Carbon nanotube film, gas diffusion layer, and polymer electrolyte fuel cell

Abstract

To provide a carbon nanotube film which can be made thin and has high gas permeability and excellent wettability.SOLUTION: The carbon nanotube film 10 has self-supporting properties. The carbon nanotube film 10 includes carbon nanotubes 1, a polyvinyl alcohol-based resin, and a sulfonated perfluoroalkyl polymer, wherein the ratio of the polyvinyl alcohol-based resin to the sulfonated perfluoroalkyl polymer is 0.6 or more and 1.4 or less on a mass basis in terms of solid content.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a carbon nanotube membrane, a gas diffusion layer, and a polymer electrolyte fuel cell. [Background technology]

[0002] BACKGROUND ART Carbon materials that exhibit electrical conductivity are used, for example, as materials for electrodes or electrode peripheral members in energy devices (such as storage batteries and power generation devices) (hereinafter referred to as energy device materials).

[0003] Patent Document 1 discloses an electrode for a polymer electrolyte fuel cell, which comprises a gas diffusion layer that is electrically conductive and breathable and is made of a fired membrane made of carbon nanotubes, and a catalyst layer that contains a metal catalyst supported on a catalyst support.

[0004] Patent Document 2 discloses a method for producing a gas diffusion layer having electrical conductivity and breathability for use in an electrode for a polymer electrolyte fuel cell. The production method comprises a dispersion process of dispersing carbon nanotubes in an organic solvent in which a binder resin and a water-soluble polymer have been dissolved, a forming process of forming the dispersion solution obtained in the dispersion process into a membrane and drying it, an elution process of eluting the water-soluble polymer from the membrane obtained in the forming process into water, and a firing process of firing the membrane from which the water-soluble polymer has been eluted in the elution process under an inert gas atmosphere to obtain a fired membrane.

[0005] Patent Document 3 discloses a gas diffusion layer in which a coating layer made of carbon powder and a water repellent agent is formed on at least one surface of a porous electrode substrate, and the contact angle of water on the surface of the coating layer is 120 to 150°. This gas diffusion layer is used as a gas diffusion layer for a polymer electrolyte fuel cell.

[0006] Patent Document 4 discloses a gas diffusion layer for a fuel cell, which includes a porous layer mainly composed of conductive particles and a binder resin, groove-shaped fluid flow paths provided on one main surface of the porous layer, and conductive wire portions having pores and formed by a layer of conductive fibers, which extend along the shape of the one main surface and the surface of the fluid flow paths. In this gas diffusion layer for a fuel cell, the length of the conductive fibers is shorter than the depth of the fluid flow paths and is also shorter than the minimum width of the fluid flow paths. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2008-210801 [Patent Document 2] Japanese Patent Application Laid-Open No. 2010-062021 [Patent Document 3] Japanese Patent Application Laid-Open No. 2018-142450 [Patent Document 4] Patent No. 6611056 Summary of the Invention [Problem to be solved by the invention]

[0008] Energy device materials are required to have both permeability to substances responsible for energy transfer and electron (electrical) conductivity. An energy device material having such properties may be, for example, a conductive porous material having a coating layer formed on a carbon fiber nonwoven fabric, the coating layer containing carbon powder and a binder resin such as a fluorine-based resin. When a conductive porous material using a water-repellent material such as a fluorine-based resin is used as an energy device material, the high water repellency may prevent the energy device from fully exhibiting its performance.

[0009] On the other hand, in some cases, a conductive porous material is provided with a coating layer containing a water-soluble resin on a carbon fiber nonwoven fabric. When a conductive porous material using a water-soluble resin is used as an energy device material, the performance of the energy device may not be fully exhibited due to its high hydrophilicity.

[0010] Carbon nanotubes are sometimes used as conductive porous materials for energy devices, and it is desirable that energy device materials using carbon nanotubes also fully exhibit the performance of the energy device. Furthermore, energy devices are sometimes required to be thin in terms of, for example, weight reduction and compactness, and in order to achieve thin energy devices, the energy device materials are sometimes required to have a reduced thickness.

[0011] The object of the present invention is to provide a carbon nanotube membrane that can be thinned, has high gas permeability, and has excellent wettability, a gas diffusion layer to which the carbon nanotube membrane is applied, and a solid polymer fuel cell equipped with a gas diffusion layer to which the carbon nanotube membrane is applied. [Means for solving the problem]

[0012] [1] A free-standing carbon nanotube film, Carbon nanotubes and a polyvinyl alcohol-based resin; a sulfonated perfluoroalkyl polymer; The ratio of the polyvinyl alcohol resin to the sulfonated perfluoroalkyl polymer is 0.6 or more and 1.4 or less by mass in terms of solid content. Carbon nanotube membrane.

[0013] [2] In the carbon nanotube film according to [1], The thickness is 20 μm or more and 50 μm or less. Carbon nanotube membrane.

[0014] [3] The carbon nanotube film according to [1] or [2], The carbon nanotubes are multi-walled carbon nanotubes. Carbon nanotube membrane.

[0015] [4] The carbon nanotube film according to any one of [1] to [3], the content of the polyvinyl alcohol-based resin is 30 parts by mass or more and 100 parts by mass or less, in terms of solid content, per 100 parts by mass of the carbon nanotubes; Carbon nanotube membrane.

[0016] [5] The carbon nanotube film according to any one of [1] to [4], The content of the sulfonated perfluoroalkyl polymer is 20 parts by mass or more and 140 parts by mass or less, based on 100 parts by mass of the carbon nanotubes, in terms of solid content. Carbon nanotube membrane.

[0017] [6] The carbon nanotube film according to any one of [1] to [5], The contact angle of water at 23°C is 130° or less. Carbon nanotube membrane.

[0018] [7] The carbon nanotube film according to any one of [1] to [6], Dry nitrogen flow rate measurement at a pressure of 200 kPa. Area: 2 cm 2 The air permeability is 3.0 x 10 -5 m Pa -1 ·S -1 That's all. Carbon nanotube membrane.

[0019] [8] A gas diffusion layer used in a polymer electrolyte fuel cell, [1] to [7], consisting of only the carbon nanotube film according to any one of [1] to [7] Gas diffusion layer.

[0020] [9] A polymer electrolyte fuel cell, a membrane electrode assembly; gas diffusion layers disposed to sandwich the membrane electrode assembly; separators arranged to sandwich the membrane electrode assembly via the gas diffusion layers, At least one of the gas diffusion layers is made solely of the carbon nanotube film according to any one of [1] to [7]. Polymer electrolyte fuel cell. [Effects of the Invention]

[0021] According to one aspect of the present invention, it is possible to provide a carbon nanotube membrane that can be thinned, has high gas permeability, and has excellent wettability, a gas diffusion layer to which the carbon nanotube membrane is applied, and a solid polymer fuel cell having a gas diffusion layer to which the carbon nanotube membrane is applied. [Brief explanation of the drawings]

[0022] [Figure 1] 1A and 1B are a schematic perspective view showing an example of a carbon nanotube film according to an embodiment of the present invention and an enlarged schematic view of the surface of the carbon nanotube film. [Figure 2] 1 is a schematic cross-sectional view illustrating an example of a polymer electrolyte fuel cell according to an embodiment of the present invention. [Figure 3] 1 is an example of an SEM image of the surface of the carbon nanotube film produced in Example 1. [Figure 4] 1 is a graph showing the results of evaluation of fuel cell characteristics of fuel cells produced using any of the carbon nanotube films produced in Example 1, Comparative Example 1, and Comparative Example 2. DETAILED DESCRIPTION OF THE INVENTION

[0023] Preferred embodiments of the present invention will be described below with reference to examples, but the present invention is not limited to the contents of the embodiments.

