Electrode catalyst and anion-exchange membrane type electrochemical cell

Abstract

The present invention provides an electrode catalyst which has high catalytic activity while maintaining good durability, substance conductivity, and electrical conductivity, and which can be manufactured at a low cost.　The present invention provides an electrode catalyst which is composed of a void-containing body having voids, wherein: the void-containing body is provided with a core part and a skin layer that covers the core part; the core part is composed of a metal; the skin layer is composed of Ni and an oxide that contains a second metal; the second metal is a metal that is different from Ni; in addition, the skin layer is loaded with transition metal nanoparticles that are dispersed on the surface of the skin layer; and the particle diameter of the transition metal nanoparticles is smaller than the particle diameter of the metal and the particle diameter of the oxide.

Description

Electrode catalyst, anion exchange membrane type electrochemical cell

[0001] The present invention relates to an electrode catalyst and an anion exchange membrane type electrochemical cell. The electrode catalyst of the present invention can be used, for example, as a catalyst for an electrochemical reaction in an anion exchange membrane type electrochemical cell. Examples of the anion exchange membrane type electrochemical cell include an anion exchange membrane type water electrolysis cell and an anion exchange membrane type fuel cell.

[0002] In the production of hydrogen by water electrolysis using an anion exchange membrane, a hydrogen reaction (H ２ O + e － →1 / 2H ２ + OH － ) occurs at the cathode, and an oxygen reduction reaction (OH － →1 / 4O ２ + 1 / 2H ２ O + e － ) occurs at the anode at the opposite position. Currently, a noble metal oxide (IrO ｘ ) is used as the electrode catalyst. However, since the amount of Ir buried is small and the cost is high, the development of an alternative non-noble metal-based catalyst is required. In Non-Patent Document 1, an IrOx oxide with a high specific surface area is used to reduce the amount of Ir used, but there are problems in gas transport. In addition, IrOx nanoparticles (Non-Patent Document 2), a catalyst in which non-noble metal Ni(OH) ２ nanoparticles are supported on conductive carbon (Patent Document 1), etc. have been proposed, but there are problems of carbon deterioration and oxidation of the carrier at the operating potential (1.7 V - 1.8 V). In addition, although Non-Patent Document 3 describes an improvement in catalytic activity when using a Ni alloy, no improvement has been observed regarding the performance as a catalyst layer (gas transport and electron conductivity). Regarding the above problems, Patent Document 2 discloses a non-noble metal-based NiFeO-based catalyst that is excellent in durability, mass conductivity, conductivity, etc., and is inexpensive.

[0003] Special Table 2017-527693 WO2021 / 251341

[0004] ACS Appl. Mat. Interfaces (2017) 44567ACS Catal. 10 (2020) 4608ACS Nano Energy 47 (2018) 172

[0005] However, there is a need to develop electrode catalysts that possess higher catalytic activity while maintaining good durability, material conductivity, and electrical conductivity.

[0006] This invention has been made in view of these circumstances, and provides an electrode catalyst that has high catalytic activity while ensuring durability, material conductivity, and electrical conductivity, and that can be manufactured at low cost.

[0007] The present invention provides the following: [1] An electrode catalyst comprising a porous body having voids, wherein the porous body comprises a core portion and a skin layer covering the core portion, the core portion being made of a metal, the skin layer being made of Ni and an oxide containing a second metal, the second metal being a metal different from Ni, and the skin layer further having transition metal nanoparticles dispersed and supported on its surface, wherein the particle size of the transition metal nanoparticles is smaller than the particle size of the metal and the particle size of the oxide. [2] An electrode catalyst according to [1], wherein the second metal has an atomic number smaller than Ni. [3] An electrode catalyst according to [1] or [2], wherein, in a spectrum measured by X-ray photoelectron spectroscopy (XPS), when I1 is the intensity at the point where the peak intensity of the oxide is maximum and I2 is the intensity at the point where the peak intensity of the transition metal nanoparticles is maximum, the peak intensity ratio I2 / I1 is 0.2 or more and 5 or less. [4] An electrode catalyst according to any one of [1] to [3], wherein the particle size of the transition metal nanoparticles, calculated from an SEM image of the electrode catalyst, is 0.1 nm to 10 nm. [5] An electrode catalyst according to any one of [1] to [4], wherein the second metal is Fe or Co. [6] An electrode catalyst according to any one of [1] to [5], wherein the transition metal nanoparticles include Ni or Cu. [7] An electrode catalyst according to any one of [1] to [6], wherein the void-containing body is composed of fine particles that act as catalysts, and the fine particles comprise the core portion and the skin layer. [8] An electrode catalyst according to any one of [1] to [7], wherein the fine particles are composed of a plurality of primary particles fused together in a chain. An electrode catalyst according to any one of [9] [1] to [8], wherein the oxide contains a transition metal with an atomic number greater than Ni. An electrode catalyst according to any one of

[10] [1] to [9], wherein the void-containing body has a porosity of 20% or more. An electrode catalyst according to any one of

[11] to

[10] , wherein the electrode catalyst is an electrode catalyst for an anion exchange membrane type electrochemical cell.

[12] An electrochemical catalyst according to

[11] , wherein the anion exchange membrane type electrochemical cell is a water electrolysis cell or a fuel cell.

[13] An electrochemical cell comprising a cathode, an anode, and an anion exchange membrane disposed between them, wherein at least one of the cathode and the anode comprises a catalyst layer composed of the electrochemical catalyst according to any one of [1] to

[12] .

[14] An electrochemical cell according to

[13] , wherein the anion exchange membrane type electrochemical cell is a water electrolysis cell or a fuel cell.

