JP7327798B2 - Cathode catalyst for metal-air battery, positive electrode for metal-air battery and metal-air battery - Google Patents

Description

The present invention relates to a positive electrode catalyst for metal-air batteries, a positive electrode for metal-air batteries, and a metal-air battery.

A metal-air battery, which is one of the candidates for innovative storage batteries, is a secondary battery that uses oxygen in the air as a positive electrode active material and a metal material for the negative electrode. Oxygen is an element that is unlikely to cause problems such as resource supply, cost, and effects on the human body. In addition, oxygen has a high oxidizing power, and high energy density is expected for metal-air batteries.

By the way, in the air electrode of a metal-air battery, hydroxide ions are generated by a four-electron reduction reaction of oxygen (active material) during discharge, while oxygen is generated by a four-electron oxidation reaction of hydroxide ions during charging. . Oxygen reduction reaction (hereinafter sometimes referred to as "ORR") and oxygen evolution reaction (hereinafter sometimes referred to as "OER") accompanied by transfer of these four electrons are kinetically very slow reactions. Therefore, a large overvoltage occurs during charging and discharging, and a highly active positive electrode catalyst capable of promoting ORR/OER is required.

In recent years, perovskite (ABO ₃ )-type transition metal oxides have been developed as positive electrode catalysts (Non-Patent Documents 1 and 2).

J. Suntivich, H.; A. Gasteiger, N.; Yabuuchi, H.; Nakanishi, J.; B. Goodenough, Y.; S. - Horn, Nat. Chem. , 3, 546-550 (2011). J. Suntivich, K.; J. May, H. A. Gasteiger, J.; B. Goodenough, Y.; S. - Horn, Science, 334, 1383-1385 (2011).

Metal-air batteries are required to have further improved input characteristics.

Accordingly, one object of the present invention is to provide a novel positive electrode catalyst for a metal-air battery that can improve the input characteristics of the metal-air battery.

As a result of intensive studies, the present inventors have found that a melilite-type composite oxide containing Fe, Co, and Ni tends to exhibit excellent OER activity as a positive electrode catalyst for metal-air batteries, and that this composite oxide is used as a positive electrode. The inventors have found that the input characteristics of a metal-air battery can be improved by using it as a catalyst, and have completed the present invention.

One aspect of the present invention relates to a positive electrode catalyst for metal-air batteries, comprising a melilite-type composite oxide represented by the following formula (1).
(Ba _w Sr _1-w ) ₂ Fe _{2-2 x} (Co _y Ni _1-y ) _x (Si _z Ge _1-z ) _1+x O ₇ (1)
[In formula (1), w, x, y and z satisfy 0≤w≤1, 0<x<1, 0<y<1 and 0≤z≤1, respectively. ]

According to the positive electrode catalyst of the aspect described above, it is possible to improve the input characteristics of the metal-air battery. That is, by using the positive electrode catalyst of the aspect described above for the positive electrode (air electrode) of the metal-air battery, a metal-air battery having excellent input characteristics can be obtained. In addition, the positive electrode catalyst of the aspect described above tends to be excellent in ORR activity, and the use of the positive electrode catalyst tends to improve the output characteristics of the metal-air battery. Furthermore, by using the positive electrode catalyst of the aspect described above, the life characteristics of the metal-air battery tend to be improved.

x in formula (1) may satisfy 0.5≦x≦0.9.

z in formula (1) may be less than one.

z in Formula (1) may be 0.1 or less.

Another aspect of the present invention relates to a positive electrode for a metal-air battery comprising a catalyst layer containing the positive electrode catalyst for a metal-air battery according to the aspect described above.

The positive electrode for a metal-air battery according to the above aspect may further include a positive electrode diffusion layer.

Another aspect of the present invention relates to a metal-air battery comprising the positive electrode for a metal-air battery according to the aspect described above, a negative electrode, and an electrolyte.

ADVANTAGE OF THE INVENTION According to this invention, the novel positive electrode catalyst for metal air batteries which can improve the input characteristic of a metal air battery can be provided.

FIG. 3 is a schematic cross-sectional view showing an experimental cell used for performance evaluation of Examples.

In this specification, a numerical range indicated using "to" indicates a range including the numerical values before and after "to" as the minimum and maximum values, respectively. When the upper limit value and lower limit value of a parameter in this specification are respectively described stepwise, the upper limit value and lower limit value of a step can be arbitrarily combined. Further, in this specification, the term "layer" includes the case where the layer is formed in the entire region when the region where the layer is present is observed, and the case where the layer is formed only in a part of the region. Also included if any.

Preferred embodiments of the present invention are described below. However, the present invention is by no means limited to the following embodiments.

