JP7799997B2 - Gas diffusion electrode manufacturing method, gas diffusion electrode, and gas diffusion electrode roll - Google Patents

Description

Fuel cells are a mechanism that electrically extracts the energy generated when hydrogen and oxygen react to produce water. Because they are highly energy efficient and produce only water as an exhaust, they are expected to become widespread as a clean energy source. This invention relates to gas diffusion electrodes used in fuel cells, and in particular to gas diffusion electrodes used in polymer electrolyte fuel cells, which are used as power sources for fuel cell vehicles and the like, and to a method for manufacturing such electrodes.

The electrodes used in polymer electrolyte fuel cells are sandwiched between two separators in the polymer electrolyte fuel cell and are constructed with a catalyst layer formed on the surface of the polymer electrolyte membrane and a gas diffusion layer formed on the outside of this catalyst layer on both sides of the polymer electrolyte membrane. Gas diffusion electrodes are available as individual components for forming the gas diffusion layer in the electrode. Performance requirements for gas diffusion electrodes include gas diffusivity, conductivity for collecting electricity generated in the catalyst layer, and drainage properties for efficiently removing moisture generated on the surface of the catalyst layer. To obtain such gas diffusion electrodes, conductive porous substrates that combine gas diffusivity and conductivity are used.

Specific examples of conductive porous substrates (hereinafter sometimes simply referred to as "substrates") that are preferably used include carbon felt, carbon paper, and carbon cloth, all of which are made from carbon fibers, with carbon paper being the most preferred from the standpoint of mechanical strength, etc.

If the above-mentioned substrate is used as a gas diffusion electrode without modification, the coarse fibers can easily generate large droplets when water vapor condenses, resulting in flooding. For this reason, a microporous layer (also called a microporous layer) can be formed by applying a microporous layer coating (hereinafter simply referred to as "coating") containing dispersed conductive particles such as carbon black to a water-repellent substrate, followed by drying and sintering. The microporous layer provides a cosmetic effect by preventing the substrate's roughness from being transferred to the electrolyte membrane, and by adequately filling the substrate's voids, reducing the contact resistance (electrical resistance) between the catalyst layer and the gas diffusion layer. To achieve this cosmetic effect, the substrate's roughness (arithmetic mean roughness) is typically 10 to 30 μm, so the microporous layer thickness on the substrate (after drying and sintering) must be 10 to 80 μm, a large thickness for a wet coating. To ensure this thickness and to prevent the coating liquid from soaking into the porous substrate, the coating liquid must have a high viscosity. Furthermore, to improve power generation efficiency, there is also a demand for fuel cells to be operated at temperatures of 90°C or higher. In such cases, it is particularly important to prevent a decline in power generation performance due to drying up of the electrolyte membrane, and it is necessary to minimize the number of fine through-holes (pinholes) with a pore size of 2 to 10 μm.

On the other hand, for fuel cell vehicles to become more widespread, it is important to reduce costs to the same level as gasoline vehicles. To achieve this, all costs, including material costs and processing costs, must be reduced, and gas diffusion layers are no exception.

As fuel cells and fuel cell vehicles become more widespread in the future, gas diffusion electrodes will also need to be made more suitable for mass production, with improvements in quality and cost reductions. Gas diffusion electrodes can be manufactured using either sheet-like production or continuous processing, but from the perspective of mass production, it is preferable to continuously manufacture long rolled gas diffusion electrodes of 1,000 m or more, in which case physical properties such as in-plane gas diffusivity must be consistent over the entire length.

As a method for preparing a coating liquid for forming a microporous layer, Patent Document 1 discloses a technique for controlling shear force applied to conductive particles and a water-repellent resin (PTFE) to disperse and suppress aggregation. Patent Document 2 discloses a technique for improving the dispersibility of conductive particles by specifying the type of disperser. Patent Document 3 discloses a technique for leaving the prepared microporous layer coating liquid in a container to cause aggregation at the bottom of the container, generating cracks in the microporous layer and improving drainage. Patent Document 4 describes a method for controlling the size and number of the through-holes.

Patent No. 5148036 Patent No. 4425364 Patent No. 5922552 JP 2018-156818 A

However, the microporous layer preparation methods described in Patent Documents 1 and 2 are insufficient to achieve high levels of quality, uniformity, and productivity for mass production of gas diffusion electrodes, and the technology described in Patent Document 3 is also insufficient to ensure uniform quality; rather, the additional step of agglomerating the coating liquid may be a factor in reducing productivity. Furthermore, the level of control of the through-holes in Patent Document 4 is insufficient to accommodate high-temperature operation of fuel cells.

The object of the present invention is to provide gas diffusion electrodes of uniform quality and to provide technology for producing gas diffusion electrodes with high productivity and low cost.

In order to solve the above problems, the method for producing a gas diffusion electrode, the gas diffusion electrode, and the rolled gas diffusion electrode of the present invention have the following configurations.
[1] A method for producing a gas diffusion electrode, comprising: a coating step of applying a microporous layer coating liquid containing conductive particles, a water-repellent resin, and a dispersion medium to a conductive porous substrate to form a microporous layer; a drying step of removing the dispersion medium from the microporous layer coating liquid; and a sintering step of sintering the microporous layer and the water-repellent resin of the conductive porous substrate,
In the coating step,
a microporous layer coating liquid stirring device having a container for stirring and an agitator comprising at least a stirring shaft and a stirring blade;
a coater for applying the microporous layer coating liquid to the conductive porous substrate;
A method for producing a gas diffusion electrode, comprising: using an agitator for a gas diffusion electrode having piping and a liquid delivery device for delivering the microporous layer coating liquid from the microporous layer coating liquid agitator to a coater; delivering the microporous layer coating liquid by the liquid delivery device and applying the coating liquid while stirring the microporous layer coating liquid with the agitator under conditions that generate vertical convection in the microporous layer coating liquid in a container; and wherein the peripheral speed of the outermost periphery of the agitator blade during this process is 0.8 m/s or less.
[2] The method for producing a gas diffusion electrode according to [1], wherein the microporous layer coating liquid is a water-based coating liquid.
[3] The method for producing a gas diffusion electrode according to [1] or [2], wherein the viscosity of the microporous layer coating liquid is 1.0 Pa·s or more.
[4] The method for producing a gas diffusion electrode according to any one of [1] to [3], wherein the microporous layer coating liquid has a shear thinning index (shear thinning index) defined in the specification of 1.5 or more.
[5] The method for producing a gas diffusion electrode according to any one of [1] to [4], wherein the shape of the stirring blade is a helical ribbon blade or a paddle blade.
[6] The method for producing a gas diffusion electrode according to any one of [1] to [5], wherein the rotation speed of the stirring blade is 20 rpm or less.
[7] A gas diffusion electrode comprising a conductive porous substrate and a microporous layer for a gas diffusion layer containing conductive particles and a water-repellent resin, wherein the conductive particles contained in the microporous layer are carbon black, the number of through-holes extending from the surface of the microporous layer through the gas diffusion electrode is less than 5 per 10 cm square (100 cm ² ), and the maximum bubble point diameter is 2.0 μm or less.
[8] The gas diffusion electrode according to [7], wherein the number of through-holes extending from the surface of the microporous layer through the gas diffusion electrode is less than 2 per 10 cm square (100 cm ² ), and the maximum bubble point diameter is 1.0 μm or less.
[9] The gas diffusion electrode according to [7] or [8], wherein the carbon black is acetylene black.
[10] A long gas diffusion electrode roll comprising a conductive porous substrate and a microporous layer for a gas diffusion layer, the microporous layer comprising conductive particles and a water-repellent resin, wherein the length of the gas diffusion electrode roll is 1000 m or more and the relative standard deviation of in-plane gas diffusivity over the entire length is 5% or less.

