JP2013016281A - Production method of catalyst paste for fuel cell - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the amount of catalyst metallic particles used by improving the catalyst being applied to a reaction layer having a structure oriented to the catalyst side (PFF type) so that the hydrophilic functional group in the side chain of a polyelectrolyte forms a hydrophilic layer on the catalyst.SOLUTION: The production method includes performing: a hydrophilic group adsorption step for making a hydrophilic group adsorb to catalyst metallic particles; a reduction step for reducing the hydrophilic group adsorbed to the catalyst metallic particles; and a pulverization step for performing wet pulverization of the catalyst following to the reduction step. Platinum particles are adopted as the catalyst metallic particles, and when NOis adopted as the hydrophilic group, heating treatment is preferably carried out in an inert gas atmosphere at a temperature of 150°C or higher, as the reduction conditions.

Description

  The present invention relates to a method for producing a catalyst paste for a fuel cell.

A fuel cell has a structure in which a reaction layer is laminated on a solid electrolyte membrane, and the characteristics of the reaction layer on the air electrode side, which is a fuel cell reaction field, greatly affect the performance of the fuel cell.
The present inventor, as the air electrode side reaction layer, has a reaction layer (hereinafter referred to as “PFF (registered trademark: the same applies hereinafter) type reaction layer”) in which the periphery of the catalyst is surrounded by a polymer electrolyte phase via an aqueous phase. (Patent Document 1). Here, the catalyst is obtained by supporting catalyst metal particles on a carrier made of carbon particles or the like.
See Patent Documents 1-3 and Non-Patent Document 1 as documents disclosing techniques related to the present invention.
Patent Document 3 discloses a method (such as homogenizing treatment) of wet-pulverizing the catalyst in order to efficiently pulverize the catalyst.

JP 2006-140061 A Japanese Patent Laid-Open No. 7-134995 JP 2011-078891 A “Carbon” p199 (Kim Kinoshita; John Wiley & Sons 1988)

For the catalytic metal particles, fine particles of expensive noble metal such as platinum are employed. In order to reduce the manufacturing cost of fuel cells and to promote the spread of fuel cells, it is important to reduce the amount of catalyst metal particles used.
An object of the present invention is to improve a catalyst applied to a PFF type reaction layer and reduce the amount of catalyst metal particles used.

As a result of intensive studies to solve the above-mentioned problems, the present inventor conducted a chemical treatment in which the surface of the catalytic metal particle was modified with a hydrophilic group, thereby stabilizing the PFF structure and improving the characteristics of the reaction layer. As a result, it has been found that the amount of catalyst metal particles used can be reduced.
On the other hand, when the catalyst is wet pulverized, the catalyst is subdivided and its specific surface area is increased, so that the amount of expensive catalyst metal particles used can be suppressed. However, after performing the above-described chemical treatment, it was found that if wet pulverization is performed on the catalyst paste and a large force is applied thereto, hydrophilic groups may be detached from the catalyst metal particles.
The present inventor thought that the adsorption state of the hydrophilic group to the catalyst metal particles should be stabilized, and then wet pulverization treatment should be performed, and the present inventor has arrived at the present invention.

That is, the first aspect of the present invention is defined as follows.
A method for producing a catalyst paste for a fuel cell by treating a catalyst having catalyst metal particles supported on a substrate,
A hydrophilic group adsorption step for adsorbing a hydrophilic group to the catalytic metal particles;
A reduction step for reducing the hydrophilic groups adsorbed on the catalytic metal particles;
After the reduction step, a pulverizing step for wet pulverizing the catalyst;
A method for producing a catalyst paste, comprising:

  According to the method for producing a catalyst paste of the first aspect thus defined, the hydrophilic groups adsorbed on the catalyst metal particles are reduced, and thereby the bond between the catalyst metal particles and the hydrophilic groups is strengthened. Therefore, even when the catalyst is wet-ground, the bond between the catalyst metal particles and the hydrophilic group is maintained, and the PFF structure is stabilized.

The second aspect of the present invention is defined as follows.
That is, in the manufacturing method according to the first aspect, the reduction step heats the catalyst containing the catalytic metal particles having the hydrophilic group adsorbed under conditions such that the hydrophilic group is reduced.
According to the manufacturing method of the 2nd situation prescribed | regulated in this way, a hydrophilic group can be reduce | restored by the simple method of heating in inert atmosphere, and the coupling | bonding with respect to a catalyst metal particle can be strengthened.
When platinum or a platinum alloy is used for the catalyst metal particles and a nitrate group is used as the hydrophilic group, the heating temperature is preferably 150 ° C. or higher (third aspect), and an inert atmosphere is a nitrogen gas atmosphere. Is preferably adopted.

