Catalyst for electrode, electrode for electrochemical device, and electrochemical device

Abstract

To provide a catalyst for an electrode, an electrode for an electrochemical device, and an electrochemical device that achieve higher activity per unit amount of the catalyst relative to activity per unit amount of the respective materials.SOLUTION: A catalyst for an electrode contains a first oxygen compound having a perovskite-type crystal structure including Mn and a second oxygen compound, wherein a change amount in valence of Mn atoms, determined by X-ray absorption spectroscopy before and after heat treatment in a reducing atmosphere, is greater than a change amount in valence of Mn atoms of the first oxygen compound, determined by X-ray absorption spectroscopy before and after heat treatment in the atmosphere.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an electrode catalyst, an electrode for an electrochemical device, and an electrochemical device. [Background technology]

[0002] Electrochemical devices in which electrochemical oxidation and reduction reactions of oxygen occur on electrodes pose a problem of energy loss due to the overvoltage of these reactions. Patent Document 1 discloses a prior art technique for combining a material (ORR material) with high activity in the oxygen reduction reaction (ORR), selected from oxygen compounds with a perovskite-type crystal structure containing Mn, and a material (OER material) with high activity in the oxygen evolution reaction (OER), as an electrode catalyst to further reduce the overvoltage. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2005-190833 Summary of the Invention [Problem to be solved by the invention]

[0004] In the prior art, the content of ORR material and OER material per catalyst is reduced depending on the compounding ratio of the materials, which results in a problem that the activity per unit amount of catalyst is lower than the activity per unit amount of each material.

[0005] The present invention has been made to solve this problem, and aims to provide an electrode catalyst, an electrode for an electrochemical device, and an electrochemical device that can increase the activity per unit amount of the catalyst compared to the activity per unit amount of each material. [Means for solving the problem]

[0006] A first aspect for achieving this object is an electrode catalyst comprising a first oxygen compound having a perovskite-type crystal structure containing Mn, and a second oxygen compound, wherein the change in valence of the Mn atom measured by X-ray absorption spectroscopy before and after heat treatment in a reducing atmosphere is greater than the change in valence of the Mn atom measured by X-ray absorption spectroscopy before and after heat treatment of the first oxygen compound in the same atmosphere.

[0007] In a second embodiment, in the first embodiment, the second oxygen compound has a layered structure.

[0008] In a third aspect, in the first or second aspect, the second oxygen compound is represented by formula A n+1 B n X 3n+1+δ It has a crystal structure expressed as (δ≦0.5).

[0009] In a fourth embodiment, in the third embodiment, the B site of the second oxygen compound contains Ni.

[0010] In a fifth aspect, in the third or fourth aspect, the crystal structure of the first oxygen compound is represented by the chemical formula ABO 3-δ When expressed as (δ≦0.5), the A site contains La, and the A site of the second oxygen compound contains La.

[0011] In a sixth aspect, in any one of the third to fifth aspects, the first oxygen compound and the second oxygen compound contain an alkali metal or alkaline earth metal at the A site.

[0012] In a seventh aspect, in any one of the third to sixth aspects, at least one of the first oxygen compound and the second oxygen compound contains Sr at the A site.

[0013] An eighth aspect is an electrode for an electrochemical device, which comprises the electrode catalyst of any one of the first to seventh aspects.

[0014] A ninth aspect is an electrochemical device, comprising the electrode catalyst of any one of the first to seventh aspects. [Effects of the Invention]

[0015] According to the present invention, the change in valence of Mn atoms, as determined by X-ray absorption spectroscopy, before and after heat treatment in a reducing atmosphere of an electrode catalyst containing a first oxygen compound and a second oxygen compound is greater than the change in valence of Mn atoms, as determined by X-ray absorption spectroscopy, before and after heat treatment in a reducing atmosphere of the first oxygen compound. It is estimated that the first oxygen compound contained in the electrode catalyst has more oxygen vacancies than the first oxygen compound before it comes into contact with the second oxygen compound. Therefore, the electrode catalyst, electrochemical device electrode, and electrochemical device can have a higher OER catalytic activity (OER overpotential reduction performance) per unit amount of catalyst than the activity per unit amount of the first oxygen compound. [Brief explanation of the drawings]

