Method for manufacturing catalyst ink and film electrode assembly

Abstract

A membrane electrode assembly (20) is provided with: a polymer electrolyte membrane (22); a pair of electrode catalyst layers (23) that are in close contact with both surfaces of the polymer electrolyte membrane (22); and a pair of gas diffusion layers (24) that are in close contact with outer surfaces of the pair of electrode catalyst layers (23). The membrane electrode assembly (20) is provided, inside the electrode catalyst layer (23) or within a predetermined range in the thickness direction from a boundary plane (27) between the electrode catalyst layer (23) and the gas diffusion layer (24), with an ionomer region (28) in which an ionomer (233) that mutually bonds carbon particles (232) carrying a metal catalyst (231) is present.

Description

[Technical Field]

[0001] The present invention relates to a film electrode assembly, a catalyst ink, and a method for manufacturing a film electrode assembly. [Background technology]

[0002] Conventionally, a method for manufacturing a membrane electrode assembly comprising an electrode catalyst layer and a gas diffusion layer mounted on both sides of an electrolyte membrane is known (see, for example, Patent Document 1). In the manufacturing method described in Patent Document 1, a catalyst ink is coated onto the electrolyte membrane, the gas diffusion layer is bonded while the catalyst ink is still wet, and the catalyst ink is dried while controlling the bonding load of the gas diffusion layer so that the thickness of the ionomer-existing region where the ionomer has entered the gas diffusion layer is 9 to 37% of the total ionomer region. [Prior art documents] [Patent Documents]

[0003] [Patent Document 1] Japanese Patent Publication No. 2013-134877 [Overview of the Initiative] [Problems that the invention aims to solve]

[0004] When manufacturing such a membrane electrode assembly, it is preferable to ensure the output performance of the electrode while suppressing ionomer consumption. However, in the manufacturing method described in Patent Document 1, a relatively large amount of ionomer is present in the gas diffusion layer, which increases the amount of ionomer consumed to ensure the desired output performance. [Means for solving the problem]

[0005] This is one aspect of the present invention. The catalyst ink contains a solvent, carbon particles on which a metal catalyst is supported, an ionomer that adheres the carbon particles together, and a dispersion stabilizer. .

[0006] The solvent is a 50-60 weight percent aqueous solution of ethanol. The carbon particle content is 4-5 weight percent. The dispersion stabilizer content relative to the ionomer is 6 weight percent or more.

[0007] Another aspect of the present invention relates to a method for manufacturing a membrane electrode assembly, which includes preparing a catalyst ink containing a solvent, carbon particles on which a metal catalyst is supported, an ionomer that adheres the carbon particles together, and a dispersion stabilizer; applying the prepared catalyst ink to a gas diffusion layer; laminating a polymer electrolyte film on the catalyst ink layer applied to the gas diffusion layer; and joining the polymer electrolyte film, the catalyst ink layer, and the gas diffusion layer by thermocompression bonding. The solvent is a 50-60 weight percent aqueous solution of ethanol. The content ratio of carbon particles to the catalyst ink is 4-5 weight percent. The content ratio of the dispersion stabilizer to the ionomer is 6 weight percent or more. [Effects of the Invention]

[0008] According to the present invention, it is possible to ensure output performance as an electrode while suppressing ionomer consumption. [Brief explanation of the drawing]

[0009] [Figure 1] A schematic perspective view showing the overall configuration of a fuel cell stack including a membrane electrode assembly according to an embodiment of the present invention. [Figure 2] Figure 1 is a perspective view showing the schematic configuration of the integrated electrode assembly included in the fuel cell stack. [Figure 3] A diagram illustrating a membrane electrode assembly according to an embodiment of the present invention. [Figure 4] A diagram illustrating a method for manufacturing a membrane electrode assembly according to an embodiment of the present invention. [Figure 5] A diagram illustrating a catalyst ink according to an embodiment of the present invention. [Figure 6] Figure 3 illustrates the relationship between the amount of ionomer contained in the electrode catalyst layer and the output performance of the fuel cell. [Figure 7] A diagram illustrating the ionomer region when the mass concentration of ethanol relative to the solvent is varied as part of the catalyst ink composition. [Figure 8]A diagram for explaining the adsorption rate of an ionomer to a metal catalyst when the mass concentration of a dispersion stabilizer for the ionomer is changed as the composition of the catalyst ink. [Figure 9] A diagram for explaining the effect of suppressing the penetration of an ionomer by the composition compatibility of the catalyst ink. [Figure 10] A diagram for explaining the effect of suppressing coating surface defects by the composition compatibility of the catalyst ink.

