OXYGEN REDUCTION ELECTRODE FOR BRINE ELECTROLYSIS AND METHOD FOR PRODUCING SAME

Description

RELATED APPLICATIONS

The present application is a National Phase of International Application No. PCT/JP2023/039065, filed Oct. 30, 2023, which claims priority to Japanese Application No. 2022-176495, filed Nov. 2, 2022.

TECHNICAL FIELD

In the brine electrolysis industry, in general, the mainstream is so-called ion-exchange membrane brine electrolysis, in which a chlorine-generating electrode is used as an anode, a hydrogen-generating electrode is used as a cathode, and the anode and the cathode are partitioned by a fluorinated cation-exchange membrane. On the other hand, replacing the cathode with an oxygen reduction electrode can lower the theoretical electrolysis voltage by 1 V or more as compared with the existing hydrogen-generating ion-exchange membrane brine electrolysis and has attracted attention as a means for energy conservation. However, an oxygen reduction reaction actually generates a high overpotential and limits the voltage reduction effect to approximately 0.7 V. Furthermore, combined with the problem that hydrogen reusable as an energy source is not produced as a by-product, brine electrolysis using an oxygen reduction electrode has not been put into practical use on a large scale.

To reduce the overpotential of an oxygen reduction electrode for brine electrolysis, the use of silver as a catalyst has been widely studied.

For example, there is disclosed an oxygen reduction electrode for brine electrolysis containing a hydrophilic catalyst, which contains at least one noble metal selected from silver, platinum and palladium and a carbon powder (Patent Literature 1).

CITATION LIST

Patent Literature

• • PTL 1: Japanese Patent No. 5178959

SUMMARY OF INVENTION

Technical Problem

Recently, there has been a desire for a brine electrolysis process that operates at a lower voltage for carbon-neutral propulsion. In Patent Literature 1, application to the operating current density of brine electrolysis (4.0 to 8.0 kA/m²) generates a high overpotential, which hinders energy conservation of the brine electrolysis process.

The present disclosure has been made in view of the above problems and aims to provide an oxygen reduction electrode for brine electrolysis that exhibits a lower overpotential than a known oxygen reduction electrode for brine electrolysis and produces an effect of contributing to energy conservation in a brine electrolysis process.

Solution to Problem

The present inventors conducted intensive studies to solve the above problems. As a result, it has been found that the above problems can be solved by using an oxygen reduction electrode for brine electrolysis containing platinum and carbon at a specific mass ratio on the electrode surface. That is, the present disclosure is as described in the appended claims, and the gist of the present disclosure is as follows:

• • [1] An oxygen reduction electrode for brine electrolysis, comprising: a porous electroconductive substrate; and a catalyst layer containing platinum and electroconductive carbon on the porous electroconductive substrate, wherein a mass of the platinum is 21% by weight or more when a total mass of the platinum and the electroconductive carbon is 100% by weight, and a mass ratio Pt/C of platinum (Pt) to carbon (C) on a surface of the catalyst layer is 0.18 or more and 1.0 or less.
  • [2] The oxygen reduction electrode for brine electrolysis according to [1], wherein the platinum loading is 0.1 mg/cm² or more and 10 mg/cm² or less per geometric area of the electrode.
  • [3] The oxygen reduction electrode for brine electrolysis according to [1] or [2], wherein the platinum has a crystallite size of 2 nm or more and 20 nm or less.
  • [4] The oxygen reduction electrode for brine electrolysis according to any one of [1] to [3], wherein a hydrogen desorption charge per geometric area of the electrode measured by cyclic voltammetry is 10 mC/cm² or more and 200 mC/cm² or less.
  • [5] The oxygen reduction electrode for brine electrolysis according to any one of [1] to [4], wherein the catalyst layer contains a fluoropolymer, and a mass ratio of the fluoropolymer to the electroconductive carbon is 1:0.1 to 10.
  • [6] The oxygen reduction electrode for brine electrolysis according to any one of [1] to [5], wherein the porous electroconductive substrate is a carbon fiber.
  • [7]A method for producing the oxygen reduction electrode for brine electrolysis according to any one of [1] to [6], comprising: an application step of applying, to the porous electroconductive substrate, a catalyst ink containing at least the platinum and the electroconductive carbon in which the mass of the platinum is 21% by weight or more when the total mass of the platinum and the electroconductive carbon is 100% by weight; a drying step at 60° C. or more and 120° C. or less; and a firing step at 250° C. or more and 400° C. or less in an inert gas atmosphere.

