JP2019215993A - Production method of coating liquid - Google Patents

Abstract

To provide a production method of a coating liquid, which efficiently removes bubble contained in the coating liquid for forming a fine porous layer of a gas diffusion electrode.SOLUTION: Provided is a production method of a coating liquid for forming a fine porous layer of a gas diffusion electrode, the coating liquid containing conductive particles and a water repellent resin, where pressure applied to the coating liquid is set by using a defoaming device provided with a means which enables defoaming under a reduced pressure, the coating liquid being sealed in the device, to 30 kPa in absolute pressure or less and a vapor pressure or more of a solvent of the coating liquid at a liquid temperature during a defoaming treatment, and where a preset temperature of the device or a liquid temperature in the device is selected in a range 15 degree C and 0 degree C.SELECTED DRAWING: None

Description

  Fuel cells are mechanisms that electrically extract energy generated when hydrogen and oxygen are reacted to produce water, and are expected to spread as clean energy because they have high energy efficiency and only water is discharged. Have been. The present invention relates to a gas diffusion electrode used for a fuel cell, and in particular, for forming a microporous layer of a gas diffusion electrode used for a polymer electrolyte fuel cell used as a power source of a fuel cell vehicle among fuel cells. The present invention relates to a method for producing a coating liquid.

  Electrodes used in a polymer electrolyte fuel cell are sandwiched between two separators in a polymer electrolyte fuel cell, and are disposed between the two separators. It has a structure consisting of a formed catalyst layer and a gas diffusion layer formed outside the catalyst layer. Gas diffusion electrodes are distributed as individual members for forming a gas diffusion layer at an electrode. The performance required of the gas diffusion electrode includes, for example, gas diffusion, conductivity for collecting electricity generated in the catalyst layer, and drainage for efficiently removing moisture generated on the surface of the catalyst layer. can give. In order to obtain such a gas diffusion electrode, generally, a conductive porous substrate having both gas diffusion ability and conductivity is used.

  As the conductive porous substrate, specifically, carbon felt made of carbon fiber, carbon paper, carbon cloth, and the like are used, and among them, carbon paper is most preferable in terms of mechanical strength and the like.

When the conductive porous substrate as described above is used as it is as a gas diffusion electrode, the fibers thereof are coarse, so that when water vapor condenses, large water droplets are generated and flooding is likely to occur. Therefore, a coating liquid in which conductive fine particles such as carbon black are dispersed is applied onto a water-repellent conductive porous base material and dried and sintered to form a layer called a microporous layer (microporous layer). (Also referred to as a layer). In addition, the role of the microporous layer is to make up the effect of preventing the roughness of the conductive porous substrate from being transferred to the electrolyte membrane. For example, the contact resistance (electric resistance) of the diffusion layer may be reduced. Since the roughness (arithmetic mean roughness) of the conductive porous substrate is usually 10 to 30 μm, the thickness of the microporous layer on the substrate should be 10 to 80 μm, which is large for wet coating, in order to obtain a makeup retouching effect. It costs. In order to ensure such a thickness and to prevent penetration into the porous substrate, the coating liquid for forming the microporous layer is required to have a high viscosity.
In addition, the coating liquid is preferably water-based from the viewpoint of reducing the environmental load and cost. When dispersing hydrophobic conductive fine particles in water as a solvent, a surfactant may be added as a dispersant. In order to increase the viscosity of the coating liquid, a surfactant may be added as a thickener. As described above, bubbles are easily generated in the water-based coating liquid to which a surfactant is added, and since the viscosity is high, bubbles generated once are difficult to be removed. If the coating is performed while bubbles are contained in the coating liquid, a defect will occur in the coating film. For this reason, the coating liquid needs to be sufficiently defoamed before coating.
As a general method of the defoaming treatment of the liquid, for example, a decompression method can be mentioned. Specifically, there is a method in which the liquid is allowed to stand in the depressurized tank, or a method in which the liquid in the depressurized tank is stirred. However, in this method, particularly in the case of a highly viscous liquid, once air bubbles are mixed in, it is very difficult to remove the air bubbles, so that a long-time processing may be required in batch processing. In addition, if the liquid is held under a reduced pressure for a long time, the component ratio of the liquid changes due to volatilization. When the composition ratio of the liquid changes, the performance of the product using the liquid also decreases.

