Oxygen reduction electrode and method for producing same

Abstract

The present invention addresses the problem of providing an oxygen reduction electrode capable of suppressing platinum elution. Disclosed is an oxygen reduction electrode comprising: a conductive base material; and a reaction layer which is formed on the conductive base material and contains platinum-supporting carbon and a hydrophobic resin, wherein the crystal diameter of platinum contained in the platinum-supporting carbon is 3 nm or more, and the BET specific surface area of the platinum-supporting carbon is 30 m2 / g or more and 300 m2 / g or less. The proportion of platinum supported by the platinum-supporting carbon is preferably less than 59 mass%.

Description

Oxygen reduction electrode and method for producing the same

[0001] The present disclosure relates to an oxygen reduction electrode and a method for manufacturing the same.

[0002] In the sodium chloride electrolysis industry, a chlorine-evolving electrode has traditionally been used as the anode and a hydrogen-evolving electrode as the cathode. However, in recent years, so-called gas diffusion-type sodium chloride electrolysis, in which the cathode is replaced with an oxygen reduction electrode, has attracted attention as a means of energy conservation toward promoting carbon neutrality. In gas diffusion-type sodium chloride electrolysis, a high overvoltage (oxygen reduction overvoltage) occurs due to the oxygen reduction reaction. Theoretically, the voltage reduction effect of the gas diffusion electrode is 1.0 V or more, but currently it is limited to about 0.7 V. To commercialize gas diffusion-type sodium chloride electrolysis on an industrial scale, further reduction of the overvoltage due to the oxygen reduction reaction is being investigated.

[0003] The use of platinum or silver as a catalyst has been investigated as a technique for reducing the overvoltage associated with the oxygen reduction reaction. For example, Patent Document 1 discloses an oxygen gas diffusion cathode that can reduce the electrolysis voltage using a silver catalyst and a silver-platinum catalyst. Also, Non-Patent Document 1 discloses a gas diffusion electrode that uses a silver catalyst and a platinum catalyst.

[0004] US2013 / 037415A1

[0005] Electrochemica Acta, 2000, vol. 45, Issue 25, P. 4251-4256

[0006] The oxygen gas diffusion cathode described in Patent Document 1 was used at an operating current of sodium chloride electrolysis (usually 4.0 to 8.0 kA / m ２ ), there is a problem that the oxygen reduction overvoltage is high and energy saving is insufficient. On the other hand, the gas diffusion electrode using a silver catalyst and a platinum catalyst described in Non-Patent Document 1 can reduce the oxygen reduction overvoltage compared to a gas diffusion electrode using only a silver catalyst. However, a cathode using a platinum catalyst dissolves platinum into the electrolyte due to continuous electrolysis, making it difficult to stably electrolyze sodium chloride for a long period of time. The present disclosure aims to provide an oxygen reduction electrode that can suppress platinum dissolution.

[0007] In this disclosure, we have focused on the fact that in gas diffusion type electrolysis of sodium chloride using a platinum catalyst, the physical properties of the platinum catalyst in the cathode have a significant effect on the elution of platinum during sodium chloride electrolysis. Furthermore, we have discovered a physical property that particularly affects the elution of platinum when platinum-supported carbon, in which platinum is supported on carbon, is used as a platinum catalyst in gas diffusion type electrolysis of sodium chloride, and have found that the above-mentioned problem can be solved by controlling this physical property. That is, the present invention is as set forth in the claims, and the gist of this disclosure is as follows.

[0008] [1] A conductive substrate and a reaction layer formed on the conductive substrate, the reaction layer containing platinum-supported carbon and a hydrophobic resin, wherein the crystallite diameter of the platinum contained in the platinum-supported carbon is 3 nm or more, and the BET surface area of ​​the platinum-supported carbon is 30 m ２ / g or more 300m ２ / g or less.

[0009] [2] The oxygen reduction electrode according to [1] above, wherein the platinum loading rate of the platinum-supported carbon is less than 59 mass%. [3] The oxygen reduction electrode according to [1] or [2] above, wherein the conductive substrate is a porous conductive substrate. [4] The oxygen reduction electrode according to [3] above, wherein the porous conductive substrate is carbon fiber. [5] The oxygen reduction electrode according to any of [1] to [4] above, wherein the platinum contained in the platinum-supported carbon is single metal platinum. [6] The oxygen reduction electrode according to any of [1] to [5] above, wherein the hydrophobic resin is a fluororesin. [7] An oxygen reduction electrode for sodium chloride electrolysis, comprising the oxygen reduction electrode according to any of [1] to [6] above. [8] A method for producing the oxygen reduction electrode according to any of [1] to [6] above, comprising a coating step of coating a catalytic ink for a reaction layer on a conductive substrate, a drying step of drying the conductive substrate carrying the catalytic ink for the reaction layer, and a calcination step of calcining the dried conductive substrate carrying the precursor of the reaction layer in an inert gas atmosphere. [9] A method for producing an oxygen reduction electrode for sodium chloride electrolysis, comprising the steps of producing the oxygen reduction electrode according to [8] above.

