JP2016126907A - Method for manufacturing core shell catalyst for fuel cell, and apparatus for manufacturing core shell catalyst for fuel cell - Google Patents

Abstract

PROBLEM TO BE SOLVED: To manufacture a core shell catalyst for a fuel cell, which is high in catalyst activity.SOLUTION: A method for manufacturing a core shell catalyst for a fuel cell comprises the steps of: preparing a device which has a reaction chamber holding therein a fluid dispersion containing core metal particles and having an action electrode, a counter electrode, a reference electrode, and a probe electrode for monitoring a potential of the core metal particles, and a potential control part; applying a potential to the action electrode while stirring the fluid dispersion containing the core metal particles until a potential of the probe electrode relative to that of the reference electrode becomes equal to or lower than a predetermined potential; supplying an electrolytic solution containing copper ions to the reaction chamber; applying a potential higher than a copper oxidation reduction potential to the core metal particles by applying to the action electrode in the electrolytic solution containing copper ions, thereby covering the surface of each core metal particle with copper; and bringing the core metal particles into contact with a liquid solution containing platinum ions, thereby substituting copper on the surface of each core metal particle with platinum to form a shell.SELECTED DRAWING: Figure 6

Description

  The present invention relates to a method for producing a fuel cell core-shell catalyst and a fuel cell core-shell catalyst production apparatus.

As an electrode catalyst for a fuel cell, a core-shell catalyst intended to reduce the amount of noble metal used such as platinum is known.
For example, Patent Document 1 describes a method for producing a core-shell catalyst by displacement plating using copper underpotential deposition (Cu-UPD).

JP 2013-215701 A

The production of a core-shell catalyst for a fuel cell is intended to remove impurities such as oxides on the surface of the core metal particles from the core metal particles before the Cu-UPD treatment in order to obtain high catalytic activity. A cleaning process is required.
Conventionally, before the Cu-UPD process, the cleaning process uses a potential control device such as a potentiostat to apply a potential to the working electrode disposed in the reaction vessel, so that the core metal particles are dispersed in the core metal particle-containing dispersion. This was done by applying a potential cycle.
However, the potential applied to the working electrode is different from the potential actually applied to the core metal particles floating in the core metal particle-containing dispersion, and is actually floating in the core metal particle-containing dispersion. It is unclear what potential is applied to the core metal particles. In addition, since the potential is not applied unless the core metal particle is in contact with the working electrode, and the efficiency of applying the potential from the working electrode to the core metal particle is determined by the number of collisions of the core metal particle with the working electrode, the structure of the working electrode And the reaction environment (temperature, pH, stirring conditions, etc.).
Therefore, in the conventional method, it cannot be determined whether or not a target potential is applied to the core metal particles, and if the potential application to the core metal particles is insufficient, the cleaning process of the core metal particles is not performed. As a result, there is a problem that a highly active core-shell catalyst for a fuel cell cannot be obtained. In addition, when the potential is sufficiently applied to the core metal particles, the potential is applied to the core metal particles over an excessive amount of time, which is inefficient.
The present invention has been accomplished in view of the above circumstances, and the object of the present invention is to efficiently and sufficiently remove impurities such as oxides on the surface of the core metal particles by applying a potential to the core metal particles. It is an object to provide a method for producing a fuel cell core-shell catalyst having high catalytic activity and a fuel cell core-shell catalyst production apparatus.

A method for producing a core-shell catalyst for a fuel cell according to the present invention is a method for producing a core-shell catalyst for a fuel cell comprising a core containing a core metal and a shell containing platinum and covering at least a part of the core,
The core metal particle-containing dispersion containing the core metal particles is contained, and the working electrode, the counter electrode, the reference electrode, and the potential of the core metal particles floating in the core metal particle-containing dispersion are monitored. A preparation step of preparing an apparatus having a reaction vessel including a probe electrode for controlling the potential of the working electrode, the counter electrode, and the reference electrode;
A potential application step of applying a potential to the working electrode until the potential of the probe electrode with respect to the reference electrode is equal to or lower than a predetermined potential while stirring the core metal particle-containing dispersion;
After the potential application step, a copper ion-containing electrolytic solution is supplied to the reaction vessel, and in the copper ion-containing electrolytic solution, a potential nobler than a copper oxidation-reduction potential is applied to the core electrode in the working electrode. A copper coating step of coating at least a part of the surface of the core metal particles with copper;
After the copper coating step, a platinum ion-containing solution is brought into contact with the core metal particles to replace copper covering at least a part of the surface of the core metal particles with platinum, thereby forming the shell. And a process.

In the method for producing a fuel cell core-shell catalyst according to the present invention, the core metal particles are preferably supported on a carrier.
In the method for producing a core-shell catalyst for a fuel cell according to the present invention, it is preferable that the reaction vessel also functions as the working electrode.
In the method for producing a fuel cell core-shell catalyst according to the present invention, in the potential application step, the potential of the probe electrode with respect to the reference electrode is not more than a predetermined potential while stirring the core metal particle-containing dispersion, and the action It is preferable to apply a potential to the working electrode until a current value flowing between the electrode and the counter electrode reaches a steady state.
In the method for producing a core-shell catalyst for a fuel cell of the present invention, the core metal is palladium,
In the potential application step, the working electrode is stirred until the potential of the probe electrode with respect to the reference electrode is within a range of 0.60 to 0.15 V (vs. RHE) while stirring the dispersion containing the core metal particles. It is preferable to apply a potential.

An apparatus for producing a core-shell catalyst for a fuel cell according to the present invention produces a core-shell catalyst for a fuel cell comprising a core containing a core metal and a shell containing platinum and covering at least a part of the core by an underpotential deposition method. A device that performs
The core metal particle-containing dispersion containing the core metal particles is contained, and the working electrode, the counter electrode, the reference electrode, and the potential of the core metal particles floating in the core metal particle-containing dispersion are monitored. A reaction vessel equipped with a probe electrode for
It has a potential control unit for controlling the potential of the working electrode, the counter electrode, and the reference electrode.

