Porous transport layer, membrane electrode assembly, water electrolysis device, composition for microporous layer, and method for producing porous transport layer

Abstract

The purpose of the present invention is to provide a porous transport layer in which puncture of a conductive porous base material is suppressed and which has excellent gas diffusibility.　The present invention is a porous transport layer comprising a microporous layer on at least one surface of a conductive porous base material containing carbon fibers, wherein the ratio of the volume of pores having pore diameters in the range of 10-100 µm to the volume of pores having pore diameters of 100-500 nm is 8-100.

Description

Porous transport layer, membrane electrode assembly, water electrolysis device, composition for microporous layer, and method for manufacturing a porous transport layer

[0001] The present invention relates to a porous transport layer used in electrolysis, a method for manufacturing the same, a membrane electrode assembly using the porous transport layer, a water electrolysis apparatus, and a composition for a microporous layer.

[0002] In recent years, methods for producing hydrogen through the electrolysis of water (water electrolysis), particularly those that use renewable energy as the power source for water electrolysis to produce clean hydrogen with a low environmental impact, have attracted attention. While there are several methods for water electrolysis, polymer electrolyte membrane (PEM) water electrolysis devices are being widely developed because they can utilize the technology of polymer electrolyte fuel cells (PEFCs) and can produce high-purity hydrogen.

[0003] The cell of a PEM-type water electrolysis apparatus has a structure in which a membrane electrode assembly (MEA) is formed by bonding catalyst layers to both sides of a solid polymer electrolyte membrane, sandwiching both sides of the solid polymer electrolyte membrane between porous transport layers (PTLs), and then sandwiching this assembly with separators. In other words, in a PEM-type water electrolysis apparatus cell, the layer sandwiched between the catalyst layer and the separator is called the porous transport layer. The solid polymer electrolyte membrane is generally an ion exchange membrane made of a fluorine-based polymer such as "Nafion®," which exhibits high ionic conductivity in a wet state, similar to PEFC. For the porous transport layer, conductive porous substrates such as carbon paper or carbon felt, or metal porous sintered bodies made of titanium, are used in terms of conductivity and mechanical strength. Metal porous sintered bodies have the advantage of being resistant to corrosion, but are expensive, so a carbon porous substrate is sometimes used on the cathode side where the potential is low. In this case, since the water electrolysis cell is fastened under high pressure, the fuzz on the surface of the substrate can damage the polymer electrolyte membrane, causing deterioration of the polymer electrolyte membrane starting from that point, which can reduce the water electrolysis performance. Therefore, to improve the durability of polymer electrolyte membranes, a porous transport layer is sometimes used that consists of a microporous layer placed on top of a conductive porous substrate.

[0004] The porous transport layer used in water electrolysis devices requires low electrical resistance to reduce electrode overvoltage and high gas diffusivity to efficiently discharge gases generated by electrolysis.

[0005] As a porous transport layer used in water electrolysis devices, Patent Document 1 discloses a porous metal gas diffusion layer characterized by being made of a porous metal sheet for solid polymer fuel cells. Patent Document 2 discloses a diffusion layer comprising a porous substrate made of a conductive material and a coating containing a metal oxide. Patent Document 3 discloses a gas diffusion layer for water electrolysis cells that includes a mesh having a filtration accuracy of 80 μm or less.

[0006] Japanese Patent Publication No. 2022-145670, Japanese Patent Publication No. 2023-76968, Japanese Patent No. 7053128

[0007] The conductive porous substrate made of a metal porous body disclosed in Patent Document 1 is high in cost and has productivity challenges. Furthermore, because it has low springiness, the stress within the plane is not uniform, resulting in areas with insufficient contact, which increases contact resistance and reduces electrolytic efficiency.

[0008] The metal oxide coating described in Patent Document 2 has the problem that it cannot suppress the fuzzing of the conductive porous substrate and damages the electrolyte membrane.

[0009] The gas diffusion layer described in Patent Document 3 has large voids in the network structure, which reduces the contact area with the catalyst layer, resulting in a decrease in conductivity and electrolytic efficiency.

[0010] The present invention aims to provide a porous transport layer that suppresses puncture of conductive porous substrates and exhibits excellent gas diffusion properties.

[0011] The present invention and its preferred embodiments employ the following means to solve the above problems: (1) A porous transport layer comprising a conductive porous substrate containing carbon fibers, having a microporous layer on at least one side thereof, wherein the ratio of the pore volume of pores with a diameter of 10 μm to 100 μm to the pore volume of pores with a diameter of 100 nm to 500 nm is 8 or more and 100 or less.

[0012] (2) The porous transport layer according to (1), having a peak within the pore diameter range of 100 nm to 1 μm in the logarithmic differential pore volume distribution.

[0013] (3) The porous transport layer according to (1) or (2), having a peak within the pore diameter range of 100 nm to 500 nm in the logarithmic differential pore volume distribution.

[0014] (4) The porous transport layer according to any one of (1) to (3), wherein the ratio of the pore volume with a pore diameter of 500 nm to 10 μm to the pore volume with a pore diameter of 100 nm to 500 nm is 0.5 or more and 10.0 or less.

[0015] (5) The porous transport layer according to any one of (1) to (4), wherein the pore volume with a pore diameter of 100 nm to 500 nm is 1.0 μL / cm ２ or more and 10.0 μL / cm ２ or less.

[0016] (6) The porous transport layer according to any one of (1) to (5), wherein the pore volume with a pore diameter of 500 nm to 10 μm is 2.0 μL / cm ２ or more and 30.0 μL / cm ２ or less.

[0017] (7) The porous transport layer according to any one of (1) to (6), wherein the pore volume with a pore diameter of 10 μm to 100 μm is 40 μL / cm ２ or more and 200 μL / cm ２ or less.

[0018] (8) The porous transport layer according to any one of (1) to (7), wherein the thickness of the conductive porous substrate is 0.4 mm or more and 5.0 mm or less.

[0019] (9) The porous transport layer according to any one of (1) to (8), wherein the microporous layer contains conductive particles made of a carbon material.

[0020] (10) The porous transport layer according to (9), wherein the microporous layer further contains a binder.

[0021] (11) The porous transport layer according to (9) or (10), wherein the primary particle diameter of the conductive particles is 20 nm or more and 100 nm or less.

[0022] (12) The basis weight of the microporous layer is 10 g / m ２ 30g / m or more ２ A porous transport layer as described in any of (1) to (11) below.

[0023] (13) The porous transport layer according to any one of (1) to (12), wherein the surface coverage rate of the microporous layer is 95% or more and 100% or less.

[0024] (14) The porous transport layer according to any one of (1) to (13), wherein the contact angle between the microporous layer and water is 0° or more and less than 120°.

[0025] (15) The porous transport layer according to any one of (1) to (14), wherein the contact angle between the microporous layer and water is 90° or more and less than 120°.

[0026] (16) A porous transport layer according to any one of (1) to (15), wherein the fluororesin content is 0.001% by mass or more and 0.100% by mass or less.

[0027] (17) The porous transport layer according to any one of (1) to (16), wherein the mass loss rate when the microporous layer is heated at 420°C for 1 hour is 5% by mass or more and 30% by mass or less.

[0028] (18) The porous transport layer according to any one of (1) to (17), wherein at least one of acetic acid, benzaldehyde, phenol, or cresol is detected in the microporous layer by analysis by pyrolysis gas chromatography-mass spectrometry.

[0029] (19) The porous transport layer according to any one of (1) to (18), wherein the gloss of the microporous layer with respect to incident light at an incident angle of 85° is 10 or more and 40 or less.

[0030] (20) The porous transport layer according to any one of (1) to (19), wherein the surface roughness Sa of the microporous layer is 0.1 μm or more and 5.0 μm or less.

[0031] (21) A membrane electrode assembly having a porous transport layer as described in any of (1) to (20).

[0032] (22) A water electrolysis apparatus having a porous transport layer as described in any of (1) to (21).

[0033] (23) A composition for forming a microporous layer used in the production of a porous transport layer having a microporous layer on at least one side of a conductive porous substrate containing carbon fibers, comprising conductive particles, water, a polymer dispersant, a binder, and a fluororesin, wherein the mass of the fluororesin contained in the composition for forming the microporous layer is 0.01% by mass or more and 0.50% by mass or less of the mass of the composition for forming the microporous layer.

[0034] (24) The microporous layer composition according to (23), wherein the mass of the polymer dispersant is 0.1% by mass or more and 5.0% by mass or less of the mass of the microporous layer composition.

[0035] (25) A method for manufacturing a porous transport layer, comprising the steps of: applying the microporous layer composition described in (23) or (24) to a conductive porous substrate containing carbon fibers; and firing the conductive porous substrate coated with the microporous layer composition at a temperature of 200°C to 400°C.

