WO2025037119A1

WO2025037119A1 - A proton-exchange membrane

Info

Publication number: WO2025037119A1
Application number: PCT/GB2024/052160
Authority: WO
Inventors: Denis Duchesne; Melissa Hoang Lan NOVY
Original assignee: Johnson Matthey Hydrogen Technologies Ltd
Current assignee: Johnson Matthey Hydrogen Technologies Ltd
Priority date: 2023-08-17
Filing date: 2024-08-16
Publication date: 2025-02-20
Anticipated expiration: 2026-02-17

Abstract

The present invention provides a proton-exchange membrane comprising a blend of first and second ionomers, the first ionomer comprising a first main chain covalently bonded to a first side chain and the second ionomer comprising a second main chain covalently bonded to a second side chain; wherein each of the first and second side chains comprise a sulfonic acid end group; wherein a relaxation modulus of a membrane formed from the first ionomer is at least 10 times less than a relaxation modulus of a membrane formed from the second ionomer, preferably at least 100 times less; and wherein the relaxation modulus of the membrane formed from the second ionomer is greater than 10,000 MPa.

Description

A proton-exchange membrane

Field of the invention

The present invention relates to a proton-exchange membrane (PEM) and a method for the manufacture thereof. In particular, the present invention relates to a PEM comprising a blend of first and second ionomers. The PEM is particularly suitable for use in electrochemical devices such as fuel cells and/or water electrolysers.

Background of the invention

A fuel cell is an electrochemical cell comprising two electrodes separated by an electrolyte. A fuel, e.g. hydrogen, an alcohol such as methanol or ethanol, or formic acid, is supplied to the anode and an oxidant, e.g. oxygen or air, is supplied to the cathode. Electrochemical reactions occur at the electrodes, and the chemical energy of the fuel and the oxidant is converted to electrical energy and heat. Electrocatalysts are used to promote the electrochemical oxidation of the fuel at the anode and the electrochemical reduction of oxygen at the cathode.

Fuel cells are usually classified according to the nature of the electrolyte employed. Often the electrolyte is a solid polymeric membrane, in which the membrane is electronically insulating but ionically conducting. In proton-exchange membrane fuel cells (PEMFCs) the membrane is proton conducting, and protons, produced at the anode, are transported across the membrane to the cathode, where they combine with oxygen to form water.

An electrolyser is an electrochemical device for electrolysing water to produce high purity hydrogen and oxygen. Electrolysers can operate in both alkaline and acidic systems. Those electrolysers that employ a PEM, are known as proton-exchange membrane water electrolysers (PEMWEs).

Conventional proton-conducting membranes used in PEMFCs or PEMWEs are generally formed from sulfonated fully-fluorinated polymeric materials (often generically referred to as perfluorinated sulphonic acid (PFSA) ionomers). As an alternative to PFSA type ionomers, it is possible to use proton-conducting membranes based on partially-fluorinated or non-fluorinated hydrocarbon polymers.

An article published by Sigma-Aldrich® on their webpage titled “Perfluorosulfonic Acid Membranes for Fuel Cell and Electrolyser Applications” (available at https://www.sigmaaldrich.com/GB/en/technical- documents/technical-article/materials-science-and-engineering/batteries-supercapacitors-and-fuel- cells/perfluorosulfonic-acid-membranes) provides an overview of PFSA ionomer types, their composition, structure and synthesis routes, as well as membrane properties and durability. Examples of such ionomers include those sold under the trade names Nation® (E.l. DuPont de Nemours and Co.), Aciplex® (Asahi Kasei), Flemion® (Asahi Glass Company), Aquivion® (Solvay Speciality Polymers), and 3M™ (3M Corporation).

Chemical Engineering Journal Advances, 2022, 12, 100372 “Advances in perfluorosulfonic acidbased proton exchange membranes for fuel cell applications: A review” provides a review of perfluorosulfonic acid-based membranes for fuel cell applications, and in particular the development of polymer composite membranes incorporating various multifunctional organic, inorganic, and hybrid fillers.

Chemical Engineering Science, 2023, 280, 119051 “Manufacturing defects in slot die coated polymer electrolyte membrane for fuel cell application” identifies degradation of PEM as being among the major hurdles for the commercialisation of PEM fuel cells identifying that manufacturing defects can significantly contribute to membrane degradation in a fuel cell environment as the initial source of degradation during use. Membranes discussed therein were cast from Nation® D-2021 onto polyethylene terephthalate (PET) film using roll-to-roll manufacturing techniques in which the quality of the cast film is disclosed as being affected by the thickness of the coating, the flow rate of the solution, width of the slot die and substrate speed.

Despite these developments, there remains a need in the art for higher quality proton-exchange membranes with fewer manufacturing defects without compromising the proton-exchange properties in order to then improve the effectiveness and durability/lifetime of electrolytic devices incorporating such PEMs.

It is therefore an object of the present invention to provide a proton-exchange membrane suitable for use in electrochemical devices such as PEMWEs and PEMFCs with improved robustness and stability over known membranes, together with a method of manufacture thereof, or at least to provide a commercially viable alternative thereto.

Description of the invention

Accordingly, a first aspect of the present invention provides a proton-exchange membrane comprising a blend of first and second ionomers, the first ionomer comprising a first main chain covalently bonded to a first side chain and the second ionomer comprising a second main chain covalently bonded to a second side chain; wherein each of the first and second side chains comprise a sulfonic acid end group; wherein a relaxation modulus of a membrane formed from the first ionomer is at least 10 times less than a relaxation modulus of a membrane formed from the second ionomer, preferably at least 100 times less; and wherein the relaxation modulus of the membrane formed from the second ionomer is greater than 10,000 MPa.

