JP7613497B2 - Electrode catalyst layer and membrane electrode assembly - Google Patents

Description

This disclosure relates to an electrode catalyst layer and a membrane electrode assembly suitable for an anion exchange membrane water electrolysis cell.

In recent years, the movement to use hydrogen as a CO2 _- free energy that can be produced from various resources as a primary energy source has been accelerating in order to achieve carbon neutrality. As a method for producing such hydrogen, a method of electrolyzing water using renewable energy is considered to be promising. As a method for electrolyzing water, alkaline water electrolysis and proton exchange membrane (PEM) water electrolysis are generally known. Alkaline water electrolysis is easy to scale up but has poor adaptability to fluctuations, while PEM water electrolysis has the disadvantage of being excellent in adaptability to fluctuations but using expensive precious metal materials. Therefore, anion exchange membrane (AEM) water electrolysis, which is expected to reduce material costs while maintaining the adaptability of PEM water electrolysis, has attracted attention.

AEM water electrolysis cells generally include a pair of main electrodes and a membrane electrode assembly disposed between the pair of main electrodes. The membrane electrode assembly includes a laminate having an anode side electrode catalyst layer disposed on one side of a solid polymer electrolyte membrane that conducts hydroxide ions, and a cathode side electrode catalyst layer disposed so as to sandwich the solid polymer electrolyte membrane together with the anode side electrode catalyst layer.

When external electricity is applied to the anode and cathode electrode catalyst layers, a water electrolysis reaction occurs on the catalyst in each electrode catalyst layer, producing oxygen at the anode and hydrogen at the cathode.

The membrane electrode assembly is obtained by forming an electrode catalyst layer on one side of a solid polymer electrolyte membrane, for example, by using a coating method (see, for example, Patent Document 1 below).

WO 2017/159820

However, the membrane electrode assembly described in Patent Document 1 may develop cracks in the electrode catalyst layer, and there is room for improvement in terms of preventing cracks from developing. The development of cracks may impede the transfer of hydroxide ions and electrons to the catalyst, reducing the water electrolysis performance.

This disclosure has been made in consideration of the above problems, and aims to provide an electrode catalyst layer and a membrane electrode assembly suitable for a water electrolysis cell that can suppress the occurrence of cracks in the electrode catalyst layer and has improved water electrolysis performance.

[1] A catalyst comprising catalyst particles, a polymer electrolyte having hydroxide ion conductivity, and a coated fiber;
The coated fiber has a fiber core and an outer circumferential coating layer that covers an outer circumferential surface of the fiber core,
The outer peripheral coating layer comprises a polymer having a Lewis acidic functional group.
[2] The coated fiber has an intermediate coating layer between the fiber core and the outer coating layer,
The electrode catalyst layer according to claim [1] or [2], wherein the intermediate coating layer contains a polymer compound having a Lewis basic functional group.
[3] The electrode catalyst layer according to [2], wherein the polymer compound of the intermediate coating layer has an azole structure.
[4] The electrode catalyst layer according to [3], wherein the polymer compound of the intermediate coating layer is polybenzimidazole.
[5] The electrode catalyst layer according to any one of [1] to [4], wherein the fiber core is a conductive fiber.
[6] The electrode catalyst layer according to [5], wherein the conductive fibers are carbon fibers.
[7] The electrode catalyst layer according to [6], wherein the carbon fiber is vapor grown carbon fiber (VGCF).
[8] The electrode catalyst layer according to any one of [1] to [7], wherein the average fiber diameter of the coated fibers is within a range of 0.1 to 1 μm.
[9] The electrode catalyst layer according to any one of [1] to [8], which is for an anion exchange membrane water electrolysis cell.
[10] The electrode catalyst layer according to [9], which is a cathode-side electrode catalyst layer.
[11] A fuel cell comprising: a polymer electrolyte membrane; and an anode-side electrode catalyst layer and a cathode-side electrode catalyst layer sandwiching the polymer electrolyte membrane;
The cathode-side electrode catalyst layer is the electrode catalyst layer according to any one of [1] to [10].

The electrode catalyst layer and membrane electrode assembly of one embodiment of the present invention can provide a laminate for a water electrolysis cell that has sufficient mechanical strength and improved water electrolysis performance using a simple method.

FIG. 1 is an exploded perspective view illustrating a membrane electrode assembly having an electrode catalyst layer for a water electrolysis cell according to one embodiment of the present invention. FIG. 2 is a cross-sectional schematic diagram of a cathode electrode catalyst layer 2 and an anode electrode catalyst layer 3 according to one embodiment of the present invention. FIG. 2 is a cross-sectional view along the axis of a covered fiber according to one embodiment of the present invention.

