JP2013164929A - Method of producing electrode catalyst for fuel cell - Google Patents

Abstract

A method for producing an electrode catalyst for a fuel cell capable of improving the catalytic performance of catalyst particles made of platinum or a platinum alloy is provided.
A method for producing an electrode catalyst for a fuel cell in which catalyst particles made of platinum or a platinum alloy are supported on carrier particles made of a metal oxide. A hydrocarbon gas and water vapor are brought into contact with the surface of the carrier particles 2 while being heated, and a carbon film 6 made of carbon derived from the hydrocarbon gas is coated on the surface of the carrier particles 2 by a steam reforming reaction. And a step of heat-treating the carrier particles 2 coated with the carbon film so as to promote crystallization of carbon of the carbon film 6.
[Selection] Figure 1

Description

  The present invention relates to a method for producing a catalyst in which catalyst particles made of platinum or a platinum alloy are supported on carrier particles made of a metal oxide, and more particularly to an electrode catalyst suitable for use in a fuel cell.

  Conventionally, as a cathode and an anode catalyst of an electrode catalyst of a polymer electrolyte (solid polymer) type fuel cell, a catalyst in which noble metal catalyst particles such as platinum or platinum alloy are supported on carrier particles such as carbon black has been used. .

  For example, platinum-supported carbon black, which is an electrode catalyst for fuel cells, is obtained by adding sodium hydrogen sulfite to a chloroplatinic acid aqueous solution and then reacting it with hydrogen peroxide solution so that the resulting platinum colloid is supported on carbon black and washed. Generally, it is produced by heat treatment as necessary.

  Using the platinum-supported carbon black thus obtained, an electrode of a polymer electrolyte fuel cell is prepared by dispersing platinum-supported carbon black in a polymer electrolyte solution to prepare an ink. It is produced by applying to a gas diffusion substrate and drying. An electrolyte membrane-electrode assembly (MEA) is assembled by sandwiching a polymer electrolyte membrane between these two electrodes and performing hot pressing.

  As such a fuel cell electrode catalyst, for example, Patent Document 1 discloses that catalyst particles made of platinum or a platinum alloy, carrier particles made of carbon supporting the catalyst particles, and poisoning of the catalyst particles are alleviated. A fuel cell electrode catalyst containing a metal oxide such as titanium oxide has been proposed.

  Patent Document 2 discloses a fuel cell electrode catalyst in which catalyst particles made of platinum or a platinum alloy are supported on carrier particles, which are titanium oxide particles coated with a carbon film. Proposed.

  Furthermore, the fuel cell electrode catalyst described in Patent Document 2 is produced as follows. First, titanium oxide particles (carrier particles) coated with a carrier particle carbon film are produced by mixing titanium oxide particles and PVA to produce a paste, firing it, and pulverizing it. Then, the titanium oxide particles coated with the carbon film are put into an ethanol and chloroplatinic acid solution, and heated and dry-distilled to carry the platinum particles on the titanium oxide particles.

JP 2009-193957 A JP 2011-175773 A

  However, in the case of a fuel cell electrode catalyst as disclosed in Patent Document 1, a metal oxide such as titanium oxide is exposed on the surface of the electrode catalyst, and the metal oxide is more conductive than carbon. Therefore, the catalyst performance of catalyst particles made of platinum or a platinum alloy may be lower than that of an electrode catalyst made of platinum-supported carbon.

  Therefore, it is considered that such a point can be improved by coating the surface of the carrier particles with a carbon film as in Patent Document 2, but the carrier particles described in Patent Document 2 are made of a paste mixed with PVA. Since it is produced by firing and crushing, there is a variation in the film thickness of the carbon film to be coated, and the metal oxide of the carrier particles may be partially exposed. Thereby, like patent document 1, the catalyst performance of the catalyst particle which consists of platinum or a platinum alloy cannot be fully exhibited.

  The present invention has been made in view of these points, and an object of the present invention is to provide a method for producing an electrode catalyst for a fuel cell capable of improving the catalyst performance of catalyst particles made of platinum or a platinum alloy. It is to provide.

