Manganese-iridium composite oxide for water decomposition catalyst, manganese-iridium composite oxide electrode material, and method for producing same

Abstract

Provided are a manganese-iridium composite oxide, which is inexpensive and has high catalytic activity, for use as an anode catalyst for oxygen generation in water electrolysis, a manganese-iridium composite oxide electrode material, and a method for producing the same. A manganese-iridium composite oxide characterized in that the metal of iridium has a composition ratio of (iridium / (manganese+iridium) of 0.1 atom% or more and 30 atom% or less, and has at least a crystal face spacing of 0.243±0.002 nm, 0.214±0.002 nm, 0.165±0.002 nm, and 0.140±0.002 nm. Also provided is a manganese-iridium composite oxide electrode material formed of an electrically conductive substrate composed of fibers at least a part of which is coated with the above-described manganese-iridium composite oxide.

Description

Technical Field

[0001] This invention relates to manganese-iridium composite oxides for water splitting catalysts, manganese-iridium composite oxide electrode materials, their manufacturing methods, and their uses. More specifically, this invention relates to manganese-iridium composite oxides, manganese-iridium composite oxide electrode materials, and their manufacturing methods for use as anode catalysts for oxygen production in industrial water electrolysis or water electrolysis using solid polymer membrane (PEM) type electrolyzers under alkaline, neutral, or acidic conditions. Background Technology

[0002] Due to the depletion of fossil fuels and environmental pollution, the utilization of hydrogen as a clean energy source and its production methods have attracted much attention. Water electrolysis is one of the effective methods for producing high-purity hydrogen from the cathode by electrolyzing water, but its characteristic is that oxygen is simultaneously produced from the anode of the counter electrode. To efficiently perform the water splitting reaction in water electrolysis, it is necessary to use an electrode catalyst with low hydrogen overvoltage at the cathode and an electrode catalyst with low oxygen overvoltage at the anode, while maintaining a low electrolysis voltage. Among these, compounds primarily composed of rare platinum group metals such as platinum (Pt), iridium (Ir), and ruthenium (Ru), as well as oxides containing these elements, have been proposed as excellent electrode catalyst materials for low oxygen overvoltage at the anode (Patent Documents 1, 2; Non-Patent Documents 1-3).

[0003] On the other hand, since electrode catalysts made of platinum group metals are very expensive, electrode catalysts using inexpensive transition metals are being developed. For example, transition metal materials made of manganese (Mn), iron (Fe), cobalt (Co), and nickel (Ni) have been proposed in recent years (Patent Documents 3 and 4, Non-Patent Documents 4-7).

[0004] However, the catalytic materials composed of transition metals proposed to date have significantly lower activity (higher oxygen overvoltage) compared to platinum group metal electrode catalysts. In other words, an oxygen-producing electrode catalyst composed of inexpensive transition metals with high catalytic activity comparable to platinum group metals such as Pt and Ir has not yet been achieved.

[0005] To address this issue, manganese oxides with oxygen-generating electrode catalytic activity equivalent to or higher than that of Pt have been discovered, but their activity does not reach that of Ir-based catalysts, which are said to exhibit the highest activity among the platinum group metals. Further development is expected (Patent Document 5).

[0006] Existing technical documents

[0007] Patent documents

[0008] Patent Document 1: Japanese Patent Application Publication No. 8-269761

[0009] Patent Document 2: Japanese Patent Publication No. 2007-514520

[0010] Patent Document 3: Japanese Patent Application Publication No. 2015-192993

[0011] Patent Document 4: International Publication (WO) 2009 / 154753A2

[0012] Patent Document 5: International Publication (WO) 2019 / 117199A1

[0013] Non-patent literature

[0014] Non-patent literature 1: S. Trasatti, G. Buzzanca, J. Electroanal. Chem., 1971, 29, A1.

[0015] Non-patent literature 2: A. Harriman, I. J. Pickering, J. M. Homas, P. A. Christensen, J. Chem. Soc., Faraday Trans. 1, 1988, 84, 2795.

[0016] Non-patent literature 3: Y. Zhao, NMVargas-Barbosa, EAHernandez-Pagan, TEMallouk, Small, 2011, 7, 2087.

[0017] Non-patent literature 4: MM Najafpour, G. Renger, M. Holynska, AN Moghaddam, E.-M. Aro, R. Carpentier, H. Nishihara, JJ Eaton-Rye, J.-R. Shen, SI Allakhverdiev, Chem. Rev., 2016, 116, 2886.

[0018] Non-patent literature 5: T. Takashima, K. Ishikawa, H. Irie, J. Phys. Chem. C, 2016, 120, 24827.

[0019] Non-patent literature 6: JB Gerken, JGMc Alpin, JYCChen, MLRigsby, WHCasey, RDBritt, SSS Stahl, J. Am. Chem. Soc., 2011, 133, 14431.

[0020] Non-patent literature 7: M. Dinca, Y. Surendranath, DGNocera, Proc. Natl. Acad. Sci. USA, 2010, 107, 10337. Summary of the Invention

[0021] The problem the invention aims to solve

[0022] The purpose of this invention is to provide: manganese-iridium composite oxide for water splitting catalyst, manganese-iridium composite oxide electrode material and manufacturing method thereof.

[0023] More specifically, the object of the present invention is to provide: a manganese-iridium composite oxide (hereinafter, sometimes referred to as the manganese-iridium composite oxide of the present invention) used as an anode catalyst for oxygen production in industrial water electrolysis or water electrolysis using a solid polymer membrane (PEM) type electrolyzer under alkaline, neutral or acidic conditions, a water splitting catalyst with low cost and high oxygen production catalytic activity, an electrode material of the manganese-iridium composite oxide for water splitting catalyst, and a method for manufacturing the same.

