Oxygen catalyst, electrode material or electrode using said oxygen catalyst, electrochemical device using said oxygen catalyst or said electrode, and production method for oxygen catalyst or electrode material having said oxygen catalyst

Abstract

The present invention relates to: an oxygen catalyst that contains a pyrochlore oxide in which the main component of an A site is trivalent bismuth ions and the main component of a B site is tetravalent ruthenium ions, said oxygen catalyst being characterized in that a Tafel slope for an oxygen reaction is 33 mV / dec or less; an electrode material, an electrode, and an electrochemical device each using the oxygen catalyst; and the oxygen catalyst or an electrode material using the oxygen catalyst.

Description

Oxygen catalyst, electrode material or electrode using said oxygen catalyst, electrochemical device using said oxygen catalyst or said electrode, and method for producing an oxygen catalyst or an electrode material having said oxygen catalyst

[0001] The present invention relates to an oxygen catalyst used in an oxygen reaction, an electrode material or electrode using the catalyst, an electrochemical device using the oxygen catalyst or the electrode, and a method for producing the oxygen catalyst or the electrode material having the oxygen catalyst.

[0002] The oxygen reaction described in this specification means a reaction that generates oxygen as an oxidation reaction, or a reaction that reduces oxygen as a reduction reaction, or both, at the interface between an electrode and an electrolyte in an electrochemical reaction. Electrochemical reactions include oxidation reactions in which a reductant transfers electrons to an electrode to become an oxidant, and reduction reactions in which an oxidant receives electrons from an electrode to become a reductant. An example of an oxygen reaction, an oxidation reaction and a reduction reaction when the electrolyte is an alkaline aqueous solution, are expressed by the following formulas (1) and (2). Note that while the oxygen reaction shown here is a four-electron reaction, there are other four-electron reactions as well as two-electron oxygen reactions, but these will be omitted here. Oxidation reaction: 4OH － →O ２ +2H ２ O+4e － ... (1) Reduction reaction: O ２ +2H ２ O+4e － →4OH － ... (2)

[0003] In these equations, the reductant is OH － (Oxygen has an oxidation number of -2) and the oxidant is O ２ (The oxidation number of oxygen is 0.) In addition, in the case of oxygen reactions in acidic aqueous solutions, the reduced product is H ２ O (oxidation number of oxygen is -2), oxidant is O ２ (The oxidation number of oxygen is 0).

[0004] Such oxygen reactions are known to be utilized in the anode reaction in water electrolysis, the cathode reaction in fuel cells, the cathode reaction at the active cathode in sodium chloride electrolysis, the charging reaction at the positive electrode of a primary air battery, and the charge / discharge reaction at the positive electrode of a secondary air battery. At the positive electrode of a secondary air battery, an oxidation reaction occurs during charging, and a reduction reaction occurs during discharging. Furthermore, in seawater electrolysis, it is desirable to generate oxygen while suppressing the generation of chlorine at the anode, and the oxygen reaction can also be utilized at the anode in such seawater electrolysis. Seawater electrolysis here also includes electrolysis using an aqueous solution obtained by extracting water from seawater and dissolving a chloride, such as NaCl, in the extracted water.

[0005] Among various electrochemical reactions, the oxygen reaction is well known as one of the most difficult to initiate. Therefore, a catalyst is required to promote the oxygen reaction, and this is called an oxygen catalyst. In electrochemical reactions, a driving force (overvoltage) is applied to cause an oxidation reaction by making the electrode potential more noble than the equilibrium electrode potential, and a driving force (overvoltage) to cause a reduction reaction by making the electrode potential more base than the equilibrium electrode potential. The initiation overvoltage is defined as the difference between the electrode potential at which an oxidation or reduction reaction actually begins and the electrode potential at equilibrium. The initiation overvoltage is positive for oxidation reactions and negative for reduction reactions, but its absolute value is important. Therefore, even for reduction reactions, the initiation overvoltage is often expressed as its absolute value without any special explanation. Hereinafter, the initiation overvoltage will be treated as an absolute value regardless of whether it is an oxidation or reduction reaction.

[0006] The ease with which an electrochemical reaction occurs can be determined by the magnitude of this onset overvoltage. Oxygen reactions are less likely to occur than other electrochemical reactions because their onset overvoltage is larger. According to electrochemical theory, the onset overvoltage varies depending on the Tafel slope and exchange current density. The Tafel slope is the slope of the linear region observed in the potential region where the current is small, with the common logarithm of the electrochemical reaction current or the current density calculated by dividing the current by the electrode area on the horizontal axis and the electrode potential on the vertical axis. Its unit is generally mV / dec (dec stands for decade and refers to the overvoltage required to increase the current tenfold). For example, the Tafel slope of the oxygen reaction on platinum, which is said to have high catalytic activity, is known to be 60 mV / dec to 120 mV / dec. The exchange current density is commonly defined as the current at equilibrium divided by the electrode surface area. Here, the current at equilibrium is based on kinetic equilibrium, which states that the equilibrium of an electrochemical reaction is zero because the oxidation and reduction reactions occur at the electrode-electrolyte interface at the same rate, i.e., with the same current. The role of the oxygen catalyst is to change the Tafel slope, the exchange current density, or both. However, reducing the Tafel slope is particularly important for increasing catalytic activity, as follows:

[0007] The difference between the Tafel slope and the exchange current density is that the Tafel slope does not depend on the reaction area of ​​the oxygen catalyst, while the exchange current density varies depending on the reaction area. Furthermore, in electrochemical reactions involving two or more electrons (hereinafter referred to as multi-electron reactions), when the overall reaction is considered to consist of multiple reaction steps, the Tafel slope varies depending on which reaction step is the rate-determining step. Generally, in multi-electron reactions, the later the reaction step in the multi-step reaction is, the smaller the Tafel slope becomes.

[0008] Here, the Tafel slope can be considered the overvoltage required to increase the current tenfold. However, in electrochemical reactions, the reaction cannot be determined to have started unless the current increases by at least three orders of magnitude relative to the exchange current density. Therefore, if the Tafel slope is 120 mV / dec, the reaction onset overvoltage is at least 360 mV, and even if the Tafel slope is 60 mV / dec, the reaction onset overvoltage must be at least 180 mV. On the other hand, if the Tafel slope is 30 mV / dec, the reaction onset overvoltage becomes 90 mV, potentially reducing the reaction onset overvoltage to 100 mV or less (0.1 V or less). In oxygen reactions, even when an oxygen catalyst is used, the reaction onset overvoltage is generally 300 mV or more, and this is also true when noble metals such as platinum are used as nanoparticles of the oxygen catalyst. Thus, the high reaction onset overvoltage is a major issue in oxygen reactions.

[0009] In response to this, the present inventors developed an oxygen catalyst capable of achieving a Tafel slope of 39 mV / dec, which they disclosed in Patent Document 1. They also developed another oxygen catalyst having a Tafel slope equivalent to this, which they disclosed in Patent Document 2. The oxygen catalysts disclosed in these documents are all oxides in which the main component of the A site in a pyrochlore structure is a +3-valent bismuth ion and the main component of the B site is a +4-valent ruthenium ion. Similar oxides having a similar structure are also disclosed in Patent Documents 3 to 9. Furthermore, oxygen catalysts made of oxides other than those having a pyrochlore structure are disclosed in, for example, Patent Documents 10 and 11.

[0010] International Publication No. WO2020 / 153401 Patent No. 6799346 Patent No. 7081762 Patent No. 7149525 JP 2018-149518 JP 2022-172734 JP 2023-26826 JP 2023-76130 JP 2023-159730 JP 2018-149518 JP 2015-046403

[0011] As mentioned above, it is necessary and important to reduce the Tafel slope in order to reduce the reaction onset overvoltage of an oxygen catalyst. However, conventional oxygen catalysts have a Tafel slope of less than 33 mV / dec, and there was a problem in that there was no oxygen catalyst that could reduce the reaction onset overvoltage to less than 100 mV. In addition to such high catalytic activity, there was also a problem in that there was no oxygen catalyst that was chemically and electrochemically stable against electrolytes and oxygen reactions. In particular, there was a problem in that there was no oxygen catalyst that was highly durable against reverse electrolysis.

[0012] Another problem is that there is no electrode material having an oxygen catalyst with the above-mentioned properties, or an electrode or electrochemical device formed from an oxygen catalyst having the above-mentioned properties or an electrode material having this oxygen catalyst.Furthermore, there is no method for producing an oxygen catalyst having the above-mentioned properties, or a method for producing an electrode material having this oxygen catalyst.

[0013] An object of the present invention is to provide an oxygen catalyst, an electrode material or electrode using the oxygen catalyst, an electrochemical device using the oxygen catalyst or the electrode, and a method for producing an oxygen catalyst or an electrode material having the oxygen catalyst.

[0014] In order to solve the above problems, the oxygen catalyst of the present invention is an oxygen catalyst containing a pyrochlore oxide in which the main component of the A site is a +3-valent bismuth ion and the main component of the B site is a +4-valent ruthenium ion, and is characterized in that the Tafel gradient for the oxygen reaction is 33 mV / dec or less. This has the effect of making it possible to reduce the reaction onset overpotential for the oxygen reaction to 100 mV or less. Note that the Tafel gradient is a positive value for oxidation reactions and a negative value for reduction reactions, but the Tafel gradient defined here is its absolute value.

