ELECTRODE AND METHOD OF PRODUCING THE SAME, AND ELECTROCHEMICAL DEVICE USING THE SAME

Abstract

An electrode used for oxygen reactions, the electrode being excellent in catalytic activity and stability, a method of producing the electrode, and an electrochemical device using the electrode are provided. This electrode includes, as an oxygen catalyst, an oxide that has peaks at positions of 2=34.881.00, 50.201.00, and 59.651.00 in an X-ray diffraction measurement using a CuK ray and has constituent elements of bismuth, ruthenium, sodium, and oxygen.

Claims

1. An electrode used for oxygen reactions, the electrode comprising, as an oxygen catalyst, an oxide that has peaks at positions of 2=34.881.00, 50.201.00, and 59.651.00 in an X-ray diffraction measurement using a CuK ray and has constituent elements of bismuth, ruthenium, sodium, and oxygen.

2. The electrode according to claim 1, wherein an atomic ratio of the oxygen to the bismuth and an atomic ratio O/Ru of the oxygen to the ruthenium are both more than 3.5.

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

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

5. The electrode according to claim 1, wherein the oxygen catalyst has a secondary particle size of 3 m or less.

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

7. The electrode according to claim 1, comprising a catalytic layer that includes the oxygen catalyst, a conducting material, and a water-repellent material as constitutional materials.

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

9. The electrode according to claim 7, being formed into a thin plate shape and having a thickness of 250 m or less.

10. The electrode according to claim 7, comprising a water-repellent layer, through which oxygen can permeate, on an atmosphere side of the catalytic layer or a gas diffusion layer.

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

12. The electrode according to claim 1, being the air electrode of an air battery, the oxygen cathode of brine electrolysis, the cathode of an alkaline fuel cell, or the anode of alkaline water electrolysis.

13. The electrode according to claim 1, comprising a non-electronically conductive reaction space divider arranged on an electrolyte side, the reaction space divider including a plurality of electrolyte holder portions consisting of a concave space configured to hold a liquid electrolyte.

14. A method of producing an electrode used for oxygen reactions, the method comprising a step 1 of synthesizing an oxygen catalyst being an oxide that has peaks at positions of 2=30.071.00, 34.881.00, 50.201.00, and 59.651.00 in an X-ray diffraction measurement using a CuK ray and has constituent elements of bismuth, ruthenium, sodium, and oxygen; and a step 2 of producing a catalytic layer that includes the oxygen catalyst, a conducting material, and a water-repellent material.

15. The method of producing an electrode according to claim 14, wherein an atomic ratio O/Bi of the oxygen to the bismuth and an atomic ratio O/Ru of the oxygen to the ruthenium are both more than 3.5.

16. The method of producing an electrode according to claim 14, wherein the oxygen catalyst has a secondary particle size of 3 m or less, in the step 1.

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

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

19. The method of producing an electrode according claim 14, comprising a step 4 of including a current collector integrated with the catalytic layer or a gas diffusion layer, applying a suspension containing a water-repellent material on a surface opposite to a side in contact with an electrolyte, and subsequently applying heat treatment.

20. An electrochemical device using the electrode according to claim 1.

21. The electrochemical device according to claim 20, being an air battery, a brine electrolyzer, an alkaline water electrolyzer, an alkaline fuel cell, or a water electrolysis and fuel cell device that uses an alkaline aqueous solution as an electrolyte.

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

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0098] In the accompanying drawings:

[0099] FIG. 1 is a diagram that illustrates the structure of an electrode according to a first embodiment;

[0100] FIG. 2 is a diagram that illustrates the structure of an electrode according to a second embodiment;

[0101] FIG. 3 is a diagram that illustrates the structure of an electrode according to a third embodiment;

[0102] FIG. 4 is a diagram that illustrates the structure of an electrode according to a fourth embodiment;

[0103] FIG. 5 is a result of the X-ray diffraction measurement for oxygen catalysts used in electrodes of Example 1 and Example 2;

[0104] FIG. 6 is a result of the X-ray diffraction measurement for an oxygen catalyst used in an electrode of Comparative Example 1;

[0105] FIG. 7 is a result of the X-ray diffraction measurement for the oxygen catalyst used in the electrode of Example 1 and an oxide of Comparative Example 2;

[0106] FIG. 8 is a SEM image illustrating a surface aspect of the oxygen catalyst used in the electrode of Example 1;

[0107] FIG. 9 is a relationship diagram between the primary particle size and the frequency of the oxygen catalyst used in the electrode of Example 1;

[0108] FIG. 10 is a schematic diagram explaining the structure of an electrode used for evaluation of catalytic activity;

[0109] FIG. 11 is a relationship diagram between the specific activity and the electrode potential for the oxygen generation of the electrodes of Example 1, Example 2, and Comparative Example 1; and

[0110] FIG. 12 is a relationship diagram between the specific activity and the electrode potential for the oxygen reduction of the electrodes of Example 1, Example 2, and Comparative Example 1.

[0111] FIG. 13 is a particle size distribution diagram of unground graphite and ground graphite;

[0112] FIG. 14 is a shape diagram of a nickel mesh;

[0113] FIG. 15 is a relationship diagram (particle size distribution) between the secondary particle size and the frequency of an oxygen catalyst before a grinding process;

[0114] FIG. 16 is a relationship diagram (particle size distribution) between the secondary particle size and the frequency of the oxygen catalyst after the grinding process;

[0115] FIG. 17 is a schematic view of a three-electrode measuring cell used for evaluation of an electrode;

[0116] FIG. 18 is a relationship diagram between the electrode potential and the current density of Example 3;

[0117] FIG. 19 is a relationship diagram between the electrode potential and the current density of Example 4;

[0118] FIG. 20 is a diagram that compares the relationships between the electrode potential and the current density of Example 3 and Example 4; and

[0119] FIG. 21 is a diagram that compares the relationship between the charging and discharging potential and the time of Example 6 with cycle number.

DETAILED DESCRIPTION

[0120] The following describes embodiments and examples of an electrode, a method of producing the electrode, and an electrochemical device according to this disclosure. The electrode, the method of producing the electrode, and the electrochemical device according to this disclosure are not limited to these embodiments or examples.

First Embodiment

[0121] The structure of an electrode according to this embodiment is described. The electrode according to this embodiment is an electrode used for oxygen reactions, and the electrode may include, as at least its oxygen catalyst, an oxide having peaks at positions of 2=30.071.00, 34.881.00, and 59.651.00 in an X-ray diffraction measurement using a CuK ray and having constituent elements of bismuth, ruthenium, sodium, and oxygen. The following describes a preferred configuration example of the electrode according to this embodiment.

[0122] FIG. 1 illustrates an electrode 101 according to this embodiment. The electrode 101 includes a catalytic layer 4, a gas diffusion layer 5, and a current collector 3. In the electrode 101, the catalytic layer 4, the gas diffusion layer and the current collector 3 are laminated in this order. In the electrode 101, the catalytic layer 4 is arranged on the side in contact with an electrolytic solution. In the electrode 101, the current collector 3 is arranged on the gas side (atmosphere side). The catalytic layer 4 includes a conducting material, an oxygen catalyst, and a water-repellent material as the constitutional materials. The gas diffusion layer includes no oxygen catalyst.

[0123] The gas diffusion layer 5 is a gas (oxygen, atmosphere)-permeable layer. The gas diffusion layer 5 is gas-permeable and includes a conducting material and a water-repellent material similar to those of the catalytic layer 4, as its constitutional materials, to have water repellency for the permeation of the electrolytic solution from the catalytic layer 4 side. The gas diffusion layer may have a double-layer structure in which two layers using the conducting material with different sizes and shapes are bonded. As the gas diffusion layer 5, a commercially available gas diffusion layer for fuel cell may be used.

