Membrane electrode assembly, and polymer electrolyte fuel cell.

Abstract

The present invention provides a membrane electrode assembly which is provided with: a polymer electrolyte membrane; a pair of electrode catalyst layers that are in contact with the surfaces of the polymer electrolyte membrane, while having the polymer electrolyte membrane sandwiched therebetween; and gas diffusion layers that are respectively superposed on the pair of electrode catalyst layers. Each of the gas diffusion layers comprises a microporous layer that is in contact with one of the electrode catalyst layers, and a diffusion base material layer that is formed of a fiber base material. The average fiber diameter of fiber materials contained in the diffusion base material layer is 10 to 150 times the average fiber diameter of fibrous substances contained in the electrode catalyst layers.

Description

Technical Field

[0001] The present disclosure relates to a membrane electrode assembly and a solid polymer fuel cell.

Background Art

[0002] A solid polymer fuel cell includes a membrane electrode assembly having a fuel electrode as an anode, an air electrode as a cathode, and a polymer electrolyte membrane sandwiched between the fuel electrode and the air electrode. Each of the fuel electrode and the air electrode includes a laminate of an electrode catalyst layer and a gas diffusion layer.

[0003] A fuel gas containing hydrogen is supplied to the fuel electrode, and an oxidant gas containing oxygen is supplied to the air electrode. From the fuel gas supplied to the fuel electrode, protons and electrons are generated by the action of a catalyst included in the electrode catalyst layer. The protons are conducted by a polymer electrolyte included in the electrode catalyst layer and the polymer electrolyte membrane, and move through the polymer electrolyte membrane to the air electrode. The electrons are taken out from the fuel electrode to an external circuit and move through the external circuit to the air electrode. At the air electrode, the oxidant gas reacts with the protons and electrons that have moved from the fuel electrode to generate water. The progress of such electrode reactions generates a current flow (see, for example, Patent Documents 1 to 6).

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Patent Document 3

Patent Document 4

Patent Document 5

Patent Document 6

[0005] Substances involved in electrode reactions flow through the pores of the electrode catalyst layer. For example, fuel gas reaches the reaction site at the fuel electrode through the pores of the electrode catalyst layer of the fuel electrode. Also, as the membrane electrode assembly is humidified, water that contributes to proton conduction reaches the polymer electrolyte membrane through the pores of the electrode catalyst layer. Furthermore, oxidant gas reaches the reaction site at the oxygen electrode through the pores of the electrode catalyst layer of the oxygen electrode. Finally, water generated at the oxygen electrode is discharged from the oxygen electrode through the pores of the electrode catalyst layer of the oxygen electrode.

[0006] If excess water accumulates in the electrode catalyst layer, the movement of substances involved in the electrode reaction is hindered, leading to a decrease in power generation performance. To suppress flooding, a phenomenon in which the electrode reaction is inhibited due to water accumulation, various improvements have been proposed, such as adjusting the pore size of the electrode catalyst layer, as described in Patent Documents 1 to 4.

[0007] On the other hand, for proper proton conduction, it is desirable for the polymer electrolyte membrane to contain an appropriate amount of water. Therefore, when operating in a high-current region where a large amount of water is generated under high-humidity conditions, it is necessary to suppress the flooding described above, while under low-humidity conditions, it is necessary to suppress the drying up, which is the decrease in the water content of the polymer electrolyte membrane. For example, Patent Document 5 attempts to suppress both flooding and drying up by providing a humidity control film between the electrode catalyst layer and the gas diffusion layer. Also, Patent Document 6 attempts to suppress both flooding and drying up by providing grooves on the surface of the electrode catalyst layer that is in contact with the gas diffusion layer.

[0008] However, in terms of achieving a membrane electrode assembly that suppresses both flooding and dry-up and provides good power generation performance under both low and high humidity conditions, there is still room for improvement in the structure of the membrane electrode assembly, through improvements in the materials of the electrode catalyst layer and gas diffusion layer. [Means for solving the problem]

[0009] A membrane electrode assembly for solving the above problems comprises a polymer electrolyte membrane, a pair of electrode catalyst layers in contact with the surface of the polymer electrolyte membrane with the polymer electrolyte membrane in between, and a gas diffusion layer laminated on each of the pair of electrode catalyst layers, the gas diffusion layer including a microporous layer in contact with the electrode catalyst layer and a diffusion substrate layer made of a fibrous substrate, wherein at least one of the pair of electrode catalyst layers is a target catalyst layer containing catalyst-supported particles, a polymer electrolyte, and a fibrous material, the diffusion substrate layer is embedded in at least a part of the microporous layer, and the average fiber diameter of the fibrous material contained in the diffusion substrate layer in the gas diffusion layer laminated on the target catalyst layer is 10 to 150 times the average fiber diameter of the fibrous material contained in the target catalyst layer.

[0010] With the above configuration, an appropriate balance can be achieved between the pore size of the target catalyst layer and the pore size of the gas diffusion layer. As a result, the moisture state of the membrane electrode assembly is favorable under both high and low humidity conditions, and good power generation performance can be obtained in the fuel cell.

[0011] In the above configuration, the gas diffusion layer has a first region consisting only of the microporous layer, a second region in which the diffusion substrate layer is embedded in the microporous layer, and a third region consisting only of the diffusion substrate layer, wherein the first region is in contact with the electrode catalyst layer, and the second region may be sandwiched between the first region and the third region in the thickness direction of the gas diffusion layer.

[0012] With the above configuration, compared to the case where the gas diffusion layer does not have a first region, the change in the size of the voids from the target catalyst layer toward the side of the gas diffusion layer opposite to the target catalyst layer progresses more finely and stepwise. Therefore, the supply and discharge of substances involved in the electrode reaction can be made smoother. As a result, the effects due to the relationship of average fiber diameter described above can also be obtained more favorably.