  In the fuel cell core-shell catalyst manufacturing apparatus of the present invention, it is preferable that the reaction vessel also functions as the working electrode.

  According to the present invention, it is possible to monitor in real time whether or not the core metal particles are at a target potential. In addition, according to the present invention, by performing potential control while monitoring the potential of the core metal particles, it is possible to efficiently and sufficiently remove impurities such as oxides on the surface of the core metal particles by applying a potential to the core metal particles. As a result, a core-shell catalyst for fuel cells with high catalytic activity can be produced.

It is a flowchart which shows an example of the manufacturing method of the core-shell catalyst for fuel cells of this invention. It is the perspective schematic diagram which showed an example of the core-shell catalyst manufacturing apparatus for fuel cells of this invention. It is the perspective schematic diagram which showed an example of the core-shell catalyst manufacturing apparatus for fuel cells of this invention. It is the figure which piled up and showed the cyclic voltammogram obtained by reference experiment example 1 and 2. FIG. FIG. 3 is a diagram showing the potential behavior of Pd / C measured by the probe electrode and the current behavior between the working electrode and the counter electrode measured by the potentiostat from the time of starting the potential application to the time of 2700 seconds in the potential application process of Example 1. is there. 2 is a bar graph comparing the specific activities of the core-shell catalysts for fuel cells of Example 1 and Comparative Example 1. FIG.

1. Manufacturing method of core shell catalyst for fuel cell The manufacturing method of a core shell catalyst for a fuel cell according to the present invention comprises: a core containing a core metal; and a shell containing platinum and covering at least a part of the core. A method for producing a catalyst, comprising:
The core metal particle-containing dispersion containing the core metal particles is contained, and the working electrode, the counter electrode, the reference electrode, and the potential of the core metal particles floating in the core metal particle-containing dispersion are monitored. A preparation step of preparing an apparatus having a reaction vessel including a probe electrode for controlling the potential of the working electrode, the counter electrode, and the reference electrode;
A potential application step of applying a potential to the working electrode until the potential of the probe electrode with respect to the reference electrode is equal to or lower than a predetermined potential while stirring the core metal particle-containing dispersion;
After the potential application step, a copper ion-containing electrolytic solution is supplied to the reaction vessel, and in the copper ion-containing electrolytic solution, a potential nobler than a copper oxidation-reduction potential is applied to the core electrode in the working electrode. A copper coating step of coating at least a part of the surface of the core metal particles with copper;
After the copper coating step, a platinum ion-containing solution is brought into contact with the core metal particles to replace copper covering at least a part of the surface of the core metal particles with platinum, thereby forming the shell. And a process.

In the preparation step in the production method of the present invention, an electrode (probe electrode) independent of an electrode (working electrode) to which a potential is applied is placed in the reaction vessel, and the potential application step is performed, whereby the core metal particle-containing dispersion liquid The potential of the core metal particle can be monitored when the core metal particle floating inside collides with the probe electrode.
In addition, by stirring the core metal particle-containing dispersion, countless core metal particles floating in the core metal particle-containing dispersion collide with the probe electrode. Therefore, by providing a probe electrode, the average potential of the core metal particles floating in the core metal particle-containing dispersion can be monitored in real time.
Therefore, according to the present invention, it is possible to monitor in real time whether or not the core metal particles are at the target potential. The monitoring can be performed without depending on the structure of the working electrode and the reaction environment (temperature, pH, stirring conditions, etc.). Furthermore, by controlling the potential while monitoring the potential of the core metal particles, it is possible to efficiently and sufficiently remove impurities such as oxides on the surface of the core metal particles by applying a potential to the core metal particles, As a result, a fuel cell core-shell catalyst having high catalytic activity can be produced.

  In the present invention, the shell covers the core as well as a form in which the entire surface of the core is covered by the shell, but at least a part of the surface of the core is covered by the shell, and a part of the surface of the core is exposed. Are also included. Furthermore, the shell may be a monoatomic layer or a polyatomic layer in which two or more atoms are stacked, but from the viewpoint of improving catalytic activity, the shell is preferably a monoatomic layer.

Hereafter, the manufacturing method of the core-shell catalyst for fuel cells of this invention is demonstrated in detail.
FIG. 1 is a flowchart showing an example of a method for producing a fuel cell core-shell catalyst of the present invention.
The method for producing a core-shell catalyst for a fuel cell according to the present invention includes (1) a preparation step, (2) a potential application step, (3) a copper coating step, and (4) a replacement step. (5) A cleaning step, (6) a drying step, and the like are included.
Hereinafter, each process is demonstrated in order.

(1) Preparatory step The preparatory step accommodates a core metal particle-containing dispersion containing core metal particles, and floats in the working electrode, the counter electrode, the reference electrode, and the core metal particle-containing dispersion. Preparing a device having a reaction vessel including a probe electrode for monitoring the potential of the core metal particles, and a potential control unit for controlling the potential of the working electrode, the counter electrode, and the reference electrode.

Examples of the solvent used in the core metal particle-containing dispersion include water and organic solvents, and water is preferable. In addition to the above solvent, the core metal particle-containing dispersion may contain, for example, an acid or the like. Specific examples of the acid that can be added to the core metal particle dispersion include nitric acid, sulfuric acid, perchloric acid, hydrochloric acid, hypochlorous acid, and the like, and sulfuric acid is preferred.
For example, when sulfuric acid is used as the acid, the concentration of the acid is preferably 0.001 mol / L or more, and particularly preferably 0.001 to 1.0 mol / L.
The method for dispersing the core metal particles is not particularly limited, and examples thereof include a method using an ultrasonic homogenizer, a magnetic stirrer, a motor with stirring blades, and the like.

The core metal is preferably palladium or a palladium alloy, and more preferably palladium.
In the case of a palladium alloy, an alloy with a metal material selected from the group consisting of tungsten, iridium, ruthenium, rhodium, iron, cobalt, nickel, silver, and gold can be used. The metal other than palladium constituting the palladium alloy is 1 Two or more species may be used.
When using a palladium alloy, it is preferable that the content rate of palladium is 30 mass% or more when the mass of the whole alloy is 100 mass%. This is because a uniform platinum-containing shell can be formed when the palladium content is 30% by mass or more.

