Gas diffusion layers for fuel cells

Abstract

A gas diffusion layer for a fuel cell is provided, which maintains the high rigidity required by the GDL and improves the discharge characteristics of water vapor generated in the fuel cell unit, thereby improving the power generation efficiency of the fuel cell. A gas diffusion layer (50) for a fuel cell has a first layer (10) formed of carbon cloth (carbon fiber fabric) and a second layer (20) formed of carbon paper (carbon fiber papermaking), and the first layer (10) and the second layer (20) are fixed to each other by a conductive material. In addition, the unit area weight of the first layer (10) can also be made greater than the unit area weight of the second layer (20).

Description

Technical Field

[0001] The present invention relates to a gas diffusion layer for use in a fuel cell mounted on a vehicle, a ship, an airplane or other transportation means. Background Art

[0002] The solid polymer fuel cell (hereinafter referred to as fuel cell or FC), which has become mainstream for home and vehicle use, has a structure formed by repeatedly stacking unit cells (hereinafter referred to as battery cells) consisting of a membrane / platinum catalyst bonding layer (hereinafter referred to as CCM layer) with electrodes bonded to both sides of a polymer membrane, a gas diffusion layer (hereinafter referred to as GDL) for introducing fuel gas and oxidant gas into the electrode reaction area, a partition with gas inlet / outlet grooves, a sealing material, etc.

[0003] Among them, GDL is generally formed into a thin sheet-like component with a thickness of less than 1 mm, and it is required to have the function of smoothly supplying two reaction gases, fuel gas containing hydrogen or oxidant gas containing oxygen from the outside, to the electrode catalyst layer. In addition, as the basic functions of GDL, 1) it has a sufficiently low resistance to efficiently extract electric energy; 2) it has sufficient air permeability for extracting large currents and good diffusivity composed of the discharge of generated water generated in the battery; 3) it has a buffering property (elasticity) that can absorb the uneven thickness and thickness fluctuation of the stacked components; 4) it is necessary to have corrosion resistance against strong acids and strong alkalis.

[0004] Since the GDL in the past is a paper structure (carbon paper) formed of carbon fibers, it has the above-mentioned problems of insufficient air permeability and low cushioning (elasticity) for absorbing uneven thickness. Therefore, in order to solve the problems of carbon paper, Patent Documents 1 to 4 propose GDLs formed by laminating carbon cloth (carbon fiber fabric) and carbon paper.

[0005] Prior art literature

[0006] Patent Literature

[0007] Patent Document 1: Japanese Patent Application Publication No. 2003-173789

[0008] Patent Document 2: Japanese Patent Application Publication No. 2003-89968

[0009] Patent Document 3: Japanese Patent Application Publication No. 2004-355991

[0010] Patent Document 4: Japanese Patent Application Publication No. 2009-295509 Summary of the invention

[0011] Technical problem to be solved by the invention

[0012] However, if high rigidity is required for the properties of carbon paper, the weight per unit area increases, and the electrical resistance of the entire GDL in the thickness direction also increases. As a result, the fuel cell itself generates heat, and the power generation efficiency decreases.

[0013] On the other hand, if the weight per unit area of ​​the carbon paper is excessively reduced, the number of fibers bridging the blanks of the mesh of the carbon cloth decreases, and the pressing force against the CCM layer decreases. As a result, the power generation performance decreases, and the carbon cloth enters the grooves of the separator, causing the ventilation characteristics of the GDL and the drainage characteristics of the separator to deteriorate. In addition, when the MPL layer (porous smooth layer) is coated on one side of the GDL, there is also a problem that the smooth surface (smoothness) required for contact with the CCM layer cannot be ensured.

[0014] In addition, in a GDL formed by simply stacking carbon paper and carbon cloth, when the carbon paper and carbon cloth are stacked separately in the assembly process of the membrane / electrode assembly, the contact resistance increases because the carbon paper and carbon cloth each apply a pressure that does not damage the CCM layer. Furthermore, the positioning process of the carbon paper and carbon cloth is required, which results in an increase in the assembly man-hours of the fuel cell unit.

[0015] Therefore, the technical problem of the present invention is to provide a gas diffusion layer for a fuel cell that improves the discharge characteristics (ventilation characteristics) of water vapor generated in a fuel cell unit while maintaining the high rigidity required of a GDL, thereby improving the power generation efficiency of the fuel cell.

[0016] Solutions for solving technical problems

[0017] In order to solve the above technical problems, the inventors re-examined the structure of GDL which is a simple stack of carbon paper and carbon cloth. As a result, it was found that the power generation efficiency of the fuel cell is affected by the structure of the interface between the carbon paper and the carbon cloth and the density (weight per unit area) of the carbon paper.

[0018] Specifically, regarding the structure of the interface between carbon paper and carbon cloth, it was found that since the power generation efficiency varies depending on the contact mode of the interface, the power generation efficiency can be improved by interposing a conductive material at the interface where carbon paper and carbon cloth are in contact.

