Catalyst ink and method for producing membrane 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

Membrane electrode assembly, catalyst ink, and method for manufacturing membrane electrode assembly

[0001] The present invention relates to a membrane electrode assembly, a catalyst ink, and a method for manufacturing the membrane electrode assembly.

[0002] Conventionally, there have been known methods for manufacturing a membrane electrode assembly in which an electrode catalyst layer and a gas diffusion layer are attached to both sides of an electrolyte membrane (see, for example, Patent Document 1). In the manufacturing method described in Patent Document 1, a catalyst ink is applied to the electrolyte membrane, a gas diffusion layer is bonded to the catalyst ink 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-containing region in the gas diffusion layer, in which the ionomer has permeated, is 9 to 37% of the total ionomer region.

[0003] JP 2013-134877 A

[0004] When manufacturing such a membrane electrode assembly, it is preferable to ensure the output performance of the electrode while suppressing the amount of ionomer consumed. 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.

[0005] A membrane electrode assembly according to one embodiment of the present invention includes a polymer electrolyte membrane, a pair of electrode catalyst layers in close contact with both surfaces of the polymer electrolyte membrane, and a pair of gas diffusion layers in close contact with the outer surfaces of the pair of electrode catalyst layers. The assembly includes an ionomer region, inside the electrode catalyst layers or within a predetermined range in the thickness direction from the interface between the electrode catalyst layers and the gas diffusion layers, in which an ionomer exists that bonds together carbon particles carrying a metal catalyst.

[0006] Another embodiment of the present invention is a catalyst ink containing a solvent, carbon particles carrying a metal catalyst, an ionomer that bonds the carbon particles together, and a dispersion stabilizer. The solvent is a 50 to 60 weight percent aqueous ethanol solution. The carbon particle content is 4 to 5 weight percent. The dispersion stabilizer content relative to the ionomer is 6 weight percent or more.

[0007] Another embodiment of the present invention relates to a method for producing a membrane electrode assembly, which includes preparing a catalyst ink containing a solvent, carbon particles carrying a metal catalyst, an ionomer that bonds the carbon particles together, and a dispersion stabilizer, applying the prepared catalyst ink to a gas diffusion layer, laminating a polymer electrolyte membrane on the layer of catalyst ink applied to the gas diffusion layer, and bonding the polymer electrolyte membrane, the catalyst ink layer, and the gas diffusion layer by thermocompression bonding. The solvent is a 50 to 60 weight percent aqueous ethanol solution. The content of the carbon particles relative to the catalyst ink is 4 to 5 weight percent. The content of the dispersion stabilizer relative to the ionomer is 6 weight percent or more.

[0008] According to the present invention, it is possible to ensure the output performance of the electrode while suppressing the amount of ionomer consumed.

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

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

[0011] FIG. 1 is a perspective view schematically illustrating 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 mutually orthogonal axial directions 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 these definitions. These directions are not necessarily the same as the front-rear direction, the left-right direction, and the up-down direction of a vehicle. For example, the front-rear direction in FIG. 1 may be the front-rear direction, the left-right direction, or the up-down direction of a vehicle.

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

[0013] The separator 3 has a pair of front and rear thin metal plates with a corrugated cross section, which are integrally formed by joining the outer peripheries of these thin plates. The separator 3 is made of a conductive material with excellent corrosion resistance, such as titanium, a titanium alloy, or stainless steel. Inside the separator 3, cooling channels through which a coolant flows are formed by press molding or the like, and the flow of the coolant cools the power generation surface of the power generation cell 1. Water, for example, can be used as the coolant. The surfaces (front and rear surfaces) of the separator 3 facing the integrated electrode assembly 2 are unevenly configured to form gas channels 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 a 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 an oxidant gas flows is formed between the cathode separator 3 and the integrated electrode assembly 2. Hydrogen gas, for example, can be used as the fuel gas, and air, for example, can be used as the oxidant gas. The fuel gas and the oxidant gas are sometimes referred to as reactant gases without any distinction being made between them.

