JP2011119209A - Catalyst layer for fuel cell and membrane-electrode assembly - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a catalyst layer for a fuel cell, assuring high-current performance under all conditions, and to provide a membrane-electrode assembly with the catalyst layer for the fuel cell. <P>SOLUTION: A cathode catalyst layer of an MEA (membrane-electrode assembly) 1 is bonded to one surface of an electrolyte membrane 11 and contains countless catalysts 90 each being formed by carrying Pt fine particles 92 on a carbon carrier 91, a polymer electrolyte 93, and a water-retentive water holding material. The water holding material contains a carbon carrier 96. The catalysts 90, the carbon carrier 96 and the polymer electrolyte 93 constitute: a catalyst complex 94 in which the catalysts 90 are coated with the polymer electrolyte 93; and a water holding complex 95 in which the carbon carrier 96 is coated with the polymer electrolyte 93. The coating of the polymer electrolyte 93 of the catalyst complex 94 is thinner than the coating of the polymer catalyst 93 of the water holding complex 95. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a fuel cell catalyst layer and a membrane electrode assembly.

  An electrode such as a cathode electrode or an anode electrode of a fuel cell includes a gas-permeable base material such as carbon cloth, carbon paper, or carbon felt, and a catalyst layer formed on one surface of the base material. The portion other than the catalyst layer is constituted by a base material, which is a diffusion layer that diffuses air and fuel into the catalyst layer on the non-electrolyte layer side. The catalyst layer contains a catalyst and a polymer electrolyte. The catalyst is formed by supporting fine catalyst particles such as platinum (Pt) on a carbon support. In addition, in this fuel cell, it is also known that a water retention material capable of holding water is included in the catalyst layer in order to prevent flooding and to facilitate an electrochemical reaction (Patent Document 1).

JP 2005-174765 A

  However, the catalyst layer for the fuel cell and the fuel cell that can achieve high current performance under all conditions by improving the catalytic activity, preventing flooding under high humidification conditions and preventing dry-up under low humidification conditions. There is a need for a membrane electrode assembly having a catalyst layer.

  The present invention has been made in view of the above-described conventional situation, and provides a fuel cell catalyst layer capable of ensuring high current performance under all conditions, and a membrane electrode assembly including the fuel cell catalyst layer. It is a problem to be solved.

  According to the test results of the inventors, the smaller the weight ratio (E / C ratio) between the polymer electrolyte and the carbon support in the catalyst layer is, as shown in FIG. The catalyst 90 on which 92 is supported covers only a small amount of the polymer electrolyte 93. That is, in the catalyst composite 94, the catalyst 91 is thinly covered with a small amount of the polymer electrolyte 93. For this reason, as shown in FIG. 1B, when the catalyst layer 81 is formed only by the catalyst composite 94 and the fuel cell is configured together with the electrolyte layer 11, oxygen and hydrogen are easily supplied to the catalyst.

  On the contrary, as the E / C ratio is larger, as shown in FIG. 2A, the catalyst complex 94 is covered with a larger amount of the polymer electrolyte 93 in the catalyst composite 94. For this reason, as shown in FIG. 2B, in the fuel cell in which the catalyst layer 81 is formed only by the catalyst composite 94, it is difficult for oxygen and hydrogen to be supplied to the catalyst.

  For this reason, in this fuel cell, as shown in FIG. 3, the catalyst activity improves as the E / C ratio decreases, and the catalyst activity decreases as the E / C ratio increases. FIG. 3 is a graph showing the relationship between the E / C ratio of the catalyst paste constituting the cathode catalyst layer and the weight specific activity of the catalyst fine particles (Pt fine particles 92). This cathode catalyst layer employs a catalyst 90 obtained by supporting Pt fine particles 92 on BLACKPEARL 880 (trade name, manufactured by CABOT, hereinafter abbreviated as BP880) as the carbon carrier 91. The weight specific activity means the value of the current flowing through the Pt fine particles 92 per gram in the cathode catalyst layer when the potential is set to 0.9 V, and is a value serving as an index of the catalyst activity. In this graph, a cell having a membrane electrode assembly (MEA) having the above-described cathode catalyst layer is prepared, the cell temperature is set to 50 ° C., the relative humidity is set to 100% RH, and the anode electrode Hydrogen and air are supplied to the cathode electrode at normal pressure.

