JP2014117652A - Method for producing catalyst fine particle, and fuel battery including catalyst fine particle produced by the same method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing a catalyst fine particle that has a smaller number of steps than the conventional method, and to provide a fuel battery including a catalyst fine particle produced by the production method.SOLUTION: The method for producing a catalyst fine particle produces a catalyst fine particle having a central particle comprising palladium and an outermost layer comprising platinum and coating the central particle. The method comprises a first electric potential treatment step of applying an electric potential in a range of 0.4-0.6 V(vs.RHE) to an acid solution in which palladium-containing particles are dispersed and a second electric potential treatment step of adding at least a platinum ion to the acid solution in which the palladium-containing particles are dispersed after the first potential treatment step and applying an electric potential in a range of 0.80-1.0 V(vs.RHE) to thereby coat the palladium-containing particle with the outermost layer containing platinum.

Description

  The present invention relates to a method for producing catalyst fine particles having fewer man-hours than before, and a fuel cell including catalyst fine particles produced by the production method.

  A fuel cell directly converts chemical energy into electrical energy by supplying fuel and an oxidant to two electrically connected electrodes, respectively, and oxidizing the fuel electrochemically. Unlike thermal power generation, fuel cells are not subject to the Carnot cycle, and thus exhibit high energy conversion efficiency. A fuel cell is usually formed by laminating a plurality of single cells having a basic structure of a membrane / electrode assembly in which an electrolyte membrane is sandwiched between a pair of electrodes.

  Conventionally, supported platinum and platinum alloy materials have been employed as electrode catalysts for anodes and cathodes of fuel cells. However, the amount of platinum required for today's electrocatalysts is still expensive for commercial realization of fuel cell mass production. Therefore, studies have been made to reduce the amount of platinum contained in fuel cell cathodes and anodes by combining platinum with less expensive metals.

In recent years, core-shell type catalyst fine particles have attracted attention as electrode catalysts for fuel cells. From the viewpoint of increasing the coverage of the shell with respect to the core, in general, in a method for producing core-shell type catalyst fine particles, a monoatomic layer is formed on the surface of the core portion in advance by an underpotential precipitation method such as a Cu-UPD method, A method of replacing an atomic layer with a shell part is known.
As a technique to which the Cu-UPD method is applied, Patent Document 1 discloses a manufacturing method of core-shell type catalyst fine particles including a core part and a shell part that covers the core part, and includes a process of covering the core part with the shell part. A manufacturing method comprising a total of two steps, that is, a step of covering a core portion with a monoatomic layer and a step of replacing the monoatomic layer with a shell portion is disclosed.

JP 2012-016684 A

Paragraph [0035] of the specification of the cited document 1 describes a method in which a copper monoatomic layer is formed on the surface of the core portion in advance by the Cu-UPD method, and then the copper monoatomic layer is replaced with a shell portion. . However, the synthesis method using the Cu-UPD method has a problem that the adjustment of the potential is very complicated, and it is difficult to scale up because a large amount of copper waste liquid is discharged.
The present invention has been accomplished in view of the above circumstances, and an object of the present invention is to provide a method for producing catalyst fine particles having fewer man-hours than before, and a fuel cell including the catalyst fine particles produced by the production method. To do.

  The method for producing catalyst fine particles of the present invention is a method for producing catalyst fine particles comprising center particles containing palladium, and an outermost layer containing platinum and covering the center particles, with respect to the acid solution in which the palladium-containing particles are dispersed, A first potential treatment step of applying a potential within the range of 0.4 to 0.6 V (vs. RHE), and at least the acid solution in which the palladium-containing particles after the first potential treatment step are dispersed; By adding platinum ions and applying a potential within a range of 0.80 to 1.0 V (vs. RHE), a second potential for coating the palladium-containing particles with the outermost layer containing platinum. A processing step.

  In the present invention, the first potential processing step and the second potential processing step may be alternately repeated twice or more.

  In the present invention, the potential treatment in the first potential treatment step is preferably within a range of 0.4 to 0.6 V (vs. RHE), more preferably 0.4 to 0.45 V (vs. RHE). ) Is a potential process for sweeping between any two potentials within the range.

  In the present invention, the potential treatment in the second potential treatment step is a potential treatment for applying an end point potential of an open circuit potential measured in advance within a range of 0.80 to 1.0 V (vs. RHE). Is preferred.

  In the present invention, the palladium-containing particles may be supported on a carrier.

  The fuel cell of the present invention comprises a single cell comprising a membrane / electrode assembly comprising an anode electrode having at least an anode catalyst layer on one side of a polymer electrolyte membrane and a cathode electrode having at least a cathode catalyst layer on the other side. A fuel cell comprising: catalyst fine particles produced by the above production method is included in at least one of the anode catalyst layer and the cathode catalyst layer.

  According to the present invention, since palladium is directly coated on palladium-containing particles, the number of man-hours can be reduced as compared with a conventional method of constructing a multi-stage core-shell structure using an underpotential deposition method. Synthesis is possible.

It is a graph which shows the waveform of the electric potential process of the 1st cycle when an electric potential is reciprocated within the range of 0.40-0.45V with respect to the acid solution containing a palladium microparticle. It is a figure which shows an example of the single cell of the fuel cell which concerns on this invention, Comprising: It is the figure which showed typically the cross section cut | disconnected in the lamination direction. It is the graph which plotted Q value of each cycle about each Pd / C of Example 1, comparative example 1, and reference example 2. 3 is a bar graph comparing the platinum ratio of catalyst fine particles of Example 1 and Comparative Example 2. FIG. 3 is a bar graph comparing the platinum surface areas of catalyst fine particles of Example 1 and Comparative Example 2. FIG. It is the bar graph which compared the palladium elution amount in 80 degreeC sulfuric acid about the catalyst fine particle of the reference example 1 and the comparative example 3. FIG. It is a graph which shows the change of the open circuit potential in the conventional method which coat | covers the platinum outermost layer on the surface of palladium fine particle.

1. Method for Producing Catalyst Fine Particles The method for producing catalyst fine particles of the present invention is a method for producing catalyst fine particles comprising central particles containing palladium and an outermost layer containing platinum and covering the central particles, wherein the palladium-containing particles are dispersed. A first potential treatment step for applying a potential in the range of 0.4 to 0.6 V (vs. RHE) to the acid solution, and an acid in which the palladium-containing particles after the first potential treatment step are dispersed. The palladium-containing particles are coated with the outermost layer by adding at least platinum ions to the solution and applying a potential in the range of 0.80 to 1.0 V (vs. RHE). And a second potential processing step.

