JP7640495B2 - Coating paste for forming microporous layer, its manufacturing method, and gas diffusion layer for fuel cell - Google Patents

Description

The present invention relates to a coating paste for forming a microporous layer, which is applied to the surface of a conductive porous substrate (gas diffusion layer electrode substrate) that constitutes a gas diffusion layer (GDL) for a fuel cell to form a microporous layer (MPL), and a manufacturing method thereof, as well as a gas diffusion layer for a fuel cell employing the same. In particular, the present invention relates to a coating paste for forming a microporous layer that can form a microporous layer with a high porosity, a manufacturing method thereof, and a gas diffusion layer for a fuel cell employing the same.

In recent years, fuel cells, particularly polymer electrolyte fuel cells (PEFCs), have been attracting much attention due to their potential for a wide range of applications, including automobiles, home power sources, and home appliances.
A polymer electrolyte fuel cell uses an ion-conductive polymer membrane as an electrolyte, and a negative electrode and a positive electrode are bonded to this to form a single cell. Multiple single cells are stacked with separators between them to generate high voltage.

Generally, one single cell constituting a polymer electrolyte fuel cell has a cathode (+) side electrode and an anode (-) side electrode constituted by an electrode catalyst layer made of a conductive material such as carbon carrying a catalyst such as platinum and an ion exchange resin, and a porous gas diffusion layer (GDL) disposed on the outside of the electrode catalyst layer, on both sides of a polymer electrolyte membrane (ion-conductive solid polymer electrolyte membrane) that selectively transmits specific ions, thereby forming a membrane-electrode-gas diffusion layer assembly (MEGA).
In a polymer electrolyte fuel cell, a separator having gas flow paths for supplying a fuel gas (anode gas) or an oxidizing gas (cathode gas) and discharging generated gas and excess gas is disposed on the outside of the gas diffusion layer constituting the membrane/electrode assembly, and the membrane/electrode assembly is sandwiched between the separators to form a stack of multiple unit cells, thereby improving power generation.

Since solid polymer fuel cells with this configuration generate energy with high efficiency and have a low environmental impact, they are increasingly being used as a power source (driving source) for automobiles, and there is a demand for higher output cells through improved performance of the various components that make up the fuel cell, in order to further popularize them as automobile fuel cells.

In the configuration of such a polymer electrolyte fuel cell, as described above, in order to efficiently react the fuel gas (anode gas) and the oxidizing gas (cathode gas), a gas diffusion layer that guides the fuel gas or oxidizing gas supplied from the gas flow path of the separator is adjacent to a catalyst layer that supports a catalyst such as platinum and performs an electrochemical reaction. In other words, the gas diffusion layer that constitutes the electrode of the single cell battery is formed to increase the diffusibility of the reactant gas, and plays the role of diffusing the fuel gas or oxidizing gas supplied from the gas flow path of the separator to the adjacent catalyst layer (gas diffusibility, gas permeability). In addition, this gas diffusion layer between the separator and the catalyst layer is required to have a conductive function and a current collecting function to efficiently move the electrons necessary for the electrochemical reaction. Furthermore, to improve and stabilize power generation performance, it is important to keep the polymer electrolyte membrane in an optimal wet state in order to obtain high proton conductivity (conductivity) in the electrodes, while quickly draining excess moisture from the catalyst layer to prevent flooding (the phenomenon in which the pores of the gas diffusion layer are blocked by water). The gas diffusion layer, which is also a moisture passageway, is required to properly take in the necessary moisture into the reaction system while also having drainage properties (water repellency, moisture permeability) to drain excess reaction water and condensation water generated by the electrochemical reaction of hydrogen and oxygen during power generation. It is also required to properly take in and drain moisture, i.e., moisture management through diffusivity.

In recent years, it has been proposed to form a coating thin film (sometimes called a microporous layer, carbon layer, or filling layer) called a microporous layer (MPL) mainly composed of a water-repellent resin such as PTFE and a conductive material such as carbon black on the conductive porous substrate (gas diffusion layer electrode substrate) that constitutes the gas diffusion layer, for example, made of carbon paper, carbon cloth, carbon felt, etc., to suppress the generation of a liquid film at the interface with the adjacent catalyst layer, promote water discharge and reverse diffusion of generated water, and improve the water management (water content control) of the solid polymer electrolyte fuel cell, thereby improving the power generation performance. In addition, by forming a microporous layer on the conductive porous substrate, the microporous layer also functions as a buffer material between the catalyst layer and the conductive porous substrate, and as a cosmetic effect that prevents the surface of the conductive porous substrate from being transferred to the electrolyte membrane. In addition, in a case where a microporous layer is formed by applying a paste for forming a microporous layer to a conductive porous substrate and drying and baking it, the fine characteristics of the gas diffusion layer can be easily controlled by adjusting the paste components.

Here, examples of this type of known technology include those disclosed in Japanese Patent Application Laid-Open No. 2003-233699 and Japanese Patent Application Laid-Open No. 2003-233699.
Patent Document 1 discloses a gas diffusion electrode substrate having a carbon sheet and a microporous layer, the carbon sheet being porous, the carbon powder contained in the microporous layer having a DBP oil absorption of 70 to 155 ml/100 g, the microporous layer having a penetration index (L/W) calculated from the basis weight (W) of the microporous layer and the thickness (L) of the microporous layer of 1.10 to 8.00, and the thickness (L) of the microporous layer being 10 to 100 μm.

Patent Document 2 discloses a paste composition for forming a microporous layer, which contains carbon black, a water-repellent resin, a dispersant, and a solvent, and uses a first carbon black having a DBP absorption amount in the range of 210 ml/100 g to 262 ml/100 g and a second carbon black having a DBP absorption amount in the range of 32 ml/100 g to 48 ml/100 g, and further discloses that the iodine adsorption amount of the first carbon black is in the range of 85 mg/g to 97 mg/g, and the iodine adsorption amount of the second carbon black is in the range of 15 mg/g to 20 mg/g.

According to the technology of Patent Document 1, by increasing the DBP oil absorption of the carbon used to form the microporous layer (hereinafter sometimes abbreviated as "MPL"), it is possible to suppress the penetration of the MPL coating liquid into the carbon sheet, and according to the technology of Patent Document 2, by combining carbon blacks with specific differences in the degree of structure development and controlling the chain structure and three-dimensional structure of the carbon particles, it is possible to suppress cracks in the MPL.

In addition, in Patent Documents 1 and 2, the carbon to be blended into the coating paste for forming the MPL is proposed to be of a high DBP absorption, because if the carbon has a small structure and a small DBP absorption, it will easily penetrate the conductive porous substrate made of a carbon sheet or the like when applied to the substrate and strike through to the back, and it will also be weak in strength and prone to cracking.

International Publication No. 2016/076132 JP 2019-040791 A (Patent No. 6932447 A)

However, carbon with a high DBP absorption capacity has structures (aggregates) that tend to become entangled with each other, causing the structures to develop excessively (with secondary agglomerate diameters of several tens of μm or more), which generates agglomerates (aggregated particles) and adsorbs and retains large amounts of resin and solvent, resulting in poor dispersibility and an increase in the viscosity of the paint, which reduces the coatability, causing surface defects in the gas diffusion electrode (reduced surface smoothness and narrowed coating width) and reducing productivity.
In addition, carbon products often contain foreign matter (impurities), such as sulfur, metals (potassium, lead, sodium, iron, etc.), calcium, chlorine, etc., and these foreign matter, particularly iron foreign matter, tend to reduce the durability and lifespan of the catalyst layer adjacent to the gas diffusion layer, thereby reducing battery performance and lifespan. When carbon with a high DBP absorption is blended into the coating paste for forming the MPL, the agglomerates cause clogging in fine-mesh filtration filters used as foreign matter removal filters for removing foreign matter in carbon products, particularly iron foreign matter, reducing passability through the filtration filter, making it difficult to remove iron foreign matter and the like that reduces the durability and lifespan of the battery.

The use of carbon with a high DBP absorption rate therefore causes aggregation and increases the viscosity of the coating paste, so there have traditionally been limitations to the use of carbon with a highly developed structure to improve the porosity (air porosity) and thereby improve the diffusivity and other performance of the gas diffusion layer.

The present invention aims to provide a coating paste for forming a microporous layer that can improve the performance of the gas diffusion layer by forming a microporous layer with a high porosity, a method for producing the same, and a gas diffusion layer for a fuel cell.

The coating paste for forming a microporous layer of the invention of claim 1 contains carbon black, a water-repellent resin, a dispersant, and a solvent, and the carbon black has a secondary agglomeration diameter that allows the carbon black, which has a DBP absorption amount in the range of 300 ml/100 g to 500 ml/100 g, to pass through a mesh with an opening size in the range of 34 to 154 μm. The coating paste is applied to the surface of a conductive porous substrate made of a carbon fiber sheet or the like, and is dried and fired to form a microporous layer of a gas diffusion layer for a fuel cell.

As the carbon black, for example, furnace black, thermal black, ketjen black, acetylene black, etc. can be used, but acetylene black is preferred since it can form a microporous layer having high gas diffusivity, strength, conductivity, etc.
The carbon black having a DBP absorption in the range of 300 ml/100 g to 500 ml/100 g and having a secondary agglomerate size that can pass through a mesh with an opening of 34 to 154 μm means that the secondary agglomerates (several tens of μm) of the carbon black having a DBP absorption in the range of 300 ml/100 g to 500 ml/100 g have been disintegrated (disaggregated) by high shear dispersion to a secondary agglomerate size (several μm) that can pass through a mesh with an opening of 34 μm to 154 μm, i.e., 100 mesh to 400 mesh. In other words, the carbon black having a DBP absorption in the range of 300 ml/100 g to 500 ml/100 g has been disintegrated to have a secondary agglomerate size that can pass through a mesh with an opening of 34 to 154 μm (100 to 400 mesh).

Here, the "DBP absorption amount" is measured in accordance with JIS K6217-4. The DBP absorption amount (oil absorption amount) utilizes the fact that the porosity between carbon black aggregates is positively correlated with the structure, and the DBP (dibutyl phthalate) is absorbed into the voids to indirectly quantify the structure based on the DBP absorption amount. The larger (higher) this value is, the more developed the carbon black structure is, and the higher the structure is, and the smaller (lower) the value is, the less developed the structure is, and the lower the structure is. The structure indicates the degree of development of chain-like aggregates (aggregates) consisting of several or several tens of carbon black particles, that is, the degree of development of the connections between carbon black particles. If the number of carbon particles constituting the aggregate is large or the aggregate has a complex shape, the DBP absorption amount is high.
In this specification and claims, the mesh openings are those specified in JIS G 3556.

The coating paste for forming a microporous layer of the invention of claim 1 has a viscosity at a shear rate of 50 [1/s] of 0.05 Pa·s or more and 1.0 Pa·s or less, more preferably 0.1 Pa·s or more and 0.9 Pa·s or less, and even more preferably 0.1 Pa·s or more and 0.5 Pa·s or less.

In the coating paste for forming a microporous layer according to the invention, the carbon black and the water-repellent resin have a solid weight ratio of 9/1≦carbon black/water-repellent resin≦5/5 .

The method for producing a coating paste for forming a microporous layer of the invention of claim 3 comprises mixing carbon black having a DBP absorption capacity in the range of 300 ml/100 g or more and 500 ml/100 g or less and a water-repellent resin in a solvent in which a dispersant has been mixed and in which the dispersant has been dissolved and mixed, and stirring the mixture by a thin film swirling method.

Here, the thin film swirling method is a dispersion method in which the material to be treated (mixed material) is rotated at high speed while being pressed against the inner wall surface of the device in a thin cylindrical shape by centrifugal force, and the shear stress generated by the centrifugal force and the speed difference between the inner wall surface of the device and the swirling flow is applied to the dispersed material (mixed material), thereby creating a high-energy turbulent state throughout the tank, dispersing the aggregated particles in the thin cylindrical processed material (mixed material). As a swirling thin film type high-speed agitator using such a high-speed swirling thin film flow, for example, the thin film swirling type high-speed mixer "Filmix" (registered trademark) series (manufactured by Primix Co., Ltd.) can be suitably used. The stirring conditions at this time are preferably a circumferential speed at the tip of the stirring blade of 25 m/s or more, and the circumferential speed is 25 m/s or more, and the upper limit is 60 m/s or less in terms of the performance limit of the machine.
As the carbon black, for example, furnace black, thermal black, ketjen black, acetylene black, etc. can be used, but acetylene black is preferred because it is capable of forming a microporous layer having high gas diffusivity, strength, conductivity, etc.

The method for producing a coating paste for forming a microporous layer of the invention of claim 4 comprises mixing carbon black having a DBP absorption in the range of 300 ml/100 g or more and 500 ml/100 g or less and a water-repellent resin into a solvent in which a dispersant has been mixed and in which the dispersant has been dissolved and mixed, and stirring the mixture at a peripheral speed of 25 m/s or more and 60 m/s or less.

Here, the stirring at a peripheral speed of 25 m/s or more refers to stirring at a peripheral speed of 25 m/s or more at the tip of the stirring blade using a high-speed stirrer such as Homomixer, Homodisper, Neokaisa (registered trademark), Neomixa (registered trademark), thin film swirl type high-speed mixer, Homomixer, etc., the upper limit being 60 m/s or less in view of the performance limit of the machine. Among them, the thin film swirl type high-speed mixer "Filmix" (registered trademark) series (manufactured by Primix Corporation) can be preferably used.
As the carbon black, for example, furnace black, thermal black, ketjen black, acetylene black, etc. can be used, but acetylene black is preferred because it is capable of forming a microporous layer having high gas diffusivity, strength, conductivity, etc.

The method for producing a coating paste for forming a microporous layer of the invention of claim 5 further comprises passing the mixture through a mesh having an opening of 34 to 154 μm (100 to 400 mesh), preferably a mesh having an opening of 45 to 154 μm (100 to 300 mesh), more preferably a mesh having an opening of 45 to 150 μm (100 to 300 mesh), after the stirring.

The method for producing a coating paste for forming a microporous layer of the invention of claim 6 comprises mixing carbon black having a DBP absorption in the range of 300 ml/100 g or more and 500 ml/100 g or less and a water-repellent resin with a solvent in which a dispersant has been mixed and in which the dispersant has been dissolved and mixed, pre-stirring with a Disper or turbine-stator type stirrer, and then stirring by a thin film swirling method or at a peripheral speed of 25 m/s or more and 60 m/s or less.

