JP2021101416A - Positive electrode catalyst for metal-air battery, positive electrode for metal-air battery, and metal-air battery - Google Patents

Abstract

To provide a novel positive electrode catalyst for a metal-air battery capable of improving the input characteristics of a metal-air battery.SOLUTION: There is provided a positive electrode catalyst for a metal-air battery, including a melilite-type composite oxide represented by the following formula (1). (BawSr1-w)2Fe2-2x(CoyNi1-y)x(SizGe1-z)1+xO7 (1) [In the formula (1), w, x, y, and z satisfy 0≤w≤1, 0<x<1, 0<y<1, and 0≤z≤1, respectively.]SELECTED DRAWING: None

Description

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

A metal-air battery, which is one of the candidates for an innovative storage battery, 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 human impact. In addition, oxygen has a high oxidizing power, and a high energy density is expected for a metal-air battery.

By the way, in the air electrode of a metal-air battery, hydroxide ions are generated by a 4-electron reduction reaction of oxygen (active material) during discharge, while oxygen is generated by a 4-electron oxidation reaction of hydroxide ions during charging. .. The oxygen reduction reaction (hereinafter, also referred to as “ORR”) and the oxygen evolution reaction (hereinafter, also referred to as “OER”) accompanied by the transfer of these four electrons are very slow reactions in terms of kinetics. Therefore, a large overvoltage is generated during charging and discharging, so a highly active positive electrode catalyst capable of promoting ORR / OER is required.

In recent years, _{the development of perovskite (ABO 3} ) type transition metal oxides has been promoted as a positive electrode catalyst (Non-Patent Documents 1 and 2).

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

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

Therefore, one of the objects of the present invention is to provide a novel positive electrode catalyst for a metal-air battery capable of improving the input characteristics of the metal-air battery.

As a result of diligent studies, the present inventors tend to show excellent OER activity as a positive electrode catalyst for a metal-air battery in a melilite-type composite oxide containing Fe, Co, and Ni, and this composite oxide tends to be a positive electrode. We 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 a metal-air battery, which comprises a melilite type composite oxide represented by the following formula (1).
_{(Ba w Sr 1-w)} 2 Fe 2-2x (Co y Ni 1-y) x (Si z Ge 1-z) 1 + x O 7 (1)
[In the 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 on the above side surface, the input characteristics of the metal-air battery can be improved. That is, by using the positive electrode catalyst on the side surface for the positive electrode (air electrode) of the metal-air battery, a metal-air battery having excellent input characteristics can be obtained. Further, the positive electrode catalyst on the side surface tends to be excellent in ORR activity, and the output characteristics of the metal-air battery tend to be improved by using the positive electrode catalyst. Further, by using the positive electrode catalyst on the above side surface, the life characteristic of the metal-air battery tends to be improved.

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

Z in the formula (1) may be less than 1.

Z in the formula (1) may be 0.1 or less.

Another aspect of the present invention relates to a positive electrode for a metal-air battery, which comprises a catalyst layer containing the positive electrode catalyst for the metal-air battery on the above side.

The positive electrode for a metal-air battery on the side surface may further include a positive electrode diffusion layer.

Another aspect of the present invention relates to a metal-air battery including a positive electrode for a metal-air battery, a negative electrode, and an electrolyte on the above side.

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

It is a schematic cross-sectional view which shows the experimental cell used for the performance evaluation of an Example.

In the present specification, the numerical range indicated by using "~" indicates a range including the numerical values before and after "~" as the minimum value and the maximum value, respectively. When the upper limit value and the lower limit value of the parameters in the present specification are described stepwise, the upper limit value and the lower limit value of a certain step can be arbitrarily combined. Further, in the present specification, the term "layer" is used only in a part of the region in addition to the case where the layer is formed in the entire region when the region in which the layer exists is observed. It is also included if there is.

Hereinafter, preferred embodiments of the present invention will be described. However, the present invention is not limited to the following embodiments.

<Positive catalyst for metal-air batteries>
The positive electrode catalyst of one embodiment is a catalyst (positive electrode catalyst for a metal-air battery) used to promote an oxygen reaction (particularly an oxygen generation reaction) in 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 2-2x (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 have excellent OER activity, and the positive electrode catalyst can improve the input characteristics of the metal-air battery. This positive electrode catalyst tends to be excellent in ORR activity, and by using the positive electrode catalyst, the output characteristics at the time of discharge also tend to be improved. Further, there is a tendency to obtain an effect that excellent input / output characteristics are maintained even when charging / discharging is repeated. That is, according to the positive electrode catalyst of the present embodiment, excellent life characteristics tend to be obtained.