[0008] The electrode catalyst of the present invention does not require carbon addition and is composed of a metal core and an oxide skin layer, thus exhibiting excellent durability. Furthermore, because this electrode catalyst is composed of a porous material, it has excellent material conductivity. Moreover, since this electrode catalyst has a core and a skin layer, and the core is made of metal, it has excellent conductivity. In addition, because this electrode catalyst does not require precious metals as essential components, it can be manufactured at low cost. Furthermore, because the surface of the skin layer is coated with transition metal nanoparticles, (1) catalytic activity is improved by increasing the number of active sites, and (2) catalytic activity is improved by the diffusion of hydroxide ions from the skin layer to the transition metal nanoparticles (reverse spillover).

[0009] This is a diagram showing the state in which the electrochemical cell 10 is performing water electrolysis. This is a diagram showing the state in which the electrochemical cell 10 is performing power generation. This is a cross-sectional view of the porous body 60 that constitutes the electrode catalyst 50. This is a cross-sectional view of the powder 70 that constitutes the electrode catalyst 50. Figure 5A is a perspective view of fine particles 80 having a beaded structure. Figure 5B is an enlarged view of region A in Figure 5A. This is a cross-sectional view of fine particles 80 having a beaded structure. This is a cross-sectional view of the manufacturing apparatus 1 for producing powder, passing through the center of the burner 2. This is an enlarged view of region X in Figure 7. This is a cross-sectional view of A-A in Figure 7. This is an enlarged view of region Y in Figure 9. Figure 11A is an example of a TEM image of oxide fine particles 80, and Figure 11B is a diagram with arrows indicating the primary particle diameter of the primary particles in the TEM image of Figure 11A superimposed. This is an SEM image of the electrode catalyst 50. This shows the configuration of the electrochemical measuring device 15 used for OER activity measurement. This is a graph showing the potential and mass activity in the OER measurement of the electrode catalysts 50 of Examples 1 to 4 and Comparative Example 1. This graph shows the relationship between the specific surface area and mass activity of the electrode catalysts 50 of Examples 1-4 and Comparative Example 1. This graph shows the XPS measurement results of the electrode catalysts 50 of Example 1 and Comparative Example 1. This figure shows the calculation method for peak intensities I1 and I2 in the XPS spectrum. This graph shows the relationship between the current density and cell voltage of a water electrolysis cell where the anode catalyst is the electrode catalyst 50 of Example 1, Comparative Example 1, and Comparative Example 2.

[0010] Embodiments of the present invention will be described below with reference to the drawings. The various features shown in the embodiments below can be combined with each other. Furthermore, each feature can constitute an invention independently. In addition, any element not specified in the claims in the embodiments below is an optional element and can be omitted. Any number of zeros (for example, one or two) may be added to the end of the numerical values ​​disclosed in the following description. For example, one or two zeros may be added after "1.4" to make it "1.40" or "1.400".

[0011] 1. Electrochemical Cell 10 Figure 1 shows the configuration of an anion exchange membrane type electrochemical cell 10 according to one embodiment of the present invention. The electrochemical cell 10 comprises a cathode 20, an anode 30, and an anion exchange membrane 40 disposed between them.

[0012] As shown in Fig. 1, when a voltage is applied between the cathode 20 and the anode 30 and water is supplied to the cathode 20, the following cathode reaction and anode reaction occur, generating hydrogen from the cathode 20 and water and oxygen from the anode. Electrons move from the anode 30 to the cathode 20 through the wiring, and OH － moves from the cathode 20 to the anode 30 through the anion exchange membrane 40. In this case, the electrochemical cell 10 becomes an anion exchange membrane type water electrolysis cell and operates in water electrolysis. Cathode reaction: H ２ O + e － → 1 / 2H ２ + OH － Anode reaction: OH － → 1 / 2H ２ O + 1 / 4O ２ + e －

[0013] Also, as shown in Fig. 2, when a load R is connected between the cathode 20 and the anode 30, water and oxygen are supplied to the cathode 20, and hydrogen is supplied to the anode 30, an electromotive force is generated by the following cathode reaction and anode reaction, and water is produced. Due to the generated electromotive force, electrons move from the anode 30 to the cathode 20 through the load R, and OH － moves from the cathode 20 to the anode 30 through the anion exchange membrane 40. In this case, the electrochemical cell 10 becomes an anion exchange membrane type fuel cell and operates in power generation. Cathode reaction: 1 / 2H ２ O + 1 / 4O ２ + e － → OH － Anode reaction: 1 / 2H ２ + OH － → H ２ O + e －

[0014] The cathode reaction in the power generation operation is the reverse reaction of the anode reaction in the water electrolysis operation, and the anode reaction in the power generation operation is the reverse reaction of the cathode reaction in the water electrolysis operation.

[0015] Thus, the electrochemical cell 10 can operate as a water electrolysis cell or a fuel cell. Therefore, for example, surplus power generated by solar power generation or the like is used to operate the electrochemical cell 10 as a water electrolysis cell to generate and store hydrogen and oxygen, and when power is required, the stored hydrogen and oxygen are used to operate the electrochemical cell 10 as a fuel cell to generate an electromotive force, thereby enabling the efficient operation of the electrochemical cell 10.

[0016] The cathode 20 preferably includes a diffusion layer 21, a microporous layer 22, and a catalyst layer 23. The anode 30 preferably includes a diffusion layer 31, a microporous layer 32, and a catalyst layer 33. The diffusion layers 21 and 31 are made of a porous body and have a function of diffusing a fluid (liquid or gas) supplied to the catalyst layers 23 and 33. The microporous layers 22 and 32 have a function of further diffusing the fluid supplied to the catalyst layers 23 and 33 or efficiently removing the liquid generated in the catalyst layers 23 and 33. The catalyst layers 23 and 33 have a function of promoting an electrochemical reaction (cathode reaction or anode reaction) by a catalyst.

