Gas diffusion layer and method for manufacturing same, membrane electrode assembly, and fuel cell

Abstract

Provided is a gas diffusion layer that exhibits an excellent mechanical strength. This gas diffusion layer is constituted of a porous member that contains conductive particles, conductive fibers, and a polymer resin. A particulate polymer resin is contained in the interior of the porous member. A part of the particulate polymer resin exists in a state in which two or more particles are fused.

Description

Gas diffusion layer and method for manufacturing the same, membrane electrode assembly, and fuel cell

[0001] The present disclosure relates to a gas diffusion layer, a method for manufacturing the same, a membrane electrode assembly, and a fuel cell.

[0002] A single cell of a polymer electrolyte fuel cell consists of a membrane electrode assembly (hereinafter abbreviated as "MEA") and a separator. The MEA consists of an electrolyte membrane and an electrode layer. The electrode layer consists of a catalyst layer and a gas diffusion layer. The gas diffusion layer is required to have gas permeability, gas diffusibility, and conductivity, and is made of a conductive porous material. In addition, the gas diffusion layer is required to have water repellency, and it is common to use one provided with a water repellent layer.

[0003] Patent Document 1 discloses a gas diffusion layer composed of a porous material containing conductive particles and a polymer resin as main components. Patent Document 2 discloses a gas diffusion layer containing conductive particles and a fluororesin, the fluororesin containing first fibers having a first average fiber diameter and second fibers having a second average fiber diameter different from the first average fiber diameter.

[0004] Patent No. 4938133 JP 2021-197245 A

[0005] However, there is a demand for a gas diffusion layer having a higher degree of fiberization of the polymer resin and improved mechanical strength.

[0006] An object of the present disclosure is to provide a gas diffusion layer that contains conductive fibers, conductive particles, and a polymer resin as its main components and has excellent mechanical strength.

[0007] The gas diffusion layer according to the present disclosure is composed of a porous material containing conductive particles, conductive fibers, and a polymer resin, and contains particulate polymer resin inside the porous material, with some of the particulate polymer resin being present in a state where two or more particles are fused together.

[0008] A membrane electrode assembly according to the present disclosure includes the gas diffusion layer, a pair of electrodes, and an electrolyte membrane.

[0009] A fuel cell according to the present disclosure includes the above membrane electrode assembly and a current collector plate.

[0010] The method for manufacturing a gas diffusion layer according to the present disclosure includes the steps of kneading conductive particles, conductive fibers, and a polymer resin in a dispersion solvent to obtain a kneaded mixture in which the polymer resin has a fibrous degree of 50% or less, and rolling the kneaded mixture to obtain a sheet in which the polymer resin has a fibrous degree of less than 50%.

[0011] The method for producing a gas diffusion layer according to the present disclosure includes the steps of: kneading conductive particles, conductive fibers, a polymer resin, and crushing aid particles having a specific gravity at least twice that of the conductive particles; crushing and dispersing the conductive particles, the conductive fibers, and the polymer resin with the crushing aid particles to obtain a kneaded mixture; and rolling the kneaded mixture into a sheet to obtain a gas diffusion layer made of the sheet.

[0012] The method for producing a gas diffusion layer according to the present disclosure includes the steps of kneading conductive particles, conductive fibers, and a polymer resin in a dispersion solution with a dispersant content of 1 wt % or more and 5 wt % or less to obtain a kneaded mixture, adding second conductive particles having a specific surface area that is 2 times or more and 20 times or less that of the conductive particles to the kneaded mixture, and kneading the mixture further, and rolling the kneaded mixture into a sheet to obtain a gas diffusion layer made of the sheet.

[0013] The gas diffusion layer according to the present disclosure has improved mechanical strength, which makes it possible to suppress deformation due to external forces such as vibrations and impacts, as well as deformation during long-term fuel cell operation, thereby extending the life of the fuel cell.

[0014] FIG. 1 is a schematic perspective view showing the configuration of a polymer electrolyte fuel cell stack according to a first embodiment of the present disclosure. FIG. 2 is a schematic cross-sectional view showing the cross-sectional structure of a polymer electrolyte fuel cell cell according to the first embodiment of the present disclosure. FIG. 3 is an enlarged schematic view of a gas diffusion layer according to the first embodiment of the present disclosure. FIG. 4 is an SEM photograph of a gas diffusion layer according to the first embodiment of the present disclosure. FIG. 5 is an enlarged schematic view of a gas diffusion layer according to the second embodiment of the present disclosure. FIG. 6 is an SEM photograph of a gas diffusion layer according to the second embodiment of the present disclosure. FIG. 7 is an enlarged schematic view of a gas diffusion layer according to the third embodiment of the present disclosure. FIG. 8 is an SEM photograph of a gas diffusion layer according to the third embodiment of the present disclosure. FIG. 9 is an SEM photograph of a gas diffusion layer according to the calculation of the PTFE fiber degree and the PTFE fiber amount according to the present disclosure. FIG. 10 is a phase separation image of C phase and CF phase by EDX according to the calculation of the PTFE fiber degree and the PTFE fiber amount according to the present disclosure. FIG. 11 is a flowchart showing a method for manufacturing a gas diffusion layer according to the first and second embodiments of the present disclosure. FIG. 12 is a flowchart showing a method for manufacturing a gas diffusion layer according to the third embodiment of the present disclosure.