[0015] This specification has the following aspects. 1. An oxygen catalyst comprising a pyrochlore oxide in which the main component of the A site is a +3-valent bismuth ion and the main component of the B site is a +4-valent ruthenium ion, the oxygen catalyst having a Tafel gradient for the oxygen reaction of 33 mV / dec or less. 2. The oxygen catalyst according to the above item 1, wherein the primary particle diameter d90 of the oxygen catalyst is 20 nm or less. 3. The oxygen catalyst according to the above item 1 or 2, wherein the oxygen catalyst contains +1-valent sodium ions, +4-valent manganese ions, or both of these ions. 4. The oxygen catalyst according to any one of the above items 1 to 3, wherein the oxygen catalyst has peaks at 2θ = 30.07° ± 1.00°, 34.88° ± 1.00°, and 50.20° ± 1.00° in X-ray diffraction measurement using CuKα radiation. 5. The oxygen catalyst in the oxygen reaction converts the oxidant into O ２ , the reduced substance is H ２ O or oxidized to O ２ , the reduced substance is OH －The oxygen catalyst according to any one of items 1 to 4 above, characterized in that the reaction is an oxidation reaction, a reduction reaction, or both of the above reactions. 6. An electrode material, characterized in that the oxygen catalyst according to any one of items 1 to 5 above is supported on or formed on a conductive material. 7. The electrode material according to item 6 above, characterized in that the conductive material is made of any of carbon, metal, alloy, and ceramic. 8. The electrode material according to item 6 or 7 above, characterized in that the conductive material is porous. 9. The electrode material according to any one of items 6 to 8 above, characterized in that the conductive material is in the form of particles having a particle size of 1 mm or less. 10. An electrode using the oxygen catalyst according to any one of items 1 to 5 above or the electrode material according to any one of items 6 to 9 above. 11. The electrode according to item 10 above, characterized in that the electrode is used as an anode in water electrolysis, a cathode in a fuel cell, an oxygen cathode in sodium chloride electrolysis, or a positive electrode in an air secondary battery. 12. An electrochemical device using the oxygen catalyst according to any one of items 1 to 5 above, or the electrode according to item 10 or 11 above. 13. The electrochemical device according to claim 12, characterized in that the electrochemical device is used in any one of water electrolysis, fuel cells, sodium chloride electrolysis, and air secondary batteries. 14. A method for producing an oxygen catalyst containing a pyrochlore oxide in which the A site mainly contains +3-valent bismuth ions and the B site mainly contains +4-valent ruthenium ions, the method comprising: Step 1: preparing a precursor of the oxygen catalyst; and Step 2: heat-treating the precursor to synthesize an oxygen catalyst having a Tafel gradient for oxygen reaction of 33 mV / dec or less. 15. The method for producing an oxygen catalyst according to claim 14, characterized in that the precursor is heat-treated at a temperature at which the primary particle size of the oxygen catalyst obtained in Step 2 is 20 nm or less. 16. The method for producing an oxygen catalyst according to claim 14 or 15, characterized in that the heat treatment temperature is 550°C or less. 17. A method for producing an electrode material according to any one of claims 14 to 16, characterized in that after Step 2, Step 3: supporting the oxygen catalyst on all or part of a conductive material.18. The method for producing an electrode material according to any one of the above items 14 to 16, characterized in that it comprises a step 4 of adhering the precursor to all or part of a conductive material between the steps 1 and 2. 19. The method for producing an electrode material according to item 18, characterized in that the surface of the conductive material is subjected to a surface treatment that improves the bonding strength between the oxygen catalyst and the conductive material.

[0016] The oxygen catalyst of the present invention reduces the initiation overvoltage of the oxygen reaction, resulting in the following effects: when used as an anode in water electrolysis, the electrolysis voltage is reduced; when used as a cathode in a fuel cell, the voltage during power generation is increased; when used as an active cathode in sodium chloride electrolysis, the electrolysis voltage is reduced; when used as an air electrode in a primary air cell, the discharge voltage is increased; and when used as an air electrode in an air secondary cell, the discharge voltage is increased and the charge voltage is reduced. These effects improve the electrolysis efficiency in water electrolysis, sodium chloride electrolysis, and seawater electrolysis, improve the power generation efficiency in fuel cells, improve the discharge output in primary air cells, and achieve both improved discharge output and improved voltage efficiency in air secondary cells. Furthermore, when the oxygen catalyst of the present invention is used as an anode in seawater electrolysis, oxygen generation is prioritized over chlorine generation from seawater, and this occurs at a low initiation overvoltage. This reduces the electrolysis voltage in seawater electrolysis and reduces the generation and disposal of harmful chlorine gas. Furthermore, because the oxygen catalyst of the present invention is chemically and electrochemically stable against oxygen reactions, the above-mentioned effects can be achieved over a long period of time. In particular, the oxygen catalyst of the present invention has catalytic activity for both oxygen generation and oxygen reduction, and even if a reverse current flows, the reverse current can be utilized in either the oxygen generation reaction or the oxygen reduction reaction. In this case, the oxygen catalyst of the present invention does not involve a change in the valence of the constituent metal elements themselves in either the oxygen generation reaction or the oxygen reduction reaction. Therefore, the problem that occurs with oxygen catalysts made of oxides of nickel, iron, cobalt, or other transition metal elements, i.e., deterioration of the oxygen catalyst or electrodes using the same due to a change in the valence of the constituent metal elements themselves due to a reverse current, is prevented. Therefore, when used in water electrolysis, salt electrolysis, seawater electrolysis, and fuel cells, the oxygen catalyst of the present invention has high durability against reverse current, prevents deterioration due to reverse current, and has the effect of preventing deterioration of the catalyst, electrodes, electrolytic cells, and stacks over a long period of time due to current flow / shutdown, such as starting and stopping of operation.

