JP7599797B2 - Membrane electrode assembly and method for producing same - Google Patents

Description

The present invention relates to a membrane electrode assembly and a method for producing the same, and more specifically to a membrane electrode assembly containing an additive that has the function of suppressing the generation of radicals that cause deterioration of a solid polymer electrolyte, and a method for producing the same.

Membrane electrode assemblies (MEAs) are used in soda electrolysis devices, water electrolysis devices, fuel cells, etc., where electrochemical reactions occur. It is said that MEAs are deteriorated due to the electrolyte being attacked by radicals generated directly by the direct reaction between oxygen and hydrogen or by electrochemical reactions, or radicals generated via hydrogen peroxide.
For example, in fuel cells, radical attack is known to increase the resistance of the electrolyte membrane, increase cross leakage, and shorten the lifespan due to thinning. Furthermore, degradation products generated by radical attack may cause catalyst poisoning, which may result in a decrease in electrolytic performance and cell performance.

In order to solve this problem, various proposals have been made in the past.
For example, Patent Document 1 discloses a membrane electrode assembly that includes an electrolyte membrane, and electrodes each consisting of a catalyst layer and a diffusion layer bonded to both sides of the electrolyte membrane, with the peripheral portion of the diffusion layer impregnated with cerium nitrate.

The same document states:
(A) Deterioration of the electrolyte membrane due to radicals occurs notably near the center of the MEA but near the periphery; and
(B) It is described that by adding Ce to the end of the diffusion layer, harmful hydrogen peroxide can be efficiently removed from within the cell without reducing the power generation performance.

Patent Document 2 describes a fuel cell in which a plurality of unit cells are stacked horizontally (i.e., a fuel cell in which a plurality of unit cells are arranged vertically),
(a) The anode-side water-repellent layer and the cathode-side water-repellent layer each contain _CeO2 as a radical inhibitor, and
(b) A fuel cell is disclosed in which the concentration of radical inhibitor (concentration of dissolved Ce ions) contained in the vertically upper portions of the anode-side water-repellent layer and the cathode-side water-repellent layer is higher than the concentration of radical inhibitor contained in the vertically lower portions.

The same document states:
(A) In a fuel cell in which a plurality of unit cells are stacked in the horizontal direction, even if scavenging control is performed to discharge the residual water after the fuel cell is stopped, the amount of residual water in the vertically lower portion of the MEA is greater than the amount of residual water in the vertically upper portion;
(B) When the radical inhibitor is uniformly contained in the water-repellent layer, the amount of radical inhibitor eluted is small in the vertically upper portion of the MEA where the residual water amount is small, so that deterioration due to hydroxyl radicals cannot be suppressed, whereas the amount of radical inhibitor eluted is excessive in the vertically lower portion of the MEA where the residual water amount is large, so that the proton transfer resistance of the electrolyte membrane increases; and
(C) It is described that by increasing the concentration of the radical inhibitor contained in the vertically upper portion of the water-repellent layer and the concentration of the radical inhibitor contained in the vertically lower portion of the water-repellent layer, it is possible to ensure the amount of radical inhibitor eluted in the vertically upper portion where the amount of residual water is small, while suppressing the amount of radical inhibitor eluted in the vertically lower portion where the amount of residual water is large.

In Patent Document 3, in a counterflow type fuel cell in which the flow of fuel gas and the flow of oxidant gas are in opposite directions,
(a) The anode (anode catalyst layer or anode diffusion layer) contains Ag, 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.

The same document states:
(A) In counterflow type fuel cells, the humidity at the outlet side of the fuel gas is lower than that at the inlet side, so degradation due to radicals is likely to occur; and
(B) It is described that by making the Ag concentration at the outlet side of the fuel gas higher than that at the inlet side, degradation due to radicals can be suppressed without reducing 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 side of a composite membrane in which an expanded PTFE membrane is impregnated with an ion exchange material.
The document describes that dispersing platinum-supported carbon in a high-strength solid polymer electrolyte improves the life of a fuel cell.

