Electrode catalyst for carbon dioxide reduction

Abstract

To provide an electrocatalyst for carbon dioxide reduction having high Faraday efficiency when reducing carbon dioxide to formic acid.SOLUTION: The electrocatalyst for carbon dioxide reduction of the present disclosure contains at least indium and chlorine and electrochemically reduces carbon dioxide. The electrocatalyst preferably contains chlorine in an amount of 0.5 or more and less than 3.0 times the amount of indium in a molar ratio. The electrocatalyst is supported on a conductive material, and preferably 5 mass% or more and less than 30 mass% indium is supported on the conductive material and / or is supported on a gas diffusion electrode.SELECTED DRAWING: Figure 4B

Description

[Technical Field]

[0001] This disclosure relates to an electrode catalyst for carbon dioxide reduction. [Background technology]

[0002] Technologies that use catalysts to reduce carbon dioxide and convert it into carbon monoxide, organic acids, alcohols, and / or hydrocarbons are attracting attention.

[0003] Methods for reducing carbon dioxide using catalysts can be broadly classified into thermodynamic reduction, photochemical reduction, and electrochemical reduction.

[0004] Patent Document 1 discloses an electrode catalyst used for electrochemically reducing carbon dioxide. Specifically, it discloses an electrode catalyst in which metallic tin and / or tin dioxide are supported on tin-doped indium oxide (ITO) and / or antimony-doped tin oxide (ATO). [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2019-173131 [Overview of the Initiative] [Problems that the invention aims to solve]

[0006] When carbon dioxide is reduced using the electrode catalyst disclosed in Patent Document 1, formic acid can be produced relatively quickly. However, although the Faraday efficiency when reducing carbon dioxide to formic acid is over 55%, the highest value was only 66%.

[0007] This disclosure was made to solve the above-mentioned problems. The purpose of this disclosure is to provide an electrode catalyst for carbon dioxide reduction that exhibits high Faraday efficiency when reducing carbon dioxide to formic acid. [Means for solving the problem]

[0008] To achieve the above objectives, the Disclosers have diligently conducted studies and completed the electrocatalyst for carbon dioxide reduction described herein. The electrocatalyst for carbon dioxide reduction described herein includes the following embodiments. <Aspect 1> Containing at least indium and chlorine, Electrochemical reduction of carbon dioxide, Electrode catalyst for carbon dioxide reduction. <Aspect 2> The electrode catalyst for carbon dioxide reduction according to aspect 1, wherein the electrode catalyst contains chlorine in a molar ratio of 0.5 times or more and less than 3.0 times the amount of indium. <Aspect 3> The electrode catalyst for carbon dioxide reduction according to aspect 1 or 2, wherein the electrode catalyst is supported on a conductive material. <Aspect 4> The electrode catalyst for carbon dioxide reduction according to aspect 3, wherein 5% by mass or more and less than 30% by mass of indium is supported on the conductive material. <Aspect 5> The electrode catalyst for carbon dioxide reduction according to any one of aspects 1 to 4, wherein the electrode catalyst is supported on a gas diffusion electrode. [Effects of the Invention]

[0009] According to this disclosure, by containing at least indium and chlorine, it is possible to provide an electrode catalyst for carbon dioxide reduction that exhibits high Faraday efficiency when reducing carbon dioxide to formic acid. [Brief explanation of the drawing]

[0010] [Figure 1] Figure 1 is a schematic diagram illustrating the general outline of an example of an electrochemical reduction apparatus. [Figure 2] Figure 2 shows the HAADF image obtained by TEM observation of the sample from Example 1. [Figure 3] Figure 3 shows the BF image of the sample from Example 1 when observed by TEM. [Figure 4A] Figure 4A shows the XPS spectrum (In 3d2 / 5) of the sample from Example 1. [Figure 4B]Figure 4B shows the XPS spectrum (Cl 2p) of the sample of Example 1. [Figure 5] Figure 5 shows the XRD pattern of the sample of Example 1. Embodiments for Carrying out the Invention

[0011] Hereinafter, embodiments of the electrode catalyst for carbon dioxide reduction of the present disclosure will be described. Note that the embodiments shown below do not limit the electrode catalyst for carbon dioxide reduction of the present disclosure.