Here, the disperser is a device for stirring the material to be treated (blended material) by generating shear through high speed rotation of stirring blades such as a disk turbine or a dissolver (dispers).
The above-mentioned turbine/stator type agitator applies shear force in the gap between the turbine and the stator to agitate the material to be treated (mixed materials).

The method for producing a coating paste for forming a microporous layer of the invention of claim 7 comprises mixing carbon black having a DBP absorption in the range of 300 ml/100 g or more and 500 ml/100 g or less and a water-repellent resin into a solvent in which a dispersant has been mixed and in which the dispersant has been dissolved and mixed, pre-stirring at a peripheral speed of 1 m/s or more and 20 m/s or less, and then stirring by a thin film swirling method or at a peripheral speed of 25 m/s or more and 60 m/s or less.

Here, the stirring at a peripheral speed of 1 m/s or more and 20 m/s or less refers to stirring at a peripheral speed at the tip of the stirring blade of an agitator of 1 m/s or more and 20 m/s or less, preferably 10 m/s or more and 20 m/s or less, and for example, a Disper or turbine-stator type agitator can be used.

In the method for producing a coating paste for forming a microporous layer according to the invention of claim 8 , the carbon black and the water-repellent resin have a solid weight ratio of 9/1≦carbon black/water-repellent resin≦5/5.

The coating paste for forming a microporous layer in the method for producing a coating paste for forming a microporous layer of the invention of claim 9 has a viscosity at a shear rate of 50 [1/s] of 0.05 Pa·s or more and 1.0 Pa·s or less, more preferably 0.1 Pa·s or more and 0.9 Pa·s or less, and even more preferably 0.1 Pa·s or more and 0.5 Pa·s or less.

According to the coating paste for forming a microporous layer according to the invention of claim 1, the coating paste contains carbon black, a water-repellent resin, a dispersant, and a solvent, and the carbon black has a DBP absorption in the range of 300 ml/100 g or more and 500 ml/100 g or less and has a secondary aggregate diameter that passes through a mesh with an opening of 34 to 154 μm. Since the secondary aggregates of carbon black with high DBP absorption are crushed, the viscosity increase of the paste is suppressed and the paste can be applied with good coating properties to the surface of the conductive porous substrate, and in the microporous layer formed by drying and baking the applied paste, the carbon black with high DBP absorption has a high porosity due to its high structure, resulting in a high porosity (air porosity).

In particular, when carbon black having a DBP absorption in the range of 300 ml/100 g to 500 ml/100 g has a secondary aggregation diameter that passes through a mesh with an opening of 34 to 154 μm, the paste has good coating properties to ensure good surface smoothness in the microporous layer, and can achieve both high porosity and coating strength that is less likely to cause cracks. That is, a microporous layer can be formed that has high porosity, good surface smoothness, few cracks, and good adhesion to the catalyst layer. Therefore, such a microporous layer is less likely to accumulate moisture and has high drainage properties, maintains high gas diffusivity, and has good durability.
In addition, when carbon black having a DBP absorption amount in the range of 300 ml/100 g to 500 ml/100 g has a secondary agglomeration diameter that passes through a mesh with an opening size in the range of 34 to 154 μm, the carbon black is of a size that can pass through a fine mesh filter that enables the removal of foreign matter such as iron that affects the durability and lifespan of the catalyst layer in which the cell reaction takes place, and is therefore unlikely to reduce the durability and lifespan of the battery.
Thus, the coating paste for forming a microporous layer according to the invention of claim 1 can form a microporous layer having a high porosity and capable of improving the gas diffusion properties and moisture management properties of the gas diffusion layer.

According to the coating paste for forming a microporous layer according to the invention of claim 1 , the viscosity at a shear rate of 50 s ^-1 is within the range of 0.05 Pa.s to 1.0 Pa.s, so that when applied to the conductive porous substrate constituting the gas diffusion layer, the paste can be prevented from striking through (the phenomenon of the paste penetrating through to the back side of the conductive porous substrate opposite to the surface to which the paste is applied), and a cured coating film of a microporous layer with good smoothness can be formed at a desired coating width, resulting in good coatability. Therefore, a smooth microporous layer can be obtained without reducing the porosity of the conductive porous substrate, and the adhesion to the catalyst layer can be improved , so that a microporous layer that can better exhibit the performance of the gas diffusion layer can be formed.

According to the coating paste for forming a microporous layer of the invention of claim 2 , the weight ratio of the carbon black to the water-repellent resin is within the range of 9/1 to 5/5, so that the adhesiveness (binding property, binder effect) of the resin in the cured coating film of the microporous layer formed is good, cracks are unlikely to occur, and clogging of voids due to excess resin is suppressed. Therefore, in addition to the effect of claim 1, a microporous layer can be formed that can achieve both coating film strength and gas and moisture permeability.

According to the method for producing a coating paste for forming a microporous layer according to the invention of claim 3 , carbon black having a DBP absorption in the range of 300 ml/100 g to 500 ml/100 g and a water-repellent resin are mixed in a solvent mixed with a dispersant and stirred by a thin film swirling method, and by dispersing the compounded materials with a high shear by the thin film swirling method, the secondary aggregates of the carbon black with a high DBP absorption can be broken down, and the viscosity increase of the paste can be suppressed. Therefore, according to the coating paste for forming a microporous layer in which the secondary aggregates of the carbon black with a high DBP absorption are broken down and the viscosity increase is suppressed, it can be applied to the surface of a conductive porous substrate with good coatability, and in the microporous layer formed by drying and baking the applied paste, the carbon black with a high DBP absorption has a high porosity due to its high structure, and therefore has a high porosity (air porosity).

In particular, when carbon black having a DBP absorption in the range of 300 ml/100 g to 500 ml/100 g has a secondary aggregation diameter that passes through a mesh with an opening of 34 to 154 μm, the good coating properties of the paste result in a microporous layer with good surface smoothness, and high porosity and coating strength that is less likely to cause cracks can be achieved. That is, a microporous layer can be formed that has high porosity, good surface smoothness, few cracks, and good adhesion to the catalyst layer. Therefore, such a microporous layer is less likely to accumulate moisture and has high drainage properties, maintains high gas diffusivity, and has good durability.
Therefore, according to the method for producing a coating paste for forming a microporous layer of the invention of claim 3 , a coating paste can be obtained that can form a microporous layer having a high porosity and that can improve the diffusion and moisture management performance of the gas diffusion layer.

According to the method for producing a coating paste for forming a microporous layer according to the invention of claim 4 , carbon black having a DBP absorption in the range of 300 ml/100 g to 500 ml/100 g and a water-repellent resin are mixed in a solvent mixed with a dispersant and stirred at a peripheral speed of 25 m/s or more and 60 m/s or less, and the compounded materials are dispersed at a high shear by stirring at a peripheral speed of 25 m/s or more and 60 m/s or less, so that secondary aggregates of carbon black with high DBP absorption can be broken down and the viscosity increase of the paste can be suppressed. Therefore, according to the coating paste for forming a microporous layer in which secondary aggregates of carbon black with high DBP absorption are broken down and the viscosity increase is suppressed, it can be applied to the surface of a conductive porous substrate with good coating properties, and in the microporous layer formed by drying and baking the applied paste, the carbon black with high DBP absorption has a high porosity due to its high structure, so that the porosity (air porosity) is high.

In particular, when carbon black having a DBP absorption in the range of 300 ml/100 g to 500 ml/100 g has a secondary aggregation diameter that passes through a mesh with an opening of 34 to 154 μm, the good coating properties of the paste result in a microporous layer with good surface smoothness, and high porosity and coating strength that is less likely to cause cracks can be achieved. That is, a microporous layer can be formed that has high porosity, good surface smoothness, few cracks, and good adhesion to the catalyst layer. Therefore, such a microporous layer is less likely to accumulate moisture and has high drainage properties, maintains high gas diffusivity, and has good durability.
Therefore, according to the method for producing a coating paste for forming a microporous layer of the invention of claim 4 , a coating paste can be obtained that can form a microporous layer having a high porosity and that can improve the diffusion and moisture management performance of the gas diffusion layer.

According to the method for producing a coating paste for forming a microporous layer according to the invention of claim 5 , the mixture is passed through a mesh having an opening of 34 to 154 μm after the stirring, so that the mixture contains less foreign matter such as iron that affects the durability and lifespan of the catalyst layer. Thus, in addition to the effects of claim 3 or 4 , a coating paste capable of forming a microporous layer that does not reduce the durability and lifespan of the battery can be obtained.

According to the method for producing a coating paste for forming a microporous layer according to the invention of claim 6 , a mixture of a dispersant, a solvent, carbon black, and a water-repellent resin is pre-mixed by a Disper or a turbine-stator type agitator before the stirring. In this way, by pre-mixing by a Disper or a turbine-stator type agitator and then stirring by a thin film rotation method or at a peripheral speed of 25 m/s or more and 60 m/s or less, the carbon black and the solvent can be made to blend well with each other by pre-mixing, and further, by stirring and degassing, it is possible to suppress the variation in the distribution of the secondary agglomeration diameter of the carbon black, and also to reduce the number of bubbles. Therefore, in addition to the effects described in claim 3 or claim 4 , a coating paste capable of forming a more uniform microporous layer in which the variation in the distribution of voids is suppressed can be obtained.

According to the method for producing a coating paste for forming a microporous layer according to the invention of claim 7 , a mixture of a dispersant, a solvent, carbon black, and a water-repellent resin is pre-mixed at a peripheral speed of 1 m/s or more and 20 m/s or less before the stirring. In this way, when the mixture is pre-mixed at a low shear and then diffused at a high shear, the pre-mixing can improve the compatibility of the carbon black with the solvent, and it is possible to suppress the variation in the distribution of the secondary agglomeration diameter of the carbon black. Therefore, in addition to the effects described in claim 3 or claim 4 , a coating paste can be obtained that can form a microporous layer in which the variation in the distribution of voids is suppressed.

According to the method for producing a coating paste for forming a microporous layer according to the invention of claim 8 , the weight ratio of the carbon black to the water-repellent resin is within the range of 9/1 to 5/5, so that the adhesiveness (binding property, binder effect) of the resin in the cured coating film of the microporous layer formed is good, cracks are unlikely to occur, and clogging of voids due to excess resin is suppressed. Thus, in addition to the effects of claim 3 or 4 , a coating paste capable of forming a microporous layer that combines coating film strength and gas and moisture permeability can be obtained.

According to the method for producing the coating paste for forming a microporous layer according to the invention of claim 9 , since the viscosity at a shear rate of 50 s ^-1 is within the range of 0.05 Pa.s to 1.0 Pa.s, the back penetration of the paste (the phenomenon of the paste penetrating to the back side of the conductive porous substrate opposite to the coated surface) can be suppressed when applied to the conductive porous substrate constituting the gas diffusion layer, and a cured coating film of a microporous layer with good smoothness can be formed at the desired coating width, resulting in good coatability. Therefore, a smooth microporous layer can be obtained without reducing the porosity of the conductive porous substrate, and adhesion to the catalyst layer can be improved, so that in addition to the effects described in claim 3 or claim 4 , a coating paste that can form a microporous layer that can better exhibit the performance of the gas diffusion layer can be obtained.

FIG. 1 is a schematic diagram showing the general configuration of a solid polymer fuel cell using a gas diffusion layer formed by applying a coating paste for forming a microporous layer according to an embodiment of the present invention to a conductive porous substrate.

Hereinafter, an embodiment of the present invention will be described with reference to the drawings.
In the embodiments, the same symbols and the same reference characters denote the same or corresponding functional parts in the embodiments, so that repeated detailed explanations will be omitted here.

[Embodiment]
First, the structure of a polymer electrolyte fuel cell (single cell) incorporating a gas diffusion layer for a fuel cell on which a microporous layer is formed by applying a coating paste for forming a microporous layer according to the present embodiment will be described with reference to the schematic diagram of Fig. 1. In the figure, the anode side is designated as A and the cathode side is designated as K.

1, the gas diffusion layers 14 (cathode-side gas diffusion layer 14K, anode-side gas diffusion layer 14A) are made of catalyst-supported carbon in which a precious metal catalyst such as platinum, gold, or palladium is supported by carbon, and ion exchange resin, and are joined to catalyst layers 13 (cathode-side catalyst layer 13K, anode-side catalyst layer 13A) with which an oxidizing gas or a fuel gas reacts, and are integrated to form electrodes 12 (cathode electrode 12K, anode electrode 12A). The gas diffusion layers 14 are disposed together with the catalyst layers 13 on both sides of a polymer electrolyte membrane 11 (core of a single cell) that selectively allows specific ions to pass therethrough, forming a membrane electrode assembly (MEGA) 10.
This membrane/electrode assembly 10 is sandwiched between a cathode side separator 20K having an oxidizing gas flow path 21K for supplying an oxidizing gas that serves as an oxidant on the outside of the cathode side gas diffusion layer 14K, and an anode side separator 20A having a fuel gas flow path 21A for supplying a fuel gas on the outside of the anode side gas diffusion layer 14A, thereby forming a single cell of the fuel cell 1.

That is, the fuel cell 1 using the gas diffusion layer 14 to which the coating paste for forming a microporous layer according to this embodiment is applied has a cathode electrode 12K composed of a cathode side catalyst layer 13K and a cathode side gas diffusion layer 14K with which an oxidizing gas such as oxygen gas reacts arranged on one surface of the polymer electrolyte membrane 11, and an anode electrode 12A composed of an anode side catalyst layer 13A and an anode side gas diffusion layer 14A with which a fuel gas such as hydrogen gas reacts arranged on the other surface, and is composed of a membrane/electrode assembly 10 constituting a power generation section, a cathode side separator 20K arranged on the surface of the cathode electrode 12K of the membrane/electrode assembly 10, and an anode side separator 20A arranged on the surface of the anode electrode 12A of the membrane/electrode assembly 10.

Here, the gas diffusion layer 14 (cathode side gas diffusion layer 14K, anode side gas diffusion layer 14A) in this embodiment is formed by applying a coating paste for forming a microporous layer, which will be described later, to the surface of the conductive porous substrate 16 (cathode side conductive porous substrate 16K, anode side conductive porous substrate 16A) and drying and firing the coating paste, thereby forming a microporous layer (fine porous layer) 15 (cathode side microporous layer 15K, anode side microporous layer 15A) on one side of the conductive porous substrate 16 (cathode side conductive porous substrate 16K, anode side conductive porous substrate 16A).
The gas diffusion layer 14 consisting of the microporous layer 15 and the conductive porous substrate 16 is assembled so that the microporous layer 15 side is adjacent to the catalyst layer 13 and the conductive porous substrate 16 side is adjacent to the separator 20, thereby forming the fuel cell 1.

