Porous conveying layer for liquid electrolysis and method of producing the same

Abstract

To provide a porous conveying layer for liquid electrolysis, satisfying all of: the uniformity of weight per unit area in the in-plane direction; the smoothness of surface; and the peeling resistance (permeation amount).SOLUTION: The porous conveying layer for liquid electrolysis, comprises a microporous layer at least on a surface of an electroconductive porous substrate including a carbon fiber, with an arithmetic average height Sa of the surface of the microporous layer being 8 μm or lower.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a porous transport layer used in a membrane electrode assembly of a liquid electrolysis device, and a method for producing the same. [Background technology]

[0002] Currently, in order to reduce greenhouse gas emissions, a method of producing hydrogen that can be stored for a long period of time by electrolyzing liquids using renewable energy is attracting attention. Because liquid electrolysis produces hydrogen using only electricity and the liquid to be electrolyzed, it has almost no environmental impact and is considered a promising means of stably storing energy. Liquids that can be electrolyzed include water, formic acid, and liquid ammonia, and research into water electrolysis in particular is currently being actively conducted. Water electrolysis devices include polymer electrolyte membrane (PEM) water electrolysis devices, alkaline water electrolysis devices, and solid oxide water electrolysis devices. In particular, PEM water electrolysis devices are being widely researched and developed because they can utilize polymer electrolyte fuel cell (PEFC) technology and can produce high-purity hydrogen.

[0003] A PEM water electrolysis cell consists of a membrane electrode assembly (MEA) sandwiched between porous transport layers (PTLs) on both sides of a solid polymer electrolyte membrane (PEEM). The resulting MEA is then sandwiched between separators. Conductive porous substrates for the porous transport layers include porous substrates made of carbon fiber (e.g., carbon paper or carbon felt) and sintered metal porous bodies (e.g., titanium) for their electrical conductivity and mechanical strength. While sintered metal porous bodies have the advantage of being corrosion-resistant, they are expensive. Therefore, porous substrates made of carbon fiber are sometimes used on the cathode side, which is relatively corrosion-resistant. However, because water electrolysis cells are fastened together under high pressure, when using porous substrates made of carbon fiber, fluff on the surface of the porous substrate can penetrate the polymer electrolyte membrane, causing short circuits and degradation of the polymer electrolyte membrane originating from the penetration points, resulting in reduced water electrolysis performance. Therefore, to prevent damage to the polymer electrolyte membrane and enhance durability, a microporous layer is sometimes provided on the conductive porous substrate as a porous transport layer.

[0004] The thickness of the microporous layer affects water electrolysis performance, durability, quality, and cost, so it must be uniformly formed on the porous substrate at the specified thickness and basis weight. The surface smoothness of the microporous layer also significantly affects adhesion to the electrolyte membrane and durability. Furthermore, the penetration of the microporous layer into the porous substrate also needs to be controlled because it affects the peel strength between the microporous layer and the porous transport layer. One method for forming a microporous layer on a porous substrate is to apply a composition for a microporous layer to the porous substrate. However, porous substrates made of carbon fibers that can be used for a porous transport layer have large thickness variations and warpage in an unpressurized state due to their structure and manufacturing process, making it difficult to apply a composition for a microporous layer to a uniform thickness. In this case, one method is to reduce the viscosity of the paint to smooth the surface of the paint after application, but this increases penetration and reduces the gas diffusion properties of the porous transport layer. Another method is to add a leveling agent, but this may deteriorate the physical properties of the porous transport layer and have a negative impact on water electrolysis performance and durability.

[0005] As a method for smoothing a composition for a microporous layer after application, Patent Document 1 discloses a method of applying microvibrations to an uncured coating film to promote smoothing of the coating film. Patent Document 2 discloses a method of obtaining a printing pattern with a high aspect ratio by applying ultrasonic vibrations to a paste during screen printing. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 5-154441 [Patent Document 2] Japanese Patent Application Laid-Open No. 2010-149301 Summary of the Invention [Problem to be solved by the invention]

[0007] The method disclosed in Patent Document 1 is a method for smoothing an uncured coating film on a void-free substrate such as glass, metal, or plastic, and does not describe or suggest application to a porous substrate such as the conductive porous substrate of the present application. Furthermore, there is no mention of penetration of the coating film into the porous substrate.

