JP2011028978A - Electrode catalyst layer of fuel cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for increasing an amount of reaction gas supplied to a catalyst metal by forming a gap inside an electrode catalyst layer included in a membrane-electrode assembly, preferably in the vicinity of the catalyst metal and improving power generation performance of the membrane-electrode assembly. <P>SOLUTION: A manufacturing method for a membrane-electrode assembly having a polymer electrolyte membrane, and the electrode catalyst layer arranged on both sides of the polymer electrolyte membrane and internally forming a plurality of gaps includes an electrode catalyst ink production process of producing electrode catalyst ink by mixing catalyst powder including the catalyst metal carried with a carbon carrier and ionomer with particles of a polymer organic compound; an electrode catalyst layer formation process of forming the electrode catalyst layer by applying the electrode catalyst ink to the surface of the polymer electrolyte membrane; and a gap formation process of removing particles of the polymer organic compound by making the membrane-electrode assembly immersed in an organic solvent and forming a plurality of gaps. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to an electrode catalyst layer of a fuel cell that forms an air gap in the vicinity of a catalyst metal, thereby increasing the amount of oxygen supplied to the catalyst metal and improving power generation performance.

  Since a fuel cell obtains electric power by electrochemically reacting hydrogen and oxygen, the product produced by power generation is in principle only water. Therefore, it is attracting attention as a clean power generation system that has little impact on the global environment.

  Fuel cells are classified into solid polymer type (PEFC), phosphoric acid type (PAFC), molten carbonate type (MCFC), and solid oxide type (SOFC) depending on the type of electrolyte. Among these, the polymer electrolyte fuel cell uses a proton conductive ion-exchange solid polymer electrolyte membrane as an electrolyte. In general, membrane electrode bonding is performed by bonding the electrode catalyst layer to a solid polymer electrolyte membrane to form a catalyst coated membrane (CCM), and then bonding a gas diffusion layer to the surface of the electrode catalyst layer. The body (membrane electrode assembly; MEA) is made. A polymer electrolyte fuel cell is produced using this membrane electrode assembly as a basic unit.

A fuel cell obtains an electromotive force by supplying a fuel gas containing hydrogen to one electrode (fuel electrode: anode) side and an oxidizing gas containing oxygen to the other electrode (air electrode: cathode) side. Here, the oxidation reaction shown in the following formula (1) proceeds on the anode side, and the reduction reaction shown in the following formula (2) progresses on the cathode side, and the reaction shown in the formula (3) progresses as a whole, and the external circuit To supply electromotive force.
_{^{H 2 → 2H + + 2e -}} (1)
^{_{(1/2) O 2 + 2H +}} + 2e - → H 2 O (2)
H ₂ + (1/2) O ₂ → H ₂ O (3)

  The above reaction does not proceed uniformly in the entire electrode catalyst layer, but in a so-called three-phase interface region where a catalytic metal as a solid phase, a polymer electrolyte membrane as a liquid phase, and a reactive gas as a gas phase are in contact. Only progress. Since the catalyst metal is not in contact with the polymer electrolyte membrane or the reaction gas supply to the catalyst metal is insufficient, the catalyst metal present at a site that does not form a three-phase interface cannot participate in the above redox reaction. In a fuel cell containing a large amount of such catalytic metals, the actual power generation performance is significantly lower than the power generation performance expected from the amount of catalyst metal used. Therefore, in order to manufacture a high-performance fuel cell that can utilize the activity of the catalytic metal contained in the electrode catalyst layer without waste, it is important to increase the three-phase interface region of the electrode catalyst layer.

