JP5831009B2 - MICROSTRUCTURE MATERIAL, PROCESS FOR PRODUCING THE SAME, AND MEMBRANE ELECTRODE ASSEMBLY FOR FUEL CELL - Google Patents

Description

  The present invention relates to a microstructure material, a method for producing the same, and a membrane electrode assembly for a fuel cell. More specifically, the present invention comprises an array of carbon-containing fibers whose entire surface or part is coated with an ionomer, and contains carbon. The present invention relates to a microstructure material having voids between fibers, a manufacturing method thereof, and a membrane electrode assembly for a fuel cell using such a microstructure material.

A carbon nanotube (CNT) is a hollow fibrous substance having a three-dimensional structure in which a graphene sheet (sheet in which carbon atoms are arranged in a hexagonal network) corresponding to one layer of graphite is rolled into a cylindrical shape and having a diameter on the order of nm. Say. Carbon nanofiber (CNF) is a solid fibrous substance made of carbon and having a diameter of the order of nm, and cup-shaped graphene having a three-dimensional structure in which the diameter of one end of cylindrical graphene is narrower than the other end. There is also a “cup stack type” fibrous material, or a “bamboo type” fibrous shape in which there is something like a bamboo node in the middle of a CNT tube, and the cavity surrounded by the inner wall of the tube is blocked in the middle A substance. “Cup stack type” and “bamboo type” fibrous materials may be classified as CNTs when the proportion of the cavity surrounded by the inner wall of the tube is large in their structure.
When carbon-containing fibers such as carbon nanotubes (CNT) and carbon nanofibers (CNF) are arranged in one direction, electron conductivity and gas permeability in the arrangement direction are improved. Therefore, application of this type of array to an electrode for a fuel cell is being studied.

Conventionally, various proposals have been made regarding the structure and manufacturing method of an array of carbon-containing fibers.
For example, Patent Document 1 discloses that
(1) One end of a CNT having a length of about 500 μm and an outer diameter of about 10 nm is bonded so as to be oriented substantially perpendicular to the membrane surface of the tubular electrolyte membrane, and a catalytic metal and a proton conductive material are supported on the surface of the CNT. A cell module for a fuel cell, and
(2) The CNTs are formed on the substrate so as to be oriented substantially perpendicular to the substrate surface, Pt is supported on the surface of the CNTs, and then a layer thickness is used using a solution in which a proton conductive material is dissolved. A method of forming a layer of about 10 nm proton conductive material is disclosed.
Further, the same document describes that the tube average diameter of CNT is preferably 10 to 50 nm.

  Patent Document 2 discloses a fuel cell electrode in which a catalyst material is supported only near the tip (electrolyte membrane side) of a CNT oriented perpendicular to the separator surface.

In Patent Document 3,
(1) A membrane electrode assembly in which CNTs having an outer diameter of 10 nm are oriented substantially perpendicularly to the membrane surface of the polymer electrolyte membrane, and an electrode catalyst metal is supported on the surface of the CNTs, and
(2) The CNTs are formed on the substrate so as to be oriented substantially perpendicular to the substrate surface, Pt is supported on the surface of the CNTs, and then a layer thickness is used using a solution in which a proton conductive material is dissolved. A method of forming a layer of about 10 nm proton conductive material is disclosed.
Further, the same document describes that the tube outer diameter of CNT is preferably 10 to 100 nm.

Patent Document 4 includes
(1) generating CNTs having a corrugated shape on a substrate so as to be oriented perpendicular to the substrate surface;
(2) The catalyst metal salt solution is dropped on the CNT, dried and baked to reduce the catalyst metal on the CNT surface,
(3) A method for producing a catalyst electrode is disclosed in which an ionomer dispersion is dropped onto a CNT carrying a catalyst metal and dried.

  Patent Document 5 does not describe a method for forming a catalyst layer for a fuel cell. However, by dissolving a fluid in a supercritical state in a solvent solution of the polymer, the solvent solution is made a poor solvent with respect to the polymer, thereby A method of forming a polymer film on a solid surface is disclosed in which a polymer is deposited on a solid surface that is deposited in the system and deposited on the solid surface.

In Patent Document 6,
(1) Enclose the vertically aligned CNT carrying the catalyst in an airtight container, the electrolyte solution, and trifluoromethane,
(2) After pressurizing the pressure in the reactor to 30 MPa, the temperature of trifluoromethane is raised to 60 ° C., so that trifluoromethane is brought into a supercritical state, and the electrolyte is dissolved in supercritical trifluoromethane,
(3) A method for producing an electrode catalyst layer in which the temperature of vertically aligned CNTs is cooled to 20 ° C. to make supercritical trifluoromethane in a liquid state and an electrolyte is deposited on the CNTs is disclosed.
According to this document, supercritical trifluoromethane has a high polarity and easily dissolves the electrolyte, so that the electrolyte can penetrate deeply between the CNTs, and the uniformity of the electrolyte formed around the CNTs can be improved. The points that can be done are described.