By using the manufacturing method of the present invention, gas diffusion electrodes with no or very few through-holes can be obtained, which is advantageous for preventing dry-up during high-temperature operation. Gas diffusion electrodes with a uniformly coated microporous layer can be produced efficiently with high yields, reducing the manufacturing costs of gas diffusion electrodes and, ultimately, fuel cells.

1 is a schematic diagram of an example of an apparatus used to stir the coating liquid when applying the microporous layer coating liquid of the present invention. 1 is a schematic diagram of an example of an apparatus for manufacturing a gas diffusion electrode of the present invention. 1 is a schematic diagram showing the configuration of a gas diffusion electrode to which the present invention is applied; FIG. 1 shows the shape of the stirring blade used in Comparative Example 2. 1 is a diagram showing the properties of the coating liquid used in the present invention. Schematic diagram of an in-plane gas diffusion evaluation device used in the examples of the present invention.

The first aspect of the present invention relates to a manufacturing method that specifies how to handle the coating liquid during application, in order to improve the uniformity of the physical properties of the gas diffusion electrode after application of a microporous layer coating liquid with shear-thinning properties used in the mass production of gas diffusion electrodes, and to increase the use efficiency of the coating liquid.

A second aspect of the present invention relates to a gas diffusion electrode comprising a conductive porous substrate and a microporous layer for gas diffusion containing conductive particles and a water-repellent resin, wherein the conductive particles are carbon black, the gas diffusion electrode has a small number of through-holes penetrating from the surface of the microporous layer through the gas diffusion electrode, and has a small maximum bubble point diameter.

The gas diffusion electrode manufactured by the manufacturing method of the first aspect of the present invention and the gas diffusion electrode of the second aspect have a microporous layer on at least one side of the conductive porous substrate. First, we will explain the conductive porous substrate.

In solid polymer fuel cells, the gas diffusion electrode must have high gas diffusivity to diffuse the gas supplied from the separator to the catalyst, high drainage properties to discharge water produced during the electrochemical reaction back to the separator, and high conductivity to extract the generated current. For this reason, the gas diffusion electrode uses a conductive porous substrate made of a porous material that is conductive and typically has a peak pore size in the 10 μm to 100 μm range.

Specific examples of conductive porous substrates that can be used include porous substrates containing carbon fibers such as carbon fiber woven fabric, carbon fiber paper, carbon fiber nonwoven fabric, carbon felt, carbon paper, and carbon cloth, as well as metal porous substrates such as foamed sintered metal, metal mesh, and expanded metal. Among these, porous substrates containing carbon fibers are preferred due to their excellent corrosion resistance. Of these, carbon paper, a substrate made by bonding carbon fiber paper with carbide, is particularly suitable due to its excellent "springiness," or ability to absorb dimensional changes in the thickness direction of the electrolyte membrane.

In order to improve the gas diffusion properties of the gas diffusion electrode and maximize the power generation performance of the fuel cell, the substrate must have a high porosity. The porosity is preferably 80% or higher, and more preferably 85% or higher. The upper limit for porosity is 95%, as this is the limit at which the substrate can maintain its structure.

Furthermore, the gas diffusion properties of the gas diffusion electrode can be improved by reducing the thickness of the substrate, such as carbon paper. Therefore, the thickness of the substrate, such as carbon paper, is preferably 220 μm or less, more preferably 150 μm or less, and even more preferably 120 μm or less. Setting the lower limit of the substrate thickness at 50 μm is preferable, as it maintains mechanical strength and facilitates handling during the manufacturing process. Reducing the thickness of the conductive substrate is also effective in reducing electrical resistance in the thickness direction when used in a fuel cell.

To efficiently manufacture a gas diffusion electrode using the above substrate, it is preferable to unwind a long roll of the substrate and form a microporous layer continuously during the process.

The substrate may be treated with a water-repellent coating to improve drainage. The water-repellent coating is preferably performed using a water-repellent resin such as a fluororesin. Examples of fluororesins include polytetrafluoroethylene (PTFE) (such as Teflon (registered trademark)), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), perfluoroalkoxy fluoride resin (PFA), ethylene-tetrafluoroethylene copolymer (ETFA), polyvinylidene fluoride (PVDF), and polyvinyl fluoride (PVF). PTFE or FEP, which exhibit strong water-repellent properties, are preferred.

The amount of water-repellent resin is not particularly limited, but is preferably 0.1% by mass or more and 20% by mass or less relative to 100% by mass of the substrate. Within this range, water repellency is fully exhibited, while the water-repellent resin is unlikely to clog pores that serve as gas diffusion or drainage paths, or increase electrical resistance.

Methods for making a substrate water-repellent include the commonly known treatment technique of immersing the substrate in a dispersion containing a water-repellent resin, as well as coating techniques such as die coating and spray coating, which apply a water-repellent resin to the substrate. Dry processes such as sputtering of fluororesin can also be used. After the water-repellent treatment, a drying process and even a sintering process can be added as needed.

Next, the microporous layer will be described. The microporous layer is a layer containing conductive particles such as carbon black, carbon nanotubes, carbon nanofibers, chopped carbon fibers, graphene, and graphite. Carbon black is preferably used as the conductive particles due to its low cost, safety, and stable product quality. Acetylene black is preferably used as the carbon black contained in the microporous layer because it contains few impurities and is less likely to reduce catalyst activity.

