CROSS REFERENCE TO RELATED APPLICATIONS

This is the U.S. national stage of application No. PCT/JP2018/034073, filed on Sep. 13, 2018. Priority under 35 U.S.C. § 119(a) and 35 U.S.C. § 365(b) is claimed from Japanese Application No. 2017-190808, filed Sep. 29, 2017, the disclosure of which is also incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to melilite-type complex oxide.

BACKGROUND ART

In order to further distribute electric vehicles (EV), the development of high-energy-density secondary batteries that attain a standard cruising distance of petrol vehicles is indispensable. Currently, metal-air batteries have attracted attention as “innovative storage batteries” exceeding the current lithium-ion secondary battery. The metal-air batteries are secondary batteries using a metal such as zinc, as a negative-electrode active material, and oxygen in the air, as a positive-electrode active material. For such metal-air batteries, there is a possibility that an extremely high theoretical energy density is attained. The research and development of the metal-air batteries, in particular, zinc-air batteries using zinc as a metal has been conducted for a long time by domestic and international research institutions (for example, Non-Patent Documents 1 and 2), but currently, the metal-air batteries have not been fully commercialized.

However, in the air electrode of the metal-air batteries, hydroxide ions are generated by a four-electron reduction reaction of oxygen (an active material) in the discharging process, and oxygen molecules are generated by a four-electron oxidation reaction of hydroxide ions in the charging process. The oxygen reduction reaction (hereinafter, also referred to as “ORR”) and the oxygen evolution reaction (hereinafter, also referred to as “OER”), involving the four-electron transfer, are kinetically sluggish, causing large overpotential both in the charging and discharging processes, and therefore, high-activity electrocatalysts that are capable of accelerating the ORR/OER are required.

Specifically, the charging and discharging reactions in the positive/negative electrodes of the metal-air batteries are as represented by Eqs. (1) to (4). Note that, in Eqs. (1) to (4), for convenience, an example is described in which zinc is used as a negative-electrode.

(Positive-Electrode)
Charging Reaction (Oxygen Evolution Reaction): 4OH−→O2+2H2O+4e−(1)
Discharging Reaction (Oxygen Reduction Reaction): O2+2H2O+4e−→4OH−(2)
(Negative-Electrode)
Charging Reaction: ZnO+H2O+2e−→Zn+2OH−(3)
Discharging Reaction: Zn+2OH−→ZnO+H2O+2e−(4)

Here, in the metal-air batteries, a highly alkaline aqueous solution of a high-concentration KOH aqueous solution is used as an electrolyte, and supplies hydroxide ions involved in the formulas (1) and (4). Then, positive-electrode catalysts are immersed in highly alkaline media, and thus, are required to have excellent chemical stability.

It is known that noble metal-based catalysts such as platinum, ruthenium oxide, and iridium oxide exhibit high ORR/OER activity, as the positive-electrode catalyst. However, the noble metals contained in the positive-electrode catalyst are rare and expensive, and thus, it is difficult to commercialize secondary batteries for vehicles or the like on a massive scale. Therefore, positive-electrode catalysts that contain elements abundant in resources, such as transition metals, as a main component, and exhibits versatile high-performance ORR/OER activity have been strongly desired to be developed.

On the other hand, recently, perovskite-type transition metal oxides (ABO3) have been developed as positive-electrode catalysts. It has been reported that when the number of egelectrons at the octahedrally-coordinated B-site (BO6) in the perovskite structure is close to unity, the ORR/OER activity is maximized (for example, Non-Patent Documents 3 and 4). However, in such a design guide, compounds having other metal-oxygen coordination structures, in particular, other than the perovskite-type oxides having BO6octahedrally-coordinated sites, are not considered at all. In particular, promising materials that are capable of standing the practical use in the environment in which the metal-air battery is operated have not been found.Non-Patent Document 1:F. Cheng, J. Chen, Chem. Soc. Rev., 41, 2172-2192(2012).Non-Patent Document 2: Y. Li, H. Dai, Chem. Soc. Rev. 43, 5257-5275(2014).Non-Patent Document 3:J. Suntivich, H. A. Gasteiger, N. Yabuuchi, H. Nakanishi, J. B. Goodenough, Y. S.-Horn, Nat. Chem., 3, 546-550(2011).Non-Patent Document 4:J. Suntivich, K. J. May, H. A. Gasteiger, J. B. Goodenough, Y. S.-Horn, Science, 334, 1383-1385(2011).

DISCLOSURE OF THE INVENTION

Problems to be Solved by the Invention

The present invention has been made in consideration of the circumstances as described above, and an object thereof is to provide a novel compound that can be used in positive-electrode catalyst for a metal-air battery.

Means for Solving the Problems

The present inventors have conducted intensive studies for attaining the object described above. As a result thereof, it has been found that a melilite-type complex oxide represented by a general formula (Ba1−z)2Sr2zCoxFe2−2x(SiyGe1−y)1+xO7(in the formula, 0≤x≤1, 0≤y≤1, and 0≤z≤1, excluding the case of x=1 and y=1, the case of x=1 and y=0, and the case of x=0 and y=0) can be used in a positive-electrode catalyst of a metal-air battery, and thus, the present invention has been completed. Specifically, the present invention provides the following.

(1) The first invention of the present invention is melilite-type complex oxide represented by a general formula (BazSr1−z)2COxFe2−2x(SiyGe1−y)1+xO7(in the formula, 0≤x≤1, 0≤y≤1, and 0≤z≤1, excluding the case of x=1, y=1, and z=0, the case of x=1, y=1, and z=1, the case of x=1, y=0, and z=0, the case of x=1, y=0, and z=1, the case of x=0, y=0, and z=0, and the case of x=0, y=0, and z=1.

(2) The second invention of the present invention is the melilite-type complex oxide according to the first invention, in which 0.5≤x≤0.9 is satisfied in the general formula.

(3) The third invention of the present invention is the melilite-type complex oxide according to the first invention or the second invention, in which 0≤y≤0.1 is satisfied in the general formula.

(4) The fourth invention of the present invention is a melilite-type complex oxide represented by a general formula (Baz1Sr1−z1−z2REz2)2Cox1Znx2Fe2−2(x1+x2)(SiyGe1−y)1+x1+x2O7(in the formula, 0≤x1≤1, 0≤x2≤0.2, 0≤y≤1, 0≤z1≤1, 0≤z2≤0.2, and at least one of x2 and z2 is greater than 0).

(5) The fifth invention of the present invention is a melilite-type complex oxide, in which RE is Y in the general formula.

Effects of the Invention

According to the present invention, it is possible to provide a novel compound which can be used for a positive-electrode catalyst of a metal-air battery.