[0008] Patent Document 2 also does not describe or suggest application to porous substrates such as conductive porous substrates, nor does it mention the penetration of the paste into porous substrates. [Means for solving the problem]

[0009] The present invention solves the above problems and employs the following means to form a microporous layer that has uniform basis weight and thickness, good smoothness, and high peel resistance. (1) A porous transport layer for liquid electrolysis, comprising a microporous layer on at least one surface of a conductive porous substrate containing carbon fibers, the microporous layer having an arithmetic mean surface height Sa of 8 μm or less. (2) The porous transport layer for liquid electrolysis according to (1), having a thickness of 100 μm or more. (3) The porous transport layer for liquid electrolysis according to (1) or (2), wherein the thickness of the microporous layer is 10 μm or more and 200 μm or less. (4) The porous transport layer for liquid electrolysis according to any one of (1) to (3), wherein the thickness of the microporous layer at the portion where it penetrates into the conductive porous substrate is 10 μm or more and 100 μm or less. (5) A membrane electrode assembly using the porous transport layer for liquid electrolysis according to any one of (1) to (4). (6) A liquid electrolysis device using the porous transport layer for liquid electrolysis according to any one of (1) to (4). (7) A method for producing a porous transport layer for liquid electrolysis, comprising: a coating step of applying a microporous layer composition to a conductive porous substrate containing carbon fiber to form a microporous layer; a vibration step of applying vibrations to the microporous layer after the coating step to smooth the surface of the microporous layer; and a sintering step of sintering the conductive porous substrate after the vibration step to form a porous transport layer for liquid electrolysis. (8) The method for producing a porous transport layer for liquid electrolysis according to (7), wherein the thickness of the part of the microporous layer that has penetrated into the conductive porous substrate is set to 10 μm or more and 100 μm or less by the vibration step. [Effects of the Invention]

[0010] According to the present invention, a porous transport layer for liquid electrolysis can be obtained which has a microporous layer with high surface smoothness, uniform thickness and basis weight, and good peeling resistance. DETAILED DESCRIPTION OF THE INVENTION

[0011] The porous transport layer for liquid electrolysis of the present invention has a microporous layer on at least one side of a conductive porous substrate containing carbon fibers. The above elements constituting the porous transport layer for liquid electrolysis are described below.

[0012] <Conductive porous base material> The conductive porous substrate used in the porous transport layer for liquid electrolysis of the present invention contains carbon fiber. Specific examples of conductive porous substrates containing carbon fiber include carbon paper, carbon felt, carbon fiber woven fabric, carbon fiber paper, and carbon fiber nonwoven fabric. Among these, substrates containing resin carbide are preferred because of their excellent "springiness," i.e., ability to absorb dimensional changes in the thickness direction of the electrolyte membrane. It is particularly preferred to use a substrate obtained by binding a carbon fiber paper with a binder, i.e., carbon paper.

[0013] The binder contained in the conductive porous substrate refers to a component other than the carbon fiber in the conductive porous substrate, and refers to a component that mainly serves to bind the carbon fibers together. As the binder, a resin composition or a carbonized product thereof is preferably used.

[0014] Examples of the resin used in the resin composition include thermosetting resins such as phenolic resins, epoxy resins, melamine resins, and furan resins.

[0015] Examples of carbon fibers include polyacrylonitrile (PAN)-based carbon fibers, pitch-based carbon fibers, and rayon-based carbon fibers. PAN-based carbon fibers and pitch-based carbon fibers are preferred due to their excellent mechanical strength. They may also be mixed with natural or synthetic fibers, such as rayon, acrylic, or cellulose fibers.

[0016] The thickness of the conductive porous substrate in the present invention is preferably 200 μm or more. A conductive porous substrate having a thickness of 200 μm or more can distribute pressure uniformly in the in-plane direction, thereby increasing the contact area between the porous transport layer for liquid electrolysis and the electrolyte membrane, reducing contact electrical resistance, and improving electrolysis performance. Furthermore, it is possible to prevent breakdown or deterioration of the electrolyte membrane due to localized pressure, thereby improving the durability of the electrolyte membrane. The thickness of the conductive porous substrate is more preferably 500 μm or more, and even more preferably 1,000 μm or more. Furthermore, the thickness of the conductive porous substrate is preferably 5,000 μm or less, and more preferably 3,000 μm or less. A conductive porous substrate having a thickness of 5,000 μm or less can reduce electrical resistance.

