Iridium-manganese oxide composite material, iridium-manganese oxide composite electrode material, and methods for producing the same

Abstract

To provide an iridium-manganese oxide composite material and an iridium-manganese oxide composite electrode material which are used as positive electrode catalysts for oxygen generation in water electrolysis, are inexpensive and have high catalytic activity, and methods for producing them.SOLUTION: In the iridium-manganese oxide composite material, iridium is arranged and distributed over a surface of a manganese oxide, and the iridium has a metallic valence of 3.1 or more and 3.8 or less. The iridium-manganese oxide composite electrode material contains an electrically conductive base material comprising fibers at least partially covered with the iridium-manganese oxide composite material.SELECTED DRAWING: Figure 5

Description

[Technical Field]

[0001] The present invention relates to an iridium-manganese oxide composite material for water splitting catalysts, an iridium-manganese oxide composite electrode material, a membrane-electrode assembly, and methods for producing these. More specifically, the present invention relates to an iridium-manganese oxide composite material, an iridium-manganese oxide composite electrode material, a membrane-electrode assembly, and methods for producing these that are used as an anode catalyst for oxygen generation in industrial water electrolysis performed under alkaline, neutral, or acidic conditions, or in water electrolysis using a polymer electrolyte membrane (PEM) electrolytic cell. [Background technology]

[0002] Due to the problems of fossil fuel depletion and environmental pollution, attention is being focused on the use of hydrogen as a clean energy source and methods for its production. Water electrolysis is an effective method for producing high-purity hydrogen gas from the cathode by electrolyzing water. However, a distinctive feature of this process is that oxygen is simultaneously generated from the anode (counter electrode). To efficiently promote the water splitting reaction in water electrolysis, it is necessary to maintain a low electrolysis voltage by using an electrode catalyst with a low hydrogen overvoltage at the cathode and an electrode catalyst with a low oxygen overvoltage at the anode. Among these, rare platinum group metals such as platinum (Pt), iridium (Ir), and ruthenium (Ru), as well as oxides and other compounds containing these elements, have been proposed as electrode catalyst materials with excellent anode oxygen overvoltage (Patent Documents 1 and 2, Non-Patent Documents 1 to 3).

[0003] Among these, iridium (Ir) is widely known as an extremely active oxygen-evolving electrode catalyst. However, compared to other precious metals, its reserves are extremely small, and its uneven distribution in specific regions means that its global production is also very low. This raises concerns that even if water electrolysis technology becomes widespread in the future, there will not be enough catalysts to meet the demand (Non-Patent Document 4). Because such platinum group metal electrode catalysts are very expensive, development of alternative electrode catalysts using inexpensive transition metals has been promoted. For example, transition metal materials composed of manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), etc. have been proposed in recent years (Patent Documents 3 and 4, Non-Patent Documents 5 to 8).

[0004] However, catalytic materials composed of transition metals that have been proposed so far have the problem of significantly lower activity (high oxygen overvoltage) compared to platinum group metal-based electrode catalysts. In other words, no oxygen evolution electrode catalyst material composed of inexpensive transition metals and having high catalytic activity comparable to platinum group metals such as Pt and Ir has been realized. To address these issues, manganese oxides have been found to have oxygen evolution electrode catalytic activity equal to or greater than that of Pt. However, these do not reach the activity of Ir-based catalysts, which are considered to have the highest activity among platinum group metal elements, and further development has been awaited (Patent Document 5). [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 8-269761 [Patent Document 2] Japan Special Publication No. 2007-514520 [Patent Document 3] Japanese Patent Publication No. 2015-192993 [Patent Document 4] International Publication (WO) 2009 / 154753 [Patent Document 5] International Publication (WO) 2019 / 117199 [Non-patent literature]

[0006] [Non-Patent Document 1] S. Trasatti, G. Buzanca, J. Electroanal.Chem.,1971,29,A1. [Non-patent document 2] A. Harriman, I. J. Pickering, J. M. Thomas, P. A. Christensen, J. Chem. Soc., Faraday Trans. 1, 1988, 84, 2795.

Non-Patent Document 3

Non-Patent Document 4

Non-Patent Document 5

Non-Patent Document 6

Non-Patent Document 7

Non-Patent Document 8

Summary of the Invention

Problems to be Solved by the Invention

[0007] An object of the present invention is to provide an iridium-manganese oxide composite material for use as a water splitting catalyst, an iridium-manganese oxide composite electrode material, a membrane-electrode assembly, and methods for producing the same. More specifically, the present invention relates to an iridium-manganese oxide composite material for water splitting catalysts (hereinafter sometimes referred to as the iridium-manganese oxide of the present invention), which is an anode catalyst material for oxygen evolution in industrial water electrolysis performed under alkaline, neutral, or acidic conditions or in water electrolysis using a polymer electrolyte membrane (PEM) electrolytic cell, and which is cheaper than current iridium catalyst systems and has high oxygen evolution catalytic activity; an iridium-manganese oxide composite electrode material for water splitting catalysts; a membrane-electrode assembly using the iridium-manganese oxide composite material; and methods for producing the same. [Means for solving the problem]

[0008] As a result of extensive research into catalyst materials for use as oxygen evolving electrode catalysts in water electrolysis, the inventors discovered that an iridium-manganese oxide composite material in which iridium is dispersed and allocated at least on the surface of a manganese oxide and the metal valence of the iridium is 3.1 or more and 3.8 or less exhibits high oxygen evolving electrode catalytic activity and excellent durability even with an extremely small amount of iridium, leading to the completion of the present invention. That is, the present invention is an iridium-manganese oxide composite material for use as an oxygen evolving electrode catalyst in water electrolysis, characterized in that iridium is dispersed and allocated at least on the surface of a manganese oxide and the metal valence of the iridium is 3.1 or more and 3.8 or less. The present inventors have found that an iridium-manganese oxide composite electrode material in which at least a portion of a conductive substrate made of conductive fibers is coated with the iridium-manganese oxide composite material of the present invention exhibits particularly high oxygen evolution electrode catalytic activity. That is, the present invention provides an iridium-manganese oxide composite electrode material for an oxygen evolution electrode in which at least a portion of a conductive substrate made of conductive fibers is coated with the iridium-manganese oxide composite material of the present invention. . That is, the gist of the present invention is as follows. [1] An iridium-manganese oxide composite material, characterized in that iridium is dispersed and allocated at least on the surface of a manganese oxide, and the metal valence of the iridium is 3.1 or more and 3.8 or less. [2] When at least a portion of a conductive substrate is coated with an iridium-manganese oxide composite material, the iridium content is 0.01 mg / cm per geometric area of ​​the conductive substrate. 2 More than 0.2mg / cm 2 The iridium-manganese oxide composite material according to [1] above, characterized in that: [3] The iridium-manganese oxide composite material according to [1] or [2] above, characterized in that the metal content ratio (iridium / (manganese+iridium)) is 0.2 atomic % or more and 10 atomic % or less. [4] The iridium-manganese oxide composite material according to any one of [1] to [3] above, characterized in that the peak position appearing in the XANES region of the Ir L3 absorption edge spectrum obtained from XAFS measurement is 11,200 eV or more and 11,230 eV or less. [5] The iridium-manganese oxide composite material according to any one of [1] to [4] above, characterized in that the peak position corresponding to the iridium-oxygen bond in the radial structure function obtained from XAFS measurement is 1.0 Å or more and 2.0 Å or less. [6] BET specific surface area is 15m 2 / g or more 100m 2 The iridium-manganese oxide composite material according to any one of [1] to [5] above, characterized in that its viscosity is not more than 100 MPa. [7] The iridium-manganese oxide composite material according to any one of [1] to [6] above, wherein the manganese oxide has a manganese metal valence of 3.5 or more and 4.0 or less. [8] When at least a portion of a conductive substrate is coated with an iridium-manganese oxide composite material, the manganese content is 0.12 mg / cm per geometric area of ​​the conductive substrate. 2 More than 14.35mg / cm 2 The iridium-manganese oxide composite material according to any one of the above [1] to [7], characterized in that: [9] The iridium-manganese oxide composite material according to any one of [1] to [8] above, wherein the manganese oxide is electrolytic manganese dioxide.

