JP2009026479A - Fuel cell and electrolyte membrane-electrode assembly - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve a battery performance in a solid polymer fuel cell. <P>SOLUTION: The fuel cell includes an electrolyte membrane 20 and an anode 21 and a cathode 22 which are electrodes containing a catalyst formed on respective faces of the electrolyte membrane 20. The electrolyte membrane 20 is composed of a first electrolyte layer 31 which is formed on one side and is supplied with an electrode active material that generates ions to conduct in the electrolyte membrane 20 and a second electrolyte layer 32 which is formed on the other side and is formed of a high ion conductive electrolyte having a higher ion conductivity than the electrolyte to form the first electrolyte layer 31. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a fuel cell and an electrolyte membrane-electrode assembly for a fuel cell.

  A polymer electrolyte fuel cell is a fuel cell including a solid polymer electrolyte membrane having proton conductivity as an electrolyte layer. The battery performance in such a fuel cell is affected by the proton conductivity as the performance of the electrolyte membrane, the rate of the electrochemical reaction that proceeds at the electrode, the water content in the electrolyte membrane, and the like. One of the attempts that have been made in the past to improve battery performance is to improve the electrode shape (see, for example, Patent Document 1). Here, by improving the electrode shape, the catalytic performance is improved and the mobility of the substance supplied to and discharged from the catalyst is improved, thereby improving the battery performance.

Japanese Patent No. 3711545

  However, the improvement of the electrode shape is accompanied by a complicated electrode shape, that is, a complicated manufacturing process. In addition, even if the electrode shape is improved, the efficiency of proton transfer from the electrolyte membrane to the catalyst does not change, so the effect may not be sufficiently obtained from the viewpoint of the mobility of the substance supplied to and discharged from the catalyst. It was. Furthermore, in the fuel cell, there are more demands such as improvement in durability, and further improvement in performance has been desired while satisfying these demands.

  The present invention has been made to solve at least one of the above-described conventional problems, and aims to improve cell performance in a polymer electrolyte fuel cell.

In order to achieve the above object, the fuel cell of the present invention comprises:
An electrolyte membrane, and an anode and a cathode that are electrodes containing a catalyst formed on each surface of the electrolyte membrane,
The electrolyte membrane is formed on one side to which an electrode active material that generates ions conducting in the electrolyte membrane is supplied, and on the other side, and the first electrolyte layer And a second electrolyte layer formed by a high ion conductive electrolyte having higher ion conductivity than the electrolyte forming the layer.

  According to the fuel cell of the present invention configured as described above, the electrolyte membrane is formed on the side opposite to the side where the electrode active material that generates ions conducting in the electrolyte membrane is supplied by the high ion conductive electrolyte. By providing the second electrolyte layer, the ion conductivity in the first electrolyte layer is improved, the ion conductivity of the entire electrolyte membrane is increased, and the performance of the fuel cell can be improved. Furthermore, by providing the second electrolyte layer, the thickness of the entire electrolyte membrane is increased, so that the strength of the electrolyte membrane can be improved and the durability of the fuel cell can be increased.

  In the fuel cell of the present invention, the electrolyte membrane is formed of a solid polymer electrolyte, the ions conducted in the electrolyte membrane are protons, the first electrolyte layer is formed on the anode side, and the cathode side Alternatively, the second electrolyte layer may be formed. In the solid polymer electrolyte membrane, the higher the proton conductivity, the higher the water content, which may cause flooding in the fuel cell. In the present invention, a layer made of a high proton conductive electrolyte is used as the second electrolyte layer. Since it only needs to be provided on the cathode side, battery performance can be improved while suppressing flooding.

  In such a fuel cell of the present invention, the second electrolyte layer has a weight of the whole solid polymer electrolyte per unit amount of ion-exchange groups related to proton conduction rather than the electrolyte constituting the first electrolyte layer. It is good also as being formed of an electrolyte having a low value. With such a configuration, battery performance is easily achieved with a simple configuration in which an electrolyte membrane is formed by stacking layers made of electrolytes having different weight values of the solid polymer electrolyte per unit amount of ion exchange groups. It becomes possible to improve.

  In the fuel cell of the present invention, the second electrolyte layer may be formed thinner than the first electrolyte layer. By providing a second electrolyte layer that is thinner than the first electrolyte layer, ion conductivity in the first electrolyte layer, which is the main part of the electrolyte membrane, is increased, and the strength of the electrolyte membrane is improved by further increasing the film thickness. It is possible to achieve both the improvement in ion conductivity of the electrolyte membrane. At this time, particularly when the electrolyte membrane is formed of a solid polymer electrolyte, the effect of suppressing flooding can be enhanced by forming the second electrolyte layer thin.