[0019] In addition, since the MPL layer is provided between the carbon paper and the CCM layer, a surface as smooth as possible is required. Therefore, as the weight per unit area of ​​the carbon paper increases, the thickness also increases. As a result, the thickness of the GDL as a whole also increases, resulting in a decrease in power generation efficiency. Therefore, in the present invention, it is found that by reducing the weight per unit area of ​​the carbon paper and making its thickness smaller than that of the carbon cloth, the decrease in power generation efficiency can be suppressed.

[0020] Specifically, the gas diffusion layer for fuel cells of the present invention is a structure in which a first layer formed of carbon cloth (carbon fiber fabric) and a second layer formed of carbon paper (carbon fiber papermaking) are fixed to each other by a conductive material. The unit area weight of the first layer (carbon fiber fabric) can also be made greater than the unit area weight of the second layer (carbon fiber papermaking). For example, the unit area weight of the first layer can be set to 20 g / m 2 In the above, the weight per unit area of ​​the second layer (excluding the weight per unit area of ​​the MPL layer) is set to 2 to 10 g / m 2 range.

[0021] The first layer of carbon cloth may have a structure in which at least one of the warp and weft yarns is substantially untwisted. Furthermore, the second layer may have a resin layer having carbon particles dispersed in a fluororesin on one side (the side opposite to the first layer).

[0022] Effects of the Invention

[0023] As described above, the gas diffusion layer for fuel cells of the present invention can take into account both the high rigidity and ventilation characteristics of GDL and can improve the power generation efficiency of fuel cells. At the same time, compared with GDL simply stacked with carbon paper and carbon cloth, the gas diffusion layer for fuel cells of the present invention does not require positioning and other operations in the assembly process of the fuel cell unit, thus also helping to simplify the assembly process. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 It is a cross-sectional view of a gas diffusion layer 50 as a first embodiment of the present invention.

[0025] Figure 2 yes Figure 1 An enlarged view of the vicinity of the central portion.

[0026] Figure 3 It is a cross-sectional view of a gas diffusion layer 80 according to a second embodiment of the present invention.

[0027] Figure 4 yes Figure 3 An enlarged view of the vicinity of the central portion.

[0028] Figure 5 Schematic diagram of the structure of the power generation performance test device used in the examples.

[0029] Figure 6 1 is a schematic diagram of the structure of a battery cell 100 used in the power generation performance test of the embodiment.

[0030] Figure 7 These are the results of the power generation performance measurement test of the examples.

[0031] Description of Reference Numerals

[0032] 10, 30: carbon cloth (carbon fiber fabric); 20, 40: carbon paper (carbon fiber papermaking); 50, 51, 52, 80: gas diffusion layer (GDL) for fuel cells; 60: polymer membrane (electrolyte membrane); 71, 72: separator; 100: evaluation battery cell. DETAILED DESCRIPTION

[0033] Hereinafter, an embodiment of the gas diffusion layer of the present invention will be described with reference to the drawings. Figure 1 FIG. 2 is a diagram showing a microscope photograph (magnification: 500 times) of a cross section of a gas diffusion layer 50 according to a first embodiment of the present invention. Figure 2 An enlarged view of the vicinity of the center of the cross-sectional view is shown in FIG. Figure 1 As shown, the gas diffusion layer (GDL) 50 of the present invention is formed of a carbon cloth (carbon fiber woven fabric: first layer) 10 and a carbon paper (carbon fiber papermaking: second layer) 20, and the layers are in close contact with each other.

[0034] In the carbon cloth (first layer) 10, as Figure 1 As shown, the warp yarn 11 is formed from the inner side to the front side of the figure, and the weft yarn 12 is formed in the left and right direction of the figure. The warp yarn 11 is in a state where multiple single yarns are basically untwisted, while the weft yarn 12 is as shown in FIG. Figure 1 The shown one is formed in a twisted state.

[0035] In addition, at the boundary portion between the carbon cloth (first layer) 10 and the carbon paper (second layer) 20, as shown in FIG. Figure 2 As shown, the single yarns (carbon fibers) 11A of the warp yarns 11 forming the carbon cloth (first layer) 10 and the carbon fibers 20A forming the carbon paper (second layer) 20 are fixedly bonded by a conductive substance (not shown).

[0036] exist Figure 3 FIG. 2 is a diagram showing a microscope photograph (magnification: 500 times) of a cross section of a gas diffusion layer 80 according to a second embodiment of the present invention. Figure 4 FIG. 2 shows an enlarged view of the center of the cross-sectional view (magnification: 1000 times). The gas diffusion layer (GDL) 80 of this embodiment is Figure 1 and Figure 2 The first embodiment shown is similarly formed of a carbon cloth (carbon fiber woven fabric: first layer) 30 and a carbon paper (carbon fiber papermaking: second layer) 40, and the layers are fixed by a conductive substance (not shown).

[0037] In the carbon cloth (first layer) 30, as Figure 3 and Figure 4 As shown, the warp yarn 31 is formed from the inner side to the front side of the figure, and the weft yarn 32 is formed in the left and right direction of the figure. The warp yarn 31 is in a state where multiple single yarns are basically untwisted, while the weft yarn 32 is as shown in FIG. Figure 3 and Figure 4 The shown one is formed in a twisted state.

