JP2003151564A - Electrode for solid high polymer fuel cell - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an electrode for solid high polymer fuel cells, which can form a catalyst layer, which has the vacancy structure of high power-generating efficiency, without adding a complicated process. SOLUTION: It has the catalyst layer, which consists of a catalyst substance, an ion conductivity substance, an electronic conductivity substance, and a hole making agent, and vacancy volume of diameters of 60 to 1000 nm in the above catalyst layer is set to 0.15 to 0.25 cm<3> /g.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electrode for a polymer electrolyte fuel cell, and more particularly to a technique for effectively functioning a catalyst layer.

[0002]

2. Description of the Related Art A polymer electrolyte fuel cell is constructed by laminating separators on both sides of a flat plate electrode structure. In the electrode structure, generally, a polymer electrolyte membrane is sandwiched between a positive electrode (cathode) side catalyst layer and a negative electrode (anode) side catalyst layer, and a gas diffusion layer is laminated outside each catalyst layer. It is a laminated body. The catalyst layer is formed by applying a catalyst paste to the electrolyte membrane and then hot pressing,
It is formed by a method in which the catalyst paste is applied to carbon paper or an FRP sheet to form an electrode sheet, and then hot pressing is performed to join the electrode sheet. The catalyst paste is a catalyst substance such as platinum, an electron conductive substance such as carbon particles, and an ion conductive substance (polymer electrolyte).
And a binder such as PTFE and a water repellent are added as necessary.

According to such a fuel cell, in the catalyst layer of the negative electrode, a reaction of taking out protons and electrons from the fuel gas (hydrogen) is carried out, and in the catalyst layer of the positive electrode, the protons, electrons and the oxidizing gas (air) are taken. There is a reaction in which and become water, and the fuel cell generates electricity by these reactions.

Therefore, in order to improve the power generation efficiency of the fuel cell, it is necessary to uniformly supply the fuel gas and the oxidizing gas to the entire catalyst layer. Therefore, various studies have been carried out on the pores of the catalyst layer serving as the gas passages, that is, the optimum pore structure in the catalyst layer.

In Japanese Patent Laid-Open No. 9-92293, pores (pores) having a diameter of 0.04 to 1.0 μm function as gas channels, and if the specific volume is 0.04 cm ³ / g or more, the gas is Is said to have good diffusivity. However, the above-mentioned void volume is controlled by the amount of ion-conducting substance or hot pressing conditions, and in order to increase the number of vacancies, the ion-conducting substance must be decreased (decrease of ion-conducting channel). Alternatively, the temperature and pressure of the hot press conditions must be lowered, which causes a problem that the adhesion between the film and the electrode deteriorates. Also, with this method,
It is difficult to set the pore density to 0.1 cm ³ / g or more.

Further, JP-A-6-203840 proposes an electrode in which the porosity of the catalyst layer is 65 to 90% by volume. However, also in this electrode, the voids are controlled by the hot pressing conditions.

Further, although the catalyst layer formed by the above method for forming a catalyst layer has some pores before hot pressing, the pores are crushed and reduced after the hot pressing step. It cannot be increased beyond the amount of holes before the hot pressing process.

Therefore, in order to solve this problem, JP-A-6-203852, JP-A-6-236762, JP-A-7-176310, and JP-A-8-18 are available.
In Japanese Patent No. 0879, Japanese Patent Application Laid-Open No. 9-199138, Japanese Patent Application Laid-Open No. 10-3929, Japanese Patent Application Laid-Open No. 10-189005, and Japanese Patent Application Laid-Open No. 10-189012, zinc powder, silica sol, ammonium hydrogen carbonate, It has been proposed to add a pore-forming agent such as water-soluble short fibers, camphor and lithium carbonate, and remove the pore-forming agent after hot pressing to increase the number of pores. However, these methods require a step of removing the pore-forming agent after hot pressing, which makes the step complicated.

Further, Japanese Patent Application Laid-Open No. 9-223503 proposes a method of increasing the number of pores by adding a solvent having a high boiling point to the catalyst paste and evaporating it in the firing step. However, even in this method, an additional step called a firing step is required, and there is a problem of complicated steps.

Further, in JP-A-11-329452, an ink C is prepared by mixing an ink A containing an ion conductive substance and an ink B containing a solvent such as methyl dodecanoate which does not dissolve an ionomer. Then, after forming a catalyst layer with this ink C, a method of increasing the number of pores by evaporating an organic solvent such as methyl dodecanoate has been proposed. However, this method
There is a problem that it is necessary to prepare various kinds of ink, and it is necessary to strictly control the working atmosphere for evaporating the organic solvent.

