Membrane electrode assembly and fuel cell

Abstract

To improve the power generation performance of fuel cells. [Solution] A membrane electrode assembly is used, comprising a cathode having a cathode catalyst layer, an anode having an anode catalyst layer, an electrolyte membrane interposed between the cathode and the anode, and a pair of gas diffusion layers laminated on the cathode and the anode opposite to the electrolyte membrane, respectively. The cathode catalyst layer has a peak in the logarithmic differential pore volume dV / d(logD) distribution in the range of pore size D from 0.01 μm to 1 μm, and the gas diffusion layers laminated on the cathode have a peak in the logarithmic differential pore volume distribution in the range of pore size D from 0.01 μm to 5 μm.

Description

Technical Field

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

Background Art

[0002] A fuel cell includes, for example, a membrane electrode assembly having an electrolyte membrane and a pair of electrodes sandwiching the electrolyte membrane. The pair of electrodes each include a catalyst layer and a gas diffusion layer in this order from the electrolyte membrane side.

[0003] As a configuration of the gas diffusion layer, Patent Document 1 discloses a gas diffusion layer in which a conductive agent containing boron-modified carbon black is filled in a fibrous porous substrate, and in a Log differential pore volume distribution graph measured by a mercury intrusion method, there is only one peak, and the peak exists in the range of a pore diameter of 0.01 to 1 μm. It is said that by having the pore diameter peak of the gas diffusion layer in the above range, water generated at the positive electrode (cathode) can be retained inside and the solid polymer membrane can be kept wet.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] However, the performance of a fuel cell depends not only on the gas diffusion layer but also on the configuration of, for example, the catalyst layer. Simply setting the pore diameter peak of the gas diffusion layer in the range of 0.01 to 1 μm may not be able to maintain the required performance.

Means for Solving the Problems

[0006] One aspect of the present disclosure relates to a membrane electrode assembly comprising: a cathode having a cathode catalyst layer; an anode having an anode catalyst layer; an electrolyte membrane interposed between the cathode and the anode; and a pair of gas diffusion layers laminated on the sides of the cathode and the anode opposite to the electrolyte membrane, wherein the cathode catalyst layer has a peak in the logarithmic differential pore volume dV / d(logD) distribution in the range of pore diameter D from 0.01 μm to 1 μm; the gas diffusion layers laminated on the cathode have a peak in the logarithmic differential pore volume dV / d(logD) distribution in the range of pore diameter D from 0.01 μm to 5 μm; and the pore diameter D1 at the peak of the cathode catalyst layer is smaller than the pore diameter D2 at the peak of the gas diffusion layer laminated on the cathode.

[0007] Another aspect of this disclosure relates to a fuel cell comprising the above-mentioned membrane electrode assembly. [Effects of the Invention]

[0008] According to this disclosure, the power generation performance of fuel cells can be improved. [Brief explanation of the drawing]

[0009] [Figure 1] This graph shows an example of measuring the logarithmic differential pore volume distribution in the gas diffusion layer of a fuel cell using the mercury intrusion method. [Figure 2] This is a schematic cross-sectional view showing the structure of a single cell of a fuel cell according to an embodiment of the present disclosure. [Modes for carrying out the invention]

[0010] A fuel cell according to an embodiment of the present disclosure includes a cathode having a cathode catalyst layer, an anode having an anode catalyst layer, an electrolyte membrane interposed between the cathode and the anode, and a pair of gas diffusion layers laminated on surfaces of the cathode and the anode opposite to the electrolyte membrane, and has a membrane electrode assembly (hereinafter also referred to as "MEA"). The fuel cell may further include, for example, a conductive cathode separator in contact with the cathode and a conductive anode separator in contact with the anode. One cell may be constituted by the MEA and the pair of separators. By laminating a plurality of cells such that the cathode separator and the anode separator are adjacent to each other, a stack in which the cells are connected in series can be formed.

