Method of producing catalyst ink for fuel cell

Abstract

To provide a catalyst ink suppressed in separation and relatively reduced in water ratio.SOLUTION: A method for producing a catalyst ink for fuel cells includes: a preparation step of preparing water, alcohol, a catalyst-carrying particle and an ionomer; a dispersion step of dispersing the catalyst-carrying particle and the ionomer in a solvent containing the water and the alcohol to obtain a first dispersion; and a re-addition step of adding the alcohol furthermore into the first dispersion to obtain a second dispersion. The mass percentage of the water relative to the sum of the water and the alcohol in the first dispersion is 3% to 20% higher than the mass percentage of the water relative to the sum of the water and the alcohol in the second dispersion.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to a method for making a catalyst ink for a fuel cell. [Background technology]

[0002] Various technologies have been proposed for catalyst inks used in the production of fuel cells. For example, Patent Document 1 discloses a method for increasing the coverage of ionomer catalyst by increasing the water ratio in the catalyst ink. Increasing the coverage not only improves power generation performance but also suppresses separation caused by settling of catalyst-supported particles during long-term storage of the catalyst ink. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 2017-188335 Summary of the Invention [Problem to be solved by the invention]

[0004] However, a high water content in the catalyst ink can increase the risk of air bubbles forming due to the surface tension of water when the catalyst ink is applied to a substrate. Furthermore, if the substrate is made of a water-repellent material such as a Teflon sheet, a high water content in the catalyst ink can make it difficult to apply the catalyst ink to the substrate. [Means for solving the problem]

[0005] The present disclosure can be realized in the following forms.

[0006] According to one aspect of the present disclosure, there is provided a method for producing a catalyst ink for a fuel cell, the method comprising: a preparation step of preparing water, an alcohol, catalyst-supported particles, and an ionomer; a dispersion step of dispersing the catalyst-supported particles and the ionomer in a solvent containing the water and the alcohol to obtain a first dispersion; and a re-addition step of adding the alcohol to the first dispersion to obtain a second dispersion, wherein the mass percentage of the water relative to the sum of the water and the alcohol in the first dispersion is 3% to 20% higher than the mass percentage of the water relative to the sum of the water and the alcohol in the second dispersion. According to this catalyst ink manufacturing method, the mass percentage of water relative to the sum of water and alcohol in the first dispersion is 3% to 20% higher than the mass percentage of water relative to the sum of water and alcohol in the second dispersion. This allows for a high adsorption rate of the ionomer to the catalyst-supported particles while maintaining a relatively low water ratio in the second dispersion. In other words, the high adsorption rate of the ionomer to the catalyst-supported particles reduces the tendency for the catalyst-supported particles to settle even during long-term storage of the second dispersion, thereby preventing separation of the second dispersion. Furthermore, the relatively low water ratio in the second dispersion prevents the generation of bubbles due to the surface tension of water when the second dispersion is applied to a substrate as a catalyst ink. Furthermore, the second dispersion can be applied more easily to a water-repellent substrate than when the water ratio of the second dispersion is relatively high. [Brief explanation of the drawings]

[0007] [Figure 1] 1 is a process diagram of a method for producing a catalyst ink for a fuel cell according to an embodiment of the present disclosure. [Figure 2] 1 is a graph showing the relationship between the water ratio and the separation rate in the first dispersion of an example, a comparative example, and a reference example. DETAILED DESCRIPTION OF THE INVENTION

[0008] A. Implementation: A1. Catalyst ink manufacturing method: 1 is a process diagram of a method for producing a catalyst ink for a fuel cell (hereinafter also referred to as a catalyst ink production method) according to one embodiment of the present disclosure. The process shown in FIG. 1 is carried out as one step for producing a fuel cell. The catalyst ink production method includes a preparation step P100, a dispersion step P200, and a re-addition step P300.

[0009] In the preparation step P100, water, alcohol, catalyst-supporting particles, and an ionomer are prepared.

[0010] Since the mass of water is used in calculations in the dispersion step P200 and re-addition step P300 described below, it is preferable to measure the mass of water in advance.

[0011] The alcohol may be any alcohol, such as, for example, methanol, ethanol, 1-propanol, and the like.