[0036] According to the present invention, it is possible to suppress puncture of conductive porous substrates and provide a porous transport layer with excellent gas diffusion properties.

[0037] The present invention will now be described in detail. In this invention, when a numerical range is expressed as "A to B", it includes the values ​​A and B at both ends, meaning "A or greater and B or less". For example, "100 nm to 1 μm" means 100 nm or greater and 1 μm or less.

[0038] The porous transport layer of the present invention comprises a conductive porous substrate containing carbon fibers, with a microporous layer on at least one side.

[0039] <Conductive Porous Substrate> The conductive porous substrate used in the porous transport layer of the present invention contains carbon fibers. Compared to metal porous sintered bodies, conductive porous substrates have the advantage of lower material costs and reduced manufacturing costs.Specific examples of conductive porous substrates containing carbon fibers include carbon paper, carbon felt, carbon fiber fabrics, carbon fiber papermaking bodies, and carbon fiber nonwoven fabrics.In particular, it is preferable to include resin carbides because they have excellent properties for absorbing dimensional changes in the thickness direction of the electrolyte membrane, i.e., "springiness," and it is preferable to use a substrate obtained by binding a carbon fiber papermaking body with resin carbides, i.e., carbon paper.

[0040] The thickness of the conductive porous substrate used in the porous transport layer of the present invention is preferably 0.4 mm or more. A thickness of 0.4 mm or more allows for uniform distribution of pressure in the in-plane direction, thereby increasing the compressive strength of the conductive porous substrate and increasing gas diffusion in the in-plane direction due to the increased number of gas diffusion paths. Furthermore, the increased contact area between the porous transport layer and the electrolyte membrane reduces contact electrical resistance, leading to improved electrolytic performance. Additionally, it prevents damage and deterioration of the electrolyte membrane due to localized pressure, improving the durability of the electrolyte membrane. The thickness of the conductive porous substrate is more preferably 0.5 mm or more, and even more preferably 1.0 mm or more. However, if the conductive porous substrate is too thick, the electrical resistance of the porous transport layer increases; therefore, the thickness of the conductive porous substrate is preferably 5.0 mm or less, and more preferably 3.0 mm or less. The thickness of the conductive porous substrate can be measured using a micrometer or the like while applying a load of 0.15 MPa. When measuring the thickness of a conductive porous substrate in a porous transport layer, the porous transport layer is heated in air at 420°C for one hour, then scraped off to remove the microporous layer, and only the conductive porous substrate is extracted, thereby allowing the thickness of the conductive porous substrate to be measured.

[0041] The conductive porous substrate used in the porous transport layer of the present invention preferably has a fluororesin content of 1.0% by mass or less. A fluororesin content of 1.0% by mass or less in the conductive porous substrate can improve the conductivity of the porous transport layer, and an improvement in electrolytic performance can be expected. The fluororesin content of the conductive porous substrate can be measured, for example, by thermal decomposition of the conductive porous substrate using pyrolysis gas chromatography-mass spectrometry (hereinafter also referred to as "pyrolysis GC / MS"). To measure the fluororesin content of the conductive porous substrate from the porous transport layer, the surface of the porous transport layer is scraped away until the microporous layer is gone, and only the conductive porous substrate is extracted. A fluororesin content of 0.1% by mass or less in the conductive porous substrate is more preferable.

[0042] Furthermore, it is more preferable that the conductive porous substrate used in the porous transport layer of the present invention is not treated with a fluororesin to repel water, that is, does not contain a fluororesin. By not using a conductive porous substrate that contains a fluororesin, the conductivity of the porous transport layer can be improved. In addition, water may accumulate in areas where the wettability between the microporous layer and the conductive porous substrate changes, which can inhibit the diffusion of the generated gas and reduce the electrolytic performance. Therefore, by using a conductive porous substrate that is not treated with a fluororesin to repel water, a decrease in electrolytic performance can be prevented.

[0043] The atomic ratio (F / C) of fluorine atoms to carbon atoms in the conductive porous substrate is preferably 0.05 or less. A F / C of 0.05 or less reduces the water repellency of the conductive porous substrate, thereby minimizing the difference in wettability between the conductive porous substrate and the microporous layer. Furthermore, because the amount of fluorine contained in the conductive porous substrate is reduced, the conductivity of the porous transport layer can be improved. A more preferable F / C of 0.03 or less is preferred, and even more preferable is 0.01 or less. The F / C of the conductive porous substrate can be controlled by adjusting the fluororesin content of the conductive porous substrate.

[0044] The fluorine atom content / carbon atom content of a conductive porous substrate can be calculated by observing the surface of the conductive porous substrate at 2,000x magnification using a scanning electron microscope (SEM), detecting and quantifying characteristic X-rays reflected from an electron beam irradiated at an accelerating voltage of 20 kV using energy-dispersive X-ray analysis (EDX), and then calculating the fluorine atom content / carbon atom content.

[0045] <Microporous Layer> The porous transport layer of the present invention has a microporous layer on at least one side of a conductive porous substrate. The microporous layer enhances the adhesion between the catalyst layer and the porous transport layer, improving conductivity, and also prevents damage to the electrolyte membrane due to fraying of the conductive porous substrate when fastened under high pressure in a water electrolysis cell. To achieve the above effects, the microporous layer has finer pores than the conductive porous substrate.

[0046] The microporous layer preferably contains conductive particles to improve conductivity. By including conductive particles, the conductivity of the porous transport layer can be improved, and an improvement in electrolytic performance can be expected.

[0047] As conductive particles, carbon materials are preferred due to their low cost, but other conductive particles may be included as needed. Examples of conductive particles made of carbon materials include carbon black, graphite particles, carbon nanotubes, carbon nanofibers, chopped carbon fibers, and graphene, with carbon black being particularly preferred due to its low impurity content. Other conductive particles include metal powders, foamed metals, and foamed metal alloys. Examples of metals constituting these include titanium, nickel, aluminum, alloys mainly composed of at least one of these metals, and stainless steel, with titanium and titanium alloys being particularly preferred. The surfaces of these materials may be coated with precious metals such as platinum as needed.

[0048] The primary particle size of the conductive particles is preferably 20 nm or more and 100 nm or less. When the primary particle size is 20 nm or more, the pore diameter of the microporous layer becomes larger when a microporous layer is formed, that is, a peak appears in the range of larger pore diameters in the logarithmic differential pore volume distribution, and the gas diffusion in the direction perpendicular to the surface of the microporous layer becomes higher, so the electrolysis performance of the water electrolysis cell using this microporous layer tends to improve. In addition, the viscosity of the composition for the microporous layer tends to decrease when forming the microporous layer, so the composition for the microporous layer tends to penetrate the conductive porous substrate more easily, and the conductivity can be increased. When the primary particle size of the conductive particles is 100 nm or less, the pores of the microporous layer are dense, so the surface quality of the microporous layer can be increased, and penetration into the electrolyte membrane by the conductive porous substrate can be suppressed. For these reasons, the primary particle size of the conductive particles is more preferably 60 nm or less, and even more preferably 40 nm or less.

[0049] Here, the primary particle size of the conductive particles is obtained by taking photographs of a cross-sectional observation sample prepared using an ion milling apparatus, magnified to more than 200,000 times using a microscope such as a scanning electron microscope, measuring the diameter of 10 randomly selected primary particles, and averaging them. For example, the IM4000 (manufactured by Hitachi High-Technologies Corporation) can be used as the ion milling apparatus.

[0050] In the porous transport layer of the present invention, the microporous layer preferably has a contact angle with water of 0° or more and less than 120°. A small contact angle between the microporous layer and water indicates that the microporous layer is not hydrophobic, and that hydrophilic liquids easily wet and spread within the microporous layer. Furthermore, although the lower limit of the contact angle with water in the present invention is 0°, a contact angle of 0° with water means that, in the measurement method described later, all the water is absorbed into the microporous layer after 1.0 second. A contact angle between the microporous layer and water of less than 120° has the effect of improving electrolytic performance. Preferably, the contact angle between the microporous layer and water is 60° or more, more preferably 90° or more, and even more preferably 100° or more; that is, the more the microporous layer has properties intermediate between hydrophilic and hydrophobic, the greater the effect of improving electrolytic performance. The present invention is not limited to theory, but the reason for this is presumed to be as follows.

