MEMBRANE ELECTRODE ASSEMBLY, ELECTROLYSIS CELL, AND ELECTROLYSIS DEVICE

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2024-200129, filed on Nov. 15, 2024; the entire contents of which are incorporated herein by reference.

FIELD

Embodiments described herein relate generally to a membrane electrode assembly, an electrolysis cell, and an electrolysis device.

BACKGROUND

In recent years, it has been desired, from viewpoints of both energy problems and environmental problems, not only to convert renewable energy, such as sunlight, into electric energy and use it but also to store the energy and convert it into a conveyable state. In response to these demands, research and development of power-to-chemical technology, which uses sunlight to produce chemical substances like photosynthesis by plants, is underway. With this technology, it becomes possible to store the renewable energy as a storable fuel, or the like, and it is expected to produce chemical substances becoming industrial raw materials to thereby create value.

As an apparatus that produces the chemical substances using the renewable energy, such as sunlight, a carbon dioxide electrolysis device that reduces carbon dioxide (CO₂) generated from, for example, a power plant or a garbage furnace is known. The CO₂ electrolysis device includes a cathode (reduction electrode), which reduces CO₂ to produce carbon compounds such as carbon monoxide (CO), and an anode (oxidation electrode), which oxidizes water (H₂O) and hydroxide ions (OH⁻). In such a carbon dioxide electrolysis device, it is effective to apply a cell structure (electrolysis cell) in which the cathode and anode are stacked through a diaphragm such as an ion-exchange membrane or porous membrane, and CO₂ can be supplied directly to a cathode catalyst layer of the electrolysis cell to quickly progress the CO₂ reduction reaction.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view illustrating an example of an electrolysis device of an embodiment.

FIG. 2 is a view illustrating another example of the electrolysis device of the embodiment.

DETAILED DESCRIPTION

A membrane electrode assembly of an embodiment includes a cathode that is arranged in contact with a reducible substance to be supplied as a raw material gas and reduces the reducible substance, an anode that is arranged in contact with an electrolytic solution containing an oxidizable substance and oxidizes the oxidizable substance, and a diaphragm separating the cathode and anode. The anode has a porous substrate having a first surface facing on the diaphragm and a second surface opposite the first surface, and an oxidation catalyst in contact with the porous substrate. An average surface roughness of the first surface is 0.1 μm or more and 17.0 μm or less.

An electrolysis cell of an embodiment and an electrolysis device using the electrolysis cell will be explained below with reference to the drawings. In each embodiment, substantially the same components are denoted by the same reference signs, and the explanation thereof will be partially omitted in some cases. The drawings are schematic, in which the relationship between the thickness and planar dimensions, a thickness ratio among the components, and so on may be different from actual ones.

In the following, an example in which the electrolysis cell and electrolysis device of the embodiment are applied to a carbon dioxide electrolysis cell and electrolysis device is mainly described, but the electrolysis cell and electrolysis device of the embodiment are not limited thereto. The electrolysis cell and electrolysis device of the embodiment may be applied to an electrolysis cell and electrolysis device that electrolyze nitrogen (N₂) to produce ammonia (NH₃), for example. It is also applicable to water electrolysis devices and fuel cells.

FIG. 1 is a sectional view illustrating a carbon dioxide (CO₂) electrolysis device 10 of an embodiment. The CO₂ electrolysis device 10 illustrated in FIG. 1 includes an electrolysis cell 20. The electrolysis cell 20 includes a cathode part 30, an anode part 40, and a diaphragm 50 arranged to separate them.

The cathode part 30 includes a cathode (reduction electrode) 31 with a metal catalyst layer, a cathode flow path 32, and a cathode current collector 33. The cathode flow path 32 is a flow path that supplies a reducible substance (a substance to be reduced) to the cathode 31 and is formed by pits (grooves) provided in a cathode flow path plate 34. Examples of the reducible substance include CO₂ gas. The cathode 31 is arranged to be in contact with CO₂ flowing in the cathode flow path 32. The anode part 40 includes an anode (oxidation electrode) 41, an anode flow path 42, and an anode current collector 43. The anode flow path 42 is an electrolytic solution flow path that supplies an electrolytic solution as an anode solution to the anode 41 and is formed by pits (grooves) provided at an anode flow path plate 44. The electrolytic solution contains water (H₂O) or hydroxide ions (OH⁻) as an oxidizable substance (a substance to be oxidized). The anode 41 is arranged to be in contact with the anode solution flowing in the anode flow path 42.

The CO₂ electrolysis device 10 includes a gas supply part 60 that supplies CO₂, the reducible substance, as a raw material gas to the electrolysis cell 20, and an electrolytic solution supply part (supply system) 70 that supplies the anode solution to the electrolysis cell 20. The gas supply part 60 includes a CO₂ storage part 61, such as a CO₂ gas cylinder, and a CO₂ adjustment part 62 that adjusts a flow rate of the CO₂ gas, and supplies the CO₂ gas from the CO₂ storage part 61 to the cathode flow path 32 through a gas pipe 63. The gas supply part 60 may include a humidifying part that humidifies the CO₂ gas. The CO₂ gas supplied to the cathode flow path 32 is not limited to single gas of CO₂ but may be gas that is mainly composed of CO₂ (for example, gas that contains 90 vol % or more CO₂). In the cathode 31 of the electrolysis cell 20 illustrated in FIG. 1, ions are supplied through the diaphragm 50, and the CO₂ gas is supplied from the cathode flow path 32. CO₂ reduction products are mainly discharged from the cathode flow path 32.

The electrolytic solution supply part 70 includes an anode solution tank 71 and a pump 72, and supplies the anode solution from the anode solution tank 71 to the anode flow path 42 through the pump 72 and an electrolytic solution pipe 73. The electrolytic solution supply part 70 may have an anode solution concentration measuring part. The anode solution circulates through the anode flow path 42 and the electrolytic solution pipe 73. The cathode current collector 33 and anode current collector 43 in the CO₂ electrolysis device 10 are connected to a power source 80. It is preferable to use a material with low chemical reactivity and high electrical conductivity for the anode flow path plate 44, which forms the anode flow path 42, and the cathode flow path plate 34, which forms the cathode flow path 32. Such materials include metal materials such as Ti and SUS, carbon, or other materials.

