Method of producing oxygen

Abstract

To provide a method of producing oxygen, capable of producing oxygen while suppressing the oxidation of a chloride ion upon electrolyzing water including a chloride ion.SOLUTION: There is provided a method of producing oxygen by electrolyzing water including a chloride ion by using an electrode supporting an oxygen generation reaction catalyst including ruthenium oxide (IV) or iridium oxide (IV) as an anode, to perform the electrolysis by setting a temperature of the water including the chloride ion to be 30°C or more.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to a method for producing oxygen by electrolysis using a catalyst for oxygen evolution reaction containing ruthenium (IV) oxide or iridium (IV) oxide. [Background technology]

[0002] Currently, 95% of the world's hydrogen is produced by steam reforming of fossil fuels such as methane. Water electrolysis using electricity derived from renewable energy sources is an ideal hydrogen production process, as it emits no CO2 at any stage. Regardless of the electrolysis conditions, the bottleneck in water electrolysis is not hydrogen production at the cathode but oxygen production at the counter electrode (anode), necessitating the development of highly efficient anode catalysts. Existing water electrolysis technologies include alkaline water electrolysis and proton exchange membrane (PEM) electrolysis. The former requires alkaline water and pure water, while the latter requires membrane materials and pure water. Given that 97% of the Earth's water resources are seawater, the process of directly electrolyzing seawater to produce hydrogen is attractive. This is particularly effective in coastal and desert regions where access to freshwater is limited and energy is readily available. Seawater can also be desalinated using reverse osmosis (RO) and then electrolyzed using conventional methods, but this poses challenges such as impurity removal and space constraints. For example, direct seawater electrolysis is preferable for applications such as offshore wind power generation, island areas, and mobile systems (including ships).

[0003] The technical challenges in seawater electrolysis are (a) the side reaction of chloride ions (Cl - The objectives are to suppress the oxidation of chloride ions (Cl) (chloride oxidation reaction), and (b) to obtain a sufficient amount of hydrogen evolution (= current density) with a small overvoltage. The oxygen evolution reaction (OER) occurs by oxidation of water. - ) oxidation (COR) to chlorine (Cl2) and hypochlorite ion (ClO -Although OER is thermodynamically favorable over the formation reaction of chloride ions, in practice, when NaCl is electrolyzed using an anodic catalyst such as IrO2 or RuO2, COR prevails. This is due to the kinetic requirement that OER is a slow reaction involving the transfer of four electrons, while COR is a fast reaction involving the transfer of only two electrons. To address issues (a) and (b), conventional methods have been used: (i) adding alkali to seawater, and (ii) coating the catalyst with a layer that blocks chloride ions. However, (i) requires the addition of a large amount of alkali to actual seawater, and (ii) while achieving OER selectivity, it suffers from the problem of low current density. Non-Patent Document 1 describes the use of sodium-doped oxygen-vacant ruthenium oxide as an OER catalyst, but does not disclose any information on suppressing COR. [Prior art documents] [Non-patent literature]

[0004] [Non-Patent Document 1] Lijie Zhang, et al., 'Sodium-Decorated Amorphous / Crystalline RuO2 with RichOxygen Vacancies:A Robust pH-Universal Oxygen Evolution Electrocatalyst', Angewandte Chemie, 2021, 60, 18821-18829 Summary of the Invention [Problem to be solved by the invention]

[0005] An object of the present invention is to provide a method for producing oxygen that can suppress oxidation of chloride ions during electrolysis of water containing chloride ions and produce oxygen. [Means for solving the problem]

[0006] The present inventors began investigations to solve the above problems. During their investigations, they found that by using ruthenium(IV) oxide or iridium(IV) oxide as an oxygen generation reaction catalyst to electrolyze water containing chloride ions and raising the water temperature during electrolysis above room temperature, catalytic activity can be increased, and even when the current density during electrolysis is increased, COR is excellently suppressed, and oxidation of chloride ions is suppressed to generate oxygen. Ruthenium(IV) oxide with a crystallite size of 2.5 to 4.0 nm was particularly suitable for achieving these effects. The present invention was thus completed.

