JP2002289201A - Fuel cell - Google Patents

Abstract

PROBLEM TO BE SOLVED: To resolve a problem that a conventional polymer electrolyte fuel cell has problem of flooding and drying up conflicting with each other. SOLUTION: A catalyst layer is made of a noble metal catalyst supported by conductive carbon particles or conductive carbon fibers without having a hydrophilic functional group on the surface, a hydrogen ion conductive polymer electrolyte, and metal oxide particles.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fuel cell using a polymer electrolyte, and more particularly to an electrode thereof.

[0002]

2. Description of the Related Art First, a general structure of a conventional polymer electrolyte fuel cell will be described. A fuel cell using a polymer electrolyte generates electricity and heat simultaneously by electrochemically reacting a fuel gas containing hydrogen and an oxidizing gas containing oxygen such as air.

FIG. 1 is a schematic sectional view for explaining a partial structure of a unit cell of a solid polymer electrolyte fuel cell.
As shown in FIG. 1, on both surfaces of a polymer electrolyte membrane 11 for selectively transporting hydrogen ions, a carbon powder carrying a platinum-based metal catalyst is used as a catalyst, and a hydrogen ion-conductive polymer electrolyte is mixed therewith. Thus, the catalyst layer 12 is formed.

At present, as the polymer electrolyte membrane 11, a polymer electrolyte membrane made of perfluorocarbon sulfonic acid (for example, Nafion membrane manufactured by DuPont, USA) is generally used. The gas diffusion layer 13 is formed on the outer surface of the catalyst layer 12 using, for example, a water-repellent carbon paper having both air permeability and electron conductivity. The catalyst layer 12 and the gas diffusion layer 13 are collectively called an electrode 14.

Next, a gas sealing material or a gasket is disposed around the electrode with a polymer electrolyte membrane interposed therebetween so that the supplied fuel gas and the oxidizing gas do not leak outside or the two types of gases do not mix with each other. I do. The sealing material and the gasket are integrated with the electrode and the polymer electrolyte membrane in advance and assembled, and this is referred to as MEA (electrolyte membrane electrode assembly) 15.

As shown in FIG. 2, a conductive separator plate 21 for mechanically fixing the MEA 15 is disposed outside the MEA 15. FIG. 2 is a schematic cross-sectional view for explaining the structure of a unit cell of the solid polymer electrolyte fuel cell. A gas flow path 22 for supplying a reaction gas to the electrode surface and carrying away generated gas and surplus gas is formed in a portion of the separator plate 21 that contacts the MEA 15. Although the gas flow path can be provided separately from the separator plate, a method is generally used in which a groove is provided on the surface of the separator to form a gas flow path.
As described above, the MEA 15 is fixed by the pair of separator plates 21, the fuel gas is supplied to one gas flow path, and the oxidant gas is supplied to the other gas flow path. Of electromotive force can be generated.

A cell in which the MEA is fixed by a pair of separators is referred to as a unit cell 23. However, normally, when a fuel cell is used as a power source, a voltage of several volts to several hundred volts is required. Therefore, in practice, the required number of cells 23 are connected in series. At this time, the gas flow paths 22 are formed on both sides of the separator 21 and the separator / MEA / separator / MEA is repeated to form a series connection configuration.

In order to supply gas to the gas flow path, a pipe jig for branching a gas supply pipe into a number corresponding to the number of separators to be used, and connecting the branch directly to a separator-like groove. Is required. This jig is referred to as a manifold, and in particular, the type directly connected from the fuel gas supply pipe as described above is referred to as an external manifold. There is a type of this manifold called an internal manifold which has a simpler structure. The internal manifold is provided with a through hole in the separator plate on which the gas flow path is formed,
The entrance and exit of the gas flow path are passed to this hole, and the gas is supplied directly to the gas flow path from this hole.

Next, the gas diffusion layer and the catalyst layer constituting the electrode of the fuel cell will be described. The gas diffusion layer must have the following three functions. The first function is to supply a fuel gas or an oxidizing gas to the catalyst particles in the catalyst layer from a gas channel formed on the outer surface of the gas diffusion layer. The second is a function of quickly discharging water generated in the catalyst layer to the gas flow path. The third is a function of conducting electrons required for the reaction or generated electrons. That is, the gas diffusion layer requires high reaction gas permeability, water vapor permeability, and electron conductivity.

