JP7340419B2 - Electrocatalyst materials for fuel cells - Google Patents

Description

The present invention relates to an electrode catalyst material, and particularly to an electrode catalyst material having high catalytic activity as an air electrode catalyst material for a fuel cell.

In recent years, from the perspective of energy conservation, there has been an increasing demand for improvements in power generation equipment and battery performance. Furthermore, from the perspective of reducing environmental impact and production costs, there is a need to develop new materials for electrodes installed in power generation devices and batteries while maintaining and improving conventional performance. Furthermore, from the viewpoint of reducing exhaust gas and greenhouse gases, it has been proposed to use power generation devices such as fuel cells to drive transportation equipment such as automobiles.

Conventionally, 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 used. Platinum is excellent as a catalyst for oxygen reduction reactions (hereinafter sometimes referred to as "ORR"), and carbon particles have excellent conductivity. It is commonly used as an air electrode catalyst material. However, since platinum is a rare metal with little reserves and is also expensive, a new catalyst material is needed to replace the catalyst material in which fine platinum particles are supported on the surface of carbon particles.

As a new electrode that can also be used as an electrode for fuel cells, there is provided a carbon nanotube composite electrode having a surface layer containing a porous oxide and carbon nanotubes on the surface of an electrode base material, wherein the carbon nanotubes are A carbon nanotube composite electrode has been proposed that is formed from an oxide and in which at least some of the carbon nanotubes are electrically connected to an electrode base material (Patent Document 1).

Patent Document 1 discloses an electrode in which carbon nanotubes generated from a porous oxide are firmly fixed to an electrode base material, thereby preventing the carbon nanotubes that exchange electrons with the electrode base material from falling off from the electrode base material. be. Further, Patent Document 1 proposes that metal fine particles, such as platinum fine particles, which are commonly used as a catalyst to be mounted on an electrode, be supported on the wall surface of a carbon nanotube fixed to an electrode base material.

In this way, in Patent Document 1, metal fine particles such as platinum fine particles are used as the metal catalyst fine particles supported on carbon nanotubes, as in the past, and a new catalyst material is used to replace the catalyst material on which platinum fine particles are supported. No materials have been proposed.

International Publication No. 2012/157506

The present invention was made in view of the above circumstances, and uses platinum as a catalyst component while having oxygen reduction reaction activity and catalytic activity equivalent to that of an electrode catalyst material in which fine platinum particles are supported on the surface of carbon particles. The purpose of the present invention is to provide an electrocatalyst material for fuel cells that does not require the use of an electrocatalyst material.

The present inventors have conducted intensive studies on the above-mentioned problem, and have found that nanocrystalline pieces of a metal oxide having catalytic activity are flaky with a main surface and end faces in which specific crystal planes are exposed. The plurality of nanocrystal pieces are connected to each other, and the plurality of nanocrystal pieces are connected to each other, and the plurality of nanocrystal pieces are connected to each other, and the plurality of nanocrystal pieces are connected to each other, and the conductive material is connected to the main surfaces of the nanocrystal pieces. Along the surface, there are planar parts where a plurality of conductive linear substances are in contact with each other and distributed in the planar direction, and the planar conductivity of the planar parts is in the direction perpendicular to the planar direction. It is a conductive structure with greater conductivity, and the main surface of the nanocrystal piece and the planar portion are in electrical contact, so that the same level of conductivity can be achieved even without using platinum as a catalyst component. It was found that oxygen reduction reaction activity and catalytic activity were obtained.

That is, the gist of the present invention is as follows.
[1] An electrode catalyst material for a fuel cell comprising a metal oxide and a conductive structure, wherein the metal oxide has a flaky shape having a main surface and end surfaces in which specific crystal planes are exposed. It is a connected aggregate in which nanocrystal pieces are interconnected,
A plurality of the nanocrystal pieces have a gap arranged between the main surfaces and open to the outside of the connected aggregate,
The conductive structure has a planar portion along the main surface of the nanocrystal piece, in which a plurality of conductive linear substances are in contact with each other and distributed in the planar direction, and the conductivity is greater than the conductivity in the direction perpendicular to the surface direction,
An electrode catalyst material in which the nanocrystal piece and the planar portion are in contact with each other.
[2] The electrode catalyst material according to [1], wherein the nanocrystalline pieces have an average thickness of 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 average dimension of the planar portion in a direction orthogonal to the surface direction is less than 10 nm.
[6] The conductive structure includes a metal oxide contact portion in which at least one conductive linear substance among the plurality of conductive linear substances electrically contacts the nanocrystal piece; The electrode catalyst material according to any one of [1] to [5], further comprising: an electrode base material contact portion in which the two conductive linear substances are electrically contacted with the electrode base material.
[7] The electrode catalyst material according to any one of [1] to [6], wherein the conductive structure further includes a gas diffusion section having gas permeability.
[8] The electrode catalyst material according to any one of [1] to [7], wherein the conductive linear substance is a carbon nanotube.
[9] The electrical conductivity of the electrocatalyst material formed on the electrode is 5.0% or more of the electrical conductivity of the layer formed of the conductive structure on the electrode [1] to [ 8].

