Oxygen reduction catalyst

Abstract

To realize an oxygen reduction catalyst that exhibits high catalytic activity and durability.SOLUTION: An oxygen reduction catalyst includes a substrate 110 that is an aggregate of tin oxide-based crystal particles 111, and platinum particles 120 supported on the substrate 110. The substrate 110 includes tin oxide-based crystal particles 111 that has flat crystal faces with a side length of 2 nm or more as determined from a bright-field image of a (scanning) transmission electron microscope, and whose crystallite size determined by the X-ray diffraction method is 3 nm or more and 20 nm or less. At least some of the platinum particles 120 supported on the substrate 110 form linked structures 125 in which three or more particles are linked linearly as determined from a dark-field image of the (scanning) transmission electron microscope.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to an oxygen reduction catalyst and a method for producing the same. [Background technology]

[0002] Fuel cells, which generate electricity by electrochemically reacting hydrogen and oxygen, use oxygen reduction catalysts. These catalysts consist of catalytic particles, such as platinum, supported on the surface of a conductive carbon substrate. However, repeated operation can lead to performance degradation, as the carbon substrate oxidizes and corrodes, causing the catalyst to fall off or the catalytic particles aggregate, reducing the effective catalytic area. For this reason, an excess amount of catalytic particles is supported on the substrate in anticipation of degradation.

[0003] In order to reduce the amount of expensive catalyst particles used, substrates that can replace conductive carbon have been investigated. Tin oxide has attracted attention as a substrate that can replace carbon (see, for example, Patent Document 1). Metal oxides such as tin oxide are stable even at high potentials, and are expected to improve durability. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2022-158480 Summary of the Invention [Problem to be solved by the invention]

[0005] However, conventional oxygen reduction catalysts based on tin oxide have the problem of low catalytic activity.

[0006] An object of the present disclosure is to improve the catalytic activity of tin oxide-based oxygen reduction catalysts. [Means for solving the problem]

[0007] One embodiment of the oxygen reduction catalyst of the present disclosure comprises a substrate which is an aggregate of tin oxide-based crystalline particles, and platinum particles supported on the substrate, wherein the substrate has flat crystal faces with a side length of 2 nm or more as determined from a bright-field image obtained by a (scanning) transmission electron microscope, and contains tin oxide-based crystalline particles with a crystallite size of 3 nm or more and 20 nm or less as determined by X-ray diffraction, and at least some of the platinum particles supported on the substrate form a connected structure in which three or more particles are linearly connected in a dark-field image obtained by a (scanning) transmission electron microscope.

[0008] It should be noted that the term "(scanning) transmission electron microscope" used in this disclosure refers to a "transmission electron microscope (TEM) or a scanning transmission electron microscope (STEM)."

[0009] In one embodiment of the oxygen reduction catalyst, the tin oxide based crystal particles may have a crystal particle diameter of 4 nm or more as determined from a bright field image under a (scanning) transmission electron microscope.

[0010] In one embodiment of the oxygen reduction catalyst, at least a portion of the connecting structure may form a mesh structure.

[0011] In one embodiment of the oxygen reduction catalyst, the tin oxide-based crystal particles may contain one or more impurity elements (doping elements) selected from antimony, niobium, and tantalum. The tin oxide-based crystal particles are particles that either contain no impurity elements (doping elements) or contain impurity elements. In the present disclosure, doping elements such as antimony, niobium, and tantalum that are contained in trace amounts are referred to as impurity elements.

[0012] One embodiment of the electrode of the fuel cell of the present disclosure comprises the oxygen reduction catalyst of the present disclosure.

[0013] One aspect of the method for producing an oxygen reduction catalyst of the present disclosure comprises a step of forming a substrate that is an aggregate of tin oxide-based crystal particles, and a step of supporting platinum particles on the aggregate of the substrate, wherein the step of forming the substrate includes a sub-step of oxidizing a raw material aqueous solution containing tin fluoride with ozone to obtain an aqueous solution containing tin oxide, and a sub-step of hydrothermally treating the obtained aqueous solution containing tin oxide to obtain an aqueous solution containing tin oxide-based crystal particles.

[0014] In one embodiment of the method for producing an oxygen reduction catalyst, the raw material aqueous solution may contain one or more impurity elements (doping elements) selected from antimony, niobium, and tantalum.