[0017] The electrode material, electrode, and electrochemical device of the present invention also achieve the above-mentioned effects. For example, an increase in the discharge voltage of an 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 an air secondary battery improves the energy density, power density, voltage efficiency, and energy efficiency. These are also maintained.

[0018] Furthermore, by reducing the electrolysis voltage in salt electrolysis, the power consumption rate and power consumption rate of the produced hydrogen, chlorine, and caustic soda are reduced, thereby reducing the power costs involved in production. Furthermore, by increasing the voltage in fuel cells, the energy density and output density are improved. Furthermore, by reducing the electrolysis voltage in hydrogen production by water electrolysis or seawater electrolysis, the power consumption rate and power consumption rate of the produced hydrogen are reduced, thereby reducing the initial costs and the power costs involved in production and maintenance costs. Furthermore, by suppressing the generation of chlorine at the anode in hydrogen production by seawater electrolysis, there is an effect that the cost of chlorine treatment is reduced or eliminated. Furthermore, a water electrolysis / fuel cell device can enjoy the effects of both water electrolysis and fuel cells.

[0019] Furthermore, the method for producing an oxygen catalyst and the method for producing an electrode material of the present invention have the advantage that the oxygen catalyst and the electrode material having the above-mentioned effects can be produced in a small number of steps, through simple and easy processes, at low cost, efficiently, and in either a batch or continuous manner.

[0020] 1 shows the particle size distribution of the oxide of Example 1 (oxygen catalyst of the present invention); FIG. 2 shows the particle size distribution of the oxide of Comparative Example 1; FIG. 3 shows a comparison of X-ray diffraction images of the oxide of Example 1 (oxygen catalyst of the present invention) and the oxide of Comparative Example 1; FIG. 4 shows a comparison of potential sweep curves of an electrode supporting the oxide of Example 1 (electrode of the present invention) and an electrode supporting the oxide of Comparative Example 1; FIG. 5 shows a potential sweep curve of an electrode of Example 2 (electrode of the present invention); FIG. 6 shows a potential sweep curve of an electrode of Comparative Example 2.

[0021] The oxygen catalyst of the present invention is an oxygen catalyst containing a pyrochlore oxide in which the main component of the A site is a +3-valent bismuth ion and the main component of the B site is a +4-valent ruthenium ion, and is characterized by a Tafel gradient for the oxygen reaction of 33 mV / dec or less. This has the effect of making it possible to reduce the reaction onset overvoltage for the oxygen reaction to 100 mV or less. Note that the Tafel gradient is positive for oxidation reactions and negative for reduction reactions, but the Tafel gradient defined here is its absolute value. Note that the "oxygen catalyst in which the main component of the A site in the pyrochlore oxide is a +3-valent bismuth ion and the main component of the B site is a +4-valent ruthenium ion" does not exclude impurities that inevitably occur during the production of the oxygen catalyst of the present invention. Note that the Tafel gradient for the oxygen reaction is preferably 32 mV / dec or less, and more preferably 31 mV / dec or less.

[0022] Furthermore, the oxygen catalyst of the present invention preferably has a primary particle diameter d90 of 20 nm or less. A diameter of 20 nm or less has the effect of reducing the Tafel slope and increasing the exchange current density. Here, the primary particle diameter refers to the major axis of the catalyst particles observed under an electron microscope at a magnification of at least 100,000 times.

[0023] The oxygen catalyst of the present invention preferably contains a +1-valent sodium ion, a +4-valent manganese ion, or both of these ions.

[0024] Furthermore, the oxygen catalyst of the present invention preferably has peaks at 2θ=30.07°±1.00°, 34.88°±1.00°, and 50.20°±1.00° in X-ray diffraction measurement using CuKα rays. 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 oxygen catalyst of the present invention is characterized in that the oxygen reaction converts the oxidant into O ２ , the reduced substance is H ２ O or oxidized to O ２ , the reduced substance is OH － The reaction is preferably an oxidation reaction, a reduction reaction, or both of these reactions.

[0025] In addition, the electrode material of the present invention preferably has the oxygen catalyst supported or formed on a conductive material. This has the effect of providing an electrode material with a small Tafel slope, a small reaction onset overvoltage, and a small ohmic resistance. In addition, in the electrode material of the present invention, the conductive material is preferably composed of carbon, a metal, or an alloy. In addition, in the electrode material of the present invention, the conductive material is preferably porous. In addition, in the electrode material of the present invention, the conductive material is preferably in the form of particles with a particle size of 1 mm or less. Here, particle size refers to the sieve diameter.

[0026] The electrode of the present invention preferably uses the above-mentioned oxygen catalyst or electrode material, and is preferably used as an anode for water electrolysis, a cathode for a fuel cell, an oxygen cathode for sodium chloride electrolysis, or a positive electrode for an air secondary battery.

