JP2023174340A - Electrode catalyst layer and membrane electrode assembly - Google Patents

Abstract

To provide an electrode catalyst layer that can suppress lowering of an electrolytic performance, and a membrane electrode assembly for an electrolytic cell.SOLUTION: An electrode catalyst layer 12 is an electrode catalyst layer that is formed on a solid polymer electrolyte membrane 11, and includes a catalyst 21 and a polymer electrolyte 23. An electrode surface porosity of an electrode surface opposite to a surface in contact with the solid polymer electrolyte membrane 11 is 1.0% or larger and 20% or smaller. The membrane electrode assembly is a membrane electrode assembly in which a cathode-side electrode catalyst layer is formed on one surface of the solid polymer electrolyte membrane and an anode-side electrode catalyst layer is formed on the other surface of the solid polymer electrolyte membrane; and at least one of the anode-side electrode catalyst layer and the cathode-side electrode catalyst layer is the electrode catalyst layer 12 of the present disclosure.SELECTED DRAWING: Figure 2

Description

The present invention relates to an electrode catalyst layer and a membrane electrode assembly suitable for electrodes for electrolyzers and the like.

In recent years, with the aim of achieving carbon neutrality, there has been an accelerating movement to use hydrogen, which can be generated from various resources and can be used as CO2-free energy, as a primary energy source. Among these, methods that use renewable energy to produce or store hydrogen, such as electrolysis of water or ammonia or dehydrogenation reactions from organic substances, are considered promising.

Alkaline water electrolysis and solid polymer water electrolysis are generally known as water electrolysis techniques. In particular, solid polymer water electrolysis is attracting attention as a method that allows miniaturization through highly efficient operation. A typical configuration of a solid polymer water electrolysis device includes a membrane electrode assembly in which an electrode catalyst layer is arranged on both sides of a proton-conducting solid polymer electrolyte membrane, and a power supply body and a current carrying plate on the outside of the membrane electrode assembly. . As an electrolysis method using a solid polymer electrolyzer, organic substance electrolysis, an electrochemical reaction between an organic substance and hydrogen or protons, is also well known.
In both water electrolysis and organic substance electrolysis, supplied water is oxidized and oxygen gas is generated by an electrochemical reaction in the electrode catalyst layer on the anode side, as shown in the reaction formula below.
Reaction formula: 2H ₂ O → O ₂ + 4H ⁺ + 4e ^-

The oxygen gas generated by this reaction passes through the pores in the anode electrode catalyst layer and exits the system. At this time, if the formation of pores in the anode-side electrode catalyst layer is insufficient, oxygen gas generated by the reaction remains near the catalyst. In this case, water, which is a reaction substrate, becomes difficult to approach the catalyst, which may increase mass transport resistance and increase cell voltage during electrolysis.
Furthermore, when the amount of oxygen gas in the anode side electrode catalyst layer increases, the pressure of the retained oxygen gas increases. In that case, the structure of the electrode catalyst layer may collapse due to oxygen gas. Specifically, cracks may occur in the electrode catalyst layer, resulting in a decrease in durability.

In the case of water electrolysis, an electrochemical reaction (see reaction formula below) occurs in the electrode catalyst layer on the cathode side, and protons that have migrated from the cathode side through the solid polymer electrolyte membrane are reduced, and hydrogen gas is generated.
Reaction formula: 2H ⁺ + 2e ^- → H ₂
Hydrogen gas generated by this reaction passes through the pores in the cathode electrode catalyst layer and exits the system. If the formation of pores in the cathode electrode catalyst layer is insufficient, the hydrogen gas generated by the reaction may remain near the catalyst, which may cause the catalyst layer structure to collapse due to pressure.
Here, Patent Document 1 proposes an electrode in which the porosity of the catalyst layer is 50 vol% or more and 95 vol% or less.

JP 2017-89007 Publication

However, in Patent Document 1, the porosity on the surface of the catalyst layer has not been sufficiently verified, and the mass transportability of the electrode catalyst layer cannot be said to be sufficient. Specifically, even if the pores inside the electrode catalyst layer are sufficient at 50 vol %, if the pore formation on the surface of the electrode catalyst layer is insufficient, gas will not be discharged from inside the electrode catalyst layer. Mass transport properties may be reduced and electrolytic performance may be reduced.
The present invention was made with attention to the above points, and an object of the present invention is to provide an electrode catalyst layer and a membrane electrode assembly for an electrolytic cell that can suppress deterioration of electrolytic performance.

The inventors of the present invention have conducted intensive studies on the pores in the electrode catalyst layer with the aim of improving the discharge of gas from inside the electrode catalyst layer. We obtained the knowledge that the emission of Based on this knowledge, the present invention was accomplished.
In order to solve the problem, an electrode catalyst layer according to one embodiment of the present invention is an electrode catalyst layer formed on a solid polymer electrolyte membrane, and includes a catalyst and a polymer electrolyte, and includes a catalyst and a polymer electrolyte. The electrode surface porosity of the electrode surface opposite to the surface in contact with the membrane is 1.0% or more and 20% or less.
Further, in a membrane electrode assembly according to another aspect of the present invention, a cathode electrode catalyst layer is formed on one surface of the solid polymer electrolyte membrane, and an anode electrode is formed on the other surface of the solid polymer electrolyte membrane. In the membrane electrode assembly in which a catalyst layer is formed, at least one of the anode-side electrode catalyst layer and the cathode-side electrode catalyst layer is an electrode catalyst layer according to one embodiment of the present invention.