[0051] In a water electrolysis cell, gas is generated during the electrolysis reaction, and the amount of gas generated is proportional to the current density. Therefore, a particularly large amount of gas is generated at high current densities. If the wetting of water is insufficient, the generated gas can obstruct the water supply, potentially drying out the electrolyte membrane and reducing electrolysis performance. Because the microporous layer is not hydrophobic, the water transport resistance within the microporous layer is reduced, allowing for a stable supply of water to the electrolyte membrane. As a result, even when a large amount of gas is generated at high current densities, the electrolyte membrane can maintain a moist state, thus improving electrolysis performance. Furthermore, if the hydrophilicity of the microporous layer is too high, the widespread wetting of water can block the gas discharge pathway, allowing the generated gas to permeate the electrolyte membrane, i.e., cross-leak, potentially causing a crossover where the generated gases mix. This crossover of generated gases triggers the generation of hydrogen peroxide through the reduction reaction of oxygen, and the generated hydrogen peroxide degrades the electrolyte membrane. Therefore, it is thought that excessive hydrophilicity in the microporous layer can reduce electrolysis performance. Furthermore, if the amount of cross-leakage of the generated gas increases, a runaway reaction between hydrogen and oxygen may occur, so it is necessary to suppress crossover from the standpoint of ensuring safety. For the reasons mentioned above, it is preferable that the microporous layer has properties intermediate between hydrophilicity and hydrophobicity.

[0052] The fluororesin content of the microporous layer of the porous transport layer of the present invention is preferably 0.1% by mass or more and 1.0% by mass or less. When the fluororesin content of the microporous layer is 1.0% by mass or less, the contact angle between the microporous layer and water can be reduced, and an improvement in electrolytic performance can be expected. In addition, since the amount of fluororesin with low conductivity can be reduced, the conductivity of the microporous layer can be improved. Furthermore, when the fluororesin content of the microporous layer is 0.1% by mass or more, the contact angle between the microporous layer and water can be increased, suppressing the crossover of generated gases and preventing a decrease in electrolytic performance. The fluororesin content of the microporous layer can be measured, for example, by thermal decomposition of the microporous layer powder scraped from the surface of the porous transport layer by thermal decomposition GC / MS.

[0053] Examples of fluororesins include polytetrafluoroethylene (PTFE), tetrafluoroethylene hexafluoropropylene copolymer (FEP), perfluoroalkoxyalkane (PFA), ethylene tetrafluoroethylene copolymer (ETFE), polyvinylidene fluoride (PVDF), polychlorotrifluoroethylene (PCTFE), polyvinyl fluoride (PVF), and perfluoropolyether (PFPE).

[0054] Furthermore, the F / C ratio of the microporous layer is preferably 0.001 or more and 0.050 or less. An F / C ratio of 0.050 or less in the microporous layer reduces the contact angle between the microporous layer and water, which is expected to improve electrolytic performance. Also, because the amount of fluorine in the microporous layer is low, the conductivity of the porous transport layer can be improved. An F / C ratio of 0.030 or less is more preferable, and 0.010 or less is even more preferable. An F / C ratio of 0.001 or more in the microporous layer increases the contact angle between the microporous layer and water, suppressing the crossover of generated gases and preventing a decrease in electrolytic performance. The F / C ratio of the microporous layer can be controlled by adjusting the fluororesin content of the microporous layer.

[0055] The F / C ratio of a microporous layer can be obtained by measuring the surface of the microporous layer using SEM and EDX, similar to how the F / C ratio of a conductive porous substrate is measured.

[0056] The thickness of the microporous layer is preferably 5 μm or more and 100 μm or less. When the thickness of the microporous layer is 5 μm or more, it can sufficiently cover the conductive porous substrate, increasing the contact area with the electrolyte membrane and thus improving electrolytic performance. In addition, it can suppress fuzzing of the conductive porous substrate, thus preventing damage to the electrolyte membrane and improving durability. The thickness of the microporous layer is more preferably 10 μm or more, and even more preferably 15 μm or more. Furthermore, when the thickness of the microporous layer is 100 μm or less, it can suppress the decrease in gas diffusion in the direction perpendicular to the surface. The thickness of the microporous layer is more preferably 75 μm or less, and even more preferably 50 μm or less.

[0057] The thickness of the microporous layer can be determined by measuring the thickness of the conductive porous substrate when it is placed on a smooth surface plate and a pressure of 0.15 MPa is applied, then similarly measuring the thickness of the porous transport layer on which the microporous layer is formed, and subtracting the thickness of the conductive porous substrate from the thickness of the porous transport layer. When measuring the thickness of the microporous layer from the completed porous transport layer, the porous transport layer is heated in air at 420°C for 1 hour, and then the microporous layer is scraped off and removed. The thickness can then be calculated from the difference in thickness before and after removal of the microporous layer, both measurements taken under a pressure of 0.15 MPa.

[0058] The microporous layer preferably contains a binder in addition to conductive particles. The binder enhances the bonding between the conductive materials in the microporous layer, suppressing the occurrence of cracks on the surface of the microporous layer during the firing process when manufacturing the porous transport layer. It also enhances the adhesion between the conductive porous substrate and the microporous layer, suppressing the peeling of the microporous layer from the conductive porous substrate.

[0059] It is preferable to use a thermoplastic resin as the binder. When forming a microporous layer by applying the microporous layer composition described later to a conductive porous substrate, heating to a temperature above the melting point of the thermoplastic resin during the drying and firing process allows the thermoplastic resin to wet and spread throughout the microporous layer. Subsequently, lowering the temperature firmly bonds the conductive particles within the microporous layer, thereby suppressing cracks and peeling of the microporous layer.

[0060] Examples of binders include carboxyl vinyl polymers, polyesters, polyether urethanes, polyvinyl alcohol, polyvinyl acetate, polyvinyl butyral, polyvinylpyrrolidone, polyacrylic acid, polyacrylamide, polyvinyl chloride, polyethyleneimine, polylactic acid, phenolic resins, polyphenylene oxide, ABS resins, epoxy resins, polycarbonates, and modified versions thereof. One or more of these may be used.

[0061] Among these, phenolic resin is preferred because it has high stability against water and can maintain good electrolytic performance without deterioration even when in contact with high-temperature water of 80-90°C during operation in a water electrolysis device for a long period of time.

[0062] Both thermoplastic and thermosetting phenolic resins can be used as the phenolic resin, but when using a thermosetting phenolic resin, it is preferable to use an uncrosslinked or partially crosslinked phenolic resin. Such phenolic resins are completely cured during the drying and firing process, firmly binding the conductive particles together. In particular, it is preferable to use a heat-melt-curable phenolic resin with a low degree of crosslinking because it has the characteristic of melting during the aforementioned drying and firing process, wetting and spreading between the conductive particles contained in the microporous layer, and then thermosetting. This characteristic can particularly improve the binding between conductive particles in the microporous layer and the adhesion between the microporous layer and the conductive porous substrate, thereby suppressing cracks and peeling of the microporous layer. The degree of crosslinking can be indicated by the weight-average molecular weight. It is preferable to use a phenolic resin with a weight-average molecular weight of 100,000 or less.

[0063] The microporous layer preferably has a mass loss rate of 5% by mass or more and 30% by mass or less when heated at 420°C for 1 hour. Heating the microporous layer at 420°C for 1 hour decomposes the binder, fluororesin, and undecomposed polymer dispersant in the microporous layer. In other words, the mass loss rate when heated at 420°C for 1 hour represents the content of these components. A mass loss rate of 5% by mass or more means that there is sufficient binder and fluororesin to bind the conductive particles in the microporous layer, and cracks and peeling of the microporous layer can be suppressed. A mass loss rate of 30% by mass or less means that the amount of binder and fluororesin added to the microporous layer can be reduced, and a decrease in conductivity can be suppressed. A mass loss rate of 25% by mass or less is more preferable, and 20% by mass or less is even more preferable. The mass loss rate can be calculated by heating the microporous layer powder scraped from the surface of the porous transport layer in an electric furnace at 420°C for 1 hour and measuring the mass before and after.

[0064] It is preferable that at least one of acetic acid, benzaldehyde, phenol or cresol is detected by pyrolysis GC / MS analysis from the microporous layer. Detection of at least one of acetic acid, benzaldehyde, phenol or cresol by pyrolysis GC / MS analysis indicates that a binder capable of firmly binding the microporous layer is contained in the micropores, thereby particularly improving the binding property between conductive particles and the adhesion between the microporous layer and the conductive porous substrate, and suppressing cracks and peeling of the microporous layer. Detection of at least one of acetic acid, benzaldehyde, phenol or cresol by pyrolysis GC / MS analysis indicates that the binder includes, for example, polyvinyl alcohol, polyvinyl acetate, polyvinyl butyral, phenol resin, polyphenylene oxide, ABS resin, epoxy resin, polycarbonate and modified products thereof.

[0065] Pyrolysis GC / MS can be measured by using a microporous layer powder obtained by scraping from the microporous layer of the porous transport layer, PY3030D (manufactured by Frontier Lab Co., Ltd.) and GCMS-QP2010 (manufactured by Shimadzu Corporation), and performing the measurement under the conditions of a heating temperature of 600 °C and a heating rate of 50 °C / min.