[0013] In the above configuration, the fibrous material included in the target catalyst layer may include at least one of an electrically conductive fiber and a polymer fiber. According to the above configuration, by including the fibrous material, it is possible to improve proton conductivity or improve electron conductivity.

[0014] The solid polymer fuel cell for solving the above problems includes the above membrane electrode assembly and a pair of separators sandwiching the membrane electrode assembly. According to the above configuration, good power generation performance can be obtained under both high humidity conditions and low humidity conditions.

Effect of the Invention

[0015] According to the present disclosure, good power generation performance can be obtained under both high humidity conditions and low humidity conditions.

Brief Description of the Drawings

[0016] [Figure 1] A diagram showing a cross-sectional structure of a membrane electrode assembly according to an embodiment. [Figure 2] A diagram schematically showing a configuration of a gas diffusion layer according to an embodiment. [Figure 3] A diagram showing a perspective structure of a solid polymer fuel cell according to an embodiment disassembled.

Mode for Carrying Out the Invention

[0017] Referring to FIGS. 1 to 3, an embodiment of a membrane electrode assembly and a solid polymer fuel cell will be described. It should be understood that the description "at least one of A and B" in this specification means "only A, or only B, or both A and B".

[0018] [Membrane Electrode Assembly] As shown in Figure 1, the membrane electrode assembly 10 comprises a polymer electrolyte membrane 11, a pair of electrode catalyst layers 12A and 12C, and a pair of gas diffusion layers 13A and 13C. The gas diffusion layer 13A includes a microporous layer 14A made of a porous material and a diffusion substrate layer 15A made of a fibrous substrate. Similarly, the gas diffusion layer 13C includes a microporous layer 14C made of a porous material and a diffusion substrate layer 15C made of a fibrous substrate.

[0019] The polymer electrolyte membrane 11 is sandwiched between the electrode catalyst layer 12A and the electrode catalyst layer 12C. The electrode catalyst layer 12A is in contact with one of the two surfaces of the polymer electrolyte membrane 11, and the electrode catalyst layer 12C is in contact with the other of the two surfaces of the polymer electrolyte membrane 11.

[0020] The gas diffusion layer 13A is laminated on the electrode catalyst layer 12A, and the gas diffusion layer 13C is laminated on the electrode catalyst layer 12C. In other words, the laminate of the polymer electrolyte membrane 11, the electrode catalyst layer 12A, and the electrode catalyst layer 12C is sandwiched between the gas diffusion layer 13A and the gas diffusion layer 13C.

[0021] The electrode catalyst layer 12A and the gas diffusion layer 13A constitute the fuel electrode, which is the anode of the polymer electrolyte fuel cell. The electrode catalyst layer 12C and the gas diffusion layer 13C constitute the air electrode, which is the cathode of the polymer electrolyte fuel cell.

[0022] When viewed from a position opposite one of the surfaces of the polymer electrolyte membrane 11, the outer shapes of the electrode catalyst layers 12A, 12C and the gas diffusion layers 13A, 13C are approximately the same and smaller than the outer shape of the polymer electrolyte membrane 11. The shape of each layer 12A, 12C, 13A, 13C and the outer shape of the polymer electrolyte membrane 11 are not particularly limited; for example, they may be rectangular.

[0023] Figure 2 is a schematic, enlarged view of the cross-sectional structure of the laminated portion of the electrode catalyst layer 12A and the gas diffusion layer 13A. In Figure 2, dots are added to indicate the location of the microporous layer 14C for easier understanding.

[0024] As shown in Figure 2, the microporous layer 14A is in contact with the electrode catalyst layer 12A, and the microporous layer 14A and the diffusion substrate layer 15A overlap in the thickness direction. The outermost surface of the gas diffusion layer 13A opposite to the electrode catalyst layer 12A is the surface of the diffusion substrate layer 15A.

[0025] Preferably, the gas diffusion layer 13A has a first region 21 consisting only of the microporous layer 14A, a second region 22 in which the microporous layer 14A and the diffusion substrate layer 15A are mixed, and a third region 23 consisting only of the diffusion substrate layer 15A. In the second region 22, the diffusion substrate layer 15A is embedded in the microporous layer 14A. In other words, the second region 22 has a structure in which the porous material constituting the microporous layer 14A is located between the fibers of the diffusion substrate layer 15A.

[0026] The first region 21, the second region 22, and the third region 23 are arranged in this order along the thickness direction of the gas diffusion layer 13A, starting from a position close to the electrode catalyst layer 12A. That is, the first region 21 is in contact with the electrode catalyst layer 12A, the third region 23 is located on the outermost edge of the gas diffusion layer 13A, and the second region 22 is sandwiched between the first region 21 and the third region 23.

[0027] Furthermore, the gas diffusion layer 13A may not have a first region 21, but may be composed of a second region 22 and a third region 23. In other words, the gas diffusion layer 13A may not have a region consisting solely of the microporous layer 14A. In this case, the second region 22 is in contact with the electrode catalyst layer 12A. That is, the microporous layer 14A and the diffusion substrate layer 15A are in contact with the electrode catalyst layer 12A.

[0028] The gas diffusion layer 13C has the same configuration as the gas diffusion layer 13A. That is, the microporous layer 14C is in contact with the electrode catalyst layer 12C, and the diffusion substrate layer 15C is embedded in at least a part of the microporous layer 14C. Preferably, the gas diffusion layer 13C has a first region consisting only of the microporous layer 14C, a second region where the microporous layer 14C and the diffusion substrate layer 15C are mixed, and a third region consisting only of the diffusion substrate layer 15C, in order of proximity to the electrode catalyst layer 12C, but the first region does not have to be present.

[0029] The following describes the materials of each layer that make up the membrane electrode assembly 10. 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. Examples of fluorine-based polymer electrolytes include Nafion (registered trademark: manufactured by DuPont), Flemion (registered trademark: manufactured by Asahi Glass Co., Ltd.), Aciplex (registered trademark: manufactured by Asahi Kasei Corporation), and Gore-Select (registered trademark: manufactured by Gore Japan LLC). Examples of hydrocarbon-based polymer electrolytes include sulfonated polyether ketone, sulfonated polyether sulfone, sulfonated polyether ether sulfone, sulfonated polysulfide, sulfonated polyphenylene, etc.

