Gas diffusion electrode, method for using same, and co2 electrolytic reduction device

Abstract

Provided is a gas diffusion electrode having excellent Faraday efficiency of CH4 when electrolytic reduction of CO2 is performed for a long time while exhibiting excellent industrial productivity and handleability. The gas diffusion electrode is used for CO2 electrolytic reduction, and is composed of a porous copper foil having a plurality of openings.

Description

Gas diffusion electrode, method of using the same, and CO2 electrolytic reduction device

[0001] The present disclosure relates to a gas diffusion electrode and a method for using the same, and a CO ２ This relates to an electrolytic reduction device.

[0002] CO ２ CO emissions are increasing year by year, causing global warming and becoming a social problem. ２ This technology involves the electrolytic reduction of CO under certain conditions. ２ and H ２ By passing electricity through O, CO ２ It is possible to reduce CO and convert it into valuable products such as methane and ethylene. ２ It is also expected that a sustainable reaction system can be constructed by using renewable energy such as solar power as the electrical energy used in the electrolytic reduction of .

[0003] Conventional CO ２ In electrolytic reduction, the cathode is immersed in an aqueous solution and CO dissolved in the aqueous solution is added. ２ However, such electrolysis devices require CO 2 soluble in aqueous solution. ２ Therefore, in recent years, the cathode and anode are separated by a hydroxide ion conductive membrane, and a gas diffusion electrode is used as the cathode. ２ An electrolytic device is used in which reactivity is improved by supplying a gas containing

[0004] Also, CO ２In the electrolytic reduction of CO₂, the product may vary depending on the reaction conditions, the electrode used, etc., so reactant selectivity is important. In this regard, it has been proposed to use copper as a catalyst capable of producing hydrocarbons such as methane and ethylene. For example, Non-Patent Document 1 (A. Inoue et al., "Ultra-high-rate CO₂ reduction reactions to multicarbon products with a current density of 1.7 A cm₂"). -2 "In neutral electrolytes", EES Catalysis Volume 1, Issue 1, pages 9-16 (2023)" discloses a gas diffusion electrode in which copper fine particles as a catalyst are slurried, coated on a carbon sheet, and dried. ２ In the reduction cell, CO ２ Furthermore, Patent Document 1 (Japanese Patent No. 5683883) discloses a cathode electrode having a porous body made of copper or a copper alloy, which is an aggregate of granular, rod-shaped, or thin-flaked bodies having an average diameter of 50 nm to 5 μm, and which is capable of efficiently producing ethane, ethylene, and the like.

[0005] The use of porous metal foil as a negative electrode current collector in an electricity storage device such as a lithium-ion secondary battery has been investigated. For example, Patent Document 2 (WO 2011 / 67957) discloses that a porous metal foil having a two-dimensional network structure composed of metal fibers can be obtained with high productivity and low cost by metal plating on a peelable layer having cracks formed therein.

[0006] Patent No. 5683883 WO2011 / 67957

[0007] A. Inoue et al., "Ultra-high-rate CO2 reduction reactions to multicarbon products with a current density of 1.7 A cm-2 in neutral electrolytes", EES Catalysis Vol 1, Issue 1, pages 9-16 (2023)

[0008] CO produced by slurrying or sintering copper fine particles as disclosed in Patent Document 1 and Non-Patent Document 1 ２ Electrodes for electrolytic reduction have problems with industrial productivity. Specifically, the production of such electrodes requires numerous manufacturing processes, making it difficult to achieve uniformity and large area in the product. Furthermore, such electrodes are prone to cracking, limiting the device shape. Furthermore, porous bodies composed of copper fine particles can experience problems such as a gradual aggregation of the copper fine particles during prolonged electrolytic reduction, resulting in reduced reactivity.

[0009] The present inventors have now discovered that by using a porous copper foil having a plurality of openings as a gas diffusion electrode, it is possible to obtain a CO ２ When electrolytic reduction of CH ４ It was also found that the Faraday efficiency was excellent.

[0010] Therefore, an object of the present invention is to provide a method for producing a CO ２ When electrolytic reduction of CH ４ The object of the present invention is to provide a gas diffusion electrode that is excellent in Faraday efficiency.