[0012] Although not bound by theory, the findings obtained by the present inventors regarding the reason why the electrode catalyst for carbon dioxide reduction of the present disclosure can obtain a high Faradaic efficiency when reducing carbon dioxide to formic acid will be described.

[0013] When reducing carbon dioxide to formic acid, hydrogen generation, which is a competing reaction, basically cannot be completely avoided. In the catalyst for carbon dioxide reduction disclosed in Patent Document 1, a large amount of hydrogen generation, which is a competing reaction, occurs, and as a result, it is considered that the Faradaic efficiency decreases.

[0014] When carbon dioxide is reduced, the selectivity of the reduction product is determined by the adsorption energy to the electrode catalyst. The present inventors found that when chlorine is present in the vicinity of indium in the electrode catalyst, the adsorption energy of hydrogen to the electrode catalyst becomes small, and the adsorption energy of the OCOH* intermediate reaction product becomes appropriate. And the present inventors found that when using an electrode catalyst containing indium and chlorine, hydrogen generation, which is a competing reaction, can be suppressed when reducing carbon dioxide to formic acid, and the Faradaic efficiency can be increased.

[0015] The constituent requirements of the electrode catalyst for carbon dioxide reduction of the present disclosure, which have been completed based on the findings described so far, will be described below.

[0016] Electrode Catalyst for Carbon Dioxide Reduction The electrocatalyst for carbon dioxide reduction described herein contains at least indium (In) and chlorine (Cl), and electrochemically reduces carbon dioxide. This electrocatalyst for carbon dioxide reduction will be described in detail below.

[0017] The electrode catalyst for carbon dioxide reduction described herein (hereinafter referred to as "electrode catalyst" unless otherwise specified) is mainly composed of indium and contains chlorine. Because indium is the main component, formic acid is selected as the reduction product, and because chlorine is present near the indium, the adsorption energy of hydrogen to the electrode catalyst is reduced, and the adsorption energy of the OCOH* intermediate reaction product becomes appropriate.

[0018] The crystalline structure of the electrode catalyst is not particularly limited, but it is preferably amorphous. Being amorphous allows for advantageous reduction of carbon dioxide.

[0019] The electrode catalyst may contain chlorine in a molar ratio of 0.5 times or more, 0.6 times or more, 0.7 times or more, 0.8 times or more, 0.9 times or more, or 1.0 times or more relative to indium, and may contain chlorine in a ratio of less than 3.0 times, 2.9 times or less, 2.8 times or less, 2.6 times or less, 2.4 times or less, 2.2 times or less, 2.0 times or less, 1.9 times or less, 1.8 times or less, 1.7 times or less, 1.6 times or less, 1.5 times or less, 1.4 times or less, 1.3 times or less, 1.2 times or less, or 1.1 times or less relative to indium. x When expressed as such, x may be 0.5 or greater, 0.6 or greater, 0.7 or greater, 0.8 or greater, 0.9 or greater, or 1.0 or greater, and may be less than 3.0, 2.9 or less, 2.8 or less, 2.6 or less, 2.4 or less, 2.2 or less, 2.0 or less, 1.9 or less, 1.8 or less, 1.7 or less, 1.6 or less, 1.5 or less, 1.4 or less, 1.3 or less, 1.2 or less, or 1.1 or less. The electrode catalyst may contain unavoidable impurities other than indium and chlorine.

[0020] It is preferable that the electrode catalyst is supported on a conductive material. Supporting the electrode catalyst on a conductive material is advantageous for increasing the reduction efficiency, and the current density (absolute value) is 300 mA / cm².2 This is particularly advantageous when the density is as high as above. Examples of conductive materials include carbon black.

[0021] It is preferable that the conductive material has 5% or more by mass, 10% or more by mass, or 15% or more by mass of indium supported on it, and it is preferable that it has less than 30% by mass, 25% or less by mass, or 20% or less by mass of indium supported on it. By having 5% or more by mass of indium supported on the conductive material, the Faraday efficiency when reducing carbon dioxide to formic acid can be particularly increased. On the other hand, by having less than 30% by mass of indium supported on the conductive material, it is possible to avoid supporting excess electrode catalyst on the conductive material even though the effect of improving the Faraday efficiency when reducing carbon dioxide to formic acid has reached saturation.