According to one aspect of the present invention, gas generated within the electrode catalyst layer during electrolysis is easily discharged from the surface of the electrode catalyst layer. As a result, it is possible to provide an electrode catalyst layer and a membrane electrode assembly that have a low cell voltage during electrolysis and are highly durable.

FIG. 1 is a schematic cross-sectional view for explaining a membrane electrode assembly for an electrolytic cell according to an embodiment of the present invention. FIG. 2 is a schematic cross-sectional view for explaining an electrode catalyst layer that constitutes a membrane electrode assembly for an electrolytic cell according to an embodiment based on the present invention.

Embodiments of the present invention will be described below with reference to the accompanying drawings.
The embodiments shown below are illustrative rather than limiting the invention, and are not limited to the features described as the embodiments. Various changes can be made to the embodiments within the technical scope defined by the claims. Further, the drawings are schematic, and the relationship between thickness and planar dimension, the ratio of thickness of each layer, etc. differ from the actual one.

"composition"
(Membrane electrode assembly)
In the membrane electrode assembly 10 of this embodiment, as shown in FIG. 1, a cathode electrode catalyst layer 12C and an anode electrode catalyst layer 12A are arranged to face each other with the solid polymer electrolyte membrane 11 in between. Furthermore, in FIG. 1, on the outside in the stacking direction, a cathode side current collector 13C and an anode side current collector 13A are sandwiched between the cathode side electrode catalyst layer 12C, the solid polymer electrolyte membrane 11, and the anode side electrode catalyst layer 12A. The figure shows a state in which they are arranged facing each other.

(polymer electrolyte membrane)
The solid polymer electrolyte membrane 11 is formed from a solid polymer.
That is, the solid polymer electrolyte membrane 11 is made of a polymer material having proton conductivity, and for example, a fluorine-based electrolyte membrane or a hydrocarbon-based polymer electrolyte membrane can be used. Examples of the fluoropolymer electrolyte membrane include Nafion (registered trademark) manufactured by DuPont, Forblue (registered trademark) manufactured by AGC Corporation, Aciplex (registered trademark) manufactured by Asahi Kasei Corporation, and Gore Select (registered trademark) manufactured by Gore Corporation. etc. can be used. Furthermore, as the hydrocarbon polymer electrolyte membrane, for example, electrolyte membranes such as sulfonated polyetherketone, sulfonated polyethersulfone, sulfonated polyetherethersulfone, sulfonated polysulfide, and sulfonated polyphenylene can be used.

(electrode catalyst layer)
When viewed from the direction in which the cathode side electrode catalyst layer 12C and the anode side electrode catalyst layer face each other (thickness direction of the solid polymer electrolyte membrane 11; plan view from the top and bottom directions in FIG. 1), for example, the cathode side electrode catalyst layer The outer shape of 12C and the outer shape of the anode side electrode catalyst layer 12A are almost the same. Alternatively, the outer shape of the electrode catalyst layer formed later is larger than the outer shape of the electrode catalyst layer formed first with respect to the surface of the solid polymer electrolyte membrane. Further, the outer shape of the solid polymer electrolyte membrane 11 is larger than the outer shape of these electrode catalyst layers 12C and 12A.
The outer shape of the solid polymer electrolyte membrane 11 and the outer shape of the electrode catalyst layer 12 are not particularly limited, and are, for example, rectangular.
Here, when the cathode side electrode catalyst layer 12C and the anode side electrode catalyst layer 12A are not distinguished, they are simply referred to as "electrode catalyst layer 12." The same description will be made regarding the catalyst 21, conductive carrier 22, polymer electrolyte 23, and fibrous material 24. Moreover, when the anode side current collector 13A and the cathode side current collector 13C are not distinguished, they are simply referred to as "current collector 13."

FIG. 2 is a cross-sectional view schematically showing the structure of the electrode catalyst layer 12 on the cathode side and the anode side of this embodiment. In this embodiment, it is assumed that the cathode side electrode catalyst layer 12C and the anode side electrode catalyst layer 12A have the same configuration.
The electrode catalyst layer 12 is provided on the surface of the solid polymer electrolyte membrane 11 and includes a catalyst 21 and a polymer electrolyte 23. The catalyst 21 of this embodiment consists of particle-shaped catalyst particles, and the catalyst particles are cathode catalyst particles for performing a reduction reaction or anode catalyst particles for performing an oxidation reaction.

One surface (lower surface in FIG. 2) of the electrode catalyst layer 12 is in contact with the solid polymer electrolyte membrane 11. When the membrane electrode assembly is incorporated into a single cell or a unit cell, the other surface of the electrode catalyst layer 12 is in contact with the current collector 13. Note that in FIGS. 1 and 2, the current collectors 13, 13A, and 13C are shown separated for clarity.
The current collector 13 may be made of any electrically conductive material. Specific examples include carbon paper, carbon nonwoven fabric, oxide and metal plates, and the like. Examples of the metal plate include a titanium sintered body. The carbon paper may be treated to be water repellent, and the oxide or metal plate may be plated with a precious metal. It is preferable that the current collector 13 is a porous body. Since the current collector 13 is a porous body, gas generated from the membrane electrode assembly due to electrolytic reaction is easily discharged to the outside of the system. This may reduce mass transport resistance during electrolysis and improve electrolysis performance.

