Method for producing water-repellent layer for fuel cell

Abstract

To provide a manufacturing method of a water-repellent layer for a fuel cell showing high peeling strength to a base material.SOLUTION: Forming a composite powder layer by forming a composite powder of conductive particles and a thermoplastic fluororesin powder into a film by a dry film forming method; compacting the composite powder layer by applying pressure; and heat-treating the compacted composite powder layer at a temperature in a range of a melting point of the thermoplastic fluororesin or more and 350 °C or less. A water-repellent layer forming step of forming a water-repellent layer for a fuel cell in which the conductive particles are bound to each other by the fluorothermoplastic resin, wherein when the consolidated composite powder layer is heat-treated, a melt viscosity of the fluorothermoplastic resin at the heat treatment temperature is in a range of 50 to 50000Pa·s.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a method for producing a water-repellent layer for a fuel cell. [Background technology]

[0002] Fuel cells typically have a membrane electrode assembly (MEA) as a basic unit, in which electrodes including catalyst layers are bonded to both sides of an electrolyte membrane. In such fuel cells, the electrodes typically have a two-layer structure consisting of a gas diffusion layer and a catalyst layer. The gas diffusion layer is used to supply reactant gases and electrons to the catalyst layer, which is the reaction site for electrode reactions. To improve the power generation performance of fuel cells, such gas diffusion layers are required to have high gas permeability and high water repellency. Conventionally, gas diffusion layers have been used that include a porous substrate such as carbon paper or carbon cloth and a water-repellent layer laminated on the surface of the porous substrate. Known examples of the water-repellent layer include those formed by a dry membrane formation method using composite particles containing conductive particles such as carbon particles and fluorine-based resin particles such as polytetrafluoroethylene (PTFE) particles (see, for example, JP 2019-121423 A (Patent Document 1) and JP 2021-2444 A (Patent Document 2)). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 2019-121423 [Patent Document 2] Patent Publication No. 2021-2444 Summary of the Invention [Problem to be solved by the invention]

[0004] However, in conventional gas diffusion layers, the peel strength of the water-repellent layer to the porous substrate is not always sufficient, and in particular, in a water-repellent layer containing PTFE, it is difficult to obtain sufficient peel strength even when heated at a temperature exceeding the melting point of PTFE.

[0005] The present invention has been made in view of the problems with the prior art described above, and has an object to provide a method for producing a water-repellent layer for a fuel cell that exhibits high peel strength to a substrate. [Means for solving the problem]

[0006] As a result of extensive research to achieve the above object, the present inventors have found that in a method for producing a water-repellent layer containing conductive particles and a thermoplastic fluororesin, a water-repellent layer exhibiting high peel strength from a substrate can be formed by heat-treating a composite powder layer containing the conductive particles and the thermoplastic fluororesin powder so that the melt viscosity of the thermoplastic fluororesin at the heat treatment temperature falls within a predetermined range, thereby completing the present invention.

[0007] That is, the present invention provides the following aspects. [1] A film-forming step in which a composite powder of conductive particles and thermoplastic fluororesin powder is formed into a film by a dry film-forming method to form a composite powder layer; a consolidation step of compressing the composite powder layer; a water-repellent layer forming step of forming a water-repellent layer for a fuel cell in which the conductive particles are bound together by the thermoplastic fluororesin by heat-treating the consolidated composite powder layer at a temperature in the range of from the melting point of the thermoplastic fluororesin to 350°C; Including, A method for manufacturing a water-repellent layer for a fuel cell, wherein when the consolidated composite powder layer is heat-treated, the melt viscosity of the thermoplastic fluororesin at the heat treatment temperature is set to be within a range of 50 to 50,000 Pa·s. [2] The method for producing a water-repellent layer for a fuel cell according to [1], wherein when the composite powder layer after the compaction is heat-treated, the melt viscosity of the thermoplastic fluororesin at the heat treatment temperature is set to be in the range of 100 to 10,000 Pa·s.

