Membrane electrode assembly for fuel cell, and fuel cell

Abstract

Provided is a membrane electrode assembly for a fuel cell, the membrane electrode assembly being capable of maintaining high gas diffusibility without allowing generated water to accumulate. The membrane electrode assembly for a fuel cell comprises: a catalyst layer having a first main surface and a second main surface; a gas diffusion layer disposed on the first main surface side; and an electrolyte membrane disposed on the second main surface side. The gas diffusion layer contains an electroconductive material and a polymer resin. The electroconductive material contains a fibrous carbon material. The strength S1 required to peel the interface between the catalyst layer and the gas diffusion layer is 31.2-1000 N·cm-2.

Description

Membrane electrode assembly for fuel cell and fuel cell

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

[0002] A fuel cell is a highly efficient, clean power generation device that generates electricity and produces water through an electrochemical reaction between a fuel and an oxidant (hereinafter, simply referred to as "gas"). A fuel cell includes, for example, an electrolyte membrane, two catalyst layers arranged to sandwich the electrolyte membrane, two gas diffusion layers arranged to sandwich the electrolyte membrane with each catalyst layer interposed therebetween, and two separators arranged to sandwich the electrolyte membrane with each gas diffusion layer interposed therebetween. Gas diffused in the plane direction by the gas diffusion layers is oxidized or reduced by the catalyst layers. Increasing the gas diffusivity in the catalyst layers improves the efficiency of this electrochemical reaction.

[0003] Patent Document 1 describes a cathode catalyst layer 7 having a solid polymer electrolyte content of 16.7 to 28.6% by weight, and a cathode catalyst layer 7 having a peel strength of 0.256 to 0.36 N cm between the cathode catalyst layer and the cathode gas diffusion layer after power generation. －２ The present invention discloses a membrane electrode assembly characterized in that:

[0004] JP 2011-65963 A

[0005] However, the peel strength of the membrane electrode assembly described in Patent Document 1 is insufficient for the adhesion strength between the catalyst layer and the gas diffusion layer, and repeated power generation by the fuel cell may cause peeling at the interface between the catalyst layer and the gas diffusion layer.

[0006] The present disclosure has been made in view of these circumstances, and aims to provide a membrane electrode assembly and a fuel cell that can maintain high gas diffusivity without generating water stagnation.

[0007] A membrane electrode assembly for a fuel cell according to the present disclosure has a catalyst layer having a first main surface and a second main surface, a gas diffusion layer disposed on the first main surface side, and an electrolyte membrane disposed on the second main surface side, wherein the gas diffusion layer contains a conductive material and a polymer resin, and the conductive material contains a fibrous carbon material, and the strength S1 required to peel the interface between the catalyst layer and the gas diffusion layer is 31.2 to 1000 N cm －２ is.

[0008] Another fuel cell membrane electrode assembly according to the present disclosure is a fuel cell membrane electrode assembly having a catalyst layer having a first main surface and a second main surface, a gas diffusion layer arranged on the first main surface side, and an electrolyte membrane arranged on the second main surface side, wherein the gas diffusion layer includes a conductive material and a polymer resin, and the conductive material includes a fibrous carbon material, and a strength S1 required to peel the interface between the catalyst layer and the gas diffusion layer and a breaking strength S2 of the gas diffusion layer satisfy the relationship S1 > S2.

[0009] Still another fuel cell membrane electrode assembly of the present disclosure comprises an electrolyte membrane, a pair of catalyst layers arranged to sandwich the electrolyte membrane, and a pair of gas diffusion layers arranged to sandwich the electrolyte membrane, with the pair of catalyst layers interposed therebetween, each of the pair of catalyst layers having a first main surface and a second main surface, with the gas diffusion layer arranged on the first main surface side and the second main surface side facing the electrolyte membrane, at least one of the pair of gas diffusion layers comprising a conductive material and a polymer resin, the conductive material comprising a fibrous carbon material, and a strength S1 required to peel the interface between the catalyst layer and the gas diffusion layer and a breaking strength S2 of the gas diffusion layer satisfy the relationship S1 > S2.

[0010] Yet another fuel cell of the present disclosure comprises a fuel cell membrane electrode assembly including an electrolyte membrane, a pair of catalyst layers arranged to sandwich the electrolyte membrane, and a pair of gas diffusion layers arranged to sandwich the electrolyte membrane with the pair of catalyst layers interposed therebetween, each of the pair of catalyst layers having a first main surface and a second main surface, with the gas diffusion layer arranged on the first main surface side and the second main surface side facing the electrolyte membrane, at least one of the pair of gas diffusion layers including a conductive material and a polymer resin, the conductive material including a fibrous carbon material, and a strength S1 required to peel the interface between the catalyst layer and the gas diffusion layer and a breaking strength S2 of the gas diffusion layer satisfy the relationship S1 > S2; and a pair of separators arranged to sandwich the fuel cell membrane electrode assembly with the pair of gas diffusion layers included in the fuel cell membrane electrode assembly interposed therebetween.

[0011] In the fuel cell membrane electrode assembly according to the present disclosure, the catalyst layer and the gas diffusion layer are adhered to each other with high strength, which reduces the contact resistance between the catalyst layer and the gas diffusion layer and reduces the gap between the catalyst layer and the gas diffusion layer to improve adhesion, thereby suppressing the accumulation of generated water inside the fuel cell membrane electrode assembly and maintaining high gas diffusivity.

[0012] Fig. 1 is a cross-sectional view schematically showing the cross-sectional structure of a single cell of a fuel cell according to Embodiment 1. Fig. 2 is a cross-sectional view schematically showing the cross-sectional structure of a membrane electrode assembly according to Embodiment 1. Fig. 3 is a table showing the results of a peel test for Examples and Comparative Example 1. Fig. 4 is a flowchart of a method for manufacturing a gas diffusion layer according to Embodiment 1.