[0007] That is, the present invention is specified by the following items. (1) A method for producing oxygen by electrolyzing water containing chloride ions using an electrode carrying a catalyst for oxygen generation reaction containing ruthenium (IV) oxide or iridium (IV) oxide as the anode, in which the electrolysis is carried out at a temperature of 30°C or higher. (2) The method for producing oxygen according to (1) above, in which electrolysis is carried out by raising the temperature of water containing chloride ions to 40°C or higher. (3) The method for producing oxygen according to (1) or (2), wherein the crystallite size of the ruthenium (IV) oxide is 2.5 to 4.0 nm. [Effects of the Invention]

[0008] The method for producing oxygen of the present invention can produce oxygen while suppressing the oxidation of chloride ions in the electrolysis of water containing chloride ions. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 shows XRD patterns of the ruthenium (IV) oxides of Examples 1 to 5. [Figure 2] FIG. 2 shows the XRD pattern of iridium (IV) oxide of Example 6. [Figure 3]FIG. 3 is a graph showing the COR Faraday efficiency when constant current electrolysis was carried out on the S-RuOx obtained in Example 1 while changing the temperature of the electrolyte and the constant current. [Figure 4] FIG. 4 is a graph showing the COR Faraday efficiency when constant current electrolysis was carried out on commercially available RuO2 in Example 2 while changing the temperature of the electrolyte and the constant current. [Figure 5] FIG. 5 is a graph showing the COR Faraday efficiency when constant current electrolysis was carried out on iridium (IV) oxide in Example 6 while changing the temperature of the electrolyte and the constant current. [Figure 6] FIG. 6 shows the results of BET measurements. [Figure 7] FIG. 7 shows the results of CV measurements. [Figure 8] FIG. 8 is a plot of the relationship between the difference Δj between the cathode current and the anode current and the sweep rate. DETAILED DESCRIPTION OF THE INVENTION

[0010] The oxygen production method of the present invention is a method for producing oxygen by electrolyzing water containing chloride ions using an electrode supported on an oxygen generation reaction catalyst containing ruthenium(IV) oxide or iridium(IV) oxide as the anode, and the electrolysis is performed at a temperature of 30°C or higher. The ruthenium(IV) oxide and iridium(IV) oxide used in the present invention are not particularly limited, and for example, catalysts generally used can be used. The oxygen generation reaction catalyst of the present invention may consist solely of ruthenium(IV) oxide or iridium(IV) oxide, or may consist of ruthenium(IV) oxide and iridium(IV) oxide, or may contain components other than ruthenium(IV) oxide and iridium(IV) oxide. The components other than ruthenium(IV) oxide and iridium(IV) oxide in the present invention are not particularly limited, and examples thereof include other catalysts, binders, carriers, etc.

[0011] The electrode of the present invention comprises a substrate on which the oxygen generating reaction catalyst of the present invention is supported. The substrate of the electrode of the present invention is not particularly limited as long as it is a conductive substrate. Examples of the material include metals such as titanium, zirconium, and tungsten; conductive glasses such as FTO glass; and carbon-based materials such as carbon fiber, graphite, and artificial graphite. Titanium and titanium alloys are preferred as the conductive substrate material because they are commonly used industrially and have high corrosion resistance. Examples of titanium alloys include alloys of titanium with zirconium, niobium, and tantalum, and alloys of titanium and palladium. The shape of the conductive substrate is not particularly limited, and examples include flat plates, curved plates, rods, meshes, and laths. The electrode of the present invention may be supported on the conductive substrate so that the oxygen generating reaction catalyst is in direct contact with the conductive substrate, or the oxygen generating reaction catalyst may be supported on the conductive substrate via another substance.