Advantageous Effects of Invention

The present disclosure can provide an oxygen reduction electrode for brine electrolysis that exhibits a lower overpotential than a known oxygen reduction electrode for brine electrolysis and contributes to energy conservation in a brine electrolysis process.

DESCRIPTION OF EMBODIMENTS

The present disclosure is described below with reference to an example of an embodiment thereof. It should be noted that the present disclosure includes any combination of the configurations and numerical values disclosed in the present description and any combination of the upper limits and lower limits of the numerical values disclosed in the present description.

An oxygen reduction electrode for brine electrolysis of the present embodiment (hereinafter also referred to as “the present electrode”) has a porous electroconductive substrate and a catalyst layer containing platinum (a platinum material) and electroconductive carbon on the porous electroconductive substrate. That is, the present electrode is an oxygen reduction electrode that includes a porous electroconductive substrate and a catalyst layer containing platinum (a platinum material) and electroconductive carbon and that has a structure in which the catalyst layer is present on the porous electroconductive substrate, preferably at least on the surface of the porous electroconductive substrate.

The porous electroconductive substrate is used for the purposes of reinforcing the strength of the oxygen reduction electrode, performing current collection and electronic conduction and rapidly supplying oxygen (O₂) gas, which is a reactant in an oxygen reduction reaction, to a catalyst component. Thus, the porous electroconductive substrate is any material with the above function, and a material constituting the porous electroconductive substrate is, for example, at least one of electroconductive carbon and a metallic material, and the porous electroconductive substrate is preferably a substrate composed of at least one of electroconductive carbon and a metallic material and is preferably a substrate composed of electroconductive carbon.

The electroconductive carbon is, for example, one or more selected from the group consisting of a carbon fiber, carbon black, graphite, activated carbon and carbon nanotube. The carbon fiber is, for example, at least one of carbon paper and carbon cloth, and the carbon black is, for example, one or more selected from the group consisting of acetylene black, furnace black and Ketjen black (registered trademark). The metallic material is, for example, at least one of an expanded metal and a mesh, and the expanded metal or the like is preferably one or more selected from the group consisting of nickel, silver, copper, iron, titanium and stainless alloy steel. Among them, the porous electroconductive substrate is preferably a carbon fiber in terms of high corrosion resistance and easy processing. Furthermore, from the perspective of ensuring long-term gas permeability, it is more preferable that a water repellent is applied to the surface (that is, the porous electroconductive substrate is at least one of a carbon fiber subjected to water repellent treatment and a carbon fiber having a water repellent on the surface).

The platinum means a platinum material, which is a main component of the catalyst. More specifically, the platinum (a platinum material) is a main component of a catalyst located in a reaction field where a gas-liquid-solid three-phase interface is formed and an oxygen reduction reaction proceeds. The platinum (a platinum material) is a catalyst with activity for an oxygen reduction reaction and is, for example, one or more selected from the group consisting of a platinum single metal (metallic platinum; Pt as a metal element) and a platinum alloy. The platinum alloy is an alloy composed of platinum (Pt) and a different metal element. The different metal element is, for example, but not limited to, one or more metal elements selected from the group consisting of Ag, Au, Co, Cu, Fe, Ir, Mn, Ni, Pd, Ru, Ti and Zr. Furthermore, in the present embodiment, the meaning of the platinum alloy can be interpreted in a broad sense and may be a platinum alloy with a multilayer structure (core-shell structure) in addition to a platinum alloy composed of a single phase. The platinum (a platinum material) with a multilayer structure is, for example, a platinum material having a core composed of at least one of metal platinum (Pt) and a platinum alloy and a shell composed of one or more metal elements selected from the group consisting of Ag, Au, Co, Cu, Fe, Ir, Mn, Ni, Pd, Ru, Ti and Zr. The platinum (a platinum material) may be a commercial product used as it is or one synthesized by a known method. The synthesis method is, for example, but not limited to, a method (wet process) of adding a reducing agent (an alcohol or the like) to a platinum compound solution (a dinitrodiammine platinum nitric acid solution or the like) to precipitate platinum (Pt) particles or a method (dry process) such as vapor deposition or sputtering.