[0015] In one embodiment of the method for producing an oxygen reduction catalyst, the substrate contains tin oxide-based crystal particles having flat crystal faces with a side length of 2 nm or more as determined from a bright-field image of a (scanning) transmission electron microscope and a crystallite size of 3 nm or more and 20 nm or less as determined by X-ray diffraction, and at least a portion of the platinum particles can be configured to form a linked structure in which three or more particles are linked linearly on the substrate. [Effects of the Invention]

[0016] The oxygen reduction catalyst of the present disclosure can improve both durability and catalytic activity. [Brief explanation of the drawings]

[0017] [Figure 1] FIG. 1 is a schematic diagram illustrating an oxygen reduction catalyst according to an embodiment. [Figure 2] These are transmission electron microscope (TEM) bright-field images showing tin oxide crystal particles. (a) SnO2 crystal particle 1 formed by ozone-assisted hydrothermal synthesis. (b) Antimony-doped SnO2 crystal particle 1 formed by ozone-assisted hydrothermal synthesis. (c) Antimony-doped SnO2 crystal particle 2 formed by ozone-assisted hydrothermal synthesis. (d) SnO2 crystal particle 2 prepared by ozone treatment. [Figure 3]1 is a diagram showing X-ray diffraction profiles for comparison between SnO2 crystal particles 1 prepared by ozone-assisted hydrothermal synthesis and SnO2 crystal particles 2 prepared by ozone treatment. [Figure 4] 1A and 1B are scanning transmission electron microscope (STEM) images of an oxygen reduction catalyst in which platinum is supported on SnO2 crystal particles 1 formed by an ozone-assisted hydrothermal synthesis method according to Example 1, where (a) is a bright-field image (BF-STEM image) and (b) is a dark-field image (HAADF-STEM image). [Figure 5] 1A and 1B are scanning transmission electron microscope (STEM) images of an oxygen reduction catalyst in which platinum is supported on antimony-doped SnO crystal particles 1 formed by an ozone-assisted hydrothermal synthesis method according to Example 2, where (a) is a bright-field image (BF-STEM image) and (b) is a dark-field image (HAADF-STEM image). [Figure 6] 1A and 1B are scanning transmission electron microscope (STEM) images of an oxygen reduction catalyst in which platinum is supported on SnO2 crystal particles 2 prepared by ozone treatment in Comparative Example 1, where (a) is a bright-field image (BF-STEM image) and (b) is a dark-field image (HAADF-STEM image). [Figure 7] 1A and 1B are scanning transmission electron microscope (STEM) images of an oxygen reduction catalyst in which platinum is supported on antimony-doped SnO crystal particles 3 prepared by ozone treatment in Comparative Example 2, where (a) is a bright-field image (BF-STEM image) and (b) is a dark-field image (HAADF-STEM image). [Figure 8] 1A and 1B are scanning transmission electron microscope (STEM) images of an oxygen reduction catalyst in which platinum is supported on antimony-doped SnO crystal particles 4 that have not been subjected to ozone treatment or hydrothermal treatment, according to Comparative Example 3; (a) is a bright-field image (BF-STEM image) and (b) is a dark-field image (HAADF-STEM image). [Figure 9] 1 is a transmission electron microscope bright-field image of an oxygen reduction catalyst in which platinum is supported on particles based on Ketjen Black according to Comparative Example 4. [Figure 10] FIG. 1 is a graph showing the change in durability of ECSA in a repeated test up to 100,000 cycles in Examples 1 and 2 and Comparative Example 4, with one cycle being 2 seconds and one cycle being 6 seconds. DETAILED DESCRIPTION OF THE INVENTION

[0018] As shown in the schematic diagram of FIG. 1, an oxygen reduction catalyst according to one embodiment includes a substrate 110, which is an aggregate of tin oxide-based crystal particles 111, and catalyst particles 120 supported on the substrate 110. The substrate 110 includes particles having flat crystal faces with a side length of 2 nm or more, and the crystallite size is 3 to 20 nm. At least some of the catalyst particles 120 supported on the substrate 110 form a connected structure 125 in which three or more particles are linearly connected. The side length of the crystal face can be determined from a bright-field image obtained by a (scanning) transmission electron microscope, which will be described later. The crystallite size can be determined by an X-ray diffraction method, which will be described later. The number of connected catalyst particles can be determined from a dark-field image obtained by a (scanning) transmission electron microscope, which will be described later.