  Therefore, in order to improve the conventional decompression method, an efficient defoaming method for a highly viscous liquid has been studied. Patent Document 1 discusses defoaming of a highly viscous liquid having a viscosity of 30 Pa · s or more. More specifically, when the liquid level reaches the exhaust pipe in a reduced pressure state, the pressure in the container is increased by closing the reduced pressure exhaust valve. Then, when the liquid level decreases, the pressure reduction is restarted. If the liquid level does not reach the exhaust pipe even if the pressure is reduced, set the stirring speed to a speed that reduces the required power per unit volume to １ ／, and reduce the pressure until the defoaming is completed. It is a defoaming method of stirring underneath. Patent Literature 2 describes a method in which a liquid in which ceramic particles are dispersed is evacuated in a container and a shear stress is applied to the liquid by shaking the container in a horizontal plane to remove bubbles.

  Patent Document 3 discloses a process of preparing a paste and maintaining the paste under a reduced pressure of 60 torr or less.

JP-A-2002-113303 JP-A-4-256404 Japanese Patent No. 5148036

However, the defoaming method described in Patent Document 1 requires that the pressure and the number of rotations in the container be changed one by one, and the operation becomes complicated.

  In addition, in some liquids having dispersed fine particles, when a shear stress is excessively applied, the dispersion structure in the liquid is changed and the viscosity is significantly reduced. In the case of treating a large amount of liquid, it is necessary to apply an excessive shear stress to the liquid, which may cause a change in the dispersion structure of the liquid.

  Further, in the method of merely keeping the paste under reduced pressure after the preparation of the paste as in Patent Document 3, although the bubbles expand according to the pressure and the rising speed of the bubbles increases, it is not sufficient as a defoaming method.

  An object of the present invention is to provide a defoaming method capable of efficiently removing bubbles contained in a high-viscosity paint in a short time.

The method for producing a coating liquid for forming a microporous layer of a gas diffusion electrode according to the present invention has the following configuration in order to solve the above problems.
(1) A method for producing a coating liquid for forming a microporous layer of a gas diffusion electrode, comprising a conductive particle and a water-repellent resin,
Using a defoaming device equipped with means capable of defoaming under reduced pressure,
The pressure applied to the coating liquid sealed in the apparatus is 30 kPa or less in absolute pressure, not less than the vapor pressure at the liquid temperature during the defoaming treatment of the solvent of the liquid, and
A method for producing a coating liquid, wherein a set temperature of the apparatus or a liquid temperature in the apparatus is selected from a range of 15 degrees or less and greater than 0 degrees.
(2) The viscosity of the coating liquid is
At a temperature of 23 degrees Celsius is 1 Pa.s or more and 30 Pa.s or less,
(1) The method for producing a coating liquid according to (1), wherein the viscosity at a temperature selected from the range of 15 degrees or less and greater than 0 degrees is 1 Pa · s or more lower than the viscosity at 23 degrees.

  By using the production method of the present invention, bubbles can be efficiently removed from the high-viscosity coating solution, and thus the production efficiency is improved. In addition, a gas diffusion electrode having no coating defect in the microporous layer of the gas diffusion layer can be obtained with a high yield, and the manufacturing cost of the gas diffusion layer and thus the fuel cell can be reduced.

Schematic diagram of a decompressible water-cooled jacketed stirring tank used in Examples of defoaming the microporous layer coating liquid of the present invention Schematic diagram of the apparatus used for applying the microporous layer coating liquid of the present invention

  The gas diffusion electrode targeted by the production method of the present invention has a microporous layer on at least one surface of a conductive porous substrate. Alternatively, only the microporous layer may be used. Regarding such a gas diffusion electrode to which the present invention is applied, a conductive porous substrate will be described first.

