JP5521959B2 - Catalyst for polymer electrolyte fuel cell, electrode and battery using the same - Google Patents

Description

  The present invention relates to a polymer electrolyte fuel cell.

A single cell of a polymer electrolyte fuel cell (PEFC) has a structure in which a membrane / electrode assembly (MEA) is sandwiched between a pair of gas flow plates. The membrane / electrode assembly is obtained by joining an anode to one surface of a polymer electrolyte membrane and a cathode to the other surface. A gas flow path is processed in the gas flow plate. For example, electric power can be obtained by supplying hydrogen as fuel to the anode and oxygen as oxidant to the cathode. The following electrochemical reaction proceeds at the anode and the cathode.
Anode: 2H ₂ → 4H ⁺ + 4e ⁻ (1)
Cathode: O ₂ + 4H ⁺ + 4e ⁻ → 2H ₂ O (2)

The electrochemical reaction proceeds at an interface where a reactant such as oxygen or hydrogen, protons (H ⁺ ), and electrons (e ⁻ ) exist. Therefore, for the anode and cathode in PEFC, a catalyst metal, a carbonaceous material serving as an electron conductor, and a mixture of a cation exchange resin that is a polymer electrolyte serving as a proton conductor are used as a catalyst layer. In the cation exchange resin phase, proton conduction paths (also called ion clusters or clusters) in which hydrophilic exchange groups are gathered together with water are formed, and not only protons but also gases (hydrogen or oxygen) that are reaction active materials In addition, it is a movement path of water which is a product of the cathode reaction.

  Patent Document 1 discloses a PEFC catalyst powder and an electrode in which fine particles of platinum, which is a catalytic metal, are mainly supported on a contact surface between a carbon particle and a cluster of a polymer electrolyte, which serve as an electrochemical reaction field. Yes. The utilization rate of expensive catalyst metals has been dramatically improved, and a high-performance fuel cell has been realized even if the amount used is small. Further, it is disclosed that the average particle diameter of a catalyst substance such as platinum fine particles is preferably 2 to 4 nm.

  According to Patent Document 2, it is preferable that the cation exchange resin covers at least a part of the surface of the carbonaceous material, and the thickness of the cation exchange resin covering the carbonaceous material is 4 nm or more and 300 nm or less. It is disclosed that it is preferable.

JP 2000-12041 A JP 2003-257439 A

  However, the PEFC using the catalyst powder disclosed in Patent Document 1 sometimes has a problem that the high output required for automobile use cannot be obtained or the durability performance is insufficient. Accordingly, an object of the present invention is to provide a catalyst capable of realizing PEFC having durability and high output, and also providing an electrode and a PEFC using the catalyst.

The catalyst for a fuel cell of the present invention includes a carbonaceous material, a cation exchange resin that is a solid polymer electrolyte, and catalytic metal particles, and the catalytic metal particles are protons of the carbonaceous material and the cation exchange resin. A fuel cell catalyst mainly supported on a contact surface with a conduction path, wherein the thickness (A) of the cation exchange resin and the particle diameter (B) of the catalyst metal particles are B ≦ A ≦ B × It satisfies the relationship 1.2.
Since the catalytic metal particles are mainly supported on the contact surface between the carbonaceous material and the proton conduction path of the cation exchange resin, the utilization rate of the expensive catalytic metal can be drastically improved, and the usage amount can be reduced. Further, when the thickness A of the cation exchange resin and the particle size B of the catalytic metal particles satisfy the relationship of B ≦ A, the durability performance is obtained, and when the relationship of A ≦ B × 1.2 is satisfied, the thickness is high. An output PEFC can be realized.

  In another embodiment of the present invention, the diameter (D) of the proton conduction path of the cation exchange resin and the particle diameter (B) of the catalytic metal particle satisfy a relationship of D ≦ B. And Thereby, the durability performance of the fuel cell can be further improved.

  In another embodiment of the present invention, the carbonaceous material is carbon particles. Further, the catalyst metal is a platinum group metal, an alloy of two or more platinum group metals, or an alloy of one or more platinum group metals and other metals.

  The fuel cell electrode of the present invention uses the fuel cell catalyst of the present invention.

  The fuel cell of the present invention uses the fuel cell electrode of the present invention.

  According to the PEFC using the PEFC catalyst or PEFC electrode of the present invention and the PEFC of the present invention, the utilization rate of the expensive catalyst metal is high, and higher output and superior durability performance can be obtained.