  As a result of intensive studies, the inventor has thought that it is important to uniformly coat a carbon film on carrier particles made of a metal oxide carrying catalyst particles. Thus, the inventors have obtained new knowledge that by utilizing the steam reforming reaction, the surface of the carrier particles can be uniformly coated with a carbon film and the thickness thereof can be controlled.

  The present invention is based on the inventors' new knowledge, and the method for producing an electrode catalyst for a fuel cell according to the present invention includes a catalyst particle made of platinum or a platinum alloy on carrier particles made of a metal oxide. A method for producing a supported electrode catalyst for a fuel cell, wherein a hydrocarbon gas is brought into contact with the surface of the support particles while being heated, and the hydrocarbon gas is applied to the surface of the support particles by a steam reforming reaction. And a step of heat-treating the carrier particles coated with the carbon film so as to promote crystallization of carbon in the carbon film. To do.

  According to the present invention, first, in the step of coating the carbon film, the surface of the carrier particles is heated to a temperature at which a steam reforming reaction occurs while bringing a hydrocarbon gas and water vapor into contact with each other. Thereby, the carbon film which consists of carbon originating in hydrocarbon gas is coat | covered on the surface of a support particle.

  In this way, the carbon derived from the hydrocarbon gas is deposited on the surface of the carrier particle by utilizing the steam reforming reaction, and this becomes a carbon film coated on the surface of the carrier particle. The carbon film is uniformly coated, and the thickness of the carbon film is substantially uniform.

  Here, the obtained carbon film is a film made of amorphous carbon, and its conductivity is lower than that of crystallized carbon. Therefore, in the heat treatment step, the carrier particles coated with the carbon film are heat-treated so as to promote carbon crystallization of the carbon film. Thereby, crystallization of the carbon film is promoted, and conductivity can be imparted to the carbon film.

  Thus, since the carbon film is uniformly coated on the surface of the carrier particles, it is possible to improve the catalyst performance of the catalyst particles made of platinum or a platinum alloy supported thereon. Furthermore, as described later, the catalyst performance can be further improved by bringing the catalyst particles supported on the uniformly coated carbon film into contact with the carrier particles.

  Here, the method of coating the carbon film is not particularly limited as long as the carbon film can be coated on the surface of the carrier particles with the carbon obtained using the steam reforming reaction described above. Absent. However, as a more preferable embodiment, before performing the step of coating the carbon film, a crystal made of at least one of Co, Fe, or Ni is used as a material for promoting the crystallization of the carbon on the surface of the carrier particles. The conversion promoting particles are supported.

  According to this aspect, in the heat treatment, since the crystallization promoting particles Co, Fe, or Ni act as a catalyst for crystallizing the amorphous carbon of the carbon film, the carbon film coated on the surface of the carrier particles Crystallization from amorphous carbon to carbon is promoted, and the conductivity of the carbon film can be increased.

  Furthermore, the timing of supporting the catalyst particles made of platinum or platinum alloy on the carrier particles is as long as the catalyst particles can be supported on the carrier particles. (1) After coating the carbon film and performing the heat treatment, (2 ) Any of the cases before coating the carbon film may be used.

  As a more preferred embodiment, after the heat treatment, the catalyst particles are supported on the surface of the carbon film covering the support particles, and the support particles supporting the catalyst particles are baked in an oxidizing atmosphere containing oxygen gas. As a result, a part of the carbon film is gasified by oxidation, and the catalyst particles are brought into contact with the metal oxide of the carrier particles.

  According to this aspect, the catalyst particles are supported on the surface of the carbon film of the carrier particles and fired in an oxygen gas atmosphere, whereby the carbon film is gasified by partial oxidation. As a result, the catalyst particles are buried in the carbon film and come into contact with the carrier particles (metal oxide) serving as the base. As a result, since the catalyst particles come into contact with the carrier particles (metal oxide), the activity and durability of the electrode catalyst can be improved.

  In another preferred embodiment, the catalyst particles are supported on the surface of the carrier particles before the step of coating the carbon film. In this case as well, since the catalyst particles come into contact with the carrier particles (metal oxide) before the carbon film is coated, the activity and durability of the electrode catalyst can be improved.

