METHOD OF OPERATING ELECTROCHEMICAL REACTION DEVICE AND ELECTROCHEMICAL REACTION DEVICE

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2024-161156, filed on Sep. 18, 2024 and Japanese Patent Application No. 2025-134015, filed on Aug. 12, 2025; the entire contents of which are incorporated herein by reference.

FIELD

Embodiments relate to a method of operating an electrochemical reaction device and the electrochemical reaction device.

BACKGROUND

In recent years, depletion of fossil fuel such as petroleum or coal has been concerned, and expectation for sustainably-usable renewable energy has been rising. As the renewable energy, a solar cell, wind power generation, and the like can be cited. These have a problem of hampering stable supply of electric power due to dependence of a power generation amount on weather and a natural situation. Thus, an attempt is being made to store the electric power generated by the renewable energy in a storage battery, and stabilize the electric power. However, the storage of electric power creates problems of requiring a cost for the storage battery, and producing losses at the time of storage.

With respect to such points, attention is focused on such a technology of converting electric energy into chemical substances (chemical energy) that water electrolysis is performed by using the electric power generated by the renewable energy to produce hydrogen from water, carbon dioxide is electrochemically reduced to produce a carbon compound such as carbon monoxide, formic acid, methanol, methane, acetic acid, ethanol, ethane, or ethylene, or nitrogen is electrochemically reduced to produce ammonia. The storage of these chemical substances in a cylinder or a tank has advantageous points that a storage cost of energy can be reduced, and a storage loss is also small as compared with the storage of the electric power (electric energy) in the storage battery.

Such an electrochemical reaction is usually performed by a device provided with an electrolysis cell or a cell stack made by stacking the electrolysis cells. Operation modes of the electrochemical reaction device can differ depending on the kind of reaction and the purpose of use. For example, the operation is performed continuously for a long time in some cases, or the start and stop are frequently repeated in some cases. The continuous operation and the start-stop operation may cause performance degradation of the electrolysis cell stack, for example, an increase in cell voltage and a decrease in product selectivity. For example, there are known an operation method for recovering the once-degraded performance of the electrolysis cell stack and an operation method for protecting an electrolysis reactor from an adverse effect due to an unexpected event such as power failure. On the other hand, for further improvement in reliability of the electrochemical reaction device, to begin with, it is important to make the performance degradation of the electrolysis cell stack caused by the continuous operation and the start-stop operation sufficiently small in a normal operation mode. Thus, a start-stop operation method of the electrochemical reaction device capable of reducing the performance degradation of the electrolysis cell stack is required.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram illustrating an example configuration of an electrochemical reaction device.

FIG. 2 is a schematic view illustrating another example configuration of an electrolytic unit.

FIG. 3 is a schematic view illustrating the other example configuration of the electrolytic unit.

FIG. 4 is a schematic diagram for explaining an example method of operating the electrochemical reaction device.

FIG. 5 is a schematic graph illustrating a relation between positions and pressures in flow paths.

FIG. 6 is a schematic graph illustrating a relation between the positions and pressures in the flow paths.

FIG. 7 is a schematic graph illustrating a relation between the positions and pressures in the flow paths.

FIG. 8 is a schematic diagram illustrating another example configuration of the electrochemical reaction device.

FIG. 9 is a schematic diagram illustrating the other example configuration of the electrochemical reaction device.

FIG. 10 is a graph illustrating experimental results of examples 9 to 13.

FIG. 11 is a graph illustrating experimental results of examples 14, 15.

DETAILED DESCRIPTION

A method of operating an electrochemical reaction device of an embodiment, the device including an electrolytic unit having a cathode, an anode, a cathode space facing on the cathode, an anode space facing on the anode, and a diaphragm provided between the cathode space and the anode space, the method including: a startup process of regulating at least one parameter selected from the group consisting of a plurality of parameters in the electrolytic unit to satisfy an operation start condition, the plurality of parameters including a temperature, a pressure, a current density, a voltage, a composition of a first fluid containing a reducible material to be supplied to the cathode space, a flow rate of the first fluid, a composition of a second fluid containing an oxidizable material to be supplied to the anode space, and a flow rate of the second fluid; an operation process of using the electrolytic unit in an operation condition range including the operation start condition and reducing the reducible material in the cathode to produce a reduction product; a shutdown process of regulating at least one parameter selected from the group consisting of the plurality of parameters to satisfy a storage start condition; and a storage process of using the electrolytic unit in a storage condition range including the storage start condition. The electrolytic unit is controlled during each of the startup process, the operation process, and the shutdown process so that a first time-averaged pressure at a first position in the cathode space is equal to or higher than a second time-averaged pressure at a second position in the anode space and a third time-averaged pressure at a third position in the cathode space is equal to or higher than a fourth time-averaged pressure at a fourth position in the anode space, the first position being closer to an inlet of the cathode space than an outlet of the cathode space, the second position being opposite the first position with the diaphragm therebetween, the third position being closer to the outlet than the inlet, and the fourth position being opposite the third position with the diaphragm therebetween.

FIG. 1 is a schematic diagram illustrating an example configuration of an electrochemical reaction device of an embodiment. FIG. 1 illustrates an example configuration of an electrochemical reaction device 1. The electrochemical reaction device 1 has an electrolytic unit 10, a flow path P1, a flow path P2, a flow path P3, a flow path P4, a cathode supply part 21, and an anode supply part 22.

The electrolytic unit 10 can perform at least one electrolytic reaction, for example. The electrolytic unit 10 has a cathode 11, an anode 12, a diaphragm 13, a cathode space 140, and an anode space 150.

The electrolytic unit 10 may have a membrane electrode assembly, for example. FIG. 2 is a schematic view illustrating another example configuration of the electrolytic unit 10. The electrolytic unit 10 may have the cathode 11, the anode 12, the diaphragm 13, a flow path plate 14, a flow path plate 15, a current collector 16, and a current collector 17, as illustrated in FIG. 2. FIG. 2 illustrates an X axis, a Y axis, and a Z axis. The X axis, the Y axis, and the Z axis perpendicularly cross one another. The Z axis is along a thickness direction of the electrolytic unit 10. FIG. 2 illustrates a part of an X-Z cross section including the X axis and the Z axis. The cathode 11, the anode 12, and the diaphragm 13 may be stacked to form an electrolysis cell 100 having the membrane electrode assembly MEA.

The cathode 11 is a reduction electrode for performing a reduction reaction of at least one reducible material (substance to be reduced) to produce a reduction product, for example. At least one reducible material includes carbon dioxide or nitrogen, for example. The cathode 11 reduces carbon dioxide to produce a carbon compound, or reduces nitrogen to produce a nitrogen compound such as ammonia, for example. Examples of the carbon compound include carbon monoxide, formic acid, methanol, methane, ethanol, ethane, ethylene, formaldehyde, ethylene glycol, acetic acid, propanol, and so on. By the cathode 11, a side reaction in which hydrogen is produced by the reduction reaction of water sometimes occurs with the reduction reaction of carbon dioxide or the reduction reaction of nitrogen.

The cathode 11 has a cathode catalyst which accelerates the reduction reaction of reducing at least one reducible material, for example. The cathode catalyst can be formed using an activation energy-reducing material for reducing at least one reducible material, for example. In other words, the cathode catalyst can be formed using a material which lowers an overvoltage at the time of producing the reduction product by the reduction reaction of at least one reducible material, for example.

The cathode 11 may have a first surface in contact with the diaphragm 13, and a second surface facing on the cathode space 140. Further, the cathode 11 may have a gas diffusion layer and a cathode catalyst layer, for example. The cathode 11 may have a porous layer denser than the gas diffusion layer between the gas diffusion layer and the cathode catalyst layer. The gas diffusion layer is disposed on the cathode space 140 side, and the cathode catalyst layer is disposed on the diaphragm 13 side. The cathode catalyst layer may enter the interior of the gas diffusion layer. The cathode catalyst layer preferably has catalyst nanoparticles, a catalyst nanostructure, or the like. The gas diffusion layer is formed of, for example, carbon paper, carbon cloth, or the like, and water repellent treatment is performed thereon. To the cathode catalyst layer, an electrolytic solution and ions are supplied from the anode 12 side via the diaphragm 13. In the gas diffusion layer, a cathode supply fluid is supplied from the cathode space 140, and the product of the reduction reaction is discharged. The reduction reaction occurs in the vicinity of the boundary of the gas diffusion layer and the cathode catalyst layer, and the gaseous product is discharged through the gas diffusion layer from a cathode flow path to the outside of the electrolysis cell 100.

As the cathode catalyst, there can be cited a metal material such as a metal such as gold (Au), silver (Ag), copper (Cu), platinum (Pt), palladium (Pd), nickel (Ni), cobalt (Co), iron (Fe), manganese (Mn), titanium (Ti), cadmium (Cd), zinc (Zn), indium (In), gallium (Ga), lead (Pb), or tin (Sn), or, an alloy or an intermetallic compound containing at least one of the above metals, a carbon material such as carbon (C), graphene, CNT (carbon nanotube), fullerene, or ketjen black, or a metal complex such as a Ru complex or a Re complex. To the cathode catalyst layer, various shapes such as a plate shape, a mesh shape, a wire shape, a particle shape, a porous shape, a thin film shape, and an island shape can be applied.

The anode 12 is an oxidation electrode for performing an oxidation reaction of at least one oxidizable material (substance to be oxidized) to thereby produce an oxidation product, for example. At least one oxidizable material includes water, for example. The anode 12 oxidizes the oxidizable material such as a substance and ions in an electrolytic solution (anode solution) to produce oxygen, for example.

The anode 12 may have a first surface in contact with the diaphragm 13, and a second surface facing on the anode space 150. The anode 12 has an anode catalyst which accelerates the oxidation reaction of oxidizing water to produce oxygen, for example. The anode catalyst can be formed using a material which reduces an activation energy at the time of oxidizing the oxidizable material, in other words, a material which lowers a reaction overvoltage, for example. Examples of the oxidation reaction by the anode 12 include a reaction of oxidizing water to produce oxygen and a hydrogen peroxide solution, a reaction of oxidizing chloride ions (Cl⁻) to produce chlorine, a reaction of oxidizing carbonate ions or hydrogen carbonate ions to produce carbon dioxide, and the like.

As examples of the anode catalyst, there can be cited a metal such as platinum (Pt), palladium (Pd), iridium (Ir), or nickel (Ni), an alloy or an intermetallic compound containing the above metals, a binary metal oxide such as a manganese oxide (Mn—O), an iridium oxide (Ir—O), a nickel oxide (Ni—O), a cobalt oxide (Co—O), an iron oxide (Fe—O), a tin oxide (Sn—O), an indium oxide (In—O), a ruthenium oxide (Ru—O), a lithium oxide (Li—O), or a lanthanum oxide (La—O), a ternary metal oxide such as Ni—Co—O, Ni—Fe—O, La—Co—O, Ni—La—O, Sr—Fe—O, or Ru—Co—O, a quaternary metal oxide such as Pb—Ru—Ir—O, La—Sr—Co—O, or Ru—Co—Sn—O, or a metal complex such as a Ru complex or an Fe complex.

