Membrane electrode assembly and catalyst layer for membrane electrode

Abstract

Provided is 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 of the anion conductive membrane, and in a catalyst layer X of at least one of the cathode catalyst layer and the anode catalyst layer, the surface free energy of a surface α in contact with the anion conductive membrane is higher than the surface free energy of a surface β on the reverse side to the surface α. Additionally provided is a catalyst layer for a membrane electrode in which the surface free energy of the two surfaces thereof differ.

Description

Membrane electrode assembly and catalyst layer for membrane electrode

[0001] The present invention relates to a membrane electrode assembly and a catalyst layer for a membrane electrode.

[0002] Hydrogen is a clean energy source that does not emit carbon dioxide and is used as a fuel for fuel cell vehicles and household fuel cells, for example. Electrolysis of water is a well-known method for producing hydrogen.

[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] To improve the efficiency of water electrolysis, it is necessary to increase the current density (the current value per electrode area). However, the inventors have conducted extensive research on water electrolysis performed at high current densities and found that increasing the current density increases the amount of hydrogen and oxygen gas generated. In particular, when water electrolysis is performed at a slow water supply rate, these gases tend to accumulate in the catalyst layer, increasing the resistance of the membrane electrode assembly and increasing the overvoltage, resulting in a decrease in the hydrogen gas production efficiency (electrolysis efficiency). More specifically, the water supply rate during water electrolysis can be changed, for example, depending on the amount of power supplied. Specifically, when the amount of power supplied by solar or wind power generation fluctuates, the water supply rate can be regulated accordingly for cost reasons. A fast water supply rate allows the generated gas to easily flow with the water, and therefore the resistance of the membrane electrode assembly is less likely to increase even when electrolysis is performed at a high current density to increase the amount of gas generated. However, it has been found that when the water supply rate is slow, a large amount of gas generated by a high current density tends to accumulate in the catalyst layer, increasing the resistance of the membrane electrode assembly and reducing the hydrogen gas production efficiency.

[0008] An object of the present invention is to provide a membrane electrode assembly that is less likely to experience a decrease in electrolysis efficiency over time even when water is electrolyzed at a high current density, and a membrane electrode catalyst layer that is suitable for forming this 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, the surface free energy of a surface α in contact with the anion conductive membrane is higher than the surface free energy of a surface β opposite to the surface α. [2] The membrane / electrode assembly according to [1], in which a volume occupancy rate Mβ of the catalyst on the surface β side of the catalyst layer X is lower than a volume occupancy rate Mα of the catalyst on the surface α side. [3] The membrane / electrode assembly according to [2], in which a difference between the volume occupancy rates Mα and Mβ is 1 to 70 volume %. [4] The membrane / electrode assembly according to any one of [1] to [3], in which the catalyst layer X contains a polymer component, and the polymer component contained on the surface β side is different from the polymer component contained on the surface α side. [5] The membrane electrode assembly according to any one of [1] to [4], wherein the fluorine content of the polymer component contained on the surface β side of the catalyst layer X is higher than the fluorine content of the polymer component contained on the surface α side. [6] The membrane electrode assembly according to any one of [1] to [4], wherein the silicon content of the polymer component contained on the surface β side of the catalyst layer X is higher than the silicon content of the polymer component contained on the surface α side. [7] A membrane electrode catalyst layer having two surfaces with different surface free energies.

[0010] The membrane electrode assembly of the present invention is less likely to experience a decrease in electrolysis efficiency over time even when water is electrolyzed at a high current density. The membrane electrode catalyst layer of the present invention is suitable for producing 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 free energy of the surface α in contact with the anion conductive membrane is higher than the surface free energy of the surface β opposite to the surface α. In other words, the catalyst layer X whose surface free energy of the surface α is higher than the surface free energy 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" means the anode catalyst layer and the cathode catalyst layer.

[0015] By increasing the surface free energy of the surface α above compared to the surface free energy of the surface β, gases generated within 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. This is because a surface with high surface free energy has low affinity for gases generated within the catalyst. Conventional catalyst layers cannot facilitate this gas transfer smoothly enough, leading to gas retention within the catalyst layer, which inhibits the supply of water and electrons to the catalyst layer and increases the resistance of the membrane / electrode assembly. While gas transfer can be promoted by increasing the rate at which water is supplied to the catalyst layer, it is difficult to suppress an increase in the resistance of the membrane / electrode assembly when the rate at which water is supplied to the catalyst layer is slow. In the membrane / electrode assembly of the present invention, the efficiency of gas diffusion from within the catalyst layer to the outside of the catalyst layer (the side opposite the anion conductive membrane) is improved, and an increase in the resistance of the membrane / electrode assembly can be suppressed even when the rate at which water is supplied to the catalyst layer is slow, thereby improving electrolysis efficiency. Furthermore, even if the water supply rate fluctuates during high-current-density 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 electrolysis reaction is also excellent in 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] If the surface free energy of the surface α is Eα and the surface free energy of the surface β is Eβ, the difference between Eα and Eβ (Eα - Eβ) is preferably 1 mN / m or more, more preferably 2 mN / m or more, even more preferably 3 mN / m or more, even more preferably 4 mN / m or more, and even more preferably 5 mN / m or more. There is no particular upper limit. For example, it can be 25 mN / m or less. The difference between Eα and Eβ is preferably 1 to 25 mN / m, more preferably 2 to 25 mN / m, even more preferably 3 to 25 mN / m, even more preferably 5 to 25 mN / m, even more preferably 5 to 18 mN / m, and even more preferably 5 to 14 mN / m. Eα is preferably 50 to 100 mN / m, more preferably 60 to 90 mN / m. Eβ is preferably 49 to 99 mN / m, more preferably 59 to 89 mN / m. The surface free energies of surfaces α and β can be measured by the method described in the Examples. The surface free energies of surfaces α and β can be controlled to the above values ​​by adjusting the concentrations, types, etc. of the components (e.g., catalyst, polymer) constituting the catalyst layer X. For example, in the case of catalyst concentration, the surface free energies of surfaces α and β can be made to satisfy the above relationship by forming a catalyst concentration gradient in the single-layer catalyst layer X such that the catalyst concentration gradually decreases from the anion conductive membrane side to the opposite side in the thickness direction. Furthermore, the catalyst layer X can have a multilayer structure consisting of multiple layers with different catalyst concentrations, and by forming a catalyst concentration gradient in the thickness direction such that the catalyst concentration gradually decreases from the anion conductive membrane side to the opposite side, the surface free energies of 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 ranging 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 ranging 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, but 70 vol% or less is practical. The difference between Mα and Mβ is preferably 1 to 70 vol%, more preferably 5 to 60 vol%, even more preferably 10 to 50 vol%, even more preferably 10 to 40 vol%, even more preferably 10 to 25 vol%, and even more preferably 10 to 20 vol%. Mα is preferably 20 to 70% by volume, more preferably 20 to 50% by volume, and Mβ is preferably 5 to 60% by volume, more preferably 10 to 35% by volume, and even more preferably 10 to 30% 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] The catalyst concentration on the surface α side of the catalyst layer X is preferably 70 to 95 mass %, more preferably 80 to 90 mass %, and even more preferably 80 to 89 mass %, based on the solid content. The catalyst concentration on the surface β side of the catalyst layer X is preferably 40 to 80 mass %, more preferably 50 to 70 mass %, and even more preferably 55 to 70 mass % based on the solid content. Here, the "catalyst concentration on the surface α side" of the catalyst layer X means the catalyst concentration in a range from the surface of the surface α to a depth of 1 μm, and similarly, the "catalyst concentration on the surface β side" means the catalyst concentration in a range from the surface of the surface β to a depth of 1 μm.

