JP3326254B2 - Fuel cell - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fuel cell for generating electricity from a fuel gas containing hydrogen and a gas containing oxygen.

[0002]

2. Description of the Related Art A polymer electrolyte fuel cell usually comprises an electrolyte membrane made of a humidified solid polymer, and a fuel electrode and an air electrode provided with the electrolyte membrane interposed therebetween. The fuel electrode and the air electrode are provided with a catalyst layer for causing an oxygen-containing gas such as a fuel gas and air to undergo an electrode reaction, a supply path for the fuel gas and an oxygen-containing gas such as air to the catalyst layer, and a current collector. It has a two-layer structure with a gas diffusion layer functioning as a body. Hydrogen ions formed by the electrode reaction in the catalyst layer from the fuel gas move in the electrolyte membrane and become water by the electrode reaction by the catalyst on the air electrode side, so that current is output. (Hereinafter, simply refers to both the electrolyte of the electrolyte membrane and the catalyst layer in the "electrolyte", distinguished from the "electrolyte membrane", "electrolyte in the catalyst layer".) The typical catalyst layer,
It is produced by adding hydrophobic binder particles (fluororesin powder) to carbon particles (carbon black) carrying a catalyst (Pt or the like) in a highly dispersed state, and firing and molding. This catalyst is dispersed in a relatively thick (100 μm or more) catalyst layer, and the supply of the reaction gas to the catalyst surface is ensured by the pores formed by the hydrophobic particles. The contact interface is hardly taken into account when forming the catalyst layer.

[0003] When the above-mentioned catalyst layer is simply joined to the electrolyte by hot pressing or the like, the electric resistance at the interface is large, and the performance is greatly reduced as the amount of current increases. Therefore, improvement measures such as applying a solution in which a polymer electrolyte is dissolved to a catalyst layer before bonding the electrolyte membrane and the catalyst layer and bonding the same are devised (J. Power Source, 22 , 359).
(1988). However, the conventional catalyst layer made of a mixture of the catalyst-supporting carbon particles and the fluororesin powder is difficult to have a thickness of about 100 μm or less in view of securing air permeability and strength. It is practically difficult to apply the electrolyte all over to form an interface between the catalyst and the electrolyte in the catalyst layer. For this reason, there is a problem that the utilization rate of the catalyst is generally low even in a battery manufactured from an electrode coated with an electrolyte, and the battery performance tends to deteriorate in a high current region. Further, it is not an easy technique to form a thin coating of the electrolyte in the catalyst layer on the surface of the catalyst in the pores of the thick catalyst layer so as not to impair the gas diffusion characteristics. Therefore, it is not easy to produce a high-performance battery with a conventional electrode in which an electrolyte is applied to a catalyst layer, having a large low concentration polarization.

In view of the drawbacks of the conventional electrode, a completely new type of catalyst layer has recently been devised. That is,
The catalyst layer is a catalyst layer composed only of carbon hydrophobizing agent carrying an electrolyte and catalyst totally not comprise such as fluorine resin particles, the solvent is evaporated from the kneaded an electrolyte solution and the catalyst supporting carbon (J. Ele
trochem. Soc. Lett. L28 (199
2). Appl. Electrochem. 22 ,
1 (1992). In this catalyst layer, the size and state of the interface between the catalyst and the electrolyte can be controlled by adjusting the mixing ratio of the catalyst and the electrolyte in the catalyst layer and the molding conditions, and can be reduced to a thickness of about 15 μm or less. . Therefore, a high-performance battery having a higher catalyst utilization rate and lower concentration polarization than the conventional battery can be obtained.

[0005] However, the above-described improvement in battery performance by making the catalyst layer thinner has a problem that the effect tends to be reduced when the thickness of the catalyst layer is 10 to 15 µm or less. This is because the electrolyte layer that fills most of the gaps between the catalyst-supporting carbon particles in the catalyst layer is almost the only water and gas transport path, and the flooding phenomenon due to liquid water generated in the electrode occurs more than in the conventional catalyst layer. This is because it has the property of being easy. When the current density, that is, the water generation rate increases, the number of catalysts that do not contribute to the electrode reaction due to flooding increases, and the ratio of the deactivated catalyst increases as the catalyst layer becomes thinner.

