Gas diffusion electrode

Abstract

A gas diffusion electrode according to one embodiment of the present invention has a microporous layer, containing carbonaceous fine particles and a fluorine-based water-repellent resin, on at least one surface of a conductive porous substrate. In the gas diffusion electrode, the ratio (surface layer F / C) of elemental fluorine to elemental carbon detected, at 1,000 times magnification and an acceleration voltage of 2.0 kV by scanning electron microscope energy dispersive X-ray spectroscopy (SEM-EDX), on the surface on the side of the microporous layer with which the conductive porous substrate is not in contact is 0.35 or more and 0.60 or less.

Description

Gas Diffusion Electrode

[0001] The present invention relates to a gas diffusion electrode used in a fuel cell, and more particularly to a gas diffusion electrode used in a polymer electrolyte fuel cell used as a power source for a fuel cell vehicle or the like.

[0002] Fuel cells are a mechanism that electrically extracts the energy generated when hydrogen and oxygen react to produce water. They are highly energy efficient and produce only water as an exhaust, making them a promising clean energy source.

[0003] Polymer electrolyte fuel cells, which supply a hydrogen-containing fuel gas to the anode and an oxygen-containing oxidizing gas to the cathode to generate electromotive force through an electrochemical reaction at both electrodes, are generally constructed by stacking a separator, a gas diffusion layer, a catalyst layer, an electrolyte membrane, a catalyst layer, a gas diffusion layer, and a separator in this order. The electrodes used in polymer electrolyte fuel cells are sandwiched between two separators in a polymer electrolyte fuel cell and arranged between them. They have a structure consisting of a catalyst layer formed on the surface of the polymer electrolyte membrane and a gas diffusion layer formed outside the catalyst layer on both sides of the polymer electrolyte membrane. Gas diffusion electrodes are used as individual components for forming the gas diffusion layers. Performance requirements for gas diffusion electrodes include, for example, gas diffusivity, electrical conductivity, and drainage. To obtain gas diffusion electrodes with excellent performance, a conductive porous substrate that combines gas diffusivity and electrical conductivity is generally used.

[0004] Fuel cells are systems that electrically extract the energy generated when hydrogen and oxygen react to produce water, resulting in a large electrical load. In other words, increasing the current output from the fuel cell generates a large amount of water (water vapor). This water vapor condenses into droplets at low temperatures, and if these droplets clog the pores of the gas diffusion electrode, the amount of gas (oxygen or hydrogen) supplied to the catalyst layer decreases. Eventually, if all the pores of the gas diffusion electrode become clogged, power generation by the fuel cell will cease (this phenomenon is called flooding). To minimize flooding, the gas diffusion electrode must have good drainage properties. Typically, a gas diffusion electrode substrate, which is a conductive porous substrate treated with a water-repellent coating, is used to enhance the drainage (water repellency) of the gas diffusion electrode.

[0005] However, if the water-repellent treated conductive porous substrate is used as a gas diffusion electrode without any treatment, the coarse fibers of the substrate will generate large water droplets when water vapor condenses, making flooding more likely to occur. For this reason, a microporous layer containing conductive particles such as carbon black may be provided on the water-repellent treated conductive porous substrate.

[0006] Because the microporous layer is a component of the gas diffusion electrode, it is required to have not only drainage but also conductivity, similar to the conductive porous substrate. However, it is known that the drainage and conductivity required for a gas diffusion electrode generally have a trade-off relationship. To overcome this trade-off, many efforts have been made to improve the drainage and conductivity of the gas diffusion electrode and improve power generation performance by changing the structure and materials in the microporous layer.

[0007] For example, Patent Document 1 proposes a gas diffusion electrode characterized by a microporous layer having a first microporous layer in contact with a conductive porous substrate and a second microporous layer, in which the first microporous layer contains more water-repellent resin than the second microporous layer, thereby providing a microporous layer with high adhesion to a catalyst layer and high conductivity.

[0008] Furthermore, Patent Document 2 proposes a gas diffusion electrode in which the electrical resistance in the thickness direction is reduced by using carbon black and graphite particles with an aspect ratio of 10 or more in the microporous layer.

[0009] Patent No. 6766650 International Publication No. 2023 / 190153

[0010] However, in the gas diffusion electrode described in Patent Document 1, although the adhesion to the catalyst layer is improved, the amount of fluororesin on the outermost surface of the microporous layer is small, resulting in insufficient drainage, which may cause flooding. In addition, the need to form two microporous layers poses a problem in terms of productivity.

[0011] In the gas diffusion electrode described in Patent Document 2, the amount of fluororesin is reduced to improve the conductivity in the microporous layer. As a result, the amount of fluororesin on the outermost surface of the microporous layer is small, resulting in insufficient drainage, which may cause flooding.

[0012] Therefore, an object of the present invention is to provide a gas diffusion electrode that can suppress flooding when used in a fuel cell by maintaining high electrical conductivity and ensuring sufficient drainage on the surface of a microporous layer.

[0013] In order to solve the above problems and achieve the object, the present invention has the configuration described in any one of [1] to [5] below.

[0014] That is, the gas diffusion electrode according to the present invention is characterized in that [1] it is a gas diffusion electrode having a microporous layer containing carbonaceous fine particles and a fluorine-based water-repellent resin on at least one surface of a conductive porous substrate, and the ratio of fluorine to carbon (surface layer F / C) detected on the surface of the microporous layer not in contact with the conductive porous substrate by scanning electron microscope energy dispersive X-ray spectroscopy (SEM-EDX) at an acceleration voltage of 2.0 kV and a 1,000x field of view is 0.35 or more and 0.60 or less.