[0030] The electrode catalyst layers 12A and 12C contain catalyst-supported particles and aggregates of polymer electrolytes. The catalyst-supported particles are carriers on which the catalytic substance is supported. Furthermore, at least one of the electrode catalyst layers 12A and 12C contains a fibrous material. In the electrode catalyst layers 12A and 12C, aggregates of polymer electrolytes and fibrous material are located around the dispersed catalyst-supported particles, and pores are formed between these components.

[0031] The catalyst material is particulate. From the viewpoint of increasing the activity of the catalyst material, the average particle size of the catalyst particles is preferably 20 nm or less, and more preferably 5 nm or less. From the viewpoint of stabilizing the activity of the catalyst particles, the average particle size of the catalyst particles is preferably 0.5 nm or more, and more preferably 1 nm or more. The material of the catalyst material is, for example, platinum group metals, non-platinum group metals, alloys of these metals, oxides of these metals, or complex oxides of these metals. Platinum group metals are platinum, palladium, ruthenium, iridium, rhodium, and osmium. Non-platinum group metals are iron, lead, copper, chromium, cobalt, nickel, manganese, vanadium, molybdenum, gallium, and aluminum. From the viewpoint of increasing the activity of the catalyst material, the material of the catalyst material is preferably platinum or a platinum alloy.

[0032] The support is a particle that is not eroded by the catalyst and is conductive. An example of a support is carbon particles. From the viewpoint of facilitating the formation of electron conduction paths, the particle size of the support is preferably 10 nm or larger. From the viewpoint of reducing the resistance of the electrode catalyst layers 12A and 12C and increasing the amount of catalyst particles supported, the particle size of the support is preferably 1000 nm or smaller, and more preferably 100 nm or smaller.

[0033] Specific examples of carriers include carbon black, graphite, graphite, activated carbon, and fullerene. Examples of carbon black include acetylene black, furnace black, and Ketjen black.

[0034] The support may be covered with a hydrophobic coating. The hydrophobic coating imparts hydrophobicity to the support and also has gas permeability, allowing fuel gas and oxidizer gas to pass through. From the viewpoint of improving gas permeability, the thickness of the hydrophobic coating is preferably 40 nm or less. From the viewpoint of improving water discharge in the electrode catalyst layers 12A and 12C, the thickness of the hydrophobic coating is preferably 2 nm or more.

[0035] An example of a hydrophobic coating material is a fluorine-based compound having at least one polar group. Examples of polar groups include hydroxyl groups, alkoxy groups, carboxyl groups, ester groups, ether groups, carbonate groups, and amide groups. Hydrophobic coatings with polar groups are easily fixed to the outermost surface of a support. An example of a non-polar group component in a fluorine-based compound is a fluoroalkyl skeleton.

[0036] The polymer electrolyte constituting the aggregate can be any polymer electrolyte having proton conductivity, such as fluorine-based polymer electrolytes or hydrocarbon-based polymer electrolytes. Examples of fluorine-based polymer electrolytes include Nafion (registered trademark: manufactured by DuPont), Flemion (registered trademark: manufactured by Asahi Glass Co., Ltd.), Aciplex (registered trademark: manufactured by Asahi Kasei Corporation), and Gore-Select (registered trademark: manufactured by Gore Japan LLC). Examples of hydrocarbon-based polymer electrolytes include sulfonated polyether ketones, sulfonated polyethersulfones, sulfonated polyetherethersulfones, sulfonated polysulfides, and sulfonated polyphenylenes.

[0037] If the polymer electrolyte constituting the aggregate and the polymer electrolyte constituting the polymer electrolyte membrane 11 are of the same type, the adhesion of the electrode catalyst layers 12A and 12C to the polymer electrolyte membrane 11 is improved.

[0038] The fibrous material is either an electronically conductive fiber or a polymer fiber. An electronically conductive fiber is a fiber made of an electronically conductive material, such as a carbon fiber. A polymer fiber is a fiber made of a polymer material, such as a proton-conducting fiber or a fiber having Lewis acidic or Lewis basic functional groups. A proton-conducting fiber is a fiber made of a polymer electrolyte that has proton conductivity. The fibrous material contained in the electrode catalyst layers 12A and 12C may be of only one type or of two or more types. For example, the fibrous material contained in the electrode catalyst layers 12A and 12C may consist only of electronically conductive fibers or only of polymer fibers. Alternatively, the fibrous material contained in the electrode catalyst layers 12A and 12C may include both electronically conductive fibers and polymer fibers.

[0039] Because the electrode catalyst layers 12A and 12C contain fibrous material, their strength is increased, making it easier to form the electrode catalyst layers 12A and 12C, and also making it less likely for cracks to occur in the electrode catalyst layers 12A and 12C. This also improves the durability of the film electrode assembly 10.

[0040] Examples of electronically conductive fibers include carbon fibers, carbon nanotubes, carbon nanohorns, and conductive polymer nanofibers. From the viewpoint of improving conductivity in the electrode catalyst layers 12A and 12C, and improving dispersibility in the coating solution for forming the electrode catalyst layers 12A and 12C, the fibrous material is preferably carbon nanofiber or carbon nanotube.

[0041] Materials for proton-conducting fibers include, for example, fluorine-based polymer electrolytes and hydrocarbon-based polymer electrolytes. Examples of fluorine-based polymer electrolytes include Nafion (registered trademark: manufactured by DuPont), Flemion (registered trademark: manufactured by Asahi Glass Co., Ltd.), Aciplex (registered trademark: manufactured by Asahi Kasei Corporation), and Gore-Select (registered trademark: manufactured by Gore Japan LLC). Examples of hydrocarbon-based polymer electrolytes include sulfonated polyether ketones, sulfonated polyethersulfones, sulfonated polyetherethersulfones, sulfonated polysulfides, and sulfonated polyphenylenes.

