JP2021077468A - Electrode catalyst material for fuel cell, catalyst ink, electrode catalyst layer and membrane electrode assembly - Google Patents

Abstract

To provide: a novel electrode catalyst material for fuel cells, having high catalytic activity in an oxygen reduction reaction without using platinum as a catalyst component; and a catalyst ink, an electrode catalyst layer and a membrane electrode assembly, in which the electrode catalyst material for fuel cells is used.SOLUTION: An electrode catalyst material for fuel cells according to the present invention includes a metal oxide and a conductive material. The metal oxide is a linked aggregate in which flaky nanocrystal pieces are linked to each other, the flaky nanocrystal pieces each having main surfaces where a specific crystal plane is exposed and an end surface. The plurality of the nanocrystal pieces has a gap arranged while opening to an outside of the linked aggregate, between the main surfaces. The conductive material is in contact with at least a part of the nanocrystal pieces.SELECTED DRAWING: Figure 1

Description

The present invention relates to an electrode catalyst material, and more particularly to an electrode catalyst material having high catalytic activity as an air catalyst material for a fuel cell, and a catalyst ink, an electrode catalyst layer, and a membrane electrode assembly using the same.

In recent years, from the viewpoint of energy saving, there is an increasing demand for improvement of power generation equipment and battery performance. Further, from the viewpoint of reducing environmental load and production cost, it is required to develop new materials for electrodes mounted on power generation devices and batteries while maintaining and improving the conventional performance. From the viewpoint of reducing exhaust gas and greenhouse gases, it has also been proposed to drive transportation equipment such as automobiles by using a power generation device such as a fuel cell.

As an air electrode catalyst material used in a fuel cell, a catalyst material in which fine particles of platinum (Pt) are supported on the surface of carbon particles has been conventionally used. Platinum is excellent as a catalyst for oxygen reduction reaction (hereinafter sometimes referred to as "ORR"), and carbon particles are excellent in conductivity. Therefore, a catalyst material in which platinum fine particles are supported on the surface of carbon particles is a fuel cell. It is generally used as an air electrode catalyst material of. However, since platinum is a rare metal having a small reserve and is also expensive, a new catalyst material is required in place of the catalyst material in which platinum fine particles are supported on the surface of carbon particles.

According to Patent Document 1, a catalyst obtained by calcining and activating a silk material and supporting a catalyst metal such as platinum on a nitrogen-containing powdery charcoal derived from the silk material has an oxygen reduction reaction ability. It is disclosed that it is useful as a catalyst for the cathode layer of a fuel cell. However, since platinum is used as the catalyst metal as in the conventional case, no new catalyst material has been proposed to replace the catalyst material using platinum.

Japanese Unexamined Patent Publication No. 2010-063952

The present invention provides a new electrode catalyst material for a fuel cell having high catalytic activity in an oxygen reduction reaction without using platinum as a catalyst component, and a catalyst ink, an electrode catalyst layer and a membrane electrode assembly using the same. With the goal.

As a result of diligent studies on the above problems, the present inventor has made a connected set in which flaky nanocrystal pieces having a main surface and an end face on which a specific crystal face is exposed are connected to each other as a catalyst component. A catalyst material comprising a body, a metal oxide having catalytic activity, and a conductive material as a conductivity-imparting component, in which at least a part of the nanocrystal pieces is in contact with the conductive material, is used as a fuel. It has been found that by using it as an electrode catalyst material for a battery, high catalytic activity can be obtained in an oxygen reduction reaction without using expensive platinum.

That is, the gist structure of the present invention is as follows.
[1] An electrode catalyst material for a fuel cell having a metal oxide and a conductive material.
The metal oxide is a linked aggregate in which flaky nanocrystal pieces having a main surface and an end face on which a specific crystal face is exposed are connected to each other.
The plurality of nanocrystal pieces have a gap between the main surfaces, which is arranged so as to be open to the outside of the connecting assembly.
An electrode catalyst material in which the conductive material is in contact with at least a part of the nanocrystal pieces.
[2] The electrode catalyst material according to [1], wherein the average thickness of the nanocrystal pieces is less than 10 nm.
[3] The electrode catalyst material according to [1] or [2], wherein the metal oxide is copper oxide.
[4] The electrode catalyst material according to [3], wherein the specific crystal plane is a (001) crystal plane.
[5] The electrode catalyst material according to any one of [1] to [4], wherein the conductive material is a carbon material having a continuous structure in which conductive substances are connected.
[6] The electrode catalyst material according to [5], wherein the conductive substance is at least one selected from fibrous carbon and carbon particles.
[7] The electrical conductivity of the electrode catalyst material formed on the electrode is 0.5% or more with respect to the electrical conductivity of the layer formed on the electrode by the conductive material [1] to [6]. ]. The electrode catalyst material according to any one of.
[8] A catalyst ink for forming an electrode catalyst layer for a fuel cell, which comprises the electrode catalyst material according to any one of [1] to [7], a polymer electrolyte, and a solvent.
[9] An electrode catalyst layer for a fuel cell formed by using the catalyst ink according to [8].
[10] A positive electrode having a positive electrode catalyst layer, a negative electrode having a negative electrode catalyst layer, a solid polymer electrolyte layer arranged between the positive electrode catalyst layer and the negative electrode catalyst layer, and a solid polymer electrolyte layer. With
A membrane electrode assembly for a fuel cell, wherein at least one of the positive electrode catalyst layer and the negative electrode catalyst layer contains the electrode catalyst material according to any one of [1] to [7]. ..

According to the aspect of the present invention, the electrode catalyst material imparts conductivity to at least a part of nanocrystal pieces having a main surface of a metal oxide having catalytic activity and a specific crystal surface is exposed. Since it is a composite material in contact with a sex material, a new electrode catalyst material for a fuel cell that exhibits high catalytic activity in an oxygen reduction reaction without using expensive platinum as a catalyst material, and an electrode catalyst material for which this is used were used. A catalyst ink, an electrode catalyst layer, and a film electrode joint can be provided.

According to the aspect of the present invention, a catalytic material in which the metal oxide is copper oxide and the specific crystal plane is the (001) crystal plane can more reliably obtain high catalytic activity in the oxygen reduction reaction.

