Catalyst ink

Abstract

To provide a catalyst ink that ensures the dispersibility of ionomer in a solvent even when the alcohol ratio of the solvent is increased.SOLUTION: Catalyst ink 10 is for forming a catalyst layer 22 when coated onto the surface of an electrolyte membrane 21 of a fuel cell 1 and dried. The catalyst ink 10 includes ionomer 4, catalyst metal particles, carbon 5, and a solvent 6. The carbon 5 includes carbon fibers 52. The proportion of carbon fibers 52 in the carbon 5 within the catalyst ink 10 is 5.0 volume percent or more and 20 volume percent or less.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a catalyst ink used to form a catalyst layer of a fuel cell. [Background technology]

[0002] In recent years, fuel cells, which generate electricity through an electrochemical reaction between hydrogen and oxygen, have been attracting attention as a new power source for automobiles, etc. Fuel cells are considered desirable because they generate electricity directly through an electrochemical reaction, resulting in high power generation efficiency.Furthermore, fuel cells are considered desirable from the perspective of environmental impact, as they only produce water during power generation.

[0003] A fuel cell includes a membrane electrode assembly that is configured by sandwiching an electrolyte membrane between a pair of catalyst layers. For example, Patent Document 1 describes a catalyst layer that includes platinum particles, carbon particles, a polymer electrolyte, etc. The catalyst layer is formed, for example, by applying a catalyst ink that includes an ionomer as an ion-conductive member, catalytic metal particles, carbon, a solvent, etc. to the surface of the electrolyte membrane and then drying the ink. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2023-10955 Summary of the Invention [Problem to be solved by the invention]

[0005] When applying a catalyst ink to an electrolyte membrane, it is desirable to reduce the ratio of alcohol to water in the solvent of the catalyst ink in consideration of the effect on the electrolyte membrane. However, reducing the ratio of alcohol to water tends to reduce the dispersibility of the ionomer.

[0006] An object of the present invention is to provide a catalyst ink that can ensure the dispersibility of an ionomer in a solvent even when the alcohol ratio in the solvent is increased. [Means for solving the problem]

[0007] (1) The catalyst ink is applied to the surface of a fuel cell electrolyte membrane and dried to form a catalyst layer, and contains an ionomer, catalytic metal particles, carbon, and a solvent, the carbon containing carbon fiber, and the proportion of the carbon fiber in the carbon in the catalyst ink is 5.0% by volume or more and 20% by volume or less.

[0008] (2) The catalyst ink according to (1), wherein the carbon fiber is vapor-grown carbon fiber.

[0009] (3) The catalyst ink according to (1) or (2), wherein the carbon fiber accounts for 5.0% by volume or more and 10% by volume or less of the carbon in the catalyst ink. [Effects of the Invention]

[0010] According to the present invention, it is possible to provide a catalyst ink that can ensure the dispersibility of an ionomer in a solvent even when the ratio of alcohol in the solvent is increased. [Brief explanation of the drawings]

