Catalyst layer

Abstract

To provide a catalyst layer that can improve the power generation performance of a polymer electrolyte fuel cell in a high current range.SOLUTION: A catalyst layer for a polymer electrolyte fuel cell includes a catalyst-supported carrier in which a catalyst is supported on a catalyst carrier, and a polymer electrolyte, and the catalyst layer has voids, from among the voids observed in the cross section in the thickness direction perpendicular to the surface of the catalyst layer, the total cross-sectional area of the voids having a cross-sectional area of 0.01 μm2 or more and 1 μm2 or less is 60% or more of the total cross-sectional area of all the voids.SELECTED DRAWING: Figure 2

Description

Technical Field

[0001] The present disclosure relates to a catalyst layer used in a solid polymer fuel cell.

Background Art

[0002] A fuel cell (FC) is composed of one single cell (hereinafter may be referred to as a cell) or a fuel cell stack in which a plurality of single cells are stacked (hereinafter may be simply referred to as a stack), and is a power generation device that extracts electrical energy through an electrochemical reaction between a fuel gas such as hydrogen and an oxidant gas such as oxygen. Fuel cells can be classified into alkaline, phosphoric acid, molten carbonate, solid polymer, solid oxide, etc. according to the type of electrolyte. Solid polymer fuel cells have attracted attention because they can be miniaturized and lightened, have a high output density at room temperature operation, and have excellent startup performance.

[0003] A single cell of a solid polymer fuel cell includes a structure in which a solid polymer electrolyte membrane (hereinafter may be simply referred to as an "electrolyte membrane") is sandwiched between a fuel electrode (anode) and an oxidant electrode (cathode). Each of the anode and the cathode usually has a catalyst layer and a gas diffusion layer in order from the side of the polymer electrolyte membrane. In the anode, hydrogen (H2) in the fuel gas supplied from the gas flow path and the gas diffusion layer is protonated by the catalytic action of the catalyst layer. The generated protons move through the electrolyte membrane to the cathode, and at the same time, the generated electrons move to the cathode through the external circuit. In the cathode, oxygen (O2) in the supplied oxidant gas reacts with protons and electrons in the catalyst layer to generate water. The generated water gives appropriate humidity to the electrolyte membrane, and the excess water permeates through the gas diffusion layer and is discharged out of the system. The catalyst layers provided in the anode and the cathode have fine voids, and these voids serve as passages for transporting substances such as gas and generated water. Therefore, attempts have been made to improve the power generation performance by controlling the size and distribution of the voids in the catalyst layer.

[0004] For example, in Patent Document 1, among the voids observed in the cross-section in the thickness direction perpendicular to the surface of the catalyst layer, 10,000 nm , , , ,

[0008] , , , , ,

[0006] , , , , , , , , , 2 , , , ,

[0007] ,

[0009] , , , The ratio occupied by the frequency of the voids having the above cross-sectional area is 13% or more and 20% or less, and the sum of the cross-sectional areas of the voids having a cross-sectional area of 10,000 nm 2 or more accounts for 40% or more and 50% or less of the total cross-sectional area of all the voids. A catalyst layer for a solid polymer fuel cell is disclosed.

[0005] In Patent Document 2, the pore area ratio, which is the ratio of the total area of pores to the area of the cross-section perpendicular to the bonding surface with the polymer electrolyte membrane, is 25.0% or more and 35.0% or less in the image obtained by a scanning electron microscope of the cross-section, and the ratio of the total area of pores having an area larger than 90,000 nm 2 to the total area of the pores is 15.0% or more and 30.0% or less. A catalyst layer is disclosed.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0007] However, in a solid polymer fuel cell, even if the size and distribution of voids in the catalyst layer are adjusted as described in Patent Document 1 or 2, the power generation performance in the high current region may be insufficient.

[0008] The present disclosure has been made in view of the above circumstances, and an object thereof is to provide a catalyst layer capable of improving the power generation performance in the high current region of a solid polymer fuel cell.