  In a polymer electrolyte fuel cell, a gas diffusion electrode has a high gas diffusivity for diffusing gas supplied from a separator to a catalyst, and a high drainage for discharging water generated by an electrochemical reaction to the separator. In order to take out the generated current and generated current, high conductivity is required. For this reason, a conductive porous substrate, which is a porous substrate having conductivity and having a pore diameter peak in a region of usually 10 μm or more and 100 μm or less, is used for the gas diffusion electrode.

  As the conductive porous substrate, specifically, for example, a porous substrate containing carbon fibers such as carbon fiber woven fabric, carbon fiber paper, carbon fiber nonwoven fabric, carbon felt, carbon paper, carbon cloth, foam firing It is preferable to use a porous metal substrate such as a binding metal, a metal mesh, or an expanded metal. Among them, it is preferable to use a porous substrate such as carbon felt containing carbon fibers, carbon paper, carbon cloth, etc., since the corrosion resistance is excellent, and in particular, the property of absorbing a dimensional change in the thickness direction of the electrolyte membrane, that is, It is preferable to use a base material obtained by binding a carbon fiber paper body with a carbide, that is, carbon paper, because of its excellent “spring properties”.

  In order to increase the gas diffusivity of the gas diffusion electrode and to maximize the power generation performance of the fuel cell, a high porosity is required for the conductive porous substrate. The porosity is preferably at least 80%, more preferably at least 85%. The upper limit of the porosity is 95% as a limit that the conductive porous substrate can keep its structure.

    Also, by reducing the thickness of the conductive porous substrate such as carbon paper, the gas diffusibility of the gas diffusion electrode can be increased, so that the thickness of the conductive porous substrate such as carbon paper is 220 μm or less. Is preferably 150 μm or less, more preferably 120 μm or less. It is preferable that the lower limit of the thickness of the conductive porous substrate be 50 μm, because mechanical strength can be maintained and handling in the manufacturing process can be facilitated. Reducing the thickness of the conductive substrate is also effective in reducing the electric resistance in the thickness direction when the fuel cell is used.

  In order to efficiently manufacture a gas diffusion electrode using the conductive porous substrate, the conductive porous substrate is unwound in a state in which the conductive porous substrate is rolled up in a long length, and is continuously wound before being wound. It is preferable to form a microporous layer.

  The conductive porous substrate may be subjected to a water-repellent treatment in order to enhance drainage. The water-repellent treatment is preferably performed using a water-repellent resin such as a fluororesin. Examples of the fluororesin include PTFE (polytetrafluoroethylene) (for example, “Teflon” (registered trademark)), FEP (ethylene tetrafluoride hexafluoropropylene copolymer), PFA (perfluoroalkoxy fluorinated resin), ETFA (ethylene Examples thereof include tetrafluoroethylene copolymer), PVDF (polyvinylidene fluoride), and PVF (polyvinyl fluoride), and PTFE or FEP exhibiting strong water repellency is preferable.

  The amount of the water-repellent resin is not particularly limited, but is preferably 0.1% by mass or more and 20% by mass or less based on 100% by mass of the whole conductive porous substrate. In this range, the water repellency is sufficiently exhibited, while the water repellent resin is less likely to block the pores serving as the gas diffusion path or the drain path, or to increase the electric resistance.

  The method of water-repellent treatment of the conductive porous base material includes, in addition to a generally known processing technique of dipping the conductive porous base material in a dispersion containing a water-repellent resin, conductive coating by die coating, spray coating, or the like. An application technique for applying a water-repellent resin to a porous substrate is also applicable. Processing by a dry process such as sputtering of a fluororesin can also be applied. After the water repellent treatment, a drying step and a sintering step may be added as necessary.

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. As the conductive particles, carbon black is preferably used from the viewpoint of low cost, safety and stability of product quality. As carbon black contained in the microporous layer, acetylene black is preferably used because it has few impurities and is hard to lower the activity of the catalyst.
In addition, the microporous layer has properties such as conductivity, gas diffusion, water drainage, 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. Since the microporous layer is required to have a property, it is preferable that the microporous layer contains a water-repellent resin such as a fluororesin in addition to the conductive particles. Examples of the fluororesin contained in the microporous layer include PTFE, FEP, PFA, ETFA and the like, similarly to the fluororesin suitably used when water repelling the conductive porous substrate. PTFE or FEP is preferred because of its particularly high water repellency.