The anode 12 may include a base material having a structure capable of moving an anode supply fluid and ions between the diaphragm 13 and the anode space 150, for example, a porous structure such as a mesh material, a punching material, a porous body, or a metal fiber sintered body. The base material may be composed of a metal material such as a metal such as titanium (Ti), nickel (Ni), or iron (Fe), or, an alloy (for example, SUS) containing at least one of these metals, or may be composed of the above-described anode catalyst. When an oxide is used as the anode catalyst, a catalyst layer is preferably formed in a manner that the anode catalyst is made to adhere to or stacked on a surface of the base material made of the above-described metal material. The anode catalyst may have nanoparticles, a nanostructure, a nanowire, or the like for the purpose of increasing the oxidation reaction. The nanostructure is a structure in which nanoscale irregularities are formed on a surface of the catalyst material.

The diaphragm 13 is provided between the cathode 11 and the anode 12. The diaphragm 13 can divide the cathode space 140 and the anode space 150. The diaphragm 13 can move ions such as hydrogen ions (H⁺), hydroxide ions (OH⁻), hydrogen carbonate ions (HCO₃⁻), or carbonate ions (CO₃²⁻). The diaphragm 13 allows formation of the electrolysis cell 100 having a two-chamber structure. The diaphragm 13 may be provided in contact with the cathode 11 and the anode 12.

The diaphragm 13 is formed of an ion exchange membrane capable of moving ions and the electrolytic solution between the cathode 11 and the anode 12, and capable of separating the cathode space 140 and the anode space 150, or the like. As the ion exchange membrane, for example, there can be cited NEOSEPTA (registered trademark) of ASTOM Corporation, Selemion (registered trademark) of AGC Inc., Aciplex (registered trademark) of Asahi Kasei Corporation, Fumasep (registered trademark) and Fumapem (registered trademark) of Fumatech GmbH, Nafion (registered trademark) which is a fluorocarbon resin made by sulfonating and polymerizing tetrafluoroethylene of Du Pont de Nemours, Inc., Lewabrane (registered trademark) of LANXESS AG, IONSEP (registered trademark) of IONTECH Inc., Mustang (registered trademark) of PALL Corporation, RALEX (registered trademark) of mega Corporation, or Gore-Tex (registered trademark) of Gore-Tex Co., Ltd., or the like. However, other than the ion exchange membrane, a glass filter, a porous polymeric membrane, a porous insulating material, or the like may be used for the diaphragm 13 as long as they are each a material capable of moving ions between the anode 12 and the cathode 11.

Other than the ion exchange membrane, for example, a porous membrane of a silicone resin, a fluorine-based resin (perfluoroalkoxyalkane (PFA), perfluoroethylene propene copolymer (FEP), polytetrafluoroethylene (PTFE), ethylene-tetrafluoroethylene copolymer (ETFE), polyvinylidene fluoride (PVDF), polychlorotrifluoroethylene (PCTFE), ethylene-chlorotrifluoroethylene copolymer (ECTFE), or the like), polyethersulfone (PES), or ceramics, a filling filled with glass filter, agar, and the like, an insulating porous body such as zeolite or an oxide, or the like can be used. In particular, a hydrophilic porous membrane never causes clogging due to air bubbles, and is thus preferable as the diaphragm 13.

The cathode space 140 is provided to face the cathode 11, and can form the cathode flow path, for example. The cathode 11 may be disposed in the cathode space 140 as illustrated in FIG. 1. The cathode space 140 has an inlet for supplying the cathode supply fluid to the cathode space 140, and an outlet for discharging a cathode discharge fluid from the cathode space 140.

The cathode supply fluid transforms its composition through the cathode space 140, and is discharged outside the electrolytic unit 10. A fluid discharged from the electrolytic unit 10 is called the cathode discharge fluid. The cathode discharge fluid contains an objective product, which is separated and recovered as necessary. The cathode discharge fluid may be subjected to gas/liquid separation to make a part of a gas phase flow together into the cathode supply fluid. This method is effective in increasing a conversion ratio of a reactant in the cathode supply fluid. Further, the cathode discharge fluid can also be subjected to the gas/liquid separation to make a part or the whole of a liquid phase flow together into the anode supply fluid.

The anode space 150 is provided to face the anode 12, and can form an anode flow path, for example. The anode space 150 has an inlet for supplying an anode supply fluid to the anode space 150, and an outlet for discharging an anode discharge fluid from the anode space 150, as illustrated in FIG. 1.

The anode supply fluid transforms its composition through the anode space 150, and is discharged outside the electrolytic unit 10. A fluid discharged from the anode space 150 is called the anode discharge fluid. The anode discharge fluid contains a gas such as oxygen produced by the anode 12. The anode discharge fluid may be subjected to the gas/liquid separation to make a part or the whole of a liquid phase flow together into the anode supply fluid. Without supplying the anode supply fluid from the external system, the liquid phase part of the anode discharge fluid is regarded as the anode supply fluid, and supplied to the anode space 150, and thereby the anode supply fluid can be circulated.

The flow path plate 14 forms the cathode space 140, for example. The cathode space 140 is provided in a surface of the flow path plate 14 to face the cathode 11, and can form the cathode flow path.

The flow path plate 15 forms the anode space 150, for example. The anode space 150 is provided in a surface of the flow path plate 15 to face the anode 12, and can form the anode flow path.

At least one of the flow path plate 14 and the flow path plate 15 preferably has at least one of a land (projection) 141 and a land 151. The land 141 and the land 151 are provided for mechanical retention and electrical continuity. The land 141 is provided in contact with the cathode 11. The land 151 is provided in contact with the anode 12. The land 141 and the land 151 are preferably provided alternately to make a flow of the fluids uniform. With the land 141 and the land 151 provided in this manner, the cathode flow path and the anode flow path each have a shape meandering along the surface.

The flow path plate 14 and the flow path plate 15 are preferably formed using a material having low chemical reactivity and high conductivity. As such a material, there can be cited a metal material such as titanium or SUS, a carbon material, or the like. Further, between each of the flow path plates and a member adjacent thereto, a member such as packing whose illustration is omitted may be sandwiched as necessary.

The current collector 16 is stacked on the opposite side of the flow path plate 14 to the cathode 11, and electrically connected to the cathode 11. The current collector 17 is stacked on the opposite side of the flow path plate 15 to the anode 12, and electrically connected to the anode 12. The current collector 16 and the current collector 17 are electrically connected to a power supply 40 via, for example, a wiring line or the like. The current collector 16 and the current collector 17 are preferably formed using a material having high conductivity.

The power supply 40 can supply electric power by supplying current or voltage to the electrolytic unit 10, for example. The power supply 40 is electrically connected to the cathode 11 and the anode 12 via the flow path plate 14, the flow path plate 15, the current collector 16, and the current collector 17, for example. The power supply 40 can feed the electric power for the occurrence of the electrolytic reaction such as the oxidation reaction and the reduction reaction to the electrolytic unit 10, and is electrically connected to the cathode 11 and the anode 12. The reduction reaction by the cathode 11 and the oxidation reaction by the anode 12 are performed using electric energy supplied from the power supply 40. The power supply 40 and the current collector 16, and, the power supply 40 and the current collector 17 are each connected therebetween by wiring, for example. Between the electrolytic unit 10 and the power supply 40, an electric device such as an inverter, a converter, or a battery may be installed as necessary. A drive system of the electrolytic unit 10 may be a constant-voltage system or may be a constant-current system.

The power supply 40 may be an ordinary commercial power source, a battery, or the like, or may be a power supply which converts renewable energy into electric energy and supplies it. As examples of such a power supply, there can be cited a power supply which converts kinetic energy or potential energy such as wind power, water power, geothermal power or tidal power into electric energy, a power supply such as a solar cell having a photoelectric conversion element which converts light energy into electric energy, a power supply such as a fuel cell or a storage battery which converts chemical energy into electric energy, and a power supply such as a device which converts vibrational energy such as sound into electric energy. The photoelectric conversion element has a function of performing charge separation by using light energy of irradiated sunlight or the like. Examples of the photoelectric conversion element include a pin-junction solar cell, a pn-junction solar cell, an amorphous silicon solar cell, a multijunction solar cell, a single crystal silicon solar cell, a polycrystalline silicon solar cell, a dye-sensitized solar cell, an organic thin-film solar cell, and the like. Further, the photoelectric conversion element may be stacked with at least one of the cathode 11 and the anode 12 inside the electrolytic unit 10.

The electrolysis cell 100 is sandwiched by a pair of non-illustrated support plates, and further fastened by bolts or the like. In the electrolysis cell 100, the membrane electrode assembly MEA may be horizontally disposed, or may be vertically disposed. For the horizontal disposition of the membrane electrode assembly MEA, one of the cathode 11 and the anode 12 may be disposed over the other.

The electrolytic unit 10 may have a cell stack formed by stacking a plurality of the electrolysis cells 100, for example. FIG. 3 is a schematic view illustrating the other example configuration of the electrolytic unit 10. FIG. 3 illustrates a part of an X-Z cross section including the X axis and the Z axis. The electrolytic unit 10 may have a plurality of the membrane electrode assemblies MEA, the flow path plate 14, the flow path plate 15, the current collector 16, the current collector 17, and a flow path plate 18, as illustrated in FIG. 3. FIG. 3 illustrates a plurality of the cathodes 11, a plurality of the anodes 12, a plurality of the diaphragms 13, a plurality of the cathode spaces 140, and a plurality of the anode spaces 150. The plurality of membrane electrode assemblies MEA are provided between the current collector 16 and the current collector 17 to form the cell stack. The formation of the cell stack increases a reaction amount of the reducible material per unit area, thus allowing an increase in a production amount of the reduction product, for example. The number of the plurality of electrolysis cells 100 to be stacked is preferably not less than 10 nor more than 150, for example.

When the electrolytic unit 10 has the plurality of electrolysis cells 100, the cathode supply fluid and the anode supply fluid supplied to each of the cells can be distributed from a smaller number of pipes than the number of the electrolysis cells 100 to each of the cell. Further, the cathode discharge fluid and the anode discharge fluid discharged from each of the cells can be collected to a smaller number of pipes than the number of the electrolysis cells 100. The distribution of the cathode supply fluid and the anode supply fluid and the collection of the cathode discharge fluid and the anode discharge fluid may be performed outside the electrolytic unit 10, or may be performed inside the electrolytic unit 10. Here, the inside of the electrolytic unit 10 indicates a range within which the plurality of electrolysis cells 100 are sandwiched by a pair of support plates, and further fastened by the bolts or the like.