According to the aspect of the present invention, since the conductive material is a carbon material having a continuous structure in which conductive substances are continuously connected, efficient electron transfer by the continuous structure is possible, and thus metal oxidation The transfer of electrons between the object and the conductive material is facilitated, and the catalytic activity can be further enhanced.

FIG. 1 is a schematic view illustrating an embodiment of an electrode catalyst material according to the present invention. FIG. 2 is an SEM image of the electrode catalyst material produced in Example 1 when observed at a magnification of 30,000. FIG. 3 shows a reflected electron image in the same field of view of the SEM image shown in FIG. FIG. 4 is an SEM image of the electrode catalyst material produced in Example 3 when observed at a magnification of 50,000 times. FIG. 5 is an SEM image of the electrode catalyst material produced in Comparative Example 1 when observed at a magnification of 30,000. FIG. 6 shows a reflected electron image in the same field of view of the SEM image shown in FIG. FIG. 7 is a graph showing the relationship between the potential and the current density in the measurement of catalytic activity.

Hereinafter, the electrode catalyst material, the catalyst ink, the electrode catalyst layer, and the membrane electrode assembly, which are the embodiments of the present invention, will be described with reference to the drawings. FIG. 1 is a schematic view illustrating an embodiment of the electrode catalyst material of the present invention.

<Electrode catalyst material>
As shown in FIG. 1, the electrode catalyst material 1 of the embodiment of the present invention has a metal oxide having catalytic activity and a conductive material 30 that imparts conductivity, and the metal oxide is a specific crystal. It is a connected assembly 20 in which flaky nanocrystal pieces 21 having a main surface 22 and an end surface 23 whose surfaces are exposed are connected to each other. The linked aggregate 20 exhibits excellent catalytic activity because it is composed of flaky nanocrystal pieces 21 having a main surface 22 on which a specific crystal plane is exposed. Further, the connected aggregate 20 has a gap G arranged between the main surfaces 22 of the plurality of nanocrystal pieces 21 so as to be open to the outside of the connected aggregate 20.

The conductive material 30 is in contact with the nanocrystal pieces 21, preferably at least a portion of the main surface 22. For example, when the electrode catalyst material 1 is mounted on the positive electrode of the fuel cell, the electrons generated in the hydrogen oxidation reaction _{of H 2} → 2H ⁺ + 2e ^{− in the negative electrode catalyst material of the fuel cell are in contact with the nanocrystal pieces 21.} It is transported through the conductive material 30. Since the conductive material 30 is in contact with at least a part of the nanocrystal pieces 21, electron transfer between the connecting aggregate 20 which is a metal oxide and the conductive material 30 is achieved. On the other hand, when the conductive material 30 covers the entire surface of the nanocrystal piece 21, particularly the entire surface of the main surface 22, the main surface 22 which is the catalytically active surface is not exposed, and the reactant at the positive electrode cannot be supplied to the catalytically active surface. , The oxygen reduction reaction is inhibited. In the electrode catalyst material 1 in which at least a part of the nanocrystal piece 21, particularly at least a part of the main surface 22, and the conductive material 30 are in contact with each other, good contact with the conductive material 30 and improvement of catalytic activity are realized. Will be done. Further, it is preferable that the conductive material 30 is in electrical contact with at least a part of the main surface 22 of the nanocrystal piece 21. As a result, electron transfer between the connecting aggregate 20 and the conductive material 30 is facilitated, and the catalytic activity is further improved. When the conductive material 30 and at least a part of the main surface 22 of the nanocrystal piece 21 are in good electrical contact, the electrical conductivity of the electrode catalyst material 1 formed on the electrode is on the electrode. It is preferably 0.5% or more with respect to the electrical conductivity of the layer formed by the conductive material 30.

The conductive material 30 may have a continuous structure 31 in which the conductive substances constituting the conductive material 30 are continuously connected to each other like beads. In this case, the continuous structure 31 of the conductive material 30 may be in contact with at least a part of the nanocrystal piece 21, for example, at least a part of the main surface 22. When the conductive material 30 has the continuous structure 31, the area where a part of the nanocrystal pieces 21 can come into contact with the continuous structure 31 of the conductive material 30 increases. As a result, the transfer of electrons between the connecting aggregate 20 and the conductive material 30 is smoother, and the catalytic activity can be further enhanced. Further, the continuous structure 31 of the conductive material 30 is supported on the exposed main surface 22 of the nanocrystal piece 21 by being in contact with at least a part of the main surface 22 of the nanocrystal piece 21. .. Therefore, for example, even if there is an impact from the outside or the like, good contact between the conductive material 30 and the nanocrystal piece 21 can be maintained. On the other hand, when the continuous structure 31 of the conductive material 30 is in electrical contact with at least a part of the end face 23 of the nanocrystal piece 21, the conductive performance can be improved without inhibiting the catalytically active surface. Therefore, in the electrode catalyst material 1, both a form of holding the conductive material 30 using the connecting aggregate 20 as a carrier and a form of holding the nanocrystal piece 21 using the continuous structure 31 of the conductive material 30 as a carrier are possible. is there.

<Metal oxide>
As shown in FIG. 1, the metal oxide is a connected aggregate 20 in which a plurality of nanocrystal pieces 21 having a main surface 22 and an end surface 23 are connected to each other, and exhibits a flower-like shape. The connection state of the plurality of nanocrystal pieces 21 is not particularly limited, and it is sufficient that the plurality of nanocrystal pieces 21 are connected to form an aggregate.

The shape of the nanocrystal piece 21 is a flaky shape in which the thickness of the end face 23 is thin with respect to the size of the main surface 22. On the outer surface of the articulated aggregate 20, a gap G is formed between the main surfaces 22 of the plurality of adjacent nanocrystal pieces 21, and the gap G is arranged so as to open to the outside of the articulated aggregate 20. There is. When the linkage assembly 20 has the gap G, the electrolyte described later is filled in the gap G, and the reactant in the oxygen reduction reaction can effectively reach the main surface which is the catalytically active surface. Therefore, the effective movement of water (moisture), which is a reaction product, is promoted.

The main surface 22 of the nanocrystal piece 21 is a surface having a large surface area among the outer surfaces constituting the flaky nanocrystal piece 21, and both surfaces forming the upper and lower end edges of the end surface 23 having a small surface area. Means. In the electrode catalyst material 1 used for the oxygen reduction reaction, a specific crystal plane is exposed on the main surface 22. Since the main surface 22 on which the specific crystal plane is exposed becomes the catalytically active surface exhibiting high catalytic activity, the larger the surface area of the main surface 22, the more efficiently the oxygen reduction reaction can be carried out.