[0012] The electrolysis in the present invention can be carried out in the same manner as conventional electrolysis, except that an electrode carrying an oxygen generation reaction catalyst containing ruthenium (IV) oxide or iridium (IV) oxide is used as the anode, and the temperature of the chloride ion-containing water during electrolysis is set to 30°C or higher. The cathode is not particularly limited, but examples include a platinum electrode, a carbon electrode, a titanium electrode, and a nickel electrode. The chloride ion concentration of the chloride ion-containing water is not particularly limited as long as it is within a range that allows electrolysis, but examples include 0.001 to 5M and 0.05 to 2M. The current density during electrolysis can be appropriately selected depending on the electrode used, the chloride ion concentration of the chloride ion-containing water, and other factors. The temperature of the chloride ion-containing water is not particularly limited as long as it is 30°C or higher, but preferred ranges include 35°C or higher, 40°C or higher, 45°C or higher, 50°C or higher, 60°C or higher, and 70°C or higher. The upper limit of the temperature of the water containing chloride ions is not particularly limited, and examples thereof include 95°C, 90°C, 80°C, and 75°C. Preferred ranges of the temperature of the water containing chloride ions include ranges between the respective lower limit values ​​and the respective upper limit values ​​(including the respective lower limit values ​​and the respective upper limit values). In the present invention, the temperature of each of the above-mentioned lower limit values ​​may be maintained during electrolysis, or may be changed between the temperature of each of the above-mentioned lower limit values ​​and a temperature equal to or higher than the lower limit value.

[0013] The ruthenium (IV) oxide in the present invention is not particularly limited, but may be ruthenium (IV) oxide having a BET specific surface area of ​​7 m 2 / g or more is preferable, and 20m 2 / g or more is preferable, and 50m 2 / g or more is more preferable, and 100m 2 / g or more is more preferable. 2 / g or less is preferable, and 180m 2 / g or less is more preferable, and 7 to 200m 2 / g is preferred, and 20 to 200m 2 / g is more preferable, and 100 to 180m 2 / g is even more preferable. The BET specific surface area in the present invention is the specific surface area measured by the BET method, and can be determined by measuring the relative pressure and the amount of adsorption when nitrogen molecules are physically adsorbed onto a sample. By setting the BET specific surface area of ​​ruthenium (IV) oxide within the above range, it is possible to further improve catalytic activity and the effect of inhibiting the oxidation of chloride ions.

[0014] The ruthenium(IV) oxide in the present invention is not particularly limited, but from the viewpoint of further improving catalytic activity and the effect of inhibiting the oxidation of chloride ions, the crystallite size is preferably 2.5 to 30.0 nm, more preferably 2.5 to 15.0 nm, and even more preferably 2.5 to 4.0 nm. The crystallite size of ruthenium(IV) oxide can be calculated using the Scherrer equation from the full width at half maximum of the peak corresponding to the 110 direction in a diffraction pattern obtained by X-ray diffraction (XRD). As the ruthenium(IV) oxide in the present invention, ruthenium(IV) oxide having a crystallite size of 2.5 to 4.0 nm is particularly preferred, with ruthenium(IV) oxide having a crystallite size of 2.5 to 3.5 nm being preferred, or ruthenium(IV) oxide having a crystallite size of 2.7 to 3.5 nm being more preferred.

[0015] The ruthenium (IV) oxide in the present invention is not particularly limited, but from the viewpoint of further improving the catalytic activity and the effect of inhibiting the oxidation of chloride ions, the electrochemically active surface area (ECSA) is preferably in the range of 60 m 2 / g or more, 100m 2 / g or more, 200m 2 / g or more, 300m 2 / g or more, 500m 2 / g or more, 700m 2 / g or more or 800m 2 / g or more, and the upper limit is, for example, 1500m 2 / g or less, 1300m 2 / g or less. Preferred ranges of the electrochemically active surface area include the ranges between the respective lower limit values ​​and the respective upper limit values ​​(including the respective lower limit values ​​and the respective upper limit values). A particularly preferred range of the ECSA of ruthenium (IV) oxide in the present invention is 200 to 1500 m 2 / g, 500-1500m 2 / g, 700-1500m 2 / g or 800~1300m 2 / g. The ECSA in the present invention can be calculated from the formula ECSA=Cdl / Cs. In the formula, Cdl is the electrochemical double layer capacitance, which can be calculated by cyclic voltammetry (CV). Cs is the specific capacitance of a sample per unit area under the same electrolyte conditions or the capacitance of an atomically smooth surface of a material. Furthermore, the ruthenium (IV) oxide in the present invention may have oxygen vacancies. When oxygen vacancies are present, the ruthenium (IV) oxide in the present invention is RuO 2-X This can be expressed as O X This indicates that oxygen defects of the order of 1 to 4 have occurred. X is preferably 0.5 or less, and more preferably 0.1 to 0.4. The oxygen defects can be determined from the area ratio of the peak derived from hydroxide groups in the O1s region to the peak derived from lattice oxygen in XPS (X-ray photoelectron spectroscopy).

