Electrode for oxidation reaction, electrochemical reaction device and artificial photosynthesis device

Abstract

To provide an oxidation electrode with excellent durability, and an electrochemical reaction device and an artificial photosynthesis device using the same.SOLUTION: The oxidation electrode constituted of a substrate and iridium oxide supported thereon as an oxidation reaction catalyst, has an amount of hydrated water and a hydroxy group contained in iridium oxide of more than 0 and less than 0.36.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to an electrode for oxidation reaction, and an electrochemical reaction device and an artificial photosynthesis device using the same. [Background technology]

[0002] Artificial photosynthesis, which synthesizes formic acid (HCOOH), carbon monoxide (CO), etc. from water (H2O) and carbon dioxide (CO2) using only solar energy, is expected to be one of the technologies for reducing CO2 emissions to prevent global warming, and research into it is being conducted very actively. In the artificial photosynthesis reaction, H2O is oxidized at the oxidation reaction electrode, supplying electrons and protons, and O2 is produced. These electrons and protons are used to reduce CO2 at the reduction reaction electrode. When a nearly neutral electrolyte is used, iridium oxide (IrOx) is used as a catalyst for the oxidation reaction.

[0003] When an electrode for oxidation reaction using IrOx as a catalyst is operated in an electrolyte, IrOx particles fall off the electrode, causing a drop in current density within a short period of time, resulting in a problem of low durability of the electrode.

[0004] Patent Document 1 describes forming a protective layer such as a glass coating or a resin coating on the current collecting electrode on the surface of a substrate used for an oxidation reaction electrode or a reduction reaction electrode. The method for producing an oxidation reaction electrode is described as coating an IrOx nanocolloid solution on a substrate and evaporating the water solvent by heating. Non-Patent Document 1 also contains a similar description.

[0005] In Patent Document 1 and Non-Patent Document 1, in the process of applying IrOx, an IrOx nanocolloid solution is dropped onto a substrate, and then the substrate is heated in air at 60°C to evaporate the water. However, the oxidation electrode manufactured by this process cannot prevent the IrOx particles from falling off from the electrode during operation. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 2019-052347 [Non-patent literature]

[0007] [Non-Patent Document 1] "A monolithic device for CO2 photoreduction to generate liquid organic substances in a single-compartment reactor", Energy Environ. Sci., 2015, 8, pp.1998-2002 Summary of the Invention [Problem to be solved by the invention]

[0008] An object of the present invention is to provide an oxidation reaction electrode having good durability, and an electrochemical reaction device and an artificial photosynthesis device using the same. [Means for solving the problem]

[0009] The present invention provides an oxidation reaction electrode in which iridium oxide is supported on a substrate as an oxidation reaction catalyst, and the amount of water of hydration and hydroxyl groups contained in the iridium oxide is: 0.029 or more It is an electrode for an oxidation reaction, having a conductivity of less than 0.36.

[0010] In the oxidation reaction electrode, the substrate is preferably a fluorine-doped tin oxide glass substrate or a titanium substrate.

[0011] In the oxidation reaction electrode, the iridium oxide is preferably iridium oxide particles made from potassium hexachloroiridate as a starting material.

[0012] The present invention is an electrochemical reaction device comprising the above-described oxidation reaction electrode, a reduction reaction electrode, and an electrolyte solution.

[0013] In the electrochemical reaction device, the reduction reaction electrode preferably contains a ruthenium complex.

[0014] In the electrochemical reaction device, the electrolyte is preferably an aqueous phosphate buffer solution having a pH of 6 to 8, and the electrochemical reaction device is preferably a device for reducing carbon dioxide.

