Carbon fiber sheet manufacturing method

Abstract

To suppress accumulation of liquid substance-derived deposit on a surface of an in-liquid guide roll in an impregnation tank in a process to impregnate a carbon fiber porous body sheet with the liquid substance by roll-to-roll process in manufacturing a carbon fiber sheet.SOLUTION: A carbon fiber sheet manufacturing method has an impregnation process to impregnate a carbon fiber porous body sheet with a liquid substance by making the carbon fiber porous body sheet pass through an in-liquid guide roll in an impregnation tank filled with the liquid substance with dispersed filler and making the carbon fiber porous body sheet continuously pass in the liquid substance. The carbon fiber sheet manufacturing method has at least one liquid current formation part inside the impregnation tank, and forms liquid current above the in-liquid guide roll by the liquid current formation part.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for producing a carbon fiber sheet suitable for use in electrochemical units using porous electrodes that require electrical conduction and gas diffusion or liquid permeability. Examples of such electrochemical units include fuel cells, water electrolysis cells, redox flow batteries, and air batteries, and the carbon fiber sheet is particularly suitable for use in polymer electrolyte fuel cells. [Background technology]

[0002] Polymer electrolyte fuel cells (PEFCs) generate electromotive force through electrochemical reactions between the anode (containing hydrogen) and cathode (containing oxygen). These components typically consist of a separator, a gas diffusion electrode, a catalyst layer, an electrolyte membrane (electrolyte membrane), another catalyst layer, a gas diffusion electrode, and a separator, stacked in this order. The gas diffusion electrode must have high gas diffusivity to diffuse gas from the separator to the catalyst layer, high drainage properties to expel water generated during the electrochemical reaction, and high conductivity to extract the generated current. Furthermore, the operating temperatures of PEFCs have risen in recent years, creating a demand for carbon fiber sheets with low electrical and thermal resistance. One method for obtaining such carbon fiber sheets is to add a large amount of filler, such as carbon particles, to obtain low electrical and thermal resistance.

[0003] When producing a carbon fiber sheet, it is known to provide a roll-to-roll impregnation step in which a resin composition is applied to a long carbon fiber sheet (Patent Document 1).

[0004] In addition, a method has been proposed in which, when impregnating a carbon fiber sheet precursor with an impregnation liquid, the pass line angle of the carbon fiber sheet precursor is adjusted so that the impregnation liquid does not appear on the upper surface of a submerged guide roll, thereby obtaining a carbon fiber sheet with good texture (Patent Document 2).

[0005] Furthermore, in order to reduce deposition of solids contained in the slurry at the bottom of the impregnation tank, an impregnation tank has been proposed in which the bottom of the tank is sloped and the liquid supply port faces the side (Patent Document 3). [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Patent No. 4591128 [Patent Document 2] Japanese Patent Application Laid-Open No. 2018-159164 [Patent Document 3] Special Publication No. 2015-527187 Summary of the Invention [Problem to be solved by the invention]

[0007] In the production of carbon fiber sheets containing a large amount of filler, when a process of adding the filler by passing a precursor sheet of a carbon fiber sheet through a liquid in which the filler is dispersed is adopted, problems arise in that the filler accumulates on the pass line during the addition process, resulting in uneven adhesion of the filler and breakage of the precursor sheet of the carbon fiber sheet.

[0008] In the invention described in Patent Document 2, since there is no impregnation liquid containing a resin composition on the upper surface of the carbon fiber sheet precursor sheet, there remains a problem that the amount of filler attached varies between the front and back of the sheet depending on the basis weight of the carbon fiber sheet precursor sheet and the composition of the resin composition. Also, in the invention described in Patent Document 3, there remains a problem that the filler accumulates on the guide roll in the liquid. [Means for solving the problem]

[0009] The present invention is configured as follows. (1) A method for producing a carbon fiber sheet, comprising an impregnation step of passing a porous carbon fiber sheet through submerged guide rolls in an impregnation tank filled with a liquid material in which a filler is dispersed, thereby continuously passing the porous carbon fiber sheet through the liquid material to impregnate the porous carbon fiber sheet with the liquid material, wherein the impregnation tank has at least one liquid flow forming unit inside, and the liquid flow forming unit forms a liquid flow above the submerged guide rolls. (2) A method for producing a carbon fiber sheet, comprising an impregnation step of passing a porous carbon fiber sheet through submerged guide rolls in an impregnation tank filled with a liquid material in which a filler is dispersed, thereby continuously passing the porous carbon fiber sheet through the liquid material to impregnate the porous carbon fiber sheet with the liquid material, wherein the impregnation tank has at least one liquid flow forming unit inside, and the liquid flow forming unit forms a liquid flow above the porous carbon fiber sheet. (3) The method for producing a carbon fiber sheet according to (1) or (2), wherein at least one liquid flow forming portion is located above the submerged guide roll or above the carbon fiber porous sheet. (4) The method for producing a carbon fiber sheet according to any one of (1) to (3), wherein the liquid flow forming unit forms a liquid flow in the axial direction of the rotation axis of the submerged guide roll above the submerged guide roll or above the carbon fiber porous sheet. (5) The method for producing a carbon fiber sheet according to any one of (1) to (4), wherein the liquid flow forming unit forms a liquid flow directed toward the surface of a submerged guide roll. (6) The method for producing a carbon fiber sheet according to any one of (1) to (5), wherein the impregnation vessel has two or more liquid flow forming units inside, and at least one of the liquid flow forming units forms a liquid flow at the bottom of the impregnation vessel. (7) The method for producing a carbon fiber sheet according to any one of (1) to (6), wherein at least two liquid flow forming portions are located above the submerged guide roll or above the carbon fiber porous sheet. (8) The method for producing a carbon fiber sheet according to any one of (1) to (7), wherein the carbon fiber sheet is a long sheet-like object. [Effects of the Invention]

