ION EXCHANGE MEMBRANE AND METHOD OF MANUFACTURING AN ION EXCHANGE MEMBRANE

Abstract

This invention relates to an an-ion exchange membrane and method for making said membrane. The membrane being intended for use in electrolysers or other AEM electrochemical devices. The membrane comprises: a thermoplastic elastomer (TPE) comprising styrene, said TPE being a polymeric backbone, wherein: the styrene content of the thermoplastic elastomer is between 30 wt % and 70 wt %, and crosslinking of a first polymeric backbone to one or more other polymeric backbones, and one or more cationic groups, and the functionalisation degree is between 1% and 50%.

Claims

1. A method of manufacturing an an-ion exchange membrane, the method comprising: purifying a thermoplastic elastomer (TPE) comprising an aromatic ring, halomethylating the purified TPE, and casting of the membrane, aminating the purified and halomethylated TPE with at least a first amine and a second amine, the amines being any two or more of: a monoamine, a diamine, and a polyamine, and preparing the resultant membrane for use or storage.

2. A method according to claim 1, wherein the TPE is a polymeric backbone, and the method includes cross-linking of a polymeric backbone to one or more other polymeric backbones and/or a side chain of a said polymeric backbone.

3. A method according to claim 2, wherein said cross-linking occurs during, or immediately after, said casting step.

4. A method according to claim 2 or claim 3, wherein said cross-linking occurs during said amination step.

5. A method according to any of claims 1 to 4, wherein the first and second amines are TMHDA and TMA respectively.

6. A method according to any of claims 1 to 5, wherein the first and second amines are any two of: N-methylimidazole, N-methylpiperidine, N-Methylpyrrolidine, Triethanolamine DABCO, TMEDA, TMHDA, and TMA.

7. A method according to any of the preceding claims, wherein halomethylation involves: dissolving the purified TPE, mixing the dissolved TPE with any of trioxane, trimethylsilyl chloride and SnCl.sub.4, placing the reactants in a reflux condenser, and heating from 0° C. to 50° C. for an extended period between 3 hours and 6 days.

8. A method according to any of the preceding claims, wherein the membrane is cast with means adapted to control the rate of evaporation of the solvent.

9. A method according to any of the preceding claims, wherein the membrane is cast by heating and extruding then purified chloromethylated polymer or by roll to roll.

10. A method according to any of the preceding claims, wherein one or more of the amines selected have a carbon chain of three or more.

11. A method according to claim 2, wherein a ratio of said first amine to said second amine is predetermined to determine cross-linking.

12. A method according to any of the preceding claims, wherein any one of, or combination of, the following fillers are present: Al.sub.2O.sub.3, SnO.sub.2, Cu phthalocyanine, Vulcan, and montmorillonite.

13. A method according to any of the preceding claims, wherein a, or each of two or more steps, is undertaken in an environment controlled for any of the following: light, cleanliness, humidity, and inert atmosphere.

14. An an-ion exchange membrane manufactured by the method according to any of the preceding claims.

15. An an-ion exchange membrane according to claim 14, comprising one or more cationic groups wherein the functionalisation degree is between 1% and 50%.

16. An an-ion exchange membrane comprising a thermoplastic elastomer (TPE) comprising an aromatic ring, said TPE being a polymeric backbone, wherein the styrene content of the TPE is between 30 wt % and 70 wt %, and wherein a first polymeric backbone is cross-linked to one or more polymeric backbones and/or side chain(s) of a polymeric backbone, the an-ion exchange membrane further comprising one or more cationic groups, wherein the functionalisation degree is between 1% and 50%.

17. An an-ion exchange membrane according to claim 15 or claim 16, wherein the functionalisation degree is in the range 3% to 35%.

18. An an-ion exchange membrane according to any of claims 15 to 17, wherein the one or more cationic groups comprise nitrogen, phosphorous, sulphur and/or a metal ion.

19. An an-ion exchange membrane manufactured by a method of claim 2 or claim 11, wherein a first polymeric backbone is cross-linked to a side chain including a cationic group.

20. An an-ion exchange membrane according to any of claims 14 to 19, wherein the styrene content of the TPE is in the range 35 wt % to 55 wt %.

21. An an-ion exchange membrane according to any of claims 14 to 20, having a thickness between 10 and 100 nm.

22. An an-ion exchange membrane according to any of claims 14 to 21, utilised in a membrane electrode assembly in any one of: electrolyser, fuel cell, electrochemical compressor, and electroosmotic device.