[0015] The gas diffusion layer according to the first aspect is composed of a porous material containing conductive particles, conductive fibers, and a polymer resin, and contains particulate polymer resin inside the porous material, with some of the particulate polymer resin existing in a state where two or more particles are fused together.

[0016] The gas diffusion layer according to the second aspect may be the same as that of the first aspect, in which a fibrous polymer resin is contained inside the porous member, and when the degree of fibrous polymer resin is defined as the proportion of the polymer resin that exists in a fibrous form, the degree of fibrous polymer resin may be 50% or more.

[0017] The gas diffusion layer according to the third aspect may be the gas diffusion layer according to the first or second aspect, which contains a fibrous polymer resin inside the porous member, and the amount of fibers as a ratio of the fibrous polymer resin to the entire porous member may be 3 wt % or more.

[0018] The gas diffusion layer according to a fourth aspect is the gas diffusion layer according to any one of the first to third aspects, which contains a particulate polymer resin inside the porous member, and a part of the particulate polymer resin may be present in the form of a film, with two or more particles being fused.

[0019] The gas diffusion layer according to a fifth aspect may be the gas diffusion layer according to any one of the first to fourth aspects, further comprising crushing auxiliary particles having a specific gravity of 2 to 20 times that of the conductive particles.

[0020] A gas diffusion layer according to a sixth aspect is the gas diffusion layer according to the fifth aspect, wherein the crushing aid particles are cerium-containing oxides.

[0021] The gas diffusion layer according to a seventh aspect is any one of the first to sixth aspects, wherein the porous member contains 5 wt % or more and less than 35 wt % of conductive particles, 35 wt % or more and 80 wt % or less of conductive fibers, 10 wt % or more and 40 wt % or less of polymer resin, and 0 wt % or more and 30 wt % or less of disintegration auxiliary particles.

[0022] A gas diffusion layer according to an eighth aspect is the gas diffusion layer according to any one of the first to seventh aspects, wherein the polymer resin may include polytetrafluoroethylene.

[0023] A membrane electrode assembly according to a ninth aspect includes the gas diffusion layer according to any one of the first to eighth aspects, a pair of electrodes, and an electrolyte membrane.

[0024] A fuel cell according to a tenth aspect includes the membrane electrode assembly according to the ninth aspect and a current collector plate.

[0025] The method for producing a gas diffusion layer according to the eleventh aspect includes the steps of kneading conductive particles, conductive fibers, and a polymer resin in a dispersion solvent to obtain a kneaded mixture in which the polymer resin has a fibrous degree of 50% or less, and rolling the kneaded mixture to obtain a sheet in which the polymer resin has a fibrous degree of less than 50%.

[0026] A twelfth aspect of the method for producing a gas diffusion layer is the eleventh aspect, wherein in the step of obtaining a kneaded mixture, the conductive particles, the conductive fibers, and the polymer resin may be kneaded in a dispersion solution having a dispersant content of 1 wt % or more and 5 wt % or less.

[0027] The method for producing a gas diffusion layer according to the thirteenth aspect may further include a step of firing the sheet in which the polymer resin has a fibrous degree of less than 50% in the eleventh or twelfth aspect to obtain a gas diffusion layer in which the polymer resin has a fibrous degree of 50% or more.

[0028] The method for producing a gas diffusion layer according to the fourteenth aspect includes the steps of: kneading conductive particles, conductive fibers, a polymer resin, and crushing aid particles having a specific gravity at least twice that of the conductive particles; crushing and dispersing the conductive particles, the conductive fibers, and the polymer resin with the crushing aid particles to obtain a kneaded mixture; and rolling the kneaded mixture into a sheet to obtain a gas diffusion layer made of the sheet.

[0029] The method for producing a gas diffusion layer according to the fifteenth aspect includes the steps of: kneading conductive particles, conductive fibers, and a polymer resin in a dispersion solution with a dispersant content of 1 wt % or more and 5 wt % or less to obtain a kneaded mixture; adding second conductive particles having a specific surface area that is 2 times or more and 20 times or less that of the conductive particles to the kneaded mixture; and rolling the kneaded mixture into a sheet to obtain a gas diffusion layer made of the sheet.

[0030] Hereinafter, a gas diffusion layer and a manufacturing method thereof, a membrane electrode assembly, and a fuel cell according to embodiments of the present disclosure will be described with reference to the accompanying drawings.

