JP2022167439A - Membrane electrode assembly and manufacturing method thereof - Google Patents

Abstract

To provide a membrane electrode assembly capable of suppressing the generation of radicals causing a deterioration in a solid polymer electrolyte.SOLUTION: A membrane electrode assembly includes an electrolyte membrane including a solid polymer electrolyte, an anode catalyst layer bonded to one side of the electrolyte membrane, a cathode catalyst layer bonded to the other side of the electrolyte membrane, and a first fine particle arranged at the interface between the electrolyte membrane and the anode catalyst layer, the first fine particle comprising a first compound having a function to decompose hydrogen peroxide. Such a membrane electrode assembly can be obtained by: adhering a first fine particle comprising a first compound having a function to decompose hydrogen peroxide, to the surface of the electrolyte membrane and/or the surface of the anode catalyst layer on an electrolyte membrane side; forming the anode catalyst layer on the surface of the electrolyte membrane on an anode side such that the side where the first fine particle is adhered faces inside and forming a cathode catalyst layer on the other side of the electrolyte membrane to form a laminate; and heat-treating the laminate.SELECTED DRAWING: Figure 1

Description

TECHNICAL FIELD The present invention relates to a membrane electrode assembly and its manufacturing method, and more particularly to a membrane electrode assembly containing an additive that has a function of suppressing the generation of radicals that cause deterioration of a solid polymer electrolyte, and its manufacturing method.

BACKGROUND ART Membrane electrode assemblies (MEAs) are used at sites where electrochemical reactions occur in soda electrolyzers, water electrolyzers, fuel cells, and the like. MEA is said to be deteriorated by attacking the electrolyte by radicals directly generated by direct reaction of oxygen and hydrogen or by electrochemical reaction, or by radicals generated via hydrogen peroxide.
For example, in a fuel cell, it is known that radical attack causes an increase in the resistance of the electrolyte membrane, an increase in cross-leakage, and a shortened lifetime due to thinning. Furthermore, deterioration products generated by radical attack may cause catalyst poisoning, resulting in deterioration of electrolytic performance and battery performance.

In order to solve this problem, various proposals have been conventionally made.
For example, Patent Document 1 discloses a membrane electrode assembly comprising an electrolyte membrane and electrodes composed of a catalyst layer and a diffusion layer bonded to both sides of the electrolyte membrane, and in which the periphery of the diffusion layer is impregnated with cerium nitrate. It is

In the same document,
(A) deterioration of the electrolyte membrane due to radicals occurs notably in the periphery of the MEA, but not in the center; and
(B) Addition of Ce to the edge of the diffusion layer makes it possible to efficiently remove harmful hydrogen peroxide from the cell without lowering power generation performance.

In Patent Document 2, in a fuel cell in which a plurality of single cells are horizontally stacked (that is, a fuel cell in which a plurality of single cells are vertically arranged),
(a) the anode-side water-repellent layer and the cathode-side water-repellent layer each contain CeO ₂ as a radical inhibitor, and
(b) Fuel in which the concentration of the radical inhibitor contained in the vertically upper portions of the anode-side water-repellent layer and the cathode-side water-repellent layer (concentration of eluted Ce ions) is higher than the concentration of the radical inhibitor contained in the vertically lower portions. A battery is disclosed.

In the same document,
(A) In a fuel cell in which a plurality of single cells are stacked horizontally, even if scavenging control for discharging residual water is executed after the fuel cell is stopped, the amount of residual water in the vertically lower part of the MEA is more than the residual water content of the upper part,
(B) When the radical inhibitor is uniformly contained in the water-repellent layer, deterioration due to hydroxyl radicals cannot be suppressed because the amount of the radical inhibitor eluted is small in the vertically upper portion of the MEA where the residual water content is small. On the other hand, the proton transfer resistance of the electrolyte membrane increases because the amount of the radical inhibitor eluted becomes excessive at the vertically lower portion of the MEA where the residual water content is large, and
(C) When the concentration of the radical inhibitor contained in the vertically upper portion of the water-repellent layer is increased to the concentration of the radical inhibitor contained in the vertically lower portion of the water-repellent layer, radicals are inhibited in the vertically upper portion where the residual water content is small. It is described that the elution amount of the radical inhibitor can be suppressed at the vertically lower part where the residual water content is large while ensuring the elution amount of the agent.

In Patent Document 3, in a counter-flow fuel cell in which the flow of fuel gas and the flow of oxidant gas are in opposing directions,
(a) Ag is contained in the anode (anode catalyst layer or anode diffusion layer), and
(b) A fuel cell is disclosed in which the Ag concentration on the fuel gas outlet side is higher than the Ag concentration on the fuel gas inlet side.

In the same document,
(A) In a counter-flow fuel cell, the outlet side of the fuel gas has lower humidity than the inlet side, so deterioration due to radicals is likely to occur;
(B) It is described that if the Ag concentration on the fuel gas outlet side is higher than that on the inlet side, deterioration due to radicals can be suppressed without lowering the power generation performance of the fuel cell.

Furthermore, Patent Document 4 discloses a solid polymer electrolyte membrane in which a second layer containing an ion exchange material and platinum-supported carbon is formed on one surface of a composite membrane in which an expanded PTFE membrane is impregnated with an ion exchange material. It is
This document describes that dispersing platinum-supported carbon in a high-strength solid polymer electrolyte improves the service life of the fuel cell.