[10] The iridium-manganese oxide composite material according to any one of [1] to [9] above, characterized in that the manganese oxide is manganese dioxide in any one of the γ-, β-, ε-, or α-crystalline phases, or in the form of a mixed crystal.

[11] An iridium-manganese oxide composite electrode material, characterized in that the iridium-manganese oxide composite material according to any one of [1] to

[10] above is coated on at least a portion of a conductive substrate made of conductive fibers.

[12] The iridium-manganese oxide composite material is 0.1 mg / cm per geometric area of ​​the conductive substrate. 2 More than 20mg / cm 2 The iridium-manganese oxide composite electrode material according to

[11] above is characterized by being coated with the following:

[13] The iridium-manganese oxide composite electrode material according to

[11] or

[12] above, wherein the conductive substrate is made of carbon, titanium, or platinum-coated titanium.

[14] A membrane-electrode assembly comprising an electrode carrying the iridium-manganese oxide composite material according to any one of [1] to

[10] above, and a polymer electrolyte membrane.

[15] A method for producing an iridium-manganese oxide composite material according to any one of [1] to

[10] above, characterized in that manganese oxide obtained by electrolysis of a mixed solution containing sulfuric acid and manganese sulfate is immersed in or brought into contact with an iridium salt solution, and then annealed.

[16] The method for producing an iridium-manganese oxide composite material according to

[15] above, wherein the sulfuric acid concentration of the mixed solution containing sulfuric acid and manganese sulfate is 5 g / L or more and 65 g / L or less.

[17] The electrolysis of the mixed solution containing the above sulfuric acid and manganese sulfate was performed at 0.3 mA / cm 2 More than 20mA / cm 2

[15] or

[16] above, characterized in that the method is carried out at a current density of The method for producing the iridium-manganese oxide composite material is as follows.

[18] The method for producing an iridium-manganese oxide composite material according to any one of

[15] to

[17] above, wherein the iridium salt in the iridium salt solution is K2IrCl6.

[19] The method for producing an iridium-manganese oxide composite material according to any one of

[15] to

[18] above, wherein the annealing treatment is carried out at a temperature higher than 100°C and not higher than 600°C for 10 minutes to 24 hours.

[20] A method for producing an iridium-manganese oxide composite electrode material according to any one of

[11] to

[13] above, comprising electrolyzing a mixed solution containing sulfuric acid and manganese sulfate to electrodeposit manganese oxide onto at least a portion of a conductive base material made of conductive fibers, followed by immersing the material in or contacting the material with an iridium salt solution to uniformly disperse and adsorb iridium onto at least the surface of the manganese oxide, and then annealing the material.

[21] The method for producing an iridium-manganese oxide composite electrode material according to

[20] above, wherein the sulfuric acid concentration of the mixed solution containing sulfuric acid and manganese sulfate is 5 g / L or more and 65 g / L or less.

[22] The electrolysis of the mixed solution containing the above sulfuric acid and manganese sulfate was performed at 0.3 mA / cm 2 More than 20mA / cm 2 The method for producing an iridium-manganese oxide composite electrode material according to

[20] or

[21] above, characterized in that the method is carried out at the following current density:

[23] The method for producing an iridium-manganese oxide composite electrode material according to any one of

[20] to

[22] above, wherein the iridium salt in the iridium salt solution is K2IrCl6.

[24] The method for producing an iridium-manganese oxide composite electrode material according to any one of

[20] to

[23] above, wherein the annealing treatment is carried out at a temperature higher than 100°C and not higher than 600°C for 10 minutes to 24 hours.

[25] An oxygen generating electrode active material for water electrolysis, comprising the iridium-manganese oxide composite material according to any one of [1] to

[10] above.

[26] An oxygen generating electrode comprising the oxygen generating electrode active material according to

[25] above.

[27] A membrane-electrode assembly comprising the oxygen generating electrode according to

[26] above and a polymer electrolyte membrane.

[28] A water electrolysis device comprising the iridium-manganese oxide composite electrode material according to any one of

[11] to

[13] above or the oxygen generating electrode according to

[26] above.

[29] A method for producing hydrogen by electrolyzing water using the iridium-manganese oxide composite electrode material according to any one of

[11] to

[13] above or the oxygen generating electrode according to

[26] above. [Effects of the Invention]

[0009] The iridium-manganese oxide composite material of the present invention and the iridium-manganese oxide composite electrode material of the present invention exhibit high activity and durability in industrial water electrolysis performed under alkaline, neutral, or acidic conditions, and in water electrolysis using a PEM-type electrolytic cell, and act as an inexpensive and excellent anode catalyst for oxygen evolution. Furthermore, by adding carbon dioxide to the electrolysis system using the iridium-manganese oxide composite material of the present invention and the iridium-manganese oxide composite electrode material of the present invention, the carbon dioxide can be reduced at the cathode to produce hydrocarbon compounds (formic acid, formaldehyde, methanol, methane, ethane, propane, etc.). [Brief explanation of the drawings]

[0010] [Figure 1] 1 is a SEM photograph of the surface of the iridium-manganese oxide composite electrode material of Example 1. [Figure 2] 1 is a cross-sectional SEM photograph of the iridium-manganese oxide composite electrode material of Example 1. [Figure 3] 1 is a photograph showing the distribution of the elements O, Ir, and Mn, corresponding to an SEM photograph of an iridium-manganese oxide composite material layer in the iridium-manganese oxide composite electrode material of Example 1. [Figure 4]XRD patterns of the iridium-manganese oxide composite electrode material of Example 1 and the platinum-coated titanium mesh conductive substrate. [Figure 5] Linear sweep voltammograms showing the relationship between current and potential (voltage) measured using a <PEM type electrolytic cell> during oxygen evolution (during water electrolysis) in Examples 1 to 3, Comparative Example 1, and Comparative Example 3. [Figure 6] Data showing the time course of the electrolytic voltage measured at 80°C and 1 A / cm² per geometric area of the conductive substrate using a <PEM type electrolytic cell> during oxygen evolution (during water electrolysis) in Examples 1 to 3. [Figure 7] Linear sweep voltammograms showing the relationship between current and potential (voltage) measured using a <PEM type electrolytic cell> during oxygen evolution (during water electrolysis) in Examples 4 to 6. [Figure 8] Data showing the time course of the electrolytic voltage measured at 80°C and 1.5 A / cm² using a <PEM type electrolytic cell> during oxygen evolution (during water electrolysis) in Examples 4 to 6.

