Carbon sheet and method for manufacturing the same, gas diffusion electrode, and fuel cell

Abstract

The present invention provides a carbon sheet favorably used in a gas diffusion electrode, it being possible to prevent said electrode from bending into a gas flow path worked into a separator without degrading electrical resistance in the plane-perpendicular direction.　This carbon sheet, with which the purpose described above is achieved, has a first surface and a second surface positioned on the opposite side to the first surface, wherein: when a specific section is divided into 20 equal parts in the thickness direction to create 20 layers, the degree of fiber orientation of the outermost surface layer on the first-surface side is 1.20–3.00 inclusive; and, taking an area of said layer composed of a continuous layer satisfying a specific condition to be a first-surface-side area and an area of said layer that is not included in the first-surface-side area to be a second-surface-side area, the thickness of the first-surface-side area is 40% or less of the thickness of the entire carbon sheet, and the difference between the average degree of fiber orientation in the second-surface-side area and the degree of fiber orientation in the outermost surface layer of on the first-surface side is greater than 0.10.

Description

[Technical Field]

[0001] This invention relates to a carbon sheet, a method for producing the same, a gas diffusion electrode, and a fuel cell. [Background technology]

[0002] A polymer electrolyte fuel cell (MSF) generates an electromotive force through an electrochemical reaction that occurs at both electrodes by supplying a hydrogen-containing fuel gas to the anode and an oxygen-containing oxidizing gas to the cathode. MFCs generally consist of a separator, gas diffusion electrode, catalyst layer, electrolyte membrane, catalyst layer, gas diffusion electrode, and separator, stacked in that order. The gas diffusion electrode requires high gas diffusivity to diffuse the gas supplied from the separator into the catalyst layer, high drainage to discharge water generated by the electrochemical reaction into the separator, and high conductivity to extract the generated current. Therefore, gas diffusion electrodes made of a conductive porous substrate of carbon fiber with a microporous layer formed on its surface are widely used.

[0003] To efficiently supply fuel gas (hydrogen gas on the anode side, air and oxygen on the cathode side) from the gas diffusion electrode to the catalyst layer, it is necessary to distribute the fuel gas across the entire surface through gas channels provided in the separator adjacent to the gas diffusion electrode. Furthermore, various proposals have been made regarding the structure of the gas diffusion electrode to efficiently diffuse the fuel gas that has reached the gas diffusion electrode from the separator within the gas diffusion electrode.

[0004] For example, a gas diffusion electrode has been proposed in which the carbon sheet constituting the gas diffusion electrode has a fiber orientation (Patent Document 1).

[0005] Furthermore, a gas diffusion electrode has been proposed in which 2 to 8 carbon sheets having fiber orientation are laminated (Patent Document 2).

[0006] Furthermore, a gas diffusion electrode has been proposed in which a carbon sheet with strong fiber orientation and a carbon sheet with weak fiber orientation are laminated (Patent Document 3).

[0007] Further, a gas diffusion electrode having a gradient of fiber orientation in the thickness direction of a single layer has been proposed (Patent Document 4). [Prior Art Documents] [Patent Documents]

[0008] [Patent Document 1] Japanese Unexamined Patent Application Publication No. 2006-222024 [Patent Document 2] United States Patent Application Publication No. 2017 / 0301923 [Patent Document 3] Japanese Unexamined Patent Application Publication No. 2013-191435 [Patent Document 4] Korean Unexamined Patent Publication No. 2016-0120060 [Summary of the Invention] [Problem to be Solved by the Invention]

[0009] However, in the invention described in Patent Document 1, fibers are strongly oriented throughout the entire carbon sheet. Although the carbon sheet has strength in the orientation direction, it has low strength in the direction perpendicular to the fiber orientation direction, which poses a problem that breakage or tearing is likely to occur during conveyance or processing. Further, when fibers are strongly oriented throughout the entire carbon sheet, the flexural rigidity in the fiber orientation direction increases. By disposing the carbon sheet such that the fiber orientation is substantially perpendicular to the gas flow path provided on the separator surface, deflection of the carbon sheet into the gas flow path can be suppressed, and an effect of reducing pressure loss during fuel gas supply can be expected. On the other hand, since the distribution of resin carbide in the carbon sheet is biased in the fiber direction, there is a problem that the electrical resistance in the thickness direction (through-plane direction) of the carbon sheet deteriorates.

[0010] In the inventions described in Patent Document 2 and Patent Document 3, since two or more carbon sheet layers are laminated, delamination between layers occurs during power generation, leading to problems such as deterioration of electrical resistance due to contact breakage and deterioration of power generation performance due to flooding at the delaminated locations. Furthermore, since a plurality of carbon sheets are manufactured and then laminated, a problem arises in that productivity deteriorates due to an increase in the number of processing steps.

[0011] In the invention described in Patent Document 4, a gradient of fiber orientation in the thickness direction (through-plane direction) is provided within a single layer, but since the region where fibers are strongly oriented does not have sufficient thickness, there is a problem that the suppression of deflection of the carbon sheet into the gas flow paths cannot be sufficiently exhibited.

[0012] An object of the present invention is, in view of the problems in the prior art, to provide a carbon sheet that, when used as a gas diffusion electrode, does not deteriorate the electrical resistance in the through-plane direction and can prevent deflection into the gas flow paths provided in a separator, as well as to provide a gas diffusion electrode including the carbon sheet and a fuel cell including the gas diffusion electrode. [Means for Solving the Problems]

[0013] In order to solve the above problems, the present invention has the following constitution. That is, The carbon sheet of the present invention is a carbon sheet having a first surface and a second surface located opposite the first surface, wherein, in a section from the plane having a 50% filling rate closest to the first surface to the plane having a 50% filling rate closest to the second surface, when the section is equally divided into 20 parts in the thickness direction to form 20 layers, the fiber orientation degree of the outermost surface layer on the first surface side is 1.20 or more and 3.00 or less, and when a region consisting of consecutive layers among the 20 layers having a fiber orientation degree with a difference within ±0.10 from the fiber orientation degree of the outermost surface layer on the first surface side is defined as a first surface side region, and a region consisting of layers among the 20 layers that are not included in the first surface side region is defined as a second surface side region, the thickness of the first surface side region is 40% or less of the total thickness of the carbon sheet, and the difference between the average fiber orientation degree of the second surface side region and the fiber orientation degree of the outermost surface layer on the first surface side is larger than 0.10. (Here, 50% filling rate refers to the filling rate of a carbon sheet measured every 3.9 μm from one surface to the other, followed by calculating the average value of the obtained filling rates, and then taking 50% of that average value. Furthermore, the layer filling rate refers to the average value obtained using the filling rates of the surfaces forming the layer.) [Effects of the Invention]

[0014] According to the present invention, it is possible to provide a carbon sheet that can be suitably used in a gas diffusion electrode without worsening the electrical resistance in the direction perpendicular to the surface, which has been difficult in the past, and without deflection into the gas flow path provided in the separator. [Brief explanation of the drawing]

[0015] [Figure 1] Schematic diagram of a fuel cell including the gas diffusion electrode of the present invention [Figure 2] Schematic diagram of the method for evaluating the deflection of a gas diffusion electrode according to the present invention. [Figure 3] A schematic diagram showing the packing density distribution in the thickness direction when the degree of fiber orientation in the thickness direction of the carbon sheet of the present invention is measured. [Figure 4] A schematic diagram showing the fiber orientation distribution of the carbon sheet of the present invention. [Figure 5] A schematic diagram showing the packing density distribution of the carbon sheet of the present invention, divided into multilayer X and multilayer Y. [Figure 6] Schematic diagram of an example of a paper machine capable of controlling the fiber orientation of carbon fiber paper. [Modes for carrying out the invention]

[0016] The embodiments of the present invention will be described below, but the present invention is not limited to these embodiments. Also, for the reader's understanding, some terms in the specification are denoted by reference numerals in the drawings, but the drawings are illustrative, and the present invention is not limited to the embodiments shown in the drawings.