[0006] As described above, in the current situation where flooding of the catalyst layer with liquid water is unavoidable in the conventional battery, the catalyst metal is simply reduced by simply thinning the layer uniformly filled with the catalyst.
It is not economical because it cannot be used effectively, and the characteristics of an ultrathin catalyst layer having a large interface between the catalyst and the electrolyte cannot be fully utilized.

[0007]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and has been developed in a polymer electrolyte fuel cell having a thin electrode catalyst layer composed of an electrolyte and a catalyst-supporting carbon. It is an object of the present invention to provide a fuel cell in which the cell performance is improved by suppressing a decrease in the catalyst utilization.

[0008]

The fuel cell of the present invention comprises an electrolyte membrane made of a solid polymer, a fuel electrode and an air electrode disposed on both sides of the electrolyte membrane, and the fuel electrode and the air electrode are In a fuel cell having a catalyst layer arranged in contact with the membrane surface of the electrolyte membrane , the catalyst layer comprises carbon particles and carbon
The catalyst metal and the electrolyte are composed of a catalyst metal supported on particles and an electrolyte, and the compounding ratio of the catalyst metal and the electrolyte to the carbon particles forming a portion of the catalyst layer opposite to the electrolyte membrane side is the same as that of the catalyst layer. It is characterized in that the mixing ratio of the catalyst metal and the solid electrolyte to the carbon particles forming the portion on the electrolyte membrane side is larger than that.

[0009] The fuel cell of the present invention is composed of an electrolyte membrane and a fuel electrode and an air electrode arranged on both sides of the electrolyte membrane made of solid polymer, respectively the air electrode and the fuel electrode electrolyte
Those having a catalyst layer disposed interview to the film surface of the film. As the electrolyte membrane, a polymer membrane exhibiting an electrolyte property that allows a charge carrier (H ⁺ ) to permeate can be used.

In the air electrode and the fuel electrode, a gas diffusion layer may be arranged on the surface of the catalyst layer opposite to the electrolyte membrane. The gas diffusion layer is formed of a porous material having high gas permeability and high electron conductivity, and uniformly supplies a gas containing oxygen such as fuel gas or air to the catalyst layer. The gas diffusion layer is usually formed by molding a mixture of carbon particles and hydrophobic particles.

The catalyst layer, which is another component of the fuel electrode and the air electrode, is composed of carbon particles, a catalyst metal supported on the carbon particles, and an electrolyte (electrolyte in the catalyst layer). The carbon particles constituting the catalyst layer carry electrons, and the electrolyte in the catalyst layer carries H ⁺ as a charge carrier. The catalytic metal fuel electrode to the hydrogen H ^+, catalytic metal of the air electrode is reacted with oxygen and H ^+. In other words, the catalyst layer contains carbon particles, a catalyst metal, and an electrolyte , and the carbon particles are the main constituent of the catalyst layer, and the abundance is large. The catalyst layer according to the present invention, the catalytic metal is supported on carbon particle surface, the carbon particle to further catalytic metal is supported
The surface of the cell is coated with an electrolyte . The area of the portion of the catalyst particles dispersed and supported on the carbon particles that is in contact with the electrolyte is referred to as the catalyst effective surface area. This catalyst effective surface area indicates the maximum surface area of the catalyst that can participate in the electrode reaction. The surface area of the catalyst that participates in the actual electrode reaction (eg, can be referred to as the effective surface area) is generally smaller than the effective surface area, and the ratio is related to the diffusion process of the reaction gas to the catalyst surface. Specifically, the internal pore structure of the catalyst layer, the amount of water generated by the reaction, and the thickness of the electrolyte covering the catalyst metal determine the diffusion rate of the reaction gas. The more porous the catalyst layer and the smaller the thickness of the electrolyte covering the catalyst metal, the closer the effective surface area becomes to the effective catalyst surface area.

The total surface area of the catalyst is calculated from the amount of saturated chemical (single-molecule) adsorption of CO at around room temperature of the catalyst-supporting carbon powder or molded body containing no electrolyte, or the average measured by observation with a transmission electron microscope. It can be determined from the particle size. In general, the results obtained by both methods are in good agreement. The effective surface area of the catalyst is determined by an electrochemical method (measurement of the hydrogen adsorption wave component of the electric double layer capacity, that is, supply of an inert gas (N ₂ ) to the catalyst layer, electrode potential (vs,
(Hydrogen electrode) can be determined by cyclic voltammetry (repetitive sweeping in the range of 0.06 to 1.4 V).