[0015] The gas diffusion electrode according to the present invention is [2] the invention described in the above [1], wherein the surface of the microporous layer that is not in contact with the conductive porous substrate is measured by X-ray photoelectron spectroscopy (ESCA) to detect monochromatic Al Kα 1,2 The ratio of fluorine to carbon elements (F / C in the outermost 10 nm layer) detected by measurement under conditions of an X-ray (1486.6 eV), an X-ray diameter of 200 μm, and a photoelectron detection angle of 45° is 0.62 or more and 0.80 or less.

[0016] The gas diffusion electrode according to the present invention is, in the invention described in [1] or [2] above, [3] characterized in that the surface of the microporous layer that is not in contact with the conductive porous substrate is measured by X-ray photoelectron spectroscopy (ESCA) to determine whether or not excited X-rays are monochromatic Al Kα 1,2 The ratio of oxygen to carbon elements (O / C in the outermost 10 nm layer) detected under the conditions of an X-ray (1486.6 eV), an X-ray diameter of 200 μm, and a photoelectron detection angle of 45° is 0.01 or less.

[0017] The gas diffusion electrode according to the present invention is, [4] the invention according to any one of the above [1] to [3], characterized in that the electrical resistance in the direction perpendicular to the surface when compressed at a pressure of 2.4 MPa is 5.5 mΩ cm 2 The present invention is characterized in that:

[0018] The gas diffusion electrode according to the present invention is [5] the invention described in any one of the above [1] to [4], characterized in that the surface of the microporous layer that is not in contact with the conductive porous substrate has a contact angle of 35° or more and 80° or less with a mixed liquid of 2-propanol and water in a mass ratio of 1:2.

[0019] The gas diffusion electrode according to the present invention can suppress flooding by sufficiently ensuring drainage on the surface of the microporous layer without impairing gas diffusibility and electrical conductivity.

[0020] Preferred embodiments of the gas diffusion electrode according to the present invention will be specifically described below, but the present invention is not limited to the following embodiments and can be practiced with various modifications depending on the purpose and application.

[0021] A gas diffusion electrode according to an embodiment of the present invention (hereinafter, sometimes referred to as the gas diffusion electrode of the present invention) is a gas diffusion electrode having a microporous layer containing carbonaceous fine particles and a fluorine-based water-repellent resin on at least one surface of a conductive porous substrate (for example, at least one of both end surfaces in the thickness direction of the conductive porous substrate). In the gas diffusion electrode of the present invention, the surface of the microporous layer not in contact with the conductive porous substrate has a surface F / C of 0.35 or more and 0.60 or less. The surface F / C is the ratio of elemental fluorine to elemental carbon detected by measuring the surface of the microporous layer using scanning electron microscope energy dispersive X-ray spectroscopy (SEM-EDX) at an acceleration voltage of 2.0 kV and a 1,000x field of view. The surface of the microporous layer is, for example, the end surface of the microporous layer not in contact with the conductive porous substrate.

[0022] As the conductive porous substrate used in the gas diffusion electrode of the present invention (hereinafter, sometimes referred to as the conductive porous substrate of the present invention), for example, porous substrates containing carbon fibers such as carbon fiber woven fabric, carbon fiber paper, carbon fiber nonwoven fabric, carbon felt, carbon paper, and carbon cloth, and metal porous substrates such as foamed sintered metal, metal mesh, and expanded metal are preferably used. Among these, porous substrates containing carbon fibers are preferably used because of their excellent corrosion resistance. In particular, it is more preferable to use a substrate containing a resin carbide, i.e., carbon paper, obtained by binding a carbon fiber paper with a resin carbide, because of its excellent property of absorbing dimensional changes in the thickness direction of the electrolyte membrane, i.e., its excellent "springiness."

[0023] In addition, as the conductive porous substrate used in the gas diffusion electrode of the present invention, a conductive porous substrate that has been subjected to a water-repellent treatment is preferably used. As a method of water-repellent treatment, a method of adding a water-repellent resin is preferred. Examples of the water-repellent resin that can be added to the conductive porous substrate of the present invention include fluorine-based water-repellent resins such as PTFE (polytetrafluoroethylene) (for example, "Teflon (registered trademark)"), FEP (tetrafluoroethylene-hexafluoropropylene copolymer), PFA (perfluoroalkoxy fluoride resin), ETFE (ethylene-tetrafluoroethylene copolymer), PVDF (polyvinylidene fluoride), and PVF (polyvinyl fluoride). Among these, PTFE or FEP, which exhibit strong water repellency, are preferred.

[0024] The amount of water-repellent resin applied in the water-repellent treatment is not particularly limited, but when the mass of the entire water-repellent treated conductive porous substrate is taken as 100 mass%, it is appropriate that the conductive porous substrate contains approximately 0.1 mass% to 20 mass% of water-repellent resin. If the content of the water-repellent resin (the ratio of the mass of the water-repellent resin to the mass of the entire conductive porous substrate) is less than 0.1 mass%, the water repellency may not be fully exhibited. If the content of the water-repellent resin exceeds 20 mass%, the pores that serve as gas diffusion paths or drainage paths may be blocked, or electrical resistance may increase.

[0025] As a method for water-repellent treatment of a conductive porous substrate, in addition to the commonly known treatment method of immersing the conductive porous substrate in a dispersion containing a water-repellent resin, a method of applying a water-repellent resin to the conductive porous substrate by die coating, spray coating, etc. can also be applied. A dry process method such as sputtering of a fluororesin can also be applied. After the water-repellent treatment, a drying step and / or a sintering step may be added as necessary.

