JP2009129853A - Membrane electrode assembly, manufacturing method thereof, and solid polymer fuel cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a membrane electrode assembly which gets little damage to an electrolyte membrane by fiber conductor carriers and can obtain a high battery performance, to provide a manufacturing method thereof, and to provide a solid polymer fuel cell using the membrane electrode assembly. <P>SOLUTION: Fiber conductor carriers are molded in nano-pores by using a mold having nano-pores arranged almost perpendicular to a surface and the surface of the mold is melted and top ends of the fiber conductor carriers are exposed out from a surface of the mold, and a solution or a melt containing a solid polymer electrolyte (A) is coated on the surface of the mold to fix the top ends of the fiber conductor carriers by a fixing membrane composed of the solid polymer electrolyte (A), and the mold and the electrolyte membrane composed of a solid polymer electrolyte (B) are joined with the fixing membrane in-between, and the mold is removed completely and catalyst particles are carried on the surface of the fiber conductor carriers, and the fiber conductor carriers on which the catalyst particles are carried and a solid polymer electrolyte (C) are composed to obtain a membrane electrode assembly. The manufacturing method thereof and a solid polymer fuel cell using the membrane electrode assembly are provided. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a membrane electrode assembly, a method for producing the same, and a polymer electrolyte fuel cell. More specifically, the present invention relates to a membrane electrode junction using a carrier made of a fibrous conductive material as a carrier for supporting a catalyst. And a solid polymer fuel cell using such a membrane electrode assembly.

  In various electrochemical devices such as a polymer electrolyte fuel cell and a water electrolysis apparatus, the polymer electrolyte is used in the form of a membrane electrode assembly (MEA) in which a membrane is formed and electrodes are bonded to both sides thereof. . In the polymer electrolyte fuel cell, the electrode generally has a two-layer structure of a diffusion layer and a catalyst layer. The diffusion layer is for supplying reaction gas and electrons to the catalyst layer, and carbon fiber, carbon paper, or the like is used. The catalyst layer is a part that becomes a reaction field for electrode reaction, and is generally composed of a composite of an electrode catalyst and a solid polymer electrolyte.

  Electrocatalysts used in such various electrochemical devices conventionally include fine particles of noble metals such as Pt (Pt black and the like), those in which noble metal fine particles such as Pt are supported on a carbonaceous carrier such as carbon black, electrolytes A noble metal thin film formed on the surface of the film by a method such as plating or sputtering is used.

  However, noble metals such as Pt exhibit high catalytic activity and high catalytic activity stability, but are expensive and limited in terms of resources. For this reason, the electrode catalyst contributes to increase the cost of various electrochemical devices. In particular, since a fuel cell is used in a state where a large number of MEAs are stacked in order to obtain a predetermined output, the amount of electrode catalyst used per fuel cell is also large, which hinders the spread of fuel cells. .

In order to solve this problem, various proposals have heretofore been made.
For example, in Non-Patent Document 1,
(1) using a thermal decomposition method, on the Si substrate was supported Fe catalyst, density: 0.18 g / cm ^2, diameter: 20 nm, length: grown 40μm vertically aligned carbon nanotubes (CNT) ,
(2) By repeating injection / H ₂ reduction method using Pt (NO ₂ ) ₂ (NH ₃ ) ₂ solution, 0.09 mg / cm ² (anode) or 0.26-0.52 mg / to support Pt of cm ² (cathode),
(3) After depositing Pt, the surface of Pt / CNT is coated with Nafion (registered trademark) to form a CNT electrode.
(4) The CNT electrode is transferred to the electrolyte membrane by hot pressing.
A method for manufacturing an MEA is disclosed.
In the same document,
(A) MEA (CNT-MEAs) using a Pt / CNT electrode is excellent in gas diffusivity, so that a higher limit current density can be obtained compared to a conventional MEA using a Pt / C electrode.
(B) The proton conduction resistance of the catalyst layer of CNT-MEAs is significantly smaller than that of conventional MEA, and
(C) The total ohmic resistance of the catalyst layer of CNT-MEAs is small under both wet and dry conditions,
Points are listed.

In addition, Patent Document 1 is not an electrode for a fuel cell,
(1) Anodized alumina film having a plurality of nanoholes oriented substantially perpendicular to the film surface is formed on the glass substrate surface,
(2) Using a mixed gas of propylene and butadiene, vapor-grown carbon nanofibers in the nanoholes of the anodized alumina film,
(3) Carbon deposited on the surface of the anodized alumina is removed by oxygen plasma etching.
(4) Anodized alumina is removed using an aqueous sodium hydroxide solution.
A method for producing a carbonaceous nanostructure is disclosed.
In the same document,
(A) When a mixed gas of propylene and butadiene is used as a raw material gas, the thermal decomposability of the carbon compound is improved, so that carbon is vapor-phase grown in nanoholes of anodized alumina at a lower temperature without using a catalytic metal. Points that can and
(B) Since the carbonaceous nanostructure obtained by such a method can emit a sufficient current at a low driving voltage, it is suitable as an electron gun or an electron-emitting device for a field emission display,
Is described.

