Electrode, method for producing said electrode, and electrochemical device using said electrode

Abstract

The present invention provides: an electrode which is used for oxygen reactions and which is excellent in catalyst activity and stability; a method for preparing the electrode; and an electrochemical device using the electrode. This electrode is provided with an oxide, as an oxygen catalyst, which has peaks at positions of 2θ = 30.07° ± 1.00°, 34.88° ± 1.00°, 50.20° ± 1.00° and 59.65° ± 1.00° in X-ray diffraction measurements using CuKα rays and which contains, as constituent elements, bismuth, ruthenium, sodium and oxygen.

Description

[Technical Field]

[0001] The present disclosure relates to an electrode used in an oxygen reaction, a method for producing the electrode, and an electrochemical device using the electrode. [Background technology]

[0002] In this book, oxygen reaction refers to the electrochemical reaction of oxygen reduction, oxygen generation, or both. For example, in an air battery that uses an alkaline aqueous solution such as lithium hydroxide, potassium hydroxide, or sodium hydroxide as an electrolyte (hereinafter referred to as alkaline aqueous solution), hydroxide ions (OH - ) is produced, and the following reaction is known in which hydroxide ions in an alkaline aqueous solution are oxidized to produce oxygen: Reduction: O2 + 2H2O + 4e - →4OH - ···(1) Oxidation: 4OH - →O2+2H2O+4e - ···(2)

[0003] In air batteries, these oxygen reactions occur at the positive electrode. In primary air batteries, the reduction reaction shown in formula (1) occurs during discharge. In secondary air batteries, the same reaction shown in formula (1) occurs during discharge as in primary air batteries, and the oxidation reaction shown in formula (2) occurs during charge. The term "air battery" is used because oxygen in the air can be used for discharge, and for the same reason, the positive electrode of an air battery is also called the air electrode. However, the oxygen used in the reaction shown in formula (1) does not necessarily have to be oxygen in the air; for example, high-purity oxygen can be used.

[0004] Furthermore, the oxygen reduction reaction at the air electrode of an air battery using an alkaline aqueous solution, as described above, is the same as the oxygen reduction reaction at the oxygen cathode of a sodium chloride electrolysis system, which produces caustic soda and chlorine through electrolysis, and the same catalyst can be used for both. In this book, the catalyst used for such an oxygen reaction is called an oxygen catalyst. Furthermore, the reaction during power generation at the cathode of an alkaline fuel cell is also oxygen reduction, and the same oxygen catalyst can be used for the air electrode of an air battery, the oxygen cathode of a sodium chloride electrolysis system, and the cathode of an alkaline fuel cell. Furthermore, the charging reaction at the air electrode of an air secondary battery is expressed as equation (2), and the oxygen evolution reaction at the anode in alkaline water electrolysis is also expressed as equation (2). Therefore, the same oxygen catalyst can be used for these reactions. As mentioned above, in this book, the catalyst for oxygen reduction, oxygen evolution, or both reactions is called an oxygen catalyst.

[0005] The aforementioned air battery, salt electrolysis, alkaline fuel cell, and alkaline water electrolysis all use alkaline aqueous solutions as electrolytes, and their operating temperatures are between room temperature and around 90°C. In other words, oxygen reactions using alkaline aqueous solutions as electrolytes are oxidation and reduction reactions between oxygen and hydroxide ions in this temperature range. There are other electrochemical reactions that reduce oxygen or generate oxygen. For example, the reaction at the cathode of a solid oxide fuel cell (abbreviated as SOFC) converts oxygen into oxide ions (O 2- ) and the reaction at the anode of a solid oxide water electrolysis device (SOEC) is an oxidation reaction of oxide ions to oxygen. Both of these reactions occur at high temperatures of around 600°C to 1000°C. As the reaction mechanism of oxygen reactions differs depending on the temperature, the oxygen catalysts suitable for them naturally differ, and as the reaction mechanism differs, the mechanism of action and effects of the oxygen catalyst also differ greatly.

[0006] For example, even if a certain oxygen catalyst is found to have high catalytic activity at high temperatures, such as 600°C or higher, this does not necessarily mean that the oxygen catalyst will have similarly high catalytic activity at temperatures below 100°C. It is extremely difficult for even those skilled in the art to make such analogical inferences or inferences. Furthermore, it is generally more difficult for electrochemical reaction catalysts to exhibit high catalytic activity at lower temperatures, such as around room temperature, and it is also difficult to find an oxygen catalyst that has high catalytic activity at lower temperatures. Furthermore, not only the catalytic activity of an oxygen catalyst but also its stability varies significantly depending on the temperature and reaction mechanism.

[0007] On the other hand, lithium-air secondary batteries are known as air batteries that operate at temperatures below 100°C, similar to air batteries that use alkaline aqueous solutions as electrolytes. While these lithium-air secondary batteries are still under development, they use lithium, which has the property of explosively reacting with water, as the negative electrode reactant. Therefore, the electrolyte typically uses an organic solvent or ionic liquid containing lithium salts, rather than an aqueous solution. However, if a lithium-ion conductive solid electrolyte is added to the lithium negative electrode, the solid electrolyte prevents direct contact between lithium and water, enabling a configuration such as lithium / solid electrolyte / alkaline aqueous solution (liquid electrolyte) / air electrode. The electrode disclosed herein can also be used as an air electrode with such a configuration. An example of such a lithium-ion conductive solid electrolyte is an oxide known as LTAP, which has a NASICON-type crystal structure. The oxygen reaction in a lithium-air secondary battery using a lithium negative electrode with a lithium-ion conductive solid electrolyte and an alkaline aqueous solution is expressed by the aforementioned equations (1) and (2).

[0008] In contrast, in lithium-air secondary batteries that use a non-aqueous electrolyte such as an organic solvent or ionic liquid to which a lithium salt has been added, the reaction at the air electrode is known to be as follows: Reduction: O2 + 2e - →O2 2- ···(3) Oxidation: O2 2- →O2+2e - ···(4) O2 shown in these formulas 2- It is known that Li2O2 (solid) does not exist as an ion in the electrolyte, but precipitates in the air electrode. 2- and O2 shown in the above equations (3) and (4). 2- are completely different ions. The reactions in equations (1) and (2) are four-electron reactions, while the reactions in equations (3) and (4) are two-electron reactions. However, all of the oxygen reactions in equations (1) to (4) proceed at temperatures below approximately 100°C. Furthermore, if a two-electron reaction is considered to have progressed further in each of equations (3) and (4), the oxidation or reduction will proceed to the four-electron state of equations (1) and (2). Therefore, oxygen catalysts with high catalytic activity for four-electron reactions such as equations (1) and (2) are expected to also exhibit high catalytic activity for the two-electron reactions of equations (3) and (4), which occur in similar temperature ranges.

[0009] Among primary air batteries using alkaline aqueous solutions as electrolytes, zinc-air primary batteries, which use zinc at the anode, have been put to practical use as power sources for hearing aids. Similar primary air batteries have been developed using metals other than zinc, such as magnesium, calcium, aluminum, and iron, at the anode. Primary air batteries using alkaline aqueous solutions as electrolytes and magnesium at the anode have been commercialized as emergency power sources. Meanwhile, no secondary air batteries using alkaline aqueous solutions as electrolytes have been put to practical use, except for mechanically rechargeable zinc-air secondary batteries, which regenerate their discharge function by mechanically (physically) replacing the anode or electrolyte. Therefore, non-mechanically rechargeable zinc-air secondary batteries and hydrogen / air secondary batteries, which use hydrogen storage alloys at the anode, have been developed. While the reactions at the anodes of these secondary batteries differ, the reactions at the cathode (air electrode) are the same, and are both represented by the reaction formulas (1) and (2). The inventor of the present disclosure previously disclosed a hydrogen / air secondary battery in Patent Document 1.

[0010] Materials that have been used or investigated as oxygen catalysts not only for the air electrodes of air batteries as described above, but also for oxygen cathodes in brine electrolysis, cathodes in alkaline fuel cells, and anodes in alkaline water electrolysis include a wide variety of materials, including precious metals such as platinum, silver, and gold, or their alloys, platinum group metals and other transition metal elements and alloys containing them, various oxides and sulfides, doped or undoped carbonaceous materials (including carbon with various crystalline structures and forms such as graphite, amorphous carbon, glassy carbon, carbon nanotubes, carbon nanofibers, and fullerenes), various nitrides, carbides, and organic metal compounds. Among these, oxides with crystalline structures called pyrochlore, perovskite, and spinel are known as oxygen catalysts, and are disclosed in, for example, Patent Documents 1 to 4.

[0011] Specifically, Patent Document 1 describes a positive electrode of an air secondary battery that uses an alkaline aqueous solution as an electrolyte, the positive electrode comprising a nickel-coated material that includes a core material that has a density lower than that of nickel and a coating layer that coats the core material and is made of nickel and / or a nickel alloy, and discloses a catalyst made of bismuth iridium oxide and / or bismuth ruthenium oxide mixed with the nickel-coated material.

[0012] Patent Document 2 describes La x Sr 3-x Fe 2-y Co y A cathode catalyst for metal-air batteries with an atomic ratio of Sr3Fe2O7 is disclosed. This catalyst is said to exhibit excellent alkaline resistance and catalytic activity because at least a portion of the Sr sites of Sr3Fe2O7 are substituted with La, or at least a portion of the Fe sites are substituted with Co.

[0013] Patent Document 3 discloses an air battery having a negative electrode, an electrolyte layer interposed between the air electrode and the negative electrode, and an electrode catalyst, and the electrode catalyst contains at least an oxide (oxide electrode catalyst) active in the oxygen reduction reaction. Preferred oxide electrode catalysts include those containing at least one metal element selected from iron, cobalt, nickel, titanium, manganese, and copper, and having a perovskite structure or a spinel structure.

[0014] Patent Document 4 discloses a method for producing an air secondary battery catalyst used in the air electrode of an air secondary battery, which includes a precursor preparation step of preparing a precursor of a pyrochlore oxide, a calcination step of calcining the precursor to form a pyrochlore oxide, and an acid treatment step of immersing the pyrochlore oxide obtained in the calcination step in an acidic aqueous solution to perform an acid treatment. Patent Document 4 lists oxygen-deficient bismuth ruthenium oxide as an example of such a pyrochlore oxide.

[0015] Patent Document 5 describes a compound of the general formula: A 2-x B 2-y O 7-z (where x, y, and z respectively satisfy the relationships 0≦x≦1, 0≦y≦1, and −0.5≦z≦0.5; A represents at least one element selected from Bi, Pb, Tb, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, Lu, Mn, Y, Zn, and Al; and B represents at least one element selected from Ru, Ir, Si, Ge, Ta, Sn, Hf, Zr, Ti, Nb, V, Sb, Rh, Cr, Re, Sc, Co, Cu, In, Ga, Cd, Fe, Ni, W, and Mo), and the air electrode catalyst for an air secondary battery includes a pyrochlore-type composite oxide represented by the formula: (where x, y, and z respectively satisfy the relationships 0≦x≦1, 0≦y≦1, and −0.5≦z≦0.5; A represents at least one element selected from Bi, Pb, Tb, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Dy, Ho, Er, Tm, Yb, Lu, Mn, Y, Zn, and Al; and B represents at least one element selected from Ru, Ir, Si, Ge, Ta, Sn, Hf, Zr, Ti, Nb, V, Sb, Rh, Cr, Re, Sc, Co, Cu, In, Ga, Cd, Fe, Ni, W, and Mo), and the pyrochlore-type composite oxide has two or more crystal structures with different oxygen amounts.

[0016] The composition of oxides with a pyrochlore structure is generally A2B2O7, with the A-site element (A), B-site element (B), and oxygen (O) in the crystal structure. However, there have been many reports that the atomic ratios obtained from analysis of actual oxides do not necessarily correspond to integers like this.

[0017] For example, bismuth ruthenium oxide (hereinafter referred to as BRO) has a pyrochlore structure with bismuth (Bi) at the A site and ruthenium (Ru) at the B site. Bi2Ru2O7 BiRuO 6.9 BiRuO 6.92 Bi 1.87 Ru2O 6.903 Bi 1.88 Ru2O 6.906 Bi 1.9 Ru2O 6.922 Bi 1.9 Ru2O 6.928 Bi 1.9 Ru2O 6.901 The atomic ratio of BRO shown in the table below is registered in the database of the International Diffraction Data Center. As such, it is known that the atomic ratio of BRO varies depending on the synthesis method and conditions, making it a compound whose composition is easily changed.

[0018] On the other hand, oxygen catalysts using alkaline aqueous solutions as electrolytes are required to have both chemical and electrochemical stability in a strongly alkaline environment and high catalytic activity for oxygen reduction and / or oxygen generation. That is, for oxygen reactions accompanied by a strongly oxidizing atmosphere that occurs in a strongly corrosive environment such as a strongly alkaline environment, the oxygen catalyst must maintain its own composition constant while reducing oxygen in the atmosphere and / or generating oxygen from hydroxide ions in the electrolyte on its surface, rather than its own oxygen. In addition, in the case of oxygen reactions using nonaqueous electrolytes, the oxygen catalyst must maintain its own composition constant while reducing oxygen in the atmosphere and / or generating O22- In the case of compounds like BRO, whose composition is easily changed, slight differences in composition (atomic ratio) can potentially result in high catalytic activity, but the composition of the catalyst itself can change due to the oxygen reaction, which can lead to a deterioration in catalytic activity, and there are many cases in which the initial catalytic activity and its continuity cannot coexist.

[0019] Furthermore, Patent Document 4 points out the problem that the discharge capacity of an air secondary battery decreases after a relatively small number of cycles due to by-products generated during the synthesis of oxygen-deficient BRO. Furthermore, to address this problem, it is disclosed that by immersing the synthesized BRO in an acidic solution such as nitric acid for a certain period of time, followed by washing with ion-exchanged water and drying (this operation is referred to as "acid treatment"), the atomic ratio of Bi to Ru decreases and the retention rate of discharge capacity with respect to charge-discharge cycles improves compared to BRO that has not been subjected to such acid treatment.