23. A membrane electrode assembly including an an-ion exchange membrane according to any of claims 14 to 21.

Description

[0126] To help understanding of the invention, a specific embodiment thereof will now be described by way of example and with reference to the accompanying drawings, in which:

[0127] FIG. 1 depicts the phase volume ratio of styrene/butadiene block copolymers, and its morphology;

[0128] FIG. 2a-c shows SEM micrographs of membranes cast in accordance with the present invention;

[0129] FIG. 3 depicts the crosslinking resultant from the utilisation of Lewis Acids; and

[0130] FIG. 4a-d shows graphs demonstrating the properties of a membrane manufactured in accordance with the present invention.

[0131] Referring to FIG. 1, there are a plurality of images, both micrography and diagrams, showing the structure of commercially available styrene butadiene products. From 1A to 1G there is an increasing wt % of styrene and a decreasing amount of butadiene. Styrene is depicted as black, and butadiene as white.

[0132] The morphology seen in 1A, depicted in another format in 2A, shows no styrene present. In 1B and 2B, with less than 15 wt % styrene shows microchannels in the butadiene. Between 15 wt % and 35 wt % styrene content, 1C, 2C the number of channels is increased such that it may be considered bicontinuous.

[0133] Unexpectedly, in the range of 35 wt %-65 wt % styrene, 1D and 2D, the structure changes from microchannels 3 of styrene to a lamellar structure with alternating layers of styrene 4 and butadiene 5. The lamellar structure allows for improved properties such as mechanical strength, conductivity, reduced hydrogen crossover, etc.

[0134] Once the amount of styrene increases to 65 wt %-85 wt %, 1E and 2E, the morphology substantially mirrors 1C and 2C with channels of butadiene present in the styrene. Above 85 wt % 1F, 2F, the morphology substantially mirrors 1B 2B, with butadiene channels being present in a majority styrene structure.

[0135] The properties of the compounds depicted in FIG. 1 vary dependent upon composition. With no styrene, 1A, the properties are rubber. Increasing up to 35 wt % styrene, it exhibits thermoplastic elastomeric properties. In the preferred range, 1D, it is a plastomer, exhibiting both elastomeric and plastic properties. Above 65% the compound becomes more brittle.

[0136] Key stages of a method of manufacturing an an-ion exchange membrane in accordance with a specific and exemplary embodiment of the present invention will now be outlined. Whilst specific compounds are named, the alternatives outlined above may be used. The steps follow, with headings added depicting each section.

Purification of SEBS

[0137] a) 10 g of SEBS, 30 wt %-70 wt % styrene, is dissolved in 200 ml chloroform at room temperature.
b) Pouring the dissolved SEBS from step 1, slowly, into 300 ml Methanol.
c) Filtering to obtain the precipitate.—steps a to c may be repeated to obtain radical inhibitor free purified SEBS.
d) Dry the precipitate above room temperature for a sufficient period of time to ensure the solvent is removed.
e) If not being used right away, the dried purified SEBS should be stored adequately.

Chloromethylation

[0138] f) Approximately 10 g of purified SEBS is dissolved in 500 ml chloroform in a flask at room temperature.
g) Add to the mixture 4 g trioxane 30 ml trimethylsilyl chloride and 3 ml SnCl.sub.4, affix a water cooled reflux condenser, and place the flask in an oil bath for heating to 50° C. for 48 hours.
h) Stop the reaction by adding 300 ml of 50:50 (vol) water and methanol from the top of the condenser.
i) The multiphase solution can be separated using a separation funnel—the chloroform phase being added, slowly, into 500 ml methanol to precipitate the chloromethylated SEBS. Filter the precipitate, and dry it.
j) Purification of the precipitated chloromethylated SEBS by dissolving it in minimal chloroform required at room temperature, and slowly adding to methanol to precipitate.
k) Filter to obtain the purified chloromethylated SEBS.
l) Dry the purified chloromethylated SEBS precipitate above room temperature for a sufficient period of time to ensure the solvent is removed.
m) If not being used right away, the dried purified chloromethylated SEBS should be stored appropriately.

Casting

[0139] n) The dried, purified, chloromethylated SEBS is dissolved in chloroform, or other organic solvent, and filtered to remove particles.
o) Casting of the membrane is done in a clean environment, into a petri dish with means to ensure slow evaporation of the solvent, such as a class bell jar with small opening.
p) Detaching the cast membrane from the petri dish after the solvent has evaporated, carefully to minimise the risk of contamination.

Amination

[0140] q) The cast membrane is soaked in a solution of TMHDA:TMA (in a ratio of 10:90) for 48-72 hours at 60° C. with a reflux condenser. The amination reaction occurs in heterogeneous phases.
r) Extract the membrane from the amination bath, and rinse.
s) Preparation of the membrane for storage, or use.

[0141] FIG. 2a shows an SEM micrograph of a membrane cast in accordance with the present invention. The membrane 30a can be seen to be substantially smooth with no major defects. This membrane sample differs to the sample shown in FIGS. 2b and 2c, wherein the membrane was cast onto an unpolished surface.