[0124] The current collector 3 and the gas diffusion layer 5 may be closely attached to keep electron conductivity or may be integrally shaped by pressing, etc. The catalytic layer 4 and the gas diffusion layer 5 may also be integrated by pressing.

[0125] The catalytic layer 4 may have a thickness of 50 to 300 m, and the gas diffusion layer 5 may have a thickness of 50 to 500 m. As the current collector 3, a nickel mesh with a mesh opening of around 100 mesh to 200 mesh is preferably used. Oxygen cannot permeate through the current collector 3 if there is no openings like a mesh. Nickel is stable for an alkaline aqueous solution and thus preferable as a material of the mesh. However, in a state where the electrolytic solution (alkaline aqueous solution) does not permeate to the current collector, the material of the mesh is not necessarily limited to nickel.

Second Embodiment

[0126] As illustrated in FIG. 2, an electrode 102 according to a second embodiment is obtained by further providing a water-repellent layer 6 to the electrode 101 according to the first embodiment. The gas diffusion layer 5 has a function of suppressing or preventing the permeation of the electrolytic solution. However, the water-repellent layer 6 is preferably provided as in the electrode 102, which is described below, to prevent the permeation or leakage of the electrolytic solution for a long time.

[0127] In the electrode 102, the catalytic layer 4, the gas diffusion layer 5, the current collector 3, and the water-repellent layer 6 are laminated in this order. In the electrode 102, the catalytic layer 4 is arranged on the side in contact with the electrolytic solution. In the electrode 102, the water-repellent layer 6 is arranged on the gas side (atmosphere side). The water-repellent layer 6 prevents the electrolytic solution from leaking to the gas side of the electrode 102. Simultaneously, oxygen required for the reactions must be able to permeate through the water-repellent layer 6. As the water-repellent layer 6, for example, a commercially available PTFE porous membrane may be used. The water-repellent layer 6 may be formed by spraying and drying PTFE particles as water-repellent material particles from the current collector 3 side. In this case, the gap between the PTFE particles becomes a path through which oxygen permeates.

[0128] The water-repellent layer 6 is not required to be conductive. Thus, the water-repellent layer 6 is not arranged between the current collector 3 and the gas diffusion layer 5 and between the current collector 3 and the catalytic layer 4. If there is a material that satisfies all of electron conductivity, oxygen permeability, water repellency, and alkaline resistance, a water-repellent layer different from the water-repellent layer 6 may be arranged on the electrolytic solution side of the current collector 3.

Third Embodiment

[0129] As illustrated in FIG. 3, an electrode 103 according to a third embodiment is obtained by removing the gas diffusion layer 5 in the electrode 102 according to the second embodiment. The electrode 103 is not required to have the gas diffusion layer 5.

Fourth Embodiment

[0130] As illustrated in FIG. 4, an electrode 104 according to a fourth embodiment is obtained by removing the gas diffusion layer 5 in the electrode 101 according to the first embodiment. When the electrode of this disclosure is used as the anode of alkaline water electrolysis by being immersed in the alkaline aqueous solution as the electrolyte, the electrode may have a configuration consisting of the catalytic layer 4 and the current collector 3 as the electrode 104 illustrated in FIG. 4. In such application as the anode, the gas is not required to be introduced from the atmosphere into the electrode 104 as the anode, and the problem of the leakage of the electrolytic solution to the gas side no longer exists. Therefore, the configuration without the gas diffusion layer 5 (refer to FIG. 1 and FIG. 2) and the water-repellent layer 6 (refer to FIG. 2 and FIG. 3) is also possible.

Example 1

[0131] The following describes this disclosure with examples. In the following procedure, an oxide to be used as an oxygen catalyst in an electrode of Example 1 (hereinafter, may be simply referred to as the oxygen catalyst) was synthesized (one 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. with a hot stirrer. The concentration was set to about 9.010.sup.1 mol/L. In the following, this solution is a TPAB solution. TPAB has a role of the aforementioned introduction promoter and stabilizer.

[0132] Next, ruthenium(III) chloride-n-hydrate (Ru content rate: 43.25%) and bismuth(III) nitrate pentahydrate (purity: 99.5%) were weighed, and each was dissolved in distilled water. The concentrations of these solutions were both about 1.810.sup.1 mol/L. The solution of bismuth(III) nitrate pentahydrate was subjected to ultrasonic agitation for about 5 minutes. Hereinafter, the respective solutions were referred to as a Ru solution and a Bi solution.

[0133] Next, the TPAB solution, the Ru solution, the Bi solution, and distilled water were mixed at 75 C. with predetermined amounts to prepare a metallic salt solution with the total amount of 500 mL. The Ru concentration and the Bi concentration in this metallic salt solution were both 7.4410.sup.3 mol/L, and the TPAB concentration was 3.7210.sup.2 mol/L. After this metallic salt solution was agitated and mixed at 75 C. for 1 hour, a separately prepared 2 mol/L NaOH (sodium hydroxide) aqueous solution of 60 mL was added. After this, with keeping the temperature at 75 C., this solution was subjected to agitation for 24 hours while blowing oxygen at 50 mL/min (while oxygen bubbling).

[0134] After the end of the agitation, this solution was allowed to stand for another 24 hours to obtain a precipitate. This precipitate was taken out and evaporated to dryness for about 2 hours in an electric furnace at 105 C. The thus obtained dried solid material was transferred to an evaporating dish and then dried in the electric furnace at 120 C. for 3 hours. The material after drying was ground in a Menor mortar and then held and calcined in the electric furnace at 600 C. for 1 hour. The material obtained after the calcination was filtered by suction using distilled water at 75 C., an aspirator, and a paper filter. The material on the paper filter was collected and then dried in the electric furnace at 120 C. for 3 hours. Thus, the oxygen catalyst (oxide) used in the electrode of Example 1 was obtained.

[0135] In the above operation, the relationship between the Ru concentration, the Bi concentration, and the TPAB concentration is important. TPAB has a role as the introduction promoter and stabilizer for sodium, but too much or too little TPAB relative to the Ru and Bi concentrations is unpreferable. Too much TPAB is unpreferable because the introduction of sodium into the metal hydroxide is inhibited. On the other hand, too little TPAB is unpreferable because the stabilization of the metal hydroxide containing sodium is insufficient and the particle size of the oxygen catalyst to be obtained increases, making it difficult to obtain the oxygen catalyst at nano-level. In the oxygen catalyst used in the electrode of Example 1, the TPAB concentration is 5 times higher than the Ru concentration. For example, if the TPAB concentration is 1 time, TPAB is insufficient, and if the TPAB concentration exceeds 20 times, TPAB is excess. Both cases are unpreferable because TPAB cannot function as the introduction promoter and stabilizer. Such introduction promoter and stabilizer are not limited to TPAB, and other materials can also be used.

Example 2

[0136] An oxygen catalyst used in an electrode of Example 2 was similarly obtained, except that the TPAB solution, the Ru solution, the Bi solution, and distilled water were mixed at 75 C. with predetermined amounts and the Bi concentration in the metallic salt solution with the total amount of 500 mL was changed from 7.4410.sup.3 mol/L to 6.9610.sup.3 mol/L, in the synthesis of the oxygen catalyst used in the electrode of Example 1.