[0017] [Carbon Sheet] The carbon sheet 1 of the present invention is a carbon sheet having a first surface 11 and a second surface 12 located on the opposite side of the first surface, and when the section from the surface 13 having a 50% filling rate 15 closest to the first surface to the surface 14 having a 50% filling rate closest to the second surface is divided into 20 equal parts in the thickness direction to form 20 layers 17, the fiber orientation of the outermost layer on the first surface side is 1.20 or more and 3.00 or less, and the region consisting of continuous layers within this region, where the difference in fiber orientation from the outermost layer on the first surface side is within ±0.10, is called the first surface side region 1-1, and the region consisting of layers not included in the first surface side region is called the second surface side region 1-2, the thickness of the first surface side region is 40% or less of the total thickness of the carbon sheet, and the difference between the average fiber orientation of the second surface side region and the fiber orientation of the outermost layer on the first surface side is greater than 0.10. In this invention, "dividing the carbon sheet into 20 equal parts in the thickness direction to form 20 layers" means, as shown in Figure 3, dividing the carbon sheet into 20 equal parts in the thickness direction 10 from the surface with a 50% filling rate closest to the first surface of the carbon sheet to the surface with a 50% filling rate closest to the second surface, and the total thickness of the carbon sheet refers to the thickness of the above-mentioned section.

[0018] The carbon sheet of the present invention is preferably porous. The porous nature of the carbon sheet allows for both excellent gas diffusion for diffusing the gas supplied from the separator to the catalyst and high drainage for discharging water generated by the electrochemical reaction back to the separator. Furthermore, to provide high conductivity for extracting the generated current, it is preferable to use a conductive porous material. As the porous material, it is preferable to use a porous material containing carbon fibers, such as a carbon fiber paperboard, carbon fiber fabric, and felt-type carbon fiber nonwoven fabric. Among these, it is preferable to use a carbon fiber paperboard as the porous material because it has excellent properties for absorbing dimensional changes perpendicular to the electrolyte membrane, i.e., "springiness," which allows for a thinner porous material and makes it easier to control the fiber orientation in the thickness direction. Furthermore, the carbon sheet of the present invention is preferably in a form in which a binder is attached to the porous material, and more preferably consists of a carbon fiber paperboard and a binder. This improves the rigidity of the carbon sheet, and the binder acts as a conductive path, thereby improving conductivity.

[0019] In this invention, the filling rate is the ratio of the constituent components of the carbon sheet to the total area of ​​a surface obtained by slicing the carbon sheet with a plane perpendicular to a certain thickness direction. In other words, it is the area occupancy rate of the portion excluding the voids inside the carbon sheet. Details of the filling rate measurement method will be described later. The 50% filling rate is defined as the value obtained by measuring the filling rate of a surface at regular thickness intervals from one surface of the carbon sheet to the other surface, then calculating the average value of the obtained surface filling rates, and finally taking 50% of the obtained average value. Furthermore, the layer filling rate is defined as the average value obtained using the filling rates of the surfaces forming the layer.

[0020] In the carbon sheet of the present invention, when the section from the surface with a 50% filling density closest to the first surface to the surface with a 50% filling density closest to the second surface is divided into 20 equal parts in the thickness direction, resulting in 20 layers, the fiber orientation of the outermost layer on the first surface side is 1.20 or more and 3.00 or less, and more preferably 1.35 or more and 2.80 or less. By setting the fiber orientation of the outermost layer on the first surface side to 1.20 or more, bending rigidity is improved and deflection into the gas flow path provided in the separator is suppressed, and by setting it to 3.00 or less, a gas diffusion electrode with excellent electrical resistance in the direction perpendicular to the surface can be made. On the other hand, if the fiber orientation of the outermost layer on the first surface side is less than 1.20, the electrical resistance will be low, but the effect of suppressing deflection will not be obtained.

[0021] When the region consisting of continuous layers having a fiber orientation degree such that the difference from the fiber orientation degree of the outermost layer on the first surface side is within ±0.10 is defined as the first surface side region, the thickness of the first surface side region is 40% or less of the total thickness of the carbon sheet, preferably 20% or less, and more preferably 10% or less. By setting the thickness of the first surface side region to 40% or less of the total thickness of the carbon sheet, a deflection suppression effect can be obtained, and a gas diffusion electrode with excellent electrical resistance can be made. On the other hand, if the thickness of the first surface side region exceeds 40%, a greater deflection suppression effect can be obtained, but it can lead to fracture due to a decrease in strength in the direction perpendicular to the fiber orientation direction, deterioration of dimensional stability during processing, and deterioration of electrical resistance during power generation.

[0022] Furthermore, in the carbon sheet of the present invention, when the region consisting of layers not included in the first surface region is defined as the second surface region, the difference between the fiber orientation of the outermost layer of the first surface region and the average fiber orientation of the second surface region is greater than 0.10. When this difference in fiber orientation is greater than 0.10, it is possible to suppress fracture due to a decrease in strength in the direction perpendicular to the fiber orientation direction and deterioration of dimensional stability during processing. In addition, when the first surface region is positioned on the separator side, deflection into the gas flow path provided in the separator can be suppressed.

[0023] Preferably, the difference between the fiber orientation of the outermost layer on the second surface side and the fiber orientation of each layer belonging to the second surface side region is within ±0.10. When this difference is within ±0.10, deterioration of electrical resistance can be prevented by a uniform mesh structure.

[0024] In the carbon sheet of the present invention, it is preferable that the average degree of orientation of the second surface region is 1.20 or less. By setting the average degree of orientation of the second surface region within the above range, intersections between fibers are secured and the number of conductive paths is increased, thereby suppressing deterioration of electrical resistance.

[0025] In the carbon sheet of the present invention, the difference between the fiber orientation angle of the outermost layer on the first surface and the fiber orientation angle of the outermost layer on the second surface is preferably 45° or less, and more preferably 30° or less. When the difference in fiber orientation angles on both sides is 45° or less, the fibers on both sides are oriented in almost the same direction, which is more effective in suppressing deflection.

[0026] In this specification, "fiber orientation degree" is defined by the intensity of the expression of the fiber orientation angle, where the angle of the most frequently occurring fiber in a two-dimensional plane within the carbon sheet is defined as the fiber orientation angle. In this invention, the fiber orientation degree and fiber orientation angle are measured using fiber orientation measurement software (image analysis software for measuring fiber orientation of paper and nonwoven fabrics (ver8.03), created by Professor Toshiharu Enomae of the Graduate School of Life and Environmental Chemistry, University of Tsukuba, HP: http: / / www.enomae.com / FiberOri / index.htm). In this fiber orientation measurement software, a fiber orientation degree close to 1.00 (lower limit) means that there is no orientation, and a larger value means that the fiber orientation is strong. Furthermore, the fiber orientation angle is 90° when most of the fibers in the image are oriented upwards on the screen, 0° when the screen is tilted to the right, and 180° when the screen is tilted to the left. For this reason, it is necessary to determine the orientation in which the image is displayed on the screen and the actual orientation of the sheet.

[0027] [Procedure for measuring fiber orientation and filling density] A carbon sheet (dimensions: 10mm x 5mm) is placed in a three-dimensional X-ray CT scanner (manufactured by Yamato Scientific, product name TDM1000H-CF), and a 3D image of "4mm x 4mm x sheet thickness [mm]" is created by scanning 360° with the vertical axis as the rotation axis. In the example described below, imaging was performed with a tube voltage of 60kV and a tube current of 60μA. At this time, each slice image constituting the 3D image is made to have a thickness of 3.9μm. Next, the 3D image is divided into 20 equal parts in the thickness direction using the image analysis software attached to the X-ray CT scanner, creating 20 layers. At this time, it is necessary to determine the sections to be divided into 20 layers. First, all slice images constituting the 3D image are output from the image analysis software attached to the X-ray CT scanner. Next, each obtained slice image is binarized to distinguish the background area where the carbon sheet does not exist from the carbon sheet. For example, using the image processing program "ImageJ (ver1.53a), HP: https: / / imagej.nih.gov / ij / )", each obtained slice image is divided into 1 to 255 brightness levels, and the point where the separation of the two peaks obtained when plotting the grayscale histogram is highest is used as the brightness threshold for binarization (Otsu's binarization method). That is, it is divided into the dark side of brightness (background) and the bright side of brightness (components of the carbon sheet). The percentage of the total area of ​​each slice image occupied by the binarized bright side is defined as the packing density [%]. Next, the packing density of each slice image is plotted from one surface of the carbon sheet to the other surface to confirm the packing density distribution 16. From the packing density distribution obtained in this way, the average value of the region where the packing density exceeds 0% is calculated, and 50% of that average value is set as the 50% packing density. After determining the surface 13 with the 50% filling rate closest to the first surface of the carbon sheet and the surface 14 with the 50% filling rate closest to the second surface, the section between the two determined surfaces with 50% filling rates is divided into 20 equal parts to form 20 layers. A plan view is created by observing each of the 20 layers perpendicular to the surface, and this plan view is input into the fiber orientation measurement software to calculate the degree of fiber orientation and the fiber orientation angle.