It is preferable that the surface of the electrode catalyst layer 12 opposite to the surface in contact with the solid polymer electrolyte membrane 11 (the upper surface in FIG. 2) has many pores. In this embodiment, the electrode surface porosity on the opposite surface is adjusted to be in the range of 1.0% to 20%.
Since there are many pores on the electrode surface on the surface opposite to the surface in contact with the solid polymer electrolyte membrane 11, the gas generated within the electrode catalyst layer 12 due to the electrolytic reaction is easily discharged from the electrode surface. The gas discharged from the electrode catalyst layer 12 passes through the current collector 13 and the like and is discharged to the outside of the system. When the electrode surface porosity is less than 1.0%, the gas generated within the electrode catalyst layer 12 tends to stay within the electrode catalyst layer 12, the gas pressure increases, and the structure of the electrode catalyst layer 12 is destroyed. Sometimes it happens. Destruction occurs, for example, when the polymer electrolyte 23 peels off from the catalyst 21 or when the conductive network formed by the catalyst 21 and the conductive carrier 22 ruptures. On the other hand, if the electrode surface porosity is too large than 20%, the contact area between the electrode catalyst layer 12 and the current collector 13 will become smaller, the cell resistance during electrolysis will increase, and the electrolytic performance will deteriorate. There is.

The above-mentioned electrode surface porosity of the electrode catalyst layer 12 can be controlled by devising the method for manufacturing the membrane electrode assembly.
Specifically, by directly coating the catalyst ink on the solid polymer electrolyte membrane 11 and then drying it, the solvent in the catalyst ink evaporates from the surface opposite to the surface in contact with the solid polymer electrolyte membrane 11. Due to its volatilization, many pores can be formed on the surface of the electrode catalyst layer 12. Further, by adding the fibrous substance 24 to the catalyst ink, the electrode catalyst layer 12 has a structure similar to a nonwoven fabric, so that many pores are formed on the electrode surface as well. A blowing agent may be added to the catalyst ink to increase the electrode surface porosity.

(catalyst)
The catalyst 21 has a particle shape, for example.
The catalyst 21 may be formed of, for example, a metal containing a platinum group element, an alloy containing a platinum group element, or an oxide containing a platinum group element. The platinum group elements are palladium (Pd), ruthenium (Ru), iridium (Ir), rhodium (Rh), and osmium (Os). Examples of alloys containing platinum group elements include iron (Fe), lead (Pt), copper (Cu), chromium (Cr), cobalt (Co), nickel (Ni), manganese (Mn), vanadium (V), It can contain molybdenum (Mo), gallium (Ga), aluminum (Al), and the like.

(Cathode catalyst)
The catalyst 21 (referred to as cathode catalyst 21) used in the cathode electrode catalyst layer 12C is preferably formed from platinum, an alloy containing platinum, or an oxide containing platinum. A cathode catalyst containing platinum is preferable from the viewpoint of low overvoltage in the cathode reduction reaction. More preferably, the cathode catalyst 21 contains an alloy of platinum and ruthenium. By using these metals, the reaction substrate is adsorbed to ruthenium during organic electrolysis, and the electrolysis efficiency may be improved.
The particle size of the cathode catalyst 21 is preferably contained within a range of 0.5 nm or more and 20 nm or less, and more preferably contained within a range of 1 nm or more and 5 nm or less. When the particle size of the cathode catalyst 21 is 0.5 nm or more, the stability of the cathode catalyst 21 is unlikely to decrease. When the particle size of the cathode catalyst 21 is 20 nm or less, the activity of the cathode catalyst 21 is less likely to decrease.

(Anode catalyst)
The catalyst 21 (referred to as anode catalyst 21) used in the anode-side electrode catalyst layer 12A is preferably formed from iridium, an alloy containing iridium, or an oxide containing iridium. This is preferable from the viewpoint of low overvoltage in the anodic oxidation reaction.
The particle size of the anode catalyst 21 is preferably contained within a range of 5 nm or more and 500 nm or less, and more preferably contained within a range of 10 nm or more and 200 nm or less. When the particle size of the anode catalyst 21 is 5 nm or more, the stability of the anode catalyst 21 is unlikely to decrease. When the particle size of the anode catalyst 21 is 20 nm or less, the activity of the anode catalyst 21 is less likely to decrease.

(conductive carrier)
The catalyst 21 may be supported on a conductive carrier 22, as shown in FIG. The cathode catalyst 21 is preferably supported on a conductive carrier 22.
The conductive carrier 22 may be, for example, carbon particles, carbon fibers, metal oxides, or the like. Among these, the metal oxide may be, for example, titanium oxide or tin oxide. The tin oxide may be doped with tungsten or niobium. This can increase the conductivity of the carrier.