[0021] The catalyst layer X usually contains a polymer component. When the catalyst layer X contains a polymer component, the polymer component contained on the surface β side of the catalyst layer X may be different from the polymer component contained on the surface α side. Details of the polymer components that can be used in the catalyst layer X will be described later. Here, "different polymer components" means that the types of polymers are different, or, when multiple types of polymers are contained, the mixing ratios of the polymers are different. Polymers with the same name but containing different ratios of elements are considered to be different types of polymers. For example, two types of ionomer resins with different fluorine contents are considered to be different types of polymers.

[0022] The catalyst layer X may have a configuration in which the fluorine content of the polymer component contained on the surface β side of the catalyst layer X is higher than the fluorine content of the polymer component contained on the surface α side. Here, the "fluorine content of the polymer component contained on the surface α side of the catalyst layer X" refers to the fluorine content of the polymer component contained within a range from the surface α to a thickness of 1 μm, and similarly, the "fluorine content of the polymer component contained on the surface α side of the catalyst layer X" refers to the fluorine content of the polymer component contained within a range from the surface β to a thickness of 1 μm. The "fluorine content of the polymer component contained on the surface α side of the catalyst layer X" and the "fluorine content of the polymer component contained on the surface α side of the catalyst layer X" can be determined by collecting a sample from a portion of the catalyst layer up to a thickness of 1 μm from the surface of the surface α or surface β. More specifically, they can be determined by the method described in the Examples. The difference (Fβ - Fα) between the fluorine content of the polymer component contained on the surface β side (Fβ) and the fluorine content of the polymer component contained on the surface α side (Fα) is not particularly limited, but is preferably 4 mass% or more, more preferably 7 mass% or more. There is no particular upper limit, and 40% by mass is practical. Therefore, Fβ-Fα is preferably 4 to 40% by mass, more preferably 7 to 30% by mass. Fβ-Fα can also be 0% by mass. The fluorine content of the polymer component contained on the surface α side is preferably 10 to 50% by mass, more preferably 20 to 45% by mass. The fluorine content of the polymer component contained on the surface β side is preferably 20 to 60% by mass, more preferably 30 to 55% by mass.

[0023] Focusing on the entire thickness direction of the catalyst layer X, the fluorine content of the polymer component throughout the thickness direction of the catalyst layer X may gradually increase or may increase stepwise from the surface α side to the surface β side. Note that, within the scope of not impairing the effects of the present invention, there may be a portion where the fluorine content of the polymer component is lower from the surface α side to the surface β side. When the catalyst layer X is divided into two equal parts in the thickness direction, it is preferable that the fluorine content of the polymer component in the half on the surface β side is higher than the fluorine content of the polymer component in the half on the surface α side. When the catalyst layer X is divided into four equal parts in the thickness direction from the surface α side to the surface β side, it is preferable that the fluorine content of the polymer component in X1 is lower than the fluorine content of the polymer component in X4, and that there is no portion where the fluorine content of the polymer component decreases from X1 to X4.

[0024] The catalyst layer X may have a higher silicon content in the polymer component contained on the surface β side of the catalyst layer X than in the polymer component contained on the surface α side. Here, the "silicon content in the polymer component contained on the surface α side of the catalyst layer X" refers to the silicon content in the polymer component contained within a range from the surface α to a thickness of 1 μm, and similarly, the "silicon content in the polymer component contained on the surface α side of the catalyst layer X" refers to the silicon content in the polymer component contained within a range from the surface β to a thickness of 1 μm. The "silicon content in the polymer component contained on the surface α side of the catalyst layer X" and the "silicon content in the polymer component contained on the surface β side of the catalyst layer X" can be determined by collecting a portion of the catalyst layer from the surface α or surface β to a thickness of 1 μm. More specifically, they can be determined by the method described in the Examples. The difference (Sβ - Sα) between the silicon content in the polymer component contained on the surface β side (Sβ) and the silicon content in the polymer component contained on the surface α (Sα) is not particularly limited, but is preferably 4 mass% or more, more preferably 7 mass% or more. There is no particular upper limit, and 40 mass% is practical. Therefore, Sβ-Sα is preferably 4 to 40 mass%, more preferably 7 to 30 mass%. Sβ-Sα can also be 0 mass%. The silicon content of the polymer component contained on the surface α side is preferably 10 to 50 mass%, more preferably 20 to 45 mass%. The silicon content of the polymer component contained on the surface β side is preferably 20 to 60 mass%, more preferably 30 to 55 mass%.