Ce ions and Ag have the effect of eliminating radicals. Therefore, as described in Patent Documents 1 to 3, the deterioration of the electrolyte membrane can be suppressed to some extent by adding more Ce ions or Ag to portions where chemical deterioration is severe.
However, Ce ions and Ag ions tend to diffuse in the in-plane direction and/or the film thickness direction of the electrolyte membrane due to the concentration gradient. Therefore, when Ce ions or Ag ions migrate from a location where chemical deterioration is severe, the effect of suppressing deterioration may not be sufficiently exhibited.

Similarly, Pt has the effect of scavenging radicals. Therefore, as described in Patent Document 4, by adding Pt-supporting carbon to a Pt electrolyte membrane, deterioration of the electrolyte membrane can be suppressed to some extent. However, Pt may also ionize in the electrolyte membrane and diffuse in the in-plane direction and/or the film thickness direction due to the concentration gradient. Furthermore, since Pt-supported carbon also has the function of generating hydrogen peroxide, Pt-supported carbon may accelerate deterioration of the electrolyte membrane depending on the position where the Pt-supported carbon is added.

JP 2008-098996 A JP 2018-022570 A JP 2019-160604 A JP 2014-139939 A

The problem to be solved by the present invention is to provide a membrane electrode assembly capable of suppressing the generation of radicals that cause deterioration of the solid polymer electrolyte.
Another problem to be solved by the present invention is to provide a membrane electrode assembly capable of suppressing deterioration of the electrolyte membrane over a long period of time.

In order to solve the above problems, the membrane electrode assembly according to the present invention comprises:
an electrolyte membrane containing a solid polymer electrolyte;
an anode catalyst layer bonded to one surface of the electrolyte membrane;
a cathode catalyst layer bonded to the other surface of the electrolyte membrane;
first fine particles arranged at an interface between the electrolyte membrane and the anode catalyst layer;
The first fine particles are composed of a first compound having a function of decomposing hydrogen peroxide.

A method for manufacturing a membrane electrode assembly according to the present invention comprises:
a first step of attaching first fine particles made of a first compound having a function of decomposing hydrogen peroxide to the surface of the electrolyte membrane and/or the surface of the anode catalyst layer facing the electrolyte membrane;
The anode catalyst layer is formed on the anode-side surface of the electrolyte membrane so that the surface to which the first fine particles are adhered faces the inside, and the cathode catalyst layer is further formed on the other surface of the electrolyte membrane. a second step of forming a laminate;
and a third step of heat-treating the laminate to obtain a membrane electrode assembly according to the present invention.

Degradation of the electrolyte membrane
(a) oxygen permeated from the cathode to the anode reacts with hydrogen on the anode catalyst to produce H ₂ O ₂ ;
(b) H ₂ O ₂ generated on the anode catalyst diffuses into the electrolyte membrane, and Fe mixed as an impurity in the electrolyte membrane reacts with H ₂ O ₂ to generate hydroxyl radicals,
(c) hydroxyl radicals decompose electrolytes;
This is thought to be caused by

Therefore, when the first fine particles having the function of decomposing hydrogen peroxide are present at the interface between the electrolyte membrane and the anode catalyst layer, the H ₂ O ₂ generated at the anode is decomposed into water and oxygen before becoming hydroxyl radicals. be. As a result, the decomposition of the electrolyte membrane is suppressed and the chemical durability of the electrolyte membrane is improved.

1 is a schematic diagram of a fuel cell having a gas channel structure for flowing fuel gas (hydrogen) and oxidant gas (air) in predetermined directions; FIG. 2A is a SEM image of a cross section of the membrane electrode assembly obtained in Example 1. FIG. 2B is a SEM image of the cross section of the membrane electrode assembly obtained in Comparative Example 2. FIG. 3A is an SEM image of the surface of the electrolyte membrane after the anode catalyst layer was peeled off from the membrane electrode assembly obtained in Example 1. FIG. 3B is an SEM image of the surface of the electrolytic membrane after the anode catalyst layer was peeled off from the membrane electrode assembly obtained in Comparative Example 2. FIG. FIG. 4 is a diagram showing application positions of solutions containing various additives and quantitative positions of W or Ce.

An embodiment of the present invention will be described in detail below.
[1. Membrane electrode assembly]
The membrane electrode assembly (hereinafter also referred to as "MEA") according to the present invention is
an electrolyte membrane containing a solid polymer electrolyte;
an anode catalyst layer bonded to one surface of the electrolyte membrane;
a cathode catalyst layer bonded to the other surface of the electrolyte membrane;
First fine particles are arranged at the interface between the electrolyte membrane and the anode catalyst layer.

[1.1. Electrolyte membrane]
The electrolyte membrane contains a solid polymer electrolyte. In the present invention, the type of solid polymer electrolyte contained in the electrolyte membrane is not particularly limited. The solid polymer electrolyte may be either a fluorine-based electrolyte or a hydrocarbon-based electrolyte. The electrolyte membrane may contain any one of these solid polymer electrolytes, or may contain two or more of them.
Also, the type of acid group in the solid polymer electrolyte is not particularly limited. Acid groups include sulfonic acid groups, carboxylic acid groups, phosphonic acid groups, sulfonimide groups, and the like. The solid polymer electrolyte may contain only one of these acid groups, or may contain two or more.