[0013] A fuel cell membrane electrode assembly according to a first aspect is a fuel cell membrane electrode assembly having a catalyst layer having a first main surface and a second main surface, a gas diffusion layer disposed on the first main surface side, and an electrolyte membrane disposed on the second main surface side, wherein the gas diffusion layer contains a conductive material and a polymer resin, and the conductive material contains a fibrous carbon material, and the strength S1 required to peel the interface between the catalyst layer and the gas diffusion layer is 31.2 to 1000 N cm －２ The combination of the catalyst layer and the gas diffusion layer may constitute either an anode or a cathode.

[0014] A fuel cell membrane electrode assembly according to a second aspect is a fuel cell membrane electrode assembly having a catalyst layer having a first main surface and a second main surface, a gas diffusion layer disposed on the first main surface side, and an electrolyte membrane disposed on the second main surface side, wherein the gas diffusion layer contains a conductive material and a polymer resin, and the conductive material contains a fibrous carbon material, and a strength S1 required to peel the interface between the catalyst layer and the gas diffusion layer and a breaking strength S2 of the gas diffusion layer satisfy the relationship S1 > S2. Note that the combination of the catalyst layer and the gas diffusion layer may constitute either an anode or a cathode.

[0015] A fuel cell membrane electrode assembly according to a third aspect is the fuel cell membrane electrode assembly of the first or second aspect, wherein an average fiber diameter D of the fibrous carbon material may be 25% or less of a thickness T of the catalyst layer.

[0016] A fuel cell membrane electrode assembly according to a fourth aspect may be the fuel cell membrane electrode assembly according to the first or second aspect, wherein the average fiber diameter D of the fibrous carbon material is 5 nm or more and 250 nm or less.

[0017] A membrane electrode assembly for a fuel cell according to a fifth aspect may be the membrane electrode assembly for a fuel cell according to the first or second aspect, wherein the thickness T of the catalyst layer is 1 μm or more and 30 μm or less.

[0018] A fuel cell membrane electrode assembly according to a sixth aspect includes an electrolyte membrane, a pair of catalyst layers arranged to sandwich the electrolyte membrane, and a pair of gas diffusion layers arranged to sandwich the electrolyte membrane via the pair of catalyst layers, each of the pair of catalyst layers having a first main surface and a second main surface, with the gas diffusion layer arranged on the first main surface side and the second main surface side facing the electrolyte membrane, at least one of the pair of gas diffusion layers containing a conductive material and a polymer resin, the conductive material containing a fibrous carbon material, and a strength S1 required to peel the interface between the catalyst layer and the gas diffusion layer and a breaking strength S2 of the gas diffusion layer satisfy the relationship S1 > S2. One of the pair of combinations of catalyst layer and gas diffusion layer constitutes one of the anode and the cathode, and the other of the pair of combinations of catalyst layer and gas diffusion layer constitutes the other of the anode and the cathode.

[0019] a pair of separators arranged to sandwich the membrane electrode assembly for a fuel cell via the pair of gas diffusion layers provided in the membrane electrode assembly for a fuel cell, the pair of catalyst layers being arranged to sandwich the electrolyte membrane, the pair of catalyst layers each having a first main surface and a second main surface, with the gas diffusion layer disposed on the first main surface side and the second main surface side facing the electrolyte membrane, at least one of the pair of gas diffusion layers comprising a conductive material and a polymer resin, the conductive material comprising a fibrous carbon material, wherein a strength S1 required to peel the interface between the catalyst layer and the gas diffusion layer and a breaking strength S2 of the gas diffusion layer satisfy the relationship S1 > S2;

[0020] A membrane electrode assembly for a fuel cell and a fuel cell according to an embodiment of the present disclosure will be described with reference to the accompanying drawings.

[0021] (Embodiment 1) <Fuel Cell> Fig. 1 is a cross-sectional view that schematically shows the cross-sectional structure of a single cell 1 of a fuel cell according to Embodiment 1. As shown in Fig. 1, the fuel cell 1 according to Embodiment 1 includes a fuel cell membrane electrode assembly 5 and a pair of separators 10, 11 that are arranged to sandwich the fuel cell membrane electrode assembly 5 therebetween.

[0022] <Fuel Cell Membrane Electrode Assembly> FIG. 2 is a cross-sectional view schematically illustrating the cross-sectional structure of a membrane electrode assembly 5 according to the first embodiment. The fuel cell membrane electrode assembly 5 according to the first embodiment includes an electrolyte membrane 4, a pair of catalyst layers 6, 8 arranged to sandwich the electrolyte membrane 4, and a pair of gas diffusion layers 7, 9 arranged to sandwich the electrolyte membrane 4 via the pair of catalyst layers 6, 8. Each of the pair of catalyst layers 6, 8 has a first main surface and a second main surface. The gas diffusion layers 7, 9 are arranged on the first main surface side, and the second main surface side faces the electrolyte membrane 4. At least one of the pair of gas diffusion layers 7, 9 includes a conductive material (first conductive material) and a polymer resin (first polymer resin), and the conductive material (first conductive material) includes a fibrous carbon material (first fibrous carbon material). The combination of the catalyst layer and the gas diffusion layer may form an anode or a cathode. In other words, in the fuel cell membrane electrode assembly 5, at least one of the gas diffusion layers 7, 9 may contain a first conductive material and a first polymer resin, and the first conductive material may contain a first fibrous carbon material. Alternatively, both gas diffusion layers 7, 9 may each contain an independent first conductive material and an independent first polymer resin, and each first conductive material may contain a first fibrous carbon material. The strength S1 required to peel the interface between the catalyst layer 6, 8 and the gas diffusion layer 7, 9 and the breaking strength S2 of the gas diffusion layer 7, 9 may satisfy the relationship S1 > S2.

[0023] When the gas diffusion layer includes a first conductive material and a first polymer resin, and the first conductive material includes a first fibrous carbon material, the mechanical strength of the gas diffusion layer is improved. Furthermore, this facilitates the design of the gas diffusion properties of the gas diffusion layer, enabling smoother gas diffusion and suppressing pressure loss. Furthermore, by properly controlling the gas diffusion properties, the humidity of the electrolyte membrane is more easily maintained.