[0011] According to the present disclosure, the following aspects are provided: [Aspect 1] CO ２A gas diffusion electrode for use in electrolytic reduction, the gas diffusion electrode being made of a porous copper foil having a plurality of openings. [Aspect 2] The gas diffusion electrode according to Aspect 1, wherein the average opening diameter, calculated as a circle equivalent diameter of the plurality of openings, is 1050 μm or less. [Aspect 3] The gas diffusion electrode according to Aspect 1 or 2, wherein the average opening diameter is 400 μm or less. [Aspect 4] The gas diffusion electrode according to any one of Aspects 1 to 3, wherein the average opening diameter is 50 μm or less. [Aspect 5] The gas diffusion electrode according to any one of Aspects 1 to 4, wherein the porous copper foil has an opening ratio of 1% to 60%. [Aspect 6] The gas diffusion electrode according to any one of Aspects 1 to 5, wherein the average opening diameter is 30 μm to 50 μm and the opening ratio is 20% to 40%. [Aspect 7] The gas diffusion electrode according to any one of Aspects 1 to 6, wherein the porous copper foil has a thickness of 1 μm to 120 μm. [Aspect 8] The gas diffusion electrode according to any one of Aspects 1 to 6, wherein the porous copper foil has a thickness of 1 μm to 120 μm. ２ CH by electrolytic reduction ４ The gas diffusion electrode according to any one of Aspects 1 to 7 is used for the production of CO. ２ A method for using a gas diffusion electrode, comprising electrolytic reduction of CO. ２ CH by electrolytic reduction ４ A method for using the gas diffusion electrode according to aspect 9 to generate CO. ２ and H ２ a step of supplying O; and a step of supplying the CO ２ The electrolytic reduction of CH ４ A method for using the gas diffusion electrode according to aspect 9 or 10, comprising: a step of generating a gas containing CO. [Aspect 12] A method for using the gas diffusion electrode according to any one of aspects 1 to 8, comprising: ２ Electrolytic reduction device.

[0012] 1 is a schematic top view showing one embodiment of a porous copper foil constituting the gas diffusion electrode of the present invention. FIG. 2 is a schematic top view showing another embodiment of a porous copper foil constituting the gas diffusion electrode of the present invention. FIG. 3 is a schematic top view showing another embodiment of a porous copper foil constituting the gas diffusion electrode of the present invention. ２1 is a schematic cross-sectional view showing one embodiment of an electrolytic reduction apparatus. 2 is a schematic cross-sectional view showing a gas phase half-cell used in measuring the Faraday efficiency in the examples.

[0013] Gas diffusion electrode The gas diffusion electrode of the present invention is ２ This gas diffusion electrode is used for electrolytic reduction. The gas diffusion electrode is made of a porous copper foil. One embodiment of the porous copper foil constituting the gas diffusion electrode is shown in FIGS. 1 and 2. As shown in FIGS. 1 and 2, the porous copper foil 10 has a plurality of openings 12. By using the porous copper foil 10 having a plurality of openings 12 as the gas diffusion electrode, it is possible to achieve a high level of CO ２ When electrolytic reduction of CH ４ The Faraday efficiency is also excellent.

[0014] That is, unlike an aggregate of copper fine particles, copper foil has excellent processability and shape conformability. Therefore, by using the porous copper foil 10, it becomes easy to fabricate a gas diffusion electrode having a desired shape, and ２ When applied to an electrolytic reduction device or the like, it is also easy to handle. Furthermore, since copper foil can be easily manufactured to be uniform and large in area, gas diffusion electrodes made of porous copper foil can be said to be excellent in industrial productivity. Furthermore, as mentioned above, when a porous body made of copper fine particles is used for electrolytic reduction for a long period of time, the copper fine particles gradually aggregate, which can cause problems such as a decrease in reactivity. In contrast, the gas diffusion electrode of the present invention uses porous copper foil 10, so aggregation of fine particles does not occur, and it has excellent long-term stability and CO ２ When electrolytic reduction of CH ４ As a specific index, when the Faraday efficiency is measured under the conditions shown in the examples described later, ２ CH at 3600 seconds after the start of the electrolytic reduction reaction ４ The faradaic efficiency of the gas diffusion electrode of the present invention is 1% or more, preferably more than 20%. ２ CH by electrolytic reduction ４ It is particularly preferred to use it for the production of

