OXYGEN CATALYST AND ELECTRODE USING SAID OXYGEN CATALYST

Abstract

Provided are: an oxygen catalyst that uses an alkaline aqueous solution as an electrolyte and has high catalytic activity; and an electrode. The oxygen catalyst according to the present invention is an oxygen catalyst in which an alkaline aqueous solution is used as an electrolyte, the oxygen catalyst being characterized by having a pyrochlore oxide structure including bismuth on an A-site and ruthenium on a B-site, and containing manganese as well as bismuth and ruthenium. The electrode according to the present invention is characterized by using the oxygen catalyst according to the present invention.

Claims

1-10. (canceled)

11. An oxygen catalyst that uses an alkaline aqueous solution as an electrolyte, the oxygen catalyst comprising a structure of a pyrochlore oxide with bismuth located at A-sites and ruthenium at B-sites, and containing manganese as well as the bismuth and the ruthenium.

12. The oxygen catalyst of claim 11, wherein the pyrochlore oxide further contains sodium.

13. The oxygen catalyst of claim 12, wherein the sodium is less than 15 atom % in an atomic ratio of four elements that are the bismuth, the ruthenium, the manganese, and the sodium.

14. The oxygen catalyst of claim 13, wherein the sodium is 11 atom % to 14 atom % in the atomic ratio of the four elements that are the bismuth, the ruthenium, the manganese, and the sodium.

15. The oxygen catalyst of claim 11, wherein the manganese is located at the B sites.

16. The oxygen catalyst of claim 12, wherein the manganese is located at the B sites.

17. The oxygen catalyst of claim 13, wherein the manganese is located at the B sites.

18. The oxygen catalyst of claim 11, wherein the manganese is 15 atom % or less in an atomic ratio of three elements that are the bismuth, the ruthenium, and the manganese.

19. The oxygen catalyst of claim 13, wherein the manganese is 15 atom % or less in an atomic ratio of three elements that are the bismuth, the ruthenium, and the manganese.

20. The oxygen catalyst of claim 15, wherein the manganese is 15 atom % or less in an atomic ratio of three elements that are the bismuth, the ruthenium, and the manganese.

21. The oxygen catalyst of claim 11, wherein the manganese is cations having a valence of +4.

22. The oxygen catalyst of claim 13, wherein the manganese is cations having a valence of +4.

23. The oxygen catalyst of claim 15, wherein the manganese is cations having a valence of +4.

24. The oxygen catalyst of claim 18, wherein the manganese is cations having a valence of +4.

25. The oxygen catalyst of claim 11, wherein the pyrochlore oxide is of an oxygen-deficient type.

26. The oxygen catalyst of claim 15, wherein the pyrochlore oxide is of an oxygen-deficient type.

27. The oxygen catalyst of claim 18, wherein the pyrochlore oxide is of an oxygen-deficient type.

28. The oxygen catalyst of claim 22, wherein the pyrochlore oxide is of an oxygen-deficient type.

29. An electrode characterized by using the oxygen catalyst of claim 11.

30. The electrode of claim 29, wherein the electrode is one of: an air electrode of an air primary battery, an air electrode of an air secondary battery, an oxygen cathode for brine electrolysis, a cathode of an alkaline fuel cell, or an anode for alkaline water electrolysis.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0024] FIG. 1 shows polarization curves for oxygen reduction of Example 1, Comparative Example 1, Comparative Example 2, and Comparative Example 3.

[0025] FIG. 2 shows polarization curves for oxygen reduction of Comparative Example 1 and Examples 2 to 6.

[0026] FIG. 3 shows polarization curves for oxygen generation of Comparative Example 1 and Examples 2 to 6.

[0027] FIG. 4 is a graph showing the relationship between the atomic ratio of manganese and the exchange current density.

DETAILED DESCRIPTION OF VARIOUS EMBODIMENTS

[0028] The present invention will be specifically described below based on examples. The present invention is not limited to these examples.