[0011] The cathode catalyst layer has a peak in the logarithmic differential pore volume dV / d(logD) distribution in the range where the pore diameter D is 0.01 μm to 1 μm. On the other hand, the gas diffusion layer laminated on the cathode (hereinafter also referred to as "cathode-side gas diffusion layer") has a peak in the logarithmic differential pore volume dV / d(logD) distribution in the range where the pore diameter D is 0.01 μm to 5 μm. The pore diameter D1 at the peak of the cathode catalyst layer is smaller than the pore diameter D2 at the peak of the cathode-side gas diffusion layer (D1 < D2). D2 / D1 is, for example, 1.1 or more and 2.5 or less, and may be 1.1 or more and 1.7 or less, or 1.1 or more and 1.5 or less. D2 - D1 is, for example, 5 nm or more and 120 nm or less, and may be 5 nm or more and 60 nm or less, or 10 nm or more and 40 nm or less.

[0012] During power generation in a fuel cell, water is generated in the cathode. This generated water is discharged from the cathode catalyst layer through the gas diffusion layer into the gas flow path. As the generated water progresses from the cathode catalyst layer to the cathode-side gas diffusion layer, the water droplets may combine and enlarge before being discharged. Therefore, by making the peak pore size D2 of the cathode-side gas diffusion layer larger than the peak pore size D1 of the cathode catalyst layer, the discharge of generated water can be made more effective. In addition, the blockage of the gas diffusion path by the generated water is suppressed, and the gas necessary for the reaction diffuses more easily to the catalyst. As a result, the reaction efficiency is improved, and the performance of the fuel cell can be easily enhanced.

[0013] The logarithmic differential pore volume dV / d(logD) distribution of the cathode catalyst layer and the cathode-side gas diffusion layer is measured by the mercury intrusion method. Here, if multiple peaks exist within a given pore diameter range in the logarithmic differential pore volume distribution, the principal peak is determined from among the multiple peak pore diameters. The principal peak is the one with the largest peak area (i.e., pore volume) among the multiple peaks within the given pore diameter range. In the cathode catalyst layer, if there are multiple peaks in the logarithmic differential pore volume distribution in the range of 0.01 μm to 1 μm, the pore diameter of the principal peak among the multiple peak pore diameters should be smaller than the peak pore diameter of the gas diffusion layer. Similarly, in the cathode-side gas diffusion layer, if there are multiple peaks in the logarithmic differential pore volume distribution in the range of 0.01 μm to 5 μm, the pore diameter of the principal peak among the multiple peak pore diameters should be larger than the peak pore diameters in the cathode catalyst layer in the range of 0.01 μm to 1 μm.

[0014] Figure 1 shows an example of a measurement of the logarithmic differential pore volume distribution. In Figure 1, the logarithmic differential pore volume distribution of the gas diffusion layer, shown by the solid line, has one peak in the range of 0.01 μm to 5 μm (D2 = 0.09 μm). On the other hand, the logarithmic differential pore volume distribution of the gas diffusion layer, shown by the dashed line, has two peaks in the range of 0.01 μm to 5 μm, at pore diameters D of 0.04 μm and 0.2 μm. Of these, the peak at 0.04 μm, with a smaller pore diameter D, has a larger peak area (i.e., a larger pore volume) than the peak at 0.2 μm, and is therefore considered dominant in the pore structure. Therefore, the pore diameter D2 = 0.04 μm of the main peak with the larger peak area is adopted as the peak pore diameter of the gas diffusion layer.

[0015] The pore size D1 at the peak of the cathode catalyst layer may be 0.05 μm or larger. In this case, the pore size D2 at the peak of the gas diffusion layer is greater than 0.05 μm. This facilitates the discharge of generated water and improves gas diffusivity.

[0016] The gas diffusion layer (at least one of the cathode-side gas diffusion layer and the anode-side gas diffusion layer) may be fabricated without a substrate. To maintain mechanical strength, the gas diffusion layer may have a configuration in which a water-repellent layer is formed on a conductive porous sheet such as carbon cloth or carbon paper. On the other hand, a configuration without a conductive substrate, mainly composed of a conductive material and a polymer resin, has also been attempted because it has good adhesion to the catalyst layer, allows for easy control of film thickness and porosity, and allows for the direct formation of channels on the gas diffusion layer. In the case of a gas diffusion layer with a substrate, the logarithmic differential pore volume distribution may have a peak in the range exceeding 5 μm due to the pore structure of the carbon fiber woven fabric and carbon paper. In contrast, a gas diffusion layer without a substrate does not have a peak in the logarithmic differential pore volume dV / d(logD) distribution in the range where the pore diameter exceeds 5 μm.