[0028] Furthermore, to evaluate the filling density and fiber orientation of the carbon sheet, multiple measurements were taken and the average value was calculated. In the example described later, 30 samples were taken from different parts of the carbon sheet and measured, and the average value was calculated.

[0029] In the above analysis method, as shown in Figure 4, the thickness [%] of the first surface region, the thickness [%] of the second surface region, and the degree of fiber orientation of each layer can be determined.

[0030] In the carbon sheet of the present invention, when the section from the surface with a 50% filling rate closest to the first surface to the surface with a 50% filling rate closest to the second surface is divided into two equal parts in the thickness direction, as shown in Figure 5, and the section is divided into two multilayer bodies, it is preferable that the filling rate of multilayer body Y is lower than that of multilayer body X when the multilayer body on the first surface side is called multilayer body X19 and the multilayer body on the second surface side is called multilayer body Y20.

[0031] Here, the packing density of each multilayer refers to the average packing density of each layer contained within each multilayer. Furthermore, multilayer X includes the 1st to 10th layers from the first surface side of the present invention, and multilayer Y includes the 11th to 20th layers.

[0032] By making the filling rate of the multilayer X higher than that of the multilayer Y, the [resin impregnation process] described later can be achieved. As described above, the amount of binder on the X side of the multilayer body increases, resulting in a rigid plane. Therefore, when using the carbon sheet of the present invention as a gas diffusion electrode in a fuel cell, arranging the X side of the multilayer body on the separator side can suppress deflection into the gas flow path provided in the separator.

[0033] Furthermore, when the section from the surface with the 50% filling rate closest to the first surface, as defined above, to the surface with the 50% filling rate closest to the second surface is divided into three sections in the thickness direction, the resulting multilayer body is more preferable when the multilayer body on the first surface side is called multilayer body X', the multilayer body on the second surface side is called multilayer body Y', and the multilayer body located between multilayer body X' and multilayer body Y' is called multilayer body Z', in which order the layers decrease in the order of multilayer body X', multilayer body Y', and multilayer body Z'. Here, multilayer body X' includes the 1st to 7th layers from the first surface side of the present invention, multilayer body Y' includes the 14th to 20th layers, and multilayer body Z' includes the 8th to 13th layers. By lowering the packing density of the multilayer Z compared to the other two multilayers, the diameter of the pores in multilayer Z' increases, improving gas diffusion. Furthermore, because multilayer X' has a higher packing density than multilayer Z', it becomes less prone to bending when the separator is pressed against it, resulting in a gas diffusion electrode that balances both power generation and mechanical properties. In addition, since multilayer X or multilayer X', which has a high packing density, is the first surface side with a high degree of fiber orientation, it can more effectively prevent bending.

[0034] In this invention, the "thickness" used when measuring the filling density and fiber orientation degree refers to the unpressurized thickness measured during X-ray CT analysis, i.e., under no pressure, and is different from the "pressurized thickness" obtained by the 0.15 MPa pressure measurement described later.

[0035] Furthermore, it is possible to separate the carbon sheet from the gas diffusion electrode and measure the degree of fiber orientation and packing density of the carbon sheet. For example, the gas diffusion electrode can be heated in air at 500°C for 60 minutes to oxidize and decompose the resin composition contained in the microporous layer within the gas diffusion electrode. After that, ultrasonic treatment in ethanol is performed and drying is carried out to remove the residue from the microporous layer and extract the carbon sheet.

[0036] Furthermore, the carbon sheet of the present invention has a density of 0.20 to 0.40 g / cm³. 3 It is preferably within the range of 0.22 to 0.35 g / cm³. 3 It is within the range of 0.24 to 0.31 g / cm³, and more preferably 0.24 to 0.31 g / cm³. 3 It is within the range. The density is 0.20 g / cm³. 3With these characteristics, water vapor diffusion is low, and drying up can be suppressed. Furthermore, the mechanical strength of the carbon sheet is improved, allowing it to adequately support the electrolyte membrane and catalyst layer when used as a gas diffusion electrode in a fuel cell. In addition, high conductivity improves power generation performance. On the other hand, the density is 0.40 g / cm³. 3 The following conditions can improve drainage and suppress flooding.

[0037] Carbon sheets having such density can be obtained by controlling the basis weight of the carbon fibers, the amount of resin component blended with the carbon fibers, and the thickness of the carbon sheet, as described in the [Method for Manufacturing Carbon Sheets] below. Here, the density of the carbon sheet can be determined by dividing the basis weight (mass per unit area) of the carbon sheet, weighed using an electronic balance, by the thickness of the carbon sheet under pressure when pressurized at a surface pressure of 0.15 MPa.

[0038] The carbon sheet of the present invention preferably has a pressurized thickness of 50 to 230 μm, more preferably 70 to 210 μm, and even more preferably 90 to 190 μm. When the pressurized thickness of the carbon sheet is 230 μm or less, the gas diffusivity is increased, and the generated water is also more easily discharged. Furthermore, the overall size of the fuel cell can be made smaller. On the other hand, when the pressurized thickness of the carbon sheet is 50 μm or more, the in-plane gas diffusivity within the carbon sheet is improved, and the power generation performance is improved.

[0039] The pressurized thickness of the carbon sheet of the present invention is determined by the following method. Specifically, the carbon sheet is placed on a smooth surface plate, and the height difference between the area with the object to be measured and the area without the object (zero point) is measured while a pressure of 0.15 MPa is applied. Ten samples are taken at different locations, and the average of the measured height differences is taken as the pressurized thickness.

[0040] Examples of carbon fibers used in the carbon sheet of the present invention include polyacrylonitrile (PAN)-based, pitch-based, and rayon-based carbon fibers. Among these, PAN-based carbon fibers and pitch-based carbon fibers are preferably used in the present invention due to their excellent mechanical strength. Flame-resistant yarn that becomes carbon fiber through a carbonization process may also be used. In addition, conventionally known natural fibers and synthetic fibers such as rayon fibers, acrylic fibers, and cellulose fibers may be mixed.

[0041] The carbon fibers used in this invention preferably have an average diameter of 3 to 20 μm, and more preferably 5 to 10 μm. When the average diameter of the single fibers is 3 μm or more, the diameter of the pores becomes larger, improving drainage and suppressing flooding. On the other hand, when the average diameter of the single fibers is 20 μm or less, it becomes easier to control the thickness within a preferred range for the carbon sheet and gas diffusion electrode.

[0042] When using carbon fiber papermaking, which is preferably used in the present invention, as a carbon sheet, the average length of the single carbon fibers is preferably in the range of 3 to 20 mm, and more preferably in the range of 5 to 15 mm. If the average length of the single fibers is 3 mm or more, the carbon sheet and gas diffusion electrode will have excellent mechanical strength, conductivity, and thermal conductivity. On the other hand, if the average length of the single fibers is 20 mm or less, a homogeneous carbon sheet and gas diffusion electrode can be obtained.

[0043] [Method for manufacturing carbon sheets] The present invention preferably includes a method for manufacturing a carbon sheet, comprising a dispersion step of uniformly dispersing a bundle of carbon fibers in a liquid to obtain a slurry, and a papermaking step of papermaking the slurry containing carbon fibers using a papermaking machine having a mechanism that allows for orientation control in the thickness direction while continuously laminating carbon fibers.

[0044] The following describes an example of a preferred manufacturing method, but the present invention is not limited to the following description.

