JP2019083181A - Membrane electrode assembly and solid polymer fuel cell - Google Patents

Abstract

To provide an electrode catalyst layer for fuel cell improving water retention under low humidification condition without disturbing removal of water produced by electrode reaction, showing high power generation performance even under low humidification condition, the electrode catalyst layer being able to be manufactured at a low cost.SOLUTION: At least one electrode catalyst layer, out of a pair of electrode catalyst layers 2, 3 sandwiching a polymer electrolyte membrane 1, contains catalyst carrier particles consisting of carbon particles carrying a catalyst, a fibrous material and a polyelectrolyte, where the ratio of fibrous material in the electrode catalyst layers, represented by ((mass of the fibrous material)/(mass of carbon particles carrying catalyst material)), decreases from the inside, i.e., the polyelectrolyte membrane, toward the outside, i.e., the electrode catalyst layer surface.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a membrane electrode assembly and a polymer electrolyte fuel cell comprising the membrane electrode assembly.

A fuel cell is a power generation system that uses a hydrogen-containing fuel gas and an oxygen-containing oxidant gas to cause the reverse reaction of water electrolysis at a catalyst-containing electrode to generate electricity simultaneously with heat. This power generation system has features such as high efficiency, low environmental load, and low noise as compared with the conventional power generation system, and is attracting attention as a future clean energy source. There are several types of fuel cells depending on the type of ion conductor used, and a cell using a proton conductive polymer membrane is called a polymer electrolyte fuel cell.
Among fuel cells, polymer electrolyte fuel cells can be used at around room temperature, and their use in automotive power supplies and home stationary power supplies is expected to be promising, and various research and development have been conducted in recent years. There is. In a polymer electrolyte fuel cell, a membrane electrode assembly (Membrane Electrode Assembly: hereinafter sometimes referred to as MEA) in which a pair of electrode catalyst layers are disposed on both sides of a polymer electrolyte membrane is sandwiched by a pair of separator plates. Battery.

In one separator plate, a gas flow path for supplying a fuel gas containing hydrogen is formed in one of the pair of electrodes, and in the other separator plate, oxidation is performed including oxygen in the other of the pair of electrodes. A gas flow path for supplying the agent gas is formed.
Here, an electrode supplied with fuel gas is a fuel electrode, and an electrode supplied with an oxidant gas is an air electrode. These electrodes are provided with an electrode catalyst layer formed by laminating a polymer electrolyte and carbon particles carrying a catalyst such as a platinum-based noble metal, and a gas diffusion layer having both gas permeability and electron conductivity. . In addition, these electrodes are disposed such that the gas diffusion layer faces the separator.

For the electrode catalyst layer, efforts have been made to improve the gas diffusivity in order to improve the power density of the fuel cell. The pores in the electrode catalyst layer are located ahead from the separator plate through the gas diffusion layer and play a role of a passage for transporting a plurality of substances. The fuel electrode not only smoothly supplies the fuel gas to the three-phase interface which is the reaction site of the oxidation reduction, but also functions to supply water for conducting the generated protons smoothly in the polymer electrolyte membrane. The air electrode, together with the supply of the oxidant gas, functions to smoothly remove the water produced by the electrode reaction.
In the polymer electrolyte fuel cell, in order to prevent a phenomenon called so-called "flooding" in which the power generation reaction is stopped due to the obstruction of the material transport in the fuel electrode and the air electrode, a configuration for improving the drainage property has been studied ( See, for example, Patent Document 1, Patent Document 2, Patent Document 3 and Patent Document 4).

Moreover, although the subject for practical use of a polymer electrolyte fuel cell includes the improvement of power density and durability, etc., the biggest subject is cost reduction (cost reduction).
One way to reduce costs is to reduce the number of humidifiers. Perfluorosulfonic acid membranes and hydrocarbon-based membranes are widely used as polymer electrolyte membranes located at the center of membrane electrode assemblies. And, in order to obtain excellent proton conductivity, water management close to a saturated water vapor pressure atmosphere is required, and at present, water is supplied from the outside by a humidifier. Therefore, for the purpose of low power consumption and simplification of the system, development of a polymer electrolyte membrane showing sufficient proton conductivity even under low humidification conditions that does not require a humidifier has been promoted. .

However, in the case of the electrode catalyst layer having enhanced drainage performance, the polymer electrolyte is dried up under low humidification conditions, so it is necessary to optimize the structure of the electrode catalyst layer to improve the water retention. Heretofore, in order to improve the water retentivity of the fuel cell under low humidification conditions, for example, a method of sandwiching a humidity control film between the electrode catalyst layer and the gas diffusion layer has been devised.
For example, Patent Document 5 describes a method in which a humidity control film composed of conductive carbonaceous powder and polytetrafluoroethylene exhibits a humidity control function to prevent dry-up.
Patent Document 6 describes a method of providing a groove on the surface of a catalyst electrode layer in contact with a polymer electrolyte membrane. In this method, a groove having a width of 0.1 to 0.3 mm is formed on the surface of the catalyst electrode layer to suppress a decrease in power generation performance under low humidification conditions.

Unexamined-Japanese-Patent No. 2006-120506 Unexamined-Japanese-Patent No. 2006-332041 JP 2007-87651A JP 2007-80726 A Unexamined-Japanese-Patent No. 2006-252948 Japanese Patent Application Publication No. 2007-141588

According to the methods described in Patent Documents 5 and 6, it is possible to enhance the drainage property of the electrode catalyst layer (do not inhibit the removal of the water generated by the electrode reaction), and at the same time, to retain water of the electrode catalyst layer under low humidification conditions. Can be expected to improve. However, fuel cells using the electrode catalyst layer obtained by these methods have room for improvement in terms of power generation performance under low humidification conditions and durability. In addition, these methods are complicated, and there is also a problem that the production cost of the electrode catalyst layer is high.
The present invention was made focusing on the above points, and the water retention under the low humidity condition is improved without inhibiting the removal of the water generated by the electrode reaction, and the low humidity condition is also achieved. However, it is an object of the present invention to provide a fuel cell membrane electrode assembly exhibiting high power generation performance and durability.

In order to solve the problem, according to one aspect of the present invention, there is provided a membrane electrode assembly in which a polymer electrolyte membrane is sandwiched between a pair of electrode catalyst layers, wherein at least one of the pair of electrode catalyst layers is electrocatalyst The layer has catalyst-supporting particles consisting of carbon particles on which a catalyst is supported, a fibrous material, and a polymer electrolyte, and the ratio of the fibrous material represented by the following formula (1) is the above-mentioned polymer electrolyte It is characterized in that it becomes smaller continuously or intermittently as it goes away from the film in the film thickness direction.
(Ratio of fibrous substances)
= {(Mass of fibrous material) / (mass of carbon particles in carbon particles carrying catalyst material)} (1)

  According to one aspect of the present invention, there is provided a membrane electrode assembly comprising an electrode catalyst layer that enhances water retention without inhibiting removal of water generated by electrode reaction and the like and exhibits high power generation characteristics even under low humidification conditions. Can. As a result, according to one aspect of the present invention, a polymer electrolyte fuel cell having high power generation characteristics can be obtained.

It is a disassembled perspective view which shows typically the membrane electrode assembly of embodiment of this invention. It is a disassembled perspective view which shows typically the structure of the polymer electrolyte fuel cell provided with the membrane electrode assembly of FIG.

Hereinafter, embodiments of the present invention will be described with reference to the attached drawings.
The present invention is not limited to the embodiments described below, and modifications such as design changes can be made based on the knowledge of those skilled in the art, and such modifications can be made. Is also within the scope of the present invention.

