JP6659916B2 - Oxygen reduction catalyst - Google Patents

Description

  The present invention relates to an oxygen reduction catalyst containing a titanium oxide.

  Titanium oxide has high catalytic properties and is used as a photocatalyst or an organic matter decomposition catalyst. However, there is no report of a titanium oxide catalyst which is stable under strong acidity and has a high oxygen reduction ability.

  Patent Literature 1 discloses a sulfur-containing titanium oxide powder used for a dispersion in which a photocatalyst is dispersed. Although the visible light absorption properties of the sulfur-containing titanium oxide powder are described, no study has been made on the oxygen reduction properties, and there is no suggestion of using the powder as an oxygen reduction catalyst.

Patent Document 2 discloses an oxygen reduction catalyst having titanium, carbon, nitrogen, and oxygen as constituent elements and satisfying a specific elemental composition and a specific relational expression for a peak intensity ratio obtained in an XRD spectrum. It is described that there may be a case where an anatase-type crystal structure is included, and that a case where a sulfur element is included. It is described that the oxygen reduction catalyst has a high cubic structure as a proportion, and thus has high durability confirmed by a change in cell voltage with time in a constant current load measurement test.
Non-Patent Document 1 discloses a partially oxidized titanium carbonitride in which the oxygen reduction activity is improved by doping with nitrogen, but the oxygen reduction onset potential does not reach 0.8 V, and the nitrogen element is further reduced. It is considered that the contained titanium oxide catalyst is unstable under strong acidic conditions during fuel cell operation, easily elutes, and has poor durability. Anatase-type titanium dioxide treated with the same nitrogen element-containing treatment is also described, but their oxygen reduction onset potential is as low as about 0.4 V.

  Non-Patent Document 2 discloses that an electrolyte membrane doped with sulfided titanium oxide is used as an electrolyte membrane of a polymer electrolyte fuel cell. It is described that the fuel cell characteristics are high even at a low relative humidity due to the increase in proton conductivity of the electrolyte membrane. However, there is no description or suggestion that sulfided titanium oxide is used as the catalyst instead of the platinum catalyst.

JP 2006-001774 A JP 2013-240785 A

M. Chisaka et al. , Electrochimica Acta, vol. 88, p697-707 (2013). M. Renzi et al. , International Journal of Hydrogen Energy, vol. 42, p16686-16694 (2017).

  The above prior art does not disclose an oxygen reduction catalyst having a crystal structure of anatase-type titanium dioxide and having an improved oxygen reduction characteristic by containing a specific amount of a sulfur atom.

  The present invention has the following configuration.

[1] The content of anatase-type titanium dioxide in titanium dioxide crystals confirmed by X-ray diffraction measurement is more than 50.0%, and the sulfur atom content is 0.1 to 3.0% by mass. An oxygen reduction catalyst which is a titanium compound.
[2] An electrode catalyst for a fuel cell comprising the oxygen reduction catalyst according to the above [1].
[3] A fuel cell electrode having a catalyst layer containing the fuel cell electrode catalyst according to the item 2 above.
[4] A membrane electrode assembly including a cathode, an anode, and a polymer electrolyte membrane disposed between the cathode and the anode, wherein the cathode is the fuel cell electrode according to the above item 3. Membrane electrode assembly.
[5] A fuel cell comprising the membrane electrode assembly according to item 4 above.

  The oxygen reduction catalyst of the present invention has high oxygen reduction ability, and for example, when used as a fuel cell catalyst for a cathode electrode, a fuel cell having high power generation characteristics can be obtained.

