JP2012074347A - Method for producing catalyst paste and apparatus thereof - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing a catalyst paste suitable for the reaction layer of a fuel cell having a PFF structure.SOLUTION: A method for producing a catalyst paste comprises a step of mixing and stirring a prepaste composed by mixing water and a catalyst, and an electrolyte solution obtained by dissolving an electrolyte into a solvent while setting the moisture content is 10 wt.% or less. A method for producing a prepaste comprises: mixing and stirring a catalyst and water by means of a hybrid mixer to obtain the prepaste, dispersing the prepaste into a large amount of water and subjecting to wet pulverization, and removing moisture from the dispersion obtained by wet pulverization.

Description

  The present invention relates to a method for producing a catalyst paste and an apparatus therefor.

A membrane electrode assembly used in a fuel cell has a structure in which a solid polymer electrolyte membrane is sandwiched between a hydrogen electrode and an air electrode. The hydrogen electrode and the air electrode are each provided with a reaction layer and a diffusion layer from the solid polymer electrolyte membrane side. It is laminated sequentially.
The reaction layer is made of a mixture of a catalyst and an electrolyte, and is required to have electrical and proton conductivity and air permeability. Here, since protons move in the form of H ₃ O ⁺ with water, it is necessary to maintain the reaction layer in a wet state. Of course, excessive moisture in the reaction layer inhibits air permeability (so-called flooding phenomenon), so the moisture in the reaction layer must always be maintained at an appropriate amount.
In order to satisfy this requirement, the present applicant has proposed a reaction film having a PFF structure (registered trademark: the same applies hereinafter) in Patent Document 1 and Patent Document 2. Here, the PFF 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 region on the catalyst. Such a PFF structure can be obtained by preparing a pre-paste in which a catalyst and water are mixed in advance, mixing the pre-paste and the electrolyte solution, and employing an appropriate stirring method. By adopting PFF structure,
The uneven distribution of water in the reaction layer is prevented. Further, even when the fuel cell is operated in a low humidified environment, water is present in this hydrophilic region, so that overdrying can be prevented. Further, in operation in a highly humidified environment, excess water is discharged to the outside (diffusion layer side) through this hydrophilic region, so that flatting can be prevented.
In addition, please refer to patent document 3 and nonpatent literatures 1-3 as literature which discloses the technique relevant to this invention.

JP 2006-140061 A JP 2006-140062 JP JP 2009-104905 A

Journal of Electrochemical Society 2005, vol.152, No.5, PP.A970-A977 MAKHARIA Rohit; MATHIAS Mark F.; BAKER Daniel R. “Measurement of catalyst layer electrolyte resistance in PEFCs using electrochemicalimpedance spectroscopy” Journal of Electroanalytical Chemistry 475, 107-123 (1999) M.Eikerling and A.A.kornyshev “electrochemical impedance of Cathode Catalyst Layer of Polymer Electrolyte Fuel Cells” "Electrochemical Impedance Method" (Maruzen, Masayuki Itagaki) 8 Electrochemical Impedance Analysis Using Distributed Constant Type Equivalent Circuit (pp133-146)

In order to secure the PFF structure, the present inventor has made various studies.
A (Improved preparation of pre-paste made by mixing catalyst and 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.
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).
For details, refer to Japanese Patent Application No. 2009-256659.

B (Preparation of electrolyte solution)
As the electrolyte, a polymer material, for example, a fluorine-based polymer such as Nafion (trade name of DuPont, hereinafter the same) 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 aforementioned fluoropolymers are also distributed in the state of an electrolyte solution dissolved in a solvent. This solvent is a mixture of water and an organic solvent.
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. 1A, water is adsorbed on the side chain E2 of the electrolyte and the electrolyte main chain E1 is aggregated in the electrolyte solution, and the viscosity of the electrolyte solution is increased. I guessed that will decline. Further, when the water concentration of the electrolyte solution becomes slightly low, the main chain E1 of the electrolyte opens in the electrolyte solution by the action of the organic solvent contained in the electrolyte solution, as shown in FIG. As a result, the viscosity of the electrolyte solution increases.

