JP2013127869A - Platinum-titanium oxide-titanium carbide composite catalyst for fuel batteries, manufacturing method thereof, and film-electrode assembly for fuel batteries with composite catalyst - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide: a platinum-titanium oxide-titanium carbide composite catalyst for fuel batteries having both excellent catalyst active property and durability; a manufacturing method thereof; and a film-electrode assembly for fuel batteries with the composite catalyst.SOLUTION: The catalyst for fuel batteries comprises: platinum fine particles; a titanium oxide layer including titanium oxide and surrounding the platinum fine particles; and titanium carbide (TiC). The platinum fine particles, and the titanium oxide layer are arranged on a surface of the titanium carbide. The titanium oxide layer has at least one electrically conducting channel through the platinum fine particles between the surface of the titanium carbide and a surface of the catalyst with the aid of electrical contact between a platinum atom in the platinum fine particle and the titanium carbide surface. In the electrically conducting channel, a platinum atom in the platinum fine particle and the titanium oxide are bonded to each other.

Description

  The present invention relates to a platinum / titanium oxide / titanium carbide composite catalyst for fuel cells having excellent catalytic activity and durability, a method for producing the same, and a membrane / electrode assembly for fuel cells using the composite catalyst.

  As various electrode catalysts and gas diffusion electrode catalysts that control energy conversion and substance conversion, generally, metal fine particle supported catalysts obtained by supporting a metal component on a conductive carrier such as carbon are used. In particular, in a fuel cell which is an energy conversion device that directly converts chemical energy into electrical energy by supplying fuel and an oxidant to two electrically connected electrodes and electrochemically oxidizing the fuel, the anode As a cathode electrode catalyst, a catalyst in which platinum fine particles are supported on a conductive carrier such as carbon is employed.

The electrode potential at the anode and the cathode varies greatly due to various causes such as continuous operation of the fuel cell over a long period of time and start / stop. Depending on the operating conditions of the fuel cell, corrosion occurs in a conductive carrier such as carbon, and the electrode performance deteriorates with time.
As a technique for preventing the corrosion of such a conductive support, Patent Document 1 includes a catalytic metal that promotes an electrode reaction, a conductive support that supports the catalytic metal, and a metal carbide. A fuel cell electrode catalyst is disclosed. In paragraph [0013] of the specification of Patent Document 1, it is described that the metal carbide has an effect of preventing the corrosion of the conductive support. In addition, in paragraph [0019] of the specification of Patent Document 1, the reaction start potential for the electrochemical oxidation reaction with water is nobler than the reaction start potential of carbon, and the metal carbide having high corrosion resistance is described. Examples include silicon carbide, zirconium carbide, cerium carbide, titanium carbide, and tantalum carbide.

JP 2004-172107 A

  In paragraph [0032] of the specification of Patent Document 1, it is described that platinum is used as a catalyst metal. However, platinum catalysts still have problems in catalyst performance and catalyst durability, and the use of platinum catalysts is a problem of the amount of resources such as reserves and mining amount, the problem of price due to being relatively scarce, In addition, since the production areas are ubiquitous, it is easy to be affected by the international situation, and it is easy to cause large price fluctuations.

On the other hand, in addition to the advantage that the amount of platinum can be reduced, the shell that covers the surface of the core particle from the viewpoint that only the surface of the catalyst promotes the catalytic reaction and the inner core does not contribute to the reactivity of the individual catalyst particles. The technology of an electrode catalyst having a core / shell structure using platinum as a base material (hereinafter sometimes referred to as a core-shell catalyst) has attracted attention. However, in the core-shell catalyst, a metal that is baser than platinum is usually contained in the core. Therefore, in the fuel cell using the core-shell catalyst, there is a problem that the core metal is eluted during discharge and the catalytic performance and catalyst durability of the core-shell catalyst are lowered.
The present invention has been accomplished as a result of diligent research in view of the above circumstances, and is a platinum / titanium oxide / titanium carbide composite catalyst for fuel cells having both excellent catalytic activity and durability, a method for producing the same, and the composite It aims at providing the membrane electrode assembly for fuel cells using a catalyst.

  The platinum / titanium oxide / titanium carbide composite catalyst for fuel cells of the present invention is a catalyst for fuel cells containing fine platinum particles, titanium oxide, and titanium carbide (TiC), and the titanium carbide is formed on the surface thereof. The platinum fine particles and the titanium oxide layer containing the titanium oxide and surrounding the platinum fine particles, and the platinum atoms in the platinum fine particles are in electrical contact with the titanium carbide surface. Thus, the titanium oxide layer includes at least one conductive channel through the platinum fine particles between the surface of the titanium carbide and the surface of the catalyst, and the platinum atoms in the platinum fine particles are included in the conductive channel. And the titanium oxide has a bond.

  In the present invention, the titanium oxide layer preferably has a thickness of 0.23 to 2 nm.

  In the present invention, the titanium carbide preferably has a beaded structure.

  In the present invention, the platinum fine particles preferably have an average particle size of 0.25 to 1.5 nm.

  The membrane / electrode assembly for a fuel cell according to the present invention is a membrane / electrode assembly for a fuel cell comprising an anode electrode on one side of a polymer electrolyte membrane and a cathode electrode on the other side. At least an anode catalyst layer is provided, the cathode electrode is provided with at least a cathode catalyst layer, and at least one of the anode catalyst layer and the cathode catalyst layer includes the platinum / titanium oxide / titanium carbide composite catalyst for the fuel cell. It is characterized by that.

  The method for producing a platinum / titanium oxide / titanium carbide composite catalyst for a fuel cell according to the present invention is a method for producing a catalyst for a fuel cell containing platinum fine particles, titanium oxide, and titanium carbide (TiC). A step of preparing a solution, a step of preparing an aqueous dispersion of the titanium carbide, a reducing agent is added to the aqueous dispersion of titanium carbide, and the surface of the titanium carbide is oxidized with water to form a titanium oxide layer. A step of mixing an aqueous dispersion of titanium carbide having a titanium oxide layer formed thereon and the platinum ion solution to prepare a mixed solution, the platinum ion solution, an aqueous dispersion of titanium carbide, and a titanium oxide. Carbon monoxide is bubbled into at least one of the aqueous dispersion of titanium carbide having a layer formed on the surface and the mixed liquid. A titanium oxide containing the titanium oxide on the surface of the titanium carbide and surrounding the platinum fine particles by adding a reducing agent to the mixed solution and precipitating platinum fine particles on the surface of the titanium carbide. And forming at least one conductive channel through the platinum fine particles between the surface of the titanium carbide and the surface of the catalyst in the titanium oxide layer, and the conductive layer It has a reduction step of forming a bond between a platinum atom and the titanium oxide in the channel.

  According to the present invention, a conventional platinum alloy catalyst or core-shell catalyst can be obtained by securing a conductive channel through platinum fine particles on the surface of titanium carbide and combining platinum atoms with titanium oxide in the conductive channel. It is possible to exhibit superior catalyst performance and catalyst durability.

It is sectional drawing which showed the typical example of the composite catalyst which concerns on this invention typically, and its enlarged view. It is a figure which shows an example of the membrane-electrode assembly for fuel cells of this invention, Comprising: It is the figure which showed typically the cross section cut | disconnected in the lamination direction. It is a cross-sectional schematic diagram of the rosary titanium carbide in each manufacture process in the manufacture example of a rosary titanium carbide. In the manufacturing method which concerns on this invention, it is the schematic diagram which showed the structure of the titanium carbide surface from the process of preparing the aqueous dispersion of titanium carbide to the reduction process. 4 is a TEM image of beaded titanium carbide of Production Example 1. FIG. 4 is a SEM image of a cross section substantially perpendicular to the surface direction of the membrane / electrode assembly of Example 3. FIG. 6 is a discharge curve of the membrane / electrode assembly of Example 3. It is a schematic diagram for demonstrating the coupling | bonding between a platinum atom and a titanium atom. It is a part of XRD spectrum of the core-shell catalyst which has baked on the conditions of calcination temperature 500 degreeC, 600 degreeC, and 700 degreeC, respectively, and has platinum shell. It is a schematic diagram of the apparatus used for CO bubbling. It is the isometric view schematic diagram which showed the apparatus which performs an electric potential process. It is an example of the X-ray-diffraction spectrum of the titanium carbide coat | covered with the titanium oxide layer. In the evaluation of the durability of the catalyst of Comparative Example 1, it is a graph in which the CV of the second cycle and the CV of the 10,000th cycle are overlapped. In the evaluation of the durability of the catalyst of Comparative Example 2, it is a graph in which the CV of the second cycle and the CV of the 10,000th cycle are overlapped. 10 is a SEM image of a cross section of the membrane / electrode assembly of Comparative Example 3 substantially perpendicular to the surface direction. 10 is an SEM image at a magnification of 10,000 times of a cross section substantially perpendicular to the surface direction of the membrane / electrode assembly of Comparative Example 4. FIG. 10 is a discharge curve of the membrane / electrode assembly of Comparative Example 3.

1. Platinum / Titanium Oxide / Titanium Carbide Composite Catalyst for Fuel Cell The platinum / titanium oxide / titanium carbide composite catalyst for fuel cell of the present invention is for a fuel cell containing platinum fine particles, titanium oxide, and titanium carbide (TiC). The titanium carbide includes a titanium oxide layer that includes the platinum fine particles and the titanium oxide and surrounds the platinum fine particles on the surface thereof, and the platinum atoms in the platinum fine particles and the titanium carbide Due to the electrical contact with the titanium carbide surface, the titanium oxide layer comprises at least one conductive channel through the platinum fine particles between the surface of the titanium carbide and the surface of the catalyst, A platinum atom in the platinum fine particle and the titanium oxide have a bond in the conductive channel. .

  Research and development of platinum alloy catalysts for the purpose of improving the catalytic activity of platinum and core-shell catalysts for the purpose of improving the utilization rate of platinum catalysts in order to overcome problems such as high cost and limited reserves of platinum It is actively done. However, since the platinum alloy catalyst and the platinum core-shell catalyst are both a combination of base metals rather than platinum, the base metals are eluted under the operating environment of the fuel cell, and the catalytic activity of platinum and the durability of the catalyst. There is a problem that the performance is lowered.

  As a result of diligent efforts, the present inventors have found that a platinum / titanium oxide / titanium carbide composite catalyst having a completely different structure from conventional platinum alloy catalysts and core-shell catalysts can achieve both excellent catalyst performance and catalyst durability. The present inventors have found that the amount of platinum used can be reduced and cost reduction can be achieved.

