ELECTROLYSIS DEVICE AND ELECTROLYSIS METHOD

Abstract

An electrolysis device of an embodiment includes: an electrolysis cell including a cathode part in which a reduction electrode is disposed, an anode part in which an oxidation electrode is disposed, and a diaphragm provided between the cathode part and the anode part; a supply power property obtaining unit that obtains a property of power that is to be supplied to the electrolysis cell; an input gas property obtaining unit that obtains a property of a gas that is to be input to the electrolysis cell; an electric property obtaining unit that obtains an electric property of the electrolysis cell; an output gas property obtaining unit that obtains a property of an output gas of the electrolysis cell; a temperature obtaining unit that obtains a temperature of the electrolysis cell; a data storage unit that stores data from the obtaining units; and a data processing unit to which the data is sent from the data storage unit and that processes the data to determine a state of the electrolysis cell.

Description

CROSS REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2022-147380, filed on Sep. 15, 2022, the entire contents of which are incorporated herein by reference.

FIELD

Embodiments disclosed herein relate to an electrolysis device and an electrolysis method.

BACKGROUND

There has been a concern about the depletion of fossil fuels such as petroleum and coal, and expectations are increasing for sustainable renewable energy. Examples of the renewable energy include those by solar power generation, hydroelectric power generation, wind power generation, and geothermal power generation. The amount of powers generated by these depends on weather, nature conditions, and so on and thus they are power sources whose outputs vary (variable power sources) and have a problem of difficulty in stably supplying the power. In light of this, it has been attempted to adjust power by combining a variable power source and a storage battery. Storing the power, however, has problems of the cost of the storage battery and loss during the power storage.

Also drawing attention as decarbonization attempts are: water electrolysis technology that electrolyzes water (H₂O) to produce hydrogen (H₂); and carbon dioxide electrolysis technology that electrolyzes carbon dioxide (CO₂) and electrochemically reduces it to convert it to a chemical substance (chemical energy) such as a carbon compound such as carbon monoxide (CO), formic acid (HCOOH), methanol (CH₃OH), methane (CH₄), acetic acid (CH₃COOH), ethanol (C₂H₅OH), ethane (C₂H₆), or ethylene (C₂H₄). Connecting a variable power source that uses renewable energy to such an electrolysis device is advantageous in that power adjustment and hydrogen production, and carbon dioxide recycling are achieved at the same time.

As a carbon dioxide electrolysis device, a structure is under consideration, for example, in which a catholyte and a CO₂ gas are in contact with a cathode and an anolyte is in contact with an anode. Such a structure will be called a carbon dioxide electrolysis cell here. For example, if a reaction of producing, for example, CO from CO₂ is caused for a long time using such an electrolysis cell by passing a constant current to the cathode and the anode, there arise problems of time-dependent deterioration in cell output such as a decrease in the amount of CO produced and an increase in cell voltage. One example of the deterioration is a phenomenon that salt originating in an electrolyte of the solution precipitates in a gas channel to obstruct the flow of the gas or the like, against which the introduction of a refresh operation to dissolve the salt is under consideration.

However, it is becoming clear that the electrolysis device of carbon dioxide (CO₂) or the like undergoes various deterioration phenomena in addition to the phenomenon of the precipitation of salt in the channel. Further, an electrolysis device of, for example, nitrogen (N₂) undergoes a deterioration phenomenon unique to N₂ electrolysis, in addition to the same deterioration phenomena as that in the CO₂ electrolysis. This necessitates appropriately setting a determination standard according to the types of an electrolyte and deterioration, such as continuing the operation or executing a work of stopping the operation and performing the maintenance of the electrolysis cell according to an electrolyte or a deteriorated place. Therefore, it is required to determine cell states such as the state and type of the deterioration of the electrolysis cell.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating an electrolysis device of an embodiment.

FIG. 2 is a view illustrating an electrolysis cell in a carbon dioxide electrolysis device of a first embodiment.

FIG. 3 is a chart illustrating a deterioration detection process by the carbon dioxide electrolysis device of the first embodiment.

FIG. 4 is a diagram illustrating an equivalent circuit model of the electrolysis cell in the carbon dioxide electrolysis device of the first embodiment.

FIG. 5 is a table illustrating equivalent circuit parameters of the equivalent circuit model illustrated in FIG. 4.

FIG. 6 is a view illustrating an electrolysis cell in a carbon dioxide electrolysis device of a second embodiment.

FIG. 7 is a diagram illustrating an equivalent circuit model of an electrolysis cell in a carbon dioxide electrolysis device of a third embodiment.

FIG. 8 is a table illustrating equivalent circuit parameters of the equivalent circuit model illustrated in FIG. 7.

FIG. 9 is a diagram illustrating an equivalent circuit model of an electrolysis cell in a nitrogen electrolysis device of a fourth embodiment.

FIG. 10 is a chart illustrating a design process of an electrolysis device according to a fifth embodiment.

FIG. 11 is a chart illustrating measurement data and simulation data of CO part current density J_CO and H₂ part current density J_H2 according to Example 1.

FIG. 12 is a chart illustrating measurement data and simulation data of cell voltage V_cell, cathode potential V_cm, and anode potential V_am according to Example 1.

FIG. 13 is a chart illustrating measurement data and simulation data of CO Faraday efficiency FE_CO and H₂ Faraday efficiency H₂ according to Example 1.

FIG. 14 is a chart illustrating measurement data and simulation data of cathode output gases (CO, H₂, and CO₂) according to Example 1.

FIG. 15 is a chart illustrating measurement data and simulation data of anode output gases (O₂ and CO₂) according to Example 1.

DETAILED DESCRIPTION

An electrolysis device of an embodiment includes: an electrolysis cell including a cathode part to be supplied with a gas or a liquid containing a substance to be reduced and in which a reduction electrode is disposed, an anode part to be supplied with a liquid containing a substance to be oxidized and in which an oxidation electrode is disposed, and a diaphragm provided between the cathode part and the anode part; a supply power property obtaining unit that obtains a property of power that is to be supplied to the electrolysis cell; an input gas property obtaining unit that obtains a property of a gas that is to be input to the electrolysis cell; an electric property obtaining unit that obtains an electric property of the electrolysis cell; an output gas property obtaining unit that obtains a property of an output gas of the electrolysis cell; a temperature control unit that controls a temperature of the electrolysis cell; a temperature obtaining unit that obtains the temperature of the electrolysis cell; a data storage unit that stores data from the supply power property obtaining unit, the input gas property obtaining unit, the electric property obtaining unit, the output gas property obtaining unit, and the temperature obtaining unit; and a data processing unit to which the data is sent from the data storage unit and that processes the data to determine a state of the electrolysis cell.

Electrolysis devices and electrolysis methods of embodiments will be hereinafter described with reference to the drawings. In the embodiments below, substantially the same constituent parts are denoted by the same reference signs and a description thereof may be partly omitted. The drawings are schematic, and the relation of thickness and planar dimension, a thickness ratio among parts, and so on may be different from actual ones.

FIG. 1 is a diagram illustrating an electrolysis device 1 of an embodiment. The electrolysis device 1 illustrated in FIG. 1 includes an electrolysis cell 2; a supply power control unit 3 that controls power that is to be supplied to the electrolysis cell 2; a supply power property obtaining unit 4 that obtains the properties of the supply power; a gas/electrolysis solution control unit 5 that controls a gas and an electrolysis solution that are to be supplied to the electrolysis cell 2; an input gas property obtaining unit 6 that obtains the properties of the input gas that is to be supplied; an electric property obtaining unit 7 that obtains the electric properties of the electrolysis cell 2; an output gas property obtaining unit 8 that obtains the properties of an output gas of the electrolysis cell 2; a temperature control unit 9 that controls the temperature of the electrolysis cell 2; a temperature obtaining unit 10 that obtains the temperature of the electrolysis cell 2; a data storage unit 11 that stores data from the supply power property obtaining unit 4, the input gas property obtaining unit 6, the electric property obtaining unit 7, the output gas property obtaining unit 8, and the temperature obtaining unit 10; a data processing unit 12 to which the data is transmitted from the data storage unit 11 and that processes the transmitted data; and a display unit 13. These parts will be described in detail below.

The electrolysis cell 2, which has a structure appropriate for a substance that is to be electrolyzed by the electrolysis device 1, includes at least a reduction electrode chamber that is supplied with a gas or a liquid containing a substance to be reduced and in which a reduction electrode is disposed, an oxidation electrode chamber that is supplied with a liquid containing a substance to be oxidized and in which an oxidation electrode is disposed, and a diaphragm provided between the reduction electrode chamber and the oxidation electrode chamber. Examples of the substance that is to be electrolyzed by the electrolysis device 1 include carbon dioxide (CO₂), nitrogen (N₂), and water (H₂O). Through the electrolysis and reduction of CO₂, a carbon compound such as carbon monoxide (CO), formic acid (HCOOH), methane (CH₄), methanol (CH₃OH), ethane (C₂H₆), ethylene (C₂H₄), ethanol (C₂H₅OH), formaldehyde (HCHO), or ethylene glycol (C₂H₆O₂) is produced. At the same time with the reduction reaction of CO₂, hydrogen (H₂) is sometimes produced through a reduction reaction of H₂O. In the case where N₂ is electrolyzed to be reduced, ammonia (NH₃) is produced.