[0017] One or both of the catalyst layers 23 and 33 are constituted by the electrode catalyst 50 of the present invention described later. When one of the catalyst layers 23 and 33 is constituted by the electrode catalyst 50, the other of the catalyst layers 23 and 33 can be constituted by any catalyst capable of promoting a desired electrochemical reaction. For example, a catalyst (e.g., IrOx) as described in the prior art can be used. When both of the catalyst layers 23 and 33 are constituted by the electrode catalyst 50, the composition and structure of the electrode catalyst 50 may be the same or different from each other.

[0018] 2. Electrode catalyst 50 The electrode catalyst 50 is constituted by a void-containing body having voids. Examples of the void-containing body include a porous body 60 as shown in FIG. 3 and a powder 70 as shown in FIG. 4. In the present invention, since the void-containing body is formed by the catalyst itself, it is not necessary to support the catalyst on a carrier as in the prior art.

[0019] The porous body 60 is a substrate 61 with numerous pores 62 formed therein, where the pores 62 form voids. The pores 62 may be regularly or irregularly arranged. The pores 62 may have a regular shape (e.g., a linear shape) or an irregular shape. The pores 62 may or may not penetrate the substrate 61. The powder 70 is an aggregate of fine particles 80, where the gaps 81 between the fine particles 80 and the gaps within the fine particles form voids. The fine particles 80 may be spherical or have other shapes. When the void-containing body is powder 70, the void-containing body is composed of the fine particles 80 themselves that act as catalysts.

[0020] The porosity of the porous material is preferably 20% or more, and preferably 50% or more. This porosity is, for example, 20 to 90%, specifically, for example, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, and 90%, and may be within the range of any two of the values ​​exemplified here. The porosity can be calculated by bulk density / true density. The porosity of the powder can be measured in a state molded using a uniaxial pressure molding machine (molded body size: 5 mm × 5 mm × 30 mm, molding pressure 2 MPa or less).

[0021] The void-containing material comprises a core portion 90 and a skin layer 91 covering the core portion 90. When the void-containing material is a porous body 60, the porous body 60 comprises a core portion 90 and a skin layer 91. When the void-containing material is a powder 70, each fine particle 80 comprises a core portion 90 and a skin layer 91.

[0022] The core portion 90 is made of metal, and the skin layer 91 is composed of an oxide containing Ni and a second metal, which is a metal different from Ni. Since the skin layer 91 contains NiO bonds, it generates NiOOH (active sites) in an alkaline aqueous solution and promotes electrochemical reactions. On the other hand, since the core portion 90 is made of metal, it has high electrical conductivity. The metal of the core portion 90 may or may not contain Ni, but if the metal of the core portion 90 contains Ni, the skin layer 91 composed of an oxide containing Ni can be formed by reducing the entire void body and then oxidizing only its surface, making manufacturing easy. The skin layer 91 may also be formed by coating the core portion 90 with an oxide containing Ni. In this case, the core portion 90 does not have to contain Ni.

[0023] The thickness of the skin layer 91 is, for example, 0.1 to 50 nm, and preferably 1 to 10 nm. Specifically, this thickness may be, for example, 0.1, 0.5, 1, 5, 10, 15, 20, 30, 40, or 50 nm, and may be within the range of any two of the values ​​exemplified here.

[0024] The metal other than Ni in the core portion 90 and the second metal in the skin layer 91 preferably include a metal with a smaller atomic number than Ni, and this metal is preferably a transition metal. Examples of such transition metals include Co, Fe, Mn, Cr, V, Ti, Sc, etc., with Co or Fe being preferred. Including such a transition metal lowers the Fermi level and promotes the electrochemical reaction. The ratio of the transition metal to the total of Ni and the transition metal is preferably 5 to 95 atomic percent, more preferably 10 to 80 atomic percent, and specifically, for example, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95 atomic percent, and may be within the range of any two of the values ​​exemplified here.

[0025] The metals other than Ni in the core portion 90 and the second metal in the skin layer 91 may include a metal with a larger atomic number than Ni, and this metal is preferably a transition metal. Mo is an example of such a transition metal. Including such a transition metal has the effect of lowering the onset potential of the catalytic reaction (particularly the oxygen evolution reaction).

[0026] As shown in Figure 5B, the skin layer 91 also has transition metal nanoparticles 92 dispersed and supported on its surface. This causes the diffusion of hydroxide ions from the skin layer 91 to the transition metal nanoparticles 92 (reverse spillover), which leads to the generation of oxygen and water at a lower potential, thus improving catalytic activity.

[0027] The transition metal nanoparticles 92 contain one or more zero-valent transition metals. In this case, a reverse spillover phenomenon occurs, improving catalytic activity. Ni or Cu are preferred as the zero-valent transition metals. When the transition metal nanoparticles 92 contain zero-valent Ni or Cu, the effect of improving catalytic activity is greater. Furthermore, the metals constituting the transition metal nanoparticles 92 may consist only of zero-valent transition metals, or they may also contain a second element with a valence of 1 or higher.

[0028] The particle size of the transition metal nanoparticles 92 is preferably smaller than the particle size of the metal constituting the core or the particle size of the oxide constituting the skin layer. The particle size of the transition metal nanoparticles 92 can be calculated as the average value of the equivalent circle diameter for any number of particles using images of the cross-section or surface of the particles observed in electron microscopy such as SEM. The particle size of the transition metal nanoparticles 92 is preferably 0.1 nm to 10 nm, more preferably 1 nm to 5 nm, specifically, for example, 0.1, 0.5, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 nm, and may be within the range of any two of the values ​​exemplified here.