The average particle diameter of the core metal particles is not particularly limited, but is preferably 10 nm or less. When the average particle diameter of the core metal particles exceeds 10 nm, the surface area per mass of platinum becomes small, and a lot of platinum is required to obtain the required activity, which is expensive. If the average particle diameter of the core metal particles is too small, the durability of the core-shell catalyst for fuel cells is lowered. Therefore, the average particle diameter of the core metal particles is preferably 3 nm or more.
The calculation method of the average particle diameter of the particles used in the present invention is as follows. That is, using a scanning electron microscope (TEM), take a TEM photograph at a magnification of 1,000,000, and calculate the diameter of a perfect circle having the same area as the projected area of the particle on the plane (equivalent particle diameter) of the particle. Considered as particle size. The measurement of the particle size by photographic observation is performed for 500 particles of the same type, and the average of the particle sizes of these particles is defined as the average particle size. Note that the broken particles observed at the edge of the photograph are excluded from the analysis.

The core metal particles are preferably supported on a carrier. Although it does not specifically limit as a support | carrier, When using the core-shell catalyst for fuel cells of this invention for the electrode catalyst layer of a fuel cell, it is preferable to use an electroconductive support | carrier from a viewpoint of ensuring electroconductivity to an electrode catalyst layer.
Specific examples of materials that can be used as the carrier for supporting the core metal particles include Ketjen Black (trade name: manufactured by Ketjen Black International), Vulcan (trade name: manufactured by Cabot), and Nolit (trade name: Norit). ), Black pearl (trade name: manufactured by Cabot), acetylene black (trade name: manufactured by Chevron), etc., conductive carbon materials such as carbon particles and carbon fibers, metal materials such as metal particles and metal fibers, perylene red And non-conductive materials such as organic pigments.

  The average particle size of the carrier is not particularly limited, but is preferably 0.01 to several hundred μm, more preferably 0.01 to 1 μm. If the average particle size of the support is less than the above range, the support may be deteriorated by corrosion, and the core metal particles supported on the support may fall off over time. Moreover, when the average particle diameter of a support | carrier exceeds the said range, a specific surface area is small and there exists a possibility that the dispersibility of a core metal particle may fall.

The specific surface area of the support is not particularly limited, preferably 50~2000m ² / g, more preferably 100~1600m ² / g. If the specific surface area of the support is less than the above range, the dispersibility of the core metal particles in the support may be reduced, and sufficient battery performance may not be exhibited. Moreover, when the specific surface area of a support | carrier exceeds the said range, there exists a possibility that the effective utilization rate of a core metal particle may fall and sufficient battery performance may not be expressed.

The supporting rate of the core metal particles by the carrier [{(core metal particle mass) / (core metal particle mass + carrier mass)} × 100%] is not particularly limited, and is generally in the range of 20 to 60%. Is preferred. If the amount of the core metal particles supported is too small, the catalytic function may not be sufficiently exhibited. On the other hand, if the amount of the core metal particles supported is too large, there may be no particular problem from the viewpoint of the catalyst function, but even if more core metal particles are supported than the necessary amount, there is an effect commensurate with the increase in manufacturing cost. It becomes difficult to obtain.
As a method for supporting the core metal particles on the carrier, a conventionally used method can be employed. For example, a method in which core metal particles are mixed in a carrier dispersion in which a carrier is dispersed, filtered, washed, redispersed in ethanol or the like, and then dried with a vacuum pump or the like. After drying, heat treatment may be performed as necessary.

The probe electrode is not particularly limited as long as it can ensure conductivity. However, it is preferable that the oxidation-reduction reaction hardly occurs and the probe electrode is not easily deformed in an acidic atmosphere, and carbon is particularly preferable. The arrangement position of the probe electrode in the reaction vessel is not particularly limited as long as the probe electrode is not in contact with the working electrode and is in contact with the floating core metal particles.
As the working electrode (WE), for example, a material that can ensure conductivity, such as a metal material such as titanium, platinum mesh, a platinum plate, or a gold plate, or a conductive carbon material such as glassy carbon or a carbon plate, can be used. Further, from the viewpoint of increasing the surface area of the working electrode involved in the reaction, it is preferable to form the reaction vessel with the conductive material and to function as the working electrode. When using a reaction vessel made of a metal material as a working electrode, it is preferable to coat the inner wall of the reaction vessel with RuO ₂ from the viewpoint of suppressing corrosion. When a carbon material reaction vessel is used as a working electrode, it can be used as it is without coating.
As the counter electrode (CE), for example, a platinum mesh plated with platinum black and conductive carbon fiber can be used.
As the reference electrode (RE), a reversible hydrogen electrode (RHE), a silver-silver chloride electrode, a silver-silver chloride-potassium chloride electrode, or the like can be used. When converting the measured value of the silver-silver chloride electrode to the reversible hydrogen electrode, the potential difference between the RHE and the silver-silver chloride electrode is measured in advance and corrected later.
As the potential controller, a potentiostat, a potentiogalvanostat, or the like can be used.

(2) Potential application step In the potential application step, the potential is applied to the working electrode until the potential of the probe electrode with respect to the reference electrode becomes equal to or lower than a predetermined potential while stirring the core metal particle-containing dispersion. It is.
By the potential application step, the core metal oxide, which is an impurity, can be removed from the surface of the core metal particles, and the platinum-containing shell can be uniformly coated on the core metal particles.

In the present invention, the “predetermined potential” means a potential at which the core metal oxide is reduced to the core metal. The potential at which the core metal oxide is reduced to the core metal can be appropriately set from a reduction wave of a cyclic voltammogram obtained by cyclic voltammetry (CV) measurement of the core metal.
When the core metal is palladium, the “predetermined potential” is preferably 0.60 V (vs. RHE) or less, particularly preferably 0.53 V (vs. RHE) or less (see Reference Experimental Example 1).
When the core metal is palladium, the potential of the probe electrode with respect to the reference electrode is not more than the “predetermined potential” and not less than 0.15 V (vs. RHE), particularly not less than 0.37 V (vs. RHE). It is preferable to apply a potential to the working electrode so as to be within the range. If it is 0.15 V (vs. RHE) or more, it is possible to suppress the occlusion of palladium by hydrogen, so that a uniform platinum-containing shell can be formed on the surface of palladium particles in the substitution step described later. Moreover, if it is 0.37V (vs.RHE) or more, in the copper coating process after an electric potential application process, the large amount precipitation of copper on the palladium particle surface can be suppressed (refer to Reference Experimental Example 2).

