Carbon fiber sheet manufacturing method

Abstract

Carbon fiber sheets with little deflection or warping can be manufactured from cellulose paper. [Solution] The method for manufacturing a carbon fiber sheet includes a sheet preparation process in which a sheet made of cellulose paper is prepared; a heat treatment process in which the sheet is heat-treated at a temperature of 200°C or higher and 300°C or lower in an oxygen atmosphere; and a carbonization process in which the sheet after the heat treatment process is sandwiched between press plates and heated to 1000°C or higher in an inert gas atmosphere while a load is applied, thereby carbonizing it.

Description

[Technical Field]

[0001] The present invention relates to a method for producing a carbon fiber sheet. [Background technology]

[0002] A polymer electrolyte fuel cell includes a fuel cell stack made up of a plurality of unit cells stacked together. Each unit cell has a membrane electrode assembly made up of a solid polymer electrolyte membrane (hereinafter referred to as the electrolyte membrane) and a pair of catalyst layers sandwiching the electrolyte membrane, a pair of gas diffusion layers sandwiching the membrane electrode assembly, and an anode separator and a cathode separator sandwiching the pair of gas diffusion layers.

[0003] An example of such a gas diffusion layer is the gas diffusion electrode disclosed in Patent Document 1. The gas diffusion electrode disclosed in Patent Document 1 includes a sheet-like porous substrate and a microporous layer (hereinafter, MPL) disposed in contact with one surface of the porous substrate. The porous substrate is carbon paper, carbon cloth, carbon nonwoven fabric, or the like. The MPL is formed by applying a paste-like coating liquid, which is a mixture of conductive fine particles, a binder, a solvent, a thickener, and the like, to one surface of the porous substrate. The conductive particles are carbon black, such as acetylene black, having an average particle size of 20 to 150 nm. The binder is, for example, polytetrafluoroethylene (PTFE).

[0004] Traditionally, carbon paper is made by heating cellulose paper to over 1000 degrees Celsius to carbonize it. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2022-42074 Summary of the Invention [Problem to be solved by the invention]

[0006] When carbon paper is formed, the cellulose fibers tend to shrink thermally, causing the paper to warp and bend. This can lead to problems such as difficulty in applying the coating liquid that forms the MPL to the surface of the porous substrate, and problems such as the gas diffusion layer breaking under the load when assembling the single cell.

[0007] This problem is not limited to the carbon paper that forms the gas diffusion layer of fuel cells; it also occurs in carbon fiber sheets formed by carbonizing cellulose paper by heating it to over 1000 degrees. [Means for solving the problem]

[0008] A method for manufacturing a carbon fiber sheet to solve the above problems includes a sheet preparation step of preparing a sheet that is cellulose paper, a heat treatment step of heat treating the sheet at a temperature of 200°C or more and 300°C or less in an oxygen atmosphere, and a carbonization step of carbonizing the sheet after the heat treatment step by sandwiching the sheet between press plates and heating it to 1000°C or more in an inert gas atmosphere while applying a load.

[0009] To produce a carbon fiber sheet with minimal deflection or warpage from a cellulose paper sheet, the sheet can be sandwiched between press plates, subjected to a load, and heated to 1000°C or higher in an inert gas atmosphere for carbonization. In this case, the cellulose fibers undergo rapid thermal contraction while restrained by the press plates. As a result, the cellulose fibers are likely to break, causing cracks in the carbon fiber sheet.

[0010] In this regard, according to the above-mentioned method, the sheet is heat-treated at a temperature of 200°C or higher and 300°C or lower in an oxidizing atmosphere before carbonization. This causes the cellulose fibers to shrink to a certain extent and harden, making the cellulose fibers less likely to deform. This prevents the cellulose fibers from rapidly shrinking due to heat during the carbonization process. This prevents the cellulose fibers from breaking and, ultimately, the carbon fiber sheet from cracking. Therefore, a carbon fiber sheet with little deflection or warpage can be produced from cellulose paper. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 1 is a cross-sectional view that schematically shows a gas diffusion layer having a carbon fiber sheet manufactured by one embodiment of the method for manufacturing a carbon fiber sheet. [Figure 2] FIG. 2 is a flowchart showing a manufacturing procedure of a carbon fiber sheet and a gas diffusion layer according to one embodiment. [Figure 3] FIG. 3 is a cross-sectional view of the inside of the electric furnace in the heat treatment step of FIG. [Figure 4] FIG. 4 is a cross-sectional view of the inside of the electric furnace in the carbonization step of FIG. DETAILED DESCRIPTION OF THE INVENTION

[0012] Hereinafter, with reference to FIGS. 1 to 4, an embodiment in which the method for producing a carbon fiber sheet is embodied as a method for producing a gas diffusion layer for a fuel cell will be described. <Gas diffusion layer 10> As shown in FIG. 1, the gas diffusion layer 10 includes a porous carbon fiber sheet 11 and a microporous layer 12 provided on a surface layer portion 11a of the carbon fiber sheet 11. The microporous layer 12 is provided so as to fill the surface layer portion 11a. The microporous layer 12 is a thin film containing a water-repellent resin and a conductive material as its main components. The water-repellent resin is, for example, polytetrafluoroethylene (PTFE). The conductive material is, for example, carbon black.

