Proton exchange membrane

Abstract

The present invention provides a proton exchange membrane comprising a blend of a first ionomer and a second ionomer, wherein 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, each of the first and second side chains comprising a sulfonic acid terminal group, the relaxation modulus of the membrane formed from the first ionomer is at least one-tenth, preferably at least one-hundredth, of the relaxation modulus of the membrane formed from the second ionomer, and the relaxation modulus of the membrane formed from the second ionomer is greater than 10,000 MPa.

Description

Technical Field

[0001] The present invention relates to a proton-exchange membrane (PEM) and a method for producing the same. Specifically, the present invention relates to a PEM comprising a blend of a first ionomer and a second ionomer. The PEM is particularly suitable for use in electrochemical devices such as fuel cells and / or water electrolysis devices. Background Art

[0002] A fuel cell is an electrochemical cell comprising two electrodes separated by an electrolyte. A fuel, for example hydrogen, an alcohol such as methanol or ethanol, or formic acid, is supplied to an anode, and an oxidant, for example oxygen or air, is supplied to a cathode. Electrochemical reactions occur at the electrodes, converting the chemical energy of the fuel and oxidant into electrical energy and heat. Electrode catalysts are used to promote the electrochemical oxidation of fuel at the anode and the electrochemical reduction of oxygen at the cathode.

[0003] Fuel cells are generally classified according to the nature of the electrolyte used. In many cases, the electrolyte is a solid polymer membrane, which is electronically insulating but ion-conductive. In a proton exchange membrane fuel cell (PEMFC), the membrane is proton-conductive, and protons generated at the anode are transported through the membrane to the cathode, where they combine with oxygen to form water.

[0004] An electrolysis device is an electrochemical device for electrolyzing water to produce high-purity hydrogen and oxygen. The electrolysis device can operate in both alkaline and acidic systems. Those electrolyzers using PEM are known as proton-exchange membrane water electrolysers (PEMWE).

[0005] Conventional proton-conducting membranes used in PEMFC or PEMWE are generally formed from sulfonated fully fluorinated polymer materials (often commonly referred to as perfluorinated sulfonic acid (PFSA) ionomers). As an alternative to PFSA-type ionomers, proton-conducting membranes based on partially fluorinated or non-fluorinated hydrocarbon polymers can be used.

[0006] An article titled "Perfluorosulfonic Acid Membranes for Fuel Cell and Electrolyser Applications" published by Sigma-Aldrich® on its webpage (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 compositions, structures, and synthesis routes, as well as membrane properties and durability.

[0007] Examples of such ionomers include those sold under the brand names Nafion® (EIDuPont de Nemours and Co.), Aciplex® (Asahi Kasei), Flemion® (Asahi Glass Co., Ltd.), Aquivion® (Solvay Speciality Polymers), and 3M® (3M Corporation).

[0008] The article "Advances in perfluorosulfonic acid-based proton exchange membranes for fuel cell applications: A review" in Chemical Engineering Journal Advances, 2022, 12, 100372, provides a review of perfluorosulfonic acid-based membranes for fuel cells, particularly the development of polymer composite membranes incorporating various multifunctional organic, inorganic, and hybrid fillers.

[0009] The article "Manufacturing defects in slot die coated polymer electrolyte membrane for fuel cell application" in Chemical Engineering Science, 2023, 280, 119051, considers PEM degradation to be one of the major obstacles to the commercialization of PEM fuel cells and identifies that manufacturing defects may be deeply involved in membrane degradation in the fuel cell environment as an initial cause of degradation in use. The membrane discussed therein was cast from Nafion® D-2021 onto polyethylene terephthalate (PET) film using roll-to-roll manufacturing technology, and the quality of the cast film is disclosed to be affected by the coating thickness, solution flow rate, slot die width, and substrate speed.

[0010] Despite these developments, the need remains in this field for higher quality proton exchange membranes with fewer manufacturing defects and without compromising proton exchange properties, in order to improve the effectiveness and durability / lifespan of electrolytic devices incorporating such PEMs.

[0011] Therefore, an object of the present invention is to provide a proton exchange membrane, which is more robust and stable than known membranes and suitable for use in electrochemical devices such as PEMWE and PEMFC, along with a method for manufacturing the same, or at least a commercially viable alternative thereto. [Overview of the project]

[0012] Therefore, a first aspect of the present invention provides a proton exchange membrane comprising a blend of a first ionomer and a second ionomer, wherein 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. Each of the first and second side chains contains a sulfonic acid terminal group. The relaxation modulus of the film formed from the first ionomer is at least one-tenth, preferably at least one-hundredth, of the relaxation modulus of the film formed from the second ionomer. The relaxation modulus of the film formed from the second ionomer is greater than 10,000 MPa.

[0013] The present disclosure is described further here. Different aspects / embodiments of the present disclosure are defined in more detail in the following sections. Each of the aspects / embodiments defined in this way may be combined with any other aspects / embodiments or more aspects / embodiments unless otherwise expressly indicated. In particular, any feature shown as preferred or advantageous may be combined with any other feature or more features shown as preferred or advantageous.

