WO2007072665A1 - Oxygen reduction electrode for direct fuel cell - Google Patents

Abstract

The present invention aims to provide an oxygen reduction electrode having excellent oxygen reduction catalytic ability in a direct fuel cell wherein a liquid fuel composed of an aqueous solution of an organic compound is oxidized at the negative electrode. Specifically disclosed is an oxygen reduction electrode for direct fuel cells, which contains a metal oxide having at least one composition ratio selected from the group consisting of ZrOx1 (0.25 < x1 < 2.0), CoOx2 (0.2 < x2 < 1.3), NbOx3 (0.3 < x3 < 2.5), TiOx4 (0.25 < x4 < 2.0) and SnOx5 (0.25 < x5 < 2.0) as an electrode active material. The oxygen reduction electrode satisfies the following relation: 0.9 ≤ (EO/ES) ≤ 1 with ES being the electrode potential in 0.1 mol/L sulfuric acid solution and EO being the electrode potential in an aqueous solution containing 0.1 mol/L of sulfuric acid and 0.1 mol/L of the organic compound at a current density from -10 μA/cm2 to -5 μA/cm2.

Description

 Specification

 Oxygen reduction electrode for direct fuel cell

 Technical field

 [0001] The present invention relates to an oxygen reduction electrode used in a direct fuel cell that directly oxidizes liquid fuel such as an alcohol aqueous solution.

 Background art

 [0002] Direct fuel cells that use liquid fuel such as methanol as a direct fuel do not require a hydrogen gas cylinder and have a simple structure. Therefore, they are being applied to portable applications, mobile power supplies, and distributed power supplies. Yes.

 A direct fuel cell has a structure in which a proton conductive polymer electrolyte membrane is sandwiched between a negative electrode and a positive electrode, a methanol aqueous solution serving as fuel is supplied to the negative electrode, and air is supplied to the positive electrode. Then, fuel is oxidized at the negative electrode, and oxygen is reduced at the positive electrode, and the electric energy is taken out to the outside.

 However, in the case of a direct fuel cell, there is a problem that fuel permeates through the electrolyte membrane and reaches the positive electrode (air electrode), and directly burns directly on the catalyst surface of the air electrode. Yes (cross leak phenomenon). When cross leakage occurs, the fuel utilization rate and the potential of the air electrode decrease, and the energy conversion efficiency is significantly reduced.

 Therefore, an electrolyte membrane that suppresses the permeation of fuel has been developed (see, for example, Patent Documents 1 to 3).

 On the other hand, as an oxygen reduction catalyst that can be used in sulfuric acid aqueous solution, ZrO with oxygen deficiency

 2-x has been reported (for example, see Non-Patent Document 1).

[0004] Patent Document 1: Japanese Patent Laid-Open No. 11 144745

 Patent Document 2: Japanese Patent Laid-Open No. 2002-184427

 Patent Document 3: Japanese Patent Laid-Open No. 2003-257453

 Non-specific literature l: Yan Liu et.al., Zirconium uxides for PRFC Cathodes, Electrochemical and solid-state Letter, 8 (8), A400— A402 (2005)

Disclosure of the invention Problems to be solved by the invention

 However, in the case of the techniques described in Patent Documents 1 to 3 described above, ionic conductivity and stability equivalent to those of conventional electrolyte membranes (for example, perfluoroethylenesulfonic acid membranes) are secured. And, it is difficult to significantly reduce only the fuel permeability. Even if an electrolyte membrane with suppressed fuel permeability is used, a certain amount of fuel permeation is inevitable because an aqueous solution is used.

 Accordingly, an object of the present invention is to provide an oxygen reduction electrode for a direct fuel cell which is excellent in oxygen reduction catalytic ability even when a liquid fuel is used.

 Means for solving the problem

[0006] The oxygen reduction electrode for a direct fuel cell of the present invention is used as a positive electrode of a direct fuel cell that oxidizes a liquid fuel composed of an aqueous solution of an organic compound at the negative electrode, and ZrO (0.25 × xl 2.0) xl

At least one selected from the group consisting of CoO (0.2 x 2 x 1.3), NbO (0.3 x x 2.5 x 2.5), TiO (0.25 x x4 x 2.0), and SnO (0.25 x x5 x2 x3 x4 x5 x 2.0) It contains a metal oxide having a composition ratio as an electrode active material, and at a current density of 10 A / cm ² to 15 A / cm ² , the electrode potential in 0.1 mol / L sulfuric acid aqueous solution is E, and sulfuric acid 0.1 mol / L and 0.1% of the organic compound

 S

 When the electrode potential in an aqueous solution containing mol / L is E, 0.9≤ (E / E) ≤1 is satisfied.