[0025] Focusing on the entire thickness direction of the catalyst layer X, the silicon content of the polymer component throughout the entire thickness direction of the catalyst layer X may gradually increase from the surface α side to the surface β side, or may increase in stages. Note that, as long as the effects of the present invention are not impaired, there may be a portion where the silicon content of the polymer component decreases 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 silicon content of the polymer component in the half on the surface β side is higher than the silicon content of the polymer component in the half on the surface α 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 silicon content of the polymer component in X1 is lower than the silicon content of the polymer component in X4, and there is no portion where the silicon content of the polymer component decreases from X1 to X4.

[0026] The catalyst layer X can be formed by preparing an ink for forming a catalyst layer 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, the catalyst layer X can be formed by preparing multiple types of ink for forming a catalyst layer with different compositions, applying each ink for forming a catalyst layer, drying, and laminating them.

[0027] 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 .

[0028] 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.

[0029] The membrane electrode assembly of the present invention will now be described in more detail.

[0030] <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 is catalyst layer X, the anode catalyst layer can be used in a normal membrane / electrode assembly, except that the surface free energies of the surfaces α and β satisfy the above-mentioned relationship. In the membrane / electrode assembly of the present invention, when the anode catalyst layer is not catalyst layer X, the anode catalyst layer can be used in a normal membrane / electrode assembly.

[0031] 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.

[0032] The anode catalyst is preferably in particulate form. In the anode catalyst layer, the particulate anode catalyst is preferably bound by the polymer.

[0033] 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)), silicon polymers (e.g., silicone rubber), and the like can also be used.

[0034] The thickness of the anode catalyst layer is not particularly limited, but 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.

[0035] <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 is catalyst layer X, the cathode catalyst layer can be a cathode catalyst layer that can be used in a normal membrane / electrode assembly, except that the surface free energies of the surfaces α and β satisfy the above-mentioned relationship. In the membrane / electrode assembly of the present invention, when the cathode catalyst layer is not catalyst layer X, the cathode catalyst layer can be a cathode catalyst layer that can be used in a normal membrane / electrode assembly.

[0036] 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.

[0037] The cathode catalyst is preferably in particulate form. In the cathode catalyst layer, the particulate cathode catalyst is preferably bound by the polymer.

[0038] 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.

[0039] The thickness of the cathode catalyst layer is not particularly limited, but 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.

[0040] <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.

[0041] <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.

[0042] 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 may then be bonded to the anion conductive membrane.

[0043] [Membrane Electrode Catalyst Layer] The membrane electrode catalyst layer of the present invention has different surface free energies on both sides. 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 side γ of the membrane electrode catalyst layer of the present invention can be the same as the side α of the catalyst layer of the membrane electrode assembly of the present invention, and the opposite side δ can be the same as the side β of the catalyst layer of the membrane electrode assembly of the present invention. That is, the surface free energies of the side γ and the side δ of the catalyst layer of the membrane electrode of the present invention can be the same as the surface free energies of the side α and the side β of the catalyst layer of the membrane electrode assembly of the present invention, and the preferred relationship (difference) and preferred range between them are also the same. All of the other details described for the membrane electrode assembly of the present invention (e.g., catalyst volume fraction, catalyst concentration, fluorine content of the polymer component, silicon content of the polymer component, preparation method, constituent materials, etc.) are applicable to the membrane electrode catalyst layer of the present invention, 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 lower surface free energy than 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.

[0044] 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.

[0045] 1. Preparation of Ink for Forming Catalyst Layer (1) Preparation of Ink a1d for Forming Anode Catalyst Layer In a 5 mL vial, 1.4 g of Nafion dispersion (5 mass% Nafion dispersion DE521 (trade name), manufactured by Fujifilm Wako Chemical Co., Ltd., containing perfluoroalkanesulfonic acid ionomer resin; hereinafter, this dispersion will be referred to as "Nafion 1") and 0.32 g of iridium oxide powder (TEC77100 (trade name), manufactured by Tanaka Kikinzoku Kogyo Co., Ltd., Ir content 75 mass%) were mixed and 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, an ink a1d for forming an anode catalyst layer with a catalyst concentration of 81 mass% was prepared.

[0046] (2) Preparation of ink a2d for forming an anode catalyst layer An ink a2d for forming an anode catalyst layer was prepared in the same manner as in the preparation of ink a1d for forming an anode catalyst layer, except that the content of iridium oxide powder in the ink a1d for forming an anode catalyst layer was changed so that the catalyst concentration was 90 mass %.

[0047] (3) Preparation of ink a3d for forming an anode catalyst layer An ink a3d for forming an anode catalyst layer was prepared in the same manner as in the preparation of ink a1d for forming an anode catalyst layer, except that the content of iridium oxide powder in the ink a1d for forming an anode catalyst layer was changed so that the catalyst concentration was 95 mass %.

[0048] (4) Preparation of Ink a4d for Forming Anode Catalyst Layer An ink a4d for forming an anode catalyst layer was prepared in the same manner as for the ink a3d for forming an anode catalyst layer, except that a Nafion dispersion (5% Nafion dispersion DE520 (product name), manufactured by Fujifilm Wako Chemical Co., Ltd.; hereinafter, this dispersion is referred to as "Nafion2") was used instead of Nafion1 in the preparation of the ink a3d for forming an anode catalyst layer.

[0049] (5) Preparation of Ink a5d for Forming Anode Catalyst Layer An ink a5d for forming an anode catalyst layer was prepared in the same manner as for the ink a1d for forming an anode catalyst layer, except that an ionomer resin (XB-7 (product name), manufactured by Dioxide Material Corporation, resin component 5% by mass, hereinafter referred to as "XB-7") was used instead of Nafion1 in the preparation of the ink a1d for forming an anode catalyst layer.