Means for Solving the Problems

[0010] In the catalyst layer of this disclosure, the catalyst support may be carbon particles.

[0011] In the catalyst layer of this disclosure, the mass ratio of the polymer electrolyte to the catalyst support (polymer electrolyte / catalyst support) may be 0.6 or more and 1.8 or less.

[0012] In the catalyst layer of this disclosure, the specific surface area of ​​the catalyst support is 600 m². 2 It may be more than / g.

[0013] The catalyst layer of this disclosure may be a cathode catalyst layer used in the cathode of a polymer electrolyte fuel cell. [Effects of the Invention]

[0014] The catalyst layer of this disclosure can improve the power generation performance of polymer electrolyte fuel cells in the high-current range. [Brief explanation of the drawing]

[0015] [Figure 1] Figure 1 schematically shows the relationship between the cumulative distribution of voids observed in the cross-section of the catalyst layer of a polymer electrolyte fuel cell and the power generation performance of the battery. [Figure 2] Figure 2 is a graph showing the relationship between the void area ratio of the cathode catalyst layer cross-sectional area of ​​0.01 to 1 μm2 and the current density at a voltage of 0.4 V for solid polymer fuel cells equipped with membrane electrode assemblies prepared in Examples 1 to 3 or Comparative Examples 1 to 2. [Figure 3] FIG. 3 is a graph showing the relationship between current density and voltage for a solid polymer fuel cell including the membrane electrode assembly fabricated in Example 1 or Comparative Example 1. [Figure 4] FIG. 4 is a graph showing the ratio of the void area with a cross-sectional area of 10,000 nm2 or more for the cathode catalyst layer included in the membrane electrode assembly fabricated in Example 1 or Comparative Example 1. MODE FOR CARRYING OUT THE INVENTION

[0016] 1. Catalyst layer The catalyst layer of the present disclosure is a catalyst layer for a solid polymer fuel cell including a catalyst-supported carrier in which a catalyst is supported on a catalyst carrier and a polymer electrolyte, where the catalyst layer has voids, and the sum of the cross-sectional areas of the voids observed in a cross-section in the thickness direction orthogonal to the surface of the catalyst layer and having a cross-sectional area of 0.01 μm 2 or more and 1 μm 2 or less is 60% or more of the sum of the cross-sectional areas of all the voids.

[0017] The catalyst layer of the present disclosure is disposed so as to be joined to a solid polymer electrolyte membrane in a cathode or an anode included in a solid polymer fuel cell. Therefore, the surface of the catalyst layer may be a joining surface with the solid polymer electrolyte membrane. In the present disclosure, a cross-section in the thickness direction orthogonal to the surface of the catalyst layer may be simply referred to as "the cross-section of the catalyst layer". Also, in the present disclosure, with respect to the sum of the cross-sectional areas of all the voids observed in the cross-section of the catalyst layer, the ratio of the sum of the cross-sectional areas of the voids having a cross-sectional area of 0.01 μm 2 or more and 1 μm 2 or less may be simply referred to as "the ratio of the void area with a cross-sectional area of 0.01 to 1 μm 2 ". Also, in the present disclosure, a solid polymer fuel cell may be simply referred to as "a fuel cell".