Generally, "melilite type _{compound", A 2 MM '2 O 7} (A 1-3 group cations, M and M' is a divalent or more transition metals or non-transition metals, M and M ' Refers to a group of compounds represented by (all of which are arranged at the tetracoordinated site), and all transition metals in the melilite type structure are arranged at the tetracoordinated site. The melilite-type composite oxide has a smaller number of oxide ions that coordinate transition metal ions than the perovskite-type composite oxide used as a positive electrode catalyst material for existing metal-air batteries. In this way, since the oxide ion is sparsely coordinated with the transition metal ion, the melilite-type composite oxide has a higher adsorption capacity as a starting point of the catalytic reaction than the perovskite-type composite oxide. It is considered that this tends to show excellent 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, the composite oxide (1) is characterized by having Ni, but Ni is arranged at a hexacoordinated octahedral site or a planar tetracoordinated site in almost all crystalline oxides. Therefore, it is a surprising result that a compound containing Ni can be synthesized with a composite oxide having a melilite-type structure in which only tetracoordinated tetrahedral sites are present, and this is the first discovery by the present inventors. ..

Furthermore, the composite oxide (1) tends to exhibit excellent OER activity and ORR activity as compared with other melilite-type composite oxides. The reason for this is not clear, but (i) the melilite-type structure has high flexibility in chemical composition, and three types of transition metals, Fe, Co, and Ni, can be dissolved in a wide composition range. And (ii) it is considered that the reason is that a large synergistic effect can be obtained among these three types of transition metals.

Further, the composite oxide (1) has excellent chemical stability, and is difficult to dissolve in an alkaline solution, for example, when immersed in an alkali. This is because the composite oxide (1) contains a very stable Si ion or Ge ion having an oxidation number of +4.

W in the formula (1) satisfies 0 ≦ w ≦ 1. That is, the ratio of Ba to Sr is w: 1-w. w may be an integer or a small 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 more, 0.7 or more, 0.9 or more, or 1.0.

X in the equation (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 small number. From the viewpoint that the OER activity is more excellent and more excellent input characteristics can be obtained, x is preferably 0.2 or more, more preferably 0.4 or more, still more preferably 0.5 or more, and particularly preferably. Is 0.6 or more. From the viewpoint that the OER activity is more excellent and more excellent input characteristics can be obtained, 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 0.4 ≦ x ≦ 0.95, and more preferably 0.5 ≦ x ≦ 0.9. It is more preferable, and it is particularly preferable to satisfy 0.6 ≦ x ≦ 0.8.

Y in the formula (1) is 0 <y <1. That is, the ratio of Co to Ni is y: 1-y. y may be an integer or a small 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. Hereinafter, it may be 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, from the viewpoint of obtaining better OER activity and better input characteristics. It is preferably 0.5 or more. When the composite oxide (1) contains Sr (ie, w is less than 1) and x is 0.7 or greater (eg, 0.7 ≦ x ≦ 0.9), it is better and better at OER activity. From the viewpoint of obtaining input characteristics, y is preferably 0.5 or more, more preferably 0.6 or more, and further preferably 0.7 or more. When the composite oxide (1) contains Sr (ie, w is less than 1) and x is less than 0.7 (eg, 0.5 ≤ x <7), it is better for OER activity and better input. From the viewpoint of obtaining the characteristics, y is preferably 0.5 or less, more preferably 0.4 or less, and further preferably 0.35 or less.

Z in the equation (1) satisfies 0 ≦ z ≦ 1. That is, the ratio of Si to Ge is z: 1-z. z may be an integer or a small 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 only one of Si and Ge, or may contain both. From the viewpoint that the OER activity is more excellent and more excellent input characteristics can be obtained, z is preferably less than 1, more preferably 0.7 or less, still more preferably 0.5 or less, and particularly preferably 0. It is .2 or less, and extremely preferably 0.1 or less. On the other hand, industrially, the larger z is, the more advantageous it is.

The shape of the composite oxide (1) is not particularly limited, and can be appropriately selected depending on 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 is 50 μm or less, 20 μm or less, 1 μm or less or 0.5 μm or less. It may be there. The particle size in the present specification is a volume average particle size measured by a transmission electron microscope (TEM).