When the reaction solution is formed by mixing the electrolyte solution containing the main chain E1 of the electrolyte (FIG. 1A) in which the aggregation has progressed, it is considered that this reaction layer is in a state as shown in FIG. That is, since the main chain E1 of the electrolyte is aggregated, the side chain E2 extends in multiple directions. Then, the side chain E2 and the water in the reaction layer are adsorbed to form the hydrophilic region W in a dispersed manner in the reaction layer. For this reason, at the location where the main chain E1 of the electrolyte is agglomerated in this reaction layer, protons and water hardly move in the reaction layer due to ionic resistance in the reaction layer. For this reason, in a low humidification state, the performance fall by drying of the electrolyte in an electrolyte membrane and each catalyst layer will be caused, and in an excessive humidification state, it will become a factor by which the performance fall by flooding will arise.
In other words, in order to make the hydrophilic group W (side chain E2) of the electrolyte face the catalyst C and to reliably form the hydrophilic region W between the two, the electrolyte in the electrolyte solution is as shown in FIG. It is preferable to be in a state. 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 of FIG. 1B is used is considered to be in the state of 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. It becomes. Therefore, as shown in FIG. 3, in this reaction layer, the electrolyte hydrophilic group E2 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. 3, 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.
For details, refer to Japanese Patent Application No. 2010-002362.

The catalyst paste obtained by executing at least one of the knowledge A concerning the preparation of the pre-paste (mixture of catalyst and water) and the knowledge B concerning the preparation of the electrolyte solution improves the output characteristics of the fuel cell. Let
Fuel cells are required to further improve output characteristics.

As a result of intensive studies to meet the above requirements, the present inventor has found that in the reaction layer having the PFF structure, it is preferable that the catalyst particles are more evenly dispersed in the electrolyte. In other words, if the catalyst particles form lumps (secondary particles, tertiary particles, etc.), no hydrophilic region is formed on the contact surface between the catalyst and the catalyst. On the other hand, generally, in the reaction layer not having the PFF structure, the catalyst particles are considered to be aggregates having different diameters. This is because it is considered that large pores are formed between the catalyst particle aggregates having different diameters and function as a water discharge path.
The present inventor has noticed that the pre-paste, which is a mixture of the catalyst and water, is wet pulverized as a measure to disperse the catalyst particles and disperse the catalyst uniformly. However, in order to perform wet grinding, the pre-paste needs to have sufficient fluidity, and for this purpose, the catalyst must be dispersed in a large amount of water.
However, as described above, the amount of water to be contained in the pre-paste is limited. That is, the amount of water should be an amount that makes the pre-paste a fluidity limit or an amount in the vicinity thereof. Therefore, in order to disassemble the agglomerates of the catalyst, the catalyst is dispersed in a large amount of water, this is wet pulverized, and then the moisture is removed so that the obtained pre-paste is at or near the fluidity limit. Adjust the amount of water.

The present inventor has conceived the present invention based on the above knowledge. That is, the first aspect of the present invention is defined as follows.
A first step of mixing a catalyst and water to obtain a pre-paste;
A second step of obtaining an electrolyte solution by dissolving an electrolyte having a hydrophilic side chain in a solvent with a water content of 10% by weight or less;
A third step of mixing and stirring the pre-paste and the electrolyte solution,
The first step includes a wet pulverization step in which the catalyst is dispersed in a large amount of water and wet pulverized.
A moisture removal step in which the moisture content of the pre-paste obtained by removing moisture from the wet-pulverized dispersion is within the range from the flow limit to the slurry state;
A method for producing a catalyst paste, comprising:

