JP7207025B2 - Paste for microporous layer and method for producing the same - Google Patents

Description

TECHNICAL FIELD The present invention relates to a microporous layer paste and a method for producing the same, and more particularly, to a microporous layer paste used for forming a microporous layer on the surface of a gas diffusion layer of a polymer electrolyte fuel cell, and the paste. It relates to a manufacturing method.

A solid polymer fuel cell is based on a membrane electrode assembly (MEA) in which electrodes including catalyst layers are joined to both sides of a solid polymer electrolyte membrane. In polymer electrolyte fuel cells, electrodes generally have a two-layer structure of a gas diffusion layer and a catalyst layer. The gas diffusion layer is for supplying reaction gas and electrons to the catalyst layer, and is made of carbon paper, carbon cloth, or the like. The catalyst layer is a portion that serves as a reaction field for electrode reactions, and is generally composed of a composite of carbon carrying an electrode catalyst such as platinum and a solid polymer electrolyte (catalyst layer ionomer).

Gas diffusion layers of polymer electrolyte fuel cells are required to have high water repellency in addition to high gas permeability. Therefore, the gas diffusion layer is generally formed by forming a microporous layer (a layer containing conductive particles and water-repellent particles and having a relatively large porosity) on the surface of a porous substrate such as carbon paper. Others are used. A microporous layer is generally formed by applying a paste containing conductive particles and water-repellent particles to the surface of a porous substrate, followed by drying and baking.
However, when the paste is applied to the surface of the porous substrate, the paste sometimes penetrates into the interior of the porous substrate (set-off). When the paste permeates, some of the pores of the porous substrate are clogged, and the gas diffusion layer has reduced gas permeability and water repellency.

In order to solve this problem, various proposals have been conventionally made.
For example, Patent Document 1 contains carbon particles, a dispersant, a polytetrafluoroethylene (PTFE) emulsion, ion-exchanged water, and a water-soluble polyethylene oxide (PEO) that thermally decomposes at less than 300°C as a thickening agent. A paste composition for forming a microporous layer is disclosed.
In the same document, the use of water-soluble polyethylene oxide, which thermally decomposes at less than 300°C, as a thickener reduces the load in the drying process, improves the fluidity of the paste, and improves the production efficiency during coating. Moreover, it is described that long-term storage is possible.

Patent Document 2 discloses a gas diffusion method containing carbon black, a fluororesin dispersion, a surfactant, and water, wherein the surfactant is a nonionic surfactant having an HLB value of 14 to 20. A layer water-repellent paste is disclosed.
In the same document,
(A) By adopting such a composition, a water-repellent paste for a gas diffusion layer having excellent flow characteristics and coatability to a substrate can be obtained;
(B) the paste may contain a water-soluble organic solvent that has a high affinity for water;
is described.

Patent Document 3 discloses a water-repellent paste for a gas diffusion layer in which a carbon slurry is mixed with a fluororesin dispersion and a solvent that is difficult to dissolve in water and a solvent that dissolves in water easily, although it is not intended to suppress the penetration of the paste. disclosed.
In the same document,
(A) A high molecular weight fluororesin with high water repellency is likely to be fibrillated by a shearing force during mixing.
(B) When the fluororesin becomes fibrous, it becomes poorly dispersed, the stability of the water-repellent paste is lowered, and this causes poor coating.
(C) A solvent that dissolves easily in water promotes fiberization of the fluororesin, whereas a solvent that dissolves poorly in water suppresses the fiberization of the fluororesin;
(D) By optimizing the ratio of both solvents in the paste, the fluororesin does not fibrillate at relatively low shear forces in pipes and pumps, and fibers form on the substrate when coated at high shear forces. The point that it is possible to convert
is described.

Furthermore, in Non-Patent Document 1, although it is not a paste for a microporous layer of a fuel cell, when a solvent B having a low affinity with the solvent A is added to a paste mainly composed of particles and a solvent A, It is described that a liquid bridging force is generated and a network structure is formed.