The specific surface area of the composite oxide (1) is not particularly limited, but is preferably 0.1 m ² / g or more, more preferably 0.5 m ² / g or more, and further preferably 0.5 m 2 / g or more from the viewpoint of being superior in 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, and is 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). It is 9 m ² / g or less. The specific surface area in the present specification refers to a specific surface area / pore distribution measuring device for a sample after pretreatment of a sample using a pretreatment device (manufactured by Microtrac BEL, BELPREP-vacII). A value measured by the BET method using (manufactured by Microtrac BEL, manufactured by BELSORP-miniII).

The positive electrode catalyst of the present embodiment may consist of only one type of composite oxide (1), and may include a plurality of types of composite oxides (1). Further, the positive electrode catalyst of the present embodiment may further include a composite oxide other than the composite oxide (1). For example, a positive electrode catalyst having both excellent ORR activity and OER activity can be obtained by combining a composite oxide having excellent ORR activity and a composite oxide having excellent 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 a ceramic material can be used. For example, a liquid phase method such as a complex polymerization method or a hydrothermal synthesis method, a solid phase method such as a sintering method, or the like can be used. Of these, according to the liquid phase method, particles with high chemical uniformity can be obtained even by low-temperature firing, and as a result, a positive electrode catalyst exhibiting higher ORR activity and OER activity with a small particle size and a high specific surface area can be obtained. Obtainable.

The composite oxide (1) can be synthesized, for example, by the amorphous metal complex method. According to this method, the firing temperature can be lowered as compared with the solid phase method. Therefore, this method is excellent in energy cost for producing the composite oxide. In this method, first, the metal source is added and dissolved in pure water so as to be similar to the chemical ratio of the metal contained in the target product, citric acid is added and stirred so as to be uniform, and the raw material solution is prepared. (Solution preparation step). Next, the raw material solution is heated and concentrated to produce a citric acid gel (gelling step). Then, the citric acid gel is heat-treated to decompose organic substances to obtain a powder precursor (precursor preparation step). By pulverizing this precursor (pulverizing step) and calcining it (calcination step), the composite oxide (1) is obtained. However, the pulverization step is an arbitrary 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 for example, germanium oxide and / or germanium complex can be used. As the germanium complex, for example, a citric acid complex, a glycolic acid complex, a lactic acid complex, an apple acid complex, a malonic acid complex, a fumaric acid complex, a maleic acid complex and the like can be used. Further, as the germanium complex, a complex of a chelating agent having a carboxy group (-COOH) and a hydroxy group (-OH) and having a plurality of these functional groups can also be used. In such a chelating agent, an ion deprotoned by a carboxy group and a hydroxy group in the molecule is easily coordinated to a cation, and by having two or more of these functional groups, the ion is coordinated so as to sandwich the cation (chelate). ) And has a high ability to form complex. Not limited to such a chelating agent, other chelating agents can be used as long as they form a complex with germanium and the germanium complex is soluble in water.

Usually, germanium oxide (IV) is used as a starting material for the germanium compound in the solid phase method. On the other hand, germanium oxide (IV) is insoluble in water, so it is not suitable as a starting material especially in the liquid phase method using water as a solvent. Further, germanium oxide (IV) is soluble in an aqueous solution of a strong base, but even if an aqueous solution of a strong base is prepared using, for example, sodium hydroxide or potassium hydroxide and the germanium is dissolved, the raw material solution is sodium. , Potassium and other metals will be included, and there is a possibility that unintended metal ions may be contained in the product. Further, as a starting material in the liquid phase method, germanium chloride (IV) can be mentioned. This germanium chloride (IV) is also soluble in glycol, but germanium oxide (IV) may precipitate, and water cannot be used as the main solvent. On the other hand, by using the above-mentioned water-soluble germanium complex, a uniform and stable aqueous solution of a germanium source can be obtained, and a uniform liquid phase synthesis of a germanium compound becomes possible. As a result, a low temperature synthetic process of melilite-type composite oxide can be provided.

Among the above-mentioned germanium complexes, 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 for example, 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, glycol, and hydrochloric acid (catalyst). it can. A more detailed preparation method is disclosed in, for example, Japanese Patent Application Laid-Open No. 2010-7032, and thus the description thereof is omitted 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. As a result, gel can be efficiently produced in the subsequent gelation step.

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

The temperature of the heat treatment in the precursor preparation step is not particularly limited as long as it is the temperature at which the organic matter is decomposed, but is preferably 250 ° C. or higher and 600 ° C., more preferably 300 ° C. or higher and 550 ° C. or lower, and 400 ° C. It is more preferably 500 ° C. or lower.

