JP2011238443A - Fuel cell - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve fuel cell performance by varying a ratio of elements contained in an electrode depending on portions while suppressing increase in usage of a catalyst metal.SOLUTION: A fuel cell comprises an electrolyte membrane, and an anode and a cathode which are a pair of porous electrodes disposed on both surfaces of the electrolyte membrane and mixed with carbon particles, a catalyst metal supported on the carbon particles, and a polymer electrolyte. At least one of the pair of the electrodes includes a plurality of layers arranged along a plane direction of the electrolyte membrane. In the plurality of the layers, layers closer to a contact surface with the electrolyte membrane is formed so that a ratio in weight of the catalyst metal against weight of the carbon particles is small and a ratio in weight of the polymer electrolyte against weight of the carbon particles is large.

Description

  The present invention relates to a fuel cell.

  In general, an electrode of a polymer electrolyte fuel cell is formed by mixing carbon carrying a catalyst metal and a polymer electrolyte. Conventionally, in order to improve the performance of the fuel cell, various configurations have been proposed as the electrode structure of the fuel cell. As a configuration of such an electrode, an electrode in which the content ratio of each component in the electrode is varied depending on the site is known. For example, in an electrode for a fuel cell, an electrode has been proposed in which the content ratio of the polymer electrolyte is gradually decreased and the content ratio of the metal catalyst-supported carbon particles is gradually increased as the distance from the electrolyte membrane is increased (for example, cited reference). 1).

JP 2006-185800 A Japanese Patent Application Laid-Open No. 07-134996

  As a method for enhancing the performance and durability of a fuel cell during power generation, it is known that it is useful to increase the amount of catalytic metal contained in a fuel cell electrode per unit area. However, simply increasing the amount of catalytic metal to improve the performance of the fuel cell is accompanied by a significant increase in the manufacturing cost of the fuel cell and is difficult to employ. Although it is a useful method for improving the performance of the electrode to vary the content ratio of each component in the electrode, it can further improve the performance of the fuel cell while suppressing an increase in the amount of catalyst metal used. An electrode was sought.

  The present invention has been made in order to solve the above-described conventional problems, and by suppressing the increase in the amount of catalyst metal used, the content ratio of each component in the electrode varies depending on the site, thereby improving the performance of the fuel cell. It aims at improving.

  SUMMARY An advantage of some aspects of the invention is to solve at least a part of the problems described above, and the invention can be implemented as the following forms or application examples.

[Application Example 1]
A fuel cell,
An electrolyte membrane;
An anode and a cathode, which are a pair of porous electrodes provided on both surfaces of the electrolyte membrane, in which carbon particles, a catalyst metal supported on the carbon particles, and a polymer electrolyte are mixed;
With
At least one of the pair of electrodes includes a plurality of layers provided along the surface direction of the electrolyte membrane, and in the plurality of layers, the closer to the contact surface with the electrolyte membrane, the more the carbon particles The fuel cell is formed such that a ratio of the weight of the catalyst metal to a weight of the catalyst metal is small and a ratio of the weight of the polymer electrolyte to the weight of the carbon particles is large.

  According to the fuel cell described in Application Example 1, in at least one of the electrodes, of the plurality of layers provided along the surface direction of the electrolyte membrane, the layer closer to the contact surface of the electrolyte membrane corresponds to the weight of the carbon particles. Since the ratio of the weight of the catalyst metal is small and the ratio of the weight of the polymer electrolyte to the weight of the carbon particles is large, the current-voltage characteristics of the fuel cell are improved, or the fuel cell The durability of can be improved.

  The present invention can be realized in various forms other than those described above. For example, the present invention can be realized in forms such as a fuel cell electrode and a method for manufacturing a fuel cell electrode.

2 is a schematic cross-sectional view illustrating a schematic configuration of a single cell 10. FIG. It is a cross-sectional schematic diagram which expands and shows the structure of MEA30. FIG. 5 is a process diagram illustrating an outline of a method for manufacturing the cathode 22. It is explanatory drawing showing the result of having compared the current-voltage characteristic. FIG. 5 is an explanatory diagram showing cathode conditions in each fuel cell shown in FIG. 4. It is explanatory drawing which shows the result of having compared the durability of a fuel cell.

A. Fuel cell configuration:
FIG. 1 is a schematic cross-sectional view showing a schematic configuration of a single cell 10 constituting a fuel cell as a preferred embodiment of the present invention. The unit cell 10 includes an electrolyte membrane 20, an anode 21 and a cathode 22 that are electrodes formed on each surface of the electrolyte membrane 20, and a gas diffusion layer 23 that sandwiches the electrolyte membrane 20 on which the electrode is formed from both sides, 24 and gas separators 25 and 26 disposed further outside the gas diffusion layers 23 and 24.