[0022] The electrode catalysts described so far can be produced, for example, as follows:

[0023] A conductive material is placed in formic acid, and ultrasonic vibrations are applied to obtain a dispersion in which the conductive material is dispersed in the formic acid. A typical conductive material is carbon black. Alternatively, indium(III) chloride tetrahydrate is dissolved in pure water to obtain an InCl3 aqueous solution. This InCl3 aqueous solution is then added dropwise to the dispersion. The prepared solution, to which the InCl3 aqueous solution has been added, is then evaporated to dry and obtain a powder. Furthermore, the obtained powder may be heat-treated in an argon gas atmosphere.

[0024] The amount of InCl3 aqueous solution to be added should be determined appropriately based on the amount of indium to be supported on the conductive material. If it is not desired to support the electrode catalyst on the conductive material, the InCl3 aqueous solution can be added dropwise to formic acid in which the conductive material is not dispersed.

[0025] The evaporation drying temperature of the prepared solution may be 80°C or higher, 90°C or higher, or 100°C or higher, and may be 150°C or lower, 140°C or lower, 130°C or lower, 120°C or lower, or 110°C or lower.

[0026] The heat treatment temperature may be 200°C or higher, 220°C or higher, 240°C or higher, 260°C or higher, or 280°C or higher, and may be 800°C or lower, 750°C or lower, 700°C or lower, 650°C or lower, 600°C or lower, 550°C or lower, 550°C or lower, 450°C or lower, 400°C or lower, 350°C or lower, or 300°C or lower. A heat treatment temperature of 200°C or higher is advantageous for chlorine to bond to indium. On the other hand, a heat treatment temperature of 800°C or lower is advantageous for the electrode catalyst supported on the conductor to be amorphous after heat treatment. If the electrode catalyst is not supported on the conductor, a heat treatment temperature of 800°C or lower is advantageous for the electrode catalyst to be amorphous.

[0027] The heat treatment time can be appropriately determined based on the mass of the powder to be heat treated, but for example, it may be 1 hour or more, 2 hours or more, or 3 hours or more, and 6 hours or less, 5 hours or less, or 4 hours or less.

[0028] The electrode catalyst may be supported on a gas diffusion electrode. Alternatively, the electrode catalyst may be supported on a conductor, and the electrode catalyst supported on the conductor may be further supported on a gas diffusion electrode, or the electrode catalyst may be directly supported on the gas diffusion electrode. In any case, by supporting the electrode catalyst on a gas diffusion electrode, the electrode catalyst exhibits catalytic function in the electrochemical reaction between the liquid phase and the gas phase, which is advantageous for improving reduction efficiency.

[0029] The gas diffusion electrode is equipped with a conductive support. The conductive support is typically carbon paper, but is not limited to carbon paper; it may be a porous metal, a porous oxide, or a porous semiconductor. The gas diffusion electrode is obtained by mixing an electrode catalyst in an organic solvent, dropping the mixture onto the aforementioned conductive support (carrier), and drying it. This is used as the working electrode.

[0030] The electrode catalyst described above can be brought into contact with carbon dioxide to electrochemically reduce the carbon dioxide and produce formic acid from it. In other words, the electrode catalyst described above can be applied to a method for producing formic acid from carbon dioxide, which involves bringing the electrode catalyst into contact with carbon dioxide and electrochemically reducing the carbon dioxide. A well-known method can be used to bring the electrode catalyst into contact with carbon dioxide.

[0031] When the electrode catalyst is supported on a gas diffusion electrode, typically, the electrode catalyst is brought into contact with carbon dioxide using the apparatus shown in Figure 1 to electrically reduce the carbon dioxide, but this is not the only method. Figure 1 is a schematic diagram illustrating an example of an electrochemical reduction apparatus. This electrochemical reduction apparatus will be described below.

[0032] The electrochemical reduction apparatus 100 comprises an anode chamber 10, a cathode chamber 20, and a gas chamber 30. The anode chamber 10 and the cathode chamber 20 are connected via a separator 50. The cathode chamber 20 and the gas chamber 30 are connected via a working electrode 60. The working electrode 60 is a gas diffusion electrode on which an electrode catalyst is supported.