[0015] The present invention is an artificial photosynthesis device including the electrochemical reaction device and a solar cell that generates electricity to be supplied to the oxidation reaction electrode and the reduction reaction electrode. [Effects of the Invention]

[0016] According to the present invention, it is possible to provide an oxidation reaction electrode having good durability, as well as an electrochemical reaction device and an artificial photosynthesis device using the same. [Brief explanation of the drawings]

[0017] [Figure 1] FIG. 10 is a diagram showing the IR spectrum of IrOx on a small anode electrode produced in Comparative Example 2. [Figure 2] FIG. 1 shows measurement locations (nine circled locations) of IR spectra in a 1 cm square IrOx-coated area on an FTO substrate in Examples and Comparative Examples. [Figure 3] 10 is a graph showing the results of CV measurement of each anode electrode. [Figure 4] 10 is a graph showing the results of it measurements for (a) Comparative Example 2 and (b) Example 2. [Figure 5] (a) Photographs showing secondary electron images of the surface of an FTO glass substrate and (b) the surface of an electrode after catalyst application (initial stage). [Figure 6] 10 is a photograph showing secondary electron images of the electrode surfaces of (a) Comparative Example 2 and (b) Example 2 after a durability test. DETAILED DESCRIPTION OF THE INVENTION

[0018] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The following describes an embodiment of the present invention. The embodiment is an example of implementing the present invention, and the present invention is not limited to the embodiment.

[0019] [Oxidation reaction electrode] The oxidation reaction electrode, which is a positive electrode (anode electrode), is an electrode used to oxidize a substance through an oxidation reaction. The oxidation reaction electrode according to this embodiment is an oxidation reaction electrode in which iridium oxide is supported on a substrate as an oxidation reaction catalyst, and the amount of hydrated water and hydroxyl groups contained in the iridium oxide (hydrated water / hydroxyl group amount) is greater than 0 and less than 0.36. The hydrated water / hydroxyl group amount is more preferably 0.029 or more and less than 0.36, even more preferably greater than 0.029 and 0.29 or less, even more preferably 0.076 or more and 0.29 or less, and particularly preferably 0.12 or more and 0.29 or less.

[0020] Here, the amount of hydration water and hydroxyl groups contained in iridium oxide (hydration water / hydroxyl group amount) is measured in the wavenumber range of 2900 to 3650 (cm) in the IR spectrum of IrOx on the electrode measured by Fourier transform infrared spectroscopy (FT-IR). -1 ) hydration water and hydroxyl group peak areas were calculated from 800 to 1200 (cm -1 ) divided by the peak area of ​​IrO.

[0021] By controlling the hydration water / hydroxyl group ratio of the oxidation reaction catalyst (IrOx particles) supported on the electrode within the above-mentioned range, durability in the oxygen generation reaction can be significantly improved. In particular, when the hydration water / hydroxyl group ratio is 0.076 to 0.29, durability can be significantly improved while maintaining high catalytic activity.

[0022] As described above, there has been a problem with IrOx particles falling off the electrode during operation in an electrolyte, resulting in a rapid decrease in current density. The inventors discovered that by controlling the amount of hydration water / hydroxyl groups contained in the IrOx coated on a substrate within a predetermined range, high catalytic activity can be maintained and catalyst particle shedding from the electrode can be suppressed. This significantly improves the durability of the oxidation electrode. This is thought to be because a high amount of hydration water contained in IrOx particles weakens the bonding strength between particles, making them more likely to fall off during operation. On the other hand, the hydroxyl groups on the surface of IrOx particles contribute to high catalytic activity. Heat treatment of the electrode at high temperatures is thought to reduce the hydroxyl groups contained in IrOx, thereby reducing activity. The processes described in Patent Document 1 and Non-Patent Document 1 are insufficient in reducing hydration water and therefore cannot suppress IrOx particle shedding from the electrode during operation.

[0023] In order to set the amount of hydrated water and hydroxyl groups (hydrated water / hydroxyl group amount) contained in iridium oxide within the above-mentioned predetermined range, there are no particular limitations as long as the hydrated water / hydroxyl group amount is within the above-mentioned predetermined range. For example, a method of drying under reduced pressure at a predetermined temperature for a period of time using a vacuum constant temperature drying apparatus or the like can be used. Conditions for vacuum constant temperature drying include, for example, a reduced pressure of 0.1 to 30 Pa, preferably a reduced pressure of 1 to 10 Pa, a temperature of 40 to 150°C, preferably a temperature of 40 to 100°C, and a time of 1 to 12 hours, preferably 4 to 8 hours. For example, it is thought that with vacuum constant temperature drying, the hydrated water is first reduced, and then the hydroxyl groups are reduced once a certain temperature is exceeded. Therefore, it is desirable to use conditions that minimize the reduction of hydrated water while minimizing the reduction of hydroxyl groups.