[0019] The tin oxide-based crystal particles are tin oxide (SnO2) crystal particles or tin oxide crystal particles doped with an impurity element. Tin oxide doped with an impurity element has higher conductivity than tin oxide not doped with an impurity element, making it preferable as a support for supporting catalyst particles. Examples of impurity elements to be doped include antimony (Sb), niobium (Nb), and tantalum (Ta). Among these, antimony-doped Sb-SnO2 is preferable because it is easy to manufacture and significantly improves conductivity. The doping amount of antimony is not particularly limited, but is preferably 3 at% or more, more preferably 5 at% or more, relative to Sn, from the viewpoint of conductivity, and is preferably 15 at% or less, more preferably 10 at% or less, from the viewpoint of maintaining the crystal structure.

[0020] The tin oxide crystal particles of this embodiment, which have flat crystal faces with sides of 2 nm or more, can be formed by ozone-assisted hydrothermal synthesis. In the ozone-assisted hydrothermal synthesis method, tin halide is reacted with ozone in water to oxidize it, followed by hydrothermal treatment. Tin oxide doped with an impurity element can be obtained by adding a halide of the impurity element when reacting tin oxide with ozone.

[0021] The tin halide can be, for example, tin fluoride (SnF2). The halide of the impurity element can be appropriately selected depending on the impurity element to be doped, but when antimony is doped, antimony chloride (SbCl3) can be used. Ozone treatment, in which the tin halide is oxidized with ozone, can be performed by bubbling ozone into an aqueous solution of tin halide such as tin fluoride. Tetramethylammonium hydroxide (TMAH) can be added before bubbling ozone to convert tin ions into hydroxides. Hydrothermal treatment can be performed, for example, under pressure at 120°C to 240°C for 6 to 36 hours. After the hydrothermal treatment, the solid matter is collected by centrifugation or the like and dried to obtain a crystalline powder of nanoparticles.

[0022] The catalyst particles can be supported on the substrate by heat-treating a mixture of tin oxide crystal particles and an ionic compound of a catalyst metal soluble in a solvent. Examples of catalyst metals include platinum (Pt), ruthenium (Ru), iridium (Ir), nickel (Ni), iron (Fe), and metal complex compounds. When platinum is used as the catalyst metal, the water-soluble catalyst metal compound can be, for example, chloroplatinic acid (HPtCl6·6H2O). Other compounds can be selected appropriately depending on the type of catalyst metal. The tin oxide crystal particles and the catalyst metal are preferably mixed at a mass ratio of approximately 2% to 50% by weight. The heat treatment is not particularly limited, but can be performed, for example, at 120°C for 1 hour. A mixed solvent of water and ethylene glycol can be used as the solvent for the heat treatment. After the catalyst metal is supported, the solid matter is recovered by centrifugation or the like and dried to obtain the oxygen reduction catalyst.

[0023] The oxygen reduction catalyst of this embodiment has very high crystallinity, and the substrate 110 contains tin oxide-based crystal particles 111 having flat crystal faces with a side length of 2 nm or more. The tin oxide-based crystal particles 111 have clear particle contours in a (scanning) transmission electron microscope bright-field image, and the particle shapes are not elliptical or rounded, with mainly curved surfaces. More than half of the particles in the bright-field image have flat crystal faces with a side length of 2 nm or more, preferably 4 nm or more, and more preferably 5 nm or more. The side length is determined by extracting 10 to 20 tin oxide-based crystal particles with a shape having flat crystal faces from the bright-field image, measuring the side size of only the flat portions of the crystal particles, excluding the curved corners, and averaging the measured side lengths. While there is no upper limit to the side length of the crystal faces from the viewpoint of the properties of the substrate, the side length of the crystal faces of the crystals obtained is typically at most about 100 nm.

[0024] Furthermore, the size of the tin oxide-based crystal particles is 3 nm to 20 nm, more preferably 4 nm to 15 nm, as calculated by the Schaller equation from the full width at half maximum of the (101) diffraction line of X-ray diffraction. Therefore, compared to a case where randomly shaped particles are aggregated, a large plane with exposed flat crystal faces is formed. As a result, catalyst particles 120 such as platinum are supported on the flat crystal faces, forming interconnected structures 125 in which adjacent catalyst particles 120 are interconnected. By forming the interconnected structures 125 with multiple catalyst particles 120, strong metal-element interactions are realized, improving the catalytic activity per catalyst particle.

