Carbon fiber sheet, fuel cell separator, and method for manufacturing fuel cell separator

Abstract

Provided is a carbon fiber sheet that tends to have stable electoconductivity and planar direction strength, and that is suitable for constituting fuel cell members including fuel cell separators and the like.　An embodiment of the present invention is a carbon fiber sheet containing carbon fibers and electroconductive particles. The carbon fiber sheet has a basis weight coefficient of variation of 0.10 or less, as calculated by the following measurement method. (Measurement Method) Test pieces of 40 mm × 40 mm are sampled at 20 random locations of the carbon fiber sheet. The mass of each test piece is measured and the basis weight is calculated. The average value and the standard deviation of the basis weight of the test pieces are determined with n = 20, and the basis weight coefficient of variation is calculated by dividing the standard deviation by the average value.

Description

Carbon fiber sheet, fuel cell separator, and method for manufacturing a fuel cell separator

[0001] The present invention relates to a carbon fiber sheet, a fuel cell separator, and a method for manufacturing a fuel cell separator.

[0002] Conventionally, a molded product containing carbon fiber and a resin component has been used as a separator for a fuel cell.

[0003] For example, Patent Document 1 discloses a fuel cell separator in which a composition containing 100 parts by mass of a porous artificial graphite material, 15 to 30 parts by mass of a thermosetting resin, and 0.1 to 1.0 part by mass of an internal mold release agent is formed into a carbon fiber sheet by compression molding, injection molding, transfer molding, or the like.

[0004] Japanese Patent Application Laid-Open No. 2007-134225

[0005] However, when carbon fiber sheets according to conventional technology are used as fuel cell components, including fuel cell separators, the electrical conductivity and strength in the planar direction are difficult to stabilize, and there are cases in which the desired strength and electrical conductivity cannot be obtained.

[0006] Therefore, an object of the present invention is to provide a carbon fiber sheet that is likely to have stable electrical conductivity and strength in the plane direction and is suitable for constituting fuel cell components, including fuel cell separators.

[0007] The present inventors have found that the above problems can be solved by using a carbon fiber sheet having a specific structure, and have completed the present invention.

[0008] One aspect of the present invention is a carbon fiber sheet. The carbon fiber sheet includes carbon fibers and conductive particles. The carbon fiber sheet has a coefficient of variation of basis weight of 0.10 or less, calculated by the following measurement method. (Measurement Method) 40 mm x 40 mm test pieces are sampled from 20 random locations on the carbon fiber sheet. The mass of each test piece is measured, and the basis weight is calculated. The average and standard deviation of the basis weights of the test pieces are determined with n=20, and the coefficient of variation of the basis weight is calculated by dividing the standard deviation by the average.

[0009] The carbon fiber sheet preferably has a coefficient of variation of the weight loss rate (nitrogen atmosphere) of 0.08 or less, as calculated by the following measurement method. (Measurement Method) 10 mg test pieces are sampled at 20 random locations on the carbon fiber sheet. Using thermogravimetric analysis, the temperature is increased in a nitrogen atmosphere between 100 and 600°C at a rate of 10°C / min, and the weight loss rate of each test piece is measured after holding at 600°C for 30 minutes. The average and standard deviation of the weight loss rates of each test piece are determined (n=20), and the coefficient of variation of the weight loss rate (nitrogen atmosphere) is calculated by dividing the standard deviation by the average.

[0010] The carbon fiber sheet preferably has a coefficient of variation of the weight loss rate (in the air atmosphere) of 0.15 or less, as calculated by the following measurement method. (Measurement Method) 10 mg test pieces are sampled at 20 random locations on the carbon fiber sheet. Using thermogravimetric analysis, the temperature is increased in the air atmosphere between 600 and 800°C at a rate of 10°C / min, and the test pieces are held at 800°C for 10 minutes, after which the weight loss rate of each test piece is measured. The average and standard deviation of the weight loss rates of each test piece are determined (n=20), and the coefficient of variation of the weight loss rate (in the air atmosphere) is calculated by dividing the standard deviation by the average.

[0011] The carbon fiber sheet preferably has a coefficient of variation of surface resistivity of 0.10 or less, as calculated by the following measurement method. (Measurement Method) Test pieces of 3 cm x 3 cm are sampled at 20 random locations on the carbon fiber sheet. The surface resistivity (Ω / cm) of each test piece is measured by the Van der Pauw method using a surface resistance measuring device. ２ The average value and standard deviation of the surface resistivity of each test piece are determined with n=20, and the standard deviation is divided by the average value to calculate the coefficient of variation of the surface resistivity.