The schematic diagram which shows the cell structure of general PEFC. The conceptual diagram which shows the state of the carbon particle surface vicinity which concerns on one Embodiment of this invention. The figure which shows the change of the cation exchange resin thickness by the difference in manufacturing conditions. The figure which shows the change of the cation exchange resin thickness by the difference in manufacturing conditions. The figure which shows the relationship between cation exchange resin thickness / catalyst metal particle diameter, and durable performance. The figure which shows the relationship between cation exchange resin thickness / catalyst metal particle diameter, and cell voltage. The figure which shows the relationship between catalyst metal particle diameter / cluster diameter and durability performance. The conceptual diagram which shows the state of the carbon particle surface vicinity of the conventional catalyst.

  FIG. 1 is a schematic diagram showing a general PEFC cell structure. The anode 3 is joined to one surface of the polymer electrolyte membrane 4 and the cathode 2 is joined to the other surface, and these are sandwiched by a pair of gas flow plates 5. The anode 3 and the cathode 2 are each composed of a catalyst layer 7 and a gas diffusion layer 6. The catalyst layer 7 includes solid polymer electrolyte 9 and carbon particles 8 supporting a noble metal as a catalyst. The noble metal-supporting carbon particles 8 are in contact with each other, and a part of the surface is covered with the polymer electrolyte 9. A gas flow path is processed in the gas flow plate 5. For example, electric power can be obtained by supplying hydrogen as a fuel to the anode 3 and oxygen as an oxidant to the cathode 2. Carbon gas or the like is used for the gas flow plate 5.

  FIG. 2 is a conceptual diagram showing the state of the surface layer of carbon particles contained in the catalyst according to one embodiment of the present invention. In the cation exchange resin phase 9 on the surface of the carbon particles 10, a fluorocarbon skeleton 12 and a cluster 13 which is a proton conduction path are formed. Cluster 13 is a region where the hydrophilic exchange groups of the cation exchange resin gather together with water. L. As described in the report of Yeager et al. (J. Electrochem. Soc., 128, 1880 (1981)) and the report of Kokumi et al. (J. Electrochem. Soc., 132, 2601 (1985)) Of course, it becomes a conduction path of gas (hydrogen or oxygen) which is a reaction active material and water which is a product of the cathode. The catalytic metal 11 is mainly supported on the contact surface between the carbon particles 10 that are electron conductors and the cluster 13.

  As the carbonaceous material used in the present invention, various carbonaceous materials having electron conductivity such as carbon black such as acetylene black, furnace black, and channel black, activated carbon, and carbon fiber can be used. The carbonaceous material may be in the form of particles or fibers, but is preferably in the form of particles from the viewpoint of ease of dispersion in the production process.

As a material for the solid polymer electrolyte in the present invention, a cation exchange resin having proton conductivity, for example, a perfluorocarbon sulfonic acid type or a styrene-divinylbenzene sulfonic acid type cation exchange resin is preferably used. it can.
The saturated water content of the cation exchange resin varies depending on the EW value. The EW value of the cation exchange resin represents the equivalent weight of the ion exchange group having proton conductivity. The equivalent weight is the dry weight of the cation exchange membrane or cation exchange resin per equivalent of ion exchange groups, and is expressed as “g / eq”. The ion exchange groups increase as the EW value decreases. That is, as the EW value decreases, the proton conductivity increases and the hydrophilicity also increases. The EW value of the cation exchange resin in the present invention is preferably in the range of 700 to 1150. When the EW value is smaller than this, the strength of the cation exchange resin is insufficient, and when the EW value is larger than this, the amount of the cation exchange group is decreased and the catalyst content is decreased.

As a method of coating the surface of carbon particles with a cation exchange resin, for example, a slurry containing carbon particles and a cation exchange resin solution is stirred, and after ultrasonic dispersion is applied to promote dispersion, the slurry is spray-dried. can do. As a result, a powder containing a cation exchange resin and carbon particles and partially covering the surface of the carbon particles with the cation exchange resin is obtained.
In many cases, the ion exchange resin does not completely dissolve in the solvent. Therefore, in this specification, the ion exchange resin solution is not only a true solution, but also includes those in which the ion exchange resin is dispersed as a colloid. Including.

