Membrane electrode assembly, method for producing same, and catalyst layer for membrane electrode

Abstract

A membrane electrode assembly includes an anion conductive membrane, a cathode catalyst layer that is in direct contact with one surface of the anion conductive membrane, and an anode catalyst layer that is in direct contact with the other surface, and in a catalyst layer X of at least one of the cathode catalyst layer and the anode catalyst layer, the surface roughness Rα of the surface α in contact with the anion conductive membrane is less than the surface roughness Rβ of the surface β on the reverse side to the surface α. Additionally provided are a method for producing the membrane electrode assembly, and a catalyst layer for a membrane electrode in which the surface roughness of the two surfaces thereof differ.

Description

Membrane electrode assembly, its manufacturing method, and catalyst layer for membrane electrode

[0001] The present invention relates to a membrane electrode assembly, a method for producing the same, and a catalyst layer for a membrane electrode.

[0002] Hydrogen is a clean energy source that does not emit carbon dioxide and is used, for example, as fuel for fuel cell vehicles and household fuel cells. Water electrolysis (electrolysis) is a well-known method for producing hydrogen. Hydrogen can be produced without carbon dioxide emissions by electrolyzing water using a renewable energy power generation system as a power source. Therefore, hydrogen is increasingly attracting attention as a fundamental energy source for a sustainable society.

[0003] Alkaline water electrolysis (AWE), which uses a highly concentrated alkaline aqueous solution as the electrolyte, is a known practical water electrolysis technology. In alkaline water electrolysis, hydrogen bubbles (2H ２ O + 2e － →H ２ +2OH － ) to the anode side, and also prevents the bubble-like oxygen (4OH) generated at the anode (positive electrode). － →O ２ +2H ２ O+4e － In order to prevent OH from migrating to the cathode side, a gas barrier separator (membrane) is disposed between the cathode and the anode. － Anion-conducting membrane water electrolysis (AEMWE) has also been proposed, which uses an anion-conducting membrane to improve the efficiency of the conduction (transfer) of hydroxy ions. AEMWE can conduct hydroxy ions from the cathode to the anode with high efficiency without using a highly concentrated alkaline aqueous solution as the electrolyte.

[0004] The electrodes (anode catalyst layer and cathode catalyst layer) of an AEMWE are generally formed by mixing a catalyst in which a noble metal, metal oxide, or the like is optionally supported on a carrier such as carbon, an anionic ionomer resin that functions as both a binder and an ion conductor, and a liquid medium, applying the mixture to a gas-permeable and electronically conductive conductive substrate (gas diffusion layer), and drying the coating to remove the liquid medium. Next, an anion conductive membrane containing a hydroxy ion-conducting polymer is sandwiched between the formed anode catalyst layer and cathode catalyst layer, and the anion conductive membrane is joined to the anode catalyst layer and the cathode catalyst layer by thermocompression bonding under pressure, thereby obtaining a membrane electrode assembly (MEA) for an AEMWE.

[0005] In water electrolysis, it is necessary to suppress overvoltage, which causes energy loss. For example, Patent Document 1 discloses the following membrane electrode assembly: a pair of electrodes having a porous power supply layer made of a conductive material; and an electrolyte membrane disposed between the pair of electrodes, wherein at least one of the pair of electrodes has a catalyst layer in the power supply layer, and in a cross section of the power supply layer having the catalyst layer, an electrolyte is present in a first region that is 80% or less of the thickness of the power supply layer from the electrolyte membrane side toward the opposite direction from the electrolyte membrane side, a catalyst layer is present on 50% or more of the outer periphery of the cross section of the conductive material in the first region, and the catalyst layer is present on 10% or less of the outer periphery of the cross section of the conductive material in a second region other than the first region. The technology described in Patent Document 1 is said to be able to suppress an increase in overvoltage.

[0006] Japanese Patent Application Laid-Open No. 2019-49043

[0007] In order to improve the efficiency of water electrolysis, it is necessary to increase the current density (the value of current flowing per electrode area). However, the inventors of the present invention have conducted extensive research into water electrolysis performed at high current densities and have found that as the current density increases, the amount of hydrogen gas and oxygen gas generated increases, and these gases accumulate in the catalyst layer, increasing the resistance of the membrane electrode assembly and increasing the overvoltage, which tends to result in a decrease in the efficiency of hydrogen gas production (electrolysis efficiency). Furthermore, when producing hydrogen by water electrolysis using a power generation system that utilizes renewable energy as a power source, there is a need to address the inevitable fluctuations in current density.

[0008] An object of the present invention is to provide a membrane electrode assembly that has excellent electrolysis efficiency during operation at a high current density and also has excellent durability against fluctuations in current density caused by repeated operation at a high current density and a low current density, a method for producing the same, and a membrane electrode catalyst layer that is suitable for forming the membrane electrode assembly.

[0009] The above-mentioned problems of the present invention are solved by the following means. [1] A membrane / electrode assembly having an anion conductive membrane, a cathode catalyst layer in direct contact with one surface of the anion conductive membrane, and an anode catalyst layer in direct contact with the other surface, wherein in at least one catalyst layer X of the cathode catalyst layer or the anode catalyst layer, a surface roughness Rα of a surface α that is in contact with the anion conductive membrane is smaller than a surface roughness Rβ of a surface β opposite to the surface α. [2] The membrane / electrode assembly according to [1], in which a volume occupancy Mβ of the catalyst on the surface β side of the catalyst layer X is lower than a volume occupancy Mα of the catalyst on the surface α side. [3] The membrane / electrode assembly according to [1] or [2], in which the particle size of the catalyst on the surface α side of the catalyst layer X is smaller than the particle size of the catalyst on the surface β side. [4] A method for producing a membrane electrode assembly according to any one of [1] to [3], comprising forming the catalyst layer X by stacking two catalyst layers having different surface roughnesses. [5] A membrane electrode catalyst layer having two surfaces with different surface roughnesses.

[0010] The membrane electrode assembly of the present invention has excellent electrolysis efficiency when operated at a high current density, and also has excellent durability against fluctuations in current density caused by repeated operation at a high current density and a low current density. The membrane electrode assembly of the present invention can be produced by the method for producing the membrane electrode assembly of the present invention. The catalyst layer for a membrane electrode of the present invention is suitable as the catalyst layer for the membrane electrode assembly of the present invention.

[0011] FIG. 1 is a longitudinal sectional view showing a schematic diagram of a basic layer structure of one embodiment of a membrane electrode assembly according to the present invention.

[0012] [Membrane electrode assembly]

[0013] The membrane electrode assembly of the present invention is suitable as a membrane electrode assembly for use in water electrolysis. The membrane electrode assembly of the present invention can be widely used as a membrane electrode assembly in water electrolysis such as alkaline water electrolysis and anion exchange membrane water electrolysis.

[0014] The membrane / electrode assembly of the present invention has an anion conductive membrane, a cathode catalyst layer in direct contact with one surface of the anion conductive membrane, and an anode catalyst layer in direct contact with the other surface of the anion conductive membrane. In the membrane / electrode assembly of the present invention, in at least one catalyst layer X of the cathode catalyst layer or the anode catalyst layer, the surface roughness Rα of the surface α that is in contact with the anion conductive membrane is smaller than the surface roughness Rβ of the surface β opposite to the surface α. In other words, the catalyst layer X in which the surface roughness Rα of the surface α is smaller than the surface roughness Rβ of the surface β may be either the cathode catalyst layer or the anode catalyst layer, or both the cathode catalyst layer and the anode catalyst layer may be catalyst layers X. In the present invention, the simple term "catalyst layer" refers to the anode catalyst layer and the cathode catalyst layer.