[0012] It is preferable that the average fiber length of the carbon fibers is 6 mm or less. The average loading amount (basis weight) of the carbon fibers in the carbon fiber sheet is 35.0 g / m ２Preferably, the conductive particles have an average particle size of 500 μm or less. Preferably, the carbon fiber sheet is for use as a fuel cell component. Preferably, the carbon fiber sheet is a precursor for a fuel cell separator or a precursor for a fuel cell gas diffusion layer.

[0013] Another aspect of the present invention is a fuel cell separator or a gas diffusion layer for a fuel cell. The fuel cell separator or gas diffusion layer includes the carbon fiber sheet and a resin component cured while being held by the carbon fiber sheet. The fuel cell separator or gas diffusion layer is molded so as to have a wavy cross section.

[0014] Yet another aspect of the present invention is a method for producing a fuel cell separator or a gas diffusion layer for a fuel cell, comprising the steps of: impregnating the carbon fiber sheet with a resin component; and press-molding the carbon fiber sheet in a state where the resin component is impregnated into the carbon fiber sheet.

[0015] According to the present invention, a carbon fiber sheet is provided which is likely to have stable electrical conductivity and strength in the planar direction and is suitable for constituting fuel cell members including fuel cell separators.

[0016] Fig. 1 is a conceptual perspective view of a fuel cell separator according to the present disclosure. Fig. 2 is a conceptual cross-sectional view of a fuel cell separator according to the present disclosure. Fig. 3 is a schematic view of a membrane electrode assembly according to the present disclosure. Fig. 4 is a schematic view of a cell stack according to the present disclosure.

[0017] Hereinafter, when an upper limit value and a lower limit value are separately described, it is considered that a numerical range combining any upper limit value and any lower limit value is substantially disclosed.

[0018] In the following, unless otherwise specified, various measurements are carried out at room temperature (23° C.).

[0019] The composition, structure / physical properties, manufacturing method, and applications / use methods of the carbon fiber sheet according to the present disclosure will be described below, but the present invention is not limited to the following.

[0020] <<Composition>> The carbon fiber sheet according to the present disclosure includes carbon fibers and conductive particles. The carbon fiber sheet according to the present disclosure may also include other components. More specifically, the carbon fiber sheet according to the present disclosure is configured by supporting conductive particles and other components on a nonwoven fabric formed from carbon fibers. Each component will be described below.

[0021] <Carbon fiber> The average fiber length of the carbon fiber is preferably 6 mm or less. The lower limit of the average fiber length of the carbon fiber is, for example, 0.1 mm. By setting the average fiber length of the carbon fiber in this range, it becomes easier to obtain a carbon fiber sheet that is excellent in strength and has uniform conductivity.

[0022] In the present disclosure, the term "carbon fiber" refers to a carbon material that can generally be considered to be fibrous. More specifically, the term refers to a carbon material having a fiber diameter of 1 μm or more and an aspect ratio (fiber length / fiber diameter) of 50 / 11 or more. Carbon materials having a fiber diameter of less than 1 μm and an aspect ratio of less than 50 / 11 may be referred to as carbon particles.

[0023] The average fiber length of the carbon fibers was calculated by measuring the fiber lengths of 50 carbon fibers constituting the carbon fiber sheet visually or using an optical microscope, and averaging the measured fiber lengths. The fiber diameter of the carbon fibers can also be measured in the same manner.

[0024] The average loading amount (basis weight) of the carbon fiber in the carbon fiber sheet is 50.0 g / m ２ Below, 35.0g / m ２ or less, or 25.0 g / m ２ The lower limit of the average loading amount is, for example, 1.0 g / m ２ , 3.0 g / m ２ , or 5.0 g / m ２By setting the loading basis weight of the carbon fibers within this range, it becomes easy to obtain a carbon fiber sheet that is excellent in strength, toughness, texture, etc. and has uniform conductivity.

[0025] The carbon fiber content in the carbon fiber sheet is not particularly limited. For example, when the entire carbon fiber sheet is taken as 100% by mass, the carbon fiber content is preferably 1% by mass or more, or 2% by mass or more, and is preferably 20% by mass or less, 15% by mass or less, or 10% by mass or less.

[0026] <Conductive Particles> The conductive particles are not particularly limited as long as they are conductive. Examples of conductive particles include metal-based materials such as gold, silver, copper, nickel, tin, lead, zinc, bismuth, and alloys thereof, as well as carbon-based materials such as carbon black, graphite, carbon nanotubes, and fullerenes.