In the potential application step, the potential of the probe electrode with respect to the reference electrode is equal to or lower than a predetermined potential while stirring the core metal particle-containing dispersion from the viewpoint of sufficiently removing impurities such as the core metal oxide from the surface of the core metal particles. In addition, it is preferable to apply a potential to the working electrode until the value of the current flowing between the working electrode and the counter electrode reaches a steady state.
In the present invention, the “steady state” is a state in which the value of the current flowing between the working electrode and the counter electrode that is observed over time by the potential control unit is within a range of ± 10% amplitude. Means.

A method of monitoring the potential of the probe electrode with respect to the reference electrode is not particularly limited, and examples thereof include a method of monitoring by providing an electrometer separately from the potential control unit.
The method for stirring the core metal particle-containing dispersion is not particularly limited, and examples thereof include a method using an ultrasonic homogenizer, a magnetic stirrer, a motor with stirring blades, and the like.
The stirring speed is not particularly limited, but is preferably 200 to 1000 rpm when a magnetic stirrer is used.

In this step, by recombining the atomic arrangement on the surface of the core metal particle, the crystal plane with high catalytic activity is exposed, and the surface activity of the core metal particle is improved, so that the potential is reciprocated several times in a certain potential range. It is preferable. When reciprocating the potential multiple times, at least, the potential of the probe electrode with respect to the reference electrode at the end of the potential application should be equal to or lower than the predetermined potential, and when repeating the application of the low potential and the application of the high potential alternately, The application of the low potential is preferably performed every cycle until the potential of the probe electrode with respect to the reference electrode becomes a predetermined potential or less. When the low potential application and the high potential application are alternately repeated, if the last potential application is a low potential application, the first potential application may be a high potential application even if it is a low potential application. There may be.
The potential applying signal pattern includes a rectangular wave, a triangular wave, a trapezoidal wave, and the like. Since it is meaningful to repeat the low potential and the high potential, the potential application signal pattern is not particularly limited.

The number of cycles of the potential can be appropriately set depending on the type of the core metal, the scale of the apparatus, and the like. When the core metal is palladium and the potential applying signal pattern is a rectangular wave, for example, 0.05 to It is preferable to perform 10 to 100 cycles, with 800 to 2400 seconds hold at 0.30 V (vs. RHE) and 800 to 2400 seconds hold at 0.90 V to 1.20 V (vs. RHE) as one cycle.
The number of potential cycles when the potential applying signal pattern is a triangular wave is not particularly limited, and is preferably 10 to 100 cycles. The potential sweep rate is, for example, 0.10 to 5.0 mV / sec. be able to.

(3) Copper coating step In the copper coating step, after the potential application step, a copper ion-containing electrolytic solution is supplied to the reaction vessel, and in the copper ion-containing electrolytic solution, a copper redox potential is applied to the core metal particles. In this step, at least a part of the surface of the core metal particle is coated with copper by applying a noble potential to the working electrode.

The copper ion-containing electrolytic solution is not particularly limited as long as it is an electrolytic solution that can coat the surface of the core metal particles with Cu-UPD. The copper ion-containing electrolytic solution is usually composed of a predetermined amount of copper salt dissolved in a solvent, but is not particularly limited to this configuration, and some or all of the copper ions are dissociated and exist in the liquid. Any electrolyte solution may be used.
Examples of the solvent used for the copper ion-containing electrolyte include water and organic solvents, but water is preferable from the viewpoint of not preventing the precipitation of copper on the surface of the core metal particles.
Specific examples of the copper salt used in the copper ion-containing electrolyte include copper sulfate, copper nitrate, copper chloride, copper chlorite, copper perchlorate, and copper oxalate.
In the electrolytic solution, the copper ion concentration is not particularly limited, but is preferably 0.01 to 1.0 mol / L.
In addition to the solvent and copper salt, the copper ion-containing electrolytic solution may contain, for example, an acid or the like. Specific examples of the acid that can be added to the electrolytic solution containing copper ions include sulfuric acid, nitric acid, hydrochloric acid, chlorous acid, perchloric acid, and oxalic acid. Note that the counter anion in the copper ion-containing electrolyte and the counter anion of the acid contained in the core metal particle-containing dispersion may be the same or different.
In addition, it is preferable that an inert gas is bubbled in advance in the electrolytic solution. This is because the oxidation of the core metal particles is suppressed and uniform coating with the platinum-containing shell becomes possible. Nitrogen gas, argon gas, etc. can be used as the inert gas.

The method for applying a potential to the core metal particles is not particularly limited, and a general method can be adopted. For example, a method in which a working electrode, a counter electrode, and a reference electrode are immersed in a copper ion-containing electrolytic solution, and a potential is applied to the working electrode.
The potential to be applied is not particularly limited as long as it is a potential at which copper can be deposited on the surface of the core metal particles, that is, a potential nobler than the oxidation-reduction potential of copper. It is preferably within the range of (vs. RHE), particularly preferably 0.38 V (vs. RHE).
The time for applying the potential is not particularly limited, but is preferably 60 minutes or more, and more preferably until the reaction current becomes steady and approaches zero.

The copper coating step is preferably performed in an inert gas atmosphere such as a nitrogen atmosphere from the viewpoint of preventing oxidation of the surface of the core metal particles and preventing oxidation of copper.
In the copper coating step, it is preferable that the copper ion-containing electrolyte is appropriately stirred as necessary. For example, when a core metal particle is immersed and dispersed in a copper ion-containing electrolytic solution in the reaction vessel using a reaction vessel that also serves as a working electrode, each core metal particle acts by stirring the copper ion-containing electrolytic solution. The electrode can be brought into contact with the surface of the reaction vessel, which is an electrode, and a potential can be applied uniformly to each core metal particle. In this case, stirring may be performed continuously or intermittently during the copper coating step.