First Embodiment
First, the first embodiment will be described.
[Membrane electrode assembly]
First, the membrane electrode assembly 11 according to the present embodiment will be described.
As shown in FIG. 1, the membrane electrode assembly 11 comprises a polymer electrolyte membrane 1 and an electrode catalyst layer 2 sandwiching the polymer electrolyte membrane 1 from the upper and lower surfaces of the polymer electrolyte membrane 1 (in FIG. And an electrode catalyst layer 3 (shown below in FIG. 1).

Here, in the following description, the ratio of the fibrous substance is a value represented by the following formula (2). The ratio of the fibrous material is defined, for example, by the ratio of the fibrous material in each cross section parallel to the surface facing the polymer electrolyte membrane 1.
(Ratio of fibrous substances)
= {(Mass of fibrous material) / (mass of carbon particles in carbon particles carrying a catalyst material)} (2)

At least one electrode catalyst layer of the pair of electrode catalyst layers 2 and 3 has a catalyst supporting particle consisting of carbon particles supporting a catalyst, a fibrous material, and a polymer electrolyte, and the ratio of the fibrous material is The distance from the polymer electrolyte membrane 1 in the film thickness direction decreases continuously or intermittently.
The fibrous substance is preferably a conductive fiber having an average fiber length of 1 μm to 15 μm. In this embodiment, it is assumed that conductive fibers having an average fiber length of 1 μm to 15 μm are used as the fibrous substance.

For example, at least one electrode catalyst layer of the pair of electrode catalyst layers 2 and 3 is configured by laminating two or more layers of electrode catalyst portions having different proportions of fibrous materials, and the stacked two or more layers of electrodes The catalyst portion is set so that the ratio of the fibrous material decreases as the electrode catalyst portion is farther from the polymer electrolyte membrane 1. In this case, the ratio of the fibrous material is set so as to be intermittently reduced as it is separated from the polymer electrolyte membrane 1 in the film thickness direction.
At this time, a value obtained by dividing the largest value of the ratio of the fibrous material in the thickness direction of the electrode catalyst layer by the smallest value of the ratio of the fibrous material is in the range of 1.5 to 25. Is preferred.

The case where an electrode catalyst layer is comprised by two layers is illustrated by FIG. Moreover, although the case where both of a pair of electrode catalyst layers 2 and 3 are comprised by 2 layers is illustrated in FIG. 1, only one electrode catalyst layer may be 2 layer structure.
An electrode catalyst layer having a configuration (hereinafter also referred to as a configuration) having catalyst-supporting particles consisting of carbon particles supporting a catalyst, a fibrous substance having an average fiber length of 1 μm to 15 μm, and a polymer electrolyte As for Nos. 2 and 3, high durability and mechanical properties such as suppression of cracking of the electrode catalyst layer which is a cause of deterioration in durability can be obtained by entanglement of highly crystalline fibrous substances. If the average fiber length of the fibrous material is less than 1 μm, the mechanical properties may be deteriorated because the entanglement of the fibrous material is weak. When the average fiber length of the fibrous material exceeds 15 μm, the fibrous material may not be dispersed as ink because the entanglement of the fibrous material is strong.

  Further, as the ratio of the fibrous material in the electrode catalyst layer is larger, the number of pores formed by the fibrous material is increased (sparse structure). On the other hand, the smaller the proportion of fibrous material, the smaller the pores formed by the fibrous material (dense structure). And, in the membrane electrode assembly 11 of the present embodiment, the structure of the electrode catalyst portion 2a (or 3a) located on the outer side (the side away from the polymer electrolyte membrane 1) is a polymer electrolyte membrane that is the inner side. The ratio of the fibrous material is reduced compared to the side electrode catalyst portion 2b (or 3b) to form a dense structure, and the structure of the electrode catalyst portion 2b (or 3b) located on the inner side is the outer electrode catalyst portion 2a Compared with (or 3a), the ratio of fibrous substances is increased to form a sparse structure. As a result, in the present embodiment, it is possible to simultaneously achieve the drainage property of the water generated by the electrode reaction and the water retentivity under the low humidification condition.

The value obtained by dividing the largest value of the ratio of the fibrous material in the film thickness direction of the electrode catalyst layer by the smallest value of the ratio of the fibrous material is preferably in the range of 1.5 or more and 25 or less.
In the electrode catalyst layers 2 and 3, the ratio of the fibrous material of the electrode catalyst portion 2b (or 3b) showing the largest value of the ratio of the fibrous material in the thickness direction of the electrode catalyst layer When the value divided by the ratio of the fibrous material of the electrode catalyst portion 2a (or 3a) showing a small value is less than 1.5, the difference between the pores formed of the electrode catalyst layer by the fibrous material is small, It may be difficult to achieve both the drainage of water generated by the electrode reaction and the water retention under low humidification conditions. In this case, the proton conductivity may be reduced. In addition, when the above-mentioned divided value exceeds 25, the difference of the pores formed by the fibrous substance becomes excessively large, and both the drainage property of the water generated by the electrode reaction and the water retentivity under the low humidification condition are compatible. It may be difficult to do.

Polymer Electrolyte Fuel Cell
Next, a polymer electrolyte fuel cell 12 including the membrane electrode assembly 11 of the embodiment will be described with reference to FIG.
As shown in FIG. 2, the polymer electrolyte fuel cell 12 is opposed to the gas diffusion layer 4 on the air electrode side disposed so as to face the electrode catalyst layer 2 of the membrane electrode assembly 11 and the electrode catalyst layer 3. And a gas diffusion layer 5 on the fuel electrode side arranged to The electrode catalyst layer 2 and the gas diffusion layer 4 form an air electrode (cathode) 6. The electrode catalyst layer 3 and the gas diffusion layer 5 form a fuel electrode (anode) 7.

In addition, a pair of separators 10 a and 10 b made of a conductive and impermeable material are disposed outside the gas diffusion layers 4 and 5, respectively. One set of separators 10a and 10b includes gas flow channels 8a and 8b for gas flow and cooling water flow channels 9a and 9b for cooling water flow.
For example, hydrogen gas is supplied as fuel gas from the gas flow path 8b of the separator 10b on the fuel electrode 7 side. On the other hand, for example, oxygen gas is supplied as an oxidant gas from the gas flow path 8a of the separator 10a on the air electrode 6 side. By causing electrode reaction between hydrogen of the fuel gas and oxygen of the oxidant gas in the presence of the catalyst, an electromotive force can be generated between the fuel electrode and the air electrode.

In the polymer electrolyte fuel cell 12, the polymer electrolyte membrane 1, the two electrode catalyst layers 2 and 3, and the gas diffusion layers 4 and 5 are sandwiched between a pair of separators 10a and 10b. The polymer electrolyte fuel cell 12 is a single cell fuel cell, but the polymer electrolyte fuel cell 12 may also be used as a polymer electrolyte fuel cell by laminating a plurality of cells via the separator 10a or 10b. good.
In the membrane electrode assembly 11 according to the present embodiment, in the electrode catalyst layers 2 and 3 formed on both sides of the polymer electrolyte membrane 1, only one of the electrode catalyst layers has a fibrous material ratio inside In the case of an electrode catalyst layer decreasing from the polymer electrolyte membrane toward the surface of the electrode catalyst layer which is the outer side, the electrode catalyst layer is on the air electrode (cathode) side where water is generated due to the electrode reaction. It is preferred to be arranged.