  In the case of a polymer electrolyte fuel cell, an electrode catalyst layer is formed by coating an electrode catalyst with an ion exchange resin (polymer electrolyte; ionomer) of the same type or different from that of the polymer electrolyte membrane, so that the polymer electrolyte membrane directly Catalytic metals present at the non-contacting sites can also be linked to the polymer electrolyte membrane via the ionomer. In contrast, since the reaction gas is supplied from the gas diffusion layer to the electrode catalyst layer, when the surface of the electrode catalyst is completely covered with the ionomer, the reaction gas is supplied to the catalyst metal existing inside the electrode catalyst layer. Therefore, there is an aspect that the reaction gas supply amount is reduced. Therefore, technological development aimed at increasing the three-phase interface region by securing the reaction gas supply path to the catalyst metal while enhancing the connection of the solid-liquid two-phase region using ionomer It was. For example, Patent Document 1 includes carbon particles having at least two distribution centers of particle diameter, and the electrode body is formed of carbon particles having a distribution center having a large particle diameter among the carbon particles. An electrode is described in which a catalyst is supported on carbon particles having a distribution center with a small particle size. According to the above-mentioned document, by adopting such a configuration, it is possible to prevent the carbon particles in the electrode from becoming clogged and to secure a fine passage of the reaction gas. Also, in Cited Document 1, an ink is prepared by mixing a pore-forming agent composed of zinc particles having an average particle size of 200 nm together with the carbon particles, and the pore-forming agent is eluted with an acid after being applied to a polymer electrolyte membrane. A method is also described.

  When forming a gap in the electrode catalyst layer to secure a reaction gas supply path, not only the space occupied volume of the gap but also the space shape and position of the gap are important factors. That is, when the space occupied volume of the void is large, or when the spatial position is in the vicinity of the joint surface with the polymer electrolyte membrane or the gas diffusion layer, the strength of the electrode catalyst layer decreases, and peeling from the joint surface occurs. Is more likely to occur. Therefore, a technique for controlling the space shape and the space position of the air gap has been developed. For example, Patent Document 2 has a plurality of pores having a constant diameter over at least twice as long as a diameter as an electrode having improved gas permeability by having elongated pores. An electrode of a fuel cell is described. The above document describes a method in which short fibers having a substantially constant diameter formed of water-soluble organic polymer are mixed with carbon to prepare an ink, and after applying to a polymer electrolyte membrane, the short fibers are eluted with warm water. It is described. Patent Document 3 discloses that a pore-forming agent is oriented by a magnetic field by applying a catalyst ink containing a pore-forming agent having magnetism in a magnetic field having a magnetic flux density distribution or by evaporating a solvent. A method of forming pores with continuously inclined orientations is described.

Japanese Patent Laid-Open No. 10-241703 Japanese Patent Laid-Open No. 8-180879 JP 2008-78075 A

  As described above, the formation of voids in the electrode catalyst layer secures a reaction gas supply path and increases the three-phase interface region in the electrode catalyst layer. This is important in order to draw out the activity as much as possible and to fully demonstrate the power generation performance of the fuel cell. In particular, the space shape and space position of the gap are not only elements that cannot be ignored in maintaining the strength of the electrode catalyst layer and preventing separation from the joint surface, but are also important elements for improving power generation performance. . That is, if the voids are formed in the vicinity of the catalyst metal, it is considered that the amount of reaction gas supplied to the catalyst metal can be increased more effectively.

  Therefore, when the improvement of power generation performance by void formation is intended, it is considered effective to localize the material for void formation in the vicinity of the catalyst metal in the step of forming the electrode catalyst layer. However, it is difficult to control the position where the void is formed in the vicinity of the catalyst metal in the prior art including the above-mentioned patent documents. Further, as in Patent Documents 1 and 3, when metal particles are used to form voids, dissolved metal ions remain in the electrode catalyst layer and bind to the ion exchange groups of the polymer electrolyte membrane, so that the polymer electrolyte There are also problems that degrade the performance of the membrane. Therefore, according to the present invention, by using a material that does not generate metal ions in the void formation step, a void is formed inside the electrode catalyst layer, preferably in the vicinity of the catalyst metal, thereby increasing the amount of reaction gas supplied to the catalyst metal. An object of the present invention is to provide a method for improving the power generation performance of a membrane electrode assembly.

  As a result of various studies on means for solving the above problems, the present inventor joined an electrode catalyst layer containing particles of a polymer organic compound to a polymer electrolyte membrane, and then removed the particles using an organic solvent. Thus, it was found that the surface area of the ionomer in the vicinity of the catalyst metal can be increased, and as a result, the power generation performance of the fuel cell can be improved, and the present invention has been completed.

