Method for manufacturing electrode catalysts

Abstract

To provide a method for producing an electrode catalyst and an electrode catalyst which achieve high catalytic activity.SOLUTION: A method for producing an electrode catalyst includes a nanowire synthesis step of synthesizing metal nanowires, and a supporting step of supporting the metal nanowires on a carbon support. The supporting step is performed in a manner so that, in a first suspension containing the carbon support and the metal nanowires, while the metal nanowires are cut into short metal nanowires each having a length equal to or less than the particle diameter of the carbon support, the short metal nanowires are attached to the carbon support.SELECTED DRAWING: Figure 3

Description

Technical Field

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[0001] The present invention relates to a method for manufacturing a catalyst for an electrode suitable for an electrode of an electrochemical cell. In the law It relates to.

Background Art

[0002] In electrodes of electrochemical cells such as fuel cells, water electrolyzers, and various sensors, the oxygen reduction reaction is known as a reaction with a relatively slow rate. In order to cause such an oxygen reduction reaction to proceed quickly, a catalyst in which an active metal such as a noble metal is supported on a carrier (for example, carbon) is used.

[0003] For example, Japanese Unexamined Patent Application Publication No. 2005-216661 (Patent Document 1) discloses a catalyst in which platinum is supported on a carbon carrier as a catalyst for a fuel cell. Further, Li et.al (Science, 2016, vol.354, Issue.6318, pp.1414-1419) (Non-Patent Document 1) reports a method for synthesizing platinum nanowires in which platinum is formed in a wire shape with a diameter of several nanometers.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Non-Patent Documents

[0005]

Non-Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0006] However, it was found that increasing the amount of platinum nanowires synthesized and supporting them on a carbon support to create a catalyst did not yield sufficient catalytic activity.

[0007] The present invention aims to solve the above-mentioned problems. [Means for solving the problem]

[0008] One aspect of the following disclosure is a method for producing an electrode catalyst, comprising a nanowire synthesis step of synthesizing metal nanowires and a supporting step of supporting the metal nanowires on a carbon support, wherein the supporting step is performed in a first suspension containing the carbon support and the metal nanowires, while cutting the metal nanowires into short fibers equal to or shorter than the particle size of the carbon support and attaching them to the carbon support. [Effects of the Invention]

[0010] Method for producing electrode catalysts from the above perspective The law This enables high catalytic activity. [Brief explanation of the drawing]

[0011] [Figure 1] Figure 1 is an electron microscope image of the electrode catalyst according to an embodiment (Example 1). [Figure 2] Figure 2 is an explanatory diagram of the nanowire synthesis step in the method for producing an electrode catalyst according to the embodiment. [Figure 3] Figure 3 is an explanatory diagram of the loading step in the method for manufacturing an electrode catalyst according to the embodiment. [Figure 4] Figure 4 shows the measurement results of the length distribution of the metal nanowires used as electrode catalysts in Example 1. [Figure 5] Figure 5 is a graph showing the results of evaluating the mass activity ratio of the electrode catalyst of Example 1 and the electrode catalyst of Comparative Example 1 using the RDE method. [Modes for carrying out the invention]

[0012] As shown in Figure 1, the electrode catalyst 10 of this embodiment has a structure in which short-fiber metal nanowires 14 are dispersed and supported on the surface of a carbon support 12. Such an electrode catalyst 10 is suitably used as an electrode in an electrochemical cell such as a fuel cell or a water electrolysis device.

[0013] The carbon support 12 is not particularly limited as long as it is a carbon material capable of supporting the metal nanowires 14. For example, the carbon support 12 can be made of carbon materials such as CA250 (product name: manufactured by Denki Kagaku Kogyo Co., Ltd.), OSAB (product name: manufactured by Denki Kagaku Kogyo Co., Ltd.), Vulcan (product name: manufactured by Cabot), Ketjenblack (product name: manufactured by Ketjenblack International), Norit (product name: manufactured by Norit), Black Pearl (product name: manufactured by Cabot), Acetylene Black (product name: manufactured by Chevron), VGCF (product name: manufactured by Resonac), etc. The carbon support 12 may also be composed of carbon nanotubes, carbon nanohorns, carbon nanowalls, carbon nanofibers, etc.

[0014] The carbon carrier 12 is dispersed in particulate form in the solution used in the loading process. In this specification, the particle size of the carbon carrier 12 is the particle size measured in the solution. In the solution, multiple carbon carriers 12 may form aggregates to constitute a single particle, and in this specification, the particle size of the carbon carrier 12 may include the particle size of the aggregates of the carbon carrier 12.