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

[0016] FIG. 3 is a diagram for explaining a membrane electrode assembly 20, conceptually illustrating the configuration of the vicinity of the surface of the membrane electrode assembly 20 facing the reactant gas flow path (anode flow path or cathode flow path; in FIG. 3 , the anode flow path). As shown in FIG. 3 , the membrane electrode assembly 20 includes an electrolyte membrane 22, a pair of electrode catalyst layers 23 in close contact with 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. FIG. 3 only shows the front electrode catalyst layer 23 and gas diffusion layer 24. The gas diffusion layer 24 is made of a gas-permeable conductive member, such as a carbon porous body, 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, the outer surface of which faces the reactant gas flow path.

[0017] A solid polymer electrolyte membrane is used as the electrolyte membrane 22, and a thin film of perfluorosulfonic acid containing water or the like can be used. The electrolyte is not limited to a fluorine-based electrolyte, and a hydrocarbon-based electrolyte can also be used. The electrolyte membrane 22 has proton conductivity and transfers hydrogen ions (H + ) while transferring electrons (e -) and gases (H2, O2, etc.)

[0018] The electrode catalyst layer 23 includes a catalytic metal that promotes the electrochemical reaction between hydrogen contained in the fuel gas and oxygen contained in the oxidant gas, a proton-conductive electrolyte, and carbon particles that are electronically conductive. More specifically, the electrode catalyst layer 23 includes carbon particles 232 such as carbon black carrying a metal catalyst 231 such as platinum or a platinum alloy, and an ionomer 233 that bonds the carbon particles 232 together. The carbon particles 232 are electronically conductive and can transfer electrons (e - The ionomer 233 has proton conductivity and transfers hydrogen ions (H + ) is moved. A polymer electrolyte (perfluorosulfonic acid) that is a high oxygen permeable ionomer (HOPI) can be used as the ionomer 233. The front electrode catalyst layer 23 constitutes an anode electrode, and the rear electrode catalyst layer 23 constitutes a cathode electrode.

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

[0020] At the cathode electrode, the oxidant gas (O) that is supplied through the cathode flow path and the gas diffusion layer 24 and reaches the electrode catalyst layer 23 and the hydrogen ions (H + ) and electrons transferred from the anode electrode (e - ) react with the electrolyte membrane 22 to generate water. +) moves to the surface of the metal catalyst 231 through the ionomer 233, and reacts with the oxidizing gas (O) and electrons (e - The water produced at this time provides an appropriate humidity to the electrolyte membrane 22, and excess water is discharged to the outside of the integrated electrode assembly 2. + +4e - → 2H2O (ii)

[0021] The frame 21 is a thin plate having a substantially rectangular shape and is made of insulating resin, rubber, or the like. A substantially rectangular opening 21a is provided in the center of the frame 21, and the membrane electrode assembly 20 is provided to cover the entire opening 21a. Three through-holes 211 to 213 that penetrate the frame 21 in the front-to-rear direction are opened in a vertically aligned manner on the left side of the opening 21a of the frame 21, and three through-holes 214 to 216 that penetrate the frame 21 in the front-to-rear direction are opened in a vertically aligned manner on the right side of the opening 21a.

[0022] As shown in FIG. 1 , the front and rear separators 3 of the integrated electrode assembly 2 are provided with through-holes 311 to 316 that penetrate the separators 3 in the front-rear direction at positions corresponding to the through-holes 211 to 216 in the frame 21. The through-holes 311 to 316 are connected to the through-holes 211 to 216 in the frame 21, respectively. The interconnected through-holes 211 to 216 and 311 to 316 collectively form flow paths PA1 to PA6 (indicated by arrows for convenience) that penetrate the cell stack 101 and extend in the front-rear direction. The flow paths PA1 to PA6 are sometimes called manifolds (exhaust manifolds). The flow paths PA1 to PA6 are connected to a manifold external to the fuel cell stack 100.

[0023] Flow path PA1 (solid arrow) extending forward through through-holes 211 and 311 is a fuel gas supply flow path. Flow path PA6 (solid arrow) extending rearward through through-holes 216 and 316 is a fuel gas discharge flow path. The fuel gas supply flow path PA1 and the fuel gas discharge flow path PA6 are connected to the anode flow path facing the front surface of the membrane electrode assembly 20, and as shown by the solid arrows, fuel gas flows left and right 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 that has been partially consumed at the anode electrode, and is sometimes referred to as fuel exhaust gas.