Embodiments for Carrying Out the Invention

[0010] Hereinafter, embodiments of the present invention will be described with reference to FIGS. 1 to 10. The membrane electrode assembly according to the embodiment of the present invention is a component of a power generation cell that constitutes a fuel cell. The power generation cell is a component of a fuel cell stack that is a component of a fuel cell. The fuel cell can be mounted on a vehicle, for example, and generate electric power for driving the vehicle. First, the overall configuration of the fuel cell stack will be schematically described.

[0011] FIG. 1 is a perspective view schematically showing the overall configuration of a fuel cell stack 100 including a membrane electrode assembly according to an embodiment of the present invention. Hereinafter, for convenience, three axial directions orthogonal to each other as shown in the figure are defined as the front-rear direction, the left-right direction, and the up-down direction, and the configuration of each part will be described according to this definition. These directions are not necessarily the same as the front-rear direction, the left-right direction, and the up-down direction of the vehicle. For example, the front-rear direction in FIG. 1 may be the front-rear direction of the vehicle, the left-right direction, or the up-down direction.

[0012] As shown in FIG. 1, the fuel cell stack 100 includes a cell stack 101 formed by stacking a plurality of power generation cells 1 in the front-rear direction, and end units 102 disposed at both front and rear ends of the cell stack 101, and has an overall substantially rectangular parallelepiped shape. The length of the cell stack 101 in the left-right direction is longer than the length in the up-down direction. For the sake of convenience, a single power generation cell 1 is shown in FIG. 1. The power generation cell 1 includes an integrated electrode assembly (UEA; Unitized Electrode Assembly) 2, which is a membrane electrode structure including an electrolyte membrane and electrodes, and separators 3, 3 disposed on both front and rear sides of the integrated electrode assembly 2 and sandwiching the integrated electrode assembly 2. The integrated electrode assembly 2 and the separator 3 are alternately arranged in the front-rear direction.

[0013] The separator 3 has a pair of front and rear metal thin plates with a corrugated cross-section, and the outer peripheries of these thin plates are joined together to form an integral structure. A conductive material with excellent corrosion resistance is used for the separator 3, and for example, titanium, titanium alloy, stainless steel, etc. can be used. Inside the separator 3, a cooling flow path through which a cooling medium flows is formed by press molding or the like, and the power generation surface of the power generation cell 1 is cooled by the flow of the cooling medium. For example, water can be used as the cooling medium. The surfaces (front surface and rear surface) of the separator 3 facing the integrated electrode assembly 2 are configured in a concave-convex shape so as to form a gas flow path between the separator 3 and the integrated electrode assembly 2.

[0014] The separator 3 on the front side of the integrated electrode assembly 2 is, for example, an anode-side separator (anode separator), and an anode flow path through which fuel gas flows is formed between the anode separator 3 and the integrated electrode assembly 2. The separator 3 on the rear side of the integrated electrode assembly 2 is, for example, a cathode-side separator (cathode separator), and a cathode flow path through which oxidant gas flows is formed between the cathode separator 3 and the integrated electrode assembly 2. For example, hydrogen gas can be used as the fuel gas, and air can be used as the oxidant gas. Sometimes, without distinguishing between the fuel gas and the oxidant gas, these are collectively referred to as reaction gases.

[0015] Figure 2 is a perspective view showing the schematic configuration of the integrated electrode assembly 2. As shown in Figure 2, the integrated electrode assembly 2 comprises a substantially rectangular membrane electrode assembly (MEA) 20 and a frame 21 that supports the membrane electrode assembly 20.

[0016] Figure 3 is a diagram illustrating the membrane electrode assembly 20, conceptually showing the configuration near the surface of the membrane electrode assembly 20 facing the reaction gas channel (anode channel or cathode channel; anode channel in Figure 3). As shown in Figure 3, the membrane electrode assembly 20 comprises an electrolyte membrane 22, a pair of electrode catalyst layers 23 in close contact with both the front and rear surfaces of the electrolyte membrane 22, and a pair of gas diffusion layers 24 in close contact with the outer surfaces of the pair of electrode catalyst layers 23. Figure 3 shows only the front electrode catalyst layer 23 and gas diffusion layer 24. The gas diffusion layer 24 is made of a gas-permeable conductive material, such as a porous carbon material, and includes a water-repellent layer 25 in close contact with the outer surface of the electrode catalyst layer 23, and a carbon paper layer 26 in close contact with the outer surface of the water-repellent layer 25, with its outer surface facing the reaction gas channel.

[0017] As the electrolyte membrane 22, a solid polymer electrolyte membrane can be used, and a thin film of perfluorosulfonic acid or the like containing water can be used. Not limited to fluorine-based electrolytes, hydrocarbon-based electrolytes can also be used. The electrolyte membrane 22 has proton conductivity and hydrogen ions (H) are transmitted from the anode side to the cathode side. + ) moves while electrons (e - ) and hinder the movement of gases (H2, O2, etc.).