[0066] The atomic ratio (O / C) of oxygen atoms to carbon atoms in the microporous layer is preferably 0.03 or more and 0.10 or less. When the O / C of the microporous layer is 0.03 or more, the microporous layer tends to be well bound, and when the O / C of the microporous layer is 0.10 or less, an increase in the electrical resistance of the microporous layer can be suppressed. More preferably, the O / C of the microporous layer is 0.04 or more and 0.08 or less.

[0067] The O / C of the microporous layer can be obtained by calculating the oxygen atom content and carbon atom content on the surface of the microporous layer by SEM and EDX, and calculating the oxygen atom content / carbon atom content.

[0068] The basis weight of the microporous layer is preferably 10 g / m ２ or more and 30 g / m ２ or less. When the basis weight of the microporous layer is 10 g / m ２The above results in a high level of suppression of fuzzing of the conductive porous substrate, and an increase in the contact area with the electrolyte membrane, which is expected to improve electrolytic performance. The basis weight of the microporous layer is 15 g / m². ２ It is more preferable that the amount be greater than or equal to 20 g / m². ２ It is even more preferable that the basis weight of the microporous layer be 30 g / m². ２ The following conditions can suppress the decrease in gas diffusivity perpendicular to the surface due to excessively long gas diffusion pathways. Furthermore, it can suppress the increase in electrical resistance of the porous transport layer due to the thickness of the microporous layer. In addition, it can prevent a decrease in surface quality due to crack formation in the microporous layer.

[0069] The surface coverage of the microporous layer is preferably 95% or more and 100% or less. Here, the surface coverage of the microporous layer refers to the area percentage on the microporous layer side surface of the porous transport layer where the microporous layer is present. A surface coverage of 95% or more improves adhesion with the catalyst layer, and an improvement in electrolytic performance can be expected. On the other hand, a surface coverage of 100% or less is a natural upper limit by definition and cannot be exceeded. The surface coverage of the microporous layer can be measured using an optical microscope such as the VHX-6000 digital microscope manufactured by Keyence Corporation. On the microporous layer side surface of the porous transport layer, areas that are not surface-covered by the microporous layer, i.e., areas where the microporous layer does not exist, are areas where the conductive porous surface is exposed due to coating gaps or cracks, or where there are voids. Therefore, the brightness differs from areas where the microporous layer is present, and these areas can be clearly distinguished. Using an optical microscope, photographs were taken at 10x magnification for 10 arbitrary fields of view. The images were then binarized using image processing software such as HALCON, and the area of ​​the low-luminance portion was calculated from the total area of ​​the image (it is difficult to consider all minute areas with low luminance, so the area is set to 10 μm). ２ The surface coverage rate in each field of view can be calculated by dividing the area obtained by subtracting the above-mentioned portion by the total area of ​​the image. The surface coverage rates for 10 fields of view are determined, and their average value is adopted as the surface coverage rate of the microporous layer.

[0070] The gloss of the microporous layer with respect to incident light at an incident angle of 85° is preferably between 10 and 40. The gloss of the microporous layer with respect to incident light at an incident angle of 85° is an indicator of the surface quality of the microporous layer. When the gloss is 10 or higher, the surface of the microporous layer has high surface quality, good adhesion with the catalyst layer is obtained, and a more effective improvement in electrolysis performance can be expected. On the other hand, when the gloss is 40 or lower, the surface of the microporous layer is not excessively smoothed and has an appropriate roughness, and the fine roughness and void structure necessary for gas and water transport are appropriately maintained. As a result, gas diffusion and water management functions are well maintained, and excellent electrolysis performance is achieved more effectively. In this invention, the gloss of the microporous layer with respect to incident light at an incident angle of 85° is measured using the specular gloss at 85° (D85) as defined in JIS K5600-4-7, for example, using a gloss meter GM-1 manufactured by Suga Test Instruments Co., Ltd.

[0071] The surface roughness Sa of the microporous layer is preferably 0.1 μm or more and 5.0 μm or less. Here, Sa is the arithmetic mean height in the reference region and is calculated according to JIS B0681-2:2018. A surface roughness Sa of 5.0 μm or less improves adhesion with the catalyst layer, and an improvement in electrolytic performance can be expected. A surface roughness Sa of 4.0 μm or less is more preferable. Furthermore, a lower surface roughness Sa of the microporous layer is preferable, but from the perspective of simplicity of the porous transport layer fabrication process, it is preferable to have a surface roughness of 0.1 μm or more.

[0072] When the porous transport layer of the present invention is used in a water electrolysis apparatus, it is preferable that the microporous layer is insoluble in water, which is the target of electrolysis. If the microporous layer dissolves in water, cracks and delamination of the microporous layer may occur starting from the dissolved portion, reducing adhesion to the electrolyte membrane and potentially degrading electrolysis performance.

[0073] <Porous Transport Layer> The porous transport layer of the present invention has a ratio of pore volume of pores with a diameter of 10 μm to 100 μm to pore volume of pores with a diameter of 100 nm to 500 nm of 8 to 100. Here, pore volume refers to the pore volume per unit area of ​​the porous transport layer. Pores with a diameter of 100 nm to 500 nm mainly reflect pores originating from the microporous layer, and pores with a diameter of 10 μm to 100 μm mainly reflect pores originating from the conductive porous substrate. Furthermore, pores with a diameter of 500 nm to 10 μm reflect pores originating from regions where the microporous layer has permeated the conductive porous substrate. These pore diameter ranges function as indicators that appropriately capture the characteristics of each component and are effective in evaluating the pore volume ratio. When the ratio is 8 or higher, more preferably 20 or higher, and even more preferably 30 or higher, the pore volume of the conductive porous substrate portion is appropriately secured, good gas diffusion in the in-plane direction is obtained, and excellent electrolytic performance can be achieved more effectively. On the other hand, when the ratio is 100 or lower, more preferably 80 or lower, and even more preferably 60 or lower, the thickness of the conductive porous substrate is appropriately maintained, and high conductivity can be maintained. Furthermore, the density of the conductive porous substrate is controlled within an appropriate range, and sufficient compressive strength is ensured, resulting in mechanical stability during cell assembly. In addition, the microporous layer can uniformly and continuously cover the surface of the conductive porous substrate, and excellent adhesion with the catalyst layer allows for more effectively achieved excellent electrolytic performance.

[0074] The porous transport layer of the present invention preferably has a peak in the logarithmic differential pore volume distribution within the range of pore diameter 100 nm to 1 μm. Pores with a diameter of 100 nm to 1 μm correspond to pores in the microporous layer and the portion in which the microporous layer has permeated the conductive porous substrate. Having a peak within the above range means that the pores are of a size that allows for efficient discharge of water and gas from the portion containing the microporous layer, and an improvement in electrolytic performance can be expected. When the pore diameter peak is 100 nm or more, more preferably 150 nm or more, and even more preferably 200 nm or more, the permeability of water and gas is appropriately maintained, and good electrolytic performance can be obtained more effectively while suppressing gas crossover. When the pore diameter peak is 1 μm or less, more preferably 500 nm or less, and even more preferably 300 nm or less, pore blockage during compression can be prevented, and the permeability of water and gas in the direction perpendicular to the surface can be maintained more effectively. Furthermore, it is possible to maintain high adhesion with the catalyst layer, reduce the resistance of the water electrolysis cell, and achieve superior electrolysis performance more effectively. When employing a manufacturing method that forms a microporous layer by coating and firing the microporous layer composition described later, the areas where the polymer dispersant, binder, and fluororesin in the microporous layer composition were present become voids due to firing, thereby generating pores in the microporous layer. The pore diameter of the microporous layer also varies depending on the type and content of the polymer dispersant, binder, and fluororesin compounds, as well as the particle size and dispersion state of the conductive particles. When only fluororesin is used as the binder component for conductive particles, the position of the peak in the logarithmic differential pore diameter distribution tends to be in the range of smaller pores, and this tendency becomes stronger as the particle size of the conductive particles decreases. The pore diameter peak can be calculated by measuring the pore diameter distribution using a mercury intrusion porosimeter.