[0017] The thickness of the conductive porous substrate can be measured using a micrometer or the like while applying a load of 0.15 MPa. The thickness of the conductive porous substrate in the porous transport layer for liquid electrolysis can be measured by heating the porous transport layer for liquid electrolysis to a temperature at which the binder remaining in the microporous layer can be removed, and then removing the microporous layer by scraping or blowing with air, etc., to obtain the conductive porous substrate. The thickness of the conductive porous substrate can then be measured using the same method as above.

[0018] <Microporous layer> In the porous transport layer for liquid electrolysis of the present invention, the microporous layer has the effect of increasing the adhesion between the catalyst layer and the porous transport layer for liquid electrolysis and improving the electrical conductivity, as well as the effect of preventing damage to the electrolyte membrane due to fluffing of the conductive porous substrate.

[0019] The microporous layer preferably contains conductive particles to improve conductivity. By containing conductive particles, the conductivity of the porous transport layer for liquid electrolysis can be improved, and improved electrolysis performance can be expected.

[0020] As the conductive particles, conductive particles made of a carbon material are preferred because of their low cost, but other conductive particles may also be included as necessary. Examples of conductive particles made of a carbon material include carbon black, graphite particles, carbon nanotubes, carbon nanofibers, chopped carbon fibers, and graphene. Carbon black is particularly preferred because it contains few impurities.

[0021] The thickness of the microporous layer is preferably 10 μm to 200 μm, more preferably 30 μm to 150 μm, even more preferably 50 μm to 100 μm, and most preferably 60 μm to 100 μm. A microporous layer thickness of 10 μm or more can adequately cover the conductive porous substrate, increasing the contact area with the electrolyte membrane and thereby improving electrolysis performance. Furthermore, damage to the electrolyte membrane due to fluffing of the conductive porous substrate can be prevented, improving durability. Furthermore, a microporous layer thickness of 200 μm or less can suppress an increase in the electrical resistance of the porous transport layer for liquid electrolysis.

[0022] The thickness of the microporous layer can be calculated by measuring the thickness of the porous transport layer for liquid electrolysis by the method described below, measuring the thickness of the conductive porous substrate by the method described above, and then calculating the difference between the two.

[0023] The microporous layer has a basis weight of 10g / m 2 More than 100g / m 2 Preferably, it is 20 g / m or less. 2 More than 80g / m 2 More preferably, it is 20 g / m or less. 2 More than 50g / m 2 It is more preferable that the basis weight of the microporous layer is 10 g / m or less. 2 When the microporous layer has a mass of 100 g / m or more, the conductive porous substrate can be sufficiently covered, and the contact area with the electrolyte membrane can be increased, which is expected to improve electrolysis performance. 2 When the content is equal to or less than this, an increase in the electrical resistance of the porous transport layer for liquid electrolysis can be suppressed.

[0024] The basis weight of the microporous layer can be calculated by measuring the basis weight of the porous transport layer for liquid electrolysis, then removing the microporous layer from the porous transport layer for liquid electrolysis to obtain a conductive porous substrate, in the same manner as when measuring the thickness of the conductive porous substrate described above, measuring the basis weight of the conductive porous substrate, and determining the difference between the basis weight of the porous transport layer for liquid electrolysis and the basis weight of the conductive porous substrate.

[0025] The gloss of the microporous layer to incident light at an incidence angle of 85° is preferably 10 or more. The gloss to incident light at an incidence angle of 85° is an index of the surface quality of the microporous layer, and a gloss of 10 or more ensures that the surface of the microporous layer has high surface quality as a porous transport layer for liquid electrolysis. In the present invention, the gloss to incident light at an incidence angle of 85° refers to the specular gloss at 85° (D85) specified in JIS K5600-4-7:1999, and can be measured, for example, using a gloss meter GM-1 manufactured by Suga Test Instruments Co., Ltd.

[0026] In the porous transport layer for liquid electrolysis of the present invention, the arithmetic mean height Sa of the surface of the microporous layer is 8 μm or less. It is preferably 5 μm or less, and more preferably 4 μm or less. A surface roughness Sa of 8 μm or less improves adhesion to the catalyst layer, and is expected to improve electrolysis performance. Here, Sa is the arithmetic mean height in the reference area, and can be measured using a 3D shape measuring device (VR-3200 manufactured by Keyence Corporation) or the like. During measurement, the porous transport layer for liquid electrolysis is fixed to the device with the microporous layer facing up, ensuring that there are no lifting or wrinkles. Thereafter, a 48 mm 2 The arithmetic mean height Sa is measured according to JIS B0681-2: 2018 at any 10 points on the surface of the microporous layer within the field of view. The average of the arithmetic mean heights Sa of the 10 points obtained is defined as the arithmetic mean height Sa of the surface of the microporous layer.