[0017] Hereinafter, an example of the structure of the fuel cell according to the present embodiment will be described while referring to FIG. 2. FIG. 2 is a cross-sectional view schematically showing the structure of a single cell arranged in a fuel cell according to an embodiment. Usually, a plurality of single cells are stacked and arranged in the fuel cell as a cell stack. In FIG. 2, for convenience, one single cell is shown.

[0018] The single cell 200 includes a membrane electrode assembly 100 having an electrolyte membrane 110, a first catalyst layer 120A and a second catalyst layer 120B arranged so as to sandwich the electrolyte membrane 110, and a first gas diffusion layer 130A and a second gas diffusion layer 130B arranged so as to sandwich the electrolyte membrane 110 through the first catalyst layer 120A and the second catalyst layer 120B, respectively. Further, the single cell 200 includes a first separator 240A and a second separator 240B that sandwich the membrane electrode assembly 100. One of the first catalyst layer 120A and the second catalyst layer 120B functions as an anode, and the other functions as a cathode. Since the electrolyte membrane 110 is slightly larger than the first catalyst layer 120A and the second catalyst layer 120B, the peripheral portion of the electrolyte membrane 110 protrudes from the first catalyst layer 120A and the second catalyst layer 120B. The peripheral portion of the electrolyte membrane 110 is sandwiched by a pair of seal members 250A and 250B.

[0019] Either one of the first catalyst layer 120A and the second catalyst layer 120B is an anode catalyst layer, and the other is a cathode catalyst layer. Here, the first catalyst layer 120A is used as the cathode catalyst layer, and the second catalyst layer 120B is used as the anode catalyst layer. In this case, the first gas diffusion layer 130A corresponds to the cathode-side gas diffusion layer, and the logarithmic differential pore volume distribution of the first catalyst layer 120A and the logarithmic differential pore volume distribution of the first gas diffusion layer 130A satisfy the relationship of the above-mentioned pore diameter peak.

[0020] (Gas Diffusion Layer) The first gas diffusion layer 130A and the second gas diffusion layer 130B may have a substrate layer structure or may not have a substrate layer structure. A structure without a substrate layer is more preferable. As a structure without a substrate layer, a microporous sheet formed from the above-described conductive material and a polymer resin can be used. As a structure with a substrate layer, for example, a structure having a substrate layer and a microporous layer provided on the catalyst layer side can be mentioned. For the substrate layer, a conductive porous sheet such as carbon cloth or carbon paper is used. For the microporous layer, a mixture of a water-repellent resin such as a fluororesin, a conductive carbon material, and a proton-conductive resin (polymer electrolyte) is used.

[0021] The substrate-less gas diffusion layer contains, for example, a conductive material and a polymer resin. The polymer resin is preferably 10 to 40 parts by mass with respect to a total of 100 parts by mass of the conductive material and the polymer resin. The conductive material includes a particulate material, and may further include a fibrous material.

[0022] Examples of the particulate material include carbon black, spherical graphite, and activated carbon. Among them, it is preferable to use carbon black in terms of high conductivity and large pore volume. As the carbon black, acetylene black, ketjen black, furnace black, etc. can be used. Examples of the fibrous material include fibrous carbon materials such as vapor-grown carbon fiber (VGCF (registered trademark)), carbon nanotube, and carbon nanofiber.

[0023] The conductive material may contain particulate material and fibrous material, and may also contain plate-like material. The plate-like material may be provided within the gas diffusion layer so as to be oriented along the plane direction (direction perpendicular to the thickness direction) of the gas diffusion layer. The plate-like material enhances the gas diffusion in the plane direction of the gas diffusion layer. The plate-like material appears particulate macroscopically, but is composed of plate-like particles microscopically. Specific examples of plate-like materials include flake graphite, crushed graphitized polyimide film, and graphene. Among these, crushed graphitized polyimide film and graphene are particularly advantageous because they easily orient themselves along the plane direction of the gas diffusion layer, making it advantageous for forming a thin gas diffusion layer, and are suitable for enhancing the gas diffusion in the plane direction of the gas diffusion layer.