[0075] In the porous transport layer of the present invention, the ratio of the pore volume of pores with a diameter of 500 nm to 10 μm to the pore volume of pores with a diameter of 100 nm to 500 nm is preferably 0.5 to 10.0, more preferably 1.0 to 5.0, and even more preferably 1.5 to 3.0. When the above-mentioned ratio of pore volumes is 0.5 or more, more preferably 1.0 or more, and even more preferably 1.5 or more, the pores in the portion of the conductive porous substrate into which the microporous layer has permeated are sufficiently larger than the pores in the portion consisting only of the microporous layer, and the discharge of water and generated gases in the path from the microporous layer to the conductive porous substrate is facilitated, thus making it easier to more effectively improve electrolytic performance. Furthermore, when the above-mentioned ratio of pore volumes is 10.0 or less, more preferably 5.0 or less, and even more preferably 3.0 or less, excessive permeation of the microporous layer into the conductive porous substrate is suppressed, and the structure from the microporous layer to the conductive porous substrate is appropriately formed. This maintains the void structure on the surface of the microporous layer and ensures gas diffusion in the in-plane direction, allowing for smooth discharge of generated gas and improving electrolysis performance.

[0076] The porous transport layer of the present invention has a pore volume of 1.0 μL / cm² with a pore size of 100 nm to 500 nm. ２ 10.0 μL / cm or more ２ The following is preferable: Pores with a diameter of 100 nm to 500 nm mainly originate from a microporous layer and function as fine pathways responsible for the discharge of water and generated gases at the interface with the catalyst layer. Having the pore volume within the above range prevents film drying and gas accumulation, contributing to the stabilization of electrolytic performance.

[0077] The porous transport layer of the present invention has a pore volume of 2.0 μL / cm² with a pore size of 500 nm to 10 μm. ２ 30.0 μL / cm or more ２ The following is preferable: Pores with a diameter of 500 nm to 10 μm are intermediate voids formed in the region where the microporous layer has permeated the conductive porous substrate, and function as pathways for the migration of water and gas from the microporous layer to the substrate. By having the pore volume within the above range, gas diffusion in the in-plane direction is ensured, and an improvement in electrolytic performance can be expected.

[0078] The porous transport layer of the present invention has a pore size of 10 μm to 100 μm and a pore volume of 40 μL / cm³. ２ More than 200μL / cm ２ The following is preferable: Pores with a diameter of 10 μm to 100 μm mainly originate from the conductive porous substrate and function as the main pathway supporting the supply and discharge of water and gas throughout the cell. By having the pore volume within the above range, pressure loss is reduced and gas management is improved, contributing to the stabilization of electrolysis performance.

[0079] The pore volume of a porous transport layer can be measured using a mercury intrusion porosimeter. Specifically, pressure is applied to intrude mercury into the pores, and the pore size distribution can be determined from the pressure and the amount of mercury intruded. These methods allow for the determination of pore volume for each pore size. The logarithmic differential pore volume distribution can be obtained by plotting the result of dividing the increase in pore volume between two points by the difference in the common logarithms of the upper and lower values ​​of the corresponding pore size against the midpoint of the increase in pore size.

[0080] Furthermore, the pore volume for pore diameters A to B can be determined by subtracting the cumulative pore volume for pore diameters B and above from the cumulative pore volume for pore diameters A and above, and then dividing the result by the area of ​​the measurement sample.

[0081] The porous transport layer of the present invention preferably has a fluororesin content of 0.001% by mass or more and 0.100% by mass or less. By having a fluororesin content of 0.001% by mass or more, more preferably 0.005% by mass or more, the hydrophilicity of the porous transport layer is appropriately adjusted, and excessive wetting and spreading of water transported through the electrolyte membrane into the porous transport layer is suppressed. As a result, an appropriate discharge path for generated gases is secured, and good electrolytic performance is maintained more effectively. On the other hand, by having a fluororesin content of 0.100% by mass or less, more preferably 0.050% by mass or less, the water repellency of the conductive porous substrate and the microporous layer is maintained within an appropriate range, and the liquid retention capacity of the porous transport layer is sufficiently ensured. As a result, appropriate moisture supply to the electrolyte membrane is maintained, and drying of the electrolyte membrane is prevented, thereby more effectively obtaining excellent electrolytic performance. Furthermore, by controlling the content of fluororesin with low conductivity within an appropriate range, the high conductivity of the porous transport layer is maintained more effectively.

[0082] The fluororesin content of the porous transport layer can be calculated by adding the masses of fluororesin contained in the conductive porous substrate and the microporous layer composition, and dividing by the mass of the porous transport layer after firing, when the porous transport layer is prepared by coating a conductive porous substrate with a microporous layer composition. If only the porous transport layer is obtained, it can be measured, for example, by thermal decomposition of the porous transport layer using pyrolysis GC / MS.

[0083] The porous transport layer of the present invention preferably has a metal leaching amount of 10 ppm by mass or less as determined by leaching tests. A metal leaching amount of 10 ppm by mass or less suppresses the degradation of the electrolyte membrane and improves the durability of the water electrolysis cell. The metal leaching amount can be measured by an ICP mass spectrometer.

[0084] The porous transport layer of the present invention is preferably used as the porous transport layer on the cathode side in a water electrolysis cell. The cathode has a lower potential compared to the anode and does not require high oxidation resistance, so the porous transport layer of the present invention is preferable because it is less likely to corrode when used as the porous transport layer on the cathode side.

[0085] <Membrane Electrode Assembly> A membrane electrode assembly is one embodiment of the present invention. The membrane electrode assembly of the present invention refers to one that uses the porous transport layer of the present invention. That is, the membrane electrode assembly of the present invention can be formed by bonding the porous transport layer of the present invention to at least one side of an electrolyte membrane having catalyst layers on both sides. Arranging the porous transport layer so that the microporous layer side is in contact with the catalyst layer is preferable because it increases the contact area between the catalyst layer and the porous transport layer, reduces contact electrical resistance, and prevents damage to the electrolyte membrane. As the electrolyte membrane, one with high proton conductivity and oxidation resistance and low gas crossover is preferred. As the electrolyte membrane, one made of fluorine-based polymers or hydrocarbon-based polymers is known, and either of these can be used.

[0086] The film electrode assembly of the present invention is preferably composed of a cathode catalyst layer and an anode catalyst layer with different compositions. For example, the cathode catalyst layer preferably uses platinum, platinum-cobalt alloy, or platinum-ruthenium alloy as catalyst particles, while the anode catalyst layer preferably uses noble metals such as iridium, ruthenium, rhodium, palladium, or osmium, their alloys, or their oxides.

[0087] <Water Electrolysis Device> A water electrolysis device is one embodiment of the present invention. The water electrolysis device of the present invention refers to one that uses the porous transport layer of the present invention. That is, it has a water electrolysis cell having separators on both sides of the membrane electrode assembly of the present invention.

[0088] <Composition for Microporous Layers> A composition for microporous layers is one embodiment of the present invention. A microporous layer can be formed and a porous transport layer can be manufactured by applying the composition for microporous layers of the present invention to a conductive porous substrate and firing it, as described later. The composition for microporous layers of the present invention comprises conductive particles, water, a polymer dispersant, and a binder. Here, the polymer dispersant and the binder represent different compounds.

[0089] By including conductive particles in the microporous layer composition, the conductivity of the porous transport layer can be improved, and an improvement in electrolytic performance can be expected.

[0090] By including a polymeric dispersant in the microporous layer composition, the dispersibility of conductive particles in the microporous layer composition can be improved, and the viscosity of the microporous layer composition can be increased, thereby improving the uniformity of the coating. The polymeric dispersant referred to here is a compound with a molecular weight of 1,000 or more. Examples of polymeric dispersants include polyalkylene glycol, carboxyl vinyl polymer, polyester, polyvinyl alcohol, polyvinyl butyral, polyvinylpyrrolidone, polyacrylic acid, polyacrylate, polystyrene sulfonate, polyalkylene polyamine, polyvinylimidazoline, cellulose derivatives, alginate, guar gum, xanthan gum, carrageenan, pectin, gelatin, starch, etc., and one or more of these may be used.

[0091] Cellulose derivatives refer to compounds in which different substituents are introduced to the hydroxyl groups contained in cellulose molecules. Examples of cellulose derivatives include methylcellulose, ethylcellulose, carboxymethylcellulose, carboxyethylcellulose, hydroxyethylcellulose, hydroxypropylcellulose, hydroxyethylmethylcellulose, hydroxypropylmethylcellulose, and acetylcellulose.

[0092] The average degree of substitution of hydroxyl groups per glucose unit in the cellulose derivative is preferably 1.0 to 2.2. Having an average degree of substitution within this range improves water solubility and enhances the dispersibility of carbon in the microporous layer composition. The degree of substitution of the cellulose derivative can be measured according to the Japanese Pharmacopoeia.

[0093] The mass of the polymer dispersant contained in the microporous layer composition is preferably 5.0% by mass or less of the total mass of the microporous layer composition. Having a polymer dispersant mass of 5.0% by mass or less can suppress the increase in the electrical resistance of the porous transport layer. Alternatively, the mass of the polymer dispersant contained in the microporous layer composition is preferably 0.1% by mass or more. Having a polymer dispersant mass of 0.1% by mass or more can improve the dispersibility of conductive particles.