The electroconductive carbon imparts electroconductivity to the electrode (oxygen reduction electrode) and also plays an important role in increasing the three-phase interface of the electrode. That is, the electroconductive carbon exhibits either hydrophilicity or hydrophobicity depending on the surface functional group (also referred to as “hydrophilic carbon” and “hydrophobic carbon”, respectively). Thus, hydrophilic carbon and hydrophobic carbon can be appropriately combined to optimize the three-phase interface in the electrode. The electroconductive carbon may be any one that is relatively stable in a high-temperature and high-concentration alkali, for example, one or more selected from the group consisting of carbon black, acetylene black, furnace black, Ketjen black (registered trademark), graphite, activated carbon and carbon nanotube, preferably carbon black or furnace black. The electroconductive carbon may be used alone or as a mixture thereof.

The platinum (a platinum material) and the electroconductive carbon may be mixed when the electrode is produced, or the platinum (a platinum material) may be loaded on the electroconductive carbon before the electrode is produced. The electroconductive carbon on which platinum (a platinum material) is loaded (platinum-loaded electroconductive carbon) may be a commercial product used as it is or a product synthesized by a known method. The synthesis method is, for example, but not limited to, a method of adding an electroconductive carbon powder serving as a carrier to a platinum compound solution, mixing them and then adding a reducing agent thereto to precipitate platinum particles directly on the electroconductive carbon.

The catalyst layer is a reaction field where a gas-liquid-solid three-phase interface is formed and an oxygen reduction reaction proceeds, and contains platinum (Pt) as a metal catalyst, electroconductive carbon and the like. For the purpose of forming a three-phase interface, the catalyst layer preferably has a structure in which a hydrophilic material, such as hydrophilic carbon, and a hydrophobic material, such as hydrophobic carbon or a fluoropolymer, are highly dispersed. Although the catalyst layer may have any thickness, an excessively large thickness stagnates the diffusion of gas or ions and increases the conduction path length, and conversely an excessively small thickness decreases the reaction surface area.

In the present electrode, the mass of the platinum (a platinum material) is 21% by weight or more when the total mass of the platinum (a platinum material) and the electroconductive carbon is 100% by weight. When the mass of the platinum (a platinum material) is less than 21% by weight, a sufficient amount of the platinum (Pt) is not introduced into the catalyst layer, and the catalytic reaction on the platinum (Pt) cannot be promoted. The mass of the platinum (a platinum material) is preferably 23% by weight or more, more preferably 25% by weight or more. The mass of the platinum (a platinum material) is preferably 85% by weight or less, more preferably 80% by weight or less. Thus, the mass of the platinum (a platinum material) is 21% by weight or more and 85% by weight or less, preferably 23% by weight or more and 80% by weight or less, more preferably 25% by weight or more and 80% by weight. The mass (% by weight) of the platinum (a platinum material) is determined from the mass (g) of the platinum (a platinum material) and the mass (g) of the electroconductive carbon used in the production of the electrode using the following formula.

Mass ⁢ of ⁢ platinum ⁢ ( platinum ⁢ material ) ⁢ ( % ⁢ by ⁢ weight ) = ( mass ⁢ of ⁢ 
 platinum ⁢ ( platinum ⁢ material ) ⁢ ( g ) / ( mass ⁢ of ⁢ platinum ⁢ ( platinum ⁢ material ) ⁢ 
 ( g ) + mass ⁢ of ⁢ electroconductive ⁢ carbon ⁢ ( g ) ) ) × 100

In the present electrode, the mass ratio Pt/C of platinum (Pt) to carbon (C) (hereinafter also referred to simply as “Pt/C”) of the surface of the catalyst layer is 0.18 or more and 1.0 or less. Pt in Pt/C means platinum as metal platinum. Pt/C is preferably 0.20 or more and 1.0 or less, more preferably 0.22 or more and 1.0 or less. At Pt/C of less than 0.18, when the electrode is used at a high electric current density, the reaction proceeds not only on the platinum as a catalyst but also on the carbon (that is, the electroconductive carbon in the catalyst layer and the carbon in the porous electroconductive substrate), which may increase the overpotential. Furthermore, in the oxygen reduction reaction on the carbon, a two-electron reaction that produces hydrogen peroxide (H₂O₂) preferentially proceeds, rather than a four-electron reaction that produces a hydroxide ion (OH⁻). The progress of the oxygen reduction reaction on the carbon results in degradation of the electrode and a decrease in production efficiency (current efficiency) of sodium hydroxide (NaOH). Pt/C of more than 1.0 may result in a lower degree of dispersion of platinum in the catalyst layer and sintering. The Pt/C of platinum (Pt) to carbon (C) of the catalyst layer surface can be calculated by using an energy dispersive X-ray spectrometer (JSM-IT500, manufactured by JEOL Ltd.) to perform an area analysis on three arbitrary regions in the direction perpendicular to a coated surface of the catalyst layer at a magnification of 100 times, performing quantitative correction by a ZAF method and then averaging the mass ratio Pt/C of the elements platinum (Pt) to carbon (C) obtained in each visual field.