 O O S Effect of invention

 [0007] According to the present invention, an oxygen reduction electrode for a direct fuel cell excellent in oxygen reduction catalytic ability can be obtained even when liquid fuel is used.

 BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present invention will be described. In addition, the potential used in the following explanations and charts is based on the reversible hydrogen electrode potential reference, and this is indicated as RHE as necessary.

[0009] Ku-Direct Fuel Cell>

The oxygen reduction electrode of the present invention is used as a positive electrode of a direct fuel cell in which a liquid fuel comprising an aqueous solution of an organic compound is oxidized at the negative electrode. In a direct fuel cell, an electrolyte membrane is sandwiched between a positive electrode and a negative electrode, the liquid fuel is supplied from the outside of the negative electrode, and oxygen is contained from the outside of the positive electrode. A gas (usually air) is supplied to extract electrical energy from the outside! The negative electrode and the positive electrode are usually formed by applying an electrode active material as a catalyst on the surface of a porous electrode substrate.

 As said organic compound, what contains a carbon atom and a hydrogen atom in chemical structures, such as alcohol and ether, can be used, for example. Specific examples of the organic compound include methanol, ethanol, glycol, acetal, dimethyl ether, etc., but the activity energy of the acid-acid reaction is particularly small. It is effective in improving the energy conversion efficiency.

 [0010] <Electrode active material>

 The electrode active material of the oxygen reduction electrode of the present invention is ZrO (0.25 <xl <2.0), CoO (0.2 <x2 <1.3

 xl x2

 ), NbO (0.3 <x3 <2.5), TiO (0.25 <x4 <2.0), and SnO (0.25 <x5 <2.0) x3 x4 x5

 A metal oxide having at least one composition ratio.

 The reason for using the above metal oxide is not clear, but the oxygen reduction reaction is thought to start from the adsorption of oxygen molecules on the oxygen-deficient part of the catalyst surface. It is considered that the product has a low oxygen reduction catalytic ability. Therefore, it is preferable that the electrode active material in the present invention is in a state where oxygen of the metal oxide is insufficient when surface analysis is performed with XPS (X-ray photoelectron spectroscopy analyzer).

 The reason why the metal oxide is inactive to alcohol is not clear, but in the case of an oxide, the alcohol is not adsorbed, so that an acid-acid reaction does not occur.

 In the present invention, an electrode active material force electrode containing at least one metal oxide is formed.

 In the above metal oxide, the reason why the coefficients xl to x5 are defined in the above range is that if each coefficient is less than the above range, the metal component becomes excessive, and the metal component becomes active and becomes unstable. It is. In addition, if each coefficient exceeds the above range (for example, when xl becomes equal to the upper limit of 2.0), it becomes a complete acid state, oxygen adsorption does not occur, and it does not act as an oxygen reduction catalyst. .

[0011] The metal oxide can be obtained, for example, by sputtering a metal or metal oxide target on a carbon electrode substrate. In addition, spray reaction method using solution containing metal salt It can also be used. The formation of metal oxides with insufficient oxygen can be achieved by controlling the gas pressure in the sputtering atmosphere when depositing by sputtering, by controlling the annealing atmosphere in the case of heat treatment, and by controlling the atmosphere in the case of the spray reaction method. It can be carried out.

 [0012] When the above-described metal oxide is used for an oxygen reduction electrode, the electrode active material does not dissolve and is stable even when used in the presence of an aqueous solution of an organic compound.

 This will be described with reference to FIGS. 1 and 2, which schematically show current-potential curves of electrodes. Fig. 1 shows the curve when the electrode according to the present invention is used, and Fig. 2 shows the curve when the Pt electrode is used.

 [0013] In Fig. 1, curve L is in an acidic solution without liquid fuel (alcoholic aqueous solution).