  According to the present invention, the catalyst performance of catalyst particles made of platinum or a platinum alloy can be improved.

It is a figure for demonstrating the manufacturing method of the electrode catalyst for fuel cells concerning 1st Embodiment of this invention, (a) is the process of making the surface of a support particle carry | support crystallization promotion particle | grains, (b) A step of coating the surface of the carrier particles with a carbon film by a steam reforming reaction, (c) is a step of heat-treating the carrier particles so as to promote carbon crystallization of the coated carbon film, and (d) FIG. 5 is a schematic conceptual diagram for explaining a step of supporting the platinum particles on the carrier particles, and (e) a step of bringing the catalyst particles into contact with the metal oxide of the carrier particles. It is a figure for demonstrating the manufacturing method of the electrode catalyst for fuel cells concerning 2nd Embodiment of this invention, (a) is the process explaining the process which carry | supports platinum particle on a support particle, (b) A step of supporting the crystallization promoting particles on the surface of the carrier particles, (c) a step of coating the surface of the carrier particles with a carbon film by a steam reforming reaction, and (d) a carbon crystal of the coated carbon film. The typical conceptual diagram for demonstrating the process of heat-processing a carrier particle so that conversion may be accelerated | stimulated. (A) is a schematic conceptual diagram of the electrode catalyst for fuel cells according to Examples 1 and 2, (b) is a schematic conceptual diagram of the electrode catalyst for fuel cell according to Comparative Example 1, (a) ) A schematic conceptual diagram of an electrode catalyst for a fuel cell according to Comparative Example 2.

The following embodiments according to the present invention will be described.
[First Embodiment]
FIG. 1 is a diagram for explaining a method for producing a fuel cell electrode catalyst according to a first embodiment of the present invention, wherein (a) is a step of supporting crystallization promoting particles on the surface of carrier particles; (b) is a step of coating the surface of the carrier particles with a carbon film by a steam reforming reaction, (c) is a step of heat-treating the carrier particles so as to promote carbon crystallization of the coated carbon film, d) is a diagram for explaining a step of supporting platinum particles on carrier particles, and (e) is a diagram for explaining a step of bringing catalyst particles into contact with metal oxides of carrier particles.

  As shown in FIG. 1A, in the first embodiment, first, the crystallization promoting particles 4 are supported on the surfaces of the carrier particles 2. Carrier particles 2 made of a metal oxide are prepared. The carrier particles 2 are particles for supporting the catalyst particles 5, can support the catalyst particles 5, and themselves have conductivity.

Here, the carrier particles 2 are made of a metal oxide. Examples of the metal oxide include iridium oxide, zirconium oxide, tungsten oxide, zinc oxide, vanadium oxide, hafnium oxide, tantalum oxide, and titanium oxide (TiO ₂ , Ti ₄ O). ₇ ), one or more transition metal oxides selected from niobium oxide, silicon oxide and tin oxide are preferably exemplified. Here, the carrier particles 2 preferably have a particle size in the range of 5 to 500 nm, and the specific surface area of the oxide measured by the BET method is preferably in the range of 10 to 200 m ² / g.

  On the surface of such carrier particles 2, crystallization promoting particles 4 made of at least one of Co, Fe, or Ni are supported as a material for promoting the crystallization of carbon described later. Specifically, the crystallization promoting particles 4 are loaded on an acid aqueous solution in which Co, Fe, or Ni is ionized, and evaporated and dried, thereby supporting the crystallization promoting particles 4 on the surfaces of the carrier particles 2. can do.

  As shown in FIG. 1 (b), the carrier particles 2 on which the crystallization promoting particles 4 are supported are brought into contact with the surface of the carrier particles 2 while heating a hydrocarbon-based gas and water vapor, and the carrier particles 2 are subjected to a steam reforming reaction. A carbon film 6 made of carbon derived from a hydrocarbon-based gas is coated on the surface.