According to the method for producing a catalyst paste of the first aspect defined as described above, since the catalyst is wet pulverized, the aggregation of the catalyst particles is solved. As a result, the contact area between the electrolyte and the catalyst is increased. Since a hydrophilic region is formed on the contact surface between the electrolyte and the catalyst, by widening the contact area between the electrolyte and the catalyst in this way, the PFF structure in the reaction layer is expanded, and thus the characteristics of the reaction layer are improved. improves.
As described above, when excess water is present when the pre-paste and the electrolyte are mixed to form the catalyst paste, the PFF structure is adversely affected. Therefore, a stable PFF structure can be constructed by removing excess water in the water removal step.
According to the study by the present inventors, the amount of water to be contained in the pre-paste is preferably within twice the fluidity limit. On the other hand, wet milling usually requires more than 5 times the minimum fluidity limit.
Moreover, it is preferable to modify the catalyst metal particles of the catalyst with hydrophilic groups (excluding hydroxyl groups) such as nitrate groups, amino groups, and sulfone groups. This modification is preferably performed on the catalyst before forming the pre-paste.

In order to carry out the above-described method for producing a catalyst paste, the following apparatus is proposed. That is, a first mixing unit for mixing a catalyst and water to obtain a pre-paste,
A second mixing and stirring unit for mixing and stirring the pre-paste and the electrolyte solution,
The first mixing unit includes a wet pulverizing unit that disperses the catalyst in water and wet pulverizes the catalyst,
A moisture content adjustment unit that adjusts the moisture content of the pre-paste obtained by removing moisture from the wet-pulverized dispersion to a range from the flow limit to the slurry state;
An apparatus for producing a catalyst paste, comprising:

FIG. 1 is a schematic diagram showing the form of the electrolyte in the electrolyte solution. FIG. 2 is a schematic diagram for explaining a PFF structure corresponding to FIG. FIG. 3 is a schematic diagram for explaining the PFF structure corresponding to FIG. FIG. 4 is a schematic diagram showing a dispersion state of particles and water. FIG. 5 relates to Experimental Example 1 and is a graph showing the relationship between the shear rate and the viscosity of the pre-paste. FIG. 6 relates to Experimental Example 1 and is a graph showing the relationship between water magnification and shear rate-viscosity gradient. FIG. 7 relates to Experimental Example 2 and is a graph showing the relationship between the shear rate and the viscosity of the pre-paste. FIG. 8 relates to Experimental Example 2 and is a graph showing the relationship between the water magnification and the shear rate-viscosity gradient. 9 is a graph showing the relationship between the shear rate and the viscosity of the pre-paste according to Experimental Example 3. FIG. FIG. 10 relates to Experimental Example 3 and is a graph showing the relationship between water magnification and shear rate-viscosity gradient. FIG. 11 (A) shows the relationship between the stirring time and the viscosity of the pre-paste and the electrolyte solution, and FIG. 11 (B) shows the relationship between the stirring time and the reaction layer resistance. FIG. 12 is a block diagram showing the configuration of the catalyst paste manufacturing apparatus according to the embodiment of the present invention. FIG. 13 shows the output characteristics of the fuel cell with the amount of water. FIG. 14 also shows the relationship between the amount of water and the reaction layer resistance.

  In the above, the catalyst means a catalyst metal particle supported on a carrier having conductivity. Common carbon black particles can be used as the carrier, but tin oxide, titanium oxide, and the like can also be used. General-purpose materials such as platinum and platinum-cobalt alloy can be used as the catalyst metal particles.