[0016] [Figure 1] 1 is a schematic diagram of an electrochemical device according to one embodiment. [Figure 2] 1 shows X-ray diffraction patterns of catalysts in an example, comparative example 1, and comparative example 2. DETAILED DESCRIPTION OF THE INVENTION

[0017] Preferred embodiments of the present invention will now be described with reference to the accompanying drawings. Fig. 1 is a schematic diagram of an electrochemical device 10 according to one embodiment. The electrochemical device 10 is a device that converts electrical energy and chemical energy, and an air secondary battery is shown as an example. An air secondary battery is a battery that uses oxygen in the air as the positive electrode active material, and includes an air electrode 11 (positive electrode) and a negative electrode 16. The air electrode 11 includes a gas diffusion layer 12 through which air (oxygen) diffuses, and a catalyst layer 14 in which an oxygen reduction reaction (ORR) occurs during discharge and an oxygen evolution reaction (OER) occurs during charge.

[0018] The gas diffusion layer 12 allows air to pass through while preventing water from entering from the outside air. The gas diffusion layer 12 is made of a sheet formed by mixing highly hydrophobic carbon such as acetylene black with a highly water-repellent polytetrafluoroethylene (PTFE) binder, a carbon mesh, a porous PTFE film, or the like. A current collector 13 is embedded in the gas diffusion layer 12. Examples of the current collector 13 include a metal mesh made of nickel or nickel-plated copper.

[0019] The catalyst layer 14 contains carbon with a large specific surface area, such as carbon black or activated carbon, a PTFE binder, and a catalyst 15 (electrode catalyst). The negative electrode 16 is made of a metal containing, for example, Li, Zn, Al, Mg, or Fe, and has the function of generating ions in the electrolyte 18. The separator 17 electrically insulates the air electrode 11 from the negative electrode 16 and also has the function of allowing the ions contained in the electrolyte 18 to pass through. The reaction in the catalyst layer 14 occurs at the three-phase interface between the solid phase (carbon and catalyst 15), the liquid phase (electrolyte 18), and the gas phase (oxygen).

[0020] Catalyst 15 contains a first oxygen compound having a perovskite-type crystal structure containing Mn, and a second oxygen compound. The oxygen compound is a compound containing oxygen in its molecule, such as an oxide or hydroxide.

[0021] The first oxygen compound with a perovskite-type crystal structure is represented by the chemical formula ABO 3-δ When expressed as (δ≦0.5), the A site contains one or more elements selected from La, Ce, Pr, and Nd, and B contains Mn. In the first oxygen compound, a portion of the elements in the A site and B site may be substituted with one or more elements selected from transition metals such as Cr, Fe, Co, Ni, and Cu. The first oxygen compound may contain an alkali metal or alkaline earth metal in the A site.

[0022] The second oxygen compound is not particularly limited, but examples include oxygen compounds with a perovskite-type crystal structure, oxygen compounds with a spinel-type crystal structure, and particularly oxygen compounds with a layered structure. Examples of oxygen compounds with a layered structure include Ruddersden-Popper type oxides, layered double hydroxides, and layered rock salt type oxides. Oxygen compounds with a layered structure are preferred because they are thought to contribute to the transfer of oxygen between the first oxygen compound and the oxygen compound.

[0023] The second oxygen compound with a perovskite-type crystal structure is represented by the chemical formula ABO 3-δ When expressed as (δ≦0.5), the A site contains one or more elements selected from La, Ce, Pr, and Nd, and B contains one or more elements selected from Cr, Mn, Fe, Co, and Ni. In the second oxygen compound, part of the elements in the A site and B site may be substituted with one or more elements selected from transition metals such as Cr, Fe, Co, Ni, and Cu. The second oxygen compound may contain an alkali metal or alkaline earth metal in the A site.