In the catalyst layer according to the present invention, the electrolysis of the catalyst layer
Carbon particles (if there is a gas diffusion layer the gas diffusion layer side) and the quality membrane side opposite the mixing ratio of the catalytic metal and the electrolyte to carbon particles forming a portion of which forms a portion of the electrolyte membrane side of the catalyst layer Is larger than the mixing ratio of the catalyst metal and the electrolyte . That is, the voltage of the catalyst layer
The catalyst effective surface area of the part on the side opposite to the decomposing membrane side is larger than the catalyst effective surface area of the part of the catalyst layer on the electrolyte membrane side.

The thickness of the catalyst layer is preferably about 10 μm or less from the viewpoint of diffusion of the introduced reaction gas.
As the thickness of the catalyst layer increases, it becomes more difficult to introduce a reaction gas, and the effective surface area decreases. Therefore, the thickness of the catalyst layer is about 10
and [mu] m, is set to electrodes 1 cm ² of about 400cm per ² area of catalytically active surface area in terms of battery performance low current region. Under these conditions, the average effective catalyst surface area in the catalyst layer needs to be about 40 cm ² or more per 1 cm ³ of the catalyst layer. on the other hand,
The loading amount of the catalyst-supporting carbon particles constituting the catalyst layer is 100% by weight of the total of the carbon particles and the catalyst metal from the viewpoint of the degree of catalyst dispersion.
(Hereinafter,% means weight% unless otherwise specified.)
In this case, the catalyst metal content is desirably 40% or less.

The following method can be adopted as means for providing a gradient of the effective surface area of the catalyst in the thickness direction of the catalyst layer. Specifically, several types of highly dispersed catalyst-supported carbon particles having different amounts of supported catalyst are formed. Each type of carbon particles is then mixed with a solution in which an electrolyte is dissolved. As a result, a mixed paste of the catalyst metal-carrying carbon particles and the solution of the electrolyte having different catalyst effective surface areas is obtained. By forming a thin layer of these solutions in the order of the effective catalyst surface area and laminating them, a catalyst layer having a gradient of the effective catalyst surface area can be obtained.

When the catalyst is supported at a high concentration, carbon particles
Since the dispersibility of the above catalyst is reduced, it is desirable to take some measures for increasing the dispersion. For example, using a high surface area carbon support (such as small particle size carbon black), expanding the surface area of the carbon support by a thermal oxidation activation method, etc., modifying the chemical properties of the carbon support surface (such as nitric acid oxidation treatment) to carbon particles Promotes adsorption or wetting of the catalyst or precursor. This is a countermeasure such as adding a third component before or after carrying the catalyst.

When carbon particles carrying 40% of platinum were mixed with an electrolyte solution to form a catalyst layer, it was found that the effective surface area of the catalyst in the layer was about 60 m ² per cm ³ of the catalyst layer. Therefore, the slope of the effective surface area of the catalyst is reduced to a thickness of 1
When the catalyst layer is provided linearly in the 0 μm catalyst layer, a layer of about 60 m ² is provided on one side end of the catalyst layer and a layer of about 10 m ^{2 is provided} on the opposite side end, and the catalyst effective surface area is sequentially reduced between the layers. Can be formed by overlapping. By the way, 10% of catalyst
When the highly dispersed and supported carbon particles are mixed with an electrolyte solution and molded, a layer having a catalyst effective surface area of about 10 m ² per 1 cm ³ can be formed.

However, what gradient is actually provided in the effective surface area of the catalyst depends on the gas that affects the drainage speed from the electrode.
It is desirable to determine in consideration of the performance of the diffusion layer and battery operating conditions. The change in the concentration of the reaction gas in the catalyst layer generally tends to be high on the gas diffusion layer side and low on the electrolyte membrane side due to the reaction and diffusion resistance in the catalyst layer. Also, the amount of liquid water that suppresses gas diffusion is large on the electrolyte membrane side . In the catalyst layer according to the present invention, when the reaction gas concentration is high in the high current density region, the catalyst effective area per unit volume is increased, and when the reaction gas concentration is decreased, the catalyst effective area per unit volume is conversely increased. The configuration is such that the area is reduced.