  Specifically, the carrier particles 2 carrying the crystallization promoting particles 4 described in FIG. 1A are spread in the reaction vessel, the inert gas is introduced, and the inside of the reaction vessel is heated. And the temperature in reaction container is hold | maintained at the heated temperature, and gas is distribute | circulated in order of hydrogen gas, water vapor | steam, and hydrocarbon gas. As a result, the carbon derived from the hydrocarbon gas is deposited on the surface of the carrier particle 2 by utilizing the steam reforming reaction, and the deposited carbon becomes the carbon film 6 coated on the surface of the carrier particle 2. The carbon film 6 is uniformly coated on the surface of the carrier particle 2, and the thickness of the carbon film 6 becomes substantially uniform.

  Examples of the hydrocarbon-based gas include methane gas, methylene, ethylene gas, acetylene gas, and the like. As long as carbon can be deposited on the surface of the carrier particle 2 by the steam reforming reaction, In particular, the type of hydrocarbon gas is not limited.

  Further, the carbon film 6 has a film thickness such that the catalyst particles 5 are exposed from the surface of the fuel cell electrode catalyst in a state where the surfaces of the catalyst particles 5 described later are in contact with the surfaces of the carrier particles 2 (FIG. 1e). It is desirable to make it as thick as possible. Accordingly, the thickness of the carbon film 6 is preferably in the range of 0.2 to 2 nm on average considering the conductivity of the carbon film 6 and the particle size of the catalyst particles 5 described later, and the reaction time and reaction of the steam reforming reaction This range can be achieved by adjusting the temperature and the concentration of hydrocarbon gas and water vapor.

  Here, the obtained carbon film 6 is a film made of amorphous carbon, and its conductivity is lower than that of crystallized carbon. Therefore, as shown in FIG. 1C, the carrier particles 2 coated with the carbon film 6 are placed under an inert gas atmosphere such as argon gas (hydrogen gas) so as to promote carbon crystallization of the carbon film 6. Alternatively, heat treatment is performed in a temperature range of 800 ° C. to 1000 ° C. in a non-oxidizing gas atmosphere such as nitrogen gas. Here, since Co, Fe, or Ni of the crystallization promoting particles 4 acts as a catalyst for crystallizing the amorphous carbon of the carbon film 6, the carrier particles 2 are obtained by performing the heat treatment within this temperature range. The crystallization of the carbon film 6 covered on the surface of the carbon film 6 from amorphous carbon to carbon is promoted, and the conductivity of the carbon film 6 can be increased.

  Next, as shown in FIG. 1 (d), the catalyst particles 5 made of platinum are supported on a carrier using various methods commonly used in the art. For example, the catalyst particles 5 are supported by immersing the carrier particles 2 in a solution obtained by adding chloroplatinic acid hexahydrate to pure water so as to have a predetermined mass%, and reducing platinum with formaldehyde. Can do.

  The catalyst particles 5 used in the fuel cell electrode catalyst 1 according to the present embodiment may be a platinum alloy made of platinum and a transition metal. Platinum is an expensive noble metal, and it is preferable that both the anode catalyst and the cathode catalyst exhibit sufficient performance with a small amount of support. In the fuel cell electrode catalyst according to the present embodiment, by using an alloy catalyst made of platinum and various transition metals, it is possible to reduce the amount of platinum used without impairing the catalytic activity.

  In the present embodiment, the transition metal constituting the platinum alloy includes the following: ruthenium (Ru), molybdenum (Mo), osnium (Os), cobalt (Co), rhodium (Rh), iridium (Ir), iron (Fe ), Nickel (Ni), titanium (Ti), tungsten (W), palladium (Pd), rhenium (Re), chromium (Cr), manganese (Mn), niobium (Nb), tantalum (Ta), copper (Cu) And at least one selected from gold (Au).

  The supported density of platinum or platinum alloy is defined as the weight percentage of the supported platinum or platinum alloy based on the total weight of the electrode catalyst. In the case of platinum, the carrying density is calculated by the following formula: platinum weight / (platinum weight + carrier weight) × 100. In the case of a platinum alloy, it is calculated by the formula of (platinum weight + transition metal weight) / (platinum weight + transition metal weight + support weight) × 100. In the fuel cell electrode catalyst of the present embodiment, the supported density of platinum or platinum alloy is preferably 5 to 60% by weight.

