JP2023084962A - Oxidation reaction electrode and electrochemical reaction device - Google Patents

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

The present invention relates to an oxidation reaction electrode and an electrochemical reaction device.

By electrically connecting the oxidation reaction electrode and the reduction reaction electrode and applying a bias voltage, the oxidation reaction electrode oxidizes water to generate oxygen, and the reduction reaction electrode produces carbon dioxide, etc. is known to produce formic acid or the like (for example, Patent Document 1).

Non-Patent Document 1 discloses an oxidation reaction electrode using iridium oxide as an oxidation catalyst.

JP 2017-125242 A

“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

Incidentally, an oxidation reaction electrode is generally produced by coating a substrate with a solution containing an oxidation catalyst such as iridium oxide. end up If the thickness of the catalyst layer is uneven, the catalyst particles are likely to fall off when operated as an electrode, resulting in a decrease in the durability of the electrode.

Accordingly, an object of the present invention is to provide an oxidation reaction electrode having excellent durability and an electrochemical reaction device provided with the oxidation reaction electrode.

An oxidation reaction electrode according to an embodiment of the present invention includes a substrate and a catalyst layer containing iridium oxide supported on the substrate, the thickness of the catalyst layer being 10 nm or more and 500 nm or less, energy Variation coefficient of the iridium atomic concentration when the iridium atomic concentration is measured at a plurality of measurement points on the catalyst layer surface at a detection depth at which the substrate material can be detected by dispersive X-ray analysis. ([standard deviation of iridium atomic concentration/average iridium atomic concentration]) is 0.1 or less.

An electrochemical reaction device according to an embodiment of the present invention is characterized by comprising the oxidation reaction electrode, the reduction reaction electrode, and the electrolytic solution.

Moreover, in the above electrochemical reaction device, the pH of the electrolytic solution is preferably 6 or more and 10 or less.

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.

It is a figure which shows an example of a structure of the electrochemical reaction device which concerns on this embodiment. 1 is a cross-sectional view showing an example of the configuration of an oxidation reaction electrode according to the present embodiment; FIG. FIG. 5 is a diagram showing the iridium atomic number concentration and the titanium atomic number concentration detected by performing measurements at five measurement points from the surface of the catalyst layer to a depth at which a substrate material can be detected. FIG. 4 is a graph showing results of current densities of the oxidation reaction electrodes of Examples and Comparative Examples with respect to energization time.

Embodiments of the present invention are described below. This embodiment is an example of implementing the present invention, and the present invention is not limited to this embodiment.

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 electrolytic solution . Moreover, the electrochemical reaction device 100 shown in FIG.

The reduction reaction electrode 102 is an electrode used for reducing 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 capable of reducing carbon compounds or protons. and a conductive layer.

The substrate is a member that structurally supports the reduction reaction electrode 102, and although the material is not particularly limited, it may be, for example, a glass substrate. The substrate may also comprise, for example, metals or semiconductors. The metal used as the substrate is not particularly limited, but silver (Ag), gold (Au), copper (Cu), zinc (Zn), indium (In), cadmium (Cd), tin (Sn). , palladium (Pd), and lead (Pb). The semiconductor used as the substrate is not particularly limited, but titanium oxide (TiO ₂ ), silicon (Si), strontium titanate (SrTiO ₃ ), zinc oxide (ZnO), tantalum oxide (Ta ₂ O ₅ ). etc. is preferable.

The conductive layer is provided to effectively collect current 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 thermal and chemical stability.

The conductor layer is composed of a conductor containing a reduction catalyst. The conductor layer can be formed by supporting a reduction catalyst on a conductor. The conductor can be made of, for example, a material containing a carbon material (C). The carbon material preferably includes at least one of carbon nanotubes, graphene and graphite, for example. 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 and heating a carbon material mixed with a liquid such as ethanol. It may be applied by spin coating instead of spraying. Alternatively, the solution may be directly dropped and dried without using spin coating.

The reduction catalyst is not particularly limited as long as it is a material having a reduction catalyst function, but is preferably a complex catalyst, for example. The complex catalyst is preferably a ruthenium complex, for example. Complex catalysts include, for example, [Ru{4,4′-di(1-H-1-pyrrolypropyl carbonate)-2,2′-bipyridine}(CO)(MeCN)Cl ₂ ], [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)(CH ₃ CN)Cl ₂ ] etc.

The conductor layer can be produced, for example, by coating a liquid obtained by dissolving a complex catalyst in an acetonitrile (MeCN) solution on the conductor. Moreover, the conductor layer can also be produced by an electrolytic polymerization method. For example, using a conductive electrode as the working electrode, a glass substrate coated with fluorine-containing tin oxide (FTO) as the counter electrode, and an Ag/ ^{Ag + electrode} as the reference electrode, On the other hand, it can be produced by passing a cathodic current so as to give a negative voltage and then passing an anodic current so as to give a positive potential to the Ag/Ag ⁺ electrode. For example, acetonitrile (MeCN) can be used as the electrolyte solution, and Tetrabutylammonium perchlorate (TBAP) can be used as the electrolyte.

