JP2011171120A - Manufacturing method of electrode catalyst - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a manufacturing method of electrode catalyst capable of superbly dispersing catalyst-carrying carbon or ionomers, while preventing aggregation with one another. <P>SOLUTION: The manufacturing method of the electrode catalyst includes (1) an electrode material preparing process, (2) a crushing dispersion process, and (3) a coating and drying process. In (2) the crushing dispersion process, when a rotor 14 is rotated with supply of catalyst ink, a drift velocity difference is generated among beads 18 of a stirring space 16. According to this, the shearing force, grinding stress, friction, or the like, are generated among the beads 18 so as to be able to crush and disperse the catalyst ink. Hence, in this process, adjoining beads 18 can be made to flow in a reverse direction, as shown in Fig.2(B). and shearing force can be mainly generated among the beads 18. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a method for producing an electrode catalyst, and more particularly to a method for producing an electrode catalyst for a fuel cell.

  Conventionally, for example, Patent Document 1 discloses a method for producing an electrode catalyst by stirring platinum-supported carbon, an alcohol solution of ionomer, and an organic solvent with a planetary ball mill or a homogenizer. When an electrode paste is applied to a porous gas diffusion substrate such as carbon paper, there is a problem that the electrode paste enters the gas diffusion substrate. When the electrode paste enters the gas diffusion base material, the pores of the gas diffusion base material are closed. For this reason, a gas diffusion base material cannot exhibit the function, As a result, possibility that the catalyst utilization rate in an electrode catalyst will fall is high. Therefore, in Patent Document 1, stirring is performed with a planetary ball mill or a homogenizer to form a structure in which a plurality of platinum-supporting carbons are connected in the form of primary particles, and these structures are highly entangled. Thereby, viscosity can be given to the electrode paste after stirring. Therefore, it is possible to prevent the electrode paste from entering the gas diffusion base material.

JP 2000-164224 A

  By the way, further improvement in power generation performance is required for fuel cells. For this reason, not only Patent Document 1 but various efforts to improve power generation performance have been made. However, in various production methods, very few have focused on the method of stirring the electrode catalyst material. Further, even in the method of Patent Document 1 that focuses on the stirring method, if the stirring is insufficient, the dispersion of the catalyst-supporting carbon and ionomer becomes poor, so that the catalyst layer resistance (proton transfer resistance) of the produced electrode catalyst becomes poor. ) May increase, leading to a decrease in the output of the fuel cell. In addition, if the stirring is excessive, the catalyst-carrying carbon and ionomer are aggregated, and the reaction gas diffusibility of the produced electrode catalyst is reduced, which may lead to a decrease in the output of the fuel cell as described above. It was.

  The present invention has been made in order to solve the above-described problems, and provides an electrode catalyst manufacturing method capable of satisfactorily dispersing the catalyst-supporting carbon and ionomer while preventing aggregation thereof. For the purpose.

In order to achieve the above object, a first invention is a method for producing an electrode catalyst,
A step of preparing an electrode catalyst material comprising a catalyst-supporting carbon and an ionomer as a solid polymer electrolyte component;
Prepared and prepared a milling device comprising a cylindrical bezel that houses a cylindrical rotor, and a milling tank having a width of 5 mm to 9 mm formed between the outer peripheral wall of the rotor and the inner peripheral wall of the bezel Charging the electrode catalyst material and beads having an average particle size of 0.1 mm or more and 0.3 mm or less into the milling tank;
Crushing and dispersing the charged electrocatalyst material by rotating the rotor at a peripheral speed of 1.5 m / s or more and less than 5.0 m / s to flow the beads;
It is characterized by providing.

  According to the first invention, in the milling tank having a width of 5 mm or more and 9 mm or less, the electrode catalyst material and beads having an average particle diameter of 0.1 mm or more and 0.3 mm or less are fed at a peripheral speed of 1.5 m / s or more and 5. By rotating the rotor at less than 0 m / s to cause the beads to flow, the charged electrode catalyst material can be crushed and dispersed. Thereby, a shearing force necessary for satisfactorily crushing and dispersing the catalyst-supporting carbon and ionomer in the electrode catalyst material can be applied. Therefore, these can be favorably dispersed while preventing aggregation of the catalyst-supporting carbon and ionomer.