[0026] As described above, the gas diffusion electrode of the present invention has a configuration in which a microporous layer is provided on at least one surface of a conductive porous substrate. In the present invention, the microporous layer is a layer having pores, and is a layer containing carbonaceous fine particles such as carbon black, carbon nanotubes, carbon nanofibers, chopped carbon fibers, graphene, and graphite. The microporous layer (hereinafter sometimes referred to as the microporous layer of the present invention) may contain other conductive materials as necessary.

[0027] Carbon black is preferably used as the carbonaceous fine particles contained in the microporous layer of the present invention because of its low cost and stable product quality. Examples of carbon black include furnace black, acetylene black, ketjen black, channel black, and lamp black. Among these carbon blacks, acetylene black is preferably used because it contains few impurities and is less likely to reduce catalyst activity. Ash content is used as a measure of the impurity content of carbon black, and it is preferable to use carbon black with an ash content of 0.1% by mass or less, assuming the total mass of the carbon black to be 100% by mass. The lower the ash content in carbon black, the better, and carbon black with an ash content of 0% by mass, i.e., ash-free carbon black, is particularly preferable.

[0028] Furthermore, the microporous layer of the present invention is required to have drainage properties. Therefore, the microporous layer of the present invention contains a fluorine-based water-repellent resin in addition to carbonaceous fine particles. Here, the fluorine-based water-repellent resin refers to a water-repellent resin containing fluorine atoms in the molecule. Examples of fluorine-based water-repellent resins include PTFE, FEP, PFA, ETFE, PVDF, and PVF. Among these, PTFE and FEP are preferred from the viewpoint of particularly high water repellency.

[0029] The mass of the fluorine-based water-repellent resin contained in the microporous layer of the present invention is preferably 20% by mass or more, more preferably 25% by mass or more, when the mass of the entire microporous layer is taken as 100% by mass. If the mass of the fluorine-based water-repellent resin contained in the microporous layer of the present invention is less than 20% by mass, the water repellency may not be sufficiently exhibited, and flooding may occur. Furthermore, the mass of the fluorine-based water-repellent resin contained in the microporous layer of the present invention is preferably 45% by mass or less, more preferably 40% by mass or less, when the mass of the entire microporous layer is taken as 100% by mass. If the mass of the fluorine-based water-repellent resin contained in the microporous layer of the present invention is more than 45% by mass, the conductivity of the microporous layer may decrease.

[0030] The mass of the fluorine-based water-repellent resin contained in the microporous layer of the present invention can be calculated, for example, by the following method. First, the surface of the microporous layer of the gas diffusion electrode is scraped off with a spatula or the like, and the resulting microporous layer powder is heated and sintered at 500°C for about 2 hours in equipment such as an electric furnace. This decomposes and removes the fluorine-based water-repellent resin from the microporous layer powder. The desired mass of the fluorine-based water-repellent resin can be calculated by dividing the difference in mass of the microporous layer powder before and after heating by the mass of the microporous layer powder before heating.

[0031] The microporous layer of the present invention is preferably formed by applying a microporous layer-forming coating liquid to a conductive porous substrate, followed by drying and sintering. The microporous layer-forming coating liquid referred to here is a liquid in which carbonaceous particles, a fluorine-based water-repellent resin, and the like to be contained in the microporous layer are dispersed in a dispersion medium.

[0032] When drying the coating liquid for forming a microporous layer, it is preferable to dry it at a temperature of 100°C or higher and 200°C or lower in order to evaporate the dispersion medium while promoting migration of the fluorine-based water-repellent resin in the coating liquid for forming a microporous layer, while maintaining the smoothness of the microporous layer surface. If the drying temperature is lower than 100°C, evaporation of the dispersion medium may be insufficient and migration may not be promoted. If the drying temperature exceeds 200°C, rapid evaporation of the dispersion medium may cause a loss of smoothness of the microporous layer surface. The lower limit of this drying temperature is more preferably 120°C or higher. The upper limit of this drying temperature is more preferably 180°C or lower.

[0033] Furthermore, when sintering the coating liquid for forming a microporous layer, in order to melt the fluorine-based water-repellent resin on the surface of the microporous layer, sintering is preferably carried out at a temperature of 325° C. or higher, more preferably at a temperature of 350° C. or higher. If the sintering temperature is lower than 325° C., the fluorine-based water-repellent resin will not melt sufficiently, which may result in reduced drainage and cause flooding.

[0034] The coating liquid for forming a microporous layer preferably contains, in addition to the above-mentioned carbonaceous fine particles and fluorine-based water-repellent resin, a dispersion medium and a dispersant to help disperse the carbonaceous fine particles. The amount of the dispersant is preferably such that the ratio of the mass of the dispersant to the mass of the carbonaceous fine particles contained in the coating liquid for forming a microporous layer is 0.1 or more. If the mass ratio of the dispersant is less than 0.1, the dispersion of the carbonaceous fine particles may not proceed well. In addition, other substances such as a thickener and an antifoaming agent may be added to the coating liquid for forming a microporous layer as needed, as long as they do not impair the dispersion of the carbonaceous fine particles.

[0035] As the dispersion medium, water is preferably used. As other dispersion medium, for example, a water-soluble organic solvent such as alcohol is used. These dispersion mediums can be used alone or in combination of two or more.

[0036] Examples of the dispersant include nonionic surfactants such as polyoxyethylene alkyl ethers, polyoxyethylene alkenyl ethers, polyoxyethylene polyoxypropylene alkyl ethers, polyoxyethylene phenyl ethers, polyoxyethylene alkylphenyl ethers, polyoxyethylene polystyrylphenyl ethers, polyoxyethylene naphthyl ethers, polyoxyethylene alkylamines, polyoxyethylene alkylamides, polyoxyethylene polyoxypropylene glycols, polyoxyethylene fatty acid esters, sorbitan fatty acid esters, polyoxyethylene sorbitan fatty acid esters, and polyoxyethylene glycerol ethers. One or more of these can be used as the dispersant. Among these, polyoxyethylene alkyl ethers, polyoxyethylene polyoxypropylene alkyl ethers, polyoxyethylene alkylphenyl ethers, and polyoxyethylene polystyrylphenyl ethers are preferred from the viewpoint of good dispersibility of carbonaceous microparticles.