  Catalyst fine particles having a core-shell structure are considered very promising for reducing the amount of rare and expensive metals such as platinum. For the construction of the core-shell structure, a synthesis method using the Cu-UPD method described above and a synthesis method in which platinum is deposited on the surface of the core material in a platinum ion solution are known.

  The synthesis method using the Cu-UPD method is to deposit copper on the surface of the core material by the Cu-UPD method, and then add platinum ions to cause a substitution action between copper and platinum. This is a method of forming a platinum layer. As a specific example of the synthesis method using the Cu-UPD method, first, a core material is applied on an electrode, and the electrode is immersed in an aqueous copper sulfate solution that is bubbled at room temperature and nitrogen, thereby applying a predetermined potential. And forming a monolayer of copper on the surface of the core material, and then immersing the electrode in a platinum ion solution to replace copper and platinum, thereby forming a monolayer of platinum on the surface of the core material. Can be mentioned.

In the synthesis method using the Cu-UPD method, how to deposit copper uniformly on the surface of the core material is a problem. This is because if copper is deposited non-uniformly on the surface of the core material, it will lead to a result of non-uniform deposition of platinum on the surface of the core material after replacing copper with platinum.
However, when the synthesis method using the Cu-UPD method is scaled up, it takes a long time to uniformly diffuse the copper ions in the solution, which may reduce the production efficiency and increase the cost. In addition, when scaled up, it becomes difficult to uniformly apply a potential to the entire core material, so a potential drop occurs locally, and as a result, copper is uniformly deposited on the core material. Therefore, the quality of the obtained catalyst fine particles may vary. Further, the scale-up of the Cu-UPD method involves a large amount of waste liquid treatment containing copper, and has a high environmental load. Furthermore, when the substitution of copper and platinum is insufficient, when the obtained catalyst fine particles are used in the membrane / electrode assembly, the copper remaining in the catalyst fine particles is eluted during the electrode reaction. The membrane / electrode assembly may deteriorate.

As a specific example of the synthesis method for depositing platinum on the surface of the core material in the platinum ion solution, the potential is lowered by heating the suspension containing the core material, and then the platinum ion solution is added to the suspension. Thus, a method of coating the core material with platinum can be mentioned.
In such a method, a large amount of oxide is present on the surface of the core material. Therefore, when the platinum shell is formed on the surface of the core material, the coverage of the platinum shell on the surface of the core material is reduced.

Further, as a premise of the catalyst fine particles having the core-shell structure, there is a problem that when the covering ratio of the shell portion to the core portion is low, the core is eluted and the performance is remarkably deteriorated. Therefore, in the method for producing catalyst fine particles having a core-shell structure, it is important how easily a core-shell catalyst having a high coverage can be synthesized.
The present inventors perform the first potential treatment step on the palladium-containing particles, and then add platinum ions and perform the second potential treatment to obtain catalyst fine particles having a core-shell structure with a high coverage. As a result, it was found that mass synthesis can be performed more easily than in the past, and the present invention was completed.

The present invention includes (1) a first potential processing step and (2) a second potential processing step. The present invention is not necessarily limited to only the above two steps, and may include, for example, a filtration step, a washing step, a drying step and the like as described later in addition to the above two steps.
Hereinafter, the steps (1) to (2) and other steps will be described in order.

1-1. First potential treatment step The first potential treatment step in the present invention is a step of applying a potential within the range of 0.4 to 0.6 V (vs. RHE) to the acid solution in which the palladium-containing particles are dispersed. is there.

As the palladium-containing particles, those previously synthesized can be used, or commercially available products can also be used. In addition, the palladium containing particle | grains said by this invention is a general term for palladium particle | grains and palladium alloy particle | grains.
As will be described later, the outermost layer contains platinum. Platinum is excellent in catalytic activity, particularly oxygen reduction reaction (ORR) activity. In addition, the lattice constant of platinum is 3.92Å, whereas the lattice constant of palladium is 3.89Å, and the lattice constant of palladium is a value within a range of ± 5% of the lattice constant of platinum. There is no lattice mismatch between platinum and palladium, and palladium is sufficiently covered with platinum.
The palladium-containing particles used in the present invention preferably contain a metal material that is less expensive than the material used for the outermost layer described later, from the viewpoint of cost reduction. Furthermore, the palladium-containing particles preferably include a metal material that can be electrically connected.
From the above viewpoints, the palladium-containing particles used in the present invention are preferably palladium particles or alloy particles of palladium such as iridium, rhodium or gold. When palladium alloy particles are used, the palladium alloy particles may contain only one type of metal in addition to palladium, or two or more types of metals.

The average particle diameter of the palladium-containing particles is not particularly limited as long as it is equal to or smaller than the average particle diameter of the catalyst fine particles described later. In addition, from a viewpoint that the ratio of the surface area with respect to the cost per palladium containing particle | grain is high, the average particle diameter of palladium containing particle | grains becomes like this. Preferably it is 30 nm or less, More preferably, it is 5-10 nm.
In addition, the average particle diameter of the palladium containing particle | grains and catalyst fine particle which are used for this invention is computed by a conventional method. An example of a method for calculating the average particle diameter of the palladium-containing particles and the catalyst fine particles is as follows. First, in a TEM image having a magnification of 400,000 to 1,000,000, a particle size is calculated for a certain particle when the particle is considered spherical. The calculation of the particle size by such TEM observation is performed on 200 to 300 particles of the same type, and the average of these particles is defined as the average particle size.

The palladium-containing particles may be supported on a carrier. In particular, when the catalyst fine particles produced according to the present invention are used in an electrode catalyst layer of a fuel cell, the carrier is preferably a conductive material from the viewpoint of imparting conductivity to the electrode catalyst layer.
Specific examples of the conductive material that can be used as a carrier include Ketjen black (trade name: manufactured by Ketjen Black International Co., Ltd.), Vulcan (product name: manufactured by Cabot), Norit (trade name: manufactured by Norit), Examples thereof include carbon particles such as black pearl (trade name: manufactured by Cabot), acetylene black (trade name: manufactured by Chevron), conductive carbon materials such as carbon fibers, and metal materials such as metal particles and metal fibers.