[0024] First Embodiment [Carbon nanotube film] The carbon nanotube membrane according to the first embodiment is a self-supporting carbon nanotube membrane. The carbon nanotube membrane includes carbon nanotubes, a polyvinyl alcohol-based resin, and a sulfonated perfluoroalkyl polymer. The ratio of the polyvinyl alcohol-based resin to the sulfonated perfluoroalkyl polymer is 0.6 to 1.4 by mass, calculated as solid content.

[0025] In this specification, the term "carbon nanotube film having self-supporting properties" refers to a film that is self-supporting by itself, and indicates that the carbon nanotube film is a film that has self-supporting properties (also referred to as a free-standing film). In other words, a free-standing carbon nanotube film is a film that can maintain its shape by itself even without a substrate or the like. The carbon nanotube film of this embodiment is flexible and can withstand molding because it has the above-mentioned configuration in addition to being self-supporting. In other words, the carbon nanotube film of this embodiment is a free-standing film that can maintain its shape by itself even without a substrate or the like, and is flexible and has mechanical strength sufficient to withstand molding.

[0026] The carbon nanotube film according to this embodiment can be used, for example, as an energy device material, a heat exchange filter, etc. In particular, the carbon nanotube film according to this embodiment is preferably applied to an energy device material. Specifically, one of the applications to which the carbon nanotube film according to this embodiment can be suitably applied is as a material applied to a gas diffusion layer (GDL) of a polymer electrolyte fuel cell (PEFC).

[0027] As an application to which the carbon nanotube membrane according to this embodiment can be applied, a gas diffusion layer (hereinafter sometimes referred to as GDL) of a polymer electrolyte fuel cell (hereinafter sometimes referred to as PEFC) will be described.

[0028] In a PEFC, hydrogen is supplied to the catalyst support layer through the GDL on the anode side, and hydrogen molecules are converted into hydrogen ions (H + ) and electrons (e - On the cathode side, oxygen is supplied to the catalyst support layer through the GDL, and hydrogen ions (H + ) and electrons (e - The binding of the hydroxyl groups with oxygen molecules generates heat and water vapor. Therefore, the GDL must have the following characteristics: (1) gas permeability (permeability of oxygen, hydrogen, nitrogen, and water vapor), (2) drainage (ability to discharge by-product water outside the cell), (3) electrical (electronic) conductivity, (4) heat resistance, and (5) water resistance.

[0029] For example, Patent Document 4 discloses a GDL having a porous layer composed of conductive particles and a binder resin, and a conductive line portion formed by a layer of conductive fibers composited with carbon nanotubes or the like. However, the technology disclosed in Patent Document 4 requires the provision of grooves in the porous layer to improve gas supply. Furthermore, the GDL disclosed in Patent Document 4 is designed to have high water repellency, with a water contact angle of 130° or more, in order to suppress flooding (clogging due to water vapor). Furthermore, Patent Document 3 discloses a GDL in which a coating layer composed of carbon powder and a water repellent agent is formed on at least one surface of a porous electrode substrate. Thus, the technologies disclosed in Patent Documents 3 and 4 have a laminated structure, making it difficult to reduce the thickness. Furthermore, the high water repellency may result in insufficient performance of the energy device, leaving room for improvement.

[0030] Furthermore, as disclosed in Patent Documents 1 and 2, for example, it has been proposed to subject a composite of carbon nanotubes and a resin to high-temperature treatment and use the resulting fired film as a GDL. However, the fired films containing carbon nanotubes disclosed in Patent Documents 1 and 2 require high-temperature treatment to obtain the fired film, and furthermore, because the thickness is about 200 μm, it is difficult to thin the fired carbon nanotube film. For this reason, it is difficult to thin the GDL using fired films containing carbon nanotubes.

[0031] In contrast, the carbon nanotube membrane according to the present embodiment contains carbon nanotubes as a primary material and further contains a hydrophilic polyvinyl alcohol-based resin and a sulfonated perfluoroalkyl polymer having both hydrophilic and hydrophobic moieties in a specific ratio. Therefore, the carbon nanotube membrane according to the present embodiment is a conductive porous membrane, has self-supporting properties, is water-resistant, and can maintain its shape even in hot water. Furthermore, the carbon nanotube membrane according to the present embodiment has the above-described configuration, allowing for thinning, high gas permeability, and excellent wettability. Because the carbon nanotube membrane according to the present embodiment has excellent gas permeability and wettability, when used as a GDL material, moisture generated by the reaction on the cathode side does not form droplets, thereby suppressing moisture accumulation in the pores of the carbon nanotube membrane. In other words, when the carbon nanotube membrane according to the present embodiment is used as a GDL material, it exhibits enhanced resistance to clogging due to water droplet formation. Therefore, it is believed that using the carbon nanotube membrane according to the present embodiment as a GDL material in a PEFC will suppress the impairment of gas permeability. As a result, when the carbon nanotube film according to this embodiment is used as a material for the GDL of a PEFC, overvoltage is suppressed, which can contribute to improving the cell performance.

[0032] In addition, a typical PEFC module is fabricated as a stack structure in which several hundred single cells are stacked. Since the carbon nanotube film according to the present embodiment can be thinned, when the carbon nanotube film according to the present embodiment is used as a material for the GDL of a PEFC, the thickness of the PEFC module can be reduced compared to when a conventional GDL is used. Therefore, by using the carbon nanotube film according to the present embodiment as a material for the GDL of a PEFC, the effect of thinning the PEFC module is enhanced.

[0033] Therefore, the carbon nanotube membrane according to the present embodiment has properties of being self-supporting, water-resistant, thin-filmable, highly gas-permeable, and excellent wettability, and therefore can be used as a GDL material exposed to high-temperature and high-humidity environments in PEFCs. Furthermore, when used as a GDL material, the carbon nanotube membrane according to the present embodiment functions as a thin GDL, enabling the thickness of the PEFC unit cell and stack structure to be reduced.

[0034] From the above, the carbon nanotube membrane according to this embodiment is useful as a carbon nanotube membrane for a gas diffusion layer of a polymer electrolyte fuel cell. While the GDL of a PEFC has been described as an example of an application of the carbon nanotube membrane according to this embodiment, the application of the carbon nanotube membrane according to this embodiment is not limited thereto. The carbon nanotube membrane according to this embodiment can also be used, for example, as a material for energy devices other than PEFCs, a heat exchange filter, etc.

[0035] Here, the carbon nanotube film according to this embodiment will be described with reference to the drawings. FIG. 1 shows a perspective view schematically illustrating an example of the carbon nanotube film according to this embodiment, and a schematic view showing an enlarged surface of the carbon nanotube film. As shown in FIG. 1, the carbon nanotube film 10 has a first main surface 11 and a second main surface 12 opposite to the first main surface 11. The carbon nanotube film 10 has a thickness T. The distance between the first main surface 11 and the second main surface 12 is the thickness T of the carbon nanotube film 10. The thickness T of the carbon nanotube film 10 is, for example, in the range of 20 μm or more and 50 μm or less. Here, the main surface refers to the largest surface of the carbon nanotube film 10, facing in the direction of the thickness T. For the sake of convenience, the terms first main surface 11 and second main surface 12 are used to clarify the positional relationship between one side and the other side of the carbon nanotube film 10, but in some cases, the terms first main surface 11 and second main surface 12 can be used interchangeably, and the terms first main surface 11 and second main surface 12 can be used interchangeably.

