JP7363645B2 - Fuel cell gas diffusion layer - Google Patents

Description

The present invention relates to a fuel cell gas diffusion layer, and more particularly to a fuel cell gas diffusion layer with excellent flooding resistance.

The basic unit of a polymer electrolyte fuel cell is a membrane electrode assembly (MEA) in which electrodes (catalyst layers) are bonded to both sides of a polymer electrolyte membrane. Furthermore, in polymer electrolyte fuel cells, a gas diffusion layer is generally arranged outside the catalyst layer. The gas diffusion layer is for supplying reaction gas and electrons to the catalyst layer, and carbon paper, carbon cloth, etc. are used for the gas diffusion layer. The catalyst layer is a reaction field for electrode reactions, and is generally made of a composite of carbon supporting an electrode catalyst such as platinum and a solid polymer electrolyte (catalyst layer ionomer).

The gas diffusion layer of a polymer electrolyte fuel cell is required to have high water repellency in addition to high gas permeability. Therefore, gas diffusion layers generally have a microporous layer (a layer containing conductive particles and water-repellent particles and having fine pores) formed on the surface of a porous base material such as carbon paper. used. A microporous layer is generally formed by applying a paste containing conductive particles and water-repellent particles to the surface of a porous base material, drying and firing.
However, since conventional gas diffusion layers have insufficient water repellency, flooding is likely to occur particularly during high-load operation.

In order to solve this problem, various proposals have been made in the past.
For example, in Patent Document 1,
(a) Apply a silane coupling agent to the surface of carbon paper, dry it,
(b) Applying a water repellent treatment liquid containing polytetrafluoroethylene (PTFE) and acetylene black to the surface of carbon paper and drying it,
(c) A diffusion layer obtained by firing at 350° C. for 8 hours is disclosed.
This document states that when the surface of carbon fibers is treated with a silane coupling agent, the affinity with the water repellent treatment liquid containing PTFE improves, and it becomes possible to uniformly disperse PTFE throughout the carbon paper. Are listed.

In Patent Document 2,
(a) Applying tetraethoxysilane to the surface of a base material made of carbon paper and hydrolyzing it with sodium hydroxide to form an intermediate film made of silica on the surface of the base material,
(b) A diffusion layer obtained by immersing carbon paper in a mixed solution containing 1H, 1H, 2H, 2H-perfluorodecyltriethoxysilane to form a water-repellent film on the surface of the intermediate film is disclosed. .
The same document states that the diffusion layer obtained in this way has improved water repellency durability due to chemical bonds between the base material and the intermediate coating and between the intermediate coating and the water-repellent coating. has been done.

Furthermore, in Patent Document 3,
(a) By dropping ammonia water into an ethanol solution of tetraethyl orthosilicate (TEOS) and C ₆ F ₁₃ CH ₂ CH ₂ Si(OCH ₃ ) ₃ , which is a type of silane coupling agent, the surface of the silica particles becomes silane. Producing hydrophobic group-modified particles modified with a coupling agent,
(b) A gas diffusion electrode base material obtained by immersing a carbon fiber nonwoven fabric in a dispersion containing hydrophobic group-modified particles and attaching the hydrophobic group-modified particles to the surface of the carbon fibers is disclosed.
This document describes that by using such a gas diffusion electrode base material, it is possible to fabricate a fuel cell that has excellent drainage properties and good power generation performance even under high humidity power generation conditions.

The fuel cell gas diffusion layer has the function of efficiently discharging water generated by electrochemical reactions. When a microporous layer containing a fluorine-based polymer such as PTFE is formed on the surface of a gas diffusion layer base material made of carbon paper or the like, the hydrophobicity of the gas diffusion layer is improved. As a result, water condensed inside the gas diffusion layer is easily discharged to the gas flow path by capillary force.
The pores in the gas diffusion layer include hydrophobic pores and hydrophilic pores. The more hydrophilic pores there are, the lower the fuel cell performance will be, so it is desirable that the water-repellent material be uniformly dispersed. However, since PTFE does not dissolve in any solvent and does not form a PTFE solution, a PTFE dispersion is used when producing a microporous layer. Therefore, the distribution of PTFE becomes non-uniform, resulting in insufficient water repellency.