Examples of fluorine-based electrolytes include Nafion (registered trademark), Flemion (registered trademark), Aquivion (registered trademark), and Aciplex (registered trademark).
The fluorinated electrolyte includes not only a fully fluorinated electrolyte containing no C—H bonds in the polymer structure, but also a partially fluorinated electrolyte containing C—H bonds and CF bonds in the polymer structure. be

Hydrocarbon-based electrolytes include, for example,
(a) a polyetheretherketone, polysulfone, polyethersulfone, polyimide, polyphenylene, polyamide, polyamideimide, or a wholly aromatic hydrocarbon-based electrolyte comprising derivatives thereof, into which an acid group such as a sulfonic acid group is introduced;
(b) a partially aromatic hydrocarbon-based electrolyte having an aromatic ring in part of the polymer chain of the aliphatic hydrocarbon-based electrolyte;
and so on.

Furthermore, the electrolyte membrane may consist of only a solid polymer electrolyte (composite electrolyte) containing various additives, or may be a composite of a composite electrolyte and a reinforcing material. In this case, the type of reinforcing material is not particularly limited.
As a reinforcing material, for example,
(a) porous films and non-woven fabrics of fluororesins such as polytetrafluoroethylene (PTFE), perfluoroalkoxyalkane (PFA), perfluoroethylenepropene copolymer (FEP), and ethylenetetrafluoroethylene copolymer (ETFE);
(b) porous films and non-woven fabrics of hydrocarbon resins such as polyethylene (PE) and polypropylene (PP);
and so on.

[1.2. Anode catalyst layer]
An anode catalyst layer is bonded to one surface of the electrolyte membrane. The anode catalyst layer includes an electrode catalyst (anode catalyst) having activity against hydrogen oxidation reaction and a catalyst layer ionomer. The anode catalyst may consist of only catalyst particles (A) having activity against hydrogen oxidation reaction, or may have catalyst particles (A) supported on the surface of a carrier.
In the present invention, materials for the catalyst layer ionomer, the catalyst particles (A), and the carrier are not particularly limited, and optimum materials can be selected according to the purpose.

The catalyst layer ionomer on the anode side may be made of the same material as the solid polymer electrolyte forming the electrolyte membrane, or may be made of a different material.
Examples of catalyst particles (A) include noble metals, alloys containing two or more noble metals, and alloys of one or two or more noble metals and one or two or more base metals.
Examples of carriers include carbon black, carbon nanotubes, carbon nanohorns, activated carbon, natural graphite, mesocarbon microbeads, and glassy carbon powder.

[1.3. Cathode catalyst layer]
A cathode catalyst layer is bonded to the other surface of the electrolyte membrane. The cathode catalyst layer includes an electrode catalyst (cathode catalyst) having activity in oxygen reduction reaction and a catalyst layer ionomer. The cathode catalyst may consist of only the catalyst particles (B) having activity for the oxygen reduction reaction, or may have the catalyst particles (B) supported on the surface of the carrier.
In the present invention, materials for the catalyst layer ionomer, the catalyst particles (B), and the carrier are not particularly limited, and optimum materials can be selected according to the purpose.

The catalyst layer ionomer on the cathode side may be made of the same material as the solid polymer electrolyte forming the electrolyte membrane, or may be made of a different material.
Examples of catalyst particles (B) include precious metals, alloys containing two or more precious metals, and alloys containing one or more precious metals and one or more base metals.
Examples of carriers include carbon black, carbon nanotubes, carbon nanohorns, activated carbon, natural graphite, mesocarbon microbeads, and glassy carbon powder.

[1.4. first microparticles]
[1.4.1. material]
The term "first microparticles" refers to microparticles made of a first compound (inorganic compound) having a function of decomposing hydrogen peroxide.
The type of the first compound is not particularly limited as long as it exhibits the function of decomposing hydrogen peroxide. Examples of the first compound include tungsten compounds and molybdenum compounds.
Among these, the first compound is preferably a tungsten compound. A tungsten compound is suitable as a material for the first fine particles because it has a higher hydrogen peroxide decomposability than other materials.

Examples of tungsten compounds include
(a) tungsten oxide (+IV, +V, +VI),
(b) tungstates;
The first fine particles may consist of any one of these, or may be a mixture containing two or more.

Tungsten oxide may be anhydride or hydrate. Tungsten oxide is particularly preferably WO3. _{( 1/3} ) _H2O or _WO3.2H2O . The 1/3 hydrate or dihydrate of WO ₃ is particularly suitable as a material for the first fine particles because it has much higher hydrogen peroxide decomposability than other tungsten oxides.
Examples of tungstates include Al ₂ (WO ₄ ) ₃ , BaWO ₄ , FeWO ₄ , Ni ₂ WO ₄ , Cu ₂ WO ₄ , ZnWO ₄ and Ce ₂ (WO ₄ ) ₃ .

[1.4.2. Position of first microparticle]
[A. Position in film thickness direction]
The first fine particles are arranged at the interface between the electrolyte membrane and the anode catalyst layer. This point is different from the conventional one.
Hydrogen peroxide is produced by reaction of oxygen permeated from the cathode to the anode with hydrogen on the anode catalyst. In order for the hydrogen peroxide generated on the anode catalyst to be decomposed into water and oxygen before becoming radicals, the position of the first fine particles should be closer to the anode side. In order to suppress deterioration due to radicals, the first fine particles are preferably arranged at the interface between the electrolyte membrane and the anode-side catalyst layer.