FIG. 1 is a sectional view showing the structure of a fuel cell according to an embodiment of the present invention. FIG. 2 is a schematic diagram of a PFF structure. 3A and 3B are schematic views showing the form of the electrolyte in the electrolyte solution. FIG. 3A shows a case where water is excessive, and FIG. 3B shows a case where water is appropriate. FIG. 4 is a schematic diagram showing the structure of the reaction layer corresponding to FIG. FIG. 5A shows the relationship between the stirring time and the viscosity of the pre-paste and the electrolyte solution, and FIG. 5B shows the relationship between the stirring time and the reaction layer resistance. It is a block diagram which shows the manufacturing apparatus of the catalyst paste of embodiment of this invention. It is a flowchart which shows the manufacturing method of the pre paste of embodiment of this invention. The adsorption characteristics of NO ₃ ⁻ (no heat treatment) on catalyst platinum particles are shown. The adsorption characteristics of NO ₃ ⁻ (with heat treatment, 130 ° C.) on catalyst platinum particles are shown. The adsorption characteristics of NO ₃ ⁻ (with heat treatment: 150 ° C.) on catalyst platinum particles are shown. The adsorption characteristics of NO ₃ ⁻ (with heat treatment, 150 ° C.) on the catalyst platinum particles after the homogenizing treatment are shown. The temperature characteristic of the fuel cell using the heat-treated catalyst is shown.

A fuel cell 1 according to an embodiment of the present invention is shown in FIG.
This fuel cell 1 has a configuration in which a solid electrolyte membrane 2 is sandwiched between a hydrogen electrode 10 and an air electrode 20.
The solid electrolyte membrane 2 may be made of a proton conductive polymer material, for example, a fluorine-based polymer such as Nafion (trade name of DuPont, hereinafter the same).

The hydrogen electrode 10 includes a reaction layer 11 and a diffusion layer 16, and is sequentially stacked on the solid electrolyte membrane 2. The reaction layer 11 is obtained by coating an electroconductive carrier such as carbon particles with catalyst metal particles such as platinum (catalyst) coated with an electrolyte. The diffusion layer 16 is made of a conductive material such as carbon paper, carbon cloth, or carbon felt, and has gas diffusibility. Tin oxide or titanium oxide can also be used as the carrier of the reaction layer 11.
The electrolyte can be arbitrarily selected as long as it allows proton movement, but Nafion is preferably used from the standpoint of durability and the like.
The reaction layer 11 is laminated on the diffusion layer 16 by applying a paste of catalyst and electrolyte constituting the reaction layer 11 to the diffusion layer 16 and drying. Such a laminate is bonded to the solid electrolyte membrane 2.
The air electrode 20 includes a reaction layer 21 and a diffusion layer 26. The basic structure and manufacturing method of the reaction layer 21 and the diffusion layer 26 are the same as those of the hydrogen electrode 10.
However, since the fuel cell reaction is performed exclusively in the reaction layer 21 of the air electrode 20, the reaction layer 21 determines the characteristics of the fuel cell 1.

The inventor has proposed a PFF structure as the reaction layer. In the present invention, a method for producing a catalyst suitable for the PFF structure is proposed.
The PFF structure will be described.
Here, the PFF (Applicant's registered trademark) structure refers to a structure in which the hydrophilic functional group of the side chain of the polymer electrolyte is oriented to the catalyst side so as to form a hydrophilic layer on the catalyst.
For example, in perfluorosulfonic acid (Nafion, etc .; registered trademark of DuPont) widely used as a polymer electrolyte, a sulfone group (SO ₃ ⁻ ) as a hydrophilic functional group is present in the side chain E2 with respect to the hydrophobic main chain E1. As shown in FIG. 2, this hydrophilic functional group is oriented toward the catalyst C, whereby a continuous hydrophilic region W is formed between the catalyst C and the electrolyte layer E. In the agglomerated catalyst C, the hydrophilic regions W on the surfaces of the catalyst particles communicate with each other. Proton (H ⁺ ) and water (H ₂ O) can move smoothly in the hydrophilic region W of the PFF structure, and as a result, the electrochemical reaction of the fuel cell is promoted.
In addition, according to the PFF structure, since water is gathered around the catalyst C, most of the water efficiently contributes to the electrochemical reaction even in a small amount of water. Capability can be prevented from decreasing. On the other hand, the continuous hydrophilic region W functions as a drainage path for excess water, and can prevent flooding even in a highly humidified state.

In the above description, the catalyst C is a catalyst in which the catalyst metal particles C2 are supported on the carrier C1 having conductivity. The carrier C1 is required to have electrical conductivity and air permeability, and porous carbon black particles can be used, but tin oxide, titanic acid compounds, and the like can also be used. The catalytic metal particles C2 are made of fine metal particles that can provide an active site for the fuel cell reaction, and a noble metal such as platinum, cobalt, ruthenium, and an alloy of the noble metal can be used.
The method of supporting the catalyst metal particles C2 on the support C1 can be appropriately selected from known methods such as an impregnation method, a colloid method, and a precipitation-precipitation method depending on the material of both and the use of the catalyst.