Because a significant effect of decreasing the overpotential can be achieved even at a high electric current density, the amount of platinum (Pt) loaded is preferably 0.1 mg/cm² or more and 10 mg/cm² or less, more preferably 0.2 mg/cm² or more and 10 mg/cm² or less, per geometric area of the electrode. The term “geometric area”, as used herein, corresponds to the projected area of the electrode, and the thickness of the electrode is not taken into consideration.

The platinum (a platinum material) preferably has a crystallite size of 2 nm or more and 20 nm or less, more preferably 2 nm or more and 15 nm or less, because the platinum (a platinum material) used as an electrode can have a high catalyst specific surface area. The crystallite size of platinum (a platinum material) can be calculated by using an X-ray diffractometer (for example, Ultima 4, manufactured by Rigaku Corporation) and applying the half-width of a diffraction line near 2θ=39.5 degrees±1.0 degree, which is a main peak (111) of platinum, in an XRD pattern obtained by measurement using CuKα radiation as a radiation source to the following Scherrer equation. K denotes the Scherrer constant (=0.94), λ denotes the wavelength of the X-ray used (for example, 0.154 nm for CuKα radiation), B denotes the peak half-width (degrees) and θ denotes the incident angle (degrees).

Platinum ⁢ crystallite ⁢ size ⁢ ( nm ) = K × λ / ( B × cos ⁢ θ )

Because the present electrode can achieve a significant effect of decreasing the overpotential even at a high electric current density, the hydrogen desorption charge per geometric area of the electrode measured by cyclic voltammetry is preferably 10 mC/cm² or more and 200 mC/cm² or less, more preferably 20 mC/cm² or more and 200 mC/cm² or less, still more preferably 30 mC/cm² or more and 200 mC/cm² or less. Since a hydrogen desorption reaction based on the cyclic voltammetry proceeds only on the platinum (Pt) surface, the hydrogen desorption charge substantially represents the total surface area of (electrochemically active) platinum (Pt) accessible to the electrolyte solution. The hydrogen desorption charge can be calculated by subtracting the double layer capacitance from the quantity of electricity of an oxidation current wave observed in the range of 50 to 450 mV (vs. RHE) when cyclic voltammetry is performed using a three-electrode cell.

The catalyst layer contains a fluoropolymer for the purpose of optimizing hydrophilicity and hydrophobicity and enhancing electrode strength, and the mass ratio of the fluoropolymer to the electroconductive carbon is preferably 1:0.1 to 10 (that is, the mass ratio [% by weight] of the electroconductive carbon to the fluoropolymer is 10% by weight or more and 1000% by weight or less), more preferably 1:0.1 to 5 (10% by weight or more and 500% by weight or less), still more preferably 1:0.5 to 2 (50% by weight or more and 200% by weight or less). The fluoropolymer may be any fluoropolymer that is relatively stable in a high-temperature and high-concentration alkali, for example, at least one of poly(vinylidene difluoride) (PVDF) and polytetrafluoroethylene (PTFE), or the like. The fluoropolymer can be a powder of a commercial product or a suspension in which a commercial product is dispersed in an aqueous solution. In the case of a fuel cell application, although the catalyst layer may contain an ionomer with proton conductivity (for example, Nafion), the catalyst layer in the present electrode may not contain an ionomer with proton conductivity.

Next, a method for producing the present electrode will be described.

To obtain a homogeneous catalyst layer, the method for producing the present electrode includes an application step of applying, to the porous electroconductive substrate, a catalyst ink containing at least the platinum (a platinum material) and the electroconductive carbon in which the mass of the platinum is 21% by weight or more when the total mass of the platinum and the electroconductive carbon is 100% by weight, a drying step at 60° C. or more and 120° C. or less (that is, a step of drying the porous electroconductive substrate having the catalyst ink at 60° C. or more and 120° C. or less) and a firing step at 250° C. or more and 400° C. or less in an inert gas atmosphere (that is, a step of firing the porous electroconductive substrate having a precursor of the dried catalyst layer at 250° C. or more and 400° C. or less in an inert gas atmosphere).

