Electrode catalyst layer, membrane electrode assembly, and solid polymer fuel cell

Abstract

The present invention provides an electrode catalyst layer, a membrane electrode assembly, and a polymer electrolyte fuel cell that can enhance power generation performance over a wide load range by suppressing catalyst poisoning and improving proton transport using ionomers. [Solution] The electrode catalyst layer 12C of the air electrode includes a catalyst support 20 which is a porous carbon support 21 supporting a catalyst substance 23, and a polymer electrolyte 24, and the ECSA ratio, which is the ratio of ECSA at a relative humidity of 30% to ECSA at a relative humidity of 100%, is 0.55 or more and 0.65 or less. The catalyst support 20 includes a first catalyst support 20a which has an ECSA ratio of 0.5 or less when used alone as the catalyst support 20 included in the electrode catalyst layer 12C, and a second catalyst support 20b which has an ECSA ratio of 0.7 or more when used alone as the catalyst support 20 included in the electrode catalyst layer 12C.

Description

[Technical Field]

[0001] This disclosure relates to an electrode catalyst layer, a membrane electrode assembly, and a polymer electrolyte fuel cell. [Background technology]

[0002] A polymer electrolyte fuel cell comprises a membrane electrode assembly having an electrode catalyst layer that constitutes the fuel electrode, which is the anode, an electrode catalyst layer that constitutes the air electrode, which is the cathode, and a polymer electrolyte membrane sandwiched between these two electrode catalyst layers.

[0003] The fuel electrode is supplied with a fuel gas containing hydrogen, and the air electrode is supplied with an oxidizing gas containing oxygen. At the fuel electrode, protons and electrons are produced from the fuel gas. The protons move to the air electrode through a polymer electrolyte membrane. The electrons are extracted from the fuel electrode into an external circuit and move to the air electrode. At the air electrode, the oxidizing gas reacts with the protons and electrons that have moved from the fuel electrode to produce water. This series of electrochemical reactions generates an electromotive force.

[0004] The electrode catalyst layer comprises a catalyst material containing a metal such as platinum, a support for the catalyst material, and a polymer electrolyte that is an ionomer. Among these, porous carbon supports have attracted attention as the support material. Porous carbon supports have excellent conductivity and gas diffusion properties, as well as a high surface area. Therefore, various configurations of porous carbon supports have been reported with the aim of further improving the power generation performance.

[0005] For example, Patent Document 1 aims to suppress the decrease in catalytic activity caused by coating of the catalytic material with an ionomer by defining the pore diameter of the porous carbon support and the relationship between the specific surface area of ​​the pores and the specific surface area of ​​the porous carbon support. Furthermore, Patent Document 2 aims to improve power generation performance by defining the ratio of the inner and outer surface areas of the porous carbon support and the proportion of the catalytic material supported on the outer surface. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Patent No. 6931808 [Patent Document 2] Japanese Patent Publication No. 2024-6372 [Overview of the Initiative] [Problems that the invention aims to solve]

[0007] By controlling the pore size and specific surface area of ​​the porous carbon support, it is possible to suppress the coating of the catalyst material by the ionomer, thereby suppressing the decrease in catalytic activity caused by this coating. However, the further the ionomer and the catalyst material are from each other, the more the transport of protons from the ionomer to the catalyst material is hindered. Therefore, from the two perspectives of suppressing catalyst poisoning by the ionomer and improving proton transport, there is a desire to improve the power generation performance of electrode catalyst layers using porous carbon supports. [Means for solving the problem]

[0008] This paper describes embodiments of an electrode catalyst layer, a membrane electrode assembly, and a polymer electrolyte fuel cell that address the above-mentioned problems. [Aspect 1] An electrode catalyst layer for use in the air electrode of a polymer electrolyte, comprising a catalyst support which is a porous carbon support supporting a catalyst material, and a polymer electrolyte, wherein the ECSA ratio, which is the ratio of ECSA at a relative humidity of 30% to ECSA at a relative humidity of 100%, is 0.55 or more and 0.65 or less, and the catalyst support includes a first catalyst support which, when used alone as the catalyst support included in the electrode catalyst layer, has an ECSA ratio of 0.5 or less, and a second catalyst support which, when used alone as the catalyst support included in the electrode catalyst layer, has an ECSA ratio of 0.7 or more.

[0009] With the above configuration, the suppression of catalyst poisoning by the ionomer and the improvement of proton transport make it possible to enhance power generation performance over a wide range from low load to high load.

[0010] [Aspect 2] The electrode catalyst layer according to [Aspect 1], wherein the pore size of the porous carbon support is 5 nm or more and 50 nm or less.

[0011] With the above configuration, sufficient gas transport into the pores of the porous carbon support is obtained, while the intrusion of polymer electrolytes into the pores is suppressed, thereby preventing excessive catalyst poisoning. Furthermore, it is easy to control the ECSA ratio to 0.5 or less or 0.7 or more when each catalyst support is used individually.

[0012] [Aspect 3] The electrode catalyst layer according to [Aspect 2], wherein the pore diameter of the porous carbon support in the first catalyst support and the pore diameter of the porous carbon support in the second catalyst support are different from each other.

[0013] With the above configuration, it is easy to control the ECSA ratio of each catalyst support to 0.5 or less or 0.7 or more when used individually, and it is also easy to obtain an ECSA ratio of 0.55 to 0.65 by combining them.

[0014] [Aspect 4] The electrode catalyst layer according to [Aspect 1], wherein the particle size distribution of the catalyst support includes a peak of 0.5 μm or more and 1.0 μm or less, and a peak of 1.5 μm or more and 5.0 μm or less. With the above configuration, by utilizing the differences in particle size of the porous carbon support, it is easy to control the ECSA ratio of each catalyst support to 0.5 or less or 0.7 or more when used individually, and it is also easy to obtain an ECSA ratio of 0.55 to 0.65 by combining these.