[0008] Although the reason why a water-repellent layer exhibiting high peel strength from a substrate is formed according to the present invention is not entirely clear, the inventors speculate as follows: In other words, in the method for producing a water-repellent layer for a fuel cell according to the present invention, when a composite powder layer containing conductive particles and a thermoplastic fluororesin powder is heat-treated, if the melt viscosity of the thermoplastic fluororesin at the heat treatment temperature is set within a predetermined range, as shown in Fig. 1 , the molten thermoplastic fluororesin 1 spreads to cover the surfaces of the conductive particles 2 or flows into the gaps between the conductive particles 2, and the thermoplastic fluororesin acts as a binder resin for binding the conductive particles together, forming a three-dimensional structure on the substrate 3 in which the conductive particles 2 are cross-linked by the thermoplastic fluororesin 1, which is presumably why the peel strength of the water-repellent layer from the substrate is high.

[0009] On the other hand, when a composite powder layer containing conductive particles and thermoplastic fluororesin powder is heat-treated, if the melt viscosity of the thermoplastic fluororesin at the heat treatment temperature is lower than a predetermined range, as shown in FIG. 2, most of the molten thermoplastic fluororesin 1a will penetrate into the substrate 3 before spreading onto the surfaces of the conductive particles 2, or even if it spreads, it will penetrate into the substrate 3 without remaining on the surfaces of the conductive particles 2. As a result, most of the thermoplastic fluororesin will not act to bond the conductive particles together, and it is presumed that this will result in a decrease in the peel strength of the water-repellent layer from the substrate.

[0010] Furthermore, when a composite powder layer containing conductive particles and thermoplastic fluororesin powder is heat-treated, if the melt viscosity of the thermoplastic fluororesin at the heat treatment temperature exceeds a predetermined range, as shown in FIG. 3, the thermoplastic fluororesin does not melt sufficiently, and the conductive particles 2 and the thermoplastic fluororesin powder 1b remain independently dispersed on the substrate 3. As a result, the thermoplastic fluororesin does not act to bond the conductive particles together, and it is presumed that the peel strength of the water-repellent layer to the substrate does not improve. [Effects of the Invention]

[0011] According to the present invention, it is possible to produce a water-repellent layer for a fuel cell that exhibits high peel strength to a substrate. [Brief explanation of the drawings]

[0012] [Figure 1] FIG. 2 is a schematic diagram showing the structure of a water-repellent layer for a fuel cell produced so that the melt viscosity of the thermoplastic fluororesin at the heat treatment temperature falls within a predetermined range. [Figure 2] FIG. 2 is a schematic diagram showing the structure of a water-repellent layer for a fuel cell produced so that the melt viscosity of the thermoplastic fluororesin at the heat treatment temperature is lower than a predetermined range. [Figure 3] FIG. 2 is a schematic diagram showing the structure of a water-repellent layer for a fuel cell produced so that the melt viscosity of the thermoplastic fluororesin at the heat treatment temperature is higher than a predetermined range. [Figure 4] 1 is a graph showing the relationship between the melt viscosity of a thermoplastic fluororesin at a heat treatment temperature and the tape peel strength of the water-repellent layers for fuel cells obtained in Examples 1 to 6 and Comparative Examples 1 to 6. DETAILED DESCRIPTION OF THE INVENTION

[0013] The present invention will be described in detail below based on preferred embodiments thereof.

[0014] The method for producing a water-repellent layer for a fuel cell of the present invention comprises the steps of: a film-forming step of forming a composite powder layer by forming a film of a composite powder of conductive particles and a thermoplastic fluororesin powder by a dry film-forming method; a consolidation step of compressing the composite powder layer; a water-repellent layer forming step of forming a water-repellent layer for a fuel cell in which the conductive particles are bound together by the thermoplastic fluororesin by heat-treating the consolidated composite powder layer at a temperature in the range of from the melting point of the thermoplastic fluororesin to 350°C; Including, This is a method in which, when the composite powder layer after the densification is heat-treated, the melt viscosity of the thermoplastic fluororesin at the heat treatment temperature is set to be in the range of 50 to 50,000 Pa·s.

[0015] First, each component used in the present invention will be described.

[0016] (Conductive particles) The conductive particles used in the present invention are not particularly limited as long as they are conductive particles, and examples thereof include carbon particles such as carbon black, carbon nanofibers, carbon nanotubes, carbon nanohorns, vapor-grown carbon fibers, etc. These conductive particles may be used alone or in combination of two or more types.