[0018] Simulation models of fuel cell power generation suggest that Knudsen diffusion is dominant in the transport of gases and generated water in the catalyst layer. The catalyst layer of this disclosure is presumed to improve the power generation performance of polymer electrolyte fuel cells in the high-current range because it contains many voids that are effectively utilized for power generation. Specifically, the cross-sectional area is 0.01 to 1 μm². 2 In voids of this size, mass transport by Knudsen diffusion is easily carried out, and since the catalyst layer of this disclosure has a large proportion of voids of this size, it is presumed that mass transport in the catalyst layer is carried out efficiently, thereby improving the power generation performance of the fuel cell. In power generation in the high current range (high load range), gas diffusion tends to be the rate-limiting factor and power generation performance tends to decrease, but when using the catalyst layer of this disclosure, the gas diffusion in the catalyst layer is good even in the high current range, so excellent power generation performance can be achieved. Figure 1 schematically shows the relationship between the cumulative distribution of voids observed in the cross-section of the catalyst layer and the power generation performance of a polymer electrolyte fuel cell. In Figure 1, the catalyst layer with the smallest void distribution is the one with a cross-sectional area of ​​0.01 μm². 2 Because there are many voids smaller than 0.01 to 1 μm², the cross-sectional area is 0.01 to 1 μm². 2 The void area ratio is less than 60%, and the catalyst layer where the distribution of voids is most skewed towards the large area side has a cross-sectional area of ​​1 μm. 2 Due to the large amount of excess void space, the cross-sectional area is 0.01 to 1 μm. 2 The void area ratio is less than 60%. Solid polymer fuel cells equipped with these catalyst layers have a cross-sectional area of ​​0.01 to 1 μm 2 Compared to polymer electrolyte fuel cells equipped with a catalyst layer having a void area ratio of 60% or more, its power generation performance is inferior, especially in the high-current range. The catalyst layer has a cross-sectional area of ​​0.01 to 1 μm 2 If the proportion of voids of this size is small, the gas diffusion resistance tends to increase, which is presumed to lead to a decrease in power generation performance. 2 If there are many voids smaller than 1 μm, drainage performance tends to deteriorate, which can also cause a decrease in power generation performance. 2If there are many excess voids, the proton resistance tends to increase, which is also thought to be a cause of reduced power generation performance. 2 If there are many excess voids, cracks are more likely to occur in the catalyst layer, which can lead to insufficient strength in the catalyst layer.

[0019] In the catalyst layer of this disclosure, the cross-sectional area is 0.01 to 1 μm 2 The void area ratio should be 60% or more, but preferably 65% ​​or more, and more preferably 70% or more, in order to further improve the power generation performance of the fuel cell. The cross-sectional area of ​​the catalyst layer is 0.01 to 1 μm. 2 There is no particular upper limit to the void area ratio, but from the viewpoint of ease of manufacture, it may be 90% or less, or 85% or less.

[0020] Furthermore, in this disclosure, the porosity of the catalyst layer is defined as the percentage of the total cross-sectional area of ​​all voids observed in the cross-section of the catalyst layer relative to the total area of ​​the cross-section. The catalyst layer of this disclosure preferably has a porosity of 30% or more, more preferably 45% or less, and more preferably 40% or less. When the porosity of the catalyst layer is within the above range, mass transport in the catalyst layer is more easily performed, which can improve the power generation performance of the fuel cell.

[0021] In this disclosure, the cross-sectional area of ​​each void observed in the cross-section of the catalyst layer is measured from a cross-sectional image of the catalyst layer observed using a scanning electron microscope (SEM) or the like, preferably at a magnification of 5,000x to 10,000x. When measuring the cross-sectional area of ​​each void, first, the voids are visualized by performing image processing on the cross-sectional image of the catalyst layer, such as cropping, filtering, or binarization. In this binarized image, pixels constituting the voids are represented as black pixels. The cross-sectional area of ​​the void can be determined by calculating the product of the number of black pixels constituting the void and the area of ​​one pixel. Cross-sectional area 0.01~1μm 2 The ratio of the void area is the sum of the cross-sectional areas of all voids present in the cross-sectional image (S A) and the cross-sectional area present in the cross-sectional image is 0.01 to 1 μm. 2 The sum of the cross-sectional areas of the gaps (S B ) Percentage ((S B / S A The answer is calculated as () × 100). The void ratio is calculated by the area of ​​the cross-section (S C The sum of the cross-sectional areas of all voids present in the cross-sectional image (S A ) Percentage ((S A / S C The answer is calculated as () × 100). In this disclosure, the cross-sectional area of ​​the catalyst layer is 0.01 to 1 μm. 2 The void area ratio and porosity are the average values ​​obtained from nine cross-sectional images taken from the catalyst layer. The cross-section of the catalyst layer can be obtained, for example, by cutting the catalyst layer in the thickness direction perpendicular to the surface using known methods such as ion milling or ultramicrotome.

