Electrocatalyst 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] The present disclosure relates to an electrode catalyst for carbon dioxide reduction. [Background technology]

[0002] 2. Description of the Related Art Attention has been focused on a technology in which carbon dioxide is reduced with a catalyst and converted into carbon monoxide, organic acids, alcohols, and / or hydrocarbons, etc.

[0003] Methods for reducing carbon dioxide using a catalyst are roughly classified into thermodynamic reduction, photochemical reduction, electrochemical reduction, and the like.

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

[0005] [Patent Document 1] JP 2019-173131 A Summary of the Invention [Problem to be solved by the invention]

[0006] When carbon dioxide is reduced using the electrode catalyst disclosed in Patent Document 1, formic acid can be produced at a relatively high speed. However, although the faradaic efficiency when reducing carbon dioxide to formic acid reaches 55% or more, the maximum value remains at 66%.

[0007] The present disclosure has been made to solve the above problems. The present disclosure has an object to provide an electrode catalyst for reducing carbon dioxide that has high faradaic efficiency when reducing carbon dioxide to formic acid. [Means for solving the problem]

[0008] The present inventors have conducted intensive research to achieve the above object, and have completed the presently disclosed electrode catalyst for reducing carbon dioxide. The presently disclosed electrode catalyst for reducing carbon dioxide includes the following aspects. <Aspect 1> Contains at least indium and chlorine, and Electrochemically reduce carbon dioxide, Electrocatalyst for carbon dioxide reduction. Aspect 2: The electrode catalyst for reducing carbon dioxide according to aspect 1, wherein the electrode catalyst contains, in terms of a molar ratio, 0.5 to less than 3.0 times chlorine relative to indium. <Aspect 3> The electrode catalyst for reducing carbon dioxide according to Aspect 1 or 2, wherein the electrode catalyst is supported on a conductive material. Aspect 4: The electrode catalyst for reducing carbon dioxide according to aspect 3, wherein 5 mass % or more and less than 30 mass % of indium is supported on the conductive material. <Aspect 5> The electrode catalyst for reducing carbon dioxide according to any one of Aspects 1 to 4, wherein the electrode catalyst is supported on a gas diffusion electrode. Effect of the Invention

[0009] According to the present disclosure, it is possible to provide an electrode catalyst for reducing carbon dioxide that contains at least indium and chlorine and thus has high faradaic efficiency when reducing carbon dioxide to formic acid. [Brief description of the drawings]

[0010] [Figure 1] FIG. 1 is an explanatory diagram that shows a schematic overview of an example of an electrochemical reduction device. [Diagram 2] FIG. 2 is an HAADF image obtained by observing the sample of Example 1 with a TEM. [Diagram 3] FIG. 3 is a BF image of the sample of Example 1 observed by TEM. [Figure 4A] FIG. 4A is an XPS spectrum (In 3d2 / 5) of the sample of Example 1. [Figure 4B]FIG. 4B is an XPS spectrum (Cl 2p) of the sample of Example 1. [Diagram 5] FIG. 5 is an XRD pattern of the sample of Example 1. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] Hereinafter, embodiments of the carbon dioxide reduction electrode catalyst of the present disclosure will be described. Note that the carbon dioxide reduction electrode catalyst of the present disclosure is not limited to the embodiments described below.

[0012] Without being bound by theory, the findings obtained by the present inventors regarding the reason why the carbon dioxide reduction electrode catalyst of the present disclosure can obtain high faradaic efficiency when reducing carbon dioxide to formic acid will be described.

[0013] When carbon dioxide is reduced to formic acid, the competing reaction of hydrogen generation cannot be completely avoided. In the carbon dioxide reduction catalyst disclosed in Patent Document 1, the competing reaction of hydrogen generation occurs frequently, which is considered to result in a decrease in the faradaic efficiency.