[0031] First Embodiment A basic configuration of a fuel cell 100 according to a first embodiment of the present disclosure will be described using Figure 1. Figure 1 is a schematic perspective view showing the configuration of a fuel cell 100 according to the first embodiment (hereinafter also referred to as a "polymer electrolyte fuel cell stack"). Note that this embodiment is not limited to polymer electrolyte fuel cells, and can be applied to various types of fuel cells.

[0032] <Fuel Cell> As shown in FIG. 1 , a fuel cell 100 is formed by stacking one or more battery cells 10, which are basic units, and compressing and fastening them together under a predetermined load using current collector plates 11, insulating plates 12, and end plates 13 arranged on both sides of the stacked battery cells 10. The current collector plates 11 are made of a gas-impermeable conductive material, such as copper or brass. Current collector terminals (not shown) are provided on the current collector plates 11, and current is extracted from the current collector terminals during power generation. The insulating plates 12 are made of an insulating material such as resin. Examples of materials used for the insulating plates 12 include fluorine-based resins and PPS resins. The end plates 13 fasten and hold the stacked battery cells 10, current collector plates 11, and insulating plates 12 together under a predetermined load using a pressure device (not shown). The end plates 13 are made of a highly rigid metal material, such as steel.

[0033] FIG. 2 is a schematic cross-sectional view showing the cross-sectional structure of a battery cell 10. In the battery cell 10, a membrane electrode assembly (hereinafter also referred to as MEA) 20 is sandwiched between an anode-side separator 4a and a cathode-side separator 4b. Hereinafter, the anode-side separator 4a and the cathode-side separator 4b will be collectively referred to as the separator 4. The same description will be used for other components when multiple components are described together. A fluid flow path 5 is formed in the separator 4. A fluid flow path 5 for fuel gas is formed in the anode-side separator 4a. A fluid flow path 5 for oxidant gas is formed in the cathode-side separator 4b. Carbon-based and metal-based materials can be used for the separator 4. The fluid flow path 5 is a groove portion formed in the separator 4. A rib portion 6 is provided around the fluid flow path 5.

[0034] <Membrane Electrode Assembly (MEA)> The membrane electrode assembly (MEA) 20 has a polymer electrolyte membrane 1, a catalyst layer 2, and a gas diffusion layer 3. An anode catalyst layer 2a and a cathode catalyst layer 2b (collectively, catalyst layers 2) are formed on both sides of the polymer electrolyte membrane 1, which selectively transports hydrogen ions, and an anode-side gas diffusion layer 3a and a cathode gas diffusion layer 3b (collectively, gas diffusion layers 3) are disposed on the outer sides thereof, respectively. For example, a perfluorocarbon sulfonic acid polymer is used for the polymer electrolyte membrane 1, but there is no particular limitation as long as it has proton conductivity. For the catalyst layer 2, a layer containing a polymer electrolyte and a carbon material supporting catalyst particles such as platinum can be used.

[0035] <Gas Diffusion Layer 3> Next, the configuration of the gas diffusion layer 3 according to the first embodiment of the present disclosure will be described in detail with reference to Figures 3A, 3B, 6A, and 6B. Figure 3A is an enlarged schematic view of a portion of the gas diffusion layer 3, and Figure 3B is an SEM photograph. The gas diffusion layer 3 is composed of conductive particles 31, conductive fibers 32, and polymer resin 33. The polymer resin 33 exists in various forms, such as individual particles 35, fibers 36, and aggregates 37 in which two or more individual particles are aggregated and fused together.

[0036] When the degree of fiberization of the polymer resin is defined as the proportion of the polymer resin 33 that exists in a fibrous form, the degree of fiberization of the polymer resin is 50% or more. More preferably, it is 60% or more. The PTFE, which is the polymer resin 33, is particulate in the raw material stage, but is fiberized by shear force, and the PTFE fibers 36 become entangled with the conductive particles 31 and the conductive fibers 32, thereby ensuring mechanical strength. Therefore, a PTFE fiberization degree of 50% or more improves the mechanical strength of the gas diffusion layer 3, providing strength that can withstand the pressure of gas and generated water during power generation. If the PTFE fiberization degree is less than 50%, the membrane will function as a free-standing membrane, but will be damaged by the pressure of gas and generated water during power generation.

[0037] Furthermore, the polymer resin fiber content of the polymer resin present in fibrous form is 3 wt% or more, more preferably 4 wt% or more. As described above, the polymer resin PTFE fibers 36 ensure the mechanical strength of the gas diffusion layer 3, so if the PTFE fiber content is low, the mechanical strength of the gas diffusion layer 3 will decrease. The fiber content of the polymer resin PTFE can be increased by increasing the degree of fiberization of the PTFE or by increasing the amount of PTFE added. If the fiber content of the polymer resin PTFE is less than 3 wt%, the mechanical strength will decrease and the gas diffusion layer will be damaged by the pressure of the gas and water produced during power generation.