[0015] By making the surface roughness Rα of the surface α smaller than the surface roughness Rβ of the surface β, gases generated in the catalyst layer (hydrogen gas in the cathode catalyst layer and oxygen gas in the anode catalyst layer) can be efficiently transferred to the side opposite the anion conductive membrane (to the gas diffusion layer if the membrane electrode assembly has a gas diffusion layer), enabling efficient discharge of the gas outside the catalyst layer. Conventional catalyst layers are unable to achieve this gas transfer smoothly enough, which tends to result in gas retention within the catalyst layer, inhibiting the supply of water and electrons to the catalyst layer and increasing the resistance of the membrane electrode assembly. In particular, water electrolysis at high current densities increases the amount of hydrogen and oxygen gas generated, which tends to reduce the electrolysis efficiency. The membrane electrode assembly of the present invention improves gas diffusion efficiency from within the catalyst layer to the outside of the catalyst layer (the side opposite the anion conductive membrane), suppressing an increase in the resistance of the membrane electrode assembly even at high current densities, and improving electrolysis efficiency. Furthermore, even if the current density fluctuates during water electrolysis, the resistance of the membrane electrode assembly is unlikely to increase, allowing the electrolysis reaction to proceed efficiently. This suppresses deterioration of the catalyst layer and the anion conductive membrane that would otherwise be caused by an increase in resistance, and the device also has excellent durability.

[0016] In the present invention, the catalyst layer X may have a single-layer structure or a multi-layer structure consisting of two or more layers. When the catalyst layer X has a multi-layer structure, the surface of the layer adjacent to the anion conductive membrane that is in contact with the anion conductive membrane is called surface α, and the surface of the layer farthest from the anion conductive membrane that is opposite to the anion conductive membrane is called surface β.

[0017] The difference between Rβ and Rα (Rβ-Rα) is preferably 0.001 μm or more, more preferably 0.005 μm or more, and even more preferably 0.010 μm or more. There is no particular upper limit. For example, it can be 1.500 μm or less. The difference between Rβ and Rα is preferably 0.001 to 1.500 μm, more preferably 0.001 to 1.000 μm, even more preferably 0.001 to 0.500 μm, even more preferably 0.005 to 0.300 μm, and even more preferably 0.010 to 0.100 μm. Rα is preferably 0.001 to 0.050 μm, and more preferably 0.003 to 0.030 μm. Rβ is preferably 0.010 to 1.500 μm, and more preferably 0.015 to 0.150 μm. The surface roughnesses Rβ and Rα can be measured by the method described in the Examples. Rβ and Rα can be controlled to the above values ​​by adjusting the particle size, volume occupancy, etc. of the catalyst contained in the catalyst layer X, adjusting the solids concentration in the catalyst layer-forming ink, or adjusting the method of forming the catalyst layer (e.g., pressurization). For example, using pressurization, the volume occupancy of the catalyst in the catalyst layer can be adjusted by applying and drying the catalyst layer ink and then pressurizing the resulting catalyst layer, thereby controlling the surface roughness R. Therefore, by forming multiple catalyst layers using the same catalyst layer-forming ink under different pressurization conditions and stacking these layers, the surface roughnesses Rα and Rβ of the surfaces α and β can be made to satisfy the above relationship. Furthermore, by forming the catalyst layer X as a multilayer structure consisting of multiple layers with different catalyst particle sizes and forming a catalyst particle size gradient so that the catalyst particle size gradually increases in the thickness direction from the anion conductive membrane side to the opposite side, the two-sided roughnesses of the surfaces α and β can be made to satisfy the above relationship. Furthermore, by forming a plurality of catalyst layers X using catalyst layer-forming inks with different solid content concentrations and laminating these, the roughnesses of both surfaces α and β can be made to satisfy the above relationship.

[0018] In the catalyst layer X, the volume occupancy of the catalyst on the surface β side (Mβ) is preferably lower than the volume occupancy of the catalyst on the surface α side (Mα). Here, "the volume occupancy of the catalyst on the surface α side" refers to the volume occupancy of the catalyst in the volume range from the surface α to a thickness of 1 μm, and similarly, "the volume occupancy of the catalyst on the surface β side" refers to the volume occupancy of the catalyst in the volume range from the surface β to a thickness of 1 μm. Mα and Mβ can be determined by the method described in the Examples. The difference between Mα and Mβ (Mα - Mβ) is preferably 1 vol% or more, more preferably 5 vol% or more, and even more preferably 10 vol% or more. There is no particular upper limit, and 40 vol% or less is practical. The difference between Mα and Mβ is preferably 1 to 40 vol%, more preferably 5 to 40 vol%, even more preferably 10 to 40 vol%, even more preferably 10 to 25 vol%, and even more preferably 15 to 25 vol%. Mα is preferably 35 to 80% by volume, more preferably 35 to 75% by volume, and Mβ is preferably 25 to 70% by volume, more preferably 30 to 65% by volume.

[0019] Focusing on the entire thickness direction of the catalyst layer X, the volume occupancy of the catalyst throughout the entire thickness direction may gradually decrease from the surface α side to the surface β side, or may decrease in stages. Note that, as long as the effects of the present invention are not impaired, there may be a portion where the volume occupancy of the catalyst increases from the surface α side to the surface β side. It is preferable that the catalyst layer X is divided into two equal parts in the thickness direction, and the volume occupancy of the catalyst in the half on the surface α side is higher than the volume occupancy of the catalyst in the half on the surface β side. It is preferable that the catalyst layer X is divided into four equal parts in the thickness direction, and is divided into layers X1, X2, X3, and X4 from the surface α side to the surface β side, and the volume occupancy of the catalyst in X1 is higher than the volume occupancy of the catalyst in X4, and there is no portion where the volume occupancy of the catalyst increases from X1 to X4.

[0020] In the catalyst layer X, the particle diameter (rα) of the catalyst on the surface α side can be smaller than the particle diameter (rβ) of the catalyst on the surface β side. In the present invention, "particle diameter" refers to the volume-based median diameter (d50). Here, "particle diameter of the catalyst on the surface α side" refers to the particle diameter of the catalyst within a range of 1 μm from the surface α, and similarly, "particle diameter of the catalyst on the surface β side" refers to the particle diameter of the catalyst within a range of 1 μm from the surface β. rα and rβ can be determined by photographing a cross-section of the catalyst layer with a scanning electron microscope (SEM) and analyzing the obtained SEM. Specifically, the process is as follows. The catalyst layer is cut with a razor or the like to expose the cross-section, and then the cross-section is smoothed using ion milling (e.g., a cross-section polisher) to obtain an SEM image. The diameters (sphere-equivalent diameters) of catalyst particles present within 1 μm of the surface α and surface β in the obtained SEM image are measured. The size of one field of view is 20 μm x 10 μm, and measurements are taken over 100 fields of view. The particle size is determined by calculating D50 from the diameter distribution. If the particle size is within a 1 μm thickness range from the surfaces α and β, it usually coincides with the particle size of the catalyst contained in the catalyst layer-forming ink used to form the catalyst layer including those surfaces. The difference between rβ and rα (rβ - rα) is preferably 0.050 μm or more, more preferably 0.100 μm or more, and even more preferably 0.500 μm or more. There is no particular upper limit, and a practical value is 1.000 μm or less. The difference between rβ and rα is preferably 0.050 to 1.000 μm, more preferably 0.100 to 1.000 μm, and even more preferably 0.500 to 1.000 μm. rα is preferably 0.005 to 1.000 μm, and more preferably 0.010 to 0.100 μm. rβ is preferably from 0.010 to 1.100 μm, more preferably from 0.010 to 0.900 μm, and even more preferably from 0.010 to 0.500 μm.