[0014] The present invention relates to a proton exchange membrane comprising a blend of a first ionomer and a second ionomer. Ionomers are well known in the art, particularly for the manufacture of proton exchange membranes for devices such as fuel cells. An ionomer is a polymer containing electrically neutral repeating units that form a main chain (i.e., a polymer backbone), where ionizable / ionized units are covalently bonded to a portion of the repeating units of the main chain, randomly or periodically along the main chain, as pendant group portions (i.e., side chains). Typically, the polymer backbone is ethylene-based (i.e., C2 repeating units based, for example, C2H4 or preferably C2F4). Thus, in the proton exchange 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., instead of, for example, a hydrogen atom or a fluorine atom). Each of the first and second side chains contains a sulfonic acid terminal group. That is, the first and second ionomers are sulfonic acid ionomers in which the side chains are terminated with a sulfonic acid portion (i.e., -SO3H or -S(=O)2-OH) that provides an ionizable group.

[0015] As described in the IUPAC definition of ionomers, ionic groups are typically present in sufficient quantities to cause microphase separation of the ionic region from the continuous polymer phase. The ionic region acts as a physical crosslink.

[0016] The first ionomer and the second ionomer exist as a blend. That is, the proton exchange membrane contains a mixture of the first ionomer and the second ionomer, preferably a substantially homogeneous blend.

[0017] The inventors have found that forming films from preferred ionomers results in significant manufacturing defects, particularly cracking. Specifically, preferred ionomers exhibit properties favorable to proton exchange, such as ion exchange capacity, proton conductivity, and / or water lift. For example, Aquivion® films exhibit high-temperature stability (resulting from increased Tg) and higher proton conductivity even at low relative humidity (made possible by the use of ionomers with lower EW), making them attractive for high-temperature, higher-performance PEMFCs and PEMWEs. These properties generally result from the use of “short side chains,” and such ionomers may be referred to as SSC ionomers. Nafion® ionomers, on the other hand, are an example of “long side chains,” i.e., LSC ionomers. The inventors found that cracking is a particular problem for high equivalent ionomers, and that the relaxation modulus of films formed from such ionomers helps characterize them, as they exhibit increased cracking tendencies during manufacturing.

[0018] As a result, according to the first embodiment, the relaxation modulus of a film formed from only one of the ionomers of the blend (i.e., the second ionomer) is greater than 10,000 MPa. Although there is no specific upper limit, the relaxation modulus of a film formed from the second ionomer may be up to 100,000 MPa, or up to 50,000 MPa, and in some embodiments up to 20,000 MPa. In some preferred embodiments, the relaxation modulus of a film formed from the second ionomer is at least 11,000 MPa, preferably at least 12,000 MPa.

[0019] To their surprise, the inventors found that, through blending with a further ionomer (i.e., the first ionomer), the relaxation modulus of the proton exchange membrane formed from the blend was lower than expected. The relaxation modulus of the membrane formed from the first ionomer is at least one-tenth, preferably at least one-hundredth, of the relaxation modulus of the membrane formed from the second ionomer. Therefore, the first ionomer is preferably an LSC ionomer.

[0020] As used herein, the relaxation modulus is the value measured at 10 minutes under a 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 film by casting. Therefore, this represents the relevant period during which cracks may occur in the film. The relaxation modulus may be referred to as the stress relaxation modulus and provides a value of stress normalized by the applied strain. Such measurement techniques, using dynamic mechanical analyzers, are so well known that they are part of the common general knowledge of those skilled in the art. Such measurements can be performed, for example, on a 20 μm thick film having a width of 6–8 mm and a gauge length of 5.5–6.5 mm.

[0021] Although the properties of the first ionomer (or the expected properties of the film formed from the first ionomer) are less desirable than those of the second ionomer, the effect on the relaxation modulus of the blended film is far greater than expected, which has been found to further reduce the likelihood of cracking during manufacturing (particularly by the preferred casting method, as will be described in more detail herein). This makes it possible to form proton-exchange films from a relatively small amount of the first ionomer (and thus retain more of the preferred second ionomer), thereby mitigating any adverse effects on the final film properties, such as proton-exchange capacity, while also significantly reducing manufacturing defects. Using such blends, the inventors have been able to produce films with minimal cracking, and in some preferred embodiments, films without cracking.

[0022] In some embodiments of the prior art, composite films are disclosed, but these composites are designed to reinforce the film to improve its resistance after expected degradation due to use. The prior art does not consider the technical features necessary to improve the quality of the film at the time of manufacture. Therefore, the proton exchange film at the time of manufacture of the present invention can provide an electrolytic device that is less susceptible to degradation and has greater durability, thus resulting in improved lifespan without the need for composite materials. However, the proton exchange film may further include composite materials as known in the art, but in some preferred embodiments, the proton exchange film consists of a blend of ionomers.

[0023] Preferably, the film containing the ionomer blend has a relaxation modulus of less than 10,000 MPa, more preferably less than 7,500 MPa. There is no specific lower limit, but the relaxation modulus of the film may be greater than 1,000 MPa, at least 2,000 MPa, and in some embodiments at least 3,000 MPa, when more preferred first ionomers are included to result in desirable film properties. More preferably, the relaxation modulus of the film is less than 6,000 MPa, as has been found to provide a film that is essentially crack-free.

