Water-repellent layer for fuel cell, manufacturing method thereof, and gas diffusion layer for fuel cell using the same

Abstract

To provide a water-repellent layer for a fuel cell that exhibits high peel strength with respect to a porous substrate.SOLUTION: A water-repellent layer for a fuel cell includes conductive particles and at least one thermoplastic fluororesin selected from the group consisting of ethylene-tetrafluoroethylene and perfluoroalkoxyalkanes, which bonds the conductive particles together.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a water-repellent layer for a fuel cell, a method for manufacturing the same, and a gas diffusion layer for a fuel cell using the same.

Background Art

[0002] A fuel cell usually includes a membrane electrode assembly (MEA) in which electrodes including catalyst layers are joined to both sides of an electrolyte membrane as a basic unit. In such a fuel cell, the electrode usually has a two-layer structure of a gas diffusion layer and a catalyst layer, and the gas diffusion layer is used to supply a reaction gas and electrons to the catalyst layer, which is a reaction field for the electrode reaction. In such a gas diffusion layer, from the viewpoint of improving the power generation performance of the fuel cell, high gas permeability and high water repellency are required. Conventionally, a gas diffusion layer including a porous base material such as carbon paper or carbon cloth and a water-repellent layer laminated on the surface of the porous base material has been used. As the water-repellent layer, a water-repellent layer formed by a dry film-forming method using composite particles containing conductive particles such as carbon particles and fluororesin particles such as polytetrafluoroethylene (PTFE) particles is known (for example, Japanese Unexamined Patent Application Publication No. 2019-121423 (Patent Document 1) and Japanese Unexamined Patent Application Publication No. 2021-2444 (Patent Document 2)).

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0004] However, in the conventional gas diffusion layer, the peel strength of the water-repellent layer with respect to the porous substrate is not always sufficient. In particular, in the water-repellent layer containing PTFE, it has been 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 of the above prior art, and an object thereof is to provide a water-repellent layer for a fuel cell that exhibits high peel strength with respect to a porous substrate, a method for manufacturing a water-repellent layer for a fuel cell that can be manufactured at low cost, and a gas diffusion layer for a fuel cell including the water-repellent layer.

Means for Solving the Problems

[0006] As a result of intensive studies to achieve the above object, the present inventors have found that in a water-repellent layer containing conductive particles and a thermoplastic fluororesin, by using at least one of ethylene tetrafluoroethylene and perfluoroalkoxyalkane as the thermoplastic fluororesin, a water-repellent layer exhibiting high peel strength with respect to a porous substrate can be obtained by firing treatment at a relatively low temperature, and thus the present invention has been completed.

[0007] That is, the present invention provides the following aspects. [1] A water-repellent layer for a fuel cell containing conductive particles and at least one thermoplastic fluororesin selected from the group consisting of ethylene tetrafluoroethylene and perfluoroalkoxyalkane that binds the conductive particles to each other. [2] The melt viscosity of the thermoplastic fluororesin (temperature: 300 ° C (in the case of ethylene tetrafluoroethylene) or 360 ° C (in the case of perfluoroalkoxyalkane or a mixture of ethylene tetrafluoroethylene and perfluoroalkoxyalkane), shear rate: 100 s -1 ) is 1 × 10 5 Pa·s or less, and the water-repellent layer for a fuel cell according to [1]. [3] The melt point of the thermoplastic fluororesin is 200 to 325 ° C, and the water-repellent layer for a fuel cell according to [1] or [2]. [4] The water-repellent layer for a fuel cell according to any one of [1] to [3], which is formed by a dry film-forming method. [5] A gas diffusion layer for a fuel cell, comprising a porous substrate and the water-repellent layer for a fuel cell according to any one of [1] to [4] laminated on the surface of the porous substrate. [6] A film-forming step of forming a composite powder layer by film-forming a composite powder of conductive particles and at least one thermoplastic fluororesin powder selected from the group consisting of ethylene tetrafluoroethylene powder and perfluoroalkoxy alkane powder by a dry film-forming method; a consolidation step of pressurizing and consolidating the composite powder layer; and a water-repellent layer forming step of firing the consolidated composite powder layer at a temperature within the range of not lower than the melting point of the thermoplastic fluororesin and not higher than 350 °C, thereby forming a water-repellent layer for a fuel cell in which the conductive particles are bound to each other by the thermoplastic fluororesin. A method for manufacturing a gas water-repellent layer for a fuel cell, comprising: [7] The method for manufacturing a gas water-repellent layer for a fuel cell according to [6], wherein the particle diameter d90 at cumulative 90% in the volume-based particle size distribution of the thermoplastic fluororesin powder is 90 μm or less. [8] The method for manufacturing a gas water-repellent layer for a fuel cell according to [7], wherein the particle diameter d90 at cumulative 90% in the volume-based particle size distribution of the thermoplastic fluororesin powder is 50 μm or less.

[0008] Incidentally, the reason why the water-repellent layer for a fuel cell of the present invention exhibits high peel strength with respect to the porous substrate is not necessarily clear, but the present inventors speculate as follows. That is, a composite powder layer is formed by film-forming a composite powder of conductive particles and thermoplastic fluororesin powder by a dry film-forming method, and after consolidating this composite powder layer, firing is performed at a temperature within the range of not lower than the melting point of the thermoplastic fluororesin and not higher than 350 °C, whereby a water-repellent layer for a fuel cell containing the conductive particles and the thermoplastic fluororesin powder is formed.