The microporous layer is required to have properties such as electrical conductivity, gas diffusion, water drainage or moisture retention, and thermal conductivity, as well as strong acid resistance on the anode side inside the fuel cell and oxidation resistance on the cathode side. Therefore, the microporous layer contains a water-repellent resin such as a fluororesin in addition to conductive particles. The fluororesins contained in the microporous layer include PTFE, FEP, PFA, ETFA, etc., similar to fluororesins that are preferably used to make substrates water-repellent. PTFE or FEP is preferred because of its particularly high water repellency.

For a gas diffusion electrode to have a microporous layer, a coating liquid for forming the microporous layer, i.e., a microporous layer coating liquid, is typically applied to the substrate. The coating liquid typically contains the above-mentioned conductive particles and a dispersant such as water or alcohol. However, from the perspective of reducing environmental impact and simplifying the coating and drying process, a water-based coating liquid using water as the dispersant is preferred. Here, a water-based coating liquid is a coating liquid in which water accounts for 50% or more by weight of the dispersant, preferably 70% or more by weight, and even more preferably 90% or more. Furthermore, a surfactant or other dispersant is often added as a dispersant to disperse the conductive particles. Furthermore, the coating liquid used in the present invention is pre-loaded with a water-repellent resin in order to incorporate the water-repellent resin into the microporous layer.

To prepare the microporous layer coating liquid, various dispersing machines and kneaders can be used, such as a planetary mixer, ball mill, bead mill, jet mill, kneader, pressure kneader, continuous kneader, homogenizer, and homomixer. Furthermore, when preparing an aqueous coating liquid to which a surfactant has been added as a dispersant, foaming is likely to occur, so an apparatus that does not incorporate air or that can perform preparation while vacuum degassing is preferred. From this perspective, multi-screw kneaders such as planetary mixers are suitable.

A preferred method for forming a microporous layer on a substrate is to apply a coating liquid to the substrate.

From the perspective of coating liquid productivity, the concentration of conductive particles in the coating liquid is preferably 5% by mass or more, and more preferably 10% by mass or more. There is no upper limit to the concentration, as long as the viscosity, dispersion stability of the conductive particles, and coatability of the coating liquid are suitable. When acetylene black is used as the conductive particles, the upper limit of the acetylene black concentration in the coating liquid for an aqueous coating liquid is preferably 25% by mass. Within this preferred range, the acetylene black particles do not re-aggregate, preventing so-called percolation, and reducing the likelihood of a sudden increase in viscosity impairing the coatability of the coating liquid.

The roles of the microporous layer include (1) protecting the catalyst, (2) providing a cosmetic effect by preventing the surface of the coarse substrate from being transferred to the electrolyte membrane, and (3) preventing condensation of water vapor generated at the cathode. In order to achieve the cosmetic effect, it is preferable that the microporous layer have a certain thickness.

The thickness of the microporous layer is preferably 10 μm or more and 60 μm or less. If the thickness of the microporous layer is less than 10 μm, the above-mentioned touch-up effect may be insufficient. To fully achieve the touch-up effect, a thickness of 20 μm or more is preferable. Furthermore, if the thickness of the microporous layer exceeds 60 μm, the gas diffusion properties of the gas diffusion electrode itself may decrease or the electrical resistance may increase. From the perspective of improving gas diffusion properties or reducing electrical resistance, the thickness of the microporous layer is more preferably 50 μm or less, and even more preferably 40 μm or less. Note that, because the microporous layer is formed by applying a coating liquid to the surface of the substrate, the coating liquid may penetrate into the pores of the substrate, forming a soaked portion 203 of the microporous layer in the substrate as shown in Figure 3. However, the thickness of each microporous layer refers to the thickness of the portion outside the substrate, excluding this soaked portion.

In the present invention, as described above, it is preferable to apply a thick coating of a microporous layer to a substrate with high porosity. In order to apply the coating liquid thickly without soaking it into a substrate with high porosity as much as possible, it is preferable that the viscosity be low at the moment of application when shear force is applied, and high when the dispersion medium is dried. In other words, it is preferable that the coating liquid have shear thinning properties. Shear thinning properties are the property whereby viscosity temporarily decreases when shear force is applied (and then recovers when the shear force is removed).

For the microporous layer coating liquid used in the present invention, a quantitative indicator of shear thinning properties (hereinafter referred to as the shear thinning index) is defined as the ratio (a/b) of the viscosity value (a) at a shear rate (shear rate) of 17 (/sec) to the viscosity value (b) at a shear rate of 127 (/sec). A high shear thinning index indicates high viscosity when no shear force is applied and low viscosity when high shear force is applied. For the microporous layer coating liquid used in the present invention, this shear thinning index is preferably 1.5 or greater, and more preferably 2.0 or greater. The upper limit of the shear thinning index is preferably 10.0 or less, and more preferably 8.0 or less.

The coating liquid is prepared by dispersing conductive particles in a dispersion medium. To disperse the conductive particles, it is preferable to use a surfactant as a dispersant. To keep this dispersion stable for a long period of time and prevent the coating liquid from separating, the amount of dispersant added is preferably 10% by mass or more of the conductive particles in the coating liquid. If the amount is less than 10% by mass, the coating liquid's dispersion stability cannot be ensured, and changes such as a decrease in the physical properties of the coating liquid are likely to occur. Furthermore, when using a water-repellent resin, it is preferable to use a dispersant that does not interfere with the dispersion of the water-repellent resin.

Furthermore, if the thickness of the microporous layer is to be greater than 10 μm as a coating film after sintering, the viscosity of the coating liquid is preferably 1.0 Pa·s or more, more preferably 3.0 Pa·s or more, and even more preferably 5.0 Pa·s or more. If the viscosity of the coating liquid is less than 1.0 Pa·s, the coating liquid may flow on the surface of the substrate, making it impossible to achieve the desired thickness, or the coating liquid may flow into the pores of the substrate, causing bleed-through. Conversely, if the viscosity of the coating liquid is too high, the coatability may be reduced, so the viscosity of the coating liquid is preferably 25 Pa·s or less, more preferably 20 Pa·s or less, and even more preferably 15 Pa·s or less.

Adding a thickener is effective in maintaining a high viscosity of the coating liquid. The thickener used here may be any commonly known thickener. For example, methyl cellulose-based, polyethylene glycol-based, or polyvinyl alcohol-based thickeners are suitable.

