JP2013073852A - Catalyst layer for fuel cell and production method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To solve such problems of a catalyst layer for fuel cell using conductive diamond-like carbon or glassy carbon as a carrier that it is difficult to attain high activity, and that the E/C ensuring high output characteristics is different in each humidification state.SOLUTION: The catalyst layer for fuel cell is produced via a step of preparing a main catalyst paste where the catalyst is coated with a first polymer electrolyte layer of first thickness by dispersing a catalyst including a first carbon catalyst composed of conductive diamond-like carbon or glassy carbon into a solution of first polymer electrolyte, a step of preparing a water-retention paste where a second catalyst is coated with a second polymer electrolyte layer of second thickness thicker than the first thickness by dispersing the second catalyst into a solution of second polymer electrolyte, and a step of mixing the main catalyst paste and the water-retention paste.

Description

  The present invention relates to an improvement in a fuel cell catalyst layer and a method for producing the same.

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 a structure that is sequentially laminated.
This reaction layer is a mixture of a catalyst and an electrolyte, and it is proposed in Patent Document 1 that conductive diamond-like carbon (hereinafter sometimes abbreviated as “DLC”) or glassy carbon can be used as a catalyst support. . Since these materials are chemically and physically stable compared to carbon black, the catalytic activity is stabilized.
A main catalyst paste having a thin coating of an electrolyte (that is, a small E / C (weight ratio of electrolyte to carbon; the same applies hereinafter)) and carrier particles (that is, not supporting catalyst metal particles) with respect to a catalyst having low activity Patent Documents 2 to 4 propose techniques for mixing with a water-retaining material paste coated with a thick electrolyte layer (that is, having a large E / C). Here, if the electrolyte membrane covering the catalyst is thinned, the oxygen permeability is improved, so that the activity inherent to the catalyst can be maximized. On the other hand, if the electrolyte is made thinner, the moisture retention function in the low humidification state and the moisture removal function in the high humidification state may be inferior. Therefore, a water retention paste having a large E / C is added to control the moisture in the reaction layer. Make it.

JP 2010-99063 A JP 2011-119208 A JP 2011-119209 A JP 2011-14209 A

Catalysts using DLC or glassy carbon as a carrier are difficult to obtain higher activity than catalysts using carbon black as a carrier.
Particularly in DLC, there was a difference in preferable E / C depending on the humidified state. For example, when E / C that can ensure high output characteristics in a low humidified state is employed, the output characteristics in a highly humidified state are sacrificed.

The present inventors have paid attention to the techniques disclosed in Patent Documents 2 to 4 described above as a countermeasure against low activity of catalysts using DLC or glassy carbon as a carrier.
That is, the first aspect of the present invention is defined as follows.
A catalyst including a first carbon support made of conductive diamond-like carbon or glassy carbon is dispersed in a first polymer electrolyte solution, and the catalyst is covered with the first polymer electrolyte layer having a first film thickness. Preparing a main catalyst paste comprising:
A second carrier is dispersed in a second polymer electrolyte solution, and the second carrier is covered with the second polymer electrolyte layer having a second film thickness larger than the second film thickness. Preparing a water-retaining material paste;
And a step of mixing the main catalyst paste and the water retention material paste.

In the catalyst layer obtained by the production method defined as described above, a portion where the solvent evaporates from the main catalyst paste (main catalyst body), and a portion where the solvent evaporates from the water retention material paste (water retention material) Are mixed. Then, the original activity of the catalyst is utilized to the maximum in the former part (main catalyst body) where the electrolyte membrane is thin, and the moisture adjustment in the catalyst layer is favorably performed in the latter part (water retention material) where the electrolyte membrane is thick. .
As a result, a fuel cell catalyst layer including a catalyst having DLC or glassy carbon as a carrier has high activity.