Comparative Example 1

[0137] An oxide used in an electrode of Comparative Example 1 was similarly obtained, except that the NaOH aqueous solution was changed to a LiOH (lithium hydroxide) aqueous solution, in the synthesis of the oxygen catalyst used in the electrode of Example 1

Comparative Example 2

[0138] An oxide synthesized in Comparative Example 2 is similar to that in Example 1, except that the following acid treatment is additionally applied to the oxygen catalyst used in the electrode of Example 1. The oxygen catalyst synthesized by the procedure described in Example 1 of 0.145 g and 0.1 mol/L nitric acid solution of 12 mL were put into a container and subjected to ultrasonic agitation for 30 minutes, and after the ultrasonic agitation, the agitated solution was allowed to stand for 1 hour (acid treatment). After this, the supernatant fluid was removed, distilled water of 12 mL was added for cleaning, the ultrasonic agitation was performed for 30 minutes, and then, the agitated solution was allowed to stand for 1 hour. Subsequently, the supernatant fluid was removed, the same amount of distilled water was added again for further cleaning, the ultrasonic agitation was performed for the same period, and then, the agitated solution was filtered by suction using distilled water until the pH of filtrate became 7. The preceding operation was performed at the room temperature. After this, the material on the paper filter was taken out and then dried at 120 C. for 3 hours, obtaining an oxide of Comparative Example 2.

X-Ray Diffraction Measurement

[0139] The oxygen catalysts used in the electrodes of Example 1 and Example 2 and the oxides of Comparative Example 1 and Comparative Example 2 were analyzed by an X-ray diffractometer (made by Rigaku, Ultima IV) using CuK ray (wavelength: 1.54 ). Measurement conditions included a voltage of 40 kV, a current of 40 mA, a range of a diffraction angle 2 (hereinafter, may be simply referred to as 2) of 10 to 90, and a step angle of 0.020. FIG. 5 and Table 1 illustrated the results of Example 1 and Example 2, FIG. 6 illustrated the result of Comparative Example 1, and FIG. 7 illustrated the comparison of the results of Example 1 and Comparative Example 2.

[0140] As illustrated in FIG. 5 and Table 1, the oxygen catalysts of Example 1 and Example 2 were found to be oxides that have diffraction peaks when the mean value of 2 values (2 mean value) is 14.82, 30.07, 34.88, 38.17, 45.88, 50.20, 59.65, 62.61, 73.80, 81.68, and 84.28 by the X-ray diffraction measurement using the CuK ray. The 2 mean value in Table 1 is presented by synthesizing a plurality of oxides for each of Example 1 and Example 2, determining respective 2 values for all of them, calculating respective mean values of 2 values for Example 1 and Example 2, and obtaining the mean value of these two mean values. That is, the 2 mean value in Table 1 is the mean value of Example 1 and Example 2.

[0141] In both of the oxygen catalyst used in the electrode of Example 1 and the oxygen catalyst used in the electrode of Example 2, the diffraction peaks when the 2 mean value is 30.07, 34.88, 50.20, and 59.65 occur with higher diffraction intensity compared with those of the other values among the above values. This is characteristic of the oxygen catalyst used in the electrode of this disclosure. Theoretically, the diffraction intensity in the X-ray diffraction measurement tends to become significantly weaker as the particle size of an object decreases, particularly, to the nano-level. Therefore, the above characteristic 2 values characterize the oxygen catalyst used in the electrode of this disclosure even if it is a particle having several tens of nanometers, as described later. The result in FIG. 5 had no diffraction peak that indicates the presence of by-products other than the oxygen catalyst used in the electrode of this disclosure.

[0142] Next, as illustrated in FIG. 6, the result of Comparative Example 1 detected diffraction lines (diffraction peaks) at numerous 2 values, which were different from the diffraction peaks of the oxygen catalysts used in the electrodes of Example 1 and Example 2. This detection result found that the oxide of Comparative Example 1 was a compound having a structure different from that of the oxygen catalyst of this disclosure. That is, it was found that the oxygen catalyst used in the electrode of this disclosure could not be obtained when replacing the sodium hydroxide aqueous solution of Example 1 with the lithium hydroxide aqueous solution of Comparative Example 1. It was also suggested that the oxygen catalyst used in the electrode of this disclosure included sodium in the crystal structure.

[0143] Furthermore, as illustrated in FIG. 7, the result of Comparative Example 2 detected diffraction lines at obviously different and numerous 2 values, compared with the result of Example 1. This detection result found that the crystal structure of the oxygen catalyst obtained in Example 1 had changed to a different crystal structure by the acid treatment. That is, it was found that the acid treatment as described in PTL 4 had no effect of removing impurities on the oxygen catalyst used in the electrode of this disclosure and rather had an influence of changing the structure of the oxygen catalyst used in the electrode of this disclosure. Such difference also found that the oxygen catalyst of this disclosure is a compound different from the BRO disclosed in a plurality of documents including PTL 4.

TABLE-US-00001 TABLE 1 Example 1 Example 2 2 mean value 2 () 2 () () 14.79 14.85 14.82 30.06 30.09 30.07 34.85 34.91 34.88 38.17 38.18 38.17 45.88 45.88 45.88 50.18 50.23 50.20 59.62 59.68 59.65 62.58 62.64 62.61 73.70 73.90 73.80 81.64 81.72 81.68 84.23 84.34 84.28

Particle Observation

[0144] For the particle size of the oxygen catalyst of this disclosure, as one example, the oxygen catalyst used in the electrode of Example 1 was observed by a scanning electron microscope (abbreviation: SEM, made by ZEISS, ULTRA 55), the long diameter of each particle was determined from the SEM image by image analysis (image processing), and this long diameter was defined as a primary particle size of the oxygen catalyst of Example 1 to obtain the frequency distribution. The long diameter is the longest distance between two parallel lines that contact the outline of a particle in the SEM image of the particle. FIG. 8 illustrates a SEM image with which the particle size was observed. FIG. 9 illustrates the frequency distribution analysis result of the particle size obtained from such SEM image. The analysis of the frequency distribution was conducted for at least 250 or more particles. Thus, the oxygen catalyst used in the electrode of Example 1 was obtained as nanoparticles having a particle size distribution being across the range of 10 to 70 nm as a whole and having higher frequencies at 20 to 30 nm. Similar observation and particle size analysis were conducted for Example 2, and the result was almost identical to that of Example 1.

Energy Dispersive X-Ray Elemental Analysis (Abbreviation: EDX)

[0145] An energy dispersive X-ray elemental analyzer (made by AMETEK, Genesis APEX2) attached to the aforementioned scanning electron microscope device obtained atomic ratios of sodium, bismuth, and ruthenium for the oxygen catalysts used in the electrodes of Example 1 and Example 2. In this case, the acceleration voltage was set to 15 kV, and the cumulative time was set to 500 seconds, the maximum time that could be set on the device.

Rutherford Backscattering Spectroscopy (Abbreviation: RBS)

[0146] Rutherford backscattering spectroscopy equipment (made by National Electrostatics, Pelletron 3SDH) obtained atomic ratios of bismuth, ruthenium, and oxygen for the oxygen catalysts used in the electrodes of Example 1 and Example 2. In this case, bismuth and ruthenium were analyzed from the measurement result using He ions as incident ions, and oxygen was analyzed from the measurement result using H ions. Based on these analysis results, each of an atomic ratio Ru/Bi of ruthenium to bismuth, an atomic ratio O/Bi of oxygen to bismuth, and an atomic ratio O/Ru of oxygen to ruthenium was obtained. Among them, Table 2 presented the atomic ratio O/Bi and the atomic ratio O/Ru.

Analysis of Atomic Ratio of Four Elements

[0147] When obtaining the atomic ratio of four elements: bismuth (Bi), ruthenium (Ru), sodium (Na), and oxygen (O) from the results of the aforementioned EDX and RBS, bismuth, ruthenium, sodium, and oxygen as atoms may be referred to as Bi, Ru, Na, and O, respectively, in the following.