[0025] The crystal particle size of tin oxide-based crystal particles can also be determined by image analysis of a (scanning) transmission electron microscope bright-field image. For image analysis, 10 to 20 crystal particles randomly selected from a (scanning) transmission electron microscope bright-field image are analyzed using standard image analysis software, and the crystal particle size is averaged. The crystal particle size determined from a (scanning) transmission electron microscope bright-field image is larger than the crystallite size determined from X-ray diffraction, and is 4 nm or more. When measurement by X-ray diffraction is unavoidably difficult, if the sample contains crystal particles with flat crystal faces with a side length of 2 nm or more, the crystallite size determined from X-ray diffraction may be determined using a value calculated using the conversion formula "crystal particle size determined from a (scanning) transmission electron microscope bright-field image × 0.8."

[0026] The connected structure 125 is made up of three or more nanoparticles of catalytic metal with a particle size of about 2 nm to 10 nm connected in a line. Furthermore, the particles may be connected to form a nanowire with a length of 100 nm or more. Furthermore, the particles may grow two-dimensionally to form a network structure. Furthermore, the particles may aggregate locally.

[0027] The interconnected state of catalytic metals such as platinum can be observed by observing the appearance of white particles in a dark-field image of a (scanning) transmission electron microscope. In a bright-field image, catalytic metals such as platinum appear as black particles, making it difficult to distinguish them from other particles. Therefore, it is preferable to use a dark-field image, which allows the interconnected state to be clearly observed.

[0028] The connected structure is a state in which the catalytic metal such as platinum is not uniformly dispersed and supported on the surface of the tin oxide-based crystal particle, but rather the particles are locally in contact with each other and connected in a nanowire-like or mesh-like manner, or are closely connected at a distance of less than half the particle diameter, or are locally closely aggregated.

[0029] Doping the tin oxide-based crystal particles 111 that make up the base material 110 with an impurity element such as antimony increases the conductivity of the base material 110, thereby further improving catalytic activity. However, the catalytic activity can also be improved by using, as the base material 110, tin oxide-based crystal particles 111 that have large crystal faces and are prepared by ozone-assisted hydrothermal synthesis, rather than being limited to particles doped with an impurity element. [Example]

[0030] The present invention will be described in more detail with reference to examples. The following examples are illustrative and are not intended to limit the present invention.

[0031] <Preparation of base particles> A reaction solution was prepared by adding 4.3 mL of 25% tetramethylammonium hydroxide (TMAH) aqueous solution to a solution of 0.784 g of tin fluoride (SnF2) dissolved in 10 mL of water. Ozone (O3) was bubbled through the reaction solution for 1 hour to perform ozone treatment. After ozone treatment, the solution was subjected to hydrothermal treatment at 240°C in an autoclave for 12 hours. After hydrothermal treatment, the solid matter was collected by centrifugation and dried at 60°C to obtain SnO2 crystal particles 1.

[0032] After the ozone treatment, the solid matter was recovered and dried without being subjected to hydrothermal treatment in an autoclave, thereby preparing SnO2 crystal particles 2.

[0033] As a reaction solution, 37.5 mL of an aqueous solution containing 0.060 g of antimony chloride (SbCl3) was further added, and the same ozone treatment and hydrothermal treatment as for SnO2 crystal particles 1 were carried out to prepare Sb-SnO2 crystal particles 1.

[0034] As a reaction solution, 37.5 mL of an aqueous solution containing 0.120 g of antimony chloride (SbCl3) was further added, and Sb-SnO2 crystal particles 2 were prepared by the same process as for Sb-SnO2 crystal particles 1.

[0035] The same reaction solution as that for Sb-SnO2 crystal particles 1 was subjected to ozone treatment, and then the solid matter was recovered and dried without being subjected to hydrothermal treatment in an autoclave, thereby preparing Sb-SnO2 crystal particles 3.

[0036] SnO2 crystal particles 3 were prepared by coprecipitation. Specifically, an aqueous solution containing tin fluoride and tetramethylammonium hydroxide was stirred at room temperature for 3 days. After stirring, the solution was transferred to an autoclave and subjected to hydrothermal treatment in an electric furnace at 240°C for 12 hours. The reaction solution was then centrifuged to wash the solid matter, and the solid matter was collected and dried to prepare SnO2 crystal particles 3.

[0037] After stirring in the same manner as for SnO2 crystal particles 3, 500 μL of hydrochloric acid was added dropwise to the solution without hydrothermal treatment, and the reaction solution was then washed by centrifugation. The solid matter was collected and dried to prepare SnO2 crystal particles 4.

[0038] The same treatment as for SnO2 crystal particles 4 was carried out using a solution containing antimony chloride, to obtain Sb-SnO2 crystal particles 4.