[0020] Here, with respect to oxides with a pyrochlore structure, when the atomic ratio of oxygen relative to the general atomic ratio A2B2O7 mentioned above is less than 7, it is called an oxygen-deficient type, and when it is greater than 7, it is called an oxygen-excess type. [Prior art documents] [Patent documents]

[0021] [Patent Document 1] Japanese Patent Application Laid-Open No. 2016-152068 [Patent Document 2] Japanese Patent Application Publication No. 2018-149518 [Patent Document 3] Japanese Patent Application Laid-Open No. 2015-046403 [Patent Document 4] Japanese Patent Application Publication No. 2019-179592 [Patent Document 5] Japanese Patent Publication No. 2020-126754 Summary of the Invention [Problem to be solved by the invention]

[0022] BRO has a pyrochlore structure composed of bismuth (Bi) at the A-site and ruthenium (Ru) at the B-site, and oxygen (O) at the B-site. As mentioned above, BRO is expected to exhibit high catalytic activity for both oxygen reduction and oxygen evolution in oxygen reactions using aqueous electrolytes, such as alkaline solutions, or nonaqueous electrolytes. Therefore, it has been developed as an oxygen catalyst for the air electrode of air secondary batteries, where both are required. However, BRO's composition is easily changed, i.e., the atomic ratio of Bi, Ru, and O varies depending on the synthesis method and synthesis conditions. Furthermore, by-products generated during synthesis react with each other, degrading the cycle performance of air secondary batteries. One factor that destabilizes the composition of BRO is the dissolution of the A-site element, Bi. Furthermore, the degradation of cycle performance is due to the generation of by-products containing Bi during BRO synthesis. In response to this, Patent Document 4 discloses a method of treating synthesized BRO with an acid. However, this involves two steps: first synthesizing the BRO and then further treating it, which increases the number of steps required to obtain the oxygen catalyst, and this treatment changes the composition and particle size (particle diameter) of the BRO.

[0023] Furthermore, for particles larger than nanoparticles, the physicochemical properties of the interior (bulk) and surface of the particle differ, but in the case of nanoparticles, this distinction is difficult to make, making it difficult to achieve bulk stability at the surface. Note that the particle size here refers to the size of so-called primary particles, and is different from the size of secondary particles formed by bonding or agglomerating primary particles. On the other hand, because the oxygen reaction occurs on the surface of the oxygen catalyst, oxygen catalyst nanoparticles are required to have compositional stability and uniformity from the surface to the bulk, but there are no oxygen catalysts, including BRO, that meet these requirements.

[0024] For the reasons mentioned above, electrodes that use BRO as an oxygen catalyst have the problem that it is difficult to maintain high catalytic activity and stability against oxygen reactions that occur at the electrode due to the problems with BRO.

[0025] Furthermore, there is a problem that there is no method for producing an electrode that can maintain high catalytic activity and stability in oxygen reactions, and there is also a problem that there is no electrochemical device equipped with an electrode that can maintain high catalytic activity and stability in oxygen reactions.

[0026] That is, to maintain high catalytic activity in electrodes used for oxygen reactions, electrodes equipped with oxygen catalysts with better compositional stability are desirable. Furthermore, electrodes equipped with oxygen catalysts that balance the charges of cations and anions throughout the oxide are needed, resolving the problems of conventional oxygen catalysts, such as reduced compositional stability and long-term maintenance. Furthermore, electrodes equipped with oxygen catalysts with smaller particle sizes, particularly at the nanoparticle level, are desirable, so that catalytic activity per unit weight is higher even with the same composition.

[0027] Furthermore, a method for producing an electrode that can maintain high catalytic activity and stability in oxygen reactions is desired, and an electrochemical device equipped with an electrode that maintains high catalytic activity and stability in oxygen reactions is also desired.

[0028] The present disclosure has been made in view of the above circumstances, and its purpose is to provide an electrode for use in oxygen reactions that has excellent catalytic activity and stability, a method for producing the electrode, and an electrochemical device that uses the electrode. [Means for solving the problem]

[0029] To solve the above problems, the electrode of the present disclosure has the following configuration. The electrode of the present disclosure is an electrode used in oxygen reactions, and is characterized by having peaks at 2θ=30.07°±1.00°, 34.88°±1.00°, 50.20°±1.00°, and 59.65°±1.00° in X-ray diffraction measurement using CuKα radiation, and is equipped with an oxide containing bismuth, ruthenium, sodium, and oxygen as constituent elements as an oxygen catalyst. Note that 2θ in X-ray diffraction measurement refers to the diffraction angle of diffracted X-rays, and hereinafter, this diffraction angle will be simply referred to as 2θ. Furthermore, the term "oxide containing bismuth, ruthenium, sodium, and oxygen as constituent elements" used herein does not exclude impurities that are inevitably generated during the production of the oxygen catalyst of the present disclosure.

[0030] This configuration, unlike BRO with a pyrochlore structure, includes sodium as a constituent element, which firstly enhances the stability of the composition in terms of charge balance, and at the same time improves the uniformity of the composition from the interior to the surface of the nanoparticles, and also suppresses the generation of by-products during synthesis, thereby providing an electrode that can stably maintain catalytic activity for oxygen reactions. In particular, the improved composition stability suppresses composition changes that occur when Bi dissolves in the oxygen catalyst, unlike electrodes that use BRO as the oxygen catalyst, in the electrode of the present disclosure.

[0031] As shown in the examples below, the oxide used as the oxygen catalyst in the electrode of the present disclosure (hereinafter simply referred to as the oxygen catalyst) can be obtained by preparing an aqueous solution in which metal salts of bismuth and ruthenium, such as metal nitrates or metal chlorides, are dissolved, mixing these, adding an aqueous sodium hydroxide solution to the resulting solution, stirring, and precipitating a metal hydroxide in the solution, and then calcining the precipitate at a predetermined temperature. In this process, the mixed solution contains an introduction-promoting / stabilizing agent (described below) that promotes the introduction of sodium ions into the metal hydroxide so that the metal hydroxide contains sodium ions along with bismuth and ruthenium ions, and stabilizes the precipitate of the metal hydroxide in this state at nanoscale.

[0032] The oxygen catalyst obtained by this method exhibits diffraction peaks at 2θ values of 14.82°, 30.07°, 34.88°, 38.17°, 45.88°, 50.20°, 59.65°, 62.61°, 73.80°, 81.68°, and 84.28° in X-ray diffraction using CuKα radiation. However, each 2θ value has a range of approximately ±1.00°. Among these, the diffraction peaks at 30.07°, 34.88°, 50.20°, and 59.65° have higher diffraction intensities than the others. In other words, the oxygen catalyst used in the electrode of the present disclosure has the crystalline structure of the X-ray diffraction pattern shown above and uses an oxide composed of bismuth, ruthenium, sodium, and oxygen.

[0033] The oxygen catalyst used in the electrode of the present disclosure can be obtained by first precipitating a metal hydroxide in a solution and then calcining the precipitate, as described above. However, in this solution process, a sodium salt solution is added to a metal salt solution to make the solution's hydrogen ion exponent (hereinafter referred to as pH) alkaline. The metal salt solution must contain a component that promotes and stabilizes the incorporation of sodium into the metal hydroxide. In other words, simply adjusting the pH of a solution containing bismuth ions, ruthenium ions, and sodium ions makes it difficult to obtain a metal oxide composed of bismuth, ruthenium, and sodium, such as the oxygen catalyst used in the electrode of the present disclosure. For example, without the sodium incorporation promoter and stabilizer described above, it may be possible to obtain BRO, but it is difficult to obtain the oxygen catalyst used in the electrode of the present disclosure, which contains sodium as well as bismuth, ruthenium, and oxygen as constituent elements and has a structure other than the pyrochlore structure.

[0034] Even if a material obtained without the addition of a sodium-incorporation promoter / stabilizer shows diffraction peaks indicating BRO using X-ray diffraction, a method commonly used for crystal structure analysis, and energy-dispersive X-ray spectroscopy, a method commonly used for analyzing elemental species and composition, reveals sodium along with bismuth and ruthenium, this does not necessarily mean that sodium is present in the BRO. This is because, in the absence of a sodium-incorporation promoter / stabilizer, sodium-containing oxides such as BiNaO3 are likely to be produced as by-products. In X-ray diffraction measurements, such by-products tend to produce no diffraction peaks if their amounts are relatively small. Energy-dispersive X-ray spectroscopy, on the other hand, can detect elements with higher sensitivity than X-ray diffraction, and can detect all elements present in the sample, even in amounts so small that X-ray diffraction does not produce peaks. Therefore, when BRO and BiNaO3 are mixed, sodium will be detected along with bismuth and ruthenium. However, these are separate compounds, not a single compound. Therefore, the elements contained in the crystal structure of a certain oxide can only be determined by analyzing the crystal structure in a finer region, for example, by X-ray absorption fine structure analysis (XAFS) as described below. Even under conditions in which a sodium introduction promoter / stabilizer is present, if the mixing and stirring of the metal salt solution is insufficient, or if the amount of metal salt solution is large and the mixing and stirring time is short relative to the amount of solution, it may not be possible to obtain an oxide composed of bismuth, ruthenium, sodium, and oxygen and having the crystal structure with the above-mentioned X-ray diffraction pattern, such as the oxygen catalyst used in the electrode of the present disclosure.

[0035] The temperature at which the metal hydroxide is calcined varies depending on various conditions other than the calcination temperature, such as the type of solvent used in the solution used to precipitate the metal hydroxide, the type of metal salt used and its concentration in the solution, and the drying method and conditions for the precipitated metal hydroxide. For example, in the method using an aqueous solution as described above, a calcination temperature lower than 300°C is generally undesirable because the structural change from the hydroxide state to the oxide is difficult to occur sufficiently. A calcination temperature higher than 800°C is undesirable because the oxide may decompose. Furthermore, in the examples described below, a temperature range of 450°C to 650°C is preferable, and 550°C to 600°C is even more preferable. However, the method for producing the oxygen catalyst used in the electrode of the present disclosure is not limited to the above-mentioned method. Various production methods can also be used, such as the sol-gel method, a method known as hydrothermal synthesis, or a method in which bismuth oxide, ruthenium oxide, and sodium oxide are prepared in advance and then mechanical, thermal, or electrical energy is applied to convert them into oxides through solid-state or semi-solid-state reactions.

[0036] The sodium salt solution in the above method is typically, but not limited to, an aqueous sodium hydroxide solution, provided that the pH can be adjusted to an alkaline level to allow precipitation of the metal hydroxide, and that the anion of the sodium salt does not inhibit the incorporation and stabilization of the sodium ion into the metal hydroxide.

[0037] The oxygen catalyst used in the electrode of the present disclosure has a crystal structure shown in the X-ray diffraction pattern described above. As will be described later, the position of bismuth in the crystal structure corresponds to the vicinity of the A site position when explained with reference to a theoretical pyrochlore structure represented by A2B2O7, the position of ruthenium corresponds to approximately the B site position in the same structure, and the position of sodium corresponds to a position near the A site or near the B site. The oxygen catalyst used in the electrode of the present disclosure is similar to a pyrochlore structure but has a different structure. Therefore, to explain the meaning of "near the site" here using the pyrochlore structure represented by A2B2O7, this means a position that is off-center from the center of the A site or B site. Note that although the position of sodium is described as near the A site or near the B site, as will be described later, a higher proportion of sodium is present near the A site. Furthermore, in the crystal structure, sodium, along with bismuth and ruthenium, is a cation, with a valence of +1, +3, and +4, respectively, as described below. The charge balance between the anion (-2) oxide ion and the cations (bismuth ion, ruthenium ion, and sodium ion) is maintained throughout the oxide, which is believed to improve composition stability and long-term stability, as well as uniformity in nanoparticles, compared to BRO. Here, "balanced charges" in metal oxides generally means that the total number of charges on the cations and the total number of charges on the anions are the same. A significant difference between these two can result in a decrease in the composition stability, long-term stability, or uniformity, as described above. In other words, the oxygen catalyst used in the electrode of the present disclosure contains sodium ions along with bismuth ions, ruthenium ions, and oxide ions, and its well-balanced charges are believed to contribute to its properties, which are significantly different from those of BRO.

[0038] The charge balance in the oxygen catalyst used in the electrode of the present disclosure will be further explained. First, as mentioned above, oxides with a pyrochlore structure are generally expressed by the atomic ratio A2B2O7. Here, if the atomic ratio of oxygen to A-site elements is O / A and the atomic ratio of oxygen to B-site elements is O / B, both atomic ratios are 3.5. On the other hand, if we consider the above-mentioned oxygen-deficient BRO in a similar manner, the atomic ratio of oxygen to Ru, O / Ru, will be smaller than 3.5.

[0039] However, in the oxygen catalyst used in the electrode of the present disclosure, both the atomic ratios O / Bi and O / Ru are greater than 3.5. Furthermore, in the oxygen catalyst of the present disclosure, it is preferable that both the atomic ratios O / Bi and O / Ru are 4 or less. Simply put, this is because the composition ratio of a general pyrochlore structure is such that the anion, oxide ion (ionic formula: O 2- ), resulting in a relatively low charge balance of cations. However, the oxygen catalyst used in the electrode of the present disclosure contains not only bismuth and ruthenium but also sodium. That is, the charge balance is achieved between these three metal ions and oxide ions. As shown in the examples described below, the total number of charges on the cations of these three metals is approximately the same as the total number of charges on the oxide ion, which is the only anion. That is, if the total charge ratio is defined as (total number of charges on anions) / (total number of charges on cations), the oxygen catalyst used in the electrode of the present disclosure has a total charge ratio of 0.9 to 1.1. Here, when the atomic ratios of bismuth, ruthenium, sodium, and oxygen in the four elements constituting the oxygen catalyst used in the electrode of the present disclosure are K, L, M, and N, respectively, the total number of charges on the anions is 2N, and the total number of charges on the cations is (3K + 4L + M), and the total charge ratio is expressed as 2N / (3K + 4L + M).