[0050] (6) Preparation of Ink a6d for Forming Anode Catalyst Layer Ink a1d for forming anode catalyst layer was prepared by using NiFe instead of iridium oxide. ２ O ４ (Nickel-iron oxide, manufactured by Kojundo Chemical Co., Ltd., hereinafter referred to as "NiFe ２ O ４ An ink a6d for forming an anode catalyst layer was prepared in the same manner as in the ink a1d for forming an anode catalyst layer, except that a 1-hydroxybenzoic acid compound (hereinafter referred to as "anode catalyst layer a6d") was used.

[0051] (7) Preparation of Ink a1u for Forming Anode Catalyst Layer In a 5 mL vial, 1.4 g of a Nafion dispersion (perfluoroalkanesulfonic acid ionomer resin, manufactured by Fujifilm Wako Chemical Co., Ltd., 5 mass% Nafion dispersion DE521 (trade name), "Nafion1") and 0.15 g of iridium oxide powder (manufactured by Tanaka Kikinzoku Kogyo K.K., TEC77100 (trade name), Ir content 75 mass%) 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, an ink a1u for forming an anode catalyst layer with a catalyst concentration of 68 mass% was prepared.

[0052] (8) Preparation of ink a2u for forming anode catalyst layer An ink a2u for forming an anode catalyst layer was prepared in the same manner as in the preparation of ink a1u for forming anode catalyst layer, except that the content of iridium oxide powder in the ink a1u for forming anode catalyst layer was changed so that the catalyst concentration was 50 mass %.

[0053] (9) Preparation of ink a3u for forming anode catalyst layer An ink a3u for forming an anode catalyst layer was prepared in the same manner as in the preparation of ink a1u for forming anode catalyst layer, except that the content of iridium oxide powder in the ink a1u for forming anode catalyst layer was changed so that the catalyst concentration was 45 mass%.

[0054] (10) Preparation of ink a4u for forming anode catalyst layer An ink a4u for forming an anode catalyst layer was prepared in the same manner as for the ink a1u for forming anode catalyst layer, except that Nafion 2 was used instead of Nafion 1 and the catalyst concentration was 76 mass %.

[0055] (11) Preparation of Ink a5u for Forming Anode Catalyst Layer An ink a5u for forming an anode catalyst layer was prepared in the same manner as for the ink a1u for forming an anode catalyst layer, except that Nafion 2 was used instead of Nafion 1 in the preparation of the ink a1u for forming an anode catalyst layer.

[0056] (12) Preparation of Ink a6u for Forming Anode Catalyst Layer In a 5 mL vial, 0.23 g of a polytetrafluoroethylene (PTFE) dispersion (Sigma-Aldrich, 665800 (trade name), PTFE 30 mass% dispersion), 1.2 g of water, and 0.15 g of iridium oxide powder (Tanaka Kikinzoku Kogyo K.K., TEC77100, Ir content 75 mass%) were mixed, and the mixture was dispersed for 5 minutes at a power of 30 W using an ultrasonic homogenizer (UH-300 (trade name), 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, an ink a6u for forming an anode catalyst layer with a catalyst concentration of 68 mass% was prepared.

[0057] (13) Preparation of Ink a7u for Forming Anode Catalyst Layer An ink a7u for forming an anode catalyst layer was prepared in the same manner as for the ink a1u for forming an anode catalyst layer, except that in the preparation of the ink a1u for forming an anode catalyst layer, the blending amount of Nafion1 was changed to 0.7 g and 0.12 g of the PTFE dispersion was further used.

[0058] (14) Preparation of Ink a8u for Forming Anode Catalyst Layer An ink a8u for forming an anode catalyst layer was prepared in the same manner as for the ink a1u for forming an anode catalyst layer, except that 0.06 g of a silicone rubber powder emulsion (KM-9729 (trade name), manufactured by Shin-Etsu Chemical Co., Ltd., an emulsion containing 50% by mass of silicone components; hereinafter referred to as "Si1") and 0.77 g of XB-7 were used instead of Nafion1 in the preparation of the ink a1u for forming an anode catalyst layer.

[0059] (15) Preparation of ink a9u for forming anode catalyst layer An ink a9u for forming an anode catalyst layer was prepared in the same manner as for the ink a8u for forming anode catalyst layer, except that 0.12 g of Si1 and 0.21 g of XB-7 were used.

[0060] (16) Preparation of Ink a10u for Forming Anode Catalyst Layer In the preparation of ink a1u for forming anode catalyst layer, NiFe was used instead of iridium oxide. ２ O ４ An ink a10u for forming an anode catalyst layer was prepared in the same manner as for the ink a1u for forming an anode catalyst layer, except that the ink a10u for forming an anode catalyst layer was used.

[0061] (17) Preparation of ink b1d for forming a cathode catalyst layer In a 10 mL vial, 3.0 g of Nafion dispersion (perfluoroalkanesulfonic acid ionomer resin, manufactured by Fujifilm Wako Chemical Co., Ltd., 5 mass% Nafion dispersion DE521 (trade name), "Nafion1") and 0.4 g of platinum carbon powder (manufactured by Tanaka Kikinzoku Kogyo Co., Ltd., TEC10E50E, Pt content 47 mass%) were mixed and 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 b1d for forming a cathode catalyst layer with a catalyst concentration of 73 mass% was prepared.

[0062] (18) Preparation of ink b1u for forming a cathode catalyst layer An ink b1u for forming a cathode catalyst layer was prepared in the same manner as in the preparation of ink b1d for forming a cathode catalyst layer, except that the content of platinum carbon powder in the ink b1d for forming a cathode catalyst layer was changed so that the catalyst concentration was 62 mass%.

[0063] Table 1 shows the compositions of the inks for forming the anode catalyst layer and the inks for forming the cathode catalyst layer obtained above.