The minimum size of the main surface 22 of the nanocrystal piece 21 is not particularly limited, but is preferably 10 nm or more and less than 1.0 μm. The average thickness t of the nanocrystal pieces 21 is not particularly limited, but is preferably 1/10 or less of the minimum dimension of the main surface 22. As a result, the area of the main surface 22 of the nanocrystal piece 21 becomes about 10 times or more larger than the area of the end face 23, and the catalytic activity per unit amount of the linked aggregate 20 becomes the catalytic activity per unit amount of the nanoparticles. Improves compared to. The average thickness of the nanocrystal pieces is preferably less than 10 nm. When the minimum size of the main surface 22 is 1.0 μm or more, it tends to be difficult to connect the nanocrystal pieces 21 at high density, and when the minimum size is less than 10 nm, a plurality of adjacent nanocrystal pieces tend to be connected. It tends to be impossible to form a sufficient gap G between the main surfaces 22 of 21. Further, in order to suppress a decrease in rigidity of the nanocrystal piece 21 in the thickness direction, the average thickness t of the nanocrystal piece 21 is preferably 1.0 nm or more. The dimensions of the main surface 22 of the nanocrystal pieces 21 are determined by measuring the nanocrystal pieces 21 separated from the connecting aggregate 20 as individual nanocrystal pieces so as not to impair the shape of the nanocrystal pieces 21. Can be done. As a specific example of the measurement method, a rectangle having the minimum area circumscribing is drawn with respect to the main surface 22 of the nanocrystal piece 21, and the short and long sides of the rectangle are set as the minimum and maximum dimensions of the nanocrystal piece 21, respectively. Measure.

The nanocrystal pieces 21 constituting the linked aggregate 20 are made of a metal oxide. Examples of the metal oxide include oxides of noble metals (excluding platinum), oxides of transition metals, oxides of their alloys, composite oxides, and the like. The noble metal and its alloy include, for example, a metal composed of one component selected from the group of palladium (Pd), rhodium (Rh), ruthenium (Ru), silver (Ag) and gold (Au), or a metal thereof. Alloys containing one or more components selected from the group can be mentioned. The transition metal and its alloy include, for example, a metal composed of one component selected from the group of copper (Cu), nickel (Ni), cobalt (Co) and zinc (Zn), or a metal thereof. Alloys containing one or more selected components can be mentioned.

Among these metal oxides, metal oxides containing one or more metals selected from the group of transition metals are preferable. Metal oxides of transition metals are abundant on the earth as metal resources and are cheaper than precious metals, so that production costs can be reduced. Among the transition metals, it is more preferable that the metal oxide contains one or more metals selected from the group of Cu, Ni, Co and Zn, and such a metal oxide contains at least copper. Is even more preferable. Examples of the metal oxide containing copper include copper oxide, Ni-Cu oxide, Cu-Pd oxide and the like, and copper oxide (CuO) is particularly preferable.

<Crystal orientation of main surface>
When the electrode catalyst material 1 of the present invention is mounted on an electrode for a fuel cell, the main surface 22 is specified because the main surface 22 on which the specific crystal plane is exposed in the nanocrystal piece 21 is the catalytically active surface. It is configured to have the crystal orientation of.

In order to configure the main surface 22 of the nanocrystal piece 21 to be a reducing catalytically active surface, the surface of the metal atom exhibiting catalytic activity in the metal oxide constituting the nanocrystal piece 21 is formed on the main surface 22. The main surface 22 may be composed of a metal atomic surface by orienting the surface so as to be located at. Specifically, it is preferable that the number ratio of the number of metal atoms to the metal atoms and oxygen atoms constituting the metal oxide existing on the main surface 22 is 80% or more.

On the other hand, in order to configure the main surface 22 of the nanocrystal piece 21 to be an oxidizing catalytically active surface, the surface of the oxygen atom exhibiting catalytic activity in the metal oxide constituting the nanocrystal piece 21 is mainly used. The main surface 22 may be composed of oxygen atom planes by orienting the surface 22 so as to be located on the surface 22. Specifically, it is preferable that the number ratio of oxygen atoms to the metal atoms and oxygen atoms constituting the metal oxide existing on the main surface 22 is 80% or more.

By adjusting the number ratio of metal atoms or oxygen atoms to the metal atoms and oxygen atoms constituting the metal oxide existing on the main surface 22 of the nanocrystal piece 21, depending on the role of the catalytically active surface, the main surface The catalytically active function of 22 can be enhanced. The electrode catalyst material 1 having such nanocrystal pieces 21 can exhibit sufficient catalytic activity.

Further, the reason why the main surface 22 of the nanocrystal piece 21 has a specific crystal orientation is that the crystal orientations abundantly present on the main surface 22 differ depending on the type of metal oxide constituting the nanocrystal piece 21. Is. Therefore, the crystal orientation of the main surface 22 is not specifically described, but for example, when the metal oxide is copper oxide (CuO), the main crystal orientation of the single crystal constituting the main surface 22, that is, that is, The specific crystal plane as the catalytically active plane is preferably the (001) crystal plane.

As a configuration in which the main surface 22 is a metal atomic surface, the metal atomic surface and the oxygen atomic surface are regularly and alternately laminated, and the metal atomic surface is located on the main surface 22 as a regular structure having regular arrangement of atoms. As such, it is preferable to construct a crystal structure of the metal oxide. Specifically, not only when the main surface 22 is composed of aggregates of single crystals having the same orientation, but also aggregates of single crystals having different crystal structures and different orientations, grain boundaries and polycrystals. Even if the structure is composed of an aggregate containing the above, the case where the metal atomic surface is present on the main surface 22 is included.