  In such a fuel cell of the present invention, the second electrolyte layer has an ionic conductivity exhibited by the electrolyte membrane composed of the electrolyte layer composed of the high ion conductive electrolyte and the first electrolyte layer in the fuel cell. The electrolyte membrane formed only by the first electrolyte layer is formed thinner than the thickness of the electrolyte layer made of the high ion conductive electrolyte when the ion conductivity is equivalent to the ion conductivity shown in the fuel cell. It is also good that it is. With such a configuration, the strength of the electrolyte membrane can be improved by increasing the thickness of the electrolyte membrane while improving the ionic conductivity of the entire electrolyte membrane.

  In the fuel cell of the present invention, the electrode is formed by mixing a solid polymer electrolyte exhibiting proton conductivity similar to that of the first electrolyte layer between conductive particles carrying a catalyst metal dispersedly on the surface. It is also good. With such a configuration, by providing the second electrolyte layer made of a high proton conductive electrolyte on the cathode side of the electrolyte membrane, the efficiency of proton transfer from the electrolyte membrane to the cathode is improved, and proton supply to the catalyst at the cathode is achieved. Efficiency can be increased. At this time, since the solid polymer electrolyte having the same proton conductivity as that of the first electrolyte layer is used as the electrolyte mixed in the cathode, even if the proton supply efficiency to the catalyst in the cathode is improved, the electrolyte included in the cathode An increase in water content can be suppressed. Therefore, it can suppress that the supply and discharge of the gas with respect to a catalyst are prevented with water.

  The present invention can be realized in various forms other than those described above. For example, the present invention can be realized in the form of an electrolyte membrane-electrode assembly for 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 fuel cell, and the electrolyte membrane 20 includes a solid polymer electrolyte that exhibits proton conductivity in a wet state. In the present embodiment, the electrolyte membrane 20 includes a first electrolyte layer 31 disposed on the anode 21 side and a second electrolyte layer 32 disposed on the cathode 22 side. 32 is formed of electrolytes having different proton conductivity. The anode 21 and the cathode 22 include, for example, platinum or a platinum alloy as a catalyst. More specifically, the anode 21 and the cathode 22 include carbon particles supporting the catalyst and an electrolyte similar to the polymer electrolyte that constitutes the electrolyte membrane 20. The anode 21 and the cathode 22 together with the electrolyte membrane 20 constitute an MEA (Membrane Electrode Assembly) 30. The detailed configuration and manufacturing process of the MEA 30 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.

  The gas separators 25 and 26 are formed of a gas impermeable conductive member, for example, a member made of compressed carbon or stainless steel. Each of the gas separators 25 and 26 has a predetermined uneven shape. Due to this uneven shape, a fuel gas flow path 27 in the single cell in which the 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 28 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 27 and the single-cell oxidizing gas flow channel 28 (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 path 27 while being subjected to an electrochemical reaction. Collect in the gas exhaust manifold. Similarly, the oxidant gas flowing through the oxidant gas supply manifold is distributed to each single cell 10, passes through the oxidant gas flow path 28 in each single cell while being subjected to an electrochemical reaction, and then collects in the oxidant gas discharge manifold. To do.

B. Configuration and manufacturing method of MEA 30:
FIG. 2 is an explanatory diagram schematically showing an enlarged configuration of the MEA 30. In FIG. 2, unlike FIG. 1, the MEA is shown in the direction in which the film surface is horizontal. As described above, the electrolyte membrane 20 includes two electrolyte layers having different electrolytes as constituent materials, that is, the first electrolyte layer 31 formed on the anode 21 side and the second electrolyte layer formed on the cathode 22 side. 32. In the present embodiment, the second electrolyte layer 32 is formed thinner than the first electrolyte layer 31. For example, the first electrolyte layer 31 can be formed to a thickness of 10 to 70 μm, and the second electrolyte layer 32 can be formed to a thickness of 0.2 to 10 μm.

  In the electrolyte membrane 20 included in the fuel cell of this example, the second electrolyte layer 32 is formed of a solid polymer having higher proton conductivity than the first electrolyte layer 31. In this embodiment, a solid polymer having an EW (Equivalent Weight) value smaller than that of the first electrolyte layer 31 is used as the solid polymer having high proton conductivity. Here, the EW value is an equivalent weight of ion exchange groups (sulfonic acid groups) in the solid polymer electrolyte, that is, a dry weight value of the whole solid polymer electrolyte per unit amount (1 mol) of ion exchange groups. Say. The solid polymer electrolyte has a structure in which a repeating unit (side chain) having a sulfonic acid group is bonded to a predetermined main chain, and proton conductivity is realized by the sulfonic acid group. Therefore, in the case of an electrolyte having the same kind of main chain, it can be said that the smaller the EW value, the higher the ratio of sulfonic acid groups to the whole solid polymer electrolyte, and the higher the proton conductivity. In the present embodiment, both the first electrolyte layer 31 and the second electrolyte layer 32 are constituted by an electrolyte including a main chain of a fluororesin and a side chain having a sulfonic acid group at the end. . Then, by forming the second electrolyte layer 32 with a solid polymer electrolyte having a smaller EW value, the proton conductivity of the second electrolyte layer 32 is made higher than that of the first electrolyte layer 31.