In order to suppress penetration of the microporous layer paste into the substrate, it is common to add a water-soluble polymer such as PEO to the paste as a thickening agent. However, in order to impart water repellency to the microporous layer using this method, it is necessary to heat the substrate at a temperature of 300° C. or higher to thermally decompose the thickening agent. Further, as described in Patent Document 1, even if PEO that thermally decomposes at less than 300° C. is used, a thermal decomposition step of the thickener is required, which poses a problem of a large heat load.

Japanese Patent No. 5851386 JP 2016-085894 A Special Meeting No. 2004-247148

"Capillary Forces in Suspension Rheology," Koos, E., et al., Science 2011, 331.6019, 897-900

The problem to be solved by the present invention is to provide a microporous layer paste capable of suppressing penetration (set-off) of the paste into a substrate without using a water-soluble polymer, and a method for producing the same. That's what it is.

In order to solve the above problems, the microporous layer paste according to the present invention has the following configuration.
(1) The microporous layer paste is
conductive particles for imparting conductivity to the microporous layer of the fuel cell;
water repellent particles for imparting water repellency to the microporous layer;
a dispersant for stabilizing the dispersed state of the conductive particles and the water-repellent particles in water;
a hydrophobic organic solvent to increase the viscosity of the paste;
Water for dispersing the conductive particles and the water-repellent particles is provided.
(2) The microporous layer paste has a viscosity of 0.05 Pa·s or more and 1 Pa·s or less at a shear rate of 1000 s ⁻¹ .

The method for producing a microporous layer paste according to the present invention comprises:
A first step of mixing and dispersing conductive particles, water-repellent particles, a dispersant, and water in a predetermined ratio;
and a second step of adding a hydrophobic organic solvent as a thickening agent to the dispersion liquid obtained in the first step and mixing the mixture.

When the conductive particles, hydrophobic particles, and dispersant are dispersed in water, the hydrophobic groups of the dispersant are adsorbed to the aggregates of the conductive particles and the surfaces of the water-repellent particles. When a hydrophobic organic solvent is added to the dispersion liquid in which such particles are dispersed, the hydrophobic organic solvent penetrates into the gaps between the conductive particles to form swollen particles. As a result, the apparent volume of the aggregates of the conductive particles increases, increasing the viscosity of the paste. Further, when the amount of the hydrophobic organic solvent added is further increased, a network structure is formed in which at least a portion of the swollen particles are linked via the liquid-bridging force of the hydrophobic organic solvent. As a result, the viscosity of the paste further increases.

Since the paste thus obtained does not contain water-soluble polymer, heat treatment for thermally decomposing the water-soluble polymer is unnecessary. Therefore, productivity of the paste is improved.

It is a figure which shows the relationship between the shear rate of a paste, and a viscosity. FIG. 4 is a diagram showing the relationship between the content of 1-octanol and viscosity. FIG. 3A is a schematic diagram of a fuel cell microporous layer paste in which X=0 wt % (X is the content of the hydrophobic organic solvent). FIG. 3B is a schematic diagram of a fuel cell microporous layer paste in which 0<X≦1.3 wt %. FIG. 3(C) is a schematic diagram of a fuel cell microporous layer paste in which X>1.3 wt %. Fig. 3 shows the relationship between the octanol/water partition coefficient (log P _ow ) and the viscosity of the paste at a shear rate of 1 s ^-1 ;

An embodiment of the present invention will be described in detail below.
[1. Paste for microporous layer]
The microporous layer paste (hereinafter also simply referred to as "paste") according to the present invention comprises conductive particles, water-repellent particles, a dispersant, a hydrophobic organic solvent, and water.

[1.1. Ingredients of the paste]
[1.1.1. Conductive particles]
[A. material]
The conductive particles are for imparting conductivity to the microporous layer. The material of the conductive particles is not particularly limited as long as it has conductivity and corrosion resistance to withstand the environment in which the fuel cell is used.
Materials for the conductive particles include, for example, carbon materials such as carbon black, carbon nanotubes, activated carbon, natural graphite, and mesoporous carbon.

[B. Average particle size]
The average particle size of the conductive particles is not particularly limited, and an optimum value can be selected depending on the purpose. In general, when the average particle size of the conductive particles is too large, the contact resistance between particles increases and the conductivity of the microporous layer decreases. Therefore, the average particle size of the conductive particles is preferably 20 μm or less. The average particle size is preferably 10 μm or less, more preferably 1 μm or less.
Here, "average particle size" refers to the median value ( _d50 ) of particle sizes measured by a laser diffraction scattering method.