[0024] From the aforementioned viewpoint, alternatively or additionally, the relaxation modulus of the film formed from the first ionomer is preferably less than 1,000 MPa, more preferably less than 100 MPa. Although there is no specific lower limit, the relaxation modulus of the film formed from the first ionomer may be at least 10 MPa.

[0025] The first ionomer and / or the second ionomer present in the blend may be partially fluorinated, but fully fluorinated (i.e., perfluorinated) ionomers are preferred. This is because such ionomers typically provide, for example, higher proton conductivity. Therefore, it is preferable that both the first ionomer and the second ionomer are fully fluorinated. As will be understood by those skilled in the art, partial fluorination refers to the symmetrical substitution of some existing C-H bonds with C-F bonds. In some embodiments, one of the main chain or the side chain may be fully fluorinated and the other may be non-fluorinated to provide a partially fluorinated ionomer.

[0026] As described herein, the blend may comprise a relatively small amount of the first ionomer, and a significant reduction in cracking in the manufactured membrane can be achieved. Preferably, the membrane comprises at least 5% by weight of the first ionomer, more preferably at least 10% by weight, based on the weight of the blend. One particular advantage of the membrane of the present invention is that benefits can be achieved with a much lower amount of the first ionomer than might be expected. In order to maintain the favorable properties resulting from the presence of the second ionomer, the membrane preferably comprises at most 40% by weight, preferably at most 25% by weight, more preferably at most 20% by weight, even more preferably at most 15% by weight of the first ionomer. Preferably, the membrane comprises at least 60% by weight, preferably at least 75% by weight, more preferably at least 80% by weight, even more preferably at least 85% by weight of the second ionomer, and / or at most 95% by weight, preferably at most 90% by weight of the second ionomer. In some preferred embodiments, the blend consists of the first ionomer and the second ionomer.

[0027] 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. Generally, it is preferred that the first ionomer has a lower equivalent weight than the second ionomer. The difference in EW may be, for example, at least 50, or at least 100. 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. Equivalent weight can be readily measured using acid titration after hydroxide exchange. For example, a membrane sample may be vacuum dried at about 110°C for 16 hours to obtain about 2 g of dried film. The film may then be immersed in about 30 mL of 0.1 M NaOH solution to replace protons in the membrane with sodium ions. Titration by neutralization is then performed using, for example, 0.1 M hydrochloric acid to determine the number of exchangeable protons, and thus EW can be calculated.

[0028] Accordingly, one preferred embodiment of a proton exchange membrane is a membrane comprising a blend of a first ionomer and a second ionomer, wherein the first ionomer comprises a first main chain covalently bonded to first side chains, and the second ionomer comprises a second main chain covalently bonded to second side chains, each of the first side chains and the second side chains comprises a sulfonic acid end group, the relaxation modulus of a membrane formed from the second ionomer is greater than 10,000 MPa, and the relaxation modulus of a membrane formed from the first ionomer is less than 1,000 MPa, the blend comprises 10 to 40% by weight of the first ionomer, each ionomer has an equivalent weight of from 450 to 1100.

[0029] The thickness of a proton exchange membrane depends on its intended application. For example, a proton exchange membrane for a water electrolysis device is typically thicker than one for a fuel cell, but this is not always the case. Typically, a proton exchange membrane has a thickness of at least about 5 micrometers at 0% relative humidity. A proton-conducting membrane may preferably have a thickness of at least about 6 micrometers, at least about 7 micrometers, at least about 8 micrometers, at least about 9 micrometers, or at least about 10 micrometers. Typically, the thickness of a proton-conducting membrane is about 200 micrometers or less at 0% relative humidity, for example, about 150 micrometers or less, about 100 micrometers or less, about 50 micrometers or less, about 30 micrometers or less, about 25 micrometers or less, or about 20 micrometers or less. The membrane thickness can be determined by analysis of scanning electron microscope (SEM) images of the membrane's cross-section. The proton-conducting film may preferably have a thickness in the range of approximately 5 to approximately 200 micrometers, approximately 6 to approximately 100 micrometers, approximately 6 to approximately 50 micrometers, approximately 7 to approximately 30 micrometers, or approximately 8 to approximately 20 micrometers (including both end values) at 0% relative humidity.

[0030] In some embodiments, the first ionomer and / or the second ionomer may be characterized independently with respect to the length of the side chains, or together with the relaxation modulus of the film formed from such ionomers. Preferably, the chain length of the first side chain between the first main chain and the end group is longer than the chain length of the second side chain between the second main chain and the end group. The chain length refers to the minimum number of atoms between the end group (a sulfur atom if the ionizable end group is a sulfonic acid) and the carbon atoms of the repeating units of the main chain to which the side chain is covalently bonded.

[0031] Preferably, the first ionomer is an LSC ionomer, and the second ionomer is an SSC ionomer. Therefore, it is preferable 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, preferably at least 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 atoms, more preferably at least 6 atoms and / or at most 8 atoms, and preferably at least 5 or 6 atoms, preferably at least 6 atoms.