<Embodiment>
Hereinafter, an embodiment of the present invention will be described with reference to the accompanying drawings.
Here, the drawings are schematic, and the relationship between thickness and planar dimensions, the thickness ratio of each layer, etc. differ from the actual ones. Furthermore, the embodiments shown below are merely examples of configurations for embodying the technical idea of the present invention, and the technical idea of the present invention is not limited to the materials, shapes, structures, etc. of the components described below. The technical idea of the present invention can be modified in various ways within the technical scope defined by the claims.

[Membrane Electrode Assembly]
As shown in FIG. 1 , the membrane electrode assembly 11 of the present embodiment includes a polymer electrolyte membrane 1, and an anode-side electrode catalyst layer 3 (shown on the upper side in FIG. 1 ) and a cathode-side electrode catalyst layer 2 (shown on the lower side in FIG. 1 ) that sandwich the polymer electrolyte membrane 1 from above and below.

(Cathode side electrode catalyst layer 2)
As shown in FIG. 2, the cathode side electrode catalyst layer 2 includes catalyst particles 12, a polymer electrolyte 13, and coated fibers 14.

(Catalyst particles)
The catalyst particles 12 are particulate. When improvement of the activity of the catalyst particles is required, the average particle size of the catalyst particles is preferably 20 nm or less, and more preferably 5 nm or less. When stabilization of the activity of the catalyst particles is required, the average particle size of the catalyst particles is preferably 0.5 nm or more, and more preferably 1 nm or more.

The catalyst particles in the cathode electrode catalyst layer are catalysts for carrying out reduction reactions. The constituent materials of the catalyst particles can be platinum group elements, metals, alloys, oxides, double oxides, etc. of these metals. Examples of platinum group elements include platinum, palladium, ruthenium, iridium, rhodium, and osmium, and examples of metals include iron, lead, copper, chromium, cobalt, nickel, manganese, vanadium, molybdenum, gallium, and aluminum. Note that a double oxide is an oxide made of two types of metals.

The catalyst particles of the cathode-side electrode catalyst layer may be supported on a carrier 15. The carrier is a particle that is conductive and is not eroded by the catalyst material. An example of the carrier is carbon particles. When an expansion of the electron conduction path is required, the particle size of the carrier is preferably 10 nm or more. When a reduction in the resistance value of the electrode catalyst layer is required, or an increase in the amount of catalyst particles supported is required, the particle size of the carrier is preferably 1000 nm or less, and more preferably 100 nm or less.
An example of the material of the carrier is at least one selected from the group consisting of carbon black, graphite, activated carbon, and fullerene. The carbon black is at least one selected from the group consisting of acetylene black, furnace black, and ketjen black.

(Polymer electrolyte)
The polymer electrolyte 13 is a polymer electrolyte having hydroxide ion conductivity, and is a so-called anion exchange membrane. The polymer electrolyte is not particularly limited as long as it conducts hydroxide ions. For example, there are fluorine-based anion exchange electrolytes and hydrocarbon-based anion exchange electrolytes. Examples of fluorine-based electrolytes include perfluorocarbon-based electrolytes, and examples of hydrocarbon-based electrolytes include styrene-based electrolytes and acrylic-based electrolytes.

(Coated fiber 14)
3, the coated fiber 14 has a fiber core 14A and an outer peripheral coating layer 14C that coats the outer peripheral surface of the fiber core 14A. It is preferable to have an intermediate coating layer 14B between the fiber core 14A and the outer peripheral coating layer 14C.

(Fiber core 14A)
The fiber core 14A is not particularly limited, and may be a non-conductive fiber, but is preferably a conductive fiber. Examples of non-conductive fibers include polymer fibers obtained by electrospinning and cellulose nanofibers. Examples of conductive fibers include carbon fibers such as carbon fibers, carbon nanotubes, and carbon nanohorns, and conductive polymer nanofibers. That is, the fiber core may be solid or hollow.

Among the conductive fiber cores, the fiber core 14A is preferably a carbon fiber, and in particular, vapor grown carbon fiber (VGCF) is suitable.
The average fiber diameter of the fiber core may be from 45 nm to 0.8 μm. The average fiber diameter of the fiber core can be measured in the same manner as the average fiber diameter of the coated fibers.