Example 1

[0029] 500 mL of solution was prepared by dissolving tetra-n-propylammonium bromide (dispersant), ruthenium(III) chloride hydrate, bismuth(III) nitrate hydrate, and manganese(II) nitrate hydrate in 75° C. distilled water. The ruthenium concentration was 7.44×10.sup.−3 mol/L and the dispersant concentration was 3.72×10.sup.−2 mol/L. The total concentration of bismuth and manganese was also 7.44×10.sup.−3 mol/L that is the same as the ruthenium concentration, and the atomic ratio of bismuth to manganese was 90:10. That is, the atomic ratio of manganese, bismuth, and ruthenium was 5:45:50. After the solution was sufficiently stirred, 60 mL of 2 mol/L NaOH aqueous solution was dropped to the solution, and the resultant solution was stirred at 75° C. for 24 hours while blowing oxygen into the solution. After the stirring was stopped, the solution was left stand for 24 hours. The supernatant liquid was then removed, and the remaining precipitate was heated at 85° C. for about 2 hours to form a paste. The paste was dried at 120° C. for 3 hours. After the resultant material was pulverized in a mortar, the pulverized material was heated from room temperature to 600° C. in an air atmosphere and then held at 600° C. for one hour. The baked product thus obtained was filtered by suction filtration using about 70° C. distilled water and then dried at 120° C. for 3 hours. The substance obtained by the above operation was analyzed using an X-ray diffractometer. The analysis showed that the substance was an oxygen-deficient pyrochlore oxide as the obtained results matched the diffraction data (registration numbers 01-073-9239) of Bi.sub.1.87Ru.sub.2O.sub.6.903 registered in the database of the International Center for Diffraction Data (ICDD). This substance was observed with a scanning electron microscope, and its particle size was analyzed by image analysis. As a result, it was found that the average particle size was 50 nm. Elemental analysis and analysis of the composition ratio were carried out using characteristic X-rays in an energy dispersive X-ray analyzer. The results showed that the atomic ratio of the three elements, namely bismuth, ruthenium, and manganese, was Bi:Ru:Mn=46.8:47.0:5.3. Characteristic X-rays of sodium were also observed, and the obtained atomic ratio of the four elements, namely bismuth, ruthenium, manganese, and sodium, was Bi:Ru:Mn:Na=40.5:41.4:4.5:13.6.

[0030] 3.7 g/L MBRO particles were added to distilled water in a sample bottle, and ultrasonic dispersion was performed using an ultrasonic generator for 2 hours to obtain a suspension of the MBRO particles. After a titanium disc (diameter: 4.0 mm, height: 4.0 mm) was placed in acetone and cleaned by ultrasound, 10 μL of the above suspension was dropped onto one side of the titanium disc and naturally dried to obtain a titanium disc uniformly carrying the MBRO particles on its one side. The amount of MBRO carried on the titanium disc was 34 μg.

[0031] The titanium disc carrying the MBRO particles thereon was attached as a working electrode to a rotating electrode device. This working electrode and a platinum plate (area: 25 cm.sup.2) were immersed in a 0.1 mol/L potassium hydroxide aqueous solution in the same container. A commercially available mercury/mercury oxide electrode immersed in a 0.1 mol/L potassium hydroxide aqueous solution was also prepared in another container. These two potassium hydroxide aqueous solutions were connected by a liquid junction filled with a 0.1 mol/L potassium hydroxide aqueous solution. By using a three-electrode electrochemical cell with such a configuration, electrochemical measurement was carried out with the temperature of the aqueous solutions adjusted to 25° C. The measurement was carried out by linear sweep voltammetry using a commercially available electrochemical measurement device and electrochemical software. This is a method in which a current flowing through the working electrode is measured while changing the potential of the working electrode at a constant sweep rate. The current flowing at this time is a current generated by a reaction that occurs in the oxygen catalyst carried on the titanium disc. Since using only the titanium disc is not enough to cause oxygen reduction or oxygen generation in a wide potential range, the reaction current generated only by the oxygen catalyst can be measured by the above measurement method. Typically, a method using a carbon disc rather than a titanium disc is often used. However, since the carbon disc itself has a catalytic action to reduce oxygen, the reaction current generated only by the oxygen catalyst cannot be measured from the current measured with the carbon disc carrying the oxygen catalyst thereon.