(4) Substitution step In the substitution step, after the copper coating step, the core metal particles are brought into contact with the platinum ion-containing solution, whereby the copper covering at least a part of the surface of the core metal particles is converted into platinum. It is a step of replacing and forming the shell.
In the substitution step, copper and platinum can be substituted due to the difference in ionization tendency by bringing the platinum ion-containing solution into contact with copper deposited on the surface of the core metal particles.

The shell in the present invention includes platinum and a platinum alloy.
Examples of the platinum alloy include an alloy with a metal material selected from the group consisting of iridium, ruthenium, rhodium, nickel and gold. The metal other than platinum constituting the platinum alloy may be one kind or two or more kinds.
The platinum alloy preferably has a platinum content of 90% by mass or more when the mass of the entire alloy is 100% by mass. This is because if the platinum content is less than 90% by mass, sufficient catalytic activity and durability cannot be obtained.

The platinum ion-containing solution is not particularly limited as long as it contains at least platinum ions, and preferably contains a reaction inhibitor. In the platinum ion-containing solution, platinum may exist as ions or may exist as a platinum compound such as a platinum complex.
The solvent that can be used for the platinum ion-containing solution can be the same as the solvent used for the copper ion-containing electrolytic solution described above. Further, the platinum ion-containing solution may contain, for example, an acid or the like in addition to the solvent and the platinum salt. As an acid, it can be made to be the same as that of the acid used for the copper ion containing electrolyte solution mentioned above.
As the platinum salt used in the platinum ion-containing solution, for example, K ₂ PtCl ₄ , K ₂ PtCl ₆ or the like can be used, and an ammonia complex such as ([PtCl ₄ ] [Pt (NH ₃ ) ₄ ]) can be used. It can also be used.
The platinum ion concentration in the platinum ion-containing solution is not particularly limited, but is preferably 0.0005 to 0.1 mol / L.
The reaction inhibitor is not particularly limited as long as it can suppress the substitution reaction between copper and platinum. Examples of the reaction inhibitor include platinum, copper deposited on the surface of the core metal particle, and a complex forming agent that forms a complex in the solution with the core metal exposed on the surface of the core metal particle.
Examples of the complexing agent include citric acid, sodium salt of citric acid, potassium salt of citric acid, ethylenediaminetetraacetic acid (hereinafter sometimes referred to as EDTA), sodium salt of EDTA, potassium salt of EDTA, and the like. Citric acid is preferred. Only one kind of the complex forming agent may be used, or two or more kinds may be mixed and used. Since these complex-forming agents form platinum and complexes with copper in solution, they suppress the substitution reaction between copper and platinum, and as a result, uniformly coat the platinum-containing shell on the surface of the core metal particles. Can do.
The concentration of the reaction inhibitor in the platinum ion-containing solution is not particularly limited, but is preferably 1 to 10 times the platinum ion concentration.
The platinum ion-containing solution is sufficiently stirred in advance, and nitrogen is preferably bubbled into the solution in advance from the viewpoint of preventing oxidation of the surface of the core metal particles and preventing oxidation of copper.
Although the temperature of a platinum ion containing solution is not specifically limited, From a viewpoint of the catalytic activity improvement of the core-shell catalyst for fuel cells, it is preferable that it is -3-10 degreeC.

In the substitution step, the temperature in the reaction system is not particularly limited, but is preferably maintained at −3 ° C. or more and 10 ° C. or less. If the temperature is lower than −3 ° C., the solution may freeze and the reaction may not proceed. If the temperature exceeds 10 ° C., sufficient platinum mass activity may not be obtained.
The method for maintaining the temperature in the reaction system is not particularly limited, and examples thereof include a method using a circulating cooling device (chiller), a cooling pipe, and the like.
The replacement time (contact time between the platinum ion-containing solution and the core metal particles) is not particularly limited, but it is preferable to ensure 10 minutes or more. As the platinum ion-containing solution is added, the potential of the reaction solution increases. Therefore, it is more preferable to substitute until the monitor potential does not change.
In addition, when performing a copper coating process and a substitution process within the same reaction container, you may add a platinum ion containing solution to the copper ion containing electrolyte solution used for the copper coating process. For example, after the copper coating step, the potential application is stopped, and the copper ion-containing core metal particles are brought into contact with the platinum ion-containing solution by adding the platinum ion-containing solution to the copper ion-containing electrolyte used in the copper coating step. You may let them.

(5) Washing step The washing step is a step of washing the fuel cell core-shell catalyst after the replacement step.
The cleaning of the fuel cell core-shell catalyst is not particularly limited as long as it is a method capable of removing impurities without impairing the core-shell structure of the produced fuel cell core-shell catalyst. As an example of the cleaning, a method of suction filtration using water, perchloric acid, dilute sulfuric acid, dilute nitric acid or the like can be mentioned.

(6) Drying process A drying process is a process of drying the core-shell catalyst for fuel cells after a substitution process.
The drying of the fuel cell core-shell catalyst is not particularly limited as long as it can remove the solvent and the like, and examples thereof include a method of maintaining a temperature of 50 to 100 ° C. for 6 to 12 hours in an inert gas atmosphere.
You may grind | pulverize the core-shell catalyst for fuel cells as needed. The pulverization method is not particularly limited as long as it is a method capable of pulverizing solids. Examples of the pulverization include pulverization using an inert gas atmosphere or mortar in the atmosphere, and mechanical milling such as a ball mill, a turbo mill, and a jet mill.