  As the catalyst substance used in the electrode of the present invention, various substances that promote the aforementioned electrochemical reaction can be used. Of these, white metal elements, that is, ruthenium, rhodium, palladium, osmium, iridium, platinum, and alloys thereof are preferably used, and platinum and alloys thereof are particularly preferably used. Examples of platinum alloys include Pt—Ru, Pt—Sn, Pt—Pd, Pt—Mn, Pt—Co, Pt—Ni, Pt—Fe alloy catalysts and the like as catalysts having high CO poisoning resistance. It has been.

  In the present invention, catalytic metal particles are supported mainly on the contact surface between the carbon particles and the cluster of the cation exchange resin. Since the electrochemical reaction of the fuel cell proceeds on the catalytic metal on the contact surface between the carbon surface and the cluster, the amount of the catalyst metal supported on the contact surface may exceed 50% by mass of the total supported metal amount, The catalyst metal utilization rate is high, which is preferable. Further, in order to exhibit high activity while having an extremely small amount of supported catalyst metal, the amount of catalyst metal supported on the contact surface between the carbon particle surface and the cluster is preferably 80% by mass or more of the total amount of supported catalyst metal. More preferably, it exceeds 90% by mass.

The catalyst metal particles are mainly supported on the contact surfaces of the carbon particles and the cation exchange resin cluster by immersing the carbon particles coated with the cation exchange resin in an aqueous solution containing a metal compound as a catalyst. In addition, it is possible to use a method in which ions containing the catalyst metal are adsorbed on the cluster part of the ion exchange resin by an ion exchange reaction, and if necessary, excess ions are washed away by water washing, and then the catalyst metal is reduced and precipitated.
Various salts and complex salts can be used as the catalyst metal compound. When an alloy composed of a plurality of elements is supported as the catalyst metal, several compounds may be mixed and used, or a double salt may be used. For example, carbon particles coated with a cation exchange resin are immersed in an aqueous solution [Pt (NH ₃ ) ₄ ] Cl ₂ , so that [Pt (NH ₃ ) ₄ ] ²⁺ ions are clustered in the cation exchange resin. Adsorbed by ion exchange reaction. Excess [Pt (NH ₃ ) ₄ ] ²⁺ ions can be washed away by washing with water, and then Pt can be reduced and precipitated. For example, when a Pt—Ru alloy is to be supported, an aqueous solution containing [Pt (NH ₃ ) ₄ ] Cl ₂ and [Ru (NH ₃ ) 5] Cl ₃ can be used.

  A reducing agent can be used as a method of chemically reducing the cation of the catalytic metal element. In this method, the cation of the metal element present near the surface of the carbonaceous material can be preferentially reduced by adjusting the type of reducing agent, reducing pressure, reducing agent concentration, reducing time, and reducing temperature. preferable. For example, when hydrogen is used as the reducing agent, by adjusting the reduction temperature, it is possible to selectively reduce metal element cations near the surface of the carbonaceous material. This phenomenon utilizes the fact that the carbonaceous material has catalytic activity for the reduction reaction of the cation of the metal element.

  Reducing agents that can be used include hydrogen, hydrogen sulfide, hydrogen iodide, non-metal ions such as iodine ions, lower oxides such as carbon monoxide and sulfur dioxide, sodium phosphinate, sodium thiosulfate, sodium borohydride, Examples include hydrides such as lithium tetrahydroaluminate, hydrazine, diimide, formic acid, aldehydes, and sugars. In particular, a gas containing hydrogen or hydrazine is preferable because it is suitable for mass production. The hydrogen is preferably used as a mixed gas (hydrogen mixed gas) with an inert gas such as nitrogen, helium or argon, and the hydrogen content of the hydrogen mixed gas is 10% by volume or more. preferable.

  Further, the reduction temperature is preferably lower than the decomposition temperature of the cation exchange resin in order to prevent deterioration of the cation exchange resin. For example, when the cation exchange resin is a perfluorocarbon sulfonic acid type cation exchange resin, deterioration of the resin can be suppressed by reducing the cation exchange resin at a temperature lower than the decomposition temperature of 280 ° C. When hydrogen gas or hydrogen mixed gas is used as the reducing agent, the reduction temperature condition is preferably 100 ° C. or higher and 200 ° C. or lower.

In order to increase the amount of noble metal particles supported, a method of repeating the steps from adsorption of noble metal complex ions to reduction treatment can be used.
After a predetermined number of reduction treatments, the noble metal-supported carbon particles are separated and recovered, washed as necessary, and dried to obtain a PEFC catalyst powder.