  The platinum / titanium oxide / titanium carbide composite catalyst for fuel cells according to the present invention (hereinafter sometimes referred to as composite catalyst) contains platinum fine particles, titanium oxide, and titanium carbide. Hereinafter, the platinum fine particles, titanium oxide, and titanium carbide used in the present invention will be described in order.

1-1. Platinum fine particles The platinum fine particles used in the present invention are present on the surface of titanium carbide. Here, the presence of the platinum fine particles on the surface of the titanium carbide includes an aspect in which the surface of the platinum fine particles and the surface of the titanium carbide are in contact with each other, and includes an aspect in which the platinum fine particles are supported on the surface of the titanium carbide.

Furthermore, the platinum atoms in the platinum fine particles and the titanium carbide surface are in electrical contact. When titanium oxide described later is used alone as a fuel cell electrode catalyst, it is generally difficult to impart sufficient conductivity to the electrode. Assuming that the titanium oxide covers the entire surface of the titanium carbide, the conductive path connecting the titanium carbide and the outside of the composite catalyst is blocked by the titanium oxide, and between the composite catalyst and other parts inside the composite catalyst and the fuel cell. Any conductivity between the members may be impaired. Therefore, when the platinum atoms in the platinum fine particles are in electrical contact with the titanium carbide surface, a conductive path, a so-called conductive channel is formed between the surface of the titanium carbide and the composite catalyst surface via the platinum fine particles, The conductivity between the composite catalysts and between the composite catalyst and other members inside the fuel cell can be ensured.
Here, “electrically in contact” means that a conductive path is ensured by the contact between the platinum atom and the titanium carbide surface, and that the platinum atom and the titanium carbide surface are located very close to each other. This includes the case where a conductive path is secured by a tunnel effect between the platinum atom and the titanium carbide surface. As an example in which the platinum atom and the titanium carbide surface are located in the very vicinity, there is a case where a nano-order titanium oxide layer exists between the platinum atom and the titanium carbide surface.

Here, the conductive channel can be rephrased as an electron conduction path formed of at least one platinum fine particle and penetrating through the titanium oxide layer and connecting the surface of the titanium carbide and the outside of the composite catalyst. At least one conductive channel is formed in the titanium oxide layer. It is considered that there are as many conductive channels as there are platinum fine particles existing on the surface of titanium carbide, but the number of conductive channels does not necessarily match the number of platinum fine particles.
Further, in the conductive channel, the platinum atom in the platinum fine particle and the titanium oxide in the titanium oxide layer have a bond. This connection will be described in detail later.

The average particle diameter of the platinum fine particles used in the present invention is 0.25 to 1.5 nm.
According to a known document (T, Ioannides et al. Journal of Catalysis 161, 560-569 (1996)) regarding charge transfer by a metal-semiconductor junction model, if the diameter of the metal particles on the metal oxide is 2 nm or less, 0 There is a description that a charge transfer of .5 electrons / (metal atom) occurs and as a result affects the entire metal particle. On the other hand, with respect to platinum fine particles having an average particle size of 1.5 nm or less measured by small angle X-ray scattering (hereinafter sometimes referred to as SAXS) measurement, the effect of electron donation (SMSI described later) Effect) can be confirmed more clearly.
The effective radius of platinum atoms is 1.39 mm (National Astronomical Observatory, “Science Chronology 2008”, Maruzen Co., Ltd., page 486). Therefore, in view of the effective radius, the lower limit of the average particle diameter of the platinum fine particles used in the present invention is 0.25 nm.
Therefore, the average particle diameter of the platinum fine particles used in the present invention is 0.25 to 1.5 nm, preferably 0.50 to 1.5 nm from the viewpoint of containing platinum atoms bonded to titanium oxide, and more Preferably it is 1.0-1.5 nm.

The average particle diameter of the particles in the present invention is calculated by a conventional method.
The average particle diameter of the particles can be determined by, for example, SAXS measurement. As SAXS measurement conditions, for example, an X-ray diffractometer (manufactured by Rigaku, model number: RINT-2500, etc.) is used. Is mentioned.
Another example of the method for calculating the average particle diameter of the particles is as follows. First, a transmission electron microscope (hereinafter referred to as TEM) with an appropriate magnification (for example, 50,000 to 1,000,000 times), an image or a scanning electron microscope (hereinafter referred to as SEM). Then, for a certain particle, the particle diameter when the particle is regarded as spherical is calculated. Calculation of the particle size by such TEM observation or SEM observation is performed for 200 to 300 particles of the same type, and the average of these particles is taken as the average particle size.

1-2. Titanium oxide The titanium oxide used in the present invention exists as a titanium oxide layer on the surface of titanium carbide. In the present invention, the titanium oxide is represented by the chemical formula TiO _x (0 <x ≦ 2).
The coverage of the titanium oxide layer with respect to titanium carbide is preferably 50% or more and less than 100%, and more preferably 50 to 99%. If the coverage of the titanium oxide layer with respect to titanium carbide is less than 50%, the proportion of the titanium oxide layer is too small and the platinum fine particles are also small. Therefore, relatively, the proportion of titanium carbide in the composite catalyst increases, and the platinum loading rate decreases. Then, when producing the membrane / electrode assembly, the composite catalyst is used more than twice compared with the case where the coverage is 100% in order to increase the amount of platinum. As a result, the catalyst layer becomes thicker and oxygen in the catalyst layer is increased. Diffusion may decrease and the discharge performance of the fuel cell may decrease. On the other hand, if the coverage of the titanium oxide layer with respect to titanium carbide is 100%, the titanium carbide layer may be blocked by the titanium oxide layer, resulting in a loss of conductivity between the composite catalysts.

The “coverage of the titanium oxide layer with respect to titanium carbide” as used herein refers to the ratio of the surface area of the titanium carbide covered with the titanium oxide layer when the total surface area of the titanium carbide is 100%. . Although it is difficult to estimate the coverage, it can be confirmed by X-ray diffraction measurement that the surface of titanium carbide is oxidized to become titanium oxide.
FIG. 12 is an example of an X-ray diffraction spectrum of titanium carbide covered with a titanium oxide layer. Note that FIG. 12 is merely an example of an X-ray diffraction spectrum, and is not necessarily an X-ray diffraction spectrum of the composite catalyst of the present invention. From the ratio of the intensity of the peak derived from titanium oxide (TiO ₂ (111) peak) and the intensity of the peak derived from titanium carbide (TiC (111) peak) in the X-ray diffraction spectrum of FIG. You can give an approximate measure of the rate.

  The titanium oxide layer surrounds the platinum fine particles described above. Here, the state in which the titanium oxide layer surrounds the platinum fine particles means that the titanium oxide constituting the titanium oxide layer surrounds the platinum fine particles, and preferably the titanium oxide layer covers a part of the surface of the platinum fine particles. However, the titanium oxide layer does not completely cover the entire platinum fine particles, and a part of the platinum fine particles appears on the surface of the composite catalyst. The titanium oxide layer surrounds the periphery of the platinum fine particles, whereby the above-described conductive channel is formed and the electrochemical specific surface area of platinum that can react with oxygen or the like (Electrochemical Surface Area; hereinafter referred to as ECSA). ) Is also secured.

  The thickness of the titanium oxide layer is preferably 0.23 to 2 nm. Although titanium carbide has conductivity, when the thickness of the titanium oxide layer existing on the surface exceeds 2 nm, the composite catalyst of the present invention is used in the catalyst layer of the fuel cell. There is a possibility that sufficient conductivity cannot be imparted. Further, when the thickness of the titanium oxide layer exceeds 2 nm, if the platinum fine particles are present also on the titanium oxide layer, the thick titanium oxide layer obstructs the platinum fine particles and the titanium carbide. There is no electrical continuity between the two. On the other hand, when the thickness of the titanium oxide layer is 2 nm or less, even if the platinum fine particles exist on the titanium oxide layer, a conductive path is formed between the platinum fine particles and titanium carbide by the tunnel effect. Is done. If the thickness of the titanium oxide layer is less than 0.23 nm, the titanium oxide may not be present as a compound in the first place.

The thickness of the titanium oxide layer is more preferably less than the average particle size of the platinum fine particles. When the thickness of the titanium oxide layer is equal to or greater than the average particle diameter of the platinum fine particles, the fine particles are completely buried in the titanium oxide layer and contain platinum fine particles that do not appear outside the composite catalyst. This is because the catalytic activity of the oxygen reduction reaction may be impaired.
The output performance of a fuel cell using a platinum catalyst is strongly influenced by the ECSA of platinum. Fuel cells, depending on the application, typically ^are required surface area of 100m 2 ^-Pt / m 2 -MEA more platinum. Depending on the amount of platinum used and the type and amount of ionomer used, if the basis weight is 0.1 mg-Pt / cm ² -MEA, the ECSA of platinum needs to be 100 m ² / g-Pt or more. When the average particle diameter of the platinum fine particles is, for example, 1.5 nm or less, the ECSA is about 200 m ² / g-Pt. When the thickness of the titanium oxide layer surrounding the platinum fine particles and blocking the contact between platinum and oxygen molecules exceeds 0.6 nm, ECSA that can participate in the oxygen reduction reaction is less than 100 m ² / g-Pt. End up.

From the viewpoint that the platinum fine particles can exhibit oxygen reducing ability without being buried in the titanium oxide layer, the thickness of the titanium oxide layer is more preferably 20 to 60% of the average particle diameter of the platinum fine particles.
The thickness of the titanium oxide layer is more preferably 0.23 to 0.60 nm. When the thickness of the titanium oxide layer exceeds 0.60 nm, the platinum fine particles are buried too much in the titanium oxide layer due to the above-mentioned relationship with the average particle diameter of the platinum fine particles, and a sufficient ECSA of platinum can be secured. There is a risk of disappearing. The thickness of the titanium oxide layer is further preferably 0.30 to 0.60 nm. From the same viewpoint, the thickness of the titanium oxide layer is more preferably a monoatomic layer or more and a tetraatomic layer or less.

