Solid polymer electrolyte membrane, membrane electrode assembly, water electrolysis device, method for producing hydrogen, and method for producing membrane electrode assembly

Abstract

The present disclosure addresses the problem of providing a solid polymer electrolyte membrane which is capable of suppressing occurrence of curl and suppressing crossover of hydrogen. The present disclosure also addresses the problem of providing: a membrane electrode assembly; a water electrolysis device; and a method for producing a membrane electrode assembly.　A solid polymer electrolyte membrane according to the present disclosure comprises: a first layer that contains a first fluorine-containing polymer which has an ion exchange group and a platinum-containing material; and a second layer that contains a second fluorine-containing polymer which has an ion exchange group. The concentration of the platinum-containing material in the second layer is lower than the concentration of the platinum-containing material in the first layer, and the ion exchange capacity of the first fluorine-containing polymer is higher than the ion exchange capacity of the second fluorine-containing polymer. This solid polymer electrolyte membrane further has a reinforcement body that is disposed closer to the first layer-side surface of the solid polymer electrolyte membrane than the center position of the solid polymer electrolyte membrane in the thickness direction.

Description

Solid polymer electrolyte membrane, membrane electrode assembly, water electrolysis device, hydrogen production method, and membrane electrode assembly production method

[0001] The present disclosure relates to a solid polymer electrolyte membrane, a membrane electrode assembly, a water electrolysis device, a method for producing hydrogen, and a method for producing a membrane electrode assembly.

[0002] The use of polymer electrolyte membrane (PEM) water electrolysis devices has been considered for power-to-gas applications, i.e., converting surplus electricity into gas for storage and utilization. In recent years, there has been a demand for further improvements in the performance of water electrolysis devices, specifically, a demand for reducing hydrogen crossover. Here, hydrogen crossover refers to the transfer of hydrogen gas generated at the cathode to the anode side through a solid polymer electrolyte membrane in a water electrolysis device. Hydrogen crossover poses a problem of reduced hydrogen gas recovery efficiency.

[0003] Patent Document 1 discloses a water electrolysis device including a membrane, a cathode, and an anode, in which the membrane has first, second, and third regions arranged at equal intervals in the thickness direction, the first region being the region closest to the first main surface, the second region being the region closest to the second main surface, and the third region being located between the first and second regions, the first and third regions being substantially free of metallic Pt and Pt oxide, respectively, and at least one of metallic Pt or Pt oxide being incorporated in the second region, the cathode including a first catalyst on the first main surface of the membrane, and the anode including a second catalyst on the second main surface of the membrane.

[0004] U.S. Patent No. 11,414,770

[0005] The present inventors further studied a water electrolysis device having a solid polymer electrolyte membrane containing a platinum-containing material with reference to Patent Document 1, and found that although hydrogen crossover can be suppressed, there is still room for improvement in the phenomenon of curling of the solid polymer electrolyte membrane.

[0006] The present disclosure has been made in view of the above-mentioned problems, and an object of one embodiment of the present invention is to provide a solid polymer electrolyte membrane that can suppress the occurrence of curling and hydrogen crossover. Another object of one embodiment of the present invention is to provide a membrane electrode assembly, a water electrolysis apparatus, a method for producing hydrogen, and a method for producing the membrane electrode assembly.

[0007] The present disclosure has the following aspects. [1] A solid polymer electrolyte membrane comprising: a first layer containing a first fluoropolymer having ion exchange groups and a platinum-containing substance; and a second layer containing a second fluoropolymer having ion exchange groups, wherein the concentration of the platinum-containing substance in the second layer is lower than the concentration of the platinum-containing substance in the first layer, and the ion exchange capacity of the first fluoropolymer is higher than the ion exchange capacity of the second fluoropolymer, and further comprising a reinforcing body disposed closer to the surface of the solid polymer electrolyte membrane closer to the first layer than to a center position in the thickness direction of the solid polymer electrolyte membrane. [2] The solid polymer electrolyte membrane according to [1], wherein the solid polymer electrolyte membrane further contains cerium oxide. [3] The solid polymer electrolyte membrane according to [1] or [2], wherein the second layer does not contain the platinum-containing substance. [4] The solid polymer electrolyte membrane according to any of [1] to [3], wherein the reinforcing body is a woven fabric. [5] The solid polymer electrolyte membrane according to [4], wherein the woven fabric is made of a material selected from the group consisting of polytetrafluoroethylene, tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer, polyether ether ketone, and polyphenylene sulfide. [6] The solid polymer electrolyte membrane according to any of [1] to [5], wherein the thickness of the first layer is thinner than the thickness of the second layer. [7] The solid polymer electrolyte membrane according to any of [1] to [6], wherein the ratio of the thickness of the first layer to the total thickness of the first layer and the second layer is 0.50 or less. [8] The solid polymer electrolyte membrane according to any of [1] to [7], wherein the total thickness of the first layer and the second layer is 30 to 400 μm. [9] The solid polymer electrolyte membrane according to any of [1] to [8], wherein the absolute value of the difference between the ion exchange capacity of the first fluoropolymer and the ion exchange capacity of the second fluoropolymer is 0.10 to 1.40 meq / g dry resin.

[10] The solid polymer electrolyte membrane according to any one of [1] to [9], wherein the ion exchange group of the first fluoropolymer is a sulfonic acid type functional group, and the ion exchange group of the second fluoropolymer is a sulfonic acid type functional group.

[11] A membrane electrode assembly comprising the solid polymer electrolyte membrane according to any one of [1] to

[10] , a cathode catalyst layer disposed on a surface side of the second layer of the solid polymer electrolyte membrane, and an anode catalyst layer disposed on a surface side of the first layer of the solid polymer electrolyte membrane.

[12] A water electrolysis device comprising the membrane electrode assembly according to

[11] , a power supply unit connected to the cathode catalyst layer side and the anode catalyst layer side of the membrane electrode assembly, and a water supply unit that supplies water to the anode catalyst layer side.

[13] A hydrogen production method comprising producing hydrogen by electrolyzing water using the water electrolysis device according to

[12] .

[14] A method for producing a membrane electrode assembly comprising the solid polymer electrolyte membrane according to any one of [1] to

[10] , a cathode catalyst layer, and an anode catalyst layer, comprising forming the cathode catalyst layer on a surface side of the solid polymer electrolyte membrane facing the second layer, and forming the anode catalyst layer on a surface side of the solid polymer electrolyte membrane facing the first layer.

[0008] According to one embodiment of the present invention, it is possible to provide a solid polymer electrolyte membrane capable of suppressing curling and hydrogen crossover, a membrane electrode assembly, a method for producing a membrane electrode assembly, a method for producing hydrogen, and a water electrolysis apparatus.

[0009] 1 is a cross-sectional view schematically showing an example of a solid polymer electrolyte membrane according to the present disclosure; 2 is a cross-sectional view schematically showing another example of a solid polymer electrolyte membrane according to the present disclosure; 3 is a cross-sectional view schematically showing an example of a membrane electrode assembly according to the present disclosure;

[0010] The definitions of the following terms apply throughout the present specification and claims unless otherwise specified. An "ion exchange group" is a group that can exchange at least a portion of the ions contained in this group with other ions, and examples thereof include the sulfonic acid functional group and carboxylic acid functional group shown below. A "sulfonic acid functional group" is a sulfonic acid group (-SO ３ Here, the form of the sulfonate group is, for example, (—SO ３ － ) Ma ＋ , (-SO ３－ ) ２ Mb ２＋ , and (-SO ３ － ) ３ Mc ３＋ (However, Ma ＋ is an alkali metal ion or a quaternary ammonium cation, and Mb ２＋ is a divalent metal ion, Mc ３＋ is a trivalent metal ion.) When there are two ligands, the number of ion exchange groups is counted as two, and when there are three ligands, the number of ion exchange groups is counted as three. "Carboxylic acid type functional group" means a carboxylic acid group (-COOH) or a carboxylic acid salt group. Here, the form of the carboxylic acid salt group can be, for example, (-COO － ) Ma ＋ , (-COO － ) ２ Mb ２＋ , and (-COO － ) ３ Mc ３＋ (However, Ma ＋ is an alkali metal ion or a quaternary ammonium cation, and Mb ２＋ is a divalent metal ion, Mc ３＋ is a trivalent metal ion.) Note that when there are two ligands, the number of ion exchange groups is counted as two, and when there are three ligands, the number of ion exchange groups is counted as three. A "precursor membrane" is a membrane containing a polymer having a group that can be converted into an ion exchange group. A "group that can be converted into an ion exchange group" means a group that can be converted into an ion exchange group by treatment such as hydrolysis or acidification. A "group that can be converted into a sulfonic acid functional group" means a group that can be converted into a sulfonic acid functional group by treatment such as hydrolysis or acidification. A "group that can be converted into a carboxylic acid functional group" means a group that can be converted into a carboxylic acid functional group by known treatment such as hydrolysis or acidification.

[0011] The term "unit" in a polymer refers to an atomic group based on one molecule of a monomer formed by polymerization of the monomer. The unit may be an atomic group formed directly by the polymerization reaction, or may be an atomic group in which a part of the atomic group is converted into a different structure by treating the polymer obtained by the polymerization reaction.

[0012] A numerical range expressed using "to" means a range that includes the numerical values ​​before and after "to" as the upper and lower limits. In the numerical ranges described in stages in this specification, the upper or lower limit described in a certain numerical range may be replaced with the upper or lower limit of another numerical range described in stages. Furthermore, in the numerical ranges described in this specification, the upper or lower limit described in a certain numerical range may be replaced with a value shown in the examples.

[0013] The present disclosure will be described in detail below with reference to the embodiments shown in the accompanying drawings. The following description of the components may be based on representative embodiments of the present disclosure, but the present disclosure is not limited to such embodiments. In the embodiments shown in each drawing, components may be drawn on a scale different from the actual scale to make them easier to see and explain.

[0014] [Electrolyte Membrane] FIG. 1 is a cross-sectional view schematically illustrating an example of the configuration of a solid polymer electrolyte membrane (hereinafter also simply referred to as "electrolyte membrane") according to the present disclosure. An electrolyte membrane 10 according to the present disclosure has a first layer 11, a second layer 12, and a reinforcing body 13. As shown in the figure, of the surfaces (main surfaces) of the electrolyte membrane 10, the surface facing the first layer 11 is referred to as "surface A," and the surface facing the second layer 12 is referred to as "surface B." In addition, the surface located at the center in the thickness direction of the electrolyte membrane 10 is referred to as "center surface C," and the interface between the first layer 11 and the second layer 12 is referred to as "interface D."

[0015] The first layer 11 contains a first fluoropolymer having ion exchange groups (hereinafter also referred to as "fluoropolymer (I-1)") and a platinum-containing substance, and the second layer 12 contains a second fluoropolymer having ion exchange groups (hereinafter also referred to as "fluoropolymer (I-2)"). In the electrolyte membrane 10 of the present disclosure, the concentration of the platinum-containing substance in the first layer 11 is higher than the concentration of the platinum-containing substance in the second layer 12. Therefore, it can be said that the platinum-containing substance is unevenly distributed on the surface A side of the electrolyte membrane 10. Furthermore, the ion exchange capacity of the fluoropolymer (I-1) contained in the first layer 11 is larger than the ion exchange capacity of the fluoropolymer (I-2) contained in the second layer 12. As shown in the figure, the reinforcing body 13 is disposed on the surface A side closer to the first layer 11 than the central plane C located at the center position in the thickness direction of the electrolyte membrane 10 (hereinafter also simply referred to as "disposed on the surface A side"). This means that the thickness T of the first layer 11 is １ is the thickness T of the second layer 12 ２ This can also be seen from the fact that the reinforcing member 13 is disposed at the interface D between the first layer 11 and the second layer 12 .

[0016] When applied to a water electrolysis device, the electrolyte membrane of the present disclosure can suppress the occurrence of curling and can also suppress hydrogen crossover. Although the details of the reason for this are not clear, it is presumed to be due to the following reasons. When the electrolyte membrane of the present disclosure is applied to a water electrolysis device, it is presumed that the ion exchange capacity of the fluoropolymer contained in the first layer is larger than the ion exchange capacity of the fluoropolymer contained in the second layer, and that hydrogen generated on the cathode side reacts with oxygen generated on the anode side, at least on the platinum-containing material contained in the first layer, to form water, thereby suppressing the movement of hydrogen to the anode side (hydrogen crossover). It is also thought that the fluoropolymer (I-1) having a relatively large ion exchange capacity has a relatively large content of ion exchange groups, and the fluoropolymer (I-2) having a relatively small ion exchange capacity has a relatively small content of ion exchange groups. It is presumed that the fluorine-containing polymers contained in the two laminated layers have different contents of ion exchange groups, which causes a difference in the dimensional change rate of each layer, resulting in curling of the laminate. In the electrolyte membrane of the present disclosure, it is believed that curling can be suppressed in an electrolyte membrane having a first layer and a second layer with different ion exchange capacities by disposing a reinforcing member that improves the strength of the electrolyte membrane on the surface A side of the first layer, which has a relatively high content of ion exchange groups. By suppressing curling in the electrolyte membrane, scratches on the surface of the catalyst layer formed on the surface of the electrolyte membrane due to contact between the edge of the electrolyte membrane and the catalyst layer can be suppressed.