<Conductive material>
As shown in FIG. 1, the electrode catalyst material 1 of the embodiment of the present invention has a connecting aggregate 20 which is a metal oxide and a conductive material 30. Further, the conductive material 30 has a continuous structure 31 in which the conductive substances constituting the conductive material 30 are continuously connected to each other like beads, and the continuous structure 31 is the main surface 22 of the nanocrystal piece 21. It may be supported on at least a part of the above. The continuous structure 31 of the conductive material 30 may be in contact with at least a part of the main surface 22 of the nanocrystal piece 21 as well as at least a part of both the main surface 22 and the end face 23. The conductive material 30 having the continuous structure 31 is supported in a state where a part of the main surface 22 of the nanocrystal piece 21 is exposed, so that the conductive material 30 comes into contact with the nanocrystal piece 21 while maintaining the exposure of the catalytically active surface. The possible range can be increased.

When the conductive material 30 has a continuous structure 31, the average length of the continuous structure 31 is such that the conductive material 30 is in contact with at least a part of the nanocrystal pieces 21 and the nanocrystal pieces 21 It suffices as long as it can suppress the decrease in rigidity in the thickness direction. Further, when the continuous structure 31 comes into contact with a part of the main surface 22 of the nanocrystal piece 21, the contact area is from the viewpoint of preventing the catalytic activity of the linked aggregate 20 which is a metal oxide from being hindered. The area of the main surface 22 of the nanocrystal piece 21 may be smaller than the area of the main surface 22 of the nanocrystal piece 21, and it is preferably 50% or less of the area of the main surface 22 of the nanocrystal piece 21. This prevents the main surface 22 of the nanocrystal piece 21 from which the specific crystal plane, which is the catalytically active surface, is exposed, from being completely covered with the continuous structure 31 of the conductive material 30, and as a result, The main surface 22 can exhibit an excellent catalytic function. The contact area represents the total area in which the continuous structure 31 is in contact with a part of the main surface 22 when the continuous structure 31 is in contact with a part of the main surface 22 of the nanocrystal piece 21. It is different from the covering area where the continuous structure 31 covers a part of the main surface 22.

Since the conductive material 30 has the continuous structure 31, electron transfer between the connecting aggregate 20 which is a metal oxide and the conductive material 30 is further facilitated. Such a conductive material 30 preferably has a one-dimensional or higher continuous structure in which conductive substances are connected, and is preferably a carbon material having a chain structure, for example. Examples of the conductive substance that can form a continuous structure other than the carbon material include copper fine wire chain nickel fine particles and scaly nickel fine particles. The conductive substance that can form a continuous structure as the carbon material is preferably at least one selected from fibrous carbon (carbon fiber) and carbon particles.

Among carbon materials, fibrous carbon and carbon particles have high conductivity and easily form a chain structure by aggregation. Such conductive materials are preferably on the order of nanometers, which allows more chain structures to be formed. The continuous structure 31 of the conductive material 30 is a wide-area continuous structure that continuously connects each nanocrystal piece 21 to the electrode in order to efficiently transport electrons to the main surface 22 of the nanocrystal piece 21 and the electrode. It is preferable to have. Since the continuous structure 31 is a wide-area continuous structure, a structure in which the polymer electrolyte described later does not exist is formed between the conductive materials 30. Since the polymer electrolyte has conductive performance, it can transport electrons like the conductive material 30, but has a higher electrical resistance than the conductive material 30. Therefore, the wide-area continuous structure in which the polymer electrolyte does not exist between the conductive materials 30 can secure the electron transport path in a wider area. When carbon particles are used as the conductive substance, it is desirable that the particle size of the carbon particles is large in order to reduce the contact resistance between the conductive materials 30. However, if the particle size of the carbon particles is too large, it becomes difficult for each nanocrystal piece 21 to come into contact with the conductive material 30 and to form a chain structure between the conductive materials 30, and the catalytically active surface of the nanocrystal piece 21. There is a risk that the entire surface of the crystal will be covered and the reaction efficiency will decrease. Therefore, the average particle size of the carbon particles is preferably 20 nm or more and 200 nm or less. Similarly, when the conductive substance is fibrous carbon (carbon fiber), the average diameter of the fibrous carbon is preferably 100 nm or less, and the average length is preferably 0.5 μm or more and 100 μm or less. ..

<Use of electrode catalyst material>
The electrode catalyst material 1 according to the embodiment of the present invention can be used as an air electrode catalyst material for a fuel cell.

<Manufacturing method of electrode catalyst material>
Next, an example of a method for producing the electrode catalyst material of the present invention will be described. Examples of a method for producing an electrode catalyst material include a metal oxide preparation step Sa for preparing a metal oxide which is a connected aggregate in which flaky nanocrystal pieces are interconnected, and conductivity to the prepared metal oxide. It has a conductive material supporting step Sb for supporting a sex material.

The metal oxide preparation step Sa includes a mixing step Sa1 and a hydrothermal synthesis step Sa2 for applying temperature and pressure.

(Mixing step Sa1)
In the mixing step, a hydrate of a compound containing a noble metal, a transition metal or an alloy thereof, which is a raw material of a metal oxide, particularly a hydrate of a metal halide, and a metal complex which is a precursor of the metal oxide are arranged. This is a step of dissolving an organic compound having a carbonate diamide skeleton constituting a position in an organic solvent such as ethylene glycol, 1,4-butanediol, or polyethylene glycol, water, or a solvent containing both of them. Examples of the hydrate of the metal halide include copper (II) chloride dihydrate, and examples of the organic compound having a carbonic acid diamide skeleton include urea.

(Hydrothermal synthesis step Sa2)
The hydrothermal synthesis step is a step of applying predetermined heat and pressure to the mixed solution obtained in the mixing step Sa1 and leaving it to stand for a predetermined time. The mixed solution is preferably heated at 100 ° C. or higher and 300 ° C. or lower. If the heating temperature is less than 100 ° C, metal oxides are not generated, and if it exceeds 300 ° C, the heat-resistant temperature of the packing for maintaining airtightness that constitutes the heat-resistant container is exceeded, airtightness cannot be maintained, and volatile gas leaks to the outside. Not preferable. The heating time is preferably 10 hours or more. If the heating time is less than 10 hours, unreacted material may remain. The predetermined pressure is preferably a pressure equal to or higher than the vapor pressure of water (1 atm) at 100 ° C. In order to apply a predetermined heat and pressure, for example, a method of heating and pressurizing using a pressure-resistant container and a closed container can be mentioned. After heating and pressurizing the mixed solution, it is cooled to room temperature and held for a certain period of time, and then the produced precipitate is recovered. The recovered precipitate is washed with methanol, pure water, etc. and dried for a predetermined time. As a result, the desired metal oxide can be obtained.