[0012] Since the alcohol is used in two steps, the dispersion step P200 and the re-addition step P300, which will be described later, it is preferable to prepare it separately. As with water, it is also preferable to measure the mass of the alcohol in advance.

[0013] The catalyst-supported particles are conductive particles that support a catalyst. The support of the catalyst-supported particles is, for example, a carbon material such as carbon black, carbon nanotubes, or carbon nanofibers, or a carbon compound such as silicon carbide. Examples of carbon black include acetylene black-based carbon black (e.g., Ketjen), furnace black-based carbon black (e.g., Vulcan), etc.

[0014] The catalyst supported on the conductive particles is, for example, platinum, a platinum alloy, palladium, rhodium, gold, silver, osmium, iridium, etc. The platinum alloy is, for example, an alloy of platinum with at least one of aluminum, chromium, manganese, iron, cobalt, nickel, gallium, zirconium, molybdenum, ruthenium, rhodium, palladium, vanadium, tungsten, rhenium, osmium, iridium, titanium, and lead.

[0015] Ionomers are proton conductors in fuel cells. Ionomers include, for example: Any ionomer such as Nafion®, Aquivion®, Aciplex®, Flemion®, etc.

[0016] The catalyst-supported particles and the ionomer are dispersed in a solvent containing water and alcohol to obtain a first dispersion (step P200). Specifically, the catalyst-supported particles and the ionomer are added to the solvent and mixed using a mixer, stirrer, or the like. The mass percentage of water relative to the sum of water and alcohol in the first dispersion (hereinafter also referred to as the water ratio) is adjusted to be 3% to 20% higher than the water ratio in the second dispersion in the re-addition step P300 described below.

[0017] Alcohol is further added to the first dispersion to obtain a second dispersion (step P300). The amount of alcohol added in this step is such that the water ratio in the first dispersion is 3% to 20% higher than the water ratio in the second dispersion. The alcohols used in the dispersion step P200 and the re-addition step P300 may be the same or different alcohols. After adding the alcohol, mixing is performed using a mixer, stirrer, etc., as in the dispersion step P200.

[0018] The resulting second dispersion is applied to a substrate such as a Teflon (registered trademark) sheet and dried to form a catalyst layer for a fuel cell. The catalyst layer can then be transferred to an electrolyte membrane.

[0019] A2. Comparison of Examples, Comparative Examples, and Reference Examples: Experiments were conducted by adjusting the difference between the water ratio in the first dispersion and the water ratio in the second dispersion to various values. In Examples 1 to 3, first dispersions with water ratios of 78.2%, 80.49%, and 82.92% were prepared. Alcohol was added again to these first dispersions to prepare second dispersions with a water ratio of 75%. That is, in Examples 1 to 3, the water ratio in the first dispersion was adjusted to be 3.2% to 7.92% higher than the water ratio in the second dispersion.

[0020] As a comparative example, a dispersion with a water ratio of 75% was prepared without adding alcohol again, i.e., the alcohol was added all at once without being divided.

[0021] As a reference example, a dispersion with a water ratio of 80% was prepared without adding alcohol again. That is, the dispersion of the reference example had a water ratio 5% higher than the second dispersions of Examples 1 to 3 and the dispersion of the comparative example. In the examples, comparative examples, and reference example, 1-propanol was used as the alcohol, and platinum-supported carbon was used as the catalyst-supported particles.

[0022] The separation speeds of the second dispersions of Examples 1 to 3, the dispersions of the Comparative Examples, and the dispersions of the Reference Examples were measured. The separation speeds were determined from the time change in the transmitted light intensity obtained by irradiating the dispersions with a laser while centrifuging at 2000 G using LUMiFuge (registered trademark) (LUM Corporation). The transmitted light intensity increases when catalyst-supported particles in the dispersion settle. Therefore, the greater the time change in the transmitted light intensity, the faster the separation speed when the dispersion is stored for a long period of time.

[0023] The experimental results for the Examples, Comparative Examples, and Reference Examples are shown in Table 1 below. Note that for Examples 1 to 3 in Table 1, the values ​​for the first dispersion are shown, and the numbers in parentheses indicate the difference from the water ratio of the second dispersion. FIG. 2 is a graph showing the relationship between the water ratio in the first dispersion and the separation speed for the Examples, Comparative Examples, and Reference Examples. The vertical axis of FIG. 2 represents the separation speed, and the horizontal axis represents the water ratio of the dispersion. In FIG. 2, Examples are represented by black circles, Comparative Examples are represented by black triangles, and Reference Examples are represented by open triangles.