According to an aspect of the present invention, at least a portion of the flaky nanocrystal piece having a main surface in which a specific crystal plane of a metal oxide having catalytic activity is exposed is a nanocrystal of a conductive structure. The electrocatalyst material, in which platinum fine particles are supported on the surface of carbon particles, is in contact with a planar region with excellent conductivity in the plane direction formed along the main surface of a nanocrystalline piece. An electrode catalyst material for fuel cells having comparable oxygen reduction reaction activity and catalytic activity can be obtained.

According to an aspect of the present invention, the metal oxide is copper oxide, and the specific crystal plane is the (001) crystal plane, so that the metal oxide has the same level of performance as an electrode catalyst material in which platinum fine particles are supported on the carbon particle surface. Oxygen reduction reaction activity and catalytic activity can be reliably obtained.

According to the aspect of the present invention, since the conductive structure has the metal oxide contact portion and the electrode base material contact portion, electron transfer between the metal oxide and the electrode base material is facilitated.

1 is a schematic diagram illustrating an embodiment of an electrode catalyst material of the present invention. It is a graph showing the relationship between potential and current density in measuring catalytic activity. 3 is a SEM image of the electrode catalyst material in Example 3.

EMBODIMENT OF THE INVENTION Hereinafter, the electrode catalyst material which is an embodiment of this invention is demonstrated using drawings. FIG. 1 is a schematic diagram illustrating an embodiment of the electrode catalyst material of the present invention.

<Electrode catalyst material>
As shown in FIG. 1, an electrode catalyst material 1 according to an embodiment of the present invention includes a metal oxide having catalytic activity and a conductive structure 31 supported on the metal oxide. The metal oxide is a connected aggregate 20 in which a plurality of thin nanocrystal pieces 21 are interconnected, each having a main surface 22 and an end surface 23 in which specific crystal planes are exposed. The connected aggregate 20 exhibits excellent catalytic activity because it is composed of flaky nanocrystal pieces 21 having a main surface 22 in which specific crystal planes are exposed. Further, the connected aggregate 20 has a gap G in which a plurality of nanocrystal pieces 21 are arranged between the main surfaces 22 and open to the outside of the connected aggregate 20 .

A conductive structure 31 is supported on a connected aggregate 20 in which flaky nanocrystal pieces 21 are assembled. In the conductive structure 31, a planar portion 32 is formed by a plurality of conductive linear substances 30 being in contact with each other and distributed in the plane direction along the main surface 22 of the nanocrystal piece 21. The conductivity of the planar portion 32 in the planar direction is greater than the conductivity in the direction orthogonal to the planar direction. Further, the main surface 22 of the nanocrystal piece 21 and the planar portion 32 are in electrical contact. Since the planar portion 32 of the conductive structure 31 has a property that the conductivity in the planar direction is greater than the conductivity in the direction orthogonal to the planar direction, the connected assembly 20 and the conductive structure 31 Electronic transfers between the parties will be facilitated.

<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 interconnected, and has 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.

Further, the shape of the nanocrystal piece 21 is a thin flake with the thickness of the end face 23 being thinner than the size of the main surface 22. On the outer surface of the connected aggregate 20, a gap G is formed between the main surfaces 22 of the plurality of adjacent nanocrystal pieces 21, and this gap G is opened to the outside of the connected aggregate 20. There is.

Here, the main surface 22 of the nanocrystal piece 21 is a surface with a large surface area among the outer surfaces constituting the flaky nanocrystal piece 21, and defines the upper and lower edges of the end face 23 with a narrow surface area. means both surfaces. In the electrode catalyst material 1 used for catalytic reactions, specific crystal planes are exposed on the main surface 22. The main surface 22 on which a specific crystal plane is exposed becomes a catalytically active surface exhibiting high catalytic activity. Therefore, the larger the surface area of the main surface 22, the more efficiently the catalytic reaction can be performed.

The minimum dimension 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, and the average thickness t of the nanocrystal piece 21 is not particularly limited, but is It is preferably 1/10 or less of the minimum dimension of No. 22. As a result, the area of the main surface 22 of the nanocrystal piece 21 becomes about 10 times larger than the area of the end face 23, and the catalytic activity per unit amount of the connected aggregate 20 is increased by the catalytic activity per unit amount of the nanoparticles. improved compared to For example, the average thickness of the nanocrystalline pieces is preferably less than 10 nm. If the minimum dimension of the main surface 22 is 1.0 μm or more, it tends to be difficult to connect the nanocrystal pieces 21 with high density, and if the minimum dimension is less than 10 nm, the adjacent nanocrystal pieces There is a tendency that it becomes impossible to form a sufficient gap G between the main surfaces 22 of 21. Furthermore, in order to suppress a decrease in the rigidity of the nanocrystalline pieces 21 in the thickness direction, the average thickness t of the nanocrystalline pieces 21 is preferably 1.0 nm or more. Note that the dimensions of the main surface 22 of the nanocrystal piece 21 can be determined by measuring the nanocrystal piece 21 separated from the connected aggregate 20 as an individual nanocrystal piece so as not to damage the shape of the nanocrystal piece 21. I can do it. As a specific example of the measurement method, a rectangle with the minimum area circumscribing the main surface 22 of the nanocrystal piece 21 is drawn, and the short and long sides of the rectangle are taken as the minimum and maximum dimensions of the nanocrystal piece 21, respectively. Measure.