  As shown in FIG. 2, the anode 21 and the cathode 22 include carbon particles 34 on the surface of which platinum (or platinum alloy) as a catalyst is dispersedly supported, and the electrolyte membrane 20 is interposed between the carbon particles 34. It is formed by mixing the electrolyte 36 similar to the electrolyte which comprises. In this embodiment, the electrolyte 36 included in the anode 21 and the cathode 22 is formed of a fluorine-based solid polymer electrolyte that exhibits the same EW value as that of the first electrolyte layer 31.

  FIG. 3 is a process diagram showing a method for manufacturing the MEA 30. FIG. 4 is an explanatory view schematically showing a part of the process shown in FIG. When manufacturing the MEA 30, first, an electrolyte membrane to be the first electrolyte layer 31 is prepared (step S100). In this embodiment, an electrolyte membrane made of a perfluorosulfonic acid electrolyte is used.

  Next, an electrolyte having a lower EW value than the first electrolyte layer 31 prepared in step S100 (hereinafter referred to as a low EW electrolyte) is prepared, and the low electrolyte is used on the first electrolyte layer 31. The second electrolyte layer 32 is formed (step S110), and the electrolyte membrane 20 is produced. In this embodiment, the low EW electrolyte is prepared as a dispersion in which the electrolyte is dispersed in an alcohol aqueous solution. And this dispersion liquid is apply | coated on the 1st electrolyte layer 31, for example by spray application, The 2nd electrolyte layer 32 is formed by making it dry and solidify after that. Thus, by applying the electrolyte dispersed in the solvent onto the first electrolyte layer 31, the surface of the first electrolyte layer 31 is slightly dissolved by the solvent, and the first electrolyte layer 31 and the second electrolyte layer 32 are dissolved. Can be fully integrated. At this time, the film thickness of the second electrolyte layer 32 can be controlled by adjusting the concentration and the coating amount of the low EW electrolyte dispersion. FIG. 4A shows a state where a low EW electrolyte dispersion is applied on the first electrolyte layer 31, and FIG. 4B shows a state where the second electrolyte layer 32 is formed on the first electrolyte layer 31. Show.

  In addition to the preparation of the electrolyte membrane 20, a catalyst paste for forming the anode 21 and the cathode 22 as electrodes is prepared (step S120). The catalyst paste is a paste containing the above-described carbon particles 34 supporting platinum and an electrolyte. The carbon particles 34 carrying platinum are, for example, carbon particles made of carbon black dispersed in a platinum compound solution (for example, a tetraammine platinum salt solution, a dinitrodiammine platinum solution, a platinum nitrate solution, or a chloroplatinic acid solution). Then, it is produced by an impregnation method, a coprecipitation method, or an ion exchange method. The platinum-supported carbon particles 34 thus produced are dispersed in an appropriate water and an organic solvent, and an electrolyte solution containing the above-described electrolyte (for example, Aldrich Chemical Co., Nafion Solution) is further mixed. A catalyst paste is obtained.

  When a catalyst paste is prepared, this catalyst paste is applied onto the electrolyte membrane 20 produced in step S110 to form an electrode, thereby completing the MEA 30 (step S130). A state in which the cathode 22 is formed on the electrolyte membrane 20 is shown in FIG. The application of the catalyst paste onto the electrolyte membrane 20 can be performed by, for example, a spray method. Moreover, it is good also as performing by screen printing on the electrolyte membrane 20 using a catalyst paste. Alternatively, it can be performed by a doctor blade method or an ink jet method. By using these methods, the catalyst paste can be applied to a desired thickness. Thus, by preparing the electrode raw material in the form of paste and applying it on the electrolyte membrane 20, the surface of the electrolyte membrane 20 is slightly dissolved by the solvent in the paste, and the electrolyte membrane 20 and the electrode are sufficiently integrated. It becomes possible to make it. After applying the catalyst paste as described above, the applied catalyst paste is dried to form a porous electrode having fine pores inside. Here, the anode 21 and the cathode 22 may have different configurations. For example, it may be configured using different catalyst pastes such as using catalyst pastes with different platinum loadings, or may be formed in different thicknesses, or may be formed by different methods.