 Ol

It is a current-potential curve of, and shows that only the oxygen reduction reaction proceeds. E ^eq is the theoretical o equilibrium potential, showing a balance between the rate of oxygen reduction and evolution. The equilibrium potential is the potential at which the oxygen electrode reaction is in equilibrium, and the electrode reaction formula: 1Z20 + 2H ++

 2

 In 2e "= H 2 O, oxygen reduction reaction from the left side to the right side and oxygen evolution from the right side to the left side

2

The equilibrium potential is the state in which the live reaction is balanced. When the potential is lower than the equilibrium potential, oxygen reduction occurs preferentially. Curved fl is a current-potential curve in liquid fuel (alcoholic aqueous solution). E ^eq is the theoretical equilibrium potential of the liquid fuel.

 Similarly, the oxidation and reduction reaction of the fuel is in a balanced state. For example, in methanol, the equilibrium state is the state where the electrode reaction formula: CH OH + H 0 = CO + 6H + + 6e— is balanced.

 3 2 2

 In the region where the potential is higher than the equilibrium potential, the acid-oxidation reaction of the fuel proceeds. Since the electrode of the present invention is inactive to the fuel acid, the current value associated with the fuel acid decreases.

 Curve L shows the voltage tl when the liquid fuel supplied to the negative electrode of the fuel cell permeates the positive electrode.

 A flow-potential curve is shown. In this case, since oxygen and liquid fuel coexist in the solution, curve L is a combination of curve L and curve L. The electrode of the present invention has a small oxidation current of fuel.

1 Ol fl

 Therefore, curve L is almost the same as curve L, and the catalytic reaction is reduced by the permeated fuel.

 It can be seen that there are few.

 [0014] In Fig. 2, curve L is in an acidic solution containing no liquid fuel (alcohol aqueous solution).

 02

It is a current-potential curve of, and shows that only the oxygen reduction reaction proceeds. The curve is the current-potential curve in f2 liquid fuel (alcohol solution). Pt electrode for fuel acid On the other hand, since it is active, the current value accompanying the oxidation of the fuel increases.

 Curve L shows the electric power t2 when the liquid fuel supplied to the negative electrode of the fuel cell permeates the positive electrode.

 A current-potential curve is shown. Curve L is a combination of curve L and curve L. Pt electrode is t2 02 f2

 Curve L showing the reaction of only oxygen reduction, which is greatly affected by the oxidation current associated with fuel oxidation

 02 It can be seen that there is a large shift to the left side (low potential side) of the figure. This indicates that the performance as an air electrode catalyst (oxygen reduction electrode) decreases.

[0015] As described above, even if the liquid fuel of the fuel cell permeates to the positive electrode side, the performance of the oxygen reduction electrode does not deteriorate. It is required to be active. In other words, an electrode having a high selectivity for the oxygen reduction reaction is required under the conditions where the fuel oxidation reaction and the electrochemical oxygen reduction reaction proceed as a competitive reaction. The present inventors have results of examining the conditions described above, - 10 at a current density of AZcm ² ~- 5 AZcm ^2, the electrode potential in 0. I mol / L aqueous solution of sulfuric acid and E, the organic compound

 S

 Relation of 0 · 9≤ (Ε / 物) ≤1 where E is the electrode potential in an aqueous solution containing 0 · lmol / L

 O O S

 When satisfying the above, it was found useful as an oxygen reduction electrode.

The reason for setting the current density in the range of 10 μAZcm ² to 15 μAZcm ² is that the oxygen reduction reaction is dominant in this range of current, and the oxygen reduction is measured by measuring the current in this region. This is because the selectivity of the reaction can be evaluated.

 And if it is less than E / E force, the selectivity of oxygen reduction reaction will be less than 90%.

 O S

 The performance of the oxygen reduction electrode is reduced.

 In an ordinary fuel cell, the air electrode (force sword) acts by supplying oxygen as a reactant to the portion where the catalyst and the electrolyte are in contact. In addition, a polymer having a sulfonic acid group is used as an electrolyte in a water-containing state. Therefore, the catalytic ability of the oxygen reduction electrode can be evaluated by substituting the electrolyte with sulfuric acid and simulating the above-described state of the air electrode.

 As described above, according to the oxygen reduction catalyst according to the embodiment of the present invention, the oxygen reduction catalyst is inactive with respect to the acid-oxidation reaction of the liquid fuel (eg, alcohol such as methanol) of the fuel cell, and oxygen reduction. Since it has sufficient catalytic activity for the reaction, it can contribute to the improvement of the performance (power generation efficiency, etc.) of the direct fuel cell.