[0027] The electrochemical device of the present invention preferably uses the oxygen catalyst or electrode described above. The electrochemical device of the present invention is preferably used for any one of water electrolysis, fuel cells, sodium chloride electrolysis, and air secondary batteries.

[0028] The method for preparing an oxygen catalyst of the present invention is a method for preparing an oxygen catalyst containing a pyrochlore oxide in which the A site is primarily composed of +3-valent bismuth ions and the B site is primarily composed of +4-valent ruthenium ions, and is characterized by comprising: step 1 of synthesizing a precursor of the oxygen catalyst; and step 2 of heat-treating the precursor to synthesize an oxygen catalyst characterized by a Tafel gradient for oxygen reaction of 33 mV / dec or less. The oxygen catalyst of the present invention is preferably prepared using the method for preparing an oxygen catalyst of the present invention. The Tafel gradient for oxygen reaction is preferably 32 mV / dec or less, more preferably 31 mV / dec or less. The method for preparing an oxygen catalyst of the present invention also preferably includes heat-treating the precursor at a temperature at which the primary particle size of the oxygen catalyst obtained in step 2 is 20 nm or less. The heat treatment temperature is preferably 550°C or less, more preferably 530°C or less. The heat treatment temperature is preferably 300°C or higher, more preferably 400°C or higher, and even more preferably 450°C or higher.

[0029] The method for producing an electrode material of the present invention preferably includes, after step 2, step 3 of supporting the oxygen catalyst on all or part of the surface of a conductive material. The method for producing an electrode material of the present invention preferably includes, between steps 1 and 2, step 4 of adhering the precursor to all or part of the surface of a conductive material. The method for producing an electrode material of the present invention preferably includes a surface treatment on the surface of the conductive material that improves the bonding strength between the oxygen catalyst and the conductive material. Such a surface treatment is not particularly limited as long as it can provide an electrode material that is the object of the present invention. For example, a surface treatment that prevents oxidation of the conductive base material and a decrease in surface conductivity can be exemplified.

[0030] EXAMPLES Hereinafter, the oxygen catalyst, electrode material, electrode, electrochemical device, and method for producing the oxygen catalyst and electrode according to the present invention will be described using examples, but the present invention is not limited to these examples.

[0031] Example 1 An oxide serving as the oxygen catalyst of Example 1 (hereinafter sometimes simply referred to as the oxygen catalyst) was prepared using the following procedure. Tetra-n-propylammonium bromide (abbreviated as TPAB, purity 98.0%) was dissolved in distilled water in a beaker, and then the solution was added to distilled water at 75°C using a hot stirrer. The concentration was adjusted to 0.093 mol / L. Hereinafter, this solution will be referred to as the TPAB solution. Next, ruthenium(III) chloride n-hydrate (Ru content 45.38%) and bismuth(III) nitrate pentahydrate (purity 99.5%) were weighed and dissolved in distilled water. The concentration of both solutions was adjusted to 0.0744 mol / L. Note that the bismuth(III) nitrate pentahydrate was ground in an agate mortar for approximately 5 minutes before dissolution, and the solution was subjected to ultrasonic stirring for approximately 5 minutes. Hereinafter, each solution will be referred to as the Ru solution and the Bi solution. A metal salt solution was prepared by mixing predetermined amounts of TPAB solution, Ru solution, Bi solution, and distilled water at 75°C to a total volume of 500 mL. The Ru and Bi concentrations in this metal salt solution were both 0.00744 mol / L, and the TPAB concentration was 0.0372 mol / L. After stirring and mixing this metal salt solution at 75°C for 1 hour, 60 mL of a separately prepared 2 mol / L NaOH (sodium hydroxide) aqueous solution was added. The mixture was then stirred for 24 hours while maintaining the temperature at 75°C and blowing oxygen at 30 mL / min (oxygen bubbling). This corresponds to step 1 in the method for preparing an oxygen catalyst of the present invention. After stirring, the mixture was allowed to stand for approximately 20 hours to obtain a precipitate. The precipitate was removed and evaporated to dryness at 200°C for 30 minutes. The resulting dried product 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 calcined in an electric furnace at 500°C for 1 hour. This step corresponds to step 2 in the method for producing the oxygen catalyst of the present invention. The material obtained after calcination was suction filtered using distilled water at 75°C, 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, the oxide of Example 1 (the oxygen catalyst of the present invention) was obtained.

[0032] Comparative Example 1 The oxide of Comparative Example 1 was obtained in the same manner as in Example 1, except that the firing temperature was changed from 500° C. to 600° C.