(Catalyst treatment)
Usually the catalyst is provided by the catalyst manufacturer. It is preferable to physically and / or chemically treat the catalyst according to the characteristics required for the fuel cell.
(Physical treatment of catalyst)
The physical treatment of the catalyst includes a pulverization treatment and a defoaming treatment.
-Grinding treatment-
In general, in a catalyst, carriers are aggregated to form secondary particles and tertiary particles. Therefore, in order to improve the surface area of the catalyst, it is preferable to pulverize the agglomerates into a fine powder. For this purpose, it is preferable to disperse the agglomerates of the catalyst in a medium and perform wet pulverization.
By adopting wet pulverization, it becomes possible to pulverize the catalyst aggregate more finely by applying higher energy to the agglomerates of the catalyst as compared with dry pulverization. In addition, recombination of the catalyst can be effectively prevented as compared with dry pulverization. As a wet pulverization method, a homogenizer, a wet jet mill, a ball mill or a bead mill can be employed.
By adopting wet pulverization, an effect of removing impurities adhering to the catalyst carrier can be obtained. Usually, water is used as the medium, but other medium (organic solvent or the like) may be used depending on the characteristics of the impurities. It is also possible to first perform wet pulverization using water as a medium, and then remove impurities from the catalyst with an organic solvent or the like.
In order to dry the wet pulverized catalyst, it is preferable to remove the medium by sublimation. Thereby, reaggregation of the catalyst can be prevented. An example of a method for sublimating the medium is a vacuum drying method. On the other hand, when the heat drying method is adopted, when the medium is moved by heating or when the medium evaporates, a capillary contraction phenomenon occurs and the catalysts recombine, and the high dispersion state obtained by wet drying is obtained. It cannot be maintained.
The wet pulverization and, if necessary, impurity removal may be performed on the catalyst support, and the catalyst metal particles may be supported on the support in a state where the support is dispersed in a medium (for example, water). Even in this case, as a drying step, it is preferable to remove the medium in which the catalyst is dispersed by sublimation.

-Defoaming treatment-
It is necessary to remove bubbles (defoaming treatment) from around the catalyst in a state where the catalyst is mixed and dispersed in water. This is because the bubbles obstruct the formation of the hydrophilic region between the catalyst and the electrolyte layer.
This defoaming treatment can be performed by using a centrifugal stirring method with a hybrid mixer (rotating / revolving centrifugal stirrer).
Of course, the present invention is not limited to the centrifugal stirring method, and other stirring methods (ball mill method, stirrer method, bead mill method, roll mill method, etc.) can also be used.
In some cases, bubbles may be removed from around the catalyst during wet pulverization, in which case independent defoaming treatment is unnecessary.

(Chemical treatment of catalyst)
The catalyst is chemically treated to modify the surface of the catalyst metal particles with specific hydrophilic groups.
By modifying the surface of the metal catalyst particles with a hydrophilic group, the hydrophilicity around the catalyst metal particles is improved, and the hydrophilicity of the hydrophilic region W between the catalyst C and the electrolyte layer E is increased.
Here, the modification means that the modifying group is present on the surface of the catalytic metal particle, and the modifying group is not separated from the catalytic metal particle even through a normal production process.
Examples of the hydrophilic group include at least one selected from a nitrate group, an amino group, a sulfonic acid group, a hydroxyl group, and a halogen group. More preferably, at least one selected from a nitric acid group, an amino group, and a sulfonic acid group can be given as a hydrophilic group.
The presence of these hydrophilic groups around the catalyst metal particles makes it easy to form a hydrophilic region around the catalyst metal particles. Since the catalyst metal particles are evenly dispersed in the support, as a result, a hydrophilic region on the surface of the catalyst is likely to be formed, and it is stabilized after the formation.
As a method of modifying the above-mentioned hydrophilic group on the catalyst metal particles, in the present invention, a complex of a metal (noble metal) that is the same as or the same type as the catalyst metal particles and includes the modification group is bonded to the catalyst metal particles. By using the complex, the hydrophilic group can be modified to the catalytic metal particles without any stress on the structure of the catalyst.

When platinum or a platinum alloy is employed as the catalyst metal particles, the modification is preferably performed with the following platinum complex solution. As such a platinum complex solution, a platinum chloride (IV) acid hydrate aqueous solution (H ₂ PtCl ₆ .nH ₂ O / H ₂ O sol.), A platinum chloride (IV) hydrochloride acid solution (H ₂ PtCl ₆ / HCl sol. ), Platinum chloride (IV) ammonium aqueous solution ((NH ₄ ) ₂ PtCl ₆ / H ₂ O sol.), Dinitrodiammine platinum (II) aqueous solution (cis- [Pt (NH ₃ ) ₂ (NO ₂ ) ₂ ] / H ₂ O sol.), Dinitrodiammine platinum (II) nitric acid solution (cis- [Pt (NH ₃ ) ₂ (NO ₂ ) ₂ ] / HNO ₃ sol.), Dinitrodiammine platinum (II) sulfuric acid solution (cis- [ Pt (NH ₃ ) ₂ (NO ₂ ) ₂ ] / H ₂ SO ₄ sol.), Potassium tetracurl platinum (II) acid aqueous solution (K ₂ PtCl ₄ ) / H ₂ O sol. ), Platinum (II) chloride aqueous solution (PtCl ₂ / H ₂ O sol.), Aqueous platinum (IV) chloride (PtCl ₄ / H ₂ O sol.), Tetraammineplatinum (II) dichloride hydrate aqueous solution ([Pt (NH ₃ ) ₄ ] Cl ₂ .H ₂ O / H ₂ O sol.), Tetraammineplatinum (II) hydroxide aqueous solution ([Pt (NH ₃ ) ₄ ] (OH) ₂ / H ₂ O sol. .), Hexaammine platinum (IV) dichloride aqueous solution ([Pt (NH ₃ ) ₆ ] Cl ₂ / H ₂ O sol.), Hexaammine platinum (IV) hydroxide aqueous solution ([Pt (NH ₃ ) ₆ ] ( OH) ₂ / H ₂ O sol.), Hexahydroxoplatinum (IV) acid aqueous solution (H ₂ [Pt (OH) ₆ ] / H ₂ O sol.), Hexahydroxoplatinum (IV) acid nitric acid solution (H ₂ [ _{Pt (OH) 6] / HNO} 3 sol.), hexahydro Seo platinum (IV) acid sulfuric acid solution _{(H 2 [Pt (OH)} 6] / H 2 SO 4 sol.), Ethanolamine platinum solution _{(H 2 [Pt (OH)} 6] / H 2 NCH 2 CH 2 OH sol .) Etc. can be adopted.