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

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

The positive electrode catalyst described above can be widely applied to an air electrode used in a known metal-air battery, and for example, it can be applied to a bifunctional air electrode for an in-vehicle water-based air battery.

<Air electrode and metal-air battery>
Hereinafter, an example of a positive electrode (air electrode) for a metal-air battery in which the positive electrode catalyst of the above embodiment is used and a metal-air battery including the empty positive electrode will be described.

The 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 arranged at least between the air electrode and the negative electrode, so that ion conduction is performed between the air electrode and the negative electrode. The metal-air battery may be composed of at least one cell (cell battery) including 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 for suppressing a short circuit between the air electrode and the negative electrode. The separator is not an essential component.

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

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

Hereinafter, each component of the metal-air battery will be described.

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

The arrangement of the air electrode current collector, the catalyst layer and the gas diffusion layer is not particularly limited, but typically, the gas diffusion layer is arranged on the outside air side and the catalyst layer is arranged on the electrolyte side. Further, the air electrode current collector may be arranged so as to be on the outermost air side (that is, on the side opposite to the catalyst layer of the gas diffusion layer), and may be arranged between the catalyst layer and the gas diffusion layer. Good.

The air electrode current collector is made of 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 improving the balance between the life characteristics and the input / output characteristics. % Or more, more preferably 60% by 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 improving the balance between the life characteristics and the input / output characteristics. % Or less, more preferably 70% by mass or less. In the present embodiment, the content of the composite oxide (1) is preferably in the above range with reference to the total mass of the catalyst layer.

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

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, and further preferably 15% by mass or less, based on the total mass of the catalyst layer, from the viewpoint of further improving the life characteristics. It is preferably 5% by mass or less.

The catalyst layer may further contain components other than the positive electrode catalyst and the conductive material. Examples of other components include a binder and the like.

Examples of the binder include a fluorine-based binder and the like, 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 has conductivity and also has a function of diffusing oxygen gas used for the reaction in the catalyst layer. Therefore, when the air electrode includes 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 contains, for example, a conductive material. Examples of the conductive material include the conductive material 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 embodiments 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 total 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 composition for forming a gas diffusion layer (for example, a slurry-like composition) is prepared by mixing a conductive material, a dispersion solvent, and a binder in a mortar. Examples of the dispersion solvent include water, alcohols such as ethanol, and the like. The content of the conductive material and the binder in the composition for forming the gas diffusion layer may be adjusted so that the content of each component in the gas diffusion layer is within the above-mentioned range.

When preparing the composition for forming a gas diffusion layer, a dispersant may be further added. The dispersant is not particularly limited as long as it can impart dispersion stability to the conductive material, and for example, a nonionic surfactant such as octylphenol ethoxylate can be used. As the octylphenol ethoxylate, Triton X-100 is preferably used. 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.

Further, a composition for forming a catalyst layer (for example, an ink composition) is prepared by mixing a positive electrode catalyst, a conductive material, and a binder. The positive electrode catalyst may be used as it is or may be used while being supported on a carrier for producing the composition for forming a catalyst layer. 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 above-mentioned conductive material.

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 content of each component in the catalyst layer is within the above-mentioned range.

When preparing the composition for forming the catalyst layer, a dispersant may be further added. The dispersant is not particularly limited as long as it can impart dispersion stability to the positive electrode catalyst and / or the conductive material, and for example, a nonionic surfactant such as octylphenol ethoxylate can be used. As the octylphenol ethoxylate, Triton X-100 is preferably used. 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 100 parts by mass of the total of the positive electrode catalyst and the conductive material.

Next, a series of operations of stretching the obtained composition for forming a gas diffusion layer and folding it in two is repeated 10 to 40 times. A general method can be applied to the method used for stretching, and the method is not particularly limited as long as the composition for forming a gas diffusion layer can be smoothly stretched. For example, a method of stretching with an acrylic cylinder is suitable. Then, it is rolled to a specified thickness by a roll press, cut, and dried on a hot plate heated to 150 to 170 ° C. for 1 to 2 hours. As a result, a gas diffusion layer sheet is obtained. The diffusion layer sheet obtained so far has a plate shape as a whole.

Next, the 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 inert gas atmosphere such as argon to obtain a gas diffusion layer and the gas diffusion layer. An air electrode laminate having a catalyst layer laminated on top can be obtained. The heat treatment is carried out, for example, at 300 to 350 ° C. for 10 to 20 minutes.