ECS Transactions, 3 (1) 277-284 (2006) JP 2007-243477 A

The CNT has a three-dimensional structure in which a graphene sheet corresponding to one layer of graphite (a sheet in which carbon atoms are arranged in a hexagonal network) is rolled into a cylindrical shape. The CNT includes a single-walled CNT made of one cylindrical graphene sheet and a multi-walled CNT in which a plurality of cylindrical graphene sheets are concentrically overlapped. In addition, the tip of the synthesized unprocessed CNT usually has a structure closed with a hemispherical graphite layer called “cap”.
CNTs have a diameter on the order of nm and a length on the order of μm to cm, have an extremely large aspect ratio, and have a feature that the radius of curvature of the tip is as small as several nm to several tens of nm. CNT is mechanically tough, has excellent chemical and thermal stability, and is characterized by being both a metal and a semiconductor depending on the helical structure of the cylindrical portion.

The catalyst layer obtained by orienting such CNTs perpendicularly to the substrate surface and transferring it to the electrolyte membrane has higher gas diffusivity, proton conductivity and electron conductivity than the catalyst layer produced by the conventional method. Indicates. Moreover, since the gas diffusibility is high, high performance is exhibited in a high current region.
However, in order to join vertically aligned CNTs to the electrolyte membrane, thermocompression bonding with a predetermined temperature and pressure is required. For this reason, if the conditions are not appropriate, there is a concern that CNT penetrates the electrolyte membrane and shorts. In addition, when transferring CNTs to the electrolyte membrane, the CNT growth catalyst may become a contaminant and remain in the catalyst layer, which may cause a reduction in fuel cell performance.

On the other hand, since the CNT synthesis technique using a template such as anodized alumina does not necessarily require the use of a catalyst, there is little possibility that the performance of the fuel cell is reduced due to the CNT synthesis catalyst. In addition, by changing the template, it is possible to control the structure of the CNT, such as the diameter of the CNT and the number of CNTs per unit area.
However, if the CNTs are transferred to the electrolyte membrane while leaving the carbon deposition layer deposited on the mold surface, the orientation structure of the CNTs can be maintained, but the diffusion of the reaction gas is inhibited by the carbon deposition layer, so that it does not hold as an electrode. . On the other hand, when the carbon deposition layer is removed, there is a problem that the alignment structure cannot be maintained because the CNTs fall apart in the process of removing the template.

The problem to be solved by the present invention is that even if a carrier made of a fibrous conductive material such as CNT is used as a carrier for supporting an electrode catalyst, the electrolyte membrane is damaged by the fibrous conductive carrier. It is an object of the present invention to provide a membrane electrode assembly and a method for producing the same, and a polymer electrolyte fuel cell using such a membrane electrode assembly.
In addition, another problem to be solved by the present invention is membrane electrode bonding in which gas diffusivity is not hindered even when a fibrous conductive support is used as a support for supporting an electrode catalyst. It is an object of the present invention to provide a solid polymer fuel cell using such a membrane electrode assembly.
Furthermore, another problem to be solved by the present invention is that it is possible to maintain the orientation structure of the fibrous conductive support, and the membrane electrode assembly having high battery performance, the method for producing the same, and such a membrane. An object of the present invention is to provide a polymer electrolyte fuel cell using an electrode assembly.

In order to solve the above problems, a method for producing a membrane electrode assembly according to the present invention includes:
Using a template having nanopores arranged substantially perpendicular to the surface, a fibrous conductive carrier forming step of forming a fibrous conductive carrier in the nanopores;
An exposure step of dissolving only the surface of the mold and exposing the tip of the fibrous conductive carrier from the surface of the mold;
A fixed film forming step of applying a solution or melt containing the solid polymer electrolyte (A) to the mold surface and fixing the tip of the fibrous conductive carrier with a fixed film made of the solid polymer electrolyte (A); ,
A bonding step of bonding the template and an electrolyte membrane made of a solid polymer electrolyte (B) through the fixed coating;
A mold removing step of removing the mold and exposing the fibrous conductive carrier;
A catalyst supporting step for supporting catalyst particles on the surface of the fibrous conductive support;
A composite step of combining the fibrous conductive support on which the catalyst particles are supported and a solid polymer electrolyte (C);
The main point is that
Moreover, the membrane electrode assembly which concerns on this invention consists of what was obtained by the method which concerns on this invention.
The gist of the polymer electrolyte fuel cell according to the present invention is that the membrane electrode assembly according to the present invention is used.

