Catalytic electrode, method for fabricating catalytic electrode, and photoelectrochemical cell

Abstract

This invention provides a catalytic electrode that has the function of electrochemically reducing carbon dioxide to produce acetic acid under light irradiation, and that exhibits high selectivity for acetic acid production and good stability. [Solution] A catalytic electrode having the function of electrochemically reducing carbon dioxide to produce acetic acid under light irradiation, wherein the catalyst is supported on a conductive substrate, and the catalyst is an oxide composed of Cu and Fe, with CuFe2O4 as its main crystal structure.

Description

[Technical Field]

[0001] The present invention relates to a catalytic electrode having the function of electrochemically reducing carbon dioxide to produce acetic acid under light irradiation, a method for producing the catalytic electrode, and a photoelectrochemical cell equipped with the catalytic electrode. [Background technology]

[0002] A catalytic electrode technology is disclosed that has the function of electrochemically reducing carbon dioxide to produce acetic acid under light irradiation.

[0003] For example, Non-Patent Document 1 describes a CuO / Cu(I)FeO2-based photoelectrode catalyst. Non-Patent Document 1 describes a photoelectrochemical reduction reaction of carbon dioxide using a CuO / CuFeO2-based photoelectrode catalyst, resulting in a potential of 0.17 V (RHE) and a current density of 0.1 mA / cm². 2 Under these conditions, the selectivity for acetic acid production (Faraday efficiency FE of acetic acid) is described as 80%. However, the method in Non-Patent Literature 1 has the problem that Fe gradually dissolves into the electrolyte, causing a significant drop in current about 10 minutes after electrolysis (see Fig. S15 in Non-Patent Literature 1).

[0004] Non-patent document 2 describes a Cu2O / Cu(I)FeO2-based nanorod photoelectrode catalyst. Non-patent document 2 describes a photoelectrochemical reduction reaction of carbon dioxide using a Cu2O / CuFeO2-based nanorod photoelectrode catalyst, resulting in a potential of 0.35 V (RHE) and a current density of 1.1 mA / cm². 2 Under these conditions, the Faraday efficiency FE of acetic acid is described as 69%. In the method described in Non-Patent Literature 2, the current density is 1.1 mA / cm². 2 Although it has improved, the Faraday efficiency (FE) of acetic acid is still low at 69%.

[0005] Thus, conventional photoelectrode catalyst materials such as CuFeO2 have problems such as catalyst instability due to Cu deposition and Fe elution, and low Faraday efficiency FE of acetic acid. [Prior art documents] [Non-patent literature]

[0006] [Non-Patent Document 1] Yang, X., Baker, LR, et. al, Photoelectrochemical CO2 reduction to Acetate on Iron-Copper Oxide Catalysts. ACS Catal., 7, 177-180 (2017). [Non-Patent Document 2] Oh, SH, et. al, Photoelectrochemical CO2 reduction via Cu2O / CuFeO2Hierarchical nanorods photocatalyst. Chem Cat Chem, 12, 5185-5191 (2020). [Overview of the Initiative] [Problems that the invention aims to solve]

[0007] The object of the present invention is to provide a catalytic electrode that has the function of electrochemically reducing carbon dioxide under light irradiation to produce acetic acid, having high selectivity for acetic acid production and good stability, a method for producing the catalytic electrode, and a photoelectrochemical cell equipped with the catalytic electrode. [Means for solving the problem]

[0008] The present invention relates to a catalytic electrode having the function of electrochemically reducing carbon dioxide under light irradiation to produce acetic acid, wherein the catalyst is supported on a conductive substrate, and the catalyst is an oxide composed of Cu and Fe, and has CuFe2O4 as its main crystal structure.

[0009] In the catalyst electrode, it is preferable that the catalyst is composed of a first catalyst phase of CuFe2O4, or composed of a first catalyst phase of CuFe2O4 and a second catalyst phase of CuO, and that the atomic composition ratio Fe / Cu in the catalyst is 2.5 or less.

[0010] In the catalyst in the catalyst electrode, the catalyst preferably includes a phase other than CuFe2O4 of the first catalyst phase, or a phase other than CuFe2O4 of the first catalyst phase and CuO of the second catalyst phase, and the content of the phase is 1% or less relative to CuFe2O4.

[0011] The present invention relates to a method for producing a catalyst electrode having the function of electrochemically reducing carbon dioxide under light irradiation to produce acetic acid, wherein the catalyst is supported on a conductive substrate, and the method for producing a catalyst electrode includes a heating step of heating CuFeO2 in an atmospheric atmosphere at 400 to 700°C to obtain the catalyst which is composed of a first catalyst phase CuFe2O4 and a second catalyst phase CuO.

[0012] The present invention relates to a photoelectrochemical cell for electrochemically reducing carbon dioxide under light irradiation to produce acetic acid, comprising: a catalyst electrode; an oxidation electrode; and an electrolyte layer between the catalyst electrode and the oxidation electrode, the electrolyte layer comprising a solvent, carbon dioxide, and an electrolyte. [Effects of the Invention]

[0013] The present invention provides a catalytic electrode that has the function of electrochemically reducing carbon dioxide under light irradiation to produce acetic acid, exhibiting high selectivity and good stability in acetic acid production, a method for producing the catalytic electrode, and a photoelectrochemical cell equipped with the catalytic electrode. [Brief explanation of the drawing]

[0014] [Figure 1] (a) A schematic diagram of the light-irradiated three-electrode cell viewed from directly above, used in the example (Source: EC Frontier product, https: / / ec-frontier.co.jp / product / cell / VB1300.php), and (b) A photograph of the light-irradiated three-electrode cell viewed from the side. [Figure 2]XRD patterns of the CuFeO2 powder and the catalyst-supported / Ti film of Example 1, showing diffraction lines of 2H-CuFeO2 (04-009-4371), cubic CuFe2O4 (04-005-8587), CuO (00-045-0937), and Ti (04-004-8487). [Figure 3] XRD pattern of the catalyst-supported / Ti film of Example 2, showing diffraction lines of cubic CuFe2O4 (04-005-8587), CuO (00-045-0937), and Ti (04-004-8487). [Figure 4] XRD pattern of the catalyst-supported / Ti film of Example 3, showing diffraction lines of cubic CuFe2O4 (04-005-8587) and Ti (04-004-8487). [Figure 5] XRD pattern of the catalyst-supported / Ti film of Example 4, showing diffraction lines of Fe2O3 (Hematite, 04-003-2900) and Ti (04-004-8487). [Figure 6] XRD pattern of the catalyst-supported / Ti film of Comparative Example 1, showing diffraction lines of 3R-CuFeO2 (04-007-2807) and Ti (04-004-8487). [Figure 7] XRD pattern of the catalyst-supported / Ti film of Comparative Example 2, showing diffraction lines of 2H-CuFeO2 (04-009-4371), 3R-CuFeO2 (04-007-2807), and Ti (04-004-8487). [Figure 8] XRD pattern of the catalyst-supported / Ti film of Comparative Example 3, showing diffraction lines of 2H-CuFeO2 (04-009-4371), cubic CuFe2O4 (04-005-8587), CuO (00-045-0937), and Ti (04-004-8487). [Figure 9] XRD pattern of the catalyst-supported / Ti film of Comparative Example 4, showing diffraction lines of cubic CuFe2O4 (04-005-8587), CuO (00-045-0937), Fe2O3 (Hematite, [04-003-2900]), and Ti (04-004-8487). [Figure 10]This figure shows the XRD patterns of the catalyst-supported / Ti film in Comparative Example 5, with diffraction lines for 3R-CuFeO2 (04-007-2807), tetragonal CuFe2O4 (04-005-8587), CuO (00-045-0937), and Ti (04-004-8487). [Figure 11] This graph shows the dependence of acetic acid (FE) on the Fe / Cu atomic composition ratio for Examples 1-4 and Comparative Examples 1-5. [Figure 12] This figure shows the XRD patterns of the 2H-CuFeO2 single-phase catalyst electrode of Comparative Example 2 before and after photoelectrochemical CO2 electrolysis. [Figure 13] This figure shows the SEM observation results of the surface of the 2H-CuFeO2 single-phase catalyst electrode of Comparative Example 2 before (left) and after (right) photoelectrochemical CO2 electrolysis. [Figure 14] The images show a photograph (left) and a schematic diagram (right) of the measurement system for in-situ XAFS measurement of the photoelectrochemical CO2 electrolysis reaction. [Figure 15] This figure shows the in-situ EXAFS spectrum during a photoelectrochemical CO2 electrolysis reaction. [Modes for carrying out the invention]