  The fuel cell of this example is a solid polymer type fuel cell, and the electrolyte membrane 20 is a proton conductive ion exchange membrane formed of a solid polymer material, for example, a fluororesin containing perfluorocarbon sulfonic acid. Good electrical conductivity when wet. The anode 21 and the cathode 22 include platinum (Pt) as a catalyst metal. More specifically, the anode 21 and the cathode 22 include carbon particles supporting the catalyst metal and a polymer electrolyte of the same type as the polymer electrolyte that constitutes the electrolyte membrane 20. The electrolyte membrane 20, the anode 21, and the cathode 22 constitute an MEA (membrane-electrode assembly) 30. Note that the fuel cell of this example is characterized by the structure of the cathode 22 composed of two layers having different composition ratios of the catalyst metal, the carbon particles, and the polymer electrolyte. This will be described in detail later.

  The gas diffusion layers 23 and 24 can be formed of a conductive member having gas permeability, such as carbon paper or carbon cloth, metal mesh, or foam metal. The gas diffusion layers 23 and 24 of the present embodiment are both formed as flat plate members. Such a gas diffusion layer 24 serves as a flow path for a gas used for an electrochemical reaction and collects current.

  A water repellent layer including a water repellent material may be provided between the anode 21 and the gas diffusion layer 23 or between the cathode 22 and the gas diffusion layer 24. The water repellent layer is, for example, a water repellent layer ink that is a mixed liquid containing carbon particles and a water repellent substance such as a fluororesin, and the conductive member constituting the gas diffusion layers 23 and 24, or the surface of the MEA. It can form by apply | coating to, performing drying and baking. By providing such a water-repellent layer, it becomes possible to improve battery performance by promoting drainage from the electrode to the gas diffusion layer or suppressing drying of the electrolyte membrane.

  The gas separators 25 and 26 are formed of a gas-impermeable conductive member, for example, a metal member made of compressed carbon, stainless steel, or the like. Each of the gas separators 25 and 26 has a predetermined uneven shape. Due to this uneven shape, an in-cell fuel gas channel 47 through which a fuel gas containing hydrogen flows is formed between the gas separator 25 and the gas diffusion layer 23. In addition, due to the uneven shape, an in-single cell oxidizing gas channel 48 through which an oxidizing gas containing oxygen flows is formed between the gas separator 26 and the gas diffusion layer 24.

  Further, a sealing member such as a gasket is disposed on the outer peripheral portion of the single cell 10 in order to ensure gas sealing performance in the single-cell fuel gas flow channel 47 and the single-cell oxidizing gas flow channel 48 (FIG. Not shown). In addition, the fuel cell of this embodiment has a stack structure in which a plurality of single cells 10 are stacked. The outer periphery of the stack structure is parallel to the stacking direction of the single cells 10 and is fuel gas or oxidation. A plurality of gas manifolds through which gas flows are provided (not shown). The fuel gas flowing through the fuel gas supply manifold among the plurality of gas manifolds is distributed to each single cell 10 and passes through each single cell fuel gas flow channel 47 while being subjected to an electrochemical reaction. Collect in the gas exhaust manifold. Similarly, the oxidizing gas flowing through the oxidizing gas supply manifold is distributed to each single cell 10 and passes through each single cell oxidizing gas flow path 48 while being subjected to an electrochemical reaction, and then gathers in the oxidizing gas discharge manifold. To do.

B. Electrode configuration:
FIG. 2 is an enlarged schematic cross-sectional view showing the configuration of the MEA 30 in the fuel cell of the present embodiment. That is, FIG. 2 is an explanatory diagram showing an enlargement of a region A surrounded by a broken line in FIG. In the present embodiment, the cathode 22 is provided along the surface direction of the electrolyte membrane 20, specifically, the membrane side layer 22 a and the diffusion layer side layer 22 b provided in parallel to the surface direction of the electrolyte membrane 20. It has. As described above, the constituent ratios of the catalyst metal, the carbon particles, and the polymer electrolyte are different between the membrane side layer 22a and the diffusion layer side layer 22b. More specifically, the ratio of the weight of the catalyst metal to the weight of the carbon particles (hereinafter referred to as Pt / C ratio) and the ratio of the weight of the polymer electrolyte to the weight of the carbon particles (hereinafter referred to as N / C ratio). ) Is different. The film side layer 22a is formed to have a smaller Pt / C ratio and a larger N / C ratio than the diffusion layer side layer 22b. In the present embodiment, the membrane-side layer 22a has a weight ratio of 0: 1: 1.5 for the catalyst metal (Pt), carbon particles, and polymer electrolyte, and the diffusion-layer-side layer 22b The weight ratio of Pt), carbon particles, and polymer electrolyte is 1: 1: 1.