  Further, Non-Patent Document 1 discloses that CNTs having a diameter of 20 nm are vertically aligned on a substrate surface and Pt is supported on the CNT surface, and then the surface of the Pt-supported CNT is Nafion (registered) using a Nafion (registered trademark) solution. A method of coating with a trademark resin is disclosed.

When vertically aligned CNT is applied to an electrode for a fuel cell, it is necessary to support an ionomer on the CNT surface. Conventionally, for carrying an ionomer, a method is used in which an ionomer solution is dropped on vertically aligned CNTs, the ionomer solution is infiltrated between the CNTs, and the solvent is removed. When the diameter of the CNT is 10 nm or more, even with such a method, the ionomer can be supported without breaking the alignment structure of the vertically aligned CNT.
However, when the outer diameter of the CNT is less than 10 nm, there is a problem in that if the solvent is removed after the ionomer solution is infiltrated between the CNTs, the CNT aggregates due to the surface tension of the solvent and the alignment structure is broken.

On the other hand, in Patent Document 6, in order to infiltrate the electrolyte deeply between the CNTs, trifluoromethane is added to the electrolyte solution to bring the trifluoromethane to the supercritical state, and the supercritical trifluoromethane is returned to the liquid state to deposit the electrolyte. Is described.
However, in the case where an ionomer is supported on an array in which carbon-containing fibers having an outer diameter of less than 10 nm are arranged in parallel, an example in which a supercritical fluid is applied in order to prevent aggregation of carbon-containing fibers is conventionally known. Absent.

Japanese Patent No. 4438525 JP-A-2005-004967 JP 2005-203332 A JP 2010-272437 A Japanese Unexamined Patent Publication No. 04-222626 JP 2011-003531 A

T. Hatanaka et al., ECS Transactions 3 (1) 277-284 (2006)

  The problem to be solved by the present invention is to suppress aggregation between fibers when an ionomer is supported on the fiber surface of an array in which carbon-containing fibers having an outer fiber diameter of less than 10 nm are arranged substantially in parallel. .

In order to solve the above problems, the gist of the microstructure material according to the present invention is as follows.
(1) The microstructure material is
Formed on the substrate surface, consisting of carbon containing fibers for several, and array of fiber axis of the carbon-containing fibers are arranged substantially parallel to,
An ionomer in contact with a side wall of the carbon-containing fiber;
E Bei and the gap does not exist carbon-containing fibers and the ionomer,
The ionomer is made of a polymer material having an ion exchange group .
(2) In an arbitrary cross section substantially perpendicular to the fiber axis direction of the array, the total area of regions where the distance between adjacent carbon-containing fibers is less than 1 μm is 50% or more of the total cross-sectional area.
(3) The carbon-containing fiber has a fiber outer diameter of 0.4 nm or more and less than 10 nm.

  The membrane electrode assembly for a fuel cell according to the present invention is obtained by bonding an electrolyte membrane to a surface substantially perpendicular to the fiber axis of the microstructure material according to the present invention.

The gist of the manufacturing method of the microstructure material according to the present invention is as follows.
(1) formed on the substrate surface, consisting of carbon containing fibers for multiple, the the array of fiber axis is arranged substantially parallel to the carbon-containing fibers, A made of a polymer material having an ion exchange group An ionomer solution impregnation step in which a solution in which an ionomer is dissolved in a solvent is brought into contact.
(2) sealing the putting array and a large excess of liquid CO ₂ into the container, the liquid CO ₂ addition step of immersing the array in the liquid CO _2.
However, “large excess” means that the ionomer can be precipitated from the ionomer solution in the closed container, and the non-aggregation rate of the carbon-containing fibers (= substantially perpendicular to the fiber axis direction of the arrayed body). In any arbitrary cross section, the carbon-containing fibers penetrate between the carbon-containing fibers so that the ratio of the total area of the regions where the distance between the adjacent carbon-containing fibers to the entire cross-sectional area is less than 1 μm is 50% or more. The amount is equal to or higher than the lower limit amount capable of removing all or part of the solvent.
(3) A supercritical process in which the inside of the closed container is temperature: 31 ° C. or higher, pressure: 7.4 MPa or higher, and the liquid CO ₂ is made supercritical.
(4) A removal step of removing supercritical CO ₂ from the sealed container.