The electrolysis cell 20 is generally sandwiched between a pair of support plates (not illustrated) and further tightened with bolts or the like. The power source 80 connected to the cathode current collector 33 and anode current collector 43 is not limited to ordinary commercial power sources, batteries, or the like, but may also be a power source that converts and supplies renewable energy into electric energy. Examples of such power sources include power sources that convert kinetic energy or positional energy such as wind, hydraulic, geothermal, and tidal power into the electric energy, power sources such as solar cells such as photoelectric conversion elements that convert light energy into the electric energy, power sources such as fuel cells and storage batteries that convert chemical energy into the electric energy, and power sources such as devices that convert vibration energy, such as sounds, into the electric energy. The use of renewable energy sources is environmentally preferable in combination with the effective use of carbon dioxide.

The cathode 31 is an electrode (reduction electrode) for causing a reduction reaction of carbon dioxide (CO₂) as reducible gas to produce carbon compounds such as carbon monoxide (CO), methane (CH₄), ethane (C₂H₆), ethylene (C₂H₄), methanol (CH₃OH), ethanol (C₂H₅OH), and ethylene glycol (C₂H₆O₂). In the cathode 31, a side reaction to generate hydrogen (H₂) through a reduction reaction of water (H₂O) sometimes occurs in addition to the reduction reaction of carbon dioxide (CO₂). The cathode 31 has a first surface in contact with the diaphragm 50 and a second surface facing the cathode flow path 32. The first surface of the cathode 31 is in contact with one surface of the diaphragm 50.

The cathode flow path 32, which is a flow path for gas containing CO₂ (hereinafter referred to as CO₂ gas), is formed by pits (grooves/recesses) provided at the cathode flow path plate 34. The cathode flow path plate 34 is provided with an inlet and outlet of gas, which are not illustrated in the figure. The CO₂ gas is introduced from the gas supply part 60 through the gas inlet or gas outlet. Furthermore, reaction product gas containing CO, H₂, or other gases is discharged through the gas inlet or gas outlet, and the discharged gas is sent to a valuable material production part, not illustrated, or recovered in a product recovery part. The cathode flow path plate 34 and the cathode flow path 32 provided thereon are in contact with the second surface of the cathode 31 opposite the first surface in contact with the diaphragm 50.

The cathode 31 has a porous structure, for example, such as a mesh material, a punched material, a porous substrate, and a metal fiber sintered compact, that enables move of ions and water between the diaphragm 50 and the cathode flow path 32. The cathode 31 preferably includes a porous base material containing carbon fibers, and a gas diffusion layer and a catalyst layer provided in sequence on the porous base material. A catalyst material that forms the catalyst layer of the cathode 31 may have nanoparticles, nanostructures, nanowires, or the like to enhance the reduction reaction. The nanostructure is a structure with nanoscale irregularities on a surface of the catalyst material or other materials. The porous base material is arranged on the cathode flow path 32 side, and the cathode catalyst layer is arranged on the diaphragm 50 side. The cathode catalyst layer may intrude into the gas diffusion layer. Thus, the cathode 31 has a porous structure. At least one of the porous base material, the gas diffusion layer, and the catalyst layer may have water repellency by forming a surface covered with a fluorocarbon resin such as Teflon (registered trademark) as a water-repellent agent. This is because a phenomenon of water clogging (flooding) in a porous material occurs due to move of humidification water or the electrolytic solution, which may degrade characteristics of the electrolysis cell 20.

The cathode catalyst layer preferably has catalyst nanoparticles, catalyst nanostructures, or the like. The porous base material is composed of, for example, carbon paper, carbon cloth, or the like, and is preferably subjected to a water-repellent treatment. The cathode catalyst layer is supplied with ions from the anode 41 through the diaphragm 50. The porous base material is supplied with CO₂ gas from the cathode flow path 32, and products of the CO₂ gas reduction reaction are discharged therefrom. The CO₂ reduction reaction occurs at a three-phase boundary of the cathode catalyst layer, and the gaseous products are discharged from the cathode flow path 32.

The cathode catalyst layer of the cathode 31 is preferably formed by a catalyst material (cathode catalyst material) that is capable of reducing CO₂ to produce carbon compounds and decreasing an overvoltage of such a reaction. Examples of the cathode catalyst material include metals such as gold (Au), silver (Ag), copper (Cu), platinum (Pt), palladium (Pd), nickel (Ni), cobalt (Co), iron (Fe), manganese (Mn), titanium (Ti), cadmium (Cd), zinc (Zn), indium (In), gallium (Ga), lead (Pb), and tin (Sn), metal materials such as alloys and intermetallic compounds containing at least one of the metals, carbon materials such as carbon (C), graphene, CNT (carbon nanotube), fullerene, and ketjen black, and metal complexes such as a Ru complex and a Re complex. Various shapes such as a plate shape, a mesh shape, a wire shape, a particle shape, a porous shape, a thin-film shape, or an island shape can be applied to the cathode catalyst layer.

The anode 41 is an electrode that causes the oxidation reaction of water (H₂O) in the anode solution to produce oxygen (O₂) and hydrogen ions (H⁺), or the oxidation reaction of hydroxide ions (OH⁻) generated in the cathode part 30 to produce oxygen and water. The anode 41 is arranged between the diaphragm 50 and the anode flow path 42, in contact with them. That is, the anode 41 has a first surface facing the diaphragm 50 and a second surface facing the anode flow path 42. The first surface of the anode 41 may be in close contact with the diaphragm 50. A solution inlet and a solution outlet (both not illustrated) are connected to the anode flow path plate 44, and the anode solution is introduced and discharged by the pump 72 through these solution inlet and solution outlet. The anode solution flows through the inside of the anode flow path 42 in a manner to be in contact with the anode 41. The anode current collector 43 is in electrical contact with a surface of the anode flow path plate 44, which forms the anode flow path 42, opposite to the anode 41.