The nanocrystal pieces 21 constituting the connected aggregate 20 are made of metal oxide. Examples of metal oxides include oxides of noble metals, oxides of transition metals, oxides of alloys thereof, and composite oxides. Examples of noble metals and alloys thereof include metals consisting of one component selected from the group of palladium (Pd), rhodium (Rh), ruthenium (Ru), silver (Ag), and gold (Au), or Mention may be made of alloys containing one or more components selected from the group. Furthermore, the transition metal and its alloy include, for example, a metal consisting of one component selected from the group of copper (Cu), nickel (Ni), cobalt (Co), and zinc (Zn), or a metal selected from these groups. Examples include alloys containing one or more components.

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

<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 of the nanocrystal piece 21 becomes a catalytically active surface. configured to have.

In order to configure the main surface 22 of the nanocrystal piece 21 to be a reducing catalytically active surface, in the metal oxide constituting the nanocrystal piece 21, the surface of metal atoms that exhibit catalytic activity is set to the main surface 22. The main surface 22 may be composed of a metal atomic plane by oriented so that the main surface 22 is located at It is preferable that the number ratio 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, in the metal oxide constituting the nanocrystal piece 21, the surface of oxygen atoms that exhibits catalytic activity is The main surface 22 may be composed of an oxygen atomic plane by oriented so as to be located on the surface 22. Specifically, the oxygen atoms occupying in the metal atoms and oxygen atoms constituting the metal oxide present on the main surface 22 may be It is preferable that the number ratio of atoms is 80% or more.

Depending on the role of the catalytic active surface, by adjusting the number ratio of metal atoms or oxygen atoms to the metal atoms and oxygen atoms that constitute the metal oxide present on the main surface 22 of the nanocrystal piece 21, the main surface The catalytic activity function of 22 can be enhanced, and sufficient catalytic activity can be exhibited as nanocrystalline pieces 21 and, ultimately, as electrode catalyst material 1.

Furthermore, the main surface 22 of the nanocrystalline piece 21 is assumed to have a specific crystal orientation because the crystal orientation that is present in large numbers on the main surface 22 differs depending on the type of metal oxide constituting the nanocrystalline piece 21. It is. Therefore, although the crystal orientation of the main surface 22 is not specifically described, 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, The catalytically active plane is preferably a (001) crystal plane.

In addition, as for the structure in which the main surface 22 is a metal atomic plane, the crystal structure of the metal oxide is changed to a regular structure in which metal atomic planes and oxygen atomic planes are stacked regularly and alternately, and the arrangement of atoms is regular. , it is preferable to configure so that the metal atomic plane is located on the main surface 22. Specifically, this applies not only when the main surface 22 has a structure composed of an aggregate of single crystals with the same orientation, but also when the main surface 22 has a structure composed of an aggregate of single crystals with different crystal structures or different orientations, or when the main surface 22 has a structure composed of an aggregate of single crystals with different crystal structures or different orientations, Even if the structure is composed of an aggregate containing crystals, there may be a case where a metal atomic plane exists on the main surface 22.

An example of the conductive linear substance 30 constituting the conductive structure 31 is a carbon nanotube (hereinafter sometimes referred to as "CNT"). CNTs are cylindrical bodies having a single-walled structure or a multi-walled structure, and are called SWNTs (single-walled nanotubes) and MWNTs (multi-walled nanotubes), respectively. The outer diameter of the CNT is several nm or less, and the length in the longitudinal direction is, for example, 500 nm to 20 μm. A CNT having a two-layer structure has a three-dimensional network structure in which two cylindrical bodies having a hexagonal lattice network structure are arranged substantially coaxially, and is called a DWNT (double-walled nanotube). The hexagonal lattice, which is a constituent unit, is a six-membered ring with a carbon atom at its apex, and these six-membered rings are adjacent to and continuously bonded to other six-membered rings. The properties of CNT are related to the chirality of the cylindrical body. Chirality is broadly classified into armchair type, zigzag type, and chiral type, with armchair type exhibiting metallic behavior, zigzag type exhibiting semiconducting and semimetallic behavior, and chiral type exhibiting semiconducting and semimetallic behavior. Therefore, the conductivity of CNTs differs depending on which chirality the cylindrical body has. In the electrode catalyst material 1, in order to further improve the conductivity of the conductive linear substance 30, it is preferable to increase the proportion of armchair-type CNTs exhibiting metallic behavior.

On the other hand, by doping chiral CNTs exhibiting semiconducting behavior with a substance (different element) having electron-donating or electron-accepting properties, the chiral CNTs exhibit metallic behavior. In addition, in general metals, doping with a different element causes scattering of conduction electrons within the metal, resulting in a decrease in conductivity. Similarly, CNTs that exhibit metallic behavior are doped with a different element. If it does, it will cause a decrease in conductivity. In this way, the doping effects on CNTs that exhibit metallic behavior and CNTs that exhibit semiconducting behavior are in a trade-off relationship from the viewpoint of conductivity, so theoretically CNTs exhibit metallic behavior. It is desirable to separately produce CNTs and CNTs exhibiting semiconducting behavior, perform doping treatment only on the CNTs exhibiting semiconducting behavior, and then combine them. When CNTs exhibiting metallic behavior and CNTs exhibiting semiconducting behavior are produced in a mixed state, it is preferable to select a CNT layer structure in which doping treatment with different elements or molecules is effective. Thereby, the conductivity of the conductive structure 31 made of a mixture of CNTs exhibiting metallic behavior and CNTs exhibiting semiconducting behavior can be further improved.