[0009] Here, when at least one of ethylene tetrafluoroethylene (ETFE) and perfluoroalkoxy alkane (PFA) is used as the thermoplastic fluororesin, when the consolidated composite powder layer is heated at a temperature within the range of not lower than the melting point of ETFE or PFA and not higher than 350°C, ETFE or PFA melts. At this time, since the melt viscosities of ETFE and PFA are low, the melted ETFE or PFA easily spreads to cover the surface of the aggregates of the conductive particles or flows into the voids within the aggregates of the conductive particles. The ETFE or PFA that covers the surface of the aggregates of the conductive particles or flows into the voids within the aggregates of the conductive particles acts as a binder resin in binding the conductive particles to each other. Therefore, in the water-repellent layer for fuel cells of the present invention containing the conductive particles and at least one of ETFE and PFA, as shown in FIG. 1, a three-dimensional structure in which the conductive particles (1) are cross-linked with ETFE or PFA (2b) is formed, and thereby, it is presumed that a high peel strength with respect to the porous substrate can be obtained.

[0010] On the other hand, when polytetrafluoroethylene (PTFE) is used as the thermoplastic fluororesin, when the consolidated composite powder layer is heated at a temperature within the range of not lower than the melting point of PTFE and not higher than 350°C, PTFE melts. However, since the melt viscosity of PTFE is high, the melted ETFE and PFA are difficult to flow and maintain their particle shapes. For this reason, PTFE does not act as a binder resin in binding the conductive particles to each other. In the conventional water-repellent layer for fuel cells containing the conductive particles and PTFE, as shown in FIG. 2, PTFE (3) remains in a particle shape and adheres to the conductive particles (1), and since the three-dimensional structure as described above is not formed, it is presumed that the peel strength with respect to the porous substrate is lower than that of the water-repellent layer for fuel cells of the present invention.

[0011] Also, when polyvinylidene fluoride (PVDF) is used as the thermoplastic fluororesin, when the consolidated composite powder layer is heated at a temperature within the range of not lower than the melting point of PVDF and not higher than 350°C, PVDF melts. Although the melt viscosity of PVDF is lower than that of PTFE, it is higher than that of ETFE or PFA. Therefore, the molten PVDF hardly covers the surface of the aggregate of conductive particles sufficiently or flows into the voids in the aggregate of conductive particles sufficiently. For this reason, in the water-repellent layer for fuel cells containing conductive particles and PVDF, since the three-dimensional structure as described above is not sufficiently formed, the peel strength with respect to the porous substrate is higher than that of the conventional water-repellent layer for fuel cells containing conductive particles and PTFE, but it is presumed to be lower than that of the water-repellent layer for fuel cells of the present invention.

[0012] Furthermore, as described above, the reason for the difference in the peel strength with respect to the porous substrate between the case where ETFE or PFA is used as the thermoplastic fluororesin and the case where PTFE or PVDF is used is that depending on the high or low melt viscosity of the thermoplastic fluororesin, whether the thermoplastic fluororesin sufficiently covers the surface of the aggregate of conductive particles or whether it sufficiently flows into the voids in the aggregate of conductive particles, as shown in FIG. 1, it is presumed that it depends on whether a three-dimensional structure in which the conductive particles are crosslinked with the thermoplastic fluororesin is formed. Therefore, it is presumed that the effect of improving the peel strength with respect to the porous substrate can be obtained without depending on the type of conductive particles.

[0013] Also, although the reason for the improvement in the peel strength against the porous substrate by using the micronized thermoplastic fluororesin powder is not necessarily clear, the inventors of the present invention speculate as follows. That is, when the composite powder of the micronized thermoplastic fluororesin powder and the conductive particles is formed into a film by a dry film-forming method, as shown in FIG. 3, it is speculated that in the obtained composite powder layer, the micronized thermoplastic fluororesin powder is in a highly dispersed state. By densifying and firing such a composite powder layer, a three-dimensional structure in which the conductive particles are cross-linked by the thermoplastic fluororesin is formed in a large amount, and the bonding strength between the conductive particles by the thermoplastic fluororesin increases. As described in paragraph

[0005] of JP-A-2002-124266, it is speculated that an improvement in the peel strength against the porous substrate due to an increase in the bonding strength is achieved.

Advantages of the Invention

[0014] According to the present invention, it is possible to obtain a water-repellent layer for a fuel cell that exhibits high peel strength against a porous substrate, and a gas diffusion layer for a fuel cell including this water-repellent layer. In addition, it is possible to manufacture the water-repellent layer for a fuel cell by firing treatment at a relatively low temperature.

Brief Description of the Drawings

[0015]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Mode for Carrying Out the Invention

[0016] Hereinafter, the present invention will be described in detail according to its preferred embodiments.

[0017] 〔Water-repellent layer for fuel cell〕 First, the water-repellent layer for a fuel cell of the present invention will be described. The water-repellent layer for a fuel cell of the present invention contains conductive particles and at least one thermoplastic fluororesin selected from the group consisting of ethylene tetrafluoroethylene and perfluoroalkoxyalkane that binds the conductive particles to each other.

[0018] (Conductive particles) The conductive particles used in the present invention are not particularly limited as long as they are conductive particles. For example, carbon particles such as carbon black, carbon nanofibers, carbon nanotubes, carbon nanohorns, and vapor-grown carbon fibers can be mentioned. These conductive particles may be used alone or in combination of two or more.

[0019] The average primary particle diameter of such conductive particles is not particularly limited, but is preferably 1 to 1000 nm, more preferably 10 to 200 nm, and even more preferably 15 to 100 nm. When the average primary particle diameter of the conductive particles is less than the above lower limit, the gas diffusibility tends to decrease. On the other hand, when the average primary particle diameter of the conductive particles exceeds the above upper limit, the electrical resistance increases and the power generation performance of the fuel cell tends to decrease.

[0020] 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 still more preferably 0.08 to 1 μm. When the average particle size of the aggregates of the conductive particles is less than the lower limit, the gas diffusibility tends to decrease. On the other hand, when the average particle size of the aggregates of the conductive particles exceeds the upper limit, the electrical resistance increases and the power generation performance of the fuel cell tends to decrease.