[0011] [Figure 1] 1 is a cross-sectional view showing a membrane electrode assembly of a fuel cell manufactured using a catalyst ink according to one embodiment of the present invention. [Figure 2] FIG. 2 is a schematic diagram showing the dispersion state of each component in a catalyst ink according to one embodiment of the present invention. [Figure 3A] 1 is a schematic diagram showing the state of penetration of a solvent into an electrolyte membrane coated with a catalyst ink having an SP value of 18 or less. [Figure 3B] 3B is a schematic diagram showing a catalyst layer formed by coating the catalyst ink in FIG. 3A. FIG. [Figure 4A] 3 is a schematic diagram showing the state of penetration of a solvent in a catalyst ink into an electrolyte membrane coated with the catalyst ink according to one embodiment of the present invention. FIG. [Figure 4B]4B is a schematic diagram showing a catalyst layer formed by coating the catalyst ink in FIG. 4A. FIG. [Figure 5] FIG. 2 is a schematic diagram showing the dispersion state of each component in a catalyst ink that does not contain carbon fiber. [Figure 6A] 1 is a graph showing the relationship between the SP value of a solvent in a catalyst ink and the solvent absorption rate of an electrolyte membrane coated with the catalyst ink. [Figure 6B] 1 is a graph showing the relationship between the SP value of a solvent in a catalyst ink and the solvent absorption rate of an electrolyte membrane coated with the catalyst ink. [Figure 7] FIG. 1 is a diagram showing a drop of catalyst ink applied to an electrolyte membrane, and is an explanatory diagram for explaining a method for determining the solvent absorption rate of an electrolyte membrane. [Figure 8] 1 is a graph showing the relationship between viscosity and shear rate for a catalyst ink according to an embodiment of the present invention and a catalyst ink that does not contain carbon fiber. [Figure 9] 1 is a graph showing particle size distributions of a catalyst ink according to an embodiment of the present invention and a catalyst ink that does not contain carbon fiber. [Figure 10] 10 is a graph showing the hardness of a catalyst layer formed using a catalyst ink according to an embodiment of the present invention and a catalyst ink that does not contain carbon fiber. [Figure 11] 1 is a graph showing the relationship between the proportion of carbon fibers in the carbon in a catalyst layer and the crack occurrence rate. DETAILED DESCRIPTION OF THE INVENTION

[0012] Hereinafter, embodiments of the present invention will be described with reference to the drawings. However, the embodiments shown below are merely examples of the present invention, and the present invention is not limited to the following embodiments.

[0013] A catalyst ink 10 according to this embodiment and a fuel cell 1 produced using the catalyst ink 10 will be described with reference to FIGS. 1 and 2. FIG. 1 is a cross-sectional view showing a membrane electrode assembly 2 of the fuel cell 1 produced using the catalyst ink 10. FIG. 2 is a schematic diagram showing the dispersion state of each component in the catalyst ink 10. First, a fuel cell 1 produced using the catalyst ink 10 of this embodiment will be described with reference to FIG. 1.

[0014] [Fuel cell] The fuel cell 1 is a polymer electrolyte fuel cell that generates electricity by reacting hydrogen and oxygen and is mainly equipped with a membrane electrode assembly 2 and a gas diffusion layer 3. Polymer electrolyte fuel cells have the advantages of low operating temperatures, short start-up times, and compact construction, and are therefore used in fields such as power sources for automobiles.

[0015] The membrane electrode assembly 2 has an electrolyte membrane 21 and a pair of catalyst layers 22 arranged opposite each other with the electrolyte membrane 21 interposed therebetween. The catalyst ink 10 is used to form the catalyst layers 22.

[0016] The electrolyte membrane 21 is a proton-conducting membrane formed from a film of a cation-exchange resin polymer with proton conductivity. Examples of the cation-exchange resin include sulfonated vinyl polymers such as polystyrene sulfonic acid, polymers in which sulfonic acid groups or phosphoric acid groups have been introduced into heat-resistant polymers such as perfluoroalkyl sulfonic acid polymers, perfluoroalkyl carboxylic acid polymers, polybenzimidazole, and polyether ether ketone, and polymers in which sulfonic acid groups have been introduced into rigid polyphenylene, which is obtained by polymerizing an aromatic compound consisting of phenylene chains, as the main component.

[0017] The catalyst layer 22 is configured to include catalytic metal particles (not shown), an ionomer 4 (not shown in FIG. 1) which is a polymer electrolyte, and carbon 5 (not shown in FIG. 1). One of the pair of catalyst layers 22 functions as the anode of the fuel cell 1, and the other functions as the cathode.

[0018] The catalytic metal particles are not particularly limited as long as they have catalytic activity, and examples thereof include platinum and platinum alloys. The use of platinum alloys can also impart stability and activity to the electrode catalyst. Platinum alloys are preferably alloys of platinum with one or more metals selected from the group consisting of platinum group metals other than platinum (ruthenium, rhodium, palladium, osmium, and iridium), cobalt, iron, titanium, gold, silver, chromium, manganese, molybdenum, tungsten, aluminum, silicon, rhenium, zinc, and tin, and the platinum alloy may contain an intermetallic compound of platinum and the metal to be alloyed.