[0017] In the electrolyte membrane 10 shown in FIG. 1, the thickness T １ is the thickness T of the second layer 12 ２ 1 , and a reinforcing body 13 is disposed at the interface D between the first layer 11 and the second layer 12. However, the electrolyte membrane of the present disclosure is not limited to the configuration shown in Fig. 1 as long as the concentration of the platinum-containing material in the first layer 11 is higher than the concentration of the platinum-containing material in the second layer 12, the ion exchange capacity of the fluoropolymer (I-1) is higher than the ion exchange capacity of the fluoropolymer (I-2), and the reinforcing body is disposed on the surface A side. For example, in the electrolyte membrane of the present disclosure, the thicknesses of the first layer and the second layer and the position of the reinforcing body can be selected.

[0018] FIG. 2 is a cross-sectional view schematically illustrating another example of the configuration of an electrolyte membrane according to the present disclosure. The electrolyte membrane 20 according to the present disclosure illustrated in FIG. 2 includes a first layer 11, a second layer 12, and a reinforcing member 13. The meanings of the symbols and the functions of the components in FIG. 2 are the same as those in FIG. 1. In the electrolyte membrane 10 illustrated in FIG. 1, the reinforcing member 13 is disposed at the interface D between the first layer 11 and the second layer 12. However, in the electrolyte membrane 20 illustrated in FIG. 2, the reinforcing member 13 is disposed inside the second layer 12, between a central plane C at the center of the thickness direction of the electrolyte membrane 20 and the interface D. The electrolyte membrane 20 having such a reinforcing member 13 can also achieve the desired effect.

[0019] The reinforcement may be disposed inside the first layer. For example, the first layer may have a thickness T １ is the thickness of the second layer T ２ In an electrolyte membrane thinner than 1000 nm, a reinforcing body may be disposed between surface A and interface D.

[0020] In the electrolyte membrane 10 shown in FIG. 1, the thickness T １ is the thickness of the second layer T ２ The thickness of the first layer T １ is the thickness of the second layer T ２ and the thickness of the second layer T ２ The thickness of the first layer T １ is the thickness of the second layer T ２ The following is preferred: the thickness of the second layer T ２ The thinner the better.

[0021] The configuration and physical properties of the electrolyte membrane of the present disclosure will be described in more detail below.

[0022] [First Layer] The first layer contains a fluoropolymer (I-1) and a platinum-containing substance. The first layer may contain components other than the fluoropolymer (I-1) and the platinum-containing substance. Examples of such components include cerium oxide.

[0023] 1 and 2 show the case where the first layer is a single layer, but the first layer may have a multi-layer structure consisting of a plurality of layers. When the first layer has a multi-layer structure, the single layers constituting the multi-layer structure may be the same or different in terms of the composition, ion exchange capacity and content of the fluorine-containing polymer (I-1), the type and content of the platinum-containing substance, and the type and content of other components.

[0024] In the electrolyte membrane, the first layer and the second layer can be distinguished, for example, by the content of the platinum-containing substance. Specifically, based on the average value of the concentration of the platinum-containing substance in the thickness direction of the electrolyte membrane, the region where the concentration of the platinum-containing substance is equal to or greater than the average value may be defined as the first layer, and the region where the concentration of the platinum-containing substance is less than the average value may be defined as the second layer. A method for measuring the concentration of the platinum-containing substance in the thickness direction of the electrolyte membrane will be described later.

[0025] <Fluoropolymer (I-1)> The first layer may contain one type of fluoropolymer (I-1) alone, or may contain two or more types of fluoropolymer (I-1). The first layer may contain a polymer other than the fluoropolymer (I-1), but preferably consists essentially of the fluoropolymer (I-1). "Consisting essentially of the fluoropolymer (I-1)" means that the content of the fluoropolymer (I-1) is 90% by mass or more relative to the total mass of the polymers in the first layer. The upper limit of the content of the fluoropolymer (I-1) is 100% by mass relative to the total mass of the polymers in the first layer.

[0026] Specific examples of polymers other than the fluorine-containing polymer (I-1) include one or more polyazole compounds selected from the group consisting of polymers of heterocyclic compounds containing one or more nitrogen atoms in the ring, and polymers of heterocyclic compounds containing one or more nitrogen atoms and an oxygen atom and / or a sulfur atom in the ring. Specific examples of polyazole compounds include polyimidazole compounds, polybenzimidazole compounds, polybenzobisimidazole compounds, polybenzoxazole compounds, polyoxazole compounds, polythiazole compounds, and polybenzothiazole compounds. Furthermore, from the viewpoint of the oxidation resistance of the electrolyte membrane, other polymers also include polyphenylene sulfide resins and polyphenylene ether resins.

[0027] The fluoropolymer (I-1) has an ion-exchange group. Specific examples of the ion-exchange group include a sulfonic acid type functional group and a carboxylic acid type functional group, and the sulfonic acid type functional group is preferred from the viewpoint of further reducing the electrolysis voltage when the electrolyte membrane is applied to a water electrolysis device. Below, mainly, embodiments of the fluoropolymer having a sulfonic acid type functional group (hereinafter, also referred to as "fluoropolymer (S)") will be described in detail.

[0028] The fluorine-containing polymer (S) preferably contains a unit based on a fluorine-containing olefin and a unit having a sulfonic acid functional group and a fluorine atom. Examples of the fluorine-containing olefin include fluoroolefins having 2 to 3 carbon atoms and having one or more fluorine atoms in the molecule. Specific examples of the fluoroolefin include tetrafluoroethylene (hereinafter also referred to as "TFE"), chlorotrifluoroethylene, vinylidene fluoride, vinyl fluoride, and hexafluoropropylene. Among these, TFE is preferred from the viewpoints of the production cost of the monomer, reactivity with other monomers, and excellent properties of the resulting fluorine-containing polymer (S). One type of fluorine-containing olefin may be used alone, or two or more types may be used in combination.

[0029] As the unit having a sulfonic acid type functional group and a fluorine atom, a unit represented by formula (1) is preferred. ２ -CF(-L-(SO３ M) ｎ )]-

[0030] L is an (n+1)-valent perfluorohydrocarbon group which may contain an etheric oxygen atom. The etheric oxygen atom may be located at a terminal of the perfluorohydrocarbon group or between carbon atoms. The (n+1)-valent perfluorohydrocarbon group preferably has 1 or more carbon atoms, more preferably 2 or more carbon atoms, and preferably 20 or less, more preferably 10 or less.

[0031] L is preferably an (n+1)-valent perfluoroaliphatic hydrocarbon group which may contain an etheric oxygen atom, more preferably a divalent perfluoroalkylene group which may contain an etheric oxygen atom, where n = 1, or a trivalent perfluoroaliphatic hydrocarbon group which may contain an etheric oxygen atom, where n = 2. The divalent perfluoroalkylene group may be either linear or branched.

[0032] M is a hydrogen atom, an alkali metal, or a quaternary ammonium cation. The n Ms may be the same or different. n is 1 or 2.

[0033] The unit represented by formula (1) is preferably a unit represented by formula (1-1), a unit represented by formula (1-2), a unit represented by formula (1-3), or a unit represented by formula (1-4). ２ -CF(-O-R ｆ１ -SO ３ M)] - Formula (1-2) - [CF ２ -CF(-R ｆ１ -SO ３ M) ]-

[0034]

[0035]

[0036] R ｆ１ is a perfluoroalkylene group which may contain an oxygen atom between carbon atoms. The number of carbon atoms in the perfluoroalkylene group is preferably 1 or more, more preferably 2 or more, and is preferably 20 or less, more preferably 10 or less.

[0037] R ｆ２ is a single bond or a perfluoroalkylene group which may contain an oxygen atom between carbon atoms. The number of carbon atoms in the perfluoroalkylene group is preferably 1 or more, more preferably 2 or more, and is preferably 20 or less, more preferably 10 or less.

[0038] R ｆ３ is a single bond or a perfluoroalkylene group which may contain an oxygen atom between carbon atoms. The number of carbon atoms in the perfluoroalkylene group is preferably 1 or more, more preferably 2 or more, and is preferably 20 or less, more preferably 10 or less.

[0039] r is 0 or 1. m is 0 or 1. M is as defined above.

[0040] As the unit represented by formula (1-1) and the unit represented by formula (1-2), a unit represented by formula (1-5) is more preferred. ２ -CF(-(CF ２ ) ｘ -(OCF ２ CFY) ｙ -O-(CF ２ ) ｚ -SO ３ M)]—x is 0 or 1, y is an integer from 0 to 2, z is an integer from 1 to 4, and Y is F or CF ３ M is as described above.

[0041] Specific examples of the unit represented by formula (1-1) include the following units. In the formula, w is an integer of 1 to 8, and v is an integer of 1 to 5. The definition of M in the formula is as described above. -[CF ２ -CF(-O-(CF ２ ) ｗ -SO ３ M)]- -[CF ２ -CF(-O-CF ２ CF (CF ３ )-O-(CF ２ ) ｗ -SO ３ M)]- -[CF ２ -CF(-(O-CF２ CF (CF ３ )) ｖ -SO ３ M) ]-

[0042] Specific examples of the unit represented by formula (1-2) include the following units. In the formula, w is an integer of 1 to 8. The definition of M in the formula is as described above. -[CF ２ -CF(-(CF ２ ) ｗ -SO ３ M)]- -[CF ２ -CF (-CF ２ -O-(CF ２ ) ｗ -SO ３ M) ]-

[0043] The unit represented by formula (1-3) is preferably a unit represented by formula (1-3-1), where M is defined as above.

[0044]

[0045] R ｆ４ is a linear perfluoroalkylene group having 1 to 6 carbon atoms, and R ｆ５ is a single bond or a linear perfluoroalkylene group having 1 to 6 carbon atoms which may contain an oxygen atom between the carbon atoms. The definitions of r and M are as described above.

[0046] Specific examples of the unit represented by formula (1-3-1) include the following.

[0047]

[0048] As the unit represented by formula (1-4), a unit represented by formula (1-4-1) is preferred. ｆ１ , R ｆ２ and M are defined as above.

[0049]

[0050] Specific examples of the unit represented by formula (1-4-1) include the following.

[0051]

[0052] The unit having a sulfonic acid type functional group and a fluorine atom may be used alone or in combination of two or more.

[0053] The fluorine-containing polymer (I-1) may contain units based on other monomers other than the units based on fluorine-containing olefin and the units having a sulfonic acid functional group and a fluorine atom. Specific examples of other monomers include CF ２ = CFR ｆ６ (However, R ｆ６ is a perfluoroalkyl group having 2 to 10 carbon atoms, CF ２ =CF-OR ｆ７ (However, R ｆ７ is a perfluoroalkyl group having 1 to 10 carbon atoms, CF ２ = CFO (CF ２ ) ｖ CF = CF ２ (wherein v is an integer of 1 to 3.) The content of units based on other monomers is preferably at most 30 mass % based on all units in the fluoropolymer (I-1), from the viewpoint of maintaining ion exchange performance.

[0054] The ion exchange capacity of the fluoropolymer (I-1) is preferably 0.90 milliequivalents / gram dry resin or more, more preferably greater than 1.10 milliequivalents / gram dry resin, even more preferably 1.20 milliequivalents / gram dry resin or more, particularly preferably 1.25 milliequivalents / gram dry resin or more, and most preferably 1.30 milliequivalents / gram dry resin or more, from the viewpoint of being able to further reduce the electrolysis voltage when the electrolyte membrane is applied to a water electrolysis device and to suppress hydrogen crossover. The ion exchange capacity of the fluoropolymer (I-1) is preferably 2.00 milliequivalents / gram dry resin or less, more preferably 1.50 milliequivalents / gram dry resin or less, and even more preferably 1.43 milliequivalents / gram dry resin or less, from the viewpoint of the strength of the membrane electrode assembly when water is contained. The ion exchange capacity of the fluoropolymer (I-1) can be adjusted by changing the content of ion exchange groups in the fluoropolymer (I-1).

