Electrode for oxidation reaction and electrochemical reaction device

Abstract

To provide an oxidation reaction electrode with excellent durability and an electrochemical reaction device.SOLUTION: An oxidation reaction electrode has a substrate 114, and a catalyst layer 116 containing iridium oxide supported on the substrate 114. A thickness of the catalyst layer 116 is 10 nm or more and 500 nm or less. When the atomic concentration of iridium is measured by energy-dispersive X-ray analysis at a plurality of measurement points on a surface of the catalyst layer 116 at a detection depth where a substance that becomes the substrate 114 can be detected, the coefficient of variation (standard deviation of the atomic concentration of iridium / average value of the atomic concentration of iridium) of the atomic concentration of iridium is 0.1 or less.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to an electrode for an oxidation reaction and an electrochemical reaction device. [Background technology]

[0002] An electrochemical reaction device is known in which an electrode for an oxidation reaction and an electrode for a reduction reaction are electrically connected and a bias voltage is applied, thereby oxidizing water at the electrode for oxidation reaction to generate oxygen and reducing carbon dioxide and the like at the electrode for reduction reaction to generate formic acid and the like (for example, Patent Document 1).

[0003] Non-Patent Document 1 discloses an electrode for oxidation reaction that uses iridium oxide as an oxidation catalyst. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2017-125242 [Non-patent literature]

[0005] [Non-Patent Document 1] “A large-size cell for solar-driven CO2 conversion with a solar-to-formate conversion efficiency of 7.2%”, Joule, 2021, 5, pp.687-705 Summary of the Invention [Problem to be solved by the invention]

[0006] Generally, an oxidation electrode is fabricated by applying a solution containing an oxidation catalyst such as iridium oxide to a substrate, but this method results in uneven thickness of the catalyst layer within the surface of the substrate. When uneven thickness of the catalyst layer occurs, catalyst particles are more likely to fall off when the electrode is operated, thereby reducing the durability of the electrode.

[0007] Therefore, an object of the present invention is to provide an oxidation reaction electrode having excellent durability and an electrochemical reaction device including the oxidation reaction electrode. [Means for solving the problem]

[0008] The electrode for oxidation reaction according to an embodiment of the present invention comprises a Ti substrate and a catalyst layer containing iridium oxide carried on the Ti substrate, the catalyst layer having a thickness of 10 nm to 500 nm, and a detection depth at which a substance that becomes the Ti substrate can be detected at a plurality of measurement points on the surface of the catalyst layer by energy dispersive X-ray analysis. at a depth from the surface of the catalyst layer that is 5 to 10 times the thickness of the catalyst layer, When the atomic concentration of iridium is measured, the coefficient of variation of the atomic concentration of iridium ([standard deviation of atomic concentration of iridium / average value of atomic concentration of iridium]) is 0.1 or less.

[0009] An electrochemical reaction device according to an embodiment of the present invention is characterized by comprising the above-described oxidation reaction electrode, reduction reaction electrode, and an electrolyte solution.

[0010] In the electrochemical reaction device, the pH of the electrolyte solution is preferably 6 or more and 10 or less. [Effects of the Invention]

[0011] According to the embodiments of the present invention, it is possible to provide an oxidation reaction electrode having excellent durability and an electrochemical reaction device including the oxidation reaction electrode. [Brief explanation of the drawings]

[0012] [Figure 1]1 is a diagram showing an example of the configuration of an electrochemical reaction device according to an embodiment of the present invention. [Figure 2] 1 is a cross-sectional view showing an example of the configuration of an electrode for oxidation reaction according to the present embodiment. [Figure 3] FIG. 10 is a graph showing the iridium atom number concentration and titanium atom number concentration detected by measuring at five measurement points at a depth from the surface of the catalyst layer at which the substrate material can be detected. [Figure 4] FIG. 1 is a graph showing the results of current density of the oxidation reaction electrodes of Examples and Comparative Examples relative to the current-carrying time. DETAILED DESCRIPTION OF THE INVENTION

[0013] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below. The embodiment is an example of implementing the present invention, and the present invention is not limited to this embodiment.