[0033] (Particle Observation) The oxide of Example 1 and the oxide of Comparative Example 1 were observed with a scanning electron microscope (abbreviated as SEM, manufactured by ZEISS, ULTRA 55), and the major axis of each particle was determined from the SEM image obtained at 150,000 magnification by image analysis (image processing). This major axis was defined as the primary particle diameter, and its frequency distribution was calculated. Note that the major axis is the longest distance between two parallel lines tangent to the outline of the particle in the SEM image of the particle. Figures 1 and 2 show the results of the frequency distribution analysis of particle diameters calculated from the SEM images of particle size observation. Note that the frequency distribution analysis was performed on at least 300 particles. From this, the primary particle size of the oxide of Example 1 was 20 nm or less in d90 as shown in FIG. 1 (cumulative 92.3% from 0 to 20 nm), whereas the oxide of Comparative Example 1 had d90 greater than 20 nm as shown in FIG. 2 (cumulative 52.0% from 0 to 20 nm, cumulative 92.8% from 0 to 30 nm), and the primary particle size ranged from 10 to 70 nm.

[0034] (X-ray Diffraction Measurement) The oxides of Example 1 and Comparative Example 1 were analyzed using an X-ray diffractometer (Ultima IV, manufactured by Rigaku) ​​using CuKα radiation (wavelength 1.54 Å). The 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°. X-ray diffraction images of the oxides of Example 1 and Comparative Example 1 are shown in Figure 1. The results for all oxides showed diffraction peaks including 2θ = 30.07° ± 1.00°, 34.88° ± 1.00°, and 50.20° ± 1.00°, and also showed diffraction peaks specific to oxides with a single pyrochlore structure, as indicated by the ● marks in Figure 3. Furthermore, these diffraction peaks were at approximately the same positions as the diffraction peaks of a bismuth ruthenium oxide with a pyrochlore structure registered under PDF card number 01-073-9239 provided by the International Center for Diffraction Data (ICDD). This indicates that the oxides of Example 1 and Comparative Example 1 are pyrochlore oxides in which a +3-valent bismuth ion is located at the A site of the pyrochlore structure and a +4-valent ruthenium ion is located at the B site.

[0035] (Rutherford backscattering analysis, ICP optical emission spectroscopy, atomic absorption spectroscopy) For the oxide of Example 1, the atomic ratios of sodium, bismuth, ruthenium, and oxygen were determined as follows. First, the concentration of bismuth in the oxide was quantified by ICP optical emission spectroscopy, and the concentration of sodium was quantified by atomic absorption spectroscopy. Next, the amounts of bismuth, ruthenium, and oxygen were quantified by Rutherford backscattering analysis. From these results, the atomic ratios of sodium, ruthenium, and oxygen were determined based on bismuth, and finally, the atomic ratios of these four elements were determined, resulting in an atomic ratio of Na:Bi:Ru:O of 4.5:17.0:16.9:61.6.

[0036] (X-ray Absorption Fine Structure Analysis) The oxide of Example 1 was measured for X-ray absorption fine structure spectrum (abbreviated as XAFS), and the absorption near edge structure (abbreviated as XANES) in the spectrum was analyzed. As a result, it was found that bismuth was +3, ruthenium was +4, and sodium was +1. In addition, in the XANES analysis of sodium, sodium sulfate (Na ２ SO ４ ) and sodium bismuthate (NaBiO ３ ) and other measurements were also performed, but the results for the oxide of Example 1 were different from those for these reference substances in the rise of the sodium K-edge absorption edge, the shape of the spectrum, and the position of the main peak. These results demonstrate that sodium in the oxide of Example 1 is not contained in a by-product or the like, but constitutes the oxygen catalyst of the present invention having a pyrochlore structure together with bismuth, ruthenium, and oxygen.

[0037] (Electrode Fabrication) Electrodes were fabricated by supporting the oxides of Example 1 and Comparative Example 1 on a titanium disk, a conductive material, using the following method. First, the oxide was crushed in a mortar. Distilled water was used as a dispersion medium, and the crushed oxide was added to a sample bottle. Ultrasonic dispersion was then performed using a homogenizer to obtain a dispersion liquid. A titanium disk (4.0 mm diameter, 4.0 mm height) was placed in acetone and ultrasonically cleaned. The dispersion liquid was then dropped onto one side of the titanium disk (one of the cylindrical bottom surfaces) and allowed to air dry for 24 hours, resulting in an electrode with the oxide supported on one side of the titanium disk. The electrode using the oxide of Example 1 is an example of an electrode of the present invention. Note that no immobilizing agent was used to immobilize the oxide on the titanium disk. Furthermore, the oxide loading amounts of Example 1 and Comparative Example 1 were adjusted to be the same, resulting in 32 μg and 31 μg, respectively. This slight difference did not result in any difference in the results of the electrochemical measurements described below.