First Embodiment

As a first embodiment, an electrolysis device 1 of carbon dioxide (CO₂) will be described with reference to FIG. 1 and FIG. 2. As illustrated in FIG. 2, out of the electrolysis cell 2 illustrated in FIG. 1, an electrolysis cell 2 (2A) for electrolyzing CO₂ includes a cathode part (reduction electrode chamber) 24 having a first storage part (storage tank) 22 for storing a first electrolysis solution 21 containing CO₂ and a reduction electrode (cathode) 23 disposed in the first storage part 22, an anode part (oxidation electrode chamber) 28 having a second storage part (storage tank) 26 for storing a second electrolysis solution 25 containing water and an oxidation electrode (anode) 27 disposed in the second storage part 26, and a diaphragm 29 disposed between the first storage part 22 and the second storage part 26. The first storage part 22, the second storage part 26, and the diaphragm 29 form a reaction tank 30.

The reaction tank 30 is divided into two chambers, the first storage part 22 and the second storage part 26, by the diaphragm 29 allowing ions such as hydrogen ions (H⁺), hydroxide ions (OH⁻), hydrogen carbonate ions (HCO₃⁻), and carbonate ions (CO₃⁻) to move therethrough. The reaction tank 30 may be formed of, for example, a white quartz glass plate, an acrylic resin (PMMA), polystyrene (PS), or the like. The reaction tank 30 may be partly formed of a light-transmitting material and the other part thereof may be formed of a resin material. Examples of the resin material include polyether ether ketone (PEEK), polyamide (PA), polyvinylidene fluoride (PVDF), polyacetal (POM) (copolymer), polyphenylene ether (PPE), an acrylonitrile-butadiene-styrene copolymer (ABS), polypropylene (PP), and polyethylene (PE).

In the first storage part 22, the reduction electrode 23 is disposed and CO₂ is further stored. In the first storage part 22, CO₂ is stored, for example, as the first electrolysis solution 21 containing the same. The first electrolysis solution 21 functions as a reduction electrode solution (catholyte) and contains carbon dioxide (CO₂) as the substance to be reduced. Here, CO₂ present in the first electrolysis solution 21 need not be gaseous and may be in a dissolved form or may be in the form of carbonate ions (CO₃²⁻), hydrogen carbonate ions (HCO₃⁻), or the like. The first electrolysis solution 21 may contain hydrogen ions, and it is preferably an aqueous solution. In the second storage part 26, the oxidation electrode 27 is disposed and the second electrolysis solution 25 containing water is further stored. The second electrolysis solution 25 functions as an oxidation electrode solution (anolyte) and contains, as the substance to be oxidized, water (H₂O), chloride ions (Cl⁻), carbonate ions (CO₃²⁻), hydrogen carbonate ions (HCO₃⁻), or the like, for instance. The second electrolysis solution 25 may be an alcohol aqueous solution or an aqueous solution of an organic substance such as amine.

Varying the amounts of water contained in the first and second electrolysis solutions 21, 25 or changing the electrolysis solution components can change reactivity to change the selectivity of the substance to be reduced or a ratio of produced chemical substances. The first and second electrolysis solutions 21, 25 may contain a redox couple as required. Examples of the redox couple include Fe³⁺/Fe²⁺ and IO³⁻/I⁻. The first storage part 22 is connected to a gas supply channel 31 that supplies a source gas containing CO₂ and a first solution supply channel 32 that supplies the first electrolysis solution 21 and is further connected to a first gas and solution discharge channel 33 that discharges a reaction gas and the first electrolysis solution 21. The second storage part 26 is connected to a second solution supply channel 34 that supplies the second electrolysis solution 25 and is further connected to a second gas and solution discharge channel 35. The first and second storage parts 22, 26 may each have a space for storing gases contained in a reactant and a product.

The pressures in the first and second storage parts 22, 26 are preferably set to pressures at which CO₂ does not liquefy. Specifically, their pressures are preferably adjusted to, for example, a range of not lower than 0.1 MPa nor higher than 6.4 MPa. If the pressures in the storage parts 22, 26 are lower than 0.1 MPa, the efficiency of the CO₂ reduction reaction may decrease. If the pressures in the storage parts 22, 26 exceed 6.4 MPa, CO₂ liquefies, and the efficiency of the CO₂ reduction reaction may decrease. A differential pressure between the first storage part 22 and the second storage part 26 may cause the breakage or the like of the diaphragm 29. Therefore, the difference between the pressures of the first storage part 22 and the second storage part 26 (differential pressure) is preferably 1 MPa or lower.

The lower the temperatures of the electrolysis solutions 21, 25, the larger the dissolution amount of CO₂, but low temperatures result in high solution resistance and a high theoretical voltage of the reaction and thus are disadvantageous from a viewpoint of the CO₂ electrolysis. On the other hand, high temperatures of the electrolysis solutions 21, 25 result in a small dissolution amount of CO₂ but are advantageous from a viewpoint of the CO₂ electrolysis. Therefore, the operating temperature condition of the electrolysis cell 2A is preferably in a mid-temperature range, for example, in a range of not lower than the atmospheric temperature nor higher than the boiling points of the electrolysis solutions 21, 25. In the case where the electrolysis solutions 21, 25 are aqueous solutions, a temperature of not lower than 10° C. nor higher than 100° C. is preferable and a temperature of not lower than 25° C. nor higher than 80° C. is more preferable. The operation under higher temperatures is allowed in the case where the source gas containing CO₂ is filled in the first storage part 22 and water vapor is filled in the second storage part 26. In this case, the operating temperature is decided in consideration of the heat resistance of members such as the diaphragm 29. In the case where the diaphragm 29 is an ion exchange membrane or the like, the maximum operating temperature is 180° C., and in the case where the diaphragm 29 is a polymeric porous membrane such as Teflon (registered trademark), the maximum temperature is 300° C.

The first electrolysis solution 21 and the second electrolysis solution 25 may be electrolysis solutions containing different substances or may be the same electrolysis solutions containing the same substance. In the case where the first electrolysis solution 21 and the second electrolysis solution 25 contain the same substance and the same solvent, the first electrolysis solution 21 and the second electrolysis solution 25 may be regarded as one electrolysis solution. Further, pH of the second electrolysis solution 25 may be higher than pH of the first electrolysis solution 21. This facilitates the movement of ions such as hydrogen ions or hydroxide ions through the diaphragm 29. This also achieves the effective progress of the redox reaction owing to a liquid junction potential due to the pH difference.

The first electrolysis solution 21 is preferably a solution high in CO₂ absorptance. CO₂ does not necessarily have to be in a dissolved form in the first electrolysis solution 21, and CO₂ in a bubble form may be mixed and present in the first electrolysis solution 21. Examples of the electrolysis solution containing CO₂ include aqueous solutions containing hydrogen carbonate or carbonate such as lithium hydrogen carbonate (LiHCO₃), sodium hydrogen carbonate (NaHCO₃), potassium hydrogen carbonate (KHCO₃), cesium hydrogen carbonate (CsHCO₃), sodium carbonate (Na₂CO₃), and potassium carbonate (K₂CO₃), phosphoric acid, boric acid, or the like. The electrolysis solution containing CO₂ may contain any of alcohols such as methanol, ethanol, and acetone or may be an alcohol solution. The first electrolysis solution 21 may be an electrolysis solution containing a CO₂ absorbent that lowers the reduction potential of CO₂, has high ion conductivity, and absorbs CO₂.

The second electrolysis solution 25 may be a solution containing water (H₂O), for example, an aqueous solution containing a desired electrolyte. This solution is preferably an aqueous solution that promotes an oxidation reaction of water. Examples of the aqueous solution containing the electrolyte include aqueous solutions containing phosphate ions (PO₄³⁻), borate ions (BO₃³⁻), sodium ions (Na⁺), potassium ions (K⁺), calcium ions (Ca²⁺), lithium ions (Li⁺), cesium ions (Cs⁺), magnesium ions (Mg²⁺), chloride ions (Cl⁻), hydrogen carbonate ions (HCO₃⁻), carbonate ions (CO₃²⁻), hydroxide ions (OH⁻), or the like.

As the aforesaid electrolysis solutions 21, 25, an ionic liquid that is composed of salt of cations such as imidazolium ions or pyridinium ions and anions such as BF₄⁻ or PF₆⁻ and that is in a liquid form in a wide temperature range, or an aqueous solution thereof is usable, for instance. Other examples of the electrolysis solutions include solutions of amine such as ethanolamine, imidazole, or pyridine and aqueous solutions thereof. Examples of the amine include primary amine, secondary amine, and tertiary amine. These electrolysis solutions may be high in ion conductivity, have properties of absorbing carbon dioxide, and have characteristics of decreasing reduction energy.