[0014] Fig. 1 is a diagram showing an example of the configuration of an electrochemical reaction device according to this embodiment. As shown in Fig. 1, the electrochemical reaction device 100 includes a reduction reaction electrode 102, an oxidation reaction electrode 104, and an electrolyte 106. The electrochemical reaction device 100 shown in Fig. 1 also includes a solar cell 108, a window material 110, and a frame material 112.

[0015] The reduction reaction electrode 102 is an electrode used to reduce carbon compounds or protons by a reduction reaction. The form of the reduction reaction electrode 102 is not particularly limited as long as it is an electrode that can reduce carbon compounds or protons, but it may have, for example, a substrate, a conductive layer disposed on the substrate, and a conductor layer disposed on the conductive layer.

[0016] The substrate is a member that structurally supports the reduction reaction electrode 102 and is not particularly limited to a material such as a glass substrate. The substrate may also include, for example, a metal or a semiconductor. The metal used as the substrate is not particularly limited, but preferably includes silver (Ag), gold (Au), copper (Cu), zinc (Zn), indium (In), cadmium (Cd), tin (Sn), palladium (Pd), or lead (Pb). The semiconductor used as the substrate is not particularly limited, but preferably includes titanium oxide (TiO), silicon (Si), strontium titanate (SrTiO), zinc oxide (ZnO), or tantalum oxide (TaO).

[0017] The conductive layer is provided to improve the efficiency of current collection at the reduction reaction electrode 102. The conductive layer is not particularly limited, but is preferably made of indium tin oxide (ITO), fluorine-doped tin oxide (FTO), zinc oxide (ZnO), or the like. In particular, it is preferable to use fluorine-doped tin oxide (FTO) in consideration of its thermal and chemical stability.

[0018] The conductor layer is composed of a conductor containing a reduction catalyst. The conductor layer can be constructed by supporting a reduction catalyst on a conductor. The conductor can be constructed, for example, from a material containing a carbon material (C). The carbon material preferably contains at least one of carbon nanotubes, graphene, and graphite. Graphene and graphite preferably have a size of 1 nm or more and 1 μm or less. Carbon nanotubes preferably have a diameter of 1 nm or more and 40 nm or less. The conductor can be formed 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 be applied without using spin coating.

[0019] The reduction catalyst is not particularly limited as long as it is a material having a reduction catalytic function, but for example, a complex catalyst is preferable. For example, a ruthenium complex is preferable. For example, the complex catalyst is [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], [Ru{4,4'-di(1-H-1-pyrrolypropyl carbonate)-2,2'-bipyridine}(CO)2] n , [Ru{4,4'-di(1-H-1-pyrrolypropyl carbonate)-2,2'-bipyridine}(CO)(CHCN)Cl], etc.

[0020] The conductive layer can be prepared by, for example, dissolving the complex catalyst in an acetonitrile (MeCN) solution and applying it to the conductive material. Alternatively, the conductive layer can be prepared by electrolytic polymerization. For example, a conductive electrode is used as the working electrode, a glass substrate coated with fluorine-containing tin oxide (FTO) is used as the counter electrode, and an Ag / Ag reference electrode is used. + Using an electrode, Ag / Ag in an electrolyte containing a complex catalyst + After applying a cathodic current to the electrode to create a negative voltage, Ag / Ag + It can be produced by passing an anodic current so that the electrode has a positive potential. For example, acetonitrile (MeCN) can be used as the electrolyte solution, and tetrabutylammonium perchlorate (TBAP) can be used as the electrolyte.

[0021] The conductive layer thus formed is carried, coated, or attached onto a conductive layer disposed on a substrate, thereby producing an electrode for a reduction reaction including the substrate, the conductive layer disposed on the substrate, and the conductive layer disposed on the conductive layer.