[0039] The Sb abundance ratio was measured using a fluorescent X-ray analyzer (MiniFlexII, manufactured by Rigaku Co., Ltd.), and it was found that the Sb abundance ratio in Sb-SnO2 crystal particles 1, 3 and 4 was 5.0 atomic %, and the Sb abundance ratio in Sb-SnO2 crystal particle 2 was 7.9 atomic %.

[0040] <Crystal size measurement> The state of the prepared SnO2-based crystal particles was observed using bright-field images from a transmission electron microscope (TEM, JEM-2100, manufactured by JEOL). As shown in Figure 2(a), SnO2 crystal particle 1 prepared by ozone-assisted hydrothermal synthesis is an aggregate of relatively uniform, flat crystals with a side length of 2 nm or more. As shown in Figures 2(b) and (c), the same is true for antimony-doped Sb-SnO2 crystal particles 1 and 2. The crystal size of Sb-SnO2 crystal particles with a higher antimony doping level is smaller.

[0041] On the other hand, SnO2 crystal particles 2 that have not been subjected to hydrothermal treatment are small in size and contain amorphous particles. Figure 2(d) shows a TEM image of SnO2 crystal particles 2 enlarged approximately twice as large as the TEM images of other crystal particles. The image shows small, rounded particles containing amorphous particles with unclear contours. In this disclosure, the term "crystal particles" refers not only to particle aggregates composed only of crystalline particles, but also to aggregates containing amorphous particles.

[0042] Figure 3 shows the results of measuring SnO2 crystal particles 1 and 2 using an X-ray diffractometer (XRD, MiniFlex II: manufactured by Rigaku). In SnO2 crystal particle 1, which was subjected to ozone oxidation and hydrothermal treatment, peaks derived from the Sn (110) plane (near 27°), (101) plane (near 34°), (200) plane (near 38°), (211) plane (near 52°), (220) plane (near 55°), (310) plane (near 62°), and (112) plane (near 68°) are clearly observed. On the other hand, in SnO2 crystal particle 2, which was not subjected to hydrothermal treatment, the peak intensity is low, indicating that it contains many amorphous particles.

[0043] SnO2 crystal particle 1, which underwent ozone oxidation and hydrothermal treatment, had a full width at half maximum of 0.017 rad and a crystallite size of 8.56 nm. SnO2 crystal particle 2, which was prepared by ozone oxidation only but not hydrothermal treatment, had a full width at half maximum of 0.100 rad and a crystallite size of 1.44 nm. SnO2 crystal particle 3, which was prepared by coprecipitation followed by hydrothermal treatment, had a full width at half maximum of 0.006 rad and a crystallite size of 22.18 nm. SnO2 crystal particle 4, which was prepared by coprecipitation but not hydrothermal treatment, had a full width at half maximum of 0.054 rad and a crystallite size of 2.66 nm. The Sb-SnO2 crystal particles 1 that had been subjected to ozone oxidation and hydrothermal treatment had a full width at half maximum of 0.025 rad and a crystallite size of 5.86 nm, and the Sb-SnO2 crystal particles 2 had a full width at half maximum of 0.034 rad and a crystallite size of 4.30 nm.

[0044] The SnO2 and Sb-SnO2 crystalline particles that underwent ozone oxidation and hydrothermal treatment had a full width at half maximum of 0.01 to 0.05 rad and a crystallite size of 3 nm to 20 nm. The crystallite size was calculated using the Scherrer equation. Table 1 summarizes the measurement results for each base particle.

[0045] [Table 1]

[0046] If the crystallite size is small, platinum particles are difficult to connect, and catalytic activity (ECSA and MA described later) may decrease, so the crystallite size is preferably 3 nm or more. Also, if the crystallite size is too large, ECSA tends to decrease, so the crystallite size is preferably 20 nm or less, more preferably 15 nm or less.

[0047] <Conductivity measurement> Thin films of SnO2 crystal particles 1, Sb-SnO2 crystal particles 1, and Sb-SnO2 crystal particles 2 were formed on glass substrates by spin coating, and the sheet resistance and conductivity were measured.

[0048] SnO2 crystal particle 1 forms a thin film with an average thickness of 939 nm, and the sheet resistance is above the measurement limit (>10 7 Ω·sq -1 ) Sb-SnO2 crystal particle 1 formed a thin film with an average thickness of 1150 nm, and the sheet resistance was 4.03 × 10 4 Ω·sq -1 , conductivity is 0.25 S·cm -1 The Sb-SnO2 crystal particles 2 formed a thin film with an average thickness of 950 nm, and the sheet resistance was 1.68 × 10 5 Ω·sq -1 , conductivity is 0.11 S cm -1 It was.