2. Fuel cell core-shell catalyst production apparatus The fuel cell core-shell catalyst production apparatus of the present invention includes a core containing a core metal and a shell covering platinum and covering at least a part of the core by an underpotential deposition method. An apparatus for producing a core-shell catalyst for a fuel cell,
The core metal particle-containing dispersion containing the core metal particles is contained, and the working electrode, the counter electrode, the reference electrode, and the potential of the core metal particles floating in the core metal particle-containing dispersion are monitored. A reaction vessel equipped with a probe electrode for
It has a potential control unit for controlling the potential of the working electrode, the counter electrode, and the reference electrode.
According to the present invention, by disposing an electrode (probe electrode) independent of an electrode (working electrode) to which an electric potential is applied in the reaction vessel, the core metal particles floating in the core metal particle-containing dispersion are obtained. It is possible to monitor the potential of the core metal particle when it collides with the probe electrode.
Therefore, according to the present invention, it is possible to monitor in real time whether or not the core metal particles are at the target potential. The monitoring can be performed without depending on the structure of the working electrode and the reaction environment (temperature, pH, stirring conditions, etc.).

FIG. 2 is a schematic perspective view showing an example of a fuel cell core-shell catalyst manufacturing apparatus according to the present invention. The fuel cell core-shell catalyst manufacturing apparatus is the same as the apparatus prepared in the (1) preparation step of “1. Manufacturing method of fuel cell core-shell catalyst”. The contents described in the (1) preparation step of “Method for producing core-shell catalyst for battery” are omitted.
In an apparatus 100 shown in FIG. 2, a core metal particle-containing dispersion liquid 22 containing core metal particles 23 and a stirrer 24 are added in a reaction vessel 21. In the reaction vessel 21, the working electrode 25, the counter electrode 26, the reference electrode 27, and the probe electrode 28 are disposed so as to be sufficiently immersed in the core metal particle-containing dispersion liquid 22, and the working electrode 25, the counter electrode 26, and the reference electrode 27 are arranged. Are electrically connected to a potential controller (not shown).
In order to prevent the core metal particles 23 in the core metal particle-containing dispersion liquid 22 from coming into contact with the counter electrode 26, the counter electrode 26 is placed in the core metal particle-containing dispersion liquid 22 while being accommodated in the glass counter electrode compartment 29. Soaked. The counter electrode compartment 29 is formed of a porous glass frit at the bottom, and the contact property between the counter electrode 26 and the core metal particle-containing dispersion liquid 22 is ensured.

As another embodiment of the fuel cell core-shell catalyst production apparatus of the present invention, there is the one shown in FIG. In FIG. 3, the same components as those in FIG. 2 are denoted by the same reference numerals, and description thereof is omitted.
In an apparatus 200 shown in FIG. 3, a core metal particle-containing dispersion liquid 22 containing core metal particles 23 and a stirrer 24 are added to a reaction vessel 21 that also functions as a working electrode 25.
In the reaction vessel 21, the counter electrode 26, the reference electrode 27, and the probe electrode 28 are disposed so as to be sufficiently immersed in the core metal particle-containing dispersion liquid 22.

(Reference Experiment Example 1)
[CV measurement of Pd / C]
0.5 g of palladium-supported carbon (hereinafter sometimes referred to as Pd / C) in which commercially available palladium particles having an average particle size of 3.0 nm are supported on carbon particles (Pd / C may be referred to as 30% by mass) is made of graphite. Was added to an oxygen-saturated 0.1 mol / L perchloric acid aqueous solution.
A graphite reaction vessel was used as a working electrode, and a counter electrode (platinum mesh) and a reference electrode (silver-silver chloride) were placed in the reaction vessel so that each was immersed in a perchloric acid aqueous solution. The working electrode, reference electrode, and counter electrode were connected to a potentiostat (HZ-5000, manufactured by Hokuto Denko Corporation), the working electrode potential was 0.05 V to 1.20 V (vs. RHE), and the potential sweep rate was 5 mV / s was subjected to a potential cycle once to perform CV measurement. In addition, the electric potential of the silver-silver chloride electrode was described in terms of RHE. The obtained cyclic voltammogram is shown in FIG.

(Reference Experiment Example 2)
[CV measurement of Pd / C during Cu-UPD]
0.5 g of Pd / C was prepared, put into a graphite reaction vessel, and 14.6 g of copper sulfate pentahydrate dissolved in 66 mL of 0.05 mol / L sulfuric acid aqueous solution was added.
A graphite reaction vessel was used as a working electrode, and a counter electrode (platinum mesh) and a reference electrode (silver-silver chloride) were placed in the reaction vessel so that each was immersed in an aqueous sulfuric acid solution. The working electrode, reference electrode, and counter electrode are connected to a potentiostat, the working electrode potential is set to 0.35 V to 0.75 V (vs. RHE), the potential sweep rate is set to 5 mV / s, and the potential is cycled once. Measurements were made. The obtained cyclic voltammogram is shown in FIG.

FIG. 4 is a diagram in which the cyclic voltammogram of Pd / C obtained in Reference Experimental Example 1 and the cyclic voltammogram of Pd / C obtained in Cu-UPD obtained in Reference Experimental Example 2 are overlapped.
A cyclic voltammogram is a current-potential curve that appears when the potential is scanned, with the vertical axis representing current and the horizontal axis representing potential. The positive current is usually defined as the oxidation current and the negative current as the reduction current. Is done. Therefore, the reduction wave of the cyclic voltammogram described later refers to a negative current wave.
As shown in FIG. 4, in the reduction wave of the cyclic voltammogram of Pd / C obtained in Reference Experimental Example 1, when the potential is 0.6 V (vs. RHE) or less, the maximum value (− 2.5 mA / cm ²⁾ of less than half (-1.0mA / cm ²⁾ weakens reduction wave peak intensity up to, it can be seen that a reduction wave peak becomes less 0.53V (vs.RHE) is almost gone . The disappearance of the reduction wave peak indicates that the palladium oxide is sufficiently reduced to palladium (PdO + 2H ⁺ + 2e ⁻ → Pd + H ₂ O).
Therefore, in the present invention, when the core metal is palladium, the “predetermined potential or less” is preferably at least 0.6 V (vs. RHE) or less, particularly 0.53 V (vs. RHE) or less. It turns out that it is preferable.
Further, in the reduction wave of the cyclic voltammogram of Pd / C obtained at the time of Cu-UPD obtained in Reference Experimental Example 2, when the potential becomes less than 0.37 V (vs. RHE), the reduction wave peak intensity increases. I understand. This indicates that copper is excessively deposited on the surface of the palladium particles.