[0055] "Ion exchange capacity" is a value corresponding to the ion exchange ability per dry mass of a fluoropolymer, calculated as follows. First, the fluoropolymer is placed in a glove box flushed with dry nitrogen for 24 hours, and the dry mass of the fluoropolymer is measured. Thereafter, the fluoropolymer is immersed in a 2 mol / L aqueous sodium chloride solution at 60°C for 1 hour. The fluoropolymer is washed with ultrapure water, then removed, and the solution in which the fluoropolymer was immersed is titrated with a 0.1 mol / L aqueous sodium hydroxide solution to determine the ion exchange capacity of the fluoropolymer. The ion exchange capacity of the fluoropolymer in the thickness direction of the electrolyte membrane, the first layer, and the second layer can be calculated, for example, by measuring a cross section of the electrolyte membrane along the thickness direction based on micro-Raman spectroscopy. More specifically, the electrolyte membrane is cut along the thickness direction, and the obtained cross section is measured along the thickness direction using a micro-Raman spectrometer. The peaks derived from TFE units and the peaks derived from ion exchange groups (e.g., SO) in the side chains, which appear in the obtained Raman spectrum, are measured. ３ The ion exchange capacity of the fluoropolymer in the thickness direction can be calculated from the intensity ratio of the peak derived from fluorine-containing polymer to the peak derived from fluorine-containing polymer X).

[0056] The content of the fluorine-containing polymer (I-1) is preferably from 80 to 99% by mass, based on the total mass of the first layer, when a reinforcing body is disposed inside the first layer, and is preferably from 90 to 99.9% by mass, based on the total mass of the first layer, when a reinforcing body is not disposed inside the first layer.

[0057] <Platinum-containing material> A platinum-containing material is a compound containing platinum atoms. Specific examples of platinum-containing materials include platinum itself, platinum oxide, platinum-containing composite metal oxides, and platinum alloys. Specific examples of platinum-containing composite oxides include M ｘ Pt ３ O ４(M is at least one metal atom selected from the group consisting of Li, Na, Mg, Ca, Zn, Cd, Co, Ni, Mn, Cu, Ag, Bi, and Ce, and x is greater than 0 and equal to or less than 1.) Specific examples of platinum alloys include alloys containing platinum and at least one metal selected from the group consisting of transition metals and noble metals other than platinum. The second layer may contain one type of platinum-containing material alone, or may contain two or more types of platinum-containing materials.

[0058] Specific examples of the shape of the platinum-containing material include particles and sheets. When the platinum-containing material is particulate, it may be a core-shell type particle. An example of a core-shell type particle is a particle in which the core is carbon or contains a metal other than platinum, and the shell contains platinum atoms. When the platinum-containing material is particulate, the average particle diameter (D50) of the platinum-containing material is preferably 1 nm or more, more preferably 5 nm or more, even more preferably 10 nm or more, particularly preferably 100 nm or more, and preferably 50 μm or less, more preferably 30 μm or less, even more preferably 14 μm or less, and particularly preferably 7 μm or less. The average particle diameter (D50) of the platinum-containing material may also be 500 nm or more.

[0059] The method for measuring the average particle size of a platinum-containing material is as follows. The average particle size of a platinum-containing material is obtained by measuring the particle size of 40,000 particles in a dry state using an image particle size distribution analyzer (for example, the "Morphologi (registered trademark)" series manufactured by Malvern Panalytic) and determining the cumulative 50% diameter (D50) of the volume-based particle size distribution cumulative curve. When particles less than 500 nm are included, the circle-equivalent diameters of 100 particles are measured using a scanning electron microscope (SEM) and the cumulative 50% diameter (D50) of the volume-based particle size distribution curve is determined. The platinum-containing material may be composed of monodispersed primary particles or secondary particles formed by agglomeration of multiple primary particles. When the platinum-containing material contains secondary particles, the particle size of the secondary particles is measured when measuring the D50.

[0060] The mass of the platinum-containing material contained in the first layer is２ 0.005 mg / cm ２ More than 0.010 mg / cm ２ More preferably, 0.015 mg / cm or more ２ More preferably, 0.050 mg / cm ２ Preferably, 0.040 mg / cm or less ２ When the mass of the platinum-containing material contained in the first layer is equal to or greater than the above lower limit, hydrogen crossover can be further suppressed. ２ If the above ratio is less than 1, the electrolysis voltage of the electrolyte membrane can be reduced, and a low-cost membrane electrode assembly and water electrolysis device can be provided.

[0061] The mass ratio of the platinum-containing material to the fluoropolymer (I-1) in the first layer (mass of platinum-containing material / mass of fluoropolymer (I-1)) is preferably 0.0005 or more, more preferably 0.004 or more, even more preferably 0.007 or more, and is preferably 0.024 or less, more preferably 0.014 or less. When the mass ratio is 0.005 or more, hydrogen crossover can be further suppressed. When the mass ratio is 0.024 or less, the electrolysis voltage of the electrolyte membrane can be further reduced, and a low-cost membrane electrode assembly and water electrolysis device can be provided.

[0062] The platinum-containing material may be supported on a carrier. Specific examples of the carrier include carbon carriers such as carbon black powder, graphitized carbon, carbon fiber, and carbon nanotubes. When the platinum-containing material is supported on a carrier, the amount of the platinum-containing material supported is preferably 10% by mass or more, more preferably 20% by mass or more, even more preferably 30% by mass or more, and preferably 50% by mass or less, based on the total mass of the platinum-containing material and the carrier.

[0063] The location of the platinum-containing substance in each layer of the electrolyte membrane and the content of the platinum-containing substance can be identified from element distribution using, for example, an energy dispersive X-ray analyzer (product name "QUANTAX FlatQUAD", manufactured by Bruker).

[0064] <Cerium oxide> The first layer may further contain cerium oxide. As will be described later, it is preferable that the electrolyte membrane further contains cerium oxide in at least one of the first layer and the second layer, in order to improve the chemical durability of the electrolyte membrane.

[0065] Cerium oxide is CeO ２ (cerium (IV) oxide), Ce ２ O ３ (cerium (III) oxide) may be used, but from the viewpoint of stability, CeO ２ Cerium oxide may be doped with polyvalent metal ions such as zirconium and praseodymium.

[0066] The cerium oxide is preferably in particulate form. When the cerium oxide is in particulate form, the average particle size (D50) of the cerium oxide is preferably 10 nm or more, more preferably 100 nm or more, even more preferably 1 μm or more, particularly preferably 3 μm or more, and is preferably 30 μm or less, more preferably 14 μm or less, and even more preferably 10 μm or less. When the average particle size of cerium oxide is 10 nm or more, aggregation of cerium oxide is suppressed, and it is easy to achieve a stable dispersion state. When the average particle size of cerium oxide is 30 μm or less, the chemical durability of the electrolyte membrane can be further improved.

[0067] The method for measuring the average particle size of cerium oxide is the same as the method for measuring the average particle size (D50) of the platinum-containing material.

[0068] 1 cm of the first layer ２ The mass of cerium oxide per ２ More than 0.010 mg / cm ２ More preferably, 0.015 mg / cm or more ２ More preferably, 0.050 mg / cm ２ Preferably, 0.030 mg / cm or less ２The following is more preferable. When the mass of cerium oxide is equal to or greater than the above-mentioned lower limit, the first layer has a whitish color, making it easier to detect foreign matter present in the first layer. As a result, when the electrolyte membrane is applied to a water electrolysis device, the electrolyte membrane can be used while avoiding the portion where foreign matter is present, thereby suppressing the occurrence of pinholes in the electrolyte membrane due to foreign matter. When the mass of cerium oxide is equal to or less than the above-mentioned upper limit, the electrolysis voltage of the electrolyte membrane can be further reduced, making it possible to provide a low-cost membrane electrode assembly and water electrolysis device.

[0069] The location of cerium oxide in the electrolyte membrane can be identified by element distribution using, for example, an energy dispersive X-ray analyzer (product name "QUANTAX FlatQUAD", manufactured by Bruker).

[0070] Thickness of the first layer T １ The thickness T of the first layer (when the first layer has a multi-layer structure, the total thickness of each layer) is preferably 5 to 60 μm, more preferably 10 to 40 μm, and even more preferably 10 to 30 μm. １ When the thickness T of the first layer is equal to or greater than the lower limit, the mechanical strength of the electrolyte membrane is improved and the current efficiency is further improved. １ If is equal to or less than the upper limit, the electrolysis voltage of the electrolyte membrane can be kept low.

[0071] Thickness of the first layer T １ and the thickness T of the second layer ２ can be measured, for example, by observing a cross section of the electrolyte membrane along the thickness direction with an optical microscope or the like and measuring the distance between a boundary line appearing as a difference in the concentration of platinum-containing substances and each surface of the electrolyte membrane. Furthermore, when the second layer contains platinum, it can also be measured by using an energy dispersive X-ray analyzer (product name "QUANTAX FlatQUAD", manufactured by Bruker) to measure the concentration of platinum-containing substances in the thickness direction based on the element distribution of a cross section along the thickness direction of the electrolyte membrane.

[0072] [Second Layer] The second layer contains a fluoropolymer (I-2) having ion exchange groups and an ion exchange capacity smaller than that of the fluoropolymer (I-1). The second layer may contain components other than the fluoropolymer (I-2). Examples of such components include a platinum-containing material and cerium oxide.

[0073] The second layer may be a single layer or may have a multi-layer structure consisting of a plurality of layers. When the second layer has a multi-layer structure, the composition, ion exchange capacity and content of the fluorine-containing polymer (I-2), and the types and contents of other components may be the same or different in the single layers constituting the multi-layer structure. When the second layer has a multi-layer structure, it is preferable that the ion exchange capacity of the fluorine-containing polymer (I-2) contained in each layer constituting the multi-layer structure is all smaller than that of the fluorine-containing polymer (I-1) contained in the first layer.

[0074] <Fluorine-containing polymer (I-2)> The second layer may contain one type of fluorine-containing polymer (I-2) alone, or may contain two or more types of fluorine-containing polymer (I-2). The second layer may contain a polymer other than the fluorine-containing polymer (I-2), but preferably consists essentially of the fluorine-containing polymer (I-2). "Substantially consists of the fluorine-containing polymer (I-2)" means that the content of the fluorine-containing polymer (I-2) is 90% by mass or more relative to the total mass of the polymers in the second layer. The upper limit of the content of the fluorine-containing polymer (I-2) is 100% by mass relative to the total mass of the polymers in the second layer. Specific examples of the polymer other than the fluorine-containing polymer (I-2) are the same as those of the polymer other than the fluorine-containing polymer (I-1) described above.

[0075] As the fluoropolymer (I-2), it is preferred to use a polymer similar to the fluoropolymer (I-1), including its preferred embodiments, except that the ion exchange capacity is different.

[0076] The ion exchange capacity of the fluoropolymer (I-2) is preferably 0.90 milliequivalents / gram dry resin or more, more preferably greater than 1.10 milliequivalents / gram dry resin, even more preferably 1.15 milliequivalents / gram dry resin or more, particularly preferably 1.20 milliequivalents / gram dry resin or more, and most preferably 1.25 milliequivalents / gram dry resin or more, from the viewpoint of further reducing the electrolysis voltage when the electrolyte membrane is applied to a water electrolysis device. The ion exchange capacity of the fluoropolymer (I-2) is preferably 2.00 milliequivalents / gram dry resin or less, more preferably 1.50 milliequivalents / gram dry resin or less, even more preferably 1.43 milliequivalents / gram dry resin or less, from the viewpoint of the strength of the membrane electrode assembly when wet.

[0077] In the electrolyte membrane of the present disclosure, the ion exchange capacity of the fluoropolymer (I-1) contained in the first layer is greater than the ion exchange capacity of the fluoropolymer (I-2) contained in the second layer. This allows the electrolysis voltage to be lowered even in the first layer, where the concentration of a platinum-containing substance is high. Furthermore, hydrogen crossover can be suppressed. The absolute value of the difference between the ion exchange capacity of the fluoropolymer (I-1) and the ion exchange capacity of the fluoropolymer (I-2) is preferably 0.10 to 1.40 milliequivalents / gram of dry resin, more preferably 0.10 to 0.70 milliequivalents / gram of dry resin, even more preferably 0.01 to 0.50 milliequivalents / gram of dry resin, and particularly preferably 0.10 to 0.30 milliequivalents / gram of dry resin, from the viewpoints of lowering the electrolysis voltage and suppressing hydrogen crossover. The ion exchange capacity of the fluoropolymer (I-2) can be adjusted by changing the content of ion exchange groups in the fluoropolymer (I-2).