[0024] When the second oxygen compound having a spinel-type crystal structure is represented by the chemical formula AB2O4, the A site contains one or more elements selected from Mn, Zn, Fe, Mo, Cd, Ti, Ni, and Co, and the B site contains one or more elements selected from Cr, Al, Fe, Ni, Co, Mn, and V. The second oxygen compound may contain an alkali metal or alkaline earth metal in the A site.

[0025] The Ruddersden-Popper oxides have a layered structure consisting of alternating perovskite layers ABO3 and rock salt layers AO. The second oxygen compound of the Ruddersden-Popper type is represented by the chemical formula A n+1 B n X 3n+1+δ When expressed as (δ≦0.5), the A site contains one or more elements selected from La, Ce, Pr, and Nd, B contains one or more elements selected from Cr, Mn, Fe, Co, and Ni, and X contains O. In the second oxygen compound, part of the elements in the A site or B site may be substituted with one or more elements selected from transition metals such as Cr, Fe, Co, Ni, and Cu. The second oxygen compound may contain an alkali metal or alkaline earth metal in the A site.

[0026] The catalyst 15 can be obtained by, for example, preparing a first oxygen compound and a second oxygen compound, mixing them, and heating them in a reducing atmosphere. The heating in a reducing atmosphere is performed in an atmosphere where the oxygen partial pressure is 5.0×10 -1 An example of this is heating at a temperature of 500°C to 800°C in an atmosphere of 100 Pa or less. The first oxygen compound and the second oxygen compound can be produced by a solid-phase method using thermal decomposition or solid-phase reaction, a liquid-phase method such as precipitation or solvent evaporation, or a gas-phase method such as a gas-phase reaction method or evaporation-condensation method.

[0027] The particle size of catalyst 15 is not particularly limited, but is exemplified as 2 nm to 10 μm. In order to utilize the activity of the first oxygen compound and the activity of the second oxygen compound, catalyst 15 preferably has a volume ratio of the first oxygen compound of 20% to 80% of the combined volume of the first oxygen compound and the second oxygen compound.

[0028] The catalyst 15 may contain other elements or compounds in addition to the first oxygen compound and the second oxygen compound. Examples of the other elements include noble metals such as Pd, Pt, Ag, and Ru. Examples of the other compounds include oxygen compounds having a perovskite crystal structure that are different from the first oxygen compound and the second oxygen compound, phthalocyanine compounds, and porphyrin compounds. [Example]

[0029] The present invention will be explained in more detail with reference to examples, but the present invention is not limited to these examples.

[0030] (Preparation of First Oxygen Compound) La(OH)3 and Mn2O3 (both 99.9% pure) were weighed out to produce LaMnO3, and the weighed raw materials and ethanol were placed in an alumina pot along with zirconia balls and ground and mixed in a ball mill for 15 hours. The slurry removed from the pot was dried and then placed in an alumina crucible and fired at 1000°C for 10 hours. The fired powder and ethanol were placed in an alumina pot along with zirconia balls and ground in a ball mill for 15 hours. The slurry removed from the pot was dried and then ground in a mortar to obtain a powder (first oxygen compound). Powder X-ray diffraction (XRD) confirmed that the resulting powder was LaMnO3 with a perovskite-type crystal structure.

[0031] (Preparation of the Second Oxygen Compound) La 1.6 Sr 0.4 La(OH)3, SrCO3, and NiO (all 99.9% pure) were weighed out to obtain NiO4, and the weighed raw materials and ethanol were placed in an alumina pot together with zirconia balls and ground and mixed in a ball mill for 15 hours. The slurry removed from the pot was dried, then placed in an alumina crucible and fired at 1250°C for 10 hours. The fired powder and ethanol were placed in an alumina pot together with zirconia balls and ground in a ball mill for 15 hours. The slurry removed from the pot was dried, then ground in a mortar to obtain a powder. The obtained powder (second oxygen compound) was identified by XRD as the Rudolfsden-Popper type oxide, La 1.6 Sr 0.4 It was confirmed to be NiO4.

