US20220298652A1 - Carbon dioxide electrolysis device and method of operating carbon dioxide electrolysis device - Google Patents
Carbon dioxide electrolysis device and method of operating carbon dioxide electrolysis device Download PDFInfo
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- US20220298652A1 US20220298652A1 US17/406,256 US202117406256A US2022298652A1 US 20220298652 A1 US20220298652 A1 US 20220298652A1 US 202117406256 A US202117406256 A US 202117406256A US 2022298652 A1 US2022298652 A1 US 2022298652A1
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- C—CHEMISTRY; METALLURGY
- C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
- C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
- C25B9/00—Cells or assemblies of cells; Constructional parts of cells; Assemblies of constructional parts, e.g. electrode-diaphragm assemblies; Process-related cell features
- C25B9/60—Constructional parts of cells
-
- C—CHEMISTRY; METALLURGY
- C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
- C25B—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES FOR THE PRODUCTION OF COMPOUNDS OR NON-METALS; APPARATUS THEREFOR
- C25B9/00—Cells or assemblies of cells; Constructional parts of cells; Assemblies of constructional parts, e.g. electrode-diaphragm assemblies; Process-related cell features
- C25B9/60—Constructional parts of cells
- C25B9/65—Means for supplying current; Electrode connections; Electric inter-cell connections
Definitions
- Embodiments relate to a carbon dioxide electrolysis device.
- renewable energy such as solar power is desirably converted into not only electrical energy for use hut also a storable and transportable resource in terms of both energy and environmental issues.
- This demand has advanced research and development of artificial photosynthesis technology, which uses sunlight to produce chemical substances like photosynthesis in plants.
- This technology has potential to store the renewable energy as storable fuel and is also expected to create value by producing chemical substances that can be used as industrial raw materials.
- a device that uses the renewable energy such as solar power to produce chemical substances include an electrochemical reaction device having a cathode and an anode, the cathode being configured to reduce carbon dioxide (CO 2 ) generated from a power plant or waste treatment plant, and the anode being configured to oxidize water (H 2 O).
- the cathode for example, reduce carbon dioxide to produce carbon compounds such as carbon monoxide (CO).
- a cell form also called an electrolysis cell
- it may be effective to fabricate it in a form similar to a fuel cell, such as a polymer electric fuel cell (PEFC).
- PEFC polymer electric fuel cell
- FIG. 1 is a schematic diagram to explain a configuration example of a carbon dioxide electrolysis device.
- FIG. 2 is a planar schematic diagram illustrating a structural example of a part of a flow path plate.
- FIG. 3 is a cross-sectional schematic diagram illustrating a structural example of a part of the flow path plate.
- FIG. 4 is a planar schematic diagram illustrating another structural example of the flow path plate.
- FIG. 5 is a cross-sectional schematic diagram illustrating another structural example of a part of the flow path plate.
- FIG. 6 is a cross-sectional schematic diagram illustrating still another structural example of a part of the flow path plate.
- FIG. 7 is a cross-sectional schematic diagram illustrating yet another structural example of a part of the flow path plate.
- FIG. 8 is a schematic diagram illustrating another configuration example of the carbon dioxide electrolysis device.
- FIG. 9 is a flowchart to explain an operating method example of the carbon dioxide electrolysis device.
- FIG. 10 is a flowchart to explain an operation example of a refresh operation step.
- FIG. 11 is a diagram illustrating a relationship between a partial current density of carbon monoxide and a utilization ratio of carbon dioxide at a cathode.
- FIG. 12 is a diagram illustrating a relationship between the partial current density of carbon monoxide and the utilization ratio of carbon dioxide at the cathode.
- a carbon dioxide electrolysis device includes: a cathode configured to reduce carbon dioxide and thus form a carbon compound; an anode configured to oxidize water and thus generate oxygen; a cathode gas flow path facing on the cathode and configured to supply gas containing carbon dioxide; an anode solution flow path facing on the anode and configured to supply an electrolytic solution containing water; and a separator provided between the anode and the cathode.
- An aspect ratio of the cathode gas flow path is greater than 1 and 3 or less, the aspect ratio being defined by a ratio of a depth of the cathode gas flow path to a width of the cathode gas flow path.
- a fluid mean depth M of the cathode gas flow path and a depth h of the cathode gas flow path satisfy a formula: h/8 ⁇ M ⁇ h/4, the fluid mean depth M being defined by a ratio of a circumferential length of the cathode gas flow path to a cross-sectional area of the cathode gas flow path.
- connection includes not only direct connection but also indirect connection, unless otherwise specified.
- FIG. 1 is a schematic diagram to explain a configuration example of a carbon dioxide electrolysis device.
- FIG. 1 illustrates a carbon dioxide electrolysis device 1 including an electrolysis cell 10 .
- the electrolysis cell 10 includes an anode part 11 , a cathode part 12 , and a separator 13 that separates the anode part 11 from the cathode part 12 .
- the electrolysis cell 10 is, for example, sandwiched between a pair of support plates and further tightened with bolts or the like.
- the anode part 11 includes an anode 111 , an anode solution flow path 112 a provided on a flow path plate 112 , and an anode current collector 113 .
- the cathode part 12 includes a cathode 121 , a cathode gas flow path 122 a provided on a flow path plate 122 , and a cathode current collector 123 .
- the anode 111 is an electrode (oxidation electrode) that promotes an oxidation reaction of water (H 2 O) in an anode solution to produce oxygen (O 2 ) and hydrogen ions (H + ), or an oxidation reaction of hydroxide ions (OH ⁇ ) generated in the cathode part 12 to produce oxygen and water.
- oxidation electrode oxidation electrode
- the anode 111 is disposed between the separator 13 and the flow path plate 112 to be in contact therewith.
- a first surface of the anode 111 is in contact with the separator 13 .
- a second surface of the anode 111 is provided on an opposite side of the first surface of the anode 111 and faces the anode solution flow path 112 a.
- the anode 111 is preferably mainly composed of a catalyst material (anode catalyst material) capable of oxidizing water (H 2 O) to produce oxygen and hydrogen ions, or oxidizing hydroxide ions (OH ⁇ ) to produce water and oxygen, and capable of decreasing an overvoltage of such reactions.
- a catalyst material anode catalyst material capable of oxidizing water (H 2 O) to produce oxygen and hydrogen ions, or oxidizing hydroxide ions (OH ⁇ ) to produce water and oxygen, and capable of decreasing an overvoltage of such reactions.
- Such catalyst materials include metals such as platinum (Pt), palladium (Pd), and nickel (Ni), alloys and intermetallic compounds containing these 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 Ru complexes and Fe complexes.
- binary metal oxides such as manganese oxide (Mn—O), iridium oxide (Ir—O),
- the anode 111 is preferably equipped with a substrate (support) having a structure that enables a movement of the anode solution and ions between the separator 13 and the anode solution flow path 112 a, such as a mesh material, a punching material, or a porous structure such as a porous member.
- the substrate having the porous structure also includes the substrate with relatively large pores, such as a metal fiber sintered compact.
- the substrate may be composed of metals such as titanium (Ti), nickel (Ni), and iron (Fe) or a metal material such as an alloy containing at least one of these metals (for example, SUS), or may be composed of the anode catalyst materials described above.
- a catalyst layer is preferably formed by attaching or laminating the anode catalyst material to a surface of the substrate made of the metal material described above.
- the anode catalyst material preferably has a shape such as nanoparticles, nanostructures, nanowires to enhance the oxidation reaction.
- the nanostructures are structures with nanoscale irregularities formed on a surface of the catalyst material.
- the oxidation catalyst does not necessarily have to be provided on the oxidation electrode.
- An oxidation catalyst layer provided other than the oxidation electrode may be electrically connected to the oxidation electrode.
- the cathode 121 is an electrode (reduction electrode) that generates reduction reactions of carbon dioxide and reduction products to produce carbon compounds.
- the carbon compounds include carbon monoxide (CO), formic acid (HCOOH), methane (CH 4 ), ethane (C 2 H 6 ), ethylene (C 2 H 4 ), methanol (CH 3 OH), acetic acid (CH 3 COOH), ethanol (C 2 H 5 OH)), formaldehyde (HCHO), propanol (C 3 H 7 OH), and ethylene glycol (C 2 H 6 O 2 ).
- the reduction reaction at the cathode 121 may include a side reaction that generates a water reduction reaction to produce hydrogen (H 2 ).
- the cathode 121 is preferably composed of an ion-conductive substance in addition to an electrode substrate and a metal catalyst supported on a carbon material.
- the ion-conductive substance exhibits an action of transferring ions between metal catalysts contained in layers and thus exerts effects of enhancing the electrode activity.
- a cation exchange resin or anion exchange resin is preferably used as the above ion-conductive substance.
- the support for the metal catalyst preferably has the porous structure.
- applicable materials include, for example, carbon blacks such as Ketjen black and Vulcan XC-72, activated carbon, carbon nanotubes, and the like.
- the catalyst layer itself formed on the substrate as well as the support preferably has the porous structure and has a large number of relatively large pores.
- a frequency of pore distribution is preferably maximized in a range of 5 ⁇ m or more and 200 ⁇ m or less in diameter. In this case, gas diffuses quickly throughout the catalyst layer, and reduction products are easily discharged outside the catalyst layer through this pathway, resulting in an efficient electrode.
- a gas diffusion layer is preferably provided on the electrode substrate supporting the catalyst layer to efficiently supply carbon dioxide to the catalyst layer.
- the gas diffusion layer is formed by a conductive porous member.
- the gas diffusion layer is preferably formed by a water-repellent porous member because amounts of water produced by the reduction reaction and water that has moved from an oxidation side can be lowered, allowing the water to drain through a reduction flow path and increasing the percentage of carbon dioxide gas in the porous member.
- a denser diffusion layer (mesoporous layer (MPL)) is more preferably provided between the gas diffusion layer and the catalyst layer to further improve diffusibility, as it changes the water repellency and porosity to promote gas diffusibility and liquid component drainage.
- the metal catalysts supported on the above support include materials that decrease activation energy for the reduction of hydrogen ions and carbon dioxide.
- metal materials that decrease the overvoltage in the reduction reaction of carbon dioxide to produce carbon compounds are included.
- at least one of copper, gold, and silver is preferably used.
- metal complexes such as ruthenium (Ru) complexes or rhenium (Re) complexes
- Ru ruthenium
- Re rhenium
- a plurality of materials may also be mixed.
- Various shapes can be applied to the metal catalyst, such as plate, mesh, wire, particle, porous, thin-film, and island shapes.
- an average diameter is preferably 1 nm or more and 15 nm or less, more preferably 1 nm or more and 10 nm or less, and even more preferably 1 nm or more and 5 nm or less. Meeting these conditions is desirable because a surface area of the metal per catalyst weight becomes larger, and a small amount of metal is required to exhibit high activity.
- the anode 111 and cathode 121 can be connected to a power supply 20 .
- the power supply 20 applies a voltage between the anode 111 and cathode 121 .
- Examples of the power supply 20 are not limited to ordinary system power supplies or batteries but may include power sources that supply power generated by renewable energy sources such as solar cells or wind power.
- the power supply 20 may further include a power controller that adjusts output of the above power supply to control the voltage between the anode 111 and cathode 121 .
- the power supply 20 may be provided outside the carbon dioxide electrolysis device 1 .
- the anode solution flow path 112 a has a function of supplying the anode solution to the anode 111 .
- the anode solution flow path 112 a is formed by pits (grooves/recesses) provided in the flow path plate 112 .
- the flow path plate 112 has an inlet and an outlet port (both not illustrated) connected to the anode solution flow path 112 a, and the anode solution is introduced and discharged by a pump (not illustrated) through these inlet and outlet ports.
- the anode solution is distributed in the anode solution flow path 112 a to be in contact with the anode.
- aqueous solution containing metal ions can be used as the anode solution.
- electrolytic solution containing metal ions
- the aqueous solution can be, for example, aqueous solutions containing phosphate ions (PO 4 2 ⁇ ), borate ions (BO 3 3 ⁇ ), sodium ions (Na + ), potassium ions (K + ), calcium ions (Ca 2+ ), lithium ions (Li + ), cesium ions (Cs + ), magnesium ions (Mg 2+ ), chloride ions (Cl ⁇ ), hydrogen carbonate ions (HCO 3 ⁇ ), carbonate ions (CO 3 2 ⁇ ), and other aqueous solutions.
- Other aqueous solutions containing LiHCO 3 , NaHCO 3 , KHCO 3 , CsHCO 3 , phosphoric acid, boric acid, and so on may also be used.
- the cathode gas flow path 122 a faces a first surface of the cathode 121 .
- the cathode gas flow path 122 a has a function of supplying gas containing carbon dioxide to the cathode 121 .
- the cathode gas flow path 122 a can be connected to a carbon dioxide supply source that supplies the gas containing carbon dioxide.
- the carbon dioxide supply source can be, for example, a power plant, waste treatment plant, or other facilities.
- the cathode gas flow path 122 a is formed by pits (grooves/recesses) provided in the flow path plate 122 .
- the flow path plate 122 has an inlet and an outlet port (both not illustrated) connected to the cathode gas flow path 122 a, and the gas is introduced and discharged by a pump (not illustrated) through these inlet and outlet ports.
- the flow path plates 112 and 122 have low chemical reactivity and high electrical conductivity.
- materials for the flow path plates 112 and 122 include, for example, metal materials such as Ti and SUS, carbon, and the like.
- the flow path plates 112 and 122 have the inlet and outlet ports for each flow path, as well as screw holes for tightening, although these are not illustrated in the drawing. Packing, not illustrated, is sandwiched between a front and back of each flow path plate as necessary.
- the flow path plates 112 and 122 are mainly formed from a single member, they may also be formed from different members and laminated together. Furthermore, hydrophilic and water-repellent functions may be added by applying surface treatment to part or all thereof.
- the flow path plate 122 can have a land in contact with the cathode 121 for electrical connection with the cathode 121 .