[0033] The interiors of the anode chamber 10 and the cathode chamber 20 are filled with electrolyte 70. A counter electrode 80 is installed in the anode chamber 10 so that a portion of it is immersed in the electrolyte 70. A reference electrode 90 is installed in the cathode chamber 20 so that a portion of it is immersed in the electrolyte 70. The reference electrode 90 allows for the precise application of a desired potential to the working electrode 60.

[0034] The gas chamber 30 is equipped with an inlet pipe 40 and an outlet pipe 42. Carbon dioxide gas is introduced into the gas chamber 30 through the inlet pipe 40. The carbon dioxide gas introduced into the gas chamber 30 comes into contact with the working electrode 60, which functions as an electrode catalyst and promotes the reduction of the carbon dioxide gas. The formic acid obtained by the reduction of carbon dioxide gas is accumulated at the bottom of the gas chamber 30. The accumulated formic acid is discharged through the outlet pipe 42.

[0035] When reducing carbon dioxide using the apparatus shown in Figure 1, the gas diffusion electrode on which the electrode catalyst is supported is set up as the working electrode 60. The working electrode 60, counter electrode 80, reference electrode 90, separator 50, and electrolyte 70 will be described below.

[0036] The working electrode 60 is used by supporting the electrode catalyst on a gas diffusion electrode (carrier) through which the electrolyte 70 does not pass but carbon dioxide does. The gas diffusion electrode (carrier) used for the working electrode 60 is as described above.

[0037] The counter electrode 80 is not particularly restricted as long as it does not hinder the efficient reduction of carbon dioxide, but typically a platinum mesh is used.

[0038] The reference electrode 90 only needs to be able to accurately measure the potential of the working electrode 60. Examples of materials that the reference electrode 90 may contain include Hg / Hg2Cl2, Ag / AgCl, and / or reversible hydrogen, with Ag / AgCl being particularly preferred.

[0039] As the separator 50, well-known materials can be used, and examples of materials contained in the separator 50 include Nafion, glass filters, Celemion, and / or Zirphon, with Nafion being particularly preferred. A typical example of the separator 50 is a Nafion membrane.

[0040] The electrolyte 70 can be any well-known electrolyte, such as KHCO3, HClO4, H2SO4, HCl, and / or HNO3, with KHCO3 being preferred.

[0041] The carbon dioxide reduction conditions can be appropriately determined by the combination of the working electrode 60, counter electrode 80, reference electrode 90, separator 50, and electrolyte 70. The potential of the working electrode 60 may be in the range of, for example, 0V vs. RHE to -3.5V vs. RHE. The current density may be, for example, -800mA / cm with respect to the working electrode. 2 More than -700mA / cm 2above, or -600 mA / cm 2 may be above, -500 mA / cm 2 or less, -400 mA / cm 2 or less, -300 mA / cm 2 or less, or -200 mA / cm 2 or less. The supply rate of carbon dioxide gas is, for example, 5 cm 3 / min or more, 10 cm 3 / min or more, or 50 cm 3 / min or more, and may be 200 cm 3 / min or less, 150 cm 3 / min or less, or 100 cm 3 / min or less.

[0042] 《Modification》 The electrode catalyst for carbon dioxide reduction of the present disclosure can be appropriately modified within the scope of the content described in the claims. For example, the electrode catalyst for carbon dioxide reduction of the present disclosure only needs to contain indium and chlorine, and does not exclude that the electrode catalyst contains substances other than indium and chlorine as long as the effects of the electrode catalyst for carbon dioxide reduction of the present disclosure are not impaired.

Examples

[0043] Hereinafter, the electrode catalyst for carbon dioxide reduction of the present disclosure will be described more specifically by way of examples and comparative examples. Note that the electrode catalyst for carbon dioxide reduction of the present disclosure is not limited to the conditions used in the following examples.

[0044] 《Preparation of Samples》 Each sample was prepared as follows.