Japanese Unexamined Patent Publication No. 10-223233 proposes to add carbon fibers to the catalyst paste so that the catalyst paste does not block the pores in the porous substrate (diffusion layer). However, since this electrode has a structure in which the catalyst layer is formed on the surface layer inside the carbon paper, it is different from the embodiment of the present invention having the catalyst layer containing no carbon paper.

[0012]

As described above, in the polymer electrolyte fuel cell, various attempts have been made to increase the power generation efficiency per volume of the catalyst layer, but satisfactory results have been obtained. The reality is that the ones that exhibit power generation efficiency have not been manufactured yet. Therefore, an object of the present invention is to provide an electrode for a polymer electrolyte fuel cell capable of forming a catalyst layer having a pore structure with high power generation efficiency without adding a complicated process.

[0013]

An electrode for a polymer electrolyte fuel cell of the present invention has a catalyst layer composed of a catalyst substance, an ion conductive substance, an electron conductive substance and a pore-forming agent. The pore volume of 60 to 1000 nm in diameter is 0.15
It is characterized by being ˜0.25 cm ³ / g.

The solid polymer fuel cell electrode of the present invention comprises:
For example, in a catalyst layer of a porous body having a large number of pores due to an electron conductive material and an ion conductive material, the catalyst material is supported on the surface of the electron conductive material, dispersed in the ion conductive material, or both of them. It is a composition included as.

According to the present invention, a diameter of 6 in the catalyst layer can be obtained by using a specific pore-forming agent when forming the above structure.
0-1000 nm pore volume 0.15-0.25 cm
³ / g, and a catalyst layer having a pore structure that functions as a good gas channel is formed.
High power generation efficiency can be realized. Furthermore, in the present invention, the pore volume is 0.17 to 0.22 cm.
^A more preferable form is ³ / g.

The pore-forming agent in the present invention has a diameter of 0.4 μm.
It is desirable that the fine fibrous substance is m or less. By adding such a fine fibrous substance to the catalyst paste as a pore-forming agent, the fibers become pillars and bear the load at the time of pressing, and carbon or solid polymer electrolyte is subjected to excessive compression load. Since the gas channel is not crushed and is held, the power generation efficiency is improved. Further, as an advantage of using such a fibrous substance, the porosity of the catalyst layer after the pressing step can be freely controlled by the addition amount of the fibrous substance.

Further, according to the present invention, since the catalyst layer is formed with the fibrous substance left in the formed catalyst layer,
It is possible to manufacture an electrode for a polymer electrolyte fuel cell that exhibits high power generation efficiency by a simplified process without requiring a pore forming agent removing process.

Examples of the above fibrous substances include inorganic fibers such as alumina whiskers and silica whiskers, carbon fibers such as vapor grown carbon (carbon whiskers), and polymer fibers such as nylon and polyimide. Among these, among these, A carbon whisker that is fine and has electronic conductivity is preferably used. The carbon whiskers are entangled with the catalyst substance that constitutes the catalyst layer and the electron conductive substance carrying the catalyst substance, so that a new conductive path is developed in addition to the conductive path due to the point contact of this electron conductive substance. The electron conductivity of the catalyst layer is improved. Further, since the carbon whiskers have electron conductivity, the catalyst substances can be supported on the surface of the carbon whiskers for the purpose of improving the area density of the catalyst substance in the electrode.

Further, the fibrous material according to the present invention preferably has a water-repellent property by itself or a surface of which is treated to be water-repellent. As described above, the fibrous substances are entangled with each other and are present in the catalyst layer, so that vacancies are easily generated, and the vacancies function as gas channels. In a fuel cell, steam is generated in the catalyst layer on the positive electrode (cathode) side as power is generated, and the steam is discharged to the outside of the system through a diffusion layer formed on the surface side of the catalyst layer. Here, when the water vapor condenses, water clogs the gas channel and remarkably lowers the fluidity of the gas.
Therefore, if the fibrous substance has water repellency or its surface is treated to be water repellent, dew condensation is prevented, pores, that is, gas channels are prevented from being closed, and gas permeability is secured.