[0013] <Method of manufacturing the carbon fiber sheet 11 and the gas diffusion layer 10> 2, the method for producing the carbon fiber sheet 11 includes a sheet preparation step, a heat treatment step, and a carbonization step. The method for producing the gas diffusion layer 10 includes a coating step and a burning step in addition to the sheet preparation step, heat treatment step, and carbonization step.

[0014] Next, each step will be described. <Seat preparation process> The sheet preparation step is a step of preparing a sheet 20 which is cellulose paper.

[0015] The sheet preparation process of this embodiment includes a fiber-opening process, a dispersion process, and a paper-making process, all of which are well known. The defibration process is a process of breaking down cellulose fibers from pulp or the like.

[0016] The dispersion step is a step in which cellulose fibers are dispersed in water containing a resin such as polyethylene (PE). The papermaking process is a process of forming cellulose paper by papermaking a dispersion containing cellulose fibers.

[0017] The sheet preparation step is not limited to forming the sheet 20, but may be a step of preparing commercially available cellulose paper. <Heat treatment process> As shown in FIG. 3, the heat treatment step is a step of heat treating the sheet 20 at a temperature of 200° C. or more and 300° C. or less in an oxygen atmosphere.

[0018] In the heat treatment step, the sheet 20 is preferably heat-treated while being sandwiched between plates 30. The plate 30 is preferably a porous body. The plate 30 in this embodiment is a wire mesh made of stainless steel.

[0019] In this embodiment, the sheet 20 is heat-treated in an electric furnace 50 while sandwiched between two plate materials 30. The lower plate material 30 is placed on a spacer 31 placed on the bottom surface of the electric furnace 50. In this embodiment, the heat treatment is performed at 250°C for 60 minutes.

[0020] The shrinkage rate of cellulose fibers during the heat treatment step is 30 to 40%. <Carbonization process> As shown in FIG. 4, the carbonization step is a step in which the sheet 20A after the heat treatment step is sandwiched between press plates 40 and heated to 1000° C. or higher in an inert gas atmosphere while a load is applied, thereby carbonizing the sheet.

[0021] The press plate 40 preferably includes a press plate body 41 that sandwiches the sheet 20A, and a weight 42 that applies a load to the press plate body 41. In this embodiment, the press plate body 41 and the weight 42 are both made of carbon.

[0022] In this embodiment, the sheet 20A is sandwiched between two press plate bodies 41 placed on spacers 43 arranged on the bottom surface of the electric furnace 60, and a weight 42 is placed on the top surface of the upper press plate body 41, and the sheet 20A is heated to 1250 degrees.

[0023] The shrinkage rate of cellulose fibers during the carbonization process is 10 to 20%. <Coating process> The coating step is a step of applying paint containing a water-repellent resin and a conductive material as main components to the sheet 20A after the carbonization step, that is, the surface layer portion 11a of the carbon fiber sheet 11.

[0024] <Firing process> The firing process is a process in which the carbon fiber sheet 11 after the coating process is fired at approximately 300 degrees to soften or melt the water-repellent resin and bond the conductive material to the surface layer 11a of the carbon fiber sheet 11 using the water-repellent resin as a binder.

[0025] <Measurement results of the height difference on the surface of carbon fiber sheet 11> When the height difference (difference between the maximum height and the minimum height) on the surface of the carbon fiber sheet 11 having a thickness of 200 μm was measured, the measurement result for the carbon fiber sheet 11 of this embodiment was 100 μm. On the other hand, the measurement result for the carbon fiber sheet 11 of the comparative example in which the press plate 40 was not used in the carbonization step was 315 μm. Note that, in manufacturing the carbon fiber sheet 11 of the comparative example, a heat treatment step similar to that of this embodiment is carried out.

[0026] <Measurement results of in-plane air permeability of carbon fiber sheet 11> The in-plane air permeability (air permeability in the plane direction) of the carbon fiber sheet 11 of this embodiment at 0.9 MPa is 53 × 10 -12 m / (Pa·s).

[0027] On the other hand, when the heat treatment step was not performed and the press plate 40 was not used in the carbonization step, the in-plane air permeability of the carbon fiber sheet 11 at 0.9 MPa was 21 × 10 -12 m / (Pa·s).

[0028] When a non-air-permeable ceramic plate is used in the heat treatment process and the press plate 40 is not used in the carbonization process, the in-plane air permeability of the carbon fiber sheet 11 at 0.9 MPa is 29 × 10 -12 m / (Pa·s).

[0029] When the same heat treatment process as in this embodiment is performed and the press plate 40 is not used in the carbonization process, the in-plane air permeability of the carbon fiber sheet 11 at 0.9 MPa is 47 × 10 -12 m / (Pa·s).

