Carbon fiber electrode material, gas diffusion electrode substrate for fuel cell, fuel cell, and method for producing carbon fiber electrode material

Abstract

To provide: a carbon fiber electrode material made of a carbon fiber knit that can be suitably used as an electrode material; and a gas diffusion electrode base material for a fuel cell and a fuel cell using the same.SOLUTION: A carbon fiber electrode material is made of carbon fiber spun yarn with a metric count of less than 1 / 60 Nm, a twist angle of 40° or less, and a yarn width of 200 μm or less, and is made of carbon fiber knitted fabric with a basis weight of 150 g / m2 or less and a thickness of 230 μm or less when pressed under 1 MPa.SELECTED DRAWING: Figure 1A

Description

[Technical Field]

[0001] The present invention relates to a carbon fiber electrode material made of knitted carbon fibers, a gas diffusion electrode substrate for a fuel cell, a fuel cell, and a method for producing a carbon fiber electrode material. [Background technology]

[0002] Electrode materials for electrochemical devices such as fuel cells, electrolysis, and redox flow batteries, as well as conductive supports for secondary battery active materials, are required to have properties such as high electrical conductivity and corrosion resistance that can withstand strong acids and alkalis. Carbon fiber materials have attracted attention as materials that possess these properties. Furthermore, as low-cost fuel cell electrode materials, knitted fabrics are generally less expensive per area than woven fabrics, and therefore, it is believed that knitted carbon fiber materials would be effective as carbon fiber electrode materials.

[0003] In the production of carbon fiber knitted fabrics, a method of using fiber bundles of polyacrylonitrile fiber, the raw fiber of carbon fiber, and then heat-treating the knitted fabric in an oxidizing atmosphere after production has been prone to runaway reactions, in which the raw fiber suddenly flares up. Therefore, the flame-retardant treatment must be carried out slowly, resulting in low productivity. Furthermore, shrinkage of the raw fiber makes it difficult to obtain a bulky material. Therefore, a method for producing a battery electrode substrate has been proposed in which a knitted fabric is produced using fiber bundles of flame-retardant fiber, the precursor fiber of carbon fiber, and then the knitted fabric is heat-treated in a non-oxidizing atmosphere to carbonize it (see, for example, Patent Document 1). Another proposed method for producing a sheet-like carbon fiber knitted fabric that can be knitted without fuzzing or thread breakage involves repeatedly feeding carbon fiber bundles while converging them by spirally winding them around a guide or passing them through a pipe guide (see, for example, Patent Document 2).

[0004] However, the battery electrode substrate produced by the manufacturing method proposed in Patent Document 1 has the problem of high electrical resistance in the thickness direction. In addition, in the technology described in Patent Document 2, the starting material is a long fiber precursor (filament), and the resulting knitted fabric is a reinforcing material for realizing the mechanical properties of the molded product, although it can be molded into fiber-reinforced plastics with complex shapes such as curved surfaces. Therefore, the expected basis weight is 100 to 700 g / m 2 Therefore, it was not suitable as an electrode material. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 63-40259 [Patent Document 2] Japanese Patent Application Laid-Open No. 2008-106391 Summary of the Invention [Problem to be solved by the invention]

[0006] Therefore, an object of the present invention is to provide a carbon fiber electrode material made of knitted carbon fibers that can be suitably used as an electrode material, a gas diffusion electrode substrate for a fuel cell using the same, and a fuel cell. [Means for solving the problem]

[0007] The carbon fiber electrode material of the present invention is made of a carbon fiber spun yarn having a metric count of less than 1 / 60 Nm, a twist angle of 40° or less, and a yarn width of 200 μm or less, and has a basis weight of 150 g / m 2 The carbon fiber knitted fabric is characterized in that it is made of a carbon fiber knitted fabric having a thickness of 230 μm or less when pressed under a pressure of 1 MPa.

[0008] The gas diffusion electrode substrate for a fuel cell of the present invention is characterized in that a microporous smooth layer containing a carbon filler and a fluorine-based resin is formed on the carbon fiber electrode material of the present invention.

[0009] In the gas diffusion electrode substrate for a fuel cell of the present invention, it is preferable that the carbon fiber knitted fabric contains at least two types of spun yarns of different thicknesses, has a double-sided knitting structure with thick yarns on one side and thin yarns on the other side, and the microporous smooth layer is formed on the side of the thick yarns.

[0010] The fuel cell of the present invention is characterized in that it incorporates the gas diffusion electrode substrate for a fuel cell of the present invention.

[0011] The method for producing a carbon fiber electrode material of the present invention includes a knitting step of producing a knitted fabric from a polyacrylonitrile spun yarn having a metric count of less than 1 / 50 Nm, or a knitted fabric from a doubling spun yarn obtained by doubling the polyacrylonitrile spun yarn with a vanishing fiber so as to untwist the polyacrylonitrile spun yarn, from which the vanishing fiber has been eliminated; a flame-retardant treatment step in which the knitted fabric obtained in the knitting step is treated at a temperature in the range of 210°C or higher and 250°C or lower for a period of 20 minutes to 12 hours; and The method is characterized in that after the flame-proofing treatment step, a carbonization firing step is performed in which the material is carbonized by being held in an inert gas at 1200°C to 2600°C for 3 minutes or more and 20 minutes or less.