[0024] The strength S1 required to peel the interface between the catalyst layer and the gas diffusion layer is 31.2 N cm －２ If the strength S1 is less than 1000 N cm, repeated power generation of the fuel cell may cause the interface between the catalyst layer and the gas diffusion layer to peel off. －２If the temperature exceeds this value, the pores of the catalyst layer and the gas diffusion layer will collapse, resulting in a decrease in gas diffusibility.

[0025] When the strength S1 required to peel the interface between the catalyst layer and the gas diffusion layer and the breaking strength S2 of the gas diffusion layer satisfy the relationship S1 > S2, the catalyst layer and the gas diffusion layer are adhered to each other with high strength, reducing the contact resistance between the catalyst layer and the gas diffusion layer. Furthermore, by suppressing peeling at the interface between the catalyst layer and the gas diffusion layer and increasing the adhesion, high gas diffusibility can be maintained without the generated water remaining.

[0026] The configuration of the gas diffusion layer will be described in detail below. (Gas Diffusion Layer) The gas diffusion layer may contain a first conductive material and a first polymeric resin, but is preferably a "porous member primarily composed of the first conductive material and the first polymeric resin." Here, "a porous member primarily composed of the first conductive material and the first polymeric resin" refers to a porous member having a structure supported by the first conductive material and the first polymeric resin (a so-called self-supporting structure) without using a woven or nonwoven fabric such as carbon paper or carbon cloth as a substrate. The gas diffusion layer may have a region without such a substrate that is sufficiently thick (e.g., 30 μm or more). The surface of the gas diffusion layer that does not contact the catalyst layer may be composed of the above-mentioned substrate. Having a substrate on the surface that does not contact the catalyst layer is not excluded from embodiments of the present disclosure.

[0027] The thickness of the gas diffusion layer is preferably thin in consideration of miniaturization of fuel cells. On the other hand, from the viewpoint of strength, it is preferable that the gas diffusion layer is not excessively thin. The thickness of the gas diffusion layer is, for example, 30 μm or more and 1000 μm or less, preferably 50 μm or more and 500 μm or less, and may be 80 μm or more and 300 μm or less.

[0028] The thickness of the gas diffusion layer is an average thickness, and can be determined, for example, by selecting any 10 points on the cross section of the gas diffusion layer in an image such as an SEM photograph, drawing a straight line from one main surface to the other main surface in the image along the thickness direction of the gas diffusion layer, measuring the distance as the thickness, and averaging the thicknesses at the 10 points.

[0029] When manufacturing a gas diffusion layer made of a porous member using a first conductive material and a first polymer resin, a surfactant and a dispersion medium may be used, as described below. In this case, the surfactant and dispersion medium are removed by baking during the manufacturing process, but they may not be sufficiently removed and may remain in the porous member. Therefore, the term "porous member mainly composed of a first conductive material and a first polymer resin" means that the residual surfactant and dispersion medium may be contained in the gas diffusion layer made of the porous member, as long as the structure is supported by the first conductive material and the first polymer resin. It also means that materials other than the first conductive material, the first polymer resin, the surfactant, and the dispersion medium may be contained in the gas diffusion layer made of the porous member.

[0030] <First conductive material> The first conductive material preferably contains a first fibrous carbon material as a main component. For example, the mass proportion of the first fibrous carbon material in the gas diffusion layer may be 10 mass% or more and 90 mass% or less, or 20 mass% or more and 75 mass% or less. In order to easily ensure sufficient gas diffusibility, a higher mass proportion of the first fibrous carbon material is desirable. On the other hand, by appropriately controlling the mass proportion of the first fibrous carbon material, it becomes easier to reduce resistance.

[0031] <First fibrous carbon material> The first fibrous carbon material has an aspect ratio of 2 or more and is electrically conductive. The aspect ratio of the first fibrous carbon material may be 3 or more, or 5 or more. The aspect ratio of the first fibrous carbon material may be 1000 or less, 500 or less, or 100 or less. The aspect ratio of the first fibrous carbon material is, for example, 3 or more and 1000 or less. The aspect ratio of the first fibrous carbon material is the ratio (L / D) of the average length L to the average fiber diameter D of the first fibrous carbon material.

[0032] Examples of the first fibrous carbon material include vapor-grown carbon fiber (VGCF (registered trademark)), single-walled or multi-walled carbon nanotubes (CNT), and carbon nanofibers. The first fibrous carbon material may have a hollow space (hollow portion) therein. Both ends of the first fibrous carbon material in the longitudinal direction may be open or closed.

[0033] The average fiber diameter D of the first fibrous carbon material may be 25% or less of the thickness T of the catalyst layer. When the average fiber diameter D of the first fibrous carbon material is 25% or less of the thickness T of the catalyst layer, that is, when the first fibrous carbon material is sufficiently thin with respect to the thickness T of the catalyst layer. In this case, by using a gas diffusion layer including the first fibrous carbon material and the first polymer resin, the stretchability or flexibility of the first polymer resin is fully exhibited, and the gas diffusion layer can flexibly plastically deform to conform to the uneven shape of the catalyst layer. This results in high strength adhesion between the catalyst layer and the gas diffusion layer, and low contact resistance between the catalyst layer and the gas diffusion layer. Furthermore, by suppressing peeling at the interface between the catalyst layer and the gas diffusion layer and increasing adhesion, high gas diffusibility can be maintained without generated water stagnation in the gas diffusion layer.

[0034] The average fiber diameter D may be 25% or less of the thickness T of the catalyst layer, but is more desirably sufficiently smaller than the thickness T of the catalyst layer. Specifically, the average fiber diameter D may be, for example, 250 nm or less, or 170 nm or less. In this case, it is also preferable in terms of ensuring a sufficiently fine gas path. The lower limit of the average fiber diameter D is not particularly limited, but may be, for example, 5 nm or more, from the viewpoint of reducing resistance by using the first fibrous carbon material as an electron path. That is, the average fiber diameter D may be 5 nm or more and 250 nm or less, or 5 nm or more and 170 nm or less.

[0035] The average fiber diameter D can be determined, for example, by randomly selecting 10 first fibrous carbon materials from the gas diffusion layer in an image such as an SEM photograph, measuring the diameters of the first fibrous carbon materials in the image, and averaging the diameters of the 10 first fibrous carbon materials. The diameter is the length in the direction perpendicular to the length direction of the first fibrous carbon materials.