The carrier on which the palladium-containing particles are supported may be prepared in advance or may be commercially available.
Conventionally used methods can be employed as a method for supporting the palladium-containing particles on the carrier. When palladium alloy particles are used, the preparation of the palladium alloy and the loading of the palladium alloy particles on the carrier may be performed simultaneously.

The acid solution used in the present invention is an acid solution having an oxidizing power sufficient to remove the oxide on the surface of the palladium-containing particles, and specifically, nitric acid, sulfuric acid, perchloric acid, hydrochloric acid, hypochlorous acid, and the like. Examples include chloric acid. In particular, sulfuric acid is preferable from the viewpoint of mainly having an oxidizing power sufficient to dissolve palladium. In addition, what is necessary is just to adjust suitably the density | concentration of an acid solution and the atmosphere control in the acid solution by bubbling for every kind of acid solution.
In the acid solution in which the palladium-containing particles are dispersed, the palladium-containing particles are uniformly dispersed in the acid solution without agglomeration from each other, from the viewpoint that the potential treatment proceeds uniformly and promptly for all the palladium-containing particles. Preferably it is.

The potential range of 0.4 to 0.6 V (vs. RHE) in the first potential treatment step is a potential range in which the oxide (oxide film) on the surface of the palladium-containing particles can be removed. When the potential is less than 0.4 V (vs. RHE), there is a possibility that the occlusion of hydrogen by palladium starts. On the other hand, at a potential exceeding 0.6 V (vs. RHE), metals such as palladium in the palladium-containing particles may start to elute. Even when the lower limit of 0.4 V (vs. RHE) is lower by about 0.2 V, the cleaning effect itself for removing oxides on the surface of the palladium-containing particles is 0.4 to 0.6 V (vs. RHE). This is equivalent to the effect of sweeping the potential range. The range of the potential applied in the first potential treatment step is preferably in the range of 0.4 to 0.45 V (vs. RHE).
In the first potential processing step, the potential processing may be executed with a predetermined potential fixed within the range of 0.4 to 0.6 V (vs. RHE), or within a predetermined potential range. May be swept once or more than once. In addition, from the viewpoint that the desorption of the adsorbed substance on the surface of the palladium-containing particles can be repeated and the oxide existing on the surface can be efficiently removed, the potential treatment in the first potential treatment step is 0.4 to 0.6 V ( (vs.RHE) is preferably a potential treatment that sweeps between any two potentials.
In the case where the potential is swept between any two potentials, the number of sweeps can be appropriately adjusted according to the reaction scale. The number of sweeps is, for example, about 1 to 1,000 cycles for 1 to 100 g of palladium-containing particles.

The time required for the first potential treatment step is not particularly limited as long as the oxide on the surface of the palladium-containing particles can be sufficiently removed, and can be appropriately adjusted depending on the synthesis scale. As an indication of the end of the first potential processing step, for example, when potential processing that sweeps between any two potentials within the range of 0.4 to 0.6 V (vs. RHE) is performed, it will be described later. The potential trajectory of the potential processing as shown in FIG. 1 substantially overlaps the trajectory of the waveform at the previous sweep, and the potential processing waveform draws substantially the same trajectory even after multiple sweeps. . In such a case, the fluctuation of the current with respect to the potential treatment becomes constant, and it can be considered that the oxide on the surface of the palladium-containing particles has almost disappeared.
The time required for the first potential treatment step is, for example, about 1 to 24 hours for 1 to 100 g of palladium-containing particles.

  A specific example of the first potential treatment process is as follows. First, carbon-supported palladium fine particle powder is added to an acid solution and dispersed in the acid solution as appropriate, and then the potential is swept back and forth within a range of 0.4 to 0.45 V (vs. RHE). At this time, the acid solution is preferably bubbled beforehand with an inert gas such as nitrogen gas or argon gas to remove oxygen or the like in the acid solution as much as possible. Note that FIG. 1 shows the waveform of the potential treatment in the first cycle when the potential is reciprocated and swept within a range of 0.4 to 0.45 V (vs. RHE) with respect to the acid solution containing palladium fine particles. It is a graph to show.

  As described above, by performing the potential treatment on the palladium-containing particles before the platinum coating, oxides such as palladium oxide adsorbed on the surface of the palladium-containing particles can be removed, and the surface of the palladium-containing particles can be cleaned. Further, the potential to be applied is within the range of 0.4 to 0.6 V (vs. RHE), preferably within the range of 0.4 to 0.45 V (vs. RHE). Since there is no fear of elution of metals such as palladium and hydrogen occlusion by palladium, there is no possibility that an oxide will newly appear on the surface of the palladium-containing particles.

Such a first potential treatment step can also be applied to repair and inspection of catalyst fine particles having a core-shell structure in which the shell is not completely covered with the core.
Conventionally, for catalyst fine particles having a core-shell structure with a low coverage, the core-shell structure is repaired by the Cu-UPD method, and the coverage has been improved. However, when the Cu-UPD method is applied to the core-shell structure, copper is deposited not only on the surface of the core material that is exposed without being covered but also on the shell portion. The inability to selectively coat the exposed core material surface results in the consumption of more metal than necessary to repair the core-shell structure, resulting in the production of catalyst particles with poor catalytic performance and increased cost. Become.
By passing through the first potential treatment step, in the catalyst fine particles having a core-shell structure with a coverage of less than 100%, the oxide on the surface of the central particles exposed on the surface of the catalyst fine particles can be removed, and the subsequent second potential is obtained. The efficiency of coating the outermost layer in the treatment process can be increased.

  A specific example in which the first potential treatment process is applied to catalyst fine particles having a core-shell structure with a coverage of less than 100% is as follows. First, the catalyst fine particles are added to an acid solution and dispersed appropriately in the acid solution, and then the potential is reciprocated and swept within a range of 0.4 to 0.6 V (vs. RHE). The conditions of the acid solution at this time are the same as those in the specific example of the first potential treatment step.

1-2. Second potential treatment step In the second potential treatment step of the present invention, at least platinum ions are added to the acid solution in which the palladium-containing particles after the first potential treatment step are dispersed, and 0.80 is added. This is a step of coating palladium-containing particles with an outermost layer containing platinum by applying a potential within a range of ˜1.0 V (vs. RHE).