[0045] 〈Example 1〉 Carbon black (Vulcan, XC-72) was placed in formic acid, and ultrasonic vibrations were applied for 30 minutes to disperse the carbon black in the formic acid. In addition, indium(III) chloride tetrahydrate (Nacalai Tesque, 95% purity) was dissolved in pure water to obtain an InCl3 aqueous solution. The InCl3 aqueous solution was added dropwise to the formic acid containing the dispersed carbon black. The amount added was adjusted so that 10% by mass of indium was supported on the carbon black. Subsequently, the prepared solution, in which the InCl3 aqueous solution was added dropwise to the formic acid containing the dispersed carbon black, was stirred overnight with a magnetic stirrer. The stirred prepared solution was then evaporated and dried at 110°C using a hot plate to obtain a powder. Furthermore, the obtained powder was heat-treated at 300°C for 3 hours in an argon gas atmosphere.

[0046] 3.4 mg of heat-treated powder was mixed with 1000 μl of isopropanol and 4.4 μl of Nafion 117 solution (Sigma-Aldrich), and dispersed using an ultrasonic cleaner. Then, 20 μl of this dispersion was dropped onto carbon paper and dried to obtain the working electrode. This working electrode was used as the sample for Example 1. The area of ​​the carbon paper was 0.5 cm². 2 That was the case.

[0047] <Example 2> The sample for Example 2 was obtained in the same manner as in Example 1, except that the amount of InCl3 aqueous solution added was adjusted so that 5% by mass of indium was supported on the carbon black.

[0048] <Example 3> The sample for Example 3 was obtained in the same manner as in Example 1, except that the amount of InCl3 aqueous solution added was adjusted so that 30% by mass of indium was supported on the carbon black.

[0049] <Example 4> The sample for Example 4 was obtained in the same manner as in Example 1, except that the amount of InCl3 aqueous solution added was adjusted so that 50% by mass of indium was supported on the carbon black.

[0050] <Example 5> The sample for Example 5 was obtained in the same manner as in Example 1, except that carbon black was not dispersed in formic acid.

[0051] <Example 6> The sample for Example 6 was obtained in the same manner as in Example 1, except that no heat treatment was performed.

[0052] <Example 7> The sample for Example 7 was obtained in the same manner as in Example 1, except that the heat treatment temperature was set to 500°C.

[0053] <Example 8> The sample for Example 8 was obtained in the same manner as in Example 1, except that the heat treatment temperature was set to 700°C.

[0054] <Comparative Example 1> Carbon black was dissolved in ethanol. Indium nitrate (In(NO3)3) was also dissolved in ethanol to obtain an In(NO3)3 solution. The In(NO3)3 solution was added dropwise to the ethanol containing the dispersed carbon black. The amount added was adjusted so that 10% by mass of indium was supported on the carbon black. The prepared solution, obtained by adding the In(NO3)3 solution dropwise to the ethanol containing the dispersed carbon black, was heated to 80°C and evaporated to obtain a powder. Furthermore, the obtained powder was heat-treated at 300°C for 3 hours in an atmosphere of a mixed gas of argon and hydrogen (hydrogen concentration: 2%). 3.4 mg of the heat-treated powder obtained in this way was mixed with 1000 μl of isopropanol and 4.4 μl of Nafion 117 solution (manufactured by Sigma-Aldrich) and dispersed using an ultrasonic cleaner. Then, 20 μl of this dispersion was added dropwise to carbon paper and dried to obtain the working electrode. This working electrode was used as the sample for Comparative Example 1. The area of ​​the carbon paper was 0.5 cm². 2 That was the case.

[0055] 《Reduction of carbon dioxide》 Carbon dioxide was reduced using the electrical reduction apparatus shown in Figure 1. A platinum mesh was used as the counter electrode, an Ag / AgCl electrode (saturated KCl) as the reference electrode, and 1M KHCO3 as the electrolyte. The working electrodes of each sample described above were placed in this electrical reduction apparatus, and 10 cm of water was placed in the gas compartment. 3 A flow rate of carbon dioxide gas was applied per minute. The current density was -300 mA / cm² relative to the working electrode. 2 This was a constant current. To obtain such a constant current, a potentiometer galvanostat (HZ-7000, manufactured by Hokuto Denko Co., Ltd.) was used. The reduction time (electrolysis time) was 30 minutes. For Example 1, -400 mA / cm was applied to the working electrode. 2 -500mA / cm 2 , and -600mA / cm 2 Even at a constant current density, carbon dioxide was reduced.