  According to the fuel cell of the present embodiment configured as described above, the electrolyte membrane 20 includes the first electrolyte layer 31 that is a layer including the surface on the anode side and the second electrolyte that is a layer including the surface on the cathode side. The second electrolyte layer 32 is formed of an electrolyte having higher ion conductivity, that is, an electrolyte having a smaller EW value in this embodiment. Therefore, the proton conductivity in the first electrolyte layer 31 is improved, the proton conductivity of the entire electrolyte membrane 20 is increased, and the battery performance can be improved. The mechanism by which the proton conductivity in the first electrolyte layer 31 is improved by providing the second electrolyte layer 32 is considered as follows. That is, when the second electrolyte layer 32 (a layer having a higher proton conductivity and a higher ability to retain protons) made of a low EW electrolyte is provided on the cathode 22 side in the electrolyte membrane 20, Protons move quickly to the electrolyte layer 32. Therefore, it is considered that the proton concentration is lower in the vicinity of the cathode side surface in the first electrolyte layer 31 than in the case where the second electrolyte layer 32 is not provided. When protons move in the electrolyte membrane 20 from the anode side to the cathode side in accordance with the electrochemical reaction, the proton transfer efficiency, that is, the proton transfer speed is determined by the magnitude of the proton concentration gradient in the electrolyte membrane 20. Depends on. Therefore, when the second electrolyte layer 32 is provided as in the present embodiment, the proton concentration in the vicinity of the cathode side surface in the first electrolyte layer 31 decreases, and the anode side in the first electrolyte layer 31 It is considered that the proton conductivity in the first electrolyte layer 31 is improved by increasing the concentration gradient with respect to the cathode side.

  In the fuel cell of this example, as described above, the proton conductivity of the entire electrolyte membrane 20 is improved by providing the low EW electrolyte layer on the cathode side of the electrolyte membrane 20. Therefore, when the electrolyte membrane 20 is manufactured, battery performance can be easily improved by a simple manufacturing process of stacking and forming electrolyte layers having different EW values.

  Further, according to the fuel cell of the present embodiment, by providing the second electrolyte layer 32, it is possible to improve the strength of the entire electrolyte membrane while improving the proton conductivity of the entire electrolyte membrane. In order to improve the proton conductivity of the electrolyte, a configuration in which the entire electrolyte membrane is formed of an electrolyte having a lower EW value may be considered, but the strength of the electrolyte membrane decreases as the EW value constituting the electrolyte membrane decreases. It is known to do. In the fuel cell of this embodiment, the second electrolyte layer 32 is formed on the first electrolyte layer 31, so that the proton conductivity of the entire electrolyte membrane is compared to, for example, the case where the electrolyte membrane is formed by only the first electrolyte layer 31. The strength of the electrolyte membrane can be increased by increasing the thickness of the entire electrolyte membrane.

  When the electrolyte membrane is composed of an electrolyte having a lower EW value, the water content of the electrolyte membrane increases, which may cause a flooding problem in the fuel cell. In a solid polymer electrolyte membrane, protons are transmitted by the sulfonic acid groups provided in the electrolyte membrane. When protons move in the electrolyte membrane in this way, the protons move in a hydrated state with water molecules. . Therefore, an electrolyte membrane made of a low EW electrolyte having a large number of sulfonic acid groups and capable of easily transmitting and retaining protons has a higher water content and flooding due to excess liquid water contained in the electrolyte membrane, specifically, an electrode There is a possibility that the supply / exhaust of gas to / from the catalyst included in the catalyst is hindered. According to the present embodiment, the electrolyte having a low EW value includes only a part of the electrolyte membrane, specifically, the second electrolyte layer 32 that is thinner than the first electrolyte layer 31 and provided on the cathode side. Proton conductivity in the electrolyte membrane can be improved while suppressing this problem.

  Furthermore, according to the fuel cell of the present embodiment, the efficiency of proton transfer from the electrolyte membrane 20 to the cathode 22 is improved by providing the second electrolyte layer 32 having a high ability to hold protons on the cathode side of the electrolyte membrane 20. It is thought that it can be improved. That is, the proton concentration in the vicinity of the cathode-side surface in the electrolyte membrane 20 increases, so that the proton concentration gradient between the electrolyte membrane 20 and the cathode 22 increases, and protons easily move from the electrolyte membrane 20 to the cathode 22. Therefore, in the cathode 22, when an electrochemical reaction proceeds with the catalyst on the carbon particles 34 and protons are consumed, the protons quickly move from the electrolyte membrane 20 to the cathode 22 via the electrolyte included in the cathode 22. Thus, the proton supply to the catalyst is continued efficiently. Thus, the battery performance can be improved by improving the proton supply efficiency to the catalyst in the cathode.