[0036] The average length L of the first fibrous carbon material is not particularly limited. The average length L of the first fibrous carbon material may be, for example, 0.2 μm or more, or 0.5 μm or more. The average length L1 of the first fibrous carbon material may be, for example, 20 μm or less, or 10 μm or less. When the average length L is within this range, the mechanical strength of the gas diffusion layer is further improved, and the design of the gas diffusion properties of the gas diffusion layer is made easier, which is advantageous for suppressing pressure loss. The average length L can be determined, for example, by arbitrarily selecting 10 pieces of the first fibrous carbon material from the gas diffusion layer in an image such as an SEM photograph, measuring their actual lengths in the image, and averaging the lengths of the 10 pieces.

[0037] The first fibrous carbon material may be a mixture of two or more materials having different average fiber diameters D and / or average lengths L. For example, the first fibrous carbon material may contain single-walled carbon nanotubes and multi-walled carbon nanotubes.

[0038] <First polymer resin> The first polymer resin functions as a binder that binds together the fibrous carbon material. From the viewpoint of suppressing water retention in the gas diffusion layer, it is preferable that 50% by mass or more, and even 90% by mass or more of the first polymer resin is a water-repellent resin. The first polymer resin may further contain a proton-conductive resin (polymer electrolyte). The mass proportion of the first polymer resin in the gas diffusion layer may be 5% by mass or more and 50% by mass or less, or 10% by mass or more and 40% by mass or less.

[0039] Examples of water-repellent resins include fluororesins. Examples of fluororesins include polytetrafluoroethylene (PTFE), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), polyvinylidene fluoride (PVdF), tetrafluoroethylene-ethylene copolymer (ETFE), polychlorotrifluoroethylene (PCTFE), and tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA). Among these, from the viewpoints of heat resistance, water repellency, and chemical resistance, the fluororesin is preferably PTFE.

[0040] The proton conductive resin is not particularly limited. Examples of the proton conductive resin include perfluorocarbon sulfonic acid polymers and hydrocarbon polymers. Among them, perfluorocarbon sulfonic acid polymers and the like are preferred because of their excellent heat resistance and chemical stability. Examples of the perfluorocarbon sulfonic acid polymer include Nafion (registered trademark).

[0041] <Others> The gas diffusion layer may contain, as a conductive material, a particulate carbon material, a plate-like carbon material, or the like.

[0042] <Particulate Carbon Material> The particulate carbon material has an aspect ratio of less than 2 and is electrically conductive. The particulate carbon material is not particularly limited, but examples thereof include carbon black, spherical graphite, and activated carbon. Among these, carbon black is preferred because of its high electrical conductivity and large pore volume. 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 (secondary particle) formed by the connection of multiple primary particles) of the carbon black may be the same as that conventionally used in gas diffusion layers of fuel cells.

[0043] The average particle size of the primary particles of the particulate carbon material is preferably 250 nm or less, and may be 170 nm or less. The average particle size of the primary particles may be determined by observing 100 primary particles of the particulate carbon material under a microscope and averaging the maximum diameters of 100 particles.

[0044] The mass proportion of the particulate carbon material in the gas diffusion layer is not particularly limited, but from the viewpoint of gas diffusibility, the mass proportion of the particulate carbon material in the gas diffusion layer may be 30 mass% or less, or may be 20 mass% or less.

[0045] <Plate-shaped carbon material> Specific examples of the plate-shaped carbon material include flake graphite, pulverized graphitized polyimide film, graphene, etc. Among them, pulverized graphitized polyimide film and graphene are easily oriented in the plane direction, which is advantageous for forming a thin gas diffusion layer and is suitable for improving gas diffusivity in the plane direction.

[0046] The thickness of the carbon material plate is preferably 250 nm or less, and may be 170 nm or less. The thickness of the carbon material plate can be determined by observing 100 particles of the carbon material plate with a scanning electron microscope (SEM) and averaging the maximum thicknesses of the 100 particles.

[0047] The mass proportion of the carbon material plate in the gas diffusion layer is not particularly limited, but from the viewpoint of gas diffusibility, the mass proportion of the carbon material plate in the gas diffusion layer may be 30 mass% or less, or may be 20 mass% or less.

[0048] <Method for Manufacturing Gas Diffusion Layer> Next, a method for manufacturing the gas diffusion layers 7 and 9 in the membrane electrode assembly according to embodiment 1 of the present disclosure will be described. Fig. 4 is a flowchart of the method for manufacturing the gas diffusion layer according to embodiment 1. Note that the method for manufacturing the gas diffusion layer according to the present disclosure is not limited to the flowchart of Fig. 4 and the manufacturing method described below, and may be modified within the scope of the present disclosure.

[0049] In step S1, the particulate carbon material, the first fibrous carbon material, and the polymer resin are stirred and kneaded with a dispersion solvent to obtain a kneaded mixture of the particulate carbon material, the first fibrous carbon material, and the polymer resin. For example, a planetary mixer, a hybrid mixer, a kneader, a roll mill, or the like can be used to knead the materials in step S1. In step S1, which is the kneading process, the particulate carbon material, the first fibrous carbon material, the surfactant, and the dispersion solvent, excluding the polymer resin, are first kneaded and dispersed, and then the polymer resin is added and stirred, thereby achieving a state in which the polymer resin is uniformly dispersed in the kneaded mixture.

[0050] In step S2, the kneaded product is baked at a temperature equal to or higher than the decomposition temperature of the dispersion solvent to remove the dispersion solvent. For example, an IR oven, a hot air oven, or the like can be used for the baking in step S2. The baking temperature is set to a temperature higher than the decomposition temperature of the surfactant and lower than the melting temperature of the polymer resin. The reason for this is as follows: If the baking temperature is lower than the decomposition temperature of the surfactant, the surfactant remains inside the gas diffusion layer, making the interior of the gas diffusion layer hydrophilic and causing water to stagnate, which may reduce the gas permeability of the gas diffusion layer. On the other hand, if the baking temperature is higher than the melting point of the polymer resin, the polymer resin may melt, which may reduce the strength of the gas diffusion layer. Specifically, for example, when PTFE is used as the polymer resin, the baking temperature is preferably 280°C or higher and 340°C or lower. The baking time is set according to the baking temperature so that the residual dispersion solvent is 1 wt% or less.