  FIG. 3 is a process diagram showing an outline of a method for manufacturing the cathode 22 of this embodiment. When the cathode 22 is manufactured, first, a first catalyst-supporting carbon for forming the membrane-side layer 22a is prepared, and a second catalyst-supporting carbon for forming the diffusion layer-side layer 22b is prepared. (Step S100). As described above, the first catalyst-supported carbon has a weight ratio of 0: 1 to the catalyst metal (Pt) and the carbon particles, and the second catalyst-supported carbon has a weight of the catalyst metal (Pt) and the carbon particles. The ratio is 1: 1. That is, in this embodiment, the first catalyst-carrying carbon is carbon particles that do not actually carry the catalyst metal (Pt). Below, the manufacturing method of the 2nd catalyst carrying | support carbon particle which actually carry | supports a catalyst metal (Pt) is demonstrated.

  When preparing the catalyst-supporting carbon, first, carbon particles (carbon powder) are prepared. Here, various carbon particles can be selected, and for example, carbon black or graphite can be used. As such carbon particles, for example, carbon particles having an average particle diameter (average particle diameter of primary particles which is the minimum unit of carbon particles) of 10 to 50 nm can be used. Next, Pt, which is a catalyst metal, is supported in the liquid on the prepared carbon particles. In order to support Pt, the carbon particles may be dispersed in a solution of a Pt compound and an impregnation method, a coprecipitation method, or an ion exchange method may be performed. As the Pt compound solution, for example, a tetraammine platinum salt solution, a dinitrodiammine platinum solution, a platinum nitrate solution, or a chloroplatinic acid solution can be used. At this time, the amount of carbon particles introduced into the liquid and Pt so that the ratio between the weight of the carbon particles and the weight of the supported Pt is a predetermined ratio according to the first or second catalyst-supporting carbon, The solution concentration of the compound and the solution amount of the Pt compound are adjusted. Hereinafter, the ratio of the weight of the supported catalyst metal (Pt) to the weight of the carbon particles (Pt / C ratio) is also referred to as the catalyst loading ratio. When carbon particles are dispersed in a Pt compound solution and Pt is supported on the carbon particles, this is then dried and fired. As a result, carbon particles carrying and supporting Pt fine particles are obtained. For example, in the case of the impregnation method, after carbon particles are dispersed in a solution containing a predetermined amount of Pt, the solvent is evaporated and dried, and a reduction treatment (firing in a reducing atmosphere) is performed. .

  Once the first and second catalyst-carrying carbons are produced, next, the first and second catalyst inks are produced using the first and second catalyst-carrying carbons (step S110). The catalyst ink is an electrolyte solution (for example, Nafion solution, manufactured by Aldrich) containing the above-described first or second catalyst-supported carbon dispersed in appropriate water and an organic solvent and containing an electrolyte having proton conductivity. Are further mixed. As described above, if the average particle diameter of the primary particles of the catalyst-supporting carbon used is 10 to 50 nm, the secondary particles of the catalyst-supporting carbon in the electrode ink (catalyst dispersion) (primary particles are aggregated to form. The average particle size of the particles) can be about 0.1 to 0.5 μm.

  Next, the second catalyst ink produced in step S110 is applied on the substrate (step S120). As the substrate, for example, a film made of polyethylene terephthalate (PET), polytetrafluoroethylene (PTFE), polyethylene naphthalate (PEN) or polyethylene (PE) can be used. The application of the second catalyst ink onto the substrate can be performed by, for example, gravure printing, spraying method, screen printing, doctor blade method, or ink jet method. According to gravure printing, screen printing, or ink-jet method, pattern application for applying catalyst ink to a desired electrode shape can be easily performed, and waste generated in catalyst ink as a material for application can be suppressed. The ratio of the amount of solvent in the catalyst ink used for coating may be appropriately adjusted depending on the coating method and solvent type to be employed. In step S120, after the second catalyst ink is applied on the substrate, the applied second catalyst ink is also dried. The temperature at which the second catalyst ink is dried may be a temperature at which the constituent material of the catalyst ink, such as a polymer electrolyte, does not deteriorate. For example, the temperature may be from room temperature (about 30 ° C.) to 100 ° C.

Next, the first catalyst ink is further applied on the layer of the second catalyst ink applied on the substrate (step S130). As the method of applying the first catalyst ink, various methods can be used as in the case of the second catalyst ink. In this example, the weight of the carbon particles contained per unit area in the layer formed by applying the second catalyst ink and the layer formed by applying the first catalyst ink are the same. The ink was applied. When the first and second catalyst inks are applied onto the substrate in this way, the weight of the catalyst metal (Pt) contained per unit area in the entire layer made of the catalyst ink (hereinafter referred to as catalyst support). For example) can be 0.035 to 0.5 mg / cm ² .