[0024] The oxidation reaction electrode is configured to include, for example, a substrate having a conductive layer formed thereon and an oxidation reaction catalyst layer formed thereon.

[0025] The substrate is a member that structurally supports the electrode. The substrate is not particularly limited in material, but examples thereof include glass substrates. The substrate may include, for example, a metal or a semiconductor. The metal used as the substrate is not particularly limited, but examples thereof include silver (Ag), gold (Au), copper (Cu), zinc (Zn), indium (In), cadmium (Cd), tin (Sn), palladium (Pd), lead (Pb), and titanium (Ti). The semiconductor used as the substrate is not particularly limited, but examples thereof include titanium oxide (TiO), tin oxide (SnO), silicon (Si), strontium titanate (SrTiO), zinc oxide (ZnO), and tantalum oxide (TaO). When the substrate includes a metal or a semiconductor, an insulating layer may be formed between the conductive layer and the substrate. The insulating layer is not particularly limited, but examples thereof include semiconductor oxides, nitrides, and resins. The metal substrate and the conductive layer may be electrically connected directly. In order to make the oxidation reaction electrode translucent, it is preferable that the substrate is made of, for example, a glass substrate or plastic. When a metal substrate is used, the conductive layer may not be present. A composite substrate of carbon fiber and carbon may be used as the substrate. This has the advantages of ensuring light weight and high conductivity and eliminating the need for a separate conductive layer.

[0026] The conductive layer is provided to improve current collection efficiency at the oxidation electrode. The conductive layer is not particularly limited, but examples thereof include transparent conductive layers such as indium tin oxide (ITO), fluorine-doped tin oxide (FTO), and zinc oxide (ZnO). In particular, fluorine-doped tin oxide (FTO) is preferred in terms of thermal and chemical stability.

[0027] Of the above, the substrate is preferably a titanium substrate from the viewpoints of chemical stability, corrosion resistance, etc. From the same viewpoint, the substrate is preferably a fluorine-doped tin oxide glass substrate in which a fluorine-doped tin oxide conductive layer, which is a transparent conductive material, is formed on a glass plate.

[0028] The oxidation reaction catalyst layer contains iridium oxide (IrOx) with oxidation catalytic function. Ruthenium oxide, for example, may also be included as a material with oxidation catalytic function. Iridium oxide or ruthenium oxide can be supported on the surface of a conductive layer or substrate as a nanocolloid solution (see T. Arai et al., Energy Environ. Sci 8, 1998 (2015)).

[0029] The iridium oxide is preferably iridium oxide particles made from potassium hexachloroiridate (K2IrCl6) as a starting material, because it has high activity, for example, high activity over a wide pH range.

[0030] [Electrochemical reactors and artificial photosynthetic devices] The electrochemical reaction device according to this embodiment is a device including the above-mentioned oxidation reaction electrode, which is an anode electrode that causes an oxidation reaction, a reduction reaction electrode, which is a cathode electrode that promotes a reduction reaction of carbon dioxide, etc., and an electrolytic solution in which an electrolyte is dissolved in a solvent. In the electrochemical reaction device, for example, the oxidation reaction electrode and the reduction reaction electrode are immersed in the electrolytic solution.

[0031] The artificial photosynthesis device of this embodiment is an electrochemical reaction device that includes the above-mentioned oxidation reaction electrode, which is an anode electrode that causes an oxidation reaction, a reduction reaction electrode, which is a cathode electrode that promotes a reduction reaction of carbon dioxide or the like, and an electrolyte solution in which an electrolyte is dissolved in a solvent, and a solar cell that generates electricity to be supplied to the oxidation reaction electrode and the reduction reaction electrode.