  In order for the gas diffusion electrode to have a microporous layer, it is common to apply a coating liquid (hereinafter, referred to as a coating liquid) for forming the microporous layer to the conductive porous substrate. The coating liquid usually contains the conductive particles and a dispersion medium such as water or alcohol, but it is preferable to use water as a solvent from the viewpoint of reducing the environmental load and simplifying the coating and drying process. Surfactants and the like are often blended as a dispersant for dispersing the conductive particles. When the water-repellent resin is contained in the microporous layer, it is preferable that the coating liquid contains the water-repellent resin in advance.

  In order to form a gas diffusion electrode only with a microporous layer, it is common to apply and peel off a film.

  The concentration of the conductive particles in the coating liquid is preferably 5% by mass or more, more preferably 10% by mass or more, from the viewpoint of coating liquid productivity. There is no upper limit on the concentration as long as the viscosity, the dispersion stability of the conductive particles, and the applicability of the coating liquid are favorable. When acetylene black is used as the conductive particles, in the case of an aqueous coating solution, the upper limit of the concentration of acetylene black in the coating solution is preferably at or around 25% by mass. Black does not re-aggregate, so-called percolation does not occur, and there is a low possibility that the applicability of the coating liquid is impaired due to a sharp increase in viscosity.

  The role of the microporous layer is as follows: (1) protection of the catalyst; (2) a makeup repair effect of preventing the surface of the coarse conductive porous substrate from being transferred to the electrolyte membrane; and (3) water vapor generated at the cathode. This is the effect of preventing condensation. Of the above, it is preferable that the microporous layer has a certain thickness in order to exhibit a makeup retouching effect. Further, in order to form a gas diffusion electrode only with a microporous layer, it is preferable to have a certain thickness from the viewpoint of strength.

  The thickness of the microporous layer targeted by the production method of the present invention is, when applied to a conductive porous substrate, the dry film thickness is more than 10 μm in consideration of the current roughness of the conductive porous substrate. It is preferably as large as 60 μm or less. If the thickness of the microporous layer is 10 μm or less, the above-described makeup repair effect may be insufficient, and if it exceeds 60 μm, the gas diffusion property (permeability) of the gas diffusion electrode itself may decrease or the electrical resistance may increase. Sometimes. The thickness of the microporous layer is preferably 50 μm or less, more preferably 40 μm or less, from the viewpoint of increasing gas diffusivity or reducing electric resistance. The microporous layer may penetrate into the pores of the conductive porous substrate, and a portion of the microporous layer may be formed in the conductive porous substrate. The evaluation is made only by the thickness of the portion existing outside the conductive porous substrate, excluding the infiltrated portion. That is, the thickness of the microporous layer is defined as a value obtained by subtracting the thickness of the conductive porous substrate from the thickness of the gas diffusion layer. When only the microporous layer is used, the thickness is preferably larger than 10 μm and 5 mm or less from the viewpoint of strength. If the microporous layer has a thickness of 10 μm or less, the strength becomes weak, the durability of the fuel cell becomes weak, and the handling becomes difficult. If it exceeds 5 mm, the gas diffusivity (permeability) may decrease or the electric resistance may increase. The thickness of the microporous layer is preferably 1 mm or less, more preferably 500 μm or less, and still more preferably 100 μm or less, from the viewpoint of increasing gas diffusivity or reducing electric resistance.