[0024] Solution for solving the problem

[0025] The inventors conducted repeated and in-depth studies on catalytic materials used as oxygen-generating electrode catalysts in water electrolysis. The results showed that manganese-iridium composite oxides containing iridium metal at a ratio of iridium / (iridium + manganese)) of 0.1 atomic% to 30 atomic% and having at least interplanar spacings of 0.243±0.002 nm, 0.214±0.002 nm, 0.165±0.002 nm, and 0.140±0.002 nm exhibit high catalytic activity for oxygen-generating electrodes. This led to the completion of this invention. Specifically, this invention provides a manganese-iridium composite oxide for use as an oxygen-generating electrode catalyst in water electrolysis, characterized by an iridium metal content of 0.1 atomic% to 30 atomic% (iridium / (manganese + iridium)) and at least interplanar spacings of 0.243±0.002 nm, 0.214±0.002 nm, 0.165±0.002 nm, and 0.140±0.002 nm.

[0026] The inventors have discovered that at least a portion of the manganese-iridium composite oxide electrode material of the present invention, which is a fiber coated with a conductive substrate, exhibits particularly high catalytic activity for oxygen generation electrodes. That is, the present invention provides a manganese-iridium composite oxide electrode material for oxygen generation electrodes, which is formed from a conductive substrate composed of fibers at least a portion coated with the manganese-iridium composite oxide of the present invention.

[0027] The effects of the invention

[0028] The manganese-iridium composite oxide of the present invention and the manganese-iridium composite oxide electrode material of the present invention exhibit high activity in industrial water electrolysis or water electrolysis using a PEM type electrolytic cell under alkaline, neutral or acidic conditions, and act as an anode catalyst for oxygen production that is inexpensive and excellent. In addition, the manganese-iridium composite oxide and the manganese-iridium composite oxide electrode material of the present invention exhibit extremely excellent durability as an oxygen production electrode catalyst.

[0029] In addition, by adding carbon dioxide or the like to the aforementioned electrolytic system using the manganese-iridium composite oxide electrode material of the present invention, the carbon dioxide or the like can also be reduced at the cathode to produce hydrocarbon compounds (formic acid, formaldehyde, methanol, methane, ethane, propane, etc.). Description of the Drawings

[0030] Figure 1 XRD spectra of the manganese-iridium composite oxides of Examples 2 to 9 and the manganese oxide of Comparative Example 1.

[0031] Figure 2 XRD spectra of the manganese-iridium composite oxides of Examples 10 to 12 and the manganese oxide of Comparative Example 1.

[0032] Figure 3 SEM photograph of a carbon paper (CP) substrate.

[0033] Figure 4 SEM photograph of the manganese-iridium composite oxide deposited on the carbon paper (CP) substrate of Example 12.

[0034] Figure 5 Linear sweep voltammogram showing the relationship between current and potential (voltage) measured using <PEM type electrolytic cell 1> during oxygen production (during water electrolysis) for Examples 13 to 16 and Comparative Examples 1 to 3.

[0035] Figure 6 Linear sweep voltammogram showing the relationship between current and potential (voltage) measured using <PEM type electrolytic cell 1> during oxygen production (during water electrolysis) for Examples 17 to 20 and Comparative Example 1.

[0036] Figure 7 Linear sweep voltammogram showing the relationship between current and potential (voltage) measured using <PEM type electrolytic cell 1> during oxygen production (during water electrolysis) for Examples 21, 22 and Comparative Example 1.

[0037] Figure 8 Linear sweep voltammogram showing the relationship between current and potential (voltage) measured using <PEM type electrolytic cell 1> during oxygen production (during water electrolysis) for Examples 18 to 20 and Comparative Example 1.

[0038] Figure 9Linear sweep voltammogram showing the relationship between current and potential (voltage) measured using <PEM electrolyzer 1> during oxygen production (during water electrolysis) for Example 22 and Comparative Example 1.

[0039] Figure 10 Data showing the time course of the electrolysis voltage measured using <PEM electrolyzer 2> at 80 °C and 0.5 A / cm during oxygen production (during water electrolysis) for Example 21. 2

[0040] Figure 11 SEM photograph of the manganese-iridium composite oxide deposited on the substrate of a platinum-coated Ti mesh for Example 24.

[0041] Figure 12 Linear sweep voltammogram showing the relationship between current and potential (voltage) measured using <PEM electrolyzer 2> during oxygen production (during water electrolysis) for Example 23 and Comparative Example 1.

[0042] Figure 13 Linear sweep voltammogram showing the relationship between current and potential (voltage) measured using <PEM electrolyzer 2> during oxygen production (during water electrolysis) for Example 24 and Comparative Example 1. Detailed Description of the Invention

[0043] Hereinafter, the present invention will be described in more detail.

[0044] First, regarding the decomposition of water based on electrolysis, an example of a reaction in an acidic environment such as PEM type water electrolysis will be described. On the cathode catalyst, as shown in Equation 1, hydrogen gas is generated by the reaction of 2 protons and 2 electrons.

[0045] 2H + + 2e - → H2 …… Equation 1

[0046] On the other hand, on the anode catalyst, as shown in Equation 2, 4 electrons and 4 protons are generated from 2 water molecules and oxygen gas is simultaneously generated.

[0047] 2H2O → O2 + 4H + + 4e - …… Equation 2

[0048] Thus, overall, as shown in Equation 3, the reaction is a reaction in which 2 water molecules generate 2 hydrogen molecules and 1 oxygen molecule.

