JP2009021049A - Cathode catalyst layer for fuel cell and fuel cell having it - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a cathode catalyst layer for fuel cell capable of suppressing the deterioration of power generation performance, and a fuel cell having the same. <P>SOLUTION: The cathode catalyst layer (12) has a catalyst and a carbon carrier to carry the catalyst, and the weight of the catalyst with respect to the weight of the carbon carrier is over 60% and the thickness of the layer is 10 μm or smaller. The catalyst layer can supply sufficient moisture to an electrolyte membrane (11), while suppressing appropriately the evaporation amount of water from the cathode catalyst layer (12). Further, since a generation reaction place of water is moved to near the electrolyte membrane (11) of the cathode catalyst layer (12), flooding can be suppressed. Thereby, the deterioration of the power generation performance of the fuel cell (100) can be suppressed. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a cathode catalyst layer for a fuel cell and a fuel cell including the same.

  A fuel cell is a device that generally obtains electric energy using hydrogen and oxygen as fuel. Since this fuel cell is excellent in terms of the environment and can realize high energy efficiency, it has been widely developed as a future energy supply system. In particular, since the polymer electrolyte fuel cell operates at a relatively low temperature among various types of fuel cells, it has a good startability. For this reason, research has been actively conducted for practical application in various fields.

  Generally, a polymer electrolyte fuel cell includes a membrane-electrode assembly (MEA) in which a cathode and an anode are provided on both sides of an electrolyte membrane made of a solid polymer electrolyte having proton conductivity. I have. When the water content of the electrolyte membrane decreases, the ionic conductivity of the electrolyte membrane decreases. Therefore, it is preferable that moisture is appropriately supplied to the electrolyte membrane.

  Therefore, it is conceivable to improve the generation density of water during power generation. For example, in order to improve the generation density of water, a technique for improving the activity of the catalyst by increasing the concentration of the catalyst in the electrode can be used (see, for example, Patent Document 1). If the concentration of the catalyst at the cathode is increased using this technique, the water generation density is improved.

JP 2005-108838 A

  However, even if the catalyst concentration at the cathode is increased, the water content in the electrolyte membrane decreases if the amount of water evaporated from the cathode is large. As a result, power generation performance may be reduced. On the other hand, when the amount of water evaporation from the cathode is small, flooding may occur at the cathode. Even in this case, the power generation performance may be reduced.

  An object of this invention is to provide the cathode catalyst layer for fuel cells which can suppress a power generation performance fall, and a fuel cell provided with the same.

  The cathode catalyst layer for a fuel cell according to the present invention comprises a catalyst and a carbon carrier supporting the catalyst, wherein the weight of the catalyst with respect to the weight of the carbon carrier is more than 60%, and the layer thickness is 10 μm or less. It is what. In the cathode catalyst layer for a fuel cell according to the present invention, water can be sufficiently supplied to the electrolyte membrane of the fuel cell while moderately suppressing the evaporation amount of water. Further, since the water generation reaction field moves to the vicinity of the electrolyte membrane of the cathode catalyst layer, flooding can be suppressed. From the above, it is possible to suppress a decrease in power generation performance of a fuel cell including the cathode catalyst layer according to the present invention. The catalyst may be platinum or a platinum alloy.

  The fuel cell according to the present invention is a solid polymer electrolyte membrane having proton conductivity, an anode catalyst layer disposed on one surface of the electrolyte membrane, and disposed on the other surface of the electrolyte membrane. Or a catalyst layer for a fuel cell as described in 2 above. In the fuel cell according to the present invention, water can be sufficiently supplied to the electrolyte membrane while appropriately suppressing the amount of water evaporation. Further, since the water generation reaction field moves to the vicinity of the electrolyte membrane of the cathode catalyst layer, flooding can be suppressed. From the above, it is possible to suppress a decrease in power generation performance of the fuel cell according to the present invention.

  According to the present invention, a decrease in power generation performance can be suppressed.

  Hereinafter, the best mode for carrying out the present invention will be described.