[0021] Furthermore, the conductive particles preferably have a bulk density of 0.01 to 0.30 g / cm 3 and more preferably 0.02 to 0.20 g / cm 3 When the bulk density of the conductive particles is less than the lower limit, the voids between the conductive particles become too large, and the binding between the conductive particles by the thermoplastic fluororesin becomes insufficient, so that the peel strength of the water-repellent layer with respect to the porous substrate tends to be difficult to improve. On the other hand, when the bulk density of the conductive particles exceeds the upper limit, the voids between the conductive particles become small, and it becomes difficult for the thermoplastic fluororesin particles to enter the voids between the conductive particles, so that the peel strength of the water-repellent layer with respect to the porous substrate tends to be difficult to improve.

[0022] (Thermoplastic fluororesin) The thermoplastic fluororesin used in the present invention is at least one selected from the group consisting of ethylene tetrafluoroethylene (ETFE) and perfluoroalkoxyalkane (PFA). Since ETFE and PFA have a lower melt viscosity than polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF), the melted ETFE and PFA are easy to flow, spread to cover the surface of the conductive particles, or flow into the voids in the aggregates of the conductive particles. As a result, the melted ETFE and PFA act as a binder resin in the binding of the conductive particles to each other, and a three-dimensional structure in which the conductive particles are crosslinked by ETFE and PFA is formed, so that the peel strength of the water-repellent layer with respect to the porous substrate becomes high.

[0023] The melt viscosity of the thermoplastic fluororesin used in the present invention is such that the temperature is 300 °C (in the case of ETFE) or 360 °C (in the case of PFA or a mixture of ETFE and PFA), and the shear rate is 100 s -1 under the conditions of, preferably 1×10 5 Pa·s or less, more preferably 1×10 4 Pa·s or less, and even more preferably 1×10 3 Pa·s or less. When the melt viscosity of the thermoplastic fluororesin exceeds the above upper limit, the fluidity of the molten thermoplastic fluororesin is low, and it cannot sufficiently cover the surface of the conductive particles or sufficiently flow into the voids in the aggregates of the conductive particles. It is difficult for the molten thermoplastic fluororesin to act as a binder resin in binding the conductive particles to each other. Therefore, a three-dimensional structure in which the conductive particles are cross-linked by the thermoplastic fluororesin is not sufficiently formed, and the peel strength of the water-repellent layer with respect to the porous substrate tends to be low. Note that there is no particular limitation on the lower limit of the melt viscosity of the thermoplastic fluororesin used in the present invention, but from the viewpoint of preventing excessive segregation of the thermoplastic fluororesin, 1 mPa·s or more is preferable.

[0024] In addition, since the thermoplastic fluororesin used in the present invention has a lower melting point than PVDF, it can be melted by heating at a relatively low temperature, enabling the production of the water-repellent layer of the present invention at low cost. The melting point of the thermoplastic fluororesin used in the present invention is preferably 140 to 350 °C, and more preferably 200 to 325 °C. When the melting point of the thermoplastic fluororesin is less than the above lower limit, during the heat press bonding treatment (120 to 160 °C) in the subsequent process related to fuel cell formation, the thermoplastic fluororesin melts and soaks into the substrate too much, and the peel strength tends to decrease. On the other hand, when the melting point of the thermoplastic fluororesin exceeds the above upper limit, it becomes difficult to melt the thermoplastic fluororesin by heating at a temperature of 350 °C or less, and a three-dimensional structure in which the conductive particles are cross-linked by the thermoplastic fluororesin is not sufficiently formed, and the peel strength of the water-repellent layer with respect to the porous substrate tends to be low.

[0025] (Water-repellent layer) The water-repellent layer for a fuel cell of the present invention contains the conductive particles and the thermoplastic fluororesin, and the thermoplastic fluororesin binds the conductive particles to each other. By binding the conductive particles to each other with the thermoplastic fluororesin, a three-dimensional structure in which the conductive particles are cross-linked with the thermoplastic fluororesin is formed. As a result, the water-repellent layer for a fuel cell of the present invention has a high peel strength with respect to the porous substrate.

[0026] In the water-repellent layer for a fuel cell of the present invention, the content ratio of the conductive particles and the thermoplastic fluororesin is preferably 97 / 3 to 3 / 97, more preferably 95 / 5 to 30 / 70, and still more preferably 90 / 10 to 40 / 60 in terms of mass ratio (conductive particles / thermoplastic fluororesin). When the mass ratio (conductive particles / thermoplastic fluororesin) is less than the lower limit, the proportion of the conductive particles decreases, so that the electrical resistance in the water-repellent layer increases and the power generation performance of the fuel cell tends to deteriorate. On the other hand, when the mass ratio (conductive particles / thermoplastic fluororesin) exceeds the upper limit, the proportion of the thermoplastic fluororesin acting as a binder resin in the binding of the conductive particles decreases, so that a three-dimensional structure in which the conductive particles are cross-linked with the thermoplastic fluororesin is not sufficiently formed, and the peel strength of the water-repellent layer with respect to the porous substrate tends to be low.

[0027] The film thickness of the water-repellent layer for a fuel cell of the present invention is not particularly limited, but is preferably 1 to 100 μm, and more preferably 5 to 50 μm. When the film thickness of the water-repellent layer is less than the lower limit, the water-repellent layer cannot be reliably sandwiched between the catalyst layer and the substrate, and the surface of the substrate tends to be exposed. On the other hand, when the film thickness of the water-repellent layer exceeds the upper limit, the electrical resistance increases, so that the power generation performance of the fuel cell tends to deteriorate.