The thickness of the titanium oxide layer can be measured and calculated by the following method, for example.
First, the composite catalyst particles are observed by TEM, and the composite catalyst particle closest to the spherical shape is selected. Next, the particle size R of the selected composite catalyst particle is measured. Subsequently, the central portion of the composite catalyst particles was subjected to spot analysis by energy dispersive X-ray spectroscopy (TEM-EDS) using a transmission electron microscope, and Pt / The Ti mass ratio is measured. The composite catalyst particles are assumed to have a structure in which titanium carbide particles having a particle diameter of r ₁ are covered with a titanium oxide layer having a thickness of r ₂ , and the thickness of the titanium oxide layer is considered to be uniform. The thickness r ₂ of the surface titanium oxide layer can be estimated from the mass ratio obtained by approximating the sphere and the following formula (1).
R = r ₁ + 2r ₂ formula (1)

The titanium oxide layer includes at least one conductive channel capable of conducting electrons between the surface of the titanium carbide and the surface of the composite catalyst via the platinum fine particles. The conductive channel is as described above.
In the conductive channel, the platinum atom in the platinum fine particle and the titanium oxide in the titanium oxide layer have a bond.
As an example of forming a platinum layer on a titanium oxide, a report by Diebold et al. (U. Diebold. Et.al, Surf. Sci., 331, 845-854, 109 (1995)) is known. However, the bond between platinum and titanium itself is not necessarily strong.
In the present invention, from the standpoint that a strong metal support interaction (hereinafter referred to as SMSI) effect is exhibited and both the specific activity and durability of platinum can be improved, oxygen is partially absorbed from a part of titanium oxide. It is preferable that the removed portion, so-called oxygen defect of titanium oxide, and the platinum atom have a bond.

The action of titanium oxide in the fuel cell operating environment will be specifically described.
As can be seen from the pH-potential diagram at 25 ° C. of the titanium-water system, under the pH-potential conditions (pH = 0-2, potential = 0.4-1.2 V) in the normal operating environment of the fuel cell In titanium, titanium exists in a state of titanium (IV) oxide (TiO ₂ ). Therefore, there is no possibility that the titanium oxide layer will elute in the normal operating environment of the fuel cell.

  Titanium oxide is a preferred compound from the chemical point of view of its catalytic activity, ease of electron donation to platinum placed in oxygen vacancies, and the ability to stably supply titanium oxide sol because of its established preparation method. It is. Titanium oxide is also preferable from an industrial point of view such as cost, reserves, output, and safety to the human body.

FIG. 8 is a schematic diagram for explaining a bond between a platinum atom and a titanium atom. An arrow 11 in FIG. 8 is an arrow representing the flow of electrons from trivalent titanium to platinum.
According to the mechanism based on the SMSI theory, the following can be understood. That is, a titanium atom having a bond with a platinum atom usually exists as a partially reduced unstable metal cation (such as Ti ³⁺ ). An unstable valence cation such as Ti ³⁺ usually changes its valence to a more stable high valence cation (Ti ⁴⁺ ), and thus supplies electrons to the platinum of the bond destination. By this electron donation, platinum is easily maintained in a metal state of zero valence, and even if oxidized, the electron is supplied from a metal cation (Ti ³⁺ or the like) having a low unstable valence to zero again. It is considered to return.
From the above principle, it is presumed that the platinum atom bonded to the oxygen defect of the titanium oxide has a weak bond with oxygen adsorbed from the gas phase as compared with ordinary bulk platinum. Therefore, oxygen adsorbed on the platinum atom is likely to be oxide ion (O ²⁻ ), and the oxide ion forms a bond with proton (H ⁺ ), thereby becoming water (H ₂ O) and easily desorbing from platinum. Release.
As described above, the composite catalyst according to the present invention is considered to have a particularly high catalytic activity for promoting the oxygen reduction reaction occurring at the cathode electrode of the fuel cell as compared with the conventional platinum catalyst.

In order to charge-compensate oxygen defects, it is preferable that the titanium ions in the titanium oxide have a plurality of valences and that the valence change of the titanium ions in the titanium oxide is small.
Table 1 below shows the activity of platinum based on the SMSI theory with the change in the valence of tin (Ti) or tin (Sn), niobium (Nb), or tantalum (Ta), which exhibits the same SMSI effect as titanium. It is the table | surface which put together the order | rank (henceforth a SMSI order | rank) and the order | rank of the activity of platinum (henceforth a first principle calculation order | rank) based on a 1st principle calculation result. In addition, the SMSI ranking in the following Table 1 ranks the stability from the difference in the electron binding energy (Eb value) of X-ray photoelectron spectroscopy (XPS). Moreover, although the first principle calculation of tin was not carried out, it was found that the specific activity of platinum by experiment was larger when titanium was used than when tin was used. Table 1 below shows that the SMSI effect when using titanium is higher than the SMSI effect when using tin, niobium, or tantalum.

The bond between the oxygen defect and the platinum atom in the titanium oxide can be confirmed by detecting unstable Ti ³⁺ cations in the composite catalyst by electron spin resonance (hereinafter referred to as ESR) measurement. The following are examples of ESR measurement conditions.
Apparatus: Electron spin resonance apparatus (manufactured by JEOL, model number: JEOL FE-3X, etc.)
Measurement temperature (T): room temperature (25 ° C.)
Microwave frequency (Fr): 9.19 GHz
Microwave output (power) (Pw): 2 mW
Central magnetic field (Fd) of the chart: 327.5 ± 7.5 mT
Sweep width from center magnetic field (SW): 327.5 ± 50 mT
Magnetic field sweep time (ST): 2.0 min
Magnetic field modulation width (MW): 0.2 mT
Magnification (gain) (G): x1000
Time constant (TC): 0.1s
AD conversion level (AC): 1
Standard sample: Mn ²⁺ / MgO

From the viewpoint that the SMSI effect is exhibited regardless of crystallinity, the titanium oxide used in the present invention may be crystalline or amorphous. In particular, when crystalline TiO ₂ is used, anatase type crystals are more preferable, but rutile type or brookite type crystals may be used.

1-3. Titanium Carbide In the present invention, titanium carbide comprises platinum fine particles and a titanium oxide layer on the surface thereof.
The titanium carbide used in the present invention may contain titanium oxide inside. That is, in the present invention, platinum fine particles and a titanium oxide layer may be present on the surface of 100% titanium carbide, or platinum fine particles and a titanium oxide may be further provided on the surface of titanium carbide containing titanium oxide. A layer may be present. Therefore, at least the outermost surface of the titanium carbide used in the present invention only needs to be covered with titanium carbide.

Hereinafter, the average particle diameter of titanium carbide will be described while comparing with conventional platinum-supported carbon and core-shell catalysts.
In platinum-supported carbon that is used as standard in fuel cells, platinum fine particles having a large average particle diameter cannot be used. This is because when the average particle size is increased, the surface area per 1 g of platinum is reduced, so that an attempt to secure the necessary platinum surface area requires more platinum.
Moreover, in the core-shell catalyst using platinum as the shell and using a noble metal such as palladium as the core, platinum is only the 1 to 3 atomic layers of the outermost layer, so the surface area per 1 g of platinum is large. However, in terms of cost, internal noble metal content must also be taken into account, and there is a limit to increasing the average particle size as with the platinum fine particles. In the case of a core-shell catalyst using a palladium core, considering the cost, the average particle size is preferably around 6 nm, and when the average particle size is 10 nm with sufficient durability, the advantages of the core-shell catalyst are fully enjoyed. Can not.

On the other hand, the cost of the titanium oxide used in the present invention is extremely low, being 1/1000 or less of the cost of the noble metal. Further, the catalytic performance of the composite catalyst according to the present invention tends to depend on the average particle diameter (platinum surface area) of the platinum fine particles, but does not depend so much on the particle diameter of the entire composite catalyst. Therefore, the composite catalyst according to the present invention is different in principle from the core-shell catalyst using a noble metal as a core, and in principle, it is possible to sufficiently exert the catalyst performance even if the overall composite catalyst has an average particle size of 10 nm or more. It is.
The average particle diameter of the composite catalyst according to the present invention is substantially determined by the average particle diameter of titanium carbide in view of the average particle diameter of the platinum fine particles and the thickness of the titanium oxide layer. The average particle size of titanium carbide is preferably 30 nm or less, more preferably 25 nm or less. However, if titanium carbide having an average particle size exceeding 30 nm is used, the average particle size of the entire composite catalyst can be further increased, but the thickness of the catalyst layer is contradictory. When the catalyst layer is thick, there is a risk that the discharge characteristics of the fuel cell will be deteriorated, particularly in a high temperature operating environment or a low temperature operating environment.

The titanium carbide preferably has a beaded structure. Here, the beaded structure means a structure in which a plurality of particles are linked in a chain.
The crystal form of titanium carbide is a face-centered cubic lattice, and titanium carbide usually forms cubic crystals. Even when platinum is supported, the crystal structure of titanium carbide is considered not to change significantly. When a composite catalyst containing particulate titanium carbide is used in the catalyst layer of the membrane / electrode assembly, cubic crystals of titanium carbide are regularly arranged and filled into the catalyst layer, resulting in voids in the catalyst layer. It is not considered. The catalyst layer is usually supplied with oxidant, fuel, water, etc. necessary for the electrode reaction. However, if there are no voids in the catalyst layer, these components necessary for the electrode reaction will not reach the catalyst layer. There is a possibility that the overvoltage will rise and the discharge performance of the membrane / electrode assembly will be lowered. This is apparent from the SEM image of FIG. 15 described later and the discharge performance evaluation of the membrane / electrode assembly of Comparative Example 3.
The conditions of the titanium carbide chemical structure and the average particle diameter described above are applied to each titanium carbide particle constituting the bead-like titanium carbide. That is, the titanium carbide particles constituting the bead-like titanium carbide may be 100% titanium carbide or may contain titanium oxide inside. Moreover, the average particle diameter of the titanium carbide particles constituting the bead-like titanium carbide is preferably 30 nm or less, and more preferably 25 nm or less.

The beaded titanium carbide preferably has a branch. Here, the term “branch” refers to a portion where a chain structure is branched as a result of one particle in a bead-like structure being bonded to three or more particles. By using a composite catalyst containing branched beaded titanium carbide in the catalyst layer of the membrane / electrode assembly, the composite catalyst can be entangled in the catalyst layer, so that more voids can be provided in the catalyst layer. .
The average number of titanium carbide particles constituting the beaded titanium carbide is preferably 10 to 100, more preferably 20 to 80. The average number of branches contained in the beaded titanium carbide is preferably 1 to 10, more preferably 3 to 7.
The average number of titanium carbide particles constituting the beaded titanium carbide and the average number of branches included in the beaded titanium carbide are calculated by the following method, for example. First, in a TEM image or SEM image at an appropriate magnification (for example, 50,000 to 1,000,000 times), a group of titanium carbide particles bonded in a chain is determined as one bead-like titanium carbide. Next, the number of titanium carbide particles in the beaded titanium carbide and the number of branches are counted. Calculation of the number of particles and the number of branches by such TEM observation or SEM observation is performed for 200 to 300 beaded titanium carbide. Let the average of the number of particles of these bead-like titanium carbides be the average of the number of titanium carbide particles constituting the bead-like titanium carbide. Moreover, let the average of the number of branches of these bead-like titanium carbides be the average of the number of branches contained in the bead-like titanium carbide.