  The coating liquid is usually prepared by dispersing conductive particles using a dispersant as described above. In order to disperse the conductive particles, the conductive particles are preferably dispersed using a surfactant as a dispersant. In order to stabilize the dispersion for a long time and prevent the viscosity of the coating liquid from increasing and prevent the liquid from being separated, the amount of the dispersant added should be 10% by mass or more of the conductive particles in the coating liquid. Is preferred. When the content is 500% by mass or more, a load may be applied during the paint sintering step, which is not preferable. If the amount is less than 10% by mass, the dispersion stability of the coating liquid cannot be secured, and a change such as a decrease in viscosity may occur. Further, as in the case of preparing a coating liquid by dispersing conductive particles using a dispersant whose viscosity decreases with a decrease in temperature, the viscosity of the coating liquid itself may exhibit a characteristic of decreasing with a decrease in temperature. The present invention exhibits a particularly remarkable effect when a coating liquid having such characteristics is used. That is, by lowering the temperature during the defoaming treatment, the viscosity of the coating liquid is reduced, and thus the effect of further improving the defoaming efficiency is confirmed.

  When the thickness of the microporous layer is made larger than 10 μm as a coating film after drying as described above, it is preferable that the viscosity of the coating liquid at the time of coating be kept at least 1 Pa · s or more. If the viscosity of the coating liquid is lower than this, the coating liquid may flow on the surface of the conductive porous substrate or film and may not have a desired thickness, or the coating liquid may flow into the pores of the conductive porous substrate. May flow in and cause strikethrough. Conversely, if the viscosity of the coating liquid is too high, the coatability may decrease, so the upper limit is 30 Pa · s. The preferred viscosity of the coating liquid is 3 Pa · s or more and 20 Pa · s or less, more preferably 5 Pa · s or more and 15 Pa · s or less. The range may be a combination of any of the above upper limits and any of the lower limits. In the present invention, after forming the first microporous layer, a coating liquid for forming the second microporous layer (hereinafter, a second coating liquid) is applied to form the second microporous layer. Although it may be preferable to form, the viscosity of the second coating liquid is preferably lower than the viscosity of the coating liquid for forming the first microporous layer (hereinafter, the first coating liquid), And it is desirable that it is 10 Pa.s or less. Further, in the case of a coating material exhibiting the property of decreasing the viscosity due to the temperature decrease as described above, it is preferable that the viscosity is within the above-mentioned viscosity range at a temperature of 23 ° C. Is preferably 1 Pa · s or more lower than the viscosity at 23 degrees. It is more preferably lower than the viscosity at 23 degrees by 2 Pa.s or more, and particularly preferably lower than the viscosity at 23 degrees by 4 Pa.s or more. It is more preferable that the above-mentioned viscosity decrease occurs in the entire temperature range of 15 degrees or less and 0 degrees. It is considered that the lower the viscosity difference, the better the defoaming property is.However, the lower limit of the viscosity is the viscosity of the solvent alone contained in the coating liquid at that temperature, and it is considered that the viscosity is lower than this in terms of physical properties. Absent.

  In order to keep the viscosity of the coating liquid high as described above, it is effective to add a thickener. The thickener used here may be a generally well-known one. For example, a methylcellulose type, a polyethylene glycol type, a polyvinyl alcohol type or the like is suitably used.