[0051] In step S3, the solid material from which the dispersion solvent has been removed is pulverized to obtain a powder for a gas diffusion layer. The pulverization in step S3 is not particularly limited as long as it is an apparatus capable of pulverizing to an average particle size of 30 μm to 300 μm, and for example, a cutter mill, a jet mill, a pin mill, or the like can be used. Furthermore, the powder for a gas diffusion layer can also be obtained by classifying the powder particles after pulverization into particles with an average particle size of 30 μm to 300 μm. A vibrating sieve, a rotary sieve, or the like can be used to classify the powder after pulverization.

[0052] In step S4, the pulverized powder is rolled into a sheet to obtain a gas diffusion layer. A horizontal roll press device with two rolls arranged horizontally can be used for the roll rolling in step S4. The powder for the gas diffusion layer prepared in step S3 is fed between the two rolls using a conveyor, ultrasonic feeder, or the like, and a shear force is applied to the polymer resin 43 with a roll press force of, for example, 0.01 ton / cm to 4 ton / cm to fibrillate it. The fibrillated polymer resin can produce a high-strength gas diffusion layer. The powder-formed sheet can be re-rolled with rolls one or more times to improve the thickness accuracy and tensile break strength of the gas diffusion layer. Note that the present disclosure is not limited to the above-described embodiment and can be implemented in various other forms.

[0053] The configuration of the catalyst layer will be specifically described below. (Catalyst Layer) The catalyst layer includes a conductive material (hereinafter also referred to as a second conductive material), catalyst particles, and a polymer resin (hereinafter also referred to as a second polymer resin). Examples of the second conductive material include materials having the same or similar physical properties as the first conductive material. In particular, it is desirable for the second conductive material to include a second fibrous carbon material having the same or similar physical properties as the first fibrous carbon material.

[0054] The content of the second conductive material in the catalyst layer may be, for example, 1 mass % or more and 85 mass % or less, and more preferably 5 mass % or more and 75 mass % or less, relative to the total of the second conductive material, catalyst particles, and second polymer resin.

[0055] It is desirable that the thickness T of the catalyst layer be as thin as possible from the viewpoints of reducing the amount of expensive catalyst used and reducing resistance. On the other hand, from the viewpoint of the strength of the catalyst layer, it is preferable that the thickness T is not excessively thin. The thickness T of the catalyst layer may be, for example, 1 μm or more, and may be 5 μm or more. The thickness T of the catalyst layer may be, for example, 30 μm or less, or 20 μm or less. The thickness T of the catalyst layer may be 1 μm or more and 30 μm or less, or 5 μm or more and 20 μm or less.

[0056] The thickness T of the catalytic layer is an average thickness, and can be determined, for example, by selecting any 10 points on the cross section of the catalytic layer in an image such as an SEM photograph, drawing a straight line from the first main surface to the second main surface in the image along the thickness direction of the catalytic layer, measuring the distance as the thickness, and averaging the thicknesses at the 10 points.

[0057] <Catalyst Particles> The catalyst particles are not particularly limited, but examples thereof include at least one or two or more alloys selected from the group consisting of Sc, Y, Ti, Zr, V, Nb, Fe, Co, Ni, Ru, Rh, Pd, Pt, Os, Ir, lanthanide series elements, and actinide series elements. For example, catalyst particles used in the anode include Pt—Ru alloys. Catalyst metals used in the cathode include Pt and Pt—Co alloys.

[0058] <Second Polymer Resin> The second polymer resin functions as a binder that binds the second conductive material together. From the viewpoint of improving reactivity, the second polymer resin preferably contains a proton conductive resin. Examples of the proton conductive resin include materials similar to those that can be used in the gas diffusion layer. The proton conductive resin is preferably contained in an amount of 50 parts by mass or more and 200 parts by mass or less per 100 parts by mass of the second conductive material contained in the catalyst layer.

[0059] (Method for manufacturing catalyst layer) The catalyst layer can be formed on the gas diffusion layer by, for example, applying a catalyst ink containing a second conductive material, catalyst particles, a second polymer resin, and a dispersion medium to a predetermined transfer substrate sheet and transferring the ink onto the gas diffusion layer. Preferred coating methods include, for example, coating using an applicator, screen printing, and coating methods using various coaters such as a blade coater, knife coater, gravure coater, and die coater. As the substrate sheet, it is preferred to use a film such as PTFE, polypropylene, polyethylene terephthalate (PET), or polyimide. Furthermore, these films may be surface-treated, for example, by applying a release agent.

[0060] (Method for manufacturing membrane electrode assembly) A membrane electrode assembly can be manufactured as follows. When a transfer base sheet is used, the catalyst layer and the gas diffusion layer are first bonded together, and then the electrolyte membrane is attached to obtain a membrane electrode assembly. (1) Specifically, a catalyst layer having a first main surface and a second main surface is formed on a base sheet having a flat surface, so that the second main surface of the catalyst layer is in contact with the flat surface of the base sheet. (2) Next, the gas diffusion layer is bonded to the catalyst layer so that it is in contact with the first main surface of the catalyst layer. (3) Thereafter, the base sheet is peeled off from the second main surface of the catalyst layer, and the electrolyte membrane is attached to the second main surface of the catalyst layer, thereby obtaining a membrane electrode assembly. This makes it possible to obtain a membrane electrode assembly in which the strength S1 required to peel the interface between the catalyst layer and the gas diffusion layer is 31.2 to 1000 N cm －２ Furthermore, the strength S1 required to peel the interface between the catalyst layer and the gas diffusion layer and the breaking strength S2 of the gas diffusion layer can satisfy the relationship S1>S2.

[0061] When joining the catalyst layer and the gas diffusion layer, they may be heated and rolled at high temperature and pressure to enhance adhesion between them. By increasing the pressure during lamination, the strength S1 required to peel the interface between the catalyst layer and the gas diffusion layer can be increased. Note that if the pressure is too high, the pores inside the catalyst layer or the gas diffusion layer will be crushed.