[0015] Embodiments of the present invention will be described below. This embodiment is just one example of how the present invention can be implemented, and the present invention is not limited to this embodiment.

[0016] [Catalytic electrode] The catalyst electrode according to this embodiment has the function of electrochemically reducing carbon dioxide under light irradiation to produce acetic acid, and is a catalyst electrode in which the catalyst is supported on a conductive substrate. The catalyst used in this catalyst electrode is an oxide composed of Cu and Fe, and has CuFe2O4 as its main crystalline structure.

[0017] As described above, conventional photoelectrode catalysts using Cu(I)FeO2, which have the function of electrochemically reducing carbon dioxide under light irradiation to produce acetic acid, have problems such as Cu deposition and Fe elution, making the catalyst unstable, and a low Faraday efficiency FE of acetic acid (hereinafter sometimes referred to as "acetic acid FE"). The inventors of the present invention considered that when carboxyl groups generated by the supply of hydrogen ions and electrons to CO2, or their decomposition product CO, are adsorbed onto the Fe surface, acetic acid is preferentially produced, and that Cu(2) CuFe oxide is more stable than Cu(1) oxide, and therefore focused on the ferrite Cu(II)Fe2O4 / CuO system. They then found that by using a catalyst that is an oxide composed of Cu and Fe and has CuFe2O4 as its main crystal structure, a catalyst electrode with high selectivity for acetic acid production and good stability can be obtained, which has the function of electrochemically reducing carbon dioxide under light irradiation to produce acetic acid.

[0018] Catalysts containing Cu(II)Fe2O4, which consists of divalent Cu and trivalent Fe, exhibit higher selectivity (acetic acid FE) for acetic acid production and better stability compared to Cu(I)FeO2, which consists of monovalent Cu and has been previously reported to photoelectrochemically reduce carbon dioxide to form acetic acid. The mechanism by which these effects are obtained is hypothesized as follows.

[0019] In Cu(I)FeO2, during the photoelectrochemical carbon dioxide reduction reaction, Fe gradually dissolves into the electrolyte, and Cu is reduced, resulting in the deposition of Cu metal, making it unstable. In contrast, in catalysts that are oxides composed of Cu and Fe and have CuFe2O4 as the main crystalline structure, such as a single-phase Cu(II)Fe2O4 catalyst or a mixed-phase catalyst of Cu(II)Fe2O4 and Cu(II)O, the reduction to Cu is suppressed during the photoelectrochemical carbon dioxide reduction reaction, and the photogenerated charge is mainly used for acetic acid production by carbon dioxide reduction, resulting in a high acetic acid FE. Furthermore, the mixed-phase catalyst of Cu(II)Fe2O4 and Cu(II)O has a higher acetic acid FE and a larger reduction current compared to the single-phase Cu(II)Fe2O4 catalyst, meaning that the acetic acid production rate is improved. This is presumed to be because the presence of Cu(II)O between Cu(II)Fe2O4 particles reduces interparticle resistance and improves the conductivity of the film, thereby increasing the reduction current.

[0020] These Cu(II)Fe2O4 single-phase catalysts, or mixed-phase catalysts of Cu(II)Fe2O4 and Cu(II)O, can be formed, for example, by heat-treating Cu(I)FeO2 in air. However, α-Fe2O3 (Hematite) may be produced during this heat treatment. α-Fe2O3 has catalytic activity to reduce water and produce hydrogen, and since the photoproductive charge is used for hydrogen production, the amount of acetate FE decreases. Therefore, it is important to use production conditions that minimize the production of α-Fe2O3 (for an example of hydrogen production by α-Fe2O3, see "https: / / pubs.rsc.org / en / content / articlehtml / 2017 / ta / c7ta00431a").

[0021] Furthermore, it was found that both catalyst electrodes containing conventional materials, Cu(I)FeO2 and Cu(II)Fe2O4, showed a significant improvement in acetic acid (FE) when electrochemical reduction was performed under light irradiation compared to electrochemical reduction by applying voltage in the dark.

[0022] The conductive substrate is a component that supports the catalyst and is not particularly limited as long as it is conductive. Examples of conductive substrates include plate-shaped members and mesh-shaped members containing metal or semiconductor. There are no particular restrictions on the metal used as the conductive substrate, but examples include titanium (Ti), silver (Ag), gold (Au), copper (Cu), zinc (Zn), indium (In), cadmium (Cd), tin (Sn), palladium (Pd), and lead (Pb). There are no particular restrictions on the semiconductor used as the conductive substrate, but examples include titanium oxide (TiO2), tin oxide (SnO2), silicon (Si), strontium titanate (SrTiO3), zinc oxide (ZnO), and tantalum oxide (Ta2O5).

[0023] The catalyst is an oxide composed of Cu and Fe, and is not particularly limited as long as it is a material whose main crystalline structure is CuFe2O4. Hereinafter, in this specification, "a material whose main crystalline structure is CuFe2O4" means a material in which the content of CuFe2O4 in the material constituting the catalyst is 50% or more, preferably 60% or more.

[0024] The CuFe2O4 content can be estimated from the ratio of the strongest diffraction peak area of ​​CuFe2O4 to the sum of the strongest diffraction peak areas of CuFe2O4 and the strongest diffraction peak areas of other crystal structures in the X-ray diffraction (XRD) pattern of the catalyst layer.

[0025] Examples of materials that are oxides composed of Cu and Fe and have CuFe2O4 as the main crystalline structure include a single phase of Cu(II)Fe2O4, a mixed phase of Cu(II)Fe2O4 and Cu(II)O, and a mixed phase of Cu(II)Fe2O4 and Cu(I)2O. The catalyst is composed of, for example, a first catalytic phase of CuFe2O4, or a first catalytic phase of CuFe2O4 and a second catalytic phase of CuO. The catalyst is defined as the first catalytic phase, the second catalytic phase, and the third phase, in order of decreasing diffraction peak area, among the crystalline phases consisting of one or more catalytic phases assigned by X-ray diffraction peaks. Furthermore, "a catalyst composed of CuFe2O4 in the first catalytic phase" includes cases where impurities other than CuFe2O4 are present, for example, impurities of 2 mol% or less, and "a catalyst composed of CuFe2O4 in the first catalytic phase and CuO in the second catalytic phase" includes cases where impurities other than "CuFe2O4 and CuO" are present, for example, impurities of 1 mol% or less.