Examples of the primary amine include methylamine, ethylamine, propylamine, butylamine, pentylamine, and hexylamine. A hydrocarbon of the amine may be replaced by alcohol, halogen, or the like. Examples of the amine whose hydrocarbon is replaced include methanolamine, ethanolamine, and chloromethylamine. Further, an unsaturated bond may be present therein. The same applies to hydrocarbons of the secondary amine and the tertiary amine.

Examples of the secondary amine include dimethylamine, diethylamine, dipropylamine, dibutylamine, dipentylamine, dihexylamine, dimethanolamine, diethanolamine, and dipropanolamine. The replaced hydrocarbons may be different. This also applies to the tertiary amine. Examples of one whose hydrocarbons are different include methylethylamine and methylpropylamine.

Examples of the tertiary amine include trimethylamine, triethylamine, tripropylamine, tributylamine, trihexylamine, trimethanolamine, triethanolamine, tripropanolamine, tributanolamine, triexanolamine, methyldiethylamine, and methyldipropylamine.

Examples of the cations of the ionic liquid include 1-ethyl-3-methylimidazolium ions, 1-methyl-3-propylimidazolium ions, 1-butyl-3-methylimidazolium ions, 1-methyl-3-pentylimidazolium ions, and 1-hexyl-3-methylimidazolium ions.

The second position of the imidazolium ion may be replaced. Examples of the cation resulting from the replacement of the second position of the imidazolium ion include a 1-ethyl-2,3-dimethylimidazolium ion, a 1,2-dimethyl-3-propylimidazolium ion, a 1-butyl-2,3-dimethylimidazolium ion, a 1,2-dimethyl-3-pentylimidazolium ion, and a 1-hexyl-2,3-dimethylimidazolium ion.

Examples of the pyridinium ions include methylpyridinium, ethylpyridinium, propylpyridinium, butylpyridinium, pentylpyridinium, and hexylpyridinium. In the imidazolium ion and the pyridinium ion, an alkyl group may be replaced, or an unsaturated bond may be present.

Examples of the anions include fluoride ions (F), chloride ions (Cl⁻), bromide ions (Br⁻), iodide ions (I⁻), BF₄⁻, PF₆⁻, CF₃COO⁻, CF₃SO₃⁻, NO₃⁻, SCN⁻, (CF₃SO₂)₃C⁻, bis(trifluoromethoxysulfonyl)imide, bis(trifluoroethoxysulfonyl)imide, and bis(perfluoroethylsulfonyl)imide. The ionic liquid may be composed of dipolar ions formed of the cations and the anions that are connected by hydrocarbons. Note that a buffer solution such as a potassium phosphate solution may be supplied to the storage parts 22, 26.

As the diaphragm 29, a membrane selectively allowing the flow of anions or cations is used. Consequently, the electrolysis solutions 21, 25 in contact with the reduction electrode 23 and the oxidation electrode 27 respectively can be electrolysis solutions containing different substances. Further, it is possible to promote a reduction reaction and an oxidation reaction owing to a difference in ionic strength, a difference in pH, and so on. The use of the diaphragm 29 can separate the first electrolysis solution 21 and the second electrolysis solution 25 from each other. The diaphragm 29 may have a function of allowing the permeation of part of the ions contained in the electrolysis solutions 21, 25 in which these electrodes 23, 27 are immersed, that is, a function of shutting off one kind of ions or more contained in the electrolysis solutions 21, 25. As a result, the two electrolysis solutions 21, 25 can be solutions different in pH, for instance.

Examples usable as the diaphragm 29 include ion exchange membranes such as NEOSEPTA (registered trademark) of ASTOM Corporation, SELEMION (registered trademark) of AGC Inc., Aciplex (registered trademark) of AGC Inc., Fumasep (registered trademark) and fumapem (registered trademark) of Fumatech BWT GmbH, Nafion (registered trademark) of DuPont, which is a fluorocarbon resin formed of sulfonated and polymerized tetrafluoroethylene, lewabrane (registered trademark) of LANXESS AG, IONSEP (registered trademark) of IONTECH Inc., Mustang (registered trademark) of PALL Corporation, relax (registered trademark) of MEGA Co., Ltd., and GORE-TEX (registered trademark) of W.L. Gore & Associates GmbH. The ion exchange membrane may be formed using a membrane whose basic structure is hydrocarbons, or in the case of anion exchange, it may be a membrane having an amine group. In the case where the first electrolysis solution 21 and the second electrolysis solution 25 are different in pH, the use of a bipolar membrane in which a cation exchange membrane and an anion exchange membrane are stacked makes it possible to keep pH of the electrolysis solutions stable when they are used.

Examples usable as the diaphragm 29 other than the ion exchange membrane include: porous membranes of a silicone resin, a fluorine-based resin such as perfluoroalkoxyalkane (PFA), a perfluoroethylene-propane copolymer (FEP), polytetrafluoroethylene (PTFE), an ethylene-tetrafluoroethylene copolymer (ETFE), polyvinylidene fluoride (PVDF), polychlorotrifluoroethylene (PCTFE), and an ethylene-chlorotrifluoroethylene copolymer (ECTFE), polyethersulfone (PES), or ceramic; and insulating porous bodies such as a glass filter, a filling filled with agar, zeolite, and oxide. A hydrophilic porous membrane is preferable as the diaphragm 29 because it is not clogged with bubbles.

The reduction electrode 23 is an electrode (cathode) that reduces carbon dioxide (CO₂) to produce a carbon compound. The reduction electrode 23 is disposed in the first storage part 22 to be immersed in the first electrolysis solution 21. The reduction electrode 23 contains a reduction catalyst for producing the carbon compound through the reduction reaction of CO₂, for instance. Examples of the reduction catalyst include a material that lowers activation energy for reducing CO₂. In other words, a material that lowers overvoltage when the carbon compound is produced through the reduction reaction of CO₂ is usable.

As the reduction electrode 23, a metal material or a carbon material is usable, for instance. As the metal material, metal such as gold, aluminum, copper, silver, platinum, palladium, zinc, mercury, indium, nickel, or titanium, or an alloy containing the aforesaid metal is usable, for instance. As the carbon material, graphene, carbon nanotube (CNT), fullerene, ketjen black, or the like is usable, for instance. The reduction catalyst is not limited to these, and a metal complex such as a Ru complex or a Re complex, or an organic molecule having an imidazole skeleton or a pyridine skeleton may be used, for instance. The reduction catalyst may be a mixture of a plurality of materials. The reduction electrode 23 may have a structure in which the reduction catalyst in a thin film form, a lattice form, a granular form, a wire form, or the like is provided on a conductive base material, for instance.

The carbon compound produced through the reduction reaction in the reduction electrode 23 differs depending on the kind of the reduction catalyst and so on, and examples thereof include carbon monoxide (CO), formic acid (HCOOH), methane (CH₄), methanol (CH₃OH), ethane (C₂H₆), ethylene (C₂H₄), ethanol (C₂H₅OH), formaldehyde (HCHO), and ethylene glycol (C₂H₆O₂). The reduction electrode 23 sometimes causes a side reaction to produce hydrogen (H₂) through a reduction reaction of water (H₂O) simultaneously with the reduction reaction of carbon dioxide (CO₂).

The oxidation electrode 27 is an electrode (anode) that oxidizes the substance to be oxidized such as the substance or ions contained in the second electrolysis solution 25. For example, it oxidizes water (H₂O) to produce oxygen or a hydrogen peroxide solution or oxidizes chloride ions (Cl⁻) to produce chlorine. The oxidation electrode 27 is disposed in the second storage part 26 to be immersed in the second electrolysis solution 25. The oxidation electrode 27 contains an oxidation catalyst for the substance to be oxidized. As the oxidation catalyst, used is a material that decreases activation energy for the oxidation of the substance to be oxidized, in other words, a material that lowers reaction overpotential.

Examples of such an oxidation catalyst material include metals such as ruthenium, iridium, platinum, cobalt, nickel, iron, and manganese. Further, binary metal oxide, ternary metal oxide, quaternary metal oxide, or the like is usable. Examples of the binary metal oxide include 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), and ruthenium oxide (Ru—O). Examples of the ternary metal oxide include Ni—Fe—O, Ni—Co—O, La—Co—O, Ni—La—O, and Sr—Fe—O. Examples of the quaternary metal oxide include Pb—Ru—Ir—O and La—Sr—Co—O. The oxidation catalyst is not limited to these and may be metal hydroxide containing cobalt, nickel, iron, manganese, or the like or a metal complex such as a Ru complex or an Fe complex. A mixture of a plurality of materials may also be used.