[0094] Furthermore, by including a binder in the microporous layer composition, the adhesive strength between conductive particles in the microporous layer after drying and firing can be improved, thereby suppressing cracks and delamination of the microporous layer.

[0095] When using a phenolic resin as a binder in the microporous layer composition of the present invention, it is preferable to use a liquid or particulate phenolic resin. When the phenolic resin is in liquid or particulate form, it can exist stably in the microporous layer composition, and precipitation due to aggregation and phase separation can be suppressed, thereby improving uniformity in coating.

[0096] When the phenolic resin is in particulate form, the particle size is preferably between 1 μm and 50 μm. A particle size of 1 μm or more allows for a stable dispersion state in the microporous layer composition, while a particle size of 50 μm or less suppresses cracks and surface roughness in the microporous layer.

[0097] The mass of the binder contained in the microporous layer composition is preferably 1.0% by mass or more and 20.0% by mass or less of the mass of the microporous layer composition. A binder mass of 1.0% by mass or more is sufficient to bind the conductive particles in the microporous layer, thereby suppressing cracks and delamination of the microporous layer. A binder mass of 20.0% by mass or less suppresses the increase in the electrical resistance of the porous transport layer. A binder mass of 2.0% by mass or more and 15.0% by mass or less is more preferable, and 3.0% by mass or more and 10.0% by mass or less is even more preferable.

[0098] The mass of the fluororesin contained in the microporous layer composition of the present invention is preferably 0.01% by mass or more and 0.50% by mass or less of the mass of the microporous layer composition. By setting the mass of the fluororesin to 0.01% by mass or more and 0.50% by mass or less, preferably 0.01% by mass or more and 0.30% by mass or less, and more preferably 0.01% by mass or more and 0.10% by mass or less of the mass of the microporous layer composition, a good balance between the liquid retention and drainage properties of the porous transport layer can be maintained, and a more effective improvement in electrolytic performance can be expected. Furthermore, the amount of fluororesin in the porous transport layer can be adjusted within the above-mentioned preferred range, and the conductivity of the porous transport layer can be more effectively improved.

[0099] <Method for Manufacturing a Porous Transport Layer> A method for manufacturing a porous transport layer is one aspect of the present invention. The method for manufacturing a porous transport layer of the present invention comprises the steps of applying the microporous layer composition of the present invention to a conductive porous substrate containing carbon fibers, and firing the conductive porous substrate coated with the microporous layer composition at a temperature of 200°C to 400°C. Before the firing step, there may be a drying step in which the conductive porous substrate is dried at a temperature below the firing temperature.

[0100] Preferred methods for applying the microporous layer composition to a conductive porous substrate include spray spraying, intaglio printing, screen printing, gravure printing, die coating, bar coating, blade coating, and knife coating.

[0101] In the above firing process, firing is carried out at a temperature of 200°C to 400°C. By firing at a temperature of 200°C or higher, the binder can be insoluble, and the peeling of the microporous layer can be suppressed. Furthermore, by firing at a temperature of 400°C or lower, the occurrence of cracks on the surface of the microporous layer and the peeling of the microporous layer due to excessive decomposition of the binder can be suppressed. A firing temperature of 250°C to 350°C is preferable.

[0102] The present invention will be described below with reference to examples and comparative examples.

[0103] [Measurement Method] (1) In an environment with a contact angle temperature of 20°C and humidity of 60%, 5 μL of deionized water droplets were dropped onto 10 randomly selected locations on the microporous layer. The contact angle was measured 1.0 second after dropping at each location using an automatic contact angle meter (DM-501, manufactured by Kyowa Interface Science Co., Ltd.). The average value of the 10 points was taken as the contact angle between the microporous layer and the water.

[0104] (2) Using a gloss meter (GM-1 manufactured by Suga Test Instruments Co., Ltd.), the specular gloss (D85) for incident light at an incident angle of 85° as defined in JIS K5600-4-7 was measured at 10 randomly selected points with the microporous layer facing upwards, and the average value of the gloss values ​​obtained at the 10 points was taken as the gloss of the microporous layer.

[0105] (3) Surface coverage was measured using an optical microscope (M205C, manufactured by Leica Microsystems Co., Ltd.) with the microporous layer facing upwards. Photographs were taken at 10x magnification for any 10 fields of view, and the images were binarized using image processing software (ImageJ). The area of ​​the entire image was measured in 10 μm increments. ２ The surface coverage ratio for each field of view was calculated by subtracting the area of ​​the low-luminance regions mentioned above and dividing the result by the total area of ​​the image. The surface coverage ratio for 10 fields of view was determined, and the average of these values ​​was taken as the surface coverage ratio of the microporous layer.

[0106] (4) Surface roughness The surface roughness was measured using a 3D shape measuring machine (VR-3200 manufactured by Keyence Corporation). The porous transport layer was fixed to the device with the microporous layer facing upwards, ensuring there were no lifting or wrinkles, and measured at 48 mm. ２ Within the specified field of view, measurements were taken at 10 arbitrary points on the surface of the microporous layer, and the arithmetic mean height Sa was calculated for all 10 points according to JIS B0681-2:2018. The average of the obtained 10 points was defined as the surface roughness of the microporous layer.

[0107] (5) Mass loss rate The mass loss rate of the microporous layer was calculated by scraping off the portion containing only the microporous layer from the surface of the porous transport layer using a spatula, ensuring that the conductive porous substrate was not included, and then heating 1.0 g of the resulting microporous layer powder in an electric furnace at 420°C for 1 hour and measuring the mass before and after heating.

[0108] (6) The position of the peak in the logarithmic differential pore volume distribution and the pore volume of the porous transport layer are 2 cm × 3 cm square and 6 cm ２ Five measurement samples were prepared by cutting the material into sections. Using the mercury intrusion method, the pore size distribution of each sample was obtained using an automated mercury porosimeter pore distribution analyzer (Shimadzu Corporation Autopore 9520) at measurement pressures ranging from 6 kPa to 123 MPa (pore size from 10 nm to 200 μm). In the resulting logarithmic differential pore volume distribution graph, the pore size of the maximum point where the logarithmic differential pore volume was maximized was determined as the position of the pore size peak among the maximum points in the region with a pore size of less than 1 μm that may contain a peak corresponding to the microporous layer. The position of the pore size peak was determined for each of the five measurement samples, and the average value was taken as the position of the pore size peak in the region with a pore size of less than 1 μm in the porous transport layer.

[0109] Furthermore, the pore volume in the range of 100 nm to 500 nm was determined by subtracting the cumulative pore volume in the range of pore diameters 500 nm and above from the cumulative pore volume in the range of pore diameters 100 nm and above, and then dividing by the area of ​​the measured sample. The pore volume was calculated for each of the five samples, and the average value was taken as the pore volume in the range of 100 nm to 500 nm in the porous transport layer. The pore volumes in the range of pore diameters 10 μm to 100 μm and the pore volumes in the range of pore diameters 500 nm to 10 μm were similarly calculated from the cumulative pore volume.

[0110] (7) Fluororesin Content The fluororesin content of the porous transport layer was calculated as follows. First, the fluororesin content in the microporous layer was calculated by multiplying the basis weight of the microporous layer by the ratio of the mass of PTFE to the mass of solid components (carbon black, binder, and PTFE) in the microporous layer after firing. When the conductive porous substrate was treated with a water-repellent coating, the fluororesin content in the conductive porous substrate was calculated from the change in mass of the conductive porous substrate before and after the water-repellent coating. The fluororesin content of the porous transport layer was calculated by adding the mass of fluororesin in the microporous layer and the mass of fluororesin in the conductive porous substrate and dividing by the mass of the porous transport layer after firing. The mass of each component after firing was determined from the mass reduction rate when each component was heated in advance under the same firing conditions of temperature and time.

[0111] (8) Liquid Retention After measuring the mass of a porous transport layer cut into 5 cm squares, it was immersed in water for 5 minutes and removed onto a cloth. Then, it was air-dried at 20°C for 2 minutes, and the mass of the porous transport layer was measured again. The water content of the porous transport layer was calculated by dividing the difference in mass before and after immersion by the area of ​​the porous transport layer. The water content value represents the degree of liquid retention; a high water content indicates a highly hydrophilic porous transport layer, and a low water content indicates a highly hydrophobic porous transport layer. An intermediate water content is preferable because it has properties intermediate between hydrophilic and hydrophobic, and the porous transport layer has an appropriate balance of liquid retention and drainage. Liquid retention was evaluated based on the water content value according to the following criteria. A is the most preferable range, B1 and B2 are the next most preferable ranges, and C1 and C2 are undesirable ranges. C1: Water content of the porous transport layer is 0.001 g / cm³ ２ Less than B1: Water content of porous transport layer is 0.001 g / cm³ ２ 0.010g / cm or more ２ Less than A: The water content of the porous transport layer is 0.010 g / cm³. ２ 0.050g / cm or more ２ Less than B2: Moisture content of porous transport layer is 0.050 g / cm³ ２ 0.100g / cm or more ２ Less than C2: The water content of the porous transport layer is 0.100 g / cm³. ２ That's all.