In order to solve this problem, it is possible to make the surface of the fibers constituting the gas diffusion layer base material hydrophobic using various methods, as described in Patent Documents 1 to 3. However, simply by making the fiber surface hydrophobic, it is not possible to suppress the decrease in water repellency caused by non-uniform dispersion of the water repellent material. Furthermore, if the water repellent component in the gas diffusion layer is increased in order to ensure water repellency, the electronic conductivity of the gas diffusion layer may decrease.

Japanese Patent Application Publication No. 2008-181834 Japanese Patent Application Publication No. 2015-056298 Japanese Patent Application Publication No. 2016-195105

The problem to be solved by the present invention is to suppress a decrease in water repellency caused by non-uniform dispersion of a water repellent material in a fuel cell gas diffusion layer.
Another problem to be solved by the present invention is to provide a fuel cell gas diffusion layer that has both relatively high electronic conductivity and relatively high water repellency.

In order to solve the above problems, the fuel cell gas diffusion layer according to the present invention includes:
a base material made of a porous conductive material (A);
A microporous layer formed on the surface of the base material and containing conductive particles made of a conductive material (B) and a water-repellent material;
A water-repellent coating derived from a silane coupling agent having a hydrophobic group is formed on the surface of the base material and the surface of the conductive particles.

When a microporous layer containing conductive particles and a water-repellent material is formed on the surface of the base material, the water repellency of the gas diffusion layer can be improved to some extent. However, since the distribution of the water-repellent material within the microporous layer tends to be uneven, the water-repellency of the gas diffusion layer also tends to be insufficient.
On the other hand, in a gas diffusion layer comprising a base material and a microporous layer, when both the surface of the base material and the surface of the conductive particles are modified with a silane coupling agent having a hydrophobic group, the water repellent material becomes Decrease in water repellency caused by non-uniform dispersion is alleviated. As a result, the flooding resistance of the gas diffusion layer is improved. Further, by optimizing the amount of treatment with the silane coupling agent, excessive decrease in electronic conductivity can be suppressed.

FIG. 2 is a schematic diagram of a fuel cell gas diffusion layer. FIG. 3 is a schematic diagram of a hydrophobization process of a gas diffusion layer. FIG. 2 is a schematic diagram of an electrical resistance measurement method. FIG. 4(A) is a schematic diagram of a steam introduction X-ray CT measurement device. FIG. 4(B) is an enlarged schematic diagram of the upper end of the sample stage (the area surrounded by the broken line in FIG. 4(A)).

FIG. 5(A) is a cross-sectional SEM image (×1500) of the gas diffusion layer obtained in Comparative Example 1. FIG. 5(B) is a cross-sectional SEM image (×1500) of the gas diffusion layer obtained in Example 2. This is the electrical resistance of the gas diffusion layers obtained in Example 2 and Comparative Example 1. FIG. 3 is a diagram showing the relationship between the content of a water-repellent coating and electrical resistance. These are the power generation test results (45° C., 165% RH) of the fuel cells obtained in Example 2 and Comparative Example 1.

These are power generation test results (60° C., 80% RH) of the fuel cells obtained in Example 2 and Comparative Example 1. FIG. 3 is a diagram showing the oxygen concentration dependence of the limiting current value. FIG. 11(A) is a two-dimensional CT image of Comparative Example 1 under steam introduction. FIG. 11(B) is a two-dimensional CT image of Example 2 under steam introduction.

Hereinafter, one embodiment of the present invention will be described in detail.
[1. Fuel cell gas diffusion layer]
The fuel cell gas diffusion layer according to the present invention includes:
a base material made of a porous conductive material (A);
A microporous layer formed on the surface of the base material and containing conductive particles made of a conductive material (B) and a water-repellent material;
A water-repellent coating derived from a silane coupling agent having a hydrophobic group, which is bonded to the surface of the base material and the surface of the conductive particles.