  When the first catalyst ink is applied, the substrate and the electrolyte membrane on which the catalyst ink is applied so that the membrane made of the solid polymer electrolyte to be the electrolyte membrane 20 and the application surface of the first catalyst ink are in contact with each other. And a layer of the catalyst ink is transferred onto the electrolyte membrane 20 by transfer accompanied by pressurization and heating (step S140). Thereafter, the substrate is peeled from the transferred catalyst ink layer (step S150), thereby completing the cathode 22 provided on the electrolyte membrane 20. By evaporating the solvent in the catalyst ink by heating, a porous cathode 22 having fine pores formed therein can be produced.

  The anode 21 can also be produced in the same manner as the cathode 22. That is, a catalyst ink for forming the anode 21 may be applied on a substrate similar to the substrate used for producing the cathode 22 and transferred onto the electrolyte membrane 20. However, unlike the cathode 22, the anode 21 of the present embodiment is configured by a single layer having a uniform Pt / C ratio and N / C ratio. The transfer of the anode 21 onto the electrolyte membrane 20 may be performed simultaneously with the transfer of the cathode 22 or may be performed separately from the transfer of the cathode 22. In this way, the MEA 30 can be manufactured by bonding the cathode 22 and the anode 21 on the electrolyte membrane 20.

  When assembling the fuel cell, a conductive porous material such as carbon cloth is laminated on each electrode surface of the MEA 30 as the gas diffusion layers 23, 24, and the gas separator 25, 26 may be further laminated to produce the single cell 10. By stacking a plurality of such single cells 10, a fuel cell having a stack structure can be assembled.

  According to the fuel cell of the present embodiment configured as described above, the membrane side layer 22a has a smaller Pt / C ratio and a larger N / C ratio than the diffusion layer side layer 22b. Since the cathode 22 is formed, the performance of the fuel cell can be improved while suppressing an increase in the amount of catalyst metal used (amount of catalyst supported). That is, the cathode 22 is formed of two layers having different Pt / C ratio and N / C ratio as described above, thereby improving the current-voltage characteristics of the fuel cell and improving the durability of the fuel cell. Effect is obtained. Hereinafter, individual effects obtained by the fuel cell of this embodiment will be described in detail.

C. Current-voltage characteristics:
When the cathode 22 is formed such that the N / C ratio of the membrane side layer 22a is larger than the N / C ratio of the diffusion layer side layer 22b, the contact resistance (proton) between the cathode 22 and the electrolyte membrane 20 is increased. Current-voltage characteristics can be improved. That is, even when the current density at the output from the fuel cell is increased, a higher voltage can be maintained.

Here, the effect of increasing the N / C ratio on the film side in the electrode will be further described. FIG. 4 shows a fuel cell including a cathode having a different N / C ratio on the membrane side and the diffusion layer side as in the embodiment (large N / C membrane side) and a cathode having a uniform N / C ratio. It is explanatory drawing showing the result of having compared the current-voltage characteristic using a fuel cell (N / C flat). These fuel cells have the same configuration as the fuel cell of the embodiment except for the N / C ratio or the Pt / C ratio. FIG. 5 is an explanatory diagram showing cathode conditions in each fuel cell shown in FIG. As shown in FIG. 5, in the cathode included in the “N / C membrane side large” fuel cell, the N / C ratio in the membrane side layer is 0.875, and the N / C ratio in the diffusion layer side layer is 0.8. 625. Further, the N / C ratio is 0.75 in the cathode included in the “N / C flat” fuel cell. However, in the “N / C membrane side large” fuel cell shown in FIGS. 4 and 5, unlike the fuel cell of the example, the catalyst loading rate (Pt / C ratio) is uniform over the entire cathode. That is, in both the “N / C membrane side large” fuel cell and the “N / C flat” fuel cell, the Pt / C ratio is fixed to 50 wt% for the entire cathode. In both the “N / C membrane side large” fuel cell and the “N / C flat” fuel cell, the amount of catalyst supported is 0.4 mg / cm ² . In both the N / C membrane side large fuel cell and the N / C flat fuel cell, the anode has a constant N / C ratio of 1 and a Pt / C ratio of 60 wt%. The catalyst loading is 0.2 mg / cm ² .

  As shown in FIG. 4, by setting the cathode to “N / C membrane side large”, in the case of a lower current density and a case of a higher current density than a fuel cell having an “N / C flat” cathode. Both showed that the voltage could be kept higher. When the N / C ratio is increased in the entire cathode, the proton transfer efficiency in the entire cathode is also increased. However, in the catalyst metal supported on the carbon particles, the ratio covered with the polymer electrolyte increases and the thickness covered with the polymer electrolyte increases. For example, when the current density is increased, the concentration overvoltage increases. The output voltage may be insufficient. Battery performance by reducing the contact resistance between the cathode and the electrolyte membrane by suppressing the disadvantage caused by the increase in the average value of N / C in the entire cathode by making the cathode “large on the N / C membrane side” Improvement effect is obtained.