[0015] A typical gas diffusion electrode (cathode electrode) typically includes a reduction catalyst layer and a gas diffusion layer provided on one side of the reduction catalyst layer. ２ In this respect, in the gas diffusion electrode of the present invention, the porous copper foil 10 can function not only as a reduction catalyst layer but also as a gas diffusion layer. ２ When a gas containing CO is supplied, the gas is uniformly diffused to the electrode surface through the plurality of openings 12 in the porous copper foil 10, and thus CO ２ Therefore, the gas diffusion electrode may not have a gas diffusion layer (except for the porous copper foil 10 itself). That is, the gas diffusion electrode may consist of only the porous copper foil 10. However, the gas diffusion electrode may further include a gas diffusion layer provided on one side of the porous copper foil 10. Preferred examples of the gas diffusion layer include, but are not limited to, a membrane filter made of a hydrophobic material such as PTFE.

[0016] Each of the multiple openings 12 in the porous copper foil 10 is a hole (through-hole) that can pass through from one surface of the copper foil to the other surface. The openings 12 may be provided in a regular pattern on the copper foil surface, or may be provided irregularly on the copper foil surface. The shape of the openings 12 is not particularly limited and may be circular, polygonal, or other irregular shapes. The arrangement pattern of the openings 12 is also not particularly limited and may be a staggered pattern (e.g., 60° staggered), a parallel pattern, or other irregular patterns. The porous copper foil 10 may have multiple openings 12 provided in a single copper foil. Alternatively, as shown in FIG. 2 , the porous copper foil 10 may have a two-dimensional network structure composed of multiple copper fibers 11. In this case, the copper fibers 11 spread throughout the two-dimensional network can exhibit high strength. In any case, the porous copper foil 10 is different from a copper porous body (i.e., an aggregate of copper fine particles) formed by slurrying or sintering copper powder.

[0017] The average opening diameter of the porous copper foil 10 is preferably 1050 μm or less, more preferably 1000 μm or less, even more preferably 400 μm or less, even more preferably 50 μm or less, particularly preferably 10 μm or more and 50 μm or less, and most preferably 30 μm or more and 50 μm or less. ２ When electrolytic reduction of CH ４ The Faraday efficiency of the porous copper foil 10 is further improved. The average opening diameter in the present invention means the average value of the circle-equivalent diameters of the plurality of openings 12 in the porous copper foil 10. The average opening diameter of the porous copper foil 10 can be measured according to the procedure shown in the examples described later.

[0018] The porous copper foil 10 preferably has an opening ratio of 1% or more and 60% or less, more preferably 10% or more and 40% or less, and even more preferably 20% or more and 40% or less. ２ When electrolytic reduction of CH ４ The aperture ratio of the porous copper foil 10 can be measured according to the procedure shown in the examples below.

[0019] According to a preferred embodiment of the present invention, the porous copper foil 10 has an average opening diameter of 30 μm or more and 50 μm or less and an opening ratio of 20% or more and 40% or less. ２ When the electrolytic reduction of CH ４ The Faraday efficiency can be further improved.

[0020] The thickness of the porous copper foil 10 is preferably 1 μm or more and 120 μm or less, more preferably 1 μm or more and 50 μm or less, even more preferably 1 μm or more and 35 μm or less, and particularly preferably 1 μm or more and 18 μm or less. This allows for a good balance between high strength and excellent processability. The thickness of the porous copper foil 10 can be measured according to the procedure shown in the examples below.

[0021] The method for manufacturing the porous copper foil 10 constituting the gas diffusion electrode is not particularly limited, and any method may be used. For example, as disclosed in Patent Document 2 (WO 2011 / 67957), a release layer is formed on a conductive substrate, cracks are generated in the release layer, and then electrolytic copper plating is performed on the cracked release layer, thereby preferably forming a porous copper foil 10 having a two-dimensional network structure composed of copper fibers 11. Preferred conditions for electrolytic copper plating will be described later.