(Pre-paste preparation)
The pre-paste is created by the following steps (1) to (3).
(1) Step of mixing and stirring the catalyst and water with a hybrid mixer Conventionally, what was obtained by this step was used as a pre-paste.
(2) Step of wet pulverization The pre-paste obtained in step (1) is wet pulverized. At this time, a large amount of water is added to the pre-paste to increase the fluidity of the pre-paste and to fully exhibit the wet pulverization function by a wet jet mill or a homogenizer.
(3) Moisture removal step Necessary moisture is removed from the wet-pulverized catalyst-water mixture.
The amount of water in step (1) can be arbitrarily set, and the pre-paste obtained in step (1) may contain a large amount of water. This is because the amount of water is finally prepared in the step (3).
If the catalyst surface can be hydrophilized by wet grinding, the mixing and stirring step (1) by the hybrid mixer can be omitted.

(Wet grinding)
When wet pulverization is performed, the catalyst is dispersed in a large amount of water. The amount (weight) of water at this time is preferably 5 times or more that of the catalyst. If the water content is less than 5 times, wet grinding may be incomplete. The upper limit of the moisture content is not particularly limited, but is preferably 100 times or less with respect to the catalyst. This is to reduce the load of the water removal step.
The wet pulverization may be performed using any type of wet pulverization method as long as the aggregate of the target catalyst can be pulverized. Can be adopted. In particular, it is preferable to use an ultrasonic homogenizer, a wet jet mill, or a wet disk mill, which are medialess, from the viewpoint of improving work efficiency. In the embodiment of the present invention, an ultrasonic homogenizer or a wet jet mill is used.

At the time of wet processing, the catalyst is dispersed in a large amount of water (more than 5 times the amount of water corresponding to the fluidity limit). If this excess water is present, there is a possibility that the PFF structure construction may be hindered 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. Accordingly, 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, or the hydrophilic function in the region is reduced (moisture content). Holding force).
In addition, excess water in the pre-paste also affects the electrolyte solution, and the electrolyte in a state where the hydrophobic groups are extended (see FIG. 1B) is in the state shown in FIG. It can be.
According to the study of the present inventor, the amount of water contained in the pre-paste is preferably set to the amount of water corresponding to the fluidity limit or within twice the amount.

Hereinafter, the moisture content and fluidity limit in the pre-paste will be described.
In the mixture of particles and water, the dispersion state of the particles and water is classified into four types as shown in “Slurry Stabilization Technology and Preparation Examples” (published by Information Technology Corporation). Hereinafter, explanation will be made based on the above-mentioned sources. As shown in FIG. 4, reference numeral 2 indicates particles, reference numeral 3 indicates water, and reference numeral 4 indicates air.
The particles 2 correspond to the above catalyst. When the amount of water 3 is small with respect to the particles 2, the solid phase of each particle 2 is in a continuous state and the liquid phase of water 3 is in a discontinuous state, as shown in FIG. For this reason, a gas phase composed of large air 4 is generated between the particles 2. This state is called a pendula state.
If water 3 is further added to this pendulum state and the amount of water 3 with respect to the particles 2 is increased, the ratio of the gas phase is reduced as shown in FIG. This state is called a funicular state. In the funicular state, the liquid phase of water 3 is continuous, but the gas phase of air 4 still remains.

If the amount of water 3 is further increased from the funicular state, the mixture of particles 2 and water 3 exceeds the plastic limit as shown in FIG. This state is called a capillary state. In the capillary state, the gap between the particles 2 is filled with water 3. That is, the solid phase of each particle 2 is in a discontinuous state, the liquid phase of water 3 is in a continuous state, and the gas phase of air 4 is not present.
If the amount of water 3 is further increased from this capillary state, as shown in FIG. 4D, each particle 2 is dispersed in water 3 and has fluidity. This state is called a slurry state.
The flow limit is the limit of the moisture content at which the mixture changes from a capillary state to a slurry state and begins to flow.
For the purpose of simply hydrophilizing the surface of the catalyst, it is considered that the effect can be obtained by mixing water excessively with respect to the catalyst. However, when water is mixed excessively with respect to the catalyst, excess water increases in the pre-paste. In that case, the hydrophilic group of the polyelectrolyte is oriented in the excess water, which prevents the formation of a continuous hydrophilic layer between the catalyst and the polymer, and the generated water during power generation is placed near the catalyst. The holding effect is impaired, and the adverse effect that the output of the fuel cell system tends to decrease in a low humidified environment occurs.