<Positive catalyst for metal-air battery>
The positive electrode catalyst of one embodiment is a catalyst (positive electrode catalyst for metal-air batteries) used to promote an oxygen reaction (especially an oxygen evolution reaction) at the positive electrode of a metal-air battery, and is represented by the following formula (1): A melilite-type composite oxide (hereinafter also referred to as “composite oxide (1)”) is provided.
(Ba _w Sr _1-w ) ₂ Fe _{2-2 x} (Co _y Ni _1-y ) _x (Si _z Ge _1-z ) _1+x O ₇ (1)
[0≤w≤1, 0<x<1, 0<y<1, 0≤z≤1]

Since the positive electrode catalyst of the present embodiment includes the composite oxide (1), it tends to be excellent in OER activity, and according to the positive electrode catalyst, the input characteristics of the metal-air battery can be improved. This positive electrode catalyst tends to be excellent in ORR activity, and the use of the positive electrode catalyst tends to improve output characteristics during discharge. Furthermore, there is a tendency to obtain the effect of maintaining excellent input/output characteristics even when charging and discharging are repeated. That is, according to the positive electrode catalyst of the present embodiment, there is a tendency to obtain excellent life characteristics.

In general, a "melilite-type compound" means A ₂ MM' ₂ O ₇ (A is a cation of Groups 1 to 3, M and M' are transition metals or non-transition metals having a valence of 2 or higher, and M and M' All of the transition metals are arranged at the tetracoordination sites. Melilite-type composite oxides have a smaller number of oxide ions to be coordinated with transition metal ions than perovskite-type composite oxides used as positive electrode catalyst materials for existing metal-air batteries. Thus, since the oxide ions are loosely coordinated with the transition metal ions, the melilite-type composite oxide has a higher adsorption capacity, which is the starting point of the catalytic reaction, compared to the perovskite-type composite oxide. It is thought that this tends to exhibit superior OER activity and ORR activity.

The composite oxide (1) has a chemical composition different from that of the melilite-type compound, but has a melilite-type structure. In particular, one of the characteristics of the composite oxide (1) is that it contains Ni. Ni is arranged at the 6-coordinated octahedral site or the planar 4-coordinated site in almost all crystalline oxides. Therefore, the fact that a compound containing Ni can be synthesized from a composite oxide having a melilite structure in which only four-coordinated tetrahedral sites are present is an unexpected result, which the present inventors have discovered for the first time. .

Furthermore, composite oxide (1) tends to exhibit superior OER activity and ORR activity compared to other melilite-type composite oxides. Although the reasons for this are not clear, (i) the melilite structure has high flexibility in terms of chemical composition, and the three transition metals of Fe, Co, and Ni can be dissolved in a wide composition range; and (ii) a large synergistic effect among these three transition metals is considered to be the reason.

Moreover, the composite oxide (1) has excellent chemical stability, and is difficult to dissolve in an alkaline solution, for example, when immersed in an alkaline solution. This is because the composite oxide (1) contains Si ions or Ge ions that have an oxidation number of +4 and are very stable.

w in Expression (1) satisfies 0≦w≦1. That is, the ratio of Ba to Sr is w:1-w. w may be an integer or a fractional number. w may be 0 or greater than 0, and may be 1 or less than 1; That is, the composite oxide (1) may contain only one of Ba and Sr, or may contain both. w may be 0.5 or less, 0.2 or less, 0.1 or less, or 0. w may be 0.5 or greater, 0.7 or greater, 0.9 or greater, or 1.0.

x in Formula (1) is 0<x<1. That is, the composite oxide (1) contains Fe, Co, and Ni, and also contains Si and/or Ge. x may be an integer or a decimal number. From the viewpoint of obtaining better OER activity and better input characteristics, x is preferably 0.2 or more, more preferably 0.4 or more, still more preferably 0.5 or more, and particularly preferably is greater than or equal to 0.6. From the viewpoint of obtaining better OER activity and better input characteristics, x is preferably 0.97 or less, more preferably 0.95 or less, still more preferably 0.9 or less, and particularly preferably is 0.8 or less. From these viewpoints, x preferably satisfies 0.2 ≤ x ≤ 0.97, more preferably satisfies 0.4 ≤ x ≤ 0.95, and satisfies 0.5 ≤ x ≤ 0.9 is more preferable, and it is particularly preferable to satisfy 0.6≦x≦0.8.

y in Equation (1) satisfies 0<y<1. That is, the ratio of Co to Ni is y:1-y. y may be an integer or a fractional number. y may be 0.1 or more, 0.2 or more, 0.3 or more, 0.4 or more, 0.5 or more, or 0.6 or more, and 0.9 or less, 0.8 or less, 0.7 0.6 or less, 0.5 or less, or 0.4 or less.