[0028] Further, the basis weight of the water-repellent layer for a fuel cell of the present invention is not particularly limited, but is preferably 0.1 to 20 mg / cm 2 and more preferably 0.5 to 5 mg / cm 2is more preferable. If the basis weight of the water-repellent layer is less than the lower limit, the water-repellent layer cannot be reliably sandwiched between the catalyst layer and the substrate, and the substrate surface tends to be exposed. On the other hand, if the basis weight of the water-repellent layer exceeds the upper limit, the electric resistance increases, and thus the power generation performance of the fuel cell tends to deteriorate.

[0029] Such a water-repellent layer for a fuel cell of the present invention can be formed, for example, as follows: a composite powder of the conductive particles and the thermoplastic fluororesin powder is formed into a film by a dry film-forming method, and after the obtained composite powder layer is consolidated, it is fired.

[0030] [Manufacturing Method of Water-Repellent Layer for Fuel Cell] Next, the manufacturing method of the water-repellent layer for a fuel cell of the present invention will be described. The manufacturing method of the water-repellent layer for a fuel cell of the present invention includes a film-forming step of forming a composite powder layer by forming a composite powder of conductive particles and at least one thermoplastic fluororesin powder selected from the group consisting of ethylene tetrafluoroethylene powder and perfluoroalkoxyalkane powder by a dry film-forming method; a consolidation step of pressurizing and consolidating the composite powder layer; a water-repellent layer forming step of firing the consolidated composite powder layer at a temperature within the range of not less than the melting point of the thermoplastic fluororesin and not more than 350 °C to form a water-repellent layer for a fuel cell in which the conductive particles are bound to each other by the thermoplastic fluororesin; and is a method including the above steps. Hereinafter, each step will be described separately.

[0031] Note that the "conductive particles", "thermoplastic fluororesin" and "water-repellent layer" in the manufacturing method of the water-repellent layer for a fuel cell of the present invention are the same as those described in the water-repellent layer for a fuel cell of the present invention (and their preferred conditions are also the same).

[0032] (Film-Forming Step) The film-forming process according to the present invention is a process of forming a composite powder of the conductive particles and at least one selected from the group consisting of the thermoplastic fluororesin powders (ETFE and PFA) by a dry film-forming method, whereby a composite powder layer is formed.

[0033] The thermoplastic fluororesin powders (ETFE powder and PFA powder) used in the present invention are the thermoplastic fluororesins (ETFE and PFA) described in the water-repellent layer for fuel cells of the present invention, and are in powder form. The average particle diameter of such thermoplastic fluororesin powders is not particularly limited, but is preferably 0.0001 to 1000 μm, and more preferably 0.01 to 100 μm. When the average particle diameter of the thermoplastic fluororesin powder is less than the above lower limit, the handleability as a resin powder significantly deteriorates and the productivity tends to decrease. On the other hand, when the average particle diameter of the thermoplastic fluororesin powder exceeds the above upper limit, uneven density of the resin is partially formed in the water-repellent layer, and three-dimensional cross-linking is not sufficiently formed in the sparse portion, and the peel strength tends to be low.

[0034] Further, the thermoplastic fluororesin powder preferably has a particle diameter d90 at the cumulative 90% in the volume-based particle size distribution of 90 μm or less, and more preferably 50 μm or less. When a thermoplastic fluororesin powder with d90 below the above upper limit is used, the thermoplastic fluororesin powder easily enters the gaps between the conductive particles, and a three-dimensional structure in which the conductive particles are cross-linked with the thermoplastic fluororesin is likely to develop, so the peel strength of the water-repellent layer against the porous substrate tends to improve. Also, the effect due to the particle diameter of such thermoplastic fluororesin powder tends to be more prominent for powders with smaller d90. Furthermore, the effect due to the particle diameter of the thermoplastic fluororesin powder tends to appear more prominently as the bulk density of the conductive particles is larger, that is, as the gaps between the conductive particles are smaller.

[0035] The composite powder contains the conductive particles and the thermoplastic fluororesin powder. In such a composite powder, the content ratio of the conductive particles to the thermoplastic fluororesin powder is preferably 97 / 3 to 3 / 97, more preferably 95 / 5 to 30 / 70, and still more preferably 90 / 10 to 40 / 60 in terms of mass ratio (conductive particles / thermoplastic fluororesin powder). When the mass ratio (conductive particles / thermoplastic fluororesin powder) is less than the lower limit, the proportion of the conductive particles decreases, so the electrical resistance in the obtained water-repellent layer increases, and the power generation performance of the fuel cell tends to deteriorate. On the other hand, when the mass ratio (conductive particles / thermoplastic fluororesin powder) exceeds the upper limit, the proportion of the thermoplastic fluororesin acting as a binder resin in the binding of the conductive particles decreases, so a three-dimensional structure in which the conductive particles are cross-linked with the thermoplastic fluororesin is not sufficiently formed, and in the obtained water-repellent layer, the peel strength with respect to the porous substrate tends to be low.

[0036] Further, the composite powder may contain a powder composed of other components other than the conductive particles and the thermoplastic fluororesin powder as long as it does not inhibit the function of the obtained water-repellent layer and the effects of the present invention.

[0037] Also, in the composite powder, when the conductive particles form aggregates, the ratio of the average particle diameter of the thermoplastic fluororesin powder to the average particle diameter (average secondary particle diameter) of the aggregates of the conductive particles ([average particle diameter of thermoplastic fluororesin powder] / [average secondary particle diameter of 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. When the ratio of the average particle diameters is less than the lower limit, the thermoplastic fluororesin covers the conductive particles and the electrical resistance tends to increase. On the other hand, when the ratio of the average particle diameters exceeds the upper limit, the distribution of the thermoplastic fluororesin becomes uneven, and three-dimensional cross-linking is not sufficiently formed in the sparse part, and the peel strength tends to be low.