  The composition of the platinum alloy is defined as the weight percentage of platinum and / or transition metal relative to the total weight of the supported platinum alloy. Such a composition is calculated by a formula of platinum weight / (platinum weight + transition metal weight) × 100. In the fuel cell electrode catalyst of the present embodiment, the platinum alloy composition is preferably 50 to 95% by weight of platinum and 5 to 50% by weight of transition metal.

  Next, as shown in FIG. 1 (e), a part of the carbon film 6 is gasified by oxidation by firing the carrier particles 2 carrying the catalyst particles 5 in an oxidizing atmosphere containing oxygen gas, The catalyst particles 5 are brought into contact with the metal oxide of the support particles 2. Specifically, the carrier particles 2 on which the catalyst particles 5 are supported are heated in an inert gas containing an oxygen concentration lower than the oxygen concentration in the atmosphere (for example, 1000 ppm) at a low temperature of about 200 ° C. Thereby, the electrode catalyst 1 for fuel cells which concerns on this embodiment can be obtained.

  In this way, the catalyst particles 5 are supported on the surface of the carbon film 6 of the carrier particles 2 and calcined in an oxygen gas atmosphere, whereby the carbon film 6 is gasified by partial oxidation. However, it is buried in the carbon film 6 and comes into contact with the carrier particles (metal oxide) 2 serving as a base. As a result, since the catalyst particles 5 come into contact with the support particles (metal oxide) 2, the activity and durability of the electrode catalyst can be improved.

[Second Embodiment]
FIG. 2 is a diagram for explaining a method for producing a fuel cell electrode catalyst according to a second embodiment of the present invention, wherein (a) is a step for explaining a step of supporting platinum particles on carrier particles; b) is a step of supporting crystallization promoting particles on the surface of the carrier particles, (c) is a step of coating the surface of the carrier particles with a carbon film by a steam reforming reaction, and (d) is a coated carbon film. It is a figure for demonstrating the process of heat-processing a carrier particle so that crystallization of carbon may be accelerated | stimulated.

  The second embodiment is different from the first embodiment in that the catalyst particles are coated on the carrier particles before the carrier particles are coated with the carbon film. Specifically, as shown in FIG. 2 (a), the same carrier particles as the carrier particles shown in the first embodiment are prepared, and the carrier is prepared in the same manner as the method shown in FIG. 1 (d). The particles 2 carry catalyst particles 5 made of platinum.

  Next, as shown in FIG. 2 (b), the crystallization promoting particles 4 are formed on the surfaces of the carrier particles 2 on which the catalyst particles 5 are supported by the same method as described in FIG. 1 (a). Furthermore, it carries.

  Next, as shown in FIG. 2 (c), a hydrocarbon-based gas is formed on the surface of the carrier particle 2 on which the crystallization promoting particles 4 are supported by the same method as the method shown in FIG. 1 (b). The carbon film 6 made of carbon derived from the hydrocarbon gas is coated on the surfaces 21 of the carrier particles 2 by a steam reforming reaction. At this time, the metal contained in the crystallization promoting particles 4 and the platinum of the catalyst particles are alloyed, so that the specific activity (SA) of the fuel cell electrode catalyst is improved.

  Finally, as shown in FIG. 2 (d), the carrier particles 2 coated with the carbon film 6 are placed under an inert gas atmosphere such as argon gas so as to promote the crystallization of the carbon of the carbon film 6. , Heat treatment. Thereby, the electrode catalyst 1 for fuel cells which concerns on this embodiment can be obtained.

  Thus, in the case of the second embodiment, the catalyst particles are brought into contact with the metal oxides of the carrier particles by low-temperature heating as in the first embodiment (FIG. 2B). The obtained fuel cell electrode catalyst 1 can bring the catalyst particles 5 into contact with the metal oxide of the carrier particles 2 in the step shown in FIG. Thereby, similarly to 1st Embodiment, the activity and durability of an electrode catalyst can be improved. Furthermore, since the metal (for example, Co) contained in the crystallization promoting particles and the platinum of the catalyst particles are alloyed by the heat treatment described above, the specific activity (SA) of the fuel cell electrode catalyst is the same as that of the first embodiment. It is further improved compared to things.