  As this graph shows, the smaller the E / C ratio in the cathode catalyst layer, that is, the smaller the weight ratio of Nafion in the ionomer solution with respect to the weight of the carbon support 91, the smaller the current flowing through the Pt fine particles 92 per gram in the cathode catalyst layer. The value of increases. That is, as shown in FIG. 1B, as the layer made of the polymer electrolyte 93 formed around the catalyst 90 becomes thinner, oxygen or hydrogen is diffused or supplied more quickly during the electrochemical reaction. The weight specific activity of is improved.

  However, since the amount of the polymer electrolyte 93 in the entire catalyst layer 81 is small, water hardly moves. For this reason, under high humidification conditions, the high current performance is degraded mainly by flooding. Moreover, under low humidification conditions, the catalyst layer is easily dried up, proton conductivity is deteriorated, and high current performance is also lowered.

  The inventors have discovered that this problem can be solved when carbon is employed as part of the water retention material.

That is, the catalyst layer for a fuel cell of the present invention comprises a myriad of catalysts bonded to one surface of an electrolyte layer and having catalyst fine particles supported on a carbon support, a polymer electrolyte, and a water retention material capable of holding water. And
The water retention material includes the carbon carrier,
The catalyst, the carbon carrier and the polymer electrolyte constitute a catalyst complex in which the catalyst is coated with the polymer electrolyte, and a water retention complex in which the carbon carrier is coated with the polymer electrolyte,
The coating thickness of the polymer electrolyte of the catalyst complex is smaller than the coating thickness of the polymer electrolyte of the water retention complex (Claim 1).

  As shown in FIGS. 4A and 4B, the fuel cell catalyst layer 82 of the present invention has a catalyst composite 94 and a water retention composite 95. As shown in FIG. 4A, the catalyst complex 94 has a catalyst 91 covered with a small amount of polymer electrolyte 93 as compared with the water retention complex 95. Thereby, when each carbon support has the same specific surface area, the average coating thickness of the polymer electrolyte 93 covering the catalyst composite 94 is smaller than that of the polymer electrolyte 93 covering the water retention composite 95. When these are mixed to form the fuel cell catalyst layer of the present invention, the amount of the polymer electrolyte 93 included in the catalyst composite 94 is less than the amount of the polymer electrolyte 93 included in the water retention composite 95, and E / The C ratio remains small. For this reason, the catalyst layer 82 ensures high catalytic activity by the catalyst complex 94 in which the catalyst 90 is thinly coated with a small amount of the polymer electrolyte 93.

  In the catalyst layer 82, the water retention complex 95 functions as a water retention material. In the water retaining composite 95, since the carbon carrier 96 is coated with a larger amount of the polymer electrolyte 93 than the catalyst composite 94, the amount of the polymer electrolyte 93 in the catalyst layer 82 is secured, Can move easily. Therefore, the catalyst layer 82 realizes flooding under high humidification conditions and prevention of dry-up under low humidification conditions.

  Therefore, according to this fuel cell catalyst layer, it is possible to ensure high current performance under all conditions.

  The same carbon carrier may be adopted as the carbon carrier constituting the catalyst and the carbon carrier constituting the water retention complex, or different carbon carriers may be adopted. When the specific surface area of the carbon support constituting the catalyst composite 94 is larger than that of the carbon support constituting the water retention composite 95, the E / C ratio is adjusted as appropriate, and the average of the polymer electrolyte 93 of the catalyst composite 94 is adjusted. The coating thickness may be made thinner than the average coating thickness of the polymer electrolyte 93 of the water retaining composite 95.

The membrane electrode assembly of the present invention is a membrane electrode assembly comprising an electrolyte layer, a cathode catalyst layer bonded to one surface of the electrolyte layer, and an anode catalyst layer bonded to the other surface of the electrolyte layer. ,
At least the cathode catalyst layer contains a myriad of catalysts in which catalyst fine particles are supported on a carbon support, a polymer electrolyte, and a water retention material capable of holding water,
The water retention material includes the carbon carrier,
The catalyst, the carbon carrier and the polymer electrolyte constitute a catalyst complex in which the catalyst is coated with the polymer electrolyte, and a water retention complex in which the carbon carrier is coated with the polymer electrolyte,
The coating thickness of the polymer electrolyte of the catalyst complex is smaller than the coating thickness of the polymer electrolyte of the water retention complex (Claim 2).

  In the membrane electrode assembly of the present invention, at least the cathode catalyst layer has the above characteristics. For this reason, according to this MEA, it is possible to ensure high current performance under all conditions.