[0015] The metal nanowire 14 can be composed of constituent elements that can be appropriately selected from any metal element according to the purpose. The metal nanowire 14 can be composed of, for example, base metal elements such as nickel (Ni), copper (Cu), iron (Fe), cobalt (Co), tin (Sn), aluminum (Al), zinc (Zn), titanium (Ti), niobium (Nb), tungsten (W), molybdenum (Mo), chromium (Cr), or vanadium (V). Further, the metal nanowire 14 can be composed of noble metal elements such as platinum (Pt), silver (Ag), gold (Au), palladium (Pd), rhodium (Rh), iridium (Ir), ruthenium (Ru), or osmium (Os). The above elements may include a single element or a compound with other elements, or may be composed of two or more of the elements listed above contained or used in combination. Also, the metal nanowire 14 may be composed of an alloy combining some of the above elements.

[0016] Generally, the metal nanowire 14 has a diameter of about 1 nm to 10 nm and has a structure in which metal atoms are connected in a wire shape. The length of the metal nanowire 14 varies depending on the type of material or the manufacturing method, but generally, it is about several hundred nanometers to several micrometers. Since the total length of the metal nanowire 14 is longer than the diameter, it has an elongated wire shape. The metal nanowire 14 may be in a state of being shorter in length and short-fiberized than after synthesis. It is preferable that the metal nanowire 14 is short-fiberized to a length that can be supported on the surface of the carbon carrier 12. When the length of the synthesized metal nanowire 14 is short, short-fiberization is unnecessary. The average value of the length of the metal nanowire 14 in the electrode catalyst 10 is preferably equal to or less than the particle size of the carbon carrier 12 (or its aggregate).

[0017] The metal nanowire 14 may contain metal particles 14a generated during the synthesis process. The metal particles 14a are located at the ends of the metal nanowire 14. The metal particles 14a are formed by the precipitation of metal in a lump at the ends of the metal nanowire 14.

[0018] The electrode catalyst 10 of the above-described embodiment is manufactured by the following method for manufacturing an electrode catalyst.

[0019] The method for manufacturing the catalyst for an electrode has a nanowire synthesis process shown in FIG. 2 and a supporting process shown in FIG. 3. The supporting process is carried out after the nanowire synthesis process. As shown in FIG. 2, in the nanowire synthesis process (reaction process), first, a raw material input step (step S1) is performed in which a precursor of the metal nanowire 14, glucose, octadecene (solvent), and oleylamine are introduced.

[0020] As the precursor of the metal nanowire 14, a salt (metal salt) of the above metal element is used. As the precursor, a substance that is reduced to a metal in the reaction with the mixed solution in the nanowire synthesis process is used.

[0021] In the nanowire synthesis process, oleylamine functions as a reducing agent and a stabilizer. That is, oleylamine becomes an electron donor at high temperature and reduces platinum or nickel of the precursor. Further, oleylamine suppresses the growth of the generated metal particles 14a by covering the surface of the metal particles 14a and stops the particle growth at the nano size. Glucose functions as a reducing agent for the precursor. Instead of glucose, a polyol can also be used. Examples of the polyol include hexadecanediol, tetraethylene glycol, propylene glycol, trimethylene glycol, diethylene glycol, ethylene glycol, or stearyl glycol.

[0022] Next, the mixture is stirred using a stirrer or the like (Step S2). After that, the mixture is further dispersed uniformly using ultrasound (Step S3). Next, a protective agent and a catalyst are added to the mixture (Step S4). The protective agent is, for example, polyvinylpyrrolidone (PVP). Polyvinylpyrrolidone has multiple CO-bonding groups in its molecule. The CO-bonding groups of polyvinylpyrrolidone adsorb to the precipitated metal, so that the chain-like polyvinylpyrrolidone molecules surround the metal. This prevents the metal from growing in a clump and causes it to grow in a wire-like manner. The catalyst is added to promote the growth of the metal wire. The catalyst may be, for example, a tungsten (W) complex, a chromium (Cr) complex, or a molybdenum (Mo) complex.

[0023] Next, the mixture containing the protective agent and catalyst is gradually heated to a predetermined temperature (step S5). This step is carried out until the temperature of the mixture reaches the predetermined temperature (e.g., 120°C).

[0024] Subsequently, a step (step S6) is performed in which the metal nanowires 14 are grown (heated) by holding them at a predetermined temperature (e.g., 120°C) for a predetermined time. The reaction temperature can be, for example, 110°C to 140°C. When the reaction temperature is low, the reaction time is required for a longer period, and the yield tends to decrease.