The material constituting the outermost layer formed in this step preferably has high catalytic activity. As used herein, the term “catalytic activity” refers to activity as a fuel cell catalyst, particularly oxygen reduction reaction (ORR) activity.
The outermost layer may contain only platinum, or may contain iridium, ruthenium, rhodium, or gold in addition to platinum. When a platinum alloy is used for the outermost layer, the platinum alloy may contain only one type of metal in addition to platinum, or two or more types.

In the second potential treatment step, at least platinum ions may be added to the acid solution in which the palladium-containing particles after the first potential treatment step are dispersed. In addition to the platinum ions, iridium ions, ruthenium ions may be added. , Rhodium ions, or gold ions may be added as appropriate. The addition ratio of these metal ions can be adjusted as appropriate in accordance with the composition of the target outermost layer.
For the addition of metal ions such as platinum ions, a metal compound itself such as a platinum compound may be added, or a metal ion solution such as a platinum ion solution may be added. In addition, when adding a metal ion solution such as a platinum ion solution, the metal ion solution is previously bubbled with an inert gas such as nitrogen gas or argon gas, and oxygen or the like in the metal ion solution is as much as possible. It is preferable to remove.
The addition amount of metal ions such as platinum ions is preferably determined from the area (or volume) of the outermost layer to be formed, which is calculated in advance from the average particle diameter of the palladium-containing particles. The average particle diameter of the palladium-containing particles can be calculated by the method described above.

  The potential within the range of 0.80 to 1.0 V (vs. RHE) is a potential at which the outermost layer containing platinum can completely cover the entire surface of the palladium-containing particles. When the potential is less than 0.80 V (vs. RHE), the surface of the palladium-containing particles has a portion that is not yet covered with the outermost layer, so that the obtained catalyst fine particles have a coverage of less than 100%. It becomes. Further, when the potential exceeds 1.0 V (vs. RHE), the catalyst performance of the catalyst fine particles may be impaired.

  In the second potential processing step, from the viewpoint of efficiently covering the outermost layer, the potential processing may be performed with a predetermined potential within a range of 0.80 to 1.0 V (vs. RHE). preferable.

The potential treatment in the second potential treatment step is more preferably a potential treatment for applying an end potential of an open circuit potential measured in advance within a range of 0.80 to 1.0 V (vs. RHE).
The end point potential of the open circuit potential can be determined by previously mixing a combination of palladium-containing particles to be used and metal ions such as platinum ions on a small reaction scale and measuring the open circuit potential (OCV). FIG. 7 is a graph showing changes in the open circuit potential (OCV) when the surface of the palladium fine particle is covered with the outermost platinum layer. In FIG. 7, the OCV at the beginning of the reaction is about 0.3 V (vs. RHE), but then rises, and after about 3,000 seconds it rises to 0.83 V (vs. RHE). Converge. In the present invention, the potential converged after a sufficient time has elapsed from the initial stage of the open circuit is set as the end point potential of the open circuit potential. The end point potential is a potential at which the process of coating the palladium fine particles with platinum is completed. The change in the open circuit potential as shown in FIG. 7 is observed in a conventional method in which the palladium particles after Cu-UPD are directly attached to the platinum ion solution and the potential is not particularly adjusted.
In a more preferred embodiment of the second potential treatment step, the coating of the outermost layer on the palladium-containing particles can be promoted by applying the end point potential from the initial stage of the reaction.

The time required for the second potential treatment step is not particularly limited as long as the outermost layer is completely covered on the surface of the palladium-containing particles for at least one layer, and can be appropriately adjusted depending on the reaction scale. As an indication of the end of the second potential treatment step, for example, the application of the potential is interrupted and the open circuit potential of the reaction solution is measured, and the open circuit potential is in the range of 0.80 to 1.0 V (vs. RHE). When it is within, preferably when the end point potential is reached.
The time required for the second potential treatment step is, for example, about 1 to 24 hours for 1 to 100 g of palladium-containing particles.

  A specific example of the second potential treatment step is as follows. First, a platinum ion solution is added to an acid solution containing carbon-supported palladium fine particle powder after the first potential treatment step. Next, a potential within the range of 0.80 to 1.0 V (vs. RHE), preferably an end-point potential of an open circuit potential, is quickly applied to the electrochemical cell, thereby forming platinum on the surface of the palladium-containing particles. Cover the monoatomic layer.

In this way, the coating of the outermost layer on the palladium-containing particles is promoted by adding at least platinum ions to the palladium-containing particles and applying a potential within the range of 0.80 to 1.0 V (vs. RHE). Can be made.
Further, in the present invention, unlike the synthesis method using the conventional Cu-UPD method, since copper is not used, there is no problem of a large amount of copper-containing drainage treatment discharged after synthesis. Furthermore, when using the catalyst fine particles produced according to the present invention for a membrane / electrode assembly, there is no concern about the adverse effect of copper remaining in the catalyst fine particles on the membrane / electrode assembly, which has been a problem in the past.
As described above, the production method of the present invention can provide catalyst fine particles having a higher coverage at a lower cost and more easily than the conventional synthesis method.

  Such a second potential treatment step can also be applied to restoration and inspection of catalyst fine particles having a core-shell structure in which the shell is not completely covered with the core. When the second potential treatment step is carried out for the repair of the catalyst fine particles having an incomplete core-shell structure, the exposed palladium-containing particles such as palladium and metal ions such as platinum ions are exposed due to the difference in ionization tendency. And are replaced. In the portion already covered with platinum or the like, the substitution reaction does not proceed, so that only the surface of the palladium-containing particles that are not coated can be selectively coated, and the consumption of metal ions such as platinum ions is minimized. Can be stopped.

  By passing through the second potential treatment step, it is possible to deposit platinum only on the defective outermost layer portion without depositing useless platinum in catalyst fine particles having a core-shell structure with a coverage of less than 100%. The outermost layer can be repaired. Such a process can also be applied to inspection from the viewpoint that the synthesized catalyst fine particles can be produced with higher quality and higher performance and are free from defects. As described above, in the method using the conventional Cu-UPD method, the thickness of the outermost layer is increased every time the process is repeated. Therefore, it is difficult to improve the quality of the catalyst fine particles by such a method.