(Embodiment)
First, an outline of a fuel cell 100 according to an embodiment of the present invention will be described. FIG. 1 is a schematic cross-sectional view of the fuel cell 100. As shown in FIG. 1, in the fuel cell 100, an oxidant gas flow path 20 and a separator 30 are sequentially provided on one surface side of the membrane-electrode assembly 10, and a fuel gas flow path 40 and a separator 50 are sequentially provided on the other surface side. It has a provided structure.

  In the membrane-electrode assembly 10, the cathode catalyst layer 12 and the gas diffusion layer 13 are sequentially provided on the separator 30 side of the electrolyte membrane 11, and the anode catalyst layer 14 and the gas diffusion layer 15 are sequentially provided on the separator 50 side of the electrolyte membrane 11. Have a structured. The electrolyte membrane 11 is made of a solid polymer electrolyte having proton conductivity, for example, a perfluorosulfonic acid type polymer.

  The cathode catalyst layer 12 is a catalyst layer that promotes the reaction between protons and oxygen. The anode catalyst layer 14 is a catalyst layer that promotes protonation of hydrogen. The cathode catalyst layer 12 and the anode catalyst layer 14 are made of catalyst-supporting carbon. Details of the cathode catalyst layer 12 will be described later. The gas diffusion layer 13 is a layer that transmits an oxidant gas (for example, air) containing oxygen. The gas diffusion layer 15 is a layer that transmits a fuel gas containing hydrogen. The gas diffusion layers 13 and 15 are made of, for example, carbon paper, carbon cloth, or the like.

  The oxidant gas flow path 20 and the fuel gas flow path 40 are porous flow paths made of, for example, foam sintered metal. Separator 30,50 consists of conductors, such as stainless steel. The separators 30 and 50 may be provided with grooves without using the oxidant gas flow path 20 and the fuel gas flow path 40, and may be used as gas flow paths.

  Fuel gas is supplied to the fuel gas flow path 40 from a fuel gas supply means (not shown). The fuel gas passes through the gas diffusion layer 15 and reaches the anode catalyst layer 14. Hydrogen contained in the fuel gas is dissociated into protons and electrons via the catalyst of the anode catalyst layer 14. Protons are conducted through the electrolyte membrane 11 and reach the cathode catalyst layer 12.

  On the other hand, the oxidizing gas channel 20 is supplied with an oxidizing gas from an oxidizing gas supply means (not shown). The oxidant gas passes through the gas diffusion layer 13 and reaches the cathode catalyst layer 12. In the cathode catalyst layer 12, protons and oxygen react through the catalyst. Thereby, power generation is performed and moisture is generated.

  Next, details of the cathode catalyst layer 12 will be described. The cathode catalyst layer 12 has a structure in which a catalyst such as platinum or a platinum alloy is supported on a carbon support. Elements that can be used in the platinum alloy are, for example, Ti, Cr, Mn, Fe, Co, Ni, Cu, Ga, Zr, Hf, Ru, Ir, Pd, Os, Au, Ag, or a mixture thereof. In the present embodiment, the power generation performance of the fuel cell 100 is reduced by adjusting the weight of the catalyst relative to the weight of the carbon support (hereinafter referred to as catalyst loading density (%)) and the thickness of the cathode catalyst layer 12. Is suppressed. Hereinafter, the principle will be described with reference to FIG.

  First, as shown in FIG. 2A, when the catalyst support density in the cathode catalyst layer is relatively small, the water generation density is relatively small. In this case, there is a possibility that sufficient moisture cannot be supplied to the electrolyte membrane. Therefore, there is a possibility that the water content in the electrolyte membrane is insufficient.

  Therefore, as shown in FIG. 2B, it is conceivable to set the catalyst carrying density high. In this case, the catalyst is sufficiently present in the vicinity of the electrolyte membrane of the cathode catalyst layer. Thereby, the water generation reaction field moves to the vicinity of the electrolyte membrane of the cathode catalyst layer. As a result, the distance between the generated water and the electrolyte membrane is shortened. In addition, the water concentration in the water production reaction field is larger than the water concentration in the electrolyte membrane. As a result, water easily moves to the electrolyte membrane. However, on the gas diffusion layer side of the cathode catalyst layer, the amount of water produced decreases because the amount of water produced decreases. Therefore, there is a possibility that water cannot be sufficiently supplied to the electrolyte membrane. As a result, power generation performance may be reduced.