[0078] <Platinum-containing substance> The concentration of the platinum-containing substance in the second layer is lower than the concentration of the platinum-containing substance in the first layer. The second layer may contain the platinum-containing substance at a lower concentration than the first layer, or may not contain any platinum-containing substance. Since the electrolysis voltage can be reduced by reducing the amount of platinum that does not contribute to ion conduction in the electrolyte membrane, it is preferable that the second layer does not contain any platinum-containing substance. Here, "not containing any platinum-containing substance" means that the platinum-containing substance is contained in an amount of 1 cm of the second membrane. ２The mass of platinum content per unit area is 0.001 mg / cm ２ This means that:

[0079] When the second layer contains a platinum-containing substance, the mass of the platinum-containing substance contained in the second layer is ２ 0.050 mg / cm ２ Preferably, 0.040 mg / cm or less ２ More preferably, 0.015 mg / cm or less ２ More preferably, it is less than 0.005 mg / cm ２ If the mass of the platinum-containing material contained in the second layer is equal to or less than the above upper limit, hydrogen crossover can be further suppressed.

[0080] The absolute value of the difference between the mass of the platinum-containing material contained in the first layer and the mass of the platinum-containing material contained in the second layer is preferably 1 / 1 cm of the electrolyte membrane, since the effects of the present disclosure are more excellent. ２ 0.010 mg / cm ２ More than 0.015 mg / cm is preferred. ２ More preferably, 0.030 mg / cm or more ２ More preferably, 0.040 mg / cm ２ More than 0.050 mg / cm is particularly preferred. ２ The following is preferred:

[0081] The second layer may further contain cerium oxide. The cerium oxide contained in the second layer is the same as the cerium oxide contained in the first layer, including its preferred embodiments.

[0082] If the second film contains cerium oxide, ２ The mass of cerium oxide per ２ More than 0.010 mg / cm ２ More preferably, 0.015 mg / cm or more ２ More preferably, 0.100 mg / cm ２ Preferably, 0.050 mg / cm or less ２ More preferably, 0.030 mg / cm or less ２The following is even more preferable: The reason why the above range of the mass of cerium oxide is preferable is as already explained for the cerium oxide contained in the first layer.

[0083] 1cm of the second layer ２ The mass of cerium oxide per 1 cm of the first layer is ２ In this case, the absolute value of the difference between the mass of cerium oxide contained in the second layer and the mass of cerium oxide contained in the first layer may be less than the mass of cerium oxide contained in the second layer per cm of the electrolyte membrane. ２ 0.005 mg / cm ２ or more, and 0.015 mg / cm ２ The second layer may not contain cerium oxide. ２ The mass of cerium oxide per ２ This means that:

[0084] In another embodiment, 1 cm of the first layer ２ The mass of cerium oxide per 1 cm of the second layer is ２ In this case, the absolute value of the difference between the mass of cerium oxide contained in the first layer and the mass of cerium oxide contained in the second layer may be less than the mass of cerium oxide contained in the first layer per cm of the electrolyte membrane. ２ 0.005 mg / cm ２ or more, and 0.015 mg / cm ２ The following are preferred: Also, the first layer may not contain cerium oxide.

[0085] Thickness of the second layer T ２ The thickness T of the second layer (when the second layer has a multi-layer structure, the total thickness) is preferably 40 to 120 μm, more preferably 40 to 110 μm, and even more preferably 40 to 90 μm. ２ When the thickness T of the second layer is equal to or greater than the lower limit, the mechanical strength of the electrolyte membrane is improved and the current efficiency is also improved. ２ If is equal to or less than the upper limit, the electrolysis voltage of the electrolyte membrane can be kept low.

[0086] Thickness of the first layer T １ is the thickness of the second layer T ２The following is preferred: the thickness of the second layer T ２ The thickness T of the first layer is preferably as thin as possible. １ and the thickness T of the second layer ２ While maintaining the total thickness of the first layer at a predetermined value or more, １ By reducing the thickness of the first layer and increasing the density of the platinum-containing material in the first layer, hydrogen crossover can be further suppressed. １ and the thickness T of the second layer ２ The thickness T of the first layer relative to the total thickness １ The ratio of (i.e., T １ / (T １ +T ２ ) is preferably 0.50 or less, more preferably 0.42 or less, even more preferably 0.33 or less, particularly preferably 0.17 or less, from the viewpoint of better formability during production of the electrolyte membrane, and is preferably 0.01 or more, more preferably 0.02 or more, and even more preferably 0.08 or more.

[0087] Thickness T of the first layer １ and the thickness T of the second layer ２ The total thickness is preferably 30 μm or more, more preferably 400 μm or less, more preferably 300 μm or less, even more preferably 200 μm or less, particularly preferably 90 μm or less, and most preferably 60 μm or less, in order to suppress hydrogen crossover.

[0088] [Reinforcing Body] The reinforcing body serves to improve the dimensional stability, strength, handleability, etc. of the electrolyte membrane. Specific examples of the reinforcing body include porous bodies, fibers, woven fabrics, and nonwoven fabrics, with woven fabrics being preferred. The reinforcing body is preferably made of warp and weft threads that are perpendicular to each other.

[0089] The aperture ratio of the reinforcement is preferably 50% or more, more preferably 55% or more, even more preferably 60% or more, and particularly preferably 70% or more, in order to further reduce the electrolysis voltage. The upper limit of the aperture ratio of the reinforcement is preferably 90% or less, more preferably 80% or less, in order to further improve the strength of the electrolyte membrane. The aperture ratio of the reinforcement is calculated using the following formula (ε) based on the average diameter R1 of the threads constituting the reinforcement and the average spacing P1 between adjacent threads (hereinafter also referred to as "pitch P1") among the threads constituting the reinforcement. Here, the average thread diameter R1 refers to the arithmetic average value of the diameters of 10 different threads arbitrarily selected based on a magnified image (e.g., 100x magnification) of the surface of the reinforcement obtained using a microscope. Furthermore, the pitch P1 refers to the arithmetic average value of the spacing between 10 different points arbitrarily selected based on a magnified image (e.g., 100x magnification) of the surface of the reinforcement obtained using a microscope. Opening ratio of the reinforcement (%) = [P1 / (P1+R1)] ２ × 100 (ε)

[0090] The denier number of the yarn constituting the reinforcing body is preferably 2 or more, and from the viewpoint of obtaining better strength and dimensional stability of the electrolyte membrane, it is more preferably 10 or more, and particularly preferably 15 or more. The upper limit of the denier number of the yarn constituting the reinforcing body is preferably 60 or less, more preferably 50 or less, and particularly preferably 20 or less, from the viewpoint of further reducing the electrolysis voltage. The denier number is the mass of 9,000 m of yarn expressed in grams (g / 9000 m).

[0091] The density of the threads constituting the reinforcing body is preferably 50 threads / inch or more, more preferably 70 threads / inch or more, and particularly preferably 90 threads / inch or more, from the viewpoint of excellent strength and dimensional stability of the electrolyte membrane, and is preferably 200 threads / inch or less, more preferably 150 threads / inch or less, and particularly preferably 100 threads / inch or less, from the viewpoint of further reducing the electrolysis voltage.

[0092] The thread constituting the reinforcing member may be either a monofilament consisting of one filament or a multifilament consisting of two or more filaments, with a monofilament being preferred.

[0093] The reinforcing body (more preferably a woven fabric) is preferably made of a material selected from the group consisting of polytetrafluoroethylene (hereinafter also referred to as "PTFE"), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (hereinafter also referred to as "PFA"), polyether ether ketone (hereinafter also referred to as "PEEK"), and polyphenylene sulfide (hereinafter also referred to as "PPS"), in view of superior thread durability. The thread constituting the reinforcing body is preferably made of a slit yarn in view of superior thread durability and strength.

[0094] When the material constituting the reinforcing body is PTFE, the weight of the reinforcing body is 20 g / m from the viewpoint of an excellent balance between the strength and the handling property of the electrolyte membrane. ２ More than 30 g / m ２ More than 40 g / m ２ When the material constituting the reinforcing body is PFA, the weight of the reinforcing body is preferably 10 g / m or less, in view of an excellent balance between the strength and the handling property of the electrolyte membrane. ２ More than 30 g / m ２ Preferably, 20 g / m or less ２ When the material constituting the reinforcing body is PEEK, the weight of the reinforcing body is preferably 5 g / m2 or less, in view of an excellent balance between the strength and the handling properties of the electrolyte membrane. ２ More than 40 g / m ２ Preferably, 30 g / m or less ２ When the material constituting the reinforcing body is PPS, the weight per unit area of ​​the reinforcing body is preferably 5 g / m or less, in view of an excellent balance between the strength and the handling properties of the electrolyte membrane. ２ More than 40 g / m ２ More than 30 g / m ２ The following is more preferred:

[0095] The content of the reinforcing material is preferably 3 mass% or more, more preferably 5 mass% or more, and is preferably 50 mass% or less, more preferably 40 mass% or less, and even more preferably 30 mass% or less, based on the total mass of the electrolyte membrane.

[0096] The reinforcing body may be disposed inside the first layer, inside the second layer, or at the interface between the first and second layers. However, from the viewpoint of suppressing curling in the electrolyte membrane, it is preferable that the reinforcing body be disposed at the interface between the first and second layers.

[0097] The electrolyte membrane may have other components in addition to the first layer, the second layer, and the reinforcing member, but an electrolyte membrane consisting of the first layer, the second layer, and the reinforcing member without any other components is preferred.

[0098] Electrolyte membrane 1cm ２ The mass of platinum content per unit area is 0.005 mg / cm ２ More than 0.010 mg / cm ２ More preferably, 0.015 mg / cm or more ２ More preferably, 0.050 mg / cm ２ Preferably, 0.040 mg / cm or less ２ It is more preferable that the mass of the platinum-containing material is 0.015 mg / cm ２ If the mass of the platinum-containing material is 0.050 mg / cm or more, hydrogen crossover can be further suppressed. ２ If the above ratio is less than 1, the electrolysis voltage can be reduced, and a low-cost membrane electrode assembly and water electrolysis device can be provided.

[0099] In the electrolyte membrane, it is preferable that the second layer does not contain a platinum-containing material in order to suppress hydrogen crossover. When the second layer does not contain a platinum-containing material, the thickness T １ is the thickness of the electrolyte membrane (the thickness of the first layer T １ and the thickness T of the second layer ２ The thickness of the electrolyte membrane is preferably 50% or less of the total thickness of the electrolyte membrane, more preferably 42% or less of the thickness of the electrolyte membrane, further preferably 33% or less of the thickness of the electrolyte membrane, and particularly preferably 17% or less of the thickness of the electrolyte membrane.

[0100] The electrolyte membrane preferably further contains cerium oxide. When the electrolyte membrane further contains cerium oxide, it is believed that the chemical durability of the electrolyte membrane is improved. This is presumably because, when the electrolyte membrane is applied to a water electrolysis device, OH radicals generated from hydrogen peroxide generated in the system during operation are quenched by cerium ions dissociated from cerium oxide contained in the electrolyte membrane, resulting in suppression of decomposition of the fluoropolymer. The cerium oxide contained in the electrolyte, including its preferred embodiments, has been described above.

[0101] The cerium oxide contained in the electrolyte membrane may be dispersed throughout the electrolyte membrane or may be unevenly distributed on one surface of the electrolyte membrane. For example, the concentration of cerium oxide in the first layer may be higher, lower, or the same as the concentration of cerium oxide in the second layer. It is preferable that the concentration of cerium oxide in the first layer is higher than the concentration of cerium oxide in the second layer, and it is more preferable that the first layer contains cerium oxide and the second layer does not contain cerium oxide.

[0102] The electrolyte membrane preferably does not substantially contain a thickener, since this increases the membrane resistance. The term "the electrolyte membrane is substantially free of a thickener" means that the content of the thickener is 0.1% by mass or less, preferably 0.01% by mass or less, and more preferably 0% by mass, relative to the total mass of the electrolyte membrane. Examples of the thickener include thickeners contained in the catalyst layers of the anode and cathode. The thickeners contained in the catalyst layers will be described later.