[0036] When the first main surface 11 of the carbon nanotube film 10 is enlarged, as shown in the enlarged view of FIG. 1 , the carbon nanotube film 10 contains carbon nanotubes 1 and has a porous structure. Because the carbon nanotube film 10 contains carbon nanotubes 1, it is electrically conductive. Furthermore, in addition to the carbon nanotubes 1, the carbon nanotube film 10 also contains a polyvinyl alcohol-based resin and a sulfonated perfluoroalkyl polymer, with the ratio of the polyvinyl alcohol-based resin to the sulfonated perfluoroalkyl polymer being in the range of 0.6 to 1.4 by mass, calculated as solid content. Because of this configuration, the carbon nanotube film 10 is self-supporting.

[0037] Although an example of the carbon nanotube film according to this embodiment has been described above with reference to FIG. 1, the carbon nanotube film according to this embodiment is not limited to this. The carbon nanotube film according to this embodiment may adopt various forms as long as the above-described effects are obtained. For example, the carbon nanotube film 10 shown in FIG. 1 is depicted as having a rectangular shape, but is not limited to this and various shapes may be adopted. Furthermore, the dispersion state of the carbon nanotubes 1 is not limited to the state shown in the enlarged schematic view of FIG. 1.

[0038] Next, materials used in the carbon nanotube film according to this embodiment will be described. In the following description, reference numerals will be omitted.

[0039] (carbon nanotubes) Carbon nanotubes are carbon structures with a cylindrically closed graphite sheet structure, with carbon six-membered rings as the main structure. Carbon nanotubes are formed by interconnecting carbon atoms to form a cylindrical structure. Carbon nanotubes can be obtained by known manufacturing methods, such as arc discharge, laser ablation, and chemical vapor deposition. The carbon nanotubes are not particularly limited, but are preferably at least one selected from the group consisting of multi-walled carbon nanotubes (MWCNTs), few-walled carbon nanotubes (FWCNTs), double-walled carbon nanotubes (DWCNTs), and single-walled carbon nanotubes (SWCNTSs). In the carbon nanotube film according to this embodiment, multi-walled carbon nanotubes are preferably included because they have good dispersibility in water and can increase the concentration of the carbon nanotube film-forming composition described below. From the same viewpoint, in the carbon nanotube film according to this embodiment, the carbon nanotubes are preferably multi-walled carbon nanotubes.

[0040] The dimensions of the carbon nanotubes are not particularly limited, and examples thereof include a diameter of 0.4 nm or more and 100 nm or less, and a length of 1 μm or more and 1000 μm or less.

[0041] The content of carbon nanotubes in the carbon nanotube film according to this embodiment is not particularly limited, and is preferably 40% by mass or more and 70% by mass or less relative to the entire carbon nanotube film (100% by mass). The content of carbon nanotubes in the carbon nanotube film is the content converted into solid content. If the content of carbon nanotubes relative to the entire carbon nanotube film is 40% by mass or more, it is easy to reduce the electrical resistance. If the carbon nanotube content in the entire carbon nanotube film is 70 mass % or less, a freestanding film is easily obtained, and the carbon nanotube content in the carbon nanotube film is more preferably 45 mass % or more. The carbon nanotube content in the carbon nanotube film is more preferably 60 mass % or less, and even more preferably 55 mass % or less.

[0042] (Polyvinyl alcohol resin) In this specification, the term "polyvinyl alcohol-based resin" encompasses resins containing 50% by mass or more of structural units derived from vinyl alcohol. The polyvinyl alcohol-based resin is not particularly limited and may be, for example, (I) polyvinyl alcohol obtained by polymerizing polyvinyl acetate from a monomer containing vinyl acetate and then saponifying the polyvinyl acetate; (II) polyvinyl alcohol copolymer resin obtained by copolymerizing vinyl alcohol units obtained by polymerizing polyvinyl acetate from a monomer containing vinyl acetate and then saponifying the polyvinyl acetate, primarily with ethylene, an α-olefin having 3 to 20 carbon atoms, vinyl ether, vinyl ester, (meth)acrylamide and its salts, or the like; or (III) polyvinyl alcohol-based resin obtained by introducing a modifying group into the polyvinyl alcohol (I) or the polyvinyl alcohol copolymer resin (II) by post-reaction. Polyvinyl alcohol-based resins are widely used industrially, and the polyvinyl alcohol (I) is preferred in terms of its excellent affinity with sulfonated perfluoroalkyl polymers. The polyvinyl alcohol may be a commercially available product or may be synthesized.

[0043] In the carbon nanotube film of this embodiment, from the viewpoint of obtaining a free-standing film, being able to be thinned, and obtaining properties of high gas permeability and excellent wettability, it is preferable that the content of the polyvinyl alcohol-based resin is 30 parts by mass or more and 100 parts by mass or less, in terms of solid content, per 100 parts by mass of the carbon nanotubes. From the same viewpoint, the content of the polyvinyl alcohol resin is more preferably 40 parts by mass or more, and even more preferably 45 parts by mass or more, per 100 parts by mass of the carbon nanotubes, calculated as solid content. From the same viewpoint, the content of the polyvinyl alcohol resin is more preferably 70 parts by mass or less, and even more preferably 60 parts by mass or less, per 100 parts by mass of the carbon nanotubes, calculated as solid content.

[0044] The degree of saponification of polyvinyl alcohol is expressed as a percentage of the number of hydroxyl groups relative to the total number of acetate groups and hydroxyl groups in the polyvinyl alcohol. The degree of saponification and degree of polymerization of polyvinyl alcohol are not particularly limited. The degree of saponification of polyvinyl alcohol may be 70% or more, 78% or more, 85% or more, or 95% or more. The degree of saponification of polyvinyl alcohol may be 100% or less, or 99% or less. The degree of polymerization of polyvinyl alcohol may be 300 or more, or 500 or more. The degree of polymerization of polyvinyl alcohol may be 3000 or less, 1000 or less, 800 or less, or 600 or less. The degree of saponification and degree of polymerization of polyvinyl alcohol are determined, for example, by the method specified in JIS K 6726:1994. Polyvinyl alcohols may be used singly or in combination of two or more.

[0045] (sulfonated perfluoroalkyl polymer) The sulfonated perfluoroalkyl polymer is, for example, a polymer having a structural unit derived from a fluoroolefin in the main chain and a perfluoroalkyl ether group having a sulfonic acid group in the side chain, and has a hydrophilic portion and a hydrophobic portion. Specific examples include a copolymer of tetrafluoroethylene and 4-(trifluoromethyl)decafluoro-3,6-dioxa-7-octene-1-sulfonic acid fluoride. For example, Nafion (registered trademark) can be used as such a copolymer.

[0046] In the carbon nanotube film of this embodiment, from the viewpoint of obtaining a free-standing film, being able to be thinned, and obtaining high gas permeability and excellent wettability, it is preferable that the content of the sulfonated perfluoroalkyl polymer is 20 parts by mass or more and 140 parts by mass or less, in terms of solid content, per 100 parts by mass of the carbon nanotubes. From the same viewpoint, the content of the sulfonated perfluoroalkyl polymer is more preferably 30 parts by mass or more, and even more preferably 40 parts by mass or more, per 100 parts by mass of the carbon nanotubes, calculated as solid content. From the same viewpoint, the content of the sulfonated perfluoroalkyl polymer is more preferably 80 parts by mass or less, and even more preferably 60 parts by mass or less, per 100 parts by mass of the carbon nanotubes, calculated as solid content.