When an aqueous solution of an electrolyte is used as the anode solution, the anode 41 is preferably mainly formed by a catalyst material (anode catalyst material) that is capable of oxidizing water (H₂O) to produce oxygen and hydrogen ions, or oxidizing hydroxide ions (OH⁻) to produce water or oxygen, and decreasing an overvoltage of such a reaction. Examples of the catalyst material include metals such as platinum (Pt), palladium (Pd), and nickel (Ni), alloys and intermetallic compounds containing those metals, binary metal oxides such as manganese oxide (Mn—O), iridium oxide (Ir—O), nickel oxide (Ni—O), cobalt oxide (Co—O), iron oxide (Fe—O), tin oxide (Sn—O), indium oxide (In—O), ruthenium oxide (Ru—O), lithium oxide (Li—O), and lanthanum oxide (La—O), ternary metal oxides such as Ni—Co—O, Ni—Fe—O, La—Co—O, Ni—La—O, and Sr—Fe—O, quaternary metal oxides such as Pb—Ru—Ir—O and La—Sr—Co—O, and metal complexes such as a Ru complex and a Fe complex.

The anode 41 includes a base material (porous base material) having a structure capable of moving the anode solution and ions between the diaphragm 50 and the anode flow path 42, for example, a porous structure such as a mesh material, a punched material, a porous sintered compact as an anode base material. The porous base material can be, for example, an aggregate or sintered compact of particles, or an aggregate or sintered compact of fibers. When the porous base material, as described above, is used for the anode base material, the structure will have large surface irregularities.

The porous substrate, which is the anode base material, has a surface 41a provided on the diaphragm 50 side and a surface 41b provided on the anode flow path 42 side, which is an opposite side of the surface 41a. The porous substrate is preferably an aggregate formed by conductive particles. Here, particles include not only nearly spherical particles (spherical particles or substantially spherical particles), but also rugby ball-shaped, fibrous, needle-shaped, and other shaped particles, and there are no restrictions on shape. In this case, not only can the surface roughness of the anode 41 be decreased, but it is also less likely to cause protrusions in the porous substrate, further decreasing unevenness described below. The conductive particles preferably contain at least one metal element selected from the group consisting of carbon (C), indium (In), tin (Sn), zinc (Zn), aluminum (Al), copper (Cu), titanium (Ti), nickel (Ni), silver (Ag), tungsten (W), cobalt (Co), and gold (Au).

An average diameter of the particles and fibers is preferably 10 nm or more and 200 μm or less, more preferably 50 nm or more and 150 μm or less, and further preferably 100 nm or more and 100 μm or less. When the average diameter is too large, voids inside the anode 41 become larger, and bubbles of oxygen and other produced gases become trapped and difficult to escape, which is undesirable because an electrolysis voltage (cell voltage) increases. When the average diameter is too small, it is undesirable because the porous substrate becomes dense, resulting in poor permeation of the electrolytic solution and an increase in the electrolysis voltage.

The porous material is preferably formed by conductive materials. In addition, it is preferably formed by Ti base materials such as Ti and Ti alloys, which are less reactive, but not limited thereto. Examples of the porous material include carbon materials such as carbon black, activated carbon, fullerene, carbon nanotubes, graphene, ketjen black, and diamond, indium tin oxide (ITO), zinc oxide (ZnO), fluorine-doped tin oxide (FTO), aluminum-doped zinc oxide (AZO), antimony-doped tin oxide (ATO) and other transparent conductive oxides, metals such as copper, aluminum, titanium, nickel, silver, tungsten, cobalt, and gold, and alloys containing at least one of these metals are preferred because of their high conductivity. An element preferably consists of at least one of C, In, Sn, Zn, Al, Cu, Ti, Ni, Ag, W, Co, and Au. The element more preferably consists of Ti, which is less reactive and stable, among the above.

Examples of the oxidation catalyst provided at the porous substrate include ruthenium (Ru), iridium (Ir), platinum (Pt), cobalt (Co), nickel (Ni), iron (Fe), manganese (Mn), lanthanum (La), lithium (Li), indium (In), tantalum (Ta), zirconium (Zr), tin (Sn), titanium (Ti), and other metals and metal oxides, binary metal oxides, ternary metal oxides, quaternary metal oxides, or the like. Examples are manganese oxide (Mn—O), iridium oxide (Ir—O), nickel oxide (Ni—O), cobalt oxide (Co—O), iron oxide (Fe—O), tin oxide (Sn—O), indium oxide (In—O), ruthenium oxide (Ru—O), nickel-iron oxide (Ni—Fe—O), nickel-cobalt oxide (Ni—Co—O), lanthanum-cobalt oxide (La—Co—O), nickel-lanthanum oxide (Ni—La—O), strontium-iron oxide (Sr—Fe—O), lead-ruthenium-iridium oxide (Pb—Ru—Ir—O), lanthanum-strontium-cobalt oxide (La—Sr—Co—O), and the like. Not limited to these materials, the oxidation catalyst may be formed using metal hydroxides containing metals such as cobalt, nickel, iron, and manganese, metal complexes such as a ruthenium complex and an iron complex. The oxidation catalyst may be formed by mixing a plurality of materials. It is preferred that at least one of the elements Ir, Ru, Ni, Fe, Co, Mn, La, Li, In, Sn, and Ti is contained to make a highly active catalyst.

The porous substrate forming the anode 41 and the oxidation catalyst need only be at least partially in contact with each other, and a form thereof is not limited. On the other hand, the oxidation catalyst is more preferably in such a form that it is coated on an entire surface of the anode base material. In this case, it is preferable because the surface of the anode base material can be widely used and adhesion between the oxidation catalyst and the anode base material is improved, resulting in a highly active anode 41 and decreasing the electrolysis voltage. When the oxidation catalyst is pre-coated and adhered to the anode base material, it is preferred because the oxidation catalyst is less likely to peel off from the anode base material and will not fill the pores of the diaphragm 50 when the diaphragm 50 is a porous membrane. The oxidation catalyst may be coated on the surface 41a. The material of the oxidation catalyst preferably has a shape such as nanoparticles, nanostructures, or nanowires to enhance the oxidation reaction. The nanostructure is a structure with nanoscale irregularities on the surface of the catalyst material.