3 is an X-ray diffraction spectrum of the oxidation-reduction catalyst (1) produced in Example 1. Among the diffraction peaks, the peak having the highest diffraction intensity among the peaks corresponding to the anatase-type titanium dioxide crystal is indicated by A. 5 is an X-ray diffraction spectrum of the oxidation-reduction catalyst (2) produced in Example 2. Among the diffraction peaks, the peak having the highest diffraction intensity among the peaks corresponding to the anatase-type titanium dioxide crystal is indicated by A. 9 is an X-ray diffraction spectrum of a redox catalyst (3) produced in Example 3. Among the diffraction peaks, the peak having the highest diffraction intensity among the peaks corresponding to the anatase-type titanium dioxide crystal is indicated by A, and the peak having the highest diffraction intensity among the peaks corresponding to the rutile-type titanium dioxide crystal is indicated by R. . 9 is an X-ray diffraction spectrum of a redox catalyst (4) produced in Example 4. Among the diffraction peaks, the peak having the highest diffraction intensity among the peaks corresponding to the anatase-type titanium dioxide crystal is indicated by A. 9 is an X-ray diffraction spectrum of a redox catalyst (5) produced in Example 5. Among the diffraction peaks, the peak having the highest diffraction intensity among the peaks corresponding to the anatase-type titanium dioxide crystal is indicated by A, and the peak having the highest diffraction intensity among the peaks corresponding to the rutile-type titanium dioxide crystal is indicated by R. . 6 is an X-ray diffraction spectrum of the oxidation-reduction catalyst (c1) produced in Comparative Example 1. Among the diffraction peaks, the peak having the highest diffraction intensity among the peaks corresponding to the rutile-type titanium dioxide crystal is indicated by R. 9 is an X-ray diffraction spectrum of a redox catalyst (c2) produced in Comparative Example 2. Among the diffraction peaks, the peak having the highest diffraction intensity among the peaks corresponding to the anatase-type titanium dioxide crystal is indicated by A, and the peak having the highest diffraction intensity among the peaks corresponding to the rutile-type titanium dioxide crystal is indicated by R. . 9 is an X-ray diffraction spectrum of a redox catalyst (c3) produced in Comparative Example 3. Among the diffraction peaks, the peak having the highest diffraction intensity among the peaks corresponding to the anatase-type titanium dioxide crystal is indicated by A, and the peak having the highest diffraction intensity among the peaks corresponding to the rutile-type titanium dioxide crystal is indicated by R. . 9 is an X-ray diffraction spectrum of a redox catalyst (c4) produced in Comparative Example 4. Among the diffraction peaks, the peak having the highest diffraction intensity among the peaks corresponding to the anatase-type titanium dioxide crystal is indicated by A, and the peak having the highest diffraction intensity among the peaks corresponding to the rutile-type titanium dioxide crystal is indicated by R. . 13 is an X-ray diffraction spectrum of the oxygen reduction catalyst (c5) prepared in Comparative Example 5. Among the diffraction peaks, the peak having the highest diffraction intensity among the peaks corresponding to the anatase-type titanium dioxide crystal is indicated by A.

  Hereinafter, the oxygen reduction catalyst of the present invention will be described in detail.

(Oxygen reduction catalyst)
In the oxygen reduction catalyst of the present invention, the content of anatase-type titanium dioxide in the titanium dioxide crystal confirmed by X-ray diffraction measurement is more than 50.0%, and the sulfur atom content is 0.1 to 3.0% by mass. Is a titanium compound. In other words, the oxygen reduction catalyst of the present invention can be said to be an oxygen reduction catalyst comprising a specific titanium compound. However, this does not strictly exclude the presence of impurities in the oxygen reduction catalyst of the present invention, and unavoidable impurities due to the raw material and / or the production process and other impurities within a range that does not deteriorate the characteristics of the catalyst. May be included in the oxygen reduction catalyst of the present invention.

  The oxygen reduction catalyst of the present invention contains titanium oxide as a main component, but may contain an oxygen-containing compound of another transition metal element. Examples of the transition metal element include Group 4 elements, Group 5 elements, Group 6 elements, and iron group elements in the periodic table. Iron group elements include iron, cobalt and nickel elemental species. Group 4 elements other than titanium include zirconium and hafnium.

(Crystal structure)
Among the titanium compounds constituting the oxygen reduction catalyst of the present invention, the titanium oxide may have a crystal structure of an anatase type titanium dioxide crystal, a rutile type titanium dioxide crystal structure, or Brookite. Crystal structure of titanium dioxide. In the X-ray diffraction spectrum obtained from the X-ray diffraction measurement, these crystal structures can be distinguished by the presence and appearance pattern of a peak characteristic of each crystal structure.
In the crystal structure of anatase type titanium dioxide, the peak of the highest diffraction intensity tends to appear at a position where 2θ is 25 ° to 26 °.
On the other hand, in the crystal structure of rutile-type titanium dioxide, the peak of the highest diffraction intensity appears at the position where 2θ is 27 ° to 28 °, but the pattern tends to have no peak at the position where 2θ is 30 ° to 31 °. There is.
In the crystal structure of the brookite-type titanium dioxide, the peak of the strongest diffraction intensity appears at a position where 2θ is 25 ° to 26 °, and a peak tends to appear at a position where 2θ is 30 ° to 31 °. Therefore, the distinction between the crystal structure of brookite-type titanium dioxide and the crystal structure of anatase-type titanium dioxide can be determined by the presence or absence of a peak at a position where 2θ is 30 ° to 31 °.
Further, as the titanium compound, a case where a cubic crystal structure represented by titanium nitride is included in the oxygen reduction catalyst may be considered. In this case, peaks tend to appear at positions where 2θ is 37 ° to 38 ° and 43 ° to 44 °, respectively.
When titanium disulfide (TiS ₂ ) is used as one of the raw materials, the obtained oxygen reduction catalyst may contain titanium sulfide. In this case, the peak of the highest diffraction intensity tends to appear at a position where 2θ is 34 ° to 35 °.

(Content of anatase type titanium dioxide)
The oxygen reduction catalyst of the present invention has an anatase-type titanium dioxide content (hereinafter, sometimes referred to as “anatase content”) of 50.0% in titanium dioxide crystals confirmed by X-ray diffraction (XRD) measurement. Many included. The anatase content is a value determined from XRD measurement, as described later. The anatase content is preferably at least 60%, more preferably at least 95%. When the anatase content is in the above range, stability under strong acidity is high.