[0017] The average primary particle size of such conductive particles is not particularly limited, but is preferably 1 to 1,000 nm, more preferably 10 to 200 nm, and even more preferably 15 to 100 nm. If the average primary particle size of the conductive particles is less than the lower limit, the gas diffusibility of the resulting water-repellent layer tends to decrease. On the other hand, if the average primary particle size of the conductive particles exceeds the upper limit, the electrical resistance of the resulting water-repellent layer tends to increase, and the power generation performance of the fuel cell tends to decrease.

[0018] Furthermore, when the conductive particles form aggregates, the average particle size (average secondary particle size) is not particularly limited, but is preferably 0.01 to 10 μm, more preferably 0.05 to 5 μm, and even more preferably 0.08 to 1 μm. If the average particle size of the conductive particle aggregates is less than the lower limit, the gas diffusibility of the resulting water-repellent layer tends to decrease. On the other hand, if the average particle size of the conductive particle aggregates exceeds the upper limit, the electrical resistance of the resulting water-repellent layer tends to increase, and the power generation performance of the fuel cell tends to decrease.

[0019] (thermoplastic fluororesin powder) The thermoplastic fluororesin powder used in the present invention is not particularly limited as long as it is a powder of thermoplastic fluororesin having a melt viscosity within a predetermined range at temperatures ranging from the melting point to 350°C, and examples thereof include powders of thermoplastic fluororesin such as polyvinylidene fluoride (PVDF, melting point: 117 to 178°C), ethylene-tetrafluoroethylene (ETFE, melting point: 160 to 230°C), perfluoroethylenepropene copolymer (FEP, melting point: 250 to 280°C), and perfluoroalkoxyalkane (PFA, melting point: 280 to 320°C). Among these thermoplastic fluororesins, PVDF and ETFE are preferred from the viewpoints of appropriate melt viscosity and easy-to-handle temperature range, and PVDF is more preferred from the viewpoint of a wide melting point range due to the various copolymers.

[0020] Furthermore, the melting point of the thermoplastic fluororesin powder is preferably in the range of 140 to 350°C, and more preferably in the range of 200 to 325°C. If the melting point of the thermoplastic fluororesin powder is below the lower limit, the thermoplastic fluororesin powder will melt and penetrate too much into the substrate during the heat-pressure bonding process (120 to 160°C) in the subsequent process of forming the fuel cell, which tends to result in a decrease in peel strength. On the other hand, if the melting point of the thermoplastic fluororesin powder is above the upper limit, it will be difficult to melt the thermoplastic fluororesin powder by heating at a temperature of 350°C or less, and the three-dimensional structure in which the conductive particles are cross-linked by the thermoplastic fluororesin will not be sufficiently formed, which tends to result in a decrease in the peel strength of the water-repellent layer from the substrate.

[0021] Furthermore, the average particle size of the thermoplastic fluororesin powder is not particularly limited, but is preferably 0.0001 to 1000 μm, and more preferably 0.01 to 100 μm. If the average particle size of the thermoplastic fluororesin powder is below the lower limit, the handleability of the resin powder tends to be significantly impaired, resulting in reduced productivity. On the other hand, if the average particle size of the thermoplastic fluororesin powder exceeds the upper limit, the resin density may vary in parts within the water-repellent layer, and three-dimensional crosslinking may not be sufficiently formed in the sparse parts, resulting in a tendency for the resulting water-repellent layer to have low peel strength from the substrate.

[0022] Next, each step in the method for producing a water-repellent layer for a fuel cell of the present invention will be described.

[0023] (Film forming process) The film-forming step according to the present invention is a step of forming a film from a composite powder of the conductive particles and the thermoplastic fluororesin powder by a dry film-forming method, thereby forming a composite powder layer.