  In this membrane electrode assembly, at least the cathode catalyst layer can be formed in a layered form by laminating a water retention layer made of a water retention complex on the opposite side of the electrolyte layer. In this case, a water retention layer is separately laminated in the cathode catalyst layer. For this reason, it is thought that this membrane electrode assembly can further enhance the above-described effects by laminating a water retention layer. The water retention layer is not limited to a single layer.

The catalyst layer for a fuel cell of the present invention may contain a water retention material other than the carbon support. As other water-retaining materials, it is possible to employ conductive resins, conductive ceramics, conductive metal oxides, water-absorbing polymers, water-absorbing fibers, and the like. Examples of the metal oxide include TiO ₂ and SiO ₂ . Examples of the water-absorbing polymer include silica gel and polyacrylic acid. Furthermore, examples of water-absorbing fibers include tinsel and chemical fibers.

It is a model enlarged view around the catalyst when the E / C ratio is small. The figure (A) shows a catalyst and a polymer electrolyte, and the figure (B) shows a part of MEA. It is a model enlarged view around the catalyst when the E / C ratio is large. The figure (A) shows a catalyst and a polymer electrolyte, and the figure (B) shows a part of MEA. It is a graph which shows the relationship between E / C ratio of the catalyst paste which comprises a cathode catalyst layer, and the weight specific activity of a catalyst fine particle. It is a model enlarged view around the catalyst of the present invention. Fig. (A) shows the catalyst, polymer electrolyte and water retention material in the mixed paste, and Fig. (B) shows a part of the MEA. It is a 3D-TEM image which shows distribution of Pt microparticles | fine-particles in the carbon support | carrier which has a pore of about 3.5 nm. The figure (A) shows a 2D-TEM image, and the figure (B) shows a three-direction slice image. It is an expansion schematic diagram which shows the electrochemical reaction in the catalyst layer which has a carbon support | carrier which has a pore of about 3.5 nm. It is a graph which shows distribution of the pore of a catalyst layer when a catalyst layer which has a catalyst which has a carbon support which has a pore of about 3.5 nm changes E / C ratio. It is a graph which shows the relationship between E / C ratio and the pore volume of a catalyst layer. It is an expansion schematic diagram which shows the electrochemical reaction in the catalyst layer which has a carbon support | carrier which has a pore of about 4 nm or more. 2 is a partially enlarged schematic view showing a part of an MEA of Example 1. FIG. It is a graph which shows the relationship between E / C ratio of a water retention material paste, weight ratio / volume ratio, and the thickness increase rate of a catalyst layer. It is a graph which shows the change of the cell current and cell voltage in a full humidification state in connection with the experiment example 2 under the difference in E / C ratio. It is a graph which shows the change of the cell current and the cell voltage in a low humidification state in connection with the experiment example 3 under the difference in E / C ratio. It is a graph which shows the change of the cell current and cell voltage in a full humidification state in connection with the example 4 of an experiment, with and without the water retaining material. It is a graph which shows the change of the cell electric current and cell voltage in a low humidification state in connection with the example 5 of an experiment, with and without the water retention material. It is a graph which shows the change of the cell current and cell voltage in a low humidification state in connection with the example 6 of an experiment, with and without the water retention material. 6 is a partial enlarged schematic view showing a part of an MEA of Example 2. FIG.

(Experimental example 1)
The inventors consider that it is necessary to improve the density at the active point of the reaction in order to improve the performance of the fuel cell, increase the specific surface area of the carbon support, and increase the dispersion rate of more catalyst fine particles on the carbon support It has been aimed to be supported by.

For example, when a carbon support having a specific surface area of 800 m ² / g or more is used and 50 wt% or more of Pt fine particles are supported thereon, the specific surface area of the Pt fine particles can be 100 m ² / g-Pt or more. Examples of such a carbon carrier include Ketjen Black EC (trade name (hereinafter, the same) manufactured by Ketjen Black International), Ketjen Black EC-600JD (hereinafter abbreviated as KB600JD), and the like. it can.

  By increasing the amount of catalyst fine particles supported in this way, the catalyst layer can be made thinner, and an MEA with high activity and low concentration overvoltage can be provided.