  On the other hand, the amount of water mixed into the catalyst is defined based on the flow limit, and by making the pre-paste a moisture state that changes from a capillary state in the vicinity of the flow limit to a slurry state, the amount of water mixed is While the surface of the catalyst is hydrophilized with water, the water content becomes the minimum amount of water that can form a continuous hydrophilic layer between the catalyst and the polymer electrolyte. For this reason, the obtained pre-paste will be in the ideal state for manufacturing MEA (membrane electrode assembly) which can obtain a high output stably.

  In the relationship between the shear rate and the viscosity of the pre-paste, an approximate straight line is obtained when the viscosity is plotted in logarithm with respect to the shear rate. The flow limit is a paste state in which the slope of the approximate straight line is −1, and 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.

{Verification 1}
About the optimal value of the amount of water contained in a pre paste, verification by the following Experimental Examples 1-3 was performed. In Experimental Examples 1 to 3, the viscosity of the pre-paste was measured by paying attention to the viscosity of the pre-paste containing the catalyst and water as a parameter for determining the optimum value of the amount of water mixed with the catalyst. In the measurement of the viscosity, an RB80 type viscosity measuring machine manufactured by Toki Sangyo was used.

(Experimental example 1)
In Experimental Example 1, the viscosity of the pre-paste was measured by changing the amount of water mixed with the catalyst. In Experimental Example 1, a catalyst in which Pt fine particles were supported at a supporting density of 60 wt% on a substantially spherical carbon support (KB600JD, manufactured by Ketjen Black International) was prepared as a catalyst. The amount of water mixed in the catalyst is determined by the ratio of the weight of water to the weight of the carbon support (H ₂ O / C) after removing Pt fine particles from the catalyst. The experimental results are shown in FIGS.

FIG. 5 shows the shear rate (log (D [1 / sec])) on the horizontal axis x and the viscosity (log (η [Pa · sec])) on the vertical axis y. The function formula of the straight line in each water quantity is as follows.
H ₂ O / C = 19: y = −1.0934x + 0.7264
H ₂ O / C = 20: y = −0.9226x + 0.7341
H ₂ O / C = 22: y = −0.8156x + 0.5516
H ₂ O / C = 26: y = −0.7314x + 0.369
H ₂ O / C = 38: y = −0.7418x−0.0975

  Thus, it can be seen that as the amount of water decreases, the viscosity of the pre-paste increases and the slope of the straight line approaches -1. This is because, when the amount of water decreases, the carbon carriers agglomerate with each other and the viscosity of the pre-paste increases, which makes the straight line slope stiff. FIG. 5 suggests that the pre-paste with the straight line slope greater than −1 exceeded the flow limit and became a capillary state.

FIG. 6 shows the water magnification (H ₂ O / C) [g / g-C] and shear rate-viscosity gradient d (log η) / d (log D) in a pre-paste having a water content of H ₂ O / C = 19. Shows the relationship. In FIG. 5, if the shear rate-viscosity gradient exceeds −1, the flow limit is indicated, and the pre-paste exceeds the flow limit. As shown in FIG. 6, when the water magnification is around 20, the gradient is -1. That is, in the catalyst used in Experimental Example 1, it can be said that the water magnification 20 is the optimum value.

(Experimental example 2)
In Experimental Example 2, similar to the catalyst used in Experimental Example 1, a catalyst in which Pt fine particles 81b are supported on KB600JD at a supporting density of 60 wt% was used, but a catalyst different from the catalyst was used. The experimental method and the like are the same as in Experimental Example 1. The results are shown in FIGS.