The flow path plate 18 is a bipolar plate having the cathode space 140 and the anode space 150, for example. The flow path plate 18 is provided between the plurality of membrane electrode assemblies MEA, and divides the plurality of electrolysis cells 100. The cathode 11 and the anode 12 adjacent to the flow path plate 18 may be electrically connected via the flow path plate 18. The plurality of electrolysis cells 100 are stacked, and sandwiched by a pair of the support plates, and further fastened by the bolts or the like to be fixed. As an installation method of the cell stack, the membrane electrode assemblies MEA may be horizontally disposed or vertically disposed. For the horizontal disposition of the membrane electrode assemblies MEA, the cathode 11 or the anode 12 may be disposed on the top surface of the cell stack.

The cathode space 140 of the flow path plate 18 is provided in a first surface of the bipolar plate, and faces on the cathode 11 of one of the plurality of MEAs, for example. An inlet of the cathode space 140 of the flow path plate 18 is connected to the flow path P1. An outlet of the cathode space 140 of the flow path plate 18 is connected to the flow path P3. The flow path P1 and the flow path P2 are each formed of, for example, a pipe.

The anode space 150 of the flow path plate 18 is provided in a second surface on the opposite side to the first surface of the bipolar plate, and faces on the anode 12 of the other of the plurality of MEAs, for example. An inlet of the anode space 150 of the flow path plate 18 is connected to the flow path P2. An outlet of the anode space 150 of the flow path plate 18 is connected to the flow path P4. The flow path P2 and the flow path P4 are each formed of, for example, a pipe.

The cathode supply fluid is a cathode supply gas or a two-phase flow of the cathode supply gas and a cathode supply liquid (gas-liquid two-phase flow), for example.

The cathode supply gas contains at least one gas of carbon dioxide, nitrogen, argon, and water vapor, for example. The cathode supply gas may be a mixed gas of at least two gases selected from carbon dioxide, nitrogen, argon, and water vapor. The cathode supply liquid may contain water, for example.

The cathode supply fluid is prepared by the cathode supply part 21, and supplied to the electrolytic unit 10, for example. The cathode supply part 21 includes a humidifier 201. The cathode supply part 21 may include a bypass pipe capable of, for no use of the humidifier 201, making the fluid flow by bypassing it. A cathode-supply-fluid supply system including the cathode supply part 21 may have a gas cylinder, a flow rate control part, a pressure control part, and so on.

The anode supply fluid contains water, for example. An existing form of water may be liquid water, or may be water vapor. When the anode supply fluid contains no gas phase, and only a liquid phase in particular, this is hereinafter also called an anode aqueous solution. As the anode aqueous solution, an aqueous solution containing an optional electrolyte can be cited. As the aqueous solution containing the electrolyte, for example, there can be cited an aqueous solution containing at least one selected from hydroxide ions (OH⁻), hydrogen ions (H⁺), potassium ions (K⁺), sodium ions (Na⁺), lithium ions (Li⁺), chloride ions (Cl⁻), bromide ions (Br⁻), iodide ions (I⁻), nitrate ions (NO₃⁻), sulfate ions (SO₄²⁻), phosphate ions (PO₄²⁻), borate ions (BO₃³⁻), carbonate ions (CO₃²⁻), and hydrogen carbonate ions (HCO₃⁻). In order to reduce an electrical resistance of the solution, as the anode aqueous solution, an alkaline solution in which an electrolyte of a potassium hydroxide, a sodium hydroxide, or the like is dissolved in high concentration may be used.

The anode supply fluid is supplied to the electrolytic unit 10 via the anode supply part 22. For example, when the anode supply fluid is the anode aqueous solution, an anode-supply-fluid supply system including the anode supply part 22 on the inlet side of the anode space 150 has devices such as a pressure control part, an anode aqueous solution tank, a flow rate control part (pump), a reference electrode, a pressure gauge, and a temperature regulating mechanism. The anode aqueous solution is subjected to control of a flow rate, a pressure, and a temperature in the anode-supply-fluid supply system, and supplied to the anode space 150.

The humidifier 201 can humidify the cathode supply fluid supplied from a cathode-supply-fluid supply source such as the gas cylinder, for example. The humidifier 201 is provided in the course of the flow path P1. In the previous stage of the humidifier 201, the cathode supply fluid is preferably only the gas phase. The humidifier 201 can mix water vapor into the cathode supply fluid supplied from the previous stage of the humidifier 201. As method examples of the humidification by using the humidifier 201, there can be cited methods such as a method of babbling the gas in liquid water, a method of providing a vaporizer in the flow path of the gas to pour the liquid water there and evaporate it, and a method of spraying and evaporating the liquid water in the gas, but these methods are not restrictive.

The humidifier 201 can preferably regulate an amount of the water vapor mixed into the cathode supply fluid supplied from the previous stage of the humidifier 201. As regulation method examples of the water vapor amount, there can be cited methods such as a method of specifying a temperature of the liquid water used for the bubbling, a method of specifying a total amount and a rate of the liquid water poured into the vaporizer, and a method of specifying a total amount and a rate of the liquid water discharged from a sprayer, but these methods are not restrictive. The humidified cathode supply fluid discharged from the humidifier 201 is preferably only the gas phase, but may contain a liquid phase.

Next, an example method of operating the electrochemical reaction device of the embodiment will be explained. FIG. 4 is a schematic diagram for explaining the example method of the embodiment. FIG. 4 illustrates a horizontal axis representing a time and a vertical axis representing a control target parameter value.

The method of operating the electrochemical reaction device 1 is roughly divided to include the following four processes.

• • (1) a startup process: at least one of a plurality of parameters including a temperature, a pressure, a current density, a voltage, a composition of the cathode supply fluid supplied to the cathode space 140, a flow rate of the cathode supply fluid, a composition of the anode supply fluid supplied to the anode space 150, and a flow rate of the anode supply fluid in the electrolytic unit 10 is regulated to satisfy an operation start condition.
  • (2) an operation process: the electrolytic unit 10 is used (operated) in an operation condition range including the operation start condition to produce the reduction product by reducing a raw material supplied to the cathode space 140 (reducible material) by the cathode 11 and to produce the oxidation product by oxidizing a raw material supplied to the anode space 150 (oxidizable material) by the anode 12.
  • (3) a shutdown process: at least one of the plurality of parameters including the temperature, the pressure, the current density, the voltage, the composition of the cathode supply fluid supplied to the cathode space 140, the flow rate of the cathode supply fluid, the composition of the anode supply fluid supplied to the anode space 150, and the flow rate of the anode supply fluid in the electrolytic unit 10 is regulated to satisfy a storage start condition.
  • (4) a storage process: the electrolytic unit 10 is used (stored) in a storage condition range including the storage start condition.

The startup process, the operation process, the shutdown process, and the storage process may be performed in order of the above numbers. That is, the operation process may be performed next to the startup process, the shutdown process may be performed next to the operation process, and the storage process may be performed next to the shutdown process. Further, the startup process may be performed next to the storage process again, and a sequence composed of the startup process, the operation process, the shutdown process, and the storage process may be repeated multiple times. At this time, the operation start condition, the operation condition range, the storage start condition, and storage condition range may differ each time. Operation contents in each process may differ each time.

The startup process, the operation process, the shutdown process, and the storage process can be distinguished from one another according to variations of the parameters to be adjusted.

At switching from the startup process to the operation process, when all of values of parameters which are control targets (control target parameter values) of the plurality of parameters including the temperature, the pressure, the current density, the voltage, the composition of the cathode supply fluid supplied to the cathode space 140, the flow rate of the cathode supply fluid, the composition of the anode supply fluid supplied to the anode space 150, and the flow rate of the anode supply fluid in the electrolytic unit 10, which are adjustable parameters, are controlled continuously over predetermined durations in ranges in which predetermined allowable widths are added to setting values of predetermined operation start conditions (operation start condition setting values) (operation start condition allowable widths), a judgment of the switching from the startup process to the operation process can be made. Further, switching from the shutdown process to the storage process is also similar. When all of values of parameters which are control targets of the adjustable parameters are controlled continuously over predetermined durations in ranges in which predetermined allowable widths are added to setting values of predetermined storage start conditions, a judgment of the switching from the shutdown process to the storage process can be made.

At switching from the operation process to the shutdown process, when at least one of values of parameters which are control targets (control target parameter values) of the plurality of parameters including the temperature, the pressure, the current density, the voltage, the composition of the cathode supply fluid supplied to the cathode space 140, the flow rate of the cathode supply fluid, the composition of the anode supply fluid supplied to the anode space 150, and the flow rate of the anode supply fluid in the electrolytic unit 10, which are the adjustable parameters, is varied to deviate from a range in which a predetermined allowable width is added to a boundary value in the predetermined operation condition range (operation condition range boundary value) (operation condition range boundary value allowable width), a judgment of the switching from the operation process to the shutdown process can be made. Similarly also at switching in shifting from the storage process to the startup process again, when at least one of values of parameters which are control targets of the adjustable parameters is varied to deviate from a range in which a predetermined allowable width is added to a boundary value of the predetermined storage condition range (storage condition range boundary value) (storage condition range boundary value allowable width), a judgment of the switching from the storage process to the startup process can be made.

As an example, a case where the temperature, the pressure, and the current density in the electrolytic unit 10 are set as the control target parameters in the startup process will be explained. The predetermined allowable width and the predetermined duration may be set for each parameter.

When the temperature in the electrolytic unit 10 is controlled continuously for the predetermined duration or longer in a range of ±5° C. relative to the setting value of the predetermined operation start condition, for example, a judgment of satisfying a switching condition from the startup process to the operation process can be made. The allowable width is more preferably ±3° C. When it is difficult to directly measure an internal temperature of the electrolytic unit 10, temperatures of the fluids in the flow paths P1, P2, P3, P4 may be measured to use any of the measured temperatures as an index in place of the temperature in the electrolytic unit 10.

When the pressure in the electrolytic unit 10 is controlled continuously for the predetermined duration or longer in a range of ±10 kPa relative to the setting value of the predetermined operation start condition, for example, a judgment of satisfying a switching condition from the startup process to the operation process can be made. The allowable width is more preferably ±5 kPa. When it is difficult to directly measure an internal pressure of the electrolytic unit 10, a pressure of the cathode supply fluid measured in the flow path P1 connected to the inlet of the cathode space 140, a pressure of the anode supply fluid measured in the flow path P2 connected to the inlet of the anode space 150, or a differential pressure calculated as a difference between them may be set as an index, for example.

When the current density is controlled continuously for the predetermined duration or longer in a range of +10 mA/cm² relative to the setting value of the predetermined operation start condition, for example, a judgment of satisfying a switching condition from the startup process to the operation process can be made. The allowable width is more preferably +5 mA/cm².

The predetermined duration can be appropriately set to a time such as, for example, 1 minute, 2 minutes, 3 minutes, 5 minutes, 10 minutes, or 15 minutes according to the performance of the electrochemical reaction device of the embodiment.

In the startup process, for example, operations (action) such as, for example, a distribution (supply) of the cathode supply fluid to the cathode space 140, a distribution (supply) of the anode supply fluid to the anode space 150, preheating of the electrolytic unit 10, a pressure rise for adjustment of an inter-electrode differential pressure to be described later, a start of current application, and a rise in the current density may be performed.