Modes for Carrying out the Invention

[0011] Hereinafter, the present invention will be described in more detail. First, regarding the decomposition of water by electrolysis, taking a reaction where the reaction field is in an acidic environment like PEM type water electrolysis as an example, it will be explained. On the cathode catalyst, as shown in Equation 1, hydrogen is generated by the reaction of two protons and two electrons. 2H + + 2e - → H2… Equation 1

[0012] On the other hand, on the anode catalyst, as shown in Equation 2, oxygen is generated together with four electrons and four protons from two water molecules. 2H2O → O2+ 4H + + 4e - … Equation 2 And overall, as shown in Equation 3, it is a reaction in which two hydrogen molecules and one oxygen molecule are generated from two water molecules. 2H2O → 2H2+ O2… Equation 3

[0013] The oxygen evolution reaction in the above formula 2 is generally considered to be the rate-limiting step of the overall reaction, and the development of a catalyst that can drive this reaction with minimal energy is of great importance in this technical field. The present invention provides an oxygen evolution electrode catalyst that has high water oxidation catalytic activity. In the iridium-manganese oxide composite material of the present invention, iridium is dispersed at least on the surface of the manganese oxide, and the metal valence of the iridium is 3.1 or more and 3.8 or less. The iridium is dispersed at least on the surface of the manganese oxide. For example, as shown in FIG. 3, the SEM image of the iridium-manganese oxide composite layer clearly shows bright contrast areas indicating the locations of the elements Mn and O. Furthermore, the XRD pattern of the iridium-manganese oxide composite electrode material in FIG. 4 clearly shows that the iridium-manganese oxide composite layer is composed of manganese oxide with a γ structure. Therefore, first, the main component is manganese oxide. Second, as shown in FIG. 3, the SEM image of the iridium-manganese oxide composite layer clearly shows bright contrast areas indicating the locations of Ir. Furthermore, since Ir is introduced after the formation of the manganese oxide by the manufacturing method described below, it is reasonable to believe that iridium is dispersed at least on the surface of the manganese oxide. If the metal valence of iridium is below 3.1, it has low chemical stability as a catalytic material, and is easily consumed as iridium ions, especially when used in an acidic environment such as PEM. Conversely, if the metal valence of iridium is above 3.8, there is little trivalent iridium to effectively act as an active center, and oxygen electrocatalytic activity decreases. It exhibits excellent oxygen electrocatalytic activity. Therefore, the metal valence of iridium is preferably 3.15 or more and 3.6 or less, and more preferably 3.2 or more and 3.5 or less.

[0014] When at least a portion of a conductive substrate is coated with the iridium-manganese oxide composite material of the present invention, the iridium content per geometric area of ​​the conductive substrate is 0.01 mg / cm 2 More than 0.2mg / cm 2 It is preferable that the iridium content is 0.01 mg / cm or less. 2 When the iridium content is below 0.2 mg / cm, the catalyst becomes similar in characteristics to that of a manganese oxide catalyst alone. 2 If the concentration exceeds 0.015 mg / cm, catalytic activity equivalent to that of iridium oxide alone is achieved, but the use of a large amount of iridium, a rare element, makes it expensive and reduces cost performance. In order to obtain excellent properties at a reasonable price, the iridium content should be 0.015 mg / cm 2 More than 0.15mg / cm 2 Less than 0.02 mg / cm is more preferable. 2 More than 0.1mg / cm 2 The following is more preferable: Here, the geometric area corresponds to the projected area of ​​the conductive substrate, and the thickness of the substrate is not taken into consideration.

[0015] In the iridium-manganese oxide composite material of the present invention, the metal content ratio of iridium (iridium / (iridium + manganese)) is preferably 0.2 atomic % or more and 10 atomic % or less. The metal content ratio of iridium is important for dispersing and distributing iridium at least on the surface of the manganese oxide. Furthermore, although the interrelationship is unclear, it is presumed to also influence the control of the metal valence range of iridium in the present invention. If the metal content ratio of iridium is below 0.2 atomic %, the catalyst will have properties comparable to those of a manganese oxide catalyst alone. On the other hand, if the metal content ratio of iridium is above 10 atomic %, catalytic activity equivalent to that of iridium oxide alone will be exhibited, but the use of a large amount of iridium, a rare element, will result in an extremely high price and poor cost performance. To obtain excellent properties at a reasonable cost, the metal content ratio of iridium is preferably 0.3 atomic % or more and 5 atomic % or less, and more preferably 0.4 atomic % or more and 2 atomic % or less.

[0016] The iridium-manganese oxide composite material of the present invention preferably has a peak position in the XANES region of the Ir L3 absorption edge spectrum obtained from XAFS measurement of 11,200 eV or more and 11,230 eV or less. Generally, the peak position in the XANES region tends to be affected by the metal valence, with the lower the metal valence, the lower the energy, and the higher the metal valence, the higher the energy. By having a peak position in the XANES region of the Ir L3 absorption edge spectrum of 11,200 eV or more, the metal valence of iridium is maintained at a higher level, thereby maintaining high chemical stability as a catalytic material and effectively preventing elution of iridium ions, particularly when used in an acidic environment such as PEM. On the other hand, by having a peak position in the XANES region of the Ir L3 absorption edge spectrum of 11,230 eV or less, the amount of iridium that effectively functions as an active center is maintained at a higher level, thereby maintaining high oxygen electrocatalytic activity. In order to achieve excellent oxygen electrocatalytic activity, it is more preferable that the peak position appearing in the XANES region of the Ir L3 absorption edge spectrum is 11210 eV or more and 11220 eV or less.

[0017] In the iridium-manganese oxide composite material of the present invention, the position of the peak corresponding to the iridium-oxygen bond in the radial structure function obtained from XAFS measurement is preferably 1.0 Å or more and 2.0 Å or less. When the value specified above is 1.0 Å or more, aggregation of iridium particles can be more effectively prevented, and the dispersion of iridium on the manganese oxide can be more highly maintained. On the other hand, when the value specified above is 2.0 Å or less, the interaction between iridium and oxygen can be more strongly strengthened, and collapse can be more effectively prevented. In order to stably disperse and allocate iridium in this iridium-manganese oxide composite material, the position of the peak corresponding to the iridium-oxygen bond in the radial structure function obtained from XAFS measurement is more preferably 1.3 Å or more and 1.6 Å or less.

[0018] The iridium-manganese oxide composite material of the present invention has a BET specific surface area of ​​15 m 2 / g or more 100m 2The BET specific surface area mainly reflects the state of manganese oxide, and a BET specific surface area of ​​15m 2 / g, the active sites for adsorbing iridium are limited, and the dispersion state of iridium that effectively acts as a catalyst decreases. 2 When the BET specific surface area of ​​the iridium-manganese oxide composite exceeds 20 m / g, the number of active sites on which iridium can be adsorbed increases, but the manganese oxide coating becomes porous, its strength decreases, and it becomes prone to collapse. 2 / g or more 70m 2 / g or less is more preferable, and 30m 2 / g or more 60m 2 / g or less is more preferable.

[0019] The manganese oxide in the iridium-manganese oxide composite material of the present invention preferably has a manganese metal valence of 3.5 or more and 4.0 or less. Manganese oxides with a metal valence below 3.5 have low chemical stability as a catalytic material, and when used in an acidic environment such as PEM, they tend to be consumed as they continue to dissolve unilaterally as divalent manganese ions. On the other hand, manganese oxides with a metal valence above 4.0 also have low chemical stability due to the inclusion of soluble pentavalent and heptavalent manganese. Therefore, in order to stably disperse and allocate iridium, the manganese oxide preferably has a manganese metal valence of 3.6 or more and 4.0 or less, and even more preferably 3.7 or more and 4.0 or less.