[0018] The electrode catalyst layer 23 contains a catalyst metal that promotes the electrochemical reaction between hydrogen contained in the fuel gas and oxygen contained in the oxidizing gas, a proton-conducting electrolyte, and electron-conducting carbon particles. More specifically, it contains carbon particles 232 such as carbon black on which a metal catalyst 231 such as platinum or a platinum alloy is supported, and an ionomer 233 that adheres the carbon particles 232 to each other. The carbon particles 232 are electron-conducting and electron-conducting (e -) is moved. The ionomer 233 has proton conductivity and moves hydrogen ions (H + ) As the ionomer 233, a polymer electrolyte (perfluorosulfonic acid) which is a high oxygen permeable ionomer (HOPI) can be used. The front electrode catalyst layer 23 constitutes the anode electrode, and the rear electrode catalyst layer 23 constitutes the cathode electrode.

[0019] At the anode electrode, the fuel gas (H2) supplied through the anode flow path and the gas diffusion layer 24 and reaching the electrode catalyst layer 23 is ionized by the action of the metal catalyst 231, passes through the electrolyte membrane 22, and moves to the cathode electrode side. At this time, the hydrogen ions (H + ) ionized on the surface of the metal catalyst 231 move to the surface of the anode side of the electrolyte membrane 22 through the ionomer 233. The electrons (e - ) generated at this time pass through the external circuit through the carbon particles 232 and are taken out as electrical energy. H2 → 2H + + 2e - (i)

[0020] At the cathode electrode, the oxidant gas (O2) supplied through the cathode flow path and the gas diffusion layer 24 and reaching the electrode catalyst layer 23 reacts with the hydrogen ions (H + ) and the electrons (e - ) conducted from the anode electrode to generate water. At this time, the hydrogen ions (H + ) reaching the surface of the cathode side from the anode side of the electrolyte membrane 22 move to the surface of the metal catalyst 231 through the ionomer 233 and react with the oxidant gas (O2) and the electrons (e - ) on the surface of the metal catalyst 231. The water generated at this time gives appropriate humidity to the electrolyte membrane 22, and the excess water is discharged to the outside of the integrated electrode assembly 2. O2 + 4H + + 4e - → 2H2O (ii)

[0021] The frame 21 is a thin plate with a roughly rectangular shape and is made of an insulating resin or rubber. A roughly rectangular opening 21a is provided in the center of the frame 21, and a film electrode assembly 20 is provided so as to cover the entire opening 21a. Three through holes 211 to 213 are opened vertically on the left side of the opening 21a, penetrating the frame 21 in the front-to-back direction, and three through holes 214 to 216 are opened vertically on the right side of the opening 21a, penetrating the frame 21 in the front-to-back direction.

[0022] As shown in Figure 1, the front and rear separators 3 of the integrated electrode assembly 2 have through-holes 311 to 316 that penetrate the separator 3 in the front-to-back direction, at positions corresponding to the through-holes 211 to 216 of the frame 21. The through-holes 311 to 316 communicate with the through-holes 211 to 216 of the frame 21, respectively. The collection of these interconnected through-holes 211 to 216 and 311 to 316 forms flow paths PA1 to PA6 (indicated by arrows for convenience) that penetrate the cell stack 101 and extend in the front-to-back direction. Flow paths PA1 to PA6 are sometimes called manifolds (discharge manifolds). Flow paths PA1 to PA6 are connected to a manifold outside the fuel cell stack 100.

[0023] The flow path PA1 (solid arrow) extending forward through through holes 211 and 311 is the fuel gas supply flow path. The flow path PA6 (solid arrow) extending backward through through holes 216 and 316 is the fuel gas discharge flow path. The fuel gas supply flow path PA1 and the fuel gas discharge flow path PA6 communicate with the anode flow path facing the front surface of the membrane electrode assembly 20, and as shown by the solid arrows, fuel gas flows in the left-right direction through the anode flow path via the fuel gas supply flow path PA1 and the fuel gas discharge flow path PA6. The fuel gas flowing through the fuel gas discharge flow path PA6 is the fuel gas after a portion has been used at the anode electrode, and this is sometimes called fuel exhaust gas.

[0024] The flow path PA4 (dotted arrow) extending forward through through holes 214 and 314 is the oxidizer gas supply flow path. The flow path PA3 (dotted arrow) extending backward through through holes 213 and 313 is the oxidizer gas discharge flow path. The oxidizer gas supply flow path PA4 and the oxidizer gas discharge flow path PA3 communicate with the cathode flow path facing the rear surface of the membrane electrode assembly 20, and as shown by the dotted arrows, the oxidizer gas flows in the left-right direction through the cathode flow path PA4 and the oxidizer gas discharge flow path PA3. The oxidizer gas flowing through the oxidizer gas discharge flow path PA3 is the oxidizer gas after a portion has been used at the cathode electrode, and this is sometimes called oxidizer exhaust gas. Sometimes, fuel exhaust gas and oxidizer exhaust gas are not distinguished and are collectively called reaction exhaust gas.