It is process drawing of the manufacturing method of embodiment. It is a figure for demonstrating the rotor type bead mill used for a crushing dispersion | distribution process. It is a figure for demonstrating a pin type bead mill. It is a figure for demonstrating the ink state before and behind the crushing dispersion | distribution process using various dispersion apparatuses. It is a graph which shows the result of a particle size distribution measurement. It is a graph which shows the result of catalyst layer resistance. It is a figure which shows the result of an IV characteristic test.

  Hereinafter, the manufacturing method of the present embodiment will be disclosed with reference to FIGS. FIG. 1 is a process diagram of the manufacturing method according to the embodiment. As shown in FIG. 1, the manufacturing method of this embodiment includes (1) an electrode material preparation step, (2) a crushing and dispersing step, and (3) a coating and drying step. Details of these steps (1) to (3) will be described.

(1) Electrocatalyst material preparation step This step is a step of preparing an electrode catalyst material containing catalyst-supporting carbon and an ionomer as a solid polymer electrolyte component.

  In this step, first, a catalyst is supported on carbon. Specifically, carbon is dispersed in a metal compound solution serving as a catalyst and supported by an impregnation method, a coprecipitation method, or an ion exchange method. Here, carbon black is most commonly used as carbon. In addition, graphite, carbon fiber, activated carbon and the like, pulverized products thereof, carbon compounds such as carbon nanofiber and carbon nanotube, and the like can be used. Moreover, these two or more types can be mixed and used. Examples of the catalyst include platinum, ruthenium, iridium, rhodium, palladium, osnium, tungsten, lead, iron, chromium, cobalt, nickel, manganese, vanadium, molybdenum, gallium, aluminum, and the like, or alloys thereof. It is done. A composite of these two or more types can also be used.

  For example, when carbon black is used as carbon and platinum is used as a catalyst, a suitable amount of tetraammine platinum salt solution, dinitrodiammine platinum solution, platinum nitrate solution, or chloroplatinic acid solution is dissolved in an alcohol such as ethanol or isopropanol. Carbon black is dispersed in the platinum salt solution. Thereafter, this platinum salt solution is heated to 200 ° C. or higher in a hydrogen atmosphere to reduce platinum. Thereby, platinum particles can be carried on carbon black.

  Subsequently, the obtained catalyst-supported carbon is dispersed in an appropriate organic solvent such as water and alcohol. Further, an ionomer alcohol solution is added. Thereby, an electrode catalyst material (hereinafter also referred to as “catalyst ink”) can be prepared. Here, the ionomer functions as a proton conducting site in the electrode catalyst, and is generally composed of a hydrocarbon-based polymer electrolyte in which a phosphate group, a sulfonate group or the like is introduced into the side chain. Examples of the ionomer alcohol solution include a Nafion (registered trademark) solution manufactured by Aldrich.

(2) Crushing and dispersing step This step is a step (1) by using a bead mill having a rotor and rotating the rotor at a peripheral speed of 1.5 m / s or more and less than 5.0 m / s to flow the beads. In this step, the prepared catalyst ink is crushed and dispersed.

  This process will be described in detail with reference to FIGS. FIG. 2 is an explanatory diagram of a rotor type bead mill, and FIG. 3 is an explanatory diagram of a pin type bead mill. FIG. 4 is an explanatory diagram of the state of the catalyst ink before and after the crushing and dispersing step using various dispersing devices.

  As shown in FIG. 2A, a rotor type bead mill 10 is used in this step. The bead mill 10 is of a type including a cylindrical bezel 12 whose center line is oriented in the vertical direction. A cylindrical rotor 14 is housed inside the bezel 12. As a result, a stirring space 16 for crushing and dispersing the catalyst ink is formed in an annular shape between the inner peripheral wall of the bezel 12 and the outer peripheral wall of the rotor 3. Here, the width of the stirring space 16 is designed to be 5 mm or more and 9 mm or less. In the bezel 12, a supply port (not shown) for supplying catalyst ink and beads is formed on the upper peripheral wall, and a discharge port (not shown) for discharging catalyst ink and beads is formed on the side wall. Therefore, the catalyst ink is pumped from the supply port into the bezel 12 by a pump or the like, flows through the stirring space 16 and is discharged from the discharge port.