As a parameter | index of the space | gap in the catalyst layer containing a bead-shaped titanium carbide, the porosity (number: 5173) described in JISC8000 is mentioned, for example. According to JISC 8000, and porosity is a value of _V 1 / _{V 2 when V 1} the total pore volume of the porous material of the electrode, the total volume of, including pores and _{V 2.}
Examples of the method for measuring the porosity include a mercury porosimetry method and a method of calculating from the physical properties. As a method of calculating from the physical properties, for example, a method of calculating the porosity from the physical properties of each material included in the catalyst layer and the measured value of the thickness of the catalyst layer can be mentioned. Specifically, from the actually measured values of the size, density and content of beaded titanium carbide, the density and content of ionomer suitably added to the catalyst layer, and the SEM cross-sectional observation, The porosity of the catalyst layer can be calculated.
The porosity in the catalyst layer containing the bead-like titanium carbide used in the present invention is preferably 30 to 70%.

By using a composite catalyst containing beaded titanium carbide in the catalyst layer of the fuel cell, innumerable voids are generated in the catalyst layer. As a result, in addition to the effects of the composite catalyst described above, transport of substances such as water and oxygen to the catalyst layer is promoted, and the concentration overvoltage of the fuel cell is reduced and the discharge performance is improved as compared with the conventional fuel cell.
The titanium carbide used in the present invention may be a mixture of beaded titanium carbide and particulate titanium carbide.

FIG. 1 is a cross-sectional view schematically showing a typical example of a composite catalyst according to the present invention and an enlarged view thereof. The double wavy line means that the drawing is omitted. Further, FIG. 1 is not necessarily a figure that faithfully reflects the particle diameter of platinum particles actually used, the thickness of the titanium oxide layer, and the particle diameter of titanium carbide. When beaded titanium carbide is used, the titanium carbide particles in the beaded titanium carbide correspond to the titanium carbide 3 in FIG.
Fig.1 (a) is a cross-sectional schematic diagram of the typical example 100 of the composite catalyst based on this invention. The typical example 100 includes platinum fine particles 1, a titanium oxide layer 2 made of titanium oxide, and titanium carbide 3. On the surface of the titanium carbide 3, platinum fine particles 1 are supported, and a titanium oxide layer 2 surrounding the platinum fine particles 1 is formed.

FIG. 1B is an enlarged schematic diagram of the frame 4 surrounded by the alternate long and short dash line in FIG. A white double arrow 5 indicates an electron conduction path (conduction path).
As shown in FIG. 1B, the platinum atoms (represented by Pt in a circle) in the platinum fine particles and the surface of the titanium carbide 3 are in electrical contact. In the titanium oxide layer, a conductive channel connecting the surface of the titanium carbide 3 and the surface of the composite catalyst is formed through platinum fine particles. Further, in the conductive channel, at least one of the oxygen (represented by O in the circle) that is normally bonded to the titanium ion (represented by Ti in the circle) in the titanium oxide is removed by partial reduction, Platinum atoms in the platinum fine particles are arranged in the removed portion, and the platinum atoms and titanium ions are adjacent to each other and have a bond.

As described above, the composite catalyst according to the present invention uses platinum fine particles to increase the ECSA of platinum, and combines the platinum atoms in the platinum fine particles with inexpensive and chemically stable titanium oxide, thereby improving the SMSI effect. By giving, the specific activity and catalyst durability superior to the conventional platinum catalyst for fuel cells are exhibited. In addition, unlike conventional platinum alloy catalysts and core-shell catalysts, there is no use of unstable metals in the fuel cell operating environment, so both excellent catalytic activity and durability can be achieved, and the amount of precious metals is further reduced to reduce manufacturing costs. Can be reduced.
Furthermore, since the composite catalyst according to the present invention uses titanium carbide having an oxidation potential higher than that of carbon, it exhibits excellent corrosion resistance. As shown in the examples described later, the compatibility between the excellent SMSI effect and the corrosion resistance effect is an effect that cannot be achieved by the conventional platinum-supported titanium carbide catalyst.

2. MEMBRANE / ELECTRODE ASSEMBLY FOR FUEL CELL The fuel cell membrane / electrode assembly of the present invention comprises an anode electrode on one side of a polymer electrolyte membrane and a cathode electrode on the other side. The anode electrode includes at least an anode catalyst layer, the cathode electrode includes at least a cathode catalyst layer, and at least one of the anode catalyst layer and the cathode catalyst layer is formed of platinum / titanium oxide for a fuel cell. It contains a product-titanium carbide composite catalyst.
FIG. 2 is a diagram showing an example of a membrane / electrode assembly for a fuel cell according to the present invention, schematically showing a cross section cut in the stacking direction. The membrane-electrode assembly 200 includes a solid polymer electrolyte membrane (hereinafter sometimes simply referred to as an electrolyte membrane) 21 and a pair of cathode electrode 26 and anode electrode 27 sandwiching the electrolyte membrane 21. Usually, the electrode is formed by laminating a catalyst layer and a gas diffusion layer in order from the electrolyte membrane side. That is, the cathode electrode 26 is formed by stacking the cathode catalyst layer 22 and the gas diffusion layer 24, and the anode electrode 27 is formed by stacking the anode catalyst layer 23 and the gas diffusion layer 25.

  The polymer electrolyte membrane is a polymer electrolyte membrane used in a fuel cell, and includes a fluorine polymer electrolyte membrane containing a fluorine polymer electrolyte such as perfluorocarbon sulfonic acid resin represented by Nafion (trade name). In addition, sulfonic acid can be added to hydrocarbon polymers such as engineering plastics such as polyetheretherketone, polyetherketone, polyethersulfone, polyphenylene sulfide, polyphenylene ether, and polyparaphenylene, and general-purpose plastics such as polyethylene, polypropylene, and polystyrene. And hydrocarbon polymer electrolyte membranes including hydrocarbon polymer electrolytes into which proton acid groups (proton conductive groups) such as groups, carboxylic acid groups, phosphoric acid groups, and boronic acid groups are introduced.

The electrode has a catalyst layer and a gas diffusion layer.
Both the anode catalyst layer and the cathode catalyst layer can be formed using the catalyst ink containing the composite catalyst and the electrode electrolyte described above. Both the anode catalyst layer and the cathode catalyst layer may contain the composite catalyst, either the anode catalyst layer or the cathode catalyst layer contains the composite catalyst, and the other is a conventional platinum catalyst, platinum alloy catalyst, Or the core-shell catalyst may be included.
As the electrode electrolyte, the same material as the polymer electrolyte membrane described above can be used.

  The method for forming the catalyst layer is not particularly limited. For example, the catalyst layer may be formed on the surface of the gas diffusion sheet by applying and drying the catalyst ink on the surface of the gas diffusion sheet, or on the surface of the electrolyte membrane. The catalyst layer may be formed on the electrolyte membrane surface by applying and drying the catalyst ink. Alternatively, a transfer sheet is prepared by applying and drying a catalyst ink on the surface of the transfer substrate, and the transfer sheet is joined to the electrolyte membrane or the gas diffusion sheet by thermocompression bonding or the like. The catalyst layer may be formed on the surface of the electrolyte membrane or the catalyst layer may be formed on the surface of the gas diffusion sheet.

  The catalyst ink is obtained by dissolving or dispersing the above-described catalyst and electrode electrolyte in a solvent. The solvent of the catalyst ink may be appropriately selected. For example, alcohols such as methanol, ethanol and propanol, organic solvents such as N-methyl-2-pyrrolidone (NMP) and dimethyl sulfoxide (DMSO), or organic solvents such as these Mixtures and mixtures of these organic solvents and water can be used. In addition to the catalyst and the electrolyte, the catalyst ink may contain other components such as a binder and a water repellent resin as necessary.

  The method for applying the catalyst ink, the drying method, and the like can be selected as appropriate. For example, examples of the coating method include a spray method, a screen printing method, a doctor blade method, a gravure printing method, and a die coating method. Examples of the drying method include vacuum drying, heat drying, and vacuum heat drying. There is no restriction | limiting in the specific conditions in reduced pressure drying and heat drying, What is necessary is just to set suitably. The thickness of the catalyst layer is not particularly limited, but may be about 1 to 50 μm.

  As the gas diffusion sheet for forming the gas diffusion layer, a gas diffusion property capable of efficiently supplying fuel to the catalyst layer, conductivity, and a strength required as a material constituting the gas diffusion layer, for example, Carbonaceous porous bodies such as carbon paper, carbon cloth, carbon felt, titanium, aluminum, nickel, nickel-chromium alloy, copper and its alloys, silver, aluminum alloy, zinc alloy, lead alloy, niobium, tantalum, iron, Examples thereof include a metal mesh composed of a metal such as stainless steel, gold or platinum, or a conductive porous material such as a metal porous material. The thickness of the conductive porous body is preferably about 50 to 500 μm.

The gas diffusion sheet may be composed of a single layer of the conductive porous body as described above, but a water repellent layer may be provided on the side facing the catalyst layer. The water-repellent layer usually has a porous structure containing conductive particles such as carbon particles and carbon fibers, water-repellent resin such as polytetrafluoroethylene (PTFE), and the like. The water-repellent layer is not necessarily required, but the drainage of the gas diffusion layer can be improved while maintaining an appropriate amount of water in the catalyst layer and the electrolyte membrane, and the catalyst layer, the gas diffusion layer, There is an advantage that the electrical contact between can be improved.
The electrolyte membrane and gas diffusion sheet on which the catalyst layer has been formed by the above-described method are appropriately overlapped and subjected to thermocompression bonding or the like, and joined together to obtain a membrane / electrode assembly.

  The membrane / electrode assembly of the present invention is preferably held by a separator having a reaction gas flow path to form a single cell. The separator has conductivity and gas sealing properties, and can function as a current collector and gas sealing body, for example, a carbon separator containing a high concentration of carbon fiber and made of a composite material with resin, metal A metal separator using a material can be used. Examples of the metal separator include those made of a metal material excellent in corrosion resistance, and those coated with a coating that enhances the corrosion resistance by coating the surface with carbon or a metal material excellent in corrosion resistance. . By appropriately compression molding or cutting such a separator, the above-described reaction gas flow path can be formed.

  The fuel cell membrane / electrode assembly of the present invention has excellent discharge characteristics by containing the composite catalyst in the catalyst layer. In addition, when a composite catalyst containing beaded titanium carbide is used, innumerable voids are generated in the catalyst layer, so that the transport of substances such as water and oxygen to the catalyst layer is promoted. The concentration overvoltage of the fuel cell is reduced and the discharge performance is improved.