- a shape of the cathode gas flow path 122 a can be adjacent to a columnar land, a serpentine shape where a thin flow path is folded or the like, but any shape with a cavity can be used.
- the cathode gas flow path 122 a is preferably constituted by a plurality of flow paths connected in parallel, a serpentine flow path, or a combination thereof because the uniformity of the gas to be supplied to the cathode 121 can be enhanced and the uniformity of an electrolytic reaction can be enhanced.
- FIG. 2 is a planar schematic diagram illustrating a structural example of a part of the flow path plate 122 .
- FIG. 2 illustrates an X-Y plane of the flow path plate 122 including an X-axis and a Y-axis orthogonal to the X-axis. In FIG. 2 , only a superposition of the flow path plate 122 and the cathode 121 is schematically illustrated.
- FIG. 3 is a cross-sectional schematic diagram illustrating a structural example of a part of the flow path plate 122 .
- FIG. 3 illustrates a Y-Z plane of the flow path plate 122 including the Y-axis and a Z-axis orthogonal to the Y-axis and X-axis.
- the Z-axis direction is a thickness direction of the flow path plate 122 .
- the flow path plate 122 has a surface 241 , a surface 242 , and the cathode gas flow path 122 a.
- the surface 241 is in contact with the cathode 121 .
- the surface 242 is provided on an opposite side of the surface 241 and in contact with the cathode current collector 123 .
- the flow path plate 122 illustrated in FIG. 2 and FIG. 3 has a rectangular parallelepiped shape.
- a three-dimensional shape of the flow path plate 122 is not limited to the rectangular parallelepiped shape.
- the cathode gas flow path 122 a faces the gas diffusion layer of the cathode 121 .
- the cathode gas flow path 122 a is connected to the inlet and outlet ports.
- the inlet port is provided for introducing the gas containing carbon dioxide into the cathode gas flow path 122 a.
- the outlet port is provided for discharging the gas containing carbon dioxide from the cathode gas flow path 122 a and for discharging products of the reduction reaction from the cathode gas flow path 122 a.
- the cathode gas flow path 122 a illustrated in FIG. 2 extends in a serpentine shape along the surface 241 .
- the cathode gas flow path 122 a may also extend in a comb-teeth or spiral shape along the surface 241 without being limited to the serpentine shape.
- the cathode gas flow path 122 a includes a space formed by, for example, a groove or opening provided at the flow path plate 122 .
- the carbon dioxide gas may be supplied in a dry state.
- a carbon dioxide concentration in the gas supplied to the cathode gas flow path 122 a does not have to be 100%. In this case, it is also possible to reduce the gas containing carbon dioxide emitted from each of various facilities, although the efficiency will decrease.
- the flow path plates 112 and 122 preferably have the same shape as each other. This can improve uniformity of the reaction.
- the flow path plates 112 and 122 may have different shapes from each other.
- the anode current collector 113 is in contact with a surface of the flow path plate 112 opposite to a contact surface with the anode 111 .
- the anode current collector 113 is electrically connected to the anode 111 .
- the anode current collector 113 preferably contains a material with low chemical reactivity and high electrical conductivity. Such materials include metal materials such as Ti and SUS, carbon, and the like.
- the cathode current collector 123 is in contact with a surface of the flow path plate 122 opposite to a contact surface with the cathode 121 .
- the cathode current collector 123 is electrically connected to the cathode 121 .
- the cathode current collector 123 preferably contains a material with low chemical reactivity and high electrical conductivity. Such materials include metal materials such as Ti and SUS, carbon, and the like.
- the separator 13 is provided between the anode 111 and the cathode 121 .
- the separator 13 is formed of an ion-exchange membrane or the like that can move ions between the anode and the cathode and can separate the anode part from the cathode part.
- ion-exchange membranes such as National and Fremion
- anion-exchange membranes such as Neosepta, Selemion, and Sustenion can be used as the ion-exchange membrane.
- the separator is preferably formed of the anion-exchange membrane.
- the ion-exchange membrane may be formed by using a membrane with a hydrocarbon basic structure or a membrane with an amine group.
- salt bridges, glass filters, porous polymer membranes, porous insulating materials, and so on may be applied to the separator as long as the material is capable of moving ions between the anode and the cathode.
- gas distribution occurs between the cathode and anode parts, circular reactions due to reoxidation of reduction products may occur. Therefore, it is preferable to have less gas exchange between the cathode and anode parts. Therefore, care should be taken when using a porous thin membrane as the separator.
- the carbon dioxide electrolysis device 1 illustrated in FIG. 1 produces carbon monoxide as a carbon compound
- the carbon compound as the reduction product of carbon dioxide is not limited to carbon monoxide.
- the reduction product, carbon monoxide may be further reduced to produce organic compounds as described above.
- the electrolysis cell 10 is preferably used when producing solution carbon compounds.
- a reaction process by the electrolysis cell 10 may be the case mainly producing hydrogen ions (H + ) or the case mainly producing hydroxide ions (OH ⁇ ) but is not limited to either of these reaction processes.
- the reaction process is described mainly for the oxidation of water (H 2 O) to produce hydrogen ions (H + ).
- H 2 O oxidation reaction of water
- H + hydrogen ions
- H + produced at the anode 111 moves through the electrolytic solution present in the anode 111 and the separator 13 to reach near the cathode 121 .
- Electrons (e ⁇ ) in accordance with the current supplied to the cathode 121 from the power supply 20 and H + that moves near the cathode 121 causes the reduction reaction of carbon dioxide (CO 2 ).
- CO 2 carbon dioxide
- carbon dioxide supplied to the cathode 121 from the cathode gas flow path 122 a is reduced to produce carbon monoxide.
- Hydrogen is also produced when hydrogen ions receive electrons, as shown in Equation (3) below. In this case, hydrogen may be produced simultaneously with carbon monoxide.
- the anode solution and ions are supplied from the separator 13 , and carbon dioxide gas is supplied from the cathode gas flow path 122 a.
- the carbon dioxide electrolysis device 1 can not only specialize in carbon dioxide reduction, but can also produce carbon dioxide reduction products and hydrogen, for example, by having carbon monoxide and hydrogen in any ratio, such as 1:2, and then producing methanol in a subsequent chemical reaction.
- the cathode gas flow path 122 a is preferably shallow in terms of supplying carbon dioxide to the gas diffusion layer.
- pressure loss increases in the cathode gas flow path 122 a, which is not desirable in terms of energy loss in the gas supply.
- the shallow flow path will be closed and prevent the carbon dioxide gas from spreading to an entire electrode surface, which may cause failure to affect durability.
- a groove is formed on an inner bottom surface of a flow path to store foreign matters (water droplets) to prevent the flow path from being blocked by the foreign matters (water droplets).
- the precipitated salts solidify near a facing surface between the cathode 121 and the cathode gas flow path 122 a, forming a groove on the inner bottom surface is not effective in preventing the blockage.
- a cross-sectional shape of the cathode gas flow path 122 a is controlled to prevent the blockage of the flow path.
- An aspect ratio of the cathode gas flow path 122 a is preferably greater than 1 and 3 or less.
- the aspect ratio of the cathode gas flow path 122 a is defined by a ratio of a depth h of the cathode gas flow path 122 a to a width W of the cathode gas flow path 122 a in the X-axis or Y-axis direction.
- the flow path may be blocked due to the salt precipitation.
- the aspect ratio exceeds 3, the flow path plate 122 needs to be thickened, which increases material and processing costs.
- the aspect ratio is more preferably 2 or more and 3 or less.
- a fluid mean depth M of the cathode gas flow path 122 a and the depth h of the cathode gas flow path 122 a preferably satisfy a formula (A) below,
- the fluid mean depth M of the cathode gas flow path 122 a is defined by a cross-sectional area Ac of the cathode gas flow path 122 a to a circumferential length S of the cathode gas flow path 122 a.
- the circumferential length S may be calculated by (width W ⁇ 2)+(depth h ⁇ 2). Even if the aspect ratio of the cathode gas flow path 122 a is large, when the fluid mean depth M of the cathode gas flow path 122 a is small, salts may precipitate in the cathode gas flow path 122 a and easily block the cathode gas flow path 122 a.
- the flow path may be blocked due to the salt precipitation.
- the fluid mean depth M is h/4 or more, a utilization ratio of carbon dioxide may decrease.
- the fluid mean depth M is more preferably h/7.9 or more and h/6 or less.
- the depth h, the width W, the circumferential length S, and the fluid mean depth M can be measured by the following method.
- a cross-section of the flow path plate 122 in a perpendicular direction (X-axis direction in FIG. 2 ) to a long direction (Y-axis direction in FIG. 2 ) of the cathode gas flow path 122 a is cut out at an arbitrary position, and the cross-section is observed with a microscope, for example, to measure each parameter.
- neutron radiography can be used to visualize an inside of the flow path plate. It is preferable to calculate these values by averaging values at multiple locations.
- the enough deep of the cathode gas flow path 122 a to meet the above conditions can form a space in a depth direction of the cathode gas flow path 122 a where the gas can be diverted.
- This is desirable in terms of eliminating blockage of gas supply during salt precipitation and makes it easier to supply the carbon dioxide gas to the entire surface of the cathode 121 even if salts precipitate at a part of the cathode gas flow path 122 a, and is therefore less prone to failure and more desirable in terms of durability.
- the decrease in the electrolysis efficiency can be prevented, and the carbon dioxide electrolysis device that is highly efficient and can be operated for a long time can be provided.
- the shape of the cathode gas flow path 122 a is not limited to the shapes illustrated in FIG. 2 and FIG. 3 . Other examples of the shape of the cathode gas flow path 122 a are described below.
- FIG. 4 is a planar schematic diagram illustrating another structural example of the flow path plate 122 .
- FIG. 4 illustrates the X-Y plane of the flow path plate 122 .
- the flow path plate 122 illustrated in FIG. 4 differs from the flow path plate 122 illustrated in FIG. 2 in that the cathode gas flow path 122 a has a plurality of flow path parts 244 connected in parallel in the X-Y plane.
- the other parts are the same as the flow path plate 122 illustrated in FIG. 2 , so the above explanation can be used as appropriate.
- the plurality of flow path parts 244 extend along the long direction (Y-axis direction in FIG. 4 ) of the cathode gas flow path 122 a.
- FIG. 4 illustrates an example of two flow path parts 244 connected in parallel for each fold of the cathode gas flow path 122 a, but the number of flow path parts 244 is not limited to the number illustrated in FIG. 4 .
- the width W of the cathode gas flow path 122 a illustrated in FIG. 4 is defined by a width of one flow path part 244 .
- FIG. 5 is a cross-sectional schematic diagram illustrating still another structural example of a part of the flow path plate 122 .
- FIG. 5 illustrates the Y-Z plane of the flow path plate 122 .
- the flow path plate 122 illustrated in FIG. 5 differs from the flow path plate 122 illustrated in FIG. 3 in that the cathode gas flow path 122 a has regions 122 a 1 and 122 a 2 in an X-Z cross-section.
- the other parts are the same as the flow path plate 122 illustrated in FIG. 3 , so the above explanation can be used as appropriate.
- the region 122 a 1 faces the cathode 121 and has an inner wall surface 246 .
- a cross-sectional shape of the region 122 a 1 illustrated in FIG. 5 is a rectangle, but the shape of the region 122 a 1 is not limited to the shape in FIG. 5 .
- the region 122 a 2 is provided between the region 122 a 1 and an inner bottom surface 245 of the cathode gas flow path 122 a and has an inner wall surface 247 .
- a cross-sectional shape of the region 122 a 2 illustrated in FIG. 5 is a rectangle, but the shape of the region 122 a 2 is not limited to the shape in FIG. 5 .
- a width W 2 in the X-axis direction of the region 122 a 2 is wider than a width W 1 in the X-axis direction of the region 122 a 1 .
- a space for the carbon dioxide gas to be diverted can be increased, and thus the blockage of the cathode gas flow path 122 a due to the salt precipitation can be prevented by making the width W 2 wider than the width W 1 .
- the width W of the cathode gas flow path 122 a illustrated in FIG. 5 is defined by the width W 1 .
- the fluid mean depth M is defined by the circumferential length of the cathode gas flow path 122 a having the shape illustrated in FIG. 5 , taking into account both the width W 1 and the width W 2 .
- FIG. 6 is a cross-sectional schematic diagram illustrating still another structural example of a part of the flow path plate 122 .
- FIG. 6 illustrates the Y-Z plane of the flow path plate 122 .
- the flow path plate 122 illustrated in FIG. 6 has a different shape of the region 122 a 2 in the X-Z cross-section.
- the other parts are the same as the flow path plate 122 illustrated in FIG. 5 , so the above explanation can be used as appropriate.
- a cross-sectional shape of the region 122 a 2 illustrated in FIG. 6 is a square, but the shape of the region 122 a 2 is not limited to the shape in FIG. 6 .
- a cross-sectional area of the region 122 a 2 is larger than that of the region 122 a 1 . This allows for a larger space for the carbon dioxide gas to be diverted and prevents the blockage of the cathode gas flow path 122 a due to the salt precipitation.
- the width W of the cathode gas flow path 122 a illustrated in FIG. 6 is defined by the width W 1 .
- the fluid mean depth M is defined by the circumferential length of the cathode gas flow path 122 a having the shape illustrated in FIG. 6 , taking into account both the width W 1 and the width W 2 .
- FIG. 7 is a cross-sectional schematic diagram illustrating yet another structural example of a part of the flow path plate 122 .
- FIG. 7 illustrates the Y-Z plane of the flow path plate 122 .
- the flow path plate 122 illustrated in FIG. 7 differs from the flow path plate 122 illustrated in FIG. 3 in that the cathode gas flow path 122 a has the regions 122 a 1 and 122 a 2 in the X-Z cross-section, the inner wall surface 246 of the region 122 a 1 is hydrophilic, and the inner wall surface 247 and the inner bottom surface 245 of the region 122 a 2 are water repellent.