For example, CNTs with a small number of layers, such as a two-layer structure or a three-layer structure, have relatively higher conductivity than CNTs with a larger number of layers. The doping effect is highest in structured CNTs. Therefore, in order to further improve the conductivity of the conductive structure 31, it is preferable to increase the proportion of CNTs having a two-layer structure or a three-layer structure. Specifically, the ratio of CNTs having a two-layer structure or a three-layer structure to the total CNTs is preferably 50% by number or more, and particularly preferably 75% by number or more.

Due to the above-mentioned structure, CNTs have a characteristic that the conductivity in the longitudinal direction is superior to that in the radial direction, that is, the CNTs have anisotropy in conductivity. From this, the conductivity in the planar direction of the planar portion 32 of the conductive structure 31 has a characteristic that is greater than the conductivity in the direction perpendicular to the planar direction (thickness direction).

<Conductive structure>
As shown in FIG. 1, the electrode catalyst material 1 according to the embodiment of the present invention has a conductive structure 31 supported on a connected assembly 20. As shown in FIG. The conductive structure 31 includes the plurality of conductive linear substances 30 described above, and the plurality of conductive linear substances 30 are dispersed and supported on the connected aggregate 20, so that the conductive structure 31 is It is carried by a connected assembly 20. The conductive linear material 30 is supported within the gap G. Further, a plurality of conductive linear substances 30 are supported on the main surface 22 of the nanocrystal piece 21 along the main surface 22 of the nanocrystal piece 21 while being in contact with each other. Furthermore, the conductive linear material 30 is supported with its longitudinal direction along the main surface 22 of the nanocrystal piece 21 . Therefore, the conductive structure 31 has a planar region 32 in which a plurality of conductive linear substances 30 are distributed planarly along the plane direction of the main surface 22 of the nanocrystal piece 21 . The plurality of conductive linear substances 30 in the planar region 32 are arranged randomly in the longitudinal direction. In the conductive structure 31, the plurality of conductive linear substances 30 do not need to be distributed in a planar manner in a portion other than the planar portion 32, and the longitudinal direction of the plurality of conductive linear substances 30 is , are randomly placed. In FIG. 1, for convenience of explanation, a conductive structure 31 (a plurality of conductive linear substances 30) is supported on a partial region of the connected assembly 20, but the conductive structure 31 (a plurality of conductive linear substances 30) is supported over the entire region of the connected assembly 20. A conductive structure 31 (a plurality of conductive linear substances 30) may be supported.

The average dimension of the planar portion 32 of the conductive structure 31 in the direction perpendicular to the planar direction (that is, the thickness direction of the planar portion 32) is such that the catalytic activity of the connected aggregate 20, which is a metal oxide, is inhibited. The thickness is preferably less than 10 nm from the viewpoint of preventing the formation of particles. Since the average dimension in the thickness direction of the planar portions 32 is less than 10 nm, the main surface 22 of the nanocrystal piece 21, which is a catalytically active surface, is completely covered with the planar portions 32 of the conductive structure 31. As a result, the main surface 22 can exhibit an excellent catalytic function. Further, the dimension in the planar direction of the planar portion 32 of the conductive structure 31 is, for example, since the planar portion 32 is a portion distributed in a planar manner along the planar direction of the main surface 22 of the nanocrystal piece 21. , is approximately the same as the dimension of the main surface 22 of the nanocrystal piece 21.

The planar region 32 of the conductive structure 31 is a region in which a plurality of CNTs (conductive linear substances 30) are distributed in a planar manner along the plane direction of the main surface 22 of the nanocrystal piece 21. The CNTs 31 are in electrical surface contact with the main surface 22 of the nanocrystal piece 21 at a planar portion 32 formed of the CNTs. Therefore, electron transfer from the planar portion 32 of the CNT to the main surface 22 of the nanocrystal piece 21 is facilitated. The metal oxide of the electrode catalyst material 1, which is a catalyst for O ₂ +4H ⁺ +4e ⁻ →2H ₂ O, which is the oxygen reduction reaction at the air electrode of the fuel cell, smoothly transfers electrons from the planar portion 32 of the CNT. , the efficiency of oxygen reduction reaction is improved.

In order for the conductive linear substance 30 to be reliably supported on the connected aggregate 20 and for the main surface 22 of the nanocrystal piece 21 to efficiently exhibit catalytic activity, the conductive linear substance 30 must be supported on the connected aggregate 20. Appropriately control the amount of supported. For example, from the viewpoint of the balance between the conductivity and catalytic activity of the electrode catalyst material 1, it is preferable that the conductive linear substance 30 is supported in an amount of 0.1 parts by mass or more and 20 parts by mass or less with respect to 100 parts by mass of the connected aggregate 20. is preferable, and it is particularly preferable that 0.2 parts by mass or more and 10 parts by mass or less are supported.