  FIG. 5 shows a 3D-TEM observation result of a catalyst in which Pt fine particles 92 are supported at a support density of 60 wt% on KB600JD as the carbon support 91. FIG. 5A shows a 2D-TEM image, and FIG. 5B shows a three-direction slice image. From the right figure, it is confirmed that Pt fine particles 92 are present inside the catalyst. Moreover, a catalyst layer is manufactured with this catalyst, and a part of MEA having this catalyst layer is schematically shown in an enlarged manner in FIG. In FIG. 6, the Pt fine particles 92a are indicated by black circles. As shown in FIGS. 5 and 6, in the catalyst 90 to be observed, about 60% of the Pt fine particles 92 are present in the carbon support 91, and as a result, about 50% of the surface of the Pt fine particles 92 serving as an active site. The area is within the carbon support 91.

  If the Pt fine particles 92 present in the carbon carrier 91 do not contribute to power generation, a considerable proportion of the supported Pt fine particles 92 exists in vain. If the amount of Pt fine particles 92 supported is sufficiently large, sufficient performance can be obtained with only the Pt fine particles 92 present on the outer surface of the carbon support 91. However, in order to reduce the amount of Pt fine particles 92 used, In order to reduce the loading amount and maintain the performance, it is necessary to reduce the ratio of the Pt fine particles 92 present in the carbon support 91 as much as possible and increase the utilization rate of the Pt fine particles 92.

FIG. 7 shows the measurement results of N ₂ adsorption in the catalyst layer. The catalyst layer is produced by changing the weight ratio (E / C ratio) between the polymer electrolyte functioning as an ionomer and the carbon support.

  According to FIG. 7, the pore volume of the carbon support that decreases when the E / C ratio is increased is mainly due to pores having a pore diameter of 4 nm or more. The pore volume due to pores having a pore diameter of less than about 4 nm in the carbon support hardly changes even when the E / C ratio is increased. This shows that the pores in the carbon support are hardly blocked by the polymer electrolyte.

  FIG. 8 is a graph showing the relationship between the E / C ratio and the pore volume of the carbon support based on the results of FIG. FIG. 8 shows that the pore volume that decreases when the E / C ratio is increased is mainly that having a pore diameter of 4 nm or more, and the pore volume due to pores having a pore diameter of less than 4 nm hardly changes.

  From the above results, it can be seen that the polymer electrolyte does not enter the pores of less than 4 nm of the carbon support, and the catalyst fine particles existing in the pores of less than 4 nm cannot contact the electrolyte. It can also be seen that a three-phase interface is not formed around such catalyst fine particles and cannot contribute to power generation.

  Based on the above findings, the inventors have filed a patent application that a catalyst obtained by supporting catalyst fine particles on a carbon support having only pores having a pore diameter of 4 nm or more is suitable as a fuel cell catalyst. (Japanese Patent Application No. 2009-013219). According to this catalyst, catalyst fine particles can be prevented from entering pores into which the polymer electrolyte cannot enter. For this reason, the utilization rate of catalyst fine particles can be increased by using this carbon carrier, and as a result, the amount of platinum used can be reduced.

  In addition, as shown in FIG. 6, the inventors of the present invention have a catalyst using a carbon carrier having pores of about 3.5 nm. If the catalyst layer is formed by increasing the E / C ratio, oxygen and hydrogen can be produced. It was confirmed that it was difficult to supply to the catalyst. On the other hand, as shown in FIG. 9, in a catalyst using a carbon support having no pores with a pore diameter of less than 4 nm, that is, a low specific surface area carbon support, the catalyst layer is formed by reducing the E / C ratio. It was also confirmed that oxygen and hydrogen were easily supplied to the catalyst.

Example 1
As shown in FIG. 10, the MEA 1 of Example 1 is joined to the electrolyte membrane 11 made of Nafion, the cathode electrode 3 joined to one surface of the electrolyte membrane 11 and supplied with air, and joined to the other surface of the electrolyte membrane 11. And an anode 5 to which a fuel such as hydrogen is supplied.

  The cathode electrode 3 includes a base material made of carbon paper and a cathode catalyst layer 10 formed on one surface of the base material. A portion other than the cathode catalyst layer 10 in the cathode electrode 3 is formed of a base material, which is a cathode diffusion layer 13 that diffuses air into the cathode catalyst layer 10 on the non-electrolyte layer side. The anode 5 is an anode catalyst layer 12 and an anode diffusion layer 14. The cathode catalyst layer 10 of the MEA 1 is formed of a catalyst paste and a water retention material paste.

  The catalyst paste constituting the catalyst composite 94 is manufactured by the following manufacturing method. First, a commercially available BP880 was prepared as the carbon carrier 91 shown in FIG. This BP880 has a primary particle size of about 15 nm. Pt fine particles 92 as catalyst fine particles were supported on BP880 at a support density of 20 wt% to obtain catalyst 90. Water was added to this catalyst 90, defoamed and stirred with a rotation / revolution centrifugal stirrer (trade name “Hybrid Mixer HM-500” manufactured by Keyence Corporation), and the catalyst 90 and water were blended together. A paste was obtained.