[0148] Here, the atomic ratio of O is given as a value with respect to Bi in the result of the RBS, and the atomic ratio of Na is given as a value with respect to Bi in the result of the EDX. On the other hand, the atomic ratios of Bi and Ru are given from each of the EDX and the RBS. Thus, there are two ways to obtain the atomic ratio of the four elements, depending on whether the result of the EDX or the RBS is used, for Bi and Ru atoms. Therefore, Table 3 presented the respective results of the atomic ratios calculated by these two ways. In the atomic ratio of Bi, Ru, Na, and O (atomic ratio of four elements) in Table 3, the case using the result of the EDX for the atomic ratios of Bi and Ru was indicated as EDX, and the case using the result of the RBS for the atomic ratios of Bi and Ru was indicated as RBS. Table 4 presented the total charge ratio calculated using these values, the atomic ratio Na/Ru, the cation atomic ratio, and the cation charge ratio. This disclosure evaluates the oxygen catalysts based on the result in Table 2, as well as the results of the RBS in Table 3 and Table 4. That is, the oxygen catalysts are evaluated based on the atomic ratios each obtained in the RBS in Table 2 to Table 4, and the results of the EDX in Table 3 and Table 4 are treated as references.

TABLE-US-00002 TABLE 2 Atomic ratio Atomic ratio O/Bi O/Ru Example 1 3.57 3.63 Example 2 3.60 3.54

TABLE-US-00003 TABLE 3 Bi:Ru:Na:O (atomic ratio of four elements) Example 1 EDX 17.26:16.41:4.68:61.64 RBS 17.17:16.87:4.65:61.31 Example 2 EDX 16.90:17.14:5.09:60.87 RBS 16.89:17.18:5.09:60.84

TABLE-US-00004 TABLE 4 Total charge Atomic ratio Cation Cation ratio Na/Ru atomic ratio charge ratio Example EDX 1.01 0.285 1.34 0.860 1 RBS 0.992 0.276 1.29 0.832 Example EDX 0.979 0.297 1.28 0.814 2 RBS 0.978 0.296 1.28 0.811

X-Ray Absorption Fine Structure Analysis

[0149] For the oxygen catalyst of Example 1, an X-ray absorption fine structure (XAFS) spectrum was measured, and information on chemical states of bismuth and ruthenium was obtained from the absorption near edge structure (X-ray Absorption Near Edge Structure (abbreviation: XANES)) in this spectrum. The measurement used equipment of High Energy Accelerator Research Organization (BL12C, NW10A) and Aichi Synchrotron Radiation Center (BL1N2). The result found that the valence was +3 from the analysis result of the XANES spectrum at the L3 edge of Bi and that the valence was +4 from the analysis result of the XANES spectrum at the K edge of Ru. The valence of the cation of Na is only +1 valence. The total charge ratio and the cation charge ratio in Table 4 were calculated using the valences of the above respective elements and the atomic ratio of the four elements presented in Table 3.

[0150] Next, information on local structure of the oxygen catalyst was obtained from the Extended X-ray Absorption Fine Structure (trivial name: EXAFS) that appeared on the side of high energy of about 100 eV or more, with respect to the absorption edge. First, as a result of comparing a FT-EXAFS spectrum obtained from the EXAFS spectrum at the L3 edge of Bi (which is equivalent to a radial distribution function and indicates the atomic distance in the crystal structure) with a FT-EXAFS spectrum obtained theoretically assuming that Bi occupied the A site in the A.sub.2B.sub.2O.sub.7 structure (hereinafter, theoretically obtained FT-EXAFS spectrum is abbreviated to theoretical spectrum), the measurement spectrum had a peak intensity smaller than that of the theoretical spectrum. The reason that the peak intensity of the measurement spectrum is thus smaller than that of the theoretical spectrum is that the actual structure is different or there is a difference in atomic distance, with respect to a crystal structure assumed when obtaining the theoretical spectrum, that is, the A.sub.2B.sub.2O.sub.7 structure, resulting in distortion. On the other hand, with respect to a peak originating from the first nearest neighbor BiO component of 1.6 to 2.2 in the theoretical spectrum, the measurement spectrum had a peak equivalent to the first nearest neighbor BiO component at 1.2 to 2.0 . These results found that, in the oxygen catalyst of Example 1, Bi existed in the vicinity of the A site when assuming the A.sub.2B.sub.2O.sub.7 structure, but at the position away from the center of this site.

[0151] Similarly, as a result of comparing a FT-EXAFS spectrum at the K edge of Ru with a FT-EXAFS spectrum obtained theoretically assuming that Ru occupied the B site in the A.sub.2B.sub.2O.sub.7 structure, the measurement spectrum and the theoretical spectrum had almost identical intensities. The theoretical spectrum had a peak originating from the first nearest neighbor RuO component at 1.2 to 1.6 , while the measurement spectrum also had a peak equivalent to the first nearest neighbor BiO component at 1.2 to 1.8 . The peak positions considered to be the second nearest neighbor RuORu other than this were also almost identical. These results found that, in the oxygen catalyst of Example 1, Ru was positioned on the B site when assuming the A.sub.2B.sub.2O.sub.7 structure.

[0152] Furthermore, a FT-EXAFS spectrum at the K edge of Na was compared with a FT-EXAFS spectrum obtained theoretically in each case when assuming Na occupied the A site or the B site in the A.sub.2B.sub.2O.sub.7 structure. As the result, the measurement spectrum had a peak intensity smaller than that of each of the A site occupying theoretical spectrum and the B site occupying theoretical spectrum. The measurement spectrum indicated a peak (1) at 1.2 to 2.0 and also a peak (2) at 2.0 to 2.8 , the longer distance side. On the other hand, the A site occupying theoretical spectrum had a peak originating from the first nearest neighbor NaO component at 1.6 to 2.6 , which was correlated to the peak (2), and the B site occupying theoretical spectrum had a peak originating from the first nearest neighbor NaO component at 0.7 to 2.1 , which was correlated to the peak (1). These results indicated that, in the oxygen catalyst of Example 1, Na exists at the position near the A site and the position near the B site when assuming the A.sub.2B.sub.2O.sub.7 structure. As described above, considering high similarity in ionic radius between the +3 valence bismuth ion and +1 valence sodium ion, Na is likely to be positioned more in the vicinity of the A site than the B site, in the structure assuming the A.sub.2B.sub.2O.sub.7 structure.

[0153] NaBiO.sub.3 measured as a reference sample was different from the catalyst of this disclosure in each of first, the rise of the absorption edge in the XANES spectrum at the K edge of Na, the shape of spectrum, and the main peak position. The FT-EXAFS spectrum at the K edge of Na for NaBiO.sub.3 did not indicate two peaks such as the above peak (1) and peak (2). The above further supported that the catalyst of this disclosure did not include by-products including sodium such as NaBiO.sub.3 and that sodium existed in the crystal structure.

[0154] The above results found that the oxygen catalyst used in the electrode of this disclosure had not the pyrochlore structure such as the BRO, but a structure similar to pyrochlore but different from that of the BRO, in which sodium was considered to be arranged in the vicinity of the A site and in the vicinity of the B site in pyrochlore. Furthermore, as a result of performing the analysis identical to the above on the oxygen catalyst used in the electrode of Example 2, a result indicating the trend identical to that of Example 1 for the valence and the atomic distance was obtained.