<Applications of electrode catalyst materials>
Electrode catalyst material 1, which is an embodiment of the present invention, can be used as an air electrode catalyst material for fuel cells.

When the electrode catalyst material 1 according to the embodiment of the present invention is used as an electrode catalyst material for a fuel cell, the conductive structure 31 includes at least one conductive linear material among the plurality of conductive linear materials 30. In an embodiment, 30 has a metal oxide contact portion in which the nanocrystal piece 21 is electrically contacted, and an electrode base material contact portion in which at least one conductive linear substance 30 is in electrical contact with the electrode base material. good. Planar portion 32 of conductive structure 31 corresponds to a metal oxide contact portion. Since the conductive structure 31 has the metal oxide contact portion and the electrode base material contact portion, electron exchange between the nanocrystal pieces 21 of the connected aggregate 20 and the electrode base material can be further facilitated.

Electrodes for fuel cells are equipped with a gas diffusion layer in order to uniformly disperse gas supplied to the electrodes. On the other hand, since the conductive structure 31 includes the plurality of conductive linear substances 30, it has excellent gas permeability. Therefore, the conductive structure 31 can further include a gas diffusion section having gas permeability. From the above, when the electrode catalyst material 1 according to the embodiment of the present invention is used as an electrode catalyst material for a fuel cell, it is clear that the conductive structure 31 further has a gas diffusion part having gas permeability. Therefore, the gas diffusion section of the conductive structure 31 can be used as a substitute for the gas diffusion layer, and as a result, there is no need to provide a separate gas diffusion layer, so the number of parts can be reduced.

<Method for producing electrode catalyst material>
Next, an example of a method for manufacturing the electrode catalyst material of the present invention will be described. An example of a method for producing an electrode catalyst material is a metal oxide preparation step Sa in which a metal oxide is a connected aggregate in which flaky nanocrystal pieces are interconnected, and a metal oxide preparation step Sa in which the prepared metal oxide is conductive. and a conductive linear material supporting step Sb of supporting a conductive linear material.

The metal oxide preparation step Sa includes a mixing step Sa1 and a hydrothermal synthesis step Sa2 in which temperature and pressure are applied.

(Mixing step Sa1)
The mixing process involves mixing a hydrate of a compound containing a noble metal, a transition metal, or an alloy thereof, which is a raw material for a metal oxide, especially a hydrate of a metal halide, and a metal complex, which is a precursor of a metal oxide. This is a step of dissolving an organic compound having a diamide carbonate skeleton constituting a ligand in an organic solvent such as ethylene glycol, 1,4-butanediol, or polyethylene glycol, water, or a solvent containing both. Examples of hydrates of metal halides include copper(II) chloride dihydrate, and examples of organic compounds having a carbonate diamide skeleton include urea.

(Hydrothermal synthesis step Sa2)
The hydrothermal synthesis step is a step of applying predetermined heat and pressure to the obtained mixed solution and leaving it for a predetermined period of 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, the reaction between urea and the metal halide cannot be completed, and if it exceeds 300°C, the reaction vessel cannot withstand the high vapor pressure generated. The heating time is preferably 10 hours or more. If the heating time is less than 10 hours, unreacted materials may remain. In order to apply predetermined heat and pressure, for example, a method of heating and pressurizing using a pressure-resistant container or a closed container can be mentioned. After the mixed solution is heated and pressurized, it is cooled to room temperature and held for a certain period of time, and then the generated precipitate is collected. The collected precipitate is washed with methanol, pure water, etc., and dried for a predetermined period of time. In this way, the desired metal oxide can be obtained.

After the metal oxide preparation step Sa, a conductive linear substance supporting step Sb is performed. The conductive linear material supporting step Sb includes a conductive linear material dispersion step Sb1 of producing a dispersion of a conductive linear material, a metal oxide dispersion step Sb2 of producing a dispersion of the prepared metal oxide, The method includes a dispersion mixing step Sb3 of mixing a conductive linear substance dispersion and a metal oxide dispersion.

(Conductive linear substance dispersion step Sb1)
In the conductive linear substance dispersion process, an organic solvent and a dispersant are added to a dispersion medium (e.g., water), a conductive linear substance is added to a mixed liquid, and then the conductive linear substance is dispersed using an ultrasonic dispersion machine or the like. This is a step of preparing a dispersion of a conductive linear substance. Examples of the organic solvent include alcohols such as ethanol and isopropyl alcohol. Examples of the dispersant include carboxymethyl cellulose, sodium dodecyl sulfate, and water-soluble xylan. The content of the conductive linear substance contained in the dispersion of the conductive linear substance is 0.05% by mass or more and 5.0% by mass or less, from the viewpoint of the balance between dispersibility of the conductive linear substance and manufacturing efficiency. is preferred, and 0.1% by mass or more and 2.0% by mass or less is particularly preferred. Note that, if necessary, an electrolyte used in a fuel cell may be further added to and dispersed in the dispersion of the conductive linear material. Examples of the electrolyte include polymer electrolytes such as Nafion (registered trademark).