  A specific example in which the second potential treatment step is applied to catalyst fine particles having a core-shell structure with a coverage of less than 100% is substantially the same as the above specific example of the second potential treatment step. Unlike the above specific example, in the case of repair or inspection, the amount of metal ions such as platinum ions added may be arbitrary. If the addition amount of metal ions is insufficient, metal ions may be appropriately added to the reaction solution. If the addition amount of metal ions is excessive, the metal solution is added to the acid solution. This is because only ions remain, and a predetermined amount or more of platinum or the like does not precipitate on the surface of the palladium-containing particles, so that the metal ions can be reused after the acid solution is recovered.

  In the present invention, the first and second potential treatment steps described above may be repeated twice or more alternately. By repeating these potential treatment steps, the coverage of the outermost layer on the palladium-containing particles can be brought close to 100%, preferably 100%.

  From the viewpoint that elution of the palladium-containing particles can be further suppressed, the coverage of the outermost layer with respect to the palladium-containing particles is preferably 80 to 100%. When the coverage of the outermost layer with respect to the palladium-containing particles is less than 80%, the center particles are eluted in the electrochemical reaction, and as a result, the catalyst fine particles may be deteriorated.

  As used herein, “the coverage of the outermost layer relative to the palladium-containing particles” refers to the percentage of the area of the palladium-containing particles covered by the outermost layer when the total surface area of the palladium-containing particles is 100%. Refers to the value (%) expressed. An example of a method for calculating the coverage will be described below. First, the platinum content (A) in the catalyst fine particles is measured by inductively coupled plasma mass spectrometry (ICP-MS) or the like. On the other hand, the average particle diameter of the catalyst fine particles is measured with a transmission electron microscope (TEM) or the like. From the measured average particle diameter, the number of atoms that the particle having the particle diameter has on the surface is estimated, and the platinum content (B) when one atomic layer on the particle surface is replaced with platinum is estimated. A value obtained by dividing the platinum content (A) by the platinum content (B) is “the coverage of the outermost layer with respect to the palladium-containing particles”.

The outermost layer formed in this step is preferably a monoatomic layer. Such catalyst fine particles have the advantage that the catalyst performance in the outermost layer is extremely high compared to the catalyst fine particles having the outermost layer of two atomic layers or more, and the advantage that the material cost is low because the coating amount of the outermost layer is small. There is.
The lower limit of the average particle diameter of the catalyst fine particles is preferably 4 nm or more, more preferably 5 nm or more, and the upper limit thereof is preferably 40 nm or less, more preferably 10 nm or less.

After the second potential treatment step, catalyst fine particles may be filtered, washed, dried, and the like.
Filtration and washing of the catalyst fine particles are not particularly limited as long as they can remove impurities without impairing the coating structure of the produced catalyst fine particles. Examples of the filtration and washing include a method of suction filtration using water, perchloric acid, dilute sulfuric acid, dilute nitric acid or the like.
The drying of the catalyst fine particles is not particularly limited as long as the method can remove the solvent and the like. Examples of the drying include a method of performing vacuum drying at room temperature for 0.5 to 2 hours and then drying under an inert gas atmosphere at a temperature of 60 to 80 ° C. for 1 to 12 hours.

2. Fuel cell The fuel cell of the present invention comprises a membrane / electrode assembly comprising an anode electrode having at least an anode catalyst layer on one side of a polymer electrolyte membrane and a cathode electrode having at least a cathode catalyst layer on the other side. A fuel cell comprising a cell, wherein the catalyst fine particles produced by the production method are contained in at least one of the anode catalyst layer and the cathode catalyst layer.

  FIG. 2 is a diagram showing an example of a single cell of the fuel cell according to the present invention, and is a diagram schematically showing a cross section cut in the stacking direction. The membrane / electrode assembly 8 includes a polymer electrolyte membrane (hereinafter sometimes simply referred to as an electrolyte membrane) 1 having hydrogen ion conductivity, and a pair of cathode electrode 6 and anode electrode 7 sandwiching the electrolyte membrane 1. . The single cell 100 includes a membrane / electrode assembly 8 and a pair of separators 9 and 10 that sandwich the membrane / electrode assembly 8 from the outside of the electrode. Gas flow paths 11 and 12 are secured at the boundary between the separator and the electrode. Usually, a laminate of a catalyst layer and a gas diffusion layer is used as an electrode in order from the electrolyte membrane side. That is, the cathode electrode 6 includes a stacked body of the cathode catalyst layer 2 and the gas diffusion layer 4, and the anode electrode 7 includes a stacked body of the anode catalyst layer 3 and the gas diffusion layer 5. The fuel cell catalyst according to the present invention is used in at least one of an anode catalyst layer and a cathode catalyst layer.

  The polymer electrolyte membrane is a polymer electrolyte membrane used in a fuel cell, and is 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 groups are used in hydrocarbon polymers such as engineering plastics such as polyether ether ketone, polyether ketone, polyether sulfone, polyphenylene sulfide, polyphenylene ether, and polyparaphenylene, and general-purpose plastics such as polyethylene, polypropylene, and polystyrene. And a hydrocarbon polymer electrolyte membrane including a hydrocarbon polymer electrolyte into which a protonic acid group (proton conductive group) such as a carboxylic acid group, a phosphoric acid group, or a boronic acid group is introduced.

The electrode includes a catalyst layer and a gas diffusion layer.
Both the anode catalyst layer and the cathode catalyst layer can be formed using a catalyst ink containing a catalyst, a conductive material, and a polymer electrolyte. As the polymer electrolyte, the same material as the polymer electrolyte membrane described above can be used. As the catalyst, the fuel cell catalyst according to the present invention is used.
The fuel cell catalyst according to the present invention may be used only for the anode catalyst layer, may be used only for the cathode catalyst layer, or may be used for both the anode catalyst layer and the cathode catalyst layer. When the fuel cell catalyst according to the present invention is used only for the anode catalyst layer, another catalyst is used for the cathode catalyst layer. Further, when the fuel cell catalyst according to the present invention is used only for the cathode catalyst layer, another catalyst is used for the anode catalyst layer.
As the other catalyst, usually, a catalyst component supported on conductive particles is used. The catalyst component is not particularly limited as long as it has catalytic activity for the oxidation reaction of the fuel supplied to the anode electrode or the reduction reaction of the oxidant supplied to the cathode electrode. What is generally used for the fuel cell can be used. For example, platinum or an alloy of platinum and a metal such as ruthenium, iron, nickel, manganese, cobalt, and copper can be used. As the conductive particles as the catalyst carrier, carbon particles such as carbon black, conductive carbon materials such as carbon fibers, and metal materials such as metal particles and metal fibers can also be used. The conductive material also plays a role of imparting conductivity to the catalyst layer.