[0045] [Dispersion process / Paper making process] As an example of a method for producing a carbon sheet according to the present invention, a method for producing a carbon fiber paper body is described, in which a slurry is prepared by dispersing carbon fiber bundles in an aqueous solution, and the prepared slurry is formed into a sheet by a wet papermaking method. The aqueous solution for dispersing the carbon fibers preferably contains an antifoaming agent, a surfactant, and a thickening agent. The antifoaming agent suppresses foaming during stirring and is not particularly limited, but urethane-based, polyoxyalkylene-based, and silicone-based agents are preferred. The surfactant is effective in defibrating the carbon fiber bundles and is not particularly limited, but polyethylene glycol-based and polyethylene oxide-based agents are preferred. The thickening agent thickens the aqueous solution and is effective in suppressing physical contact between the carbon fibers and is not particularly limited, but polyethylene oxide-based and polyacrylic acid-based agents are preferred. The viscosity of the aqueous solution for dispersing the carbon fibers is preferably 5 to 50 mPa·s, and particularly preferably 8 to 20 mPa·s, considering the dispersibility of the carbon fibers and the drainage during papermaking. Below 5 mPa·s, the viscosity is low and re-aggregation of carbon fibers may occur. Above 50 mPa·s, re-aggregation of carbon fibers can be suppressed, but the rate of unraveling the carbon fiber bundles slows down, and the water drainage during papermaking deteriorates, leading to excess moisture, which can cause breakage during transport and poor adhesion of the binder resin. Slurry is obtained by dispersing the carbon fiber bundles in the above aqueous solution.

[0046] As a method for obtaining a wet material that will serve as the base material for a carbon fiber papermaking body from the above-mentioned slurry, a wet papermaking method is used in which the carbon fibers and aqueous solution in the slurry are separated using a papermaking machine such as a long-wire papermaking machine, a cylinder papermaking machine, or an inclined papermaking machine, and the carbon fibers are formed into a sheet. However, it is particularly preferable to use an inclined papermaking machine, as it is easier to control the fiber orientation and re-aggregation can be suppressed by diluting the carbon fiber concentration in the slurry. In the wet papermaking method, the basis weight and thickness of the carbon fiber papermaking body can be adjusted by adjusting the slurry concentration, slurry flow rate, and papermaking wire speed to achieve the desired basis weight and thickness. The wet papermaking method includes single-layer papermaking and multi-layer papermaking, in which two or more layers of wet material are laminated, but single-layer papermaking is preferred considering production stability and production costs.

[0047] In the wet papermaking method, fiber orientation can be controlled using the fiber orientation control method described later, thereby imparting fiber orientation to the carbon fiber papermaking structure.

[0048] An example of a method for controlling fiber orientation is described below. The fiber orientation of a carbon fiber paper is determined by the speed at which the fiber slurry flows in the paper machine (slurry velocity) and the speed of the papermaking wire 23, which stacks the fibers to form a wet body 24. By controlling these speeds, the fiber orientation can be controlled, and in the papermaking process, the control conditions are set using the "jet wire ratio," which is the ratio of the slurry velocity to the speed of the papermaking wire. Jet wire ratio = Slurry flow velocity [m / min] ÷ Papermaking wire speed [m / min] (Equation 1) Generally, the closer the jet wire ratio is to 1, the weaker the fiber orientation becomes, and the further it is from 1, the stronger the fiber orientation becomes. To strengthen the fiber orientation, there are two methods: making the jet wire ratio greater than 1 or making it smaller than 1. However, a jet wire ratio less than 1 is preferable because it allows for miniaturization of the fluid delivery equipment such as pumps.

[0049] One method for adjusting the fiber orientation distribution 18 in the thickness direction of the carbon fiber papermaking body is to change the jet wire ratio at each stage in which the fibers are laminated on the papermaking wire. For example, in the part of the papermaking machine where the slurry is fed, multiple slurry discharge ports 21 are arranged in the height direction, and the slurry flow rate supplied from each slurry discharge port is individually adjusted. A schematic diagram is shown in Figure 6. By designing as shown in Figure 6, the jet wire ratio can be finely adjusted in the thickness direction of the carbon fiber papermaking body, and the degree of fiber orientation and distribution in the thickness direction of the carbon fiber papermaking body can be controlled. For example, if the fiber orientation of one side of the carbon fiber papermaking body is to be strengthened, the slurry flow rate of the slurry discharge port located at the top or bottom of the papermaking machine can be adjusted to make the jet wire ratio less than 1, thereby strengthening the fiber orientation. For this reason, one method of orienting the fibers in the same direction by making the difference between the fiber orientation angle of the outermost layer on the first surface side and the fiber orientation angle of the outermost layer on the second surface side 45° or less can be achieved by adjusting the slurry flow rate located at the top and bottom of the papermaking machine.

[0050] Furthermore, by diluting the slurry with a dispersion aqueous solution before sending it to each slurry outlet to adjust the concentration and flow rate, the same basis weight can be maintained even if the jet wire ratio changes. Also, as shown in Figure 6, by extending the partition plate 22 to separate adjacent slurry outlets, the slurry flows from each outlet are less likely to interfere with each other, allowing for more precise control of fiber orientation and distribution.

[0051] In order to finely adjust the degree of fiber orientation and distribution in the thickness direction, it is preferable to have 3 to 15 slurry discharge ports, and more preferably 5 to 10. If there are fewer than 3 slurry discharge ports, it may be difficult to finely control the orientation in the thickness direction, and if there are more than 15 ports, the equipment may become larger and the operation may become complicated.

[0052] In addition to the control methods exemplified above, fiber orientation can also be controlled by controlling the liquid level of the slurry flowing through the paper machine and by controlling the viscosity of the slurry.

[0053] It is common practice to apply a binder resin to the wet material obtained by papermaking in order to maintain its shape as a carbon fiber papermaking body. This application can be done by, for example, coating the wet material with the binder resin, using various commercially available coating devices. Coating methods such as spray coaters, curtain coaters, and die coaters can be used. The coating methods exemplified above are merely examples and are not necessarily limited to these.

[0054] It is preferable to dry the wet material coated with binder resin at a temperature of 100 to 180°C. Common drying methods such as multi-cylinder dryers, Yankee dryers, and hot air drying can be used. The drying methods exemplified above are merely examples and are not necessarily limited to these. Furthermore, it is preferable to transport the material while supporting it with a heat-resistant belt, heat-resistant mesh, or heat-resistant felt, and then perform hot air drying.

[0055] In the present invention, the carbon fiber paper body preferably has a basis weight of carbon fiber of 10 to 50 g / m 2 , more preferably 15 to 35 g / m 2 , and still more preferably 20 to 30 g / m 2 . When the basis weight of carbon fiber in the carbon fiber paper body is 10 g / m 2 or more, the resulting carbon sheet has excellent mechanical strength. When the basis weight of carbon fiber is 50 g / m 2 or less, the resulting carbon sheet has excellent in-plane gas diffusivity and drainage performance.

[0056] Here, the basis weight of carbon fiber in the carbon fiber paper body is determined by: cutting a 10 cm square piece of the carbon fiber paper body, holding it in an electric furnace at 450°C under a nitrogen atmosphere for 15 minutes to remove organic matters, and then dividing the mass of the obtained residue by the area (0.01 m 2 ) of the carbon fiber paper body.

[0057] The method for producing the carbon sheet of the present invention preferably comprises: the carbon sheet obtained by the aforementioned dispersing step and papermaking step, that is, a resin impregnation step of applying a resin to the carbon fiber paper body, a heating and pressing step of adjusting the thickness by heating and pressing, and a firing step of carbonizing the resin-applied carbon sheet. Each step is described below.

[0058] [Resin Impregnation Step] As a method for producing the carbon sheet of the present invention, impregnating a porous body containing carbon fibers with a resin composition serving as a binder is one preferred embodiment.

[0059] In the present invention, the binder in the carbon sheet refers to components other than carbon fibers in the carbon sheet, and mainly functions to bind carbon fibers to each other. Examples of the binder include a resin composition impregnated into a porous body containing carbon fibers or a carbide thereof. In the present invention, a porous body containing carbon fibers impregnated with a resin composition serving as a binder may sometimes be referred to as a "pre-impregnated body".

[0060] In the present invention, the resin composition used to prepare the pre-impregnated body contains a resin component and a solvent as needed. Here, the resin component includes a resin such as a thermosetting resin or a thermoplastic resin, and further includes additives such as carbon powder or a surfactant as needed.