Further, it should be noted that in a fuel cell using a catalyst with DLC as a support as a catalyst layer, the E / C at which high output characteristics can be obtained differs for each humidified state.
On the other hand, according to the catalyst layer manufactured by the manufacturing method of the first aspect, even if a catalyst using DLC as a carrier is employed, by using this together with a water retaining material, a predetermined E / C can be obtained. In terms of the ratio, high output characteristics were obtained in all humidification states from the low humidification state to the high humidification state.

FIG. 1 is a schematic view showing a mixed paste according to an embodiment of the present invention. FIG. 2 shows IV characteristics when a fuel cell provided with catalyst layers of Examples and Comparative Examples is operated under high humidification conditions. FIG. 3 shows IV characteristics when a fuel cell provided with catalyst layers of Examples and Comparative Examples is operated under low humidification conditions. FIG. 4 is a schematic diagram for explaining a PFF structure corresponding to FIG. FIG. 5 is a schematic diagram showing the form of the electrolyte in the electrolyte solution, and shows a case where the water concentration of the electrolyte solution is high (A) and a case where it is low (B). FIG. 6 is a schematic diagram for explaining the PFF structure corresponding to FIG. FIG. 7 (A) shows the relationship between the stirring time and the viscosity of the pre-paste and the electrolyte solution, and FIG. 7 (B) shows the relationship between the stirring time and the reaction layer resistance. FIG. 8 shows a schematic diagram of an apparatus for producing a catalyst paste.

In this invention, the main catalyst paste and the water retention material paste are separately prepared and mixed to obtain the catalyst paste.
Here, the main catalyst paste disperses the catalyst including the first carbon support in the first polymer electrolyte solution, and coats the catalyst with the first polymer electrolyte layer.
DLC or glassy carbon can be used for the first carbon support.

Here, DLC is a conductive carbon among carbons having an amorphous structure in which both diamond bonds (SP ₃ hybrid orbital bonds between carbons) and graphite bonds (SP ₂ hybrid orbital bonds between carbons) are mixed. Means 1000 Ωcm or less. However, in addition to the amorphous structure, those having a phase composed of a crystal structure partially composed of a graphite structure (that is, a hexagonal crystal structure composed of SP ₂ hybrid orbital bonds) and thereby exhibiting conductivity are also included. . Diamond-like carbon having properties intermediate between graphite and diamond adjusts the conductivity by adjusting the ratio of SP ₂ hybrid orbital bonds and SP ₃ hybrid orbital bonds of the carbon atoms constituting diamond-like carbon during film formation. be able to.
Glassy carbon refers to glassy carbon that is generated when carbon is denatured at a high temperature in a reducing atmosphere.
The metal catalyst particles supported on these are not particularly limited as long as they can contribute to the fuel cell reaction, but fine particles such as platinum and platinum alloys can be adopted.

Perfluorosulfonic acid is generally used for the first polymer electrolyte. This first polymer electrolyte is dissolved in a mixed solvent (water + alcohol) and mixed with a main catalyst pre-paste (catalyst + water) to obtain a main catalyst paste.
This main catalyst paste preferably employs a PFF structure. The PFF structure will be described in detail at the end of this specification.

In order to ensure oxygen permeability, the first polymer electrolyte layer covering the catalyst including the first carbon support can be made as thin as possible. The optimum weight ratio (E / C) of the electrolyte and carbon varies depending on the specific surface area of the carbon support used, the pore volume, etc., but the specific surface area such as DLC or glassy carbon used here is 300 m 2 / g or less. In such a low specific surface area carrier, according to the study by the present inventors, it is preferable that the E / C of the main catalyst body is about 0.15 to 0.60.
If E / C is less than 0.15, the first electrolyte membrane becomes too thin and the PFF structure becomes unstable. On the other hand, if E / C is larger than 0.60, the film thickness (first film thickness) of the first electrolyte film becomes too thick, and oxygen permeability is lowered, and as a result, the output characteristics are greatly lowered.