[0010] As a conventional technique, in order to impart gas permeability, a conductive porous substrate such as carbon fine powder, pore-forming material, carbon paper and carbon cloth having a developed structure is used for a gas diffusion layer. Is being done. Further, in order to impart water vapor permeability, a water-repellent polymer such as a fluororesin is dispersed in a gas diffusion layer. Further, in order to provide electron conductivity, a gas diffusion layer is made of an electron conductive material such as carbon fiber, metal fiber, and carbon fine powder.

[0011] The catalyst layer must have the following four functions. The first function is to supply the fuel particles or the oxidizing gas supplied from the gas diffusion layer to the catalyst particles. The second function is to quickly transfer hydrogen ions required for the reaction on the catalyst or generated hydrogen ions to the electrolyte membrane. The third is a function of conducting electrons required for the reaction or generated electrons. Fourth, high catalytic performance for prompt reaction and a wide reaction area. That is, the catalyst layer needs to have high reaction gas permeability, hydrogen ion permeability, electron conductivity and catalytic performance.

As a conventional technique, a carbon fine powder or a pore former having a well-developed structure is used for a catalyst layer in order to impart gas permeability. Further, in order to provide hydrogen ion permeability, a polymer electrolyte is dispersed in the vicinity of a catalyst in a catalyst layer to form a hydrogen ion network. Further, in order to impart electron conductivity, a catalyst carrier is made of an electron conductive material such as carbon fine powder or carbon fiber. Furthermore, in order to improve the catalyst performance, a highly reactive metal catalyst represented by platinum is supported on carbon fine powder as very fine particles having a particle size of several nm, and is highly dispersed in the catalyst layer. That is being done.

[0013]

The wettability of the catalyst layer and the gas diffusion layer increases with the operation time of the fuel cell. When the wettability increases, a so-called flooding phenomenon occurs in which the pores used as the reaction gas supply passages in the catalyst layer and the gas diffusion layer are blocked by generated water and moisture in the reaction gas. For this reason, there is a problem that the reaction gas is not sufficiently supplied and the battery performance is reduced.

One of the factors causing this problem is the hydrophilicity of the carbon particles supporting the noble metal catalyst. That is, the carbon particles for supporting the catalyst particles used for the catalyst layer of the fuel cell are:
Those having a large specific surface area are generally used. However, currently used carbon with a large specific surface area,
In the manufacturing process, hydrophilic functional groups such as -OH and -COOH remain on the surface. For this reason, the carbon particles are made hydrophilic.

On the other hand, when the humidity of the supply gas is lowered or when the operation of the battery is stopped for a long time, the conductivity of the hydrogen ions in the electrodes decreases because the polymer electrolyte and the electrolyte membrane in the electrodes dry. However, there is a problem that battery performance is reduced. The cause of such wetting and drying of the electrode is due to insufficient water retention and humidity control of the conventionally used electrode.
Until now, the following efforts have been made to solve this problem. For example, JP-A-10-334922 discloses that a catalyst layer contains a water retention agent composed of sulfuric acid or phosphoric acid, and JP-A-2000-251910, JP-A-2001-15137, and JP-T-98-052.
No. 242 discloses a configuration in which a sheet-like water-absorbing material such as nylon, cotton, polyester / rayon, polyester / acryl, rayon / polychlor, etc., which covers the surface of a conductive plate outside an electrode, is disposed. Also,
Japanese Patent Application Laid-Open No. 2000-340247 discloses a configuration in which a moisture-permeable waterproof sheet layer using a water-absorbing polymer gel such as sodium polyacrylate, which is in contact with a catalyst layer of a negative electrode, is disposed.

However, in the above-mentioned conventional method, since the humidity is adjusted from outside the catalyst layer, the effect of adjusting the humidity in details inside the catalyst layer and the diffusion layer is small. Further, there is a problem that sufficient humidity control performance cannot be obtained due to a problem that sulfuric acid and phosphoric acid are scattered. Furthermore, in the case of using sulfuric acid or phosphoric acid, the problem of corrosion of constituent members such as arrangement of a stack or a fuel cell system due to their scattering is inevitable.