Ce ions and Ag have the effect of scavenging radicals, and therefore, as described in Patent Documents 1 to 3, by adding more Ce ions or Ag to a portion where chemical deterioration is severe, deterioration of the electrolyte membrane can be suppressed to some extent.
However, Ce ions and Ag ions tend to diffuse in the in-plane direction and/or in the thickness direction of the electrolyte membrane due to the concentration gradient, and therefore, if Ce ions or Ag ions move from a site where chemical deterioration is severe, the deterioration suppression effect may not be fully exerted.

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

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

An object of the present invention is to provide a membrane electrode assembly capable of suppressing the generation of radicals that cause deterioration of a solid polymer electrolyte.
Another problem to be solved by the present invention is to provide a membrane electrode assembly capable of suppressing deterioration of an 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 including 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;
a first fine particle disposed at an interface between the electrolyte membrane and the anode catalyst layer,
The first fine particles are made of a first compound having a function of decomposing hydrogen peroxide.

The method for producing a membrane electrode assembly according to the present invention includes the steps of:
a first step of attaching first fine particles made of a first compound having a function of decomposing hydrogen peroxide to a surface of an electrolyte membrane and/or a surface of an anode catalyst layer facing the electrolyte membrane;
a second step of forming the anode catalyst layer on the anode side surface of the electrolyte membrane such that the surface to which the first fine particles are attached faces inward, and further forming a cathode catalyst layer on the other side of the electrolyte membrane to form a laminate;
and a third step of heat-treating the laminate to obtain the membrane electrode assembly of the present invention.

Deterioration of the electrolyte membrane is
(a) Oxygen that has permeated from the cathode to the anode reacts with hydrogen on the anode catalyst to produce _{H2O2 ;}
(b) _H2O2 produced on the anode _catalyst diffuses into the electrolyte membrane, and reacts with _Fe , which is present as an impurity in the _electrolyte membrane, to produce hydroxyl radicals.
(c) Hydroxyl radicals decompose electrolytes;
This is thought to be caused by the

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, _H2O2 generated at the anode is decomposed into water and oxygen before _it becomes hydroxyl radicals, which suppresses the decomposition of the electrolyte membrane and improves the chemical durability of the electrolyte membrane.

1 is a schematic diagram of a fuel cell having a gas flow path structure that allows fuel gas (hydrogen) and oxidant gas (air) to flow in a predetermined direction. Fig. 2(A) is a SEM image of a cross section of the membrane electrode assembly obtained in Example 1. Fig. 2(B) is a SEM image of a cross section of the membrane electrode assembly obtained in Comparative Example 2. Fig. 3(A) is 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. 3(B) is an SEM image of the electrolyte membrane surface after the anode catalyst layer was peeled off from the membrane electrode assembly obtained in Comparative Example 2. FIG. 1 is a diagram showing application positions of solutions containing various additives and quantitative determination 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 including 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;
The electrochemical cell further comprises first fine particles disposed 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 the 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.
The type of acid group of the solid polymer electrolyte is not particularly limited. Examples of the acid group include a sulfonic acid group, a carboxylic acid group, a phosphonic acid group, and a sulfonimide group. The compound may contain only one kind of the acid group mentioned above, or may contain two or more kinds of the acid group mentioned above.

Examples of fluorine-based electrolytes include Nafion (registered trademark), Flemion (registered trademark), Aquivion (registered trademark), and Aciplex (registered trademark).
The fluorine-based electrolyte includes not only fully fluorine-based electrolytes that do not contain C—H bonds in the polymer structure, but also partially fluorine-based electrolytes that contain C—H bonds and C—F bonds in the polymer structure.

Examples of the hydrocarbon electrolyte include:
(a) A wholly aromatic hydrocarbon-based electrolyte made of polyether ether ketone, polysulfone, polyether sulfone, polyimide, polyphenylene, polyamide, polyamide imide, or a derivative thereof, into which an acid group such as a sulfonic acid group has been introduced;
(b) a partially aromatic hydrocarbon-based electrolyte having an aromatic ring in a part of the polymer chain of an aliphatic hydrocarbon-based electrolyte;
etc.