[0029] The shape of the transition metal nanoparticles 92 is not particularly limited and may be spherical, plate-shaped, rod-shaped, cubic, or other shapes. Preferably, the transition metal nanoparticles 92 include a cluster structure. The thickness of one cluster is about one atomic layer to several atoms, and the number of transition metal atoms forming one cluster is about 1000. The type of cluster structure is not particularly limited and can take the form of hexagonal crystal structures, cubic crystal structures, honeycomb structures, etc., and two or more of these structures may be present.

[0030] The particle size and shape of the transition metal nanoparticles 92, the cluster structure, etc., can be adjusted by selecting and concentrating the metal source, heating temperature, heating conditions (stirring speed, etc.), reaction time, etc., in the transition metal nanoparticle loading process described later.

[0031] The presence of the transition metal nanoparticles 92 can be confirmed, for example, by X-ray photoelectron spectroscopy (XPS). XPS is a measurement technique that obtains information about elements present at a depth of a few nanometers on the surface of a sample by irradiating the sample with X-rays and measuring the kinetic energy distribution of the emitted photoelectrons. As mentioned above, the particle size of the transition metal nanoparticles 92 present on the surface of the skin layer 91 is a few nanometers, so the presence of the transition metal nanoparticles 92 can be confirmed by XPS measurement. When the transition metal nanoparticles 92 are supported on the surface of the electrode catalyst 50, a peak of the zero-valent transition metal (hereinafter also referred to as the "transition metal nanoparticle 92 peak") is observed in the XPS measurement.

[0032] XPS can use a known X-ray photoelectron spectroscopy analyzer. The X-ray photoelectron spectroscopy analyzer may be a commercially available product (for example, an X-ray photoelectron analyzer (product name "KRATOS ULTRA2", Shimadzu Corporation), a photoelectron spectrometer (model number "JPS-9030", JEOL Ltd.), a scanning X-ray photoelectron spectroscopy analyzer (model number "PHI Quantera II", ULVAC-PHI, Inc.), etc.). As the X-ray source, AlKα rays, MgKα rays, AgLα rays, etc. can be used.

[0033] In the spectrum measured by X-ray photoelectron spectroscopy (XPS), the electrode catalyst 50 preferably has a peak intensity ratio I2 / I1 of 0.5 to 2.0, and more preferably 0.8 to 1.5, where I1 is the intensity at which the peak intensity of the oxide constituting the skin layer 91 is maximum, and I2 is the intensity at which the peak intensity of the transition metal nanoparticles 92 is maximum. The peaks of the oxide constituting the skin layer 91 are usually observed on the higher energy side than the peaks of the transition metal nanoparticles 92. Specifically, I2 / I1 can be, for example, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2.0, 3.0, 4.0, 5.0, and may also be within the range of any two of the values ​​exemplified here.

[0034] Figure 17 shows the calculation methods for I1 and I2. In an XPS spectrum obtained with the photoelectron binding energy (eV) to the atomic nucleus on the horizontal axis and the photoelectron intensity on the vertical axis, a linear function equation is calculated connecting the start and end points of the peak. By substituting the x-axis value of the peak related to I1 into the linear function equation, the baseline of the peak intensity (y-axis value) is obtained. I1 is obtained by subtracting the baseline of the intensity from the absolute value of the peak intensity. I2 can be calculated in the same way. For example, if there are local minima before and after the peak, the x-axis value of the local minima can be used as the start or end point.

[0035] The specific surface area of ​​the electrode catalyst 50 is 5 to 100 m². ２ / g is preferred, and 10 to 50 m ２ A value of / g is even more preferable. Specifically, this specific surface area is, for example, 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 100 m². ２ It is / g, and may be within the range of any two of the numerical values ​​exemplified here.

[0036] The electrical conductivity of the electrode catalyst 50 is preferably 0.001 S / cm, more preferably 0.01 S / cm or higher, and even more preferably 0.1 S / cm or higher. This electrical conductivity is, for example, 0.001 to 1 S / cm, specifically, for example, 0.001, 0.002, 0.005, 0.01, 0.02, 0.05, 0.1, 0.2, 0.5, and 1 S / cm, and may be within the range of any two of the values ​​exemplified here. The electrical conductivity can be determined, for example, by measuring the resistance value when the electrode catalyst 50 is pressurized using the DC two-terminal method.

[0037] The fine particles 80 preferably have a structure having a chain-like portion 83 formed by the fusion bonding of multiple primary particles 82 in a chain-like manner, as shown in Figure 5A (hereinafter referred to as a "beaded structure"). In this case, the region surrounded by the chain-like portion 83 becomes a void 84. Furthermore, the beaded structure preferably has a branched structure in which the chain-like portion 83 is branched at a branching point 85. In this case, voids 84 are more easily formed. Also, since the beaded structure is formed by the catalyst itself, it is not necessary to support the catalyst on a carrier as in the conventional method.

[0038] As shown in Figure 6, it is preferable that each of the multiple primary particles 82 constituting the chain portion 83 has a core portion 90 and a skin layer 91, and that the core portions 90 of adjacent primary particles 82 are connected to each other. In this case, the conductivity is particularly good.

[0039] The average size of the primary particles 82 is preferably 1 to 100 nm, more preferably 5 to 40 nm, and even more preferably 10 to 20 nm. Specifically, this average size is, for example, 1, 5, 10, 15, 20, 25, 30, 35, 40, 50, and 100 nm, and may be within the range of any two of the values ​​exemplified here. The average size of the primary particles 82 can be determined by the arithmetic mean of the primary particle diameters of 10 or more primary particles. The primary particle diameter can be measured using a TEM image as shown in Figure 11. In the TEM image shown in Figure 11A, the darker areas indicate the overlap of multiple primary particles. When measuring the primary particle diameter, focus on particles with relatively light color and visible outer circumferences, and define the value at which the distance between two points on the outer circumference of that particle is maximized (the length of the arrow in Figure 11B) as the primary particle diameter.