[1.1.2. Water-repellent particles]
[A. material]
The water repellent particles are for imparting water repellency to the microporous layer. The material of the water-repellent particles is not particularly limited as long as it has water repellency and corrosion resistance to withstand the environment in which the fuel cell is used.
Materials for the water-repellent particles include, for example, polytetrafluoroethylene (PTFE), ethylene tetrafluoride, propylene hexafluoride copolymer (FED), perfluoroalkoxy fluororesin (PFA), polyvinylidene fluoride (PVdF), poly Chlorotrifluoroethylene (PCTFE) and the like.
Any one of these may be used for the water-repellent particles, or two or more of them may be used in combination.

[B. Average particle size]
The average particle size of the water-repellent particles is not particularly limited, and an optimum value can be selected depending on the purpose. In general, when the average particle size of the water-repellent particles is too large, it becomes impossible to uniformly impart water repellency to the entire layer. Therefore, the average particle size of the water-repellent particles is preferably 10 μm or less. The average particle size is preferably 5 μm or less, more preferably 1 μm or less.

[1.1.3. Dispersant]
The dispersant is for stabilizing the dispersed state of the solid content (that is, the conductive particles and the water-repellent particles) in water. For this purpose, the dispersant must have a hydrophilic group and a hydrophobic group in its molecule. The hydrophobic groups of the dispersant are adsorbed on the surface of the water-repellent particles. Further, when the conductive particles are hydrophobic, the hydrophobic groups of the dispersant are adsorbed on the surfaces of the conductive particles.

Dispersants include, for example, nonionic surfactants, cationic surfactants, and anionic surfactants. Either may be used in the present invention.
Among these, ionic surfactants generate ions in the solvent, and these ions may impair the power generation characteristics of the fuel cell. On the other hand, nonionic surfactants are suitable as dispersants because they do not generate ions in solvents.
The molecular structure and molecular weight of the dispersant are not particularly limited, and an optimum one can be selected according to the purpose.

[1.1.4. Hydrophobic organic solvent]
The hydrophobic organic solvent is for increasing the viscosity of the paste. Here, "hydrophobic organic solvent" refers to an organic compound that is insoluble in water and liquid at room temperature.
In order to obtain a high thickening effect, the hydrophobic organic solvent should have relatively high hydrophobicity. The degree of hydrophobicity can be represented by octanol/water partition coefficient (log P _ow ) (hereinafter also simply referred to as “partition coefficient”). In order to obtain a high thickening effect, the partition coefficient of the hydrophobic organic solvent is preferably 1.65 or more. The partition coefficient is preferably 2.0 or higher, more preferably 3.0 or higher.

Examples of hydrophobic organic solvents that satisfy these conditions include 1-octanol, 1-hexanol, xylene, styrene, toluene, benzene, and pentane.
Any one of these may be used as the hydrophobic organic solvent, or two or more thereof may be used in combination.

[1.1.5. water]
Water is used to disperse solids contained in the paste (that is, conductive particles and water-repellent particles) and to provide the paste with fluidity necessary for coating.

[1.2. Composition of paste]
The paste contains predetermined amounts of conductive particles, water-repellent particles, a dispersing agent, and a hydrophobic organic solvent, and the remainder consists of water and unavoidable impurities. As for the content of each component, it is preferable to select the optimum content according to the purpose.

[1.2.1. Content of conductive particles]
The content of the conductive particles contained in the paste affects the coatability of the paste and the properties of the microporous layer. In general, if the content of the conductive particles in the paste is too low, the viscosity necessary for suppressing penetration of the paste cannot be obtained. Also, if the content of the conductive particles is too small compared to the water-repellent particles, the conductivity of the microporous layer will be lowered. Therefore, the content of the conductive particles in the paste is preferably 5 wt % or more. The content is preferably 7.5 wt% or more, more preferably 10 wt% or more.