  After forming the fibrous conductive carrier in the nanopores of the template, only the surface of the template is dissolved, and only the tip of the fibrous conductive carrier is fixed with the fixed coating made of the solid polymer electrolyte (A), the fiber The fibrous conductive carrier can be transferred to the electrolyte membrane while maintaining the orientation structure of the fibrous conductive carrier. In addition, since the tip of the fibrous conductive carrier is covered with the fixed coating, damage to the electrolyte membrane due to the fibrous conductive carrier can be suppressed when joining the electrolyte membrane. Furthermore, since there is no carbon deposition layer at the interface between the fibrous conductive support and the electrolyte membrane, gas diffusivity is not hindered.

Hereinafter, an embodiment of the present invention will be described in detail.
[1. Manufacturing method of membrane electrode assembly]
The manufacturing method of the membrane electrode assembly (MEA) according to the present invention includes a fibrous conductive carrier forming step, an exposing step, a fixing step, a joining step, a template removing step, a catalyst supporting step, a composite step, It has.

[1.1. Fibrous conductive carrier forming step]
The fibrous conductive carrier forming step is a step of forming a fibrous conductive carrier in the nanopores using a template having nanopores arranged substantially perpendicular to the surface.
The term “substantially perpendicular to the surface” includes not only the case where the nanopores are oriented completely perpendicular to the surface, but also the case where the nanopores are slightly tilted. This represents a state in which nanopores are arranged in a specific direction.
“Fibrous conductive support” refers to a support made of carbonaceous fibers (carbon nanotubes (CNT), carbon nanofibers (CNF), etc.) and having electron conductivity. In particular, vapor-grown CNT is suitable as a fibrous conductive support because an electrode excellent in gas diffusibility, proton conductivity, and electron conductivity can be obtained.

Examples of the template having nanopores arranged substantially perpendicular to the surface include anodized alumina and anodized silicon.
Anodized alumina is produced by immersing an aluminum substrate or a substrate having an aluminum thin film formed on the surface thereof in an acidic solution (for example, a 20% aqueous sulfuric acid solution) and applying a voltage using the aluminum substrate or the aluminum thin film as an anode. be able to. At this time, when the applied voltage is controlled, the diameter and density of the nanopores can be arbitrarily controlled. In the case of anodized alumina, nanopores having a diameter of about 10 to 300 nm and a density of about 5 × 10 ⁸ to 2 × 10 ¹¹ pieces / cm ² can be formed by controlling the applied voltage.
Anodized silicon can be produced by electrolytic polishing at a low current density in an HF solution using a silicon substrate as an anode. At this time, the nanopore structure can be controlled by controlling the current density. In the case of anodized silicon, an irregular branched structure having a pore diameter of 10 to 500 nm can be formed by controlling the current density.

The fibrous conductive support is obtained by supplying a carbon source to the mold surface and thermally decomposing the carbon source on the mold surface (vapor phase growth method). When such a vapor phase growth method is used, a fibrous conductive support can be formed in the nanopore. In the nanopores, a catalyst for synthesis of the fibrous conductive support may be supported in advance. However, if a template having nanopores is used, the fibrous pores are not contained in the nanopores without using the catalyst for synthesis. A conductive carrier can be formed.
As the carbon source, hydrocarbon gases such as methane, ethane, and propane, monoolefin compounds such as propylene, and diolefin compounds such as butadiene can be used. In particular, a mixture of a monoolefin compound and a diolefin compound is particularly suitable as a carbon source because a fibrous conductive support can be formed in the nanopores at a relatively low temperature.

Specifically, the fibrous conductive carrier is formed as follows. That is, the template is placed in a reaction vessel and the pressure is reduced to normal pressure or a predetermined pressure (for example, 10 ⁻⁵ Torr (1.3 × 10 ⁻³ Pa or less)). Next, the mold is heated up to the synthesis temperature using a heating device. The synthesis temperature is preferably 500 to 900 ° C. When the mold has reached the synthesis temperature, if a carrier gas and a carbon source are flowed while adjusting the pressure at a predetermined flow rate ratio for several minutes to several hours using a reaction gas supply device, the fibrous conductive material flows into the nanopores. Sex carriers can be grown.
When the carbon source is thermally decomposed on the mold surface, not only a fibrous conductive support is formed in the nanopores, but also a carbon deposition layer is usually formed on the mold surface. The carbon deposition layer is removed using a method such as oxygen plasma etching.