[0024] The polymer resin functions as a binder that binds conductive materials together. From the viewpoint of suppressing water accumulation in the pores within the gas diffusion layer, it is preferable that 50% or more, and more preferably 90% or more, of the polymer resin be a water-repellent fluororesin. Examples of fluororesins include PTFE (polytetrafluoroethylene), FEP (tetrafluoroethylene-hexafluoropropylene copolymer), PVdF (polyvinylidene fluoride), ETFE (tetrafluoroethylene-ethylene copolymer), PCTFE (polychlorotrifluoroethylene), and PFA (polyfluoroethylene-perfluoroalkyl vinyl ether copolymer). Among these, PTFE is preferred as the fluororesin from the viewpoint of heat resistance, water repellency, and chemical resistance.

[0025] The gas diffusion layer can be fabricated, for example, as follows: First, a mixture containing a conductive material, a polymer resin, a surfactant, and a dispersion medium is prepared. A kneader or mixer can be used as the mixing device. At this time, it is preferable to put the conductive material, surfactant, and dispersion medium into the mixing device and uniformly disperse the conductive material in the dispersion medium, and then add the polymer resin and disperse it further. It is preferable to apply an appropriate shear force to the polymer resin to fibrillate it. Examples of dispersion media include water, alcohol, and glycols. Examples of surfactants include polyoxyethylene alkyl ethers and alkylamine oxides.

[0026] Next, the obtained mixture is formed into a sheet by a molding method such as extrusion molding. The obtained sheet may be further rolled. A roll press can be used for rolling. The conditions for the roll press are not particularly limited, but rolling at a linear pressure of 0.001 ton / cm to 4 ton / cm makes it easier to obtain a gas diffusion layer with high strength.

[0027] Next, the sheet is fired to remove the surfactant and dispersion medium, resulting in a fired sheet. The firing temperature should be such that the polymer resin does not deteriorate and the surfactant and dispersion medium decompose or volatilize. When PTFE is used as the polymer resin, a firing temperature of 280 to 340°C is preferred. The firing atmosphere should be an inert atmosphere, such as a nitrogen or argon atmosphere, or a reduced pressure atmosphere. It is sufficient that the majority of the surfactant and dispersion medium are removed from the sheet; complete removal is not necessarily required.

[0028] (catalyst layer) The first catalyst layer 120A (cathode catalyst layer) includes, for example, a conductive material, catalyst particles, and a proton-conducting resin. The catalyst particles are supported on the conductive material. The conductive material includes particulate conductive members and / or fibrous conductive members. By including fibrous conductive members, the gas diffusivity of the catalyst layer can be improved.

[0029] (Fibrous conductive material) Examples of fibrous conductive members include fibrous carbon materials such as vapor-grown carbon fibers, carbon nanotubes, and carbon nanofibers. The diameter D of the fibrous conductive member... F The diameter of the fibrous conductive member is not particularly limited, but is preferably 200 nm or less, more preferably 5 nm to 200 nm, and even more preferably 10 nm to 170 nm. In this case, the volume ratio of the fibrous conductive member in the catalyst layer can be reduced while ensuring sufficient gas pathways and improving gas diffusivity. Diameter D of the fibrous conductive member F This is determined by randomly selecting 10 fibrous conductive members from the catalyst layer and averaging their diameters. The diameter is the length perpendicular to the length direction of the fibrous conductive member.

[0030] Length L of the fibrous conductive member F The length of the fibrous conductive member is not particularly limited, but is preferably 0.2 μm to 20 μm, and more preferably 0.2 μm to 10 μm. In this case, at least a portion of the fibrous conductive member is oriented along the thickness direction of the catalyst layer, making it easier to secure a gas diffusion path. Length L of the fibrous conductive member F This is the average fiber length, which can be determined by arbitrarily selecting 10 fibrous conductive members from the catalyst layer and averaging the fiber lengths of these fibrous conductive members. Note that, in the case of a substantially straight fibrous conductive member, the fiber length of the fibrous conductive member refers to the length of the straight line connecting one end of the fibrous conductive member to the other end.