[0032] In the measurement of an oxygen reduction current, nitrogen was first blown into an aqueous solution with the working electrode immersed therein at a flow rate of 30 mL/min for 2 hours or more to remove oxygen dissolved in the aqueous solution, and then the current was measured. Thereafter, oxygen was blown into the aqueous solution at the same flow rate for 2 hours or more, and the current was measured again while continuing to blow oxygen into the aqueous solution. Subsequently, an oxygen reduction current was obtained by subtracting the current measured after blowing nitrogen from the current measured while blowing oxygen. An oxygen reduction current density was also obtained by dividing this oxygen reduction current by the surface area of the titanium disc carrying the MBRO thereon. The result showing the relationship between the potential of the working electrode and the oxygen reduction current density (hereinafter referred to as the polarization curve) was thus obtained. The working electrode used was rotated at 1600 rpm during the above measurement. Such measurement is called a rotating electrode method. The sweep rate at which the potential is changed (amount of change in electrode per second) was 1 mV/s. The obtained polarization curve was plotted according to the usual method with the abscissa representing the common logarithm of the oxygen reduction current density and the ordinate representing the potential (hereinafter this result will be referred to as the Tafel plot), and the slope of a linear part of the Tafel plot, that is, a Tafel slope, was obtained. For the results obtained as described above, the polarization curve is shown in FIG. 1, and the Tafel slope is shown in Table 1.

Comparative Example 1

[0033] Synthesis was performed in the same manner as Example 1 except that manganese(II) nitrate hydrate was not dissolved in 75° C. distilled water and the bismuth concentration was 7.44×10.sup.−3 mol/L that is the same as the ruthenium concentration. The substance thus obtained was examined using an X-ray diffractometer. The examination showed that, as in Example 1, the substance was an oxygen-deficient pyrochlore oxide as the obtained results matched the diffraction data of Bi.sub.1.87Ru.sub.2O.sub.6.903. This substance was observed with a scanning electron microscope, and its particle size was analyzed by image analysis. As a result, it was found that the average particle size was 28 nm. These results showed that a bismuth ruthenium oxide (BRO) with an oxygen-deficient pyrochlore structure was obtained.

[0034] The BRO particles were used to obtain a titanium disc uniformly carrying the BRO particles on its one side by the same method as Example 1. The amount of BRO carried on the titanium disc was 36 μg. A polarization curve and a Tafel slope were obtained by carrying out the same measurement as Example 1 using the titanium disc carrying the BRO particles thereon as a working electrode. The results are shown in FIG. 1 and Table 1.

Comparative Example 2

[0035] Synthesis was performed in the same manner as Example 1 except that manganese(II) nitrate hydrate was replaced with aluminum(III) nitrate hydrate. The substance thus obtained was examined using an X-ray diffractometer. The examination showed that, as in Example 1, the substance was an oxygen-deficient pyrochlore oxide as the obtained results matched the diffraction data of Bi.sub.1.87Ru.sub.2O.sub.6.903. This substance was observed with a scanning electron microscope. As a result, it was found that the average particle size was almost the same as Comparative Example 1. These results showed that an oxygen-deficient pyrochlore oxide (ABRO) containing 5 atom % of aluminum as well as bismuth and ruthenium was obtained.

[0036] The ABRO particles were used to obtain a titanium disc uniformly carrying the ABRO particles on its one side by the same method as Example 1. The amount of ABRO carried on the titanium disc was 28 μg. A polarization curve and a Tafel slope were obtained by carrying out the same measurement as Example 1 using the titanium disc carrying the ABRO particles thereon as a working electrode. The results are shown in FIG. 1 and Table 1.

Comparative Example 3

[0037] Synthesis was performed in the same manner as Example 1 except that manganese(II) nitrate hydrate was replaced with lead(II) nitrate. The substance thus obtained was examined using an X-ray diffractometer. The examination showed that, as in Example 1, the substance was an oxygen-deficient pyrochlore oxide as the obtained results matched the diffraction data of Bi.sub.1.87Ru.sub.2O.sub.6.903. A diffraction line matching the composition formula of Bi.sub.2Ru.sub.2O.sub.7.3 (registration number 00-026-0222) was also observed although the diffraction peak intensity was very low. This substance was observed with a scanning electron microscope. As a result, it was found that the average particle size was almost the same as Comparative Example 1. These results showed that an oxygen-deficient pyrochlore oxide (PBRO) containing 5 atom % of lead as well as bismuth and ruthenium was obtained.