[0038] <Method for Calculating the Degree of Fiberization and Fiber Amount of the Polymer Resin in the Gas Diffusion Layer 3> Here, a method for calculating the degree of fiberization and fiber amount of the polymer resin in the gas diffusion layer 3 will be described using FIGS. 6A and 6B . The degree of fiberization of the polymer resin in the gas diffusion layer 3 is calculated by phase separation in the EDX image measurement results of the surface or cross-section of the gas diffusion layer 3 using a SEM, as shown in FIGS. 6A and 6B . For example, phase separation can be performed using Oxford Instrument's AZtec 4.3 software. Here, we will describe phase analysis. The EDX image measurement results have a spectrum for each pixel. By performing calculations on the spectrum of each pixel obtained by EDX image measurement, a phase analysis image ( FIG. 6B ) can be obtained, in which pixels with similar spectra are grouped. From this phase analysis image, the area ratio of each phase in the image field and the composition ratio can be calculated through quantitative analysis of the spectrum. In this first embodiment, we focused on the fluorine (F) contained in the polymer resin PTFE and the carbon (C) contained in the conductive particles and conductive fibers, which are carbon materials, and grouped the phases into two phases: a fluorine (F)-based CF phase and a carbon (C)-based C phase. In FIG. 6B, the CF phase is shown in black, while the surrounding area is C phase. PTFE fibers are so thin that they are difficult to observe using an SEM, and the amount detected per area is small, so the F concentration in the C phase, which is mainly C, correlates with the amount of PTFE fiber. Therefore, the fiber amount and fiberization degree are calculated using the following formula: (fiber amount) = C phase area ratio × C phase F concentration (fiberization degree) = C phase area ratio × C phase F concentration / phase analysis image F concentration

[0039] The gas diffusion layer 3 is composed of a porous material containing conductive particles 31, conductive fibers 32, and a polymer resin 33. The porous material contains particulate polymer resin 35, and some of the particulate polymer resin 35 may exist as agglomerates 37 in which two or more particles are fused together. As described above, PTFE, which is a polymer resin, is in a particulate form as a raw material. In the process of kneading conductive particles, conductive fibers, and particulate PTFE to disperse the PTFE particles, some of the PTFE particles do not disperse into individual particles but exist in a state of agglomerates of two or more particles. The gas diffusion layer 3 is then manufactured through a sheet-forming process and a sintering process. In the sintering process, two or more agglomerated PTFE particles are fused together and adhere to the conductive particles and conductive fibers, thereby improving the strength of the gas diffusion layer 3.

[0040] <Types of Conductive Particles 31, Conductive Fibers 32, and Polymer Resins 33> Examples of the conductive particles 31 include carbon materials such as carbon black, graphite, and activated carbon. Among these, it is preferable to use carbon black, which has high conductivity and a large pore volume. Furthermore, acetylene black, ketjen black, furnace black, and vulcan can be used as carbon black. Among these, it is preferable to use acetylene black, which has a low impurity content, or ketjen black, which has a large specific surface area and high conductivity. Furthermore, fullerenes such as C60 fullerene may be used as the conductive particles. The size of the conductive particles is, for example, D50, which is 10 nm or more and 5 μm or less. It may also be, for example, 10 nm or more and 500 nm or less, or even 10 nm or more and 100 nm or less. When the conductive particles are carbon black, for example, the primary particle diameter may be 10 nm or more and 500 nm or less, or may be 10 nm or more and 100 nm or less, and the size of the aggregates (primary agglomerates) may be, for example, 100 nm or more and 500 nm or less. When the conductive particles are graphite or activated carbon, D50 is, for example, 1 μm or more and 5 μm or less.

[0041] The conductive fibers 32 contribute to improving the conductivity and mechanical strength of the gas diffusion layer 3. The material of the conductive fibers 32 is not particularly limited, but carbon fibers such as carbon nanotubes can be used, for example. The average fiber diameter of the conductive fibers 32 is preferably 50 nm or more and 300 nm or less. Having an average fiber diameter of 50 nm or more contributes more effectively to improving the conductivity of the gas diffusion layer 3 and can further increase the mechanical strength of the gas diffusion layer 3. This allows the gas diffusion layer 3 to have sufficient strength as a self-supporting membrane. Furthermore, having an average fiber diameter of the conductive fibers 32 of 300 nm or less prevents the diameter from becoming too large, making it easier to ensure a sufficient pore volume in the porous member 30. This further improves the gas diffusibility of the gas diffusion layer 3.

[0042] The average fiber length of the conductive fibers 32 is preferably 0.5 μm or more and 50 μm or less. When the average fiber length of the conductive fibers 32 is 0.5 μm or more, this more effectively contributes to improving the conductivity of the gas diffusion layer 3 and can further increase the mechanical strength of the gas diffusion layer 3. When the average fiber length of the conductive fibers 32 is 50 μm or less, the fibers do not become too long, and therefore the conductive fibers 32 can be crushed without forming lumps during production, and the gas diffusibility of the gas diffusion layer 3 can further be improved.