These dispersants and thickeners may have the same substance have two or more functions, and materials suitable for each function may be selected. However, when the thickener and the dispersant are separately selected, it is preferable to select one that does not break the dispersion of the conductive particles and the dispersion of the fluororesin that is the water-repellent resin. The dispersant and the thickener are collectively referred to herein as a surfactant. In order to maintain high viscosity and dispersion stability of the coating liquid, the total amount of the surfactant is preferably 10% by mass or more, more preferably 50% by mass or more, even more preferably 100% by mass or more of the added mass of the conductive particles. is there. The preferable upper limit of the amount of the surfactant added is usually 500% by mass or less of the added mass of the conductive particles, and within this preferable range, it is difficult to generate steam or decomposition gas in the subsequent sintering step, and the safety is increased. , Ensuring productivity.
The coating liquid containing these components can be dispersed with various dispersing devices, but the coating liquid containing the components as described above often decreases in viscosity if dispersion is excessively advanced, Even if the viscosity is adjusted in minutes, the coating liquid tends to seep into the porous substrate. For this reason, it may be necessary to keep the variance at a low level and keep it. Therefore, it is preferable that a shearing force that changes the dispersion state of the conductive particles is not applied as much as possible after the preparation of the coating solution and before the application to the substrate.
As described above, since the prepared coating liquid contains a surfactant in an aqueous system, bubbles are easily generated by operations such as stirring. If air bubbles are applied to the substrate while being contained in the coating liquid, a coating film is not formed in the area where the air bubbles are present, and a phenomenon called so-called coating loss occurs, which deteriorates the function of the microporous layer. There is a risk of doing it. Therefore, the coating liquid must be sufficiently defoamed before coating.
In the defoaming method of the present invention, a defoaming device including at least a device capable of defoaming the coating liquid under reduced pressure is used.
The device capable of defoaming the coating solution under reduced pressure may be any device capable of defoaming from the coating solution under reduced pressure.For example, a decompressible container or a stirring blade in a decompressible container may be used. Examples include a so-called stirring tank, a planetary stirring device (rotating revolving mixer), a continuous defoaming machine, and a defoaming pump having a used structure. Means for adjusting the pressure of a device capable of defoaming from a coating solution under reduced pressure is a pressure applied to the coating solution sealed in the device, in other words, a pressure contributing to defoaming, Any pressure can be used as long as the pressure can be set to 30 kPa or less and the vapor pressure at the temperature of the liquid during the defoaming treatment of the solvent in the liquid in the above-described apparatus, and examples thereof include a vacuum pump and an aspirator. As the means for controlling the temperature of the liquid in the apparatus or the apparatus, any means can be used as long as the temperature of the liquid at the moment when the defoaming treatment is performed can be controlled. And using a water-cooled jacket and chiller, a Peltier element, and a pre-cooled coating liquid.
In the defoaming method of the present invention, the temperature of the device capable of defoaming from the coating liquid under reduced pressure is maintained at 15 ° C. or lower and higher than 0 ° C. using a means for controlling the temperature. However, since relatively long time may be required for defoaming, the set temperature in the above apparatus may be set in the above range, and as a result, the liquid temperature can be considered to be in the above range. On the other hand, the temperature of the defoaming device does not necessarily need to be set in the above range.For example, the coating liquid may be transferred to the defoaming device in another device or container in the above range, It is sufficient that the temperature of the coating liquid does not deviate from the above range during defoaming by adjusting the atmosphere or the like.
Regarding the reason for setting the liquid temperature, if the temperature exceeds 15 degrees, the temperature is too high and the efficiency improvement effect is small, and if it is 0 degrees or less, there is a possibility of freezing. More preferably, it is 10 degrees or less and 4 degrees or more. The range may be a combination of any of the above upper limits and any of the lower limits.
Further, the pressure contributing to the defoaming of the above-described apparatus is, as an absolute pressure, equal to or higher than the vapor pressure at the temperature during the defoaming of the solvent contained in the coating liquid. It is good for suppressing that it changes. For the above reasons, when a plurality of types of solvents are contained in the coating liquid, it is preferable that the vapor pressure be equal to or higher than the vapor pressure of all the solvents. On the other hand, if the pressure is too high, the efficiency of defoaming is impaired. Therefore, the absolute pressure is preferably 30 kPa or less, more preferably 30 kPa or less and the vapor pressure +10 kPa or less. More preferably, the vapor pressure is equal to or lower than +5 kPa, and further preferably, the vapor pressure is equal to the vapor pressure. Further, since the vapor pressure also changes depending on the temperature of the coating liquid, the pressure may be changed according to the liquid temperature at the time of defoaming.
The coating liquid prepared and defoamed as described above is applied to a conductive porous substrate or peeled off after being applied to a film to produce a gas diffusion electrode.

  The application of the coating liquid to the conductive porous substrate or film can be performed using various commercially available coating apparatuses. As the coating method, screen printing, rotary screen printing, spray spraying, intaglio printing, gravure printing, die coater coating, bar coating, blade coating, roll knife coater coating, etc. can be used, but the surface roughness of the conductive porous substrate can be used. The application by a die coater is preferable because the amount of application can be quantified regardless of this. Further, when a gas diffusion electrode is incorporated in a fuel cell, when smoothness of the coated surface is required to enhance adhesion to the catalyst layer, coating with a blade coater or a roll knife coater is preferably used. The application methods described above are for illustration only, and are not necessarily limited thereto.