[0040] Furthermore, the oxygen catalyst used in the electrode of the present disclosure contains sodium ions in its crystal structure together with bismuth ions and ruthenium ions, which improves composition stability and long-term maintainability compared to BRO, and also improves uniformity in nanoparticles. This will be explained based on the coordination number and ionic radius of the cation.

[0041] In pyrochlore structures such as BRO, the coordination number of the A-site cation is 8. A cation with a coordination number of 8 means that the cation is surrounded by its eight nearest anions. The coordination number of the B-site cation is 6. The radius of an ion in a solid crystal generally varies depending on the element, oxidation number, and coordination number, and this data is available in various publications and databooks. For example, even though the bismuth ion has the same valence of +3, its ionic radius is 0.96 Å for a coordination number of 5, 1.03 Å for a coordination number of 6, and 1.17 Å for a coordination number of 8. The bismuth ion in BRO is +3 and in the A-site, so as mentioned above, its coordination number is 8, and its ionic radius is 1.17 Å. The ruthenium ion is +4 and in the B-site, so its coordination number is 6, and its ionic radius is 0.62 Å. On the other hand, sodium ions only have a +1 valence, but their ionic radii are 0.99 Å for a coordination number of 4, 1.00 Å for a coordination number of 5, 1.02 Å for a coordination number of 6, 1.12 Å for a coordination number of 7, 1.18 Å for a coordination number of 8, 1.24 Å for a coordination number of 9, and 1.39 Å for a coordination number of 12. The ionic radius of the sodium ion at a coordination number of 8 (1.18 Å) is less than 1% different from the ionic radius of the bismuth ion at a coordination number of 8 (1.17 Å). Therefore, considering only the ionic radius, it is predicted that sodium ions can occupy the position of bismuth ions in BRO. However, because sodium ions are +1 valent and bismuth ions are +3 valent, it is expected that it would be difficult to simply replace some of the bismuth ions with sodium ions while maintaining the pyrochlore structure. In other words, the oxygen catalyst used in the electrode of the present disclosure does not have a pyrochlore structure in which some of the bismuth ions in BRO are simply replaced with sodium ions, but is similar to but different from the pyrochlore structure. Thus, in forming a structure similar to pyrochlore, the sodium ions contribute to stabilizing the resulting oxide structurally and in terms of charge balance.

[0042] Furthermore, it is important that the oxygen catalyst used in the electrode of the present disclosure contains sodium as its constituent elements in addition to bismuth, ruthenium, and oxygen. If other elements classified as alkali metals, like sodium, are used, it is difficult to obtain an oxygen catalyst with a structure similar to the pyrochlore structure of the oxygen catalyst used in the electrode of the present disclosure. This will be demonstrated in the comparative examples described below, but the reason for this difficulty with other alkali metal elements can also be understood from their ionic radii. For example, the alkali metal elements lithium has an ionic radius of 0.92 Å when it is +1 and has a coordination number of 8; potassium has an ionic radius of 1.51 Å when it is +1 and has a coordination number of 8; rubidium has an ionic radius of 1.61 Å when it is +1 and has a coordination number of 8; and cesium has an ionic radius of 1.74 Å when it is +1 and has a coordination number of 8. In other words, structurally, with a coordination number of 8, only the sodium ion has an ionic radius close to that of the +3valent bismuth ion, while the lithium ion has an ionic radius that is more than 20% smaller than that of the +3valent bismuth ion, and the ionic radius differs by more than 29% from that of other +1valent cations, so it is estimated that it would be extremely difficult for them to form a stable crystal structure.

[0043] Furthermore, the oxygen catalyst used in the electrode of the present disclosure preferably has an atomic ratio of sodium to ruthenium, Na / Ru, of 0.285±0.015. Having the atomic ratio Na / Ru within this range has the effect of suppressing structural distortion that occurs when sodium ions, which have an ionic radius similar to that of bismuth ions, are contained in a crystal structure similar to pyrochlore. The atomic ratio Na / Ru is preferably 0.286±0.013, and more preferably 0.275 to 0.297.

[0044] Furthermore, the oxygen catalyst used in the electrode of the present disclosure preferably has an atomic ratio (Bi + Na) / Ru (cation atomic ratio) of the sum of bismuth and sodium to ruthenium of 1.285±0.010. This has the effect of further suppressing the above-mentioned structural distortion. Furthermore, the oxygen catalyst used in the electrode of the present disclosure preferably has a ratio (3Bi + Na) / 4Ru (cation charge ratio) of the sum of the total number of charges on bismuth and the total number of charges on sodium (3Bi + Na) to the total number of charges on ruthenium (4Ru) of 0.820±0.020. This brings the balance between the total number of charges on cations and the total number of charges on anions closer to 1, thereby further improving the structural stability in terms of the charge balance of the entire crystal structure. The cation charge ratio (3Bi + Na) / 4Ru is preferably 0.821±0.015, more preferably 0.810 to 0.833.

[0045] Furthermore, it is desirable that the electrode of the present disclosure be used in a system in which an alkaline aqueous solution is used as an electrolyte. That is, it is desirable that the electrode of the present disclosure be used in a system in which an alkaline aqueous solution is used as an electrolyte. The oxygen reaction occurring in an alkaline aqueous solution is a four-electron reaction such as that shown in formula (1) or (2). It is difficult to exhibit high catalytic activity for formula (1) or (2) compared to two-electron reactions such as formula (3) or (4). However, the electrode of the present disclosure has the effect of being able to stably maintain high catalytic activity even for such four-electron reactions.

[0046] In addition, in the electrode of the present disclosure, it is desirable that the primary particle diameter of the oxygen catalyst is 100 nm or less, which increases the specific surface area of the oxygen catalyst and improves the catalytic activity per unit weight or per unit volume, thereby enabling the electrode to exhibit high catalytic activity even when its weight or volume is reduced.

[0047] Furthermore, in the electrode of the present disclosure, the secondary particle diameter of the oxygen catalyst is preferably 3 μm or less. Here, the difference between primary particle diameter and secondary particle diameter will be explained. The primary particle diameter can be determined as the size of the crystal particles when observing the oxygen catalyst using an electron microscope, etc., while the secondary particle diameter can be determined as the size of the crystal particles bonded or aggregated to form a single mass when observed using an electron microscope, etc. Secondary particle diameter is commonly measured using methods such as laser scattering. Specifically, the catalytic activity per unit weight or per unit volume of an oxygen catalyst increases as the primary particle diameter decreases. However, even if the primary particle diameter is small, the catalytic activity does not increase if the mass of bonded or aggregated particles increases. This is because the proportion of oxygen catalyst crystal particles present on the surface of the oxygen catalyst mass decreases, while the proportion of oxygen catalyst crystal particles inside the mass that do not contribute to the oxygen reaction increases. Therefore, catalytic activity is significantly affected by both primary particle diameter and secondary particle diameter, and therefore the catalytic activity of the electrode. Therefore, a secondary particle diameter of 3 μm or less can exhibit high catalytic activity. The primary particle size and secondary particle size mentioned here do not necessarily refer to perfectly spherical particles, but also include oxygen catalysts with non-spherical or asymmetric shapes. When evaluating (measuring) particle size by image analysis, the major axis can be used as the particle size, for example. Alternatively, the particle size may be determined by a laser diffraction particle size distribution analyzer.

[0048] Furthermore, in the air electrode of an air battery, the oxygen cathode of sodium chloride electrolysis, the cathode of an alkaline fuel cell, the anode of alkaline water electrolysis, etc., a material that is cheaper than an oxygen catalyst may be used as a conductive support, and an oxygen catalyst having a particle size smaller than that of the support may be supported on this support, as will be described later. In this case, if the secondary particle size of the oxygen catalyst is set to 3 μm or less, as in the electrode disclosed herein, it becomes easy to disperse and support the oxygen catalyst on the support, and this has the effect of suppressing aggregation of the oxygen catalyst and improving the utilization efficiency of the oxygen catalyst in the electrode.

[0049] Furthermore, the electrode of the present disclosure preferably includes a catalyst layer containing an oxygen catalyst, a conductive material, and a water-repellent material as constituent materials. The conductive material imparts electronic conductivity to the entire electrode and also functions as a carrier capable of dispersing and supporting the oxygen catalyst. Furthermore, the water-repellent material forms a flow path through which oxygen flows inside the electrode, thereby facilitating the intake of oxygen and its reduction within the electrode, or the generation of oxygen within the electrode and its release to the outside of the electrode.

[0050] Here, carbon materials, metals, ceramics, etc. can be used as conductive materials. Examples of carbon materials include, but are not limited to, graphite, glassy carbon, fullerene, carbon nanotubes, carbon nanofibers, and other carbon structures. Furthermore, metals are preferably chemically and electrochemically stable in electrolytes and are less susceptible to changes in their chemical state or structure due to oxygen reactions. For example, nickel and titanium are preferred regardless of the type of electrolyte, whether aqueous or non-aqueous, but are not particularly limited thereto. Furthermore, ceramics are preferably highly stable in electrolytes and oxygen reactions, and also have high electronic conductivity. Furthermore, even if silica and alumina themselves have low electronic conductivity, ceramics with a conductive film of nickel or nickel alloy formed on their surface can also be used. For example, the surface of silica particles can be coated with a thin film of metal or alloy using electroless plating techniques, and particles with such a conductive film can be used as conductive materials in the same way as carbon materials and metals. Furthermore, the core of such conductive particles can be made of not only ceramics but also resin. It should be noted that the candidate materials for the conductive material shown above are all examples, and the use of other materials as the conductive material is not excluded.

[0051] Water-repellent materials include the traditionally used polytetrafluoroethylene (PTFE), as well as fluororesin materials such as tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), polychlorotrifluoroethylene (PCTFE), tetrafluoroethylene-ethylene copolymer (ETFE), chlorotrifluoroethylene-ethylene copolymer (ECTFE), and polyvinylidene fluoride (PVDF). However, the candidate water-repellent materials listed above are all examples and do not exclude other materials.

[0052] Furthermore, polymers other than those mentioned above, organic / inorganic hybrid materials, etc. can also be used as the water repellent. Usable polymers include linear polymers such as polystyrene, polymethyl methacrylate, polyethylene, polypropylene, polyethylene terephthalate, polybutylene terephthalate, polysulfone, polycarbonate, and polyamide; divinylbenzene, hexatriene, divinyl ether, divinyl sulfone, diallyl carbinol, alkylene diacrylate, oligo- or polyalkylene glycol diacrylate, alkylene triacrylate, alkylene tetraacrylate, alkylene trimethacrylate, alkylene tetramethacrylate, alkylene bisacrylamide, alkylene bismethacrylamide, and polymers having both ends acrylic-modified. Examples of the polymerizable monomer include, but are not limited to, network polymers obtained by polymerizing a hydroxylated polybutadiene oligomer, either alone or with other polymerizable monomers; thermosetting resins such as phenol formaldehyde resin, melamine formaldehyde resin, benzoguanamine formaldehyde resin, and urea formaldehyde resin; resins obtained by polymerizing a silane-containing monomer, such as γ-(meth)acryloxypropyltrimethoxysilane, trimethoxysilylstyrene, and vinyltrimethoxysilane, either alone or with other polymerizable monomers; polymer fine particles in which the hydrolyzable silyl groups of these monomers are crosslinked after hydrolysis; and organopolysiloxane resins such as dimethylpolysiloxane.

[0053] Furthermore, the electrode of the present disclosure preferably uses graphite with different particle sizes as the conductive material. Graphite has excellent oxidation resistance and is therefore less susceptible to oxidative consumption, thereby enhancing its stability as a conductive material. The different particle sizes also have the effect that larger graphite particles contribute to the formation of oxygen flow paths, while smaller graphite particles, taking advantage of their high specific surface area, contribute to highly dispersed support of the oxygen catalyst.

[0054] The electrode of the present disclosure preferably includes a gas diffusion layer. Here, the gas diffusion layer is provided on the side of the electrode used in the oxygen reaction into which oxygen flows, i.e., on the side opposite the electrolyte, and the gas diffusion layer does not use an oxygen catalyst or is not required to function as a reaction field for the oxygen reaction.

[0055] The material and configuration of the gas diffusion layer, which is different from the catalyst layer, are not particularly limited, and may be, for example, the aforementioned conductive material, or a conductive material and a water-repellent material. For example, a thin film formed by mixing or kneading carbon powder such as graphite with PTFE particles, or a thin film formed by supporting PTFE particles on carbon fiber, may be used. Furthermore, a gas diffusion layer with a multi-layer structure may be used, such as a two-layer structure in which a layer made of a conductive material without a water-repellent material such as PTFE particles is further laminated on the thin film of such a structure.

[0056] The gas diffusion layer is conductive, but differs from the catalyst layer in that it does not use an oxygen catalyst or function as a reaction field for oxygen reactions, as described above. The gas diffusion layer is positioned between the catalyst layer and the current collector (described in the next paragraph), providing a flow path for smoothly supplying oxygen to the catalyst layer or smoothly releasing oxygen generated in the catalyst layer to the outside of the electrode. Furthermore, because the gas diffusion layer is conductive, integrating the current collector (described in the next paragraph) with the gas diffusion layer and then integrating the gas diffusion layer with the catalyst layer eliminates oxygen reactions on the surface of the current collector, as occurs when the catalyst layer and current collector are integrated. This prevents alkaline corrosion of the current collector due to the electrolyte and oxidation corrosion due to oxygen generated during charging. Furthermore, if the gas diffusion layer contains a water-repellent material, it can prevent liquid electrolyte from leaking out of the electrode.

[0057] The configuration of the electrode of the present disclosure as described above will be further explained. An electrode equipped with a catalyst layer may have a current collector integrated with the catalyst layer for conducting current between the catalyst layer and an external circuit. As described in the examples below, such a current collector may be a metal mesh such as nickel mesh. A metal plate or metal wire may be used, or a mesh, plate, wire, foil, etc. made of a conductive material other than metal may be used. Furthermore, when an electrode of the present disclosure is equipped with a gas diffusion layer together with a catalyst layer, the layers may be stacked in the order of catalyst layer / gas diffusion layer / current collector and then integrated. In this case, if the electrode of the present disclosure is an air electrode, the catalyst layer is used so that it faces the electrolyte and the current collector faces the atmosphere.