[0042] If the polymer electrolytes constituting the polymer electrolyte membrane 11, the polymer electrolyte aggregate, and the proton-conducting fibers are of the same type, the adhesion of the electrode catalyst layers 12A and 12C to the polymer electrolyte membrane 11 can be improved.

[0043] Polymer fibers having the above-mentioned functional groups contain Lewis acidic or Lewis basic functional groups in the molecular structure of the material. This makes it easier for polymer electrolytes to be present around the fibrous material. Examples of fibrous materials having Lewis acidic functional groups include hydrophilic carbon fibers and polymer fibers having hydroxyl groups, carbonyl groups, sulfonic acid groups, and phosphite groups. Examples of fibrous materials having Lewis basic functional groups include polymer fibers having imide structures or azole structures. Hydrogen bonding occurs between Lewis acidic functional groups in fibrous materials and proton-conducting sites such as sulfonyl groups contained in polymer electrolytes, making it easier for polymer electrolytes to be present around the fibrous material. On the other hand, if a fibrous material has Lewis basic functional groups, the Lewis basic functional groups and acidic proton-conducting sites such as sulfonyl groups contained in polymer electrolytes bond as acid and base, making it easier for polymer electrolytes to be present around the fibrous material. Since acid-base bonding is stronger than hydrogen bonding, it is preferable for fibrous materials to contain Lewis basic functional groups. In particular, it is preferable for fibrous materials to have an azole structure. An azole structure is a heterogeneous five-membered ring structure containing one or more nitrogen atoms, such as an imidazole structure or an oxazole structure. Substances containing nitrogen atoms preferably have a benzoazole structure, such as a benzimidazole structure or a benzoxazole structure. Specific examples of substances containing nitrogen atoms include polymers such as polybenzimidazole and polybenzoxazole.

[0044] The average fiber diameter dm of the fibrous material is preferably between 10 nm and 300 nm, and more preferably between 100 nm and 200 nm. If the average fiber diameter dm is within the above range, pores are accurately formed within the electrode catalyst layers 12A and 12C.

[0045] The microporous layers 14A and 14C are composed of a mixture of a water-repellent resin and a conductive material. The water-repellent resin is a fluororesin such as polytetrafluoroethylene (PTFE), perfluoroethylene-propene copolymer (FEP), or tetrafluoroethylene-ethylene copolymer resin (ETFE). The conductive material is a carbon material such as carbon black or graphite.

[0046] The diffusion substrate layers 15A and 15C are gas-permeable layered bodies that are conductive. The diffusion substrate layers 15A and 15C are, for example, substrates made of carbon fibers. The diffusion substrate layers 15A and 15C are, for example, carbon cloth, carbon paper, carbon felt, etc.

[0047] The average fiber diameter dg of the fibrous material contained in the diffusion substrate layers 15A and 15C is preferably 0.1 μm or more and 45 μm or less, and more preferably 1 μm or more and 30 μm or less. If the average fiber diameter dg is within the above range, suitable gas diffusion properties can be obtained in the gas diffusion layers 13A and 13C.

[0048] The average fiber diameter dg of the fibrous material contained in the diffusion substrate layers 15A and 15C is between 10 and 150 times the average fiber diameter dm of the fibrous material contained in the electrode catalyst layers 12A and 12C. Fine voids are formed within the electrode catalyst layers 12A, 12C and the gas diffusion layers 13A, 13C, serving as pathways for gases and water involved in the electrode reaction. The average fiber diameter dm of the fibrous material is a major factor in controlling the size of the pores (voids) formed within the electrode catalyst layers 12A, 12C. Similarly, the average fiber diameter dg of the diffusion substrate layers 15A, 15C is a major factor in controlling the size of the pores (voids) formed within the gas diffusion layers 13A, 13C. In particular, the average fiber diameter dg of the diffusion substrate layers 15A, 15C is a factor in controlling the size of the pores (voids) formed within the regions where the diffusion substrate layers 15A, 15C penetrate the microporous layers 14A, 14C.

[0049] Specifically, by making the average fiber diameter dg of the fibrous material contained in the diffusion substrate layers 15A and 15C larger than the average fiber diameter dm of the fibrous material contained in the electrode catalyst layers 12A and 12C, the pores formed inside the gas diffusion layers 13A and 13C become larger than the pores formed inside the electrode catalyst layers 12A and 12C. Furthermore, in the region where the diffusion substrate layers 15A and 15C are embedded in the microporous layers 14A and 14C, the pores formed within that region become smaller than the pores formed inside the gas diffusion layers 13A and 13C, and larger than the pores formed inside the electrode catalyst layers 12A and 12C. Therefore, the change in pore size in the electrode catalyst layers 12A and 12C and the gas diffusion layers 13A and 13C is preferable.

[0050] By ensuring that the average fiber diameter dg in the diffusion substrate layers 15A and 15C is between 10 and 150 times the average fiber diameter dm in the electrode catalyst layers 12A and 12C, an appropriate balance is achieved between the pore size of the electrode catalyst layers 12A and 12C and the pore size of the gas diffusion layers 13A and 13C. As a result, the moisture state of the membrane electrode assembly 10 is favorable under both high and low humidity conditions, leading to high power generation performance.

[0051] In detail, for example, the following principle can be inferred: If the difference in pore size between the electrode catalyst layers 12A,12C and the gas diffusion layers 13A,13C is too small, when the fuel cell is operated in the high-current region under high-humidity conditions, the drainage of the generated water will be insufficient, and excess water will tend to accumulate. On the other hand, if the average fiber diameter dg is 10 times or more the average fiber diameter dm, a sufficient difference in pore size between the electrode catalyst layers 12A,12C and the gas diffusion layers 13A,13C can be obtained, and it is thought that the generated water will be properly drained even under high-humidity conditions. Therefore, the inhibition of gas diffusion is suppressed, and the decrease in power generation performance can be suppressed.