[0038] (Electrochemical Measurement) The above-mentioned electrode was attached to a rotating electrode device, which served as a working electrode. This working electrode and a platinum plate (with an area of ​​25 cm ２) were immersed in a 0.1 mol / L potassium hydroxide aqueous solution in the same container. The pH of the potassium hydroxide aqueous solution was 13 or higher. This constituted a water electrolysis device using an alkaline aqueous solution (an example of an electrochemical device of the present invention). A commercially available mercury / mercury oxide electrode immersed in the same 0.1 mol / L potassium hydroxide aqueous solution was also prepared in a separate container. These two potassium hydroxide aqueous solutions were connected via a liquid junction filled with the same 0.1 mol / L potassium hydroxide aqueous solution. Using this three-electrode electrochemical cell, electrochemical measurements were performed with the aqueous solution temperature adjusted to 25°C. A commercially available electrochemical measurement device and electrochemical software were used to measure potential sweep curves. A potential sweep curve is a method of measuring the current flowing through the working electrode while changing the potential of the working electrode at a constant sweep 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 oxygen reduction and oxygen generation do not occur over a wide potential range using a titanium disk alone, the above measurement method allows for the measurement of only the current resulting from the oxygen reaction occurring at the oxide. During the above measurement, the working electrode was rotated at 1600 rpm (min －１ ) and used. Specifically, the titanium disk was attached to a rotating disk electrode device with the oxygen catalyst-fixed side facing downward, and the device was rotated at a constant speed with the oxygen catalyst pyrochlore oxide immersed in a potassium hydroxide aqueous solution. This type of measurement is called the rotating disk electrode method or RDE (Rotating Disk Electrode) method. The scan rate, which means the amount of change in potential per unit time, was 1 mV / s. In this way, the oxygen evolution current was measured in the electrolyte in an open-to-air state.

[0039] (Tafel Slope and Initiation Overvoltage) The potential scan curves of the electrodes of Example 1 and Comparative Example 1 obtained by the above method are shown in FIG. 4 for comparison. The electrode potential on the horizontal axis in this figure is the value after ohmic loss correction for the measured electrode potential. As shown in this figure, in both cases, when the electrode potential was scanned in the noble direction, a positive current flowed from a certain electrode potential, indicating that oxygen generation occurred. However, the onset of oxygen generation was more than 50 mV more noble in Example 1 than in Comparative Example 1, indicating that the initiation overvoltage was reduced accordingly. Furthermore, from the results of FIG. 4 , the common logarithm of the current density for oxygen generation was plotted on the horizontal axis and the electrode potential on the vertical axis according to a standard method (the result of such plotting is called a Tafel plot), and the slope of the linear portion of the Tafel plot, i.e., the Tafel slope, was determined. As a result, the Tafel slope of Example 1 was 30 mV / dec and the Tafel slope of Comparative Example 1 was 44 mV / dec. Thus, the Tafel slope of Example 1 was smaller than 35 mV / dec, thereby significantly reducing the onset overpotential for oxygen evolution. Furthermore, in the Tafel plot, the electrode potential at the intersection of the extrapolated line of the Tafel plot and the extrapolated line of the charging current of the electric double layer in a potential region less noble than that at which oxygen evolution begins was determined. This method is commonly used to determine the onset potential for oxygen evolution. As a result, the onset potential for oxygen evolution in Example 1 was 0.384 V, the difference from the equilibrium potential (0.293 V) being 0.091 V, achieving an onset overpotential of 0.1 V or less (100 mV or less), which had previously been considered difficult to achieve. As described above, it was found that the oxygen catalyst of the present invention has a Tafel slope of 35 mV / dec for the oxygen reaction, thereby resulting in a very small onset overpotential.

[0040] Example 2 An electrode material and electrode of the present invention were prepared as follows. 0.08 g of graphite particles (Toyo Tanso, MH-18, particle size 1 mm or less), a conductive material, was added to 3 mL of distilled water and dispersed for 1 minute using a homogenizer at 72 W power. 0.16 g of the oxide of Example 1 was added to the dispersion thus obtained, and then dispersed for 1 minute using a homogenizer at 72 W power. The dispersion thus obtained was spread on filter paper, which was placed in a dryer and kept at 80°C for 60 minutes, after which the residue on the filter paper was recovered. Observation of this recovered material using a scanning electron microscope revealed that the oxide of Example 1 was supported on the surface of the graphite particles (hereinafter referred to as catalyst-supported graphite). This catalyst-supported graphite is an example of an electrode material of the present invention. 0.019 g of this catalyst-supported graphite was added to 5 mL of hexanol and dispersed for 1 minute using a homogenizer at 48 W power. 8 μL of the dispersion thus obtained was dropped onto one side (one of the cylindrical bottom surfaces) of a titanium disk (diameter 4.0 mm, height 4.0 mm) that had been ultrasonically cleaned in acetone, and the disk was allowed to dry naturally for 24 hours to obtain an electrode in which 34 μg of catalyst-supported graphite was supported on one side of the titanium disk (an example of an electrode of the present invention). The above process corresponds to step 3 of the present invention. Note that no immobilizing agent was used when supporting the catalyst-supported graphite on the titanium disk.

[0041] (Comparative Example 2) 0.019 g of graphite particles (Toyo Tanso, MH-18, particle size 1 mm or less) was added to 5 mL of hexanol and dispersed for 1 minute at 48 W using a homogenizer. 10 μL of the dispersion thus obtained was dropped onto one side (one bottom surface of the cylindrical shape) of a titanium disk (diameter 4.0 mm, height 4.0 mm) that had been ultrasonically cleaned in acetone, and the disk was allowed to air dry for 24 hours, yielding an electrode in which 37 μg of graphite was supported on one side of the titanium disk. Note that no immobilizing agent was used when supporting the graphite on the titanium disk.