[0049] 2H2O → 2H2 + O2 …… Equation 3

[0050] The oxygen production reaction in Equation 2 above is generally considered to be the rate-limiting process of all reactions. In this technical field, the development of catalysts that can carry out the reaction with minimal energy is of great importance. The present invention provides an oxygen production electrode catalyst with high catalytic ability for water oxidation.

[0051] The manganese-iridium composite oxide of the present invention controls the iridium metal content ratio (iridium / (manganese + iridium)) to be 0.1 atomic% or more and 30 atomic% or less. If the iridium metal content ratio is less than 0.1 atomic%, it exhibits the characteristic of catalytic activity of manganese oxide alone; on the other hand, if the iridium metal content ratio exceeds 30 atomic%, it exhibits the catalytic activity of iridium oxide, but the extensive use of iridium as a rare element makes it extremely expensive, thus compromising cost-effectiveness. To exhibit superior characteristics, the iridium metal content ratio is preferably 0.2 atomic% or more and 27 atomic% or less, more preferably 0.2 atomic% or more and 20 atomic% or less, further preferably 0.3 atomic% or more and 10.5 atomic% or less, and particularly preferably 1 atomic% or more and 7 atomic% or less.

[0052] The XRD of the manganese-iridium composite oxide of the present invention has characteristic diffraction lines. The d values, which represent the interplanar spacing, are at least 0.243±0.002 nm, 0.214±0.002 nm, 0.165±0.002 nm, and 0.140±0.002 nm, respectively, starting from the diffraction line on the low-angle side.

[0053] No examples of manganese-iridium composite oxides with such interplanar spacing have been reported before. The X-ray diffraction pattern is similar to that of ε-type manganese dioxide, which appears to have an irregular structure due to the formation of numerous microtwins.

[0054] By loading the manganese-iridium composite oxide of the present invention onto an electrode, the manganese-iridium composite oxide of the present invention becomes the active material for the oxygen-generating electrode in water electrolysis, thereby endowing the oxygen-generating electrode with catalytic ability in the water splitting reaction. A laminate is formed by stacking an oxygen-generating electrode containing this active material, a polymeric electrolyte membrane, and an electrode endowed with a hydrogen-generating catalyst. Here, the polymeric electrolyte membrane refers to, for example, a fluororesin-based cation exchange membrane, and the hydrogen-generating catalyst refers to, for example, fine platinum particles. The present invention, by having this oxygen-generating electrode, forms a water electrolysis device, which can produce hydrogen by using this oxygen-generating electrode to perform water electrolysis.

[0055] The following describes the method for manufacturing the manganese-iridium composite oxide of the present invention.

[0056] The manganese-iridium composite oxide of the present invention can be manufactured, for example, by electrolytic extraction using a mixed solution of sulfuric acid, manganese sulfate, and iridium salt as the electrolyte. Alternatively, it can be manufactured by first electrolytically extracting manganese oxide using a mixed solution of sulfuric acid and manganese sulfate, and then electrolytically extracting iridium oxide onto the manganese oxide using a mixed solution of sulfuric acid and iridium salt.

[0057] The concentration of each component in these sulfuric acid-manganese sulfate-iridium salt mixed solutions, sulfuric acid-manganese sulfate mixed solutions, and sulfuric acid-iridium salt mixed solutions, in terms of sulfuric acid concentration, is preferably controlled in the range of more than 4 g / L and less than 65 g / L, more preferably more than 20 g / L and less than 50 g / L.

[0058] The manganese concentration in the above-mentioned mixed solution is not particularly limited as long as it is below the solubility, but preferably 5 g / L or more and 50 g / L or less, more preferably 10 g / L or more and 30 g / L or less. The iridium concentration in the mixed solution is not particularly limited as long as it is below the solubility, but preferably 0.1 g / L or more and 10 g / L or less, more preferably 0.3 g / L or more and 5 g / L or less. The ratio of manganese concentration to iridium concentration in the mixed solution affects the metal composition of the manganese-iridium composite oxide deposited during electrolytic oxidation; therefore, the iridium / manganese molar ratio in the mixed solution is preferably 0.002 or more and 0.05 or less, more preferably 0.005 or more and 0.02 or less.

[0059] To maintain the component concentration of the above mixed solution, it is effective to add manganese sulfate and iridium salt equivalent to the manganese and iridium consumed by electrolytic oxidation, or solutions of manganese sulfate and iridium salt, or to continuously supply the mixed solution.

[0060] It should be noted that the sulfuric acid concentration in the above-mentioned mixed solutions of sulfuric acid-manganese sulfate-iridium salt, mixed solutions of sulfuric acid-manganese sulfate, and mixed solutions of sulfuric acid-iridium salt refers to the value excluding the divalent anion (sulfate ion) of manganese sulfate.

[0061] In the electrolytic manufacturing method of manganese-iridium composite oxide, the electrolytic current density is not particularly limited, but preferably 0.2 A / dm³. 2 Above and 0.9A / dm 2 Therefore, the manganese-iridium composite oxide of the present invention can be easily, efficiently, and stably produced by electrolysis. To obtain the manganese-iridium composite oxide of the present invention more stably, an electrolysis current density of 0.3 A / dm³ is preferred. 2 Above and 0.88A / dm 2 The following is a further preferred value: 0.5A / dm 2 Above and below 0.8A / dm 2 .

[0062] The electrolysis temperature can be, for example, above 93°C and below 98°C. The higher the electrolysis temperature, the higher the efficiency of electrolytic production of the deposited manganese-iridium complex oxide; therefore, an electrolysis temperature of at least 94°C is preferred.