[0024] The composite powder contains the conductive particles and the thermoplastic fluororesin powder. In such a composite powder, the ratio of the conductive particles to the thermoplastic fluororesin powder is preferably 97 / 3 to 3 / 97 by mass (conductive particles / thermoplastic fluororesin powder), more preferably 95 / 5 to 30 / 70, and even more preferably 90 / 10 to 40 / 60. When the mass ratio (conductive particles / thermoplastic fluororesin powder) is less than the lower limit, the proportion of conductive particles decreases, which increases the electrical resistance of the resulting water-repellent layer and tends to reduce the power generation performance of the fuel cell. On the other hand, when the mass ratio (conductive particles / thermoplastic fluororesin powder) exceeds the upper limit, the proportion of thermoplastic fluororesin acting as a binder resin to bind the conductive particles decreases, which prevents the formation of a sufficient three-dimensional structure in which the conductive particles are cross-linked by the thermoplastic fluororesin. This tends to result in a water-repellent layer with low peel strength from the substrate.

[0025] The composite powder may also contain a powder of components other than the conductive particles and the thermoplastic fluororesin powder, as long as the function of the resulting water-repellent layer and the effects of the present invention are not impaired.

[0026] When the conductive particles in the composite powder form aggregates, the ratio of the average particle size of the thermoplastic fluororesin powder to the average particle size (average secondary particle size) of the conductive particle aggregates ([average particle size of the thermoplastic fluororesin powder] / [average secondary particle size of the conductive particles]) is preferably 0.001 μm / 10 μm to 1000 μm / 0.01 μm, and more preferably 0.01 μm / 1 μm to 100 μm / 1 μm. If the average particle size ratio is below the lower limit, the conductive particles tend to be coated with the thermoplastic fluororesin, resulting in increased electrical resistance. On the other hand, if the average particle size ratio exceeds the upper limit, the thermoplastic fluororesin distribution will vary in density, and three-dimensional crosslinking will not be sufficiently formed in the sparse areas, resulting in reduced peel strength.

[0027] The composite powder can be obtained by mixing the conductive particles, the thermoplastic fluororesin powder, and, if necessary, powders of other components in a predetermined ratio. There are no particular limitations on the method for mixing these powders, and any known method that can sufficiently uniformly disperse and mix the conductive particles, the thermoplastic fluororesin powder, and, if necessary, powders of other components, such as a method of stirring and mixing using a commercially available stirrer, can be appropriately adopted. Furthermore, there are no particular limitations on the stirring conditions, and they can be set appropriately, but mixing under shear conditions (shear mixing) is preferred.

[0028] In the film-forming step according to the present invention, the composite powder is formed into a film by a dry film-forming method to form a layer (composite powder layer) made of the composite powder. By forming the composite powder into a film by the dry film-forming method, the conductive particles and the thermoplastic fluororesin powder in the composite powder before film-forming are maintained in the obtained composite powder layer.

[0029] The dry film-forming method is not particularly limited, and known dry film-forming methods such as electrostatic screen printing and electrostatic coating can be used as appropriate. However, electrostatic screen printing is preferred because it allows composite powder having a desired particle size to be more easily applied (formed into a film) and the thickness of the composite powder layer obtained after coating can be more easily adjusted to the desired thickness. Therefore, electrostatic screen printing is more preferred, in which a screen is placed above the substrate on which a coating film is to be formed, the composite powder is placed on the screen, and then the composite powder is rubbed into the screen using a pressing member (squeegee, etc.). When electrostatic screen printing is used as the dry film-forming method, various film-forming conditions such as the type of screen (mesh) and the magnitude of voltage can be appropriately set depending on the design of the water-repellent layer to be produced, and conditions used in known electrostatic screen printing methods can be appropriately used.

[0030] In the film-forming step, the substrate on which the composite powder is formed can be appropriately selected depending on the intended use and is not particularly limited, but for example, when the laminate of the substrate and the water-repellent layer obtained after the water-repellent layer is finally formed is used as a gas diffusion layer, it is preferable to use a porous substrate for a gas diffusion layer as the substrate on which the film is formed. Examples of such porous substrates include carbon paper and carbon cloth.

[0031] In the film-forming step, the coating amount of the composite powder is not particularly limited, but the coating amount of the resulting water-repellent layer is preferably 0.1 to 20 mg / cm. 2 is preferred, and 0.5 to 5 mg / cm 2 If the basis weight of the water-repellent layer is less than the lower limit, the water-repellent layer cannot be securely sandwiched between the catalyst layer and the substrate, and the surface of the substrate tends to be exposed. On the other hand, if the basis weight of the water-repellent layer exceeds the upper limit, the electrical resistance increases, and the power generation performance of the fuel cell tends to decrease.