[0021] The catalyst layer X may have an embodiment in which the particle size (rα) of the catalyst on the surface α side is the same as the particle size (rβ) of the catalyst on the surface β side. In this embodiment, the particle size of the catalyst is preferably 0.005 to 1.000 μm, more preferably 0.008 to 0.800 μm, and even more preferably 0.008 to 0.500 μm.

[0022] Focusing on the entire thickness direction of the catalyst layer X, the particle size of the catalyst throughout the thickness direction may gradually decrease from the surface β side to the surface α side, or may decrease in stages. Note that, as long as the effects of the present invention are not impaired, there may be a portion where the particle size of the catalyst increases from the surface β side to the surface α side. It is preferable that the catalyst layer X is divided into two equal parts in the thickness direction, and the particle size of the catalyst in the half on the surface α side is smaller than the particle size in the half on the surface β side. It is preferable that the catalyst layer X is divided into four equal parts in the thickness direction, and is divided into layers X1, X2, X3, and X4 from the surface α side to the surface β side, and the particle size of the catalyst in X1 is smaller than the particle size of the catalyst in X4, and there is no portion where the particle size of the catalyst decreases from X1 to X4.

[0023] The catalyst layer X can be formed by preparing a catalyst layer-forming ink containing an anode catalyst or a cathode catalyst, a polymer, and, if necessary, a solvent, etc., and applying and drying the ink. When the catalyst layer X has a multi-layer structure, it can be formed by preparing multiple types of catalyst layer-forming inks with different compositions, applying each catalyst layer-forming ink, drying, and laminating them. A more specific method for forming the catalyst layer X will be described later.

[0024] FIG. 1 shows a preferred embodiment of the membrane electrode assembly of the present invention. In the membrane electrode assembly 4 of FIG. 1, a cathode catalyst layer 2c is formed on one surface of an anion conductive membrane 1, an anode catalyst layer 2a is formed on the other surface, and a gas diffusion layer 3 is formed on each of these catalyst layers on the surface opposite the anion conductive membrane 1. This membrane electrode assembly 4 has a layered structure in which the gas diffusion layer 3 - the cathode catalyst layer 2c - the anion conductive membrane 1 - the anode catalyst layer 2a - and the gas diffusion layer 3 are layered in this order. The anode catalyst layer 2a contains a particulate anode catalyst 21 and a polymer 23 that also functions as a binder, and the anode catalyst 21 is bound by the polymer 23. The cathode catalyst layer 2c contains a particulate cathode catalyst 22 and a polymer 23 that also functions as a binder, and the cathode catalyst 22 is bound by the polymer 23. In FIG. 1, a bipolar plate 5 is further formed on the surface of the two gas diffusion layers 3 of the membrane electrode assembly 4 opposite to the catalyst layer, thereby forming a water electrolysis cell 10 .

[0025] The membrane electrode assembly of the present invention can be suitably used for producing hydrogen by incorporating it into a system (apparatus) such as the water electrolysis cell 10. The membrane electrode assembly of the present invention can be used, for example, in the form of a water electrolysis system in which the membrane electrode assembly of the present invention is further combined with components commonly used in water electrolysis, such as bipolar plates.

[0026] The membrane electrode assembly of the present invention will now be described in more detail.

[0027] <Anode catalyst layer> The anode catalyst layer preferably contains an anode catalyst and a polymer. In the membrane / electrode assembly of the present invention, when the anode catalyst layer satisfies the above-mentioned relationship between plane α and plane β with respect to the anion conductive membrane, the anode catalyst layer can be used in a normal membrane / electrode assembly, except for satisfying the relationship between plane α and plane β. In the membrane / electrode assembly of the present invention, when the anode catalyst layer does not satisfy the above-mentioned relationship between plane α and plane β with respect to the anion conductive membrane, the anode catalyst layer can be used in a normal membrane / electrode assembly.

[0028] The anode catalyst can be selected depending on the application of the membrane electrode assembly of the present invention. When the membrane electrode assembly of the present invention is a membrane electrode assembly intended for water electrolysis, the anode catalyst may be one that electrolyzes water to produce oxygen and water. Examples of such anode catalyst include iridium oxide, iridium oxide-coated titanium, iridium ruthenium cobalt oxide, iridium ruthenium tin oxide, iridium ruthenium iron oxide, iridium ruthenium nickel oxide, iridium tin oxide, iridium zirconium oxide, ruthenium titanium oxide, ruthenium zirconium oxide, ruthenium tantalum oxide, ruthenium titanium cerium oxide, oxides such as nickel iron oxide, barium iron oxide, and strontium iron oxide, and hydroxides such as nickel iron hydroxide.

[0029] The anode catalyst is preferably in particulate form. In the anode catalyst layer, the particulate anode catalyst is preferably bound by the polymer.

[0030] The polymer contained in the anode catalyst layer is preferably, for example, an ionomer resin. The ionomer resin is preferably an anionic ionomer resin. Examples of the ionomer resin include sulfonated plastic electrolytes such as perfluoroalkanesulfonic acid, sulfonated polyether ketone, sulfonated polyether sulfone, sulfonated polyether ether sulfone, sulfonated polysulfone, sulfonated polysulfide, and sulfonated polyphenylene, and sulfoalkylated plastic electrolytes such as sulfoalkylated polyether ether ketone, sulfoalkylated polyether sulfone, sulfoalkylated polyether ether sulfone, sulfoalkylated polysulfone, sulfoalkylated polysulfide, and sulfoalkylated polyphenylene. Furthermore, fluoropolymers other than ionomer resins (e.g., polytetrafluoroethylene (PTFE)), silicone polymers, and the like can also be used.

[0031] The thickness of the anode catalyst layer is not particularly limited as long as it is thicker than the particle size of the catalyst contained in the catalyst layer, and is preferably 2 to 80 μm, more preferably 3 to 50 μm, even more preferably 5 to 40 μm, and still more preferably 5 to 30 μm.

[0032] <Cathode catalyst layer> The cathode catalyst layer preferably contains a cathode catalyst and a polymer. In the membrane / electrode assembly of the present invention, when the cathode catalyst layer satisfies the above-mentioned relationship between plane α and plane β with respect to the anion conductive membrane, the cathode catalyst layer can be the same as the cathode catalyst layer used in a normal membrane / electrode assembly, except for the relationship between plane α and plane β. In the membrane / electrode assembly of the present invention, when the cathode catalyst layer does not satisfy the above-mentioned relationship between plane α and plane β with respect to the anion conductive membrane, the cathode catalyst layer can be the same as the cathode catalyst layer used in a normal membrane / electrode assembly.

[0033] The cathode catalyst can be selected depending on the application of the membrane electrode assembly of the present invention. When the membrane electrode assembly of the present invention is a membrane electrode assembly intended for water electrolysis, the cathode catalyst may be one that electrolyzes water to produce hydrogen (gas). Examples of such cathode catalysts that can be used include platinum-supported carbon particles, platinum-coated titanium, palladium-supported carbon particles, cobalt glyoxime, and nickel glyoxime.

[0034] The cathode catalyst is preferably in particulate form. In the cathode catalyst layer, the particulate cathode catalyst is preferably bound by the polymer.

[0035] As the polymer contained in the cathode catalyst layer, for example, the polymers described above as the polymer contained in the anode catalyst layer can be used.