(Content of rutile titanium dioxide)
The content of rutile-type titanium dioxide contained in the oxygen reduction catalyst of the present invention (hereinafter sometimes referred to as “rutile content”), which is obtained in the same manner as the above-mentioned method of obtaining the content of anatase-type titanium dioxide, is determined by the present invention. The content of rutile type titanium dioxide in titanium dioxide crystals confirmed by X-ray diffraction measurement of the oxygen reduction catalyst is preferably less than 50.0%, more preferably 40.0% or less. When the content of the rutile-type titanium dioxide is within the above range, stability under strong acidity is high.

(Content of titanium compound having a cubic crystal structure)
The content of the titanium compound having a cubic crystal structure contained in the oxygen reduction catalyst of the present invention, which is determined in the same manner as the method for determining the content of the anatase type titanium dioxide described above (hereinafter, referred to as “cubic content”) Is preferably less than 30%, more preferably 20% or less, in the titanium compound crystals confirmed by X-ray diffraction measurement of the oxygen reduction catalyst of the present invention. When the content of the cubic titanium compound is within the above range, stability under strong acidity is high.

(Content of sulfur-containing titanium compound)
When a sulfur-containing titanium compound such as titanium disulfide is used as one of the raw materials, the resulting oxygen reduction catalyst may contain the raw material sulfur-containing titanium compound. The content of the sulfur-containing titanium compound as a raw material contained in the obtained oxygen reduction catalyst (hereinafter, sometimes referred to as “sulfur-containing titanium compound content”) is determined in the same manner as in the above-described method of determining the content of anatase-type titanium dioxide. Desired. In the titanium compound crystal confirmed by the X-ray diffraction measurement of the oxygen reduction catalyst of the present invention, the content is preferably less than 10%, more preferably 5% or less, more preferably 1% or less. When the content of the sulfur-containing titanium compound is within the above range, stability under strong acidity is high.

(Sulfur atom content)
The sulfur atom content of the oxygen reduction catalyst of the present invention is in the range of 0.1 to 3.0% by mass. The lower limit of the sulfur atom content is preferably 0.4% by mass, and more preferably 0.5% by mass. The upper limit of the sulfur atom content is preferably 2.0% by mass, and more preferably 1.5% by mass. The state in which the sulfur atom content is smaller than the lower limit is a state in which titanium oxide is insufficiently doped with sulfur, and the amount of active sites as a catalyst tends to be insufficient. The state where the sulfur atom content is larger than the above upper limit includes the state where the sulfur atom is not doped in the titanium oxide, and such a sulfur atom does not contribute to the oxygen reduction property.

(Electrodes, membrane electrode assemblies, fuel cells)
The above-described oxygen reduction catalyst of the present invention is not particularly limited in use, but can be suitably used for an electrode catalyst for a fuel cell, an electrode catalyst for an air cell, and the like.

(Fuel cell electrode)
One preferred embodiment of the present invention is a fuel cell electrode having a catalyst layer containing the above-described oxygen reduction catalyst of the present invention. In this aspect, the fuel cell electrode includes the fuel cell electrode catalyst comprising the oxygen reduction catalyst of the present invention.
The catalyst layer constituting the fuel cell electrode includes an anode catalyst layer and a cathode catalyst layer, and any of the oxygen reduction catalysts of the present invention can be used. Since the oxygen reduction catalyst of the present invention has a high oxygen reduction ability, it is preferably used for the cathode catalyst layer.
Here, the catalyst layer preferably further includes a polymer electrolyte. The polymer electrolyte is not particularly limited as long as it is generally used in a fuel cell catalyst layer. Specifically, a perfluorocarbon polymer having a sulfo group (for example, Nafion (NAFION (registered trademark)), a hydrocarbon polymer compound having a sulfo group, a polymer compound doped with an inorganic acid such as phosphoric acid, Examples thereof include an organic / inorganic hybrid polymer partially substituted with a proton conductive functional group, and a proton conductor in which a polymer matrix is impregnated with a phosphoric acid solution or a sulfuric acid solution, etc. Among these, NAFION (registered trademark) As a supply source of Nafion (NAFION (registered trademark)) for forming the catalyst layer, a 5% Nafion (NAFION (registered trademark)) solution (DE521, DuPont) and the like can be mentioned.
Further, the catalyst layer may further include electron conductive particles made of carbon, a conductive polymer, a conductive ceramic, a metal, or a conductive inorganic oxide such as tungsten oxide or iridium oxide, if necessary. Good.
The method for forming the catalyst layer is not particularly limited, and a known method can be appropriately employed.
The fuel cell electrode may further include a porous support layer in addition to the catalyst layer.
The porous support layer is a layer that diffuses gas (hereinafter, also referred to as “gas diffusion layer”). As the gas diffusion layer, any material may be used as long as it has electron conductivity, high gas diffusivity, and high corrosion resistance. Materials are used.