  In the present invention, the thickness (A) of the polymer electrolyte and the particle diameter (B) of the catalyst metal particles satisfy the relationship of B ≦ A ≦ B × 1.2. In a preferred embodiment of the present invention, the proton conduction path diameter (D) of the solid electrolyte and the catalyst metal particle diameter (B) have a relationship of D ≦ B. In the following, how these values are determined will be described.

In the present invention, the thickness (A) of the ion exchange resin layer coated with the carbonaceous material refers to a thickness determined by the method described below.
First, the electric double layer capacity of the electrode is measured to determine the carbon coverage of the ion exchange resin. A membrane / electrode assembly (MEA) in which a target electrode (WE) is joined to one surface of an ion exchange membrane (solid polymer membrane, electrolyte) and a counter electrode (CE) is joined to the other surface is provided. A fuel cell was manufactured, and nitrogen and hydrogen adjusted to a predetermined relative humidity by changing the humidification temperature were supplied to WE and CE, while using a potentiostat, the sweep rate was 0.05 V / s, and 0.05 V to 0 Sweep the potential of WE with respect to CE in the range of .8V vs RHE to obtain a cyclic voltammogram. The electric double layer capacity (unit: F) at a predetermined relative humidity is obtained by dividing the current value (average value of anode current and cathode current) (unit: A) at 0.4 V vs RHE by the sweep speed (unit: V / s). ) In addition, the present inventors measured using the Hokuto Denko Co., Ltd. make and the electrochemical measurement system HZ-3000.
The carbon coverage (CR) of the ion exchange resin is obtained by dividing the electric double layer capacity at 30% relative humidity by the electric double layer capacity at 100% relative humidity.

The thickness A (unit: cm) of the ion exchange resin layer is obtained by the following formula.
A = (WP × DP) / (WC × SC × CR)
Here, WP is the amount of ion-exchange resin contained per 1 cm ² of electrode (unit: g).
DP is the density of the ion exchange resin (unit: cm ³ / g)
WC is the amount of carbon carrier contained per 1 cm ² of electrode (unit: g)
SC is the BET specific surface area of the carbon support (unit: cm ² / g)
CR represents the carbon coverage (unit: dimensionless) of the ion exchange resin obtained from the measurement of the electric double layer capacity.
The BET specific surface area is measured by adsorption of N ₂ gas by the BET method. In the measurement, as a pretreatment, it is necessary to remove the physically adsorbed substance from the sample surface by degassing treatment. The pretreatment conditions can be determined by thermogravimetric analysis or preliminary experiments using degassing conditions with different time and temperature.

  Here, the present inventors have found that the value of the thickness A of the ion exchange resin layer varies greatly depending on manufacturing conditions, and will be described below.

For various carbon powders, the relationship between the ultrasonic irradiation time and the thickness of the polymer electrolyte on the carbon surface was investigated. Table 1 shows the main experimental conditions, and FIG. 3 shows the results.
Samples were prepared as follows. As carbon particles, C1 (manufactured by Tokai Carbon Co., Ltd., particle diameter 25 nm, BET specific surface area 90 m ² / g), C2 (manufactured by Cabot Corporation, particle diameter 30 nm, BET specific surface area 254 m ² / g), C3 (Ketjen Black) -International Co., Ltd. product, particle diameter 30nm, BET specific surface area 800m < ² > / g) were used. As an ion exchange resin solution, P1 (5% by mass solution, the balance being ethanol 75% by mass, water 20% by mass) was diluted 2 times with ethanol and used. Carbon particles and ion exchange resin solution are weighed so as to have a predetermined mass ratio, placed in a 200 ml beaker, stirred with a magnetic stirrer for 1 minute, irradiated with ultrasonic waves for a predetermined time, and then the slurry is spray dried. did.
A commercially available ultrasonic disperser can be used for ultrasonic irradiation. The present inventors used UH-600SR (oscillation frequency 20 kHz ± 10%, output 600 W) manufactured by SMT Corporation. Unless otherwise stated, the irradiation conditions were intermittent irradiation with 0.5 second irradiation and 0.5 second pause. When the irradiation time is shown below, it is shown including the pause time of intermittent irradiation.
From FIG. 3, it was found that the thickness A of the ion exchange resin varies greatly depending on the type of carbon particles, the concentration of the polymer electrolyte solution, and the ultrasonic dispersion treatment time.