[0024] A flow path PA4 (dotted arrow) extending forward via the through-holes 214 and 314 is an oxidant gas supply flow path. A flow path PA3 (dotted arrow) extending rearward via the through-holes 213 and 313 is an oxidant gas discharge flow path. The oxidant gas supply flow path PA4 and the oxidant 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 oxidant gas flows left and right through the cathode flow path via the oxidant gas supply flow path PA4 and the oxidant gas discharge flow path PA3. The oxidant gas flowing through the oxidant gas discharge flow path PA3 is the oxidant gas after a portion has been used at the cathode electrode, and is sometimes referred to as oxidant exhaust gas. The fuel exhaust gas and the oxidant exhaust gas are sometimes referred to as reaction exhaust gas without distinction between them.

[0025] A flow path PA5 (indicated by a dashed-dotted arrow) extending forward through through-holes 215 and 315 is a coolant supply flow path. A flow path PA2 (indicated by a dashed-dotted arrow) extending rearward through through-holes 212 and 312 is a coolant discharge flow path. The coolant supply flow path PA5 and the coolant discharge flow path PA2 are connected to the cooling flow path inside separator 3, and the coolant flows through the cooling flow path via the coolant supply flow path PA5 and the coolant discharge flow path PA2.

[0026] The end units 102, arranged on both the front and rear sides of the cell stack 101, each have a terminal plate 4, an insulating plate 5, and an end plate 6. The front end unit 102 is sometimes called the dry-side end unit, and the rear end unit 102 is sometimes called the wet-side end unit. The pair of front and rear terminal plates 4, 4 are arranged on both the front and rear sides of the cell stack 101, sandwiching the cell stack 101 between them. The pair of front and rear insulating plates 5, 5 are arranged on both the front and rear sides of the terminal plates 4, 4. The pair of front and rear end plates 6, 6 are arranged on both the front and rear sides of the insulating plates 5, 5, sandwiching the insulating plates 5, 5.

[0027] The terminal plate 4 is a generally rectangular metal plate-like member that has terminals for extracting the electric power generated by the electrochemical reaction in the cell stack 101. The insulating plate 5 is a generally rectangular non-conductive resin or rubber plate-like member that electrically insulates the terminal plate 4 from the end plates 6. The end plates 6 are metal or high-strength resin plate-like members, and are fixed to the end plates 6 with bolts, for example, by connecting members that are elongated in the front-to-rear direction and connect the front and rear end plates 6, 6 to each other. The fuel cell stack 100 is held in a state where it is pressed in the front-to-rear direction by the end plates 6, 6 via the connecting members.

[0028] The rear end unit 102 is provided with a plurality of through holes 102a-102f penetrating the end unit 102 in the front-to-rear direction. The through holes 102a-102f include a through hole penetrating the terminal plate 4, a through hole penetrating the insulating plate 5, and a through hole penetrating the end plate 6, but for convenience, these are collectively shown as through holes 102a-102f in FIG. 1 . The through hole 102a is opened on an extension of the fuel gas supply channel PA1 and communicates with the fuel gas supply channel PA1. The through hole 102b is opened on an extension of the coolant discharge channel PA2 and communicates with the coolant discharge channel PA2. The through hole 102c is opened on an extension of the oxidant gas discharge channel PA3 and communicates with the oxidant gas discharge channel PA3. The through hole 102d is opened on an extension of the oxidant gas supply channel PA4 and communicates with the oxidant gas supply channel PA4. The through-hole 102e is opened on an extension of the coolant supply channel PA5 and communicates with the coolant supply channel PA5. The through-hole 102f is opened on an extension of the fuel gas discharge channel PA6 and communicates with the fuel gas discharge channel PA6.

[0029] More specifically, a fuel gas tank storing high-pressure fuel gas is connected to the through-hole 102a via an ejector, an 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 through the ejector and supplied again to the fuel cell stack 100. The separated water is discharged to the outside through a drain passage.

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

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

[0032] Fig. 4 is a diagram illustrating a method for manufacturing the membrane electrode assembly 20, and Fig. 5 is a diagram illustrating a catalyst ink 230 used in manufacturing the membrane electrode assembly 20. As shown in Fig. 4, in manufacturing 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 carrying the metal catalyst 231 shown in Fig. 3, an ionomer 233 that bonds the carbon particles 232 together, and a solvent containing water or alcohol.