  In addition, in order to improve the proton transfer efficiency in the cathode, a configuration using an electrolyte having a lower EW value as an electrolyte constituting the cathode is also conceivable. However, when the cathode is composed of a low EW electrolyte, the water content in the cathode increases, so flooding, specifically, the pores formed in the cathode are blocked by liquid water, and gas supply to and discharge from the catalyst is prevented. The problem of being disturbed can arise. According to this embodiment, since it is not necessary to reduce the EW value of the electrolyte constituting the cathode, it is possible to improve the proton transfer efficiency at the cathode while suppressing flooding.

C. Evaluation results of fuel cells of Examples:
FIG. 5 shows a fuel cell according to the present embodiment, in which fuel cells with different thicknesses of the second electrolyte layer 32 are manufactured while keeping the thickness of the first electrolyte layer 31 constant. It is explanatory drawing which shows the result of the comparison. Here, a conventional fuel cell having an electrolyte membrane made only of the first electrolyte layer 31 is used as a comparative example.

  The first electrolyte layer 31 in the electrolyte membrane included in each fuel cell used for examining the power generation performance is formed using a perfluorosulfonic acid polymer having an EW value of 1000 so that the thickness is 30 μm. did. The second electrolyte layer 32 is made of a perfluorosulfonic acid polymer having an EW value of 700 and has a thickness in the range of 0.1 to 10 μm. The electrode was prepared using platinum-supported carbon particles 34 having platinum supported on carbon black and having a platinum support ratio of 50 wt%. In addition, a platinum carrying rate means the ratio of the weight of the supported platinum with respect to the carbon particle weight. Using such a catalyst paste containing platinum-supported carbon particles 34, the cathode side, which is the surface of the second electrolyte layer 32, or the anode side, which is the surface of the first electrolyte layer 31, is applied by a spray method, and the thickness is 10 μm. A cathode 22 and an anode 21 having a thickness of 10 μm were formed.

Using each MEA produced as described above, a fuel cell was produced in a single cell state as shown in FIG. 1 and connected to a load to generate power. The condition of the gas supplied to each fuel cell during power generation was that humidification was performed using a bubbler at a flow rate of 1000 ml / min in both hydrogen gas supplied to the anode and air supplied to the cathode. The operating temperature of the fuel cell during power generation was 80 ° C. In FIG. 5, the horizontal axis indicates the thickness of the second electrolyte layer 32 made of a low EW electrolyte, and the vertical axis indicates the measurement of the output voltage when the output current is 1.0 A / cm ² in each fuel cell. The value is shown.

  As shown in FIG. 5, when the second electrolyte layer 32 made of an electrolyte having an EW value smaller than that of the first electrolyte layer 31 is formed on the first electrolyte layer 31, the thickness of the second electrolyte layer 32 has a predetermined value. In this case, the output voltage value is the highest and the battery performance is the highest. The thickness of the second electrolyte layer 32 when the output voltage reaches a peak is about 0.5 μm when the thickness of the first electrolyte layer 31 is 30 μm as shown in FIG. That is, when the thickness of the second electrolyte layer 32 is about 1/60 of the thickness of the first electrolyte layer 31, the output voltage is about 28 mV higher than the comparative example, and the battery performance is the highest. . Thus, it is considered that the reason why the output voltage peaks when the thickness of the second electrolyte layer 32 is a predetermined value is that the film resistance increases as the film thickness increases. That is, as the thickness of the second electrolyte layer 32 increases, the second electrolyte layer 32 can attract protons and increase proton conductivity of the entire electrolyte membrane. However, the thickness of the second electrolyte layer 32 is greater than the predetermined value. In the case of exceeding the above, it is considered that the effect of providing the second electrolyte layer 32 made of the low EW electrolyte is canceled by the increase in the membrane resistance, and the output voltage gradually decreases.

  Further, as shown in FIG. 5, when the thickness of the first electrolyte layer 31 is 30 μm, the thickness of the second electrolyte layer 32 is about 5 μm (the thickness of the second electrolyte layer 32 is the thickness of the first electrolyte layer 31). The output voltage value was almost the same as that of the fuel cell of the comparative example (a fuel cell not provided with the second electrolyte layer 32). Therefore, if the thickness of the second electrolyte layer 32 is up to about 5 μm, the proton conductivity of the entire electrolyte membrane 20 can be improved by providing the second electrolyte layer 32, and the entire membrane is made thicker. It can be said that the effect of improving the film strength (improving the durability of the fuel cell) can be obtained. That is, the ionic conductivity exhibited by the electrolyte membrane 20 in the fuel cell is composed of a highly ionic conductive electrolyte when the electrolyte membrane formed by only the first electrolyte layer is equivalent to the ionic conductivity exhibited in the fuel cell. If the second electrolyte layer 32 is formed so as to be thinner than the thickness of the electrolyte layer, it can be said that both the effect of improving the battery performance and the effect of improving the film strength can be achieved.