[Method of producing membrane electrode assembly]
Next, an example of a method of manufacturing the membrane electrode assembly 11 having the above configuration (a) will be described.
The membrane electrode assembly 11 having the above configuration (a) is manufactured by a method including the following first step, second step and third step.
The first step is a step of producing a catalyst ink comprising catalyst-supporting particles consisting of carbon particles supporting a catalyst, a fibrous material, a polymer electrolyte, and a solvent. In the first step, at least two types of catalyst inks having different proportions of fibrous materials are produced.

  In the second step, when the substrate to which the catalyst ink is applied is a gas diffusion layer or a transfer sheet, the ratio of the fibrous material from the catalyst ink having the smallest ratio of the fibrous material is the largest on the substrate. By sequentially applying and drying toward a large catalyst ink, the electrode catalyst layers 2 and 3 having a multilayer structure in which the pore volume changes sequentially are formed on the substrate. When the substrate to which the catalyst ink is applied is a polymer electrolyte membrane, the catalyst ink having the largest ratio of the fibrous material to the catalyst ink having the smallest ratio of the fibrous material is provided on the substrate. By sequentially applying and drying, the electrode catalyst layers 2 and 3 having a multilayer structure in which the pore volume changes sequentially are formed on the substrate. When the substrate is a polymer electrolyte membrane, the third step is also carried out by performing the second step.

In the third step, when the substrate is a gas diffusion layer or a transfer sheet, the electrode catalyst layer formed on the substrate is laminated on the surface of the polymer electrolyte membrane.
In the second step of the method of manufacturing a membrane electrode assembly, in forming the electrode catalyst layer of two or more layers, a drying step of removing the solvent in the coating film is provided as necessary.
At this time, a first layer catalyst ink is applied on a substrate to form a coating film, and then the coating film is dried to form a first layer electrode catalyst layer, and a second layer catalyst ink is formed in the first layer electrode catalyst layer After coating on the top, the coated film is dried to form a second electrode catalyst layer, whereby an electrode catalyst layer having a multilayer structure can be formed.

In addition, a first layer of catalyst ink is applied to form a coating, and a drying step is not performed. A second layer of catalyst ink is subsequently applied to form a coating, and these coatings are dried to form a multilayer structure. An electrode catalyst layer can also be formed.
In addition, the first layer catalyst ink is applied to form a coating film, and the coating film is dried to leave a part of the solvent in the coating film to make it semi-dry, and then the second layer catalyst ink is applied and coated. It is also possible to form a membrane and dry these coatings to form a multi-layered electrocatalyst layer. By doing so, the ratio of fibrous material can be continuously changed along the film thickness direction.
In the method of manufacturing a membrane electrode assembly, the drying step of drying the coating can be changed as needed.

[Detailed description]
Hereinafter, the membrane electrode assembly 11 and the polymer electrolyte fuel cell 12 will be described in more detail.
The polymer electrolyte membrane 1 may be one having proton conductivity, and a fluorine-based polymer electrolyte membrane or a hydrocarbon-based polymer electrolyte membrane can be used. As an example of a fluorine-based polymer electrolyte membrane, Nafion (registered trademark) manufactured by DuPont, Flemion (registered trademark) manufactured by Asahi Glass Co., Ltd., Aciplex (registered trademark) manufactured by Asahi Kasei Corp., Gore Select (registered trademark) manufactured by Gore Etc. can be used.

Further, as the hydrocarbon-based polymer electrolyte membrane, an electrolyte membrane of sulfonated polyether ketone, sulfonated polyether sulfone, sulfonated polyether ether sulfone, sulfonated polysulfide, sulfonated polyphenylene, or the like can be used. In particular, Nafion (registered trademark) -based materials manufactured by DuPont can be suitably used as the polymer electrolyte membrane 1.
The electrode catalyst layers 2 and 3 are formed on both sides of the polymer electrolyte membrane 1 using a catalyst ink. The catalyst ink for the electrode catalyst layers 2 and 3 contains catalyst supporting particles, a polymer electrolyte and a solvent. In addition, the catalyst ink used for at least one of the electrode catalyst layers 2 and 3 contains catalyst-supporting particles, a fibrous substance, a polymer electrolyte, and a solvent, and has two kinds of compositions different in the mass ratio of fibrous substance to carbon particles. Use.

  The polymer electrolyte contained in the catalyst ink may be any one having proton conductivity, and the same material as that of the polymer electrolyte membrane 1 can be used, and a fluorine-based polymer electrolyte and a hydrocarbon-based polymer electrolyte can be used. It can be used. As an example of a fluorine-based polymer electrolyte, Nafion (registered trademark) -based material manufactured by DuPont can be used. Further, as the hydrocarbon-based polymer electrolyte, electrolytes such as sulfonated polyether ketone, sulfonated polyether sulfone, sulfonated polyether ether sulfone, sulfonated polysulfide, sulfonated polyphenylene and the like can be used. In particular, Nafion (registered trademark) manufactured by DuPont can be suitably used as the fluorine-based polymer electrolyte.

As a catalyst used in the present embodiment (hereinafter sometimes referred to as catalyst particles or catalyst), platinum group elements, metals, alloys or oxides of the metals, composite oxides, etc. can be used. The platinum group elements include platinum, palladium, ruthenium, iridium, rhodium and osmium. Examples of the metal include iron, lead, copper, chromium, cobalt, nickel, manganese, vanadium, molybdenum, gallium, aluminum and the like. The term "multiple oxide" as used herein means an oxide composed of two types of metals.
When the catalyst particles are one or more metals selected from platinum, gold, palladium, rhodium, ruthenium, and iridium, they are excellent in electrode reactivity and efficiently and stably perform electrode reactions. it can. When the catalyst particles are one or more metals selected from platinum, gold, palladium, rhodium, ruthenium, and iridium, the polymer electrolyte fuel cell 12 provided with the electrode catalyst layers 2 and 3 is It is preferable because it exhibits high power generation characteristics.

Moreover, 0.5 nm or more and 20 nm or less are preferable, and, as for the average particle diameter of the catalyst particle mentioned above, 1 nm or more and 5 nm or less are more preferable. Here, the average particle size is an average particle size obtained from an X-ray diffraction method if it is a catalyst supported on a carrier such as carbon particles. In addition, if the catalyst is not supported on the carrier, it is the arithmetic mean particle size obtained from particle size measurement. When the average particle diameter of the catalyst particles is in the range of 0.5 nm or more and 20 nm or less, the activity and stability of the catalyst are preferably improved.
Carbon particles are generally used as the electron conductive powder (support) supporting the above-mentioned catalyst. The type of carbon particles is not limited as long as it is particulate and conductive and is not dependent on a catalyst, but carbon black, graphite, graphite, activated carbon, carbon fiber, carbon nanotubes, fullerene may be used Can.

The average particle diameter of the carbon particles is preferably about 10 nm or more and 1000 nm or less, and more preferably 10 nm or more and 100 nm or less. Here, the average particle size is an average particle size obtained from a SEM image. When the average particle diameter of the carbon particles is in the range of 10 nm to 1000 nm, the activity and stability of the catalyst are improved, which is preferable. It is preferable because an electron conduction path is easily formed, and the gas diffusivity of the two electrode catalyst layers 2 and 3 and the utilization of the catalyst are improved.
As the fibrous material, conductive fibers can be used. Examples of conductive fibers include carbon nanofibers, carbon nanotubes, and vapor grown carbon fibers. As an example of the vapor grown carbon fiber, VGCF (registered trademark) manufactured by Showa Denko K. K. can be used. In addition, an electrolyte fiber obtained by processing a polymer electrolyte into a fiber may be used. The proton conductivity can be improved by using an electrolyte fiber. These fibrous substances may be used alone or in combination of two or more, and conductive fibers and electrolyte fibers may be used together.