(Metal oxide dispersion step Sb2)
In the metal oxide dispersion process, the metal oxide prepared in the metal oxide preparation process is added to a mixed liquid in which an organic solvent is added to a dispersion medium (for example, water), and then the metal oxide prepared in the metal oxide preparation process is dispersed using an ultrasonic dispersion machine or the like. In this step, a metal oxide dispersion is prepared. Examples of the organic solvent include monoalcohols such as methanol, ethanol, n-propyl alcohol, and isopropyl alcohol. The content of metal oxide contained in the metal oxide dispersion is preferably 0.05% by mass or more and 5.0% by mass or less, and 0.1% by mass or less, from the viewpoint of balance between dispersibility of metal oxide and production efficiency. Particularly preferably % by mass or more and 2.0% by mass or less. Note that, if necessary, an electrolyte used in a fuel cell may be further added to and dispersed in the metal oxide dispersion. Examples of the electrolyte include polymer electrolytes such as Nafion (registered trademark).

(Dispersion liquid mixing step Sb3)
The dispersion mixing step is a step in which a dispersion of a conductive linear material and a dispersion of a metal oxide are subjected to a dispersion treatment using an ultrasonic disperser or the like and mixed. In the dispersion mixing step, from the viewpoint of the balance between the conductivity and catalytic activity of the electrode catalyst material, the area covered by the conductive linear substance on 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 linear substance are adjusted. When copper oxide nanocrystal pieces are used as the metal oxide and CNTs are used as the conductive linear substance, a preferable coverage area can be obtained by containing the metal oxide and the conductive linear substance in equal mass. Through such steps, the electrode catalyst material 1 is produced.

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

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

(Example 1)
Preparation of Metal Oxide As a metal oxide, a connected aggregate in which flaky nanocrystal pieces having a main surface in which the (001) crystal plane of copper oxide was exposed was interconnected. Specifically, after mixing 2.0 g of copper (II) chloride dihydrate (manufactured by Junsei Kagaku Co., Ltd.) and 1.6 g of urea (manufactured by Junsei Kagaku Co., Ltd.), 180 ml of ethylene glycol ( (manufactured by Junsei Kagaku Co., Ltd.) and 120 ml of water were added and further mixed. The obtained mixed solution of copper chloride and urea was poured into a pressure-resistant glass container with an internal volume of 500 ml, and heat-treated at 180° C. for 24 hours in a sealed atmosphere inside the container. Thereafter, the mixed solution was cooled to room temperature and held for one day, and then a thin film-shaped precipitate was collected from the sealed container, washed with methanol and pure water, and then heated under vacuum for 70 minutes. It was vacuum dried at ℃ for 10 hours to obtain a connected aggregate in which copper oxide nanocrystal pieces were interconnected.

Preparation of dispersion liquid of conductive linear material CNT was used as the conductive linear material. The CNT dispersion liquid was prepared using a dispersion medium containing purified water (60 mass%), ethanol (20 mass%), and isopropyl alcohol (20 mass%) so that the CNT content was 1.0 mass%. . As a dispersant used in the CNT dispersion, carboxymethyl cellulose (CMC) was added in an equal mass to CNT. A CNT dispersion liquid was prepared by adding CNTs and carboxymethyl cellulose to the above dispersion medium, and then performing a dispersion treatment at 20 to 40° C. for 1 hour using an ultrasonic disperser. It was confirmed that the CNT dispersion liquid was stably dispersed, as the dynamic light scattering particle diameter of CNTs in the CNT dispersion liquid was 150 to 300 nm.

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 isopropyl alcohol, and 15 μL of a 5% by mass solution of Nafion as a polymer electrolyte was added. A dispersion liquid of a connected aggregate of copper oxide was prepared by performing a dispersion treatment at 20 to 40° C. for 1 hour using an ultrasonic disperser.

Mixing of the dispersion 20 mg of the CNT dispersion obtained as above was added to the dispersion of the connected aggregate of copper oxide obtained as above, and the mixture was heated to 20 to 40°C using an ultrasonic disperser. A dispersion treatment was performed for 10 minutes to produce an electrode catalyst material in which a conductive structure formed of a plurality of CNTs was supported on the interconnected aggregate of copper oxide of Example 1.

(Example 2)
As the CNT dispersion, ZERONANO SG101 dispersion (dispersion medium: water, CNT concentration 0.5% by mass) manufactured by Zeon Nano Technology Co., Ltd. was used in place of the CNT dispersion prepared in Example 1. 4 mg of a linked aggregate of copper oxide prepared in the same manner as in Example 1 was added to a mixed solution of 1600 μL of purified water and 800 μL of isopropyl alcohol, and 15 μL of a 5% by mass solution of Nafion was added as a polymer electrolyte, followed by ultrasonic dispersion. Dispersion treatment was carried out in a machine at 20 to 40°C for 1 hour to prepare a dispersion of connected aggregates of copper oxide. 16 mg of the above CNT dispersion was added to the dispersion of the linked aggregate of copper oxide obtained as above, and a dispersion treatment was performed at 20 to 40°C for 10 minutes using an ultrasonic disperser. An electrode catalyst material was manufactured in which a conductive structure formed of a plurality of CNTs was supported on a connected aggregate of copper oxide of No. 2.