[0025] The flow path PA5 (dotted arrow) extending forward through the through holes 215 and 315 is a cooling medium supply flow path. The flow path PA2 (dotted arrow) extending backward through the through holes 212 and 312 is a cooling medium discharge flow path. The cooling medium supply flow path PA5 and the cooling medium discharge flow path PA2 are in communication with the cooling flow path inside the separator 3, and the cooling medium flows through the cooling flow path via the cooling medium supply flow path PA5 and the cooling medium discharge flow path PA2.

[0026] The end units 102, positioned on both the front and rear sides of the cell laminate 101, each have a terminal plate 4, an insulating plate 5, and an end plate 6. The front end unit 102 is sometimes referred to as the dry end unit, and the rear end unit 102 as the wet end unit. The front and rear pair of terminal plates 4, 4 are positioned on either side of the cell laminate 101. The front and rear pair of insulating plates 5, 5 are positioned on either side of the terminal plates 4, 4. The front and rear pair of end plates 6, 6 are positioned on either side of the insulating plates 5, 5.

[0027] The terminal plate 4 is a roughly rectangular plate-shaped member made of metal and has terminals for extracting the electricity generated by the electrochemical reaction in the cell stack 101. The insulating plate 5 is a roughly rectangular plate-shaped member made of non-conductive resin or rubber and electrically insulates the terminal plate 4 from the end plate 6. The end plate 6 is a plate-shaped member made of metal or a high-strength resin, and a long, narrow connecting member in the front-to-back direction, for example, connecting the front and rear end plates 6, 6 together, is fixed to the end plate 6 by bolts. The fuel cell stack 100 is held in a state where it is pressed in the front-to-back direction by the end plates 6, 6 via the connecting member.

[0028] The rear end unit 102 has multiple through holes 102a to 102f that penetrate the end unit 102 in the front-to-back direction. Note that the through holes 102a to 102f include through holes that penetrate the terminal plate 4, through holes that penetrate the insulating plate 5, and through holes that penetrate the end plate 6, respectively, but in Figure 1, for convenience, these are shown collectively as through holes 102a to 102f. Through hole 102a opens on the extension of the fuel gas supply passage PA1 and communicates with the fuel gas supply passage PA1. Through hole 102b opens on the extension of the cooling medium discharge passage PA2 and communicates with the cooling medium discharge passage PA2. Through hole 102c opens on the extension of the oxidizer gas discharge passage PA3 and communicates with the oxidizer gas discharge passage PA3. Through hole 102d opens on the extension of the oxidizer gas supply passage PA4 and communicates with the oxidizer gas supply passage PA4. The through-hole 102e opens on the extension of the cooling medium supply passage PA5 and communicates with the cooling medium supply passage PA5. The through-hole 102f opens on the extension of the fuel gas exhaust passage PA6 and communicates with the fuel gas exhaust passage PA6.

[0029] More specifically, a fuel gas tank containing high-pressure fuel gas is connected to the through-hole 102a via an ejector, injector, etc., and the fuel gas in the fuel gas tank is supplied to the fuel cell stack 100 through the through-hole 102a. A gas-liquid separator is connected to the through-hole 102f, and the fuel gas (fuel exhaust gas) discharged through the through-hole 102f is separated into fuel gas and water by the gas-liquid separator. The separated fuel gas is sucked in via an ejector and supplied again to the fuel cell stack 100. The separated water is discharged to the outside via a drain channel.

[0030] A compressor for supplying oxidant gas is connected to the through-hole 102d, and the oxidant gas compressed by the compressor is supplied to the fuel cell stack 100 through the through-hole 102d. Oxidant gas (oxidant exhaust gas) flows out to the outside from the through-hole 102c. A pump for supplying cooling medium is connected to the through-hole 102b, and the cooling medium is supplied to the fuel cell stack 100 through the through-hole 102b. The cooling medium is discharged from the through-hole 102e. The discharged cooling medium is cooled by heat exchange in the radiator and supplied back to the fuel cell stack 100 through the through-hole 102b.

[0031] The above is a general overview of the fuel cell stack 100. The fuel cell stack 100 is housed in a roughly box-shaped case and mounted on the vehicle.