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

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

[0035] Next, the electrolyte membrane 22, the layer of catalyst ink 230, and the gas diffusion layer 24 are bonded together by thermocompression (thermocompression bonding step S4). In the thermocompression bonding step S4, the membrane electrode assembly semi-finished product 20A is thermocompression bonded using, for example, a heated press, to bond the electrolyte membrane 22, the layer of catalyst ink 230, 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 FIG. 3 , the 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 tightly adhered to one another.

[0036] 6 is a diagram for explaining the relationship between the amount of ionomer 233 contained in the electrode catalyst layer 23 and the output performance as an electrode of a fuel cell. As shown in FIG. 6, when an appropriate amount of ionomer 233 is contained in the electrode catalyst layer 23, hydrogen ions (H + ) moves smoothly, and the electrode reactions of formulas (i) and (ii) proceed smoothly. In this case, the output performance of the fuel cell electrode 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] 5 , part of the catalyst ink 230 containing the ionomer 233 permeates into the gas diffusion layer 24 through the voids between the porous particles constituting the water-repellent layer 25 and 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 membrane electrode assembly 20, it becomes necessary to use an excessive amount of ionomer 233 in consideration of the amount that permeates from the layer of the catalyst ink 230 in the membrane electrode assembly semi-finished product 20A into the gas diffusion layer 24, which leads to an increase in costs.

[0038] Furthermore, the presence of a hydrophilic substance, ionomer 233, in the gas diffusion layer 24 (water-repellent layer 25) reduces water repellency. In this case, impurities such as ferrous ions derived from the metal separator 3 are more likely to reach the electrolyte membrane 22 along with the water produced by the electrode reaction of formula (ii) and moisture contained in the reaction gas. That is, some of the oxygen in the oxidant gas flowing through the cathode flow path permeates the electrolyte membrane 22 and moves to the anode electrode, where it reacts with the hydrogen ions produced by the electrode reaction of formula (i) and the adsorbed hydrogen before ionization to generate hydrogen peroxide. When this hydrogen peroxide reacts with impurities such as ferrous ions, hydroxyl radicals (.OH) that decompose and deteriorate the electrolyte membrane 22 are generated, reducing the durability of the electrolyte membrane 22.

[0039] Therefore, in this embodiment, the membrane electrode assembly 20 is configured so that the gas diffusion layer 24 does not contain the ionomer 233, so that the output performance can be ensured while suppressing the consumption of the ionomer 233 and the durability of the electrolyte membrane 22 can be improved.

[0040] 3 , the membrane electrode assembly 20 includes an ionomer region 28 in which an ionomer 233 exists inside the electrode catalyst layer 23 or within a predetermined range in the thickness direction from the boundary surface 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 so as to cover the carbon particles 232 carrying the metal catalyst 231 that constitute the electrode catalyst layer 23, and a portion of the ionomer 233 exists so as to adhere to the surfaces of the electrolyte membrane 22 and the gas diffusion layer 24 (water-repellent layer 25) that face the electrode catalyst layer 23. The center line CL of the ionomer region 28 in the thickness direction is located at least inside the electrode catalyst layer 23 and roughly coincides with the center line of the electrode catalyst layer 23 in the thickness direction.

[0041] This embodiment can achieve the following advantageous effects. (1) The membrane electrode assembly 20 includes 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 ( FIG. 3 ). The membrane electrode assembly 20 includes an ionomer region 28, in which ionomer 233 exists, bonding together carbon particles 232 carrying metal catalysts 231, inside the electrode catalyst layer 23 or within a predetermined range in the thickness direction from an interface 27 between the electrode catalyst layer 23 and the gas diffusion layer 24 ( FIG. 3 ). This makes it possible to ensure the output performance of the membrane electrode as a fuel cell electrode while suppressing consumption of the ionomer 233, and improve the durability of the electrolyte membrane 22.

[0042] (2) The center line CL of the ionomer region 28 in the thickness direction is located inside the electrode catalyst layer 23 ( FIG. 3 ). By configuring the center line CL of the ionomer region 28 to be located at least inside the electrode catalyst layer 23, hydrogen ions (H + ) and the amount of ionomer 233 that does not contribute to output performance can be reduced.