[0015] [Aspect 5] A membrane electrode assembly comprising a polymer electrolyte membrane and a pair of electrode catalyst layers sandwiching the polymer electrolyte membrane, wherein one of the pair of electrode catalyst layers is the electrode catalyst layer described in any one of [Aspect 1] to [Aspect 4]. According to the above configuration, a film electrode assembly can be obtained that can achieve high power generation performance over a wide range from low load to high load.

[0016] A solid polymer fuel cell comprising the membrane electrode assembly according to [Aspect 6] [Aspect 5] and a pair of separators sandwiching the membrane electrode assembly. According to the above configuration, the power generation performance is enhanced over a wide range from low load to high load.

Effects of the Invention

[0017] According to the present disclosure, the power generation performance can be enhanced over a wide load range by suppressing catalyst poisoning by the ionomer and improving proton transport.

Brief Description of the Drawings

[0018] [Figure 1] FIG. 1 is a diagram showing a cross-sectional structure of a membrane electrode assembly according to an embodiment. [Figure 2] FIG. 2 is a diagram showing the internal structure of an air electrode catalyst layer according to an embodiment. [Figure 3] FIG. 3 is a diagram showing the internal structure of an air electrode catalyst layer according to the first example. [Figure 4] FIG. 4 is a diagram showing the internal structure of an air electrode catalyst layer according to the second example. [Figure 5] FIG. 5 is a diagram showing a perspective structure of a solid polymer fuel cell according to an embodiment in an exploded manner. [Figure 6] FIG. 6 is a diagram showing the relationship between the ECSA ratio of Comparative Examples 1 to 7 and the output voltage at low load and high load.

Modes for Carrying Out the Invention

[0019] Referring to the drawings, an embodiment of an electrode catalyst layer, a membrane electrode assembly, and a solid polymer fuel cell will be described. [Configuration of Membrane Electrode Assembly and Electrode Catalyst Layer] As shown in FIG. 1, the membrane electrode assembly 10 includes a polymer electrolyte membrane 11 and a pair of electrode catalyst layers, namely a fuel electrode catalyst layer 12A and an air electrode catalyst layer 12C.

[0020] The polymer electrolyte membrane 11 is sandwiched between the fuel electrode catalyst layer 12A and the air electrode catalyst layer 12C in the thickness direction. The fuel electrode catalyst layer 12A is in contact with one of the two surfaces of the polymer electrolyte membrane 11, and the air electrode catalyst layer 12C is in contact with the other of the two surfaces of the polymer electrolyte membrane 11.

[0021] The fuel electrode catalyst layer 12A constitutes the fuel electrode, which is the anode of the polymer electrolyte fuel cell. The air electrode catalyst layer 12C constitutes the air electrode, which is the cathode of the polymer electrolyte fuel cell. When viewed from a position opposite one of the surfaces of the polymer electrolyte membrane 11, the external shapes of the fuel electrode catalyst layer 12A and the air electrode catalyst layer 12C are approximately the same, and their external shapes are smaller than those of the polymer electrolyte membrane 11. The external shapes of the catalyst layers 12A, 12C and the polymer electrolyte membrane 11 are not particularly limited; for example, they may be rectangular.

[0022] The polymer electrolyte membrane 11 contains a polymer electrolyte. The polymer electrolyte used in the polymer electrolyte membrane 11 can be any polymer electrolyte having proton conductivity, such as a fluorine-based polymer electrolyte or a hydrocarbon-based polymer electrolyte. As a fluorine-based polymer electrolyte, a polymer electrolyte having a tetrafluoroethylene skeleton can be used. An example of a fluorine-based polymer electrolyte is Nafion (registered trademark: manufactured by DuPont). Examples of hydrocarbon-based polymer electrolytes are sulfonated polyether ketone, sulfonated polyether sulfone, sulfonated polyether ether sulfone, sulfonated polysulfide, sulfonated polyphenylene, etc. Among these, a polymer electrolyte membrane made of perfluorosulfonic acid polymer is preferably used because it has high proton conductivity and can provide stable performance even under the power generation environment of a fuel cell.

[0023] The ion exchange capacity of the polymer electrolyte membrane 11 is preferably 0.9 mm equivalent / g dry resin or more from the viewpoint of enhancing proton conductivity, and preferably 2.0 mm equivalent / g dry resin or less from the viewpoint of suppressing swelling due to water content and thus minimizing dimensional changes.

[0024] The catalyst layers 12A and 12C contain a catalytic material, a conductive carrier, and a polymer electrolyte. The catalyst layers 12A and 12C may further contain a fibrous material. Referring to Figure 2, the structure of the air electrode catalyst layer 12C will be described in detail. Figure 2 schematically shows the internal structure of the air electrode catalyst layer 12C. As shown in Figure 2, the air electrode catalyst layer 12C includes a catalyst support 20 consisting of a catalyst material 23 and a porous carbon support 21 which is a conductive support, and a polymer electrolyte 24 which is an ionomer. The catalyst material 23 is supported on the porous carbon support 21.

[0025] The porous carbon carrier 21 is a carbon material having numerous pores 22. The porous carbon carrier 21 supports a catalyst substance 23 within the pores 22. The porous carbon carrier 21 may also support the catalyst substance 23 on its outer surface. The shape of the porous carbon carrier 21 is not particularly limited, but may be particulate, for example. The particle size of the particulate porous carbon carrier 21 is, for example, 200 nm or more. The above particle size is the mode diameter of the particle size distribution of the porous carbon carrier 21.

[0026] The pore volume of the porous carbon support 21 is 1.0 cm³. 3 / g or more 3.0cm 3 It is preferable that the amount is less than or equal to / g. The pore volume is 1.0 cm³. 3 If the amount is greater than / g, a large amount of catalyst material 23 can be supported within the pores 22. 3 If the value is less than / g, the strength of the porous carbon support 21 can be increased. The pore volume is determined by analyzing the measurement data of nitrogen adsorption and desorption isotherms using methods such as the BJH method.