[0038] The PBRO particles were used to obtain a titanium disc uniformly carrying the PBRO particles on its one side by the same method as Example 1. The amount of PBRO carried on the titanium disc was 35 μg. A polarization curve and a Tafel slope were obtained by carrying out the same measurement as Example 1 using the titanium disc carrying the PBRO particles thereon as a working electrode. The results are shown in FIG. 1 and Table 1.

[0039] The polarization curve in FIG. 1 shows the current density when the potential of the working electrode was changed in the negative direction at a constant rate. The current density takes a negative value for the reduction current. This means that the larger the current density in the negative direction, the larger the reduction current. When the potential is the same, the larger the reduction current, the higher the catalytic activity. When the reduction current density is the same, the higher the potential (the more on the right the potential is on the abscissa in the figure), the higher the catalytic activity. That is, it can be said that as a larger reduction current flows at a higher potential, an overvoltage for the reduction reaction is lower and therefore the catalytic activity is higher. Accordingly, the four oxygen catalysts are, in descending order of catalytic activity, Example 1, Comparative Example 1, Comparative Example 2, and Comparative Example 3. MBRO had higher catalytic activity than BRO and had higher catalyst activity than ABRO and PBO containing elements other than bismuth and ruthenium like MBRO. As described above, not all pyrochlore oxides containing elements other than bismuth and ruthenium had higher catalytic activity for oxygen reduction than BRO, and MBRO containing manganese had higher catalytic activity than BRO.

[0040] The difference in catalytic activity revealed by the polarization curves was examined by comparing the Tafel slopes. Since the Tafel slope is the amount of change in potential required to increase the current density by 10 times, the Tafel slope is a value that is not affected even if the substantial reaction surface area of the oxygen catalyst is different. It is therefore not necessary to consider the difference in amount of catalyst carried on the titanium disc when comparing the four oxygen catalysts. The smaller the Tafel slope, the more the current density increases with a lower overvoltage. That is, in the reduction current density of the polarization curve, the smaller the Tafel slope, the larger the reduction current is at the potential more on the right in the figure.

[0041] As shown in Table 1, these four oxygen catalysts are, in ascending order of the Tafel slope, MBRO, BRO, ABRO, and PBRO, and the higher the catalytic activity in the polarization curve, the smaller the Tafel slope. In particular, the Tafel slope of MBRO was −30 mV/dec that is smaller than −40 mV/dec.

TABLE-US-00001 TABLE 1 Tafel Slope (mV/dec) Example 1 −39 Comparative Example 1 −43 Comparative Example 2 −49 Comparative Example 3 −67

Example 2

[0042] An oxygen catalyst of Example 2 was synthesized by the following method. 500 mL of solution was prepared by dissolving tetra-n-propylammonium bromide (dispersant), ruthenium(III) chloride hydrate, bismuth(III) nitrate hydrate, and manganese(II) nitrate hydrate in 75° C. distilled water. The ruthenium concentration and the manganese concentration were as shown in Table 2, and bismuth was added to the solution to the atomic ratio shown in Table 2. Bi:(Ru+Mn) shown in Table 2 represents the ratio of the bismuth concentration to the total concentration of ruthenium and manganese in the prepared solution in atom %. In Example 2, the atomic ratio of ruthenium to manganese in the prepared solution was 95:5, and the atomic ratio of bismuth, ruthenium, and manganese in the prepared solution was 48.3:49.1:2.6. After the solution was sufficiently stirred, 60 mL of 2 mol/L NaOH aqueous solution was dropped to the solution, and the resultant solution was stirred at 75° C. for 24 hours while blowing oxygen into the solution. After the stirring was stopped, the solution was left stand for 24 hours. The supernatant liquid was then removed, and the remaining precipitate was heated at 105° C. for about 2 hours to form a paste. The paste was dried at 120° C. for 3 hours. After the resultant material was pulverized in a mortar, the pulverized material was heated from room temperature to 600° C. in an air atmosphere and then held at 600° C. for one hour. The baked product thus obtained was filtered by suction filtration using about 75° C. distilled water and then dried at 120° C. for 3 hours. The substance obtained by the above operation was analyzed using an X-ray diffractometer. The analysis showed that the substance was an oxygen-deficient pyrochlore oxide as the obtained results matched the diffraction data (registration numbers 01-073-9239) of Bi.sub.1.87Ru.sub.2O.sub.6.903 registered in the database of the International Center for Diffraction Data (ICDD). Moreover, according to the results of energy dispersive elemental analysis, the obtained pyrochlore oxide contained sodium as well as bismuth, ruthenium, and manganese, and the atomic ratio of the three elements other than sodium and the atomic ratio of the four elements including sodium were as shown in Table 3. The results thus showed that an oxygen-deficient pyrochlore oxide containing the four elements was obtained. As described in Example 1, for the atomic ratios in Table 3, Bi:Ru:Mn is the atomic ratio of the three components, namely bismuth, ruthenium, and manganese, in atom %, and Bi:Ru:Mn:Na is the atomic ratio of the four components, namely bismuth, ruthenium, manganese, and sodium. In Table 3, the analysis results of the oxygen catalyst of Example 1 are also shown for comparison.