Production of Electrode

[0155] It was produced an electrode in which each of the oxygen catalysts of Example 1 and Example 2 and the oxide of Comparative Example 1 was supported on a titanium disk, which was a conducting material, in the following method. First, the oxygen catalyst or the oxide was ground in a mortar. Then, the ground powder was added to a sample bottle to be 3.77 g/L using distilled water as a dispersion medium, and ultrasonic dispersion was performed at an ultrasonic generator for 2 hours to obtain the suspension. As illustrated in FIG. 10, a cylindrically formed titanium disk 10 (diameter d of 4.0 mm, height h of 4.0 mm, hereinafter, referred to as the titanium disk 10) was put in acetone to be subjected to ultrasonic cleaning, and then, the above suspension was dropped with 10 L on one surface 11 (one-side bottom of the cylinder) of the titanium disk and naturally dried for 24 hours, obtaining an electrode 100 in which the oxygen catalyst or the oxide was supported in the form of a uniform film on the one surface 11 of the titanium disk 10. FIG. 10 indicates the oxygen catalyst or the oxide, which is supported in the film form, as an oxide layer C. No fixative was used when fixing the oxygen catalyst or the oxide to the titanium disk 10.

Electrochemical Measurement

[0156] The aforementioned electrode was mounted on a rotating electrode device to be a working electrode. This working electrode and a platinum plate (area: 25 cm.sup.2) were immersed in 0.1 mol/L potassium hydroxide aqueous solution in an identical container. The pH of the potassium hydroxide aqueous solution was 13 or more. In another container, a commercially available mercury/mercury oxide electrode similarly immersed in 0.1 mol/L potassium hydroxide aqueous solution was prepared. These two potassium hydroxide aqueous solutions were connected one another through a liquid junction similarly filled with 0.1 mol/L potassium hydroxide aqueous solution. Using a three-electrode electrochemical cell having such configuration, the electrochemical measurement was performed by adjusting the temperature of the aqueous solution at 25 C. The measurement was performed by the linear sweep voltammetry, using a commercially available electrochemical measurement device and electrochemical software. The linear sweep voltammetry is a method of measuring the current flowing through the working electrode while changing the potential of the working electrode at a constant scanning rate. The current flowing during this measurement is a current of the reaction that occurs in the oxygen catalyst supported on the electrode. That is, the titanium disk alone does not cause the reduction of oxygen and the generation of oxygen in a wide potential range, and the above measurement method thus can measure the reaction current that occurs only in the oxide layer C.

[0157] The measurement of an oxygen reduction current was performed as follows. First, nitrogen was blown into the aqueous solution in which the working electrode was immersed at a flow rate of 30 mL/min for 2 hours or more. Then, after removing dissolved oxygen, the measurement was performed while blowing nitrogen. After this, oxygen was blown at the identical flow rate for 2 hours or more, and the measurement was performed again while further blowing oxygen. Then, a value obtained by subtracting the current measured while blowing nitrogen from the current measured while blowing oxygen was set as the reduction current of oxygen. A value obtained by dividing this oxygen reduction current by the surface area of the titanium disk was set as an oxygen reduction current density. The thus obtained result indicating the relationship between the potential of the working electrode and the oxygen reduction current density was used for creating the Tafel plot described later. During the above measurement, the working electrode was used with being rotated at 1600 rpm (min.sup.1). Specifically, the electrode was mounted on the rotating disk electrode device with the surface (one surface 11), to which the oxygen catalyst of the titanium disk 10 was fixed, downward (one example of the rotating disk electrode), and it was rotated at a constant rate in a state in which the oxygen catalyst was immersed in the electrolytic solution. Such measurement is called a Rotating Disk Electrode (RDE) method. The scanning rate that means an amount of change of the potential per unit of time was set to 1 mV/s.

[0158] After performing the above-described measurement of the oxygen reduction current, an oxygen generation current was measured. The measurement of the oxygen generation current was performed under atmosphere open conditions without blowing nitrogen and oxygen. The oxygen generation is a reaction that oxygen is generated from hydroxide ions, and thus it has no relation with the blowing of nitrogen and oxygen. The measurement of the oxygen generation current was also performed by the linear sweep voltammetry at a scanning rate of 1 mV/s, while rotating the working electrode at 1600 rpm, similarly to that of the oxygen reduction current.

Specific Activity

[0159] For the linear sweep voltammogram obtained in the above method (the result indicating the relationship between the potential and the current of the working electrode, which was obtained by the linear sweep voltammetry), in order to remove the influence by the difference in amount of the supported oxygen catalyst, a value obtained by dividing a current value (A) during the oxygen reduction or the oxygen generation by the catalyst weight (g) was used as a specific activity. The unit of the specific activity is A/g. The supported amount of the oxygen catalyst was 35 g to 43 g. FIG. 11 illustrates the thus created result of the electrode potential and the specific activity for the oxygen generation. FIG. 12 illustrates the result of the electrode potential and specific activity for the oxygen reduction. The short dashed line, the solid line, and the long dashed line in FIG. 11 and FIG. 12 represent Example 1, Example 2, and Comparative Example 1, respectively.

[0160] When comparing the specific activities at an electrode potential of 0.6 V from the result in FIG. 11, it was found that Example 1 was 14.5 times and Example 2 was 29.0 times as high as Comparative Example 1, and thus Example 1 and Example 2 had catalytic activities 10 times or higher than that of Comparative Example 1. Similarly, comparing the specific activities at an electrode potential of 0.1 V from the result in FIG. 12, it was found that Example 1 was 6.6 times and Example 2 was 6.1 times as high as Comparative Example 1, and thus Example 1 and Example 2 had catalytic activities 6 times or higher than that of Comparative Example 1 also in the oxygen reduction.

[0161] As above, it was found that the electrode of this disclosure had high catalytic activity both in the oxygen generation and the oxygen reduction.

Tafel Slope

[0162] From the aforementioned linear sweep voltammogram, according to the rule, by organizing the common logarithm of the current density of the oxygen reduction or the oxygen generation on the horizontal axis and the potential on the vertical axis (hereinafter, such organized result is referred to as the Tafel plot), the slope at a part of the straight line in the Tafel plot, that is, the Tafel slope was obtained. The Tafel slope is an amount of change of the potential required to increase the current 10 times for various electrochemical reactions in addition to the reduction of oxygen and the generation of oxygen, which is normally represented using V/dec (dec is an abbreviation of decade that means times) as a unit. Here, the Tafel slope is a positive value in the oxidation reaction and a negative value in the reduction reaction. In each case, the smaller absolute value means the higher catalytic activity. In the following, the magnitude of the Tafel slope is written in terms of the absolute value.

[0163] On the other hand, the reactions of the oxygen generation and the oxygen reduction are known as the reactions that have large Tafel slopes and are less likely to occur, among the electrochemical reactions. For example, even in the case of platinum that is known to have high catalytic activity, the absolute value of the Tafel slope is 60 mV/dec or more in each of the oxygen generation and the oxygen reduction. Very few catalysts exhibit smaller Tafel slopes than that of platinum. However, the Tafel slope in the oxygen catalyst of Example 1 was 44 mV/dec in the oxygen generation and 43 mV/dec in the oxygen reduction, and the Tafel slope in the oxygen catalyst of Example 2 was 39 mV/dec in the oxygen generation and 41 mV/dec in the oxygen reduction. The Tafel slopes in the oxygen catalysts of Example 1 and Example 2 were decreased by 25% or more in both oxygen generation and oxygen reduction, with respect to the Tafel slopes of platinum, which exhibited that the oxygen catalysts of Example 1 and Example 2 had extremely high catalytic activities.

[0164] For the electrodes of Example 1 and Example 2, the aforementioned measurement of the current for the oxygen generation and the oxygen reduction was repeatedly performed about 10 times, but there was no change in measurement result. However, for the oxygen catalyst of Comparative Example 1, when the measurement was repeated 2 to 3 times, the current decreased, and the potassium hydroxide aqueous solution used in the measurement was colored, which suggested dissolution of the constituent elements from the oxygen catalyst of Comparative Example 1. The results of Comparative Example 1 illustrated in FIG. 11 and FIG. 12 are both initial measurement results.