Further, the fibrous substance according to the present invention has hydrophilicity on its own or has its surface hydrophilized from the viewpoint of preventing clogging of the gas channel due to dew condensation as described above. Good. According to this form,
When the water vapor generated by the power generation is condensed, the water spreads to the fibrous substance due to the capillary phenomenon and no droplets are generated. For this reason, the projected area of the water becomes small, and at the same time, the water moves to the dry portion, and the blockage of the gas channel is prevented. For example, on the downstream side of the gas channel, the humidity is high and dew condensation is likely to occur. However, dew condensation is prevented even in such a place, and the power generation performance is less likely to deteriorate. In addition, due to the capillary phenomenon, rapid movement of water from a place where water is excessive to a place where water is insufficient occurs, thereby eliminating spontaneous water shortage inside the electrode. as a result,
The effect of suppressing the occurrence of voltage fluctuations according to the amount of humidification is exhibited.

The content of the fibrous substance according to the present invention in the catalyst layer is preferably 5 to 25% by weight based on the total amount of the catalyst layer. The reason is that if the content is less than 5% by weight, each of the above-mentioned effects becomes difficult to be exhibited, while if it exceeds 25% by weight, the absolute amount of catalytic reaction sites per volume is reduced and the power generation efficiency is reduced. Because it invites.

In the present invention, as the electron conductive material,
For example, carbon black particles can be used, and a platinum group metal such as platinum or palladium can be used as the catalyst substance. Further, as the ion conductive material, a fluororesin ion exchange resin can be used.

The catalyst material dispersed in the ion conductive material is
It is desirable to be smaller than the catalyst material supported on the electron conducting material. That is, when the finer catalyst material is dispersed in the ion conductive material, the number of points at which the fuel gas is activated is increased and the utilization rate of the catalyst material is improved. The average particle size of the catalyst material dispersed in the ion conductive material is 0.
It is preferably 5 to 5 nm, and more preferably 1 to 3 nm. Further, the average particle size of the catalyst material supported on the electron conductive material is preferably 1 to 8 nm, and more preferably 3 to 5 nm.

The amount of the catalyst substance dispersed in the ion conductive material is preferably 1 to 80% by weight based on the total amount of the catalyst substance. When the amount of the catalyst substance is less than 1%, the activation overvoltage becomes high and the voltage that can be used decreases, and it becomes difficult to obtain the advantage over the case where the catalyst substance is covered only by the catalyst-supporting particles. Further, when the amount of the catalyst substance dispersed in the ion conductive substance exceeds 80% by weight, most of the catalyst substance is dispersed in the ion conductive substance, and in consideration of durability, the amount of the catalyst substance required for power generation is supported. Will be difficult. For example, when the catalyst substance is introduced only by replacement / reduction of the catalyst ion, the amount of the catalyst substance is determined by the ion exchange capacity of the ion conductive substance. Increasing the amount of substance can be mentioned. However, in the former case, the particle size of the catalyst substance grows, and in the latter case, there is a problem that the gas diffusivity in the electrode is lowered. Therefore, the amount of the catalytic material dispersed in the ion conductive material is 3 to 50% by weight of the total amount of the catalytic material.
Is more preferable, and 3 to 20% by weight is further preferable. Further, by increasing the amount of the catalyst substance supported on the electron conductive substance, the catalyst substance can be unevenly distributed on the contact surface between the ion conductive substance and the electron conductive substance and in the vicinity thereof, thereby increasing the utilization rate of the catalyst. You can Furthermore, an effective electron conduction network can be constructed by uniformly dispersing the catalyst substance in the ion conductive substance.

The weight ratio of the ion conductive material to the electron conductive material is preferably 1.2 or less. When the amount of ionic conductive material is small, the porosity increases and the gas diffusivity improves, but on the other hand, the platinum-supported electron conductive material is not sufficiently covered, and the point at which the fuel gas is activated decreases. The utilization rate of the catalytic substance is reduced.

The fuel cell electrode of the present invention can be manufactured as follows. First, an electron conductive material having a catalytic material supported on the surface or an electron conductive material having no catalytic material, an ion conductive material, and a pore-forming agent are mixed, and the mixture is treated with a solution containing the catalytic material to produce ions. Replace. For example, when the ion conductive material has a sulfone group,
The protons of the sulphonic groups are replaced by cations containing the catalytic material. Next, the mixture after ion substitution is exposed to a reducing atmosphere to obtain a catalyst paste in which fine catalyst substances are dispersed in an ion conductive substance.

The reduction method can be roughly divided into a gas phase method (dry method) using a reducing gas such as hydrogen and carbon monoxide, and a liquid phase method (wet method) using NaBH ₄ , formaldehyde, glucose, hydrazine and the like. . In the present invention, any reduction method can be adopted, but the liquid phase method is preferable. The reason is that in the reduction by the liquid phase method, all the catalytic metal ions in the ion conductive material are reduced, and the catalytic material is uniformly dispersed in the ion conductive material.