[0027] The BET specific surface area of ​​the porous carbon support 21 is 500 m². 2 It is preferable that the BET specific surface area is 500 m² or more. 2 If the amount is 1 / g or more, the catalyst material 23 can be dispersed and supported. The BET specific surface area is the total surface area per unit mass of the porous carbon support 21, and is determined using the Brunauer-Emmett-Teller (BET) method.

[0028] The catalyst material 23 is a particle containing a metal that functions as a catalyst for the electrochemical reaction in the fuel cell. Preferably, the catalyst material 23 contains one or more elements from platinum, rhodium, palladium, gold, and iridium. For example, the catalyst material 23 may be a particle of pure platinum, an alloy particle of platinum and cobalt, or a core-shell particle in which platinum particles are coated on a palladium core.

[0029] In particular, the catalyst material 23 is preferably platinum particles with a crystallite size (1,1,1) of 8 nm or less as determined by XRD, and more preferably platinum particles with a crystallite size (1,1,1) of 5 nm or less. The above (1,1,1) represents the Miller index. If the platinum particles have a crystallite size (1,1,1) of 8 nm or less, the catalytic activity is high and the output voltage of the fuel cell can be increased. Furthermore, if the platinum particles have a crystallite size (1,1,1) of 2 nm or more and 3 nm or less, high catalytic activity can be easily and stably obtained.

[0030] The polymer electrolyte 24 can be any polymer electrolyte having proton conductivity, such as a fluorine-based polymer electrolyte or a hydrocarbon-based polymer electrolyte. As the polymer electrolyte 24, the polymer electrolytes exemplified as materials for the polymer electrolyte membrane 11 can be used.

[0031] The electrolyte constituting the polymer electrolyte membrane 11 and the polymer electrolyte 24 may be the same electrolyte or may be different electrolytes.

[0032] The mass of the polymer electrolyte 24 contained in the air electrode catalyst layer 12C is preferably 0.8 times or more and 1.3 times or less the mass of the porous carbon support 21 contained in the air electrode catalyst layer 12C. If the content of the polymer electrolyte 24 is 0.8 times or more the mass of the porous carbon support 21, sufficient proton conductivity can be obtained in the air electrode catalyst layer 12C, thus suppressing an increase in resistance. If the content of the polymer electrolyte 24 is 1.3 times or less the mass of the porous carbon support 21, suitable voids can be secured in the air electrode catalyst layer 12C, and sufficient drainage can be obtained, thus suppressing a decrease in power generation performance.

[0033] The air electrode catalyst layer 12C includes a first catalyst support 20a and a second catalyst support 20b as the catalyst support 20. The first catalyst support 20a is a catalyst support 20 in which, when the first catalyst support 20a is used alone as the catalyst support 20 contained in the air electrode catalyst layer 12C, the ECSA ratio, which is the ratio of the ECSA (Electrochemical Surface Area) at a relative humidity of 30% to the ECSA at a relative humidity of 100%, is 0.5 or less.

[0034] The second catalyst support 20b is a catalyst support 20 in which, when used alone as the catalyst support 20 included in the air electrode catalyst layer 12C, the ECSA ratio is 0.7 or higher.

[0035] When using the air electrode catalyst layer 12C of this embodiment, in which the first catalyst support 20a and the second catalyst support 20b are combined, the ECSA ratio is 0.55 or more and 0.65 or less. ECSA is the surface area of ​​the electrochemically active catalyst 23, that is, the surface area of ​​the catalyst 23 that is effectively used in the electrochemical reaction of a fuel cell.

[0036] For measuring ECSA at 100% relative humidity, a membrane electrode assembly equipped with an air electrode catalyst layer containing the target catalyst support is used. The assembly is humidified to 100% relative humidity, and hydrogen is flowed on the fuel electrode side and nitrogen on the air electrode side. A cyclic voltammogram is then acquired at a scanning speed of 50 mV / s in the voltage range of 0.05 V to 0.9 V, and the ECSA is obtained by calculating the catalyst surface area from the hydrogen adsorption charge. The hydrogen adsorption charge is calculated by creating a Tafel plot from the voltage and current in the range of 0.05 V to 0.06 V and subtracting the charge corresponding to the hydrogen generation current. When measuring ECSA at a relative humidity of 30%, the measurement is performed under the same conditions as when the relative humidity is 100%, except that the humidification conditions are changed to achieve a relative humidity of 30%.

[0037] The fact that the ECSA ratio of the first catalyst support 20a when used alone is 0.5 or less indicates that when the relative humidity is 30%, i.e., at low humidity when water is unlikely to be interposed between the constituent materials of the air electrode catalyst layer 12C, proton transport to the catalyst substance 23 is restricted and the ECSA is small, while when the relative humidity is 100%, i.e., at high humidity when water is likely to be interposed between the constituent materials of the air electrode catalyst layer 12C, proton transport to the catalyst substance 23 becomes favorable and the ECSA is large. This suggests that the distance between the catalyst substance 23 and the polymer electrolyte 24 is large.

[0038] The fact that the ECSA ratio of the second catalyst support 20b is 0.7 or higher when used alone indicates that proton transport to the catalyst material 23 is not easily restricted even at low humidity, and that the decrease in ECSA is suppressed. This suggests that the catalyst material 23 and the polymer electrolyte 24 are close together.

[0039] Thus, the ECSA ratio functions as an indicator of the distance between the catalyst 23 and the polymer electrolyte 24. The greater the distance between the catalyst 23 and the polymer electrolyte 24, the more effectively catalyst poisoning by the ionomer can be suppressed, but proton transport becomes more restricted. The closer the distance between the catalyst 23 and the polymer electrolyte 24, the more favorable proton transport becomes, but catalyst poisoning by the ionomer becomes more likely.

[0040] The first catalyst support 20a, which has excellent properties for suppressing catalyst poisoning by ionomers, contributes to improving power generation performance at low loads. The second catalyst support 20b, which has excellent properties for proton transport, contributes to improving power generation performance at high loads.