- the other parts are the same as the flow path plate 122 illustrated in FIG. 3 , so the above explanation can be used as appropriate.
- a contact angle with water at the hydrophilic inner wall surface 246 is, for example, more than 0 degrees and 90 degrees or less.
- the hydrophilic inner wall surface 246 can be formed using a flow path layer containing, for example, a hydrophilic material.
- the hydrophilic inner wall surface 246 may also be formed by applying a hydrophilic treatment to the flow path layer containing a material applicable to the flow path plate 122 .
- a contact angle with water at the water-repellent inner wall surface 247 is, for example, 100 degrees or more and less than 180 degrees.
- the water-repellent inner wall surface 247 can be formed using a flow path layer containing, for example, a water-repellent material.
- the water-repellent inner wall surface 247 may also be formed by applying a water-repellent treatment to the flow path layer containing the material applicable to the flow path plate 122 .
- a thickness (length in the Z-axis direction) of the region 122 a 1 is not particularly limited, but is preferably half the depth h of the cathode gas flow path 122 a or more, for example.
- the cathode gas flow path 122 a illustrated in FIG. 7 for example, when metal ions in the anode solution flow into the cathode gas flow path 122 a, the anode solution tends to flow into the region 122 a 1 by forming the hydrophilic inner wall surface 246 and the water-repellent inner wall surface 247 . Therefore, the salt precipitation can be prevented in the region 122 a 2 , and the blockage of the cathode gas flow path 122 a due to the salt precipitation can be prevented.
- the structure illustrated in FIG. 7 may be combined with the stricture illustrated in FIG. 5 or FIG. 6 as appropriate.
- FIG. 8 is a schematic diagram illustrating another configuration example of the carbon dioxide electrolysis device.
- the carbon dioxide electrolysis device 1 illustrated in FIG. 8 includes the electrolysis cell 10 , an anode solution supply system 100 that supplies an anode solution to the electrolysis cell 10 , a gas supply system 300 that supplies carbon dioxide (CO 2 ) gas to the electrolysis cell 10 , a product collection system 400 that collects products produced by a reduction reaction in the electrolysis cell 10 , a control system 500 that detects types and amounts of the collected products as well as controls the products and a refresh operation, a waste solution collection system 600 that collects a waste solution of the anode solution, and a refresh material supply 700 that recovers the anode, cathode, and so on of the electrolysis cell 10 .
- Components necessary for the refresh operation do not necessarily have to be provided.
- the electrolysis cell 10 corresponds to the electrolysis cell 10 illustrated in FIG. 1 .
- the explanation of each component of the electrolysis cell 10 in the first embodiment can be used as appropriate.
- the power supply 20 that applies current to the anode 111 and the cathode 121 is provided.
- the power supply 20 is connected to the anode current collector 113 and the cathode current collector 123 through a current introduction member.
- the power supply 20 is not limited to an ordinary system power supply, batteries, and the like, but may also have a power source that supplies power generated by renewable energy sources such as solar cells and wind power.
- the power supply 20 may have the above power source, a power controller that adjusts output of the power source to control a voltage between the anode 111 and the cathode 121 , or the like.
- the anode solution is supplied as an electrolytic solution from the anode solution supply system 100 to the anode solution flow path 112 a of the anode part 11 .
- the anode solution supply system 100 circulates the anode solution so that the anode solution is distributed in the anode solution flow path 112 a.
- the anode solution supply system 100 has a pressure controller 101 , an anode solution tank 102 , a flow rate controller (pump) 103 , a reference electrode 104 , and a pressure gauge 105 , and is configured such that the anode solution circulates in the anode solution flow path 112 a.
- the anode solution tank 102 is connected to a gas component collection unit, not illustrated, which collects oxygen (O 2 ) and other gas components contained in the circulating anode solution.
- the anode solution is introduced into the anode solution flow path 112 a with the flow rate and pressure controlled in the pressure controller 101 and flow rate controller 103 .
- the CO 2 gas is supplied to the cathode gas flow path 122 a from the gas supply system 300 .
- the gas supply system 300 has a CO 2 gas cylinder 301 , a flow rate controller 302 , a pressure gauge 303 , and a pressure controller 304 .
- the CO 2 gas is introduced into the cathode gas flow path 122 a with the flow rate and pressure controlled in the flow rate controller 302 and pressure controller 304 .
- the gas supply system 300 is connected to the product collection system 400 that collects products in the gas distributed through the cathode gas flow path 122 a.
- the product collection system 400 has a gas-liquid separation unit 401 and a product collection unit 402 . Reduction products such as CO and H 2 contained in the gas distributed through the cathode gas flow path 122 a are accumulated in the product collection unit 402 through the gas-liquid separation unit 401 .
- the anode solution circulates in the anode solution flow path 112 a during the electrolytic reaction operation as described above.
- the anode solution is discharged into the waste solution collection system 600 so that the anode 111 and the anode solution flow path 112 a are exposed from the anode solution.
- the waste solution collection system 600 has a waste solution collection tank 601 connected to the anode solution flow path 112 a.
- a waste solution of the anode solution is collected in the waste solution collection tank 601 by opening and closing not-illustrated valves. The opening and closing of the valves and other operations are collectively controlled by the control system 500 .
- the waste solution collection tank 601 also functions as a collection unit for a rinse solution supplied from the refresh material supply 700 . Furthermore, gaseous substances supplied from the refresh material supply 700 and containing some liquid substances are also collected in the waste solution collection tank 601 as necessary.
- the refresh material supply 700 includes a gaseous substance supply system 710 and a rinse solution supply system 720 .
- the rinse solution supply system 720 can be omitted in some cases.
- the gaseous substance supply system 710 has a gas tank 711 that serves as a supply source for gaseous substances such as air, carbon dioxide, oxygen, nitrogen, argon, and a pressure controller 712 that controls a supply pressure of the gaseous substances.
- the rinse solution supply system 720 has a rinse solution tank 721 that serves as a supply source of the rinse solution such as water, and a flow rate controller (pump) 722 that controls a supply flow rate, and the like of the rinse solution.
- a flow rate controller (pump) 722 that controls a supply flow rate, and the like of the rinse solution.
- the gaseous substance supply system 710 and the rinse solution supply system 720 are connected to the anode solution flow path 112 a and the cathode gas flow path 122 a through pipes.
- the gaseous substances and rinse solution are supplied to the anode solution flow path 112 a and the cathode gas flow path 122 a by opening arid closing not-illustrated valves. The opening and closing of the valves, and other operations are collectively controlled by the control system 500 .
- a part of the reduction products accumulated in the product collection unit 402 is sent to a reduction performance detection unit 501 of the control system 500 .
- a reduction performance detection unit 501 In the reduction performance detection unit 501 , a production amount and a proportion of each product such as CO or H 2 in the reduction products, are detected.
- the detected production amount and proportion of each product are input into a data collection and controller 502 of the control system 500 .
- the data collection and controller 502 collects electrical data such as a cell voltage, a cell current, a cathode potential, and an anode potential, and data such as pressure and pressure loss inside the anode solution flow path 112 a and the cathode gas flow path 122 a as part of cell performance of the electrolysis cell 10 , and sends the data to a refresh controller 503 .
- the data collection and controller 502 is electrically connected, through bi-directional signal lines whose illustration is partially omitted, to the power supply 20 , the pressure controller 101 and the flow rate controller 103 of the anode solution supply system 100 , the flow rate controller 302 and the pressure controller 304 of the gas supply system 300 , and the pressure controller 712 and the flow rate controller 722 of the refresh material supply 700 , in addition to the reduction performance detection unit 501 , and these are collectively controlled.
- Each pipe is provided with a not-illustrated valve, and an opening/closing operation of the valve is controlled by a signal from the data collection and controller 502 .
- the data collection and controller 502 may control operations of the above components during the electrolysis operation, for example.
- the refresh controller 503 is electrically connected, through bi-directional signal lines whose illustration is partially omitted, to the power supply 20 , the flow rate controller 103 of the anode solution supply system 100 , the flow rate controller 302 of the gas supply system 300 , and the pressure controller 712 and flow rate controller 722 of the refresh material supply 700 , and these are collectively controlled.
- Each pipe is provided with a not-illustrated valve, and an opening/closing operation of the valve is controlled by a signal from the refresh controller 503 .
- the refresh controller 503 may control operations of the above components during the electrolysis operation, for example.
- the refresh controller 503 and the data collection and controller 502 may be configured by a single controller.
- FIG. 9 is a flowchart to explain an operating method example of the carbon dioxide electrolysis device 1 .
- a startup step S 101 of the carbon dioxide electrolysis device 1 is performed.
- the following operations are performed.
- the anode solution supply system 100 the anode solution is introduced into the anode solution flow path 112 a after its flow rate and pressure are controlled by the pressure controller 101 and the flow rate controller 103 .
- the CO 2 gas is introduced into the cathode gas flow path 122 a after its flow rate and pressure are controlled by the flow rate controller 302 and the pressure controller 304 .
- a CO 2 electrolysis operation step S 102 is performed.
- the CO 2 electrolysis operation step S 102 application of an electrolysis voltage by the power supply 20 of the electrolysis device 1 that has been subjected to the startup step S 101 is started, and a current is supplied by applying a voltage between the anode 111 and the cathode 121 .
- a current is supplied by applying a voltage between the anode 111 and the cathode 121 .
- an oxidation reaction near the anode 111 and a reduction reaction near the cathode 121 occur, which will be described below.
- the explanation of the oxidation and reduction reactions in the first embodiment can be used as appropriate.
- the electrolysis operation may cause salts to precipitate in the cathode gas flow path 122 a, resulting in decreased cell performance.
- ions move between the anode 111 and the cathode 121 through the separator 30 and the ion-exchange membrane, and the ions react with gas components.
- potassium hydroxide solution is used as the anode solution and the carbon dioxide gas is used as the cathode gas
- potassium ions move from the anode 111 to the cathode 121 and react with carbon dioxide to produce salts such as potassium hydrogen carbonate and potassium carbonate.
- the salts When the salts are solubility or less in the cathode gas flow path 122 a, the salts precipitate in the cathode gas flow path 122 a.
- the salt precipitation prevents uniform gas flow throughout the cell and decreases the cell performance.
- the decrease in the cell performance is particularly noticeable when a plurality of cathode gas flow paths 122 a are provided.
- the performance of the cell itself can be improved by partially increasing a gas flow rate. This is because the cell performance is improved by increasing a gas pressure, which increases gas components and the like supplied to a catalyst, or by increasing gas diffusibility.
- a step S 103 that determines whether the cell performance meets request criteria is performed to detect such a decrease in the cell performance.
- the data collection and controller 502 periodically or continuously collects, for example, the production amount and the proportion of each product, the cell performance such as the cell voltage, the cell current, the cathode potential, and the anode potential of the electrolysis cell 10 , the pressure inside the anode solution flow path 112 a, the pressure inside the cathode gas flow path 122 a, and the like.
- the data collection and controller 502 has predetermined request criteria for the cell performance, and it is determined whether the collected data meets the set request criteria. When the collected data meets the set request criteria, the CO 2 electrolysis operation is continued without stopping the CO 2 electrolysis (S 104 ). When the collected data does not meet the set request criteria, a refresh operation step S 105 is performed.
- the cell performance collected by the data collection and controller 502 is defined by parameters such as, for example, an upper limit value of the cell voltage when a constant current is applied to the electrolysis cell 10 , a lower limit value of the cell current when a constant voltage is applied to the electrolysis cell 10 , and Faradaic efficiency of the carbon compounds produced by the reduction reaction of CO 2 .
- the Faradaic efficiency is defined as a proportion of a current that contributed to the production of the desired carbon compound to a total current that flowed in the electrolysis cell 10 .
- the refresh operation step S 105 be performed when the upper limit value of the cell voltage reaches 150% or more, preferably 120% or more of the set value when the constant current is applied.
- the refresh operation step S 105 may be performed when the lower limit value of the cell current when the constant voltage is applied reaches 50% or less, preferably 80% or less, of the set value. To maintain the production amount of the reduction products such as the carbon compounds, it is recommended that the refresh operation step S 105 be performed when the Faradaic efficiency of the carbon compounds reaches 50% or less, preferably 80% or less, of the set value.
- the cell performance is determined as not meeting the request criteria when at least one of the following parameters, for example, the cell voltage, the cell current, the Faradaic efficiency of the carbon compounds, the pressure inside the anode solution flow path 112 a, and the pressure inside the cathode gas flow path 122 a, does not meet the request criteria, and the refresh operation step S 105 is performed.
- Two or more of the above parameters may be combined to set the request criteria for the cell performance.
- the refresh operation step S 105 may be performed when both the cell voltage and the Faradaic efficiency of the carbon compounds do not meet the request criteria.
- the refresh operation step S 105 is performed when at least one of the cell performances does not meet the request criteria.
- the refresh operation step S 105 is preferably performed at intervals of, for example, one hour or more to stably perform the CO 2 electrolysis operation step S 102 .
- the electrolysis cell 10 mainly produces CO
- the leak in the electrolysis cell 10 is not limited to a gas leak between the anode 111 and the cathode 121 , but includes, for example, a gas leak from between the cathode 121 and the cathode gas flow path 122 a. This gas leak is likely to occur, for example, when the electrolysis cell 10 with salt precipitated is operated for a long time under conditions of high pressure in the cathode gas flow path 122 a.
- FIG. 10 is a flowchart to explain an operation example of the refresh operation step S 105 .
- the application of the electrolysis voltage by the power supply 20 is stopped to stop the reduction reaction of CO 2 (S 201 ).
- the application of the electrolysis voltage does not necessarily have to be stopped.
- the supply of the gas to the cathode gas flow path 122 a is stopped, the supply of the anode solution to the anode solution flow path 112 a is stopped, and the anode solution is discharged from the anode solution flow path 112 a (S 202 ).