[0112] (9) Cut the short-circuit current density porous transport layer to a size of 40 mm x 40 mm, and place a high-density polyethylene film (50 mm x 50 mm, 10 μm thick) on top of the cut porous transport layer so that the centers of the porous transport layer and the high-density polyethylene film coincide, and then place these on top of each other with gold-plated smooth metal rigid electrodes (each with an area of ​​9 cm²). ２ The electrodes were sandwiched together, and an average pressure of 4.0 MPa was applied. In this state, when a current of 2.0 V was passed through the upper and lower electrodes, the current between the upper and lower electrodes was measured, and the obtained value was measured over an electrode area of ​​9 cm². ２ The short-circuit current density was calculated by dividing by the given value and judged based on the following criteria. A lower short-circuit current density indicates a porous transport layer that can prevent punctures. A: Short-circuit current density of 30 mA / cm² ２ Less than B: Short-circuit current density of 30 mA / cm² ２ 100mA / cm or more ２ Less than C: Short-circuit current density of 100 mA / cm² ２ That's all.

[0113] (10) The measurement was performed using a permeable pressure capillary flow porometer for through-pores (Porometter 3G, manufactured by Anton Paar GmbH). A sample of the porous transport layer punched out to a diameter of 25 mm was immersed in water and placed in the cylinder of the device using a special holder with a 1 / 2 inch diameter hole. The air pressure above the sample was increased, and the pressure at which water was first discharged from the bottom of the porous transport layer sample was defined as the permeable pressure. The smaller the permeable pressure value, the more easily water and gas can diffuse in the direction perpendicular to the surface of the porous transport layer. The following criteria were used to determine the permeable pressure value: A: Permeable pressure less than 300 kPa B: Permeable pressure 300 kPa or more and less than 400 kPa C: Permeable pressure 400 kPa or more.

[0114] (11) In-plane air permeability A porous transport layer punched out in the shape of a donut with an outer diameter of φ40 mm and an inner diameter of φ10 mm was subjected to a pressure of 2.0 MPa using a press device. Air was then flowed into the porous transport layer from the inside of the punched holes using a mass flow controller, and the pressure was measured as the flow rate was increased by 0.1 L / min increments. The ratio of the change in air flow rate to pressure was calculated from the following equation 1: I = ΔQ / ΔP × μ × ln(r０ / r ｉ ) / (2π × 60) × 10 １２ ... (Equation 1) In Equation 1, I is the in-plane air permeability (μm ３ Q is the airflow rate (L / min), P is the pressure (kPa), and μ is the air viscosity (1.8 × 10⁻¹⁰). －５ kg / (m・s)), r ０ The outer diameter of the sample (m), r ｉ The values ​​of , , and indicate the inner diameter (m) of the sample. Also, ΔQ / ΔP indicates the slope when the airflow rate and pressure are linearly approximated. A larger value of in-plane air permeability I indicates a porous transport layer that allows water and gas to diffuse easily in the in-plane direction. The following criteria were used to determine the value of in-plane air permeability I: A: In-plane air permeability I is 10,000 μm ３ Above B: In-plane air permeability I is 1,000 μm ３ 10,000 μm or more ３ Less than C: In-plane air permeability I is 1,000 μm ３ less than.

[0115] (12) A peel-off adhesive tape (Scotch® Mending Tape manufactured by 3M Company) was attached to the microporous layer surface of the porous transport layer using a pressure roller, and peeled off at a speed of 100 mm / min such that the angle between the tape attached to the microporous layer surface and the tape being pulled for peeling was 60 degrees. The peel-off area was defined as the value obtained by dividing the area where the conductive porous substrate was exposed after the microporous layer was peeled off from the porous transport layer by the area adhered to the tape.

[0116] (13) Electrolytic Performance In one embodiment of the present invention, a method for preparing and evaluating a membrane electrode assembly (MEA) for evaluating the electrolytic performance of a water electrolytic cell is shown below. First, to prepare a catalyst coating solution for the anode, 1.0 g of iridium oxide (TEC77100, manufactured by Tanaka Kikinzoku Kogyo Co., Ltd.) was sequentially mixed with 2.2 g of 5% by mass Nafion® dispersion solution (manufactured by Sigma-Aldrich Co., LLC) and 8.0 g of a mixed solvent of water:isopropyl alcohol = 1:1 to obtain a homogeneous catalyst coating solution. Similarly, as a catalyst coating solution for the cathode, 1.0 g of platinum-supported carbon catalyst (TEC10E50E, manufactured by Tanaka Kikinzoku Kogyo Co., Ltd., supported amount: 50% by mass) was sequentially mixed with 6.6 g of 5% by mass Nafion® dispersion solution and 5.7 g of a mixed solvent of water:isopropyl alcohol = 1:1 to obtain a homogeneous coating solution.

[0117] Each of the obtained catalyst coatings was applied to a PTFE sheet cut to 50 mm x 50 mm and dried at room temperature, resulting in an iridium loading of 1.0 mg / cm². ２ The platinum load amounts were 0.5 mg / cm³, respectively. ２ A PTFE sheet with a catalyst layer was prepared. Next, NR117 (manufactured by The Chemours Company, film thickness: 183 μm) was used as the electrolyte membrane and cut to 80 mm x 80 mm. The obtained electrolyte membrane was sandwiched between the above-mentioned PTFE sheets with anode and cathode catalyst layers and pressed for 5 minutes under conditions of 130°C and 4 MPa to transfer the catalyst layer to both sides of the electrolyte membrane. After pressing, the PTFE sheets were peeled off to obtain a solid polymer electrolyte membrane having catalyst layers on both sides.

[0118] A membrane electrode assembly (MEA) was fabricated using this solid polymer electrolyte membrane with a catalyst layer. A titanium sintered body Ti-67 / 200 (manufactured by NV Bekaert SA, 0.20 mm thick) was used as the electrode on the anode side, and the porous transport layer described above was used on the cathode side. The porous transport layer was arranged so that the microporous layer was in contact with the catalyst layer. The MEA was obtained by pressing this laminate at 130°C and 1 MPa for 5 minutes.

[0119] The obtained MEA was incorporated into a single cell for water electrolysis evaluation, and purified water was supplied only to the anode side at a flow rate of 100 mL / min. The cell temperature was set to 80°C, and the current density was 2.0 A / cm². ２ The system was operated continuously for 48 hours, and the cell voltage at the end of the 48-hour operation was recorded as an indicator of electrolytic performance.

[0120] [Example 1] A composition for a microporous layer was prepared using carbon black (CB) (primary particle size 24 nm) as conductive particles, carboxymethylcellulose (CMC) (DN-400H; manufactured by Daicel Mirise Co., Ltd.) as a polymer dispersant, polyvinyl alcohol (PVA) (polyvinyl alcohol 1,000% partially saponified type; manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) as a binder, and polytetrafluoroethylene (PTFE) ("Polyflon®" D-210C; manufactured by Daikin Industries, Ltd.; containing 60% by mass of PTFE in the dispersion medium (water)) and purified water as a water repellent. At this time, the composition was prepared by adjusting the mass ratio of CB / polymer dispersant / binder / PTFE / water to 7.0 parts by mass / 2.5 parts by mass / 4.0 parts by mass / 0.08 parts by mass / 86.42 parts by mass. The above microporous layer composition is applied to 2.0 mm thick carbon paper (conductive porous substrate, basis weight 1,050 g / m²). ２ The amount of solid content (CB, binder, and PTFE) after firing is 25 g / m². ２ The porous transport layer was obtained by coating with a slit die coater, drying at 120°C, and then firing at 320°C. The measurement results for the obtained porous transport layer are shown in Table 1. The water permeability pressure was A.

[0121] [Example 2] A porous transport layer was obtained in the same manner as in Example 1, except that a thermosetting phenolic resin (Phenolite® TD-2250; manufactured by DIC Corporation) was used as a binder. The measurement results are shown in Table 1.

[0122] [Example 3] A porous transport layer was obtained in the same manner as in Example 1, except that a thermoplastic phenolic resin (Tamanol® 7509; manufactured by Arakawa Chemical Co., Ltd.) was used as a binder. The measurement results are shown in Table 1. The water permeability pressure was B.