When the composite oxide (1) contains Ba (that is, w is greater than 0), y is preferably 0.4 or more, and more Preferably it is 0.5 or more. When the composite oxide (1) contains Sr (that is, w is less than 1) and x is 0.7 or more (for example, 0.7 ≤ x ≤ 0.9), the OER activity is better, and the From the viewpoint of obtaining input characteristics, y is preferably 0.5 or more, more preferably 0.6 or more, and still more preferably 0.7 or more. Better OER activity and better input From the viewpoint of obtaining properties, y is preferably 0.5 or less, more preferably 0.4 or less, and still more preferably 0.35 or less.

z in Formula (1) satisfies 0≦z≦1. That is, the ratio of Si to Ge is z:1-z. z may be an integer or a fractional number. z may be 0 or greater than 0, and may be 1 or less than 1. That is, the composite oxide (1) may contain either one of Si and Ge, or both. From the viewpoint of better OER activity and better input characteristics, z is preferably less than 1, more preferably 0.7 or less, even more preferably 0.5 or less, and particularly preferably 0. .2 or less, and most preferably 0.1 or less. On the other hand, industrially, the larger z is more advantageous.

The shape of the composite oxide (1) is not particularly limited, and can be appropriately selected according to the specifications of the metal-air battery to be used. The shape of the composite oxide (1) is, for example, particulate or bulk, preferably particulate.

The particle size of the composite oxide (1) is not particularly limited, and may be 0.01 μm or more, 0.02 μm or more, 1 μm or more, or 2 μm or more, and may be 50 μm or less, 20 μm or less, 1 μm or less, or 0.5 μm or less. It's okay. In addition, the particle size in this specification is the volume average particle size measured by a transmission electron microscope (TEM).

Although the specific surface area of the composite oxide (1) is not particularly limited, it is preferably 0.1 m ² /g or more, more preferably 0.5 m 2 /g or more, still more preferably 0.5 m ² /g or more from the viewpoint of better catalytic activity. It is 0.7 m ² /g or more, and particularly preferably 1 m ² /g or more. The specific surface area of the composite oxide (1) may be 100 m ² /g or less, preferably 10 m 2 /g or less, more preferably 10 m ² /g or less from the viewpoint of durability (for example, difficulty in dissolving in alkali). 9 m ² /g or less. In addition, the specific surface area in this specification refers to pretreatment of the sample using a pretreatment device (manufactured by Microtrac BEL, BELPREP-vac II), and then the sample after the treatment. (manufactured by MicrotracBEL, BELSORP-miniII) and measured by the BET method.

The positive electrode catalyst of the present embodiment may consist of only one type of composite oxide (1), or may include a plurality of types of composite oxides (1). Moreover, the positive electrode catalyst of the present embodiment may further comprise a composite oxide other than the composite oxide (1). For example, a composite oxide excellent in ORR activity and a composite oxide excellent in OER activity can be combined to obtain a positive electrode catalyst excellent in both ORR activity and OER activity. The content of the composite oxide (1) in the positive electrode catalyst is 50% by mass or more, 70% by mass or more, 90% by mass or more, 97% by mass or more, or 100% by mass, based on the total mass of the positive electrode catalyst. good.

The method for producing the composite oxide (1) in the positive electrode catalyst of the above embodiment is not particularly limited, and various methods for producing ceramic materials can be used. For example, a complex polymerization method, a liquid phase method such as a hydrothermal synthesis method, a solid phase method such as a sintering method, and the like can be used. Among them, the liquid-phase method makes it possible to obtain particles with high chemical uniformity even at low-temperature sintering. Obtainable.

Composite oxide (1) can be synthesized, for example, by an amorphous metal complex method. According to this method, the sintering temperature can be lowered as compared with the solid phase method. Therefore, this method is excellent in energy cost for producing a composite oxide. In this method, first, a metal source is added to and dissolved in pure water so that the stoichiometric ratio of the metals contained in the target product is the same, citric acid is added, and the raw material solution is stirred so as to be uniform. is obtained (solution preparation step). Next, the raw material solution is heated and concentrated to produce a citric acid gel (gelation step). Thereafter, the citric acid gel is heat-treated to decompose the organic matter, thereby obtaining a powdery precursor (precursor preparing step). Composite oxide (1) is obtained by pulverizing this precursor (pulverizing step) and firing (firing step). However, the pulverization step is an optional step, and it is not always necessary to pulverize the precursor.

The Sr source, Ba source, Fe source, Co source and Ni source used in the solution preparation step are not particularly limited, and for example nitrates or acetates of these metals can be used.

The Ge source used in the solution preparation step is not particularly limited, and germanium oxide and/or germanium complex can be used, for example. Examples of germanium complexes that can be used include citric acid complexes, glycolic acid complexes, lactic acid complexes, malic acid complexes, malonic acid complexes, fumaric acid complexes, maleic acid complexes, and the like. As the germanium complex, a complex of a chelating agent having a carboxy group (--COOH) and a hydroxyl group (--OH) and having a plurality of these functional groups can also be used. In such a chelating agent, an ion obtained by deprotonating a carboxy group and a hydroxyl group in the molecule easily coordinates with a cation. ) and has a high ability to form complexes. The chelating agent is not limited to such chelating agents, and other chelating agents can be used as long as they form a complex with germanium and the germanium complex is soluble in water.

