Oxygen reduction reaction catalyst, electrode for oxygen reduction, and battery

Abstract

To provide an oxygen reduction reaction catalyst that maintains an inexpensive configuration based on manganese oxide while achieving superior oxygen reduction reaction (ORR) catalytic activity.SOLUTION: An oxygen reduction reaction catalyst comprises manganese oxide and silver nanoparticles having a diameter of 1-50 nm that are supported on a surface of the manganese oxide.SELECTED DRAWING: Figure 1A

Description

[Technical Field]

[0001] The present disclosure relates to an oxygen reduction reaction catalyst, an electrode for oxygen reduction, and a battery. [Background technology]

[0002] Manganese oxides have been extensively investigated as inexpensive alternatives to expensive oxygen reduction reaction (ORR) catalysts, such as precious metals and precious metal / carbon supports. Manganese oxides are used, for example, as cathode (air electrode) materials in metal-air secondary batteries, where they act as catalysts for the oxygen reduction reaction (ORR) that occurs during discharge. This reaction has a large activation energy, which is a factor in the increase in overvoltage of metal-air secondary batteries. Therefore, improving the catalytic performance is desirable for device development.

[0003] Non-Patent Document 1 (Yuanyuan Ma et al., Journal of Power Sources 280 (2015) 526-532) reports that by varying the ratio of the raw materials KMnO4 and MnSO4 and performing hydrothermal treatment, tremella-like δ-MnO2 (T-MnO2), a mixture of tremella-like and short belt-like α-MnO2 (M-MnO2), and long belt-like α-MnO2 (B-MnO2) were prepared; and that of the three resulting forms, B-MnO2 showed the highest oxygen reduction reaction (ORR) activity in alkaline media.

[0004] Meanwhile, metal-air secondary batteries have been proposed that have separate discharge and charge catalyst layers. For example, Patent Document 1 (WO2022 / 209010) discloses an air electrode / separator assembly for a metal-air secondary battery, which includes a hydroxide ion conductive separator, a pair of air electrode catalyst layers covering both sides of the hydroxide ion conductive separator, and a pair of gas diffusion electrodes disposed on the outer sides of the pair of catalyst layers, where one of the pair of catalyst layers is a discharge catalyst layer and the other of the pair of catalyst layers is a charge catalyst layer. This document lists examples of discharge catalysts, such as (i) nickel, (ii) platinum group elements such as palladium and platinum, (iii) perovskite oxides containing transition metals such as cobalt, manganese, and iron, (iv) noble metal oxides such as ruthenium and palladium, and (v) manganese oxide. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] WO2022 / 209010 [Non-patent literature]

[0006] [Non-Patent Document 1] Yuanyuan Ma et al., "Control of MnO2 nanocrystal shape from tremella to nanobelt for ehancement of the oxygen reduction reaction activity", Journal of Power Sources 280 (2015) 526-532 Summary of the Invention

[0007] There is a need for inexpensive catalysts that exhibit high catalytic activity for the oxygen reduction reaction (ORR). However, there are currently no catalysts that exhibit sufficient performance. Therefore, a need exists for a catalyst that is inexpensive and based on manganese oxide, yet exhibits excellent ORR catalytic activity.

[0008] The present inventors have now discovered that a catalyst exhibiting excellent ORR catalytic activity can be provided by supporting silver nanoparticles with a diameter of 1 to 50 nm on the surface of a manganese oxide.

[0009] Therefore, an object of the present invention is to provide a catalyst that has an inexpensive manganese oxide-based structure and exhibits excellent oxygen reduction reaction (ORR) catalytic activity.

[0010] According to the present disclosure, the following aspects are provided. [Aspect 1] Manganese oxide, silver nanoparticles having a diameter of 1 to 50 nm supported on the surface of the manganese oxide; An oxygen reduction reaction catalyst comprising: [Aspect 2] 2. The oxygen reduction reaction catalyst according to aspect 1, wherein the average valence of Mn in the manganese oxide is within a range of 2.05 to 3.0. [Aspect 3] 3. The oxygen reduction reaction catalyst according to claim 1 or 2, wherein the manganese oxide is Mn2O3, Mn3O4, or a mixture thereof. [Aspect 4] 4. The oxygen reduction reaction catalyst according to any one of aspects 1 to 3, wherein the manganese oxide is in the form of particles having a diameter of 5 to 100 nm. [Aspect 5] 5. The oxygen reduction reaction catalyst according to any one of Aspects 1 to 4, wherein the silver nanoparticles are present in the form of metallic Ag. [Aspect 6] An electrode for oxygen reduction, comprising the oxygen reduction reaction catalyst according to any one of aspects 1 to 5. [Aspect 7] A battery comprising the oxygen reduction reaction catalyst according to any one of aspects 1 to 5. [Brief explanation of the drawings]