These dispersants and thickeners may be a single substance that functions as both a dispersant and a thickener, or they may have other functions, such as antifungal properties. Surfactants are preferably used as substances that function as both dispersants and thickeners. Alternatively, different substances suitable for each function may be selected as dispersants and thickeners. In such cases, it is preferable to select a thickener that does not interfere with the dispersion of the conductive particles or water-repellent resin. To maintain the viscosity and dispersion stability of the coating liquid, the total amount of dispersant and thickener is preferably 10% by weight or more of the added weight of conductive particles, more preferably 50% by weight or more, and even more preferably 100% by weight or more. The preferred upper limit of the total amount of dispersant and thickener is typically 500% by weight or less of the added weight of conductive particles. At 500% by weight or less, steam and decomposition gases are less likely to be generated during the subsequent sintering process, ensuring safety and productivity.

The above coating liquid can be dispersed using various dispersing devices, but excessive dispersion often results in a decrease in viscosity, and even if the viscosity is adjusted with a thickener, the coating liquid tends to soak into the porous substrate. For this reason, it may be necessary to keep the dispersion at a low level and maintain it. In other words, after preparing the coating liquid, it is preferable to avoid applying shear forces that could change the dispersion state of the conductive particles as much as possible until it is applied to the substrate.

The above-mentioned coating liquids are prone to generating bubbles during stirring and other operations, particularly when the dispersion medium is aqueous and surfactants are added as dispersants or thickeners. When a coating liquid containing bubbles is applied to a substrate, a coating film will not form in the areas where the bubbles exist, resulting in a phenomenon known as coating gaps, which can reduce the functionality of the microporous layer. Therefore, the coating liquid must be thoroughly degassed before application.

The gas diffusion electrode manufacturing apparatus of the present invention includes at least a microporous layer coating liquid agitator having a container and agitator for preparing the microporous layer coating liquid, a coater for applying the microporous layer to the substrate, and piping and a liquid delivery device for sending the microporous layer coating liquid from the microporous layer agitator to the coater. The coating liquid is applied to the substrate by the coater, thereby producing a gas diffusion electrode.

As mentioned above, it is preferable to avoid generating bubbles in the coating liquid during transfer from the container to the applicator or during application, as this can lead to coating defects. Stirring in the container may be stopped to prevent bubbles from forming. However, stopping for long periods of time, in particular, can result in a distribution of the coating liquid's density or viscosity within the container. That is, the closer to the bottom of the container, the higher the density and viscosity of the coating liquid tend to be. If a long-length gas diffusion electrode is coated with a distribution of these fluids, even if the coating volume per unit time is set to achieve the desired coating weight and thickness based on the high-density and high-viscosity coating liquid near the bottom of the container at the start of coating, as coating progresses, a coating liquid with a lower density and viscosity than the coating liquid at the top of the container will be applied. If coating is continued at the same volumetric rate, the coating weight will decrease and the viscosity will decrease, making it easier to penetrate, resulting in a smaller thickness of the gas diffusion electrode. This tendency is particularly pronounced when continuously coating large volumes of coating liquid in mass production. In other words, when continuously manufacturing gas diffusion electrodes by long-length coating, it is important to ensure that the composition of the coating liquid is uniform in the container in which it is prepared and to prevent bubbles from forming in the coating liquid.

In addition, it is preferable to use a microporous layer coating liquid with a high viscosity in the present invention. However, when a highly viscous coating liquid is extracted from an extraction port at the bottom of a large-capacity container using a pump or the like, once the liquid level drops to a certain level, the liquid flows only in the area directly above the extraction port, which is likely to result in the coating liquid near the wall surface not being transported, as shown in Figure 5, and the usage rate of the microporous layer coating liquid decreases.

As a result of investigations into solving these problems, the inventors discovered that by operating the agitator so that vertical convection occurs in the coating liquid in the container when the liquid is sent to the coater, the coating liquid composition in the container can be maintained uniform to the end, the physical properties of the microporous layer in the long gas diffusion electrode can be made uniform from start to finish, and the utilization efficiency of the coating liquid in the container can be increased.

We will now describe in detail one example of a method for manufacturing a gas diffusion electrode, which is the first aspect of the present invention, but the present invention is not limited to the following description.

In the microporous layer coating liquid stirring device of the present invention, as illustrated in Figure 1, a coating liquid 4 is placed in a vessel 1 equipped with an agitator 2 having an agitator blade 21 and an agitator shaft 22, and is stirred by the agitator blade 21. The vessel 1 is connected to a pressure reducing means (not shown) via a pressure reducing means connection port 3, and the internal pressure of the vessel 1 is controlled by the pressure reducing means, such as a vacuum pump. A baffle plate may be provided inside the vessel 1 to adjust the agitation state of the coating liquid.

The container is designed to be airtight, and its internal pressure is controlled by a pressure reducing means during degassing. There are no particular restrictions on the container's capacity, but for mass production applications, it is preferably 200 liters or more, more preferably 300 liters or more, and even more preferably 1000 liters or more. Considering installation space and the efficiency of external temperature control, the upper limit on capacity is thought to be around 3000 liters.

The shape of the container can be, for example, a tank with a cylindrical straight body and a mirror-shaped bottom, but in order to use as much of the coating liquid as possible, it is preferable that the bottom be bowl-shaped. There are no restrictions on the shape of the container's top, and a flat or mirror-shaped top can be used. There are no restrictions on the top having a lid-like structure, and it can be openable, closable, or removable. There are also no particular restrictions on the material of the container, and it can be freely selected depending on the properties of the coating liquid.

The container preferably has a pressure reduction means connection port for connecting to pressure reduction means. By connecting the pressure reduction means connection port to pressure reduction means (e.g., a vacuum pump), the pressure inside the container can be reduced to below atmospheric pressure. There are no restrictions on the size or shape of the pressure reduction means connection port. Furthermore, the pressure reduction means is not limited to a vacuum pump; any known device capable of controlling the pressure inside the container can be used.

The container is provided with an outlet for sending the coating liquid to the coating device. A coating liquid supply section may also be provided. A separate gas supply connection port may also be provided for supplying gas when returning the pressure from a reduced pressure state to atmospheric pressure. If temperature control of the coating liquid is required, it is possible to configure the container to be temperature controllable by immersing a coiled heat transfer medium pipe in the coating liquid, or by giving the container a jacket structure and running a heat transfer medium through this jacket, or by attaching a heater to the side of the container, etc.

The coating fluid used in the present invention may exhibit significant temperature dependence of physical properties such as viscosity. In such cases, it is preferable to control the temperature of the coating fluid constant from the degassing process, through the transfer of the coating fluid to the coating device, and until the moment the coating fluid is discharged from the coater and applied to the substrate. It is also desirable to maintain the same temperature in the coating fluid disperser, the transfer section from the disperser to the mixing vessel, and even the building section in which these devices are installed. The temperature fluctuation range is ±5°C from the center value, preferably ±3°C, and more preferably ±1°C. The more constant the temperature control, the better in terms of uniforming the fluid properties, but because the control mechanism is expensive, a practical limit is around ±1°C.