3. Method for Producing Platinum / Titanium Oxide / Titanium Carbide Composite Catalyst for Fuel Cell The method for producing a platinum / titanium oxide / titanium carbide composite catalyst for fuel cell according to the present invention comprises platinum fine particles, titanium oxide, and titanium carbide (TiC). A method for producing a catalyst for a fuel cell comprising: a step of preparing a platinum ion solution; a step of preparing an aqueous dispersion of the titanium carbide; adding a reducing agent to the aqueous dispersion of titanium carbide; A step of oxidizing the surface of the carbide with water to form a titanium oxide layer, a step of mixing an aqueous dispersion of titanium carbide having the titanium oxide layer formed on the surface and the platinum ion solution to prepare a mixed solution; The platinum ion solution, the aqueous dispersion of titanium carbide, the aqueous dispersion of titanium carbide having a titanium oxide layer formed on the surface thereof, And bubbling carbon monoxide in at least one of the mixed solution, and adding a reducing agent to the mixed solution to precipitate platinum fine particles on the surface of the titanium carbide. Forming a titanium oxide layer containing the titanium oxide and surrounding the platinum fine particles, and the platinum between the surface of the titanium carbide and the surface of the catalyst in the titanium oxide layer. It has a reduction step of forming at least one conductive channel through fine particles and forming a bond between a platinum atom and the titanium oxide in the conductive channel.

The present invention includes (1) a step of preparing a platinum ion solution, (2) a step of preparing an aqueous dispersion of titanium carbide, (3) a step of forming a titanium oxide layer, and (4) a step of preparing a mixed solution. , (5) a CO bubbling step, and (6) a reduction step. The present invention is not necessarily limited to the above six steps. In addition to the above six steps, for example, a filtration / washing step, a drying step, a pulverizing step, a heating step, an acid treatment step, and an electric potential as described below. You may have a processing process etc.
Hereinafter, the steps (1) to (6) and other steps will be described in order.

3-1. Step of Preparing Platinum Ion Solution The platinum ion solution used in the production method is not particularly limited, but the pH may be adjusted according to the liquidity of an aqueous dispersion of titanium carbide described later. That is, if the aqueous dispersion of titanium carbide is acidic, the platinum ion solution may be previously acidic (pH: about 1 to 5). If the titanium carbide aqueous dispersion is basic, the platinum ion solution may be basic (pH: about 9 to 13) in advance. Thus, aggregation of platinum fine particles and titanium carbide can be suppressed by matching the liquidity of the aqueous dispersion of titanium carbide with the liquidity of the platinum ion solution. If the liquidity of the aqueous dispersion of titanium carbide and the liquidity of the platinum ion solution are different, aggregation of platinum fine particles and titanium carbide may occur due to heat of neutralization. A platinum ion solution prepared in advance may be used, or a commercially available one may be used.
The platinum ion concentration in the platinum ion solution is preferably adjusted after calculating in advance the amount of platinum in the composite catalyst to be produced.
A typical example of this step is as follows. That is, the platinum complex crystal is dissolved in an aqueous solution having a desired pH to prepare a platinum ion solution. Examples of the platinum complex that can be used in this step include hexahydroxyplatinum (IV) acid (H ₂ [Pt (OH) ₆ ]), chloroplatinic acid (H ₂ PtCl ₆ .6H ₂ O), and tetrahydroxyplatinum (IV ) Acid (H ₂ [Pt (OH) ₄ ]), platinum chloride (IV) acid (H ₂ [PtCl ₄ ]) and the like.

3-2. Step of preparing an aqueous dispersion of titanium carbide The titanium carbide used in the present invention may be a pre-synthesized one or a commercially available one. In addition, as a titanium carbide, a 100% titanium carbide can also be used and the titanium carbide which contains a titanium oxide inside can also be used. The titanium carbide used in the present invention is preferably the above-mentioned beaded titanium carbide, but may be a titanium carbide crystal or an amorphous titanium carbide.
As the aqueous dispersion of titanium carbide, one prepared in advance may be used, or a commercially available one may be used. The concentration of titanium carbide in the aqueous dispersion is preferably adjusted after previously calculating the total mass of titanium carbide and titanium oxide in the composite catalyst to be produced.
A typical example of this step is a step of dispersing titanium carbide in an aqueous solution having a desired pH. In this step, in order to further improve the dispersibility of titanium carbide in the aqueous dispersion, homogenization treatment may be performed using an ultrasonic homogenizer or the like. In addition, when the liquid temperature of an aqueous dispersion rises by a homogenization process, since there exists a possibility that a titanium carbide may aggregate, it is preferable to cool after a homogenization process.

A production example of beaded titanium carbide is as follows. In addition, the manufacture example of the bead-shaped titanium carbide used suitably for this invention is not limited only to the following method.
First, a compound containing a titanium atom is dispersed in a dispersion medium. The compound containing a titanium atom is not particularly limited as long as it is a titanium compound that can be appropriately dispersed in a dispersion medium. For example, titanium (IV) alkoxide (Ti (OR) ₄ ; R is an alkyl group), titanium chloride (III) (TiCl ₃ ), titanium (IV) chloride (TiCl ₄ ) and the like. Examples of the dispersion medium include alcohols such as methanol, ethanol, 1-propanol, and 2-propanol; or aqueous solutions of these alcohols. Only one type of alcohol may be used, or two or more types of alcohol may be used. The alcohol ratio in the aqueous alcohol solution is preferably 50% or more.
Next, beaded titania is produced using the prepared titanium dispersion. The method for producing the beaded titania is not particularly limited, and examples thereof include a method of firing with a burner while spraying a titanium dispersion. As an apparatus for realizing such a manufacturing method, for example, an oxide nanoparticle synthesizer (manufactured by Hosokawa Micron, trade name: FCM-MINI) can be cited.

Subsequently, the beaded titania is appropriately washed with an acid or the like, collected by a filter and isolated. The isolated beaded titania is mixed with a carbon source and fired. The carbon source is not particularly limited as long as it can introduce carbon in place of oxygen in titania, and examples thereof include carbon such as activated carbon and ketjen black; saccharides such as cellulose and methylcellulose. The carbon source is preferably added in an excess amount with respect to the beaded titania, and for example, it is preferably added 3 times as much as the beaded titania.
The firing conditions are preferably performed in a reducing atmosphere, and specifically, firing in a hydrogen atmosphere is preferred. The firing temperature is preferably 600 to 1200 ° C, more preferably 700 to 1100 ° C.
By the firing, beaded titanium carbide is synthesized. Depending on the firing conditions, the inside of the beaded titania may not be reduced, and a beaded structure containing titania inside the titanium carbide may be obtained. Such beaded titanium carbide containing titania inside can also be used in the present invention.

FIG. 3 is a schematic cross-sectional view of a beaded titanium carbide in each manufacturing process in the above manufacturing example. However, this figure shows only one aspect of this production example. Further, FIG. 3 is not necessarily a figure that faithfully reflects the number of particles and the number of branches contained in the beaded titania and the beaded titanium carbide actually used.
Fig.3 (a) is a cross-sectional schematic diagram of the rosary titania used as a raw material. The beaded titania is formed by connecting a plurality of particles made of titania 3a in a chain.
FIG. 3B is a schematic cross-sectional view of a beaded titania coated with a carbon source. As shown in FIG. 3B, the carbon source 3b is coated on each particle made of titania 3a.
FIG. 3 (c ₁ ) and FIG. 3 (c ₂ ) are schematic cross-sectional views of a beaded titanium carbide obtained after firing. FIG. 3 (c ₁ ) is a schematic cross-sectional view of a bead-like titanium carbide composed of particles made of only titanium carbide 3c. Further, FIG. 3 (c ₂₎ is a cross-sectional schematic view of a beaded titanium carbide containing particles containing titania 3a inside the titanium carbide 3c. The bead-like titanium carbide shown in FIG. 3 (c ₂ ) is obtained when the firing temperature is lower than that of the bead-like titanium carbide shown in FIG. 3 (c ₁ ) and the inside of titania is not completely reduced. Any of the beaded titanium carbides shown in FIG. 3 (c ₁ ) and FIG. 3 (c ₂ ) can be used in the present invention.

3-3. Step of Forming Titanium Oxide Layer This step is a step of adding a reducing agent to the above-described titanium carbide aqueous dispersion and oxidizing the surface of titanium carbide with water to form a titanium oxide layer.
The mechanism of this process is as follows. First, the surface of titanium carbide is reduced by a reducing agent to become titanium. The produced titanium is oxidized by water as a dispersion medium, and finally a titanium oxide layer is formed on the surface of titanium carbide.

The reducing agent that can be used in this step is not particularly limited as long as it is a reducing agent having a strong reducing power. For example, sodium borohydride (NaBH ₄ ), hydrogen, hydrazine, sodium thiosulfate, citric acid, sodium citrate, Examples include L-ascorbic acid and formaldehyde. These reducing agents may be used alone or in combination of two or more.

3-4. Step of preparing a mixed solution This step is a step of preparing a mixed solution by mixing an aqueous dispersion of titanium carbide having a titanium oxide layer formed on the surface and a platinum ion solution.
In this step, the mixing speed of the titanium carbide aqueous dispersion and the platinum ion solution is preferably as slow as possible. For example, when the aqueous dispersion of titanium carbide is acidic and the platinum ion solution is basic, the titanium carbide and the platinum ions may be aggregated due to the heat of neutralization generated during mixing. As a method of slowing the mixing speed of the titanium carbide aqueous dispersion and the platinum ion solution, for example, a method of slowly dropping the platinum ion solution into the titanium carbide aqueous dispersion, or vice versa, titanium is added to the platinum ion solution. For example, a method of slowly dropping an aqueous dispersion of carbide may be used.

3-5. CO bubbling step This step consists of a platinum ion solution, an aqueous dispersion of titanium carbide, an aqueous dispersion of titanium carbide having a titanium oxide layer formed on the surface, and the above-mentioned mixed liquid (these solutions and dispersions are referred to below). , May be referred to as a solution, etc.) at least one of them is bubbled with carbon monoxide (CO).
By bubbling CO into the solution or the like before the reduction step and saturating the CO into the solution or the like, the CO can be coordinated to platinum ions that are present in the solution or the like and are to be added to the solution or the like. As a result, platinum ions can be prevented from aggregating with each other, and platinum fine particles having a smaller particle diameter can be obtained.
The CO bubbling may be performed on any of a platinum ion solution, an aqueous dispersion of titanium carbide, an aqueous dispersion of titanium carbide having a titanium oxide layer formed on the surface, and the mixed liquid described above. Further, CO bubbling may be performed twice or more in different stages.
The CO concentration is not particularly limited, but is preferably about 10 to 30% from the viewpoint of safety. The CO bubbling time is not particularly limited, but is preferably about 30 minutes to 2 hours from the viewpoint of productivity.