A porosity of the anode 41 is preferably 10% or more and 60% or less. When the porosity is too small, the material becomes a dense porous substrate, which is undesirable because the permeation of the electrolytic solution becomes poor, and the electrolysis voltage increases. When the porosity is too large, conductivity decreases, and the electrolysis voltage increases. It is also undesirable because in such cases, an amount of the electrolytic solution moving across the anode 41 from the anode flow path 42 increases, resulting in that the cathode 31 is excessively covered with the electrolytic solution, and a reaction in which hydrogen is subsidiary produced (side reaction) becomes dominant over the reduction reaction of target raw material gas. The porosity is more preferably 10% or more and 55% or less, and further preferably 10% or more and 50% or less. The porosity is concretely determined using X-ray CT and accompanying software. For example, a volume of the anode 41 is calculated from a thickness and area, and the porosity is calculated from a weight and specific gravity of constituent materials of the anode 41. A maximum peak in a pore size distribution of the anode 41 is preferably 0.1 μm or more and 100 μm or less.

A thickness of the anode 41 is preferably 40 μm or more and 500 μm or less. When the anode 41 is too thin, it is undesirable because mechanical strength of the anode 41 decreases. When the anode 41 is too thick, it is undesirable because bubbles of oxygen and other produced gases are difficult to escape, and the electrolysis voltage increases. The thickness of the anode 41 is more preferably 40 μm or more and 400 μm or less, and further preferably 40 μm or more and 300 μm or less. For example, the thickness can be measured concretely by using a micrometer to measure thicknesses of four corners and a center of the anode 41 to be used and then calculating an average value.

An electrolytic solution, such as an aqueous solution containing any electrolyte, can be used as the anode solution. Examples of the aqueous solution containing the electrolyte include aqueous solutions containing phosphate ions (PO₄²⁻), borate ions (BO₃³⁻), sodium ions (Na⁺), potassium ions (K⁺), calcium ions (Ca²⁺), lithium ions (Li⁺), cesium ions (Cs⁺), magnesium ions (Mg²⁺), chloride ions (Cl⁻), hydrogen carbonate ions (HCO₃⁻), carbonate ions (CO₃²⁻), and so on. The pH of the anode solution is not limited, but considering the environment of use, the pH is preferably 6 or more and 10 or less. Furthermore, the anode solution is more preferably an alkaline solution containing HCO₃⁻, CO₃²⁻, LiHCO₃, NaHCO₃, KHCO₃, and the like.

The diaphragm 50 is provided between the cathode 31 and anode 41 and is stacked with the cathode 31 and anode 41 to form a membrane electrode assembly (MEA). The diaphragm 50 is formed by a material capable of moving ions between the anode 41 and cathode 31 and separating the anode part 40 and cathode part 30, concretely an ion-exchange membrane such as an anion-exchange membrane or cation-exchange membrane, or a porous membrane of organic polymer material. Examples of the ion-exchange membrane used for the diaphragm 50 include, for example, the cation-exchange membranes such as Nafion (registered trademark) and Flemion (registered trademark), and the anion-exchange membranes such as Neosepta (registered trademark), Selemion (registered trademark), Sustenio (registered trademark), and PiperION (registered trademark). The organic polymer materials forming the porous membrane are not limited, but examples include fluorocarbon resins such as Teflon (registered trademark) and polyvinylidene fluoride, hydrocarbon polymers such as polyether, polysulfone, polyethylene, polypropylene, and polyethersulfone, cellulose, and the like.

When the above porous membrane is used as the diaphragm 50, an amount of crossover of produced gas to a counter electrode is higher than that of the ion-exchange membrane. Here, crossover is a phenomenon in which substances such as a reducible substance, an oxidizable substance, a reduction product, and an oxidation product are unnecessarily moved between the anode part 40 and cathode part 30 through the diaphragm 50, and gases react with each other due to this phenomenon.

Electrolysis devices that supply raw materials and conduct electrolysis for a target product, for example, carbon dioxide and nitrogen electrolysis devices, produce hydrogen as a reaction by-product on the cathode 31 side. The produced hydrogen reacts with oxygen produced on the anode 41 side to produce active oxygen species, including hydrogen peroxide. The active oxygen species cause the oxidation reaction of polymers that form the porous membrane. As a result, a composition and shape of the diaphragm 50 change, which may cause degradation of the function of the diaphragm 50 and, in the worst case, may cause a short circuit between electrodes and damage the electrolysis cell.

Hydrogen peroxide is easily formed, for example, by using acidic and neutral electrolytic solutions. When hydrogen peroxide is not easily self-decomposed, this is particularly problematic, for example, in the carbon dioxide and nitrogen electrolysis devices and when the porous membrane is used for the diaphragm 50.

The inventors diligently studied and found that the structure of the anode base material of the anode 41 is an important factor that largely determines a degree of degradation of the diaphragm 50 resulting from the hydrogen production. In this embodiment, roughness of the surface of the anode base material in contact with the diaphragm 50 is controlled to make it a smooth surface, thereby preventing the degradation of the diaphragm 50 in contact with the anode base material.

When the electrolysis cell 20 is assembled using the anode 41, which has large surface irregularities, for example, made of fibrous Ti, and the anode 41 is pressed strongly against the diaphragm 50, which is made of organic material, unevenness in a contact surface between the anode 41 and diaphragm 50 is generated due to the irregularities of the surface 41a of the anode 41. The unevenness includes, for example, in-plane unevenness at a portion that is not exposed to an adhered portion between the anode 41 and diaphragm 50, unevenness in a film-thickness direction caused by thin deformation of the pressed diaphragm 50 at the adhered portion, and unevenness in voltage and current density caused by the unevenness, and the like. Such unevenness promotes partial degradation of the diaphragm 50 by the active oxygen species and causes functional degradation.