[0027] The shape of the conductive particles is not particularly limited and may be, for example, spherical, plate-like, linear, flake-like, or scale-like.

[0028] The average particle size of the conductive particles is preferably 500 μm or less, or 200 μm or less. The lower limit of the average particle size of the conductive particles is not particularly limited. By setting the average particle size of the conductive particles within this range, particle detachment is easily prevented, and a carbon fiber sheet with excellent conductivity, strength, etc. can be easily obtained.

[0029] The average particle diameter of the conductive particles was calculated by measuring the particle diameters (maximum diameters) of 50 conductive particles contained in the carbon fiber sheet using an optical microscope or an electron microscope, and then calculating the average value.

[0030] When the entire carbon fiber sheet is taken as 100 mass%, the content of the conductive particles is preferably 30 mass% or more, 40 mass% or more, or 50 mass% or more, and is preferably 95 mass% or less, 90 mass% or less, or 85 mass% or less. By setting the content of the conductive particles within such ranges, it becomes easier to obtain a carbon fiber sheet with excellent conductivity, strength, etc.

[0031] The ratio of the conductive particle content to the carbon fiber content in the carbon fiber sheet [(conductive particle content) / (carbon fiber content)] is preferably 1 or more, 5 or more, or 8 or more, and is preferably 100 or less, 80 or less, 50 or less, or 30 or less.

[0032] <Other Components> The carbon fiber sheet may contain other components within the scope of not impairing the effects of the present invention. Examples of other components include surfactants, dispersants, thickeners, antifoaming agents, paper strength agents, inorganic binders, organic binders, aggregation aids, and fiber materials other than carbon fiber.

[0033] The carbon fiber sheet preferably includes a fiber material. The fiber material may be organic or inorganic. The organic fiber may be synthetic or natural. Examples of materials constituting the synthetic fiber include olefin-based resins such as polyethylene fiber and polypropylene fiber, fluorine-based resins such as PTFE, polyamide, polyimide, polyacrylonitrile, styrene and its copolymers, and acrylic ester and its copolymers.

[0034] The content of other components is, for example, 0.1 mass% or more, 0.5 mass% or more, 1 mass% or more, 5 mass% or more, or 10 mass% or more, and 60 mass% or less, 50 mass% or less, 40 mass% or less, 30 mass% or less, or 20 mass% or less, when the entire carbon fiber sheet is taken as 100 mass%.

[0035] Here, when the carbon fiber sheet contains organic fibers, the ratio of the conductive particle content to the organic fiber content in the carbon fiber sheet [(conductive particle content) / (organic fiber content)] is preferably 0.1 or more, 0.5 or more, 1 or more, or 2 or more, and is also preferably 100 or less, 50 or less, 20 or less, or 10 or less.

[0036] <<Structure / Physical Properties>> <Change in Basis Weight> The carbon fiber sheet according to the present disclosure preferably has a coefficient of variation in basis weight calculated by the following measurement method of 0.10 or less, or 0.09 or less. The coefficient of variation in basis weight is measured according to the following measurement method. The lower limit of the coefficient of variation in basis weight is not particularly limited, but is, for example, 0.01 or 0.02.

[0037] (Measurement method) 40 mm x 40 mm test pieces are sampled at 20 random locations on the carbon fiber sheet. The mass of each test piece is measured and the basis weight is calculated. The average and standard deviation of the basis weight of each test piece are determined with n = 20, and the standard deviation is divided by the average to calculate the coefficient of variation of the basis weight.

[0038] When a fuel cell separator is molded using a carbon fiber sheet, a process of impregnating the carbon fiber sheet with a resin component is carried out, as described below. The inventors discovered that the variation in basis weight of the carbon fiber sheet in the in-plane direction significantly affects the degree of impregnation with the insulating resin and, further, significantly affects the conductivity and other properties of the resulting molded product (fuel cell separator). Based on this discovery, the inventors further discovered that by keeping the variation in basis weight of the carbon fiber sheet in the in-plane direction within a predetermined range, the physical properties of the carbon fiber sheet, such as its conductivity, can be improved. Furthermore, by keeping the variation in basis weight of the carbon fiber sheet in the in-plane direction within a predetermined range, the density variation in the in-plane direction is reduced in the hot compression molding process following the resin impregnation. This reduces the variation in the gas barrier properties required for fuel cell separators, and is expected to improve the performance of the fuel cell. Furthermore, uniform pressure is more easily applied in the pressing process described below, making it easier to obtain a separator with excellent performance.