[0010] The carbon fiber sheet obtained by the method for producing a carbon fiber sheet of the present invention has good quality stability, and the quality of a fuel cell or water electrolysis device using the carbon fiber sheet is also stable and the performance is improved. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 2 is a schematic side view of an example of the impregnation step in the present invention. [Figure 2] FIG. 2 is a schematic view showing the upper side of a submerged guide roll in the present invention. [Figure 3] FIG. 2 is a top view schematic diagram showing an example of a liquid flow formed above the submerged guide roll in the axial direction of the rotation shaft of the submerged guide roll in the impregnation step of the present invention. [Figure 4] FIG. 2 is a side view showing an example of a liquid flow formed above a submerged guide roll in the impregnation step of the present invention, directed toward the surface of the submerged guide roll. [Figure 5] 1A and 1B are a perspective view and a side view illustrating a first embodiment of the present invention; [Figure 6] 1A and 1B are a schematic perspective view and a schematic side view illustrating Example 2 which is an embodiment of the present invention. [Figure 7] 10A and 10B are a perspective view and a side view illustrating a third embodiment of the present invention. [Figure 8] 1A and 1B are a perspective view and a side view illustrating Comparative Example 1. FIG. [Figure 9] FIG. 2 is a schematic view showing the upper side of a carbon fiber porous sheet according to the present invention. [Figure 10] 10A to 10C are a schematic perspective view, a schematic side view, and a schematic front view illustrating Example 4, which is one embodiment of the present invention. [Figure 11] FIG. 1 is a side view schematic diagram showing an example in which a liquid flow forming section is provided at a position close to the contact point between a submerged guide roll and a carbon fiber porous sheet in the impregnation step of the present invention. [Figure 12] 10A and 10B are a perspective view and a side view illustrating Example 5, which is an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0012] Hereinafter, a method for producing a carbon fiber sheet of the present invention and a method for producing a gas diffusion electrode substrate using the carbon fiber sheet of the present invention will be described.

[0013] [Carbon fiber sheet] The carbon fiber sheet obtained by the carbon fiber sheet manufacturing method of the present invention contains carbon fibers and a filler. The filler is a particulate solid that, when supported on the carbon fiber surfaces or at the intersections between the carbon fibers of the carbon fiber sheet, improves the electrical conductivity and durability of the carbon fiber sheet.

[0014] The carbon fiber sheet obtained by the manufacturing method of the present invention contains a filler as an essential component, but may also contain a carbonized resin composition or a resin composition as a binder. The binder refers to a component other than the carbon fibers and filler in the carbon fiber sheet that plays a role in firmly binding the carbon fibers together. The inclusion of both a filler and a binder is preferable because the filler is supported by the binder and the amount of filler attached is stable.

[0015] In the manufacturing method of the present invention, it is preferable to use a carbon fiber porous sheet containing carbon fibers as a raw material. As the carbon fiber porous sheet, a carbon fiber paper sheet, a carbon fiber woven fabric, a carbon fiber nonwoven fabric, etc. can be used. However, a carbon fiber paper sheet is preferred because it has an excellent property of absorbing dimensional changes in the thickness direction of the electrolyte membrane, i.e., excellent "springiness." In other words, a preferred embodiment of the carbon fiber sheet is carbon paper obtained by binding carbon fiber paper sheets with a binder carrying a filler.

[0016] Examples of carbon fibers that can be used in 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 because of their excellent mechanical strength.

[0017] The carbon fibers used in the present invention preferably have an average single fiber diameter in the range of 3 to 20 μm, more preferably in the range of 5 to 10 μm. When the average single fiber diameter is 3 μm or more, the pore size of the carbon fiber sheet increases, improving drainage and making it easier to suppress flooding. On the other hand, when the average single fiber diameter is 20 μm or less, it is preferable because it is easy to control the thickness of the carbon fiber sheet to the preferred range described below.

[0018] When a carbon fiber paper sheet is used as the carbon fiber porous sheet, the average length of the carbon fibers used in the carbon fiber paper sheet is preferably within the range of 3 to 20 mm, more preferably within the range of 5 to 15 mm. When the average length of the single fibers is 3 mm or more, a carbon fiber sheet with excellent mechanical strength, electrical conductivity, and thermal conductivity can be obtained. On the other hand, when the average length of the single fibers is 20 mm or less, the dispersion of the carbon fibers during papermaking is excellent, and a homogeneous carbon fiber sheet can be obtained.

[0019] The average diameter and average length of single fibers in carbon fibers, and the cross-sectional area of carbon fibers described below are usually measured by directly observing the carbon fibers that are raw materials, but they can also be measured by observing a carbon fiber sheet.

[0020] The carbon fiber porous sheet may contain natural fibers such as pulp or synthetic fibers as fibers other than carbon fibers.

[0021] When a carbon fiber paper sheet is used as the carbon fiber porous sheet, it is preferable that the carbon fibers are randomly dispersed in a two-dimensional plane in order to maintain isotropic in-plane electrical and thermal conductivities.

[0022] The carbon fiber sheet of the present invention preferably contains a resin charcoal as a binder. However, when the carbonization step described below is not applied, a resin composition can also be used.

[0023] Examples of fillers contained in the carbon fiber sheet of the present invention include conductive fillers and inorganic oxide particles such as metal oxide particles. The inclusion of a conductive filler is preferred because it can reduce the electrical and thermal resistance of the carbon fiber sheet, and carbon powder is particularly preferred because it is less likely to cause catalyst degradation.

[0024] Examples of carbon powder that can be used include carbon black such as furnace black, acetylene black, lamp black, and thermal black, graphite such as flake graphite, flaky graphite, amorphous graphite, artificial graphite, expanded graphite, and flake graphite, carbon nanotubes, linear carbon, and milled carbon fiber. Graphite having an average particle size of 2 μm or more and 8 μm or less and an aspect ratio of 20 or more and 60 or less is particularly preferred.