Example 3

[0043] An oxygen catalyst of Example 3 was synthesized by the same oxygen catalyst synthesis method as Example 2 except that the ruthenium concentration and the manganese concentration were as in Table 2 and bismuth was added to the ratio shown in Table 2. That is, the atomic ratio of ruthenium to manganese in the prepared solution was 90:10 and the atomic ratio of bismuth, ruthenium, and manganese in the prepared solution was 50:45:5. The substance thus obtained was analyzed using an X-ray diffractometer. The analysis showed that the substance was an oxygen-deficient pyrochlore oxide as the obtained results matched the diffraction data (registration numbers 01-073-9239) of Bi.sub.1.87Ru.sub.2O.sub.6.903 registered in the database of the International Center for Diffraction Data (ICDD). Moreover, according to the results of energy dispersive elemental analysis, the obtained pyrochlore oxide contained sodium as well as bismuth, ruthenium, and manganese, and the atomic ratio of the three elements other than sodium and the atomic ratio of the four elements including sodium were as shown in Table 3. The results thus showed that an oxygen-deficient pyrochlore oxide containing the four elements was obtained.

Example 4

[0044] An oxygen catalyst of Example 4 was synthesized by the same oxygen catalyst synthesis method as Example 2 except that the ruthenium concentration and the manganese concentration were as in Table 2 and bismuth was added to the molar ratio shown in Table 2. That is, the atomic ratio of ruthenium to manganese in the prepared solution was 85:15 and the atomic ratio of bismuth, ruthenium, and manganese in the prepared solution was 50:42.5:7.5. The substance thus obtained was analyzed using an X-ray diffractometer. The analysis showed that the substance was an oxygen-deficient pyrochlore oxide as the obtained results substantially matched the diffraction data (registration numbers 01-073-9239) of Bi.sub.1.87Ru.sub.2O.sub.6.903 registered in the database of the International Center for Diffraction Data (ICDD). However, the 20 values of the diffraction peaks of (222), (400), and (440) planes were higher by about 0.2 deg to 0.35 deg than those of the peak positions of the diffraction data in the database. This is theoretically reasonable for the following reason. Ruthenium having a valence of +4 has an ionic radius of 0.62 angstroms, while manganese having a valence of +4 has an ionic radius of 0.53 angstroms. Manganese thus has a smaller ionic radius. Accordingly, when manganese is considered to have been substituted for ruthenium located at the B-sites, the oxygen catalyst has reduced lattice spacing and diffraction peaks shifted to higher angles. Moreover, according to the results of energy dispersive elemental analysis, the obtained pyrochlore oxide contained sodium as well as bismuth, ruthenium, and manganese, and the atomic ratio of the three elements other than sodium and the atomic ratio of the four elements including sodium were as shown in Table 3. The results thus showed that an oxygen-deficient pyrochlore oxide containing the four elements was obtained.

Example 5

[0045] An oxygen catalyst of Example 5 was synthesized by the same oxygen catalyst synthesis method as Example 2 except that the ruthenium concentration and the manganese concentration were as in Table 2 and bismuth was added to the molar ratio shown in Table 2. That is, the atomic ratio of ruthenium to manganese in the prepared solution was 80:20 and the atomic ratio of bismuth, ruthenium, and manganese in the prepared solution was 50:40:10. The substance thus obtained was analyzed using an X-ray diffractometer. The analysis showed that the substance was an oxygen-deficient pyrochlore oxide as the obtained results substantially matched the diffraction data (registration numbers 01-073-9239) of Bi.sub.1.87Ru.sub.2O.sub.6.903 registered in the database of the International Center for Diffraction Data (ICDD). However, the 2θ values of the diffraction peaks of (222), (400), and (440) planes were higher than those of the peak positions of the diffraction data in the database as in Example 4. Moreover, according to the results of energy dispersive elemental analysis, the obtained pyrochlore oxide contained sodium as well as bismuth, ruthenium, and manganese, and the atomic ratio of the three elements other than sodium and the atomic ratio of the four elements including sodium were as shown in Table 3. The results thus showed that an oxygen-deficient pyrochlore oxide containing the four elements was obtained.