[0039] <Weight Loss Rate (Nitrogen Atmosphere)> From the viewpoint of improving electrical conductivity, etc., the carbon fiber sheet according to the present disclosure preferably has a coefficient of variation of the weight loss rate (nitrogen atmosphere) calculated by the following measurement method of 0.12 or less, 0.08 or less, or 0.06 or less. The coefficient of variation of the weight loss rate (nitrogen atmosphere) is measured according to the following measurement method.

[0040] (Measurement Method) 10 mg test pieces are sampled from 20 random locations on a carbon fiber sheet. Using thermogravimetric analysis, the temperature is raised in a nitrogen atmosphere between 100 and 600°C at a temperature rise rate of 10°C / min, and the weight loss rate of each test piece is measured after holding at 600°C for 30 minutes. The average and standard deviation of the weight loss rates for each test piece are determined (n=20), and the coefficient of variation of the weight loss rate (nitrogen atmosphere) is calculated by dividing the standard deviation by the average.

[0041] <Weight Loss Rate (Air Atmosphere)> From the viewpoint of improving electrical conductivity, etc., the carbon fiber sheet according to the present disclosure preferably has a coefficient of variation of the weight loss rate (air atmosphere) calculated by the following measurement method of 0.20 or less, 0.15 or less, 0.10 or less, or 0.08 or less. The coefficient of variation of the weight loss rate (air atmosphere) is measured according to the following measurement method.

[0042] (Measurement Method) 10 mg test pieces are sampled from 20 random locations on a carbon fiber sheet. Using thermogravimetric analysis, the temperature is raised in an air atmosphere between 600 and 800°C at a rate of 10°C / min, and the test pieces are held at 800°C for 10 minutes, after which the weight loss rate of each test piece is measured. The average and standard deviation of the weight loss rates for each test piece are determined (n=20), and the coefficient of variation of the weight loss rate (air atmosphere) is calculated by dividing the standard deviation by the average.

[0043] <Surface Resistivity> The carbon fiber sheet according to the present disclosure preferably has a coefficient of variation of surface resistivity calculated by the following measurement method of 0.15 or less, 0.12 or less, 0.10 or less, or 0.08 or less.

[0044] (Measurement Method) Test pieces of 3 cm x 3 cm are sampled at 20 random locations on the carbon fiber sheet. The surface resistivity (Ω / cm) of each test piece is measured by the Van der Pauw method. ２) is measured. For measuring the surface resistivity using the Van der Pauw method, the method disclosed in WO2018 / 131658 and the like can be referenced, for example. The average value and standard deviation of the surface resistivity of each test piece are determined with n=20, and the standard deviation is divided by the average value to calculate the coefficient of variation of the surface resistivity. (Measuring device) DC current supply device Manufacturer: KENWOOD Product number: PA250-0.25A Voltage measuring device Manufacturer: KEITHLEY Product number: 617 programmable electrometer

[0045] The thickness and basis weight of the carbon fiber sheet may be adjusted as appropriate depending on the application.

[0046] <<Manufacturing Method>> An example of a method for manufacturing a carbon fiber sheet according to the present disclosure will be described below.

[0047] The carbon fiber sheet according to the present disclosure can be produced by wet papermaking. More specifically, the carbon fiber sheet according to the present disclosure can be obtained by carrying out a raw material preparation process, a papermaking process, and a drying process. Each process will be described below.

[0048] <Raw Material Preparation Step> The raw materials (carbon fiber, conductive particles, other components, liquid medium, etc.) are uniformly dispersed to prepare a raw material slurry. The carbon fiber, conductive particles, and other components are as described above.

[0049] The liquid medium is usually water. The liquid medium may be a liquid medium other than water that can be used for papermaking (for example, an aqueous medium such as alcohol), or a mixture of water and a liquid medium other than water. The solids concentration in the raw material slurry may be appropriately set taking into consideration the ease of production, production costs, etc., and is not particularly limited.

[0050] <Papermaking Process> The raw material slurry is made into paper using a known papermaking machine to form a moisture-containing fiber sheet (wet sheet).

[0051] The paper machine used in the papermaking process is not particularly limited and may be any paper machine that is applied to general papermaking techniques, such as a Fourdrinier paper machine, a short wire paper machine, a cylinder paper machine, a tilting paper machine, a twin-wire paper machine, or a combination paper machine formed by combining the same or different types of paper machines among these.