(Cathode conductive carrier)
It is particularly preferable that the conductive carrier 22 (referred to as cathode conductive carrier 22) supporting the cathode catalyst 21 is carbon particles. By selecting carbon particles as the carrier, the surface area of the carrier can be increased compared to when other carriers are selected, so the cathode catalyst 21 can be supported at a high density. Thereby, the catalytic activity per cathode conductive carrier 22 can be improved. Note that the cathode conductive carrier 22 may be a carrier other than the above-mentioned carbon particles, carbon fibers, and metal oxides. In this case, the cathode conductive carrier 22 may be any material as long as it is a conductive fine particle and is not affected by the cathode catalyst 21.

The average particle diameter of the cathode conductive carrier 22 is preferably in the range of 10 nm or more and 3 μm or less, more preferably in the range of 10 nm or more and 2 μm or less. When the particle size of the cathode conductive carrier 22 is 10 nm or more, the cathode conductive carrier 22 is prevented from being too small, thereby preventing the formation of electron conduction paths. Furthermore, since the particle size of the cathode conductive carrier 22 is 3 μm or less, the cathode conductive carrier 22 is prevented from being too large, and therefore the electrode catalyst layer is prevented from becoming thick. This suppresses an increase in the resistance of the electrode catalyst layer, and as a result, suppresses an increase in cell voltage during electrolysis of the membrane electrode assembly 10 including the electrode catalyst layer.

The specific surface area of the cathode conductive carrier 22 is preferably included in the range of 50 m ² /g or more and 2000 m ² /g or less, and more preferably included in the range of 100 m ² /g or more and 1500 m ² /g or less. When the specific surface area of the cathode conductive carrier 22 is 50 m ² /g or more, the cathode catalyst 21 is supported at a high density, and thereby the activity of the cathode catalyst 21 can be increased. Further, since the specific surface area of the cathode conductive carrier 22 is 2000 m ² /g or less, the amount of micropores that the cathode conductive carrier 22 has can be suppressed, so that the substrate diffusivity within the cathode conductive carrier 22 can be improved. I can do it. This suppresses an increase in the mass transport resistance of the electrode catalyst layer, and as a result, suppresses an increase in cell voltage during electrolysis of the membrane electrode assembly 10 including the electrode catalyst layer.

(Anode conductive carrier)
The anode catalyst 21 may be supported on a conductive carrier 22 (referred to as an anode conductive carrier 22). When the anode catalyst 21 is supported on the anode conductive carrier 22, it is particularly preferable that the anode conductive carrier 22 is titanium oxide. By selecting titanium oxide as the anode conductive carrier 22, the stability of the anode conductive carrier 22 during electrolysis can be made higher than when other carriers are selected, so the durability of the anode side electrode catalyst layer 12A is increased. It is possible to increase the sex. Furthermore, the shape of titanium oxide is preferably particles or fibers, and fibers are particularly preferred. By making the anode conductive carrier 22 into a fiber shape, the electrode surface porosity of the anode side electrode catalyst layer 12A can be increased. Note that the anode conductive carrier 22 may be any material as long as it has conductivity and is not affected by the anode catalyst 21.

(polymer electrolyte)
The polymer electrolyte 23 may be, for example, a polymer substance having proton conductivity. The polymeric substance having proton conductivity may be, for example, a fluororesin or a hydrocarbon resin. The fluororesin may be, for example, Nafion (manufactured by DuPont, registered trademark), Aquivion (manufactured by Solvay, registered trademark), or the like. The hydrocarbon resin may be, for example, an engineering plastic or a resin obtained by introducing a sulfonic acid group into a copolymer of an engineering plastic. Further, different polymer electrolytes may be added to the cathode side electrode catalyst layer and the anode side electrode catalyst layer, respectively. Two or more types of polymer electrolytes may be added to each of the cathode side electrode catalyst layer and the anode side electrode catalyst layer.
In the polymer electrolyte 23, the dry mass value (equivalent weight: EW) per mole of proton donating group is preferably 600 or more, more preferably 700 or more. When the equivalent weight EW is 600 or more, expansion of the polymer electrolyte due to absorption of water is suppressed, pores in the electrode catalyst layer are less likely to decrease, and an increase in cell voltage during electrolysis is suppressed.

(fibrous substance)
Fibrous material 24 may be, for example, carbon fibers, polymer fibers, oxide fibers, and metal fibers. Examples of carbon fibers include carbon fibers, carbon nanotubes, carbon nanohorns, and the like. Examples of the polymer fibers include nanofibers made of amine polymers such as imide structures and azole structures. The polymer fibers are more preferably engineering plastic nanofibers. The fibrous material 24 preferably does not dissolve or be corroded by the solution used for electrolysis and has heat resistance at temperatures higher than 120°C. According to the above physical properties, stability of the fibrous material 24 in the electrode catalyst layer can be expected.

The average fiber diameter of the fibrous substance 24 is preferably 300 nm or less, more preferably 200 nm or less. If the average fiber diameter of the fibrous substance 24 is 300 nm or less, an appropriate fineness is ensured as a fibrous material to be included in the electrode catalyst layer 12. Moreover, if the average fiber diameter of the fibrous material 24 is 50 nm or more, an appropriate thickness is ensured, the strength of the electrode catalyst layer 12 is increased, and the effect of suppressing the occurrence of cracks is obtained.
The average fiber length of the fibrous substance 24 is preferably included in the range of 0.7 μm or more and 40 μm or less, and more preferably included in the range of 2 μm or more and 30 μm or less. When the fiber length is within this range, the strength of the electrode catalyst layer 12 can be increased, so the occurrence of cracks can be suppressed, and as a result, the durability of the electrode catalyst layer 12 against physical impact can be improved. improves. Furthermore, the number of pores formed in the electrode catalyst layer 12 can be increased, thereby reducing mass transport resistance during electrolysis.