[0022] The following describes the materials contained in the catalyst layer of this disclosure. The catalyst layer of this disclosure comprises a catalyst support on which a catalyst is supported, and a polymer electrolyte, and may further contain other materials as long as they do not impair the effects of this disclosure. The catalyst support is a catalyst support on which a catalyst is supported.

[0023] As a catalyst, for example, at least one selected from platinum and platinum alloys can be used. Examples of platinum alloys include alloys of platinum with a metallic material such as cobalt or nickel.

[0024] The catalyst support can be any material that is conductive and capable of supporting the catalyst without being affected by it, and is not particularly limited, but carbon supports can be preferably used. Examples of carbon supports include carbon particles such as carbon black, graphite, activated carbon, and fullerene, and carbon fibers such as carbon nanotubes and carbon nanofibers. Among these, carbon particles are preferred as the catalyst support in order to improve the power generation performance of the fuel cell. Commercially available catalyst supports may be used. Examples of commercially available carbon particles that can be used as carbon supports include Ketjenblack (trade name, manufactured by Ketjenblack International), Vulcan (trade name, manufactured by Cabot), Norit (trade name, manufactured by Norit), Black Pearl (trade name, manufactured by Cabot), and Acetylene Black (trade name, manufactured by Chevron).

[0025] The specific surface area of ​​the catalyst support is not particularly limited, but it is preferably 600 m² because it allows for high-density catalyst support and improves catalytic activity. 2 / g or more, comfortably 1000m 2 / g or more, more preferably 1300m 2 The amount is 1600 m² or more. The upper limit of the specific surface area of ​​the catalyst support is not particularly limited, for example, 1600 m². 2 The concentration may be less than or equal to / g. The specific surface area of ​​the catalyst support is determined by BET adsorption measurement.

[0026] From the standpoint of improving the power generation performance of fuel cells, the catalyst support is preferably in particulate form, more preferably with an average particle size of 10 nm to 10 μm, and even more preferably with an average particle size of 10 nm to 1 μm. If the average particle size of the catalyst support is above the lower limit, the gas diffusivity of the catalyst layer can be improved, and if it is below the upper limit, the dispersibility of the catalyst support can be improved, and the occurrence of cracks in the catalyst layer can be suppressed. An example of the method for calculating the average particle size in this disclosure is as follows: First, for a single particle, the particle size is calculated assuming the particle is spherical, using a transmission electron microscope (TEM) or scanning electron microscope (SEM) image at an appropriate magnification (e.g., 50,000 to 1,000,000 times). This calculation of particle size using TEM or SEM observation is performed for 200 to 300 particles of the same type, and the average of these particles is taken as the average particle size.

[0027] As the polymer electrolyte, a resin having proton conductivity can be used, and is not particularly limited; however, an ionomer can be preferably used from the viewpoint of improving the power generation performance of the fuel cell. The ionomer may be a fluororesin or the like. As a fluororesin, for example, a perfluorosulfonic acid resin such as Nafion® may be used.

[0028] In the catalyst layer of this disclosure, the mass ratio of the polymer electrolyte to the catalyst support (polymer electrolyte / catalyst support) is not particularly limited, but from the viewpoint of improving the power generation performance of the fuel cell, it is preferably 0.6 or more, more preferably 0.8 or more, and even more preferably 0.9 or more, while preferably 1.8 or less, more preferably 1.5 or less, and even more preferably 1.2 or less. Furthermore, in the catalyst layer of this disclosure, it is preferable that the polymer electrolyte is an ionomer, the catalyst support is a carbon support, and the mass ratio (I / C) of the ionomer to the carbon support is within the above range.

[0029] The thickness of the catalyst layer in this disclosure is not particularly limited, but may be, for example, 5 μm or more and 50 μm or less. If the thickness of the catalyst layer is above the lower limit, variations in the thickness of the catalyst layer are less likely to occur, thus improving the uniformity of performance. If the thickness of the catalyst layer is below the upper limit, gas diffusion in the catalyst layer can be improved, thus improving power generation performance.

