JP2013071053A - Method for manufacturing catalyst, and catalyst - Google Patents

Abstract

PROBLEM TO BE SOLVED: To efficiently utilize catalyst metal particles of a catalyst for fuel cell reaction.SOLUTION: Water is confined inside an electrolyte layer in a PFF structure forming a hydrophilic area between the surface of the catalyst and the electrolyte. By previously giving an acidic functional group to the surface of a carrier, the acidic functional group is always in contact with the water and protons are supplied from the acidic functional group into the water. Thus, even in an environment that the electrolyte cannot go around and into micropores or the like of the catalyst, protons are supplied from the acidic functional group to the catalyst metal particles present on the peripheral surfaces of the micropores and the catalyst metal particles contribute to fuel cell reaction.

Description

  The present invention relates to a method for producing a catalyst for a reaction layer of a fuel cell having a PFF structure, and a catalyst obtained by this production method.

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. Here, the catalyst is obtained by dispersing catalyst metal particles such as platinum on the surface of a conductive carrier such as carbon.
PFF
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 cause the fuel cell reaction, it is necessary to supply electrons, protons and oxygen to the catalyst metal particles of the catalyst. However, in the catalyst carrier made of porous carbon, etc. Since the electrolyte cannot go around, protons are not supplied into the micropores. Therefore, the catalytic metal particles supported on the inner peripheral surface of the micropore cannot contribute to the fuel cell reaction.
Similarly, when the catalyst constitutes multi-particulate particles, the electrolyte cannot wrap around the minute gaps formed between the catalyst particles, so the catalyst metal particles appearing in the gaps contribute to the fuel cell reaction. I don't get it.
Accordingly, an object of the present invention is to make it possible to efficiently utilize catalytic metal particles of a catalyst by a fuel cell reaction.

As a result of extensive studies to achieve the above object, in the PFF structure in which a hydrophilic region is formed between the surface of the catalyst and the electrolyte, water is confined inside the electrolyte layer. If an acidic functional group is added, since this acidic functional group is always in contact with water, protons are added to the catalytic metal particles, and the catalytic metal particles do not contribute to the fuel cell reaction. I came up with this invention.
That is, the first aspect of the present invention is defined as follows.
A method for producing a catalyst for a reaction layer of a fuel cell having a PFF structure,
Including an acidic functional group imparting step of imparting an acidic functional group to the support of the catalyst,
A method for producing a catalyst.

  According to the method for producing a catalyst for a fuel cell of the first aspect thus defined, an acidic functional group is imparted to the catalyst support. Thereby, the surface exposed to the inner peripheral surface of the micropores of the carrier and the gap between the catalyst particles is also modified with the acidic functional group. In the PFF structure, the inside of the electrolyte membrane covering the catalyst is filled with generated water, so that the micropores of the carrier and the gaps between the catalyst particles are also filled with water. Therefore, even if the electrolyte does not flow into the micropores or gaps, acidic functional group protons are released into water, so the catalytic metal particles that appear on the micropore inner peripheral surfaces and gaps contribute to the fuel cell reaction. become able to.

The second aspect of the present invention is defined as follows. That is,
In the method for producing a catalyst defined in the first aspect, the acidic functional group imparting step includes a first imparting step for imparting a weakly acidic acidic functional group to the carrier, and a strong acidity following the first imparting step. A second imparting step of imparting an acidic functional group of
By adding an acidic functional group in two steps in this way, an acidic functional group that can supply sufficient protons without giving large stress to the catalyst support, in other words, while maintaining the characteristics of the support is provided. It becomes possible.
In addition, by bringing the catalyst into contact with hydrogen peroxide, the surface of the carrier (including the inner peripheral surface of the micropore) is modified with a hydroxyl group, a carboxyl group, or a carbonyl group as a weakly acidic acidic functional group. .
On the other hand, by bringing the catalyst into contact with an aqueous solution of sulfuric acid, nitric acid, phosphoric acid, or a mixed acid thereof, the surface of the catalyst (including the inner peripheral surface of the micropores) is a sulfonic acid group as a strongly acidic acidic functional group, It will be coated with nitro groups and phosphate groups.