Next, the influence of the mixing ratio of the carbon particles and the ion exchange resin and the concentration of the ion exchange resin solution on the thickness of the polymer electrolyte was investigated. Table 2 shows the main experimental conditions, and FIG. 4 shows the results.
Samples were prepared as follows. C1 was used as the carbon particles, and a 5% by mass solution of P1 or 2.5% by mass diluted twice with ethanol was used as the ion exchange resin solution. Carbon and ion exchange resin are weighed to a predetermined mass ratio, put in a 200 ml beaker, stirred with a magnetic stirrer for 1 minute, and irradiated with ultrasonic waves for 1 second per 1 ml of slurry. The slurry was spray dried.
From FIG. 4, the ion exchange resin solution concentration increases to 2.5% by mass and 5% by mass, and the ion exchange resin thickness increases as the ion exchange resin compounding ratio increases. I can see that it is not. Moreover, even if the blending ratio of the ion exchange resin is the same, the ion exchange resin thickness increases as the polymer electrolyte solution concentration increases, but it is understood that the two are not in a simple proportional relationship.

  As described above, the thickness (A) of the ion exchange resin greatly depends on the production conditions such as the type of carbon particles, the blending ratio of carbon and ion exchange resin, the concentration of the ion exchange resin solution, and the time of ultrasonic dispersion. . Furthermore, the dependency cannot be given by simple functions. Therefore, it is understood that the thickness of the ion exchange resin layer cannot be predicted only by showing the type of carbon, the blending ratio of the ion exchange resin, and simple mixing conditions. In other words, it is understood that the thickness of the actually obtained ion exchange resin layer cannot be easily predicted unless actually measured.

ここで、ｄは結晶子径（単位：ｎｍ）
Ｋはシェラーの定数（＝０．９４とする）
λはＸ線の波長（ＣｕＫα線では０．１５４１８４１ｎｍ）
βは半価幅（単位：ラジアン）
θは回折Ｘ線のブラッグ角（単位：度）
である。 In the present invention, the particle diameter (B) of the catalytic metal particles refers to the crystallite diameter obtained from the result of X-ray diffraction (XRD) measurement using Scherrer's equation (Equation 1).
The carbon particles carrying the catalyst noble metal were subjected to XRD measurement under the conditions of a scanning range of 60 to 75 degrees (2θ) and a scanning speed of 0.2 degrees / minute using CuKα rays as a light source. ) The position and half-value width of the peak attributed to the (110) plane of the hexagonal dense structure. For example, in the case of platinum, a face-centered cubic structure is adopted. Ask. In the case of ruthenium and osmium, a hexagonal close-packed structure is adopted, and the peak position (2θ = 69.4 degrees for ruthenium, 2θ = 68.6 degrees for osmium) and the half-value width are obtained. Here, the half width refers to the full width at half maximum (FWHM). From the above measurement results, the crystallite diameter d is obtained by the Scherrer equation, and this is used as the particle diameter B of the noble metal in the present invention.

Where d is the crystallite size (unit: nm)
K is Scherrer's constant (= 0.94)
λ is the wavelength of X-rays (0.1541841 nm for CuKα rays)
β is half width (unit: radians)
θ is the Bragg angle of diffraction X-ray (unit: degree)
It is.

In the present invention, the cluster diameter refers to a value measured by the X-ray small angle scattering method (SAXS).
The cluster diameter is T.D. D. According to a study by Gierke et al. (J. Membrane Sci., 13, 307 (1983)), a typical perfluorocarbon sulfonic acid type ion exchange resin membrane (DuPont, Nafion-112) is a proton conduction path. The diameter of the cluster is about 4 nm. The cluster diameter can be controlled by changing the type of resin serving as the skeleton of the ion exchange resin and the ion exchange capacity.

  A well-known method can be used for manufacture of a battery. For example, a catalyst powder containing carbon particles, polymer electrolyte, and catalyst metal particles obtained as described above is coated on both surfaces of a polymer electrolyte membrane to produce a membrane / electrode assembly (MEA). What is necessary is just to pinch with the gas flow plate. Known materials can be used for the polymer electrolyte membrane and the gas flow plate.