The present disclosure will now be described further. In the following passages, different aspects/embodiments of the disclosure are defined in more detail. Each aspect/embodiment so defined may be combined with any other aspect/embodiment or aspects/embodiments unless clearly indicated to the contrary. In particular, any feature indicated as being preferred or advantageous may be combined with any other feature or features indicated as being preferred or advantageous.

The present invention relates to a proton-exchange membrane comprising a blend of first and second ionomers. Ionomers are well-known in the art, particularly for the manufacture of proton-exchange membranes for devices such as fuel cells. Ionomers are polymers containing electrically neutral repeating units forming a main chain (i.e. polymer backbone) wherein ionisable/ionised units are covalently bonded to a fraction of the repeating units of the main chain as pendant group moieties (i.e. side chains), either randomly or periodically along the main chain. Typically, the polymer backbone is ethylene based (i.e. C2 repeating units, e.g. based on C2H4 or preferably C2F4). As such, in the protonexchange membrane of the present invention, the first ionomer comprises a first main chain covalently bonded to a first side chain and the second ionomer comprises a second main chain covalently bonded to a second side chain. One end of each chain is covalently bonded to a carbon atom of the main chain (i.e. in place of a hydrogen or fluorine atom, for example). Each of the first and second side chains comprise a sulfonic acid end group. That is, the first and second ionomers are sulfonic acid ionomers in which the side chains are terminated with a sulfonic acid moiety (i.e. -SO3H or -S(=O)2-OH) providing the ionisable group.

As noted in the IUPAC definition of an ionomer, ionic groups are usually present in sufficient amounts to cause micro-phase separation of ionic domains from the continuous polymer phase. The ionic domains act as physical crosslinks.

The first and second ionomers are present as a blend. That is, the proton-exchange membrane comprises a mixture of first and second ionomers, preferably a substantially homogenous blend.

The present inventors have found that forming membranes from preferred ionomers resulted in significant manufacturing defects, in particular cracking. Specifically, the preferred ionomers are those which exhibit favourable properties for proton-exchange, such as ion exchange capacity, proton conductivity and/or water uptake. For example, Aquivion® membranes exhibit high thermal stability (arising from the increased Tg) and higher proton conductivity even at low relative humidity (enabled by use of lower EW ionomers), making them attractive for high temperature, higher performance PEMFCs and PEMWEs. The properties generally result from the use of a “short side chain” such ionomers may be referred to as SSC ionomers. On the other hand, Nation® ionomer is an example of a ’’long side chain”, or LSC, ionomer. The inventors found that cracking was a particular problem for high equivalent weight ionomers and found that the relaxation modulus of a membrane formed from such an ionomer served to characterise those ionomers which exhibit an increased propensity to cracking on manufacture.

Consequently, in accordance with the first aspect, the relaxation modulus of the membrane formed from only one of the ionomers of the blend (i.e. the second ionomer) is greater than 10,000 MPa. Whilst there is no specific upper limit, the relaxation modulus of the membrane formed from the second ionomer may be at most 100,000 MPa, or at most 50,000 MPa, and in some embodiments at most 20,000 MPa. In some preferred embodiments, the relaxation modulus of the membrane formed from the second ionomer is at least 11 ,000 MPa, preferably at least 12,000 MPa.

The inventors were surprised to find that through a blend with a further ionomer (i.e. the first ionomer), the relaxation modulus of the proton-exchange membrane formed from the blend exhibits a lower than expected relaxation modulus. The relaxation modulus of a membrane formed from the first ionomer is at least 10 times less than a relaxation modulus of a membrane formed from the second ionomer, preferably at least 100 times less. Preferably, the first ionomer is therefore an LSC ionomer.

As used herein, relaxation modulus is the value measured at 10 minutes, under application of 2.5% strain at 80 °C and 0% relative humidity (RH). 10 minutes is the typical duration of the solvent evaporation step when preparing a membrane by casting. This therefore represents a relevant period of time within which membrane cracking may occur. The relaxation modulus may be referred to as the stress relaxation modulus and provides the value of the stress normalised by the applied strain. Such measurement techniques are well-known using a dynamic mechanical analyser so as to be part of the skilled person’s common general knowledge. Such measurements may be performed on a 20 pm thick membrane having a width of from 6 to 8 mm and a gage length of from 5.5 to 6.5 mm, for example.

Whilst the properties of the first ionomer (or those expected of a membrane formed from a first ionomer) are less favourable than those of the second ionomer, the impact on the relaxation modulus of the blended membrane was much greater than expected which in turn was found to reduce the likelihood of cracking on manufacture (particularly by the preferred method of casting as described in further detail herein). This allows for the proton-exchange membrane to be formed from a relatively small amount of the first ionomer (and therefore maintain a greater quantity of the preferred second ionomer) thereby mitigating any adverse effect on the final membrane properties, such as protonexchange capacity, whilst providing a significant reduction in manufacturing defects. Through use of such a blend, the inventors have been able to manufacture membranes with minimal cracking, and in some preferred embodiments, crack-free membranes.

Whilst in some embodiments in the prior art, there are disclosed composite membranes, these composites are designed to reinforce the membrane to improve its resilience after the expected degradation through use. The prior art fails to consider the technical features necessary to improve the quality of the as-manufactured membrane. The as-manufactured proton-exchange membrane of the present invention is therefore less susceptible to degradation and can provide electrolytic devices with greater durability and therefore improved lifetime without requiring composite materials.

However, the proton-exchange membrane may still further comprise such composite materials as are known in the art though in some preferred embodiments, the proton-exchange membrane consists of the blend of ionomers.