[0032] The electrochemical reaction device is, for example, a device in which an electrolytic solution obtained by dissolving an electrolyte in a solvent is contained in a container, and the oxidation reaction electrode that causes an oxidation reaction and the reduction reaction electrode that promotes a reduction reaction of carbon dioxide or the like are immersed in the electrolytic solution, and the oxidation reaction electrode and the reduction reaction electrode are electrically connected. The electrolytic solution contains a reaction substrate such as carbon dioxide. The reaction substrate such as carbon dioxide may be saturated in the electrolytic solution in advance, or a reaction substrate gas such as carbon dioxide gas may be flowed through the system.

[0033] When an appropriate bias voltage is applied between the oxidation electrode and the reduction electrode, an oxidation reaction occurs at the oxidation electrode, where carbon dioxide (CO2) is reduced to produce carbon monoxide (CO) and formic acid (HCOOH).

[0034] The oxidation reaction electrode and the reduction reaction electrode are electrically connected to each other, and an appropriate bias voltage is applied. The means for applying the bias voltage is not particularly limited, and examples thereof include a chemical battery (including a primary battery, a secondary battery, etc.), a constant voltage source, a solar cell, etc. In this case, it is sufficient that the positive electrode is connected to the oxidation reaction electrode and the negative electrode is connected to the reduction reaction electrode.

[0035] By using a solar cell as a means for applying a bias voltage, an artificial photosynthesis device can be provided that includes the electrochemical reaction device and a solar cell that generates power to be supplied to the oxidation reaction electrode and the reduction reaction electrode. When a solar cell is used as a means for applying a bias voltage, the positive electrode of the solar cell can be connected to the oxidation reaction electrode, and the negative electrode can be connected to the reduction reaction electrode. In the artificial photosynthesis device of this embodiment, the oxidation reaction electrode and the reduction reaction electrode of the electrochemical reaction device are connected via a solar cell, and the device is powered by sunlight as an energy source.

[0036] When synthesizing formic acid (HCOOH) from carbon dioxide (CO2), water (H2O) is oxidized to provide electrons and protons to carbon dioxide (CO2). At a pH of around 7, the oxidation potential of water (H2O) is 0.82 V, and the reduction potential is -0.41 V (both on the standard hydrogen electrode (NHE)). The reduction potentials of carbon dioxide (CO2) to carbon monoxide (CO), formic acid (HCOOH), and methyl alcohol (CH3OH) are -0.53 V, -0.61 V, and -0.38 V, respectively. Therefore, the potential difference between the oxidation and reduction potentials is 1.20 to 1.43 V. When reducing carbon dioxide (CO2), a carbon compound, a suitable solar cell configuration is one in which four to six crystalline silicon solar cells are connected in series, or an amorphous silicon triple-junction solar cell.

[0037] (electrode for reduction reaction) The reduction reaction electrode, which is a negative electrode (cathode electrode), is an electrode used to reduce a substance by a reduction reaction. The reduction reaction electrode is configured, for example, to include a conductive layer and a conductor layer formed in this order on a substrate.

[0038] The substrate is a member that structurally supports the electrode. The substrate is not particularly limited in material, but examples thereof include glass substrates. The substrate may include, for example, a metal or a semiconductor. The metal used as the substrate is not particularly limited, but examples thereof include silver (Ag), gold (Au), copper (Cu), zinc (Zn), indium (In), cadmium (Cd), tin (Sn), palladium (Pd), lead (Pb), and titanium (Ti). The semiconductor used as the substrate is not particularly limited, but examples thereof include titanium oxide (TiO), tin oxide (SnO), silicon (Si), strontium titanate (SrTiO), zinc oxide (ZnO), and tantalum oxide (TaO). When the substrate includes a metal or semiconductor, an insulating layer may be formed between the conductive layer and the substrate. The insulating layer is not particularly limited, but examples thereof include semiconductor oxides, nitrides, and resins. The metal substrate may be electrically connected directly to the conductive layer and the conductor layer, or when a metal substrate is used, the conductive layer may not be provided.