[0040] The average particle size of the fine particles 80 is 0.1 μm to 4 μm, and preferably 0.5 μm to 2 μm. The average particle size of the fine particles 80 can be measured by a laser diffraction / scattering particle size distribution analyzer.

[0041] The specific surface area of ​​powder 70 is 5 m². ２ A value of 1 / g or more is preferable. This specific surface area is, for example, 5 to 100 m². ２ This is per gram, specifically for 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85, 90, 100 m ２ It is / g, and may be within the range of any two of the numerical values ​​exemplified here.

[0042] The powder 70 preferably has an angle of repose of 50 degrees or less, and more preferably 45 degrees or less. In this case, the powder has a fluidity similar to that of wheat flour and is easy to handle. This angle of repose is, for example, 20 to 50 degrees, specifically, for example, 20, 25, 30, 35, 40, 45, and 50 degrees, and may be within the range of any two of the values ​​exemplified here. The angle of repose can be determined by the drop volume method.

[0043] 3. Method for Manufacturing the Electrode Catalyst 50 When the electrode catalyst 50 is composed of a powder 70 which is an aggregate of fine particles 80 having a beaded structure, the electrode catalyst 50 can be manufactured by a method comprising a powder formation step, a reduction and surface oxidation step, and a transition metal nanoparticle support step. Each step will be described in detail below.

[0044] 3-1. Powder Formation Process First, a manufacturing apparatus 1 that can be used for powder production will be described using Figures 7 to 10. The manufacturing apparatus 1 comprises a burner 2, a raw material supply unit 3, a reaction cylinder 4, a recovery unit 5, and a gas storage unit 6. The raw material supply unit 3 comprises an outer cylinder 13 and a raw material flow cylinder 14.

[0045] The burner 2 is cylindrical, and the raw material supply unit 3 is located inside the burner 2. Burner gas 2a flows between the burner 2 and the outer cylinder 13. The burner gas 2a is used to form a flame 7 at the tip of the burner 2 by ignition. The flame 7 forms a high-temperature region of 1000°C or higher. The burner gas 2a preferably contains a flammable gas such as propane, methane, acetylene, hydrogen, or nitrous oxide. In one example, a mixed gas of oxygen and propane can be used as the burner gas 2a. The temperature of the high-temperature region is, for example, 1000 to 2000°C, specifically, for example, 1000, 1100, 1200, 1300, 1400, 1500, 1600, 1700, 1800, 1900, and 2000°C, and may be within the range of any two of the values ​​exemplified here.

[0046] A raw material solution 14a for generating powder is circulated through the raw material distribution tube 14. The raw material solution 14a used contains a Ni compound and a compound of a second metal (e.g., Co, Fe). Examples of compounds include fatty acid metal salts (e.g., fatty acid Ni, fatty acid Co, fatty acid Fe). The number of carbon atoms in the fatty acid is, for example, 2 to 20, preferably 4 to 15, and more preferably 6 to 12. Octylic acid is preferred as the fatty acid.

[0047] In the raw material solution 14a, it is preferable that the fatty acid metal salt is dissolved or dispersed in a non-aqueous solvent. Examples of non-aqueous solvents include organic solvents such as turpentine. If water is present in the raw material solution 14a, the fatty acid metal salt may be hydrolyzed and deteriorate.

[0048] A misting gas 13a, used to atomize the raw material solution 14a, flows between the outer cylinder 13 and the raw material flow cylinder 14. When the misting gas 13a and the raw material solution 14a are ejected together from the tip of the raw material supply unit 3, the raw material solution 14a is atomized. The mist 14b of the raw material solution 14a is sprayed into the flame 7, and the fatty acid metal salt in the raw material solution 14a undergoes a thermal decomposition reaction in the flame 7, generating oxide powder, which is an aggregate of oxide fine particles 9 having chain-like portions formed by the fusion bonding of oxide crystallites 8 in a chain-like manner. These crystallites 8, oxide fine particles 9, and oxide powder undergo reduction and surface oxidation processes to become primary particles 82, fine particles 80, and powder 70, respectively. In one example, the misting gas 13a is oxygen.

[0049] The reaction cylinder 4 is located between the recovery unit 5 and the gas storage unit 6. A flame 7 is formed inside the reaction cylinder 4. The recovery unit 5 is equipped with a filter 5a and a gas discharge unit 5b. Negative pressure is applied to the gas discharge unit 5b. As a result, an airflow directed towards the gas discharge unit 5b is generated inside the recovery unit 5 and the reaction cylinder 4.

[0050] The gas storage section 6 is cylindrical and includes a cooling gas introduction section 6a and a slit 6b. Cooling gas 6g is introduced into the gas storage section 6 from the cooling gas introduction section 6a. Since the cooling gas introduction section 6a is oriented in a direction along the tangent to the inner circumferential wall 6c of the gas storage section 6, the cooling gas 6g introduced into the gas storage section 6 through the cooling gas introduction section 6a swirls along the inner circumferential wall 6c. A burner insertion hole 6d is provided in the center of the gas storage section 6. A burner 2 is inserted through the burner insertion hole 6d. The slit 6b is provided adjacent to the burner insertion hole 6d and surrounds the burner insertion hole 6d. Therefore, when the burner 2 is inserted through the burner insertion hole 6d, the slit 6b surrounds the burner 2. The cooling gas 6g in the gas storage section 6 is driven by the negative pressure applied to the gas discharge section 5b and discharged from the slit 6b toward the reaction cylinder 4. The cooling gas 6g can be any gas capable of cooling the generated metal oxide, and an inert gas is preferred, such as air.