Preferably, the membrane comprising the blend of ionomers has a relaxation modulus of less than 10,000 MPa, more preferably less than 7,500 MPa. Whilst there is no particular lower limit, the relaxation modulus of the membrane may be more than 1 ,000 MPa in comprising a greater quantity of the preferred first ionomer to deliver the preferred membrane properties, and may be at least

2,000 MPa, and in some embodiments at least 3,000 MPa. More preferably, the relaxation modulus of the membrane is less than 6,000 MPa as this was found to provide an essentially crack-free membrane.

In view of the foregoing, it is preferred that alternatively, or additionally, the relaxation modulus of the membrane formed from the first ionomer is less than 1 ,000 MPa, preferably less than 100 MPa. Whilst there is no specific lower limit the relaxation modulus of the membrane formed from the first ionomer may be at least 10 MPa.

The first and/or second ionomers present in the blend may be partially-fluorinated, though fully- fluorinated (i.e. perfluorinated) is preferred since such ionomers typically provide greater proton conductivities and the like. As such, it is preferred that both first and second ionomers are fully fluorinated. As will be appreciated by those skilled in the art, partial fluorination refers to the formal replacement of a fraction of the C-H bonds present with C-F. In some embodiments, one of the main chain or the side chain may be fully-fluorinated and the other non-fluorinated to provide a partially- fluorinated ionomer.

As described herein, the blend may comprise a relatively small amount of the first ionomer and a significant reduction in membrane cracking of manufacture may be achieved. Preferably, the membrane comprises at least 5 wt% of the first ionomer by weight of the blend, more preferably at least 10 wt%. One particular advantage of the membrane of the present invention is that a benefit may be achieved with a much lower amount of the first ionomer than could have been anticipated. In order to maintain the preferred properties resulting from the presence of the second ionomer, it is preferred that the membrane comprises at most 40 wt% of the first ionomer, preferably at most 25 wt%, more preferably at most 20 wt%, even more preferably at most 15 wt%. Preferably, the membrane comprises at least 60 wt% of the second ionomer, preferably at least 75 wt%, more preferably at least 80 wt%, even more preferably at least 85 wt%; and/or at most 95 wt% of the second ionomer, preferably at most 90 wt%. In some preferred embodiments, the blend consists of the first and second ionomers.

Typically, each ionomer has an equivalent weight (EW) of at most 1 ,100 and/or at least 450. Preferably, the second ionomer has a relatively high equivalent weight, such as at least 850, or at least 900. It is generally preferred that the first ionomer has a lower equivalent weight than the second ionomer. The difference in EW may be at least 50, or at least 100, for example. In some preferred embodiments, the first ionomer has an equivalent weight of from 600 to 850, and the second ionomer has an equivalent weight of from 850 to 1 ,100. The equivalent weight may be readily measured using an acid titration following a hydroxide exchange. For example, a membrane sample may be vacuum dried at about 110 °C for 16 hours to obtain about 2 g of the dried film. The film may then be immersed in about 30 mL of a 0.1 M NaOH solution to substitute sodium ions for protons in the membrane. Then titration by neutralisation is carried out, for example using 0.1 M hydrochloric acid, to determine the number of exchangeable protons, and therefore the EW may be calculated.

As such, one preferred embodiment of the proton-exchange membrane is a membrane comprising a blend consisting of first and second ionomers, the first ionomer comprising a first main chain covalently bonded to a first side chain and the second ionomer comprising a second main chain covalently bonded to a second side chain; wherein each of the first and second side chains comprise a sulfonic acid end group; wherein a relaxation modulus of a membrane formed from the second ionomer is greater than 10,000 MPa and wherein a relaxation modulus of a membrane formed from the first ionomer is less than 1 ,000 MPa; wherein the blend comprises from 10 to 40 wt% of the first ionomer; and wherein each ionomer has an equivalent weight of from 450 to 1 ,100.

The thickness of the proton-exchange membrane will depend on its intended use. For example, a proton-exchange membrane for a water electrolyser will typically be thicker than for a fuel cell but that may not always be the case. Typically, the proton-exchange membrane has a thickness at 0% relative humidity of at least about 5 micrometres. It may be preferred that the proton-conducting membrane has a thickness of at least about 6 micrometres, at least about 7 micrometres, at least about 8 micrometres, at least about 9 micrometres or at least about 10 micrometres. Typically, the thickness of the proton-conducting membrane at 0% relative humidity is less than or equal to about 200 micrometres, such as less than or equal to about 150 micrometres, less than or equal to about 100 micrometres, less than or equal to about 50 micrometres, less than or equal to about 30 micrometres, less than or equal to about 25 micrometres, or less than or equal to about 20 micrometres. The thickness of the membrane may be determined by analysis of a scanning electron microscope (SEM) image of a cross section of the membrane. It may be preferred that the proton-conducting membrane has a thickness at 0% relative humidity in the range of and including about 5 micrometres to about 200 micrometres, about 6 to about 100 micrometres, about 6 to about 50 micrometres, about 7 to about 30 micrometres, or about 8 to about 20 micrometres.

In some embodiments, the first and/or second ionomers may be characterised with respect to the length of the side chain either independently or together with the relaxation modulus of a membrane formed from such an ionomer. Preferably, a chain length of the first side chain between the first main chain and the end group is longer than a chain length of the second side chain between the second main chain and the end group. A chain length refers to the minimum number of atoms between the end group (the sulfur atom where the ionisable end group is sulfonic acid) and the carbon atom of the repeating unit of the main chain to which the side chain is covalently bonded.