[0103] [Physical Properties of Electrolyte Membrane] The thickness of the electrolyte membrane is preferably 30 μm or more, and is preferably 400 μm or less, more preferably 300 μm or less, even more preferably 200 μm or less, particularly preferably 90 μm or less, and most preferably 60 μm or less. The thickness of the electrolyte membrane is the thickness T of the first layer described above. １ and the thickness T of the second layer ２ The total thickness may be:

[0104] The thickness of the electrolyte membrane is determined by measuring the thickness at any 20 points using an enlarged image (for example, 100x) of a cross section of the electrolyte membrane taken in the thickness direction using an optical microscope (product name "BX-51", manufactured by Olympus Corporation), and the arithmetic mean value of the obtained thicknesses is defined as the thickness of the electrolyte membrane. If the electrolyte membrane has unevenness on its surface, the thickness of 10 recessed portions on the electrolyte membrane and the thickness of 10 raised portions on the electrolyte membrane are measured, and the arithmetic mean value of the thicknesses at a total of 20 points is defined as the thickness of the electrolyte membrane. However, if the raised portions include portions that constitute reinforcing bodies, the thickness of the raised portions is defined as the value obtained by subtracting the thickness of the above-mentioned portions present in the raised portions.

[0105] [Method for Producing Electrolyte Membrane] An example of a method for producing an electrolyte membrane is as follows. First, a polymer (hereinafter also referred to as "fluoropolymer (I'-1)") is prepared, which is a polymer of a fluorine-containing monomer having a group that can be converted to an ion-exchange group, and in which the content of the group that can be converted to an ion-exchange group is adjusted so that a fluoropolymer (I-1) is obtained. Further, a polymer (hereinafter also referred to as "fluoropolymer (I'-2)") is prepared, which is a polymer of a fluorine-containing monomer having a group that can be converted to an ion-exchange group, and in which the content of the group that can be converted to an ion-exchange group is adjusted so that a fluoropolymer (I-2) is obtained. Next, a kneaded product A' containing the fluoropolymer (I'-1) and a platinum-containing substance is extruded into a film by a melt extrusion method, to obtain a precursor membrane A. Similarly, a kneaded product B' containing at least the fluoropolymer (I'-2) is extruded into a film by a melt extrusion method, to obtain a precursor membrane B. Here, by using a kneaded product B' that does not contain a platinum-containing substance, a precursor membrane B that does not contain a platinum-containing substance is obtained. When using a kneaded material B' further containing a platinum-containing material, the content of the platinum-containing material in the kneaded material B' is adjusted so that the concentration of the platinum-containing material in the precursor film B is lower than the concentration of the platinum-containing material in the precursor film A. When producing the precursor films A and B, the thickness T Ａ is the thickness T of precursor film B ＢThe thickness of the first layer is adjusted to be thinner than the thickness of the second layer. Next, the precursor membrane A, the reinforcing member, and the precursor membrane B are arranged in this order, and laminated using a lamination roll, a vacuum laminating device, or a hot press device to obtain a laminate. Next, groups convertible to ion exchange groups in the precursor membranes A and B are converted into ion exchange groups to obtain an electrolyte membrane having a first layer, a reinforcing member, and a second layer as shown in FIG. 1. In the electrolyte membrane obtained in this manner, the concentration of the platinum-containing material in the second layer is lower than the concentration of the platinum-containing material in the first layer, the ion exchange capacity of the fluoropolymer (I-1) contained in the first layer is greater than the ion exchange capacity of the fluoropolymer (I-2) contained in the second layer, and the reinforcing member is arranged closer to the surface A than the center position in the thickness direction of the electrolyte membrane.

[0106] Other examples of the method for producing an electrolyte membrane include the following method. First, a group convertible to an ion-exchange group in a fluoropolymer (I'-1) is converted into an ion-exchange group to obtain a fluoropolymer (I-1). Next, an electrolyte membrane A (corresponding to the first layer) is obtained by a casting method using a dispersion A in which the fluoropolymer (I-1) and a platinum-containing substance are dispersed. Furthermore, a group convertible to an ion-exchange group in a fluoropolymer (I'-2) is converted into an ion-exchange group to obtain a fluoropolymer (I-2). Next, an electrolyte membrane B (corresponding to the second layer) is obtained by a casting method using a dispersion B in which the fluoropolymer (I-2) is dispersed. Here, by using a dispersion B that does not contain a platinum-containing substance, an electrolyte membrane B that does not contain a platinum-containing substance can be obtained. Furthermore, when a dispersion B that further contains a platinum-containing substance is used, the content of the platinum-containing substance in dispersion B is adjusted so that the concentration of the platinum-containing substance in electrolyte membrane B is lower than the concentration of the platinum-containing substance in electrolyte membrane A. Note that when producing electrolyte membranes A and B, the thickness T of the electrolyte membrane A is Ａ is the thickness T of the electrolyte membrane B Ｂ Next, the electrolyte membrane A, the reinforcing body, and the electrolyte membrane B are arranged in this order, and these are laminated using a lamination roll, a vacuum lamination device, or a heat press device, thereby obtaining an electrolyte membrane having a first layer, a reinforcing body, and a second layer as shown in FIG.

[0107] As another example of the method for producing an electrolyte membrane, the following method can be mentioned. First, electrolyte membrane A and electrolyte membrane B are obtained according to the above method. Next, a reinforcing body is impregnated with dispersion B in which a fluoropolymer (I-2) is dispersed, and then dried to obtain an electrolyte membrane C containing the fluoropolymer (I-2) and the reinforcing body (corresponding to the second layer containing the reinforcing body therein). When producing the electrolyte membranes A, B, and C, the thickness T Ａ and the thickness T of the electrolyte membrane C Ｃ The sum of these is the thickness T of the electrolyte membrane B. Ｂ Next, the electrolyte membrane A, the electrolyte membrane C, and the electrolyte membrane B are arranged in this order, and are laminated using a lamination roll, a vacuum lamination device, or a heat press device, thereby obtaining an electrolyte membrane having a first layer, a reinforcing body, and a second layer as shown in FIG.

[0108] Still another example of the method for producing an electrolyte membrane is the following method. First, a group convertible to an ion-exchange group in the fluoropolymer (I'-1) is converted into an ion-exchange group to obtain a fluoropolymer (I-1). Next, a reinforcing body is impregnated with a dispersion A in which the fluoropolymer (I-1) and a platinum-containing substance are dispersed, and then dried to obtain an electrolyte membrane A containing the fluoropolymer (I-1) and the reinforcing body (corresponding to the first layer containing the reinforcing body therein). Furthermore, an electrolyte membrane B is obtained in accordance with the above method. Note that when producing the electrolyte membranes A and B, the thickness T Ａ is the thickness T of the electrolyte membrane B Ｂ Next, the electrolyte membrane A containing the reinforcing body therein and the electrolyte membrane B are laminated using a lamination roll, a vacuum lamination device, or a heat press device, thereby obtaining an electrolyte membrane having the first layer, the reinforcing body, and the second layer.

[0109] In the above production method, by adding cerium oxide to at least one of the kneaded material A' and the kneaded material B', or to at least one of the dispersion A and the dispersion B, an electrolyte membrane containing cerium oxide can be obtained.

[0110] <Fluoropolymers (I'-1) and (I'-2)> The fluoropolymer (I'-1) and the fluoropolymer (I'-2) differ in the content of groups that can be converted into ion-exchange groups. The fluoropolymer (I'-1) and the fluoropolymer (I'-2) may be the same or different in structure other than the content of groups that can be converted into ion-exchange groups. Specifically, the type of groups that can be converted into ion-exchange groups, and the type and content of each unit may be the same or different between the fluoropolymer (I'-1) and the fluoropolymer (I'-2). Hereinafter, both the fluoropolymer (I'-1) and the fluoropolymer (I'-2) will also be collectively referred to as "fluoropolymer (I')."

[0111] Examples of the fluoropolymer (I') include a polymer (hereinafter also referred to as "fluoropolymer (S')") of a fluoromonomer having a group that can be converted into a sulfonic acid type functional group (hereinafter also referred to as "fluoromonomer (S')"), and a polymer of a fluoromonomer having a group that can be converted into a carboxylic acid type functional group, of which the fluoropolymer (S') is preferred, and a copolymer of a fluorine-containing olefin and a monomer having a fluorine atom and a group that can be converted into a sulfonic acid type functional group is more preferred. The fluoropolymer (S') will be described in detail below.

[0112] As the method for copolymerizing the fluoropolymer (S'), known methods such as solution polymerization, suspension polymerization and emulsion polymerization can be adopted.

[0113] Examples of the fluorine-containing olefin include those exemplified above, and TFE is preferred from the viewpoints of the production cost of the monomer, the reactivity with other monomers, and the excellent properties of the resulting fluorine-containing polymer (S). One type of fluorine-containing olefin may be used alone, or two or more types may be used in combination.

[0114] The fluorine-containing monomer (S') may be a compound having one or more fluorine atoms in the molecule, an ethylenic double bond, and a group that can be converted into a sulfonic acid functional group. As the fluorine-containing monomer (S'), a compound represented by formula (2) is preferred in terms of the production cost of the monomer, reactivity with other monomers, and excellent properties of the resulting fluorine-containing polymer (S). Formula (2) CF ２ =CF-L-(A) ｎ The definitions of L and n in formula (2) are the same as those of L and n in formula (1) above. A is a group that can be converted into a sulfonic acid functional group. The group that can be converted into a sulfonic acid functional group is preferably a functional group that can be converted into a sulfonic acid functional group by hydrolysis. Specific examples of groups that can be converted into a sulfonic acid functional group include -SO ２ F, -SO ２ Cl, —SO ２ Br. The n A's may be the same or different.

[0115] The compound represented by formula (2) is preferably a compound represented by formula (2-1), a compound represented by formula (2-2), a compound represented by formula (2-3), or a compound represented by formula (2-4). ２ =CF-O-R ｆ１ -A Formula (2-2) CF ２ =CF-R ｆ１ -A

[0116]

[0117]

[0118] R in the formula ｆ１ , R ｆ２ , R ｆ３ The definitions of r and m are as follows: ｆ１ , R ｆ２ , R ｆ３ , r, and m are the same as those in the formula (2).

[0119] As the compound represented by formula (2-1) and the compound represented by formula (2-2), a compound represented by formula (2-5) is preferred. ２ =CF-(CF ２ ) ｘ -(OCF ２ CFY) ｙ -O-(CF ２ ) ｚ -SO ３ The definitions of x, y, z and Y in the formula F are the same as those of x, y, z and Y in the formula (1-5) above.

[0120] Specific examples of the compound represented by formula (2-1) include the following compounds. In the formula, w is an integer of 1 to 8, and v is an integer of 1 to 5. CF ２ =CF-O-(CF ２ ) ｗ -SO ２ FCF ２ =CF-O-CF ２ CF (CF ３ )-O-(CF ２ ) ｗ -SO ２ FCF ２ =CF-[O-CF ２ CF (CF ３ )] ｖ -SO ２ F

[0121] Specific examples of the compound represented by formula (2-2) include the following compounds: In the formula, w is an integer of 1 to 8. CF ２ =CF-(CF ２ ) ｗ -SO ２ FCF ２ =CF-CF ２ -O-(CF ２ ) ｗ -SO ２ F

[0122] The compound represented by formula (2-3) is preferably a compound represented by formula (2-3-1).

[0123]

[0124] R in the formula ｆ４ , Rｆ５ , r and A are as defined above.

[0125] Specific examples of the compound represented by formula (2-3-1) include the following.

[0126]

[0127] The compound represented by formula (2-4) is preferably a compound represented by formula (2-4-1).

[0128]

[0129] R in the formula ｆ１ , R ｆ２ and A are defined as above.

[0130] Specific examples of the compound represented by formula (2-4-1) include the following.

[0131]

[0132] The fluorine-containing monomer (S') may be used alone or in combination of two or more. In the production of the fluorine-containing polymer (S'), in addition to the fluorine-containing olefin and the fluorine-containing monomer (S'), other monomers may be further used. Examples of other monomers include those exemplified above.

[0133] The ion exchange capacities of the fluoropolymer (I-1) and the fluoropolymer (I-2) can be adjusted by changing the content of groups convertible to ion exchange groups in the fluoropolymer (I'-1) and the fluoropolymer (I'-2), respectively.

[0134] In the above-described method for producing an electrolyte membrane, specific examples of a method for converting groups in the precursor membrane that can be converted to ion-exchange groups to ion-exchange groups include a method of subjecting the precursor membrane or a laminate having the precursor membrane to a hydrolysis treatment or an acid-form treatment. Among these, a method of contacting the precursor membrane or a laminate having the precursor membrane with an alkaline aqueous solution is preferred. The treatment of the precursor membrane described below also includes a treatment of the laminate having the precursor.