[10] A method for producing an oxygen reduction electrode for sodium chloride electrolysis, comprising: a conductive base material; and a reaction layer formed on the conductive base material, the reaction layer containing platinum-supported carbon and a hydrophobic resin, wherein the crystallite diameter of platinum contained in the platinum-supported carbon is 3 nm or more, and the BET specific surface area of ​​the platinum-supported carbon is 30 m ２ / g or more 300m ２ / g or less as an oxygen reduction electrode.

[11] The use of the electrode according to the above

[10] as an oxygen reduction electrode for sodium chloride electrolysis.

[0010] According to the present disclosure, an oxygen reduction electrode capable of suppressing platinum elution can be provided.

[0011] The present disclosure will be described below by showing an example of an embodiment. The present disclosure includes any combination of the configurations and numerical values ​​disclosed herein, as well as any range of the upper and lower limits of the numerical values ​​disclosed herein.

[0012] [Oxygen reduction electrode] The oxygen reduction electrode of this embodiment comprises a conductive substrate and a reaction layer formed on the conductive substrate, the reaction layer containing platinum-supported carbon and a hydrophobic resin, wherein the crystallite diameter of platinum contained in the platinum-supported carbon is 3 nm or more, and the BET surface area of ​​the platinum-supported carbon is 30 m ２ / g or more 300m ２ / g or less. The term "on a conductive substrate" means that the reaction layer is formed directly on the conductive substrate, and that the reaction layer is formed indirectly via another layer such as a gas diffusion layer, and has a structure in which the conductive substrate and the reaction layer are laminated. Each component will be described in detail below.

[0013] <Conductive substrate> The conductive substrate reinforces the strength of the oxygen reduction electrode, collects current and conducts electrons, and is a reactant in the oxygen reduction reaction. ２ ) gas to the catalyst component. The material constituting the conductive substrate is not particularly limited as long as it has the above-mentioned function, and examples thereof include at least one of conductive carbon and metal materials. The conductive substrate is preferably a substrate made of at least one of conductive carbon and metal materials, and more preferably a substrate made of conductive carbon. Furthermore, the conductive substrate is preferably a substrate made of oxygen (O ２ ) From the viewpoint of good gas permeability, a porous conductive substrate is preferred.

[0014] The conductive carbon, which is a material constituting the conductive substrate, may be one or more selected from the group consisting of carbon fiber (also called carbon cloth), carbon black, graphite, activated carbon, and carbon nanotubes. The carbon fiber may be at least one of carbon paper and carbon cloth. The carbon black may be one or more selected from the group consisting of acetylene black, furnace black, and Ketjen Black (registered trademark).

[0015] The metal material constituting the conductive substrate may be at least one of an expanded metal and a mesh. The material of the expanded metal and the mesh is preferably one or more selected from the group consisting of nickel, silver, copper, iron, titanium, and stainless steel alloy.

[0016] The conductive substrate is preferably carbon fiber because of its excellent corrosion resistance and ease of processing. In addition, in order to ensure long-term gas permeability, it is preferable that the surface of the conductive substrate is coated with a water repellent agent. For example, the conductive substrate is preferably at least one of carbon fiber that has been subjected to a water repellent treatment and carbon fiber having a water repellent agent on its surface.

[0017] <Gas Diffusion Layer> The oxygen reduction electrode of the present embodiment may have a gas diffusion layer formed by applying a suspension for a gas diffusion layer containing conductive carbon and a hydrophobic resin onto a conductive substrate and drying the applied suspension. By having the gas diffusion layer, oxygen (O ２ The conductive carbon contained in the gas diffusion layer is not particularly limited as long as it imparts conductivity to the gas diffusion electrode, is stable to the extent that its structure and composition do not change upon reaction with an aqueous solution containing at least one of an alkali metal element and an alkaline earth metal element (hereinafter also referred to as an "alkaline aqueous solution"), and is hydrophobic, but examples thereof include one or more selected from the group consisting of acetylene black, furnace black, Ketjen Black (registered trademark), graphite, activated carbon, carbon nanotubes, and carbon nanofibers, and is preferably one or more selected from the group consisting of acetylene black, Ketjen Black, furnace black, and graphite. The hydrophobic resin contained in the gas diffusion layer is not particularly limited as long as it is stable enough that its structure and composition do not change upon reaction with an alkaline aqueous solution, and examples thereof include one or more selected from the group consisting of PVDF (polyvinylidene fluoride), PTFE (polytetrafluoroethylene), PFA (tetraperfluoroalkoxyalkane), PVDF (polyvinylidene fluoride), and FEP (tetrafluoroethylene-hexafluoropropylene copolymer).