[0061] The carbonization yield of the resin components in the above resin composition is preferably 40% by mass or more. Carbonization is the state in which carbon remains when a substance is burned in the absence of oxygen. The carbonization yield is expressed as the ratio of the mass of the remaining carbon to the mass of the original resin components. When the carbonization yield is 40% by mass or more, the carbon sheet tends to have excellent mechanical properties, electrical conductivity, and thermal conductivity. There is no particular upper limit to the carbonization yield of the resin components in the resin composition, but it is usually around 60% by mass.

[0062] The resins constituting the resin components in the above resin composition are preferably thermosetting resins such as phenolic resins, epoxy resins, melamine resins, and furan resins. Among these, phenolic resins are particularly preferred due to their high carbonization yield.

[0063] Furthermore, as an additive that may be added as needed to the resin component in the above resin composition, carbon powder can be used for the purpose of improving the mechanical properties, electrical conductivity, and thermal conductivity of the carbon sheet. Here, as carbon powder, carbon black such as furnace black, acetylene black, lamp black, and thermal black; graphite such as flake graphite, scale graphite, earthy graphite, artificial graphite, expanded graphite, and flake graphite; carbon nanotubes, carbon nanofibers, and milled carbon fibers can be used.

[0064] The above resin composition can be used as is, or, if necessary, may contain various solvents to enhance its impregnation into porous materials such as carbon fiber paper. Here, methanol, ethanol, and isopropyl alcohol can be used as solvents.

[0065] The above resin composition is preferably in liquid form at 25°C and 0.1 MPa. When the resin composition is in liquid form, it has excellent impregnation properties into porous materials such as carbon fiber paper, and the resulting carbon sheet has excellent mechanical properties, electrical conductivity, and thermal conductivity.

[0066] When impregnating, it is preferable to impregnate the pre-impregnated material with the resin composition such that the resin component is 30 to 400 parts by mass per 100 parts by mass of carbon fibers, and more preferably 50 to 300 parts by mass. If the amount of resin component impregnated per 100 parts by mass of carbon fibers in the pre-impregnated material is 30 parts by mass or more, the carbon sheet will have excellent mechanical properties, electrical conductivity, and thermal conductivity. On the other hand, if the amount of resin component impregnated per 100 parts by mass of carbon fibers in the pre-impregnated material is 400 parts by mass or less, the carbon sheet will have excellent gas diffusion in the in-plane direction and gas diffusion in the direction perpendicular to the plane.

[0067] In the present invention, methods for impregnating a porous body containing carbon fibers with a resin composition include immersing the porous body containing carbon fibers in a resin composition containing a solvent, coating the porous body containing carbon fibers with a resin composition containing a solvent, and forming a layer made of the resin composition on a release film and transferring the layer made of the resin composition to the porous body containing carbon fibers. Among these, the method of immersing the porous body containing carbon fibers in a resin composition with added solvent is particularly preferred due to its excellent productivity. By adhering the resin composition to the entire pre-impregnated body, a binder can be attached to the entire carbon sheet obtained, thereby further improving the strength of the carbon sheet.

[0068] As described above, in the carbon sheet of the present invention, it is preferable that the packing rate of the multilayer Y is lower than that of the multilayer X. Such a carbon sheet can be obtained, for example, by impregnating the porous body with a resin composition, such that the amount of resin composition impregnated into the multilayer Y is less than the amount impregnated into the multilayer X. For this reason, after uniformly impregnating the porous body containing carbon fibers with a resin composition that acts as a binder by immersion or the like, the amount of resin composition adhering excessively from one side is removed before drying, thereby controlling the distribution of the resin composition perpendicular to the surface of the carbon sheet and thus controlling the packing rate of each multilayer.

[0069] For example, after obtaining a pre-impregnated body by immersing a porous body containing carbon fibers in a solution containing a resin composition, the amount of resin composition solution absorbed from one surface before drying, or by squeezing the pre-impregnated body between rolls with different surface structures, can be used to adhere a binder to the entire body while varying the amount of resin composition adhering to one surface relative to the amount adhering to the other surface.

[0070] Another example is that after obtaining a pre-impregnated body by immersing a porous body containing carbon fibers in a solution containing a resin composition, the filling density of one surface of the carbon sheet can be controlled to be different by applying the resin composition to only one surface of the pre-impregnated body using a spray or gravure roll, or by drying one side first after resin immersion.

[0071] [Heating and pressurizing process] In this invention, it is preferable to heat and pressurize the pre-impregnated material. By heating and pressurizing, the resin composition in the pre-impregnated material can be thickened and partially crosslinked, allowing the carbon sheet to be adjusted to the desired thickness and density. As a method of heating and pressurizing, a method using a heated hot plate, roll, or belt can be used. Winding and unwinding devices may be provided before and after this heating and pressurizing device. By providing such devices, the pre-impregnated material can be continuously heated and pressurized. Since this promotes the thickening and crosslinking of the resin composition in the pre-impregnated material, additional heat treatment with hot air or the like may be applied.

[0072] [Firing process] In the present invention, it is preferable to impregnate a porous body containing carbon fibers with a resin composition to create a pre-impregnated body, and then perform firing in an inert atmosphere to carbonize the resin composition. This firing can be carried out using either a batch-type or a continuous-type heating furnace.

[0073] The maximum firing temperature is preferably within the range of 1300 to 3000°C. If the maximum temperature is 1300°C or higher, the carbonization of the resin components in the pre-impregnated material progresses, resulting in a carbon sheet with excellent conductivity and thermal conductivity. On the other hand, if the maximum temperature is 3000°C or lower, the operating cost of the heating furnace is reduced.

[0074] In this invention, a carbonized pre-impregnated material may be referred to as a "carbon fiber calcined body." In other words, a carbon fiber calcined body corresponds to a carbon sheet. Both a carbon fiber calcined body before water-repellent treatment and a carbon fiber calcined body after water-repellent treatment are considered to be carbon sheets.

[0075] [Water-repellent treatment] In the present invention, it is preferable to apply a water-repellent treatment to the carbon fiber sintered body in order to improve drainage. The water-repellent treatment can be performed by applying a water-repellent material to the carbon fiber sintered body and heat-treating it. By applying the water-repellent treatment, a carbon sheet containing the water-repellent material as a binder can be obtained.

[0076] As a water-repellent material, it is preferable to use a fluorine-based polymer due to its excellent corrosion resistance. Examples of fluorine-based polymers include polytetrafluoroethylene (PTFE), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), and tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA).

[0077] [Gas diffusion electrode] Next, the gas diffusion electrode of the present invention will be described.

[0078] The gas diffusion electrode of the present invention has a microporous layer 2 on the second surface side of the carbon sheet of the present invention. The side having the microporous layer is designated as the second surface of the gas diffusion electrode, and the side without the microporous layer is designated as the first surface.

[0079] A microporous layer can be formed by applying a coating solution to the second surface of a carbon sheet. The coating solution is a dispersion of conductive filler in a dispersion medium such as water or an organic solvent. The coating solution may also contain a dispersion aid such as a surfactant. Water is preferred as the dispersion medium, and a nonionic surfactant is preferred as the dispersion aid. A water-repellent material may also be included.

[0080] Furthermore, in the present invention, the microporous layer may also preferably contain a conductive filler, and carbon powder is preferred as the conductive filler because it is physically and chemically stable.

[0081] The coating liquid can be applied to the carbon sheet using various commercially available coating devices, and it is also preferable to dry the coating liquid using a drying facility after application.

[0082] There is no particular upper limit to the basis weight of the microporous layer, but it is 50 g / m². 2 The following is preferable, and more preferably, 30 g / m² 2 The following is more preferably 25 g / m² 2 The following applies. Furthermore, the lower limit is 10g / m². 2 Preferably, it is 14 g / m² or more.2 More preferably, and even more preferably, 16 g / m² 2 That's all.

[0083] The basis weight of the microporous layer is 10 g / m². 2 With the above configuration, the entire second surface of the carbon sheet can be covered with a microporous layer, further promoting the back diffusion of generated water and suppressing drying up. Furthermore, the basis weight of the microporous layer is 50 g / m². 2 The following conditions will further improve drainage and reduce flooding.