[0025] The carbon fiber sheet obtained by the production method of the present invention preferably contains 10 to 100 parts by mass of filler per 100 parts by mass of carbon fiber. When the filler is 10 parts by mass or more, the carbon fiber sheet has excellent mechanical properties, electrical conductivity, and thermal conductivity. On the other hand, when the filler is 100 parts by mass or less, the carbon fiber sheet has excellent gas diffusivity in the in-plane direction and in the thickness direction. When the carbon fiber sheet further contains a binder, it preferably contains 10 to 200 parts by mass of binder per 100 parts by mass of carbon fiber. When the binder is 10 parts by mass or more, the carbon fiber sheet has even more excellent mechanical properties. On the other hand, when the binder is 200 parts by mass or less, the carbon fiber sheet can be used over a wide range of filler loading amounts without impairing the gas diffusivity in the in-plane direction and in the thickness direction.

[0026] The carbon fiber sheet obtained by the production method of the present invention is preferably a carbon fiber sheet consisting of a single layer sheet, which allows for reduced production costs compared to carbon fiber sheets formed by laminating multiple precursor sheets.

[0027] [Method of manufacturing carbon fiber sheets] Next, a method for producing a carbon fiber sheet of the present invention will be described. The method for producing a carbon fiber sheet will be described in the order of an impregnation step of impregnating a porous carbon fiber sheet with a liquid, a heating and pressurizing step, and a carbonization step. However, the heating and pressurizing step and the carbonization step are not necessarily required and can be combined as appropriate.

[0028] [Impregnation process] The method for producing a carbon fiber sheet of the present invention includes an impregnation step. The impregnation step in the present invention uses a method in which a porous carbon fiber sheet is continuously passed through a liquid material in which a filler is dispersed, thereby impregnating the sheet with the liquid material. The porous carbon fiber sheet impregnated with the liquid material may be referred to as a "pre-impregnated body."

[0029] The liquid material may contain a resin component in addition to the filler. The resin component is preferably a resin component that is carbonized to form a conductive charcoal when fired. Examples of resins constituting the resin component include thermosetting resins such as phenolic resin, epoxy resin, melamine resin, and furan resin. Among these, phenolic resin is preferably used because of its high carbonization yield. Furthermore, the resin component may be either soluble or insoluble in the dispersion medium described below. When an insoluble resin is used, the resin component is dispersed in the dispersion medium.

[0030] The liquid material in which the filler is dispersed contains a dispersion medium in addition to the filler. Examples of the dispersion medium include water, methanol, ethanol, isopropyl alcohol, etc. Multiple dispersion media may be mixed as needed.

[0031] In the impregnation step, if the mass of the filler or resin component is reduced in a subsequent step, the mass reduction rate can be confirmed in advance, and the content of each component in the carbon fiber sheet can be controlled by impregnating the carbon fiber sheet with a mass of each component calculated by dividing the mass of each component to be contained in the carbon fiber sheet by the mass reduction rate.

[0032] The impregnation step in the method for producing a carbon fiber sheet of the present invention is a roll-to-roll production step. The roll-to-roll production step includes a roll-out section that holds a long, rolled-up carbon fiber porous sheet before processing and continuously pays out the carbon fiber porous sheet to an impregnation tank, and a roll-up section that winds up the pre-impregnated body after impregnation, and is a step in which the carbon fiber porous sheet is conveyed at a predetermined speed while applying a predetermined tension to the carbon fiber porous sheet between the roll-out section and the roll-up section. In the present invention, after the impregnation step, the pre-impregnated body may be cut into sheet substrates and sent to a subsequent step, or the long sheet-like body may be passed through a heating and pressurizing step or a carbonization step by a roll-to-roll method.

[0033] Preferred embodiments of the present invention will be described below with reference to the drawings. However, the present invention is not limited to these embodiments. Furthermore, the descriptions of preferred configurations and numerical ranges in the descriptions of individual embodiments can also be interpreted as descriptions of the method for producing a carbon fiber sheet of the present invention as a general concept.

[0034] As shown in Fig. 1, the impregnation step of the present invention involves passing a porous carbon fiber sheet 6 through an impregnation tank 1 containing a liquid 5. At this time, a submerged guide roll 2 is installed in the impregnation tank 1 to change the direction of travel of the porous carbon fiber sheet 6, and at least one liquid flow forming unit forms a liquid flow above the submerged guide roll 2 or above the porous carbon fiber sheet, preventing the filler from remaining on the surface of the submerged guide roll 2.

[0035] The submerged guide roll 2 may be completely immersed in the liquid 5, or a portion of the submerged guide roll 2 may be exposed above the liquid surface. Complete immersion is preferable because it makes it easier for deposits such as filler that have accumulated on the surface to be dispersed and dissolved again in the liquid 5. Furthermore, it is also possible to provide a mechanism for stirring the liquid 5 inside the impregnation tank 1, a mechanism for supplying the liquid 5 so as to maintain a constant liquid level, and a squeezing mechanism for removing excess dispersion medium contained in the impregnated porous carbon fiber sheet 6. Examples of squeezing mechanisms include a method in which a roll is pressed against the impregnated porous carbon fiber sheet 6 from one side, and a method in which the porous carbon fiber sheet 6 is passed between two rolls to apply pressure from both sides.

[0036] Next, the carbon fiber porous sheet impregnated with the liquid is dried at a temperature of 80 to 200°C to obtain a pre-impregnated body.

[0037] In the past, filler gradually accumulated on the surface of the submerged guide roll 2 in the impregnation tank 1, which caused a gradual change in the filler concentration in the liquid material 5 in the impregnation tank 1, and the accumulated filler was transferred from the submerged guide roll 2 to the porous carbon fiber sheet 6, resulting in an unstable amount of filler adhesion in the pre-impregnated body. The carbon fiber sheet obtained from such a pre-impregnated body had problems such as deviations from the target values in terms of basis weight and physical properties, or large variations. Furthermore, if the amount of deposits on the surface of the submerged guide roll 2 locally increased, abnormal tension was applied to the porous carbon fiber sheet 6 from that point, which could lead to meandering or breakage of the porous carbon fiber sheet 6.