[0039] The substrate may contain carbon fiber. The substrate may be, for example, a composite substrate of carbon and carbon heat-treated at high temperature, such as carbon paper or carbon cloth. Carbon paper is prepared by impregnating organic fibers such as polyacrylonitrile (PAN) fibers in a dispersion of polyvinyl alcohol and an aqueous medium, carbonizing the fibers at approximately 2000°C, and binding the fibers together to form a sheet. Carbon paper may also contain approximately 25% by mass of Teflon (registered trademark)-based materials. Carbon cloth is woven from carbon fibers obtained by baking and carbonizing organic fibers. Carbon paper and carbon cloth are porous and have countless pores measuring several tens of micrometers (approximately 10 μm to 100 μm). The thickness of the carbon paper or carbon cloth is, for example, in the range of 0.1 mm to 0.4 mm per sheet. This composite substrate may be multilayered (e.g., 5 to 10 layers) to a thickness of approximately 10 times, in the range of 1 mm to 4 mm. A multilayered composite substrate can be formed, for example, by laminating multiple pieces of carbon paper, carbon cloth, etc. using a carbon-based adhesive containing a conductive carbon material, such as a polymer (e.g., an acrylic polymer binder) containing graphite or graphene as the adhesive.

[0040] The conductive layer is provided to improve current collection efficiency at the reduction reaction electrode. The conductive layer is not particularly limited, but examples include transparent conductive layers such as indium tin oxide (ITO), fluorine-doped tin oxide (FTO), and zinc oxide (ZnO). In particular, fluorine-doped tin oxide (FTO) is preferred in terms of thermal and chemical stability.

[0041] The conductor layer is composed of a conductor containing a material with reduction catalytic function. The conductor can be composed of a material containing a carbon material (C). The size of the single unit of the carbon material structure is preferably 1 nm or more and 1 μm or less. Examples of the carbon material include at least one of carbon nanotubes such as multi-walled carbon nanotubes (MWCNTs), graphene, and graphite. Graphene and graphite are preferably sized 1 nm or more and 1 μm or less. Carbon nanotubes are preferably sized 1 nm or more and 40 nm or less. The conductor can be formed, for example, by spraying a carbon material mixed with a liquid such as ethanol and heating it. Spin coating may be used instead of spraying. Alternatively, the solution may be directly dripped and dried to apply the coating without using spin coating.

[0042] A complex catalyst or the like can be used as a material having a reduction catalytic function. The complex catalyst is preferably, for example, a ruthenium complex. Examples of the complex catalyst include [Ru{4,4'-di(1-H-1-pyrrolypropyl carbonate)-2,2'-bipyridine}(CO)(MeCN)Cl2], [Ru{4,4'-di(1-H-1-pyrrolypropyl carbonate)-2,2'-bipyridine}(CO)2Cl2], and [Ru{4,4'-di(1-H-1-pyrrolypropyl carbonate)-2,2'-bipyridine}(CO)2]. nand [Ru{4,4'-di(1-H-1-pyrrolypropyl carbonate)-2,2'-bipyridine}(CO)(CHCN)Cl]. Furthermore, the ruthenium complex may be a Ru complex polymer in which a Ru complex containing a Ru complex monomer and a polymerization initiator (e.g., pyrrole and an oxidizing agent (e.g., an Fe-based oxidizing agent such as iron chloride) is polymerized. The reduction reaction electrode is, for example, an electrode containing a carbon-based material (carbon nanotubes such as multi-walled carbon nanotubes (MWCNTs) and graphite) and containing a Ru complex polymer as a reduction catalyst and an Fe-based oxidizing agent (FeCl).