When the method according to the present invention is used, the ionomer can be supported on the surface of the carbon-containing fiber without breaking the alignment structure of the array even if the fiber outer diameter of the carbon-containing fiber is less than 10 nm. This is considered to be due to the following reason.
That is, when liquid CO ₂ , which is a poor solvent for ionomer, is added while the array is in contact with the ionomer solution, the ionomer is deposited on the surface of the carbon-containing fiber, and at the same time, the liquid CO ₂ is formed between the carbon-containing fibers. invade. In this state, when the inside of the sealed container is set to a predetermined temperature and pressure, the liquid CO ₂ between the fibers becomes supercritical CO ₂ . Supercritical CO ₂ has a lower surface tension than liquid CO ₂ or a solvent of an ionomer solution. For example, the surface tension of ethanol is 21.82 mN / m (25 ° C.), the surface tension of 1-propanol is 23.28 mN / m (25 ° C.), and the surface tension of 2-propanol is 21.22 mN / m (25 ° C.). The surface tension of liquid CO ₂ is 0.59 mN / m (25 ° C.), whereas the surface tension of supercritical CO ₂ is almost zero. Generally, the smaller the surface tension of the fluid, the more the fiber aggregation is suppressed when the fluid is removed from the fiber in contact with the fluid. Therefore, if supercritical CO ₂ is removed from between the fibers without returning the supercritical CO ₂ to liquid CO ₂ , fiber aggregation is suppressed. As a result, the ionomer can be supported on the surface of the carbon-containing fiber without breaking the orientation structure of the array.

It is a schematic diagram of a cross section substantially perpendicular to the fiber axis direction of the microstructure material in which the aggregation of the carbon-containing fibers has occurred. It is a field emission scanning electron microscope (FESEM) image of the side surface (cleavage cross section) of vertical alignment CNT. It is a FESEM image of the surface (left column), surface edge part (middle column), and cleavage section (right column) of the ionomer carrying vertically aligned CNTs prepared in Examples 1 and 2 and Comparative Example 1. It is obtained from the FESEM image (left figure) of the cleaved cross section of the ionomer-supported vertically aligned CNT produced in Example 1 and the upper part (upper right figure), the center part (middle right figure), and the lower part (lower right figure) of the sectional FESEM image. EDX spectrum. It is a current-voltage characteristic of the membrane electrode assembly produced in Example 3 and Comparative Example 2.

Hereinafter, an embodiment of the present invention will be described in detail.
[1. Microstructure material]
The microstructure material according to the present invention has the following configuration.
(1) The microstructure material is
An array of a plurality of carbon-containing fibers, wherein the fiber axes of the carbon-containing fibers are arranged substantially in parallel;
An ionomer in contact with a side wall of the carbon-containing fiber;
The carbon-containing fiber and the void where the ionomer is not present.
(2) In an arbitrary cross section substantially perpendicular to the fiber axis direction of the array, the total area of the regions where the distance between adjacent carbon-containing fibers is less than 1 μm is greater than 20% of the total cross-sectional area.
(3) The carbon-containing fiber has a fiber outer diameter of 0.4 nm or more and less than 10 nm.

[1.1. Array]
The “array” refers to a structure composed of a plurality of carbon-containing fibers in which the fiber axes of the carbon-containing fibers are arranged substantially in parallel.
“Carbon-containing fiber” refers to a fibrous material containing carbon as a main constituent element. The carbon-containing fiber may be made of only carbon or may contain other elements. Examples of other elements include nitrogen, oxygen, hydrogen, and boron.
The carbon-containing fiber may be a hollow fiber or a solid fiber. The carbon-containing fiber may be a straight fiber, or may be a corrugated fiber that is curved at a predetermined period along the longitudinal direction of the fiber.

“Fiber axis” refers to an axis substantially parallel to the longitudinal direction passing through the center of the carbon-containing fiber when the carbon-containing fiber is viewed microscopically (region having a length of about several μm). In the case of a corrugated fiber, the “center” refers to the center of a pair of parallel lines in contact with both side surfaces of the carbon-containing fiber when the carbon-containing fiber is viewed microscopically. The fiber axis of the carbon-containing fiber is a straight line when viewed microscopically, but is not always a straight line when viewed macroscopically, and may be curved at one or more locations.
“The fiber axes are arranged substantially in parallel” means that when the cross section in the direction parallel to the fiber axis of the carbon-containing fiber is viewed, it can be visually recognized that the fiber axes are oriented in substantially the same direction. It means that the fibers are lined up and does not have to be strictly parallel. This includes the case where the fiber axis has an inclination angle with respect to the substrate and the case where the fiber axis is curved in substantially the same direction.