[1.2. Exposure process]
The exposing step is a step of dissolving only the surface of the mold and exposing the tip of the fibrous conductive carrier from the surface of the mold.
The tip of the fibrous conductive carrier is exposed by immersing only the mold in a solution that can be dissolved. For example, when anodized alumina is used as the template, it is preferable to use a sodium hydroxide aqueous solution, an HF aqueous solution, phosphoric acid, or the like as the template solution. Further, when anodized silicon is used as a template, it is preferable to use an HF aqueous solution or the like as the template solution.
By optimizing the concentration of the dissolving solution, the immersion time of the template in the dissolving solution, the temperature of the dissolving solution, etc., only the surface of the template is dissolved, and the tip of the fibrous conductive carrier can be exposed from the surface of the template.
The exposed length of the tip of the fibrous conductive carrier can be arbitrarily selected according to the purpose. In general, if the exposed length is too short, the fibrous conductive carrier is not sufficiently fixed by the fixed coating. On the other hand, when the exposed length is too long, the disorder of the orientation structure of the fibrous conductive support increases, and the electrode performance may be deteriorated.

[1.3. Fixed film formation process]
The fixed film forming process is a process in which a solution or melt containing the solid polymer electrolyte (A) is applied to the mold surface, and the tip of the fibrous conductive carrier is fixed with a fixed film made of the solid polymer electrolyte (A). is there.
The solid polymer electrolyte (A) used for the fixed coating is usually the same material as the solid polymer electrolyte (B) used for the electrolyte membrane, but a different material may be used.

The solid polymer electrolyte (A) is composed of a hydrocarbon electrolyte containing a C—H bond in a polymer chain and no C—F bond, and a fluorine electrolyte containing a C—F bond in a polymer chain. Either may be sufficient. Further, the fluorine-based electrolyte may be a partial fluorine-based electrolyte containing both C—H bonds and C—F bonds in the polymer chain, or contains C—F bonds in the polymer chain, and A perfluorinated electrolyte containing no C—H bond may be used.
In addition to the fluorocarbon structure (—CF ₂ —, —CFCl—), the fluorine-based electrolyte includes a chlorocarbon structure (—CCl ₂ —) and other structures (eg, —O—, —S—, —C ( ═O) —, —N (R) —, etc. provided that “R” may be an alkyl group). The molecular structure of the solid polymer electrolyte (A) is not particularly limited, and may be linear or branched, or may have a cyclic structure.

  Further, the type of acid group provided in the solid polymer electrolyte (A) is not particularly limited. Examples of the acid group include a sulfonic acid group, a carboxylic acid group, a phosphonic acid group, and a sulfonimide group. The solid polymer electrolyte (A) may contain only one of these electrolyte groups, or may contain two or more. Furthermore, these acid groups may be directly bonded to the linear solid polymer compound, or may be bonded to either the main chain or the side chain of the branched solid polymer compound.

Specifically, as the hydrocarbon electrolyte,
(1) Polyamide, polyacetal, polyethylene, polypropylene, acrylic resin, polyester, polysulfone, polyether, etc. in which an acid group such as a sulfonic acid group is introduced into any of the polymer chains, and derivatives thereof (aliphatic hydrocarbons) System electrolyte),
(2) Polystyrene having an acid group such as a sulfonic acid group introduced into any of the polymer chains, polyamide having an aromatic ring, polyamideimide, polyimide, polyester, polysulfone, polyetherimide, polyethersulfone, polycarbonate, and the like, and These derivatives (partial aromatic hydrocarbon electrolytes),
(3) Polyetheretherketone, polyetherketone, polysulfone, polyethersulfone, polyimide, polyetherimide, polyphenylene, polyphenylene ether, polycarbonate, in which an acid group such as a sulfonic acid group is introduced into any of the polymer chains, Polyamide, polyamideimide, polyester, polyphenylene sulfide, etc., and derivatives thereof (fully aromatic hydrocarbon electrolytes),
and so on.

Further, as the partial fluorine-based electrolyte, specifically, a polystyrene-graft-ethylenetetrafluoroethylene copolymer (hereinafter referred to as “PS”) in which an acid group such as a sulfonic acid group is introduced into any of the polymer chains. -G-ETFE "), polystyrene-graft-polytetrafluoroethylene, and derivatives thereof.
Further, as the perfluorinated electrolyte, specifically, Nafion (registered trademark) manufactured by DuPont, Aciplex (registered trademark) manufactured by Asahi Kasei Co., Ltd., Flemion (registered trademark) manufactured by Asahi Glass Co., Ltd., etc. There are derivatives.
Among these, fluorinated electrolytes, particularly perfluorinated electrolytes, have a C—F bond in the polymer chain and are excellent in oxidation resistance. Therefore, if the present invention is applied thereto, A membrane electrode assembly and a polymer electrolyte fuel cell excellent in oxidation resistance and durability can be obtained.

  The fixed coating is formed by dissolving the solid polymer electrolyte (A) or a derivative thereof in a suitable solvent to form a solution, applying this solution to the surface of the mold exposing the tip of the fibrous conductive carrier, This is done by removing. Alternatively, the solid polymer electrolyte (A) or a derivative thereof may be melted, and the melt may be applied to the surface of the mold and solidified.