D. Electrode durability:
When the cathode 22 is formed such that the Pt / C ratio of the membrane side layer 22a is smaller than the Pt / C ratio of the diffusion layer side layer 22b, elution of Pt in the cathode into the electrolyte membrane is suppressed. As a result, the durability of the fuel cell can be improved. That is, when the fuel cell is used for a long time, the performance degradation of the fuel cell due to the dissolution of Pt in the cathode into the electrolyte membrane can be suppressed.

  Here, in the fuel cell, it is known that the amount of catalyst metal in the electrode, particularly the amount of catalyst metal in the cathode, gradually decreases and the cell performance deteriorates during repeated power generation. Such a phenomenon is considered to be because the catalytic metal is eluted into the electrolyte membrane, and the effective surface area, which is the area of the catalytic metal where the electrochemical reaction proceeds, gradually decreases. Below, the result of investigating the relationship between the degree of battery performance degradation due to elution of catalyst metal and the Pt / C ratio at the cathode is shown.

FIG. 6 shows the result of comparing the durability of a plurality of fuel cells provided with a cathode having a uniform structure and having different Pt / C ratios and catalyst loadings (Pt loadings per unit area). It is explanatory drawing. Sample (1) shown in FIG. 6 has a Pt / C ratio of 20 wt% and a catalyst loading of 0.5 mg / cm ² . Sample (2) has a Pt / C ratio of 45 wt% and a catalyst loading of 0.5 mg / cm ² . Sample (3) has a Pt / C ratio of 20 wt% and a catalyst loading of 0.1 mg / cm ² . Sample (4) has a Pt / C ratio of 45 wt% and a catalyst loading of 0.1 mg / cm ² . In any of samples (1) to (4), the N / C ratio is constant at 0.75. In any of samples (1) to (4), the anode had a Pt / C ratio of 50 wt%, a catalyst loading amount of 0.2 mg / cm ² , and an N / C ratio of 0.75. is there.

  In FIG. 6, when power generation is performed using the above-described four types of fuel cells (single cells), a cycle of predetermined power generation conditions for changing the output voltage is repeatedly executed, and effective for the catalytic metal (Pt) included in the cathode. The surface area maintenance rate was examined. The conditions for one cycle when generating power were 0.7 V (4 seconds) and 1 V (4 seconds). At this time, a large excess amount of fuel gas and oxidizing gas were supplied to each fuel cell. The effective surface area maintenance ratio in the catalytic metal of the cathode represents the ratio of the effective surface area of the catalytic metal in the cathode of the fuel cell that has generated power in a predetermined cycle to the effective surface area of the catalytic metal in the cathode of the fuel cell at the beginning of manufacture. In FIG. 6, the effective surface area of the catalyst metal at the beginning of manufacture is set to 1, and the degree of change in the effective surface area thereafter is shown. The effective surface area of the catalytic metal was measured by cyclic voltammetry.

  As shown in FIG. 6, if the catalyst loading amount (Pt loading amount per unit area) is the same, the fuel cell having a smaller Pt / C ratio maintains the effective surface area retention ratio higher for a longer time. I was able to. Since elution of the catalytic metal from the cathode to the electrolyte membrane proceeds mainly in the vicinity of the contact surface with the electrolyte membrane, by reducing the Pt / C ratio in the cathode, particularly in the region including the contact surface with the electrolyte membrane, It is possible to enhance the effect of suppressing deterioration of battery performance due to metal elution (improving the durability of the fuel cell). In general, as the amount of catalyst supported increases, the performance of the fuel cell improves. However, by suppressing the Pt / C ratio in the vicinity of the electrolyte membrane as in the embodiment, long-term durability of the fuel cell can be achieved while suppressing the amount of catalyst supported. As a result, the performance of the fuel cell can be improved.

  Note that, in the electrode, if the Pt / C ratio is lowered while securing the amount of catalytic metal, the thickness of the electrode increases, so that gas diffusion into the electrode may be suppressed and the concentration overvoltage may increase. As in the fuel cell of the present example, the Pt / C ratio in the diffusion layer side layer 22b is increased while suppressing the Pt / C ratio in the membrane side layer 22a, thereby suppressing an increase in the thickness of the entire electrode. A decrease in battery performance due to an increase in thickness can be suppressed.

  In particular, the fuel cell of this example has a configuration in which no catalyst metal (Pt) is disposed in the membrane side layer 22a, which is in the vicinity of the contact surface with the electrolyte membrane. Therefore, the effect of suppressing the elution of the catalyst metal to the electrolyte membrane can be enhanced. Further, by providing the membrane side layer 22a having carbon particles not supporting the catalyst metal, the membrane side layer 22a can block the elution of the catalyst metal included in the diffusion layer side layer 22b into the electrolyte membrane. The effect of suppressing elution of the electrolyte into the electrolyte membrane can be further enhanced.