(Peripheral coating layer 14C)
The outer coating layer 14C contains a polymer having a Lewis acidic functional group in its molecular structure.
Lewis acidic functional groups refer to functional groups that can accept an electron pair. Lewis acidic functional groups may be functional groups that can donate a proton. Examples of Lewis acidic functional groups include -SO ₃ H, - COOH. An example of a polymer containing a Lewis acid functional group is a cation exchange electrolyte having proton conductivity. An example of a fluorine-based cation exchange electrolyte is Nafion (registered trademark, manufactured by DuPont Co., Ltd.). At least one selected from the group consisting of Flemion (registered trademark, manufactured by Asahi Glass Co., Ltd.), Aciplex (registered trademark, manufactured by Asahi Kasei Corporation), and Gore-Select (registered trademark, manufactured by Gore Japan, LLC). An example of the hydrocarbon-based cation exchange electrolyte is at least one selected from the group consisting of sulfonated polyether ketone, sulfonated polyether sulfone, sulfonated polyether ether sulfone, sulfonated polysulfide, and sulfonated polyphenylene. be.

When the outer coating layer 14C contains a polymer containing a Lewis acid functional group in its molecular structure, a polymer electrolyte having hydroxide ion conductivity is adsorbed to the polymer (outer coating layer) having Lewis acid functional groups, and thus the occurrence of cracks in the electrode catalyst layer is suppressed, and durability is improved by improving the mechanical properties of the electrode catalyst layer. In addition to the improvement in mechanical properties, a conductive path for hydroxide ions is formed by adsorbing a polymer electrolyte having hydroxide ion conductivity onto the coating fiber 14, and high electrolysis performance is obtained. In other words, the polymer electrolyte having hydroxide ion conductivity can be strongly adsorbed to the coating fiber 14 via the outer coating layer 14C containing a polymer having Lewis acid functional groups, rather than directly adsorbing a polymer electrolyte having hydroxide ion conductivity onto the fiber core 14A.

On the other hand, if the outer coating layer 14C containing a polymer with Lewis acidic functional groups is not used, the mechanical properties of the electrode catalyst layer are insufficient, making it difficult to suppress the occurrence of cracks and making it impossible to improve durability. In addition, since the polymer electrolyte with hydroxide ion conductivity is dispersed, the formation of a conduction path for hydroxide ions is inhibited, making it impossible to improve electrolysis performance.

It is preferable that the outer coating layer 14C contains a polymer containing a Lewis acid functional group as a main component. The main component means that it accounts for 50% by mass or more. The mass proportion of the polymer containing a Lewis acid functional group in the outer coating layer 14C may be 70% by mass, 80% by mass, or 90% by mass.

There is no particular limit to the thickness of the outer coating layer 14C, but it may be, for example, approximately 20 to 100 nm.

(Intermediate coating layer 14B)
The intermediate coating layer 14B contains a polymer having a Lewis basic functional group in its molecular structure. The Lewis basic functional group refers to a functional group capable of donating an electron pair. Examples of the Lewis basic functional group are an imide structure and/or an azole structure. An example of a polymer having an imide structure is a polyimide such as an aromatic polyimide. An example of the azole structure is a benzoazole structure, and examples of the benzoazole structure are a benzimidazole structure, a benzoxazole structure, and a benzothiazole structure. Examples of these polymers include polybenzimidazole, polybenzoxazole, and polyenzothiazole.

Polybenzimidazole is one of the polymers suitable for coating carbon fibers because its molecular structure has a basic portion that exhibits interaction with the polymer (peripheral coating layer) containing Lewis acidic functional groups contained in the peripheral coating layer 14C, and a benzene ring portion that exhibits interaction with the carbon of the fiber core 14A. That is, by having the intermediate coating layer 14B between the fiber core 14A, which is a carbon fiber, and the peripheral coating layer 14C, the adhesion between the fiber core 14A, which is a carbon fiber, and the intermediate coating layer 14B is improved. In addition, the peripheral coating layer 14C containing a polymer containing a Lewis acidic functional group is provided on the intermediate coating layer 14B containing a polymer having a Lewis basic functional group, so that the adhesion between the intermediate coating layer 14B and the peripheral coating layer 14C is also improved.
As a result, the polymer electrolyte having hydroxide ion conductivity can be strongly adsorbed onto the coated fiber 14 .

It is preferable that the intermediate coating layer 14B contains a polymer containing a Lewis basic functional group as a main component. The main component means that it accounts for 50% by mass or more. The mass proportion of the polymer containing a Lewis basic functional group in the intermediate coating layer 14B may be 70% by mass, 80% by mass, or 90% by mass.