[0018] <Reaction Layer> The oxygen reduction electrode of this embodiment has a reaction layer formed on a conductive substrate. The reaction layer is a reaction field where a gas-liquid-solid three-phase interface is formed and the oxygen reduction reaction proceeds. The reaction layer preferably has a structure in which a hydrophilic material such as hydrophilic carbon and a hydrophobic material such as hydrophobic carbon or a fluororesin are highly dispersed in order to form the three-phase interface. The reaction layer in the oxygen reduction electrode of this embodiment contains platinum-supported carbon and a hydrophobic resin. As will be described in detail later, the reaction layer is obtained by applying a catalyst ink for the reaction layer, which contains platinum-supported carbon and a hydrophobic resin, to a conductive substrate, followed by drying and baking.

[0019] (Platinum-supported carbon) The platinum-supported carbon contained in the reaction layer of the oxygen reduction electrode of this embodiment is platinum supported on conductive carbon. Hereinafter, the platinum-supported carbon will be described in detail.

[0020] - Platinum - In this disclosure, the "platinum" in platinum-supported carbon refers to a platinum material, which is the main component of the catalyst. Hereinafter, it will be referred to as "platinum (platinum material)." More specifically, platinum (platinum material) is the main component of the catalyst located at the reaction site where a gas-liquid-solid three-phase interface is formed and the oxygen reduction reaction proceeds. Platinum (platinum material) is a catalyst active in the oxygen reduction reaction, and is preferably at least one of platinum alone (metallic platinum; Pt as the metal) and a platinum alloy. A platinum alloy is an alloy consisting of metallic platinum (Pt) and a different metal element. The different metal element is not particularly limited, and examples include one or more selected from the group consisting of Ag, Au, Co, Cu, Fe, Ir, Mn, Ni, Pd, Ru, Ti, and Zr.

[0021] In the present disclosure, the term "platinum alloy" can be broadly interpreted, and can refer to not only a platinum alloy consisting of a single phase, but also a multilayer structure (core-shell structure). Examples of platinum (platinum materials) having a multilayer structure include platinum materials whose core material is at least one of metallic platinum (Pt) and a platinum alloy, and whose shell material is one or more selected from the group consisting of Ag, Au, Co, Cu, Fe, Ir, Mn, Ni, Pd, Ru, Ti, and Zr. These platinums (platinum materials) may be commercially available or synthesized according to known methods. The synthesis method is not particularly limited, and examples include a wet method in which a reducing agent (such as alcohol) is added to a platinum compound solution (such as a dinitrodiammine platinum nitrate solution) to precipitate platinum material particles, and at least one of vapor deposition and sputtering (dry method).

[0022] Conductive Carbon—The conductive carbon contained in the platinum-supported carbon not only imparts conductivity to the electrode (oxygen reduction electrode) but also plays an important role in increasing the three-phase interface of the electrode. That is, the conductive carbon exhibits either hydrophilic or hydrophobic properties depending on the surface functional groups (hereinafter, the former will be referred to as "hydrophilic carbon" and the latter as "hydrophobic carbon"). Therefore, it is possible to optimize the three-phase interface in the electrode by appropriately combining hydrophilic and hydrophobic carbons. The conductive carbon in the platinum-supported carbon is not particularly limited as long as it is relatively stable in high-temperature, high-concentration alkali environments. Examples include one or more selected from the group consisting of carbon black, acetylene black, furnace black, Ketjen Black (registered trademark), graphite, activated carbon, and carbon nanotubes, with carbon black and furnace black being preferred. The conductive carbon contained in the platinum-supported carbon may be a single carbon or a mixture thereof.

[0023] Platinum-supported carbon may be commercially available or may be synthesized according to a known method. The synthesis method is not particularly limited, and examples include at least one of a method in which a carbon powder carrier is added to a platinum compound solution and mixed, followed by the addition of a reducing agent to deposit platinum (platinum material) directly on the carbon, and a method in which platinum (platinum material) is supported on the surface of a carbon material surface-modified with functional groups such as carboxyl groups and phenolic hydroxyl groups. By appropriately selecting the raw material for platinum (platinum material) and the method for supporting the carbon material, the platinum support rate of platinum (platinum material), the crystallite size of the platinum material, and the particle size of the platinum material can be controlled to appropriate values.

[0024] - Platinum Loading Ratio - The platinum loading ratio of the platinum-loaded carbon is preferably less than 59% by mass, more preferably 55% by mass or less, and even more preferably 51% by mass or less. The platinum loading ratio of the platinum-loaded carbon is preferably greater than 46% by mass. When the platinum loading ratio of the platinum-loaded carbon is within the above range, the catalytic active sites are increased and aggregation of platinum (platinum material) is suppressed, thereby improving catalytic activity. Here, the "platinum" in the platinum loading ratio refers to platinum (platinum material) and includes platinum as a single metal and platinum alloys, as described above.