[0062] Furthermore, when laminating the catalyst layer and the gas diffusion layer, it is desirable to have a solvent for the second polymer resin (proton-conductive resin) near the interface between them. For example, it is advisable to spray a solvent such as ethanol onto the catalyst layer or the gas diffusion layer immediately before laminating the catalyst layer and the gas diffusion layer. This dissolves and softens the proton-conductive resin in the catalyst layer near the interface, and in this state, the catalyst layer comes into contact with the gas diffusion layer, thereby further improving adhesion.

[0063] The configuration of the fuel cell will be specifically described below.

[0064] [Fuel Cell] As shown in FIG. 1 , the fuel cell 1 includes a first separator 10 and a second separator 11 arranged to sandwich the membrane electrode assembly 5 with a first gas diffusion layer 7 and a second gas diffusion layer 9 interposed therebetween. Specifically, the fuel cell 1 includes a membrane electrode assembly (hereinafter, MEA) 5 having an electrolyte membrane 4, a cathode 3 in contact with one main surface of the electrolyte membrane 4, and an anode 2 in contact with the other main surface of the electrolyte membrane 4, a conductive cathode separator (first separator) 11 in contact with the cathode 3, and a conductive anode separator (second separator) 10 in contact with the anode 2. The MEA 5 and the pair of separators 10, 11 constitute one cell 1. Typically, a stack in which the cells are connected in series is formed by stacking multiple cells such that the cathode separator 11 of one cell is adjacent to the anode separator 10 of another cell.

[0065] 1 , the cell 1 has a membrane electrode assembly (MEA) 5 including an anode 2, a cathode 3, and an electrolyte membrane 4 interposed between the anode 2 and the cathode 3. The MEA 5 is sandwiched between an anode separator 10 and a cathode separator 11. The anode 2 includes an anode catalyst layer (first catalyst layer) 6 in contact with the electrolyte membrane 4 and an anode gas diffusion layer (first gas diffusion layer) 7 in contact with the anode separator 10. The cathode 3 includes a cathode catalyst layer (second catalyst layer) 8 in contact with the electrolyte membrane 4 and a cathode gas diffusion layer (second gas diffusion layer) 9 in contact with the cathode separator 11. A gasket 14 is disposed on one side of the MEA 5 to seal the anode 2, and a gasket 15 is disposed on the other side to seal the cathode 3.

[0066] The anode separator 10 has a fuel flow path 12 that supplies fuel to the anode 2. The cathode separator 11 has an oxidant flow path 13 that supplies an oxidant to the cathode 3. A stack is formed by electrically stacking a plurality of cells such as those shown in FIG. 1 in series. In the stack, one surface of a single separator may serve as an anode separator, and the other surface as a cathode separator. Alternatively, one surface of a single separator may serve as an anode separator or a cathode separator, and the other surface may be used as a coolant flow path. Current collector plates 16, 17 may be disposed on the outside of the cell or stack, respectively, and end plates 18, 19 may be disposed and fastened together to secure the fuel cell.

[0067] In the illustrated example, the fuel flow channel 12 is formed on the main surface of the anode separator 10, and the oxidant flow channel 13 is formed on the main surface of the cathode separator 11, but this is not limiting. For example, the fuel flow channel may be formed on the main surface of the anode gas diffusion layer, and the oxidant flow channel may be formed on the main surface of the cathode gas diffusion layer.

[0068] The present disclosure will be described in detail below based on examples, but the present disclosure is not limited to the following examples.

[0069] Example 1 A membrane electrode assembly (MEA) was produced as shown in Fig. 2. The MEA 5 includes an electrolyte membrane 4, an anode catalyst layer 6 and a cathode catalyst layer 8 disposed so as to sandwich the electrolyte membrane 4, an anode gas diffusion layer 7 disposed on the outer side of the anode catalyst layer 6, and a cathode gas diffusion layer 9 disposed on the outer side of the cathode catalyst layer 8.

[0070] (1) Preparation of Gas Diffusion Layer The gas diffusion layer of Example 1 was produced by the following method. First, 15 wt % of particulate carbon material, 65 wt % of the first fibrous carbon material, and 20 wt % of polymer resin were blended, and then a surfactant and a dispersion solvent were added and kneaded using a planetary mixer. Next, the kneaded mixture was baked in a hot air baking oven at 300°C for 4 hours to remove the surfactant and dispersion solvent. At this time, it was confirmed using TG / DTA that the surfactant residue was 1% or less. The baked mixture was then pulverized using a cutter mill to produce powder for the gas diffusion layer.

[0071] The powder for gas diffusion layers was then placed in a roll press and pressed at a pressure of 0.2 ton / cm to form a sheet, and unnecessary portions were cut off to prepare a gas diffusion layer having a thickness of 200 μm. The resulting sheet was then cut into the desired shape to obtain a cathode gas diffusion layer and an anode gas diffusion layer.

[0072] (2) Preparation of Cathode Catalyst Ink A second conductive material carrying catalyst particles (Pt—Co alloy) was added to an appropriate amount of water and then stirred to disperse. The second conductive material consisted of 40 parts by mass of CNT (average fiber diameter 0.15 μm, average length 6 μm) as a second fibrous carbon material and 100 parts by mass of carbon black (average particle size 40 nm) as a particulate carbon material. An appropriate amount of ethanol was added to the resulting dispersion while stirring, and then 80 parts by mass of a proton-conductive resin (Nafion®) was added to 100 parts by mass of the second particulate carbon material carrying 30 parts by mass of catalyst particles to prepare a cathode catalyst ink.

[0073] (3) Preparation of anode catalyst ink The second conductive material carrying catalyst particles (Pt—Ru alloy) was added to an appropriate amount of water and then stirred to disperse. Carbon black (average particle size 40 nm), a particulate carbon material, was used as the second conductive material. An appropriate amount of ethanol was added to the resulting dispersion while stirring, and then 80 parts by mass of a proton-conductive resin (Nafion (registered trademark)) was added to 100 parts by mass of the second particulate carbon material carrying 30 parts by mass of catalyst particles to prepare an anode catalyst ink.