[0036] The thickness of the cathode catalyst layer is not particularly limited as long as it is thicker than the particle size of the catalyst contained in the catalyst layer. The thickness of the cathode catalyst layer is preferably 2 to 80 μm, more preferably 3 to 50 μm, even more preferably 5 to 40 μm, and still more preferably 5 to 30 μm.

[0037] <Anion Conducting Membrane> The anion conducting membrane may be any membrane having anion conductivity, and a typical anion conducting membrane used in water electrolysis may be used. The ions conducted in the anion conducting membrane are preferably hydroxy ions. The anion conducting membrane may be a solid polymer membrane, a porous membrane, or the like. The polymer constituting the anion conducting membrane may be the same polymer as that described above for the anode catalyst layer.

[0038] <Gas Diffusion Layer> The membrane / electrode assembly of the present invention may have a gas diffusion layer on the surface of the catalyst layer opposite to the anion conductive membrane. The gas diffusion layer may be a layer capable of transporting gas and moisture and having electronic conductivity. The gas diffusion layer may be a gas diffusion layer used in a typical membrane / electrode assembly. Examples of materials that can be used for the gas diffusion layer include carbon fiber nonwoven fabric, carbon paper, carbon plate, stainless steel (SUS) fiber nonwoven fabric, stainless steel sintered compact, titanium (Ti) fiber nonwoven fabric, titanium sintered compact, nickel (Ni) fiber nonwoven fabric, nickel sintered compact, and materials in which each of these substrates is coated with platinum or gold.

[0039] [Method for manufacturing membrane electrode assembly] The membrane electrode assembly can be formed by applying a catalyst layer-forming ink to an anion conductive membrane, or by laminating a pre-formed catalyst layer and an anion conductive membrane. When a gas diffusion layer is provided on the membrane electrode assembly, the catalyst layer-forming ink may be applied to one side of the gas diffusion layer to form a catalyst layer-gas diffusion layer laminate, which is then bonded to the anion conductive membrane.

[0040] In the method for producing a membrane electrode assembly, the catalyst layer X can also be formed by separately forming two catalyst layers with different surface roughnesses R and then stacking them together to form a single layer. For example, the catalyst layer can be formed by applying a catalyst layer-forming ink I to one side of a gas diffusion layer to form a catalyst layer-gas diffusion layer laminate, applying another catalyst layer-forming ink II to one side of an anion conductive membrane to form a catalyst layer-anion conductive membrane laminate, and stacking these laminates so that the catalyst layers are in contact with each other. (In this case, a membrane electrode assembly having a catalyst layer X with a two-layer structure is obtained.) Alternatively, a catalyst layer can be formed by applying and drying a catalyst layer-forming ink III to an anion conductive membrane or a gas diffusion layer as a support, and then applying and drying another catalyst layer-forming ink IV on top of this catalyst layer, thereby forming a catalyst layer X with a two-layer structure. When forming the catalyst layer, the catalyst layer can be pressurized to compress the polymer component in the catalyst layer, thereby changing the volume fraction of the catalyst in the catalyst layer and thereby controlling the surface roughness R. When controlling the surface roughness R by applying pressure, catalyst layers with different surface roughness R can be formed using the same catalyst layer-forming ink. Pressurization may be applied to a catalyst layer formed on a support, or to catalyst layers that are superimposed when joining multiple catalyst layers together. The pressure applied when applying pressure to form the catalyst layer X is not particularly limited as long as it can achieve the desired surface roughness R, and can be, for example, 0.5 to 7.0 MPa. Although the catalyst content in the catalyst layer-forming ink depends on the particle size, reducing the applied pressure tends to reduce the surface roughness R. When applying pressure, it is preferable to heat the catalyst layer.

[0041] [Membrane Electrode Catalyst Layer] The membrane electrode catalyst layer of the present invention has a surface roughness R different between one surface γ and the opposite surface δ. The membrane electrode catalyst layer of the present invention is the same as the membrane electrode assembly of the present invention, except that it does not have an anion conductive membrane. Here, one surface γ of the membrane electrode catalyst layer of the present invention can be the same as the surface α of the catalyst layer of the membrane electrode assembly of the present invention, and the opposite surface δ can be the same as the surface β of the catalyst layer of the membrane electrode assembly of the present invention. That is, the surface roughness Rγ of the surface γ of the membrane electrode catalyst layer of the present invention and the surface roughness Rδ of the surface δ can be the same as the surface roughness Rα of the surface α of the catalyst layer of the membrane electrode assembly of the present invention and the surface roughness Rβ of the surface β of the catalyst layer of the membrane electrode assembly of the present invention, and the preferred relationship (difference) and preferred range thereof are also the same. All other details (e.g., catalyst volume occupancy, preparation method, constituent materials, etc.) described above for the membrane electrode assembly of the present invention are applicable, except for the details related to the anion conductive membrane. The membrane electrode catalyst layer of the present invention can also be integrated with a gas diffusion layer. In this embodiment, the surface δ, which has a surface roughness R greater than that of the surface γ, is the surface in contact with the gas diffusion layer. The catalyst layer for membrane electrodes of the present invention can be suitably used for forming the membrane electrode assembly of the present invention.

[0042] The present invention will be described in more detail below based on examples, but the present invention should not be construed as being limited thereto.

[0043] 1. Preparation of Ink for Forming Catalyst Layer (1) Preparation of Ink a1 for Forming Anode Catalyst Layer In a 5 mL vial, 0.2 g of Nafion dispersion (perfluoroalkanesulfonic acid ionomer resin, manufactured by Fujifilm Wako Chemical Co., Ltd., 5 mass% Nafion dispersion DE521 (trade name)), 0.41 g of iridium oxide powder (manufactured by Furuya Metal Co., Ltd., iridium oxide, particle size 0.010 μm), and 1.5 g of deionized water were mixed, and the mixture was dispersed for 5 minutes at a power of 30 W using an ultrasonic homogenizer (UH-300 (trade name), manufactured by SMT Co., Ltd.), cooled at 5° C. for 10 minutes, and then dispersed again for 5 minutes at a power of 30 W. In this way, Ink a1 for forming an anode catalyst layer was prepared.

[0044] (2) Preparation of Ink a2 for Forming Anode Catalyst Layer In a 5 mL vial, 0.2 g of Nafion dispersion (perfluoroalkanesulfonic acid ionomer resin, manufactured by Fujifilm Wako Chemical Co., Ltd., 5 mass% Nafion dispersion DE521 (trade name)), 0.41 g of iridium oxide powder (manufactured by Kojundo Chemical Co., Ltd., iridium oxide, particle size 0.800 μm), and 0.5 g of deionized water were mixed, and the mixture was dispersed for 5 minutes at a power of 30 W using an ultrasonic homogenizer (UH-300 (trade name), manufactured by SMT Co., Ltd.), cooled at 5° C. for 10 minutes, and then dispersed again for 5 minutes at a power of 30 W. In this way, Ink a2 for forming an anode catalyst layer was prepared.

[0045] (3) Preparation of Ink a3 for Forming Anode Catalyst Layer In a 45 mL zirconia container, 20 φ3 mm zirconia balls, 0.2 g of Nafion dispersion (perfluoroalkanesulfonic acid ionomer resin, manufactured by Fujifilm Wako Chemical Co., Ltd., 5 mass% Nafion dispersion DE521 (trade name)), 0.41 g of iridium oxide powder (manufactured by Kojundo Chemical Co., Ltd., iridium oxide, particle size 0.800 μm), and 1.0 g of deionized water were mixed and dispersed at 200 rpm for 30 minutes using a planetary ball mill (P-7 (trade name), manufactured by Fritsch Japan KK). The zirconia balls were then separated. In this way, Ink a3 for forming an anode catalyst layer, in which the particle size of the iridium oxide was 0.100 μm, was prepared.