The present invention will be described based on the following examples.
Example 1
In Example 1, an electrode catalyst for a fuel cell was produced according to the first embodiment described above. Specifically, an electrode catalyst for a fuel cell was produced according to the procedure shown in the following (1) to (6).
(1) Cobalt nitrate is used on the surface of carrier particles made of titanium oxide (TiO ₂ ) having a particle size of 16 nm and a specific surface area of 100 m ² / g so that the amount of cobalt is 2.0% by mass with respect to the obtained electrode catalyst. Then, crystallization promoting particles made of cobalt were supported by evaporation to dryness and pulverized with a mortar and blender. As a result, a Co / TiO ₂ powder composed of carrier particles (TiO ₂ particles) carrying crystallization promoting particles (Co particles) was obtained.
(2) The Co / TiO ₂ powder obtained in (1) is spread thinly in a tubular flow reactor, Ar gas is allowed to flow at 6.36 × 10 ⁻⁴ mol / s, and the temperature in the reaction vessel is increased to 773K. Gas, hydrogen gas (H ₂ ) 1.61 × 10 ⁻⁶ mol, water vapor (H ₂ O) 7.87 × 10 ⁻⁵ mol, methane gas (CH ₄ ) 7.87 × 10 ⁻⁵ mol in this order. For 60 minutes to coat the carrier particles with a carbon film made of carbon derived from methane gas.
(3) The carrier particles coated with the carbon film obtained in (2) were heated in an argon atmosphere by holding at 800 ° C. for 2 hours. This promoted carbon crystallization of the carbon film and increased conductivity.
(4) The carrier particles coated with the carbon film obtained in (3) are dispersed in pure water so that the supported Pt particles are 15% by mass with respect to the obtained electrode catalyst. Hydrate was added. Next, by reducing platinum with formaldehyde, catalyst particles (Pt particles) made of platinum were supported on the surface of a carbon film coated with carrier particles to obtain an electrode catalyst.
(5) The electrode catalyst obtained in (4) is held at 200 ° C. for 30 minutes in an oxidizing gas atmosphere in which oxygen gas is mixed with argon gas so that the oxygen gas becomes 1000 ppm. partially gasified by the oxidation of, contacting the Pt particles of TiO ₂ TiO ₂ particles.
(6) The electrode catalyst obtained in (5) was heated at 400 ° C. for 1 hour in a gas atmosphere in which argon gas was mixed with 21% by volume of hydrogen gas.

Thus, according to the procedure shown in the above (1) to (6), as shown in FIG. 3A, the surface of the TiO ₂ particles is coated with a carbon film having conductivity, and further, TiO 2 An electrode catalyst carrying Pt particles so as to come into contact with the surface of the _two particles was obtained.

(Example 2)
In Example 2, an electrode catalyst for a fuel cell was produced according to the second embodiment described above. Specifically, an electrode catalyst for a fuel cell was produced by the procedure shown in the following (1) to (4).
(1) A carrier particle made of titanium oxide (TiO ₂ ) having a particle size of 16 nm and a specific surface area of 100 m ² / g is dispersed in pure water, so that the Pt particle supported on the surface of the carrier particle is 15 Chloroplatinic acid hexahydrate was added so that it might become the mass%. Next, the catalyst particles (Pt particles) made of platinum were supported on the surface of the carrier particles by reducing platinum with formaldehyde.
(2) Crystallization promotion consisting of cobalt so that the carrier particles carrying the Pt particles obtained in (1) may be 2.0% by mass with respect to the obtained electrode catalyst using cobalt nitrate. The particles were supported by evaporation to dryness and pulverized with a mortar and blender. As a result, a powder composed of carrier particles (TiO ₂ particles) carrying catalyst particles (Pt particles) and crystallization promoting particles (Co particles) was obtained.
(3) After the powder obtained in (2) is spread thinly in a tubular flow reactor, Ar gas is allowed to flow at 6.36 × 10 ⁻⁴ mol / s, and the temperature in the reaction vessel is increased to 773 K. Hydrogen gas (H ₂ ) 1.61 × 10 ⁻⁶ mol, water vapor (H ₂ O) 7.87 × 10 ⁻⁵ mol, methane gas (CH ₄ ) 7.91 × 10 ⁻⁵ mol in this order for 60 minutes. Then, the carrier particles were coated with a carbon film.
(4) The carrier particles carrying the Pt particles coated with the carbon film obtained in (3) were heated at 800 ° C. for 2 hours in an argon atmosphere. This promoted carbon crystallization of the carbon film and increased conductivity.