[0016] Ruthenium (IV) oxide with a crystallite size of 2.5 to 4.0 nm can be produced, for example, by placing a ruthenium salt, an oxidizing agent, and a solvent in a pressure-resistant container and heating the mixture. Specifically, for example, the ruthenium salt and the oxidizing agent are dissolved or dispersed in a solvent, and the resulting solution or dispersion is placed in a pressure-resistant container and heated to perform hydrothermal synthesis. After cooling, the synthesized ruthenium (IV) oxide is recovered by centrifugation or the like, and the recovered product is washed with distilled water or ethanol, if necessary, and dried. If necessary, the dried product may be heated. The ruthenium salt is not particularly limited, but examples thereof include trivalent ruthenium salts, such as ruthenium (III) chloride, which may also be hydrates. The oxidizing agent is not particularly limited as long as it can oxidize ruthenium in the ruthenium salt, and examples thereof include potassium peroxodisulfate (KSO), sodium peroxodisulfate (NaSO), and potassium permanganate. The solvent is not particularly limited as long as it can dissolve or disperse the ruthenium salt and the oxidizing agent, and examples thereof include water. The heating temperature in the hydrothermal synthesis is not particularly limited, and examples thereof include 100 to 200°C. The heating time in the hydrothermal synthesis can be appropriately selected in relation to the heating temperature and the like, and examples thereof include 6 to 48 hours. When the ruthenium(IV) oxide recovered and dried after the hydrothermal synthesis is further heated, the heating temperature is preferably 500°C or less, more preferably 300°C or less, more preferably 100 to 500°C, and more preferably 100 to 300°C. The heating time in this case can be appropriately selected in relation to the heating temperature and the like, and examples thereof include 1 to 12 hours, more preferably 2 to 4 hours. The compounding ratio of the ruthenium salt to the oxidizing agent is not particularly limited, but for example, a molar ratio of ruthenium salt:oxidizing agent of 1:6 to 1:2 is preferred.

[0017] The iridium (IV) oxide in the present invention is not particularly limited, but from the viewpoint of further improving the catalytic activity and the effect of inhibiting the oxidation of chloride ions, it is preferable to use an iridium (IV) oxide having a BET specific surface area of ​​5 m 2 / g or more is preferable, and 10m 2 / g or more is more preferable, and 20m 2 The upper limit is not particularly limited, but for example, 200 m 2 / g, 100m 2 / g, 50m 2 / g, and the range of the BET specific surface area is, for example, 5 to 200 m 2 / g, 10-100m 2 / g, 20-50m 2 / g, etc. From the viewpoint of further improving the catalytic activity and the effect of inhibiting the oxidation of chloride ions, the crystallite size is preferably 50 to 300 nm, more preferably 70 to 200 nm, and even more preferably 100 to 150 nm.