[0032] Typically, the side chains of the first and second ionomers are, independently, linear or branched alkyl groups (preferably fully fluorinated, as described herein). In addition, one or more non-adjacent non-terminal carbon atoms of the alkyl group may be substituted with oxygen atoms. Preferably, the side chains are covalently bonded to the main chain via oxygen atoms (oxygen atom substitutions, if present, are understood to provide an ether, and may not include, for example, a ketone or ester group). In one preferred embodiment, the first side chain is branched.

[0033] In some embodiments, the first side chain is represented by the following formula: -O-CR2-CR2-O-(CR2) a -SO3H In the formula, 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 (alkyl, perfluoroalkyl, perchloroalkyl and perfluorochloroalkyl may have 1 to 10 carbon atoms, for example 1 to 3, and in some embodiments may be 1), preferably selected from the group consisting of H, F and perfluoroalkyl, for example, as shown in the formula below. -O-CX2-CX(CF2X)-O-(CX2) b -(CF2)c -SO3H In the equation, X is either F or Cl, and b and c are each greater than 1 (b+c=a).

[0034] In some embodiments, the second side chain is represented by the following equation: -O-(CR'2) d -SO3H In the formula, d is 1 to 4, preferably 1 to 3, preferably 2. Preferably, R' is as described above for R, or selected from H or F.

[0035] Therefore, in one preferred embodiment, the first side chain is -O-CR2-CR(CR3)-O-(CR)2)2-SO3H and the second side chain is -O-(CR')2)2-SO3H, where each R and R' is independently selected from H and F, and preferably R=R'=F.

[0036] Proton exchange membranes containing ionomer blends as described herein may be further characterized with respect to the molar and / or weight ratio of side chains and main chains provided by each ionomer.

[0037] In some embodiments, the molar ratio of the first side chain to the second side chain is at least 0.07:1, for example at least 0.15:1, for example at least 0.2:1. In some embodiments, the molar ratio of the first side chain to the second side chain is at most 1:1, for example at most 0.5:1.

[0038] In some embodiments, the ionomer blend includes 5 to 60% by weight of a first side chain, for example 10 to 50% by weight, or for example 20 to 40% by weight, relative to the weight of the side chains. In some embodiments, the ionomer blend includes 1 to 20% by weight of a first side chain, for example 2 to 15% by weight, relative to the weight of the blend. In some embodiments, the ionomer blend includes 95 to 40% by weight of a second side chain, for example 90 to 50% by weight, or for example 80 to 60% by weight, relative to the weight of the side chains. In some embodiments, the ionomer blend includes 10 to 20% by weight of a second side chain, preferably 15 to 18% by weight, relative to the weight of the ionomer blend. In some embodiments, the ionomer blend includes 60 to 89% by weight of a first main chain and a second main chain, preferably 67 to 83% by weight, relative to the weight of the ionomer blend. If the ionomer blend consists of a first ionomer and a second ionomer, the sum of the first side chains and the second side chains, as well as the first and second main chains, is equal to 100% by weight.

[0039] In another embodiment, the present invention provides a proton exchange membrane comprising a blend of a first ionomer and a second ionomer, wherein 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. Each of the first and second side chains contains a sulfonic acid terminal group. The length of the first side chain between the first main chain and the terminal group is longer than the length of the second side chain between the second main chain and the terminal group, and the length of the second side chain is at most 5 atoms.

[0040] The present invention also provides a catalyst-coated film including a proton exchange film as described herein, having a cathode catalyst layer applied to a first surface of the film and / or an anode catalyst layer applied to a second surface of the film.

[0041] The catalyst layer comprises one or more electrode catalysts. Each of the one or more electrode catalysts is independently a finely pulverized unsupported metal powder or a supported catalyst, and small nanoparticles can be dispersed on a conductive fine-particle carbon support. The electrode catalyst metal is preferably, (i) One or more platinum group metals (platinum, palladium, rhodium, ruthenium, iridium, and osmium), (ii) Gold or silver, (iii) base metals; Alternatively, an alloy or mixture containing one or more of these metals or their oxides may be selected.

[0042] A preferred electrode catalyst metal is platinum, which can be alloyed with other noble or base metals. Base metals are tin or transition metals that are not noble metals. Noble metals are platinum group metals (platinum, palladium, rhodium, ruthenium, iridium, or osmium), gold, or silver. Preferred 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 electrode catalyst is a supported catalyst, the amount of metal particles packed on the carbon support material is preferably in the range of 10 to 90% by weight, more preferably 15 to 75% by weight, of the weight of the resulting electrode catalyst. The exact electrode catalyst used will depend on the reaction intended to be catalyzed, and its selection is within the capabilities of those skilled in the art.

[0043] The catalyst layer may further contain additional components. Such additional components include, but are not limited to, catalysts that facilitate oxygen evolution and are therefore beneficial in cell inversion conditions and high potential shifts, or hydrogen peroxide decomposition catalysts. Examples of such catalysts and any other additives suitable for inclusion in the catalyst layer are known to those skilled in the art.