[0037] Examples of the thickener that can be used include water-soluble polymers such as methyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, carboxymethyl cellulose, polyvinyl alcohol, polyvinylpyrrolidone, polyethylene glycol, polyacrylic acid, polylactic acid, guar gum, xanthan gum, and starch. Examples of the antifoaming agent that can be used include polyorganosiloxanes and phosphate esters.

[0038] The method for applying the coating liquid for forming a microporous layer onto the conductive porous substrate of the present invention is preferably a coating method using screen printing, rotary screen printing, spray atomization, intaglio printing, gravure printing, die coater printing, bar coating, blade coating, knife coater, or the like.

[0039] The thickness of the conductive porous substrate of the present invention is preferably 120 μm or more and 200 μm or less. When the thickness of the conductive porous substrate of the present invention is preferably 120 μm or more, more preferably 130 μm or more, the in-plane gas diffusivity is easily improved and the mechanical strength of the gas diffusion electrode can be increased. Furthermore, when the thickness of the conductive porous substrate of the present invention is preferably 200 μm or less, more preferably 160 μm or less, the conductive path is easily connected and the conductivity can be effectively increased. The thickness of the conductive porous substrate of the present invention can be measured using a digital micrometer.

[0040] The thickness of the microporous layer of the present invention is preferably 10 μm or more and 120 μm or less. When the thickness of the microporous layer of the present invention is preferably 120 μm or less, more preferably 100 μm or less, and even more preferably 80 μm or less, the gas diffusion electrode itself has excellent gas and water diffusibility (permeability and drainage), and electrical resistance can be effectively reduced. Furthermore, when the thickness of the microporous layer of the present invention is preferably 10 μm or more, more preferably 20 μm or more, it is possible to cover the roughness of the conductive porous substrate. The thickness of the microporous layer of the present invention can be calculated by measuring the thickness of the gas diffusion layer and the thickness of the conductive porous substrate using a digital micrometer and subtracting the thickness of the conductive porous substrate from the thickness of the gas diffusion layer obtained.

[0041] The basis weight (mass per unit area) of the microporous layer of the present invention is 10 g / m 2 30g / m or more 2 The microporous layer of the present invention preferably has a basis weight of 10 g / m 2 When the surface area of ​​the microporous layer is 14 g / m or more, the surface of the conductive porous substrate can be reliably covered, and the back diffusion of the generated water is promoted, thereby improving the power generation performance. 2 More preferably, it is 16 g / m or more. 2 It is more preferable that the basis weight of the microporous layer of the present invention is 30 g / m or more. 2 When the weight per unit area of ​​the microporous layer of the present invention is 26 g / m or less, clogging of recesses and voids is suppressed, and drainage properties are further improved. 2 More preferably, it is 24 g / m or less.2 The weight of the gas diffusion electrode of the present invention having the microporous layer formed thereon is more preferably 30 g / m or less. 2 90g / m or more 2 It is preferable that the content is within the following range.

[0042] The density of the conductive porous substrate of the present invention is 0.15 g / cm 3 0.50g / cm or more 3 The density of the conductive porous substrate of the present invention is preferably within the following range: 3 When the density of the conductive porous substrate of the present invention is 0.50 g / cm or more, the strength of the conductive porous substrate can be made sufficient and the durability can be effectively improved. 3 By satisfying the condition of not more than 100%, it is possible to prevent a decrease in drainage property and gas diffusion property.

[0043] In the gas diffusion electrode of the present invention, the ratio of elemental fluorine to elemental carbon (surface layer F / C) detected by scanning electron microscope energy dispersive X-ray spectroscopy (SEM-EDX) at an acceleration voltage of 2.0 kV and a 1,000x magnification on the surface of the microporous layer not in contact with the conductive porous substrate is 0.35 or more and 0.60 or less.

[0044] Here, the method for measuring the surface layer F / C in the present invention, i.e., SEM-EDX, will be described in more detail. In SEM-EDX, first, of the entire surface (e.g., both end faces in the layer thickness direction) of a microporous layer coated with platinum to a film thickness of 1.0 nm, the surface that is not in contact with the conductive porous substrate is observed at 1,000x magnification using a scanning electron microscope (SEM). The observed surface is irradiated with an electron beam at an acceleration voltage of 2.0 kV using energy dispersive X-ray analysis (EDX), and characteristic X-rays reflected from the observed surface are detected. Based on the detected characteristic X-rays, the fluorine content and carbon content of the observed surface are measured, and the fluorine content / carbon content ratio is calculated based on the obtained measured values, thereby obtaining the surface layer F / C. In this case, the detection depth is several hundred nanometers to several micrometers, so the surface layer F / C measured by SEM-EDX quantifies the contents of elemental fluorine and elemental carbon in the surface layer several hundred nanometers to several micrometers from the surface of the microporous layer that is not in contact with the conductive porous substrate. This is an index showing the degree of migration of the fluorine-based water-repellent resin. Migration of the fluorine-based water-repellent resin is a phenomenon in which, after the microporous layer-forming coating liquid is applied to the conductive porous substrate, the fluorine-based water-repellent resin migrates to the surface of the microporous layer that is not in contact with the conductive porous substrate as the dispersion medium in the microporous layer-forming coating liquid dries.