[0012] In the method for producing a carbon fiber electrode material of the present invention, in the flame-proofing treatment step, the knitted fabric obtained in the knitting step is preferably subjected to the flame-proofing treatment under conditions in which thermal shrinkage in two dimensions is restricted to within a range of 75% to 95% of the length and width dimensions before the oxidation firing. [Effects of the Invention]

[0013] The carbon fiber electrode material of the present invention can be suitably used as an electrode material, and by using the carbon fiber electrode material of the present invention, a gas diffusion electrode substrate for a fuel cell and a fuel cell having excellent power generation performance can be provided at low cost. [Brief explanation of the drawings]

[0014] [Figure 1A] FIG. 1A is a micrograph of the surface of the knitted carbon fiber fabric of Example 1 of the present invention. [Figure 1B] FIG. 1B is a micrograph of the surface of the knitted carbon fiber fabric of Example 2 of the present invention. [Figure 1C] FIG. 1C is a micrograph of the surface of the knitted carbon fiber fabric of Example 3 of the present invention. [Figure 2] FIG. 2 is a schematic structural diagram of an evaluation cell 100 used in the power generation performance test in the examples. [Figure 3] FIG. 3 is a graph showing the results of the power generation performance test. [Figure 4] FIG. 4 is a schematic diagram illustrating the characteristics of the carbon fiber electrode material of the present invention. [Figure 5] FIG. 5 is a schematic cross-sectional view of an example of a baking (carbonization) device used in producing the carbon fiber electrode material of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0015] Hereinafter, embodiments of the present invention will be described with reference to the drawings. However, the present invention is not limited to or restricted by the following examples. Note that the drawings referred to below are schematic, and the dimensional ratios of objects depicted in the drawings may differ from the dimensional ratios of actual objects. The dimensional ratios of objects may also differ between drawings.

[0016] Fibers that can be used as starting materials for carbon fiber include natural cellulose (e.g., cotton, bamboo fiber), regenerated cellulose (e.g., rayon, acetate), polyacrylonitrile, pitch, polynosic, phenolic resin, polyparaphenylene terephthalamide, and mixtures thereof. While requiring a flame-retardant treatment process, fibers primarily composed of polyacrylonitrile (PAN) are desirable as starting materials due to their marketability and strength. Gas diffusion electrode substrates for fuel cells are typically stacked, so compressive and flexural strengths are more important than tensile strength. Therefore, the strength required for carbon fiber as a structural reinforcement material need not be excessive. Therefore, knitting acrylic fibers and then performing a flame-retardant treatment and carbonization process allows for stable production of fine spun yarns, reducing total energy costs compared to knitting flame-retardant fibers.

[0017] [Spun yarn] The spun yarn used as the material for knitted fabrics can be produced by combining a spun yarn with a primary twist number A and twist direction Z with a vanishing fiber that is to be eliminated after knitting. In this case, the primary twist number should be approximately the same as A, with a twist direction S (the direction in which the primary twist is reversed). The vanishing fiber should contribute to the stabilization of the knitting process, be easily eliminated with water or organic solvents, and be low-cost. It can be in the form of a filament or spun yarn, and its raw material can be PVA (polyvinyl alcohol) fiber or starch, which can be washed off after knitting. Alternatively, alkali-soluble SST polymer fiber (pentasodium dimethyl sulfoisophthalate) can be used. Alternatively, spun yarn fixed with a starch paste that dissolves in warm water can be untwisted and knitted. Alternatively, a composite yarn can be used in which the vanishing fiber is wrapped around the spun sliver after drafting. That is, any material that can be dissolved and removed after knitting is acceptable. However, PVA fiber is preferred due to its environmental impact, cost, and stable technology.

[0018] Spun yarns may be either two-ply yarns or single fibers, but two-ply yarns generally have higher tensile strength than single fibers. Even when two-ply yarns are used as knitted materials, the fibers can be easily flattened by appropriately selecting the conditions. In the present invention, polyacrylonitrile spun yarns are used that have a twist angle of 40° or less after calcination, and the number of final twists of the starting white yarn is 300 to 1,000 times per meter of fiber length, preferably 800 times per meter or less. The twist angle is preferably 30° or less, and no twist is more preferable. A small number of first twists tends to increase the number of fluffs, while a large number of first twists increases the probability of yarn breakage during twisting and may increase thickness unevenness. Here, if the number of fluffs per unit length is high, it is desirable to include a calcining process. If the number of top twists is approximately equal to the number of first twists and twisted in the opposite direction to form a yarn, the twist of each single yarn will be unwound, and the yarn bundles of both single yarns will be wrapped around each other.Although this cannot be said to be essentially untwisted, the single yarns will be wrapped around each other and will be aligned roughly parallel within the longitudinal range of the pitch, and will be flattened in the thickness direction during the compression process after knitting, making it easier to obtain straight sections in the outline of the yarn bundle cross section.

[0019] On the other hand, knitted fabrics made from single yarns with a twist that gives them tensile strength (a twist angle of more than 40° after baking) tend to be difficult to flatten, resulting in thinning and a small contact area, resulting in high electrical resistance. Generally, the twist angle is increased to obtain strong spun yarn, but if the twist angle exceeds 45° after baking shrinkage, the fabric will become double-woven, which will actually reduce strength and cause the weave to become distorted. Furthermore, curling may occur after baking, significantly impairing workability in subsequent processes.