It is preferable that the intermediate coating layer 14B is mainly composed of polybenzimidazole. The main component means that it accounts for 50% by mass or more. The mass proportion of polybenzimidazole may be 70% by mass, 80% by mass, or 90% by mass.

There are no particular limitations on the polybenzimidazole as long as it contains a benzimidazole structure in the repeating unit.

There is no particular limit to the thickness of the intermediate coating layer 14B, but it may be, for example, approximately 1 to 5 nm.

(Mechanism of action)
The inventors of the present application have confirmed that the electrode catalyst layer having the above-mentioned configuration has high mechanical properties and also exhibits high electrolytic performance. The detailed mechanism is speculated as follows, but the present invention is not limited to the following mechanism.

In the coated fiber 14, a polymer electrolyte having hydroxide ion conductivity is adsorbed to the polymer having Lewis acid functional groups in the outer coating layer 14C, which improves the mechanical properties of the electrode catalyst layer and improves durability, such as suppressing the occurrence of cracks in the electrode catalyst layer. In addition to improving the mechanical properties, a conductive path for hydroxide ions is formed by adsorbing the side polymer electrolyte having hydroxide ion conductivity onto the coated fiber 14, resulting in high electrolysis performance.

On the other hand, if a polymer having a Lewis acid functional group is not used in the outer coating layer 14C, the mechanical properties of the electrode catalyst layer are insufficient, making it difficult to suppress the occurrence of cracks and making it impossible to improve durability. In addition, since a polymer electrolyte having hydroxide ion conductivity is dispersed, the formation of a conduction path for hydroxide ions is inhibited, making it impossible to improve electrolysis performance.

In the coated fiber 14, by having an intermediate coating layer 14B between the fiber core 14A and the outer coating layer 14C, the polymer containing Lewis acidic functional groups contained in the outer coating layer 14C is adsorbed to the polymer containing Lewis basic functional groups in the intermediate coating layer 14B, improving the adhesion of the outer coating layer 14C and further improving durability. In addition, by using a polymer mainly composed of polybenzimidazole for the intermediate coating layer 14B, the adhesion between the carbon fiber of the fiber core 14A and the outer coating layer 14C is improved, resulting in further improvement in durability.

In addition, if the fiber core 14A of the coated fiber 14 is vapor-grown carbon fiber (VGCF), it is assumed that the carbon surface is more likely to have a tree-ring structure in which the carbon surface is stacked in a concentric cylindrical shape, which tends to increase the affinity with the intermediate coating layer 14B.

The average fiber diameter of the coating fiber 14 is not particularly limited, but is preferably 50 nm to 1 μm, and more preferably 0.1 to 0.4 μm. In this case, the occurrence of cracks in the electrode catalyst layer is more likely to be suppressed. In addition, the adhesion between the polymer electrolyte membrane and the electrode catalyst layer can be improved. This makes it possible to suppress the occurrence of voids due to peeling between the polymer electrolyte membrane and the electrode catalyst layer, and to further suppress the increase in resistance of the membrane electrode assembly caused by these voids. If the average fiber diameter of the coating fiber is less than 50 nm, it is estimated that the mechanical properties may be difficult to improve. Also, if the average fiber diameter of the coating fiber is more than 1 μm, it is estimated that the coating fiber may not be able to disperse as an ink.

The average fiber diameter refers to the average diameter measured for the cross-section of the exposed coated fiber when the cross-section of the electrode catalyst layer is observed using a scanning electron microscope (SEM). When the coated fiber is cut diagonally to its major axis, an elliptical cross-section is obtained, in which case the diameter refers to the diameter of a perfect circle fitted along the minor axis of the ellipse. When the cross-section of the electrode catalyst layer is observed using an SEM, the surface of the coated fiber may be exposed instead of the cross-section of the coated fiber. In that case, the diameter refers to the width of the fiber perpendicular to the major axis of the exposed coated fiber. The average fiber diameter of the coated fiber refers to the arithmetic mean value of the fiber diameters obtained by similar measurements at at least 20 observation points.

To expose the cross section of the electrode catalyst layer, known methods such as ion milling and ultramicrotome can be used.

The average fiber length of the coated fibers is not particularly limited, but is preferably 500 nm or more, and more preferably 1 μm or more. In this case, the coated fibers become entangled, forming pores of appropriate size in the electrode catalyst layer, and the mechanical properties of the electrode catalyst layer can be improved. However, the average fiber length of the coated fibers is preferably 100 μm or less, and more preferably 40 μm or less.