On the other hand, if the content of the conductive particles in the paste becomes excessive, the viscosity of the paste becomes excessively high, and coating itself may become difficult. Also, when the content of the conductive particles is excessive compared to the water-repellent particles, the water repellency of the microporous layer is lowered. Therefore, the content of the conductive particles in the paste is preferably 30 wt % or less. The content is preferably 25 wt% or less, more preferably 20 wt% or less.

[1.2.2. Content of water-repellent particles]
The content of the water-repellent particles contained in the paste affects the coatability of the paste and the properties of the microporous layer. In general, if the content of the water-repellent particles in the paste is too low, the viscosity necessary for suppressing penetration of the paste cannot be obtained. In addition, if the content of the water-repellent particles is too small compared to the conductive particles, the water repellency of the microporous layer is lowered. Therefore, the content of the water-repellent particles in the paste is preferably 0.5 wt % or more. The content is preferably 1 wt % or more, more preferably 3 wt % or more.

On the other hand, if the content of the water-repellent particles in the paste becomes excessive, the viscosity of the paste becomes excessively high, and coating itself may become difficult. Moreover, when the content of the water-repellent particles is excessive compared to the conductive particles, the conductivity of the microporous layer is lowered. Therefore, the content of the water-repellent particles in the paste is preferably 20 wt % or less. The content is preferably 10 wt% or less, more preferably 5 wt% or less.

[1.2.3. Dispersant content]
The content of the dispersant contained in the paste affects the coatability of the paste and the properties of the microporous layer. In general, if the content of the dispersant in the paste is too low, the conductive particles and the water-repellent particles will not be sufficiently dispersed. Therefore, the content of the dispersant in the paste is preferably 1 wt % or more. The content of the dispersant is preferably 3 wt % or more, more preferably 5 wt % or more.

On the other hand, if the content of the dispersant in the paste becomes excessive, the firing load for decomposing the dispersant increases. Therefore, the content of the dispersant in the paste is preferably 15 wt% or less. The content of the dispersant is preferably 12.5 wt% or less, more preferably 10 wt% or less.

[1.2.4. Content of Hydrophobic Organic Solvent]
The content of the hydrophobic organic solvent affects the coatability of the paste and the properties of the microporous layer. In general, if the content of the hydrophobic organic solvent in the paste is too low, the viscosity required to suppress penetration of the paste cannot be obtained. Therefore, the content of the hydrophobic organic solvent in the paste is preferably 0.5 wt % or more. The content is preferably 1 wt % or more, more preferably 2 wt % or more.

On the other hand, if the content of the hydrophobic organic solvent in the paste becomes excessive, the viscosity will increase excessively, making coating itself difficult. Therefore, the content of the hydrophobic organic solvent in the paste is preferably 3.0 wt% or less. The content is preferably 2.5 wt% or less.

[1.3. viscosity]
The paste according to the present invention is characterized by having a viscosity suitable for coating and less permeation into a substrate. In addition, when the composition of the paste is optimized, it exhibits pseudoplastic flow with low viscosity at high shear rates and high viscosity at low shear rates.

Specifically, when the composition of the paste is optimized, the viscosity at a shear rate of 1000 s ⁻¹ becomes 0.05 Pa·s or more and 1 Pa·s or less. Further optimization of the composition of the paste resulted in a viscosity of 0.5 at a shear rate of 1000 s ^-1 . lPa·s or more and 0.5 Pa·s or less.

Further, when the composition of the paste is optimized, the viscosity at a shear rate of 0.1 s ⁻¹ becomes 10 Pa·s or more and 1000 Pa·s or less. If the composition of the paste is further optimized, the viscosity at a shear rate of 0.1 s ⁻¹ will be 10 Pa·s or more and 100 Pa·s or less.

[1.4. Structure of paste]
By adding a hydrophobic organic solvent to an aqueous dispersion containing conductive particles, hydrophobic particles, and a dispersing agent, a paste having a viscosity suitable for coating and less infiltration into a substrate can be obtained. In particular, the content of the hydrophobic organic solvent has a large effect on the viscosity of the paste.