Specifically, as carbon-containing fiber,
(1) Single-walled carbon nanotubes made of single-layered cylindrical graphene or multi-walled carbon nanotubes in which two or more layers of tubular graphene are nested,
(2) carbon nanofiber made of carbon,
and so on.
In particular, single-walled CNTs or multi-walled CNTs are mechanically tough, have excellent chemical and thermal stability, and can be either metal or semiconductor depending on the helical structure of the cylindrical part. Suitable as a contained fiber.

In the present invention, the carbon-containing fiber is composed of a fiber having an outer diameter of 0.4 nm or more and less than 10 nm. This point is different from the conventional one.
The length of the carbon-containing fiber is not particularly limited and can be arbitrarily selected according to the purpose. In general, as the ratio of the fiber length to the fiber outer diameter (aspect ratio) increases, the fibers tend to aggregate after supporting the ionomer. However, when the method according to the present invention is used, the aspect ratio is relatively large. Can also suppress aggregation between fibers.

[1.2. Ionoma]
“Ionoma” refers to a polymer material having an ion exchange group. The ionomer may be a fluorine ionomer in which an ion exchange group is bonded to a fluorine polymer, or may be a hydrocarbon ionomer in which an ion exchange group is bonded to a hydrocarbon polymer. .
The ionomer is in contact with the sidewall of the carbon-containing fiber. The ionomer may be in contact with the carbon-containing fiber so as to cover the entire side wall of the carbon-containing fiber, or may be in contact with the carbon-containing fiber so as to cover a part of the side wall of the carbon-containing fiber. May be.

[1.3. Air gap]
There are voids between the carbon-containing fibers whose whole or part of the side wall is coated with ionomer. Air gap, as will be described later, when precipitating the ionomer in the side wall of the carbon-containing fibers, liquid CO ₂ to penetrate between the fibers, are formed by removing the liquid CO ₂ as the supercritical CO _2. The amount of voids contained in the array can be controlled by the amount of ionomer supported, the deposition conditions, and the like.

[1.4. Catalytic metal particles]
Further, catalytic metal particles may be supported on the surface of the carbon-containing fiber. The catalytic metal particles are supported between the carbon-containing fiber and the ionomer. The catalytic metal particles may be completely covered with an ionomer, or the tip thereof may be exposed from the ionomer surface.
Examples of the catalytic metal particles include Pt particles, alloy particles of Pt such as PtCo and other metal elements, Pt shell particles using Pd or Au as a core material, and hollow Pt shell particles. Alternatively, particles other than Pt include Pd and its alloys.
The particle diameter of the catalyst metal particles is not particularly limited, and can be arbitrarily selected according to the purpose. In general, only the atoms on the surface of the catalyst metal particles function as a catalyst. Therefore, the use efficiency of the catalyst metal improves as the particle size of the catalyst metal particles decreases. The particle diameter of the catalyst metal particles is usually less than 5 nm, although it depends on the loading method.

[1.5. Non-aggregation rate]
In the case where an ionomer is supported on an array body in which carbon-containing fibers are arranged substantially in parallel, if the ionomer support conditions are not appropriate, aggregation between fibers occurs. FIG. 1 shows a schematic diagram of a cross section substantially perpendicular to the fiber axis direction of a fine structure material in which aggregation of carbon-containing fibers has occurred.
As shown in FIG. 1, when agglomeration occurs between carbon-containing fibers arranged substantially in parallel, the density of carbon-containing fibers is increased in the vertical cross section of the microstructure material. In FIG. 1, a hatched region A represents a dense region where carbon-containing fibers are densely packed. On the other hand, the white area B represents a blank area in which almost no carbon-containing fiber exists.

A region where the distance between adjacent carbon fibers is less than 1 μm is defined as “dense region A”, and a region other than the dense region A is defined as “non-dense region B”.
Further, in an arbitrary cross section substantially perpendicular to the fiber axis direction of the array, the ratio of the total area of the dense region A to the total cross sectional area (total area of the dense region A + total area of the non-dense region B) is expressed as “non-aggregation”. Rate ".
The microstructure material according to the present invention is characterized in that the non-aggregation rate is greater than 20%. When the manufacturing conditions are optimized, the non-aggregation rate is 50% or more, 70% or more, or 90% or more.

[2. Membrane electrode assembly for fuel cells]
The membrane electrode assembly for a fuel cell according to the present invention is obtained by bonding an electrolyte membrane to a surface substantially perpendicular to the fiber axis of the microstructure material according to the present invention.

[2.1. Microstructure material]
The microstructure material is
An array of a plurality of carbon-containing fibers, wherein the fiber axes of the carbon-containing fibers are arranged substantially in parallel;
An ionomer in contact with a side wall of the carbon-containing fiber;
Voids in which the carbon-containing fiber and the ionomer are not present;
Catalytic metal particles supported between the surface of the carbon-containing fiber and the ionomer.
The details of the array, the ionomer, the voids, and the catalytic metal particles are as described above, and thus the description thereof is omitted.