[1.4. Joining process]
A joining process is a process of joining a casting_mold | template and the electrolyte membrane which consists of a solid polymer electrolyte (B) through a fixed film.
The solid polymer electrolyte (B) used for the electrolyte membrane may be the same material as the solid polymer electrolyte (A) used for the fixed coating, or may be a different material. Since the details of the solid polymer electrolyte (B) are the same as those of the solid polymer electrolyte (A), the description thereof is omitted.
The joining is performed by superposing and hot pressing the mold and the electrolyte membrane through a fixed coating. Hot press conditions are not particularly limited, and optimum conditions are selected according to the materials of the solid polymer electrolytes (A) and (B). Alternatively, it is possible to superimpose in a swollen state with an alcohol such as 1-propanol and then dry.

[1.5. Mold removal process]
The mold removing step is a step of removing the mold and exposing the fibrous conductive carrier.
For removing the template, a solution similar to the solution used in the exposure step can be used. The dissolution conditions are not particularly limited as long as the template can be completely dissolved.

[1.6. Catalyst loading process]
The catalyst supporting step is a step of supporting catalyst particles on the surface of the fibrous conductive support.
In the present invention, the composition of the catalyst is not particularly limited, and any material that contributes to the electrode reaction can be used as the catalyst. Specifically, as a catalyst,
(1) noble metals such as platinum, palladium, rhodium, ruthenium, iridium, gold, silver, or alloys containing any two or more thereof,
(2) An alloy of one or more kinds of noble metals and one or more kinds of transition metal elements made of cobalt, nickel, iron, chromium, etc.
and so on.

The particle size of the catalyst particles affects the fuel cell performance. In general, when the particle size of the catalyst particles becomes too small, the catalytic activity is reduced. On the other hand, when the particle size of the catalyst particles becomes too large, the specific surface area of the catalyst decreases, and the utilization factor of the catalyst decreases. Accordingly, it is preferable to select an optimum particle size of the catalyst particles according to the composition of the catalyst particles, required characteristics, and the like.
In addition, the amount of catalyst supported on the surface of the fibrous conductive support also affects the fuel cell performance. In general, if the loading density of catalyst particles is too low, a high output cannot be obtained. In order to obtain a high output, the loading density of the catalyst particles is preferably as large as possible.

Specifically, the catalyst is supported as follows.
[1.6.1. Hydrophilization process]
First, the surface of the fibrous conductive carrier is subjected to a hydrophilic treatment (a hydrophilic treatment step). Hydrophilic treatment is not always necessary, but if the fibrous conductive support is hydrophilized in advance, there is an advantage that the catalyst can be supported very easily.
Any hydrophilic treatment may be used as long as the surface of the fibrous conductive carrier can be uniformly hydrophilized. Specific examples of the hydrophilic treatment method include UV ozone treatment, water plasma treatment, oxygen plasma treatment, and the like. In particular, the UV ozone treatment is convenient and can be hydrophilized without being affected by the water repellency of the fibrous conductive carrier, and thus is suitable as a hydrophilization treatment method.

[1.6.2. Immersion process]
Next, if necessary, the surface of the fibrous conductive carrier is hydrophilized, and then the electrolyte membrane to which the fibrous conductive carrier is bonded is immersed in a solution in which catalytic metal ions are dissolved in a polar solvent ( Dipping process). Thereby, a solution penetrate | invades in the space | gap of a fibrous conductive support | carrier.
Water, ethanol, methanol, 1-propanol, 2-propanol, butanol, ethylene glycol, propylene glycol, or the like can be used as the polar solvent for dissolving the catalytic metal ion. In particular, water is particularly suitable as a solvent because there is no risk of poisoning the catalyst metal.

The noble metal source and the transition metal source only need to be soluble in the polar solvent described above.
Specific examples of the noble metal source include the following. These may be used alone or in combination of two or more.
(1) Hexachloroplatinum (IV) acid hexahydrate, dinitrodiammineplatinum (II), hexaammineplatinum (IV) chloride, tetraammineplatinum (II) chloride, bis (acetylacetonato) platinum (II), etc. Pt compound.
(2) Pd compounds such as palladium sulfate, palladium chloride, palladium nitrate, dinitrodiammine palladium (II), bis (acetylacetonato) palladium (II), and trans-dichlorobis (triphenylphosphine) palladium (II).
(3) Rhodium chloride, rhodium sulfate, rhodium nitrate, rhodium acetate (II), tris (acetylacetonato) rhodium (III), chlorotris (triphenylphosphine) rhodium (I), acetylacetonatodicarbonyl rhodium (I), Rh compounds such as tetracarbonyldi-μ-chlorodirhodium (I).
(4) Ru compounds such as ruthenium chloride, ruthenium nitrate, dodecacarbonyltriruthenium (0), and tris (acetylacetonato) ruthenium (III).
(5) Ir compounds such as hexachloroiridium (IV) acid hexahydrate, dodecacarbonyltetriiridium (0), carbonylchlorobis (triphenylphosphine) iridium (I), tris (acetylacetonato) iridium (III) and the like.
(6) Au compounds such as hexachlorogold (IV) acid hexahydrate and gold gold cyanide.
(7) Ag compounds such as silver nitrate and silver cyanide.