In the operation process, operations (action) such as, for example, an operation at a constant current, an operation at a constant voltage, and fine adjustment of operation conditions accompanying performance degradation of the electrolytic unit 10 may be performed.

In the shutdown process, operations (action) such as, for example, a decrease in the current density, a stop of the current application, a stop of the distribution of the cathode supply fluid to the cathode space 140, a stop of the distribution of the anode supply fluid to the anode space 150, a purge by making a gas flow through the cathode space 140 and the anode space 150, depressurization for adjustment of the inter-electrode differential pressure, and cooling of the electrolytic unit 10 may be performed.

In the storage process, for example, an operation (action) of sealing the electrolytic unit 10 by operating valves provided in the flow paths P1, P2, P3, P4 to leave the electrochemical reaction device 1 still may be performed. Alternatively, an operation (action) of holding a differential pressure between the cathode space 140 and the anode space 150 (inter-electrode differential pressure) in a state of supplying the cathode supply fluid at a constant flow rate to the cathode space 140 and supplying the anode supply fluid at a constant flow rate to the anode space 150 may be performed.

Here, attention is focused on pressures of the interior of the electrolytic unit 10 as a factor of affecting electrolytic cell performance, specifically, the selectivity of the product and a cell voltage.

The pressure in the cathode space 140 is different depending on positions. When the cathode space 140 forms the cathode flow path in particular, a pressure loss is produced between an upstream portion (for example, a portion closer to the inlet than the outlet) and a downstream portion (for example, a portion closer to the outlet than the inlet) of the cathode flow path, so that the upstream portion is higher in pressure than the downstream portion. The magnitude of the pressure loss in the cathode space 140 is different depending on a shape of the cathode space 140, and also different depending on the operation conditions of the electrolytic unit 10. For example, the larger the flow rate of the cathode supply fluid supplied to the cathode space 140 is, the larger a value of the pressure loss is. Similarly, the pressure in the anode space 150 is different depending on positions. When the anode space 150 forms the anode flow path in particular, a pressure loss is produced between an upstream portion (for example, a portion closer to the inlet than the outlet) and a downstream portion (for example, a portion closer to the outlet than the inlet) of the anode flow path, so that the upstream portion is higher in pressure than the downstream portion. The magnitude of the pressure loss in the anode space 150 is different depending on a shape of the anode space 150, and also different depending on the operation conditions of the electrolytic unit 10. For example, the larger the flow rate of the anode supply fluid supplied to the anode space 150 is, the larger a value of the pressure loss is. Further, when the aqueous solution (anode aqueous solution) is supplied to the anode space 150, a gas such as oxygen is produced by the anode 12 accompanying the operation of the electrolytic unit 10, and a gas-liquid two-phase flow is formed in the anode space 150, thereby increasing the pressure loss in the anode space 150. The value of the pressure loss in the anode space 150 is affected by a gas production amount in the anode 12. In other words, the value of the pressure loss in the anode space 150 is affected by the current density.

During each of the startup process, the operation process, and the shutdown process, the electrolytic unit 10 is preferably controlled so that a first time-averaged pressure at a first position in the cathode space 140 and closer to the inlet of the cathode space 140 than the outlet of the cathode space 140 is equal to or higher than a second time-averaged pressure at a second position in the anode space 150 and opposite the first position with the diaphragm 13 therebetween, or higher than it, and a third time-averaged pressure at a third position in the cathode space 140 and closer to the outlet than the inlet is equal to or higher than a fourth time-averaged pressure at a fourth position in the anode space 150 and opposite the third position with the diaphragm 13 therebetween, or higher than it. Moreover, also in the storage process, it is preferable that, similarly, the first time-averaged pressure is equal to or higher than the second time-averaged pressure, or higher than it, and the third time-averaged pressure is equal to or higher than the fourth time-averaged pressure, or higher than it.

The first time-averaged pressure can be specified by a value of a first pressure at the first position which is obtained by averaging over a necessary time in each process. The second time-averaged pressure can be specified by a value of a second pressure at the second position which is obtained by averaging over the necessary time in each process. The third time-averaged pressure can be specified by a value of a third pressure at the third position which is obtained by averaging over the necessary time in each process. The fourth time-averaged pressure can be specified by a value of a fourth pressure at the fourth position which is obtained by averaging over the necessary time in each process.

Further, the difference in pressure between the cathode space 140 and the anode space 150 is referred to as the inter-electrode differential pressure. The pressures in the cathode space 140 and in the anode space 150 are different depending on the positions as previously described. Therefore, the inter-electrode differential pressures are also different depending on the positions. Thus, in the following, attention is focused on a difference between a pressure at an arbitrary position (position A) in the cathode space 140 and a pressure at a position (position B) in the anode space 150 and opposite the position A with the diaphragm 13 therebetween, and a value obtained by subtracting the pressure on the anode 12 side from the pressure on the cathode 11 side is defined as the difference (inter-electrode differential pressure) between the pressure at the position A and the pressure at the position B.

For example, when the gas of the cathode supply fluid is supplied to the cathode space 140 and the liquid of the anode supply fluid is supplied to the anode space 150 respectively to perform the electrochemical reaction, it is preferable to perform the operation to set the inter-electrode differential pressure at the positions to zero or higher, that is, to set the pressure on the cathode 11 side (pressure in the cathode space 140) to be equal to or higher than the pressure on the anode 12 side (pressure in the anode space 150) at least in the startup process, the operation process, and the shutdown process. Setting the inter-electrode differential pressure to zero kPa or higher allows the liquid on the anode 12 side to be restrained from excessively moving to the cathode 11 side via the diaphragm 13. The movement of the liquid on the anode 12 side to cathode 11 side is necessary to some extent in terms of forming a reaction field inside the cathode 11, but may have an adverse effect when an amount of the moving liquid is excess. The excessively moved liquid can enter pores provided in the cathode catalyst layer and the gas diffusion layer of the cathode 11 to clog the pores. The clogging of the pores inhibits the gas which is the reactant from being supplied to the reaction field near a surface of the cathode catalyst to cause the performance degradation of the electrolytic unit 10 such as an increase in cell resistance and a decrease in product selectivity to the objective products. The excessive movement of the liquid can be restrained and the performance degradation of the electrolysis cell can be reduced by setting the inter-electrode differential pressure to zero or higher. The degradation in performance of the electrolytic unit 10 in the second and subsequent operation processes can be reduced in performing the start and stop multiple times by keep the inter-electrode differential pressure at zero or higher not only in the operation process but also at least in the startup process and the shutdown process.

FIG. 5, FIG. 6, FIG. 7 are each a schematic graph illustrating a relation between the positions and pressures in the flow paths. FIG. 5, FIG. 6, FIG. 7 each have a horizontal axis representing the positions of the flow paths and a vertical axis representing the pressures in the flow paths. The inter-electrode differential pressures are different depending on the positions as described above, and thus a minimum value of the inter-electrode differential pressure is preferably zero or higher. Further, the minimum value of the inter-electrode differential pressure is more preferably positive. When the gas of the cathode supply fluid is supplied to the cathode space 140 and the liquid of the anode supply fluid is supplied to the anode space 150, the anode space 150 is often larger in the pressure loss than the cathode space 140. For example, when the cathode flow path inlet (cathode inlet) and the anode flow path inlet (anode inlet) are at positions opposite each other with the diaphragm 13 therebetween, the cathode flow path outlet (cathode outlet) and the anode flow path outlet (anode outlet) are at positions opposite each other with the diaphragm 13 therebetween, and the cathode fluid and the anode fluid are made to flow in the same direction, the inter-electrode differential pressure at the outlet sides of the flow paths is higher than the inter-electrode differential pressure at the inlet sides of the flow paths (FIG. 5). Further, as illustrated in FIG. 6, a case where the pressure at the outlet of the cathode flow path is lower than the pressure at the inlet of the anode flow path is also applicable. Also in this case, all the inter-electrode differential pressures at the positions in the flow paths are zero or higher. When the cathode fluid and the anode fluid are made to flow in reverse directions, the inter-electrode differential pressure at positions where the outlet of the cathode flow path and the inlet of the anode flow path are opposite each other with the diaphragm 13 therebetween becomes smallest in the electrolytic unit 10 (FIG. 7).

It is difficult to actually measure the pressure and the inter-electrode differential pressure at each of positions inside the electrolytic unit 10, and thus it is useful to be estimated from pressure measured values at the outlets and inlets of the two poles of the cell. A case where the cathode space 140 forms the cathode flow path and the anode space 150 forms the anode flow path respectively, the cathode flow path inlet and the anode flow path inlet are at the positions opposite each other with the diaphragm 13 therebetween, the cathode flow path outlet and the anode flow path outlet are at the positions opposite each other with the diaphragm 13 therebetween, and the cathode fluid and the anode fluid are made to flow in the same direction is explained as an example. For example, pressure gauges are provided in the flow path P1, the flow path P2, the flow path P3. the flow path P4 connected at the outlets and the inlets of the cathode flow path and the anode flow path to measure the pressures. The pressure gauges are preferably provided at the positions as close to the cathode space 140 and the anode space 150 as possible. The respective pressure losses of the cathode flow path and the anode flow path can be obtained from the pressures measured in such a manner. Further, it is possible to find the inter-electrode differential pressure at the flow path inlets by subtracting the pressure at the anode flow path inlet from the pressure at the cathode flow path inlet and the inter-electrode differential pressure at the flow path outlets by subtracting the pressure at the anode flow path outlet from the pressure at the cathode flow path outlet respectively. For example, when it is assumed that the inter-electrode differential pressure inside the electrolytic unit 10 varies along the flow paths, and the inter-electrode differential pressure at the flow path inlets and the inter-electrode differential pressure at the flow path outlets can be approximated by a linear function as end points, the inter-electrode differential pressure at each of the positions can be estimated to be a value obtained by internally dividing values of the end points by a length of the flow paths. Further, a spatial average value of the inter-electrode differential pressures can be estimated by a simple average value of the inter-electrode differential pressure at the flow path inlets and the inter-electrode differential pressure at the flow path outlets, for example.

The pressure in the cathode space 140, the pressure in the anode space 150, and the inter-electrode differential pressure in each process vary with time. Thus, at least in the startup process, the operation process, and the shutdown process, the operation can be performed so that a spatial minimum value of the inter-electrode differential pressure is time-averaged by the necessary time in the corresponding process and the obtained time-averaged value (time-averaged inter-electrode differential pressure) is zero or higher. The value of the time-averaged inter-electrode differential pressure is more preferably positive.

For example, when the electrochemical reaction device 1 is controlled so that the spatial minimum value of the inter-electrode differential pressure is a positive value close to zero, the time when the spatial minimum value of the inter-electrode differential pressure is negative also occurs depending on accuracy of the control. Also in such a case, as long as the time-averaged value of the spatial minimum value of the inter-electrode differential pressure through the processes is zero or higher, the above-described effect of reducing the performance degradation can be promising.