The average fiber length of the coated fibers refers to the arithmetic average value of the fiber lengths obtained by measuring the lengths of at least 10 coated fibers. The average fiber length of the coated fibers in the electrode catalyst layer can be determined by performing particle size distribution measurement using a solution in which the electrode catalyst layer is dissolved in a solvent. Specifically, the correlation between the average fiber length determined by an electron microscope and the peak position in the particle size distribution measurement is determined in advance, and the average fiber length of the coated fibers in the electrode catalyst layer is determined based on this correlation and the peak position determined by the particle size distribution measurement.

There are no particular limitations on the composition of the cathode electrode catalyst layer 2, but the mass of the catalyst particles can be 1, the mass of the polymer electrolyte (solid content) can be 0.1 to 0.4, and the mass of the coated fiber can be 0.025 to 0.25.

(Anode side electrode catalyst layer 3)
2, the anode electrode catalyst layer 3 includes catalyst particles 22 and a polymer electrolyte 23. The anode electrode catalyst layer 3 includes the coated fibers 14, similar to the cathode electrode catalyst layer 2, thereby improving the mechanical properties of the electrode catalyst layer and improving durability. In addition to the improved mechanical properties, the polymer electrolyte having hydroxide ion conductivity is adsorbed onto the coated fibers 14, forming a conduction path for hydroxide ions, thereby achieving high electrolysis performance.

(Catalyst particles)
The catalyst particles 22 are particulate. When improvement of the activity of the catalyst particles is required, the average particle size of the catalyst particles is preferably 20 nm or less, and more preferably 5 nm or less. When stabilization of the activity of the catalyst particles is required, the average particle size of the catalyst particles is preferably 0.5 nm or more, and more preferably 1 nm or more.

The catalyst particles in the anode-side electrode catalyst layer are catalysts for carrying out oxidation reactions. The constituent materials of the catalyst particles can be metals in the platinum group, metals other than the platinum group, or alloys, oxides, double oxides, and carbides of these metals. Among these, ruthenium, rhodium, palladium, iridium, platinum, and alloys containing at least one of these metals are preferred because of their high catalytic activity. The catalyst particles may be one of the above examples, or a combination of two or more of them.

The catalyst particles of the anode-side electrode catalyst layer may be supported on a carrier 25. The carrier is a particle that is conductive and is not eroded in an oxidizing atmosphere. An example of the carrier is an oxide particle containing titanium, tin, or zirconium. When an expansion of the electron conduction path is required, the particle size of the carrier is preferably 10 nm or more. When a reduction in the resistance value of the electrode catalyst layer is required, or an increase in the amount of catalyst particles supported is required, the particle size of the carrier is preferably 1000 nm or less, and more preferably 100 nm or less.

(Anode side polymer electrolyte)
The polymer electrolyte 23 may be a polymer electrolyte 23 having hydroxide ion conductivity similar to that of the polymer electrolyte in the cathode side electrode catalyst layer.

(Polymer electrolyte membrane 1)
An example of the polymer electrolyte membrane 1 is a polymer electrolyte membrane having hydroxide ion conductivity, that is, a so-called anion exchange resin membrane. There are no particular limitations as long as the electrolyte membrane is a fluorine-based anion exchange electrolyte membrane or a hydrocarbon-based anion exchange electrolyte membrane. The fluorine-based electrolyte membrane is a perfluorocarbon-based electrolyte membrane, and the hydrocarbon-based electrolyte membrane is a Examples of the electrolyte membrane include a styrene-based electrolyte membrane and an acrylic-based electrolyte membrane.

When it is required to improve the adhesion between the electrode catalyst layer and the polymer electrolyte membrane, if the constituent material of the polymer electrolyte of the electrode catalyst layer is a fluorine-based polymer electrolyte, it is preferable that the constituent material of the polymer electrolyte membrane is also a fluorine-based polymer electrolyte. Furthermore, if the constituent material of the polymer electrolyte of the electrode catalyst layer is a hydrocarbon-based polymer electrolyte, it is preferable that the constituent material of the polymer electrolyte membrane is also a hydrocarbon-based polymer electrolyte. Furthermore, it is preferable that the constituent material of the polymer electrolyte membrane is the same as the constituent material of the polymer electrolyte of the electrode catalyst layer.

[Method for producing membrane electrode assembly]
Next, an example of a method for producing the membrane electrode assembly having the above configuration will be described.

The membrane electrode assembly is manufactured by a method including the following first to third steps.

The first step is to form a polymer (intermediate coating layer) containing Lewis basic functional groups such as polybenzimidazole on the surface of vapor grown carbon fiber (VGCF) or the like.