[0142] FIG. 2b depicts a second membrane, 30b, with FIG. 2c being a close up of the cross section of membrane 30b. Ridges 31 can be seen, and are attributable to how smooth the surface upon which the membrane was cast. In FIG. 2c, the cross section of membrane 30b is more easily viewed. It can be seen that the membrane has lamellar planes 32, the planes being sustainably perpendicular to the orientation of the membrane.

[0143] FIG. 3 depicts the reaction occurring during the Lewis acid crosslinking. Either AlCl.sub.3, SnCl.sub.4 or another Lewis acid may be used. The crosslinking process occurs on the styrene blocks of the polymer and the halomethylene group react in an alkylation reaction (Friedel-Craft), with the Lewis acid as a catalyst, and a methylene bridge between the two aromatic rings of the polymer.

[0144] FIGS. 4a and b depict graphs demonstrating the properties of a membrane made in accordance with the present invention comprising SEBS with 30% styrene using a ratio of monoamine and diamine.

[0145] In FIG. 4a the conductivity and water uptake are shown as the ratio of diamine mol % increases. The conductivity falls significantly between 0% and 10% diamine from 9 mS/cm to 6.5 mS/cm. As the diamine ratio increases to 70% there is a sustained decrease to approximately 1.7 mS/cm. Between 70% and 90% diamine, the conductivity remains substantially, falling to 1.5 mS/cm.

[0146] The water uptake sees a similar pattern inversed for the 30% SEBS sample. Starting at approximately 68% uptake (wt) at 0% amine, there is little difference when the ratio of diamine increases to 10%. Between 10% to 70% diamine the water uptake falls to 40%. The final slope of the graph is most significant, with approximately 30% water uptake when 90% diamine is used.

[0147] FIG. 4b shows the ion exchange capacity (IEC) of the same membrane. Interestingly, the IEC remains substantially constant between 1.1 and 1.2 mEq/g, independent of the ratio of diamine.

[0148] FIGS. 4c and 4d show the same graphs as 4a and 4b respectively, but for a membrane made in accordance with the present invention using SEBS with a styrene content of 67%. In FIG. 4c it is shown that the conductivity falls from 11 mS/cm to 10.5 mS/cm as the diamine increases from 0% to 5%. The slope remains gentle decreasing to approximately 10 mS/cm at 40% diamine. This differs to the water uptake, which is far higher in this embodiment falling from 157% to approximately 148% as the diamine % increased from 0% to 5%. The water uptake drops drastically as the diamine % is increased to 40%, falling to 60% water uptake.

[0149] FIG. 4d shows the IEC of the membrane comprising 67% SEBS. As the diamine % increases from 0% to 40%, the IEC remains substantially the same around 1.5 mEq/g.

[0150] The IEC is noted as being slightly higher than the embodiment with 30% SEBS. However, the IEC remains substantially constant for both embodiments independent of the ratio used.

[0151] The difference is more pronounced when comparing the conductivity and water uptake. The drop in conductivity is greater in the sample with 30% SEBS, falling to under 4 mS/cm at a comparable 40% diamine, whereas the sample with 67% SEBS remains at above 10 mS/cm. Conversely, the water uptake is generally much higher in the sample with more styrene in the SEBS.

[0152] Different applications for a membrane require different properties to be optimised for, these include conductivity, cross-over and mechanical strength, amongst others. Water uptake is linked to mechanical strength, and this parameter varied greatest.

[0153] The invention is not intended to be restricted to the details of the above described embodiments, and it will be apparent to a person skilled in the art, from the foregoing description, that modifications and variations can be made to the described embodiments without departing from the scope of the invention as defined by the appended claims. For instance, different parameters may be altered to improve different characteristics, such as conductivity, cross-over and mechanical strength.

[0154] The above invention is not intended to be limited to a specific type of electrochemical device. In fact, the present invention may be used with any device or process which may require an AEM.

[0155] The invention is not intended to be limited to monoamine, diamines and triamines, similar results may be achieved by using other polyamines.

[0156] Halomethylation to achieve a halomethyl group on the aromatic ring may be achieved by alternate means, such as, but not limited to, polyphenylene oxide. Such routes are not intended to be excluded by the present invention.

[0157] Amination is used to refer to the stage of introducing cationic groups. Amination as a term is not intended to exclude other suitable compounds.

[0158] Whilst polymer is used extensively in this document, its use is intended to include oligomers.

[0159] Various stages require heating, the present invention is not intended to be limited to the use of an oil bath, as any suitable heating means may be used.

[0160] As used herein, AEM is intended to refer to anion exchange membranes and an AEM electrolyser being an electrolyser utilising an AEM. Conversely, PEM refers to proton exchange membrane, and a PEM electrolyser is an electrolyser with a PEM.