That is, the gist of the present invention is as follows.
(1) A method for producing a membrane electrode assembly comprising a polymer electrolyte membrane and an electrode catalyst layer disposed on both sides of the polymer electrolyte membrane and having a plurality of voids formed therein,
An electrode catalyst ink preparation step of preparing an electrode catalyst ink by mixing a catalyst powder containing a catalyst metal supported on a carbon carrier and an ionomer with particles of a polymer organic compound;
Electrocatalyst layer formation step of forming an electrode catalyst layer by applying an electrode catalyst ink to the surface of the polymer electrolyte membrane:
A void forming step of immersing the membrane electrode assembly in an organic solvent to remove particles of the polymer organic compound and forming a plurality of voids;
The method comprising the steps of:
(2) The method according to (1) above, wherein the particle size ratio of the polymer organic compound particles to the carbon support is 1: 2 to 2: 1.
(3) The method of (1) or (2) above, wherein the polymer organic compound particles are made of polystyrene.
(4) The method according to any one of (1) to (3) above, wherein the surface of the polymer organic compound particles is subjected to a hydrophilic treatment by introducing a sulfonic acid group.
(5) The membrane electrode assembly manufactured by the method of any one of said (1)-(4).
(6) A fuel cell comprising the membrane electrode assembly of (5).

  According to the present invention, it is possible to improve the power generation performance of the membrane electrode assembly by forming voids in the electrode catalyst layer, preferably in the vicinity of the catalyst metal, thereby increasing the amount of reaction gas supplied to the catalyst metal.

It is sectional drawing for demonstrating the whole structure of suitable embodiment of the membrane electrode assembly of this invention. It is a schematic diagram for demonstrating the outline of the electrode catalyst layer contained in the membrane electrode assembly of this invention. It is a schematic diagram for demonstrating the manufacturing method of the membrane electrode assembly of this invention. It is a figure which compares the BET surface area of the ionomer contained in each electrode catalyst layer about the membrane electrode assembly of an Example and a comparative example. In the figure, each value of Examples 1 and 2 is a value normalized by setting the value of the comparative example to 1. It is a figure which compares the oxygen diffusion resistance in the ionomer contained in each electrode catalyst layer about the membrane electrode assembly of an Example and a comparative example. In the figure, each value of Examples 1 and 2 is a value normalized by setting the value of the comparative example to 1. It is a figure which compares the output voltage in 1.0 A / cm < ² > current density about the fuel cell produced using the membrane electrode assembly of an Example and a comparative example. In the figure, each value of Examples 1 and 2 is a value normalized by setting the value of the comparative example to 1.

Hereinafter, preferred embodiments of the present invention will be described in detail.
1. Membrane / electrode assembly The overall structure of the membrane / electrode assembly of the present invention is shown in FIG.
As shown in FIG. 1, the membrane electrode assembly 11 of the present invention includes a polymer electrolyte membrane 12 and electrode catalyst layers 13 disposed on both surfaces of the polymer membrane 12. A plurality of voids 17 formed therein are included. In general, in order to operate a fuel cell at a high output, it is necessary to increase the supply amount of reaction gas (oxygen and hydrogen) corresponding to the increase in the reaction amount. Factors that determine the amount of reactant gas supplied to the catalytic metal surface contained in the electrode catalyst layer include the amount of reactant gas dissolved in the ionomer and the diffusion resistance of the reactant gas in the ionomer. Here, the amount of the reaction gas dissolved in the ionomer can be increased by increasing the surface area of the ionomer exposed to the reaction gas. Therefore, the membrane electrode assembly 11 of the present invention can increase the surface area of the ionomer by forming the voids 17 in the electrode catalyst layer 13, resulting in an increase in the amount of reaction gas supplied to the catalyst metal. It becomes possible.

  In this specification, the “membrane electrode assembly” means a member including the above-described configuration, and not only the catalyst coating film having the above three-layer structure, but also gas diffusion layers on both sides of the catalyst coating film. The joined member is also included in the membrane electrode assembly.

Each member included in the membrane electrode assembly of the present invention will be described below.
1-1. Below the electrode catalyst layer, the structure of the electrode catalyst layer, will be described with reference to FIG.
The electrode catalyst layer 23 used in the membrane electrode assembly of the present invention includes a catalyst powder containing a catalyst metal 25 supported on a carbon support 24 and an ionomer 26 covering the catalyst powder. The electrode catalyst layer 23 of the present invention has a configuration in which a plurality of voids 27 formed by removing particles of the polymer organic compound are distributed in the vicinity of the catalyst metal 25, so that the ionomer 26 in the vicinity of the catalyst metal 25 is provided. It is possible to contribute to an increase in the surface area of the substrate.