3-6. Reduction step In this step, a reducing agent is added to the liquid mixture described above, and platinum fine particles are precipitated on the surface of the titanium carbide, so that the titanium oxide contains titanium oxide on the surface of the titanium carbide and surrounds the platinum fine particles. And forming at least one conductive channel through platinum fine particles between the surface of titanium carbide and the surface of the composite catalyst in the titanium oxide layer, and within the conductive channel, platinum is formed. It is a reduction process for forming a bond between an atom and titanium oxide.

In this step, the addition (preferably loading) of the platinum fine particles on the titanium carbide surface proceeds by the addition of the reducing agent.
Although details about the mechanism by which the platinum fine particles adhere to the surface of the titanium carbide on which the titanium oxide layer is formed are unknown, several modes are conceivable. For example, there are an aspect in which platinum fine particles are deposited and adhered on the surface of the titanium carbide in the gap between the titanium oxide layers, and an aspect in which the platinum fine particles are deposited so as to penetrate the titanium oxide layer once formed.
In addition, when the titanium oxide layer is sufficiently thin as 2 nm or less, even if the platinum fine particles are not in direct contact with the titanium carbide surface, conduction of electrons between the platinum fine particles and the titanium carbide is ensured by the tunnel effect. The

As shown in FIG. 1 described above, platinum fine particles adhere to the surface of the titanium carbide, conductivity between the titanium carbide and platinum fine particles is ensured, and the titanium oxide layer is formed so as to surround the platinum fine particles. Thus, it means that a conductive channel is formed in the titanium oxide layer. This conductive channel is a path that ensures electrical conduction between the surface of the titanium carbide and the surface of the composite catalyst.
Furthermore, in the conductive channel, a bond between platinum atoms and titanium oxide is formed. The bond, particularly the bond between the platinum atom and the oxygen defect of the titanium oxide is as described in the section “1-2. Titanium oxide”.

However, in this step, not all platinum fine particles deposited on the surface of the titanium carbide are surrounded by the titanium oxide layer and do not necessarily have a bond with the titanium oxide. That is, it is considered that only platinum fine particles are deposited on the surface of the titanium carbide, and there are portions where the titanium oxide layer is not formed in the vicinity of the platinum fine particles.
However, titanium oxide is amorphous and has a large number of functional groups on its surface, for example, a peroxide group (—OOH), a hydroxyl group (—OH), and the like. Accordingly, platinum ions are more likely to be adsorbed to titanium oxide than titanium carbide, and it is considered that platinum fine particles are preferentially deposited on the surface of titanium carbide on which the titanium oxide layer is formed and on the surface of titanium carbide near the titanium oxide layer. .
In this step, only the platinum fine particles are deposited, and the portion where the titanium oxide layer is not formed in the vicinity of the platinum fine particles is preferably 5% or less of the total surface area of titanium carbide, and preferably 1% or less. It is more preferable.
In addition, since titanium carbide has conductivity, electrical conduction can be established between the titanium carbide and the platinum fine particles regardless of the presence or absence of the titanium oxide layer.

The reducing agent that can be used in this step is as described above.
In this step, CO bubbling may be performed from the viewpoint of enhancing the dispersibility of the precipitated platinum fine particles together with the use of a reducing agent. The CO concentration and CO bubbling time used for CO bubbling are as described above.

  In this production method, a step of reducing platinum in the presence of titanium carbide is performed instead of the step of carrying platinum on carbon as in the conventional production method of platinum-supported carbon. Therefore, the manufacturing process becomes simpler than before. As a result, the platinum that does not come into contact with the carrier and cannot be electrically connected as in the conventional method for producing platinum-supported carbon is reduced, and the ECSA of platinum can be increased.

3-7. Other Steps After the reduction step, the obtained composite catalyst may be subjected to filtration / washing, drying, pulverization, heating, acid treatment, potential treatment, and the like.
The filtration / washing of the composite catalyst is not particularly limited as long as it is a method capable of removing impurities without impairing the structure of the produced composite catalyst. Examples of the filtration / washing include a method of separating by suction filtration using a filter paper (Whatman, # 42), PTFE membrane filter (0.25 μm), etc. using distilled water as a solvent. Moreover, the obtained filtrate, further distilled water and _may be washed with _{(NH 4)} 2 CO ₃ aqueous solution and the like.
The drying of the composite catalyst is not particularly limited as long as the method can remove the solvent and the like. Examples of the drying include a method of vacuum drying for 10 to 24 hours under a temperature condition of 60 to 100 ° C.
The pulverization of the composite catalyst is not particularly limited as long as the solid catalyst can be pulverized. Examples of the pulverization include pulverization using a mortar and the like, and mechanical milling such as a ball mill, a turbo mill, a mechanofusion, and a disk mill.

The heating of the composite catalyst is not particularly limited as long as it can remove impurities without impairing the structure of the manufactured composite catalyst. The heating performed in this step is preferably baking.
Specific examples of the firing are as follows. The heating conditions are not limited to the following.
Atmosphere: Purge with inert gas (such as argon) for 30-120 minutes.
Temperature: 150-1000 ° C, preferably 200-900 ° C
Temperature raising condition: The temperature is raised from room temperature to the above temperature over 30 to 120 minutes.
Holding conditions: Hold at the above temperature for 30 to 120 minutes.

The composite catalyst obtained by the present invention may be subjected to acid treatment or potential treatment in order to activate the platinum fine particles.
FIG. 9 (a) is a part of the XRD spectrum of a core-shell catalyst having a platinum shell, which was fired under conditions of firing temperatures of 500 ° C., 600 ° C. and 700 ° C., respectively. As can be seen from FIG. 9A, the peak at 2θ = 40 ° in the spectrum at the firing temperature of 500 ° C. (the peak representing Pt (111)) has almost disappeared in the spectrum at the firing temperature of 700 ° C. On the other hand, from FIG. 9 (a), a peak at 2θ = 30 ° (a peak representing PtS (002) (101)) that does not appear at all in the spectrum at the firing temperature of 500 ° C. is strong in the spectrum at the firing temperature of 700 ° C. You can see it. These results show that a part of platinum becomes PtS and platinum is oxidized by raising the firing temperature. Therefore, it can be seen that platinum needs to be cleaned in order to perform a correct electrochemical evaluation of the catalyst after the high-temperature calcination.

The acid that can be used for the acid treatment is not particularly limited as long as it does not impair the structure of the obtained composite catalyst. For example, perchloric acid, hydrochloric acid, nitric acid and the like can be used. The acid treatment is completed by immersing the composite catalyst in these acids for a predetermined time.
A normal electrochemical cell can be used for the potential treatment. The electrochemical cell preferably uses a rotating disk electrode. The acid used for the said acid treatment can be used for an electrochemical cell.
FIG. 9B shows a cyclic voltammogram showing the result of sweeping the potential for 120 cycles at a potential sweep range of 0.05 to 1.2 V (vs RHE) and a potential sweep rate of 100 mV / sec (hereinafter sometimes referred to as CV). It is. In FIG. 9B, the outer voltammogram represents a voltammogram having a larger number of cycles. As can be seen from FIG. 9B, it can be seen that the platinum peak becomes clear by repeating the potential treatment.
Note that if the potential treatment is extended after the platinum cleaning is completed, the platinum fine particles may be dissolved.

Hereafter, the manufacturing method of this invention is demonstrated using figures. FIG. 4 is a schematic view showing the structure of the titanium carbide surface from the step of preparing an aqueous dispersion of titanium carbide to the reduction step in the production method according to the present invention. However, this figure shows only one aspect of this manufacturing method. Further, FIG. 4 is not necessarily a figure that faithfully reflects the sizes of titanium carbide, metal (or ions thereof), and compounds actually used. When beaded titanium carbide is used, the titanium carbide particles in the beaded titanium carbide correspond to the titanium carbide 3 in FIG. For the sake of illustration, the reducing agent, dispersion medium and other additives are omitted.
FIG. 4A is a schematic diagram showing the structure of the titanium carbide surface. The surface of the titanium carbide 3 is composed of titanium atoms (represented by Ti in circles) and carbon atoms (represented by C in circles).
FIG. 4B is a schematic diagram showing the structure of the titanium carbide surface after a predetermined time has elapsed after mixing a reducing agent with titanium carbide. The titanium carbide surface is oxidized by water and converted into titanium oxide (oxygen atoms are represented by O in a circle).
FIG.4 (c) is the schematic diagram which showed the structure of the titanium carbide surface after mixing a platinum ion solution further. Platinum ions (represented by Pt ⁴⁺ in a circle) are adsorbed on the surface of the titanium oxide layer.
FIG. 4D is a schematic view showing the structure of the titanium carbide surface after CO bubbling. Bubbled CO (carbon atoms are represented by C in a circle) coordinates to platinum, and a carbonyl complex of platinum is temporarily generated, so that aggregation of platinum fine particles hardly occurs.
FIG. 4E is a schematic view showing the structure of the titanium carbide surface after the reduction step. When the platinum ions are reduced to platinum, oxygen defects (—O—Ti) and platinum atoms (Pt) of the titanium oxide are bonded to each other as shown in the figure, and the composite catalyst according to the present invention is generated.

  Specific embodiments of the present invention will be described below in more detail with reference to examples. However, the present invention is not limited to these examples unless it exceeds the gist.

1. Production of beaded titanium carbide [Production Example 1]
First, tetra (2-ethylhexyl) titanate was dispersed in an aqueous ethanol solution. Next, the prepared dispersion was nanoparticulated using an oxide nanoparticle synthesizer (manufactured by Hosokawa Micron, trade name: FCM-MINI). The obtained nanoparticles were diluted and dispersed in dilute nitric acid, and then collected with a molybdenum mesh and observed with a TEM. As a result, it was confirmed that bead-like TiO ₂ could be synthesized. To the beaded TiO _2, methylcellulose as a carbon source, mixed 3 times the mass of the beaded TiO _2. When the mixture was baked at 800 ° C. or higher in a hydrogen atmosphere, the beaded titanium carbide of Production Example 1 in which at least the surface of the beaded TiO ₂ was converted to TiC was obtained.

2. TEM observation of beaded titanium carbide The structure of the beaded titanium carbide of Production Example 1 was confirmed by TEM observation. Detailed measurement conditions are as follows.
Using a field emission transmission electron microscope (manufactured by JEOL, model number: JEM-2100F, with Cs correction), dark field STEM observation (Scanning Transmission Electron Microscope, hereinafter referred to as STEM) at an acceleration voltage of 200 kV and a magnification of 600,000 times Called).