The surface roughness of the anode 41 can be specified by an average surface roughness Ra. To prevent the degradation of the diaphragm 50 caused by the aforementioned unevenness, the average surface roughness Ra of the surface 41a is preferably 17.0 μm or less. The average surface roughness Ra of the surface 41a is more preferably 6 μm or less, and further preferably 3 μm or less. In this case, the unevenness is prevented, and the degradation of the diaphragm 50 can be prevented, resulting in the electrolysis cell 20 that can withstand operation for a long period of time. When this average surface roughness Ra is too small, it is undesirable because the porous substrate becomes dense, resulting in poor permeation of the electrolytic solution and an increase in the electrolysis voltage. Therefore, the average surface roughness Ra of the surface 41a is preferably 0.1 μm or more, more preferably 0.2 μm or more, and further preferably 0.3 μm or more. The average surface roughness Ra of the surface 41a is preferably, for example, 0.1 μm or more and 17.0 μm or less. The average surface roughness Ra is concretely calculated using an X-ray CT (computed tomography) device, scanning electron microscope (SEM), microscope, laser microscope, or other devices capable of observing images of cross-sections, as well as image analysis software. For example, a cross-sectional image of the anode 41 is obtained using the X-ray CT device (Zeiss, Xradia 620Versa), and a surface roughness curve is obtained using ImageJ (open source image processing software). The roughness curve is partially extracted, and absolute values of deviations from an average line of the extracted portion to the roughness curve are summed, and the average value is set as Ra. A size of the roughness curve to be extracted is, for example, a range of 350 μm along a direction perpendicular to the thickness direction of the anode 41.

The shape and material of the diaphragm 50 are not limited, but the porous membrane has a larger amount of crossover of the produced gas to the counter electrode than the ion-exchange membrane, so the degradation of the diaphragm 50 resulting from hydrogen is more likely to progress. Therefore, it is easier to obtain the effect of preventing the degradation of the diaphragm 50 by decreasing the surface roughness, and it works more effectively in the case of the porous membrane.

The active oxygen species, such as hydrogen peroxide, which is one of the degradation factors of the diaphragm 50, easily attack an ether group or a sulfonyl group and degrade the diaphragm 50. Therefore, when the diaphragm 50 contains a polymer compound cross-linked with at least one functional group of ether and sulfonyl groups, there is a great advantage in preventing the degradation of the diaphragm 50 by lowering the average surface roughness of the anode 41.

In an electrolysis device that supplies and reduces raw materials containing substances to be reduced, such as a carbon dioxide electrolysis device and a nitrogen electrolysis device, the electrolysis voltage tends to increase, and in such cases, the active oxygen species such as hydrogen peroxide are produced in large quantities and the degradation of the diaphragm 50 is likely to progress. Therefore, in these electrolysis devices, it is very effective to use the anode 41 with a smooth surface to prevent the reaction acceleration of the active oxygen species, which is a degradation factor.

A thickness of the diaphragm 50 is preferably 3 or more and 1000 or less times the average surface roughness Ra of the surface 41a, more preferably 4 or more and 900 or less times, and further preferably 5 or more and 800 or less times. When the thickness is less than 3 times, a distance between the cathode 31 and anode 41 at some place will be short when the surface 41a of the anode 41 intrudes into the diaphragm 50 when the electrolysis cell is assembled. In this case, the crossover of hydrogen produced at the cathode 31 to the anode 41, which is the cause of the degradation of the diaphragm 50, tends to progress, which is undesirable. When the thickness is thicker than 1000 times, it is also undesirable because a solution resistance of the electrolysis cell increases, and the voltage of the electrolysis cell 20 increases.

To prevent the above unevenness, it is effective to provide a layer (also called irregularity mitigation layer or intermediate layer) 51 on the surface of the diaphragm 50 as illustrated in FIG. 2, in addition to the configuration illustrated in FIG. 1. The layer 51 can mitigate the irregularities of the surface 41a, for example, by being provided at the surface on the surface 41a side of the diaphragm 50. A thickness of the layer 51 is preferably ⅓ or more of the average surface roughness Ra of the surface 41a and more preferably ½ or more so that the irregularities of the surface 41a of the anode 41 can be mitigated. The thickness of layer 51 can be checked by a cross-sectional SEM image.

A maximum peak pore size of the layer 51 is preferably 10 nm or more and 50 μm or less, and more preferably 20 nm or more and 10 μm or less. When the pore size is too large, the electrolytic solution present in the anode 41 enters the layer 51 more than necessary and crosses over to the cathode 31 through the diaphragm 50, causing flooding. When the pore size is too small, too little electrolytic solution permeates into the diaphragm 50, causing the diaphragm 50 to dry out or, when using the porous membrane, a solution-film formation to be insufficient, resulting in an increase in the electrolysis voltage.

When the porous membrane is used as the diaphragm 50, the average pore size of the diaphragm 50 is preferably concretely 10 nm or more and 0.3 μm or less, and more preferably 20 nm or more and 0.2 μm or less. When the pore size is too small, there is no balance between the supply of the electrolytic solution from the anode 41 and the consumption of water in the electrolytic solution in the diaphragm 50, resulting in drying of the diaphragm 50, causing the crossover when the porous membrane is used, and the increase in the electrolysis voltage when the ion-exchange membrane is used. A relationship among the average pore sizes of the anode 41, diaphragm 50, and layer 51 is preferably such that the average pore size of the layer 51 is smaller than that of the anode 41 and the average pore size of the diaphragm 50 is smaller than that of the layer 51. For example, the smallest average pore size of the diaphragm 50 allows the electrolytic solution to be supplied smoothly and without excessive supply from the anode 41 to the layer 51 and the diaphragm 50. The average pore size of the anode 41 can be measured, for example, by a mercury intrusion method.

A thickness of the layer 51 is preferably 90 μm or less. When the thickness exceeds 90 μm, durability is maintained, but the cell characteristics deteriorate due to higher electrolysis voltage. A lower limit of the thickness of the layer 51 is not limited, but is, for example, 1 μm or more. The thickness of the layer 51 can be checked by the cross-sectional SEM image.

An inorganic material with high environmental resistance is preferred as a main constituent of the layer 51. The layer 51 is porous because the electrolytic solution and gas must permeate it for the reaction. The inorganic material can be selected as a particulate or fibrous aggregate, a porous membrane of the inorganic material, or the like to form the layer 51. Examples of the inorganic material include oxides, nitrides, carbon, and the like, which are stable to oxidation and alkali states that fluctuate during electrolysis and are highly resistant to hydrogen peroxide and radicals that may be produced as by-products. Chemical species can be common oxides such as aluminum oxide, zirconium oxide, and silicon oxide, for example, or cerium oxide, titanium oxide, and manganese oxide.