[0046] (4) Preparation of Ink b1 for Forming Cathode Catalyst Layer In a 10 mL vial, 3.1 g of Nafion dispersion (perfluoroalkanesulfonic acid ionomer resin, manufactured by Fujifilm Wako Chemical Co., Ltd., 5 mass% Nafion dispersion DE521 (trade name)) and 0.41 g of platinum carbon powder (manufactured by Tanaka Kikinzoku Kogyo Co., Ltd., TEC10E50E (trade name), Pt content 47 mass%, particle size 0.008 μm) were mixed and dispersed for 5 minutes using an ultrasonic homogenizer (UH-300 (trade name), manufactured by SMT Co., Ltd.) at a power of 30 W. The mixture was cooled at 5° C. for 10 minutes and then dispersed again for 5 minutes at a power of 30 W. In this way, Ink b1 for forming a cathode catalyst layer was prepared.

[0047] 2. Preparation of Water Electrolysis Cells Using the inks a1 to a3 for forming anode catalyst layers and ink b1 for forming cathode catalyst layers prepared above, water electrolysis cells including membrane electrode assemblies having an anode catalyst layer, an anion conductive membrane, and a cathode catalyst layer were prepared as follows. In Examples 1 to 4, and 7 and Comparative Examples 1 and 2, catalyst layers X were formed by controlling the surface roughness R on both sides of the catalyst layer by applying pressure. In Examples 5 and 6, catalyst layers X were formed by controlling the surface roughness R on both sides of the catalyst layer by using catalysts with different particle sizes. Table 1 shows the inks for forming anode catalyst layers and inks for forming cathode catalyst layers used, as well as the numbers of the resulting catalyst layers.

[0048] (1) Preparation of water electrolysis cell of Example 1 (a) Preparation of anion conductive membrane-anode catalyst layer-gas diffusion layer laminate MAAG Anode catalyst layer-forming ink a1 was applied to a polytetrafluoroethylene film (NITOFLON No. 900UL (product name), manufactured by Nitto Denko Corporation) using an applicator in an amount of 0.75 mg / cm. ２ The anode catalyst layer AC1a was applied to the anion conductive membrane under pressure at 25°C, and the resulting laminate was naturally dried. The resulting laminate was overlaid on an anion conductive membrane (X37-50 (trade name), manufactured by Dioxide Material Corporation, an anion exchange membrane having a PTFE support) so that the anode catalyst layer was in contact with the anion conductive membrane. The entire surface was pressed at a pressure of 5.0 MPa and 25°C to bond the anode catalyst layer and the anion conductive membrane, and then the polytetrafluoroethylene film was peeled off. In this manner, the anode catalyst layer AC1a was transferred to the anion conductive membrane under pressure, and a laminate MA was obtained. The ink a1 for forming an anode catalyst layer was applied to a gas diffusion layer (GDL-39BB (trade name), manufactured by SGL, a carbon fiber nonwoven fabric having a microporous layer) using an applicator, with the catalyst coating amount being 0.75 mg / cm. ２ The coated film was coated so that the anode catalyst layer was in contact with the anode catalyst layer AC1b, and then allowed to dry naturally. In this way, a gas diffusion layer-anode catalyst layer laminate AG having an anode catalyst layer AC1b was obtained. This laminate AG and the laminate MA were laminated so that the anode catalyst layers were in contact with each other, and the entire surfaces were pressed together at a pressure of 0.5 MPa and 25°C. In this way, an anion conduction membrane-anode catalyst layer-gas diffusion layer laminate MAAG having a two-layer structure of anode catalyst layer AC1 was produced.

[0049] (b) Preparation of cathode catalyst layer-gas diffusion layer laminate xCG The cathode catalyst layer-forming ink b1 was applied to a gas diffusion layer (manufactured by SGL, GDL-39BB, a carbon fiber nonwoven fabric having a microporous layer) using an applicator in a catalyst coating amount of 1.5 mg / cm. ２ The coating was then allowed to dry naturally. In this manner, a cathode catalyst layer-gas diffusion layer laminate xCG having a single-layer cathode catalyst layer xC was obtained. The cathode catalyst layer xC had the same surface roughness R of 0.030 μm on both surfaces α and β.

[0050] (c) Preparation of a water electrolysis cell The anion conductive membrane-anode catalyst layer-gas diffusion layer laminate MAAG and the cathode catalyst layer-gas diffusion layer laminate xCG obtained above were each separated into 1 cm ２ The laminate was punched out to the same shape as the laminate MAAG, and the laminate was placed on top of the laminate MAAG so that the cathode catalyst layer of the laminate xCG was in contact with the surface of the anion conductive membrane of the laminate MAAG opposite the anode catalyst layer. The entire surface was pressurized at 0.5 MPa and 25°C. The pressed laminate was sandwiched between Ni bipolar plates having flow channels and constrained with bolts to a confining pressure of 0.5 MPa. In this way, a water electrolysis cell was obtained having the same layer structure as water electrolysis cell 10 shown in FIG. 1 , which has a layer structure of bipolar plate-gas diffusion layer-anode catalyst layer-anion conductive membrane-cathode catalyst layer-gas diffusion layer-bipolar plate. This water electrolysis cell includes a membrane electrode assembly having an anion conductive membrane, and an anode catalyst layer AC1 and cathode catalyst layer xC that are in direct contact with the anion conductive membrane. In this membrane electrode assembly, the anode catalyst layer AC1 has a multilayer structure consisting of two catalyst layers: an anode catalyst layer AC1a formed at a pressure of 5.0 MPa and an anode catalyst layer AC1b formed at a pressure of 0.5 MPa, and has surfaces α and β with different surface roughnesses R, and is in contact with the anion conductive membrane at surface α. On the other hand, the cathode catalyst layer xC is formed by pressing from both sides at a pressure of 0.5 MPa when bonding to the anion conductive membrane, and surfaces α and β have the same surface roughness R.

[0051] (2) Preparation of water electrolysis cell of Example 2 A water electrolysis cell including a membrane electrode assembly having an anode catalyst layer AC2 was prepared in the same manner as in the preparation of the water electrolysis cell of Example 1, except that in the preparation of the anion conductive membrane-anode catalyst layer-gas diffusion layer laminate MAAG, the pressure applied when preparing the laminate MA was changed to 3.0 MPa.

[0052] (3) Preparation of water electrolysis cell of Example 3 A water electrolysis cell including a membrane electrode assembly having an anode catalyst layer AC3 was prepared in the same manner as in the preparation of the water electrolysis cell of Example 1, except that in the preparation of the anion conductive membrane-anode catalyst layer-gas diffusion layer laminate MAAG, the pressure used to join the laminate AG and the laminate MA was changed to 0.1 MPa.

[0053] (4) Preparation of water electrolysis cell of Example 4 A water electrolysis cell including a membrane electrode assembly having an anode catalyst layer AC4 was prepared in the same manner as in the preparation of the water electrolysis cell of Example 1, except that in the preparation of the anion conductive membrane-anode catalyst layer-gas diffusion layer laminate MAAG, the pressure used to join the laminate AG and the laminate MA was changed to 3.0 MPa.