With this cell configuration, when oxidizing gas is supplied from the outside to the oxidizing gas flow path 21K of the cathode-side separator 20K, a portion of the oxidizing gas flowing along the oxidizing gas flow path 21K penetrates into the inside from the conductive porous substrate 16K side surface of the cathode-side gas diffusion layer 14K. The remaining unreacted oxidizing gas flows along the oxidizing gas flow path 21K and is discharged to the outside of the fuel cell 1. Similarly, when fuel gas is supplied from the outside to the fuel gas flow path 21A of the anode-side separator 20A, a portion of the fuel gas flowing along the fuel gas flow path 21A penetrates into the inside from the conductive porous substrate 16A side surface of the anode-side gas diffusion layer 14A. The remaining unreacted fuel gas flows directly along the fuel gas flow path 21A and is discharged to the outside of the fuel cell 1. Then, the oxidizing gas and the fuel gas react with each other, and electricity is taken out between the cathode-side separator 20K and the anode-side separator 20A. In addition, a microporous layer 15 may be formed on both the front and back sides of the conductive porous substrate 16.

Next, we will explain the coating paste for forming the microporous layer 15 that is applied to the fuel cell gas diffusion layer 14 (cathode side gas diffusion layer 14K, anode side gas diffusion layer 14A) that is incorporated into the fuel cell 1 having such a configuration.

The coating paste for forming the microporous layer in this embodiment (hereinafter sometimes abbreviated as "MPL coating paste") contains carbon black, which is a conductive material, a water-repellent resin, a solvent, a dispersant for dispersing the carbon black and the water-repellent resin, etc.

The carbon black blended into the MPL coating paste of this embodiment has a DBP absorption in the range of 300 ml/100 g or more and 500 ml/100 g or less, preferably 300 ml/100 g or more and 480 ml/100 g or less, and more preferably 300 ml/100 g or more and 450 ml/100 g or less. Such carbon black with high DBP absorption has an average primary particle size in the range of about 25 nm to 45 nm, and preferably in the range of 30 nm to 45 nm.

As the carbon black, furnace black, thermal black, acetylene black, ketjen black, etc. can be used, but acetylene black is preferable. Acetylene black is highly structured, has high purity, has few impurities such as metals, has few surface functional groups, and is highly crystalline, so that a microporous layer 15 (hereinafter sometimes abbreviated as "MPL15") having high gas diffusivity, strength, and conductivity can be formed.
In addition, such carbon black has a structure in which primary particles are connected in a chain shape, i.e., a structure of aggregates (primary agglomerates) formed by fusing primary particles, and further, agglomerates (secondary agglomerates) formed by bonding aggregates with van der Waals forces are aggregated to exist as a powder or granular aggregate structure. Incidentally, carbon black such as acetylene black and ketjen black exhibits electrical conductivity by the movement of π electrons of condensed benzene rings.
Furthermore, carbon black is less expensive than other types of carbon, so the material cost is also low.

In addition, examples of water-repellent resins that can be used in the MPL coating paste include fluorine-based resins such as polytetrafluoroethylene (PTFE), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), perfluoroalkoxy fluorine resin (PFA), polychlorotrifluoroethylene (PCTFE), ethylene-tetrafluoroethylene copolymer (ETFE), ethylene-chlorotrifluoroethylene copolymer (ECTFE), and polyvinylidene fluoride (PVdF), as well as silicone resins. These can be used alone or in appropriate combination of two or more.

Among these, fluorine-based polymer materials that have excellent water repellency and corrosion resistance during electrode reactions are preferred, and in particular, PTFE, a fluororesin that provides high water repellency, is preferred. PTFE as a water-repellent resin may be a homopolymer of tetrafluoroethylene, or may be a modified PTFE that contains units derived from other fluorine-based monomers such as halogenated olefins such as chlorotrifluoroethylene and hexafluoropropylene, and perfluoro(alkyl vinyl ether).

The water-repellent resin preferably has an average molecular weight in the range of 50,000 to 1,000,000. If a resin with too low an average molecular weight is used, the binding ability as a binder for binding carbon black particles is low, and cracks are likely to occur in the MPL15 formed. On the other hand, if a resin with too large an average molecular weight is used, it is likely to become fibrous when mixed with other raw materials, resulting in poor dispersion and a decrease in paste stability, or it may become fibrous and solidify due to the shear force during stirring, causing changes in concentration, clogging of pipes, pumps, foreign matter removal filters, etc., poor adhesion of the coating film, and surface defects. In addition, if the water-repellent resin is a fluoropolymer, it is difficult to measure the average molecular weight by melt viscosity because the fluoropolymer is difficult to dissolve in a solvent. For this reason, the specific gravity method is applied, which determines the average molecular weight from the relationship between specific gravity and number average molecular weight.

Since water-repellent resins such as fluororesins do not usually disperse in water as is, they are dispersed in water using a suitable dispersant (surfactant), for example, in the form of an emulsion in which a water-repellent resin such as a fluororesin is emulsified, such as a PTFE emulsion, or in the form of a dispersion in which a water-repellent resin such as a fluororesin is dispersed. In other words, the emulsion of the water-repellent resin to be mixed as a composition raw material may already contain a dispersant.

Incidentally, an emulsion is also called an emulsion, and its original meaning is a system in which liquid particles are in the form of colloidal particles or larger particles that form a milk-like consistency in a liquid (dispersed system) (Nagakura Saburo et al., eds., Iwanami Physics and Chemistry Dictionary (5th Edition), p. 152, published February 20, 1998 by Iwanami Shoten Co., Ltd.), but in this specification, the term "emulsion" is used in the broader sense of the word, meaning "a dispersion of solid or liquid particles in a liquid."

In addition, in the MPL coating paste of this embodiment, a dispersant is used to disperse and stabilize the compounding materials such as carbon black and water-repellent resin in the solvent. Examples of the dispersant include nonionic surfactants such as polyoxyethylene tridecyl ether, polyoxyethylene alkylphenyl ether (Triton X-100, etc.), polyoxyethylene lauryl ether, polyoxyethylene distyrenated phenyl ether, polyoxyethylene alkylene alkyl ether, and polyethylene glycol alkyl ether; cationic surfactants such as alkyltrimethylammonium salt, dialkyldimethylammonium chloride, and alkylpyridinium chloride; anionic surfactants such as polyoxyethylene fatty acid ester and acidic group-containing structurally modified polyacrylate; and thickeners such as polyethylene oxide, methylcellulose, hydroxyethylcellulose, polyethylene glycol (e.g., ethers of alkylphenol and polyethylene glycol, ethers of higher aliphatic alcohol and polyethylene glycol, etc.), and polyvinyl alcohol.

Among these, nonionic surfactants are preferred because they have a low metal ion content and are therefore less likely to cause a decrease in battery performance due to short circuits, etc. In particular, when carbon black and a PTFE emulsion as a water-repellent resin are dispersed in a solvent, polyoxyethylene lauryl ether, polyoxyethylene tridecyl ether, polyoxyethylene alkyl phenyl ether (Triton X-100), etc. are preferably used to improve the wettability of the carbon black particles and PTFE particles and increase their dispersibility.
The type of dispersant, such as a surfactant or a thickener, is selected depending on the properties required of MPL15 and the ingredients used, and the dispersant may have both dispersing and thickening functions.
Furthermore, if necessary, a thickener such as a polyvinyl alcohol-based thickener may be added to adjust the viscosity.

In this embodiment, an aqueous solvent such as ion-exchanged water is used as the solvent for the MPL coating paste. Aqueous solvents such as ion-exchanged water provide better dispersion of water-repellent resins such as PTFE emulsions than organic solvents, and the water-repellent resins do not separate, resulting in good film-forming properties. In addition, the workability during coating and firing is good and there is no risk of fire. Furthermore, it is environmentally friendly and low cost.

In the MPL coating paste of this embodiment, the carbon black, which has a DBP absorption in the range of 300 ml/100 g or more and 500 ml/100 g or less, is preferably mixed in an amount of 55 to 90 mass %, more preferably 70 to 85 mass %, and even more preferably 75 to 80 mass %, the water-repellent resin is preferably mixed in an amount of 10 to 45 mass %, more preferably 15 to 30 mass %, and even more preferably 20 to 25 mass %, and the dispersant is preferably mixed in an amount of 5 to 30 mass parts, more preferably 8 to 20 mass parts, and even more preferably 10 to 15 mass parts per 100 mass parts of carbon.

The MPL coating paste of this embodiment is obtained through a dispersant mixing/dissolving process in which a solvent and a dispersant are mixed and stirred, a stirring process in which carbon black (usually powder or granular) with a DBP absorption amount in the range of 300 ml/100 g or more and 500 ml/100 g or less and a water-repellent resin (usually a resin emulsion or resin dispersion) are mixed with the solvent in which the dispersant has been mixed and dissolved, and the mixture is stirred at a peripheral speed of 25 m/s or more using a thin film rotary stirrer or the like, and a filtration process in which the paste in which the compounding materials have been dispersed by stirring is filtered through a mesh filter of a specified size to remove foreign matter.

That is, in this embodiment, a dispersant mixing and dissolving step is first carried out in which the dispersant is mixed and dissolved in the solvent by mixing and stirring the dispersant and the solvent. The stirring means and stirring conditions at this time are not particularly limited as long as the dispersant can be uniformly mixed and dissolved in the solvent. For example, if the dispersant is soluble in water, a uniform solution can be obtained by stirring with a stirrer (for example, stirring conditions of rotation speed: 100 to 1500 rpm (revolutions/minute), stirring time: about 1 to 15 minutes).

Next, carbon black having a DBP absorption amount in the range of 300 ml/100 g or more and 500 ml/100 g or less and a water-repellent resin are mixed into the solvent in which the dispersant has been mixed and dissolved,
A stirring step is carried out using a thin film rotary stirrer or the like at a peripheral speed of 25 m/s or more.
In the above-mentioned stirring step at a peripheral speed of 25 m/s or more, the mixture is stirred at a peripheral speed of 25 m/s or more at the tip of the stirring blade using a high-speed stirrer such as Homomixer, Homodisper, Neokaisa (registered trademark), Neomixa (registered trademark), thin film swirling type high-speed mixer, or Homomixer.
In particular, the stirring by the thin film swirling type agitator is called the thin film swirling method, and the entire inside of the agitator tank is made into a turbulent state of high energy. According to this stirring method, the agglomerated particles are pressed against the tank wall surface by the swirling flow that creates a turbulent state, and perform a swirling motion along the wall surface, at which time a swirling thin film is formed that is stretched thin by the rotation of the turbine and the liquid flow of the solvent, and the carbon black agglomerates are disintegrated by the shear stress of the centrifugal force pressed against the wall surface and the force that moves by the swirling flow. Furthermore, in the stirring by such a thin film swirling method, the re-agglomeration (flocculation) of the secondary agglomerates is prevented by stabilizing the droplet interface, etc., and the disintegration state of the secondary agglomerates is stabilized. Carbon black with a high DBP absorption can be highly dispersed and the dispersion can be stabilized by a simple process without spending a lot of time. It is also possible to prevent the re-agglomeration (flocculation) of the secondary agglomerates that have been finely dispersed by such strong shearing, and to stabilize the disintegration state of the secondary agglomerates, using a dispersant.

The inventors' experimental research has confirmed that if the peripheral speed of the stirring section is too slow or the stirring time is too short, the carbon black aggregates are not sufficiently broken down, making it difficult to pass through a mesh filter with a specified mesh size under specified pressure conditions. Therefore, it is preferable that the peripheral speed of the stirring section is 25 m/s or more, and the stirring time is 20 seconds or more, more preferably 25 seconds or more, and even more preferably 30 seconds or more. The upper limit of the peripheral speed is a finite value at the performance limit (maximum peripheral speed) of the stirrer, which is about 60 m/s, but the inventors' experimental research has confirmed that the desired viscosity characteristics can be obtained for a specified stirring time even at a peripheral speed of 50 m/s or more. In particular, in diffusion using this thin film swirling method, it is possible to control the secondary aggregate diameter by changing the peripheral speed and stirring time conditions, and it is also possible to design the secondary aggregate diameter of carbon black according to the desired characteristics of MPL15.

Here, if the stirring time with a thin film swirling stirrer or the like is too long, the secondary agglomeration diameter of the carbon black becomes finer, and the viscosity of the paste decreases significantly, and when the paste is applied to the conductive porous substrate 16 constituting the gas diffusion layer 14, the paste is likely to penetrate the conductive porous substrate 16 and pass through the thickness. For this reason, it becomes necessary to increase the viscosity by adding a large amount of thickener. However, if a large amount of thickener is added, the load of decomposing and removing the thickener in the baking and drying process after the paste application increases. In other words, if the thickener is not sufficiently decomposed by the heat in the baking and drying process, it deteriorates the water repellency, so if a large amount of thickener is added, it becomes necessary to take a long drying and baking time, which increases the load of the baking and drying process.
If the peripheral speed of the stirring part is preferably 25 m/s or more and the stirring time is preferably 20 seconds to 200 seconds, more preferably 25 seconds to 150 seconds, and even more preferably about 30 seconds to 100 seconds, it is possible to obtain the desired viscosity characteristics in the range of 0.05 to 1.0 Pa·S at a shear rate of 50 ^S-1 without the need to add a large amount of thickener.

Then, by stirring for a predetermined time using a thin film rotation method or the like, it is sufficient if the secondary agglomerates of carbon black can be disintegrated to a size that allows the paste to pass through a mesh with an opening of 154 μm or less, i.e., 100 mesh or more, which can remove metal foreign matter contained in the carbon black product. However, since disintegration to a size that allows the paste to pass through a filter with too fine a mesh reduces productivity, it is sufficient if the secondary agglomerates of carbon black can be dispersed by the above stirring to a size that allows the paste to pass through a mesh with an opening of 34 μm or more and 154 μm or less, i.e., 100 mesh or more and 400 mesh or less, preferably, a mesh with an opening of 45 μm or more and 154 μm or less, i.e., 100 mesh or more and 300 mesh or less, more preferably, a mesh with an opening of 45 μm or more and 150 μm or less, i.e., 100 mesh or more and 300 mesh or less.