[0032] Furthermore, in the film-forming step, it is desirable to form the composite powder layer so that the thickness of the resulting water-repellent layer is preferably 1 to 100 μm, more preferably 5 to 50 μm. If the thickness of the resulting water-repellent layer is less than the lower limit, it becomes difficult to securely sandwich the water-repellent layer between the catalyst layer and the substrate, which tends to reduce the peel strength of the water-repellent layer from the substrate and increase electrical resistance, which tends to reduce the power generation performance of the fuel cell. On the other hand, if the thickness of the resulting water-repellent layer exceeds the upper limit, the electrical resistance increases, which tends to reduce the power generation performance of the fuel cell.

[0033] (consolidation process) The consolidation step according to the present invention involves compressing the composite powder layer. The pressure applied during the compression step is not particularly limited, but is preferably 0.2 to 5 MPa, more preferably 0.3 to 4.5 MPa, and even more preferably 0.4 to 4 MPa. If the pressure is below the lower limit, the composite powder layer cannot be sufficiently compressed, the contact between the conductive particles decreases, and electrical resistance increases, making it difficult to manufacture a structure of the desired design. On the other hand, if the pressure exceeds the upper limit, the structure of the substrate (e.g., carbon fibers constituting the porous substrate) is destroyed, and the properties of the structure are degraded by compression. As a result, the resulting water-repellent layer tends to have reduced peel strength from the substrate and reduced power generation performance of the fuel cell.

[0034] The pressing time for pressing the composite powder layer is not particularly limited, but is preferably 0.1 to 600 seconds, and more preferably 0.5 to 300 seconds. If the pressing time is less than the lower limit, the contact between the conductive particles decreases, increasing electrical resistance, and the resulting water-repellent layer tends to have reduced peel strength from the substrate and reduced power generation performance of the fuel cell. On the other hand, if the pressing time exceeds the upper limit, the structure of the substrate (e.g., carbon fibers constituting the porous substrate) is destroyed, and the properties of the structure are reduced due to compaction, so the resulting water-repellent layer tends to have reduced peel strength from the substrate and reduced power generation performance of the fuel cell.

[0035] The method for pressing the composite powder layer is not particularly limited, and any known pressing method (pressing method) such as plate pressing or roll pressing can be appropriately used. There are also no particular limitations on the temperature conditions when pressing the composite powder layer. Furthermore, in the present invention, hot pressing, in which pressing is performed while heating, may be used as the method for pressing the composite powder layer.

[0036] The conditions such as the thickness and porosity of the composite powder layer thus compacted may be appropriately set depending on the types of the conductive particles and the thermoplastic fluororesin powder so that the water-repellent layer finally obtained has the desired properties, and the pressing conditions such as the pressure, pressing time, and pressing temperature during compaction may be appropriately adjusted accordingly.

[0037] (Water-repellent layer formation process) The water-repellent layer forming process according to the present invention involves heat-treating the consolidated composite powder layer at a temperature ranging from the melting point of the thermoplastic fluororesin to 350°C (preferably 300°C or lower), thereby obtaining a water-repellent layer for a fuel cell having a three-dimensional structure in which the conductive particles are bonded (cross-linked) by the thermoplastic fluororesin. If the heat treatment temperature is below the lower limit, it becomes difficult to sufficiently melt the thermoplastic fluororesin, and the three-dimensional structure in which the conductive particles are cross-linked by the thermoplastic fluororesin is not sufficiently formed, resulting in a decrease in the peel strength of the resulting water-repellent layer from the substrate. On the other hand, if the heat treatment temperature exceeds the upper limit, energy efficiency decreases and production costs increase.