[0074] (4) Lamination of Each Layer The cathode catalyst ink was applied to the surface of a PTFE substrate sheet having a flat surface using an applicator, and then dried to form a cathode catalyst layer (thickness: 10 μm). ２ The cathode gas diffusion layer sprayed with ethanol was placed on the cathode catalyst layer, and a pressure of 100 kgf / cm ２ The cathode catalyst layer was transferred to one side of the cathode gas diffusion layer by heating and rolling at 120° C. for 1 minute and then peeling off the substrate sheet, thereby forming a cathode.

[0075] The anode catalyst ink was applied to the surface of a PTFE substrate sheet having a flat surface using an applicator and dried to form an anode catalyst layer (thickness: 5 μm). ２ The anode gas diffusion layer onto which ethanol was sprayed and the anode catalyst layer were stacked, and subjected to a pressure of 100 kgf / cm ２ The substrate sheet was then peeled off, and the anode catalyst layer was transferred to one side of the anode gas diffusion layer to form an anode.

[0076] Next, the electrolyte membrane was sandwiched between the anode and the cathode so that the anode catalyst layer and the cathode catalyst layer were in contact with the electrolyte membrane, respectively, and a pressure of 10 kgf / cm ２ The resultant was heated and rolled at 140° C. for 5 minutes to obtain a membrane electrode assembly (MEA) A1. As the electrolyte membrane, a Nafion® membrane having a thickness of 50 μm and a size slightly larger than that of the cathode was used.

[0077] [Evaluation 1] In the peel test, a probe was lowered and pressed against the sample, and the sample surface was adhered to the probe. The probe was then raised to measure the maximum load at which the sample broke (or peeled off). The broken portion was observed and its area was measured. The same test was performed on five points of the same membrane electrode assembly (MEA).

[0078] Specifically, the peel test was performed as follows. (1) A portion of the membrane electrode assembly (MEA) A1 (including at least the cathode gas diffusion layer, cathode catalyst, and electrolyte membrane) was cut into a square with sides of 20 mm as a sample, and attached to a glass slide with double-sided tape so that the cathode gas diffusion layer faced outward. (2) A probe with a circular, flat tip with a diameter of 2 mm was used, and double-sided tape was attached to cover the probe tip. (3) A square cut (cross cut) with sides of 3 mm was made in the sample, and the glass slide with the sample attached was fixed to the device. Note that the cross cut of the sample was performed, for example, with reference to JIS K5600-5-6:1999, "Method for preparing test pieces for cross-cut test of coating films." (4) The sample was moved to a position such that the probe tip would contact the center of the square cut when the probe was lowered. (5) The probe tip was lowered at 1 mm / min until it touched the sample, and once it touched the sample, the probe was pressed against the sample with a load of 250 gf for 10 seconds. This caused the double-sided tape on the probe tip to adhere to the cathode gas diffusion layer on the sample. (6) The probe was then raised at 600 mm / min to break (or peel) the sample. (7) The load required to raise the probe was measured, and the maximum value was taken as the break (or peel) load. The broken portion was also observed and its area was measured. Similar tests were performed at five points on the membrane electrode assembly (MEA) A1.

[0079] Observation of the fractured areas in the peel test revealed that the interior of the cathode gas diffusion layer had fractured at all five points in the membrane electrode assembly (MEA) A1. Here, fracture within the gas diffusion layer is defined as 95% or more of the fractured area being covered by the gas diffusion layer. This indicates that the cathode catalyst layer and the cathode gas diffusion layer are in close contact with each other with high strength, and the strength required to peel the interface between the cathode catalyst layer and the cathode gas diffusion layer is greater than the fracture strength of the cathode gas diffusion layer. Furthermore, because the cathode catalyst layer and the cathode gas diffusion layer are in close contact with each other with high strength, only the area touching the tip of the probe fractured. In other words, the area of ​​the fractured area nearly matched the area of ​​the tip of the probe. The fracture strength of the cathode gas diffusion layer was calculated from the maximum load and the area of ​​the fractured area. The results are shown in the table in Figure 3. For the membrane electrode assembly (MEA) A1, the fracture load was 31.2 N cm at all five points. －２ That's all.

[0080] Comparative Example 1 (1) Preparation of Gas Diffusion Layers A cathode gas diffusion layer and an anode gas diffusion layer were prepared in the same manner as in Example 1.

[0081] (2) Preparation of Cathode Catalyst Ink A cathode catalyst ink was prepared in the same manner as in Example 1.

[0082] (3) Preparation of Anode Catalyst Ink An anode catalyst ink was prepared in the same manner as in Example 1.

[0083] (4) Lamination of Each Layer The cathode catalyst ink was applied to the surface of a flat PTFE substrate sheet using an applicator and dried to form a cathode catalyst layer (thickness 10 μm).

[0084] The anode catalyst ink was applied to the surface of a PTFE substrate sheet having a flat surface using an applicator, and then dried to form an anode catalyst layer (thickness: 5 μm).

[0085] Next, the same electrolyte membrane as in Example 1 was prepared, and the electrolyte membrane was sandwiched between the cathode catalyst layer and the substrate sheet, and between the anode catalyst layer and the substrate sheet so that the anode catalyst layer and the cathode catalyst layer were in contact with the electrolyte membrane, respectively, and subjected to a pressure of 100 kgf / cm ２The cathode and anode catalyst layers were then transferred to both sides of the electrolyte membrane by heating and rolling at 120°C for 1 minute and peeling off the substrate sheets on both sides. Thereafter, a cathode gas diffusion layer was laminated facing the cathode catalyst layer, and an anode gas diffusion layer was laminated facing the anode catalyst layer. ２ The sheet was then heated and rolled at 140° C. for 5 minutes to obtain a membrane electrode assembly (MEA) B1.