Any carbon carrier can be used as the second carrier. From the viewpoint of production cost, it is preferable to use general-purpose carbon black.
The material of the second carrier is not particularly limited as long as it has conductivity and can ensure durability equal to or higher than that of carbon black.

Perfluorosulfonic acid is also used for the second polymer electrolyte. This second polymer electrolyte is mixed with water and alcohol to form a mixed solution, and this is mixed with a water retention material pre-paste (second carrier + water) to obtain a water retention material paste.
The same or the same type as the first polymer electrolyte can be employed as the second polymer electrolyte.
In addition, it is preferable to insolubilize the second polymer electrolyte so that the second polymer electrolyte on the water retention material paste side does not move to the main catalyst paste side. The insolubilization treatment can be performed by drying the water retention material paste and heating the water retention material paste at a temperature equal to or higher than the glass transition temperature of the second polymer electrolyte. For details, see Patent Document 4.
For the same purpose, the viscosity of the second polymer electrolyte can be made higher than that of the first polymer electrolyte.

The mixing of the main catalyst paste and the water retaining material paste is adjusted so that the entire E / C is within the target range.
According to the study by the present inventors, when DLC is used as a catalyst carrier, the E / C in the mixed paste of the main catalyst paste and the water retaining material paste is preferably 0.4 to 0.5. More preferably, it is 0.45. Within this range, high output characteristics can be maintained regardless of the humidification state, that is, both in the high humidification state and in the low humidification state.

Examples of the present invention will be described below.
Model number S-7 (trade name: SCM fine diamond) manufactured by Sumiishi Materials Co., Ltd. was used as DLC. Using this DLC powder as a carrier, platinum particles as catalyst metal particles were supported by a conventional method. The supported amount of platinum is 20 wt%.
The catalyst thus prepared is dispersed in water to obtain a main catalyst pre-paste.
On the other hand, a mixed solution (2-propanol: electrolyte solution = 1: 1 (weight ratio)) of Aciplex SS-900 / 10 manufactured by Asahi Kasei Chemicals and 2-propanol was prepared as a first polymer electrolyte solution. The mixing ratio of the mixed solution and the main catalyst pre-paste was adjusted so that E / C after mixing was 0.35.

On the other hand, a water retention paste was prepared as follows.
Carbon black (Vulcan-XC72R (trade name of CABOT)) as a second carrier is dispersed in an appropriate amount of water in the same manner as the main catalyst paste to obtain a water retaining material pre-paste. The second polymer electrolyte is the same as the electrolyte solution used in the main catalyst paste, and mixed with the water retention material pre-paste as a mixed solution of 2-propanol: electrolyte solution = 1: 1 (weight ratio). Thus, a water retention paste is obtained. At this time, the amount of the mixed solution is adjusted so that the E / C of the water retention material paste becomes 1.0.

  The main catalyst paste and water retention material paste thus obtained were mixed for 4 minutes with a hybrid mixer to obtain a catalyst paste (mixed paste) of the example. The blending weight ratio of the main catalyst paste and the water retention material paste is main catalyst paste: water retention material paste = 0.812: 0.188, and the overall E / C is 0.45.

The mixed paste thus obtained is applied to the gas diffusion substrate 1 made of carbon paper (FIG. 1). Reference numeral 3 represents a main catalyst paste, and reference numeral 7 represents a water retention paste.
The mixed paste is dried to obtain an air electrode and a fuel electrode.
The polymer electrolyte membrane is sandwiched between these electrodes, and these are joined by hot pressing to form a membrane electrode assembly (MEA).
FIG. 2 (full humidification (50 ° C.) and FIG. 3 (low humidification (70 ° C.-40% RH)) show IV characteristics of a fuel cell provided with such a membrane electrode assembly.
2 and 3, Comparative Examples 1 to 4 are results obtained when the catalyst layer is obtained only from the main catalyst paste using DLC as a carrier.
From the results of FIGS. 2 and 3, in this example in which a main catalyst paste using DCL as a carrier and a water retention material paste are mixed to obtain a catalyst paste, Comparative Examples 1 to 1 including only the main catalyst paste using DCL as a carrier are used. It can be seen that a higher IV characteristic can be obtained compared to 4.
Furthermore, in Comparative Examples 1 to 4, even under the same humidification conditions, the IV characteristics change according to E / C. In the case of FIG. 2 which is a low humidification condition, the highest IV characteristic is exhibited when E / C = 0.35, but when the humidification condition is high (see FIG. 3), the case where E / C = 0.35 is obtained. It does not show the highest IV characteristics. On the other hand, in the case of an Example, in E / C = 0.45, it has the IV characteristic which was excellent in both the low humidification conditions and the high humidification conditions.