[0014] When carbon dioxide is reduced, the selectivity of the reduction product is determined by the adsorption energy to the electrode catalyst. The inventors have found that when chlorine is present near 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. The inventors have also found that when an electrode catalyst containing indium and chlorine is used, the competing reaction of hydrogen generation can be suppressed when carbon dioxide is reduced to formic acid, and the Faraday efficiency can be increased.

[0015] The constituent elements of the carbon dioxide reduction electrode catalyst of the present disclosure, which has been completed based on the findings and the like described thus far, will be described below.

[0016] 《Electrode catalyst for carbon dioxide reduction》 The carbon dioxide reduction electrode catalyst of the present disclosure contains at least indium (In) and chlorine (Cl) and electrochemically reduces carbon dioxide. Hereinafter, this carbon dioxide reduction electrode catalyst will be described in detail.

[0017] The carbon dioxide reduction electrode catalyst of the present disclosure (hereinafter, unless otherwise specified, referred to as "electrode catalyst") 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 in the vicinity of indium, the adsorption energy of hydrogen to the electrode catalyst is small, and the adsorption energy of the OCOH* intermediate reaction product becomes appropriate.

[0018] The crystal structure of the electrode catalyst is not particularly limited, but is preferably amorphous. Being amorphous allows carbon dioxide to be reduced advantageously.

[0019] The electrocatalyst may contain 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 chlorine relative to indium, and may contain 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 chlorine. This means that the electrocatalyst is InCl x When expressed by, x may be 0.5 or more, 0.6 or more, 0.7 or more, 0.8 or more, 0.9 or more, or 1.0 or more, 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 inevitable impurities other than indium and chlorine.

[0020] The electrode catalyst is preferably supported on a conductive material. Supporting the electrode catalyst on a conductive material is advantageous in increasing the reduction efficiency, and the current density (absolute value) is 300 mA / cm2 It is particularly advantageous when the density is equal to or higher than this. The conductive material may be, for example, carbon black.

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

[0022] The electrode catalysts described above can be produced, for example, as follows.

[0023] A conductive material is placed in formic acid, and ultrasonic vibration is applied to the conductive material to obtain a dispersion in which the conductive material is dispersed in the formic acid. A typical example of the conductive material is carbon black. Indium (III) chloride tetrahydrate is dissolved in pure water to obtain an InCl3 aqueous solution. The InCl3 aqueous solution is then dripped into the dispersion. The InCl3 aqueous solution is then dripped into the dispersion, and the resulting solution is evaporated and dried to obtain a powder. The obtained powder may then be heat-treated in an argon gas atmosphere.

[0024] The amount of the InCl3 aqueous solution to be dropped may be appropriately determined 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 may be dropped into formic acid in which the conductive material is not dispersed.

[0025] The evaporation drying temperature of the adjustment liquid may be 80° C. or more, 90° C. or more, or 100° C. or more, and may be 150° C. or less, 140° C. or less, 130° C. or less, 120° C. or less, or 110° C. or less.

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

[0027] The heat treatment time may be appropriately determined based on the mass of the powder to be heat treated, and may be, for example, 1 hour or more, 2 hours or more, or 3 hours or more, and may be 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. 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 a gas diffusion electrode. In either case, by supporting the electrode catalyst on the gas diffusion electrode, the electrode catalyst exerts a catalytic function for an electrochemical reaction between a liquid phase and a gas phase, which is advantageous for improving the reduction efficiency.

[0029] The gas diffusion electrode includes a conductive support. The conductive support is typically carbon paper, but is not limited thereto and may be a metal porous body, an oxide porous body, a semiconductor porous body, or the like. The gas diffusion electrode is obtained by mixing an electrode catalyst in an organic solvent, dropping the mixture onto the conductive support (carrier) described above, and drying the mixture. 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 the carbon dioxide. That is, the electrode catalyst described above can be applied to a method for producing formic acid from carbon dioxide, which includes bringing the electrode catalyst into contact with carbon dioxide and electrochemically reducing the carbon dioxide. A well-known method can be applied as a method for bringing the electrode catalyst into contact with carbon dioxide.