(Membrane electrode assembly)
The membrane electrode assembly of the present invention is a membrane electrode assembly having a cathode, an anode, and a polymer electrolyte membrane disposed between the cathode and the anode, wherein at least one of the cathode and the anode One of them is the above-described fuel cell electrode of the present invention. At this time, a conventionally known fuel cell electrode, for example, a fuel cell electrode containing a platinum-based catalyst such as platinum-supported carbon can be used as the electrode that does not employ the fuel cell electrode of the present invention. As an example of a preferred embodiment of the membrane electrode assembly of the present invention, at least the cathode is the fuel cell electrode of the present invention.
Here, when the fuel cell electrode of the present invention has a gas diffusion layer, in the membrane electrode assembly of the present invention, this gas diffusion layer is disposed on the opposite side of the catalyst layer when viewed from the polymer electrolyte membrane.
As the polymer electrolyte membrane, for example, an electrolyte membrane using a perfluorosulfonic acid-based membrane or a hydrocarbon-based electrolyte membrane is generally used, and a membrane in which a microporous polymer membrane is impregnated with a liquid electrolyte or a porous membrane is used. A membrane or the like in which a body is filled with a polymer electrolyte may be used.
The membrane electrode assembly of the present invention can be appropriately formed using a conventionally known method.

(Fuel cell)
A fuel cell according to the present invention includes the above-described membrane electrode assembly. Here, in a typical embodiment of the present invention, the fuel cell of the present invention further includes two current collectors with the membrane electrode assembly interposed therebetween. The current collector may be a conventionally known one generally adopted for a fuel cell.

(Production method of oxygen reduction catalyst)
The method for producing the oxygen reduction catalyst of the present invention is not particularly limited as long as an oxygen reduction catalyst having the above configuration is obtained. For example, a method in which a titanium oxide powder obtained by using a sol-gel method and a sulfur-containing substance are mixed and calcined in an oxygen gas-containing atmosphere (manufacturing method 1), or a sulfur-containing titanium compound is mixed in an oxygen gas-containing atmosphere. A firing method (manufacturing method 2) is exemplified. Hereinafter, these two methods will be described in detail.

(Manufacturing method 1)
The production method 1 includes a precursor preparation step of preparing a titanium oxide powder as a titanium oxide precursor using a sol-gel method, a mixing step of mixing the titanium oxide precursor and a sulfur-containing substance, Baking the mixture obtained in the step under an atmosphere containing oxygen gas.

(Precursor preparation step)
In the precursor preparation step, a titanium oxide powder to be used as a titanium oxide precursor is produced using a sol-gel method. As the sol-gel method, a known method can be used. That is, it can be obtained by hydrolyzing titanium-containing compounds such as alkoxides, organic acid salts, nitrates and chlorides of titanium. The specific titanium-containing compound is not particularly limited, but titanium tetramethoxide, titanium tetraethoxide, titanium tetrapropoxide, titanium tetraisopropoxide, titanium tetrabutoxide, titanium tetraisobutoxide, titanium tetrapentoxide, titanium tetra acetyl acetonate, titanium oxy diacetyl acetonate, tris (acetylacetonato) first Nichitan chloride _{([Ti (acac) 3]} 2 [TiC1 6]), titanium tetrachloride, titanium trichloride, titanium oxychloride, four Examples thereof include titanium compounds such as titanium bromide, titanium tribromide, titanium oxybromide, titanium tetraiodide, titanium triiodide, and titanium oxyiodide. These may be used alone or in combination of two or more.
The method of hydrolysis is not particularly limited. For example, the titanium-containing compound is dissolved in an organic solvent such as ethanol to form a titanium-containing compound solution, and the titanium-containing compound solution is hydrolyzed by adding water to the titanium-containing compound solution. Sol can be deposited.
The titanium oxide powder can be obtained as a titanium oxide precursor by removing the solvent from the solution in which the titanium oxide sol is precipitated, washing with water, and drying.