Preferably, the first ionomer is an LSC ionomer and the second ionomer is an SSC ionomer. Accordingly, it is preferred that the chain length of the second side chain is at most 5 atoms, preferably at most 4 atoms. Preferably, the second side chain has a chain length of at least 2 atoms and/or at most 4 atoms, and preferably has a chain length of 3 atoms. Preferably, the chain length of the first side chain is at least 4 atoms, preferably at least 5 atoms, more preferably at least 6 atoms and/or at most 10 atoms, preferably at least 5, more preferably at least 6, and/or at most 8 atoms, and preferably has a chain length of 5 or 6 atoms, preferably 6 atoms.

Typically, the side chains of the first and second ionomers are each independently linear or branched alkyls (and as described herein preferably fully-fluorinated). Additionally, one or more non-adjacent non-terminal carbon atoms of the alkyl may be substituted with an oxygen atom. Preferably, the side chains are covalently bonded to the main chain via an oxygen atom (oxygen atom substitution, where present, will be appreciated as providing an ether, and no ketone or ester groups, for example, may be present). In one preferred embodiment, the first side chain is branched.

In some embodiments, the first side chain is represented by the formula:

-O-CR2-CR2-O-(CR₂)a-SO₃H wherein a is 2 or more (preferably up to 6, preferably 2, 3 or 4), and each R is independently selected from the group consisting of H, F, Cl, alkyl, perfluoroalkyl, perchloroalkyl and perfluorochloroalkyl (wherein the alkyl, perfluoroalkyl, perchloroalkyl and perfluorochloroalkyl may have a carbon number of from 1 to 10, such as from 1 to 3, and in some embodiments may be 1), preferably selected from the group consisting of H, F and perfluoroalkyl, for example:

-O-CX2-CX(CF₂X)-O-(CX2)b-(CF2)c-SO₃H wherein X is F or Cl, wherein b and c are each greater than 1 (wherein b + c = a). In some embodiments, the second side chain is represented by the formula:

-O-(CR’₂)d-SO₃H wherein d is from 1 to 4, preferably from 1 to 3, preferably 2. Preferably R’ is as described above for R, or is selected from H or F.

Accordingly, in one preferred embodiment, the first side chain is -O-CR2-CR(CR₃)-O-(CR2)2-SO₃H and the second side chain is -O-(CR’2)2-SO₃H, wherein each R and R’ are each independently selected from H and F, preferably wherein R = R’ = F.

The proton-exchange membrane comprising a blend of ionomers as described herein may be further characterised with respect to the molar and/or weight ratios of the side chains and main chains provided by each ionomer.

In some embodiments, a molar ratio of the first side chain to the second side chain is at least 0.07:1 , such as at least 0.15:1 , such as at least 0.2:1 . In some embodiments, a molar ratio of the first side chain to the second side chain is at most 1 :1 , such at most 0.5:1 .

In some embodiments, the blend of ionomers comprises from 5 to 60 wt% of the first side chain by weight of the side chains, such as from 10 to 50 wt%, such as from 20 to 40 wt%. In some embodiments, the blend of ionomers comprises from 1 to 20 wt% of the first side chain by weight of the blend, such as from 2 to 15 wt%. In some embodiments, the blend of ionomers comprises from 95 to 40 wt% of the second side chain by weight of the side chains, such as from 90 to 50 wt%, such as from 80 to 60 wt%. In some embodiments, the blend of ionomers comprises from 10 to 20 wt% of the second side chain by weight of the blend of ionomers, preferably from 15 to 18 wt%. In some embodiments, the blend of ionomers comprises from 60 to 89 wt% of the first and second main chains by weight of the blend of ionomers, preferably from 67 to 83 wt%. Where the blend of ionomers consists of the first and second ionomers, the total of the first and second side chains and first and second main chains equals 100 wt%.

In another aspect, the present invention provides a proton-exchange membrane comprising a blend of first and second ionomers, the first ionomer comprising a first main chain covalently bonded to a first side chain and the second ionomer comprising a second main chain covalently bonded to a second side chain; wherein each of the first and second side chains comprise a sulfonic acid end group; and wherein a chain length of the first side chain between the first main chain and the end group is longer than a chain length of the second side chain between the second main chain and the end group, wherein the chain length of the second side chain is at most 5 atoms. Also provided is a catalyst-coated membrane comprising a proton-exchange membrane as described herein, with a cathode catalyst layer applied to a first face of the membrane and/or an anode catalyst layer applied to a second face of the membrane.

The catalyst layer comprises one or more electrocatalysts. The one or more electrocatalysts may be independently a finely divided unsupported metal powder, or a supported catalyst wherein small nanoparticles are dispersed on electrically conducting particulate carbon supports. The electrocatalyst metal is suitably selected from

(i) one or more platinum group metals (platinum, palladium, rhodium, ruthenium, iridium and osmium);

(ii) gold or silver;

(iii) a base metal; or an alloy or mixture comprising one or more of these metals or their oxides.

The preferred electrocatalyst metal is platinum, which may be alloyed with other precious metals or base metals. A base metal is tin or a transition metal which is not a noble metal. A noble metal is a platinum group metal (platinum, palladium, rhodium, ruthenium, iridium or osmium), gold or silver. Suitable base metals include copper, cobalt, nickel, zinc, iron, titanium, molybdenum, vanadium, manganese, niobium, tantalum, chromium and tin. Preferred base metals are nickel, copper, cobalt, and chromium. More preferred base metals are nickel, cobalt and copper. If the electrocatalyst is a supported catalyst, the loading of metal particles on the carbon support material is suitably in the range 10-90 wt%, preferably 15-75 wt% of the weight of resulting electrocatalyst. The exact electrocatalyst used will depend on the reaction it is intended to catalyse and its selection is within the capability of the skilled person.