  FIG. 5 is a TEM image of the beaded titanium carbide of Production Example 1 obtained under the above observation conditions. In FIG. 5, white circles having a diameter of about 10 nm indicate titanium carbide fine particles. From FIG. 5, one bead-shaped titanium carbide is formed by connecting about 5 to 160 titanium carbide fine particles, and the connection structure includes 3 to 20 branch portions. It was confirmed.

3. Production of platinum / titanium oxide / titanium carbide composite catalyst for fuel cell [Example 1]
First, 5 g of titanium carbide (TiC) (Nano Titanium Carbide powder, NaBond Technologies Co., Ltd.) was added to 200 mL of ultrapure water to prepare a TiC aqueous dispersion. Next, while stirring the TiC aqueous dispersion, it was treated with an ultrasonic homogenizer at 300 W for 30 minutes to thoroughly disperse TiC in the aqueous dispersion. Furthermore, 11.9 g of sodium borohydride (NaBH ₄ ; hereinafter may be referred to as SBH) was added to the TiC aqueous dispersion, and the mixture was stirred for 3 hours.

FIG. 10 is a schematic diagram showing a reactor used for CO bubbling.
CO was supplied from a CO gas cylinder 31 stored in the CO gas cabinet 30. The CO gas cylinder 31 was connected to a valve 34 in the draft 33 via a SUS line 32. The draft shutter was lowered to prevent gaps between the shutter and the experimental table. In addition, a CO detector 35 was installed in the draft 33, and a blow-off 36 was provided in the lower part of the shutter to prepare for CO gas leakage from the draft 33. Based on the display of the gas flow meter 37, the CO gas flow rate was appropriately adjusted by the valve 34. CO gas was introduced into the reaction vessel 39 on the stirrer 38 through a flexible hose and the like, and CO bubbling into the reaction solution was performed. The CO after the bubbling was exhausted to the outside of the draft through the bubbler 40 through the exhaust port above the draft. Note that the bubbler 40 was constantly monitored during CO bubbling, and when gas bubbles in the bubbler 40 changed, the CO gas leakage was warned.
Using an apparatus shown in FIG. 10, 20% CO was bubbled in a TiC aqueous dispersion to which SBH was added for 60 minutes.

On the other hand, crystals of hexahydroxyplatinic acid (Cataler) were dissolved in a basic solution to prepare a platinum ion solution (pH: about 9 to 12).
While stirring the COC bubbling TiC aqueous dispersion, 22.43 g of platinum ion solution (corresponding to 1.10 g of Pt) was slowly dropped dropwise with a dropping funnel to prepare a mixed solution. SBH 11.9g was added to the liquid mixture, and also it stirred for 60 minutes, carrying out CO bubbling. Thereafter, the mixed solution was filtered through a PTFE membrane filter (0.25 μm), and the obtained filtrate was added to 200 mL of (NH ₄ ) ₂ CO ₃ aqueous solution (concentration: 6.43 mg / L), and the mixture was heated without heating. Stir for minutes. The neutralized filtrate was repeatedly washed with ultrapure water at 80 ° C., and the resulting solid was vacuum-dried for 1 day.

  The vacuum-dried solid was added to the quartz cell and placed in a tubular furnace. The inside of the tube was replaced with 100% G1 argon gas (flow rate = 750 mL / min, replacement time: 60 minutes). After the replacement with argon, the temperature in the tube was increased to 250 ° C. over 60 minutes and maintained at 250 ° C. for 60 minutes. After calcination, the mixture was cooled to room temperature, and a platinum / titanium oxide / titanium carbide composite catalyst for Example 1 was produced.

[Example 2]
In Example 1, the same method as in Example 1 except that 5 g of titanium carbide (TiC) (Nano Titanium Carbide powder, NaBond Technologies Co., Ltd.) was replaced with 5 g of beaded titanium carbide of Production Example 1. Thus, a platinum / titanium oxide / titanium carbide composite catalyst for a fuel cell of Example 2 was produced.

4). Production of platinum-supported titanium carbide catalyst [Comparative Example 1]
First, 5 g of titanium carbide (TiC) (Nano Titanium Carbide powder, NaBond Technologies Co., Ltd.) was dispersed in 500 mL of ethanol to prepare a TiC ethanol dispersion. Next, 3 g of hexachloroplatinic acid (manufactured by Ishifuku Metal Industry Co., Ltd.) was dissolved in the TiC ethanol dispersion. The obtained solution was refluxed with ethanol at 80 ° C. for 3 hours to reduce platinum ions to platinum. After refluxing, the solution was filtered, and the recovered powder was repeatedly washed with 80 ° C. ultrapure water until the pH reached 6-7, and the resulting solid was vacuum dried. The dried powder was calcined at 400 ° C. in an argon 100% atmosphere to produce a platinum-supported titanium carbide catalyst of Comparative Example 1.

5. Evaluation of catalyst Platinum / titanium oxide / titanium carbide composite catalyst for fuel cell of Example 1 (hereinafter sometimes referred to as catalyst of Example 1), platinum-supported titanium carbide catalyst of Comparative Example 1 (hereinafter, Comparative Example) 1), and a catalyst (Pt / C, hereinafter referred to as a catalyst of Comparative Example 2) in which platinum is supported on a carbon support (Ketjen Black), a rotating disk electrode was used. The catalyst performance and catalyst durability were evaluated by measurement.

5-1. Potential Treatment for Catalyst The potential treatment was performed on the catalysts of Example 1, Comparative Example 1 and Comparative Example 2 for the purpose of cleaning platinum before measurement with a rotating disk electrode.
FIG. 11 is a schematic perspective view illustrating an apparatus that performs potential processing.
An aqueous perchloric acid solution 52 was added to the glass cell 51, and a rotating disk electrode 54 coated with a catalyst slurry 53 was set. The rotating disk electrode 54 is connected to a tachometer 55. In the perchloric acid aqueous solution 52, in addition to the rotating disk electrode 54, a counter electrode 56 and a reference electrode 57 are disposed so as to be sufficiently immersed in the perchloric acid aqueous solution 52. It is electrically connected to the analyzer. Argon introduction pipe 58 is disposed so as to be immersed in perchloric acid aqueous solution 52, and argon is bubbled into perchloric acid aqueous solution 52 at room temperature for a certain period of time from an argon supply source (not shown) installed outside the cell. As a result, argon was saturated in the perchloric acid aqueous solution 52. A circle 59 indicates an argon bubble.
The details of the apparatus are as follows.
-Perchloric acid aqueous solution: 0.1 mol / L HClO ₄
・ Rotating disk electrode: electrode made of glassy carbon ・ Rotometer: Hokuto Denko, HR-201
・ Counter electrode: Platinum electrode (made by Hokuto Denko)
・ Reference electrode: Hydrogen electrode (manufactured by KM Lab)
Dual electrochemical analyzer: ALS700C manufactured by BAS
With the apparatus shown in FIG. 11, the potential was swept for 120 cycles at a potential sweep range of 0.05 to 1.2 V (vs RHE) and a potential sweep rate of 100 mV / sec to activate platinum.

5-2. Measurement using rotating disk electrode (a) Calculation of ECSA For the catalysts of Example 1, Comparative Example 1 and Comparative Example 2, the ECSA of the catalyst was calculated.
With the apparatus shown in FIG. 11, the potential was swept for two cycles at a potential sweep range of 0.05 to 1.2 V (vs RHE) and a potential sweep rate of 50 mV / sec. ECSA was calculated from CV at the second cycle.

(B) Measurement of specific activity and mass activity Electrochemical measurement was performed on the catalysts of Example 1, Comparative Example 1 and Comparative Example 2, and the oxygen reduction reaction (hereinafter referred to as ORR) activity of the catalyst. Specific activity was measured as an index of
In the apparatus shown in FIG. 11, while bubbling oxygen into the perchloric acid aqueous solution 52 in the glass cell 51, the potential is 2 at a potential sweep range of 0.1 to 1.05 V (vs RHE), a potential sweep rate of 10 mV / sec. The cycle was swept. An activity-dominated current (hereinafter referred to as IK) was calculated from a current value of 0.9 V in the ORR curve at the second cycle. The specific activity was obtained by dividing the IK by the above-mentioned ECSA.

(C) Durability Evaluation Electrochemical measurements were performed on the catalysts of Example 1, Comparative Example 1, and Comparative Example 2 to evaluate the durability. The detailed conditions of the electrochemical measurement are as follows.
In the apparatus shown in FIG. 11, a rectangular wave potential cycle of 0.65 / 5 sec to 1.2 V / 5 sec is set to 10,000 cycles (vsRHE) while bubbling oxygen into the perchloric acid aqueous solution 52 in the glass cell 51. ) Swept. After sweeping 10,000 cycles, CV was performed in the same manner as described in the section “(a) Calculation of ECSA”.

FIG. 14 is a graph in which the CV of the second cycle and the CV of the 10,000th cycle are overlapped in the evaluation of the durability of the catalyst of Comparative Example 2.
As can be seen from FIG. 14, the current value of the 10,000th cycle CV is significantly smaller than that of the second cycle CV for both the oxidation wave and the reduction wave. In particular, the magnitude of the current peak in the range of 0.05 to 0.3 V (vs RHE) corresponding to the adsorption and desorption reaction of hydrogen atoms on platinum, and 0 corresponding to the adsorption and desorption reaction of oxygen atoms on platinum. The magnitude of the current peak in the range of 0.7 to 1.2 V (vsRHE) is smaller in the 10,000th cycle CV than in the second cycle CV. This result shows that the ECSA in the catalyst of Comparative Example 2 is much smaller at the 10,000th cycle than at the second cycle. Also, the current value in the range of 0.3 to 0.4 V (vsRHE) corresponding to the electric double layer is smaller in the 10,000th cycle CV than in the second cycle CV. This result shows that the electric double layer has decreased as a result of the carbon support in the catalyst of Comparative Example 2 being oxidized during 10,000 cycles.

FIG. 13 is a graph in which the CV of the second cycle and the CV of the 10,000th cycle are overlapped in the evaluation of the durability of the catalyst of Comparative Example 1.
As can be seen from FIG. 13, there is not much difference in current value between the 10,000th cycle CV and the second cycle CV for both the oxidation wave and the reduction wave. In particular, the current value in the range of 0.3 to 0.4 V (vsRHE) corresponding to the electric double layer hardly changes between the CV at the 10,000th cycle and the CV at the second cycle. This result shows that even after 10,000 cycles, the titanium carbide in the catalyst of Comparative Example 1 is hardly oxidized and there is no change in the electric double layer.
By comparing FIG. 13 and FIG. 14, it can be seen that titanium carbide has higher oxidation resistance (corrosion resistance) than the carbon support. In Example 1, since titanium carbide is used as a carrier, Example 1 is considered to show the same result as Comparative Example 1 in that there is no change in the electric double layer after 10,000 cycles.