[0051] After the oxide particles 9 leave the flame 7, they are immediately cooled by the cooling gas 6g, thus maintaining their chain-like structure. The cooled oxide particles 9 are captured and collected by the filter 5a. The captured oxide particles 9 may be heat-treated at 400 to 1000°C to adjust them to a desired primary particle size.

[0052] 3-2. Reduction and Surface Oxidation Process The oxide fine particles 9 that make up the oxide powder obtained in the above process are entirely oxide, so their conductivity is not good. Therefore, in this process, the oxide fine particles 9 are reduced to a state in which the entire material is metal, and then only the surface is oxidized to form a metal core 90 and an oxide skin layer 91.

[0053] The oxide fine particles 9 can be reduced by heat treatment in a hydrogen-containing atmosphere. The hydrogen-containing atmosphere is an atmosphere containing hydrogen, preferably an atmosphere in which hydrogen is diluted with an inert gas (e.g., nitrogen). The hydrogen content in the atmosphere is, for example, 0.5 to 50%. The heat treatment temperature is preferably 20 to 500°C. Through this process, the oxide fine particles 9 are reduced to metal fine particles. Next, after reduction, the surface of the metal fine particles can be oxidized by holding them in nitrogen containing a small amount of oxygen while slowly cooling to room temperature. The oxygen concentration is preferably 0.5 to 50 ppm.

[0054] 3-3. Transition Metal Nanoparticle Supporting Process The method for supporting the transition metal nanoparticles 92 on the surface of the oxide skin layer 91 obtained in the above process is not particularly limited. The transition metal nanoparticles 92 can be supported, for example, by impregnation. In the impregnation method, the powder of surface-oxidized metal fine particles obtained in the above process is added to an aqueous solution of chloride, hydrate, etc., containing a transition metal, and after heating and stirring, it is dried. Next, the dried powder is heat-treated in a hydrogen-containing atmosphere so that the transition metal nanoparticles 92 are supported on the surface of the skin layer 91. This gives an electrode catalyst 50 composed of powder 70 which is an aggregate of fine particles 80 having a beaded structure.

[0055] The electrode catalysts were manufactured using the method described below and various evaluations were performed. 1. Manufacturing of Electrode Catalyst 50 The electrode catalysts of Examples 1 to 4 and Comparative Examples 1 to 2 shown in Table 1 were manufactured according to the method described below and various evaluations were performed. <Example 1 (Ni / Ni ０．８ Fe ０．２ O)> 1-1. Powder Formation Process An electrode catalyst 50 was manufactured using the manufacturing apparatus 1 shown in Figures 7 to 10. As the burner gas 2a, a mixture of oxygen at 5 L / min and propane gas at 1 L / min was used, and this gas was ignited to form a flame (chemical flame) 7 of 1600°C or higher at the tip of the burner 2. As the raw material solution 14a, Ni octoyl acid and Fe octoyl acid were mixed in mineral split turpentine such that the atomic ratio x of Fe to the total of Ni and Fe was 0.2, and dissolved. As the atomizing gas 13a, oxygen was used. 9 L / min of atomizing gas 13a and 3 g / min of raw material solution 14a were mixed and sprayed from the tip of the raw material supply unit 3, which is a spray nozzle (atomizer), to the center of the flame, and burned to generate oxide powder, which is an aggregate of oxide fine particles 9. At that time, by creating a negative pressure in the gas discharge section 5b, air was drawn in from the slit 6b at a flow rate of 170 L / min, and the generated powder was collected in the recovery unit 5 (with filter 5a). The raw material supply section 3 has a double-pipe structure (total length 322.3 mm), with oxygen gas supplied from the outer cylinder 13 and the raw material solution 14a supplied to the raw material flow tube 14. The tip of the raw material flow tube 14 has a fluid nozzle and an air nozzle, where the raw material solution 14a is converted into mist 14b.

[0056] 1-2. Reduction and Surface Oxidation Process Next, the oxide powder obtained in the above process was subjected to reduction and surface oxidation to form a metal core 90 and an oxide skin layer 91.

[0057] Reduction was performed by heat-treating oxide fine particles 9 at 500°C for 1 hour in a hydrogen-containing atmosphere (a mixed gas atmosphere of hydrogen and nitrogen with a hydrogen content of 4%). Surface oxidation was performed after reduction by holding the mixture in nitrogen containing a small amount of oxygen while slowly cooling it to room temperature. The oxygen concentration was 5 ppm.

[0058] 1-3. Transition metal nanoparticle loading process: NiCl ２ 6H ２O was dissolved in 3 mL of MeOH to prepare a MeOH solution. NiCl ２ 6H ２ The amount of O is Ni ０．８ Fe ０．２ The solution was adjusted to 15 wt% relative to O. 0.3 g of the surface-oxidized metal nanoparticles obtained above was added to the aqueous solution, and the resulting mixed solution was stirred at 120°C on a hot stirrer. Stirring was carried out at 120°C for 1 hour to evaporate the solvent MeOH and obtain a solid in which Ni nanoparticles were adsorbed on the surface of the metal nanoparticles. Next, the obtained solid was heat-treated at 100°C for 1 hour in a hydrogen-containing atmosphere (a mixed gas atmosphere of hydrogen and nitrogen; hydrogen content 5%) to reduce the surface of the Ni nanoparticles. This yielded an electrode catalyst 50 composed of powder 70, which is an aggregate of nanoparticles 80 having a beaded structure.