(Cathode fibrous material)
As the fibrous material 24 (referred to as cathode fibrous material 24) used in the cathode side electrode catalyst layer 12C, carbon nanofibers or carbon nanotubes are particularly suitable in terms of conductivity and dispersibility. The amount of carbon fiber blended is preferably within a range of 0.3 to 1.5 times the mass of the cathode conductive carrier 22 supporting the cathode catalyst 21 in the form of particles. If the carbon fiber content is within the above numerical range, pores of a suitable size can be formed in a suitable amount in the cathode side electrode catalyst layer.
When the cathode fibrous material 24 is a polymer fiber, the amount of the polymer fiber is 0.05 times or more and 0.3 times or less the mass of the cathode conductive carrier 22 supporting the cathode catalyst 21 in the form of particles. It is preferably within the range. If the polymer fiber content is within the above numerical range, pores of a suitable size can be formed in a suitable amount in the cathode side electrode catalyst layer 12C.

(Anode fibrous material)
The fibrous material 24 (referred to as anode fibrous material 24) used in the anode side electrode catalyst layer 12A is preferably a polymer fiber or an oxide fiber. When the anode fibrous material 24 is carbon fiber, the carbon fiber is eroded during electrolysis, and the conductivity within the electrode catalyst layer of the anode side electrode catalyst layer 12A may be lost.
The mass % of the cathode fibrous material 24 in the cathode electrode catalyst layer 12C used in the above configuration is preferably higher than the mass % of the anode fibrous material 24 in the anode electrode catalyst layer 12A. According to this configuration, the mechanical strength of the cathode side electrode catalyst layer 12C is increased, and when forming the anode side electrode catalyst layer 12A after forming the cathode side electrode catalyst layer 12C on the surface of the solid polymer electrolyte membrane 11. Furthermore, deformation and cracking of the solid polymer electrolyte membrane 11 can be suppressed.

(Method for manufacturing electrode catalyst layer)
The electrode catalyst layer 12 of this embodiment can be manufactured by preparing an electrode catalyst layer slurry (catalyst ink), coating it on a base material, etc., and drying it.

(Preparation of slurry for electrode catalyst layer)
A catalyst ink is prepared by mixing each component constituting the electrode catalyst layer, that is, the catalyst 21 or the conductive carrier 22 supporting the catalyst 21, and the polymer electrolyte 23 using a dispersion medium.
The solvent used as the catalyst ink dispersion medium is particularly limited as long as it does not corrode each component constituting the electrode catalyst layer and can dissolve the polymer electrolyte 23 in a highly fluid state or disperse it as a fine gel. It's not something you can do. However, it is desirable that the solvent contains at least a volatile organic solvent. The solvent used as a dispersion medium for the catalyst ink may be water, alcohols, ketone solvents, ether solvents, polar solvents, and the like. Specifically, water, alcohols such as methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, 2-butanol, isobutyl alcohol, tert-butyl alcohol, acetone, methyl ethyl ketone, methyl butyl ketone, methyl Ketones such as isobutyl ketone, methyl amyl ketone, pentanone, heptanone, cyclohexanone, methyl cyclohexanone, acetonyl acetone, diethyl ketone, dipropyl ketone, diisobutyl ketone, tetrahydrofuran, dioxane, diethylene glycol dimethyl ether, anisole, methoxytoluene, dibutyl ether, etc. Ether solvents and polar solvents such as dimethylformamide, dimethylacetamide, N-methylpyrrolidone, ethylene glycol, diethylene glycol, diacetone alcohol, and 1-methoxy-2-propanol can be used as appropriate. Further, the solvent may be a mixed solvent in which two or more of the above-mentioned materials are mixed.

Moreover, since a dispersion medium using a lower alcohol has a high risk of ignition, when a lower alcohol is used as a dispersion medium, it is preferable to use a mixed solvent of a lower alcohol and water. Furthermore, the dispersion medium may contain water that is compatible with the polymer electrolyte 23, that is, water that has a high affinity for the polymer electrolyte 23. The amount of water added to the dispersion medium is not particularly limited as long as the polymer electrolyte 23 does not separate and become cloudy or gel. A dispersant may be included in the catalyst ink in order to disperse the catalyst 21 or the conductive carrier 22 on which the catalyst 21 is supported in the catalyst ink.
Further, the catalyst ink may be subjected to a dispersion treatment if necessary. The dispersion treatment is not particularly limited as long as it is a method that can disperse each component contained in the catalyst ink. Examples include treatment with a planetary ball mill and roll mill, treatment with a shear mill, treatment with a wet mill, ultrasonic dispersion treatment, and treatment with a homogenizer.