Therefore, the present invention solves the above-mentioned conventional problems, and simultaneously solves the above-mentioned conflicting problems of flooding and dry-up by optimizing the humidity control properties of the catalyst layer and the gas diffusion layer. An object of the present invention is to provide an electrode for a fuel cell exhibiting performance and a polymer electrolyte fuel cell using the electrode.

[0018]

In order to solve the above-mentioned problems, a fuel cell according to the present invention comprises a hydrogen ion conductive polymer electrolyte membrane and a pair of hydrogen ion conductive polymer electrolyte membranes disposed on both sides of the hydrogen ion conductive polymer electrolyte membrane. And a pair of conductive separators that form a gas supply groove for supplying and discharging hydrogen-containing fuel gas to one of the electrodes and supplying and discharging an oxidizing gas to the other of the electrodes. In the fuel cell sandwiching the unit cell, the electrode has a catalyst layer in contact with the hydrogen ion conductive polymer electrolyte membrane, and a gas diffusion layer in contact with the catalyst layer and the conductive separator, The catalyst layer comprises a conductive carbon particle having no hydrophilic functional group on the surface or a noble metal catalyst supported on conductive carbon fibers, a hydrogen ion conductive polymer electrolyte, and metal oxide particles. Fuel cell.

The metal oxide is preferably an oxide of at least one metal selected from silicon, titanium, aluminum, zirconium, magnesium and chromium. At this time, the content of the metal oxide is desirably 10 to 1000 ppm by weight.

By optimizing the humidity control performance of the electrode with the above-mentioned metal oxide, the deterioration with time can be suppressed.
It is possible to provide a polymer electrolyte fuel cell, a liquid fuel cell, and an electrode exhibiting higher performance while maintaining high gas diffusion ability and hydrogen ion conductivity over a long period of time.

[0021]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention is characterized in that noble metal particles are supported on conductive carbon particles having no hydrophilic functional group on the surface as a catalyst, and a moisturizing component comprising a metal oxide is used as an electrode. It is to arrange in. The electrode includes a polymer electrolyte membrane, an anode and a cathode sandwiching the polymer electrolyte membrane, an anode-side conductive separator plate having a gas flow path for supplying a fuel gas to the anode, and an oxidant gas supplied to the cathode. In particular, a polymer electrolyte fuel cell including a cathode-side conductive separator plate having a gas flow path exhibiting excellent characteristics.

As a method for producing conductive carbon particles having no hydrophilic functional group on the surface, a conventional graphite production method can be used. In this method, the higher the firing temperature, the higher the degree of graphitization, and in proportion to this, the number of hydrophilic functional marks such as -OH and -COOH remaining on the surface decreases. However, since the surface area decreases as the firing temperature increases, it must be finely ground in nitrogen gas after firing. In addition, a conductive carbon fiber can be produced by using a chemical vapor deposition method (CVD) without leaving -OH or -COOH but leaving -CF3 groups having strong water repellency.

On the other hand, the effect of arranging a moisturizing component composed of a metal oxide on an electrode is different from that of a conventional method using a polymer water-absorbing sheet. Because of the dispersion, more microscopic humidity control can be performed. Further, in the present invention, since the metal oxide is used for the particles having the moisturizing property, conventionally used nylon, cotton, polyester / rayon, polyester /
It is possible to make finer particles than polymers such as acrylic and rayon / polyclar and sodium polyacrylate.

For example, it can be placed very close to the polymer electrolyte in contact with the catalyst requiring humidification,
Therefore, drying of the polymer electrolyte can be prevented.
In addition, since it is possible to dispose it in the electrode functioning as a flow path of the reaction gas very close to the micropores, it is possible to absorb generated water or excessive humidification water and prevent the gas flow path from being blocked. As described above, it is optimal for realizing the above-mentioned micro humidity control.

Also, since very chemically stable substances such as silica, titania, alumina, zirconia, magnesia and chromium oxide are employed as the above-mentioned metal oxide, the solvent used in the electrode or MEA coating is not used. Little effect,
Unlike the case where a polymer is used, it does not dissolve in a solvent. Also, in the heat treatment for removing the solvent and the surfactant, it is necessary to set the temperature at which the polymer does not deteriorate or decompose.
There is no problem even if the treatment is performed at a high temperature of not less than ° C. Therefore, according to the present invention, there are few restrictions on the selection of conditions in these various manufacturing processes.