[0064] 2. Preparation of catalyst layer-gas diffusion layer laminates Using the anode catalyst layer-forming inks a1d to a6d, a1u to a10u, and cathode catalyst layer-forming inks b1d and b1u prepared above, the following catalyst layer-gas diffusion layer laminates were prepared as follows. Table 2 shows the anode catalyst layer-forming inks and cathode catalyst layer-forming inks used, as well as the numbers of the resulting catalyst layers. The gas diffusion layer numbers are omitted.

[0065] (1) Preparation of anode catalyst layer-gas diffusion layer laminate AC1G Two types of catalyst layer-forming inks were applied to a nonwoven fabric that would become a gas diffusion layer, to obtain a catalyst layer with a two-layer structure. In this case, the catalyst layer-forming ink for forming a catalyst layer constituent layer having a surface β was applied to the nonwoven fabric first, and then the catalyst layer-forming ink for forming a catalyst layer constituent layer having a surface α was applied. The procedure is explained in more detail below. The anode catalyst layer-forming ink a1u was applied to a gas diffusion layer (manufactured by SGL, GDL-39BB (trade name), carbon fiber nonwoven fabric having a microporous layer) using an applicator, with the catalyst coating amount being 0.68 mg / cm. ２ After air drying, the anode catalyst layer forming ink a1d was applied using an applicator so that the catalyst coating amount was 0.81 mg / cm ２ The coating was then applied so that the thickness of the anode catalyst layer AC1G was 1 / 2 μm, and the anode catalyst layer AC1G was naturally dried. In this way, an anode catalyst layer-gas diffusion layer laminate AC1G having a two-layer anode catalyst layer AC1 was obtained. In this catalyst layer-gas diffusion layer laminate, the anode catalyst layer AC1 was in contact with the gas diffusion layer at surface β (the surface of the catalyst layer formed with the anode catalyst layer-forming ink a1u). The thickness of the obtained anode catalyst layer AC1 was 12 μm.

[0066] (2) Anode catalyst layer-gas diffusion layer laminate AC2G In the preparation of AC1G, the ink a1u for forming an anode catalyst layer was replaced with the ink a2u for forming an anode catalyst layer at a catalyst coating amount of 0.50 mg / cm ２ The anode catalyst layer forming ink a2d was used in place of the anode catalyst layer forming ink a1d in a catalyst coating amount of 0.90 mg / cm ２ An anode catalyst layer-gas diffusion layer laminate AC2G having an anode catalyst layer AC2 of a two-layer structure was obtained in the same manner as in AC1G, except that the anode catalyst layer AC2 was used so as to satisfy the following conditions:

[0067] (3) Anode catalyst layer-gas diffusion layer laminate AC3G In the preparation of AC1G, the ink a1u for forming an anode catalyst layer was replaced with the ink a3u for forming an anode catalyst layer at a catalyst coating amount of 0.45 mg / cm ２ The ink a3d for forming an anode catalyst layer was used in place of the ink a1d for forming an anode catalyst layer in a catalyst coating amount of 0.95 mg / cm ２ An anode catalyst layer-gas diffusion layer laminate AC3G having an anode catalyst layer AC3 of a two-layer structure was obtained in the same manner as in AC1G, except that the anode catalyst layer AC3 was used so as to satisfy the following conditions:

[0068] (4) Anode catalyst layer-gas diffusion layer laminate AC4G In the preparation of AC3G, the ink a4d for forming an anode catalyst layer was used in place of the ink a3d for forming an anode catalyst layer, with a catalyst coating amount of 0.95 mg / cm ２ An anode catalyst layer-gas diffusion layer laminate AC4G having an anode catalyst layer AC4 of a two-layer structure was obtained in the same manner as in AC3G, except that the anode catalyst layer AC4 was used so as to satisfy the following conditions:

[0069] (5) In the preparation of anode catalyst layer-gas diffusion layer laminates AC5G and AC1G, the ink a4u for forming an anode catalyst layer was used in place of the ink a1u for forming an anode catalyst layer, with a catalyst coating amount of 0.76 mg / cm ２ An anode catalyst layer-gas diffusion layer laminate AC5G having a two-layer structure anode catalyst layer AC5 was obtained in the same manner as AC1G, except that the anode catalyst layer AC5 was used so as to satisfy the following conditions:

[0070] (6) In the preparation of anode catalyst layer-gas diffusion layer laminates AC6G and AC1G, the ink a5u for forming an anode catalyst layer was used in place of the ink a1u for forming an anode catalyst layer, with a catalyst coating amount of 0.68 mg / cm ２ An anode catalyst layer-gas diffusion layer laminate AC6G having an anode catalyst layer AC6 of a two-layer structure was obtained in the same manner as in AC1G, except that the anode catalyst layer AC6 was used so as to satisfy the following conditions:

[0071] (7) Anode catalyst layer-gas diffusion layer laminate AC7G In the preparation of AC1G, the ink a6u for forming an anode catalyst layer was used in place of the ink a1u for forming an anode catalyst layer, with a catalyst coating amount of 0.68 mg / cm ２ An anode catalyst layer-gas diffusion layer laminate AC7G having a two-layer structure anode catalyst layer AC7 was obtained in the same manner as AC1G, except that the anode catalyst layer AC7 was used so as to satisfy the following conditions:

[0072] (8) Anode catalyst layer-gas diffusion layer laminate AC8G In the preparation of AC1G, the ink a7u for forming an anode catalyst layer was used in place of the ink a1u for forming an anode catalyst layer, with a catalyst coating amount of 0.68 mg / cm ２ An anode catalyst layer-gas diffusion layer laminate AC8G having a two-layer structure anode catalyst layer AC8 was obtained in the same manner as AC1G, except that the anode catalyst layer AC8 was used so as to satisfy the following conditions:

[0073] (9) Anode catalyst layer-gas diffusion layer laminate AC9G In the preparation of AC1G, the ink a8u for forming an anode catalyst layer was used in place of the ink a1u for forming an anode catalyst layer, with a catalyst coating amount of 0.68 mg / cm ２ The ink a5d for forming an anode catalyst layer was used in place of the ink a1d so that the catalyst coating amount was 0.81 mg / cm ２ An anode catalyst layer-gas diffusion layer laminate AC9G having an anode catalyst layer AC9 of a two-layer structure was obtained in the same manner as in AC1G, except that the anode catalyst layer AC9 was used so as to satisfy the following conditions:

[0074] (10) In the preparation of the anode catalyst layer-gas diffusion layer laminates AC10G and AC9G, the ink a9u for forming an anode catalyst layer was used in place of the ink a8u for forming an anode catalyst layer, with a catalyst coating amount of 0.68 mg / cm ２An anode catalyst layer-gas diffusion layer laminate AC10G having an anode catalyst layer AC10 of a two-layer structure was obtained in the same manner as in AC9G, except that the anode catalyst layer AC10 was used so as to satisfy the following conditions:

[0075] (11) Anode catalyst layer-gas diffusion layer laminate AC11G In the preparation of AC1G, the ink a1u for forming an anode catalyst layer was replaced with the ink a10u for forming an anode catalyst layer at a catalyst coating amount of 0.68 mg / cm ２ The ink a6d for forming an anode catalyst layer was used in place of the ink a1d for forming an anode catalyst layer, with a catalyst coating amount of 0.81 mg / cm ２ An anode catalyst layer-gas diffusion layer laminate AC11G having an anode catalyst layer AC11 of a two-layer structure was obtained in the same manner as in AC1G, except that the anode catalyst layer AC11 was used so as to satisfy the following conditions:

[0076] (12) Anode catalyst layer-gas diffusion layer laminate x AC1G In the preparation of AC1G, the ink a1d for forming an anode catalyst layer was used in place of the ink a1u for forming an anode catalyst layer, with a catalyst coating amount of 0.75 mg / cm ２ An anode catalyst layer-gas diffusion layer laminate xAC1G having a two-layer structure of the anode catalyst layer xAC1 was obtained in the same manner as in AC1G, except that the anode catalyst layer xAC1 was used so as to satisfy the following conditions:

[0077] (13) In the preparation of cathode catalyst layer-gas diffusion layer laminates CC1G and AC1G, the ink b1u for forming a cathode catalyst layer was used in place of the ink a1u for forming an anode catalyst layer, with a catalyst coating amount of 0.62 mg / cm ２ The ink b1d for forming a cathode catalyst layer was used in place of the ink a1d for forming an anode catalyst layer, with the catalyst coating amount being 0.73 mg / cm ２ A cathode catalyst layer-gas diffusion layer laminate CC1G having a two-layer cathode catalyst layer CC1 was obtained in the same manner as AC1G, except that the cathode catalyst layer CC1 was used so as to satisfy the following conditions:

[0078] (14) Cathode catalyst layer-gas diffusion layer laminate x CC1G In the preparation of CC1G, the cathode catalyst layer-forming ink b1d was applied in a catalyst coating amount of 1.5 mg / cm ２A cathode catalyst layer-gas diffusion layer laminate xCC1G having a single-layer cathode catalyst layer xCC1 was obtained in the same manner as CC1G, except that the cathode catalyst layer-forming ink b1u was used so that the cathode catalyst layer-forming ink b1u was not used. In Table 2, for convenience, the same cathode catalyst layer-forming inks are listed in the "Surface α" and "Surface β" columns, respectively.

[0079] 3. Preparation of water electrolysis cell (1) Preparation of water electrolysis cell AH1 The anode catalyst layer-gas diffusion layer laminate AC1G and the cathode catalyst layer-gas diffusion layer laminate xCC1G were each prepared to a thickness of 1 cm. ２ The plates were punched out to the same shape so that they were stacked with each catalyst layer side facing inward, sandwiching an anion conduction membrane (X37-50 (trade name), Dioxide, manufactured by Material Corporation, an anion exchange membrane having a PTFE support), and then pressed at a surface pressure of 1.5 MPa. The pressed plate was sandwiched between two Ni bipolar plates having flow channels and constrained with bolts to maintain a confining pressure of 1 MPa. In this manner, a water electrolysis cell AH1 was obtained, which has 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 conduction membrane-cathode catalyst layer-gas diffusion layer-bipolar plate. This water electrolysis cell AH1 includes a membrane electrode assembly having an anion conduction membrane and two catalyst layers in direct contact with the anion conduction membrane. In this membrane electrode assembly, the anode catalyst layer AC1 has a surface α and a surface β with different surface free energies, and the anode catalyst layer AC1 is in contact with the anion conduction membrane at the surface α.

[0080] (2) Preparation of water electrolysis cells AH2 to AH11 and xAH1 Water electrolysis cells AH2 to AH11 and xAH1 were prepared in the same manner as the water electrolysis cell AH1, except that an anode catalyst layer-gas diffusion layer laminate having an anode catalyst layer listed in Table 3 was used as the anode catalyst layer-gas diffusion layer laminate.

[0081] (3) Preparation of water electrolysis cells CH1 and xCH1 Water electrolysis cells CH1 and xCH1 were each prepared in the same manner as water electrolysis cell AH1, except that the cathode catalyst layer-gas diffusion layer laminate having each of the cathode catalyst layers listed in Table 3 was used as the cathode catalyst layer-gas diffusion layer laminate, and xAC1 was used as the anode catalyst layer-gas diffusion layer laminate. As shown in Table 3, water electrolysis cell xAH1 and water electrolysis cell xCH1 have the same anode catalyst layer and cathode catalyst layer. For convenience, they are referred to as xAH1 and xCH1 in Table 3.

[0082] The properties of each catalyst layer of the obtained water electrolysis cell were confirmed as follows.