[0022] Another example of a preferred method for producing the porous copper foil 10 is described below. First, a support for producing the porous copper foil is prepared. When producing the porous copper foil 10 by electrolytic copper plating, the support may be a substrate having sufficient conductivity to be plated. Inorganic materials, organic materials, laminates, and materials with a metal surface can all be used, but metal is preferred. Preferred examples of such metals include metals such as copper, nickel, cobalt, iron, chromium, tin, zinc, indium, silver, gold, aluminum, and titanium, as well as alloys containing at least one of these metal elements. More preferred are copper, copper alloys, nickel, nickel alloys, titanium, titanium alloys, and stainless steel. However, when producing the porous copper foil by electroless copper plating, the support may also be a substrate that does not have electrical conductivity.

[0023] A plurality of plating masks (e.g., photoresist) are formed on the surface of the prepared support. The areas covered by these plating masks correspond to the openings 12 in the porous copper foil 10. Therefore, by adjusting the size of the plating masks and the coverage of the support surface by the plating masks, the average opening diameter and opening ratio of the porous copper foil 10 can be controlled. The size (circular equivalent diameter) of each plating mask is preferably 5 μm to 1100 μm, more preferably 5 μm to 440 μm, even more preferably 5 μm to 55 μm, and particularly preferably 25 μm to 55 μm. The coverage of the support surface by the plating masks is preferably 1% to 60%, more preferably 15% to 45%. Furthermore, by changing the shape and arrangement pattern of each plating mask, the shape and pattern of the openings 12 in the porous copper foil 10 can be controlled. The shape of each plating mask is not particularly limited and may be circular, polygonal (e.g., rectangular), or any other shape. The arrangement pattern of the plating mask is not particularly limited, and examples of possible patterns include a staggered pattern (e.g., 60° staggered), a parallel pattern, etc. The type and method of forming the plating mask are not particularly limited, but it can be preferably formed, for example, by laminating a photosensitive resin on the surface of the support, followed by exposure and development in a predetermined pattern. Alternatively, an insulating layer made of an insulating inorganic material such as DLC (Diamond-Like Carbon) can be formed in a predetermined pattern on the surface of the support. The plating mask may be formed directly on the surface of the support using various printing methods such as inkjet printing.

[0024] Copper plating is applied to the support on which the plating mask is formed, and after removing the plating mask as necessary, the copper plating is peeled off from the support, thereby forming a porous copper foil 10 having a plurality of openings 12. The copper plating may be either electrolytic copper plating or electroless copper plating, but electrolytic copper plating is preferred. This electrolytic copper plating is carried out using a solution having a copper concentration of 5 g / L to 200 g / L (more preferably 10 g / L to 100 g / L, and even more preferably 15 g / L to 30 g / L) and a sulfuric acid concentration of 50 g / L to 260 g / L (more preferably 60 g / L to 230 g / L, and even more preferably 150 g / L to 230 g / L), at a solution temperature of 10° C. to 50° C. (more preferably 20° C. to 45° C., and even more preferably 20° C. to 30° C.) and a current density of 0.5 A / dm ２ 50A / dm or more ２ less than (more preferably 1.0 A / dm ２ 30.0A / dm or more ２ or less, more preferably 1.0 A / dm ２ 10A / dm or more ２ It is preferable to perform the process under the conditions described below. This facilitates the formation of a porous copper foil having a predetermined average opening diameter and opening ratio. The porous copper foil 10 thus obtained may be used as a gas diffusion electrode as is, or may be processed by a known method to form a gas diffusion electrode having a desired shape.

[0025] CO ２ During electrolytic reduction, CH ４ From the viewpoint of efficiently generating the oxide film, it is preferable that copper is exposed on the surface of the porous copper foil 10. Therefore, if an oxide film, an anti-rust film, or the like is present on the surface of the porous copper foil 10, it is preferable to remove these films. Examples of removal methods include polishing, acid cleaning, and electrical cleaning.

[0026] Method for Using the Gas Diffusion Electrode The gas diffusion electrode of the present invention is ２ That is, according to a preferred embodiment of the present invention, the above-mentioned gas diffusion electrode is used to electrolytically reduce CO ２A method for using a gas diffusion electrode according to this embodiment is provided, which comprises performing electrolytic reduction of CO ２ CH by electrolytic reduction ４ Specifically, the method of using the gas diffusion electrode is as follows: (1) adding CO ２ and H ２ (2) supplying O; ２ The steps (1) and (2) are described below.