Specific examples of the transition metal source include the following. These may be used alone or in combination of two or more.
(1) Nitrates such as Co (NO ₃ ) ₂ , Ni (NO ₃ ) ₂ , Fe (NO ₃ ) ₂ , and Cr (NO ₃ ) ₂ .
(2) Hydroxides such as Co (OH) ₂ .
(3) Sulfates such as CoSO ₄ , NiSO ₄ , FeSO ₄ , CrSO ₄ .
(4) Chlorides such as CoCl ₂ , NiCl ₂ , FeCl ₂ , and CrCl ₂ .
Among these, nitrates and sulfates are particularly suitable as transition metal sources because they are highly soluble in polar solvents, are not easily oxidized even under anaerobic conditions, and do not contain ions that cause catalyst poisoning.

  In general, if the concentration of the noble metal source and the transition metal source in the solution is too low, the catalyst particle deposition reaction becomes inefficient. On the other hand, if the concentrations of the noble metal source and the transition metal source are too high, it becomes difficult to deposit catalyst particles uniformly and finely. Therefore, it is preferable to select the optimum concentration of the noble metal source and the transition metal source dissolved in the solvent according to the kind of the noble metal source and the transition metal source, the composition of the catalyst particles, and the like. The amount of the solution may be an amount sufficient to immerse the template / electrolyte membrane assembly.

[1.6.3 Reduction step]
Next, the catalytic metal ions injected into the fibrous conductive support are reduced to deposit catalyst particles on the surface of the fibrous conductive support (reduction step).
As the reduction method, a method (wet reduction method) is used in which the template / electrolyte membrane assembly is immersed in a solution containing catalytic metal ions and then a reducing agent is added to the solution to reduce the catalytic metal ions.

When the catalytic metal ion is wet-reduced, an optimum reducing agent is selected according to the type of the catalytic metal ion. Specific examples of the reducing agent include ethanol, methanol, formic acid, sodium borohydride (NaBH ₄ ), hydrazine, ethylene glycol, and propylene glycol. These may be used alone or in combination of two or more.

When the reducing agent has a relatively strong reducing power, the reduction reaction proceeds only by adding the reducing agent to the solution. However, it is not preferable that the reducing agent has too strong reducing power because the probability that the catalyst particles precipitate in the liquid is higher than the surface of the fibrous conductive support.
On the other hand, when a reducing agent having a relatively low reducing power is used, the reduction reaction hardly proceeds only by adding the reducing agent to the solution. Therefore, in such a case, it is preferable to increase the reaction rate of the reduction reaction by increasing the temperature of the solution.
In both cases of using a reducing agent having a relatively strong reducing power and using a reducing agent having a relatively low reducing power, the reduction reaction is performed while convection of catalytic metal ions in the presence of the reducing agent. preferable. When the reduction reaction is performed while convection is performed, the noble metal source and the transition metal source are forcibly supplied toward the porous carbon film, so that the catalyst particles can be efficiently supported on the surface of the fibrous conductive support.
In particular, when a reducing agent having relatively weak reducing power is used and the temperature is raised while convection with a solution containing catalytic metal ions in the presence of the reducing agent, the catalyst particles are uniformly and finely formed on the surface of the fibrous conductive support. It can be supported. Suitable reducing agents for such methods include ethanol, methanol, ethylene glycol, propylene glycol and the like.

The temperature, reaction time, amount of reducing agent used during the reduction reaction, etc. should be optimized according to the type of noble metal source, transition metal source and reducing agent used, the concentration of noble metal source and transition metal source in the solution, etc. select. In general, the catalyst particles can be uniformly and finely supported on the surface of the fibrous conductive support when the reduction reaction proceeds under mild conditions.
In addition, the reduction reaction rate can be controlled by optimizing the amount and timing of use of the reducing agent. Generally, as the amount of the reducing agent used is relatively small, the reduction reaction proceeds more slowly, so that the catalyst particle diameter can be reduced. On the other hand, when the reduction reaction proceeds to some extent and the concentration of catalytic metal ions remaining in the solution becomes low, the reaction rate of the reduction reaction becomes extremely slow. Therefore, a relatively small amount of reducing agent (specifically, 1 to 20% of the total amount of reducing agent used) is added at the beginning of the reaction to promote nucleation of the catalyst particles, When a reducing agent is added, catalyst particles having a small particle size can be produced efficiently.