- the rinse solution is supplied (S 203 ) to the anode solution flow path 112 a and the cathode gas flow path 122 a for cleaning.
- a refresh voltage may be applied between the anode 111 and the cathode 121 while the rinse solution is being supplied. This can remove ions and impurities attached to the cathode catalyst layer.
- the refresh voltage is applied so that the process is mainly an oxidation process, ions, impurities such as organic matters on a catalyst surface are oxidized and removed.
- ions substituted in an ion exchange resin when the ion-exchange membrane is used as the separator 30 can be removed by performing this process in the rinse solution.
- the refresh voltage is preferably ⁇ 2.5 V or more and 2.5 V or less, for example. Since energy is used for the refresh operation, the refresh voltage range is preferably as narrow as possible, and more preferably between ⁇ 1.5 V or more and 1.5 V or less, for example.
- the refresh voltage may be applied cyclically so that the oxidation process and the reduction process of ions and impurities are alternately performed. This can accelerate a regeneration of the ion exchange resin and the catalyst.
- a voltage of a value equivalent to the electrolysis voltage during the electrolysis operation may be applied as the refresh voltage to perform the refresh operation. In this case, a configuration of the power supply 20 can be simplified.
- the gas is supplied to the anode solution flow path 112 a and the cathode gas flow path 122 a (S 204 ) to dry the cathode 121 and the anode 111 .
- Supplying the rinse solution to the anode solution flow path 112 a and the cathode gas flow path 122 a increases a water saturation level in the gas diffusion layer and causes a decrease in output due to gas diffusibility.
- the gas is preferably supplied immediately after the distribution of the rinse solution, or at least within 5 minutes after the end of the rinse solution supply. This is because the decrease in the output due to the increase in the water saturation level is significant. For example, when the refresh operation is performed every hour, the output during a 5-minute refresh operation may be 0 V or significantly less, resulting in a loss of 5/60 of the output.
- the anode solution is introduced into the anode solution flow path 112 a and the CO 2 gas is introduced into the cathode gas flow path 122 a (S 205 ). Then, the application of the electrolysis voltage between the anode 111 and the cathode 121 by the power supply 20 is resumed to restart the CO 2 electrolysis operation (S 206 ). When the application of the electrolysis voltage has not been stopped in S 201 , the restart operation is not performed.
- the gas may be used, or the rinse solution may be used to discharge the anode solution from the anode solution flow path 112 a.
- the supply and flow of the rinse solution (S 203 ) are performed to prevent precipitation of electrolytes contained in the anode solution and to clean the cathode 121 , the anode 111 , the anode solution flow path 112 a, and the cathode gas flow path 122 a. Therefore, water is preferable as the rinse solution, further, water with the electrical conductivity of 1 mS/m or less is more preferable, and water with an electrical conductivity of 0.1 mS/m or less is even more preferable.
- An acid rinse solution such as low-concentration sulfuric acid, nitric acid, hydrochloric acid may be supplied to remove the electrolytes and other precipitates at the cathode 121 and the anode 111 , and the like, and the electrolytes may be thereby dissolved.
- a step of supplying a water rinse solution is performed in a subsequent step.
- the water rinse solution supply step is preferably performed to prevent additives contained in the rinse solution from remaining.
- FIG. 8 illustrates the rinse solution supply system 720 having one rinse solution tank 721 , but when a plurality of rinse solutions are used, such as the water and acid rinse solutions, a plurality of rinse solution tanks 721 are used accordingly.
- An acid or alkaline rinse solution is particularly desirable for refreshing the ion exchange resin. This has the effect of discharging cations and anions that have been substituted for protons and OH ⁇ in the ion exchange resin. For this reason, it is preferable to alternately distribute the acid and alkaline rinse solutions, to combine the rinse solution with water having the electrical conductivity of 1 mS/m or less, and to supply the gas in between the supply of the plurality of rinse solutions to prevent mixing of the rinse solutions.
- the gas used for the gas supply and flow step S 204 preferably contains at least one of air, carbon dioxide, oxygen, nitrogen, and argon. Furthermore, it is preferable to use the gas with low chemical reactivity. In this regard, air, nitrogen, and argon are preferably used. Nitrogen and argon are even more preferable.
- the rinse solution and gas for refreshing is not limited to be supplied to only the anode solution flow path 112 a and the cathode gas flow path 122 a, but also to the cathode gas flow path 122 a to clean a surface of the cathode 121 in contact with the cathode gas flow path 122 a. It is effective to supply the gas to the cathode gas flow path 122 a to dry the cathode 121 from the surface side in contact with the cathode gas flow path 122 a as well.
- the CO 2 electrolysis operation step S 102 should be continued or the refresh operation step S 105 should be performed in accordance with whether the cell performance of the electrolysis cell 10 meets the request criteria.
- Supplying the rinse solution and gas for refreshing in the refresh operation step S 105 prevents uneven distribution of ions and residual gas near the anode 111 and cathode 121 , which are factors in the deterioration of the cell performance, and removes the electrolyte precipitation, and the like in the cathode 121 , the anode 111 , the anode solution flow path 112 a, and the cathode gas flow path 122 a.
- the cell performance of the electrolysis cell 10 can be recovered by restarting the CO 2 electrolysis operation step S 102 . after the refresh operation step S 105 .
- the CO 2 electrolysis performance by the electrolysis device 1 can be maintained for a long time by repeating the CO 2 electrolysis operation step S 102 and the refresh operation step S 105 in accordance with the request criteria of the cell performance.
- the refresh operation of the electrolysis cell is performed by temporarily flowing the rinse solution through the flow path when salts precipitate, which can prevent the blockage of the flow path. Therefore, the decrease in the electrolysis efficiency of the carbon dioxide electrolysis device can be prevented.
- the region 122 a 1 with the hydrophilic inner wall surface 246 and the region 122 a 2 with the water-repellent inner wall surface 247 are formed in the cathode gas flow path 122 a as illustrated in FIG. 7 , which enables the rinse solution to easily bypass the salts and flow through the region 122 a 2 , which is near the salts and therefore the salts are easily dissolved, because the hydrophilic inner wall surface 246 is near the salt even if the salts precipitate.
- the rinse solution flows into the region 122 a 2 until the salts are dissolved and can be supplied to the entire surface of the cathode 121 , thus efficiently removing the salts.
- a carbon dioxide electrolysis device was fabricated as follows. Iridium oxide was formed on a surface of titanium mesh as an oxidation catalyst. Carbon paper with a catalyst layer was fabricated by spraying carbon, which supports 10.2% by mass gold on the carbon paper with MPL. A membrane electrode composite (catalyst area 4 cm square) was prepared by laminating this carbon paper and titanium mesh with iridium oxide sandwiched by ion-exchange membranes.
- a cathode gas flow path and an anode solution flow path were formed of titanium and were each a serpentine-shaped flow path containing two parallel-connected flow path parts, with a land width of 0.8 mm, a flow path width W of 1 mm, and a flow path depth h of 3 mm.
- An aspect ratio was 3, and a fluid mean depth M was 0.38.
- An electrolysis cell was assembled by sandwiching the membrane electrode composite between the anode solution flow path and the cathode gas flow path.
- a 0.1 M potassium hydrogen carbonate solution was supplied to the anode solution flow path at 10 mL/min as an electrolytic solution.
- Carbon dioxide gas was supplied to the cathode gas flow path at a flow rate of 320 ccm.
- a current was passed between the anode and the cathode with a stepwise increase in a current value, and gas generated from a cathode side was collected to measure its flow rate and conversion efficiency of carbon dioxide. The gas generated was sampled and identified and quantified by gas chromatography.
- the current value at this time was measured with an ammeter.
- a partial current density of carbon monoxide (CO) which is an indicator of the percentage used for carbon monoxide production out of a total current density flowed, was found from a conversion efficiency from carbon dioxide to carbon monoxide.
- a utilization ratio of carbon dioxide at the cathode was found from the conversion efficiency from carbon dioxide to carbon monoxide and a flow rate of the gas generated from the cathode side.
- a relationship between the partial current density of carbon monoxide and the utilization ratio of carbon dioxide at the cathode was evaluated in accordance with the above.
- Example 1 In the carbon dioxide electrolysis device of Example 1, the electrolysis cell was assembled in the same way as in Example 1, except that the cathode gas flow path was made with the land width of 0.8 mm, the flow path width W of 1 mm, and the flow path depth h of 0.5 mm.
- the cathode gas flow path had the aspect ratio of 0.5 and the fluid mean depth M of 0.17.
- the relationship between the partial current density of carbon monoxide and the utilization ratio of carbon dioxide at the cathode was evaluated as in Example 1.
- Example 1 In the carbon dioxide electrolysis device of Example 1, the electrolysis cell was assembled in the same way as in Example 1, except that the cathode gas flow path was made with the land width of 0.8 mm, the flow path width W of 1 mm, and the flow path depth h of 1 mm.
- the cathode gas flow path had the aspect ratio of 1 and the fluid mean depth of 0.25.
- the relationship between the partial current density of carbon monoxide and the utilization ratio of carbon dioxide at the cathode was evaluated as in Example 1.
- Example 1 In the carbon dioxide electrolysis device of Example 1, the electrolysis cell was assembled in the same way as in Example 1, except that the cathode gas flow path was made with the land width of 0.8 mm, the flow path width W of 1 mm, and the flow path depth h of 2 mm.
- the cathode gas flow path had the aspect ratio of 2 and the fluid mean depth of 0.33.
- the relationship between the partial current density of carbon monoxide and the utilization ratio of carbon dioxide at the cathode was evaluated as in Example 1.
- Example 1 In the carbon dioxide electrolysis device of Example 1, the electrolysis cell was assembled in the same way as in Example 1, except that the cathode gas flow path was made with the land width of 0.49 mm, the flow path width W of 0.49 mm, and the flow path depth h of 1 mm.
- the cathode gas flow path had the aspect ratio of 2 and the fluid mean depth of 0.16.
- the relationship between the partial current density of carbon monoxide and the utilization ratio of carbon dioxide at the cathode was evaluated as in Example 1.
- FIG. 11 presents the relationships between the partial current density of carbon monoxide and the utilization ratio of carbon dioxide at the cathode in Example 1, Comparative Example 1, Comparative Example 2, and Comparative Example 3.
- FIG. 12 presents the relationships between the partial current density of carbon monoxide and the utilization ratio of carbon dioxide at the cathode in Comparative Example 3 and Comparative Example 4.
Abstract
A carbon dioxide electrolysis device includes: a cathode configured to reduce carbon dioxide and thus form a carbon compound; an anode configured to oxidize water and thus generate oxygen; a cathode gas flow path facing on the cathode and configured to supply gas containing carbon dioxide; an anode solution flow path facing on the anode and configured to supply an electrolytic solution containing water; and a separator provided between the anode and the cathode. An aspect ratio of the cathode gas flow path is greater than 1 and 3 or less. In a cross-section along a direction perpendicular to a facing surface between the cathode and the cathode gas flow path in the cathode gas flow path, a fluid mean depth M of the cathode gas flow path and a depth h of the cathode gas flow path satisfy a formula: h/8≤M<h/4.
Description
- This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2021-044811, filed on Mar. 18, 2021; the entire contents of which are incorporated herein by reference.
- Embodiments relate to a carbon dioxide electrolysis device.
- In recent years, renewable energy such as solar power is desirably converted into not only electrical energy for use hut also a storable and transportable resource in terms of both energy and environmental issues. This demand has advanced research and development of artificial photosynthesis technology, which uses sunlight to produce chemical substances like photosynthesis in plants. This technology has potential to store the renewable energy as storable fuel and is also expected to create value by producing chemical substances that can be used as industrial raw materials.
- Known examples of a device that uses the renewable energy such as solar power to produce chemical substances, include an electrochemical reaction device having a cathode and an anode, the cathode being configured to reduce carbon dioxide (CO2) generated from a power plant or waste treatment plant, and the anode being configured to oxidize water (H2O). The cathode, for example, reduce carbon dioxide to produce carbon compounds such as carbon monoxide (CO). When such an electrochemical reaction device is fabricated in a cell form (also called an electrolysis cell), it may be effective to fabricate it in a form similar to a fuel cell, such as a polymer electric fuel cell (PEFC). The directly supply of carbon dioxide to a catalyst layer of the cathode, rapidly processes a carbon dioxide reduction reaction.
- However, such a cell form has a problem similar to that of the PEFC arises. In other words, it is necessary to stably supply carbon dioxide to the cathode catalyst layer to fabricate the electrolysis cell that is resistant to failure and durable and to improve efficiency of carbon compound production.
-
FIG. 1 is a schematic diagram to explain a configuration example of a carbon dioxide electrolysis device. -
FIG. 2 is a planar schematic diagram illustrating a structural example of a part of a flow path plate. -
FIG. 3 is a cross-sectional schematic diagram illustrating a structural example of a part of the flow path plate. -
FIG. 4 is a planar schematic diagram illustrating another structural example of the flow path plate. -
FIG. 5 is a cross-sectional schematic diagram illustrating another structural example of a part of the flow path plate. -
FIG. 6 is a cross-sectional schematic diagram illustrating still another structural example of a part of the flow path plate. -
FIG. 7 is a cross-sectional schematic diagram illustrating yet another structural example of a part of the flow path plate. -
FIG. 8 is a schematic diagram illustrating another configuration example of the carbon dioxide electrolysis device. -
FIG. 9 is a flowchart to explain an operating method example of the carbon dioxide electrolysis device. -
FIG. 10 is a flowchart to explain an operation example of a refresh operation step. -
FIG. 11 is a diagram illustrating a relationship between a partial current density of carbon monoxide and a utilization ratio of carbon dioxide at a cathode. -
FIG. 12 is a diagram illustrating a relationship between the partial current density of carbon monoxide and the utilization ratio of carbon dioxide at the cathode. - A carbon dioxide electrolysis device includes: a cathode configured to reduce carbon dioxide and thus form a carbon compound; an anode configured to oxidize water and thus generate oxygen; a cathode gas flow path facing on the cathode and configured to supply gas containing carbon dioxide; an anode solution flow path facing on the anode and configured to supply an electrolytic solution containing water; and a separator provided between the anode and the cathode. An aspect ratio of the cathode gas flow path is greater than 1 and 3 or less, the aspect ratio being defined by a ratio of a depth of the cathode gas flow path to a width of the cathode gas flow path. In a cross-section along a direction perpendicular to a facing surface between the cathode and the cathode gas flow path in the cathode gas flow path, a fluid mean depth M of the cathode gas flow path and a depth h of the cathode gas flow path satisfy a formula: h/8≤M<h/4, the fluid mean depth M being defined by a ratio of a circumferential length of the cathode gas flow path to a cross-sectional area of the cathode gas flow path.