[0027] (1) CO ２ and H ２ In this step, CO is supplied to the gas diffusion electrode. ２ and H ２ Supplying O to the gas diffusion electrode ２ The supply of CO ２ It is preferable to use a gas containing 50% or more (more preferably 99% or more) of H at a temperature of 0°C to 90°C. The gas may be humidified before being supplied to the electrode. ２ The supply of O is CO ２ As will be described later with respect to the electrolytic reduction device, H ２ It is preferable to carry out the reaction through a hydroxide ion conductive membrane that is permeable to O. That is, by supplying a gas containing water vapor to the anode, ２ O passes through the anode and the hydroxide ion conducting membrane to reach the surface of the gas diffusion electrode. ２ O may be supplied by bringing an electrolyte into contact with one surface of the gas diffusion electrode. The type of electrolyte is not particularly limited, but examples thereof include KHCO ３ Aqueous solution, especially 1 mol / L KHCO ３ An aqueous solution of KHCO can be preferably used. ３ The electrolyte, such as an aqueous solution, ２ It is preferable to saturate the gas phase CO ２ It is possible to effectively prevent the dissolution of in the electrolyte solution.

[0028] (2) CO ２In this process, the copper constituting the gas diffusion electrode is used as a catalyst to reduce CO ２ This allows the electrolytic reduction of CH ４ A gas containing CO can be produced. ２ The electrolytic reduction of CO can be promoted by applying a predetermined voltage to the gas diffusion electrode. ２ and H ２ By supplying O, CO ２ The applied voltage is preferably −3.0 V or more and 0.0 V or less based on the reversible hydrogen electrode potential, more preferably −3.0 V or more and −1.0 V or less, and even more preferably −3.0 V or more and −2.0 V or less in order to obtain high faradaic efficiency. On the other hand, in order to obtain high overall efficiency of the cell, it is preferable to adopt the applied voltage of −1.0 V or more and 0.0 V or less.

[0029] CO ２ Electrolytic reduction device The gas diffusion electrode of the present invention is ２ That is, according to a preferred embodiment of the present invention, a CO 2 gas diffusion electrode is used as a cathode of an electrolytic reduction device. ２ According to this embodiment, an electrolytic reduction device is provided. ２ A preferred embodiment of the electrolytic reduction device is shown in Figure 3. ２ The electrolytic reduction device 100 includes a cathode 102, an anode 104, and a hydroxide ion conducting membrane 106. The cathode 102 is the gas diffusion electrode described above. The anode 104 is disposed opposite the cathode 102. The anode 104 typically includes a catalyst layer and a gas diffusion layer. The hydroxide ion conducting membrane 106 connects the cathode 102 and the anode 104 to each other in a manner that is capable of conducting hydroxide ions and capable of conducting H ２ The hydroxide ion conductive membrane 106 is made of, for example, a solid electrolyte. ２ The electrolytic reduction device 100 preferably further includes an external power supply 108 for applying a predetermined voltage to the cathode 102 and the anode 104. ２ The electrolytic reduction device 100 supplies CO ２It is preferable that the catalyst further includes a flow path for supplying a gas containing CO and a flow path for discharging the gas subjected to the reduction reaction at the cathode 102 together with the reaction product gas. ２ The electrolytic reduction device 100 preferably further includes a flow path for supplying a gas containing water vapor to the anode 104, and a flow path for discharging the gas subjected to the oxidation reaction at the anode 104 together with the reaction product gas. ２ The electrolytic reduction device 100 may have any known configuration, except that it includes the above-described gas diffusion electrode as the cathode 102 .

[0030] The present invention will be explained in more detail by the following examples, but the present invention is not limited to the following examples.