[0026] The Fe / Cu atomic composition ratio in the catalyst is preferably 2.5 or less, and more preferably 2.2 or less. If the Fe / Cu atomic composition ratio in the catalyst exceeds 2.5, the FE of products other than acetic acid, such as formic acid, hydrogen, and carbon monoxide, may increase. The Fe / Cu atomic composition ratio in the catalyst can be measured by inductively coupled plasma emission spectrometry (ICP-OES). The Fe / Cu atomic composition ratio in the catalyst can be adjusted, for example, by changing the molar ratio of the Cu compound and Fe compound used in the synthesis of the raw material CuFeO2.

[0027] In the catalyst, if a phase other than CuFe2O4 in the first catalytic phase, or a phase other than CuFe2O4 in the first catalytic phase and CuO in the second catalytic phase (third phase) is present, it is preferable that the content of this phase is 1% or less relative to CuFe2O4, and more preferably 0.5% or less. If the content of the above phase exceeds 1%, the amount of acetic acid (FE) may decrease.

[0028] This content can be estimated from the ratio of the peak area of ​​the second catalytic phase to the peak area of ​​the first catalytic phase CuFe2O4 in the X-ray diffraction (XRD) pattern of the catalyst layer, and from the ratio of the peak areas of the phase other than the first catalytic phase and the second catalytic phase (third phase). This allows us to estimate the content of the second catalytic phase and the third phase relative to the first catalytic phase CuFe2O4.

[0029] The catalyst electrode according to this embodiment can achieve a selectivity for acetic acid production (Faraday efficiency FE of acetic acid) of 65% or more, preferably 90% or more, by electrochemically reducing carbon dioxide under light irradiation to produce acetic acid. Furthermore, the current retention rate, which is an indicator of the stability of the catalyst electrode and is expressed as the ratio of the current i after 1 hour of potential application to the current i after 0.1 hours of potential application in it measurement, can be achieved at 60% or more, preferably 70% or more.

[0030] [Method for fabricating catalyst electrodes] The method for producing a catalyst electrode according to this embodiment is a method for producing a catalyst electrode having the function of electrochemically reducing carbon dioxide under light irradiation to produce acetic acid, wherein the catalyst is supported on a conductive substrate, and includes a heating step of heating CuFeO2 in an atmospheric atmosphere at 400 to 700°C to obtain a catalyst composed of a first catalyst phase CuFe2O4 and a second catalyst phase CuO.

[0031] While a single phase of Cu(II)Fe2O4 can be formed by heating Cu(I)FeO2 in air, the inventors have found that a catalyst composed of a first catalytic phase of CuFe2O4 and a second catalytic phase of CuO can be obtained by heating under atmospheric conditions of 400-700°C.

[0032] The heat treatment temperature is, for example, in the range of 400 to 700°C, and preferably in the range of 500 to 650°C. If the heat treatment temperature is below 400°C, Cu(I)FeO2 may not react and become the first catalytic phase, and if it exceeds 700°C, the substrate may oxidize, and the conductivity of the substrate may decrease.

[0033] The heating time depends on the heating temperature, but is preferably in the range of 1 to 24 hours, for example, and preferably in the range of 1 to 2 hours. If the heating time is less than 1 hour, Cu(II)Fe2O4 may not be formed, and if it exceeds 24 hours, the substrate may oxidize, and the conductivity of the substrate may decrease.

[0034] The heat treatment atmosphere may be an atmospheric atmosphere, a nitrogen atmosphere, or an oxygen-free atmosphere (for example, with an oxygen content of 10% or less). An atmospheric atmosphere is preferred for the heat treatment.

[0035] The raw material, CuFeO2, can be obtained by known methods such as hydrothermal synthesis or citric acid complex polymerization (see References A and B in the Examples section).

[0036] For example, a catalyst composed of a first catalytic phase of CuFe2O4 and a second catalytic phase of CuO can be obtained by heat-treating CuFeO2 supported on a conductive substrate in an atmospheric environment at 400 to 700°C, preferably 500 to 650°C.

[0037] [Photoelectrochemical cell] The photoelectrochemical cell according to this embodiment is a photoelectrochemical cell for producing acetic acid by electrochemically reducing carbon dioxide under light irradiation. The photoelectrochemical cell comprises a catalyst electrode that promotes the reduction reaction of carbon dioxide by light irradiation, an oxidation electrode that promotes the oxidation reaction, and an electrolyte layer between the catalyst electrode and the oxidation electrode that contains a solvent, carbon dioxide, and an electrolyte. The photoelectrochemical cell may also include a translucent housing that houses the catalyst electrode, the oxidation electrode, and the electrolyte layer, and through which light is irradiated onto the catalyst electrode. In the photoelectrochemical cell, for example, the oxidation electrode and the reduction electrode are immersed in an electrolyte solution, which is the electrolyte layer housed in the housing.

[0038] A photoelectrochemical cell is a cell in which, for example, an electrolyte solution, obtained by dissolving an electrolyte in a solvent, is housed in a casing to form an electrolyte layer, and a catalytic electrode that promotes the reduction reaction of carbon dioxide and an oxidation electrode that promotes the oxidation reaction are immersed in the electrolyte solution, with the reduction electrode and the oxidation electrode being electrically connected. The electrolyte solution contains carbon dioxide. The carbon dioxide may be pre-saturated in the electrolyte solution, or carbon dioxide gas may be bubbled or flowed into the system.

[0039] An oxidation electrode is an electrode used to oxidize a substance through an oxidation reaction. For example, an oxidation electrode is composed of a conductive substrate and an oxidation catalyst layer formed on it.

[0040] The same conductive substrate as that used for catalyst electrodes can be used.

[0041] The oxidation catalyst constituting the oxidation catalyst layer can be any material having oxidation catalyst function; there are no particular restrictions. Examples of materials with oxidation catalyst function include materials containing iridium oxide (IrOx). Iridium oxide can be supported on the surface of a conductive substrate as a nanocolloidal solution (see T. Arai et.al, Energy Environ. Sci 8, 1998 (2015)).

[0042] For example, nanocolloidal iridium oxide (IrOx) is synthesized. Next, 50 mL of 2 mM potassium iridium(IV) chloride (K2IrCl6) aqueous solution is mixed with 10 wt% sodium hydroxide (NaOH) aqueous solution to adjust the pH to 13, and this yellow solution is heated at 90°C for 20 minutes using a hot stirrer. The resulting blue solution is cooled in ice water for 1 hour. Then, 3 M nitric acid (HNO3) is added dropwise to the cooled solution (20 mL) to adjust the pH to 1, and the mixture is stirred for 80 minutes to obtain an aqueous solution of iridium oxide (IrOx) nanocolloids. Furthermore, 1-2 mL of 1.5 wt% NaOH aqueous solution is added dropwise to this solution to adjust the pH to 12. The resulting aqueous solution of iridium oxide (IrOx) nanocolloids is coated onto a conductive substrate at pH 12 and dried in a drying oven at 60°C for 40 minutes. After drying, the precipitated salt can be washed with ultrapure water to form an oxide electrode. Furthermore, the application and drying of the iridium oxide (IrOx) nanocolloid aqueous solution may be repeated multiple times.