- Hereinafter, embodiments will be described with reference to the drawings. The drawings are schematic, and dimensions such as a thickness and a width of each component are sometimes different from actual ones. In each embodiment presented below, substantially the same components are denoted by the same reference signs, and a description thereof is sometimes partially omitted.
- In this specification, “connection” includes not only direct connection but also indirect connection, unless otherwise specified.
-
FIG. 1 is a schematic diagram to explain a configuration example of a carbon dioxide electrolysis device.FIG. 1 illustrates a carbondioxide electrolysis device 1 including anelectrolysis cell 10. - The
electrolysis cell 10 includes ananode part 11, acathode part 12, and aseparator 13 that separates theanode part 11 from thecathode part 12. Theelectrolysis cell 10 is, for example, sandwiched between a pair of support plates and further tightened with bolts or the like. - The
anode part 11 includes ananode 111, an anodesolution flow path 112 a provided on aflow path plate 112, and an anodecurrent collector 113. - The
cathode part 12 includes acathode 121, a cathodegas flow path 122 a provided on aflow path plate 122, and a cathodecurrent collector 123. - The
anode 111 is an electrode (oxidation electrode) that promotes an oxidation reaction of water (H2O) in an anode solution to produce oxygen (O2) and hydrogen ions (H+), or an oxidation reaction of hydroxide ions (OH−) generated in thecathode part 12 to produce oxygen and water. - The
anode 111 is disposed between theseparator 13 and theflow path plate 112 to be in contact therewith. A first surface of theanode 111 is in contact with theseparator 13. A second surface of theanode 111 is provided on an opposite side of the first surface of theanode 111 and faces the anodesolution flow path 112 a. - Compounds produced by the oxidation reaction of the
anode 111 are different depending on types of oxidation catalysts and other factors. When an electrolytic solution is used as the anode solution, theanode 111 is preferably mainly composed of a catalyst material (anode catalyst material) capable of oxidizing water (H2O) to produce oxygen and hydrogen ions, or oxidizing hydroxide ions (OH−) to produce water and oxygen, and capable of decreasing an overvoltage of such reactions. Such catalyst materials include metals such as platinum (Pt), palladium (Pd), and nickel (Ni), alloys and intermetallic compounds containing these 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 Ru complexes and Fe complexes. - The
anode 111 is preferably equipped with a substrate (support) having a structure that enables a movement of the anode solution and ions between theseparator 13 and the anodesolution flow path 112 a, such as a mesh material, a punching material, or a porous structure such as a porous member. The substrate having the porous structure also includes the substrate with relatively large pores, such as a metal fiber sintered compact. The substrate may be composed of metals such as titanium (Ti), nickel (Ni), and iron (Fe) or a metal material such as an alloy containing at least one of these metals (for example, SUS), or may be composed of the anode catalyst materials described above. When oxides are used as the anode catalyst material, a catalyst layer is preferably formed by attaching or laminating the anode catalyst material to a surface of the substrate made of the metal material described above. The anode catalyst material preferably has a shape such as nanoparticles, nanostructures, nanowires to enhance the oxidation reaction. The nanostructures are structures with nanoscale irregularities formed on a surface of the catalyst material. The oxidation catalyst does not necessarily have to be provided on the oxidation electrode. An oxidation catalyst layer provided other than the oxidation electrode may be electrically connected to the oxidation electrode. - The
cathode 121 is an electrode (reduction electrode) that generates reduction reactions of carbon dioxide and reduction products to produce carbon compounds. Examples of the carbon compounds include carbon monoxide (CO), formic acid (HCOOH), methane (CH4), ethane (C2H6), ethylene (C2H4), methanol (CH3OH), acetic acid (CH3COOH), ethanol (C2H5OH)), formaldehyde (HCHO), propanol (C3H7OH), and ethylene glycol (C2H6O2). Along with the reduction reaction of carbon dioxide, the reduction reaction at thecathode 121 may include a side reaction that generates a water reduction reaction to produce hydrogen (H2). - The
cathode 121 is preferably composed of an ion-conductive substance in addition to an electrode substrate and a metal catalyst supported on a carbon material. The ion-conductive substance exhibits an action of transferring ions between metal catalysts contained in layers and thus exerts effects of enhancing the electrode activity. A cation exchange resin or anion exchange resin is preferably used as the above ion-conductive substance. - The support for the metal catalyst preferably has the porous structure. In addition to the above materials, applicable materials include, for example, carbon blacks such as Ketjen black and Vulcan XC-72, activated carbon, carbon nanotubes, and the like. By having the porous structure, an area of an active surface that contributes to the oxidation-reduction reaction can be increased, thus increasing conversion efficiency.
- The catalyst layer itself formed on the substrate as well as the support preferably has the porous structure and has a large number of relatively large pores. Specifically, in terms of a pore size distribution of the catalyst layer measured by a mercury intrusion method, a frequency of pore distribution is preferably maximized in a range of 5 μm or more and 200 μm or less in diameter. In this case, gas diffuses quickly throughout the catalyst layer, and reduction products are easily discharged outside the catalyst layer through this pathway, resulting in an efficient electrode.
- A gas diffusion layer is preferably provided on the electrode substrate supporting the catalyst layer to efficiently supply carbon dioxide to the catalyst layer. The gas diffusion layer is formed by a conductive porous member. The gas diffusion layer is preferably formed by a water-repellent porous member because amounts of water produced by the reduction reaction and water that has moved from an oxidation side can be lowered, allowing the water to drain through a reduction flow path and increasing the percentage of carbon dioxide gas in the porous member.
- When a thickness of the gas diffusion layer is extremely small, uniformity on a cell surface is impaired, which is not desirable. On the other hand, when the thickness is extremely large, a member cost increases and efficiency decreases due to the increase in gas diffusion resistance, which is not desirable. A denser diffusion layer (mesoporous layer (MPL)) is more preferably provided between the gas diffusion layer and the catalyst layer to further improve diffusibility, as it changes the water repellency and porosity to promote gas diffusibility and liquid component drainage.
- The metal catalysts supported on the above support include materials that decrease activation energy for the reduction of hydrogen ions and carbon dioxide. In other words, metal materials that decrease the overvoltage in the reduction reaction of carbon dioxide to produce carbon compounds are included. For example, it is preferred to use at least one metal selected from the group consisting of gold (Au), silver (Ag), copper (Cu), platinum (Pt), palladium (Pd), nickel (Ni), (Co), iron (Fe), manganese (Mn), titanium (Ti), cadmium (Cd), zinc (Zn), indium (In), gallium (Ga), lead (Pb), and tin (Sn), and metal oxide or alloys containing these metals. For example, at least one of copper, gold, and silver is preferably used. For example, metal complexes such as ruthenium (Ru) complexes or rhenium (Re) complexes, can also be used as reduction catalysts without being limited to the above. A plurality of materials may also be mixed. Various shapes can be applied to the metal catalyst, such as plate, mesh, wire, particle, porous, thin-film, and island shapes.
- When metal nanoparticles are applied to the metal catalyst, an average diameter is preferably 1 nm or more and 15 nm or less, more preferably 1 nm or more and 10 nm or less, and even more preferably 1 nm or more and 5 nm or less. Meeting these conditions is desirable because a surface area of the metal per catalyst weight becomes larger, and a small amount of metal is required to exhibit high activity.
- The
anode 111 andcathode 121 can be connected to apower supply 20. Thepower supply 20 applies a voltage between theanode 111 andcathode 121. Examples of thepower supply 20 are not limited to ordinary system power supplies or batteries but may include power sources that supply power generated by renewable energy sources such as solar cells or wind power. Thepower supply 20 may further include a power controller that adjusts output of the above power supply to control the voltage between theanode 111 andcathode 121. Thepower supply 20 may be provided outside the carbondioxide electrolysis device 1. - The anode
solution flow path 112 a has a function of supplying the anode solution to theanode 111. The anodesolution flow path 112 a is formed by pits (grooves/recesses) provided in theflow path plate 112. Theflow path plate 112 has an inlet and an outlet port (both not illustrated) connected to the anodesolution flow path 112 a, and the anode solution is introduced and discharged by a pump (not illustrated) through these inlet and outlet ports. The anode solution is distributed in the anodesolution flow path 112 a to be in contact with the anode. - An aqueous solution containing metal ions (electrolytic solution) can be used as the anode solution. By using the electrolytic solution containing metal ions, the electrolysis efficiency can be increased. The aqueous solution can be, for example, aqueous solutions containing phosphate ions (PO4 2−), borate ions (BO3 3−), 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 (CO3 2−), and other aqueous solutions. Other aqueous solutions containing LiHCO3, NaHCO3, KHCO3, CsHCO3, phosphoric acid, boric acid, and so on may also be used.
- The cathode
gas flow path 122 a faces a first surface of thecathode 121. The cathodegas flow path 122 a has a function of supplying gas containing carbon dioxide to thecathode 121. For example, the cathodegas flow path 122 a can be connected to a carbon dioxide supply source that supplies the gas containing carbon dioxide. The carbon dioxide supply source can be, for example, a power plant, waste treatment plant, or other facilities. The cathodegas flow path 122 a is formed by pits (grooves/recesses) provided in theflow path plate 122. Theflow path plate 122 has an inlet and an outlet port (both not illustrated) connected to the cathodegas flow path 122 a, and the gas is introduced and discharged by a pump (not illustrated) through these inlet and outlet ports. - It is preferable to use materials for the
flow path plates flow path plates flow path plates - The
flow path plate 122 can have a land in contact with thecathode 121 for electrical connection with thecathode 121. A shape of the cathodegas flow path 122 a can be adjacent to a columnar land, a serpentine shape where a thin flow path is folded or the like, but any shape with a cavity can be used. The cathodegas flow path 122 a is preferably constituted by a plurality of flow paths connected in parallel, a serpentine flow path, or a combination thereof because the uniformity of the gas to be supplied to thecathode 121 can be enhanced and the uniformity of an electrolytic reaction can be enhanced. -
FIG. 2 is a planar schematic diagram illustrating a structural example of a part of theflow path plate 122.FIG. 2 illustrates an X-Y plane of theflow path plate 122 including an X-axis and a Y-axis orthogonal to the X-axis. InFIG. 2 , only a superposition of theflow path plate 122 and thecathode 121 is schematically illustrated.FIG. 3 is a cross-sectional schematic diagram illustrating a structural example of a part of theflow path plate 122.FIG. 3 illustrates a Y-Z plane of theflow path plate 122 including the Y-axis and a Z-axis orthogonal to the Y-axis and X-axis. The Z-axis direction is a thickness direction of theflow path plate 122. - The
flow path plate 122 has asurface 241, asurface 242, and the cathodegas flow path 122 a. Thesurface 241 is in contact with thecathode 121. Thesurface 242 is provided on an opposite side of thesurface 241 and in contact with the cathodecurrent collector 123. Theflow path plate 122 illustrated inFIG. 2 andFIG. 3 has a rectangular parallelepiped shape. A three-dimensional shape of theflow path plate 122 is not limited to the rectangular parallelepiped shape. - The cathode
gas flow path 122 a faces the gas diffusion layer of thecathode 121. The cathodegas flow path 122 a is connected to the inlet and outlet ports. The inlet port is provided for introducing the gas containing carbon dioxide into the cathodegas flow path 122 a. The outlet port is provided for discharging the gas containing carbon dioxide from the cathodegas flow path 122 a and for discharging products of the reduction reaction from the cathodegas flow path 122 a. - The cathode
gas flow path 122 a illustrated inFIG. 2 extends in a serpentine shape along thesurface 241. The cathodegas flow path 122 a may also extend in a comb-teeth or spiral shape along thesurface 241 without being limited to the serpentine shape. The cathodegas flow path 122 a includes a space formed by, for example, a groove or opening provided at theflow path plate 122. - The carbon dioxide gas may be supplied in a dry state. A carbon dioxide concentration in the gas supplied to the cathode
gas flow path 122 a does not have to be 100%. In this case, it is also possible to reduce the gas containing carbon dioxide emitted from each of various facilities, although the efficiency will decrease. - The
flow path plates flow path plates - The anode
current collector 113 is in contact with a surface of theflow path plate 112 opposite to a contact surface with theanode 111. The anodecurrent collector 113 is electrically connected to theanode 111. The anodecurrent collector 113 preferably contains a material with low chemical reactivity and high electrical conductivity. Such materials include metal materials such as Ti and SUS, carbon, and the like. - The cathode
current collector 123 is in contact with a surface of theflow path plate 122 opposite to a contact surface with thecathode 121. The cathodecurrent collector 123 is electrically connected to thecathode 121. The cathodecurrent collector 123 preferably contains a material with low chemical reactivity and high electrical conductivity. Such materials include metal materials such as Ti and SUS, carbon, and the like. - The
separator 13 is provided between theanode 111 and thecathode 121. Theseparator 13 is formed of an ion-exchange membrane or the like that can move ions between the anode and the cathode and can separate the anode part from the cathode part. For example, cation-exchange membranes such as Nation and Fremion, and anion-exchange membranes such as Neosepta, Selemion, and Sustenion can be used as the ion-exchange membrane. When an alkaline solution is used as the electrolytic solution and the moving of mainly hydroxide ions (OH−) is mainly assumed, the separator is preferably formed of the anion-exchange membrane. The ion-exchange membrane may be formed by using a membrane with a hydrocarbon basic structure or a membrane with an amine group. In addition to the ion-exchange membrane, salt bridges, glass filters, porous polymer membranes, porous insulating materials, and so on may be applied to the separator as long as the material is capable of moving ions between the anode and the cathode. However, when gas distribution occurs between the cathode and anode parts, circular reactions due to reoxidation of reduction products may occur. Therefore, it is preferable to have less gas exchange between the cathode and anode parts. Therefore, care should be taken when using a porous thin membrane as the separator. - Next, an operation example of the carbon dioxide electrolysis device of the embodiment will be explained. Here, the case where the carbon
dioxide electrolysis device 1 illustrated inFIG. 1 produces carbon monoxide as a carbon compound is mainly explained, but the carbon compound as the reduction product of carbon dioxide is not limited to carbon monoxide. The reduction product, carbon monoxide, may be further reduced to produce organic compounds as described above. Theelectrolysis cell 10 is preferably used when producing solution carbon compounds. A reaction process by theelectrolysis cell 10 may be the case mainly producing hydrogen ions (H+) or the case mainly producing hydroxide ions (OH−) but is not limited to either of these reaction processes. - The reaction process is described mainly for the oxidation of water (H2O) to produce hydrogen ions (H+). When the current is supplied from the
power supply 20 between theanode 111 andcathode 121, the oxidation reaction of water (H2O) occurs at theanode 111 in contact with the anode solution. Specifically, as shown in Equation (1) below, H2O contained in the anode solution is oxidized to produce oxygen (O2) and hydrogen ions (H+). -
2H2O→4H++O2+4e (1) - H+ produced at the
anode 111 moves through the electrolytic solution present in theanode 111 and theseparator 13 to reach near thecathode 121. Electrons (e−) in accordance with the current supplied to thecathode 121 from thepower supply 20 and H+ that moves near thecathode 121 causes the reduction reaction of carbon dioxide (CO2). Specifically, as shown in Equation (2) below, carbon dioxide supplied to thecathode 121 from the cathodegas flow path 122 a is reduced to produce carbon monoxide. Hydrogen is also produced when hydrogen ions receive electrons, as shown in Equation (3) below. In this case, hydrogen may be produced simultaneously with carbon monoxide. -
CO2+2H++2e−→CO+H2O (2) -
2H++2e−→H2 (3) - Next, a reaction process when carbon dioxide (CO2) is mainly reduced to produce hydroxide ions (OH−) is described. When the current is supplied from the
power supply 20 between theanode 111 and thecathode 121, water (H2O) and carbon dioxide (CO2) are reduced near thecathode 121 to produce carbon monoxide (CO) and hydroxide ions (OH−), as shown in Equation (4) below. In addition, hydrogen is produced when water receives electrons as shown in Equation (5) below. At this time, hydrogen may be produced simultaneously with carbon monoxide. The hydroxide ions (OH−) produced by these reactions diffuse near theanode 111, and as shown in Equation (6) below, hydroxide ions (OH−) are oxidized to produce oxygen (O2). -
2CO2+2H2O+4e−→2CO+4OH− (4) -
2H2O+2e−→H2+2OH− (5) -
4OH−→2H2O+O2+4e− (6) - In the
electrolysis cell 10 illustrated inFIG. 1 , the anode solution and ions are supplied from theseparator 13, and carbon dioxide gas is supplied from the cathodegas flow path 122 a. - The carbon
dioxide electrolysis device 1 can not only specialize in carbon dioxide reduction, but can also produce carbon dioxide reduction products and hydrogen, for example, by having carbon monoxide and hydrogen in any ratio, such as 1:2, and then producing methanol in a subsequent chemical reaction. - Since hydrogen is an inexpensive and readily available raw material from water electrolysis and fossil fuels, it is not necessary to have a large ratio of hydrogen. From these points of view, a ratio of carbon monoxide to hydrogen of at least 1 or more, and preferably 1.5 or more, is preferable in terms of economic efficiency and environmental friendliness.