[0059] Figure 12 shows an SEM image of the electrode catalyst 50 according to Example 1. Numerous white dots of a few nanometers in size are scattered on the surface of the catalyst, indicating that transition metal nanoparticles 92 are dispersed and supported on the surface of the skin layer 91.

[0060] <Example 2> In the above transition metal nanoparticle loading process, NiCl ２ 6H ２ The amount of O, Ni ０．８ Fe ０．２ The electrode catalyst 50 of Example 2 was manufactured under the same conditions as in Example 1, except that the amount of O was 3 wt%.

[0061] <Example 3> In the above transition metal nanoparticle loading process, NiCl ２ 6H ２ The amount of O, Ni ０．８ Fe ０．２ The electrode catalyst 50 of Example 3 was manufactured under the same conditions as in Example 1, except that the amount of O was 25 wt%.

[0062] <Example 4> In the above transition metal nanoparticle loading process, NiCl ２ 6H ２ The amount of O, Ni ０．８ Fe ０．２ The electrode catalyst 50 of Example 4 was manufactured under the same conditions as in Example 1, except that the amount of O was 30 wt%.

[0063] <Comparative Example 1 (Ni ０．８ Fe ０．２ O), Comparative Example 2 (Ni ０．８ Co ０．２ O)> In Comparative Example 1, the electrode catalyst 50 of Comparative Example 1 was manufactured under the same conditions as in Example 1, except that the "1-3. Transition metal nanoparticle loading step" described above was not performed. In Comparative Example 2, the electrode catalyst 50 was manufactured under the same conditions as in Comparative Example 1, except that octic acid Co was used instead of octic acid Fe.

[0064]

[0065] 2. BET specific surface area of ​​electrode catalyst 50 The BET specific surface area of ​​the electrode catalysts 50 of Examples 1 to 4 and Comparative Examples 1 to 2 obtained by the above method was measured by the following method. The results are shown in Table 1. As shown in Table 1, the BET specific surface area of ​​the electrode catalysts 50 of Examples 1 to 4 was larger than that of the comparative examples.

[0066] (Method for measuring BET specific surface area) A 0.2 g sample of electrode catalyst 50 was weighed into a measuring glass cell and dehydrated for 1 to 2 hours at 130°C under reduced pressure until the Torr was 30 mm or less. Then, it was slowly cooled to room temperature and purged with nitrogen. Next, the BET specific surface area was determined by the BET method using an Antonpaar gas adsorption analyzer (Autosorp iQ) under relative pressure conditions of 0.01 to 0.30.

[0067] 3. Measurement of OER Activity of Electrode Catalyst 50 The oxygen evolution reaction (hereinafter, "OER") activity of the electrode catalysts 50 in the water electrolysis operation of Examples 1 to 4 and Comparative Examples 1 to 2 was measured using a three-electrode electrochemical measuring device 15 shown in Figure 13. The device 15 comprises a glass cell 15a, a working electrode 15b, a counter electrode 15c, and a reference electrode 15d. The potential of the working electrode 15b relative to the reference electrode 15d can be adjusted by a potentiostat (not shown). A KOH solution 15e with a concentration of 0.1 mol / L is contained in the glass cell 15a. Nitrogen or oxygen can be blown into the KOH solution 15e. The working electrode 15b is made of glassy carbon (GC), is cylindrical, and the electrode catalyst 50 is coated on its lower surface. The lower surface of the working electrode 15b and the counter electrode 15c are immersed in the KOH solution 15e. The reference electrode 15d is liquid-junctioned with the KOH solution 15e by a salt bridge 15f.

[0068] The electrode catalyst 50 was dispersed in a mixed solution of 80 wt% water and 20 wt% ethanol, coated onto the underside of the working electrode 15b, and dried. Before measurement, nitrogen was blown into the KOH solution 15e to purge it. During measurement, oxygen was blown in at a flow rate of 100 ml / min, and the reference electrode 15d was rotated around its central axis. In this state, the current value was measured while changing the potential of the working electrode 15b relative to the reference electrode 15d (Potential / V vs RHE).

[0069] The mass activity of the electrode catalyst 50 was determined by substituting the current value I obtained from the above measurement into the following formula: Mass activity [A / mg] = Specific surface area of ​​the electrode catalyst [m²] ２ [ / mg] × Current value I (= OER activity) [A / m] ２ ]...(1)

[0070] Figure 14 shows the measurement results of mass activity for Examples 1 to 4 and Comparative Example 1. Compared to Comparative Example 1, Examples 1 to 4 all showed increased mass activity, and this improvement was particularly noticeable when the voltage was 1.6V or higher.

[0071] Furthermore, Figure 15 shows the correlation between the mass activity of the electrode catalyst 50 at a voltage of 1.7 V and the specific surface area of ​​the electrode catalyst 50. Comparing Example 2 and Comparative Example 1, although Example 2 and Comparative Example 1 have almost the same specific surface area, the mass activity is higher in Example 2. In other words, it can be seen that the catalytic activity improvement effect in Examples 1 to 4 is not solely due to the increase in specific surface area. In Examples 1 to 4, it is thought that the OER activity was improved by the reverse spillover effect due to the support of transition metal nanoparticles 92 on the surface of the skin layer 91.