[1.1. Base material]
[1.1.1. material]
In a polymer electrolyte fuel cell, current collectors each having a gas flow path are arranged on both sides of a membrane electrode gas diffusion layer assembly (MEGA). The base material of the gas diffusion layer has enough gas permeability (porosity) to supply the reaction gas (fuel gas, oxidant gas) to the catalyst layer, and a gap between the catalyst layer and the current collector. A level of conductivity is required that allows the exchange of electrons. Therefore, a porous conductive material (A) is used for the base material.

Examples of the conductive material (A) include carbon and graphite.
Furthermore, examples of the base material include carbon paper, carbon cloth, and graphite foam.
The base material is particularly preferably an aggregate of conductive fibers. This is because the pores formed by the gaps between the fibers have a high degree of communication, so that good gas diffusivity can be obtained.

[1.1.2. Base material thickness]
The thickness of the base material affects the drainage and water retention properties of the gas diffusion layer. Generally, if the thickness of the substrate becomes too thin, it becomes difficult for liquid to condense. Therefore, dry-up tends to occur under low humidity conditions. Therefore, the thickness of the base material is preferably 90 μm or more. The thickness is preferably 120 μm or more.
On the other hand, if the thickness of the base material becomes too thick, water will condense and liquid water will tend to accumulate. Therefore, flooding is likely to occur under high humidity conditions. Therefore, the thickness of the base material is preferably 250 μm or less. The thickness is preferably 220 μm or less.

[1.2. Microporous layer]
The microporous layer (MPL) is for promoting discharge of water generated in the catalyst layer, and includes conductive particles made of a conductive material (B) and a water-repellent material. The MPL is formed on the surface of the substrate (ie, at the interface between the catalyst layer and the substrate).

[1.2.1. Conductive particles]
The conductive material (B) constituting the conductive particles may be the same as or different from the conductive material (A) constituting the base material. Further, the shape of the conductive particles is not particularly limited. Examples of the conductive particles include carbon black, carbon fiber, graphite, and activated carbon.

[1.2.2. Water repellent material]
The water-repellent material is used to impart water repellency to the MPL and at the same time bind the conductive particles. The water-repellent material is not particularly limited as long as it has such a function.
Examples of water-repellent materials include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), polyvinyl fluoride (PVF), and polyhexafluoropropylene (FEP). The water-repellent material may be made of any one of these materials, or may be made of two or more of these materials.

[1.2.3. Composition of MPL]
The optimum composition of MPL is selected depending on the purpose. Generally, the higher the proportion of water repellent material contained in MPL, the higher the water repellency. On the other hand, when the proportion of the water-repellent material becomes excessive, the conductivity decreases. The optimum proportion of the water-repellent material varies depending on the type of the water-repellent material and the conductive material (B), but is usually about 1 mass% to 50 mass%. The proportion of the water-repellent material is preferably 3 mass% to 40 mass%, more preferably 10 mass% to 30 mass%.

[1.2.4. MPL thickness]
The optimal thickness of the MPL is selected depending on the purpose. Generally, if the thickness of MPL is too thin, sufficient water repellency cannot be obtained. Therefore, the thickness of MPL is preferably 10 μm or more. The thickness of MPL is preferably 20 μm or more.
On the other hand, if the thickness of the MPL becomes too thick, the gas diffusivity or electron conductivity of the MPL will decrease. Therefore, the thickness of MPL is preferably 60 μm or less. The thickness of MPL is preferably 50 μm or less.

[1.3. Water repellent film]
The water-repellent film consists of a film derived from a silane coupling agent having a hydrophobic group. The water-repellent coating is formed not only on the surface of the base material of the gas diffusion layer but also on the surface of the conductive particles in the MPL. This point is different from the conventional method.

[1.3.1. Silane coupling agent]
The silane coupling agent having a hydrophobic group is represented by the general formula: R _4-n Si(-OR") _n . However, R is a hydrophobic group, -OR" is an alkoxy group, and n is 1 or more and 3 It is an integer below. Furthermore, when two or more hydrophobic groups (R) are bonded to the Si atom, each R may be the same or different from each other.
In addition, when two or more -OR'' are bonded to a Si atom, the silane coupling agent may be bonded to the base material or conductive particles via one -Si-O- bond. Alternatively, they may be bonded via two or more -Si-O- bonds.