  On the other hand, as shown in FIG. 2C, it is conceivable to set the thickness of the cathode catalyst layer small. In this case, since the water generation reaction field is the entire cathode catalyst layer, the amount of water evaporated from the cathode catalyst layer is reduced, and flooding may occur in the cathode catalyst layer. Thereby, supply of the reaction gas to the cathode catalyst layer is suppressed. As a result, power generation performance may be reduced.

  Therefore, in the present embodiment, as shown in FIG. 2D, the catalyst support density in the cathode catalyst layer 12 is set to a value exceeding 60%, and the layer thickness of the cathode catalyst layer 12 is set to 10 μm or less. To do. In this case, it is possible to sufficiently supply water to the electrolyte membrane 11 while appropriately suppressing the evaporation amount of water from the cathode catalyst layer 12. Moreover, since the water generation reaction field moves to the vicinity of the electrolyte membrane 11 of the cathode catalyst layer 12, flooding can be suppressed, and a decrease in the diffusibility of the reaction gas to the cathode catalyst layer 12 can be suppressed. From the above, a decrease in power generation performance of the fuel cell 100 can be suppressed.

  Note that the higher the catalyst support density in the cathode catalyst layer 12, the better. The upper limit of the cathode catalyst layer 12 is not particularly limited, but is about 70%, for example. Further, the lower limit of the thickness of the cathode catalyst layer 12 is not particularly limited, but is, for example, about 1 μm. This is because the required layer thickness is 1 μm when platinum black of 100% Pt is used for the cathode catalyst layer 12.

The cathode catalyst layer 12 preferably has a small basis weight. This is because when the basis weight decreases, the amount of water produced per unit volume of the cathode catalyst layer 12 increases, and the water generation reaction field in the cathode catalyst layer 12 becomes closer to the electrolyte membrane 11. Here, the basis weight means the amount (mg / cm ² ) of the catalyst per unit area of the cathode catalyst layer 12.

  In the present embodiment, the cathode catalyst layer 12 corresponds to the cathode catalyst layer for a fuel cell according to the present invention.

  Hereinafter, the fuel cell according to the above embodiment was manufactured, and the characteristics were examined.

Example 1
In Example 1, the fuel cell 100 of FIG. 1 was produced. The electrolyte membrane 11 is made of a perfluorosulfonic acid polymer material and has a thickness of 15 μm to 45 μm. Platinum-supported carbon was used as the cathode catalyst layer 12. In the cathode catalyst layer 12, the catalyst support density was 70%, the layer thickness was 10 μm, and the basis weight was 0.5 mg / cm ² . The anode catalyst layer 14 was made of platinum-supported carbon and had a layer thickness of 5 μm. As the gas diffusion layers 13 and 15, those made of carbon paper were used.

(Example 2)
In Example 2, the same one as in Example 1 was used except that the basis weight was 0.4 mg / cm ² .

(Comparative example)
In the comparative example, the same thing as Example 1 was used except the layer thickness of the cathode catalyst layer 12 having been 15 micrometers and the catalyst carrying density having been 60%.

(analysis)
The fuel cells according to Examples 1 and 2 and the comparative example were caused to generate power, and the cell voltage and cell resistance were measured. A fuel gas having a dew point Dp of 45 ° C., a back pressure of 40 kPaG, and a stoichiometry of 1.2 was supplied to the fuel gas channel 40. An oxidant gas having a dew point Dp of 55 ° C. and a back pressure of 45 kPaG was supplied to the oxidant gas flow path 20. The stoichiometry of the oxidant gas was changed within a predetermined range. The cell temperature was set to 80 ° C., and the current density was set to 2.0 A / cm ² . The results are shown in FIGS. 3 (a) to 3 (c).