  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 catalyst ink to the surface of the gas diffusion sheet and drying, or the polymer electrolyte membrane. A catalyst layer may be formed on the surface of the polymer electrolyte membrane by applying a catalyst ink on the surface and drying. Alternatively, a transfer sheet is prepared by applying a catalyst ink to the surface of the transfer substrate and drying, and the transfer sheet is bonded to the polymer electrolyte membrane or the gas diffusion sheet by thermocompression bonding or the like. A catalyst layer may be formed on the surface of the polymer electrolyte membrane or a catalyst layer may be formed on the surface of the gas diffusion sheet by a method of peeling the material film.

  The catalyst ink is obtained by dispersing the above catalyst, electrode electrolyte, and the like 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, a die coating method, and the like. Examples of the drying method include reduced pressure drying, heat drying, and reduced pressure 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 and alloys thereof, nickel, nickel-chromium alloy, copper and alloys thereof, silver, 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 material, 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 always necessary, but it can improve the drainage of the gas diffusion layer while maintaining an appropriate amount of liquid water in the catalyst layer and the polymer electrolyte membrane. There is an advantage that the electrical contact between the layers can be improved.
The polymer electrolyte membrane and the gas diffusion sheet on which the catalyst layer is formed by the above-described method are appropriately overlapped and subjected to thermocompression bonding or the like, and bonded together to obtain a membrane / electrode assembly.

  The produced membrane / electrode assembly 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. . The reaction gas flow path described above can be formed by appropriately compression molding or cutting such a separator.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples and comparative examples, but the present invention is not limited only to these examples.

1. Production of catalyst fine particles [Example 1]
1-1. First potential treatment step First, carbon-supported palladium fine particle powder (Cataler, 20% Pd / C) was prepared.
Next, the carbon-supported palladium fine particle powder was immersed in an acid solution, and the potential was reciprocated and swept within the range of 0.40 to 0.45 V (vs. RHE). Specifically, first, sulfuric acid (0.05 mol / L H ₂ SO ₄ ) was prepared as an acid solution, and after adding sulfuric acid to the electrochemical cell, nitrogen bubbling was performed on the sulfuric acid for 60 minutes. Next, 3 g of carbon-supported palladium fine particle powder was put into sulfuric acid to prepare a suspension. Subsequently, the electrochemical cell was swept by reciprocating the potential within a range of 0.40 to 0.45 V for 360 cycles, and the carbon-supported palladium fine particle powder was subjected to a potential treatment for 1 hour.
FIG. 1 is a graph showing a waveform of potential processing in the first cycle when the potential is swept back and forth within a range of 0.40 to 0.45 V (vs. RHE).

1-2. Second Potential Treatment Step First, a platinum ion solution (K ₂ PtCl ₄ aqueous solution) was prepared. The concentration of the platinum ion solution was calculated in advance from the average particle diameter of the carbon-supported palladium fine particle powder based on the amount of platinum necessary to form the platinum outermost layer. In addition, about the platinum ion solution, nitrogen bubbling was performed beforehand.
Next, a platinum ion solution is further added to the electrochemical cell after the first potential treatment step, and a potential of 0.83 V (vs. RHE) is quickly applied to the electrochemical cell, and platinum is applied to the surface of the palladium fine particles. A monoatomic layer was coated. The application of the potential was performed for 1.5 hours. Note that the potential of 0.83 V (vs. RHE) corresponds to the end potential of the open circuit potential measured in advance in the combination of the palladium fine carbon powder on carbon and the aqueous solution of K ₂ PtCl ₄ .

1-3. Cleaning, drying, and pulverization of catalyst fine particles By filtering the reaction solution in the electrochemical cell, catalyst fine particles containing palladium particles supported on carbon and covered with a platinum monoatomic layer were obtained. Thereafter, about 4 L of pure water at room temperature (15 to 30 ° C.) was added in 10 portions to the catalyst fine particles, and filtered and washed each time.
The washed catalyst fine particles were dried at 60 ° C. for 12 hours. The catalyst fine particles after drying were appropriately pulverized using an agate mortar and pestle to produce catalyst fine particles of Example 1.

[Comparative Example 1]
In the first potential treatment process of the first embodiment, the range of the potential to be swept back and forth is from 0.40 to 0.45 V (vs. RHE) to 0.10 to 0.15 V (vs. RHE). The first potential treatment step was performed in the same manner as in Example 1 except that the range was changed.
Thereafter, in the same manner as in Example 1, the second potential treatment step was performed, and the obtained catalyst fine particles were washed, dried, and pulverized to produce catalyst fine particles of Comparative Example 1.

[Comparative Example 2]
First, as in Example 1, the first potential treatment process was performed.
Next, the same platinum ion solution as in Example 1 was prepared. Subsequently, a platinum ion solution was added to the electrochemical cell after the first potential treatment step, and the surface of the palladium fine particles was coated with the platinum outermost layer. In addition, in the platinum outermost layer coating of Comparative Example 2, no potential was particularly applied to the electrochemical cell. In Comparative Example 2, the open circuit potential (OCV) was measured. FIG. 7 is a graph showing the OCV change when the platinum outermost layer is coated on the surface of the palladium fine particles.
Thereafter, the catalyst fine particles were washed, dried, and pulverized in the same manner as in Example 1 to produce catalyst fine particles of Comparative Example 2.