The second step is to coat the surface of the conductive fiber core coated with the intermediate coating layer obtained in the first step with a polymer (outer coating layer) containing Lewis acid functional groups to form a coated fiber.

The third step is a step of producing a cathode-side catalyst ink containing catalyst particles, a polymer electrolyte, the coated fiber obtained in the second step, and a solvent, and a step of producing an anode-side catalyst ink containing catalyst particles, a polymer electrolyte, and a solvent.

The fourth step is to apply the anode side catalyst ink and cathode side catalyst ink obtained in the third step onto the polymer electrolyte membrane and dry the solvent to form an anode side electrode catalyst layer and a cathode side electrode catalyst layer on both sides of the polymer electrolyte membrane.

Detailed Description
(First step)
A polymer containing a Lewis basic functional group such as polybenzimidazole is dissolved or dispersed in a solvent to prepare a polymer dispersion/solution. An example of the solvent is dimethylacetamide. A fiber core is added to this polymer dispersion/solution, and ultrasonic dispersion treatment is performed as necessary. Then, the solid content is collected by filtration and dried, so that an intermediate coating layer can be formed on the surface of the fiber core.

(Second step)
A polymer containing a Lewis basic functional group such as a cation exchange electrolyte is dissolved or dispersed in a solvent to prepare a polymer dispersion/solution. An example of the solvent is alcohol. The conductive fiber core coated with the intermediate coating layer obtained in the first step is added to this polymer dispersion/solution, and ultrasonic dispersion treatment is performed as necessary. Then, the solid content is collected by filtration and dried, so that a peripheral coating layer can be formed on the surface of the intermediate coating layer.

(Third process)
The solvent constituting the catalyst ink does not corrode the catalyst particles, the polymer electrolyte, and the coated fiber, and dissolves the polymer electrolyte or disperses it as a fine gel. An example of the solvent constituting the catalyst ink is at least one selected from the group consisting of alcohols, ketone-based solvents, ether-based solvents, and polar solvents. An example of the alcohols is at least one selected from the group consisting of methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, 2-butanol, isobutyl alcohol, tert-butyl alcohol, and pentanol. An example of the ketone-based solvent is at least one selected from the group consisting of acetone, methyl ethyl ketone, pentanone, methyl isobutyl ketone, heptanone, cyclohexanone, methyl cyclohexanone, acetonyl acetone, and diisobutyl ketone. An example of the ether-based solvent is at least one selected from the group consisting of tetrahydrofuran, dioxane, diethylene glycol dimethyl ether, anisole, methoxytoluene, and dibutyl ether. An example of the polar solvent is at least one selected from the group consisting of dimethylformamide, dimethylacetamide, N-methylpyrrolidone, ethylene glycol, diethylene glycol, diacetone alcohol, and 1-methoxy-2-propanol. The solvent constituting the catalyst ink may contain water, which has a high affinity for the polymer electrolyte.

When improved dispersibility of catalyst particles is required, it is preferable that the catalyst ink contains a dispersant. Examples of dispersants are anionic surfactants, cationic surfactants, amphoteric surfactants, and nonionic surfactants. When improved dispersibility of the catalyst ink is required, it is preferable to carry out a dispersion treatment in the production of the catalyst ink. Examples of dispersion treatments are stirring with a ball mill and a roll mill, stirring with a shear mill, stirring with a wet mill, stirring by applying ultrasonic waves, and stirring with a homogenizer.

When it is required to suppress the occurrence of cracks on the surface of the electrode catalyst layer, the solid content of the catalyst ink is preferably 50% by mass or less. When it is required to improve the film formation rate of the electrode catalyst layer, the solid content of the catalyst ink is preferably 1% by mass or more.

(Fourth step)
Examples of the method for applying the catalyst ink onto the substrate include a doctor blade method, a dipping method, a screen printing method, or a roll coating method. The substrate to which the catalyst ink is applied is a transfer sheet. Examples of the constituent material of the transfer sheet include a fluorine-based resin or an organic polymer compound other than a fluorine-based resin. Examples of the fluorine-based resin include an ethylene tetrafluoroethylene copolymer (ETFE), a tetrafluoroethylene-hexafluoropropylene copolymer (FEP), a tetrafluoroperfluoroalkylvinyl ether copolymer (PFA), or a polytetrafluoroethylene (PTFE). Examples of the organic polymer compound include a polyimide, a polyethylene terephthalate, a polyamide, a polysulfone, a polyethersulfone, a polyphenylene sulfide, a polyether ether ketone, a polyetherimide, a polyarylate, and a polyethylene naphthalate.