According to the inventor's knowledge, it is preferable to select a nitrate group as a hydrophilic group for modifying the catalytic metal particles made of platinum or a platinum alloy. Nitroplatinum complex solution for that purpose includes dinitrodiammineplatinum (II) nitric acid solution (cis- [Pt (NH ₃ ) ₂ (NO ₂ ) ₂ ] / HNO ₃ sol.), NO ₃ ⁻ as a hydrophilic ion, hexa hydroxo Pt (IV) acid nitric acid solution _{((H 2 Pt (OH)} 6) / HNO 3 sol.), hexahydroxoplatinate and SO ₄ ^{2-hydrophilic} ion source platinum (IV) acid sulfate solution ((H ₂ Pt ( _{OH) 6) / H 2 SO} 4 sol.), tetraammineplatinum (II) hydroxide aqueous solution of NH ₄ ⁺ and hydrophilic ion _{([Pt (NH 3) 4} (OH) 2] / H 2 O sln. ) Etc. can be employed.

The method of modifying the hydrophilic group on the catalyst metal particles can be appropriately selected according to the characteristics of the catalyst metal particles and the hydrophilic group. For example, when the catalyst metal particles are made of platinum or a platinum alloy, the catalyst is treated with a platinum complex solution. And may be stirred if necessary. When a nitric acid group is selected, the raw material catalyst is put into an aqueous nitric acid solution of dinitrodiamine platinum (complex) and stirred to adsorb the platinum complex (dinitrodiamine platinum) to the catalyst platinum particles of the raw material catalyst. Further, a nitric acid aqueous solution of dinitrodiamine platinum (complex) may be added and stirred in a state where the raw material catalyst is dispersed in water. Here, the stirring is not limited to mechanical stirring using a blade or a stirrer, and can be performed by circulating two solutions through one pipe line.
In addition, in order to stabilize the coupling | bonding of a hydrophilic group and catalyst metal particle, it is preferable to heat-process.
When platinum particles are used as the catalyst metal particles, it is preferable to reduce the nitrate groups (—NO ₃ ) adsorbed thereon to (NO ₂ ). Although the reduction method is not particularly limited, a catalyst having catalytic platinum particles having adsorbed nitrate groups may be heated in an inert atmosphere. The catalyst subjected to such stabilization treatment does not release the hydrophilic group from the catalyst metal even if a strong force is applied later by physical treatment such as a homogenizer.

(Physical and chemical treatment order for catalyst)
In order to efficiently modify the catalytic metal particles in the catalyst with a hydrophilic group, it is preferable to perform physical treatment prior to chemical treatment. This is because, by physically treating the catalyst, more catalyst metal particles can come into contact with the treatment liquid containing a hydrophilic group. In a fuel cell including a catalyst obtained by performing physical treatment (particularly wet pulverization) prior to chemical treatment, temperature characteristics are improved as compared with a fuel cell provided with a catalyst that does not perform such treatment. That is, the output at the same temperature is improved, and furthermore, power generation at a higher temperature is possible.
In addition, when there exists a possibility that a catalyst may re-aggregate by a chemical process, it is preferable to perform a physical process again after performing a chemical process.
Of course, the chemical treatment for the catalyst may be performed first, followed by the physical treatment.
In order to obtain a PFF structure, it is necessary to perform a defoaming process on at least the catalyst.

(Pre-paste preparation)
The water content is adjusted in a pre-paste obtained by dispersing the catalyst in water.
In order to obtain a hydrophilic region between the electrolyte and the catalyst by causing the hydrophilic group of the electrolyte to face the surface of the catalyst, the catalyst and water are mixed to form a water layer in advance on the surface of the catalyst (catalyst hydrophilicity). Process).
According to the study of the present inventor, the mixing ratio of the catalyst and water should be appropriately selected according to the type of catalyst (especially the type of catalyst carrier and the particle size), but the mixture of catalyst and water ( Moisture state (flowability limit) where the pre-paste changes from a capillary state (water is present around the catalyst particles but no fluidity) to a slurry state (water is present around the catalyst particles and fluidity) ) And the moisture state in the vicinity thereof. Such an amount of water is an optimum amount capable of forming a continuous hydrophilic region between the catalyst and the electrolyte while making the surface of the catalyst hydrophilic.
Here, the flow limit refers to the limit of the moisture content at which the mixture changes from the capillary state to the slurry state and begins to flow.