[0063] The manganese-iridium composite oxide, electrolyzed on electrodes such as pure titanium plates, is stripped from the electrodes and coarsely pulverized using a jaw crusher or similar equipment. The manganese-iridium composite oxide monomers are then pulverized using a roller mill, vertical mill, Loesche mill, or spray mill, and adjusted to a specified average secondary particle size. Next, the produced manganese oxide undergoes a washing and neutralization process to remove residual electrolyte, and is then dried using a flash drying apparatus. During this flash drying, submicron-sized manganese-iridium composite oxide powder, a byproduct of over-pulverization during the pulverization process, can be recovered and separated using a bag filter or similar device. Furthermore, a calcination process at 200°C or higher and 500°C is sometimes further performed to obtain the manganese-iridium composite oxide of this invention.

[0064] Next, the manganese-iridium composite oxide electrode material of the present invention will be described.

[0065] The manganese-iridium composite oxide electrode material of the present invention is formed from a conductive substrate, which is composed of fibers at least partially coated with the manganese-iridium composite oxide of the present invention. In this case, the coating amount of the manganese-iridium composite oxide of the present invention is preferably 0.1 mg / cm² per unit geometric area of ​​the conductive substrate. 2 Above and 25mg / cm 2 Below. Here, geometric area refers to the projected area equivalent to the conductive substrate, without considering the substrate thickness.

[0066] When the coating amount of the manganese-iridium composite oxide of the present invention is within the above-mentioned range, although it is also affected by the diameter and porosity of the fibers constituting the conductive substrate, the manganese oxide can be coated on the fibers in the form of islands or covering the entire outer surface of the fibers, so that the average coating thickness is about 25 μm or less. It should be noted that since the manganese-iridium composite oxide coated on the fibers is composed of secondary particles, the average coating thickness is usually consistent with the average secondary particle size of the manganese-iridium composite oxide constituting it.

[0067] In the manganese-iridium composite oxide electrode material of the present invention, the following relationship exists: the average thickness of the manganese-iridium composite oxide coating on the conductive substrate fibers increases with the amount of manganese-iridium composite oxide coating. More preferably, the coating amount of the manganese-iridium composite oxide is 0.1 mg / cm². 2 Above and 20mg / cm 2 The following, and more preferably, is 0.2 mg / cm³. 2 Above and 15mg / cm 2The following, particularly preferred, dosage is 0.5 mg / cm³. 2 Above and 12mg / cm 2 The following should be noted: The thickness of the manganese-iridium composite oxide coating can be determined by subtracting, for example, the diameter of the conductive fibers that form the building blocks of the conductive substrate, from a scanning electron microscope (SEM) image.

[0068] The manganese-iridium composite oxide electrode material of the present invention can be obtained as follows: using a mixed solution of sulfuric acid, manganese sulfate, and iridium salt, the manganese-iridium composite oxide is electrolytically deposited onto a conductive substrate, such as carbon paper, titanium mesh, or a platinum-coated titanium mesh, instead of the pure titanium plate, as an electrode substrate. In this case, the concentration is 0.1 mg / cm² per unit geometric area. 2 Above and 25mg / cm 2 The following precipitation amounts lead to the electrolytic deposition of manganese-iridium composite oxides: less than 0.1 mg / cm² per unit geometric area. 2 If the amount of manganese-iridium composite oxide precipitated is too small, it will not adhere sufficiently to the substrate and will easily detach; or if the amount of catalyst is too small, it may not be able to fully demonstrate its performance. If it exceeds 25 mg / cm³, the precipitated amount may be insufficient. 2 If the thickness of the manganese-iridium composite oxide coating is too thick, it will hinder the movement of electrons to the conductive substrate, or the gaps between the conductive fibers of the conductive substrate will become narrower, thus restricting the movement of water and oxygen as reaction products, which will become rate limiting and sometimes prevent the catalytic performance from being fully realized.

[0069] The conductive substrate is preferably, for example, a plate-shaped substrate in which conductive fibers such as carbon or titanium with a wire diameter of 100 μm or less are formed or sintered to make the substrate thickness 1 mm or less. The porosity of the conductive substrate is preferably 40% or more, more preferably 50% to 90%. Here, porosity is defined by the volume of the conductive substrate that does not contain conductive fibers.

[0070] For conductive substrates, acid treatment with hydrochloric acid, sulfuric acid, nitric acid, oxalic acid, etc., before electrodeposition of manganese-iridium composite oxides is effective in removing the passivation film and hydrophilizing the substrate surface. On the other hand, to control the electrodeposition site of manganese-iridium composite oxides within the conductive substrate, or to impart gas diffusion characteristics important for actual use as electrodes in water electrolysis, it is also effective to impregnate the conductive substrate with water-repellent agents such as dispersions of fluorinated resins.

[0071] As for the conditions for electrodepositing the manganese-iridium composite oxide of the present invention onto a conductive substrate, for example, the sulfuric acid concentration, manganese concentration, iridium concentration, electrolytic current density, electrolytic temperature, etc., of the mixed solution of sulfuric acid-manganese sulfate-iridium salt, mixed solution of sulfuric acid-manganese sulfate, and mixed solution of sulfuric acid-iridium salt are selected from the ranges described above. The electrolysis time is in the range of 5 minutes to 120 minutes. After the electrolysis is completed, the solution is washed with water and dried, thereby producing the manganese-iridium composite oxide electrode material of the present invention.

[0072] During the electrodeposition of the manganese-iridium composite oxide, if one side of the conductive substrate is masked by a resinous film or the like, the manganese-iridium composite oxide electrodeposition film can be intentionally segregated so that it is preferentially electrodeposited only on one side, while almost no manganese-iridium composite oxide is electrodeposited on the other side.