In the present invention, the type of hydrophobic group (R) contained in the silane coupling agent is not particularly limited, and the most suitable one can be selected depending on the purpose.
Examples of the hydrophobic group (R) include a fluoroalkyl group and an alkyl group. In particular, a fluoroalkyl group has higher hydrophobicity than other hydrophobic groups, and is therefore suitable as the hydrophobic group (R) contained in the silane coupling agent.

In the present invention, the type of silane coupling agent that modifies the surfaces of the base material and conductive particles is not particularly limited, and the most suitable one can be selected depending on the purpose.
As the silane coupling agent, for example,
1H,1H,2H,2H-heptadecafluorodecyltrimethoxysilane,
1,1,1-trifluoro-3-(trimethoxysilyl)propane,
1H,1H,2H,2H-nonafluorohexyltrimethoxysilane,
1H,1H,2H,2H-nonafluorohexyltriethoxysilane,
trimethoxy(1H,1H,2H,2H-perfluoro-n-octyl)silane,
Examples include 1,1,1-trifluoro-3-[dimethoxy(methyl)silyl]propane.

[1.3.2. Content of water-repellent film]
The "content of the water-repellent coating" refers to the ratio of the mass of the water-repellent coating to the total mass of the fuel cell gas diffusion layer. The content of the water-repellent coating affects the performance of the gas diffusion layer. Generally, if the content of the water-repellent coating becomes too low, the water repellency will be insufficient. Therefore, the content of the water-repellent coating is preferably 1 mass% or more. The content is preferably 3 mass% or more, more preferably 5 mass% or more.
On the other hand, when the content of the water-repellent coating becomes excessive, the electrical resistance of the gas diffusion layer increases excessively. Therefore, the content of the water-repellent coating is preferably 20 mass% or less. The content is preferably 15 mass% or less, more preferably 10 mass% or less.

[2. Manufacturing method of fuel cell gas diffusion layer]
The fuel cell gas diffusion layer according to the present invention includes:
(a) preparing a gas diffusion layer precursor in which MPL is formed on the surface of a base material,
(b) Obtained by immersing the gas diffusion layer precursor in an ethanol solution containing a silane coupling agent, and hydrolyzing and polycondensing the silane coupling agent.

[2.1. 1st process]
First, a gas diffusion layer precursor having an MPL formed on the surface of a base material is prepared. The method for producing the gas diffusion layer precursor is not particularly limited. The gas diffusion layer precursor is usually
(a) Water-repellent treatment of a base material made of carbon paper or the like with a PTFE dispersion,
(b) Applying an MPL forming paste containing conductive powder and a water-repellent material such as PTFE particles to the surface of the base material,
(c) Obtained by drying and baking the coating film.

[2.2. 2nd process]
Next, the gas diffusion layer precursor is immersed in an ethanol solution containing a silane coupling agent, and the silane coupling agent is hydrolyzed and polycondensed (second step). As a result, the hydrophilic groups on the surfaces of the base material and conductive particles condense with the alkoxy groups of the silane coupling agent, and a water-repellent film derived from the silane coupling agent is formed on the surfaces of the base material and conductive particles. Ru.

Specifically, the water-repellent coating is formed as follows. That is, when ion-exchanged water is dropped into an ethanol solution containing a silane coupling agent, the alkoxy group is hydrolyzed and the silane coupling agent is converted into silanol. The following formula (1) shows an example of the hydrolysis reaction when the silane coupling agent is R-Si(OCH ₃ ) ₃ .
R-Si(OCH ₃ ) ₃ +3H ₂ O → R-Si(OH) ₃ +3CH ₃ OH…(1)

Next, the silanol group (-Si-OH) of the silane coupling agent and the hydrophilic group (eg, hydroxy group, carboxyl group) on the surface of the base material or conductive particles are condensed. The following formula (2) shows an example of the condensation reaction when the base material is R'-OH.
R-Si(OH) ₃ +3R'-OH → R-Si-(OR') ₃ +3H ₂ O...(2)