[0019] The catalyst layer 22 is formed by applying the catalyst ink 10 to the surface of the electrolyte membrane 21 and drying it. The method for applying the catalyst ink 10 is not particularly limited, and various application methods can be used. Examples of application methods include bar coater application, knife coater application, die coat application, doctor blade method, screen printing, and spray application. The ionomer 4 and carbon 5 are contained in the catalyst ink 10, and their details will be described later.

[0020] The gas diffusion layer 3 is made of a porous sheet-like material. The sheet-like material may be, for example, nickel foam or carbon cloth. The gas diffusion layer 3 is disposed on the surface of the catalyst layer 22 opposite to the electrolyte membrane 21.

[0021] [Catalyst ink] Next, the catalyst ink 10 will be described with reference to Fig. 2. The catalyst ink 10 is a mixed liquid containing the above-mentioned catalyst metal particles, an ionomer 4, carbon 5, and a solvent 6.

[0022] The ionomer 4 is used in the electrode reaction in the catalyst layer 22 and conducts the ions generated. Examples of the ionomer 4 include a fluorine-based polymer in which at least a portion of the polymer skeleton is fluorinated, and a hydrocarbon-based polymer that does not contain fluorine in the polymer skeleton and has an ion exchange group. The ionomer 4 is preferably a fluorine-based ionomer 4 that is a fluorine-based polymer. The type of ion exchange group is not particularly limited and can be selected as desired depending on the application. Examples of the ion exchange group include sulfonic acid, carboxylic acid, and phosphonic acid. Note that in FIG. 2, the ionomer 4 is shown both as a series of solid circles and a series of dashed circles, for the sake of convenience, so that it is clear that multiple ionomers 4 are dispersed.

[0023] The carbon 5 may be carbon black, graphite, carbon fiber, activated carbon, or pulverized products thereof, or carbon compounds such as carbon nanofiber, carbon nanotube, carbon nitride, carbon sulfide, and carbon phosphide.

[0024] Carbon 5 is broadly classified into non-fibrous carbon particles 51 such as carbon black, and fibrous carbon fibers 52. Carbon particles 51 function as a conductive support that supports catalytic metal particles. There are no particular limitations on the conductive support as long as it has conductivity and appropriate corrosion resistance, but it is desirable for it to have a sufficient specific surface area for highly dispersing catalytic metal particles and sufficient electronic conductivity.

[0025] The carbon 5 carrying catalytic metal particles may be prepared or commercially available. When preparing the carbon 5 carrying catalytic metal particles, the preparation method is not particularly limited, and conventionally known methods can be used. Examples of conventionally known preparation methods include adding a solution or suspension of a platinum compound to a support powder, evaporating the mixture to dryness, insolubilizing the mixture with an acid or alkali, and then performing a reduction treatment to activate the supported components. In this embodiment, the catalytic metal particles are supported only on the carbon particles 51, but the catalytic metal particles may be supported on both the carbon particles 51 and the carbon fibers 52.

[0026] The carbon fibers 52 preferably have a fiber diameter of 100 nm to 200 nm and a fiber length of 10 μm to 50 μm, for example. The carbon fibers 52 are preferably multi-walled carbon nanotubes, for example. The multi-walled carbon nanotubes are preferably vapor grown carbon fibers (VGCF). Examples of multi-walled carbon nanotubes other than VGCF include multi-walled carbon nanotubes such as NTDWK010 (manufactured by Hamamatsu Carbonix Co., Ltd.).

[0027] The proportion of carbon fiber 52 in the carbon 5 in the catalyst ink 10 may be, for example, greater than 0 vol% and less than 20 vol%, or may be 5.0 vol% or greater and 20 vol% or less. This can improve the dispersibility of ionomer 4 in the catalyst ink 10. From the perspective of improving the dispersibility of ionomer 4, the proportion of carbon fiber 52 in the carbon 5 in the catalyst ink 10 is preferably greater than 0 vol% and less than 10 vol%, and more preferably greater than 5.0 vol% and less than 10 vol%.