Incidentally, when carrying out the present invention, pre-mixing (pre-dispersion) may be carried out, if necessary, by mixing with a disperse type or turbine stator type mixer before mixing with a thin film swirl type mixer or the like.
By stirring with a Disper or turbine stator type agitator before stirring with a thin film swirl agitator, the compounding materials are roughly dispersed, and it is possible to suppress the variation in the distribution of secondary agglomeration diameters in the carbon black after stirring with a thin film swirl agitator (sharply align the particle size distribution) and stabilize the dispersion, resulting in high stability. It is also possible to suppress the variation in the distribution of voids in MPL15, thereby obtaining stable performance and uniform diffusibility. This is thought to be because, by stirring with a Disper or turbine stator type agitator at low shear before stirring with a thin film swirl agitator at high shear, the wettability of the carbon black and the affinity between the carbon black and the water solvent are improved, and the dispersibility due to high shear stirring can be improved. And, if the carbon black has a highly developed structure, it can be assumed that it is easy to adsorb water and wet due to its large specific surface area. It is also possible to improve the wettability of the carbon black by using a dispersant.

Incidentally, the Disper type agitator generates shear force by rotating high-speed agitating blades called disk turbines or dissolvers at high speed. The turbine-stator type agitator is composed of a turbine blade (homogenizer) that rotates at high speed and a stator (fixed ring) that is formed to surround the periphery and does not rotate, and applies shear force through the gap (clearance) between them. For example, homogenizers, comb-tooth turbine-stators, and in-line types (turbine-stator types installed inside a casing) can be used. A composite agitator that combines the Disper type and the turbine-stator side can also be used. All of these generate shear force by inputting energy locally.
In particular, if a disperse type or turbine-stator type agitator is used, it is possible to prepare a paste that can form a uniform coating film with few bubbles by agitating and degassing the mixture.

The stirring conditions for a Disper-type or turbine-stator-type stirrer are a peripheral speed of 1 m/s or more and 20 m/s or less, preferably 10 m/s or more and 20 m/s or less. If the peripheral speed (rotation speed) of the stirrer is too low, the wettability of the carbon black particles cannot be sufficiently improved, and the variation in the distribution of the secondary agglomeration diameter of the carbon black cannot be effectively suppressed. On the other hand, if the rotation speed of the stirrer is higher than a certain level, even if the peripheral speed (rotation speed) or stirring time is too long, idling (vortexing) occurs, which does not affect the improvement of wettability and dispersibility, but only increases the load and energy consumption of the stirrer. If it is within the above range, the variation in the distribution of the secondary agglomeration diameter of the carbon black can be effectively suppressed with good productivity and low energy consumption.
If a large amount of bubbles are generated by the stirring, a defoaming step may be performed before or after the stirring.

In this embodiment, after stirring with the thin film rotary stirrer, a filtration step is carried out in which the paste after stirring is filtered through a mesh filter with a predetermined mesh size to remove foreign matter such as iron.
In this filtration step, the paste after stirring is passed through a filter having a mesh size of 154 μm or less, i.e., a mesh size of 100 mesh or more, using a ram press (pressure extrusion device) or the like. This removes foreign matter contained in the paste blending materials, particularly metal foreign matter such as iron contained in carbon black products that causes short circuits and adversely affects the durability of the catalyst layer 13 and the life of the battery 1. Since a filter with too fine mesh reduces productivity, it is preferable to use a mesh size of 34 μm or more and 154 μm or less, i.e., 100 mesh or more and 400 mesh or less, preferably a mesh size of 45 μm or more and 154 μm or less, i.e., 100 mesh or more and 300 mesh or less, and more preferably a mesh size of 45 μm or more and 150 μm or less, i.e., 100 mesh or more and 300 mesh or less.

In this case, if the viscosity of the paste after stirring exceeds 1.0 Pa·S at a shear rate of 50 S ^-1 , the pressure loss increases when passing through a fine mesh for removing foreign matter, resulting in reduced productivity. For this reason, if the viscosity of the paste after stirring exceeds 1.0 Pa·S at a shear rate of 50 S ^-1 , it is preferable to adjust the viscosity by adding a solvent to reduce the viscosity to 1.0 Pa·S or less at a shear rate of 50 ^{S-1 before} passing through a specified filter. More preferably, the viscosity is adjusted to 0.9 Pa·S or less, and even more preferably to 0.5 Pa·S or less at a shear rate of 50 S-1.

Furthermore, in order to prevent the MPL coating paste from striking through when applied to the conductive porous substrate 16, the viscosity of the MPL coating paste applied to the conductive porous substrate 16 at a shear rate of 50 S ^-1 is preferably 0.05 Pa·S or more, and more preferably 0.1 Pa·S or more.
In other words, if the viscosity of a paste that has been passed through a filter with a specified mesh size is too low, when it is applied to the conductive porous substrate 16 that constitutes the gas diffusion layer 14, the paste will penetrate into the conductive porous substrate 16, pass through the conductive porous substrate 16, and reach the back surface opposite the coated surface, causing the paste components to clog the voids in the conductive porous substrate 16 and reduce the permeability to gas and moisture.

For this reason, if the viscosity of the paste after filtration through a filter with a predetermined mesh size is less than 0.05 Pa·S at a shear rate of 50 S ^{- 1} , the viscosity is adjusted by adding a thickener so that the viscosity at a shear rate of 50 S ^-1 is preferably in the range of 0.05 Pa·S or more and 1.0 Pa·S or less. If the viscosity at a shear rate of 50 S-1 is increased too much by adding a thickener, the coatability of the MPL coating paste will decrease, and as described above, the load of the firing and drying process for decomposing the thickener will increase. Therefore, the viscosity of the MPL coating paste applied to the conductive porous substrate 16 at a shear rate of 50 S ^-1 is preferably in the range of 0.05 Pa·S or more and 1.0 Pa·S or less, more preferably 0.1 Pa·S or more and 0.9 Pa·S or less, and even more preferably 0.1 Pa·S or more and 1.0 Pa·S or less.

If the viscosity of the MPL coating paste applied to the conductive porous substrate 16 at a shear rate of 50 S ^-1 is within the range of 0.05 Pa.S or more and 1.0 Pa.S or less, the paste can be prevented from penetrating through when applied to the conductive porous substrate 16, allowing the conductive porous substrate 16 to exhibit good performance, and the load of the firing and drying process is not increased. Since the coating property (surface smoothness, etc.) on the conductive porous substrate 16 is good, adhesion with the catalyst layer 13 is also good, and good moisture management and gas diffusivity by the MPL 15 are exhibited. Note that the solid content will vary from the viscosity setting aimed at.

The MPL coating paste prepared in this manner is applied to the surface of the conductive porous substrate 16 that constitutes the gas diffusion layer 14 for the fuel cell 1, and then dried and fired to form a cured coating that constitutes the MPL 15. As shown in FIG. 1, the gas diffusion layer 14 of the fuel cell electrode is constituted by the conductive porous substrate 16 as the gas diffusion layer electrode substrate and the MPL 15 as the cured coating formed on one or both sides of the substrate.

Here, as the conductive porous substrate 16 to which the MPL coating paste of this embodiment is applied, those generally used in the gas diffusion layer 14 of conventional fuel cells 1 (particularly solid polymer fuel cells 1) can be used, for example, those composed of carbon materials such as metals and graphite (including nanocarbon materials) in the form of porous bodies such as fibers, particles, woven fabrics, nonwoven fabrics, meshes, lattices, punched bodies, and foams, and a porous substrate having gas permeability and conductivity is used. In particular, from the viewpoint of gas permeability, etc., carbon fibers (also called carbon fibers, carbon-based fibers, and carbon-based fibers) are preferred, and polyacrylonitrile (PAN)-based carbon fibers, pitch-based carbon fibers, rayon-based carbon fibers, phenol-based carbon fibers, etc. are used as graphite fibers, etc. Carbon fiber sheets such as carbon paper, carbon cloth, and carbon felt (carbon nonwoven fabric) are preferably used as the form of the substrate. These are selected in consideration of the usage environment and operating conditions of the fuel cell 1. Such a conductive porous substrate 16 is typically a porous body with a peak pore size in the range of 10 μm or more and 100 μm or less.

If necessary, the conductive porous substrate 16 such as carbon paper, carbon cloth, or carbon felt is subjected to a water-repellent treatment in order to improve the water repellency of the gas diffusion layer 14. The method of water-repellent treatment is not particularly limited, and the water-repellent treatment can be performed, for example, by immersing the conductive porous substrate 16 in an emulsion or dispersion containing a water-repellent agent, applying a water-repellent agent to the conductive porous substrate 16 by die coating or spray coating, or processing by a dry process such as sputtering of a fluororesin. After the water-repellent treatment, drying and firing may be performed as necessary. The water-repellent agent used in this water-repellent treatment is the same as the above-mentioned water-repellent resin that can also be used in the MPL coating paste. The amount of the water-repellent agent is not particularly limited, but from the viewpoint of achieving both water repellency and good gas diffusion paths and drainage paths, it is preferably within the range of 0.1% by mass to 20% by mass in the entire conductive porous substrate 16 (100% by mass).

The thickness of the conductive porous substrate 16 constituting the gas diffusion layer 14 for the fuel cell 1 is set taking into consideration the operating conditions of the fuel cell 1 and the required performance. However, if the conductive porous substrate 16 is too thin, the strength is insufficient, and the gas diffusion layer 14 performance such as stable gas diffusion, moisture management, and conductivity cannot be obtained, or the handling (handling ability) is poor and misalignment with the catalyst layer 13 occurs, resulting in a decrease in cell performance. Also, if the conductive porous substrate 16 is too thick, the gas diffusion decreases due to an increase in resistance, and high cell performance cannot be obtained. For this reason, for example, a thickness of 2 μm or more and 500 μm or less, preferably 10 μm or more and 250 μm or less, and more preferably 70 μm or more and 150 μm or less is used.

In addition, examples of coating methods for applying the MPL coating paste of this embodiment to the conductive porous substrate 16 include brush coating, paint brush coating, roll coater method, bar coater method, die coater method, blade method, knife coater method, spin coater method, screen printing, rotary screen printing, curtain coating method, dip coater method, spray coater method, gravure coater method, applicator, spray atomization, etc. In particular, the die coater method is preferable because it allows the amount of coating to be quantified regardless of the surface roughness of the conductive porous substrate 16, but is not necessarily limited to this.

The MPL coating paste of this embodiment is applied to the surface of the conductive porous substrate 16, dried and fired, and the solvent is dried and removed to form MPL 15 on the surface of the conductive porous substrate 16. The drying and firing temperature at this time may be any temperature that can decompose and remove dispersants such as surfactants and thickeners, and does not thermally decompose the water-repellent resin, and is, for example, heated for 5 to 20 minutes within the range of 250 to 400°C.
If the drying and baking temperature is too low, even if the drying and baking time is extended, the dispersant and the like cannot be sufficiently pyrolyzed and volatilized to be removed, and the dispersant remains and the hydrophilic groups capture moisture, so that the gas diffusion layer 14 to be formed cannot exhibit sufficient water repellency. On the other hand, if the drying and baking temperature is too high, the coating film components such as the water-repellent resin are pyrolyzed to deteriorate the components of the MPL 15, or the deformation of the MPL 15 increases due to a large temperature change, resulting in a decrease in smoothness and cracks, making it difficult to form the MPL 15 that meets the desired required performance. Furthermore, the load in the drying and baking process and the load on the natural environment increase, and the manufacturing cost also increases. By drying and baking at a predetermined temperature range, the dispersant and thickener such as surfactant contained in the MPL coating paste can be removed to prevent the inhibition of water repellency by the dispersant and thickener such as surfactant, and the carbon black can be bound by melting the water repellent to form the MPL 15 with the desired strength.

For example, if the dispersant is polyoxyethylene tridecyl ether, polyoxyethylene alkylphenyl ether (Triton X-100), etc., the drying and baking temperature is set in the range of 250 to 350°C. Within this range, the dispersant etc. is sufficiently decomposed and volatilized, so that the water repellency, which is an important characteristic of the gas diffusion layer 14 that greatly affects the power generation efficiency, can be secured, and the MPL 15 with the desired characteristics can be formed, and good bonding with the conductive porous substrate 16 can be secured. In addition, in such a drying and baking process involving heat treatment, the order of drying and baking is not particularly important, and drying and baking may be performed simultaneously. In addition, it is also possible to perform each heat treatment separately by changing the temperature and time conditions for the heat treatment mainly for decomposing and removing the dispersant and the heat treatment mainly for bonding by melting the water-repellent resin.

The coating thickness when the MPL coating paste of this embodiment is applied to the conductive porous substrate 16 is set in consideration of the operating conditions of the fuel cell 1 and the performance required of the MPL 15. For example, the thickness of the cured coating film after the MPL coating paste is applied to the conductive porous substrate 16 and dried and fired, that is, the thickness of the MPL 15 which is the dry film thickness, is set to be within the range of 1 to 300 μm, preferably within the range of 10 to 100 μm.
If the thickness of the MPL 15 (the dry film thickness of the cured coating film) is too thin, the effect of suppressing the formation of a liquid film at the interface with the catalyst layer 13 is small, and the effect of absorbing the unevenness of the conductive porous substrate 16 or preventing the fibers of the conductive porous substrate 16 from piercing the electrolyte membrane 11 cannot be obtained. On the other hand, if the coating thickness of the MPL coating paste is too thick, the surface smoothness is likely to decrease due to shrinkage caused by drying and baking, and the contact resistance with the catalyst layer 13 increases, leading to a decrease in moisture management due to a decrease in adhesion and a decrease in conductivity. In addition, if the dry film thickness is too thick, the electrical resistance in the thickness direction increases, and further, the gas diffusion resistance also increases.

If the thickness of the cured coating film after applying the MPL coating paste to the conductive porous substrate 16 and drying and baking, i.e., the dry film thickness of MPL 15, is within the range of 1 to 300 μm, MPL 15 with high surface smoothness can be obtained, and good gas and moisture permeability and conductivity performance can be ensured. It is preferably within the range of 10 to 100 μm. In addition, it is desirable that the thickness of MPL 15 is 50% or less of the thickness of the conductive porous substrate 16. If the difference in thickness is within a moderate range, the shrinkage stress between the conductive porous substrate 16 and MPL 15 due to drying and baking is small, and high bonding strength can be obtained. In addition, the smoothness of the entire gas diffusion layer 14 is high and the contact resistance with the catalyst layer 13 is small, so that good adhesion with the catalyst layer 13 exhibits good moisture management and conductivity.