[0049] When the absorbance of the obtained thin film was measured, absorption peaks were observed in the infrared region for Sb-SnO2 crystal particle 1 and Sb-SnO2 crystal particle 2 that were not observed for SnO2 crystal particle 1, confirming that donor levels were formed in SnO2 by doping.

[0050] The sheet resistance and conductivity of the base particles are summarized in Table 2.

[0051] [Table 2]

[0052] <Preparation of oxygen reduction catalyst> A redox catalyst was prepared by loading platinum nanoparticles onto substrate particles. First, 1 g of hexachloroplatinic acid hexahydrate was dissolved in 50 mL of ethanol to prepare a platinum precursor solution. 81 mg of substrate particles were placed in a flask and mixed with 135 mL of a 2:1 ethylene glycol (EG):water mixture at room temperature. While stirring the solution with a magnetic stirrer at room temperature, 2.688 mL of the platinum precursor solution was added. After sonication, the mixture was stirred at room temperature for 12 hours. This was then transferred to an oil bath and heated at 125 °C for 2 hours with stirring to reduce the platinum. The reaction mixture was centrifuged with distilled water and dried overnight in an oven at 60 °C to obtain a powder of the oxygen reduction catalyst with Pt loaded on the substrate.

[0053] <Checking catalyst loading status> The shape and size of the tin oxide crystal particles and the distribution of the catalyst particles in the prepared oxygen reduction catalyst were observed using a scanning transmission electron microscope (STEM, Talos F200i, Thermo Fisher Scientific) at an accelerating voltage of 200 kV. The tin oxide crystal particles were observed using bright-field images (BF-STEM images), and the catalyst particles were observed using high-angle annular dark field (HAADF) dark-field images (HAADF-STEM images).

[0054] <Evaluation of catalytic activity> Cyclic voltammetry (CV) and linear sweep voltammetry (LSV) were measured using a rotating disk electrode system (HR-500, HOKUTO DENKO CORPORATION) in combination with an electrochemical device (HZ-7000, manufactured by HOKUTO DENKO CORPORATION).

[0055] The working electrode has a surface area of ​​0.196 cm 2 A glassy carbon disk electrode (GCDE, HR2-D1-GC5: manufactured by HOKUTO DENKO) was used as the reference electrode, a reversible hydrogen electrode (RHE, manufactured by Interchemi) was used as the counter electrode, a platinum wire electrode (HX-C13A: manufactured by HOKUTO DENKO) was used as the counter electrode, and a 0.1M HClO4 aqueous solution was used as the electrolyte.

[0056] A 10 μL drop of a solution (catalyst ink) containing each oxygen reduction catalyst was placed on a glassy carbon electrode and dried in a thermostatic chamber at 60°C to deposit a catalyst layer. The amount of catalyst deposited on the glassy carbon electrode was determined so that the amount of platinum on the glassy carbon electrode surface was 17.3 μg Pt / cm. 2 The amount was set to be:

[0057] The CV measurement was performed with the rotating disk electrode stationary under nitrogen saturation by sweeping the voltage range from 0.05 V to 1.2 V at a rate of 50 mV / s for 50 cycles. The hydrogen adsorption charge (Q) from 0.05 V to 0.4 V at the 50th cycle was H ) and platinum loading (L Pt The effective surface area (ECSA) per gram of platinum was calculated from the calculated value. The higher the ECSA, the larger the surface area of ​​platinum exposed to the electrolyte and functioning as a catalyst. ECSA was calculated using the following formula (1).

[0058]

number

[0059] The LSV was measured under oxygen saturation by sweeping the voltage range from 0.05 V to 1.2 V at a rate of 10 mV / s. The LSV was measured while rotating the working electrode at rotation speeds of 100 rpm, 400 rpm, 900 rpm, 1600 rpm, and 2500 rpm.

[0060] The oxygen reduction reaction activity (MA), which is the current value per gram of platinum, was calculated from the rotation speed of the working electrode and the current value at 0.9 V in the LSV measurement by correcting for mass transfer using the Koutecky-Levich equation. The larger the MA, the less platinum is needed to generate the same current value.