Germanium (IV) oxide is usually used as a starting material for germanium compounds in the solid-phase method. On the other hand, since germanium (IV) oxide is insoluble in water, it is not particularly suitable as a starting material for a liquid phase method using water as a solvent. Germanium (IV) oxide is soluble in a strong base aqueous solution. , potassium, etc., and there is a possibility that unintended metal ions are contained in the product. Germanium (IV) chloride can also be used as a starting material in the liquid phase method. Germanium (IV) chloride is also soluble in glycol, but germanium (IV) oxide may precipitate, and water cannot be used as a main solvent. On the other hand, by using the water-soluble germanium complex described above, a uniform and stable aqueous solution of the germanium source can be obtained, and a uniform liquid-phase synthesis of the germanium compound becomes possible. As a result, it is possible to provide a process for low-temperature synthesis of melilite-type composite oxides.

Among the germanium complexes described above, it is preferable to use a citric acid complex from the viewpoint of cost, solubility in water, and the like. The citric acid complex can be prepared by dissolving germanium oxide in an aqueous citric acid solution.

The Si source used in the solution preparation step is not particularly limited, and glycol-modified silanes such as propylene glycol-modified silane, ethylene glycol-modified silane, and polyethylene glycol-modified silane can be used. Such glycol-modified silanes can be prepared by mixing tetraalkoxysilanes such as tetramethoxysilane, tetraethoxysilane, tetrapropoxysilane, and tetraisopropoxysilane with glycol and hydrochloric acid (catalyst). can. A more detailed preparation method is disclosed, for example, in JP-A-2010-7032, and therefore will not be described here. As the tetraalkoxysilane, it is preferable to use tetramethoxysilane from the viewpoint of miscibility with other metal sources.

The amount of citric acid added to the raw material solution used in the solution preparation step is preferably 3 to 5 times the molar ratio of all metal ions in the raw material solution. Thereby, the gel can be efficiently generated in the subsequent gelation step.

The method of heating and concentrating in the gelation step is not particularly limited, and for example, a constant temperature bath or a constant temperature furnace can be used. The temperature for the heat concentration is not particularly limited, but for example, heating at 80° C. or higher and 150° C. or lower is preferable, and heating at 90° C. or higher and 140° C. or lower is more preferable.

The temperature of the heat treatment in the precursor preparation step is not particularly limited as long as it is a temperature at which the organic substance decomposes. More preferably, the temperature is at least 500° C. and not more than 500° C.

In the pulverization step, a conventionally known pulverizer can be used. The pulverization step can be carried out so that the particle size of the composite oxide (1) becomes the particle size described above.

The firing temperature in the firing step is not particularly limited, and is preferably, for example, 800° C. or higher and 1200° C. or lower, more preferably 850° C. or higher and 1150° C. or lower, and even more preferably 900° C. or higher and 1100° C. or lower. .

The positive electrode catalyst described above can be widely applied to air electrodes used in known metal-air batteries.

<Air electrode and metal-air battery>
An example of a positive electrode (air electrode) for a metal-air battery using the positive electrode catalyst of the above-described embodiment and a metal-air battery including the air electrode will be described below.

A metal-air battery includes at least an air electrode (positive electrode), a negative electrode, and an electrolyte. In a metal-air battery, an electrolyte is disposed at least between the air electrode and the negative electrode, thereby ionic conduction between the air electrode and the negative electrode is performed. A metal-air battery may be composed of one cell (single cell) including at least an air electrode (positive electrode), a negative electrode, and an electrolyte, or may be composed of a plurality of cells.

The metal-air battery may further include a separator between the air electrode and the negative electrode to prevent short circuits. A separator is not a required component.

The air electrode, negative electrode, electrolyte and separator may be housed in a housing (battery case). Specific examples of the shape of the housing include a coin shape, a flat plate shape, a cylindrical shape, a laminate shape, and the like. The housing may be an open-air battery case or a closed battery case. An air-open type battery case is a battery case having a structure that allows at least the air electrode to sufficiently contact the air. On the other hand, when the housing is a sealed battery case, it is preferable to provide a gas (air) introduction pipe and an exhaust pipe in the sealed battery case. In this case, the gas to be introduced and exhausted preferably has a high oxygen concentration, more preferably pure oxygen. Moreover, it is preferable to increase the oxygen concentration during discharging and decrease the oxygen concentration during charging.

A metal-air battery can be obtained by, for example, placing an air electrode in a housing, injecting an electrolyte into the housing, and immersing the negative electrode and the separator in the electrolyte.

Each component of the metal-air battery will be described below.