[0032] (Preparation of catalysts in the examples) The same volumes of the first oxygen compound and the second oxygen compound (first oxygen compound:second oxygen compound = 1:1 (volume ratio)) were placed in an alumina pot together with ethanol and zirconia balls, and were milled and mixed in a ball mill for 15 hours. The slurry removed from the pot was dried and then milled in a mortar to obtain powder. The oxygen partial pressure was 5.0 × 10 -1The powder was heated from room temperature to 800°C under a nitrogen atmosphere of 1 Pa, heated at 800°C for 1 hour, and then cooled from 800°C to room temperature. This yielded a catalyst in this example in which the first oxygen compound and the second oxygen compound were combined.

[0033] (Preparation of catalyst in Comparative Example 1) The first oxygen compound (powder) is heated to a temperature of 5.0 × 10 -1 The temperature was increased from room temperature to 800° C. under a nitrogen atmosphere of 1 Pa, heated at 800° C. for 1 hour, and then decreased from 800° C. to room temperature. This gave a catalyst in Comparative Example 1.

[0034] (Preparation of catalyst in Comparative Example 2) The second oxygen compound (powder) was added to the gas at an oxygen partial pressure of 5.0 × 10 -1 The temperature was increased from room temperature to 800° C. under a nitrogen atmosphere of 1 Pa, heated at 800° C. for 1 hour, and then decreased from 800° C. to room temperature. This gave a catalyst in Comparative Example 2.

[0035] 2 shows X-ray diffraction patterns of the catalysts in the Examples, Comparative Examples 1 and 2. The X-ray diffraction pattern of the catalyst in the Examples was a pattern obtained by combining the X-ray diffraction pattern of the catalyst in Comparative Example 1 and the X-ray diffraction pattern of the catalyst in Comparative Example 2. It was confirmed that the catalyst in the Examples did not produce any new products resulting from the reaction between the first oxygen compound and the second oxygen compound.

[0036] (Preparation of ink for producing air electrodes) A 5 wt% Nafion dispersion (Fujifilm Wako DE521) was mixed with a KOH (0.1M) aqueous solution to prepare a 3.33 wt% Nafion dispersion, and then 1-propanol (Fujifilm Wako) was added to this dispersion to prepare a solution. Each of the catalysts in Examples and Comparative Examples 1 and 2, along with acetylene black (Denka Li-250), a conductive material, were dispersed in this solution to prepare three types of ink. The ratio (mass ratio) of catalyst, conductive material, and Nafion in the ink was catalyst:conductive material:Nafion = 5:1:1. The three prepared inks were applied to a rotating disk electrode at approximately 250 μg / cm. 2The catalyst was then evaporated to dryness at room temperature to obtain three types of electrodes intended as air electrodes.

[0037] (Electrochemical measurements) Electrochemical measurements were performed using a rotating disk electrode apparatus (BAS RRDE-3A) to evaluate the OER activity of the electrode (air electrode). The rotating disk electrode (RDE) was used as the working electrode, Hg / HgO (internal solution: 0.1 M KOH aqueous solution) as the reference electrode, and a Pt coil as the counter electrode. Linear sweep voltammetry (LSV) was performed using a potentiostat in an oxygen-saturated 0.1 M KOH aqueous solution, and the current was measured when sweeping the voltage range of 1.20–1.92 V vs. the reversible hydrogen electrode (RHE) at 10 mV / s. During the measurements, the RDE was rotated at 1600 rpm to prevent charge transfer rate limitation and bubble adhesion. Based on the current measurements using LSV, the surface area of ​​the electrode was determined to be 0.126 cm. 2 The current density is 0.1mA / cm 2 The potential at this time was calculated, and the results are shown in Table 1.