[0026] By such an effect, the carbon fine powder is gradually wetted by the water vapor or the generated water added to the reaction gas in order to impregnate the polymer electrolyte. As a result, dew is condensed in the pores of the catalyst layer and gas diffusion layer, closing the gas flow path,
The phenomenon of reducing the gas supply capacity and the generated water discharge capacity is suppressed, and high gas permeability and generated water discharge capacity can be obtained. On the other hand, the polymer electrolyte or the electrolyte membrane in the electrode is humidified by the moisture retained by the metal oxide species even under conditions such as lowering the humidity of the supply gas or supplying the gas without humidification, and stopping for a long time. In addition, a decrease in hydrogen ion conductivity is suppressed. As a result, higher cell performance is exhibited over a long period of time, and the life characteristics of the fuel cell are improved.

Next, the electrode of the present invention comprises a gas diffusion layer and a catalyst layer, and contains the above-mentioned metal oxide in at least one layer. Generally, the gas diffusion layer is obtained by dispersing carbon powder in a conductive porous substrate such as carbon paper. Further, the catalyst layer is obtained by applying a paste composed of a polymer electrolyte and a catalyst to the polymer electrolyte membrane or the gas diffusion layer.

Hereinafter, an electrode according to the present invention and a fuel cell including the same will be described with reference to examples.

[0029]

Example 1 First, acetylene black (denka black, manufactured by Denki Kagaku Kogyo Co., Ltd.)
Particle diameter 35 nm) with polytetrafluoroethylene (PT
FE) aqueous dispersion (Daikin Industries, D
1) to prepare a water-repellent ink containing 20% by weight of PTFE as a dry weight. This ink is applied to a carbon paper (TGPH060, manufactured by Toray Co., Ltd.) as a base material of the gas diffusion layer
H) on top of, impregnated with hot air dryer at 300 ° C
To form a gas diffusion layer.

Next, graphite particles, which are conductive carbon particles having substantially no hydrophilic functional groups on the surface, were prepared. The production was performed using a phenol resin as a raw material and a final firing temperature of 2200 ° C. After baking, the powder was pulverized in a nitrogen atmosphere using a zirconia mortar to make the average particle size 220 nm. When the -OH groups and -COOH groups remaining on the surface of the graphite particles were quantified, the number of oxygen was about 0.003 per 6 carbons constituting the graphite.

Platinum fine particles (average particle size 30
66% by weight of a catalyst supporting 50% by weight of Å), and 33 parts by weight (as solid content) of a perfluorocarbon sulfonic acid ionomer (a 5% by weight Nafion dispersion manufactured by Aldrich, USA) as a hydrogen ion conductor. A catalyst ink was formed by mixing 100 ppm of zirconia (ZrO ₂ ). This catalyst ink was applied to one side of the gas diffusion layer and dried to form a catalyst layer on the gas diffusion layer.

The gas diffusion layer on which the catalyst layer obtained as described above was formed was joined to both surfaces of a polymer electrolyte membrane (Napion 112, manufactured by DuPont, USA) to obtain an ME having the structure shown in FIG.
A was prepared.

A gasket plate made of rubber was joined to the outer peripheral portion of the polymer electrolyte membrane of the MEA manufactured as described above, and a manifold hole for circulation of cooling water, fuel gas and oxidizing gas was formed.

A conductive separator plate made of a phenol resin-impregnated graphite plate having an outer dimension of 20 cm × 32 cm × 1.3 mm and having a gas channel and a cooling water channel having a depth of 0.5 mm was prepared. . Using two of these separator plates, a unit cell was obtained by laminating a separator plate having an oxidizing gas channel formed on one surface of the MEA and a separator plate having a fuel gas channel formed on the back surface.

Two unit cells were stacked and sandwiched with a separator plate formed with a cooling channel groove so that the groove was positioned on the MEA side to obtain a two-cell stacked battery. This pattern was repeated to obtain a 100-cell stacked battery. (Stack) was prepared.
At this time, a current collector plate made of stainless steel, an insulating plate made of an electrically insulating material and an end plate were arranged at both ends of the stack, and were fixed with fastening rods. The fastening pressure at this time was 15 kgf / cm ² per area of the separator. This battery was designated as battery X1 of the example.