[0058] The electrode of the present disclosure is formed in a thin plate shape, and its thickness is preferably 250 μm or less. A thickness of 250 μm or less allows oxygen to be supplied to oxygen catalysts located at various locations inside the electrode without becoming a rate-limiting factor, and also allows oxygen generated inside the electrode to be smoothly released outside the electrode.

[0059] Furthermore, the electrode of the present disclosure preferably includes a water-repellent layer that allows oxygen to pass through on the atmospheric side of the catalyst layer or the gas diffusion layer, and the water-repellent layer is preferably composed of water-repellent particles. The water-repellent material for this water-repellent layer can be any of the materials described above. For example, PTFE particles can be used, but the present disclosure is not limited to this.

[0060] The water-repellent layer does not have both electrical conductivity and oxygen catalysis like the catalyst layer, nor does it have electrical conductivity like the gas diffusion layer. The water-repellent layer has the function of preventing leakage of liquid electrolyte such as alkaline aqueous solution from the electrode, while allowing sufficient oxygen to permeate toward the catalyst layer or gas diffusion layer.

[0061] Furthermore, the particle size of the water-repellent particles is preferably 1 μm or less, and more preferably 0.01 μm or more. When the particle size of the water-repellent particles is within this range, leakage of the alkaline solution (electrolyte) can be prevented for a long period of time, and the water-repellent layer can be provided with sufficient oxygen permeability. If the particle size of the PTFE particles is large, the voids in the water-repellent layer become large, which is undesirable because leakage of the alkaline solution (electrolyte) cannot be prevented for a long period of time. If the particle size of the PTFE particles is small, the voids in the water-repellent layer become small, which reduces the area through which oxygen can permeate the water-repellent layer and limits the permeation rate, which is undesirable.

[0062] The catalyst layer or gas diffusion layer provided with the water-repellent layer may be integrated with the current collector. In this case, the side where the catalyst layer or gas diffusion layer and the current collector are integrally formed faces the atmosphere, and the water-repellent layer is provided on one side of such an integrated structure.

[0063] For example, an electrode provided with the water-repellent layer of the present disclosure may have a structure in which a catalyst layer / gas diffusion layer / current collector are laminated and integrated, and then a water-repellent layer is formed on the current collector side. In this way, by covering not only the surface of the gas diffusion layer but also the surface of the current collector with a water-repellent material, the water-repellent material of the water-repellent layer can prevent leakage of the liquid electrolyte even in areas where gaps are likely to occur between different materials, such as the interface between the current collector and the conductive material of the gas diffusion layer, while providing the function of an oxygen-permeable liquid electrolyte leakage prevention layer that can supply sufficient oxygen to the gas diffusion layer or catalyst layer.

[0064] The gaps at the interface between the current collector and the conductive material of the gas diffusion layer may arise due to differences in physical properties such as rigidity and ductility, for example, when a metal commonly used as the current collector and graphite commonly used as the conductive material are used in applications of the electrodes disclosed herein. The size of these gaps may range from submillimeters to tens of nanometers. The water-repellent material of the water-repellent layer can effectively fill these gaps to prevent leakage. When the current collector is a metal and the liquid electrolyte is an alkaline aqueous solution, the metal surface is hydrophilic and can act as a place that guides the flow of the liquid electrolyte. Therefore, the water-repellent layer suppresses the flow of the liquid electrolyte due to its water repellency, thereby functioning particularly effectively to prevent leakage of the liquid electrolyte.

[0065] Furthermore, the electrode of the present disclosure is preferably an air electrode for an air battery, an oxygen cathode for sodium chloride electrolysis, a cathode for an alkaline fuel cell, or an anode for alkaline water electrolysis.

[0066] Furthermore, the electrode of the present disclosure preferably includes a non-electron-conductive reaction space-restricting portion disposed on the electrolyte side, and the reaction space-restricting portion preferably includes an electrolyte retention portion consisting of a plurality of recessed spaces capable of retaining a liquid electrolyte. This has the effect of suppressing, when the electrode of the present disclosure is used in an air battery together with a metal negative electrode, the growth of metal dendrites occurring in the metal negative electrode, and the uneven distribution of the metal reactant and its oxidation products in the negative electrode over charge-discharge cycles, which can cause an internal short circuit and disable the battery, or a significant decrease in battery capacity due to an increase in the number of charge-discharge cycles.

[0067] The electrode manufacturing method of the present disclosure is a method for manufacturing an electrode to be used in an oxygen reaction, and is characterized by comprising: step 1 of synthesizing an oxygen catalyst that has peaks at 2θ=30.07°±1.00°, 34.88°±1.00°, 50.20°±1.00°, and 59.65°±1.00° in X-ray diffraction measurement using CuKα rays, and is characterized by being an oxide containing bismuth, ruthenium, sodium, and oxygen as constituent elements; and step 2 of manufacturing a catalyst layer containing the oxygen catalyst, a conductive material, and a water-repellent material.

[0068] In the method for producing an electrode according to the present disclosure, it is desirable that the atomic ratio of oxygen to bismuth, O / Bi, and the atomic ratio of oxygen to ruthenium, O / Ru, are both greater than 3.5.

[0069] In the method for producing an electrode according to the present disclosure, it is preferable that in step 1, the secondary particle diameter of the oxygen catalyst is 3 μm or less.

[0070] In the method for producing an electrode according to the present disclosure, it is preferable that graphite having different particle sizes is used as the conductive material in step 2.

[0071] The method for producing an electrode according to the present disclosure preferably includes a step 3 of forming a gas diffusion layer on the catalyst layer or integrating the catalyst layer with a gas diffusion layer.

[0072] Furthermore, the method for producing an electrode according to the present disclosure preferably includes step 4, in which a current collector integrated with the catalyst layer or gas diffusion layer is coated on the surface opposite the side in contact with the electrolyte with a suspension containing a water-repellent material, followed by heat treatment. In the case of electrodes that undergo oxygen reactions, such as those disclosed herein, relatively hydrophilic materials, such as metals, are often used for the current collector. When a metal current collector is porous, such as a mesh, the current collector is integrated with the catalyst layer or gas diffusion layer, and the hydrophilic portion in contact with the metal may serve as a path for leakage of aqueous electrolytes or liquid electrolytes other than aqueous solutions. To maintain the long-term stability of the electrode according to the present disclosure, it is desirable to suppress even the slight possibility of leakage. By coating the electrode with a water-repellent material as described above and then heat-treating it to support it, leakage can be effectively suppressed for a long period of time.

[0073] Various methods of application can be used, including dripping, brushing, and spraying. The water-repellent material can be any of the materials mentioned above, but is not limited to the materials described herein. If the amount of water-repellent material carried is too small, the effect of suppressing leakage as described above will be weakened, while if it is too large, it will clog the areas in the catalyst layer or gas diffusion layer where oxygen flows, which is undesirable. For example, if PTFE is used as the water-repellent material in electrodes configured as in Examples 5 and 6 described below, the amount of water-repellent material carried per electrode area is 0.001 g / cm. 2 greater than 0.12 g / cm 2 A range smaller than this is desirable.

[0074] According to the method for producing an electrode of the present disclosure, the above-described electrode can be realized.

[0075] The present disclosure also provides an electrochemical device that uses the electrode described above, and is any one of an air battery, a salt electrolysis device, an alkaline water electrolysis device, an alkaline fuel cell, and a water electrolysis / fuel cell device that uses an alkaline aqueous solution as an electrolyte.

[0076] In the electrochemical device of the present disclosure, the active material of the negative electrode of the air battery is preferably any one of hydrogen, lithium, sodium, potassium, magnesium, calcium, and zinc.

[0077] According to the electrochemical device of the present disclosure, it is possible to enjoy the effects of the electrodes described above. [Effects of the Invention]

[0078] The electrode of the present disclosure uses an oxygen catalyst that contains sodium as a constituent element, unlike BRO with a pyrochlore structure, and thereby achieves the following effects: The oxygen catalyst has high compositional stability in terms of charge balance, improves long-term maintainability, and improves compositional uniformity from the interior to the surface even when nanoparticles are formed. This makes it possible to provide an electrode that can exhibit high catalytic activity for oxygen reduction and / or oxygen generation over long periods of use, regardless of the type of liquid electrolyte.

[0079] Furthermore, by having an oxygen catalyst with improved composition stability, long-term maintainability, and composition uniformity, and with suppressed generation of by-products, high catalytic activity and stability for oxygen reduction and / or oxygen generation can be simultaneously imparted, thereby reducing reaction overvoltage at the air electrode of an air battery, the oxygen cathode of sodium chloride electrolysis, the cathode of an alkaline fuel cell, and the anode of alkaline water electrolysis. Furthermore, the discharge voltage of an air primary battery is increased. Furthermore, the discharge voltage of an air secondary battery is increased and the charge voltage is decreased. Furthermore, the electrolysis voltage in sodium chloride electrolysis is decreased. Furthermore, the voltage of an alkaline fuel cell is increased. Furthermore, the electrolysis voltage in alkaline water electrolysis is decreased. These are maintained.

[0080] Furthermore, an increase in the discharge voltage of the air primary battery improves the energy density and power density of the air battery. Furthermore, an increase in the discharge voltage and a decrease in the charge voltage of the air secondary battery improves the energy density, power density, voltage efficiency, and energy efficiency, and these are also maintained.

[0081] Furthermore, a decrease in the electrolysis voltage in salt electrolysis reduces the power consumption and energy consumption of the chlorine and caustic soda produced, thereby reducing the electricity costs involved in production. In alkaline fuel cells, an increase in voltage improves the energy density and output density. In alkaline water electrolysis, a decrease in the electrolysis voltage reduces the power consumption and energy consumption of the hydrogen produced, thereby reducing the electricity costs involved in production. Furthermore, a water electrolysis / fuel cell device that uses an alkaline aqueous solution as the electrolyte can enjoy the benefits of alkaline water electrolysis and alkaline fuel cells described above.

[0082] Furthermore, the electrode disclosed herein does not require a post-process, such as acid treatment to remove by-products from the synthesis product obtained after BRO synthesis using an oxygen catalyst, as disclosed in Reference 4. This allows for further reductions in the manufacturing costs of primary air cells and secondary air cells, the manufacturing costs of chlorine and caustic soda produced by salt electrolysis, the manufacturing costs of alkaline fuel cells, and the manufacturing costs of hydrogen by alkaline water electrolysis for air electrodes, oxygen cathodes in sodium chloride electrolysis, fuel cell cathodes, and anodes in alkaline water electrolysis that use a BRO catalyst. Furthermore, the price of sodium is less than 1 / 1000 of that of bismuth and ruthenium, significantly reducing the raw material cost compared to BRO, thereby providing a more affordable electrode.

[0083] Furthermore, the electrode fabrication method of the present disclosure can provide an electrode having the above-described excellent properties. Also, it is possible to reduce the number of steps required to fabricate an electrode having the above-described excellent properties, thereby reducing the fabrication costs.

[0084] Furthermore, the electrode of the present disclosure can provide electrochemical devices such as air batteries, salt electrolysis devices, alkaline water electrolysis devices, alkaline fuel cells, and water electrolysis / fuel cell devices that use an alkaline aqueous solution as an electrolyte, which have the effects described above. [Brief explanation of the drawings]

[0085] [Figure 1]2A and 2B are diagrams illustrating the structure of an electrode according to the first embodiment. [Figure 2] 10A and 10B are diagrams illustrating the structure of an electrode according to a second embodiment. [Figure 3] 10A and 10B are diagrams illustrating the structure of an electrode according to a third embodiment. [Figure 4] 10A and 10B are diagrams illustrating the structure of an electrode according to a fourth embodiment. [Figure 5] 1 shows the results of X-ray diffraction measurements of the oxygen catalysts used in the electrodes of Examples 1 and 2. [Figure 6] 1 shows the results of X-ray diffraction measurement of the oxygen catalyst used in the electrode of Comparative Example 1. [Figure 7] 1 shows the results of X-ray diffraction measurements of the oxygen catalyst used in the electrode of Example 1 and the oxide of Comparative Example 2. [Figure 8] 1 is an SEM image showing the surface morphology of an oxygen catalyst used in the electrode of Example 1. [Figure 9] 1 is a graph showing the relationship between the primary particle size and frequency of the oxygen catalyst used in the electrode of Example 1. FIG. [Figure 10] FIG. 2 is a schematic diagram illustrating the structure of an electrode used in evaluating catalytic activity. [Figure 11] FIG. 2 is a graph showing the relationship between the specific activity of oxygen evolution and the electrode potential of the electrodes of Example 1, Example 2, and Comparative Example 1. [Figure 12] FIG. 2 is a graph showing the relationship between the specific activity for oxygen reduction and the electrode potential of the electrodes of Example 1, Example 2, and Comparative Example 1. [Figure 13] FIG. 1 is a particle size distribution diagram of unpulverized graphite and pulverized graphite. [Figure 14] FIG. 1 is a diagram showing the shape of a nickel mesh. [Figure 15] 1 is a diagram showing the relationship between the secondary particle size and frequency (particle size distribution) of an oxygen catalyst before pulverization treatment. [Figure 16] 1 is a graph showing the relationship between the secondary particle size and frequency of the oxygen catalyst after pulverization (particle size distribution). [Figure 17] FIG. 1 is a schematic diagram of a three-electrode measuring cell used to evaluate electrodes. [Figure 18] FIG. 10 is a graph showing the relationship between electrode potential and current density in Example 3. [Figure 19]FIG. 10 is a graph showing the relationship between electrode potential and current density in Example 4. [Figure 20] FIG. 10 is a graph comparing the relationship between electrode potential and current density in Example 3 and Example 4. [Figure 21] FIG. 10 is a graph comparing the relationship between charge / discharge potential and time in Example 6 by the number of cycles. DETAILED DESCRIPTION OF THE INVENTION

[0086] Hereinafter, embodiments and examples of the electrode, electrode fabrication method, and electrochemical device according to the present disclosure will be described. The electrode, electrode fabrication method, and electrochemical device according to the present disclosure are not limited to these embodiments or examples.