[0024] [Table 1]

[0025] As shown in Table 1 and Figure 2, the separation rate in the Reference Example was less than 0.01 × 10 (% / h), which is thought to be due to the fact that the adsorption rate between the ionomer and the catalyst-supported particles increased as a result of increasing the water ratio.

[0026] The separation speed in Examples 1 to 3 was 0.01 × 10 -3 (% / h). In contrast, the separation rate of the comparative example was 0.05256×10 -3 (% / h). Thus, although the water ratio of the dispersions in Examples 1 to 3 and the Comparative Example was the same at 75%, a large difference in separation speed was observed. This is thought to be because, as in Examples 1 to 3, the water ratio in the first dispersion was temporarily increased to increase the adsorption rate of the ionomer to the catalyst-supported particles, and then alcohol was added again to adjust the water ratio to the desired level, thereby lowering the water ratio while maintaining the increased adsorption rate. This suggests that the ionomer and catalyst-supported particles have the property of being difficult to separate from each other once adsorbed. In contrast, in the Comparative Example, the dispersion was prepared without increasing the water ratio, and therefore the separation speed was thought to be faster than in Examples 1 to 3.

[0027] The above experiments suggest that by preparing a second dispersion via a first dispersion in which the water ratio is temporarily increased, as in Examples 1 to 3, it is possible to prepare a second dispersion in which the water ratio is lower than in the Reference Example while increasing the adsorption rate of the ionomer to the catalyst-supported particles and suppressing separation of the dispersion compared to the Comparative Example.

[0028] Furthermore, even if the water ratio of the first dispersion is increased by about 20% compared to the water ratio of the second dispersion and then alcohol is added again to obtain the second dispersion, it is believed that the second dispersion will have a relatively high adsorption rate of the ionomer to the catalyst-supported particles and a relatively low water ratio. Furthermore, it is believed that similar effects can be obtained even when an alcohol other than 1-propanol and catalyst-supported particles other than platinum-supported carbon are used.

[0029] According to the manufacturing method of the catalyst ink for fuel cells of the embodiment described above, the mass percentage of water relative to the sum of water and alcohol in the first dispersion is 3% to 20% higher than the mass percentage of water relative to the sum of water and alcohol in the second dispersion. Therefore, the adsorption rate of the ionomer to the catalyst-supported particles can be increased while maintaining a relatively low water ratio in the second dispersion. That is, the catalyst-supported particles are less likely to settle even when the second dispersion is stored for a long period of time, and separation of the second dispersion can be suppressed. Furthermore, because the water ratio in the second dispersion is relatively low, the generation of bubbles due to the surface tension of water can be suppressed when the second dispersion is applied to a substrate as a catalyst ink. Furthermore, the second dispersion can be applied more easily to a substrate that repels water than when the water ratio of the second dispersion is high.

[0030] The present disclosure is not limited to the above-described embodiments and can be realized in various configurations without departing from the spirit thereof. For example, the technical features in the embodiments corresponding to the technical features in each aspect described in the Summary of the Invention section can be appropriately replaced or combined to solve some or all of the above-described problems or achieve some or all of the above-described effects. Furthermore, if a technical feature is not described as essential in this specification, it can be appropriately deleted. [Explanation of symbols]

[0031] P100…preparation process, P200…dispersion process, P300…re-addition process

Claims

[Claim 1] A method for producing a catalyst ink for a fuel cell, comprising the steps of: a preparation step of preparing water, alcohol, catalyst-supported particles, and an ionomer; a dispersing step of dispersing the catalyst-supporting particles and the ionomer in a solvent containing the water and the alcohol to obtain a first dispersion; a re-addition step of further adding the alcohol to the first dispersion to obtain a second dispersion, the weight percentage of the water relative to the sum of the water and the alcohol in the first dispersion is 3% to 20% higher than the weight percentage of the water relative to the sum of the water and the alcohol in the second dispersion; A method for producing catalyst ink for fuel cells.

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