Note that when ion-exchanged water is dropped into an ethanol solution containing a silane coupling agent, in addition to the reactions of formulas (1) and (2) described above, the reaction (self-condensation) represented by the following formula (3) occurs. It may happen. Self-condensation products hardly contribute to making the water repellency of the gas diffusion layer uniform, so it is preferable to suppress self-condensation. Such self-condensation can be suppressed by adjusting the pH in the solution.
2R-Si(OH) ₃ → R-Si(OH) ₂ -O-Si(OH) ₂ -R+H ₂ O...(3)

[3. Effect]
FIG. 1 shows a schematic diagram of a fuel cell gas diffusion layer. A fuel cell gas diffusion layer is generally formed by applying a microporous layer-forming paste containing conductive powder and a water-repellent material (for example, PTFE particles) to the surface of a base material such as carbon paper that has been treated with a PTFE dispersion to make it water-repellent. It is manufactured by drying and baking the coating film. In the gas diffusion layer thus obtained, there are pores having hydrophobicity and pores having hydrophilicity. The more hydrophilic pores there are, the worse the fuel cell performance is, so it is desirable that the water-repellent material is uniformly dispersed within the gas diffusion layer.

However, water-repellent materials such as PTFE are usually insoluble in solvents and are therefore used in the form of fine particle dispersions. Therefore, when the surface of carbon paper is treated with water repellency using such a fine particle dispersion, or when a microporous layer is formed using a paste containing fine particles made of conductive particles and a water repellent material, the gas diffusion layer The distribution of the water-repellent material within the container tends to be non-uniform. As a result, flooding occurs locally within the gas diffusion layer, reducing fuel cell performance. On the other hand, if the content of the water-repellent material is increased in order to avoid this, the electronic conductivity of the gas diffusion layer will decrease.

On the other hand, in a gas diffusion layer comprising a base material and a microporous layer, when both the surface of the base material and the surface of the conductive particles are modified with a silane coupling agent having a hydrophobic group, the water repellent material becomes Decrease in water repellency caused by non-uniform dispersion is alleviated. As a result, the flooding resistance of the gas diffusion layer is improved. Further, by optimizing the amount of treatment with the silane coupling agent, excessive decrease in electronic conductivity can be suppressed.

(Examples 1 to 4, Comparative Example 1)
[1. Preparation of sample]
[1.1. Preparation of gas diffusion layer]
[1.1.1. Examples 1 to 4]
FIG. 2 shows a schematic diagram of the hydrophobization process of the gas diffusion layer. A commercially available product (SIGRACET (registered trademark) SGL29BC manufactured by SGL) was used for the gas diffusion layer. 1H,1H,2H,2H-heptadecafluorodecyltrimethoxysilane (manufactured by Tokyo Chemical Industry Co., Ltd., T2917) was used as the silane coupling agent. Four types of ethanol solutions containing the above-mentioned silane coupling agent and having different concentrations of the silane coupling agent were used as the treatment liquid for hydrophobizing the gas diffusion layer.

A treatment liquid was placed in a beaker, and a gas diffusion layer fixed to a jig was immersed in the treatment liquid. In order to promote hydrolysis of the silane coupling agent, ion-exchanged water was added dropwise to the treatment liquid. Further, in order to suppress self-condensation of the silane coupling agent, the pH was adjusted with acetic acid. In this state, the treatment liquid was heated at 60° C. for 4 hours. After heating, the gas diffusion layer was taken out from the treatment solution, washed with ethanol, and dried. The content of the water-repellent coating is 1.5 mass% (Example 1), 3.0 mass% (Example 2), 12.0 mass% (Example 3), or 24.0 mass% (Example 4). there were.

[1.1.2. Comparative example 1]
The untreated gas diffusion layer was subjected to the test as is.

[1.2. Production of fuel cell]
A membrane electrode assembly was produced in which catalyst layers containing a Pt catalyst were formed on both sides of a Nafion (registered trademark) membrane. Next, the gas diffusion layer obtained in Example 2 or Comparative Example 1 was placed on the air electrode side of the membrane electrode assembly, and the untreated gas diffusion layer (commercially available) was placed on the hydrogen electrode side of the membrane electrode assembly. The MEGA was obtained. Further, current collectors having straight groove channels were placed on both sides of the MEGA to obtain a fuel cell.