The obtained catalyst powder can be evaluated by a power generation test and measurement of an active surface area.
The catalyst powder was applied and bonded to both surfaces of the ion exchange membrane so that the amount of Pt per unit area was 0.4 mg / cm ² , incorporated into a PEFC holder, and then 100 mA / cm in H ₂ / air. ^Two power generation tests were conducted, and the cell voltage was measured as an output index.
The power generation test method will be described in detail. The cell temperature was adjusted to 70 ° C. with an external heater, and the anode was humidified with hydrogen (gas utilization rate 70%, back pressure 0.1 MPa), and the cathode at 65 ° C. Humidified air (oxygen gas utilization rate 40%, back pressure 0.1 MPa) was supplied, the generated current density was adjusted to 300 mA / cm ² with an electronic load, and the cell voltage was measured.

Furthermore, a potential cycle test (0.6 to 1.0 V vs RHE, 30,000 cycles, room temperature) was performed using the cell, and the active surface area before and after the cycle test was measured as an index of durability performance.
The measurement of the active surface area is as follows. Nitrogen and hydrogen produced by preparing a fuel cell comprising an MEA having a target electrode (WE) bonded to one surface of an ion exchange membrane and a counter electrode (CE) bonded to the other surface, adjusted to 25 ° C. and a relative humidity of 100% The electric potential of WE to CE in the range of 0.05V to 0.2VvsRHE at a sweep rate of 0.05V / s using Hokuto Denko Co., Ltd., electrochemical measurement system (HZ-3000). In contrast, a cyclic voltammogram is obtained. The initial active surface area (AS ₀ ) of platinum was calculated by the following formula using the electric quantity resulting from desorption of adsorbed hydrogen appearing in 50 to 200 mV vs RHE of the voltammogram.
AS = (Q × 1000) / (210 × M × S)
Here, AS is the electrochemically active surface area of platinum (unit: cm ² / mg)
Q is the amount of hydrogen desorption electricity (unit: μC)
210 is the amount of electricity by which monoatomic hydrogen is adsorbed on the surface of a smooth platinum plate of 1 cm ² (unit: μC / cm ² ).
M is the amount of platinum supported per electrode area (unit: mg / cm ² )
S is the electrode area (unit: cm ² )
Represents.
Subsequently, after performing sweeping 30,000 potentials with respect to CE in the range of 0.60 V to 1.0 Vvs RHE at a sweep rate of 0.50 V / s, the active surface area (AS ₃₀₀₀₀ ) is again obtained. Was measured in the same manner as the initial active surface area (AS ₀ ). Further, the maintenance ratio of the active surface area was determined by the following formula.
Maintenance ratio of active surface area (%) = (AS ₃₀₀₀₀ / AS ₀ ) × 100

[Catalyst having a Pt particle size of 6 nm]
80 g of a solution obtained by diluting 3 g of carbon powder (C1) and cation exchange resin (P1) with ethanol twice (carbon / ion exchange resin blend ratio = 60/40) was put in a beaker having a volume of 200 ml, and a magnetic stirrer After kneading for 1 minute and irradiating with ultrasonic waves for a predetermined time to disperse, the slurry was spray-dried to obtain a powder containing carbon and a cation exchange resin. Here, by changing the ultrasonic irradiation time, the thickness of the cation exchange resin layer was controlled, and six types of powders having different cation exchange resin thicknesses were obtained.
The powder was immersed in 500 ml of an aqueous solution of tetraammineplatinum (II) dichloride [Pt (NH ₃ ) ₄ ] Cl _{2 at} a concentration of 50 mmol / liter for 6 hours, and [Pt (NH ₃ ) ₄ ] ²⁺ ions were exchanged with a cation exchange resin. It was adsorbed on the ion cluster. The powder was collected by suction filtration, washed with 0.5 mol / liter sulfuric acid, washed with pure water, and dried at 80 ° C. This was kept in a hydrogen stream at 150 ° C. for 4 hours to reduce and deposit Pt. Here, the process from adsorption to reduction was repeated until the Pt particle size reached 6 nm.

Next, 3 g of the obtained powder and 1 g of N-methyl-2-pyrrolidone (NMP) were mixed, applied to a fluororesin film, then dried at 80 ° C. for 30 minutes, and the electrolyte membrane was heated with a hot press. A membrane / electrode assembly (MEA) was prepared by bonding to both surfaces.
The MEA was produced in the same manner for the following various catalysts.

[Catalyst having a Pt particle size of 3 nm]
30 g of a solution obtained by diluting 3 g of carbon powder (C1) and cation exchange resin (P1) with ethanol twice (carbon / ion exchange resin blending ratio = 80/20) was used, and then [Pt particle size was 6 nm. The catalyst was treated in the same manner as in [Catalyst] to obtain catalyst powder in which Pt fine particles were supported on six kinds of powders having different cation exchange resin thicknesses. However, the processes from adsorption to reduction were repeated until the Pt particle size reached 3 nm.