[0087] (First embodiment) The structure of the electrode according to this embodiment will be described. The electrode according to this embodiment is an electrode used in oxygen reactions, and has peaks at 2θ=30.07°±1.00°, 34.88°±1.00°, 50.20°±1.00°, and 59.65°±1.00° in X-ray diffraction measurement using CuKα radiation, and is provided with at least an oxide containing bismuth, ruthenium, sodium, and oxygen as constituent elements as its oxygen catalyst. A preferred configuration example of the electrode according to this embodiment will be described below.

[0088] FIG. 1 shows an electrode 101 according to this embodiment. The electrode 101 includes a catalyst layer 4, a gas diffusion layer 5, and a current collector 3. In the electrode 101, the catalyst layer 4, the gas diffusion layer 5, and the current collector 3 are laminated in this order. In the electrode 101, the catalyst layer 4 is disposed on the side that contacts the electrolyte. In addition, in the electrode 101, the current collector 3 is disposed on the gas side (atmosphere side). The catalyst layer 4 contains a conductive material, an oxygen catalyst, and a water-repellent material as constituent materials. The gas diffusion layer does not contain an oxygen catalyst.

[0089] The gas diffusion layer 5 is a layer that is permeable to gases (oxygen, air). The gas diffusion layer 5 contains, as its constituent materials, a conductive material and a water-repellent material similar to those of the catalyst layer 4, so that the gas diffusion layer 5 is permeable to gases and has water-repellent properties against the permeation of the electrolyte solution from the catalyst layer 4 side. The gas diffusion layer 5 may have a two-layer structure in which two layers using conductive materials with different sizes and shapes are bonded together. The gas diffusion layer 5 may be a commercially available gas diffusion layer for use in fuel cells.

[0090] The current collector 3 and the gas diffusion layer 5 may be in close contact with each other to maintain electronic conductivity, or may be integrally formed by pressing, etc. The catalyst layer 4 and the gas diffusion layer 5 may also be integrally formed by pressing.

[0091] The thickness of the catalyst layer 4 may be 50 to 300 μm, and the thickness of the gas diffusion layer 5 may be 50 to 500 μm. It is preferable to use a nickel mesh with an opening size of approximately 100 to 200 mesh as the current collector 3. If the current collector 3 does not have openings like a mesh, oxygen cannot pass through. Nickel is stable in alkaline aqueous solutions, so it is a preferable material for the mesh. However, as long as the electrolyte (alkaline aqueous solution) does not pass through to the current collector, the material for the mesh does not have to be limited to nickel.

[0092] Second Embodiment 2, the electrode 102 according to the second embodiment is the electrode 101 according to the first embodiment, further provided with a water-repellent layer 6. The gas diffusion layer 5 has the function of suppressing or preventing permeation of the electrolyte solution, but in order to prevent permeation or leakage of the electrolyte solution over a long period of time, it is preferable to provide the water-repellent layer 6 as in the electrode 102 described below.

[0093] In the electrode 102, a catalyst layer 4, a gas diffusion layer 5, a current collector 3, and a water-repellent layer 6 are laminated in this order. In the electrode 102, the catalyst layer 4 is disposed on the side that contacts the electrolyte. In addition, in the electrode 102, the water-repellent layer 6 is disposed on the gas side (atmosphere side). The water-repellent layer 6 is intended to prevent leakage of the electrolyte to the gas side of the electrode 102, and at the same time, it must be permeable to oxygen necessary for the reaction. For example, a commercially available porous membrane made of PTFE may be used as the water-repellent layer 6. The water-repellent layer 6 may be formed by spraying PTFE particles as water-repellent particles from the current collector 3 side and drying them. In this case, the gaps between the PTFE particles also serve as paths for oxygen to permeate.

[0094] The water-repellent layer 6 does not need to be conductive, and therefore the water-repellent layer 6 is not disposed between the current collector 3 and the gas diffusion layer 5 or between the current collector 3 and the catalyst layer 4. If a material is available that satisfies all of the requirements of electron conductivity, oxygen permeability, water repellency, and alkali resistance, a water-repellent layer other than the water-repellent layer 6 may be disposed on the electrolyte side of the current collector 3.

[0095] (Third embodiment) 3, the electrode 103 according to the third embodiment is the electrode 102 according to the second embodiment in which the gas diffusion layer 5 is omitted. The electrode 103 does not necessarily have to have the gas diffusion layer 5.

[0096] (Fourth embodiment) As shown in FIG. 4, the electrode 104 according to the fourth embodiment does not include the gas diffusion layer 5 of the electrode 101 according to the first embodiment. When the electrode according to the present disclosure is used as an anode for alkaline water electrolysis by immersing it in an alkaline aqueous solution, which serves as an electrolyte, it may be configured to include a catalyst layer 4 and a current collector 3, as in the electrode 104 shown in FIG. 4. In such an application as an anode, there is no need to introduce gas from the atmosphere into the electrode 104 as an anode, and the problem of electrolyte leakage to the gas side is eliminated in the first place. Therefore, the electrode may be configured without the gas diffusion layer 5 (see FIGS. 1 and 2) or the water-repellent layer 6 (see FIGS. 2 and 3).

[0097] Example 1 The present disclosure will be explained below with reference to examples. The oxide used as the oxygen catalyst for the electrode in Example 1 (hereinafter, sometimes simply referred to as the oxygen catalyst) was synthesized by the following procedure (an example of step 1). First, tetra-n-propylammonium bromide (abbreviation: TPAB, purity 98.0%) was dissolved in distilled water in a beaker, and then added to distilled water at 75°C using a hot stirrer. The concentration was approximately 9.0 x 10 -1 The concentration was set at mol / L. Hereafter, this solution will be referred to as the TPAB solution. TPAB acts as the aforementioned introduction promoter and stabilizer.

[0098] Next, ruthenium(III) chloride n-hydrate (Ru content 43.25%) and bismuth(III) nitrate pentahydrate (purity 99.5%) were weighed and dissolved in distilled water. The concentration of each solution was approximately 1.8 × 10 -1 The bismuth (III) nitrate pentahydrate solution was subjected to ultrasonic stirring for approximately 5 minutes. Hereafter, each solution will be referred to as the Ru solution and the Bi solution.

[0099] Next, predetermined amounts of the TPAB solution, Ru solution, Bi solution, and distilled water were mixed at 75°C to prepare a metal salt solution with a total volume of 500 mL. The Ru and Bi concentrations in this metal salt solution were both 7.44 × 10 -3 mol / L, the concentration of TPAB is 3.72 × 10 -2 The metal salt solution was stirred and mixed at 75°C for 1 hour, and then 60 mL of a separately prepared 2 mol / L aqueous solution of NaOH (sodium hydroxide) was added. After this, the temperature remained at 75°C and the mixture was stirred for 24 hours while oxygen was blown in at 50 mL / min (oxygen bubbling).

[0100] After stirring was completed, the mixture was allowed to stand for another 24 hours to obtain a precipitate. This precipitate was removed and evaporated to dryness in an electric furnace at 105°C for approximately 2 hours. The dried product thus obtained was transferred to an evaporating dish and dried in an electric furnace at 120°C for 3 hours. The dried material was pulverized in an agate mortar and then fired in an electric furnace at 600°C for 1 hour. The fired material was suction-filtered using 75°C distilled water, an aspirator, and filter paper. The material on the filter paper was collected and dried in an electric furnace at 120°C for 3 hours. In this way, an oxygen catalyst (oxide) to be used in the electrode of Example 1 was obtained.

[0101] In the above procedure, the relationship between the Ru concentration, Bi concentration, and TPAB concentration is important. TPAB acts as an incorporation promoter and stabilizer for sodium, but using too much or too little TPAB relative to the Ru or Bi concentration is undesirable. Too much TPAB inhibits the incorporation of sodium into the metal hydroxide, which is undesirable. On the other hand, too little TPAB is undesirable because it insufficiently stabilizes the sodium-containing metal hydroxide and increases the particle size of the resulting oxygen catalyst, making it difficult to obtain nano-level oxygen catalysts. In the oxygen catalyst used in the electrode in Example 1, the TPAB concentration is 5 times that of Ru. For example, if this is 1:1, there is insufficient TPAB, and if it is more than 20:1, there is an excess. In either case, it is undesirable because it will not function as an incorporation promoter and stabilizer. Note that the incorporation promoter and stabilizer is not limited to TPAB; other substances may also be used.

[0102] Example 2 In the synthesis of the oxygen catalyst used for the electrode of Example 1, predetermined amounts of TPAB solution, Ru solution, Bi solution, and distilled water were mixed at 75°C to obtain a metal salt solution with a total volume of 500 mL. The Bi concentration in the metal salt solution was 7.44 × 10 -3 mol / L to 6.96 x 10 -3 The oxygen catalyst used in the electrode of Example 2 was obtained in the same manner except for changing the concentration to mol / L.

[0103] (Comparative Example 1) The oxide used in the electrode of Comparative Example 1 was obtained in the same manner as in the synthesis of the oxygen catalyst used in the electrode of Example 1, except that the NaOH aqueous solution was changed to an LiOH (lithium hydroxide) aqueous solution.

[0104] (Comparative Example 2) The oxide synthesized in Comparative Example 2 was the same as in Example 1, except that the oxygen catalyst used in the electrode in Example 1 was additionally subjected to the following acid treatment. 0.145 g of the oxygen catalyst synthesized using the procedure described in Example 1 and 12 mL of 0.1 mol / L nitric acid solution were placed in a container and ultrasonically stirred for 30 minutes. After ultrasonic stirring, the mixture was allowed to stand for 1 hour (acid treatment). The supernatant was then removed, and 12 mL of distilled water was added for cleaning. The mixture was ultrasonically stirred for 30 minutes and then allowed to stand for 1 hour. The supernatant was then removed, and the same amount of distilled water was added again for cleaning. The mixture was ultrasonically stirred for the same amount of time, and then suction filtered with distilled water until the pH of the filtrate reached 7. All of these operations were performed at room temperature. The material on the filter paper was then removed and dried at 120°C for 3 hours to obtain the oxide of Comparative Example 2.

[0105] (X-ray diffraction measurement) The oxygen catalysts used in the electrodes of Examples 1 and 2 and the oxides of Comparative Examples 1 and 2 were analyzed with an X-ray diffractometer (Ultima IV, manufactured by Rigaku) using CuKα radiation (wavelength 1.54 Å). Measurement conditions were a voltage of 40 kV, a current of 40 mA, a diffraction angle 2θ (hereinafter sometimes simply referred to as 2θ) range of 10 to 90°, and a step angle of 0.020°. The results of Examples 1 and 2 are shown in FIG. 5 and Table 1, the results of Comparative Example 1 in FIG. 6, and a comparison of the results of Example 1 and Comparative Example 2 in FIG. 7.

[0106] As shown in Figure 5 and Table 1, the oxygen catalysts of Examples 1 and 2 were found to be oxides with diffraction peaks at average 2θ values (2θ averages) of 14.82°, 30.07°, 34.88°, 38.17°, 45.88°, 50.20°, 59.65°, 62.61°, 73.80°, 81.68°, and 84.28° by X-ray diffraction measurement using CuKα radiation. The 2θ averages in Table 1 were calculated by synthesizing multiple oxides for each of Examples 1 and 2, determining the 2θ values for all of them, and then calculating the average of the 2θ values for each of Examples 1 and 2. The 2θ averages in Table 1 are then averaged over these two averages. In other words, the 2θ averages in Table 1 are the averages for Examples 1 and 2.

[0107] The oxygen catalyst used in the electrode of Example 1 and the oxygen catalyst used in the electrode of Example 2 both exhibit diffraction peaks at 30.07°, 34.88°, 50.20°, and 59.65° with higher diffraction intensities than the others among the above 2θ values, which are characteristic of the oxygen catalyst used in the electrode of the present disclosure. Theoretically, the diffraction intensity in X-ray diffraction measurements tends to be significantly weaker as the particle size of the object decreases, especially at the nano-level. Therefore, as will be described later, the above characteristic 2θ values characterize the oxygen catalyst used in the electrode of the present disclosure, even for particles of several tens of nanometers. The results in Figure 5 did not show any diffraction peaks indicating the presence of by-products other than the oxygen catalyst used in the electrode of the present disclosure.

[0108] Next, as shown in FIG. 6, the results of Comparative Example 1 are different from those of the oxygen catalysts used in the electrodes of Examples 1 and 2, and diffraction lines (diffraction peaks) were detected at a large number of 2θ values. These detection results revealed that the oxide of Comparative Example 1 is a compound with a structure different from that of the oxygen catalyst of the present disclosure. In other words, it was found that if the sodium hydroxide aqueous solution in Example 1 is replaced with a lithium hydroxide aqueous solution as in Comparative Example 1, the oxygen catalyst used in the electrode of the present disclosure cannot be obtained. It was also suggested that the oxygen catalyst used in the electrode of the present disclosure contains sodium in its crystal structure.

[0109] Furthermore, as shown in FIG. 7, the results of Comparative Example 2 clearly differ from those of Example 1, and diffraction lines are detected at a significantly higher number of 2θ values. These detection results indicate that the crystal structure of the oxygen catalyst obtained in Example 1 was changed to a different crystal structure by the acid treatment. In other words, it was revealed that the acid treatment described in Patent Document 4 does not have the effect of removing impurities from the oxygen catalyst used in the electrode of the present disclosure, and that it actually has the effect of changing the structure of the oxygen catalyst used in the electrode of the present disclosure. These differences also reveal that the oxygen catalyst of the present disclosure is a different compound from BRO, which is disclosed in multiple documents, including Patent Document 4.