[0027] The microporous layer is preferably insoluble in the liquid to be electrolyzed. If the microporous layer dissolves in the liquid, cracks or peeling of the microporous layer may occur from the dissolved portion, reducing adhesion to the electrolyte membrane and potentially reducing electrolytic performance.

[0028] To enhance the diffusibility of gases perpendicular to and within the surface of the conductive porous substrate, it is preferable that the microporous layer does not penetrate too deeply into the conductive porous substrate. Specifically, the thickness of the microporous layer that penetrates into the conductive porous substrate is preferably 10 μm or more and 100 μm or less, more preferably 10 μm or more and 50 μm or less, and even more preferably 10 μm or more and 30 μm or less. When the thickness of the penetrated portion is 100 μm or less, the diffusibility of gases perpendicular to and within the surface of the conductive porous substrate can be enhanced. On the other hand, if the amount of penetration is too small, the adhesion between the substrate and the microporous layer may be weakened. Therefore, the thickness of the microporous layer that penetrates into the conductive porous substrate is preferably 10 μm or more.

[0029] The thickness of the part of the microporous layer that has penetrated into the conductive porous substrate is calculated by subtracting the thickness of the porous transport layer from the sum of the thickness of the microporous layer when applied and sintered under the same conditions on a substrate that has no penetration (such as a polyimide film ("Kapton (registered trademark)" manufactured by Toray DuPont Co., Ltd.)) and the thickness of the conductive porous substrate.

[0030] <Porous transport layer for liquid electrolysis> The thickness of the porous transport layer for liquid electrolysis of the present invention is preferably 100 μm or more to achieve both electrical resistance and spring properties, and is preferably 5,000 μm or less, more preferably 3,000 μm or less.

[0031] <Membrane electrode assembly> The membrane electrode assembly of the present invention can be formed by bonding the porous transport layer for liquid electrolysis of the present invention to at least one side of an electrolyte membrane having catalyst layers on both sides. Arranging the microporous layer side of the porous transport layer for liquid electrolysis on the catalyst layer side is preferred because it increases the contact area between the catalyst layer and the porous transport layer for liquid electrolysis, reduces contact electrical resistance, and prevents damage to the electrolyte membrane.

[0032] The electrolyte membrane is preferably one that has high proton conductivity and oxidation resistance and small gas crossover. Specifically, those made of fluorine-based polymers and hydrocarbon-based polymers are known, and either of these can be used.

[0033] In the membrane electrode assembly of the present invention, the cathode catalyst layer and the anode catalyst layer preferably have different compositions. For example, the cathode catalyst layer preferably uses platinum as catalyst particles, while the anode catalyst layer preferably uses a noble metal such as iridium, ruthenium, rhodium, or palladium, or an oxide thereof.

[0034] <Liquid electrolyzer> The liquid electrolysis device of the present invention includes the porous transport layer for liquid electrolysis of the present invention, that is, it has a liquid electrolysis cell having separators on both sides of the above-mentioned membrane electrode assembly.

[0035] <Method of manufacturing porous transport layer for liquid electrolysis> The method for producing a porous transport layer for liquid electrolysis of the present invention includes a coating step of applying a microporous layer composition to a conductive porous substrate containing carbon fibers to form a microporous layer, a vibration step of applying vibrations to the microporous layer after the coating step to smooth the surface of the microporous layer, and a sintering step of sintering the conductive porous substrate after the vibration step to form a porous transport layer for liquid electrolysis.

[0036] The composition for a microporous layer to be applied in the coating step of the present invention will be described. The composition for a microporous layer of the present invention is a dispersion in which raw materials to be contained in the microporous layer are dispersed in a dispersion medium. That is, the composition for a microporous layer preferably contains conductive particles, a binder, and a dispersion medium, and also preferably contains a dispersant to disperse the conductive particles and binder well. In the composition for a microporous layer of the present invention, the dispersant refers to a compound different from the binder.

[0037] The inclusion of a dispersant in the composition for a microporous layer improves the dispersibility of conductive particles in the composition for a microporous layer and increases the viscosity of the composition for a microporous layer, thereby improving uniformity during application. Examples of dispersants include polyalkylene glycols, carboxyl vinyl polymers, polyesters, polyvinyl alcohols, polyvinyl butyrals, polyvinylpyrrolidones, polyacrylic acids, polyacrylates, polystyrene sulfonates, polyalkylene polyamines, polyvinylimidazolines, cellulose derivatives, alginates, guar gum, xanthan gum, carrageenans, pectin, gelatin, and starches, and these may be used alone or in combination. Nonionic surfactants are preferred as dispersants because they contain little metal components, such as polyoxyethylene octylphenyl ether "Triton®" X-100 (manufactured by Nacalai Tesque, Inc.).