[0020] Furthermore, in the case of long fibers, if the knitted fabric is made from yarns with a low number of interlacings, it goes without saying that the resulting carbon fiber knitted fabric is essentially untwisted and easily deformed into a flat shape. However, while the acrylic long fibers mentioned above are a desirable starting material, long fibers up to 250 dtex have limited uses and low marketability. Furthermore, if the number of interlacings is low, there are frequent single-yarn breakages and other price issues. This is why spun yarns, which are more marketable than long fibers, are more desirable.

[0021] Furthermore, the thickness of the spun yarn, expressed in metric count, is at most 1 / 50 Nm or less, preferably 1 / 100 Nm or less, and the thinner side is 1 / 200 Nm or more, preferably 1 / 300 Nm or more. By using a spun yarn having a thickness of 1 / 50 Nm or less, expressed in metric count, it is possible to obtain a carbon fiber spun yarn having a thickness of 1 / 60 Nm or less (thinner than 1 / 60 Nm) after sintering. In the case of two-ply yarn, the thickest thickness, expressed in metric count, is usually 2 / 64 Nm or less, preferably 2 / 100 Nm or less, and the thinner side is usually 2 / 200 Nm or more, preferably 2 / 300 Nm or more.

[0022] The twist number of the spun yarn complies with JIS L1095 (General Spun Yarn Testing Methods). The twist number for single yarns is usually 300 to 1000 turns per meter of fiber length, but the lower the twist number, the more desirable it is for the cross section of the yarn bundle to be flat. Here, a spun yarn with a twist number so low that it does not become a yarn is defined as a substantially non-twisted yarn, and by compressing it in a process after obtaining a knitted fabric made from this fiber, the cross section of the yarn bundle made up of many single fibers can be deformed into a flat shape.

[0023] Conventionally, when producing carbon fiber knitted fabrics, flame-resistant yarn (also called precursor yarn) is knitted and then carbonized and graphitized. However, with this method, the precursor yarn (mainly PAN spun yarn) of carbon fiber could not be thinned because its strength decreases during the flame-resistant treatment process. Specifically, a polymer blended for carbon fiber is spun into yarn, and the resulting sliver-like fiber bundle, approximately 10,000 times thicker than the fiber used in the present invention, is flame-resistant in one direction and then spun into yarn. This reduces the tensile strength of the yarn, and it cannot be made thinner than, for example, 1 / 34 Nm in metric count. As a result, the knitted fabric must be thick, resulting in high resistance in the thickness direction, making it unsuitable for use as an electrode material. On the other hand, for electrodes, a compressive strength of, for example, 2 MPa is sufficient, and tensile strength greater than necessary is not required. Therefore, in the present invention, PAN fiber (using a polymer blended for carbon fiber) manufactured for carbon fiber use is not required, and ultra-fine PAN spun yarn mass-produced for clothing can be used. By knitting this material, then flame-proofing and carbonizing it in a planar form, an ultrathin carbon fiber knitted fabric is obtained, which reduces electrical resistance in the thickness direction and improves electrode performance. Since the knitted fabric is then flame-proofed and carbonized, even thin threads can be easily knitted. Furthermore, even if the polymer is not blended for carbon fiber, stretching the material during flame-proofing to orient the polymer makes it possible to ensure compressive strength during actual use as an electrode material, as described below.

[0024] [Knitting process] The carbon fiber knitted fabric of the present invention has a basis weight of 150 g / m 2The thickness of the electrode substrate must be 230 μm or less and the thickness at 1 MPa must be 230 μm or less. The knitting structure can be plain knitting, rib knitting, purl knitting, tuck knitting, float knitting, double knitting, terry knitting, tricot knitting, raschel knitting, jacquard knitting, denbigh knitting, half knitting, punched roman knitting, or corrugated knitting, and can be selected depending on the purpose of the electrode. Rib knitting is preferred because the formed ribs act as electrolyte flow paths, facilitating electrolyte flow and improving reaction efficiency. Furthermore, punched roman knitting and corrugated knitting allow for different yarn thicknesses on the front and back. Therefore, when used as a gas diffusion electrode substrate for a fuel cell, it is desirable to have a thicker yarn on the catalyst-coated membrane (CCM) side and a thinner yarn on the separator side. Furthermore, applying the essentially untwisted or nearly untwisted yarn to the CCM surface increases the contact area, making it easier to apply the MPL described below, which is desirable.

[0025] [Carbonization process (flameproofing process and carbonization baking process)] Over 95% of commercially available carbon fibers are acrylic fibers that have been flame-retarded, carbonized, and graphitized. However, for fuel cells, which require good electrical properties and corrosion resistance, graphitization to increase rigidity is not necessary; rather, commercially available, low-cost cellulose or acrylic fibers are desirable. Other materials mentioned in the starting materials section can also be used, and these can be carbonized in an inert gas atmosphere at 1200°C to 2600°C. In the case of acrylic fibers, the fibers can be knitted and flame-retarded in an oxidizing atmosphere at 210°C to 250°C for 20 minutes to 12 hours, allowing for shrinkage. Then, the fibers can be carbonized in an inert gas atmosphere at 1200°C to 2600°C for 3 minutes to 20 minutes.