[0043] Water or other solvents can be used for the electrolyte layer.

[0044] Potassium dihydrogen phosphate (KH2PO4), dipotassium hydrogen phosphate (K2HPO4), potassium bicarbonate (KHCO3), potassium carbonate (K2CO3), sodium bicarbonate (NaHCO3), sodium carbonate (Na2CO3), etc., can be used as the electrolyte for the electrolyte layer. The electrolyte layer is preferably a phosphate buffer solution or a borate buffer solution.

[0045] Carbon dioxide (CO2) can be incorporated into the electrolyte layer as a reaction substrate by bubbling or other methods.

[0046] A separator (membrane) that separates the liquid and gas and allows protons to move may be provided between the reducing electrode and the oxidizing electrode. The separator can be made of any material that separates the liquid and gas and allows protons to move, and there are no particular restrictions, but for example, a solid polymer electrolytic membrane such as Nafion® can be used.

[0047] The housing is a component that houses the catalyst electrode, the oxidation electrode, and the electrolyte layer. The housing is made of a material that has the mechanical strength necessary to constitute a photoelectrochemical cell. For example, the housing can be made of metal, plastic, or the like. Preferably, the housing is a translucent housing that allows light to be irradiated onto the catalyst electrode.

[0048] When the reduction electrode is irradiated with appropriate light, such as sunlight, containing light with a wavelength of 250-1200 nm, carbon dioxide (CO2) is reduced at the reduction electrode to produce acetic acid (CH3COOH). The light intensity is, for example, 1-200 mW / cm². 2 It is within the range of [the specified range]. [Examples]

[0049] The present invention will be described in more detail below with reference to examples and comparative examples, but the present invention is not limited to the following examples.

[0050] [Fabrication and evaluation of catalyst electrodes] <Example 1> Hexagonal 2H-CuFeO2 powder was synthesized as follows, following the preparation process described in reference A (Y. Jin, G. Chumanov, RSC Adv., 2016, 6, 26392-26397).

[0051] The raw materials, CuI (0.7 mmol) and FeCl3 (0.7 mmol), were dissolved in ultrapure water (conductivity 18 MW, 3.5 mL) that had been pre-bubbled with nitrogen (N2) for 30 minutes. Then, KOH (55.5 mmol) solid was added and the mixture was bubbled with N2 for 15 minutes. An exothermic reaction between the raw materials and KOH yielded a dark brown slurry solution. The slurry solution was then heated in an oven at 120°C for 24 hours, after which ultrapure water was added, and the by-product salt (KCl) was removed by vacuum filtration using a vacuum filter and a filter (made of Teflon®, pore size 200 nm). Finally, after vacuum drying (80°C, 15 hours), the powder was finely ground in a mortar to obtain 2H-CuFeO2 powder.

[0052] 20 mg of the obtained single-phase 2H-CuFeO2 powder was mixed with 2 mL of 2-propanol, and a 2H-CuFeO2 slurry solution was prepared by repeating a cycle of 30 seconds of operation followed by 30 seconds of rest 20 times using an ultrasonic homogenizer (Biorepter BR-II, 300 W output). Masking tape was applied to 5 mm widths on all four sides of a 25 mm square Ti substrate, and the 2H-CuFeO2 slurry solution was dropped onto the center (15 mm square) of the Ti surface. This process of vacuum drying at room temperature (25°C) was repeated 15 times, resulting in a catalyst concentration of 2.7 mg / cm³. 2 The material was then supported. After drying at 100°C for 10 minutes, it was heat-treated at 650°C for 1 hour (in an air atmosphere) to obtain a catalyst electrode.

[0053] X-ray diffraction (XRD) measurements were performed on the powder and catalyst-supported samples on a Ti substrate using an X-ray diffraction (XRD) instrument (Rigaku, Ultima IV) under the following measurement conditions. (Measurement conditions) 2θ / θ scan: 20°~80° Sweep speed: 10° / min X-ray intensity: 40kV, 40mA X-ray wavelength (CuKα): 1.5418 Å

[0054] The CO2 electrolytic reduction characteristics under light irradiation were evaluated using a three-electrode cell with a light irradiation window (EC Frontier, VB1300), a potentiostat (Biologic, VMP3), and a high-brightness xenon lamp light source (Asahi Spectroscopy, HAL-C100) as shown in Figure 1. A catalyst electrode (Ti substrate with catalyst film) prepared as described above was used as the working electrode, a Pt wire was used as the counter electrode, and Hg / Hg2SO4 was used as the reference electrode. A diaphragm (Furukawa Agency, Nafion® N117) was placed between the working electrode and the counter electrode, and a 0.2M KH2PO4 + 0.2M K2HPO4 aqueous solution (60 mL) was used as the electrolyte. CO2 gas (100%) was bubbled into the electrolyte on the working electrode side.

[0055] Electrochemical measurements were performed in the following order: Light (1 sun = 100 mW / cm²) was applied to the working electrode. 2The sample was irradiated with light, and linear sweep voltammetry (LSV) was measured. Then, LSV was measured again in the dark, followed by light irradiation and current-time (it) measurement under constant potential control (1 hour). Finally, it measurement was performed in the dark (1 hour). After each it measurement under light irradiation and in the dark, the electrolyte was collected and acetate ions were quantified using an ion chromatograph (ThermoFisher Scientific, Integrion RFIC). Based on the reaction equation (1) below for the production of acetic acid from CO2, the Faraday efficiency FE of acetate was calculated using equation (2) below. As shown in equation (3) below, the ratio of the current i after 1 hour of potential application to the current i after 0.1 hours of potential application was used as the current maintenance rate and as an indicator of stability. 2CO2 + 8H + +8e - →CH3COOH+2H2O (1) Acetic acid FE = Aa × 8 / (C / F) × 100 (%) (2) (Aa is the amount of acetic acid produced (mol), C is the charge, and F is the Faraday constant (96485.3 C / mol).) Current maintenance rate (%) = 100 × (current (after 1 hour) / current (after 0.1 hour)) (3)

[0056] Figure 2 shows the X-ray diffraction (XRD) patterns of CuFeO2 powder and the catalyst-supported / Ti film from Example 1. From the X-ray diffraction (XRD) patterns shown in Figure 2, the diffraction peak position of the CuFeO2 powder coincided with that of 2H-CuFeO2. After heat treatment, the diffraction peak of 2H-CuFeO2 disappeared from the catalyst film, and diffraction peaks of tetragonal CuFe2O4 and CuO appeared, indicating that the catalyst film is composed of a mixed phase of tetragonal CuFe2O4 and CuO. From the ratio of the peak area of ​​the second catalyst phase CuO to the peak area of ​​the strongest diffraction peak of the first catalyst phase CuFe2O4, the CuO content relative to CuFe2O4 was estimated to be 10%. Using inductively coupled plasma atomic emission spectrometry (ICP-OES) (Hitachi High-Tech Science, PS3520UVDDII), the sample was dissolved by acid decomposition to prepare the test solution. Iron (Fe) and copper (Cu) were quantitatively analyzed, and the atomic composition ratio (Fe / Cu) of the catalyst was measured. The results for the atomic composition ratio (Fe / Cu), content, and electrochemical properties are shown in Table 1.