[2. Test method]
[2.1. Morphological observation]
The surface of the gas diffusion layer and the inside of the base material were observed visually and using an optical microscope. Furthermore, the inside of the microporous layer was observed using a FIB-SEM (NB5000, manufactured by Hitachi High-Tech Corporation). When observing the inside of the microporous layer using FIB-SEM, a cross section of the test piece was prepared by irradiating Ga ions at an accelerating voltage of 40 kV, and a secondary electron image (5 kV) was obtained.

[2.2. Electrical resistance measurement]
FIG. 3 shows a schematic diagram of the electrical resistance measurement method. Electrodes were placed on both sides of the test piece, and a pressure of 1.0 MPa was applied. In this state, the electrical resistance of the test piece was measured using a tester.

[2.3. Power generation test]
Pure hydrogen is supplied to the fuel electrode of the fuel cell, and oxygen diluted with nitrogen (1%, 5%, or 10%) is supplied to the air electrode at flow rates of 500 mL/min and 2000 mL/min, respectively, via a humidifier. The power generation performance was evaluated. The power generation conditions are
(a) Cell temperature: 60°C, relative humidity: 80%RH, or (b) Cell temperature: 45°C, relative humidity: 165%RH
And so.

[2.4. Steam introduction X-ray CT measurement]
FIG. 4(A) shows a schematic diagram of a steam introduction X-ray CT measurement device. FIG. 4(B) shows an enlarged schematic diagram of the upper end of the sample stage (the area surrounded by the broken line in FIG. 4(A)). In FIG. 4, the steam introduction X-ray CT measurement apparatus 10 includes a rotation stage 12, a Peltier element 14, a sample stage 16, a heat insulating material 18, and an upper cover 20.

A Peltier element 14 for heating the sample to a predetermined temperature is mounted on the upper surface of the rotation stage 12. A sample stage 16 is erected on the surface of the Peltier element 14, and the sample stage 16 is surrounded by a heat insulating material 18.
The upper cover 20 includes an inner cover 22 that opens toward the tip of the sample stage 16, a heater 24 that covers the outer periphery of the inner cover 22, and a Kapton (registered trademark) heater 24 that is installed at the tip of the opening of the inner cover 22. A film 26 is provided. The upper cover 20 is provided with a steam supply port 28 that penetrates the inner lid 22 and the heater 24.

A gas diffusion layer with a microporous layer formed on the surface of the base material was placed on the tip of the sample stage 16 with the microporous layer facing upward, and the upper surface of the gas diffusion layer was fixed with a PTFE washer 30 (see FIG. 4). See B). Steam was supplied to the upper surface of the gas diffusion layer from the steam supply port 28 using the Peltier element 14 and the heater 24 while maintaining the gas diffusion layer at a predetermined temperature. X-rays were irradiated from the outside of the Kapton (registered trademark) film 26 toward the gas diffusion layer, and a two-dimensional CT image near the surface of the micropolar layer was obtained using the X-ray camera 32.

[3. result]
[3.1. Morphological observation]
When a silane coupling agent is subjected to condensation polymerization, lumps may be formed due to side reactions (self-condensation). Regarding the surface of the gas diffusion layer and the inside of the base material, it was confirmed by visual inspection and using an optical microscope that no lumps were formed.
Regarding the inside of the microporous layer, the presence or absence of lumps was confirmed by cross-sectional SEM observation. FIG. 5(A) shows a cross-sectional SEM image (×1500) of the gas diffusion layer obtained in Comparative Example 1. FIG. 5(B) shows a cross-sectional SEM image (×1500) of the gas diffusion layer obtained in Example 2. From FIG. 5, no lumps of silane coupling agent formed by self-condensation were observed, suggesting that only the reaction of formula (2) described above proceeded.