[0041] For example, when using a catalyst support with an ECSA ratio of around 0.5 alone, high power generation performance can be obtained at low loads, but low power generation performance at high loads. On the other hand, when using a catalyst support with an ECSA ratio of around 0.7 alone, high power generation performance can be obtained at high loads, but low power generation performance at low loads. And when using a catalyst support with an ECSA ratio of 0.55 to 0.65 alone, power generation performance is moderate at both low and high loads.

[0042] In contrast, in this embodiment, by using a combination of the first catalyst support 20a and the second catalyst support 20b, it is possible to improve power generation performance from low load to high load. When using an air electrode catalyst layer 12C containing a first catalyst support 20a and a second catalyst support 20b, the ECSA ratio is 0.55 or more and 0.65 or less, as long as the ratio of the first catalyst support 20a to the second catalyst support 20b is not particularly limited. The ratio of the total mass of the first catalyst support 20a to the total mass of the catalyst support 20 contained in the air electrode catalyst layer 12C may be, for example, 0.1 or more and 0.9 or less.

[0043] The ECSA ratio of the catalyst support 20 when used alone can be controlled by the average particle size, pore diameter, pore volume, specific surface area, etc., of the porous carbon support 21. The less the polymer electrolyte 24 approaches the catalyst substance 23 due to physical or chemical action, the smaller the ECSA ratio becomes.

[0044] The composition of the fuel electrode catalyst layer 12A may be the same as or different from that of the air electrode catalyst layer 12C. The fuel electrode catalyst layer 12A may contain the same materials as those exemplified as the constituent materials of the air electrode catalyst layer 12C, as the catalyst material, conductive carrier, and polymer electrolyte. Furthermore, the conductive carrier contained in the fuel electrode catalyst layer 12A may be a non-porous carbon material.

[0045] The output of a fuel cell depends more on the oxygen reduction activity of the air electrode than on the fuel electrode, and the ECSA ratio depends on the configuration of the catalyst support 20 in the air electrode catalyst layer 12C. Therefore, if the air electrode catalyst layer 12C contains a first catalyst support 20a and a second catalyst support 20b, it is possible to improve power generation performance regardless of the configuration of the fuel electrode catalyst layer 12A.

[0046] The fuel electrode catalyst layer 12A may, for example, contain a first catalyst support 20a and a second catalyst support 20b similar to those in the air electrode catalyst layer 12C, or it may contain a single type of catalyst support. The configuration of the catalyst support in the fuel electrode catalyst layer 12A is not limited as long as the ECSA ratio measured using the membrane electrode assembly 10 is 0.55 or more and 0.65 or less.

[0047] Furthermore, the catalyst layers 12A and 12C may contain fibrous material. The fibrous material preferably has electron conductivity or proton conductivity. The inclusion of fibrous material enhances electron and proton conductivity and improves the strength of the catalyst layers 12A and 12C.

[0048] Examples of electronically conductive fibers include carbon fibers, carbon nanotubes, carbon nanohorns, and conductive polymer nanofibers. Examples of proton-conducting fibers are resin fibers having structures or functional groups in their backbone that can interact with protons. Proton-conducting fibers may also be fibers composed of polymer electrolytes.

[0049] The average fiber diameter of the fibrous material is preferably between 2 nm and 400 nm. If the average fiber diameter of the fibrous material is within the above range, the voids in the catalyst layers 12A and 12C are formed well, which improves power generation performance. Furthermore, the average fiber length of the fibrous material is preferably between 100 nm and 20 μm. If the average fiber length of the fibrous material is within the above range, the strength of the catalyst layers 12A and 12C can be increased, thereby suppressing the occurrence of cracks in the catalyst layers 12A and 12C. In addition, the voids are formed well, which improves power generation performance.

[0050] Referring to Figures 3 and 4, a specific example of the configuration of the first catalyst support 20a and the second catalyst support 20b will be explained. Figure 3 shows the air electrode catalyst layer 12C of the first example. In the first example, the pore diameter of the porous carbon support 21 in the first catalyst support 20a and the pore diameter of the porous carbon support 21 in the second catalyst support 20b are both between 5 nm and 50 nm. If the pore diameter of the porous carbon support 21 is 5 nm or more, sufficient gas transport into the pores 22 can be obtained, thereby improving power generation performance. If the pore diameter of the porous carbon support 21 is 50 nm or less, the penetration of the polymer electrolyte 24 into the pores 22 is suppressed, thereby suppressing excessive catalyst poisoning.

[0051] Furthermore, the pore size of the porous carbon support 21 of the first catalyst support 20a is smaller than the pore size of the porous carbon support 21 of the second catalyst support 20b. For example, the pore size of the porous carbon support 21 of the first catalyst support 20a is 5 nm or more and less than 25 nm, while the pore size of the porous carbon support 21 of the second catalyst support 20b is 25 nm or more and 50 nm or less.

[0052] By controlling the pore size, the ease with which the polymer electrolyte 24 penetrates into the pores 22 can be controlled, thereby allowing for favorable control of the ECSA ratio. If the pore size of the porous carbon support 21 is between 5 nm and 50 nm, it is easy to obtain a first catalyst support 20a with an ECSA ratio of 0.5 or less when used alone, and a second catalyst support 20b with an ECSA ratio of 0.7 or more when used alone.

[0053] Furthermore, by using catalyst support carriers 20 with different pore sizes in the porous carbon support 21 as the first catalyst support carrier 20a and the second catalyst support carrier 20b, it is easy to obtain an ECSA ratio of 0.55 to 0.65 by combining the first catalyst support carrier 20a and the second catalyst support carrier 20b.

[0054] The pore diameter of the porous carbon support 21 is the mode diameter, which is determined by analyzing the measurement data of nitrogen adsorption and desorption isotherms using methods such as the BJH method. The pore diameter distribution of the entire catalyst support 20 contained in the air electrode catalyst layer 12C has at least two peaks corresponding to the pore diameters of the first catalyst support 20a and the second catalyst support 20b, respectively.