[0135] Specific examples of the method for contacting the precursor film with the alkaline aqueous solution include immersing the precursor film in the alkaline aqueous solution and spraying the alkaline aqueous solution onto the surface of the precursor film. The temperature of the alkaline aqueous solution is preferably 30° C. or higher, more preferably 40° C. or higher, and preferably 100° C. or lower. The contact time between the precursor film and the alkaline aqueous solution is preferably 3 minutes or longer, more preferably 5 minutes or longer, and preferably 150 minutes or shorter, more preferably 50 minutes or shorter.

[0136] The alkaline aqueous solution preferably contains an alkali metal hydroxide, a water-soluble organic solvent, and water. Examples of alkali metal hydroxides include sodium hydroxide and potassium hydroxide. In this specification, the water-soluble organic solvent refers to an organic solvent that is easily soluble in water. Specifically, an organic solvent having a solubility of 0.1 g or more in 1000 ml of water (20°C) is preferred, and an organic solvent having a solubility of 0.5 g or more is particularly preferred. The water-soluble organic solvent preferably contains at least one selected from the group consisting of aprotic organic solvents, alcohols, and aminoalcohols, and particularly preferably contains an aprotic organic solvent. One water-soluble organic solvent may be used alone, or two or more may be used in combination.

[0137] Specific examples of aprotic organic solvents include dimethyl sulfoxide, N,N-dimethylformamide, N,N-dimethylacetamide, N-methyl-2-pyrrolidone, and N-ethyl-2-pyrrolidone, with dimethyl sulfoxide being preferred. Specific examples of alcohols include methanol, ethanol, isopropanol, butanol, methoxyethoxyethanol, butoxyethanol, butylcarbitol, hexyloxyethanol, octanol, 1-methoxy-2-propanol, and ethylene glycol. Specific examples of aminoalcohols include ethanolamine, N-methylethanolamine, N-ethylethanolamine, 1-amino-2-propanol, 1-amino-3-propanol, 2-aminoethoxyethanol, 2-aminothioethoxyethanol, and 2-amino-2-methyl-1-propanol.

[0138] The concentration of the alkali metal hydroxide in the alkaline aqueous solution is preferably 1% by mass or more, more preferably 3% by mass or more, and preferably 60% by mass or less, more preferably 55% by mass or less. The content of the water-soluble organic solvent in the alkaline aqueous solution is preferably 1% by mass or more, more preferably 3% by mass or more, and preferably 60% by mass or less, more preferably 55% by mass or less. The concentration of water in the alkaline aqueous solution is preferably 39 to 80% by mass.

[0139] After the precursor film is brought into contact with the alkaline aqueous solution, a treatment for removing the alkaline aqueous solution may be carried out. As a method for removing the alkaline aqueous solution, for example, a method for washing the precursor film that has been brought into contact with the alkaline aqueous solution with water may be mentioned.

[0140] After contacting the precursor membrane with the alkaline aqueous solution, the resulting membrane may be contacted with an acidic aqueous solution to convert the ion exchange groups to an acid form. Specific examples of methods for contacting the precursor membrane with the acidic aqueous solution include immersing the precursor membrane in the acidic aqueous solution and spraying the acidic aqueous solution onto the surface of the precursor membrane. The acidic aqueous solution preferably contains an acid component and water. Specific examples of the acid component include hydrochloric acid and sulfuric acid.

[0141] [Membrane Electrode Assembly] The membrane electrode assembly of the present disclosure includes the electrolyte membrane, a cathode catalyst layer disposed on one side of the electrolyte membrane, and an anode catalyst layer disposed on the other side of the electrolyte membrane. When the electrolyte membrane is applied to a water electrolysis device, it is preferable to dispose the surface A of the electrolyte membrane on the first layer side on the anode side, in order to further suppress hydrogen crossover. In this case, it is preferable that the catalyst contained in the catalyst layer of the anode contains iridium oxide, in order to achieve the above-mentioned effect more effectively.

[0142] Fig. 3 is a cross-sectional view schematically illustrating an example of a membrane electrode assembly according to the present disclosure. The membrane electrode assembly 30 shown in Fig. 3 includes an anode 32 having a catalyst layer 36 and a gas diffusion layer 38, a cathode 34 having the catalyst layer 36 and the gas diffusion layer 38, and an electrolyte membrane 10 disposed between the anode 32 and the cathode 34 in contact with the catalyst layer 36. In the electrolyte membrane 10, a first layer 11 and a second layer 12 are disposed in this order from the anode 32 side toward the cathode 34 side, and a reinforcing member 13 is disposed on the surface A side.

[0143] In the example of FIG. 3, the electrolyte membrane 10 has a two-layer structure, but the electrolyte membrane of the membrane electrode assembly may have a three-layer or more structure.

[0144] <Anode and Cathode> The anode and cathode each have a catalyst layer. In the example of Figure 3, the anode 32 and cathode 34 each have a catalyst layer 36 and a gas diffusion layer 38.

[0145] Specific examples of the catalyst layer include a layer containing a catalyst and a polymer having an ion exchange group. Specific examples of the catalyst include a supported catalyst in which a carbon support supports a catalyst containing platinum, a platinum alloy, or a platinum having a core-shell structure, an iridium oxide catalyst, a composite oxide catalyst containing iridium and other metal elements, an alloy containing iridium oxide, and a catalyst containing iridium oxide having a core-shell structure. An example of the carbon support is carbon black powder. Examples of the polymer having an ion exchange group include a fluorine-containing polymer having an ion exchange group, and for example, the above-mentioned fluorine-containing polymer (I-1) or (I-2) can be used.

[0146] Catalyst layer 1cm ２ The mass of the catalytic metal per ２ More than 0.05 mg / cm ２ More preferably, 0.2 mg / cm ２ More preferably, 4 mg / cm ２ Preferably, less than 2 mg / cm ２ More preferably, 1 mg / cm or less ２The following is more preferred: The mass ratio of the catalyst to the polymer having an ion-exchange group in the catalyst layer (mass of catalyst / mass of polymer having an ion-exchange group) is preferably 2 to 6.

[0147] The catalyst layer preferably contains a thickener from the viewpoint of improving the dispersibility of the catalyst and suppressing aggregation. Specific examples of thickeners include urethane-based thickeners, polyacrylic-based thickeners, polyamide-based thickeners, cellulose-based thickeners, and clay minerals such as bentonite. Zeorora (registered trademark) H (manufactured by Zeon Corporation) can also be used as a thickener. When the catalyst layer contains a thickener, the content of the thickener is preferably 2% by mass or more, more preferably 3% by mass or more, and preferably 7% by mass or less, more preferably 6% by mass or less, relative to the total mass of the coating liquid for forming the catalyst layer described below.

[0148] The gas diffusion layer functions to rapidly diffuse gas generated from the catalyst layer out of the catalyst layer and also functions as a current collector. Specific examples of gas diffusion layers include carbon paper, carbon cloth, carbon felt, sintered titanium oxide fibers, and sintered titanium oxide particles. The anode side has a high potential, and using carbon materials would result in oxidation. Therefore, it is preferable to use sintered titanium oxide fibers or sintered titanium oxide particles. The sintered titanium oxide may be plated with platinum or other metals as needed. The cathode gas diffusion layer may be treated with PTFE or other materials for water repellency. While the membrane electrode assembly in FIG. 3 has a gas diffusion layer 38, the gas diffusion layer is an optional component and need not be included in the membrane electrode assembly.

[0149] The thickness of the anode and the cathode is preferably 5 μm or more, and is preferably 100 μm or less, more preferably 50 μm or less, even more preferably 30 μm or less, and particularly preferably 15 μm or less. The thickness of the anode and the cathode is measured using an image obtained by measuring a cross section of the membrane electrode assembly cut in the thickness direction with a laser microscope, and is the arithmetic average value at any 20 points.

[0150] <Method for Manufacturing Membrane Electrode Assembly> Examples of methods for manufacturing a membrane electrode assembly include forming a cathode catalyst layer on the surface B side of an electrolyte membrane and forming an anode catalyst layer on the surface A side of the electrolyte membrane. One example of a method for manufacturing a membrane electrode assembly includes using a laminate having an anode catalyst layer and a releasable substrate (e.g., an ETFE sheet) and another laminate having a cathode catalyst layer and a releasable substrate (e.g., an ETFE sheet), bonding catalyst layers to both sides of the electrolyte membrane, and then peeling off the releasable substrate. The laminate may include a gas diffusion layer between the catalyst layer and the releasable substrate. In this case, the gas diffusion layer can be formed on the catalyst layer opposite the electrolyte membrane. Examples of methods for manufacturing a catalyst layer include applying a catalyst layer-forming coating liquid to a predetermined position (e.g., the surface of the releasable substrate) and drying it as necessary. The catalyst layer-forming coating liquid is a liquid in which a polymer having ion exchange groups and a catalyst are dispersed in a dispersion medium.

[0151] <Applications> The membrane electrode assembly of the present disclosure is suitably used in a solid polymer water electrolysis device.

[0152] [Water Electrolysis Apparatus] The water electrolysis apparatus of the present disclosure includes the membrane electrode assembly described above, a water supply unit that supplies water to the anode catalyst layer side, and a power supply unit that is electrically connected to the anode catalyst layer side and the cathode catalyst layer side. In the water electrolysis apparatus of the present disclosure, when a DC voltage is applied by the power supply unit while water is supplied to the anode catalyst layer side by the water supply unit, water is decomposed on the anode catalyst layer side to generate oxygen and protons. On the cathode catalyst layer side, protons that have migrated to the cathode catalyst layer side through the electrolyte membrane gain electrons to generate hydrogen. The water electrolysis apparatus of the present disclosure may have the same configuration as known water electrolysis apparatuses (e.g., an oxygen recovery member that recovers the generated oxygen, a hydrogen recovery member that recovers the generated hydrogen), except for the components described above.

[0153] [Method for Producing Hydrogen] The method for producing hydrogen according to the present disclosure is a method for producing hydrogen by electrolyzing water (electrolyte) using the water electrolysis device described above. The method for producing hydrogen according to the present disclosure is thought to be able to efficiently produce hydrogen because it uses the water electrolysis device described above.

[0154] The present invention will be described in detail below with reference to examples. Examples 1 to 7 are working examples, and Examples 8 to 10 are comparative examples. However, the present invention is not limited to these examples.

[0155] [Measurements] The physical properties of the electrolyte membrane of each example were measured by the following methods. The measurement results are shown in the table below.

[0156] <Ion exchange capacity of each fluoropolymer> The fluoropolymer was left in a glove box filled with dry nitrogen for 24 hours, and the dry mass of the fluoropolymer was measured. Thereafter, the fluoropolymer was immersed in a 2 mol / L aqueous sodium chloride solution at 60°C for 1 hour. The fluoropolymer was washed with ultrapure water and then removed, and the liquid in which the fluoropolymer had been immersed was titrated with a 0.1 mol / L aqueous sodium hydroxide solution to determine the ion exchange capacity (milliequivalent / gram of dry resin (also referred to as "meq / g" in the tables below)) of the fluoropolymer used in producing the electrolyte membrane of each example.

[0157] <Thickness of Electrolyte Membrane, First Layer, and Second Layer> Thickness T of First Layer １ , the thickness of the second layer T ２ , and the thickness T of the first layer １ and the thickness T of the second layer ２ The total thickness T １ +T ２ was measured by observing an enlarged image of a cross section along the thickness direction of the electrolyte membrane according to the above-mentioned method under conditions of a temperature of 23° C. and a relative humidity of 50% RH.

[0158] <Mass of Platinum-Containing Material and Cerium Oxide> The mass of the platinum-containing material and the mass of cerium oxide contained in the first layer, and the mass of the platinum-containing material and the mass of cerium oxide contained in the second layer were measured by observing an enlarged image of a cross section along the thickness direction of the electrolyte membrane using an energy dispersive X-ray analyzer (product name "QUANTAX FlatQUAD", manufactured by Bruker) according to the method described above. The unit of the mass of the platinum-containing material and the mass of cerium oxide is per cm of the first or second layer. ３ mg per cm ２ )

[0159] [Evaluation] The electrolyte membrane or membrane electrode assembly produced in each example was evaluated for the following items. The results of each evaluation are shown in the table below.

[0160] <Occurrence of scratches in catalyst layer> A catalyst layer-forming composition having the following composition was applied by die coating to one surface of the electrolyte membrane obtained in each example, and then dried to form a catalyst layer. The electrolyte membrane was cut to a size of 210 × 297 mm, and the coating area was set to 160 × 247 mm so that the edges were equidistant from each side of the electrolyte membrane. During application and drying of the coating, the four corners of the electrolyte membrane were held in place with polyimide heat-resistant tape (Kapton (registered trademark) adhesive tape, manufactured by Teraoka Seisakusho Co., Ltd.). A catalyst layer was also formed on the surface opposite the one surface in the same manner.