[0031] Example 1 Porous copper foil was produced as follows. First, a copper foil with a thickness of 35 μm was prepared as a conductive substrate. Chromium plating was performed on this copper foil as a release layer according to the following procedure. Specifically, the copper foil was immersed in an acidic cleaner for printed wiring boards (PAC-200, manufactured by Murata Corporation) adjusted to 120 ml / L by adding water for 2 minutes at 40°C. The copper foil thus cleaned was acid-activated by immersing it in 50 ml / L sulfuric acid at room temperature for 1 minute. The acid-activated copper foil was immersed in a chromium plating bath containing 180 g / L Econochrome 300 (manufactured by Meltex Corporation) and 1 g / L of purified concentrated sulfuric acid, at a temperature of 50°C and a current density of 20 A / dm ２ Chrome plating was carried out under the following conditions. The copper foil on which the chrome plating was formed was washed with water and dried. The thickness of the obtained chrome plating was measured by XRF (X-ray fluorescence analysis) to be about 3 μm, and numerous cracks, which were thought to have been generated by plating stress, were confirmed on the surface of the chrome plating.

[0032] Copper sulfate plating was performed on the chrome-plated surface where the cracks had occurred. This copper sulfate plating was performed by immersing the chrome-plated copper foil in a copper sulfate plating bath containing 250 g / L of copper sulfate pentahydrate (copper concentration: approximately 64 g / L) and 80 g / L of sulfuric acid, at a current density of 20 A / dm ２The test was performed under the following conditions: anode: DSE (insoluble electrode for oxygen generation), bath temperature: 40°C. At this time, since current flows more easily in the cracked portion than in the outermost surface of the chrome plating, copper particles grew from the cracks as starting points. As a result, a two-dimensional mesh structure composed of copper fibers was formed on the chrome plating as a porous copper foil of a predetermined thickness. The porous copper foil was physically peeled from the chrome plating to obtain a separated porous copper foil. The separated porous copper foil was subjected to curl correction by heating at a temperature of 200°C for 1 hour under vacuum, and then the surface of the porous copper foil was acid-washed to obtain an evaluation sample.

[0033] Examples 2 to 14 A dry film (RY-3625, manufactured by Hitachi Chemical Co., Ltd.) was thermally laminated onto a titanium electrode. This thermal lamination was performed under conditions of a temperature of 110°C, a pressure of 0.4 MPa, and a transport speed of 0.5 m / min. A plating mask (resist) was formed by exposing and developing circular dots in a 60° staggered pattern on the dry film on the electrode. The pattern was adjusted so that the dot diameter of the plating mask and the coverage of the electrode surface by the plating mask were as shown in Table 1. The electrode on which the plating mask was formed was immersed in a copper sulfate plating bath containing 75 g / L of copper sulfate pentahydrate (copper concentration of approximately 19 g / L) and 220 g / L of sulfuric acid, and the plating mask was applied at a current density of 2 A / dm ２ A current was applied to the electrode to obtain a predetermined thickness under the following conditions: anode: DSE (insoluble electrode for oxygen generation), bath temperature: 25°C. In this way, copper plating was formed on the portion of the electrode where no plating mask was formed. After the copper plating was formed, the plating mask was peeled off from the electrode using a stripping solution (R-100S, manufactured by Mitsubishi Gas Chemical Company, Inc.) at a temperature of 50°C, thereby obtaining a porous copper foil with openings in the portion where the plating mask was formed. Thereafter, the porous copper foil was physically peeled off from the electrode to obtain a separated porous copper foil. The separated porous copper foil was subjected to curl correction by heating at a temperature of 200°C for 1 hour under vacuum, and then the surface of the porous copper foil was acid-washed to obtain an evaluation sample.

[0034] Example 15 A commercially available porous copper foil having a plurality of openings was prepared, and the surface of the porous copper foil was coated with 85% H. ３ P.O. ４The samples were then electrically cleaned in a vacuum to prepare evaluation samples.

[0035] Example 16 (Comparison) 0.5 g of copper powder (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., Wako Grade 1) as a catalyst component, 5% Nafion (registered trademark) dispersion solution DE521 CS type (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., Wako Grade 1) as a binder 0.25 g, and 99.5 wt% ethanol (manufactured by Fujifilm Wako Pure Chemical Industries, Ltd., Wako Grade 1) as a dispersion medium was placed in a glass vial and stirred for 2 hours using a polytetrafluoroethylene (PTFE) stirrer to prepare a copper powder dispersion. Then, ultrasonic dispersion was performed on the copper powder dispersion for 10 minutes. Immediately after ultrasonic dispersion, 65 μL of the copper powder dispersion (density 1.105 g / mL, equivalent to 71.8 mg) was sucked up with a pipetter and dropped onto 20 mm diameter carbon paper (manufactured by Toray Industries, Inc., TGP-H-060, water repellency 50% added product). The copper powder dispersion was spread using a polypropylene tip and then evacuated for 5 minutes in a vacuum desiccator to obtain a copper powder-integrated gas diffusion electrode as an evaluation sample. Example 16 had a copper content of 9.5 g / cm so that it was approximately the same as Examples 1 to 15. ２ It was adjusted to be.