[0052] On the other hand, if the difference in pore size between the electrode catalyst layers 12A, 12C and the gas diffusion layers 13A, 13C is too large, water retention will be insufficient under low humidity conditions. In contrast, if the average fiber diameter dg is 150 times or less of the average fiber diameter dm, the difference in pore size between the electrode catalyst layers 12A, 12C and the gas diffusion layers 13A, 13C will not become excessively large, and the water content of the polymer electrolyte membrane 11 can be appropriately maintained even under low humidity conditions. Therefore, the decrease in proton conductivity can be suppressed, and thus the decrease in power generation performance can be suppressed.

[0053] If the gas diffusion layers 13A and 13C have a first region consisting only of microporous layers 14A and 14C, a second region in which diffusion substrate layers 15A and 15C are embedded in the microporous layers 14A and 14C, and a third region consisting only of diffusion substrate layers 15A and 15C, the following effects can be obtained. That is, the size of the voids in the gas diffusion layers 13A and 13C increases in the order of the first region, the second region, and the third region. Therefore, compared to the case where the gas diffusion layers 13A and 13C do not have a first region, the change in the size of the voids from the electrode catalyst layers 12A and 12C toward the side of the gas diffusion layers 13A and 13C opposite to the electrode catalyst layers 12A and 12C proceeds more finely and stepwise. Thus, the supply and discharge of substances involved in the electrode reaction can be made smoother. As a result, the above-mentioned effects due to the average fiber diameter dg being 10 to 150 times the average fiber diameter dm can also be more favorably obtained.

[0054] Furthermore, if only one of the electrode catalyst layer 12A or electrode catalyst layer 12C contains a fibrous material, then in the gas diffusion layer laminated on the electrode catalyst layer containing the fibrous material, the average fiber diameter dg should be between 10 and 150 times the average fiber diameter dm.

[0055] [Polymer electrolyte fuel cell] Referring to Figure 3, the configuration of a polymer electrolyte fuel cell equipped with the membrane electrode assembly 10 described above will be explained.

[0056] As shown in Figure 3, the polymer electrolyte fuel cell 30 comprises a membrane electrode assembly 10 and a pair of separators 31A and 31C. The membrane electrode assembly 10 is sandwiched between separators 31A and 31C.

[0057] Separators 31A and 31C are made of a conductive and gas-impermeable material. Separator 31A faces the gas diffusion layer 13A, and separator 31C faces the gas diffusion layer 13C. On separator 31A, a gas channel 32A is formed on the surface facing the gas diffusion layer 13A, and a cooling water channel 33A is formed on the surface opposite to the gas diffusion layer 13A. Similarly, on separator 31C, a gas channel 32C is formed on the surface facing the gas diffusion layer 13C, and a cooling water channel 33C is formed on the surface opposite to the gas diffusion layer 13C.

[0058] When the polymer electrolyte fuel cell 30 is in use, a fuel gas such as hydrogen flows through the gas channel 32A of separator 31A, and an oxidizing gas such as oxygen flows through the gas channel 32C of separator 31C. Cooling water also flows through the cooling water channels 33A and 33C of separators 31A and 31C, respectively. When the fuel gas is supplied to the fuel electrode from the gas channel 32A and the oxidizing gas is supplied to the air electrode from the gas channel 32C, the electrode 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.

[0059] The polymer electrolyte fuel cell 30 may be used in the form of a single cell as shown in Figure 3, or multiple polymer electrolyte fuel cells 30 may be stacked and connected in series to form a single fuel cell. In addition to the above components, the polymer electrolyte fuel cell 30 may also be equipped with gaskets or the like to suppress gas leakage.

[0060] [Method for manufacturing a membrane electrode assembly] A method for manufacturing the membrane electrode assembly 10 will be described. The electrode catalyst layers 12A and 12C are formed by applying a coating solution containing the materials for the electrode catalyst layers 12A and 12C to a substrate to form a coating film, and then drying the coating film.

[0061] The solvent of the coating solution does not erode the catalyst-supported particles, polymer electrolytes, and fibrous materials, and dissolves the polymer electrolytes or disperses them as a fine gel. Examples of solvents include alcohols, ketone solvents, ether solvents, and polar solvents. Examples of alcohols include methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, 2-butanol, isobutyl alcohol, tert-butyl alcohol, and pentanol. Examples of ketone solvents include acetone, methyl ethyl ketone, pentanone, methyl isobutyl ketone, heptanone, cyclohexanone, methylcyclohexanone, acetonylacetone, and diisobutyl ketone. Examples of ether solvents include tetrahydrofuran, dioxane, diethylene glycol dimethyl ether, anisole, methoxytoluene, and dibutyl ether. Examples of polar solvents include dimethylformamide, dimethylacetamide, N-methylpyrrolidone, ethylene glycol, diethylene glycol, diacetone alcohol, and 1-methoxy-2-propanol. The solvent may contain water, which has a high affinity for the polymer electrolyte.

[0062] When improved dispersibility of catalyst-supported particles is required, the coating solution preferably contains a dispersant. Examples of dispersants include anionic surfactants, cationic surfactants, amphoteric surfactants, and nonionic surfactants. Furthermore, when improved dispersibility in the coating solution is required, it is preferable to perform a dispersion treatment during the manufacturing process of the coating solution. Examples of dispersion treatments include stirring with a ball mill or roll mill, stirring with a shear mill, stirring with a wet mill, stirring by applying ultrasonic waves, and stirring with a homogenizer.

[0063] From the viewpoint of suppressing the occurrence of cracks on the surface of electrode catalyst layers 12A and 12C, the solid content of the coating solution is preferably 50% by mass or less. From the viewpoint of increasing the film formation rate of electrode catalyst layers 12A and 12C, the solid content of the coating solution is preferably 1% by mass or more.