(Coating and drying on base material)
After applying catalyst ink (slurry for electrode catalyst layer) onto a base material, the electrode catalyst layer 12 is formed by performing a drying process to volatilize the dispersion medium.
As the base material used when forming the electrode catalyst layer 12, for example, a transfer base material that is peeled off after transferring the electrode catalyst layer 12 to the solid polymer electrolyte membrane 11 is used. As the transfer base material, for example, a resin film is used. Moreover, the solid polymer electrolyte membrane 11 may be used as a base material used when forming the electrode catalyst layer 12.

Examples of the method for applying the slurry for an electrode catalyst layer to the substrate include, but are not particularly limited to, a doctor blade method, a die coating method, a dipping method, a screen printing method, a laminator roll coating method, a spray method, and the like.
Examples of methods for drying the electrode catalyst layer slurry applied to the base material include hot air drying and IR drying. The drying temperature of the slurry for electrode catalyst layer may be within the range of 40°C or higher and 200°C or lower, preferably within the range of 40°C or higher and 120°C or lower. The drying time of the slurry for the electrode catalyst layer may be from 0.5 minutes to 1 hour, preferably from 1 minute to 30 minutes.

If the manufacturing method uses the solid polymer electrolyte membrane 11 as a base material for forming the electrode catalyst layer 12, the electrode catalyst layer 12 is formed directly on the surface of the solid polymer electrolyte membrane 11. Therefore, the adhesion between the solid polymer electrolyte membrane 11 and the electrode catalyst layer 12 is increased, and since pressurization for joining the electrode catalyst layer 12 is not required, crushing of the electrode catalyst layer 12 is also suppressed. Therefore, it is preferable to use the solid polymer electrolyte membrane 11 as the base material for forming the electrode catalyst layer 12.

Here, since the solid polymer electrolyte membrane 11 generally has a characteristic of large degrees of swelling and contraction, when the solid polymer electrolyte membrane 11 is used as a base material, it is different from when a transfer base material is used as a base material. In comparison, the volume change of the base material during the drying process of the coating film that will become the electrode catalyst layer 12 is large. Therefore, if the electrode catalyst layer 12 does not include the fibrous material 24, cracks are likely to occur in the electrode catalyst layer 12. On the other hand, with the electrode catalyst layer 12 of this embodiment, even if the volume of the solid polymer electrolyte membrane 11, which is the base material, changes greatly during the manufacturing process of the electrode catalyst layer 12, the fibrous material Since the inclusion of 24 suppresses the occurrence of cracks, a manufacturing method using the solid polymer electrolyte membrane 11 as a base material for forming the electrode catalyst layer 12 can be used.

(effect)
According to this embodiment, gas is easily discharged from the surface of the electrode catalyst layer. Therefore, when the present invention is applied to the anode side electrode catalyst layer, it is possible to provide an electrode catalyst layer and a membrane electrode assembly in which the mass transport resistance is reduced and the cell voltage is improved in the anode side electrode catalyst layer.
Further, when the present invention is applied to an anode-side electrode catalyst layer and a cathode-side electrode catalyst layer, gas is easily discharged from the surface of the electrode catalyst layer in the anode-side electrode catalyst layer and the cathode-side electrode catalyst layer. As a result, the catalyst layer is less likely to break due to gas pressure, and it is possible to provide an electrode catalyst layer and a membrane electrode assembly in which the durability of the electrode catalyst layer is improved.

(others)
The present disclosure can also take the following configuration.
(1) An electrode catalyst layer formed on a solid polymer electrolyte membrane,
comprising a catalyst and a polymer electrolyte;
An electrode catalyst layer characterized in that the electrode surface porosity of the electrode surface opposite to the surface in contact with the solid polymer electrolyte membrane is 1.0% or more and 20% or less.
(2) The electrode catalyst layer contains a fibrous material.
(3) The fibrous substance is a polymer fiber.
(4) A membrane electrode assembly in which a cathode electrode catalyst layer is formed on one surface of a solid polymer electrolyte membrane, and an anode electrode catalyst layer is formed on the other surface of the solid polymer electrolyte membrane, ,
A membrane electrode assembly, wherein at least one of the anode-side electrode catalyst layer and the cathode-side electrode catalyst layer is the electrode catalyst layer of the present disclosure.
(5) the anode-side electrode catalyst layer and the cathode-side electrode catalyst layer each consist of the electrode catalyst layer of the present disclosure;
A membrane electrode assembly, wherein the mass % concentration of the fibrous material in the cathode electrode catalyst layer is higher than the mass % concentration of the fibrous material in the anode electrode catalyst layer.

Examples based on this embodiment will be described below, but the present invention is not limited to the following examples and comparative examples.

(Example 1)
In Example 1, platinum-ruthenium alloy catalyst-supported carbon (TEC61E54 (manufactured by Tanaka Kikinzoku Kogyo Co., Ltd.)) powder was used as the cathode catalyst supported on the cathode conductive carrier, and 10 g of the platinum-ruthenium alloy catalyst-supported carbon powder was 10 g of Nafion (trademark registered) dispersion liquid (manufactured by Fuji Film Wako Pure Chemical Industries, Ltd.) with a concentration of 20% by mass as a polymer electrolyte and 5 g of carbon fiber (VGCF-H (manufactured by Showa Denko)) as a fibrous material were added. A catalyst ink (slurry for cathode side electrode catalyst layer) was prepared using an appropriate solvent. The blended amount of the fibrous material was set to be 1.0 times the mass of the conductive carrier. It was confirmed that the average fiber diameter of the carbon fibers was 150 nm.
The obtained slurry for the cathode side electrode catalyst layer was applied to one side of a Nafion (registered trademark) membrane, which is a polymer electrolyte membrane, so that the total amount of platinum and ruthenium supported was 0.5 mg/cm2 per electrode area. , coated using the die coating method. Thereafter, by drying in an oven at 80° C., a laminate in which an electrode catalyst layer was formed on one side of the polymer electrolyte membrane was obtained.