  10 operations of the bead mill will be described with reference to FIGS. When the rotor 14 is rotated while supplying the catalyst ink, a difference in flow rate occurs between the beads 18 in the stirring space 16. Thereby, a shearing force, a shear stress, friction, etc. are generated between the beads 18, and the catalyst ink can be crushed and dispersed. In particular, according to this step, as shown in FIG. 2 (B), adjacent beads 18 can flow in the opposite direction. Therefore, a shearing force can be mainly generated between the beads 18. As a result, an electrode catalyst in which the catalyst-carrying carbon particles are well dispersed in the ionomer can be produced, as will be clarified in Examples described later.

  As a comparison with the rotor type bead mill, FIG. 3 shows a pin type bead mill. As shown in FIG. 3A, the pin type bead mill 20 includes a cylindrical bezel 22, and a cylindrical rotor 24 is housed in the bezel 22. A plurality of pins 25 having a central axis formed in a direction orthogonal to the central axis of the bezel 22 are provided on the outer peripheral wall of the rotor 24. For this reason, the stirring space 26 formed between the inner peripheral wall of the bezel 22 and the outer peripheral wall of the rotor 24 is generally formed wider than the stirring space 16 in FIG.

  By providing the plurality of pins 25, a stronger stirring force can be applied to the catalyst ink when the peripheral speed is the same as that of the rotor type bead mill. In particular, around the plurality of pins 25, the beads that have been pumped from above the pins 25 can be made to flow in a direction different from the pumping direction. Therefore, when a pin-type bead mill is used, adjacent beads 28 can collide with a strong force in the vertical direction as shown in FIG. Such a strong collision in the vertical direction is also possible by a disk type bead mill in which a disk is provided instead of a pin. However, the pulverization by such a strong collision has a problem that the collision energy is too strong. If the collision energy is too strong, the crushed carbon particles may reaggregate.

  In addition, when a stirring device other than a pin type bead mill is used, other problems arise. Furthermore, even when a rotor type bead mill is used, dispersion may not always be good if the conditions for stirring are different. These phenomena will be described with reference to FIG.

  As shown in FIG. 4, before the crushing and dispersing step, the catalyst-supporting carbon, the solvent, and the ionomer are simply present in the same system. That is, carbon exists separately, and ionomer exists separately as ionomer. On the other hand, after the crushing and dispersing step, the catalyst-supporting carbon particles are coated with the ionomer.

  FIG. 4A shows the state of the catalyst ink after the crushing and dispersing step using an ultrasonic homogenizer. As shown in FIG. 4A, when the crushing dispersion is performed using an ultrasonic homogenizer, the carbon particles can be coated with the ionomer as the dispersion progresses. However, as shown in the figure, the ionomers are separated from each other. When the ionomers are separated from each other, protons cannot be conducted to remote ionomers. In addition, as shown in the figure, the ionomers that are not coated with carbon particles may aggregate together. When the ionomers are aggregated, protons can be conducted. However, since there is no catalyst in the ionomer, an electrochemical reaction cannot occur.

  FIG. 4C shows a state of the catalyst ink after performing a crushing and dispersing process by stirring with a strong force using a roller type bead mill. As shown in FIG. 4C, even when a roller-type bead mill is used, energy is excessive when stirred with a strong force. For this reason, the carbon particles coated with the ionomer are excessively dispersed. As a result, the crushed carbon particles reaggregate. Even if the carbon particles are re-aggregated, protons can be conducted and transported. However, re-aggregation may cause a plurality of carbon particles to form secondary particles and inhibit diffusion of the reaction gas dissolved in the ionomer. Therefore, for example, when the fuel cell is operated at a high load, the output is reduced despite an increase in the supply amount of the reaction gas. Further, these re-aggregated carbon particles may be separated as individual aggregates. In this case, since no ionomer is present between the aggregates, protons cannot be conducted to the separated aggregates.

  On the other hand, as shown in FIG. 4B, in the crushing and dispersing step of the present embodiment, stirring is performed with a weak force using a roller type bead mill. As a result, the carbon particles are coated with the ionomer, and at the same time, the ionomers are stretched by shearing. Therefore, as shown in the figure, it is possible to construct a network in which ionomers are intertwined in a fibrous form.