The oxidation electrode 27 may be formed of a composite material containing both the oxidation catalyst and a conductive material. Examples of the conductive material include carbon materials such as carbon black, activated carbon, fullerene, carbon nanotube, graphene, ketjen black, and diamond, transparent conductive oxides such as indium tin oxide (ITO), zinc oxide (ZnO), fluorine-doped tin oxide (FTO), aluminum-doped zinc oxide (AZO), and antimony-doped tin oxide (ATO), metals such as Cu, Al, Ti, Ni, Ag, W, Co, and Au, and an alloy containing at least one of these metals. For example, the oxidation electrode 27 may have a structure in which the oxidation catalyst in a thin film form, a lattice form, a granular form, a wire form, or the like is provided on a conductive base material. As the conductive base material, a metal material containing titanium, a titanium alloy, or stainless steel is used, for instance.

The configurations and operations of the data processing unit 12 and so on in the electrolysis device 1 illustrated in FIG. 1 will be described. The supply power control unit 3 controls power for causing a redox reaction in the electrolysis cell 2A and is electrically connected to the reduction electrode 23 and the oxidation electrode 27 of the electrolysis cell 2A. A not-illustrated power source is connected to the supply power control unit 3. In the supply power control unit 3, electric devices such as a DC-AC converter, a DC-DC converter, an AC-DC converter, an inverter, a converter, and switches are installed. The power source connected to the supply power control unit 3 may be a power source that converts renewable energy to electric energy to supply it or may be a typical commercial power source, battery, or the like. Examples of the power source using the renewable energy include a power source that converts kinetic energy or potential energy such as wind power, water power, geothermal energy, or tidal power to electric energy, a power source such as a solar cell having a photoelectric conversion element that converts light energy to electric energy, a power source such as a fuel cell or a storage battery that converts chemical energy to electric energy, and a power source that converts vibrational energy such as sound to electric energy.

The supply power property obtaining unit 4 obtains the properties of the power, that is, a voltage and a current, that is to be supplied to the electrolysis cell 2A. The supply power properties obtained by the supply power property obtaining unit 4 are transmitted to the data storage unit 11 through a signal line. The supply power control unit 3 and the supply power property obtaining unit 4 may be independent structures or may be an integrated structure.

The gas/electrolysis solution control unit 5 controls the flow rates of the CO₂-containing gas and the electrolysis solutions that are to be input to the electrolysis cell 2A. It may further control the dew point, temperature, pressure, and so on of the gas and the pressure, temperature, composition, pH, and so on of the electrolysis solutions. The input gas property obtaining unit 6 obtains the flow rate and composition of the CO₂-containing gas that is to be input to the electrolysis cell 2A. It may have a function of obtaining the properties such as the dew point, temperature, pressure, and so on of the CO₂-containing gas. The obtained input gas properties are transmitted to the data storage unit 11 through a signal line. The gas/electrolysis solution control unit 5 and the input gas property obtaining unit 6 may be independent structures or may be an integrated structure.

The electric property obtaining unit 7 obtains the electric properties such as cell voltage and cell current of the electrolysis cell 2A. To improve the accuracy of equivalent circuit parameters, it is preferable to assemble a reference electrode in the electrolysis cell 2A to obtain the potentials of the cathode 23 and the anode 27 relative to the reference electrode. The electric property obtaining unit 7 may have a function of obtaining the impedance of the electrolysis cell 2A. The electric properties are transmitted to the data storage unit 11 through a signal line.

The output gas property obtaining unit 8 obtains the flow rate of the gas output from the cathode 23 of the electrolysis cell 2A and the concentrations of CO₂ and various gases produced through the CO₂ reduction reaction. It may further have a function of obtaining the concentrations of H₂ and other gases produced through a side reaction. It may also have a function of obtaining the flow rates of O₂ and CO₂ which are output from the anode 27, the concentrations of the gases, and so on. The output gas properties are transmitted to the data storage unit 11 through a signal line.

The temperature control unit 9 controls the temperature of the electrolysis cell 2A to a predetermined value and has a function of controlling the heating by a heater assembled in the electrolysis cell 2A and the flow of a refrigerant to a cooling water channel. The temperature obtaining unit 10 obtains the temperature of the electrolysis cell 2A. The obtained temperature is transmitted to the data storage unit 11 through a signal line. The temperature control unit 9 and the temperature obtaining unit 10 may be independent structures or may be an integrated structure.

The data storage unit 11 includes a control device such as a computer and has a function of storing data in a recording medium such as a memory, a hard disk, or SSD and a data transceiving function. The display unit 13 is a display and has a function of displaying information sent from the data storage unit 11 and a deterioration detecting unit. The data storage unit 11 and the display unit 13 may be independent structures or may be an integrated structure such as a computer.

The data processing unit 12 includes a computer such as PC or a microcomputer, for instance, and based on the data transmitted from the data storage unit 11, calculates an equivalent circuit model and equivalent circuit parameters. The data processing unit 12 performs the inference of a deteriorated place, the calculation of a deterioration degree, and so on using the equivalent circuit parameters. It further determines whether to continue the operation of the CO₂ electrolysis cell 2A, whether to execute its refresh operation, or whether to stop its operation, based on the information of the deteriorated place and the deterioration degree, and transmits commands to the supply power control unit 3, the gas/electrolysis solution control unit 5, and the temperature control unit 9. The data processing unit 12 may be installed near the electrolysis cell 2A or may be installed in the cloud to perform remote diagnosis. Installing the data processing unit 12 in the cloud enables the integrated management of stored data of the electrolysis cells 2A installed at various places to improve the accuracy of deterioration detection. Because of this, the data processing unit 12 is preferably installed in the cloud.

Next, a deterioration detecting method of the electrolysis device 1 will be described with reference to FIG. 3. First, database of an equivalent circuit model and equivalent circuit parameters of the electrolysis cell 2A at design time is created and stored in the data processing unit 12. Another method to create the database is to test-operate the electrolysis cell 2A using the supply power control unit 3 and the gas/electrolysis solution control unit 5, and calculate an equivalent circuit model before real operation, obtain its circuit parameters, and create their database (S1). The real operation of the electrolysis cell (or the electrolysis cell stack) 2 is started, the supply power properties, the input gas properties, and the electric properties, output gas properties, and temperature of the electrolysis cell 2A are obtained, and these data are stored in the data storage unit 11 (S2). These data are transmitted to the data processing unit 12, and the data processing unit 12 performs an arithmetic operation to calculate an equivalent circuit model and equivalent circuit parameters (S3).

The data processing unit 12 compares the equivalent circuit model and its equivalent circuit parameters at the design time or before the real operation which are obtained at S1 with the equivalent circuit model and the equivalent circuit parameters during the real operation to infer a deteriorated place and calculate a deterioration degree. From the development of the deterioration degree, it further predicts a life span up to the stop of the operation (S4). It is determined whether or not the deterioration degree exceeds an operation stop standard (S5), and in the case where the deterioration degree exceeds the operation stop standard, the real operation of the electrolysis cell (or the electrolysis cell stack) 2 is stopped (S6). The maintenance appropriate for the deteriorated place is performed or the electrolysis cell 2A is changed when the operation is stopped (S7). The various property data stored in the data storage unit 11 at S2, the equivalent circuit model and the equivalent circuit parameters which are calculated at S3, and the deteriorated place, the deterioration degree, and the life span up to the operation stop which are calculated at S4 are transmitted to the display unit 13, and are displayed on the display unit 13 (S8).

Next, the operation of the electrolysis device 1 of CO₂ will be described. The description here is about the case where using an aqueous solution containing CO₂ and an aqueous potassium hydrogen carbonate (KHCO₃) solution as the electrolysis solutions 21, 25, mainly carbon monoxide (CO) is produced through the reduction of CO₂ and oxygen is produced through the oxidation of water (H₂O) or hydroxide ions (OH⁻). The reduction reaction of CO₂ is not limited to the reaction of producing CO and may be a reaction of producing C_xH_yO_z, specifically, a carbon compound such as formic acid (HCOOH), methane (CH₄), methanol (CH₃OH), ethane (C₂H₆), ethylene (C₂H₄), ethanol (C₂H₅OH), formaldehyde (HCHO), or ethylene glycol (C₂H₆O₂).

When a voltage equal to or higher than an electrolysis voltage is applied across the reduction electrode (cathode) 23 and the oxidation electrode (anode) 27, a reduction reaction of CO₂ occurs near the reduction electrode 23 in contact with the first electrolysis solution 21. As shown by the following equation (1), electrons (e−) supplied from the power source reduce CO₂ contained in the first electrolysis solution 21 to produce CO and OH⁻. As shown by the equation (2) and the equation (3), part of the produced OH⁻ reacts with CO₂, resulting in the production of hydrogen carbonate ions (HCO₃⁻) or carbonate ions (CO₃²⁻). The voltage across the reduction electrode 23 and the oxidation electrode 27 causes part of OH⁻, HCO₃⁻, and CO₃²⁻ to move into the second electrolysis solution 25 through the diaphragm 29.