Hereinafter, the PFF structure will be described (ver. 110915).
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 sulfonic acid group (SO ₃ ⁻ ) as a hydrophilic functional group is a side chain with respect to the hydrophobic main chain E1. As shown in FIG. 2, this hydrophilic functional group is oriented toward the catalyst C, so that 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 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 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 a 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, the aggregation of the catalyst particles is loosened, and more catalyst metal particles can come into contact with the treatment liquid containing a hydrophilic group. Furthermore, air is defoamed by physical treatment, that is, the air layer covering the catalyst surface is removed, so that more catalyst metal particles can come into contact with the treatment liquid containing hydrophilic groups. become. When a large force is applied as a physical treatment by a homogenizer or the like, a hydrophilic group (for example, —NO ³ ) or the like may be separated from the catalyst metal particles from the catalyst metal particles.
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. This is to prevent precipitation.

(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. 5A, 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 is slightly lowered, the electrolyte main chain E1 is opened 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 layer is formed by mixing the electrolyte solution in the state of the electrolyte (A in FIG. 5), 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. 5B 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. 5B 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, in the fuel cell reaction layer, hydrophilic ion exchange groups (sulfonic acid groups, also referred to as sulfo groups) are pre-paste. The water inside will be adsorbed. For this reason, as shown in FIG. 4, 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 area | region W is continuously formed around the catalyst C, and it is formed in the mutually connected state because a sulfonic acid group adsorb | sucks with the water in a pre paste as mentioned above. For this reason, in the reaction layer using this catalyst paste, as shown in FIG. 4, 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. 7A, 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. 7B shows the relationship between stirring time (= viscosity) and 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.
7A and 7B, 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. 5B 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 apparent from FIG. 7A, when the stirring of the mixture exceeds a certain time (4 minutes in the example of FIG. 7A), 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 (EMA) is obtained by sandwiching a solid 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. 8 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.

In the present invention, DLC and / or glassy carbon is employed as a catalyst support.
In order to maximize the catalytic activity, the electrolyte membrane covering the catalyst in the PFF structure is thinned.
On the other hand, a water retention material is prepared to adjust the water retention and water discharge functions of the catalyst layer.

  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.

C catalyst C1 carrier C2 catalyst metal particle E electrolyte layer W fresh water region

Claims (2)

A catalyst comprising a carrier comprising conductive diamond-like carbon or glassy carbon is dispersed in a first polymer electrolyte solution, and the catalyst is coated with the first polymer electrolyte layer having a first film thickness. Preparing a catalyst paste;
A second carbon carrier is dispersed in a second polymer electrolyte solution, and the second carbon carrier is coated with the second polymer electrolyte layer having a second film thickness that is greater than the first film thickness. Preparing a water retention paste comprising:
And a step of mixing the main catalyst paste and the water retention paste. A main catalyst body formed by coating a catalyst comprising a carrier made of conductive diamond-like carbon or glassy carbon with a first polymer electrolyte layer having a first film thickness;
A fuel cell catalyst layer in which a water retention material formed by coating a second carbon carrier with a second polymer electrolyte layer having a second film thickness larger than the first film thickness is mixed.

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