Furthermore, the electrolyte membrane may be made 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 the reinforcing material is not particularly limited.
Examples of reinforcing materials include:
(a) Porous membranes and nonwoven fabrics of fluorine-based resins such as polytetrafluoroethylene (PTFE), perfluoroalkoxyalkane (PFA), perfluoroethylenepropene copolymer (FEP), and ethylenetetrafluoroethylene copolymer (ETFE);
(b) Porous membranes and nonwoven fabrics of hydrocarbon resins such as polyethylene (PE) and polypropylene (PP);
etc.

[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) active in hydrogen oxidation reactions and a catalyst layer ionomer. The anode catalyst may be composed only of catalyst particles (A) active in hydrogen oxidation reactions, or the catalyst particles (A) may be supported on the surface of a carrier.
In the present invention, the materials of the catalyst layer ionomer, the catalyst particles (A), and the carrier are not particularly limited, and the most suitable materials can be selected depending on the purpose.

The catalyst layer ionomer on the anode side may be made of the same material as the solid polymer electrolyte constituting the electrolyte membrane, or may be made of a different material.
The catalyst particles (A) may be, for example, a precious metal, an alloy containing two or more precious metals, or an alloy of one or more precious metals and one or more base metals.
Examples of the carrier 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) active in the oxygen reduction reaction and a catalyst layer ionomer. The cathode catalyst may be composed only of catalyst particles (B) active in the oxygen reduction reaction, or the catalyst particles (B) may be supported on the surface of a carrier.
In the present invention, the materials of the catalyst layer ionomer, the catalyst particles (B), and the carrier are not particularly limited, and the most suitable materials can be selected depending on the purpose.

The catalyst layer ionomer on the cathode side may be made of the same material as the solid polymer electrolyte constituting the electrolyte membrane, or may be made of a different material.
Examples of the 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 the carrier include carbon black, carbon nanotubes, carbon nanohorns, activated carbon, natural graphite, mesocarbon microbeads, and glassy carbon powder.

[1.4. First fine particles]
[1.4.1. material]
The "first fine particles" refer to fine particles made of a first compound (inorganic compound) having the function of decomposing hydrogen peroxide.
The type of the first compound is not particularly limited as long as it has the function of decomposing hydrogen peroxide. Examples of the first compound include a tungsten compound and a molybdenum compound.
Among these, the first compound is preferably a tungsten compound, which has a higher hydrogen peroxide decomposition ability than other materials and is therefore suitable as a material for the first fine particles.

Examples of tungsten compounds include:
(a) tungsten oxide (+IV, +V, +VI),
(b) Tungstates, etc.
The first fine particles may be made of any one of these, or may be a mixture containing two or more of them.

Tungsten oxide may be anhydrous or hydrated. Tungsten oxide is particularly preferably _WO3 .( _{1/3)H2O or WO3.2H2O} . _WO3 1/3 hydrate or dihydrate has a significantly higher hydrogen peroxide decomposition ability than other tungsten oxides, and is therefore particularly suitable as a material for the first fine particles.
Examples of _tungstates include _Al2 ( _WO4 ) ₃ , _BaWO4 , _FeWO4 , _{Ni2WO4 , Cu2WO4} , _ZnWO4 , and _Ce2 ( _WO4 ) ₃ .

[1.4.2. Position of first particle]
[A. Position in Film Thickness Direction]
The first fine particles are disposed at the interface between the electrolyte membrane and the anode catalyst layer, which is different from the conventional technology.
Hydrogen peroxide is generated when oxygen that has permeated from the cathode to the anode reacts with hydrogen on the anode catalyst. In order for hydrogen peroxide generated on the anode catalyst to be decomposed into water and oxygen before it becomes radicals, the closer the position of the first fine particles is to the anode side, the better. In order to suppress deterioration due to radicals, the first fine particles are preferably disposed at the interface between the electrolyte membrane and the anode-side catalyst layer.