[0028] The solvent 6 is a medium for dispersing the ionomer 4 and the carbon 5. The solvent 6 is preferably a mixed solvent containing water and an alcohol. Examples of the alcohol include a mixture of at least one type or two or more types selected from lower alcohols such as methanol, ethanol, 1-propanol, and 1-butanol. The solvent 6 of this embodiment is a mixed solvent containing water, ethanol, and 1-propanol.

[0029] The solvent 6 in this embodiment is adjusted so that its solubility parameter (SP value) exceeds 18. This makes the solvent 6 more hydrophilic and reduces its affinity with the electrolyte membrane 21, which is mainly composed of polymers, thereby reducing swelling or dissolution of the electrolyte membrane 21 due to the solvent 6 in the catalyst ink 10 applied to the surface of the electrolyte membrane 21.

[0030] [Table 1]

[0031] Table 1 shows the relationship between the composition of solvent 6 in catalyst ink 10 and the SP value. Solvent 6 shown in Table 1 is composed of water, ethanol, and 1-propanol. As shown in Table 1, when the ratio of alcohol to water in solvent 6 is 50% by mass, it can be confirmed that the SP value exceeds 18 when the ethanol content is 35% by mass or more.

[0032] Here, the relationship between the permeability of the solvent 6 of the catalyst ink 10 into the electrolyte membrane 21 and cracks that occur in the electrolyte membrane 21 will be described with reference to FIGS. 3A to 4B.

[0033] FIG. 3A is a schematic diagram showing the state of permeation of solvent 6 into an electrolyte membrane 21 coated with catalyst ink having an SP value of 18 or less. FIG. 3B is a schematic diagram showing the state of a catalyst layer 22 formed by drying the catalyst ink coated in FIG. 3A. FIG. 4A is a schematic diagram showing the state of permeation of solvent 6 into an electrolyte membrane 21 coated with catalyst ink 10 having an SP value of solvent 6 exceeding 18. FIG. 4B is a schematic diagram showing the state of a catalyst layer 22 formed by drying the catalyst ink 10 coated in FIG. 4A. In FIGS. 3A to 4B, dots indicate solvent 6 that has permeated into the electrolyte membrane 21.

[0034] 3A, a catalyst ink in which the SP value of the solvent 6 is 18 or less has a high affinity with the electrolyte membrane 21, and a larger amount of the solvent 6 tends to penetrate from the surface to the inside of the electrolyte membrane 21. At this time, the electrolyte membrane 21 is swollen and soft, and therefore, during the drying process to form the catalyst layer 22, the contraction stress of the catalyst ink (for example, a force acting in the direction of the arrow shown in FIG. 3B) causes the catalyst layer 22 to slip at the interface with the electrolyte membrane 21, resulting in cracks in the catalyst layer 22 as shown in FIG. 3B.

[0035] On the other hand, catalyst ink 10 in which the SP value of solvent 6 is 18 or higher has a low affinity with electrolyte membrane 21, as shown in Fig. 4A, which reduces penetration of solvent 6 from the surface of electrolyte membrane 21 and suppresses swelling and dissolution of the surface of electrolyte membrane 21. This suppresses a decrease in the hardness of the surface of electrolyte membrane 21 and prevents slippage of catalyst layer 22 at the interface with electrolyte membrane 21 due to shrinkage of catalyst ink 10 during drying, as shown in Fig. 4B. This suppresses the occurrence of cracks in catalyst layer 22.