  The solvent used as a dispersion medium for the catalyst ink is not particularly limited as long as it can dissolve the polymer electrolyte in a highly fluid state or disperse as a fine gel without eroding the catalyst material-supporting particles and the polymer electrolyte. It is not a thing. However, the solvent preferably contains at least a volatile organic solvent. Examples of the solvent used as a dispersion medium for the catalyst ink include methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, 2-butanol, alcohols such as isobutyl alcohol, tert-butyl alcohol and pentanol, and acetone Ketone solvents such as methyl ethyl ketone, pentanone, methyl isobutyl ketone, heptanone, cyclohexanone, methyl cyclohexanone, acetonyl acetone, diisobutyl ketone, etc., ether solvents such as tetrahydrofuran, dioxane, diethylene glycol dimethyl ether, anisole, methoxytoluene, dibutyl ether, etc. Dimethylformamide, dimethylacetamide, N-methylpyrrolidone, ethylene glycol, diethylene glycol, diacetone alcohol , It can be used polar solvents such as 1-methoxy-2-propanol. Further, as the solvent, a mixed solvent in which two or more of the above-mentioned materials are mixed may be used.

Moreover, since the thing using a lower alcohol as a solvent used as a dispersion medium of a catalyst ink has a high danger of ignition, when using a lower alcohol, it is preferable to set it as the mixed solvent with water. Furthermore, water that is compatible with the polymer electrolyte (water with high affinity) may be included. The amount of water added is not particularly limited as long as the polymer electrolyte is not separated to cause white turbidity or gelation.
A dispersant may be contained in the catalyst ink in order to disperse the catalyst material supporting particles. As a dispersing agent, an anionic surfactant, a cationic surfactant, an amphoteric surfactant, a nonionic surfactant etc. can be mentioned.

  Examples of anionic surfactants include alkyl ether carboxylates, ether carboxylates, alkanoyl sarcosines, alkanoyl glutamates, acyl glutamates, oleic acid / N-methyl taurine, potassium oleate / diethanolamine salts, alkyl ether sulfates / triols Ethanolamine salt, polyoxyethylene alkyl ether sulfate triethanolamine salt, amine salt of specially modified polyether ester acid, amine salt of higher fatty acid derivative, amine salt of specially modified polyester acid, amine salt of high molecular weight polyether ester acid Amine salts of specially modified phosphoric esters, high molecular weight polyester acid amidoamine salts, amidoamine salts of special fatty acid derivatives, alkylamine salts of higher fatty acids, amides of high molecular weight polycarboxylic acids Minates, carboxylic acid type surfactants such as sodium laurate, sodium stearate, sodium oleate, dialkyl sulfosuccinates, dialkyl sulfosuccinates, 1,2-bis (alkoxycarbonyl) -1-ethane sulfonates, Alkyl sulfonates, alkyl sulfonates, paraffin sulfonates, α-olefin sulfonates, linear alkyl benzene sulfonates, alkyl benzene sulfonates, polynaphthyl methane sulfonates, polynaphthyl methane sulfonates, naphthalene sulfonate-formalin condensates, alkyl naphthalene sulfonates, alkanoyl Methyl taurine, lauryl sulfate sodium salt, cetyl sulfate sodium salt, stearyl sulfate sodium salt, oleyl sulfate S Sulfonic acid type surfactants such as ter sodium salt, lauryl ether sulfuric acid ester salt, sodium alkyl benzene sulfonate, oil-soluble alkyl benzene sulfonic acid salt, α-olefin sulfonic acid salt, alkyl sulfuric acid ester salt, sulfuric acid alkyl salt, alkyl sulfate, alkyl Ether sulfate, polyoxyethylene alkyl ether sulfate, alkyl polyethoxy sulfate, polyglycol ether sulfate, alkyl polyoxyethylene sulfate, sulfated surfactant such as sulfated oil, highly sulfated oil, phosphoric acid (mono or Di) alkyl salts, (mono or di) alkyl phosphates, (mono or di) alkyl phosphoric acid ester salts, phosphoric acid alkylpolyoxyethylene salts, alkyl ether phosphates, alkyl polyethoxy. Acid salt, polyoxyethylene alkyl ether, phosphoric acid alkylphenyl polyoxyethylene salt, alkylphenyl ether phosphate, alkylphenyl polyethoxy phosphate, polyoxyethylene alkylphenyl ether phosphate, higher alcohol phosphoric acid monoester Disodium salt, higher alcohol phosphoric acid diester disodium salt, phosphoric acid ester type surfactants such as zinc dialkyl dithiophosphate, etc. may be mentioned.

  Examples of cationic surfactants include benzyldimethyl {2- [2- (P-1,1,3,3-tetramethylbutylphenoxy) ethoxy] ethyl} ammonium chloride, octadecylamine acetate, tetradecylamine acetate Octadecyl trimethyl ammonium chloride, tallow trimethyl ammonium chloride, dodecyl trimethyl ammonium chloride, coconut trimethyl ammonium chloride, hexadecyl trimethyl ammonium chloride, behenyl trimethyl ammonium chloride, coconut dimethyl benzyl ammonium chloride, tetradecyl dimethyl benzyl ammonium chloride, octadecyl dimethyl benzyl ammonium chloride , Dioleyl dimethyl ammonium chloride, 1-hydroxyethyene -2- tallow imidazoline quaternary salt, 2-heptadecenyl-hydroxyethyl imidazoline, stearamide ethyl diethylamine acetate, stearamide ethyl diethylamine hydrochloride, triethanolamine monostearate formate, alkyl pyridinium salt, higher alkyl amine Ethylene oxide adduct, polyacrylamide amine salt, modified polyacrylamide amine salt, perfluoroalkyl quaternary ammonium iodide and the like can be mentioned.

  As an example of amphoteric surfactants, dimethyl coconut betaine, dimethyl lauryl betaine, sodium lauryl aminoethyl glycine, sodium lauryl amino propionate, stearyl dimethyl betaine, lauryl dihydroxyethyl betaine, amido betaine, imidazolinium betaine, lecithin, 3- [ Examples include sodium ω-fluoroalkanoyl-N-ethylamino] -1-propanesulfonic acid, N- [3- (perfluorooctanesulfonamido) propyl] -N, N-dimethyl-N-carboxymethylene ammonium betaine and the like.

  Examples of nonionic surfactants include coconut fatty acid diethanolamide (1: 2 type), coconut fatty acid diethanolamide (1: 1 type), bovine fatty acid diethanolamide (1: 2 type), bovine fatty acid diethanolamide (1: 1 type) Type), oleic acid diethanolamide (1: 1 type), hydroxyethyl lauryl amine, polyethylene glycol lauryl amine, polyethylene glycol coyl amine, polyethylene glycol stearyl amine, polyethylene glycol tallow amine, polyethylene glycol tallow propylene diamine, polyethylene glycol dioleyl amine, Dimethyl lauryl amine oxide, dimethyl stearyl amine oxide, dihydroxyethyl lauryl amine oxide, perfluoroalkyl amine oxide, polyvinyl pyro Don, higher alcohol ethylene oxide adduct, alkylphenol ethylene oxide adduct, fatty acid ethylene oxide adduct, polypropylene glycol ethylene oxide adduct, fatty acid ester of glycerin, fatty acid ester of pentaerythritol, fatty acid ester of Sorbit, fatty acid ester of sorbitan, The fatty acid ester of sugar etc. are mentioned.