[0045] The surface layer F / C can also be controlled by adjusting the mass of the fluorine-based water-repellent resin in the microporous layer, but if the mass of the fluorine-based water-repellent resin is high, the conductivity of the microporous layer may be reduced. Therefore, a preferred method for controlling the surface layer F / C is to control the migration of the fluorine-based water-repellent resin by adjusting the drying conditions or the viscosity of the coating liquid for forming the microporous layer. A preferred method for controlling the migration of the fluorine-based water-repellent resin is to control the drying conditions, such as the drying temperature and drying time. Another preferred method is to control the migration of the fluorine-based water-repellent resin by adjusting the viscosity of a mixed solution containing carbonaceous fine particles and components other than the fluorine-based water-repellent resin in the coating liquid for forming the microporous layer, thereby changing the resistance to migration of the fluorine-based water-repellent resin. It is more preferred to use both of these two methods to control the migration of the fluorine-based water-repellent resin.

[0046] When the surface layer F / C is 0.35 or more, preferably 0.40 or more, a large amount of the fluorine-based water-repellent resin is present in the surface layer of the microporous layer, thereby effectively improving drainage.When the surface layer F / C is 0.60 or less, preferably 0.55 or less, the electrical conductivity of the microporous layer can be sufficiently maintained.

[0047] In the gas diffusion electrode of the present invention, the F / C of the outermost 10 nm of the microporous layer on the surface not in contact with the conductive porous substrate is preferably 0.62 to 0.80. The F / C of the outermost 10 nm is determined by X-ray photoelectron spectroscopy (ESCA) of the surface of the microporous layer using monochromatic Al Kα excitation X-rays. 1,2 The ratio is the ratio of fluorine element to carbon element detected by measurement under the conditions of an X-ray (1486.6 eV), an X-ray diameter of 200 μm, and a photoelectron detection angle of 45°.

[0048] Here, the method for measuring the F / C of the outermost 10 nm layer in the present invention, that is, X-ray photoelectron spectroscopy (ESCA), will be described in more detail.

[0049] In ESCA, first, the entire surface of the microporous layer (for example, both end surfaces in the layer thickness direction) that is not in contact with the conductive porous substrate is subjected to excitation X-rays with monochromatic Al Kα 1,2 X-rays are irradiated under conditions of a 1486.6 eV X-ray diameter of 200 μm and a photoelectron detection angle of 45°, and photoelectrons emitted from the surface are detected with an analyzer. The detected data is smoothed using 9-point smoothing, and the horizontal axis correction is performed with the C1s main peak (CHx, C-C, C=C) at 284.6 eV, and the data is processed. The value (%) of the number ratio of fluorine elements obtained in this way is divided by the value (%) of the number ratio of carbon elements, thereby allowing the F / C of the outermost 10 nm layer to be calculated. The detection depth of ESCA can be controlled by the photoelectron detection angle, and by using the above conditions, the contents of fluorine elements and carbon elements up to a depth of 10 nm in the outermost layer of the surface of the microporous layer not in contact with the conductive porous substrate can be quantified. The F / C in the outermost 10 nm layer obtained by the above measurement can be an index representing the amount of fluorine in the outermost 10 nm layer itself, but can also be an index representing an increase in surface area due to melting of the fluorine-based water-repellent resin due to high temperature, etc. In other words, even if the same amount of fluorine-based water-repellent resin is contained in the microporous layer, if the F / C in the outermost 10 nm layer is large, the surface of the microporous layer that is not in contact with the conductive porous substrate will exhibit high water repellency.

[0050] The F / C ratio of the outermost 10 nm layer can be increased by adjusting the mass of the fluorine-based water-repellent resin in the microporous layer, but simply increasing the mass of the fluorine-based water-repellent resin may reduce the conductivity of the microporous layer. Therefore, similar to the method for adjusting the surface layer F / C described above, it is preferable to adjust the F / C ratio of the outermost 10 nm layer by controlling the migration of the fluorine-based water-repellent resin. In addition, it is more preferable to increase the surface area of ​​the fluorine-based water-repellent resin in the outermost 10 nm layer by applying sufficient heat to the fluorine-based water-repellent resin that has migrated to the surface layer side to melt it. This is because the F / C ratio of the outermost 10 nm layer can be further improved.

[0051] The F / C of the outermost 10 nm layer is preferably 0.62 or more, more preferably 0.66 or more. If the F / C of the outermost 10 nm layer is less than 0.62, the amount of fluorine-based water-repellent resin at the outermost surface of the microporous layer will be small, which may result in insufficient drainage. The F / C of the outermost 10 nm layer is preferably 0.80 or less, more preferably 0.76 or less. If the F / C of the outermost 10 nm layer exceeds 0.80, the fluorine-based water-repellent resin at the outermost surface of the microporous layer will obstruct the conductive path, which may result in reduced conductivity.

[0052] In the gas diffusion electrode of the present invention, the O / C ratio of the outermost 10 nm of the microporous layer on the surface not in contact with the conductive porous substrate is preferably 0.01 or less. The O / C ratio of the outermost 10 nm of the microporous layer is determined by X-ray photoelectron spectroscopy (ESCA) using monochromatic Al Kα excitation X-rays. 1,2 The O / C ratio is the ratio of oxygen to carbon detected by measurement under conditions of an X-ray (1486.6 eV), an X-ray diameter of 200 μm, and a photoelectron detection angle of 45°. The O / C ratio of the outermost 10 nm layer can be calculated by obtaining the number ratio (%) of fluorine and the number ratio (%) of oxygen under the same conditions as when measuring the F / C ratio of the outermost 10 nm layer described above. An O / C ratio of the outermost 10 nm layer of greater than 0.01 indicates that dispersants, thickeners, and antifoamers in the coating liquid for forming a microporous layer remain on the surface of the microporous layer, which may reduce drainage. The O / C ratio of the outermost 10 nm layer can be reduced by reducing the contents of dispersants, thickeners, and antifoamers in the coating liquid for forming a microporous layer, or by increasing the sintering time and temperature to promote their decomposition.