[0038] In the water-repellent layer forming process according to the present invention, when the consolidated composite powder layer is heat-treated, the melt viscosity of the thermoplastic fluororesin at the heat treatment temperature is set to be within the range of 50 to 50,000 Pa·s (preferably 100 to 10,000 Pa·s, more preferably 150 to 3,000 Pa·s, and even more preferably 180 to 1,000 Pa·s). By setting the melt viscosity of the thermoplastic fluororesin at the heat treatment temperature within this range, a water-repellent layer with excellent peel strength to the substrate can be obtained. On the other hand, if the melt viscosity of the thermoplastic fluororesin at the heat treatment temperature is below the lower limit, the thermoplastic fluororesin does not remain on the surface of the conductive particles but penetrates into the substrate, and the thermoplastic fluororesin does not act to bond the conductive particles together, resulting in a decrease in peel strength to the substrate. On the other hand, if the melt viscosity of the thermoplastic fluororesin at the heat treatment temperature exceeds the upper limit, the conductive particles and the thermoplastic fluororesin powder are independently dispersed, and the thermoplastic fluororesin does not act to bond the conductive particles together, resulting in a decrease in peel strength to the substrate.

[0039] Examples of methods for ensuring that the melt viscosity of the thermoplastic fluororesin at the heat treatment temperature falls within the above range include a method in which a thermoplastic fluororesin powder to be used is selected depending on various conditions (e.g., the required peel strength of the water-repellent layer) and the heat treatment temperature is determined so that the melt viscosity falls within the above range; and a method in which the heat treatment temperature is determined depending on various conditions (e.g., the heat resistance of the substrate) and a thermoplastic fluororesin is selected whose melt viscosity at the heat treatment temperature falls within the above range.

[0040] The heat treatment time is not particularly limited, but is preferably 0.1 to 120 minutes, more preferably 0.5 to 60 minutes. If the heat treatment time is less than the lower limit, it becomes difficult to sufficiently melt the thermoplastic fluororesin, and the three-dimensional structure in which the conductive particles are crosslinked by the thermoplastic fluororesin is not sufficiently formed, and the peel strength of the resulting water-repellent layer to the substrate tends to decrease. On the other hand, if the heat treatment time exceeds the upper limit, energy efficiency tends to decrease and production costs tend to increase.

[0041] The gas atmosphere during the heat treatment is not particularly limited, and may be, for example, an oxidizing gas atmosphere containing oxygen or an inert gas atmosphere such as nitrogen, but an air atmosphere is preferred from the viewpoints of reducing costs and improving workability.Furthermore, the pressure conditions during the heat treatment are not particularly limited, but atmospheric pressure (normal pressure) is preferred from the viewpoints of reducing costs and improving workability.

[0042] The heating means used for such heat treatment is not particularly limited, and examples thereof include known heating furnaces such as hot air furnaces and electric furnaces.

[0043] In this way, by heat treating the mixed powder layer after the compaction, a water-repellent layer having a three-dimensional structure in which the conductive particles are bonded (cross-linked) to each other by the thermoplastic fluororesin can be obtained. [Example]

[0044] The present invention will be described in more detail below based on examples and comparative examples, but the present invention is not limited to the following examples.

[0045] Example 1 First, carbon black (Denka Black Granules, manufactured by Denka Co., Ltd., average primary particle size: 35 nm) and polyvinylidene fluoride (PVdF) powder (Arkema Kynar Superflex 2501, melting point: 120°C, average particle size: 80 μm) were weighed out in a mass ratio of 60 / 40 and mixed for 5 minutes at 20,000 rpm using a laboratory mill (Osaka Chemical Co., Ltd. Labo Mill 2), after which the mixture was vacuum dried at 120°C for 1 hour to remove moisture, yielding a composite powder for MPL.

[0046] Next, the obtained composite powder for MPL was applied to the surface of a porous substrate, carbon paper ("Torayca Carbon Paper" manufactured by Toray Industries, Inc., thickness: 200 μm), by electrostatic screen printing (dry film formation method) to form a film with a basis weight of 2 mg / cm. 2The electrostatic screen printing method used an electrostatic screen printing device (manufactured by Berg Kogyo Co., Ltd., product name: T-1) and a screen mesh (manufactured by Berg Kogyo Co., Ltd., product name: Electrostatic Screen), with the carbon paper and the screen mesh at a distance of 6 mm and a voltage of 1 to 3 kV applied between them, and the composite powder was placed on the screen mesh and rubbed with a squeegee, causing the composite powder to fall from the mesh onto the carbon paper, thereby forming a film.