Example 6

[0046] An oxygen catalyst of Example 6 was synthesized by the same oxygen catalyst synthesis method as Example 2 except that the ruthenium concentration and the manganese concentration were as in Table 2 and bismuth was added to the molar ratio shown in Table 2. That is, the atomic ratio of ruthenium to manganese in the prepared solution was 70:30 and the atomic ratio of bismuth, ruthenium, and manganese in the prepared solution was 50:35:15. The substance thus obtained was analyzed using an X-ray diffractometer. The analysis showed that the substance was an oxygen-deficient pyrochlore oxide as the obtained results substantially matched the diffraction data (registration numbers 01-073-9239) of Bi.sub.1.87Ru.sub.2O.sub.6.903 registered in the database of the International Center for Diffraction Data (ICDD). However, the 2θ values of the diffraction peaks of (222), (400), and (440) planes were higher than those of the peak positions of the diffraction data in the database as in Example 4. Moreover, according to the results of energy dispersive elemental analysis, the obtained pyrochlore oxide contained sodium as well as bismuth, ruthenium, and manganese, and the atomic ratio of the three elements other than sodium and the atomic ratio of the four elements including sodium were as shown in Table 3. The results thus showed that an oxygen-deficient pyrochlore oxide containing the four elements was obtained.

TABLE-US-00002 TABLE 2 Ruthenium Bismuth Concentration Concentration Bi:(Ru + Mn) (mol/L) (mol/L) (atom %) Example 2 3.53 × 10.sup.−3 1.86 × 10.sup.−4 48.3:51.7 Example 3 3.35 × 10.sup.−3 3.72 × 10.sup.−4 48.3:51.7 Example 4 3.16 × 10.sup.−3 5.58 × 10.sup.−4 50:50 Example 5 2.98 × 10.sup.−3 7.44 × 10.sup.−4 50:50 Example 6 2.60 × 10.sup.−3 1.12 × 10.sup.−3 50:50

TABLE-US-00003 TABLE 3 Bi:Ru:Mn Bi:Ru:Mn:Na (atom %) (atom %) Example 1 46.8:47.9:5.3 40.5:41.4:4.5:13.6 Example 2 49.7:47.9:2.5 43.1:41.5:2.1:13.3 Example 3 49.5:45.3:5.2 43.7:39.9:4.6:11.8 Example 4 50.5:42.1:7.4 44.6:37.1:6.6:11.7 Example 5 50.7:39.7:9.6 44.1:34.4:8.4:13.1 Example 6  50.5:36.0:13.5  44.4:31.7:11.9:12.0

[0047] For each of the oxygen catalysts of Examples 2 to 6, a titanium disc carrying the MBRO particles thereon was obtained by a method similar to Example 1. By using each of the titanium discs carrying the MBRO particles thereon, linear sweep voltammetry was performed by the same method as Example 1 to measure a polarization curve for oxygen reduction. A polarization curve for oxygen generation was also measured by linear sweep voltammetry at the same sweep rate as the measurement of polarization for oxygen reduction. In addition to these measurements, cyclic voltammetry was also performed at 5 mV/s to measure a charging current of an electrical double layer, and the charged ampere-hour Cp (unit: C/cm.sup.2) of the electrical double layer was obtained from the measurement result of the charging current. A Tafel slope was also obtained by the same method as Example 1 from the results of the linear sweep voltammetry, and the exchange current density was obtained from the intersection of the Tafel plot. The relationship between the specific activity iw that is the oxygen reduction current divided by the weight of the catalyst carried on the titanium disc and the potential was obtained from the relationship between the potential and the oxygen reduction current obtained by the linear sweep voltammetry. The results are shown in FIG. 2. The specific activity iw was used instead of the oxygen reduction current for the following reason. The oxygen reduction reaction occurs at the three-phase boundary where the catalyst, the alkaline aqueous solution, and oxygen contact each other. Accordingly, when the amount of catalyst carried is large, the three-phase boundary is also large. In order to compare the catalysts with different element composition ratios, it is therefore suitable to perform normalization using the amount of catalyst carried. The result for the oxygen catalyst of Comparative Example 1 is also shown in FIG. 2 for comparison. According to the results of FIG. 2, each of the oxygen catalysts MBRO of Examples 2 to 6 containing manganese generated an oxygen reduction current from a higher potential (potential more on the right in the figure) and had a larger maximum value of specific activity shown in FIG. 2 as compared to the oxygen catalyst BRO of Comparative Example 1 that does not contain manganese. That is, MBRO had higher catalytic activity for oxygen reduction than BRO. Moreover, comparison of Examples 2 to 6 shows that, like Example 6, as the atomic ratio of manganese increased, the oxygen reduction current flowed from a higher potential, and the maximum value of specific activity tended to be larger. It was therefore found that the high atomic ratio of manganese improved the oxygen activity for oxygen reduction.