[0043] The support of a reduction catalyst can be produced, for example, by dissolving a metal complex (catalyst) in an acetonitrile (MeCN) solution on a carbon-based material such as carbon paper or carbon cloth, and then drying the solution. For example, the support of a Ru complex polymer can be produced by dissolving a Ru complex monomer, a polymerization initiator (e.g., pyrrole, and an oxidant (e.g., an Fe-based oxidant such as iron chloride)) in a solvent such as acetonitrile (MeCN) to form a solution (Ru complex polymer solution) on a carbon-based material such as carbon paper or carbon cloth, and then drying the solution. The support of a reduction catalyst can also be produced by electropolymerization. For example, a carbon-based material electrode can be used as the working electrode, a glass substrate coated with fluorine-containing tin oxide (FTO) as the counter electrode, and an Ag / Ag reference electrode can be used. + Using an electrode, Ag / Ag was used in an electrolyte containing a reduction catalyst. + After applying a cathodic current to the electrode to create a negative voltage, Ag / Ag + By passing an anode current so that the electrode is at a positive potential, the carbon-based material can be supported as a reduction catalyst. For example, acetonitrile (MeCN) can be used as the electrolyte solution, and tetrabutylammonium perchlorate (TBAP) can be used as the electrolyte.

[0044] (electrolyte) Examples of the electrolyte include an aqueous phosphate buffer solution and an aqueous borate buffer solution, and from the viewpoint that a neutral state is suitable as a practical electrolyte solution, an aqueous phosphate buffer solution having a pH of 6 to 8 is preferred.

[0045] (reaction substrate) The reaction substrate may be, for example, a carbon compound, such as carbon dioxide (CO2).

[0046] (container) The container is a member that supports the reduction reaction electrode and the oxidation reaction electrode and also contains the electrolyte. The container is made of a material that has the mechanical strength required to configure the electrochemical reaction device as a cell. For example, the container can be made of metal, plastic, etc. [Example]

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

[0048] <Examples 1 to 6, Comparative Examples 1 and 2> 1. Experimental Method 1.1. Preparation of FTO / IrOx anode electrode First, we synthesized iridium oxide (IrOx) nanocolloids. A 50 mL solution of 2 mM potassium chloroiridate(IV) (K2IrCl6) was combined with 10% by weight of sodium hydroxide (NaOH) to adjust the pH to 13, and the solution was heated at 90°C for 20 minutes using a hot stirrer. This solution was cooled in ice water for 1 hour, and then 3 M nitric acid (HNO3) was added dropwise to adjust the pH to 1. After stirring for 80 minutes, an IrOx nanocolloid aqueous solution was obtained. A 1.5% by weight of NaOH aqueous solution (1-2 mL) was added dropwise to this solution to adjust the pH to 12.

[0049] Next, the catalyst was applied to an FTO glass substrate. A 1.5 cm x 2 cm FTO glass substrate was masked with tape, leaving a 1 cm square area for application. 125 μL of the above-mentioned IrOx nanocolloid aqueous solution was dropped onto this area. The substrate was heated in a 60°C oven for 1 hour to evaporate the water, and the electrode surface was washed with pure water to remove the precipitated salt. This process was repeated six times to produce an anode electrode.

[0050] 1.2. Heat treatment conditions The IrOx-coated anode electrodes were subjected to vacuum heating treatment for 6 hours using a rectangular constant-temperature vacuum dryer (Yamato Scientific Co., Ltd., DP610) at 40°C (Example 1), 60°C (Example 2), 80°C (Example 3), 100°C (Example 4), 120°C (Example 5), and 150°C (Example 6). Furthermore, an electrode was prepared by heating in air at 500°C for 30 minutes (Comparative Example 1), and a conventional electrode was prepared by coating with IrOx and storing it in a desiccator at room temperature (25±2°C) for one week (Comparative Example 2). The anode electrode in Comparative Example 2 was prepared using the same process as in Patent Document 1 and Non-Patent Document 1.

[0051] 1.3. Quantification of the amount of hydration water / hydroxyl groups of IrOx on the electrode The equipment used was a Fourier transform infrared spectrophotometer (FT-IR) (Thermo Fisher, NICOLET iS50). The measurement conditions were ATR (using SMART-iTR (Ge crystal)), measurement area: φ3 mm, wavenumber resolution: 4 cm. -1 , Number of accumulations: 64, Detector: TDGS KBr, 600-4000 cm -1 The IR spectrum was measured in the wave number range of 1 cm. Figure 1 shows the IR spectrum of the electrode of Comparative Example 2. As shown in Figure 2, measurements were made at nine locations on a 1 cm square IrOx coated area.