[0084] [Membrane electrode assembly] In the present invention, a membrane electrode assembly can be formed by bonding the gas diffusion electrode to at least one side of a solid polymer electrolyte membrane having catalyst layers on both sides. In this case, positioning the second surface side of the gas diffusion electrode on the catalyst layer side is preferable because it facilitates back diffusion of the generated water, increases the contact area between the catalyst layer and the gas diffusion electrode, and reduces contact electrical resistance. The catalyst layer consists of a layer containing a solid polymer electrolyte and catalyst-supported carbon. Platinum is usually used as the catalyst. The membrane electrode assembly shown in Figure 1, consisting of a gas diffusion electrode 3 and a catalyst-supported electrolyte membrane 4, is an example in which the gas diffusion electrodes of the present invention are arranged on both sides, but the gas diffusion electrode of the present invention may be used on only one side, and a conventional gas diffusion electrode may be used on the other side.

[0085] [Fuel cell] The fuel cell of the present invention includes the gas diffusion electrode of the present invention as a component and is a fuel cell in which a separator, gas diffusion electrode, catalyst layer, electrolyte membrane, catalyst layer, gas diffusion electrode, and separator are stacked in that order. That is, as shown in Figure 1, the fuel cell is constructed by arranging separators 5 on both sides of the above membrane electrode assembly. The separator is a plate-shaped member of a highly conductive material such as metal or graphite, and has grooves formed on the surface that contacts the membrane electrode assembly. It plays a role in enabling the exchange of electricity between the gas diffusion electrode and the external circuit, as well as the supply of fuel gas or oxidizing gas and the drainage of generated water. In order to distribute the fuel gas across the entire surface of the gas diffusion electrode, the separator used has multiple parallel straight grooves on its surface that serve as gas flow channels. Examples of gas flow channel structures include parallel groove type and meandering groove type (serpentine type).

[0086] In the fuel cell of the present invention, it is preferable that the carbon sheet is arranged such that the fiber orientation direction of the first surface of the carbon sheet is substantially perpendicular to the direction of the gas flow path provided in the separator when the sheet is placed on the separator side. Here, if the gas flow path of the separator has multiple different directions, the direction of the gas flow path of the separator is set to the direction of the gas flow path of the separator. By arranging it in this way, the gas diffusion electrode is less likely to bend in the gas flow path provided in the separator, preventing pressure loss of the fuel gas flowing in the separator and allowing the fuel gas to be efficiently supplied to the entire surface of the gas diffusion electrode. Typically, a polymer electrolyte fuel cell is constructed by stacking multiple such membrane electrode assemblies, each sandwiched between separators with gaskets in between. [Examples]

[0087] Next, the present invention will be specifically described with reference to examples. The materials used in the examples, the carbon sheets (carbon fiber paper bodies, carbon fiber fired bodies), and the methods for producing the gas diffusion electrodes are shown below.

[0088] <Preparation of carbon fiber papermaking> To 100 parts by mass of deionized water, 0.01 parts by mass of the defoaming agent KM-73 (manufactured by Shin-Etsu Chemical Co., Ltd.), 0.01 parts by mass of the thickening agent "Noptex®" E-R060 (manufactured by Sunopco Co., Ltd.), and 0.01 parts by mass of the surfactant "Alcox®" CP-B1 (manufactured by Meisei Chemical Industry Co., Ltd.) were added, and the mixture was stirred in a disperser for 10 minutes to prepare an aqueous solution for dispersion.

[0089] Next, 0.15 parts by mass of PAN-based carbon fiber "Torayca®" T300 (manufactured by Toray Industries, Inc.) (average single fiber diameter: 7 μm), cut to a length of 6 mm, were added to 100 parts by mass of the dispersion aqueous solution, and the mixture was stirred in a disperser for 5 minutes to prepare a slurry. Then, papermaking was carried out using the obtained slurry in a paper machine with 10 slurry discharge ports arranged in the height direction, as shown in Figure 6. When the obtained slurry was sent from the disperser to each slurry discharge port, the slurry was diluted in the dispersion aqueous solution in each pipe to adjust the slurry concentration so that a similar basis weight could be obtained even with the different jet wire ratios described later. Next, the slurry flow rate at each slurry discharge port was adjusted to obtain the target jet wire ratio, and papermaking was carried out continuously while the diluted slurry was sent from each slurry discharge port to the inclined paper machine. At this time, a dewatering mechanism installed at the bottom of the papermaking wire promoted the separation of fibers in the slurry. Furthermore, the slurry discharge port located at the top of the paper machine was designated as slurry discharge port No. 1, and the slurry discharge port at the bottom of the paper machine was designated as slurry discharge port No. 10. In addition, a 10% by mass aqueous solution of polyvinyl alcohol was applied to the wet material coming out of the paper machine as a binder resin, and the material was heated at 180°C for 2 minutes to dry it, thereby producing a long carbon fiber paper machine. The amount of polyvinyl alcohol applied was 22 parts by mass per 100 parts by mass of the carbon fiber paper machine.

[0090] <Preparation of water-repellent treated carbon fiber sintered bodies> A resin mixture was prepared by mixing 10 parts by mass of thermosetting resin (resol-type phenolic resin KP-743K (manufactured by Arakawa Chemical Industries, Ltd.) and novolac-type phenolic resin "Tanomaru®" 759 (manufactured by Arakawa Chemical Industries, Ltd.) in a 1:1 mass ratio), 5 parts by mass of flake-like graphite BF-5A (average particle size 5 μm, aspect ratio 1:5) manufactured by Chuetsu Graphite Industry Co., Ltd., and 85 parts by mass of solvent (methanol). The mixture was stirred for 1 minute using an ultrasonic dispersion device to obtain a uniformly dispersed resin composition.

[0091] Next, a long carbon fiber paper roll was continuously immersed in an impregnation solution of resin composition filling a tank, and the amount of resin adhering was adjusted by sandwiching it between two rolls. At this time, a certain clearance was left between the two rolls as the pre-impregnated material passed through, thereby adjusting the overall amount of resin composition adhering. One of the two rolls was a smooth metal roll, and the other roll was a gravure roll with a textured surface. By placing the metal roll on one side of the pre-impregnated material and the gravure roll on the other side, a difference in the amount of resin composition adhering to one surface of the pre-impregnated material was created. After that, the pre-impregnated material was heated and dried at a temperature of 100°C for 5 minutes to produce the pre-impregnated material. Next, the pre-impregnated material was continuously heated and pressurized at a temperature of 180°C for a total of 5 minutes while being transported in stages and pressurized with parallel hot plates.

[0092] This pre-impregnated body, subjected to heat and pressure treatment, was introduced into a heating furnace maintained in a nitrogen gas atmosphere at a maximum temperature of 2400°C to obtain a carbon fiber sintered body.

[0093] The carbon sheet (carbon fiber sintered body) prepared as described above was cut to 15 cm x 12.5 cm and impregnated with a water-repellent material by immersion in an aqueous dispersion of polytetrafluoroethylene resin (an aqueous dispersion of "Polyflon®" PTFE Dispersion D-201C (manufactured by Daikin Industries, Ltd.)). Subsequently, it was heated and dried in a drying oven at 100°C for 5 minutes to produce a water-repellent carbon fiber sintered body with uniform adhesion of the water-repellent material, which was then used as the carbon sheet for the following <Preparation of Gas Diffusion Electrode>.

[0094] <Fabrication of gas diffusion electrodes> To 100 parts by mass of deionized water, 9.2 parts by mass of carbon powder "Denka Black®" (manufactured by Denki Kagaku Kogyo Co., Ltd.), 5.5 parts by mass of the water repellent "Polyflon®" PTFE Dispersion D-1E (aqueous dispersion containing 60 parts by mass of PTFE resin, manufactured by Daikin Industries, Ltd.), and 18.5 parts by mass of the surfactant "TRITON®" X-100 (manufactured by Nakalai Tesque Co., Ltd.) were added and mixed using a disperser to form a filler-containing coating solution. This filler-containing coating solution was applied to the entire second surface of a water-repellent carbon sheet using a slit die coater, and then heated at 120°C for 10 minutes, followed by heating at 380°C for 10 minutes. In this way, a microporous layer was formed on the second surface side of the water-repellent carbon sheet, and a gas diffusion electrode was fabricated.

[0095] <Measurement of basis weight of carbon sheets and gas diffusion electrodes> Cut a 10cm square piece of the carbon sheet or gas diffusion electrode to be measured to create a sample, and measure its mass [g] over the area of ​​the sample (0.01m²). 2 It was found by dividing by ).