[0032] Figure 4 is a diagram illustrating the manufacturing method of the membrane electrode assembly 20, and Figure 5 is a diagram illustrating the catalyst ink 230 used in the manufacturing of the membrane electrode assembly 20. As shown in Figure 4, in the manufacturing of the membrane electrode assembly 20, the catalyst ink 230 is first prepared (preparation step S1). The catalyst ink 230 is prepared by uniformly mixing and dispersing carbon particles 232 on which the metal catalyst 231 shown in Figure 3 is supported, an ionomer 233 that adheres the carbon particles 232 to each other, and a solvent containing water or alcohol.

[0033] Next, the prepared catalyst ink 230 is applied to the gas diffusion layer 24 (coating step S2). In coating step S2, the catalyst ink 230 is applied directly to the gas diffusion layer 24, for example, by a spray coating device. Alternatively, the catalyst ink 230 may be applied indirectly to the gas diffusion layer 24 by applying the catalyst ink 230 to a sheet (not shown) and transferring the catalyst ink 230 applied to the sheet to the gas diffusion layer 24.

[0034] Next, the electrolyte membrane 22 is laminated onto the layer of catalyst ink 230 applied to the gas diffusion layer 24 (lamination step S3). As shown in Figure 5, the laminate of the electrolyte membrane 22, the catalyst ink 230 layer and the gas diffusion layer 24, in which the electrolyte membrane 22 is laminated onto the layer of catalyst ink 230 applied to the gas diffusion layer 24, is conveniently referred to as a semi-finished membrane electrode assembly 20A. In the semi-finished membrane electrode assembly 20A, the catalyst ink 230 layer is located at the position of the electrode catalyst layer 23 in Figure 3. In this catalyst ink 230 layer, a metal catalyst 231, carbon particles 232 and an ionomer 233 are dispersed in the solvent.

[0035] Next, the electrolyte membrane 22, the catalyst ink 230 layer, and the gas diffusion layer 24 are joined by thermocompression bonding (thermocompression bonding step S4). In thermocompression bonding step S4, the semi-finished membrane electrode assembly 20A is thermocompressed using, for example, a heated press, to join the electrolyte membrane 22, the catalyst ink 230 layer, and the gas diffusion layer 24. At this time, the solvent contained in the catalyst ink 230 is dried and removed by high temperature. As a result, as shown in Figure 3, a membrane electrode assembly 20 is completed in which the electrolyte membrane 22, the electrode catalyst layer 23 from which the solvent has been removed from the catalyst ink 230, and the gas diffusion layer 24 are in close contact with each other.

[0036] Figure 6 illustrates the relationship between the amount of ionomer 233 contained in the electrode catalyst layer 23 and the output performance as an electrode of the fuel cell. As shown in Figure 6, when the electrode catalyst layer 23 contains an appropriate amount of ionomer 233, hydrogen ions (H) between the surface of the electrolyte membrane 22 and the surface of the metal catalyst 231 +The movement of ) proceeds smoothly, and the electrode reactions of equations (i) and (ii) proceed smoothly. In this case, the output performance as an electrode of the fuel cell is ensured. The amount of ionomer 233 to be contained in the electrode catalyst layer 23 is determined according to the required output performance of the fuel cell.

[0037] Incidentally, depending on the composition of the catalyst ink 230, in the state of the semi-finished film electrode assembly 20A shown in Figure 5, a portion of the catalyst ink 230 containing the ionomer 233 may penetrate into the gas diffusion layer 24 through the voids between the porous particles constituting the water-repellent layer 25 and through the pores of the porous material. In this case, in order to leave an appropriate amount of ionomer 233 in the electrode catalyst layer 23 of the completed film electrode assembly 20, it becomes necessary to use an excess amount of ionomer 233, taking into account the amount that penetrates from the catalyst ink 230 layer of the semi-finished film electrode assembly 20A into the gas diffusion layer 24, which leads to increased costs.

[0038] Furthermore, if the hydrophilic substance ionomer 233 is present in the gas diffusion layer 24 (water-repellent layer 25), the water repellency decreases. In this case, there is a higher risk that impurities such as divalent iron ions originating from the metallic separator 3 will reach the electrolyte membrane 22 along with the water produced by the electrode reaction of formula (ii) and the moisture contained in the reaction gas. That is, some of the oxygen in the oxidizing gas flowing through the cathode channel permeates the electrolyte membrane 22 and moves to the anode electrode, where it reacts with hydrogen ions generated by the electrode reaction of formula (i) and adsorbed hydrogen before ionization to generate hydrogen peroxide. When this hydrogen peroxide reacts with impurities such as divalent iron ions, hydroxyl radicals (·OH) are generated, which decompose and degrade the electrolyte membrane 22, reducing the durability of the electrolyte membrane 22.

[0039] Therefore, in this embodiment, the membrane electrode assembly 20 is configured such that the gas diffusion layer 24 does not contain ionomer 233, in order to ensure output performance while suppressing the consumption of ionomer 233 and to improve the durability of the electrolyte membrane 22.