[2.2. Electrolyte membrane]
The electrolyte membrane is bonded to a surface substantially perpendicular to the fiber axis of the microstructure material.
The material for the electrolyte membrane is not particularly limited, and may be either a fluorine-based electrolyte or a hydrocarbon-based electrolyte.

[2.3. Joining]
As a method of joining the electrolyte membrane and the microstructure material,
(1) A method of applying an electrolyte solution to the surface of a microstructure material and drying it,
(2) There is a method in which a microstructure material and an electrolyte membrane are superposed and hot pressed.
When joining an electrolyte membrane and a fine structure material, when a method that does not apply excessive pressure to the fine structure material (for example, a coating method) is used, a membrane electrode assembly having an oriented structure of the fine structure material is obtained. It is done.
On the other hand, when a method in which an excessive pressure is applied to the fine structure material at the time of joining (for example, a hot press method), the orientation structure of the fine structure material may collapse depending on the degree of pressurization. However, even in such a case, the cell voltage in the high current density region is improved as compared with the case where the conventional electrode is used.

[3. Manufacturing method of microstructure material]
The method for producing a microstructure material according to the present invention includes a catalyst metal particle supporting step, an ionomer solution impregnation step, a liquid CO ₂ addition step, a supercritical step, and a removal step.

[3.1. Catalyst metal particle loading process]
The catalyst metal particle supporting step is a step of supporting the catalyst metal particles on an array composed of a plurality of carbon-containing fibers and in which the fiber axes of the carbon-containing fibers are arranged substantially in parallel. Depending on the use of the microstructure material, the catalyst metal particle supporting step may be omitted.

The array can be produced by various methods.
For example, when the array is a vertically aligned CNT, the vertically aligned CNT has a fine catalyst (for example, an Fe—Ti—O-based catalyst) densely supported on the substrate surface, introduces a carbon source on the substrate surface, and carbon It can be produced by pyrolyzing the source.
In addition, for example, when the array is vertically aligned CNF, the vertically aligned CNF deposits a Ni-based catalyst on the substrate surface and introduces a carbon source (for example, a hydrocarbon gas such as methane) to the substrate surface. It can be manufactured by a vapor deposition method.

The array on which the catalytic metal particles are supported is, for example,
(1) An array is impregnated with an ethanol solution of a noble metal complex (for example, platinum dinitrodiammine complex) and naphthalene,
(2) Water, which is a poor solvent for naphthalene, is dropped into the array to precipitate naphthalene fine particles between the fibers,
(3) Depressurize the array to evaporate ethanol and water, and sublimate naphthalene;
(4) It can be produced by a method of reducing a noble metal (hereinafter referred to as “naphthalene method”).

  Alternatively, catalytic metal particles can be supported on the surface of the carbon-containing fibers of the array by a vapor phase method. As the gas phase method, for example, a catalyst metal target is installed in a vacuum chamber, a rare gas element (for example, Ar) ionized by application of a high voltage is collided with the target, and the catalyst is blown off from the target. There is a method of depositing metal on the array (sputtering method).

[3.2. Ionomer solution impregnation process]
The ionomer solution impregnation step is a step of bringing a solution in which an ionomer is dissolved in a solvent into contact with an array composed of a plurality of carbon-containing fibers and in which the fiber axes of the carbon-containing fibers are arranged substantially in parallel.

  The solvent for dissolving the ionomer is not particularly limited, and an optimum solvent can be selected according to the purpose. Since carbon-containing fibers are usually hydrophobic, it is preferable to use ethanol, propanol, cyclohexanol or the like as the solvent.

The ionomer concentration in the ionomer solution is not particularly limited, and an optimum concentration can be selected according to the purpose. In general, the higher the ionomer concentration, the greater the amount of ionomer supported on the carbon-containing fiber surface. However, if the ionomer concentration becomes excessive, the ionomer may be deposited thickly on the surface of the array.
Therefore, the ionomer concentration is preferably 10% by weight or less. The ionomer concentration is more preferably 5% by weight or less, and still more preferably 1% by weight or less.

As a method of contacting the array and the ionomer solution,
(1) A method of dripping an appropriate amount of ionomer solution onto the surface of the array and allowing the ionomer solution to penetrate between the fibers;
(2) A method of immersing the array in an appropriate amount of ionomer solution,
and so on. Any method may be used in the present invention.
The contact with the ionomer solution may be performed in a closed container used for supercritical processing, or may be performed in a separate container.
In the case where the array and the ionomer solution are contacted in a closed container used for supercritical processing, if the excess ionomer solution remains in the sealed container, the next step is to leave the excess ionomer solution remaining. You may transfer to a process, or you may transfer to the next process, after discarding an excess ionomer solution.