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

[0032] The electrochemical reduction device 100 includes 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 that supports an electrode catalyst.

[0033] The inside of the anode chamber 10 and the inside of the cathode chamber 20 are filled with an electrolyte 70. A counter electrode 80 is installed in the anode chamber 10 so that a portion of the counter electrode 80 is immersed in the electrolyte 70. A reference electrode 90 is installed in the cathode chamber 20 so that a portion of the reference electrode 90 is immersed in the electrolyte 70. The reference electrode 90 allows a desired potential to be accurately applied to the working electrode 60.

[0034] An inlet pipe 40 and an outlet pipe 42 are installed in the gas chamber 30. Carbon dioxide gas is introduced into the gas chamber 30 from 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 to promote the reduction of the carbon dioxide gas. Then, formic acid obtained by the reduction of the carbon dioxide gas is accumulated at the bottom of the gas chamber 30. The accumulated formic acid is discharged from the outlet pipe 42.

[0035] 1, a gas diffusion electrode carrying an electrode catalyst is installed 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] A gas diffusion electrode (support), which is impermeable to the electrolytic solution 70 but permeable to carbon dioxide, carrying an electrode catalyst is used as the working electrode 60. The gas diffusion electrode (support) used for the working electrode 60 is as described above.

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

[0038] The reference electrode 90 may be any material capable of accurately measuring the potential of the working electrode 60. Examples of materials contained in the reference electrode 90 include Hg / Hg2Cl2, Ag / AgCl, and / or reversible hydrogen, with Ag / AgCl being particularly preferred.

[0039] Well-known materials can be used for the separator 50, and examples of materials contained in the separator 50 include Nafion, glass filter, Selemion, and / or Zilphon, with Nafion being particularly preferred. A typical example of the separator 50 is a Nafion membrane.

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

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

[0042] 《Transform》 The carbon dioxide reduction electrode catalyst of the present disclosure can be modified as appropriate within the scope of the claims. For example, the carbon dioxide reduction electrode catalyst of the present disclosure only needs to contain indium and chlorine, and the electrode catalyst may contain substances other than indium and chlorine as long as the effect of the carbon dioxide reduction electrode catalyst of the present disclosure is not impaired. EXAMPLES

[0043] The carbon dioxide reduction electrode catalyst of the present disclosure will be described in more detail below with reference to examples and comparative examples. Note that the carbon dioxide reduction electrode catalyst of the present disclosure is not limited to the conditions used in the following examples.

[0044] <Sample preparation> Each sample was prepared as follows.

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

[0046] 3.4 mg of the 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, which was then dried to obtain a working electrode. This working electrode was used as the sample of Example 1. The area of ​​the carbon paper was 0.5 cm 2 It was.

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

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

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

[0050] Example 5 A sample of Example 5 was obtained similarly to Example 1, except that no carbon black was dispersed in the formic acid.

[0051] Example 6 A sample of Example 6 was obtained in the same manner as in Example 1, except that no heat treatment was performed.

[0052] Example 7 A sample of Example 7 was obtained in the same manner as in Example 1, except that the heat treatment temperature was 500°C.

[0053] Example 8 A sample of Example 8 was obtained in the same manner as in Example 1, except that the heat treatment temperature was 700°C.

[0054] Comparative Example 1 Carbon black was placed in ethanol, and the carbon black was dispersed in the ethanol. Indium nitrate (In(NO3)3) was dissolved in ethanol to obtain an In(NO3)3 solution. The In(NO3)3 solution was dropped into the ethanol in which the carbon black was dispersed. The amount of the drop was adjusted so that 10% by mass of indium was supported on the carbon black relative to the carbon black. Then, the In(NO3)3 solution was dropped into the ethanol in which the carbon black was dispersed, and the adjusted solution was heated to 80°C and evaporated to dryness 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 gas and hydrogen gas (hydrogen concentration: 2%). 3.4 mg of the heat-treated powder thus obtained was mixed with 1000 μl of isopropanol and 4.4 μl of Nafion 117 solution (manufactured by Sigma-Aldrich Co.), and dispersed using an ultrasonic cleaner. Then, 20 μl of this dispersion was dropped onto carbon paper, which was dried to obtain a working electrode. This working electrode was used as the sample of Comparative Example 1. The area of ​​the carbon paper was 0.5 cm 2 It was.