The conductive layer thus formed is carried, applied or adhered onto the conductive layer arranged on the substrate. As a result, a reduction reaction electrode including a substrate, a conductive layer disposed on the substrate, and a conductive layer disposed on the conductive layer is produced.

The oxidation reaction electrode 104 is an electrode used to oxidize water by 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 arranged so as to face the conductor layer of the reduction reaction electrode 102 .

The substrate 114 is, for example, a conductive substrate having a metal surface. Specifically, a substrate made of metal, a substrate formed by forming a metal layer on the surface of a base substrate, or the like can be mentioned. Here, the metal forming the substrate 114 preferably has corrosion resistance. For example, a metal that does not corrode in an electrolytic solution and at a potential equal to or higher than the oxidation-reduction potential of water is preferred.

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., particularly , Ti.

A method for forming a metal layer on the surface of the underlying substrate is not particularly limited, but examples thereof include electroplating, hot-dip plating, vacuum deposition, and sputtering. Among these, sputtering is preferable because it is easy to thin the metal layer. The base substrate is not particularly limited, and may be a glass substrate, a plastic substrate, a glass substrate coated with indium tin oxide (ITO), fluorine-doped tin oxide (FTO), zinc oxide (ZnO), or the like, or a plastic substrate. mentioned. Further, the underlying substrate may be, for example, a substrate such as Ti, Au, Pt, Ru, Ir, Sn, Rh, Cu, Ag, or the like.

The catalyst layer 116 contains an oxidation catalyst. The oxidation catalyst is a material having an oxidation catalyst function and contains iridium oxide. The oxidation catalyst may contain, for example, ruthenium oxide in addition to iridium oxide.

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 is lowered, or it becomes difficult to prepare the catalyst layer 116 having a uniform thickness within the substrate 114 plane. Further, when the thickness of the catalyst layer 116 exceeds 500 nm, the oxidation catalyst tends to come off when the oxidation reaction electrode 104 is operated by applying a voltage. The thickness of the catalyst layer 116 is an average value, and the measuring method will be described in the Examples section.

Further, in the present embodiment, when the atomic number concentration of iridium is measured at a plurality of measurement points on the surface of the catalyst layer 116 by energy dispersive X-ray analysis at a detection depth at which the substance that becomes the substrate 114 can be detected, the The variation coefficient of iridium atomic concentration ([standard deviation of iridium atomic concentration/average iridium atomic concentration]) is 0.1 or less, preferably 0.08 or less. Here, the lower the variation coefficient of the iridium atomic concentration, the more uniform the thickness of the catalyst layer 116 formed in the plane of the substrate 114 .

When the variation coefficient of the atomic number concentration of iridium is 0.1 or less as in the present embodiment, the catalyst layer 116 having a uniform thickness is formed in the plane of the substrate 114, so that the oxidation reaction electrode 104 is When operated, the shedding of catalyst particles in the catalyst layer 116 is suppressed. As a result, the oxidation reaction electrode 104 of this embodiment exhibits excellent durability. Further, in the oxidation reaction electrode 104 of the present embodiment, in which the catalyst layer 116 having a uniform thickness is formed in the plane of the substrate 114, there is little variation in the current characteristics in the plane of the electrode, and the amount of uniformity in the plane of the electrode is small. Oxygen can be produced.

The measurement conditions of the energy dispersive X-ray analysis, the method of calculating the coefficient of variation of the iridium atomic number concentration, and the like in this embodiment will be described in the following Examples section.

An example of a method for producing the oxidation reaction electrode 104 of this embodiment will be described. The oxidation reaction electrode 104 is produced, for example, by a method of supporting iridium oxide on the substrate 114 by electrodeposition. In one example of the electrodeposition method, the substrate 114 is first immersed in a liquid containing iridium oxide. In the liquid, iridium oxide is charged with a predetermined polarity. Next, a predetermined voltage is applied between the substrate 114 and a counter electrode immersed in the same liquid. At this time, the substrate 114 is set to a polarity opposite to that of the charged iridium oxide. As a result, iridium oxide can be supported on the substrate 114, and the oxidation reaction electrode 104 having the catalyst layer 116 containing iridium oxide formed on the substrate 114 can be manufactured. In addition, by adopting the electrodeposition method, the coefficient of variation of the iridium atomic number concentration calculated based on energy dispersive X-ray analysis ([standard deviation of iridium atomic number concentration / average value of iridium atomic number concentration ]) is 0.1 or less. In addition, the electrodeposition method can strengthen the adhesion between the catalyst layer 116 and the substrate 114, so that the oxidation reaction electrode 104 can be manufactured with superior durability.