[3.3. Liquid CO ₂ addition process]
The liquid CO ₂ addition step is a step in which the array body and a large excess of liquid CO ₂ are placed in an airtight container and the array body is immersed in the liquid CO ₂ .
When liquid CO ₂ , which is a poor solvent for ionomer, is added to the array in contact with the ionomer solution, the ionomer solution and liquid CO ₂ are compatible with each other, and ionomer is deposited on the surface of the carbon-containing fiber. At the same time, all or part of the solvent remaining between the carbon-containing fibers is replaced with liquid CO ₂ .
Here, “large excess” means that the ionomer can be precipitated from the ionomer solution in the sealed container, and the carbon-containing fiber penetrates between the carbon-containing fibers so that the non-aggregation rate exceeds 20%. An amount equal to or higher than the lower limit amount capable of removing all or a part of the solvent.

[3.4. Supercritical process]
The supercritical process is a process of making the liquid CO ₂ supercritical by setting the inside of the sealed container to a temperature of 31 ° C. or higher and a pressure of 7.4 MPa or higher.
When the liquid CO ₂ is supercritical CO ₂ , its surface tension is smaller than that of the liquid CO ₂ or the solvent of the ionomer solution.
In order to change the liquid CO ₂ to supercritical CO ₂ , it is necessary to set the temperature in the sealed container to 31 ° C. or higher and the pressure to 7.4 MPa or higher. However, an increase in temperature and / or application of pressure more than necessary does not have a difference in effect and has no actual benefit.

[3.4. Removal process]
The removing step is a step of removing supercritical CO ₂ from the sealed container.
Supercritical CO ₂ is removed by lowering the pressure to normal pressure while keeping the temperature in the sealed container constant.
After the liquid CO ₂ between the carbon-containing fibers with supercritical CO _2, lowering the pressure, the supercritical CO ₂ between fibers is vaporized directly without supercritical CO ₂ is returned to the liquid CO _2. This point is different from the conventional one. Since supercritical CO ₂ has a low surface tension, the non-aggregation rate of the carbon-containing fiber is higher than when vaporizing liquid CO ₂ as it is.

[4. Microstructure material, manufacturing method thereof, and action of membrane electrode assembly for fuel cell]
Carbon-containing fibers (particularly CNTs) have an electronic state close to that of graphite when the fiber outer diameter is thick, but have a unique electronic state when the outer diameter is thin. Therefore, when a carbon-containing fiber having a small fiber outer diameter is used as, for example, a catalyst carrier of an electrode for a fuel cell, an improvement in electron mobility can be expected. In addition, a thinner fiber outer diameter is advantageous for increasing the reaction interface area and reducing the size of the fuel cell device.
However, when an ionomer is supported on an array of carbon-containing fibers having an outer fiber diameter of less than 10 nm by impregnation and drying with an ionomer solution, aggregation between fibers occurs when the solvent volatilizes.

On the other hand, when the method according to the present invention is used, the ionomer can be supported on the surface of the carbon-containing fiber without breaking the orientation structure of the array even if the fiber outer diameter of the carbon-containing fiber is less than 10 nm. . This is considered to be due to the following reason.
That is, when liquid CO ₂ , which is a poor solvent for ionomer, is added while the array is in contact with the ionomer solution, the ionomer is deposited on the surface of the carbon-containing fiber, and at the same time, the liquid CO ₂ is formed between the carbon-containing fibers. invade. In this state, when the inside of the sealed container is set to a predetermined temperature and pressure, the liquid CO ₂ between the fibers becomes supercritical CO ₂ . Supercritical CO ₂ has a smaller surface tension than liquid CO ₂ or the solvent of ionomer solution. Therefore, if supercritical CO ₂ is removed from between the fibers without returning the supercritical CO ₂ to liquid CO ₂ , fiber agglomeration occurs. It is suppressed. As a result, the ionomer can be supported on the surface of the carbon-containing fiber without breaking the orientation structure of the array.

  Furthermore, a membrane electrode assembly using a microstructure material with relaxed aggregation for the cathode catalyst layer has better power generation characteristics than when a microstructure material with severe aggregation is used. This is thought to be due to the improved mobility of oxygen gas, protons, and the like.