  A 5 wt% ionomer solution (Nafion solution (5 mass% solution)) was added to the pre-paste. At this time, the ionomer solution was added to the catalyst 90 so that (weight of Nafion in the ionomer solution) / (weight of the carbon support 91) (E / C ratio) was 0.15. Thereafter, these were further stirred by a rotation / revolution centrifugal stirrer to obtain a catalyst paste. The catalyst paste constitutes a catalyst composite 94 in which the catalyst 90 is coated with a thin polymer electrolyte 93.

  On the other hand, the water retention material paste constituting the water retention complex 95 is manufactured by the manufacturing method shown below. First, Vulcan-XC72R (trade name, manufactured by CABOT) was prepared as a carbon carrier 96 having water retention. To this Vulcan-XC72R, a solvent and a 5% by weight Nafion solution were added. At this time, the Nafion solution was added so that (weight of Nafion in Nafion solution) / (weight of carbon) (E / C ratio) was 1.0. Then, these were stirred with a rotation / revolution centrifugal stirrer to obtain a water retention paste. The water retention paste constitutes a water retention complex 95 in which a carbon carrier 96 is coated with a thick polymer electrolyte 93.

  The obtained catalyst paste and water retention material paste were mixed at a solid content weight ratio of 0.65: 0.35 to obtain a mixed paste. By mixing these at this ratio, the E / C ratio of the entire mixed paste becomes 0.35.

  The mixed paste was screen-printed on a substrate and then dried to obtain a cathode electrode 3. The printed portion is the cathode catalyst layer 10. In addition, as a method of applying the mixed paste to the substrate, it can be performed by means such as a spray method or an ink jet method in addition to screen printing. Moreover, the anode catalyst layer 12 of the anode electrode 5 was obtained by screen-printing the above catalyst paste on a substrate.

FIG. 11 is a graph showing the relationship between the E / C ratio, the weight ratio / volume ratio, and the catalyst layer thickness increase rate of the water retention material paste. In the catalyst layer, the supported amount of the Pt fine particles 92 is 0.1 mg / cm ² .

  In FIG. 11, when the optimum value of each numerical value is obtained under the condition that the entire cathode catalyst layer 10 does not become extremely thick while securing the network for the proton path in the water retention material, The volume ratio of the solid content of the paste is in the range of 0.15 to 0.25.

  In FIG. 11, when the rate of increase in the thickness of the cathode catalyst layer 10 is 2 or less and the E / C ratio of the water retention material paste is determined, the E / C ratio of the water retention material paste is in the range of 0.6 to 1.2. It becomes. Note that the rate of increase in the thickness of the cathode catalyst layer 10 (catalyst layer thickness increase rate) refers to the thickness when the cathode catalyst layer 10 is manufactured using only the catalyst paste, and the cathode catalyst layer 10 as a catalyst paste and a water retaining material paste. It is a ratio with the thickness at the time of manufacturing with.

  From the above conditions, when the E / C ratio of the water retention material paste is 1.0, the weight ratio of the solid content in the water retention material paste is 0.35 and the volume ratio is 0.17. Further, the thickness increase rate of the cathode catalyst layer 10 is 1.4.

  The cathode electrode 3 and the anode electrode 5 thus obtained are joined to the electrolyte layer 11 to obtain the MEA 1 shown in FIG.

  In the obtained MEA 1, the cathode catalyst layer 10 constituting the cathode electrode 3 is formed of a catalyst paste and a water retention material paste. For this reason, as shown in FIG. 4B, the cathode catalyst layer 10 includes a water retention complex 95 capable of retaining water. The water retention complex 95 includes a polymer electrolyte 93. And the water retention material paste which comprises this water retention composite body 95 is mixing Vulcan-XC72R and the polymer electrolyte 93 based on the relationship shown by said FIG. Therefore, in the cathode catalyst layer 10, as shown in FIG. 4, the cathode catalyst layer 10 has a proton path and an electron path simultaneously with the water path due to the polymer electrolyte 93 in the water retention complex 95. Network is secured. For this reason, in the cathode catalyst layer 10, flooding is unlikely to occur in a high current region, and the smooth progress of the electrochemical reaction is hardly hindered. Further, the cathode catalyst layer 10 is difficult to dry during an electrochemical reaction in a low humidification environment, and the smooth progress of the electrochemical reaction is difficult to be prevented even in a low humidification environment.