[0020] The iridium-manganese oxide composite material of the present invention preferably has an energy of 6520 eV or more and 6600 eV or less, corresponding to 0.5 when the edge jump in the XANES region of the Mn K absorption edge spectrum obtained from XAFS measurement is normalized to 1. As mentioned above, the energy position of the spectrum appearing in the XANES region tends to be affected by the metal valence. By setting the value specified above to 6520 eV or more, the chemical stability as a catalytic material can be maintained at a higher level, and elution of divalent manganese ions can be more effectively prevented, particularly when used in an acidic environment such as PEM. On the other hand, by setting the value specified above to 6600 eV or less, elution of pentavalent and heptavalent manganese ions can be prevented, thereby maintaining higher chemical stability. In order to stably disperse and allocate iridium, the manganese oxide more preferably has an energy of 6530 eV or more and 6560 eV, corresponding to 0.5 when the edge jump in the XANES region of the Mn K absorption edge spectrum is normalized to 1.

[0021] When at least a portion of a conductive substrate is coated with the iridium-manganese oxide composite material of the present invention, the manganese content per geometric area of ​​the conductive substrate is 0.12 mg / cm 2 More than 14.35mg / cm 2 It is preferable that the manganese content is 0.12 mg / cm or less. 2 By keeping the manganese content at 14.35 mg / cm or more, the amount of iridium that can be adsorbed on the manganese oxide can be maintained, and therefore the catalytic activity can be maintained at a higher level. 2 By keeping the manganese content below 0.30 mg / cm, the resistance of the manganese oxide can be kept lower and the catalytic activity can be kept higher. 2 More than 2.40mg / cm 2 Less than 0.60 mg / cm is more preferable. 2 More than 1.20mg / cm 2 The following is more preferable: Here, the geometric area corresponds to the projected area of ​​the conductive substrate, and the thickness of the substrate is not taken into consideration.

[0022] In the iridium-manganese oxide composite material of the present invention, the position of the peak corresponding to the manganese-oxygen bond in the radial structure function obtained from XAFS measurement is preferably 1.0 Å or more and 2.0 Å or less. When the value specified above is 1.0 Å or more, aggregation of manganese molecules can be more effectively prevented, and the dispersibility of iridium can be maintained at a higher level. On the other hand, when the value specified above is 2.0 Å or less, the interaction between manganese and oxygen, and manganese and manganese can be more strongly strengthened, and collapse can be more effectively prevented. Furthermore, it is more preferable that the position of the peak corresponding to the manganese-oxygen bond in the radial structure function obtained from XAFS measurement is 1.3 Å or more and 1.7 Å or less.

[0023] The manganese oxide in the iridium-manganese oxide composite material of the present invention may be, for example, electrolytic manganese dioxide obtained by an electrolytic method or manganese dioxide obtained by a chemical method, with electrolytic manganese dioxide being preferred. Furthermore, the manganese oxide in the iridium-manganese oxide composite material of the present invention may be a crystalline phase having a basic γ-type, β-type, ε-type, or α-type crystal structure, or may be a mixed crystal manganese dioxide in which these crystal structures are mixed.

[0024] By supporting the iridium-manganese oxide composite material of the present invention on an electrode, the iridium-manganese oxide composite material of the present invention serves as an oxygen generating electrode active material for water electrolysis, thereby imparting catalytic activity to the oxygen generating electrode in a water splitting reaction. A membrane-electrode assembly is formed by stacking an oxygen generating electrode containing this oxygen generating electrode active material, a polymer electrolyte membrane, and an electrode to which a hydrogen generating catalyst has been applied. Examples of the polymer electrolyte membrane include a fluororesin-based cation exchange membrane, and examples of the hydrogen generating catalyst include platinum fine particles. In the present invention, the inclusion of this oxygen generating electrode constitutes a water electrolysis device, and hydrogen can be produced by water electrolysis using this oxygen generating electrode.

[0025] A method for producing the iridium-manganese oxide composite material of the present invention will be described below. The iridium-manganese oxide composite material of the present invention can be obtained, for example, by electrolytically depositing manganese oxide onto an electrode substrate such as a pure titanium plate using a mixed solution containing sulfuric acid and manganese sulfate as an electrolyte, immersing the electrode substrate in or contacting it with an iridium salt solution, and then annealing the resulting material.

[0026] Manganese oxide may be electrolytically deposited using a mixed solution containing sulfuric acid and manganese sulfate, and then peeled off from the electrode substrate and pulverized to form a powder. Regarding the concentration of each component in the mixed solution containing sulfuric acid and manganese sulfate, the sulfuric acid concentration is preferably controlled to 5 g / L or more and 65 g / L or less, and more preferably 20 g / L or more and 50 g / L or less.

[0027] The concentration of manganese (manganese ions of manganese sulfate) in the mixed solution is not particularly limited as long as it is below the solubility, but is preferably 5 g / L or more and 50 g / L or less, and more preferably 10 g / L or more and 30 g / L or less. In order to maintain the component concentrations of the mixed solution, it is effective to appropriately add manganese sulfate equivalent to the manganese consumed in the electrolytic oxidation, or to continuously supply a manganese sulfate solution.

[0028] The sulfuric acid concentration in the above-mentioned sulfuric acid-manganese sulfate mixed solution is a value excluding the divalent anion (sulfate ion) of manganese sulfate. In the method for electrolytic deposition of manganese oxide for the iridium-manganese oxide composite material of the present invention, the electrolytic current density is not particularly limited, but is preferably 0.3 mA / cm per geometric area of ​​the conductive substrate. 2 More than 20mA / cm 2 It is preferable that the electrolytic current density is 1 mA / cm or less. This allows manganese oxide to be deposited efficiently and stably. In order to obtain the iridium-manganese oxide composite material of the present invention more stably, the electrolytic current density should be 1 mA / cm or less. 2 More than 10mA / cm 2 Less than 3mA / cm is more preferable. 2 More than 8mA / cm 2The following is more preferable: Here, the geometric area corresponds to the projected area of ​​the conductive substrate, and the thickness of the substrate is not taken into consideration.

[0029] In the method for electrolytic deposition of manganese oxide for the iridium-manganese oxide composite material of the present invention, the electrolysis temperature can be, for example, 93° C. or higher and 98° C. or lower. The higher the electrolysis temperature, the higher the efficiency of electrolytic production of deposited manganese oxide, so the electrolysis temperature is preferably above 94° C. Manganese oxide electrolytically deposited on an electrode substrate such as a pure titanium plate is peeled from the electrode substrate, then coarsely crushed using a jaw crusher or the like, and then pulverized as a simple manganese oxide to a predetermined average secondary particle size using a roller mill, vertical mill, Roesche mill, jet mill, or the like. The produced manganese oxide then undergoes a washing process and a neutralization process to remove residual electrolyte, etc., and is then dried using a flash dryer or the like. During this flash drying, submicron manganese oxide powder produced as a by-product due to excessive crushing in the crushing process can be collected and separated using a dust collector bag filter or the like. In some cases, manganese oxide is obtained by a further calcination process at 200°C to 500°C.

[0030] The oxide of manganese is then immersed in a container containing an iridium salt solution, or the oxide of manganese is brought into contact with the iridium salt solution. Examples of the iridium salt in the iridium salt solution include potassium hexachloroiridate (K2IrCl6) and hexachloroiridate (H2IrCl6). The iridium concentration in the iridium salt solution is not particularly limited as long as it is below the solubility, but is preferably 0.1 g / L or more and 10 g / L or less, and more preferably 0.3 g / L or more and 5 g / L or less.