[0026] When the fabric is thermally shrunk during the flame-resistant treatment process, the polymer molecular orientation can be made to proceed in the direction of the fiber, ensuring sufficient strength for the electrode. One method for achieving this is to keep the twist angle of the spun yarn after carbonization to 40° or less. Here, "twist angle" refers to the angle of the single yarn relative to the center line of the spun yarn. If the twist angle exceeds 45° during the flame-resistant treatment and carbonization processes, the twist will tighten, causing microcracks to form inside the single yarn, reducing strength.

[0027] Furthermore, the strength can be improved by promoting the molecular orientation of the PAN spun yarn in each process. The knitted fabric obtained in the knitting process is subjected to a flame-retardant treatment under conditions where thermal shrinkage in two dimensions is restricted to between 75% and 95% of the longitudinal and lateral dimensions before oxidation baking. Specifically, a rectangular heat-resistant thin plate (heat-resistant plate) with dimensions approximately the same as the dimensions of the fabric after flame-retardant treatment is first prepared. Next, the knitted fabric before flame-retardant treatment is machine-sewn into a bag shape so that it encases the heat-resistant plate. It is desirable to perform the machine sewing so that the longitudinal and lateral dimensions of the heat-resistant plate are 75 to 95% of the longitudinal and lateral dimensions of the knitted fabric. Next, with the heat-resistant plate encased approximately in the center of the perforations, the knitted fabric is placed horizontally on a wire mesh and heat-shrunk under the above-mentioned flame-retardant treatment conditions. The fibers constituting the knitted fabric thermally shrink over the heating period, and after it adheres to the heat-resistant plate, the thermal contraction tensile stress acts, promoting molecular orientation in the fiber direction and improving the tensile strength of the fibers.

[0028] Furthermore, by sandwiching the white fabric between heat-resistant plates with smooth surfaces during heat treatment and placing shims at the four corners so that the gap between the heat-resistant plates is 70% to 200% of the knitted fabric's thickness, frictional forces are generated during two-dimensional contraction, promoting the orientation of the PAN molecules in the fiber direction. This orientation ensures the compressive strength required for electrodes, even with commercially available spun acrylic fibers for clothing. Furthermore, because the raw materials are mass-produced commercially, the cost of the starting material can be reduced. Furthermore, by subjecting the fabric to heat treatment while sandwiched between the smooth surfaces of the heat-resistant plates, the surface of the knitted fabric after heat treatment becomes smooth, thereby reducing the electrical resistance of the resulting carbon fiber knitted fabric.

[0029] Furthermore, the following manufacturing method is effective for reducing the electrical resistance of the resulting carbon fiber knitted electrode material. First, PAN spun yarn is doubling with a vanishing fiber (e.g., PVA) to untwist it. In this process, a loose twist is preferable, so that the resulting carbon fiber cannot be measured with a twist meter; even more preferable is a no-twist state. The doubling yarn is knitted, and the vanishing fiber is dissolved in a hot water washing process (85°C or higher), followed by flame retardation and carbonization to obtain a carbon fiber knitted fabric. This results in a carbon fiber knitted fabric formed from spun yarn that is essentially no-twist, even though it is spun yarn. When compressed against a flat plate, the cross section of the yarn bundle composed of single yarns deforms flat, increasing the contact area and reducing the contact resistance in the thickness direction.

[0030] The carbonization process for the carbon fiber knitted fabric (carbon fiber electrode material) obtained in the knitting process and the flame-resistant treatment process described above will be described in detail with reference to FIG. 5. The baking (carbonization) device 200 consists of a baking box 11 with an upper opening 11a. A flat, heat-resistant plate 16, preferably with a flatness of 0.1 mm or less, is placed on the bottom plate 11b. The carbon fiber knitted fabric (hereinafter referred to as the knitted fabric) 20 is placed on top of the flat, smooth plate, ensuring that no metal, such as iron, is interposed between the plate and the knitted fabric, preventing wrinkles and protrusion. Similar heat-resistant plates and knitted fabrics 20 are stacked on top of the flat plate, and a weight 17 is placed on the top layer to ensure a uniform load. The baking box 11 is housed in the main body 2 of a baking furnace 1 with an upper opening 11a. The upper opening 11a of the baking furnace 1 is closed by a lid 3, which is hermetically sealed by bolts 5a and nuts 5b, sandwiching the lid 3 via a graphite sheet seal 4. Alternatively, instead of bolting, a heat-resistant gas seal method or a vacuum high-temperature furnace with exhaust gas treatment may be used.

[0031] Next, a gas supply pipe 6 is connected to the main body 2 of the calcination furnace 1, allowing inert gas to be supplied from a gas source (not shown). A gas outlet 7 is also connected to the main body 2, which is connected to an exhaust gas trap 31 through a pipe 8. Water 34 is placed in the main body 32 of the exhaust gas trap 31, and the tip 8a of the pipe 8 is submerged in the water 34. The main body 32 of the exhaust gas trap 31 is sealed with a lid 33 having an exhaust gas trap outlet 35. The exhaust gas trap outlet 35 is connected to a purification device 37 or the like via a pipe 36, allowing harmless exhaust gas to be discharged to the outside. It is recommended that the exhaust gas trap 31 be equipped with a system that allows the water level to be visually confirmed or its components to be detected, and that valves be installed to check the temperature and pressure within the calcination furnace and analyze the gases generated by decomposition. It is also recommended that an exhaust gas combustion device be installed. The carbonization process using such an apparatus configuration involves introducing an inert gas into the baking box 11, heating it to a temperature of 1200°C to 2600°C at which carbonization is possible while discharging the gases generated by decomposition in the baking box 11, holding it for 3 to 20 minutes, and then cooling it.