Comparative Example 1 In Example 1, as a conductive carbon particle carrying platinum particles, graphite subjected to a predetermined process was used, and a predetermined amount of zirconia was further mixed in the catalyst layer. On the other hand, when a catalyst layer is formed as a comparative example, the hydrophilic functional groups -OH and -COO are formed on the surface.
Using Ketjen black (AKZO Chemie, the Netherlands, average primary particle diameter: 30 nm), which is conductive carbon particles in which H remains, one without the addition of zirconia as a water retention agent was prepared. When -OH and -COOH remaining on the surface of the Ketjen Black used at this time were quantified, the number of oxygen was about 0.3 with respect to 6 carbons of the Ketjen Black.

Except for this, a battery Y1 of a comparative example was prepared using the same materials and the same method as in Example 1.

(Evaluation Test) With respect to the battery X1 of the embodiment and the battery Y1 of the comparative example, pure hydrogen gas was supplied to the fuel electrode and air was supplied to the air electrode. Uf) was set to 70%, air utilization rate (Uo) was set to 40%, and gas humidification was performed by passing a fuel gas through a bubbler at 70 ° C. and air through a bubbler at 50 ° C. to perform a discharge test of the battery. The result is shown in FIG.

FIG. 3 is a graph showing the relationship between the operating time and the battery voltage per unit cell. 0.8 A /
cm ² , the initial voltage of the battery is 52 for the battery X1.
The battery Y1 was 5 mV and the battery Y1 was 572 mV, and the battery Y1 of the comparative example was superior in initial characteristics. However, after operation 150
With respect to the voltage after 0 hours, the characteristics were reversed with X1 being 478 mV and Y1 being 445 mV, and it was confirmed that the batteries of the examples had better durability performance than the batteries of the comparative examples.

[0040]

As described above, a catalyst comprising a noble metal catalyst supported on conductive carbon particles or conductive carbon fibers having no hydrophilic functional group on the surface, a hydrogen ion conductive polymer electrolyte and metal oxide particles. By forming the layers, a fuel cell having excellent durability was obtained.

[Brief description of the drawings]

FIG. 1 is a schematic sectional view showing a configuration of an MEA which is a component of a polymer electrolyte fuel cell according to an embodiment of the present invention.

FIG. 2 is a schematic cross-sectional view showing a configuration of a unit cell which is a component of a polymer electrolyte fuel cell according to an embodiment of the present invention.

FIG. 3 is a graph showing voltage-operation time characteristics of the batteries of the first embodiment and the comparative example of the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 11 Polymer electrolyte membrane 12 Catalyst layer 13 Gas diffusion layer 14 Electrode 15 MEA 21 Separator plate 22 Gas flow path 23 Single cell

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Junji Arakura 1006 Odomo Kadoma, Kadoma City, Osaka Prefecture Matsushita Electric Industrial Co., Ltd. F term (reference) 5H018 AA06 AS01 BB05 BB06 BB08 BB12 CC06 DD05 DD08 EE03 EE06 EE08 EE12 EE18 EE19 5H026 AA06 CC03 CX05 EE02 EE05 EE12 EE18 5H027 AA06

Claims (1)

[Claims] 1. A unit cell comprising a hydrogen ion conductive polymer electrolyte membrane and a pair of electrodes arranged on both sides of the hydrogen ion conductive polymer electrolyte membrane, wherein one of the electrodes contains hydrogen. In a fuel cell in which the unit cell is sandwiched between a pair of conductive separators that supply and discharge a fuel gas and form a gas supply groove for supplying and discharging an oxidizing gas to the other of the electrodes, the electrode includes the hydrogen ion A catalyst layer in contact with the conductive polymer electrolyte membrane, and a gas diffusion layer in contact with the catalyst layer and the conductive separator, wherein the catalyst layer has conductive carbon particles having no hydrophilic functional group on the surface or A fuel cell comprising: a noble metal catalyst supported on conductive carbon fibers; a hydrogen ion conductive polymer electrolyte; and metal oxide particles.