[0052] The hydration amount / hydroxyl group amount of IrOx was quantified using the analysis software OMNIC (version: 9.12.923). The measured IR spectrum was opened and the peak area range and background range were calculated using the peak area tool from 2900 to 3650 cm. -1A background line is created from the selected background range, and the area of ​​the peak area range is calculated. The displayed value of the peak area is read. -1 The same procedure was performed for the IR spectrum measured at nine points. Wavenumber range: 2900-3650 cm -1 The peak area (sum of nine points) is calculated from 800 to 1200 cm -1 The value obtained by dividing the peak area by the sum of the nine peak areas was calculated as the quantitative value of the amount of water of hydration / hydroxyl groups in IrOx.

[0053] 1.4.Method for measuring the amount of Ir on the electrode The equipment used was an X-ray fluorescence analyzer (XRF) (Rigaku Corporation, ZSX Primus II). The measurement area was a measurement diameter of φ20 mm, and the X-ray intensity of iridium (Ir) was measured over the entire area coated with iridium oxide (IrOx). The measurement ray for iridium (Ir) was Ir-Lα ray. The peak angle was set to 39,200°, where the X-ray intensity is highest. In addition, the background angles were set to 38,000° and 40,500°, where the X-ray intensity is flat on both ends of the Ir-Lα ray peak and is not affected by other X-rays. The measurement time was 60 seconds for each sample. Other conditions were those automatically selected when Ir-Lα ray was selected.

[0054] 1.5. Scanning Electron Microscope (SEM) Observation of IrOx on Anode Electrode The equipment used was a scanning electron microscope (Hitachi, S-4300). Measurement conditions were accelerating voltage: 5 kV, working distance (WD): 5 mm, and acquisition: secondary electron image (SEI). The center of the area where the catalyst was applied was observed.

[0055] 1.6. Activity evaluation and durability test by electrochemical measurement Measurements were performed using a potentio / galvanostat (BIOLOGIC, VMP3) with a three-electrode cell. The working electrode was the anode electrode (prepared for each treatment), the counter electrode was a platinum wire, and the reference electrode was Ag / AgCl. Measurements were performed in a phosphate buffer solution with CO2 gas bubbling through as the reaction substrate. The phosphate buffer solution was prepared by mixing K2HPO4 and KH2PO4 in a 1:1 molar ratio and adding pure water to achieve a phosphate concentration of 0.6 M. Catalytic activity was evaluated by cyclic voltammogram (CV) measurements over a potential range of 0 to 2.0 V (vs. Ag / AgCl). Durability tests were performed by current-time (it) measurements at a fixed potential of 1.2 V (vs. Ag / AgCl) for 6 hours.

[0056] 2.Results 2.1. Quantification of the amount of hydration water / hydroxyl groups contained in IrOx on each anode electrode For each anode electrode, the amount of hydrated water / hydroxyl groups on the IrOx electrode was quantified using FT-IR as described in 1.3 above. The results are shown in Table 1. Compared to the untreated electrode (Comparative Example 2), the amount of hydrated water / hydroxyl groups on the IrOx decreased as the heat treatment temperature increased. On the other hand, the amount was 0 for the electrode heated at 500°C in air (Comparative Example 1).

[0057] [Table 1]

[0058] 2.2. Initial catalytic activity of each anode electrode through electrochemical characterization As described in 1.6 above, a three-electrode cell was used to perform CV measurements on each anode electrode at potentials of 0 to 2.0 V (vs. Ag / AgCl), and the results are shown in FIG. 3. The current density at a potential of 2.0 V is also shown in Table 2. Examples 1 to 6 exhibited superior catalytic activity, higher than that of Comparative Example 2. The significant decrease in catalytic activity in Comparative Example 2 is thought to be due to the fact that the amount of water of hydration / hydroxyl groups became zero as a result of heat treatment at high temperature.