(Examples 1-2, Comparative Example 1)
[1. Preparation of sample]
[1.1 Fabrication of vertically aligned CNT]
Vertically aligned CNTs were produced according to the technique described in the literature (Japanese Patent Laid-Open No. 2007-268319). A vertically aligned CNT film was prepared by thermal chemical vapor deposition (CVD) using a 1 cm square Si substrate carrying Fe—Ti—O catalyst fine particles as a catalyst substrate. The CVD conditions were as follows: hydrogen flow rate: 45 sccm, acetylene flow rate: 30 sccm, temperature: 600 ° C., pressure: 400 MPa.
FIG. 2 shows a field emission scanning electron microscope (FESEM) image of the side surface (cleavage cross section) of the obtained vertically aligned CNT. From FIG. 2, the tube outer diameter of the CNT was about 5 nm or more and less than 10 nm, and CNT having a wave shape (wavelength: several 100 nm to 1 μm) and linear CNT were mixed.

[1.2. Ionoma carrying]
[1.2.1. Example 1]
A commercially available ionomer solution (Nafion (registered trademark) solution DE2020, manufactured by DuPont, ionomer concentration 20 to 22 wt%) was dissolved in ethanol to prepare an ionomer solution having an ionomer concentration of about 0.5 wt%. Next, 30 μL of this solution was impregnated into vertically aligned CNT (film thickness: 150 to 200 μm). After putting the sample in a pressure vessel, liquid CO ₂ was introduced into the vessel. Subsequently, the inside of the container was set to about 37 ° C. and about 7.5 MPa to supercriticalize liquid CO _2, and then supercritical CO ₂ was removed from the inside of the container.

[1.2.2. Example 2]
Except for producing an ionomer solution having an ionomer concentration of about 5 wt%, the ionomer was supported on the vertically aligned CNT in the same manner as in Example 1.
[1.2.3. Comparative Example 1]
In the same manner as in Example 1, an ionomer solution having an ionomer concentration of about 0.5 wt% was prepared. Next, 30 μL of this solution was impregnated into vertically aligned CNT (film thickness: 150 to 200 μm). Subsequently, the solvent was naturally evaporated in the atmosphere.

[2. Test method]
[2.1. FESEM observation]
The surface of the ionomer-supported vertically aligned CNT, the surface edge, and the cleaved cross section were observed with FESEM.
[2.2. EDX analysis]
The cleavage cross section of the ionomer-supported vertically aligned CNT was analyzed by energy dispersive X-ray spectroscopy (EDX).

[3. result]
[3.1. FESEM observation]
The forms of the ionomer-supported vertically aligned CNTs produced in Examples 1 and 2 and Comparative Example 1 were each observed with FESEM. FIG. 3 shows FESEM images of the surface (left column), surface end (middle column), and cleaved cross section (right column) of each sample.
The sample of Example 1 had pores with a size of several μm, but the aggregation was small and the vertical alignment was maintained (FIG. 3A). It is presumed that the pores were formed by agglomeration during impregnation with the ionomer solution. In the cross section perpendicular to the fiber axis direction of the vertically aligned CNTs, the area of the pores was 1% or less of the total area (non-aggregation rate: more than 99%). On a microscopic scale, there were contact points between adjacent ionomer-carrying CNTs and parts separated by gaps at intervals of up to about 500 nm (FIG. 3C).

Even in the sample of Example 2, the vertical alignment was maintained with little aggregation (FIG. 3D). Further, a film-like ionomer was deposited to cover the upper part of the vertically aligned CNT (FIG. 3 (e)).
In the sample of Comparative Example 1, CNTs aggregated vigorously due to the surface tension acting during solvent evaporation to form a network structure (FIG. 3 (g)). In this network structure, in the cross section perpendicular to the fiber axis direction of the ionomer-supported vertically aligned CNT, the area of the uppermost wall composed of the end portions of the ionomer-supported vertically aligned CNT occupied about 15% of the total area (non-aggregated) Rate: about 15%).

[3.2. EDX analysis]
FIG. 4 shows an FESEM image (left figure) of the cleaved cross section of the ionomer-supported vertically aligned CNT produced in Example 1, and an upper part (upper right figure), a central part (middle right figure), and a lower part (lower right figure) of the sectional FESEM image. The EDX spectrum obtained from FIG.
FIG. 4 shows that fluorine (F) contained in the ionomer is detected from any of the upper part, the central part, and the lower part of the cleavage cross section of the sample.

(Example 3, Comparative Example 2)
[1. Preparation of sample]
[1.1. Vertically aligned CNT supported on Pt and ionomer]
[1.1.1. Example 3]
30 μL of the ionomer solution having an ionomer concentration of 0.5 wt% obtained in Example 1 was added to Pt-supported vertically aligned CNT (film thickness: 40 to 60 μm, Pt particle size <5 nm, Pt support amount: 0.04 to 0.06 mg). Impregnated. After putting the sample in a pressure vessel, liquid CO ₂ was introduced into the vessel. Subsequently, after supercriticalizing the liquid CO ₂ at about 37 ° C. and about 7.5 MPa, the supercritical CO ₂ was removed from the container.