The coating liquid in the container is stirred by a stirrer. It is preferable that the container be equipped with a stirring blade, which is a component of the stirring mechanism. There are no particular limitations on the stirrer, as long as it can generate vertical convection in the highly viscous, shear-thinning coating liquid inside and stir the inside uniformly. Any known stirring device can be used. For example, a stirring mechanism with stirring blades such as paddle blades or turbine blades fixed to the stirring shaft can be used. The stirring blades may be provided in a single stage on the stirring shaft, or in two or more stages. However, for the coating liquids used in this invention, it is preferable to use stirring blades known as helical ribbon blades. Furthermore, stirring blades commonly known as paddle blades, such as the "Maxblend®" blade (Sumitomo Heavy Industries Process Equipment Co., Ltd.) or the "Fullzone®" blade (Kobe Eco-Solutions Co., Ltd.), may also be used, due to their simple structure and ease of cleaning when switching product types.

Furthermore, there are no particular restrictions on the location of the stirring mechanism within the container, as long as it can stir the coating liquid. There are also no particular restrictions on the material of the stirring mechanism, and it can be selected appropriately depending on the properties of the coating liquid.

The occurrence of vertical convection can be confirmed, for example, by placing beads having an apparent density equivalent to that of the coating liquid in the liquid and visually observing the movement of the beads when the liquid is stirred. If vertical convection is occurring, beads that have fallen to the liquid surface will sink with the current toward the bottom of the container due to stirring, then reappear on the liquid surface with the rising current, and this process will be repeated. If vertical convection is not occurring, the beads will continue to float near the liquid surface or will sink and not rise to the liquid surface. The beads used here should have a density approximately equivalent to that of the coating liquid (preferably within ±0.1 g/cm ³ of the liquid density) and be insoluble in the coating liquid.

A baffle plate can also be installed inside the vessel. There are no particular restrictions on the shape of the baffle plate, and any known shape such as a flat plate or a cylindrical shape can be used.

The coating liquid in the stirring device is extracted from an outlet near the bottom of the container and pumped toward the coater using a pump or other means for generating a pressure differential from upstream to downstream. If necessary, the container may be pressurized to pump the coating liquid. During pumping, it is preferable to operate the stirring blades at a rotation speed or peripheral speed that does not generate bubbles or entrain them into the coating liquid, thereby generating vertical convection within the container and maintaining uniform composition and physical properties within the container. The generation of bubbles can be investigated, for example, using the following method: Two glass plates, each 15 cm on a side and 0.5 cm thick, are prepared. Fluorine tape (ASF-110FR, manufactured by Chukoh Chemical Industry Co., Ltd.) is attached to the four corners of one plate. 0.2 g of the liquid sample is sandwiched between them and allowed to stand for 30 minutes. The sample observed using the Otsuka Optical Co., Ltd. ILLMINATED MAGNIFIERS OSL-1 can then be photographed with a digital camera or other device, and the number of bubbles in the image can be visually counted, allowing for a judgment.

The rotation speed of the stirring blade depends on the physical properties of the coating liquid, such as the viscosity, but as long as the peripheral speed of the outermost periphery of the stirring blade (tip speed) is 0.8 m/s or less, bubbles will not be generated and the coating liquid properties can be maintained at a nearly uniform level. Because the coating liquid used in the present invention has shear-thinning properties, rotating it at high speed may result in a decrease in viscosity due to shear with the wall or bottom of the container. If the coating liquid is applied at this viscosity, there is a possibility of significant penetration if the substrate has high porosity. Therefore, a low tip speed is preferable, with a tip speed of 0.5 m/s or less, and even 0.1 m/s or less being preferable. The lower limit of the tip speed is not particularly limited as long as it generates movement in the stirring blade and generates vertical convection, but it is preferable to set it to 0.01 m/s or more from the perspective of stable operation of the stirring mechanism. Here, the tip speed can be calculated using the diameter (m) of the stirring blade and the rotation speed (rpm) of the stirrer using the following formula:
Impeller tip speed (m/s) = stirrer rotation speed (rpm) / 60 × impeller diameter (m) × pi Here, the impeller diameter is the diameter of the impeller's plane parallel to the liquid surface if it is circular, or if it is not circular, the diameter is the diameter of the circumscribed circle of the path that the impeller traces as it rotates.

Also, even if the peripheral speed of the outermost periphery of the stirring blades is 0.8 m/s or less, if the blade diameter is small and there is a possibility of air bubble entrapment, it is advisable to reduce the rotation speed. In this case, 20 rpm or less is preferable, and 15 rpm or less is even more preferable.

The delivered coating liquid can be applied to the substrate using a variety of commercially available coating devices. Coating methods include screen printing, rotary screen printing, spray atomization, intaglio printing, gravure printing, die coater coating, bar coating, blade coating, and roll knife coater coating. However, die coater coating is preferred because it allows for quantification of the coating amount regardless of the substrate's surface roughness. Furthermore, blade coater and roll knife coater coating are preferred when a smooth coating surface is required to improve adhesion with the catalyst layer when the gas diffusion electrode is incorporated into a fuel cell. The coating methods listed above are merely examples and are not necessarily limited to these. The various coating methods listed above are described in numerous existing publications, such as "All About Converting" (edited by Processing Technology Research Institute, Inc., March 2014).

After the coating liquid is applied, the dispersion medium of the coating liquid is dried and removed as necessary. When the dispersion medium is water, the drying temperature after application is preferably 20°C or higher and 150°C or lower, and more preferably 60°C or higher and 140°C or lower. The drying temperature may be within a range that combines any of the upper and lower limits listed above. This drying of the dispersion medium may be performed all at once in the subsequent sintering process.

After the coating liquid is applied, sintering is generally carried out to remove the dispersant and thickener used in the coating liquid and to dissolve the water-repellent resin and bind the conductive particles.

The sintering temperature depends on the boiling point or decomposition temperature of the added dispersant and thickener, but is preferably 250°C or higher and 400°C or lower. It is even more preferably 300°C or higher and 380°C or lower. Sintering temperatures within this preferred range ensure sufficient removal of the dispersant and thickener, while also reducing the likelihood of decomposition of the water-repellent resin.

Once sintering is complete, the gas diffusion electrode is wound onto a winding core by a winding machine. The core material used for winding can be a paper tube, plastic, metal, or other material. The outer diameter of the core should be in the range of 80 mm to 180 mm, as a too small diameter can cause the substrate to crack, while a too large diameter can reduce handling. During winding, a paper insert can be wound around the gas diffusion electrode to protect its surface.