[0030] The catalyst layer of this disclosure is for use in polymer electrolyte fuel cells and may be a cathode catalyst layer used in the cathode or an anode catalyst layer used in the anode. From the viewpoint of improving the power generation performance of the fuel cell, the catalyst layer of this disclosure is preferably used as at least a cathode catalyst layer in a polymer electrolyte fuel cell.

[0031] 2. Method for manufacturing the catalyst layer The catalyst layer of this disclosure comprises, for example, the step of preparing a catalyst slurry by mixing a catalyst, a catalyst support, a polymer electrolyte, a solvent, and other materials that may be further added as needed. The steps include applying the catalyst slurry onto a support, It can be manufactured by a method comprising the step of forming a catalyst layer by removing the solvent from the catalyst slurry.

[0032] The catalyst, catalyst support, and polymer electrolyte contained in the catalyst slurry are as described above. The catalyst and catalyst support added when preparing the catalyst slurry may be catalyst-supported supports on which the catalyst has been pre-loaded.

[0033] The solvent contained in the catalyst slurry is appropriately selected depending on the type of polymer electrolyte, etc., and is not particularly limited, but for example, water, methanol, ethanol, propanol, propylene glycol, N,N-dimethylformamide, N,N-diethylformamide, N,N-dimethylacetamide, N,N-diethylacetamide, etc. may be used. Alternatively, a mixed solvent obtained by mixing two or more solvents may be used as the solvent. The amount of solids in the catalyst slurry is not particularly limited and may be, for example, 5% by mass or more and 15% by mass or less.

[0034] The mixing method for preparing the catalyst slurry is not particularly limited, and known methods such as homogenizers, ball mills, shear mixers, and roll mills can be used. Cross-sectional area 0.01~1 μm 2 Since a catalyst layer with a void area ratio of 60% or more is easily formed, it is preferable to mix the catalyst slurry using a homogenizer.

[0035] The method for applying the catalyst slurry is not particularly limited, and known methods such as the doctor blade method, metal mask printing method, electrostatic coating method, dip coating method, spray coating method, roll coating method, gravure coating method, and screen printing method can be employed.

[0036] The support to which the catalyst slurry is applied may be a solid polymer electrolyte membrane or a peelable transfer sheet. The catalyst layer formed on the transfer sheet can be transferred and bonded to a solid polymer electrolyte membrane or various other substrates. As the transfer sheet, a self-supporting material can be appropriately selected and used. For example, metal foils such as Cu and Al, or resin sheets such as polytetrafluoroethylene (PTFE) can be used.

[0037] The method for drying the catalyst slurry applied to the support is not particularly limited, and known methods such as sound air drying and IR drying can be employed. The drying temperature is not particularly limited, but for a cross-sectional area of ​​0.01 to 1 μm 2 The preferred temperature is 50°C to 120°C, more preferably 75°C to 85°C, as this facilitates the formation of a catalyst layer with a void area ratio of 60% or more. The drying time is not particularly limited and can range from, for example, 3 seconds to 1 hour.

[0038] 3. Polymer electrolyte fuel cell The polymer electrolyte fuel cell of the present disclosure comprises a polymer electrolyte membrane, an anode including an anode catalyst layer bonded to one side of the polymer electrolyte membrane, and a cathode including a cathode catalyst layer bonded to the other side of the polymer electrolyte membrane, wherein at least one of the anode catalyst layer and the cathode catalyst layer is the catalyst layer of the present disclosure described above.

[0039] In the polymer electrolyte fuel cell of this disclosure, from the viewpoint of improving power generation performance in the high current range, it is preferable that at least the cathode catalyst layer is the catalyst layer of this disclosure, and both the anode catalyst layer and the cathode catalyst layer may be the catalyst layer of this disclosure. If either the anode catalyst layer or the cathode catalyst layer is not a catalyst layer of the present disclosure, a known catalyst layer conventionally used in polymer electrolyte fuel cells can be used as the catalyst layer that is not a catalyst layer of the present disclosure.