The catalyst ink is a slurry produced by uniformly dispersing a constituent material of the catalyst layer in a solvent, that is, a composition composed of the constituent material of the catalyst layer and the solvent. The solvent is, for example, but not limited to, one or more selected from the group consisting of water, alcohol and naphtha, which may be used alone or in combination. Furthermore, the catalyst ink may contain a surfactant for the purpose of dispersing the constituent material of the catalyst layer in the solvent. The surfactant is, for example, but not limited to, a nonionic surfactant, such as octylphenol ethoxylate.

The catalyst ink contains at least the platinum (a platinum material) and the electroconductive carbon, and the mass of the platinum (a platinum material) is 21% by weight or more when the total mass of the platinum (a platinum material) and the electroconductive carbon is 100% by weight. When the mass of the platinum (a platinum material) is less than 21% by weight, a sufficient amount of platinum is not introduced into the catalyst layer. The mass of the platinum (a platinum material) is preferably 23% by weight or more, more preferably 25% by weight or more, and preferably 85% by weight, more preferably 80% by weight. Thus, the mass of the platinum (a platinum material) is 21% by weight or more and 85% by weight or less, preferably 23% by weight or more and 80% by weight or less, more preferably 25% by weight or more and 80% by weight.

The application method in the application step may be any method of uniformly applying the catalyst ink on the porous electroconductive substrate and may be at least one of manual coating and machine coating. For example, in the case of manual coating, a method of coating with one or more selected from the group consisting of a brush, a spatula and a roller can be exemplified, and in the case of mechanical coating, a method of coating by one or more selected from the group consisting of screen printing, spraying, die coater coating and blade coating can be exemplified.

The method for producing the present electrode includes a drying step at 60° C. or more and 120° C. or less, that is, a drying step of drying the porous electroconductive substrate having the catalyst ink at 60° C. or more and 120° C. or less. In the drying step, the solvent of the catalyst ink applied in the application step is removed by drying. When the drying temperature is lower than 60° C., it takes a long time to remove the solvent, resulting in a decrease in production efficiency, and when the drying temperature is higher than 120° C., there is a risk of combustion of carbon due to platinum (Pt). The drying temperature is preferably 70° C. or more and 120° C. or less, more preferably 80° C. or more and 120° C. or less. As long as the solvent can be removed, the drying time may be appropriately changed depending on the size of the porous electroconductive substrate, the characteristics of the dryer and the like and may be, for example, 10 minutes or more and 12 hours or less. The atmosphere of the drying step is not particularly limited as long as the removal of the solvent is not inhibited, and is, for example, one or more atmospheres selected from the group consisting of air, oxygen, nitrogen, helium, argon, neon, argon, krypton and xenon, preferably air.

The application step and the drying step may be repeated a plurality of times (for example, 2 times or more and 10 times or less) until the mass of the catalyst layer reaches the target mass, that is, until the amount of platinum (Pt) loaded reaches the above-described amount.

The method for producing the present electrode includes the firing step at 250° C. or more and 400° C. or less in an inert gas atmosphere, that is, a firing step of firing the porous electroconductive substrate having the precursor of the dried catalyst layer at 250° C. or more and 400° C. or less in an inert gas atmosphere. The firing step is performed after the mass of the catalyst layer reaches the target mass in the drying step for the purpose of removing the surfactant remaining in the catalyst layer and for the purpose of once dissolving and binding the fluoropolymer. When the firing temperature is lower than 250° C., the dissolution of the fluoropolymer does not sufficiently proceed, and when the firing temperature is higher than 400° C., the fluoropolymer disappears due to volatilization or thermal decomposition. The firing temperature is preferably 260° C. or more and 390° C. or less, more preferably 270° C. or more and 380° C. or less. The firing time may be appropriately changed depending on the size of the porous electroconductive substrate, the characteristics of the furnace and the like and is, for example, 1 minute or more and 24 hours or less. As long as the combustion of carbon by platinum (Pt) can be avoided, the inert gas is not particularly limited and is, for example, one or more selected from the group consisting of helium, neon, argon, krypton, xenon and nitrogen, which may be used alone or in combination, particularly preferably nitrogen.