[0044] The present invention also provides a membrane electrode assembly comprising a proton exchange membrane as described herein and a gas diffusion electrode and / or porous transport layer on a first and / or second surface of the proton exchange membrane. The present invention also provides a membrane electrode assembly comprising a catalyst-coated proton-conducting membrane and a gas diffusion layer or porous transport layer present on at least one of the catalyst layers. The anode and cathode gas diffusion layers are preferably based on conventional gas diffusion substrates. Typical substrates include nonwoven paper or webs containing a network of carbon fibers and a thermosetting resin binder (e.g., TGP-H series carbon fiber paper available from Toray Industries Inc., Japan, or H2315 series available from Freudenberg FCCT KG, Germany, or Sigracet® series available from SGL Technologies GmbH, Germany, or Ballard Power Systems). Examples include the AvCarb® series from Inc., or carbon fiber cloth. Carbon paper, web, or cloth can be further processed and incorporated into the MEA to enhance either wettability (hydrophilicity) or moisture resistance (hydrophobicity). The properties of any treatment depend on the type of fuel cell and the operating conditions under which it is used. The substrate can be made more wettable by incorporating materials such as amorphous carbon black through impregnation from a liquid suspension, or its hydrophobicity can be enhanced by impregnating the pore structure of the substrate with a colloidal suspension of a polymer such as PTFE or polyfluoroethylene propylene (FEP), followed by drying and heating above the melting point of the polymer. In applications such as PEMFCs, a microporous layer can be applied to the gas diffusion substrate on the surface that will come into contact with the catalyst layer. The microporous layer typically contains a mixture of carbon black and a polymer such as polytetrafluoroethylene (PTFE). The porous transport layer is preferably based on a conventional porous transport substrate such as titanium mesh.

[0045] In further embodiments, 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 may be a fuel cell, such as a proton exchange membrane fuel cell. The electrochemical device may be an electrolytic device, such as a water electrolytic device.

[0046] In a further embodiment, the present invention provides a method for forming a proton exchange membrane as described herein, the method being (i) Dispersing the first ionomer and the second ionomer in a solvent to form a dispersion, (ii) Coating the surface of the substrate with the dispersion to a wet thickness of 50 μm to 1 mm, (iii) Evaporating the solvent and annealing the coated substrate to form a film, (iv) including removing the film from the substrate.

[0047] The inventors found that cracking is a particular problem when forming proton exchange films using solution casting, and that cracking is more pronounced when using high EW ionomers, especially when using relatively polar solvents. Such problems related to manufacturing can be mitigated by various parameters, such as solvent evaporation (drying) and annealing temperature profiles, solvent selection based on volatility, and modification of coating thickness. In the case of continuous roll-to-roll casting, it is also possible to modify, for example, the solution flow rate and substrate speed.

[0048] However, the inventors have surprisingly found that this problem can be addressed by simply forming films using blends of ionomers, specifically blends containing ionomers that individually form films having completely different relaxation moduli. Thus, the films described herein may or may be obtained by the methods described herein.

[0049] This method involves dispersing each of the first and second ionomers in a solvent to form a dispersion. The ionomers may be soluble, or the ionomer dispersion may contain a fine suspension of ionomer particles, preferably a homogeneous dispersion. Dispersions of PFSA ionomers are conventional in the art. Many different ionomers suitable for use in the present invention are commercially available and may be supplied as a dry solid or as a dispersion (generally in water or an alcohol-water mixture). Therefore, in some embodiments, this method may involve drying the aqueous dispersion to increase the ionomer concentration or completely drying the ionomer. Alternatively, suitable ionomers may be produced by conventional organic synthesis techniques. Those skilled in the art can easily and clearly verify the values ​​of the relaxation modulus, as described herein, for films formed from any ionomer.

[0050] Preferred solvents are polar protic solvents, typically aqueous solvents and / or alcoholic solvents. Preferably, the solvent consists of an alcohol and optionally water. Preferably, the solvent contains an alcohol having 1 to 5 carbon atoms. Ethanol (i.e., 2 carbon atoms) is particularly preferred. Preferably, the solvent contains 50 to 95% by weight, more preferably 60 to 80% by weight, of the alcohol relative to the weight of the solvent.

[0051] In some preferred embodiments, the dispersion contains the first ionomer and the second ionomer in a total amount of 10 to 40% by weight, preferably 20 to 30% by weight, relative to the weight of the dispersion. Preferably, the dispersion contains 1 to 6% by weight of the first ionomer and / or 9 to 34% by weight of the second ionomer, relative to the weight of the dispersion.

[0052] This method further includes coating the surface of the substrate with a dispersion to a wet thickness of 50 μm to 1 mm. The dispersion is preferably 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 application apparatus, thereby applying the coating to the substrate and then passing it through a gap between the knife and the support roller. R2R coating techniques may include, for example, slot die coating or gravure coating, in which the coating is squeezed onto the substrate by gravity or under pressure through a slot.

[0053] In some preferred embodiments, the surface of the substrate is formed of fluorinated ethylene propylene (FEP) or poly(4,4'-oxydiphenylene-pyromelliimide). Such surfaces have low adhesion and facilitate subsequent removal of the film. The substrate may include a polyester backing with an FEP coating, such as those available from Diacel®. Poly(4,4'-oxydiphenylene-pyromelliimide) substrates are available from Kapton® and may be known as such. The surface may be 10 to 200 μm thick.