- The cathode
gas flow path 122 a is preferably shallow in terms of supplying carbon dioxide to the gas diffusion layer. On the other hand, when the cathodegas flow path 122 a is narrow, pressure loss increases in the cathodegas flow path 122 a, which is not desirable in terms of energy loss in the gas supply. Furthermore, when salts are precipitated due to a reaction between metal ions in the anode solution and the carbon dioxide gas, and the salts solidify at a boundary with the gas diffusion layer of the cathodegas flow path 122 a, the shallow flow path will be closed and prevent the carbon dioxide gas from spreading to an entire electrode surface, which may cause failure to affect durability. - in one example of a conventional fuel cell, it is known that a groove is formed on an inner bottom surface of a flow path to store foreign matters (water droplets) to prevent the flow path from being blocked by the foreign matters (water droplets). However, in the case of the carbon dioxide electrolysis device, since the precipitated salts solidify near a facing surface between the
cathode 121 and the cathodegas flow path 122 a, forming a groove on the inner bottom surface is not effective in preventing the blockage. - On the other hand, in the carbon dioxide electrolysis device of this embodiment, a cross-sectional shape of the cathode
gas flow path 122 a is controlled to prevent the blockage of the flow path. An aspect ratio of the cathodegas flow path 122 a is preferably greater than 1 and 3 or less. The aspect ratio of the cathodegas flow path 122 a is defined by a ratio of a depth h of the cathodegas flow path 122 a to a width W of the cathodegas flow path 122 a in the X-axis or Y-axis direction. - When the aspect ratio is less than 1, the flow path may be blocked due to the salt precipitation. When the aspect ratio exceeds 3, the
flow path plate 122 needs to be thickened, which increases material and processing costs. The aspect ratio is more preferably 2 or more and 3 or less. - A fluid mean depth M of the cathode
gas flow path 122 a and the depth h of the cathodegas flow path 122 a preferably satisfy a formula (A) below, -
A formula: h/8≤M<h/4 (A) - The fluid mean depth M of the cathode
gas flow path 122 a is defined by a cross-sectional area Ac of the cathodegas flow path 122 a to a circumferential length S of the cathodegas flow path 122 a. The circumferential length S may be calculated by (width W×2)+(depth h×2). Even if the aspect ratio of the cathodegas flow path 122 a is large, when the fluid mean depth M of the cathodegas flow path 122 a is small, salts may precipitate in the cathodegas flow path 122 a and easily block the cathodegas flow path 122 a. - When the fluid mean depth M is less than h/8, the flow path may be blocked due to the salt precipitation. When the fluid mean depth M is h/4 or more, a utilization ratio of carbon dioxide may decrease. The fluid mean depth M is more preferably h/7.9 or more and h/6 or less.
- The depth h, the width W, the circumferential length S, and the fluid mean depth M can be measured by the following method. A cross-section of the
flow path plate 122 in a perpendicular direction (X-axis direction inFIG. 2 ) to a long direction (Y-axis direction inFIG. 2 ) of the cathodegas flow path 122 a is cut out at an arbitrary position, and the cross-section is observed with a microscope, for example, to measure each parameter. Also, as a non-destructive inspection method, for example, neutron radiography can be used to visualize an inside of the flow path plate. It is preferable to calculate these values by averaging values at multiple locations. - The enough deep of the cathode
gas flow path 122 a to meet the above conditions can form a space in a depth direction of the cathodegas flow path 122 a where the gas can be diverted. This is desirable in terms of eliminating blockage of gas supply during salt precipitation and makes it easier to supply the carbon dioxide gas to the entire surface of thecathode 121 even if salts precipitate at a part of the cathodegas flow path 122 a, and is therefore less prone to failure and more desirable in terms of durability. Thus, the decrease in the electrolysis efficiency can be prevented, and the carbon dioxide electrolysis device that is highly efficient and can be operated for a long time can be provided. - The shape of the cathode
gas flow path 122 a is not limited to the shapes illustrated inFIG. 2 andFIG. 3 . Other examples of the shape of the cathodegas flow path 122 a are described below. -
FIG. 4 is a planar schematic diagram illustrating another structural example of theflow path plate 122.FIG. 4 illustrates the X-Y plane of theflow path plate 122. Theflow path plate 122 illustrated inFIG. 4 differs from theflow path plate 122 illustrated inFIG. 2 in that the cathodegas flow path 122 a has a plurality offlow path parts 244 connected in parallel in the X-Y plane. The other parts are the same as theflow path plate 122 illustrated inFIG. 2 , so the above explanation can be used as appropriate. - The plurality of
flow path parts 244 extend along the long direction (Y-axis direction inFIG. 4 ) of the cathodegas flow path 122 a.FIG. 4 illustrates an example of twoflow path parts 244 connected in parallel for each fold of the cathodegas flow path 122 a, but the number offlow path parts 244 is not limited to the number illustrated inFIG. 4 . The width W of the cathodegas flow path 122 a illustrated inFIG. 4 is defined by a width of oneflow path part 244. -
FIG. 5 is a cross-sectional schematic diagram illustrating still another structural example of a part of theflow path plate 122.FIG. 5 illustrates the Y-Z plane of theflow path plate 122. Theflow path plate 122 illustrated inFIG. 5 differs from theflow path plate 122 illustrated inFIG. 3 in that the cathodegas flow path 122 a hasregions 122 a 1 and 122 a 2 in an X-Z cross-section. The other parts are the same as theflow path plate 122 illustrated inFIG. 3 , so the above explanation can be used as appropriate. - The
region 122 a 1 faces thecathode 121 and has aninner wall surface 246. A cross-sectional shape of theregion 122 a 1 illustrated inFIG. 5 is a rectangle, but the shape of theregion 122 a 1 is not limited to the shape inFIG. 5 . - The
region 122 a 2 is provided between theregion 122 a 1 and aninner bottom surface 245 of the cathodegas flow path 122 a and has aninner wall surface 247. A cross-sectional shape of theregion 122 a 2 illustrated inFIG. 5 is a rectangle, but the shape of theregion 122 a 2 is not limited to the shape inFIG. 5 . - A width W2 in the X-axis direction of the
region 122 a 2 is wider than a width W1 in the X-axis direction of theregion 122 a 1. In theflow path plate 122 illustrated inFIG. 5 , a space for the carbon dioxide gas to be diverted can be increased, and thus the blockage of the cathodegas flow path 122 a due to the salt precipitation can be prevented by making the width W2 wider than the width W1. The width W of the cathodegas flow path 122 a illustrated inFIG. 5 is defined by the width W1. The fluid mean depth M is defined by the circumferential length of the cathodegas flow path 122 a having the shape illustrated inFIG. 5 , taking into account both the width W1 and the width W2. -
FIG. 6 is a cross-sectional schematic diagram illustrating still another structural example of a part of theflow path plate 122.FIG. 6 illustrates the Y-Z plane of theflow path plate 122. Compared to theflow path plate 122 illustrated inFIG. 5 , theflow path plate 122 illustrated inFIG. 6 has a different shape of theregion 122 a 2 in the X-Z cross-section. The other parts are the same as theflow path plate 122 illustrated inFIG. 5 , so the above explanation can be used as appropriate. - A cross-sectional shape of the
region 122 a 2 illustrated inFIG. 6 is a square, but the shape of theregion 122 a 2 is not limited to the shape inFIG. 6 . InFIG. 6 , a cross-sectional area of theregion 122 a 2 is larger than that of theregion 122 a 1. This allows for a larger space for the carbon dioxide gas to be diverted and prevents the blockage of the cathodegas flow path 122 a due to the salt precipitation. The width W of the cathodegas flow path 122 a illustrated inFIG. 6 is defined by the width W1. The fluid mean depth M is defined by the circumferential length of the cathodegas flow path 122 a having the shape illustrated inFIG. 6 , taking into account both the width W1 and the width W2. -
FIG. 7 is a cross-sectional schematic diagram illustrating yet another structural example of a part of theflow path plate 122.FIG. 7 illustrates the Y-Z plane of theflow path plate 122. Theflow path plate 122 illustrated inFIG. 7 differs from theflow path plate 122 illustrated inFIG. 3 in that the cathodegas flow path 122 a has theregions 122 a 1 and 122 a 2 in the X-Z cross-section, theinner wall surface 246 of theregion 122 a 1 is hydrophilic, and theinner wall surface 247 and theinner bottom surface 245 of theregion 122 a 2 are water repellent. The other parts are the same as theflow path plate 122 illustrated inFIG. 3 , so the above explanation can be used as appropriate. - A contact angle with water at the hydrophilic
inner wall surface 246 is, for example, more than 0 degrees and 90 degrees or less. The hydrophilicinner wall surface 246 can be formed using a flow path layer containing, for example, a hydrophilic material. The hydrophilicinner wall surface 246 may also be formed by applying a hydrophilic treatment to the flow path layer containing a material applicable to theflow path plate 122. - A contact angle with water at the water-repellent
inner wall surface 247 is, for example, 100 degrees or more and less than 180 degrees. The water-repellentinner wall surface 247 can be formed using a flow path layer containing, for example, a water-repellent material. The water-repellentinner wall surface 247 may also be formed by applying a water-repellent treatment to the flow path layer containing the material applicable to theflow path plate 122. - A thickness (length in the Z-axis direction) of the
region 122 a 1 is not particularly limited, but is preferably half the depth h of the cathodegas flow path 122 a or more, for example. - In the cathode
gas flow path 122 a illustrated inFIG. 7 , for example, when metal ions in the anode solution flow into the cathodegas flow path 122 a, the anode solution tends to flow into theregion 122 a 1 by forming the hydrophilicinner wall surface 246 and the water-repellentinner wall surface 247. Therefore, the salt precipitation can be prevented in theregion 122 a 2, and the blockage of the cathodegas flow path 122 a due to the salt precipitation can be prevented. The structure illustrated inFIG. 7 may be combined with the stricture illustrated inFIG. 5 orFIG. 6 as appropriate. -
FIG. 8 is a schematic diagram illustrating another configuration example of the carbon dioxide electrolysis device. The carbondioxide electrolysis device 1 illustrated inFIG. 8 includes theelectrolysis cell 10, an anodesolution supply system 100 that supplies an anode solution to theelectrolysis cell 10, agas supply system 300 that supplies carbon dioxide (CO2) gas to theelectrolysis cell 10, aproduct collection system 400 that collects products produced by a reduction reaction in theelectrolysis cell 10, acontrol system 500 that detects types and amounts of the collected products as well as controls the products and a refresh operation, a wastesolution collection system 600 that collects a waste solution of the anode solution, and arefresh material supply 700 that recovers the anode, cathode, and so on of theelectrolysis cell 10. Components necessary for the refresh operation do not necessarily have to be provided. - The
electrolysis cell 10 corresponds to theelectrolysis cell 10 illustrated inFIG. 1 . The explanation of each component of theelectrolysis cell 10 in the first embodiment can be used as appropriate. - In
FIG. 8 , thepower supply 20 that applies current to theanode 111 and thecathode 121 is provided. Thepower supply 20 is connected to the anodecurrent collector 113 and the cathodecurrent collector 123 through a current introduction member. Thepower supply 20 is not limited to an ordinary system power supply, batteries, and the like, but may also have a power source that supplies power generated by renewable energy sources such as solar cells and wind power. Thepower supply 20 may have the above power source, a power controller that adjusts output of the power source to control a voltage between theanode 111 and thecathode 121, or the like. - The anode solution is supplied as an electrolytic solution from the anode
solution supply system 100 to the anodesolution flow path 112 a of theanode part 11. The anodesolution supply system 100 circulates the anode solution so that the anode solution is distributed in the anodesolution flow path 112 a. The anodesolution supply system 100 has apressure controller 101, ananode solution tank 102, a flow rate controller (pump) 103, areference electrode 104, and apressure gauge 105, and is configured such that the anode solution circulates in the anodesolution flow path 112 a. Theanode solution tank 102 is connected to a gas component collection unit, not illustrated, which collects oxygen (O2) and other gas components contained in the circulating anode solution. The anode solution is introduced into the anodesolution flow path 112 a with the flow rate and pressure controlled in thepressure controller 101 and flowrate controller 103. - The CO2 gas is supplied to the cathode
gas flow path 122 a from thegas supply system 300. Thegas supply system 300 has a CO2 gas cylinder 301, aflow rate controller 302, apressure gauge 303, and apressure controller 304. The CO2 gas is introduced into the cathodegas flow path 122 a with the flow rate and pressure controlled in theflow rate controller 302 andpressure controller 304. Thegas supply system 300 is connected to theproduct collection system 400 that collects products in the gas distributed through the cathodegas flow path 122 a. Theproduct collection system 400 has a gas-liquid separation unit 401 and aproduct collection unit 402. Reduction products such as CO and H2 contained in the gas distributed through the cathodegas flow path 122 a are accumulated in theproduct collection unit 402 through the gas-liquid separation unit 401. - The anode solution circulates in the anode
solution flow path 112 a during the electrolytic reaction operation as described above. During the refresh operation of theelectrolysis cell 10, described below, the anode solution is discharged into the wastesolution collection system 600 so that theanode 111 and the anodesolution flow path 112 a are exposed from the anode solution. - The waste
solution collection system 600 has a wastesolution collection tank 601 connected to the anodesolution flow path 112 a. A waste solution of the anode solution is collected in the wastesolution collection tank 601 by opening and closing not-illustrated valves. The opening and closing of the valves and other operations are collectively controlled by thecontrol system 500. The wastesolution collection tank 601 also functions as a collection unit for a rinse solution supplied from therefresh material supply 700. Furthermore, gaseous substances supplied from therefresh material supply 700 and containing some liquid substances are also collected in the wastesolution collection tank 601 as necessary. - The