[0025] The reaction time in step S6 is preferably 1 hour or more from the viewpoint of increasing the yield. The reaction time may also be, for example, 8 hours or more. The longer the reaction time, the longer the length of the synthesized metal nanowire 14.

[0026] Next, a cooling step (step S7) is performed to gradually cool the mixture. The cooling step ends when the mixture returns to room temperature. Steps S5 to S7 may be performed while stirring (fluidizing) the mixture. Moderate stirring of the mixture leads to an increase in the yield of metal nanowires 14.

[0027] Next, a washing step of the metal nanowires 14 is performed (step S8). The washing step is carried out by extracting the metal nanowires 14 by a method such as filtration, and then washing the metal nanowires 14 with a solvent containing, for example, acetone, cyclohexane, and ethanol. The washing step may be repeated multiple times.

[0028] Next, the metal nanowires 14 are recovered by centrifugation (step S9). This step further removes residues such as precursors and catalysts from the metal nanowires 14.

[0029] The metal nanowires 14 obtained in the above process can be synthesized surrounded by polyvinylpyrrolidone. The polyvinylpyrrolidone surrounding multiple metal nanowires 14 form hydrophobic bonds through the interaction of their nonpolar groups. As a result, the metal nanowires 14 are dispersed in the solvent in bundles, aggregated via the polyvinylpyrrolidone. If such metal nanowires 14 are dried, the condensation of the metal nanowires 14 progresses further, reducing their dispersibility and making it difficult to disperse them onto the carbon support 12.

[0030] Therefore, a preservation step (step S10) is performed in which a preservation solvent is added to the metal nanowire 14. This preservation step prevents the metal nanowire 14 from drying out by adding the preservation solvent. The preservation solvent is, for example, ethanol. Furthermore, ultrasound is applied to the preservation solvent and the metal nanowire 14 to uniformly disperse the metal nanowire 14 in the preservation solvent.

[0031] Subsequently, the metal nanowires 14 are dispersed in a storage solvent and used in the loading process (Figure 3).

[0032] As shown in Figure 3, the loading process first involves a suspension preparation step (step S11) in which a first suspension (metal nanowire dispersion) is prepared by mixing the carbon support 12, the dispersion solvent, and the metal nanowires 14. The metal nanowires 14 are added to the dispersion solvent along with the storage solvent. In one example, 30% by weight of the metal nanowires 14 is added. The amount of metal nanowires 14 added is not limited to this example, and 50% by weight may be added.

[0033] The dispersion solvent is, for example, a solvent containing ethanol and cyclohexane. While not particularly limited, the dispersion solvent may also contain a dispersant to improve the dispersibility of the metal nanowires 14. As the dispersant, a surfactant such as a cationic surfactant can be used. Oleylamine is preferred as a dispersant because it can efficiently disperse the metal nanowires 14, which are aggregated in bundles with polyvinylpyrrolidone.

[0034] Next, a short-fiberization process (step S12) is performed in which the first suspension is treated with an ultrasonic homogenizer. In the short-fiberization process, ultrasound is applied to the first suspension through the ultrasonic homogenizer. This separates the bundled metal nanowires 14, and further shortens the separated metal nanowires 14 into shorter fibers. The length of the metal nanowires 14 depends on the frequency and output of the ultrasonic homogenizer. The shortened metal nanowires 14 adhere to the surface of the carbon support 12 in a dispersed state, and the electrode catalyst 10 is generated.

[0035] If the treatment is not performed with an ultrasonic homogenizer, the dispersion of the bundled metal nanowires 14 does not progress, and the metal nanowires 14 cannot be supported on the carbon support 12, resulting in insufficient catalytic activity.

[0036] Next, a centrifugation step (step S13) is performed to extract the electrode catalyst 10 from the first suspension. Through centrifugation, the first suspension is separated into a precipitate layer containing the electrode catalyst 10 and a supernatant layer. By removing the supernatant layer (dispersion solvent) and recovering the precipitate layer, a concentrated suspension of the electrode catalyst 10 is obtained.

[0037] Next, a drying step (step S14) is performed in which the suspension of the concentrated electrode catalyst 10 is dried under reduced pressure. The solvent is removed from the electrode catalyst 10 by the drying step.

[0038] Subsequently, a calcination process (step S15) is performed to remove organic matter adhering to the electrode catalyst 10. The calcination process is carried out under a reducing atmosphere with an inert gas containing, for example, about 3% hydrogen gas. The calcination process includes a heat treatment at a temperature of 200°C to 500°C, more preferably 250°C or higher.

[0039] The electrode catalyst 10 of this embodiment is manufactured by the process described above.