The method for producing the oxidation reaction electrode 104 of the present embodiment is not limited to the electrodeposition method. Any method can be used as long as the coefficient of variation of number concentration can be 0.1 or less, and examples thereof include sputtering and vacuum deposition.

Conventionally, as a method for producing an oxidation reaction electrode, there is a method of producing an oxidation reaction electrode by coating a substrate with a slurry in which iridium oxide is dispersed. However, with this method, when the thickness of the catalyst layer is 10 nm or more and 500 nm or less, it is difficult to make the variation coefficient of the iridium atomic number concentration calculated based on the energy dispersive X-ray analysis 0.1 or less.

The electrolytic solution 106 is preferably an aqueous phosphate buffer solution, an aqueous borate buffer solution, or the like, in terms of suppressing pH fluctuations during the oxidation-reduction reaction. When reducing a carbon compound in the reduction reaction electrode 102 , a carbon compound such as carbon dioxide is dissolved in the electrolytic solution 106 . In the electrochemical reaction device 100 shown in FIG. 1, for example, a tank for supplying the electrolytic solution 106 is provided, and the electrolytic solution 106 in the tank is provided between the reduction reaction electrode 102 and the oxidation reaction electrode 104 by a pump. feed into the gap.

Further, the pH of the electrolytic solution 106 is preferably 4 or more and 10 or less from the viewpoint of suppressing metal corrosion of the substrate 114 of the oxidation reaction electrode 104 when voltage is applied, for example.

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, by connecting the oxidation reaction electrode 104 to the positive electrode of the solar cell 108 and connecting the reduction reaction electrode 102 to the negative electrode of the solar cell 108, a bias voltage is applied to both electrodes. A device that applies a bias voltage is not limited to the solar battery cell 108, and includes, for example, chemical batteries (including primary batteries, secondary batteries, etc.), constant voltage sources, and the like.

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

The reduction reaction electrode 102 , oxidation reaction electrode 104 , solar cell 108 and window member 110 are structurally supported by a frame member 112 .

In the electrochemical reaction device 100 shown in FIG. 1 , the reduction reaction electrode 102 and the oxidation reaction electrode 104 are connected to each other by the solar cell 108 while the electrolytic solution 106 is supplied to the surfaces of the reduction reaction electrode 102 and the oxidation reaction electrode 104 . 104 is applied. As a result, at the oxidation reaction electrode 104, water in the electrolyte solution 106 is oxidized to generate oxygen (formula (1)), and at the reduction reaction electrode 102, a carbon compound, such as CO ₂ , in the electrolyte solution 106 is is reduced to produce carbon monoxide, formic acid, and the like (formulas (2) and (3)), and protons are reduced to produce hydrogen (formula (4)). Products generated by 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 a voltage at which an oxidation-reduction reaction occurs at both electrodes. 1.5 V to 2.2 V is preferred when protons are reduced to produce hydrogen.
Oxidation reaction: 2H ₂ O→O ₂ +4H ⁺ +4e ⁻ (1)
Reduction reaction: CO ₂ +2H ⁺ +2e ⁻ →CO+H ₂ O (2)
: CO ₂ +2H ⁺ +2e ⁻ →HCOOH (3)
: 2H ⁺ +2e ⁻ →H ₂ (4)

EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples, but the present invention is not limited to the following examples.

<Example>
<Preparation of electrode for oxidation reaction>
Based on the literature (Y.Zhao et.al., "A High Yield Synthesis of Ligand-Free ilridium Oxide Nanoparticles with High Electrocatalytic Activity", J.Phys.Chem.Lett., 2012, 2, P402-406), pH 12 An iridium oxide nanocolloid solution adjusted to A specific preparation method is as follows. A 10 wt % sodium hydroxide (NaOH) aqueous solution was added to 25 ml of a 1 mM potassium iridate (IV) chloride (K ₂ IrCl ₆ ) aqueous solution to obtain a solution adjusted to pH 13. After cooling the solution with ice water for 1 hour, a 3M nitric acid (HNO ₃ ) aqueous solution was added dropwise to adjust the pH to 1 to obtain an iridium oxide (IrOx) nanocolloid aqueous solution. A 1.5 wt % NaOH aqueous solution was added dropwise to this solution to adjust the pH to 12.

The pH 12 iridium oxide nanocolloid solution prepared above was used as an electrodeposition bath and poured into a glass container. Then, a Ti substrate as a working electrode, a Pt substrate as a counter electrode, and an Ag/AgCl electrode as a reference electrode were immersed in the electrodeposition bath filled in the container to construct a three-electrode electrochemical cell. Each electrode was connected to an electrochemical analyzer (ALS610), and 400 cycles of cyclic voltammetry were performed in the voltage range of -0.195 to 0.895 V at a sweep rate of 100 mV/s. As a result, an electrode was obtained in which an iridium oxide catalyst layer was formed on a Ti substrate. The electrode was taken out from the electrodeposition bath, washed with deionized water, and then naturally dried at room temperature. This electrode was used as an oxidation reaction electrode in the examples. The thickness of the catalyst layer of the oxidation reaction electrode of the example was about 100 nm.