[0042] (Electrochemical Measurement) The electrodes of Example 2 and Comparative Example 2 were evaluated using the same method and conditions as those for the electrochemical measurement described above. The potential sweep curves obtained in Example 2 and Comparative Example 2 are shown in FIG. 5 and FIG. 6, respectively. The electrode potential on the horizontal axis in these figures represents the measured electrode potential after ohmic loss correction. The electrode potential and current density ranges in FIGS. 5 and 6 are the same. In FIG. 5, a positive current flows from an electrode potential of approximately 0.43 V, indicating the occurrence of oxygen evolution. However, in FIG. 6, no positive current corresponding to oxygen evolution was observed even when the electrode potential was scanned beyond this value. These results demonstrate that an electrode (electrode of the present invention) carrying a material in which the oxygen catalyst of the present invention is supported on graphite particles (electrode material of the present invention) has extremely high catalytic activity for oxygen evolution.

[0043] The oxygen catalyst, electrode material, or electrode of the present invention can be used as an anode for water electrolysis, a cathode for fuel cells, an air electrode for primary air cells or secondary air cells, an oxygen cathode for brine electrolysis (device), an anode for seawater electrolysis, or in batteries, electrolysis devices, and sensors that utilize oxygen reactions. The electrochemical device of the present invention 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. [Related Applications] This application claims priority under Article 4 of the Paris Convention or Article 41 of the Japanese Patent Act, based on Japanese Patent Application No. 2024-054479 filed in Japan on March 28, 2024. The contents of this basic application are incorporated herein by reference.

Claims

1. An oxygen catalyst containing a pyrochlore oxide in which the main component of the A site is a +3 valent bismuth ion and the main component of the B site is a +4 valent ruthenium ion, characterized in that the Tafel gradient for the oxygen reaction is 33 mV / dec or less.

2. The oxygen catalyst according to claim 1, characterized in that the primary particle diameter d90 of the oxygen catalyst is 20 nm or less.

3. The oxygen catalyst according to claim 1, wherein the oxygen catalyst contains a +1 valent sodium ion, a +4 valent manganese ion, or both of these ions.

4. The oxygen catalyst described in claim 1, characterized in that the oxygen catalyst has peaks at 2θ = 30.07° ± 1.00°, 34.88° ± 1.00°, and 50.20° ± 1.00° in X-ray diffraction measurement using CuKα radiation.

5. The oxygen reaction converts the oxidant to O ２ , the reduced substance is H ２ O or oxidized to O ２ , the reduced substance is OH － 2. The oxygen catalyst according to claim 1, wherein the reaction is an oxidation reaction, a reduction reaction, or both of the reactions.

6. An electrode material in which the oxygen catalyst according to claim 1 is supported or formed on a conductive material.

7. The electrode material according to claim 6, wherein the conductive material is made of any one of carbon, metal, alloy, and ceramic.

8. The electrode material according to claim 6, wherein the conductive material is a porous material.

9. The electrode material according to claim 6, wherein the conductive material is in the form of particles with a particle size of 1 mm or less.

10. An electrode using the oxygen catalyst according to claim 1 or the electrode material according to claim 6.

11. The electrode according to claim 10, which is used as an anode in water electrolysis, a cathode in a fuel cell, an oxygen cathode in sodium chloride electrolysis, or a positive electrode in an air secondary battery.

12. An electrochemical device using the electrode according to claim 10.

13. The electrochemical device according to claim 12, which is used in any one of water electrolysis, fuel cells, salt electrolysis, and air secondary batteries.

14. A method for producing an oxygen catalyst containing a pyrochlore oxide in which the main component of the A site is a +3-valent bismuth ion and the main component of the B site is a +4-valent ruthenium ion, the method comprising: step 1 of preparing a precursor of the oxygen catalyst; and step 2 of heat-treating the precursor to synthesize an oxygen catalyst characterized in that the Tafel gradient for the oxygen reaction is 33 mV / dec or less.

15. A method for producing an oxygen catalyst according to claim 14, characterized in that the precursor is heat-treated at a temperature at which the primary particle size of the oxygen catalyst obtained in step 2 becomes 20 nm or less.

16. The method for producing an oxygen catalyst according to claim 14, wherein the heat treatment temperature is 550°C or less.

17. The method for producing an electrode material according to claim 14, further comprising, after step 2, step 3 of supporting the oxygen catalyst on all or part of a conductive material.

18. The method for producing an electrode material according to claim 14, characterized in that it comprises, between step 1 and step 2, step 4 of depositing the precursor onto all or part of a conductive material.

19. The method for producing an electrode material according to claim 18, wherein the surface of the conductive material is subjected to a surface treatment that improves the bonding strength between the oxygen catalyst and the conductive material.

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