Among the above-described surfactants, a sulfonic acid type surfactant can be suitably used as a dispersant, in consideration of the dispersion effect of carbon, the change of the catalyst performance due to the remaining of the dispersant, and the like. Examples of sulfonic acid type surfactants include alkyl benzene sulfonic acid, oil soluble alkyl benzene sulfonic acid, α-olefin sulfonic acid, sodium alkyl benzene sulfonate, oil soluble alkyl benzene sulfonate, α-olefin sulfonate and the like.
Pore volume generally decreases as the amount of polyelectrolyte in the catalyst ink is increased. On the other hand, when the amount of carbon particles in the catalyst ink is increased, the pore volume is increased. Also, the use of dispersants reduces the pore volume.

  In addition, the catalyst ink is subjected to dispersion processing as needed. The viscosity of the catalyst ink and the size of the particles contained in the catalyst ink can be controlled by the conditions of the dispersion process of the catalyst ink. Distributed processing can be performed by employing various devices. In particular, the method of distributed processing is not limited. For example, as the dispersion treatment, treatment with a ball mill or roll mill, treatment with a shear mill, treatment with a wet mill, ultrasonic dispersion treatment and the like can be mentioned. Moreover, you may employ | adopt the homogenizer etc. which stir by a centrifugal force. The pore volume is reduced as the dispersion time of the dispersion time is broken, the aggregates of the catalyst supporting particles are broken.

  If the solid content in the catalyst ink is too large, the viscosity of the catalyst ink becomes high, so that the surface of the electrode catalyst layers 2 and 3 is easily cracked. On the other hand, when the solid content in the catalyst ink is too small, the film forming rate is very slow, and the productivity is lowered. Therefore, the solid content in the catalyst ink is preferably 1% by mass (wt%) or more and 50% by mass or less. The solid content is composed of catalyst material-supporting particles and a polymer electrolyte, but when the content of catalyst material-supporting particles in the solid content is increased, the viscosity becomes high even with the same solid content. On the other hand, when the content of the catalyst substance-supporting particles in the solid content is reduced, the viscosity decreases even at the same solid content. Therefore, the proportion of the catalyst substance-supporting particles in the solid content is preferably 10% by mass to 80% by mass. The viscosity of the catalyst ink is preferably about 0.1 cP to 500 cP (0.0001 Pa · s to 0.5 Pa · s), preferably 5 cP to 100 cP (0.005 Pa · s to 0.1 Pa · s). Is more preferred. The viscosity can also be controlled by adding a dispersant at the time of dispersing the catalyst ink.

In addition, the catalyst ink may contain a pore forming agent. The pore forming agent can be removed after the formation of the electrode catalyst layer to form pores. Examples thereof include acids and alkalis, substances soluble in water, substances to be sublimated such as camphor, and substances to be thermally decomposed. If the pore forming agent is a substance soluble in warm water, it may be removed by water generated at the time of power generation.
Examples of the pore-forming agent soluble in acid, alkali and water include acid-soluble inorganic salts, inorganic salts soluble in alkaline aqueous solution, metals soluble in acid or alkali, water-soluble inorganic salts, water-soluble organic compounds and the like. Examples of acid-soluble inorganic salts include calcium carbonate, barium carbonate, magnesium carbonate, magnesium sulfate, magnesium oxide and the like. Alumina, silica gel, silica sol etc. are mentioned as an inorganic salt soluble in alkaline aqueous solution. Examples of metals soluble in acid or alkali include metals soluble in acid or alkali such as aluminum, zinc, tin, nickel and iron. Examples of water-soluble inorganic salts include sodium chloride, potassium chloride, ammonium chloride, sodium carbonate, sodium sulfate, monosodium phosphate and the like. Examples of water-soluble organic compounds include polyvinyl alcohol and polyethylene glycol. Moreover, although the pore making agent mentioned above may be used individually by 1 type or in combination of 2 or more types, it is preferable to use in combination of 2 or more types.

As a coating method for coating the catalyst ink on the substrate, a doctor blade method, a dipping method, a screen printing method, a roll coating method, or the like can be adopted.
A transfer sheet can be used as a base material used for manufacture of the electrode catalyst layers 2 and 3.
The transfer sheet used as the substrate may be made of a material having good transferability, and, for example, a fluorine-based resin can be used. As a fluorine resin, ethylene tetrafluoroethylene copolymer (ETFE), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), tetrafluoroperfluoroalkyl vinyl ether copolymer (PFA), polytetrafluoroethylene (PTFE) Etc. can be used. Moreover, a polymer sheet and a polymer film can be used as a transfer sheet. As materials of polymer sheet and polymer film, polyimide, polyethylene terephthalate, polyamide (nylon (registered trademark)), polysulfone, polyether sulfone, polyphenylene sulfide, polyether / ether ketone, polyether imide, poly arylate, Polyethylene naphthalate or the like can be used. When a transfer sheet is used as the substrate, the transfer sheet is peeled off after the polymer electrolyte membrane 1 is bonded to the electrode membrane which is a coating after solvent removal, and the electrode catalyst is formed on both sides of the polymer electrolyte membrane 1. The membrane electrode assembly 11 can be provided with the layers 2 and 3.

As the gas diffusion layers 4 and 5, a material having gas diffusivity and conductivity can be used. For example, as the gas diffusion layers 4 and 5, porous carbon materials such as carbon cloth, carbon paper, and non-woven fabric can be used.
As the separator 10 (10a, 10b), a carbon type or metal type can be used. The gas diffusion layers 4 and 5 and the separators 10 (10a and 10b) may be integrally formed. When the separators 10 (10a, 10b) or the electrode catalyst layers 2, 3 perform the function of the gas diffusion layers 4, 5, the gas diffusion layers 4, 5 may be omitted. The polymer electrolyte fuel cell 12 can be manufactured by assembling a gas supply device, a cooling device, and other associated devices.

Hereinafter, the method for producing a membrane / electrode assembly for a polymer electrolyte fuel cell in the present embodiment will be described with reference to specific examples and comparative examples, but the present embodiment is limited by the following examples and comparative examples. It is not something to be done.
As described above, the membrane electrode assembly of the present embodiment is a membrane electrode assembly in which a polymer electrolyte membrane is sandwiched between a pair of electrode catalyst layers, and the electrode catalyst layer is formed of carbon particles carrying a catalyst. Catalyst-supporting particles, a fibrous substance, and a polymer electrolyte, and the average fiber length of the fibrous substance is 1 μm or more and 15 μm or less, and {(mass of fibrous substance) / (in the above electrode catalyst layer) The ratio of the fibrous material represented by the mass of carbon particles in the carbon particles on which the catalyst material is supported is continuous or discontinuous from the inner polymer electrolyte membrane to the outer electrode catalyst layer surface) In addition, the electrode catalyst layer is formed by laminating at least two layers of electrode catalyst portions having different ratios of fibrous materials, and the largest value of the ratio of fibrous materials in the thickness direction of the electrode catalyst layer can be obtained. Maximum ratio of fibrous material Also, the value divided by the small value is in the range of 1.5 or more and 25 or less.

According to this configuration, the ratio of the fibrous material in the thickness direction of the catalyst electrode layer is improved, and the ratio of the fibrous material in the thickness direction is increased from the outside toward the polymer electrolyte membrane. The structure of the electrode catalyst layer changes from a dense to a sparse structure toward the molecular electrolyte membrane, and the water retention of the electrode catalyst layer is prevented without inhibiting the diffusivity of the reaction gas and the removal of water generated by the electrode reaction. It can be enhanced.
Furthermore, unlike the application of the conventional humidity control film and the response to low humidification by the formation of grooves on the surface of the electrode catalyst layer, no deterioration in power generation characteristics due to the increase in interface resistance is observed, and the conventional membrane electrode assembly The polymer electrolyte fuel cell comprising the membrane electrode assembly of the present invention exhibits a remarkable effect of showing high power generation characteristics even under low humidification conditions, as compared with the polymer electrolyte fuel cell comprising the above.