[0053] The gas diffusion electrode of the present invention has an electrical resistance of 5.5 mΩ cm in the direction perpendicular to the surface when compressed at a pressure of 2.4 MPa. 2 It is preferable that the electrical resistance is 5.5 mΩ cm or less. 2 If the temperature exceeds this range, when the gas diffusion electrode is used as a component of a fuel cell, the electrical conductivity may be impaired, which may result in a decrease in the initial power generation performance.

[0054] In the present invention, the contact angle of the surface of the microporous layer that is not in contact with the conductive porous substrate with a mixed solution of 2-propanol and water in a mass ratio of 1:2 is preferably 35° or more and 80° or less. If the contact angle is less than 35°, the drainage of the gas diffusion electrode may be poor, resulting in flooding. If the contact angle exceeds 80°, the drainage may be too high, resulting in a dry environment for the gas electrode substrate, which may result in a decrease in power generation performance. The contact angle can be controlled by controlling the above-mentioned surface layer F / C and the outermost 10 nm F / C.

[0055] The present invention will be described below using examples and comparative examples, but the present invention is not limited to the following examples.

[0056] [Measurement Method] (1) Surface Layer F / C In measuring the surface layer F / C, first, the surface of the microporous layer not in contact with the conductive porous substrate was platinum-coated to a thickness of 1.0 nm, and the surface was observed at 1,000 times magnification using an SEM (Hitachi High-Tech Corporation, cold cathode field emission scanning electron microscope S-4800) to perform element distribution analysis. In the element distribution analysis, an EDX (Bruker AXS QUANTAX Flat QUAD system Wflash 5060FQ) was used to detect and quantify characteristic X-rays reflected from an electron beam irradiated at an acceleration voltage of 2.0 kV, thereby calculating the fluorine element content and carbon element content on the surface. The calculated fluorine element content was divided by the carbon element content to calculate the desired surface layer F / C.

[0057] (2) F / C in the outermost 10 nm layer In measuring the F / C in the outermost 10 nm layer, first, the surface of the microporous layer not in contact with the conductive porous substrate was measured by X-ray photoelectron spectroscopy (ESCA) using a Quantera SXM (manufactured by PKI). The measurement conditions were: excitation X-rays were monochromatic Al Kα 1,2The X-ray energy was 1,486.6 eV, the X-ray diameter was 200 μm, and the photoelectron detection angle was 45°. Data processing was performed using 9-point smoothing, and the horizontal axis correction was performed using the C1s main peak (CHx, C-C, C=C) at 284.6 eV. The target F / C ratio of the outermost 10 nm layer was calculated by dividing the value (%) of the number ratio of fluorine elements obtained from the ESCA measurement by the value (%) of the number ratio of carbon elements.

[0058] (3) O / C in the outermost 10 nm layer The O / C in the outermost 10 nm layer was measured by X-ray photoelectron spectroscopy (ESCA) in the same manner as in (2) F / C in the outermost 10 nm layer. The target O / C in the outermost 10 nm layer was calculated by dividing the number ratio (%) of oxygen element obtained from the ESCA measurement by the number ratio (%) of carbon element.

[0059] (4) Electrical Resistance in the Normal Direction of the Gas Diffusion Electrode In this measurement, the gas diffusion electrode was cut into a size of 20 mm x 20 mm, and the cut gas diffusion electrode was sandwiched between smooth, gold-plated rigid metal electrodes on the top and bottom. An average pressure of 2.4 MPa was then applied to the gas diffusion electrode via these rigid electrodes. A current of 1 A was applied to the upper and lower rigid electrodes in this state, and the voltage between the upper and lower rigid electrodes was measured to calculate the electrical resistance in the normal direction per unit area of ​​the target gas diffusion electrode.

[0060] (5) Contact Angle of Gas Diffusion Electrode In this measurement, first, in an environment of 20°C temperature and 60% humidity, 5 μL of a mixed solution of 2-propanol and water in a mass ratio of 1:2 was dropped onto 10 randomly selected locations on the surface of the microporous layer that was not in contact with the conductive porous substrate. After that, for each drop on the surface, the contact angle between the surface and the water droplet was measured using an automatic contact angle meter DM-501 (manufactured by Kyowa Interface Science Co., Ltd.). The average of the obtained contact angles was taken as the contact angle of the gas diffusion electrode.

[0061] [Example 1] (Conductive porous substrate) In Example 1, first, polyacrylonitrile carbon fiber (Toray Industries, Inc., "Torayca (registered trademark)" T300) having an average diameter of 7 μm was cut to an average length of 12 mm, and the cut carbon fiber was dispersed in water and continuously made into paper by a wet papermaking method.

[0062] The paper body obtained as described above was coated with a 10% by mass aqueous solution of polyvinyl alcohol as a binder and dried to obtain a carbon fiber weight of 15 g / m 2 At this time, the amount of polyvinyl alcohol attached was 20 parts by mass per 100 parts by mass of the carbon fiber.

[0063] Next, a phenolic resin obtained by mixing a resol-type phenolic resin and a novolac-type phenolic resin so that the nonvolatile content was 1:1 by mass ratio, flake graphite powder (average particle size 5 μm), and methanol were mixed in a mass ratio of 10 / 5 / 85 and uniformly dispersed to obtain a resin composition.