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

The method for producing an air electrode is not limited to the above method, and for example, a method of filling an air electrode current collector with a composition for forming a gas diffusion layer and a composition for forming a catalyst layer without forming them in a sheet shape (for example, special feature). It can also be produced by the method described in Japanese Patent Application Laid-Open No. 2014-49304).

(Negative electrode)
As the negative electrode, a negative electrode of a general metal-air battery can be used. 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 electrode lead) is connected to the negative electrode current collector. A negative electrode terminal is provided at the tip of the negative electrode lead.

The negative electrode current collector is made of 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, compounds and the like containing a base metal having a high reducing power (for example, a metal having a standard electrode potential lower than that of hydrogen) as an element. Examples of the base metal include lithium (Li), zinc (Zn) and iron (Fe) in addition to zinc.

(Electrolytes)
The electrolyte is not particularly limited, and a known electrolyte can be used depending on the type of negative electrode active material used for the negative electrode. The form of the electrolyte is not particularly limited, and may be a liquid electrolyte, a gel electrolyte, a solid electrolyte, or the like. Specifically, for example, an aqueous potassium hydroxide solution is preferably used. The concentration of hydroxide ions ([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 a short circuit between the two electrodes. Specific examples thereof include a porous film or a resin non-woven fabric made of a resin material such as a polyolefin such as polyethylene and polypropylene, and a fluororesin such as polytetrafluoroethylene and polyvinylidene fluoride.

In the metal-air battery described above, the air electrode terminal and the negative electrode terminal are used to input and output a current during charging and discharging. Since the metal-air battery includes an 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.

Hereinafter, the contents of the present invention will be described in more detail with reference to 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 raw materials were used. Germanium citrate, which is a Ge source, _{was prepared by dissolving GeO 2} (99.99%, Institute of High Purity Chemistry) in an aqueous citric acid solution.
Ba source: Ba (CH ₃ COO) ₂ (purity 99.9%, Wako Pure Chemical Industries, Ltd.)
Sr source: SrNO ₃ (purity 99.5%, Wako Pure Chemical Industries, Ltd.)
Co _{source: Co (CH 3 COO) 2} · 4H2O ( purity 99%, manufactured by Wako Pure Chemical Industries)
Fe _{source: Fe (NO 3) 3 ·} 9H 2 O ( 99.9% purity, manufactured by Wako Pure Chemical Industries)
Ni _{source: Ni (NO 3) 2 ·} 6H 2 O ( purity 99.95%, Kanto Chemical Co., Inc.)
Ge source: Citric acid Germanium 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: Citric acid C ₆ H ₈ O ₇ (purity 98%, Wako Pure Chemical Industries, Ltd.)

(Examples 1 to 10)
In the target product (merylite type composite oxide) of the chemical formula shown in Table 1, each metal source is made into pure water so that the target product is 1 mmol at the same charge ratio as the chemical ratio of the metal ions in the chemical formula. After dissolution, citric acid was added in an amount of 3 times the total amount of cations and stirred to be uniform to obtain a raw material solution. The raw material solution was allowed to stand in a constant temperature bath set at 120 ° C. and concentrated by heating. The supersaturated citrate gel, which lost its fluidity and became a gel, was heat-treated at 450 ° C. to decompose organic substances to obtain a powder precursor. The precursor thus obtained was pulverized and calcined in the air at 1000 ° C. for 12 hours using a box furnace. As a result, the target products shown in Table 1 were obtained as positive electrode catalysts for metal-air batteries.

(Comparative Example 1)
Dissolve each metal source in pure water at the same charge ratio as the chemical ratio of the metal ions of the target product so that the target product La _0.5 Ca _0.5 CoO _{3 becomes 2 mmol, and citric acid.} Was added in an amount of 3 times the total amount of cations, and the mixture was stirred and mixed to obtain a uniform solution. The mixed raw material solution was allowed to stand in a constant temperature bath set at 120 ° C. and concentrated by heating. A supersaturated citric acid gel that lost its fluidity and became a gel was heat-treated at 450 ° C. to decompose organic substances to obtain a powder precursor. The precursor thus obtained was pulverized and calcined in the 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: Evaluation of OER activity>
Using a rotating electrode device (RRDE-3A, manufactured by BAS) and a potentiostat (SP-300, manufactured by Bio-Logic), a positive electrode for a metal-air battery of Examples and Comparative Examples was used by a convection voltammetry (Rotating Disk Electrode) method. The OER activity of the catalyst was evaluated. The specific evaluation method is shown below. The following electrodes were used.
Working electrode (WE): 5 mmφ glassy carbon (glassy carbon; GC) electrode pair electrode (CE): coiled platinum (Pt) electrode reference electrode (RE): alkali reference electrode (Hg / HgO / 4M KOH)

(Ink preparation)
First, acetylene black (99.99%, STREM CHEMICALS) was ultrasonically dispersed in nitric acid for 30 minutes, and then heated and stirred at 80 ° C. overnight. 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 obtained neutralized solution and ethanol were mixed at a volume ratio of 3:47. Obtained a solvent for ink.