[Catalyst having a Pt particle diameter of 6 nm by a conventional method]
80 g of a catalyst in which Pt is supported on carbon particles in advance (Pt particle size = 6 nm, Pt loading = 50 mass%) and a cation exchange resin (P1) diluted twice with ethanol (carbon / ion exchange resin blend) (Rate = 60/40) was put in a beaker having a volume of 200 ml, kneaded with a magnetic stirrer for 1 minute, irradiated with ultrasonic waves for a predetermined time and dispersed, and then the slurry was spray-dried. Here, by changing the ultrasonic irradiation time, the thickness of the cation exchange resin layer was controlled, and six types of powders having different cation exchange resin thicknesses were obtained.

Durability and cell voltage were evaluated for the above samples.
FIG. 5 shows the influence of the ratio of the ion exchange resin thickness (A) / Pt particle diameter (B) on the active surface area retention rate after the potential cycle test. In the figure, for a catalyst having a Pt particle diameter of 6 nm, a catalyst having a Pt particle diameter of 3 nm, and a catalyst having a Pt particle diameter of 6 nm according to the conventional method, the maintenance ratio when A / B = 1 is 1.0. Shown in the figure.
From FIG. 5, in the case of the catalyst in which the Pt particles are mainly supported on the contact surfaces of the clusters of carbon particles and the ion exchange resin, the active surface area retention ratio is high when A / B ≧ 1, and A / B is When it becomes smaller than 1, it is lowered rapidly. Qualitatively, the smaller the A / B, the lower the surface activity retention rate, because it is easier to elute as a result of the exposure of some of the Pt particles located in the ion exchange resin cluster from the ion exchange resin. Can be explained. Further, even when the Pt particles are not exposed from the ion exchange resin, it is considered that a part of the Pt is repeatedly eluted and re-deposited. However, when the distance from the Pt particles to the ion exchange resin surface is short, the Pt particles are eluted. It is considered that Pt diffuses out of the ion exchange resin and the ratio of not reprecipitating increases.
On the other hand, no significant effect is observed in the conventional catalyst. This is because, in the catalyst according to the conventional method, the covering state of the ion exchange resin on Pt is loose, and Pt is not firmly contained in the resin originally. As a result of simple diffusion from the gap, it is considered that the influence of the A / B ratio is small (see FIG. 8).
That is, the A / B ratio strongly influences the active surface area maintenance rate after the potential cycle test. Such a relationship is that the catalytic metal particles are mainly supported on the contact surfaces of the carbon particles and the cluster of the cation exchange resin. This is a phenomenon peculiar to the catalyst.

FIG. 6 shows the influence of the ratio of ion exchange resin thickness (A) / Pt particle diameter (B) on the cell voltage for the same sample as FIG.
From FIG. 6, in the case of the catalyst in which the Pt particles are mainly supported on the contact surfaces of the clusters of the carbon particles and the ion exchange resin, the cell voltage is high when A / B ≦ 1.2, and A / B is When it becomes larger than 1.2, it decreases rapidly. The cell voltage decreases as A / B increases. This is because Pt particles located in the ion exchange resin cluster are largely buried from the surface of the ion exchange resin, so that oxygen as a reaction gas becomes Pt particles as a reaction field. This is thought to be due to the difficulty of reaching the surface.
On the other hand, no significant effect is observed in the conventional catalyst. This is because, in the catalyst according to the conventional method, the coating state of the ion exchange resin on Pt is loose, and since Pt is not firmly included in the resin originally, oxygen can directly reach the Pt surface. It is considered that the influence of the A / B ratio is small (see FIG. 8).
That is, the A / B ratio strongly affects the cell voltage. This relationship is a phenomenon peculiar to the catalyst in which the catalytic metal particles are mainly supported on the contact surface between the carbon particles and the cluster of the cation exchange resin. It can be said that there is.