The linear function formula shown in FIG. 7 is as follows.
H ₂ O / C = 27: y = −1.0436x + 0.8981
H ₂ O / C = 28: y = −0.9077x + 0.7343
H ₂ O / C = 31: y = −0.7585x + 0.5736

As shown in FIG. 8, in these pre-pastes, the water magnification is around 27 and the gradient exceeds -1.
7 and 8, in the catalyst used in Experimental Example 2, the pre-paste is close to the flow limit by setting the water magnification to 28. That is, it can be seen that even if the catalyst using KB600JD having the same loading density of 60 wt% is used, if the manufacturer is different, there is a difference in the amount of water that is the optimum amount of water.

(Experimental example 3)
In Experimental Example 3, the viscosity of the pre-paste when the water magnification was changed was measured using a catalyst in which Pt fine particles were supported at a support density of 20 wt% on BP800 (manufactured by CABOT). The experimental method and the like are the same as in Experimental Example 1. The results are shown in FIGS.
The function function of the straight line shown in FIG. 9 is as follows.
H ₂ O / C = 3.2: y = −1.1138x + 0.8268
H ₂ O / C = 3.5: y = −0.9418x + 0.5033
H ₂ O / C = 4: y = −0.8924x + 0.1093
H ₂ O / C = 5: y = −0.6826x−0.2232

As shown in FIG. 10, in these pre-pastes, the gradient exceeds −1 in the vicinity of the water magnification of 3.2. That is, in the catalyst of Experimental Example 3, a water magnification of 3.5 is the optimum water addition amount.
Thus, compared with Experimental Example 1, the amount of water with a gradient of −1 is smaller in Experimental Example 3 because the specific surface area of BP800 is about one-sixth of KB600JD in Experimental Example 1. That is, the optimum amount of water for BP880 is about 1/6 that of Experimental Example 1.

  From Experimental Examples 1 to 3, it has been found that the optimum value of the amount of water mixed in each catalyst varies depending on the type of each catalyst, more specifically, the type of carbon support in each catalyst and the pulverization status. And it turns out that it is preferable to determine the quantity of the water mixed with each catalyst based on the flow limit of a pre paste. Moreover, according to the function formula of each straight line shown in FIG.5, FIG7 and FIG.9, as for the amount of water used as the flow limit of each pre paste, the inclination of a straight line is -0.8--1.

(Preparation of electrolyte solution)
The electrolyte used in the present invention is made of a polymer compound, and has a hydrophobic main chain 100 and a side chain 101 having a hydrophilic ion exchange group, as shown in FIG. The hydrophilic ion exchange group is composed of, for example, a sulfone group (SO ₃ ⁻ ).
Generally, the electrolyte is distributed in a state dissolved in a solvent (water / organic solvent). The amount of water contained in such a general-purpose electrolyte solution is adjusted. Since a general-purpose electrolyte solution contains more than 10% by weight of water, it is heated to evaporate it, and water and an organic solvent are added as necessary to prepare an electrolyte solution.
As described above, the water content in the electrolyte solution should be 10% by weight or less, more preferably 5% by weight or less. As a result, the electrolyte in the electrolyte solution assumes the state shown in FIG.

  The organic solvent contained in the electrolyte solution is appropriately selected according to the characteristics of the electrolyte, but according to the study of the present inventors, the organic solvent is at least one of a secondary alcohol and a tertiary alcohol. It is preferable. 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. According to the inventors' test, when the secondary alcohol and the tertiary alcohol are mixed, the solid content of the electrolyte in the electrolyte solution is further unraveled.