[0038] In contrast, in the present invention, by forming a liquid flow above the submerged guide roll 2 or above the porous carbon fiber sheet, it is possible to prevent the filler from accumulating on the surface of the submerged guide roll 2, thereby suppressing the above problem. Furthermore, when a liquid flow is formed above the submerged guide roll 2, even if filler accumulates on the surface of the submerged guide roll 2, the accumulated filler can be removed by the liquid flow. Here, "above the submerged guide roll" refers to the range above a line extending ±45° from a line 8 extending vertically upward from the lowest point 7 of the submerged guide roll when viewed from the axial direction of the submerged guide roll 2, as shown in FIG. 2. Furthermore, "above the porous carbon fiber sheet" refers to the area above the porous carbon fiber sheet 6, i.e., the range indicated by the shaded area, when the impregnation process is viewed from the axial direction of the submerged guide roll 2, as shown in FIG. The upper part 17 of the carbon fiber porous sheet may be an area overlapping with the upper part of the submerged guide roll 2 as shown in Figure 9(a), or may include an area that is not above the submerged guide roll 2 as shown in Figures 9(b) and 9(c).

[0039] The liquid flow forming unit in the present invention is not limited to any particular mechanism capable of generating a liquid flow in the liquid material, and examples thereof include a rotor such as a propeller, a stirring plate, a stirring rod, and a nozzle. A liquid flow can be formed by rotating the liquid material in the impregnation tank with a rotor, stirring the liquid material in the impregnation tank with a stirring plate or a stirring rod, or discharging or suctioning the liquid material from a nozzle. The position of the liquid flow can also be controlled by combining these with a straightening plate. Because the liquid flow formed in the liquid flow forming unit propagates and spreads to the surrounding liquid material, even if the liquid flow forming unit is not close to the submerged guide roll, it can be configured to prevent filler from accumulating on the submerged guide roll. Furthermore, as described below, forming the liquid flow so that it strikes the surface of the submerged guide roll can more effectively prevent filler from accumulating on the submerged guide roll.

[0040] In the present invention, it is preferable that the liquid flow forming section is located above the submerged guide roll or above the porous carbon fiber sheet. As in the case of the liquid flow described above, "above the submerged guide roll" refers to the range shown in Figure 2. As in the case of the liquid flow described above, "above the porous carbon fiber sheet" refers to the range shown in Figure 9. By having the liquid flow forming section located above the submerged guide roll or above the porous carbon fiber sheet, a liquid flow can be easily formed above the submerged guide roll or above the porous carbon fiber sheet without the need to provide a straightening plate or the like.

[0041] Furthermore, the direction of the liquid flow formed above the submerged guide roll is preferably the axial direction of the rotation axis of the submerged guide roll or the direction toward the surface of the submerged guide roll, and it is also preferable for liquid flows to be formed in both directions.

[0042] A method for forming a liquid flow in the axial direction of the rotation axis of the submerged guide roll can be to arrange a liquid flow forming unit so as to form a liquid flow in the axial direction of the rotation axis of the submerged guide roll. For example, a stirrer such as a propeller, a discharge nozzle, or a suction nozzle can be provided on the side surface of the end of the submerged guide roll in the impregnation tank and above the submerged guide roll, and oriented so as to form a liquid flow toward the opposite side surface, thereby forming a liquid flow in the axial direction of the rotation axis of the submerged guide roll. Specific examples are shown in Figures 5 and 6. Even if the nozzle is provided in a location other than the above, a liquid flow can also be formed in the axial direction of the rotation axis of the submerged guide roll by using a straightening plate. Note that the axial direction of the rotation axis of the submerged guide roll in this invention is not limited to a direction parallel to the rotation axis, but also includes a direction in which the angle between the rotation axis and a line parallel to the rotation axis is less than 45°.

[0043] One method for forming a liquid flow toward the surface of the submerged guide roll is to arrange a liquid flow forming unit so as to form a liquid flow toward the surface of the submerged guide roll. For example, a liquid flow toward the surface of the submerged guide roll can be formed by installing a stirrer such as a propeller or a discharge nozzle above the submerged guide roll in the impregnation tank and pointing it toward the surface of the submerged guide roll. A specific example is shown in Figure 7. Even if the nozzle is provided in a different location from the above, a liquid flow toward the surface of the submerged guide roll can also be formed by using a straightening plate. Here, a liquid flow toward the surface of the submerged guide roll refers to the case where the liquid flow hits the surface of the submerged guide roll.

[0044] Another preferred embodiment is to form a liquid flow in the axial direction of the rotation axis of the submerged guide roll and toward the surface of the submerged guide roll. A method for forming a liquid flow in the axial direction of the rotation axis of the submerged guide roll and toward the surface of the submerged guide roll is to arrange a liquid flow forming unit so that a liquid flow is formed in the axial direction of the rotation axis of the submerged guide roll and toward the surface of the submerged guide roll. For example, a stirrer such as a propeller, a discharge nozzle, or a suction nozzle is provided on the side surface of the end of the submerged guide roll in the impregnation tank and above the submerged guide roll, so that the angle between the direction of the liquid flow and a line parallel to the rotation axis of the submerged guide roll is less than 45° and the liquid flow is directed toward the surface of the submerged guide roll. This can form a liquid flow in the axial direction of the rotation axis of the submerged guide roll and toward the surface of the submerged guide roll. A specific example is shown in FIG. 10. If the angle 19 between the direction of the liquid flow and a line parallel to the rotation axis of the submerged guide roll is less than 30°, a uniform liquid flow is likely to be formed across the surface width of the submerged guide roll, which is preferable because it is possible to more effectively prevent the filler from accumulating on the submerged guide roll.