Next, the characteristics of the gas diffusion electrode of the second embodiment will be further described. The gas diffusion electrode of the present invention has a microporous gas diffusion layer containing conductive particles and a water-repellent resin on at least one surface of a conductive porous substrate. Furthermore, a preferred method for producing the gas diffusion electrode of the second embodiment of the present invention is the manufacturing method of the first embodiment of the present invention.

The gas diffusion electrode of the second embodiment of the present invention has the characteristic of having few through-holes. Typically, a microporous layer contains countless fine pores with diameters of less than 1.0 μm, which contribute to gas diffusion in the direction perpendicular to the surface. However, if the diameters are specifically larger than 2.0 μm and form through-holes, when an electrode (catalyst layer) is formed using the so-called GDE method in which a catalyst ink is applied to the microporous layer side of the gas diffusion electrode, the catalyst ink may leak out of the through-holes, potentially reducing the catalyst's contribution to power generation. Furthermore, during power generation at high temperatures, water vapor may leak out of the through-holes, potentially causing so-called dry-up. In the gas diffusion electrode of the second embodiment of the present invention, the number of through-holes is less than 5 per 10 cm square (100 cm ² ). More than 5 through-holes may result in reduced power generation performance at high temperatures. The number of through-holes is preferably less than 2, more preferably less than 1.

To check the number of through-holes in a gas diffusion electrode, use an optical microscope (stereomicroscope) and, with the microporous layer of the gas diffusion electrode facing up, shine light from below and count the areas through which the light passes as through-holes.

The diameter of the through holes can also be determined by the bubble point method. The pore diameter measured by the bubble point method is called the bubble point diameter (hereinafter sometimes referred to as the BP diameter). The measurement method will be described later. In the second aspect of the present invention, the maximum bubble point diameter is 2.0 μm or less, preferably 1.0 μm or less, and more preferably 0.5 μm or less.

The porosity of the microporous layer is preferably less than 80% to prevent drying-up at high temperatures. However, if the porosity is too low, gas diffusion properties are impaired, so the lower limit is 60%. When such a gas diffusion electrode is incorporated into a fuel cell, drying-up can be suppressed and power generation performance at high temperatures is improved.

Next, we will explain the third aspect of the present invention, the gas diffusion electrode roll.

A third aspect of the present invention is a roll-shaped material suitable for mass production of gas diffusion electrodes, characterized in that the variation (relative standard deviation) of in-plane gas diffusivity over the entire length is within 5%. A preferred method for producing the roll-shaped gas diffusion electrode of the third aspect is the manufacturing method of the first aspect.

When mass-producing gas diffusion electrodes, longer rolls are preferred, and the length of the effective portion of the gas diffusion electrode roll of the present invention must be 1,000 m or more, preferably 3,000 m or more, and more preferably 5,000 m or more. Since the thickness of a gas diffusion electrode is at most about 60 μm, the upper limit for length is about 100,000 m from the standpoint of ease of handling.

The most basic characteristics of a gas diffusion electrode are its thickness and the basis weight of the microporous layer, which can be set to any value as a design factor for the fuel cell. However, if the thickness and basis weight of the microporous layer are not uniform, problems such as uneven tightening pressure may occur when the electrode is incorporated into a fuel cell. Therefore, it is preferable that the variation (relative standard deviation) in thickness and basis weight of the microporous layer be within 5%.

If the physical properties (viscosity) of the microporous layer coating liquid are not uniform, when attempting to maintain a constant basis weight of the microporous layer, the degree of penetration of the microporous layer coating liquid into the highly porosity conductive porous substrate will vary, making it difficult to maintain a constant thickness at the same basis weight. The in-plane gas diffusivity of the gas diffusion electrode is an indicator of the degree of this penetration, and variation in in-plane gas diffusivity needs to be kept within 5%, preferably within 3%. To achieve such uniformity of physical properties over the entire length of the roll, it is preferable to apply the manufacturing method of the first embodiment when applying the microporous layer coating liquid, i.e., to generate vertical convection in the coating liquid while stirring and then send it to the coater.

The present invention will be explained in more detail below using examples, but the present invention is not limited to these examples in any way.

(a) Conductive porous substrate Carbon paper having a thickness of 100 μm, a width of 550 mm, a length of 5000 m, a porosity of 85%, and a density of 0.25 g/ ^cm3 was prepared, and a water-repellent treatment was performed in advance using a die coater with a diluted solution of PTFE dispersion ("Polyflon (registered trademark)" D-210C manufactured by Daikin Industries, Ltd.), and the dispersion medium was dried.

(b) Preparation of Coating Liquid 10 parts by mass of "Denka Black (registered trademark)" manufactured by Denki Kagaku Kogyo Kabushiki Kaisha, 6 parts by mass of PTFE dispersion ("Polyflon (registered trademark)" D-210C manufactured by Daikin Industries, Ltd.), 10 parts by mass of surfactant ("TRITON (registered trademark)" X-100 manufactured by Nacalai Tesque, Inc.), and 74 parts by mass of ion-exchanged water were kneaded in a planetary mixer to a working volume of 200 liters.

(c) Viscosity Measurement In the viscosity measurement mode of a Bohlin rotational rheometer manufactured by Spectris, a circular cone plate with a diameter of 40 mm and an inclination of 2° was used to measure stress while increasing the rotation speed of the plate (while increasing the shear rate). At this time, the viscosity value a at a shear rate of 17 (1/s) was read and used as the viscosity of the coating liquid. The viscosity value b at a shear rate of 127 (1/s) was also measured, and the ratio a/b of the two was used as the shear thinning index.

The viscosity of the coating liquid used in the examples and comparative examples was 8.8 to 9.8 Pa·s, and the shear thinning index (a/b) was within the range of 4.3 to 4.5, and it was determined that the liquid properties were nearly identical.

(d) Production of Gas Diffusion Electrode A long gas diffusion electrode roll was produced using the gas diffusion electrode production apparatus shown in FIG. 2 . That is, the substrate (101) prepared in (a) above was unwound from the unwinder (102), and the coating liquid prepared in (b) above was applied to the entire length of the substrate using a coater (die coater, 104). During this process, the coating liquid was stirred in the container 1 and sent to the coater from the outlet below the tank using a pump. The pump flow rate was set so that the basis weight of the microporous layer after sintering would be 15 g/ ^m² , and the temperature of the coating liquid was maintained at 23±1°C. Next, the substrate coated with the coating liquid was passed through a dryer (107) and a sinterer (108) and then wound up as a gas diffusion electrode by a winder (109). The application of the coating liquid was started after the coating liquid in the container 1 was sufficiently degassed, and continued until air was sucked in from the bottom of the container during the transfer of the coating liquid, causing coating voids and a large amount of bubbles to appear in the coating film.