[0011] [Figure 1A] 1 shows a STEM image (BF image) of the oxygen reduction reaction catalyst synthesized in Example 1. [Figure 1B]This is a STEM image (HAADF image) of the oxygen reduction reaction catalyst synthesized in Example 1, observed in the same field of view as in Figure 1A. DETAILED DESCRIPTION OF THE INVENTION

[0012] The oxygen reduction reaction catalyst (hereinafter referred to as ORR catalyst) of the present invention comprises manganese oxide and silver nanoparticles with a diameter of 1 to 50 nm supported on the surface of the manganese oxide. By supporting silver nanoparticles with a diameter of 1 to 50 nm on the surface of the manganese oxide in this manner, a catalyst exhibiting excellent ORR catalytic activity can be provided. Figures 1A and 1B show BF-STEM and HAADF-STEM images, respectively, of an example of an ORR catalyst of the present invention. In Figure 1A (BF-STEM image), the black particles represent silver particles, while the gray areas represent manganese oxide. In Figure 1B (HAADF-STEM image), the white particles represent silver particles, while the gray areas represent manganese oxide.

[0013] As mentioned above, there is a demand for inexpensive, high-performance ORR catalysts, but none currently exhibit sufficient performance. In this regard, the present invention provides a catalyst that exhibits excellent ORR catalytic activity by supporting silver nanoparticles with a diameter of 1 to 50 nm on the surface of a manganese oxide. Specifically, the inventors conducted extensive research into methods for producing manganese oxide / silver composites used as ORR catalysts and discovered that a manganese oxide / silver composite can be synthesized in which highly conductive silver particles are highly dispersed within the manganese oxide. Specifically, the inventors discovered that by co-precipitation of a silver-manganese composite precipitate from a solution obtained by mixing an aqueous manganese salt solution with a silver salt, and then supporting the resulting precipitate on an electrode surface and subjecting it to a reduction potential for surface reconstruction, the silver component is reduced to metallic silver particles (specifically, silver nanoparticles with a diameter of 1 to 50 nm), which are highly dispersed on the surface of the manganese oxide. This configuration allows the ORR catalytic performance of manganese oxide to be fully exhibited, enabling it to exhibit high ORR catalytic performance equivalent to that of ORR catalysts based on expensive precious metal elements, precious metals on carbon supports, etc. Specifically, when the ORR catalyst of the present invention is used as an ORR catalyst (discharge catalyst) in the air electrode of a metal-air battery, improved ORR catalytic activity increases the reaction rate of the charge and discharge reactions in charge and discharge evaluations, thereby enabling a reduction in charge and discharge overvoltage.

[0014] The manganese oxide can be an oxide of manganese (Mn) with various valences. The average valence of Mn in the manganese oxide is preferably within the range of 2.05 to 3.0, more preferably 2.3 to 3.0, even more preferably 2.5 to 3.0, and particularly preferably 2.7 to 3.0. The valence of Mn can be measured by XAFS (X-ray Absorption Fine Structure) and STEM-EELS (Electron Energy-Loss Spectroscopy), as described in detail in the Examples below. The average valence in the manganese oxide can be calculated from the abundance ratio of Mn with each valence. From the viewpoint of providing the above average valence, the manganese oxide is preferably Mn2O3, Mn3O4, or a mixture thereof. The shape and size of the manganese oxide are not particularly limited, but manganese oxide is typically particulate. The shape of the manganese oxide particles is not particularly limited and can be various shapes, whether spherical or non-spherical. The diameter of the manganese oxide particles is preferably 5 to 100 nm, and can be, for example, 5 to 80 nm. The diameter of the manganese oxide particles here refers to the maximum size of each individual particle.

[0015] Silver nanoparticles are supported on the surface of manganese oxide. The silver nanoparticles are preferably present in the form of metallic Ag, as this allows for high ORR catalytic activity. The diameter of the silver nanoparticles is preferably 1 to 50 nm, more preferably 1 to 40 nm, even more preferably 1 to 30 nm, and particularly preferably 1 to 15 nm. The shape of the silver nanoparticles is not particularly limited and can be various shapes, including spherical and non-spherical. The diameter of the silver nanoparticles herein refers to the maximum size of each individual particle.