[0040] (Example 1) In this example, metal nanowires 14 consisting solely of platinum were synthesized and supported on a carbon support 12. The composition of the mixed solution used in the nanowire synthesis process was 18.17 mmol of platinum(II) acetylacetonate (Pt(acac)2), 22.5 mmol of glucose, 0.225 mmol of polyvinylpyrrolidone (PVP), 0.15 mmol of hexacarbonyltungsten (W(CO)6), 30 ml of 1-octadecene, and 45 ml of oleylamine.

[0041] The mixture of the above preparation composition was reacted at 120°C for 3 hours while stirring (step S6 in Figure 2). This process produced the metal nanowire 14 according to Example 1. Subsequently, a loading process was performed on the metal nanowire 14 of Example 1.

[0042] The composition of the first suspension in the loading process was 9630 mg of carbon support 12 (Vulcan XC series), 107 mg of metal nanowire 14, 200 ml of ethanol, 200 ml of cyclohexane, and 20 ml of oleylamine. The first suspension was treated with an ultrasonic homogenizer for 60 minutes. Through the above treatment, the electrode catalyst 10 of this example was obtained.

[0043] The electrode catalyst 10 of this example was evaluated using a transmission electron microscope. As a result of the evaluation, the length distribution of the metal nanowires 14 was obtained as shown in Figure 4. The average length of the metal nanowires 14 in Example 1 was 29.4 nm. The average length of the metal nanowires 14 was equal to or less than the particle size (average value) of the carbon support 12.

[0044] (Comparative Example 1) Comparative Example 1 is an electrode catalyst obtained by contacting a carbon support 12 with metal nanowires 14, which were manufactured in the same manner as in Example 1, without shortening them using an ultrasonic homogenizer.

[0045] Next, the catalytic activity of the electrode catalyst 10 of Example 1 and the electrode catalyst of Comparative Example 1 was measured by the RDE method. The results are shown in Figure 5. Figure 5 shows the relative mass activity ratio when the mass activity ratio of a conventional catalyst in which platinum nanoparticles are supported on a carbon support 12 (hereinafter referred to as catalyst (Pt / C)) is set to 1. If this value is greater than 1, it indicates that the catalytic activity per unit mass of platinum is higher than that of catalyst (Pt / C).

[0046] As shown in Figure 5, in the electrode catalyst of Comparative Example 1, the metal nanowires 14 were not sufficiently dispersed, resulting in lower catalytic activity than that of the catalyst (Pt / C). In contrast, the electrode catalyst 10 of Example 1 achieved 1.7 times the catalytic activity of the catalyst (Pt / C), confirming that high catalytic activity can be obtained.

[0047] In addition to the disclosures mentioned above, the following further notes are disclosed.

[0048] (Note 1) One perspective is a method for producing an electrode catalyst, comprising a nanowire synthesis step for synthesizing metal nanowires (14) and a supporting step for supporting the metal nanowires on a carbon support (12), wherein the supporting step is performed in a first suspension containing the carbon support and the metal nanowires, while cutting the metal nanowires into short fibers equal to or shorter than the particle size of the carbon support and attaching them to the carbon support. This method for producing an electrode catalyst can effectively disperse bundled metal nanowires, and therefore can produce an electrode catalyst with high catalytic activity.

[0049] (Note 2) The above-described method for manufacturing an electrode catalyst may include, in the supporting step, a suspension preparation step of mixing the metal nanowires, the carbon support, and a solvent to prepare the first suspension, and a short-fiberization step of applying ultrasound to the first suspension through an ultrasonic homogenizer to cut the metal nanowires and attach them to the carbon support. This method for manufacturing an electrode catalyst can effectively short-fiberize metal nanowires.

[0050] (Note 3) The above-described method for producing an electrode catalyst may include a surfactant in the first suspension. This method for producing an electrode catalyst can effectively disperse bundled metal nanowires.

[0051] (Note 4) The above-described method for manufacturing an electrode catalyst comprises a nanowire synthesis step, in which a reaction step is performed to precipitate metal nanowires in which metal atoms are linked in a wire-like manner by reacting a mixture containing a precursor of the metal nanowire and a protective agent, and a storage step in which the precipitated metal nanowires are dispersed in a storage solvent without drying and stored, and the metal nanowires dispersed in the storage solvent may be used in the loading step. The above-described method for manufacturing an electrode catalyst can improve the dispersibility of the metal nanowires in the loading step by not drying the synthesized metal nanowires.