[0096] <Measurement of the thickness of carbon sheets and gas diffusion electrodes under pressure> Using a constant-pressure thickness gauge, the carbon sheet or gas diffusion electrode to be measured was placed on a smooth surface plate, and the difference in height between the area with and without the object (carbon sheet or gas diffusion electrode) was measured under a pressure of 0.15 MPa. Ten samples were taken at different locations, and the average of the measured height differences was defined as the thickness under pressure.

[0097] <Measurement of the average diameter of a single carbon fiber> The average diameter of individual carbon fibers was determined by using a scanning electron microscope (SEM) to photograph carbon fibers on one surface of a carbon sheet at 1000x magnification. Thirty randomly selected individual fibers were measured, and their diameters were calculated. The average value was then calculated. The same procedure was followed for individual carbon fibers on the other surface of the carbon sheet. A Hitachi S-4800 scanning electron microscope was used.

[0098] <Measurement of fiber orientation and filling density> The degree of fiber orientation, fiber orientation angle, and filling density were measured using the procedure described above in [Measurement Procedure for Fiber Orientation and Filling Density].

[0099] <Measurement of electrical resistance of gas diffusion electrodes> The electrical resistance under pressure was measured using the electrical resistance test mode of the "Autograph®" AGS-X manufactured by Shimadzu Corporation. A sample of a gas diffusion electrode, cut to 22.4 mm square, was placed with the microporous layer facing upwards in two electrical resistance measuring fixtures mounted 20.0 cm apart. The upper electrical resistance measuring fixture was lowered and a load was applied until it reached 1.0 MPa. After reaching the predetermined load, a current of 1.0 ampere was applied using a DC power supply connected to the electrical resistance measuring fixture. After the current was applied, the voltage value was read from a digital multimeter 20 seconds later, and the electrical resistance value was calculated from the measured value and the measured area.

[0100] In this invention, the electrical resistance value is 7 mΩ·cm. 2 Performance was evaluated using this as the baseline value.

[0101] <Measurement of deflection of gas diffusion electrode> As shown in Figure 2, a compression unit 8 was prepared by placing a gas diffusion electrode (3 cm square) with a microporous layer facing upwards on the grooved surface of a grooved block 6 (3 cm square) which had 15 grooves with a rib width of 1 mm, a groove width of 1 mm, and a depth of 1 mm on one surface, and then placing a flat block 7 (3 cm square) on top of the gas diffusion electrode, thereby sandwiching the gas diffusion electrode between two types of blocks. The compression unit was set between the flat jigs of a universal testing machine (AGX-5kN, manufactured by Shimadzu Corporation) with flat jigs set on the top and bottom, and the gas diffusion electrode was compressed and held under a load that applied an average surface pressure of 1 MPa. The gas diffusion electrode was imaged from the side of the compression unit using a digital microscope at a magnification that resulted in a field of view width of 1.5 to 2.0 mm. On the analysis screen of the digital microscope, the length of the perpendicular from the reference rib surface to the maximum deflection in the groove was measured as the deflection amount of 9 [μm]. For each gas diffusion electrode, three of the 15 grooves were measured, and the same test was performed on six gas diffusion electrodes. A total of 18 deflection values ​​were measured, and the average value was used for evaluation.

[0102] In this invention, the performance of the gas diffusion electrode was evaluated based on an average deflection of 50 μm.

[0103] (Example 1) Following the method described in <Preparation of carbon fiber papermaking bodies> above, the papermaking wire of the inclined papermaking machine is driven at a papermaking speed of 5 m / min, and 0.07 m of slurry, which has been appropriately diluted from the original slurry, is discharged into the No. 1 slurry outlet. 3 At a rate of / min, 0.10 ml of the original slurry, diluted appropriately, is dispensed into slurry outlets 2 through 9. 3 The liquid is dispensed at a rate of / min, with a basis weight of 30g / m². 2 A carbon fiber paper was obtained with a thickness of 250 μm under pressure. The obtained carbon fiber paper was treated according to the method described in <Preparation of water-repellent carbon fiber fired body> above, and the basis weight was 45 g / m². 2 A carbon sheet with a thickness of 150 μm under pressure was obtained. At this time, the degree of fiber orientation, packing density, and their distribution of the obtained carbon sheet are shown in Table 1. Furthermore, using the carbon sheet obtained here, a basis weight of 62 g / m² was obtained according to the method described in <Preparation of Gas Diffusion Electrode> above. 2A gas diffusion electrode with a thickness of 160 μm under pressure was obtained. The obtained gas diffusion electrode was measured according to the methods described in <Measurement of Electrical Resistance of Gas Diffusion Electrode> and <Measurement of Deflection of Gas Diffusion Electrode> above. This gas diffusion electrode had an electrical resistance of 6.0 mΩ·cm. 2 The deflection amount was 42 μm, and both were good.

[0104] (Examples 2-4) In the above-described <Preparation of carbon fiber paper>, the gas diffusion electrode was prepared in the same manner as in Example 1, except that the slurry concentration and slurry flow rate were adjusted as appropriate, and the ratio of the thickness of the first surface region and the second surface region to the total thickness of the carbon sheet was changed. In all examples, the gas diffusion electrode had an electrical resistance of 7 mΩ·cm. 2 The following samples all showed good results, with a deflection of 50 μm or less.

[0105] (Examples 5-7) In the above-described <Preparation of carbon fiber papermaking material>, the gas diffusion electrode was prepared in the same manner as in Example 1, except that the slurry concentration and slurry flow rate were adjusted as appropriate to change the fiber orientation of the outermost layer on the first surface side. In all examples, the gas diffusion electrode had an electrical resistance of 7 mΩ·cm. 2 The following samples all showed good results, with a deflection of 50 μm or less.

[0106] (Examples 8 and 9) In the above-described <Preparation of carbon fiber papermaking bodies>, gas diffusion electrodes were prepared in the same manner as in Example 1, except that the slurry concentration and slurry flow rate were adjusted as appropriate, and the average values ​​of the fiber orientation of the outermost layer on the first surface side and the fiber orientation of the region on the second surface side were changed. In all examples, the electrical resistance of the gas diffusion electrodes was 7 mΩ·cm. 2 The following samples all showed good results, with a deflection of 50 μm or less.

[0107] (Example 10) In the above-mentioned <Preparation of carbon fiber papermaking material>, a gas diffusion electrode was prepared in the same manner as in Example 8, except that the slurry concentration and slurry flow rate were adjusted as appropriate, and the ratio of the thickness of the first surface region and the second surface region to the total thickness of the carbon sheet and the average value of the fiber orientation of the second surface region were changed. This gas diffusion electrode had an electrical resistance of 6.6 mΩ·cm. 2 The deflection amount was 37 μm, and both were good.

[0108] (Example 11) In the above-mentioned <Preparation of carbon fiber papermaking material>, a gas diffusion electrode was prepared in the same manner as in Example 9, except that the slurry concentration and slurry flow rate were adjusted as appropriate, and the ratio of the thickness of the first surface region and the second surface region to the total thickness of the carbon sheet and the average value of the fiber orientation of the second surface region were changed. This gas diffusion electrode had an electrical resistance of 6.7 mΩ·cm. 2 The deflection amount was 35 μm, and both were good.

[0109] (Example 12) In the above-described "Preparation of a water-repellent carbon fiber fired body," a gas diffusion electrode was prepared in the same manner as in Example 1, except that two rolls were used as metal rolls and impregnated with resin, and the filling rate of the binder on the first and second surface sides was made equal. This gas diffusion electrode had an electrical resistance of 6.0 mΩ·cm. 2 The deflection amount was 44 μm, and both were good.

[0110] (Example 13) In the above-described "Preparation of a water-repellent carbon fiber fired body," a gas diffusion electrode was prepared in the same manner as in Example 1, except that the clearance between the two rolls was adjusted as appropriate to increase the difference in the filling density of the binder between the multilayer body X and the multilayer body Y. This gas diffusion electrode had an electrical resistance of 6.1 mΩ·cm. 2 The deflection amount was 39 μm, and both were good.