  After applying the coating solution, the dispersion medium (water in the case of an aqueous system) of the coating solution is dried and removed as necessary. When the dispersion medium is water, the drying temperature after coating is preferably from room temperature (about 20 ° C.) to 150 ° C. or lower, more preferably from 60 ° C. to 120 ° C. The range may be a combination of any of the above upper limits and any of the lower limits. The drying of the dispersion medium (for example, water) may be performed collectively in a subsequent sintering step.

  After applying the coating liquid, sintering is generally performed for the purpose of removing the surfactant used in the coating liquid and for dissolving the water-repellent resin once to bind the conductive particles.

  The sintering temperature depends on the boiling point or decomposition temperature of the added surfactant, but is preferably from 250 ° C. to 400 ° C. When the sintering temperature is in this preferred range, the removal of the surfactant can be sufficiently achieved, and the possibility that the water-repellent resin is decomposed is low. When only a microporous layer is used, a high heat resistance is required for a substrate film in order to perform sintering. For example, a Kapton film is used.

    Hereinafter, the present invention will be described in more detail by way of examples, but the present invention is not limited to these examples.

(A) Preparation of coating liquid 15% by mass of "Denka Black" (registered trademark) manufactured by Electrochemical Industry Co., Ltd., 5% by mass of PTFE dispersion ("Polyflon" (registered trademark) D-210C manufactured by Daikin Industries, Ltd.) 5% by mass of a surfactant (manufactured by Nacalai Tesque, Inc., "TRITON" (registered trademark) X-100) and 75% by mass of ion-exchanged water were kneaded with a planetary mixer. The viscosity of the coating liquid was 10 Pa · s at a temperature of 23 degrees, 7 Pa · s at a temperature of 15 degrees, and 3 Pa · s at a temperature of 5 degrees.