To measure the physical properties of the manufactured gas diffusion electrode, the wound gas diffusion electrode was unwound and samples were cut out every 200 m to measure the coating thickness, microporous layer weight, and in-plane gas diffusivity. For measurements of surface quality (number of through-holes, bubble point diameter) and power generation performance, samples were taken from the start of the coating, i.e., close to the core of the wound gas diffusion electrode.

(e) Measurement of coating thickness The difference between the thickness of the substrate prepared in (a) above and the thickness of the gas diffusion electrode manufactured in (d) above was taken as the coating thickness. For the measurement, a contact type thickness meter (Mitutoyo Digimicro) was used, and the contact pressure was set to 0.15 MPa.

(f) Calculation of coating liquid usage efficiency After producing a gas diffusion electrode as described in the above (d), the difference between the remaining amount of coating liquid after completion and the initial amount of coating liquid was subtracted to obtain the initial amount of coating liquid, and the value (%) obtained by dividing this by the initial amount of coating liquid was defined as the coating liquid usage efficiency.

(g) Measurement of basis weight of microporous layer The difference in basis weight between a 5 cm x 5 cm test piece of the gas diffusion electrode produced in (d) above and the substrate before the coating liquid was applied was measured and recorded as the basis weight (g/ ^m2 ) of the microporous layer.

(h) In-plane gas diffusion property A water vapor gas water vapor permeation diffusion evaluation device (MVDP-200C) manufactured by Seika Corporation was used as an in-plane gas diffusion evaluation device. In a piping system such as that shown in FIG. 6, first, only valve A (303) was opened, and valve B (305) was closed. Nitrogen gas 313 was flowed into the primary piping A (302), and a predetermined amount of gas (190 cc/min) was flowed into the mass flow controller (301). The pressure controller (304) was adjusted so that the gas pressure was 5 kPa relative to atmospheric pressure. The gas diffusion electrode sample (308) manufactured in (d) above was set on the sealing material (312) located between gas chamber A (307) and gas chamber B (309). Next, valve A (303) was closed, valve B (305) was opened, and nitrogen gas was allowed to flow into piping B (306). Nitrogen gas flowing into gas chamber A (307) moves through the gaps in the gas diffusion electrode sample (308) to gas chamber B (309), passes through pipe C (310), and then passes through the gas flow meter (311) before being released into the atmosphere. The flow rate (cc/min) of the nitrogen gas flowing through the gas flow meter (311) at this time was measured, and this value was taken as the in-plane gas permeability.

(i) Measurement of the number of through-holes The surface of the microporous layer of the gas diffusion electrode sample was observed in epi-illumination mode using a stereomicroscope (Leica MC206) with a 1x objective lens and a 2x intermediate lens, illuminating the sample from below, and the number of through-holes (light passing through) was counted in a rectangular field of view of 6.8 mm x 4.5 mm on an image processor. This was performed for 330 fields of view, and the number of through-holes per 100 ^cm2 was determined.

(j) Bubble Point Diameter Using a Perm Porometer manufactured by Porous Materials, Inc., a gas diffusion electrode sample punched to a diameter of 25 mm is immersed in a low surface tension liquid (Galwick (USA registered trademark)), and is clamped in a dedicated holder with a 1/2 inch diameter hole and set in the cylinder of the device. As the air pressure above the sample is increased, the low surface tension liquid filling the through-holes of the sample is gradually expelled downward, starting with the larger diameter holes. The diameter of the hole corresponding to the pressure at which the liquid is first expelled (bubble point pressure) is taken as the bubble point diameter. The relationship between bubble point pressure (kPa) and bubble point diameter (μm) is given by the following formula:
Bubble point diameter (μm) = 45.6 ÷ bubble point pressure (kPa)
This method complies with JIS K3832 and ASTM F316-86.
Sixteen circular samples with a diameter of 25 mm were punched out from a 10 cm x 10 cm gas diffusion electrode, and the bubble point pressure (diameter) was measured. The minimum bubble point pressure and the maximum bubble point diameter were taken as the values for that gas diffusion electrode. Incidentally, if pure water is used instead of the low surface tension liquid ("Galwick (USA)"), the so-called "water permeation pressure" is measured, but with the gas diffusion electrode of the present invention, the pressure became too high (over 150 kPa) and was not measurable.

(k) Porosity The porosity of the conductive porous substrate can be expressed as the average value of 20 randomly selected different locations in the microporous layer of the gas diffusion electrode from the perpendicular cross section, magnified by about 20,000 times, photographed, and binarized to measure the porosity of each image. The cross section of the conductive porous substrate can be prepared using an ion milling device IM4000 manufactured by Hitachi High-Technologies Corporation or its equivalent.

(l) Power generation performance Using the obtained gas diffusion electrodes, an electrolyte membrane/catalyst layer integrated product (a Gore Select (registered trademark) electrolyte membrane manufactured by Gore Japan with PRIMEA (registered trademark) catalyst layers manufactured by Gore Japan formed on both sides) was sandwiched so that the catalyst layer and the microporous layer were in contact with each other, and hot-pressed at 130°C to produce a membrane electrode assembly (MEA). This membrane electrode assembly was incorporated into a single fuel cell, and electricity was generated using a high-boiling point solvent as a refrigerant at a cell temperature of 120°C, a gas pressure of 3 atmospheres, and humidification at a relative humidity of 90%, and the voltage value at a current density of 1.5 A/ ^cm2 was used as an index of dry-up resistance.

Example 1
A gas diffusion electrode was manufactured using the method described in (d) above. A vessel equipped with a double helical ribbon impeller as shown in FIG. 1 was used as vessel 1. The impeller diameter was 0.90 m, the stirring speed was 16 rpm, and the peripheral speed of the outer periphery of the impeller was 0.75 m/s. Polypropylene (PP) beads were added to the coating solution during stirring, and the movement of the beads was visually observed. The beads that had once sunk to the bottom of the vessel reappeared on the surface of the liquid, confirming the presence of vertical convection. The agitator was operated continuously until the coating solution in the tank could no longer be delivered to the applicator and bubbles appeared on the coating surface. From the start of coating of the coating solution on the substrate to the end of coating, the length of the carbon paper coated with the coating solution (coating yield) was 3,400 m, and the coating solution usage efficiency calculated using the method described in (f) above was 96%.