[0056] "evaluation" The products obtained by reducing carbon dioxide using the working electrode of each sample were subjected to qualitative and quantitative analysis using gas chromatography and ion chromatography. The selectivity of the reduction products was evaluated by Faraday efficiency.

[0057] The Faraday efficiency (FE) was calculated using equation (A) below. The Faraday efficiency also indicates the degree to which a particular reaction process is kinetically favorable. FE = αnF / Q ···(A) Here, α is the number of electrons required to form one molecule of the target product, n is the amount of the target product, F is the Faraday constant, and Q is the total charge.

[0058] For each sample, EDS analysis was performed after heat treatment (for Example 6, after evaporation drying), and InCl XThe x-value was measured. For the sample of Example 1, TEM (Transmission Electron Microscope) observation, XPS (X-ray Photoelectron Spectroscopy) analysis, and XRD (X-Ray Diffraction) analysis were performed after heat treatment. High-Angle Annular Dark Field scanning (HAADF) and Bright-Field (BF) images were obtained from the TEM observation.

[0059] The results are shown in Table 1. The Cl / In ratio in Table 1 is InCl x This corresponds to x. Figure 2 is the HAADF image obtained by TEM observation of the sample from Example 1. Figure 3 is the BF image obtained by TEM observation of the sample from Example 1. Figure 4A is the XPS spectrum (In 3d² / 5) of the sample from Example 1. Figure 4B is the XPS spectrum (Cl 2p) of the sample from Example 1. Figure 5 is the XRD pattern of the sample from Example 1.

[0060] [Table 1]

[0061] Table 1 confirms that the Faraday efficiency in reducing carbon dioxide to formic acid was very high in the samples of Examples 1 to 8. Furthermore, Figures 2 and 3 show that the nanoparticles were very well dispersed and supported on the carbon surface. Figures 4A and 4B show that peaks for indium and chlorine are observed in the XPS spectra. From these findings, it is considered that if the catalyst electrode is mainly composed of indium and contains chlorine, it contributes to improving the Faraday efficiency in reducing carbon dioxide to formic acid. Also, from Figure 5, although a peak for C (carbon) is observed in the XRD pattern, a peak for In (indium) is not observed. From this, it is considered that the electrode catalyst supported on the carbon black in the sample of Example 1 is amorphous. And since the sample of Example 1 showed particularly high Faraday efficiency in reducing carbon dioxide to formic acid, it can be understood that an amorphous electrode catalyst is advantageous for improving the Faraday efficiency in reducing carbon dioxide to formic acid.

[0062] On the other hand, compared to the samples of Examples 1 to 8, the Faraday efficiency in reducing carbon dioxide to formic acid was lower in the sample of Comparative Example 1. This is thought to be because the electrode catalyst of the sample of Comparative Example 1 contains indium but does not contain chlorine.

[0063] Based on these results, the effectiveness of the electrode catalyst for carbon dioxide reduction described herein was confirmed. [Explanation of Symbols]

[0064] 10 Anode Rooms 20 Cathode Chambers 30 Gas chambers 40 Introductory pipe 42 Discharge pipe 50 Separators 60 Working electrode 70 Electrolyte 80 Opposite poles 90 Reference pole 100 Electrochemical Reduction Apparatus

Claims

1. It contains at least indium and chlorine, It contains chlorine in a molar ratio of 0.5 times or more and less than 3.0 times the amount of indium, and Electrochemical reduction of carbon dioxide, Electrode catalyst for carbon dioxide reduction.

2. The electrode catalyst for carbon dioxide reduction according to claim 1, wherein the electrode catalyst is supported on a conductive material.

3. The electrode catalyst for carbon dioxide reduction according to claim 2, wherein 5% by mass or more and less than 30% by mass of indium is supported on the conductive material.

4. The electrode catalyst for carbon dioxide reduction according to claim 1, wherein the electrode catalyst is supported on a gas diffusion electrode.

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