[0047] (Ratio of "polyvinyl alcohol resin / sulfonated perfluoroalkyl polymer") In the carbon nanotube membrane according to this embodiment, the ratio of polyvinyl alcohol resin to sulfonated perfluoroalkyl polymer (polyvinyl alcohol resin / sulfonated perfluoroalkyl polymer) is 0.6 or more and 1.4 or less by mass in terms of solid content. From the viewpoint of obtaining a self-supporting membrane, enabling thinning, and making it easier to obtain high gas permeability and excellent wettability, the ratio of the polyvinyl alcohol-based resin to the sulfonated perfluoroalkyl polymer is more preferably 0.7 or more, and even more preferably 0.8 or more. From the same viewpoint, the ratio of the polyvinyl alcohol resin to the sulfonated perfluoroalkyl polymer is more preferably 1.3 or less, and even more preferably 1.2 or less.

[0048] (Thickness) The thickness of the carbon nanotube film according to this embodiment is preferably 20 μm or more and 50 μm or less, from the viewpoint of making it easier to obtain a free-standing film and to make it easier to thin it. If the thickness of the carbon nanotube film is 20 μm or more, a free-standing film is easily obtained. If the thickness of the carbon nanotube film is 50 μm or less, the carbon nanotube film is thinned, and when the carbon nanotube film is used as a GDL, it can be made thinner. From the viewpoint of facilitating the production of a free-standing film, the thickness of the carbon nanotube film according to this embodiment is preferably 25 μm or more, and more preferably 30 μm or more. From the viewpoint of facilitating thinning, the thickness of the carbon nanotube film according to this embodiment is preferably 48 μm or less, and more preferably 45 μm or less.

[0049] The carbon nanotube film according to this embodiment may be a single-layer film, or may be a multi-layer film in which two or more carbon nanotube films according to this embodiment are stacked. In order to enable a thinner carbon nanotube film when used as a GDL, the thickness of the multi-layer carbon nanotube film is preferably 20 μm or more and 50 μm or less.

[0050] (volume resistivity) From the viewpoint of excellent electronic (electrical) conductivity, the volume resistivity of the carbon nanotube film according to this embodiment is preferably 150 mΩ·cm or less, and more preferably 100 mΩ·cm or less. The lower limit of the volume resistivity is not particularly limited, and may be, for example, 1 mΩ·cm or more, and is usually 50 mΩ·cm or more. Volume resistivity can be measured using the following method. First, a piece of polyvinyl chloride insulating tape with a 13 mm diameter circular hole is attached to a first smooth copper plate. Next, each test piece cut into a 10 mm diameter circle with a cutting blade is placed in the hole in the tape, and the piece is evenly pressed down with a second smooth copper plate. The electrical resistance Rv between the upper and lower copper plates is measured using the four-probe method with a resistance meter (Hioki EE, RM3545-02). The volume resistivity ρv [mΩ·cm] of each test piece can be calculated using the following formula (F1) based on the obtained Rv [mΩ], the test piece diameter d [cm], and the test piece thickness t [cm]. ρv=(π·d 2 ) / 4t×Rv [mΩ·cm]···(F1)

[0051] (Air permeability) The air permeability of the carbon nanotube membrane according to this embodiment is measured at a pressure of 200 kPa when measuring the dry nitrogen flow rate, and the air permeability is measured at an area of ​​2 cm2 The air permeability is 3.0 x 10 -5 m Pa -1 ·S -1 It is preferable that this is equal to or greater than this. In terms of improving the gas permeability of the carbon nanotube membrane, the air permeability of the carbon nanotube membrane is 3.2 × 10 -5 m Pa -1 ·S -1 More preferably, it is 3.4×10 or more. -5 m Pa -1 ·S -1 More preferably, it is equal to or greater than this. The upper limit of the air permeability is not particularly limited, and is, for example, 1.0 × 10 -2 m Pa -1 ·S -1 The standard deviation is 3.0×10 -3 m Pa -1 ·S -1 The following is the result. The air permeability of the carbon nanotube membrane is 3.0×10 -5 m Pa -1 ·S -1 If the gas permeability is above this level, the carbon nanotube membrane has excellent gas permeability, and when used as a GDL, it is easy to suppress stagnation of gas supply into the fuel cell, and the power generation performance of the PEFC is easy to improve. The air permeability can be measured by the method described in the Examples section below.

[0052] (Water contact angle) In the carbon nanotube film according to this embodiment, the contact angle of water at 23° C. is preferably 130° or less. The contact angle of water at 23° C. may be less than 130°. From the viewpoint of improving the wettability of the carbon nanotube film, the contact angle of water is more preferably 125° or less, even more preferably 120° or less, and even more preferably 115° or less. The lower limit of the water contact angle is not particularly limited, and may be, for example, 50° or more. If the water contact angle of a carbon nanotube membrane is 130° or less, it has excellent wettability, and when the carbon nanotube membrane is used as a GDL in a PEFC, water accumulation in the pores of the carbon nanotube membrane is more likely to be suppressed, which increases resistance to clogging due to water droplet formation and more likely to suppress inhibition of gas permeability, thereby improving the battery characteristics of the PEFC. A method for measuring the water contact angle will be explained in the Examples section below.

[0053] [Method of manufacturing carbon nanotube film] A preferred method for producing the carbon nanotube film according to this embodiment will be described. The method for obtaining the carbon nanotube film according to this embodiment is not particularly limited. The carbon nanotube film according to this embodiment is preferably obtained by a production method including, for example, the following steps.

[0054] (Step P1): A step of preparing carbon nanotubes, a polyvinyl alcohol-based resin, and a sulfonated perfluoroalkyl polymer. (Step P2): A step of preparing a carbon nanotube film-forming composition containing the carbon nanotubes, the polyvinyl alcohol-based resin, and the sulfonated perfluoroalkyl polymer so that the ratio of the polyvinyl alcohol-based resin to the sulfonated perfluoroalkyl polymer is 0.6 or more and 1.4 or less by mass in terms of solid content. (Step P3): A step of applying the carbon nanotube film-forming composition to the release surface of a release material to form a coating film, and then heating and drying the coating film to form a dry coating film. (Step P4): A step of extracting the dried coating film with hot water and drying it by heating.

[0055] (Process P1) Step P1 is a step of preparing the materials for forming the carbon nanotube film described above. Specific examples of the carbon nanotubes, polyvinyl alcohol resin, and sulfonated perfluoroalkyl polymer are as described above.

[0056] The carbon nanotubes may be prepared as a carbon nanotube dispersion in which the carbon nanotubes are dispersed in a solvent. The concentration of the carbon nanotube dispersion is not particularly limited as long as the concentration allows dispersion of the carbon nanotubes. The dispersion medium of the carbon nanotube dispersion is preferably a dispersion medium that is highly miscible with water.

[0057] The polyvinyl alcohol resin may be prepared as a solution of the polyvinyl alcohol resin in an aqueous solvent (e.g., water). The solution of the polyvinyl alcohol resin is preferably an aqueous solution. The concentration of the polyvinyl alcohol resin solution is not particularly limited as long as it is a concentration at which the polyvinyl alcohol resin can be dissolved.