[B. In-plane position]
The first microparticles may be uniformly present over the entire interface between the electrolyte membrane and the anode catalyst layer, or may be unevenly distributed in the in-plane direction (that is, present in part of the interface).

A gas diffusion layer is typically placed on the outside of the MEA. Further, a separator (also called a "current collector") having a gas flow path is arranged outside the gas diffusion layer.
There are various structures for the structure of the gas flow path (that is, the method for flowing the fuel gas and the oxidant gas). On the other hand, the structure of the gas flow path not only determines the direction of the gas flow, but also affects the temperature distribution and humidity distribution within the cell. Furthermore, when the temperature distribution, humidity distribution, etc. in the cell change, the deterioration rate of the electrolyte membrane also changes depending on the position. Therefore, it is preferable to disperse the first microparticles unevenly in positions where deterioration of the electrolyte membrane is likely to occur, rather than dispersing them uniformly in the in-plane direction.

FIG. 1 shows a schematic diagram of a fuel cell having a gas channel structure for flowing fuel gas (hydrogen) and oxidant gas (air) in predetermined directions. The numbers shown in FIG. 1 represent the regions of the divided MEA.
In FIG. 1, the fuel gas is introduced from the end of region 1 in the x-axis direction. The hydrogen gas introduced into the region 1 flows through the regions 1, 4, 7, 8, 5, 2, 3, 6 and 9 in this order and is discharged from the end of the region 9 in the x-axis direction. On the other hand, the oxidizing gas is introduced from the ends of the regions 3, 6 and 9 in the y-axis direction. The oxidant gas introduced into the regions 3, 6 and 9 flows in the y-axis direction and is discharged from the ends of the regions 1, 4 and 7 in the y-axis direction.

In the case of the example shown in FIG. 1, regions 1, 3, 6, and 9 have lower humidity in the cell than the other regions, and deterioration of the electrolyte membrane is more likely to occur. Therefore, it is preferable to make the content of the first fine particles added to these regions higher than the content of the other regions.
Specifically, it is preferable to place 50 to 100% of the total amount of the first fine particles present at the interface in the region where deterioration is likely to occur.

[1.4.3. Average Content of First Fine Particles]
"Average content" means the solid polymer electrolyte contained in the electrolyte membrane in the region where the first fine particles are arranged when the first fine particles are arranged at the interface between the electrolyte membrane and the anode catalyst layer. and the ratio of the mass of the first fine particles to the total mass of the first fine particles.

The average content of the first fine particles affects the durability of the membrane electrode assembly. As the average content of the first fine particles increases, the durability of the membrane electrode assembly improves. In order to obtain such effects, the average content of the first fine particles is preferably 0.01 mass % or more. The average content is more preferably 0.05 mass% or more, more preferably 0.1 mass% or more.

When the first fine particles are dispersed in the electrolyte membrane, if the average content of the first fine particles is excessive, the electrolyte membrane may crack. However, when the first fine particles are arranged at the interface, even if the average content becomes excessive, the electrolyte membrane will not crack. However, if the average content of the first microparticles becomes excessive, the proton conductivity of the electrolyte membrane may decrease. Therefore, the average content of the first fine particles is preferably 20.0 mass% or less. The average content is more preferably 10.0 mass% or less, more preferably 5.0 mass% or less.

[1.4.4. Average particle diameter of the first fine particles]
"Average particle size of the first fine particles" means the average value of the maximum size of 20 or more randomly selected first fine particles under microscope observation.

The average particle size of the first fine particles is not particularly limited, and an optimum average particle size can be selected according to the purpose. In general, if the average particle size of the first fine particles becomes too large, the first fine particles may break through the film, causing short circuits and leaks. Therefore, the average particle size of the first fine particles is preferably 1/10 or less of the thickness of the electrolyte membrane. The average particle size is more preferably 1/100 or less, more preferably 1/5000 or less of the thickness of the electrolyte membrane.
On the other hand, if the average particle size of the first microparticles is too small, the first microparticles are likely to dissolve. Therefore, the average particle size of the first fine particles is preferably 0.1 nm or more. The average particle size is more preferably 1.0 nm or more, more preferably 2.0 nm or more.

[1.5. Second Metal Element Ions and/or Second Fine Particles]
[1.5.1. material]
"Second metal element" refers to a metal element having the function of eliminating radicals.
A "second compound" refers to a compound containing a second metal element.
"Second microparticles" refer to microparticles composed of a second compound.

Compounds and ions containing certain metal elements mainly have the effect of scavenging radicals. Therefore, by adding an appropriate amount of fine particles of a compound containing such a metal element and/or ions of such a metal element to the electrolyte membrane, deterioration of the electrolyte membrane can be further suppressed.

Examples of the second metal element include Ce, Ag, Cs, Sn, Mn, Ti, Co, Ni, Zn, Zr, Ru, Rh, Pd, Pt, and Pr. Any one of these may be contained in the electrolyte membrane, or two or more thereof may be contained.
Among these, the second metal element is preferably at least one element selected from the group consisting of Ce, Ag, Cs, Sn, and Mn. Ce is particularly preferable as the second metal element. This is because all of them have a large effect of suppressing generation of radicals or of eliminating radicals.