[0043] Examples of the polymer resin 33 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 polymer resin 33 from the viewpoints of heat resistance, water repellency, and chemical resistance. Examples of raw material forms for PTFE include dispersion and powder. Among these, dispersion is preferred due to its excellent dispersibility. PTFE has the property of becoming fibrous when shear force is applied. When shear force is applied to the PTFE material during the mixing / dispersion process and sheet-forming process in the production of the gas diffusion layer 3, the PTFE becomes fibrous.

[0044] The polymer resin 33 functions as a binder that binds the conductive particles 31 and the conductive fibers 32 together. Furthermore, because the polymer resin 33 is water-repellent, it also serves to prevent water from remaining in the pores inside the gas diffusion layer 3 and inhibiting gas permeation. Furthermore, in the gas diffusion layer 3, the conductive particles 31 are present in the gaps between the conductive fibers 32, and the fibrous polymer resin 33 can satisfactorily bind the conductive fibers 32 and the conductive particles 31, so that the gas diffusion layer 3 can have sufficient strength.

[0045] 4A and 4B , the configuration of a gas diffusion layer 3 according to a second embodiment of the present disclosure will be described in detail. FIG. 4A is a schematic diagram of an enlarged portion of the gas diffusion layer 3, and FIG. 4B is an SEM photograph. The gas diffusion layer 3 according to the second embodiment is composed of conductive particles 31, conductive fibers 32, and a polymer resin 33. The polymer resin 33 exists in the form of individual particles 35, fibers 36, aggregates 37 in which two or more individual particles are aggregated and fused together, and a film-like body 38 in which individual particles or two or more particles are fused together. As described above, in the firing process, two or more aggregated PTFE particles are fused together, but some of the aggregated PTFE particles further soften and change from particle-like to film-like, filling gaps between the conductive particles and the conductive fibers, thereby further improving the strength of the gas diffusion layer 3.

[0046] 5A and 5B , the configuration of a gas diffusion layer 3 according to a third embodiment of the present disclosure will be described in detail. Fig. 5A is an enlarged schematic view of a portion of the gas diffusion layer 3 according to the third embodiment, and Fig. 5B is an SEM photograph. The gas diffusion layer 3 according to the third embodiment is composed of conductive particles 31, conductive fibers 32, a polymer resin 33 containing particulate, fibrous, or aggregates of two or more particles, and disintegration assisting particles 34. The disintegration assisting particles 34 rub against the conductive particles 31 and conductive fibers 32 during the material mixing process during manufacturing to disintegrate and uniformly disperse the aggregates, and also impart shear force to the particulate polymer resin 33 at the raw material stage, thereby increasing the degree of fibrousness of the polymer resin.

[0047] <Types of Crushing Aid Particles 34> The gas diffusion layer 3 may further include crushing aid particles 34 having a specific gravity at least twice that of the conductive particles 31. When the specific gravity of the crushing aid particles 34 is at least twice that of the conductive particles 31, crushing of the conductive particles 31 is promoted in the mixing and dispersion step during the production of the gas diffusion layer 3, increasing the surface area and the shear force applied to the polymer resin particles, thereby improving the mechanical strength of the gas diffusion layer 3. The crushing aid particles 34 may be any water-insoluble particle, such as a rare earth oxide, an alkaline earth metal oxide, or a ceramic. More specific examples include a cerium-containing oxide, a manganese-containing oxide, zirconia, and steatite. Among these, it is preferable to use a cerium-containing oxide as the crushing aid particle 34 from the viewpoint of being expected to have a radical quenching effect.

[0048] <Method 1 for Manufacturing Gas Diffusion Layer 3> Next, a method for manufacturing the gas diffusion layer 3 according to the first and second embodiments will be described with reference to FIG. 7 . In step S1 of FIG. 7 , conductive particles 31, conductive fibers 32, polymer resin 33, surfactant, and dispersion solvent are kneaded together. For example, a planetary mixer, a planetary 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 step, the conductive particles 31, conductive fibers 33, surfactant, and dispersion solvent are first kneaded and dispersed, and then the polymer resin 33 is added and stirred, thereby achieving a state in which the polymer resin 33 is uniformly dispersed in the kneaded mixture.

[0049] Furthermore, by kneading the conductive particles 31, conductive fibers 32, and polymer resin 33 in a dispersion solution containing a dispersant in an amount of 1 wt% to 5 wt% (both inclusive), the mixture is kneaded into a paste state, improving dispersibility. If the dispersant amount is less than 1 wt%, the conductive particles 31, conductive fibers 32, and disintegration aid particles 34 are not uniformly dispersed in the dispersion solvent, which may result in uneven structure of the gas diffusion layer 3 and reduced strength. On the other hand, if the dispersant amount exceeds 5 wt%, the dispersant is excessive relative to the conductive particles 31, conductive fibers 32, and disintegration aid particles 34. As a result, when the kneaded mixture is rolled into a sheet in step S2, the materials slide against each other, resulting in insufficient shear force, which may reduce the fiberization of the polymer resin 33 and reduce strength. Furthermore, the fiberization degree of the PTFE polymer resin in the kneaded mixture is less than 50%. If the degree of fiberization of PTFE is 50% or more, the fiberized PTFE will become clumpy (lumpy) during the sheet-forming process, which may cause cracks.