[0058] The sulfonated perfluoroalkyl polymer may be prepared as a dispersion of the sulfonated perfluoroalkyl polymer in a solvent, such as a dispersion of Nafion (registered trademark).

[0059] (Process P2) In step P2, a carbon nanotube film-forming composition is prepared, which is a mixture of the carbon nanotubes prepared in step P1, a polyvinyl alcohol resin, and a sulfonated perfluoroalkyl polymer. The carbon nanotube film-forming composition contains carbon nanotubes as the main material, and the polyvinyl alcohol resin and the sulfonated perfluoroalkyl polymer are contained so that the ratio of the polyvinyl alcohol resin to the sulfonated perfluoroalkyl polymer (polyvinyl alcohol resin / sulfonated perfluoroalkyl polymer) is within the range of 0.6 to 1.4 by mass, converted into solid content. The term "main material" refers to the material that is contained in the largest amount in the carbon nanotube film-forming composition.

[0060] The carbon nanotube content in the carbon nanotube film-forming composition is preferably in the range of 40% by mass or more and 70% by mass or less, calculated as solid content, in the carbon nanotube film-forming composition. Specific examples of the contents of the carbon nanotubes, polyvinyl alcohol-based resin, and sulfonated perfluoroalkyl polymer in the carbon nanotube film-forming composition are as described above for the carbon nanotube film. The carbon nanotubes, polyvinyl alcohol-based resin, and sulfonated perfluoroalkyl polymer can be mixed, for example, using a device capable of mixing these materials (e.g., a stirrer, etc.). The order in which the carbon nanotubes, polyvinyl alcohol-based resin, and sulfonated perfluoroalkyl polymer are mixed is not particularly limited.

[0061] The carbon nanotube film-forming composition may contain other components, such as a solvent such as water, a dispersant, and the like, as needed, in addition to the carbon nanotubes, the polyvinyl alcohol-based resin, and the sulfonated perfluoroalkyl polymer.

[0062] (Process P3) In step P3, the carbon nanotube film-forming composition prepared in step P2 is applied to the release surface of the release material (hereinafter sometimes referred to as the release surface) to form a coating film, and then the coating film is heated and dried to form a dry coating film. The dry coating film becomes a precursor film of the carbon nanotube film.

[0063] Examples of release materials include (i) a member that has release properties in itself, (ii) a member that has been subjected to a release treatment, or (iii) a member that has a release agent layer laminated thereon. Among these, the release material is preferably the member (iii) in terms of versatility, and specific examples of the member (iii) include a release material that includes a release substrate and a release agent layer formed by applying a release agent onto the release substrate.

[0064] Examples of release substrates include paper substrates (such as high-quality paper and coated paper), laminated paper in which a thermoplastic resin such as polyethylene is laminated to a paper substrate, and plastic films such as polyesters (such as polyethylene terephthalate, polybutylene terephthalate, and polyethylene naphthalate) and polyolefins (such as polypropylene and polyethylene). Examples of the release agent include silicone-based, fluorine-based, and long-chain alkyl-based agents.

[0065] Methods for applying the carbon nanotube film-forming composition to the release surface of the release material include, for example, spin coating, spray coating, bar coating, knife coating, roll coating, roll knife coating, blade coating, die coating, gravure coating, and air knife coating.

[0066] A dried coating film formed from the carbon nanotube film-forming composition can be formed, for example, by applying the carbon nanotube film-forming composition onto a release material, and then heating and drying the resulting coating film while it is still on the release material. A dried coating film of the carbon nanotube film-forming composition can also be formed, for example, by the following process.

[0067] First, a carbon nanotube film-forming composition is applied to the release surface of a first release material, for example, by the coating method exemplified above, to form a coating film. Next, a gap is formed, and the release surface of a second release material is laminated (gap lamination) so that it contacts the coating film formed on the first release material. The gap is formed by adjusting a predetermined clearance that allows passage between the first release material, the coating film of the carbon nanotube film-forming composition provided on the first release material, and the second release material provided on the coating film. The first release material and the second release material are each composed of the above-mentioned release material. The first release material and the second release material may be the same or different. From the perspective of releasability from the coating film, it is preferable that the first release material be a heavy release type release material and the second release material be a light release type release material. Next, the gap is removed by releasing the clearance, and the second release material is removed, after which the coating film is heated and dried. In this way, a dried coating film is formed on the release surface of the first release material. In this specification, a method of laminating a laminate formed by laminating a first release material, a coating film of a carbon nanotube film-forming composition, and a second release material in this order, by passing the laminate through a gap is sometimes referred to as gap lamination. The gap can be formed, for example, by using a laminator capable of adjusting the clearance and adjusting the gap at both ends of the laminator to form a substantially uniform clearance. The gap can also be eliminated by adjusting the gap at both ends of the laminator to release the clearance. A laminator capable of adjusting the clearance has, for example, two rolls or two plates, and the gap can be formed or eliminated by adjusting the clearance between these two rolls or two plates.

[0068] The dried coating film of the carbon nanotube film-forming composition is formed to have a thickness within the range of the carbon nanotube film described above. When a gap lamination process is employed to form the dried coating film of the carbon nanotube film-forming composition, the thickness of the dried coating film of the carbon nanotube film-forming composition is adjusted depending on the gap width.

[0069] The conditions for heating and drying the coating film of the carbon nanotube film-forming composition are not particularly limited, and any conditions that allow the formation of a dried coating film may be selected. Specific examples of the drying temperature include a temperature of 50° C. or higher and 120° C. or lower. The drying time may be, for example, 1 minute or higher and 180 minutes or lower.

[0070] (Process P4) In step P4, the dried coating film formed in step P3 is subjected to hot water extraction and then heated and dried. This step P4 results in the carbon nanotube film according to this embodiment. It is believed that by subjecting the dried coating film to hot water extraction and then heated and dried, components soluble in hot water (e.g., polyvinyl alcohol resin) in the obtained carbon nanotube film can be removed. By undergoing this step, the obtained carbon nanotube film is self-supporting and water-resistant, and can maintain its shape even in hot water.

[0071] The temperature for the hot water extraction is not particularly limited and may be, for example, in the range of 40°C or higher and 100°C or lower. The temperature for the hot water extraction is more preferably 45°C or higher, even more preferably 50°C or higher, and even more preferably 60°C or higher. The time for the hot water extraction is preferably, for example, 10 minutes to 48 hours. Specifically, the operation in step P4 involves immersing the dried coating film, which is the precursor film of the carbon nanotube film obtained in step P3, in deionized water, and performing hot water extraction by heating and stirring at 40°C to 100°C for 10 minutes to 48 hours, followed by heat drying. The heat drying after the hot water extraction treatment may be performed, for example, under conditions where the drying temperature is 50°C to 80°C and the drying time is 10 minutes to 240 minutes.

[0072] When forming a carbon nanotube film in multiple layers, for example, in step P3, a coating film (first coating film) applied to the release surface of the first release material is heated and dried to form a dried coating film (first dried coating film), and the carbon nanotube film-forming composition prepared in step P2 is applied to the surface of the dried coating film (first dried coating film) to form a coating film (second coating film), and this second coating film is heated and dried to form a dried coating film (second dried coating film) on the first dried coating film, and then step P4 is carried out to obtain a multilayer film consisting only of carbon nanotube films. Even when forming a multilayer film consisting only of carbon nanotube films, the gap lamination process can be used.