The second compound may be a water-soluble compound or a sparingly soluble compound. When the water-soluble second compound is used, the second compound is dissociated in the electrolyte membrane, and some of the protons of the acid groups of the solid polymer electrolyte are replaced with ions of the second metal element. On the other hand, when the sparingly soluble second compound is used, the second compound is dispersed in the electrolyte membrane.
Examples of the second compound include nitrates, sulfates, carbonates, formates, acetates, citrates, perchlorates, phosphates, chlorides, fluorides, hydroxides, acetylacetonate compounds, and the like. There is

[1.5.2. Content]
When the electrolyte membrane contains the ions of the second metal element and/or the second fine particles, the electrolyte membrane preferably satisfies the following formula (1).
0.0005≦M/A≦0.15 (1)
however,
M/A is the ratio of the number of moles (M) of the second metal element to the number of moles (A) of acid groups contained in the solid polymer electrolyte.

Even when only the first fine particles are arranged at the interface of the electrolyte membrane/anode catalyst layer, deterioration of the electrolyte membrane can be effectively suppressed. However, when the second compound and/or the ions of the second metal element are further added to the electrolyte membrane in addition to the first fine particles, the synergistic effect of these further suppresses deterioration of the electrolyte membrane. In order to obtain such effects, the M/A ratio is preferably 0.0005 or more. The M/A ratio is preferably 0.001 or more, more preferably 0.005 or more.

On the other hand, if the M/A ratio becomes too large, the conductivity of the electrolyte membrane may decrease. Therefore, the M/A ratio should be 0.15 or less. The M/A ratio is preferably 0.10 or less, more preferably 0.05 or less, still more preferably 0.04 or less, still more preferably 0.03 or less.

[1.5.3. Average particle size]
When the second fine particles are added to the electrolyte membrane, the average particle size of the second fine particles is not particularly limited, and an optimum average particle size can be selected according to the purpose. Other points regarding the average particle diameter of the second fine particles are the same as the average particle diameter of the first fine particles, so the description is omitted.

[1.5.4. Position of Second Metal Element Ions or Second Fine Particles]
The positions where the ions of the second metal element and the second fine particles are added are not particularly limited. Since radicals are not necessarily generated on the anode side, the ions of the second metal element and the second fine particles may be added on the cathode side.

[2. Method for manufacturing membrane electrode assembly]
A method for manufacturing a membrane electrode assembly according to the present invention comprises:
a first step of attaching first fine particles made of a first compound having a function of decomposing hydrogen peroxide to the surface of the electrolyte membrane and/or the surface of the anode catalyst layer facing the electrolyte membrane;
The anode catalyst layer is formed on the anode-side surface of the electrolyte membrane so that the surface to which the first fine particles are adhered faces the inside, and the cathode catalyst layer is further formed on the other surface of the electrolyte membrane. a second step of forming a laminate;
and a third step of heat-treating the laminate to obtain a membrane electrode assembly according to the present invention.

[2.1. First step]
First, first fine particles made of a first compound having a function of decomposing hydrogen peroxide are attached to the surface of the electrolyte membrane and/or the surface of the anode catalyst layer facing the electrolyte membrane (first step).

A method for adhering the first fine particles is not particularly limited, and an optimum method can be selected according to the purpose. As a method for attaching the first fine particles, for example,
(a) A method of spraying a dispersion liquid in which the first fine particles are dispersed on the surface of the electrolyte membrane or the surface of the anode catalyst layer and drying the dispersion liquid;
(b) a method of applying the first fine particles to the surface of the electrolyte membrane or the surface of the anode catalyst layer using a dry coating method such as an electrostatic screen printing method;
and so on.

The first fine particles may adhere to either one of the surface of the electrolyte membrane and the surface of the anode catalyst layer, or may adhere to both.
Also, the first fine particles may adhere to the entire surface of the electrolyte membrane and/or the entire surface of the anode catalyst layer, or may adhere to only a portion of the surface.

As a method of adding the ions of the second metal element or the second fine particles to the electrolyte membrane, for example,
(a) a method of adding second fine particles or a solvent-soluble compound containing a second metal element to a casting solution for producing an electrolyte membrane;
(b) a method of applying a solution containing ions of the second metal element to the surface of the electrolyte membrane after the electrolyte membrane is produced, and diffusing the ions of the second metal element into the electrolyte membrane;
(c) After preparing the electrolyte membrane, a solution containing ions of the second metal element is applied to the surface of the electrolyte membrane, the ions of the second metal element are diffused into the electrolyte membrane, and the second compound is formed in the electrolyte membrane. a method of precipitating;
and so on.

[2.2. Second step]
Next, the anode catalyst layer is formed on the anode-side surface of the electrolyte membrane so that the surface to which the first fine particles are adhered faces the inside, and a cathode catalyst layer is formed on the other surface of the electrolyte membrane. is formed to form a laminate (second step).

A method for forming the catalyst layer is not particularly limited. As a method for forming the catalyst layer, for example,
(a) a method of forming a catalyst layer on the substrate surface and transferring the catalyst layer to the surface of the electrolyte membrane;
(b) a method of applying a paste for forming a catalyst layer on the surface of the electrolyte membrane and drying it;
and so on.
When the catalyst layer is formed using a transfer method, the first fine particles may be attached to the surface of the catalyst layer, or may be attached to the surface of the electrolyte membrane.