[0038] In the coating step of the present invention, examples of methods for applying the microporous layer composition to the conductive porous substrate include spraying, intaglio printing, gravure printing, screen printing, lamination printing, knife coating, die coating, bar coating, and blade coating. However, conductive porous substrates containing carbon fibers have large thickness variations and warpage in the surface direction, and coating methods such as knife coating, die coating, bar coating, and blade coating may not be able to uniformly apply the microporous layer with a uniform basis weight because the distance between the coating device and the conductive porous substrate greatly affects the coating amount. On the other hand, coating methods such as intaglio printing, gravure printing, and screen printing are so-called transfer coating methods in which the microporous layer composition is filled into the recesses or mesh of the coating device and then pressed against the conductive porous substrate. Therefore, they have the advantage of being less susceptible to thickness variations and warpage in the surface direction of the conductive porous substrate and being able to uniformly apply the microporous layer with a uniform basis weight in the surface direction, but transfer marks (gravure patterns, mesh patterns) may remain on the coated surface. One method for solving the problem of transfer marks is to reduce the viscosity of the composition for the microporous layer, but this method has problems such as making it difficult to fill the coating device with the microporous layer composition during coating, and causing the microporous layer to soak too much into the conductive porous substrate, thereby deteriorating the gas diffusivity of the porous transport layer for liquid electrolysis.

[0039] In the present invention, by vibrating the coated microporous layer after the coating step, coating unevenness of the microporous layer in the surface direction can be reduced and transfer traces can be smoothed. That is, this is preferred as a method for solving the problem of transfer traces remaining on the coating surface, particularly in the above-mentioned transfer coating method. Note that even when a coating method other than the transfer coating method is used, this is preferred because it can reduce coating unevenness of the microporous layer in the surface direction and control the thickness of the soaked portion, as described below.

[0040] To achieve both gas diffusivity and peel strength between the microporous layer and the conductive porous substrate, it is preferable to make the thickness of the part of the microporous layer that has penetrated into the conductive porous substrate 10 μm or more and 100 μm or less by the vibration process.

[0041] Examples of devices that impart vibration in the vibration step include an ultrasonic generator, a speaker, a vibration motor, a cam drive device, etc., and it is preferable to use an ultrasonic generator. By using an ultrasonic generator, high shear is applied to the composition for the microporous layer, thereby reducing the viscosity and facilitating penetration of the composition into the conductive porous substrate.

[0042] When an ultrasonic generator is used in the vibration step, the vibration frequency is preferably 10 kHz or more and 1 MHz or less. Furthermore, since a shorter takt time during production is better, the vibration time is preferably 0.1 seconds or more and 180 seconds or less, and more preferably 1 second or more and 60 seconds or less. As described above, by controlling the vibration frequency and time, it is possible to control the thickness of the microporous layer that has penetrated into the conductive porous substrate, and to achieve desired ranges for the uniformity of the in-plane basis weight and surface smoothness.

[0043] The manufacturing method of the present invention includes a sintering step of sintering the conductive porous substrate after the vibration step, and may include a step of drying the conductive porous substrate at a temperature lower than the sintering temperature before the sintering step.

[0044] The sintering is preferably carried out at a temperature of 200°C or higher and 400°C or lower. When a binder is contained in the microporous layer, a sintering temperature of 200°C or higher can insolubilize the binder, thereby preventing peeling of the microporous layer. Furthermore, a sintering temperature of 400°C or lower can prevent cracks on the surface of the microporous layer of the porous transport layer and peeling of the microporous layer, which would otherwise be caused by excessive decomposition of the binder. [Example]

[0045] An example of an embodiment of the present invention will be described in detail below using examples and comparative examples.

[0046] [Measurement method] (1) Arithmetic mean height Sa Using a 3D shape measuring machine (VR-3200 manufactured by Keyence Corporation), the porous transport layer for liquid electrolysis was fixed to the device with the microporous layer facing up, ensuring that there were no lifting or wrinkles. 2 The arithmetic mean height Sa was measured at any 10 points on the surface of the microporous layer within a field of view according to JIS B0681-2:2018, and the average of the arithmetic mean heights Sa obtained at the 10 points was defined as the arithmetic mean height Sa of the surface of the microporous layer.