[0050] In step S2 of FIG. 7 , the kneaded material is rolled into a sheet. For example, a rolling mill can be used for the rolling in step S2. For example, the rolling is performed once or multiple times under a pressure of 0.001 ton / cm or more and 4 ton / cm or less, applying shear force to the polymer resin 33 to fiberize it. As described above, the kneaded material obtained in step S1 has the polymer resin 33 uniformly dispersed therein, resulting in the formation of polymer resin fibers 36 within the gas diffusion layer 3. Furthermore, by adjusting the pressure and number of times the kneaded material is rolled, a portion of the polymer resin 33 remains unfiberized as polymer resin particles 35. The PTFE fiberization rate of the sheet produced in this process is, for example, less than 50%.

[0051] In step S3 of FIG. 7 , the sheet-shaped kneaded material is baked to remove the surfactant and dispersion solvent from the kneaded material. For example, an IR oven, a hot air oven, or the like can be used for the baking in step S3. 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 33. 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 3, making the inside of the gas diffusion layer 3 hydrophilic and making it more likely for water to remain, which may reduce the gas permeability of the gas diffusion layer 3. On the other hand, if the baking temperature is higher than the decomposition temperature of the polymer resin 33, the polymer resin 33 decomposes, which may reduce the mechanical strength of the gas diffusion layer 3. Specifically, for example, when PTFE is used as the polymer resin 32, the baking temperature is preferably 280°C or higher and 340°C or lower.

[0052] In step S4 of Fig. 7, the sheet-like kneaded product from which the surfactant and dispersion solvent have been removed is re-rolled using a roll press to adjust the thickness. This allows the gas diffusion layer 3 according to the first embodiment of the present disclosure to be manufactured. For example, a roll press can be used for the re-rolling in step S4. For example, the thickness and porosity of the gas diffusion layer 3 can be adjusted by performing re-rolling once or multiple times under roll press conditions of a pressure of 0.01 ton / cm or more and 4 ton / cm or less. The PTFE fiberization degree of the manufactured gas diffusion layer 3 is 50% or more.

[0053] <Method 2 of Manufacturing Gas Diffusion Layer 3> Next, a method of manufacturing a gas diffusion layer 3 according to a third embodiment will be described with reference to FIG. 8 . In step S1 of FIG. 8 , conductive particles 31, conductive fibers 32, polymer resin 33, disintegration aid particles 34 having a specific gravity at least twice that of the conductive particles, a surfactant, and a dispersion solvent are kneaded together. To knead the materials in step S1, for example, a planetary mixer, a planetary mixer, a kneader, a roll mill, or the like can be used. First, the conductive particles 31, conductive fibers 32, disintegration aid particles 34, surfactant, and dispersion solvent are added, stirred, and kneaded together. The disintegration aid particles 34, which have a specific gravity greater than that of the conductive particles, act as abrasives, promoting the disintegration of the conductive particles 31 and the conductive fibers 32 during stirring through the action of friction and centrifugal force. Then, polymer resin 33 is added, and the mixture is stirred and kneaded again to obtain a mixture in which the polymer resin 33 is uniformly dispersed throughout the mixture.

[0054] Furthermore, by kneading the conductive particles 31, conductive fibers 32, polymer resin 33, and disintegration aid particles 34 in a dispersion solution containing 1 wt % to 5 wt % of dispersant, the mixture becomes paste-like, improving dispersibility. If the dispersant content is less than 1 wt %, the conductive particles 31, conductive fibers 32, and disintegration aid particles 34 are not uniformly dispersed in the dispersion solvent, which can result in uneven structure of the gas diffusion layer 3 and reduced strength. On the other hand, if the dispersant content exceeds 5 wt %, the dispersant content is excessive relative to the conductive particles 31, conductive fibers 32, and disintegration aid particles 34. Therefore, when rolling the mixture into a sheet in step S2, the materials slide against each other, resulting in insufficient shear force, which can reduce the fiberization of the polymer resin 33 and reduce strength. Furthermore, a clay-like mixture can be obtained by adding second conductive particles having a specific surface area between 2 and 20 times that of the conductive particles 31, for example, 10 times or more, to the kneaded mixture and kneading it again. By adjusting the viscosity of the kneaded mixture by selectively adsorbing the dispersant onto the second conductive particles with a high specific surface area, the shear force applied to the polymer resin 33 can be increased and the strength can be increased when the polymer resin 33 is rolled into a sheet in step S2.