[0048] The relationship between the specific activity ic that is the oxygen generation current divided by the charged ampere-hour of the electrical double layer and the potential was obtained from the relationship between the potential and the oxygen generation current obtained by the linear sweep voltammetry. The results are shown in FIG. 3. The specific activity ic was used instead of the oxygen generation current for the following reason. It is known that an oxygen generation reaction occurs at the two-phase boundary where the catalyst and the alkaline aqueous solution contacts each other and that the surface area of the two-phase boundary that functions for oxygen generation (hereinafter referred to as the reaction surface area) is proportional to the charged ampere-hour of the electrical double layer. It is also possible to compare the catalysts based on the specific activity iw that is the amount of catalyst carried divided by the current. However, by using the specific activity ic, the catalysts can be compared based on the difference in catalytic activity that reflects the difference in particle size of the catalyst. Accordingly, in order to consider the activity in view of the reaction surface area that depends on the two-phase boundary, the specific activity ic is more suitable than the specific activity iw. The result for the oxygen catalyst of Comparative Example 1 is also shown in FIG. 3 for comparison. According to the results of FIG. 3, for the oxygen catalyst BRO of Comparative Example 1 that does not contain manganese and the oxygen catalysts MBRO of Examples 2 to 6 containing manganese, the potential at the maximum specific activity value 8 A/C in the figure was 0.568 V in Example 2 in which the oxygen generation current flowed at the lowest potential, that is, the overvoltage was the lowest, 0.580 V in Comparative Example 1, and 0.585 V in Example 4 in which the overvoltage was the highest. That is, the difference between Example 2 with the lowest overvoltage and Example 4 with the highest overvoltage was 0.017 V, and the differences between Comparative Example 1 and Example 2 and between Comparative Example 1 and Example 4 were smaller than this value. The differences between Comparative Example 1 and Examples 2 to 6 were thus smaller than the differences in catalytic activity for oxygen reduction shown in FIG. 3. That is, it was found from the results of the examples in the present invention that the oxygen catalysts of the present invention exhibit substantially the same properties as BRO for oxygen generation.

[0049] The Tafel slopes for oxygen reduction and oxygen generation were obtained from the slopes of the Tafel plots of Examples 2 to 6. The results are shown in FIG. Table 4. In this table, Example 2 had the smallest Tafel slope for oxygen reduction. As the atomic ratio of manganese increased from Example 2 to Example 6, the Tafel slope increased accordingly. The Tafel slope for oxygen generation did not have such a fixed tendency for the atomic ratio of manganese, and was in the range from a minimum value of 38 mV/dec to at most 41 mV/dec. The Tafel slope for oxygen reduction of Comparative Example 1 was −43 mV/dec as shown in Table 1, but the Tafel slope for oxygen generation of Comparative Example 1 was 40 mV/dec.