[0031] The conditions for immersing manganese oxide in an iridium salt solution or contacting manganese oxide with an iridium salt solution are not particularly limited, but immersion or contact is preferably carried out for 30 minutes to 24 hours at a temperature of 20° C. to 100° C. By keeping the immersion or contact time and immersion or contact temperature within the above ranges, the amount of iridium adsorbed onto the manganese oxide can be controlled.

[0032] Next, an annealing treatment is performed. The annealing conditions are not particularly limited, but examples include a temperature of more than 100°C and not more than 600°C in air or a nitrogen stream, and an annealing time of 10 minutes to 24 hours. The annealing temperature is preferably 300°C to 550°C, more preferably 350°C to 500°C. The annealing time is preferably 1 hour to 16 hours, more preferably 2 hours to 8 hours. Although the effect of this annealing treatment is not clear, it is presumed that by selecting the annealing conditions, the interaction between iridium and manganese oxide can be enhanced and the metal valence of iridium can be controlled within a more desirable range.

[0033] Next, the iridium-manganese oxide composite electrode material of the present invention will be described. The iridium-manganese oxide composite electrode material of the present invention is one in which the iridium-manganese oxide composite material of the present invention is coated on at least a portion of a conductive substrate made of conductive fibers. In this case, the amount of the iridium-manganese oxide composite material of the present invention coated is 0.1 mg / cm per geometric area of ​​the conductive substrate. 2 More than 20mg / cm 2 The following is preferred: Here, the geometric area corresponds to the projected area of ​​the conductive substrate, and the thickness of the substrate is not taken into consideration.

[0034] When the coating amount of the iridium-manganese oxide composite material of the present invention is within the above range, the iridium-manganese oxide composite material will coat the fibers in an island-like manner or in a form that completely covers the outer surface of the fibers, although this will depend on the diameter and porosity of the fibers that make up the conductive substrate, and the average coating thickness can be approximately 25 μm or less. Note that the iridium-manganese oxide composite material that coats the fibers is composed of secondary particles, so the average coating thickness usually coincides with the average secondary particle size of the iridium-manganese oxide composite material that constitutes it.

[0035] In the iridium-manganese oxide composite electrode material of the present invention, the average thickness of the iridium-manganese oxide composite material coating the fibers of the conductive substrate increases depending on the amount of the coating iridium-manganese oxide composite material. The coating amount of the manganese oxide composite material was 0.2 mg / cm 2 More than 10mg / cm 2 Less than 0.3 mg / cm is more preferable. 2 More than 7mg / cm 2 More preferably, 0.5 mg / cm 2 More than 5mg / cm 2 The following is particularly preferred: The thickness of the coating layer of the iridium-manganese oxide composite material can also be determined, for example, from a scanning electron microscope (SEM) image by subtracting the wire diameter of the conductive fibers that are the constituent units of the conductive substrate.

[0036] In the iridium-manganese oxide composite electrode material of the present invention, the conductive substrate is preferably composed of carbon, titanium, or platinum-coated titanium. Examples of carbon include carbon paper composed of conductive carbon fibers, examples of titanium include titanium mesh composed of fibrous conductive titanium metal wires and sintered titanium, and examples of platinum-coated titanium include platinum-coated titanium mesh and sintered titanium in which the surface of fibrous conductive titanium metal wires is platinum-coated.

[0037] The iridium-manganese oxide composite electrode material of the present invention can be obtained by using a conductive substrate, such as carbon, titanium, or platinum-coated titanium, instead of the pure titanium plate electrode substrate, electrolyzing a mixed solution containing sulfuric acid and manganese sulfate to electrodeposit manganese oxide onto at least a portion of the conductive substrate composed of conductive fibers, followed by immersion in or contact with an iridium salt solution to uniformly disperse and adsorb iridium onto at least the surface of the manganese oxide, and then annealing the electrolytic solution. In this case, it is preferable that the coating amount of iridium-manganese oxide composite material per geometric area of ​​the conductive substrate be within the above-mentioned preferred range. In producing the iridium-manganese oxide composite electrode material of the present invention, the manganese oxide obtained by electrolysis of a mixed solution containing sulfuric acid and manganese sulfate is immersed in or contacted with an iridium salt solution, followed by annealing. The iridium-manganese oxide composite material is deposited on the conductive substrate, yielding the iridium-manganese oxide composite material of the present invention.

[0038] Regarding the concentration of each component in the mixed solution containing sulfuric acid and manganese sulfate used in the production of the iridium-manganese oxide composite electrode material of the present invention, the sulfuric acid concentration is preferably 5 g / L or more and 65 g / L or less, and more preferably 20 g / L or more and 50 g / L or less. The concentration of manganese (manganese ions of manganese sulfate) in the mixed solution is not particularly limited as long as it is below the solubility, but is preferably 5 g / L or more and 50 g / L or less, and more preferably 10 g / L or more and 30 g / L or less. In order to maintain the component concentrations of the mixed solution, it is effective to appropriately add manganese sulfate equivalent to the manganese consumed in the electrolytic oxidation, or to continuously supply a manganese sulfate solution.

[0039] The sulfuric acid concentration in the above-mentioned sulfuric acid-manganese sulfate mixed solution is a value excluding the divalent anion (sulfate ion) of manganese sulfate. In the method for electrolytic deposition of manganese oxide for the iridium-manganese oxide composite electrode material of the present invention, the electrolytic current density is not particularly limited, but is preferably 0.3 mA / cm per geometric area of ​​the conductive substrate. 2 More than 20mA / cm 2 It is preferable that the electrolytic current density is 1 mA / cm or less. This allows manganese oxide to be efficiently and stably deposited electrolytically. In order to obtain the iridium-manganese oxide composite electrode material of the present invention more stably, the electrolytic current density is 1 mA / cm or less. 2 More than 10mA / cm 2 Less than 3mA / cm is more preferable. 2 More than 8mA / dm 2 The following is even more preferred:

[0040] The electrolysis temperature in the method for electrolytic deposition of manganese oxide for the iridium-manganese oxide composite electrode material of the present invention can be, for example, 93° C. or higher and 98° C. or lower. The higher the electrolysis temperature, the higher the efficiency of electrolytic production of deposited manganese oxide, so the electrolysis temperature is preferably above 94° C. The type of iridium salt in the iridium salt solution is potassium hexachloroiridate. (K2IrCl6) or hexachloroiridic acid (H2IrCl6). The iridium concentration of the iridium salt solution is not particularly limited as long as it is below the solubility, but is preferably 0.1 g / L or more and 10 g / L or less, and more preferably 0.3 g / L or more and 5 g / L or less.

[0041] The conductive substrate is preferably a plate-shaped substrate having a thickness of 1 mm or less, obtained by molding or sintering conductive fibers such as carbon or titanium metal having a wire diameter of 100 μm or less. The porosity of the conductive substrate is preferably 40% or more, and more preferably 50% to 90%. Here, the porosity is defined as the volume of the space in the conductive substrate that is free of conductive fibers, etc.