[0055] Furthermore, the degree of fiberization of the PTFE, which is the polymer resin of the kneaded product, is less than 50%. If the degree of fiberization of the PTFE is 50% or more, the fiberized PTFE will become clumpy during the sheet-forming process, which will cause cracks.

[0056] In step S2 of FIG. 8 , the kneaded material is rolled and stretched into a sheet. For example, a rolling mill can be used for the rolling in step S2. For example, the rolling is performed once or multiple times under a pressure of 0.001 ton / cm or more and 4 ton / cm or less, applying shear force to the polymer resin 33 to fiberize it. As described above, the kneaded material obtained in step S1 has the polymer resin 33 uniformly dispersed by the disintegration aid particles 34, resulting in the formation of polymer resin fibers 36 within the gas diffusion layer 3. Furthermore, by adjusting the ratio of conductive particles 31, conductive fibers 32, polymer resin 33, and disintegration aid particles 34 in the kneaded material, as well as the pressure and number of times the kneaded material is rolled, a portion of the polymer resin 33 remains unfiberized as polymer resin particles 35. The PTFE fiberization rate of the sheet produced in this process is less than 50%.

[0057] In step S3 of FIG. 8 , the sheet-shaped kneaded material is baked to remove the surfactant and dispersion solvent from the kneaded material. For example, an IR oven, a hot air oven, or the like can be used for the baking in step S3. 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 33. 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 3, making the inside of the gas diffusion layer 3 hydrophilic and making it more likely for water to remain, which may reduce the gas permeability of the gas diffusion layer 3. On the other hand, if the baking temperature is higher than the decomposition temperature of the polymer resin 33, the polymer resin 33 decomposes, which may reduce the mechanical strength of the gas diffusion layer 3. Specifically, for example, when PTFE is used as the polymer resin 32, the baking temperature is preferably 280° C. or higher and 340° C. or lower.

[0058] In step S4 of Fig. 8, the sheet-like kneaded product from which the surfactant and dispersion solvent have been removed is re-rolled using a roll press to adjust the thickness. This allows the gas diffusion layer 3 according to the first embodiment of the present disclosure to be manufactured. For example, a roll press can be used for the re-rolling in step S4. For example, the thickness and porosity of the gas diffusion layer 3 can be adjusted by re-rolling once or multiple times under roll press conditions of a pressure of 0.01 ton / cm or more and 4 ton / cm or less. The PTFE fiberization degree of the manufactured gas diffusion layer 3 is 50% or more.

[0059] The present disclosure is not limited to the above-described first to third embodiments, and can be implemented in various other modes.

[0060] [Examples] Examples of the present disclosure will be described below. The following materials were used, and the evaluations were carried out using the following methods. [Conductive particles 31] Acetylene black (hereinafter referred to as AB) (Denka Black powder, manufactured by Denki Kagaku Kogyo Co., Ltd.) [Conductive fibers 32] VGCF (VGCF-H, manufactured by Showa Denko K.K.) [Polymer resin 33] PTFE dispersion (manufactured by Daikin Kogyo Co., Ltd.), average particle size 0.25 μm [Crushing aid particles 34] Cerium-containing oxide (Cerium oxide-S, manufactured by Taiyo Koko Co., Ltd.) [Dispersion solvent] Surfactant (Newcoal, manufactured by Nippon Nyukazai Co., Ltd.)

[0061] (Production of Gas Diffusion Layers in Examples 1 to 5 and Comparative Example 1) The gas diffusion layers in Examples 1 to 5 were produced as follows. (1) First, conductive particles, conductive fibers, polymer resin, and disintegration aid particles were blended in the proportions shown in the raw material composition in Table 1, and kneaded using a planetary mixer with the amount of dispersion solvent added shown in Table 1. (2) Next, using a rolling mill, the kneaded mixture was rolled five times under the rolling condition of 0.1 ton / cm. (3) Thereafter, the rolled sheet was placed in an IR furnace and fired for 0.5 hours at the temperature shown in Table 1. (4) The fired sheet was re-rolled 3 to 10 times using a roll press under the rolling condition of 1 ton / cm to obtain a gas diffusion layer with a thickness of 100 μm.

[0062] (Evaluation Test) The tensile breaking strength of the gas diffusion layer was measured in Examples 1 to 5 and Comparative Example 1. The raw material conditions and evaluation results in Examples 1 to 5 and Comparative Example are shown in Table 1. The tensile breaking strength was measured by punching the gas diffusion layer into a dumbbell test piece (dumbbell shape No. 4) specified in JIS K6251 using a Thomson die, and using a tension and compression tester (SVZ-200NB model manufactured by Imada Seisakusho).

[0063] (Measurement of Degree of Fibrosis and Degree of Aggregation) In Examples 1 to 5 and Comparative Example 1, the degree of fibrousness and the amount of fiber of the gas diffusion layer were calculated by the method described in <Method for calculating the degree of fibrousness and amount of fiber of polymer resin of gas diffusion layer 3>. The calculation results in Examples 1 to 5 and Comparative Example are shown in Table 1.