  The void 27 included in the electrode catalyst layer 23 of the present invention is formed by a void formation step that will be described in detail below. The gaps 27 are preferably a plurality of spherical or substantially spherical shapes, and the gaps 27 may be independent from each other, or adjacent gaps 27 may be in communication with each other. The voids 27 may be formed in the electrode catalyst layer 23 so as to be dispersed at substantially uniform intervals, but are formed in the vicinity of the catalyst metal 25 supported on the carbon support 24. More preferably.

  The inner diameter and volume of the void 27 can be controlled by the particle size of the polymer organic compound particles used. In the membrane electrode assembly of the present invention, the ratio of the inner diameter of the gap 27 to the particle size of the carbon carrier 24 is preferably 1: 2 to 2: 1. For example, when the particle size of the carbon support 24 is 10 to 50 nm, the inner diameter of the void 27 is preferably 5 to 100 nm, and more preferably 10 to 50 nm. The volume of the gap 27 is preferably 10 to 100% by volume with respect to the total volume of the carbon support 24. By including the void 27 having the above-mentioned preferable configuration, the membrane electrode assembly of the present invention can increase the surface area of the ionomer and increase the supply amount of the reaction gas to the catalyst metal.

  The particle diameters of the polymer organic compound particles and the carbon support mean values measured by the method described in the section 2-1 below.

  The carbon support 24 used in the membrane electrode assembly of the present invention functions not only as a support for supporting the catalyst component but also as an electrode. Therefore, the catalyst component is not particularly limited as long as the catalyst component can be supported and the catalyst component itself has conductivity. Various carbon carriers commonly used for fuel cell electrodes can be used. The shape of the carbon carrier 24 is preferably spherical or substantially spherical. In this case, the carbon support 24 has at least one set of particle size distribution centers. In an embodiment having a plurality of particle size distribution centers, the term “carbon support particle size” as used herein means the largest distribution center among the plurality of distribution centers. The particle size of the carbon support 24 used in the membrane electrode assembly of the present invention is preferably 10 to 100 nm, and more preferably 10 to 50 nm. The carbon support suitable for the membrane electrode assembly of the present invention is not limited, but for example, Ketjen EC (manufactured by Ketjen Black International), acetylene black (manufactured by Ketjen Black International), Vulcan XC-72R (manufactured by Cabot) Carbon powders such as Denka Black (made by DENKA), Printex XE2 (made by Degussa) and Printex XE2-B (made by Degussa).

  The catalytic metal 25 used in the membrane electrode assembly of the present invention is supported on the carbon support 24 and used to obtain an electromotive force by catalyzing a redox reaction on the surface thereof. The catalyst metal used in the fuel cell electrode catalyst of the present invention is platinum, or platinum and cobalt, nickel, ruthenium, rhodium, vanadium, copper, molybdenum, palladium, gold, chromium, zinc, tungsten, osmium, silver, manganese A platinum alloy made of a transition metal such as zirconium, iridium, titanium, iron or niobium is preferable. Platinum is an expensive noble metal, and it is preferable that both the anode catalyst and the cathode catalyst exhibit sufficient performance with a small amount of support. In the membrane / electrode assembly of the present invention, the amount of platinum used can be reduced without impairing the catalytic activity by forming voids 27 in the electrode catalyst layer 23 to increase the three-phase interface region.

  The catalyst metal loading density is defined as the weight percentage of the catalyst metal supported relative to the total weight of the electrode catalyst. When the catalyst metal is platinum, the loading density is calculated by a calculation formula of platinum weight / (platinum weight + support weight) × 100. Further, when the catalyst metal is a platinum alloy, it is calculated by a formula of (platinum weight + transition metal weight) / (platinum weight + transition metal weight + support weight) × 100. In the membrane electrode assembly of the present invention, the catalyst metal loading density is preferably 10 to 60% by weight.

  When a platinum alloy is used as the catalyst metal, its composition is defined by the weight percent of platinum and / or transition metal relative to the total weight of the supported platinum alloy. Such a composition is calculated by a formula of platinum weight / (platinum weight + transition metal weight) × 100. In the membrane electrode assembly of the present invention, the composition of the platinum alloy is preferably 90 to 98% by weight of platinum and 2 to 10% by weight of transition metal.