[0057] <Example 2> According to the production process of the citric acid polymerization method in Document B ("Powders and Powder Metallurgy", Vol. 46, No. 2, pp. 185-188), rhombohedral 3R-CuFeO2 powder was synthesized as follows.

[0058] After dissolving 40 mmol of raw material citric acid monohydrate and 100 g of water at a set liquid temperature of 60 °C, CuCO3·Cu(OH)2 (7.5 mmol) and iron citrate (FeC6H5O7·nH2O) (15 mmol) were added and stirred for 1 hour. Further, water was removed using a hot stirrer (set liquid temperature of 100 °C, about 1 hour) to obtain a precursor gel. Then, the gel was calcined at 350 °C for 5 hours in an oven, and then fired in an electric furnace at 700 °C for 12 hours (in a nitrogen (N2) atmosphere) to obtain a mixed-phase powder of 3R-CuFeO2 and CuFe2O4.

[0059] 24.3 mg of the obtained mixed-phase powder of 3R-CuFeO2 and CuFe2O4 was mixed with 2-propanol (2.43 mL), and the ultrasonic homogenizer (Biorepter BR-II, output 300 W) was operated for 30 seconds and stopped for 30 seconds, repeating 20 cycles to prepare a 3R-CuFeO2 slurry solution. Masking tape was pasted on the 5 mm width of the four sides of a 25 mm square Ti substrate, and the 3R-CuFeO2 slurry solution was dropped onto the central part (15 mm square) of the Ti surface, and the process of vacuum drying at room temperature was repeated 15 times to support the catalyst at 2.7 mg / cm 2 After that, it was dried at 100 °C for 10 minutes and then heat-treated in the air at 500 °C for 1 hour to obtain a catalyst electrode.

[0060] In the same manner as in Example 1, X-ray diffraction measurement of the catalyst-supported sample on the Ti substrate was performed. Further, in the same manner as in Example 1, photoelectrochemical CO2 electrolytic reduction was performed using a three-electrode cell with a light irradiation window. The atomic composition ratio (Fe / Cu) of the catalyst was measured using a fluorescence X-ray analysis (XRF) apparatus (manufactured by Rigaku, ZSX PrimusII).

[0061] Figure 3 shows the XRD pattern of the catalyst-supported / Ti film in Example 2. From the X-ray diffraction (XRD) pattern shown in Figure 3, the catalytic film after heat treatment coincides with the diffraction peak positions of tetragonal CuFe2O4 and CuO, indicating that the catalytic film is composed of a mixed phase of tetragonal CuFe2O4 and CuO. The CuO content relative to CuFe2O4 was estimated to be 25% from the ratio of the peak area of ​​the second catalytic phase CuO to the strongest diffraction peak area of ​​the first catalytic phase CuFe2O4. The atomic composition ratio (Fe / Cu), content, and electrochemical properties are shown in Table 1.

[0062] <Example 3> Following the citrate polymerization process described in reference B, tetragonal CuFe2O4 powder was synthesized as follows.

[0063] The raw material, citric acid monohydrate (40 mmol), was dissolved in 100 g of water at a liquid temperature of 60°C. Then, CuCO3·Cu(OH)2 (5 mmol) and iron citrate (FeC6H5O7·nH2O) (20 mmol) were added and the mixture was stirred for 1 hour. Furthermore, the water was removed using a hot stirrer (liquid temperature set to 100°C, approximately 1 hour) to obtain the precursor gel. After that, the gel was calcined in an oven at 350°C for 5 hours, and then fired in an electric furnace at 800°C for 12 hours (in an air atmosphere) to obtain CuFe2O4 powder.

[0064] 24.3 mg of the obtained CuFe2O4 powder was mixed with 2-propanol (2.43 mL), and a CuFe2O4 slurry solution was prepared by repeating a cycle of 30 seconds of operation followed by 30 seconds of rest 20 times using an ultrasonic homogenizer (Biorepter BR-II, output 300 W). Masking tape was applied to 5 mm widths on all four sides of a 25 mm square Ti substrate, and the CuFe2O4 slurry solution was dropped onto the center (15 mm square) of the Ti surface. This process of vacuum drying at room temperature was repeated 15 times, resulting in a catalyst concentration of 2.7 mg / cm³. 2 The material was then supported. After drying at 100°C for 10 minutes, the catalyst electrode was heat-treated in air at 500°C for 1 hour.

[0065] X-ray diffraction measurements were performed on catalyst-supported samples on a Ti substrate in the same manner as in Example 1. Furthermore, photoelectrochemical CO2 electrolytic reduction was performed using a three-electrode cell with a light irradiation window, in the same manner as in Example 1. The atomic composition ratio (Fe / Cu) of the catalyst was measured by X-ray fluorescence analysis (XRF), in the same manner as in Example 2.

[0066] Figure 4 shows the XRD pattern of the catalyst-supported / Ti film in Example 3. From the X-ray diffraction (XRD) pattern shown in Figure 4, the diffraction peak position of the catalyst film after heat treatment coincides with that of tetragonal CuFe2O4, indicating that the catalyst film is composed of a single phase of tetragonal CuFe2O4. Table 1 shows the atomic composition ratio (Fe / Cu), content, and electrochemical properties.

[0067] <Example 4> 24.3 mg of commercially available CuFe2O4 powder (Aldrich) was mixed with 2.43 mL of 2-propanol, and a CuFe2O4 slurry solution was prepared by repeating a cycle of 30 seconds of operation followed by 30 seconds of rest 20 times using an ultrasonic homogenizer (Biorepter BR-II, 300 W output). Masking tape was applied to 5 mm widths on all four sides of a 25 mm square Ti substrate, and the CuFe2O4 slurry solution was dropped onto the center of the Ti surface (15 mm square). This process of vacuum drying at room temperature was repeated 15 times, resulting in a catalyst concentration of 2.7 mg / cm³. 2 The material was then supported. After drying at 100°C for 10 minutes, it was heat-treated at 500°C for 1 hour (in an air atmosphere) to obtain a catalyst electrode.

[0068] X-ray diffraction measurements were performed on catalyst-supported samples on a Ti substrate in the same manner as in Example 1. Furthermore, photoelectrochemical CO2 electrolytic reduction was performed using a three-electrode cell with a light irradiation window, in the same manner as in Example 1. The atomic composition ratio (Fe / Cu) of the catalyst was measured by X-ray fluorescence analysis (XRF), in the same manner as in Example 2.

[0069] Figure 5 shows the XRD pattern of the catalyst-supported / Ti film of Example 4. From the X-ray diffraction (XRD) pattern shown in Figure 5, the diffraction peak positions of the catalyst film after heat treatment coincide with those of tetragonal CuFe2O4, CuO, and Fe2O3 (hematite). Therefore, the catalyst film is composed of a first catalytic phase of tetragonal CuFe2O4, a second catalytic phase of CuO, and a third phase of Fe2O3. From the ratio of the peak areas of the second catalytic phase CuO and the third catalytic phase Fe2O3 to the strongest diffraction peak area of ​​the first catalytic phase CuFe2O4, the content of the second catalytic phase CuO was estimated to be 3% and the content of the third phase Fe2O3 was estimated to be 1% relative to CuFe2O4. The atomic composition ratio (Fe / Cu), content, and electrochemical properties are shown in Table 1.