[0022] The oxidation reaction electrode 104 is an electrode used to oxidize water through an oxidation reaction. FIG. 2 is a cross-sectional view showing the configuration of the oxidation reaction electrode according to this embodiment. As shown in FIG. 2, the oxidation reaction electrode 104 has a substrate 114 and a catalyst layer 116. In the electrochemical reaction device 100 shown in FIG. 1, the catalyst layer 116 of the oxidation reaction electrode 104 is disposed so as to face the conductor layer of the reduction reaction electrode 102.

[0023] Substrate 114 is, for example, a conductive substrate with a metal surface, and specifically includes a substrate made of metal, a substrate made of a base substrate with a metal layer formed on the surface, etc. Here, the metal constituting substrate 114 is preferably corrosion-resistant, and is preferably, for example, a metal that does not corrode in an electrolyte solution at a potential equal to or higher than the oxidation-reduction potential of water.

[0024] The metal constituting the substrate 114 preferably contains at least one selected from the group consisting of Ti, Au, Pt, Ru, Ir, Sn, and Rh, in terms of high conductivity, high corrosion resistance, etc., and it is particularly preferable that it contains Ti.

[0025] Methods for forming a metal layer on the surface of a base substrate include, but are not limited to, electroplating, hot-dip plating, vacuum deposition, sputtering, etc. Among these, sputtering is preferred because it allows for easy thinning of the metal layer. The base substrate is not particularly limited, but includes, but is not limited to, glass substrates, plastic substrates, and glass or plastic substrates coated with indium tin oxide (ITO), fluorine-doped tin oxide (FTO), zinc oxide (ZnO), etc. Furthermore, the base substrate may be, for example, a substrate of Ti, Au, Pt, Ru, Ir, Sn, Rh, Cu, Ag, etc.

[0026] The catalyst layer 116 includes an oxidation catalyst. The oxidation catalyst is a material having an oxidation catalytic function, and includes iridium oxide. In addition to iridium oxide, the oxidation catalyst may also include, for example, ruthenium oxide.

[0027] The thickness of the catalyst layer 116 may be 10 nm or more and 500 nm or less, preferably 20 nm or more and 450 nm or less, and more preferably 50 nm or more and 400 nm or less. If the thickness of the catalyst layer 116 is less than 10 nm, the oxidation catalyst function may be reduced, or it may be difficult to produce a catalyst layer 116 with a uniform thickness across the surface of the substrate 114. Furthermore, if the thickness of the catalyst layer 116 exceeds 500 nm, the oxidation catalyst may easily fall off when a voltage is applied to operate the oxidation reaction electrode 104. The thickness of the catalyst layer 116 is an average value, and the measurement method will be explained in the Examples section.

[0028] Furthermore, in this embodiment, when the atomic concentration of iridium is measured by energy dispersive X-ray analysis at multiple measurement points on the surface of catalyst layer 116 at a detection depth at which the substance that will become substrate 114 can be detected, the coefficient of variation of the atomic concentration of iridium ([standard deviation of atomic concentration of iridium / average value of atomic concentration of iridium]) is 0.1 or less, and preferably 0.08 or less. Here, a lower coefficient of variation of the atomic concentration of iridium indicates that catalyst layer 116 having a more uniform thickness within the surface of substrate 114 is formed.

[0029] When the coefficient of variation of the iridium atomic concentration is 0.1 or less as in this embodiment, the catalyst layer 116 is formed with a uniform thickness within the surface of the substrate 114, and therefore, when the oxidation reaction electrode 104 is operated, catalyst particles in the catalyst layer 116 are suppressed from falling off. As a result, the oxidation reaction electrode 104 of this embodiment exhibits excellent durability. Furthermore, the oxidation reaction electrode 104 of this embodiment, in which the catalyst layer 116 with a uniform thickness within the surface of the substrate 114 is formed, has little variation in current characteristics within the electrode surface, and can generate a uniform amount of oxygen within the electrode surface.

[0030] The measurement conditions for energy dispersive X-ray analysis in this embodiment, the method for calculating the coefficient of variation of the atomic number concentration of iridium, and the like will be explained in the Examples section below.