The method for producing the present electrode may include a pressing step after the firing step in order to improve the electroconductivity associated with the improvement of the electrode density of the present electrode. The pressing method is, for example, uniaxial pressing or roll pressing, preferably roll pressing from the perspective of suppressing platinum elution due to a decrease in the surface area of the electrode. The pressing pressure may be, for example, but not limited to, 1 kgf/cm² or more and 500 kgf/cm² or less.

EXAMPLES

The present disclosure will be described in more detail below with reference to examples, but the present disclosure should not be construed as being limited to these examples.

<Measurement of Mass Ratio Pt/C of Surface>

A surface of an oxygen reduction electrode for brine electrolysis thus produced was analyzed with an energy dispersive X-ray spectrometer (apparatus name: JSM-IT500, manufactured by JEOL Ltd.). The area analysis was performed at an accelerating voltage of 15.00 kV and at a magnification of 100 times. The scan time was 100 seconds three times. The measurement was performed in three arbitrary regions, and the final value was calculated by averaging the mass ratio Pt/C of platinum (Pt) to carbon (C) obtained in each visual field.

<Measurement of Crystallite Size>

Powder X-ray diffraction of an oxygen reduction electrode for brine electrolysis thus produced was measured with the X-ray diffractometer (apparatus name: Ultima 4, manufactured by Rigaku Corporation). CuKα radiation (λ=1.5405 angstroms) was used as a radiation source, the measurement mode was a step scan, the scan condition was a sampling width 2θ=0.04 degrees, the measurement time was 4 seconds, and the measurement range was 10 degrees to 80 degrees in terms of 2θ. The half-width (full width at half maximum: FWHM) of a diffraction line near 2θ=39.5 degrees±1.0 degree was determined by peak fitting using analysis software PDXL-2 attached to the X-ray diffractometer.

<Measurement of Hydrogen Desorption Charge>

An oxygen reduction electrode for brine electrolysis thus produced was used as a working electrode, a Ni coil was used as a counter electrode, a mercury oxide electrode was used as a reference electrode, and in 32% by weight sodium hydroxide at a liquid temperature of 88° C. cyclic voltammetry was performed for three cycles at a scanning potential of 50 to 1200 mV (vs. RHE) and at three scanning speeds of 30, 50 and 100 mV/s while pure nitrogen gas was supplied at 0.5 L/min from the back side of the working electrode. The quantity of electricity of an oxidation current wave (hydrogen desorption) observed in the range of 50 to 450 mV (vs. RHE) was calculated using the current value caused by the double layer capacitance observed in the range of 450 to 550 mV (vs. RHE) as a baseline in a waveform of the third cycle. The quantity of electricity was calculated using analysis software HZ-5000-ANA. Finally, the quantity of electricity determined at each scanning speed was averaged to calculate the hydrogen desorption charge of the electrode.

<Measurement of Cathode Overpotential>

An oxygen reduction electrode for brine electrolysis thus produced was used as a working electrode, a Ni coil was used as a counter electrode, a mercury oxide electrode was used as a reference electrode, and in 32% by weight sodium hydroxide at a liquid temperature of 88° C. electrolysis was performed at 8 kA/m² while pure oxygen gas was supplied at 0.5 L/min from the back side of the working electrode. The potential of the working electrode measured during electrolysis was divided into an overpotential component and a resistance component by a current interrupter method after the equilibrium potential was subtracted.

Example 1

60 mg of carbon black (trade name: TEC10E50E, manufactured by Tanaka Kikinzoku Kogyo K.K.) loaded with 45% by weight of platinum (a platinum material), 24 μL of 60% PTFE suspension (trade name: 31-JR, manufactured by Chemours-Mitsui Fluoroproducts Co., Ltd.), 50 μL of a surfactant (trade name: Triton (registered trademark) X-100, manufactured by Union Carbide Corporation) and 250 μL of pure water were mixed using a planetary mixer (trade name: AR-100, manufactured by Thinky Corporation) to prepare a catalyst ink. The mass of the platinum (a platinum material) in the catalyst ink was 45% by weight, and the mass ratio of the fluoropolymer to the electroconductive carbon was 1:1.5 (150% by weight as the ratio of the mass of the electroconductive carbon to the mass of the fluoropolymer).