[0032] The amount of inert gas introduced is preferably the gas volume in the baking furnace 1 per 3 minutes at a pressure of 1 kPa, and the oxygen concentration is preferably 5 ppm or less at 150°C or higher. The load applied to the knitted fabric 20 by the flat plate 16 is 0.1 to 5 N / cm 2 and more preferably 0.4 to 2 N / cm 2 Furthermore, the flatness (smoothness) of the flat plate 16 is preferably within 0.1 mm per A4 size, and a surface precision equivalent to that of a cold-rolled plate without scratches is sufficient. Here, if a shim that is 70 to 200% thinner than the finished thickness of the knitted fabric 20 after flame retardation is sandwiched around the knitted fabric 20, thickness unevenness is reduced and the problem of cracking due to thermal shrinkage is also reduced. Furthermore, the knitted fabric 20 shrinks and deforms during carbonization, and at this time, it is important that the outline of the single fibers in the cross section of the yarn bundle is crushed so that it has a straight portion, and that the single fibers are heat-set so that they are aligned in the fiber longitudinal direction.

[0033] During the heat treatment of each step in the carbon fiber knitting process, the appropriate tension is applied in both the warp and weft directions, and the molecules are oriented while generating the appropriate thermal shrinkage stress. This makes it possible to ensure that even ultra-fine spun yarns for clothing that are not acrylic fibers blended for carbon fiber have the strength and toughness required for use as electrodes.

[0034] The carbon fiber knitted fabric of the present invention is a fabric in which the yarn width of the carbon fiber spun yarn constituting the knitted fabric is 200 μm or less, and the weight of the knitted fabric is 150 g / m 2 The yarn width is preferably within a range of 40 μm to 150 μm. The number of single yarns constituting the spun yarn is preferably within a range of 65 to 200, and the diameter of a single yarn constituting the carbon fiber spun yarn is preferably 8 μm or less. The basis weight of the knitted fabric is preferably 40 g / m 2 ~150g / m 2 It is preferable that the temperature is in the range of

[0035] The carbon fiber knitted fabric of the present invention preferably contains at least two types of spun yarns of different thicknesses and has a double-knit structure with thick yarns on one side and thin yarns on the other. When a knitted structure is created using thin and thick yarns, with the thin yarns on the back and the thick yarns on the front, moisture can easily move from the thin yarn side to the thick yarn side by capillary action, resulting in a knitted fabric with good drainage. Furthermore, by knitting the knitted fabric in a state where it is combined with the vanishing fiber, improvements in the knitting process and the capillary action can be expected due to the fact that the yarn becomes untwisted after the vanishing fiber is lost. When the carbon fiber knitted fabric of the present invention is used as a gas diffusion electrode substrate for a fuel cell, it is preferable to use a carbon fiber knitted fabric of this structure and form a microporous smooth layer, as described below, on the side of the thick yarn.

[0036] The gas diffusion electrode substrate for a fuel cell of the present invention is characterized in that a conductive microporous smooth layer containing a carbon filler and a fluorine-based resin, i.e., a microporous layer (MPL), is formed on at least one side of the carbon fiber electrode material of the present invention. The microporous smooth layer has a basis weight of 10 to 30 g / m 2It is preferable that the thickness is within the range of . The MPL is provided to protect the thin electrolyte membrane for polymer membrane fuel cells and to improve adhesion. The provision of the MPL can improve power generation performance, for example, by about 10%, but the performance is basically determined by the morphology of the substrate. The provision of a microporous smooth layer covers and smooths the surface irregularities of the carbon fiber electrode material, thereby reducing the electrical resistance between the catalyst layer and the membrane-electrode assembly when the fuel cell is constructed. Damage to the solid polymer electrolyte membrane can also be more reliably prevented.

[0037] The microporous smooth layer is preferably hydrophobic to improve the drainage of the gas diffusion electrode substrate, and a fluororesin can be suitably used. Examples of the fluororesin include polychlorotrifluoroethylene resin (PCTFE), polytetrafluoroethylene resin (PTFE), polyvinylidene fluoride resin (PVDF), a copolymer of tetrafluoroethylene and hexafluoropropylene (FEP), a copolymer of tetrafluoroethylene and perfluoropropyl vinyl ether (PFA), and a copolymer of tetrafluoroethylene and ethylene (ETFE). Among these, polytetrafluoroethylene resin (PTFE) is preferred from the viewpoints of versatility and ease of handling.

[0038] Examples of carbon fillers that can be used include carbon black, carbon nanotubes, carbon nanofibers, milled carbon fiber, and graphite. Carbon black, which has good electrical conductivity, is preferably used as the carbon filler. The amounts of fluororesin and carbon filler added are preferably 3 to 4.5 parts by mass of carbon filler per 1 part by mass of fluororesin, and more preferably 3.5 to 4 parts by mass. These are used as a water-soluble ink, which is then layered on a carbon fiber fabric using a spray coater, screen printing, film transfer, or other methods, and the binder resin is thermally fixed.