[0016] Since silver nanoparticles are supported on the surface of manganese oxide, they are desirably smaller than the manganese oxide particles. For example, the average particle size of silver nanoparticles is typically 0.68 times or less, more typically 0.52 times or less, and even more typically 0.27 times or less, of the average particle size of manganese oxide particles. The average particle size of silver nanoparticles or manganese oxide particles can be determined as the average diameter (maximum diameter) of 100 particles observed in a STEM image, as will be described in the Examples below.

[0017] The ORR catalyst of the present invention may contain any additive. Examples of such additives include conductive materials such as carbon, binders such as polymers, etc. Such additives may remain in the ORR catalyst due to components added during the production process of the ORR catalyst (e.g., preparation of a precursor or reduction treatment).

[0018] The ORR catalyst according to the present invention is preferably applied to an oxygen reduction electrode to utilize its excellent ORR catalytic activity. Therefore, according to a preferred embodiment of the present invention, an oxygen reduction electrode including the ORR catalyst according to the present invention is provided. The oxygen reduction electrode may include an ORR catalyst, a conductive material such as carbon, and a binder such as a polymer. The oxygen reduction electrode can be applied to various applications in which the ORR catalytic activity can be utilized. Examples of such applications include electrodes for batteries such as metal-air batteries and fuel cells, and oxygen sensors, with battery electrodes being preferred. Therefore, according to another preferred embodiment of the present invention, a battery including the ORR catalyst according to the present invention is provided. The battery may be either a primary battery or a secondary battery. Preferred examples of batteries include metal-air batteries and fuel cells, more preferably metal-air batteries, and particularly preferably metal-air secondary batteries. Examples of metal-air batteries include zinc-air batteries, lithium-air batteries, iron-air batteries, aluminum-air batteries, and hydrogen-air batteries. The zinc-air battery may be a zinc-air battery having a regeneration section for regenerating zinc and a storage section for storing zinc, separately provided. For example, metal-air secondary batteries having separate discharge catalyst layers and charge catalyst layers are known (see, for example, Patent Document 1 (WO2022 / 209010)), and the ORR catalyst of the present invention can be used as the discharge catalyst.

[0019] Manufacturing method The ORR catalyst of the present invention can be produced by (1) co-precipitation of a silver-manganese composite precipitate (manganese oxide / silver composite precursor) from a solution obtained by mixing an aqueous manganese salt solution with a silver salt, and then (2) supporting the resulting precipitate on an electrode surface and subjecting it to a reduction potential (surface reconstruction) to reduce the silver component and precipitate highly dispersed silver nanoparticles on the surface of the manganese oxide. Each step is described below.

[0020] (1) Preparation of manganese oxide / silver composite precursor A mixed solution is prepared by mixing an aqueous manganese salt solution with a silver salt. The manganese salt is not particularly limited, and known manganese salts such as manganese nitrate and manganese acetate may be used. The silver salt is also not particularly limited, and known silver salts such as silver nitrate and silver acetate may be used. A silver-manganese composite precipitate (manganese oxide / silver composite precursor) is precipitated from the resulting mixed solution by coprecipitation. The coprecipitation method may be performed based on a known technique and is not particularly limited, but is preferably performed by adding an alkaline aqueous solution such as an aqueous potassium hydroxide solution to the mixed solution at a liquid temperature of 0 to 100°C. The resulting silver-manganese composite precipitate is preferably purified by centrifugation or the like and then dried.

[0021] (2) Reduction treatment (surface reconstruction treatment) The manganese oxide / silver composite precursor is mixed with a conductive material such as carbon, a binder such as a polymer, a solvent, and other components to obtain a mixture for an electrode. Examples of carbon include acetylene black and carbon black. Conductive materials such as carbon (e.g., acetylene black) do not affect catalytic activity but are useful components for ensuring electrode functionality. The resulting mixture is applied to the surface of an electrode (e.g., a glassy carbon electrode) and dried repeatedly to form a film containing the manganese oxide / silver composite precursor on the electrode. The electrode with the film formed on it is attached to a rotating disk electrode (RDE) measurement device. In the presence of an electrolyte, a reference electrode, and a counter electrode, the electrode is rotated at a speed of preferably 400 to 4,000 rpm, more preferably 1,000 to 2,000 rpm, while a reduction potential is applied using cyclic voltammetry (CV) measurement. In this way, the silver component of the manganese oxide / silver composite precursor is reduced, yielding an oxygen reduction reaction catalyst in which silver nanoparticles with a diameter of 1 to 50 nm are highly dispersed on the surface of the manganese oxide. [Example]

[0022] The present invention will be explained in more detail by the following examples, but the present invention is not limited to the following examples.