In this way, the MPL coating paste is applied to the conductive porous substrate 16, and then dried and fired, thereby removing the dispersant, thickener, and solvent (moisture) in the MPL coating paste, and a cured coating film of MPL 15 in which carbon black is bound by the water-repellent resin is formed on the conductive porous substrate 16. At this time, the MPL coating paste is dried and fired together with the conductive porous substrate 16, so that part of the water-repellent resin in the MPL coating paste melts and bonds the carbon black together with the water-repellent resin, and also bonds the carbon black to the conductive material such as the carbon fiber of the conductive porous substrate 16, improving adhesion to the conductive porous substrate 16 and making the formed MPL 15 strong.

In this way, the MPL coating paste containing carbon black, water-repellent resin, dispersant, and solvent is applied to the conductive porous substrate 16, and then dried and baked to form a cured coating film MPL15 on the conductive porous substrate 16. The cured coating film MPL15 is made of carbon black and water-repellent resin, and is an extremely fine porous layer with pores of about 0.1 to 0.5 μm in diameter. The MPL15 has electrical conductivity due to the carbon black, water repellency due to the water-repellent resin, and moisture and gas permeability due to the pores (voids).

Then, the MPL coating paste is applied to the surface of the conductive porous substrate 16, and then dried and baked to form the MPL 15 on the conductive porous substrate 16 to form the gas diffusion layer 14 for the fuel cell. In the fuel cell 1 incorporating the gas diffusion layer 14, the presence of the MPL 15 makes it difficult for moisture to accumulate at the boundary with the adjacent catalyst layer 13, improving gas diffusion and moisture management, and stabilizing power generation performance. In addition, by adjusting the components of the MPL 15, it is possible to obtain fine control of performance according to the usage environment and operating conditions of the battery, which is difficult to achieve with the conductive porous substrate 16 alone. Furthermore, the MPL 15 absorbs the undulations of the conductive porous substrate 16 to obtain a smooth surface. In other words, the MPL 15 can prevent the occurrence of short circuits caused by fraying or fluffing of the carbon fibers of the conductive porous substrate 16 piercing or contacting the catalyst layer 13 or the polymer electrolyte membrane 11.

In particular, according to the MPL coating paste of this embodiment, secondary aggregates of carbon black with a high DBP absorption amount, which is in the range of 300 ml/100 g to 500 ml/100 g, are crushed, so that the viscosity increase of the paste is suppressed, and the paste can be applied to the surface of the conductive porous substrate 16 in a desired wide coating width, and coating streaks are unlikely to occur, resulting in good coating properties and good productivity. Furthermore, the MPL 15 formed from the MPL coating paste of this embodiment has few unevenness or undulations such as coating streaks, good surface smoothness, good adhesion to the catalyst layer 13, little penetration into the catalyst layer 13, and little mechanical stress. In addition, secondary aggregates of carbon black with a high DBP absorption amount are crushed by high shear dispersion using a thin film rotation method or the like at a peripheral speed of 25 m/s or more, so that the aggregates are unlikely to re-agglomerate and have good storage stability.

And because the carbon black with high DBP absorption has a high porosity due to its high structure, the formed MPL15 has a high porosity (air porosity) and the pores can be distributed uniformly. The high structure allows for the formation of gas and moisture passages, which improves the diffusibility and drainage of gas and moisture. This makes it difficult for flooding (the phenomenon in which generated water accumulates in the MPL) to occur, preventing the inhibition of gas diffusibility due to flooding, and also preventing the inhibition of gas diffusibility and deterioration of the MPL15 due to the freezing of moisture accumulated at the boundary with the catalyst layer 13, thereby stabilizing the battery output.

In addition, when carbon black with DBP absorption in the range of 300 ml/100 g to 500 ml/100 g has a secondary aggregation diameter that passes through a mesh with an opening of 34 μm or more and 154 μm or less (100 mesh or more and 400 mesh or less), the good coating property of the paste ensures good surface smoothness in the MPL 15, and high porosity and coating strength that is less likely to crack can be achieved at the same time. Therefore, by forming an MPL 15 that has high porosity but good surface smoothness, is less likely to crack, and has good adhesion to the catalyst layer 13, the MPL 15 is less likely to accumulate moisture, i.e., is less likely to cause flooding, and stable drainage and gas diffusion properties are obtained, and the coating film is less likely to chip and has good durability. Good adhesion to the catalyst layer 13 can reduce contact resistance at the contact surface, and also has good electrical conductivity with less chance of cracking, which can improve power generation efficiency. Furthermore, the material can withstand the clamping force from the separators during stacking, and the gaps are unlikely to be crushed even by the clamping force from the separators.
Therefore, it is possible to form an MPL 15 with a high porosity, improve the gas and moisture permeability of the MPL 15, and improve the gas diffusion and moisture management performance of the gas diffusion layer 14 in which the MPL 15 is formed.

Carbon black with a DBP absorption capacity in the range of 300 ml/100 g to 500 ml/100 g has a secondary agglomeration diameter that passes through a mesh with an opening of 34 μm or more and 154 μm or less (100 mesh or more and 400 mesh or less), and is of a size that can pass through a fine mesh filter that allows the removal of foreign matter such as iron that affects the durability and lifespan of the catalyst layer 13 in which the cell reaction occurs. This results in good yield, and the removal of foreign matter such as iron by filtration through a filter with an opening of 34 μm or more and 154 μm or less (100 mesh or more and 400 mesh or less) makes it less likely to reduce the durability and lifespan of the battery.

Next, examples of the MPL coating paste according to this embodiment and examples of the gas diffusion layer (gas diffusion electrode) 14 for a fuel cell produced using the MPL coating paste according to this embodiment will be described together with comparative examples.
The MPL coating paste of the present embodiment and the comparative example is a blend of carbon black, a water-repellent resin, a dispersant, and a solvent.

As the carbon black, acetylene black (for example, Denka Black (registered trademark) manufactured by Denki Kagaku Kogyo Co., Ltd.) was used, as the water-repellent resin, an emulsion of PTFE, a fluororesin (POLYFLON PTFE D-210C manufactured by Daikin Industries, Ltd. [solids (resin) 61%]), as the dispersant, the surfactant Triton X-100 was used (10% carbon blend), and as the solvent, ion-exchanged water was used. The PTFE emulsion also contains a non-ionic surfactant (dispersant) to stabilize the PTFE fine particles as the water-repellent resin.

In each example and comparative example, the solvent and dispersant were first mixed and stirred using a stirrer at a stirring speed of 300 rpm for about 10 minutes to dissolve the dispersant in the solvent (dispersant mixing and dissolving process). Next, the solvent in which the dispersant was dissolved was mixed with the specified carbon black and PTFE emulsion shown in Table 1, and stirred at 3000 rpm for about 30 minutes using a homomixer ("Homomixer MARK II 2.5 type" manufactured by Primix Corporation), which is a turbine-stator type high-speed stirrer, and then stirred under the specified conditions shown in Table 1 using Filmix R ("Filmix R56-L type" manufactured by Primix Corporation), which is a thin film swirling high-speed stirrer (mixing process).

In Examples 1 to 4, Example 7, Comparative Example 1 and Comparative Example 2, the viscosity of the paste after stirring by the thin film swirling method was measured, and for pastes having a viscosity in the range of 0.5 Pa·s or less at a shear rate of 50 ^S-1 , the paste was passed through a filter with a mesh size of 45 μm (300 mesh) under a pressure (pressing force) of 0.45 MPa using a ram press machine without adjusting the viscosity as it is, thereby performing filter filtration to remove iron foreign matter and the like (filtration process). Furthermore, the viscosity of the paste after passing through the filter was measured, and for those having a viscosity in the range of 0.1 Pa·s or more and 0.5 Pa·s or less at a shear rate of 50 S ^-1 , the paste was used as it is as the MPL coating paste with the finished viscosity, and for those having a viscosity less than 0.1 Pa·s, a thickener was added to adjust the viscosity so that the finished viscosity was in the range of 0.1 Pa·s or more and 0.5 Pa·s or less, and an MPL coating paste with a finished viscosity in the range of 0.1 Pa·s or more and 0.5 Pa·s or less was obtained.

On the other hand, when the viscosity of the paste after stirring by the thin film swirling method exceeded 0.5 Pa·s at a shear rate of 50 S ^-1 , a solvent (ion-exchanged water) was added to adjust the viscosity to within the range of 0.1 Pa·s or more and 0.5 Pa·s or less, and then the paste was passed through a filter with a mesh opening of 45 μm (300 mesh) under a pressure (pressing force) of 0.45 MPa using a ram press machine to perform filter filtration to remove iron foreign matter, etc. (filtration process). Furthermore, the viscosity of the paste after passing through the filter was measured, and as described above, those having a viscosity at a shear rate of 50 S ^-1 in the range of 0.1 Pa.s or more and 0.5 Pa.s or less were used as the MPL coating paste with the final viscosity as is, and those having a viscosity of less than 0.1 Pa.s were viscosity-adjusted by adding a thickener so that the final viscosity was in the range of 0.1 Pa.s or more and 0.5 Pa.s or less, thereby obtaining an MPL coating paste with a final viscosity in the range of 0.1 Pa.s or more and 0.5 Pa.s or less.

For Example 5, after stirring by the thin film swirling method, the mixture was passed through a filter with a mesh opening of 45 μm (300 mesh) using a ram press under a pressure (pressing force) of 0.45 MPa to remove iron foreign matter and the like (filtration process), and then a solvent was added so that the final viscosity was 0.05 Pa·s at a shear rate of 50 S ^{- 1} , to obtain an MPL coating paste with a final viscosity of 0.05 Pa·s at a shear rate of 50 S-1.
For Example 6, after stirring by the thin film swirling method, a thickener was added so that the finished viscosity would be 1.0 Pa·s at a shear rate of 50 S ^-1 , and then the mixture was passed through a filter with a mesh opening of 45 μm (300 mesh) under a pressure (pressing force) of 0.45 MPa using a ram press to perform filter filtration to remove iron foreign matter and the like (filtration process), thereby obtaining an MPL coating paste with a finished viscosity of 1.0 Pa·s at a shear rate of 50 S ^-1 .

In Examples 1 to 6, the MPL coating paste was prepared using carbon black A ("Denka Black Li-250" manufactured by Denki Kagaku Kogyo Co., Ltd., average primary particle diameter: 37 nm, specific surface area: 58 ^m2 /g, iodine adsorption amount: 82 mg/g) (hereinafter abbreviated as "Carbon A") with a DBP absorption capacity of 315 ml/100 g.

Example 1 is an MPL coating paste that uses carbon A with a DBP absorption of 315 ml/100 g, has a carbon/fluororesin (solid content) blending ratio of 9/1, is stirred for 30 seconds at a peripheral speed of 25 m/s by the thin film swirling method, and has a finished viscosity of 0.5 Pa·s at a shear rate of 50 S ^-1 . Note that the MPL coating paste according to Example 1 has a solid content ratio of carbon black, fluororesin, dispersant, etc. of 15%.
Example 2 is an MPL coating paste that uses carbon A with a DBP absorption of 315 ml/100 g, has a carbon/fluororesin (solid content) blending ratio of 9/1, and is stirred for 30 seconds at a peripheral speed of 50 m/s by the thin film swirling method, with a finished viscosity of 0.1 Pa·s at a shear rate of 50 S ^-1 . The solid content ratio of the MPL coating paste according to Example 2 is also 15%.
Example 3 is an MPL coating paste that uses carbon A with a DBP absorption of 315 ml/100 g, has a carbon/fluororesin (solid content) blending ratio of 5/5, is stirred for 100 seconds at a peripheral speed of 25 m/s by the thin film swirling method, and has a finished viscosity of 0.1 Pa·s at a shear rate of 50 S ^-1 . The solid content ratio of the MPL coating paste according to Example 3 is also 15%.

Example 4 is an MPL coating paste that uses carbon A with a DBP absorption of 315 ml/100 g, has a carbon/fluororesin (solid content) blending ratio of 5/5, is stirred for 30 seconds at a peripheral speed of 25 m/s by the thin film swirling method, and has a finished viscosity of 0.5 Pa·s at a shear rate of 50 S ^-1 . The solid content ratio of the MPL coating paste according to Example 4 is 20%.

Example 5 is an MPL coating paste that uses carbon A with a DBP absorption of 315 ml/100 g, has a carbon/fluororesin (solid content) blending ratio of 9/1, is stirred for 30 seconds at a peripheral speed of 25 m/s by the thin film swirling method, and has a finished viscosity of 0.05 Pa·s at a shear rate of 50 S ^-1 . The solid content ratio of the MPL coating paste according to Example 5 is 12%.
Example 6 is an MPL coating paste that uses carbon A with a DBP absorption of 315 ml/100 g, has a carbon/fluororesin (solid content) blending ratio of 9/1, is stirred for 30 seconds at a peripheral speed of 25 m/s by the thin film swirling method, and has a finished viscosity of 1.0 Pa·s at a shear rate of 50 S ^-1 . The solid content ratio of the MPL coating paste according to Example 6 is 15%.

Furthermore, in Example 7, an MPL coating paste was prepared using carbon black B ("Denka Black Li-100" manufactured by Denki Kagaku Kogyo Co., Ltd., average primary particle diameter: 35 nm, specific surface area: 68 ^m2 /g, iodine adsorption amount: 92 mg/g) (hereinafter abbreviated as "carbon B") having a DBP absorption amount of 425 ml/100 g.
Example 7 is an MPL coating paste that uses carbon B with a DBP absorption of 425 ml/100 g, has a carbon/fluororesin (solid content) blending ratio of 9/1, is stirred for 30 seconds at a peripheral speed of 25 m/s by the thin film swirling method, and has a finished viscosity of 0.5 Pa·s at a shear rate of 50 S ^-1 . The solid content ratio of the MPL coating paste according to Example 7 is 8%.