Second Embodiment
Next, a second embodiment will be described.
The basic configuration of the membrane electrode assembly of the second embodiment is the same as that of the first embodiment. However, the fibrous material used is different.
The fibrous material used in the second embodiment consists of fibers having proton conductivity.
The fiber having proton conductivity is formed, for example, by dissolving a perfluorosulfonic acid-based polymer in an organic solvent and fiberizing using an electrospinning method. The perfluorosulfonic acid polymer may be used alone or in combination of two or more.

The average fiber diameter of the fibrous substance of the second embodiment is preferably 1 μm or less. By setting the average fiber diameter of the fibrous substance to 1 μm or less, it is possible to increase the contact area between the catalyst-supporting particles consisting of carbon particles supporting the catalyst and the fibrous substance, thereby further increasing the power generation performance of the fuel cell. It can be improved. If the diameter is larger than 1 μm, the fiber spacing of the fibrous material becomes narrow, the catalyst-supporting particles can not be sufficiently filled, and the necessary power generation capacity may not be secured.
The average fiber diameter of the fibrous substance of the second embodiment is preferably 50 nm or more. If the diameter is smaller than 50 nm, the strength of the fibrous material may not be sufficiently obtained, and the proton conductivity of the fibrous material may be reduced.

In addition, it has a structure (hereinafter referred to as (A) structure) having catalyst-supporting particles consisting of carbon particles supporting a catalyst, a fibrous substance having an average fiber diameter of 50 nm to 1 μm and proton conduction performance, and a polymer electrolyte The electrode catalyst layers 2 and 3 having high durability and mechanical properties can be obtained by entanglement of highly crystalline fibrous materials, such as suppressing cracking of the electrode catalyst layer causing deterioration in durability. .
When the average fiber diameter of the fibrous material is less than 50 nm, the mechanical properties may be reduced because the entanglement of the fibrous material is weak. When the average fiber diameter of the fibrous substance exceeds 1 μm, the fibrous substance may not be dispersed as ink because the entanglement of the fibrous substance is strong.

In the present embodiment, a value obtained by dividing the largest value of the ratio of the fibrous material by the smallest value of the ratio of the fibrous material in the thickness direction of the electrode catalyst layer is within the range of 1.3 to 26. It is preferable to do.
In the electrode catalyst layers 2 and 3, the ratio of the fibrous material of the electrode catalyst portion 2b (or 3b) showing the largest value of the ratio of the fibrous material in the thickness direction of the electrode catalyst layer When the value divided by the ratio of the fibrous material of the electrode catalyst portion 2a (or 3a) showing a small value is less than 1.3, the difference between the pores formed of the electrode catalyst layer by the fibrous material is small, It may be difficult to achieve both the drainage of water generated by the electrode reaction and the water retention under low humidification conditions. In this case, the proton conductivity may be reduced. In addition, when the above-mentioned divided value exceeds 26, the difference of the pores formed by the fibrous substance becomes excessively large, and both the drainage property of the water generated by the electrode reaction and the water retentivity under the low humidity condition are compatible. It may be difficult to do.

The other configuration and the operation and effect of the configuration are the same as those of the first embodiment.
Further, as to the manufacturing method, the same manufacturing method as that of the first embodiment may be adopted.
As described above, the membrane electrode assembly of the present embodiment is a membrane electrode assembly in which a polymer electrolyte membrane is sandwiched between a pair of electrode catalyst layers, and the electrode catalyst layer is formed of carbon particles carrying a catalyst. Catalyst-supporting particles, a fibrous substance having proton conductivity, and a polymer electrolyte, and the average fiber diameter of the fibrous substance is 50 nm or more and 1 μm or less, and {(fibrous substance) in the electrode catalyst layer The ratio of the fibrous material represented by the mass of (the mass of carbon particles in the carbon particles carrying the catalyst substance)} is from the inner polymer electrolyte membrane toward the surface of the electrode catalyst layer which is the outer side The ratio of fibrous material in the thickness direction of the electrode catalyst layer is a continuous or intermittent decrease and the electrode catalyst layer is formed by laminating at least two layers of electrode catalyst parts having different ratios of fibrous material. The largest of Divided by the smallest value of the ratio value of the fibrous material is characterized in that in the range of 1.3 to 26.

According to this configuration, the ratio of the fibrous material in the thickness direction of the catalyst electrode layer is improved, and the ratio of the fibrous material in the thickness direction is increased from the outside toward the polymer electrolyte membrane. The structure of the electrode catalyst layer changes from a dense to a sparse structure toward the molecular electrolyte membrane, and the water retention of the electrode catalyst layer is prevented without inhibiting the diffusivity of the reaction gas and the removal of water generated by the electrode reaction. It can be enhanced.
Furthermore, unlike the application of the conventional humidity control film and the response to low humidification by the formation of grooves on the surface of the electrode catalyst layer, no deterioration in power generation characteristics due to the increase in interface resistance is observed, and the conventional membrane electrode assembly The polymer electrolyte fuel cell comprising the membrane electrode assembly of the present invention exhibits a remarkable effect of showing high power generation characteristics even under low humidification conditions, as compared with the polymer electrolyte fuel cell comprising the above.

"1st Example"
First, an example based on the first embodiment will be described.
[Adjustment of catalyst ink]
25 mass% polymer electrolyte solution containing a platinum-supporting carbon catalyst having a mass density of 50 mass%, a fibrous material having an average fiber length of 1.5 μm, and a polymer electrolyte having an ion exchange capacity of 1.4 meq / g The mixture was mixed in a solvent and dispersed for 30 minutes in a planetary ball mill to prepare a first catalyst ink. In the first catalyst ink, the composition ratio of the carbon particles, the fibrous substance and the polymer electrolyte was 1: 0.2: 1.2 in mass ratio, as the composition ratio of the ratio of the fibrous substance is 0.2. .

25 mass% polymer electrolyte solution containing a platinum-supporting carbon catalyst having a mass density of 50 mass%, a fibrous material having an average fiber length of 1.5 μm, and a polymer electrolyte having an ion exchange capacity of 1.4 meq / g The mixture was mixed in a solvent, and dispersed for 30 minutes in a planetary ball mill, to prepare a second catalyst ink. The second catalyst ink has a composition ratio of a fibrous substance represented by {(mass of fibrous substance) / (mass of carbon particles in carbon particles carrying the catalytic substance)} is 1.2, The compounding ratio of the carbon particles to the fibrous substance to the polymer electrolyte was 1: 1.2: 2.2 in mass ratio.
The solvent of each catalyst ink was 1: 1 in the volume ratio of ultrapure water and 1-propanol. In addition, the solid content in each catalyst ink was adjusted to be 8% by mass.
As a result, a catalyst ink having a value obtained by dividing the largest value of the ratio of the fibrous material in the thickness direction of the electrode catalyst layer by the smallest value of the ratio of the fibrous material is 6.0 is obtained.