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

(Application of ink to working electrode)
Ink was applied onto the working electrode by dropping 20 μL (catalyst amount: 0.2 mg) of an evaluation sample onto glassy carbon washed with ultrapure water and EtOH and completely drying the sample.

(Cyclic voltammetry measurement)
The working electrode of the rotating electrode device (RRDE-3A, manufactured by BAS) is rotated at 1600 rpm, connected to a potentiostat (SP-300, manufactured by Bio-Logic), and a 4M KOH aqueous solution is used as the electrolytic solution, and cyclic voltammetry (cyclic voltammetry). CV) measurement was performed. The measurement was carried out for 3 cycles at a sweep rate of 1 mV / s in the range of 1.10 to 1.70 V (vs reversible hydrogen catalyst; RHE), and a comparison of the current densities at 1.7 V in the third cycle. The OER activity of the catalyst was evaluated with. The reference electrode actually used is mercury-mercury oxide reference electrode, but for convenience, the potential was converted to the RHE reference electrode reference.

<Making metal-air batteries>
(Examples 11 to 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., Ltd., 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 dairy pot to prepare a mixed slurry which is a composition for forming a gas diffusion layer. A series of operations of stretching the obtained mixture slurry to a thickness of 0.5 mm and folding it in two were repeated 40 times. Then, it was rolled to a thickness of 0.35 mm by a roll press, 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., Ltd., 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) as a composition for forming a catalyst layer. did. As the positive electrode catalyst, the catalyst shown in Table 2 was used.

The ink obtained above was applied to the gas diffusion layer sheet obtained above by a spray method and fired under argon at 335 ° C. for 13 minutes to obtain a laminate for an air electrode. A nickel mesh was crimped onto the obtained laminate to obtain an air electrode.

Using each of the obtained air electrodes, the experimental cell shown in FIG. 1 was prepared 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 close the opening (6). Next, after injecting an 8 mol / L potassium hydroxide aqueous solution as the electrolyte (4) into the housing (5), a platinum plate is used as the counter electrode (negative electrode) (2), and a mercury-mercury oxide electrode is used as the reference electrode (3). The experimental cell (10) was prepared by immersing it in the electrolyte (4) in the housing (5). The reference electrode (3) was arranged between the air electrode (1) and the negative electrode (2). In addition, (12) in FIG. 1 shows a gas diffusion layer, and (13) shows a current collector (nickel mesh).

<Evaluation of metal-air battery>
The input / output characteristics and life characteristics of the cells prepared above were evaluated by the methods shown below. The 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 , the OER and the ORR are each 1 hour, and the rest time between the OER and the ORR is 3 minutes. The charge / discharge cycle of was repeated 100 times. Then, OER or ORR with a constant current density of 40 mA / cm ² was carried out for 1 minute, and the air electrode potential at the elapse of 1 minute was defined as the OER or ORR potential. The air pole potential is defined by the potential difference between the air pole and the reference pole. The reference electrode actually used is mercury-mercury oxide reference electrode, but for convenience, the potential was converted to the RHE reference electrode reference. The input / output characteristics were evaluated by comparing the above OER or ORR potentials.

[Evaluation of life characteristics]
First, in an environment of 25 ° C., OER current value + 40 mA / cm ² , ORR current value -40 mA / cm ² , OER and ORR are 1 hour each, 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 by repeating the charge / discharge cycle of.

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 2-2x (Co y Ni 1-y) x (Si z Ge 1-z) 1 + x O 7 (1)
[In the formula (1), w, x, y and z satisfy 0 ≦ w ≦ 1, 0 <x <1, 0 <y <1 and 0 ≦ z ≦ 1, respectively. ] The positive electrode catalyst for a metal-air battery according to claim 1, wherein x in the formula (1) satisfies 0.5 ≦ x ≦ 0.9. The positive electrode catalyst for a metal-air battery according to claim 1 or 2, 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 the 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 diffusion layer. A metal-air battery comprising the positive electrode, the negative electrode, and the electrolyte for the metal-air battery according to claim 5 or 6.

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