In the relationship between the shear rate and the viscosity of the pre-paste, the flow limit is a paste state in which the slope of the approximate line is −1 when the approximate straight line is obtained by plotting the viscosity with logarithm against the shear rate. The slurry state is a paste state in which the slope of the approximate straight line is −0.8.
The slope of the approximate straight line in the relationship of the viscosity to the shear rate is −1 or more, that is, the slope becomes gentle and the slurry state becomes highly fluid. Since the state containing excessive moisture leads to a decrease in MEA performance, the amount of water added in a range from the flow limit to the slurry state, that is, in the range of slope −1 to −0.8, is the optimum amount. Thereby, an ideal pre-paste can be obtained. In the pre-paste, it is important to define the minimum amount of water added based on the slope of this approximate line. On the other hand, in the capillary state where the inclination is less than -1 (the inclination becomes tight), the fluidity of the mixture is lost, so that more energy is required at the time of mixing, and the stirring of water and the catalyst tends to be insufficient, and a suitable pre-paste Is not suitable as a condition for obtaining.

Even when the amount of water is greater than the optimum amount, water exists around the catalyst, so that the surface of the catalyst can be hydrophilized. However, such excessive moisture may interfere with the construction of the PFF structure when the pre-paste is mixed with the electrolyte solution (pre-solution). Excess water leaves the catalyst and attracts the electrolyte's hydrophilic groups in a region away from the catalyst. Therefore, the hydrophilic group of the electrolyte facing the catalyst is reduced, and as a result, the hydrophilic region to be formed between the catalyst and the electrolyte is narrowed or divided, and the hydrophilic function in the region is reduced ( Decrease in water retention).
When the catalyst is wet pulverized in water, the catalyst is dispersed in a large amount of water. Here, the amount of water is preferably 5 to 100 in weight ratio to the catalyst. Thereafter, the water is removed to obtain a water content suitable for the pre-paste. For removing water, a method such as hot water bathing can be employed.

(Preparation of electrolyte solution)
As the electrolyte, the above-mentioned perfluorosulfonic acid is generally used. This electrolyte is dissolved in a mixed solvent of water and an organic solvent and mixed with the above-described pre-paste.
The organic solvent is appropriately selected according to the characteristics of the electrolyte, but according to the study of the present inventor, the organic solvent is preferably at least one of a secondary alcohol and a tertiary alcohol. With primary alcohols such as methanol and ethanol, the viscosity of the electrolyte solution does not increase even when the water concentration is reduced. If a secondary alcohol such as isopropyl alcohol (IPA) or a tertiary alcohol such as tertiary butyl alcohol (TBA) is mixed, the solid content of the electrolyte in the electrolyte solution is more undissolved. Further, according to the inventor's study, when the secondary alcohol and the tertiary alcohol are mixed, the solid content of the electrolyte in the electrolyte solution is further unraveled.
As a result of studying optimization of the electrolyte solution used in the above-described PFF structure, the present inventor has found that the optimal amount of water to be contained in the electrolyte solution is 10% by weight or less, more preferably 5% by weight or less of the electrolyte solution. I realized that there was.

There is the following relationship between the electrolyte and the amount of water.
It has been found that when the concentration of water in the electrolyte solution is reduced, the viscosity of the electrolyte solution increases even when the concentration of the electrolyte in the electrolyte solution is the same. Conversely, when the concentration of water is increased, the viscosity of the electrolyte solution decreases. . The reason is estimated as follows.
That is, when the concentration of water in the electrolyte solution is high, as shown in FIG. 3A, water is adsorbed on the side chain E2 of the electrolyte, the main chain E1 of the electrolyte contracts in the electrolyte solution, and the electrolyte is separated. It was assumed that the viscosity of the electrolyte solution was lowered. Further, when the water concentration of the electrolyte solution becomes slightly low, the main chain E1 of the electrolyte opens in the electrolyte solution as shown in FIG. 3B due to the action of the organic solvent contained in the electrolyte solution. As a result, the viscosity of the electrolyte solution increases.

When a reaction layer is formed by mixing the electrolyte solution in the state of the electrolyte (A in FIG. 3), it is considered that this reaction layer is in a state as shown in FIG. That is, since the main chain of the electrolyte is shrunk and the electrolytes are separated from each other, when this is mixed with the pre-paste, there is a high possibility that the hydrophilic region W is formed in a dispersed manner.
In other words, in order to make the hydrophilic side chain E2 of the electrolyte face the catalyst and to reliably form a hydrophilic region between the two, the electrolyte is in the state of FIG. 3B in the electrolyte solution. It is preferable to do. For this purpose, as described above, the amount of water contained in the electrolyte solution is set to 10% by weight or less of the electrolyte solution.

The cathode catalyst layer when the electrolyte in the state shown in FIG. 3B is used is considered to be in the state shown in FIG.
The side chain E2 of the electrolyte is in a state extending in one direction. Therefore, in the catalyst paste, that is, the fuel cell reaction layer, hydrophilic ion exchange groups (sulfone groups) adsorb water in the pre-paste. Become. For this reason, as shown in FIG. 2, in this reaction layer, the electrolyte hydrophilic group E <b> 2 faces the surface of the catalyst C, and a hydrophilic region W is formed between the electrolyte layer E and the catalyst C. And it is thought that the hydrophilic region W is continuously formed around the catalyst C and is formed in a state of communicating with each other by adsorbing the sulfone group with the water in the pre-paste as described above. For this reason, in the reaction layer using this catalyst paste, as shown in FIG. 2, protons and water easily move, and the electrochemical reaction proceeds smoothly. A fuel cell having such a reaction layer can have a high power generation capacity regardless of whether it is in a low humidified state or an excessively humidified state.