[0054] (5) Preparation of water electrolysis cell of Example 5 (a) Preparation of anode catalyst layer-gas diffusion layer laminate AG5 Anode catalyst layer-forming ink a2 was applied to a gas diffusion layer (manufactured by SGL, GDL-39BB (product name), a carbon fiber nonwoven fabric having a microporous layer) using an applicator in an amount of 0.75 mg / cm ２ The ink a1 for forming an anode catalyst layer was further applied onto this dried film (catalyst layer) using an applicator so that the catalyst coating amount was 0.75 mg / cm. ２ In this way, an anode catalyst layer-gas diffusion layer laminate AG5 having an anode catalyst layer AC5 of a two-layer structure was obtained.

[0055] (b) Preparation of water electrolysis cell The anode catalyst layer-gas diffusion layer laminate AG5 and the cathode catalyst layer-gas diffusion layer laminate xCG were each separated into 1 cm ２The plates were punched out into the same shape so that each plate was stacked with the catalyst layer side facing inward, sandwiching an anion conduction membrane (an anion exchange membrane having a PTFE support, manufactured by Dioxide Material Corporation, X37-50 (product name)), and the entire surface was pressurized at 25°C with a pressure of 0.5 MPa to obtain a formed assembly (membrane electrode assembly) having a gas diffusion layer-anode catalyst layer-anion conduction membrane-cathode catalyst layer-gas diffusion layer configuration. The pressurized formed assembly was sandwiched between Ni bipolar plates having flow channels and constrained with bolts to apply a constraining pressure of 0.5 MPa. In this way, a water electrolysis cell having the same layer configuration as water electrolysis cell 10 shown in FIG. 1 , which has a layer configuration of bipolar plate-gas diffusion layer-anode catalyst layer-anion conduction membrane-cathode catalyst layer-gas diffusion layer-bipolar plate. In the membrane electrode assembly of this water electrolysis cell, the anode catalyst layers AC5a and AC5b constituting the anode catalyst layer AC5 and the cathode catalyst layer xC are all formed by applying a pressure of 0.5 MPa.

[0056] (6) Preparation of water electrolysis cell of Example 6 A water electrolysis cell of Example 6 including a membrane electrode assembly having an anode catalyst layer AC6 was prepared in the same manner as in the preparation of the water electrolysis cell of Example 5, except that in the preparation of the anode catalyst layer-gas diffusion layer laminate AG5, the anode catalyst layer-forming ink a2 was changed to the anode catalyst layer-forming ink a3.

[0057] (7) Preparation of water electrolysis cell of Comparative Example 1 (a) Preparation of anode catalyst layer-gas diffusion layer laminate xAG Anode catalyst layer-forming ink a1 was applied to a gas diffusion layer (manufactured by SGL, GDL-39BB, a carbon fiber nonwoven fabric having a microporous layer) using an applicator in an amount of 1.5 mg / cm. ２ After air drying, the mixture was pressurized at a pressure of 5.0 MPa and 80°C. In this manner, an anode catalyst layer-gas diffusion layer laminate xAG having an anode catalyst layer xAC1 with a single-layer structure was obtained. In Table 1, for convenience, the same ink for forming an anode catalyst layer is listed in the "Surface α" and "Surface β" columns, respectively.

[0058] (b) Preparation of water electrolysis cell The anode catalyst layer-gas diffusion layer laminate xAG and the cathode catalyst layer-gas diffusion layer laminate xCG were each formed into 1 cm ２ The plates were punched out into the same shape so that each plate was stacked with the catalyst layer side facing inward, sandwiching an anion conduction membrane (X37-50 (trade name), manufactured by Dioxide Material Corporation, an anion exchange membrane having a PTFE support), and the entire surface was pressurized at 25°C with a pressure of 0.5 MPa to obtain a formed assembly having a gas diffusion layer-anode catalyst layer-anion conduction membrane-cathode catalyst layer-gas diffusion layer configuration. The pressurized formed assembly was sandwiched between Ni bipolar plates having flow channels and constrained with bolts to a constraining pressure of 0.5 MPa. In this manner, a water electrolysis cell having the same layer configuration as the water electrolysis cell 10 shown in FIG. 1 , which has a layer configuration of bipolar plate-gas diffusion layer-anode catalyst layer-anion conduction membrane-cathode catalyst layer-gas diffusion layer-bipolar plate configuration. In the membrane electrode assembly of this water electrolysis cell, the anode catalyst layer xAC1 and the cathode catalyst layer xC have the same surface roughness R of the surface α and the surface β.

[0059] (8) Preparation of water electrolysis cell of Example 7 (a) Preparation of anion conductive membrane-cathode catalyst layer-gas diffusion layer laminate MCCG A cathode catalyst layer-forming ink b1 was applied to a polytetrafluoroethylene film (manufactured by Nitto Denko Corporation, NITOFLON No. 900UL (product name)) using an applicator in a catalyst coating amount of 0.75 mg / cm. ２ After air drying, the cathode catalyst layer CC1a was placed on top of an anion conductive membrane (X37-50 (trade name), manufactured by Dioxide Material Co., Ltd.) and pressed against the entire surface at 25°C under a pressure of 5.0 MPa to bond them together, and then the polytetrafluoroethylene film was peeled off. In this way, the cathode catalyst layer CC1a was transferred under pressure to the anion conductive membrane, thereby obtaining a laminate MC. The ink b1 for forming a cathode catalyst layer was applied to a gas diffusion layer (GDL-39BB (trade name), manufactured by SGL Co., Ltd., a carbon fiber nonwoven fabric having a microporous layer) using an applicator so that the catalyst coating amount was 0.75 mg / cm. ２The coating was applied so that the thickness of the cathode catalyst layer CC1b was 1 / 3, and the coating was allowed to dry naturally. In this way, a gas diffusion layer-cathode catalyst layer laminate CG having a cathode catalyst layer CC1b was obtained. This laminate CG and a laminate MC were laminated so that the cathode catalyst layers of each laminate were in contact with each other, and the entire surfaces were pressed together at a pressure of 0.5 MPa and 25°C. In this way, an anion conductive membrane-cathode catalyst layer-gas diffusion layer laminate MCCG having a two-layer structure of cathode catalyst layer CC1 was produced.

[0060] (b) Preparation of anode catalyst layer-gas diffusion layer laminate x AG2 Anode catalyst layer-forming ink a1 was applied to a gas diffusion layer (manufactured by SGL, GDL-39BB (trade name), a carbon fiber nonwoven fabric having a microporous layer) using an applicator in a catalyst coating amount of 1.5 mg / cm ２ The anode catalyst layer xA was coated so that the thickness of the anode catalyst layer-gas diffusion layer laminate xAG2 was 0.02 μm, and the anode catalyst layer xA was naturally dried. In this manner, an anode catalyst layer-gas diffusion layer laminate xAG2 having a single-layer structure was obtained. The anode catalyst layer xA had the same surface roughness R of 0.02 μm on both surfaces α and β.

[0061] (c) Preparation of water electrolysis cell The anion conductive membrane-cathode catalyst layer-gas diffusion layer laminate MCCG and the anode catalyst layer-gas diffusion layer laminate xAG2 obtained above were each separated into 1 cm ２The laminate was punched out to the same shape as shown in FIG. 1, and the laminates were stacked together so that the anode catalyst layer of the laminate xAG2 was in contact with the surface of the anion conductive membrane of the laminate MCCG opposite the cathode catalyst layer, and pressed at a surface pressure of 0.5 MPa and 25°C. The pressed laminate was sandwiched between Ni bipolar plates having flow channels and constrained with bolts to a confining pressure of 0.5 MPa. In this way, a water electrolysis cell was obtained having the same layer structure as the water electrolysis cell 10 shown in FIG. 1 , which has a layer structure of bipolar plate-gas diffusion layer-anode catalyst layer-anion conductive membrane-cathode catalyst layer-gas diffusion layer-bipolar plate. This water electrolysis cell includes a membrane electrode assembly having an anion conductive membrane and two catalyst layers in direct contact with the anion conductive membrane. In this membrane electrode assembly, the cathode catalyst layer CC1 has a multilayer structure consisting of two catalyst layers: a cathode catalyst layer CC1a formed at a pressure of 5.0 MPa and a cathode catalyst layer CC1b formed at a pressure of 0.5 MPa, and has surfaces α and β with different surface roughnesses R, and is in contact with the anion conductive membrane at surface α. On the other hand, the anode catalyst layer xA is formed by pressing from both sides at a pressure of 0.5 MPa when bonding to the anion conductive membrane, and surfaces α and β have the same surface roughness R.