  The results of evaluating the mass transfer (diffusion) coefficient when the fuel cell generates power are shown below. Cyclic voltammetry is known as a method for analyzing the state of the electrochemical reaction, and when the electron transfer rate is high and mass transfer (proton transfer) is reaction-controlled, it is cyclic. The peak current in the voltammogram is expressed by the Randle-Sevcik equation shown as equation (1) below.

I _p = kn ^3/2 AD ^1/2 CV ^1/2 (1)
However,
I _p : peak current value k: constant 2.69 × 10 ⁵ (298K)
n: Number of mobile electrons A: Electrode area (cm ² )
D: Diffusion coefficient (cm ² / sec)
V: Potential scanning speed (V / s)
C: Concentration (mol / cm ³ )

  From the above equation (1), it can be said that the peak current value Ip is proportional to the square root of the potential scanning speed (sweep speed) V. Then, a cyclic voltammogram is created for each sweep speed with different sweep speeds, and a peak current value Ip is obtained for each voltammogram. Is a value that reflects the magnitude of the diffusion coefficient D. FIG. 6 shows the above inclination as a diffusion constant for a fuel cell having an electrolyte membrane in which the thickness of the first electrolyte layer 31 is 30 μm and the thickness of the second electrolyte layer 32 is variously changed, as in FIG. It is explanatory drawing which shows the result of having calculated | required. Here, in order to evaluate the proton mobility in the electrolyte membrane, a fuel cell having an electrolyte membrane with different thicknesses of the second electrolyte layer 32 is used, and the sweep rate of the electrode potential is varied for each sweep rate. A cyclic voltammogram was created, the reduction peak current value was derived as the peak current value Ip, and the slope was determined. In FIG. 6, the horizontal axis indicates the thickness of the second electrolyte layer 32 made of a low EW electrolyte, and the vertical axis indicates when the slope obtained from the cyclic voltammogram for the fuel cell of the comparative example is 1. The ratio of the value of the slope obtained from the cyclic voltammogram for the fuel cell of each example is shown.

  As shown in FIG. 6, the graph in which the slope obtained based on the cyclic voltammogram is plotted with respect to the thickness of the second electrolyte layer 32 is output with respect to the thickness of the second electrolyte layer 32 shown in FIG. 5. It closely matched the shape of the graph plotting the voltage. That is, when the thickness of the second electrolyte layer 32 was changed, the evaluation result of the battery performance based on the output voltage and the tendency of the size of the diffusion coefficient agreed well. When the thickness of the second electrolyte layer 32 was about 0.5 μm, the inclination was improved by about 10% as compared with the fuel cell of the comparative example not having the second electrolyte layer 32. Thus, when the 2nd electrolyte layer 32 was provided, it was confirmed that battery performance improves by improving the proton transfer efficiency with respect to the catalyst with which a cathode is provided.

  In the case where the layer made of the low EW electrolyte is formed on the anode side instead of the cathode side on the first electrolyte layer 31, the output voltage is lowered unlike the case where it is formed on the cathode side (battery performance). (Data not shown). This is presumably because the efficiency of proton supply from the anode to the first electrolyte layer 31 is reduced by providing an electrolyte layer having a high ability to retain protons on the anode side of the first electrolyte layer 31.

D. 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.

D1. Modification 1:
In the embodiment, the second electrolyte layer 32 is formed by applying a dispersion containing a low EW electrolyte on the first electrolyte layer 31, but may have a different configuration. For example, the second electrolyte layer 32, which is an electrolyte membrane made of a low EW electrolyte, is prepared separately from the first electrolyte layer 31, and then the two are bonded together by, for example, thermocompression bonding, thereby producing the electrolyte membrane 20. Also good. Even with such a configuration, the same effect can be obtained that increases the proton conductivity of the entire electrolyte membrane.

D2. Modification 2:
In the embodiment, the cathode 22 is formed by applying a catalyst paste to the surface of the electrolyte membrane 20 on the second electrolyte layer 32 side, but may have a different configuration. For example, the cathode 22 may be formed in advance on a substrate different from the electrolyte membrane 20, and then the cathode 22 may be transferred onto the second electrolyte layer 32. Below, such a manufacturing method is demonstrated as the modification 2. FIG. FIG. 7 is an explanatory view schematically showing a part of the MEA manufacturing process according to the second modification.