[0111] (Example 14) In the above-described "Preparation of a water-repellent carbon fiber fired body," a gas diffusion electrode was fabricated in the same manner as in Example 1, except that the positions of the two rolls were swapped during resin impregnation, and the relative filling ratios of the multilayer body X and multilayer body Y were reversed. This gas diffusion electrode had an electrical resistance of 6.2 mΩ·cm. 2 The deflection amount was 44 μm, and both were good.

[0112] (Comparative Example 1) In the above-described <Preparation of carbon fiber papermaking material>, a gas diffusion electrode was prepared in the same manner as in Example 1, except that the slurry concentration and slurry flow rate were adjusted as appropriate to change the fiber orientation of the outermost layer on the first surface side. This gas diffusion electrode had an electrical resistance of 5.1 mΩ·cm. 2 Although the performance was good, the deflection amount was 54 μm, resulting in insufficient performance. In addition, due to the low degree of fiber orientation, the strength was weak, and there was a problem of breakage due to tension during processing.

[0113] (Comparative Example 2) In the above-described <Preparation of carbon fiber papermaking material>, a gas diffusion electrode was prepared in the same manner as in Example 1, except that the slurry concentration and slurry flow rate were adjusted as appropriate to change the fiber orientation degree of the outermost layer on the first surface side, the fiber orientation degree of the outermost layer on the second surface side, and the average orientation degree of the second surface side region. This gas diffusion electrode had an electrical resistance of 5.3 mΩ·cm. 2 Although the performance was good, the deflection amount was 52 μm, resulting in insufficient performance. Similar to Comparative Example 1, the low degree of fiber orientation resulted in low strength, and there was a problem of breakage due to tension during processing.

[0114] (Comparative Example 3) In the above-mentioned <Preparation of carbon fiber papermaking material>, a gas diffusion electrode was prepared in the same manner as in Example 1, except that the slurry concentration and slurry flow rate were adjusted as appropriate to change the fiber orientation of the outermost layer on the first surface side and the average orientation of the region on the second surface side. This gas diffusion electrode had an extremely good deflection amount of 31 μm, but its electrical resistance was 8.0 mΩ·cm. 2 As a result, the performance was insufficient. In addition, because the fiber orientation of the first surface region of the carbon sheet was high, it tended to curl slightly in the direction perpendicular to the orientation direction, which made handling during processing somewhat difficult.

[0115] (Comparative Example 4) In the above-described <Preparation of carbon fiber paper>, a gas diffusion electrode was prepared in the same manner as in Example 1, except that the slurry concentration and slurry flow rate were adjusted as appropriate to maintain a constant fiber orientation of 1.10 throughout the sheet. This gas diffusion electrode had an electrical resistance of 5.0 mΩ·cm. 2 Although the performance was good, the deflection amount was 55 μm, resulting in insufficient performance. Similar to Comparative Example 1, the low degree of fiber orientation resulted in low strength, and there was a problem of frequent breakage due to tension during processing.

[0116] (Comparative Example 5) In the above-mentioned <Preparation of carbon fiber paper>, a gas diffusion electrode was prepared in the same manner as in Example 1, except that the slurry concentration and slurry flow rate were adjusted as appropriate to keep the fiber orientation of the entire sheet constant at 3.10. This gas diffusion electrode had an extremely good deflection of 28 μm, but its electrical resistance was 9.5 mΩ·cm. 2 The performance deteriorated significantly, resulting in insufficient performance. Furthermore, due to the high degree of fiber orientation throughout the carbon sheet, it was extremely prone to curling in the direction perpendicular to the orientation, making handling during processing quite difficult.

[0117] (Comparative Example 6) In the above-mentioned <Preparation of carbon fiber paper>, a gas diffusion electrode was prepared in the same manner as in Example 1, except that the slurry concentration and slurry flow rate were adjusted as appropriate, and the ratio of the thickness of the first surface region and the second surface region to the total thickness of the carbon sheet was changed. The deflection amount of this gas diffusion electrode was 37 μm, which was somewhat good, but the electrical resistance was 7.5 mΩ·cm. 2 The performance deteriorated somewhat, resulting in insufficient performance. It is presumed that the deterioration in electrical resistance was due to an increase in the proportion of the first surface region relative to the overall thickness of the carbon sheet.

[0118] (Comparative Example 7) In the above-mentioned <Preparation of carbon fiber paper>, a gas diffusion electrode was prepared in the same manner as in Example 1, except that the slurry concentration and slurry flow rate were adjusted as appropriate, and the ratio of the thickness of the first surface region and the second surface region to the total thickness of the carbon sheet was changed. This gas diffusion electrode had a good deflection of 33 μm, but its electrical resistance was 8.3 mΩ·cm. 2 The performance deteriorated, resulting in insufficient performance. It is presumed that the electrical resistance worsened further because the proportion of the first surface region to the total thickness of the carbon sheet increased even more than in Comparative Example 6.

[0119] [Table 1]

[0120] [Table 2] [Industrial applicability]

[0121] The carbon sheet of the present invention is suitably used in fuel cells, particularly polymer electrolyte fuel cells. [Explanation of Symbols]

[0122] 1: Carbon sheet 1-1: 1st surface side area 1-2: 2nd surface side area 2: Microporous layer 3: Gas diffusion electrode 4: Catalyst-coated electrolyte membrane 5: Separator 6: Grooved block 7: Flat Blocks 8: Compression Unit 9: Deflection Amount 10: Thickness direction of the carbon sheet 11: 1st surface side 12: 2nd surface side 13: The surface with the 50% filling rate that is closest to the first surface. 14: The surface with the 50% filling rate that is closest to the second surface. 15:50% filling rate 16: Distribution of filling density 17:20 layer 18: Fiber orientation distribution 19: Multilayered X 20: Multilayered Y 21: Slurry discharge port 22: Partition board 23: Papermaking wire 24: Wet body

Claims

1. A carbon sheet having a first surface and a second surface located opposite the first surface, wherein when the section from the surface with a 50% filling density closest to the first surface to the surface with a 50% filling density closest to the second surface is divided into 20 equal parts in the thickness direction to form 20 layers, the fiber orientation of the outermost layer on the first surface side is 1.20 or more and 3.00 or less, and the region consisting of continuous layers among the layers having a fiber orientation of ±0.10 or less from the fiber orientation of the outermost layer on the first surface side is defined as the first surface side region, and the region consisting of layers not included in the first surface side region is defined as the second surface side region, the thickness of the first surface side region is 40% or less of the total thickness of the carbon sheet, and the difference between the average fiber orientation of the second surface side region and the fiber orientation of the outermost layer on the first surface side is greater than 0.

10. (Here, 50% filling rate refers to the filling rate of a carbon sheet measured every 3.9 μm from one surface to the other, then the average value of the obtained filling rates is calculated, and finally 50% of that average value is taken.) Furthermore, the packing efficiency of a layer refers to the average value obtained using the packing efficiency of the surfaces that form the layer.

2. The carbon sheet according to claim 1, wherein the difference between the fiber orientation angle of the outermost layer on the first surface and the fiber orientation angle of the outermost layer on the second surface is within 45°.

3. The carbon sheet according to claim 1 or 2, wherein, in the section from the surface having a 50% filling rate closest to the first surface to the surface having a 50% filling rate closest to the second surface, the section is divided into two equal parts in the thickness direction to form two multilayer bodies, and when the multilayer body on the first surface side is called multilayer body X and the multilayer body on the second surface side is called multilayer body Y, the filling rate of multilayer body Y is lower than the filling rate of multilayer body X.

4. A carbon sheet according to any one of claims 1 to 3, comprising a carbon fiber paper body and a binder.

5. A carbon sheet according to any one of claims 1 to 4, wherein the average fiber orientation degree of the second surface region is 1.20 or less.

6. A gas diffusion electrode having a microporous layer on the second surface side of a carbon sheet according to any one of claims 1 to 5.

7. A fuel cell comprising the gas diffusion electrode described in claim 6 as a component, wherein the fuel cell is stacked in the order of separator, gas diffusion electrode, catalyst layer, electrolyte membrane, catalyst layer, gas diffusion electrode, and separator.

8. The fuel cell according to claim 7, wherein the carbon sheet is arranged such that the fiber orientation direction of the first surface of the carbon sheet is substantially perpendicular to the direction of the gas flow path provided in the separator when the carbon sheet is placed on the separator side.

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