2CO₂+2H₂O+4e⁻→2CO+4OH⁻  (1)

2CO₂+2OH⁻→2HCO₃⁻  (2)

2HCO₃⁻+2OH⁻→CO₃²⁻+H₂O  (3)

Near the oxidation electrode 27 in contact with the second electrolysis solution 25, an oxidation reaction of water (H₂O) occurs. As shown by the following equation (4), the oxidation reaction of H₂O contained in the second electrolysis solution 25 occurs, electrons are lost, and oxygen (O₂) and hydrogen ions (H⁺) are produced.

2H₂O→4H+O₂+4e⁻  (4)

As shown by the following equation (5) to equation (7), part of the produced hydrogen ions (H⁺) reacts with part of hydroxide ions (OH⁻), hydrogen carbonate ions (HCO₃⁻), or carbonate ions (CO₃²⁻) which have moved through the diaphragm 29, resulting in the production of H₂O and CO₂.

2H⁺+CO₃²⁻→H₂O+CO₂  (5)

2H⁺+2HCO₃⁻→2H₂O+2CO₂  (6)

H⁺+OH⁻→H₂O  (7)

The above describes the operation based on the production of OH⁻ in the reduction electrode 23, but the operation may be based on the production and movement of H⁺ in the oxidation electrode 27. When a voltage equal to or higher than the electrolysis voltage is applied across the reduction electrode 23 and the oxidation electrode 27, an oxidation reaction of water (H₂O) occurs near the oxidation electrode 27 in contact with the second electrolysis solution 25. As shown by the following equation (8), the oxidation reaction of H₂O contained in the second electrolysis solution 25 occurs, electrons are lost, and oxygen (O₂) and hydrogen ions (H⁺) are produced. The produced hydrogen ions (H⁺) partly move into the first electrolysis solution 21 through the diaphragm 29.

2H₂O→4H⁺+O₂+4e⁻  (8)

When the hydrogen ions (H⁺) produced in the oxidation electrode 27 side reach the vicinity of the reduction electrode 23 and electrons (e⁻) are supplied to the reduction electrode 23 from the power source, a reduction reaction of carbon dioxide (CO₂) occurs. As shown by the following equation (9), the hydrogen ions (H⁺) having moved to the vicinity of the reduction electrode 23 and the electrons (e⁻) supplied from the power source reduce CO₂ contained in the first electrolysis solution 21 to produce carbon monoxide (CO).

2CO₂+4H⁺+4e⁻→2CO+2H₂O  (9)

The data storage unit 11 illustrated in FIG. 1 is configured to store data from the supply power property obtaining unit 4, the input gas property obtaining unit 6, the electric property obtaining unit 7, the output gas property obtaining unit 8, and the temperature obtaining unit 10. The data processing unit 12 is configured to receive the aforesaid data from the data storage unit 11 and process the data to determine the state of the electrolysis cell 2A. Specifically, the data processing unit 12 calculates the equivalent circuit model and the equivalent circuit parameters of the electrolysis cell 2A based on the processing results of the data and determines the state of the electrolysis cell 2A based on the calculation results of the equivalent circuit model and the equivalent circuit parameters. One specific example of the determination of the state of the electrolysis cell 2A is to infer a deteriorated place or the like of the electrolysis cell 2A, and another specific example thereof is to calculate a deterioration degree of the deteriorated place.

Next, a method of calculating the equivalent circuit model and the equivalent circuit parameters, a method of inferring the deteriorated place, and a method of calculating the deterioration degree by the data processing unit 12 will be described with reference to FIG. 4. FIG. 4 illustrates an example of the equivalent circuit model of the CO₂ electrolysis cell 2A. In the description here, the case where a CO₂ reduction product (C_xH_yO_z) is produced through the reduction reaction of CO₂ will be taken as an example. In the case where CO is produced, C_xH_yO_z can be replaced by CO. In a cathode part 24 in the equivalent circuit model, a C_xH_yO_z producing part and a side reaction H₂ producing part are connected in parallel, and cathode resistance is connected in series to them. A diaphragm part 29 has diaphragm resistance. In an anode part 28, an O₂ producing part and anode resistance are connected in series. The cathode part 24, the diaphragm part 29, and the anode part 29 are connected in series.

The current densities J_A of the C_xH_yO_z producing part, the H₂ producing part, and the O₂ producing part are represented by the Tafel equation in the following formula (10), for instance.

$\begin{array}{cc}{J}_A=J_{0,A}\exp \left[-\frac{\ln \left(10\right)}{B_A}{\eta }_A\right]& \left(10\right)\end{array}$

In the subscript A, C_xH_yO_z ER (C_xH_yO_z Evolution Reaction) is entered in the case of C_xH_yO_z production, HER (Hydrogen Evolution Reaction) is entered in the case of H₂ production, and OER (Oxygen Evolution Reaction) is entered in the case of O₂ production. J₀, A represents exchange current density, B_A represents Tafel slope, and η_A represents overvoltage. Note that J_{0, A} and B_A are parameters that vary with temperature T. The cathode resistance is represented by R_cathode, the diaphragm resistance is represented by R_membrane, the anode resistance is represented by R_anode. These are parameters that vary with temperature.

In the case where the flow rate of CO₂ introduced to the CO₂ electrolysis cell 2A is high enough, the formula (10) is usable. In the case where the CO₂ flow rate is low and consideration is given to that the C_xH_yO_z production current density is restricted by the CO₂ flow rate, that is, has a limit, the current density J_{CxHyOz ER include low CO2} of the C_xH_yO_z producing part is represented by a relational formula including, as variables, the Tafel equation in the formula (10) and the C_xH_yO_z production limit current density J_{CxHyOz ER, L} as shown by the following formula (11). Here, f1 indicates a function.

J_{CxHyOz ER include low CO2}=f₁(J_{CxHyOz ER}, J_{CxHyOz ER, L})  (11)

As shown by the equation (2), the equation (3), the equation (5), and the equation (6), CO₂ in the cathode side is converted to HCO₃⁻ and CO₃²⁻ and they move to the anode side and are converted again to CO₂. Here, the flow rate of CO₂ moving from the cathode side to the anode side as a result of the aforesaid ion movement is represented by flow_CO2 from cathode to anode. Since the flow rate of CO₂ usable for the production of C_xH_yO_z decreases by an amount corresponding to the flow rate of CO₂ moving from the cathode side to the anode side, the C_xH_yO_z production limit current density J_{CxHyOz ER, L} is represented by a relational formula including, as variables, the flow rate flow_{CO2 cathode, input} of CO₂ introduced to the cathode part and the flow rate flow_{CO2 from cathode to anode} of CO₂ moving from the cathode side to the anode side, as shown by the following formula (12). Here, f2 indicates a function.

J_{CxHyOz ER, L}=f₂(flow_{cathode, input}, flow_{CO2 from cathode to anode})  (12)

The current I_cell flowing in the electrolysis cell illustrated in FIG. 4 has the following relation with the current J_{CxHyOz ER include low CO2} (this may be J_{CxHyOz ER}) used for the production of C_xH_yO_z and the current J_HER used for the production of H₂.

I_cell=(J_{CxHyOz ER include low CO2}+J_HER)A_electrode  (13)

“Aelectrode” is an electrode area of the cathode or the anode, and typically, the cathode and the anode have the same electrode area in many cases. Further, the cell voltage Veen is represented as follows.

$\begin{array}{cc}{V}_{\mathrm{Cell}}=E_{\mathrm{OER}}^0-E_{\mathrm{CxHyOz}\mathrm{ER}}^0+{\eta }_{\mathrm{CxHyOz}\mathrm{ER}}+{\eta }_{\mathrm{OER}}+\left(R_{\mathrm{cathode}}+R_{\mathrm{membrane}}+R_{\mathrm{anode}}\right)I_{\mathrm{cell}}& \left(14\right)\end{array}$

E⁰_OER and E⁰_CxHyOz are theoretical potentials of O₂ production and C_xH_yO_z production and vary with temperature. R_cathode is the cathode resistance, R_membrane is the diaphragm resistance, and R_anode is the anode resistance.

Next, FIG. 5 illustrates the equivalent circuit parameters of the equivalent circuit model illustrated in FIG. 4. Using one of or two or more of the supply power properties, the input gas properties, and the electric properties, output gas properties, and temperature of the CO₂ electrolysis cell which are collected before the real operation, the equivalent circuit parameters of these are calculated by fitting. For the fitting, spreadsheet software or a circuit simulator is usable. Using the data sent from the data storage unit 11 to the data processing unit 12 during the real operation, the equivalent circuit parameters are periodically calculated, and the equivalent circuit parameters before the real operation or at the design time and the equivalent circuit parameters during the real operation are compared, whereby it is possible to infer a deteriorated place.

Further, for each of the equivalent circuit parameters, the deterioration degree D can be calculated based on the following formula (15).