(Mixing process)
In the mixing step, the titanium oxide precursor prepared in the precursor preparation step and the sulfur-containing substance are mixed. The sulfur-containing material is not particularly limited, but is preferably a simple sulfur or a solid or liquid compound from the viewpoint of easy mixing with the titanium oxide precursor and enhancing the reactivity and catalytic activity. In order to prevent metal impurities other than titanium from being mixed into the oxygen reduction catalyst obtained after the calcination step described later, the sulfur-containing material is preferably a sulfur-containing material containing no metal element other than titanium.
Examples of the sulfur-containing substance include sulfur, carbon sulfide, sulfur chloride, sulfides, thioureas, thioamides, thioalcohols, thioaldehydes, thiazils, mercaptals, thiols, and thiocyanes. Also, in detail, thiourea, sulfoacetic acid, thiophenol, thiophene, benzothiophene, dibenzothiophene, thiobenzophenone, bithiophene, phenothiazine, sulfolane, thiazine, thiazole, thiadiazole, thiazoline, thiazolidine, thianthrene, thian, thioacetanilide, thioacetamide , Thiobenzamide, thioanisole, thionine, dimethyl sulfide, methylphenyl sulfide, diallyl sulfide, thiocyanate, sulfuric acid, sulfonic acid, sulfonamide, sulfinic acid, sulfoxide, sulfine, sulfane, and, if applicable, titanium salts thereof. . These may be used alone or in combination of two or more.
As a mixing method, if the sulfur-containing substance is a solid, for example, the titanium oxide precursor and the sulfur-containing substance can be mixed by a dry mixing method or a wet mixing method. From the viewpoint of cost reduction and simplification of the mixing step, the dry mixing method is more preferable. For example, it is possible to mix using a ball mill, a roll rolling mill, a bead mill, a medium stirring mill, an air pulverizer, a mortar or an automatic kneading mortar, and from the viewpoint of uniformity of mixing and cost, a ball mill, a bead mill, an automatic kneading machine. Mortars are preferred, and ball mills or automatic kneading mortars are more preferred. The mixing time in these cases is, for example, 1 to 10 hours. As the solid sulfur-containing material, sulfur or thiourea is preferable, and thiourea is more preferable. If the sulfur-containing substance is a liquid, the titanium oxide precursor can be dispersed and mixed in the sulfur-containing substance. When the sulfur-containing substance can be dissolved in a solvent to form a solution, a sulfur-containing substance solution may be used. Examples of the solvent for dissolving the sulfur-containing substance include water, ethanol, ethylene glycol, and the like, depending on the type of the sulfur-containing substance. The above-mentioned sulfur-containing substances may be used alone or in combination of two or more. When the mixture contains a solvent, the mixture is appropriately heated and the solvent is removed to obtain a mixture in which the titanium oxide precursor and the sulfur-containing material solution are mixed. The mixing ratio of the titanium oxide precursor and the sulfur-containing substance is preferably such that the molar ratio of titanium atoms to sulfur atoms contained in the mixture is in the range of 1: 3 to 1: 9.

(Firing process)
In the firing step, the mixture obtained in the mixing step is fired. The firing atmosphere of the mixture is preferably an oxygen gas-containing atmosphere, and more preferably a mixed gas atmosphere of nitrogen gas and / or argon gas and oxygen gas. The oxygen gas content in the oxygen gas-containing atmosphere is preferably from 10% by volume to 30% by volume. The firing can be performed in an air atmosphere. The firing temperature and time are each preferably 400 to 800 ° C, more preferably 500 to 700 ° C, preferably 1 to 5 hours, and more preferably 2 to 4 hours. The firing temperature and time are adjusted according to each other. When calcining under the conditions in the above-described range, the obtained oxygen reduction catalyst contains a necessary and sufficient amount of sulfur atoms, and the anatase content is more than 50.0%, which is preferable.

(Manufacturing method 2)
Production method 2 comprises a step of firing a sulfur-containing titanium compound under an atmosphere containing oxygen gas.
Examples of the sulfur-containing titanium compound used as a raw material include titanium disulfide, titanium monosulfide, titanium sulfate, and titanium sulfite. It is preferable to use titanium disulfide for easy handling. The oxygen gas-containing atmosphere is more preferably a mixed gas atmosphere of nitrogen gas and / or argon gas and oxygen gas. The oxygen gas content of the oxygen-containing atmosphere is preferably 0.1 to 10.0% by volume, more preferably 0.1 to 1.0% by volume, and still more preferably 0.1 to 0.5% by volume. The firing temperature is preferably higher than 500 ° C and lower than 800 ° C, more preferably 650 to 750 ° C. The firing time is preferably 1 to 5 hours, and more preferably 2 to 4 hours. The firing time and temperature are adjusted to each other. When heated under the conditions described above, for example, when titanium disulfide is used, titanium disulfide is completely decomposed, and the obtained oxygen reduction catalyst contains a necessary and sufficient amount of sulfur atoms and has an anatase content of 50%. More than 0.0% is preferable.

  Hereinafter, the present invention will be specifically described based on examples. The present invention is not limited only to these examples.

Example 1
(1) Production of oxygen reduction catalyst The production of the oxygen reduction catalyst was carried out by the above-mentioned production method 1. Details are described below.
(Precursor preparation step)
26 mL of titanium (IV) isopropoxide (manufactured by Junsei Chemical) is added to 250 mL of dehydrated ethanol (manufactured by Wako Pure Chemical Industries), 25 mL of ultrapure water is slowly added with stirring, and then the titanium oxide sol is stirred by stirring for 2 hours. Was deposited. The solvent from which the titanium oxide sol was precipitated was filtered to remove the solvent, then washed with water and dried to obtain a titanium oxide powder as a titanium oxide precursor (hereinafter referred to as “titanium oxide precursor (1)”). 0.0 g was obtained.
(Mixing process)
7.0 g of the titanium oxide precursor (1) obtained in the precursor preparation step and 27.4 g of thiourea (manufactured by Wako Pure Chemical Industries) were mixed using a mortar to obtain a mixture. The mixing ratio is 4 mol of sulfur atoms to 1 mol of titanium atoms.
(Firing process)
The mixture obtained in the mixing step is placed in a quartz tube furnace, heated to 500 ° C. at a rate of 10 ° C./min in an air atmosphere (gas flow rate 300 mL / min), and fired at 500 ° C. for 3 hours. As a result, 10 g of an oxygen reduction catalyst (1) was obtained.