[0070] <Comparative Example 1> 24.3 mg of commercially available CuFeO2 powder (manufactured by Furuchi Chemical Co., Ltd.) was mixed with 2-propanol (2.43 mL), and a CuFeO2 slurry solution was prepared by repeating a cycle of 30 seconds of operation followed by 30 seconds of rest 20 times using an ultrasonic homogenizer (Biorepter BR-II, output 300 W). Masking tape was applied to 5 mm widths on all four sides of a 25 mm square Ti substrate, and the CuFeO2 slurry solution was dropped onto the center of the Ti surface (15 mm square). This process of vacuum drying at room temperature was repeated 15 times, resulting in a catalyst concentration of 2.7 mg / cm³. 2 The material was then supported. After drying at 100°C for 10 minutes, it was heat-treated at 500°C for 1 hour (in an air atmosphere) to obtain a catalyst electrode.

[0071] X-ray diffraction measurements were performed on catalyst-supported samples on a Ti substrate in the same manner as in Example 1. Furthermore, photoelectrochemical CO2 electrolytic reduction was performed using a three-electrode cell with a light irradiation window, in the same manner as in Example 1. The atomic composition ratio (Fe / Cu) of the catalyst was measured by X-ray fluorescence analysis (XRF), in the same manner as in Example 2.

[0072] Figure 6 shows the XRD pattern of the catalyst-supported / Ti film of Comparative Example 1. From the X-ray diffraction (XRD) pattern shown in Figure 6, the diffraction peak position of the catalyst film after heat treatment coincides with that of rhombohedral 3R-CuFeO2, indicating that the catalyst film is composed of a single phase of 3R-CuFeO2 as the first catalytic phase. Table 1 shows the atomic composition ratio (Fe / Cu), content, and electrochemical properties.

[0073] <Comparative Example 2> Following the preparation process described in reference A, hexagonal 2H-CuFeO2 powder was synthesized as follows.

[0074] The raw materials, CuI (15 mmol) and FeCl3·6H2O (15 mmol), were dissolved in ultrapure water (conductivity 18 MW, 70 mL) that had been pre-bubbled with nitrogen (N2) for 30 minutes. Then, NaOH (1.1 mol) solid was added, and the mixture was bubbled with N2 for 1 hour. The mixture was then stirred in a sealed container for 1 hour. An exothermic reaction between the raw materials and NaO yielded a dark brown slurry solution. The slurry solution was then heated in an oven at 150°C for 24 hours, after which ultrapure water was added and the mixture was centrifuged three times for washing. Finally, the mixture was freeze-dried to obtain 2H-CuFeO2 powder.

[0075] 20 mg of the obtained 2H-CuFeO2 powder was mixed with 2 mL of 2-propanol, and a 2H-CuFeO2 slurry solution was prepared by repeating a cycle of 30 seconds of operation followed by 30 seconds of rest 20 times using an ultrasonic homogenizer (Biorepter BR-II, 300 W output). Masking tape was applied to 5 mm widths on all four sides of a 25 mm square Ti substrate, and the 2H-CuFeO2 slurry solution was dropped onto the center (15 mm square) of the Ti surface. This process of vacuum drying at room temperature was repeated 15 times, resulting in a catalyst concentration of 2.7 mg / cm³. 2 The material was then supported. After drying at 100°C for 10 minutes, it was heat-treated at 500°C for 1 hour (in an air atmosphere) to obtain a catalyst electrode.

[0076] X-ray diffraction measurements were performed on catalyst-supported samples on a Ti substrate in the same manner as in Example 1. Furthermore, photoelectrochemical CO2 electrolytic reduction was performed using a three-electrode cell with a light irradiation window, in the same manner as in Example 1. The atomic composition ratio (Fe / Cu) of the catalyst was measured by X-ray fluorescence analysis (XRF), in the same manner as in Example 2.

[0077] Figure 7 shows the XRD pattern of the catalyst-supported / Ti film of Comparative Example 2. From the X-ray diffraction (XRD) pattern shown in Figure 7, the diffraction peak position of the catalyst film after heat treatment coincides with that of 2H-CuFeO2, indicating that the catalyst film is composed of a single phase of 2H-CuFeO2 as the first catalyst phase. The atomic composition ratio (Fe / Cu), content, and electrochemical properties are shown in Table 1.

[0078] <Comparative Example 3> Following the preparation process described in reference A, hexagonal 2H-CuFeO2 powder was synthesized as follows.

[0079] The raw materials, CuI (0.7 mmol) and FeCl3 (0.7 mmol), were dissolved in ultrapure water (conductivity 18 MW, 3.5 mL) that had been pre-bubbled with nitrogen (N2) for 30 minutes. Then, KOH (55.5 mmol) solid was added, and the mixture was bubbling with N2 for 15 minutes. An exothermic reaction between the raw materials and KOH yielded a dark brown slurry solution. The slurry solution was then heated in an oven at 120°C for 24 hours. After adding ultrapure water, the by-product salt (KCl) was removed by vacuum filtration using a vacuum filter and a filter (made of Teflon®, pore size 200 nm). Finally, after vacuum drying (80°C, 15 hours), the powder was finely ground in a mortar to obtain 2H-CuFeO2 powder.

[0080] 20 mg of the obtained 2H-CuFeO2 powder was mixed with 2 mL of 2-propanol, and a 2H-CuFeO2 slurry solution was prepared by repeating a cycle of 30 seconds of operation followed by 30 seconds of rest 20 times using an ultrasonic homogenizer (Biorepter BR-II, 300 W output). Masking tape was applied to 5 mm widths on all four sides of a 25 mm square Ti substrate, and the 2H-CuFeO2 slurry solution was dropped onto the center (15 mm square) of the Ti surface. This process of vacuum drying at room temperature was repeated 15 times, resulting in a catalyst concentration of 2.7 mg / cm³. 2 The material was then supported. After drying at 100°C for 10 minutes, it was heat-treated at 500°C for 1 hour (in an air atmosphere) to obtain a catalyst electrode.

[0081] X-ray diffraction measurements were performed on catalyst-supported samples on a Ti substrate in the same manner as in Example 1. Furthermore, photoelectrochemical CO2 electrolytic reduction was performed using a three-electrode cell with a light irradiation window, in the same manner as in Example 1. The atomic composition ratio (Fe / Cu) of the catalyst was measured by X-ray fluorescence analysis (XRF), in the same manner as in Example 2.

[0082] Figure 8 shows the XRD pattern of the catalyst-supported / Ti film of Comparative Example 3. From the X-ray diffraction (XRD) pattern shown in Figure 8, diffraction peaks for tetragonal CuFe2O4, 2H-CuFeO2, and CuO appeared in the catalyst film after heat treatment, indicating that the catalyst film is composed of a mixed phase of tetragonal CuFe2O4 as the first catalytic phase, 2H-CuFeO2 as the second catalytic phase, and CuO as the third catalytic phase. Based on the ratios of the peak areas of the second catalytic phase 2H-CuFeO2 and the third catalytic phase CuO to the strongest diffraction peak area of ​​the first catalytic phase CuFe2O4, the content of 2H-CuFeO2 and CuO relative to CuFe2O4 was estimated to be 24% and 10%, respectively. The atomic composition ratio (Fe / Cu), content, and electrochemical properties are shown in Table 1.

[0083] <Comparative Example 4> Following the preparation process described in reference A, hexagonal 2H-CuFeO2 powder was synthesized as follows.