[0043] In the above embodiment, an example was described in which a thin film of perfluorosulfonic acid was 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 was formed of carbon particles 232, such as carbon black, on which a metal catalyst 231, such as platinum or a platinum alloy, was supported. However, 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 was formed of a porous carbon material. However, the gas diffusion layer may be any conductive material that is gas permeable and is not limited to the example. In the above embodiment, an example was described in which a highly oxygen-permeable ionomer was used as the ionomer 233. However, any material that has proton conductivity and bonds carbon particles supporting a metal catalyst to each other may be used. This is not limited to the example.

[0044] The above description is merely an example, and the present invention is not limited to the above-described embodiment and modifications as long as the features of the present invention are not impaired. One or more of the above-described embodiment and modifications can be arbitrarily combined, and modifications can also be combined with each other.

[0045] 7 is a diagram illustrating the ionomer region 28 when the mass concentration of ethanol relative to the solvent is changed as the composition of the catalyst ink 230. Carbon particles 232 were mixed into the catalyst ink 230 so that the content of the carbon particles 232 relative to the 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 into the ionomer 233.

[0046] As shown in FIG. 7 , when the ethanol concentration in the solvent was set to 60 wt % or less, the ionomer region 28, where the ionomer 233 that permeated from the electrode catalyst layer 23 to the gas diffusion layer 24 exists, was able to be kept within a predetermined range (within 5% of the thickness of the water-repellent layer 25). Because 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, the solvent containing the ionomer 233 in the catalyst ink 230 spreads and easily permeates into the gas diffusion layer 24 in the application step S2 and lamination step S3 in FIG. 4 . It is believed that setting the ethanol concentration in the solvent to 60 wt % or less sufficiently increases the surface tension of the catalyst ink 230 to such an extent that the solvent containing the ionomer 233 is retained on the surfaces of the carbon particles 232, thereby sufficiently suppressing permeation into the gas diffusion layer 24.

[0047] The solvent contained in the catalyst ink 230 is dried and removed at high temperatures in the thermocompression bonding step S4 in Fig. 4. Because water is less volatile than ethanol, the lower the ethanol concentration and the greater the amount of water, the longer it takes to remove the solvent or the higher the temperature required, which reduces the production efficiency of the membrane electrode assembly 20 and increases energy consumption. Therefore, the ethanol concentration in the solvent is preferably about 50 to 60 wt%.

[0048] FIG. 8 is a diagram illustrating the adsorption rate of the ionomer 233 to the catalyst particles (carbon particles 232 carrying a metal catalyst 231) in the electrode catalyst layer 23 of FIG. 3 when the mass concentration of the dispersion stabilizer relative to the ionomer 233 is changed as the catalyst ink 230 composition. 5 wt % of the carbon particles 232 was mixed into the catalyst ink 230. The ethanol concentration in the solvent was set to 50 wt %. The adsorption rate of the ionomer 233 was estimated based on the quantitative results obtained by recovering the supernatant containing the non-adsorbed ionomer from the catalyst ink 230 by centrifugation and quantifying the fluorine atoms in the supernatant by 19F-NMR measurement.

[0049] 8, when the concentration of the dispersion stabilizer in the ionomer 233 was set to 6 wt % or higher, the adsorption rate of the ionomer 233 to the catalyst particles in the catalyst ink 230 improved. Furthermore, when the concentration of the dispersion stabilizer in the ionomer 233 was set to 15 wt % or higher, the adsorption rate of the ionomer 233 to the catalyst particles in the catalyst ink 230 became saturated. Therefore, it is preferable that the concentration of the dispersion stabilizer in the ionomer 233 be 6 wt % or higher, and more preferably 15 wt % or higher.

[0050] FIG. 9 is a diagram for explaining the permeation suppression effect of the ionomer 233 due to the compositional suitability of the catalyst ink 230, and shows the results of quantifying the sulfur content (the S content in the sulfonic acid group of perfluorosulfonic acid) contained in the ionomer 233 in the membrane electrode assembly 20 of FIG. 3.

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

[0052] [Example] Carbon particles 232 were mixed into the catalyst ink 230 so that the content of the carbon particles 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 was a 50 wt % aqueous ethanol solution. As shown in Figure 9, it was confirmed that the ionomer 233 did not permeate the gas diffusion layer 24.