Example 3

[0165] As described below, an electrode of Example 3 was produced. For the electrode of Example 3, graphite as the conducting material and PTFE (polytetrafluoroethylene) as the water-repellent material were used (one example of step 2). First, a zirconia container (made by Fritsch, capacity of 45 mL) was charged with 66.5 g of zirconia ball with a diameter of 5 mm and 0.4 g of natural graphite (made by SEC CARBON), set in a ball mill (made by Fritsch, PL-7), and processed at a rate of 1000 rpm for 10 minutes. After this process, the ground graphite was cooled and then taken out. Thu thus processed graphite is ground graphite, and the graphite that is not ground is unground graphite.

[0166] These ground graphite and unground graphite are measured with a laser diffraction/scattering particle size distribution analyzer (made by HORIBA, LA-960V2), and FIG. 13 presents the results. In FIG. 13, the horizontal axis represents the particle size, and the vertical axis represents the frequency. In the graph in FIG. 13, the solid line represents the particle size distribution of the ground graphite, and the dashed line represents the particle size distribution of the unground graphite. The 10% particle size of the unground graphite is 1.10 m, the 50% particle size is 1.96 m, and the 90% particle size is 3.74 m. The 10% particle size of the ground graphite is 0.30 m, the 50% particle size is 3.00 m, and the 90% particle size is 6.32 m.

[0167] The particle size of the unground graphite formed a particle size distribution curve having a single peak near about 2m. The particle size of the ground graphite formed a particle size distribution curve having two peaks at about 0.2 m and about 4 m. That is, the grinding process obtained submicron order ground graphite having a peak at about 0.2 m.

[0168] For the electrode of Example 3, as such graphite having different particle sizes, two types of graphite: the above unground graphite and ground graphite were used. Furthermore, as a result of observing these unground graphite and ground graphite with a SEM (made by ZEISS, ULTRA55), the unground graphite was squamous, while the ground graphite was particulate and had an uneven surface aspect due to the break of the basal plane. The basal plane is flat and smooth and unlikely to support catalyst particles. However, it was found that the basal plane was broken to become uneven in the ground graphite, which caused the catalyst to be uniformly supported in more highly dispersed state to increase the catalyst area with small catalyst amount.

[0169] As such, the submicron order graphite particle in the ground graphite had an effect on the reduction in catalyst supported amount required per electrode unit area, for high dispersion and support of the catalyst, improvement in usage efficiency, and high catalytic activity. Graphite having different particle sizes was mixed to contain graphite particles with a large particle size, which contributed to the formation of a flow pass, which is suitable for supplying the inside of the electrode with oxygen and releasing oxygen generated inside the electrode to the atmosphere side, inside the electrode by the graphite particle with a large particle size.

[0170] Next, 0.0947 g of an oxygen catalyst synthesized by the method and conditions described in Example 1, the oxygen catalyst that passed a sieve with a mesh opening of 20 m (the one with a secondary particle size of less than m), and 0.0474 g of the ground graphite were weighed and then put in an agate mortar to be mixed with an agate pestle for 10 minutes. 0.0947 g of the oxygen catalyst and 0.0925 g of the unground graphite (one example of graphite having a particle size different from that of the ground graphite) were added to the mixture to be further mixed for 10 minutes. 0.0823 g of a PTFE suspension (made by DAIKIN INDUSTRIES, D-210C, PTFE content of 60 weight %, PTFE mean particle size of 0.25 m), 125 L of a liquid paraffin (made by FUJIFILM Wako Pure Chemical, Wako 1st Grade reagent), and 125 L of distilled water were added to this mixture in this order using a micropipette and kneaded until becoming clayey for 10 minutes, finally obtaining a kneaded product with a size of around 1 cm3 cm.

[0171] The produced kneaded product was sandwiched between two stainless plates (made by The Nilaco, SUS-304, thickness of 0.1 mm, 5 cm5 cm) and then pressed to be a thin plate with a heat roll press machine (made by YURI ROLL MACHINE, TSC-220) at a roll pressure of 1 MPa and a roll speed of 0.5 m/min, with a clearance between the rolls of 250 m.

[0172] A gas diffusion layer (made by SGL, GDL-22-BB) and a nickel mesh (made by The Nilaco, wire diameter of 0.10 mm, 100 mesh), which will be the current collector 3 (refer to FIG. 14), were laminated in this order on the thus pressed kneaded product (one example of the catalytic layer). The nickel mesh, which had been preliminarily cut into a shape as in FIG. 14, was used. Of the kneaded product and the gas diffusion layer, parts protruding from the part of 23 mm23 mm (current collecting portion 31) of the nickel mesh were cut out with a cutter. The part of 3 mm35 mm (lead wire portion 32) in FIG. 14 is a lead part to be connected to a measurement device.

[0173] The laminated product was sandwiched between the above two stainless plates and then pressed at a roll pressure of 1 MPa, a roll speed of 0.5 m/min, and a roll temperature of 80 C., with a clearance between the rolls of 275 !dm. Thus, the kneaded product, the gas diffusion layer, and the nickel mesh ware integrated (one example of step 3) like the electrode 101 in the above-described first embodiment (refer to FIG. 1) and then taken out from the stainless plates. The laminated body of the kneaded product, the gas diffusion layer, and the nickel mesh, which had been integrated as described above, was immersed in a turpentine oil (made by KANTO CHEMICAL, Extra pure) for 1 hour and then immersed in ethanol (made by KANTO CHEMICAL) for 1 hour. Furthermore, the laminated body was taken out from the ethanol to be naturally dried.

[0174] The laminated body processed as described above was subjected to heat treatment at 370 C. in an electric furnace (made by DENKEN, KDF-S70) for 13 minutes in a nitrogen atmosphere. After this, the laminated body was held in the electric furnace to be naturally cooled to 110 C. and then taken out from the electric furnace to be further cooled.

[0175] The thickness of the obtained electrode was 0.20 m, and the catalyst amount per unit area of the electrode was 5.23 mg/cm.sup.2.

Example 4

[0176] An electrode of Example 4 was produced in the same way as in Example 3, except that the oxygen catalyst of Example 1 was subjected to grinding process as described below and the mix ratio of the kneaded product was changed.

[0177] First, a zirconia container (made by Fritsch, capacity of 80 mL) was charged with 100 g of zirconia bead with a diameter of 1 mm, 5.0 g of an oxygen catalyst synthesized by the method described in Example 1, and 8.5 mL of distilled water, set in a ball mill device (made by Fritsch, PL-7), and processed at a rate of 800 rpm for 10 minutes. After this process and cooling, the solution containing the ground oxygen catalyst was separated from the zirconia bead, and the solution was then allowed to stand to remove the supernatant fluid. After this, the residue containing the oxygen catalyst was transferred to a glass dish. This glass dish was put into a vacuum drier (made by AS ONE, AVO-200SB) and subjected to vacuum drying at 80 C. for around 5 hours.

[0178] The particle size of the oxygen catalyst subjected to the grinding process as described above and the particle size of the oxygen catalyst before the grinding process were measured with a laser diffraction particle size distribution analyzer (made by Fritsch, ANALYSETTE 22). As the result, the oxygen catalyst before the grinding process had a secondary particle size of about 150 m at a maximum and a volume mean diameter of 26 m as in FIG. 15, while the oxygen catalyst subjected to the grinding process had a secondary particle size of 99.9% particle of 3 m at a maximum and a volume mean diameter of 0.67 m as in FIG. 16. In FIG. 15 and FIG. 16, the horizontal axis represents the particle size, the left-hand vertical axis represents the accumulation, and the right-hand vertical axis represents the frequency.