[0040] The polymer electrolyte fuel cell of this disclosure may consist of only one single cell, or it may be a fuel cell stack which is a stack of multiple single cells. Each single cell includes a membrane electrode gas diffusion layer assembly formed by sandwiching a polymer electrolyte membrane between an anode and a cathode, and may further include a pair of separators that sandwich the membrane electrode gas diffusion layer assembly. The separators may have supply holes, discharge holes, and flow paths for circulating the reaction gas and refrigerant in the stacking direction of the single cells. When the polymer electrolyte fuel cell of this disclosure is a fuel cell stack, the number of stacked single cells is not particularly limited and may be, for example, 2 to several hundred, 2 to 300, or 2 to 200. The fuel cell stack may be provided with end plates at both ends in the stacking direction of the single cells.

[0041] The anode and cathode of a membrane electrode gas diffusion layer assembly typically have a catalyst layer and a gas diffusion layer, in that order from the solid polymer electrolyte membrane side. That is, a membrane electrode gas diffusion layer assembly typically has an anode-side gas diffusion layer, an anode catalyst layer, a solid polymer electrolyte membrane, a cathode catalyst layer, and a cathode-side gas diffusion layer in this order. A microporous layer (MPL) or other layer may be present between the catalyst layer and the gas diffusion layer. As the microporous layer, for example, a layer containing a mixture of a water-repellent resin such as PTFE and a conductive material such as carbon black can be used.

[0042] As the gas diffusion layer, a known gas diffusion layer conventionally used in polymer electrolyte fuel cells can be used, and is not particularly limited; for example, it may be a conductive material having gas permeability. Examples of conductive materials used in the gas diffusion layer include carbon porous materials such as carbon cloth and carbon paper, and metal porous materials such as metal mesh and foamed metal.

[0043] Examples of solid polymer electrolyte membranes include fluorine-based electrolyte membranes such as thin films of perfluorosulfonic acid containing water, and hydrocarbon-based electrolyte membranes. The solid polymer electrolyte membrane may also be, for example, a Nafion® membrane.

[0044] As the separator, a known separator conventionally used in polymer electrolyte fuel cells can be used, and is not particularly limited; for example, a gas-impermeable conductive material may be used. Examples of conductive materials used in the separator include dense carbon, which is compressed to be gas-impermeable, and metal plates formed by press molding of metals such as iron, aluminum, or stainless steel. Furthermore, the separator may also have a current collection function.

[0045] In the polymer electrolyte fuel cell of this disclosure, the reaction gas supplied to the anode is a fuel gas, and the reaction gas supplied to the cathode is an oxidizing gas. The fuel gas is mainly a gas containing hydrogen, and may be hydrogen. The oxidizing gas may be oxygen, air, dry air, etc. Furthermore, in the polymer electrolyte fuel cell of this disclosure, a mixed solution of ethylene glycol and water can be used as the refrigerant for cooling in order to prevent freezing at low temperatures. [Examples]

[0046] (Example 1) A membrane electrode assembly (MEA) was fabricated by transferring a cathode catalyst layer to one side of a solid polymer electrolyte membrane (perfluorosulfonic acid (PFSA) membrane) and transferring an anode catalyst layer to the other side. The cathode catalyst layer was formed by the following method. First, a catalyst slurry was prepared by stirring and mixing a platinum-cobalt alloy-supported carbon carrier (PtCo / C), an ionomer (perfluorosulfonic acid-based resin), and a solvent containing water and alcohol using a homogenizer. Next, the obtained catalyst slurry was applied onto a transfer sheet (PTFE sheet) and dried at 80°C for 5 minutes to form a cathode catalyst layer with a thickness of 9 μm. The carbon carrier had a specific surface area of ​​1600 m². 2 Carbon particles of / g were used. In the cathode catalyst layer, the mass ratio of ionomer to carbon support (I / C) was set to 0.95. The anode catalyst layer was formed using a composite of a platinum-supported carbon support (Pt / C) and an ionomer (perfluorosulfonic acid-based resin). In the anode catalyst layer, the mass ratio of the ionomer to the carbon support (I / C) was set to 1.0.