Thus, according to the procedure shown in the above (1) to (4), similarly to Example 1, the surface of the TiO ₂ particles is coated with the carbon film having conductivity, and the TiO ₂ particles A fuel cell electrode catalyst carrying Pt particles so as to be in contact with the surface was obtained (see FIG. 3A).

(Comparative Example 1)
In Comparative Example 1, a fuel cell electrode catalyst was prepared according to the following procedures (1) and (2).
(1) Support particles (carbon particles) of Ketjen Black EC 300 having a specific surface area of 800 m ² / g as carbon black are dispersed in pure water, and on the surface of the support particles, an electrocatalyst that provides supported Pt particles is obtained. The chloroplatinic acid hexahydrate was added so that it might become 15 mass%. Next, the catalyst particles (Pt particles) made of platinum were supported on the surface of the carrier particles by reducing platinum with formaldehyde.
(2) The carrier particles (platinum-carrying carbon particles) carrying the Pt particles obtained in (1) were heated at 400 ° C. for 2 hours in an argon atmosphere.

  Thus, a fuel cell electrode catalyst in which Pt particles were supported on the surface of carbon particles was obtained by the procedure shown in (1) and (2) described above (see FIG. 3B).

(Comparative Example 2)
In Comparative Example 2, a fuel cell electrode catalyst was produced according to the following procedures (1) and (2).
(1) Carrier particles made of titanium oxide (TiO ₂ ) having a particle size of 16 nm and a specific surface area of 100 m ² / g are dispersed in pure water to obtain supported Pt particles on the surface of the carrier particles coated with a carbon film. Chloroplatinic acid hexahydrate was added so that it might be 15 mass% with respect to the electrode catalyst obtained. Next, the catalyst particles (Pt particles) made of platinum were supported on the surface of the carrier particles by reducing platinum with formaldehyde.
(2) The carrier particles (platinum-supported TiO ₂ particles) on which the Pt particles obtained in (1) were supported were heated at 400 ° C. for 2 hours in an argon atmosphere.

Thus, a fuel cell electrode catalyst in which Pt particles were supported on the surface of TiO ₂ particles was obtained by the procedure shown in (1) and (2) described above (see FIG. 3C).

(Electrocatalyst evaluation test)
The RDE electrode was coated with the fuel cell electrode catalyst according to Examples 1 and 2 and Comparative Examples 1 and 2, and the following electrochemical evaluation was performed.

<Initial performance test>
The RDE electrode was immersed in 0.1 M HClO ₄ saturated with nitrogen gas, and CV measurement was carried out under the conditions of a temperature of 27 ° C. and 50 mV / sec. From this result, ECA (m / g− Pt) was calculated. The results are shown in Table 1 below.

The RDE electrode was closely related to 0.1M HClO ₄ in a state in which oxygen gas was saturated, and LSV measurement was performed under conditions of a temperature of 27 ° C., 10 mV / sec, and 1600 trpm. From this result, MA (A / g-Pt), was calculated SA ^(a / m 2 -Pt) is Pt specific activity. The results are shown in Table 1 below.

<Durability test>
The RDE electrode was immersed in 0.1 M HClO ₄ saturated with nitrogen gas, and CV measurement was performed for 5000 cycles under the conditions of a temperature of 27 ° C., 50 mV / sec, and 0.4-1.2 (V. vs. RHE). Measurements similar to the CV measurement and LSV measurement described above were performed, and the ECA maintenance rate (ECA after 5000 cycles) / (initial ECA) × 100% was calculated from these results. The results are shown in Table 1 below.