[0110] [Table 1]

[0111] (particle observation) Regarding the particle size of the oxygen catalyst of the present disclosure, as an example, the oxygen catalyst used in the electrode of Example 1 was observed with a scanning electron microscope (abbreviated as SEM, manufactured by ZEISS, ULTRA 55). The major axis of each particle was determined from the SEM image by image analysis (image processing). This major axis was defined as the primary particle diameter of the oxygen catalyst of Example 1, and its frequency distribution was calculated. The major axis is the longest distance between two parallel lines tangent to the particle outline in the SEM image of the particle. Figure 8 shows an SEM image of particle size observation. Figure 9 shows the results of a frequency distribution analysis of particle sizes obtained from such SEM images. The frequency distribution analysis was performed on at least 250 particles. The oxygen catalyst used in the electrode of Example 1 was obtained as nanoparticles with a particle size distribution ranging from 10 to 70 nm overall, with a high frequency of 20 to 30 nm. Similar observations and particle size analysis were also performed on Example 2, and the results were nearly identical to those of Example 1.

[0112] (Energy dispersive X-ray elemental analysis: EDX) An energy dispersive X-ray elemental analyzer (Genesis APEX2, manufactured by AMETEK) attached to the above-mentioned scanning electron microscope was used to determine the atomic ratios of sodium, bismuth, and ruthenium for the oxygen catalysts used in the electrodes of Examples 1 and 2. In this case, the acceleration voltage was set to 15 kV, and the accumulation time was set to 500 seconds, which is the maximum time that can be set on the analyzer.

[0113] (Rutherford Backscattering Analysis: RBS) The atomic ratios of bismuth, ruthenium, and oxygen were determined for the oxygen catalysts used in the electrodes of Examples 1 and 2 using a Rutherford backscattering analyzer (Pelletron 3SDH, manufactured by National Electrostatics). Bismuth and ruthenium were analyzed from the results of measurements using He ions as incident ions, and oxygen was analyzed from the results of measurements using H ions. Based on these analytical results, the atomic ratio of ruthenium to bismuth (Ru / Bi), the atomic ratio of oxygen to bismuth (O / Bi), and the atomic ratio of oxygen to ruthenium (O / Ru) were determined. Table 2 shows the atomic ratios O / Bi and O / Ru.

[0114] (Analysis of atomic ratios of four elements) From the results of EDX and RBS described above, the atomic ratios of the four elements bismuth (Bi), ruthenium (Ru), sodium (Na), and oxygen (O) are calculated. In the following, the atoms of bismuth, ruthenium, sodium, and oxygen may be referred to as Bi, Ru, Na, and O, respectively.

[0115] Here, the atomic ratio of O is given by the RBS results relative to Bi, and the atomic ratio of Na is given by the EDX results relative to Bi. Meanwhile, the atomic ratios of Bi and Ru are given by EDX and RBS, respectively. Therefore, there are two ways to calculate the atomic ratios of the four elements: either using the EDX or RBS results for the Bi and Ru atoms. Therefore, the atomic ratios were calculated using these two methods, and the results are shown in Table 3. In Table 3, the atomic ratios of Bi, Ru, Na, and O (atomic ratios of the four elements) are shown as EDX when the EDX results for the Bi and Ru atomic ratio are used, and RBS when the RBS results are used. The total charge ratio, the atomic ratio Na / Ru, the cation atomic ratio, and the cation charge ratio calculated using these values are also shown in Table 4. In this disclosure, oxygen catalysts are evaluated based on the RBS results in Tables 3 and 4, as well as the results in Table 2. That is, in Tables 2 to 4, oxygen catalysts are evaluated based on the atomic ratios determined by RBS, and the EDX results in Tables 3 and 4 are treated as reference values.

[0116] [Table 2]

[0117] [Table 3]

[0118] [Table 4]

[0119] (X-ray absorption fine structure analysis) For the oxygen catalyst of Example 1, X-ray absorption fine structure (XAFS) spectra were measured, and information on the chemical states of bismuth and ruthenium was obtained from the X-ray absorption near-edge structure (XANES) in the spectra. Measurements were performed using equipment at the High Energy Accelerator Research Organization (BL12C, NW10A) and the Aichi Synchrotron Light Center (BL1N2). Analysis of the L3-edge XANES spectrum of Bi revealed a +3 valence, and analysis of the K-edge XANES spectrum of Ru revealed a +4 valence. Note that the valence of Na cations is +1 only. The total charge ratio and cation charge ratio in Table 4 were calculated using the valence of each element described above and the atomic ratios of the four elements shown in Table 3.

[0120] Next, we obtained information about the local structure of the oxygen catalyst from the extended X-ray absorption fine structure (EXAFS), which appears approximately 100 eV higher than the absorption edge. First, we compared the FT-EXAFS spectrum (equivalent to the radial distribution function, which indicates the interatomic distances in the crystal structure) obtained from the EXAFS spectrum of the Bi L3 edge with the FT-EXAFS spectrum theoretically obtained assuming Bi occupies the A site in the A2B2O7 structure (hereinafter, the theoretically obtained FT-EXAFS spectrum is abbreviated as the theoretical spectrum). The peak intensity of the measured spectrum was smaller than that of the theoretical spectrum. The reason for this smaller peak intensity in the measured spectrum compared to the theoretical spectrum could be that the actual structure is different from the assumed crystal structure, i.e., the A2B2O7 structure, or that there is a difference in the interatomic distances, resulting in distortion. On the other hand, while the theoretical spectrum had a peak at 1.6 to 2.2 Å attributable to the first-nearest neighbor Bi-O component, the measured spectrum had a peak at 1.2 to 2.0 Å corresponding to the first-nearest neighbor Bi-O component. These results indicate that in the oxygen catalyst of Example 1, Bi is present in the vicinity of the A site assuming an A2B2O7 structure, but at a position away from the center of the site.

[0121] Similarly, the FT-EXAFS spectrum of the Ru K-edge was compared with the theoretically calculated FT-EXAFS spectrum assuming that Ru occupies the B site in the A2B2O7 structure. The measured spectrum had almost the same intensity as the theoretical spectrum. The theoretical spectrum had a peak at 1.2 to 1.6 Å due to the first-nearest-neighbor Ru-O component, while the measured spectrum also had a peak at 1.2 to 1.8 Å corresponding to the first-nearest-neighbor Bi-O component. The positions of other peaks considered to be second-nearest-neighbor Ru-O-Ru were also almost identical. These results indicate that Ru in the oxygen catalyst of Example 1 is located at the B site in the assumed A2B2O7 structure.

[0122] Furthermore, the FT-EXAFS spectrum of the Na K-edge was compared with theoretically calculated FT-EXAFS spectra assuming Na occupies either the A or B site in the A2B2O7 structure. The measured spectrum had smaller peak intensities than both the theoretical spectrum for A-site occupation and the theoretical spectrum for B-site occupation. The measured spectrum also showed peak (1) at 1.2 to 2.0 Å, and peak (2) at a longer distance of 2.0 to 2.8 Å. Meanwhile, the theoretical spectrum for A-site occupation showed a peak at 1.6 to 2.6 Å due to the first-nearest-neighbor Na-O component, which correlated with peak (2). The theoretical spectrum for B-site occupation showed a peak at 0.7 to 2.1 Å due to the first-nearest-neighbor Na-O component, which correlated with peak (1). These results indicated that Na exists in the oxygen catalyst of Example 1 at positions close to the A or B site in the assumed A2B2O7 structure. As mentioned above, considering the high similarity in the ionic radii of +3 bismuth ions and +1 sodium ions, it is highly likely that in the assumed A2B2O7 structure, there are more ions located near the A site than near the B site.

[0123] Regarding NaBiO3 measured as a reference sample, the rise of the absorption edge, the shape of the spectrum, and the position of the main peak in the Na K-edge XANES spectrum were all different from those of the catalyst of the present disclosure. Furthermore, the FT-EXAFS spectrum of the Na K-edge for NaBiO3 did not show two peaks like the above-mentioned peaks (1) and (2). These findings further support the idea that the catalyst of the present disclosure does not contain sodium-containing by-products like NaBiO3, but rather that sodium is present in the crystal structure.

[0124] From the above results, it was revealed that the oxygen catalyst used in the electrode of the present disclosure does not have a pyrochlore structure like BRO, but has a structure similar to pyrochlore, but sodium is thought to be located near the A site or B site in the pyrochlore, making it a structure different from BRO. Furthermore, the same analysis as above was performed on the oxygen catalyst used in the electrode of Example 2, and the results showed the same trends as in Example 1 regarding valence and interatomic distance.

[0125] (Electrode preparation) Electrodes were fabricated by supporting the oxygen catalysts of Examples 1 and 2 and the oxide of Comparative Example 1 on a conductive titanium disk using the following method. First, the oxygen catalyst or oxide was crushed in a mortar. The crushed powder was then added to a sample bottle at a concentration of 3.77 g / L using distilled water as a dispersion medium, and ultrasonic dispersion was performed for 2 hours using an ultrasonic generator to obtain a suspension. As shown in FIG. 10 , a cylindrical titanium disk 10 (diameter d: 4.0 mm, height h: 4.0 mm, hereinafter referred to as titanium disk 10) was placed in acetone and ultrasonically cleaned. Then, 10 μL of the suspension was dropped onto one side 11 of the titanium disk (one bottom surface of the cylinder) and allowed to dry naturally for 24 hours, obtaining an electrode 100 in which the oxygen catalyst or oxide was supported in a uniform film form on one side 11 of the titanium disk 10. In FIG. 10 , the oxygen catalyst or oxide supported in a film form is shown as oxide layer C. Note that no immobilizing agent was used to immobilize the oxygen catalyst or oxide on the titanium disk 10.

[0126] (Electrochemical measurements) The above-mentioned electrode was attached to a rotating electrode device, which served as the working electrode. This working electrode and a platinum plate (with an area of 25 cm) were then connected. 2 ) were immersed in a 0.1 mol / L potassium hydroxide solution in the same container. The pH of the potassium hydroxide solution was 13 or higher. In a separate container, a commercially available mercury / mercury oxide electrode was also immersed in a 0.1 mol / L potassium hydroxide solution. These two potassium hydroxide solutions were connected through a liquid junction filled with the same 0.1 mol / L potassium hydroxide solution. Using this three-electrode electrochemical cell, electrochemical measurements were performed at a temperature of 25°C. Measurements were performed using a commercially available electrochemical measurement device and electrochemical software, using linear sweep voltammetry. Linear sweep voltammetry is a method for measuring the current flowing through the working electrode while varying the potential of the working electrode at a constant scan rate. The current flowing during this measurement is the current resulting from the reaction occurring at the oxygen catalyst supported on the electrode. In other words, because the titanium disk alone does not produce oxygen reduction or oxygen generation over a wide potential range, the above measurement method allows us to measure the reaction current resulting from the oxide layer C alone.

[0127] The oxygen reduction current was measured as follows. First, nitrogen was bubbled through the aqueous solution in which the working electrode was immersed at a flow rate of 30 mL / min for more than 2 hours to remove dissolved oxygen, and then measurements were made while bubbled with nitrogen. Oxygen was then bubbled through at the same flow rate for more than 2 hours, and measurements were made again while continuing the bubble. After this, the oxygen reduction current was determined by subtracting the current measured while bubbled with nitrogen from the current measured while bubbled with oxygen. The oxygen reduction current density was determined by dividing this oxygen reduction current by the surface area of the titanium disk. In this way, results showing the relationship between the potential of the working electrode and the oxygen reduction current density were obtained and used to create the Tafel plots described below. During the above measurements, the working electrode was rotated at 1600 rpm (min -1) and used. Specifically, the titanium disk 10 was attached to a rotating disk electrode device (an example of a rotating disk electrode) with the oxygen catalyst fixed side (one side 11) facing downward, and rotated at a constant speed with the oxygen catalyst immersed in the electrolyte. This type of measurement is called the rotating disk electrode method or RDE (Rotating Disk Electrode) method. The scanning rate, which means the amount of change in potential per unit time, was 1 mV / s.

[0128] After measuring the oxygen reduction current described above, the oxygen evolution current was measured. The oxygen evolution current was measured under open-to-air conditions without nitrogen or oxygen aeration. Because oxygen evolution is a reaction in which oxygen is generated from hydroxide ions, it is not related to the aeration of nitrogen or oxygen. As with the oxygen reduction current, the oxygen evolution current was measured using linear sweep voltammetry at a scan rate of 1 mV / s while rotating at 1600 rpm.

[0129] (specific activity) In order to eliminate the influence of differences in the amount of oxygen catalyst supported on the linear sweep voltammograms obtained by the above method (results showing the relationship between the potential and current of the working electrode obtained by linear sweep voltammetry), the current value (A) during oxygen reduction or oxygen generation was divided by the catalyst weight (g) to use the specific activity. The unit of specific activity is A / g. The amount of oxygen catalyst supported ranged from 35 μg to 43 μg. The results of the electrode potential and specific activity for oxygen generation prepared in this manner are shown in FIG. 11. The results of the electrode potential and specific activity for oxygen reduction are also shown in FIG. 12. The short-dashed line, solid line, and long-dashed line in FIGS. 11 and 12 represent Example 1, Example 2, and Comparative Example 1, respectively.

[0130] Comparing the specific activities at an electrode potential of 0.6 V from the results in Fig. 11, it was found that Example 1 was 14.5 times and Example 2 was 29.0 times higher than Comparative Example 1, indicating that the catalytic activity was more than 10 times higher. Similarly, comparing the specific activities at an electrode potential of -0.1 V from the results in Fig. 12, it was found that Example 1 was 6.6 times and Example 2 was 6.1 times higher than Comparative Example 1, indicating that the catalytic activity was more than 6 times higher even in oxygen reduction.

[0131] As described above, it was found that the electrode of the present disclosure has high catalytic activity for both oxygen evolution and oxygen reduction.