[0018] According to the method for producing oxygen of the present invention, an electrode carrying an oxygen evolution reaction catalyst containing ruthenium (IV) oxide or iridium (IV) oxide is used as the anode, and the temperature of water containing chloride ions is raised to 30°C or higher to electrolyze the water containing the chloride ions. This increases the catalytic activity of ruthenium (IV) oxide and iridium (IV) oxide compared to electrolysis at room temperature. Furthermore, while the oxidation reaction of chloride ions increases and becomes dominant as the current density increases at room temperature, the present invention is excellent in suppressing COR even when the current density in electrolysis is increased, and oxygen can be generated while suppressing the oxidation of chloride ions. In the present invention, the indicator of chloride ion oxidation is not CER (chlorine evolution reaction), which indicates the generation of chlorine (Cl), but rather the oxidation of chloride ions to Cl, ClO. -The chloride oxidation reaction (COR) is an index that includes the reaction that produces HClO. The degree of COR can be determined by the Faraday efficiency of the COR. Furthermore, in the present invention, any method that generates oxygen by electrolyzing water containing chloride ions using an anode supported on an oxygen generation reaction catalyst containing ruthenium (IV) oxide or iridium (IV) oxide is included in the oxygen production method of the present invention. Therefore, for example, even if water containing chloride ions is electrolyzed for the purpose of producing hydrogen, the method is also included in the oxygen production method of the present invention if oxygen is generated as a by-product. Furthermore, the present invention can be used as a method for suppressing the oxidation of chloride ions when electrolyzing water containing chloride ions, and can produce oxygen by electrolyzing water containing chloride ions, such as seawater, while suppressing the oxidation of chloride ions or the generation of chlorine and hypochlorite ions in the aqueous solution. [Example]

[0019] The present invention will be specifically described below with reference to examples of the present invention, but the technical scope of the present invention is not limited to these examples.

[0020] [Example 1] Ruthenium(III) chloride n-hydrate (99.9%, Fujifilm Wako Pure Chemical Industries, Ltd.) 0.31 2 g of ruthenium dioxide and 1.161 g of potassium peroxodisulfate (95%, Fujifilm Wako Pure Chemical Industries, Ltd.) were added to 30 mL of ion-exchanged water and ultrasonically dispersed. The resulting mixed aqueous solution was transferred to a Teflon-lined autoclave and kept at 160°C for 24 hours. After 24 hours, it was cooled and the precipitate was centrifuged and washed three times with distilled water and ethanol. The supernatant was discarded, and the resulting precipitate was vacuum-dried at room temperature for 12 hours. In this way, ruthenium (IV) oxide of Example 1 was synthesized.

[0021] [Example 2] Commercially available ruthenium(IV) oxide (99.9% Sigma-Aldrich, CAS: 12036-10-1) was used as the ruthenium(IV) oxide in Example 2.

[0022] [Examples 3 to 5] The ruthenium oxide obtained in Example 1 was heat-treated in air for 3 hours. The ruthenium oxide heat-treated at 200°C was used as the ruthenium oxide of Example 3, the ruthenium oxide heat-treated at 400°C was used as the ruthenium oxide of Example 4, and the ruthenium oxide heat-treated at 600°C was used as the ruthenium oxide of Example 5.

[0023] [Example 6] Commercially available iridium(IV) oxide (STREM Chemicals, INC.) was used as the iridium(IV) oxide in Example 3.

[0024] [Structural analysis] The crystal structures of the ruthenium oxides of Examples 1 to 5 and the iridium oxide of Example 6 were examined by X-ray diffraction (X-ray diffractometer: Ultima IV, Rigaku Corporation, CuKα radiation used). The results for the ruthenium oxides of Examples 1 to 5 are shown in FIG. 1, and the results for the iridium oxide of Example 6 are shown in FIG. 2. In FIG. 1, the ruthenium oxide of Example 1 is represented as S-RuOx, the ruthenium oxide of Example 2 is represented as commercially available RuO2, and the ruthenium oxides of Examples 3 to 5 are represented as H200-RuOx, H400-RuOx, and H600-RuOx, respectively. Hereinafter, these names may be used to refer to the ruthenium oxides of each Example. In FIG. 2, the iridium oxide of Example 6 is represented as commercially available IrO2. Hereinafter, this name may be used to refer to the iridium oxide of Example 6.