[0039] A membrane-electrode assembly is constructed by bonding the gas diffusion electrode substrate for fuel cells obtained by the present invention to at least one side of a solid polymer electrolyte membrane having catalyst layers on both sides. When a microporous smooth layer is formed on only one side, the membrane-electrode assembly is preferably constructed so that the microporous smooth layer is in contact with the catalyst layer. A solid polymer fuel cell can be constructed by stacking multiple such membrane-electrode assemblies, each sandwiched between separators via gaskets. The catalyst layer is composed of a layer containing a solid polymer electrolyte and catalyst-supported carbon. Platinum is typically used as the catalyst. In fuel cells in which a reformed gas containing carbon monoxide is supplied to the anode side, platinum and ruthenium are preferably used as the anode-side catalyst. The solid polymer electrolyte is preferably a perfluorosulfonic acid-based polymer material with high proton conductivity, oxidation resistance, and heat resistance. The configuration of such fuel cell units and fuel cells themselves is well known. [Example]

[0040] The present invention will be specifically explained below with reference to Table 1 using examples.

[0041] [Example 1] A cardboard knit fabric was produced using a yarn made from a 1 / 100 Nm thick PAN spun yarn and a PVA fiber, and another yarn made from a 1 / 200 Nm thin PAN spun yarn and a PVA fiber. The knitting needles were placed at a pitch of 28 needles per inch. After knitting, the PVA fiber was washed away with hot water. The resulting knit fabric was flame-retarded using a thin heat-resistant plate that resulted in a shrinkage of 95% of the pre-treatment length and width. It was then carbonized by heating at 1200°C for 5 minutes in nitrogen gas to obtain a carbon fiber electrode material knit fabric. The twist angle of the carbon fiber spun yarn constituting the resulting knitted carbon fiber fabric was 0° (untwisted yarn), and the yarn width on one side (the side with the thick yarn) was 150 μm, while the yarn width on the other side (the side with the thin yarn) was 75 μm. Figure 1A is a micrograph of the thick yarn side of the resulting knitted carbon fiber fabric.

[0042] [Example 1-1] (thick outer knit) The resulting knitted carbon fiber electrode material was used to fabricate a gas diffusion electrode substrate for fuel cells. The thick-fiber side of the knitted fabric was used as the surface to form the MPL layer, and in the power generation performance test, the thin-fiber side was used to form the separator surface. The results are shown in Table 1. The MPL layer was formed by applying a water-based ink containing 0.5 μm carbon black and PTFE particles in a 3.5:1 ratio.

[0043] [Example 1-2] (thin outer surface, thick inner surface) The resulting knitted carbon fiber electrode material was used to prepare a gas diffusion electrode substrate for a fuel cell. An MPL layer was formed on the surface of the thin yarn of the knitted fabric in the same manner as in Example 1-1, and the cell was assembled so that the thick yarn surface faced the separator surface during the power generation performance test. The results are shown in Table 1.

[0044] [Example 2] A punched roma knit fabric was produced using a yarn made by doubling PAN spun yarn (1 / 100 Nm in metric count) with PVA fiber and another yarn made by doubling PAN spun yarn (1 / 200 Nm in metric count) with PVA fiber (thick knit on the front and fine knit on the back). After knitting, the PVA fiber was washed away with hot water. The resulting knitted fabric was flame-retarded using a thin heat-resistant plate that resulted in a shrinkage of 82% of the pre-treatment length and width. It was then carbonized by holding it in nitrogen gas at 1200°C for 5 minutes, yielding a knitted carbon fiber electrode material. The twist angle of the spun carbon fiber yarns constituting the resulting knitted carbon fiber fabric was 0° (untwisted yarn), and the yarn widths were 150 μm and 75 μm. Figure 1B is a micrograph of the resulting knitted carbon fiber fabric.

[0045] When a gas diffusion electrode substrate for a fuel cell was produced using the knitted carbon fiber electrode material of this example, an MPL layer was formed on the thick yarn side of the knitted fabric in the same manner as in Example 1-1, and in the power generation characteristic test, the cell was assembled so that the thin yarn side was in contact with the separator surface. The results are shown in Table 1. The power generation performance was almost as good as in Example 1.

[0046] [Example 3] A smooth knit fabric was produced using a PAN spun yarn with a metric count of 1 / 100 Nm and a PAN spun yarn with a metric count of 1 / 200 Nm (same front and back, alternating thick and thin knit). The resulting knit fabric was flame-resistant and carbonized under the same conditions as in Example 1 using a thin heat-resistant plate that resulted in a shrinkage rate of 78% of the pre-treatment dimensions, to obtain a knitted carbon fiber electrode material fabric. The twist angle of the carbon fiber spun yarn constituting the resulting knitted carbon fiber fabric was 30°, and the yarn widths were 100 μm and 60 μm. Figure 1C is a micrograph of the resulting knitted carbon fiber fabric.