  In the MEA manufacturing method according to the second modification, first, the electrolyte membrane 20 including the first electrolyte layer 31 and the second electrolyte layer 32 is manufactured as in steps S100 and S110 of the embodiment (see FIG. 7A). ). In addition to this, an implementation for forming the cathode 22 on another substrate 35 (for example, a substrate made of polyethylene terephthalate (PET) or polytetrafluoroethylene (PTFE)) different from the electrolyte membrane 20 is performed. The same catalyst paste as in the example is applied (see FIG. 7B). Thereafter, the catalyst paste applied on the base material 35 is hot-pressure transferred onto the second electrolyte layer 32 of the electrolyte membrane 20 (see FIG. 7C), and the base material 35 is peeled off and removed (see FIG. 7). 7 (D)), the cathode 22 can be formed on the electrolyte membrane 20.

D3. Modification 3:
Further, in the manufacturing method in which the cathode 22 is formed on the substrate 35 different from the electrolyte membrane 20, the second electrolyte layer 32 may be further formed on the cathode 22 on the substrate 35. Below, such a manufacturing method is demonstrated as the modification 3. FIG. FIG. 8 is an explanatory view schematically showing a part of the manufacturing process of the MEA of the third modification.

  In the MEA manufacturing method of Modification 3, first, as in the manufacturing method of Modification 2, a catalyst paste for forming the cathode 22 is applied on the substrate 35 (see FIG. 8A). Thereafter, a low EW electrolyte dispersion is applied onto the cathode 22 formed on the substrate 35 in the same manner as in Step S110 of the embodiment (see FIG. 8B), and the second electrolyte layer 32 is formed on the cathode 22. It is formed (see FIG. 8C). The cathode 22 and the second electrolyte layer 32 thus formed are separately provided on the first electrolyte layer 31 prepared in the same manner as in step S100 of the embodiment, and the first electrolyte layer 31 and the second electrolyte layer 32 Are arranged so as to be in contact with each other, and are transferred by heat and pressure (see FIG. 8D). Then, the cathode 22 can be formed on the electrolyte membrane 20 by peeling and removing the substrate 35.

D4. Modification 4:
Alternatively, the cathode 22 may be formed on the gas diffusion layer 24. For example, the cathode 22 is formed on the conductive porous body used as the gas diffusion layer 24 by applying a low EW electrolyte dispersion in the same manner as in Step S110 of the embodiment. Then, such a porous body and the electrolyte membrane 20 produced in the same manner as steps S100 and S110 of the embodiment may be superposed and hot-pressure bonded so that the second electrolyte layer 32 and the cathode 22 are in contact with each other.

D5. Modification 5:
In the embodiment, the first electrolyte layer 31 and the second electrolyte layer 32 are made of the same type of fluorine-based resin and have different EW values. However, the first electrolyte layer 31 and the second electrolyte layer 32 may have different configurations. The two electrolyte layers 32 may be formed of different types of electrolytes. For example, one electrolyte layer may be formed of the same fluorine-based solid polymer electrolyte as in the embodiment, and the other electrolyte layer may be formed of a hydrocarbon-based solid polymer electrolyte. Even when different types of electrolytes are used, the higher the proton conductivity, the higher the proportion of functional groups (sulfonic acid groups) having proton conductivity, so the strength of the electrolyte membrane decreases and the water content decreases. To increase. Therefore, regardless of the type of electrolyte used, the same effect as the embodiment can be obtained by providing a layer made of an electrolyte having higher proton conductivity on the cathode side in the electrolyte membrane.

D6. Modification 6:
In the embodiment, the electrolyte membrane 20 is formed of a solid polymer electrolyte. However, the electrolyte membrane 20 may have a different configuration. If the fuel cell is provided with a solid electrolyte membrane, the invention of the present application can be applied to other types of fuel cells. Can also be applied. For example, the present invention may be applied to a solid oxide fuel cell. The solid oxide fuel cell is a fuel cell including an electrolyte membrane made of a solid oxide exhibiting proton conductivity or oxide ion conductivity. In a fuel cell including a solid oxide exhibiting proton conductivity, a layer made of a solid oxide electrolyte having higher proton conductivity than the anode side may be provided as the cathode-side electrolyte layer, as in the embodiment. In a fuel cell including a solid oxide exhibiting oxide ion conductivity, a layer made of a solid oxide electrolyte having higher oxide ion conductivity than the cathode side may be provided as the electrolyte layer on the anode side. Thus, in the electrolyte membrane, by providing a high ion conductive electrolyte layer having higher ion conductivity on the side opposite to the side to which the electrode active material that generates ions conducting in the electrolyte membrane is supplied, the electrolyte membrane The ion conduction efficiency of portions other than the high ion conductive electrolyte layer in can be improved, and the battery performance can be improved. Here, as the solid oxide electrolyte generally increases in ionic conductivity, the distortion of the crystal structure increases and the strength decreases. Therefore, in addition to the effect of increasing the ionic conductivity of the entire electrolyte membrane by applying the present invention to a solid oxide fuel cell and providing a high ion conductive electrolyte layer, the entire electrolyte membrane is similar to the embodiment. The effect of ensuring the strength of the steel is also obtained.