[0123] [Example 4] A porous transport layer was obtained in the same manner as in Example 1, except that polyether urethane (Riken Resin NPH-76; manufactured by Miki Riken Kogyo Co., Ltd.) was used as a binder. The measurement results are shown in Table 1.

[0124] [Example 5] A porous transport layer was obtained in the same manner as in Example 1, except that polyphenylene oxide (XYRON® 300H; manufactured by Asahi Kasei Corporation) was used as a binder. The measurement results are shown in Table 1.

[0125] [Example 6] A porous transport layer was obtained in the same manner as in Example 1, except that the mass of the binder was 8.0 parts by mass. The measurement results are shown in Table 2. The water permeability pressure was A.

[0126] [Example 7] When applying the composition for the microporous layer, the basis weight of the solid content after firing was 70 g / m². ２ A porous transport layer was obtained in the same manner as in Example 1, except that the amount of coating was adjusted accordingly. The measurement results are shown in Table 2. The water permeability pressure was B.

[0127] [Example 8] When applying the composition for the microporous layer, the basis weight of the solid content after firing was 50 g / m². ２ A porous transport layer was obtained in the same manner as in Example 1, except that the amount of coating was adjusted accordingly. The measurement results are shown in Table 2.

[0128] [Example 9] When applying the composition for the microporous layer, the basis weight of the solid content after firing should be 8 g / m². ２ A porous transport layer was obtained in the same manner as in Example 1, except that the amount of coating was adjusted accordingly. The measurement results are shown in Table 2. The water permeability pressure was A.

[0129] [Example 10] A 0.40 mm thick carbon paper (basis weight 190 g / m²) was used as the conductive porous substrate. ２ A porous transport layer was obtained in the same manner as in Example 1, except that the material used was [material name]. The measurement results are shown in Table 2. The water permeability pressure was A.

[0130] [Example 11] A porous transport layer was obtained in the same manner as in Example 1, except that it did not contain a binder and the mass ratio of the water repellent was 4.0 parts by mass. The measurement results are shown in Table 3. The water permeability pressure was C.

[0131] [Examples 12-14] A porous transport layer was obtained in the same manner as in Example 1, except that the composition of the microporous layer composition was changed as shown in Table 1. The measurement results are shown in Table 3.

[0132] [Example 15] A porous transport layer was obtained in the same manner as in Example 1, except that carbon black (CB) (primary particle size 75 nm) was used as conductive particles, no binder was used, and the mass of the water repellent was 4.0 parts by mass. The measurement results are shown in Table 4. The water permeability pressure was A.

[0133] [Example 16] Before applying the microporous layer composition, the carbon paper was immersed in a diluted aqueous solution of PTFE and then dried at 120°C to provide a water-repellent treatment. Otherwise, a porous transport layer was obtained in the same manner as in Example 1. After the water-repellent treatment, the amount of PTFE attached to the carbon paper was 5.0% by mass relative to 100.0% by mass of the carbon paper after the water-repellent treatment. The measurement results are shown in Table 4.

[0134] [Comparative Example 1] A conductive porous substrate was made of carbon paper with a thickness of 0.15 mm (basis weight 40 g / m²). ２ A porous transport layer was obtained in the same manner as in Example 1, except that the material used was [material name]. The measurement results are shown in Table 4. The water permeability pressure was A.

[0135] [Comparative Example 2] Carbon paper with a thickness of 2.0 mm (basis weight 1,050 g / m²) ２ Without applying a microporous layer composition to the carbon paper, the carbon paper itself was treated as the porous transport layer. The measurement results are shown in Table 4. At this time, no measurement items related to the microporous layer were performed. Although the contact angle is originally a measurement item related to the microporous layer, in this Comparative Example 2, the value measured on the conductive porous substrate (carbon paper) is recorded. Regarding the electrolytic evaluation, the 48h measurement could not be performed because the voltage limit was exceeded during the measurement. Also, the water permeability pressure was A.

[0136]

[0137]

[0138]

[0139]

[0140] The porous transport layer of the present invention suppresses penetration of the electrolyte membrane by the conductive porous substrate, and enables the creation of electrodes for a water electrolysis apparatus with excellent gas diffusion properties, thereby providing a water electrolysis apparatus that exhibits high electrolysis performance. Furthermore, the porous transport layer of the present invention is CO ２ It can also be used in electrolytic devices or ammonia electrolytic synthesis devices, etc. The resulting electrodes are particularly suitable for use as electrodes in PEM-type water electrolytic devices.

Claims

1. A porous transport layer comprising a conductive porous substrate containing carbon fibers, having a microporous layer on at least one side, wherein the ratio of the volume of pores with a diameter of 10 μm to 100 μm to the volume of pores with a diameter of 100 nm to 500 nm is 8 or more and 100 or less.

2. The porous transport layer according to claim 1, wherein the logarithmic differential pore volume distribution has a peak in the range of pore diameters from 100 nm to 1 μm.

3. The porous transport layer according to claim 2, wherein the logarithmic differential pore volume distribution has a peak in the range of pore diameters from 100 nm to 500 nm.

4. The porous transport layer according to claim 1 or 2, wherein the ratio of the pore volume of pores with a diameter of 500 nm to 10 μm to the pore volume of pores with a diameter of 100 nm to 500 nm is 0.5 or more and 10.0 or less.

5. Pore volume of 1.0 μL / cm² for pores with a diameter of 100 nm to 500 nm. ２ 10.0 μL / cm or more ２ The porous transport layer according to claim 1 or 2, which is as follows:

6. Pore volume of 2.0 μL / cm² for pores with a diameter of 500 nm to 10 μm. ２ 30.0 μL / cm or more ２ The porous transport layer according to claim 1 or 2, which is as follows:

7. Pore volume of 40 μL / cm³ for pores with a diameter of 10 μm to 100 μm. ２ More than 200μL / cm ２ The porous transport layer according to claim 1 or 2, which is as follows:

8. The porous transport layer according to claim 1 or 2, wherein the thickness of the conductive porous substrate is 0.4 mm or more and 5.0 mm or less.

9. The porous transport layer according to claim 1 or 2, wherein the microporous layer comprises conductive particles made of carbon material.

10. The porous transport layer according to claim 9, wherein the microporous layer further comprises a binder.

11. The porous transport layer according to claim 9, wherein the primary particle size of the conductive particles is 20 nm or more and 100 nm or less.

12. The basis weight of the microporous layer is 10 g / m². ２ 30g / m or more ２ The porous transport layer according to claim 1 or 2, which is as follows:

13. The porous transport layer according to claim 1 or 2, wherein the surface coverage rate of the microporous layer is 95% or more and 100% or less.

14. The porous transport layer according to claim 1 or 2, wherein the contact angle between the microporous layer and water is 0° or more and less than 120°.

15. The porous transport layer according to claim 14, wherein the contact angle between the microporous layer and water is 90° or more and less than 120°.

16. The porous transport layer according to claim 1 or 2, wherein the fluororesin content is 0.001% by mass or more and 0.100% by mass or less.

17. The porous transport layer according to claim 1 or 2, wherein the mass loss rate when the microporous layer is heated at 420°C for 1 hour is 5% by mass or more and 30% by mass or less.

18. The porous transport layer according to claim 1 or 2, wherein at least one of acetic acid, benzaldehyde, phenol, or cresol is detected in the microporous layer by analysis by pyrolysis gas chromatography-mass spectrometry.

19. The porous transport layer according to claim 1 or 2, wherein the glossiness of the microporous layer with respect to incident light at an incident angle of 85° is 10 or more and 40 or less.

20. The porous transport layer according to claim 1 or 2, wherein the surface roughness Sa of the microporous layer is 0.1 μm or more and 5.0 μm or less.

21. A membrane electrode assembly having the porous transport layer according to claim 1 or 2.

22. A water electrolysis apparatus having the porous transport layer described in claim 1 or 2.

23. A composition for forming a microporous layer used in the production of a porous transport layer having a microporous layer on at least one side of a conductive porous substrate containing carbon fibers, comprising conductive particles, water, a polymer dispersant, a binder, and a fluororesin, wherein the mass of the fluororesin contained in the composition for forming the microporous layer is 0.01% by mass or more and 0.50% by mass or less of the mass of the composition for forming the microporous layer.

24. The composition for a microporous layer according to claim 23, wherein the mass of the polymer dispersant is 0.1% by mass or more and 5.0% by mass or less of the mass of the composition for a microporous layer.

25. A method for manufacturing a porous transport layer, comprising the steps of: applying the microporous layer composition described in claim 23 to a conductive porous substrate containing carbon fibers; and firing the conductive porous substrate coated with the microporous layer composition at a temperature of 200°C to 400°C.

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