[0083] 4. Measurement of Surface Free Energy The contact angle on each surface of the catalyst layer was measured to determine the surface free energy. Each water electrolysis cell prepared above was disassembled to extract the catalyst layer. Specifically, the water electrolysis cell was disassembled to extract a structure including a gas diffusion layer, catalyst layer, anion conductive membrane, catalyst layer, and gas diffusion layer. This structure was then cut and removed using a surface / interface property analyzer (manufactured by Daipla Wintes Co., Ltd., SAICAS (product name)) to a depth of 0.2 μm from the interface between the catalyst layer and the layer in contact with the catalyst layer (gas diffusion layer or anion conductive membrane), exposing the catalyst layer surface for surface preparation to evaluate the contact angle. The cutting process did not substantially affect the surface free energy of the catalyst layer. Pure water (dispersive component: 22 mN / m, polar component: 51 mN / m) and diiodomethane (dispersive component: 51 mN / m, polar component: 0 mN / m) were used as liquids for contact angle measurement. The contact angles of these liquids with respect to the surface of the catalyst layer of each laminate were measured. Specifically, 2 μL of each liquid was dropped onto the surface of the catalyst layer in a 25°C environment to create a droplet. 15 seconds after landing, the contact radius r (μm) and the height h (μm) of the droplet were measured and applied to tan θ / 2 = h / r to calculate the contact angle θ. A contact angle meter (DMo-501 (product name), manufactured by Kyowa Interface Science Co., Ltd.) was used for the measurement. The obtained contact angle, the dispersive component value and the polar component value of the solvent were applied to the Owens and Wendt theoretical formula to calculate the surface free energy (=dispersive component value + polar component value) of each surface of the catalyst layer. The surface free energy of the side of the resulting catalyst layer that had been in contact with the anion conductive membrane was calculated as the surface free energy of surface α. Next, the surface free energy of the side of the catalyst layer that had been in contact with the gas diffusion layer was calculated as the surface free energy of surface β. The results are shown in Table 2. The surface free energy of surface α is shown as "surface Eα," and the surface free energy of surface β is shown as "surface Eβ." The unit is mN / m.

[0084] 5. Measurement of Fluorine Content and Silicon Content The fluorine content (F content) and silicon content (Si content) of the polymer component contained on the surface α side and the surface β side of the catalyst layer were determined as follows. The results are shown in Table 2. As in the measurement of surface free energy described above, the catalyst layer was removed from each water electrolysis cell and used as a test specimen. A 1 μm depth range of the catalyst layer was scraped from each surface of the test specimen, and a 50 mg sample was placed in a 20 mmφ mold and pressed to produce a 20 mmφ pellet. The element ratios of this pellet were evaluated using a fluorescent X-ray analyzer (EDX-800HS (trade name), manufactured by Shimadzu Corporation). From the obtained results, the fluorine content and silicon content in the polymer component were determined, excluding the catalytic metal elements.

[0085] 6. 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 2. As in the surface free energy measurement described above, the catalyst layer was removed from each water electrolysis cell and used as a test specimen. The test specimen was impregnated with epoxy resin (NER-814 (trade 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-sections were polished using a cross-section polisher. Cross-sectional SEM images (the field of view was set to a field of view that allowed the catalyst area to be confirmed, e.g., 20 μm × 10 μm) were obtained for the cross-sections thus obtained. The catalyst area per 1 μm thickness on the surface α and surface β sides of the cross-section was measured in each field of view, and the ratio of the total area of ​​all catalysts to the area of ​​the catalyst layer in that field of view was calculated, which was defined as the catalyst ratio. This catalyst ratio was determined for 100 fields of view. The above process from cross-section extraction to calculation of the catalyst ratio in 100 fields of view was carried out for a total of five cross sections of the same test specimen, and the catalyst ratio was determined for each of the five cross sections in 100 fields of view. The arithmetic mean value of the catalyst ratios in these cross sections was calculated, and this mean value was used as the volume occupancy rate of the catalyst.

[0086] The following evaluations were carried out using each of the obtained water electrolysis cells.

[0087] 7. Evaluation of Electrolysis Efficiency Under Standard Flow Rate Conditions During water electrolysis at a high current density, the electrolysis efficiency at a water supply rate of 20 mL / min (standard conditions) was evaluated using overpotential (voltage loss) as an index. (1) Conditioning Operation For each water electrolysis cell, a 1 M KOH aqueous solution heated to 50°C was supplied to each gas diffusion layer in contact with the anode catalyst layer and the cathode catalyst layer at a flow rate of 20 mL / min. The water electrolysis cell was then heated with a heater to adjust the internal cell temperature to 50°C. This water electrolysis cell was operated for 2 hours at a current of 0.1 A and for 2 hours at a current of 1 A while supplying 1 M KOH heated to 50°C to each gas diffusion layer in contact with the anode catalyst layer and the cathode catalyst layer at a flow rate of 20 mL / min, thereby performing a pre-test conditioning operation. (2) Water electrolysis Thereafter, 1 M KOH heated to 50°C was supplied to each of the gas diffusion layers in contact with the anode catalyst layer and the cathode catalyst layer at a flow rate of 20 mL / min, while water electrolysis was carried out at a current value of 2.5 A, 0.002 A, or 0.01 A for 6 hours, and the voltage after 6 hours was measured for each. ２ ) for 6 hours. The voltage after 6 hours is V ２．５Ａ 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 2.5 A was calculated using the following formula, and Vx and V ２．５Ａ The difference between ２．５Ａ The difference (overvoltage when operated at 2.5 A) was evaluated according to the following evaluation criteria: Vx = (V ０．０１Ａ -V ０．００２Ａ ) / (ln0.01 - ln0.002) x ln2.5 + (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