[0061] The area specific activity (SA) was calculated by dividing MA by ECSA, eliminating the platinum mass term. The larger the SA, the higher the catalytic activity per surface area.

[0062] Example 1 SnO2 crystal particles 1 (SnO21), prepared by ozone-assisted hydrothermal synthesis, were used as the substrate particles. Figure 4(a) shows a bright-field image (BF-STEM image), and (b) shows a dark-field image (HAADF-STEM image). The tin oxide crystal particles have clear particle contours and flat crystal faces. The length of one side of the flat crystal face determined from the bright-field image (BF-STEM image) was 6.27 nm. Furthermore, image analysis of the bright-field image (BF-STEM image) determined the crystal particle diameter to be 11.19 nm. The white platinum particles in the dark-field image (HAADF-STEM image) are clearly visible and connected.

[0063] The ECSA value is 60.3m 2 / g, MA value is 98.2A / g, SA value is 1629μA / m 2 showed high catalytic activity.

[0064] Example 2 The substrate particles used were Sb-SnO2 crystal particles 1 (Sb-SnO21) prepared by ozone-assisted hydrothermal synthesis. Figure 5(a) shows a bright-field image (BF-STEM image), and (b) a dark-field image (HAADF-STEM image). The tin oxide crystal particles have clear particle contours and flat crystal faces. The length of one side of the flat crystal face determined from the bright-field image (BF-STEM image) was 5.05 nm. Furthermore, image analysis of the bright-field image (BF-STEM image) determined the crystal particle diameter to be 6.63 nm. It can be seen that the platinum particles, which appear white in the dark-field image (HAADF-STEM image), are not uniformly dispersed but are present in locally connected, continuous regions.

[0065] The ECSA value is 61.7m 2 / g, MA value is 178.3A / g, SA value is 2890μA / m 2 showed high catalytic activity.

[0066] (Comparative Example 1) SnO2 crystal particles 2 (SnO22) prepared by ozone oxidation were used as the base particles. Figure 6(a) shows a bright-field image (BF-STEM image), and (b) shows a dark-field image (HAADF-STEM image). The tin oxide crystal particles have unclear particle contours and almost no flat crystal faces, so the length of one side of the flat crystal face could not be measured. Furthermore, image analysis of the bright-field image (BF-STEM image) revealed that the crystal particle diameter was 3.99 nm. The platinum particles, which appear white in the dark-field image (HAADF-STEM image), are independent and uniformly dispersed.

[0067] (Comparative Example 2) Sb-SnO2 crystal particles 3 (Sb-SnO23) prepared by ozone oxidation were used as the base particles. Figure 7(a) shows a bright-field image (BF-STEM image), and (b) shows a dark-field image (HAADF-STEM image). The tin oxide crystal particles have unclear particle contours and almost no flat crystal faces, so the length of one side of the flat crystal face could not be measured. Furthermore, image analysis of the bright-field image (BF-STEM image) revealed that the crystal particle diameter was 3.14 nm. The platinum particles, which appear white in the dark-field image (HAADF-STEM image), are independent and uniformly dispersed.

[0068] The ECSA value is 5.9m. 2 / g, MA value is 0.1A / g, SA value is 16.9μA / m 2 Compared with Examples 1 and 2, the catalytic activity was low.

[0069] (Comparative Example 3) Sb-SnO2 crystal particles 4 (Sb-SnO24) prepared by coprecipitation were used as the base particles. Figure 8(a) shows a bright-field image (BF-STEM image), and (b) shows a dark-field image (HAADF-STEM image). The tin oxide crystal particles have unclear particle contours and almost no flat crystal faces, so the length of one side of the flat crystal face could not be measured. Furthermore, image analysis of the bright-field image (BF-STEM image) revealed that the crystal particle diameter was 4.18 nm. The platinum particles, which appear white in the dark-field image (HAADF-STEM image), are independent and uniformly dispersed.

[0070] The ECSA value is 6.7m 2 / g, MA value is 10.9A / g, SA value is 1627μA / m 2 Compared with Examples 1 and 2, the ECSA and MA were low.

[0071] Comparative Example 4 Commercially available Ketjen Black was used as the base particles. A bright-field image taken with a transmission electron microscope is shown in Figure 9. The base particles are round, and the platinum particles, which appear black in the bright-field image, are each independent and uniformly dispersed.

[0072] The ECSA value is 82.3m 2 / g, MA value is 149.3A / g, SA value is 1814μA / m 2 It was.