In the above embodiment, the coated fiber is provided on the electrode catalyst layer on the cathode side, but it can also be provided on the electrode catalyst layer on the anode side.

The electrode catalyst layer and membrane electrode assembly for the water electrolysis device in this embodiment will be described below with specific examples and comparative examples, but this embodiment is not limited to the following examples and comparative examples.

<Other Effects>
According to this embodiment, it is possible to manufacture a membrane electrode assembly for a water electrolysis system having an electrode catalyst layer with high mechanical properties and excellent electrolysis performance and durability, without using any complicated steps.

Example 1
[Production of coated fibers]
Polybenzimidazole of the following formula was dissolved in dimethylacetamide to prepare a polybenzimidazole dispersion. Next, carbon fiber (VGCF-H (registered trademark), Showa Denko K.K.: vapor-grown carbon fiber) with an average fiber diameter of 0.15 μm was added to the polybenzimidazole dispersion, and ultrasonic dispersion treatment was performed. After that, filtration and drying were performed to obtain polybenzimidazole-coated carbon fiber. The thickness of the polybenzimidazole coating layer was as thin as a few nm, and the average fiber diameter of the obtained polybenzimidazole-coated carbon fiber was substantially the same as that of the core fiber.

A fluorine-based polymer electrolyte (Nafion (registered trademark) dispersion) was dissolved as a cation exchange electrolyte in a mixed solvent of ultrapure water and 1-propanol to prepare a fluorine-based polymer electrolyte dispersion. The volume ratio of ultrapure water to 1-propanol was 1:1. The solid content in the dispersion was adjusted to 1 mass%. Next, polybenzimidazole-coated carbon fiber was added to this fluorine-based polymer electrolyte dispersion, and ultrasonic dispersion processing was performed. After that, filtration and drying were performed to obtain a coated fiber with carbon fiber as the conductive fiber core, polybenzimidazole as the intermediate coating layer, and a fluorine-based polymer electrolyte as the outer coating layer. The average fiber diameter of the obtained coated fiber was about 0.25 μm.

[Preparation of anode-side catalyst ink]
The catalyst particles and polymer electrolyte shown below were mixed in a solvent and dispersed for 30 minutes using a planetary ball mill to prepare a catalyst ink. A mixed solvent of ultrapure water and 1-propanol was used as the solvent for the catalyst ink. The volume ratio of ultrapure water to 1-propanol was 1:1. The catalyst ink was adjusted so that the solid content in the catalyst ink was 10% by mass.
Catalyst particles: iridium oxide Polymer electrolyte: hydrocarbon-based anion exchange electrolyte Blending ratio: In the catalyst ink, the mass of the catalyst particles was 1, and the mass of the polymer electrolyte (solid content) was 0.2.

[Preparation of cathode-side catalyst ink]
The catalyst particles, polymer electrolyte, and coated fibers shown below were mixed in a solvent and dispersed for 30 minutes using a planetary ball mill to prepare a catalyst ink. A mixed solvent of ultrapure water and 1-propanol was used as the solvent for the catalyst ink. The volume ratio of ultrapure water to 1-propanol was 1:1. The catalyst ink was adjusted so that the solid content in the catalyst ink was 10% by mass.
Catalyst particles: Pt-supported carbon particles Ionomer: Hydrocarbon-based anion exchange electrolyte Blending ratio: In the catalyst ink, the mass of the catalyst particles was 1, the mass of the polymer electrolyte (solid content) was 0.3, and the mass of the coated fiber was 0.05.

[Preparation of Membrane Electrode Assembly]
The prepared cathode catalyst ink was applied to one side of a polymer electrolyte membrane (hydrocarbon-based anion exchange electrolyte membrane) using a die coater to form a rectangular coating film measuring 50 mm in length and 50 mm in width. The amount of the cathode catalyst ink applied was set to an amount that would result in a catalyst amount of 0.5 mg/cm2. A drying process was then performed using an oven at 60°C to volatilize the dispersion medium contained in the coating film, thereby forming a cathode electrode catalyst layer.
Next, the prepared anode side catalyst ink was applied to the surface of the polymer electrolyte membrane opposite to the surface on which the cathode side electrode catalyst layer was formed, to form a rectangular coating film measuring 50 mm long x 50 mm wide. The amount of the anode side catalyst ink applied was set to an amount that would result in 0.5 mg/cm2 of catalyst. Then, a drying process was performed using an oven at 60°C to volatilize the dispersion medium contained in the coating film, thereby forming an anode side electrode catalyst layer, thereby obtaining a membrane electrode assembly. Both electrode catalyst layers in Example 1 were free of cracks and did not peel off from the polymer electrolyte membrane.