[0025] From Figure 1, the peaks obtained for commercial RuO2 (Example 2) matched the XRD pattern of typical tetragonal RuO2. Broad peaks were observed for S-RuOx (Example 1), and the XRD pattern resembled that of commercial RuO2. H200-RuOx (Example 3), H400-RuOx (Example 4), and H600-RuOx (Example 5) showed sharper peaks as the heat treatment temperature increased, and a typical RuO2 diffraction pattern was observed for H600-RuOx. The full width at half maximum (FWHM) of the peak corresponding to the 110 direction in each crystal was 110) and calculated the crystallite size (L). Specifically, the full width at half maximum B and Bragg angle θ were estimated from the lowest angle diffraction peak (110), and the crystallite size was calculated using the Scherrer formula (L = Kλ / B cosθ). Here, Scherrer's constant K = 0.9, and the X-ray wavelength λ = 0.154051 nm. The crystallite size of S-RuOx was 3.02 nm, that of H200-RuOx was 3.22 nm, that of H400-RuOx was 3.75 nm, and that of H600-RuOx was 11.5 nm. The crystallite size of commercially available RuO2 was 24.1 nm. The crystallite size of iridium oxide (commercially available IrO2) in Example 6 was calculated from Figure 2 and was 110 nm. The crystallite size of commercially available IrO2 was calculated by estimating the full width at half maximum B and the Bragg angle θ from the diffraction peak (111) at 2θ = 41° and using the Scherrer equation (L = Kλ / Bcosθ), where K is the Scherrer constant and λ is the X-ray wavelength.

[0026] [Electrochemical property evaluation] (Preparation of catalyst ink) Commercially available RuO2, S-RuOx, and the iridium oxide of Example 6 were used as catalysts. 0.005 g of catalyst, 0.005 g of carbon black, 95 μL of Nafion (5 wt % alcohol dispersion), 350 μL of water, and 350 μL of ethanol were mixed and ultrasonically dispersed for 1 hour to prepare catalyst inks. (Electrode preparation) 10 μL of each of the above catalyst inks was weighed out and cast onto the disk part (Φ5.0 mm, glassy carbon) of a rotating ring-disk electrode (RRDE). The solvent was removed to prepare a catalyst-loaded electrode (disk electrode). The catalyst loading on the electrode was 0.32 mg / cm. 2 The ring part (ring electrode) was made of platinum. (Electrochemical measurements) Using the obtained electrode, the COR faradaic efficiency was calculated based on constant current electrolysis and the electrochemically active surface area (ECSA) was measured.

[0027] (Calculation of COR Faraday efficiency based on constant current electrolysis) The electrodes were immersed in the electrolyte and the current was 10 mA / cm 2 , 20mA / cm 2 , 40mA / cm 2 , 60mA / cm 2 , 80mA / cm 2 and 100mA / cm 2 Constant current electrolysis was performed at each constant current. Constant current electrolysis was performed in a 0.5M NaCl aqueous solution, and the electrodes were rotated at 1600 rpm during measurement. In both cases, the amount of electricity passing was 40 mC / cm. 2 The galvanostatic electrolysis was terminated when the electrolytic solution reached 100°C. This galvanostatic electrolysis was performed by varying the temperature of the electrolyte (0.5M NaCl aqueous solution) in which the electrodes were immersed. The electrolyte was heated before the measurement (Cool Stirrer CPS-300, Synix Co., Ltd.) and stirred at 200 rpm using a stirrer while maintaining the temperature at 25°C, 40°C, 50°C, and 70°C until the end of the electrolysis. The amount of residual chlorine species in the electrolyte after the electrolysis was measured by the diethyl paraphenylenediamine method (DPD method). N,N-diethylbenzene-1,4-diamine (DPD) was added to the electrolyte after the electrolysis, and the amount of residual chlorine species was determined based on the absorption peak intensity at 551 nm of the solution (measured by JASCO V-670DS, JASCO Corporation). The amount of residual chlorine species ([ClO ― ] (mol / L)), the COR faradaic efficiency was calculated using the following formula: COR is the Faraday efficiency based on the amount of residual chlorine species, V is the volume of the electrolyte (L), n is the number of reaction electrons (COR is 2), F is the Faraday constant (96,485 C / mol), and Q is the amount of electricity passed during electrolysis (C / cm 2 ), A is the geometric area of ​​the electrode (cm 2 At the pH of the electrolyte used in this measurement, the oxidation product of chloride ions is almost entirely ClO - Therefore, the residual chlorine species amount is [ClO ― ] (mol / L).