For example, in the catalyst layer of the air electrode, the catalyst effective surface area on the side in contact with the gas diffusion layer for introducing gas into the catalyst layer is increased, and the catalyst effective surface area is reduced toward the electrolyte membrane. The catalyst layers having different catalyst effective surface areas are sequentially stacked so as to form a slope to form a catalyst layer. As a result, the amount of the catalyst affected by the generated liquid water at the air electrode is reduced, and the electrode reaction with the reaction gas introduced into the catalyst layer can proceed efficiently.
Therefore, a decrease in output voltage in a high current density region can be minimized.

[0020]

In the fuel cell of the present invention, the effective area of the catalyst in the catalyst layer is controlled so as to reduce the amount of the catalyst in the portion where the stagnation of liquid water is unavoidable without changing the total amount of the catalyst in the catalyst layer.
This allows most of the catalyst to contribute to the electrode reaction. That is, in the conventional fuel cell, all the catalysts contribute to the reaction at a low current density, but when the current density increases, the liquid water in the catalyst layer becomes excessive and the utilization rate of the catalyst generally decreases. However, according to the present invention, since the reduction in the catalyst utilization rate is minimized, the electrode reaction can be suppressed and reduced even in the high current region without changing the performance in the low current region. For this reason, it is possible to maintain a high voltage output in a wide current range. As a result, the catalyst utilization rate is improved, and the catalyst can be saved.

Particularly on the air electrode side, it is effective to increase the effective surface area of the catalyst from the electrolyte membrane side to the gas diffusion layer side.

[0022]

The present invention will be specifically described below with reference to examples. Preparation of catalyst layer A, B, and A shown in Table 1 by impregnation method, colloid dispersion method, etc.
Platinum-supported carbon powders having different loading amounts of the four types of platinum catalysts C and D were prepared. The four kinds of platinum-supported carbon powders were mixed with an alcohol solution in which the electrolyte (trade name: NAFION) was dissolved in an amount shown in Table 1 to prepare four kinds of mixed pastes having different effective surface areas per unit volume in almost equal amounts. .

Next, an electrode gas diffusion layer made of a porous material prepared by applying a mixture of carbon particles and hydrophobic particles to a carbon cloth is prepared, and the paste is applied to one side of the electrode gas diffusion layer in the order of the size of the catalyst effective surface area (D). ← C ← B ← A) and laminated in layers. The effective surface area of the catalyst is 98 c per unit area of the electrode.
m ² , C is 79 cm ² , B is 46 cm ² , A is 27 cm
^In FIG. ² , it becomes smaller in order from the gas diffusion layer. The overall catalytic effective surface area is 250 cm ² . At the time of this lamination, the evaporation amount of the solvent (such as alcohol) in the paste at the time of forming each layer was adjusted with respect to the drying time and the external conditions (temperature and atmosphere) so that only the interface portions of each layer were mixed.

The formation of the catalyst layer can be carried out by hand such as brushing, but it is desirable to carry out the method by spraying or screen printing for forming a doctor plate from the viewpoint of ensuring uniformity. Further, in order to form the gradient of the catalyst layer more continuously, a sedimentation transfer method using a specific gravity difference of the catalyst-supporting carbon particles or a spray coating method while continuously changing the paste composition is desirable.

[0025]

[Table 1] {Whole area ABCD} ────────────────────────────────% Pt / C 10% 20% 30% 40% − Pt + C (mg ) 0.40 0.30 0.40 0.45 1.55 Pt (mg) 0.04 0.06 0.12 0.18 0.40 ───────────────────────────────────添加 Electrolyte NAFION Addition and mixing from solution to Pt / C powder in each area (mg) 0.02 0.03 0.06 0.09 0.20 Catalyst layer thickness (μm) 2 2 2 2 8 ─────────────── ───────────────────── Pt effective surface (cm ² ) 27 46 79 98 250 ───────────────── ────────────────── Finally, the anode (catalyst layer has no slope) with the electrolyte membrane separately prepared and the cathode (contact (Media layer + gas diffusion layer), sandwiched between the catalyst layers, fixed in a press jig, and applied a pressure of 30 to 150 kg / cm ² (at this stage, a small amount of solvent still exists in the electrode catalyst layer). The battery is preferably hot-pressed at a temperature as high as possible (120 to 150 ° C.) for a short time (within 15 minutes) within a range where deterioration of the electrolyte membrane does not occur.