[0047] Next, the composite powder layer laminated on the surface of the carbon paper was compressed at room temperature for 1 minute at a pressure of 3 MPa using a flat press. The compressed composite powder layer was then heat-treated at 140°C for 30 minutes at normal pressure using an electric furnace to form a water-repellent layer on the carbon paper in which the carbon black particles were bound together by the PVdF, thereby obtaining a laminate of the carbon paper and the water-repellent layer (gas diffusion layer for a fuel cell).

[0048] Example 2 A water-repellent layer in which the carbon black particles were bound together by the PVdF was formed on carbon paper in the same manner as in Example 1, and a laminate of the carbon paper and the water-repellent layer (gas diffusion layer for a fuel cell) was obtained. The water-repellent layer was formed on carbon paper in the same manner as in Example 1, except that a PVdF powder (Arkema's "Kynar 721", melting point: 170°C, average particle size: 10 μm) was used instead of the PVdF powder (Arkema's "Kynar Superflex 2501") and the heat treatment temperature after compaction was changed to 200°C.

[0049] Example 3 A water-repellent layer in which the carbon black particles were bound together by the PVdF was formed on carbon paper in the same manner as in Example 1, and a laminate of the carbon paper and the water-repellent layer (gas diffusion layer for a fuel cell) was obtained. The water-repellent layer was formed on carbon paper in the same manner as in Example 1, except that a PVdF powder (Arkema's "Kynar Flex 2821", melting point: 140-145°C, average particle size: 10 μm) was used instead of the PVdF powder (Arkema's "Kynar Superflex 2501") and the heat treatment temperature after compaction was changed to 180°C.

[0050] Example 4 A water-repellent layer in which the carbon black particles were bound together by the ETFE was formed on carbon paper in the same manner as in Example 1, and a laminate of the carbon paper and the water-repellent layer (gas diffusion layer for a fuel cell) was obtained. The water-repellent layer was formed on carbon paper in the same manner as in Example 1, except that ethylene tetrafluoroethylene (ETFE) powder ("Neoflon ETFE EC-6520" manufactured by Daikin Industries, Ltd., melting point: 220°C, average particle size: 40 μm) was used instead of the PVdF powder and the heat treatment temperature after compaction was changed to 260°C.

[0051] Example 5 A water-repellent layer in which the carbon black particles were bound together by the ETFE was formed on the carbon paper in the same manner as in Example 4, except that the heat treatment temperature after compaction was changed to 230°C, and a laminate of the carbon paper and the water-repellent layer (gas diffusion layer for fuel cells) was obtained.

[0052] Example 6 A water-repellent layer in which the carbon black particles were bound together by the ETFE was formed on the carbon paper in the same manner as in Example 4, except that the heat treatment temperature after compaction was changed to 250°C, and a laminate of the carbon paper and the water-repellent layer (gas diffusion layer for fuel cells) was obtained.

[0053] (Comparative Example 1) A water-repellent layer in which the carbon black particles were bound together by the ETFE was formed on the carbon paper in the same manner as in Example 1, except that the heat treatment temperature after compaction was changed to 200°C, and a laminate of the carbon paper and the water-repellent layer (gas diffusion layer for fuel cells) was obtained.

[0054] (Comparative Example 2) A water-repellent layer in which the carbon black particles were bound together by the ETFE was formed on the carbon paper in the same manner as in Example 1, except that the heat treatment temperature after compaction was changed to 260°C, and a laminate of the carbon paper and the water-repellent layer (gas diffusion layer for fuel cells) was obtained.

[0055] (Comparative Example 3) A water-repellent layer in which the carbon black particles were bound together by the PVdF was formed on carbon paper in the same manner as in Example 1, except that a PVdF powder ("Kynar 705" manufactured by Arkema, melting point: 170°C, average particle size: 10 μm) was used instead of the PVdF powder ("Kynar Superflex 2501" manufactured by Arkema), and a laminate of the carbon paper and the water-repellent layer (gas diffusion layer for fuel cells) was obtained.

[0056] Comparative Example 4 A water-repellent layer in which the carbon black particles were bound together by the ETFE was formed on the carbon paper in the same manner as in Comparative Example 3, except that the heat treatment temperature after compaction was changed to 160°C, and a laminate of the carbon paper and the water-repellent layer (gas diffusion layer for fuel cells) was obtained.