The amount of water in the electrolyte solution is determined by evaporating water from the electrolyte solution using, for example, a hot water bath, and then adding water appropriately.
When water is evaporated from the electrolyte solution, the organic solvent contained in the electrolyte solution is also volatilized. Accordingly, an organic solvent is also added as necessary.

(Mixing of pre-paste and electrolyte solution)
A catalyst paste is obtained by mixing the pre-paste and the electrolyte solution.
The pre-paste prepared as described above has a high viscosity because it is in the vicinity of the fluidity limit. The viscosity of the electrolyte solution increases as the amount of water contained therein decreases.
In any case, when the pre-paste obtained under the condition of increasing the viscosity and the electrolyte solution are mixed and stirred, as shown in FIG. 5 (A), the viscosity of the mixture decreases with time, and then stabilizes at a constant value. .

The inventor paid attention to the behavior of the viscosity of the mixture when the mixture of the pre-paste and the electrolyte solution was stirred.
FIG. 5B shows the relationship between the stirring time (= viscosity) and the reaction layer resistance.
A fuel cell was constructed using the catalyst paste obtained by changing the stirring time (= viscosity), and the impedance of the reaction layer was measured.
5A and 5B, it can be seen that when the viscosity decreases with stirring, the impedance of the reaction layer increases so as to be inversely proportional thereto. An increase in impedance means a decrease in proton movement in the reaction layer.

From the above, when preparing the catalyst paste by mixing the pre-paste and the electrolyte solution, it is preferable to quickly stir and complete the uniform mixing of both before the viscosity of the mixture decreases and stabilizes. Recognize. In other words, when the pre-paste and the electrolyte solution are agitated, the viscosity of the mixture of both is monitored, and the agitation is stopped before the viscosity stabilizes to a low level.
When the mixture of the pre-paste and the electrolyte solution is stirred, the periphery of the catalyst of the pre-paste is covered with the electrolyte. At this time, the electrolyte in an open state as shown in FIG. 3B is oriented with its hydrophilic group facing the catalyst to construct a PFF structure. However, if stirring is performed after the PFF structure is constructed (hereinafter sometimes referred to as “over-stirring”), the electrolyte facing the catalyst is separated from the catalyst, and then water on the catalyst surface is taken away from the catalyst surface. break away. Since the electrolyte separated from the catalyst surface is accompanied by water on the catalyst surface, the electrolyte easily takes the form of FIG. Therefore, it is considered that the viscosity of the electrolyte solution component in the catalyst paste decreases, which causes a decrease in the viscosity of the catalyst paste itself. Also, the PFF structure becomes brittle when the electrolyte is detached from the catalyst surface, and the function of the hydrophilic region formed between the catalyst and the electrolyte is lowered. This is expected to increase the reaction layer resistance.
Therefore, the viscosity of the mixture of the pre-paste and the electrolyte solution is adjusted to a predetermined viscosity. Thereby, both over-stirring can be prevented. That is, the over-stirred mixture lowers its viscosity as described above, so that it is possible to prevent over-stirring of the mixture by stopping stirring when the viscosity of the mixture exhibits a predetermined behavior. By preventing over-stirring, a stable PFF structure can always be constructed.

A rotating / revolving centrifugal stirrer is preferably used for mixing and stirring the pre-paste and the electrolyte solution, but a general ball mill, bead mill, stirrer, homogenizer, or the like having a mixing and stirring function can also be employed.
The viscosity of the mixture of the pre-paste and the electrolyte solution varies depending on each material, the blending ratio, the environmental temperature, and the like. Therefore, the viscosity of the mixture is monitored and its behavior (not the absolute value of the viscosity) is detected and evaluated.
The behavior of the viscosity of the mixture refers to the change in viscosity over time before the viscosity of the mixture is stabilized to a low level. For example, the rate of decrease in viscosity per unit time or the rate of decrease in viscosity with respect to the initial viscosity can be employed.
As is clear from FIG. 5A, when the stirring of the mixture exceeds a certain time (4 minutes in the example of FIG. 5A), the rate of decrease in viscosity per time increases. Therefore, stirring can be stopped when the rate of decrease in the viscosity of the mixture accompanying stirring exceeds a predetermined value.
In order to manage the viscosity in the process of producing the catalyst paste, it is preferable to keep the rotation speed of the hybrid mixer constant. Furthermore, it is preferable to perform stirring at a constant temperature.
In order to more accurately manage the viscosity, the viscosity of the mixture can be measured in real time during stirring. For example, the mixing of the pre-paste and the electrolyte solution and the viscosity measurement can be simultaneously performed using a rotor rotation control viscometer. In addition, a method of using a bead mill, a homogenizer, or the like for mixing the pre-paste and the electrolyte solution and incorporating a viscometer capable of real-time measurement such as a tuning fork type vibration viscometer into the paste circulation line is also applicable.
In any method, it is preferable to perform stirring and viscosity measurement at a constant temperature.