[0017] Hereinafter, the present invention will be described more specifically with reference to examples. It is not limited to examples.

 Example 1

<Preparation of oxygen reduction electrode>

A cylindrical glassy carbon having a diameter of 5.2 mm was used as a base material, and a ZrO thin film was formed on the bottom surface by sputtering as an electrode material. Sputtering conditions are as follows: He partial pressure is 1 X 10 ^_3 Pa or less

2

 ZrO with a stoichiometric composition was used as the target.

 2

 The thickness of the obtained thin film was measured with a quartz vibration type film thickness meter, and found to be 30 nm. As a result of analyzing the chemical composition of the thin film using XPS (X-ray photoelectron spectrometer), O / Zr was 1.81. This means that the thin film produced is ZrO lacking oxygen compared to ZrO.

 2 1.81.

[0019] Evaluation of electrode reaction>

 Prepare an aqueous solution containing 0.1 mol / L of H 2 SO and CH 2 OH as the electrolyte.

 2 4 3

 Electrolyte 1 was used. Prepare an aqueous solution containing 0.1 mol / L HSO alone as the electrolyte.

 2

Electrolyte 2 was used. Each of the electrolytes 1 and 2 was filled in an electrolytic cell using a reversible hydrogen electrode as a reference electrode, a platinum electrode with platinum black as a counter electrode, and the oxygen reduction electrode as a working electrode. The electrode reaction was evaluated by scanning the potential at ⁵ mV / s in an oxygen atmosphere at 30 ° C.

 The electrode reaction in the electrolyte 1 mimics the state in which liquid fuel (methanol) permeates the positive electrode (air electrode) of the fuel cell. In addition, the electrode reaction in the electrolyte 2 simulates a state in which liquid fuel (methanol) does not permeate the positive electrode (air electrode) of the fuel cell.

[0020] Fig. 3 shows a current-potential curve when the oxygen reduction electrode is used (the electrolyte 1 shows a broken line in the figure, and the electrolyte 2 shows a solid line. The current-potential curve in the other figures. The line is the same). When the electrolytes 1 and 2 were used, the current-potential curves were almost the same, and even when the electrolyte 1 was used, the current indicating the acidity of CH OH was strong. From this,

 Three

 It can be seen that the electrode of this example has very poor catalytic activity for CH OH. Na

 Three

 The negative current on the vertical axis in Fig. 3 indicates the rate of the oxygen reduction reaction. An electrode that can obtain a large oxygen reduction current when the potential on the horizontal axis is high is more active.

In addition, as already explained in Fig. 1, the curve obtained by synthesizing the curve using electrolyte 2 is the actual curve. Force representing an electrode reaction in a fuel cell Because the acid current of methanol is small, the electrode of Example 1 does not decrease the oxygen reduction potential (that is, the performance of the oxygen reduction electrode does not decrease;) Wow.

 Example 2

 [0021] Except that Co 0 having a stoichiometric composition was used as a sputtering target, it was completely the same as Example 1.

 3 4

 Similarly, an oxygen reduction electrode having a catalyst thickness of 30 nm was prepared. Next, a sample with a film thickness of 200 nm was prepared under the same conditions, and XRD diffraction was performed. Figure 4 shows the XRD diffraction chart. In the chart, peaks of Co 0 and CoO are observed, indicating that the thin film of Example 2

 3 4

 Co 0 and CoO can also be mixed, so that not only the highest oxidation number of Co 0 but also oxygen-deficient acids

3 4 3 4

 Indicate that some things are included.

 For this oxygen reduction electrode, the electrode reaction was evaluated in the same manner as in Example 1. FIG. 5 shows a current-potential curve when the oxygen reduction electrode is used. The current-potential curves when using electrolytes 1 and 2 are almost the same, and even when electrolyte 1 is used, CH OH

 3 No current indicating oxidation was observed. From this, the electrode of this example is not compatible with CH OH.

 It can be seen that the catalytic activity is very poor.

 Example 3

 [0022] Exactly the same as Example 1 except that SnO having a stoichiometric composition was used as a sputtering target.

 2

 In this way, an oxygen reduction electrode having a catalyst thickness of 30 nm was prepared. This electrode was also considered to contain oxygen-deficient oxides.