[0040] As shown in Figure 3, the membrane electrode assembly 20 includes an ionomer region 28 where the ionomer 233 exists, either inside the electrode catalyst layer 23 or within a predetermined range in the thickness direction from the interface 27 between the electrode catalyst layer 23 and the gas diffusion layer 24 (for example, within 5% of the thickness of the water-repellent layer 25). That is, most of the ionomer 233 exists to cover the carbon particles 232 on which the metal catalyst 231 constituting the electrode catalyst layer 23 is supported, and a portion exists to adhere to the surface of the electrolyte membrane 22 and the gas diffusion layer 24 (water-repellent layer 25) facing the electrode catalyst layer 23. The center line CL in the thickness direction of the ionomer region 28 is located at least inside the electrode catalyst layer 23 and generally coincides with the center line in the thickness direction of the electrode catalyst layer 23.

[0041] This embodiment can provide the following effects and advantages. (1) The membrane electrode assembly 20 comprises an electrolyte membrane 22, a pair of electrode catalyst layers 23 in close contact with both sides of the electrolyte membrane 22, and a pair of gas diffusion layers 24 in close contact with the outer surfaces of the pair of electrode catalyst layers 23 (Figure 3). The membrane electrode assembly 20 includes an ionomer region 28 within a predetermined thickness range from the inside of the electrode catalyst layer 23 or the interface 27 between the electrode catalyst layer 23 and the gas diffusion layer 24, where an ionomer 233 that adheres the metal catalyst 231 to the carbon particles 232 is present (Figure 3). This makes it possible to ensure output performance as an electrode of a fuel cell while suppressing the consumption of the ionomer 233, and also improves the durability of the electrolyte membrane 22.

[0042] (2) The center line CL in the thickness direction of the ionomer region 28 is located inside the electrode catalyst layer 23 (Figure 3). By configuring the ionomer region 28 so that the center line CL is located at least inside the electrode catalyst layer 23, hydrogen ions (H) that are present outside the electrode catalyst layer 23 are prevented from being present. + This allows for a reduction in the amount of ionomer 233, which does not contribute to the movement and output performance of the ionomer.

[0043] In the above embodiment, an example was described in which a thin film of perfluorosulfonic acid is used as the electrolyte membrane 22, but the polymer electrolyte membrane is not limited to this. In the above embodiment, an example was described in which the electrode catalyst layer 23 is composed of carbon particles 232 such as carbon black on which a metal catalyst 231 such as platinum or a platinum alloy is supported, but the metal catalyst and carbon particles are not limited to these. In the above embodiment, an example was described in which the gas diffusion layer 24 is composed of a carbon porous body, but the gas diffusion layer may be any conductive material that has gas permeability and is not limited to the examples given. In the above embodiment, an example was described in which a highly oxygen-permeable ionomer is used as the ionomer 233, but it may be any material that has proton conductivity and adheres the carbon particles on which the metal catalyst is supported, and is not limited to the examples given.

[0044] The above description is merely an example, and the present invention is not limited by the embodiments and modifications described above, as long as the features of the present invention are not impaired. It is also possible to arbitrarily combine one or more of the above embodiments and modifications, and to combine modifications with each other. [Examples]

[0045] Figure 7 illustrates the ionomer region 28 when the mass concentration of ethanol relative to the solvent is varied as the composition of the catalyst ink 230. Carbon particles 232 were mixed into the catalyst ink 230 so that the content ratio of carbon particles 232 to catalyst ink 230 was 5 wt%. In addition, to stably disperse the carbon particles 232 in the ionomer 233, 15 wt% of a dispersion stabilizer (oleylamine) was mixed with the ionomer 233.

[0046] As shown in Figure 7, by reducing the ethanol concentration in the solvent to 60 wt% or less, the ionomer region 28 where the ionomer 233 that has penetrated from the electrode catalyst layer 23 into the gas diffusion layer 24 is located could be kept within a predetermined range (within 5% of the thickness of the water-repellent layer 25). Since the surface tension of water is greater than that of ethanol, the lower the ethanol concentration and the greater the amount of water, the greater the surface tension of the catalyst ink 230. When the surface tension of the catalyst ink 230 is low, in the coating process S2 and lamination process S3 in Figure 4, the solvent containing the ionomer 233 in the catalyst ink 230 spreads and penetrates the gas diffusion layer 24 more easily. By reducing the ethanol concentration in the solvent to 60 wt% or less, the surface tension of the catalyst ink 230 is sufficiently increased to the extent that the solvent containing the ionomer 233 is retained on the surface of the carbon particles 232, and it is considered that penetration into the gas diffusion layer 24 is sufficiently suppressed.