[0084] The raw materials, CuI (2.1 mmol) and FeCl3 (4.2 mmol), were dissolved in ultrapure water (conductivity 18 MW, 10.5 mL) that had been pre-bubbled with nitrogen (N2) for 30 minutes. Then, KOH (166 mmol) solid was added, and the mixture was bubbling with N2 for 15 minutes. An exothermic reaction between the raw materials and KOH yielded a dark brown slurry solution. The slurry solution was then heated in an oven at 130°C for 24 hours. After adding ultrapure water, the by-product salt (KCl) was removed by vacuum filtration using a vacuum filter and a filter (made of Teflon®, pore size 200 nm). Finally, after vacuum drying (80°C, 15 hours), the powder was finely ground in a mortar to obtain 2H-CuFeO2 powder.

[0085] 20 mg of the obtained single-phase 2H-CuFeO2 powder was mixed with 2 mL of 2-propanol, and a 2H-CuFeO2 slurry solution was prepared by repeating a cycle of 30 seconds of operation followed by 30 seconds of rest 20 times using an ultrasonic homogenizer (Biorepter BR-II, 300 W output). Masking tape was applied to 5 mm widths on all four sides of a 25 mm square Ti substrate, and the 2H-CuFeO2 slurry solution was dropped onto the center (15 mm square) of the Ti surface. This process of vacuum drying at room temperature was repeated 15 times, resulting in a catalyst concentration of 2.7 mg / cm³. 2 The material was then supported. After drying at 100°C for 10 minutes, it was heat-treated at 500°C for 1 hour (in an air atmosphere) to obtain a catalyst electrode.

[0086] X-ray diffraction measurements were performed on catalyst-supported samples on a Ti substrate in the same manner as in Example 1. Furthermore, photoelectrochemical CO2 electrolytic reduction was performed using a three-electrode cell with a light irradiation window, in the same manner as in Example 1. The atomic composition ratio (Fe / Cu) of the catalyst was measured by X-ray fluorescence analysis (XRF), in the same manner as in Example 2.

[0087] Figure 9 shows the XRD pattern of the catalyst-supported / Ti film of Comparative Example 4. From the X-ray diffraction (XRD) pattern in Figure 9, diffraction peaks for tetragonal CuFe2O4, Fe2O3 (Hematite), and CuO appeared in the catalyst film after heat treatment. Therefore, the catalyst film is composed of a mixed phase of tetragonal CuFe2O4 as the first catalytic phase, Fe2O3 (Hematite) as the second catalytic phase, and CuO as the third catalytic phase. From the ratio of the peak areas of the second catalytic phase Fe2O3 (Hematite) and the third catalytic phase CuO to the strongest diffraction peak area of ​​the first catalytic phase CuFe2O4, the content of Fe2O3 (Hematite) and CuO relative to CuFe2O4 was estimated to be 84% and 22%, respectively. The atomic composition ratio (Fe / Cu), content, and electrochemical properties are shown in Table 1.

[0088] <Comparative Example 5> Following the citrate polymerization process described in reference B, rhombohedral 3R-CuFeO2 powder was synthesized as follows.

[0089] The raw materials, citric acid monohydrate (40 mmol) and 100 g of water, were dissolved at a liquid temperature of 60°C. Then, CuCO3·Cu(OH)2 (7.5 mmol) and iron citrate (FeC6H5O7·nH2O) (15 mmol) were added, and the mixture was stirred for 1 hour. After removing the water with a hot stirrer (liquid temperature set to 100°C, approximately 1 hour), a precursor gel was obtained. Subsequently, the gel was calcined in an oven at 350°C for 5 hours, and then fired in an electric furnace at 800°C for 12 hours (in a nitrogen (N2) atmosphere) to obtain 3R-CuFeO2 powder.

[0090] 24.3 mg of the obtained 3R-CuFeO2 powder was mixed with 2-propanol (2.43 mL), and a 3R-CuFeO2 slurry solution was prepared by repeating a cycle of 30 seconds of operation followed by 30 seconds of rest 20 times using an ultrasonic homogenizer (Biorepter BR-II, 300 W output). Masking tape was applied to 5 mm widths on all four sides of a 25 mm square Ti substrate, and the 3R-CuFeO2 slurry solution was dropped onto the center (15 mm square) of the Ti surface. This process of vacuum drying at room temperature was repeated 15 times, resulting in a catalyst concentration of 2.7 mg / cm³. 2The material was then supported. After drying at 100°C for 10 minutes, it was heat-treated at 500°C for 1 hour (in an air atmosphere) to obtain a catalyst electrode.

[0091] X-ray diffraction measurements were performed on the catalyst-supported sample on a Ti substrate in the same manner as in Example 1. Furthermore, photoelectrochemical CO2 electrolytic reduction was performed using a three-electrode cell with a light irradiation window, in the same manner as in Example 1. The atomic composition ratio (Fe / Cu) of the catalyst was measured by X-ray fluorescence analysis (XRF), in the same manner as in Example 2.

[0092] Figure 10 shows the XRD pattern of the catalyst-supported / Ti film of Comparative Example 5. From the X-ray diffraction (XRD) pattern shown in Figure 10, diffraction peaks for rhombohedral 3R-CuFeO2, tetragonal CuFe2O4, and CuO appeared in the catalyst film after heat treatment. Therefore, the catalyst film is composed of a mixed phase of the first catalytic phase 3R-CuFeO2, the second catalytic phase tetragonal CuFe2O4, and the third catalytic phase CuO. Based on the ratio of the peak areas of the second catalytic phase tetragonal CuFe2O4 and the third catalytic phase CuO to the strongest diffraction peak area of ​​the first catalytic phase 3R-CuFeO2, the content of tetragonal CuFe2O4 and CuO relative to 3R-CuFeO2 was estimated to be 93% and 24%, respectively. The atomic composition ratio (Fe / Cu), content, and electrochemical properties are shown in Table 1.

[0093] [Table 1]

[0094] From the atomic composition ratio (Fe / Cu) and electrochemical properties in Table 1, it was found that in the catalyst films / Ti using tetragonal CuFe2O4 as the first catalytic phase in Examples 1 to 4, the acetic acid FE content was 69% to 92%, which is higher than the acetic acid FE content of the 3R-CuFeO2 single phase in Comparative Example 1 and the 2H-CuFeO2 single phase in Comparative Example 2, which was 51% to 59%.

[0095] In Examples 1 and 2, where the first catalytic phase tetragonal CuFe2O4 was mixed with the second catalytic phase CuO, the current density was higher compared to Example 3. This indicates that the second catalytic phase CuO has the effect of improving the current density while maintaining the acetic acid FE.

[0096] In Example 4, the third phase contained approximately 1% Fe2O3. However, it was found that if the Fe2O3 (Hematite) content could be controlled to 1% or less, a relatively higher level of acetic acid FE could be expressed compared to the 3R-CuFeO2 single phase in Comparative Example 1 and the 2H-CuFeO2 single phase in Comparative Example 2.

[0097] Furthermore, the current retention rates of Examples 1 to 4 were 72-103%, which is higher than the current retention rates of 46-58% for the 3R-CuFeO2 single-phase in Comparative Example 1 and the 2H-CuFeO2 single-phase in Comparative Example 2. This demonstrates that the catalyst films exhibit superior stability.