[0023] Example 1 (1) Preparation of manganese oxide / silver composite precursor A raw material aqueous solution was prepared by adding 0.510 g of silver nitrate (AgNO3, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) and 0.86 g of manganese nitrate (Mn(NO3)2·6H2O, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) to 30 mL of ion-exchanged water. While stirring the resulting raw material aqueous solution at 300 rpm in an ice bath at 10°C, 3 mL of a 4 mol / L potassium hydroxide solution was added dropwise. After stirring the resulting solution for an additional 15 minutes, the resulting precipitate was centrifuged and purified using water and ethanol. The precipitate was then dried at 40°C for at least 10 hours (overnight) to obtain a manganese oxide / silver composite precursor.

[0024] (2) Reduction treatment (surface reconstruction treatment) 50 mg of manganese oxide / silver composite precursor, 10 mg of acetylene black (manufactured by Strem Chemicals), and polymer dispersion (Nafion with a polymer concentration of 5 wt%) TM Electrode ink was prepared by mixing 5 mL of PFSA polymer dispersion (Fujifilm Wako Pure Chemical Industries, Ltd.), sodium hydroxide (NaOH, Fujifilm Wako Pure Chemical Industries, Ltd.), and ethanol (EtOH, Amakasu Chemical Industry Co., Ltd.). 5 μL of this electrode ink was applied dropwise to the surface of a glassy carbon electrode (BAS, 5 mm diameter) for a rotating disk electrode (RDE) measurement device, followed by application and drying. This process of applying 5 μL of electrode ink, applying, and drying was repeated three more times (i.e., a total of four times), resulting in a total of 0.2 mg of manganese oxide / silver composite precursor. The resulting electrode was then attached to the RDE device, and the tip of the glassy carbon electrode was immersed in 100 mL of 4 mol / L KOH aqueous solution. A Hg / HgO reference electrode (BAS) and a Pt counter electrode (BAS) were attached to the RDE device. The manganese oxide / silver composite precursor was subjected to a reduction treatment by applying a reduction potential (0.176 V to -0.324 V vs. Hg / HgO) using cyclic voltammetry (CV) measurements while rotating the glassy carbon electrode at 1600 rpm. This reduction treatment was carried out under slow CV conditions of 80 cycles at a potential sweep rate of 1 mV / s. In this way, an oxygen reduction reaction catalyst was obtained in which silver nanoparticles were highly dispersed on the surface of the manganese oxide.

[0025] Example 2 An oxygen reduction reaction catalyst was synthesized in the same manner as in Example 1, except that the reduction treatment was carried out under high-speed CV conditions of 100 cycles at a potential sweep rate of 100 mV / s.

[0026] Example 3 (comparison) An oxygen reduction reaction catalyst was synthesized in the same manner as in Example 1, except that silver nitrate was not added in the preparation of the manganese oxide / silver composite precursor.

[0027] Various evaluations The oxygen reduction reaction catalysts prepared in Examples 1 to 3 were subjected to various evaluations as follows.

[0028] <stem-edx> For the oxygen reduction reaction catalysts prepared in each example, microstructure observation by scanning transmission electron microscopy (STEM) and elemental analysis (surface analysis) by energy dispersive X-ray analysis (EDX) were performed. The devices and measurement conditions used are as follows.

[0029] (SETM Observation) · Scanning transmission electron microscope: manufactured by Hitachi High-Technologies Corporation, HD-2700 · Observation conditions: acceleration voltage: 200 kV

[0030] (EDX Elemental Analysis (Surface Analysis)) · Scanning transmission electron microscope: manufactured by Hitachi High-Technologies Corporation, HD-2700 · Acceleration voltage: 200 kV ​​​​​​​​​​​​​​​​To measure the particle size of the silver particles in the catalysts synthesized in Examples 1 and 2, the catalysts were observed with a STEM (scanning transmission electron microscope (HD-2700, manufactured by Hitachi High-Technologies Corporation)), and particle size analysis was performed based on the obtained STEM images. For this particle size analysis, the results of the STEM observation were used to perform length measurement and particle size analysis on 100 silver particles observed in the catalyst for each example. The results of the particle size analysis are shown in Table 1.

[0033] [Table 1]

[0034] <Manganese oxide particle size analysis> To measure the particle size of the manganese oxide particles in the catalysts synthesized in Examples 1 and 2, the catalysts were observed with a STEM (scanning transmission electron microscope (HD-2700, manufactured by Hitachi High-Technologies Corporation)) and particle size analysis was performed based on the obtained STEM images. The results showed that the particle size of the manganese oxide particles was within the range of 5 to 100 nm.