[0025] The platinum loading rate of platinum-loaded carbon is a value measured by the following measurement method. 0.0500 g of a platinum-loaded carbon sample of this embodiment was weighed and heated in air at 800°C for 60 minutes to burn and remove the carbon. Next, the mass of the sample after heating was measured, and the platinum loading rate can be calculated using the following formula: Platinum loading rate (mass%) = Mass of sample after heating (g) / Mass of sample before heating (= 0.0500 g) × 100

[0026] -Crystallite diameter of platinum material- In the present disclosure, the crystallite diameter of platinum (platinum material) in platinum-supported carbon is 3 nm or more. If the crystallite diameter of platinum (platinum material) in platinum-supported carbon is less than 3 nm, a large amount of platinum will be eluted from the oxygen reduction electrode during electrolysis. The crystallite diameter of platinum (platinum material) is preferably 5 nm or more, more preferably 7 nm or more, and even more preferably 8 nm or more. Furthermore, in consideration of catalytic activity, the crystallite diameter of platinum (platinum material) is preferably 20 nm or less, more preferably 15 nm or less, and even more preferably 12 nm or less. The crystallite diameter of platinum (platinum material) in platinum-supported carbon is preferably 3 nm or more and 20 nm or less, more preferably 5 nm or more and 20 nm or less, even more preferably 7 nm or more and 15 nm or less, particularly preferably 8 nm or more and 12 nm or less.

[0027] The crystallite diameter of platinum (platinum material) can be calculated by applying the half-width of a diffraction line having a peak top at 2θ = 39.5 ° ± 1.0 ° corresponding to the main peak (111) of platinum in an XRD pattern obtained by measurement using an X-ray diffractometer (e.g., Ultima 4, manufactured by Rigaku Corporation) and CuKα radiation as a radiation source to the following Scherrer formula: Platinum crystallite diameter (nm) = K × λ / (B × cos θ) K: Scherrer constant (= 0.94) λ: wavelength of X-ray used (e.g., 0.154 nm in the case of CuKα radiation) B: half-width (°) θ: angle of incidence (°)

[0028] In the present disclosure, the particle size of platinum (platinum material) in platinum-supported carbon generally coincides with the crystallite size of the platinum material described above. Therefore, the above-described description of the crystallite size of the platinum material applies appropriately to the particle size of platinum (platinum material).

[0029] -BET specific surface area- The BET specific surface area of ​​platinum-supported carbon is 30 m ２ / g or more 300m ２ / g or less. The BET specific surface area of ​​the platinum-supported carbon is 300 m ２ If the specific surface area is greater than 250 m / g, the amount of platinum eluted from the oxygen reduction electrode during electrolysis increases. ２ / g or less, 200m２ / g or less, 150m ２ / g or less or 110m ２ / g or less is preferable. ２ If the ratio is less than 35m / g, the activity as an oxygen reduction electrode catalyst decreases. ２ / g or more, 40m ２ / g or more, or 45m ２ The BET specific surface area of ​​the platinum-supported carbon is preferably 30 m ２ / g or more 300m ２ / g or less, 35m ２ / g or more 250m ２ / g or less, 40m ２ / g or more 200m ２ / g or less, 45m ２ / g or more 150m ２ / g or less or 45m ２ / g or more 110m ２ / g or less.

[0030] The BET specific surface area of ​​the platinum-supported carbon is measured by the following method: Using a chemical / physical adsorption measuring device (NOVA-4200e, manufactured by Yuasa Ionics Co., Ltd.), the BET specific surface area S of the platinum-supported carbon sample after pretreatment is calculated by the BET multipoint method (constant volume method) using the following formula: ＢＥＴ (m ２ / g) can be calculated. ＢＥＴ (m ２ / g) = 21485 × B × σ / (C−A), where A is the mass (g) of the glass cell used for measurement, and B is the monolayer N. ２ Gas adsorption amount (g) C: Mass (g) of the measurement glass cell containing the platinum-supported carbon sample after pretreatment σ: Cross-sectional area of ​​the adsorbed gas molecule (= 0.162 nm ２ The BET specific surface area may be measured after pretreatment in which 0.0500 g of the platinum-supported carbon sample of this embodiment is placed in a glass cell for measurement and subjected to dehydration treatment in a vacuum at 100° C. for 30 minutes.

[0031] The BET specific surface area of ​​platinum-supported carbon is adjusted by controlling the BET specific surface area of ​​the carbon supporting platinum and / or the BET specific surface area of ​​platinum supported on the carbon. From the viewpoint of more effectively suppressing elution of platinum from the oxygen reduction electrode during electrolysis, it is preferable to control the BET specific surface area of ​​the carbon. To control the BET specific surface area of ​​the carbon, for example, the type of carbon used and the particle size may be appropriately selected.

[0032] (Hydrophobic Resin) Considering that the reaction layer is used for sodium chloride electrolysis, it is preferable to use a fluorine-based resin that is chemically stable in a high-temperature, high-concentration alkaline aqueous solution, such as one or more selected from the group consisting of PTFE, PFA, PVDF, and FEP.