[0038] [Table 1]

[0039] Since a lower potential indicates a higher OER activity, it was confirmed that the OER activity of the catalyst in the example was higher than the OER activity of the catalysts in comparative examples 1 and 2.

[0040] (X-ray absorption near-edge structure analysis) For the first oxygen compound and the catalysts in the Examples and Comparative Example 1, spectra in the Mn X-ray absorption near-edge structure (XANES) region (hereinafter referred to as "XANES spectra") were measured by X-ray absorption spectroscopy. The position of the peak in the XANES spectrum shifts toward lower energy as the valence of the element decreases, and shifts toward higher energy as the valence of the element increases. Utilizing this, the amount of change in Mn valence due to heat treatment was determined based on the peak shifts in the XANES spectrum of the first oxygen compound (before heat treatment), the XANES spectrum of the catalysts in the Examples (after heat treatment), and the XANES spectrum of the catalyst in Comparative Example 1 (after heat treatment).

[0041] The change in Mn valence of the catalyst in the Example was 0.4, while the change in Mn valence of the catalyst in Comparative Example 1 was 0.2. Because the Mn valence decreases as the number of oxygen vacancies increases, it was estimated that the number of oxygen vacancies in the catalyst in the Example was greater than that in the catalyst in Comparative Example 1. It was estimated that the catalyst in the Example was formed by combining the first oxygen compound and the second oxygen compound and then heat-treating the catalyst in a reducing atmosphere, which resulted in oxygen being transferred from the first oxygen compound to the second oxygen compound, and the OER catalytic activity (OER overvoltage reduction performance) per unit amount of the catalyst was higher than the activity per unit amount of the first oxygen compound.

[0042] The present invention has been described above based on the embodiments, but the present invention is not limited to the above embodiments, and it can be easily inferred that various improvements and modifications are possible within the scope of the present invention.

[0043] In the embodiments, the electrochemical device 10 has been described using a metal-air battery as an example, but is not necessarily limited to this. The electrode catalyst in the embodiments can be applied to other electrochemical devices. Examples of other electrochemical devices include fuel cells and electrolyzers. Examples of electrodes for other electrochemical devices include air electrodes of fuel cells and electrolyzers. [Explanation of symbols]

[0044] 10 Electrochemical Devices 11 Air electrode (electrode for electrochemical devices) 15 Catalyst (air electrode catalyst)

Claims

1. An electrode catalyst comprising a first oxygen compound having a perovskite-type crystal structure containing Mn, and a second oxygen compound, An electrode catalyst in which the change in valence of Mn atoms, as determined by X-ray absorption spectroscopy, before and after heat treatment in a reducing atmosphere is greater than the change in valence of Mn atoms, as determined by X-ray absorption spectroscopy, before and after heat treatment of the first oxygen compound in the same atmosphere.

2. 2. The electrode catalyst according to claim 1, wherein the second oxygen compound has a layered structure.

3. The second oxygen compound has the formula A ｎ＋１ B ｎ X ３ｎ＋１＋δ 3. The electrode catalyst according to claim 2, which has a crystal structure represented by (δ≦0.5).

4. 4. The electrode catalyst according to claim 3, wherein the B site of said second oxygen compound contains Ni.

5. The crystal structure of the first oxygen compound is represented by the chemical formula ABO ３－δ When expressed as (δ≦0.5), the A site contains La, 5. The electrode catalyst according to claim 4, wherein the A site of the second oxygen compound contains La.

6. 6. The electrode catalyst according to claim 5, wherein the first oxygen compound and the second oxygen compound contain an alkali metal or alkaline earth metal at the A site.

7. 6. The electrode catalyst according to claim 5, wherein at least one of the first oxygen compound and the second oxygen compound contains Sr at the A site.

8. An electrode for an electrochemical device, comprising the electrode catalyst according to claim 1 .

9. An electrochemical device comprising the electrode catalyst according to any one of claims 1 to 7.

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