[B. In-plane position]
The first fine particles may be present uniformly 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 only in a part of the interface).

A gas diffusion layer is usually disposed on the outside of the MEA, and a separator (also called a "current collector") having gas flow paths is further disposed on the outside of the gas diffusion layer.
There are various structures for the gas flow path (i.e., the method of flowing the fuel gas and the oxidant gas). Meanwhile, the structure of the gas flow path not only determines the direction of the gas flow, but also affects the temperature distribution, humidity distribution, etc. in 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 unevenly distribute the first fine particles in positions where deterioration of the electrolyte membrane is likely to occur, rather than dispersing them uniformly in the in-plane direction.

A schematic diagram of a fuel cell having a gas flow channel structure that allows a fuel gas (hydrogen) and an oxidant gas (air) to flow in a predetermined direction is shown in Fig. 1. The numbers shown in Fig. 1 represent the regions of a divided MEA.
1 , fuel gas is introduced from the end of region 1 in the x-axis direction. Hydrogen gas introduced into region 1 flows through regions 1, 4, 7, 8, 5, 2, 3, 6, and 9 in this order, and is discharged from the end of region 9 in the x-axis direction. On the other hand, oxidant gas is introduced from the ends of regions 3, 6, and 9 in the y-axis direction. The oxidant gas introduced into regions 3, 6, and 9 flows in the y-axis direction, and is discharged from the ends of regions 1, 4, and 7 in the y-axis direction.

In the example shown in Fig. 1, the humidity in the cell is lower in the regions 1, 3, 6, and 9 than in the other regions, and the electrolyte membrane is more likely to deteriorate in these regions. Therefore, it is preferable to add the first fine particles to these regions in a larger amount than the other regions.
Specifically, it is preferable to dispose 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]
The "average content" refers to the ratio of the mass of the first microparticles to the total mass of the solid polymer electrolyte and the first microparticles contained in the electrolyte membrane in the region where the first microparticles are disposed, when the first microparticles are disposed at the interface between the electrolyte membrane and the anode catalyst layer.

The average content of the first fine particles affects the durability of the membrane electrode assembly. The higher the average content of the first fine particles, the more the durability of the membrane electrode assembly improves. To achieve this effect, 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, and even 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 disposed at the interface, the electrolyte membrane will not crack even if the average content is excessive. However, if the average content of the first fine particles is 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, and even more preferably 5.0 mass% or less.

[1.4.4. Average particle size of first fine particles]
The term "average particle size of the first fine particles" refers to the average value of the maximum dimension of 20 or more first fine particles randomly selected under observation with a microscope.

The average particle size of the first fine particles is not particularly limited, and an optimal average particle size can be selected depending on the purpose. In general, if the average particle size of the first fine particles is too large, the first fine particles may break through the membrane, causing a short circuit or a leak. Therefore, the average particle size of the first fine particles is preferably 1/10 or less of the membrane thickness of the electrolyte membrane. The average particle size is more preferably 1/100 or less, and even more preferably 1/5000 or less of the membrane thickness of the electrolyte membrane.
On the other hand, if the average particle size of the first fine particles is too small, the first fine particles tend 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, and even more preferably 2.0 nm or more.

[1.5. Ions of second metal element and/or second fine particles]
[1.5.1. material]
The "second metal element" refers to a metal element having a function of scavenging radicals.
The "second compound" refers to a compound containing a second metallic element.
The "second fine particles" refer to fine particles made of a second compound.

Compounds and ions containing certain metal elements mainly act to eliminate radicals. Therefore, adding an appropriate amount of fine particles of compounds containing such metal elements and/or ions of such metal elements to the electrolyte membrane can further suppress deterioration of the electrolyte membrane.

Examples of the second metal element include Ce, Ag, Cs, Sn, Mn, Ti, Co, Ni, Zn, Zr, Ru, Rh, Pd, Pt, Pr, etc. The electrolyte membrane may contain any one of these elements, or may contain two or more of them.
Among these, the second metal element is preferably at least one element selected from the group consisting of Ce, Ag, Cs, Sn, and Mn. The second metal element is particularly preferably Ce, because these elements have a large effect of suppressing the generation of radicals or a large effect of scavenging radicals.