2 is a schematic cross-sectional view showing a schematic configuration of a single cell 10. FIG. It is explanatory drawing which expands and shows typically the structure of MEA30. 5 is a process diagram illustrating a method for manufacturing MEA 30. FIG. It is explanatory drawing which shows typically the one part mode of the manufacturing process of MEA30. It is explanatory drawing which shows the result of having compared the power generation performance of the fuel cell. It is explanatory drawing which shows the result of having evaluated the proton mobility in an electrolyte membrane. FIG. 10 is an explanatory diagram schematically showing a part of the MEA manufacturing process according to Modification 2; FIG. 10 is an explanatory diagram schematically showing a part of the manufacturing process of the MEA of Modification 3;

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 ... Single cell 20 ... Electrolyte membrane 21 ... Anode 22 ... Cathode 23, 24 ... Gas diffusion layer 25, 26 ... Gas separator 27 ... Fuel gas flow path in single cell 28 ... Oxidation gas flow path in single cell 30 ... MEA
31 ... 1st electrolyte layer 32 ... 2nd electrolyte layer 34 ... Platinum carrying | support carbon particle 35 ... Base material 36 ... Electrolyte

Claims (10)

A fuel cell,
An electrolyte membrane;
An anode and a cathode, which are electrodes containing a catalyst formed on each surface of the electrolyte membrane;
With
The electrolyte membrane is formed on one side to which an electrode active material that generates ions conducting in the electrolyte membrane is supplied, and on the other side, and the first electrolyte layer A fuel cell comprising: a second electrolyte layer formed by a high ion conductive electrolyte having higher ion conductivity than the electrolyte forming the layer. The fuel cell according to claim 1, wherein
The electrolyte membrane is formed of a solid polymer electrolyte,
Ions that conduct in the electrolyte membrane are protons,
A fuel cell in which the first electrolyte layer is formed on the anode side and the second electrolyte layer is formed on the cathode side. The fuel cell according to claim 2, wherein
The second electrolyte layer is formed of an electrolyte having a lower weight value of the entire solid polymer electrolyte per unit amount of ion exchange groups related to proton conduction than the electrolyte constituting the first electrolyte layer. Fuel cell. A fuel cell according to any one of claims 1 to 3,
The second electrolyte layer is formed thinner than the first electrolyte layer. Fuel cell. The fuel cell according to claim 4, wherein
The second electrolyte layer is formed by only the first electrolyte layer having an ionic conductivity exhibited in the fuel cell by the electrolyte membrane made of the high ion conductive electrolyte and the first electrolyte layer. The fuel cell is formed thinner than the thickness of the electrolyte layer made of the high ion conductive electrolyte when the electrolyte membrane has ion conductivity equivalent to the ion conductivity shown in the fuel cell. The fuel cell according to claim 2 or 3, wherein
The electrode is a fuel cell in which a solid polymer electrolyte having proton conductivity similar to that of the first electrolyte layer is mixed between conductive particles carrying a catalyst metal dispersed on the surface thereof. An electrolyte membrane-electrode assembly for constituting a fuel cell,
Formed by forming an electrode containing a catalyst on the surface of the electrolyte membrane,
The electrolyte membrane has a first electrolyte layer formed on one surface side and a high ion conductivity which is formed on the other surface side and has higher ionic conductivity than the electrolyte forming the first electrolyte layer. An electrolyte membrane-electrode assembly, comprising: a second electrolyte layer formed of a conductive electrolyte. The electrolyte membrane-electrode assembly according to claim 7,
The electrolyte membrane is formed of a solid polymer electrolyte,
Ions that conduct in the electrolyte membrane are protons. Electrolyte membrane-electrode assembly. The electrolyte membrane-electrode assembly according to claim 8,
The second electrolyte layer is formed of an electrolyte having a lower weight value of the entire solid polymer electrolyte per unit amount of ion exchange groups related to proton conduction than the electrolyte constituting the first electrolyte layer. Electrolyte membrane-electrode assembly. An electrolyte membrane-electrode assembly according to any one of claims 7 to 9,
The second electrolyte layer is formed thinner than the first electrolyte layer. Electrolyte membrane-electrode assembly.

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