The average value, standard deviation, and relative standard deviation of the microporous layer basis weight, coating thickness, and in-plane gas permeability of the gas diffusion electrode, measured every 200 m from the start to the end of coating using the methods described above in (e), (g), and (h), are shown in Table 1.

In addition, gas diffusion electrode samples were taken from a portion of the wound gas diffusion electrode roll near the start of coating, and the number of through-holes, bubble point diameter, and power generation performance at high temperature (120°C) were measured using the methods described above in (i), (j), and (l). These values are also shown in Table 1.

Example 2
A gas diffusion electrode was manufactured in the same manner as in Example 1, except that the rotation speed of the stirring blade was set to 5 rpm and the peripheral speed of the stirring blade's outer periphery was set to 0.24 m/s. During stirring, the presence of vertical convection was confirmed, as in Example 1. The coating liquid usage efficiency and longitudinal uniformity of each physical property were almost equivalent to those in Example 1. The number of through-holes, bubble point diameter, and power generation performance were also equivalent to those in Example 1.

Example 3
A gas diffusion electrode was manufactured in the same manner as in Example 1, except that the rotation speed of the stirring blade was set to 2 rpm and the peripheral speed of the outer periphery of the stirring blade was set to 0.09 m/s. During stirring, the presence of vertical convection was confirmed as in Example 1, and almost the same coating liquid usage efficiency and longitudinal uniformity of each physical property as in Example 1 were obtained. The number of through-holes, bubble point diameter, and power generation performance were also similar to those in Example 1.

(Comparative Example 1)
A gas diffusion electrode was produced in the same manner as in Example 1, except that the rotation speed of the stirring blade was set to 19 rpm and the peripheral speed of the stirring blade at its outer periphery was set to 0.89 m/s. During stirring, the presence of vertical convection was confirmed, as in Example 1.

(Comparative Example 2)
After degassing the coating liquid, the operation of the stirrer was stopped, and the coating liquid was fed and applied. Except for this, a gas diffusion electrode was produced in the same manner as in Example 1.

(Comparative Example 3)
A gas diffusion electrode was manufactured in the same manner as in Example 2, except that the shape of the stirring blade was changed to the anchor blade shown in Figure 4. In this example, polypropylene (PP) beads were added to the coating solution during stirring, and their movement was visually observed. Once the beads sank, they could not be seen again on the liquid surface, and it was determined that no vertical convection was present. In addition, some foaming was observed in the coating solution in the container.

Comparative Examples 1 to 3 showed a lower coating fluid usage efficiency and larger variations (relative standard deviations) in the physical property values compared to Examples 1 to 3. Furthermore, the number of through-holes on the microporous layer surface was greater in the comparative examples than in the examples, and power generation performance at high temperatures also deteriorated.

The manufacturing method of the present invention is effective for producing long gas diffusion electrodes, improving the efficiency of use of the microporous layer coating liquid and enabling the production of gas diffusion electrodes with uniform physical properties related to the microporous layer, thereby reducing manufacturing costs and achieving uniform quality.

REFERENCE SIGNS LIST 1 Container 2 Stirrer 21 Stirring blade 22 Stirring shaft 3 Pressure reducing means connection port 4 Coating liquid 5 Stirring blade diameter 6 Tank inner diameter 7 Discharge port 101 Conductive porous substrate 102 Unwinder 103 Guide roll (non-driven)
104 Coating machine 106 Back roll (drive)
107 Dryer 108 Sintering furnace 109 Winder 110 Insertion paper 111 Insertion paper unwinder 113 Liquid feed pump 114 Filter 202 Microporous layer 203 Permeation of microporous layer into conductive porous substrate 204 Thickness of microporous layer 205 Thickness of conductive porous substrate 301 Mass flow controller 302 Primary side piping A
303 Valve A
304 Pressure controller 305 Valve B
306 Piping B
307 Gas Chamber A
308 Gas diffusion electrode sample 309 Gas chamber B
310 Piping C
311 Gas flow meter 312 Sealing material 313 Nitrogen gas

Claims (9)

A method for producing a gas diffusion electrode, comprising: a coating step of applying a microporous layer coating liquid containing conductive particles, a water-repellent resin, and a dispersion medium to a conductive porous substrate to form a microporous layer; a drying step of removing the dispersion medium from the microporous layer; and a sintering step of sintering the microporous layer and the water-repellent resin of the conductive porous substrate,
In the coating step,
a microporous layer coating liquid stirring device having a container for stirring and an agitator comprising at least a stirring shaft and a stirring blade;
a coater for applying the microporous layer coating liquid to the conductive porous substrate;
A method for producing a gas diffusion electrode, comprising: using an agitator for a gas diffusion electrode having piping and a liquid delivery device for delivering the microporous layer coating liquid from a microporous layer coating liquid agitator to a coater; delivering the microporous layer coating liquid by the liquid delivery device and applying the coating liquid while stirring the microporous layer coating liquid with the agitator under conditions that generate vertical convection in the microporous layer coating liquid in a container; and wherein the peripheral speed of the outermost periphery of the agitating blade during this process is 0.8 m/s or less. The method for manufacturing a gas diffusion electrode according to claim 1, characterized in that the microporous layer coating liquid is a water-based coating liquid. The method for manufacturing a gas diffusion electrode according to claim 1 or 2, characterized in that the viscosity of the microporous layer coating liquid is 1.0 Pa·s or more. The method for manufacturing a gas diffusion electrode according to any one of claims 1 to 3, characterized in that the shear thinning index (the ratio (a/b) of the viscosity value (a) at a shear rate (shear rate) of 17 (/sec) to the viscosity value (b) at a shear rate of 127 (/sec)) of the microporous layer coating liquid is 1.5 or greater. The method for manufacturing a gas diffusion electrode according to any one of claims 1 to 4, wherein the shape of the stirring blade is a helical ribbon blade or a paddle blade. A method for manufacturing a gas diffusion electrode according to any one of claims 1 to 5, characterized in that the rotation speed of the stirring blade is 20 rpm or less. A gas diffusion electrode comprising a conductive porous substrate and a microporous layer for a gas diffusion layer containing conductive particles and a water-repellent resin, wherein the conductive particles contained in the microporous layer are carbon black, the number of through-holes extending from the surface of the microporous layer through the gas diffusion electrode is less than 5 per 10 cm square (100 cm ² ), and the maximum bubble point diameter is 2.0 μm or less. 8. The gas diffusion electrode according to claim 7, wherein the number of through-holes extending from the surface of the microporous layer through the gas diffusion electrode is less than 2 per 10 cm square (100 cm ² ), and the maximum bubble point diameter is 1.0 μm or less. A gas diffusion electrode as described in claim 7 or 8, characterized in that the carbon black is acetylene black.