[Comparative Example 3]
First, a carbon-supported palladium fine particle powder (Cataler, 20% Pd / C) was prepared.
Next, a copper monoatomic layer was coated on the palladium fine particles by the Cu-UPD method. Specifically, first, 0.5 g of carbon-supported palladium fine particle powder and 0.2 g of Nafion (trade name) were dispersed in an alcohol aqueous solution, and a catalyst ink obtained by filtration was applied to a glassy carbon electrode. The details of the electrochemical device used are as follows.
Copper ion solution: mixed solution of 0.05 mol / L CuSO ₄ and 0.05 mol / L H ₂ SO ₄ (nitrogen was previously bubbled)
・ Electrode: Electrode with glassy carbon ・ Counter electrode: Platinum electrode (manufactured by Hokuto Denko)
Reference electrode: Silver-silver chloride electrode (manufactured by Cypress)
Dual electrochemical analyzer: ALS700C manufactured by BAS

Subsequently, a 0.005M K ₂ PtCl ₄ solution was prepared, and nitrogen was bubbled into the solution in advance. After the copper monoatomic layer was deposited on the surface of the palladium fine particles by the above method, the electrode provided with the glassy carbon was quickly immersed in the K ₂ PtCl ₄ solution for a predetermined time in a glove box under a nitrogen atmosphere. By the immersion, copper atoms and platinum ions precipitated by Cu-UPD were substituted one-on-one, and carbon-supported catalyst fine particles in which a platinum monoatomic layer was deposited on the surface of palladium fine particles were produced.
It was confirmed by TEM observation that the catalyst fine particles produced by the above method had a platinum monoatomic layer coverage of less than 100% on the surface of the palladium fine particles.
Thereafter, in the same manner as in Example 1, the catalyst fine particles were washed, dried, and pulverized to produce catalyst fine particles of Comparative Example 3. That is, in Comparative Example 3, the first potential processing step and the second potential processing step as in Example 1 were not performed.

[Reference Example 1]
First, as in Comparative Example 3, catalyst fine particles in which the coverage of the platinum monoatomic layer on the surface of the palladium fine particles was less than 100% were produced.
Next, the catalyst fine particles were immersed in an acid solution and swept by reciprocating the potential within a range of 0.40 to 0.45V. The acid solution used, the potential sweeping method, and the like were the same as those in the first potential treatment step in Example 1.
Thereafter, in the same manner as in Example 1, the second potential treatment step, washing of the catalyst fine particles, drying, and pulverization were performed to produce catalyst fine particles of Reference Example 1.

2. 2. Evaluation of catalyst fine particles 2-1. About First Potential Treatment Step Carbon-supported palladium fine particle powder after the first potential treatment step in Example 1 (hereinafter sometimes referred to as Pd / C in Example 1), first potential in Comparative Example 1 Carbon-supported palladium fine particle powder (hereinafter sometimes referred to as Pd / C of Comparative Example 1) after the treatment step, and carbon-supported palladium fine particle powder (hereinafter referred to as Pd / C of Reference Example 2) as the raw material of Example 1 40 cycle potential was swept by a rotating disk electrode (Rotating Disk Electrode: RDE) method. Moreover, the surface area of the palladium fine particle in each Pd / C was measured for every cycle. Details of the measurement method are as follows.
Electrolytic solution: 0.1 M perchloric acid working electrode: glassy carbon Counter electrode: platinum wire Reference electrode: hydrogen electrode Measuring device: Dual electrochemical analyzer (manufactured by BAS, ALS700C)
Bubbling: Nitrogen bubbling Potential: 0.05-0.6V
Scan speed: 50 mV / sec

  FIG. 3 is a graph in which the Q value of each cycle is plotted for each Pd / C of Example 1, Comparative Example 1, and Reference Example 2. FIG. 3 is a graph in which the vertical axis represents the Q value (%) and the horizontal axis represents the number of cycles. In this example, the Q value refers to the surface area of the palladium fine particles. The Q value in FIG. 3 is shown as a relative value with the Q value of Reference Example 2 after 40 cycles as 100%. In FIG. 3, the square plot shows the data of Example 1, the triangular plot shows the data of Comparative Example 1, and the diamond plot shows the data of Reference Example 2. The Q value is also an index that quantitatively shows sites that cannot adsorb hydrogen, such as an oxide film, and the larger the Q value, the smaller the number of sites and the easier it is to coat platinum.

As can be seen from FIG. 3, in Pd / C of Reference Example 2, the Q value in the first cycle is 84%. Therefore, there is an increase in Q value of 16% until it reaches 100% at the 40th cycle. Therefore, it can be seen that in conventional Pd / C, there are many oxides on the surface of the palladium particles, and the Q value is low.
Further, as can be seen from FIG. 3, in Pd / C of Comparative Example 1, the Q value in the first cycle is 87%. This result is not much different from the reference example 2 which is the conventional Pd / C. Therefore, in the first potential treatment step, when the range of potentials swept back and forth is set to a range of 0.10 to 0.15 V (vs. RHE), the Q value is slightly higher than that of the conventional Pd / C. Although improved, it can be seen that there are still many oxides on the surface of the palladium particles and the Q value is low.
On the other hand, as can be seen from FIG. 3, in Pd / C of Example 1, the Q value in the first cycle is 97%. This result is comparable to the Q value of the 15th cycle of Reference Example 2 and the Q value of the 11th cycle of Comparative Example 1. Therefore, in the first potential treatment step, in the production method of the present invention in which the range of the potential swept back and forth is in the range of 0.40 to 0.45 V (vs. RHE), the palladium particle surface is initially applied from the initial stage. It can be seen that there is very little oxide and the Q value is very high.

2-2. Measurement of platinum ratio and surface area of platinum on catalyst fine particle surface For the catalyst fine particles of Example 1 and Comparative Example 2, the ratio of platinum and the platinum surface area were measured, respectively.
The measuring method of the ratio of platinum is as follows. First, the palladium particle size measured by XRD was assumed to be a sphere. Assuming that platinum atoms are on the surface of the sphere, the amount of platinum necessary to cover the palladium particles with one layer (monoatomic layer) of platinum theoretically was calculated. In addition, the following physical property values were used for the calculation.
Radius of palladium particles: Value obtained by dividing palladium particle diameter measured by XRD by 2 Radius of platinum atoms: 135 pm (reference value)
Platinum density: 2,145 kg / m ³
Palladium density: 12,023 kg / m ³
Using the above physical property values, the Pt / Pd ratio in the case where one platinum layer was coated on the palladium particle surface was theoretically calculated.
The amount of platinum and the amount of palladium on the carbon were calculated as measured values (mass%) by ICP measurement. Therefore, the number of platinum layers coated on the surface of the palladium particles was calculated from these measured values and theoretical values.
The ratio of platinum may be calculated by the method described in paragraph [0057] of the specification of Japanese Patent Application No. 2012-111402.

  On the other hand, the platinum surface area was calculated by dividing the activation dominant current (Ik) by the electrochemical surface area (ECSA). Among these, the activation dominant current (Ik) was calculated by the method described in paragraphs [0045] and [0046] of the specification of Japanese Patent Application No. 2012-054624. The electrochemical surface area (ECSA) was calculated by the method described in paragraph [0036] of the specification of Japanese Patent Application No. 2012-104504.