[0054] This method further includes evaporating a solvent and annealing the coated substrate to form a film, and removing the film from the substrate (for example, by peeling the film off the substrate).

[0055] The evaporation of the solvent and annealing may involve a series of heating steps. Evaporation may involve 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 involve 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 involves heating the coated substrate to a temperature of 120°C to 240°C, such as 140°C to 200°C. The coated substrate may be annealed at such temperatures for 1 to 20 minutes. [Brief explanation of the drawing]

[0056] The present invention will now be further described with reference to the following non-limiting figures. [Figure 1] Examples of fluorinated ionomers suitable for use in the film of the present invention are provided below. [Figure 2] Exemplary perfluorinated ionomers suitable for use as the first ionomer in the film of the present invention are illustrated below. [Figure 3] Exemplary perfluorinated ionomers suitable for use as a second ionomer in the film of the present invention are illustrated below. [Figure 4] This is a scatter plot of the relaxation modulus measured at 10 minutes for films manufactured according to the examples. [Modes for carrying out the invention]

[0057] Figure 1 illustrates the chemical structure of ionomer 100. Ionomer 100 contains a main-chain polymer backbone based on repeating units of tetrafluoroethylene (i.e., C2F4), where some of the repeating units are substituted with side chains 105 (indicated as "SC" in Figure 1). The degree of substitution is determined by the number of unsubstituted repeating units, indicated by "m", where the value of m determines the equivalent. An ionomer is a polymer containing a large number of repeating units, tetrafluoroethylene and side-chain substituted tetrafluoroethylene units, indicated by "n" in the case of ionomer 100.

[0058] Figure 2 illustrates an exemplary perfluorinated ionomer 200 suitable for use as the first ionomer in the film of the present invention. Ionomer 200 comprises a main chain polymer backbone 205 (indicated as "MC" in Figure 2), to which the structure of a "long side chain" 210 is illustrated (for example, a side chain 105 as shown in Figure 1, in which case ionomer 100 / 200 illustrates the AGC PFSA ionomer used in the following examples). The long side chain 210 is covalently bonded to the main chain 205, particularly via an oxygen atom, and further atoms in the chain 210 are oxygen atoms that provide the ether. The side chain 210 containing the -CF3 moiety is branched. The long side chain 210 contains a sulfonic acid terminal 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 terminal group 215 and the main chain 205.

[0059] Figure 3 illustrates an exemplary perfluorinated ionomer 300 suitable for use as a second ionomer in the film of the present invention. Ionomer 300 comprises a main chain polymer backbone 305 (indicated as "MC" in Figure 3) to which the structure of a "short side chain" 210 is illustrated (for example, side chain 105 as shown in Figure 1, in which case ionomer 100 / 300 illustrates the Solvay PFSA ionomer used in the following examples). The short side chain 310 is covalently bonded to the main chain 305 specifically via an oxygen atom. The short side chain 310 contains a sulfonic acid terminal group 315. The chain length 320 is 3 atoms.

[0060] Figure 4 is a scatter plot of the relaxation modulus measured at 10 minutes for films produced according to examples with various concentrations (wt%) of the first ionomer relative to the total weight of the first and second ionomers. [Examples]

[0061] Membrane preparation Perfluorosulfonic acid (PFSA) dispersions were obtained from Solvay and AGC. The AGC PFSA ionomer is an LSC ionomer as shown in Figure 2, with an equivalent weight of approximately 720. The Solvay PFSA ionomer is an SSC ionomer as shown in Figure 3, with an equivalent weight of approximately 980. Each PFSA dispersion was poured into a Teflon-lined dish and allowed to evaporate the solvent under a weak airflow at room temperature for approximately 3 days. The solid PFSA mass remaining after solvent evaporation was left overnight in a 30°C oven under vacuum to remove residual moisture. PFSA dispersions for film casting were prepared by combining various amounts of AGC solid PFSA with the remainder, which was Solvay solid PFSA, as shown in Table 1 below. The solid PFSA was combined with 70% by weight ethanol solvent (the remainder being water) and a total of 25% by weight PFSA. Glass vials containing Solvay PFSA solid, AGC PFSA solid, and solvent were placed on a 60 rpm roller table for one day to completely disperse the PFSA. The blended PFSA dispersion was used within one week of preparation.

[0062] Approximately 1.5 mL of blended PFSA dispersion was coated onto a 75 μm thick Diacel FEP-coated polyester backing film using an Elcometer 4340 electric film laminating apparatus and a steel coating knife. The Diacel backing film was thoroughly washed with 35% 2-propanol (the remainder being water) and a lint-free wipe before coating. The drawdown rate was 10 mm / s, the coating knife gap was 300 μm, and the coating temperature was 20°C. After coating, the solvent was evaporated at room temperature for 10 minutes, and then the PFSA-coated Diacel backing film was placed in a 100°C binder oven for 10 minutes to further evaporate the solvent. The PFSA-coated Diacel backing film was then placed in another oven at 160°C for 12 minutes for an annealing step. The blended PFSA film was obtained by peeling the PFSA coating from the Diacel backing film. The final thickness of the PFSA film, measured using a Fischer magnetic induction thickness gauge at ambient temperature and relative humidity, was 20 ± 2 μm. The relaxation modulus of each film was measured according to the method described herein.