[0132] (Tafel gradient) The linear sweep voltammograms described above were plotted, according to a standard method, with the common logarithm of the current density for oxygen reduction or oxygen evolution on the horizontal axis and the potential on the vertical axis (hereafter referred to as a Tafel plot). The slope of the linear portion of the Tafel plot, i.e., the Tafel slope, was calculated. The Tafel slope is the amount of potential change required to increase the current tenfold for various electrochemical reactions, including oxygen reduction and oxygen evolution, and is usually expressed in units of V / dec (dec is an abbreviation for decade, meaning 10 times). Here, the Tafel slope is positive for oxidation reactions and negative for reduction reactions; in either case, the smaller the absolute value, the higher the catalytic activity. In the following, the magnitude of the Tafel slope is expressed in terms of its absolute value.

[0133] On the other hand, oxygen evolution and oxygen reduction reactions are known to be electrochemical reactions with large Tafel slopes and difficult to occur. For example, even with platinum, which is known for its high catalytic activity, the absolute values of the Tafel slopes for both oxygen evolution and oxygen reduction are 60 mV / dec or more. There are very few catalysts that exhibit Tafel slopes smaller than that of platinum. However, the Tafel slopes for the oxygen catalyst of Example 1 were 44 mV / dec for oxygen evolution and −43 mV / dec for oxygen reduction, while the Tafel slopes for the oxygen catalyst of Example 2 were 39 mV / dec for oxygen evolution and −41 mV / dec for oxygen reduction. The Tafel slopes for the oxygen catalysts of Examples 1 and 2 were reduced by 25% or more compared to the Tafel slopes of platinum for both oxygen evolution and oxygen reduction, demonstrating their extremely high catalytic activity.

[0134] For the electrodes of Examples 1 and 2, the current measurement for the oxygen evolution and oxygen reduction was repeated about 10 times, but no change was observed in the measurement results. However, for the oxygen catalyst of Comparative Example 1, the current decreased after repeating the measurement two or three times, and coloration was observed in the potassium hydroxide aqueous solution used for the measurement, suggesting dissolution of constituent elements from the oxygen catalyst of Comparative Example 1. The results of Comparative Example 1 shown in Figures 11 and 12 are both the results of the initial measurement.

[0135] Example 3 The electrode of Example 3 was fabricated as follows. The electrode of Example 3 used graphite as the conductive material and PTFE (polytetrafluoroethylene) as the water-repellent material (an example of step 2). First, 66.5 g of zirconia balls with a diameter of 5 mm and 0.4 g of natural graphite (SEC Carbon) were placed in a zirconia container (manufactured by Fritsch, capacity 45 mL), which was then placed in a ball mill (manufactured by Fritsch, PL-7) and treated at a rotation speed of 1000 rpm for 10 minutes. After the treatment, the mixture was cooled and the crushed graphite was removed. The graphite treated in this manner was referred to as crushed graphite, and the graphite that was not crushed was referred to as uncrushed graphite.

[0136] The pulverized graphite and unpulverized graphite were measured using a laser diffraction / scattering particle size distribution analyzer (LA-960V2, manufactured by Horiba, Ltd.), and the results are shown in Figure 13. In Figure 13, the horizontal axis represents particle size and the vertical axis represents frequency. In the graph in Figure 13, the solid line represents the particle size distribution of the pulverized graphite, and the dashed line represents the particle size distribution of the unpulverized graphite. The 10% particle size of the unpulverized graphite is 1.10 μm, the 50% particle size is 1.96 μm, and the 90% particle size is 3.74 μm. The 10% particle size of the pulverized graphite is 0.30 μm, the 50% particle size is 3.00 μm, and the 90% particle size is 6.32 μm.

[0137] The particle size distribution curve for unpulverized graphite had a single peak at approximately 2 μm. The particle size distribution curve for pulverized graphite had two peaks at approximately 0.2 μm and 4 μm. In other words, the pulverization process yielded submicron-order pulverized graphite with a peak at approximately 0.2 μm.

[0138] For the electrode of Example 3, two types of graphite with different particle sizes were used: unpulverized graphite and pulverized graphite. Furthermore, when the unpulverized graphite and pulverized graphite were observed with an SEM (ULTRA55, manufactured by ZEISS), the unpulverized graphite was found to be flaky, while the pulverized graphite was particulate, with the basal plane broken and resulting in an irregular surface. The basal plane is smooth, making it difficult for catalyst particles to be supported, but with pulverized graphite, the basal plane is broken and results in an irregular surface, allowing the catalyst to be more highly dispersed and uniformly supported, resulting in a large catalyst area even with a small amount of catalyst.

[0139] Thus, the submicron-order graphite particles in the crushed graphite were effective in highly dispersing and supporting the catalyst, improving utilization efficiency, and reducing the amount of catalyst required per unit area of the electrode for high catalytic activity. Furthermore, by mixing graphite particles of different particle sizes and incorporating larger graphite particles, the larger graphite particles contributed to the formation of flow paths within the electrode that were suitable for supplying oxygen to the electrode and for releasing oxygen generated within the electrode to the atmosphere.

[0140] Next, 0.0947 g of the oxygen catalyst synthesized using the method and conditions described in Example 1, which passed through a 20 μm sieve (secondary particle diameter less than 20 μm), and 0.0474 g of crushed graphite were weighed and placed in an agate mortar and mixed for 10 minutes using an agate pestle. 0.0947 g of the oxygen catalyst and 0.0925 g of uncrushed graphite (an example of graphite with a particle size different from crushed graphite) were added to the mixture and mixed for another 10 minutes. 0.0823 g of PTFE suspension (Daikin Industries, Ltd., D-210C, PTFE content 60 wt%, PTFE average particle size 0.25 μm), 125 μL of liquid paraffin (Fujifilm Wako Pure Chemical Industries, Ltd., Wako First Grade Reagent), and 125 μL of distilled water were added to the mixture using a micropipette, in that order, and the mixture was kneaded for 10 minutes until a clay-like mixture was formed. The final kneaded mixture was approximately 1 cm x 3 cm in size.

[0141] The kneaded material was sandwiched between two stainless steel plates (Nilaco, SUS-304, thickness 0.1 mm, 5 cm x 5 cm) and pressed into a thin plate using a heat roll press (Yuri Roll Machine, TSC-220) at a roll pressure of 1 MPa, a roll speed of 0.5 m / min, and a clearance between the rolls of 250 μm.

[0142] On top of the pressed mixture (an example of a catalyst layer), a gas diffusion layer (GDL-22-BB, manufactured by SGL) and a nickel mesh (Nilaco, wire diameter 0.10 mm, 100 mesh) which would become the current collector 3 (see Figure 14) were layered in this order. The nickel mesh was pre-cut into the shape shown in Figure 13, and any portions of the mixture and gas diffusion layer that protruded beyond the 23 mm x 23 mm portion of the nickel mesh (current collecting portion 31) were cut off with a cutter. The 3 mm x 35 mm portion (lead wire portion 32) in Figure 13 is the lead portion for connecting to a measuring device.

[0143] This was sandwiched between two of the above-mentioned stainless steel plates and pressed at a roll pressure of 1 MPa, a roll speed of 0.5 m / min, a roll temperature of 80°C, and a clearance between the rolls of 275 μm to integrate the kneaded material, gas diffusion layer, and nickel mesh (an example of step 3), as in the electrode 101 of the first embodiment described above (see FIG. 1 ), and then removed from the stainless steel plates. The laminate of the integrated kneaded material, gas diffusion layer, and nickel mesh was immersed in turpentine oil (manufactured by Kanto Chemical, Grade 1) for 1 hour, and then immersed in ethanol (manufactured by Kanto Chemical) for 1 hour. The laminate was then removed from the ethanol and air-dried.

[0144] The laminate processed as described above was heat-treated in an electric furnace (Denken, KDF-S70) in a nitrogen atmosphere at 370°C for 13 minutes, then naturally cooled to 110°C in the electric furnace, and then removed from the electric furnace and further cooled.

[0145] The thickness of the obtained electrode was 0.20 μm, and the amount of catalyst per unit area of the electrode was 5.23 mg / cm 2 It was.

[0146] Example 4 The electrode of Example 4 was produced in the same manner as in Example 3, except that the oxygen catalyst of Example 1 was subjected to the following pulverization treatment and the mixing ratio of the kneaded material was changed.

[0147] First, 100 g of 1 mm diameter zirconia beads, 5.0 g of the oxygen catalyst synthesized by the method described in Example 1, and 8.5 mL of distilled water were placed in a zirconia container (Fritsch, 80 mL capacity), which was then placed in a ball mill (Fritsch, PL-7) and milled at 800 rpm for 10 minutes. After cooling, the solution containing the crushed oxygen catalyst was separated from the zirconia beads. The solution was then allowed to stand, the supernatant liquid was removed, and the residue containing the oxygen catalyst was transferred to a glass petri dish. This glass petri dish was placed in a vacuum dryer (As One, AVO-200SB) and vacuum dried at 80°C for approximately 5 hours.

[0148] The particle size of the oxygen catalyst pulverized as described above and the oxygen catalyst before and after pulverization was measured using a laser diffraction particle size analyzer (ANALYSETTE22, manufactured by Fritsch). As a result, the oxygen catalyst before pulverization had a maximum secondary particle size of approximately 150 μm and a volume average diameter of 26 μm, as shown in Figure 15, while the oxygen catalyst after pulverization had a maximum secondary particle size of 3 μm for 99.9% of the particles and a volume average diameter of 0.67 μm, as shown in Figure 16. In Figures 15 and 16, the horizontal axis represents particle size, the vertical axis on the left side represents accumulation, and the vertical axis on the right side represents frequency.

[0149] An electrode of Example 4 was fabricated using this pulverized oxygen catalyst. When initially mixing the oxygen catalyst and pulverized graphite in an agate mortar, 0.0947 g of unpulverized oxygen catalyst and 0.0474 g of pulverized graphite were used in Example 3, whereas 0.0947 g of pulverized oxygen catalyst and 0.0474 g of pulverized graphite were used in Example 4. Furthermore, to this mixture, 0.0947 g of unpulverized oxygen catalyst and 0.0925 g of unpulverized graphite were added in Example 3 and mixed for a further 10 minutes, whereas in Example 4, no oxygen catalyst was added, and only 0.0925 g of unpulverized graphite was added and mixed for 10 minutes. Example 4 was fabricated under the same conditions as Example 3, except for the above. The thickness of the resulting electrode of Example 4 was 0.18 mm, and the catalyst amount per unit area of the electrode was 0.68 mg / cm. 2 The catalyst amount per unit area was reduced by 87% compared to the electrode of Example 3. Since the electrode of Example 4 has the same area as the electrode of Example 3 and is thinner, the catalyst amount in the entire electrode was reduced by 88% in Example 4 compared to Example 3.

[0150] (Electrode evaluation) The electrode characteristics of the electrodes of Examples 3 and 4 were evaluated using a three-electrode measuring cell 200 and an electrochemical measuring device shown in FIG.

[0151] The three-electrode measuring cell 200 used has a conventional configuration, which will be described below. A rectangular cylindrical first container 28 contains a liquid electrolyte 24 and a counter electrode 22. The counter electrode 22 is immersed in the liquid electrolyte 24. The electrode 21 to be evaluated is positioned to cover a through-hole 28a in the sidewall of the first container 28, with one side in contact with the electrolyte 24 and the other side in contact with the atmosphere. The electrode 21 serves as the working electrode in the measurement. A second container 29 is located in addition to the first container 28, and the second container 29 contains a reference electrode 23 and a liquid electrolyte 24. The reference electrode 23 is immersed in the electrolyte 24. The electrolytes 24 in the first container 28 and the second container 29 are connected by a liquid junction 25. The liquid junction 25 is a tube shaped as shown in Figure 17 and filled with the electrolyte 24. One end of the liquid junction 25 is inserted into the electrolyte 24 in the first container 28, and the other end is inserted into the electrolyte 24 in the second container 29. The electrode 21 is sealed so that the electrolyte 24 does not leak from the contact surface with the first container 28 .

[0152] The electrode of Example 3 or Example 4 was placed as electrode 21 in Fig. 17, with the nickel mesh side facing the atmosphere and the opposite side in contact with a 6 mol / L KOH aqueous solution (electrolyte 24). In this case, the opening on the atmospheric side of the electrode of the present disclosure was 20 mm x 20 mm, and the electrode area based on this was 4 cm 2 When using this value to calculate the current density described later, the current is calculated for an electrode area of 4 cm 2 The value was calculated by dividing the KOH solution by 1 / 2. A platinum plate (5 cm x 5 cm x 0.1 mm) was immersed in this KOH aqueous solution as the counter electrode 22. Furthermore, a mercury / mercury oxide electrode in a 6 mol / L KOH aqueous solution was used as the reference electrode 23, and this reference electrode 23 and the KOH solution in contact with the electrode of the present disclosure were connected by a liquid junction 25 filled with a 6 mol / L KOH aqueous solution. The potential and current of the electrode of the present disclosure were measured at room temperature using cyclic voltammetry, which measures the current while changing the potential of the electrode of the present disclosure relative to the reference electrode 23 at a rate of 50 mV / s. During the measurement, oxygen was blown at 400 mL / min from the atmosphere side of the electrode of the present disclosure.

[0153] The relationship between the potential and current obtained with the electrodes of Examples 3 and 4 is shown in Figures 18 and 19, respectively. In Figures 18 and 19, the horizontal axis represents the electrode potential (hereinafter sometimes simply referred to as potential), and the vertical axis represents the measured current as a function of the electrode area (4 cm 2 ) is the current density calculated by dividing the ohmic loss by the ohmic loss. In Figures 18 and 19, the first cycle is indicated by a solid line and the 50th cycle by a dashed line. Measurements were performed over the same potential range in Figures 18 and 19, and the potentials were calculated by correcting for ohmic loss. Ohmic loss is caused by the solution resistance Rs of the liquid junction connecting the electrode of the present disclosure and the reference electrode. If the current flowing through the electrode of the present disclosure is I, a potential equivalent to I × Rs is generated in the electrode of the present disclosure. This potential equivalent to I × Rs is unavoidable in measurements using a three-electrode measurement cell and varies depending on the length of the liquid junction and the concentration of the KOH solution; therefore, it is not generated by the electrode of the present disclosure itself. Therefore, in electrochemical measurements such as those described above, the catalytic activity of an electrode is generally evaluated based on the relationship between the ohmic loss-corrected potential and the current or current density. This is essential when the current flow is large and cannot be ignored, such as when the value of I×Rs (unit: volts) is 100 mV or more, and the measurement results of the electrodes in Examples 3 and 4 also fell into this category, so the relationship between potential and current density after correcting for ohmic loss is shown in Figures 18 and 19.