[0052] <Drying Step> The moisture contained in the wet sheet is dried to form a carbon fiber sheet.

[0053] The drying device used in the drying step is not particularly limited and may be any device commonly used for drying carbon fiber sheets, such as a Yankee dryer, rotary dryer, hand dryer, air dryer, cylinder dryer, suction drum dryer, or infrared dryer.

[0054] The drying temperature is not particularly limited, and may be set to 100 to 300°C.

[0055] The drying time is not particularly limited and can be adjusted so that the moisture content in the wet sheet is sufficiently low.

[0056] Here, the coefficient of variation of the basis weight and weight loss rate of the carbon fiber sheet can be adjusted, for example, as follows: During the papermaking process, by generating turbulence in the slurry supply pipe in a state in which the conductive particles are sufficiently aggregated and fixed to fibers other than carbon fiber (for example, organic fibers), uneven concentration and pressure are eliminated, and structural variations in the inorganic fiber sheet (coefficient of variation of the basis weight and weight loss rate of the carbon fiber sheet) are reduced.

[0057] <<Applications / Method of Use>> The carbon fiber sheet according to the present disclosure can be used for various applications. The carbon fiber sheet according to the present disclosure can be preferably used as a component constituting a fuel cell (fuel cell component). In particular, the carbon fiber sheet according to the present disclosure is preferably used as a precursor for a fuel cell separator or a fuel cell gas diffusion layer, and is particularly preferably used as a precursor for a fuel cell separator. Hereinafter, a fuel cell separator or a fuel cell gas diffusion layer obtained using the carbon fiber sheet according to the present disclosure will be described.

[0058] <Fuel Cell Separator> The fuel cell separator includes a carbon fiber sheet and a resin component that is hardened while being held by the carbon fiber sheet.

[0059] A conceptual perspective view of a fuel cell separator is shown in Fig. 1. As shown in Fig. 1, the fuel cell separator is molded so as to have a wavy shape when viewed in cross section.

[0060] In other words, a fuel cell separator is a molded product obtained by processing a carbon fiber sheet into a predetermined shape (a shape that appears wavy when viewed in cross section) while it is impregnated with an uncured resin component, and then curing the resin component.

[0061] The wave shape referred to here means any waveform such as a sine wave shape as shown in FIG. 2(a), a triangular wave shape as shown in FIG. 2(b), or a square wave shape as shown in FIG. 2(c).

[0062] The carbon fiber sheet has been described above and will not be described here.

[0063] The resin component may be any known resin component that can be used to form a fuel cell separator, such as a thermosetting resin such as an epoxy resin, a phenol resin, a polyimide resin, a melamine resin, or a urethane resin.

[0064] The amount of resin component impregnated into the carbon fiber sheet (or the content of the resin component in the fuel cell separator) is not particularly limited, as long as the fuel cell separator contains a sufficient amount of resin component to have gas barrier properties (to the extent that gases cannot or have difficulty passing through).

[0065] The fuel cell separator can be produced, for example, by carrying out a step of press-molding a carbon fiber sheet in a state where the carbon fiber sheet is impregnated with a resin component.

[0066] When the resin component is a thermosetting resin, the resin component is hardened while being held by the carbon fiber sheet by hot pressing during press molding or by heating the resin component after press molding, thereby forming a fuel cell separator. By adjusting the mold for such press molding, the shape of the fuel cell separator (the wave shape when viewed in cross section) can be adjusted.

[0067] The pressure and time of press molding, the temperature and time for thermally curing the resin component, etc. can be changed as appropriate depending on the material used.

[0068] <Gas Diffusion Layer for Fuel Cell> As the gas diffusion layer for the fuel cell, it is possible to use one having the same structure as that explained for the fuel cell separator.

[0069] <Membrane Electrode Assembly / Cell Stack> Next, an example of a membrane electrode assembly constituting a fuel cell and a cell stack obtained by stacking a plurality of membrane electrode assemblies will be described as a specific article including a fuel cell separator.

[0070] As shown in FIG. 3 , the membrane electrode assembly 100 includes an electrolyte membrane 30, an anode 10 laminated on one main surface of the electrolyte membrane 30, an air electrode 20 laminated on the other main surface of the electrolyte membrane 30 (the main surface opposite to the main surface on which the anode 10 is laminated), a separator 40 (first separator 41) laminated on the surface of the anode 10 opposite to the surface on which the electrolyte membrane 30 is laminated, and a separator 40 (second separator 42) laminated on the surface of the air electrode 20 opposite to the surface on which the electrolyte membrane 30 is laminated.