FIG. 1 is a schematic diagram illustrating a PFF structure corresponding to FIG. FIG. 2 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. 3 is a schematic diagram for explaining the PFF structure corresponding to FIG. FIG. 4A shows the relationship between the stirring time and the viscosity of the pre-paste and the electrolyte solution, and FIG. 4B shows the relationship between the stirring time and the reaction layer resistance. FIG. 5 shows a schematic diagram of an apparatus for producing a catalyst paste. It is a graph which shows the IV characteristic of the fuel cell using the catalyst of the Example of this invention.

First, 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 on the side of the hydrophobic main chain E1. As shown in FIG. 1, 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 nitro group, an amino group, a sulfonic acid group, a phosphoric acid group, a hydroxyl group, and a halogen group. More preferred examples include at least one selected from a nitro group and a sulfonic acid group as the 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.), Dinitrodiamine platinum (II) aqueous solution (cis- [Pt (NH ₃ ) ₂ (NO ₂ ) ₂ ] / H ₂ O sol.), Dinitrodiamine platinum (II) nitric acid solution (cis- [Pt (NH ₃ ) ₂ (NO ₂ ) ₂ ] / HNO ₃ sol.), Dinitrodiamine platinum (II) sulfuric acid solution (cis- [ Pt (NH ₃ ) ₂ (NO ₂ ) ₂ ] / H ₂ SO ₄ sol.), Aqueous potassium tetrachloroplatinate (II) (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 nitro group as a hydrophilic group for modifying catalytic metal particles made of platinum or a platinum alloy. As the platinum complex solution for that purpose, dinitrodiamine platinum (II) nitric acid solution (cis- [Pt (NH ₃ ) ₂ (NO ₂ ) ₂ ] / HNO ₃ sol.) Having NO ₃ ^- as a hydrophilic ion, hexahydroxo platinum (IV) acid nitric acid solution _{((H 2 Pt (OH)} 6) / HNO 3 sol.), hexahydroxoplatinum to SO ₄ ^{2-hydrophilic} ion (IV) acid sulfate solution ((H ₂ Pt (OH ₆ ) / H ₂ SO ₄ sol.), Tetraammineplatinum (II) hydroxide aqueous solution ([Pt (NH ₃ ) ₄ (OH) ₂ ] / H ₂ O sln.) Having NH ₄ ⁺ as hydrophilic ions Etc. can be adopted.

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 nitro 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.
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, nitrate ions (NO ₃ ⁻ ) adsorbed thereon are preferably reduced to nitro groups (—NO ₂ ). Although the reduction method is not particularly limited, a catalyst having catalytic platinum particles adsorbed with nitrate ions 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.
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.

(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. 2A, water is adsorbed on the side chain E2 of the electrolyte, and the electrolyte main chain E1 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 main chain E1 of the electrolyte 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 a reaction layer is formed by mixing an electrolyte solution in the state of the electrolyte (A in FIG. 2), 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. 2B 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 of (B) in FIG. 2 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, 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. 1, in this reaction layer, the hydrophilic group E2 of the electrolyte 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. 1, 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. 4A, 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. 4B 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.
4 (A) and 4 (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 an open state as shown in FIG. 2B is oriented with its hydrophilic group facing the catalyst to construct a PFF structure. However, if stirring is performed after the PFF structure is constructed (hereinafter sometimes referred to as “over-stirring”), the electrolyte facing the catalyst is separated from the catalyst, and then water on the catalyst surface is taken away from the catalyst surface. break away. Since the electrolyte separated from the catalyst surface is accompanied by water on the catalyst surface, the electrolyte easily takes the form of FIG. Therefore, it is considered that the viscosity of the electrolyte solution component in the catalyst paste decreases, which causes a decrease in the viscosity of the catalyst paste itself. Also, the PFF structure becomes brittle when the electrolyte is detached from the catalyst surface, and the function of the hydrophilic region formed between the catalyst and the electrolyte is lowered. This is expected to increase the reaction layer resistance.
Therefore, the viscosity of the mixture of the pre-paste and the electrolyte solution is adjusted to a predetermined viscosity. Thereby, both over-stirring can be prevented. That is, the over-stirred mixture lowers its viscosity as described above, so that it is possible to prevent over-stirring of the mixture by stopping stirring when the viscosity of the mixture exhibits a predetermined behavior. By preventing over-stirring, a stable PFF structure can always be constructed.