[0047] The solvent contained in the catalyst ink 230 is dried and removed by high temperature in the thermocompression bonding process S4 shown in Figure 4. Since the volatility of water is lower than that of ethanol, the lower the ethanol concentration and the greater the amount of water, the longer the time required to remove the solvent, or the higher the temperature required, which reduces the manufacturing efficiency of the film electrode assembly 20 and increases energy consumption. Therefore, it is preferable that the ethanol concentration in the solvent be around 50-60 wt%.

[0048] Figure 8 illustrates the adsorption rate of ionomer 233 to catalyst particles (carbon particles 232 on which metal catalyst 231 is supported) in the electrode catalyst layer 23 shown in Figure 3, when the mass concentration of the dispersion stabilizer relative to ionomer 233 is varied as part of the composition of catalyst ink 230. 5 wt% of carbon particles 232 were mixed into the catalyst ink 230. The ethanol concentration in the solvent was set to 50 wt%. The adsorption rate of ionomer 233 was estimated by quantifying the unadsorbed ionomer by collecting the supernatant containing unadsorbed ionomer from the catalyst ink 230 by centrifugation and quantifying the fluorine atoms in the supernatant by 19F-NMR measurement.

[0049] As shown in Figure 8, when the concentration of the dispersion stabilizer in ionomer 233 was increased to 6 wt% or more, the adsorption rate of ionomer 233 to the catalyst particles in catalyst ink 230 improved. Furthermore, when the concentration of the dispersion stabilizer in ionomer 233 was increased to 15 wt% or more, the adsorption rate of ionomer 233 to the catalyst particles in catalyst ink 230 reached a saturated state. Therefore, it is preferable that the concentration of the dispersion stabilizer in ionomer 233 be 6 wt% or more, more preferably 15 wt% or more.

[0050] Figure 9 is a diagram illustrating the effect of optimizing the composition of catalyst ink 230 to suppress the penetration of ionomer 233, and shows the quantitative results of the sulfur content (S content in the sulfonic acid group of perfluorosulfonic acid) contained in ionomer 233 in the membrane electrode assembly 20 of Figure 3.

[0051] [Comparative Example 1] Carbon particles 232 were mixed into the catalyst ink 230 such that the carbon particle content of 232 relative to the catalyst ink 230 was 3 wt%. In addition, 6 wt% of a dispersion stabilizer was mixed with the ionomer 233. The solvent used was a 70 wt% aqueous solution of 1-propanol. As shown in Figure 9, it was confirmed that the ionomer 233 penetrated into the gas diffusion layer 24 and reached the carbon paper layer 26.

[0052] [Examples] Carbon particles 232 were mixed into the catalyst ink 230 so that the carbon particle content 232 relative to the catalyst ink 230 was 4 wt% or 5 wt%. In addition, 15.4 wt% of a dispersion stabilizer was mixed with the ionomer 233. The solvent used was a 50 wt% aqueous ethanol solution. As shown in Figure 9, it was confirmed that the ionomer 233 did not penetrate into the gas diffusion layer 24.

[0053] Figure 10 illustrates the effect of the catalyst ink 230 compositional suitability in suppressing coating surface defects in the electrode catalyst layer 23, and shows an image of the surface (coated surface) of the electrode catalyst layer 23 observed with an electron microscope. The electron microscope observation of the coated surface was performed by observing the surface of the electrode catalyst layer 23 after it had been dried without the electrolyte film 22 being laminated.

[0054] [Comparative Example 2] Carbon particles 232 were mixed into the catalyst ink 230 so that the carbon particle content 232 relative to the catalyst ink 230 was 3 wt%. In addition, 6 wt% of a dispersion stabilizer was mixed with the ionomer 233. The solvent used was a 50 wt% aqueous ethanol solution. As shown in Figure 10, many defects were observed on the coated surface of the electrode catalyst layer 23. The polymer electrolyte constituting the electrolyte membrane 22 and the electrode catalyst layer 23 expands and contracts in response to temperature and other factors during the use of the fuel cell. During such expansion and contraction, stress is generated in the defects of the electrode catalyst layer 23, so the more defects there are in the electrode catalyst layer 23, the higher the risk of damage to the electrolyte membrane 22 and the electrode catalyst layer 23.

[0055] [Examples] As shown in Figure 10, it was confirmed that coating surface defects were significantly suppressed in the electrode catalyst layer 23 of the example compared to Comparative Example 2. The amount of metal catalyst 231 and carbon particles 232 to be included in the electrode catalyst layer 23 is determined according to the required output performance of the fuel cell. Therefore, the higher the concentration of carbon particles 232 in the catalyst ink 230, the less catalyst ink 230 is applied to the gas diffusion layer 24 in the coating process S2 in Figure 4, and the thinner the layer thickness (coating thickness, wet thickness) of the catalyst ink 230 in the semi-finished film electrode assembly 20A in Figure 5 becomes. It is thought that by increasing the concentration of carbon particles 232 in the catalyst ink 230 and suppressing the coating thickness, the volume change when removing the solvent in the thermocompression bonding process in Figure 4 is suppressed, and coating surface defects occurring in the electrode catalyst layer 23 are suppressed. The concentration of carbon particles 232 in the catalyst ink 230 is preferably 4-5 wt%.