On the other hand, in Comparative Example 1, an MPL coating paste was prepared using carbon black C ("Denka Black Li-400" manufactured by Denki Kagaku Kogyo Co., Ltd., average primary particle diameter: 48 nm, specific surface area: 39 ^m2 /g, iodine adsorption amount: 52 mg/g) (hereinafter abbreviated as "Carbon C") with a DBP absorption capacity of 220 ml/100 g.
Comparative Example 1 is an MPL coating paste that uses carbon C with a DBP absorption of 220 ml/100 g, has a carbon/fluororesin (solid content) blending ratio of 9/1, is stirred for 30 seconds at a peripheral speed of 10 m/s by the thin film swirling method, and has a finished viscosity of 0.5 Pa·s at a shear rate of 50 S ^-1 . The solid content ratio of the MPL coating paste according to Comparative Example 1 is 13%.

In addition, in Comparative Example 2, an MPL coating paste was prepared using Carbon D ("Denka Black Li-435" manufactured by Denki Kagaku Kogyo Co., Ltd., average primary particle diameter: 23 nm, specific surface area: 133 ^m2 /g, iodine adsorption amount: 180 mg/g) (hereinafter abbreviated as "Carbon D") with a DBP absorption capacity of 510 ml/100 g.
This comparative example 2 is an MPL coating paste that uses carbon D with a DBP absorption of 510 ml/100 g, has a carbon/fluororesin (solid content) blending ratio of 5/5, and is stirred for 30 seconds at a peripheral speed of 10 m/s by the thin film swirling method, and has a finished viscosity of 0.5 Pa·s at a shear rate of 50 S ^-1 . The solid content ratio of the MPL coating paste according to comparative example 2 is 13%.

The DBP absorption (ml/100 g) of the carbon black was determined in accordance with JIS K 6217-4 by detecting the torque generated due to the change in viscosity characteristics when DBP (dibutyl phthalate) is added to carbon black using an S-500 absorption measuring device (manufactured by Asahi Research Institute Co., Ltd.) and converting the amount of DBP added at 70% of the maximum torque into a value per 100 g of sample.
The viscosity was measured using an E-type viscometer (manufactured by Toki Sangyo Co., Ltd.).

Here, the MPL coating pastes of each Example and Comparative Example were evaluated for passability through a predetermined filter for removing iron foreign matter, etc. As described above, the MPL coating pastes of each Example and Comparative Example were extruded and passed through a 300-mesh filter with an opening of 45 μm using a ram press machine at a pressing force of 0.45 MPa. The MPL coating pastes that could pass through the 300-mesh filter with an opening of 45 μm without any deformation at that time at an extrusion pressure of 0.45 MPa suppressed clogging of the filter due to carbon black aggregates (aggregated particles), and could easily pass through a fine-mesh filter for removing iron foreign matter, etc. contained in carbon black products, and had good productivity. In addition, the viscosity characteristics (flow characteristics) when applied to the conductive porous substrate 16 were good, and the coating properties with good smoothness were evaluated as ◎. Filters that were slightly deformed during extrusion of the MPL coating paste were evaluated as ◯, and filters that were torn were evaluated as × due to reduced productivity and reduced removal rate of iron foreign matter, etc. The evaluation results at this time are shown in the lower part of Table 1.

Furthermore, the MPL coating paste according to each Example and Comparative Example was applied to ^one side of a nonwoven carbon fiber sheet impregnated with a fluororesin solution and treated for water repellency as the conductive porous substrate 16 using a thin film coating tool (die head) at a moving speed of 1.0 m/min and a basis weight of 3.5 mg/cm2, and then dried and baked for 10 minutes at 300 ° C. As a result, a cured coating film that becomes MPL 15 is formed from the MPL coating paste applied to the surface of the water-repellent treated nonwoven carbon fiber sheet as the conductive porous substrate 16, and a gas diffusion layer 14 composed of the conductive porous substrate 16 made of the nonwoven carbon fiber sheet and the MPL 15 made of the cured coating film formed from the MPL coating paste was produced.

At this time, for the MPL coating paste according to each Example and Comparative Example, the viscosity characteristics of the paste when applied to a water-repellent treated nonwoven carbon fiber sheet as the conductive porous substrate 16 were observed from the state of strike-through (the phenomenon in which the paste penetrates to the back side of the conductive porous substrate 16 opposite the coating surface). In the MPL coating paste of each Example and Comparative Example, when the paste is applied to a water-repellent treated nonwoven carbon fiber sheet, even if the MPL coating paste penetrates into the water-repellent treated nonwoven carbon fiber sheet, if the circumscribed circle of the MPL coating paste soaked in is 6 mm ² / piece or less on the surface opposite the coating surface of the nonwoven carbon fiber sheet, the components of the MPL coating paste fill the voids (holes) of the conductive porous substrate 16, thereby suppressing the decrease in moisture and gas permeability, and the paste components have a moderately high viscosity characteristic (flow characteristic) that can suppress strike-through. The paste was evaluated as ◎, and the number of circumscribed circles exceeding 6 mm ² / piece was 3 pieces / m ² or less.

Furthermore, the MPL coating paste of each Example and Comparative Example was applied to a water-repellent treated nonwoven carbon fiber sheet, and the cured coating film as MPL15 formed on the nonwoven carbon fiber sheet by drying and baking was examined for the presence or absence of surface cracks using an optical microscope. Specifically, optical microscope photographs of 10 mm x 10 mm size were taken at 10 arbitrary locations of the cured coating film as MPL15 formed on the nonwoven carbon fiber sheet, and if there were 2 or less cracks (cracks in the coating film) with a major axis length of 0.5 mm or more at 9 or more of the arbitrary 10 locations, the coating film was judged to have excellent coating strength and rated as ◎, and if there were 3 or more cracks with a major axis length of 0.5 mm or more at 8 or more locations, the coating film was judged to be impractical and rated as ×.

In addition, the MPL coating paste of each Example and Comparative Example was applied to a water-repellent treated nonwoven carbon fiber sheet, and dried and baked to form a cured coating on the nonwoven carbon fiber sheet. The gas diffusion layer 14 was composed of a conductive porous substrate 16 made of a nonwoven carbon fiber sheet and an MPL 15 made of a cured coating formed from the MPL coating paste. The gas diffusion resistance was measured to evaluate the gas diffusivity. Specifically, a circular sample having a diameter of 4.7 cm was cut out from the prepared gas diffusion layer 14, and air was passed from the MPL 15 side of the gas diffusion layer sample to the opposite side at a flow rate of 58 cc/min/cm ^2. The gas diffusion resistance was examined by measuring the differential pressure between the MPL 15 side and the opposite side (the surface of the nonwoven carbon fiber sheet) with a differential pressure gauge. It can be said that the smaller the pressure difference at this time, the smaller the gas diffusion resistance and the higher the gas diffusivity (gas permeability). Those with a gas diffusion resistance of 25 mmAq or less were judged to have excellent gas diffusivity and were rated as ◎, and those with a gas diffusion resistance of more than 25 mmAq but not exceeding 50 mmAq were rated as ×.
The evaluation results are shown in the lower part of Table 1.

As shown in Table 1, the MPL coating pastes of Examples 1 to 7 were prepared by using carbon black with a DBP absorption in the range of 300 ml/100 g to 500 ml/100 g, specifically carbon A with a DBP absorption of 315 ml/100 g and carbon B with a DBP absorption of 425 ml/100 g, and mixing the compounded materials with a turbine-stator type mixing system, followed by mixing with a high shear at a peripheral speed of 25 m/s or more using the thin film rotation method. All of the pastes could easily pass through a 45 μm mesh (300 mesh) filter at a pressing force of 0.45 MPa without significantly deforming the filter, i.e., without causing clogging.

In addition, all of the MPL coating pastes of Examples 1 to 7 had little bleed-through when applied to a nonwoven carbon fiber sheet as the conductive porous substrate 16, and had moderate viscosity characteristics and excellent coatability.
Furthermore, the MPL15 cured coating film formed by applying the MPL coating paste of Examples 1 to 7 to a water-repellent treated nonwoven carbon fiber sheet and then drying and baking had few cracks on the surface and excellent coating strength.

Furthermore, according to the MPL coating pastes of Examples 1 to 7, which used carbon black with a DBP absorption amount in the range of 300 ml/100 g to 500 ml/100 g, a gas diffusion layer 14 with excellent gas diffusion properties could be obtained.
That is, the MPL 15 consisting of a cured coating film produced by applying the MPL coating paste of Examples 1 to 7, which uses carbon black having a DBP absorption within the range of 300 ml/100 g to 500 ml/100 g, to a water-repellent treated nonwoven carbon fiber sheet and drying and baking it, and the gas diffusion layer 14 consisting of the conductive porous substrate 16 consisting of a nonwoven carbon fiber sheet had a smaller differential pressure, improved gas permeability, and excellent gas diffusivity, compared to Comparative Example 1, which uses carbon black having a DBP absorption of 220 ml/100 g.
This is because carbon black with a DBP absorption in the range of 300 ml/100 g to 500 ml/100 g has a well-developed structure, so that even when dispersed at a high shear, the primary aggregates maintain a high degree of porosity, thereby making it possible to increase the porosity of MPL15 formed as a cured coating film of the MPL coating paste.

In contrast, in the MPL coating paste of Comparative Example 1, which used carbon black having a DBP absorption of less than 300 ml/100 g, specifically, carbon C having a DBP absorption of 220 ml/100 g, the resulting gas diffusion layer 14 had poor gas diffusivity even though the finished viscosity at a shear rate of 50 ^S was 0.5 Pa·S.

Furthermore, in the cured coating film of MPL15 formed from the MPL coating paste of Comparative Example 2, which used carbon black with a DBP absorption of more than 500 ml/100 g, specifically, carbon D with a DBP absorption of 510 ml/100 g, many cracks were observed and the coating film strength was poor.
This is thought to be because carbon black with a DBP absorption of more than 500 ml/100 g has a highly developed structure, so even when it is dispersed at a high shear and the secondary agglomerates are broken down, the primary agglomerates maintain a high porosity; and because carbon black with a DBP absorption of more than 500 ml/100 g has an excessively high porosity for MPL15, which reduces the coating strength and causes many cracks.

That is, if the DBP absorption amount is too high, the porosity becomes too high, and the cured coating film of MPL15 becomes brittle, but if the cured coating film of MPL15 is brittle, the adhesion with the catalyst layer 13 also decreases, and flooding in which generated water accumulates in MPL15 occurs easily, and gas diffusion and drainage properties decrease, resulting in a decrease in cell performance. That is, if there are many surface cracks in MPL15, the adhesion between the catalyst layer 13 and the gas diffusion layer 14 decreases when the MPL15 is incorporated into the fuel cell 1, and voids are generated between the layers, and liquid water accumulates there, resulting in a decrease in moisture management. On the other hand, it is possible to obtain a predetermined strength by increasing the thickness of the coating film, but if the thickness of MPL15 increases, the gas diffusion resistance increases, reducing gas diffusion properties, etc., resulting in a decrease in cell performance.
In contrast, the MPL coating pastes of Examples 1 to 7 provide a coating strength that makes the MPL 15 less susceptible to cracking, and have good adhesion to the catalyst layer 13, thereby suppressing flooding, making it possible to obtain stable battery performance without reducing gas and moisture permeability or moisture management.

In this way, in the MPL coating pastes of Examples 1 to 7, carbon black with DBP absorption in the range of 300 ml/100 g to 500 ml/100 g is used, and the carbon black and fluororesin as a water-repellent resin are mixed with water in which a dispersant is dissolved, and they are stirred at high shear with a thin-film swirling type high-speed mixer. As a result, even carbon black with a highly developed structure has its secondary aggregates broken down to several μm, and can pass through a fine mesh filter with a mesh opening of 45 μm (300 mesh) that can remove foreign matter such as metals under a pressing force of 0.45 MPa without clogging. Therefore, it is possible to remove foreign matter such as metals from carbon black products that reduce the durability and life of batteries without reducing productivity, and the durability of the batteries can be improved.

Carbon black with a DBP absorption in the range of 300 ml/100 g to 500 ml/100 g has a high porosity in its structure, even when its secondary aggregates are broken down to a few μm. When an MPL coating paste containing the carbon black is applied to a conductive porous substrate 16 and then dried and fired to form a cured coating film, MPL 15, the high porosity of the carbon black with high DBP absorption results in excellent gas and moisture permeability, as well as excellent gas diffusion and moisture management.

In particular, carbon black with a DBP absorption rate in the range of 300 ml/100 g to 500 ml/100 g can achieve both high porosity and crack resistance. That is, if the porosity of MPL 15 is too high, cracks will easily occur in MPL 15, and if MPL 15 has cracks, not only will durability be low and the properties of MPL 15 not be fully exhibited, but moisture will easily accumulate between the catalyst layer 13, resulting in reduced moisture control and making it difficult to obtain stable cell performance. In addition, water that accumulates at the interface between the cracks and catalyst layer 13 will freeze below freezing, causing MPL 15 and catalyst layer 13 to peel off and lose parts of them, which may adversely affect the performance of fuel cell 1 and shorten the life of fuel cell 1.

Carbon black with DBP absorption in the range of 300ml/100g to 500ml/100g has both high porosity and coating strength, and the good coating strength provides good adhesion to the catalyst layer 13, reduces uneven contact resistance, and allows for a uniform thickness, resulting in good conductivity. In addition, cracks and flooding in the MPL 15, which causes water to accumulate, are unlikely to occur, and the gas diffusion layer 14 exhibits stable and excellent gas diffusion and moisture management properties, resulting in stable and high battery performance. Furthermore, as described above, the carbon black can be passed through a fine-mesh filter that can remove foreign matter such as metal without reducing productivity, and foreign matter such as metal can be removed there, resulting in good battery life and durability. Furthermore, carbon black with high DBP absorption can be expected to improve conductivity due to a highly developed structure network.

According to the inventors' experimental research, the ratio of carbon black to water-repellent resin (solid content) is preferably within the range of 9/1 to 5/5 by weight. If the blending ratio of water-repellent resin (solid content) to carbon black is too low, the binding of carbon black by the resin is weak, cracks are likely to occur in the cured coating film of MPL15, and the coating film strength is reduced. Furthermore, the binding between MPL15 and the conductive porous substrate 16 is also reduced, and the contact resistance is increased. On the other hand, if the blending ratio of water-repellent resin (solid content) to carbon black is too high, the resin content becomes too high, and the resin content blocks the voids (gaps between carbon particles), resulting in a decrease in porosity and gas diffusion. If the ratio of carbon black to water-repellent resin (solid content) is preferably within the range of 9/1 to 5/5 by weight, a cured coating film of a microporous layer that is less likely to crack, has high coating film strength, and has high gas diffusion properties can be obtained.