(air electrode)
The air electrode includes a current collector (air electrode current collector), and a catalyst layer and a gas diffusion layer provided on the current collector. However, the gas diffusion layer is not an essential component.

The arrangement of the cathode current collector, the catalyst layer and the gas diffusion layer is not particularly limited, but typically the gas diffusion layer is on the outside air side and the catalyst layer is on the electrolyte side. In addition, the air electrode current collector may be arranged so as to be closest to the outside air (that is, on the opposite side of the gas diffusion layer from the catalyst layer), or may be arranged between the catalyst layer and the gas diffusion layer. good.

The air electrode current collector is made of, for example, a conductive metal material such as stainless steel, copper, or nickel. The shape of the current collector is not particularly limited, and is, for example, a mesh shape. A lead (air electrode lead) is connected to the air electrode current collector. An air electrode terminal (positive electrode terminal) is provided at the tip of the air electrode lead.

The catalyst layer contains the positive electrode catalyst of the above embodiment. The catalyst layer may further contain a conductive material. It is preferable that the positive electrode catalyst and the conductive material are uniformly dispersed in the catalyst layer. In the catalyst layer, the positive electrode catalyst may be supported on a conductive material.

The content of the positive electrode catalyst in the catalyst layer is preferably 20% by mass or more, more preferably 40% by mass, based on the total mass of the catalyst layer, from the viewpoint of better balance between life characteristics and input/output characteristics. % or more, more preferably 60 mass % or more. The content of the positive electrode catalyst in the catalyst layer is preferably 90% by mass or less, more preferably 80% by mass, based on the total mass of the catalyst layer, from the viewpoint of better balance between life characteristics and input/output characteristics. % or less, more preferably 70 mass % or less. In the present embodiment, the content of the composite oxide (1) is preferably within the above range based on the total mass of the catalyst layer.

Examples of conductive materials include graphite, carbon black, and ketjen black. When graphite, carbon black, ketjen black, or the like is used as the conductive material, it is possible to achieve both longevity characteristics and input/output characteristics at a high level while reducing manufacturing costs.

The content of the conductive material in the catalyst layer is preferably 30% by mass or more, more preferably 50% by mass or more, based on the total mass of the catalyst layer, from the viewpoint of further improving the input/output characteristics, More preferably, it is 60% by mass or more. The content of the conductive material in the catalyst layer is preferably 30% by mass or less, more preferably 15% by mass or less, based on the total mass of the catalyst layer, from the viewpoint of further improving life characteristics. Preferably, it is 5% by mass or less.

The catalyst layer may further contain components other than the positive electrode catalyst and the conductive material. Other components include, for example, a binder and the like.

Examples of the binder include fluorine-based binders, and polytetrafluoroethylene (PTFE) is preferably used. The content of the binder may be, for example, 1% by mass or more and 30% by mass or less based on the total mass of the catalyst layer.

The thickness of the catalyst layer may be, for example, 0.5 μm or more and 500 μm or less.

The gas diffusion layer is electrically conductive and has the function of diffusing oxygen gas used in the reaction in the catalyst layer. Therefore, when the air electrode has a gas diffusion layer, the efficiency of supplying the gas to the catalyst layer is improved, and the battery characteristics (for example, input/output characteristics) tend to be improved.

The gas diffusion layer includes, for example, an electrically conductive material. Examples of the conductive material include the conductive materials used for the catalyst layer described above. The content of the conductive material in the gas diffusion layer may be, for example, 65% by mass or more and 99% by mass or less based on the total mass of the gas diffusion layer.

The gas diffusion layer may further contain other components such as a binder that may be contained in the catalyst layer. Examples and preferred aspects of these components are the same as for the catalyst layer. The content of the binder may be, for example, 5% by mass or more and 35% by mass or less based on the total mass of the gas diffusion layer.

The thickness of the gas diffusion layer may be, for example, 0.5 μm or more and 500 μm or less.

The sum of the thickness of the catalyst layer and the thickness of the gas diffusion layer may be, for example, 1 μm or more and 1000 μm or less.

The air electrode can be produced, for example, as follows.

First, a conductive material, a dispersion solvent, and a binder are mixed in a mortar to prepare a composition for forming a gas diffusion layer (for example, a slurry composition). Examples of dispersion solvents include water and alcohols such as ethanol. The contents of the conductive material and binder in the gas diffusion layer-forming composition may be adjusted so that the contents of each component in the gas diffusion layer are within the ranges described above.

A dispersant may be further added during the preparation of the gas diffusion layer-forming composition. The dispersant is not particularly limited as long as it can impart dispersion stability to the conductive material. For example, nonionic surfactants such as octylphenol ethoxylate can be used. Triton X-100 is preferably used as the octylphenol ethoxylate. The amount of the dispersant added may be, for example, 10 parts by mass or more and 60 parts by mass or less with respect to 100 parts by mass of the conductive material.