Example 2
Except for using carbon fibers having an average fiber diameter of 0.8 μm as the fiber core in the covering fiber, a membrane electrode assembly of Example 2 was obtained in the same manner as in Example 1. Both electrode catalyst layers of Example 2 were free of cracks and did not peel off from the polymer electrolyte membrane.

<Comparative Example 1>
A membrane/electrode assembly of Comparative Example 1 was obtained in the same manner as in Example 1, except that carbon fibers (VGCF-H (registered trademark), Showa Denko K.K.: vapor grown carbon fibers) having an average fiber diameter of 0.15 μm were used without being coated instead of the coated fibers.

In Comparative Example 1, when the cathode-side electrode catalyst layer was formed, there were no cracks and no peeling from the polymer electrolyte membrane. However, when the anode-side electrode catalyst layer was formed, cracks occurred in the cathode-side electrode catalyst layer and partial peeling from the polymer electrolyte membrane was observed.

<Comparative Example 2>
A membrane/electrode assembly of Comparative Example 2 was obtained in the same manner as in Example 1, except that no coated fiber was used.
In Comparative Example 2, significant cracks were generated during the formation of the cathode-side electrode catalyst layer, and significant peeling from the polymer electrolyte membrane was observed, making it impossible to apply the anode-side catalyst ink.

When the membrane electrode assemblies obtained in Examples 1-2 and Comparative Examples 1-2 were immersed in water, the electrode catalyst layers of both Examples 1-2 did not peel off from the polymer electrolyte membrane, but the cathode side electrode catalyst layer of Comparative Example 1 peeled off from the polymer electrolyte membrane. In addition, the water electrolysis performance was evaluated using each membrane electrode assembly, and the membrane electrode assemblies obtained in Examples 1-2 showed good electrolysis performance, but the membrane electrode assembly obtained in Comparative Example 1 showed a significant decrease in electrolysis performance due to repeated electrolysis. From the results of the electrolysis performance of the membrane electrode assemblies produced in the Examples and the membrane electrode assemblies produced in the Comparative Examples, it was confirmed that the adhesion strength between the cathode side electrode catalyst layer and the electrolyte membrane was sufficient in the membrane electrode assemblies produced in the Examples, and good electrolysis performance was obtained.

Although the embodiments of the present invention have been described in detail above, the present invention is not limited to the above embodiments, and even if the changes are made within the scope of the gist of the present invention, they are included in the present invention.

1...polymer electrolyte membrane, 2...cathode side electrode catalyst layer, 3...anode side electrode catalyst layer, 11...membrane electrode assembly.

Claims (10)

The present invention relates to a catalyst particle, a polymer electrolyte having hydroxide ion conductivity, and a coated fiber.
The coated fiber has a fiber core and an outer circumferential coating layer that covers an outer circumferential surface of the fiber core,
The outer peripheral coating layer comprises a polymer having a Lewis acidic functional group, and the outer peripheral coating layer comprises an electrode catalyst layer for an anion exchange membrane water electrolysis cell . The coated fiber has an intermediate coating layer between the fiber core and the outer coating layer,
The electrode catalyst layer according to claim 1 , wherein the intermediate coating layer contains a polymer compound having a Lewis basic functional group. The electrode catalyst layer according to claim 2, wherein the polymer compound of the intermediate coating layer has an azole structure. The electrode catalyst layer according to claim 2, wherein the polymer compound of the intermediate coating layer is polybenzimidazole. The electrode catalyst layer according to claim 1 or 2, wherein the fiber core is a conductive fiber. The electrode catalyst layer according to claim 5, wherein the conductive fibers are carbon fibers. The electrode catalyst layer according to claim 6, wherein the carbon fibers are vapor grown carbon fibers (VGCF). The electrode catalyst layer according to claim 1 or 2, wherein the average fiber diameter of the coated fibers is within the range of 50 nm to 1 μm. The electrode catalyst layer according to claim 1 or 2 , which is a cathode-side electrode catalyst layer. a polymer electrolyte membrane; and an anode-side electrode catalyst layer and a cathode-side electrode catalyst layer sandwiching the polymer electrolyte membrane;
The cathode-side electrode catalyst layer is the electrode catalyst layer according to claim 1 .

Images