 For this oxygen reduction electrode, the electrode reaction was evaluated in the same manner as in Example 1. Figure 6 shows the current-potential curve when the oxygen reduction electrode is used. The current-potential curves when using electrolytes 1 and 2 are almost the same, and even when electrolyte 1 is used, CH OH

 3 No current indicating oxidation was observed. From this, the electrode of this example is CH OH

 It can be seen that the catalytic activity for 3 is very poor.

 Example 4

 [0023] Except for using Nb 0 having a stoichiometric composition as a sputtering target, completely the same as Example 1.

 twenty five

Similarly, an oxygen reduction electrode having a catalyst thickness of 30 nm was prepared. Next, a sample with a film thickness of 200 nm was prepared under the same conditions, and XRD diffraction was performed. XRD diffraction chart in Figure 7 Show. In the chart, Nb 0 and NbO peaks are observed, indicating that the thin film of Example 4

 2 5 2

 It consists of a mixture of Nb 0 and NbO, and not only the highest oxidation number Nb 0 but also oxygen-deficient acid

2 5 2 2 5

 Indicate that some things are included.

 For this oxygen reduction electrode, the electrode reaction was evaluated in the same manner as in Example 1.

 FIG. 8 shows a current-potential curve when the oxygen reduction electrode is used. The current-potential curves when using electrolytes 1 and 2 are almost the same, and even when electrolyte 1 is used, CH OH

 3 No current indicating oxidation was observed. From this, the electrode of this example is not compatible with CH OH.

 It can be seen that the catalytic activity is very poor.

 Example 5

 [0024] Exactly the same as Example 1 except that a sputter having a stoichiometric composition was used as a sputtering target.

 2

 In this way, an oxygen reduction electrode having a catalyst thickness of 30 nm was prepared. Next, a sample with a film thickness of 200 nm was prepared under the same conditions, and XRD diffraction was performed. Figure 9 shows the XRD diffraction chart. In the chart, a peak of TiO and 0 is observed, indicating that the thin film of Example 5 is TiO

 2 3 5 2 and 0 can also be mixed, not only the highest oxidation number but also the oxygen-deficient acid

3 5 2 Indicates that some are included.

 For this oxygen reduction electrode, the electrode reaction was evaluated in the same manner as in Example 1.

 FIG. 10 shows a current-potential curve when the oxygen reduction electrode is used. The current-potential curves when using electrolytes 1 and 2 are almost the same, and even when electrolyte 1 is used, CH OH

 No current indicating oxidation of 3 was found. From this, the electrode of this example is

 It can be seen that the catalytic activity for 3 is very poor.

<Comparative Example>

 A Pt electrode having a catalyst thickness of 30 nm was prepared in the same manner as in Example 1 except that Pt was used as the sputtering target.

 The electrode reaction of this Pt electrode was evaluated in the same manner as in Example 1.

FIG. 11 shows a current-potential curve when the above Pt electrode is used (only in this figure, electrolyte 1 shows a thin line in the figure, and electrolyte 2 shows a thick line in the figure.) _{0 Using} electrolyte 1 When the potential was less than 0.62 V, the acid current was detected when the force was 0.62 V or more, which is the current value indicating the acid current. On the other hand, when electrolyte 2 was used, no oxidation current occurred. Therefore As already explained in Fig. 2, the curve obtained by synthesizing the curve using electrolyte 2 represents the electrode reaction in the actual fuel cell, and the oxygen reduction potential of the Pt electrode decreases due to the oxidation current of methanol ( That is, it can be seen that the performance of the oxygen reduction electrode is reduced).

 Thus, when the oxygen reduction potential of the Pt electrode decreases, the actual battery voltage decreases, making it unsuitable for practical use.

 [0026] <E / E Measurement>

 o s

Next, based on the current-potential curves of the examples and comparative examples, E and E were obtained at current densities of −10 ^ A / cm ² and 5 AZcm ² and E / E) was calculated.

 S O O S

Table 1 shows E and E (unit V) at current densities of −lO .u A / cm ² and 5 / z AZcm ² from the current-potential curves of FIGS. In the table, “With MeOH” so

 Indicates E, “no MeOH” indicates E.

 o s

 [0027] [Table 1]

 [0028] Table 2 shows Ε / Ε calculated from Table 1.