Also, a composition for forming a catalyst layer (for example, an ink composition) is produced by mixing a positive electrode catalyst, a conductive material, and a binder. In preparing the composition for forming a catalyst layer, the positive electrode catalyst may be used as it is, or may be used after being supported on a carrier. That is, a supported catalyst containing a carrier and a positive electrode catalyst supported on the carrier may be used. The carrier in the supported catalyst may be the electrically conductive material described above.

The contents of the positive electrode catalyst, the conductive material and the binder in the composition for forming the catalyst layer may be adjusted so that the contents of each component in the catalyst layer are within the ranges described above.

A dispersant may be further added at the time of preparing the composition for forming a catalyst layer. The dispersant is not particularly limited as long as it can impart dispersion stability to the positive electrode catalyst and/or the conductive material. For example, nonionic surfactants such as octylphenol ethoxylate can be used. Triton X-100 is preferably used as the octylphenol ethoxylate. The amount of the dispersant added may be, for example, 5 parts by mass or more and 40 parts by mass or less with respect to the total of 100 parts by mass of the positive electrode catalyst and the conductive material.

Then, a series of operations of stretching and folding the obtained composition for forming a gas diffusion layer is repeated 10 to 40 times. A general method can be used for stretching, and there is no particular limitation as long as the method can smoothly stretch the composition for forming a gas diffusion layer. For example, a method of stretching with an acrylic cylinder is suitable. Then, it is rolled by a roll press to a specified thickness, cut, and dried on a hot plate heated to 150 to 170° C. for 1 to 2 hours. A gas diffusion layer sheet is thus obtained. The diffusion layer sheet thus obtained has a plate shape as a whole.

Next, a composition for forming a catalyst layer is applied to the obtained gas diffusion layer sheet by, for example, a spray method, and heat-treated in an atmosphere of an inert gas such as argon to form a gas diffusion layer and the gas diffusion layer. An air electrode laminate having a catalyst layer laminated thereon is obtained. The heat treatment is performed, for example, at 300-350° C. for 10-20 minutes.

Then, the air electrode is obtained by joining the air electrode current collector to the air electrode laminate by pressing or the like.

The method for manufacturing the air electrode is not limited to the above method. It can also be produced by the method described in JP-A-2014-49304).

(negative electrode)
A negative electrode of a general metal-air battery can be used for the negative electrode. The negative electrode includes, for example, a current collector (negative electrode current collector) and a negative electrode active material layer provided on the current collector. A lead (negative lead) is connected to the negative electrode current collector. A negative terminal is provided at the tip of the negative lead.

The negative electrode current collector is made of, for example, a conductive metal material such as stainless steel, copper, or nickel. The shape of the current collector is not particularly limited, and is, for example, a mesh shape.

The negative electrode active material layer contains a negative electrode active material. Examples of the negative electrode active material include simple metals, alloys, and compounds containing base metals with high reducing power (for example, metals whose standard electrode potential is less base than hydrogen) as elements. In addition to zinc, base metals include, for example, lithium (Li), zinc (Zn) and iron (Fe).

(Electrolytes)
The electrolyte is not particularly limited, and known electrolytes can be used according to the type of negative electrode active material used for the negative electrode. The form of the electrolyte is also not particularly limited, and may be a liquid electrolyte, a gel electrolyte, a solid electrolyte, or the like. Specifically, for example, a potassium hydroxide aqueous solution is preferably used. The hydroxide ion concentration ([OH-]) is preferably 1 to 10 mol/L or more.

(separator)
The structure and material of the separator are not particularly limited as long as they do not inhibit metal ion conduction between the air electrode and the negative electrode and can suppress short-circuiting between the two electrodes. Specifically, for example, a porous film or resin nonwoven fabric made of a resin material such as polyolefin such as polyethylene and polypropylene, fluororesin such as polytetrafluoroethylene and polyvinylidene fluoride, and the like can be mentioned.

In the metal-air battery described above, the air electrode terminal and the negative electrode terminal are used to input and output current during charging and discharging. Since the metal-air battery includes the air electrode containing the positive electrode catalyst of the above embodiment, it tends to exhibit excellent input characteristics, excellent output characteristics, and excellent life characteristics.

EXAMPLES The content of the present invention will be described in more detail below using examples and comparative examples, but the present invention is not limited to the following examples.