[2.3. Third step]
Next, the laminate is heat-treated (third step). Thereby, a membrane electrode assembly according to the present invention is obtained.
The heat treatment conditions are not particularly limited, and optimum conditions can be selected according to the purpose. The heat treatment is usually carried out by heating at 25°C to 200°C for 1 minute to 20 minutes while applying a pressure of 1 kgf/cm ² (9.8 × 10 ^-2 MPa) to 100 kgf/cm ² (9.8 MPa). performed by

[3. action]
Degradation of the electrolyte membrane
(a) oxygen permeated from the cathode to the anode reacts with hydrogen on the anode catalyst to produce H ₂ O ₂ ;
(b) H ₂ O ₂ generated on the anode catalyst diffuses into the electrolyte membrane, and Fe mixed as an impurity in the electrolyte membrane reacts with H ₂ O ₂ to generate hydroxyl radicals,
(c) hydroxyl radicals decompose electrolytes;
This is thought to be caused by

Therefore, when the first fine particles having the function of decomposing hydrogen peroxide are present at the interface between the electrolyte membrane and the anode catalyst layer, the H ₂ O ₂ generated at the anode is decomposed into water and oxygen before becoming hydroxyl radicals. be. As a result, the decomposition of the electrolyte membrane is suppressed and the chemical durability of the electrolyte membrane is improved.
In particular, tungsten oxides and tungstates are not soluble in the fuel cell environment (acidity, potential) and can exist in the form of fine particles at the electrolyte membrane/anode catalyst layer interface. Therefore, the dissolved ions do not undergo ion exchange with the sulfonic acid groups of the electrolyte, and high proton conductivity can be maintained, so that the battery performance does not deteriorate.
Among tungsten oxides, WO ₃ .(1/3)H ₂ O or WO ₃ .2H ₂ O exhibits remarkably high decomposition of hydrogen peroxide compared to other materials. Therefore, by arranging this at the electrolyte membrane/anode catalyst layer interface, the chemical durability of the electrolyte membrane is improved.

Also, the first fine particles, which are stable under the operating environment of the fuel cell, do not move inside the membrane electrode assembly. Therefore, if the method according to the present invention is used to selectively dispose the first fine particles at locations where the electrolyte membrane is severely deteriorated, the deterioration of the electrolyte membrane can be efficiently suppressed, and the deterioration can be prevented over a long period of time. The inhibitory effect can be sustained.

Furthermore, in addition to arranging the first fine particles at the interface of the electrolyte membrane/anode catalyst layer, if the second fine particles and/or the ions of the second metal element are further added to the electrolyte membrane, the amounts of these additions are relatively Even with a small amount, high durability can be obtained. this is,
(a) The first fine particles have the effect of decomposing hydrogen peroxide to make it harmless, and even if the amount added is small, they exhibit a high hydrogen peroxide decomposition effect.
(b) Since the ions of the second fine particles and the second metal element have the effect of eliminating radicals,
and/or
(d) Even if radicals are generated from hydrogen peroxide that has not been decomposed by the first fine particles, the ions of the second fine particles and/or the second metal element effectively eliminate the radicals.
it is conceivable that.

(Example 1, Comparative Examples 1 and 2)
[1. Preparation of sample]
[1.1. Preparation of electrolyte membrane]
[1.1.1. Example 1]
Electrolyte solution (D2020, Nafion (registered trademark) dispersion, 1000 EW, 20 mass%, manufactured by Chemours), 1-propanol, and ultrapure water were mixed together, electrolyte solution: 1-propanol: water = 10: 7: 3 (mass ratio ) was mixed. Further, the mixed solution was irradiated with ultrasonic waves for 10 minutes to disperse the electrolyte, thereby obtaining a casting solution.

Casting solution: 1.20 g was sampled in a flat petri dish of φ35 mm and left in a constant temperature room atmosphere (25° C.) for several days to dry and solidify. Then, the plate was annealed at 140° C. for 15 minutes. After annealing, the film was washed with ultrapure water five times. Furthermore, the membrane was peeled off from the petri dish and air-dried.

Next, an ethanol solution was prepared in which WO ₃ was dispersed at a rate of 1 mass%. An ethanol solution was applied to the surface of the electrolyte membrane using an atomizer spray to obtain a composite electrolyte membrane. The application area of WO ₃ was 4 cm ² . _In addition, the amount of WO3 to be applied is such that the _average content of WO3 in the application area ₍ the ratio of the mass of WO3 to the _total mass of the solid polymer electrolyte and WO3 contained in the application area) is 20% by mass. did.

[1.1.2. Comparative Examples 1 and 2]
Electrolyte solution (D2020, Nafion (registered trademark) dispersion, 1000 EW, 20 mass%, manufactured by Chemours), 1-propanol, and ultrapure water were mixed together, electrolyte solution: 1-propanol: water = 10: 7: 3 (mass ratio ), and then more WO ₃ was added. The amount of WO3 added is such that the _average content of the entire membrane (the ratio of the mass of WO3 to the _total mass of the solid polymer electrolyte and WO3 contained in the entire membrane) is ₂₀ mass% (Comparative Example 1), or The amount (Comparative Example 2) was set to 3 mass%. Further, the mixed solution was irradiated with ultrasonic waves for 10 minutes to disperse the electrolyte, thereby obtaining a casting solution.