[0036] The catalyst ink 10 preferably has a solids concentration of 2% by mass or more and 6% by mass or less relative to its total weight. A solids concentration of 2% by mass or more eliminates the need for thickening the coating during application, avoiding conditions that could lead to cracking, and reduces the amount of solvent 6 applied to the electrolyte membrane 21, thereby reducing the amount of solvent 6 that penetrates into the electrolyte membrane 21. Furthermore, a solids concentration of 6% by mass or less makes it difficult for dispersibility to decrease during kneading, for example, when using a ball mill or bead mill, thereby preventing a decrease in coatability and defects in the formation of the catalyst layer. In this specification, the solids of the catalyst ink refer to the catalytic metal particles, ionomer 4, and carbon 5 contained in the catalyst ink 10. Furthermore, the solids concentration in this specification refers to the mass percent concentration of the combined mass of the catalytic metal particles, ionomer 4, and carbon 5 relative to the total mass of the catalyst ink 10.

[0037] The concentration of solids can be determined, for example, by heating the catalyst ink 10 to evaporate the solvent and calculating the amount of the remaining solvent after heating relative to the mass of the catalyst ink 10 before heating. The concentration of solids may be measured, for example, using a moisture meter (MOC63u, manufactured by Shimadzu Corporation).

[0038] The ratio of the ionomer 4 to the total amount of carbon fibers 52 in the catalyst ink 10 (ionomer / total carbon fibers 52) is not particularly limited, but is preferably 0.003 to 0.2 from the viewpoint of performance and layer formation.

[0039] The dispersion state of each component in the catalyst ink 10 according to this embodiment will now be described with reference to Figures 2 and 5. Figure 5 is a schematic diagram showing the dispersion state of ionomer 4 in catalyst ink 10A that does not contain carbon fiber 52. Note that in Figure 5, for the sake of convenience, the ionomers 4 are shown as both multiple continuous solid circles and multiple continuous dashed circles so that it is clear that multiple ionomers 4 are dispersed.

[0040] In catalyst ink 10A that does not contain carbon fiber 52, ionomers 4 tend to bond together to form localized aggregate particles, as shown in Figure 5. If the diameter of an aggregate particle made up of multiple ionomers 4, etc., becomes large, cracks will occur starting from the aggregate particle, which will cause a decrease in the strength and functionality of catalyst layer 22.

[0041] In contrast, in the catalyst ink 10 containing the carbon fibers 52, as shown in Fig. 2, multiple ionomers 4 are connected to each other via the carbon fibers 52, and the ionomers 4 are uniformly entangled throughout the catalyst ink 10. In other words, the carbon fibers 52 increase the dispersibility of the ionomer 4 in the solvent 6 compared to Fig. 5.

[0042] Here, if the alcohol-to-water ratio of the solvent 6 in the catalyst ink 10 is reduced and the SP value is increased, the dispersibility of the ionomer 4 in the solvent 6 tends to decrease, as shown in FIG. 5. On the other hand, if the SP value of the solvent 6 in the catalyst ink 10 is increased above 18, the penetration of the solvent 6 into the electrolyte membrane 21 is suppressed, as shown in FIGS. 4A and 4B, and swelling and dissolution of the electrolyte membrane 21 can be reduced. In this embodiment, by adding carbon fibers 52 to the catalyst ink 10, high dispersibility of the ionomer 4 can be ensured even in a solvent composition with a high SP value that tends to reduce the dispersibility of the ionomer 4. This makes it possible to more reliably form a high-quality catalyst layer 22.

[0043] [Method of manufacturing catalyst ink] Next, an example of a method for producing the catalyst ink 10 will be described.

[0044] First, an ionomer 4 and solvents 6, such as an alcohol (e.g., ethanol or 1-propanol) and water, are weighed and mixed to prepare a mixed solution. Next, carbon particles 51 carrying catalytic metal particles and carbon fibers 52 are weighed and added to the mixed solution, which is then stirred using a propeller stirrer. At this time, the components are mixed and stirred while adjusting the proportion of carbon fibers 52 in the carbon 5 and the SP value of the solvent 6. Next, the mixed solution in which the proportion of carbon fibers 52 and the SP value of the solvent 6 have been adjusted is kneaded using zirconia in a ball mill or bead mill. The kneading time may be, for example, 24 hours when kneading using a ball mill.

[0045] The catalyst ink 10 according to the embodiment described above provides the following effects.