[0055] Figure 4 shows the air electrode catalyst layer 12C of the second example. In the second example, the particle size of the porous carbon support 21 in the first catalyst support 20a is larger than the particle size of the porous carbon support 21 in the second catalyst support 20b.

[0056] The particle size distribution across the entire catalyst support 20 contained in the air electrode catalyst layer 12C has at least two peaks. The two peaks are a first peak corresponding to the mode diameter of the porous carbon support 21 of the first catalyst support 20a, and a second peak corresponding to the mode diameter of the porous carbon support 21 of the second catalyst support 20b.

[0057] The first peak is between 1.5 μm and 5.0 μm, and the second peak is between 0.5 μm and 1.0 μm. When a porous carbon support 21 corresponding to the first peak is used, the large particle size of the porous carbon support 21 causes the polymer electrolyte 24 to be further away from the catalyst substance 23 in the pores 22, resulting in a tendency for the ECSA ratio to be small. Therefore, a first catalyst support 20a with an ECSA ratio of 0.5 or less when used alone is preferably obtained.

[0058] When a porous carbon support 21 corresponding to the second peak is used, the small particle size of the porous carbon support 21 brings the polymer electrolyte 24 closer to the catalyst substance 23 in the pores 22, which tends to increase the ECSA ratio. Therefore, a second catalyst support 20b with an ECSA ratio of 0.7 or higher when used alone can be preferably obtained.

[0059] Thus, the ECSA ratio can be suitably controlled by controlling the particle size. Furthermore, by using catalyst supports 20 with different particle sizes of porous carbon support 21 as the first catalyst support 20a and the second catalyst support 20b, it is easy to obtain an ECSA ratio of 0.55 to 0.65 by combining the first catalyst support 20a and the second catalyst support 20b.

[0060] The pore size of the porous carbon support 21 may be approximately the same for the first catalyst support 20a and the second catalyst support 20b. For example, by crushing one type of catalyst support 20, first catalyst support 20a and second catalyst support 20b with different particle sizes may be obtained.

[0061] [Polymer electrolyte fuel cell] Referring to Figure 5, the configuration of a polymer electrolyte fuel cell equipped with the membrane electrode assembly 10 described above will be explained.

[0062] As shown in Figure 5, the polymer electrolyte fuel cell 30 comprises a membrane electrode assembly 10, a pair of gas diffusion layers 31A and 31C, and a pair of separators 32A and 32C. The membrane electrode assembly 10 is sandwiched between the gas diffusion layer 31A and the gas diffusion layer 31C, with the gas diffusion layer 31A in contact with the fuel electrode catalyst layer 12A and the gas diffusion layer 31C in contact with the air electrode catalyst layer 12C.

[0063] The gas diffusion layers 31A and 31C are layers for uniformly diffusing the supplied gas and possess gas diffusivity and conductivity. The gas diffusion layer 31A, together with the fuel electrode catalyst layer 12A, constitutes the fuel electrode, and the gas diffusion layer 31C, together with the air electrode catalyst layer 12C, constitutes the air electrode.

[0064] The gas diffusion layers 31A and 31C include, for example, a porous material such as carbon cloth, carbon paper, or nonwoven fabric. The gas diffusion layers 31A and 31C may have a water-repellent layer on the surface in contact with the catalyst layers 12A and 12C. Providing a water-repellent layer can improve drainage. The water-repellent layer includes, for example, a conductive material such as carbon black and a water-repellent resin such as polytetrafluoroethylene.

[0065] The laminate of the film electrode assembly 10 and the gas diffusion layers 31A and 31C is sandwiched between separators 32A and 32C. Separators 32A and 32C are gas-impermeable and conductive. The materials of separators 32A and 32C are, for example, carbon-based or metallic materials.

[0066] Separator 32A faces the gas diffusion layer 31A, and separator 32C faces the gas diffusion layer 31C. On separator 32A, a gas channel 33A is formed on the surface facing the gas diffusion layer 31A, and a cooling water channel 34A is formed on the surface opposite to the gas diffusion layer 31A. Similarly, on separator 32C, a gas channel 33C is formed on the surface facing the gas diffusion layer 31C, and a cooling water channel 34C is formed on the surface opposite to the gas diffusion layer 31C.

[0067] When the polymer electrolyte fuel cell 30 is in use, a fuel gas such as hydrogen flows through the gas channel 33A of separator 32A, and an oxidizing gas such as oxygen flows through the gas channel 33C of separator 32C. Cooling water also flows through the cooling water channels 34A and 34C of each separator 32A and 32C. When the fuel gas is supplied to the fuel electrode from the gas channel 33A and the oxidizing gas is supplied to the air electrode from the gas channel 33C, an electrochemical reaction proceeds, and an electromotive force is generated between the fuel electrode and the air electrode. Organic fuel such as methanol may be supplied to the fuel electrode.

[0068] The polymer electrolyte fuel cell 30 may be used in the single-cell state shown in Figure 5, or multiple polymer electrolyte fuel cells 30 may be stacked and connected in series to form a single fuel cell. The polymer electrolyte fuel cell 30 can be used by assembling it with a gas supply device, a cooling device, and other ancillary devices.

[0069] In addition to the above-mentioned components, the polymer electrolyte fuel cell 30 may also be equipped with components such as gaskets to suppress gas leakage. The gaskets are arranged to surround the outer periphery of the catalyst layers 12A and 12C. Furthermore, the gas diffusion layer 31A and the separator 32A may be an integrated structure, and the gas diffusion layer 31C and the separator 32C may be an integrated structure. Alternatively, the gas diffusion layers 31A and 31C may be components that constitute the membrane electrode assembly 10.

[0070] [Method for manufacturing a membrane electrode assembly] A method for manufacturing the membrane electrode assembly 10, including the method for manufacturing the catalyst layers 12A and 12C, will be described. First, a catalyst ink, which is a coating liquid for forming catalyst layers 12A and 12C, is prepared. The catalyst ink comprises a conductive carrier supporting a catalyst material, a polymer electrolyte, and a dispersion medium. At least the catalyst ink for forming the air electrode catalyst layer 12C includes a first catalyst carrier 20a and a second catalyst carrier 20b as the catalyst carrier 20. The catalyst ink may also contain fibrous material.