[0047] (2) Thickness of the carbon paper, microporous layer, and liquid electrolytic porous transport layer The thickness of the raw carbon paper and the porous transport layer for liquid electrolysis was measured using a micrometer by applying a surface pressure of 0.15 MPa in the thickness direction of the object to be measured. A contact-type thickness meter (Mitutoyo Digimicro) was used for the measurements.

[0048] The difference between the thickness of the porous substrate for liquid electrolysis and the thickness of the carbon paper measured as described above was taken as the thickness of the microporous layer.

[0049] [Example 1] A composition for a microporous layer was prepared using acetylene black (Denka Black (registered trademark) manufactured by Denka Co., Ltd.) as conductive particles, Triton (registered trademark) X-100 (manufactured by Nacalai Tesque, Inc.) as a dispersant, polyester resin (Vylonal (registered trademark) MD-1480 manufactured by Toyobo MC Co., Ltd.) as a binder, and water. The mass ratio of conductive particles / dispersant / binder / water was adjusted to 7.0 parts by mass / 2.0 parts by mass / 2.0 parts by mass / 89.0 parts by mass.

[0050] The composition for the microporous layer was applied to carbon paper so that the basis weight of the solid content (acetylene black and binder) was 20 g / m 2 Before coating, the thickness of the carbon paper was measured according to the above (2) Thickness of the carbon paper, microporous layer, and liquid electrolyte porous transport layer.

[0051] Next, the cleaning tank of an AS ONE ultrasonic cleaner MCS-2 was filled about halfway with water, a cylindrical sample holder was placed in the center of the cleaning tank, and carbon paper coated with the composition for a microporous layer was placed on top of it, and vibrations of 40 kHz were applied for 1 minute.

[0052] After vibration, the carbon paper coated with the microporous layer composition was dried at 120°C and then sintered at 320°C to obtain a porous transport layer for liquid electrolysis. The obtained porous transport layer for liquid electrolysis was measured according to (1) the arithmetic mean height Sa and (2) the thicknesses of the carbon paper, microporous layer, and liquid electrolysis porous transport layer, and the results are shown in Table 1.

[0053] [Example 2] A porous transport layer for liquid electrolysis was obtained in the same manner as in Example 1, except that the time for applying 40 kHz vibration to the carbon paper coated with the composition for a microporous layer was changed to 3 minutes. The results are shown in Table 1.

[0054] [Comparative Example 1] A porous transport layer for liquid electrolysis was obtained in the same manner as in Example 1, except that the composition for a microporous layer was applied to carbon paper, and then dried and sintered without vibration. The results are shown in Table 1.

[0055] [Table 1] [Industrial Applicability]

[0056] The porous transport layer and its manufacturing method of the present invention can provide a liquid electrolysis device that exhibits high electrolysis performance. The obtained electrode is suitable for use as an electrode in a liquid electrolysis device, particularly a PEM-type water electrolysis device.

Claims

1. A porous transport layer for liquid electrolysis, comprising a microporous layer on at least one surface of a conductive porous substrate containing carbon fibers, wherein the arithmetic mean height Sa of the surface of the microporous layer is 8 μm or less.

2. 2. The porous transport layer for liquid electrolysis according to claim 1, which has a thickness of 100 μm or more.

3. 2. The porous transport layer for liquid electrolysis according to claim 1, wherein the thickness of the microporous layer is 10 μm or more and 200 μm or less.

4. 2. The porous transport layer for liquid electrolysis according to claim 1, wherein the thickness of the portion of the microporous layer that has penetrated into the conductive porous substrate is 10 μm or more and 100 μm or less.

5. A membrane electrode assembly using the porous transport layer for liquid electrolysis according to claim 1.

6. A liquid electrolysis device using the porous transport layer for liquid electrolysis according to claim 1.

7. A method for producing a porous transport layer for liquid electrolysis, comprising: a coating step of applying a microporous layer composition to a conductive porous substrate containing carbon fiber to form a microporous layer; a vibration step of applying vibrations to the microporous layer after the coating step; and a sintering step of sintering the conductive porous substrate after the vibration step to form a porous transport layer for liquid electrolysis.

8. 8. The method for producing a porous transport layer for liquid electrolysis according to claim 7, wherein the thickness of the part of the microporous layer that has penetrated into the conductive porous substrate is set to 10 μm or more and 100 μm or less by the vibration step.

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