Also in the storage process, the operation may be performed with a minimum value of the inter-electrode differential pressure set to zero or higher. The pressure in the cathode space 140, the pressure in the anode space 150, and the inter-electrode differential pressure in the storage process vary with time. Thus, the electrolytic unit 10 can be operated so that a spatial minimum value of the inter-electrode differential pressure is time-averaged by the necessary time in the storage process and the obtained time-averaged value (time-averaged inter-electrode differential pressure) is zero or higher. The time-averaged value is more preferably positive. This prevents the liquid on the anode 12 side from excessively moving to the cathode 11 side via the diaphragm 13 to allow the reduction of the degradation in performance of the electrolytic unit 10 in the second and subsequent operation processes.

The inter-electrode differential pressure in the operation process may be set to be equal to or higher than the inter-electrode differential pressure in the storage process. The inter-electrode differential pressures are different depending on the positions, and also depend on the time. Thus, in consideration of the value obtained by time-averaging the spatial minimum value of the inter-electrode differential pressure in the each process, this value may be set in the storage process to be equal or lower than that in the operation process. Further, in consideration of a value obtained by further time-averaging, in each process, a value obtained by spatially averaging the inter-electrode differential pressure in the electrolytic unit 10, this value may be set in the storage process to be equal or lower than that in the operation process Further, regarding a first time-averaged inter-electrode differential pressure between the first pressure at the first position and the second pressure at the second position and a second time-averaged inter-electrode differential pressure between the third pressure at the third position and the fourth pressure at the fourth position, the first and second time-averaged inter-electrode differential pressures in the storage process are preferably set to be equal to or lower than the first and second time-averaged inter-electrode differential pressures in the operation process respectively.

In the startup process, supplying the humidified gas of the cathode supply fluid to the cathode space 140 is effective in the reduction of the performance degradation of the electrolytic unit 10. In particular, when the start and stop are repeated, the humidified gas is preferably supplied to the cathode space 140 in the second and subsequent startup processes. For example, when the gas of the cathode supply fluid is supplied to the cathode space 140 and the aqueous solution of the anode supply fluid is supplied to the anode space 150 to perform electrolysis, at the time of the second and subsequent startup processes, the aqueous solution moved from the anode space 150 to the cathode space 140 during the operation previous to them is present in the cathode space 140. When a dry gas is supplied to the cathode in the state, there is a possibility that water is evaporated from the aqueous solution in the cathode space 140, and the aqueous solution is highly concentrated or solid salt is precipitated. The high-concentration aqueous solution and the solid salt in the cathode space 140 can accelerate the movement of the aqueous solution from the anode space 150 to the cathode space 140. Further, the solid salt itself inhibits diffusion of the gas which is the reactant. Such an influence causes the degradation in performance of the electrolytic unit 10 in the operation process thereafter. During the startup process, supplying the humidified gas to the cathode space 140 increases a relative temperature in the cathode space 140 as compared with a case of supplying the dry gas to prevent the electrolytic solution present in the cathode space 140 from being highly concentrated and the solid salt from being precipitated, resulting in contributing to maintaining the performance of the electrolytic unit 10.

A dew point of the humidified gas supplied to the cathode space 140 is preferably 0° C. or higher, and further preferably room temperature (25° C.) or higher in the startup process. It is more preferably 40° C. or higher. On the other hand, when the dew point of the supplied gas is equal to or higher than a temperature in the pipe (flow path P1) connected to the inlet of the cathode space 140 and a temperature in the cathode space 140, water vapor in the supply gas condenses to produce liquid water, resulting in that this can also inhibit the diffusion of the gas which is the reactant. Therefore, the temperature in the flow path P1 and the temperature in the cathode space 140 may be measured to control the dew point of the supplied gas to these temperatures or lower.

In the startup process, the electrolytic unit 10 may be preheated. In the startup process, the operation process, the shutdown process, and the storage process, when the dew point of the gas supplied to the cathode space 140 is higher than the temperature in flow path P1 and the temperature in the cathode space 140, the condensed water may be produced. The condensed water can enter the interior of the cathode space 140, particularly a porous structure of the cathode catalyst layer and the gas diffusion layer to inhibit the diffusion of the reactant gas and a product gas, resulting in causing the performance degradation of the electrolytic unit 10. In response to this, the condensation in the cathode space 140 of water vapor in the gas supplied to the cathode space 140 can be reduced by preheating the electrolytic unit 10 in the startup process. Further, the condensation of the water vapor in various pipes can also be reduced by preheating not only the electrolytic unit 10 but also various pipes including the flow path P1. The preheating can be performed by connecting, for example, a heater which heats the electrolytic unit 10 and a temperature regulator to the electrolytic unit 10.

The temperatures in the cathode space 140 and the flow path P1 are preferably increased to and above the dew point of the gas supplied to the cathode space 140 in the operation process by the preheating operation in the startup process. Further, also during the startup process, the temperature in the cathode space 140 and the temperature in the flow path P1 are preferably equal to or higher than the dew point of the supplied gas. For example, in a state where the temperature in the cathode space 140 and the temperature in the flow path P1 are low before a preheating start, the dew point of the cathode supply gas can be increased by lowering the dew point of the cathode supply gas, and thereafter waiting for the temperature in the cathode space 140 and the temperature in the flow path P1 to rise by the preheating operation. The dew point of the cathode supply gas may be continuously varied to respond to a rise in the temperature in the cathode space 140 and the temperature in the flow path P1.

Further, the preheating operation may be performed before applying current to the electrolytic unit 10 in the startup process. The startup process includes an operation of starting the current application and increasing a current value to a value specified in the operation start condition (current application operation). It is possible to perform the preheating operation first, and at the same time, increase the dew point of the gas supplied to the cathode 11 to a predetermined value of the operation start condition while suppressing the condensed water production in the flow path P1 and the cathode space 140 to thereafter perform the current application operation. Such a manner allows an electrolytic reaction during the current application operation to be performed under conditions close to the operation start condition, and allows prevention of an unintended adverse effect on the electrolytic unit 10.

As methods of preheating the electrolytic unit 10, for example, there are a method of making an aqueous solution with a temperature higher than a temperature in the electrolytic unit 10 flow through the anode space 150, a method of operating a heater attached to the electrolytic unit 10, and the like, but these are not restrictive. Further, regarding a preheating time, there are a method of ending the preheating in a predetermined time, a method of measuring a temperature at a specific point in the electrolytic unit 10 and continuing the preheating until the temperature exceeds a prescribed value, and the like, but these are not restrictive.

In the shutdown process, in order to change the composition of the fluid in the cathode space 140 and the composition of the fluid in the anode space 150, a purge operation can be performed. For example, when the humidified gas containing carbon dioxide is supplied to the cathode space 140 and the aqueous solution is supplied to the anode space 150 to perform the electrolysis, a mixed gas of the objective product, hydrogen which is a by-product, and unreacted humidified carbon dioxide is present in the cathode space 140, and the gas-liquid two-phase flow of the aqueous solution and a produced gas is present in the anode space 150 in the operation process. In the shutdown process, when the humidified carbon dioxide keeps being supplied as it is to the cathode space 140 after stopping current, the fluid in the cathode space 140 is replaced with only the humidified carbon dioxide. Also regarding the anode space 150, when the supply of the aqueous solution is stopped after stopping current, an inert gas such as, for example, an argon gas is made to flow through in place thereof, the fluid including the gas-liquid two-phase flow present in the anode space 150 is discharged outside the anode space 150 to be replaced with the argon gas in the anode space 150.

When the liquid is present in the anode space 150 in the storage process, a part of the liquid can move from the anode space 150 via the diaphragm 13 to the cathode space 140 and enter the porous structure of the cathode catalyst layer and the gas diffusion layer. Then, the liquid can inhibit the diffusion of the reactant gas and the product gas at the time of the next start to cause the performance degradation of the electrolytic unit 10. By discharging the liquid from the interior of the anode space 150, the effect of suppressing such a phenomenon to maintain the performance of the electrolytic unit 10 is promising.

In the purge operation, the gas supplied to the cathode space 140 is preferably the humidified gas. A dew point of the gas is preferably 0° C. or higher, and further preferably room temperature or higher. It is more preferably 40° C. or higher. On the other hand, when the dew point of the gas supplied for the purge to the cathode space 140 is equal to or higher than the temperature in the flow path P1 and the temperature in the cathode space 140, water vapor in the gas condenses to produce liquid water, resulting in that this can also inhibit the diffusion of the gas which is the reactant at the time of the next start. Therefore, the temperature in the flow path P1 and the temperature in the cathode space 140 may be measured to control the dew point of the supplied gas to these temperatures or lower.

In the operation process, the flow rate of the reducible material to be supplied to the cathode space 140 and the flow rate of the oxidizable material to be supplied to the anode space 150 may be constantly controlled to be equal to or more than theoretical amounts calculated from a current applied to the electrolytic unit 10.

Here, the theoretical amount calculated from the current applied to the electrolytic unit 10 is defined as follows. When the reduction reaction in the cathode space is considered, in setting a current applied to the electrolytic unit 10 as I [A], a Faraday constant as F [C/mol], the number of electrons required for the reduction product to produce one molecule as k, and the number of molecules of the reducible material required for the production of one molecule of the reduction product as m, a theoretical amount N [mol/s] of the reducible material is found as N=(mI)/(kF). The theoretical amount Nis appropriately converted into another unit such as a volume flow rate. Also regarding the oxidation reaction in the anode space 150, a theoretical amount N of the oxidizable material is found by a similar formula by setting the number of electrons required for the oxidation product to produce one molecule as k and the number of molecules of the oxidizable material as m.

When the supply flow rates of the reducible/oxidizable materials are smaller than the theoretical amounts, a part of the flowing current is used for a side reaction, resulting in a decrease in selectivities of the intended reduction/oxidation products. Thus, in order to produce the intended reduction/oxidation products efficiently or produce them in high concentrations, the flow rates of the reducible/oxidizable materials may be set to be equal to or more than the theoretical amounts. From the viewpoint of suppressing the side reaction, the flow rates of the reducible/oxidizable materials are more preferably 105% or more of the theoretical amounts, and further preferably 110% or more of the theoretical amounts.

Also in the startup process and the shutdown process, the flow rate of the reducible material to be supplied to the cathode space 140 and the flow rate of the oxidizable material to be supplied to the anode space 150 may be constantly controlled to be equal to or more than the theoretical amounts calculated from the current applied to the electrolytic unit 10. In the startup process and the shutdown process, for example, when a side reaction product adversely affects the performance and durability of the electrolytic unit 10, the side reaction is also required to be suppressed in the startup process and the shutdown process. Thus, the flow rates of the reducible/oxidizable materials may be set to be equal to or more than the theoretical amounts. From the viewpoint of suppressing the side reaction, the flow rates of the reducible/oxidizable materials are more preferably 105% or more of the theoretical amounts, and further preferably 110% or more of the theoretical amounts.