 O S

[0029] [Table 2]

[0030] From Table 2, in each example, 実 施 / repulsive force is more than, while in the comparative example, Ε

 O S o

Less than / E power. As a result, the electrode of each example having an E / E force or more is a fuel.

S O S

 It is clear that the oxygen reduction reaction has a high selectivity due to the low oxidation reaction of the sample, and therefore exhibits excellent oxygen reduction catalytic activity. That is, the electrode of each example has high oxygen reduction selectivity even when used in the condition where the fuel coexists.

 <Reference example>

 Lack of oxygen, な ZrO

 An experiment showing that 2 has no oxygen reduction catalytic ability is shown as a reference example. First, commercially available ZrO powder was used as a catalyst, and O.lg was weighed and dispersed in 5 ml of water. That

 2

After that, the dispersion is stirred and suspended with ultrasonic waves, 30 L is taken from this solution, and dropped onto a circular part of a glassy carbon electrode (diameter: 5.2 mm) so that the catalyst in the solution is uniformly dispersed and dried. It was. In order to cover the catalyst, a naphthion (registered trademark) solution was further dropped on the catalyst, and heat treatment was performed at 120 ° C. in a nitrogen atmosphere to solidify the naphthion (registered trademark) to obtain an electrode. The obtained electrode was immersed in a sulfuric acid solution of O.lmol / dm ³ and the oxygen reduction catalytic ability was evaluated at 30 ° C. and atmospheric pressure. A reversible hydrogen electrode in the same concentration sulfuric acid solution was used as a reference electrode. The current density is displayed per geometric area.

For this electrode, the electrode reaction was evaluated in the same manner as in Example 1. FIG. 12 shows a current-potential curve when the above electrode is used and electrolyte 1 (without methanol) is used. Compared to the curve in Fig. 3, it was found that the oxygen reduction catalytic ability with a small reduction current was inferior.

Brief Description of Drawings

FIG. 1 is a diagram schematically showing a current-potential curve of an electrode of the present invention.

FIG. 2 is a diagram schematically showing a current-potential curve of a Pt electrode.

 FIG. 3 is a diagram showing a current-potential curve when an oxygen reduction electrode according to an embodiment of the present invention is used.

 FIG. 4 is a diagram showing an XRD diffraction chart of the oxygen reduction electrode according to the embodiment of the present invention.

FIG. 5 is another diagram showing a current-potential curve when the oxygen reduction electrode according to the embodiment of the present invention is used.

 FIG. 6 is still another diagram showing a current-potential curve when the oxygen reduction electrode according to the embodiment of the present invention is used.

 FIG. 7 is still another view showing an XRD diffraction chart of the oxygen reduction electrode according to the embodiment of the present invention.

 FIG. 8 is another diagram showing a current-potential curve when the oxygen reduction electrode according to the embodiment of the present invention is used.

 FIG. 9 is another diagram showing an XRD diffraction chart of the oxygen reduction electrode according to the embodiment of the present invention.

 FIG. 10 is another diagram showing a current-potential curve when the oxygen reduction electrode according to the embodiment of the present invention is used.

 FIG. 1 l is a diagram showing a current-potential curve when a Pt electrode is used for V.

 [Fig. 12] Another graph showing the current-potential curve when oxygen is insufficient and the ZrO electrode is used V

 2

FIG.

Claims

The scope of the claims  [1] An oxygen reduction electrode used as a positive electrode of a direct fuel cell in which a liquid fuel consisting of an aqueous solution of an organic compound is oxidized at the negative electrode,  ZrO (0.25 〈xl 〈2.0), CoO (0.2 〈x2 〈1.3), NbO (0.3 〈x3 〈2.5), TiO (0.25 <x4 <2.0) xl x2 x3 x4 And a group strength of SnO (0.25 x5 2.0) Metallic acid x5 having at least one selected composition ratio  A compound as an electrode active material, At a current density of 10 / z AZcm ² to 1/5 / z AZcm ² , the electrode potential in 0.1 mol / L sulfuric acid aqueous solution is assumed to be E, 0.1 mol / L of sulfuric acid is contained, and the organic compound is 0.1 mol / L. Include  S  Direct shape combustion satisfying the relationship of 0.9≤ (E / E) ≤1, where E is the electrode potential in aqueous solution  O O S  Oxygen reduction electrode for battery.