E. Suppressing power generation concentration:
In the cathode, by reducing the Pt / C ratio in the region including the contact surface with the electrolyte membrane, it is possible to further suppress the power generation concentration in the region near the electrolyte membrane and improve the battery performance. In the electrode, the shorter the distance between the electrolyte membrane and the catalyst metal, the easier it is for protons to exchange between the catalyst metal and the electrolyte membrane, and the easier the electrochemical reaction to proceed, so place it in the area near the contact surface with the electrolyte membrane. The generated catalytic metal tends to concentrate power generation. In the electrode, if power generation concentration occurs in the region near the contact surface with the electrolyte membrane, the concentration overvoltage increases due to insufficient gas supply to the catalytic metal where power generation concentration occurs, and the battery performance decreases. . As in the fuel cell of the example, by reducing the Pt / C ratio in the vicinity of the contact surface with the electrolyte membrane, the power generation concentration in the catalyst metal disposed in the vicinity of the contact surface with the electrolyte membrane is suppressed, and the power generation concentration is reduced. The resulting deterioration in battery performance can be suppressed. In particular, power generation concentration in the region near the contact surface with the electrolyte membrane is more likely to occur as the catalyst loading amount (Pt loading amount per unit area) on the electrode is smaller. Therefore, when reducing the amount of catalyst supported on the electrode, the effect of improving battery performance by suppressing the power generation concentration by reducing the Pt / C ratio in the vicinity of the contact surface with the electrolyte membrane is particularly remarkable. .

  As described above, in the electrode, when the Pt / C ratio in the layer on the electrolyte membrane side is reduced and the Pt / C ratio in the layer on the gas diffusion layer side is increased, the electrochemical reaction in the layer on the gas diffusion layer side Will progress more. At this time, like the fuel cell of the example, by reducing the N / C ratio in the gas diffusion layer side layer, the polymer electrolysis mass covering the catalyst-supporting carbon is suppressed in the gas diffusion layer side layer, Since the diffusion resistance when the gas permeates through the polymer electrolyte to the catalytic metal is reduced, the effect of improving the battery performance is enhanced.

F. Improved drying resistance:
As described above, when the amount of catalyst supported is constant, the electrode becomes thicker as the Pt / C ratio is smaller, and the electrode is thinner as the Pt / C ratio is larger. The thicker the electrode, the larger the void volume inside the electrode. At the cathode, water is generated with power generation. However, the larger the Pt / C ratio and the thinner the electrode, the smaller the void volume inside the electrode, so that the generated water easily fills the inside of the electrode and the water retention capacity of the electrode increases. . As in the fuel cell of the embodiment, by increasing the Pt / C ratio of the diffusion layer side layer 22b as compared with the membrane side layer 22a, the water retention capacity of the diffusion layer side layer 22b is increased to increase the gas diffusion layer 24 from the cathode side. It becomes possible to suppress the movement of water into the cathode and maintain the water retention amount of the cathode 22. Thus, by increasing the water retention amount of the cathode 22, drying of the electrolyte membrane 20 can be suppressed. This makes it possible to reduce the amount of humidification of the gas supplied to the fuel cell or to omit the humidification when generating power from the fuel cell, thereby improving the energy efficiency of the entire fuel cell system. Become.

  In the fuel cell of the present embodiment, the Pt / C ratio of the membrane side layer 22a is made smaller than the Pt / C ratio of the diffusion layer side layer 22b, and the N / C ratio of the membrane side layer 22a is set to that of the diffusion layer side layer 22b. All of the effects described above can be obtained by combining with a configuration in which the N / C ratio is made larger. That is, an effect of improving the current-voltage characteristics of the fuel cell, an effect of increasing the durability of the cathode and the fuel cell, an effect of suppressing power generation concentration in the vicinity of the electrolyte membrane, and an effect of improving the drying resistance can be obtained. Therefore, the performance of the fuel cell can be improved while suppressing the amount of catalyst supported (the amount of Pt supported per unit area), that is, the amount of catalyst metal used.

  As another configuration for increasing the amount of polymer electrolysis in the vicinity of the electrolyte membrane and increasing the amount of catalytic metal on the gas diffusion layer side in the electrode, for example, the Pt / C ratio (catalyst loading) is constant throughout the electrode. It is also possible to adopt a configuration that makes That is, catalyst-supporting carbon having a constant Pt / C ratio is used for the entire electrode, and in the region closer to the electrolyte membrane, the weight of the catalyst-supporting carbon with respect to the weight of the entire electrode material is reduced, and the weight of the polymer electrolyte with respect to the weight of the entire electrode material A configuration that increases the size of the image can also be considered. However, when the catalyst supporting rate is constant in this way, the distance between the catalytic metals supported on the carbon particles is constant throughout the electrode material, and thus the effect of suppressing power generation concentration in the vicinity of the electrolyte membrane is insufficient. There is a case. Further, when the catalyst supporting rate is made constant, the number of carbon particles is small (the density is low) in the vicinity of the electrolyte membrane, so that the strength of the electrode structure having carbon particles as a skeleton may be lowered. As in this example, when the ratio of the catalyst metal and the ratio of the polymer electrolyte in the electrode are defined as the ratio with respect to the weight of the carbon particles, these disadvantages do not occur.