(Example 3)
As the CNT dispersion, instead of the CNT dispersion prepared in Example 1, a multilayer CNT dispersion MW-I (dispersion medium: water, CNT concentration 2% by mass) manufactured by Meijo Nano Carbon Co., Ltd. was used. 4 mg of a linked aggregate of copper oxide prepared in the same manner as in Example 1 was added to a mixed solution of 1600 μL of purified water and 800 μL of isopropyl alcohol, and 15 μL of a 5% by mass solution of Nafion was added as a polymer electrolyte, followed by ultrasonic dispersion. Dispersion treatment was carried out in a machine at 20 to 40°C for 1 hour to prepare a dispersion of connected aggregates of copper oxide. 2 mg of the above CNT dispersion was added to the dispersion of the linked aggregate of copper oxide obtained as above, and a dispersion treatment was performed at 20 to 40°C for 10 minutes using an ultrasonic disperser. An electrode catalyst material was manufactured in which a conductive structure formed of a plurality of CNTs was supported on a connected aggregate of copper oxide of No. 3.

(Comparative example 1)
4 mg of a linked aggregate of copper oxide prepared in the same manner as in Example 1 was added to a mixed solution of 1700 μL of purified water and 800 μL of isopropyl alcohol, and 15 μL of a 5% by mass solution of Nafion was added as a polymer electrolyte, followed by ultrasonic dispersion. Dispersion treatment was carried out in a machine at 20 to 40°C for 1 hour to prepare a dispersion of connected aggregates of copper oxide. 4 mg of carbon black (Vulcan Carbon, manufactured by Cabot) was added to the dispersion of the linked aggregate of copper oxide obtained as described above, and the mixture was dispersed for 10 minutes at 20 to 40°C using an ultrasonic disperser. The treatment was carried out to produce an electrocatalyst material of Comparative Example 1.

(Comparative example 2)
A comparative example was carried out in the same manner as in Example 1, except that commercially available copper oxide nanoparticles (manufactured by Sigma-Aldrich Japan LLC, 544868 Copper (II) oxide) were used in place of the linked aggregate of copper oxide in Example 1. Electrocatalyst materials of No. 2 were manufactured.

(Comparative example 3)
4 mg of commercially available copper oxide nanoparticles (manufactured by Sigma-Aldrich Japan LLC, 544868 Copper (II) oxide) were added to a mixed solution of 1700 μL of purified water and 800 μL of isopropyl alcohol, and 15 μL of a 5% by mass solution of Nafion as a polymer electrolyte was added. Then, dispersion treatment was performed at 20 to 40° C. for 1 hour using an ultrasonic dispersion machine to prepare a dispersion of copper oxide nanoparticles. 4 mg of carbon black (Vulcan Carbon, manufactured by Cabot) was added to the dispersion of copper oxide nanoparticles obtained as above, and a dispersion treatment was performed at 20 to 40°C for 10 minutes using an ultrasonic dispersion machine. The electrode catalyst material of Comparative Example 3 was produced.

(Example 4)
A CNT dispersion liquid was prepared in the same manner as in Example 1, and 4 mg of the copper oxide connected aggregate prepared in the same manner as in Example 1 was added to 0.8 mg of the CNT dispersion liquid, and this was mixed with 1700 μL of purified water and isopropyl alcohol. The electrode catalyst material of Example 4 was added to 800 μL of the mixed solution, and 15 μL of a 5% by mass solution of Nafion as a polymer electrolyte was added, followed by dispersion treatment for 1 hour at 20 to 40° C. using an ultrasonic dispersion machine. was manufactured.

(Electrode preparation and ORR activity evaluation)
15 μL of the electrode catalyst material obtained as described above was collected with a micropipette, dropped onto a 5 mmΦ glassy carbon of a rotating electrode, and heated and dried in a constant temperature bath at 60° C. for 30 minutes. After repeating this dropping operation three times, the surface of the rotating electrode was observed with a stereomicroscope, and after confirming that a catalyst layer of the electrode catalyst material was homogeneously formed on the glassy carbon, ORR activity evaluation was performed. . Specifically, ORR activity was evaluated by convective voltammetry. Using a rotating ring disk electrode device manufactured by PINE INSTRUMENT, a potentiostat (HSV-110), and a 0.1M KOH aqueous solution as the electrolyte, after confirming stability with cyclic voltammetry (CV) measurement, linear sweep voltammetry was performed. Metry (L
The catalytic activity of the electrode catalyst material was evaluated by SV). A glassy carbon electrode with 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) _O2 bubbling (30 minutes)
(3) CV measurement (in _O2 ) +0.2V to -1.0V, sweep rate: 10mV/s, 3 cycles (4) LSV measurement (in _O2 ) 0.0V to -0.8V, sweep rate: 1mV/s, 3 cycles, rotation speed 2000rpm

From the data obtained as described above, the relationship between potential and current density was illustrated as shown in FIG. 2, and the catalytic activity was evaluated. The catalytic activity was evaluated using the following two types of criteria, and the following evaluation was [B] or higher for both of the two types of criteria, and the following evaluation for at least one of the criteria was [A], and the catalytic activity was evaluated as good.
(1) Evaluation of ORR starting potential: At a potential of -5.0×10 ^-5 A, the absolute value is 15% of the loss amount relative to the theoretical electromotive force (1.23V) in the fuel cell. A value of 0.19V or less was evaluated as a pass [B], and a value exceeding 0.19V was evaluated as a fail [C]. Among those that passed, those with an absolute value of 10% of 0.12 V or less, which is the amount of loss with respect to the theoretical electromotive force (1.23 V), were evaluated as particularly excellent characteristics [A]. (2) Comparison with current value of Pt-C catalyst: Absolute value of current at -0.7V, current value of Alfa Aesar Pt-C catalyst (20 mass% Pt) measured under the same conditions 1. A current value of 1.22 mA or more, which is 80% or more with respect to 52 mA, was evaluated as a pass [B], and a current value of less than 1.22 mA was evaluated as a fail [C]. Among those that passed, 90% (90%) of 1.37 mA or more were evaluated as particularly excellent characteristics [A].