[0064] As a substrate for forming the electrode catalyst layers 12A and 12C, for example, a transfer substrate is used that is peeled off after transferring the electrode catalyst layers 12A and 12C 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. Examples of fluororesins include ethylene tetrafluoroethylene copolymer (ETFE), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), tetrafluoroperfluoroalkyl vinyl ether copolymer (PFA), and polytetrafluoroethylene (PTFE). Examples of organic polymer compounds include polyimide, polyethylene terephthalate, polyamide, polysulfone, polyethersulfone, polyphenylene sulfide, polyetheretherketone, polyetherimide, polyarylate, and polyethylene naphthalate.

[0065] Furthermore, the substrate for forming the electrode catalyst layers 12A and 12C may be the polymer electrolyte membrane 11 or the gas diffusion layers 13A and 13C. The method of applying the coating liquid to the substrate is not particularly limited. Examples of application methods include the doctor blade method, dipping method, screen printing method, and roll coating method.

[0066] The gas diffusion layers 13A and 13C are formed by applying a coating solution containing the materials for the microporous layers 14A and 14C to the surface of the diffusion substrate layers 15A and 15C, and then drying the coating film. For example, the microporous layers 14A and 14C can be formed by applying a coating solution containing carbon particles, a fluororesin dispersion, a solvent, and a surfactant to the surface of the diffusion substrate layers 15A and 15C and then drying it.

[0067] Alternatively, the diffusion substrate layers 15A and 15C may be formed by applying a coating solution containing a fluororesin dispersion, a solvent, and a surfactant to a carbon fiber substrate and then drying it. When the substrate for forming the electrode catalyst layers 12A and 12C is a transfer substrate, the electrode 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 electrode catalyst layers 12A and 12C. Then, gas diffusion layers 13A and 13C are laminated onto the electrode catalyst layers 12A and 12C on the polymer electrolyte membrane 11. This forms the membrane electrode assembly 10.

[0068] When the substrate for forming the electrode catalyst layers 12A and 12C is the gas diffusion layer 13A and 13C, the electrode catalyst layers 12A and 12C supported by the gas diffusion layer 13A and 13C are joined to the polymer electrolyte membrane 11 by thermocompression bonding, thereby forming the membrane electrode assembly 10.

[0069] When the substrate for forming the electrode catalyst layers 12A and 12C is a polymer electrolyte membrane 11, the electrode catalyst layers 12A and 12C are formed directly on the surface of the polymer electrolyte membrane 11, and then the gas diffusion layers 13A and 13C are laminated onto the electrode catalyst layers 12A and 12C. This forms the membrane electrode assembly 10.

[0070] [Examples] The membrane electrode assembly and polymer electrolyte fuel cell described above will be explained using specific examples and comparative examples.

[0071] (Example 1) <Manufacturing of electrode catalyst layers> A coating solution for forming an electrode catalyst layer was prepared by mixing the catalyst-supported particles, polymer electrolyte, and fibrous material shown below in a solvent and dispersing them in a planetary ball mill for 30 minutes. A mixed solvent of ultrapure water and 1-propanol was used as the solvent for the coating solution. The volume ratio of ultrapure water to 1-propanol in the mixed solvent was 1:1. The mixing ratio of each material in the coating solution was such that the mass of the catalyst-supported carrier was 1, the mass of the fibrous material was 0.3, and the mass of the polymer electrolyte was 0.8. The coating solution was prepared so that the solid content in the coating solution was 12% by mass.

[0072] • Catalyst-supported particles: Platinum-supported carbon particles (TEC10E50E, manufactured by Tanaka Kikinzoku Kogyo Co., Ltd.) • Fibrous material: Carbon fiber (average fiber diameter: 0.15 μm) • Polymer electrolyte: Fluorine-based polymer electrolyte (Nafion® dispersion, manufactured by Wako Pure Chemical Industries, Ltd.)

[0073] A polytetrafluoroethylene (PTFE) sheet was used as the transfer substrate, and the coating solution was applied to the transfer substrate using the doctor blade method. The resulting coating film was dried in an air atmosphere at 80°C. This yielded an electrode catalyst layer. Furthermore, when forming the electrode catalyst layer of the fuel electrode, the amount of platinum supported is 0.1 mg / cm³. 2 The amount of coating solution applied was adjusted accordingly. Furthermore, when forming the electrode catalyst layer of the air electrode, the platinum load was set to 0.3 mg / cm³. 2 The amount of coating solution applied was adjusted accordingly.

[0074] <Manufacturing of membrane electrode assemblies> The electrode catalyst layer on the transfer substrate was punched out in a square shape with sides of 5 cm. The punched-out electrode catalyst layer of the fuel electrode was transferred from the transfer substrate to one side of the polymer electrolyte membrane, and the punched-out electrode catalyst layer of the air electrode was transferred from the transfer substrate to the other side of the polymer electrolyte membrane. During the transfer of the electrode catalyst layers, a temperature of 130°C and 5.0 × 10⁻⁶ were used. 6Hot pressing was performed at a pressure of Pa. A fluorine-based polymer electrolyte (Nafion® 211, manufactured by DuPont) was used as the polymer electrolyte membrane. The thickness of the polymer electrolyte membrane was 25 μm.

[0075] A gas diffusion layer, in which a microporous layer is coated onto carbon paper, which serves as the diffusion substrate layer, was bonded to the electrode catalyst layers of the fuel electrode and the air electrode, respectively. The average fiber diameter of the carbon paper is 3 μm, and the fiber diameter ratio (dg / dm), which is the ratio of the average fiber diameter dg of the diffusion substrate layer in the gas diffusion layer to the average fiber diameter dm of the fibrous material in the electrode catalyst layer, is 20. The conductive material contained in the microporous layer is the same carbon particles as the carrier of the catalyst-supported particles. This yielded the film electrode assembly of Example 1.