refresh material supply 700 includes a gaseoussubstance supply system 710 and a rinse solution supply system 720. The rinse solution supply system 720 can be omitted in some cases. The gaseoussubstance supply system 710 has agas tank 711 that serves as a supply source for gaseous substances such as air, carbon dioxide, oxygen, nitrogen, argon, and apressure controller 712 that controls a supply pressure of the gaseous substances. The rinse solution supply system 720 has a rinsesolution tank 721 that serves as a supply source of the rinse solution such as water, and a flow rate controller (pump) 722 that controls a supply flow rate, and the like of the rinse solution. The gaseoussubstance supply system 710 and the rinse solution supply system 720 are connected to the anodesolution flow path 112 a and the cathodegas flow path 122 a through pipes. The gaseous substances and rinse solution are supplied to the anodesolution flow path 112 a and the cathodegas flow path 122 a by opening arid closing not-illustrated valves. The opening and closing of the valves, and other operations are collectively controlled by thecontrol system 500. - A part of the reduction products accumulated in the
product collection unit 402 is sent to a reductionperformance detection unit 501 of thecontrol system 500. In the reductionperformance detection unit 501, a production amount and a proportion of each product such as CO or H2 in the reduction products, are detected. The detected production amount and proportion of each product are input into a data collection and controller 502 of thecontrol system 500. Furthermore, the data collection and controller 502 collects electrical data such as a cell voltage, a cell current, a cathode potential, and an anode potential, and data such as pressure and pressure loss inside the anodesolution flow path 112 a and the cathodegas flow path 122 a as part of cell performance of theelectrolysis cell 10, and sends the data to a refresh controller 503. - The data collection and controller 502 is electrically connected, through bi-directional signal lines whose illustration is partially omitted, to the
power supply 20, thepressure controller 101 and theflow rate controller 103 of the anodesolution supply system 100, theflow rate controller 302 and thepressure controller 304 of thegas supply system 300, and thepressure controller 712 and theflow rate controller 722 of therefresh material supply 700, in addition to the reductionperformance detection unit 501, and these are collectively controlled. Each pipe is provided with a not-illustrated valve, and an opening/closing operation of the valve is controlled by a signal from the data collection and controller 502. The data collection and controller 502 may control operations of the above components during the electrolysis operation, for example. - The refresh controller 503 is electrically connected, through bi-directional signal lines whose illustration is partially omitted, to the
power supply 20, theflow rate controller 103 of the anodesolution supply system 100, theflow rate controller 302 of thegas supply system 300, and thepressure controller 712 and flowrate controller 722 of therefresh material supply 700, and these are collectively controlled. Each pipe is provided with a not-illustrated valve, and an opening/closing operation of the valve is controlled by a signal from the refresh controller 503. The refresh controller 503 may control operations of the above components during the electrolysis operation, for example. The refresh controller 503 and the data collection and controller 502 may be configured by a single controller. - An operating operation of the carbon
dioxide electrolysis device 1 of the embodiment will be described.FIG. 9 is a flowchart to explain an operating method example of the carbondioxide electrolysis device 1. First, as illustrated inFIG. 9 , a startup step S101 of the carbondioxide electrolysis device 1 is performed. In the startup step S101 of the carbondioxide electrolysis device 1, the following operations are performed. In the anodesolution supply system 100, the anode solution is introduced into the anodesolution flow path 112 a after its flow rate and pressure are controlled by thepressure controller 101 and theflow rate controller 103. In thegas supply system 300, the CO2 gas is introduced into the cathodegas flow path 122 a after its flow rate and pressure are controlled by theflow rate controller 302 and thepressure controller 304. - Next, a CO2 electrolysis operation step S102 is performed. In the CO2 electrolysis operation step S102, application of an electrolysis voltage by the
power supply 20 of theelectrolysis device 1 that has been subjected to the startup step S101 is started, and a current is supplied by applying a voltage between theanode 111 and thecathode 121. When the current is applied between theanode 111 and thecathode 121, an oxidation reaction near theanode 111 and a reduction reaction near thecathode 121 occur, which will be described below. The explanation of the oxidation and reduction reactions in the first embodiment can be used as appropriate. - The electrolysis operation may cause salts to precipitate in the cathode
gas flow path 122 a, resulting in decreased cell performance. This is because ions move between theanode 111 and thecathode 121 through theseparator 30 and the ion-exchange membrane, and the ions react with gas components. For example, when a potassium hydroxide solution is used as the anode solution and the carbon dioxide gas is used as the cathode gas, potassium ions move from theanode 111 to thecathode 121 and react with carbon dioxide to produce salts such as potassium hydrogen carbonate and potassium carbonate. When the salts are solubility or less in the cathodegas flow path 122 a, the salts precipitate in the cathodegas flow path 122 a. The salt precipitation prevents uniform gas flow throughout the cell and decreases the cell performance. The decrease in the cell performance is particularly noticeable when a plurality of cathodegas flow paths 122 a are provided. In some cases, the performance of the cell itself can be improved by partially increasing a gas flow rate. This is because the cell performance is improved by increasing a gas pressure, which increases gas components and the like supplied to a catalyst, or by increasing gas diffusibility. A step S103 that determines whether the cell performance meets request criteria is performed to detect such a decrease in the cell performance. - As mentioned above, the data collection and controller 502 periodically or continuously collects, for example, the production amount and the proportion of each product, the cell performance such as the cell voltage, the cell current, the cathode potential, and the anode potential of the
electrolysis cell 10, the pressure inside the anodesolution flow path 112 a, the pressure inside the cathodegas flow path 122 a, and the like. In addition, the data collection and controller 502 has predetermined request criteria for the cell performance, and it is determined whether the collected data meets the set request criteria. When the collected data meets the set request criteria, the CO2 electrolysis operation is continued without stopping the CO2 electrolysis (S104). When the collected data does not meet the set request criteria, a refresh operation step S105 is performed. - The cell performance collected by the data collection and controller 502 is defined by parameters such as, for example, an upper limit value of the cell voltage when a constant current is applied to the
electrolysis cell 10, a lower limit value of the cell current when a constant voltage is applied to theelectrolysis cell 10, and Faradaic efficiency of the carbon compounds produced by the reduction reaction of CO2. Here, the Faradaic efficiency is defined as a proportion of a current that contributed to the production of the desired carbon compound to a total current that flowed in theelectrolysis cell 10. To maintain the electrolysis efficiency, it is recommended that the refresh operation step S105 be performed when the upper limit value of the cell voltage reaches 150% or more, preferably 120% or more of the set value when the constant current is applied. The refresh operation step S105 may be performed when the lower limit value of the cell current when the constant voltage is applied reaches 50% or less, preferably 80% or less, of the set value. To maintain the production amount of the reduction products such as the carbon compounds, it is recommended that the refresh operation step S105 be performed when the Faradaic efficiency of the carbon compounds reaches 50% or less, preferably 80% or less, of the set value. - The cell performance is determined as not meeting the request criteria when at least one of the following parameters, for example, the cell voltage, the cell current, the Faradaic efficiency of the carbon compounds, the pressure inside the anode
solution flow path 112 a, and the pressure inside the cathodegas flow path 122 a, does not meet the request criteria, and the refresh operation step S105 is performed. Two or more of the above parameters may be combined to set the request criteria for the cell performance. For example, the refresh operation step S105 may be performed when both the cell voltage and the Faradaic efficiency of the carbon compounds do not meet the request criteria. The refresh operation step S105 is performed when at least one of the cell performances does not meet the request criteria. The refresh operation step S105 is preferably performed at intervals of, for example, one hour or more to stably perform the CO2 electrolysis operation step S102. - When the
electrolysis cell 10 mainly produces CO, for example, it can be determined that the request criteria of the cell performance are not met when hydrogen increases to at least 2 times, preferably 1.5 times or more of a normal level. For example, in the case of CO, it can be determined that the request criteria of the cell performance are not met when CO decreases to at least 0.8 times or less, preferably 0.9 times or less, of a normal level. - When salts are detected, the salts are discharged by the rinse solution. However, when a mass transfer amount does not change evert after the salts are discharged, it may be determined that a leak has occurred in the
electrolysis cell 10. The leak in theelectrolysis cell 10 is not limited to a gas leak between theanode 111 and thecathode 121, but includes, for example, a gas leak from between thecathode 121 and the cathodegas flow path 122 a. This gas leak is likely to occur, for example, when theelectrolysis cell 10 with salt precipitated is operated for a long time under conditions of high pressure in the cathodegas flow path 122 a. -
FIG. 10 is a flowchart to explain an operation example of the refresh operation step S105. First, the application of the electrolysis voltage by thepower supply 20 is stopped to stop the reduction reaction of CO2 (S201). At this time, the application of the electrolysis voltage does not necessarily have to be stopped. Next, the supply of the gas to the cathodegas flow path 122 a is stopped, the supply of the anode solution to the anodesolution flow path 112 a is stopped, and the anode solution is discharged from the anodesolution flow path 112 a (S202). Next, the rinse solution is supplied (S203) to the anodesolution flow path 112 a and the cathodegas flow path 122 a for cleaning. - A refresh voltage may be applied between the
anode 111 and thecathode 121 while the rinse solution is being supplied. This can remove ions and impurities attached to the cathode catalyst layer. When the refresh voltage is applied so that the process is mainly an oxidation process, ions, impurities such as organic matters on a catalyst surface are oxidized and removed. In addition to refreshing the catalyst, ions substituted in an ion exchange resin when the ion-exchange membrane is used as theseparator 30 can be removed by performing this process in the rinse solution. - The refresh voltage is preferably −2.5 V or more and 2.5 V or less, for example. Since energy is used for the refresh operation, the refresh voltage range is preferably as narrow as possible, and more preferably between −1.5 V or more and 1.5 V or less, for example. The refresh voltage may be applied cyclically so that the oxidation process and the reduction process of ions and impurities are alternately performed. This can accelerate a regeneration of the ion exchange resin and the catalyst. A voltage of a value equivalent to the electrolysis voltage during the electrolysis operation may be applied as the refresh voltage to perform the refresh operation. In this case, a configuration of the
power supply 20 can be simplified. - Next, the gas is supplied to the anode
solution flow path 112 a and the cathodegas flow path 122 a (S204) to dry thecathode 121 and theanode 111. Supplying the rinse solution to the anodesolution flow path 112 a and the cathodegas flow path 122 a increases a water saturation level in the gas diffusion layer and causes a decrease in output due to gas diffusibility. By supplying the gas, the water saturation level is lowered, thus recovering the cell performance and increasing a refreshing effect. The gas is preferably supplied immediately after the distribution of the rinse solution, or at least within 5 minutes after the end of the rinse solution supply. This is because the decrease in the output due to the increase in the water saturation level is significant. For example, when the refresh operation is performed every hour, the output during a 5-minute refresh operation may be 0 V or significantly less, resulting in a loss of 5/60 of the output. - After the refresh operation is completed, the anode solution is introduced into the anode
solution flow path 112 a and the CO2 gas is introduced into the cathodegas flow path 122 a (S205). Then, the application of the electrolysis voltage between theanode 111 and thecathode 121 by thepower supply 20 is resumed to restart the CO2 electrolysis operation (S206). When the application of the electrolysis voltage has not been stopped in S201, the restart operation is not performed. The gas may be used, or the rinse solution may be used to discharge the anode solution from the anodesolution flow path 112 a. - The supply and flow of the rinse solution (S203) are performed to prevent precipitation of electrolytes contained in the anode solution and to clean the
cathode 121, theanode 111, the anodesolution flow path 112 a, and the cathodegas flow path 122 a. Therefore, water is preferable as the rinse solution, further, water with the electrical conductivity of 1 mS/m or less is more preferable, and water with an electrical conductivity of 0.1 mS/m or less is even more preferable. An acid rinse solution such as low-concentration sulfuric acid, nitric acid, hydrochloric acid may be supplied to remove the electrolytes and other precipitates at thecathode 121 and theanode 111, and the like, and the electrolytes may be thereby dissolved. When the low-concentration acid rinse solution is used, a step of supplying a water rinse solution is performed in a subsequent step. Immediately before the gas supply step, the water rinse solution supply step is preferably performed to prevent additives contained in the rinse solution from remaining.FIG. 8 illustrates the rinse solution supply system 720 having one rinsesolution tank 721, but when a plurality of rinse solutions are used, such as the water and acid rinse solutions, a plurality of rinsesolution tanks 721 are used accordingly. - An acid or alkaline rinse solution is particularly desirable for refreshing the ion exchange resin. This has the effect of discharging cations and anions that have been substituted for protons and OH− in the ion exchange resin. For this reason, it is preferable to alternately distribute the acid and alkaline rinse solutions, to combine the rinse solution with water having the electrical conductivity of 1 mS/m or less, and to supply the gas in between the supply of the plurality of rinse solutions to prevent mixing of the rinse solutions.