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

<Comparative example>
125 μL of the prepared iridium oxide nanocolloid solution having a pH of 12 was applied onto a Ti substrate and dried in a drying oven at 60° C. for 1 hour. After drying, the electrode in which iridium oxide was applied on the Ti substrate was washed with deionized water. This application, drying and washing operation was repeated six times. However, the final drying was performed at 60°C for 3 hours. The electrode thus obtained was used as an oxidation reaction electrode of a comparative example.

(Energy dispersive X-ray analysis)
Using a scanning electron microscope (manufactured by Hitachi High-Tech Corporation, SU3500) equipped with an energy dispersive X-ray analyzer, the entire surface of the catalyst layer of the example fits in the observation field, and the range of measurement points in the observation field is 2 mm. Observation was made at an observation magnification of 2 mm, and the iridium atomic number concentration was measured for this observation field. As for the measurement conditions, the acceleration voltage was set to 10 to 15 kV, and the detection depth was set to a depth from the surface of the catalyst layer at which the substrate substance can be detected (set to 500 nm to 1 μm). The measurement range was an observation magnification in which the range of measurement points in the observation field was 2 mm × 2 mm, and arbitrary five points from one end of the catalyst layer to the other end on the opposite side were set as measurement points (measurement points should be 5 or more and 10 or less). Comparative examples were also measured in the same manner.

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

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 obtained, and from these values, the coefficient of variation of the iridium atomic number concentration ([iridium standard deviation of atomic number concentration/average value of iridium atomic number concentration]) was obtained. As a result, the variation coefficient of the iridium atomic number concentration of the example was 0.079, and the variation coefficient of the iridium atomic number concentration of the comparative example was 0.53.

(Evaluation of durability)
The current flowing through the oxidation reaction electrodes of Examples and Comparative Examples was measured by a three-electrode method using an electrochemical analyzer (ALS610). In the three-electrode method, a container is filled with an electrolytic solution, and a three-electrode electrochemical cell in which the oxidation reaction electrode prepared above as a working electrode, a platinum electrode as a counter electrode, and Ag/AgCl as a reference electrode are immersed in the electrolytic solution. It was used. The electrolytic solution used was a 0.4 mol/L phosphate buffer (K ₂ HPO ₄ +KH ₂ PO ₄ ) with a pH of 6.3. Each electrode was connected to an electrochemical analyzer, energized at a voltage of 1.2 V, and the current value flowing through the oxidation reaction electrode was measured. The energization time was set to 6 hours a day, and it was repeated for several days.

FIG. 4 is a graph showing the results of the current density of the oxidation reaction electrodes of Examples and Comparative Examples with respect to the energization time. It is a current density (mA/cm ² ), which is a value obtained by dividing the current value measured above by the area of the oxidation reaction electrode.

In the comparative example, the current density of the oxidation reaction electrode was almost 0 after 18 hours of energization, so it is considered that the oxidation reaction by the oxidation reaction electrode almost stopped at this point. On the other hand, in the example, the current density of the oxidation reaction electrode was detected even when the current was applied for 30 hours, so the oxidation reaction of the oxidation reaction electrode took longer than in the comparative example. From this result, it was concluded that the oxidation reaction electrode having a coefficient of variation of iridium atomic number concentration calculated based on energy dispersive X-ray analysis as in the example of 0.1 or less has excellent durability. I can say

100 electrochemical reaction device, 102 reduction reaction electrode, 104 oxidation reaction electrode, 106 electrolytic solution, 108 solar cell, 110 window material, 112 frame material, 114 substrate, 116 oxidation catalyst layer.

Claims (3)

having a substrate and a catalyst layer containing iridium oxide supported on the substrate;
The thickness of the catalyst layer is 10 nm or more and 500 nm or less,
When the atomic number concentration of iridium is measured at a plurality of measurement points on the surface of the catalyst layer by energy dispersive X-ray analysis at a detection depth at which the substrate material can be detected, fluctuations in the atomic number concentration of the iridium. An oxidation reaction electrode characterized by having a coefficient ([standard deviation of iridium atomic concentration/average value of iridium atomic concentration]) of 0.1 or less. An electrochemical reaction device comprising the oxidation reaction electrode according to claim 1, a reduction reaction electrode, and an electrolytic solution. 3. The electrochemical reaction device according to claim 2, wherein the electrolyte has a pH of 6 or more and 10 or less.

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