[0072] 4. XPS Measurement of Electrode Catalyst 50 XPS measurements were performed on the electrode catalyst 50 according to the examples and comparative examples. A JEOL X-ray photoelectron spectrometer was used for the measurements. Monochromatic AlKα was used as the X-ray source, and narrow-scan analysis was performed. Figure 16 shows the Ni2p obtained for the electrode catalyst 50 of Example 1 and Comparative Example 1. ３／２ The spectrum is shown. Peaks in the range of 852–853.5 eV are attributed to zero-valent Ni atoms. Within this range, a high-intensity peak was obtained in Example 1, but no high-intensity peak was observed in Comparative Example 1. Therefore, it was confirmed that zero-valent Ni atoms were present on the surface of the electrode catalyst 50 (to a depth of several nm) in Example 1. In addition to the zero-valent Ni atom peaks mentioned above, peaks were observed at 854–856 eV in Example 1 and at 856–858 eV in Comparative Example 1. These are all attributed to the Ni oxide constituting the skin layer 91. In Comparative Example 1, the peaks shifted to the higher energy side due to the charge-up of the sample.

[0073] I1 and I2 were calculated from the XPS peaks obtained above. The results are shown in Table 1. In Example 1, I1 was calculated from the peaks in the range of 854 to 856 eV using the method described above, and I2 was similarly calculated from the peaks in the range of 852 to 853.5 eV. In Comparative Example 1, I1 was calculated from the peaks in the range of 856 to 858 eV, and I2 was calculated from the peaks slightly observed in the range of 852 to 854 eV. In Example 1, because zero-valent Ni atoms were present, the I2 / I1 ratio was larger than in Comparative Example 1.

[0074] 5. Voltage-Voltage Characteristic Evaluation of Water Electrolysis Cells In the water electrolysis cell shown in Figure 1, the cathode catalyst was the electrode catalyst of Comparative Example 1, and the anode catalysts were the electrode catalysts 50 of Example 1, Comparative Example 1, and Comparative Example 2, respectively. At 80°C, the relationship between the voltage applied between the anode and cathode and the current flowing during the water electrolysis reaction was measured and the results are shown in Figure 18. As shown in Figure 18, the water electrolysis cell using the electrode catalyst 50 of Example 1 had a higher water electrolysis reaction rate and superior catalyst performance compared to the water electrolysis cells using the electrode catalysts 50 of Comparative Examples 1 and 2.

[0075] 1: Manufacturing equipment, 2: Burner, 2a: Burner gas, 3: Raw material supply section, 4: Reaction cylinder, 5: Recovery unit, 5a: Filter, 5b: Gas discharge section, 6: Gas storage section, 6a: Cooling gas introduction section, 6b: Slit, 6c: Inner peripheral wall, 6d: Burner insertion hole, 6g: Cooling gas, 7: Flame, 8: Crystallite, 9: Oxide fine particles, 10: Anion exchange membrane type electrochemical cell, 13: Outer cylinder, 13a: Misting gas, 14: Raw material flow cylinder, 14a: Raw material solution, 14b: Mist, 15: Electrochemical measuring device, 15a: Glass cell, 15 b: Working electrode, 15c: Counter electrode, 15d: Reference electrode, 15e: KOH solution, 15f: Salt bridge, 20: Cathode, 21: Diffusion layer, 22: Microporous layer, 23: Catalyst layer, 30: Anode, 31: Diffusion layer, 32: Microporous layer, 33: Catalyst layer, 40: Anion exchange film, 50: Electrocatalyst, 60: Porous body, 61: Substrate, 62: Pore, 70: Powder, 80: Fine particles, 81: Gap, 82: Primary particles, 83: Chain portion, 84: Void, 85: Branch point, 90: Core portion, 91: Skin layer, 92: Transition metal nanoparticles

Claims

1. An electrode catalyst comprising a porous body having voids, wherein the porous body comprises a core portion and a skin layer covering the core portion, the core portion being made of a metal, the skin layer being made of Ni and an oxide containing a second metal, the second metal being a metal different from Ni, and the skin layer further having transition metal nanoparticles dispersed and supported on its surface, the particle size of the transition metal nanoparticles being smaller than the particle size of the metal and the particle size of the oxide.

2. An electrode catalyst according to claim 1, wherein the second metal has an atomic number smaller than Ni.

3. An electrode catalyst according to claim 1, wherein, in a spectrum measured by X-ray photoelectron spectroscopy (XPS), when I1 is the intensity at the point where the peak intensity of the oxide is maximum and I2 is the intensity at the point where the peak intensity of the transition metal nanoparticle is maximum, the peak intensity ratio I2 / I1 is 0.2 or more and 5 or less.

4. An electrode catalyst according to claim 1, wherein the particle size of the transition metal nanoparticles, calculated from an SEM image of the electrode catalyst, is 0.1 nm to 10 nm.

5. An electrode catalyst according to claim 1, wherein the second metal is Fe or Co.

6. An electrode catalyst according to claim 1, wherein the transition metal nanoparticles include Ni or Cu.

7. An electrode catalyst according to claim 1, wherein the void-containing body is composed of fine particles that act as catalysts, and the fine particles comprise the core portion and the skin layer.

8. An electrode catalyst according to claim 1, wherein the fine particles are composed of a plurality of primary particles fused together in a chain-like manner.

9. An electrode catalyst according to claim 1, wherein the oxide contains a transition metal with an atomic number greater than Ni.

10. An electrode catalyst according to claim 1, wherein the void-containing body has a void ratio of 20% or more.

11. An electrode catalyst according to claim 1, wherein the electrode catalyst is an electrode catalyst for an anion exchange membrane type electrochemical cell.

12. An electrode catalyst according to claim 11, wherein the anion exchange membrane type electrochemical cell is a water electrolysis cell or a fuel cell.

13. An electrochemical cell comprising a cathode, an anode, and an anion exchange membrane disposed between them, wherein at least one of the cathode and the anode comprises a catalyst layer composed of the electrode catalyst described in any one of claims 1 to 12.

14. An anion exchange membrane type electrochemical cell according to claim 13, wherein the anion exchange membrane type electrochemical cell is a water electrolysis cell or a fuel cell.

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