After the metal oxide preparation step Sa, the conductive material mixing step Sb is carried out. The conductive material mixing step Sb includes (A) a metal oxide dispersion step Sb1 for preparing a prepared metal oxide dispersion, a conductive material dispersion step Sb2 for preparing a conductive material dispersion, or both. The step of preparing the liquid and (B) adding the conductive material to the dispersion liquid of the metal oxide and mixing, adding the prepared metal oxide to the dispersion liquid of the conductive material and mixing, or metal It has a dispersion treatment step Sb3 in which a dispersion liquid of an oxide and a dispersion liquid of a conductive material are mixed.

(Metal oxide dispersion step Sb1)
In the metal oxide dispersion step, the metal oxide prepared in the metal oxide preparation step Sa is added to a mixed solution in which an organic solvent is added to a dispersion medium (for example, water), and then dispersion treatment is performed with an ultrasonic disperser or the like. This is a step of preparing a dispersion liquid of a metal oxide. Examples of the organic solvent include monoalcohols such as methanol, ethanol, n-propanol and isopropanol. The content of the metal oxide contained in the dispersion liquid of the metal oxide is preferably 0.05% by mass or more and 5.0% by mass or less from the viewpoint of the balance between the dispersibility of the metal oxide and the production efficiency, and is 0.1. Particularly preferably, it is mass% or more and 1.0 mass% or less. If necessary, the electrolyte used for the fuel cell may be further added and dispersed in the metal oxide dispersion liquid. Examples of the electrolyte include polymer electrolytes such as Nafion (registered trademark).

(Conductive material dispersion step Sb2)
In the conductive material dispersion step, an organic solvent is added to a dispersion medium (for example, water), the conductive material is added to the mixed solution, and then the dispersion treatment is performed with an ultrasonic disperser or the like to disperse the conductive material. Is the process of producing. Examples of the organic solvent include alcohols such as ethanol and isopropyl alcohol. The content of the conductive material contained in the dispersion liquid of the conductive material is preferably 0.05% by mass or more and 5.0% by mass or less from the viewpoint of the balance between the dispersibility of the conductive material and the production efficiency, and is 0.1. Particularly preferably, it is mass% or more and 1.0 mass% or less. If necessary, the electrolyte used for the fuel cell may be further added and dispersed in the dispersion liquid of the conductive material. Examples of the electrolyte include polymer electrolytes such as Nafion (registered trademark).

(Dispersion processing step Sb3)
In the dispersion treatment step, a conductive material is added to the dispersion liquid of the metal oxide prepared in the metal oxide dispersion step Sb1, or a metal oxide preparation step is added to the dispersion liquid of the conductive material prepared in the conductive material dispersion step Sb2. The metal oxide prepared in Sa is added, or the dispersion liquid of the metal oxide prepared in the metal oxide dispersion step Sb1 and the dispersion liquid of the conductive material prepared in the conductive material dispersion step Sb2 are mixed. This is a step of performing dispersion processing with an ultrasonic disperser or the like. In the dispersion treatment step, from the viewpoint of the balance between the conductivity and the catalytic activity of the electrode catalyst material, it is preferable that the area of the conductive material covering the catalytically active surface of the metal oxide in the composition of the electrode catalyst material is 50% or less. Therefore, the contents of the metal oxide and the conductive material are adjusted. In the case of nanocrystal pieces of copper oxide as the metal oxide and spherical powder of carbon as the conductive material, a preferable coating area can be obtained by containing the metal oxide and the conductive material in equal masses. Through such a step, the electrode catalyst material 1 is produced.

<Catalyst ink>
The catalyst ink according to the embodiment of the present invention contains the above-mentioned electrode catalyst material 1, the polymer electrolyte, and a solvent, and is used to form an electrode catalyst layer for a fuel cell. Such a catalyst ink is produced by subjecting a solution in which these materials are mixed to a dispersion treatment using a disperser such as a homomixer, a disper, an ultrasonic disperser, a homogenizer, or a milder. As the disperser, an ultrasonic disperser is preferable. The content of the electrode catalyst material 1 in the catalyst ink is preferably 0.1% by mass or more and 10% by mass or less, and more preferably 0.2% by mass or more and 2% by mass or less with respect to the catalyst ink. The content of the polymer electrolyte in the catalyst ink is preferably 0.01% by mass or more and 0.2% by mass, and more preferably 0.01% by mass or more and 0.1% by mass or less with respect to the catalyst ink.

Examples of the polymer electrolyte include solid polymer electrolytes such as perfluorocarbon materials, and Nafion (registered trademark) is preferable in terms of actual results and conductivity. The polymer means that the mass average molecular weight (Mw) is 10,000 or more. Examples of the solvent include water; monoalcohols such as methanol, ethanol, n-propanol and isopropanol; saturated hydrocarbon solvents such as n-pentane, n-hexane, cyclohexane and methylcyclohexane; aromatics such as toluene and xylene. Group solvents; halogen-based solvents such as chloroform and dichloromethane; heteroelement-containing solvents such as acetone, diethyl ether, acetonitrile, tetrahydrofuran, N, N-dimethylformamide and the like can be mentioned. Among these, monoalcohols such as water, methanol, ethanol, n-propanol, and isopropanol are preferable because they are easy to dry. The solvent may be any one of these or a mixture containing two or more of them.

The viscosity of the catalyst ink is 0.5 mPa · s or more and 40 mPa · s or less at 25 ° C. as the viscosity by the viscometer from the viewpoint of improving the coatability on the electrode when forming the electrode catalyst layer. preferable.

<Electrode catalyst layer>
The electrode catalyst layer according to the embodiment of the present invention is formed by using the above-mentioned electrode catalyst material 1, a polymer electrolyte, and a catalyst ink containing a solvent, and is effective as an electrode catalyst layer for a fuel cell. Such an electrode catalyst layer is formed by applying the catalyst ink prepared as described above onto the electrodes and then drying. The method of applying the catalyst ink, the method of drying, and the like can be appropriately selected. For example, examples of the coating method include a spray method, an inkjet method, a drop casting method, and a die coating method. Moreover, as a drying method, for example, vacuum drying, heat drying, vacuum heat drying and the like can be mentioned. Specific conditions for vacuum drying and heat drying are not particularly limited and can be set as appropriate. The film thickness of the electrode catalyst layer is not particularly limited, but may be 1 μm or more and 20 μm or less. The electrode catalyst layer is a matrix in which the electrode catalyst material 1 and the polymer electrolyte are appropriately mixed, and the electrode reaction is performed at the interface between the electrode catalyst material 1 and the polymer electrolyte.