[0045] In addition to being located directly above the central axis of the submerged guide roll as shown in Figures 5 to 7 and 10, another preferred embodiment is to provide the liquid flow forming unit near the contact point between the submerged guide roll 2 and the porous carbon fiber sheet when viewing the submerged guide roll 2 in the axial direction, as shown in Figure 11. When the liquid flow forming unit is located close to the submerged guide roll and the porous carbon fiber sheet, the liquid flow advances through a narrow space, making it difficult for the liquid flow to diffuse and attenuate, thereby more effectively preventing filler from accumulating on the submerged guide roll. Furthermore, as shown in Figures 11(b) and 11(c), if the pass line of the porous carbon fiber sheet 6 is not perpendicular to the liquid surface, filler may accumulate on the porous carbon fiber sheet, and the filler accumulated on the porous carbon fiber sheet may be transferred by pressure to the submerged guide roll 2, resulting in filler accumulation on the submerged guide roll. When the liquid flow forming section is located close to the submerged guide roll and the carbon fiber porous sheet, not only can the filler be effectively prevented from stagnation on the submerged guide roll, but also from accumulating on the carbon fiber porous sheet.

[0046] While the porous carbon fiber sheet is passing through the liquid, it is subjected to shear force from the liquid, etc., and is therefore prone to breakage. The liquid flow in the axial direction of the rotation axis of the submerged guide roll passes over the submerged guide roll and is therefore unlikely to interfere with the porous carbon fiber sheet, and even if the liquid flow is present, the porous carbon fiber sheet is unlikely to break. Furthermore, the liquid flow toward the surface of the submerged guide roll is unlikely to interfere with the porous carbon fiber sheet, and the porous carbon fiber sheet is unlikely to break, because the flow rate decreases when the liquid flow collides with the surface of the submerged guide roll.

[0047] In the present invention, one preferred embodiment is that the impregnation tank has two or more liquid flow forming units, and at least one of the liquid flow forming units forms a liquid flow at the bottom of the impregnation tank. Forming a liquid flow at the bottom of the impregnation tank can suppress filler deposition at the bottom of the impregnation tank. When two or more liquid flow forming units are provided, they are preferably arranged so that the liquid flows formed by each unit do not interfere with each other and cause attenuation of the liquid flow.

[0048] Furthermore, in the present invention, it is preferable that at least two liquid flow forming sections are located above the submerged guide roll or above the porous carbon fiber sheet. The two liquid flow forming sections may each include one or more liquid flow forming sections that form a liquid flow in the axial direction of the rotation shaft of the submerged guide roll and one or more liquid flow forming sections that form a liquid flow toward the surface of the submerged guide roll, or each may be located in two or more places. For example, by providing multiple liquid flow forming sections that form a liquid flow in the axial direction of the rotation shaft of the submerged guide roll along the rotation shaft direction of the submerged guide roll, effects such as suppressing filler deposition can be obtained even when a wider porous carbon fiber sheet is used.

[0049] [Heat and pressure process] In one preferred embodiment, after forming the pre-impregnated body, a heating and pressurizing step is performed in which the pre-impregnated body is pressurized and heat-treated. When the carbonization step is included in the method for producing a carbon fiber sheet, it is preferable to provide the heating and pressurizing step before the carbonization step.

[0050] By carrying out the heat and pressure treatment, the thickness of the carbon fiber sheet can be controlled more accurately.

[0051] Furthermore, multiple pre-impregnated bodies can be laminated together by stacking multiple pre-impregnated bodies and then subjecting them to a heat and pressure treatment. In this case, multiple pre-impregnated bodies having the same properties can be laminated together, or multiple types of pre-impregnated bodies having different properties can be laminated together.

[0052] [Carbonization process] When a pre-impregnated body is prepared by impregnating a liquid material containing a resin component, a carbonization step is preferably provided to carbonize the resin component. The carbonization step is a step of calcining the pre-impregnated body by treating it at a high temperature, and the calcination is preferably carried out in an inert atmosphere. For the calcination, a batch-type heating furnace or a continuous-type heating furnace can be used, but a continuous-type heating furnace is preferred because it increases the speed of the carbonization treatment and reduces production costs.

[0053] The maximum firing temperature is preferably within the range of 1,300 to 3,000°C. If the maximum temperature is 1,300°C or higher, the resin component in the pre-impregnated body is carbonized, resulting in a carbon fiber sheet with excellent electrical and thermal conductivity. On the other hand, if the maximum temperature is 3,000°C or lower, the operating cost of the heating furnace is reduced.

[0054] From the viewpoint of reducing the production costs in the process of producing and using a carbon fiber sheet, the carbon fiber sheet obtained by the carbon fiber sheet production method of the present invention is preferably a long sheet-like product. In the present invention, "long" means that the length of the carbon fiber sheet in the longitudinal direction is 10 m or more.

[0055] [Gas diffusion electrode substrate] For the purpose of improving drainage, the carbon fiber sheet of the present invention can be subjected to a water-repellent treatment or a microporous layer can be formed on at least one surface of the carbon fiber sheet to form a gas diffusion electrode substrate. The microporous layer may be formed after the carbon fiber sheet is subjected to a water-repellent treatment.

[0056] The water-repellent treatment can be carried out by coating or impregnating the carbon fiber sheet with a water-repellent material and then heat treating it.

[0057] As the water-repellent material, it is preferable to use a fluorine-based polymer because of its excellent corrosion resistance. Examples of fluorine-based polymers include polytetrafluoroethylene (PTFE), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), and tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA).