Next, 5 g of a 20% concentration Nafion (trademark) dispersion as a polymer electrolyte and 0.06 g of polymer fibers as a fibrous substance were added to 5 g of iridium oxide powder as an anode catalyst, and an appropriate solvent was used. A catalyst ink (slurry for anode side electrode catalyst layer) was produced. The blending amount of the fibrous material was 1% by mass based on the mass of the electrode catalyst layer. It was confirmed that the average fiber diameter of the fibrous material was 300 nm.
The obtained slurry for the anode side electrode catalyst layer was applied to the surface of the Nafion (registered trademark) membrane on which the cathode side electrode catalyst layer was not formed, so that the total amount of iridium oxide supported was 1.0 mg/cm2 per electrode area. It was applied using the die coating method. Thereafter, the membrane electrode assembly of Example 1 was obtained by drying in an oven at 80°C.

(Example 2)
Example 2 was produced under the same conditions as Example 1, except that the amount of fibrous material added to the ink for the anode side electrode catalyst layer was set to a concentration of 20% by mass with respect to the mass of the electrode catalyst layer. A membrane electrode assembly was obtained.
(Example 3)
A membrane electrode assembly of Example 3 was obtained under the same conditions as Example 1 except that no fibrous material was added to the ink for the anode side electrode catalyst layer.
(Example 4)
A membrane electrode assembly of Example 3 was obtained under the same conditions as Example 1 except that the mass of the fibrous material added to the ink for the cathode side electrode catalyst layer was 0.05 g.

(Comparative example 1)
Comparative Example 1 A membrane electrode assembly was obtained.
(Comparative example 2)
In Comparative Example 2, platinum-ruthenium alloy catalyst-supported carbon (TEC61E54 (manufactured by Tanaka Kikinzoku Kogyo Co., Ltd.)) powder was used as the cathode catalyst supported on a conductive carrier, and 10 g of the platinum-ruthenium alloy catalyst-supported carbon powder was 10 g of Nafion (trademark registered) dispersion liquid (manufactured by Fuji Film Wako Pure Chemical Industries, Ltd.) with a concentration of 20% by mass as a polymer electrolyte and 5 g of carbon fiber (VGCF-H (manufactured by Showa Denko)) as a fibrous material were added. A catalyst ink (slurry for cathode side electrode catalyst layer) was prepared using an appropriate solvent. The blended amount of the fibrous material was set to be 1.0 times the mass of the conductive carrier. It was confirmed that the average fiber diameter of the carbon fibers was 150 nm.
The obtained slurry for the cathode side electrode catalyst layer was applied to a PTFE film (Nitoflon (manufactured by Nitto Denko)) using a die coating method so that the total amount of platinum and ruthenium supported was 0.5 mg/cm2 per electrode area. Coated. Thereafter, the cathode side electrode catalyst layer was obtained by drying in an oven at 80°C.

Next, a catalyst ink (slurry for the anode side electrode catalyst layer) was prepared by adding 5 g of iridium oxide powder as an anode catalyst and 5 g of a 20% concentration Nafion (trademark) dispersion as a polymer electrolyte using an appropriate solvent. .
The obtained slurry for the anode side electrode catalyst layer was coated on a PTFE film (Nitoflon (manufactured by Nitto Denko)) using a die coating method so that the total amount of iridium oxide supported was 1.0 mg/cm2 per electrode area. I worked on it. Thereafter, the anode side electrode catalyst layer was obtained by drying in an oven at 80°C.
A cathode electrode catalyst layer and an anode electrode catalyst layer were placed so as to sandwich the Nafion membrane, and a pressure of 5 MPa was maintained at 140° C. for 10 minutes using a heat press device. Thereafter, the membrane electrode assembly of Comparative Example 2 was obtained by peeling off the PTFE films attached to the cathode side electrode catalyst layer and the anode side electrode catalyst layer.

(Evaluation method)
The following measurements and evaluations were performed on the membrane electrode assemblies of each Example and Comparative Example.
(Measurement of electrode surface porosity)
The membrane electrode assembly was cut into a size of approximately 1 cm x 1 cm, and the surface on which the electrode surface porosity was to be measured was observed using a scanning electron microscope (SEM (SU-8020, manufactured by Hitachi High-Technology)). At this time, platinum sputtering was performed on the electrode surface for 10 seconds. The SE of the Upper detector was used as a detector, the acceleration voltage was 10.0 kV, the emission was 10 uA, and the observation was performed at a magnification of 10,000 times. By using the auto contrast function during observation, it is possible to obtain natural SEM images.
The obtained SEM image was binarized using 50 out of 256 gradations as a threshold. At this time, the black portions become holes. The electrode surface porosity was determined by calculating the ratio of the number of black pixels to the total number of pixels. The electrode surface porosity was determined from the average value of the values calculated from SEM images of three or more different fields of view. For this operation, software such as ImageJ or the Python OpenCV library can be used.