  In order to stir with a weak force, it is necessary to set the circumferential speed of the rotor 14 low. Specifically, the circumferential speed of the rotor 14 is set to 1.5 m / s or more and less than 5.0 m / s. If the peripheral speed of the rotor 14 is set within the above range, reaggregation and poor dispersion of carbon particles can be prevented.

  In addition, in order to prevent the carbon particles and ionomers from agglomerating in the stirring space 16 and to disperse them well, fine beads 18 are used. Specifically, the particle diameter (average) of the beads 18 is 0.1 mm or more and 0.3 mm or less. If the particle diameter of the beads 18 is within the above range, energy generation due to impact of adjacent beads 18 can be suppressed, and reaggregation of carbon particles can be prevented.

(3) Application and drying step This step is a step in which the catalyst ink obtained by the above (2) crushing and dispersing step is applied on a predetermined substrate and then dried. Examples of the predetermined substrate include polyethylene, polyethylene terephthalate, and polytetrafluoroethylene. A catalyst ink is applied onto such a substrate by, for example, gravure printing, spraying, applicator method, screen printing, doctor blade method, or ink jet method. Subsequently, the solvent in the catalyst ink is removed by drying. The drying temperature and drying time can be appropriately changed according to the boiling point of the solvent, the thickness of the catalyst ink, and the like. Thereby, an electrode catalyst is formed on the substrate.
This step may be a step of applying the catalyst ink on the electrolyte membrane instead of on a predetermined substrate, and then drying and removing the solvent in the catalyst ink.

  Hereinafter, the present invention will be specifically described based on examples. In addition, this invention is not limited only to these Examples.

<Preparation of sample>
First, as a catalyst ink sample, 3.5 g of Pt-supported carbon in which 30 wt% of Pt was supported on carbon black, 44 g of water and 20 g of ethanol as solvents were weighed. The ionomer was weighed so that I / C (ionomer / carbon weight in Pt-supported carbon) = 0.75. A weighed Pt-supported carbon and water were mixed, and ethanol and ionomer were further added and stirred to prepare a sample.

[Example 1]
First, a rotor type bead mill in which the width of the stirring space was adjusted to about 7 mm was prepared. Next, the prepared sample was put into the bead mill. A catalyst ink was prepared by dispersing at a bead diameter of 0.1 mm and a rotor peripheral speed of 2.5 m / s.
Then, using the applicator, the produced catalyst ink was coated on the PTFE sheet. After drying, the catalyst ink surface was thermocompression bonded to the electrolyte membrane surface, and then the PTFE sheet was peeled to form MEA.

[Comparative Example 1]
Using a homogenizer, the prepared sample was dispersed to prepare a catalyst ink. After production, it was converted to MEA in the same manner as in Example 1.

[Comparative Example 2]
Using the bead mill used in Example 1, the prepared sample was dispersed at a bead diameter of 0.3 mm and a rotor peripheral speed of 10 m / s to prepare a catalyst ink. After production, it was converted to MEA in the same manner as in Example 1.

<Evaluation method>
Evaluation was performed by (i) measurement of particle size distribution in each catalyst ink after dispersion, (ii) measurement of catalyst layer resistance in each MEA, and (iii) observation of IV characteristics in each MEA.
Note that (i) was evaluated by calculating a value (D50) of 50% of the particle size distribution integrated fraction using a laser diffraction particle size distribution measuring device. (ii) was evaluated by a value measured using an AC impedance measuring device (FRA manufactured by NF Circuit Design Block). Specifically, the following experimental conditions; temperature = 80 ° C., dew point An / Ca = 40 ° C./40° C., Bias = 0.5 V, An gas (flow rate) / Ca gas (flow rate) = H ₂ (0.5 L / Min) / N ₂ (0.5 L / min), Cole-Cole plot is performed with a flow amplitude of ± 0.05 V and a frequency region of 0.1 Hz to 10 kHz. An equivalent circuit is assembled with the Z plot of Scribner's software, and fitting is performed. The catalyst layer resistance thus determined was evaluated. The characteristic diagram of (iii) was created by measuring each IV characteristic under low humidification conditions.