[1.1.2. Comparative Example 2]
30 μL of the ionomer solution having an ionomer concentration of 0.5 wt% obtained in Example 1 was added to Pt-supported vertically aligned CNT (film thickness: 40 to 60 μm, Pt particle size <5 nm, Pt support amount: 0.04 to 0.06 mg). Impregnated. Subsequently, the solvent was naturally evaporated in the atmosphere.

[1.2. Membrane electrode assembly for fuel cells]
Membrane electrode assemblies (MEA) for fuel cells each containing the Pt and ionomer-supported vertically aligned CNTs produced in Example 3 and Comparative Example 2 as cathode catalyst layers were produced. The vertically aligned CNTs supporting Pt and ionomer and the electrolyte membrane are joined by thermocompression bonding (120 ° C., 50 kgf / cm ² (4.9 MPa)), the tube axis is oriented substantially perpendicular to the electrolyte membrane, and the tube Pt and ionomer-supporting vertically aligned CNTs were arranged so that the ends were joined to the electrolyte membrane. As the anode catalyst layer, a conventional catalyst layer made of Pt / C + ionomer was used.

[2. Test method and results]
Current-voltage characteristics (IV characteristics) were evaluated as the power generation performance of each MEA. Cell temperature: 80 ° C., anode electrode atmosphere: H ₂ , cathode electrode atmosphere: air (21% O ₂ ), pressure at both electrodes: 0.14 MPa.
FIG. 5 shows the IV characteristics. In FIG. 5, the solid line represents the cell voltage, and the broken line represents the resistance. FIG. 5 shows that the MEA using the cathode catalyst layer prepared in Example 3 has a higher cell voltage in the high current region and the power generation performance is superior to the MEA using the cathode catalyst layer prepared in Comparative Example 2. I understand that.

  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 microstructure material and the manufacturing method thereof according to the present invention can be used as a catalyst layer of a membrane electrode assembly for a fuel cell and a manufacturing method thereof.

Claims (5)

A microstructure material with the following structure.
(1) The microstructure material is
Formed on the substrate surface, consisting of carbon containing fibers for several, and array of fiber axis of the carbon-containing fibers are arranged substantially parallel to,
An ionomer in contact with a side wall of the carbon-containing fiber;
E Bei and the gap does not exist carbon-containing fibers and the ionomer,
The ionomer is made of a polymer material having an ion exchange group .
(2) In an arbitrary cross section substantially perpendicular to the fiber axis direction of the array, the total area of regions where the distance between adjacent carbon-containing fibers is less than 1 μm is 50% or more of the total cross-sectional area.
(3) The carbon-containing fiber has a fiber outer diameter of 0.4 nm or more and less than 10 nm.   2. The microstructure material according to claim 1, wherein the carbon-containing fiber includes a single-walled carbon nanotube made of one layer of cylindrical graphene or a multi-walled carbon nanotube in which two or more layers of the cylindrical graphene are nested.   The microstructure material according to claim 1 or 2, wherein catalytic metal particles are supported between a surface of the carbon-containing fiber and the ionomer.   A membrane electrode assembly for a fuel cell obtained by joining an electrolyte membrane to a surface substantially perpendicular to the fiber axis of the microstructure material according to claim 3. A manufacturing method of a microstructure material having the following configuration.
(1) formed on the substrate surface, a plurality of carbon-containing fibers, the fiber axis is arranged body are substantially parallel arrangement of the carbon-containing fibers, A ionomers made of a polymer material having an ion exchange group An ionomer solution impregnation step in which a solution obtained by dissolving a sol is dissolved in a solvent.
(2) sealing the putting array and a large excess of liquid CO ₂ into the container, the liquid CO ₂ addition step of immersing the array in the liquid CO _2.
However, “large excess” means that the ionomer can be precipitated from the ionomer solution in the closed container, and the non-aggregation rate of the carbon-containing fibers (= substantially perpendicular to the fiber axis direction of the arrayed body). In any arbitrary cross section, the carbon-containing fibers penetrate between the carbon-containing fibers so that the ratio of the total area of the regions where the distance between the adjacent carbon-containing fibers to the entire cross-sectional area is less than 1 μm is 50% or more. The amount is equal to or higher than the lower limit amount capable of removing all or part of the solvent.
(3) A supercritical process in which the inside of the closed container is temperature: 31 ° C. or higher, pressure: 7.4 MPa or higher, and the liquid CO ₂ is made supercritical.
(4) A removal step of removing supercritical CO ₂ from the sealed container.

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