The inorganic material may be the chemical species (also called quencher or inhibitor) that decompose, capture (retain), or inactivate the active oxygen species produced by the electrolysis device 10, or may contain the chemical species. The chemical species can prevent the degradation of the diaphragm 50. The active oxygen species are highly reactive oxygen species such as hydrogen peroxide, superoxide anion radicals, hydroxyl radicals, and singlet oxygen.

The chemical species may be cerium (Ce), manganese (Mn), cobalt (Co), platinum (Pt), ruthenium (Ru), and tungsten (W), and metals, metal oxides, or metal hydroxides containing at least one element of tin (Sn), iridium (Ir), cesium (Cs), silver (Ag), iron (Fe), aluminum (Al), and titanium (Ti). Preferred examples of metals are Ce, Mn, Co, Pt, Ru, W, or Sn. Particularly preferred examples are Ce, Mn, or Co. Examples of metal oxides include MnO, Mn₃O₄, MnO₂, MnO₃, Mn₂O₇, CoO, Co₂O₃, Co₃O₄, and CeO₂. Examples of metal hydroxides include Mn(OH)₂, MnO(OH), MnO(OH)₂, and Ce(OH)₄.

The decomposition of the active oxygen species is achieved, for example, by converting the active oxygen species to water and oxygen through radical reactions.

The capture of the active oxygen species is achieved, for example, by stable radical compounds or the like, which react with the generated active oxygen species and convert them into inert reactants.

The inactivation of the active oxygen species is achieved, for example, by preventing radicals generated from the active oxygen species.

The layer 51 preferably has the average pore size smaller than the irregularities and average pore size of the surface 41a of the anode 41 to mitigate the irregularities of the surface of the anode 41 and to allow good diffusion of the electrolytic solution and gas. For example, the average pore size of the layer 51 is preferably smaller than that of the anode 41. When the average pore size of the layer 51 is smaller than that of the anode 41, the electrolytic solution in the anode 41 does not permeate into the diaphragm 50 more than necessary, flooding due to water moving to the cathode 31 is prevented, and the formation of the three-phase boundary at the cathode catalyst is appropriate, resulting in high reaction selectivity and durability. The average pore size of the layer 51 can be measured, for example, by the mercury intrusion method or gas adsorption method. It is preferable to measure a pore size distribution by the mercury intrusion method, which is less affected by surface conditions. When it is difficult to separate the layer 51 from the diaphragm 50, the average pore size of the layer 51 can be checked from a difference between the average pore size of the entire diaphragm 50 with the layer 51 and the average pore size of the diaphragm 50 only.

The inorganic particles being the constituent are preferably fine, and an average particle diameter is preferably 5 nm or more and 5 μm or less, and further preferably 7 nm or more and 1 μm or less to form the small pore size. Here, particles include not only nearly spherical particles (spherical particles or substantially spherical particles), but also rugby ball-shaped, fibrous, needle-shaped, and other shaped particles, and there are no restrictions on shape. Smaller average particle diameter also increases a specific surface area of the inorganic material, which is effective in decomposing, capturing, or inactivating the generated active oxygen species. Particulate inorganic material can also be fibrous, mixed with inorganic material of different shapes such as fibrous, mixed with a plurality of particles of different diameters or of different compositions, or mixed with organic materials such as binders to bind the inorganic materials together. When mixing organic materials such as binders, a mixing ratio of less than 70 weight percent in the layer 51 is preferred. In other words, the percentage of the inorganic material in the layer 51 is preferably, for example, 30 weight percent or more. When a mixture of organic or other materials that are not the inorganic material is contained in 70 weight percent or more, the function of promoting the decomposition, capture, or inactivation of the active oxygen species may not be sufficient. In this case, a surface of the inorganic material is preferably exposed as much as possible. When particles that are not fibrous or spherical are used for the inorganic material, the above average particle diameter value indicates the fiber diameter or the smaller diameter.

The diaphragm 50 may be formed using the ion-exchange membrane or the porous membrane, but the porous membrane is more effective in installing the layer 51. When the porous membrane is used, the gases produced at both electrodes cross over more easily than the ion-exchange membrane, which is thought to facilitate the production of the active oxygen species, including hydrogen peroxide and radicals. Since the production of the active oxygen species accelerates the degradation of the diaphragm 50, it is effective to provide the layer 51 on the surface of the diaphragm 50 not only to physically protect the diaphragm 50 but also to protect the diaphragm 50 from the active oxygen species by using the inorganic material that can decompose, capture, or inactivate the active oxygen species. In this case, a material supporting a metal such as Pt or Co, or oxide particles that can decompose, capture, or inactivate the active oxygen species may be used as the inorganic material. When using the porous membrane, the inorganic material may permeate into the pores of the diaphragm 50, but the inorganic material can permeate into the pores as long as the pores are not filled and through holes are not lost.

The material used as the diaphragm 50 can be either the ion-exchange membrane or the porous membrane, but the porous membrane has through holes. The through holes in the porous membrane are filled with liquid, such as the anode solution, when the electrolysis cell 20 is in use, and the filled anode solution or the like allows the move of ions. In other words, the porous membrane with through holes functions as the diaphragm 50 capable of moving ions between the anode 41 and cathode 31. Furthermore, the anode 41 and cathode 31 are wet-sealed by filling the inside of the through holes of the porous membrane with the anode solution or the like during the use of the electrolysis cell 20. That is, the gas and liquid flow between the anode part 40 and cathode part 30, and the reaction between gases (crossover) caused by such flow can be prevented.

The porous membrane made of organic polymeric materials for the diaphragm 50, as described above, can be manufactured, for example, as follows. There are various methods of manufacturing the porous organic polymer membrane, including phase separation, melt-quenching, extraction, chemical treatment, stretching, irradiation etching, melting, foaming, composite, fiber hollowing, and other methods, and the manufacturing method is not limited. Among them, a nonsolvent-induced phase separation method (NIPS), in which a uniform film-forming solution of organic polymer dissolved in a solvent is brought into contact with a coagulation liquid containing a nonsolvent to form a concentration gradient between the solvent in the film-forming solution and the nonsolvent in the coagulation liquid, which is a driving force that causes the nonsolvent to replace the solvent in the film-forming solution to proceed a phase separation phenomenon, a manufacturing method utilizing a thermally-induced phase separation phenomenon in which phase separation is induced by cooling a polymer solution dissolved at high temperature to make it porous, or a method in which for such a material likely to become fibers as a fluorocarbon resin, fine pores are provided in the membrane by applying shear force, or other methods can be selected. The above methods can also be combined to obtain a predetermined porous structure. The inorganic material can be compounded with the above membrane, or a coating can be applied to the surface to control hydrophilic properties. A plurality of membranes may be superimposed.