The catalyst ink was applied to a carbon cloth (manufactured by ElectroChem) subjected to water-repellent finishing in advance and was dried in the air at 100° C. for 20 minutes. An electrode thus produced was fired in a nitrogen atmosphere at 305° C. for 15 minutes to produce an oxygen reduction electrode for brine electrolysis.

In the oxygen reduction electrode for brine electrolysis, the mass ratio Pt/C of the surface thereof was 0.68, the platinum loading was 1.14 mg/cm², the platinum (a platinum material) had a crystallite size of 2.7 nm, and the hydrogen desorption charge was 108 mC/cm².

When the overpotential was measured using the oxygen reduction electrode for brine electrolysis, an overpotential of 0.36 V was observed at 8 kA/m² with respect to the equilibrium potential.

Example 2

An oxygen reduction electrode for brine electrolysis was produced in the same manner as in Example 1 except that the mass of the electroconductive carbon loaded with 45% by weight of platinum was 51 mg, the volume of the 60% PTFE suspension was 26 μL, and 7 mg of furnace black (trade name: Valcan XC-72, manufactured by Cabot Corporation) was added during the production of the catalyst ink. At this time, the mass of platinum was 40% by weight, and the mass ratio of the fluoropolymer to the electroconductive carbon was 1:1.5 (150% by weight as the ratio of the mass of the electroconductive carbon to the mass of the fluoropolymer).

In the oxygen reduction electrode for brine electrolysis, the mass ratio Pt/C of the surface thereof was 0.54, the platinum loading was 1.24 mg/cm², the platinum had a crystallite size of 3.5 nm, and the hydrogen desorption charge was 94 mC/cm².

When the overpotential was measured using the oxygen reduction electrode for brine electrolysis, an overpotential of 0.37 V was observed at 8 kA/m² with respect to the equilibrium potential.

Example 3

An oxygen reduction electrode for brine electrolysis was produced in the same manner as in Example 2 except that the mass of the electroconductive carbon loaded with 45% by weight of platinum was 44 mg, the volume of the 60% PTFE suspension was 27 μL, and the mass of the furnace black was 13 mg. At this time, the mass of platinum was 35% by weight, and the mass ratio of the fluoropolymer to the electroconductive carbon was 1:1.5 (150% by weight as the ratio of the mass of the electroconductive carbon to the mass of the fluoropolymer).

In the oxygen reduction electrode for brine electrolysis, the mass ratio Pt/C of the surface thereof was 0.41, the platinum loading was 0.95 mg/cm², the platinum had a crystallite size of 3.9 nm, and the hydrogen desorption charge was 55 mC/cm².

When the overpotential was measured using the oxygen reduction electrode for brine electrolysis, an overpotential of 0.38 V was observed at 8 kA/m² with respect to the equilibrium potential.

Example 4

An oxygen reduction electrode for brine electrolysis was produced in the same manner as in Example 2 except that the mass of the electroconductive carbon loaded with 45% by weight of platinum was 37 mg, the volume of the 60% PTFE suspension was 29 μL, and the mass of the furnace black was 19 mg. At this time, the mass of platinum was 30% by weight, and the mass ratio of the fluoropolymer to the electroconductive carbon was 1:1.5 (150% by weight as the ratio of the mass of the electroconductive carbon to the mass of the fluoropolymer).

In the oxygen reduction electrode for brine electrolysis, the mass ratio Pt/C of the surface thereof was 0.34, the platinum loading was 0.78 mg/cm², the platinum had a crystallite size of 3.6 nm, and the hydrogen desorption charge was 53 mC/cm².

When the overpotential was measured using the oxygen reduction electrode for brine electrolysis, an overpotential of 0.40 V was observed at 8 kA/m² with respect to the equilibrium potential.

Example 5

An oxygen reduction electrode for brine electrolysis was produced in the same manner as in Example 2 except that the mass of the electroconductive carbon loaded with 45% by weight of platinum was 30 mg, the volume of the 60% PTFE suspension was 30 μL, and the mass of the furnace black was 25 mg. At this time, the mass of platinum was 25% by weight, and the mass ratio of the fluoropolymer to the electroconductive carbon was 1:1.5 (150% by weight as the ratio of the mass of the electroconductive carbon to the mass of the fluoropolymer).

In the oxygen reduction electrode for brine electrolysis, the mass ratio Pt/C of the surface thereof was 0.25, the platinum loading was 1.29 mg/cm², the platinum had a crystallite size of 3.2 nm, and the hydrogen desorption charge was 45 mC/cm².