[0064] Next, the carbon fiber sheet was continuously immersed in the resin composition, and the immersed carbon fiber sheet was subjected to a resin impregnation process in which it was sandwiched between rolls and squeezed, and then wound into a roll to obtain a precursor-attached carbon fiber sheet. The rolls were smooth metal rolls with a structure that allowed excess resin composition to be removed with a doctor blade, and two rolls were arranged horizontally with a certain clearance between them. The precursor-attached carbon fiber sheet was passed horizontally between these two rolls and then pulled up vertically, thereby adjusting the amount of resin composition attached to the entire precursor-attached carbon fiber sheet. The amount of phenolic resin attached to the precursor-attached carbon fiber sheet was 130 parts by mass per 100 parts by mass of the carbon fiber sheet.

[0065] The hot plates were set parallel to each other on a flat plate press, and a spacer was placed on the lower of the set upper and lower hot plates to adjust the distance between the upper and lower hot plates to a constant value. The precursor-attached carbon fiber sheet was then sandwiched between release papers from above and below, and the sandwiched precursor-attached carbon fiber sheet was subjected to a heat and pressure treatment. The precursor-attached carbon fiber sheet was then introduced into a heating furnace maintained in a nitrogen gas atmosphere and fired and carbonized to obtain the conductive porous substrate of Example 1. The thickness of the obtained conductive porous substrate under a pressure of 0.15 MPa was 140 μm, and the density was 0.30 g / cm. 3 It was.

[0066] (Water-repellent treatment) In Example 1, the conductive porous substrate was coated with a diluted aqueous solution of PTFE dispersion ("Polyflon (registered trademark)" D-210C; manufactured by Daikin Industries, Ltd.; containing 60% by mass of PTFE in a dispersion medium (water)) using a slit die coater so that PTFE was 5.0 parts by mass relative to 100.0 parts by mass of the conductive porous substrate. Then, the coating liquid on the conductive porous substrate was dried at 120 ° C., thereby subjecting the conductive porous substrate to water-repellent treatment.

[0067] (Coating liquid for forming a microporous layer) Next, a coating liquid for forming a microporous layer was prepared using acetylene black (Denka Black (registered trademark) manufactured by Denka Corporation) as the carbon black, PTFE dispersion (Polyflon (registered trademark) D-210C) as the water repellent, Triton X-100 (manufactured by Nacalai Tesque, Inc.) as the dispersant, and purified water. The mass ratio of acetylene black / water repellent / dispersant / purified water was adjusted to 8.0 parts by mass / 7.0 parts by mass / 7.0 parts by mass / 78.0 parts by mass to prepare a coating liquid for forming a microporous layer. The mass ratio of PTFE to the solids (acetylene black and PTFE) in the obtained coating liquid for forming a microporous layer was 0.34.

[0068] (Coating and Drying) The thus obtained coating liquid for forming a microporous layer was applied to the above-mentioned water-repellent conductive porous substrate using a slit die coater. At this time, the basis weight of the solid content in the coating liquid for forming a microporous layer was 14 g / m 2The coating liquid for forming a microporous layer was then applied so that the coating liquid for forming a microporous layer on the conductive porous substrate became as follows:

[0069] (Sintering) Subsequently, the conductive porous substrate and the dried components of the coating liquid for forming a microporous layer were sintered at a temperature of 365° C., thereby obtaining the gas diffusion electrode of Example 1.

[0070] The gas diffusion electrode of Example 1 thus obtained was measured for (1) the surface F / C, (2) the outermost 10 nm F / C, (3) the outermost 10 nm O / C, (4) the electrical resistance in the direction perpendicular to the surface of the gas diffusion electrode, and (5) the contact angle of the gas diffusion electrode according to the respective measurement methods. The evaluation results of Example 1 are shown in Table 1 below.

[0071] In Example 2, a gas diffusion electrode was obtained in the same manner as in Example 1, except that in preparing the microporous layer-forming coating liquid, the mass ratio of acetylene black / water repellent / dispersant / purified water was adjusted to 8.0 parts by mass / 7.0 parts by mass / 9.0 parts by mass / 76.0 parts by mass. The evaluation results of Example 2 are shown in Table 1.

[0072] In Example 3, a gas diffusion electrode was obtained in the same manner as in Example 1, except that in preparing the microporous layer-forming coating liquid, the mass ratio of acetylene black / water repellent / dispersant / purified water was adjusted to 8.0 parts by mass / 7.0 parts by mass / 11.0 parts by mass / 74.0 parts by mass. The evaluation results of Example 3 are shown in Table 1.

[0073] In Example 4, a gas diffusion electrode was obtained in the same manner as in Example 1, except that in preparing the microporous layer-forming coating liquid, the mass ratio of acetylene black / water repellent / dispersant / purified water was adjusted to 8.0 parts by mass / 7.0 parts by mass / 13.0 parts by mass / 72.0 parts by mass. The evaluation results of Example 4 are shown in Table 1.

[0074] In Example 5, a gas diffusion electrode was obtained in the same manner as in Example 1, except that in preparing the microporous layer-forming coating liquid, the mass ratio of acetylene black / water repellent / dispersant / purified water was adjusted to 8.0 parts by mass / 11.0 parts by mass / 7.0 parts by mass / 74.0 parts by mass. The evaluation results of Example 5 are shown in Table 1.

[0075] In Example 6, a gas diffusion electrode was obtained and various measurements were carried out in the same manner as in Example 1, except that the sintering temperature was set to 320° C. The evaluation results of Example 6 are shown in Table 1.