In the startup process, the operation process, the shutdown process, and the storage process, a current flowing through the electrolytic unit 10 basically includes a period in a direction of flowing from the cathode 11 through an external circuit including the power supply 40 into the anode 12. In other words, a current flows from a positive electrode of the power supply 40 to the anode 12 of the electrolytic unit 10, and a current flows from the cathode 11 of the electrolytic unit 10 to a negative electrode of the power supply 40. With this direction set to be positive, the operation can be performed with the current density constantly set to −5 mA/cm² or higher. This corresponds to its magnitude being 5 mA/cm² or lower even though the reverse current flows. In particular, when a current is stopped in the shutdown process, the reverse current may flow. When a value of the reverse current is large, an unintended reaction, for example, corrosion of an electrode material can progress in the cathode and the anode to lead to the performance degradation of the electrolytic unit 10. The current density is more preferably constantly zero or higher in the startup process, the operation process, the shutdown process, and the storage process.

As described above, the reverse current can become a main cause of leading to the corrosion of the electrodes, or the like to degrade the performance of the electrolytic unit 10. On the other hand, the reverse current also plays a role in refreshing the electrolytic unit 10 in another aspect. Thus, in at least one of the startup process and the shutdown process, control may be performed to include a period during which the current density flowing through the electrolytic unit 10 is lower than zero with the direction of flowing from the cathode 11 through the external circuit including the power supply 40 into the anode set to be positive.

For example, when carbon dioxide is made to flow through the cathode space 140 and the aqueous solution is made to flow through the anode space 150 to perform the electrolysis, a phenomenon in which the aqueous solution moves from the anode space 150 via the diaphragm 13 to the cathode space 140 is accelerated by the current in the positive direction. Excessive movement of the aqueous solution inhibits the diffusion of the reactant gas and the product gas to lead to the performance degradation of the electrolytic unit 10. By making the reverse current flow through the electrolytic unit 10, the effect of returning a part of the electrolytic solution moved to the cathode space 140 to the diaphragm 13 and the anode space 150 is promising.

In the startup process, the operation process, the shutdown process, and the storage process, the operation may be performed so that an electric potential of the cathode 11 based on a standard hydrogen electrode reference is constantly +1.5 V or less. The electric potential of the cathode 11 is preferably +1.0 V or less, and more preferably +0.5 V or less based on the standard hydrogen electrode reference. It is further preferably 0 V or less. For example, in a case of using metal nanoparticles supported by carbon as a cathode catalyst, when the electric potential of the cathode 11 becomes high, oxidation of the carbon support, elution and reprecipitation of the metal component, and the like may progress, resulting in a decrease in catalytic activity due to a change in catalyst structure and an increase in metal particle size. In order to suppress such a phenomenon, the electric potential of the cathode 11 is preferably controlled to be low. A phenomenon in which the electric potential of cathode 11 varies in a spike shape in a noble direction is likely to occur in stopping a current in the shutdown process, so that it is effective to control the electric potential of the cathode 11 in the shutdown process in particular.

In the storage process, a current may be set to zero. This allows a production speed of the product in the storage process to be set to zero, which is suitable for using the electrochemical reaction device. On the other hand, in the storage process, a current may be set to a value larger than zero. This allows the suppression of such phenomena as the variation in electrode potential in a spike shape and the reverse current likely to occur in setting a current to zero, and the effect of suppressing the performance degradation of the electrolytic unit 10 at the time of repeated start and stop is promising.

In the startup process, an increase rate of the current density is preferably controlled to be 1 mA/cm² per second or less at the maximum. When the current density is increased rapidly, an overshoot of a cell voltage in the electrolytic unit 10 may occur, so that the electrode potential may have an unintended value. Further, in a case of supplying the electrolytic solution to the anode space 150 to perform the electrolysis, when a current is applied rapidly, a large amount of gas is suddenly generated by the anode 12, thus leading to a rapid rise in pressure in the anode space 150 to make control of the inter-electrode differential pressure difficult. In order to suppress such a situation, an increase in current density in the startup process is preferably slowly performed.

FIG. 8 is a schematic diagram illustrating another example configuration of the electrochemical reaction device of the embodiment. The electrochemical reaction device 1 may further have a power supply 40, a temperature regulator 50, a pressure regulator 60, and a controller 70 in addition to the electrolytic unit 10.

The power supply 40 can supply voltage or current to the electrolytic unit 10. The power supply 40 may be connected to the current collector 16 and the current collector 17 via a wiring line, for example. The other explanation of the power supply 40 can be appropriately quoted from the explanation of the power supply 40 illustrated in FIG. 2.

The temperature regulator 50 can regulate the temperature of the electrolytic unit 10. The temperature regulator 50 may have, for example, a heater or a cooler. The preheating of the electrolytic unit 10 may be performed by heating the electrolytic unit 10 with the temperature regulator 50.

The pressure regulator 60 can regulate the pressure in the electrolytic unit 10. The pressure regulator 60 may control the inter-electrode differential pressure by, for example, appropriately regulating the pressures in the flow path P1, the flow path P2, the flow path P3, and the flow path P4, and regulating the pressure in the cathode space 140 and the pressure in the anode space 150.

The controller 70 can control the electrolytic unit 10, the power supply 40, the temperature regulator 50, and the pressure regulator 60 to control the action of the operation of the electrochemical reaction device by the above-described various operation methods.

The electrochemical reaction device of the embodiment may further have at least one detector. As examples of the detector, there can be cited a thermometer, a pressure gauge, a dew-point meter, an ammeter, a voltmeter, a gas flowmeter, a gas composition analyzer, and so on, but these detectors are not restrictive. FIG. 9 is a schematic diagram illustrating the other example configuration of the electrochemical reaction device of the embodiment. The electrochemical reaction device 1 may further have a detector D1, a detector D2, a detector D3, and a detector D4, as illustrated in FIG. 9.

The detector D1 is in the course of the flow path P1, and is provided, for example, in the previous stage of the electrolytic unit 10. The detector D1 can detect at least one parameter of the cathode supply fluid, for example. Examples of at least one parameter include a temperature, a pressure, a dew point, a current, a voltage, a flow rate, composition, and so on. As examples of the detector D1, there can be cited the thermometer, the pressure gauge, the dew-point meter, the ammeter, the voltmeter, the gas flowmeter, the gas composition analyzer, and so on, but these detectors are not restrictive.

The detector D2 is in the course of the flow path P2, and is provided, for example, in the previous stage of the electrolytic unit 10. The detector D2 can detect at least one parameter of the anode supply fluid, for example. Examples of at least one parameter include a temperature, a pressure, a dew point, a current, a voltage, a flow rate, composition, and so on. As examples of the detector D2, there can be cited the thermometer, the pressure gauge, the dew-point meter, the ammeter, the voltmeter, the gas flowmeter, the gas composition analyzer, and so on, but these detectors are not restrictive.

The detector D3 is in the course of the flow path P3, and is provided, for example, in the subsequent stage of the electrolytic unit 10. The detector D3 can detect at least one parameter of the cathode discharge fluid from the electrolytic unit 10, for example.

Examples of at least one parameter include a temperature, a pressure, a dew point, a current, a voltage, a flow rate, composition, and so on. As examples of the detector D3, there can be cited the thermometer, the pressure gauge, the dew-point meter, the ammeter, the voltmeter, a flowmeter, a composition analyzer, and so on, but these detectors are not restrictive.

The detector D4 is in the course of the flow path P4, and is provided, for example, in the subsequent stage of the electrolytic unit 10. The detector D4 can detect at least one parameter of the anode discharge fluid from the electrolytic unit 10, for example. Examples of at least one parameter include a temperature, a pressure, a dew point, a current, a voltage, a flow rate, composition, and so on. As examples of the detector D4, there can be cited the thermometer, the pressure gauge, the dew-point meter, the ammeter, the voltmeter, the flowmeter, the composition analyzer, and so on, but these detectors are not restrictive.

The electrochemical reaction device 1 may have the detector in the electrolytic unit 10. The detector of the electrolytic unit 10 can detect at least one parameter related to operation conditions of the electrolytic unit 10 and at least one parameter related to states of the cathode supply fluid, the cathode discharge fluid, the anode supply fluid, and the anode discharge fluid flowing through any of the above-described components, for example. Examples of at least one parameter of these include a temperature, a pressure, a dew point, a current, a voltage, a flow rate, composition, and so on. As examples of the detectors for these, there can be cited the thermometer, the pressure gauge, the dew-point meter, the ammeter, the voltmeter, the flowmeter, the composition analyzer, and so on, but these detectors are not restrictive. These parameters are each sent to a receiving unit 71 as a detection signal (data signal).

The controller 70 includes the receiving unit 71 which receives the detection signal from at least one of the detector D1, the detector D2, the detector D3, the detector D4, the detector provided in the electrolytic unit 10, the power supply 40, the temperature regulator 50, and the pressure regulator 60, a processing unit 72 which performs arithmetic processing based on the detection signal, and a controlling unit 73 which generates control signals to control operations of the electrolytic unit 10, the power supply 40, the temperature regulator 50, and the pressure regulator 60 based on results of the arithmetic processing. The controller 70 may be configured using hardware using a processor or the like, for example. Each operation may be stored as an operation program in a computer-readable storage medium such as a memory, and each operation may be executed by appropriately reading the operation program stored in the storage medium by the hardware.

The receiving unit 71 receives the detection signal from at least one of the detectors D1, D2, D3, D4, the detector in the electrolytic unit 10, the power supply 40, the temperature regulator 50, and the pressure regulator 60 to transmit it to the processing unit 72. The processing unit 72 includes, for example, the computer implemented with calculation algorithm. The processing unit 72 performs the arithmetic processing (calculation) based on information on the received detection signal, and determines how the electrolytic unit 10 are operated. Then, determination results thereof are transmitted to the controlling unit 73. The controlling unit 73 may transmit control signals to the electrolytic unit 10, the power supply 40, the temperature regulator 50, and the pressure regulator 60 based on the determination results (arithmetic results) received from the processing unit 72, and they may be controlled to be in desired operation conditions.

The receiving unit 71, the processing unit 72, and the controlling unit 73 may be mounted in different computers, or may be mounted in one computer. The processing in the processing unit 72 may be automatically performed, or may be performed to include the determination by an operator. For example, it is also applicable that a part of the result obtained by analyzing the signal in the processing unit 72 is displayed to the operator by a user interface, the operator looks at it and inputs an instruction into the processing unit 72, the processing unit 72 executes additional processing based on the instruction or based on both the received signal and the input instruction, and the result is output to the controlling unit 73.

The calculation algorithm implemented in the processing unit 72 can be created based on the finding of a preliminary experiment or the like. Further, the algorithm may be updated according to stored operation data as is a machine learning model.

A refresh process may be performed during the operation process. The refresh process can be performed for performance recovery when the cell performance in the electrolytic unit 10 degrades, or can be performed for prevention of the degradation in cell performance. The refresh process is neither included in the operation process nor included in the startup process, the shutdown process, or the storage process. A shift from the operation process to the refresh process can be judged when at least one of parameters which are control targets of the adjustable parameters is varied to deviate from a range in which a predetermined allowable width is added to a boundary value of the predetermined operation condition range. Further, a return from the refresh process to the operation process can be judged when all of the parameters which are the control targets of the adjustable parameters are controlled continuously over the predetermined duration in the ranges in which the predetermined allowable widths are added to the setting values of the predetermined operation start conditions.