  Further, in the cathode catalyst layer 10, as described above, the E / C ratio of the catalyst paste is 0.15, and the E / C ratio of the water retention material paste is 1.0. For this reason, the amount of the polymer electrolyte 93 included in the catalyst complex 94 is smaller than the amount of the polymer electrolyte 93 included in the water retention complex 95. For this reason, the catalyst composite 94 is thinned by the polymer electrolyte 93 formed around the catalyst 90. In particular, since the E / C ratio of the catalyst paste is as low as 0.15, the cathode catalyst layer 10 is sufficiently supplied with oxygen during the electrochemical reaction, and the weight specific activity of the Pt fine particles 92 is high. Therefore, the electrochemical reaction in the low current region proceeds smoothly and the MEA performance is enhanced (see FIG. 9).

  Therefore, the MEA 1 of Example 1 can ensure high current performance under all conditions. In addition, since the MEA 1 can reduce the amount of Pt fine particles 92 in the catalyst 90, the manufacturing cost can be reduced.

{Verification}
Next, in order to grasp the voltage characteristics of the MEA 1 of Example 1, verification was performed by the following experiments using a cell including the MEA 1 of Example 1 and a cell including the MEA of Comparative Examples 1 to 5. . In addition, the structure and manufacturing method of the cell provided with MEA1 of Example 1 and MEA of Comparative Examples 1-5 are the same as a well-known structure and method.

The cathode catalyst layers of the MEAs in Comparative Examples 1 to 4 are all single catalyst layers that do not contain the water retaining composite 95. Moreover, the E / C ratio in the cathode catalyst layer (catalyst paste) of each MEA is as follows.
Comparative Example 1: E / C ratio = 0.10
Comparative Example 2: E / C ratio = 0.15
Comparative Example 3: E / C ratio = 0.35
Comparative Example 4: E / C ratio = 0.60
The MEA of Comparative Example 2 has the same conditions as the cathode catalyst layer 10 in the MEA 1 of Example 1 except for the presence or absence of the water retention complex 95. Other configurations and manufacturing methods of the MEAs of Comparative Examples 1 to 4 are the same as those of the MEA of Example 1.

(Experimental example 2)
In Experimental Example 2, the temperature of each cell including the MEAs of Comparative Examples 1 to 3 was set to 50 ° C., and the anode 5 was measured under a measurement environment (hereinafter referred to as a fully humidified state) with a humidity of 100% RH. The change in the current of each cell and the voltage of each cell was measured when hydrogen was passed through the cathode electrode 3 and air was passed through the cathode 3. The measurement results are shown in FIG.

  In FIG. 15, the horizontal axis indicates the cell current (A), and the vertical axis indicates the cell voltage (V) (the same applies to FIGS. 13 to 16 shown below). As shown in FIG. 12, in a fully humidified state, a cell (MEA) having a cathode catalyst layer having a high E / C ratio can exhibit a higher voltage in a high current region. In addition, a cell (MEA) having a cathode catalyst layer having a high E / C ratio has a smaller degree of voltage change with respect to current change.

(Experimental example 3)
In Experimental Example 3, the temperature of each cell including the MEAs of Comparative Examples 2, 3, and 4 was set to 70 ° C., and the anode was measured in a measurement environment (hereinafter referred to as a low humidified state) with a humidity of 40% RH. Changes in current and voltage of each cell were measured when hydrogen was supplied to the electrode 5 and air was supplied to the cathode electrode 3. The measurement results are shown in FIG.

  As shown in FIG. 13, even in a low humidified state, a cell having a cathode catalyst layer having a high E / C ratio can exhibit a high voltage in a high current region. Even in this environment, the cell with the cathode catalyst layer having a high E / C ratio has a smaller degree of voltage change with respect to the current change.

  These experiments revealed the following points. That is, when the E / C ratio of the cathode catalyst layer (catalyst paste) is reduced as in the MEAs of Comparative Examples 1 and 2, the layer made of the polymer electrolyte 93 formed around the catalyst 90 as described above becomes thin. . For this reason, the weight specific activity of the Pt fine particles 92 can be increased during the electrochemical reaction (see FIG. 3). On the other hand, in the MEA of Comparative Example 1, since the layer made of the polymer electrolyte 93 becomes thin, the water path becomes insufficient. For this reason, as shown in FIG. 12, in the high current region under the fully humidified condition (highly humidified condition), the water generated in the cathode catalyst layer is not rapidly moved, and the catalyst layer including the cathode catalyst layer is included. Overall, flooding occurs. For this reason, in the MEA of Comparative Example 1 having a low E / C ratio, the smooth progress of the electrochemical reaction is hindered, and the voltage in the high current region is greatly reduced. For this reason, the performance of a fuel cell will fall.