G. Variations of the cathode configuration:
In the embodiment, the film side layer 22a and the diffusion layer side layer 22b are formed to have the same thickness, but may have different configurations. Even when the thicknesses of the two layers are made different, the same effect as in the embodiment can be obtained by making the Pt / C ratio and the N / C ratio different between the two layers as described above.

  In the example, the weight ratio of the catalyst metal (Pt), the carbon particles, and the polymer electrolyte in the membrane side layer 22a is 0: 1: 1.5, and the catalyst metal (Pt) and the carbon particles in the diffusion layer side layer 22b are The weight ratio of the polymer electrolyte is 1: 1: 1, but may be different. For example, the weight ratio of catalyst metal (Pt), carbon particles, and polymer electrolyte in the membrane side layer 22a is 0.2: 1: 1.5, and the catalyst metal (Pt) and carbon particles in the diffusion layer side layer 22b are The weight ratio of the polymer electrolyte may be 0.5: 1: 1. With such a configuration, the membrane side layer 22a includes the catalytic metal, and thus the electrochemical reaction can be sufficiently advanced in the entire electrode.

  In the embodiment, the cathode 22 has a two-layer structure including the membrane side layer 22a and the diffusion layer side layer 22b. However, the cathode 22 may be composed of three or more layers to further improve the fuel cell performance. good. For example, when the cathode 22 has a two-layer structure, if the amount of catalyst metal in the membrane-side layer 22a occupying half of the cathode is 0 as in the embodiment, the effect of improving the durability is enhanced, but from the beginning of manufacture. In some cases, it may be difficult to achieve sufficient power generation performance. In addition, when the N / C ratio is increased so that the contact resistance with the electrolyte membrane can be sufficiently reduced in the membrane side layer 22a, the membrane side layer 22a occupying half of the cathode has a high height that covers the catalyst metal. Increasing the area and thickness of the molecular electrolyte can increase the concentration overvoltage to an undesirable level. On the other hand, when the cathode has a multilayer structure of three or more layers, even if the Pt / C ratio in the layer in contact with the electrolyte membrane is sufficiently small as 0, it may be insufficient for power generation to proceed. The region where the amount of catalytic metal is so small is limited to the region near the electrolyte membrane. In addition, even if the N / C ratio in the layer in contact with the electrolyte membrane is sufficiently large, the region where the catalyst metal is covered with the polymer electrolyte to such an extent that proton permeation can be insufficient is limited to the region near the electrolyte membrane. Will be. Therefore, it is possible to obtain a sufficient effect by making the Pt / C ratio and the N / C ratio different for each layer while bringing the cathode as a whole close to a balanced component ratio in order to exhibit battery performance. .

  As described above, the cathode composed of three or more layers can be produced in the same manner as the cathode 22 of the embodiment. That is, in place of step S110 in FIG. 3, a process of producing a type of catalyst ink according to the number of layers to be formed so that the layer closer to the electrolyte membrane has a smaller Pt / C and a larger N / C. Just do it. Then, in place of steps S120 and S130, a process of applying the prepared catalyst ink on the base material in a predetermined order and drying it may be repeated a number of times according to the type of the catalyst ink. By increasing the number of catalyst inks used and increasing the number of steps of applying and drying the catalyst ink, it is also possible to produce a cathode whose composition ratio gradually changes from the electrolyte membrane side to the gas diffusion layer side. The weight ratio of the catalyst metal (Pt), the carbon particles, and the polymer electrolyte in each layer constituting the cathode is 0 to 0.2: 1: 0.75 to 1.5 for the layer including the contact surface with the electrolyte membrane. Yes, the layer including the contact surface with the gas diffusion layer may be 0.5 to 1: 1: 0.5 to 1. Within such a range, if the Pt / C ratio is smaller and the N / C ratio is larger as the layer on the electrolyte membrane side, the same effect as the embodiment can be obtained.

H. Variations:
The present invention is not limited to the above-described examples and embodiments, and can be implemented in various modes without departing from the gist thereof. For example, the following modifications are possible.

H1. Modification 1:
In the embodiment, the content of the carbon particles is made uniform in each layer constituting the cathode 22, but different structures may be adopted. The carbon content (carbon content density) is within a range in which the effects described above can be obtained by varying the Pt / C ratio and the N / C ratio in the film thickness direction within a predetermined numerical range. It may be different for each layer.