[0161] The surface of the catalyst layer after coating and drying was observed to visually check for any scratches on the surface of the catalyst layer. Based on the observation results, the resistance to scratches on the surface of the catalyst layer was evaluated according to the following criteria. In practice, an A rating, a B rating, or a C rating is preferred. The evaluation results for resistance to scratches are shown in the table below. The less scratches there are on the surface of the catalyst layer, the better the performance in suppressing curling in the electrolyte membrane. A: When no scratches have occurred on the surface of the catalyst layer in the coated area B: When scratches have occurred in less than three places on the surface of the catalyst layer in the coated area C: When scratches have occurred in 3 or more but less than 5 places on the surface of the catalyst layer in the coated area D: When scratches have occurred in 5 or more places on the surface of the catalyst layer in the coated area

[0162] <Hydrogen concentration in oxygen> The membrane electrode assembly obtained in each example was sandwiched between platinum-plated titanium fiber sintered bodies (manufactured by Bekaert) with a thickness of 0.25 mm and a porosity of 60%, and a platinum-plated titanium plate with a straight flow path was used as a separator. The electrode area was 16 cm. ２The membrane electrode assembly was incorporated into a single cell. The membrane electrode assembly was clamped so that a pressure of 1.5 MPa was applied to the electrode portions when sandwiching it. Next, in order to sufficiently hydrate the electrolyte membrane and both electrode ionomers, pure water with a conductivity of 1.0 μS / cm or less, a temperature of 60°C, and atmospheric pressure was supplied to the anode and cathode sides at a flow rate of 50 mL / min for 8 hours. Thereafter, pure water with a conductivity of 1.0 μS / cm or less and a temperature of 60°C was supplied to the anode side at a flow rate of 50 mL / min, and with the back pressure maintained at atmospheric pressure for both the anode and cathode, a current of 16 A (current density 1 A / cm) was applied using a large current potentio / galvanostat HCP-803 (manufactured by Biologic). ２ ) for 4 hours as a break-in operation, water electrolysis was carried out. After that, the main measurement was carried out at a current density of 0 to 32 A (current density 0 to 2 A / cm ２ The current was increased stepwise by 2 A in the range of 0.2 A / cm. Thereafter, pure water with a conductivity of 1.0 μS / cm or less, a temperature of 60°C, and atmospheric pressure was supplied to the cell at 50 mL / min. The back pressure was kept at atmospheric pressure for both the anode and cathode. A current of 3.2 A (current density 0.2 A / cm) was applied using a large current potentio / galvanostat HCP-803 (manufactured by Biologic). ２ After 12 hours had passed, water was separated from the gas discharged from the anode side, and the hydrogen concentration in the gas was measured using a micro GC (Agilent 490, manufactured by Agilent). ２ Medium H ２ The density was evaluated according to the following criteria: ２ Medium H ２ The smaller the concentration value, the better the performance of the electrolyte membrane in suppressing hydrogen crossover. A: Less than 0.03 vol%. B: 0.03 vol% or more and less than 0.05 vol%. C: 0.05 vol% or more and less than 0.10 vol%. D: 0.10 vol% or more.

[0163] [Example 1] <Production of precursor film A1> CF ２ =CF ２and a monomer (X) represented by the following formula (X) were copolymerized to obtain a fluoropolymer (I'-1) (ion exchange capacity: 1.40 meq / g dry resin). The ion exchange capacity in parentheses represents the ion exchange capacity of the fluoropolymer obtained when the fluoropolymer (I'-1) is hydrolyzed by the procedure described below. CF ２ =CF-O-CF ２ CF (CF ３ )-O-CF ２ CF ２ -SO ２ F...(X)

[0164] Fluorine-containing polymer (I'-1), platinum black (Tanaka Kikinzoku Kogyo K.K.'s "TEC90300", platinum-containing material), and cerium oxide (Fujifilm Wako Pure Chemical Industries, Ltd., primary particle size: 10 to 50 nm, CeO ２ ) was prepared. The kneaded product A1 was adhered by melt extrusion to a substrate made of a linear low-density polyethylene (LLDPE) film (melting point: 110 to 120°C), to obtain a precursor film-attached substrate X1 in which a precursor film A1 (film thickness: 10 µm) containing a fluorine-containing polymer (I'-1), platinum black, and cerium oxide was formed on the substrate. When preparing the kneaded product A1, the amounts of platinum black and cerium oxide added to the kneaded product A1 were adjusted so that the masses of the platinum-containing material and cerium oxide contained in the first layer obtained by hydrolyzing the precursor film A1 in the procedure described below would be the values ​​shown in the table below.

[0165] <Production of precursor membrane B1> CF ２ =CF ２and a monomer (X) represented by the above formula (X) were copolymerized to obtain a fluoropolymer (I'-2) (ion exchange capacity: 1.25 meq / g dry resin). The ion exchange capacity in parentheses represents the ion exchange capacity of the fluoropolymer obtained when the fluoropolymer (I'-2) was hydrolyzed by the procedure described below. A kneaded product B1 containing the obtained fluoropolymer (I'-2) and containing neither platinum black nor cerium oxide was prepared. The kneaded product B1 was adhered by melt extrusion to a substrate made of a linear low-density polyethylene (LLDPE) film (melting point: 110 to 120°C) to obtain a precursor film-coated substrate Y1 in which a precursor film B1 (film thickness: 50 μm) containing the fluoropolymer (I'-2) was formed on the substrate.

[0166] <Production of woven fabric> Woven fabric A1 was obtained by plain weaving 18.6 denier PFA yarns as warp and weft yarns so that the density of the PFA yarns was 100 threads / inch. The basis weight of woven fabric A1 was 16.3 g / m ２ It was.

[0167] <Production of electrolyte membrane 1> The precursor membrane-coated substrate X1 / woven fabric A1 / precursor membrane-coated substrate Y1 were fed into a roll press so that they were stacked in this order. The precursor membrane-coated substrate X1 was positioned so that the precursor membrane A1 was in contact with the woven fabric A1, and the precursor membrane-coated substrate Y1 was positioned so that the precursor membrane B1 was in contact with the woven fabric A1. Using the roll press, the mixture was heat-pressed at 150°C and 1.5 MPa for 2 minutes to obtain a precursor laminate 1 in which the precursor membrane A1, woven fabric A1, and precursor membrane B1 were arranged in this order, with both surfaces sandwiched between the substrates. After heat pressing, the substrates were peeled off from the precursor laminate 1.

[0168] The precursor laminate 1 was immersed in a solution of dimethyl sulfoxide / potassium hydroxide / water = 30 / 5.5 / 64.5 (mass ratio) at 95°C for 30 minutes to hydrolyze the groups in each precursor membrane that could be converted to sulfonic acid functional groups, converting them to K-type sulfonic acid functional groups, and then washed with water. The resulting membrane was then immersed in 1 M sulfuric acid to convert the terminal groups from K-type to H-type, and then dried to obtain an electrolyte membrane 1 of Example 1 having a first layer, a woven fabric, and a second layer.

[0169] In the obtained electrolyte membrane 1, the second layer does not contain a platinum-containing substance, and therefore the concentration of the platinum-containing substance in the second layer is lower than the concentration of the platinum-containing substance in the first layer. Furthermore, the ion exchange capacity of the fluoropolymer (I-1) contained in the first layer is greater than the ion exchange capacity of the fluoropolymer (I-2) contained in the second layer. Furthermore, the reinforcing body (woven fabric A1) of the electrolyte membrane 1 is located at the interface between the first and second layers, closer to the surface A on the first layer side than the central position in the thickness direction of the electrolyte membrane 1.

[0170] <Production of membrane electrode assembly> CF ２ =CF ２ and the above-mentioned monomer (X), and the resulting polymer (ion exchange capacity: 1.10 meq / g dry resin) was converted to an acid form through hydrolysis and acid treatment, and the resulting polymer was dispersed in a water / ethanol solvent of 40 / 60 (mass%) at a solids concentration of 26.0% to obtain a dispersion (hereinafter also referred to as "Dispersion Y"). Ethanol (18.06 g) and Zeorora-H (manufactured by Zeon Corporation) (10.58 g) were added to the resulting dispersion Y (33.0 g), and the mixture was mixed for 5 minutes at 2,200 rpm in a planetary centrifugal mixer (Thinky, Awatori Rentaro). Ethanol (46.44 g) and water (75.75 g) were added to the mixed composition (54.06 g), and a mixture having a specific surface area of ​​100 m containing 74.8 mass% iridium was further mixed. ２ 40.0 g of an iridium oxide catalyst (manufactured by Tanaka Kikinzoku Kogyo Co., Ltd.) was added to the mixture. The resulting mixture was processed in a planetary bead mill (rotational speed: 300 rpm) for 90 minutes to obtain an anode catalyst ink with a solids concentration of 22 mass %. The anode catalyst ink was applied to an ETFE sheet so that the iridium concentration was 1.0 mg / cm. ２ The anode catalyst layer decal was obtained by applying the coating with an applicator so that the coating became smooth, followed by drying at 80° C. for 10 minutes and then heat treating at 150° C. for 15 minutes.

[0171] Water (59.4 g) and ethanol (39.6 g) were added to 11 g of a supported catalyst ("TEC10E50E" manufactured by Tanaka Kikinzoku Kogyo Co., Ltd.) in which 46% by mass of platinum was supported on carbon powder, and the mixture was mixed and pulverized using an ultrasonic homogenizer to obtain a catalyst dispersion. To the catalyst dispersion, a mixture (29.2 g) of dispersion Y (20.1 g), ethanol (11 g), and Zeorola-H (manufactured by Zeon Corporation) (6.3 g) was premixed and kneaded. Furthermore, water (3.66 g) and ethanol (7.63 g) were added to the resulting dispersion and mixed for 60 minutes using a paint conditioner to obtain a cathode catalyst ink with a solids concentration of 10.0% by mass. The cathode catalyst ink was applied to an ETFE sheet using a die coater, dried at 80°C, and further heat-treated at 150°C for 15 minutes, resulting in a platinum content of 0.4 mg / cm. ２ As a result, a cathode catalyst layer decal of 1000 nm was obtained.

[0172] The surface of the anode catalyst layer decal on which the catalyst layer is located is placed opposite the first layer side of the electrolyte membrane 1 obtained above, and the surface of the cathode catalyst layer decal on which the catalyst layer is located is placed opposite the second layer side of the electrolyte membrane 1. The anode catalyst layer, electrolyte membrane 1, and cathode catalyst layer are bonded together by hot pressing under conditions of a pressing temperature of 150°C for 10 minutes and a pressure of 3 MPa. After the temperature is lowered to 70°C, the pressure is released and the assembly is taken out. The ETFE sheets of the anode catalyst layer decal and the cathode catalyst layer decal are peeled off, and an electrode with an area of ​​16 cm is obtained. ２ Thus, a membrane electrode assembly 1 was obtained.

[0173] [Example 2] An electrolyte membrane 2 and a membrane electrode assembly 2 of Example 2 were obtained in the same manner as in Example 1, except that the kneaded material A1 used in producing the precursor membrane A1 was replaced with a kneaded material A2 prepared by adjusting the amount of platinum black added so that the mass of the platinum-containing material contained in the first layer would be the value shown in the table below.

[0174] Example 3 Production of precursor membranes B31 and B32 In the same manner as in the production method of precursor membrane B1 in Example 1, a precursor membrane-attached substrate Y31 in which a precursor membrane B31 containing a fluoropolymer (I'-2) and having a thickness of 10 μm was formed on a substrate, and a precursor membrane-attached substrate Y32 in which a precursor membrane B32 containing a fluoropolymer (I'-2) and having a thickness of 40 μm was formed on a substrate were obtained.