[0031] An example of a method for fabricating the oxidation reaction electrode 104 of this embodiment will be described. The oxidation reaction electrode 104 is fabricated by, for example, a method of supporting iridium oxide on a substrate 114 by electrodeposition. In one example of the electrodeposition method, the substrate 114 is first immersed in a solution containing iridium oxide. In the solution, the iridium oxide is charged to a predetermined polarity. Next, a predetermined voltage is applied between the substrate 114 and a counter electrode immersed in the same solution. At this time, the substrate 114 is set to a polarity opposite to that of the charged iridium oxide. In this way, iridium oxide can be supported on the substrate 114, and an oxidation reaction electrode 104 can be fabricated in which a catalyst layer 116 containing iridium oxide is formed on the substrate 114. Furthermore, by employing the electrodeposition method, it becomes easy to fabricate an oxidation reaction electrode 104 in which the coefficient of variation of the iridium atomic concentration ([standard deviation of iridium atomic concentration / average value of iridium atomic concentration]) calculated based on energy dispersive X-ray analysis is 0.1 or less. Furthermore, by employing the electrodeposition method, it is possible to strengthen the adhesion between the catalyst layer 116 and the substrate 114, thereby fabricating an oxidation reaction electrode 104 that exhibits superior durability.

[0032] The method for producing the oxidation reaction electrode 104 of this embodiment is not limited to electrodeposition, and any method may be used as long as it can produce a catalyst layer 116 with a thickness of 10 nm or more and 500 nm or less and can make the coefficient of variation of the atomic concentration of iridium calculated based on energy dispersive X-ray analysis 0.1 or less, and examples thereof include sputtering and vacuum deposition.

[0033] Conventionally, an electrode for oxidation reaction has been fabricated by applying a slurry containing dispersed iridium oxide onto a substrate. However, with this method, it is difficult to achieve a catalyst layer thickness of 10 nm to 500 nm and a coefficient of variation of the iridium atomic concentration of 0.1 or less, as calculated based on energy dispersive X-ray analysis.

[0034] The electrolyte 106 is preferably, for example, a phosphate buffer solution or a borate buffer solution, in order to suppress pH fluctuations during the oxidation-reduction reaction. When a carbon compound is reduced at the reduction reaction electrode 102, a carbon compound such as carbon dioxide is dissolved in the electrolyte 106. In the electrochemical reaction device 100 shown in FIG. 1 , for example, a tank for supplying the electrolyte 106 is provided, and the electrolyte 106 in the tank is supplied to the gap provided between the reduction reaction electrode 102 and the oxidation reaction electrode 104 by a pump.

[0035] The pH of the electrolytic solution 106 is preferably 4 or more and 10 or less, for example, in order to suppress metal corrosion of the substrate 114 of the oxidation reaction electrode 104 when a voltage is applied.

[0036] The solar cell 108 shown in Fig. 1 is a device that applies an appropriate bias voltage between the reduction reaction electrode 102 and the oxidation reaction electrode 104. As shown in Fig. 1, the oxidation reaction electrode 104 is connected to the positive electrode of the solar cell 108, and the reduction reaction electrode 102 is connected to the negative electrode of the solar cell 108, thereby applying a bias voltage to both electrodes. The device that applies the bias voltage is not limited to the solar cell 108, and examples include chemical batteries (including primary batteries, secondary batteries, etc.), constant voltage sources, etc.

[0037] 1 is a member that protects the solar cell 108. It is preferable to provide the window member 110 on the light-receiving surface side of the solar cell 108. The window member 110 is a member that transmits light of a wavelength that contributes to power generation in the solar cell 108, and can be made of, for example, glass, plastic, or the like.

[0038] The reduction reaction electrode 102 , the oxidation reaction electrode 104 , the solar cell 108 and the window material 110 are structurally supported by a frame material 112 .