[0031] The fibrous conductive member may have a hollow space (hollow portion) inside. In this case, both ends of the fibrous conductive member in the longitudinal direction may be open within the catalyst layer. The fact that both ends of the fibrous conductive member in the longitudinal direction are open means that the hollow portion is in communication with the outside through these openings. That is, the openings at both ends of the fibrous conductive member are not blocked by either the electrolyte membrane or the gas diffusion layer, and gas can enter and exit from both ends. The side wall of a fibrous conductive member having a hollow portion may be provided with through holes that connect the hollow portion to the outside. Catalyst particles can be arranged and fixed to the side wall of the fibrous conductive member so as to block at least a portion of the through holes. Catalyst particles supported on the side wall so as to block at least a portion of the through holes can come into more efficient contact with the reaction gas, significantly increasing the reaction efficiency of the catalyst layer.

[0032] (Particulate conductive material) The particulate conductive material is not particularly limited, but carbon black is preferred due to its excellent conductivity. Examples of carbon black include acetylene black, Ketjen black, thermal black, furnace black, and channel black. The particle size (or the length of the structure composed of multiple linked primary particles) is not particularly limited, and those conventionally used in the catalyst layer of fuel cells can be used.

[0033] (Catalyst particles) While not particularly limited, catalyst particles can be alloys or elements selected from elements such as Sc, Y, Ti, Zr, V, Nb, Fe, Co, Ni, Ru, Rh, Pd, Pt, Os, Ir, lanthanide series elements, or actinide series elements. For example, catalyst particles used in the anode include Pt and Pt-Ru alloys. Catalyst particles used in the cathode include Pt and Pt-Co alloys. At least a portion of the catalyst particles are supported on a particulate conductive member. Preferably, the catalyst particles are supported on a fibrous conductive member in addition to the particulate conductive member. This is because it allows the catalyst particles to come into contact with the gas more easily, increasing the efficiency of the gas oxidation or reduction reaction.

[0034] From the viewpoint of immobilizing catalyst particles, the diameter X of the catalyst particles is preferably 1 nm or more and 10 nm or less, and more preferably 2 nm or more and 5 nm or less. When X is 1 nm or more, a sufficient catalytic effect from the catalyst particles can be obtained. When X is 10 nm or less, the catalyst particles can be easily supported on the sidewall of the fibrous conductive member.

[0035] The diameter X of the catalyst particle can be determined as follows: For any single catalyst particle observed in the TEM image of the catalyst layer, the particle size is calculated assuming the particle is spherical. This process is repeated for 100 to 300 catalyst particles observed in the TEM image, and the particle size of each is calculated. The average of these particle sizes is defined as the diameter X of the catalyst particle.

[0036] (Proton-conducting resin) The proton-conducting resin is not particularly limited, but examples include perfluorocarbon sulfonic acid polymers and hydrocarbon polymers. Among these, perfluorocarbon sulfonic acid polymers are preferred due to their excellent heat resistance and chemical stability. An example of a perfluorocarbon sulfonic acid polymer is Nafion®. The proton-conducting resin coats at least a portion of the particulate conductive member, the fibrous conductive member, and / or the catalyst particles.

[0037] The second catalyst layer 120B (anode catalyst layer) can employ known materials and known configurations. The anode catalyst layer, like the cathode catalyst layer, may include a conductive material, catalyst particles supported on the conductive material, and a proton-conductive resin. The conductive material may also include particulate conductive members and / or fibrous conductive members.

[0038] Although the anode catalyst layer is not exposed to an environment as highly oxidizing as the cathode catalyst layer, it tends to be in a less humid environment than the cathode catalyst layer because water is not produced in the reaction. As a result, its proton conductivity tends to decrease. The composition and content ratios of the particulate conductive material, fibrous conductive material, and proton conductive resin can be modified to obtain higher proton conductivity than the cathode catalyst layer.

[0039] From the viewpoint of miniaturizing the fuel cell and maintaining low proton resistance to obtain high output, the thickness of the catalyst layer should be as thin as possible. On the other hand, from the viewpoint of strength, it is preferable that it not be excessively thin. In general, as the proportion of fibrous conductive material increases, the thickness of the catalyst layer tends to increase.