[1.7. Combined process]
The compounding step is a step of compounding the fibrous conductive support on which the catalyst particles are supported and the solid polymer electrolyte (C).
The solid polymer electrolyte (C) is a so-called electrolyte in the catalyst layer, and is composited so as to cover the surfaces of the catalyst particles. The solid polymer electrolyte (C) used as the electrolyte in the catalyst layer is usually the same material as the solid polymer electrolyte (B) used for the electrolyte membrane, but a different material may be used. Since the details of the solid polymer electrolyte (c) are the same as those of the solid polymer electrolyte (A), the description thereof is omitted.

  Specifically, the composite of the solid polymer electrolyte (C) is prepared by dissolving the solid polymer electrolyte (C) in an appropriate solvent and dropping this solution on a fibrous conductive carrier carrying a catalyst. This is done by removing the solvent. Thereby, the MEA according to the present invention is obtained.

In addition, although the electrode which comprises MEA may consist only of a catalyst layer, it usually takes a two-layer structure of a catalyst layer and a diffusion layer. In the MEA according to the present invention, when the electrode has a two-layer structure, a diffusion layer is further bonded or pressure-bonded on the catalyst layer made of catalyst / fibrous conductive support / solid polymer electrolyte (C). Good.
The diffusion layer is for supplying and receiving a reaction gas to the catalyst layer at the same time as transferring electrons to and from the catalyst layer. Generally, carbon paper, carbon cloth, or the like is used for the diffusion layer. In order to increase water repellency, a surface of carbon paper or the like coated with a mixture of water repellent polymer powder such as polytetrafluoroethylene and carbon powder (water repellent layer) is used as a diffusion layer. Also good.

[2. MEA and polymer electrolyte fuel cell]
The MEA according to the present invention is obtained by the method according to the present invention. The gist of the polymer electrolyte fuel cell according to the present invention is that the MEA according to the present invention is used. Since the MEA manufacturing method according to the present invention is as described above, the description thereof is omitted.
In the MEA according to the present invention, the fibrous conductive carrier having an oriented structure and the electrolyte membrane are joined via the fixed coating, so that the electrolyte membrane is less damaged by the fibrous conductive carrier. Therefore, the cross leak probability can be 5% or less.
The “cross leak probability” refers to the ratio (%) of the number of MEAs in which cross leak has occurred to the total number of MEAs manufactured under the same conditions. Further, “a cross leak has occurred” means that the offset current from the origin of the double layer capacitance region at the time of CV measurement is 5 mA / cm ² or more.

[3. MEA, production method thereof, and action of polymer electrolyte fuel cell]
FIG. 1 shows an example of a method for producing an electrode when anodized alumina is used as a template.
First, a voltage is applied using an aluminum substrate as an anode to form an anodized alumina coating having nanopores vertically aligned on the surface of the aluminum substrate. Using this anodized alumina coating as a mold, a carbon source is supplied to the mold surface, and the carbon source is thermally decomposed (fibrous conductive support forming step). Thereby, a carbonaceous fibrous conductive support is formed in the nanopore. The carbon deposit layer formed on the outer surface of the mold is removed by an appropriate method.

  Next, the mold is immersed in an aqueous NaOH solution to partially dissolve the alumina, and the tip of the fibrous conductive carrier is exposed from the mold surface (exposure process). Next, an electrolyte solution is applied to the mold surface to volatilize the solvent (fixed film forming step). Further, the fixed coating and the electrolyte membrane are overlapped and both are joined by hot pressing (joining step). After the joining, the aluminum substrate and the anodized alumina are completely removed (template removing step). An electrode catalyst such as platinum is supported on the exposed surface of the fibrous conductive support (catalyst support step), and further combined with the electrolyte in the catalyst layer (composite step).

In the present invention, since the tip of the fibrous conductive carrier is fixed with the fixed coating, the fibrous conductive carrier does not fall apart even if the template is removed. Therefore, the fibrous conductive carrier can be transferred to the electrolyte membrane while maintaining the orientation structure of the fibrous conductive carrier.
In addition, since the tip of the fibrous conductive carrier is covered with the fixed coating, damage to the electrolyte membrane due to the fibrous conductive carrier can be suppressed when joining the electrolyte membrane. Therefore, the cross leak probability can be greatly reduced as compared with the conventional method.
Furthermore, since there is no carbon deposition layer at the interface between the fibrous conductive support and the electrolyte membrane, gas diffusivity is not hindered. In addition, since the electrode and the electrolyte membrane are securely bonded via the fixed coating, the bondability between the electrode (catalyst layer) and the electrolyte membrane is improved, and the battery performance is high.