Example 1
[Preparation process]
First, 50 g of Pd / C was prepared, put into a graphite reaction vessel, and 10 L of 0.05 mol / L sulfuric acid aqueous solution was added.
And Pd / C was disperse | distributed with the ultrasonic homogenizer, and the core metal particle containing dispersion liquid was obtained.
Using a graphite reaction vessel as a working electrode, a counter electrode (platinum mesh), a reference electrode (silver-silver chloride), and a probe electrode (graphite piece) were placed so as to be immersed in a sulfuric acid aqueous solution. Then, the working electrode, the reference electrode, and the counter electrode were connected to a potentiostat as a potential control unit to prepare a device.

[Potential application process]
The reaction vessel was sealed, and the core metal particle-containing dispersion was bubbled with nitrogen gas (flow rate: 1 L / min) for 30 minutes.
Then, 16 cycles of applying a rectangular wave signal pattern of 2700 seconds at 0.05 V (vs. RHE) and 2700 seconds at 1.20 V (vs. RHE) were applied to the working electrode. In contrast, a potential of 0.05 V (vs. RHE) was applied for 2700 seconds.
The potential of Pd / C measured using the probe electrode and the working electrode measured using the potentiostat from the start of the 0.05 V potential application in the first cycle in the potential application step to the time 2700 seconds elapsed. FIG. 5 shows the current flowing between the electrode and the counter electrode.
As shown in FIG. 5, the Pd / C potential measured using the probe electrode is less than 0.6 V (vs. RHE) when 800 seconds have elapsed from the start of potential application, and is 0 when 1800 seconds have elapsed. It can be seen that it is lower than .5V (vs. RHE). Therefore, it can be seen that after 1800 seconds, the oxide is sufficiently removed.
Further, it can be seen that the value of the current flowing between the working electrode and the counter electrode measured using a potentiostat is in a steady state after 1800 seconds from the start of potential application.

[Copper coating process]
Subsequently, 300 g of copper sulfate pentahydrate dissolved in 1 L of 0.05 mol / L sulfuric acid aqueous solution was added to the reaction vessel to obtain a copper ion-containing electrolytic solution.
And the electric potential of the working electrode was fixed to 0.40 V (vs. RHE), and the palladium ion surface was coated with copper by colliding Pd / C with the working electrode by stirring the copper ion-containing electrolyte.

[Replacement process]
A platinum ion-containing solution prepared by dissolving 24 g of K ₂ PtCl ₄ and 450 g of citric acid monohydrate in 1 L of 0.05 mol / L sulfuric acid aqueous solution was prepared.
The application of the potential was stopped, and the platinum ion-containing solution was added dropwise using a tube pump over 80 minutes while stirring the copper ion-containing electrolyte in the reaction vessel with a stirrer. Thereafter, the mixed solution was continuously stirred for 3 hours to replace the copper on the surface of the palladium particles with platinum.
The temperature of the mixed solution at this time was maintained at 10 ° C. The K ₂ PtCl ₄ platinum ion-containing solution was previously bubbled with nitrogen before being introduced into the reaction vessel.

[Washing process]
After the substitution step, the solution in the reaction vessel was filtered to recover the powder. The recovered powder was added with 4 L of room temperature pure water in 10 portions, filtered and washed each time.
[Drying process]
Then, it dried at 60 degreeC for 12 hours, and grind | pulverized using the agate mortar and pestle, and obtained the core-shell catalyst for fuel cells.

(Comparative Example 1)
In the preparatory step, the probe electrode is not installed in the reaction vessel, and in the subsequent potential application step, the working electrode is 0.05 V (vs. RHE) for 15 seconds and 1.20 V (vs. RHE). A core-shell catalyst for a fuel cell was produced in the same manner as in Example 1 except that 2500 cycles of applying a potential of a rectangular wave signal pattern for 15 seconds were performed.

[Mass activity (MA) evaluation]
30 mg of each of the fuel cell core-shell catalysts obtained in Example 1 and Comparative Example 1 were collected, and each fuel cell core-shell catalyst was 131 μL of 5% Nafion (registered trademark) dispersion, 30 mL of ultrapure water, and isopropanol. A catalyst ink was prepared by dispersing in 7.5 mL of the mixed solution. The catalyst ink was applied onto a glassy carbon electrode of a rotating disk electrode (RDE) and allowed to dry naturally.
And the oxygen reduction reaction (ORR) measurement was performed about each core-shell catalyst for fuel cells.
The ORR measurement conditions are shown below.
Electrolyte solution: 0.1 mol / L perchloric acid aqueous solution (pre-bubbled with oxygen gas and oxygen saturated)
-Atmosphere: oxygen atmosphere-Sweep speed: 10 mV / sec-Potential sweep range: 1.05-0.05 V (vs. RHE)
・ Rotation speed: 1600 rpm
From the oxygen reduction wave obtained by the ORR measurement, the catalytic activity per unit mass of platinum in each core cell catalyst for fuel cells (hereinafter sometimes referred to as platinum mass activity) was measured.
Specifically, in the oxygen reduction wave obtained by ORR measurement, the current value of 0.9 V (vs. RHE) in the second cycle is changed to oxygen reduction current (I _0.9 ), 0.35 V (vs. RHE). _{Is the} diffusion limit current (I _lim ), the activation dominant current (Ik) is obtained from the following equation (1), and the amount of platinum (g) contained in the core shell catalyst for fuel cells coated on the glassy carbon electrode The catalytic activity per unit mass of platinum (A / g-Pt) was calculated by dividing Ik (A).
[Formula (1)]
Ik = (I _lim × I _0.9 ) / (I _lim −I _0.9 )
In the above formula (1), the meaning of each symbol is as follows.
Ik: Activation dominant current (A)
I _lim : diffusion limit current (A)
I _0.9 : Oxygen reduction current (A)
The platinum mass activity of the fuel cell core-shell catalyst was 580 A / g-Pt in Example 1 and 540 A / g-Pt in Comparative Example 1.