[0028]

number

[0029] When S-RuOx was used as the catalyst, the electrolyte temperature was set to 40°C (Example 1(40)), 50°C (Example 1(50)), 70°C (Example 1(70)), and 25°C (Comparative Example 1). Figure 3 shows the relationship between the constant current value and the COR Faraday efficiency when S-RuOx was used as the catalyst. When commercially available RuO2 was used as the catalyst, the electrolyte temperature was set to 40°C (Example 2(40)), 50°C (Example 2(50)), 70°C (Example 2(70)), and 25°C (Comparative Example 2). Figure 4 shows the relationship between the constant current value and the COR Faraday efficiency when commercially available RuO2 was used as the catalyst. When the iridium oxide of Example 6 was used as the catalyst, the temperature of the electrolyte was set to 40°C (Example 6(40)), 50°C (Example 6(50)), 70°C (Example 6(70)), and 25°C (Comparative Example 3). Figure 5 shows the relationship between the constant current value and the COR Faraday efficiency when the iridium oxide of Example 6 was used as the catalyst.

[0030] (BET specific surface area measurement) The BET specific surface areas of S-RuOx, H200-RuOx, 600-RuOx, and commercially available RuO2 were measured using a flow-type automatic surface area analyzer (FlowSorb III 2305, Micrometrics). 0.1 g of sample was placed in a column, the column was evacuated, and the column was heated at 100°C for 1 hour. The column was then immersed in liquid nitrogen to bring the temperature inside the column to the same temperature as liquid nitrogen. Nitrogen gas was then introduced into the column, and the relative pressure and adsorption amount were measured. The BET specific surface area was calculated using the multipoint BET method from multiple points (4-5 points) at relative pressures between 0.05 and 0.30. Figure 6 shows the measurement results. The BET specific surface area was 153.23 m for S-RuOx. 2 / g, H200-RuOx 145.39m 2 / g, H600-RuOx 19.03m 2 / g, and 9.67m for commercial RuO2 2 The BET specific surface area of ​​commercially available IrO2 was also measured in the same way, and it was 25 m 2 / g.

[0031] (Measurement of electrochemically active surface area (ECSA)) The ECSA of S-RuOx, H200-RuOx, H400-RuOx, 600-RuOx, and commercial RuO2 was calculated using the formula Cdl / Cs × (catalyst loading = 0.32 mg / cm), respectively. 2 ) was calculated as a value per gram of catalyst. Cdl is the electrochemical double layer capacitance and was calculated by cyclic voltammetry (CV). Figure 7 shows the results of CV measurements. Figure 7(a) shows the results for S-RuOx, Figure 7(b) shows the results for H200-RuOx, Figure 7(c) shows the results for H400-RuOx, Figure 7(d) shows the results for 600-RuOx, and Figure 7(e) shows the results for commercial RuO2. CV was performed using a three-electrode cell with the same electrode as used in the calculation of the COR Faraday efficiency based on galvanostatic electrolysis as the working electrode, a platinum mesh as the control electrode, and a Hg / HgO as the reference electrode. The electrolyte used was a 1.0M KOH aqueous solution purged with N2 for 30 minutes. The potential range of the CV was 1.05–1.15 V vs. RHE, which is the non-Faraday region, and the sweep rate was 2–10 mV / s. Cdl was calculated from the plot (Fig. 8) of the relationship between the difference Δj between the cathodic and anodic currents at a specific potential (1.10 V vs. RHE) for each sample and the sweep rate. The slope of the fitted line obtained from the plot corresponds to Cdl, and the values ​​written along the line in Fig. 8 are the Cdl values. Cs is the specific capacitance of the sample per unit area under the same electrolyte conditions or the capacitance of an atomically smooth surface of the material. Cs is the specific capacitance of an atomically clean metal in a 1.0 M KOH aqueous solution, and is 0.04 mF / cm, which is a typical value for nickel or platinum. 2 The ECSA value was 1229.30m for S-RuOx. 2 / g, H200-RuOx 888.98m 2 / g, H400-RuOx 315.47m 2 / g, H600-RuOx 150.39m 2 / g, and 71.72m for commercial RuO2 2 / g.