(Formation of reaction layer)
The catalyst paste obtained as described above is applied to a gas diffusion substrate to form a reaction layer. Carbon cloth, carbon paper, carbon felt, etc. can be adopted as the gas diffusion base material. It is preferable to form a water repellent layer on the surface (reaction layer side surface) of the gas diffusion substrate. This water repellent layer can be formed from carbon black treated with water repellent with PTFE, for example. As a method for applying the catalyst paste, any method such as screen printing, spraying or ink jetting can be adopted.
In the above, the reaction layer using the catalyst paste having a low viscosity can be provided in a portion where the electrode is easily flooded, for example, in the vicinity of the air outlet, in the vicinity of the hydrogen outlet, in the outer periphery of the electrode, in the vicinity of the cooling plate. As a result, high performance is stably exhibited even in a high humidity atmosphere.
Further, a reaction layer using a high-viscosity catalyst paste may be provided in a portion where the electrode is easily dried, for example, in the vicinity of the air inlet, in the vicinity of the hydrogen inlet, in the center of the electrode, or in a portion away from the cooling plate. As a result, high performance is stably exhibited even in a low humidified atmosphere.
By applying and drying the catalyst paste on the gas diffusion base material a predetermined number of times, an air electrode (gas diffusion base material + reaction layer) and a hydrogen electrode (gas diffusion base material + reaction layer) are formed. A membrane electrode assembly (MEA) is obtained by sandwiching a polymer electrolyte membrane between the air electrode and the hydrogen electrode and joining them by hot pressing or the like. The membrane electrode stack is sandwiched between separators to constitute a fuel cell that is a minimum power generation unit.

In the above, the manufacturing method and the material used for manufacture of a catalyst paste have been demonstrated exclusively.
FIG. 6 is a block diagram showing an apparatus for producing a catalyst paste.
The catalyst, water, noble metal complex, and electrolyte that are the raw materials of the catalyst paste are prepared in the catalyst storage unit 1001, the water storage unit 1021, the noble metal complex solution storage unit 1025, and the electrolyte solution storage unit 1041, respectively. Note that an organic solvent for cleaning organic substances from the catalyst is prepared in the organic solvent container 1023. A tank formed of a capacity and a material corresponding to the object to be accommodated can be used as each accommodating part.
The catalyst processing unit 1003 includes a physical processing unit 1005 and a chemical processing unit 1007. The physical processing unit 1005 includes a wet pulverization unit 1009 and a defoaming unit 1011. A homogenizer, a wet jet mill, or the like can be used as the wet pulverization unit 1009. A hybrid mixer or the like can be used for the defoaming portion 1011. The chemical processing unit 1007 can be a general-purpose stirring device provided with stirring blades. When a noble metal complex having high reactivity with the metal catalyst particles is employed, it is possible to complete the chemical reaction by injecting the noble metal complex solution into a conduit through which the catalyst slurry flows.

In the catalyst processing unit, the catalyst is dispersed in a large amount of water to form a slurry-like pre-paste. Therefore, the water content adjusting unit 1031 adjusts the water content of the pre-paste.
In this case, since moisture is removed from the slurry-like pre-paste, a well-known concentration method (for example, a heating evaporation device, a filtration device, or a centrifuge device) can be used. Moreover, since the water content can be specified from the specific gravity of the pre-paste, the water content adjusting unit preferably includes a specific gravity measuring device. In addition, assuming that the moisture content of the pre-paste becomes too small, it is preferable to provide a moisture supply device.

The electrolyte solution moisture adjusting unit 1043 preferably includes a heating evaporation device and a moisture replenishing device. Since the water content can be specified from the specific gravity, it is preferable to further include a specific gravity measuring device.
The mixing and stirring unit 1051 mixes and stirs the pre-paste and the electrolyte solution each having a controlled amount of water. For example, a hybrid mixer can be used, but is not limited thereto. In order to avoid over-stirring, it is preferable to attach a viscometer 1061 to the mixing and stirring unit 1051.

<Details of Embodiment of the Invention>
In the above, the invention proposed in this application stabilizes the bonding of the hydrophilic group to the catalytic metal particles.
Therefore, in this invention, after couple | bonding a hydrophilic group to catalyst metal particle, this is reduce | restored and the coupling | bonding of both is strengthened.
FIG. 7 shows a flow of a method for producing the catalyst paste of the embodiment.
In step 1 and step 3, a catalyst and an aqueous solution containing a hydrophilic group to be adsorbed on the catalyst metal particles of the catalyst are prepared.
A catalyst in which platinum metal particles are supported on carbon black is used. A hexahydroxoplatinic acid nitric acid solution (H ₂ Pt (OH) ₆ / HNO ₃ ) is used as the aqueous solution.
In step 5, both are put into a reaction vessel, mixed, and stirred with a stirrer. At the time of mixing, the nitric acid concentration was 0.07 wt% (0.01 M), and the platinum content was 0.05 g / 150 ml. Stirring was performed at room temperature for 5 hours.
The mixture of Step 5 was concentrated by filtration (Step 7), and the resulting cake was dried at 60 ° C. for 3 hours (Step 9).