[0039] In the electrochemical reaction device 100 shown in Fig. 1, an electrolyte 106 is supplied to the surfaces of the reduction reaction electrode 102 and the oxidation reaction electrode 104, and a bias voltage is applied between the reduction reaction electrode 102 and the oxidation reaction electrode 104 by the solar cell 108. As a result, at the oxidation reaction electrode 104, water in the electrolyte 106 is oxidized to generate oxygen (Equation (1)), and at the reduction reaction electrode 102, carbon compounds, such as CO2, in the electrolyte 106 are reduced to generate carbon monoxide, formic acid, etc. (Equations (2) and (3)), and protons are reduced to generate hydrogen (Equation (4)). The products generated at both electrodes are discharged from the electrochemical reaction device 100 and collected in an external collection tank. The voltage applied to both electrodes may be any voltage that causes an oxidation-reduction reaction at both electrodes. For example, when carbon dioxide is reduced to produce formic acid, a voltage of 1.5 V to 3.0 V is preferable, and when protons are reduced to produce hydrogen, a voltage of 1.5 V to 2.2 V is preferable. Oxidation reaction: 2H2O → O2 + 4H + +4e - (1) Reduction reaction: CO2 + 2H + +2e - →CO+H2O (2) :CO2+2H + +2e - →HCOOH (3) :2H + +2e - →H2(4) [Example]

[0040] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples.

[0041] <Example> <Preparation of electrodes for oxidation reaction> Based on the literature (Y. Zhao et al., "A High Yield Synthesis of Ligand-Free Irridium Oxide Nanoparticles with High Electrocatalytic Activity", J. Phys. Chem. Lett., 2012, 2, pp. 402-406), an iridium oxide nanocolloidal solution adjusted to pH 12 was prepared. The specific preparation method is as follows: 25 ml of 1 mM potassium chloroiridate(IV) (K2IrCl6) aqueous solution was added with 10 wt% sodium hydroxide (NaOH) aqueous solution to obtain a solution adjusted to pH 13. After cooling the solution in ice water for 1 hour, 3 M nitric acid (HNO3) aqueous solution was added dropwise to adjust the pH to 1, yielding an iridium oxide (IrOx) nanocolloidal solution. A 1.5 wt% NaOH aqueous solution was added dropwise to this solution to adjust the pH to 12.

[0042] The prepared iridium oxide nanocolloid solution (pH 12) was poured into a glass container as an electrodeposition bath. A Ti substrate (working electrode), a Pt substrate (counter electrode), and an Ag / AgCl electrode (reference electrode) were immersed in the electrodeposition bath to construct a three-electrode electrochemical cell. Each electrode was connected to an electrochemical analyzer (ALS610), and 400 cycles of cyclic voltammetry were performed at a sweep rate of 100 mV / s over a voltage range of -0.195 to 0.895 V. This resulted in an electrode with an iridium oxide catalytic layer formed on the Ti substrate. The electrode was removed from the electrodeposition bath, washed with ion-exchanged water, and then air-dried at room temperature. This electrode was used as the oxidation reaction electrode in the examples. The catalytic layer of the oxidation reaction electrode in the examples had a thickness of approximately 100 nm.

[0043] The thickness of the catalyst layer was calculated from the mass of the catalyst layer and the theoretical density of the catalyst.

[0044] <Comparative Example> 125 μL of the prepared iridium oxide nanocolloid solution with a pH of 12 was applied to a Ti substrate and dried in a drying oven at 60°C for 1 hour. After drying, the electrode with iridium oxide applied to the Ti substrate was washed with ion-exchanged water. This process of application, drying, and washing was repeated six times. However, the final drying was performed at 60°C for 3 hours. The electrode obtained in this manner was used as the oxidation reaction electrode of the comparative example.

[0045] (Energy dispersive X-ray analysis) Using a scanning electron microscope (SU3500, manufactured by Hitachi High-Technologies Corporation) equipped with an energy dispersive X-ray analyzer, the catalyst layer surface of the example was observed at a magnification of 2 mm × 2 mm, so that the entire surface fit within the observation field, and the measurement point range within the observation field was measured. The iridium atom concentration within this observation field was measured. The measurement conditions were an acceleration voltage of 10 to 15 kV, and the detection depth was set to a depth from the surface of the catalyst layer at which the substrate material could be detected (500 nm to 1 μm). The measurement range was set at a magnification of 2 mm × 2 mm, with five arbitrary measurement points between one end of the catalyst layer and the other end (it is desirable to have five or more and ten or fewer measurement points). Similar measurements were also performed for the comparative example.