[0179] The electrode of Example 4 was produced using this oxygen catalyst subjected to the grinding process. In this case, when the oxygen catalyst was first mixed with the ground graphite with an agate mortar, 0.0947 g of the oxygen catalyst that was not subjected to the grinding process and 0.0474 g of the ground graphite were used in Example 3, while 0.0947 g of the oxygen catalyst subjected to the grinding process and 0.0474 g of the ground graphite were used in Example 4. To each mixture, 0.0947 g of the oxygen catalyst that was not subjected to the grinding process and 0.0925 g of the unground graphite were added to be further mixed for 10 minutes in Example 3, while only 0.0925 g of the unground graphite was added without adding the oxygen catalyst to be mixed for 10 minutes in Example 4. The electrode of Example 4 was produced under the conditions same as those in Example 3 except for the above. The obtained electrode of Example 4 had a thickness of 0.18 mm and a catalyst amount per unit area of the electrode of 0.68 mg/cm.sup.2. The catalyst amount per unit area reduced by 87% with respect to the electrode of Example 3. The electrode of Example 4 has the same area as and is thinner than the electrode of Example 3. When the catalyst amounts in the entire electrodes were compared, Example 4 was 88% less than Example 3.

Electrode Evaluation

[0180] The electrode characteristics of the electrodes of Example 3 and Example 4 were evaluated using a three-electrode measuring cell 200 illustrated in FIG. 17 and an electrochemical measurement device.

[0181] The used three-electrode measuring cell 200 has a common configuration, but the following describes this configuration. A rectangular cylindrical first container 28 contain a liquid electrolyte 24 and a counter electrode 22, and the counter electrode 22 is immersed in the liquid electrolyte 24. An electrode 21 to be evaluated is arranged to cover a through hole 28a at the side wall portion of the first container 28, one side of the electrode 21 is in contact with the electrolyte 24, while the other side is in contact with the atmosphere. The electrode 21 is a working electrode in the measurement. There is a second container 29 different from the first container 28, and the second container 29 contains a reference electrode 23 and the liquid electrolyte 24. The reference electrode 23 is immersed in the electrolyte 24. The electrolyte 24 in the first container 28 is connected to the electrolyte 24 in the second container 29 through a liquid junction 25. The liquid junction 25 is a tube formed into a shape as illustrated in FIG. 17, and this tube is filled with the electrolyte 24. One end of the liquid junction 25 is inserted into the electrolyte 24 in the first container 28, while the other end is inserted into the electrolyte 24 in the second container 29. The electrode 21 was sealed to prevent the electrolyte 24 from leaking from the contact surface of the first container 28.

[0182] The electrode of Example 3 or Example 4 was arranged as the electrode 21 in FIG. 17 to cause the nickel mesh side to be on the atmosphere side and the opposite side to be in contact with 6 mol/L of a KOH aqueous solution (electrolyte 24). At this time, the opening on the atmosphere open side of the electrode of this disclosure was 20 mm20 mm, and the electrode area based on this was 4 cm.sup.2. When the current density, which is described later, is determined using this value, the current density was obtained by dividing the current by the electrode area of 4 cm.sup.2. A platinum plate (5 cm5 cm0.1 mm) as the counter electrode 22 was immersed in this KOH aqueous solution. Furthermore, a mercury/mercury oxide electrode with 6 mol/L of the KOH aqueous solution was used as the reference electrode 23, and this reference electrode 23 was connected to the KOH solution in contact with the electrode of this disclosure through the liquid junction 25 filled with 6 mol/L of the KOH aqueous solution. The potential of the electrode of this disclosure and the current flowing through this electrode were measured at room temperature by cyclic voltammetry, which measures the current while changing the potential of the electrode of this disclosure with respect to the reference electrode 23 at a rate of 50 mV/s. During the measurement, 400 mL/min of oxygen was blown from the atmosphere side of the electrode of this disclosure.

[0183] FIG. 18 and FIG. 19 illustrate respective relationship diagrams between the potentials and the currents obtained in the electrodes of Example 3 and Example 4. In FIG. 18 and FIG. 19, the horizontal axis represents the electrode potential (hereinafter, may be simply referred to as the potential), and the vertical axis represents the current density obtained by dividing the measured current by the electrode area (4 cm.sup.2). In FIG. 18 and FIG. 19, the solid line represents the first cycle, and the dashed line represents the 50th cycle. In FIG. 18 and FIG. 19, the measurements were performed in the identical potential range, and the potentials obtained by correcting the ohmic losses according to the results were used. The ohmic loss occurs due to a solution resistance Rs in the liquid junction that connects the electrode of this disclosure to the reference electrode. When the current flowing through the electrode of this disclosure is I, the potential equivalent to IRs occurs in the electrode of this disclosure. The potential equivalent to IRs is inevitable in the measurement at the three-electrode measuring cell and varies depending on the length of the liquid junction or the concentration of the KOH solution, and is thus not caused by the electrode of this disclosure itself. Thus, in electrochemical measurement as described above, the catalytic activity of the electrode is usually evaluated from the relationship between the potential obtained by correcting the ohmic loss, and the current or the current density. This is inevitable when the flowing current is large and cannot be ignored such that the value of IRs (unit: volt) is 100 mV or more, and the measurement results of the electrodes of Example 3 and Example 4 also corresponded to this. Thus, the relationship between the potential after correcting the ohmic loss and the current density was illustrated as in FIG. 18 and FIG. 19.

[0184] The results in FIG. 18 and FIG. 19 each demonstrate that the oxygen generation current (positive current in the drawing) and the oxygen reduction current (negative current in the drawing) of the electrode of this disclosure do not change between the first cycle and the 50th cycle. The maximum current density illustrated in the drawing is 600 mA/cm.sup.2 or more, and the potential difference required from the potential at which the current starts to flow to such a high current density is very small, around 100 mV for the oxygen generation (between 500 mV and 600 mV at electrode potential) and around 200 mV even for the oxygen reduction (between 100 mV and 300 mV at electrode potential). That is, both of the electrodes of this disclosure described in Example 3 and Example 4 have high catalytic activity and stability thereof.

[0185] Next, FIG. 20 illustrates the results of the first cycles in the electrodes of Example 3 and Example 4 in comparison. In FIG. 20, the solid line represents the result of Example 3, and the dashed line represents the result of Example 4. In FIG. 20, similarly to in FIG. 18 and FIG. 19, the horizontal axis represents the electrode potential, and the vertical axis represents the current density. This demonstrates that there was little difference in the relationship between the oxygen generation current and the oxygen reduction current, and the potential. On the other hand, as described above, the catalyst amount of the electrode of Example 4 is 87 to 88% less than that of Example 3. In other words, on a per unit mass basis, the catalytic activity of the oxygen catalyst subjected to the grinding process is 8 or more times higher than that of the oxygen catalyst that is not subjected to the grinding process. This is due to the grinding process, which reduced the particle size of the oxygen catalyst to increase the surface area per unit mass.

[0186] As described above, it was found that the catalytic activity of the electrode can be significantly improved using the oxygen catalyst with a secondary particle size of 3 m or less as in Example 4.