  The ionomer 26 used in the membrane electrode assembly of the present invention is used to coat the catalyst powder and thereby increase the three-phase interface region in the electrode catalyst layer. Therefore, the ionomer 26 used in the present invention is preferably a material having a high affinity with both the catalyst powder and the polymer electrolyte membrane 22 and capable of uniform coating, and is the same type as the polymer electrolyte membrane 22 used. Particularly preferred are macromolecular organic compounds having a skeleton structure and an ion exchange group or analogs thereof. Suitable ionomers are macromolecular organic compounds having perfluorosulfonic acid groups such as, but not limited to, Nafion® DE2020 and DE2029 (DuPont).

1-2. Polymer Electrolyte Membrane The polymer electrolyte membrane 22 used in the membrane electrode assembly of the present invention separates the fuel electrode and the air electrode and functions as an electrolyte for proton conduction that occurs between the two electrodes. Therefore, the polymer electrolyte membrane 22 used in the present invention is preferably a membrane having high proton conductivity (conductivity) and high hydrophilicity. The material constituting the polymer electrolyte membrane 22 is not limited, but is a polymer having a strong acidic functional group such as a sulfonic acid group, a phosphoric acid group, or a phosphonic acid group, or a weak acidic functional group such as a carboxyl group. Organic compounds are preferred. By having an acidic functional group, the polymer electrolyte membrane can exhibit proton conductivity (conductivity) in a state containing water.

  In addition, since the polymer electrolyte membrane 22 used in the present invention is closely bonded to the electrode catalyst layer 23, it is preferable that the polymer electrolyte membrane 22 has high flexibility and high affinity with the electrode catalyst layer 23. The polymer electrolyte membrane 22 used in the membrane electrode assembly of the present invention is not limited, but may be a polymer electrolyte membrane such as Nafion (registered trademark) N112 and N111 (manufactured by DuPont). preferable.

2. Method for Producing Membrane / Electrode Assembly The method for producing a membrane / electrode assembly of the present invention is to form a plurality of voids by eluting and removing particles of the polymer organic compound contained in the electrode catalyst layer after forming the electrode catalyst layer. It is characterized by. This can contribute to an increase in the surface area of the ionomer in the vicinity of the catalytic metal.

  Hereinafter, each process of the manufacturing method of the membrane electrode assembly of this invention is demonstrated based on FIG. The polymer electrolyte membrane 32, the electrode catalyst layer 33, the carbon carrier 34, the catalyst metal 35, and the ionomer 36 are described above for the polymer electrolyte membrane 22, the electrode catalyst layer 23, the carbon carrier 24, the catalyst metal 25, and the ionomer 26, respectively. It shall have features.

2-1. Electrocatalyst ink production process This process comprises mixing the catalyst powder containing the catalytic metal 35 supported on the carbon carrier 34 and the ionomer 36 with the particles 38 of the polymer organic compound, so that the electrode catalyst ink is in a suitable dispersed state. It aims at producing. Here, the “preferred dispersion state” means that the catalyst powder and the polymer organic compound particles in the electrode catalyst ink are preferably agglomerated separately, preferably in the vicinity of the catalyst metal supported on the surface of the carbon support. Means a state in which particles of a polymer organic compound are arranged in a dispersed state. By producing the electrode catalyst ink in such a state, it is possible to obtain an electrode catalyst layer in which voids are formed in the vicinity of the catalyst metal in the following steps.

  The shape of the polymer organic compound particles 38 used in the method for producing a membrane / electrode assembly of the present invention is not limited, but is preferably spherical or substantially spherical. When spherical or substantially spherical beads are used as the polymer organic compound particles 38, the particle size of the polymer organic compound particles 38 should be the same as or substantially the same as the particle size of the carbon support 34 contained in the catalyst powder. The particle size ratio of the polymer organic compound particles 38 to the carbon carrier 34 is more preferably 1: 2 to 2: 1. For example, when the particle size of the carbon support 34 is 10 to 50 nm, the particle size of the polymer organic compound particles 38 is preferably 5 to 100 nm, and more preferably 10 to 50 nm. By using the polymer organic compound particles 38 with the above configuration, the surface area of the ionomer exposed to the reactive gas in the resulting membrane electrode assembly is increased, and the reactive gas supply to the three-phase interface is increased. Will be.