As the operation of the refresh process, there can be cited operations (action) such as, for example, lowering the current density, reducing the current density to zero, making the reverse current flow, lowering a potential difference between the cathode 11 and the anode 12, reducing the potential difference to zero, reversing the electric potentials of the cathode and the anode, making a rinse solution flow through the cathode space 140, and drying the cathode space 140, but these are not restrictive. In the refresh process, it is acceptable that a state where a temporal-spatial average value of inter-electrode differential pressures becomes negative, that is, a pressure in the anode space 150 is higher than a pressure in the cathode space 140 occurs.

EXAMPLES

Examples 1 to 8, Comparative Examples 1, 2

Examples and comparative examples of an electrochemical reaction device which performs a reduction reaction of carbon dioxide will be explained below. In an experiment, a unit cell with an electrode area of 400 cm² was used. A carbon-supported gold catalyst was used for the cathode 11, an iridium oxide catalyst was used for the anode 12, and a porous polymeric membrane was used for the diaphragm 13. The cathode space 140 forms a cathode flow path, the anode space 150 forms an anode flow path, and fluids flow through both the flow paths in the same direction. A carbon dioxide gas was supplied to the cathode flow path, and the presence/absence of humidification was controlled by the humidifier 201. The carbon dioxide gas in the examples 1, 2, 5 to 8, and the comparative examples 1, 2 is a dry gas. The carbon dioxide gas in the examples 3, 4 is a humidified gas. An aqueous potassium hydrogen carbonate solution was supplied as an electrolytic solution to the anode flow path. A DC stabilized power supply was used for the power supply 40.

In each example, parameters targeted for control were evaluated, and a CO selectivity maintenance ratio (maintenance ratio of a produced CO amount) during the tenth operation process was evaluated as an index of comparing performance in each example. This is defined by a ratio obtained by dividing a CO selectivity in the tenth operation process in repeating a startup process, an operation process, a shutdown process, and a storage process by a CO selectivity in the first operation process. Table 1 lists values of the parameters in each process and the CO selectivity maintenance ratios. The CO selectivity represents a ratio of a production amount of CO in a reduction product. Further, a maximum increase rate of a current supplied to the cell was 1 mA/cm² per second or less. A current density in the operation process was 1.25 in the examples 5 to 8 when values thereof in the examples 1 to 4 and the comparative examples 1, 2 were each set to 1 (arbitrary unit (a.u.)). In the examples 1 to 8 and the comparative example 1, 2, an inter-electrode differential pressure in each process is a time-averaged inter-electrode differential pressure in each process. In each example, a current density in the storage process was set to 0 mA/cm².

In the examples 1 to 4 and the comparative examples 1, 2, a CO₂ flow rate in the startup process and a CO₂ flow rate in the shutdown process were each 125% or more relative to a theoretical amount calculated from the current density. Further, in the examples 5 to 7, a CO₂ flow rate in the startup process and a CO₂ flow rate in the shutdown process were each 150% or more relative to the theoretical amount calculated from the current density. Moreover, in the example 8, a CO₂ flow rate in the startup process and a CO₂ flow rate in the shutdown process were each varied in a range of 0% to 150% relative to the theoretical amount calculated from the current density.

In the examples 1 to 4, 6 to 8 and the comparative examples 1, 2, the electrolytic unit 10 was preheated. The preheating of the electrolytic unit 10 was performed by a method of making the heated electrolytic solution flow through the anode flow path.

In the shutdown process, the current density flowing through the electrolytic unit 10 was −5 mA/cm² or higher with a direction of flowing from the cathode 11 into the anode 12 set to be positive. In the shutdown process, an electric potential of the cathode 11 based on a standard hydrogen electrode reference (cathode potential vs SHE) was constantly +1.5 V or less.

Further, in the shutdown process in the examples 1 to 3, 5 to 8 and the comparative examples 1, 2, a purge operation was performed by supplying a CO₂ gas to the flow paths of both the electrodes. Further, in the shutdown process in the example 4, no purge operation was performed. The CO₂ gas supplied to the cathode flow path in the purge operation is the humidified gas in the examples 1 to 3, 5, 6, 8 and the comparative examples 1, 2, and is the dry gas in the example 7.

In the example 1, in the startup process, a time-averaged value of an inter-electrode differential pressure at flow path inlets calculated using fluid pressures measured in the flow paths P1, P2 was 0 kPa or higher. The gas flowed through the cathode flow path and the liquid flowed through the anode flow path to make the anode flow path larger in pressure loss, and thus as long as the time-averaged value of the inter-electrode differential pressure was 0 kPa or higher at the flow path inlets, the inter-electrode differential pressure became higher downstream therefrom, so that the time-averaged value of the inter-electrode differential pressure was also 0 kPa or higher as the whole of the cell. In the operation process, a time-averaged value of an inter-electrode differential pressure was about 90 kPa at the flow path inlets, and about 130 kPa at the flow path outlets. Also in the shutdown process, an inter-electrode differential pressure time-averaged value at the flow path inlets was 0 kPa or higher similarly to the startup process, and was thus also 0 kPa or higher as the whole of the cell. A CO selectivity maintenance ratio during the tenth operation process was 90% in the example 1.

In contrast to this, a time-averaged value of an inter-electrode differential pressure at the flow path inlets in the startup process was lower than zero in the comparative example 1. The other conditions are the same as those in the example 1. A CO selectivity maintenance ratio during the tenth operation process in the comparative example 1 was 78% to be lower than that in the example 1. The inter-electrode differential pressure increases along the flow paths as previously described, and thus it is considered that an inter-electrode differential pressure at the flow path outlets in the startup process was also 0 kPa or higher in the comparative example 1, but the inter-electrode differential pressure became negative near the flow path inlets, and thereby the movement of the electrolytic solution from the anode 12 to the cathode 11 progressed, so that gas diffusion in a cathode catalyst layer was inhibited to lower the CO selectivity.

In the comparative example 2, a time-averaged value of an inter-electrode differential pressure at the flow path inlets was lower than zero in the shutdown process. A CO selectivity maintenance ratio during the tenth operation process was 76% to be lower than that in the example 1 also in this case. This is considered as the effect in which the movement of the electrolytic solution from the anode flow path to the cathode flow path progressed near the flow path inlets.

In the example 2, an inter-electrode differential pressure time-averaged value in the storage process was lower than zero. The other conditions are the same as those in the example 1. A CO selectivity maintenance ratio during the tenth operation process in this case was 82% to be also lower than that in the example 1 here.

In the example 3, the gas supplied to the cathode flow path during the startup process was changed from the dry gas to the humidified gas. The other conditions are the same as those in the example 1. A CO selectivity maintenance ratio during the tenth operation process in this case was 97% to be improved from 90% in the example 1 by the change to the humidified gas.

In the example 4, the cell was stored with an inter-electrode differential pressure maintained in a state of keeping the humidified carbon dioxide gas flowing through the cathode and the aqueous potassium hydrogen carbonate solution flowing through the anode without performing the purge operation of the fluids in both the electrodes after stopping the application of current in the shutdown process. The other conditions are the same as those in the example 1. This caused a CO selectivity maintenance ratio during the tenth operation process to be 100%, resulting in that a deterioration was not seen in a range of significant figures.

The example 5 and the example 6 are different in whether or not the preheating of the cell was performed in the startup process. The example 6 in which the cell preheating was performed was slightly higher in a CO selectivity maintenance ratio during the tenth operation process as compared with the example 5 in which the cell preheating was not performed.

The example 6 and the example 7 are different in whether or not carbon dioxide flowing through the cathode flow path was humidified in the purge operation of the fluids in both the electrodes which was performed in the shutdown process. The example 7 in which the dry gas was used resulted in a decrease in a CO selectivity maintenance ratio during the tenth operation process as compared with the example 6 in which the humidified gas was used.

In the example 8, in the startup process and the shutdown process, a supply amount of carbon dioxide to the cathode flow path was controlled so that a time zone in which it was less than the theoretical amount calculated from the current density was present. As a result, a CO selectivity maintenance ratio during the tenth operation process was 85% to be lower as compared with 96% in the example 6 in which a carbon dioxide supply amount in the startup process and the shutdown process was constantly the theoretical amount or more.

	TABLE 1
	Startup process	Operation process

	Inter-electrode			CO2 flow rate		Inter-electrode
	differential	Cell		(theoretical	Current	differential
	pressure (kPa)	preheating	CO2 humidification	amount ratio)	density (a.u.)	pressure (kPa)
Example 1	≥0	Presence	Absence	≥125%	1	90-130
Comparative example 1	<0	Presence	Absence	≥125%	1	90-130
Comparative example 2	≥0	Presence	Absence	≥125%	1	90-130
Example 2	≥0	Presence	Absence	≥125%	1	90-130
Example 3	≥0	Presence	Presence	≥125%	1	90-130
Example 4	≥0	Presence	Presence	≥125%	1	90-130
Example 5	≥0	Absence	Absence	≥150%	1.25	90-130
Example 6	≥0	Presence	Absence	≥150%	1.25	90-130
Example 7	≥0	Presence	Absence	≥150%	1.25	90-130
Example 8	≥0	Presence	Absence	0-150%	1.25	90-130

			Storage
	Operation process	Shutdown process	process

	CO2 flow	Inter-	CO2 flow		Inter-	Characteristic
	rate	electrode	rate		electrode	CO selectivity
	(theoretical	differential	(theoretical		differential	maintenance
	amount	pressure	amount	Purge CO2	pressure	ratio
	ratio)	(kPa)	ratio)	humidification	(kPa)	(%)
Example 1	125%	≥0	≥125%	Presence	0	90
Comparative example 1	125%	≥0	≥125%	Presence	0	78
Comparative example 2	125%	<0	≥125%	Presence	0	76
Example 2	125%	≥0	≥125%	Presence	<0	82
Example 3	125%	≥0	≥125%	Presence	0	97
Example 4	125%	≥0	≥125%	Absence of purge	90-130	100
Example 5	150%	≥0	≥150%	Presence	0	95
Example 6	150%	≥0	≥150%	Presence	0	96
Example 7	150%	≥0	≥150%	Dry gas	0	89
Example 8	150%	≥0	0-150%	Presence	0	85

Examples 9 to 15

The other examples of the electrochemical reaction device which performs the reduction reaction of carbon dioxide will be explained below. In an experiment, the unit cell with the electrode area of 400 cm² was used. The carbon-supported gold catalyst was used for the cathode 11, the iridium oxide catalyst was used for the anode 12, and the porous polymeric membrane was used for the diaphragm 13. The cathode space 140 forms the cathode flow path, the anode space 150 forms the anode flow path, and the fluids flow through both the flow paths in the same direction. The carbon dioxide gas was supplied to the cathode flow path, and humidified by the humidifier 201. The aqueous potassium hydrogen carbonate solution was supplied as the electrolytic solution to the anode flow path. The DC stabilized power supply was used for the power supply 40.