(2) Electrochemical measurement (catalyst electrode preparation)
A fuel cell electrode provided with a catalyst layer containing an oxygen reduction catalyst (hereinafter referred to as "catalyst electrode") was produced as follows. A solution containing 15 mg of the obtained oxygen reduction catalyst (1), 1.0 mL of 2-propanol, 1.0 mL of ion-exchanged water, and 62 μL of Nafion (NAFION (registered trademark), a 5% aqueous solution of Nafion, manufactured by Wako Pure Chemical Industries) was added. The suspension was stirred by irradiating a sound wave to obtain a suspension. 20 μL of this suspension was applied to a glassy carbon electrode (manufactured by Tokai Carbon Co., Ltd., diameter: 5.2 mm) and dried at 70 ° C. for 1 hour to obtain a catalyst electrode for measuring oxygen reduction catalyst activity.
(Oxygen reduction catalyst activity measurement)
The electrochemical evaluation of the oxygen reduction activity of the oxygen reduction catalyst (1) was performed as follows. The catalyst electrode prepared in the above “Catalyst electrode preparation” was polarized in a 0.5 mol / dm ³ aqueous sulfuric acid solution at 30 ° C. at a potential scanning speed of 5 mV / sec in an oxygen gas atmosphere and a nitrogen gas atmosphere, respectively. -A potential curve was measured. In addition, a natural potential (open circuit potential) in a non-polarized state in an oxygen gas atmosphere was obtained. At that time, a reversible hydrogen electrode in a sulfuric acid aqueous solution of the same concentration was used as a reference electrode.
The electrode potential at 10 μA (hereinafter also referred to as electrode potential) was obtained from the difference between the reduction current curve in an oxygen gas atmosphere and the reduction current curve in a nitrogen gas atmosphere among the current-potential curves obtained in the electrochemical evaluation. . Further, the oxygen reduction catalytic ability of the oxygen reduction catalyst (1) was evaluated using the electrode potential and the natural potential. Table 1 shows the spontaneous potential obtained as an index of the oxygen reduction activity.

(3) Powder X-ray Diffraction (XRD) Measurement The powder X-ray diffraction measurement of the sample was performed using a powder X-ray diffractometer Panaritical MPD (manufactured by Spectris Co., Ltd.). As X-ray diffraction measurement conditions, measurement was performed using Cu-Kα rays (output 45 kV, 40 mA) in a diffraction angle range of 2θ = 10 to 70 °, and an X-ray diffraction spectrum of the oxygen reduction catalyst (1) was obtained. . FIG. 1 shows the obtained X-ray diffraction (XRD) spectrum.
Peak height (Ha) of the strongest diffraction intensity among the peaks corresponding to the anatase-type titanium dioxide crystal, height (Hr) of the strongest diffraction intensity among the peaks corresponding to the rutile-type titanium dioxide crystal, Brookite Height of the highest diffraction intensity (Hb) among the peaks corresponding to the titanium dioxide crystal, the height (Hc) of the highest diffraction intensity among the peaks corresponding to cubic titanium nitride, and When there is a sulfur-containing titanium compound, the height (Hs) of the peak having the highest diffraction intensity among the peaks corresponding to the sulfur-containing titanium compound is determined, and the anatase in the prepared oxygen reduction catalyst is calculated by the following formula. The content of titanium dioxide (anatase content) was determined. The peak height of the strongest diffraction intensity was determined by performing background specification processing (processing conditions, background specification: automatic, granularity: 20, granularity: 13) using software HighScore Plus attached to the apparatus. Peak height.
Anatase content (%) = {Ha / (Ha + Hr + Hb))} × 100
Rutile content (%) = {Hr / (Ha + Hr + Hb))} × 100
Cubic content (%) = {Hc / (Ha + Hr + Hb + Hc + Hs))} × 100
Sulfur-containing titanium compound content (%) = {Hs / (Ha + Hr + Hb + Hc + Hs))} × 100
In the XRD spectrum of the oxygen reduction catalyst (1), only anatase type titanium dioxide was observed, and it was confirmed that the anatase content was 100%. The rutile content was determined to be 0%. Table 1 shows the crystal structure confirmed by the XRD measurement, together with the anatase content and the spontaneous potential.