  The particle size of the polymer compound particles and the carbon support can be measured by a method such as dynamic light scattering, laser diffraction scattering, or transmission electron microscope (TEM) observation. The particle size of the polymer compound particles is preferably measured by a dynamic light scattering method, and the particle size of the carbon support is preferably measured by TEM observation.

  The polymer organic compound particles 38 used in the method for producing a membrane / electrode assembly of the present invention are insoluble in the solvent used in the electrode catalyst ink preparation step and easily dissolved in the organic solvent used in the void formation step. If it is melted, it is not particularly limited. Polymer organic compound particles made of various materials can be used. Suitable are particles of a macromolecular organic compound such as polystyrene. Such polymer organic compound particles may be used as they are. However, since the surface of the polymer organic compound particles generally has a hydrophobic property, sulfonic acid groups, sulfate groups, phosphate groups, phosphones are used in advance. More preferably, the surface of the particles of the polymer organic compound is hydrophilized by introducing a dissociative functional group such as an acid group or a carboxyl group. As an example, when using polymer organic compound particles made of polystyrene, the hydrophilic treatment is performed by immersing the polymer organic compound particles in a hydrophilic functional group introducing agent such as chlorosulfonic acid or fuming sulfuric acid. can do. In the above case, the hydrophilic functional group introducing agent is preferably at a concentration of 0.01 to 0.5 mol / L. The immersion treatment is preferably performed at 0 to 60 ° C. The immersion treatment is preferably performed for 24 to 72 hours.

  The content of the polymer organic compound particles 38 used in this step can be appropriately set according to the desired void volume. In order to increase the amount of the reaction gas dissolved in the ionomer in the electrode catalyst layer, the void volume is preferably large. However, when the void volume increases, undesirable effects such as a decrease in strength of the electrode catalyst layer, peeling from the polymer electrolyte membrane, and an increase in electrical resistance may occur. Therefore, the polymer organic compound particles 38 used in this step are preferably 10 to 100% by volume with respect to the total volume of the carbon support. Increasing the amount of reactive gas dissolved in the ionomer without adding undesirable effects such as peeling of the electrode catalyst layer or increase in electrical resistance by adding particles of the polymer organic compound within the above preferred range Is possible.

  A catalyst ink is prepared by mixing the catalyst powder and ionomer 36 described above with polymeric organic compound particles 38. In order to obtain a dispersion having a desired ink viscosity, the mixing treatment is preferably performed in a solvent that does not substantially affect the above-described components, particularly the polymer organic compound particles 38. Suitable solvents include, but are not limited to, water, ethanol, cyclohexanone, propanol and mixtures thereof. By mixing each component with the above configuration, an electrode catalyst ink in a suitable dispersed state can be produced.

2-2. Electrode catalyst layer forming step This step consists in applying an electrode catalyst ink to the surface of the polymer electrolyte membrane 32, and catalyst powder containing the catalyst metal 35 supported on the carbon carrier 34, ionomer 36 and polymer organic compound particles 38. It is an object to form an electrode catalyst layer 33 containing. The application of the catalyst ink is preferably performed by various methods such as a transfer method such as a screen printing method, spray coating, and brush coating, and is particularly preferably performed by the transfer method. By applying the catalyst ink produced in the above-described electrode catalyst ink production process by these methods, it is possible to form the electrode catalyst layer 33 containing the catalyst powder and the polymer organic compound particles 38 in a suitable dispersion state. Become.

2-3. Void formation step This step aims to remove the polymer organic compound particles 38 contained in the electrode catalyst layer 33 bonded in the above-mentioned step and to form a plurality of voids inside the electrode catalyst layer 33. . As already described, the polymer organic compound particles 38 used in the production method of the present invention are composed of a polymer organic compound that is readily soluble in an organic solvent. On the other hand, the ionomer 36 and the polymer electrolyte membrane 32 are also made of a polymer organic compound. Therefore, the organic solvent used in this step can elute and remove the polymer organic compound particles 38, and other members constituting the membrane electrode assembly, such as the polymer electrolyte membrane 32 and the ionomer 36. Those that do not have a substantial effect on are selected. Suitable are organic solvents such as chloroform, acetone and dimethyl sulfoxide (DMSO).