[0059] [Table 2]

[0060] 2.3. Durability test by evaluating the electrochemical characteristics of each anode electrode As described in 1.6 above, a durability test was conducted with the potential fixed at 1.2 V (vs. Ag / AgCl). Figure 4 shows the results of the it measurements for Comparative Example 2 and Example 2. In the untreated Comparative Example 2, the current value significantly decreased with the passage of operating time (Figure 4(a)). In contrast, in Example 2, almost no decrease in current was observed, and stable operation was obtained (Figure 4(b)). The current retention rate was calculated for each anode electrode from the current density at the initial stage and after 6 hours of operation. The initial current density was taken as the current value after 30 minutes of operation. The results are shown in Table 3. It was found that the current retention rate was significantly improved in Examples 1 to 6 compared to Comparative Example 2. Note that Comparative Example 1 was excluded from the calculation because almost no current was generated at a potential of 1.2 V (vs. Ag / AgCl).

[0061] [Table 3]

[0062] 2.4. SEM observation of anode electrode after durability test As mentioned in 1.5 above, secondary electron images of the FTO glass substrate before catalyst application and the electrode surface after catalyst application (before durability testing) are shown in Figure 5. The surface of the FTO glass substrate can be seen to be covered with FTO crystal particles (Figure 5(a)). In contrast, the electrode surface coated with the catalyst is covered with IrOx particles, and the FTO crystal particles of the substrate cannot be seen (Figure 5(b)).

[0063] Next, secondary electron images of the electrode surfaces of Comparative Example 2 and Example 2 after the durability test are shown (Fig. 6). In Comparative Example 2, which is a conventional product, IrOx particles had fallen off the electrode surface, and FTO crystal particles on the substrate were observed (Fig. 6(a)). In contrast, in Example 2, it was observed that IrOx particles were supported on the electrode surface, just as they were initially (Fig. 6(b)). This confirmed that by controlling the amount of hydration water / hydroxyl groups of IrOx within a specified range, it was possible to suppress the detachment of IrOx particles, which is a cause of deterioration, while maintaining high catalytic activity.

[0064] 2.5. Comparison of Ir content retention rate after durability test As described in 1.4 above, the Ir content of the entire catalyst on the electrode was measured using XRF analysis, and the Ir content retention rate before and after the durability test was calculated. The results are shown in Table 4. Compared to Comparative Example 2, which is a conventional product, the Ir content retention rate was significantly improved in Examples 1 to 6, confirming that the detachment of catalyst particles, a cause of deterioration in operation, was suppressed. Note that Comparative Example 1 showed almost no current at a potential of 1.2 V (vs. Ag / AgCl), so it was excluded from the calculations, as in Table 3.

[0065] [Table 4]

[0066] As described above, the oxidation reaction electrodes of the examples had good durability.

Claims

1. An oxidation reaction electrode in which iridium oxide is supported on a substrate as an oxidation reaction catalyst, wherein the amount of water of hydration and hydroxyl groups contained in the iridium oxide is 0.029 or more and less than 0.

36.

2. The electrode for oxidation reactions according to claim 1, The electrode for oxidation reactions is characterized in that the substrate is a fluorine-doped tin oxide glass substrate or a titanium substrate.

3. The electrode for oxidation reactions according to claim 1 or 2, The oxidation reaction electrode is characterized in that the iridium oxide is iridium oxide particles made from potassium hexachloroiridate as a starting material.

4. 4. An electrochemical reaction device comprising: the electrode for oxidation reaction according to claim 1; an electrode for reduction reaction; and an electrolyte.

5. The electrochemical reaction device according to claim 4, The electrochemical reaction device, wherein the reduction reaction electrode contains a ruthenium complex.

6. The electrochemical reaction device according to claim 4 or 5, the electrolyte is a phosphate buffer solution having a pH of 6 to 8, The electrochemical reaction device is a device for reducing carbon dioxide.

7. The electrochemical reaction device according to any one of claims 4 to 6, a solar cell that generates power to be supplied to the oxidation reaction electrode and the reduction reaction electrode; An artificial photosynthesis device comprising:

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