Casting solution: 1.20 g was sampled in a flat petri dish of φ35 mm and left in a constant temperature room atmosphere (25° C.) for several days to dry and solidify. Then, the plate was annealed at 140° C. for 15 minutes. After annealing, the film was washed with ultrapure water five times. Furthermore, the membrane was peeled off from the petri dish and air-dried.

[1.2. Production of MEA]
A catalyst layer was thermally transferred (145° C., 5 minutes, 60 kgf/cm ² (5.9 MPa)) to both surfaces of the electrolyte membrane to prepare an MEA.

[2. Test method]
[2.1. Visual evaluation of cracks]
Cracking of the electrolyte membrane was visually evaluated before fabricating the MEA.
[2.2. Scanning transmission electron microscope (SEM) observation]
Using a scanning transmission electron microscope (SEM), the cross section of the obtained MEA and the surface of the electrolyte membrane after peeling off the anode catalyst layer from the MEA were observed.

[3. result]
[3.1. Visual evaluation of cracks]
Table 1 shows the results. Comparative Example 1 had cracks in the electrolyte membrane. On the other hand, in Example 1 and Comparative Example 2, no cracks were observed in the electrolyte membrane.

[3.2. SEM observation]
FIG. 2A shows a SEM image of the cross section of the membrane electrode assembly obtained in Example 1. FIG. FIG. 2B shows a SEM image of the cross section of the membrane electrode assembly obtained in Comparative Example 2. As shown in FIG. In the MEA prepared by the spray method (Example ₁ ), WO3 was present at the interface between the electrolyte membrane and the anode catalyst layer. On the other hand, in the MEA produced by the casting method (Comparative Example ₂ ), WO3 was segregated on the anode side in the electrolyte membrane _, but no WO3 layer was observed at the interface between the electrolyte membrane and the anode catalyst layer. .

FIG. 3A shows an SEM image of the electrolyte membrane surface after the anode catalyst layer was peeled off from the membrane electrode assembly obtained in Example 1. FIG. Further, FIG. 3B shows an SEM image of the electrolytic membrane surface after the anode catalyst layer was peeled off from the membrane electrode assembly obtained in Comparative Example 2. As shown in FIG.
In the case of Comparative Example 2, most of the surface of the electrolyte membrane after peeling off the anode catalyst layer consisted of the electrolyte, and almost no WO ₃ exposed on the surface of the electrolyte membrane was observed. On the other hand, in the case of Example 1 _, only WO3 was confirmed on the surface of the electrolyte membrane after the anode catalyst layer was peeled off.

(Example 2, Comparative Example 3)
[1. Preparation of sample]
[1.1. Example 2]
In the same manner as in Example 1, an electrolyte membrane was prepared by coating the anode-side surface of the electrolyte membrane with WO ₃ powder (coating area: 4 cm ² ).

[1.2. Comparative Example 3]
In the same manner as in Example 1, a cast membrane consisting of only the electrolyte was produced.
Next, an aqueous solution was prepared in which cerium nitrate was dispersed at a rate of 1% by mass. An aqueous solution was applied to the surface of the electrolyte membrane using an atomizer spray to obtain a composite electrolyte membrane. The Ce coating area was 4 cm ² . Also, the amount of Ce applied was set to an amount corresponding to 20% of the amount of sulfonic acid groups of the electrolyte membrane in the applied area.

[2. Test method]
The obtained electrolyte membrane was left under conditions of 80° C. and 80% RH for 72 hours. After that, the amount of W or Ce contained in the electrolyte membrane was quantified, and the values before and after the test were compared.
FIG. 4 shows the application positions of solutions containing various additives and the quantitative positions of W or Ce. As shown in FIG. 4, various additives were applied approximately in the center of the electrolyte membrane. In addition, W or Ce was quantified at two points, a region near the center of the additive-coated region (quantification position 1) and a region where no additive was applied (quantification position 2).

[3. result]
Table 2 shows the results. In the case of Example 2, the amount of W at the quantification position 1 was almost unchanged before and after the test. Moreover, the amount of W at the quantitative position 2 was zero both at the initial stage and after the test.
On the other hand, in the case of Comparative Example 3, the amount of Ce at the quantification position 1 decreased significantly after the test. At the quantification position 2, a small amount of Ce was already detected even in the initial state, and the amount of Ce increased after the test.
From the above results, it was found that selective addition of WO ₃ to a portion where deterioration is to be suppressed can maintain the effect of suppressing deterioration over a long period of time.

(Examples 4 to 10, Comparative Example 4)
[1. Test method]
0.067 mmol of various additives were dispersed in 10 mL of water to form a dispersion, to which 4.41 mmol of hydrogen peroxide was further added. The following additives were used.
(a) Aluminum tungstate (Al2 ₍ WO4) ₃ ) (Example ₄ )
(b) Barium tungstate ( _BaWO4 ) (Example 5)
(c) Iron tungstate ( _FeWO4 ) (Example 6)
( _d ) Nickel tungstate ( _Ni2WO4 ) (Example 7)
(e) Copper tungstate ( _Cu2WO4 ) (Example ₈ )
(f) Zinc tungstate ( _ZnWO4 ) (Example 9)
(g) _WO2 (Example 10)
(h) No additive (Comparative Example 4)

After holding the resulting dispersion at 80° C. for 1 hour, the concentration of hydrogen peroxide remaining in the dispersion was measured. Furthermore, the hydrogen peroxide decomposition rate was calculated from the following formula (2).
Hydrogen peroxide decomposition rate (%) = (C ₀ - C) x 100/C ₀ (2)
however,
C ₀ is the concentration of hydrogen peroxide contained in the original dispersion,
C is the concentration of hydrogen peroxide contained in the dispersion after being held at 80° C. for 1 hour.