Addition of a hydrophobic organic solvent to the paste increases the viscosity of the paste. This is because in the paste, particles in which the hydrophobic organic solvent has penetrated into the gaps between the aggregated conductive particles (hereinafter, such particles are also referred to as “swollen particles”) are generated, and the conductive particles This is probably because the apparent volume of aggregates increases.
Also, if the amount of the hydrophobic organic solvent added exceeds a certain critical value, the viscosity of the paste will further increase. This is thought to be due to the formation of a network structure in which at least a portion of the swollen particles are linked via the liquid bridge force of the hydrophobic organic solvent.

[2. Method for producing paste for microporous layer]
The method for producing a microporous layer paste according to the present invention comprises:
A first step of mixing and dispersing conductive particles, water-repellent particles, a dispersant, and water in a predetermined ratio;
and a second step of adding a hydrophobic organic solvent as a thickening agent to the dispersion liquid obtained in the first step and mixing the mixture.

[2.1. First step]
First, conductive particles, water-repellent particles, a dispersant, and water are mixed in a predetermined ratio and dispersed (first step). A method of mixing these raw materials is not particularly limited, and an optimum method can be selected according to the purpose.
The details of the conductive particles, the water-repellent particles, and the dispersing agent, and the details of the amount of these to be added are as described above, so the description is omitted.

[2.2. Second step]
Next, a hydrophobic organic solvent is added as a thickening agent to the dispersion liquid obtained in the first step and mixed (second step). A method for mixing the hydrophobic organic solvent is not particularly limited, and an optimum method can be selected depending on the purpose.

Addition of a hydrophobic organic solvent to the dispersion obtained in the first step increases the viscosity. In addition, when the blending amount of the hydrophobic organic solvent exceeds a certain critical value, the viscosity further increases and at the same time, pseudoplastic flow is exhibited.
The hydrophobic organic solvent is preferably an organic solvent having an octanol/water partition coefficient (log P _ow ) of 1.65 or more. The hydrophobic organic solvent is particularly preferably 1-octanol.

Furthermore, the first step and the second step are
5 wt% or more and 30 wt% or less of the conductive particles;
0.5 wt % or more and 20 wt % or less of the water-repellent particles;
1 wt% or more and 15 wt% or less of the dispersant, and
0.5 wt% or more and 3.0 wt% or less of the hydrophobic organic solvent
with the balance being water and unavoidable impurities.
The details of the hydrophobic organic solvent and the details of its addition amount are as described above, so the description is omitted.

[3. action]
When the conductive particles, hydrophobic particles, and dispersant are dispersed in water, the hydrophobic groups of the dispersant are adsorbed to the aggregates of the conductive particles and the surfaces of the water-repellent particles. When a hydrophobic organic solvent is added to the dispersion liquid in which such particles are dispersed, the hydrophobic organic solvent penetrates into the gaps between the conductive particles to form swollen particles. As a result, the apparent volume of the aggregates of the conductive particles increases, increasing the viscosity of the paste. Further, when the amount of the hydrophobic organic solvent added is further increased, a network structure is formed in which at least a portion of the swollen particles are linked via the liquid-bridging force of the hydrophobic organic solvent. As a result, the viscosity of the paste further increases.

Since the paste thus obtained does not contain water-soluble polymer, heat treatment for thermally decomposing the water-soluble polymer is unnecessary. Therefore, productivity of the paste is improved.

(Examples 1-5, Comparative Examples 1-6)
[1. Preparation of sample]
[1.1. Preparation of paste]
[1.1.1. Examples 1-5]
A microporous layer paste was prepared using conductive particles, water-repellent particles, a dispersant, and water as main components. Carbon black (Vulcan (registered trademark) XC-72R, manufactured by Cabot Corporation) was used as the conductive particles. PTFE dispersion (31-JR, concentration: 55%, manufactured by Mitsui-DuPont Fluorochemical Co., Ltd.) was used as the water-repellent particles. A nonionic surfactant (Triton X-100, manufactured by Kishida Chemical Co., Ltd.) was used as a dispersant.