[0098] In Comparative Examples 3 and 4, the first catalyst phase was tetragonal CuFe2O4, but the second catalyst phase was 2H-CuFeO2 or Fe2O3 (Hematite), resulting in a decrease in acetic acid (FE) compared to Examples 1 and 2. This indicates that using tetragonal CuFe2O4 as the first catalyst phase and CuO as the second catalyst phase is effective.

[0099] In Comparative Example 5, since the first catalyst phase was 3R-CuFeO2, even when the second catalyst phase was a mixture of tetragonal CuFe2O4 and the third catalyst phase was CuO, the acetic acid FE decreased. This indicates that tetragonal CuFe2O4 is effective as the first catalyst phase.

[0100] Figure 11 shows the dependence of acetic acid (FE) on the Fe / Cu atomic composition ratio for Examples 1-4 and Comparative Examples 1-5. As shown in Figure 11, it was found that a tetragonal CuFe2O4 first catalyst phase and an Fe / Cu atomic composition ratio of 2.2 or less are effective in obtaining high-quality acetic acid (FE).

[0101] [XRD before and after photoelectrochemical CO2 electrolysis of the 2H-CuFeO2 single-phase catalytic electrode in Comparative Example 2] Figure 12 shows the XRD patterns of the 2H-CuFeO2 single-phase catalyst electrode of Comparative Example 2 before and after photoelectrochemical CO2 electrolysis. As shown in Figure 12, the crystalline structure of 2H-CuFeO2 is observed before electrolysis, but after electrolysis, the diffraction peak intensity of 2H-CuFeO2 decreases and the diffraction peak of Cu appears, confirming that Cu precipitates and becomes unstable after electrolysis.

[0102] [SEM before and after photoelectrochemical CO2 electrolysis of the 2H-CuFeO2 single-phase catalytic electrode in Comparative Example 2] Figure 13 shows the SEM observation results of the surface of 2H-CuFeO2 of Comparative Example 2 before (left) and after (right) photoelectrochemical CO2 electrolysis. As shown in Figure 13, it was confirmed that while the catalyst was plate-like particles before electrolysis, spherical fine particles were deposited after electrolysis.

[0103] [Electrolyte analysis after photoelectrochemical CO2 electrolysis using a 2H-CuFeO2 single-phase catalytic electrode in Comparative Example 2] Table 2 shows the results of inductively coupled plasma emission spectrometry (ICP-OES) (Hitachi High-Tech Science, PS3520UVDDII) analysis of the electrolyte after photoelectrochemical CO2 electrolysis of 2H-CuFeO2 in Comparative Example 2. The sample was stirred to prepare the test solution, and the amount of Cu and Fe eluted (μg / mL), as well as the elution ratio (%) relative to the amount of Cu and Fe in the CuFeO2 film (set to 100%), were analyzed.

[0104] [Table 2]

[0105] As shown in Table 2, the elution of Cu and Fe was confirmed in the electrolyte after photoelectrochemical CO2 electrolysis of CuFeO2, clearly indicating that 2H-CuFeO2 is unstable.

[0106] [Analysis of state changes before and after photoelectrochemical CO2 electrolysis of CuFeO2 catalyst electrodes and CuFe2O4-CuO mixed catalyst electrodes] The state changes of the catalysts during photoelectrochemical CO2 electrolytic reduction using a CuFeO2 catalyst electrode and a CuFe2O4-CuO mixed catalyst electrode were investigated using the X-ray absorption fine structure (XAFS) instrument at the Toyota beamline of SPRing-8. XAFS data was acquired by fluorescence at a measurement interval of 50 seconds / spectrum (60-second intervals). Figure 14 shows the measurement system for in-situ XAFS measurement of the photoelectrochemical CO2 electrolytic reaction (photograph (left) and schematic diagram (right)). In the measurement, the (Z-1) filter was omitted because the elastic scattered X-rays were weak. Since the Ti Kα and Kβ lines were strong compared to the Cu Kα or Fe Kα lines, an Al foil filter was inserted in front of the detector. A lead shield was attached to the detector tube.

[0107] Using a three-electrode cell with a light irradiation window (EC Frontier, VB1300), X-rays were incident from a 45-degree angle to the perpendicular direction of the substrate onto the Ti substrate side of the CuFeO2 catalyst electrode and the CuFe2O4-CuO mixed catalyst electrode supported on a Ti foil (5 μm), and the reflected fluorescent X-rays were detected by a detector. Figure 15 shows the in-situ EXAFS spectrum for the photoelectrochemical CO2 electrolysis reaction.

[0108] As shown in Figure 15, the EXAFS spectrum demonstrated that while CuFeO2 changed to a Cu0 valency during electrolysis under light irradiation, the CuFe2O4-CuO mixed system remained almost unchanged, demonstrating its stability.

[0109] Thus, the examples demonstrated that a catalytic electrode with high selectivity for acetic acid production and good stability was obtained, which has the function of electrochemically reducing carbon dioxide to produce acetic acid under light irradiation.

Claims

1. A catalytic electrode having the function of electrochemically reducing carbon dioxide to produce acetic acid under light irradiation, wherein the catalyst is supported on a conductive substrate, The catalyst is an oxide composed of Cu and Fe, and CuFe ２ O ４ A catalytic electrode characterized by having as its main crystalline structure.

2. A catalyst electrode according to claim 1, The catalyst is the first catalytic phase CuFe ２ O ４ Composed of, or the first catalyst phase CuFe ２ O ４ It is composed of a second catalytic phase of CuO, A catalyst electrode characterized in that the atomic composition ratio Fe / Cu in the catalyst is 2.5 or less.

3. A catalyst electrode according to claim 2, The catalyst is a phase other than CuFe of the first catalyst phase, or includes a phase other than CuFeO of the first catalyst phase and a phase other than CuO of the second catalyst phase, and the content ratio of the phase is 1% or less with respect to CuFeO, and is a catalyst electrode characterized by this. ２ O ４ The catalyst is a phase other than CuFe of the first catalyst phase, or includes a phase other than CuFeO of the first catalyst phase and a phase other than CuO of the second catalyst phase, and the content ratio of the phase is ..... ２ O ４ The catalyst is a phase other than CuFe of the first catalyst phase, or includes a phase other than CuFeO of the first catalyst phase and a phase other than CuO of the second catalyst phase, and the content ratio of the phase is ..... ２ O ４ The catalyst is a phase other than CuFe of the first catalyst phase, or includes a phase other than CuFeO of the first catalyst phase and a phase other than CuO of the second catalyst phase, and the content ratio of the phase is 1% or less with respect to CuFeO, and is a catalyst electrode characterized by this.

4. A method for producing a catalyst electrode having the function of electrochemically reducing carbon dioxide under light irradiation to produce acetic acid, wherein the catalyst is supported on a conductive substrate, CuFeO ２ The first catalyst phase CuFe is heated in an atmospheric environment at 400-700°C. ２ O ４ A method for producing a catalyst electrode, characterized by including a heating step to obtain the catalyst composed of a second catalyst phase of CuO.

5. A photoelectrochemical cell for electrochemically reducing carbon dioxide under light irradiation to produce acetic acid, A catalytic electrode according to any one of claims 1 to 3, Oxidizing electrode and, An electrolyte layer comprising a solvent, carbon dioxide, and an electrolyte between the catalyst electrode and the oxidizing electrode, A photoelectrochemical cell characterized by comprising the following features.

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