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

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

In the above, the manufacturing method and the material used for manufacture of a catalyst paste have been demonstrated exclusively.
FIG. 5 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, the catalyst treatment, particularly chemical treatment, is improved in the production process of the PFF.
That is, an acidic functional group is imparted to the surface of the catalyst support.
Here, as the acidic functional group, one or more of a hydroxyl group, a carboxyl group, a carbonyl group, a sulfonic acid group, a nitro group, and a phosphoric acid group can be used.
These acidic functional groups are imparted to the entire surface of the catalyst support. As a result, acidic functional groups are also imparted to the micropore collection surface.
The method for imparting the acidic functional group to the catalyst can be arbitrarily selected according to the characteristics of the acidic functional group dissolved in the solvent and the carrier, but is basically performed by bringing both into contact.
When the acidic functional group is imparted, it is preferable to employ a carrier on which the catalyst metal particles are not supported. Of course, even if the acidic functional group is brought into contact with the carrier carrying the catalyst metal particles (that is, the catalyst). Good.
In the PFF structure, the inside of the electrolyte membrane covering the catalyst is filled with generated water, so that the micropores of the carrier and the gaps between the catalyst particles are also filled with water. Therefore, if there is an acidic functional group on the surface of the catalyst carrier constituting the micropores or gaps between the catalyst particles, protons are supplied from the acidic functional group to water. Therefore, even if the electrolyte does not sufficiently penetrate into the micropores and the gaps between the catalyst particles, this proton can contribute to the fuel cell reaction on the catalyst metal particles.
Thereby, the utilization efficiency of catalytic metal particles made of an expensive noble metal such as platinum is improved, and as a result, the efficiency of the fuel cell reaction can be improved.

Examples of the present invention will be described below.
(Creation of solid acid carrier)
Add 200mL of 30% hydrogen peroxide to 1g of carrier carbon and stir at room temperature for 48h.
As a result, a hydroxyl group, a carboxyl group, and a carbonyl group are formed on the surface of the carrier carbon and become hydrophilic.
In addition to the hydrophilization treatment using an oxidizing agent such as hydrogen peroxide, heat treatment may be performed in O ₂ 1% N ₂ gas at 120 to 180 ° C. for 2 hours.
Then, it is filtered and vacuum-dried at 60 ° C. for 3 hours. It is preferable to add a more acidic functional group as a pretreatment.

That is, the carbon support obtained by the above pretreatment is stirred in 200 mL of 1.0 mol / L nitric acid for 24 hours at room temperature.
By attaching a functional group in the pretreatment, it is possible to attach a more acidic acidic functional group under milder conditions, that is, without damaging the support.
Not only nitric acid but also 0.5 mol / L sulfuric acid, 0.3 mol / L phosphoric acid, or nitric acid or a mixture of phosphoric acid and sulfuric acid can be used.
Then, it is filtered and vacuum dried at 60 ° C. for 3 hours. Furthermore, heat treatment is performed in N2 flow at 150 ° C. for 2 hours to fix the functional group on the carbon. Furthermore, excess acid is removed by hydrothermal treatment at 200 ° C for 2 hours.