The electrode catalyst layer is formed on the positive electrode (air electrode) of the fuel cell that requires an oxygen reduction reaction, and serves as the electrode catalyst layer for the positive electrode.

<Membrane electrode assembly>
The film electrode joint according to the embodiment of the present invention is arranged between a positive electrode having a positive electrode catalyst layer, a negative electrode having a negative electrode catalyst layer, and a positive electrode catalyst layer and the negative electrode catalyst layer. The solid polymer electrolyte layer is provided. Further, at least one of the electrode catalyst layer for the positive electrode and the electrode catalyst layer for the negative electrode contains the above-mentioned electrode catalyst material 1, and such a membrane electrode assembly is effective as a membrane electrode assembly for a fuel cell. Is.

When the positive electrode catalyst layer is an electrode catalyst layer containing the above-mentioned electrode catalyst material 1, the negative electrode catalyst layer may be another electrode catalyst layer containing another catalyst material. As another catalyst layer, for example, a conventional electrode catalyst layer containing Pt (platinum) / C (carbon) or the like can be mentioned.

The solid polymer electrolyte layer has at least one electrolyte membrane containing an electrolyte resin. Examples of the electrolyte membrane include a fluorine-based polymer electrolyte membrane containing a fluorine-based polymer electrolyte such as Flemion (registered trademark) manufactured by AGC and Nafion (registered trademark) manufactured by DuPont, and an aromatic block copolymer polymer. Can be mentioned. The structure and material of the positive electrode and the negative electrode are not particularly limited, and known forms of the positive electrode and the negative electrode can be used.

The electrode catalyst layer for the positive electrode may have a multilayer structure in which a gas diffusion layer is laminated. The gas diffusion layer has a role of transferring electrons to the electrode catalyst material 1 contained in the electrode catalyst layer for the positive electrode and supplying gas, and a conductive porous material is used. Further, the electrode catalyst layer for the negative electrode may also have a multilayer structure in which gas diffusion layers are laminated, if necessary. As such a gas diffusion layer, a sheet made of a carbon material such as carbon paper or carbon cloth is preferable from the viewpoint of maintaining good catalytic performance in the oxygen reduction reaction and the hydrogen oxidation reaction. When the positive electrode catalyst layer and the gas diffusion layer are laminated, the surface of the gas diffusion layer, particularly the surface on the positive electrode catalyst layer side, may be a water-repellent layer in which the carbon material is densified, if necessary. Good.

The electrode catalyst layer for the negative electrode may also have a multilayer structure in which a gas diffusion layer is laminated. As the gas diffusion layer, a sheet made of a carbon material such as carbon paper or carbon cloth is preferable from the viewpoint of maintaining good catalytic performance in the hydrogen reduction reaction. When the positive electrode catalyst layer and the gas diffusion layer are laminated, the surface of the gas diffusion layer, particularly the surface on the positive electrode catalyst layer side, may be a water-repellent layer in which the carbon material is densified, if necessary. Good.

A positive electrode having a positive electrode catalyst layer, particularly a positive electrode having a multilayer structure in which a positive electrode catalyst layer and a gas diffusion layer are laminated, and a negative electrode having a negative electrode catalyst layer, particularly a positive electrode catalyst layer and gas diffusion. The negative electrode having a multilayer structure in which the layers are laminated and the solid polymer electrolyte layer arranged between the positive electrode catalyst layer for the positive electrode and the electrode catalyst layer for the negative electrode are appropriately overlapped and heat-bonded to each other. By joining, a film electrode joint can be formed. Further, a fuel cell can be manufactured by arranging separators on both outer sides of each electrode.

Although the embodiments of the present invention have been described above, the present invention is not limited to the above embodiments, but includes all aspects included in the concept of the present invention and claims, and varies within the scope of the present invention. Can be modified to.

Next, the present invention will be described in more detail based on examples, but the present invention is not limited thereto.

(Example 1)
<Making metal oxides>
As a metal oxide, a linked aggregate was prepared in which flaky nanocrystal pieces having a main surface in which the (001) crystal plane of copper oxide was exposed were connected to each other. Specifically, after mixing 2.0 g of copper (II) chloride dihydrate (manufactured by Junsei Chemical Co., Ltd.) and 1.6 g of urea (manufactured by Junsei Chemical Co., Ltd.), 180 ml of ethylene glycol (manufactured by Junsei Chemical Co., Ltd.) (Manufactured by Junsei Chemical Co., Ltd.) and 120 ml of water were added and further mixed. The obtained mixed solution of copper chloride and urea was injected into a pressure-resistant glass container having an internal volume of 500 ml, and heat treatment was performed at 180 ° C. for 24 hours in a closed atmosphere inside the container. Then, the mixed solution was cooled to room temperature and held for 1 day, and then the thin-film-shaped precipitate formed from the closed container was recovered. Next, the precipitate was washed with methanol and pure water and vacuum dried at 70 ° C. for 10 hours under vacuum to obtain a linked aggregate in which nanocrystal pieces of copper oxide were interconnected.

<Preparation of metal oxide dispersion>
4 mg of the copper oxide linked aggregate obtained as described above was added to a mixed solution of 1700 μL of purified water and 800 μL of isopropanol, and 15 μL of a 5% by mass solution of Nafion® as a polymer electrolyte was added. The obtained mixed liquid was dispersed at 20 to 40 ° C. for 1 hour with an ultrasonic disperser to prepare a dispersion liquid of a connected aggregate of copper oxide.

<Preparation of electrode catalyst material and catalyst ink>
4 mg of VULCAN XC-72 (registered trademark), which is a conductive carbon black manufactured by Cabot Corporation, was added to the dispersion liquid of the connected aggregate of copper oxide obtained as described above, and 20 by an ultrasonic disperser. A dispersion treatment was carried out at ~ 40 ° C. for 10 minutes to prepare a catalyst ink containing an electrode catalyst material in which carbon particles were supported on a connected aggregate of copper oxide.