[0042] It is also effective to treat the conductive substrate with an acid such as hydrochloric acid, sulfuric acid, nitric acid, or oxalic acid before electrolytic deposition of manganese oxide to remove the passive film on the substrate surface and to make the substrate hydrophilic. On the other hand, it is also effective to immerse the conductive substrate in a dispersion liquid of a fluororesin or the like to make it water-repellent, in order to control the electrodeposition position of manganese oxide within the conductive substrate or to impart gas diffusion properties, which are important when the substrate is actually used as an electrode for water electrolysis.

[0043] Conditions for electrolytically depositing manganese oxide from the iridium-manganese oxide composite material of the present invention onto a conductive substrate include, for example, selecting the ranges of the sulfuric acid concentration, manganese concentration, electrolysis current density, and electrolysis temperature of the sulfuric acid-manganese sulfate mixed solution, as described above, and carrying out electrolysis for 5 to 120 minutes. After electrolytically depositing manganese oxide onto the conductive substrate, the substrate is washed with water and dried. The conductive substrate with the electrolytically deposited manganese oxide is then immersed in a container containing an iridium salt solution, or the conductive substrate with the electrolytically deposited manganese oxide is brought into contact with an iridium salt solution, thereby adsorbing iridium onto at least the surface of the manganese oxide. Finally, the substrate is annealed in air or a nitrogen atmosphere at a temperature above 100°C and not exceeding 600°C for more than 10 minutes but not exceeding 24 hours, thereby producing the iridium-manganese oxide composite electrode material of the present invention.

[0044] The conditions for immersing manganese oxide electrodeposited on at least a portion of a conductive substrate made of conductive fibers in an iridium salt solution or for contacting manganese oxide with an iridium salt solution are not particularly limited, but immersion or contact is preferably performed for 30 minutes to 24 hours at a temperature of 20° C. to 100° C. By keeping the immersion or contact time and immersion or contact temperature within the above ranges, the amount of iridium adsorbed onto the manganese oxide can be controlled. The annealing temperature is preferably 300° C. to 550° C., more preferably 350° C. to 500° C. The annealing time is preferably 1 hour to 16 hours, more preferably 2 hours to 8 hours.

[0045] In the case of the iridium-manganese oxide composite electrode material of the present invention, if one side of the conductive substrate is shielded with a resin film or the like during the electrolytic deposition of manganese oxide, it is possible to preferentially coat only one side with the iridium-manganese oxide composite material, while leaving the other side almost uncoated with the iridium-manganese oxide composite material, thereby enabling intentional uneven coating of the iridium-manganese oxide composite material. Furthermore, it is believed that the iridium-manganese oxide composite electrode material of the present invention has favorable effects such as enhanced interaction between iridium and manganese oxide during annealing, making it possible to control the metal valence of iridium within a desirable range, and further improving adhesion between the iridium-manganese oxide composite material and conductive fibers, or further increasing the crystallinity of the iridium-manganese oxide composite material.

[0046] The iridium-manganese oxide composite electrode material, polymer electrolyte membrane, and hydrogen evolution catalyst of the present invention In the present invention, the iridium-manganese oxide composite electrode material of the present invention is used to form a water electrolysis device, and hydrogen can be produced by electrolyzing water using this iridium-manganese oxide composite electrode material. [Example]

[0047] The present invention will be described in detail below with reference to examples and comparative examples, but the present invention is not limited to these examples.

[0048] <Metal concentration analysis of sulfuric acid-manganese sulfate mixed solution or iridium salt solution> The sulfuric acid-manganese sulfate mixed solution was diluted, and the manganese element was quantitatively measured using ICP-AES (PerkinElmer Optima 8300). In addition, the iridium salt solution was diluted, and the iridium element concentration was quantitatively measured using a UV-Vis spectrometer (Shimadzu UV-2550).

[0049] <SEM surface observation and composition analysis of iridium-manganese oxide composite material and iridium-manganese oxide composite electrode material> Surface morphology, iridium dispersion, and cross-sectional elemental analysis were performed using a SEM-EDX instrument (JSF-7800F, manufactured by Hoskin Scientific). When performing cross-sectional elemental analysis, conductive carbon tape was used to prevent tailing.

[0050] <Calculation of metal valence of iridium-manganese oxide composite material and iridium-manganese oxide composite electrode material> The metal valences of iridium and manganese were determined using an XPS analyzer (ULVAC PHI 5000 Versa Prove II). The source was AlKα (1486.6 eV), and the C1s spectrum at 284.6 eV was used as the reference binding energy. A survey scan was performed with a pass energy of 187.85 eV. A low pass energy of 11.75 eV was used for high-resolution analysis of Ir4f, while a pass energy of 23.5 eV was used for analysis of other elements. The spectra were analyzed using CasaXPS software. The fitting model proposed by Pfeifer was used for fitting the Ir4f spectrum, and the model proposed by Eugene S. Ilton was used for fitting the Mn2p spectrum.

[0051] <XAFS analysis of iridium-manganese oxide composite material> XAFS measurements were performed using the beamline BL14B2 at the SPring-8 large-scale synchrotron radiation facility. The Mn K absorption edge spectrum was measured by the transmission method using the Si(111) surface of a double-crystal monochromator. The Ir L3 absorption edge spectrum was measured by the fluorescence method using the Si(311) surface of a double-crystal monochromator.

[0052] <Calculation of the BET specific surface area of ​​iridium-manganese oxide composite material and iridium-manganese oxide composite electrode material> The BET specific surface area was measured by nitrogen adsorption using the BET single-point method. A gas adsorption specific surface area analyzer (FlowSorb III, manufactured by Shimadzu Corporation) was used. Prior to measurement, the measurement sample was degassed by heating at 150°C for 40 minutes. When measuring the iridium-manganese oxide composite electrode material, the BET specific surface area of ​​the conductive substrate, such as a carbon mesh or titanium mesh, or a platinum-coated titanium mesh, was measured first, and the BET specific surface area of ​​the iridium-manganese oxide composite material alone was determined by subtracting the BET specific surface area of ​​the conductive substrate.

[0053] <XRD measurement of iridium-manganese oxide composite electrode material> An X-ray diffraction device (Rigaku Ultima+) was used, and CuKα radiation was used as the radiation source. XRD measurements were carried out using a wavelength of 1.5418Å (λ = 1.5418Å) at an operating potential of 40 kV and an operating current of 40 mA. In the XRD measurements of the iridium-manganese oxide composite electrode material, diffraction lines of Pt and Ti derived from the conductive substrate were also detected.

[0054] <Measurement of the amount of manganese oxide electrodeposited and the amount of coating of iridium-manganese oxide composite material> The amount of manganese oxide electrodeposited and the coating amount of iridium-manganese oxide composite material were measured according to the following methods. Before electrolytic deposition, weight 1 of the substrate (electrode substrate such as titanium or conductive substrate) was measured on a balance, and after electrolytic deposition, weight 2 of the substrate on which manganese oxide was electrodeposited was measured on a balance. The amount of manganese oxide electrodeposited was calculated from the difference between weight 1 and weight 2 (weight 2 - weight 1). The coating amount of the iridium-manganese oxide composite material in the iridium-manganese oxide composite electrode material was determined by measuring the weight 3 of the substrate coated with the iridium-manganese oxide composite material after electrolytic deposition of manganese oxide, contact with the iridium salt solution, and annealing treatment on a balance, and calculating the difference between weight 1 of the substrate and weight 3 (weight 3 - weight 1).