[0076] (Example 2) The film electrode assembly of Example 2 was obtained using the same materials and process as in Example 1, except that the diffusion substrate layer was changed to carbon paper with an average fiber diameter of 6 μm, and the fiber diameter ratio (dg / dm) was set to 40.

[0077] (Example 3) The film electrode assembly of Example 3 was obtained using the same materials and process as in Example 1, except that the diffusion substrate layer was changed to carbon paper with an average fiber diameter of 22 μm, and the fiber diameter ratio (dg / dm) was set to 146.

[0078] (Example 4) The membrane electrode assembly of Example 4 was obtained using the same materials and process as in Example 1, except that the fibrous material contained in the electrode catalyst layer was changed to a proton-conducting electrolyte fiber with an average fiber diameter of 0.2 μm, and the fiber diameter ratio (dg / dm) was set to 15. The material of the electrolyte fiber is a fluorine-based polymer electrolyte (Nafion®, manufactured by DuPont).

[0079] (Example 5) The film electrode assembly of Example 5 was obtained using the same materials and process as in Example 4, except that the diffusion substrate layer was changed to carbon paper with an average fiber diameter of 6 μm and the fiber diameter ratio (dg / dm) was set to 30.

[0080] (Example 6) The film electrode assembly of Example 6 was obtained using the same materials and process as in Example 4, except that the diffusion substrate layer was changed to carbon paper with an average fiber diameter of 22 μm, and the fiber diameter ratio (dg / dm) was set to 110.

[0081] (Example 7) The film electrode assembly of Example 7 was obtained using the same materials and process as in Example 1, except that the fibrous material contained in the electrode catalyst layer was changed to polymer fibers (with sulfonic acid groups) having cation exchange groups with an average fiber diameter of 0.2 μm, and the fiber diameter ratio (dg / dm) was set to 15. The polymer fiber material had the property of adsorbing ionomers, and it was confirmed that the amount of ionomer adsorbed was 10 mg or more per gram of fiber.

[0082] (Example 8) The film electrode assembly of Example 8 was obtained using the same materials and process as in Example 7, except that the diffusion substrate layer was changed to carbon paper with an average fiber diameter of 6 μm, and the fiber diameter ratio (dg / dm) was set to 30.

[0083] (Example 9) The film electrode assembly of Example 9 was obtained using the same materials and process as in Example 7, except that the diffusion substrate layer was changed to carbon paper with an average fiber diameter of 22 μm, and the fiber diameter ratio (dg / dm) was set to 110.

[0084] (Example 10) The membrane electrode assembly of Example 10 was obtained using the same materials and process as in Example 1, except that the fibrous material contained in the electrode catalyst layer was changed to polymer fibers (amino group-added) having cation exchange groups with an average fiber diameter of 0.2 μm, and the fiber diameter ratio (dg / dm) was set to 15. The polymer fiber material had the property of adsorbing ionomers, and it was confirmed that the amount of ionomer adsorbed was 10 mg or more per gram of fiber.

[0085] (Example 11) The film electrode assembly of Example 11 was obtained using the same materials and process as in Example 10, except that the diffusion substrate layer was changed to carbon paper with an average fiber diameter of 6 μm and the fiber diameter ratio (dg / dm) was set to 30.

[0086] (Example 12) The film electrode assembly of Example 12 was obtained using the same materials and process as in Example 10, except that the diffusion substrate layer was changed to carbon paper with an average fiber diameter of 22 μm, and the fiber diameter ratio (dg / dm) was set to 110.

[0087] (Example 13) The membrane electrode assembly of Example 13 was obtained using the same materials and process as in Example 1, except that the fibrous material contained in the electrode catalyst layer was changed to polymer fibers (with hydroxyl groups) having cation exchange groups with an average fiber diameter of 0.2 μm, and the fiber diameter ratio (dg / dm) was set to 15. The polymer fiber material had the property of adsorbing ionomers, and it was confirmed that the amount of ionomer adsorbed was 10 mg or more per gram of fiber.

[0088] (Example 14) The film electrode assembly of Example 14 was obtained using the same materials and process as in Example 13, except that the diffusion substrate layer was changed to carbon paper with an average fiber diameter of 6 μm and the fiber diameter ratio (dg / dm) was set to 30.

[0089] (Example 15) The film electrode assembly of Example 15 was obtained using the same materials and process as in Example 13, except that the diffusion substrate layer was changed to carbon paper with an average fiber diameter of 22 μm, and the fiber diameter ratio (dg / dm) was set to 110.

[0090] (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 the diffusion substrate layer was changed to carbon paper with an average fiber diameter of 1.2 μm, and the fiber diameter ratio (dg / dm) was set to 8.

[0091] (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 the diffusion substrate layer was changed to carbon paper with an average fiber diameter of 35 μm, and the fiber diameter ratio (dg / dm) was set to 233.

[0092] (Comparative Example 3) A film electrode assembly of Comparative Example 3 was obtained using the same materials and process as in Example 4, except that the diffusion substrate layer was changed to carbon paper with an average fiber diameter of 1.2 μm, and the fiber diameter ratio (dg / dm) was set to 6.

[0093] (Comparative Example 4) A film electrode assembly of Comparative Example 4 was obtained using the same materials and process as in Example 4, except that the diffusion substrate layer was changed to carbon paper with an average fiber diameter of 35 μm, and the fiber diameter ratio (dg / dm) was set to 175.

[0094] (Method for evaluating power generation performance) For each example and comparative example, a membrane electrode assembly was placed in a power generation evaluation cell to form a polymer electrolyte fuel cell, and current and voltage measurements were performed using a fuel cell measurement device under two different operating conditions with varying humidification levels. The current and voltage measurement conditions and operating conditions are shown below.