[0047] (Examples 2-3 and Comparative Example 2) In Example 1, when forming the cathode catalyst layer, carbon particles having the specific surface area shown in Table 1 were used as the carbon support, and the mass ratio of the ionomer to the carbon support (I / C) was changed according to Table 1. Otherwise, the MEAs of Examples 2-3 and Comparative Example 2 were prepared in the same manner as in Example 1.

[0048] (Comparative Example 1) In Example 1, when forming the cathode catalyst layer, carbon particles having the specific surface area shown in Table 1 were used as the carbon support, the mass ratio of the ionomer to the carbon support (I / C) was changed according to Table 1, and the peripheral speed of the homogenizer was also changed. In addition, the MEA of Comparative Example 1 was prepared in the same manner as in Example 1.

[0049] [State of the catalyst layer] The cathode catalyst layer of the film electrode assemblies manufactured in Examples 1-3 and Comparative Examples 1-2 was prepared by the following procedure, with a cross-sectional area of ​​0.01-1 μm². 2 The void area ratio and void ratio were measured. First, small pieces approximately 5 mm square were cut from three locations on each film electrode assembly. These pieces were then processed using an ion milling apparatus to expose the cross-section perpendicular to the surface of the cathode catalyst layer (the junction with the PFSA film). For each piece, three cross-sections of the cathode catalyst layer were imaged at 5000x magnification using a field emission scanning electron microscope (FE-SEM), acquiring a total of nine cross-sectional images of 1280 pixels (24 μm) × 960 pixels (18 μm). For each cross-sectional image, image processing was performed using the image processing software "Image J" to obtain a list of the cross-sectional areas of each void. Images with fewer than 5 consecutive black pixels were considered noise and were excluded. For each cross-sectional image, based on the list of cross-sectional areas of each void, the sum of the cross-sectional areas of all voids (S A ), and cross-sectional area 0.01~1μm 2 The sum of the cross-sectional areas of the gaps (S B ) was calculated. Furthermore, S A S for B The percentage of ((S B / S A By calculating () × 100), the cross-sectional area can be calculated from 0.01 to 1 μm. 2 Calculate the ratio of the void area and the area of ​​the cross-section (S C The sum of the cross-sectional areas of the entire void (S) A ) Percentage ((S A / S C The porosity was determined by calculating () × 100). The cross-sectional area was determined from 9 cross-sectional images, ranging from 0.01 to 1 μm. 2 The average values ​​of the void area ratio and the average values ​​of the porosity are given for the cross-sectional area of ​​the cathode catalyst layer, respectively, from 0.01 to 1 μm. 2 These values ​​were defined as the void area ratio and void ratio. These values ​​are shown in Table 1.

[0050] [Evaluation of power generation performance] For solid polymer fuel cells (evaluation single cells) equipped with membrane electrode assemblies manufactured in Examples 1-3 or Comparative Examples 1-2, IV measurements were performed in accordance with the "Cell Evaluation and Analysis Protocol" of the New Energy and Industrial Technology Development Organization (NEDO) under conditions of a cell temperature of 60°C and humidity of 80%RH. The current density at a voltage of 0.4V is 4.2A / cm². 2 In the above cases, the power generation performance is evaluated as "〇" and the output is 4.2 A / cm². 2 If the value was less than the specified value, the power generation performance was evaluated as "×".

[0051] [Table 1]

[0052] As shown in Table 1, the cross-sectional area is 0.01 to 1 μm². 2 Fuel cells equipped with membrane electrode assemblies of Comparative Examples 1 and 2, which included a cathode catalyst layer with a void area ratio of less than 60%, exhibited inferior power generation performance. In contrast, fuel cells with a cross-sectional area of ​​0.01 to 1 μm 2 The fuel cells equipped with the membrane electrode assemblies of Examples 1 to 3, which include a cathode catalyst layer with a void area ratio of 60% or more, exhibited excellent power generation performance.