[0063] Stress relaxation measurement Transient tensile stress relaxation measurements were performed using a TA Instruments Discovery DMA 850 dynamic mechanical analyzer equipped with a film tension clamp and relative humidity (RH) unit. Film strips were cut mechanically using a punching die, with typical strip widths of 6–8 mm. Film thickness was measured at at least five locations using a Fischer MMS inspection DFT magnetic induction thickness gauge and was found to be 20 ± 2 μm. The film strips were clamped to the DMA 850 film tension clamp with a torque of 3 inch-pounds and a gauge length of 5.6–6 mm. Each sample was equilibrated for 45 minutes at 80°C and 0% RH under a preload of 50 mN. Tensile stress relaxation measurements were then performed by applying a 2.5% step strain to the sample for 60 minutes, while measuring the force required to maintain the sample at a 2.5% strain as a function of time t. The stress relaxation modulus E(t) was obtained by normalizing the measured force F(t) with respect to the cross-sectional area A of the sample and the applied strain ε0: E(t) = F(t) / Aε0.

[0064] [Table 1]

[0065] These results are plotted in Figure 4. These examples advantageously demonstrate the surprising effect that only a small amount of the first ionomer is required to provide a greater-than-expected reduction in the relaxation modulus of the blended film compared to the relaxation modulus of films formed individually from each ionomer. This is beneficial because it allows the blend to maintain a higher proportion of the more desirable SSC ionomer in order to preserve the useful proton exchange properties of the film. Thus, 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 crack size in the final film. Furthermore, the inventors found that cracking could be completely avoided with as little as 15 wt% of the first ionomer.

[0066] When used herein, the singular forms “a,” “an,” and “the” include multiple referents unless the context clearly indicates otherwise. The use of the term “comprising” is intended to be interpreted as including such features but not excluding other features, and is intended to include a selection of features that are necessarily limited to those described. In other words, the term also includes the limitations of “essentially consisting of” (intended to mean that certain further components may exist on the condition that they do not substantially affect the essential nature of the described feature) and “consisting of” (intended to mean that if the components are expressed as percentages by their proportions, these together make up 100%, while explaining any unavoidable impurities, but not including any other features).

[0067] The numerical lower and upper limits of the features described herein may preferably be combined to provide a closed range.

[0068] Terms such as "first," "second," etc., may be used herein to describe various features (e.g., ionomers), but it should be understood that features should not be limited by these terms. These terms are used only to distinguish one feature from another or further essential features. If a film contains more than two ionomers, it will be understood that the third or further ionomers individually satisfy the requirements described herein with respect to either the first or second ionomer. In other words, the first ionomer may, for example, include a blend of the first ionomers, the blend present in the quantities described herein, and each ionomer in the blend individually satisfies the parameters described for the first ionomer (e.g., relaxation modulus).

[0069] The detailed description above is provided for illustrative and illustrative purposes only and is not intended to limit the scope of the appended claims. Many modifications of the currently preferred embodiments illustrated herein will be obvious to those skilled in the art and remain within the scope of the appended claims and their equivalents.

[0070] To avoid any ambiguity, the entire contents of all documents found herein are incorporated herein by reference.