[0064] (Observation of the State of PTFE) The surface or cross section was observed using an SEM to observe the state in which PTFE was present.

[0065]

[0066] The gas diffusion layer according to the present disclosure is particularly useful as a component for use in fuel cells, and can be used in applications such as domestic cogeneration systems, fuel cells for automobiles, mobile fuel cells, and backup fuel cells.

[0067] REFERENCE SIGNS LIST 100 Fuel cell 1 Polymer electrolyte membrane 2 Catalyst layer 2a Anode catalyst layer 2b Cathode catalyst layer 3 Gas diffusion layer 3a Anode-side gas diffusion layer 3b Cathode gas diffusion layer 4 Separator 4a Anode-side separator 4b Cathode-side separator 5 Fluid flow path 6 Rib portion 10 Battery cell 11 Current collector plate 12 Insulating plate 13 End plate 20 Membrane electrode assembly 31 Conductive particle 32 Conductive fiber 33 Polymer resin 34 Disintegration aid particle 35 Single particle polymer resin 36 Fibrous polymer resin 37 Polymer resin formed by agglomeration and fusion of two or more single particles 38 Polymer resin formed by melting polymer resin particles to form a film

Claims

1. A gas diffusion layer comprising a porous member containing conductive particles, conductive fibers, and a polymer resin, the porous member containing particulate polymer resin, and a portion of the particulate polymer resin being present in the form of two or more fused particles.

2. The gas diffusion layer according to claim 1, wherein the porous member contains a fibrous polymer resin, and when the degree of fiberization of the polymer resin is defined as the proportion of the polymer resin that exists in a fibrous form, the degree of fiberization of the polymer resin is 50% or more.

3. The gas diffusion layer according to claim 1, wherein the porous member contains fibrous polymer resin, and the amount of fibers as a ratio of the fibrous polymer resin to the entire porous member is 3 wt % or more.

4. The gas diffusion layer according to claim 1, wherein the porous member contains particulate polymer resin, and a portion of the particulate polymer resin is present in the form of a film in the form of two or more melted particles.

5. The gas diffusion layer according to claim 1, further comprising crushing auxiliary particles having a specific gravity of 2 to 20 times that of the conductive particles.

6. The gas diffusion layer according to claim 5, wherein the crushing aid particles are cerium-containing oxides.

7. The gas diffusion layer according to claim 1, wherein the porous member contains 5 wt % or more and less than 35 wt % of the conductive particles, 35 wt % or more and 80 wt % or less of the conductive fibers, 10 wt % or more and 40 wt % or less of the polymer resin, and 0 wt % or more and 30 wt % or less of the disintegration auxiliary particles.

8. The gas diffusion layer according to claim 1, wherein the polymer resin includes polytetrafluoroethylene.

9. A membrane electrode assembly comprising: the gas diffusion layer according to any one of claims 1 to 8; a pair of electrodes; and an electrolyte membrane.

10. A fuel cell comprising the membrane electrode assembly according to claim 9 and a current collector plate.

11. A method for manufacturing a gas diffusion layer, comprising: kneading conductive particles, conductive fibers, and a polymer resin in a dispersion solvent to obtain a kneaded product in which the polymer resin has a fiberization degree of 50% or less; and rolling the kneaded product to obtain a sheet in which the polymer resin has a fiberization degree of less than 50%.

12. The method for producing a gas diffusion layer according to claim 11, wherein in the step of obtaining the kneaded mixture, the conductive particles, the conductive fibers and the polymer resin are kneaded in a dispersion solution having a dispersant content of 1 wt % or more and 5 wt % or less.

13. The method for producing a gas diffusion layer according to claim 11, further comprising the step of firing the sheet having a polymer resin fibrous degree of less than 50% to obtain a gas diffusion layer having a polymer resin fibrous degree of 50% or more.

14. A method for manufacturing a gas diffusion layer, comprising: a step of kneading conductive particles, conductive fibers, a polymer resin, and crushing auxiliary particles having a specific gravity at least twice that of the conductive particles, and crushing and dispersing the conductive particles, the conductive fibers, and the polymer resin using the crushing auxiliary particles to obtain a kneaded mixture; and a step of rolling the kneaded mixture obtained by kneading and forming it into a sheet, to obtain a gas diffusion layer made of the sheet.

15. A method for manufacturing a gas diffusion layer, comprising the steps of: kneading conductive particles, conductive fibers, and a polymer resin in a dispersion solution having a dispersant addition amount of 1 wt % or more and 5 wt % or less to obtain a kneaded mixture; adding second conductive particles having a specific surface area that is 2 times or more and 20 times or less than that of the conductive particles to the kneaded mixture obtained by kneading, and kneading the mixture; and rolling the kneaded mixture into a sheet to obtain a gas diffusion layer made of a sheet.