[0154] 18 and 19, it can be seen that the oxygen evolution current (positive current in the figure) and oxygen reduction current (negative current in the figure) of the electrode of the present disclosure did not change between the 1st and 50th cycles. Furthermore, the maximum current density shown in the figure was ±600 mA / cm. 2 As described above, the potential difference required from the potential at which current begins to flow to such a high current density is very small, about 100 mV for oxygen evolution (between 500 mV and 600 mV in electrode potential) and about 200 mV for oxygen reduction (between -100 mV and -300 mV in electrode potential). In other words, it can be seen that the electrodes of the present disclosure shown in Examples 3 and 4 both have high catalytic activity and stability.

[0155] Next, Figure 20 compares the results of the first cycle for the electrodes of Example 3 and Example 4. In Figure 20, the results of Example 3 are shown by a solid line, and the results of Example 4 are shown by a dashed line. In Figure 20, as in Figures 18 and 19, the horizontal axis represents electrode potential, and the vertical axis represents current density. This shows almost no difference in the relationship between the oxygen evolution current, oxygen reduction current, and potential. Meanwhile, as mentioned above, the catalyst amount in the electrode of Example 4 is 87 to 88% less than that of Example 3. In other words, in terms of oxygen catalyst per unit mass, the crushed oxygen catalyst has catalytic activity that is more than eight times higher than that of the uncrushed oxygen catalyst. This is because the particle size of the oxygen catalyst is reduced by the crushing process, increasing the surface area per unit mass.

[0156] As described above, it was found that the catalytic activity of the electrode can be significantly improved by using an oxygen catalyst having a secondary particle size of 3 μm or less as in Example 4.

[0157] Example 5 The electrode of Example 5 was produced in the same manner as in Example 4, except that the following steps were performed in the electrode production process of Example 4. In Example 4, as in Example 3, the laminate was heat-treated in an electric furnace (KDF-S70, manufactured by Denken) at 370°C for 13 minutes in a nitrogen atmosphere, and then held in the electric furnace in a state of natural cooling to 110°C, after which it was removed from the electric furnace and further cooled. Before the heat treatment in the electric furnace, the electrode of Example 5 was spray-coated on the current collector side of the laminate with a PTFE suspension (D-210C, manufactured by Daikin Industries, Ltd., PTFE content 60% by weight) using an airbrush (HP-CS, manufactured by Anest Iwata), and then heat-treated and cooled in the electric furnace (an example of step 4). As a result, 0.065 g (0.012 g / cm per electrode area) of PTFE suspension was applied to the current collector side of the electrode of Example 5. 2 ) PTFE particle layer (an example of a water-repellent layer) was supported. That is, the electrode of Example 5 has the structure of the electrode 102 described in the second embodiment above. The electrode of Example 5 obtained in this manner was subjected to a 50-cycle test by cyclic voltammetry in the same manner as in Example 4. As a result, no leakage of liquid was observed on the current collector side of the electrode of Example 5 even after the test.

[0158] Example 6 The electrode of Example 6 was produced in the same manner as in Example 4, except that the following steps were performed in the electrode production process of Example 4. In Example 4, as in Example 3, the laminate was heat-treated in an electric furnace (KDF-S70, manufactured by Denken) at 370°C for 13 minutes in a nitrogen atmosphere, and then held in the electric furnace in a state of natural cooling to 110°C, after which it was removed from the electric furnace and further cooled. For the electrode of Example 6, after the heat treatment in the electric furnace, a PTFE suspension (D-210C, manufactured by Daikin Industries, Ltd., PTFE content 60% by weight) was sprayed onto the current collector side of the laminate using an airbrush (HP-CS, manufactured by Anest Iwata), and then the electrode was heat-treated in the electric furnace and cooled (an example of step 4). As a result, 0.0395 g (0.0075 g / cm per electrode area) of PTFE suspension was applied to the current collector side of the electrode of Example 6. 2 ) PTFE particle layer (an example of a water-repellent layer) was supported.

[0159] The electrode of Example 6 thus obtained was subjected to a charge-discharge cycle test in the following manner. For the charge-discharge cycle test, the same three-electrode measurement cell as in Examples 3 and 4 shown in FIG. 17 was used, and the same counter electrode, electrolyte, and reference electrode were also used. The current density for both charge and discharge was 100 mA / cm. 2 The current application time was 60 seconds, and the current application was paused for 10 seconds between charge and discharge, or between discharge and charge. This constitutes one cycle, and the electrode potential of Example 6 was recorded relative to the reference electrode while continuously performing charge and discharge cycles. Note that in this charge and discharge cycle test, oxygen, which is a reactant, was not forcibly supplied to the electrode by blowing air or oxygen onto the electrode, and the electrode potential was measured at 100 mA / cm under open-to-air conditions. 2 The charge-discharge cycle test was conducted under extremely harsh conditions that are likely to cause electrode deterioration, as the current density was applied at 100 s. Furthermore, the fact that the current was applied for 60 seconds, repeated with a 10-second break in between, is also likely to accelerate electrode deterioration, and durability experiments were conducted under extremely harsh conditions.

[0160] Figure 21 shows the results of comparing the potential curves of representative cycles obtained during the charge-discharge cycle test. As shown in Figure 21, the charge potential (approximately 0.6 V) and discharge potential (approximately -0.3 V) remained almost unchanged even after 500 charge-discharge cycles.

[0161] Furthermore, during the charge-discharge cycle test up to the end of 500 cycles, no leakage of the KOH aqueous solution was observed from the atmospheric side of the electrode, i.e., from the PTFE particle layer.

[0162] From the above results, it was revealed that the electrode of Example 6 was able to maintain high catalytic activity and had high durability even under extremely severe charge-discharge test conditions by providing a PTFE particle layer as a water-repellent layer on the atmosphere side of the gas diffusion layer that was integrated with the current collector.

[0163] Example 7 The oxygen catalyst was pulverized under the same conditions as in Example 4, except that the pulverization time in Example 4 was changed to 5 minutes, and observed by SEM. Furthermore, this pulverized oxygen catalyst was supported on a titanium disk under the same conditions as in Examples 1 and 2, to prepare an electrode of Example 7. The oxygen reduction current and oxygen evolution current of the electrode of Example 7 were measured in the same manner as in Examples 1 and 2. Furthermore, the value (specific activity) was calculated by dividing each current value by the catalyst weight. The amount of oxygen catalyst supported in Example 7 was within the same range as in Examples 1 and 2.

[0164] As a result of SEM observation of the oxygen catalyst pulverized for 5 minutes, particles of 70 nm or less were observed, similar to those in Examples 1 and 2 shown in Figure 9, but particles with a primary particle size of greater than 100 nm, which were not observed in Examples 1 and 2, were also observed, and some particles had a primary particle size reaching 200 nm. In other words, it was found that the maximum primary particle size in Example 7 became greater than 100 nm as a result of the pulverization treatment.

[0165] Next, the specific activities of oxygen evolution and oxygen reduction were compared for the electrodes of Example 2 and Example 7. The specific activities of oxygen evolution at 0.56 V were 23 A / g for Example 2 and 23 A / g for Example 7, which were the same. However, the specific activities of oxygen reduction at −0.1 V were −4.5 A / g for Example 2 and −1.4 A / g for Example 7. In other words, the difference in the primary particle size of the oxygen catalyst used to prepare the electrode did not affect the specific activity of oxygen evolution. In contrast, the specific activity of oxygen reduction in Example 7 was 31% of that in Example 2, indicating that the larger primary particle size reduced the specific activity for oxygen reduction. This result is thought to be due to the difference in the reaction sites for oxygen evolution and oxygen reduction, which are the two-phase interface in Example 7 and the three-phase interface in Example 2, and the fact that the reaction site for oxygen reduction is more strongly influenced by the primary particle size. From these results, it can be seen that the preferred primary particle size of the oxygen catalyst is 100 nm or less. [Industrial Applicability]

[0166] The electrode of the present disclosure can be used as an air electrode in primary air cells or secondary air cells, an oxygen cathode in brine electrolysis (device), a cathode in alkaline fuel cells (device), or an anode in alkaline water electrolysis. It can also be used as an electrode for oxygen generation, oxygen reduction, or both reactions in batteries, electrolysis devices, and sensors that use an alkaline aqueous solution as an electrolyte and utilize oxygen reduction, oxygen generation, or both reactions. It can also be used as an electrode for oxygen generation, oxygen reduction, or both reactions at temperatures below 100°C in oxygen reactions using electrolytes other than alkaline aqueous solutions. For example, it can be used as a cathode in a polymer electrolyte fuel cell. The electrochemical device of the present disclosure can be used for power generation and / or storage in various electrical devices and products, such as mobile devices, electronic devices, electrical appliances, bicycles, automobiles, trains, ships, aircraft, and drones, as well as for hydrogen production, oxygen production, chlorine production, and caustic soda production. [Explanation of symbols]

[0167] 10: Disc (titanium disc, titanium) 11: Single side (front) 100: Electrode 101: Electrode 102: Electrode 103: Electrode 104: Electrode C: Oxide layer (oxygen catalyst, oxide) 21: Electrode 22: Opposite 23:Reference electrode 24: Electrolyte 25: Liquid junction 28:First container 29:Second container 200: Measuring cell 3: Current collector 31: Current collecting part 32: Lead wire section 4: Catalyst layer 5: Gas diffusion layer 6: Water-repellent layer

Claims

1. An electrode for use in an oxygen reaction, characterized in that the electrode has peaks at 2θ=30.07°±1.00°, 34.88°±1.00°, 50.20°±1.00°, and 59.65°±1.00° in X-ray diffraction measurement using CuKα rays, and comprises an oxide containing bismuth, ruthenium, sodium, and oxygen as constituent elements as an oxygen catalyst.

2. 2. The electrode according to claim 1, wherein the atomic ratio of oxygen to bismuth, O / Bi, and the atomic ratio of oxygen to ruthenium, O / Ru, are both greater than 3.

5.

3. 3. The electrode according to claim 1, wherein the oxygen reaction occurs in an alkaline aqueous solution as an electrolyte.

4. 4. The electrode according to claim 1, wherein the oxygen catalyst has a primary particle size of 100 nm or less.

5. 5. The electrode according to claim 1, wherein the secondary particle diameter of the oxygen catalyst is 3 μm or less.

6. 6. The electrode according to claim 1, further comprising a gas diffusion layer.

7. 7. The electrode according to claim 1, further comprising a catalyst layer containing the oxygen catalyst, a conductive material, and a water-repellent material as constituent materials.

8. 8. The electrode according to claim 7, wherein the conductive material is graphite having different particle sizes.

9. 9. The electrode according to claim 7, which is formed in a thin plate shape and has a thickness of 250 μm or less.

10. 10. The electrode according to claim 7, further comprising a water-repellent layer that is oxygen-permeable on the atmospheric side of the catalyst layer or the gas diffusion layer.

11. 11. The electrode according to claim 10, wherein the water-repellent layer is made of water-repellent particles.

12. 12. The electrode according to claim 1, which is an air electrode for an air battery, an oxygen cathode for sodium chloride electrolysis, a cathode for an alkaline fuel cell, or an anode for alkaline water electrolysis.

13. 13. The electrode according to claim 1, further comprising a non-electron-conductive reaction space restricting portion disposed on the electrolyte side, the reaction space restricting portion comprising an electrolyte retaining portion consisting of a plurality of concave spaces capable of retaining a liquid electrolyte.

14. A method for producing an electrode to be used in an oxygen reaction, the method comprising: step 1 of synthesizing an oxygen catalyst, the oxygen catalyst having peaks at 2θ=30.07°±1.00°, 34.88°±1.00°, 50.20°±1.00°, and 59.65°±1.00° in X-ray diffraction measurement using CuKα rays, and characterized in that the oxygen catalyst is an oxide containing bismuth, ruthenium, sodium, and oxygen as constituent elements; and step 2 of producing a catalyst layer containing the oxygen catalyst, a conductive material, and a water-repellent material.

15. 15. The method for producing an electrode according to claim 14, wherein both the atomic ratio of oxygen to bismuth, O / Bi, and the atomic ratio of oxygen to ruthenium, O / Ru, are greater than 3.

5.

16. 16. The method for producing an electrode according to claim 14, wherein in step 1, the secondary particle diameter of the oxygen catalyst is set to 3 μm or less.

17. 17. The method for producing an electrode according to claim 14, wherein graphite having different particle sizes is used as the conductive material in step 2.

18. 18. The method for producing an electrode according to claim 14, further comprising step 3 of forming a gas diffusion layer on the catalyst layer or integrating the catalyst layer with a gas diffusion layer.

19. 19. The method for producing an electrode according to claim 14, further comprising a step 4 of applying a suspension containing a water-repellent material to a surface of a current collector that is integrated with the catalyst layer or the gas diffusion layer, the surface being opposite to the surface that comes into contact with the electrolyte, followed by a heat treatment.

20. An electrochemical device using the electrode according to any one of claims 1 to 13.

21. 21. The electrochemical device according to claim 20, which is an air battery, a salt electrolysis device, an alkaline water electrolysis device, an alkaline fuel cell, or a water electrolysis / fuel cell device using an alkaline aqueous solution as an electrolyte.

22. 22. The electrochemical device according to claim 21, wherein the active material of the negative electrode of the air battery is hydrogen, lithium, sodium, potassium, magnesium, calcium, or zinc.

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