[0046] The catalyst ink 10 of this embodiment is a catalyst ink 10 that forms a catalyst layer 22 by being applied to the surface of an electrolyte membrane 21 of a fuel cell 1, and contains an ionomer 4, catalytic metal particles, carbon 5, and a solvent 6, wherein the carbon 5 contains carbon fibers 52, and the proportion of the carbon fibers 52 in the carbon 5 in the catalyst ink 10 is 5.0 volume % or more and 20 volume % or less.

[0047] When the alcohol to water ratio of the solvent 6 in the catalyst ink 10 is reduced and the SP value is increased, the dispersibility of the ionomer 4 in the solvent 6 tends to decrease, as shown in Figure 5. On the other hand, when the SP value of the solvent 6 in the catalyst ink 10 is increased to more than 18, the penetration of the solvent 6 into the electrolyte membrane 21 is suppressed, as shown in Figures 4A and 4B, and swelling and dissolution of the electrolyte membrane 21 can be reduced.

[0048] This allows the ionomer 4 in the catalyst ink 10 to be connected to each other via the carbon fibers 52, improving the dispersibility of the ionomer 4 throughout the catalyst ink 10. Therefore, stable dispersibility of the ionomer 4 in the solvent 6 can be ensured even when the alcohol content of the solvent 6 is increased. This more reliably reduces the occurrence of cracks and wrinkles in the catalyst layer 22 formed using the catalyst ink 10. Furthermore, the improved dispersibility of the ionomer 4 leads to the formation of more crosslinks of the ionomer 4 in the catalyst ink 10, improving the strength and elastic modulus within the catalyst layer 22 and the durability of the catalyst layer 22, thereby enabling the production of a high-quality catalyst layer 22. Furthermore, the generation of bubbles in the catalyst ink 10 that are difficult to degas due to reduced dispersibility of the ionomer 4 can be prevented, thereby suppressing the occurrence of factors that hinder productivity improvement in the manufacturing process of the catalyst ink 10. Furthermore, the need for additional equipment or stirring processes to improve the dispersibility of the ionomer 4 in the catalyst ink 10 is eliminated, thereby reducing manufacturing costs.

[0049] In the catalyst ink 10 according to this embodiment, the carbon fibers 52 are vapor-grown carbon fibers.

[0050] This further improves the dispersibility of the ionomer 4 in the catalyst ink 10 .

[0051] In the catalyst ink 10 according to this embodiment, the proportion of the carbon fibers 52 in the carbon 5 in the catalyst ink 10 is 5.0% by volume or more and 10% by volume or less.

[0052] This makes it possible to more efficiently improve the dispersibility of the ionomer 4 in the catalyst ink 10 .

[0053] Although the preferred embodiment of the present invention has been described above, the present invention is not limited to the above embodiment and can be modified as appropriate. [Example]

[0054] Next, the present invention will be described in more detail based on examples, but the present invention is not limited to these examples.

[0055] [Solvent absorption rate evaluation test] The solvent absorption rate of the electrolyte membrane relative to the SP value of the catalyst ink solvent was examined. Figures 6A and 6B show the evaluation results of the solvent absorption rate of the electrolyte membrane relative to the SP value of the catalyst ink solvent. The vertical axis of Figures 6A and 6B shows the solvent absorption rate (%), and the horizontal axis shows the SP value of each catalyst ink solvent. The SP values ​​shown in Figures 6A and 6B are weighted averages of the SP values ​​of each solvent in the catalyst ink, with the proportion of each solvent used as weights. Figure 6A shows the evaluation results of the solvent absorption rate of electrolyte membrane B, and Figure 6B shows the evaluation results of the solvent absorption rate of electrolyte membrane C. Both electrolyte membranes B and C are formed by reinforcing layers sandwiched between ionomer layers on both sides in the thickness direction. The reinforcing layers are made of expanded polytetrafluoroethylene (ePTFE).