In step 11, the dried cake was placed in a furnace and heated at 150 ° C. for 2 hours in a nitrogen gas atmosphere. The heating temperature 150 ° C. is the temperature of the sample itself.
The heating time and heating temperature in step 11 can be arbitrarily selected depending on the type of the catalytic metal particles and the hydrophilic group as long as the hydrophilic group can be reduced. Of course, an inert gas other than nitrogen can be selected.
When a reducing gas such as hydrogen gas is supplied to the hydrophilic group adsorbed on the catalyst metal particles, the nitrate group is excessively reduced to ammonia or the like, which is not preferable.
In other words, a measure other than heating can be adopted as long as the hydrophilic group itself can be reduced.

The catalyst thus obtained (the catalyst platinum particles are modified with nitric acid groups) is dispersed in water (step 13), and centrifugally stirred with a hybrid mixer (step 15). Next, wet pulverization is performed by adding water and adjusting the amount of water (step 17). A homogenizer is selected as an apparatus for wet pulverization, and the catalyst is pulverized until particles of 10 μm or more in the catalyst slurry become 1% or less (step 19).
Since a large amount of water was required at the time of wet pulverization, in step 21, moisture is removed by a method such as precipitation and filtration to obtain a moisture content suitable for the pre-paste. That is, the fluidity limit and the water content in the vicinity thereof.
When the pre-paste is centrifugally stirred (step 23), its viscosity is monitored (step 25) to prevent over-stirring of the pre-paste.
An electrolyte is mixed with the pre-paste thus obtained to obtain a catalyst paste.

The following test examples of the present invention will be described.
(Test Example 1)
FIG. 8 shows the XPS (X-ray photoelectron spectroscopy) measurement results of the catalyst (Comparative Example 1) that has been processed up to Step 9 in FIG. 7 and the catalyst (Comparative Example 2) after homogenizing the catalyst.
From the results of FIG. 8, when hexahydroxoplatinic acid is mixed with the catalyst, NO ₃ ⁻ is certainly adsorbed on the catalyst platinum particles as shown in FIG. However, if the heat treatment is not performed after the adsorption, the adsorption force is weak and easily desorbed by the homogenizing process which is a subsequent process.

FIG. 9 shows the XPS measurement results of the catalyst that was similarly processed up to Step 9 (no heat treatment) and the catalyst that was heat-treated at 130 ° C. for 2 hours under a nitrogen gas content.
FIG. 9 shows that NO ₃ ⁻ is reduced to NO ₂ by the heat treatment. Note that reduction is insufficient under these heat treatment conditions (see FIG. 12).

FIG. 10 shows the XPS measurement result of the catalyst after Step 11 in FIG. 7, and the same FIG. 11 shows the XPS measurement result after the catalyst is homogenized.
From the results of FIGS. 10 and 11, suction NO ₃ ^- most is the reduction of, but is converted to NO ₂ strong adsorption force to Pt of NO _2, it can be seen that not desorbed by homogenizing process.

FIG. 12 shows the temperature specification (Test Example 1) of the fuel cell provided with the catalyst that has gone through Step 11 in FIG. 7 (ie, after heat treatment at 150 ° C.) and the test used in FIG. 9 (ie, heat treatment at 130 ° C.). ) Temperature characteristics of a fuel cell including a catalyst (Test Example 1).
The operating conditions of the fuel cell when the results of FIG. 12 are obtained are as follows.
Load: 1.6 A / cm ² (const.)
Humidification temperature: 60 ° C
Hydrogen supply rate: 0.8L / min (0.1MPa-G)
Air supply rate: 3L / min (0.1MPa-G)
Electrode area: 20.25 cm ²
Platinum loading: 0.3 mg / cm ²
From the results in FIG. 12, it can be seen that the temperature characteristics of the fuel cell are improved when the chemically treated catalyst is appropriately heat-treated to reduce the hydrophilic group.

  The present invention is not limited to the description of the embodiments and examples of the invention described above. Various modifications are also included in the present invention as long as those skilled in the art can easily conceive without departing from the scope of the claims.

DESCRIPTION OF SYMBOLS 1 Fuel cell, 2 Solid electrolyte membrane, 10 Hydrogen electrode, 11 Reaction layer, 16 Diffusion layer, 20 Air electrode, 21 Reaction layer, 31 Pipe line, 33, 37 Pump, 35 Injection pipe, 38 Water tank, 39 Syringe pump C Catalyst, C1 carrier, C2 catalyst metal particle E electrolyte layer, E1 main chain, E2 side chain W hydrophilic region

Claims (3)

A method for producing a catalyst paste for a fuel cell by treating a catalyst having catalyst metal particles supported on a substrate,
A hydrophilic group adsorption step for adsorbing a hydrophilic group to the catalytic metal particles;
A reduction step for reducing the hydrophilic groups adsorbed on the catalytic metal particles;
After the reduction step, a pulverizing step for wet pulverizing the catalyst;
A method for producing a catalyst paste, comprising:   2. The production method according to claim 1, wherein in the reduction step, a catalyst including catalytic metal particles having the hydrophilic group adsorbed is heated under a condition in which the hydrophilic group is reduced. The manufacturing method according to claim 2, wherein the catalytic metal particles are made of platinum or a platinum alloy, the hydrophilic group is a nitrate group, and is heated at a temperature of 150 ° C. or higher.

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