[0033] (Mass ratio of hydrophobic resin to conductive carbon) In order to optimize the hydrophilicity / hydrophobicity of the reaction layer and enhance the electrode strength, the mass ratio of the hydrophobic resin to the conductive carbon is preferably 1:0.1 to 10 (i.e., the mass ratio [mass %] of carbon to the hydrophobic resin is 10 mass % or more and 1000 mass % or less), and more preferably 1:0.1 to 5 (same, 10 mass % or more and 500 mass % or less).

[0034] The thickness of the reaction layer is not particularly limited, but may be adjusted from the viewpoint of easiness of diffusion of gas or ions, length of conductive path, and ensuring reaction surface area.

[0035] <Platinum Loading Amount> The amount of platinum material loaded in the oxygen reduction electrode of this embodiment (hereinafter referred to as "platinum loading amount") is 0.1 mg / cm per geometric area of ​​the electrode. ２ 10mg / cm or more ２ Preferably, 0.2 mg / cm or less ２ 10mg / cm or more ２ The following is more preferable. Here, the geometric area corresponds to the projected area of ​​the electrode, and the thickness of the electrode is not taken into consideration. When the platinum loading amount of the oxygen reduction electrode of this embodiment is within the above range, a significant oxygen reduction overpotential reduction effect can be obtained even at a high electrolysis current density.

[0036] <Platinum elution rate> The oxygen reduction electrode of this embodiment can have a platinum elution rate of less than 34%, more preferably 30% or less, even more preferably 10% or less, and particularly 1% or less. A platinum elution rate within the above range means that elution of platinum, which acts as a catalyst, is suppressed. Suppression of platinum elution makes it possible to obtain an oxygen reduction electrode that is energy-efficient and allows safe long-term operation, and an electrolysis device using this oxygen reduction electrode. The platinum elution rate of the oxygen reduction electrode of this embodiment is determined by the measurement method in the examples described below.

[0037] [Method for manufacturing an oxygen reduction electrode] A method for manufacturing an oxygen reduction electrode includes, for example, a coating step of coating a catalytic ink for a reaction layer onto a conductive substrate, a drying step of drying the conductive substrate carrying the catalytic ink for a reaction layer, and a firing step of firing the conductive substrate carrying the dried precursor of the reaction layer in an inert gas atmosphere. By this manufacturing method, a homogeneous reaction layer can be obtained.

[0038] (Catalyst Ink for Reaction Layer) The catalyst ink for reaction layer is an ink produced by uniformly dispersing the materials constituting the reaction layer in a solvent, and may be, for example, in the form of a slurry. That is, in this embodiment, the catalyst ink for reaction layer includes platinum-supported carbon, a hydrophobic resin, and a solvent. The platinum-supported carbon and the hydrophobic resin may be the same as those described above. The solvent is not particularly limited and may be, for example, one or more selected from the group consisting of water, ethanol, methanol, isopropyl alcohol, and naphtha, and may be used alone or in combination. To improve the dispersibility of the catalyst ink for reaction layer, the catalyst ink for reaction layer may contain a surfactant. The surfactant is not particularly limited and may, for example, be a nonionic surfactant such as octylphenol ethoxylate.

[0039] The coating step, drying step, and baking step will be described below.

[0040] (Coating Step) The coating step is a step of coating the catalytic ink for the reaction layer onto the conductive substrate. The coating method is not particularly limited as long as it can uniformly coat the catalytic ink for the reaction layer onto the conductive substrate, and may be at least one of manual coating and mechanical coating. For example, in the case of manual coating, examples of the coating method include one or more methods selected from the group consisting of a brush, a spatula, and a roller. In the case of mechanical coating, examples of the coating method include one or more methods selected from the group consisting of screen printing, spray atomization, die coater coating, and blade coating.

[0041] (Drying Step) The drying step is a step of drying the conductive substrate having the catalyst ink for reaction layer obtained in the coating step. That is, it is a step of drying and removing the solvent contained in the catalyst ink for reaction layer coated in the coating step. The drying temperature is preferably 60°C or higher and 120°C or lower, more preferably 70°C or higher and 120°C or lower, and even more preferably 80°C or higher and 120°C or lower. A drying temperature of 60°C or higher is a temperature sufficient for removing the solvent, improving production efficiency. Furthermore, a drying temperature of 120°C or lower can suppress the combustion reaction of carbon by the platinum material. The drying time is sufficient to remove the solvent contained in the catalyst ink for reaction layer, and can be changed appropriately depending on the size of the conductive substrate and the characteristics of the dryer, and can be, for example, 10 minutes to 12 hours. The atmosphere in the drying step is not particularly limited as long as it does not inhibit the removal of the solvent, and examples thereof include at least one of an oxidizing atmosphere and an inert gas atmosphere, such as one or more selected from the group consisting of air, oxygen, nitrogen, helium, argon, neon, krypton, and xenon, with air being preferred.