[Formation of electrode catalyst layer]
The prepared first catalyst ink was applied by a doctor blade method to a substrate of a polytetrafluoroethylene (PTFE) sheet, and dried at 70 ° C. for 2 minutes in an air atmosphere. Thereafter, a second catalyst ink was applied thereon in the same manner, and dried at 80 ° C. for 10 minutes in the air atmosphere to prepare an electrode catalyst layer having a two-layer structure. The coating amount per unit area of the first catalyst ink and the second catalyst ink was 1: 4 in mass ratio. The coating amount of the catalyst ink is adjusted so that the supported amount of platinum is 0.05 mg / cm ² as the fuel electrode (anode), and the supported amount of platinum is 0.2 mg / cm ² as the air electrode (cathode). Formed.
[Fabrication of membrane electrode assembly]
The substrate on which the electrode catalyst layer is formed is punched into 5 cm × 5 cm, disposed on both sides of the polymer electrolyte membrane 1, and hot pressed under the conditions of a transfer temperature of 140 ° C. and a transfer pressure of 5.0 × 10 ⁶ Pa. The joined body 11 was obtained.

Comparative Example
[Adjustment of catalyst ink]
25 mass including a platinum-supported carbon catalyst having a mass density of 50% by mass, a fibrous substance having an average fiber length of 1.5 μm, and a polymer electrolyte having an ion exchange capacity of 1.4 meq / g as a polymer electrolyte solution % Polymer electrolyte solution was mixed in a solvent, and dispersion treatment was performed for 30 minutes with a planetary ball mill to prepare a catalyst ink. The catalyst ink has a composition ratio of a fibrous substance represented by {(mass of fibrous substance) / (mass of carbon particles in carbon particles on which the catalytic substance is supported)} and a ratio of 1 to that of carbon particles and fibrous The compounding ratio of the substance to the polymer electrolyte was 1: 1: 2 in mass ratio. The solvent of the catalyst ink was 1: 1 in the volume ratio of ultrapure water and 1-propanol. In addition, the solid content in the catalyst ink was adjusted to 8% by mass.
Thus, a catalyst ink was obtained. This catalyst ink mixes all the components of the first catalyst ink and the second catalyst ink in the example at one time such that the mass ratio of the first catalyst ink to the second catalyst ink is 1: 4. And were obtained by dispersion.

[Formation of electrode catalyst layer]
The prepared catalyst ink was applied to a base material of a polytetrafluoroethylene (PTFE) sheet by a doctor blade method, and dried at 80 ° C. for 10 minutes in an air atmosphere. The coating amount of the catalyst ink is adjusted so that the supported amount of platinum is 0.05 mg / cm ² as the fuel electrode (anode), and the supported amount of platinum is 0.2 mg / cm ² as the air electrode (cathode). Formed.
[Fabrication of membrane electrode assembly]
The substrate on which each electrode catalyst layer is formed is punched out to 5 cm × 5 cm, and this electrode catalyst layer is disposed on both sides of the polymer electrolyte membrane, under conditions of a transfer temperature of 140 ° C. and a transfer pressure of 5.0 × 10 ⁶ Pa. Hot pressing was performed to obtain a membrane electrode assembly.

<Evaluation>
[Power generation characteristics]
A carbon paper is pasted together as a gas diffusion layer so as to sandwich each membrane electrode assembly obtained in the example and the comparative example, and installed in a power generation evaluation cell, and a current voltage measurement is performed using a fuel cell measuring device. went. The cell temperature at the time of measurement was 80 ° C., and the operating conditions were high humidification and low humidification shown below. In addition, hydrogen was used as the fuel gas, air was used as the oxidant gas, and flow control was performed with a constant utilization rate. The back pressure was 50 kPa.
[Operating conditions]
Condition 1 (high humidity): relative humidity anode 100% RH, cathode 100% RH
Condition 2 (low humidity): relative humidity anode 30% RH, cathode 30% RH

〔Measurement result〕
The membrane electrode assembly produced in the example exhibited superior power generation performance under the low humidification operating condition than the membrane electrode assembly produced in the comparative example. Moreover, the membrane electrode assembly produced in the Example had the power generation performance at the same level as that under the high humidification operation even under the low humidification operation condition. In particular, the power generation performance near the current density of 0.5 A / cm ² was improved. The cell voltage at a current density of 0.5 A / cm ² of the membrane electrode assembly produced in the example is 0.22 V compared to the cell voltage at a current density of 0.5 A / cm ² of the membrane electrode assembly produced in the comparative example. It showed high power generation characteristics.

From the results of the power generation characteristics of the membrane electrode assembly manufactured in the example and the membrane electrode assembly manufactured in the comparative example, the membrane electrode assembly of the example has a higher water retention, and the power generation characteristic under the low humidification operating condition is It was confirmed that the power generation characteristics equivalent to the high humidification operating conditions were exhibited.
Also, under the high humidification operating conditions, the cell voltage at a current density of 0.5 A / cm ² of the membrane electrode assembly produced in the example is 0.5 A / cm of the current density of the membrane electrode assembly produced in the comparative example. ^The power generation characteristics were 0.19 V higher than the cell voltage at ² .
From the results of the power generation characteristics of the membrane electrode assembly fabricated in the example and the membrane electrode assembly fabricated in the comparative example, the membrane electrode assembly fabricated in the example has high diffusivity of the reaction gas and is generated by the electrode reaction. It was confirmed that the removal of water was not inhibited.

"2nd Example"
Next, an example based on the second embodiment will be described.
[Adjustment of catalyst ink]
25 containing a platinum-supported carbon catalyst with a supported density of 50% by mass, a fibrous substance with an average fiber diameter of 100 nm fiberized using an electrospinning method, and a polymer electrolyte with an ion exchange capacity of 1.4 meq / g The mass% polymer electrolyte solution was mixed in a solvent, and dispersion treatment was performed for 30 minutes with a planetary ball mill to prepare a first catalyst ink. The first catalyst ink has a composition ratio of a fibrous material represented by {(mass of fibrous material) / (mass of carbon particles in carbon particles carrying the catalytic material)} is 0.1, The compounding ratio of the carbon particles, the fibrous substance and the polymer electrolyte was 1: 0.1: 0.4 in mass ratio.

25 containing a platinum-supported carbon catalyst with a supported density of 50% by mass, a fibrous substance with an average fiber diameter of 100 nm fiberized using an electrospinning method, and a polymer electrolyte with an ion exchange capacity of 1.4 meq / g The mass% polymer electrolyte solution was mixed in a solvent, and dispersion treatment was performed for 30 minutes in a planetary ball mill, to prepare a second catalyst ink. The second catalyst ink has a composition ratio of a fibrous substance represented by {(mass of fibrous substance) / (mass of carbon particles in carbon particles carrying the catalytic substance)} is 6.0, The compounding ratio of the carbon particles to the fibrous substance to the polymer electrolyte was 1: 0.6: 0.4 in mass ratio.
The solvent of each catalyst ink was 1: 1 in the volume ratio of ultrapure water and 1-propanol. In addition, the solid content in each catalyst ink was adjusted to be 8% by mass.
As a result, a catalyst ink having a value obtained by dividing the largest value of the ratio of the fibrous material in the thickness direction of the electrode catalyst layer by the smallest value of the ratio of the fibrous material is 4.0 is obtained.