[0047] When a gas diffusion electrode substrate for a fuel cell was produced using the knitted carbon fiber electrode material of this example, an MPL layer was formed on one side of the knitted fabric in the same manner as in Example 1-1, and in the power generation performance test, the cell was assembled so that the opposite side faced the separator. The results are shown in Table 1. The overhumidification power generation performance was inferior to that of the other examples, but was better than that of each comparative example.

[0048] [Comparative Example 1] A cell was assembled using commercially available carbon paper with MPL (24BC) manufactured by SGL as the gas diffusion electrode substrate for fuel cells. The power generation performance is shown in Table 1. The basis weight and electrical resistance values ​​were measured when the MPL was applied.

[0049] Comparative Example 2 A cell was assembled using a carbon fiber fabric (without MPL) manufactured by ZOLTEK having the physical properties shown in Table 1 as the gas diffusion electrode substrate for a fuel cell. The power generation performance is shown in Table 1.

[0050] Comparative Example 3 A cell was assembled using a gas diffusion electrode substrate for a fuel cell, which was prepared by forming an MPL layer in the same manner as in Example 1-1 on the carbon fiber fabric manufactured by ZOLTEK, which was used in Comparative Example 2. The power generation performance is shown in Table 1.

[0051] The evaluation items in the examples and comparative examples shown in Table 1 are as follows.

[0052] [Regulated shrinkage rate during flame-resistant firing] This is the shrinkage rate when the knitted fabric sewn together with sewing thread is restricted from shrinking by the enclosed heat-resistant plate during flame retardation. It shows the average value of the ratio (%) of the heat-resistant plate's vertical dimension to the sewing width's vertical dimension and the ratio (%) of the heat-resistant plate's horizontal dimension to the sewing width's horizontal dimension.

[0053] [Thread width after baking] The resulting carbon fiber knitted fabric was magnified 250 times, and the width of five points for the same type of yarn was measured on the screen, and the average value (μm) was displayed.

[0054] [Post-firing weight] The obtained carbon fiber knitted fabric was cut into a size of 8 cm x 8 cm, the mass was measured, and 2 The "weight" in this application is a unit of mass per unit area of ​​woolen fabrics, etc., as defined in the Japanese Industrial Standards (JIS) L02028. 2 This is synonymous with "grams per serving."

[0055] [Electrical resistance value through the thickness] The resulting carbon fiber knitting was cut into 1 cm 2 The resistance value in the thickness direction was measured by applying a pressure of 100 N to the silver plates. 2 )

[0056] [Thickness when pressed at 1Mpa] The resulting carbon fiber knitting was cut into 1 cm 2 The thickness when pressed with a pressure plate of 1 MPa was measured using a digital thickness measuring instrument (manufactured by Mitutoyo) and the value (μm / MPa) was measured.

[0057] [Over-humidification power generation characteristics] MPL with a basis weight of 18g / m 2 The gas diffusion electrode substrate for fuel cells was coated so that the thickness was the same for both electrodes, 1 cm 2A test piece (gas diffusion electrode substrate) of the above was used. The evaluation cell 100 used in this measurement test, as shown in FIG. 2, has a structure in which a catalyst-loaded polymer membrane 40 is sandwiched between gas diffusion electrode substrates 21 and 22 and grooved separators 51 and 52 on both sides. The polymer membrane 40 (electrolyte membrane: 40 μm thick) was attached to each of the gas diffusion electrode substrates 21 and 22 after a catalyst layer was attached to both the anode and cathode. The thickness of each gas diffusion electrode substrate incorporated in the evaluation cell for this measurement was set by adjusting a shim so that it would be the thickness at a surface pressure of 1 MPa, as shown in Table 1. Humidified hydrogen at 500 cc / min and air at 1000 cc / min were fed at 60°C for a certain period of break-in. Then, nitrogen with a gas humidity of 165% and an oxygen concentration of 1% was flowed through both electrodes of the fuel cell at 45°C, and a voltage between 0.9 V and 0.2 V was cycled five times, and the current density (A / cm) at the final 0.2 V was measured. 2 ) was measured and used as the superhumidification power generation performance (superhumidification gas diffusion resistance value S / M). Here, "break-in" refers to a test run of the cell prior to performance evaluation, in which the power generation capacity is improved to a steady state while changing the power generation pattern (voltage / current conditions and duration) to improve compatibility between the polymer membrane, catalyst layer, and gas diffusion electrode substrate. The average back pressure of the air electrode in this evaluation cell was 0.15 MPa (abs), and the average back pressure of the hydrogen electrode was 0.10 MPa (abs).

[0058] [Large current power generation characteristics] After the measurement of the overhumidification characteristics after a certain period of running-in, air with a temperature of 60°C, a gas humidity of 80%, and an oxygen concentration of 21% was passed through both electrodes, and the voltage was repeated five times from 0.9 V to 0.2 V. The current density (A / cm2) at the final voltage of 0.2 V was measured. 2 ) was measured and used as the large current power generation characteristics.

[0059] [Table 1]

[0060] In Example 1, the overhumidification power generation characteristics were better in Example 1-1 than in Example 1-2. The only difference was that in Example 1-1, MPL was applied to the thick yarn side of the knitted fabric, while in Example 1-2, MPL was applied to the thin yarn side of the knitted fabric; all other conditions were the same. The overhumidification power generation characteristics of Example 1-2 were 14% lower than those of Example 1-1. This is thought to be because the configuration of Example 1-1 provides good drainage, i.e., the space expands in the thickness direction when viewed from the electrolyte membrane side, allowing the generated water vapor to easily diffuse and be discharged through the separator without condensing. To characterize the drainage performance, a 1% oxygen / 99% nitrogen gas mixture was supplied, and the results are compared between the two examples, as shown in Figure 3.