Furthermore, according to the experimental research of the present inventors, the solid content ratio of the MPL coating paste is preferably 5% by mass or more and 40% by mass or less. If the solid content ratio of carbon, water-repellent resin, etc. is too small, the shrinkage stress during MPL film formation may become large, making the cured coating film of MPL15 more likely to crack and reducing the coating film strength. On the other hand, if the solid content ratio is too large, the porosity decreases and the gas diffusion properties decrease. If the solid content ratio of the MPL coating paste is preferably 5% by mass or more and 40% by mass or less, a cured coating film of MPL15 that is less likely to crack, has high coating film strength, and has high gas diffusion properties can be obtained. More preferably, it is 5% by mass or more and 30% by mass or less, and even more preferably, it is 5% by mass or more and 25% by mass or less.

In addition, according to the inventors' experimental research, in the stirring by the thin film swirling method, the peripheral speed is preferably 25 m/s or more. If the peripheral speed is too slow, the secondary aggregates of carbon with a high DBP absorption amount are not sufficiently disintegrated, and when passing through a fine filter with a mesh size of 34 μm or more and 154 μm or less (100 mesh or more and 400 mesh or less) for removing iron foreign matter that will impair the durability of the catalyst layer 13, i.e., the durability and life of the battery, clogging occurs, productivity decreases, and it becomes difficult to pass through the filter, which reduces the durability and life of the battery. In addition, the coating property decreases, the cured coating film has a poor surface, and the adhesion with the catalyst layer 13 decreases, so that the generated water accumulates in the MPL 15, which makes it easy for flooding to occur and the drainage property decreases. On the other hand, as shown in Example 2, even if the peripheral speed is 50 m/s, the paste is unlikely to run through the back when applied to the conductive porous substrate 16, and the gas diffusion property is also excellent. In particular, by increasing the peripheral speed, it is possible to reduce the secondary agglomeration diameter and make the secondary agglomeration diameter uniform, which allows for a uniform cured coating film.

Considering the performance limits of the high-speed agitator for the thin film swirling method, if the peripheral speed during stirring using the thin film swirling method is 25 m/s or more and 60 m/s or less, the secondary agglomerates of carbon with a high DBP absorption capacity are sufficiently broken down to a secondary agglomerate size that can pass through a fine filter with a mesh size of 34 μm or more and 154 μm or less (100 mesh or more and 400 mesh or less), and good viscosity characteristics (flow characteristics) are obtained, and good coatability is obtained that is unlikely to cause surface defects during coating and ensures a sufficient coating width, resulting in good adhesion to the catalyst layer 13 and stable moisture management, resulting in stable battery performance, and also ensuring good battery durability and lifespan.

Furthermore, according to the inventors' experimental research, in the stirring at the above peripheral speed by the thin film swirling method, the stirring time is preferably 20 seconds or more, more preferably 25 seconds or more, and even more preferably 30 seconds or more. If the stirring time is too short, the secondary aggregates of carbon with a high DBP absorption amount are not sufficiently broken down, and when passing through a fine filter with a mesh size of 34 μm or more and 154 μm or less (100 mesh or more and 400 mesh or less) for removing iron foreign matter that will impair the durability of the catalyst layer 13, i.e., the durability and life of the battery, clogging occurs, productivity decreases, and it becomes difficult to pass through the filter, which reduces the durability and life of the battery. On the other hand, if the stirring time is too long, the viscosity will be excessively reduced, causing the paste components to run through the back when applied to the conductive porous substrate 16. The viscosity can be increased by adding a thickener, but the use of a thickener will lengthen the firing time to decompose it, leading to high costs and reduced productivity due to the use of a thickener and the high load of firing. If the stirring time in the thin film swirling method is preferably 20 seconds or more and 200 seconds or less, more preferably 25 seconds or more and 150 seconds or less, and even more preferably 30 seconds or more and 100 seconds or less, the secondary aggregates of carbon with a high DBP absorption amount are sufficiently disintegrated to a secondary aggregate size that can pass through a fine filter with a mesh size of 34 μm or more and 154 μm or less (100 mesh or more and 400 mesh or less) without causing a decrease in productivity or an increase in cost. This results in good viscosity characteristics (flow characteristics) that are unlikely to cause surface defects during application, and good coatability that ensures a sufficient coating width. This results in good adhesion to the catalyst layer 13, and stable moisture management, resulting in stable battery performance, and also ensures good battery durability and life.

In addition, according to the experimental research of the present inventors, the viscosity of the MPL coating paste at a shear rate of 50 S ^-1 is preferably 0.05 Pa.s or more and 1.0 Pa.s or less, more preferably 0.1 Pa.s or more and 0.9 Pa.s or less, and even more preferably 0.1 Pa.s or more and 0.5 Pa.s or less. If the viscosity is too low, the paste will penetrate through the conductive porous substrate 16 when applied to the conductive porous substrate 16, blocking the voids in the conductive porous substrate 16 and reducing the permeability of gas and moisture. On the other hand, if the viscosity is too high, the surface smoothness when applied to the conductive porous substrate 16 will decrease, and coating at the desired coating width will become difficult, productivity will decrease, and good coating properties will not be obtained. Furthermore, when passing through a fine filter with a mesh opening of 34 μm or more and 154 μm or less (100 mesh or more and 400 mesh or less) that can remove foreign matter such as metal components that will impair the durability of the catalyst layer 13, the pressure loss will increase and it will be difficult to pass through the filter. The viscosity of the MPL coating paste at a shear rate of 50 S ^-1 is preferably 0.05 Pa.s or more and 1.0 Pa.s or less, more preferably 0.1 Pa.s or more and 0.9 Pa.s or less, and even more preferably 0.1 Pa.s or more and 0.9 Pa.s or less, so that good viscosity characteristics that allow the paste to pass through a fine filter with a mesh size of 34 μm or more and 154 μm or less (100 mesh or more and 400 mesh or less) can be obtained, and productivity is good. In addition, good coatability is obtained in which surface defects are unlikely to occur during coating and a sufficient coating width can be ensured, and good adhesion to the catalyst layer 13 is also achieved, and stable moisture controllability is ensured, resulting in stable battery performance, and further, the durability and life of the battery can be well ensured.

As described above, the coating paste for forming a microporous layer in the above embodiment is a coating paste for forming a microporous layer that contains carbon black, a water-repellent resin, a dispersant, and a solvent, and the carbon black has a DBP absorption amount in the range of 300 ml/100 g or more and 500 ml/100 g or less and has a secondary aggregation diameter that passes through a mesh with an opening of 34 μm or more and 154 μm or less (100 mesh or more and 400 mesh or less).

Therefore, according to the coating paste for forming the microporous layer of this embodiment, the secondary aggregates of carbon black with high DBP absorption are crushed, so that the viscosity increase of the paste is suppressed, and MPL 15 can be formed with good coating properties on the surface of the conductive porous substrate 16. In the MPL 15, the carbon black with high DBP absorption has a high porosity due to its high structure, so that the porosity (air porosity) is high and the pores are uniformly distributed, and the highly structured structure forms gas and moisture passages, which can improve the diffusion and drainage of gas and moisture. In addition, by making the MPL 15 have good smoothness, good adhesion with the catalyst layer 13 can be obtained. Therefore, flooding is less likely to occur, and the gas diffusion and moisture management of the gas diffusion layer 14 can be improved, stabilizing the battery output and power generation performance, and improving the battery performance.

In particular, when carbon black with DBP absorption in the range of 300 ml/100 g to 500 ml/100 g has a secondary aggregation diameter that passes through a mesh with an opening of 34 μm or more and 154 μm or less (100 mesh or more and 400 mesh or less), the paste's good coatability ensures good surface smoothness in MPL15, and it is possible to achieve both high porosity and coating strength that is less susceptible to cracking. Therefore, by forming an MPL15 that has high porosity but good surface smoothness, is less susceptible to cracking, and has good adhesion to the catalyst layer 13, the MPL15 is less likely to accumulate moisture, has high drainage properties, maintains high gas diffusion properties, and also has good durability.

In addition, when carbon black having a DBP absorption amount within the range of 300 ml/100 g to 500 ml/100 g has a secondary agglomeration diameter that passes through a mesh with an opening size of 34 μm or more and 154 μm or less (100 mesh or more and 400 mesh or less), the carbon black is of a size that can pass through a fine mesh filter that enables the removal of foreign matter such as iron that affects the durability and lifespan of the catalyst layer 13 in which the cell reaction takes place, and is therefore unlikely to reduce the durability and lifespan of the battery.
Therefore, it is possible to form an MPL 15 with a high porosity, thereby improving the gas and moisture permeability of the MPL 15, and improving the gas diffusion and moisture management performance of the gas diffusion layer 14 in which the MPL 15 is formed.

Furthermore, in the coating paste for forming the microporous layer of the above embodiment, if the weight ratio of carbon black to water-repellent resin is within the range of 9/1 to 5/5, the adhesiveness (binding properties) of the resin in the cured coating film of MPL15 formed is good and cracks are unlikely to occur, and clogging of voids due to excess resin is suppressed, so that the coating film strength of MPL15 and gas and moisture permeability can be achieved at the same time, and the high gas diffusion and moisture management properties of the gas diffusion layer 14 can be better exhibited.

In addition, in the coating paste for forming the microporous layer of the above embodiment, if the viscosity at a shear rate of 50 s ^-1 is within the range of 0.05 Pa.s to 1.0 Pa.s, the back penetration of the paste (the phenomenon of the paste penetrating to the back side of the conductive porous substrate opposite to the coated surface) can be suppressed when applied to the conductive porous substrate 16 constituting the gas diffusion layer 14, and a cured coating film of MPL 15 with good smoothness can be formed at the desired coating width, resulting in good coatability. Therefore, a smooth MPL 15 can be obtained without reducing the porosity of the conductive porous substrate 16, and adhesion to the catalyst layer 13 can be increased, so that the gas diffusion property and moisture management property of the gas diffusion layer 14 are more stabilized.

The above embodiment can also be considered as an invention of a method for producing a coating paste for forming a microporous layer, which contains carbon black, a water-repellent resin, a dispersant, and a solvent, in which carbon black and a water-repellent resin having a DBP absorption amount in the range of 300 ml/100 g to 500 ml/100 g are mixed into a solvent containing a dispersant, and the mixture is stirred by a thin film swirling method.

According to the manufacturing method of the coating paste for forming a microporous layer in the above embodiment, the secondary agglomerates of carbon black with high DBP absorption can be broken down by dispersing the compounding materials with a high shear by the thin film swirling method, and the increase in viscosity of the paste can be suppressed. Preferably, the secondary agglomerates of carbon black with high DBP absorption can be broken down with high efficiency by dispersing the compounding materials with stirring at a peripheral speed of 25 m/s to 60 m/s.

Therefore, the coating paste for forming the microporous layer, in which the secondary aggregates of carbon black with high DBP absorption are broken down and the increase in viscosity is suppressed, can be applied to the surface of the conductive porous substrate 16 with good coating properties, and in the MPL 15 formed by drying and baking the applied paste, the carbon black with high DBP absorption has a high porosity due to its high structure, and the voids are high and uniformly distributed. Therefore, the diffusion and drainage of gas and moisture can be improved by forming gas and moisture passages due to the high structure structure. In addition, by making the MPL 15 have good smoothness, good adhesion with the catalyst layer 13 can be obtained. Therefore, flooding is less likely to occur, and the gas diffusion and moisture management of the gas diffusion layer 14 can be improved, and the battery output and power generation performance can be stabilized.

In particular, when carbon black having a DBP absorption in the range of 300 ml/100 g to 500 ml/100 g has a secondary aggregation diameter that passes through a mesh with an opening of 34 μm or more and 154 μm or less (100 mesh or more and 400 mesh or less), the good coatability of the paste gives MPL15 good surface smoothness, and it is possible to achieve both high porosity and coating strength that is less susceptible to cracking. Therefore, by forming MPL15 that has high porosity but good surface smoothness, is less susceptible to cracking, and has good adhesion to catalyst layer 13, MPL15 is less likely to accumulate moisture, has high drainage properties, maintains high gas diffusivity, and also has good durability.
Therefore, the MPL coating paste can form the MPL 15 that has a high porosity and can improve the diffusivity and moisture management performance of the gas diffusion layer 14.

The above embodiment can also be considered as an invention of a method for producing a coating paste for forming a microporous layer, which contains carbon black, a water-repellent resin, a dispersant, and a solvent, in which carbon black and a water-repellent resin having a DBP absorption amount in the range of 300 ml/100 g to 500 ml/100 g are mixed into a solvent containing a dispersant, and the mixture is stirred at a peripheral speed of 25 m/s or more and 60 m/s or less.

According to the manufacturing method of the coating paste for forming a microporous layer in the above embodiment, by dispersing the compounding material with a high shear at a peripheral speed of 25 m/s or more and 60 m/s or less, secondary agglomerates of carbon black with a high DBP absorption can be broken down and an increase in the viscosity of the paste can be suppressed. Preferably, by dispersing the compounding material with stirring at a peripheral speed of 25 m/s to 60 m/s, secondary agglomerates of carbon black with a high DBP absorption can be broken down with high efficiency.

Therefore, the coating paste for forming the microporous layer, in which the secondary aggregates of carbon black with high DBP absorption are broken down and the increase in viscosity is suppressed, can be applied to the surface of the conductive porous substrate 16 with good coating properties, and in the MPL 15 formed by drying and baking the applied paste, the carbon black with high DBP absorption has a high porosity due to its high structure, and the voids are high and uniformly distributed. Therefore, the diffusion and drainage of gas and moisture can be improved by forming gas and moisture passages due to the high structure structure. In addition, by making the MPL 15 have good smoothness, good adhesion with the catalyst layer 13 can be obtained. Therefore, flooding is less likely to occur, and the gas diffusion and moisture management of the gas diffusion layer 14 can be improved, and the battery output and power generation performance can be stabilized.