Predetermined amounts of conductive particles, water-repellent particles, and dispersants were added to the water. The weight ratio of PTFE (the ratio of the weight of PTFE to the total weight of carbon and PTFE) was set to 20 wt%. After the paste was subjected to ultrasonic dispersion treatment, it was further subjected to mixing and dispersion treatment for a predetermined time using a stirring/defoaming device.
A predetermined amount of thickening agent was added to the obtained paste, and mixed with a stirrer to adjust the viscosity of the paste. A water-insoluble organic solvent (1-octanol, manufactured by Tokyo Kasei Kogyo Co., Ltd.) was used as the thickening agent. Also, the amount of the thickening agent added was 0.7 to 3.0 wt % with respect to the total weight of the paste.

[1.1.2. Comparative Examples 1 to 4]
Except that the amount of thickener added was 0 wt% (Comparative Example 1), 0.3 wt% (Comparative Example 2), 5.0 wt% (Comparative Example 3), or 20.0 wt% (Comparative Example 4) , a paste was prepared in the same manner as in Example 1.
[1.1.3. Comparative Examples 5-6]
Polyethylene oxide (Alcox (registered trademark) E-300, aqueous solution concentration: 0.5 wt%, manufactured by Meisei Chemical Industry Co., Ltd.), which is a water-soluble polymer, is used as a thickener, and the amount of thickener added is 10. A paste was prepared in the same manner as in Example 1 except that the content was 0 wt % (Comparative Example 5) or 20.0 wt % (Comparative Example 6).

[1.2. Preparation of Diffusion Layer]
The resulting paste was applied onto carbon paper with a thin film application tool (applicator). Then, it was dried and baked at a predetermined temperature for 10 minutes to prepare a diffusion layer forming a microporous layer. The firing temperature was 300°C or 350°C.

[2. Test method]
[2.1. Paste dispersibility evaluation]
In order to confirm the dispersion state of the paste, the appearance of the paste was observed. In the following evaluation results, "◯" indicates that the paste is well dispersed. “X” indicates that the paste is poorly dispersed (for example, carbon particles are agglomerated and sedimented).

[2.2. Paste coatability evaluation]
In order to confirm the coatability of the paste, the fluidity and uniformity during the coating process on the carbon paper were evaluated. In the following evaluation results, "◯" indicates that the coated surface is uniform and free of fading and strike-through. "X" indicates a state in which there is faintness or strike-through on the coating surface.

[2.3. Water repellency evaluation of microporous layer]
In order to confirm the water repellency of the surface of the microporous layer on the surface of the diffusion layer, ion-exchanged water was dropped onto the surface of the microporous layer using a dropper while the diffusion layer was tilted at 15°. Water repellency was evaluated based on whether or not the water droplets fell.
In the following evaluation results, "◯" indicates that water droplets fall. "X" indicates that the water droplets do not fall off and remain or permeate into the diffusion layer.

[2.4. Paste viscosity evaluation]
In order to confirm the thickening effect of the addition of the water-insoluble organic solvent, the shear rate dependence of the viscosity of each paste was measured. The measurement was carried out using a rheometer (ARES-G2) manufactured by TA Instruments using cones and plates of φ50 mm and 0.02 rad.

[3. result]
[3.1. Dispersibility, coatability, and water repellency]
Table 1 shows the dispersibility and coatability of the paste, and the water repellency of the microporous layer. Table 1 also shows the composition of each paste. Table 1 shows the following.

(1) When 1-octanol was used as a thickener and the amount of the thickener added was small (Comparative Examples 1 and 2), the dispersibility was good, but the coatability was poor. rice field. In addition, baking at 350° C. was necessary to impart water repellency to the microporous layer.
(2) When 1-octanol was used as a thickening agent and the amount of the thickening agent added was excessive (Comparative Examples 3 and 4), the dispersibility was poor. Moreover, the coating itself could not be performed because the viscosity increased excessively.
(3) When PEO was used as a thickener (Comparative Examples 5 and 6), both the dispersibility and coatability were good. However, baking at 350° C. was necessary to impart water repellency to the microporous layer.
(4) When 1-octanol is used as a thickener, when the amount of thickener added is appropriate (Examples 1 to 5), dispersibility, coatability, and water repellency are all improved. It was good.