(Mixing and stirring of pre-paste and electrolyte solution)
The pre-paste prepared by wet pulverization and water is further mixed with the electrolyte solution.
A rotating / revolving centrifugal stirrer (hybrid mixer) is preferably used for mixing and stirring the pre-paste and the electrolyte solution, but a general ball mill, bead mill, stirrer, homogenizer, etc. having a mixing and stirring function should be employed. You can also.
When the pre-paste and the electrolyte solution are mixed and stirred, as shown in FIG. 11A, the viscosity of the mixture decreases with time, and then stabilizes at a constant value.
FIG. 11B 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.
11 (A) and 11 (B), 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 the open state as shown in FIG. 1B is oriented with its hydrophilic group facing the catalyst to construct a PFF structure. However, if stirring is performed after the PFF structure is built (hereinafter, sometimes referred to as “over-stirring”), the electrolyte facing the catalyst is separated from the catalyst, and at that time, water on the catalyst surface is taken away and detached from the catalyst surface. To do. 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. Moreover, 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.

The viscosity of the mixture of the pre-paste and the electrolyte solution varies depending on the respective materials, blending ratio, 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 the diagram of FIG. 11A, when the stirring of the mixture exceeds a certain time (4 minutes in the example of FIG. 11A), 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.

The catalyst paste thus obtained is applied to a diffusion layer made of, for example, carbon cloth, carbon paper, carbon felt, and the like, and attached to a polymer electrolyte membrane to constitute a membrane electrode laminate (MEA). The dried catalyst paste layer becomes the reaction layer. The membrane electrode stack is sandwiched between separators to constitute a fuel cell that is a minimum power generation unit.
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.

In order to obtain an effective PFF structure, the catalyst is preferably subjected to the following treatment.
Referring to FIG. 1, catalyst C is a catalyst in which catalyst metal particles C2 are supported on a 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 preferable 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 knowledge of the inventors, 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 added to the platinum complex solution And may be stirred if necessary. When a nitric acid group is selected, the raw material catalyst is put into a nitric acid aqueous 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.

(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 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 preferable to perform a defoaming treatment on the catalyst.

In summary, the catalyst paste production apparatus shown in FIG. 12 is proposed.
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.

Examples of the present invention will be described below.
(Pre-paste preparation)
A catalyst in which Pt fine particles were supported at a supporting density of 50 wt% using carbon black as a support was prepared. Then, the catalyst and water were mixed in an amount of water corresponding to 10 g of water having a fluidity limit with respect to 1 g of the catalyst, and the mixture was centrifuged with a hybrid mixer for 4 minutes.
Next, 90 g of water was added, and wet pulverization was performed for 10 minutes using a Branson (model number: Sonifier Model 450) as an ultrasonic homogenizer. A wet jet mill may be employed as the wet pulverization.
The wet pulverized liquid was allowed to stand for 1 hour or longer to precipitate the catalyst, and the supernatant liquid was removed.
The residue was boiled at 90 ° C., and adjusted to be 0.6 times, 1.0 times, 2.0 times and 3.0 times with respect to the fluidity limit amount of water.

(Preparation of electrolyte solution)
10 g of DE2020 (DuPont product name, the same applies hereinafter) was bathed at 85 ° C., and the contents of water and NPA in DE2020 were both reduced to 2 g. Next, 34 g of IPA (isopropyl alcohol) was mixed with this DE2020 and stirred for 3 minutes by a hybrid mixer. Thereby, the content of water and NPA in DE2020 is equivalent to 5% in terms of the concentration in the Nafion solution of sample E. Thus, an electrolyte solution was obtained.

(Mixing and stirring of pre-paste and electrolyte solution)
To 11 g of the pre-paste obtained as described above (when the amount of water is 1.0 times the flow limit amount), 10 g of the electrolyte solution is put into a hybrid mixer and mixed and stirred for 4 minutes to obtain a catalyst paste.
Each catalyst paste described above was applied to a gas diffusion layer substrate to form a reaction layer. The gas diffusion layer base material is obtained by laminating carbon black subjected to water repellent treatment with PTFE and carbon cloth as a base material having electron conductivity. A gas diffusion layer substrate in which a water repellent layer made of a mixture of carbon black and PTFE is provided on a substrate made of carbon paper or the like can also be employed. In addition, as a method of applying the catalyst paste 41 to the gas diffusion layer base material, it is possible to adopt means such as screen printing, a spray method, and an ink jet method.