The catalyst paste may be formed into a sheet and then subjected to the above-mentioned ion substitution. Alternatively, after the catalyst paste is prepared, it is dried and solidified, pulverized, and subjected to ion substitution / reduction in a powdered state,
It can also be made into a paste and formed into a sheet. Also, ion substitution / reduction can be performed after the paste is prepared. In order to form a sheet, a known manufacturing method such as a method of applying the catalyst paste to a film that is to be peeled off after manufacturing the membrane electrode composite, or a method of applying the catalyst paste to carbon paper or an electrolytic membrane is used. You can

For ion substitution, when the catalyst metal is platinum, Pt (NH ₃ ) ₄ (OH) ₂ and Pt (NH ₃ ) ₄ C
A solution such as l ₂ or PtCl ₄ can be used. The catalytic metal ion to be ion-substituted may be a metal ion such as Pt ⁺ itself or a complex ion such as Pt (NH ₃ ) ₄ ²⁺ . The catalyst substance can be dispersed in the ion conductive substance without using ion substitution. For example, Pt (NH ₃ ) ₂ (NO ₂ ) ₂ and H ₂ P
It is also possible to prepare a catalyst-containing polymer electrolyte by thoroughly mixing tCl ₆ , H ₂ Pt (OH) _{6 and the} like with an ion conductive material and then reducing the catalyst metal ions. The catalyst metal ions include not only the catalyst metal ions but also ions containing a catalyst substance such as complex ions.

[0030]

Next, the present invention will be described in detail with reference to specific examples. <Samples 1 to 6> Ion conductive polymer solution (Nafio
n SE5112, manufactured by Dupont), and 10% [Pt (NO ₂ ) ₂ (N
H ₃ ) ₂ ] nitric acid aqueous solution 27.4 g, carbon particles (Ketjen Black EC) 5.0 g, and carbon whiskers (VGCF (registered trademark of Showa Denko KK), Showa Denko KK) 0 as a pore-forming agent After being mixed with 4.4 g (0 to 25% by weight in terms of solid content), an ethanol solution was added to this mixture as a reducing agent to precipitate Pt, and a catalyst paste in which the amount of carbon whiskers added was variously changed. Was prepared.

Next, the above catalyst paste was added to a tetrafluoroethylene-hexafluoropropylene copolymer (F
EP) sheet is applied and dried, and the catalyst layer thickness is 20 μm.
Of the electrode sheet. The platinum coating amount of this electrode sheet was 0.30 mg / cm ² . Next, the electrode sheet was decal-coated with a polymer electrolyte membrane (Nafion 112, D
(manufactured by Uppont) and transferred to both sides to obtain membrane electrode assemblies (MEA) of Samples 1 to 6 having a film thickness of 50 μm. The transfer by the decal method means that the FEP sheet is peeled after the electrode sheet is thermocompression-bonded to the polymer electrolyte membrane.

Amount of carbon whiskers added and porosity With respect to the catalyst layers of the electrode sheets of Samples 1 to 6 above, the porosity of the catalyst layers was examined by the mercury intrusion method. Further, the porosity of the catalyst layer was examined by the same method even after the pressing step was performed on these electrode sheets. The results are shown in FIG.

As is clear from FIG. 1, the porosity of the catalyst layer increases before and after the pressing step as the amount of carbon whiskers added to the catalyst layer increases.
Since the voids are formed by the entanglement of the added carbon whiskers, it is presumed that the more the carbon whiskers, the higher the porosity. Also, because the carbon whisker fibers become pillars to receive the load during pressing,
It was found that the porosity of the catalyst layer after the pressing step can be freely controlled within the range of 60 to 70% by the amount of carbon whiskers added.

Relationship between the addition amount of carbon whiskers and the differential pore volume In the catalyst layers of the electrode sheets of Samples 1 to 6 above, the addition amount of carbon whiskers was 0% by weight, 15% by weight in terms of solid content,
For 20% by weight, the change in differential pore volume was measured by the mercury penetration method. The result is shown in FIG.

As is clear from FIG. 2, by adding carbon whiskers, the volume of pores having a diameter of 60 nm or more, which are pores effective for reducing the concentration overvoltage, is significantly increased. Around 90nm diameter, 2
With 0% by weight, a large increase peak was observed around a diameter of 200 nm. As described above, it was found that it is possible to increase or decrease the number of pores having a desired diameter by changing the addition amount of carbon whiskers as a pore-forming agent.