[0040] Thickness T of the cathode catalyst layer C For example, the thickness of the anode catalyst layer T is between 4 μm and 15 μm. A For example, the thickness of the catalyst layer is between 2 μm and 12 μm. C and T A This is the average thickness, and it can be obtained by averaging the distances of straight lines drawn along the thickness direction of the catalyst layer from one main surface to the other at any 10 points on the cross-section of the catalyst layer.

[0041] Regarding the blending ratio of fibrous conductive material in the catalyst layer, gas diffusion can be improved by including fibrous conductive material in an amount of 20% or more by mass relative to particulate conductive material in both the anode and cathode catalyst layers. On the other hand, increasing the amount of fibrous conductive material tends to increase the thickness of the catalyst layer, which tends to increase proton transfer resistance. It also makes the catalyst layer more prone to cracking. From the viewpoint of suppressing the increase in proton transfer resistance and suppressing cracking, the amount of fibrous conductive material blended in the anode and / or cathode catalyst layers may be 50% or less by mass relative to particulate conductive material.

[0042] The catalyst layer is fabricated, for example, as follows: First, catalyst particles and particulate conductive material are mixed in a dispersion medium (e.g., water, ethanol, propanol, etc.). Next, while stirring the resulting dispersion, a proton-conducting resin and fibrous carbon material are added sequentially to obtain a catalyst dispersion. The proton-conducting resin may be added in two or more separate additions. In this case, the second and subsequent additions of the proton-conducting resin may be carried out together with the fibrous carbon material. After that, the obtained catalyst dispersion is applied to the surface of an electrolyte membrane or a suitable transfer substrate sheet to a uniform thickness and dried to obtain a catalyst layer.

[0043] Coating methods include conventional coating methods such as spraying, screen printing, and coating methods using various coaters such as blade coaters, knife coaters, and gravure coaters. For the transfer substrate sheet, it is preferable to use a sheet with a smooth surface, such as polyethylene terephthalate (PET) or polypropylene. When using a transfer substrate sheet, the resulting catalyst layer is transferred to an electrolyte membrane or gas diffusion layer, as described later.

[0044] The transfer of the catalyst layer to the electrolyte membrane or gas diffusion layer is performed by bringing the side of the catalyst layer that was facing the transfer substrate sheet into contact with the electrolyte membrane or gas diffusion layer. By bringing the smooth surface of the catalyst layer into contact with the electrolyte membrane or gas diffusion layer, the interfacial resistance with the catalyst layer is reduced, improving the performance of the fuel cell. The catalyst dispersion may also be applied directly to the electrolyte layer.

[0045] (electrolyte membrane) A polymer electrolyte membrane is preferably used as the electrolyte membrane 110. Examples of materials for the polymer electrolyte membrane include the polymer electrolytes exemplified as proton-conducting resins. The thickness of the electrolyte membrane is, for example, 5 to 30 μm.

[0046] (Separator) The first separator 240A and the second separator 240B only need to have airtightness, electronic conductivity, and electrochemical stability, and their material is not particularly limited. Preferred materials include carbon materials and metallic materials. Metallic materials may be coated with carbon. For example, the first separator 240A and the second separator 240B can be obtained by punching a metal plate into a predetermined shape and applying a surface treatment.

[0047] In this embodiment, a gas channel 260A is formed on the surface of the first separator 240A that is in contact with the first gas diffusion layer 130A. On the other hand, a gas channel 260B is formed on the surface of the second separator 240B that is in contact with the second gas diffusion layer 130B. The shape of the gas channel is not particularly limited and may be formed as a straight type, serpentine type, or the like.

[0048] (Sealing material) The sealing members 250A and 250B are made of an elastic material and prevent fuel and / or oxidizer from leaking from the gas passages 260A and 260B. The sealing members 250A and 250B have a frame-like shape that, for example, loops around the periphery of the first catalyst layer 120A and the second catalyst layer 120B. Known materials and known configurations can be used for the sealing members 250A and 250B, respectively.

[0049] The present disclosure will be described in further detail below based on examples. However, the present disclosure is not limited to the following examples.