[0095] <Measurement conditions> Fuel gas: Hydrogen Oxidizing gas: air Cell temperature: 80℃ • Controlled back pressure: 50kPa <Driving Conditions 1> • Relative humidity of the fuel electrode: 100%RH • Relative humidity at the oxygen electrode: 100%RH <Driving Conditions 2> • Relative humidity of the fuel electrode: 30%RH • Relative humidity at the oxygen electrode: 30%RH

[0096] Furthermore, the gas flow rate was controlled to maintain a constant fuel utilization rate in both operating conditions 1 (high-load operation) and 2 (low-load operation). The fuel utilization rate is the ratio of the flow rate of the reacting fuel gas to the flow rate of the supplied fuel gas. The fuel utilization rate is calculated by dividing the hydrogen consumption rate, which is derived from the current value, by the hydrogen flow rate of the supplied fuel gas.

[0097] (Evaluation results) Table 1 shows the material and average fiber diameter dm of the fibrous material, the average fiber diameter dg of the diffusion substrate layer in the gas diffusion layer, the fiber diameter ratio (dg / dm), and the evaluation results of the power generation performance for each example and comparative example. In the evaluation of power generation performance, the current density was 2.0 A / cm². 2 If the voltage is 0.65V or higher, it is marked with "○", and the current density is 2.0A / cm². 2 If the voltage was less than 0.65V, it was marked as "×".

[0098] [Table 1]

[0099] As shown in Table 1, in Examples 1 to 15, where the fiber diameter ratio (dg / dm) was between 10 and 150, good power generation performance was obtained under both operating conditions 1 (high humidity conditions) and operating conditions 2 (low humidity conditions).

[0100] On the other hand, in Comparative Examples 1 and 3, where the fiber diameter ratio (dg / dm) was less than 10, good power generation performance was obtained under low humidity conditions, but poor power generation performance was observed under high humidity conditions. This suggests that in Comparative Examples 1 and 3, the small difference in pore size between the electrode catalyst layer and the gas diffusion layer resulted in insufficient drainage under high humidity conditions, inhibiting gas diffusion and degrading performance.

[0101] Furthermore, in Comparative Examples 2 and 4, where the fiber diameter ratio (dg / dm) exceeded 150, power generation performance was poor under low humidity conditions. This suggests that in Comparative Examples 2 and 4, the large difference in pore size between the electrode catalyst layer and the gas diffusion layer resulted in insufficient water retention under low humidity conditions, inhibiting proton conduction and degrading performance. Although good power generation performance was obtained under high humidity conditions in Comparative Example 4, power generation performance was poor even under high humidity conditions in Comparative Example 2, which had a particularly large fiber diameter ratio (dg / dm). This suggests that if the difference in pore size between the electrode catalyst layer and the gas diffusion layer is too large, generated water may not flow smoothly from the electrode catalyst layer to the gas diffusion layer under high humidity conditions, potentially leading to water stagnation.

[0102] As described above using the examples, the membrane electrode assembly and polymer electrolyte fuel cell of the above embodiment provide the following effects. (1) The average fiber diameter dg of the fibrous material in the gas diffusion layers 13A and 13C is between 10 and 150 times the average fiber diameter dm of the fibrous material contained in the electrode catalyst layers 12A and 12C. This ensures an appropriate balance between the pore size of the electrode catalyst layers 12A and 12C and the pore size of the gas diffusion layers 13A and 13C. As a result, the moisture state of the membrane electrode assembly 10 is favorable under both high and low humidity conditions, leading to good power generation performance.

[0103] (2) The gas diffusion layers 13A and 13C have a first region consisting only of the microporous layers 14A and 14C, a second region in which the diffusion substrate layers 15A and 15C are embedded in the microporous layers 14A and 14C, and a third region consisting only of the diffusion substrate layers 15A and 15C. As a result, the change in the size of the voids in the electrode catalyst layers 12A and 12C and the gas diffusion layers 13A and 13C proceeds more finely and in stages. Therefore, the supply and discharge of substances involved in the electrode reaction can be made smoother. Consequently, the effect of (1) above can also be obtained more favorably. [Explanation of symbols]

[0104] 10...Membrane electrode assembly 11...Polymer electrolyte membrane 12A,12C…electrode catalyst layer 13A, 13C… Gas diffusion layer 14A, 14C... Microporous layer 15A, 15C... Diffusion substrate layer 21...First area 22…Second area 23...Third area 30...Polymer fuel cell 31A, 31C... Separators

Claims

1. Polymer electrolyte membrane, A pair of electrode catalyst layers are in contact with the surface of the polymer electrolyte membrane, with the polymer electrolyte membrane in between. A gas diffusion layer laminated on each of the pair of electrode catalyst layers, comprising a gas diffusion layer containing a microporous layer in contact with the electrode catalyst layer and a diffusion substrate layer made of a fibrous substrate, At least one of the pair of electrode catalyst layers is a target catalyst layer comprising catalyst-supported particles, a polymer electrolyte, and a fibrous material. The diffusion substrate layer is embedded in at least a portion of the microporous layer. In the gas diffusion layer laminated on the target catalyst layer, the average fiber diameter of the fibrous material contained in the diffusion substrate layer is 10 to 150 times the average fiber diameter of the fibrous substance contained in the target catalyst layer. Membrane electrode assembly.

2. The gas diffusion layer comprises a first region consisting only of the microporous layer, a second region in which the diffusion substrate layer is embedded in the microporous layer, and a third region consisting only of the diffusion substrate layer. The first region is in contact with the electrode catalyst layer, and the second region is sandwiched between the first region and the third region in the thickness direction of the gas diffusion layer. The membrane electrode assembly according to claim 1.

3. The fibrous material contained in the target catalyst layer includes at least one of electronically conductive fibers and polymer fibers. The membrane electrode assembly according to claim 1 or 2.

4. The membrane electrode assembly according to claim 1, A pair of separators sandwiching the aforementioned film electrode assembly, A polymer electrolyte fuel cell equipped with the following features.

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