[0036]

[0037] Evaluation The porous copper foil or copper powder-integrated gas diffusion electrodes prepared in Examples 1 to 16 were subjected to the following various evaluations.

[0038] (a) Measurement of Average Opening Diameter and Opening Ratio The deposit surfaces of the porous copper foils produced in Examples 1 to 15 were observed using an optical microscope (Keyence Corporation, VHX-8000) with a ring light source, a shutter speed of 25 (auto), and a gain of 0 dB (manual), at a measurement magnification of 100x. Next, the image was binarized using an automatic area measurement (particle count) function from the observed screen, and the average opening diameter and opening ratio were measured. This measurement was performed by selecting image brightness as the extraction method, and extracting dark areas as openings with brightness adjustment of -25 and brightness unevenness removal (strong). No image adjustment was performed on the extracted openings. Then, for the extracted openings, the average value of the obtained circle equivalent diameters was taken as the average opening diameter, and the total area ratio was taken as the opening ratio. The results were as shown in Table 2.

[0039] (b) Thickness Measurement The thickness of the porous copper foils produced in Examples 1 to 15 was measured using a microscope as follows. First, the porous copper foil was coated with a resin to prevent deformation due to cutting stress, etc. The resin-coated porous copper foil was cut perpendicularly (in the thickness direction) using a microtome (manufactured by Leica, RM2265). Then, the copper foil thickness was measured at three points on the cut surface of the porous copper foil using a microscope (manufactured by Leica, Leica DM LM) at a measurement magnification of 20 times, and the average value was taken as the thickness of the porous copper foil. The results were as shown in Table 2.

[0040] (c) Measurement of Faraday Efficiency To evaluate the Faraday efficiency, a gas-phase half-cell 200 shown in FIG. 4 was constructed. The gas-phase half-cell 200 included a tank 202, an electrolyte 204 injected into the tank 202, a working electrode 206 forming the bottom of the tank 202 so as to be in contact with the electrolyte 204, a PTFE membrane filter (pore diameter: 0.45 μm) 207 provided on the surface of the working electrode 206 opposite the electrolyte 204, a counter electrode 208 and a reference electrode 210 immersed in the electrolyte 204, a supply tube 212 a for supplying gas to the working electrode 206, and a recovery tube 212 b for recovering the supplied gas and the gas generated during the reduction reaction. The specific configurations of the electrolyte 204, working electrode 206, counter electrode 208, and reference electrode 210 were as follows: - Electrolyte 204: CO ２1 M KHCO saturated with ３ Aqueous solution Working electrode 206: porous copper foil with a diameter of approximately 3 cm or copper powder integrated gas diffusion electrode (evaluation sample) Counter electrode 208: carbon rod Reference electrode 210: Ag / AgCl

[0041] The copper powder-integrated gas diffusion electrode prepared in Example 16 was placed in an electrolytic reduction apparatus (gas-phase half-cell 200) and then subjected to cyclic voltammetry as a pretreatment for catalyst activation. In this cyclic voltammetry, a 0.1 mol / L potassium hydroxide aqueous solution was used as the electrolyte. Furthermore, gaseous nitrogen was supplied from the lower inlet of the cathode electrode (working electrode 206) during the cyclic voltammetry. The potential was started from the open-circuit potential, and a potential application of +1.2 V (vs. RHE) to the oxidation side and a potential application of −0.4 V (vs. RHE) to the reduction side were repeated 10 times. After the final potential application to the reduction side, the potential was returned to the open-circuit potential. The rate of the above potential application was 100 mV / s. After cyclic voltammetry, the electrolyte was discarded, and the interior of the tank 202 was filled with CO ２ 1 M KHCO saturated with ３ The mixture was washed with aqueous solution several times.