When the overpotential was measured using the oxygen reduction electrode for brine electrolysis, an overpotential of 0.43 V was observed at 8 kA/m² with respect to the equilibrium potential.

Comparative Example 1

An oxygen reduction electrode for brine electrolysis was produced in the same manner as in Example 2 except that the mass of the electroconductive carbon loaded with 45% by weight of platinum was 23 mg, the volume of the 60% PTFE suspension was 31 μL, and the mass of the furnace black was 30 mg. At this time, the mass of platinum was 20% by weight, and the mass ratio of the fluoropolymer to the electroconductive carbon was 1:1.5 (150% by weight as the ratio of the mass of the electroconductive carbon to the mass of the fluoropolymer).

In the oxygen reduction electrode for brine electrolysis, the mass ratio Pt/C of the surface thereof was 0.17, the platinum loading was 0.78 mg/cm², the platinum had a crystallite size of 2.8 nm, and the hydrogen desorption charge was 23 mC/cm².

When the overpotential was measured using the oxygen reduction electrode for brine electrolysis, an overpotential of 0.50 V was observed at 8 kA/m² with respect to the equilibrium potential.

Comparative Example 2

An oxygen reduction electrode for brine electrolysis was produced in the same manner as in Example 2 except that the mass of the electroconductive carbon loaded with 45% by weight of platinum was 17 mg, the volume of the 60% PTFE suspension was 32 μL, and the mass of the furnace black was 35 mg. At this time, the mass of platinum was 15% by weight, and the mass ratio of the fluoropolymer to the electroconductive carbon was 1:1.5 (150% by weight as the ratio of the mass of the electroconductive carbon to the mass of the fluoropolymer).

In the oxygen reduction electrode for brine electrolysis, the mass ratio Pt/C of the surface thereof was 0.12, the platinum loading was 0.62 mg/cm², the platinum had a crystallite size of 2.8 nm, and the hydrogen desorption charge was 16 mC/cm².

When the overpotential was measured using the oxygen reduction electrode for brine electrolysis, an overpotential of 0.54 V was observed at 8 kA/m² with respect to the equilibrium potential.

Comparative Example 3

An oxygen reduction electrode for brine electrolysis was produced in the same manner as in Example 2 except that 20 mg of a silver particle powder (trade name: AgC-153, manufactured by Fukuda Metal Foil & Powder Co., Ltd.) was used instead of the electroconductive carbon loaded with 45% by weight of platinum, the volume of the 60% PTFE suspension was 27 μL, and the mass of the furnace black was 37 mg. The mass of the silver was 35% by weight when the total mass of the silver and the electroconductive carbon was 100% by weight, and the mass ratio of the fluoropolymer to the electroconductive carbon was 1:1.5 (150% by weight as the ratio of the mass of the electroconductive carbon to the mass of the fluoropolymer).

No Pt was detected on the surface of the oxygen reduction electrode for brine electrolysis, the amount of silver loaded was 0.90 mg/cm², the silver had a crystallite size of 37.5 nm, and desorption of hydrogen was not observed.

When the overpotential was measured using the oxygen reduction electrode for brine electrolysis, an overpotential of 0.55 V was observed at 8 kA/m² with respect to the equilibrium potential.

Table 1 shows the electrode physical properties and electrode performances of the oxygen reduction electrodes for brine electrolysis of Examples 1 to 5 and Comparative Examples 1 to 3.

Examples 1 to 5 and Comparative Examples 1 to 3 showed that the oxygen reduction electrodes for brine electrolysis in which the mass ratio Pt/C of the surface was 0.18 or more and 1.0 or less had a lower oxygen reduction overpotential than the oxygen reduction electrodes for brine electrolysis that did not satisfy these conditions.

While the present invention has been described in detail with reference to specific embodiments, it is apparent to a person skilled in the art that various alterations and modifications may be made to the embodiments without departing from the essence and scope of the present invention.

The description, claims and abstract of Japanese Patent Application No. 2022-176495 filed on Nov. 2, 2022 are incorporated herein by reference in its entirety as the disclosure of the description of the present invention.

INDUSTRIAL APPLICABILITY

The present electrode can be used, for example, as a cathode for brine electrolysis and can be used to provide a power-saving and industrially good brine electrolysis process.