[0071] The electrolyte membrane 30 may be, for example, a solid polymer electrolyte membrane such as a fluorine-based electrolyte membrane or a hydrocarbon-based electrolyte membrane. The electrolyte membrane 30 may be an oxygen ion conductor or a hydrogen ion conductor.

[0072] The fuel electrode 10 includes a first catalyst layer 11 in contact with the electrolyte membrane 30 and a first gas diffusion layer 12 laminated on the first catalyst layer 11 .

[0073] The air electrode 20 includes a second catalyst layer 21 in contact with the electrolyte membrane 30 and a second gas diffusion layer 22 laminated on the second catalyst layer 21 .

[0074] Conventionally known gas diffusion layers can be used for the first gas diffusion layer 12 and the second gas diffusion layer 22. Examples of gas diffusion layers include conductive porous materials such as carbon fiber sheets.

[0075] Conventionally known catalyst layers can be used for the first catalyst layer 11 and the second catalyst layer 21. The catalyst layer is, for example, a porous layer made of a catalyst (e.g., a granular catalyst material). Examples of the catalyst material include metal materials such as platinum, palladium, ruthenium, iridium, rhodium, osmium, iron, lead, copper, chromium, cobalt, nickel, manganese, vanadium, molybdenum, gallium, and aluminum, as well as materials containing carbon materials.

[0076] The first separator 41 and the second separator 42 are separators (fuel cell separators) obtained by molding the carbon fiber sheet according to the present disclosure. As described above, the first separator 41 and the second separator 42 are molded so as to have a wavy shape when viewed in cross section. The uneven portions along the wavy shape of the first separator 41 and the second separator form flow paths that guide gas (a gas containing hydrogen or a gas containing oxygen) into the membrane electrode assembly 100.

[0077] In a fuel cell using the membrane electrode assembly 100 shown in FIG. 3 , reactions occur as follows. Hydrogen is guided to the first gas diffusion layer 12 through grooves in the first separator 41. The hydrogen introduced into the first gas diffusion layer 12 is uniformed in the surface direction and reaches the first catalyst layer 11. Similarly, oxygen is introduced into the second gas diffusion layer 22 through grooves in the second separator 42. The oxygen introduced into the second gas diffusion layer 22 is uniformed in the surface direction and reaches the second catalyst layer 21. When the electrolyte membrane 30 is a hydrogen ion conductor, hydrogen ions and electrons are generated from hydrogen in the first catalyst layer 11. The generated hydrogen ions pass through the electrolyte membrane 30 and reach the second catalyst layer 21, where the hydrogen ions, oxygen, and electrons react to generate water. This water is discharged to the outside through the fuel electrode 10. When the electrolyte membrane 30 is an oxygen ion conductor, oxygen ions are generated from oxygen and electrons in the second catalyst layer 21, and the generated oxygen ions pass through the electrolyte membrane 30 to reach the first catalyst layer 11. In the first catalyst layer 11, hydrogen and the oxygen ions react to generate water and electrons. This water is discharged to the outside via the air electrode 20.

[0078] Furthermore, as shown in FIG. 4, a cell stack 200 including a plurality of membrane electrode assemblies 100 may be formed.

[0079] The cell stack 200 is configured by sequentially stacking a separator 40, an anode 10, an electrolyte membrane 30, and a cathode 20, and further stacking a plurality of such stacks. The separator 40 interposed between the anode 10 and the cathode 20 is molded so as to have a corrugated shape when viewed in cross section, and therefore has irregularities on both the front and back principal surfaces, and can function as the separator 40 in both adjacent membrane electrode assemblies (e.g., membrane electrode assembly 101 and membrane electrode assembly 102). That is, as shown in FIG. 4 , one separator 40 can be configured to serve as the separator 40 on the cathode 20 side of a given membrane electrode assembly 101, and also as the separator 40 on the anode 10 side of a membrane electrode assembly 102 adjacent to the membrane electrode assembly 101.

[0080] When the membrane electrode assembly 100 or the cell stack 200 is incorporated into a fuel cell, the fuel cell may further include other components such as a current collector, a gas supply device, a cooling device, and an outer shell.

[0081] The above describes a membrane electrode assembly and a cell stack including a fuel cell separator using a carbon fiber sheet according to the present disclosure and a conventionally known gas diffusion layer for a fuel cell.