[0073] Furthermore, for the manganese-iridium composite oxide electrode material of the present invention, any one of acid impregnation, heating, or both, is effective as a post-treatment. For example, acid impregnation can be used to impregnate the manganese-iridium composite oxide electrode material in 0.5 mol / L to 5 mol / L sulfuric acid for approximately 30 minutes to 2 hours, followed by washing with water and drying. Alternatively, heating can be used to obtain the manganese-iridium composite oxide electrode material by heating it in an air or nitrogen atmosphere at 180°C to 500°C for 30 minutes to 8 hours. That is, although heating in an air or nitrogen atmosphere at 180°C to 300°C for 30 minutes to 2 hours is also possible, heating in an air or nitrogen atmosphere at a temperature exceeding 300°C and below 500°C for more than 2 hours and less than 8 hours is more preferable.

[0074] It is presumed that these post-treatments improve the adhesion between the manganese-iridium composite oxide and the conductive fiber, or enhance the crystallinity of the manganese-iridium composite oxide.

[0075] By stacking the manganese-iridium composite oxide electrode material of the present invention, the polymer electrolyte membrane, and the electrode with a hydrogen production catalyst, a laminate is formed. In the present invention, a water electrolysis device is formed by having the manganese-iridium composite oxide electrode material of the present invention, and hydrogen can be produced by using the manganese-iridium composite oxide electrode material for water electrolysis.

[0076] Example

[0077] The present invention will now be described in detail with reference to embodiments and comparative examples, but the present invention is not limited to these embodiments.

[0078] Metal concentration analysis of mixed solutions of sulfuric acid, manganese sulfate, and iridium salt; mixed solutions of sulfuric acid and manganese sulfate; and mixed solutions of sulfuric acid and iridium salt.

[0079] The mixed solutions of sulfuric acid-manganese sulfate-iridium salt, sulfuric acid-manganese sulfate, and sulfuric acid-iridium salt were diluted, and the concentrations of manganese and iridium were quantitatively determined using ICP-AES (Optima 8300 manufactured by PerkinElmer).

[0080] <SEM Surface Observation and Composition Analysis of Manganese-Iridium Composite Oxide and Manganese-Iridium Composite Oxide Electrode Materials>

[0081] Surface condition observation and elemental analysis (detected elements: B to U) were performed using standard SEM-EPMA apparatuses (Hitachi S-4500 and Horiba EX-250 (EDS)). The accelerating voltage was set to 15 kV.

[0082] <Calculation of surface spacing (d-value) of manganese-iridium composite oxide and manganese-iridium composite oxide electrode materials based on XRD measurement>

[0083] The diffraction lines of manganese-iridium composite oxides and manganese-iridium composite oxide electrode materials were measured using a standard X-ray diffraction apparatus (Ultima IV, Rigaku Corporation). CuKα rays were used as the X-ray source. The measurement mode was set to step scan, the scan speed to 4.0000° per minute, the step size to 0.02°, and the measurement range to 2θ, with measurements performed within the range of 5° to 80°. Gaussian processing was applied to the diffraction lines of the obtained XRD pattern to determine the 2θ of the peaks. The value of d was then calculated using the Bragg equation (nλ = 2dsinθ, n = 1) and used as the surface spacing.

[0084] Determination of Electrodeposition Amount of Manganese-Iridium Composite Oxides

[0085] The electrodeposition amount of manganese-iridium composite oxides was determined according to the following method.

[0086] Before electrodeposition of manganese-iridium composite oxide, the weight of the substrate (electrode substrate such as titanium, conductive substrate) is measured with a balance (1). After electrodeposition, the weight of the substrate after electrodeposition of manganese-iridium composite oxide is measured with a balance (2). The amount of manganese-iridium composite oxide electrodeposition is calculated from the difference between weight (1) and weight (2) (weight (2) - weight (1)).

[0087] Construction of a PEM-type electrolyzer for evaluating the properties of oxygen-generating electrode catalysts

[0088] The following describes the construction of a PEM-type electrolyzer using an electrode material with a conductive substrate containing a manganese-iridium composite oxide catalyst. The electrode material (flat mesh shape: 3cm × 3cm) was used as the working electrode. A 20wt% 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. Conductive catalyst ink was prepared, carbon paper was coated, and the counter electrode was fabricated by air drying. A Nafion membrane (Nafion 117, Sigma-Aldrich) was used as the electrolyte membrane. The electrolyte membrane was washed and protonated (pretreatment) by boiling in 3% hydrogen peroxide solution, pure water, 1M sulfuric acid aqueous solution, and pure water for 1 hour each. Next, the electrolyte membrane is sandwiched between the catalyst-coated surfaces of the working / counter electrode and hot-pressed for 10 minutes at 135°C and a clamping force of 600 kg using a hot press (A-010D, manufactured by FC-R&D Co., Ltd.), thereby producing a membrane / electrolyte assembly (MEA). This MEA, with the aid of two stainless steel meshes (#100), maintains improved adhesion even during electrolysis operation and is installed in the housing of either PEM-type electrolyzer 1 (3036, manufactured by FC-R&D Co., Ltd.) or PEM-type electrolyzer 2 (WE-4S-RICW, manufactured by FC Development Co., Ltd.).

[0089] <Electrochemical Measurement 1: Determination of Current-Voltage Curves (Using PEM Electrolytic Cell 1)>

[0090] To evaluate the catalytic oxidation activity of water in a real-world device, a PEM-type electrolyzer 1, constructed using an electrode material with a conductive substrate exhibiting manganese-iridium composite oxide deposition, was used. Current-voltage curves were measured at an operating temperature of 25°C. In this measurement, a two-electrode system with only a working electrode and a counter electrode was used. The applied voltage was slowly increased to measure the current-voltage curve. Pure water was supplied to the PEM-type electrolyzer. The voltage increase rate was set to 5 mV / s to facilitate easy determination of the starting voltage of the current. In the current-voltage curve measurements, a higher current at the same voltage indicates higher catalytic activity of the oxygen electrode.