FIG. 4 is a bar graph comparing the platinum ratios of the catalyst fine particles of Example 1 and Comparative Example 2, and the vertical axis represents the ratio when the platinum ratio of the catalyst fine particles of Comparative Example 2 is 1.0. FIG. 5 is a bar graph comparing the platinum surface areas of the catalyst fine particles of Example 1 and Comparative Example 2, and the vertical axis represents the ratio when the platinum surface area of the catalyst fine particles of Comparative Example 2 is 1.0.
As can be seen from FIG. 4, when the platinum ratio of the catalyst fine particles of Comparative Example 2 is 1.0, the platinum ratio of the catalyst fine particles of Example 1 is 1.15. As a result, in the second potential treatment step, when a potential of 0.83 V (vs. RHE) was quickly applied to the electrochemical cell (Example 1), the potential was applied to the electrochemical cell. It shows that the amount of platinum adsorbed increased 1.15 times as compared with the case where it did not (Comparative Example 2).
As can be seen from FIG. 5, when the platinum surface area of the catalyst fine particles of Comparative Example 2 is 1.0, the platinum surface area of the catalyst fine particles of Example 1 is 1.05. As a result, in the second potential treatment step, when a potential of 0.83 V (vs. RHE) was quickly applied to the electrochemical cell (Example 1), the potential was applied to the electrochemical cell. It shows that the platinum surface area increased 1.05 times compared with the case where it did not (Comparative Example 2). When palladium is exposed on the surface, the ratio of the surface area of platinum to the surface area of the entire catalyst fine particle is reduced, so that the platinum surface area is also reduced accordingly. The results in FIG. 5 also show that the palladium exposed area decreases with increasing platinum surface area.

  For the catalyst fine particles of Example 1, MA = 550 A / gPt was obtained. This is a value equivalent to MA of core-shell catalyst fine particles synthesized on a relatively small reaction scale using the conventional Cu-UPD method. Therefore, in the present invention, it has become clear that sufficiently high MA can be obtained even in mass synthesis.

2-3. Examination of repair of catalyst fine particles The catalyst fine particles of Reference Example 1 and Comparative Example 3 were measured for the amount of palladium eluted in sulfuric acid at 80 ° C. The measuring method is as follows.
First, 50 mg of each catalyst fine particle was weighed. Next, the weighed catalyst fine particles were added to 300 mL of 0.1 M sulfuric acid, heated to 80 ° C., and stirred for 1 hour while bubbling oxygen. Thereafter, the solution was recovered, and ICP analysis was performed to measure the palladium elution amount.

  In addition, with respect to the catalyst fine particles of Reference Example 1 and Comparative Example 3, the amounts of platinum and palladium on the surfaces of the catalyst fine particles were quantified by a known method by inductively coupled plasma mass spectrometry (ICP-MS). .

FIG. 6 is a bar graph comparing the palladium elution amount (mass%) in sulfuric acid at 80 ° C. for the catalyst fine particles of Reference Example 1 and Comparative Example 3. Table 1 below shows ICP-MS measurement results for the catalyst fine particles of Reference Example 1 and Comparative Example 3.
As can be seen from FIG. 6, the palladium elution amount in sulfuric acid at 80 ° C. is 10.6% by mass in the catalyst fine particles of Comparative Example 3, whereas it is 4.5% by mass in the catalyst fine particles of Reference Example 1. . From these results, the catalyst fine particles of Reference Example 1 that passed through the first and second potential treatment steps had a palladium elution amount of about 40% compared to the catalyst fine particles of Comparative Example 3 that did not go through these potential treatment steps. It turns out that it can be suppressed.
Further, as can be seen by comparing Reference Example 1 and Comparative Example 3 in Table 1 below, palladium exposed on the surface of the catalyst fine particles of Reference Example 1 is eluted by the first and second potential treatment steps. It can be seen that platinum is covered by the amount of the above.
From the above results, even in the catalyst fine particles where the platinum monoatomic layer coverage with respect to the surface of the palladium fine particles is less than 100%, the difference in ionization tendency can be used to replace the palladium exposed on the surface with platinum ions. As a result, it was proved that the elution amount of palladium can be greatly reduced.

DESCRIPTION OF SYMBOLS 1 Polymer electrolyte membrane 2 Cathode catalyst layer 3 Anode catalyst layer 4, 5 Gas diffusion layer 6 Cathode electrode 7 Anode electrode 8 Membrane / electrode assembly 9, 10 Separator 11, 12 Gas flow path 100 Single cell of fuel cell

Claims (6)

A method for producing catalyst fine particles comprising center particles containing palladium and outermost layers containing platinum and covering the center particles,
A first potential treatment step of applying a potential within a range of 0.4 to 0.6 V (vs. RHE) to the acid solution in which the palladium-containing particles are dispersed; and
At least platinum ions are added to the acid solution in which the palladium-containing particles are dispersed after the first potential treatment step, and a potential in the range of 0.80 to 1.0 V (vs. RHE) is applied. A second potential treatment step of covering the palladium-containing particles with the outermost layer containing platinum;
A method for producing catalyst fine particles, comprising:   The method for producing catalyst fine particles according to claim 1, wherein the first potential treatment step and the second potential treatment step are alternately repeated twice or more.   The potential treatment in the first potential treatment step is a potential treatment that sweeps between any two potentials within a range of 0.4 to 0.6 V (vs. RHE). A method for producing catalyst fine particles.   The potential treatment in the second potential treatment step is a potential treatment for applying an end potential of an open circuit potential measured in advance within a range of 0.80 to 1.0 V (vs. RHE). 4. The method for producing catalyst fine particles according to any one of 3 above.   The method for producing catalyst fine particles according to any one of claims 1 to 4, wherein the palladium-containing particles are supported on a carrier. A fuel cell comprising a single cell comprising a membrane / electrode assembly comprising an anode electrode comprising at least an anode catalyst layer on one side of a polymer electrolyte membrane and comprising a cathode electrode comprising at least a cathode catalyst layer on the other side,
6. A fuel cell comprising catalyst fine particles produced by the production method according to claim 1 in at least one of the anode catalyst layer and the cathode catalyst layer.

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