[0086] [Evaluation 2] A peel test similar to that in Evaluation 1 was performed on five points of the membrane electrode assembly (MEA) B1. Observation of the fractured portions revealed fracture (peel) at the interface between the cathode catalyst layer and the cathode gas diffusion layer at all five points of the membrane electrode assembly (MEA) B1. This indicates that the cathode catalyst layer and the cathode gas diffusion layer were not in close contact with each other and that the strength required to peel the interface between the cathode catalyst layer and the cathode gas diffusion layer was less than the fracture strength of the cathode gas diffusion layer. Furthermore, because the cathode catalyst layer and the cathode gas diffusion layer were not in close contact with each other with high strength, the entire incised square portion with sides of 3 mm peeled off. The fracture strength of the cathode gas diffusion layer was calculated from the maximum load and the area of ​​the fractured portion. The results are shown in the table in Figure 3.

[0087] Comparative Example 2 (1) Preparation of Gas Diffusion Layers A cathode gas diffusion layer and an anode gas diffusion layer were prepared in the same manner as in Example 1.

[0088] (2) Preparation of Cathode Catalyst Ink A cathode catalyst ink was prepared in the same manner as in Example 1.

[0089] (3) Preparation of Anode Catalyst Ink An anode catalyst ink was prepared in the same manner as in Example 1.

[0090] (4) Lamination of Each Layer The cathode catalyst ink was applied to the surface of a PTFE substrate sheet having a flat surface using an applicator, and then dried to form a cathode catalyst layer (thickness: 10 μm). Thereafter, the cathode gas diffusion layer and the cathode catalyst layer were stacked without spraying ethanol, and the cathode catalyst layer was subjected to a pressure of 100 kgf / cm. ２ The substrate sheet was then heated and rolled at 120° C. for 1 minute in an attempt to peel off the substrate sheet, but the cathode catalyst layer did not peel off from the substrate sheet and did not adhere to the cathode gas diffusion layer.

[0091] The anode catalyst ink was applied to the surface of a PTFE substrate sheet having a flat surface using an applicator and dried to form an anode catalyst layer (thickness: 5 μm). Thereafter, the anode gas diffusion layer and the anode catalyst layer were stacked without spraying ethanol, and the anode catalyst layer was subjected to a pressure of 100 kgf / cm. ２ The substrate sheet was then heated and rolled at 120° C. for 1 minute in an attempt to peel off the substrate sheet, but the anode catalyst layer did not peel off from the substrate sheet and did not adhere to the anode gas diffusion layer.

[0092] The fuel cell according to the present disclosure can be suitably used particularly as a power source for vehicles, a power source for stationary household cogeneration systems, etc. The fuel cell according to the present disclosure is suitable for application to polymer electrolyte fuel cells, but is not limited thereto, and can be applied to fuel cells in general.

[0093] REFERENCE SIGNS LIST 1 cell 2 anode 3 cathode 4 electrolyte membrane 5 membrane electrode assembly (MEA) 6 anode catalyst layer 7 anode gas diffusion layer 8 cathode catalyst layer 9 cathode gas diffusion layer 10 anode separator 11 cathode separator 12 fuel flow path 13 oxidant flow path 14, 15 gasket 16, 17 current collector plate 18, 19 end plate

Claims

1. A membrane electrode assembly for a fuel cell, comprising: a catalyst layer having a first main surface and a second main surface; a gas diffusion layer disposed on the first main surface side; and an electrolyte membrane disposed on the second main surface side, wherein the gas diffusion layer contains a conductive material and a polymer resin, and the conductive material contains a fibrous carbon material, and the strength S1 required to peel the interface between the catalyst layer and the gas diffusion layer is 31.2 to 1000 N cm －２ A membrane electrode assembly for a fuel cell.

2. A membrane electrode assembly for a fuel cell, comprising: a catalyst layer having a first main surface and a second main surface; a gas diffusion layer disposed on the first main surface side; and an electrolyte membrane disposed on the second main surface side, wherein the gas diffusion layer comprises a conductive material and a polymer resin, and the conductive material comprises a fibrous carbon material, and a strength S1 required to peel the interface between the catalyst layer and the gas diffusion layer and a breaking strength S2 of the gas diffusion layer satisfy the relationship S1 > S2.

3. The fuel cell membrane electrode assembly according to claim 1 or 2, wherein the average fiber diameter D of the fibrous carbon material is 25% or less of the thickness T of the catalyst layer.

4. A membrane electrode assembly for a fuel cell according to claim 1 or 2, wherein the average fiber diameter D of the fibrous carbon material is 5 nm or more and 250 nm or less.

5. The membrane electrode assembly for a fuel cell according to claim 1 or 2, wherein the thickness T of the catalyst layer is 1 μm or more and 30 μm or less.

6. A membrane electrode assembly for a fuel cell, comprising: an electrolyte membrane; a pair of catalyst layers arranged to sandwich the electrolyte membrane; and a pair of gas diffusion layers arranged to sandwich the electrolyte membrane, with the pair of catalyst layers interposed therebetween, wherein the pair of catalyst layers each have a first main surface and a second main surface, each of the gas diffusion layers is arranged on the first main surface side, and the second main surface side is arranged facing the electrolyte membrane, at least one of the pair of gas diffusion layers comprises a conductive material and a polymer resin, and the conductive material comprises a fibrous carbon material, and a strength S1 required to peel the interface between the catalyst layer and the gas diffusion layer and a breaking strength S2 of the gas diffusion layer satisfy the relationship S1 > S2.

7. A membrane electrode assembly for a fuel cell, comprising: an electrolyte membrane; a pair of catalyst layers arranged to sandwich the electrolyte membrane; and a pair of gas diffusion layers arranged to sandwich the electrolyte membrane with the pair of catalyst layers interposed therebetween, wherein the pair of catalyst layers each have a first main surface and a second main surface, and the gas diffusion layer is arranged on the first main surface side with the second main surface side facing the electrolyte membrane, at least one of the pair of gas diffusion layers comprises a conductive material and a polymer resin, and the conductive material comprises a fibrous carbon material, wherein a strength S1 required to peel the interface between the catalyst layer and the gas diffusion layer and a breaking strength S2 of the gas diffusion layer satisfy the relationship S1 > S2; and a pair of separators arranged to sandwich the membrane electrode assembly for a fuel cell with the pair of gas diffusion layers included in the membrane electrode assembly for a fuel cell interposed therebetween.

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