[0175] <Production of Electrolyte Membrane 3> The precursor membrane Y31, woven fabric A1, and precursor membrane-attached substrate Y32 were fed into a roll press so that they were stacked in this order. The precursor membrane-attached substrate Y1 was positioned so that the precursor membrane B31 was in contact with the woven fabric A1, and the precursor membrane-attached substrate Y32 was positioned so that the precursor membrane B32 was in contact with the woven fabric A1. Next, the precursor laminate 3A was obtained by heat pressing at 150°C and 1.5 MPa for 2 minutes using a roll press, in which the precursor membrane B31, woven fabric A1, and precursor membrane B32 were arranged in this order, with both surfaces sandwiched between substrates. Next, the substrate in contact with the precursor membrane B31 was peeled from the precursor laminate 3A, and the precursor membrane-attached substrate X1 and the precursor laminate 3A were stacked and fed into the roll press. The precursor membrane-attached substrate X1 and the precursor laminate 3A were arranged so that the precursor membrane A1 was in contact with the precursor membrane B31. Using a roll press, the mixture was hot-pressed at 150°C and 1.5 MPa for 2 minutes to obtain a precursor laminate 3 in which the precursor membrane A1, the precursor membrane B31, the woven fabric A1, and the precursor membrane B32 were arranged in this order, with both surfaces sandwiched between substrates. After hot-pressing, the substrates were peeled off from the precursor laminate 3. An electrolyte membrane 3 and a membrane electrode assembly 3 of Example 3 were obtained in the same manner as in Example 1, except that the precursor laminate 3 was used instead of the precursor laminate 1.

[0176] [Example 4] Thickness T of the first layer １ A precursor membrane-coated substrate X4, in which a precursor membrane A4 containing a fluoropolymer (I'-1) was formed on a substrate, was obtained in the same manner as the production method for precursor membrane A1 in Example 1, except that the amounts of platinum black and cerium oxide added to kneaded material A1 and the amount of kneaded material A1 adhered to the substrate were adjusted so that the masses of the platinum-containing material and cerium oxide contained in the first layer were the values ​​shown in the tables below. Furthermore, a precursor membrane-coated substrate Y4, in which a precursor membrane B4 containing a fluoropolymer (I'-2) and having a thickness of 40 μm was formed on a substrate, was obtained in the same manner as the production method for precursor membrane B1 in Example 1. An electrolyte membrane 4 and a membrane electrode assembly 4 of Example 4 were obtained in the same manner as in Example 1, except that precursor membrane-coated substrate X4 and precursor membrane-coated substrate Y4 were used instead of precursor membrane-coated substrate X1 and precursor membrane-coated substrate Y1, respectively.

[0177] [Example 5] CF ２ =CF ２ and a monomer (X) represented by the following formula (X) were copolymerized to obtain a fluoropolymer (I'-3) (ion exchange capacity: 1.95 meq / g dry resin). The ion exchange capacity in parentheses represents the ion exchange capacity of the fluoropolymer obtained when the fluoropolymer (I'-3) was hydrolyzed by the procedure described below. An electrolyte membrane 5 and a membrane electrode assembly 5 of Example 5 were obtained in the same manner as in Example 1, except that the fluoropolymer (I'-3) was used instead of the fluoropolymer (I'-1).

[0178] [Example 6] A substrate X6 with a precursor film A6 (film thickness: 10 μm) formed on a substrate was obtained in the same manner as in Example 1, except that the kneaded material A1 used to produce the precursor film A1 was replaced with the kneaded material A2 prepared by adjusting the amount of platinum black added so that the mass of the platinum-containing material contained in the first layer was the value shown in the table below. Instead of the kneaded material B1 used to produce the precursor film B1, a fluorine-containing polymer (I'-2), platinum black ("TEC90300" manufactured by Tanaka Kikinzoku Kogyo Kogyo K.K., platinum-containing material), and cerium oxide (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., primary particle size: less than 10 nm, CeO ２ )) was used, a precursor membrane-attached substrate Y6 was obtained in the same manner as in Example 1, in which a precursor membrane B6 (film thickness: 50 μm) containing fluorine-containing polymer (I'-2), platinum black, and cerium oxide was formed on a substrate. When preparing the kneaded substance B6, the amounts of platinum black and cerium oxide added to the kneaded substance B6 were adjusted so that the masses of the platinum-containing material and cerium oxide contained in the first layer obtained by hydrolyzing the precursor membrane B6 would be the values ​​shown in the table below. An electrolyte membrane 6 and a membrane electrode assembly 6 of Example 6 were obtained in the same manner as in Example 1, except that precursor membrane-attached substrate X6 and precursor membrane-attached substrate Y6 were used instead of precursor membrane-attached substrate X1 and precursor membrane-attached substrate Y1, respectively.

[0179] [Example 7] A precursor membrane-attached substrate Y7 was obtained in which a precursor membrane B7 containing the fluoropolymer (I'-2) and having a thickness of 80 μm was formed on a substrate in the same manner as in the production method of the precursor membrane B1 in Example 1. An electrolyte membrane 7 and a membrane electrode assembly 7 of Example 7 were obtained in the same manner as in Example 1, except that the precursor membrane-attached substrate Y7 was used instead of the precursor membrane-attached substrate Y1.

[0180] [Example 8] A precursor membrane-coated substrate X8 was obtained in the same manner as in Example 1, except that fluoropolymer (I'-2) was used instead of fluoropolymer (I'-1), in which a precursor membrane A8 (thickness: 10 μm) containing fluoropolymer (I'-2), platinum black, and cerium oxide was formed on a substrate. A precursor membrane-coated substrate Y81 was obtained in the same manner as in Example 1, in which a precursor membrane B81 containing fluoropolymer (I'-2) and having a thickness of 20 μm was formed on a substrate, and a precursor membrane-coated substrate Y82 was obtained in which a precursor membrane B82 containing fluoropolymer (I'-2) and having a thickness of 30 μm was formed on a substrate. An electrolyte membrane 8 and a membrane electrode assembly 8 of Example 8 were obtained in the same manner as in Example 3, except that precursor membrane-coated substrate X8, precursor membrane-coated substrate Y81, and precursor membrane-coated substrate Y82 were used instead of precursor membrane-coated substrate X1, precursor membrane-coated substrate Y31, and precursor membrane-coated substrate Y32, respectively. In the obtained electrolyte membrane 8, the ion exchange capacity of the fluoropolymer (I-1) contained in the first layer was equal to the ion exchange capacity of the fluoropolymer (I-2) contained in the second layer. In the electrolyte membrane 8, the reinforcing body (woven fabric A1) was disposed at the center position in the thickness direction of the electrolyte membrane 8.

[0181] Example 9 An electrolyte membrane 9 and a membrane electrode assembly 9 of Example 9 were obtained in the same manner as in Example 1, except that a precursor membrane-coated substrate X8 having the precursor membrane A8 (thickness: 10 μm) produced in Example 8 formed thereon was used instead of the precursor membrane-coated substrate X1. In the obtained electrolyte membrane 9, the ion exchange capacity of the fluoropolymer (I-1) contained in the first layer was equal to the ion exchange capacity of the fluoropolymer (I-2) contained in the second layer.

[0182] An electrolyte membrane 10 and a membrane electrode assembly 10 of Example 10 were obtained in the same manner as in Example 3, except that the precursor membrane-coated substrate Y81 and the precursor membrane-coated substrate Y82 produced in Example 8 were used instead of the precursor membrane-coated substrate Y31 and the precursor membrane-coated substrate Y32, respectively. In the obtained electrolyte membrane 10, the reinforcing body (woven fabric A1) was disposed at the center position in the thickness direction of the electrolyte membrane 10.

[0183] [Results] Table 1 shows the structure of the electrolyte membrane produced in each example, as well as the evaluation results of the resistance to scratches when forming the catalyst layer and the hydrogen concentration in oxygen. The thickness of the electrolyte membrane (unit: μm) shown in the "Thickness (μm)" column of "Electrolyte membrane" is the thickness T １ and the thickness T of the second layer ２ The numerical value shown in the "Distance from surface A of reinforcing body (μm)" column indicates the distance between the reinforcing body disposed inside each electrolyte membrane and surface A on the first layer side. When the distance from surface A of the reinforcing body is half the thickness of the electrolyte membrane, this means that the reinforcing body is disposed at the center position in the thickness direction of the electrolyte membrane, and when the distance from surface A of the reinforcing body is shorter than half the thickness of the electrolyte membrane, this means that the reinforcing body is disposed closer to surface A on the first layer side than the center position in the thickness direction of the electrolyte membrane.

[0184]

[0185] As shown in Table 1, electrolyte membranes having a first layer containing a fluorine-containing polymer (I-1) having ion exchange groups and a platinum-containing material, a second layer containing a fluorine-containing polymer (I-2) having ion exchange groups, and a reinforcing body, in which the concentration of the platinum-containing material in the second layer is lower than that in the first layer, the ion exchange capacity of the fluorine-containing polymer (I-1) is higher than that of the fluorine-containing polymer (I-2), and the reinforcing body is disposed on the surface A side, were confirmed to have excellent performance in suppressing curling and excellent performance in suppressing hydrogen crossover (Examples 1 to 7). In Examples 8 and 9, the ion exchange capacities of the fluorine-containing polymer contained in the first layer and the fluorine-containing polymer contained in the second layer were equal, which is thought to have prevented hydrogen crossover from being sufficiently reduced. Furthermore, in Examples 8 and 10, the reinforcing body was disposed at the center in the thickness direction of the solid polymer electrolyte membrane rather than at the interface between the first and second layers, which is thought to have made the electrolyte membrane more prone to curling.

[0186] The entire contents of the specification, claims, drawings and abstract of Japanese Patent Application No. 2023-222220, filed on December 28, 2023, are hereby incorporated by reference as the disclosure of the present invention.

[0187] 10, 20 Electrolyte membrane (solid polymer electrolyte membrane) 11 First layer 12 Second layer 13 Reinforcement 30 Membrane electrode assembly 32 Anode 34 Cathode 36 Catalyst layer 38 Gas diffusion layer A, B Surface C Central surface D Interface T １ , T ２ Thickness

Claims

1. A solid polymer electrolyte membrane, comprising a first layer containing a first fluorine-containing polymer having an ion exchange group and a platinum-containing substance, and a second layer containing a second fluorine-containing polymer having an ion exchange group, wherein the concentration of the platinum-containing substance in the second layer is lower than the concentration of the platinum-containing substance in the first layer, the ion exchange capacity of the first fluorine-containing polymer is larger than the ion exchange capacity of the second fluorine-containing polymer, and further comprising a reinforcing body disposed on the surface side of the first layer side of the solid polymer electrolyte membrane rather than the central position in the thickness direction of the solid polymer electrolyte membrane.

2. The solid polymer electrolyte membrane according to claim 1, further comprising cerium oxide.

3. The solid polymer electrolyte membrane according to claim 1, wherein the second layer does not contain the platinum-containing substance.

4. The solid polymer electrolyte membrane according to claim 1, wherein the reinforcing body is a woven fabric.

5. The solid polymer electrolyte membrane according to claim 4, wherein the woven fabric is composed of a material selected from the group consisting of polytetrafluoroethylene, tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer, polyether ether ketone, and polyphenylene sulfide.

6. The solid polymer electrolyte membrane according to claim 1, wherein the thickness of the first layer is thinner than the thickness of the second layer.

7. The solid polymer electrolyte membrane according to claim 1, wherein the ratio of the thickness of the first layer to the total thickness of the thicknesses of the first layer and the second layer is 0.50 or less.

8. The solid polymer electrolyte membrane according to claim 1, wherein the total thickness of the thicknesses of the first layer and the second layer is 30 to 400 μm.

9. The solid polymer electrolyte membrane according to claim 1, wherein the absolute value of the difference between the ion exchange capacity of the first fluorine-containing polymer and the ion exchange capacity of the second fluorine-containing polymer is 0.10 to 1.40 milliequivalents / gram of dry resin.

10. The solid polymer electrolyte membrane according to claim 1, wherein the ion exchange group of the first fluorine-containing polymer is a sulfonic acid type functional group, and the ion exchange group of the second fluorine-containing polymer is a sulfonic acid type functional group.

11. A membrane electrode assembly comprising the solid polymer electrolyte membrane according to any one of claims 1 to 10, a cathode catalyst layer disposed on the surface side of the second layer of the solid polymer electrolyte membrane, and an anode catalyst layer disposed on the surface side of the first layer of the solid polymer electrolyte membrane.

12. A water electrolysis device comprising the membrane electrode assembly according to claim 11, a power supply unit connected to the cathode catalyst layer side and the anode catalyst layer side in the membrane electrode assembly, and a water supply unit for supplying water to the anode catalyst layer side.

13. A method for producing hydrogen by electrolyzing water with the water electrolysis device according to claim 12.

14. A method for producing a membrane electrode assembly comprising the solid polymer electrolyte membrane according to any one of claims 1 to 10, a cathode catalyst layer, and an anode catalyst layer, the method comprising forming the cathode catalyst layer on the surface side of the second layer side of the solid polymer electrolyte membrane and forming the anode catalyst layer on the surface side of the first layer side of the solid polymer electrolyte membrane.

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