(Example 1, Comparative Examples 1-2)
[1. Production of MEA]
Regarding the production of CNTs using an anodized alumina mold, Patent Document 1 was referred to. The size of the anodized alumina mold was 1 × 1 cm ² . After generating CNTs in the nanopores by vapor deposition, the carbon deposition layer adhering to the mold surface was removed by oxygen plasma. Further, the surface of the anodized alumina mold was partially dissolved by treating with an aqueous NaOH solution, and a Nafion (registered trademark) solution was dropped and dried thereon. This was hot-pressed on a Nafion (registered trademark) film under predetermined conditions. Further, the template / electrolyte membrane assembly was immersed in an aqueous NaOH solution to remove all the template.
Subsequently, the joined body was immersed in a dinitrodiammine Pt nitric acid solution and subjected to reduction treatment in an aqueous NaBH ₄ solution to carry Pt. For the purpose of removing Na, it was immersed in an aqueous solution of H ₂ SO _{4 at} 80 ° C., sufficiently washed with water, and dried at room temperature. A predetermined amount of 5% Nafion (registered trademark) solution was dropped on the Pt / CNT layer, and then dried at room temperature (completed cathode catalyst layer).
Further, an anode catalyst layer made of Pt / C was prepared using a known method, and transferred to the other surface of the electrolyte membrane by hot pressing (completion of MEA (Example 1)).
As a comparison, MEA (Comparative Examples 1 and 2) was produced using the method described in Non-Patent Document 1.

[2. Test method]
[2.1. Battery performance]
Battery performance was evaluated using the obtained MEA. The test conditions were cell temperature: 80 ° C., gas pressure: 0.2 MPa, H ₂ : 0.5 L / min, Air: 2.0 L / min, and full humidification conditions. The battery performance was indicated by “◯” when the current value at 0.6 V exceeded 1 A / cm ² and “Δ” when inferior.
[2.2. Cross leak probability]
For each of the produced 10 MEAs, the presence or absence of cross leak was evaluated, and the cross leak probability was calculated. Test conditions are: cell temperature: 80 ° C., gas pressure: 0.2 MPa, H ₂ : 0.5 L / min, N ₂ : 1.0 L / min, full humidification condition, potential range: 0.09 to 1.2 V, Potential speed: 50 mV / s.

[3. result]
Table 1 shows the results. In the case of MEA (Comparative Example 2) hot-pressed under the condition of 120 ° C. × 10 min using the method described in Non-Patent Document 1, the cross leak probability was good, but the battery performance was lowered. This is presumably because the bonding between the CNT and the electrolyte membrane is insufficient and the proton conduction resistance at the interface is large. On the other hand, in the case of MEA hot-pressed under conditions of 140 ° C. × 10 min (Comparative Example 1), the battery performance was good, but the cross leak probability increased. This is probably because a part of the CNT penetrated the electrolyte membrane by hot pressing at a higher temperature.
On the other hand, in the case of MEA (Example 1) produced by the method according to the present invention, the cross leak probability is good and the battery performance is high. This is because the tip of the fibrous conductive carrier is fixed with a fixed coating in advance, so that damage to the electrolyte membrane due to the fibrous conductive carrier is suppressed, and the fibrous conductive carrier maintains the orientation structure. This is probably because it is transferred to the electrolyte as it is.

  Although the embodiments of the present invention have been described in detail above, the present invention is not limited to the above-described embodiments, and various modifications can be made without departing from the gist of the present invention.

  The membrane electrode assembly and the production method thereof according to the present invention are particularly suitable as an MEA used in a polymer electrolyte fuel cell and the production method thereof, but the application of the present invention is not limited thereto. MEA for various electrochemical devices (for example, solar cell, lithium battery, water electrolysis device, hydrohalic acid electrolysis device, salt electrolysis device, oxygen and / or hydrogen concentrator, humidity sensor, gas sensor, photocatalyst, etc.) and its It can be used as a manufacturing method.

It is process drawing which shows the preparation methods of the electrode which used the anodized alumina as a casting_mold | template.

Claims (4)

Using a template having nanopores arranged substantially perpendicular to the surface, a fibrous conductive carrier forming step of forming a fibrous conductive carrier in the nanopores;
An exposure step of dissolving only the surface of the mold and exposing the tip of the fibrous conductive carrier from the surface of the mold;
A fixed film forming step of applying a solution or melt containing the solid polymer electrolyte (A) to the mold surface and fixing the tip of the fibrous conductive carrier with a fixed film made of the solid polymer electrolyte (A); ,
A bonding step of bonding the template and an electrolyte membrane made of a solid polymer electrolyte (B) through the fixed coating;
A mold removing step of removing the mold and exposing the fibrous conductive carrier;
A catalyst supporting step for supporting catalyst particles on the surface of the fibrous conductive support;
A composite step of combining the fibrous conductive support on which the catalyst particles are supported and a solid polymer electrolyte (C);
The manufacturing method of the membrane electrode assembly provided with this.   A membrane electrode assembly obtained by the method according to claim 1.   The membrane electrode assembly according to claim 2, wherein the cross leak probability is 5% or less.   A polymer electrolyte fuel cell using the membrane electrode assembly according to claim 2.

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