[3.2. viscosity]
FIG. 1 shows the relationship between paste shear rate and viscosity. Comparative Example 2, in which the amount of 1-octanol added was 0.3 wt%, exhibited the same viscosity behavior as Comparative Example 1, in which no thickening agent was added. On the other hand, Example 1, in which the amount of 1-octanol added was 0.7 wt%, showed almost the same viscosity behavior as Comparative Example 5, in which the amount of PEO added was 10.0 wt%, and a thickening effect was recognized. Furthermore, when the amount of 1-octanol added was increased, the viscosity increased over the entire range of measured shear rates up to 1.3 wt% (Example 2).

However, at 1.6 wt% or more (Examples 3 to 6), the viscosity at a shear rate of 1000 s ^-1 did not increase, and the viscosity increased in the low shear rate range, indicating pseudoplastic flow. However, when the added amount was 5 wt % or more (Comparative Examples 3 and 4), part of the sample fell out of the jig of the rheometer during the viscosity measurement, so the viscosity could not be measured correctly.
From the above results, the appropriate amount of 1-octanol to be added is 0.5 wt% or more and 3.0 wt% or less. It is preferable from the viewpoint of coatability such as prevention.

FIG. 2 shows the relationship between the 1-octanol content and the viscosity. As shown in FIG. 3, it is one of the characteristics of the present invention that the viscosity at a high shear rate (1000 s ^-1 ) does not increase even if the amount added is 1.5 wt % or more.

[3.3. Paste structure]
[3.3.1. When X = 0 wt%]
FIG. 3(A) is a schematic diagram of the fuel cell microporous layer paste when X=0 wt % (X is the content of the hydrophobic organic solvent), which is the paste structure inferred from the viscosity measurement. show. 3A, the paste 10a includes conductive particles 12, 12, . . . , water-repellent particles 14, 14, .

The conductive particles 12, 12... consist of aggregates of a plurality of primary particles. Also, the dispersants 16, 16, . . . are adsorbed to the surfaces of the conductive particles 12, 12, . Therefore, the conductive particles 12, 12, . . . and the water-repellent particles 14, 14, .

[3.3.2. 0 < X ≤ 1.3 wt%]
FIG. 3(B) shows a schematic diagram of the fuel cell microporous layer paste having a paste structure inferred from the viscosity measurement when 0<X≦1.3 wt %. 3B, the paste 10b includes conductive particles 12, 12, . . . , water-repellent particles 14, 14, . there is

When the hydrophobic organic solvent 20 is added to the paste 10a shown in FIG. 3A, since the hydrophobic organic solvent 20 is insoluble in the water 18, it enters the gaps between the conductive particles 12, 12 . structure. As a result, as shown in FIG. 3B, swollen particles are produced in which the hydrophobic organic solvent 20 enters the gaps between the aggregated conductive particles 12, 12, . . . . When the swollen particles are generated, the apparent volume of the conductive particles 12, 12... increases. When the amount of 1-octanol added is 1.3 wt % or less, the slight increase in viscosity at a high shear rate (1000 s -1 ) is thought to be due to such swollen particles.

Such a phenomenon can occur even when the conductive particles 12, 12, . . . are hydrophilic. This is because the hydrophobic organic solvent 20 is insoluble in the water 18, so it is more stable to enter the gaps of the conductive particles 12, 12, which are less hydrophilic than water, rather than dispersing in the water 18. It is considered to be for

[3.3.3. When X>1.3 wt%]
FIG. 3(C) shows a schematic diagram of the fuel cell microporous layer paste when X>1.3 wt %, which is the paste structure inferred from the viscosity measurement. 3(C), the paste 10c includes conductive particles 12, 12, . . . , water-repellent particles 14, 14, . there is

As the amount of the hydrophobic organic solvent 20 added increases, the diameter of the swollen particles increases. Furthermore, when the added amount of the hydrophobic organic solvent 20 exceeds a certain critical value (1.3 wt % in the example of 1-octanol), the viscosity of the paste 10c further increases. This is thought to be due to the formation of a network structure in which at least a portion of the swollen particles are linked via the liquid bridge force of the hydrophobic organic solvent 20, as shown in FIG. 3(C). The pseudoplastic flow that appears when a certain amount or more of 1-octanol is added is thought to originate from the formation of a network structure.