[0032] 3 to 5, in Comparative Examples 1 to 3, in which the temperature of the electrolyte was 25°C, the COR Faradaic efficiency increased with increasing current, resulting in COR selectivity. On the other hand, Examples 1 (40) to (70), Examples 2 (40) to (70), and Examples 6 (40) to (70) showed lower COR Faradaic efficiencies than Comparative Examples 1 to 3, and the increase in COR Faradaic efficiency with increasing current was also suppressed. As can be seen from FIG. 3, when S-RuOx was used as a catalyst, Examples 1 (40) to (70) showed lower COR Faradaic efficiencies than Comparative Example 1 at almost all current values, and Example 1 (70) showed a COR Faradaic efficiency of 80 mA / cm. 2 Maintain 0% OER selectivity up to 100 mA / cm 2 In Example 1 (50), the COR Faraday efficiency was only about 13.4%. 2 Up to 0% and 80mA / cm 2 Even in Example 1 (40), the COR Faraday efficiency in the high current range was significantly lower than that of Comparative Example 1. From Figure 4, even when commercially available RuO2 was used as a catalyst, Examples 2 (40) to (70) showed a COR of 10 to 40 mA / cm 2 In particular, Example 2 (70) exhibited a COR Faraday efficiency of 10 mA / cm 2 The COR Faraday efficiency is about 10% at 10-60mA / cm 2 In the region of 1000 mA / cm, the COR Faraday efficiency was significantly lower than that of Comparative Example 2. H200-RuOx, H400-RuOx, and H600-RuOx have structures between S-RuOx and commercially available RuO2, and are therefore considered to have properties between S-RuOx and commercially available RuO2. In the present invention, the temperature of the electrolyte and the ruthenium (IV) oxide used can be appropriately selected according to the operating environment, such as the current density. From Figure 5, when commercially available IrO2 was used as a catalyst, the COR Faraday efficiency was 10 mA / cm 2 The COR Faraday efficiency at 20 mA / cm was approximately 55% in Comparative Example 3, while it was almost 0% in Example 6 (70), approximately 20% in Example 6 (50), and approximately 35% in Example 6 (40). 2In the above cases, the COR Faraday efficiency was 100% or more, but it was significantly lower than that of Comparative Example 3 in all of Examples 6(40) to (70). [Industrial Applicability]

[0033] The oxygen producing method of the present invention can suppress the oxidation of chloride ions in the electrolysis of water, etc., and therefore can be suitably used in the electrolysis of aqueous solutions containing chloride ions, for example, seawater, etc. Furthermore, the oxygen producing method of the present invention can be suitably used as a method for producing oxygen in the electrolysis of seawater, etc., and suppressing the oxidation of chloride ions or the generation of chlorine and hypochlorite ions in the aqueous solution.

Claims

1. An oxygen production method in which oxygen is produced by electrolyzing water containing chloride ions using an electrode supported on an oxygen generation reaction catalyst containing ruthenium (IV) oxide or iridium (IV) oxide as an anode, wherein the electrolysis is carried out at a temperature of the water containing chloride ions of 30°C or higher.

2. 2. The method for producing oxygen according to claim 1, wherein the electrolysis is carried out at a temperature of 40° C. or higher in the water containing chloride ions.

3. 3. The method for producing oxygen according to claim 1, wherein the crystallite size of the ruthenium (IV) oxide is 2.5 to 4.0 nm.

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