The CO₂ gas supplied to the cathode flow path 32 may be supplied in a dry state, but a humidified state is preferable. When the porous membrane of the organic polymer material is applied to the diaphragm 50, the use of the CO₂ gas in a humidified state facilitates the formation of a wet seal on the porous membrane of the organic polymer material. The wet seal prevents the crossover between the anode part 40 and cathode part 30, as described above. The gas supply part 60 may be equipped with a humidifying part that humidifies the CO₂ gas flowing through the gas pipe 63 to humidify the CO₂ gas. The humidifying part (humidifying device) is, for example, equipped with a humidification water tank and is configured to humidify the CO₂ gas by vaporizing humidification water using an ultrasonic oscillator or the like. Further, the CO₂ gas may be humidified by being bubbled in the humidification water tank and passed therethrough.

Next, an operation of the carbon dioxide electrolysis device 10 illustrated in FIG. 1 and FIG. 2 will be described. Here, a case of producing carbon monoxide (CO) as the carbon compound is mainly described. However, the carbon compound as the reduction product of carbon dioxide is not limited to carbon monoxide, and may be methane (CH₄), ethane (C₂H₆), ethylene (C₂H₄), methanol (CH₃OH), ethanol (C₂H₅OH), ethylene glycol (C₂H₆O₂), or the like, and moreover, carbon monoxide that is the reduction product may be further reduced to produce the above-described organic compounds.

First, the reaction process when water (H₂O) is oxidized mainly to produce hydrogen ions (H⁺) is described. When current is supplied from the power source 80 between the anode 41 and cathode 31, an oxidation reaction of water (H₂O) occurs in the anode 41, with which the anode solution is brought into contact. Concretely, as expressed in Formula (1) below, H₂O contained in the anode solution is oxidized, and oxygen (O₂) and hydrogen ions (H⁺) are produced.

H⁺ produced in the anode 41 moves in the electrolytic solution present in the anode 41 and the diaphragm 50 to reach near the cathode 31. The reduction reaction of carbon dioxide (CO₂) occurs by using electrons (e⁻) based on the current supplied from the power source 80 to the cathode 31 and H⁺ moved near the cathode 31. Concretely, as expressed in Formula (2) below, CO² supplied from the cathode flow path 32 to the cathode 31 is reduced to produce CO. Further, as expressed in Formula (3) below, hydrogen ions (H⁺) receive electrons, thereby producing hydrogen. At this time, hydrogen may be produced simultaneously with the production of carbon monoxide.

Next, the reaction process when carbon dioxide (CO₂) is reduced mainly to produce hydroxide ions (OH⁻) is described. When current is supplied from the power source 80 between the anode 41 and the cathode 31, in the vicinity of the cathode 31, as expressed in Formula (4) below, water (H₂O) and carbon dioxide (CO₂) are reduced to produce carbon monoxide (CO) and hydroxide ions (OH⁻). Further, as expressed in Formula (5) below, water receives electrons, thereby producing hydrogen. At this time, hydrogen may be produced simultaneously with the production of carbon monoxide. The hydroxide ions (OH⁻) produced by these reactions diffuse in the vicinity of the anode 41, and as expressed in Formula (6) below, the hydroxide ions (OH⁻) are oxidized to produce oxygen (O₂).

The carbon dioxide electrolysis device 10 of the embodiment as described above is not only specialized in only the reduction of carbon dioxide, but also, for example, can produce carbon monoxide and hydrogen at 1:2, and produce a carbon dioxide reduction product and hydrogen at such an optional ratio as to produce methanol in a chemical reaction thereafter. Hydrogen is a material that is inexpensive and easy to obtain through the electrolysis of water and from fossil fuel, and thus a ratio of hydrogen is not required to be large. A ratio of carbon monoxide to hydrogen is at least 1 or more, and desirably 1.5 or more, from these viewpoints, which is preferable from economical and environmental viewpoints.

As described above, according to the carbon dioxide electrolysis device 10 of the embodiment, the unevenness of the diaphragm 50 formed by pressing the surface irregularities of the anode 41 against the surface of the diaphragm 50 can be mitigated by providing the layer 51 on the surface of the membrane used as the diaphragm 50. This allows, for example, uniform contact of the anode 41 with the diaphragm 50 and prevents the degradation of the diaphragm 50 during operation of the electrolysis cell 20. Thus, the carbon dioxide electrolysis device 10 can be stably operated over a long period of time.

Not only in the carbon dioxide electrolysis device described above, but also in electrolysis devices that perform ammonia synthesis from nitrogen and water electrolysis, fuel cells, and the like, the active oxygen species derived from by-product gases are thought to cause damage to the diaphragm 50. In such cases, the rough surface derived from the anode base material with large irregularities is pressed against the diaphragm 50, which accelerates the degradation of the diaphragm 50. The installation of the layer 51 is particularly effective in the case of carbon dioxide electrolysis and nitrogen electrolysis, because the overvoltage is high at the anode 41 and cathode 31, and hydrogen generation and oxygen generation by side reactions easily produce the active oxygen species such as hydrogen peroxide and radicals.

EXAMPLES

Next, examples and evaluation results thereof will be described.