H2. Modification 2:
In the embodiment, the cathode 22 is produced by transferring the catalyst ink applied on the substrate to the electrolyte membrane, but it may be produced using a different method. For example, the catalyst ink may be directly applied on the electrolyte membrane. Alternatively, the cathode may be formed by applying an electrode material containing catalyst-supporting carbon on the electrolyte membrane by an electrostatic method. Alternatively, the cathode may be formed by applying a catalyst ink on the gas diffusion layer and superimposing and bonding the coated surface with the electrolyte membrane. Regardless of which method is used, the catalyst metal (Pt), the carbon particles, and the polymer electrolyte have a plurality of layers having different weight ratios, and the Pt / C ratio is smaller at the electrolyte membrane side layer. By producing the cathode so as to be formed by a plurality of layers having a large C ratio, the same effects as in the embodiment can be obtained.

H3. Modification 3:
In the embodiment, only the cathode is an electrode composed of a plurality of layers having different weight ratios of the catalyst metal (Pt), the carbon particles, and the polymer electrolyte, but may have different configurations. The configuration of only the anode or both the anode and the cathode may be the same as the configuration of the cathode described above. In any electrode, the Pt / C ratio is decreased as the membrane side layer is increased, and the N / C ratio is increased, or further, the weight ratio of the catalyst metal (Pt), the carbon particles, and the polymer electrolyte is changed to the electrolyte membrane. 0 to 0.2: 1: 0.75 to 1.5 in the layer including the contact surface with the gas, and 0.5 to 1: 1: 0.5 in the layer including the contact surface with the gas diffusion layer. By setting it to 1, the same effect can be obtained. That is, in both the cathode and the anode, the above configuration can provide the effect of suppressing the contact resistance with the electrolyte membrane and improving the current-voltage characteristics and the effect of suppressing the power generation concentration in the vicinity of the electrolyte membrane. .

  Since elution of the catalyst metal into the electrolyte membrane proceeds mainly at the cathode, at least the cathode is preferably configured as described above in order to improve the durability of the fuel cell. Moreover, the effect which raises the water retention power of the layer by the side of a gas diffusion layer, suppresses the movement of the water to the gas diffusion layer side, and improves drought resistance is acquired in any electrode. However, since water is generated at the cathode in accordance with power generation, the effect of improving the drying resistance in the fuel cell can be particularly enhanced by at least the cathode having the above-described configuration. When both electrodes have the above-described configuration, the water retention capacity of the gas diffusion layer side layer can be increased in both electrodes. Therefore, a layer having a low water retention ability and an electrolyte membrane are sandwiched between layers having a high water retention ability, which is particularly desirable because the effect of improving the drying resistance can be particularly enhanced.

H4. Modification 4:
In the embodiment, the catalyst metal included in the electrode is platinum, but a different configuration may be used. For example, a platinum alloy or a noble metal other than platinum may be used as the catalyst metal. Even in such a case, the effect similar to that of the example can be obtained by reducing the ratio of the weight of the catalytic metal to the weight of the carbon particles in the electrode closer to the electrolyte membrane. In that case, the weight ratio of the catalyst metal, the carbon particles, and the polymer electrolyte is set to 0 to 0.2: 1: 0.75 to 1.5 in the layer including the contact surface with the electrolyte membrane, In the layer including the contact surface, it may be 0.5 to 1: 1: 0.5 to 1.

H5. Modification 5:
In the examples, the same fluorine-based polymer electrolyte was used as the polymer electrolyte constituting the electrolyte membrane and the polymer electrolyte included in the electrode, but different types of polymer electrolytes may be used. For example, the electrolyte membrane is made of a hydrocarbon-based polymer electrolyte, and the same fluorine-based electrolyte as in the embodiment can be used as the polymer electrolyte included in the electrode. Even with such a configuration, the same effect as the embodiment can be obtained. For example, in a layer closer to the electrolyte membrane in the electrode, by increasing the ratio of the weight of the polymer electrolyte to the weight of the carbon particles, a similar effect such as enhancing the current-voltage characteristics of the fuel cell can be obtained.

DESCRIPTION OF SYMBOLS 10 ... Single cell 20 ... Electrolyte membrane 21 ... Anode 22 ... Cathode 22a ... Membrane side layer 22b ... Diffusion layer side layer 23, 24 ... Gas diffusion layer 25, 26 ... Gas separator 30 ... MEA
47 ... Fuel gas flow path in single cell 48 ... Oxidation gas flow path in single cell

Claims (1)

A fuel cell,
An electrolyte membrane;
An anode and a cathode, which are a pair of porous electrodes provided on both surfaces of the electrolyte membrane, in which carbon particles, a catalyst metal supported on the carbon particles, and a polymer electrolyte are mixed;
With
At least one of the pair of electrodes includes a plurality of layers provided along the surface direction of the electrolyte membrane, and in the plurality of layers, the closer to the contact surface with the electrolyte membrane, the more the carbon particles The fuel cell is formed such that a ratio of the weight of the catalyst metal to a weight of the catalyst metal is small and a ratio of the weight of the polymer electrolyte to the weight of the carbon particles is large.

Images