The second compound may be a water-soluble compound or a poorly soluble compound. When a water-soluble second compound is used, the second compound dissociates 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 a poorly soluble second compound is used, the second compound is dispersed in the electrolyte membrane.
The second compound may be, for example, a nitrate, sulfate, carbonate, formate, acetate, citrate, perchlorate, phosphate, chloride, fluoride, hydroxide, or acetylacetonate compound.

[1.5.2. Content]
When the electrolyte membrane contains ions of the second metal element and/or second fine particles, it is preferable that the electrolyte membrane 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.

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

On the other hand, if the M/A ratio is too large, the conductivity of the electrolyte membrane may decrease. Therefore, the M/A ratio must be 0.15 or less. The M/A ratio is preferably 0.10 or less, more preferably 0.05 or less, even more preferably 0.04 or less, and even 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 optimal average particle size can be selected depending on the purpose. Other points regarding the diameter are the same as those regarding the average particle diameter of the first fine particles, and therefore the explanation will be omitted.

[1.5.4. Position of second metal element ions or second fine particles]
The position where the ions of the second metal element and the second fine particles are added is 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. Manufacturing method of membrane electrode assembly]
The method for producing a membrane electrode assembly according to the present invention includes the steps of:
a first step of attaching first fine particles made of a first compound having a function of decomposing hydrogen peroxide to a surface of an electrolyte membrane and/or a surface of an anode catalyst layer facing the electrolyte membrane;
a second step of forming the anode catalyst layer on the anode side surface of the electrolyte membrane such that the surface to which the first fine particles are attached faces inward, and further forming a cathode catalyst layer on the other side of the electrolyte membrane to form a laminate;
and a third step of heat-treating the laminate to obtain the membrane electrode assembly of the present invention.

[2.1. 1st process]
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).

The method for adhering the first fine particles is not particularly limited, and an optimal method can be selected depending on the purpose. Examples of the method for adhering the first fine particles include
(a) A method of spraying a dispersion liquid having first fine particles dispersed therein onto a surface of an electrolyte membrane or a surface of an anode catalyst layer, and drying the dispersion liquid;
(b) A method of applying first fine particles onto a surface of an electrolyte membrane or a surface of an anode catalyst layer by using a dry application method such as an electrostatic screen printing method;
etc.

The first fine particles may be attached to only one of the surface of the electrolyte membrane or the surface of the anode catalyst layer, or may be attached to both.
The first fine particles may be attached to the entire surface of the electrolyte membrane and/or the entire surface of the anode catalyst layer, or may be attached only to a part of the surface.

The method of adding the ions of the second metal element or the second fine particles to the electrolyte membrane can be, 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 in which after preparing an electrolyte membrane, a solution containing ions of a second metal element is applied to the surface of the electrolyte membrane, and the ions of the second metal element are diffused into the electrolyte membrane;
(c) a method in which after preparing an electrolyte membrane, a solution containing ions of a second metal element is applied to a surface of the electrolyte membrane, the ions of the second metal element are diffused into the electrolyte membrane, and a second compound is precipitated in the electrolyte membrane;
etc.

[2.2. 2nd process]
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 attached faces inward, and further, a cathode catalyst layer is formed on the other surface of the electrolyte membrane. is formed into a laminate (second step).

The method for forming the catalyst layer is not particularly limited. Examples of the method for forming the catalyst layer include
(a) A method of forming a catalyst layer on a substrate surface and transferring the catalyst layer onto the surface of an electrolyte membrane;
(b) A method of applying a paste for forming a catalyst layer on the surface of an electrolyte membrane and drying the paste;
etc.
When the catalyst layer is formed by the 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 obtaining a membrane/electrode assembly according to the present invention.
The heat treatment conditions are not particularly limited, and the optimum conditions can be selected depending on the purpose. The heat treatment is usually performed under a pressure of 1 kgf/cm ² (9.8×10 ⁻² MPa) to 100 kgf/cm The process is carried out by heating at 25° C. to 200° C. for 1 to 20 minutes while applying a pressure of ^1.2 (9.8 MPa).