[0055] <Construction of a PEM-type electrolytic cell for evaluating the properties of oxygen-evolving electrode catalysts> A PEM-type electrolytic cell was constructed using an electrode material made of a conductive substrate coated with an iridium-manganese oxide composite material as follows. The electrode material (flat mesh shape: 1 cm x 1 cm) was used as the working electrode, and a 20 wt% platinum-supported carbon catalyst (20% Platinum on Vulcan XC-72, Item #PTC20-1, Fuel Cell Earth) was used as the catalyst for the counter electrode. A conductive catalyst ink was prepared, applied to carbon paper, and air-dried to form the counter electrode. A Nafion membrane (Nafion 115, Sigma-Aldrich) was used as the electrolyte membrane. The electrolyte membrane was cleaned and protonated (pretreated) by boiling it in 3% hydrogen peroxide, pure water, and 1M sulfuric acid solution, followed by pure water, for 1 hour each. Next, the electrolyte membrane was sandwiched between the catalyst-coated surfaces of the working and counter electrodes and heated at 135°C with a clamping force of 400 kg / cm using a hot press (A-010D, FC-R&D). 2 A membrane / electrolyte assembly (MEA) was fabricated by hot pressing for 3 minutes at 400 K. This MEA was fitted with a stainless steel mesh (#100) on the cathode side and a titanium mesh (#100) on the anode side to improve adhesion even during electrolysis operation, and was then attached to the housing of a PEM electrolyzer (WE-4S-RICW, manufactured by FC Development Co., Ltd.).

[0056] <Electrochemical Measurement 1: Measurement of current-voltage curve> To evaluate the water oxidation catalytic activity in an actual device, a PEM electrolytic cell constructed using an electrode material made of a conductive substrate coated with an iridium-manganese oxide composite was used to measure the current-voltage curve at an operating temperature of 80°C. In this measurement, a two-electrode system consisting of only a working electrode and a counter electrode was used, and the current-voltage curve was measured by gradually increasing the applied voltage. Pure water was supplied to the PEM electrolytic cell. The voltage increase rate was set to 5mV / s, so that the voltage at which the current rose could be easily identified.

[0057] <Electrochemical Measurement 2: Measurement of electrolysis voltage stability> To evaluate the stability of the water oxidation catalytic activity in an actual device, electrolysis voltage was measured at an operating temperature of 80°C using a PEM electrolytic cell constructed using an electrode material of a conductive substrate coated with an iridium-manganese oxide composite. In this measurement, a two-electrode system consisting of only a working electrode and a counter electrode was used, and the current density applied between the two electrodes was 1 A / cm per geometric area of ​​the conductive substrate. 2 The time change in electrolysis voltage was measured while maintaining the temperature at 100°C. Pure water was supplied to the PEM electrolytic cell.

[0058] Example 1 Electrolysis was carried out in an electrolytic cell containing a sulfuric acid-manganese sulfate mixed solution with a concentration of 35 g / L of sulfuric acid and 31 g / L of manganese sulfate, and manganese oxide was electrolytically deposited on a conductive substrate of platinum-coated Ti mesh (ADL-414302-5056, manufactured by FC Development Co., Ltd.). The conductive substrate on which the manganese oxide had been electrodeposited was immersed in an iridium salt solution bath containing 2.5 g / L of potassium iridate (K2IrCl6) and 0.5 g / L of sulfuric acid at 95 °C for 12 hours to allow iridium to adsorb onto the manganese oxide surface. The liquid in the iridium salt solution bath before and after iridium adsorption was measured using a UV-Vis spectrometer (Shimadzu UV-2550) to confirm that all of the iridium had been adsorbed onto the manganese oxide and none remained in the iridium salt solution bath. The substrate was then annealed in air at 400 °C for 5 hours to prepare an iridium-manganese oxide composite electrode material, in which an iridium-manganese oxide composite material was deposited on the conductive substrate. The synthesis conditions are shown in Table 1.

[0059] [Table 1] The SEM photograph of the surface of this electrode material is shown in Figure 1, and the SEM photograph of the cross section is shown in Figure 2. It was confirmed that the iridium-manganese oxide composite electrode material consisted of a catalyst layer of iridium-manganese oxide composite material deposited on platinum-coated Ti mesh fibers. Next, SEM-EDX data of a cross section of the iridium-manganese oxide composite catalyst layer is shown in Figure 3. Figure 3 confirms that iridium is uniformly dispersed at least on the surface of the manganese oxide.

[0060] The XRD pattern of the iridium-manganese oxide composite electrode material obtained in Example 1 is shown in Figure 4. From Figure 4, in addition to the diffraction lines of Pt and Ti derived from the conductive substrate, diffraction lines attributed to γ-type MnO2 were observed, but iridium was in such a small amount that it was not detected as a diffraction line. The amount of manganese oxide electrodeposited in this iridium-manganese oxide composite electrode material was 3.80 mg / cm. 2 , the weight of the iridium-manganese oxide composite is 3.88 mg / cm 2 Therefore, the iridium content of the iridium-manganese oxide composite material was 0.08 mg / cm 2 The metal content ratio of iridium (iridium / (manganese + iridium)) was 0.94 atomic %. Furthermore, this iridium-manganese oxide composite electrode material was measured and analyzed by XPS, and the average metal valence of iridium was calculated to be 3.3 and the average metal valence of manganese was calculated to be 3.7. The BET specific surface area of ​​this iridium-manganese oxide composite material was 42 m 2 The results of these evaluations are shown in Table 2.

[0061] [Table 2] This iridium-manganese oxide composite electrode material was cut into a 1 cm x 1 cm piece, and a PEM electrolytic cell was constructed according to the method in <Construction of a PEM electrolytic cell for evaluation of oxygen evolution electrode catalyst properties>. The oxygen evolution electrode catalyst properties were evaluated according to <Electrochemical measurement 1: Measurement of current-voltage curve>. The results are shown in Table 2 and Figure 5. In addition, the time change in electrolysis voltage was measured according to <Electrochemical measurement 2: Measurement of current-voltage curve>. The results are shown in Figure 6.

[0062] XAFS measurements were performed on this iridium-manganese oxide composite material, and the energy peak position (peak position 1) in the XANES region of the Ir L3 absorption edge spectrum was 11,215 eV, and the energy peak position (peak position 2) corresponding to 0.5 when the edge jump in the XANES region of the Mn K absorption edge spectrum is normalized to 1 was 6,550 eV. XAFS analysis of the iridium-manganese oxide composite material revealed that the peak position (peak position 3) corresponding to the manganese-oxygen bond in the radial structure function was 1.5 Å. Similarly, EXAFS analysis of the iridium-manganese oxide composite material revealed that the peak position (peak position 4) corresponding to the iridium-oxygen bond in the radial structure function was calculated to be 1.6 Å. These evaluation results are shown in Table 3. [Table 3]

[0063] Examples 2 and 3 An iridium-manganese oxide composite electrode material was produced according to the synthesis conditions of Example 1, except that the electrolyte composition, electrodeposition time, and iridium salt solution composition for manganese oxide electrodeposition were changed. These synthesis conditions are shown in Table 1, and the physical properties and characteristic values ​​of the iridium-manganese oxide composite electrode material are shown in Table 2. The results of the oxygen evolution electrode catalyst performance evaluation are shown in Figure 5, and the results of measuring the time course of the electrolysis voltage are shown in Figure 6.