  By immersing the membrane / electrode assembly produced in the above electrode catalyst layer forming step in an organic solvent, the polymer organic compound particles 38 can be eluted and removed from the electrode catalyst layer 33. When using said suitable organic solvent, it is preferable that immersion temperature is 0-60 degreeC. Moreover, it is preferable that immersion time is 24-72 hours. After being immersed under the above conditions, the membrane / electrode assembly is removed from the organic solvent, washed with a water-soluble organic solvent such as ethanol and isopropyl alcohol, and then further washed with water. By carrying out the process under the above conditions, the polymer organic compound particles 38 can be removed without damaging the membrane electrode assembly.

  As described above, the method for producing a membrane electrode assembly according to the present invention improves the efficiency of the reaction gas supply to the catalyst metal by forming a void in the vicinity of the catalyst metal in the electrode catalyst layer, thereby achieving high output. It is possible to suppress a voltage drop during operation. Further, by using the method for producing a membrane electrode assembly of the present invention, it becomes possible to increase the three-phase interface area of the catalyst metal used in the electrode catalyst layer, and consequently reduce the amount of catalyst metal used. Is possible.

Hereinafter, the present invention will be described in more detail with reference to Examples and Comparative Examples.
[Production of electrode catalyst]
A fuel cell electrode catalyst in which a catalyst metal including an alloy of platinum and a transition metal was supported on a carbon support was produced according to the following steps.

  A predetermined amount of platinum and a transition metal salt were supported on a carbon support (particle size: 30 nm) by a wet method. Thereafter, using a reducing agent, platinum and transition metal salt supported on the carbon support are reduced to be metallized, and further heat-treated in an inert gas, whereby platinum and transition metal supported on the carbon support are alloyed. Turned into. Thereafter, an unalloyed transition metal was removed by acid cleaning to obtain an electrode catalyst powder in which a catalyst metal containing an alloy of platinum and a transition metal was supported on a carbon support.

[Preparation of catalyst ink]
Table 1 shows the compositions of the comparative example and each example sample. In the table, “sulfonated polystyrene beads” in Example 2 is a material obtained by hydrophilizing the surface of the same polystyrene beads as in Example 1, and the polystyrene beads are made of 0.1 mol / L chlorosulfonic acid. It was prepared by immersing in an ethanol solution at room temperature with stirring for 24 hours.

  Each sample was prepared by the following procedure. For the comparative sample, 1.0 g of electrocatalyst powder (platinum loading density 40 wt%) and 0.45 g (dry weight) of ionomer (Nafion® DE2020 (DuPont)) were also used. In addition to these, 0.4 g of polymer organic compound particles (particle size 50 nm; nanobeads (trade name) (manufactured by Techno Chemical Co., Ltd.) (Example 1) or a sulfonated product thereof (Example 2) ) Were added to pure water and ethanol, respectively, to prepare a mixture. In this case, the content of the polymer organic compound particles corresponds to 50% by volume with respect to the total volume of the carbon support contained in the electrode catalyst powder. These mixtures were sonicated to prepare catalyst inks.

[Production of membrane electrode assembly]
The catalyst inks of the comparative example and each example were applied to both surfaces of a polymer electrolyte membrane (Nafion (registered trademark) N112 (manufactured by DuPont)) by a transfer method, and electrode catalyst layers were provided on both surfaces of the polymer electrolyte membrane. A catalyst-coated film was prepared. Next, a gas diffusion layer was joined to the electrode catalyst layers on both sides of the catalyst coated film to produce a membrane electrode assembly (MEA). The obtained MEAs of Comparative Examples and Examples were soaked in chloroform at room temperature for 24 hours to elute and remove the polymer organic compound particles contained in the electrode catalyst layer to form a plurality of voids. Thereafter, each MEA was taken out from chloroform and washed successively with ethanol and water.

[Evaluation]
Regarding the MEAs of the comparative examples and the examples obtained by the above method, the BET surface area of the ionomer, the oxygen diffusion resistance in the ionomer, and the power generation performance of each MEA were measured. The BET surface area of the ionomer was calculated by the BET multipoint method after measuring the adsorption / desorption isotherm by nitrogen using the constant volume method. The oxygen diffusion resistance in the ionomer was calculated by an overvoltage separation test in which IV characteristics were measured when air-hydrogen was supplied and when oxygen-hydrogen was supplied. MEA power generation performance of the temperature 80 ° C., 40% relative humidity, was evaluated by measurement of the output voltage at 1.0 A / cm ^2. The results are shown in FIGS. In addition, the value in a figure is a relative value which normalized the measured value of the comparative example as "1".