[Formation of electrode catalyst layer]
The prepared first catalyst ink was applied by a doctor blade method to a substrate of a polytetrafluoroethylene (PTFE) sheet, and dried at 70 ° C. for 2 minutes in an air atmosphere. Thereafter, a second catalyst ink was applied thereon in the same manner, and dried at 80 ° C. for 10 minutes in the air atmosphere to prepare an electrode catalyst layer having a two-layer structure. The coating amount per unit area of the first catalyst ink and the second catalyst ink was 1: 4 in mass ratio. The coating amount of the catalyst ink is adjusted so that the supported amount of platinum is 0.05 mg / cm ² as the fuel electrode (anode), and the supported amount of platinum is 0.2 mg / cm ² as the air electrode (cathode). Formed.
[Fabrication of membrane electrode assembly]
The substrate on which the electrode catalyst layer is formed is punched into 5 cm × 5 cm, disposed on both sides of the polymer electrolyte membrane 1, and hot pressed under the conditions of a transfer temperature of 120 ° C. and a transfer pressure of 5.0 × 10 ⁶ Pa. The joined body 11 was obtained.

Comparative Example
[Adjustment of catalyst ink]
A platinum-supporting carbon catalyst having a supported density of 50% by mass, a fibrous substance having an average fiber diameter of 100 nm fiberized using an electrospinning method, and a high ion exchange capacity of 1.4 meq / g as a polymer electrolyte solution A 25 mass% polymer electrolyte solution containing a molecular electrolyte was mixed in a solvent, and dispersion treatment was performed for 30 minutes in a planetary ball mill, to prepare a catalyst ink. The catalyst ink has a composition ratio of a fibrous substance represented by {(mass of fibrous substance) / (mass of carbon particles in carbon particles on which the catalytic substance is supported)} and a ratio of 1 to that of carbon particles and fibrous The compounding ratio of the substance to the polymer electrolyte was 5: 1: 2 in mass ratio. The solvent of the catalyst ink was 1: 1 in the volume ratio of ultrapure water and 1-propanol. In addition, the solid content in the catalyst ink was adjusted to 8% by mass.
Thus, a catalyst ink was obtained. This catalyst ink mixes all the components of the first catalyst ink and the second catalyst ink in the example at one time such that the mass ratio of the first catalyst ink to the second catalyst ink is 1: 4. And were obtained by dispersion.

[Formation of electrode catalyst layer]
The prepared catalyst ink was applied to a base material of a polytetrafluoroethylene (PTFE) sheet by a doctor blade method, and dried at 80 ° C. for 10 minutes in an air atmosphere. The coating amount of the catalyst ink is adjusted so that the supported amount of platinum is 0.05 mg / cm ² as the fuel electrode (anode), and the supported amount of platinum is 0.2 mg / cm ² as the air electrode (cathode). Formed.
[Fabrication of membrane electrode assembly]
The substrate on which each electrode catalyst layer is formed is punched out to 5 cm × 5 cm, and this electrode catalyst layer is disposed on both sides of the polymer electrolyte membrane, under conditions of a transfer temperature of 120 ° C. and a transfer pressure of 5.0 × 10 ⁶ Pa Hot pressing was performed to obtain a membrane electrode assembly.

<Evaluation>
[Power generation characteristics]
A carbon paper is pasted together as a gas diffusion layer so as to sandwich each membrane electrode assembly obtained in the example and the comparative example, and installed in a power generation evaluation cell, and a current voltage measurement is performed using a fuel cell measuring device. went. The cell temperature at the time of measurement was 80 ° C., and the operating conditions were high humidification and low humidification shown below. In addition, hydrogen was used as the fuel gas, air was used as the oxidant gas, and flow control was performed with a constant utilization rate. The back pressure was 50 kPa.
[Operating conditions]
Condition 1 (high humidity): relative humidity anode 100% RH, cathode 100% RH
Condition 2 (low humidity): relative humidity anode 30% RH, cathode 30% RH

〔Measurement result〕
The membrane electrode assembly produced in the example exhibited superior power generation performance under the low humidification operating condition than the membrane electrode assembly produced in the comparative example. Moreover, the membrane electrode assembly produced in the Example had the power generation performance at the same level as that under the high humidification operation even under the low humidification operation condition. In particular, the power generation performance near the current density of 0.5 A / cm ² was improved. The cell voltage at a current density of 0.5 A / cm ² of the membrane electrode assembly produced in the example is 0.18 V compared to the cell voltage at a current density of 0.5 A / cm ² of the membrane electrode assembly produced in the comparative example. It showed high power generation characteristics.

From the results of the power generation characteristics of the membrane electrode assembly manufactured in the example and the membrane electrode assembly manufactured in the comparative example, the membrane electrode assembly of the example has a higher water retention, and the power generation characteristic under the low humidification operating condition is It was confirmed that the power generation characteristics equivalent to the high humidification operating conditions were exhibited.
Also, under the high humidification operating conditions, the cell voltage at a current density of 0.5 A / cm ² of the membrane electrode assembly produced in the example is 0.5 A / cm of the current density of the membrane electrode assembly produced in the comparative example. ^It showed a 0.15 V higher power generation characteristics than the cell voltage at ² .
From the results of the power generation characteristics of the membrane electrode assembly fabricated in the example and the membrane electrode assembly fabricated in the comparative example, the membrane electrode assembly fabricated in the example has high diffusivity of the reaction gas and is generated by the electrode reaction. It was confirmed that the removal of water was not inhibited.

DESCRIPTION OF SYMBOLS 1 ... polymer electrolyte membrane 2 ... electrode catalyst layer 2a ... electrode catalyst part 2b with a small ratio of fibrous material ... electrode catalyst part 3 with a large ratio of fibrous material ... ... electrode catalyst layer 3a ... an electrode with a small ratio of fibrous material Catalyst portion 3b: Electrode catalyst portion 4 having a large ratio of fibrous material: Gas diffusion layer 5: Gas diffusion layer 6: Air electrode (cathode)
7 ... Fuel electrode (anode)
8, 8a, 8b: gas flow channels 9, 9a, 9b: cooling water flow channels 10, 10a, 10b: separators 11: membrane electrode assembly 12: solid polymer fuel cell

Claims (5)

A membrane electrode assembly in which a polymer electrolyte membrane is sandwiched between a pair of electrode catalyst layers,
At least one of the electrode catalyst layers of the pair of electrode catalyst layers has catalyst supporting particles consisting of carbon particles supporting a catalyst, a fibrous substance, and a polymer electrolyte, and is represented by the following formula (1) What is claimed is: 1. A membrane / electrode assembly characterized in that the ratio of the fibrous material is continuously or intermittently decreased as it is separated from the polymer electrolyte membrane in the film thickness direction.
(Ratio of fibrous substances)
= {(Mass of fibrous material) / (mass of carbon particles in carbon particles carrying catalyst material)} (1)   The electrode catalyst layer is formed by laminating two or more layers of electrode catalyst portions different in the ratio of the fibrous substance, and the stacked two or more layers of electrode catalyst portions are separated from the polymer electrolyte membrane The membrane electrode assembly according to claim 1, wherein the ratio of the fibrous substance is smaller as the electrode catalyst portion. The fibrous substance is a conductive fiber having an average fiber length of 1 μm to 15 μm,
The value obtained by dividing the largest value of the ratio of the fibrous material in the film thickness direction of the electrode catalyst layer by the smallest value of the ratio of the fibrous material is within the range of 1.5 to 25. The membrane electrode assembly according to claim 1, wherein the membrane electrode assembly is characterized. The fibrous substance is a fiber having a proton conductivity of 50 nm to 1 μm in average fiber diameter,
The value obtained by dividing the largest value of the ratio of the fibrous material in the film thickness direction of the electrode catalyst layer by the smallest value of the ratio of the fibrous material is in the range of 1.3 to 26. The membrane electrode assembly according to claim 1, wherein the membrane electrode assembly is characterized.   A polymer electrolyte fuel cell comprising the membrane electrode assembly according to any one of claims 1 to 4.

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