[0038] [Example]

[0039] Next, since a test to measure power generation performance was conducted using samples (hereinafter referred to as evaluation cells) in which the GDL of the present invention (hereinafter referred to as the invention product) and conventional GDL (hereinafter referred to as the existing product) were built, the measurement results are described. Figure 5 The overall structure of the device for this measurement test is shown in FIG. Figure 6 Schematic diagram showing the structure of the evaluation battery cell 100 is shown in FIG.

[0040] like Figure 6 As shown, the evaluation cell 100 used in the present measurement test has a structure in which a polymer membrane 60 with a catalyst is sandwiched between two types of GDLs 51 and 52 from both sides by grooved separators 71 and 72. After the catalyst layer is closely attached to both sides of the anode and the cathode, the polymer membrane 60 (electrolyte membrane: thickness 40 μm) is in contact with each GDL 51 and 52.

[0041] The GDLs 51 and 52 used in the evaluation cell 100 were: the hydrogen electrode (anode side) was a commercially available GDL 52 (manufactured by SGL, product number 24BC), and the air electrode (cathode side) was the above-mentioned invention product or a conventional product (both with an effective area of ​​1 cm 2 ) of each GDL 51.

[0042] Here, the carbon paper used in the invention product and the conventional product is obtained by curing both sides of the paper made of carbon fiber as a raw material with a thermosetting resin, and curing both sides of the carbon fiber fabric with a thermosetting resin to such an extent that it does not sink into the groove of the partition. In addition, the invention product has unified the structure, material and unit area weight (weight per unit area) of the carbon fiber paper and carbon fiber fabric of the conventional product.

[0043] The weight per unit area of ​​the thermosetting resin bonded to the carbon fiber fabric and carbon fiber paper is the same as the total weight per unit area added to the existing product. The thickness of each GDL loaded into the evaluation battery cell for this measurement is set to the thickness when the surface pressure is 1MPa, and the gasket is adjusted so that the hydrogen electrode side is 170μm and the air electrode side is 130μm.

[0044] After the common run-in operation of the above evaluation battery cells was fully performed, the critical current value (I lim :A / cm 2Here, "running-in operation" refers to the test operation of the battery cell in the early stage of performance evaluation, which is to improve the affinity between the polymer membrane, catalyst layer, and GDL while changing the power generation mode (VI conditions and their time) and increasing the power generation capacity to a steady state.

[0045] For the evaluation battery cells after the running-in operation, such as Figure 6 As shown, hydrogen gas is pressurized and delivered to the separator 72 on the anode side (hydrogen electrode) at a flow rate of 0.5 L / min, and oxygen diluted with nitrogen is pressurized and delivered to the separator 71 on the cathode side (air electrode) at a flow rate of 1 L / min. Gas is supplied to both the hydrogen electrode and the air electrode at two conditions of gas temperature of 45°C and 60°C. The two gases are supplied to the evaluation battery cell with the temperature of the power generation section at 45°C (overhumidification condition) and 60°C (humidification condition) and the operating conditions are set.

[0046] In addition, the average back pressure of the air electrode in the evaluation battery cell is set to 0.15 MPa (abs), and the average back pressure of the hydrogen electrode is set to 0.10 MPa (abs). Under the above conditions, the IV (current-voltage) characteristics of the battery cell are measured, and the current value when the voltage is 0.2 V is recorded as the critical current value. Figure 7 The results of this power generation performance test are shown in FIG.

[0047] The result of this test is that the 0.2V limit current value of the evaluation battery cell using the invention product is 186mA when the gas overhumidification temperature is 45°C, and 333mA when the gas overhumidification temperature is 60°C. In contrast, the 0.2V limit current value of the battery cell using the comparative product is 116mA when the gas overhumidification temperature is 45°C, and 328mA when the gas overhumidification temperature is 60°C. From the above test results, it can be seen that the power generation performance of the invention product can be 1.6 times that of the comparative product in the gas overhumidification atmosphere.

[0048] It should be noted that in this application, "weight per unit area" is a "unit indicating the mass per unit area of ​​wool fabrics, etc." in Japanese Industrial Standard (JIS) L02028, which is different from "weight per 1m 2 Synonymous with "grams".

Claims

1. A gas diffusion layer for a fuel cell, characterized in that: have: a first layer formed of carbon cloth; a second layer formed of carbon paper; and A conductive material comprising a thermosetting resin, The first layer and the second layer are fixedly connected to each other through the conductive material. The weight per unit area of ​​the first layer is greater than the weight per unit area of ​​the second layer, The weight per unit area of ​​the first layer is 20 g / m 2 above, The weight per unit area of ​​the second layer is 2 g / m 2 ~10g / m 2 range.

2. The gas diffusion layer for a fuel cell according to claim 1, characterized in that: The first layer is a carbon cloth in which at least one of the warp yarns and the weft yarns is substantially untwisted.

3. The gas diffusion layer for a fuel cell according to claim 1, characterized in that: The second layer has a resin layer having carbon particles dispersed in a fluororesin on one surface thereof.

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