[0042] The coating step and drying step may be repeated multiple times (for example, 2 to 10 times) until the oxygen reduction electrode of this embodiment has a desired amount of supported platinum.

[0043] (Baking Process) The baking process is performed to remove the surfactant remaining in the reaction layer and to dissolve the hydrophobic resin contained in the reaction layer to bond the conductive substrate and the reaction layer, or the conductive substrate, the gas diffusion layer, and the reaction layer. The baking process includes baking the conductive substrate having the precursor of the reaction layer obtained in the drying process in an inert gas atmosphere. The baking temperature is preferably 250°C or higher and 400°C or lower. A baking temperature of 250°C or higher allows sufficient dissolution of the hydrophobic resin. Furthermore, a baking temperature of 400°C or lower prevents the hydrophobic resin from disappearing due to volatilization or thermal decomposition. The baking temperature is more preferably 260°C or higher and 390°C or lower, and even more preferably 270°C or higher and 380°C or lower. The baking time can be appropriately adjusted depending on the size of the conductive substrate and the characteristics of the baking furnace, and can be, for example, 1 minute to 24 hours. The inert gas is not particularly limited as long as it does not cause a carbon combustion reaction with the platinum material. The inert gas may be one or more selected from the group consisting of helium, neon, argon, krypton, xenon, and nitrogen, and nitrogen is particularly preferred. The inert gas may be used alone or in combination of two or more.

[0044] The method for producing an oxygen reduction electrode according to the present embodiment may include a pressing step after the firing step in order to improve the conductivity associated with an increase in electrode density. Examples of the pressing method include uniaxial pressure pressing and roll pressing. Roll pressing is preferred from the viewpoint of suppressing platinum elution due to a reduction in the surface area of ​​the electrode. The pressing pressure is not particularly limited, and may be, for example, 1 kgf / cm. ２ More than 500kgf / cm ２ The following are included:

[0045] The present disclosure will be described in more detail below by way of examples, but the present disclosure should not be construed as being limited to these examples.

[0046] <Measurement of crystallite size> Using an X-ray diffractometer (device name "Ultima4", manufactured by Rigaku Corporation), powder X-ray diffraction measurements of the oxygen reduction electrode for sodium chloride electrolysis were carried out. A CuKα ray (λ = 1.5405 Å) was used as the radiation source, the measurement mode was step scan, the scan conditions were sampling width 2θ = 0.04 °, the measurement time was 4 seconds, and the measurement range was 2θ in the range of 10 ° to 80 °. Of the obtained XRD pattern, the half width (full width at half maximum: FWHM) of the diffraction line near 2θ = 39.5 ° ± 1.0 ° corresponding to the main peak of platinum (111) was determined by peak fitting using the analysis software PDXL-2 attached to the X-ray diffractometer. The crystallite size was calculated by applying the half width determined in this way to the following Scherrer formula. Platinum crystallite diameter (nm) = K × λ / (B × cos θ) K: Scherrer constant (= 0.94) λ: wavelength of X-ray used (for example, 0.154 nm for CuKα rays) B: half-width (°) θ: angle of incidence (°)

[0047] <BET Specific Surface Area> 0.0500 g of the platinum-supported carbon samples of the present examples and comparative examples were placed in a glass cell for measurement and pretreated by dehydration treatment at 100°C for 30 minutes under vacuum. Using a chemical / physical adsorption measuring device (NOVA-4200e, manufactured by Yuasa Ionics Co., Ltd.), the BET specific surface area S was determined by the BET multipoint method (constant volume method) using the following formula: ＢＥＴ (m ２ / g) was calculated. ＢＥＴ (m ２ / g) = 21485 × B × σ / (C−A), where A is the mass (g) of the glass cell used for measurement, and B is the monolayer N. ２ Gas adsorption amount (g) C: Mass (g) of the measurement glass cell containing the platinum-supported carbon sample after pretreatment σ: Cross-sectional area of ​​the adsorbed gas molecule (= 0.162 nm ２ )

[0048] <Platinum Support Rate> 0.0500 g of the platinum-supported carbon samples of the present examples and comparative examples was weighed and heated in air at 800°C for 60 minutes to burn and remove the carbon. Next, the mass of the sample after heating was measured, and the platinum support rate was calculated using the following formula: Platinum support rate (mass%) = Mass of sample after heating (g) / Mass of sample before heating (= 0.0500 g) × 100

[0049] <Platinum Dissolution Rate> The platinum dissolution rate of the oxygen reduction electrodes for sodium chloride electrolysis of the Examples and Comparative Examples was measured as follows.