[0038] The composite powder can be obtained by mixing the conductive particles, the thermoplastic fluororesin powder, and, if necessary, a powder composed of other components so as to have a predetermined ratio. There are no particular restrictions on the method of mixing these powders. For example, a known method capable of sufficiently and uniformly dispersing and mixing the conductive particles, the thermoplastic fluororesin powder, and, if necessary, a powder composed of other components, such as a method of stirring and mixing using a commercially available stirrer, can be appropriately employed. Also, the stirring conditions are not particularly restricted and can be set as appropriate, but it is preferable to mix under shear conditions (shear mixing).

[0039] 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) composed of the composite powder. By forming the composite powder into a film by the dry film-forming method, in the obtained composite powder layer, the uniform dispersion state of the conductive particles and the thermoplastic fluororesin powder in the composite powder before film formation is maintained.

[0040] There are no particular restrictions on the dry film-forming method. For example, known dry film-forming methods such as an electrostatic screen printing method and an electrostatic coating method can be appropriately employed. However, the electrostatic screen printing method is preferable because a composite powder having a desired particle size can be more easily applied (formed into a film), and moreover, the thickness of the composite powder layer obtained after application can be more easily adjusted to a desired thickness. Therefore, a screen is disposed above a film-forming substrate (such as a porous substrate for a gas diffusion layer) for forming a coating film, the composite powder is disposed on the screen, and then the composite powder is rubbed into the screen using a pressing member (such as a squeegee), whereby the electrostatic screen printing method of applying and forming the composite powder into a film is more preferable. When the electrostatic screen printing is employed as the dry film-forming method, various film-forming conditions such as the type of screen (mesh) and the magnitude of the voltage can be appropriately set according to the design of the water-repellent layer to be manufactured, and the conditions employed in the known electrostatic screen printing method can be appropriately employed.

[0041] In the film-forming step, the film-forming substrate on which the composite powder is formed can be appropriately selected according to the intended use, and there is no particular limitation. For example, when a laminate of the substrate obtained after finally forming the water-repellent layer and the water-repellent layer is used as the gas diffusion layer, it is preferable to use a porous substrate for the gas diffusion layer as the film-forming substrate. Examples of such a porous substrate include carbon paper and carbon cloth.

[0042] Furthermore, in the film-forming step, the coating amount of the composite powder is not particularly limited, but in terms of the basis weight of the obtained water-repellent layer, 0.1 to 20 mg / cm 2 is preferable, and 0.5 to 5 mg / cm 2 is more preferable. When the basis weight of the water-repellent layer is less than the lower limit, the water-repellent layer cannot be reliably sandwiched between the catalyst layer and the substrate, and the substrate surface tends to be exposed. On the other hand, when the basis weight of the water-repellent layer exceeds the upper limit, the electric resistance increases, and thus the power generation performance of the fuel cell tends to decrease.

[0043] (Consolidation step) The consolidation step according to the present invention is a step of pressurizing and consolidating the composite powder layer. The pressure during pressurization is not particularly limited, but 0.2 to 5 MPa is preferable, 0.3 to 4.5 MPa is more preferable, and 0.4 to 4 MPa is even more preferable. When the pressure is less than the lower limit, the composite powder layer cannot be sufficiently consolidated, the contact between the conductive particles decreases, the electric resistance increases, and it tends to be difficult to manufacture a structure with a desired design. On the other hand, when the pressure exceeds the upper limit, the carbon fibers constituting the porous substrate are broken, and the properties of the structure deteriorate due to consolidation, so both the peel strength of the water-repellent layer and the power generation performance of the fuel cell tend to decrease.

[0044] The pressurization time when pressurizing the composite powder layer is not particularly limited, but is preferably 0.1 to 600 seconds, more preferably 0.5 to 300 seconds. When the pressurization time is less than the lower limit, the contact between the conductive particles decreases, the electrical resistance increases, and both the peel strength of the water-repellent layer and the power generation performance of the fuel cell tend to decrease. On the other hand, when the pressurization time exceeds the upper limit, the carbon fibers constituting the porous base material are broken, and the characteristics of the structure deteriorate due to densification, so both the peel strength of the water-repellent layer and the power generation performance of the fuel cell tend to decrease.

[0045] The method for pressurizing the composite powder layer is not particularly limited, and for example, known pressurization methods (pressing methods) such as a flat press and a roll press can be appropriately employed. There are also no particular limitations on the temperature conditions when pressurizing the composite powder layer. Furthermore, in the present invention, as a method for pressurizing the composite powder layer, a hot press in which pressurization (pressing) is performed while heating may be employed.

[0046] The conditions such as the thickness and porosity of the composite powder layer densified in this way may be appropriately set according to the types of the conductive particles and the thermoplastic fluororesin powder so that the finally obtained water-repellent layer has desired characteristics, and accordingly, the pressurization conditions such as the pressure, pressurization time, and pressurization temperature during densification may be appropriately adjusted.

[0047] (Water-repellent layer forming step) The water-repellent layer forming step according to the present invention is a step of baking the composite powder layer after densification at a temperature within the range of not less than the melting point of the thermoplastic fluororesin and not more than 350 ° C (preferably not more than 300 ° C), whereby a water-repellent layer for a fuel cell having a three-dimensional structure in which the conductive particles are bound (crosslinked) by the thermoplastic fluororesin is obtained. When the baking temperature is less than the lower limit, it becomes difficult to sufficiently melt the thermoplastic fluororesin, and a three-dimensional structure in which the conductive particles are crosslinked by the thermoplastic fluororesin is not sufficiently formed, and in the obtained water-repellent layer, the peel strength with respect to the porous base material decreases. On the other hand, when the baking temperature exceeds the upper limit, the energy efficiency decreases and the manufacturing cost increases.