  Similarly, when the E / C ratio of the cathode catalyst layer is reduced as in the MEA of Comparative Example 2, the water retention function of the entire catalyst layer is lowered. For this reason, as shown in FIG. 13, in the high current region under the low humidification state, the entire catalyst layer including the cathode catalyst layer is dried, resulting in dry up. For this reason, as in the fully humidified state, the smooth progress of the electrochemical reaction is hindered, and the voltage drop in the high current region increases. For this reason, the performance of a fuel cell will fall like the above.

(Experimental example 4)
In Experimental Example 4, for the cell including the MEA 1 of Example 1 and the cell including the MEA of Comparative Example 2, hydrogen was supplied to the anode electrode 5 and air was supplied to the cathode electrode 3 under a fully humidified state. Changes in the current and voltage of each cell were measured. The measurement results are shown in FIG.

  As shown in FIG. 14, in the fully humidified state, the cell with MEA 1 of Example 1 can exhibit higher voltage in the high current region than the cell with MEA of Comparative Example 2. is there. Further, the cell including the MEA 1 of the first embodiment has a small degree of voltage change with respect to current change.

(Experimental example 5)
In Experimental Example 5, the cell including the MEA 1 of Example 1 and the cell including the MEA of Comparative Example 2 were subjected to a flow of hydrogen to the anode electrode 5 and air to the cathode electrode 3 in a low humidified state. Changes in the current and voltage of each cell were measured. The measurement results are shown in FIG.

  As shown in FIG. 15, even in a low humidified state, the cell provided with MEA 1 of Example 1 can exhibit a higher voltage in the high current region than the cell provided with MEA of Comparative Example 2. It is. Even in this environment, the cell including the MEA 1 of Example 1 has a smaller degree of voltage change relative to the current change than the cell including the MEA of Comparative Example 2.

  As a result of these experiments, the cell including the MEA 1 of Example 1 exhibited higher voltage characteristics with respect to current in both the fully humidified state and the low humidified state as compared with the cell including the MEA of Comparative Example 2. That is, it was proved that the MEA 1 of Example 1 has higher power generation stability, that is, robustness with respect to changes in the environment and current than the MEA of Comparative Example 2.

  The reason for this is considered to be the effect of the water retention complex 95 included in the cathode catalyst layer 10 as shown in FIG. That is, in the MEA 1 of Example 1, in the cathode catalyst layer 10, a low specific surface area carbon that increases the utilization rate of catalyst fine particles is used, and the E / C ratio of the catalyst paste is set to a low value of 0.15. By keeping the layer made of the polymer electrolyte 93 formed around the catalyst 90 thin, oxygen or hydrogen can be diffused or supplied quickly during the electrochemical reaction, so that the weight specific activity of the catalyst fine particles, ie, low The fuel cell performance in the current region is improved. Further, the water retention complex 95 is added to the cathode catalyst layer 10 to increase the E / C of the cathode catalyst layer 10 as a whole to 0.35, thereby ensuring a sufficient water path in the cathode catalyst layer 10. Thereby, it was possible to suppress flooding in a fully humidified state and dry-up in a lowly humidified state, which are problems at low E / C. As a result, the MEA 1 of Example 1 can exhibit high performance in all states compared to the MEA of Comparative Example 2 that does not contain the water retention complex 95.

(Experimental example 6)
As a reference experiment, in Experimental Example 6, the MEA of Comparative Example 5 was prepared. In this MEA, like the MEA of Example 1, the E / C ratio of the catalyst paste constituting the cathode catalyst layer is 0.15. In addition, the MEA of Comparative Example 5 is different from the case of Example 1 in the mixing ratio of the catalyst paste and the water retention material paste, and the E / C ratio of the entire MEA including the water retention complex is 0.60. ing. In this Experimental Example 6, when the cell provided with the MEA of Comparative Example 5 and the cell provided with the MEA of Comparative Example 2 were supplied with hydrogen flowing into the anode electrode 5 and air flowing into the cathode electrode 3 in a low humidified state. The change in the current and voltage of each cell was measured. The measurement results are shown in FIG.