[0035] <xafs eels> The catalysts synthesized in Examples 1 and 2 were subjected to XAFS (X-ray Absorption Fine Structure) and STEM-EELS (Electron Energy-Loss Spectroscopy) measurements under the following measurement conditions to evaluate the valence of Mn. The apparatus and measurement conditions used are as follows:

[0036] (XAFS measurement conditions) Experimental facility: Aichi Synchrotron Light Center (AichiSR) Experimental station: BL11S2 ·Spectrometer: Si(111)2 crystal spectrometer Absorption edge: Mn K (6539 eV) absorption edge Detection method: Transmission method Detector: Ion chamber

[0037] (STEM-EELS measurement conditions) ·Analyzer: Atomic resolution analytical electron microscope (manufactured by JEOL, JEM-ARM200F) EELS(Electron Energy-Loss Spectroscopy) Gatan Imaging Filter:GIF Quantum Acceleration voltage: 200kV, Beam spot diameter: approx. 0.20 nm

[0038] The results of XAFS and STEM-EELS measurements revealed that the manganese oxide contained in the catalysts synthesized in Examples 1 and 2 was composed mainly of Mn2O3, with Mn3O4 mixed in. The average valence of Mn in the manganese oxide in Examples 1 and 2 was calculated from the abundance ratio of Mn2O3 and Mn3O4 in the obtained results, and was 2.9 in both cases.

[0039] <Evaluation of catalytic activity> The catalysts synthesized in Examples 1 to 3 were evaluated for catalytic performance in the oxygen reduction reaction (ORR) by a rotating ring disk electrode (RRDE) measurement method using the following measurement equipment, electrodes, and electrolyte. (Measuring equipment, etc.) Measurement equipment: BAS rotating ring-disk electrode device Electrode: BAS glassy carbon (GC) electrode ·Electrolyte: 4.0M KOH aqueous solution

[0040] Nafion with a polymer concentration of 5 wt% TM 4 mL of PFSA polymer dispersion (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) was mixed with 2 mL of NaOH / ethanol solution containing 0.1 mol% sodium hydroxide, and the mixture was diluted with ethanol to prepare 100 mL of Nafion TM Nafion / ethanol solution was prepared. TM 50 mg of the catalyst obtained in each example and 10 mg of acetylene black were mixed with 5 mL of the ethanol solution to obtain a test solution. 20 μL of this test solution was cast onto a disk electrode and dried, after which convective voltammetry measurements were performed in an oxygen atmosphere at a rotation speed of 1600 rpm and a chiller temperature of 25°C. The onset potential was determined from the relationship between the potential and current density relative to the hydrogen electrode (RHE). The onset potential was measured when the current density was -0.05 mA / cm. 2 The results are shown in Table 2.

[0041] [Table 2]

[0042] The results shown in Table 2 indicate that the catalysts synthesized in Examples 1 and 2, which are working examples, have a higher onset potential and therefore are closer to the redox potential of water (1.23 V vs. RHE) than the catalyst synthesized in Comparative Example 3. In other words, the electrode activity or catalytic activity of the air electrode is higher when the overpotential is smaller during both charge and discharge, relative to the redox potential of water (1.23 V vs. RHE). Furthermore, since ORR catalysts (oxygen reduction reaction catalysts) generally have a reaction onset potential lower than 1.23 V vs. RHE, the closer the onset potential is to the reference 1.23 V vs. RHE (i.e., the higher the onset potential), the higher the ORR catalytic activity. Therefore, the catalysts of Examples 1 and 2 have higher ORR catalytic activity than the catalyst of Example 3 (Comparative Example).< / xafs>

Claims

1. Manganese oxide, Silver nanoparticles having a diameter of 1 to 50 nm supported on the surface of the manganese oxide; An oxygen reduction reaction catalyst comprising:

2. 2. The oxygen reduction reaction catalyst according to claim 1, wherein the average valence of Mn in the manganese oxide is within a range of 2.05 to 3.

0.

3. The manganese oxide is Mn ２ O ３ , Mn ３ O ４ 3. The oxygen reduction reaction catalyst according to claim 1, wherein the catalyst is a hydroxybenzoate, a hydroxybenzoate, or a mixture thereof.

4. 3. The oxygen reduction reaction catalyst according to claim 1, wherein the manganese oxide is in the form of particles having a diameter of 5 to 100 nm.

5. 3. The oxygen reduction reaction catalyst according to claim 1, wherein the silver nanoparticles are present in the form of metallic Ag.

6. An electrode for oxygen reduction, comprising the oxygen reduction reaction catalyst according to claim 1 or 2.

7. A battery comprising the oxygen reduction reaction catalyst according to claim 1 or 2.

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