(4) Sulfur atom content 10 mg of the oxygen reduction catalyst (1) is weighed into a ceramic crucible, and an appropriate amount of tungsten powder and tin powder is added as a combustion aid, and a carbon sulfur analyzer (model number: EMIA-920V, manufactured by HORIBA, Ltd.) And the temperature was measured under an oxygen gas flow by using an infrared absorption method. Table 1 also shows the obtained sulfur atom content (% by mass).

Example 2:
(Preparation of oxygen reduction catalyst)
An oxygen reduction catalyst (2) was obtained in the same manner as in Example 1, except that the firing temperature was changed to 600 ° C.
(Electrochemical measurement, XRD measurement, sulfur atom content)
Electrochemical measurement, XRD measurement and sulfur atom content were measured and analyzed in the same manner as in Example 1. FIG. 2 shows the obtained XRD spectrum.
In the XRD spectrum of the oxygen reduction catalyst (2), only anatase type titanium dioxide was observed, and it was confirmed that the anatase content was 100%. The rutile content was determined to be 0%. Table 1 shows the crystal structure confirmed by the XRD measurement together with the anatase content, the sulfur atom content, and the spontaneous potential.

Example 3
(Preparation of oxygen reduction catalyst)
An oxygen reduction catalyst (3) was obtained in the same manner as in Example 1, except that the firing temperature was changed to 700 ° C.
(Electrochemical measurement, XRD measurement, sulfur atom content)
Electrochemical measurement, XRD measurement and sulfur atom content were measured and analyzed in the same manner as in Example 1. FIG. 3 shows the obtained XRD spectrum.
In the XRD spectrum of the oxygen reduction catalyst (3), only anatase type titanium dioxide and rutile type titanium dioxide were observed, and it was confirmed that the anatase content was 66.5%. The rutile content was determined to be 33.5%. Table 1 shows the crystal structure confirmed by the XRD measurement together with the anatase content, the sulfur atom content, and the spontaneous potential.

Example 4:
(Preparation of oxygen reduction catalyst)
The amount of thiourea was changed to 61.7 g with respect to 7.0 g of the titanium oxide precursor (1) to be mixed in the mixing step of the preparation of the oxygen reduction catalyst, and the mixing ratio was changed to 9 mol of sulfur atoms with respect to 1 mol of titanium atoms. Except that, an oxygen reduction catalyst (4) was obtained in the same manner as in Example 1.
(Electrochemical measurement, XRD measurement, sulfur atom content)
Electrochemical measurement, XRD measurement and sulfur atom content were measured and analyzed in the same manner as in Example 1. FIG. 4 shows the obtained X-ray diffraction (XRD) spectrum.
In the XRD spectrum of the oxygen reduction catalyst (4), only anatase type titanium dioxide was observed, and it was confirmed that the anatase content was 100%. The rutile content was determined to be 0%. Table 1 shows the crystal structure confirmed by the XRD measurement together with the anatase content, the sulfur atom content, and the spontaneous potential.

Comparative Example 1:
(Preparation of oxygen reduction catalyst)
An oxygen reduction catalyst (c1) was obtained in the same manner as in Example 1, except that the firing temperature was changed to 900 ° C.
(Electrochemical measurement, XRD measurement, sulfur atom content)
Electrochemical measurement, XRD measurement and sulfur atom content were measured and analyzed in the same manner as in Example 1. FIG. 6 shows the obtained XRD spectrum.
In the XRD spectrum of the oxygen reduction catalyst (c1), only rutile-type titanium dioxide was observed, and it was confirmed that the rutile content was 100%. The anatase content was determined to be 0%. Table 1 shows the crystal structure confirmed by the XRD measurement together with the anatase content, the sulfur atom content, and the spontaneous potential.

Example 5:
(Preparation of oxygen reduction catalyst)
The production of the oxygen reduction catalyst was carried out by the above-mentioned production method 2. Details are described below.
0.3 g of titanium disulfide powder (manufactured by Alfa Aesar, purity 99.8% by mass based on titanium, 200 mesh) was weighed and placed in an inner case made of quartz, and was rotated using a rotary firing furnace (manufactured by Motoyama). In a mixed gas flow of nitrogen gas (gas flow rate 100 mL / min) and oxygen gas (gas flow rate 0.5 mL / min), the temperature was raised to 700 ° C at a rate of 10 ° C / min, and calcined at 700 ° C for 3 hours. Thus, 0.2 g of an oxygen reduction catalyst (5) was obtained.
(Electrochemical measurement, XRD measurement, sulfur atom content)
Electrochemical measurement, XRD measurement and sulfur atom content were measured and analyzed in the same manner as in Example 1. FIG. 5 shows the obtained XRD spectrum.
In the XRD spectrum of the oxygen reduction catalyst (5), only anatase type titanium dioxide and rutile type titanium dioxide were observed, and it was confirmed that the anatase content was 52.5%. The rutile content was determined to be 47.5%. The sulfur-containing titanium compound content was determined to be 0%. Table 1 shows the crystal structure confirmed by the XRD measurement together with the anatase content, the sulfur atom content, and the spontaneous potential.