[0048] The firing time during the firing is not particularly limited, but for example, 0.1 to 120 minutes is preferable, and 0.5 to 60 minutes is more preferable. When the firing time is less than the lower limit, it becomes difficult to sufficiently melt the thermoplastic fluororesin, and a three-dimensional structure in which the conductive particles are cross-linked by the thermoplastic fluororesin is not sufficiently formed. In the resulting water-repellent layer, the peel strength with respect to the porous substrate tends to decrease. On the other hand, when the firing time exceeds the upper limit, the energy efficiency tends to decrease and the manufacturing cost tends to increase.

[0049] The gas atmosphere during the firing is not particularly limited. For example, it may be an oxidizing gas atmosphere containing oxygen or an inert gas atmosphere such as nitrogen. However, from the viewpoints of cost reduction and workability improvement, an air atmosphere is preferable. Also, the pressure conditions during the firing are not particularly limited, but from the viewpoints of cost reduction and workability improvement, atmospheric pressure (normal pressure) is preferable.

[0050] The heating means used for such firing is not particularly limited, and examples include known heating furnaces such as hot blast furnaces and electric furnaces.

[0051] In this way, by firing the mixed powder layer after the densification, a water-repellent layer having a three-dimensional structure in which the conductive particles are bound (cross-linked) by the thermoplastic fluororesin can be obtained.

[0052] The film thickness of the water-repellent layer is not particularly limited, but 1 to 100 μm is preferable, and 5 to 50 μm is more preferable. When the film thickness of the water-repellent layer is less than the lower limit, it becomes difficult to reliably sandwich the water-repellent layer between the catalyst layer and the substrate. Therefore, the peel strength of the water-repellent layer tends to decrease, and the electrical resistance increases, so the power generation performance of the fuel cell tends to decrease. On the other hand, when the film thickness of the water-repellent layer exceeds the upper limit, the electrical resistance increases, so the power generation performance of the fuel cell tends to decrease.

[0053] Also, the basis weight of the water-repellent layer is not particularly limited, but is preferably 0.1 to 20 mg / cm 2 and more preferably 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 reliably sandwiched between the catalyst layer and the substrate, and the substrate surface tends to be exposed. On the other hand, if the basis weight of the water-repellent layer exceeds the upper limit, the electric resistance increases, and thus the power generation performance of the fuel cell tends to deteriorate.

[0054] 〔Gas Diffusion Layer for Fuel Cell〕 Next, the gas diffusion layer for a fuel cell of the present invention will be described. The gas diffusion layer for a fuel cell of the present invention includes a porous substrate and the water-repellent layer for a fuel cell of the present invention laminated on the surface of the porous substrate.

[0055] The porous substrate is not particularly limited as long as it can be used for a gas diffusion layer for a fuel cell, and examples thereof include carbon paper and carbon cloth. The thickness of the porous substrate is not particularly limited as long as it can be adopted in a fuel cell and can be appropriately set, and is usually 100 to 300 μm.

[0056] In addition, as long as the gas diffusion layer for a fuel cell of the present invention includes the porous substrate and the water-repellent layer for a fuel cell of the present invention laminated on the surface of the porous substrate, other configurations (for example, other layers other than the porous substrate and the water-repellent layer) and characteristics (for example, conditions such as the BET surface area and pore volume of the entire gas diffusion layer) are not particularly limited.

[0057] Such a gas diffusion layer for a fuel cell of the present invention can be produced by laminating the water-repellent layer for a fuel cell of the present invention on the surface of such a porous substrate, for example, by the method for producing the water-repellent layer for a fuel cell of the present invention. Such a gas diffusion layer for a fuel cell of the present invention is particularly useful, for example, as a gas diffusion layer used for an electrode of a fuel cell.

Examples

[0058] Hereinafter, the present invention will be described more specifically based on Examples and Comparative Examples, but the present invention is not limited to the following Examples. The melt viscosity of the thermoplastic fluororesin used in the Examples and Comparative Examples was measured using an Ubbelohde viscometer (manufactured by Shimadzu Corporation) at a temperature of 300 °C (in the case of ETFE) or 360 °C (in the case of PFA) and a shear rate of 100 s -1 under the conditions described above.

[0059] (Example 1) First, carbon black (Denka Black Li-400, manufactured by Denka Co., Ltd., average primary particle diameter 48 nm, bulk density: 0.15 g / cm 3 ) and ethylene tetrafluoroethylene (ETFE) powder (Neoflon ETFE EC-6520, manufactured by Daikin Industries, Ltd., melting point: 220 °C, melt viscosity: 2×10 3 Pa·s, average particle diameter: 40 μm) were weighed at a mass ratio of 85 / 15, and shear mixed at 10,000 rpm for 30 minutes using a multi-pass mixer (MP mixer, manufactured by Nippon Coke & Engineering Co., Ltd.) to obtain a composite powder for MPL.

[0060] Next, the obtained composite powder for MPL was applied onto the surface of carbon paper (manufactured by Chemix Co., Ltd., trade name: TGP-H-060H, PTFE: 5% by mass, film thickness: about 200 μm), which is a porous substrate, by an electrostatic screen printing method (dry film forming method) to a basis weight of 2 mg / cm 2 to form a film and form a composite powder layer. As the electrostatic screen printing method, an electrostatic screen printing apparatus (manufactured by Bell Kogyo Co., Ltd., trade name: T-1) and a screen mesh (manufactured by Bell Kogyo Co., Ltd., trade name: electrostatic screen) were used. The distance between the carbon paper and the screen mesh was set to 6 mm, and a voltage of 1 to 3 kV was applied therebetween. The composite powder was placed on the screen mesh and rubbed with a squeegee to drop the composite powder from the mesh onto the carbon paper to form a film.