[0088] 8. Evaluation of electrolysis efficiency at low flow rates During water electrolysis at high current densities, the electrolysis efficiency at a water supply rate of 2 mL / min (low rate) was evaluated using overpotential (voltage loss) as an index. For each water electrolysis cell after the conditioning operation in (1) of 7. Evaluation of electrolysis efficiency at standard flow rates, 1 M KOH heated to 50°C was supplied to each of the gas diffusion layers in contact with the anode catalyst layer and the cathode catalyst layer at a flow rate of 2 mL / min, while a current of 2.5 A (current density: 2.5 A / cm ２ ) for 6 hours, and the voltage V ｌｏｗ The voltage V ｌｏｗ and the difference between the Vx values ​​(V ｌｏｗ -Vx) (overvoltage when operated at a low flow rate of 2.5 A) was determined. The obtained difference (overvoltage) was evaluated according to the following evaluation criteria. -Evaluation criteria- A: Less than 0.05 V B: 0.05 V or more and less than 0.10 V C: 0.10 V or more and less than 0.15 V D: 0.15 V or more and less than 0.20 V E: 0.20 V or more

[0089] 8. Durability test Durability was evaluated using voltage change as an index when water electrolysis was performed at a high current density for a long period of time while varying the water supply rate. For each water electrolysis cell after the conditioning operation in (1) of 7. Evaluation of electrolysis efficiency under standard flow rate conditions, 1 M KOH heated to 50°C was supplied to each of the gas diffusion layers in contact with the anode catalyst layer and the cathode catalyst layer at a flow rate of 20 mL / min, while a current of 2.5 A (current density: 2.5 A / cm ２) for 6 hours, and the voltage after 6 hours was defined as the "initial voltage." Next, water electrolysis was performed at a current of 2.5 A for 6 hours while supplying 1 M KOH heated to 50°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 20 mL / min, and then the flow rate was changed to 2 mL / min and water electrolysis was performed for 6 hours. 200 cycles were performed. Water electrolysis was then performed at a current of 2.5 A for 6 hours, and the voltage after 6 hours was defined as the "voltage after durability test." As a durability index, the voltage change value was calculated using the following formula and evaluated according to the following evaluation criteria: voltage change value = voltage after durability test - initial voltage. The water electrolysis conditions in this test were conditions that simulated fluctuations in the water supply rate during actual operation of a water electrolysis cell. It is considered that the larger the voltage change value, the more deteriorated the membrane electrode assembly. -Evaluation criteria- A: Less than 0.10V B: 0.10V or more and less than 0.15V C: 0.15V or more and less than 0.20V D: 0.20V or more and less than 0.30V E: 0.30V or more

[0090]

[0091] <Notes for Table 1> "Catalyst concentration" indicates the catalyst concentration in the solid content of the catalyst layer-forming ink. The anode catalyst layer-forming ink a8u and the anode catalyst layer-forming ink a9u differ in the blending ratio of Si1 and XB-7. "Pt / C" refers to the platinum carbon powder described above.

[0092]

[0093]

[0094] When a membrane electrode assembly having an anode catalyst layer xAC1 in which the surface free energy of the surface α in contact with the anion conductive membrane is the same as the surface free energy of the opposite surface β, the assembly exhibited a high overvoltage of 0.20 V or more when water was electrolyzed at a slow water supply rate, resulting in poor electrolysis efficiency. Furthermore, the voltage change after long-term water electrolysis while varying the water supply rate was high, resulting in a high voltage change of 0.30 V or more, resulting in poor durability. In contrast, when a membrane electrode assembly having anode catalyst layers AC1 to AC11 in which the surface free energy of the surface α in contact with the anion conductive membrane is higher than the surface free energy of the opposite surface β, the assembly exhibited a low overvoltage of less than 0.15 V when water was electrolyzed at a slow water supply rate, resulting in excellent electrolysis efficiency. Furthermore, the voltage change after water electrolysis while varying the water supply rate was low, resulting in excellent durability. The same effect as above was also obtained when a cathode catalyst layer CC1 was used, in which the surface free energy of the surface α in contact with the anion conductive membrane was higher than the surface free energy of the opposite surface β. When these water electrolysis cells were visually observed during the evaluation of electrolysis efficiency with water supplied at the above-mentioned low rate, gas was not retained in the anode catalyst layer or the cathode catalyst layer, but was efficiently discharged to the gas diffusion layer side. Therefore, the excellent electrolysis efficiency as described above is thought to be due to the efficient gas transfer from the catalyst layer to the gas diffusion layer. Nickel iron oxide (NiFe) was used as a catalyst. ２ O ４ Even with a membrane electrode assembly having a catalyst layer (AC11) using iridium oxide containing iridium, which is a precious metal, as a catalyst, the same level of effect was obtained as with a membrane electrode assembly having a catalyst layer (AC1) using iridium oxide, which contains iridium, a precious metal, as a catalyst. Furthermore, it is clear that the use of the membrane electrode catalyst layer of the present invention makes it possible to form a membrane electrode assembly that can be used as a water electrolysis cell that has excellent electrolysis efficiency when electrolyzing water at a low flow rate and is also excellent in durability.

[0095] 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.

[0096] This application claims priority based on Japanese Patent Application No. 2024-054241, filed on March 28, 2024, the contents of which are incorporated herein by reference as part of the present specification.

[0097] 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, the surface free energy of a surface α in contact with the anion conductive membrane is higher than the surface free energy 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 2, wherein the difference between the volume occupancy Mα and the volume occupancy Mβ is 1 to 70 volume %.

4. The membrane electrode assembly according to claim 1, wherein the catalyst layer X contains a polymer component, and the polymer component contained on the surface β side is different from the polymer component contained on the surface α side.

5. The membrane electrode assembly according to claim 1, wherein the fluorine content of the polymer component contained on the surface β side of the catalyst layer X is higher than the fluorine content of the polymer component contained on the surface α side.

6. The membrane electrode assembly according to claim 1, wherein the silicon content of the polymer component contained on the surface β side of the catalyst layer X is higher than the silicon content of the polymer component contained on the surface α side.

7. A catalyst layer for a membrane electrode, with both sides having different surface free energies.

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