[0071] The dispersion medium is not particularly limited as long as it does not erode the individual materials contained in the catalyst ink and can dissolve the polymer electrolyte in a highly fluid state or disperse it as a fine gel. The dispersion medium preferably contains a volatile organic solvent. Examples of dispersion mediums include alcohols, ketone solvents, ether solvents, and other polar solvents. The dispersion medium may be a mixture or may contain water. The catalyst ink may also contain a dispersant or a pore-forming agent. Furthermore, the catalyst ink may be subjected to a dispersion treatment.

[0072] Catalyst layers 12A and 12C are formed by applying catalyst ink to a substrate to form a coating film and then drying the coating film. Known methods for applying the catalyst ink can include, for example, the doctor blade method, dipping method, screen printing method, die coating method, and roll coating method.

[0073] As the substrate, for example, a transfer substrate is used that is peeled off after the catalyst layers 12A and 12C are transferred to the polymer electrolyte membrane 11. The material of the transfer substrate is, for example, a fluororesin or an organic polymer compound other than a fluororesin. Furthermore, the substrate for forming the catalyst layers 12A and 12C may be the polymer electrolyte membrane 11 or the gas diffusion layers 31A and 31C.

[0074] If the above substrate is a transfer substrate, the catalyst layers 12A and 12C are bonded to the polymer electrolyte membrane 11 by thermocompression bonding, and then the transfer substrate is peeled off from the catalyst layers 12A and 12C. This forms the membrane electrode assembly 10.

[0075] When the substrate is a gas diffusion layer 31A, 31C, the catalyst layers 12A, 12C supported by the gas diffusion layers 31A, 31C are bonded to the polymer electrolyte membrane 11 by thermocompression bonding, thereby forming a membrane electrode assembly 10. When the substrate is a polymer electrolyte membrane 11, the catalyst layers 12A, 12C are formed directly on the surface of the polymer electrolyte membrane 11. This forms the membrane electrode assembly 10.

[0076] [Examples] The membrane electrode assembly and polymer electrolyte fuel cell equipped with the electrode catalyst layer described above will be explained using specific examples and comparative examples.

[0077] (Example 1) A mixed solution of water, NPA (n-propyl alcohol), and ethanol was used as a dispersion medium. Catalyst support P1, catalyst support P2, a polymer electrolyte (nafion dispersion, manufactured by Sigmar-Aldrich), and a fibrous material, carbon fiber (VGCF-H, manufactured by Resonaq), were added to the dispersion medium, and the mixture was dispersed using a ball mill to produce an air electrode catalyst ink, which is a catalyst ink for forming the air electrode catalyst layer.

[0078] The catalyst support P1 is a porous carbon support on which platinum, the catalytic material, is supported. The catalyst load ratio is expressed as platinum:porous carbon support = 50:50 by mass ratio. The pore size of the porous carbon support of catalyst support P1 is 50 nm.

[0079] Catalyst support P2 is a porous carbon support on which platinum, the catalytic material, is supported. The catalyst load ratio is expressed as platinum:porous carbon support = 50:50 by mass ratio. The pore size of the porous carbon support of catalyst support P2 is 5 nm. The pore size of the porous carbon support was determined by the BJH method from measurement data of nitrogen adsorption and desorption isotherms.

[0080] In the air electrode catalyst ink, the mass ratio of catalyst support P1 to catalyst support P2 is 50:50, the mass ratio of polymer electrolyte to porous carbon support is 1.1, the fibrous material content is 6% by mass relative to the total solid content, and the solid content concentration is 6.5% by mass.

[0081] Next, a fuel electrode catalyst ink was prepared, which is a catalyst ink for forming the fuel electrode catalyst layer, in the same manner as the air electrode catalyst ink, except that catalyst support P8 was used alone as the catalyst support, and the mass ratio of the polymer electrolyte to the carbon support was set to 1.0. Catalyst support P8 is a carbon support on which platinum, the catalytic material, is supported, and the catalyst support ratio is expressed as platinum:carbon support = 30:70 by mass ratio.

[0082] On one side of the polymer electrolyte membrane (Nafion membrane, manufactured by DuPont), the catalytic material content per unit area is 0.125 mg / cm². 2 To achieve this, an air electrode catalyst ink was applied to form a coating film. Then, the coating film was dried at 80°C for 5 minutes to form the air electrode catalyst layer.

[0083] Next, on the other side of the polymer electrolyte membrane, the catalytic substance content per unit area is 0.025 mg / cm². 2 To achieve this, a fuel electrode catalyst ink was applied to form a coating. Then, the coating was dried at 80°C for 5 minutes to form the fuel electrode catalyst layer.

[0084] This yielded the membrane electrode assembly of Example 1. The ECSA ratio measured using the membrane electrode assembly of Example 1, i.e., the ratio of ECSA at a relative humidity of 30% to ECSA at a relative humidity of 100%, was 0.60.

[0085] (Example 2) The film electrode assembly of Example 2 was obtained using the same materials and process as in Example 1, except that the air electrode catalyst ink was prepared using catalyst supports P3 and P4 as catalyst support carriers.

[0086] Catalyst support P3 is a porous carbon support on which platinum, the catalytic material, is supported. The catalyst load ratio is expressed as platinum:porous carbon support = 50:50 by mass ratio. The pore size of the porous carbon support of catalyst support P3 is 20 nm. In particle size distribution measurements, catalyst support P3 showed a single peak with a peak value of 2 μm.

[0087] Catalyst support P4 was obtained by grinding catalyst support P3 using a ball mill. In particle size distribution measurements, catalyst support P4 had a single peak with a peak value of 0.7 μm.

[0088] In the air electrode catalyst ink, the mass ratio of catalyst support P3 to catalyst support P4 is catalyst support P3:catalyst support P4 = 50:50. The ECSA ratio measured using the film electrode assembly of Example 2 was 0.62.