- The gas used for the gas supply and flow step S204 preferably contains at least one of air, carbon dioxide, oxygen, nitrogen, and argon. Furthermore, it is preferable to use the gas with low chemical reactivity. In this regard, air, nitrogen, and argon are preferably used. Nitrogen and argon are even more preferable. The rinse solution and gas for refreshing is not limited to be supplied to only the anode
solution flow path 112 a and the cathodegas flow path 122 a, but also to the cathodegas flow path 122 a to clean a surface of thecathode 121 in contact with the cathodegas flow path 122 a. It is effective to supply the gas to the cathodegas flow path 122 a to dry thecathode 121 from the surface side in contact with the cathodegas flow path 122 a as well. - The above describes the case where the rinse solution and gas for refreshing are supplied to both the
anode part 11 and thecathode part 12, but the rinse solution and gas for refreshing can be supplied to thecathode part 12 only. - As mentioned above, it is determined whether the CO2 electrolysis operation step S102 should be continued or the refresh operation step S105 should be performed in accordance with whether the cell performance of the
electrolysis cell 10 meets the request criteria. Supplying the rinse solution and gas for refreshing in the refresh operation step S105 prevents uneven distribution of ions and residual gas near theanode 111 andcathode 121, which are factors in the deterioration of the cell performance, and removes the electrolyte precipitation, and the like in thecathode 121, theanode 111, the anodesolution flow path 112 a, and the cathodegas flow path 122 a. Therefore, the cell performance of theelectrolysis cell 10 can be recovered by restarting the CO2 electrolysis operation step S102. after the refresh operation step S105. The CO2 electrolysis performance by theelectrolysis device 1 can be maintained for a long time by repeating the CO2 electrolysis operation step S102 and the refresh operation step S105 in accordance with the request criteria of the cell performance. - As mentioned above, in the carbon dioxide electrolysis device of this embodiment, the refresh operation of the electrolysis cell is performed by temporarily flowing the rinse solution through the flow path when salts precipitate, which can prevent the blockage of the flow path. Therefore, the decrease in the electrolysis efficiency of the carbon dioxide electrolysis device can be prevented.
- When performing the refresh operation, the
region 122 a 1 with the hydrophilicinner wall surface 246 and theregion 122 a 2 with the water-repellentinner wall surface 247 are formed in the cathodegas flow path 122 a as illustrated inFIG. 7 , which enables the rinse solution to easily bypass the salts and flow through theregion 122 a 2, which is near the salts and therefore the salts are easily dissolved, because the hydrophilicinner wall surface 246 is near the salt even if the salts precipitate. In addition, the rinse solution flows into theregion 122 a 2 until the salts are dissolved and can be supplied to the entire surface of thecathode 121, thus efficiently removing the salts. - A carbon dioxide electrolysis device was fabricated as follows. Iridium oxide was formed on a surface of titanium mesh as an oxidation catalyst. Carbon paper with a catalyst layer was fabricated by spraying carbon, which supports 10.2% by mass gold on the carbon paper with MPL. A membrane electrode composite (catalyst area 4 cm square) was prepared by laminating this carbon paper and titanium mesh with iridium oxide sandwiched by ion-exchange membranes.
- A cathode gas flow path and an anode solution flow path were formed of titanium and were each a serpentine-shaped flow path containing two parallel-connected flow path parts, with a land width of 0.8 mm, a flow path width W of 1 mm, and a flow path depth h of 3 mm. An aspect ratio was 3, and a fluid mean depth M was 0.38. An electrolysis cell was assembled by sandwiching the membrane electrode composite between the anode solution flow path and the cathode gas flow path.
- A 0.1 M potassium hydrogen carbonate solution was supplied to the anode solution flow path at 10 mL/min as an electrolytic solution. Carbon dioxide gas was supplied to the cathode gas flow path at a flow rate of 320 ccm. A current was passed between the anode and the cathode with a stepwise increase in a current value, and gas generated from a cathode side was collected to measure its flow rate and conversion efficiency of carbon dioxide. The gas generated was sampled and identified and quantified by gas chromatography.
- The current value at this time was measured with an ammeter. A partial current density of carbon monoxide (CO), which is an indicator of the percentage used for carbon monoxide production out of a total current density flowed, was found from a conversion efficiency from carbon dioxide to carbon monoxide. Furthermore, a utilization ratio of carbon dioxide at the cathode was found from the conversion efficiency from carbon dioxide to carbon monoxide and a flow rate of the gas generated from the cathode side. A relationship between the partial current density of carbon monoxide and the utilization ratio of carbon dioxide at the cathode was evaluated in accordance with the above.
- In the carbon dioxide electrolysis device of Example 1, the electrolysis cell was assembled in the same way as in Example 1, except that the cathode gas flow path was made with the land width of 0.8 mm, the flow path width W of 1 mm, and the flow path depth h of 0.5 mm. The cathode gas flow path had the aspect ratio of 0.5 and the fluid mean depth M of 0.17. The relationship between the partial current density of carbon monoxide and the utilization ratio of carbon dioxide at the cathode was evaluated as in Example 1.
- In the carbon dioxide electrolysis device of Example 1, the electrolysis cell was assembled in the same way as in Example 1, except that the cathode gas flow path was made with the land width of 0.8 mm, the flow path width W of 1 mm, and the flow path depth h of 1 mm. The cathode gas flow path had the aspect ratio of 1 and the fluid mean depth of 0.25. The relationship between the partial current density of carbon monoxide and the utilization ratio of carbon dioxide at the cathode was evaluated as in Example 1.
- In the carbon dioxide electrolysis device of Example 1, the electrolysis cell was assembled in the same way as in Example 1, except that the cathode gas flow path was made with the land width of 0.8 mm, the flow path width W of 1 mm, and the flow path depth h of 2 mm. The cathode gas flow path had the aspect ratio of 2 and the fluid mean depth of 0.33. The relationship between the partial current density of carbon monoxide and the utilization ratio of carbon dioxide at the cathode was evaluated as in Example 1.
- In the carbon dioxide electrolysis device of Example 1, the electrolysis cell was assembled in the same way as in Example 1, except that the cathode gas flow path was made with the land width of 0.49 mm, the flow path width W of 0.49 mm, and the flow path depth h of 1 mm. The cathode gas flow path had the aspect ratio of 2 and the fluid mean depth of 0.16. The relationship between the partial current density of carbon monoxide and the utilization ratio of carbon dioxide at the cathode was evaluated as in Example 1.
-
FIG. 11 presents the relationships between the partial current density of carbon monoxide and the utilization ratio of carbon dioxide at the cathode in Example 1, Comparative Example 1, Comparative Example 2, and Comparative Example 3. -
FIG. 12 presents the relationships between the partial current density of carbon monoxide and the utilization ratio of carbon dioxide at the cathode in Comparative Example 3 and Comparative Example 4. - From
FIG. 11 , it can be seen that when the aspect ratio of the cathode gas flow path is greater than 1 and 3 or less, and the fluid mean depth M and depth h of the cathode gas flow path satisfy a formula: h/8≤M<h/4, a high CO2 utilization ratio of 30% or more can be achieved at a high CO partial current density of 400 mA/cm2 or more. It can also be seen fromFIG. 12 that even if the aspect ratios are the same, the larger the fluid mean depth M is, the higher the CO2 utilization ratio can be obtained at the high the CO partial current density. - The above embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, those embodiments 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 forms or modifications as would fall within the scope and spirit of the inventions.
Claims (7)
1. A carbon dioxide electrolysis device, comprising:
a cathode configured to reduce carbon dioxide and thus form a carbon compound;
an anode configured to oxidize water and thus generate oxygen;
a cathode gas flow path facing on the cathode and configured to supply gas containing carbon dioxide;
an anode solution flow path facing on the anode and configured to supply an electrolytic solution containing water; and
a separator provided between the anode and the cathode, wherein
an aspect ratio of the cathode gas flow path is greater than 1 and 3 or less, the aspect ratio being defined by a ratio of a depth of the cathode gas flow path to a width of the cathode gas flow path, and
in a cross-section along a direction perpendicular to a facing surface between the cathode and the cathode gas flow path in the cathode gas flow path, a fluid mean depth M of the cathode gas flow path and a depth h of the cathode gas flow path satisfy a formula: h/8≤M<h/4, the fluid mean depth M being defined by a ratio of a circumferential length of the cathode gas flow path to a cross-sectional area of the cathode gas flow path.
2. The device according to claim 1 , wherein
the cathode gas flow path includes:
a first region facing on the cathode; and
a second region provided between the first region and an inner bottom surface of the cathode gas flow path, wherein
a width of the second region is wider than a width of the first region.
3. The device according to claim 1 , wherein
the cathode gas flow path includes:
a first region facing on the cathode and having a hydrophilic first inner wall surface; and
a second region provided between the first region and an inner bottom surface of the cathode gas flow path and having a water-repellent second inner wall surface.
4. The device according to claim 1 , wherein
the electrolytic solution contains a metal ion.
5. The device according to claim 1 , wherein
the cathode contains at least one catalyst selected from the group consisting of copper, gold, and silver.
6. The device according to claim 1 , further comprising:
an electrolysis cell including the cathode, the anode, the cathode gas flow path, the anode solution flow path, and the separator;
a gas supply configured to supply the gas to the cathode gas flow path;
a solution supply configured to supply the electrolytic solution to the anode solution flow path;
a power supply configured to apply a voltage between the anode and the cathode;
a refresh material supply including a solution supply source configured to supply a rinse solution to the cathode gas flow path; and
a controller configured to control operations of stopping the supply of the gas by the gas supply, stopping the supply of the electrolytic solution by the solution supply, and supplying the rinse solution to the cathode by the refresh material supply in accordance with request criteria of performance of the electrolysis cell.
7. A method of operating a carbon dioxide electrolysis device,
the carbon dioxide electrolysis device including:
a cathode configured to reduce carbon dioxide and thus form a carbon compound;
an anode configured to oxidize water and thus generate oxygen;
a cathode gas flow path facing on the cathode and configured to supply gas containing carbon dioxide;
an anode solution flow path facing on the anode and configured to supply an electrolytic solution containing water; and
a separator provided between the anode and the cathode, wherein
an aspect ratio of the cathode gas flow path is greater than 1 and 3 or less, the aspect ratio being defined by a ratio of a depth of the cathode gas flow path to a width of the cathode gas flow path, and
in a cross-section along a direction perpendicular to a facing surface between the cathode and the cathode gas flow path in the cathode gas flow path, a fluid mean depth M of the cathode gas flow path and a depth h of the cathode gas flow path satisfy a formula: h/8≤M<h/4, the fluid mean depth M being defined by a ratio of a circumferential length of the cathode gas flow path to a cross-sectional area of the cathode gas flow path,
the method comprising:
supplying gas containing carbon dioxide to the cathode gas flow path and supplying an electrolytic solution to the anode solution flow path;
applying a voltage between the anode and the cathode to reduce carbon dioxide near the cathode of the electrolysis cell to form a carbon compound and to oxidize water or hydroxide ions near the anode to generate oxygen; and
stopping the supply of the gas and the electrolytic solution and supplying a rinse solution to the cathode gas flow path, in accordance with request criteria of performance of the electrolysis cell.
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