[0073] The carbon nanotube film according to this embodiment can be obtained by the above steps. According to the preferred method for producing the carbon nanotube film according to this embodiment, high-temperature treatment such as that required for producing a fired film of carbon nanotubes is not required.

[0074] Second Embodiment [Gas diffusion layer] The gas diffusion layer according to the second embodiment is composed solely of the carbon nanotube membrane described in the first embodiment. That is, the gas diffusion layer according to this embodiment is composed of a self-supporting carbon nanotube membrane containing carbon nanotubes, a polyvinyl alcohol-based resin, and a sulfonated perfluoroalkyl polymer, where the ratio of the polyvinyl alcohol-based resin to the sulfonated perfluoroalkyl polymer is 0.6 to 1.4, based on the mass of the solid content. Specific examples of the carbon nanotube membrane are as described in the first embodiment. The gas diffusion layer according to this embodiment may have a single-layer structure or a multilayer structure. That is, the gas diffusion layer according to this embodiment may be composed of a single-layer membrane consisting of only the carbon nanotube membrane, or may be composed of a multi-layer membrane consisting of two or more layers consisting of only the carbon nanotube membrane. The gas diffusion layer according to this embodiment is preferably used as a gas diffusion layer for a polymer electrolyte fuel cell. In one aspect, the gas diffusion layer according to this embodiment is composed solely of the carbon nanotube membrane 10 described in FIG. 1. In this case, the carbon nanotube membrane 10 serves as a gas diffusion layer (for example, the gas diffusion layer 30 shown in FIG. 2, which will be described later). That is, one aspect of the gas diffusion layer according to this embodiment is a gas diffusion layer (GDL) used in a polymer electrolyte fuel cell (PEFC) described in a third embodiment below. The gas diffusion layer according to this embodiment can be applied not only as a gas diffusion layer for a polymer electrolyte fuel cell but also to, for example, an energy device using a gas diffusion layer.

[0075] Third Embodiment [Polymer electrolyte fuel cell] A polymer electrolyte fuel cell according to a third embodiment includes a membrane electrode assembly, gas diffusion layers sandwiching the membrane electrode assembly, and separators sandwiching the membrane electrode assembly via the gas diffusion layers. At least one of the gas diffusion layers is composed solely of the carbon nanotube membrane described in the first embodiment. That is, a polymer electrolyte fuel cell according to this embodiment includes a membrane electrode assembly, gas diffusion layers sandwiching the membrane electrode assembly, and separators sandwiching the membrane electrode assembly via the gas diffusion layers. At least one of the gas diffusion layers is composed of a self-supporting carbon nanotube membrane containing carbon nanotubes, a polyvinyl alcohol-based resin, and a sulfonated perfluoroalkyl polymer, wherein the ratio of the polyvinyl alcohol-based resin to the sulfonated perfluoroalkyl polymer is 0.6 to 1.4, in terms of solid content, by mass. In the polymer electrolyte fuel cell according to this embodiment, at least one of the gas diffusion layers may be composed of a single-layer membrane consisting of only the carbon nanotube membrane, or may be composed of a multi-layer membrane consisting of two or more layers of only the carbon nanotube membrane.

[0076] Here, a polymer electrolyte fuel cell according to this embodiment will be described with reference to the drawings. Fig. 2 shows a cross-sectional view schematically illustrating an example of a polymer electrolyte fuel cell according to this embodiment. As shown in Fig. 2, a polymer electrolyte fuel cell 100 includes a membrane electrode assembly 20, two gas diffusion layers 30, and two separators 40.

[0077] The membrane electrode assembly 20 includes an electrolyte membrane 21 and two catalyst layers 22. Each of the two catalyst layers 22 includes a catalyst support layer, which is a region that supports a catalyst. In the solid polymer fuel cell 100, each of the two catalyst layers 22 is composed only of a catalyst support layer, which is a region that supports a catalyst. The two catalyst layers 22 are in direct contact with the electrolyte membrane 21, and a cathode catalyst layer 22A provided on one side of the electrolyte membrane 21 and an anode catalyst layer 22B provided on the other side of the electrolyte membrane 21 are disposed so as to sandwich the electrolyte membrane 21. In the solid polymer fuel cell 100, a known material is used for the membrane electrode assembly 20. An example of a material that can be used for the electrolyte membrane 21 of the membrane electrode assembly 20 is a solid polymer membrane, which is a proton-conductive ion-exchange membrane formed of a fluorine-based resin. The cathode catalyst layer 22A and the anode catalyst layer 22B of the membrane electrode assembly 20 are each made of a known material. Examples of materials used for the cathode catalyst layer 22A and the anode catalyst layer 22B include a carbon material carrying platinum or an alloy containing platinum and another metal. The cathode catalyst layer 22A and the anode catalyst layer 22B may be made of the same material or different materials.

[0078] The two gas diffusion layers 30 are disposed on the surfaces of the cathode catalyst layer 22A and the anode catalyst layer 22B opposite the electrolyte membrane 21 side. The two gas diffusion layers 30 are disposed so that the membrane electrode assembly 20 is sandwiched between the cathode gas diffusion layer 30A and the anode gas diffusion layer 30B. The cathode gas diffusion layer 30A is in direct contact with the cathode catalyst layer 22A, and the anode gas diffusion layer 30B is in direct contact with the anode catalyst layer 22B. In the solid polymer fuel cell 100, the two gas diffusion layers 30, the cathode gas diffusion layer 30A and the anode gas diffusion layer 30B, are both made of the gas diffusion layer described in the second embodiment. That is, the cathode gas diffusion layer 30A and the anode gas diffusion layer 30B are composed solely of the carbon nanotube membrane described in the first embodiment.

[0079] The two separators 40 are disposed on the surface of the gas diffusion layer 30 opposite the membrane electrode assembly 20 side. The two separators 40 are disposed between the cathode-side separator 40A and the anode-side separator 40B, sandwiching the membrane electrode assembly 20 with the gas diffusion layer 30 interposed therebetween. The cathode-side separator 40A is in direct contact with the cathode-side gas diffusion layer 30A, and the anode-side separator 40B is in direct contact with the anode-side gas diffusion layer 30B. The cathode-side separator 40A and the anode-side separator 40B have irregularities on the cathode-side gas diffusion layer 30A side and the anode-side gas diffusion layer 30B side, respectively, that serve as gas flow paths. Known materials are used for the separators 40. Examples of materials used for the separators 40 include separators made of metal or carbon. The cathode side separator 40A and the anode side separator 40B may be made of the same material or different materials.

[0080] 2, the cathode-side separator 40A, cathode-side gas diffusion layer 30A, membrane electrode assembly 20 (cathode catalyst layer 22A, electrolyte membrane 21, anode catalyst layer 22B), anode-side gas diffusion layer 30B, and anode-side separator 40B are arranged in this order from the cathode side to the anode side. The gas diffusion layers described in the second embodiment are used for both the cathode-side gas diffusion layer 30A and the anode-side gas diffusion layer 30B.

[0081] The polymer electrolyte fuel cell 100 shown in Figure 2 is an example of a single cell, and is used as a stack structure in which single cells are stacked according to power generation performance. The gas diffusion layer 30 is composed only of the gas diffusion layer described in the second embodiment, i.e., the carbon nanotube film described in the first embodiment, so that the single cell can be made thinner and the thickness of the stack structure in which the single cells are stacked can also be reduced. As a result, the polymer electrolyte fuel cell 100 has improved cell performance.