<Preparation of positive electrode catalyst>
A sample as a positive electrode catalyst was prepared by the method shown below. The following materials were used as raw materials. Germanium citrate, which is a Ge source, was prepared by dissolving GeO ₂ (99.99%, Kojundo Chemical Laboratory) in an aqueous citric acid solution.
Ba source: Ba(CH ₃ COO) ₂ (purity 99.9%, Wako Pure Chemical Industries)
Sr source: SrNO ₃ (purity 99.5%, Wako Pure Chemical Industries)
Co source: Co(CH ₃ COO) ₂ 4H2O (purity 99%, Wako Pure Chemical Industries)
Fe source: Fe(NO ₃ ) _{3.9H 2} O (purity 99.9%, Wako Pure Chemical Industries)
Ni source: Ni(NO ₃ ) _{2.6H 2} O (purity 99.95%, Kanto Kagaku)
Ge source: Germanium citrate La source: La(NO ₃ ) _{3.6H 2} O (99.9%, Wako Pure Chemical Industries)
Ca source: Ca(NO ₃ ) _{2.4H 2} O (99.9%, Wako Pure Chemical Industries)
Gelling agent: _C6H8O7 citric acid ( _purity 98% _, Wako Pure Chemical Industries)

(Examples 1 to 10)
In the target product (melilite type composite oxide) of the chemical formula shown in Table 1, each metal source is added to pure water so that the target product is 1 mmol at the same charging ratio as the stoichiometric ratio of the metal ions in the chemical formula. After dissolving, citric acid was added in an amount 3 times the molar amount of the total amount of cations, and the mixture was stirred to obtain a raw material solution. The raw material solution was allowed to stand in a constant temperature bath set at 120° C., and heated and concentrated. The supersaturated citric acid gel that lost fluidity and became gel was heat-treated at 450° C. to decompose the organic matter to obtain a powdery precursor. The precursor thus obtained was pulverized and calcined in air at 1000° C. for 12 hours using a box furnace. As a result, the intended product shown in Table 1 was obtained as a positive electrode catalyst for a metal-air battery.

(Comparative example 1)
Each metal source was dissolved in pure water at a charge ratio similar to the stoichiometric ratio of the metal ions of the target product so that La _0.5 Ca _0.5 CoO ₃ as the target product was 2 mmol, and citric acid was added. was added in an amount 3 times the total amount of cations and mixed by stirring so as to form a uniform solution. The mixed raw material solution was allowed to stand in a constant temperature bath set at 120° C., and heated and concentrated. The supersaturated citric acid gel, which lost fluidity and became gel, was heat-treated at 450° C. to decompose the organic matter to obtain a powdery precursor. The precursor thus obtained was pulverized and calcined in air at 1000° C. for 12 hours using a box furnace. As a result, La _0.5 Ca _0.5 CoO ₃ was obtained as a positive electrode catalyst for a metal-air battery.

<Evaluation of positive electrode catalyst: OER activity evaluation>
Using a rotating electrode device (RRDE-3A, manufactured by BAS) and a potentiostat (SP-300, manufactured by Bio-Logic), the convection voltammetry (Rotating Disk Electrode; RDE) method was used to obtain positive electrodes for metal-air batteries of Examples and Comparative Examples. The OER activity of the catalyst was evaluated. A specific evaluation method is shown below. In addition, the following were used as an electrode.
Working electrode (WE): 5 mmφ glassy carbon (glassy carbon; GC) electrode counter electrode (CE): coiled platinum (Pt) electrode Reference electrode (RE): alkaline reference electrode (Hg/HgO/4M KOH)

(Ink preparation)
First, acetylene carbon black (99.99%, STREM CHEMICALS) was ultrasonically dispersed in nitric acid for 30 minutes, and then heated and stirred at 80° C. overnight. The acetylene black was then pretreated by filtering, drying and grinding.

Next, a 5% Nafion (registered trademark) dispersion (Wako Pure Chemical Industries, Ltd.) was neutralized with a sodium hydroxide-ethanol (EtOH) solution, and the resulting neutralized solution and ethanol were mixed at a volume ratio of 3:47. A solvent for the ink was obtained.

Next, the ink solvent, acetylene black, and positive electrode catalyst obtained above were placed in a sample bottle at a ratio of 5 mL:10 mg:50 mg, and ultrasonically dispersed. As a result, an ink (an ink-like evaluation sample) was obtained.

(Application of ink to working electrode)
20 μL (catalyst amount: 0.2 mg) of an evaluation sample was dropped onto glassy carbon washed with ultrapure water and EtOH, and dried completely to apply ink onto the working electrode.

(Cyclic voltammetry measurement)
Rotate the working electrode of the rotating electrode device (RRDE-3A, manufactured by BAS) at 1600 rpm, connect to the potentiostat (SP-300, manufactured by Bio-Logic), use a 4 M KOH aqueous solution as the electrolyte, and perform cyclic voltammetry ( CV) measurements were taken. The measurement was performed in the range of 1.10 to 1.70 V (vs reversible hydrogen electrode (RHE)) at a sweep rate of 1 mV / s for 3 cycles, and the current density at 1.7 V in the 3rd cycle was compared. The OER activity of the catalyst was evaluated by Although the actually used reference electrode was a mercury-mercury oxide reference electrode, the potential was converted to the RHE reference electrode standard for convenience.