[0042] Gas was supplied from the supply tube 212a of the gas-phase half-cell 200 under the following conditions, and the reduction reaction of carbon dioxide was carried out at −1.4 V. ＲＨＥ The reduction reaction was continued for 3600 seconds. <Supply gas conditions> Composition: CO ２ (Purity 99.99%) - Flow rate: 5 mL / min - Temperature: 25°C - Humidification: Dry

[0043] The supply gas and the gas generated during the reduction reaction were collected by the collection tube 212b and directly supplied to a gas chromatograph, thereby analyzing the reaction products at any time during voltage application. <Analysis conditions> - Apparatus: Gas chromatograph (Shimadzu Corporation, GC-2030) - Detector: Barrier discharge ionization detector (BID) (Shimadzu Corporation, GC-2030 built-in option) - Column: Shinwa Kako Co., Ltd., Micropacked-ST, length 1.0 m, inner diameter 1.0 mm - Sample injection method: Split method (split ratio 4:1) - Sample injection amount: 1 mL (online measurement with an inflow of carbon dioxide gas for reaction at 5 ml / min) - Injection port temperature: 150°C - Detector temperature: 280°C

[0044] As a result of the gas chromatography analysis, methane was detected as a product in Examples 1 to 15, but not in Example 16. The amount of substance produced for the detected methane was determined, and the Faraday efficiency at 3,600 seconds after the start of the reduction reaction was calculated using the following formula: FE = 100 x z x n x F / Q (where FE is the Faraday efficiency (%), z is the number of electrons required to produce one molecule of product, n is the number of moles (mol) of product, F is the Faraday constant (96,486 C / mol), and Q is the amount of charge applied (C)). The results are shown in Table 2. The amount of charge Q can be calculated by applying a predetermined voltage, detecting the current I that flows, and multiplying this by the predetermined application time t (= I x t). The number of electrons z in methane is as follows: - methane (z = 8): CO ２ +8H ＋ +8e － →CH ４ +2H ２ O

[0045]

[0046] 10 Porous copper foil 11 Copper fiber 12 Opening 100 CO ２Electrolytic reduction device 102 cathode 104 anode 106 hydroxide ion conductive membrane 108 external power supply 200 gas phase half cell 202 tank 204 electrolyte 206 working electrode 207 membrane filter 208 counter electrode 210 reference electrode 212a supply tube 212b recovery tube

Claims

1. CO ２ A gas diffusion electrode for use in electrolytic reduction, the gas diffusion electrode being made of a porous copper foil having a plurality of openings.

2. The gas diffusion electrode according to claim 1, wherein the average equivalent circle diameter of said plurality of openings is 1050 μm or less.

3. The gas diffusion electrode according to claim 2, wherein the average opening diameter is 400 μm or less.

4. The gas diffusion electrode according to claim 2, wherein the average opening diameter is 50 μm or less.

5. The gas diffusion electrode according to any one of claims 1 to 4, wherein the porous copper foil has an opening ratio of 1% or more and 60% or less.

6. The gas diffusion electrode according to claim 5, wherein the average opening diameter is 30 μm or more and 50 μm or less, and the opening ratio is 20% or more and 40% or less.

7. The gas diffusion electrode according to any one of claims 1 to 4, wherein the thickness of the porous copper foil is 1 μm or more and 120 μm or less.

8. The above CO ２ CH by electrolytic reduction ４ The gas diffusion electrode according to any one of claims 1 to 4, which is used in the manufacture of a gas diffusion electrode.

9. CO using the gas diffusion electrode according to any one of claims 1 to 4 ２ A method of using a gas diffusion electrode, comprising performing electrolytic reduction.

10. The above CO ２ CH by electrolytic reduction ４ 8. A method for using the gas diffusion electrode according to claim 7 to produce 11. CO is introduced into the gas diffusion electrode. ２ and H ２ a step of supplying O; and a step of supplying the CO ２ The electrolytic reduction of CH ４ 8. A method of using the gas diffusion electrode of claim 7, comprising:

12. A CO2 gas diffusion electrode comprising the gas diffusion electrode according to any one of claims 1 to 4 as a cathode. ２ Electrolytic reduction device.

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