[0053] Figure 2 shows the results of the above IV measurement, for fuel cells equipped with membrane electrode assemblies prepared in Examples 1-3 or Comparative Examples 1-2, with a cathode catalyst layer cross-sectional area of ​​0.01-1 μm². 2 This graph shows the relationship between the void area ratio and the current density at a voltage of 0.4V. Figure 2 shows the cross-sectional area of ​​the cathode catalyst layer from 0.01 to 1 μm². 2 If the void area ratio is 60% or more, the power generation performance is excellent, and the cross-sectional area of ​​the cathode catalyst layer is 0.01 to 1 μm. 2 It was shown that the larger the proportion of void area, the better the power generation performance. Furthermore, as shown in Figure 2, the cross-sectional area of ​​the cathode catalyst layer is 0.01 to 1 μm². 2The coefficient of determination R between the void area ratio and the current density at a voltage of 0.4V 2 The value was 0.9851, revealing a very high correlation.

[0054] Figure 3 is a graph showing the relationship between current density and voltage for the fuel cell equipped with the membrane electrode assembly of Example 1 and the fuel cell equipped with the membrane electrode assembly of Comparative Example 1, based on the results of the above IV measurement. Comparing these two batteries, the difference in power generation performance increased as the current density increased, indicating that the fuel cell equipped with the membrane electrode assembly of Example 1 had superior power generation performance in the high current range. Similarly, the fuel cells equipped with the membrane electrode assemblies of Examples 2 and 3 also showed superior power generation performance in the high current range. Figure 4 shows the cathode catalyst layer of the film electrode assembly of Example 1 and the cathode catalyst layer of the film electrode assembly of Comparative Example 1, with a cross-sectional area of ​​10,000 nm. 2 The graph above shows the void area ratio. As shown in Figure 4, the cathode catalyst layer of the film electrode assembly of Example 1 has a cross-sectional area of ​​10,000 nm. 2 The void area ratio is 88%, and the cathode catalyst layer of the film electrode assembly of Comparative Example 1 has a cross-sectional area of ​​10,000 nm. 2 The void area ratio was 95%. The cross-sectional area of ​​the catalyst layer was 10,000 nm. 2 The above void area ratio is obtained from the cross-sectional image of the catalyst layer, by summing the cross-sectional areas of all voids (S A ) with a cross-sectional area of ​​10,000 nm 2 The sum of the cross-sectional areas of the above gaps (S D ) Percentage ((S D / S A This was determined by calculating () × 100). The method for obtaining cross-sectional images of the catalyst layer is the same as the method for examining the "state of the catalyst layer" described above. In the cathode catalyst layer, the index used in Patent Document 1 is 10,000 nm 2 Figures 3 and 4 clearly show that even with similar void area ratios, fuel cell power generation performance can vary.

Claims

1. A catalyst layer for a polymer electrolyte fuel cell, comprising a catalyst support having a catalyst supported on a catalyst support, and a polymer electrolyte, The catalyst layer has voids, Of the voids observed in the cross-section in the thickness direction perpendicular to the surface of the catalyst layer, 0.01 μm ２ 1 μm or more ２ A catalyst layer in which the sum of the cross-sectional areas of the voids having the following cross-sectional areas is 60% or more of the sum of the cross-sectional areas of all the voids.

2. The catalyst layer according to claim 1, wherein the catalyst support is carbon particles.

3. The catalyst layer according to claim 1 or 2, wherein the mass ratio of the polymer electrolyte to the catalyst support (polymer electrolyte / catalyst support) is 0.6 or more and 1.8 or less.

4. The specific surface area of ​​the catalyst support is 600 m². ２ The catalyst layer according to claim 1 or 2, wherein the amount is 1 / g or more.

5. The catalyst layer according to claim 1 or 2, which is a cathode catalyst layer used in the cathode of a polymer electrolyte fuel cell.

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