[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 the above-mentioned samples were placed in such an electrical reduction apparatus, and the gas compartment was filled with 10 cm 3 Carbon dioxide gas was flowed at a flow rate of -300mA / cm2 per minute. 2 In order to obtain such a constant current, a potentiogalvanostat (HZ-7000 manufactured by Hokuto Denko Corporation) was used. The reduction time (electrolysis time) was 30 minutes. In Example 1, the current flowing through the working electrode was −400 mA / cm 2 , -500mA / cm 2 , and -600mA / cm 2 Carbon dioxide was reduced even at a constant current density of 1000 .mu.m.

[0056] "evaluation" The products obtained by reducing carbon dioxide using the working electrodes of each sample were analyzed qualitatively and quantitatively by gas chromatography and ion chromatography. The selectivity of the reduction products was evaluated by the Faraday efficiency.

[0057] The Faraday efficiency (FE) was calculated using the following formula (A): Faraday efficiency also indicates how kinetically favorable a particular reaction process is. 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 the heat treatment (for Example 6, after evaporation and drying). XThe x of the sample 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 the heat treatment. From the TEM observation, a HAADF (High-Angle Annular Dark Field Scanning) image and a BF (Bright-Field) image were obtained.

[0059] The results are shown in Table 1. The Cl / In ratio in Table 1 is InCl x FIG. 2 is a HAADF image of the sample of Example 1 observed by TEM. FIG. 3 is a BF image of the sample of Example 1 observed by TEM. FIG. 4A is an XPS spectrum (In 3d2 / 5) of the sample of Example 1. FIG. 4B is an XPS spectrum (Cl 2p) of the sample of Example 1. FIG. 5 is an XRD pattern of the sample of Example 1.

[0060] [Table 1]

[0061] From Table 1, it was confirmed that the samples of Examples 1 to 8 had very high Faraday efficiency when reducing carbon dioxide to formic acid. Also, from FIG. 2 and FIG. 3, it can be seen that the nanoparticles are very well dispersed and supported on the carbon surface. From FIG. 4A and FIG. 4B, it can be seen that indium and chlorine peaks are observed in the XPS spectrum. From these, it is considered that when the catalyst electrode contains indium as the main component and chlorine, it contributes to improving the Faraday efficiency when reducing carbon dioxide to formic acid. Also, from FIG. 5, although a peak of C (carbon) is observed in the XRD pattern, a peak of In (indium) is not observed. From this, it is considered that the electrode catalyst supported on the carbon black of the sample of Example 1 is amorphous. And, since the sample of Example 1 has a particularly high Faraday efficiency when reducing carbon dioxide to formic acid, it can be seen that the electrode catalyst being amorphous is advantageous for improving the Faraday efficiency when reducing carbon dioxide to formic acid.

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

[0063] From the above results, the effect of the carbon dioxide reduction electrode catalyst of the present disclosure was confirmed. [Explanation of symbols]

[0064] 10 Anode chamber 20 Cathode chamber 30 Gas Chamber 40 Introductory tube 42 Discharge pipe 50 Separator 60 Working electrode 70 Electrolyte 80 Opposite 90 Reference pole 100 Electrochemical reduction device

Claims

1. Contains at least indium and chlorine, Electrochemically reduce carbon dioxide, Electrocatalyst for carbon dioxide reduction.

2. 2. The electrode catalyst for reducing carbon dioxide according to claim 1, wherein the electrode catalyst contains chlorine in a molar ratio of 0.5 to less than 3.0 times that of indium.

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

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

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

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