[0061] When comparing the carbon fiber knitted fabrics of Examples 2 and 3 with the carbon paper of Comparative Example 1 and the thick-fiber carbon fiber woven fabric of Comparative Example 2, there was little difference in basis weight or thickness, but the knitted structures of Examples 2 and 3 had good drainage of generated water in the high current range and good superhumidification power generation characteristics. The reason for this will be explained with reference to FIG. 4. FIG. 4(A) is a diagram schematically showing the positional relationship between the stitches of the carbon fiber knitted fabric 21A and the separator 51. FIG. 4(B) is a partially enlarged view of the region from the separator 51 to the MPL layer 21B in cross section II of FIG. 4(A). In FIG. 4(A), the shaded area is the convex portion of the separator 51, which is in contact with the carbon fiber knitted fabric 21A. As shown in Figure 4(A), when air is passed through the groove formed by the present carbon fiber knitted fabric 21A and separator 51 in the direction of the arrow, the knitted fibers are shaped like a V in the direction of the air flow (a shape that gradually narrows along the flow path), reducing the cross-sectional area of ​​the flow path and increasing the flow rate. This is thought to promote electrochemical reactions in that area, increasing power generation efficiency, and also making it easier for water vapor generated at the interface between the electrolyte membrane catalyst layer and the MPL layer to be drawn toward the separator. Matching the pitch of the separator and the pitch of the knitted fabric increases the contact area between the separator protrusions and the knitted fabric, thereby reducing resistance.

[0062] In terms of manufacturing process, the carbon fiber knitted fabrics obtained in Examples 1, 2, and 3 are more energy-efficient and process-efficient than the carbon paper of Comparative Example 1 and the carbon fiber woven fabric made from thick yarn of Comparative Example 2, resulting in reduced CO2 emissions during manufacturing. Specifically, commercially available PAN spun yarn can be knitted before flame-proofing, allowing the use of inexpensive thin yarns. Furthermore, the flame-proofing process requires less equipment space because heating can be performed in a planar form rather than individually. Furthermore, because the flame-proofing treatment reduces strength, conventional methods of knitting thin spun yarns afterward have not been able to produce knitted carbon fiber fabrics. However, in the manufacturing method of the present invention, white yarn (yarn before flame-proofing and carbonization treatment) is first knitted and then flame-proofed in two dimensions, ensuring the shape of the knitted fabric even if the yarn strength is weakened. In fact, by orienting the fibers two-dimensionally during the flame-proofing baking process, tensile strength can be ensured. [Explanation of symbols]

[0063] 21,22 Gas diffusion electrode substrate 21A Carbon fiber knitting (carbon fiber electrode material) 21B MPL layer (microporous smooth layer) 40 Polymer membrane (electrolyte membrane) 51,52 Separator 100 evaluation cells

Claims

1. The carbon fiber spun yarn has a metric count of 1 / 60 Nm or less, a twist angle of 40° or less, and a yarn width of 200 μm or less, Weight per unit area is 150g / m ２ A carbon fiber electrode material comprising a carbon fiber knitted fabric having a compressive strength of 1 MPa or less and a thickness of 230 μm or less when pressed under pressure of 1 MPa.

2. 2. A gas diffusion electrode substrate for a fuel cell, comprising the carbon fiber electrode material of claim 1, and a microporous smooth layer containing a carbon filler and a fluorine-based resin formed on the carbon fiber electrode material.

3. the carbon fiber knitted fabric includes at least two types of spun yarns having different thicknesses, and has a double-sided knitting structure in which thick yarns appear on one side and thin yarns appear on the other side; 3. The gas diffusion electrode substrate for a fuel cell according to claim 2, wherein the microporous smooth layer is formed on the surface of the thick thread side.

4. 4. A fuel cell comprising the gas diffusion electrode substrate for a fuel cell according to claim 2 or 3 incorporated therein.

5. a knitting step for producing a knitted fabric made of a polyacrylonitrile spun yarn having a metric count of less than 1 / 50 Nm, or a knitted fabric made of a doubling spun yarn obtained by doubling the polyacrylonitrile spun yarn with a vanishing fiber so as to untwist the polyacrylonitrile spun yarn, from which the vanishing fiber has been eliminated; a flame-resistant treatment step of treating the knitted fabric obtained in the knitting step at a temperature in the range of 210°C or higher and 250°C or lower for a period of 20 minutes to 12 hours; and A method for producing a carbon fiber electrode material, comprising, after the flame-proofing treatment step, a carbonization-firing step of carbonizing the carbon fiber by holding the carbon fiber in an inert gas at 1200°C to 2600°C for 3 minutes or more and 20 minutes or less.

6. 6. The method for producing a carbon fiber electrode material according to claim 5, wherein in the flame-retardant treatment step, the knitted fabric obtained in the knitting step is subjected to the flame-retardant treatment under conditions in which thermal shrinkage in two dimensions is restricted to within a range of 75% to 95% of the length and width of the fabric before the oxidation firing.

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