[0056] <Ink preparation> The inks used in the solvent absorption rate confirmation test were prepared as follows. First, an aqueous dispersion of ionomer, ethanol, 1-propanol, and water were weighed and placed in a container. The solution in the container was then mixed and stirred using a propeller stirrer to prepare the inks used in the solvent absorption rate evaluation test. The amounts of ethanol and 1-propanol added were adjusted to achieve the SP values ​​of each ink shown in Figures 6A and 6B.

[0057] <Evaluation method> Figure 7 shows a drop of ink applied to an electrolyte membrane, and is an explanatory diagram for explaining a method for determining the solvent absorption rate of an electrolyte membrane. The amount of solvent absorbed into the electrolyte membrane was determined by dropping the prepared ink onto the surfaces of electrolyte membranes B and C, and observing the dimensions of the droplets on electrolyte membranes B and C using an optical microscope or the like. Then, based on the observed droplet dimensions, the change in droplet volume was calculated using the following formula (1), and the solvent absorption rate A of the electrolyte membrane was calculated using the following formula (2). A1=V0-V1-V2...Formula (1) A=(A1 / V0)×100...Equation (2)

[0058] Note that A1 is the amount absorbed by the electrolyte membrane (the area indicated by the second densest dots in Figure 7), V0 is the volume immediately after dropping onto the electrolyte membrane (the area from the densest dots to the sparsest dots in Figure 7), V1 is the volume 60 seconds after dropping (the area indicated by the densest dots in Figure 7), and V2 is the amount of solvent 6 that evaporates in 60 seconds after dropping.

[0059] <Evaluation results> As shown in Figures 6A and 6B, it was confirmed that the solvent absorption rates of both electrolyte membranes B and C were high when the SP value of the ink was in the range of more than 16 and less than 18. It was also confirmed that the solvent absorption rates of both electrolyte membranes B and C decreased significantly when the SP value exceeded 18.

[0060] [Viscosity and particle size evaluation test] The effect of carbon fiber on the viscosity and particle size of the catalyst ink was determined.

[0061] <Preparation of catalyst ink> The catalyst ink used in the viscosity and particle size evaluation tests was prepared as follows. First, an aqueous ionomer dispersion and ethanol, 1-propanol, and water as solvents were weighed and placed in a container. Next, carbon particles carrying catalytic metal particles and carbon fibers were weighed and placed in a container, and mixed and stirred together with the aqueous ionomer dispersion and each solvent using a propeller stirrer under a nitrogen atmosphere. Next, the mixed and stirred mixture was kneaded using zirconia in a ball mill or bead mill. VGCF (manufactured by Showa Denko K.K.) was used as the carbon fiber. Sample 1 was prepared by adjusting the carbon fiber in the catalyst ink to 10% by volume of the total carbon, including the carbon particles, and the SP of the solvent to 17.97. Sample 2 was the same as Sample 1 except that it did not contain carbon fiber.

[0062] <Viscosity evaluation method> Regarding viscosity, Sample 1 and Sample 2 were stirred at shear rates of 100 [1 / s], 450 [1 / s], 600 [1 / s], and 700 [1 / s], and then the viscosity was measured using a cone-plate viscometer (rheometer).

[0063] <Method for evaluating particle size> Regarding the particle size, the particle size of each of Sample 1 and Sample 2 was measured using a particle size meter (manufactured by Shimadzu Corporation).

[0064] <Evaluation results> FIG. 8 is a graph showing the viscosity evaluation results for Samples 1 and 2. The vertical axis of FIG. 8 represents the viscosity [mPas] of the catalyst ink, and the horizontal axis represents the shear rate [1 / s]. As shown in FIG. 8, it was confirmed that the catalyst ink of Sample 1, which contains carbon fiber, has a lower viscosity at all shear rates compared to Sample 2, which does not contain carbon fiber. FIG. 9 is a graph showing the particle size distribution evaluation results for Samples 1 and 2. The vertical axis of FIG. 9 represents the particle size (μm) of the catalyst ink, and the horizontal axis represents D50, D75, and D90. As shown in FIG. 9, it was confirmed that the catalyst ink of Sample 1, which contains carbon fiber, has a smaller particle size. Therefore, the viscosity and particle size evaluation tests confirmed that carbon fiber improves the dispersibility of the catalyst ink.