  As shown in FIG. 4, the BET surface areas of the ionomers of Examples 1 and 2 were 1.3 times and 1.7 times that of the comparative example, respectively. By adding the polymer organic compound particles, the electrode catalyst layer was formed. It was found that the BET surface area of ionomers increased. Moreover, when Examples 1 and 2 were compared, it became clear that this effect becomes more remarkable as the surface hydrophilicity of the particles of the polymer organic compound to be added increases.

  As shown in FIG. 5, the oxygen diffusion resistance in the ionomers of Examples 1 and 2 was 0.8 times and 0.6 times that of the comparative example, respectively, in contrast to the comparison of the ionomer BET surface areas shown in FIG. As a result.

  As shown in FIG. 6, the power generation performance of the MEAs of Examples 1 and 2 was 1.2 times and 1.4 times that of the comparative example, respectively, and showed a high correlation with the ionomer BET surface area results (FIG. 4).

  Consider the above results. The fuel cell obtains an electromotive force by an oxidation-reduction reaction on the surface of the catalyst contained in the anode and cathode electrode catalyst layers. For this reason, in order to continuously generate power, it is necessary to continuously supply oxygen and hydrogen, which are reaction gases, and the supply amount of oxygen and hydrogen increases corresponding to the increase in reaction amount, especially at high output. It is necessary to let Factors that define the amount of reactant gas supplied to the catalyst surface contained in the electrode catalyst layer include the amount of reactant gas dissolved in the ionomer and the diffusion resistance of the reactant gas in the ionomer. Among these, it was speculated that the amount of the reaction gas dissolved in the ionomer can be increased by increasing the surface area of the ionomer exposed to the reaction gas. As mentioned above, the BET surface areas of the ionomers of Examples 1 and 2 and the power generation performance of their MEAs were both improved relative to those of the comparative examples (FIGS. 4 and 6). From this, it is surmised that the improvement in the power generation performance of the MEAs of Examples 1 and 2 is due to the increase in the BET surface area of the ionomers of Examples 1 and 2 due to the addition and removal of the polymer organic compound particles. Is done.

  According to the method for manufacturing a membrane electrode assembly of the present invention, it is possible to improve the efficiency of supply of the reaction gas to the catalyst metal and suppress the voltage drop during the high output operation. As a result, the catalytic activity of the catalytic metal used for the electrode catalyst layer can be effectively utilized, and as a result, the amount of catalytic metal used can be reduced and the cost can be reduced.

DESCRIPTION OF SYMBOLS 11 ... Membrane electrode assembly 12, 22, 32 ... Polymer electrolyte membrane 13, 23, 33 ... Electrode catalyst layer 24, 34 ... Carbon carrier 25, 35 ... Catalyst metal 26, 36 ... Ionomer 17, 27 ... Void 38 ... High Molecular organic compound particles

Claims (6)

A method for producing a membrane electrode assembly comprising a polymer electrolyte membrane and an electrode catalyst layer disposed on both sides of the polymer electrolyte membrane and having a plurality of voids formed therein,
An electrode catalyst ink preparation step of preparing an electrode catalyst ink by mixing a catalyst powder containing a catalyst metal supported on a carbon carrier and an ionomer with particles of a polymer organic compound;
Electrocatalyst layer formation step of forming an electrode catalyst layer by applying an electrode catalyst ink to the surface of the polymer electrolyte membrane:
A void forming step of immersing the membrane electrode assembly in an organic solvent to remove particles of the polymer organic compound and forming a plurality of voids;
The method comprising the steps of:   2. The method according to claim 1, wherein the particle size ratio of the polymer organic compound particles to the carbon support is 1: 2 to 2: 1.   3. The method according to claim 1, wherein the particles of the macromolecular organic compound are made of polystyrene.   The method according to any one of claims 1 to 3, wherein the surface of the particles of the polymer organic compound is hydrophilized by introducing a sulfonic acid group.   The membrane electrode assembly manufactured by the method of any one of Claims 1-4.   A fuel cell comprising the membrane electrode assembly according to claim 5.

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