[0050] [Examples] (1) Preparation of dispersion for cathode catalyst layer A particulate conductive material (carbon black) supporting catalyst particles (Pt-Co alloy) was added to an appropriate amount of water and stirred to disperse it. After adding an appropriate amount of ethanol to the resulting dispersion while stirring, 35 parts by mass of a fibrous conductive material (vapor-grown carbon fiber, average diameter 150 nm, average fiber length 10 μm) and 100 parts by mass of a proton-conductive resin (perfluorocarbon sulfonic acid polymer) were added to 100 parts by mass of the particulate conductive material supporting the catalyst particles, and the mixture was stirred to prepare a catalyst dispersion for the cathode catalyst layer.

[0051] (2) Preparation of dispersion for the anode catalyst layer A particulate conductive material (carbon black) supporting catalyst particles (Pt) was added to an appropriate amount of water and stirred to disperse it. After adding an appropriate amount of ethanol to the resulting dispersion while stirring, 35 parts by mass of a fibrous conductive material (vapor-grown carbon fiber, average diameter 150 nm, average fiber length 10 μm) and 120 parts by mass of a proton-conductive resin (perfluorocarbon sulfonic acid polymer) were added to 100 parts by mass of the particulate conductive material supporting the catalyst particles, and the mixture was stirred to prepare a catalyst dispersion for the anode catalyst layer.

[0052] (3) Preparation of a gas diffusion layer Particulate conductive material (carbon black), fibrous conductive material (carbon nanotubes, fiber diameter 50 nm to 300 nm, fiber length 1 μm to 50 μm), surfactant, and dispersion medium were placed in a stirring device, stirred, and kneaded to uniformly disperse the materials. Then, PTFE as a polymer resin was added and uniformly dispersed to obtain a kneaded mixture. Next, the kneaded mixture was extruded and stretched into a sheet and dried. The obtained sheet was fired at 310°C to remove the surfactant and dispersion medium, and a gas diffusion layer was obtained.

[0053] The content of particulate conductive material, fibrous conductive material, and polymer resin in the gas diffusion layer was adjusted to range from 5% to 35% by mass for particulate conductive material, 35% to 80% by mass for fibrous conductive material, and 10% to 40% by mass for polymer resin, respectively. This allowed for the creation of five types of gas diffusion layers A1 to A5 with different pore structures.

[0054] (4) Preparation of single cells Two PET sheets were prepared. Using a screen printing method, the catalyst dispersion for the cathode catalyst layer obtained was applied to the smooth surface of one PET sheet to a uniform thickness, and the catalyst dispersion for the anode catalyst layer obtained was applied to the smooth surface of the other PET sheet to a uniform thickness. After drying, the two catalyst layers were formed. The thickness of the cathode catalyst layer was 6 μm, and the thickness of the anode catalyst layer was 4.5 μm.

[0055] When the pore size distribution of the cathode catalyst layer was measured by the mercury intrusion method, a peak was observed in the logarithmic differential pore volume dV / dlogD at a pore size D of 80 nm.

[0056] The catalyst layers obtained were transferred to both main surfaces of a 15 μm thick electrolyte membrane, forming a cathode on one surface of the electrolyte membrane and an anode on the other. Then, two gas diffusion layers A1 to A5 were prepared, and one of the two layers was brought into contact with the anode and the other with the cathode to fabricate five sets of membrane electrode assemblies.

[0057] Next, a frame-shaped sealing member was placed to surround the anode and cathode. The entire structure was then sandwiched between a pair of stainless steel plates (separators) having gas channels in the portion in contact with the gas diffusion layer, thereby completing test single cells X1 using gas diffusion layer A1, X2 using gas diffusion layer A2, X3 using gas diffusion layer A3, X4 using gas diffusion layer A4, and X5 using gas diffusion layer A5.

[0058] <Rating> A single cell was heated to 80°C, and fuel gas with 100% relative humidity was supplied to the anode, while oxidizer gas (air) with 100% relative humidity was supplied to the cathode. The fuel gas and oxidizer gas were supplied under pressure of 70-90 kPa at the cell inlet gas pressure for each current density. The load control device was controlled to maintain a constant current flow, and the voltage (initial voltage) and resistance of the single cell were measured while varying the current density relative to the electrode areas of the anode and cathode.