The catalyst layer may further comprise additional components. Such additional components include, but are not limited to, a catalyst which facilitates oxygen evolution and therefore will be of benefit in cell reversal situations and high potential excursions, or a hydrogen peroxide decomposition catalyst. Examples of such catalysts and any other additives suitable for inclusion in the catalyst layer will be known to those skilled in the art.

Also provided is a membrane electrode assembly comprising a proton-exchange membrane as described herein, and a gas diffusion electrode and/or a porous transport layer on a first and/or second face of the proton-exchange membrane. Also provided is a membrane electrode assembly comprising a catalyst-coated proton-conducting membrane and a gas diffusion layer or porous transport layer present on the at least one of the catalyst layers. The anode and cathode gas diffusion layers are suitably based on conventional gas diffusion substrates. Typical substrates include nonwoven papers or webs comprising a network of carbon fibres and a thermoset resin binder (e.g. the TGP-H series of carbon fibre paper available from Toray Industries Inc., Japan or the H2315 series available from Freudenberg FCCT KG, Germany, or the Sigracet® series available from SGL Technologies GmbH, Germany or AvCarb® series from Ballard Power Systems Inc., or woven carbon cloths. The carbon paper, web or cloth may be provided with a further treatment prior to being incorporated into a MEA either to make it more wettable (hydrophilic) or more wet-proofed (hydrophobic). The nature of any treatments will depend on the type of fuel cell and the operating conditions that will be used. The substrate can be made more wettable by incorporation of materials such as amorphous carbon blacks via impregnation from liquid suspensions, or can be made more hydrophobic by impregnating the pore structure of the substrate with a colloidal suspension of a polymer such as PTFE or polyfluoroethylenepropylene (FEP), followed by drying and heating above the melting point of the polymer. For applications such as the PEMFC, a microporous layer may also be applied to the gas diffusion substrate on the face that will contact the catalyst layer. The microporous layer typically comprises a mixture of a carbon black and a polymer such as polytetrafluoroethylene (PTFE). The porous transport layer is suitably based on conventional porous transport substrates, such as a titanium mesh.

In a further aspect, the present invention provides an electrochemical device comprising a proton-exchange membrane, a catalyst-coated membrane, or a membrane-electrode assembly as described herein. The electrochemical device can be a fuel cell, such as a proton exchange membrane fuel cell. The electrochemical device can be an electrolyser, such as a water electrolyser.

In a further aspect the present invention provide a method of forming a proton-exchange membrane as described herein, the method comprising:

(i) dispersing a first and a second ionomer in a solvent to form a dispersion;

(ii) coating the surface of a substrate with the dispersion to a wet thickness of from 50 pm to 1 mm;

(iii) evaporating the solvent and annealing the coated substrate to form the membrane; and

(iv) removing the membrane from the substrate.

The inventors found that cracking was a particular problem when forming proton-exchange membranes using solution casting methods, with cracking more pronounced when using high EW ionomers, especially when using relatively polar solvents. Such a problem with manufacture may be mitigated through modification of various parameters, for example solvent evaporation (drying) and annealing temperature profiles, choice of solvent based on volatility, and the thickness of the coating. With continuous roll-to-roll casting methods, it is also possible to modify the flow rate of the solution and substrate speed, for example.

However, the inventors were surprised to find that the problem may be addressed by simply employing a blend of ionomers to form a membrane, specifically a blend comprising ionomers which individually form membrane with disparate relaxation moduli. As such, the membrane described herein may be obtainable, or obtained, from the method described. The method comprises dispersing each of the first and second ionomers in a solvent to form a dispersion. The ionomers may be dissolved, or the ionomer dispersion may comprise a fine suspension of ionomer particles, and preferably be a homogenous dispersion. Dispersions of PFSA ionomers are conventional in the art. Many various ionomers suitable for use in the present invention are commercially available, and may be provided as dry solids or as dispersions (commonly in water or alcohol-water mixtures). In some embodiments, the method may therefore comprise drying an aqueous dispersion to increase the ionomer concentration, or completely dry the ionomer. Alternatively, suitable ionomers may be manufactured by conventional synthetic organic chemistry techniques. A skilled person may easily and unambiguously verify the value of the relaxation modulus as described herein for a membrane formed from any ionomer.

Preferred solvents are polar protic solvents, typically aqueous and/or alcoholic solvents. Preferably, the solvent consists of alcohol and optionally water. Preferably, the solvent comprises an alcohol having a carbon number of from 1 to 5. Ethanol (i.e. carbon number of 2) is particularly suitable. Preferably, the solvent comprises from 50 to 95 wt% alcohol by weight of the solvent, more preferably from 60 to 80 wt%.

In some preferred embodiments, the dispersion comprises the first and second ionomers in a total amount of from 10 to 40 wt% by weight of the dispersion, preferably from 20 to 30 wt%. Preferably, the dispersion comprises from 1 to 6 wt% of the first ionomer by weight of the dispersion and/or from 9 to 34 wt% of the second ionomer by weight of the dispersion.

The method further comprises coating the surface of a substrate with the dispersion to a wet thickness of from 50 pm to 1 mm. It is preferred that the dispersion is coated onto the substrate by gap coating or roll-to-roll (R2R) coating. Gap coating techniques may include using a coating knife or doctor blade of a film applicator whereby the coating is applied to the substrate then passes through a split between the knife and a support roller. R2R coating techniques may include slot-die coating whereby the coating is squeezed out by gravity or under pressure via a slot onto the substrate, or gravure coating, for example.