[Electrochemical surface area (ECSA)]
Cyclic voltammetry (CV) measurement was performed on the fuel cell core-shell catalyst obtained in Example 1 and Comparative Example 1, and the electrochemical surface area (ECSA) of the fuel cell core-shell catalyst was calculated.
A catalyst ink was prepared by the same method as the mass activity evaluation, and the catalyst ink was applied to a glassy carbon electrode (RDE) and dried, and CV measurement was performed.
CV measurement conditions are shown below.
Electrolyte: 0.1 mol / L perchloric acid aqueous solution (Ar gas was bubbled in advance at 30 mL / min for 30 minutes and saturated with Ar)
-Atmosphere: Under Ar atmosphere-Sweep speed: 50 mV / sec-Potential sweep range: 0.05 to 1.2 V (vs. RHE)
From the obtained cyclic voltammogram, the charge amount (C) of the hydrogen desorption peak was integrated.
Further, the mass (g) of platinum was calculated from the concentration and coating amount of the catalyst ink applied to the glassy carbon electrode.
From the value obtained by dividing the charge amount (C) of the hydrogen desorption peak by the charge amount (C / m ² ) per unit active surface area of platinum and the mass (g) of platinum, the electrochemical surface area (m ² / g-Pt) is calculated. Calculated.
Electrochemical surface area of the fuel cell core-shell catalyst, Example 1 113m ² / g-Pt, Comparative Example 1 was 131m ² / g-Pt.

[Specific activity (SA)]
About the core-shell catalyst for fuel cells obtained in Example 1 and Comparative Example 1, the catalytic activity per unit mass of platinum (A / g-Pt) is divided by the electrochemical surface area of platinum (m ² / g-Pt). The specific activity (catalytic activity per platinum surface area (A / m ² )) was calculated. The results are shown in FIG.
As shown in FIG. 6, the specific activity of the fuel cell core-shell catalyst, Example 1 is 5.1A / ^{m 2,} Comparative Example 1 was 4.1A / ^{m 2.}

[Comparison with conventional methods]
The specific activities (A / m ² ) of the fuel cell core-shell catalysts obtained in Example 1 and Comparative Example 1 were compared.
In Comparative Example 1, it is considered that impurities such as palladium oxide on the surface of the palladium particles were not sufficiently removed, and it was considered that the copper coating on the surface of the palladium particles was not sufficiently performed in the copper coating step, as shown in FIG. It is considered that the specific activity (SA) has become low.
On the other hand, in Example 1, a specific activity improvement of about 20% was confirmed compared with Comparative Example 1.
Therefore, the potential application step in the present invention can efficiently and sufficiently remove impurities such as the core metal oxide from the surface of the core metal particles as compared with the conventional method, and as a result, the core-shell catalyst for fuel cells having high catalytic activity. Can be said to have been manufactured.

21 reaction vessel 22 core metal particle-containing dispersion 23 core metal particle 24 stirrer 25 working electrode 26 counter electrode 27 reference electrode 28 probe electrode 29 counter electrode compartment 100 apparatus 200 apparatus

Claims (7)

A method for producing a core-shell catalyst for a fuel cell, comprising: a core containing a core metal; and a shell containing platinum and covering at least a part of the core,
The core metal particle-containing dispersion containing the core metal particles is contained, and the working electrode, the counter electrode, the reference electrode, and the potential of the core metal particles floating in the core metal particle-containing dispersion are monitored. A preparation step of preparing an apparatus having a reaction vessel including a probe electrode for controlling the potential of the working electrode, the counter electrode, and the reference electrode;
A potential application step of applying a potential to the working electrode until the potential of the probe electrode with respect to the reference electrode is equal to or lower than a predetermined potential while stirring the core metal particle-containing dispersion;
After the potential application step, a copper ion-containing electrolytic solution is supplied to the reaction vessel, and in the copper ion-containing electrolytic solution, a potential nobler than a copper oxidation-reduction potential is applied to the core electrode in the working electrode. A copper coating step of coating at least a part of the surface of the core metal particles with copper;
After the copper coating step, a platinum ion-containing solution is brought into contact with the core metal particles to replace copper covering at least a part of the surface of the core metal particles with platinum, thereby forming the shell. And a process for producing a core-shell catalyst for a fuel cell.   The manufacturing method of the core-shell catalyst for fuel cells of Claim 1 with which the said core metal particle is carry | supported by the support | carrier.   The manufacturing method of the core-shell catalyst for fuel cells of Claim 1 or 2 with which the said reaction container functions also as said working electrode.   In the potential application step, while stirring the core metal particle-containing dispersion, the potential of the probe electrode with respect to the reference electrode is equal to or lower than a predetermined potential, and a current value flowing between the working electrode and the counter electrode is steady. The manufacturing method of the core-shell catalyst for fuel cells as described in any one of Claims 1 thru | or 3 which applies an electric potential to the said working electrode until it will be in a state. The core metal is palladium;
In the potential application step, the working electrode is stirred until the potential of the probe electrode with respect to the reference electrode is within a range of 0.60 to 0.15 V (vs. RHE) while stirring the dispersion containing the core metal particles. The manufacturing method of the core-shell catalyst for fuel cells as described in any one of Claims 1 thru | or 4 which applies an electric potential to. An apparatus for producing a core shell catalyst for a fuel cell, comprising a core containing a core metal and a shell containing platinum and covering at least a part of the core by an underpotential deposition method,
The core metal particle-containing dispersion containing the core metal particles is contained, and the working electrode, the counter electrode, the reference electrode, and the potential of the core metal particles floating in the core metal particle-containing dispersion are monitored. A reaction vessel equipped with a probe electrode for
An apparatus for producing a core shell catalyst for a fuel cell, comprising: a potential control unit that controls a potential of the working electrode, the counter electrode, and the reference electrode.   The core-shell catalyst manufacturing apparatus for a fuel cell according to claim 6, wherein the reaction vessel also functions as the working electrode.