[0082] Although an example of a polymer electrolyte fuel cell according to this embodiment has been described above with reference to Fig. 2, the polymer electrolyte fuel cell according to this embodiment is not limited to this. Various components used in polymer electrolyte fuel cells can be applied to the polymer electrolyte fuel cell according to this embodiment. The polymer electrolyte fuel cell according to this embodiment can adopt various configurations as long as the above-described effects can be obtained.

[0083] For example, in the solid polymer fuel cell 100, the gas diffusion layer 30 includes the gas diffusion layer described in the second embodiment for both the cathode-side gas diffusion layer 30A and the anode-side gas diffusion layer 30B. However, the present invention is not limited to this. The gas diffusion layer described in the second embodiment may be used for either the cathode-side gas diffusion layer 30A or the anode-side gas diffusion layer 30B. For example, the gas diffusion layer described in the second embodiment may be used for the cathode-side gas diffusion layer 30A, and a gas diffusion layer other than the gas diffusion layer described in the second embodiment may be used for the anode-side gas diffusion layer 30B. Even if the gas diffusion layer described in the second embodiment is used for either the cathode-side gas diffusion layer 30A or the anode-side gas diffusion layer 30B in the solid polymer fuel cell 100, the cell performance of the solid polymer fuel cell 100 is improved.

[0084] [Modifications of the embodiment] The present invention is not limited to the above-described embodiment, and includes modifications and improvements within the scope of achieving the object of the present invention. [Example]

[0085] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples.

[0086] Measurements and evaluations in the following examples and comparative examples were carried out by the methods shown below.

[0087] [Evaluation of independence] The carbon nanotube film produced in each example was evaluated for its self-supporting property according to the following evaluation criteria. (Evaluation criteria) A: A flexible, self-supporting film that can withstand molding can be obtained. F: A free-standing film is obtained, but the obtained free-standing film is brittle and does not become a free-standing film that is flexible and can withstand molding. F - : Because there is no film-forming property or the film-forming property is low, a free-standing film cannot be obtained.

[0088] [Air permeability] The air permeability was measured for the carbon nanotube membranes produced in each example that were rated "A" for freestanding membranes, and for the carbon fiber nonwoven fabrics of each reference example. The air permeability was measured using a capillary perm porometer (CFP-1200-AECX-P, manufactured by Porous Materials Inc.) at 2 cm under a pressure of 200 kPa when measuring the dry nitrogen flow rate. 2 Airflow per m 3 s -1 ] was measured, and the airflow rate was calculated from the measured airflow rate using the following formula (F2). Air permeability [m Pa -1 ·s -1 ] = gas flow rate [m 3 ·s -1 ] / (Area [m 2 ] × pressure [Pa]) (F2)

[0089] [Water contact angle] The carbon nanotube films produced in each example that were rated "A" as freestanding films and the carbon fiber nonwoven fabrics of each reference example were measured for their water contact angles at 23°C. The water contact angles were measured in an environment of 23°C and 50% RH using a method conforming to the sessile drop method specified in JIS R 3257 "Test method for wettability of substrate glass surfaces."

[0090] [Fuel cell fabrication and fuel cell performance evaluation] (1) Fuel cell fabrication First, use a Thomson blade to measure an area of ​​1cm 2 Platinum carbon catalyst layer (manufactured by Chemix, Pt coverage 0.3 mg / cm) cut into squares2 Two platinum-carbon catalyst layers were prepared. Next, the prepared platinum-carbon catalyst layers were thermocompression bonded (compression pressure: 1 kN, compression temperature: 135°C, compression time: 5 minutes) to both sides of a proton conductive membrane (DuPont, Nafion NR211) to prepare a membrane electrode assembly. The membrane electrode assembly prepared above was then incorporated into a polymer electrolyte fuel cell (FC Development Co., Ltd., JARI Cell 2). A carbon nanotube membrane with a self-supporting membrane rating of "A" was incorporated as the cathode GDL (air electrode side), and a commercially available GDL (SGL Carbon, 22BB, carbon fiber nonwoven fabric with a microporous layer) was incorporated as the anode GDL (fuel electrode side). A 25 μm-thick polytetrafluoroethylene (PTFE) sheet was used as a gasket on the air electrode side, and a 180 μm-thick PTFE sheet was used on the fuel electrode side. In this manner, a polymer electrolyte fuel cell (fuel cell) was prepared.

[0091] (2) Fuel cell characteristic evaluation Using a fuel cell evaluation device (Toyo Corporation, AutoPEM), hydrogen gas was passed through the fuel electrode side of the fabricated fuel cell, and oxygen gas was passed through the air electrode side. These hydrogen gas and oxygen gas were passed through at predetermined flow rates (0.02 and 0.1 liters per minute, respectively) and humidified to predetermined humidities (90% RH and 30% RH, respectively). The fuel cell was heated to 80°C during this process. Current and voltage measurement cables from an electronic load device were connected to both the fuel electrode side and the air electrode side, and the voltage was measured at predetermined current values ​​to obtain a current-voltage curve.

[0092] Example 1 (1) Preparation of carbon nanotube film-forming composition (A) A carbon nanotube dispersion (K1010M, manufactured by KJ Specialty Paper Co., Ltd., solid content concentration 10.2% by mass) was prepared as carbon nanotubes, (B) a polyvinyl alcohol aqueous solution (22-88, manufactured by Kuraray Co., Ltd., solid content 20% by mass) was prepared as polyvinyl alcohol, a polyvinyl alcohol-based resin, and (C) a Nafion dispersion (manufactured by Merck, solid content 20% by mass) was prepared as a sulfonated perfluoroalkyl polymer.

[0093] In terms of apparent amounts, 23.9 parts by mass of an aqueous polyvinyl alcohol solution (hereinafter referred to as an aqueous PVA solution) and 25.0 parts by mass of a Nafion dispersion were added to 100 parts by mass of a carbon nanotube dispersion, and mixed using a planetary centrifugal mixer to obtain a carbon nanotube film-forming composition (hereinafter simply referred to as a mixture). When the mass of carbon nanotubes converted into solids is taken as 100 parts by mass, the masses of polyvinyl alcohol and Nafion converted into solids are as shown in Table 1. In Table 1, carbon nanotubes are abbreviated as CNT, polyvinyl alcohol as PVA, and Nafion as a sulfonated perfluoroalkyl polymer.

[0094] (2) Preparation of free-standing membrane The resulting mixture was applied with an applicator to the release-treated surface of a heavy-release release sheet (manufactured by Lintec Corporation, SP-PET752150), one side of which had been treated with a silicone-based release agent on a polyethylene terephthalate film, to form a coating film of the mixture on the heavy-release release sheet. Next, both ends of the laminator were adjusted to form a gap, and then a light-release release sheet (manufactured by Lintec Corporation, SP-PET381031), one side of which had been treated with a silicone-based release agent on a polyethylene terephthalate film, was gap-laminated so that the release-treated surface contacted the coating film formed on the release-treated surface of the heavy-release release sheet. The gap was then removed by adjusting both ends of the laminator, and the light-release release sheet was removed to expose the coating film, which was then heated and dried at 110°C for 5 minutes to form a dry coating film. The dried coating film was then peeled off from the heavy-release release sheet, yielding a free-standing film of the dry coating film. This free-standing film was extracted in hot water at 95°C for 24 hours, and then heated and dried in a hot air drying oven in the atmosphere at 65°C for 3 hours to obtain a carbon nanotube film. The thickness of the obtained carbon nanotube film was 40 μm. The thickness of the carbon nanotube film was measured using a constant pressure thickness measuring instrument (PG-02, manufactured by Teclock Corporation) in accordance with JIS K7130.