-Relationship between the amount of carbon whiskers added and the pore volume With respect to the catalyst layers of the electrode sheets of Samples 1 to 6 above, pores having a diameter of 10 to 60 nm and pores having a diameter of 60 to 1000 nm were formed in the catalyst layer by the mercury intrusion method. The specific volume of the holes was investigated.
The results are shown in FIG.

As is apparent from FIG. 3, the diameter is 10 to 60.
The pore volume of nm is almost flat regardless of the increase in the amount of carbon whiskers added, whereas the pore volume of 60 to 1000 nm is significantly increased. This diameter 60-1000
nm holes are effective as gas channels,
It was found that the effective amount of gas channels can be controlled by increasing or decreasing the amount of carbon whiskers, which are pore-forming agents, added.

-Void volume of 60 to 1000 nm and terminal voltage Regarding the membrane electrode assemblies of Samples 1 to 6 above, hydrogen gas was supplied to one catalyst layer on the negative electrode side and the other catalyst on the positive electrode side. Air is supplied to the bed to generate electricity, and the current density is 1A
The terminal voltage with respect to the specific pore volume at / cm ² was measured.
Hydrogen gas has a temperature of 80 ° C, humidity of 25% RH, and utilization rate of 50.
Supplied in%. Air has a temperature of 80 ° C and a humidity of 45%.
It was supplied at RH and a utilization rate of 50%. The results are shown in Fig. 4.

As is apparent from FIG. 4, the current density is 1 A /
It was found that the terminal voltage at cm ² shows a high value when the pore specific volume of the pores having a diameter of 60 to 1000 nm is 0.15 to 0.25 cm ³ / g. Therefore, in the polymer electrolyte fuel cell electrode of the present invention, the pore volume of the catalyst layer having a diameter of 60 to 1000 nm is 0.15 to 0.
It has been found suitable to be 25 cm ³ / g.

[0040]

As described above, according to the polymer electrolyte fuel cell of the present invention, a catalyst substance, an ion conductive substance,
In the catalyst layer composed of the electron conductive substance and the pore former,
By using a fibrous substance as a pore-forming agent, for example, carbon whiskers, a diameter of 6 can be obtained without adding a complicated process.
0-1000 nm pore volume, 0.15-0.25c
With m ³ / g, the effect of realizing highly efficient power generation is achieved.

[Brief description of drawings]

FIG. 1 is a diagram showing the relationship between the amount of carbon whiskers added and the porosity of the catalyst layer in the polymer electrolyte fuel cell electrode of the present invention.

FIG. 2 is a diagram showing the relationship between the amount of carbon whiskers added and the differential pore volume in the polymer electrolyte fuel cell electrode of the present invention.

FIG. 3 is a diagram showing the relationship between the amount of carbon whiskers added and the specific pore volume of pores with a diameter of 10 to 60 nm and pores with a diameter of 60 to 1000 nm in the polymer electrolyte fuel cell electrode of the present invention. is there.

FIG. 4 is a diagram showing a relationship between a pore pore specific volume having a diameter of 60 to 1000 nm and a terminal voltage at a current density of 1 A / cm ² in the electrode for a polymer electrolyte fuel cell of the present invention.

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Katsuhiko Takayama             1-4-1 Chuo 1-4-1 Wako City, Saitama Prefecture             Inside Honda Research Laboratory F-term (reference) 5H018 AA06 AS02 AS03 BB03 BB06                       BB08 BB12 BB17 CC06 DD05                       EE03 EE05 EE08 EE11 EE17                       EE18 HH00 HH01 HH04                 5H026 AA06 CX02 EE05 HH01 HH02                       HH04

Claims (4)

[Claims] 1. A catalyst layer comprising a catalyst substance, an ion conductive substance, an electron conductive substance and a pore forming agent, wherein the pore volume of the catalyst layer having a diameter of 60 to 1000 nm is 0.15 to 0.15.
It is 0.25 cm < ³ > / g and the electrode for polymer electrolyte fuel cells characterized by the above-mentioned. 2. The pore volume is 0.17 to 0.22c.
The electrode for a polymer electrolyte fuel cell according to claim 1, wherein the electrode is m ³ / g. 3. The polymer electrolyte fuel cell electrode according to claim 1, wherein the pore-forming agent is a fine fibrous substance having a diameter of 0.4 μm or less. 4. The electrode for a polymer electrolyte fuel cell according to claim 3, wherein the fibrous material is carbon whiskers.