(Example 2)
<Preparation of dispersion liquid of conductive material>
4 mg of VULCAN XC-72 (registered trademark), which is a conductive carbon black manufactured by Cabot Corporation used in Example 1, was added to a mixed solution of 1700 μL of purified water and 800 μL of isopropanol, and Nafion (registered trademark) was added as a polymer electrolyte. ) 15 μL of a 5 mass% solution was added. The obtained mixed liquid was dispersed at 20 to 40 ° C. for 1 hour with an ultrasonic disperser to prepare a dispersion liquid of a conductive material.

<Preparation of electrode catalyst material and catalyst ink>
To the dispersion liquid of the conductive material obtained as described above, 4 mg of the linked aggregate of copper oxide obtained in Example 1 was added, and the mixture was dispersed at 20 to 40 ° C. for 10 minutes with an ultrasonic disperser. The treatment was carried out to prepare a catalyst ink containing an electrode catalyst material in which carbon particles were supported on a connected aggregate of copper oxide.

(Example 3)
<Preparation of dispersion liquid of conductive material>
4 mg of VULCAN XC-72 (registered trademark), which is a conductive carbon black manufactured by Cabot Corporation used in Example 1, was added to a mixed solution of 850 μL of purified water and 400 μL of isopropanol, and Nafion (registered trademark) was added as a polymer electrolyte. ) 8 μL of a 5 mass% solution was added. The obtained mixed liquid was dispersed at 20 to 40 ° C. for 1 hour with an ultrasonic disperser to prepare a dispersion liquid of a conductive material.

<Preparation of metal oxide dispersion>
4 mg of the copper oxide linked aggregate obtained in Example 1 was added to a mixed solution of 850 μL of purified water and 400 μL of isopropanol, and 7 μL of a 5% by mass solution of Nafion® as a polymer electrolyte was added. The obtained mixed liquid was dispersed at 20 to 40 ° C. for 1 hour with an ultrasonic disperser to prepare a dispersion liquid of a connected aggregate of copper oxide.

<Preparation of electrode catalyst material and catalyst ink>
The dispersion liquid of the conductive material obtained as described above and the dispersion liquid of the connected aggregate of copper oxide are mixed and subjected to a dispersion treatment at 20 to 40 ° C. for 10 minutes with an ultrasonic disperser to obtain copper oxide. A catalyst ink containing an electrode catalyst material in which carbon particles were supported on the connected aggregate of the above was prepared.

(Example 4)
<Preparation of electrode catalyst material and catalyst ink>
A powder prepared by weighing 4 mg of the copper oxide linked aggregate prepared in Example 1 and 4 mg of VULCAN XC-72 (registered trademark), which is a conductive carbon black manufactured by Cabot Corporation, was mixed in a dairy pot, and 1700 μL of purified water and isopropanol were mixed. It was added to 800 μL of the mixed solution, and 15 μL of Nafion® 5% by mass solution was further added as a polymer electrolyte. The obtained mixed solution was dispersed at 20 to 40 ° C. for 1 hour with an ultrasonic disperser to prepare a catalyst ink containing an electrode catalyst material.

(Comparative Example 1)
Except for the fact that commercially available copper oxide nanoparticles (544868 Copper (II) oxide manufactured by Sigma Aldrich Japan GK) were prepared in place of the copper oxide linked aggregate prepared in Example 1 and used as an electrode catalyst material. Made a catalyst ink containing an electrode catalyst material in the same manner as in Example 1.

(Comparative Example 2)
Except for the fact that commercially available copper oxide nanoparticles (544868 Copper (II) oxide manufactured by Sigma Aldrich Japan GK) were prepared in place of the copper oxide linked aggregate prepared in Example 2 and used as an electrode catalyst material. Made a catalyst ink containing an electrode catalyst material in the same manner as in Example 2.

(Comparative Example 3)
Except for the fact that commercially available copper oxide nanoparticles (544868 Copper (II) oxide manufactured by Sigma Aldrich Japan GK) were prepared in place of the copper oxide linked aggregate prepared in Example 3 and used as an electrode catalyst material. Made a catalyst ink containing an electrode catalyst material in the same manner as in Example 3.

<Preparation of electrodes>
15 μL of catalyst ink containing each electrode catalyst material obtained in the above Examples and Comparative Examples was collected with a micropipette, dropped onto 5 mmΦ glassy carbon of a rotating electrode, and heated in a constant temperature bath at 60 ° C. for 30 minutes. And dried. After repeating this dropping operation three times, the surface of the rotating electrode was observed with a stereomicroscope, and it was confirmed that the catalyst layer (electrode catalyst layer) of the electrode catalyst material was uniformly formed on the glassy carbon.

<Evaluation of catalytic activity in redox reaction>
Then, the oxygen reduction reaction (ORR) activity was evaluated for each electrode catalyst material. Specifically, the ORR activity was evaluated by the convection voltammetry method. Stability was confirmed by cyclic voltammetry (CV) measurement using a rotating ring disk electrode device manufactured by PINE INSTRUMENT, a potentiostat (HSV-110), and a 0.1 M KOH aqueous solution as an electrolytic solution. Then, the catalytic activity was evaluated by linear sweep voltammetry (LSV). A glassy carbon electrode having a diameter of 5 mm was used as the working electrode (WE), a coiled platinum electrode was used as the counter electrode (CE), and a silver / silver chloride comparison electrode was used as the reference electrode (RE). The measurement conditions are as follows.

(1) Ar bubbling (30 minutes)
(2) O ₂ bubbling (30 minutes)
(3) CV measurement (in O ₂ )
+ 0.2V~-1.0V, sweep rate: 10mV / s, 3 cycles (4) LSV measurement (in _{O 2)}
0.0V to -0.8V, sweep speed: 1mV / s, 3 cycles, rotation speed: 2000rpm

From the data obtained as described above, the relationship between the potential and the current density was illustrated as shown in FIG. 7, and the catalytic activity was evaluated. The catalytic activity was evaluated according to the following two criteria.

(1) Evaluation of the starting potential of ORR The ^{absolute value of the potential at -5.0 x 10-5} A, which corresponds to 15% of the loss amount with respect to the theoretical electromotive force (1.23 V) in the fuel cell. .185V or less was evaluated as acceptable, and more than 0.185V was evaluated as unacceptable.