10.0 L of the prepared coating liquid was transferred to a stirring tank. As the stirring blade, a helical ribbon blade was used. The mixture was stirred at atmospheric pressure at a blade tip speed of 1.5 m / s for 30 minutes to obtain a coating liquid containing bubbles. When the number of bubbles of this coating solution was measured by the method shown in (c) described later, it was 15 to 25 / 0.2 g (see Table 1).
(B) Viscosity measurement In a viscosity measurement mode of a spectrin borin rotary rheometer, using a circular cone plate having a diameter of 40 mm and an inclination of 2 °, arbitrarily sampling while increasing the number of rotations of the plate (while increasing the shear rate). The shear stress of the applied coating solution is measured. At this time, the value of the viscosity of the coating liquid at a shear rate of 17 (1 / s) was read. The measurement was carried out at N = 3 while changing the measurement sample, and the arithmetic average value was defined as the viscosity of the coating liquid.
(C) Measurement of Number of Bubbles in Coating Liquid Two glass plates each having a square shape of 15 cm on a side and a thickness of 0.5 cm are prepared, and a fluorine tape (ASF-110FR manufactured by Chuko Kasei Kogyo Co., Ltd.) is attached to each of four corners. Then, 0.2 g of the coating liquid arbitrarily sampled was allowed to stand for 30 minutes between upper and lower sides. Thereafter, the number of bubbles in an image taken by a digital camera or the like was visually counted for a sample observed using ILLMINATED MAGNIFIERS OSL-1 manufactured by Otsuka Optics. This operation was performed 20 times, and the arithmetic average value of the obtained numbers was defined as the number of bubbles in the coating liquid.
(D) Confirmation of air bubbles in actual application The prepared microporous layer coating solution is charged into a tank as shown in FIG. 1, a piping system as shown in FIG. 2 is assembled, and a die coater as shown in FIG. Using a PET film or carbon paper (width: 240 mm) with a width of 300 mm, a thickness of 150 μm when pressed in the thickness direction at a pressure of 0.15 MPa, a density of 0.45 g / cm ³ , and a porosity of 80%, a coating liquid flow rate of 500 g / The coating was performed at a substrate transport speed of 5 m / min for 10 minutes, and it was visually confirmed whether or not defects due to bubbles were generated on the coating film.
(Example 1)
10 L of a coating solution containing air bubbles was charged into a stirring tank equipped with a water-cooling jacket capable of decompression (the stirring blade was a double helical ribbon), and the water-cooling jacket was cooled by a chiller to adjust the coating solution temperature to 5 degrees. Using a vacuum pump, the pressure in the vessel is changed from the atmospheric pressure state to an absolute pressure of 1 kPa over 0.5 minutes, the peripheral speed of the stirring blade is rotated at about 1 m / s, and defoaming is performed for 20 minutes in that state. Carried out. Here, the peripheral speed is the tip speed of the stirring blade, and is defined by the following equation.
Blade tip speed (m / s) = number of rotations (rpm) / 60 x diameter of stirring blade d (m) x pi The number of bubbles in the coating liquid after the defoaming treatment was measured.
(Example 2)
The defoaming treatment was performed under the same conditions as in Example 1 except that the pressure was 10 kPa in absolute pressure, and the measurement of the number of bubbles in the coating liquid was performed thereafter.
Example 3
The defoaming treatment was performed under the same conditions as in Example 2 except that the temperature of the coating liquid was changed to 15 degrees, and the subsequent measurement of the number of bubbles in the coating liquid was performed.
(Comparative Example 1)
The defoaming treatment was performed under the same conditions as in Example 1 except that the pressure was 40 kPa in absolute pressure, and the number of bubbles in the coating liquid was measured thereafter.
(Comparative Example 2)
The defoaming treatment was performed under the same conditions as in Example 2 except that the coating liquid temperature was set to 23 ° C., and the number of bubbles in the coating liquid was measured thereafter.
(Example 4)
0.5 L of the coating solution containing air bubbles was charged into a dedicated container for the planetary stirring device, and allowed to stand in a room at a temperature of 5 ° C. for 24 hours, so that the temperature of the coating solution was about 5 ° C. The coating solution was defoamed with a planetary stirrer ("ARV-930TWIN" manufactured by Sinky Co., Ltd.) in a DEFOM mode at 1200 rpm and a pressure of 30 kPa in absolute pressure for 1 minute. The number of bubbles in the coating solution after the defoaming treatment was measured.
(Comparative Example 3)
The treatment was carried out under the same conditions as in Example 4 except that the temperature of the coating liquid was kept at about 23 ° C. for 24 hours in a room where the temperature was maintained at 23 ° C., and the subsequent measurement of the number of bubbles in the coating liquid Was carried out.

  ADVANTAGE OF THE INVENTION This invention can defoam the coating liquid for microporous layers of the gas diffusion layer for fuel cells easily and at low cost, and can contribute to the reduction of fuel cell manufacturing cost.

101 conductive porous substrate (carbon paper)
102 Unwinding machine 103 Guide roll (not driven)
104 Die coater A
105 Die coater B
106 Back roll (drive)
107 dryer 108 sintering furnace 109 take-up machine 110 interleaf paper 111 interleaf paper unwinder 112 coating liquid tank 113 liquid feed pump 114 filter 115 immersion tank

Claims (2)

Including a conductive particle and a water-repellent resin, a method for producing a coating liquid for forming a microporous layer of a gas diffusion electrode,
Using a defoaming device equipped with means capable of defoaming under reduced pressure,
The pressure applied to the coating liquid sealed in the apparatus is 30 kPa or less in absolute pressure, not less than the vapor pressure at the liquid temperature during the defoaming treatment of the solvent of the liquid, and
A method for producing a coating liquid, wherein a set temperature of the apparatus or a liquid temperature in the apparatus is selected from a range of 15 degrees or less and greater than 0 degrees. The viscosity of the coating liquid,
At a temperature of 23 degrees Celsius is 1 Pa.s or more and 30 Pa.s or less,
2. The method for producing a coating liquid according to claim 1, wherein the viscosity at a temperature selected from a range of 15 degrees or less and greater than 0 degrees is 1 Pa · s or less lower than the viscosity at 23 degrees.

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