D=[(EQUIVALENT CIRCUIT PARAMETERS BEFORE REAL OPERATION)−(EQUIVALENT CIRCUIT PARAMETERS BEFORE REAL OPERATION OR AT DESIGN)]/(EQUIVALENT CIRCUIT PARAMETERS BEFORE REAL OPERATION OR AT DESIGN)  (15)

Setting a determination standard for the aforesaid deterioration degree D enables the determination on operation stop, refresh operation, or maintenance. Further, the development over time of the deterioration degree D can be represented by a regression formula which is a linear function shown in the following formula (16), for instance.

D(t)=at+b  (16)

In the math expression, “a” is a variation of D per unit time and “b” is an intercept. The use of the formula (16) enables the estimation of the remaining time up to the determination standard. An approximate formula of the development over time of the deterioration degree D is not limited to the formula (16) and may be a quadratic formula or a polynomial. Further, the remaining time up to the determination standard may be estimated using machine learning based on database of other electrolysis cell's equivalent circuit parameters and deterioration degrees stored in the data processing unit 12.

Second Embodiment

Next, the configuration and a deterioration detecting system of a carbon dioxide electrolysis device of a second embodiment will be described with reference to FIG. 1 and FIG. 6. The deterioration detecting system of the carbon dioxide electrolysis device 1 of the second embodiment is the same as the deterioration detecting system of the first embodiment. In the carbon dioxide electrolysis device 1 of the second embodiment, a contact form of a gas containing CO₂ (sometimes referred to as a CO₂ gas) with a reduction electrode 23 and a contact form of a second electrolysis solution (anolyte) containing water with an oxidation electrode 27 in an electrolysis cell 2B are different from those in the electrolysis cell 2A of the first embodiment. The electrolysis cell 2B of the carbon dioxide electrolysis device 1 of the second embodiment differs in the configuration from the electrolysis cell 2A according to the first embodiment. Except for the above, the configurations of its parts, for example, the specific configurations of the reduction electrode 23, the oxidation electrode 27, a diaphragm 29, the second electrolysis solution, and so on are the same as those of the first embodiment.

The electrolysis cell 2B according to the second embodiment includes the reduction electrode 23, the oxidation electrode 27, the diaphragm 29, a first channel 36 in which the gas containing CO₂ flows, a second channel 37 in which the second electrolysis solution (anolyte) containing water flows, a first current collector plate 38 electrically connected to the reduction electrode 23, and a second current collector plate 39 electrically connected to the oxidation electrode 27. The reduction electrode 23 and the first channel 36 facing it form a cathode part (reduction electrode chamber) 24. The oxidation electrode 27 and the second channel 37 facing it form an anode part (oxidation electrode chamber) 28.

In the second embodiment, a first electrolysis solution containing CO₂ instead of the gas containing CO₂ may flow in the first channel 36. Another adoptable configuration is to provide a not-illustrated channel between the reduction electrode 23 and the diaphragm 29, have the gas containing CO₂ flow in the first channel 36, and have the first electrolysis solution flow in the channel between the reduction electrode 23 and the diaphragm 29. The first electrolysis solution used in this case may contain CO₂ or may be one not containing CO₂. Further, instead of the second electrolysis solution containing water, a gas containing water vapor is also usable.

During the operation of the electrolysis cell 2B, the supply of the gas containing CO₂ is sometimes stopped because the first channel 36 is clogged when a reduction product of CO₂ or a component of the second electrolysis solution having moved to the reduction electrode 23 side solidifies to precipitate in the first channel 36. Therefore, in order to inhibit the formation of the precipitates, the gas containing CO₂ preferably contains moisture. However, too large a moisture content in the gas containing CO₂ is not preferable because this results in the supply of a large amount of moisture to the surface of a catalyst in the reduction electrode 23 to easily cause the production of hydrogen. Therefore, the moisture content in the gas containing CO₂ is preferably 20% to 90% and more preferably 30% to 70% in terms of relative humidity.

A first supply channel 31 that supplies the gas containing CO₂ and a first discharge channel 33 that discharges a produced gas are connected to the first channel 36. A second supply channel 34 that supplies the electrolysis solution containing water and a second discharge channel 35 are connected to the second channel 37. The first channel 36 is disposed to face the reduction electrode 23. The first channel 36 is connected to the first supply channel 31 and is supplied with the gas containing CO₂ from the first supply channel 31. The CO₂ gas or the first electrolysis solution (catholyte) comes into contact with the reduction electrode 23 when it flows in the first channel 36. CO₂ in the CO₂ gas or the catholyte passing through the reduction electrode 23 is reduced by the reduction electrode 23. A gas or solution containing a reduction reaction product of CO₂ is discharged from the first discharge channel 33.

The second channel 37 is disposed to face the oxidation electrode 27. A not-illustrated solution tank or the like is connected to the second channel 37, and the anolyte comes into contact with the oxidation electrode 27 when it flows in the second channel 37. H₂O in the anolyte passing through the oxidation electrode 27 is oxidized by the oxidation electrode 27.

In the deterioration detecting system of the carbon dioxide electrolysis device of the second embodiment, the equivalent circuit model illustrated in FIG. 4 can be employed as in the first embodiment, and the equivalent circuit parameters illustrated in FIG. 5 are calculated by fitting. Using data sent from the data storage unit 11 to the data processing unit 12 during operation, the equivalent circuit parameters are periodically calculated, and equivalent circuit parameters before the real operation or at design time are compared with the equivalent circuit parameters during the real operation, whereby a deteriorated place can be inferred. Further, by setting a determination standard for a deterioration degree D, it is possible to determine whether to stop the operation, whether to execute a refresh operation, and whether to perform maintenance.

Third Embodiment

The configuration and a deterioration detecting system of a carbon dioxide electrolysis device of a third embodiment will be described with reference to FIG. 1, FIG. 6, and FIG. 7. The carbon dioxide electrolysis device of the third embodiment includes two H₂ production equivalent circuits. The electrolysis device of the third embodiment has the same configuration and deterioration detecting system as those of the electrolysis device of the first embodiment or the second embodiment. An electrolysis cell according to the third embodiment has the same configuration as that of the electrolysis cell according to the second embodiment, for instance. However, an equivalent circuit model used in the data processing unit according to the third embodiment is different from the equivalent circuit model according to the first embodiment. The equivalent circuit model used in the data processing unit 12 according to the third embodiment will be described with reference to FIG. 7.

FIG. 7 illustrates an example of the equivalent circuit model of the carbon dioxide electrolysis cell. In the description here, the case where C_xH_yO_z is produced through a CO₂ reduction reaction is taken as an example. In the case of CO production, C_xH_yO_z can be replaced by CO. In a cathode part of the equivalent circuit model, a C_xH_yO_z producing part and two side reaction H₂ producing parts are connected in parallel, and cathode resistance is connected thereto in series. The H₂ producing parts are a H₂ producing part employed for a low current density region (low current density) and a H₂ producing part employed for a high current density region (high current density), and they are connected in parallel. A diaphragm part is diaphragm resistance. In an anode part, an O₂ producing part and anode resistance are connected in series. The cathode part, the diaphragm part, and the anode part are connected in series.

In the parameter of the Tafel equation in the formula (10), in the case where the subscript A is the H₂ producing part (low current density), “HER low” is used, and in the case where it is the H₂ producing part (high current density), “HER high” is used. FIG. 8 illustrates equivalent circuit parameters of the equivalent circuit model according to the third embodiment. Using the supply power properties, the input gas properties, and the electric properties, output gas properties, and temperature of the electrolysis cell which are collected before real operation, these equivalent circuit parameters are calculated by fitting. For the fitting, spreadsheet software or a circuit simulator is usable. Using data sent from the data storage unit 11 to the data processing unit 12 during the real operation, the equivalent circuit parameters are periodically calculated, and the equivalent circuit parameters before the real operation or at design time are compared with the equivalent circuit parameters during the real operation, whereby a deteriorated place can be inferred. Further, by setting a determination standard for a deterioration degree D, it is possible to determine whether to stop the operation, whether to execute a refresh operation, or whether to perform maintenance.

Fourth Embodiment

The configuration and a deterioration detecting system of an electrolysis device of a fourth embodiment will be described with reference to FIG. 1, FIG. 2, FIG. 6, and FIG. 9. The electrolysis device of the fourth embodiment is a device that electrolyzes and reduces nitrogen (N₂) to produce ammonia (NH₃). The electrolysis device of the fourth embodiment is the same in the device configuration itself as the electrolysis device 1 of the first embodiment illustrated in FIG. 1 though being different in an electrolyte and an electrolysis product. Further, in the electrolysis device of the fourth embodiment, an electrolysis cell is also the same as the electrolysis cell 2A illustrated in FIG. 2. In the electrolysis device of the fourth embodiment, an electrolysis cell having the same configuration as that of the electrolysis cell 2B illustrated in FIG. 6 may also be used.

In the fourth embodiment, a substance to be reduced in the cathode part of the electrolysis cell illustrated in FIG. 2, which is nitrogen (N₂), and an equivalent circuit model are different from those in the first embodiment. The first electrolysis solution stored in the cathode part contains N₂ as the substance to be electrolyzed. Alternatively, in the electrolysis cell illustrated in FIG. 6, a N₂ gas may be supplied as the substance to be electrolyzed to the cathode part.