[0095] FIG. 3 shows an example of an SEM image of the surface of the carbon nanotube film produced in Example 1, taken with a scanning electron microscope (SEM). The scale bar shown in FIG. 3 is 100 nm. The SEM image shown in FIG. 3 was taken at an accelerating voltage of 5 kV. As can be seen from the SEM image shown in FIG. 3, the carbon nanotube film produced in Example 1 is a porous film.

[0096] <Comparative Examples 1 and 2> A self-supporting membrane was prepared in the same manner as in Example 1, except that the amount of the PVA aqueous solution and the amount of the Nafion dispersion were changed according to Table 1 relative to 100 parts by mass of the carbon nanotube dispersion in terms of apparent amount.

[0097] <Comparative Example 3> A mixture was prepared in the same manner as in Example 1, except that the apparent amounts were 57.8 parts by mass of the PVA aqueous solution, 5.0 parts by mass of the Nafion dispersion, and 25.0 parts by mass of (D) titanium catalyst (referred to as Ti catalyst in Table 1) (Matsumoto Fine Chemical Co., Ltd., TC-310, solids concentration 44% by mass) per 100 parts by mass of the carbon nanotube dispersion. However, because the resulting dried coating film was self-supporting, it was difficult to obtain a flexible, self-supporting film that could withstand molding processing.

[0098] <Comparative Examples 4 and 5> The procedure for producing a free-standing film was carried out in the same manner as in Example 1, except that the amounts of the PVA aqueous solution and the Nafion dispersion were changed according to Table 1 relative to 100 parts by mass of the carbon nanotube dispersion in terms of apparent amounts. However, the obtained dried coating film had no or low film-forming properties, and it was therefore impossible to obtain a free-standing film.

[0099] <Comparative Example 6> A self-supporting film was prepared in the same manner as in Example 1, except that the amounts of the PVA aqueous solution and the Nafion dispersion were changed according to Table 1 relative to 100 parts by mass of the carbon nanotube dispersion in terms of apparent amounts. However, although the obtained dried coating film was self-supporting, it was highly brittle, making it difficult to obtain a self-supporting film that was flexible and could withstand molding.

[0100] <Reference examples 1 and 2> A carbon fiber nonwoven fabric with a microporous layer (manufactured by SGL Carbon Co., Ltd., 28BA, thickness 190 μm) was prepared as Reference Example 1. A carbon fiber nonwoven fabric with a microporous layer (manufactured by SGL Carbon Co., Ltd., 22BB, thickness 215 μm) was prepared as Reference Example 2.

[0101] [Table 1]

[0102] [Table 2]

[0103] The carbon nanotube film obtained in Example 1 was 40 μm thick, and a thin film was achieved. The carbon nanotube film obtained in Example 1 has an air permeability of 3.5 × 10 -5 m Pa -1 ·s -1 The water contact angle measured at 23°C was 112.1°. On the other hand, the carbon nanotube films obtained in Comparative Examples 1 and 2, which were rated "A" in the evaluation of the freestanding film, had lower air permeability and wettability than Example 1. Therefore, it was confirmed that the carbon nanotube film according to one embodiment of the present invention can be thinned and has excellent gas permeability and wettability. Furthermore, the carbon nanotube film obtained in Example 1 exhibited better wettability than the commercially available carbon fiber nonwoven fabrics of Reference Examples 1 and 2.

[0104] A graph showing the results of the fuel cell characteristic evaluation is shown in Figure 4. From the graph shown in Figure 4, it can be seen that the results when the carbon nanotube membrane obtained in Example 1 was used as a gas diffusion layer of a polymer electrolyte fuel cell were superior in cell voltage and voltage drop was suppressed in the high current density range compared to the results when the carbon nanotube membranes obtained in Comparative Examples 1 and 2 were used as gas diffusion layers of a polymer electrolyte fuel cell. This result is thought to be due to the carbon nanotube membrane obtained in Example 1 having superior gas permeability and flooding resistance compared to the carbon nanotube membranes obtained in Comparative Examples 1 and 2, and therefore the high drainage properties of the carbon nanotube membrane obtained in Example 1. Therefore, it was confirmed that the polymer electrolyte fuel cell using the carbon nanotube membrane according to one embodiment of the present invention contributes to improving the cell characteristics. In addition, among the polymer electrolyte fuel cells produced for the fuel cell characteristic evaluation, the polymer electrolyte fuel cell using the carbon nanotube membrane obtained in Example 1 as a gas diffusion layer can also contribute to achieving a thinner fuel cell. [Explanation of symbols]

[0105] 1...carbon nanotube, 10...carbon nanotube membrane, 11...first main surface, 12...second main surface, 20...membrane electrode assembly, 21...electrolyte membrane, 22, 22A, 22B...catalyst layer, 30, 30A, 30B...gas diffusion layer, 40, 40A, 40B...separator, 100...solid polymer fuel cell, T...thickness

Claims

1. A free-standing carbon nanotube film, Carbon nanotubes and a polyvinyl alcohol-based resin; a sulfonated perfluoroalkyl polymer; a ratio of the polyvinyl alcohol-based resin to the sulfonated perfluoroalkyl polymer, calculated on a mass basis of solid content, of 0.6 or more and 1.4 or less; Carbon nanotube membrane.

2. The carbon nanotube film according to claim 1 , The thickness is 20 μm or more and 50 μm or less. Carbon nanotube membrane.

3. The carbon nanotube film according to claim 1 or 2, The carbon nanotubes are multi-walled carbon nanotubes. Carbon nanotube membrane.

4. The carbon nanotube film according to claim 1 or 2, the content of the polyvinyl alcohol-based resin is 30 parts by mass or more and 100 parts by mass or less, in terms of solid content, per 100 parts by mass of the carbon nanotubes; Carbon nanotube membrane.

5. The carbon nanotube film according to claim 1 or 2, The content of the sulfonated perfluoroalkyl polymer is 20 parts by mass or more and 140 parts by mass or less, in terms of solid content, per 100 parts by mass of the carbon nanotubes. Carbon nanotube membrane.

6. The carbon nanotube film according to claim 1 or 2, The contact angle of water at 23°C is 130° or less. Carbon nanotube membrane.

7. The carbon nanotube film according to claim 1 or 2, Area measured at a pressure of 200 kPa when measuring the dry nitrogen flow rate: 2 cm ２ The air permeability is 3.0 x 10 －５ mPa －１ ・S －１ That's all. Carbon nanotube membrane.

8. A gas diffusion layer used in a polymer electrolyte fuel cell, The carbon nanotube film according to claim 1 or 2 is composed of only the carbon nanotube film. Gas diffusion layer.

9. A polymer electrolyte fuel cell, a membrane electrode assembly; gas diffusion layers disposed to sandwich the membrane electrode assembly; separators arranged to sandwich the membrane electrode assembly via the gas diffusion layers, At least one of the gas diffusion layers is made of only the carbon nanotube film according to claim 1 or 2. Polymer electrolyte fuel cell.

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