TABLE-US-00004 TABLE 4 Tafel Slope for Tafel Slope for Oxygen Reduction Oxygen Generation (mV/dec) (mV/dec) Example 2 −39 39 Example 3 −41 41 Example 4 −43 38 Example 5 −44 38 Example 6 −47 41

[0050] The exchange current was obtained from the intersection of the Tafel plot, and a value i0 (unit: μA/g) that is the exchange current divided by the amount of catalyst carried on the titanium disc and the average of the values i0 were calculated. The results for Comparative Example 1 and Examples 2 to 6 are shown in FIG. 4. The atomic ratio of manganese on the abscissa of the figure is zero for Comparative Example 1 as Comparative Example 1 does not contain manganese. For Examples 2 to 6, the atomic ratio of manganese is shown based on the atomic ratio of two components, namely ruthenium and manganese, in the solution during synthesis of the catalyst. The smaller the Tafel slope and the larger the exchange current density, the higher the catalytic activity. According to the results of FIG. 4, as the atomic ratio of manganese increases, the exchange current density increases. The exchange current density increases particularly at an atomic ratio higher than 15 atom %, and the exchange current density of Example 6 is about four times that of Comparative Example 1. Based on these results together with the results of the Tafel slope, the Tafel slope for oxygen reduction tends to increase as the atomic ratio of manganese increases. However, the increase in exchange current density more dominantly affects the catalytic activity than this increase in Tafel slope does. This shows that the catalytic activity for oxygen reduction of Examples 2 to 6 dramatically improved over Comparative Example 1. It was thus found that manganese can not only reduce the Tafel slope but also increase the exchange current density.

Comparative Example 4

[0051] An oxygen catalyst of Comparative Example 4 was synthesized by the same oxygen catalyst synthesis method as Example 2 except that the atomic ratio of Bi:(Ru+Mn) was 50:50 and the atomic ratio of Ru:Mn was 60:40 with the atomic ratio of Mu relatively higher than Example 6. That is, the atomic ratio of ruthenium to manganese in the prepared solution was 60:40 and the atomic ratio of bismuth, ruthenium, and manganese in the prepared solution was 50:30:20. The substance thus obtained was analyzed using an X-ray diffractometer. The analysis showed that not only an oxygen-deficient pyrochlore oxide was synthesized as a large number of diffraction peaks different from the diffraction data (registration number 01-073-9239) of Bi.sub.1.87Ru.sub.2O.sub.6.903 registered in the database of the International Center for Diffraction Data (ICDD) were observed in addition to diffraction peaks substantially matching the diffraction data. That is, the results showed that a compound containing a byproduct was obtained in addition to a pyrochlore oxide due to the high atomic ratio of manganese to bismuth or ruthenium used for the synthesis.

[0052] (EXAFS Structural Analysis)

[0053] For the oxygen catalysts of Examples 2 and 3, an X-ray absorption fine structure (EXAFS) spectrum was measured, and information regarding the valences and structures of bismuth, ruthenium, manganese, and sodium was obtained from the X-ray absorption near edge structure (commonly called XANES) in the spectrum. Information regarding the local structure of the oxygen catalyst (atomic species neighboring a certain atom, valence, and inter-atomic distance) was also obtained from the extended X-ray absorption fine structure (commonly called EXAFS) appearing in the region from about 100 eV or more above the absorption edge in the spectrum.

[0054] The results for both Example 2 and Example 3 showed that bismuth was cations having a valence of +3 and located at the A-sites of the pyrochlore structure, ruthenium was cations having a valence of +4 and located at the B-sites of the pyrochlore structure, and manganese was cations having a valence of +4 and located at the B-sites of the pyrochlore structure. The results also showed that sodium is cations having a valence of +1 and is likely to be located at both A-sites and B-sites.

CONCLUSION

[0055] The oxygen catalyst of the present invention can be used not only in air electrodes of an air primary battery and an air secondary battery, an oxygen cathode for brine electrolysis, a cathode of an alkaline fuel cell, and an anode for alkaline water electrolysis, but also as a catalyst for oxygen generation or oxygen reduction or both in a battery, electrolyzer, and sensor that use oxygen reduction or oxygen generation or both by using an alkaline aqueous solution as an electrolyte. The electrode of the present invention can be used not only as air electrodes of an air primary battery and an air secondary battery, an oxygen cathode for brine electrolysis, a cathode of an alkaline fuel cell, and an anode for alkaline water electrolysis but also as a positive electrode, negative electrode, anode, or cathode in a battery, electrolyzer, and sensor that use oxygen reduction or oxygen generation or both as an electrode reaction by using an alkaline aqueous solution as an electrolyte.