(result)
The electrocatalysts of Examples 1 and 2 had higher mass activity (MA) than that of Comparative Example 2. Furthermore, the electrocatalysts of Examples 1 and 2 had a higher ECA maintenance rate and higher durability than those of Comparative Example 1.

  From the above, in Examples 1 and 2, since the mass activity (MA) is higher than that in Comparative Example 2, the amount of catalyst particles (Pt particles) to be supported on the carrier particles is larger than that in Comparative Example 2. It is thought that the performance of the catalyst can be improved with a small amount. In addition, when specific activity (SA) and ECA are high, mass activity will improve, but the electrocatalyst of Example 1 has high specific activity (SA) compared with the thing of the comparative example 1, but since ECA is low, mass activity is high. Are considered to be equivalent.

  On the other hand, in Example 2, the specific activity (SA) is considered to be improved as compared with the others by the contact between the Pt particles and the supported particles and the securing of conductivity. Furthermore, Co in the added crystallization promoting particles remains in the carbon film due to the promotion of carbon crystallization, and when this Co is heated at 800 ° C., it is alloyed with platinum of Pt particles (Pt—Co alloy and It is considered that the specific activity (SA) was further improved.

In addition, the ECA maintenance rate decreased in Comparative Example 1 is considered to be due to Pt aggregation due to Pt dissolution reprecipitation due to potential fluctuations and Pt migration on carbon. Furthermore, it is considered that the ECA of Comparative Example 2 is small because Pt is coarsened because Pt particles are supported on low surface area carrier particles (TiO ₂ ). Further, the ECA of Example 2 is small because it is considered that Pt is coarsened by treatment at 800 ° C. or higher after supporting Pt.

From the above, in the case of Examples 1 and 2, the interaction between the Pt particles (Pt) and the carrier particles (TiO ₂ ) due to contact (improvement of potential of Pt electronic state modification) and the conductivity of the carbon film. It is thought that the specific activity was improved by improving the potential by securing. Thus, it is considered that the fuel cell electrode catalysts of Examples 1 and 2 can improve the catalytic performance of catalyst particles made of platinum or a platinum alloy and have improved durability.

  Although the embodiments of the present invention have been described in detail above, the present invention is not limited to the above-described embodiments, and various designs can be made without departing from the spirit of the present invention described in the claims. It can be changed.

  For example, in the first and second embodiments, the crystallization promoting particles are used. However, if the crystallization of the carbon film can be promoted by selecting the heat treatment conditions, the crystallization promoting particles are not necessarily used. The conversion promoting particles may not be used.

DESCRIPTION OF SYMBOLS 1 ... Fuel cell electrode catalyst, 2 ... Carrier particle, 4 ... Crystallization promotion particle, 5 ... Catalyst particle, 6 ... Carbon film,

Claims (4)

A method for producing an electrode catalyst for a fuel cell in which catalyst particles made of platinum or a platinum alloy are supported on carrier particles made of a metal oxide,
A hydrocarbon gas and water vapor are brought into contact with the surface of the carrier particles while heating, and a carbon film made of carbon derived from the hydrocarbon gas is coated on the surface of the carrier particles by a steam reforming reaction. Process,
And a step of heat-treating the carrier particles coated with the carbon film so as to promote crystallization of carbon of the carbon film.   Before carrying out the step of coating the carbon film, crystallization promoting particles made of at least one of Co, Fe, or Ni are supported on the surface of the carrier particles as a material for promoting the crystallization of the carbon. The method for producing a fuel cell electrode catalyst according to claim 1. After the heat treatment, the catalyst particles are supported on the surface of the carbon film covering the carrier particles,
The carrier particles carrying the catalyst particles are calcined in an oxidizing atmosphere containing oxygen gas, whereby a part of the carbon film is gasified by oxidation, and the catalyst particles are brought into contact with the metal oxide of the carrier particles. A method for producing an electrode catalyst for a fuel cell according to claim 1 or 2, wherein:   The method for producing an electrode catalyst for a fuel cell according to claim 1 or 2, wherein the catalyst particles are supported on the surface of the carrier particles before the step of coating the carbon film.

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