[0057] (Comparative Example 5) A water-repellent layer in which the carbon black particles were bound together by the PVdF was formed on carbon paper in the same manner as in Example 1, except that a PVdF powder ("Kynar 711" manufactured by Arkema, melting point: 170°C, average particle size: 10 μm) was used instead of the PVdF powder ("Kynar Superflex 2501" manufactured by Arkema), and a laminate of the carbon paper and the water-repellent layer (gas diffusion layer for fuel cells) was obtained.

[0058] (Comparative Example 6) A water-repellent layer in which the carbon black particles were bound together by the ETFE was formed on the carbon paper in the same manner as in Example 4, except that the heat treatment temperature after compaction was changed to 175°C, and a laminate of the carbon paper and the water-repellent layer (gas diffusion layer for fuel cells) was obtained.

[0059] <Melt viscosity> For each thermoplastic fluororesin used in the examples and comparative examples, the melt viscosity at the heat treatment temperature after compaction in each example and comparative example was measured using a high-speed flow tester (manufactured by Shimadzu Corporation) at a shear rate of 100 s -1 The results are shown in Table 1.

[0060] <Tape peel strength> The gas diffusion layers for fuel cells obtained in the Examples and Comparative Examples were cut into 2 cm widths, and tape (Mending Tape manufactured by 3M Limited) was applied to the surface of the water-repellent layer. The tape was then peeled off perpendicularly to the surface of the water-repellent layer at a rate of 30 mm / min to conduct a 90° peel test. The average tensile strength of the peeled portion of the tape between 5 mm and 30 mm in length was calculated and used as the tape peel strength. The results are shown in Table 1.

[0061] [Table 1]

[0062] Based on the results shown in Table 1, the tape peel strength was plotted against the melt viscosity of the thermoplastic fluororesin at the heat treatment temperature. The results are shown in Figure 4. As shown in Figure 4, it was confirmed that the gas diffusion layers for fuel cells (Examples 1 to 6) manufactured so that the melt viscosity of the thermoplastic fluororesin at the heat treatment temperature was within a predetermined range were superior in tape peel strength compared to the gas diffusion layers for fuel cells manufactured so that the melt viscosity of the thermoplastic fluororesin at the heat treatment temperature was lower than the predetermined range (Comparative Examples 1 and 2) or higher than the predetermined range (Comparative Examples 3 to 6). [Industrial Applicability]

[0063] As described above, according to the present invention, it is possible to produce a water-repellent layer for a fuel cell that exhibits high peel strength from a substrate. Therefore, a method for producing a gas diffusion layer for a fuel cell using the method for producing a water-repellent layer for a fuel cell of the present invention is useful as a method for producing a gas diffusion layer for a fuel cell that is highly durable and in which the porous substrate and the water-repellent layer are not easily peeled off. [Explanation of symbols]

[0064] 1: Molten thermoplastic fluororesin 1a: Thermoplastic fluororesin impregnated into the substrate 1b: Thermoplastic fluororesin powder 2: Conductive particles 3: Base material

Claims

1. a film-forming step of forming a composite powder layer by forming a film of a composite powder of conductive particles and a thermoplastic fluororesin powder by a dry film-forming method; a consolidation step of compressing the composite powder layer; a water-repellent layer forming step of forming a water-repellent layer for a fuel cell in which the conductive particles are bound together by the thermoplastic fluororesin by heat-treating the consolidated composite powder layer at a temperature in the range of from the melting point of the thermoplastic fluororesin to 350°C; Including, A method for manufacturing a water-repellent layer for a fuel cell, characterized in that when the consolidated composite powder layer is heat-treated, the melt viscosity of the thermoplastic fluororesin at the heat treatment temperature is set to be within a range of 50 to 50,000 Pa·s.

2. The method for manufacturing a water-repellent layer for a fuel cell according to claim 1, characterized in that when the composite powder layer after compaction is heat-treated, the melt viscosity of the thermoplastic fluororesin at the heat treatment temperature is set to be in the range of 100 to 10,000 Pa·s.

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