[0089] (Example 3) The film electrode assembly of Example 3 was obtained using the same materials and process as in Example 1, except that the mass ratio of catalyst support P1 to catalyst support P2 in the air electrode catalyst ink was changed to catalyst support P1:catalyst support P2 = 40:60. The ECSA ratio measured using the film electrode assembly of Example 3 was 0.55.

[0090] (Example 4) The film electrode assembly of Example 4 was obtained using the same materials and process as in Example 1, except that the mass ratio of catalyst support P1 to catalyst support P2 in the air electrode catalyst ink was changed to catalyst support P1:catalyst support P2 = 60:40. The ECSA ratio measured using the film electrode assembly of Example 4 was 0.65.

[0091] (Comparative Example 1) A film electrode assembly of Comparative Example 1 was obtained using the same materials and process as in Example 1, except that catalyst support P1 was used alone as the catalyst support to prepare the air electrode catalyst ink. The BET specific surface area of ​​the porous carbon support of catalyst support P1 is 1.6 times that of the porous carbon support of catalyst support P3. The ECSA ratio measured using the membrane electrode assembly of Comparative Example 1 was 0.80.

[0092] (Comparative Example 2) A film electrode assembly of Comparative Example 2 was obtained using the same materials and process as in Example 1, except that catalyst support P2 was used alone as the catalyst support to prepare the air electrode catalyst ink. The BET specific surface area of ​​the porous carbon support of catalyst support P2 is 0.8 times that of the porous carbon support of catalyst support P3. The ECSA ratio measured using the film electrode assembly of Comparative Example 2 was 0.40.

[0093] (Comparative Example 3) A film electrode assembly of Comparative Example 3 was obtained using the same materials and process as in Example 1, except that catalyst support P4 was used alone as the catalyst support to prepare the air electrode catalyst ink. The ECSA ratio measured using the film electrode assembly of Comparative Example 3 was 0.75.

[0094] (Comparative Example 4) A film electrode assembly of Comparative Example 4 was obtained using the same materials and process as in Example 1, except that catalyst support P3 was used alone as the catalyst support to prepare the air electrode catalyst ink. The ECSA ratio measured using the film electrode assembly of Comparative Example 4 was 0.45.

[0095] (Comparative Example 5) A film electrode assembly of Comparative Example 5 was obtained using the same materials and process as in Example 1, except that catalyst support P5 was used alone as the catalyst support to prepare the air electrode catalyst ink.

[0096] Catalyst support P5 is a porous carbon support on which platinum, the catalytic material, is supported. The catalyst load ratio is expressed as platinum:porous carbon support = 50:50 by mass ratio. The pore size of the porous carbon support of catalyst support P5 is 20 nm. The BET specific surface area of ​​the porous carbon support of catalyst support P5 is 1.3 times that of the porous carbon support of catalyst support P3. The ECSA ratio measured using the film electrode assembly of Comparative Example 5 was 0.62.

[0097] (Comparative Example 6) A film electrode assembly of Comparative Example 6 was obtained using the same materials and process as in Example 1, except that catalyst support P6 was used alone as the catalyst support to prepare the air electrode catalyst ink.

[0098] The catalyst-supporting carrier P6 is a porous carbon carrier supporting platinum as a catalyst substance, and the catalyst-supporting ratio is platinum:porous carbon carrier = 50:50 in terms of mass ratio. The pore diameter of the porous carbon carrier of the catalyst-supporting carrier P6 is 15 nm. The BET specific surface area of the porous carbon carrier of the catalyst-supporting carrier P6 is 1.3 times that of the porous carbon carrier of the catalyst-supporting carrier P3. The ECSA ratio measured using the membrane electrode assembly of Comparative Example 6 was 0.55.

[0099] (Comparative Example 7) A membrane electrode assembly of Comparative Example 7 was obtained by the same materials and processes as in Example 1, except that a catalyst-supporting carrier P7 was used alone as the catalyst-supporting carrier to prepare an air electrode catalyst ink.

[0100] The catalyst-supporting carrier P7 is a porous carbon carrier supporting platinum as a catalyst substance, and the catalyst-supporting ratio is platinum:porous carbon carrier = 50:50 in terms of mass ratio. The pore diameter of the porous carbon carrier of the catalyst-supporting carrier P7 is 20 nm. The BET specific surface area of the porous carbon carrier of the catalyst-supporting carrier P7 is 1.5 times that of the porous carbon carrier of the catalyst-supporting carrier P3. The ECSA ratio measured using the membrane electrode assembly of Comparative Example 7 was 0.65.

[0101] (Power generation performance evaluation) For the membrane electrode assemblies of each Example and each Comparative Example, gas diffusion layers and gaskets were arranged on both sides of the membrane electrode assembly and assembled into a Japan Automobile Research Institute (JARI) standard cell (electrode area 5 cm × 5 cm). Then, an IV evaluation test was carried out under the conditions that the cell temperature was 80 °C, the stoichiometric ratio was fuel electrode: 1.5, air electrode: 1.8, and the relative humidity was fuel electrode: 60%RH, air electrode: 60%RH. In the test, the current density was changed from 20 mA / cm 2 to 2000 mA / cm 2 while maintaining each current density for 5 minutes and switching, and voltage measurement was carried out.

[0102] In the power generation performance evaluation, when the current density was 20 mA / cm 2The low-load condition was defined as follows: a low-load voltage of 0.82V or higher was classified as particularly good ("A"), a low-load voltage of 0.81V or higher but less than 0.82V was classified as good ("B"), a low-load voltage of 0.80V or higher but less than 0.81V was classified as slightly poor ("C"), and a low-load voltage of less than 0.80V was classified as poor ("D").

[0103] Furthermore, the current density is 2000 mA / cm². 2 A high load condition was defined as the following: a high load voltage of 0.67V or higher was classified as particularly good ("A"), a high load voltage of 0.65V or higher but less than 0.67V was classified as good ("B"), a high load voltage of 0.62V or higher but less than 0.65V was classified as slightly poor ("C"), and a high load voltage of less than 0.62V was classified as poor ("D").