[0059] The maximum power density W1 was evaluated for each of the single cells X1 to X5. The evaluation results are shown in Table 1. In Table 1, the maximum power density W1 is shown as a relative value with the measured maximum power density W1 of single cell X5 set to 100. In addition, Table 1 also shows the pore diameter D2 at which the logarithmic differential pore volume distribution takes its main peak for the gas diffusion layers A1 to A5 used in single cells X1 to X5, in the range of 0.01 μm to 5 μm.

[0060] As can be seen from Table 1, the pore size D2 at the main peaks of gas diffusion layers A1 to A4 is larger than the pore size peak D1 (80 nm) of the cathode catalyst layer. In contrast, in single cell X5, the pore size D2 at the main peak of gas diffusion layer A5 is smaller than the pore size peak (80 nm) of the cathode catalyst layer. Single cells X1 to X4 have improved drainage of water generated at the cathode, and the maximum power density is higher than that of cell X5.

[0061] [Table 1] [Industrial applicability]

[0062] The fuel cell relating to this disclosure can be suitably used as a power source for stationary household cogeneration systems or as a power source for vehicles. While this disclosure is suitable for application to polymer electrolyte fuel cells, it is not limited thereto and can be applied to fuel cells in general. [Explanation of symbols]

[0063] 100: Membrane electrode assembly, 110: Electrolyte membrane, 120: Catalyst layer, 120A: First catalyst layer, 120B: Second catalyst layer, 130A: First gas diffusion layer, 130B: Second gas diffusion layer, 200: Fuel cell (single cell), 240A: First separator, 240B: Second separator, 250A, 250B: Sealing member, 260A, 260B: Gas flow path

Claims

1. A cathode having a conductive material, catalyst particles supported on the conductive material, and a cathode layer containing a proton-conducting resin, An anode having an anode layer, The system comprises an electrolyte membrane interposed between the cathode and the anode, The conductive material includes a particulate conductive member and a fibrous conductive member. The cathode layer has a first layer on the side in contact with the electrolyte membrane, and a second layer laminated on the first layer on the side opposite to the electrolyte membrane. The first layer comprises at least the catalyst particles and the fibrous conductive member, and has a peak in the logarithmic differential pore volume dV / d(logD) distribution in the range of 0.01 μm to 1 μm for the pore diameter D, The second layer has a peak in the logarithmic differential pore volume dV / d(logD) distribution in the range of pore diameter D from 0.01 μm to 5 μm, and does not have a peak in the logarithmic differential pore volume dV / d(logD) distribution in the range of pore diameter greater than 5 μm. Pore ​​diameter D at the peak of the first layer １ However, the pore size D at the peak of the second layer ２ A membrane electrode assembly that is smaller than this.

2. Pore ​​diameter D at the peak of the first layer １ The film electrode assembly according to claim 1, wherein the thickness is 0.05 μm or more.

3. The film electrode assembly according to claim 1, wherein in the first layer, the catalyst particles are supported on at least the particulate conductive member among the particulate conductive member and the fibrous conductive member.

4. The film electrode assembly according to claim 3, wherein the fibrous conductive member comprises at least one fibrous carbon material selected from the group consisting of vapor-grown carbon fibers, carbon nanotubes, and carbon nanofibers.

5. The film electrode assembly according to claim 3, wherein the diameter of the fibrous conductive member is 5 nm or more and 200 nm or less.

6. The film electrode assembly according to claim 3, wherein the length of the fibrous conductive member is 0.2 μm or more and 20 μm or less.

7. The film electrode assembly according to claim 3, wherein the proton-conducting resin coats at least a portion of the particulate conductive member, the fibrous conductive member, and / or the catalyst particles.

8. The film electrode assembly according to claim 3, wherein the content ratio of the fibrous conductive member in the first layer is 20% or more and 50% or less by mass relative to the particulate conductive member.

9. The film electrode assembly according to any one of claims 1 to 8, wherein the second layer does not have a conductive substrate.

10. A fuel cell comprising a membrane electrode assembly according to any one of claims 1 to 9.

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