Example 1, Example 2, Comparative Example 1

The electrolysis devices of Example 1, Example 2, and Comparative Example 1 were fabricated, respectively. To evaluate the degradation caused by the active oxygen species during hydrogen byproduction, gas containing nitrogen, an inert gas, humidified with pure water, was supplied to the cathode flow path, and an electrolytic solution containing potassium hydrogen carbonate with a concentration of 0.1 mol/L was supplied to the anode flow path as the electrolytic solution. Each electrolysis device was operated at a current density of 700 mA/cm² under the following conditions. By controlling the voltage with a power source, a current was made to flow between the cathode and anode at a current density of 700 mA/cm² to produce hydrogen by reducing water at the cathode and oxygen by oxidizing water at the anode. In addition, a fluid discharged from the cathode flow path was collected, and the hydrogen production Faraday efficiency was calculated by analyzing the fluid. The following components of the electrolysis cell were prepared The cathode to be used was prepared by mixing catalytic particles, which are carbon particles supporting Au nanoparticles (metal catalyst) with an average diameter of 2 nm, with Nafion solution (trade name, DuPont) as ion-conductive material (ion-exchange resin) and coating the resultant on carbon paper, which has a diffusion layer with a microporous layer. A porous membrane composed of polyethersulfone with a thickness of 100 μm was used for the diaphragm. The anode and cathode were cut out to 4 cm×4 cm (16 cm² electrode area).

In Example 1, a porous titanium anode coated with iridium oxide as an oxidation catalyst was used as the anode. The average surface roughness Ra of the surface of this anode in contact with the diaphragm, calculated from the X-ray CT image, was 1.2 μm. The porosity was 34%. The electrolysis device illustrated in FIG. 1 was fabricated using this anode. When the electrolysis device of Example 1 was operated under the conditions described above, the initial electrolysis voltage was 2.8 V. The hydrogen Faraday efficiency did not decrease, and no degradation of the diaphragm was observed even after 160 h of continuous operation.

In Example 2, a different porous titanium material from Example 1, coated with iridium oxide as the oxidation catalyst, was used as the anode. The average surface roughness Ra of the surface of this anode in contact with the diaphragm was 10.3 μm. The porosity was 48%. The electrolysis device illustrated in FIG. 1 was fabricated using this anode. When the electrolysis device of Example 2 was operated under the conditions described above, the initial electrolysis voltage was 2.9 V. The hydrogen Faraday efficiency did not decrease, and no degradation of the diaphragm was observed even after 40 h of continuous operation.

In Comparative Example 1, another porous titanium material, coated with the same amount of iridium oxide as in Example 1 as the oxidation catalyst, was used as the anode. The average surface roughness Ra of the surface of this anode in contact with the diaphragm, calculated from the X-ray CT image, was 17.4 μm. The porosity was 56%. The electrolysis device illustrated in FIG. 1 was fabricated using this anode. When the electrolysis device of Comparative Example 1 was operated under the conditions described above, the initial electrolysis voltage was 3.1 V. A decrease in hydrogen Faraday efficiency was observed after 25 h of continuous operation, and a crossover of the produced gas due to the degradation of the diaphragm was observed.

As described above, it became clear that the use of the anode with a small average surface roughness Ra prevented the degradation of the diaphragm and improved the durability, thereby prolonging the life of the electrolysis device.

Note that the configurations of the above-described embodiments are applicable in combination. Further, parts thereof are replaceable. While certain embodiments of the present invention have been described above, these embodiments have been presented by way of example only and are not intended to limit the scope of the invention. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions, and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such embodiments or modifications as would fall within the scope and spirit of the inventions.

The above-described embodiments can be summarized into the following clauses.

(Clause 1)

A membrane electrode assembly including:

• • a cathode that is arranged to be in contact with a reducible substance supplied as a raw material gas and reduces the reducible substance;
  • an anode that is arranged to be in contact with an electrolytic solution containing an oxidizable substance and oxidizes the oxidizable substance; and
  • a diaphragm separating the cathode and the anode, wherein
  • the anode includes a porous substrate having a first surface facing on the diaphragm and a second surface opposite to the first surface, and
  • an oxidation catalyst on the porous substrate, and
  • an average surface roughness of the first surface is 0.1 μm or more and 17.0 μm or less.

(Clause 2)

The membrane electrode assembly according to clause 1, wherein the porous substrate is an aggregate of conductive particles.

(Clause 3)

The membrane electrode assembly according to clause 2, wherein

• • an average diameter of the conductive particles is 10 nm or more and 200 μm or less.

(Clause 4)

The membrane electrode assembly according to clause 1, wherein

• • the conductive particles contain at least one selected from the group consisting of carbon, indium, tin, zinc, aluminum, copper, titanium, nickel, silver, tungsten, cobalt, and gold.

(Clause 5)

The membrane electrode assembly according to any one of clauses 1 to 4, wherein

• • a porosity of the anode is 10% or more and 60% or less.

(Clause 6)

The membrane electrode assembly according to any one of clauses 1 to 5, wherein

• • a thickness of the anode is 40 μm or more and 500 μm or less.

(Clause 7)

The membrane electrode assembly according to any one of clauses 1 to 6, wherein

• • the oxidation catalyst contains at least one selected from the group consisting of iridium, ruthenium, nickel, iron, cobalt, manganese, lanthanum, lithium, indium, tin, and titanium.

(Clause 8)

The membrane electrode assembly according to any one of clauses 1 to 7, wherein

• • the oxidation catalyst is coated on the first surface.

(Clause 9)

The membrane electrode assembly according to any one of claims 1 to 8, wherein

• • the diaphragm is a porous membrane.

(Clause 10)

The membrane electrode assembly according to any one of clauses 1 to 9, wherein a thickness of the diaphragm is 3 times or more and 1000 times or less the average surface roughness of the first surface.

(Clause 11)

The membrane electrode assembly according to any one of clauses 1 to 10, wherein

• • the diaphragm contains a polymer compound cross-linked with at least one functional group selected from the group consisting of an ether group and a sulfonyl group.

(Clause 12)

The membrane electrode assembly according to any one of clauses 1 to 11, wherein

• • the reducible substance is gaseous.

(Clause 13)

The membrane electrode assembly according to clause 12, wherein

• • the reducible substance is carbon dioxide or nitrogen.

(Clause 14)

An electrolysis cell, including

• • the membrane electrode assembly according to any one of clauses 1 to 13;
  • a cathode part that has a cathode flow path supplying the raw material gas to the cathode; and
  • an anode part that has an anode flow path supplying the electrolytic solution to the anode.

(Clause 15)

An electrolysis device, including:

• • the electrolysis cell according to clause 14:
  • a gas supply part that supplies the raw material gas to the cathode flow path; and
  • an electrolytic solution supply part that supplies the electrolytic solution to the anode flow path.