In the case where nitrogen (N₂) is reduced, the first electrolysis solution preferably contains an ammonia production catalyst and a reducing agent for the production of ammonia through the N₂ reduction, separately from an electrochemical reaction. As the reducing agent, a halide (II) or the like of a lanthanoid metal is used. Examples of the lanthanoid metal include lanthanum (La), cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu), among which Sm is preferable. Examples of halogen include chlorine (Cl), bromide (Br), and iodine (I), among which iodine is preferable. As the halide (II) of the lanthanoid metal, samarium(II) iodide (SmI₂) is more preferable.

The ammonia production catalyst promotes the production of ammonia from nitrogen under the presence of the reducing agent, and is, but not limited to, a molybdenum complex, for instance. Examples of the ammonia production catalyst include molybdenum complexes (A) to (D) listed below, for instance.

A first example is (A) a molybdenum complex having, as a PCP ligand, N,N-bis(dialkyl-phosphinomethyl)dihydrobenzo imidazolidine (where the two alkyl groups may be identical or may be different, and at least one hydrogen atom of the benzene ring may be replaced by an alkyl group, an alkoxy group, or a halogen atom).

A second example is (B) a molybdenum complex having, as a PNP ligand, 2-6-bis(dialkyl-phosphinomethyl)pyridine (where the two alkyl groups may be identical or may be different, and at least one hydrogen atom of the pyridine ring may be replaced by an alkyl group, an alkoxy group, or a halogen atom).

A third example is (C) a molybdenum complex having, as a PPP ligand, bis(dialkyl-phosphinomethyl)arylphosphine (where the two alkyl groups may be identical or may be different).

A fourth example is (D) a molybdenum complex represented by trans-Mo(N₂)₂(R1R2R3P)₄ (where R1, R2, and R3 are alkyl groups or aryl groups that may be identical or may be different, and two R3's may be linked to form an alkylene chain).

In the above molybdenum complexes, the alkyl group may be, for example, a methyl group, an ethyl group, a propyl group, a butyl group, a pentyl group, a hexyl group, or a straight-chain or branched alkyl group such as a structural isomer of any of these, or may be a cyclic alkyl group such as a cyclopropyl group, a cyclobutyl group, a cyclopentyl group, or a cyclohexyl group. The carbon number of the alkyl group is preferably 1 to 12, and more preferably 1 to 6. The alkoxy group may be, for example, a methoxy group, an ethoxy group, a propoxy group, a butoxy group, a pentoxy group, a hexyloxy group, or a straight-chain or branched alkoxy group such as a structural isomer of any of these, or may be a cyclic alkoxy group such as a cyclopropoxy group, a cyclobutoxy group, a cyclopentoxy group, or a cyclohexyloxy group. The carbon number of the alkoxy group is preferably 1 to 12, and more preferably 1 to 6. Examples of the halogen atom include a fluorine atom, a chlorine atom, a bromine atom, and an iodine atom.

The amount of the ammonia production catalyst used may be appropriately selected within a range of 0.00001 to 0.1 mol/L equivalent weight, and is preferably 0.0001 to 0.05 mol/L equivalent weight, and more preferably 0.0005 to 0.01 mol/L equivalent weight, relative to the electrolysis solution.

Next, the operation of the electrolysis device to produce ammonia through the N₂ reduction reaction will be described. When a voltage equal to or higher than an electrolysis voltage is applied across the reduction electrode (cathode) and the oxidation electrode (anode), an oxidation reaction of water (H₂O) or hydroxide ions (OH⁻) in the second electrolysis solution occurs electrochemically in the oxidation electrode. For example, in the case where the second electrolysis solution has a hydrogen ion concentration of 7 or less (pH 7), H₂O is oxidized and O₂ and H⁺ are produced based on the following equation (17). In the case where the hydrogen ion concentration of the second electrolysis solution is larger than 7 (pH>7), OH⁻ is oxidized and O₂ and H₂O are produced based on the following equation (18).

3H₂O→3/2O₂+6H⁺+6e⁻  (17)

6OH⁻→3/2O₂+3H₂O+3H₂O+6e⁻  (18)

In the first storage part (first electrolysis tank) 22, separately from the electrochemical reaction, nitrogen (N₂) in the first electrolysis solution is reduced by the ammonia production catalyst and the reducing agent, resulting in the production of ammonia (NH₃). In the case where, for example, SmI₂ is used as the reducing agent, N₂ in the first electrolysis solution is reduced, resulting in the production of ammonia (NH₃) based on the following equation (19).

N₂+6SmI₂+6H₂O→2NH₃+6SmI₂(OH)  (19)

As shown by the above equation (19), as a result of the production of NH₃, the reducing agent SmI₂ is oxidized, and if this state is left as it is, SmI₂ loses the function as the reducing agent. That is, in the case where the reduction reaction of N₂ in the first electrolysis solution is caused in a first electrolysis tank not having a reduction electrode that electrochemically causes the reduction reaction, the reduction reaction of N₂ stops and the production of NH₃ finishes at an instant when the reducing agent in the amount put into the first electrolysis tank in the initial state is consumed by the reduction reaction of N₂. Regarding this point, in the electrolysis device of the embodiment, since the reduction electrode that causes the electrochemical reduction reaction is disposed in the first electrolysis tank, the reducing agent resulting from the oxidation by the reduction electrode, that is, SmI₂(OH), can be reduced to be regenerated based on the following equation (20). This enables the reduction reaction of N₂ to continuously last. The amount of the reducing agent used is preferably 0.01 to 2 mol/L, more preferably 0.1 to 1 mol/L relative to the first electrolysis solution in order to promote its reaction with the ammonia production catalyst.

6SmI₂(OH)+6e⁻→6SmI₂+6OH⁻  (20)

FIG. 9 illustrates an equivalent circuit model of the electrolysis cell that produces NH₃ using SmI₂ as the reducing agent. As shown by the aforesaid equation (20), SmI₂(OH) is electrochemically regenerated into SmI₂. For this purpose, in a cathode part of the equivalent circuit model, a SmI₂ regenerating part and a side reaction H₂ producing part are connected in parallel. The current density of the SmI₂ regenerating part is represented by the Tafel equation in the aforesaid formula (10). In the subscript Ain the formula (10), SmI₂ RR (SmI₂ regeneration reaction) is entered.

Fifth Embodiment

A method of designing an electrolysis device and an electrolysis system of a fifth embodiment will be described with reference to FIG. 10. The electrolysis system of the fifth embodiment is the same as the electrolysis system of the first and second embodiment. In the method of designing the electrolysis system of the fifth embodiment, the system is designed using the equivalent circuit model and the equivalent circuit parameters of any of the first to fourth embodiments.

First, an electrolysis cell serving as a reference (reference electrolysis cell) is operated, the supply power properties, the input gas properties, the electric properties, the output gas properties, and the temperature properties are obtained, and their measurement data are stored in the data storage unit (S1). Before the measurement data are obtained, it is preferable to execute an aging operation of passing a current in advance to stabilize cell properties. Since the cell properties are more stabilized as the time of the aging operation is longer, the time of the aging operation is preferably one hour or longer, and more preferably two hours or longer. The data processing unit selects a candidate for the equivalent circuit model of the electrolysis cell (S2).

The data processing unit calculates parameters of the equivalent circuit model by fitting such that a square error between the measurement data of the reference electrolysis cell obtained at S1 and simulation data of the equivalent circuit model becomes small (S3). A determination standard for the square error between the measurement data and the simulation data of the equivalent circuit model is set in advance, and when the square error is larger than the determination standard, the candidate for the equivalent circuit model at S2 is changed. In the case where the square error is smaller than the determination standard, it is determined that the equivalent circuit model selected at S2 is valid (S4). The electrolysis system is designed using the equivalent circuit model determined as valid and the parameters of the equivalent circuit model (S5).

EXAMPLE

Next, an example and its evaluation results will be described.

Example 1

The carbon dioxide electrolysis cell whose configuration is illustrated in FIG. 6 was manufactured. The carbon dioxide electrolysis cell was operated by the deterioration detecting system of the electrolysis device illustrated in FIG. 1. As the reduction electrode in the carbon dioxide electrolysis cell, an electrode in which gold nanoparticle-carrying carbon particles were applied on carbon paper was used. The average particle size of the gold nanoparticles was 2 nm, and their carried amount was 10% by mass. As the oxidation electrode, an electrode in which IrO₂ nanoparticles were applied on a Ti mesh was used. As the diaphragm, an anion exchange membrane was used. The reduction electrode and the oxidation electrode cut to have a 16 cm² electrode area were used. As in the carbon dioxide electrolysis cell whose structure is illustrated in FIG. 6, the first current collector plate, the first channel, the reduction electrode, the diaphragm, the oxidation electrode, the second channel, and the second current collector plate were stacked in order from the left, and the resultant was sandwiched by an insulating plate, a cooling water channel, and a support plate, which are not illustrated, to form the carbon dioxide electrolysis cell. Further, to simply monitor a reduction electrode potential and an oxidation electrode potential, a not-illustrated Pt foil as a reference electrode was brought into contact with a reduction electrode side of the diaphragm.