[0056] This embodiment can produce the following effects. (3) The catalyst ink 230 (Figure 5) contains a solvent, carbon particles 232 on which a metal catalyst 231 is supported, an ionomer 233 that adheres the carbon particles 232 together, and a dispersion stabilizer. The solvent is a 50-60 wt% aqueous solution of ethanol. The content ratio of carbon particles 232 to catalyst ink 230 is 4-5 wt%. The content ratio of dispersion stabilizer to ionomer 233 is 6 wt% or more, more preferably 15 wt% or more.

[0057] Thus, by using an ethanol aqueous solution of 60 wt% or less as the solvent for the catalyst ink 230, sufficient surface tension of the catalyst ink 230 can be ensured, and the penetration of the solvent containing the ionomer 233 into the gas diffusion layer 24 can be suppressed. In this case, the output performance as an electrode of the fuel cell can be ensured while suppressing the consumption of the ionomer 233, and the durability of the electrolyte membrane 22 can be improved. Furthermore, by using an ethanol aqueous solution of 50 wt% or more as the solvent for the catalyst ink 230, the time and energy consumption required in the thermocompression bonding process S4 in Figure 4 can be suppressed. In addition, by setting the content ratio of carbon particles 232 to catalyst ink 230 to 4-5 wt%, coating surface defects occurring in the thermocompression bonding process S4 in Figure 4 can be suppressed, and the durability of the electrolyte membrane 22 can be improved.

[0058] Although the present invention has been described above as a membrane electrode assembly 20 and a catalyst ink 230, the present invention can also be used as a method for manufacturing the membrane electrode assembly 20. The method for manufacturing the membrane electrode assembly 20 includes preparing a catalyst ink 230 containing a solvent, carbon particles 232 on which a metal catalyst 231 is supported, an ionomer 233 that adheres the carbon particles 232 together, and a dispersion stabilizer (preparation step S1), coating the prepared catalyst ink 230 onto a gas diffusion layer 24 (coating step S2), laminating an electrolyte membrane 22 onto the layer of catalyst ink 230 coated on the gas diffusion layer 24 (lamination step S3), and joining the electrolyte membrane 22, the catalyst ink 230 layer, and the gas diffusion layer 24 by thermocompression bonding (thermocompression bonding step S4) (Figures 4 and 5). The solvent is a 50-60 wt% aqueous ethanol solution. The content ratio of carbon particles 232 to catalyst ink 230 is 4-5 wt%. The proportion of dispersion stabilizers in Ionoma 233 is 6 wt% or more. [Explanation of Symbols]

[0059] 1 Power generation cell, 2 Integrated electrode assembly, 3 Separator, 20 Membrane electrode assembly, 20A Membrane electrode assembly semi-finished product, 21 Frame, 22 Electrolyte membrane, 23 Electrode catalyst layer, 24 Gas diffusion layer, 25 Water-repellent layer, 26 Carbon paper layer, 27 Interface, 28 Ionomer region, 100 Fuel cell stack, 230 Catalyst ink, 231 Metal catalyst, 232 Carbon particles, 233 Ionomer, CL Centerline

Claims

1. A solvent, Carbon particles supported with a metal catalyst, The ionomer that adheres the carbon particles together, It contains a dispersion stabilizer, The solvent is a 50-60 weight percent aqueous solution of ethanol. The carbon particle content is 4 to 5 weight percent. A catalyst ink characterized in that the content ratio of the dispersion stabilizer to the ionomer is 6% by weight or more.

2. In the catalyst ink according to Claim 1, A catalyst ink characterized in that the content ratio of the dispersion stabilizer to the ionomer is 15% by weight or more.

3. A catalyst ink is prepared containing a solvent, carbon particles on which a metal catalyst is supported, an ionomer that adheres the carbon particles together, and a dispersion stabilizer. The prepared catalyst ink is applied to the gas diffusion layer. A polymer electrolyte film is laminated onto the layer of catalyst ink applied to the gas diffusion layer. The process includes joining the polymer electrolyte membrane, the catalyst ink layer, and the gas diffusion layer by thermocompression bonding. The solvent is a 50-60 weight percent aqueous solution of ethanol. The content ratio of the carbon particles in the catalyst ink is 4 to 5 weight percent. A method for manufacturing a membrane electrode assembly, characterized in that the content ratio of the dispersion stabilizer to the ionomer is 6% by weight or more.

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