Table 2 below summarizes the specific activities of the catalysts of Example 1 and Comparative Example 1. The “5 nm Pt ratio” in Table 2 below is the ratio of the catalytic activity of Example 1 or Comparative Example 1 to the specific activity of 303 μA / cm ² -Pt of platinum fine particles having an average particle diameter of 5 nm.

From the above Table 2, the specific activity of the catalyst of Comparative Example 1 is 330 μA / cm ² . Therefore, the catalyst of Comparative Example 1 in which platinum is simply supported on a TiC support exhibits only a specific activity comparable to that of platinum fine particles having an average particle diameter of 5 nm.
On the other hand, from Table 2 above, the specific activity of the catalyst of Example 1 is 489 μA / cm ² . Therefore, the platinum / titanium oxide / titanium carbide composite catalyst for fuel cells of the present invention is 1.5 times more specific than a conventional catalyst in which platinum is merely supported on a TiC carrier.

6). Preparation of membrane / electrode assembly [Example 3]
An anode electrode catalyst paste containing a platinum / titanium oxide / titanium carbide composite catalyst for fuel cell of Example 2 and an electrolyte having proton conductivity on one surface of a solid polymer electrolyte membrane (Nafion (registered trademark) membrane) The cathode electrode catalyst paste containing the platinum / titanium oxide / titanium carbide composite catalyst for fuel cell of Example 2 and the electrolyte having proton conductivity was applied to the other surface. The membrane / electrode assembly of Example 3 was produced by sandwiching the solid polymer electrolyte membrane after application of the electrode catalyst paste between a pair of gas diffusion sheets (carbon paper) and thermocompression bonding.

[Comparative Example 3]
In Example 3, in place of the platinum / titanium oxide / titanium carbide composite catalyst for fuel cell of Example 2, the same method as in Example 3 except that the platinum-supported titanium carbide catalyst of Comparative Example 1 was used. The membrane / electrode assembly of Comparative Example 3 was obtained.

[Comparative Example 4]
In Example 3, in place of the platinum / titanium oxide / titanium carbide composite catalyst for fuel cell of Example 2, the platinum-supported carbon catalyst of Comparative Example 2 was used, and the same method as in Example 3 was used. A membrane / electrode assembly of Comparative Example 4 was obtained.

7). Evaluation of membrane / electrode assembly 7-1. SEM Observation SEM observation was performed on cross sections substantially perpendicular to the plane direction of the membrane / electrode assemblies of Example 3, Comparative Example 3, and Comparative Example 4.
The SEM observation conditions are as follows. That is, using a scanning electron microscope (Hitachi, S-5500), SEM observation was performed at an acceleration voltage of 30 kV and a magnification of 1,000 to 50,000 times.

FIG. 16 is an SEM image at a magnification of 10,000 times of a cross section substantially perpendicular to the surface direction of the membrane / electrode assembly of Comparative Example 4. As can be seen from FIG. 16, the catalyst layer containing the platinum-supported carbon catalyst of Comparative Example 2 has innumerable voids.
FIG. 15 is an SEM image of a cross section of the membrane / electrode assembly of Comparative Example 3 substantially perpendicular to the surface direction. 15A is an SEM image at a magnification of 1,500 times, FIG. 15B is an enlarged SEM image at a magnification of 5,000 times in FIG. 15A, and FIG. 15C is an image of FIG. Further, the SEM image at a magnification of 10,000 times and FIG. 15D are SEM images at a magnification of 50,000 times that are further enlarged from FIG. 15C.
The layer having a thickness of about 10 μm shown in the center of FIG. 15A is a catalyst layer containing the platinum-supported titanium carbide catalyst of Comparative Example 1. As can be seen from FIGS. 15 (b) to 15 (d) in which the portion of the electrode is enlarged, the catalyst layer including the platinum-supported titanium carbide catalyst of Comparative Example 1 has almost no void necessary for the catalyst layer. . By comparing FIG. 16 with the same magnification as FIG. 15C, it is easily understood that there is no void in the catalyst layer containing the platinum-supported titanium carbide catalyst of Comparative Example 1.

6 is an SEM image of a cross section substantially perpendicular to the surface direction of the membrane / electrode assembly of Example 3. FIG.
FIG. 6A is an SEM image at a magnification of 1,000 times, and FIG. 6B is an SEM image at a magnification of 5,000 times that is an enlargement of FIG. 6A. The layer having a thickness of about 10 μm shown in the center of FIG. 6A is a catalyst layer containing the platinum / titanium oxide / titanium carbide composite catalyst for fuel cell of Example 2. As can be seen from FIG. 6B in which the portion of the catalyst layer is enlarged, the catalyst layer containing the catalyst of Example 2 has numerous voids. This is considered that when the catalyst layer is formed using the catalyst of Example 2 containing beaded titanium carbide, the catalyst of Example 2 is irregularly entangled in the catalyst layer, so that voids are generated.

7-2. Evaluation of Discharge Performance of Membrane / Electrode Assembly The membrane / electrode assemblies of Example 3 and Comparative Example 3 were subjected to discharge performance evaluation. The evaluation conditions are as follows.
Evaluation device: Fuel cell evaluation device (manufactured by Toyo Technica)
Humidification conditions: Bipolar full humidification conditions Measurement temperature: 80 ° C
Measurement potential: 1.0 to 0.2V
Measurement current density: 0 to 1.0 A / cm ²

FIG. 17 is a discharge curve of the membrane / electrode assembly of Comparative Example 3. FIG. 17 is a graph in which the vertical axis represents the cell voltage (V) and the horizontal axis represents the current density (A / cm ² ).
As can be seen from FIG. 17, the membrane / electrode assembly of Comparative Example 3 hardly discharges. As described above, this is considered to be because the transport of gas or water required for the electrode reaction is almost impossible in the catalyst layer containing the catalyst of Comparative Example 1.
FIG. 7 is a discharge curve of the membrane / electrode assembly of Example 3. As can be seen by comparing FIG. 7 and FIG. 17, the membrane-electrode assembly of Example 3 has a lower concentration overvoltage and is superior in discharge performance. As described above, in the catalyst layer containing the catalyst of Example 2, in addition to the high specific activity of the catalyst itself, voids are generated due to the bead-like structure of the support, which is necessary for the electrode reaction. This is because transportation of gas and water is promoted.
From the above, the membrane / electrode assembly of Example 3 using the platinum / titanium oxide / titanium carbide composite catalyst for fuel cells of the present invention is the membrane / electrode of Comparative Example 3 using a conventional platinum-supported titanium carbide catalyst. Compared to the joined body, it was proved that the discharge performance was extremely high due to the structure of the catalyst.

DESCRIPTION OF SYMBOLS 1 Platinum fine particle 2 Titanium oxide layer 3 Titanium carbide 3a Titania 3b Carbon source 3c Titanium carbide 4 The frame 5 enclosed with the dashed-dotted line 5 The double arrow 11 which shows the conduction direction of an electron 11 The arrow showing the flow of an electron from trivalent titanium to platinum 21 solid polymer electrolyte membrane 22 cathode catalyst layer 23 anode catalyst layers 24, 25 gas diffusion layer 26 cathode electrode 27 anode electrode 30 CO gas cabinet 31 CO gas cylinder 32 SUS line 33 draft 34 valve 35 CO detector 36 blow-off 37 gas flow rate Total 38 Stirrer 39 Reaction vessel 40 Bubbler 51 Glass cell 52 Perchloric acid aqueous solution 53 Slurry of catalyst 54 Rotating disk electrode 55 Tachometer 56 Counter electrode 57 Reference electrode 58 Argon introduction tube 59 Argon bubble 100 Platinum for fuel cell according to the present invention Titanium oxide / Cita Typical examples of carbide composite catalyst 200 membrane-electrode assembly

Claims (6)

A catalyst for a fuel cell containing platinum fine particles, titanium oxide, and titanium carbide (TiC),
The titanium carbide includes, on the surface thereof, a titanium oxide layer containing the platinum fine particles and the titanium oxide and surrounding the platinum fine particles,
Since the platinum atom in the platinum fine particle is in electrical contact with the titanium carbide surface, the titanium oxide layer is interposed between the surface of the titanium carbide and the surface of the catalyst. At least one conductive channel,
A platinum / titanium oxide / titanium carbide composite catalyst for a fuel cell, wherein a platinum atom in the platinum fine particle and the titanium oxide have a bond in the conductive channel.   The platinum / titanium oxide / titanium carbide composite catalyst for fuel cells according to claim 1, wherein the titanium oxide layer has a thickness of 0.23 to 2 nm.   The platinum / titanium oxide / titanium carbide composite catalyst for fuel cells according to claim 1, wherein the titanium carbide has a bead-like structure.   4. The platinum / titanium oxide / titanium carbide composite catalyst for a fuel cell according to claim 1, wherein the platinum fine particles have an average particle diameter of 0.25 to 1.5 nm. A fuel cell membrane / electrode assembly comprising an anode electrode on one side of a polymer electrolyte membrane and a cathode electrode on the other side,
The anode electrode comprises at least an anode catalyst layer;
The cathode electrode includes at least a cathode catalyst layer,
5. The platinum / titanium oxide / titanium carbide composite catalyst for a fuel cell according to claim 1, wherein at least one of the anode catalyst layer and the cathode catalyst layer includes the composite catalyst for a fuel cell according to claim 1. , Fuel cell membrane / electrode assembly. A method for producing a fuel cell catalyst containing platinum fine particles, titanium oxide, and titanium carbide (TiC),
Preparing a platinum ion solution;
Preparing an aqueous dispersion of the titanium carbide;
Adding a reducing agent to the aqueous dispersion of titanium carbide, oxidizing the surface of the titanium carbide with water, and forming a titanium oxide layer;
Mixing an aqueous dispersion of titanium carbide having a titanium oxide layer formed on the surface thereof and the platinum ion solution to prepare a mixed solution;
Bubbling carbon monoxide in at least one of the platinum ion solution, the titanium carbide aqueous dispersion, the titanium carbide aqueous dispersion having a titanium oxide layer formed on the surface thereof, and the mixed liquid; And
By adding a reducing agent to the mixture, and precipitating platinum fine particles on the surface of the titanium carbide,
Forming a titanium oxide layer containing the titanium oxide and surrounding the platinum fine particles on the surface of the titanium carbide; and
Forming at least one conductive channel through the platinum fine particles between the surface of the titanium carbide and the surface of the catalyst in the titanium oxide layer; and
A method for producing a platinum / titanium oxide / titanium carbide composite catalyst for a fuel cell, comprising a reduction step of forming a bond between a platinum atom and the titanium oxide in the conductive channel.