[0058] Furthermore, the carbon fiber sheet of the present invention having a microporous layer formed on at least one surface thereof can be preferably used as a gas diffusion electrode substrate. The microporous layer is composed of conductive fine particles and a microporous layer resin that binds the conductive fine particles. The microporous layer can be formed by applying a coating liquid containing the conductive fine particles and the microporous layer resin to the surface of the carbon fiber sheet. The conductive fine particles are preferably nanocarbon powder such as acetylene black, and the microporous layer resin is preferably a fluororesin such as PTFE or FEP. Microcarbon powder such as graphite may be mixed as the conductive fine particles to improve the conductivity of the microporous layer, and non-conductive fine particles may be mixed to improve the durability of the catalyst.

[0059] The coating liquid may contain a dispersion medium such as water or an organic solvent, and 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.

[0060] The microporous layer can be formed by applying the coating liquid to a carbon fiber sheet. Coating methods such as screen printing, rotary screen printing, spraying, intaglio printing, gravure printing, die coater coating, bar coating, and blade coating can be used for the coating. It is also preferable to subsequently dry the sheet at a temperature of 80 to 180°C. Furthermore, after drying, it is also preferable to heat the sheet at a temperature of about 300 to 400°C, preferably 340 to 390°C, to melt the water-repellent material.

[0061] The upper limit of the basis weight of the microporous layer is not particularly limited, but is preferably 50 g / m 2 It is preferable that the content is 30 g / m or less, and more preferably 30 g / m 2 or less, and more preferably 25 g / m 2 The lower limit is 10 g / m 2 It is preferable that the content is 14 g / m or more. 2 More preferably, it is 16 g / m or more, and even more preferably, it is 16 g / m or more. 2 The weight of the microporous layer is 10 g / m 2When the microporous layer has a mass per unit area of 50 g / m or more, the entire surface of the carbon fiber sheet can be covered with the microporous layer, which further promotes back-diffusion of generated water and suppresses drying-up. 2 If the thickness is less than this, the drainage property is further improved and flooding can be suppressed.

[0062] [Membrane electrode assembly] A membrane electrode assembly can be formed by bonding the above-mentioned carbon fiber sheet or gas diffusion electrode substrate to at least one side of a solid polymer electrolyte membrane having catalyst layers on both sides. The catalyst layer consists of a layer containing a solid polymer electrolyte and catalyst-supported carbon. Platinum is usually 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. When a gas diffusion electrode substrate having a microporous layer is used, arranging the microporous layer of the gas diffusion electrode substrate on the catalyst layer side not only facilitates back diffusion of generated water, but also increases the contact area between the catalyst layer and the gas diffusion electrode substrate, thereby reducing contact electrical resistance.

[0063] [Fuel cell] A fuel cell can be constructed by placing separators on both sides of the membrane electrode assembly. Typically, a polymer electrolyte fuel cell is constructed by stacking multiple such membrane electrode assemblies, each sandwiched between separators via a gasket. [Example]

[0064] Next, the method for producing a carbon fiber sheet of the present invention will be specifically described with reference to examples.

[0065] <Measuring the thickness of a carbon fiber sheet> The thickness of the carbon fiber sheet was measured by applying a load of 0.15 MPa using a micrometer with a measuring probe having a diameter of 5 mm at the measuring surface. The thickness was measured at five positions, and the average value was taken as the thickness of the carbon fiber sheet.

[0066] <Measurement of the basis weight of carbon fiber sheets> The carbon fiber sheet to be measured was cut into a 10 cm square to prepare a sample. The mass [g] of the sample was measured and divided by the area (0.01 m) of the sample. 2 ) was divided by

[0067] <Evaluation of uniformity of graphite particles in pre-impregnated body> A 20 cm square sample was cut from each of the pre-impregnated sections, 50 cm long at the beginning and end of processing, and dissolved in methanol. The phenolic resin dissolved in methanol, while the carbon fiber and flake graphite did not. The carbon fiber was removed from the solution, and the solution was dried at 90°C for 1 hour. The resulting mass was measured to determine the combined mass of the phenolic resin and flake graphite. The dried material was then redissolved in methanol. The solution was suction-filtered through a PTFE membrane filter with a 1 μm pore size to separate the flake graphite from the solution. The flake graphite was then dried at 90°C for 1 hour and its mass was measured to determine the mass of the flake graphite. The difference between these masses gave the mass of the phenolic resin. Finally, the mass ratio of flake graphite to phenolic resin was calculated. This ratio was calculated for the beginning and end of processing of the pre-impregnated section, and any change in this ratio was used as a measure of the uniformity of the impregnation process.

[0068] Example 1 Toray Industries, Inc.'s polyacrylonitrile carbon fiber "TORAYCA (registered trademark)" T300 (average diameter: 7 μm) was cut to a length of 12 mm, dispersed in water, and continuously made into paper. The paper was then impregnated with a 10% by mass aqueous solution of polyvinyl alcohol, dried, and made into a paper with a basis weight of 30 g / m. 2A porous carbon fiber sheet having a width of 700 mm and a length of 500 m was obtained. The amount of polyvinyl alcohol attached was 20 parts by mass per 100 parts by mass of the porous carbon fiber sheet.

[0069] Next, a liquid was prepared by mixing flake graphite (average particle size: 5 μm), phenolic resin (a mixture of resol-type phenolic resin and novolac-type phenolic resin in a mass ratio of 1:1), and methanol in a mass ratio of 5:5:90.

[0070] A porous carbon fiber sheet 6 was continuously passed through an impregnation tank 1 filled with a liquid in a roll-to-roll manner and then dried at 100°C for 5 minutes to obtain a pre-impregnated body containing the liquid. The impregnation tank 1 used here had the configuration shown in Figure 5. The submerged guide roll 2 in the impregnation tank was made of stainless steel and had a diameter of 20 cm. A propeller-type agitator 13 was installed on one side of the impregnation tank as a liquid flow generator, creating a liquid flow in the axial direction of the rotation axis of the submerged guide roll 2. A second liquid flow generator, a discharge port 12, was installed in the center of the upstream side of the impregnation tank, through which the liquid was supplied. A notch 14 was installed in the upper part of the downstream side of the impregnation tank to allow the liquid to overflow. The overflowed liquid was collected and resupplied through the discharge port 12, allowing the liquid to circulate. Even after obtaining a 500 m pre-impregnated body, no flake graphite or phenolic resin deposits were found on the submerged guide roll 2 in the impregnation tank.