[0046] Figure 3 shows the iridium atom number concentration and titanium atom number concentration detected by measuring at five measurement points at a depth from the surface of the catalyst layer where the substrate material can be detected. As shown in Figure 3, in the example, the iridium atom number concentration hardly changed at any of the measurement points, but in the comparative example, the iridium atom number concentration varied depending on the measurement point. In the comparative example, the iridium atom number concentration was high at both ends of the catalyst layer and low in the center.

[0047] In the examples and comparative examples, the average value of the iridium atomic number concentration measured at five measurement points and the standard deviation of the iridium atomic number concentration were determined, and the coefficient of variation of the iridium atomic number concentration ([standard deviation of iridium atomic number concentration / average value of iridium atomic number concentration]) was calculated from these values. As a result, the coefficient of variation of the iridium atomic number concentration in the example was 0.079, and the coefficient of variation of the iridium atomic number concentration in the comparative example was 0.53.

[0048] (Durability evaluation) The current flowing through the oxidation electrode in each of the examples and comparative examples was measured using an electrochemical analyzer (ALS610) in a three-electrode format. In the three-electrode format, a container was filled with an electrolyte, and a three-electrode electrochemical cell was used, in which the above-prepared oxidation electrode was immersed as the working electrode, a platinum electrode as the counter electrode, and an Ag / AgCl electrode as the reference electrode. The electrolyte used was a 0.4 mol / L phosphate buffer solution (K2HPO4 + KH2PO4) with a pH of 6.3. Each electrode was connected to the electrochemical analyzer, and a voltage of 1.2 V was applied to measure the current flowing through the oxidation electrode. The application of electricity was repeated for six hours per day for several days.

[0049] FIG. 4 is a graph showing the current density of the electrodes for oxidation reactions of the examples and comparative examples relative to the current application time. 2 ) and is a value obtained by dividing the measured current value by the area of ​​the oxidation reaction electrode.

[0050] In the comparative example, the current density of the oxidation reaction electrode became almost 0 after 18 hours of current application, and it is believed that the oxidation reaction by the oxidation reaction electrode almost stopped at this point. In contrast, in the example, the current density of the oxidation reaction electrode was detected even after 30 hours of current application, and the oxidation reaction by the oxidation reaction electrode continued for a longer period of time than in the comparative example. From these results, it can be said that the oxidation reaction electrode in the example, in which the coefficient of variation of the iridium atomic number concentration calculated based on energy dispersive X-ray analysis is 0.1 or less, has excellent durability. [Explanation of symbols]

[0051] 100 electrochemical reaction device, 102 electrode for reduction reaction, 104 electrode for oxidation reaction, 106 electrolyte, 108 solar cell, 110 window material, 112 frame material, 114 substrate, 116 oxidation catalyst layer.

Claims

1. A Ti substrate and a catalyst layer containing iridium oxide supported on the Ti substrate, The thickness of the catalyst layer is 10 nm or more and 500 nm or less, an oxidation reaction electrode characterized in that, when the atomic concentration of iridium is measured by energy dispersive X-ray analysis at a detection depth at which a substance that becomes the Ti substrate can be detected at a plurality of measurement points on the surface of the catalyst layer, the detection depth being 5 to 10 times the thickness of the catalyst layer from the surface of the catalyst layer, the coefficient of variation of the atomic concentration of iridium ([standard deviation of atomic concentration of iridium / average value of atomic concentration of iridium]) is 0.1 or less.

2. An electrochemical reaction device comprising the oxidation reaction electrode according to claim 1, a reduction reaction electrode, and an electrolyte.

3. 3. The electrochemical reaction device according to claim 2, wherein the pH of the electrolyte is 6 or more and 10 or less.

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