[0062] (9) Preparation of water electrolysis cell of Comparative Example 2 (a) Preparation of cathode catalyst layer-gas diffusion layer laminate xCG2 The ink b1 for forming a cathode catalyst layer was applied to a gas diffusion layer (manufactured by SGL, GDL-39BB, a carbon fiber nonwoven fabric having a microporous layer) using an applicator in an amount of 1.5 mg / cm. ２ After air drying, the laminate was pressurized at 5.0 MPa and 80°C. In this way, a cathode catalyst layer-gas diffusion layer laminate xCG2 having a cathode catalyst layer xCC1 was obtained. The cathode catalyst layer xCC1 had the same surface roughness R of 0.009 μm on both surfaces α and β.

[0063] (b) Preparation of water electrolysis cell The cathode catalyst layer-gas diffusion layer laminate xCG2 and the anode catalyst layer-gas diffusion layer laminate xAG2 were each formed into 1 cm ２The sheets were punched out to the same shape, and stacked with each catalyst layer facing inward so as to sandwich an anion conduction membrane (X37-50 (trade name) manufactured by Dioxide Material Corporation), and pressurized at a surface pressure of 0.5 MPa at 25°C to obtain a formed body having a gas diffusion layer-anode catalyst layer-anion conduction membrane-cathode catalyst layer-gas diffusion layer configuration. The pressurized formed body was sandwiched between Ni bipolar plates having flow channels and constrained with bolts to a constraining pressure of 0.5 MPa. In this way, a water electrolysis cell having the same layer configuration as water electrolysis cell 10 shown in FIG. 1 , which has a layer configuration of bipolar plate-gas diffusion layer-anode catalyst layer-anion conduction membrane-cathode catalyst layer-gas diffusion layer-bipolar plate.

[0064] 3. Surface Roughness (R) Measurement Method The surface roughness R of the surfaces α and β of each catalyst layer in contact with the anion conductive membrane was calculated from cross-sectional SEM images of the catalyst layer portion. Specifically, the method was as follows. Each water electrolysis cell prepared as described above was disassembled, and the gas diffusion layer-anode catalyst layer-anion conductive membrane-cathode catalyst layer-gas diffusion layer composite was removed and cut into a test specimen measuring 1 cm long x 3 cm wide. The test specimen was impregnated with epoxy resin (NER-814 (product name) manufactured by Nissin EM Co., Ltd.) and left in the atmosphere for 50 hours to harden the epoxy resin. The catalyst layer was then cut perpendicular to the surfaces α and β, and the resulting cross section was then processed using a cross-section polisher. The cross section thus obtained was observed using a scanning electron microscope (SEM) at a magnification of, for example, 10,000x, at which the crystal grain size could be observed. Elemental analysis of the cross section was performed using energy dispersive X-ray spectroscopy (EDX), and the locations where the composition discontinuously changed were identified as the interface between the catalyst layer and the gas diffusion layer and the interface between the catalyst layer and the anion conductive membrane. In a field of view including the surface α (more specifically, the observation field showed up to half the thickness of the catalyst layer on the surface α side, with the surface of the catalyst layer parallel to the lower edge of the observation field), the width of the observation field was L, and the observation position (measurement point) in the width direction of the observation field was x. The catalyst layer thickness T (the length of the catalyst layer in the direction perpendicular to the surface α) within the observation field was measured every 0.1 μm. The size of each observation field was adjusted depending on the shape and morphology of the catalyst particles, and approximately 100 measurement points were included within each observation field. The average catalyst layer thickness Tav was calculated from the catalyst layer thickness T at each measurement point, and the difference (T - Tav) between the catalyst layer thickness T at each measurement point and the average catalyst layer thickness Tav was calculated. The absolute value (|T-Tav|) of the difference (T-Tav) at measurement point x was defined as f(x), and the surface roughness R was calculated based on the following formula. The surface roughness R was determined in the same manner for a total of 20 fields of view, and the arithmetic mean value of the 20 obtained values ​​was calculated and defined as the surface roughness Rα. The surface roughness Rβ was calculated in the same manner as for the surface roughness Rα, except that the surface α was changed to the surface β in the method for determining the surface roughness Rα.

[0065]

[0066] 4. Measurement of Catalyst Volume Occupancy Rate The catalyst volume occupancy rate Mα on the surface α side of the catalyst layer and the catalyst volume occupancy rate Mβ on the surface β side were determined as follows. The results are shown in Table 1. As in the measurement of surface roughness R, the catalyst layer was removed from each water electrolysis cell and used as a test specimen. After cross-section processing was performed in the same manner as above, cross-sectional SEM images (field size: 20 μm × 10 μm) were obtained. In each field, the catalyst area per 1 μm of surface thickness on each of the surface α and surface β sides of the cross section was measured, and the ratio of the total area of ​​all catalysts to the area of ​​the catalyst layer in that field was calculated, which was defined as the catalyst ratio. This catalyst ratio was determined for 100 fields. The above-mentioned processes from cross-section processing to calculation of the catalyst ratio for 100 fields were performed on a total of five cross sections of the same test specimen, and the catalyst ratio for each of the five cross sections was determined for 100 fields. The arithmetic average of the catalyst ratios in these cross sections was calculated, and this average was defined as the catalyst volume occupancy rate.

[0067] 5. Electrolysis Efficiency Under Standard Current Density Conditions The electrolysis efficiency during water electrolysis at a normal current density (standard conditions) was evaluated using overvoltage (voltage loss) as an index. (1) Conditioning Operation A 1.2 M KOH aqueous solution heated to 60°C was supplied to each of the gas diffusion layers in contact with the anode catalyst layer and the cathode catalyst layer of the water electrolysis cell at a flow rate of 10 mL / min. The water electrolysis cell was then heated with a heater to adjust the internal cell temperature to 60°C. A pre-test conditioning operation was performed on the water electrolysis cell by operating it at a current of 0.1 A for 2 hours and at a current of 1 A for 2 hours while supplying the 1.2 M KOH aqueous solution heated to 60°C to each of the gas diffusion layers in contact with the anode catalyst layer and the cathode catalyst layer at a flow rate of 10 mL / min. (2) Water electrolysis Thereafter, water electrolysis was carried out for 6 hours at current values ​​of 1.5 A, 0.002 A, and 0.01 A while supplying a 1.2 M KOH aqueous solution heated to 60°C to each of the gas diffusion layers in contact with the anode catalyst layer and the cathode catalyst layer at a flow rate of 10 mL / min, and the voltage after 6 hours was measured for each. ２ ) for 6 hours. The voltage after 6 hours is V １．５Ａ and a current value of 0.002 A (current density: 0.002 A / cm ２) for 6 hours. The voltage after 6 hours is V ０．００２Ａ and a current value of 0.01 A (current density: 0.01 A / cm ２ ) for 6 hours. The voltage after 6 hours is V ０．０１Ａ The initial overvoltage Vx during electrolysis at a current value of 1.5 A was calculated using the following formula, and Vx and V １．５Ａ The difference between １．５Ａ The difference (overvoltage) obtained was evaluated according to the following evaluation criteria: Vx = (V ０．０１Ａ -V ０．００２Ａ ) / (ln0.01 - ln0.002) x ln1.5 + (ln0.01 x V ０．００２Ａ -ln0.002×V ０．０１Ａ ) / (ln0.01-ln0.002) -Evaluation criteria- A: Less than 0.20V B: 0.20V or more and less than 0.25V C: 0.25V or more and less than 0.30V D: 0.30V or more and less than 0.40V E: 0.40V or more