[0064] Examples 4 to 6 An iridium-manganese oxide composite electrode material was produced according to the synthesis conditions of Example 1, except that the electrodeposition time and annealing temperature for manganese oxide electrodeposition were changed. These synthesis conditions are shown in Table 4, and the physical properties and characteristic values ​​of the iridium-manganese oxide composite electrode material are shown in Table 5. The results of the oxygen evolution electrode catalyst characteristic evaluation are shown in Figure 7, and the results of measuring the time change in electrolysis voltage are shown in Figure 8.

[0065] [Table 4]

[0066] [Table 5]

[0067] Comparative Example 1 A commercially available iridium oxide catalyst (manufactured by Elyst) was used, with an iridium content of 0.08 mg / cm 2 The anode and membrane-electrode assembly were fabricated to construct a PEM-type electrolytic cell, and electrochemical measurements were carried out. The oxygen evolution electrode catalyst was evaluated according to the current-voltage curve measurement. The characteristic evaluation results are shown in Table 2 and Figure 5.

[0068] Comparative Example 2 Iridium content is 1 mg / cm 2 An anode and a membrane-electrode assembly were prepared in the same manner as in Comparative Example 1, except that the above-mentioned conditions were met, and the oxygen evolution electrode catalyst was evaluated. The results of the characteristic evaluation are shown in Table 2.

[0069] Comparative Example 3 Electrolysis was performed in an electrolytic cell containing a sulfuric acid-manganese sulfate mixed solution with 35 g / L sulfuric acid and 31 g / L manganese sulfate. Manganese oxide was electrolytically deposited onto a conductive substrate of platinum-coated titanium mesh (ADL-414302-5056, FC Development Co., Ltd.). This was followed by annealing in air at 450°C for 5 hours to produce a manganese oxide electrode material. The synthesis conditions are shown in Table 1, and the physical properties of this manganese oxide composite material are shown in Table 2. Next, a PEM-type electrolytic cell was constructed using this manganese oxide electrode material, and its oxygen evolution electrode catalyst was evaluated according to the Electrochemical Measurements: Current-Voltage Curve Measurements. The characteristic evaluation results are shown in Table 2 and Figure 5.

[0070] As shown in Figures 5 and 6, the iridium-manganese oxide composite material and iridium-manganese oxide composite electrode material of the present invention have a structure that, in principle, increases energy conversion efficiency, and have been shown to exhibit extremely good oxygen evolution electrode catalytic activity and durability while significantly reducing the amount of iridium typically used by more than 90% compared to commercially available iridium-based catalysts in PEM-type electrolytic cells, for which non-noble metal catalysts are desired.

[0071] As shown in Figures 7 and 8, the iridium-manganese oxide composite material and iridium-manganese oxide composite electrode material of the present invention have a manganese oxide deposition amount of 1 mg / cm 2 It was revealed that even at temperatures below this, the electrode exhibited extremely good oxygen evolution electrode catalytic activity and durability. [Industrial Applicability]

[0072] The iridium-manganese oxide composite material and iridium-manganese oxide composite electrode material of the present invention use a significantly smaller amount of iridium than conventional iridium-based catalysts, yet have high oxygen evolution electrode catalytic activity comparable to conventional noble metal-based catalysts. Therefore, by using them as oxygen evolution anode catalysts in industrial water electrolysis performed under alkaline or neutral conditions, or in water electrolysis using a PEM-type electrolytic cell, it becomes possible to obtain hydrogen and oxygen at extremely low production costs. Furthermore, by making carbon dioxide present in the reaction system of the above-mentioned water electrolysis or the like, the carbon dioxide or the like can be reduced at the cathode to produce hydrocarbon compounds (formic acid, formaldehyde, methanol, methane, ethane, propane, etc.). The entire contents of the specification, claims, drawings, and abstract of Japanese Patent Application No. 2021-99336, filed on June 15, 2021, are hereby incorporated by reference as the disclosure of the specification of the present invention.

Claims

1. An iridium-manganese oxide composite material in which iridium is dispersed and arranged at least on the surface of manganese oxide is coated on at least a part of a conductive substrate made of conductive fibers, and the iridium content of the iridium-manganese oxide composite material is 0.01 mg / cm per geometric area of ​​the conductive substrate. ２ 0.2mg / cm or more ２ An iridium-manganese oxide composite electrode material characterized by the following:

2. The iridium-manganese oxide composite material is present in an amount of 0.1 mg / cm per geometric area of ​​the conductive substrate. ２ 20mg / cm or more ２ The iridium-manganese oxide composite electrode material according to claim 1, characterized in that it is coated with the following:

3. 3. The iridium-manganese oxide composite electrode material according to claim 1, wherein the conductive substrate is made of carbon, titanium, or platinum-coated titanium.

4. 3. The iridium-manganese oxide composite electrode material according to claim 1, wherein the iridium-manganese oxide composite material has a metal content ratio (iridium / (manganese+iridium)) of 0.2 atomic % or more and 10 atomic % or less.

5. The iridium-manganese oxide composite electrode material according to claim 1 or 2, characterized in that the iridium-manganese oxide composite material has a peak position corresponding to a bond between iridium and oxygen in a radial structure function obtained from an XAFS measurement of 1.0 Å or more and 2.0 Å or less.

6. The iridium-manganese oxide composite material has a BET specific surface area of ​​15 m ２ / g or more 100m ２ 3. The iridium-manganese oxide composite electrode material according to claim 1, wherein the iridium-manganese oxide composite electrode material has a Cr content of 0.15 or less.

7. 3. The iridium-manganese oxide composite electrode material according to claim 1, wherein the manganese metal valence of the manganese oxide in the iridium-manganese oxide composite material is 3.5 or more and 4.0 or less.

8. 3. The iridium-manganese oxide composite electrode material according to claim 1, wherein the manganese oxide in the iridium-manganese oxide composite material is electrolytic manganese dioxide.

9. The iridium-manganese oxide composite electrode material according to claim 1 or 2, characterized in that the manganese oxide in the iridium-manganese oxide composite material is manganese dioxide in any one of a γ-type, β-type, ε-type, or α-type crystalline phase, or mixed crystal.

10. A membrane-electrode assembly comprising an electrode carrying the iridium-manganese oxide composite material according to claim 1 or 2, and a polymer electrolyte membrane.

11. 3. A method for producing an iridium-manganese oxide composite electrode material according to claim 1 or 2, comprising electrolyzing a mixed solution containing sulfuric acid and manganese sulfate to electrodeposit manganese oxide onto at least a portion of a conductive substrate made of conductive fibers, followed by immersing the substrate in or contacting the substrate with an iridium salt solution to uniformly disperse and adsorb iridium onto at least the surface of the manganese oxide, and then annealing the resulting material.

12. 12. The method for producing an iridium-manganese oxide composite electrode material according to claim 11, wherein the sulfuric acid concentration of the mixed solution containing sulfuric acid and manganese sulfate is 5 g / L or more and 65 g / L or less.

13. The electrolysis of the mixed solution containing sulfuric acid and manganese sulfate is carried out at a current of 0.3 mA / cm ２ 20mA / cm or more ２ The method for producing an iridium-manganese oxide composite electrode material according to claim 11, characterized in that the method is carried out at a current density of:

14. The iridium salt in the iridium salt solution is K ２ IrCl ６ 12. The method for producing an iridium-manganese oxide composite electrode material according to claim 11, wherein

15. The method for producing an iridium-manganese oxide composite electrode material according to claim 11, wherein the annealing treatment is carried out at a temperature higher than 100° C. and not higher than 600° C. for 10 minutes or more and not longer than 24 hours.

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