[3. Action]
Deterioration of the electrolyte membrane is
(a) Oxygen that has permeated from the cathode to the anode reacts with hydrogen on the anode catalyst to produce _{H2O2 ;}
(b) _H2O2 produced on the anode _catalyst diffuses into the electrolyte membrane, and reacts with _Fe , which is present as an impurity in the _electrolyte membrane, to produce hydroxyl radicals.
(c) Hydroxyl radicals decompose electrolytes;
This is thought to be caused by the

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, _H2O2 generated at the anode is decomposed into water and oxygen before _it becomes hydroxyl radicals, which suppresses the decomposition of the electrolyte membrane and improves the chemical durability of the electrolyte membrane.
In particular, tungsten oxide and tungstate do not dissolve in the fuel cell environment (acidity, potential) and can exist in the form of fine particles at the interface between the electrolyte membrane and the anode catalyst layer. Therefore, dissolved ions do not exchange with sulfonic acid groups of the electrolyte, and high proton conductivity can be maintained, so that the cell performance does not decrease.
Furthermore, among tungsten oxides, _WO3 .(1/3) _H2O and _WO3.2H2O have a remarkably high hydrogen peroxide decomposition ability compared to other materials, and therefore, when they are placed at the electrolyte membrane/ _anode catalyst layer interface, the chemical durability of the electrolyte membrane is improved.

In addition, the first fine particles, which are stable under the fuel cell operating environment, do not move inside the membrane electrode assembly. Therefore, by selectively placing the first fine particles in areas of the electrolyte membrane where deterioration is severe using the method of the present invention, deterioration of the electrolyte membrane can be efficiently suppressed, and the deterioration suppression effect can be maintained for a long period of time.

Furthermore, when second fine particles and/or ions of a second metal element are further added to the electrolyte membrane in addition to disposing the first fine particles at the electrolyte membrane/anode catalyst layer interface, high durability can be obtained even if the amounts of these added are relatively small.
(a) The first fine particles have the effect of decomposing hydrogen peroxide to render it harmless, and even if the amount of the first fine particles added is small, a high hydrogen peroxide decomposition effect is exhibited.
(b) The second fine particles and the ions of the second metal element have the effect of scavenging radicals,
(c) When the first fine particles are coexisted with the second fine particles and/or ions of the second metal element, the hydrogen peroxide decomposition effect of the first fine particles is improved, and/or
(d) Even if radicals are generated from hydrogen peroxide that is not decomposed by the first fine particles, the second fine particles and/or the ions of the second metal element effectively eliminate the radicals,
It is thought that.

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

Casting solution: 1.20 g was placed in a φ35 mm flat petri dish and left in a thermostatic room atmosphere (25°C) for several days to dry out. Next, the petri dish was annealed at 140°C for 15 minutes. After annealing, the membrane was washed five times with ultrapure water. The membrane was then peeled off from the petri dish and air-dried.

Next, an ethanol solution in which _WO3 was dispersed at a ratio of 1 mass% was prepared. The 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 _WO3 was 4 ^cm2 . The application amount of _WO3 was set to an amount 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) was 20 mass%.

[1.1.2. Comparative Examples 1 to 2]
An electrolyte solution (D2020, Nafion (registered trademark) dispersion, 1000EW, 20 mass%, manufactured by Chemours), 1-propanol, and ultrapure water were mixed so that the electrolyte solution: 1-propanol: water = 10: 7: 3 (mass ratio), and then _WO3 was added. The amount of _WO3 added was set to an amount such that the average content in the entire membrane (the mass ratio of _WO3 to the total mass of the solid polymer electrolyte and _WO3 contained in the entire membrane) was 20 mass% (Comparative Example 1) or 3 mass% (Comparative Example 2). Furthermore, the electrolyte was dispersed by irradiating the mixed solution with ultrasonic waves for 10 minutes to obtain a cast solution.