[0061] Next, the composite powder layer laminated on the surface of the carbon paper was consolidated by pressing at a pressure of 3 MPa for 1 minute at room temperature using a flat plate press. The consolidated composite powder layer was heated (fired) at 250 °C for 30 minutes under 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 ETFE, and a laminate of the carbon paper and the water-repellent layer (gas diffusion layer for fuel cell) was obtained.

[0062] (Example 2) Instead of the ETFE powder, perfluoroalkane (PFA) powder (Neoflon PFA ACX-31 manufactured by Daikin Industries, Ltd., melting point: 320 °C, melt viscosity: 3×10 4 Pa·s, average particle diameter: 25 μm) was used, and in the same manner as in Example 1 except that the heating (firing) temperature of the consolidated composite powder layer was changed to 350 °C, a water-repellent layer in which the carbon black particles were bound together by the PFA was formed on the carbon paper, and a laminate of the carbon paper and the water-repellent layer (gas diffusion layer for fuel cell) was obtained.

[0063] (Comparative Example 1) Instead of the ETFE powder, polytetrafluoroethylene (PTFE) powder (KTL-500F manufactured by Kitamura Co., Ltd., melting point: 330 °C, melt viscosity: 1×10 10 Pa·s, average particle diameter: 500 nm) was used, and in the same manner as in Example 1 except that the heating temperature of the consolidated composite powder layer was changed to 350 °C, a water-repellent layer in which the carbon black particles were bound together by the PTFE was formed on the carbon paper, and a laminate of the carbon paper and the water-repellent layer (gas diffusion layer for fuel cell) was obtained.

[0064] (Comparative Example 2) Instead of the ETFE powder, polyvinylidene fluoride (PVDF) powder (Kynar HSV900F manufactured by Arkema, melting point: 172 °C, melt viscosity: 1×10 6Using (Pa·s, average particle diameter: 200 nm), except that the heating temperature of the consolidated composite powder layer was changed to 350 °C, in the same manner as in Example 1, a water-repellent layer in which the carbon black particles were bound to each other by the PVDF was formed on the carbon paper, and a laminate of the carbon paper and the water-repellent layer (gas diffusion layer for fuel cell) was obtained.

[0065] (Preparation Example 1) Ethylene tetrafluoroethylene (ETFE) powder (Neoflon ETFE EC-6520 manufactured by Daikin Industries, Ltd., melting point: 220 °C, melt viscosity: 2×10 3 Pa·s, average particle diameter: 40 μm) and zirconia beads (diameter 0.5 mm) were mixed at a volume ratio of 1:10, and the resulting mixture was put into a freeze-crushing bead mill (a prototype machine manufactured by Aimex Co., Ltd.) and subjected to a crushing treatment at -196 °C for 60 minutes in a liquid nitrogen atmosphere to obtain fine ETFE powder. The volume-based particle size distribution (frequency distribution) of this fine ETFE powder and the ETFE powder before crushing was measured using a particle size distribution measuring device (CAMSIZER X2 manufactured by Microtrac Bell Co., Ltd.). The results are shown in Fig. 4. Also, based on the obtained volume-based particle size distribution (frequency distribution), the particle diameter d50 at a cumulative 50% and the particle diameter d90 at a cumulative 90% in the volume-based particle size distribution were determined. These results are shown in Table 1.

[0066]

Table 1

[0067] (Example 3) Instead of carbon black (Denka Black Li-400 manufactured by Denka Co., Ltd.), carbon black (Denka Black AB granular product manufactured by Denka Co., Ltd., average primary particle diameter 35 nm, bulk density: 0.25 g / cm 3) was used, and instead of the ETFE powder (average particle diameter: 40 μm), the fine ETFE powder prepared in Preparation Example 1 (d50: 6.0 μm, d90: 33.2 μm) was used. A composite powder for MPL was prepared in the same manner as in Example 1 except that carbon black and the fine ETFE powder were mixed at a mass ratio of 85 / 15. Further, using this composite powder for MPL, a water-repellent layer in which the carbon black particles were bound to each other by the ETFE was formed on the carbon paper, and a laminate of the carbon paper and the water-repellent layer (gas diffusion layer for fuel cell) was obtained.

[0068] (Example 4) A composite powder for MPL was prepared in the same manner as in Example 3 except that the mass ratio of carbon black to the fine ETFE powder was changed to 90 / 10. Further, using this composite powder for MPL, a water-repellent layer in which the carbon black particles were bound to each other by the ETFE was formed on the carbon paper, and a laminate of the carbon paper and the water-repellent layer (gas diffusion layer for fuel cell) was obtained.

[0069] (Example 5) A composite powder for MPL was prepared in the same manner as in Example 3 except that the ETFE powder (Neoflon ETFE EC-6520 manufactured by Daikin Industries, Ltd., average particle diameter: 40 μm, d50: 37.6 μm, d90: 78.4 μm) was used as it was instead of the fine ETFE powder (d50: 6.0 μm, d90: 33.2 μm). Further, using this composite powder for MPL, a water-repellent layer in which the carbon black particles were bound to each other by the ETFE was formed on the carbon paper, and a laminate of the carbon paper and the water-repellent layer (gas diffusion layer for fuel cell) was obtained.

[0070] (Example 6) A composite powder for MPL was prepared in the same manner as in Example 5 except that the mass ratio of carbon black to the ETFE powder was changed to 90 / 10. Further, using this composite powder for MPL, a water-repellent layer in which the carbon black particles were bound to each other by the ETFE was formed on the carbon paper, and a laminate of the carbon paper and the water-repellent layer (gas diffusion layer for fuel cell) was obtained.