[0071] The obtained 500 m pre-impregnated body was evaluated as described above in <Evaluation of uniformity of graphite particles in pre-impregnated body>. The ratio of phenolic resin to flake graphite was 5:5 at both the beginning and end of the processing of the pre-impregnated body, and the amount of attached flake graphite was uniform throughout the entire length. The beginning and end of the processing of the pre-impregnated body were then cut into pieces measuring 50 cm in the longitudinal direction and 50 cm in the width direction to obtain sheets of pre-impregnated body.

[0072] Next, the pre-impregnated sheets obtained as described above were sandwiched between upper and lower hot plates in a press molding machine and subjected to a heat compression treatment at 180°C for 5 minutes. During this treatment, release paper was placed between the paper sheet and the hot plates to prevent the hot plates from adhering to the paper sheet, and spacers were placed around the edges of the upper and lower hot plates to adjust the thickness of the pre-impregnated sheets after heat compression. The sheets were then heated to 2,000°C in a nitrogen atmosphere in a heating furnace and carbonized, yielding two carbon fiber sheets, one at the beginning and one at the end of processing. The carbon fiber sheets were measured as described above in <Measurement of Carbon Fiber Sheet Thickness> and <Measurement of Carbon Fiber Sheet Basis Weight>. The carbon fiber sheet at the beginning of processing had a thickness of 160 μm and a basis weight of 50.0 g / m. 2 The carbon fiber sheet at the end of the processing had a thickness of 160 μm and a weight per unit area of 49.5 g / m 2 The difference in the weight of the carbon fiber sheet between the beginning and end of processing was 0.5 g / m 2 A uniform carbon fiber sheet was obtained.

[0073] Example 2 As shown in FIG. 6 , a carbon fiber sheet was produced in the same manner as in Example 1, except that the position of the discharge port 12 in the impregnation step was changed to the bottom of the impregnation tank 1. When a 500 m pre-impregnated body was obtained, no deposits of flake graphite or phenolic resin had formed on the submerged guide roll 2 in the impregnation tank, and no deposits had formed at the bottom of the impregnation tank 1. The obtained 500 m pre-impregnated body was evaluated as described above in <Evaluation of uniformity of graphite particles in pre-impregnated body>. The ratio of phenolic resin to flake graphite was 5:5 at both the beginning and end of processing of the pre-impregnated body, and the amount of attached flake graphite was uniform throughout the entire length. Furthermore, when the carbon fiber sheet was measured as described above in <Measurement of thickness of carbon fiber sheet> and <Measurement of basis weight of carbon fiber sheet>, the carbon fiber sheet at the beginning of processing had a thickness of 160 μm and a basis weight of 50.0 g / m 2 The carbon fiber sheet at the end of the processing had a thickness of 160 μm and a weight of 49.7 g / m 2The difference in the weight of the carbon fiber sheet between the beginning and end of processing was 0.3 g / m 2 A uniform carbon fiber sheet was obtained.

[0074] Example 3 As shown in FIG. 7 , in the impregnation process, a pipe 15 having a slit outlet was provided above the submerged guide roll 2 instead of the propeller-type stirrer 13. The pipe 15 had a downward slit outlet 16, and a liquid was discharged from the slit outlet 16 to form a liquid flow toward the surface of the submerged guide roll 2. Apart from the liquid flow forming section, a carbon fiber sheet was produced in the same manner as in Example 2. When a 500 m pre-impregnated body was obtained, no deposits of flake graphite or phenolic resin had formed on the submerged guide roll 2 in the impregnation tank, and no deposits had formed at the bottom of the impregnation tank 1. The obtained 500 m pre-impregnated body was evaluated as described above in <Evaluation of uniformity of graphite particles in the pre-impregnated body>. The ratio of phenolic resin to flake graphite was 5:5 at both the beginning and end of processing of the pre-impregnated body, and the amount of attached flake graphite was uniform throughout the entire length. Furthermore, when the carbon fiber sheet was measured according to the above <Measurement of the thickness of the carbon fiber sheet> and <Measurement of the basis weight of the carbon fiber sheet>, the carbon fiber sheet at the beginning of processing had a thickness of 160 μm and a basis weight of 49.5 g / m 2 The carbon fiber sheet at the end of the processing had a thickness of 160 μm and a weight per unit area of 49.2 g / m 2 The difference in the weight of the carbon fiber sheet between the beginning and end of processing was 0.3 g / m 2 A uniform carbon fiber sheet was obtained. Example 4 As shown in Figure 10, a carbon fiber sheet was produced in the same manner as in Example 2, except that the angle 19 between the direction of the liquid flow formed by the propeller-type agitator 13 and a line parallel to the rotation axis of the submerged guide roll during the impregnation process was changed to 10°. When a 500 m pre-impregnated body was obtained, no deposits of flake graphite or phenolic resin had formed on the submerged guide roll 2 in the impregnation tank, and no deposits had formed on the bottom of the impregnation tank 1. The obtained 500 m pre-impregnated body was evaluated as described above in <Evaluation of uniformity of graphite particles in pre-impregnated body>. The ratio of phenolic resin to flake graphite was 5:5 at both the beginning and end of the pre-impregnated body processing, and the amount of attached flake graphite was uniform throughout the entire length. Furthermore, when the carbon fiber sheet was measured as described above in <Measurement of thickness of carbon fiber sheet> and <Measurement of basis weight of carbon fiber sheet>, the carbon fiber sheet at the beginning of processing had a thickness of 160 μm and a basis weight of 50.4 g / m 2 The carbon fiber sheet at the end of the processing had a thickness of 160 μm and a weight of 50.2 g / m 2 The difference in the weight of the carbon fiber sheet between the beginning and end of processing was 0.2 g / m 2 A uniform carbon fiber sheet was obtained. Example 5 As shown in FIG. 12 , a carbon fiber sheet was produced in the same manner as in Example 2, except that a suction nozzle 20 was provided above the submerged guide roll 2 on the side of the end of the submerged guide roll opposite the propeller stirrer 13 during the impregnation process. When a 500 m pre-impregnated body was obtained, no deposits of flake graphite or phenolic resin had formed on the submerged guide roll 2 in the impregnation tank, and no deposits had formed at the bottom of the impregnation tank 1. The obtained 500 m pre-impregnated body was evaluated as described above in <Evaluation of uniformity of graphite particles in pre-impregnated body>. The ratio of phenolic resin to flake graphite was 5:5 at both the beginning and end of the pre-impregnated body, and the amount of attached flake graphite was uniform throughout the entire length. Furthermore, when the carbon fiber sheet was measured as described above in <Measurement of thickness of carbon fiber sheet> and <Measurement of basis weight of carbon fiber sheet>, the carbon fiber sheet at the beginning of the processing had a thickness of 160 μm and a basis weight of 49.9 g / m2 The carbon fiber sheet at the end of the processing had a thickness of 160 μm and a weight per unit area of 49.8 g / m 2 The difference in the weight of the carbon fiber sheet between the beginning and end of processing was 0.1 g / m 2 A uniform carbon fiber sheet was obtained.