Using the gas/electrolysis solution control unit, CO₂ was introduced to the first channel of the carbon dioxide electrolysis cell at a flow rate of 80 sccm, and a 0.1 M KHCO₃ electrolysis solution was introduced to the second channel at a flow rate of 10 mL/min. Further, using the temperature control unit and the temperature obtaining unit, the carbon dioxide electrolysis cell was temperature-controlled to 40° C. while a heater and a cooling water channel, which are not illustrated, were in close contact with the carbon dioxide electrolysis cell. As the supply power control unit, the supply power property obtaining unit, and the electric property obtaining unit, used was a potentiostat/galvanostat in which their functions are integrated. As the output gas property obtaining unit, a volumetricflow meter or a gas chromatograph was used.

A current was passed to the carbon dioxide electrolysis cell, and current density dependences of the supply power properties (supplied current and voltage), the input gas properties (the flow rate of a gas input to the cathode), the cell temperature, the electric properties (cell current, cell voltage, cathode potential, anode potential, cell resistance), and the output gas properties (the flow rates of gases output from the cathode and the anode, the concentrations of various gases) were obtained. In the cathode, Co was produced through a CO₂ reduction reaction and H₂ was produced through a side reaction. In the anode, O₂ was produced through an oxidation reaction of water. Since the behavior of a H₂ production reaction in a low current density region and that in a high current density region were different, the circuit illustrated in FIG. 7 in which the CO producing part and the two H₂ producing parts are connected in parallel was used as the equivalent circuit model.

A CO part current density (current density contributing to CO production) and a H₂ part current density (current density contributing to H₂ production) were calculated from the flow rate of the gas output from the cathode and the gas concentrations of CO and H₂, and equivalent circuit parameters were decided such that errors between measurement data and simulation data of the CO part current density J_CO, the H₂ part current density J_H2, the cell voltage V_cell, the cathode potential V_cm, and the anode potential V_am became small as illustrated in FIG. 11 and FIG. 12. As illustrated in FIG. 13, FIG. 14, and FIG. 15, the simulation data of CO Faraday efficiency (FE_CO), H₂ Faraday efficiency (FE_H2), cathode output gases (CO, H₂, and CO₂), and anode output gases (O₂ and CO₂) well reproduce the measurement data. Therefore, using the equivalent circuit model in FIG. 7 to study changes in the equivalent circuit parameters during operation enables the detection of deterioration.

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

Claims

1. An electrolysis device comprising: an electrolysis cell including a cathode part to be supplied with a gas or a liquid containing a substance to be reduced and in which a reduction electrode is disposed, an anode part to be supplied with a liquid containing a substance to be oxidized and in which an oxidation electrode is disposed, and a diaphragm provided between the cathode part and the anode part;a supply power property obtaining unit that obtains a property of power that is to be supplied to the electrolysis cell;an input gas property obtaining unit that obtains a property of a gas that is to be input to the electrolysis cell;an electric property obtaining unit that obtains an electric property of the electrolysis cell;an output gas property obtaining unit that obtains a property of an output gas of the electrolysis cell;a temperature control unit that controls a temperature of the electrolysis cell;a temperature obtaining unit that obtains the temperature of the electrolysis cell;a data storage unit that stores data from the supply power property obtaining unit, the input gas property obtaining unit, the electric property obtaining unit, the output gas property obtaining unit, and the temperature obtaining unit; anda data processing unit to which the data is sent from the data storage unit and that processes the data to determine a state of the electrolysis cell.

2. The electrolysis device according to claim 1, wherein the data processing unit is configured to calculate an equivalent circuit parameter by fitting using measurement data from at least one of the supply power property obtaining unit, the input gas property obtaining unit, the electric property obtaining unit, the output gas property obtaining unit, and the temperature obtaining unit and simulation data of an equivalent circuit model of the electrolysis cell, and to detect deterioration based on information of the equivalent circuit parameter.

3. The electrolysis device according to claim 2, wherein the data processing unit is configured to infer a deteriorated place by comparing the equivalent circuit parameter before real operation or at design time with the equivalent circuit parameter during the real operation.

4. The electrolysis device according to claim 3, wherein the data processing unit is configured to find a deterioration degree of the equivalent circuit parameter, and to determine whether to stop the operation of the electrolysis cell by setting a determination standard for the deterioration degree, the deterioration degree being represented by [(the equivalent circuit parameter during the real operation)−(the equivalent circuit parameter before the real operation or at the design time)]/(the equivalent circuit parameter before the real operation or at the design time).

5. The electrolysis device according to claim 2, wherein the data processing unit is configured to calculate an equivalent circuit as the equivalent circuit model, the equivalent circuit including: a cathode part having a carbon dioxide reduction substance producing part and a hydrogen producing part that are connected in parallel and to which a series resistance is connected in series; a diaphragm part; and an anode part having an oxygen producing part and a series resistance that are connected in series, the cathode part, the diaphragm part, and the anode part being connected in series.

6. The electrolysis device according to claim 5, wherein the data processing unit is configured to calculate a current density of the carbon dioxide reduction substance producing part of the equivalent circuit model based on a relational formula including, as variables, a current density represented by a Tafel equation and a production limit current density of the carbon dioxide reduction substance.

7. The electrolysis device according to a claim 6, wherein the data processing unit is configured to calculate the production limit current density of the carbon dioxide reduction substance based on a relational formula including, as variables, a flow rate of the substance to be reduced introduced to the cathode part and a flow rate of the substance to be reduced that has moved to the anode part from the cathode part.

8. The electrolysis device according to claim 1, wherein the data processing unit is installed in a cloud and is configured to determine the state of the electrolysis cell remotely.

9. The electrolysis device according to claim 1, wherein the electrolysis cell is configured to produce a carbon compound by supplying carbon dioxide as the substance to be reduced, or to produce ammonia by supplying nitrogen as the substance to be reduced.

10. An electrolysis method comprising: supplying a gas or a liquid containing a substance to be reduced to a cathode part of an electrolysis cell, supplying a liquid containing a substance to be oxidized to an anode part of the electrolysis cell, and operating the electrolysis cell, the electrolysis cell including the cathode part in which a reduction electrode is disposed, the anode part in which an oxidation electrode is disposed, and a diaphragm provided between the cathode part and the anode part;obtaining property data of power that is to be supplied to the electrolysis cell, property data of a gas that is to be input to the electrolysis cell, electric property data of the electrolysis cell, property data of an output gas of the electrolysis cell, and temperature data of the electrolysis cell, all the data being obtained during the operation of the electrolysis cell; andprocessing the property data of the power, the property data of the gas, the electric property data, the property data of the output gas, and the temperature data to obtain an equivalent circuit model and an equivalent circuit parameter of the electrolysis cell, and determining a state of the electrolysis cell, using the equivalent circuit model and the equivalent circuit parameter of the electrolysis cell.

11. The electrolysis method according to claim 10, wherein the determining the state of the electrolysis cell comprises determining a deterioration state of the electrolysis cell, using the equivalent circuit model and the equivalent circuit parameter.

12. The electrolysis method according to claim 11, wherein the determining the state of the electrolysis cell comprises calculating the equivalent circuit parameter by fitting, using the obtained data and simulation data of the equivalent circuit model, and detecting a deterioration of the electrolysis cell based on information of the equivalent circuit parameter.

13. The electrolysis method according to claim 11, wherein the determining the state of the electrolysis cell comprises inferring a deteriorated place by comparing the equivalent circuit parameter before real operation or at design time with the equivalent circuit parameter during the real operation.

14. The electrolysis method according to claim 11, wherein the determining the state of the electrolysis cell comprises finding a deterioration degree of the equivalent circuit parameter, and determining whether to stop the operation of the electrolysis cell by setting a determination standard for the deterioration degree, the deterioration degree being represented by [(the equivalent circuit parameter during the real operation)−(the equivalent circuit parameter before the real operation or at the design time)]/(the equivalent circuit parameter before the real operation or at the design time).

15. The electrolysis method according to claim 10, wherein the obtaining the data of the electrolysis cell comprises obtaining data of a reference electrolysis cell, andwherein the electrolysis cell is designed based on the equivalent circuit model and the equivalent circuit parameter that are derived from the data of the reference electrolysis cell.

16. The electrolysis method according to claim 15, further comprising: selecting a candidate for the equivalent circuit model;calculating the equivalent circuit parameter by fitting such that a square error between the data of the reference electrolysis cell and simulation data of the selected equivalent circuit model becomes small; anddetermining whether the equivalent circuit parameter is valid, using the square error between the data of the reference electrolysis cell and the simulation data of the equivalent circuit parameter; anddesigning the electrolysis cell using the equivalent circuit model and the equivalent circuit parameter determined as valid.