[0082] The membrane electrode assembly and the cell stack may include a fuel cell gas diffusion layer using the carbon fiber sheet according to the present disclosure as the fuel cell gas diffusion layer. In this case, the fuel cell separator may be a fuel cell separator using the carbon fiber sheet according to the present disclosure, or may be a conventionally known fuel cell separator.

[0083] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following in any way.

[0084] <<Preparation of Carbon Fiber Sheet>> Carbon fibers, conductive particles, and other components (aramid fibers) were mixed so that the content and basis weight of each component in the resulting carbon fiber sheet would be the proportions shown in Tables 1 and 2, to prepare a raw material slurry.

[0085] A paper sheet was obtained from the raw material slurry using an inclined paper machine.

[0086] The paper sheet was dried using a Yankee dryer to obtain carbon fiber sheets according to each of the examples and comparative examples.

[0087] During papermaking, the following methods 1 to 4 were used to control the structural variation in the resulting carbon fiber sheet. The methods used in each example are shown in Tables 1 and 2.

[0088] (Method 1) The slurry is supplied so that the flow rate / concentration variations are small, with the conductive particles sufficiently coagulated and fixed to the aramid fibers. Specifically, turbulence is generated in the slurry supply pipe immediately before the papermaking head box to eliminate concentration and pressure variations. The slurry is supplied to the head box as is, and dewatered to prevent the slurry from moving across the width of the head box, to form a web. (Method 2) The slurry is supplied without controlling the flow rate / concentration variations, or with the conductive particles firmly coagulated and fixed to the aramid fibers to form coarse flocs. (Method 3) The slurry is supplied so that the flow rate / concentration variations are small, with the conductive particles not sufficiently coagulated and fixed to the aramid fibers, or with the slurry not sufficiently dispersed. (Method 4) The slurry is supplied so that the flow rate / concentration variations are small, with the conductive particles not sufficiently coagulated and fixed to the aramid fibers, or with the conductive particles firmly coagulated and fixed to the aramid fibers to form coarse flocs.

[0089] <<Structure / Physical Properties>> The coefficient of variation of the basis weight, the coefficient of variation of the weight loss rate (in a nitrogen atmosphere), the coefficient of variation of the weight loss rate (in an air atmosphere), and the coefficient of variation of the surface resistivity of the obtained carbon fiber sheet were measured using the methods described above. The measurement results are shown in Tables 1 and 2.

[0090] <<Evaluation>> <Variation in Electrical Resistivity> Variation in electrical resistivity was evaluated based on the following measurement method and evaluation criteria. The evaluation results are shown in Tables 1 and 2.

[0091] (Evaluation method) Carbon fiber sheet (basis weight: 190 g / m ２100 parts by mass of the carbon fiber sheet was impregnated with 24 parts by mass of phenolic resin, and then hot press molded at 180°C to a sheet thickness of 0.15 mm, and cut into a size of 30 mm x 30 mm to obtain an evaluation sample. Surface resistance (sheet resistance) was measured by the Van der Pauw method using the following surface resistance measurement device. Multiple evaluation samples were taken from the carbon fiber sheet, and the coefficient of variation of surface resistivity (n = 20) was calculated. DC current supply device (manufacturer: KENWOOD, product number: PA250-0.25A) Voltage measurement device (manufacturer: KEITHLEY, product number: 617 programmable electrometer)

[0092] (Evaluation Criteria) Evaluation was made according to the coefficient of variation of the surface resistivity as follows: ⊚: 0.07 or less ◯: more than 0.07 and 0.10 or less △: more than 0.10

[0093] <Strength Variation in Sheet Plane Direction> The strength variation in the sheet plane direction was evaluated based on the following measurement method and evaluation criteria. The evaluation results are shown in Tables 1 and 2.

[0094] (Evaluation method) Carbon fiber sheet (basis weight: 190 g / m ２ 100 parts by mass of the carbon fiber sheet was impregnated with 24 parts by mass of phenolic resin, and then hot press molded at 180°C to a sheet thickness of 0.15 mm, and cut into a size of 80 mm in length and 10 mm in width to obtain an evaluation sample. The bending strength of the evaluation sample was measured in accordance with ISO 178. Multiple evaluation samples were taken from the carbon fiber sheet, and the coefficient of variation (n = 20) of bending strength was determined.