A method for supporting the catalyst metal particles on the solid acid support thus obtained will be described below.
(Supporting catalyst metal particles)
(1) To 1 g of hexahydroxo Pt acid, add 8 mL of nitric acid to 1 L of water and warm to 60 ° C.
(2) Add 12 g of NaHSO ₄ and 2 L of water to (1) and warm to 60 ° C.
(3) Adjust pH to 5 with 0.6M Na ₂ CO ₃ in (2).
(4) Add 75 mL of hydrogen peroxide 35% to (3), adjust to pH 5 with 5% NaOH, and maintain 40 ° C.
(5) Disperse 0.5 g of solid acid carrier in 200 mL of water, add to (4), and stir with an ultrasonic homogenizer for 30 min.
(6) Keep (5) at 50 ° C. and stir with a 24-hour stirrer.
(7) Filter (6), wash with water, and dry at 60 ° C. for 15 h.
(8) The catalyst is reduced by heat-treating (7) in a hydrogen atmosphere at 200 ° C. for 3 hours.

A catalyst paste is produced by the following method using the catalyst thus obtained.
(A) Add 6g of water to 0.5g of catalyst (Pt: 50wt%), and centrifuge for 4min with a hybrid mixer.
(B) Add 90g of water and treat with an ultrasonic homogenizer for 10 min.
(C) Let stand for 1 hour or more to precipitate the catalyst, and remove the supernatant.
(D) The supernatant is removed so that the catalyst: water ratio of the pre-paste is 1:12.
(E) The solvent of the polymer electrolyte solution (DE2020) is removed by heating, and the polymer is dissolved again in the IPA: TBA = 1: 1 solution to form a 5% solution.
(F) To the catalyst + water paste of (D), 5 g of (E) polymer electrolyte solution is added and mixed and stirred with a hybrid mixer for 4 minutes.

The catalyst paste thus obtained is applied to carbon paper as a diffusion layer by screen printing, and further dried with hot air to obtain an electrode. This electrode was thermocompression bonded to the polymer electrolyte membrane at 140 ° C. and 40 kg / cm ^{2 to} obtain a fuel cell of the example.
FIG. 6 shows the IV characteristics of the fuel cell when both the anode and the cathode are under humidified conditions of 80 ° C. and 40% RH. In FIG. 6, the comparative example is the result when no acidic functional group is imparted to the carrier (when the production of the solid acid carrier is omitted).
From the results of FIG. 6, it can be seen that the IV characteristics of the examples are high under low humidification conditions.

Electrochemical surface area (ECSA) was measured by cyclic voltammetry at 80 ° C 40% RH low humidity and 80 ° C 100% RH high humidity for both anode and cathode.
The ratio of ECSA at the time of low humidification and ECSA at the time of high humidification (low humidification ECSA / high humidification ECSA) is respectively Example: 0.64
Comparative example: 0.54.
Thus, the values of the low humidified ECSA / highly humidified ECSA are higher in the Examples, and it is understood that the wetness in the vicinity of the catalyst is maintained even at the low humidity.

  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 (5)

A method for producing a catalyst for a reaction layer of a fuel cell having a PFF structure,
Including an acidic functional group imparting step of imparting an acidic functional group to the support of the catalyst,
A method for producing a catalyst.   The acidic functional group imparting step includes a first imparting step for imparting a weakly acidic acidic functional group to the carrier, and a second imparting step for imparting a strongly acidic acidic functional group following the first imparting step. The method for producing a catalyst according to claim 1, comprising: The first step is performed by contacting the carrier with hydrogen peroxide solution,
The method for producing a catalyst according to claim 2, wherein the second step is performed by bringing the carrier into contact with an aqueous nitric acid solution, an aqueous sulfuric acid solution, or a mixed aqueous solution thereof.   4. Production of a catalyst characterized by applying the acidic functional group imparting step according to any one of claims 1 to 3 to a support not supporting catalyst metal particles, and then supporting the catalyst metal particles on the support. Method. A catalyst for a reaction layer of a fuel cell having a PFF structure, comprising a porous carrier and catalytic metal particles supported on the carrier,
A catalyst characterized in that the inner peripheral surface of the pores of the carrier is modified with an acidic functional group.

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