Comparative Example 2:
(Preparation of oxygen reduction catalyst)
Example 5 Example 5 was repeated except that the calcination temperature for producing the oxygen reduction catalyst in Example 5 was set to 500 ° C, and the mixed gas stream was changed to nitrogen gas (gas flow rate 100 mL / min) and oxygen gas (gas flow rate 1.0 mL / min). In the same manner as in Example 5, an oxygen reduction catalyst (c2) was obtained.
(Electrochemical measurement, XRD measurement, sulfur atom content)
Electrochemical measurement, XRD measurement and sulfur atom content were measured and analyzed in the same manner as in Example 1. FIG. 7 shows the obtained XRD spectrum.
In the XRD spectrum of the oxygen reduction catalyst (c2), only anatase type titanium dioxide and rutile type titanium dioxide were observed, and it was confirmed that the anatase content was 66.1%. The rutile content was determined to be 33.9%. The sulfur-containing titanium compound content was determined to be 0%. Table 1 shows the crystal structure confirmed by the XRD measurement together with the anatase content, the sulfur atom content, and the spontaneous potential.

Comparative Example 3:
(Preparation of oxygen reduction catalyst)
An oxygen reduction catalyst (c3) was obtained in the same manner as in Example 5, except that the firing temperature for producing the oxygen reduction catalyst in Example 5 was changed to 600 ° C.
(Electrochemical measurement, XRD measurement, sulfur atom content)
Electrochemical measurement, XRD measurement and sulfur atom content were measured and analyzed in the same manner as in Example 1. FIG. 8 shows the obtained XRD spectrum.
In the XRD spectrum of the oxygen reduction catalyst (c3), only anatase type titanium dioxide and rutile type titanium dioxide were observed, and it was confirmed that the anatase content was 55.1%. The rutile content was determined to be 44.9%. The sulfur-containing titanium compound content was determined to be 0%. Table 1 shows the crystal structure confirmed by the XRD measurement together with the anatase content, the sulfur atom content, and the spontaneous potential.

Comparative Example 4:
(Preparation of oxygen reduction catalyst)
An oxygen reduction catalyst (c4) was obtained in the same manner as in Example 5, except that the firing temperature for producing the oxygen reduction catalyst in Example 5 was changed to 800 ° C.
(Electrochemical measurement, XRD measurement, sulfur atom content)
Electrochemical measurement, XRD measurement and sulfur atom content were measured and analyzed in the same manner as in Example 1. FIG. 9 shows the obtained XRD spectrum.
In the XRD spectrum of the oxygen reduction catalyst (c4), only anatase type titanium dioxide and rutile type titanium dioxide were observed, and it was confirmed that the anatase content was 7.9%. The rutile content was determined to be 92.1%. The sulfur-containing titanium compound content was determined to be 0%. Table 1 shows the crystal structure confirmed by the XRD measurement together with the anatase content, the sulfur atom content, and the spontaneous potential.

Comparative Example 5:
(Preparation of oxygen reduction catalyst)
Anatase type titanium dioxide powder (model number: F-6, manufactured by Showa Denko) was used as it was as an oxygen reduction catalyst (c5) without heat treatment.
(Electrochemical measurement, XRD measurement, sulfur atom content)
Electrochemical measurement, XRD measurement and sulfur atom content were measured and analyzed in the same manner as in Example 1. FIG. 10 shows the obtained XRD spectrum. In the XRD spectrum of the oxygen reduction catalyst (c5), only anatase type titanium dioxide was observed, and it was confirmed that the anatase content was 100%. Table 1 shows the crystal structure confirmed by the XRD measurement together with the anatase content, the sulfur atom content, and the spontaneous potential.

  According to the results of the examples, the oxygen reduction catalyst having an anatase content of more than 50.0% and a sulfur atom content in the range of 0.1 to 3.0% by mass has a high spontaneous potential.

  The oxygen reduction catalyst of the present invention has a high acid resistance due to a large content of anatase type titanium dioxide, and a high spontaneous potential in a non-polarized state in an oxygen gas atmosphere. It can be suitably used for a battery electrode.

Claims (5)

  The content of anatase-type titanium dioxide in the titanium dioxide crystal confirmed by X-ray diffraction measurement is more than 50.0%, the sulfur atom content is 0.1 to 3.0% by mass, and does not contain a fluorine atom. An oxygen reduction catalyst, which is a titanium oxide.   An electrode catalyst for a fuel cell comprising the oxygen reduction catalyst according to claim 1.   An electrode for a fuel cell, comprising a catalyst layer containing the electrode catalyst for a fuel cell according to claim 2.   A membrane electrode assembly comprising a cathode, an anode, and a polymer electrolyte membrane disposed between the cathode and the anode, wherein the cathode is the fuel cell electrode according to claim 3. Joint. A fuel cell comprising the membrane electrode assembly according to claim 4.

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