FIG. 1 schematically shows the state of lamination of the catalyst layer on the air electrode side formed by the above method. (Comparative Example) As Comparative Example, the same battery as that of the example was prepared except that four types of catalyst layers having no gradient of the effective surface area of Pt were used in the catalyst layers shown in Table 2. Except for Comparative Example 4, the effective surface area of Pt was larger than 250 cm ² (overall) of the example.

The relationship between the current density at normal pressure (H ₂ / Air) of 70 ° C. and the battery output voltage of the five batteries of this example and Comparative Examples 1 to 4 was examined. The results are shown in FIG. Comparative Examples 1 to
In No. 4, the battery performance was more governed by the amount of Pt-supported carbon, that is, the thickness of the catalyst layer, than the Pt effective surface area, and the decrease in output voltage due to an increase in current density was caused by Comparative Examples 1 → 2 → 3 → 4
The order is getting better. This is because the gas permeability decreases as the catalyst layer becomes thicker, and the catalyst sinks into liquid water and is easily deactivated.

[0028]

[Table 2] 触媒 Catalyst layer Pt effective table Pt / C powder ( mg) Pt (mg) Electrolyte (mg) Thickness μm Area (cm ² ) ────────────────────────────────比較 Comparative Example 1 10% Pt / C (4.0) 0.4 NAFION (0.2) 24 270 Comparative Example 2 20% Pt / C (2.0) 0.4 NAFION (0.2) 11 307 Comparative Example 3 30% Pt / C (1.3 ) 0.4 NAFION (0.2) 6 262 Comparative Example 4 40% Pt / C (1.0) 0.4 NAFION (0.2) 4 218 ────────────────────────で は In this example, the catalyst layer is comparatively high in Comparative Example 3 as a whole.
Have a thickness (Pt / C amount) and a platinum effective surface area substantially equal to In addition, unlike the comparative example, the effect of the gradient of the Pt effective surface area in the catalyst layer allows the reaction to proceed at a high gas concentration and keeps the catalyst utilization high, so that it exhibits higher performance than comparative example 4 in a wide current range. Was.

[0029]

According to the structure of the catalyst layer of the present invention, the catalyst effective surface area in contact with the electrolyte is increased at a place where the reaction gas concentration where the catalyst utilization is high and the reaction gas concentration is high, and the catalyst effective surface area is sequentially reduced. Even in a region where the current density is high, it is possible to minimize a decrease in the catalyst utilization rate. Therefore, the battery performance at a high current density can be remarkably improved as compared with a conventional battery having the same catalyst amount. In addition, this battery improves the catalyst utilization rate, so that the catalyst used can be saved.

[Brief description of the drawings]

FIG. 1 is a schematic diagram of a catalyst layer (divided into four regions) on the air electrode side of the present embodiment in which a catalyst effective surface area is inclined.

FIG. 2 is a graph showing a relationship between a battery output voltage and a current density of a battery.

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Takanao Suzuki 41-cho, Yokomichi, Nagakute-cho, Aichi-gun, Aichi Prefecture Inside Toyota Central R & D Laboratories Co., Ltd. 41, Yokomichi, Toyota Central Research Institute, Inc. (72) Inventor Katsuji Abe 41, Yoji, Chukuji, Nagakute-cho, Aichi-gun, Aichi Prefecture 1 within Toyota Central Research Institute, Inc. (72) Inventor Tatsuya Kawahara Toyota, Aichi Prefecture 1 Toyota Town Inside Toyota Motor Corporation (56) References JP-A-5-251086 (JP, A) JP-A-4-233164 (JP, A) JP-A-62-195855 (JP, A) (58) ) Fields surveyed (Int.Cl. ⁷ , DB name) H01M 4/86-4/98 H01M 8/00-8/24 JICST file (JOIS)

Claims (1)

(57) [Claims] 1. A becomes more a fuel electrode and an air electrode arranged on both sides of the electrolyte membrane and the electrolyte membrane made of solid polymer, the fuel electrode and the air Kikyoku are arranged interview to the film surface of the electrolyte membrane in the fuel cell having the catalyst layer, the catalyst layer is composed of a catalyst metal supported on carbon particles and carbon particles and the electrolyte, opposite to the electrolyte membrane side <br/> of the catalyst layer Wherein the mixing ratio of the catalyst metal and the electrolyte to the carbon particles forming the portion of the catalyst layer is larger than the mixing ratio of the catalyst metal and the electrolyte to the carbon particles forming the portion of the catalyst layer on the electrolyte membrane side. And the fuel cell.