[0076] In Example 7, a gas diffusion electrode was obtained in the same manner as in Example 1, except that in preparing the microporous layer-forming coating liquid, the mass ratio of acetylene black / water repellent / dispersant / purified water was adjusted to 8.0 parts by mass / 7.0 parts by mass / 15.0 parts by mass / 70.0 parts by mass, and the drying temperature after applying the microporous layer-forming coating liquid was set to 180° C. The evaluation results of Example 7 are shown in Table 1.

[0077] In Example 8, a gas diffusion electrode was obtained in the same manner as in Example 1, except that in preparing the microporous layer-forming coating liquid, the mass ratio of acetylene black / water repellent / dispersant / purified water was adjusted to 8.0 parts by mass / 9.0 parts by mass / 15.0 parts by mass / 68.0 parts by mass. The evaluation results of Example 8 are shown in Table 1.

[0078] In Example 9, a gas diffusion electrode was obtained and various measurements were carried out in the same manner as in Example 1, except that the drying temperature after application of the microporous layer-forming coating liquid was 180° C. The evaluation results of Example 9 are shown in Table 2 below.

[0079] In Example 10, a gas diffusion electrode was obtained in the same manner as in Example 1, except that in preparing the microporous layer-forming coating liquid, the mass ratio of acetylene black / water repellent / dispersant / purified water was adjusted to 8.0 parts by mass / 11.0 parts by mass / 15.0 parts by mass / 66.0 parts by mass. The evaluation results for Example 10 are shown in Table 2.

[0080] In Comparative Example 1, a gas diffusion electrode was obtained in the same manner as in Example 1, except that in preparing the microporous layer-forming coating liquid, the mass ratio of acetylene black / water repellent / dispersant / purified water was adjusted to 8.0 parts by mass / 7.0 parts by mass / 15.0 parts by mass / 70.0 parts by mass. The evaluation results of Comparative Example 1 are shown in Table 2.

[0081] In Comparative Example 2, a gas diffusion electrode was obtained and various measurements were carried out in the same manner as in Example 1, except that in preparing the microporous layer-forming coating liquid, the mass ratio of acetylene black / water repellent / dispersant / purified water was adjusted to 8.0 parts by mass / 7.0 parts by mass / 15.0 parts by mass / 70.0 parts by mass, and the drying temperature after applying the microporous layer-forming coating liquid was set to 100° C. The evaluation results of Comparative Example 2 are shown in Table 2.

[0082] In Comparative Example 3, a gas diffusion electrode was obtained in the same manner as in Example 1, except that in preparing the microporous layer-forming coating liquid, the mass ratio of acetylene black / water repellent / dispersant / purified water was adjusted to 8.0 parts by mass / 2.0 parts by mass / 7.0 parts by mass / 83.0 parts by mass. The evaluation results of Comparative Example 3 are shown in Table 2.

[0083] In Comparative Example 4, a gas diffusion electrode was obtained in the same manner as in Example 1, except that in preparing the microporous layer-forming coating liquid, the mass ratio of acetylene black / water repellent / dispersant / purified water was adjusted to 8.0 parts by mass / 25.0 parts by mass / 7.0 parts by mass / 60.0 parts by mass. The evaluation results of Comparative Example 4 are shown in Table 2.

[0084] In Comparative Example 5, a gas diffusion electrode was obtained in the same manner as in Example 1, except that in preparing the microporous layer-forming coating liquid, the mass ratio of acetylene black / water repellent / dispersant / purified water was adjusted to 8.0 parts by mass / 7.0 parts by mass / 20.0 parts by mass / 65.0 parts by mass. The evaluation results of Comparative Example 5 are shown in Table 2.

[0085]

[0086]

[0087] As described above, the gas diffusion electrode of the present invention is suitable for use as an electrode for fuel cells, particularly polymer electrolyte fuel cells used as power sources for transportation equipment such as fuel cell vehicles and fuel cell aircraft.

Claims

1. A gas diffusion electrode having a microporous layer containing carbonaceous particles and a fluorine-based water-repellent resin on at least one surface of a conductive porous substrate, wherein the ratio of fluorine to carbon (surface layer F / C) detected on the surface of the microporous layer not in contact with the conductive porous substrate by scanning electron microscope energy dispersive X-ray spectroscopy (SEM-EDX) at an acceleration voltage of 2.0 kV and a 1,000x field of view is 0.35 or more and 0.60 or less.

2. The surface of the microporous layer that is not in contact with the conductive porous substrate is subjected to X-ray photoelectron spectroscopy (ESCA) to determine whether or not the excited X-rays are monochromatic Al Kα. 1,2 2. The gas diffusion electrode according to claim 1, wherein the ratio of fluorine to carbon (F / C in the outermost 10 nm layer) detected by measurement under conditions of a fluorine-containing ion beam (1486.6 eV), an X-ray diameter of 200 μm, and a photoelectron detection angle of 45° is 0.62 or more and 0.80 or less.

3. The surface of the microporous layer that is not in contact with the conductive porous substrate is subjected to X-ray photoelectron spectroscopy (ESCA) to detect monochromatic Al Kα 1,2 2. The gas diffusion electrode according to claim 1, wherein the ratio of oxygen to carbon (O / C in the outermost 10 nm layer) detected under conditions of an X-ray (1486.6 eV), an X-ray diameter of 200 μm, and a photoelectron detection angle of 45° is 0.01 or less.

4. When compressed at a pressure of 2.4 MPa, the electrical resistance in the perpendicular direction is 5.5 mΩ cm 2 2. The gas diffusion electrode according to claim 1, wherein:

5. The gas diffusion electrode according to claim 1, wherein the surface of the microporous layer that is not in contact with the conductive porous substrate has a contact angle of 35° or more and 80° or less with a mixed liquid obtained by mixing 2-propanol and water at a mass ratio of 1:2.

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