<Production of metal-air battery>
(Examples 11-12 and Comparative Example 2)
Carbon (manufactured by Tokai Carbon Co., Ltd., trade name: TOKABLACK#3855) 3.3 g, PTFE dispersion (solid content concentration: 60% by mass, manufactured by Mitsui DuPont Fluorochemical Co., trade name: PTFE31-JR) 0.909 mL, Triton 1.33 mL of X-100 (octylphenol ethoxylate) and 4.0 mL of water were mixed in a mortar to prepare a mixture slurry as a composition for forming a gas diffusion layer. A series of operations of stretching the mixture slurry obtained to a thickness of 0.5 mm and folding it in two were repeated 40 times. Next, the sheet was rolled with a roll press until the thickness became 0.35 mm, cut, and dried on a hot plate heated to 160° C. for 1 hour to obtain a gas diffusion layer sheet.

Carbon (manufactured by Tokai Carbon Co., Ltd., trade name: TOKABLACK#3855) 0.132 g, positive electrode catalyst 0.264 g, PTFE dispersion (solid content concentration: 60% by mass, manufactured by Mitsui DuPont Fluorochemical Co., trade name: PTFE31-JR ) 0.110 mL, Triton X-100 (octylphenol ethoxylate) 0.089 mL, propanol 0.381 mL, and water 2.727 mL are mixed to prepare an ink (ink-like evaluation sample) that is a composition for forming a catalyst layer. did. The catalyst shown in Table 2 was used as the positive electrode catalyst.

The gas diffusion layer sheet obtained above was coated with the ink obtained above by a spray method, and baked at 335° C. for 13 minutes under argon to obtain an air electrode laminate. A nickel mesh was press-bonded to the obtained laminate to obtain an air electrode.

Using each of the obtained air electrodes, an experimental cell shown in FIG. 1 was produced by the following method.

First, the air electrode (1) was adhered to the acrylic housing (5) provided with the opening (6) from the catalyst layer (11) side so as to block the opening (6). Next, after injecting an 8 mol/L potassium hydroxide aqueous solution as the electrolyte (4) into the casing (5), a platinum plate as the counter electrode (negative electrode) (2) and a mercury-mercury oxide electrode as the reference electrode (3). An experimental cell (10) was prepared by immersing it in the electrolyte (4) in the housing (5). A reference electrode (3) was placed between the air electrode (1) and the negative electrode (2). In addition, (12) in FIG. 1 indicates a gas diffusion layer, and (13) indicates a current collector (nickel mesh).

<Evaluation of metal-air battery>
The input/output characteristics and life characteristics of the cell produced above were evaluated by the following methods. Evaluation results are shown in Table 2.

[Evaluation of input/output characteristics]
First, in an environment of 25° C., the OER current value is 40 mA/cm ² , the ORR current value is −40 mA/cm ² , OER and ORR are each 1 hour, and the pause time between OER and ORR is 3 minutes. was repeated 100 times. After that, OER or ORR was performed at a constant current density of 40 mA/cm ² for 1 minute, and the air electrode potential after 1 minute was taken as the OER or ORR potential. The air electrode potential was defined by the potential difference between the air electrode and the reference electrode. Although the actually used reference electrode was a mercury-mercury oxide reference electrode, the potential was converted to the RHE reference electrode standard for convenience. Input/output characteristics were evaluated by comparing the OER or ORR potentials.

[Evaluation of life characteristics]
First, in an environment of 25° C., the OER current value is +40 mA/cm ² , the ORR current value is −40 mA/cm ² , OER and ORR are each 1 hour, and the rest time between OER and ORR is 3 minutes. The cycle life of the battery was defined as the time when the OER potential became 1.726 V or more or the ORR potential became 0.526 V or less.

Claims (7)

A positive electrode catalyst for a metal-air battery, comprising a melilite-type composite oxide represented by the following formula (1).
(Ba _w Sr _1-w ) ₂ Fe _{2-2 x} (Co _y Ni _1-y ) _x (Si _z Ge _1-z ) _1+x O ₇ (1)
[In formula (1), w, x, y and z satisfy 0≤w≤1, 0<x<1, 0<y<1 and 0≤z≤1, respectively. ] 2. The positive electrode catalyst for a metal-air battery according to claim 1, wherein x in said formula (1) satisfies 0.5≤x≤0.9. 3. The positive electrode catalyst for a metal-air battery according to claim 1, wherein z in the formula (1) is less than 1. The positive electrode catalyst for a metal-air battery according to any one of claims 1 to 3, wherein z in the formula (1) is 0.1 or less. A positive electrode for a metal-air battery, comprising a catalyst layer containing the positive electrode catalyst for a metal-air battery according to any one of claims 1 to 4. The positive electrode for a metal-air battery according to claim 5, further comprising a gas diffusion layer. A metal-air battery comprising the positive electrode for a metal-air battery according to claim 5 or 6, a negative electrode, and an electrolyte.

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