In particular, when carbon black having a DBP absorption in the range of 300 ml/100 g to 500 ml/100 g has a secondary aggregation diameter that passes through a mesh with an opening of 34 μm or more and 154 μm or less (100 mesh or more and 400 mesh or less), the good coatability of the paste gives MPL15 good surface smoothness, and it is possible to achieve both high porosity and coating strength that is less susceptible to cracking. Therefore, by forming MPL15 that has high porosity but good surface smoothness, is less susceptible to cracking, and has good adhesion to catalyst layer 13, MPL15 is less likely to accumulate moisture, has high drainage properties, maintains high gas diffusivity, and also has good durability.
Therefore, the MPL coating paste can form the MPL 15 that has a high porosity and can improve the diffusivity and moisture management performance of the gas diffusion layer 14.

Furthermore, in the manufacturing method of the coating paste for forming the microporous layer of the above embodiment, if a mixture of a dispersant, a solvent, carbon black, and a water-repellent resin is pre-mixed with a mixer such as a Disper or a turbine-stator type mixer at a peripheral speed of 1 m/s to 20 m/s before stirring by the thin film swirling method or the like or at a peripheral speed of 25 m/s to 60 m/s, that is, after dispersing at a low shear speed of 1 m/s to 20 m/s with a mixer such as a Disper or a turbine-stator type mixer, and then dispersing at a high shear speed of 25 m/s to 60 m/s by the thin film swirling method or the like, it is possible to suppress the variation in the distribution of the secondary agglomeration diameter of the carbon black, and it is possible to form a homogeneous MPL15 by suppressing the variation in the distribution of voids, and to obtain stable gas and moisture permeability. Therefore, it becomes an MPL coating paste that can form an MPL15 that can suppress the variation in the distribution of voids and obtain stable performance. In particular, if a mixer such as a Disper or turbine-stator type mixer is used for pre-mixing, a paste with fewer air bubbles can be obtained by mixing and degassing.

In addition, according to the manufacturing method of the coating paste for forming the microporous layer of the above embodiment, after stirring at a peripheral speed of 25 m/s to 60 m/s by a thin film swirling method or the like, the mixture is passed through a mesh with an opening of 34 μm or more and 154 μm or less (100 mesh or more and 400 mesh or less), so that the catalyst layer 13 has less foreign matter such as iron that affects its durability and lifespan. Therefore, the durability of the fuel cell 1 can be well maintained. In other words, the MPL coating paste can form an MPL 15 that does not reduce the durability and lifespan of the fuel cell 1.

In addition, in the manufacturing method of the coating paste for forming the microporous layer, if the weight ratio of carbon black to water-repellent resin is within the range of 9/1 to 5/5, the adhesiveness (binding property) of the resin in the cured coating film of MPL15 formed is good, cracks are unlikely to occur, and clogging of voids due to excess resin is suppressed. Therefore, the MPL coating paste can form MPL15 that can achieve both coating film strength and gas and moisture permeability.

In addition, in the above-mentioned method for producing a coating paste for forming a microporous layer, if the viscosity at a shear rate of 50 s ^-1 is within the range of 0.05 Pa·s to 1.0 Pa·s, then it is possible to suppress strike-through of the paste when it is applied to the conductive porous substrate 16 constituting the gas diffusion layer 14, and to form a cured coating film of MPL 15 with good smoothness at the desired coating width, resulting in good coatability. Thus, a smooth MPL 15 can be obtained without reducing the porosity of the conductive porous substrate 16, and adhesion to the catalyst layer 13 can be improved, resulting in an MPL coating paste that can form an MPL 15 that ensures stable performance.

Furthermore, the above embodiment can be considered as an invention of a gas diffusion layer 14 for a fuel cell that is composed of a conductive porous substrate 16 made of conductive fibers or the like, and an MPL 15 made of carbon black and a water-repellent resin formed on the surface of the conductive porous substrate 16, in which the carbon black in the MPL 15 has a secondary aggregate diameter that allows carbon black with a DBP absorption amount in the range of 300 ml/100 g to 500 ml/100 g to pass through a mesh with an opening of 34 μm or more and 154 μm or less (100 mesh or more and 400 mesh or less).

According to the gas diffusion layer 14 for fuel cells of this embodiment, the carbon black of the MPL 15, which is made of carbon black and water-repellent resin formed on the surface of the conductive porous substrate 16 made of conductive fibers or the like, has a secondary aggregate diameter that passes through a mesh with an opening of 34 μm or more and 154 μm or less (100 mesh or more and 400 mesh or less) with a DBP absorption amount in the range of 300 ml/100 g to 500 ml/100 g, and the secondary aggregates of the carbon black with high DBP absorption amount are crushed, so that the viscosity increase of the paste forming the MPL 15 is suppressed, and in the MPL 15 formed with good coating properties on the surface of the conductive porous substrate 16, the carbon black with high DBP absorption amount has a high porosity due to the high structure, and the porosity (air porosity) is high and the pores are uniformly distributed. Therefore, the formation of gas and moisture passages due to the high structure structure can improve the diffusion and drainage of gas and moisture. In addition, by making the MPL 15 have good smoothness, good adhesion with the catalyst layer 13 can be obtained. Therefore, flooding is less likely to occur, and the gas diffusion and moisture management of the gas diffusion layer 14 can be improved, stabilizing the battery output and power generation performance.

In particular, when carbon black with DBP absorption in the range of 300 ml/100 g to 500 ml/100 g has a secondary aggregation diameter that passes through a mesh with an opening of 34 μm or more and 154 μm or less (100 mesh or more and 400 mesh or less), MPL15 has good surface smoothness due to the good coatability of the paste, and can achieve both high porosity and coating strength that is less susceptible to cracking. Therefore, MPL15, which has high porosity but good surface smoothness, is less susceptible to cracking, and has good adhesion to the catalyst layer 13, is less likely to accumulate moisture, has high drainage properties, maintains high gas diffusion properties, and also has good durability.

In addition, when carbon black having a DBP absorption amount within the range of 300 ml/100 g to 500 ml/100 g has a secondary agglomeration diameter that passes through a mesh with an opening size of 34 μm or more and 154 μm or less (100 mesh or more and 400 mesh or less), the carbon black is of a size that can pass through a fine mesh filter that enables the removal of foreign matter such as iron that affects the durability and lifespan of the catalyst layer 13 in which the cell reaction takes place, and is therefore unlikely to reduce the durability and lifespan of the battery 1.
Therefore, according to the gas diffusion layer 14 for the fuel cell of the above embodiment, since it has the MPL 15 with a high porosity, it is possible to improve the permeability of gas and moisture, and it is possible to improve the performance of gas diffusion and moisture management.

Furthermore, in the gas diffusion layer 14 for the fuel cell of the above embodiment, if the weight ratio of the carbon black and water-repellent resin in the MPL15 is within the range of 9/1 to 5/5, the adhesiveness (binding) of the resin in the cured coating film of the MPL15 is good, cracks are less likely to occur, and clogging of voids due to excess resin is suppressed. Therefore, the gas diffusion layer 14 exhibits good moisture and gas permeability.

In the gas diffusion layer for a fuel cell of the above embodiment, the carbon black in the microporous layer formed on the surface of the conductive porous substrate and made of carbon black and water-repellent resin has a secondary aggregate diameter that allows the carbon black to pass through a mesh with an opening size of 34 to 154 μm and has a DBP absorption capacity in the range of 300 ml/100 g to 500 ml/100 g.

The conductive porous substrate is a gas diffusion layer electrode substrate, which is conductive and has a porous body with an average pore size larger than that of the microporous layer. For example, a porous substrate made of carbon fiber (e.g., graphite fiber, etc.) such as carbon fiber woven fabric, carbon fiber paper, carbon fiber nonwoven fabric, carbon felt, carbon paper, and carbon cloth, and a metal porous substrate such as foamed sintered metal, metal mesh, and expanded metal are used.
The microporous layer is formed by applying a coating paste for forming a microporous layer, containing carbon black and a water-repellent resin, to the surface of the conductive porous substrate, followed by drying and baking, so that the carbon black is bound with the water-repellent resin. The carbon black has a DBP absorption in the range of 300 ml/100 g to 500 ml/100 g and has a secondary agglomeration diameter that allows the carbon black to pass through a mesh with an opening of 34 to 154 μm.

Examples of the carbon black that can be used herein include furnace black, thermal black, ketjen black, and acetylene black. Acetylene black is preferred because it is capable of forming a microporous layer that has high gas diffusivity, strength, electrical conductivity, and the like.
The carbon black having a DBP absorption in the range of 300 ml/100 g to 500 ml/100 g and having a secondary agglomerate size that can pass through a mesh with an opening of 34 to 154 μm means that the secondary agglomerates (several tens of μm) of the carbon black having a DBP absorption in the range of 300 ml/100 g to 500 ml/100 g have been disintegrated (disaggregated) by high shear dispersion to a secondary agglomerate size (several μm) that can pass through a mesh with an opening of 34 μm to 154 μm, i.e., 100 mesh to 400 mesh. In other words, the carbon black having a DBP absorption in the range of 300 ml/100 g to 500 ml/100 g has been disintegrated to have a secondary agglomerate size that can pass through a mesh with an opening of 34 to 154 μm (100 to 400 mesh).

According to the above embodiment of the gas diffusion layer for a fuel cell, the carbon black in the microporous layer made of carbon black and water-repellent resin formed on the surface of a conductive porous substrate made of conductive fibers or the like has a secondary agglomerate diameter that passes through a mesh with an opening of 34 μm or more and 150 μm or less, and the carbon black has a DBP absorption within the range of 300 ml/100 g to 500 ml/100 g, and the secondary agglomerates of carbon black with high DBP absorption are crushed, so that an increase in viscosity of the paste forming the microporous layer is suppressed, and the microporous layer is formed with good coatability on the surface of the conductive porous substrate, and the carbon black with high DBP absorption has a high porosity due to its high structure, resulting in a high porosity (air porosity).

In particular, when carbon black with a DBP absorption in the range of 300 ml/100 g to 500 ml/100 g has a secondary aggregation diameter that passes through a mesh with an opening of 34 to 154 μm, the microporous layer has good surface smoothness due to the good coatability of the paste, and high porosity and coating strength that is less likely to cause cracks can be achieved at the same time. Therefore, a microporous layer with high porosity, good surface smoothness, few cracks, and good adhesion to the catalyst layer is less likely to accumulate moisture, has high drainage properties, maintains high gas diffusivity, and has good durability.
In addition, when carbon black having a DBP absorption amount in the range of 300 ml/100 g to 500 ml/100 g has a secondary agglomeration diameter that passes through a mesh with an opening size of 34 μm or more and 150 μm or less, the carbon black is of a size that can pass through a fine mesh filter that enables the removal of foreign matter such as iron that affects the durability and lifespan of the catalyst layer in which the cell reaction takes place, and is therefore unlikely to reduce the durability and lifespan of the battery.
Thus, the gas diffusion layer for a fuel cell according to the above embodiment has a microporous layer with a high porosity, so that the gas diffusion layer can have improved gas diffusivity and moisture management performance.

The carbon black and the water-repellent resin in the microporous layer of the gas diffusion layer for the fuel cell in the above embodiment have a solid weight ratio of 9/1≦carbon black/water-repellent resin≦5/5.

According to the gas diffusion layer for fuel cells of the above embodiment, the weight ratio of carbon black to water-repellent resin is within the range of 9/1 to 5/5, so that the adhesion (binding, binder effect) of the resin in the cured coating of the microporous layer is good, cracks are less likely to occur, and clogging of voids due to excess resin is suppressed. Therefore, the microporous layer can achieve both coating strength and gas and moisture permeability.

The MPL coating paste of this embodiment and the fuel cell gas diffusion layer 14 produced using the same are not limited to the solid polymer fuel cell 1, but can also be applied to other fuel cells, such as direct methanol fuel cells.
In addition, when implementing the present invention, the composition, ingredients, blending amounts, materials, and other manufacturing processes of the MPL coating paste and other parts of the fuel cell gas diffusion layer 14 are not limited to those in this embodiment.
Furthermore, the above-mentioned numerical ranges are not strict, and are of course approximate values including errors due to measurements, etc., and errors of several tens of percent are not denied. Furthermore, the numerical values given in the embodiments of the present invention do not indicate critical values, but indicate appropriate values suitable for implementation, so even if the numerical values are slightly changed, the implementation is not denied.

1 fuel cell 14 gas diffusion layer 15 microporous layer 16 conductive porous substrate

Claims (9)

A coating paste for forming a microporous layer, comprising carbon black, a water-repellent resin, a dispersant, and a solvent,
The carbon black has a DBP absorption amount in the range of 300 ml/100 g to 500 ml/100 g and has a secondary agglomeration size that allows it to pass through a mesh with an opening size in the range of 34 to 154 μm,
The coating paste for forming a microporous layer is characterized in that the viscosity at a shear rate of 50 s ⁻¹ is within the range of 0.05 Pa·s to 1.0 Pa·s . The coating paste for forming a microporous layer according to claim 1, characterized in that the weight ratio of the carbon black to the water-repellent resin is within the range of 9/1 to 5/5. A method for producing a coating paste for forming a microporous layer, comprising:
A method for producing a coating paste for forming a microporous layer, comprising mixing carbon black having a DBP absorption amount in the range of 300 ml/100 g to 500 ml/100 g and a water-repellent resin in a solvent containing a dispersant, and stirring the mixture by a thin film swirling method. A method for producing a coating paste for forming a microporous layer, comprising:
A method for producing a coating paste for forming a microporous layer, comprising mixing carbon black having a DBP absorption amount in the range of 300 ml/100 g to 500 ml/100 g and a water-repellent resin in a solvent containing a dispersant, and stirring the mixture at a peripheral speed of 25 m/s or more and 60 m/s or less. The method for producing a coating paste for forming a microporous layer according to claim 3 or 4 , characterized in that after the stirring, the mixture is passed through a mesh having an opening of 34 to 154 μm. The method for producing a coating paste for forming a microporous layer according to claim 3 or 4 , characterized in that pre-stirring is performed with a disperser or turbine stator type stirrer before the stirring. The method for producing a coating paste for forming a microporous layer according to claim 3 or 4 , characterized in that, before the stirring, pre-stirring is performed at a peripheral speed of 1 m/s or more and 20 m/s or less. The method for producing a coating paste for forming a microporous layer according to claim 3 or 4 , characterized in that the weight ratio of the carbon black to the water-repellent resin is within the range of 9/1 to 5/5. The method for producing a coating paste for forming a microporous layer according to claim 3 or 4 , characterized in that the coating paste for forming a microporous layer has a viscosity in the range of 0.05 Pa·s to 1.0 Pa·s at a shear ^rate of 50 s −1 .

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