[0050] (Method for measuring platinum elution rate) An electrolytic cell was prepared using the oxygen reduction electrode for sodium chloride electrolysis prepared in the Examples and Comparative Examples as the working electrode (oxygen reduction electrode), a Ni coil as the counter electrode, and a mercury oxide electrode as the reference electrode, and sodium chloride electrolysis was carried out under the following conditions: -Conditions for sodium chloride electrolysis- Electrolysis temperature: 88°C Electrolyte: 32 mass% aqueous sodium hydroxide solution Current density: 8 kA / m ２ Electrolysis time: 3 hours. During electrolysis, pure oxygen gas was supplied from the back of the oxygen reduction electrode. After electrolysis, the amount of platinum in the electrolyte was measured using an ICP optical emission spectrometer (device name: Optima 5300DV, manufactured by PerkinElmer). The measurement wavelength was 265.945 nm.

[0051] The platinum elution rate (percentage) was calculated by dividing the amount of platinum in the electrolyte after electrolysis, measured with the ICP atomic emission spectrometer, by the amount of platinum in the oxygen reduction electrode before electrolysis (hereinafter also referred to as "initial platinum amount").

[0052] Example 1 An oxygen reduction electrode for sodium chloride electrolysis was prepared according to the following procedure. <Preparation of Gas Diffusion Electrode> A suspension for a gas diffusion layer was prepared by mixing 50 mg of hydrophobic carbon black (product name "Li-100", manufactured by Denka Co., Ltd.), 37 μL of a 60 mass % PTFE suspension (product name "31-JR", manufactured by Mitsui-Chemours Fluoroproducts Co., Ltd.), 288 μL of a surfactant (product name "Triton X-100", manufactured by Union Carbide Corporation) and 1.44 mL of pure water in a planetary centrifugal mixer (product name "AR-100", manufactured by Thinky Corporation). The resulting suspension for a gas diffusion layer was applied to one surface of a porous conductive substrate (water-repellent treated carbon cloth, 30 mm x 30 mm, manufactured by ElectroChem, Inc.) and then dried in the air at 100°C for 20 minutes to obtain an electrode precursor. The resulting electrode precursor was baked in a nitrogen atmosphere at 305° C. for 1 hour to obtain a gas diffusion electrode having a gas diffusion layer on its surface.

[0053] <Preparation of platinum catalyst ink for reaction layer> A platinum catalyst ink having a crystallite diameter of 6.2 nm and a BET specific surface area of ​​92 m was prepared so that the mass ratio of carbon to PTFE was 1.5:1. ２ To 120 mg of platinum-loaded carbon (trade name "TEC10EA50E-HT", manufactured by Tanaka Kikinzoku Kogyo K.K.) having a platinum loading of 50 mass % and a molecular weight of 1000 kJ / g, 44 μL of a 60 mass % PTFE suspension, 100 μL of a surfactant (trade name "Triton X-100", manufactured by Union Carbide Corporation), and 500 μL of pure water were added, and the mixture was mixed using a planetary centrifugal mixer to prepare a platinum catalyst ink for the reaction layer.

[0054] <Preparation of oxygen reduction electrode for sodium chloride electrolysis> The obtained platinum catalyst ink for the reaction layer was applied to the surface of the gas diffusion layer of the gas diffusion electrode so that the platinum loading was 1.0 mg / cm ２ The electrode was dried in an air atmosphere at 100°C for 20 minutes, and then baked in a nitrogen atmosphere at 305°C for 15 minutes. This resulted in an oxygen reduction electrode for sodium chloride electrolysis, in which the porous conductive substrate, the gas diffusion layer, and the reaction layer were laminated in this order. Before using this oxygen reduction electrode for sodium chloride electrolysis, i.e., before electrolysis, a 1.3 cm ２ Cut into a circle of the size of 1.13 cm ２The electrode surface area where the electrolysis reaction occurs is 1.13 cm ２ The initial platinum amount in this example was 1.13 mg (=1.0 mg / cm ２ ×1.13cm ２ ) was.

[0055] [Example 2] Platinum-supported carbon having a platinum crystallite diameter of 8.6 nm and a BET specific surface area of ​​45 m ２ A platinum catalyst ink for the reaction layer was prepared under the same conditions as in Example 1, except that platinum-loaded carbon (manufactured by Tanaka Kikinzoku Kogyo K.K.) with a platinum loading of 50 mass % was used, and an oxygen reduction electrode for sodium chloride electrolysis was obtained. The platinum loading of the oxygen reduction electrode for sodium chloride electrolysis in this example was 1.0 mg / cm. ２ , and the initial platinum amount was 1.13 mg (= 1.0 mg / cm ２ ×1.13cm ２ ) was.

[0056] [Example 3] Platinum-supported carbon having a platinum crystallite diameter of 9.2 nm and a BET specific surface area of ​​48 m ２ A platinum catalyst ink for the reaction layer was prepared under the same conditions as in Example 1, except that platinum-loaded carbon (manufactured by Tanaka Kikinzoku Kogyo K.K.) with a platinum loading of 51 mass % was used. The platinum loading of the oxygen reduction electrode for sodium chloride electrolysis in this example was 1.3 mg / cm. ２ , and the initial platinum amount was 1.47 mg (= 1.3 mg / cm ２ ×1.13cm ２ ) was.