[0071] Some embodiments of the present invention are described below in the numbered clauses. 1. A proton exchange membrane comprising a blend of a first ionomer and a second ionomer, wherein 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. Each of the first and second side chains contains a sulfonic acid terminal group. The relaxation modulus of the film formed from the first ionomer is at least one-tenth, preferably at least one-hundredth, of the relaxation modulus of the film formed from the second ionomer. A proton exchange membrane in which the relaxation modulus of the membrane formed from the second ionomer is greater than 10,000 MPa. 2. The proton exchange membrane according to Clause 1, 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 Clause 1 or 2, wherein the relaxation modulus of the membrane formed from the first ionomer is less than 1,000 MPa, preferably less than 100 MPa. 4. A proton exchange membrane according to any one of clauses 1 to 3, wherein the membrane has a thickness of less than 200 μm. 5. The membrane, At least 5% by weight, preferably at least 10% by weight, of the first ionomer, and / or A proton exchange membrane according to any one of the claims 1 to 4, comprising up to 40% by weight, preferably up to 25% by weight, of the first ionomer. 6. The membrane, At least 60% by weight, preferably at least 75% by weight, of a second ionomer, and / or A proton exchange membrane according to any one of the clauses 1 to 5, comprising up to 95% by weight, preferably up to 90% by weight, of a second ionomer. 7. A proton exchange membrane according to any one of the claims 1 to 6, wherein the first ionomer and / or the second ionomer are partially fluorinated, preferably fully fluorinated. 8. A proton exchange membrane according to any one of clauses 1 to 7, wherein the first ionomer has an equivalent weight of 600 to 850 and the second ionomer has an equivalent weight of 850 to 1,100. 9. The proton exchange membrane according to any one of the claims 1 to 8, wherein the first ionomer has a lower equivalent weight than the second ionomer, and preferably the difference in equivalent weight between the first ionomer and the second ionomer is at least 50. 10. An electrochemical device, preferably a water electrolyzer or a fuel cell, comprising a proton exchange membrane as described in any one of clauses 1 to 9. 11. A method for forming a proton exchange membrane as described in any one of clauses 1 to 9, wherein the method is (i) Dispersing the first ionomer and the second ionomer in a solvent to form a dispersion, (ii) Coating the surface of the substrate with the dispersion to a wet thickness of 50 μm to 1 mm, (iii) Evaporating the solvent and annealing the coated substrate to form a film, (iv) A method comprising removing a film from a substrate. 12. The method according to Clause 11, wherein the dispersion contains the first ionomer and the second ionomer in an amount of 10 to 40% by weight, preferably 20 to 30% by weight, relative to the weight of the dispersion. 13. The method according to clause 11 or 12, wherein the dispersion contains 1 to 6% by weight of a first ionomer and / or 9 to 34% by weight of a second ionomer relative to the weight of the dispersion. 14. The method according to any one of the claims 11 to 13, wherein the solvent consists of an alcohol and optionally water, and preferably the alcohol has 1 to 5 carbon atoms. 15. The method according to Clause 14, wherein the solvent contains 50 to 95% by weight, preferably 60 to 80% by weight, of alcohol relative to the weight of the solvent. 16. The method according to any one of claims 11 to 15, wherein the surface of the substrate is preferably formed of fluorinated ethylene propylene (FEP) or poly(4,4'-oxydiphenylene-pyromellitimide) having a thickness of 10 to 200 μm. 17. The method according to any one of the claims 11 to 16, wherein the dispersion is coated onto a substrate by gap coating or roll-to-roll coating. 18. The method according to any one of the claims 11 to 17, wherein annealing comprises heating the coated substrate to a temperature of 120°C to 240°C, preferably for 1 to 20 minutes.

Claims

1. A proton exchange membrane comprising a blend of a first ionomer and a second ionomer, wherein 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. Each of the first and second side chains contains a sulfonic acid terminal group, The relaxation modulus of the film formed from the first ionomer is at least one-tenth, preferably at least one-hundredth, of the relaxation modulus of the film formed from the second ionomer. A proton exchange membrane 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 claim 1, 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 claim 1 or 2, 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 one of claims 1 to 3, wherein the membrane has a thickness of less than 200 μm.

5. The aforementioned film, At least 5% by weight, preferably at least 10% by weight, of the first ionomer, and / or A proton exchange membrane according to any one of claims 1 to 4, comprising up to 40% by weight, preferably up to 25% by weight, of the first ionomer.

6. The aforementioned film, A proton exchange membrane according to any one of claims 1 to 5, comprising at least 5% by weight of the first ionomer.

7. The aforementioned film, A proton exchange membrane according to any one of claims 1 to 6, comprising up to 15% by weight of the first ionomer.

8. The aforementioned film, At least 60% by weight, preferably at least 75% by weight, of the second ionomer, and / or A proton exchange membrane according to any one of claims 1 to 7, comprising up to 95% by weight, preferably up to 90% by weight, of the second ionomer.

9. The aforementioned film, A proton exchange membrane according to any one of claims 1 to 8, comprising at least 85% by weight of the second ionomer.

10. The aforementioned film, A proton exchange membrane according to any one of claims 1 to 9, comprising up to 95% by weight of the second ionomer.

11. The proton exchange membrane according to any one of claims 1 to 10, wherein the first ionomer and / or the second ionomer are partially fluorinated, preferably fully fluorinated.

12. The proton exchange membrane according to any one of claims 1 to 11, wherein the first ionomer has an equivalent amount of 600 to 850, and the second ionomer has an equivalent amount of 850 to 1,100.

13. The proton exchange membrane according to any one of claims 1 to 12, wherein the first ionomer has a lower equivalent weight than the second ionomer, and preferably the difference in equivalent weight between the first ionomer and the second ionomer is at least 50.

14. The proton exchange membrane according to any one of claims 1 to 13, 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 one of claims 1 to 14, wherein the second ionomer is a short-side-chain ionomer, and the length of the second side chain is at most four atoms.

16. An electrochemical device, preferably a water electrolyzer or a fuel cell, comprising a proton exchange membrane according to any one of claims 1 to 15.

17. A method for forming a proton exchange membrane according to any one of claims 1 to 15, wherein the method is (i) Dispersing the first ionomer and the second ionomer in a solvent to form a dispersion, (ii) Coating the surface of the substrate with the dispersion to a wet thickness of 50 μm to 1 mm, (iii) Evaporating the solvent and annealing the coated substrate to form the film, (iv) A method comprising removing the film from the substrate.

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

19. The method according to claim 17 or 18, wherein the dispersion contains 1 to 6% by weight of the first ionomer and / or 9 to 34% by weight of the second ionomer based on the weight of the dispersion.

20. The method according to any one of claims 17 to 19, wherein the solvent comprises an alcohol and optionally water, and preferably the alcohol has 1 to 5 carbon atoms.

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

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

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

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