[0030] <Operation of this embodiment> To produce a carbon fiber sheet with little deflection or warpage from cellulose paper sheet 20, it is conceivable to sandwich sheet 20 between press plates 40, apply a load, and heat it to 1000°C or higher in an inert gas atmosphere to carbonize it. In this case, the cellulose fibers undergo rapid thermal contraction while being restrained by press plates 40. As a result, the cellulose fibers break, easily causing cracks in the carbon fiber sheet.

[0031] According to this embodiment, before carbonizing the sheet 20, the sheet 20 is heat-treated in an oxidizing atmosphere at a temperature of 200°C or higher and 300°C or lower. This causes the cellulose fibers to shrink to a certain extent (30 to 40%) and harden, making the cellulose fibers less likely to deform. This prevents the cellulose fibers from rapidly shrinking due to heat during the carbonization process. This prevents the cellulose fibers from breaking and, ultimately, the carbon fiber sheet 11 from cracking (this is effect 1).

[0032] However, there is a risk that the sheet 20A may bend or warp due to thermal shrinkage of the cellulose fibers in the heat treatment step, so there is room for improvement in terms of suppressing bending and warping of the carbon fiber sheet 11.

[0033] In this regard, according to this embodiment, in the heat treatment step, the sheet 20 is heat treated while being sandwiched between the plate materials 30, so that the occurrence of bending or warping in the sheet 20 can be suppressed (above, effect 2).

[0034] In particular, because the plates 30 sandwiching the sheet 20 in the heat treatment process are porous, oxygen is easily supplied to the sheet 20 sandwiched between the plates 30 through the pores in the plates 30. Cellulose fiber, a naturally occurring material, contains impurities. When the sheet 20 is heat-treated at a temperature of 200°C or higher and 300°C or lower in the heat treatment process, the impurities are burned away, resulting in voids in the cellulose fiber. These voids remain in the carbon fibers that make up the carbon fiber sheet 11 (above, effect 3).

[0035] <Effects of this embodiment> (1) The method for producing the carbon fiber sheet 11 includes a sheet preparation step, a heat treatment step, and a carbonization step. This method achieves the above-mentioned effect 1, and therefore, it is possible to produce the carbon fiber sheet 11 from cellulose paper with little deflection or warpage.

[0036] The reduction in deflection and warpage of the carbon fiber sheet 11 makes it easier to handle the carbon fiber sheet 11 during transportation, etc. Also, the microporous layer 12 can be easily applied to the carbon fiber sheet 11. Furthermore, the occurrence of cracks in the gas diffusion layer 10 due to load when assembling the unit cells of the fuel cell can be suppressed.

[0037] (2) In the heat treatment step, the sheet 20 is heat treated while sandwiched between the plates 30. This method achieves the above-mentioned effect 2, and can further prevent the carbon fiber sheet 11 from being warped or bent.

[0038] (3) The plate material 30 is a porous body. According to this method, the above-mentioned effect 3 is achieved, and the air permeability of the carbon fiber sheet 11 is improved. (4) The press plate 40 includes a press plate body 41 that sandwiches the sheet 20A and a weight 42 that applies a load to the press plate body 41.

[0039] According to this method, the press plate 40 includes the press plate body 41 and the weight 42, so that the load acting on the sheet 20A can be easily changed by appropriately changing the mass of the weight 42.

[0040] <Modification> This embodiment can be modified as follows: This embodiment and the following modifications can be combined and implemented within the scope of technical compatibility.

[0041] The press plate body 41 and the weight 42 are not limited to being made of carbon, but may be made of other materials that are heat resistant to the heating temperature in the carbonization process.

[0042] The press plate 40 may not include the weight 42. In other words, the press plate 40 may be configured by the press plate body 41 only. The plate material 30 is not limited to a porous material, and may have no holes.

[0043] The sheet 20A may not be sandwiched between the plate members 30 if the bending or warping of the sheet 20A can be suppressed. [Explanation of symbols]

[0044] 10...Gas diffusion layer 11...Carbon fiber sheet 11a...surface layer 12...Microporous layer 20…sheets 30...Plate material 31...Spacer 40...Press plate 41...Press plate body 42...Weight 43...Spacer 50,60...electric furnace

Claims

1. a sheet preparation step of preparing a sheet of cellulose paper; a heat treatment step of heat treating the sheet at a temperature of 200°C or more and 300°C or less in an oxygen atmosphere; a carbonization step in which the sheet after the heat treatment step is sandwiched between press plates and heated to 1000°C or higher in an inert gas atmosphere while a load is applied, thereby carbonizing the sheet. Manufacturing method of carbon fiber sheet.

2. In the heat treatment step, the sheet is heat-treated in a state where the sheet is sandwiched between plate materials. The method for producing the carbon fiber sheet according to claim 1.

3. The plate material is a porous body. The method for producing the carbon fiber sheet according to claim 2.

4. The press plate includes a press plate body that sandwiches the sheet, and a weight that applies a load to the press plate body. The method for producing the carbon fiber sheet according to any one of claims 1 to 3.

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