[0057] [Comparative Example 1] Platinum-supported carbon having a platinum crystallite diameter of 2.6 nm and a BET specific surface area of ​​327 m ２ A platinum catalyst ink for the reaction layer was prepared under the same conditions as in Example 1, except that platinum-loaded carbon (manufactured by Tanaka Kikinzoku Kogyo K.K.) with a platinum loading of 46 mass % was used. The platinum loading of the oxygen reduction electrode for sodium chloride electrolysis in this comparative example was 0.9 mg / cm. ２ , and the initial platinum amount was 1.02 mg (= 0.9 mg / cm ２ ×1.13cm ２ ) was.

[0058] [Comparative Example 2] Platinum-supported carbon having a platinum crystallite diameter of 2.2 nm and a BET specific surface area of ​​95 m ２ A platinum catalyst ink for the reaction layer was prepared under the same conditions as in Example 1, except that platinum-loaded carbon (manufactured by Tanaka Kikinzoku Kogyo K.K.) with a platinum loading of 46 mass % was used. The platinum loading of the oxygen reduction electrode for sodium chloride electrolysis in this comparative example was 0.9 mg / cm. ２ , and the initial platinum amount was 1.02 mg (= 0.9 mg / cm ２ ×1.13cm ２ ) was.

[0059] [Comparative Example 3] Platinum-supported carbon having a platinum crystallite diameter of 4.5 nm and a BET specific surface area of ​​367 m ２ A platinum catalyst ink for the reaction layer was prepared under the same conditions as in Example 1, except that platinum-loaded carbon (manufactured by Tanaka Kikinzoku Kogyo K.K.) with a platinum loading of 51 mass % was used, and an oxygen reduction electrode for sodium chloride electrolysis was obtained. The platinum loading of the oxygen reduction electrode for sodium chloride electrolysis in this comparative example was 0.9 mg / cm. ２ , and the initial platinum amount was 1.02 mg (= 0.9 mg / cm ２ ×1.13cm ２ ) was.

[0060] The results of the examples and comparative examples are shown in the table below.

[0061]

[0062] As shown in Table 1, the platinum crystallite diameter is 3 nm or more and the BET specific surface area is 30 m ２ / g or more 300m ２ It was confirmed that the oxygen reduction electrode for sodium chloride electrolysis of the example containing platinum-supported carbon of which the platinum elution rate was 1 / g or less had a significantly reduced platinum elution rate compared to the oxygen reduction electrode for sodium chloride electrolysis of the comparative example.

[0063] The oxygen reduction electrode of the present disclosure can be used, for example, as an oxygen reduction electrode for sodium chloride electrolysis. Use of the oxygen reduction electrode of the present disclosure makes it possible to provide an industrially excellent sodium chloride electrolysis process that requires less power.

Claims

1. A conductive substrate and a reaction layer formed on the conductive substrate, the reaction layer containing platinum-supported carbon and a hydrophobic resin, wherein the crystallite diameter of the platinum contained in the platinum-supported carbon is 3 nm or more, and the BET specific surface area of ​​the platinum-supported carbon is 30 m ２ / g or more 300m ２ / g or less.

2. The oxygen reduction electrode according to claim 1, wherein the platinum loading rate of the platinum-loaded carbon is less than 59 mass %.

3. The oxygen reduction electrode according to claim 1 or 2, wherein the conductive substrate is a porous conductive substrate.

4. The oxygen reduction electrode according to claim 3, wherein the porous conductive substrate is carbon fiber.

5. The oxygen reduction electrode according to claim 1 or 2, wherein the platinum contained in said platinum-supported carbon is platinum alone.

6. The oxygen reduction electrode according to claim 1 or 2, wherein the hydrophobic resin is a fluorine-based resin.

7. An oxygen reduction electrode for sodium chloride electrolysis, comprising the oxygen reduction electrode according to claim 1 or 2.

8. A method for producing an oxygen reduction electrode according to claim 1 or 2, comprising a coating step of coating a catalytic ink for a reaction layer onto a conductive substrate, a drying step of drying the conductive substrate carrying the catalytic ink for a reaction layer, and a firing step of firing the dried conductive substrate carrying the precursor of the reaction layer in an inert gas atmosphere.

9. A method for producing an oxygen reduction electrode for sodium chloride electrolysis, comprising the steps of producing the oxygen reduction electrode according to claim 8.

10. A conductive substrate and a reaction layer formed on the conductive substrate, the reaction layer containing platinum-supported carbon and a hydrophobic resin, wherein the crystallite diameter of the platinum contained in the platinum-supported carbon is 3 nm or more, and the BET specific surface area of ​​the platinum-supported carbon is 30 m ２ / g or more 300m ２ / g or less as an oxygen reduction electrode.

11. The use according to claim 10 as an oxygen reduction electrode for salt electrolysis.

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