[0091] <Electrochemical Measurement 2: Determination of Current-Voltage Curves (Using PEM Electrolytic Cell 2)>

[0092] To evaluate the catalytic oxidation capability of water in a real device, a PEM-type electrolyzer 2, constructed using an electrode material with a conductive substrate exhibiting manganese-iridium composite oxide deposition, was used. Current-voltage curves were measured at operating temperatures of 25°C and 80°C. In this measurement, a two-electrode system with only a working electrode and a counter electrode was used, and the applied voltage was increased slowly to determine the current-voltage curve. Pure water was supplied to the PEM-type electrolyzer. The voltage increase rate was set to 5 mV / s to facilitate easy determination of the starting voltage of the current.

[0093] <Electrochemical Measurement 3: Determination of Electrolysis Voltage Stability (Using PEM Electrolytic Cell 2)>

[0094] To evaluate the stability of the water oxidation catalytic activity in a real device, a PEM-type electrolyzer 2, constructed using an electrode material with a conductive substrate exhibiting manganese-iridium composite oxide deposition, was used. The electrolysis voltage was measured at an operating temperature of 80°C. In this measurement, a two-electrode system with only a working electrode and a counter electrode was used, while maintaining a current density of 0.5 A / cm² applied between the electrodes. 2 The change in electrolysis voltage over time was measured. Pure water was supplied to the PEM-type electrolyzer. The smaller the change in electrolysis voltage over time, the higher the durability, that is, the ability to stably maintain the catalytic activity of the oxygen electrode.

[0095] Example 1

[0096] Electrolysis was performed in an electrolytic cell containing a mixed solution of 35 g / L sulfuric acid, 51.3 g / L manganese sulfate, and 2.0 g / L potassium hexachloroiridate (K₂IrCl₆) to electrodeposit manganese-iridium composite oxide onto carbon paper (TGP-H-060, Toray Corporation). The electrolytic current density was set to 0.7 A / dm³. 2 The electrolysis temperature was set to 95℃, and electrolysis was carried out for 60 minutes. After electrolysis, the material was washed with water, air-dried, and cut into 3cm×3cm pieces to prepare the electrode material. The composition analysis and XRD determination of the manganese-iridium composite oxide electrode material were performed, and the results are shown in Table 1.

[0097] Examples 2-12

[0098] The manganese sulfate concentration, iridium salt type, substrate type, and electrolysis temperature were as specified in Table 1. Except for these parameters, a mixed solution was prepared in the same manner as in Example 1, and electrolysis was performed to electrodeposit the manganese-iridium composite oxide onto carbon paper (TGP-H-060, Toray Corporation) or a platinum-coated Ti mesh (ADL-414302-5056, FC Development Co, Ltd.). The electrolysis current density was set to 0.7 A / dm³. 2Electrolysis was performed for 60 minutes. After electrolysis, the material was washed with water, air-dried, and cut into 3cm × 3cm pieces to prepare electrode materials. Compositional analysis and XRD measurements were performed on these manganese-iridium composite oxide electrode materials, and the results are shown in Table 1 below. Figure 1 and Figure 2 .

[0099] Additionally, the SEM image of the electrode material in Example 12 ( Figure 4 As shown, the carbon fibers (coated to form carbon paper) were confirmed. Figure 3 () manganese-iridium composite oxide.

[0100] Comparative Example 1

[0101] Electrolysis was performed in an electrolytic cell containing a mixed solution of 32 g / L sulfuric acid and 75 g / L manganese sulfate to electrodeposit manganese oxide onto carbon paper (TGP-H-060, Toray Corporation). The electrolytic current density was set to 0.68 A / dm³. 2 The electrolysis temperature was set to 94℃, and electrolysis was carried out for 16 minutes. After electrolysis, the material was washed with water, air-dried, and cut into 3cm × 3cm pieces to prepare the electrode material. XRD analysis of this manganese oxide electrode material was performed, and the results are shown in Table 1 below. Figure 1 and Figure 2 .

[0102] like Figure 1 , 2 As shown, the XRD pattern of the embodiment, compared with Comparative Example 1, has the following characteristics if the diffraction lines shown by the carbon paper substrate are excluded: there are basically no diffraction lines near 22 degrees, and the other four main diffraction lines are shifted to the lower angle side.

[0103] Furthermore, for the manganese oxide electrode material of Comparative Example 1, a PEM-type electrolyzer was constructed according to the method described in "Construction of a PEM-type Electrolyzer for Evaluating the Characteristics of Oxygen-Generating Electrode Catalysts." The characteristics of the oxygen-generating electrode catalyst were evaluated according to "Electrochemical Measurement 1: Measurement of Current-Voltage Curves (PEM-type Electrolyzer)" and "Electrochemical Measurement 2: Measurement of Current-Voltage Curves (PEM-type Electrolyzer 2)." The results are shown in Table 2 below. Figures 5-9 and Figures 12-13 .

[0104] Comparative Example 2

[0105] Conductive catalyst ink was prepared using a mixture of 20 wt% platinum-supported carbon catalyst (20% Platinum on Vulcan XC-72, Item#PTC20-1, Fuel Cell Earth) and diluted Nafion dispersion (10 wt.% aqueous solution, 527106-25 mL, Sigma-Aldrich). A PEM-type electrolyzer was constructed, and the characteristics of the oxygen-generating electrode catalyst were evaluated according to <Electrochemical Measurement 1: Determination of Current-Voltage Curves (PEM-type Electrolyzer)>. The results are shown in Table 2 below. Figure 5 .