Casting solution: 1.20 g was placed in a φ35 mm flat petri dish and left in a thermostatic room atmosphere (25°C) for several days to dry out. Next, the petri dish was annealed at 140°C for 15 minutes. After annealing, the membrane was washed five times with ultrapure water. The membrane was then peeled off from the petri dish and air-dried.

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

2. Test Method
[2.1. Visual evaluation of cracks]
Before the MEA was produced, the electrolyte membrane was visually evaluated for cracks.
[2.2. Scanning Transmission Electron Microscope (SEM) Observation]
A cross section of the obtained MEA and the surface of the electrolyte membrane after the anode catalyst layer was peeled off from the MEA were observed using a scanning transmission electron microscope (SEM).

3. Results
[3.1. Visual evaluation of cracks]
The results are shown in Table 1. In Comparative Example 1, cracks were found in the electrolyte membrane. On the other hand, in Example 1 and Comparative Example 2, no cracks were found in the electrolyte membrane.

[3.2. SEM Observation]
Fig. 2(A) shows an SEM image of a cross section of the membrane electrode assembly obtained in Example 1. Fig. 2(B) shows an SEM image of a cross section of the membrane electrode assembly obtained in Comparative Example 2. In the case of the MEA produced 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 case of the MEA produced by the cast 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. 3(A) 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. Furthermore, Fig. 3(B) shows an SEM image of the electrolyte membrane surface after the anode catalyst layer was peeled off from the membrane electrode assembly obtained in Comparative Example 2.
In the case of Comparative Example 2, most of the surface of the electrolyte membrane after the anode catalyst layer was peeled off was composed of electrolyte, and almost _{no WO3} was found exposed on the surface of the electrolyte membrane, whereas in the case of Example 1, only _WO3 was found on the surface of the electrolyte membrane after the anode catalyst layer was peeled off.

(Example 2, Comparative Example 3)
1. Preparation of Samples
[1.1. Example 2]
In the same manner as in Example 1, an electrolyte membrane was prepared in which WO ₃ powder was applied (application area: 4 cm ² ) to the anode side surface of the electrolyte membrane.

[1.2. Comparative Example 3]
In the same manner as in Example 1, a cast membrane consisting of only the electrolyte was prepared.
Next, an aqueous solution was prepared in which cerium nitrate was dispersed at a rate of 1 mass%. The aqueous solution was applied to the surface of the electrolyte membrane using an atomizer spray to obtain a composite electrolyte membrane. The application area of Ce was 4 ^cm2 . The amount of Ce applied was an amount equivalent to 20% of the amount of sulfonic acid groups in the electrolyte membrane in the application area.

2. Test Method
The obtained electrolyte membrane was left for 72 hours under conditions of 80° C. and 80% RH. Thereafter, the amount of W or Ce contained in the electrolyte membrane was quantified, and the values before and after the test were compared.
Figure 4 shows the application positions of the solutions containing various additives and the quantitative determination positions of W or Ce. As shown in Figure 4, the various additives were applied to approximately the center of the electrolyte membrane. The quantitative determination of W or Ce was performed at two locations: an area approximately in the center of the additive application area (quantitative determination position 1) and an area where the additive was not applied (quantitative determination position 2).

3. Results
The results are shown in Table 2. In the case of Example 2, the amount of W at the quantitative position 1 hardly changed before and after the test. Moreover, the amount of W at the quantitative position 2 was zero both initially and after the test.
On the other hand, in the case of Comparative Example 3, the amount of Ce at the quantification position 1 was greatly reduced after the test. Also, 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 by selectively adding WO ₃ to a portion where it is desired to suppress deterioration, the effect of suppressing deterioration in that portion can be maintained for a long period of time.