[0071] <Peeling Strength> Regarding the laminates of the carbon paper and the water-repellent layer obtained in the Examples and Comparative Examples, a 90-degree peeling test of the water-repellent layer was performed using an electric measurement stand ("MX2-500N-L-FA" manufactured by IMADA Co., Ltd.), and the peeling strength (unit: N / m) of the water-repellent layer with respect to the carbon paper was measured. The results are shown in Tables 2 to 3 and Figures 5 to 6.

[0072]

Table 2

[0073]

Table 3

[0074] As shown in Table 2 and Figure 5, when comparing Examples 1 to 2 and Comparative Examples 1 to 2 that use the same conductive particles and have the same mass ratio of conductive particles to resin, when forming a water-repellent layer containing conductive particles and a thermoplastic fluororesin by the dry film-forming method, when at least one of ETFE and PFA is used as the thermoplastic fluororesin (Examples 1 to 2), a water-repellent layer with a higher peeling strength can be obtained by heating (firing) treatment at a relatively low temperature compared to the case of using PTFE (Comparative Example 1) or PVDF (Comparative Example 2).

[0075] Also, as shown in Table 3 and Figure 6, when comparing Example 3 with Example 5 and Example 4 with Example 6, which have the same mass ratio of conductive particles to resin, when forming a water-repellent layer containing conductive particles and a thermoplastic fluororesin by the dry film-forming method, when using fine powder with a small particle diameter as the thermoplastic fluororesin powder (Examples 3 to 4), a water-repellent layer with a higher peeling strength can be obtained compared to the case of using powder with a large particle diameter (Examples 5 to 6). This is considered to be because fine resin powder with a small particle diameter is more likely to enter the gaps between conductive particles than resin powder with a large particle diameter, and a three-dimensional structure in which conductive particles are crosslinked with a thermoplastic fluororesin is more likely to develop.

[0076] Incidentally, although the reason why the water-repellent layer obtained in Example 1 exhibited a higher peel strength than the water-repellent layer obtained in Example 5 is not necessarily clear, the present inventors speculate as follows. That is, the water-repellent layer obtained in Example 1 was produced using conductive particles with a lower bulk density than the water-repellent layer obtained in Example 5. When the bulk density of the conductive particles decreases, the voids between the conductive particles become larger, so it is speculated that the thermoplastic fluororesin powder easily enters between the conductive particles. As a result, in the water-repellent layer obtained in Example 1, it is speculated that the adhesive force between the conductive particles due to the thermoplastic fluororesin is improved compared to the water-repellent layer obtained in Example 5, showing a high peel strength.

Industrial Applicability

[0077] As described above, according to the present invention, it is possible to obtain a water-repellent layer for a fuel cell that exhibits a high peel strength with respect to a porous substrate. Therefore, since the gas diffusion layer for a fuel cell of the present invention includes such a water-repellent layer, it is useful as a gas diffusion layer for a fuel cell in which the porous substrate and the water-repellent layer are difficult to peel off and which has excellent durability.

[0078] Further, according to the present invention, the water-repellent layer for a fuel cell of the present invention can be manufactured by firing treatment at a relatively low temperature. Therefore, the method for manufacturing the water-repellent layer for a fuel cell of the present invention is useful as a method capable of manufacturing the gas diffusion layer for a fuel cell of the present invention at low cost.

Explanation of Reference Numerals

[0079] 1: Conductive particles 2a: ETFE particles or PFA particles 2b: ETFE or PFA (binder resin) 3: PTFE particles 11, 13: Composite particle layer 12, 14: Water-repellent layer

Claims

1. A water-repellent layer for a fuel cell, comprising conductive particles and at least one thermoplastic fluororesin selected from the group consisting of ethylene-tetrafluoroethylene and perfluoroalkoxyalkanes, which bonds the conductive particles together.

2. Melt viscosity of the thermoplastic fluororesin (temperature: 300° C. (in the case of ethylene-tetrafluoroethylene) or 360° C. (in the case of perfluoroalkoxyalkane or in the case of a mixture of ethylene-tetrafluoroethylene and perfluoroalkoxyalkane), shear rate: 100 s －１ ) is 1 x 10 ５ 2. The water-repellent layer for a fuel cell according to claim 1, having a viscosity of not more than Pa·s.

3. 2. The water-repellent layer for a fuel cell according to claim 1, wherein the melting point of the thermoplastic fluororesin is 200 to 325°C.

4. 2. The water-repellent layer for a fuel cell according to claim 1, which is formed by a dry film formation method.

5. A gas diffusion layer for a fuel cell, comprising: a porous substrate; and the water-repellent layer for a fuel cell according to claim 1 laminated on a surface of the porous substrate.

6. a film-forming step of forming a composite powder layer by forming a composite powder of conductive particles and at least one thermoplastic fluororesin powder selected from the group consisting of ethylene-tetrafluoroethylene powder and perfluoroalkoxyalkane powder by a dry film-forming method; a consolidation step of compressing the composite powder layer; a water-repellent layer forming step of firing the densified composite powder layer at a temperature in the range of from the melting point of the thermoplastic fluororesin to 350° C. to form a water-repellent layer for a fuel cell in which the conductive particles are bound together by the thermoplastic fluororesin; A method for producing a water-repellent layer for a fuel cell, comprising:

7. 7. The method for producing a water-repellent layer for a fuel cell according to claim 6, wherein the thermoplastic fluororesin powder has a cumulative 90% particle diameter d90 of 90 μm or less in a volume-based particle size distribution.

8. 8. The method for producing a water-repellent layer for a fuel cell according to claim 7, wherein the thermoplastic fluororesin powder has a cumulative 90% particle diameter d90 in a volume-based particle size distribution of 50 μm or less.

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