[0106] Comparative Example 3

[0107] A PEM-type electrolyzer was constructed using commercially available iridium oxide catalyst (manufactured by Elyst). The oxygen-generating electrode catalyst was evaluated according to <Electrochemical Measurement 1: Determination of Current-Voltage Curves (PEM-type Electrolyzer)>, and the evaluation results are shown in Table 2 below. Figure 5 .

[0108] Examples 13-20

[0109] The manganese sulfate concentration, iridium salt type, substrate type, and electrolysis temperature were as specified in Table 2. Except for these parameters, similar to Example 1, electrolysis was performed in an electrolytic cell containing a mixed solution of potassium hexachloroiridate (K₂IrCl₆) at a concentration of 2.0 g / L to electrodeposit the manganese-iridium composite oxide onto carbon paper (TGP-H-060, Toray Corporation) or a platinum-coated Ti mesh (ADL-414302-5056, FC Development Co, Ltd.). The electrolysis current density was set to 0.7 A / dm³. 2 Electrolysis was performed for 60 minutes. After electrolysis, the material was washed with water, air-dried, and cut into 3cm × 3cm pieces to prepare electrode materials. Using these electrode materials, a PEM-type electrolytic cell was constructed according to the method described in "Construction of PEM-type Electrolytic Cell for Evaluating the Characteristics of Oxygen-Generating Electrode Catalysts." The characteristics of the oxygen-generating electrode catalyst were evaluated according to either "Electrochemical Measurement 1: Determination of Current-Voltage Curves (PEM-type Electrolytic Cell 1)" or "Electrochemical Measurement 2: Determination of Current-Voltage Curves (PEM-type Electrolytic Cell 2)." The results are shown in Table 2 below. Figure 6 .

[0110] Examples 21-22

[0111] The manganese sulfate concentration, iridium salt type, substrate type, and electrolysis temperature were as specified in Table 2. Except for these parameters, similar to Example 1, electrolysis was performed in an electrolytic cell containing a mixed solution of potassium hexachloroiridate (K₂IrCl₆) at a concentration of 2.0 g / L to electrodeposit the manganese-iridium composite oxide onto carbon paper (TGP-H-060, Toray Corporation) or a platinum-coated Ti mesh (ADL-414302-5056, FC Development Co, Ltd.). The electrolysis current density was set to 0.7 A / dm³. 2 Electrolysis was performed for 60 minutes. After electrolysis, the material was washed with water, air-dried, and annealed at 230°C for 2 hours. It was then cut into 3cm × 3cm pieces to prepare electrode materials. Using these electrode materials, a PEM-type electrolytic cell was constructed according to the method described in "Construction of a PEM-type Electrolytic Cell for Evaluating the Characteristics of Oxygen-Generating Electrode Catalysts." The characteristics of the oxygen-generating electrode catalyst were evaluated according to "Electrochemical Measurement 2: Determination of Current-Voltage Curves (PEM-type Electrolytic Cell 2)." The results are shown in Table 2 below. Figure 7 and Figure 9 .

[0112] Furthermore, in the electrode material of Example 21, a PEM-type electrolyzer constructed according to the method of <Construction of PEM-type Electrolyzer for Evaluating the Characteristics of Oxygen-Generating Electrode Catalysts> was used, and the time-varying change of the electrolysis voltage was measured according to <Electrochemical Measurement 3: Measurement of Electrolysis Voltage Stability (PEM-type Electrolyzer 2)>. The results are shown below. Figure 10 .

[0113] like Figures 5-9 As shown, the manganese-iridium composite oxide and manganese-iridium composite oxide electrode materials of the present invention are structures with higher energy conversion efficiency in principle. In PEM-type electrolyzers with non-precious metal catalysts, they also exhibit high oxygen-generating electrode catalytic activity comparable to commercially available iridium oxide catalysts.

[0114] In addition, such as Figure 10 As shown, the manganese-iridium composite oxide and manganese-iridium composite oxide electrode materials of the present invention exhibit excellent durability as oxygen-generating electrode catalysts.

[0115] [Table 1]

[0116]

[0117] [Table 2]

[0118]

[0119] Example 23

[0120] Electrolysis was performed in an electrolytic cell containing a mixed solution of 35 g / L sulfuric acid, 2.71 g / L manganese sulfate, and 0.48 g / L potassium hexachloroiridate (K₂IrCl₆) to electrodeposit manganese-iridium composite oxide onto a platinum-coated Ti mesh (ADL-414302-5056, manufactured by FC Development Co., Ltd.). The electrolytic current density was set to 0.7 A / dm³. 2 Electrolysis was performed at 94℃ for 60 minutes. After electrolysis, the material was washed with water, air-dried, and then heat-treated at 350℃ for 5 hours. It was then cut into 3cm × 3cm pieces to prepare electrode materials. Using this electrode material, a PEM-type electrolyzer was constructed according to the method described in "Construction of PEM-type Electrolyzer for Evaluating the Characteristics of Oxygen-Generating Electrode Catalysts." The characteristics of the oxygen-generating electrode catalyst were evaluated according to "Electrochemical Measurement 2: Determination of Current-Voltage Curves (PEM-type Electrolyzer 2)." The results are shown in Tables 3, 4, and 5 below. Figure 12 .