[2. result]
Table 3 shows the H ₂ O ₂ decomposition rate of various additives. It was found that H ₂ O ₂ does not undergo thermal self-decomposition under the experimental conditions (Comparative Example 4). On the other hand, not only WO ₂ of Example 10 but also H ₂ O ₂ was decomposed when various tungstates of Examples 4 to 9 were used.
From the above, it is expected that chemical deterioration is suppressed by arranging the tungstate at the interface between the electrolyte and the anode-side catalyst layer.

(Examples 11-12, Comparative Example 5)
[1. Preparation of sample]
[1.1. Example 11]
An MEA was produced in the same manner as in Example 1, except that iron sulfate was added to the casting solution in such an amount that 1% of the sulfonic acid groups of the electrolyte membrane were replaced with Fe ions.
[1.2. Comparative Example 5]
An MEA was produced in the same manner as in Comparative Example 2, except that iron sulfate was added to the casting solution in such an amount that 1% of the sulfonic acid groups of the electrolyte membrane were replaced with Fe ions.
[1.3. Example 12]
Iron sulfate in an amount such that 1% of the sulfonic acid groups of the electrolyte membrane are replaced with Fe ions and cerium nitrate in an amount such that 4.4% of the sulfonic acid groups of the electrolyte membrane are replaced with Ce ions are added to the casting solution. An MEA was produced in the same manner as in Example 1, except that additional components were added.

[2. Test method]
Gas diffusion layers were placed on both sides of the obtained MEA, and assembled into a fuel cell. The electrode area was 0.64 cm ² . Next, using the obtained fuel cell, an open circuit (OC) endurance test was conducted for 90 hours under the following conditions.
Cell temperature: 95°C
Humidity: 30% RH for both anode and cathode
Gas flow rate: 300 cc/min ₍ anode: H2, cathode: _O2 )
Back pressure: 133kPa

The humidified gas under test discharged from the fuel cell was condensed and the resulting water was collected every 24 hours. In the collected water, F ions, which are chemically degraded and decomposed in the electrolyte membrane, are dissolved. This was quantified by ion chromatography, and the total amount of F ions up to 90 hours was calculated.

[3. result]
Table 4 shows the results. From Table 4, it can be seen that Example 11 has a smaller F elution amount than Comparative Example 5. This is probably because hydrogen peroxide was efficiently decomposed by uneven distribution of WO ₂ at the electrolyte membrane/anode catalyst layer interface. Furthermore, in Example 12, the F elution amount was further reduced than in Example 11, and the effect of combined addition of Ce and W was recognized.

Although the embodiments of the present invention have been described in detail above, the present invention is by no means limited to the above-described embodiments, and various modifications are possible without departing from the gist of the present invention.

The membrane electrode assembly according to the present invention can be used for various electrochemical devices such as polymer electrolyte fuel cells and polymer electrolyte water electrolysis devices.

Claims (10)

an electrolyte membrane containing a solid polymer electrolyte;
an anode catalyst layer bonded to one surface of the electrolyte membrane;
a cathode catalyst layer bonded to the other surface of the electrolyte membrane;
first fine particles arranged at an interface between the electrolyte membrane and the anode catalyst layer;
The membrane electrode assembly, wherein the first fine particles are composed of a first compound having a function of decomposing hydrogen peroxide. 2. The membrane electrode assembly according to claim 1, wherein said first compound comprises a tungsten compound. 3. The membrane electrode assembly according to claim 1, wherein said first compound comprises tungsten oxide (+IV, +V, +VI) or tungstate. The membrane electrode assembly according to any one of claims 1 to 3, wherein the average content of the first fine particles is 0.01 mass% or more and 20.0 mass% or less. 5. The membrane electrode assembly according to any one of claims 1 to 4, wherein the average particle diameter of said first fine particles is 1/10 or less of the film thickness of said electrolyte membrane. and/or
(b) The membrane electrode assembly according to any one of claims 1 to 5, further comprising second fine particles made of a second compound containing the second metal element and dispersed in the electrolyte membrane. 7. The membrane electrode assembly according to claim 6, wherein said second metal element is Ce. 8. The membrane electrode assembly according to claim 6, which satisfies the following formula (1).
0.0005≦M/A≦0.15 (1)
however,
M/A is the ratio of the number of moles (M) of the second metal element to the number of moles (A) of the acid groups contained in the solid polymer electrolyte. The membrane electrode assembly according to any one of claims 1 to 8, wherein the first fine particles are unevenly distributed in the in-plane direction. a first step of attaching first fine particles made of a first compound having a function of decomposing hydrogen peroxide to the surface of the electrolyte membrane and/or the surface of the anode catalyst layer facing the electrolyte membrane;
The anode catalyst layer is formed on the anode-side surface of the electrolyte membrane so that the surface to which the first fine particles are adhered faces the inside, and the cathode catalyst layer is further formed on the other surface of the electrolyte membrane. a second step of forming a laminate;
A method for manufacturing a membrane electrode assembly, comprising a third step of heat-treating the laminate to obtain the membrane electrode assembly according to any one of claims 1 to 9.

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