[0095] (Evaluation Criteria) Evaluation was made according to the coefficient of variation of bending strength as follows: ⊚: 0.10 or less ◯: more than 0.10 and 0.13 or less △: more than 0.13 and 0.15 or less ×: more than 0.15

[0096]

[0097]

[0098] The carbon fiber sheet according to the present invention is preferably used as a component for a fuel cell, including a fuel cell separator, because the electrical conductivity and strength in the plane direction are likely to be stable. CROSS-REFERENCE TO RELATED APPLICATIONS

[0099] This application claims priority based on Japanese Patent Application No. 2023-186444, filed with the Japan Patent Office on October 31, 2023, the entire disclosure of which is incorporated herein by reference in its entirety.

[0100] REFERENCE SIGNS LIST 10 fuel electrode 11 first catalyst layer 12 first gas diffusion layer 20 air electrode 21 second catalyst layer 22 second gas diffusion layer 30 electrolyte membrane 40 separator 100 membrane electrode assembly 200 cell stack

Claims

1. A carbon fiber sheet containing carbon fibers and conductive particles, characterized in that the coefficient of variation of the basis weight of the carbon fiber sheet calculated by the following measurement method is 0.10 or less. (Measurement method) Sample 40 mm x 40 mm test pieces at 20 random locations on the carbon fiber sheet. Measure the mass of each test piece and calculate the basis weight. Determine the average and standard deviation of the basis weight of each test piece (n=20), and calculate the coefficient of variation of the basis weight by dividing the standard deviation by the average.

2. The carbon fiber sheet according to claim 1, wherein the coefficient of variation of the weight loss rate (nitrogen atmosphere) calculated by the following measurement method is 0.08 or less. (Measurement method) 10 mg test pieces are sampled at 20 random locations on the carbon fiber sheet. Using thermogravimetric analysis, the temperature is increased in a nitrogen atmosphere from 100 to 600°C at a heating rate of 10°C / min, and the weight loss rate of each test piece is measured after holding at 600°C for 30 minutes. The average and standard deviation of the weight loss rates of each test piece are calculated with n=20, and the standard deviation is divided by the average to calculate the coefficient of variation of the weight loss rate (nitrogen atmosphere).

3. The carbon fiber sheet according to claim 1 or 2, wherein the coefficient of variation of the weight loss rate (air atmosphere) calculated by the following measurement method is 0.15 or less. (Measurement method) 10 mg test pieces are sampled at 20 random locations on the carbon fiber sheet. Using thermogravimetric analysis, the temperature is increased in the temperature range of 600 to 800°C at a heating rate of 10°C / min in an air atmosphere, and the weight loss rate of each test piece is measured after holding at 800°C for 10 minutes. The average and standard deviation of the weight loss rate of each test piece are calculated with n=20, and the standard deviation is divided by the average to calculate the coefficient of variation of the weight loss rate (air atmosphere).

4. The carbon fiber sheet according to claim 1 or 2, wherein the coefficient of variation of surface resistivity calculated by the following measurement method is 0.10 or less. (Measuring Method) Test pieces of 3 cm x 3 cm are sampled at 20 random locations on a carbon fiber sheet. Using a surface resistance measuring device, the surface resistivity (Ω / cm ２ The average value and standard deviation of the surface resistivity of each test piece are determined, with n=20, and the standard deviation is divided by the average value to calculate the coefficient of variation of the surface resistivity.

5. The carbon fiber sheet according to claim 1 or 2, wherein the carbon fibers have an average fiber length of 6 mm or less.

6. The average loading amount (basis weight) of the carbon fiber in the carbon fiber sheet is 35.0 g / m ２ 3. The carbon fiber sheet according to claim 1 or 2, wherein:

7. The carbon fiber sheet according to claim 1 or 2, wherein the conductive particles have an average particle size of 500 μm or less.

8. The carbon fiber sheet according to claim 1 or 2, which is used as a fuel cell component.

9. The carbon fiber sheet according to claim 1 or 2, which is a precursor of a separator for a fuel cell or a precursor of a gas diffusion layer for a fuel cell.

10. A separator for a fuel cell or a gas diffusion layer for a fuel cell, comprising the carbon fiber sheet according to claim 1 or 2 and a resin component that is hardened while held by the carbon fiber sheet, and is molded so as to have a wavy shape when viewed in cross section.

11. A method for manufacturing a separator for a fuel cell or a gas diffusion layer for a fuel cell, comprising: a step of impregnating the carbon fiber sheet according to claim 1 or 2 with a resin component; and a step of press-molding the carbon fiber sheet in a state where the carbon fiber sheet is impregnated with the resin component.

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