  The results of Example 1 and Comparative Examples 1 and 2 are shown in FIGS. FIG. 5 is a diagram showing the D50 result for each catalyst ink after dispersion. FIG. 6 is a diagram showing the results of diffusion layer resistance in each MEA. FIG. 7 is a diagram illustrating IV characteristics in each MEA.

  FIG. 5 shows that the D50 value decreases in the order of the catalyst inks of Comparative Example 2, Example 1, and Comparative Example 1. The smaller the D50 value, the finer the particle size. The reason why the particle size of the catalyst ink of Comparative Example 1 became fine is that the network of ionomers has not been constructed. The reason why the particle size of the catalyst ink of Comparative Example 2 became coarse is that ionomers and platinum-supported carbon particles reaggregated as a result of increasing the peripheral speed of the rotor.

  From FIG. 6, it can be seen that the catalyst layer resistance of Comparative Example 1 is extremely higher than the catalyst layer resistances of Example 1 and Comparative Example 2. The reason why the catalyst layer resistance is high is that a network of ionomers is not constructed. On the other hand, it can be seen that the catalyst layer resistance of Example 1 and Comparative Example 2 is lower than that of Comparative Example 1.

  From the result of the catalyst layer resistance shown in FIG. 6, since the MEA of Comparative Example 2 has a low catalyst layer resistance like the MEA of Example 1, it was considered that the power generation performance was good for both. However, as shown in FIG. 7, at the same current density, the voltage value of the MEA of Example 1 was higher than the voltage value of Comparative Example 2. This was thought to be due to the fact that the ionomer and platinum-supported carbon particles were re-agglomerated as a result of increasing the peripheral speed of the rotor, so that the diffusibility of oxygen in the ionomer decreased.

[Example 2]
Using the bead mill used in Example 1, the prepared sample was charged together with beads having an average particle diameter of 0.3 mm. A catalyst ink was prepared by dispersing at a rotor peripheral speed of 2.5 m / s. After production, it was converted to MEA in the same manner as in Example 1.

[Comparative Example 3]
Using the bead mill used in Example 1, the prepared sample was charged together with beads having an average particle diameter of 0.3 mm. A catalyst ink was prepared by dispersing at a rotor peripheral speed of 5.0 m / s. After production, it was converted to MEA in the same manner as in Example 1.

[Comparative Example 4]
Using the bead mill used in Example 1, the prepared sample was charged together with beads having an average particle diameter of 0.5 mm. A catalyst ink was prepared by dispersing at a rotor peripheral speed of 2.5 m / s. After production, it was converted to MEA in the same manner as in Example 1.

<Evaluation method>
Evaluation was performed about the produced Examples 1 and 2 and Comparative Examples 2 to 4. In addition to the above (i) and (ii), the evaluation was based on (iv) current density at 0.4V. (iv) is obtained by creating an IV characteristic curve for each MEA. The results are shown in the table below.

As can be seen from the table, when the bead diameter is 0.3 mm (Example 2), an MEA having a lower catalyst layer resistance than Example 1 is obtained, and an MEA having a good current density at 0.4 V is also obtained. Obtained. On the other hand, when the bead diameter was 0.5 mm (Comparative Example 4), the current density at 0.4 V was low, indicating that the power generation performance was reduced.
When the rotor peripheral speed is 5.0 m / s (Comparative Example 3), an MEA with a low catalyst layer resistance is obtained as in Comparative Example 2, but the current density at 0.4 V is low, and the power generation performance As a result, it has been shown that the decrease.

10 Bead mill 12 Bezel 14 Rotor 16 Stirring space 18 Beads

Claims (1)

A step of preparing an electrode catalyst material comprising a catalyst-supporting carbon and an ionomer as a solid polymer electrolyte component;
Prepared and prepared a milling device comprising a cylindrical bezel that houses a cylindrical rotor, and a milling tank having a width of 5 mm to 9 mm formed between the outer peripheral wall of the rotor and the inner peripheral wall of the bezel Charging the electrode catalyst material and beads having an average particle size of 0.1 mm or more and 0.3 mm or less into the milling tank;
Crushing and dispersing the charged electrocatalyst material by rotating the rotor at a peripheral speed of 1.5 m / s or more and less than 5.0 m / s to flow the beads;
A method for producing an electrode catalyst, comprising:

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