[0065] [Catalyst layer hardness evaluation test] The effect of carbon fiber on the hardness of the catalyst layer was confirmed.

[0066] <Preparation of catalyst layer> The catalyst layers used in the hardness evaluation test were prepared as follows: Sample 1, which served as catalyst ink, was applied to the surface of the electrolyte membrane and dried in a batch oven to prepare Sample 3, which served as catalyst layer; and Sample 2, which served as catalyst ink, was applied to the surface of the electrolyte membrane and dried in a batch oven to prepare Sample 4, which served as catalyst layer.

[0067] <Hardness evaluation method> The hardness of the catalyst layers of Samples 3 and 4 was measured by a microindentation test using a nanoindenter (manufactured by Anton Paar).

[0068] <Evaluation results> FIG. 10 is a graph showing the hardness of Sample 3 and Sample 4. The vertical axis of FIG. 10 represents the hardness [Mpa] of the catalyst layer. As shown in FIG. 10, it was confirmed that the catalyst layer of Sample 3, which was formed using a catalyst ink containing carbon fiber, had a higher hardness than the catalyst layer formed using a catalyst ink that did not contain carbon fiber. This is presumably because the improved dispersibility of ionomer 4 due to the carbon fiber resulted in the formation of more crosslinks of ionomer 4 in catalyst ink 10.

[0069] [Evaluation test of the rate of crack occurrence in the catalyst layer] The effect of carbon fiber on the rate of crack occurrence in the catalyst layer was confirmed.

[0070] <Preparation of catalyst layer> The catalyst layer used in the crack occurrence rate evaluation test was prepared as follows: A catalyst ink prepared in the same manner as Sample 1 above, except for the amount of carbon fiber added, was applied to an electrolyte membrane and dried in a batch oven to prepare a catalyst layer. Three types of catalyst layers were prepared using catalyst inks in which the carbon fiber content of the carbon in the catalyst ink was 0%, 10%, and 20% by volume.

[0071] <Crack occurrence rate evaluation method> The crack occurrence rate of the catalyst layer was evaluated as follows. First, the prepared catalyst layer was placed on a light board and photographed from directly above. Next, the area of ​​the catalyst layer was cut out from the obtained image, and the image was binarized using ImageJ to calculate the area occupied by the cracks. The crack occurrence rate was determined as the ratio of the area occupied by the cracks to the area of ​​the cut-out catalyst layer.

[0072] <Evaluation results> Figure 11 is a graph showing the relationship between the proportion of carbon fiber in the carbon in the catalyst layer and the crack occurrence rate. As shown in Figure 11, the catalyst layer formed with catalyst ink containing no carbon fiber had a crack occurrence rate of 13% or more, whereas the catalyst layer formed with catalyst ink containing 10% carbon fiber was able to reduce the crack occurrence rate to approximately 2% or less. It was also confirmed that the catalyst layer formed with catalyst ink in which the proportion of carbon fiber in the carbon was increased to 20% had approximately the same crack occurrence rate as when the proportion of carbon fiber was 10%. [Explanation of symbols]

[0073] 1 fuel cell 4 Ionomer 5 Carbon 6 Solvents 10 Catalyst ink 52 Carbon Fiber

Claims

1. A catalyst ink that is applied to a surface of an electrolyte membrane of a fuel cell and dried to form a catalyst layer, The catalyst includes an ionomer, catalytic metal particles, carbon, and a solvent; The carbon includes carbon fiber, A catalyst ink in which the proportion of the carbon fiber in the carbon in the catalyst ink is 5.0% by volume or more and 20% by volume or less.

2. The catalyst ink of claim 1 , wherein the carbon fibers are vapor-grown carbon fibers.

3. 3. The catalyst ink according to claim 1, wherein the carbon fiber accounts for 5.0% by volume or more and 10% by volume or less of the carbon in the catalyst ink.

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