[0075] (Comparative Example 1) As shown in FIG. 8, an attempt was made to manufacture a carbon fiber sheet in the same manner as in Example 1, except that the propeller-type stirring bar 13 was not provided.

[0076] During the production of the pre-impregnated body, deposits of flake graphite and phenolic resin gradually deposited on the surface of the submerged guide roll 2 in the impregnation tank 1. When the porous carbon fiber sheet 6 had passed 400 m, the porous carbon fiber sheet 6 broke due to the force of the deposits deposited on the submerged guide roll 2, and the production of the pre-impregnated body was interrupted. The deposits were deposited on the submerged guide roll 2 over a width of 700 mm, the width through which the porous carbon fiber sheet 6 passed, and had grown to a thickness of 3 cm around the entire circumference of the submerged guide roll 2. The obtained 400 m pre-impregnated body was evaluated as described above in <Evaluation of uniformity of graphite particles in the pre-impregnated body>. The ratio of phenolic resin to flake graphite was 5:5 at the beginning of processing of the pre-impregnated body, but was 6:4 at the end of processing, confirming that the amount of graphite particles attached had changed during the continued production of the pre-impregnated body. [Explanation of symbols]

[0077] 1 Impregnation tank 2 Submerged guide roll 3, 4 Transport roll 5 Liquids 6. Carbon fiber porous sheet 7. The lowest point of the guide roll in the liquid 8. A straight line extending vertically upward from the lowest point of the submerged guide roll 9 Line indicating the range above the submerged guide roll 10 Liquid flow in the axial direction of the rotation axis of the submerged guide roll 11 Liquid flow toward the surface of the submerged guide roll 12 Outlet 13 Propeller stirrer 14 Cutout 15 Pipe with slit outlet 16 Slit outlet 17 Above the carbon fiber porous sheet 18 Line parallel to the rotation axis of the submerged guide roll 19 Angle between the direction of the liquid flow and a line parallel to the rotation axis of the submerged guide roll 20 suction nozzle [Industrial Applicability]

[0078] The present invention can be used in electrochemical units that use porous electrodes, such as fuel cells, water electrolysis cells, redox flow batteries, and air batteries, and is particularly suitable for use in polymer electrolyte fuel cells.

Claims

1. A method for producing a carbon fiber sheet, comprising an impregnation step of passing a porous carbon fiber sheet through submerged guide rolls in an impregnation tank filled with a liquid material in which a filler is dispersed, thereby continuously passing the porous carbon fiber sheet through the liquid material to impregnate the porous carbon fiber sheet with the liquid material, wherein the impregnation tank has at least one liquid flow forming unit inside, and the liquid flow forming unit forms a liquid flow above the submerged guide rolls.

2. A method for producing a carbon fiber sheet, comprising an impregnation step of passing a porous carbon fiber sheet through submerged guide rolls in an impregnation tank filled with a liquid material in which a filler is dispersed, thereby continuously passing the porous carbon fiber sheet through the liquid material, thereby impregnating the porous carbon fiber sheet with the liquid material, wherein the impregnation tank has at least one liquid flow forming unit inside, and the liquid flow forming unit forms a liquid flow above the porous carbon fiber sheet.

3. 3. The method for producing a carbon fiber sheet according to claim 1, wherein at least one liquid flow forming portion is located above the submerged guide roll or above the carbon fiber porous sheet.

4. The method for producing a carbon fiber sheet according to claim 1 or 2, wherein the liquid flow forming unit forms a liquid flow in the axial direction of the rotation axis of the submerged guide roll above the submerged guide roll or above the carbon fiber porous sheet.

5. The method for producing a carbon fiber sheet according to claim 1 or 2, wherein the liquid flow forming unit forms a liquid flow directed toward the surface of the submerged guide roll.

6. 3. The method for producing a carbon fiber sheet according to claim 1, wherein the impregnation vessel has two or more liquid flow forming portions inside, and at least one of the liquid flow forming portions forms a liquid flow at the bottom of the impregnation vessel.

7. 3. The method for producing a carbon fiber sheet according to claim 1, wherein at least two liquid flow forming portions are located above the submerged guide roll or above the carbon fiber porous sheet.

8. The method for producing a carbon fiber sheet according to claim 1 or 2, wherein the carbon fiber sheet is a long sheet-like object.

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