[0068] 7. Electrolysis efficiency at high current density The electrolysis efficiency at high current density was evaluated using overvoltage (voltage loss) as an index. (1) For the evaluation of electrolysis efficiency under the standard current density condition, for each water electrolysis cell after the conditioning operation, water electrolysis was performed at a current value of 3 A for 6 hours while supplying a 1.2 M KOH aqueous solution heated to 60°C to each gas diffusion layer in contact with the anode catalyst layer and the cathode catalyst layer at a flow rate of 10 mL / min. After 6 hours, the voltage V ３Ａ The initial overvoltage Vy during water electrolysis at a current value of 3 A was calculated using the following formula, and the ratio between Vy and V ３Ａ The difference between ３Ａ The difference (overvoltage) obtained was evaluated according to the following evaluation criteria: Vy = (V ０．０１Ａ -V ０．００２Ａ ) / (ln0.01 - ln0.002) x ln3.0 + (ln0.01 x V ０．００２Ａ -ln0.002×V ０．０１Ａ ) / (ln0.01-ln0.002) -Evaluation criteria- A: Less than 0.60V B: 0.60V or more and less than 0.65V C: 0.65V or more and less than 0.70V D: 0.70V or more and less than 0.80V E: 0.80V or more

[0069] [Durability Test] Durability when water electrolysis was performed at a high current density for a long period of time while varying the current density was evaluated using voltage change as an index. For each water electrolysis cell after the conditioning operation (1) for evaluating electrolysis efficiency under the standard current density condition, water electrolysis was performed for 6 hours at a current of 1 A while supplying a 1.2 M KOH aqueous solution heated to 60°C to each gas diffusion layer in contact with the anode catalyst layer and the cathode catalyst layer at a flow rate of 10 mL / min. The voltage after 6 hours was defined as the "initial voltage." Next, 200 cycles were performed, each cycle consisting of supplying a 1.2 M KOH aqueous solution heated to 60°C to each gas diffusion layer in contact with the anode catalyst layer and the cathode catalyst layer at a flow rate of 10 mL / min while performing water electrolysis at a current of 3 A for 6 hours, and then changing the current to 0.03 A to perform water electrolysis for 6 hours. Thereafter, water electrolysis was performed for 6 hours at a current value of 1 A while supplying a 1.2 M KOH aqueous solution heated to 60°C to each of the gas diffusion layers in contact with the anode catalyst layer and the cathode catalyst layer at a flow rate of 10 mL / min, and the voltage after 6 hours was defined as the "voltage after long-term operation." The voltage change value was calculated using the following formula and evaluated according to the following evaluation criteria: Voltage change value = Voltage after long-term operation - Initial voltage - Evaluation criteria - A: Less than 0.10 V B: 0.10 V or more but less than 0.15 V C: 0.15 V or more but less than 0.20 V D: 0.20 V or more but less than 0.30 V E: 0.30 V or more

[0070]

[0071] <Notes to the table> "Catalyst particle size" indicates the particle size of the catalyst used when preparing each ink for forming an anode catalyst layer and each ink for forming a cathode catalyst layer.

[0072]

[0073] The membrane electrode assembly included in the water electrolysis cell of Comparative Example 1, which had an anode catalyst layer with the same surface roughness Rα of the surface α in contact with the anion conductive membrane and the same surface roughness Rβ of the opposite surface β, exhibited a high overvoltage of 0.80 V or more during water electrolysis at a high current density when incorporated into the water electrolysis cell, resulting in poor electrolysis efficiency. Furthermore, the voltage change after long-term water electrolysis with varying current density was high, being 0.30 V or more, resulting in poor durability. In contrast, each membrane electrode assembly having an anode catalyst layer with a smaller surface roughness Rα of the surface α in contact with the anion conductive membrane than the surface roughness Rβ of the opposite surface β exhibited a low overvoltage of less than 0.70 V during water electrolysis at a high current density when incorporated into the water electrolysis cell, resulting in excellent electrolysis efficiency (Examples 1 to 6). Furthermore, the voltage change after long-term water electrolysis with varying current density was low, being less than 0.20 V, resulting in excellent durability. The same effect as above was also obtained when a cathode catalyst layer was used in which the surface roughness Rα of the surface α in contact with the anion conductive membrane was smaller than the surface roughness Rβ of the opposite surface β (Example 7). When the water electrolysis cells of Examples 1 to 7 were visually observed during the evaluation of electrolysis efficiency at high current densities, gas was not retained in the anode catalyst layer and the cathode catalyst layer having the above surface roughness R relationship, and was efficiently discharged to the gas diffusion layer side. Therefore, the excellent electrolysis efficiency described above is thought to be due to efficient gas transfer from the catalyst layer to the gas diffusion layer. It was also found that the membrane electrode assembly of the present invention can be formed by the manufacturing method for the membrane electrode assembly of the present invention. Furthermore, it was found that the use of the membrane electrode catalyst layer of the present invention can form a membrane electrode assembly that can be used to form a water electrolysis cell with excellent electrolysis efficiency during water electrolysis at a low flow rate and excellent durability.

[0074] While the present invention has been described in connection with embodiments thereof, we do not intend to limit our invention to any of the details of the description unless otherwise specified, and believe that the claims should be construed broadly without departing from the spirit and scope of the invention as set forth in the appended claims.

[0075] This application claims priority based on Japanese Patent Application No. 2024-054242, filed on March 28, 2024, the contents of which are incorporated herein by reference as part of the present specification.

[0076] REFERENCE SIGNS LIST 1 anion conductive membrane 2a anode catalyst layer 21 anode catalyst 23 ionomer resin 2c cathode catalyst layer 22 cathode catalyst 23 ionomer resin 3 gas diffusion layer 4 membrane electrode assembly 5 bipolar plate 10 water electrolysis cell

Claims

1. A membrane electrode assembly comprising an anion conductive membrane, a cathode catalyst layer in direct contact with one surface of the anion conductive membrane, and an anode catalyst layer in direct contact with the other surface of the anion conductive membrane, wherein in at least one catalyst layer X of the cathode catalyst layer or the anode catalyst layer, a surface α in contact with the anion conductive membrane has a surface roughness Rα that is smaller than a surface roughness Rβ of a surface β opposite to the surface α.

2. The membrane electrode assembly according to claim 1, wherein the volume fraction Mβ of the catalyst on the surface β side of the catalyst layer X is lower than the volume fraction Mα of the catalyst on the surface α side.

3. The membrane electrode assembly according to claim 1, wherein the particle size of the catalyst on the surface α side of the catalyst layer X is smaller than the particle size of the catalyst on the surface β side.

4. A method for producing a membrane electrode assembly according to claim 1, comprising forming the catalyst layer X by stacking two catalyst layers having different surface roughnesses.

5. A catalyst layer for a membrane electrode with different surface roughness on both sides.

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