[0121] Example 24

[0122] In an electrolytic cell containing a mixed solution of 35 g / L sulfuric acid and 54.4 g / L manganese sulfate, the electrolytic current density was set to 0.7 A / dm³. 2 Electrolysis was performed at 94°C for 10 minutes to deposit manganese oxide onto a platinum-coated Ti mesh (ADL-414302-5056, manufactured by FC Development Co., Ltd.). Then, in an electrolytic cell containing a mixed solution of 35 g / L sulfuric acid and 0.16 g / L potassium hexachloroiridate (K₂IrCl₆), the electrolytic current density was set to 0.7 A / dm³. 2 Electrolysis was performed for 60 minutes to deposit iridium oxide onto manganese oxide. After electrolysis, the material was washed with water, air-dried, and then heat-treated at 400°C for 5 hours. It was then cut into 3cm × 3cm pieces to prepare the electrode material. SEM images of this electrode material are shown below. Figure 11 Using this electrode material, a PEM-type electrolyzer was constructed according to the method described in "Construction of a PEM-type Electrolyzer for Evaluating the Characteristics of Oxygen-Generating Electrode Catalysts." The characteristics of the oxygen-generating electrode catalyst were evaluated according to "Electrochemical Measurement 2: Determination of Current-Voltage Curves (PEM-type Electrolyzer 2)." The results are shown in Tables 3, 4, and 5 below. Figure 13 .

[0123] like Figure 12 , Figure 13As shown, the manganese-iridium composite oxide and manganese-iridium composite oxide electrode materials of the present invention have a structure with higher energy conversion efficiency in principle. By implementing appropriate heat treatment as a post-treatment, they exhibit particularly good oxygen-generating electrode catalytic activity. In PEM-type electrolyzers where the desired catalyst is non-precious metal, they also exhibit high oxygen-generating electrode catalytic activity comparable to commercially available iridium oxide catalysts.

[0124] [Table 3]

[0125]

[0126] [Table 4]

[0127]

[0128] [Table 5]

[0129]

[0130] Industrial availability

[0131] The manganese-iridium composite oxide and manganese-iridium composite oxide electrode materials of the present invention have high oxygen-generating electrode catalytic activity comparable to existing noble metal catalysts. Therefore, by using them as anode catalysts for oxygen generation in industrial water electrolysis under alkaline or neutral conditions or in water electrolysis using a PEM-type electrolyzer, hydrogen and oxygen can be obtained at extremely low manufacturing costs.

[0132] Furthermore, by presenting carbon dioxide in the aforementioned reaction system such as water electrolysis, hydrocarbon compounds (formic acid, formaldehyde, methanol, methane, ethane, propane, etc.) can be produced by reducing carbon dioxide at the cathode.

[0133] It should be noted that the entire contents of the specification, claims, drawings and abstract of Japanese Patent Application No. 2020-54589, filed on March 25, 2020, are incorporated herein as part of the disclosure of this invention.

Claims

1. A manganese-iridium composite oxide, characterized in that, The metallic content of iridium, i.e., iridium / (manganese + iridium) is more than 0.1 atomic% and less than 30 atomic%, and has interplanar spacings of at least 0.243±0.002nm, 0.214±0.002nm, 0.165±0.002nm and 0.140±0.002nm.

2. A manganese-iridium composite oxide electrode material, characterized in that, It is formed of a conductive substrate, which is composed of fibers at least a portion of which are coated with the manganese-iridium composite oxide of claim 1.

3. The manganese-iridium composite oxide electrode material according to claim 2, wherein, The manganese-iridium composite oxide is coated with 0.1 mg / cm² per unit geometric area of ​​the conductive substrate. 2 Above and 25mg / cm 2 the following.

4. The manganese-iridium composite oxide electrode material according to claim 2 or 3, wherein, The conductive substrate is composed of carbon, titanium, or titanium coated with platinum.

5. A laminate comprising the manganese-iridium composite oxide electrode material and the polymer electrolyte membrane as described in any one of claims 2 to 4.

6. A manufacturing method, characterized in that, It is a method for manufacturing the manganese-iridium composite oxide electrode material according to any one of claims 2 to 4, wherein the manganese-iridium composite oxide according to claim 1 is electrodeposited on the fibers constituting the conductive substrate by electrolysis of a mixed solution containing sulfuric acid-manganese sulfate-iridium salt.

7. A manufacturing method, characterized in that, It is a method for manufacturing the manganese-iridium composite oxide electrode material according to any one of claims 2 to 4, wherein, after electrolysis with a sulfuric acid-manganese sulfate mixed solution on the fiber constituting the conductive substrate, electrolysis is performed with a mixed solution containing sulfuric acid-iridium salt, thereby coating the manganese-iridium composite oxide according to claim 1.

8. The manufacturing method according to claim 6 or 7, characterized in that, Heat treatment is performed.

9. The manufacturing method according to claim 8, characterized in that, Heat treatment at 180℃~500℃ for 30 minutes to 8 hours.

10. The manufacturing method according to claim 6, wherein, The manganese-iridium composite oxide of claim 1 is such that it has a density of 0.1 mg / cm² per unit geometric area. 2 Above and 25mg / cm 2 The following amount of deposited material was electrolytically removed.

11. An oxygen-generating electrode active material for water electrolysis, comprising the manganese-iridium composite oxide as described in claim 1.

12. An oxygen-generating electrode comprising the oxygen-generating electrode active material of claim 11.

13. A laminate having the oxygen-generating electrode and polymeric electrolyte membrane as described in claim 12.

14. A water electrolysis device comprising the manganese-iridium composite oxide electrode material as described in any one of claims 2 to 4 or the oxygen-generating electrode as described in claim 12.

15. A method for producing hydrogen, comprising water electrolysis using the manganese-iridium composite oxide electrode material according to any one of claims 2 to 4 or the oxygen-generating electrode according to claim 12.

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