In some preferred embodiments, the surface of the substrate is formed of fluorinated ethylene propylene (FEP) or poly(4,4'-oxydiphenylene-pyromellitimide). Such surfaces have low adhesion and facilitate subsequent removal of the membrane. The substrate may comprise a polyester backing provided with a FEP coating, such as those available from Diacel®. Poly(4,4'-oxydiphenylene- pyromellitimide) substrates are available from, and may be known as, Kapton®. The surface may a thickness of from 10 to 200 pm.

The method further comprises evaporating the solvent and annealing the coated substrate to form the membrane, and removing the membrane from the substrate (for example by peeling the membrane from the substrate). Evaporation of the solvent and annealing may comprise sequential heating steps. Evaporation may include allowing the solvent to evaporate under ambient conditions (i.e. without heating and/or under a flow of gas such as air or nitrogen). Evaporation may also comprise heating the substrate to a temperature of up to 120 °C, for example up to 100 °C in order to substantially dry the coated dispersion. Preferably annealing comprises heating the coated substrate to a temperature of from 120 °C to 240 °C, such as from 140 °C to 200 °C. The coated substrate may be annealed at such temperatures for from 1 to 20 minutes.

Description of the Figures

The present invention will now be described further with reference to the following non-limiting Figures, in which:

Figure 1 illustrates a fluorinated ionomer suitable for use in the membrane of the present invention.

Figure 2 illustrates an exemplary perfluorinated ionomer suitable use as the first ionomer in the membrane of the present invention.

Figure 3 illustrates an exemplary perfluorinated ionomer suitable for use as the second ionomer in the membrane of the present invention.

Figure 4 is a scatter plot of the measured relaxation modulus at 10 minutes for membranes manufactured in accordance with the Examples.

Figure 1 illustrates the chemical structure of an ionomer 100. Ionomer 100 comprises a main chain polymer backbone which is based on repeating units of tetrafluoroethylene (i.e. C2F4) whereby a fraction of the repeating units are substituted with a side chain 105 (shown as “SC” in Figure 1). The extent of substitution is determined by the number of non-substituted repeating units as shown by “m” - the value of m determines the equivalent weight. Ionomers are polymers comprising numerous repeating units - units of tetrafluoroethylene and side chain substituted tetrafluoroethylene as shown by “n” in the case of ionomer 100.

Figure 2 illustrates an exemplary perfluorinated ionomer 200 suitable use as the first ionomer in the membrane of the present invention. Ionomer 200 comprises a main chain polymer backbone 205 (shown as “MC” in Figure 2) to which the structure of a “long side chain” 210 is illustrated (for example, the side chain 105 as shown in Figure 1 in which case ionomer 100/200 then illustrates the AGC PFSA ionomer as used in the Examples below). The long side chain 210 is covalently bonded to the main chain 205, specifically via an oxygen atom with a further atom within the chain 210 being an oxygen atom providing an ether. The side chain 210 is branched comprising a -CF3 moiety. The long side chain 210 comprises a sulfonic acid end group 215. The chain length 220 is 6 atoms, as determined by the minimum number of atoms in the (fluoro)alkyl chain between the sulfur atom of the end group 215 and the main chain 205.

Figure 3 illustrates an exemplary perfluorinated ionomer 300 suitable for use as the second ionomer in the membrane of the present invention. Ionomer 300 comprises a main chain polymer backbone 305 (shown as “MC” in Figure 3) to which the structure of a “short side chain” 210 is illustrated (for example, the side chain 105 as shown in Figure 1 in which case ionomer 100/300 then illustrates the Solvay PFSA ionomer as used in the Examples below). The short side chain 310 is covalently bonded to the main chain 305, specifically via an oxygen atom. The short side chain 310 comprises a sulfonic acid end group 315. The chain length 320 is 3 atoms.

Figure 4 is a scatter plot of the measured relaxation modulus at 10 minutes for membranes manufactured in accordance with the Examples with varying concentrations in wt% of the first ionomer with respect to the total weight of first and second ionomers.

Examples

Membrane Preparation

Perfluorosulfonic acid ionomer (PFSA) dispersions were obtained from Solvay and AGC. The PFSA ionomer from AGC is an LSC ionomer in accordance with Figure 2 having an equivalent weight of about 720. The PFSA ionomer from Solvay is an SSC ionomer in accordance with Figure 3 having an equivalent weight of about 980. Each PFSA dispersion was poured into a shallow dish lined with Teflon and placed under a weak stream of air to evaporate the solvent at room temperature over the course of approximately three days. The solid PFSA chunks remaining after the solvent evaporation were placed overnight in a 30 °C oven under vacuum to remove any residual moisture. PFSA dispersions for membrane casting were prepared by combining varying quantities of AGC solid PFSA, with the balance being Solvay solid PFSA as described in Table 1 below. The solids PFSAs were combined with a 70 wt% ethanol solvent (balance water) at a total of 25 wt% PFSA. Glass vials containing the Solvay and AGC PFSA solids and solvent were placed on a roller table at 60 rpm for one day to fully disperse the PFSAs. The blended PFSA dispersions were used within one week of preparation.

Approximately 1 .5 mL of blended PFSA dispersion was coated onto 75 pm thick Diacel FEP-coated polyester backing film using an Elcometer 4340 motorised film applicator and steel coating knife. The Diacel backing film was thoroughly cleaned with 35% 2-propanol (balance water) and lint-free wipes prior to coating. The draw-down speed was 10 mm/s, the coating knife gap was 300 pm, and the coating temperature was 20 °C. After coating, the solvent was allowed to evaporate at room temperature for 10 min, then the PFSA-coated Diacel backing film was placed in a Binder oven at 100 °C for 10 min to further evaporate the solvent. The PFSA-coated Diacel backing film was then placed in another oven at 160 °C for a 12 minute annealing step. Blended PFSA membranes were obtained by peeling the PFSA coating from the Diacel backing film. The final PFSA membrane thickness was 20 ± 2 pm measured using a Fischer magnetic induction thickness gauge at ambient temperature and relative humidity. The relaxation modulus of each membrane was measured in accordance with the method described herein.