(Examples 4 to 10, Comparative Example 4)
1. Test Method
0.067 mmol of each of the additives was dispersed in 10 mL of water to prepare a dispersion, and 4.41 mmol of hydrogen peroxide was further added to this dispersion.
(a) Aluminum tungstate ( _Al2 ( _WO4 ) ₃ ) (Example 4)
(b) Barium tungstate ( _BaWO4 ) (Example 5)
(c) Iron tungstate (FeWO ₄ ) (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 additives (Comparative Example 4)

The resulting dispersion was kept at 80° C. for 1 hour, and then 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)×100/C ₀ (2)
however,
C ₀ is the concentration of hydrogen peroxide in the original dispersion,
C is the concentration of hydrogen peroxide contained in the dispersion after being kept at 80° C. for 1 hour.

2. Results
Table 3 shows _{the H2O2} decomposition rate of various additives. It was found that _H2O2 does not self-decompose due to heat under these experimental conditions (Comparative Example 4). On the other hand _{, H2O2 decomposed} not only when _WO2 in Example 10 was used, but also when various tungstates in Examples 4 to ₉ were used.
From the above, it is expected that chemical degradation can be suppressed by disposing tungstate at the interface between the electrolyte and the anode-side catalyst layer.

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

2. Test Method
Gas diffusion layers were placed on both sides of the obtained MEA, and the resulting MEA was assembled into a fuel cell. The electrode area was 0.64 cm ^2. Next, the obtained fuel cell was subjected to a 90-hour open circuit (OC) durability test 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: 133 kPa

The humidified gas discharged from the fuel cell during the test was condensed, and the resulting water was collected every 24 hours. The collected water contained dissolved F ions that had been produced by chemical degradation and decomposition of the electrolyte membrane. These were quantified using ion chromatography, and the cumulative amount of F ions up to 90 hours was calculated.

3. Results
The results are shown in Table 4. From Table 4, it can be seen that the amount of F elution in Example 11 is smaller than that in Comparative Example 5. This is believed to be because hydrogen peroxide was efficiently decomposed by distributing _WO2 unevenly at the electrolyte membrane/anode catalyst layer interface. Furthermore, the amount of F elution in Example 12 was even smaller than that in Example 11, confirming the effect of the combined addition of Ce and W.

Although the embodiment of the present invention has been described in detail above, the present invention is not limited to the above embodiment, and various modifications are possible without departing from the gist of the present invention.

The membrane electrode assembly of the present invention can be used in various electrochemical devices such as solid polymer fuel cells and solid polymer water electrolysis devices.

Claims (10)

an electrolyte membrane including 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;
a first fine particle present at an interface between the electrolyte membrane and the anode catalyst layer,
The first fine particles of the membrane electrode assembly are made of a first compound having a function of decomposing hydrogen peroxide. The membrane electrode assembly according to claim 1, wherein the first compound is a tungsten compound. The membrane electrode assembly according to claim 1 or 2, wherein the first compound is 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 microparticles is 0.01 mass% or more and 20.0 mass% or less. The membrane electrode assembly according to any one of claims 1 to 4, wherein the average particle size of the first microparticles is 1/10 or less of the membrane thickness of the electrolyte membrane. (a) ions of a second metal element having a radical scavenging function, which are ion-exchanged with protons of the acid groups of the solid polymer electrolyte, and/or
6. The membrane/electrode assembly according to claim 1, further comprising: (b) second fine particles dispersed in the electrolyte membrane and made of a second compound containing the second metal element. The membrane electrode assembly according to claim 6, wherein the second metal element is Ce. The membrane/electrode assembly according to claim 6 or 7, 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 microparticles 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 a surface of an electrolyte membrane and/or a surface of an anode catalyst layer facing the electrolyte membrane;
a second step of forming the anode catalyst layer on the anode side surface of the electrolyte membrane such that the surface to which the first fine particles are attached faces inward, and further forming a cathode catalyst layer on the other side of the electrolyte membrane to form a laminate;
A method for producing a membrane electrode assembly, comprising: a third step of heat-treating the laminate to obtain the membrane electrode assembly according to claim 1 .

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