[0052] (Note 5) In the above-described method for producing electrode catalysts, the nanowire synthesis step may be carried out by reacting the mixture at a temperature of 120°C for 1 hour or more. This method for producing electrode catalysts can produce metal nanowires in high yield.

[0053] (Note 6) In the above-described method for producing electrode catalysts, the nanowire synthesis step may be carried out while the mixture is flowing. This method for producing electrode catalysts can further increase the yield of metal nanowires.

[0054] (Note 7) The above-described method for producing an electrode catalyst is also applicable, wherein in the nanowire synthesis step, the precursor may be a metal salt containing at least one element selected from nickel (Ni), copper (Cu), iron (Fe), cobalt (Co), tin (Sn), aluminum (Al), zinc (Zn), titanium (Ti), niobium (Nb), tungsten (W), molybdenum (Mo), chromium (Cr), and vanadium (V), platinum (Pt), silver (Ag), gold (Au), palladium (Pd), rhodium (Rh), iridium (Ir), ruthenium (Ru), and osmium (Os). This method for producing an electrode catalyst can produce an electrode catalyst that exhibits high catalytic activity.

[0055] (Note 8) Another perspective is an electrode catalyst comprising metal nanowires and a carbon support on which the metal nanowires are supported, wherein the average length of the metal nanowires is equal to or smaller than the average particle size of the carbon support. This electrode catalyst exhibits high catalytic activity.

[0056] (Note 9) The electrode catalyst described above may have a structure in which the carbon support to which the metal nanowires are attached is dispersed on the carbon support while the metal nanowires are shortened into short fibers by applying ultrasound to a mixture of the metal nanowires, the carbon support, and a solvent through an ultrasonic homogenizer. This electrode catalyst has excellent catalytic activity.

[0057] (Note 10) In the electrode catalyst described above, the metal nanowires may be synthesized in the liquid phase and then mixed with the carbon support and solvent without being dried. This electrode catalyst has fewer bundled metal nanowires and exhibits excellent catalytic activity.

[0058] Furthermore, the present invention is not limited to the disclosures described above, and can adopt various configurations without departing from the spirit of the invention. [Explanation of Symbols]

[0059] 10…Electrode catalyst 12…Carbon support 14…Metal nanowires 14a…Metal particles

Claims

1. The nanowire synthesis process for synthesizing metal nanowires, A storage step is performed in which, after washing the synthesized metal nanowires, a storage solvent containing ethanol is added to the metal nanowires, and ultrasound is applied to uniformly disperse the metal nanowires in the storage solvent. A method for producing an electrode catalyst, comprising: a supporting step of applying ultrasound to a first suspension obtained by mixing a carbon support with the storage solvent containing the metal nanowires, thereby cutting the metal nanowires into short fibers of 30 nm or less, while simultaneously attaching the metal nanowires to the carbon support.

2. A method for manufacturing an electrode catalyst according to claim 1, wherein the supporting step is: A suspension preparation step involves mixing the metal nanowire, the carbon support, and a solvent to prepare the first suspension. A process of shortening the metal nanowires by applying ultrasound to the first suspension through an ultrasonic homogenizer to cut them and attach them to the carbon support, A method for producing an electrode catalyst having the following characteristics.

3. A method for producing an electrode catalyst according to claim 2, wherein the first suspension contains a surfactant.

4. A method for producing an electrode catalyst according to claim 1, The aforementioned nanowire synthesis process is, The reaction step involves reacting a mixture containing a precursor of the metal nanowire and a protective agent to precipitate the metal nanowire, in which metal atoms are linked together in a wire-like structure. The aforementioned preservation step is carried out by dispersing the precipitated metal nanowires in the preservation solvent without drying them. The aforementioned loading step uses the metal nanowire dispersed in the storage solvent. A method for manufacturing electrode catalysts.

5. A method for producing an electrode catalyst according to claim 4, The nanowire synthesis step is a method for producing an electrode catalyst, in which the mixture is reacted at a temperature of 120°C for one hour or more.

6. A method for producing an electrode catalyst according to claim 5, wherein the nanowire synthesis step is performed while the mixed liquid is flowing.

7. A method for producing an electrode catalyst according to claim 5, wherein in the nanowire synthesis step, the precursor is a metal salt containing at least one element selected from nickel (Ni), copper (Cu), iron (Fe), cobalt (Co), tin (Sn), aluminum (Al), zinc (Zn), titanium (Ti), niobium (Nb), tungsten (W), molybdenum (Mo), chromium (Cr), and vanadium (V), platinum (Pt), silver (Ag), gold (Au), palladium (Pd), rhodium (Rh), iridium (Ir), ruthenium (Ru), and osmium (Os).

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