Stress Relaxation Measurements

A TA Instruments dynamic mechanical analyser, Discovery DMA 850, equipped with a film tension clamp and relative humidity (RH) unit was used to carry out transient tensile stress relaxation measurements. Strips of membrane were cut in the machine direction using a punch die, with typical strip widths between 6 and 8 mm. Membrane thickness was measured in at least five locations using a Fischer MMS Inspection DFT magnetic induction thickness gauge to be 20 ± 2 pm. Membrane strips were clamped in the DMA 850 film tension clamp with 3 inch-pounds of torque and gage length between 5.6 to 6 mm. Each sample was equilibrated at 80 °C and 0% RH under 50 mN of preload force for 45 min. Then, a tensile stress relaxation measurement was conducted by applying a step strain of 2.5% to the sample for 60 min while measuring the force required to maintain the sample at 2.5% strain as a function of time, t. The stress relaxation modulus, E(t), was obtained by normalizing the measured force, F(t), by the sample cross-sectional area, A, and the applied strain, so: E(t) = F(t)/Aso.

Table 1 :

These results are plotted in Figure 4. These examples demonstrated the surprising effect that, advantageously, only a small amount of the first ionomer is needed to provide a greater than anticipated reduction in relaxation modulus of the blended membrane when compared to the relaxation modulus of membranes formed from each ionomer individually. This is beneficial since this allows for the blend to maintain a higher proportion of the more desirable SSC ionomer in order to maintain the useful proton-exchange properties of the membrane. Accordingly, as little as 5 wt% of the first ionomer is sufficient to cause a very large reduction in the blended relaxation modulus which results in a reduction in the extent of cracking in the final membrane. Furthermore, the inventors found that cracking may be avoided altogether with as little as 15 wt% of the first ionomer.

As used herein, the singular form of “a”, “an” and “the” include plural references unless the context clearly dictates otherwise. The use of the term “comprising” is intended to be interpreted as including such features but not excluding other features and is also intended to include the option of the features necessarily being limited to those described. In other words, the term also includes the limitations of “consisting essentially of’ (intended to mean that specific further components can be present provided they do not materially affect the essential characteristic of the described feature) and “consisting of’ (intended to mean that no other feature may be included such that if the components were expressed as percentages by their proportions, these would add up to 100%, whilst accounting for any unavoidable impurities), unless the context clearly dictates otherwise.

Numerical lower and upper limits of features described herein may preferably be combined to provide a closed range.

It will be understood that, although the terms "first", "second", etc. may be used herein to describe various features (e.g. ionomers), the features should not be limited by these terms. These terms are only used to distinguish one feature from another, or a further, feature. Where the membrane comprises more than two ionomers, it will be appreciated that third or further ionomers will individually meet the requirements described herein in respect of either the first or second ionomers. In other words, a first ionomer, for example, may comprise a blend of first ionomers wherein the blend is present in the amounts described herein and each ionomer of the blend individually meets the parameters described for the first ionomer (e.g. relaxation modulus).

The foregoing detailed description has been provided by way of explanation and illustration, and is not intended to limit the scope of the appended claims. Many variations of the presently preferred embodiments illustrated herein will be apparent to one of ordinary skill in the art, and remain within the scope of the appended claims and their equivalents.

For the avoidance of doubt, the entire contents of all documents acknowledged herein are incorporated herein by reference. Some embodiments of the invention are set out in the following numbered clauses:

1 . A proton-exchange membrane comprising a blend of first and second ionomers, the first ionomer comprising a first main chain covalently bonded to a first side chain and the second ionomer comprising a second main chain covalently bonded to a second side chain; wherein each of the first and second side chains comprise a sulfonic acid end group; wherein a relaxation modulus of a membrane formed from the first ionomer is at least 10 times less than a relaxation modulus of a membrane formed from the second ionomer, preferably at least 100 times less; and wherein the relaxation modulus of the membrane formed from the second ionomer is greater than 10,000 MPa.

2. The proton-exchange membrane according to any preceding clause, wherein the membrane has a relaxation modulus of less than 10,000 MPa, preferably less than 7,500 MPa.

3. The proton-exchange membrane according to any preceding clause, wherein the relaxation modulus of the membrane formed from the first ionomer is less than 1 ,000 MPa, preferably less than 100 MPa.

4. The proton-exchange membrane according to any preceding clause, wherein the membrane has a thickness of less than 200 pm.

5. The proton-exchange membrane according to any preceding clause, wherein the membrane comprises: at least 5 wt% of the first ionomer, preferably at least 10 wt%; and/or at most 40 wt% of the first ionomer, preferably at most 25 wt%.

6. The proton-exchange membrane according to any preceding clause, wherein the membrane comprises: at least 60 wt% of the second ionomer, preferably at least 75 wt%; and/or at most 95 wt% of the second ionomer, preferably at most 90 wt%.

7. The proton-exchange membrane according to any preceding clause, wherein the first and/or second ionomers are partially-fluorinated, preferably fully-fluorinated.

8. The proton-exchange membrane according to any preceding clause, wherein the first ionomer has an equivalent weight of from 600 to 850, and wherein the second ionomer has an equivalent weight of from 850 to 1 ,100. 9. The proton-exchange membrane according to any preceding clause, wherein the first ionomer has a lower equivalent weight than the second ionomer, preferably wherein the difference in the equivalent weights of the first and second ionomers is at least 50.