[0053] 10 is a diagram for explaining the effect of suppressing defects in the coating surface of the electrode catalyst layer 23 by adapting the composition of the catalyst ink 230, and shows an image of the surface (coating surface) of the electrode catalyst layer 23 observed with an electron microscope. The observation of the coating surface with an electron microscope was performed by observing the surface of the electrode catalyst layer 23 that had been dried without the electrolyte membrane 22 being laminated thereon.

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

[0055] [Example] As shown in FIG. 10 , it was confirmed that the electrode catalyst layer 23 of the example significantly reduced coating surface defects compared to Comparative Example 2. The amounts of metal catalyst 231 and carbon particles 232 to be contained in the electrode catalyst layer 23 are determined based on the required output performance of the fuel cell. Therefore, the higher the concentration of carbon particles 232 in the catalyst ink 230, the smaller the amount of catalyst ink 230 applied to the gas diffusion layer 24 in the application step S2 of FIG. 4 , resulting in a smaller layer thickness (coating thickness, wet thickness) of the catalyst ink 230 in the membrane electrode assembly semi-finished product 20A of FIG. 5 . It is believed that increasing the concentration of carbon particles 232 in the catalyst ink 230 and reducing the coating thickness suppresses volume change during solvent removal in the thermocompression bonding step of FIG. 4 , thereby suppressing coating surface defects in the electrode catalyst layer 23. The concentration of carbon particles 232 in the catalyst ink 230 is preferably 4 to 5 wt %.

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

[0057] In this way, 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 permeation of the solvent containing the ionomer 233 into the gas diffusion layer 24 can be suppressed. In this case, the output performance of the fuel cell electrode 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 for the thermocompression bonding step S4 in FIG. 4 can be reduced. Furthermore, by setting the content of the carbon particles 232 in the catalyst ink 230 to 4 to 5 wt %, coating surface defects that occur in the thermocompression bonding step S4 in FIG. 4 can be suppressed and the durability of the electrolyte membrane 22 can be improved.

[0058] While the present invention has been described above in terms of the membrane electrode assembly 20 and the catalyst ink 230, it can also be used as a method for manufacturing the membrane electrode assembly 20. The method for manufacturing the membrane electrode assembly 20 includes the steps of: preparing a catalyst ink 230 containing a solvent, carbon particles 232 carrying a metal catalyst 231, an ionomer 233 for bonding the carbon particles 232 together, and a dispersion stabilizer (preparation step S1); applying the prepared catalyst ink 230 to a gas diffusion layer 24 (application step S2); laminating an electrolyte membrane 22 on the layer of catalyst ink 230 applied to the gas diffusion layer 24 (lamination step S3); and bonding the electrolyte membrane 22, the layer of catalyst ink 230, and the gas diffusion layer 24 by thermocompression (thermocompression bonding step S4) ( FIGS. 4 and 5 ). The solvent is a 50 to 60 wt % aqueous ethanol solution. The carbon particles 232 are contained in the catalyst ink 230 at a ratio of 4 to 5 wt %. The content of the dispersion stabilizer relative to the ionomer 233 is 6 wt % or more.

[0059] REFERENCE SIGNS LIST 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 Center line

Claims

1. A solvent, Carbon particles carrying a metal catalyst; an ionomer that bonds the carbon particles together; A dispersion stabilizer is contained, the solvent is a 50 to 60 weight percent aqueous solution of ethanol; The content of the carbon particles is 4 to 5 weight percent, A catalyst ink characterized in that the content of the dispersion stabilizer relative to the ionomer is 6 weight percent or more.

2. The catalyst ink according to claim 1, A catalyst ink characterized in that the content of the dispersion stabilizer relative to the ionomer is 15 weight percent or more.

3. A catalyst ink is prepared containing a solvent, carbon particles carrying a metal catalyst, an ionomer that bonds the carbon particles together, and a dispersion stabilizer; Applying the prepared catalyst ink to a gas diffusion layer; laminating a polymer electrolyte membrane on the layer of the catalyst ink applied to the gas diffusion layer; bonding the polymer electrolyte membrane, the catalyst ink layer, and the gas diffusion layer by thermocompression bonding; the solvent is a 50 to 60 weight percent aqueous solution of ethanol; the content of the carbon particles in the catalyst ink is 4 to 5 weight percent; A method for producing a membrane electrode assembly, wherein the content of the dispersion stabilizer relative to the ionomer is 6 weight percent or more.