tetrafluoroborate salt is EMIM tetrafluoroborate salt.

12. A method as claimed in claim 7 characterised in that supercapacitor comprises a plastic pouch containing the carbon-containing electrodes and the ionic liquid(s).

Description

EXAMPLE 1

(1) A square polymer pouch was manufactured by heat-sealing two square flexible sheets of polyethene along three of the four corresponding sides. Thereafter a pair of anode and cathode electrodes were introduced into the pouches along with an intermediate polyethene and a salt comprising EMIM tetrafluoroborate. Each electrode comprised an aluminium foil current collector on which was disposed an electrode layer consisting of a mixture of carbon nanotubes, graphene, activated carbon (85% by total weight), conductive carbon (5%) embedded in 10% by weight of a matrix comprising styrene-butadiene rubber and carboxymethyl cellulose (50:50). The pouch was then temporarily sealed and subjected to ten charge-discharge cycles from 0 to 3.5 v at a current of 4 amps. At the end of this time the pouch was opened, re-sealed and the electrodes subjected to a further 200 cycles. No expansion of the pouches corresponding to significant outgassing was observed in this subsequent period of cycling.

(2) Comparative Test

(3) In a comparative experiment the pouches were sealed immediately after the components had been introduced. Thereafter after 50 charge-discharge cycles at the conditions mentioned above and without opening and resealing so much expansion of the pouch due to outgassing had occurred as to make it essentially unfit for use.

EXAMPLE 2

(4) The method of Example 1 was repeated except that after the first ten cycles the tetrafluoroborate salt was replaced with the ionic liquid EMIM TFSI before re-sealing took place. Again no significant inflation was seen to occur after a further 200 cycles.