  As shown in FIG. 16, in the low humidification state, the cell including the MEA of Comparative Example 5 can exhibit a high voltage in a high current region. Further, since the E / C ratio in the MEA of Comparative Example 5 is 0.60, even when compared with the MEA 1 of Example 1 in which the E / C ratio shown in FIG. The degree of change in voltage is small, indicating higher performance.

(Example 2)
As shown in FIG. 17, in the MEA 31 of Example 2, the cathode catalyst layer 30 in the cathode electrode 3 is formed by sequentially laminating a first layer 42 and a second layer 43. The first layer 42 is made of the mixed paste of Example 1, and the second layer is made of the water retention material paste of Example 1.

  This cathode catalyst layer 30 is obtained by the following method. First, the water-retaining material paste is screen-printed on a substrate and then dried. In this way, the second layer 43 is formed. Thereafter, the mixed paste is screen-printed on the surface of the second layer 43 and dried. In the cathode catalyst layer 30 thus obtained, the first layer 42 and the second layer 43 are sequentially formed in layers on the electrolyte membrane 11. That is, the second layer 43 is the opposite side of the electrolyte layer 11, that is, the non-electrolyte layer side. Other configurations and manufacturing processes in the MEA 21 are the same as those in the first embodiment, and the same configurations are denoted by the same reference numerals and detailed description of the configurations is omitted.

  As described above, in the MEA 31, the first layer 42 and the second layer 43 are formed on the cathode catalyst layer 30. As described above, the first layer 42 is obtained by a mixed paste in which a catalyst paste and a water retention material paste are mixed. That is, the MEA 31 has a water retention complex 95 in both the first layer 42 and the second layer 43. For this reason, it is considered that this MEA 31 can obtain a higher effect than MEA 1 of the first embodiment.

  Therefore, the MEA 31 of Example 2 can achieve both reduction in the amount of Pt fine particles 92 in the catalyst 90 and high output.

  In the above, the present invention has been described with reference to the first and second embodiments. However, the present invention is not limited to the first and second embodiments, and can be appropriately modified and applied without departing from the spirit of the present invention. Needless to say.

  For example, in Examples 1 and 2, the anode catalyst layer 12 may have the same configuration as the cathode catalyst layer 10.

  INDUSTRIAL APPLICABILITY The present invention can be used for a fuel cell system such as a moving power source for an electric vehicle, an outdoor stationary power source, and a portable power source.

91 ... Carbon support 92 ... Pt fine particles (catalyst fine particles)
90 ... Catalyst 93 ... Nafion (Polymer electrolyte)
94 ... Catalyst complex 95 ... Water retention complex 11 ... Nafion membrane (electrolyte layer)
DESCRIPTION OF SYMBOLS 10, 30 ... Cathode catalyst layer 12 ... Anode catalyst layer 1, 31 ... MEA (membrane electrode assembly)

Claims (3)

Containing an infinite number of catalysts bonded to one surface of an electrolyte layer, in which catalyst fine particles are supported on a carbon carrier, a polymer electrolyte, and a water retention material capable of holding water;
The water retention material includes the carbon carrier,
The catalyst, the carbon carrier and the polymer electrolyte constitute a catalyst complex in which the catalyst is coated with the polymer electrolyte, and a water retention complex in which the carbon carrier is coated with the polymer electrolyte,
The catalyst layer for a fuel cell, wherein the coating thickness of the polymer electrolyte of the catalyst composite is thinner than the coating thickness of the polymer electrolyte of the water retention composite. A membrane electrode assembly comprising an electrolyte layer, a cathode catalyst layer bonded to one surface of the electrolyte layer, and an anode catalyst layer bonded to the other surface of the electrolyte layer,
At least the cathode catalyst layer contains a myriad of catalysts in which catalyst fine particles are supported on a carbon support, a polymer electrolyte, and a water retention material capable of holding water,
The water retention material includes the carbon carrier,
The catalyst, the carbon carrier and the polymer electrolyte constitute a catalyst complex in which the catalyst is coated with the polymer electrolyte, and a water retention complex in which the carbon carrier is coated with the polymer electrolyte,
The membrane electrode assembly, wherein the coating thickness of the polymer electrolyte of the catalyst composite is thinner than the coating thickness of the polymer electrolyte of the water retention composite.   The membrane electrode assembly according to claim 2, wherein at least the cathode catalyst layer is formed in a layered manner by laminating a water retention layer made of the water retention complex on the opposite side of the electrolyte layer.

Images