(Evaluation of electrical conductivity)
The electrical conductivity of the electrode catalyst material formed on the glassy carbon was measured using a 4-probe probe of a resistance meter RM3545 manufactured by Hioki Electric Co., Ltd. Note that the electrical conductivity of the electrode catalyst material was evaluated as a ratio to the electrical conductivity (assumed to be 100%) of a reference electrode prepared as follows.

15 μL of the CNT dispersion prepared in Example 1 was collected with a micropipette, dropped onto a 5 mmΦ glassy carbon of a rotating electrode, and dried by heating in a constant temperature bath at 60° C. for 30 minutes. This dropping operation was performed once and used as a reference electrode for electrical conductivity.

(organizational observation)
The crystal structure of the electrode catalyst material was observed using a scanning electron microscope (SEM, manufactured by JEOL Ltd.). FIG. 3 is a SEM image of the electrode catalyst material in Example 3 observed at a magnification of 20,000 times. A two-dimensional conductive surface is formed by folding thin line-shaped one-dimensional crystalline CNTs, and a connected aggregate of copper oxide is placed on top of the two-dimensional conductive surface. It was confirmed that the electrode catalyst material was as designed.

The evaluation results of Examples 1 to 4 and Comparative Examples 1 to 3 are shown in Table 1 below.

From Table 1 above, in Examples 1 to 3 in which CNTs were supported on a connected aggregate of copper oxide, both the ORR starting potential and the comparison with the current value of the Pt-C catalyst were rated A, indicating that they were excellent catalysts. It showed activity. In Example 4, in which the process of supporting CNTs on the connected aggregate of copper oxide was not carried out, the comparison with the current value of the Pt--C catalyst was rated A, but the starting potential of ORR was rated B.

On the other hand, Comparative Example 1 in which carbon black was supported on a connected aggregate of copper oxide instead of CNT, Comparative Example 2 in which CNT was supported on copper oxide nanoparticles instead of a connected aggregate of copper oxide, and Copper oxide nano In Comparative Example 3 in which carbon black was supported on particles, there was no A rating in comparison with the starting potential of ORR and the current value of the Pt--C catalyst, and good catalytic activity could not be obtained.

Furthermore, among Examples 1 to 4, the electrical conductivity of the electrode catalyst materials of Examples 1 to 3 was higher than that of the reference electrode (a thin film made of the CNT dispersion prepared in Example 1 on glassy carbon). Since the electrical conductivity of the electrode catalyst material of Example 4 was 3.2% with respect to the electrical conductivity of the reference electrode, Examples 1 to 3 had an excellent Corresponding to the catalytic activity exhibited, an electrode catalyst material with excellent electrical contact was obtained.

1 Electrocatalyst material 20 Connected aggregate 21 Nanocrystal piece 22 Main surface 23 End surface 30 Conductive linear substance 31 Conductive structure 32 Planar site

Claims (9)

An electrode catalyst material for a fuel cell, comprising a metal oxide and a conductive structure,
The metal oxide is a connected aggregate in which flaky nanocrystal pieces having main surfaces and end faces with specific crystal planes are interconnected,
A plurality of the nanocrystal pieces have a gap arranged between the main surfaces and open to the outside of the connected aggregate,
The connected aggregate has a flower-like shape in which the nanocrystal pieces corresponding to petals are connected and aggregated,
The conductive structure has a planar portion along the main surface of the nanocrystal piece, in which a plurality of conductive linear substances are in contact with each other and distributed in the planar direction, and the conductivity is greater than the conductivity in the direction perpendicular to the surface direction,
An electrode catalyst material in which the nanocrystal piece and the planar portion are in contact with each other. The electrocatalyst material according to claim 1, wherein the nanocrystalline pieces have an average thickness of 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 average dimension of the planar portion in a direction perpendicular to the planar direction is less than 10 nm. The conductive structure includes a metal oxide contact portion where at least one of the plurality of conductive linear substances electrically contacts the nanocrystal piece, and at least one of the conductive linear substances. The electrode catalyst material according to any one of claims 1 to 5, further comprising an electrode base material contact portion in which the linear substance is in electrical contact with the electrode base material. The electrode catalyst material according to any one of claims 1 to 6, wherein the conductive structure further includes a gas diffusion section having gas permeability. The electrode catalyst material according to any one of claims 1 to 7, wherein the conductive linear substance is a carbon nanotube. Any one of claims 1 to 8, wherein the electrical conductivity of the electrode catalyst material formed on the electrode is 5.0% or more with respect to the electrical conductivity of the layer formed on the electrode with the conductive structure. Electrocatalyst material according to item 1.

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