[0073] <Durability evaluation> Under nitrogen saturation, a test was conducted in which the voltage was changed at a rate of 0.5 V / s within a voltage range of 1.0 V to 1.5 V, with one cycle lasting 2 seconds, for 100,000 cycles, and another test in which the voltage was changed between 0.6 V and 0.95 V every 3 seconds, with one cycle lasting 6 seconds, for 100,000 cycles, and the ECSA values ​​before and after the tests were compared. This conforms to the durability test protocol established by the Fuel Cell Council of Japan (FCCJ) as a method for evaluating fuel cell start-stop cycles.

[0074] 10 shows the change in ECSA over time relative to the ECSA value before the test, which is set to 100%. (a) shows the results of a 2-second cycle, and (b) shows the results of a 6-second cycle. In the case of the oxidation-reduction catalyst of Example 1, the ECSA after the test was 92% of the value before the test in a 2-second cycle, and 82% of the value before the test in a 6-second cycle.

[0075] In the case of the oxidation-reduction catalyst of Example 2, the ECSA after the test in which one cycle was 2 seconds was 80% of that before the test, and in the test in which one cycle was 6 seconds, the ECSA after the test was 73% of that before the test.

[0076] In the case of the oxidation-reduction catalyst of Comparative Example 4, the ECSA after the test in which one cycle was 2 seconds was 48% of the value before the test, and the ECSA after the test in which one cycle was 6 seconds was 65% of the value before the test. Compared with Examples 1 and 2, Comparative Example 4 showed a large decrease in ECSA and was found to have low durability.

[0077] The evaluation results of catalytic activity and durability are summarized in Table 3. By using tin oxide crystal particles with large crystal faces prepared by ozone-assisted hydrothermal synthesis as the base material, an oxygen reduction catalyst with high catalytic activity and durability can be realized.

[0078] [Table 3] [Industrial Applicability]

[0079] The oxygen reduction catalyst of the present disclosure exhibits high catalytic activity and durability, and is useful in various fields including fuel cells and the like. [Explanation of symbols]

[0080] 110 Base material 111 Tin oxide crystal particles 120 catalytic particles 125 Connected structure

Claims

1. a substrate that is an aggregate of tin oxide-based crystal particles; platinum particles supported on the substrate; the substrate contains tin oxide-based crystal particles having a flat crystal face with a side length of 2 nm or more as determined from a bright-field image of a (scanning) transmission electron microscope and a crystallite size of 3 nm or more and 20 nm or less as determined by X-ray diffraction; An oxygen reduction catalyst, wherein at least a portion of the platinum particles supported on the substrate form a connected structure in which three or more particles are connected linearly in a dark-field image obtained by a (scanning) transmission electron microscope.

2. 2. The oxygen reduction catalyst according to claim 1, wherein the tin oxide based crystal particles have a crystal particle diameter of 4 nm or more as determined from a bright field image under a (scanning) transmission electron microscope.

3. 2. The oxygen reduction catalyst according to claim 1, wherein at least a portion of the connecting structure forms a two-dimensional network structure.

4. 2. The oxygen reduction catalyst according to claim 1, wherein the tin oxide-based crystal particles contain one or more impurity elements (doping elements) selected from antimony, niobium, and tantalum.

5. An electrode for a fuel cell, comprising the oxygen reduction catalyst according to any one of claims 1 to 4.

6. forming a substrate that is an aggregate of tin oxide-based crystal particles; and supporting platinum particles on the aggregate of the substrates, The step of forming the substrate includes: A method for producing an oxygen reduction catalyst, comprising: a sub-step of oxidizing a raw material aqueous solution containing tin fluoride with ozone to obtain an aqueous solution containing tin oxide; and a sub-step of hydrothermally treating the obtained aqueous solution containing tin oxide to obtain an aqueous solution containing tin oxide-based crystal particles.

7. 7. The method for producing an oxygen reduction catalyst according to claim 6, wherein the raw material aqueous solution contains one or more impurity elements (doping elements) selected from antimony, niobium, and tantalum.

8. the substrate contains tin oxide-based crystal particles having a flat crystal face with a side length of 2 nm or more as determined from a bright-field image of a (scanning) transmission electron microscope and a crystallite size of 3 nm or more and 20 nm or less as determined by X-ray diffraction; 8. The method for producing an oxygen reduction catalyst according to claim 6, wherein at least a portion of the platinum particles form a connected structure in which three or more platinum particles are connected linearly on the substrate in a dark-field image obtained by a (scanning) transmission electron microscope.

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