[3.2. Electrical resistance measurement]
FIG. 6 shows the electrical resistance of the gas diffusion layers obtained in Example 2 and Comparative Example 1. From FIG. 6, it can be seen that Example 2 in which a water-repellent film was formed had a higher electrical resistance than Comparative Example 1 in which a water-repellent film was not formed.
FIG. 7 shows the relationship between the content of the water-repellent coating and the electrical resistance. In order to obtain desired power generation performance in a fuel cell, the resistance value of the gas diffusion layer is preferably 13 mΩ·cm ² or less. From FIG. 7, it can be seen that when the content of the water-repellent coating (that is, the amount of silane coupling agent added) is set to 20 mass% or less, the resistance value of the gas diffusion layer becomes 13 mΩ·cm ² or less.

[3.3. Power generation test]
FIG. 8 shows the power generation test results (45° C., 165% RH) of the fuel cells obtained in Example 2 and Comparative Example 1. FIG. 9 shows the power generation test results (60° C., 80% RH) of the fuel cells obtained in Example 2 and Comparative Example 1. Furthermore, FIG. 10 shows the oxygen concentration dependence of the limiting current value. Note that the "limit current value" refers to the current density when the corrected cell voltage is 0.2V. The following can be seen from FIGS. 8 to 10.

(1) Example 2 showed higher performance than Comparative Example 1, and the output performance improved as the oxygen concentration increased. (See Figure 10). It is thought that the higher the oxygen concentration, the more water generated in the membrane electrode assembly, which is why the difference in water repellency in the fuel cell gas diffusion layer became more pronounced.
(2) The difference in limiting current values between Example 2 and Comparative Example 1 under high temperature and low humidity conditions (60°C, 80% RH) is larger than that under low temperature and high humidity conditions (45°C, 165% RH) .

[3.4. Steam introduction X-ray CT measurement]
In order to understand the mechanism by which fuel cell power generation performance improves when the water repellency of the fuel cell gas diffusion layer increases, steam was introduced into the fuel cell gas diffusion layer using the steam introduction X-ray CT measurement device 10 shown in FIG. At the same time, X-ray CT measurements were performed. FIG. 11(A) shows a two-dimensional CT image of Comparative Example 1 under steam introduction. FIG. 11(B) shows a two-dimensional CT image of Example 2 under steam introduction. From FIG. 11, it can be seen that the size of the droplets generated on the surface of the microporous layer in Example 2 is smaller than that in Comparative Example 1. When the diameter of the droplet was measured by image analysis, it was 2.5 μm in Comparative Example 1, while it was 1.4 μm in Example 2.

Considering that there was no segregation of the silane coupling agent as confirmed in the cross-sectional SEM image, small droplets were formed inside the microporous layer as well as on the surface due to the silane coupling agent treatment. It is assumed that this will be done. The smaller the size of the formed droplet, the easier it is for the droplet to move through the pores inside the microporous layer. The reason why the output performance of Example 2 was improved over Comparative Example 1 is that by treating the gas diffusion layer with a microporous layer with a silane coupling agent, the size of the droplets generated inside the gas diffusion layer became smaller, and the gas This is thought to be because water is more likely to be discharged outside the diffusion layer.

Although the embodiments of the present invention have been described in detail above, the present invention is not limited to the above embodiments, and various modifications can be made without departing from the gist of the present invention.

The fuel cell gas diffusion layer according to the present invention can be used as a gas catalyst layer of a polymer electrolyte fuel cell.

Claims (3)

a base material made of a porous conductive material (A);
A microporous layer formed on the surface of the base material and containing conductive particles made of a conductive material (B) and a water-repellent material;
A fuel cell gas diffusion layer comprising a water-repellent coating derived from a silane coupling agent having a hydrophobic group and formed on the surface of the base material and the surface of the conductive particles. The fuel cell gas diffusion layer according to claim 1, wherein a content of the water-repellent coating (=ratio of the mass of the water-repellent coating to the total mass of the fuel cell gas diffusion layer) is 1 mass% or more and 20 mass% or less. The fuel cell gas diffusion layer according to claim 1 or 2, wherein the hydrophobic group is a fluoroalkyl group.

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