10. An electrochemical device comprising the proton-exchange membrane according to any preceding clause, preferably a water electrolyser or a fuel cell.

11. A method of forming a proton-exchange membrane according to any of clauses 1 to 9, the method comprising:

(i) dispersing a first and a second ionomer in a solvent to form a dispersion;

(iv) removing the membrane from the substrate.

12. The method according to clause 11 , wherein the dispersion comprises the first and second ionomers in a total amount of from 10 to 40 wt% by weight of the dispersion, preferably from 20 to 30 wt%.

13. The method according to clause 11 or clause 12, wherein the dispersion comprises from 1 to 6 wt% of the first ionomer by weight of the dispersion and/or from 9 to 34 wt% of the second ionomer by weight of the dispersion.

14. The method according to any of clauses 11 to 13, wherein the solvent consists of alcohol and optionally water, preferably wherein the alcohol has a carbon number of from 1 to 5.

15. The method according to clause 14, wherein the solvent comprises from 50 to 95 wt% alcohol by weight of the solvent, preferably from 60 to 80 wt%.

16. The method according to any of clauses 11 to 15, wherein the surface of the substrate is formed of fluorinated ethylene propylene (FEP) or poly(4,4'-oxydiphenylene-pyromellitimide), preferably having a thickness of from 10 to 200 pm.

17. The method according to any of clauses 11 to 16, wherein the dispersion is coated onto the substrate by gap coating or roll-to-roll coating.

18. The method according to any of clauses 11 to 17, wherein annealing comprises heating the coated substrate to a temperature of from 120 °C to 240 °C, preferably for from 1 to 20 minutes.

Claims

Claims:

2. The proton-exchange membrane according to any preceding claim, wherein the membrane has a relaxation modulus of less than 10,000 MPa, preferably less than 7,500 MPa.

3. The proton-exchange membrane according to any preceding claim, wherein the relaxation modulus of the membrane formed from the first ionomer is less than 1 ,000 MPa, preferably less than 100 MPa.

4. The proton-exchange membrane according to any preceding claim, wherein the membrane has a thickness of less than 200 pm.

5. The proton-exchange membrane according to any preceding claim, wherein the membrane comprises: at least 5 wt% of the first ionomer, preferably at least 10 wt%; and/or at most 40 wt% of the first ionomer, preferably at most 25 wt%.

6. The proton-exchange membrane according to any preceding claim, wherein the membrane comprises: at least 5 wt% of the first ionomer.

7. The proton-exchange membrane according to any preceding claim, wherein the membrane comprises: at most 15 wt% of the first ionomer.

8. The proton-exchange membrane according to any preceding claim, wherein the membrane comprises: at least 60 wt% of the second ionomer, preferably at least 75 wt%; and/or at most 95 wt% of the second ionomer, preferably at most 90 wt%.

9. The proton-exchange membrane according to any preceding claim, wherein the membrane comprises: at least 85 wt% of the second ionomer.

10. The proton-exchange membrane according to any preceding claim, wherein the membrane comprises: at most 95 wt% of the second ionomer.

11 . The proton-exchange membrane according to any preceding claim, wherein the first and/or second ionomers are partially-fluorinated, preferably fully-fluorinated.

12. The proton-exchange membrane according to any preceding claim, wherein the first ionomer has an equivalent weight of from 600 to 850, and wherein the second ionomer has an equivalent weight of from 850 to 1 ,100.

13. The proton-exchange membrane according to any preceding claim, wherein the first ionomer has a lower equivalent weight than the second ionomer, preferably wherein the difference in the equivalent weights of the first and second ionomers is at least 50.

14. The proton-exchange membrane according to any preceding claim, wherein the first ionomer is a long side-chain ionomer and the length of the first side chain is at least 6 atoms.

15. The proton-exchange membrane according to any preceding claim, wherein the second ionomer is a short side-chain ionomer and the length of the second side chain is at most 4 atoms.

16. An electrochemical device comprising the proton-exchange membrane according to any preceding claim, preferably a water electrolyser or a fuel cell.

17. A method of forming a proton-exchange membrane according to any of claims 1 to 15, the method comprising:

(i) dispersing a first and a second ionomer in a solvent to form a dispersion;

(iv) removing the membrane from the substrate.

18. The method according to claim 17, wherein the dispersion comprises the first and second ionomers in a total amount of from 10 to 40 wt% by weight of the dispersion, preferably from 20 to 30 wt%.

19. The method according to claim 17 or claim 18, wherein the dispersion comprises from 1 to

6 wt% of the first ionomer by weight of the dispersion and/or from 9 to 34 wt% of the second ionomer by weight of the dispersion.

20. The method according to any of claims 17 to 19, wherein the solvent consists of alcohol and optionally water, preferably wherein the alcohol has a carbon number of from 1 to 5.

21 . The method according to claim 20, wherein the solvent comprises from 50 to 95 wt% alcohol by weight of the solvent, preferably from 60 to 80 wt%.

22. The method according to any of claims 17 to 21 , wherein the surface of the substrate is formed of fluorinated ethylene propylene (FEP) or poly(4,4'-oxydiphenylene-pyromellitimide), preferably having a thickness of from 10 to 200 pm.

23. The method according to any of claims 17 to 22, wherein the dispersion is coated onto the substrate by gap coating or roll-to-roll coating.

24. The method according to any of claims 17 to 23, wherein annealing comprises heating the coated substrate to a temperature of from 120 °C to 240 °C, preferably for from 1 to 20 minutes.