  FIG. 3A is a diagram showing the relationship between the oxidant gas stoichiometry and the cell voltage. In FIG. 3A, the horizontal axis represents the oxidant gas stoichiometry, and the vertical axis represents the cell voltage of each fuel cell. In FIG. 3A, the cell voltage of each fuel cell is normalized based on the cell voltage of the fuel cell according to the comparative example. As shown in FIG. 3A, the cell voltages of the fuel cells according to Examples 1 and 2 were larger than the cell voltage of the fuel cell according to the comparative example. This is presumably because the decrease in the water content of the electrolyte membrane 11 was suppressed in the fuel cell according to the example. In addition, since the difference was not produced in the cell voltage of the fuel cell which concerns on Example 1, 2, it turned out that there is no influence on a cell voltage even if the catalyst amount of a catalyst differs.

  FIG. 3B is a diagram showing the relationship between the oxidant gas stoichiometry and the cell resistance. In FIG. 3B, the horizontal axis indicates the oxidant gas stoichiometry, and the vertical axis indicates the cell resistance of each fuel cell. In FIG. 3B, the cell resistance of each fuel cell is normalized based on the cell resistance of the fuel cell according to the comparative example. Here, as the oxidant gas stoichiometry increases, the oxidant gas flow rate increases. Therefore, the larger the oxidant gas stoichiometry, the easier the electrolyte membrane 11 is dried. As a result, the resistance of the electrolyte membrane 11 may increase.

  As shown in FIG. 3B, the cell resistances of the fuel cells according to Examples 1 and 2 were smaller than the cell resistance of the fuel cell according to the comparative example. This is presumably because the decrease in the water content of the electrolyte membrane 11 was suppressed. In addition, compared with Example 1, in Example 2, cell resistance became still smaller. This is considered to be because the water generation reaction field becomes closer to the electrolyte membrane 11 due to the decrease in the basis weight, and the amount of water supply to the electrolyte membrane 11 increases.

  FIG. 3C is a diagram showing the relationship between the oxidant gas stoichiometry and the resistance correction voltage. In FIG.3 (c), a horizontal axis shows oxidant gas stoichiometry and a vertical axis | shaft shows the resistance correction voltage of each fuel cell. In FIG. 3C, the resistance correction voltage of each fuel cell is normalized based on the resistance correction voltage of the fuel cell according to the comparative example. The resistance correction voltage means cell voltage + current × cell resistance. Therefore, in a high current density region, the higher the resistance correction voltage, the better the gas diffusibility generally.

  As shown in FIG. 3C, the resistance correction voltages of the fuel cells according to Examples 1 and 2 were larger than the resistance correction voltage of the fuel cell according to the comparative example. This is presumably because in the fuel cells according to Examples 1 and 2, the decrease in the water content of the electrolyte membrane 11 was suppressed, the flooding in the cathode catalyst layer 12 was suppressed, and the decrease in gas diffusivity was suppressed. In addition, since the difference was not produced in the resistance correction voltage of the fuel cell which concerns on Example 1, 2, it turned out that there is no influence on the diffusivity of a reactive gas even if the basis weight of a catalyst differs.

  As described above, by setting the catalyst supporting density in the cathode catalyst layer 12 to a value exceeding 60% and setting the layer thickness of the cathode catalyst layer 12 to 10 μm or less, it is possible to suppress a decrease in power generation performance of the fuel cell. It was.

1 is a schematic cross-sectional view of a fuel cell according to an embodiment of the present invention. It is a figure for demonstrating the principle which suppresses the power generation performance fall of a fuel cell. It is a figure which shows the electric power generation voltage and cell resistance of each fuel cell.

Explanation of symbols

DESCRIPTION OF SYMBOLS 11 Electrolyte membrane 12 Cathode catalyst layer 13,15 Gas diffusion layer 14 Anode catalyst layer 20 Oxidant gas flow path 30, 50 Separator 40 Fuel gas flow path 100 Fuel cell

Claims (3)

A catalyst,
A carbon support for supporting the catalyst,
The weight of the catalyst with respect to the weight of the carbon support exceeds 60%,
A cathode catalyst layer for a fuel cell, wherein the layer thickness is 10 μm or less.   The cathode catalyst layer for a fuel cell according to claim 1, wherein the catalyst is platinum or a platinum alloy. A solid polymer electrolyte membrane having proton conductivity;
An anode catalyst layer disposed on one surface of the electrolyte membrane;
A fuel cell comprising: a cathode catalyst layer for a fuel cell according to claim 1 or 2 disposed on the other surface of the electrolyte membrane.

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