[Catalyst having a cluster diameter of 3.5 nm]
Carbon powder (C1) 3g and cation exchange resin (P1) 57g diluted solution with ethanol (carbon / ion exchange resin compounding ratio = 68/32) was put in a beaker with a volume of 200ml. After kneading for 1 minute and irradiating with ultrasonic waves for a predetermined time to disperse, the slurry was spray-dried to obtain a powder containing carbon and a cation exchange resin. The obtained powder had an ion exchange resin thickness of 3.8 nm by the electric double layer capacity measurement method described above, and a cluster diameter of 3.5 nm by the SAXS measurement described above.
The powder was immersed in 500 ml of an aqueous solution of tetraammineplatinum (II) dichloride [Pt (NH ₃ ) ₄ ] Cl _{2 at} a concentration of 50 mmol / liter for 6 hours, and [Pt (NH ₃ ) ₄ ] ²⁺ ions were exchanged with a cation exchange resin. It was adsorbed on the ion cluster. The powder was collected by suction filtration, washed with 0.5 mol / liter sulfuric acid, washed with pure water, and dried at 80 ° C. This was kept in a hydrogen stream at 150 ° C. for 4 hours to reduce and deposit Pt. Here, six types of powders having different Pt particle sizes were obtained by changing the number of repetitions of the treatment from adsorption to reduction.

[Catalyst having a cluster diameter of 5 nm]
80 g of carbon powder (C1) and cation exchange resin P2 (5% by mass solution, the balance being 48% by mass of 1-propanol, 45% by mass of water, and 2% by mass of other components) diluted twice with ethanol (carbon / carbon) Ion exchange resin compounding ratio = 60/40) is placed in a beaker having a volume of 200 ml, kneaded with a magnetic stirrer for 1 minute, dispersed by irradiation with ultrasonic waves for a predetermined time, and then the slurry is spray-dried to obtain carbon and A powder containing a cation exchange resin was obtained. The obtained powder had an ion exchange resin thickness of 5.3 nm by the electric double layer capacity measurement method described above, and a cluster diameter of 5 nm by the SAXS measurement described above.
Thereafter, the same treatment as [Catalyst having a cluster diameter of 3.5 nm] was performed, and the number of repetitions of the treatment from adsorption to reduction was changed to obtain six types of powders having different Pt particle sizes.

FIG. 7 shows the influence of the ratio of the Pt particle size (B) / cluster size (D) on the active surface area retention rate after the potential cycle test. Both are catalysts in which Pt particles are mainly supported on the contact surfaces of carbon particle and ion exchange resin clusters, and those having a cluster diameter of 3.5 nm and those having a diameter of 5 nm are maintained when B / D = 1. The rate is 1.0 in the figure.
As shown in FIG. 7, the active surface area retention rate is high when B / D ≧ 1.0. This means that when B ≧ D, the Pt ions dissolved and generated in the cluster are adsorbed on the ion exchange groups provided on the cluster wall surface, and are then precipitated and reduced again on the fine particles. In contrast, when B <D, the ions adsorbed on the cluster wall surface and the Pt surface are separated from each other, so that reduction / reprecipitation onto the fine particles is relatively difficult. Presumed.

DESCRIPTION OF SYMBOLS 1 Polymer electrolyte fuel cell 2 Cathode 3 Anode 4 Polymer electrolyte membrane 5 Gas flow plate 6 Gas diffusion layer 7 Catalyst layer 8 Noble metal carrying carbon particle 9 Polymer electrolyte (ion exchange resin)
10 Carbon Particles 11 Precious Metal Fine Particles 12 Fluorocarbon Skeleton 13 Proton Conduction Path (Cluster)

Claims (6)

A carbonaceous material, a cation exchange resin that is a solid polymer electrolyte, and catalytic metal particles,
The catalyst metal particle is a catalyst for a fuel cell mainly supported on a contact surface between the carbonaceous material and a proton conduction path of the cation exchange resin,
The fuel cell catalyst, wherein the thickness (A) of the cation exchange resin and the particle diameter (B) of the catalytic metal particles satisfy a relationship of B ≦ A ≦ B × 1.2. 2. The fuel cell catalyst according to claim 1, wherein a proton conduction path diameter (D) of the cation exchange resin and a particle diameter (B) of the catalytic metal particles satisfy a relationship of D ≦ B. . The fuel cell catalyst according to claim 1 or 2, wherein the carbonaceous material is carbon particles. 4. The catalyst metal according to claim 1, wherein the catalyst metal is a platinum group metal, an alloy of two or more platinum group metals, or an alloy of one or more platinum group metals and another metal. The catalyst for fuel cells as described.   The electrode for fuel cells using the catalyst for fuel cells described in any one of Claims 1-4.   A fuel cell using the fuel cell electrode according to claim 5.

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