(2) Comparison with the current value of the Pt-C catalyst The current value of the Alfa Aesar Pt-C catalyst (20% by mass Pt) measured under the same conditions as the absolute value of the current at -0.7V. A current value of 1.216 mA or more of 80% or more with respect to 52 mA was evaluated as acceptable, and a current value of less than 1.216 mA was evaluated as rejected.

If both of the above evaluations (1) and (2) are acceptable, it can be evaluated that the ORR exhibits high catalytic activity. The evaluation results of Examples 1 to 4 and Comparative Examples 1 to 3 are shown in Table 1 below.

<Evaluation of electrical conductivity>
The electrical conductivity of the electrode catalyst material formed on the glassy carbon was measured with a 4-probe probe of a resistance meter RM3545 manufactured by HIOKI. The electrical conductivity of the electrode catalyst material was evaluated as a ratio to the electrical conductivity (assumed to be 100%) of the reference electrode prepared as follows.

15 μL of the dispersion liquid of the conductive material prepared in Example 3 was collected with a micropipette, dropped onto 5 mmΦ glassy carbon of a rotating electrode, heated in a constant temperature bath at 60 ° C. for 30 minutes, and dried. This dropping operation was repeated three times to serve as a reference electrode for electrical conductivity.

From Table 1, in Examples 1 to 4 having the connected aggregate of copper oxide and carbon particles as the conductive material, both the starting potential of ORR and the comparison with the current value of the Pt—C catalyst are acceptance criteria, and the ORR Demonstrated high catalytic activity in. On the other hand, in Comparative Examples 1 to 3 in which copper oxide nanoparticles were used instead of the linked aggregate of copper oxide, both the starting potential of ORR and the current value of the Pt—C catalyst were rejected criteria. High catalytic activity could not be obtained in ORR. Further, among the examples 1 to 4, the electrode catalyst materials of Examples 1 to 3 are the electrical conductivity of the reference electrode (a thin film prepared by preparing a dispersion liquid containing the conductive material produced in Example 3 on glassy carbon). On the other hand, it was an electrode catalyst material that exhibited an electrical conductivity of 1.0% or more and was excellent in electrical contact.

Moreover, the crystal structure of the electrode catalyst material was observed using a scanning electron microscope (SEM, SU8020 manufactured by JEOL Ltd.). FIG. 2 is an SEM image when the electrode catalyst material produced in Example 1 is observed at a magnification of 30,000 times, and FIG. 3 is a reflection of the SEM image shown in FIG. 2 in the same field of view. It is an electron image, and the white part indicates copper oxide. From FIGS. 2 and 3, in the electrode catalyst material produced in Example 1, copper oxide is dispersed and arranged, and a crystal plane exhibiting catalytic activity in ORR without agglomeration of the connected aggregate of copper oxide is formed. It was confirmed that it was secured. Further, the carbon particles are supported on the connected aggregate of copper oxide, and the carbon particles are in contact with the main surface of the nanocrystal shards of the connected aggregate of copper oxide, and the carbon particles are connected to each other in a continuous structure. Had. As a result, electrons can be efficiently transferred to the crystal plane, so that the obtained electrode catalyst material showed high catalytic activity in ORR.

FIG. 4 is an SEM image of the electrode catalyst material produced in Example 3 as a representative, when observed at a magnification of 50,000 times. As shown in FIG. 4, a continuous structure in which carbon particles were connected to each other was formed on the outward end face of the copper oxide connected aggregate. As a result, electrons can efficiently move to the crystal plane of copper oxide, so that the obtained electrode catalyst material showed high catalytic activity in ORR.

In Comparative Example 1, the crystal structure of the electrode catalyst material was observed using a scanning electron microscope (SEM, SU8020 manufactured by JEOL Ltd.) as in Example 1. FIG. 5 is an SEM image when the electrode catalyst material produced in Comparative Example 1 is observed at a magnification of 30,000 times, and FIG. 6 is a reflected electron image in the same field of view of the SEM image shown in FIG. Yes, the white part indicates copper oxide. From FIGS. 5 and 6, in the electrode catalyst material produced in Comparative Example 1, copper oxide nanoparticles are aggregated and the distance between the reaction surface of copper oxide and the carbon particles is long, so that the structure is difficult to transfer electrons. It was confirmed that there was.

1 Electrode catalyst material 20 Connecting aggregate 21 Nanocrystal pieces 22 Main surface 23 End face 30 Conductive material 31 Continuous structure

Claims (10)

An electrode catalyst material for a fuel cell having a metal oxide and a conductive material.
The metal oxide is a linked aggregate in which flaky nanocrystal pieces having a main surface and an end face on which a specific crystal face is exposed are connected to each other.
The plurality of nanocrystal pieces have a gap between the main surfaces, which is arranged so as to be open to the outside of the connecting assembly.
An electrode catalyst material in which the conductive material is in contact with at least a part of the nanocrystal pieces. The electrode catalyst material according to claim 1, wherein the average thickness of the nanocrystal pieces is less than 10 nm. The electrode catalyst material according to claim 1 or 2, wherein the metal oxide is copper oxide. The electrode catalyst material according to claim 3, wherein the specific crystal plane is a (001) crystal plane. The electrode catalyst material according to any one of claims 1 to 4, wherein the conductive material is a carbon material having a continuous structure in which conductive substances are connected. The electrode catalyst material according to claim 5, wherein the conductive substance is at least one selected from fibrous carbon and carbon particles. Any one of claims 1 to 6 in which the electrical conductivity of the electrode catalyst material formed on the electrode is 0.5% or more with respect to the electrical conductivity of the layer formed on the electrode by the conductive material. The electrode catalyst material according to the section. A catalyst ink for forming an electrode catalyst layer for a fuel cell, which comprises the electrode catalyst material according to any one of claims 1 to 7, a polymer electrolyte, and a solvent. An electrode catalyst layer for a fuel cell formed by using the catalyst ink according to claim 8. It has a positive electrode having a positive electrode catalyst layer, a negative electrode having a negative electrode catalyst layer, and a solid polymer electrolyte layer arranged between the positive electrode catalyst layer and the negative electrode catalyst layer. ,
A membrane electrode assembly for a fuel cell, wherein at least one of the positive electrode catalyst layer and the negative electrode catalyst layer contains the electrode catalyst material according to any one of claims 1 to 7. ..

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