Example 5

[0187] An electrode of Example 5 was produced in the same way as in Example 4, except that the following step was performed in the step of producing the electrode of Example 4. In Example 4, similarly to in Example 3, the laminated body was subjected to heat treatment at 370 C. in an electric furnace (made by DENKEN, KDF-S70) for 13 minutes in a nitrogen atmosphere, and then held in the electric furnace to be naturally cooled to 110 C. and then taken out from the electric furnace to be further cooled. In the electrode of Example 5, before this heat treatment in the electric furnace, a PTFE suspension (made by DAIKIN INDUSTRIES, D-210C, PTFE content of 60 weight%) was sprayed and applied to the current collector side of the laminated body using an airbrush (made by ANEST IWATA, HP-CS), and then the above heat treatment in the electric furnace and cooling were performed (one example of step 4). Thus, 0.065 g (0.012 g/cm.sup.2 per electrode area) of a PTFE particle layer (one example of the water-repellent layer) was supported on the surface on the current collector side of the electrode of Example 5. That is, the electrode of Example 5 has the structure of the electrode 102 described in the above second embodiment. The thus obtained electrode of Example 5 was subjected to the test with 50 cycles by cyclic voltammetry in the same way as in Example 4. As the result, the electrode of Example 5 did not indicate any liquid leakage on the current collector side of the electrode even after the test.

Example 6

[0188] An electrode of Example 6 was produced in the same way as in Example 4, except that the following step was performed in the step of producing the electrode of Example 4. In Example 4, similarly to in Example 3, the laminated body was subjected to heat treatment at 370 C. in an electric furnace (made by DENKEN, KDF-S70) for 13 minutes in a nitrogen atmosphere, and then held in the electric furnace to be naturally cooled to 110 C. and then taken out from the electric furnace to be further cooled. In the electrode of Example 6, after this heat treatment in the electric furnace, a PTFE suspension (made by DAIKIN INDUSTRIES, D-210C, PTFE content of 60 weight %) was sprayed and applied to the current collector side of the laminated body using an airbrush (made by ANEST IWATA, HP-CS), and then the above heat treatment in the electric furnace and cooling were performed (one example of step 4). Thus, 0.0395 g (0.0075 g/cm.sup.2 per electrode area) of a PTFE particle layer (one example of the water-repellent layer) was supported on the surface on the current collector side of the electrode of Example 6.

[0189] Charge and discharge cycle test was conducted on the thus obtained electrode of Example 6 by the following method. In the charge and discharge cycle test, the three-electrode measuring cell in FIG. 17 same as in Example 3 and Example 4 was used, and the identical counter electrode, electrolyte, and reference electrode were used. The charge and the discharge were both performed with a current density of 100 mA/cm.sup.2 and a current applying time of 60 seconds, and the current application was paused for 10 seconds between the charge and the discharge or between the discharge and the charge. This was considered one cycle, and the potential in the electrode of Example 6 was recorded with respect to that in the reference electrode while continuously performing the charge and discharge cycle. In this charge and discharge cycle test, the electrode was energized with a current density of 100 mA/cm.sup.2 under an atmosphere open condition without any forced supply of oxygen, the reactant, to the electrode by blowing air and oxygen to the electrode, etc. This was a very strict condition under which the electrode is likely to deteriorate, as the condition of the charge and discharge cycle test. Repeating the current application for 60 seconds with a pause for 10 second is also likely to accelerate the deterioration of the electrode. Thus, the experiment for durability was performed under extremely strict conditions.

[0190] FIG. 21 illustrates the results of a comparison of the potential curves during the charge and discharge in respective cycles obtained in the above charge and discharge cycle test, by extracting the potential curves in representative cycles. As illustrated in FIG. 21, the charge potential (about 0.6 V) and the discharge potential (about 0.3 V) hardly changed during 500 cycles of charge and discharge.

[0191] During the charge and discharge cycle test up to the end of 500 cycles, no leakage of the KOH aqueous solution was observed from the atmosphere open side of the electrode, that is, the PTFE particle layer.

[0192] The above results demonstrated that the electrode of Example 6 can maintain high catalytic activity and has high durability under extremely strict charge and discharge test conditions, by including the PTFE particle layer as the water-repellent layer, on the atmosphere side of the gas diffusion layer, which is integrated with the current collector.

Example 7

[0193] An oxygen catalyst was subjected to the grinding process and observed with a SEM under the conditions same as those in Example 4, except for changing the grinding process time in Example 4 to 5 minutes. This oxygen catalyst subjected to the grinding process was supported on a titanium disk under the conditions same as those in Example 1 and Example 2 to produce an electrode of Example 7. Then, for the electrode of Example 7, the oxygen reduction current and the oxygen generation current were measured by the method same as that of Example 1 and Example 2. Furthermore, a value (specific activity) was obtained by dividing each current value by the catalyst weight. The supported amount of the oxygen catalyst in Example 7 was in the range same as that of Example 1 and Example 2.

[0194] As the result of SEM observation of the oxygen catalyst that was subjected to the grinding process for 5 minutes, although particles of 70 nm or less, which are similar to those in Example 1 and Example 2 illustrated in FIG. 9, were observed, particles with a primary particle size of more than 100 nm, which were not observed in Example 1 and Example 2, were also observed, and particles with a primary particle size reaching 200 nm also existed. That is, it was found that the grinding process increased the maximum of the primary particle size to more than 100 nm in Example 7.

[0195] Next, the specific activities of the oxygen generation and the oxygen reduction in the electrode of Example 2 and the electrode of Example 7 were compared. The specific activities of the oxygen generation at 0.56 V were 23 A/g in Example 2 and 23 A/g in Example 7, which were identical. However, the specific activities of the oxygen reduction at 0.1 V were 4.5 A/g in Example 2 and 1.4 A/g in Example 7. That is, the difference in the primary particle sizes of the oxygen catalysts used in the production of the electrodes had no effect on the specific activities of the oxygen generation. In contrast, the specific activity of the oxygen reduction in Example 7 is 31% less than that in Example 2, and an increase in the primary particle size reduced the specific activity for the oxygen reduction. Such results are thought to be due to the fact that the reaction sites of the oxygen generation and the oxygen reduction are different, a two-phase interface in Example 7 and a three-phase interface in Example 2, and the reaction site of the oxygen reduction is more susceptible to the effect the primary particle size. The above results demonstrate that the preferable primary particle size of the oxygen catalyst is 100 nm or less.

INDUSTRIAL APPLICABILITY

[0196] The electrode of this disclosure can be used as the electrode for the oxygen generation, the oxygen reduction, or both reactions, in a battery, an electrolytic device, and a sensor that use the oxygen reduction, the oxygen generation, or both reactions using an alkaline aqueous solution as an electrolyte, in addition to the air electrodes of the air primary battery and the air secondary battery, the oxygen cathode of the brine electrolysis (device), the cathode of the alkaline fuel cell (device), and the anode of the alkaline water electrolysis. The electrode of this disclosure can be also used for an electrode for the oxygen generation, the oxygen reduction, or both reactions at a temperature less than 100 C., in the oxygen reactions using an electrolyte other than the alkaline aqueous solution. The electrode of this disclosure can be used as, for example, the cathode of a solid polymer fuel cell. The electrochemical device of this disclosure can be used for power generation and/or power storage in various kinds of devices and various products that use electricity, such as mobile devices, electronic devices, electronic products, bicycles, automobiles, trains, ships, aircrafts, and drones; hydrogen production; oxygen production; chlorine production; and caustic soda production.

REFERENCE SIGNS LIST

[0197] 10 disk (titanium disk, titanium) [0198] 11 one surface (surface) [0199] 100 electrode [0200] 101 electrode [0201] 102 electrode [0202] 103 electrode [0203] 104 electrode [0204] C oxide layer (oxygen catalyst, oxide) [0205] 21 electrode [0206] 22 counter electrode [0207] 23 reference electrode [0208] 24 electrolyte [0209] 25 liquid junction [0210] 28 first container [0211] 29 second container [0212] 200 measuring cell [0213] 3 current collector [0214] 31 current collecting portion [0215] 32 lead wire portion [0216] 4 catalytic layer [0217] 5 gas diffusion layer [0218] 6 water-repellent layer