(IV performance evaluation)
The electrolytic performance of the membrane electrode assembly was evaluated by IV performance. The IV measurement was carried out using the method described below. Carbon paper (GDL 29BC (manufactured by SGL)) was placed as a cathode current collector on the cathode side electrode catalyst layer of the membrane electrode assembly, and a titanium sintered body with a platinum coating was placed on the anode side electrode catalyst layer. It was assembled into a commercial cell (manufactured by FC Co., Ltd.) and fixed with bolts at a tightening pressure of 4 Nm.
Water was flowed from the channel ends on both sides of the cell at a flow rate of 1 mL per minute, the cell temperature was set to 80°C, and the voltage was applied while changing from 1.4 V to 2.0 V, and the current density at this time was measured. .

[Evaluation criteria]
The I-V performance was evaluated as follows from the current density value at 1.9V.
・When the current density is greater than 1.0A/ ^cm2 : "〇"
・When the current density is less than 1.0A/ ^cm2 : "×"
Note that even if the determination is "x", there is no practical problem.

(Durability performance evaluation)
A voltage of 1.9 V was applied to the water electrolysis cell having the above configuration for 5 hours, and the change in current density at this time was measured.
Durability performance was evaluated as follows based on the difference in current density at 1.9 V before and after voltage application for 5 hours.
When the difference in current density values is less than 0.3A/ ^cm2 : "〇"
When the difference in current density values is greater than 0.3A/ ^cm2 : "x"
Note that even if the determination is "x", there is no practical problem.
The above evaluation results are shown in Table 1.

As shown in Table 1, all of Examples 1 to 4 had good IV performance and durability. This can be considered to be because the optimization of the electrode surface porosity of the electrode catalyst layer made it easier for the gas produced by the electrolytic reaction to be discharged.

On the other hand, in Comparative Example 1, the IV performance deteriorated because the anode electrode surface porosity was too large. When the electrode surface porosity was too large, the conductivity with the current collecting material decreased and the electrolytic performance decreased. Furthermore, if the pores are too large, the thickness of the electrode catalyst layer becomes too thick, which may reduce the conductivity and mass transportability within the electrode catalyst layer.
Furthermore, in Comparative Example 2, the I-V performance and durability performance decreased because the anode electrode surface porosity was too small. When observing the cross section of the membrane electrode assembly after durability performance evaluation, it was confirmed that the anode side electrode catalyst layer structure had collapsed. This is thought to be because the inside of the anode-side electrode catalyst layer was deformed due to the pressure of gas generated during electrolysis.

As explained above, according to this embodiment, the electrode catalyst layer has a high electrode surface porosity. As a result, the gas discharge performance from the electrode catalyst layer is improved, so that the increase in cell voltage resistance due to electrolysis is small, and an increase in cell voltage can be suppressed. Furthermore, it is possible to provide a solid polymer membrane electrode assembly for an electrolytic cell that maintains stable electrolytic performance even during long-time operation and has high electrolytic performance and high durability.

Since the electrode surface pores of the electrode catalyst layer are large, the present invention has high electrolytic performance and durability, and has high industrial utility value. For example, it is extremely suitable for manufacturing membrane electrode assemblies for electrolytic cells.

10 Membrane electrode assembly 11 Polymer electrolyte membrane 12 Electrode catalyst layer 12C Cathode side electrode catalyst layer 12A Anode side electrode catalyst layer 13C Cathode side current collector 13A Anode side current collector 21 Catalyst (anode catalyst, cathode catalyst)
22 Conductive carrier (anode conductive carrier, cathode conductive carrier)
23 Polyelectrolyte 24 Fibrous substance (anode fibrous substance, cathode fibrous substance)

Claims (5)

An electrode catalyst layer formed on a solid polymer electrolyte membrane,
comprising a catalyst and a polymer electrolyte;
The electrode surface porosity of the electrode surface opposite to the surface in contact with the solid polymer electrolyte membrane is 1.0% or more and 20% or less,
An electrode catalyst layer characterized by: The electrode catalyst layer according to claim 1, comprising a fibrous material. the fibrous substance is a polymer fiber;
The electrode catalyst layer according to claim 2, characterized in that: A membrane electrode assembly in which a cathode electrode catalyst layer is formed on one surface of a solid polymer electrolyte membrane, and an anode electrode catalyst layer is formed on the other surface of the solid polymer electrolyte membrane,
At least one of the anode side electrode catalyst layer and the cathode side electrode catalyst layer is the electrode catalyst layer according to any one of claims 1 to 3,
Membrane electrode assembly. A membrane electrode assembly in which a cathode electrode catalyst layer is formed on one surface of a solid polymer electrolyte membrane, and an anode electrode catalyst layer is formed on the other surface of the solid polymer electrolyte membrane,
The anode side electrode catalyst layer and the cathode side electrode catalyst layer each consist of the electrode catalyst layer according to claim 2 or claim 3,
The mass % concentration of the fibrous substance in the cathode side electrode catalyst layer is higher than the mass % concentration of the fibrous substance in the anode side electrode catalyst layer.
Membrane electrode assembly.

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