(Example 6: Examination of organic solvent)
[1. Preparation of sample]
Examples except that methanol (M), 1-propanol (Pr), 1-pentanol (Pe), or 1-hexanol (H) was used as the organic solvent instead of 1-octanol (O). A paste was prepared in the same manner as in 4. The amount of the organic solvent added was 2 wt % in each case. Table 2 shows the composition of each paste. Table 2 also shows the composition of the paste using 1-octanol (O) as the organic solvent (No. 5, Example 4). In addition, Table 3 shows physical property values of each solvent.

[2. Test method and results]
The viscosity of the paste was measured in the same manner as in Example 1. FIG. 4 shows the relationship between the octanol/water partition coefficient (log P _ow ) and the paste viscosity at a shear rate of 1 s ⁻¹ . From FIG. 4, it was found that the lower the affinity with water (the larger the log P _OW ), the higher the viscosity. The minimum viscosity required to exhibit the water-repellent function is 0.05 Pa·s (from Example 1). Therefore, organic solvents that can be expected to exhibit a thickening effect are considered to be organic solvents having an octanol/water partition coefficient (logP _OW ) of 1.65 or more.

Although the embodiments of the present invention have been described in detail above, the present invention is by no means limited to the above-described embodiments, and various modifications are possible without departing from the gist of the present invention.

The paste according to the present invention is used to form a microporous layer formed on the surface of the diffusion layer of a fuel cell.

Claims (11)

A microporous layer paste having the following composition:
(1) The microporous layer paste is
conductive particles for imparting conductivity to the microporous layer of the fuel cell;
water repellent particles for imparting water repellency to the microporous layer;
a dispersant for stabilizing the dispersed state of the conductive particles and the water-repellent particles in water;
a hydrophobic organic solvent to increase the viscosity of the paste;
Water for dispersing the conductive particles and the water-repellent particles is provided.
(2) The microporous layer paste has a viscosity of 0.05 Pa·s or more and 1 Pa·s or less at a shear rate of 1000 s ⁻¹ . The paste for a microporous layer according to claim 1, which has a viscosity of 10 Pa·s or more and 1000 Pa·s or less at a shear rate of 0.1 s ^-1 . 5 wt% or more and 30 wt% or less of the conductive particles;
0.5 wt % or more and 20 wt % or less of the water-repellent particles;
1 wt% or more and 15 wt% or less of the dispersant, and
0.5 wt% or more and 3.0 wt% or less of the hydrophobic organic solvent
3. The paste for a microporous layer according to claim 1 or 2, comprising water and unavoidable impurities. The paste for a microporous layer according to any one of claims 1 to 3, wherein the hydrophobic organic solvent comprises an organic solvent having an octanol/water partition coefficient (log P _ow ) of 1.65 or more. The paste for a microporous layer according to any one of claims 1 to 4, wherein the hydrophobic organic solvent is 1-octanol. 6. The paste for a microporous layer according to any one of claims 1 to 5, comprising swollen particles in which the hydrophobic organic solvent penetrates into the gaps between the conductive particles. 7. The paste for a microporous layer according to claim 6, further comprising a network structure in which at least part of said swollen particles are linked via the liquid bridging force of said hydrophobic organic solvent. A first step of mixing and dispersing conductive particles, water-repellent particles, a dispersant, and water in a predetermined ratio;
A method for producing a microporous layer paste, comprising a second step of adding a hydrophobic organic solvent as a thickening agent to the dispersion liquid obtained in the first step and mixing the mixture. The first step and the second step are
5 wt% or more and 30 wt% or less of the conductive particles;
0.5 wt % or more and 20 wt % or less of the water-repellent particles;
1 wt% or more and 15 wt% or less of the dispersant, and
0.5 wt% or more and 3.0 wt% or less of the hydrophobic organic solvent
9. The method for producing a microporous layer paste according to claim 8, wherein the paste is blended so as to obtain a paste containing water and unavoidable impurities as the balance. The method for producing a microporous layer paste according to claim 8 or 9, wherein the hydrophobic organic solvent comprises an organic solvent having an octanol/water partition coefficient ( _logPow ) of 1.65 or more. The method for producing a microporous layer paste according to any one of claims 8 to 10, wherein the hydrophobic organic solvent is 1-octanol.

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