[0104] (Evaluation results) Table 1 shows the catalyst support used in the air electrode catalyst layer, the ECSA ratio, and the evaluation results of the power generation performance for each example and comparative example.

[0105] [Table 1]

[0106] As shown in Comparative Examples 1 and 3 in Table 1, when catalyst supports P1 and P4 are used individually, the ECSA ratio is 0.7 or higher. In this case, power generation performance is high at high loads, but low at low loads. Also, as shown in Comparative Examples 2 and 4, when catalyst supports P2 and P3 are used individually, the ECSA ratio is 0.5 or lower. In this case, power generation performance is high at low loads, but low at high loads. Furthermore, as shown in Comparative Examples 5 to 7, when catalyst supports P5, P6, and P7, each with an ECSA ratio of 0.55 to 0.65, are used individually, power generation performance is moderate at both low and high loads.

[0107] Figure 6 shows the relationship between the ECSA ratio and the output voltage at low and high loads when comparative examples 1 to 7, i.e., catalyst supports P1 to P7, are used individually. As shown in Figure 6, when a catalyst support is used individually, if a catalyst support with an ECSA ratio within the range R1 of 0.55 to 0.65 is used, a good output voltage can be obtained at both low and high loads. However, if only low loads or only high loads are considered, the output power cannot be obtained as high as when a catalyst support with an ECSA ratio of 0.5 or less or 0.7 or more is used.

[0108] In contrast, as shown in Table 1, in Examples 1 to 4, where an ECSA ratio of 0.55 to 0.65 is obtained by combining a catalyst support with an ECSA ratio of 0.5 or less with a catalyst support with an ECSA ratio of 0.7 or more, high power generation performance is obtained at both low and high loads. Furthermore, at least one of the low and high loads yields higher output than when a catalyst support with an ECSA ratio of 0.55 to 0.65 is used alone. In particular, in Examples 1 and 2, where the ECSA ratio is 0.58 to 0.62, high output is obtained at both low and high loads, comparable to when a catalyst support with an ECSA ratio of 0.5 or less or 0.7 or more is used alone.

[0109] As described above using the examples, the electrode catalyst layer, membrane electrode assembly, and polymer electrolyte fuel cell of the above embodiment provide the following effects. (1) By combining a first catalyst support 20a, which has an ECSA ratio of 0.5 or less when used alone, with a second catalyst support 20b, which has an ECSA ratio of 0.7 or more when used alone, an ECSA ratio of 0.55 to 0.65 is obtained. This improves power generation performance from low load to high load. In other words, by suppressing catalyst poisoning and improving proton transport with the ionomer, power generation performance can be improved over a wide load range.

[0110] (2) If the pore size of the porous carbon support 21 is between 5 nm and 50 nm, sufficient gas transport into the pores 22 can be obtained, while the intrusion of the polymer electrolyte 24 into the pores 22 is suppressed, thereby preventing excessive catalyst poisoning. Furthermore, it is easy to control the ECSA ratio of the catalyst supporters 20a and 20b to 0.5 or less or 0.7 or more.

[0111] (3) If the pore sizes of the porous carbon support 21 are different between the first catalyst support 20a and the second catalyst support 20b, it is easy to control the ECSA ratio of the catalyst support 20a and 20b to 0.5 or less or 0.7 or more, and it is also easy to obtain an ECSA ratio of 0.55 or more and 0.65 or less by combining these.

[0112] (4) If the particle size distribution of the catalyst support 20 includes a peak between 0.5 μm and 1.0 μm and a peak between 1.5 μm and 5.0 μm, the particle size of the porous carbon support 21 will be different between the first catalyst support 20a and the second catalyst support 20b. Therefore, it is easy to control the ECSA ratio of the catalyst support 20a and 20b to 0.5 or less or 0.7 or more, and it is also easy to obtain an ECSA ratio of 0.55 or more or 0.65 or less by combining these. [Explanation of Symbols]

[0113] 10...Membrane electrode assembly 11...Polymer electrolyte membrane 12A…Fuel electrode catalyst layer 12C...Air electrode catalyst layer 20, 20a, 20b... Catalyst support carriers 21…Porous carbon carrier 22…Pore 23...catalyst material 24...polymer electrolyte 30...Polymer fuel cell 31A, 31C… Gas diffusion layer 32A, 32C... Separators

Claims

1. A catalyst support, which is a porous carbon support on which a catalytic material is supported, and an electrode catalyst layer, which contains a polymer electrolyte and is used in the air electrode of a polymer electrolyte fuel cell, The ECSA ratio, which is the ratio of the ECSA at a relative humidity of 30% to the ECSA at a relative humidity of 100%, is between 0.55 and 0.

65. The catalyst support comprises a first catalyst support having an ECSA ratio of 0.5 or less when used alone as the catalyst support included in the electrode catalyst layer, and a second catalyst support having an ECSA ratio of 0.7 or more when used alone as the catalyst support included in the electrode catalyst layer. Electrocatalyst layer.

2. The pore size of the porous carbon support is 5 nm or more and 50 nm or less. The electrode catalyst layer according to claim 1.

3. The pore diameter of the porous carbon support in the first catalyst support and the pore diameter of the porous carbon support in the second catalyst support are different from each other. The electrode catalyst layer according to claim 2.

4. The particle size distribution of the catalyst support includes a peak between 0.5 μm and 1.0 μm, and a peak between 1.5 μm and 5.0 μm. The electrode catalyst layer according to claim 1.

5. Polymer electrolyte membrane, The system comprises a pair of electrode catalyst layers sandwiching the polymer electrolyte membrane, One of the pair of electrode catalyst layers is the electrode catalyst layer according to any one of claims 1 to 4. Membrane electrode assembly.

6. The membrane electrode assembly according to claim 5, The system comprises a pair of separators that sandwich the aforementioned membrane electrode assembly. Polymer electrolyte fuel cell.

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