The substrate coated with the catalyst paste is dried. The air electrode and the hydrogen electrode thus formed were laminated on a solid polymer electrolyte membrane (Nafion membrane: NR211), and bonded by hot pressing at 140 ° C. and a pressure of 40 kgf / cm ² .
Thus, the power generation characteristics of the membrane electrode assembly were performed under the following high humidification operation conditions.
Operating conditions: Cell temperature 50 ° C., air (normal pressure) and hydrogen (normal pressure) humidified (bubbler temperature 50 ° C.).
The results are shown in FIG.
From the results of FIG. 13, it can be seen that the amount of water to be included in the pre-paste is preferably 1 to 2 times the amount of water corresponding to the fluidity limit.
The preferable amount of moisture is also applied to a pre-paste that has not been wet pulverized.

(Measurement of reaction layer resistance)
The cell temperature was 50 ° C., nitrogen (normal pressure) was passed to the air electrode, hydrogen (normal pressure) was humidified (bubbler temperature 50 ° C.) and passed through the hydrogen electrode, and the reaction layer resistance on the air electrode side was measured.
The results are shown in FIG. In FIG. 14, the horizontal axis indicates the water content when the water content corresponding to the fluidity limit is 1, and the method of measuring the resistance value on the vertical axis is based on Non-Patent Documents 1 to 3.
The result of FIG. 14 also shows that the amount of water to be included in the pre-paste is preferably 1 to 2 times the amount of water corresponding to the fluidity limit. A range smaller than 1 is not preferable because the hydrophilization treatment of the catalyst may be incomplete (see FIG. 4).

  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.

81 catalyst 81a carrier 81b platinum catalyst fine particle 82 electrolyte 83 hydrophilic region

Claims (7)

A first step of mixing a catalyst and water to obtain a pre-paste;
A second step of obtaining an electrolyte solution by dissolving an electrolyte having a hydrophilic side chain in a solvent with a water content of 10% by weight or less;
A third step of mixing and stirring the pre-paste and the electrolyte solution,
The first step includes a wet pulverization step in which the catalyst is dispersed in water and wet pulverized.
A moisture removal step in which the moisture content of the pre-paste obtained by removing moisture from the wet-pulverized dispersion is within the range from the flow limit to the slurry state;
A method for producing a catalyst paste, comprising:   The method for producing a catalyst paste according to claim 1, wherein the moisture content of the pre-paste obtained in the moisture removal step is 1 to 2 times the moisture content corresponding to the fluidity limit.   The method for producing a catalyst paste according to claim 1 or 2, wherein the catalyst metal particles of the catalyst are modified with a hydrophilic group (excluding a hydroxyl group) in the first step. A first mixing unit for mixing the catalyst and water to obtain a pre-paste;
A second mixing and stirring unit for mixing and stirring the pre-paste and the electrolyte solution,
The first mixing unit includes a wet pulverizing unit that disperses the catalyst in water and wet pulverizes the catalyst,
A moisture content adjustment unit that adjusts the moisture content of the pre-paste obtained by removing moisture from the wet-pulverized dispersion to a range from the flow limit to the slurry state;
An apparatus for producing a catalyst paste, comprising:   The apparatus for producing a catalyst paste according to claim 4, wherein the wet pulverization unit is a wet jet mill.   6. The catalyst paste manufacturing apparatus according to claim 5, wherein the second mixing and stirring unit is a rotation / revolution centrifugal stirring device.   A solution containing portion containing a solution containing a hydrophilic group capable of modifying the catalyst metal particles of the catalyst; a catalyst containing portion containing the catalyst; and mixing the solution and the catalyst to convert the hydrophilic group into the catalyst metal particles The catalyst paste manufacturing apparatus according to claim 5, further comprising a third mixing unit to be modified.

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