In the examples 9 to 15 (also called Ex. 9 to Ex. 15), in the first startup process, a time-averaged value of an inter-electrode differential pressure at the flow path inlets calculated using fluid pressures measured in the flow paths P1, P2 was 0 kPa or higher. The gas flowed through the cathode flow path and the liquid flowed through the anode flow path to make the anode flow path larger in pressure loss, and thus as long as the time-averaged value of the inter-electrode differential pressure was 0 kPa or higher at the flow path inlets, the inter-electrode differential pressure became higher downstream therefrom, so that the time-averaged value of the inter-electrode differential pressure was also 0 kPa or higher as the whole of the cell. In the operation process, a time-averaged value of an inter-electrode differential pressure was about 100 kPa at the flow path inlets, and about 105 kPa at the flow path outlets. In the shutdown process, the cell was stored with an inter-electrode differential pressure maintained in a state of keeping the humidified carbon dioxide gas flowing through the cathode 11 and the aqueous potassium hydrogen carbonate solution flowing through the anode 12 without performing the purge operation of the fluids in both the electrodes after adjusting current and voltage. In the second and subsequent startup processes, an inter-electrode differential pressure was maintained from the storage process to shift to the operation process. Thus, the inter-electrode differential pressure was constantly maintained at the same level as that in the operation process excluding the first startup process.

In the examples 9 to 13, CO₂ flow rates in the startup process, the operation process, the shutdown process, and the storage process were each 125% or more relative to the theoretical amount calculated from the current density. In particular, it was 125% in the operation process. On the other hand, in the examples 14, 15, CO₂ flow rates in the startup process, the operation process, the shutdown process, and the storage process were each 100% or more relative to the theoretical amount calculated from the current density. In particular, it was 100% in the operation process.

In the examples 9 to 15, the electrolytic unit 10 was preheated in the first startup process. The preheating of the electrolytic unit 10 was performed by the method of making the heated electrolytic solution flow through the anode flow path. In the second and subsequent startup processes, the electrolytic solution was made to flow through the anode in the storage processes immediately therebefore, and thus the heating operation was not performed.

In the examples 9 to 15, an effect on a characteristic of the electrochemical reaction device at the time of repeated start and stop was evaluated by varying a maximum value of a cathode potential. The examples 9 to 13 were a series of tests in which the cathode potential maximum value was varied at five levels under a condition in which a CO₂ supply amount in the operation process was at a theoretical amount ratio of 125%, and the examples 14, 15 were a series of tests in which the cathode potential maximum value was varied at two levels under a condition in which a CO₂ supply amount in the operation process was at a theoretical amount ratio of 100%. The maximum value of the cathode potential was varied by changing a set value of a current density or a cell voltage in the storage process. The set value of the current density or the cell voltage in the storage process was set to be smaller than a value of a current density or a cell voltage in the operation process. In the shutdown process, a current density or a cell voltage was varied toward the set value in the storage process from the value in the operation process, and the cathode potential varied in a noble direction at this time. In particular, the variation in cathode potential was significant at a timing when the current density or the cell voltage reached the set value in the storage process. The closer to the value of the current density or the cell voltage in the operation process the set value of the current density or the cell voltage in the storage process was, the smaller the variation in cathode potential at the time of shutdown was, and thus the maximum value of the cathode potential also exhibited a decreasing tendency. An experiment at each of the levels was performed to prevent an influence of the experiments at the other levels by reassembling the cell each time. Further, the current density in the operation process was also controlled to the same predetermined value in the experiment at any of the levels. The current density in the storage process was 0 mA/cm² in the examples 9, 14, a value equal to or less than a measurement lower limit in the examples 10, 11, and a value larger than 0 mA/cm² in the examples 12, 13, 15. In any case, the density of the current flowing through the electrolytic unit 10 in the shutdown process was-5 mA/cm² or more with a direction of flowing from the cathode 11 into the anode 12 set as a positive. Further, in the shutdown process, the electric potential of the cathode 11 based on the standard hydrogen electrode reference (cathode potential vs SHE) was constantly +1.5 V or less. A maximum increase rate of a current supplied to the cell in the startup process was 1 mA/cm² per second or less. Table 2 lists the main conditions and the measured maximum values of the cathode potential (also each called the cathode potential maximum value) in the examples. In Table 2, each current density during the storage process is shown as a relative value to the current density during the operation process.

	TABLE 2
	Shutdown process

Startup process

Operation process

Cathode

	CO2 flow rate	Current	CO2 flow rate	CO2 flow rate	potential	Storage process
	(theoretical	density	(theoretical	(theoretical	maximum value	Current density
	amount ratio)	(a.u.)	amount ratio)	amount ratio)	(V vs. SHE)	(a.u.)

Example 9	≥125%	1	125%	≥125%	+0.8	0
Example 10	≥125%	1	125%	≥125%	−0.1	Measurement lower
						limit or less
Example 11	≥125%	1	125%	≥125%	−0.6	Measurement lower
						limit or less
Example 12	≥125%	1	125%	≥125%	−0.9	0.01
Example 13	≥125%	1	125%	≥125%	−1.3	0.14
Example 14	≥100%	1	100%	≥100%	+0.7	0
Example 15	≥100%	1	100%	≥100%	−1.3	0.14

Any of CO selectivity maintenance ratios in the tenth operation process in the examples 9, 11, 12, 13 was 100%, and a decrease in CO selectivity was not seen as compared with the first one in any case, and besides an effect due to a difference in conditions was not seen either. A value of a CO selectivity in the tenth operation process failed to be acquired due to a temporary failure of a measuring instrument in the example 10. Thus, in the examples 9 to 13, a CO selectivity maintenance ratio and a cell voltage maintenance ratio during the 100th operation process were evaluated as indexes of comparing the performance of the electrochemical reaction device. These are defined by ratios obtained by dividing a CO selectivity and a cell voltage in the 100th operation process in repeating the startup process, the operation process, the shutdown process, and the storage process by a CO selectivity and a cell voltage in the first operation process respectively. In addition, a value obtained by dividing the CO selectivity maintenance ratio by the cell voltage maintenance ratio (selectivity/voltage) was also evaluated. This value corresponds to a maintenance ratio of energy efficiency. The energy efficiency is a rate effectively used for CO production in the fed electric energy, and the maintenance ratio of the energy efficiency is a ratio of how much the energy efficiency changes between the first one and the 100th one.

FIG. 10 illustrates a graph in which the CO selectivity maintenance ratios, the cell voltage maintenance ratios, and ratios of them during the 100th operation process in the examples 9 to 13 are plotted with respect to the maximum values of the cathode potential. The cathode potential in the operation process (Vx) in the examples 9 to 13 was about-1.8 V based on the standard hydrogen electrode reference, and there was seen a tendency in which the closer to it the maximum value of the cathode potential was, the closer to 100% the CO selectivity maintenance ratios, the cell voltage maintenance ratios, and the ratios of them during the 100th operation process came. That is, there was seen the tendency in which the closer to a value of the cathode potential in the operation process the maximum value of the cathode potential recorded at the time of shutdown was, the more the characteristic of the electrochemical reaction device was maintained.

CO selectivity maintenance ratios in the tenth operation process in the examples 14, 15 were 94%, 103% respectively. A decrease in CO selectivity was seen in the example 14, whereas the decrease in CO selectivity was not seen at the tenth time in the example 15. In the examples 14, 15, a CO selectivity maintenance ratio and a cell voltage maintenance ratio during the 50th operation process were further evaluated as indexes of comparing the performance of the electrochemical reaction device. These are defined by ratios obtained by dividing a CO selectivity and a cell voltage in the 50th operation process in repeating the startup process, the operation process, the shutdown process, and the storage process by a CO selectivity and a cell voltage in the first operation process respectively. In addition, a value obtained by dividing the CO selectivity maintenance ratio by the cell voltage maintenance ratio was also evaluated. This value corresponds to a maintenance ratio of energy efficiency, and indicates how much the energy efficiency changes between the first one and the 50th one.

FIG. 11 illustrates a graph in which the CO selectivity maintenance ratios, the cell voltage maintenance ratios, and ratios of them in the examples 14, 15 are plotted with respect to the maximum values of the cathode potential. The cathode potential in the operation process (Vx) in the examples 14, 15 varied during repeating start and stop, and was in a range from about-2.6 V to about-1.9 V based on the standard hydrogen electrode reference. Also here, there was seen a tendency in which the smaller the maximum value of the cathode potential was, the closer to 100% the CO selectivity maintenance ratios, the cell voltage maintenance ratios, and ratios of them during the 50th operation process came.

It is found from the examples 9 to 15 that the electric potential of the cathode 11 based on the standard hydrogen electrode reference in the startup process, the shutdown process, and the storage process is preferably the value in the operation process or more and 0 V or less, and more preferably the value in the operation process or more and −0.5 V or less. It is further preferably the value in the operation process or more and −0.75 V or less. However, in a case of reducing the maximum value of the electric potential of the cathode 11 based on the standard hydrogen electrode reference, the current density and the cell voltage in the storage process come close to values of those in the operation process. Thus, power consumption in the storage process increases, and besides an electrolytic reaction product produced in the storage process is also required to be appropriately treated. Therefore, to what level the maximum value of the cathode potential is controlled is preferably determined in consideration of a wide range of viewpoints of a function required for the electrochemical reaction device, constraint on system aspect and cost aspect, and the like other than durability with respect to the repeated start and stop.

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

The above embodiment can be summarized into the following clauses.

(Clause 1)

A method of operating an electrochemical reaction device,

• • the device including an electrolytic unit having a cathode, an anode, a cathode space facing on the cathode, an anode space facing on the anode, and a diaphragm provided between the cathode space and the anode space,
  • the method including:
  • a startup process of regulating at least one parameter selected from the group consisting of a plurality of parameters in the electrolytic unit to satisfy an operation start condition, the plurality of parameters including a temperature, a pressure, a current density, a voltage, a composition of a first fluid containing a reducible material to be supplied to the cathode space, a flow rate of the first fluid, a composition of a second fluid containing an oxidizable material to be supplied to the anode space, and a flow rate of the second fluid;
  • an operation process of using the electrolytic unit in an operation condition range including the operation start condition and reducing the reducible material in the cathode to produce a reduction product;
  • a shutdown process of regulating at least one parameter selected from the group consisting of the plurality of parameters to satisfy a storage start condition; and
  • a storage process of using the electrolytic unit in a storage condition range including the storage start condition, wherein
  • the electrolytic unit is controlled during each of the startup process, the operation process, and the shutdown process so that a first time-averaged pressure at a first position in the cathode space is equal to or higher than a second time-averaged pressure at a second position in the anode space and a third time-averaged pressure at a third position in the cathode space is equal to or higher than a fourth time-averaged pressure at a fourth position in the anode space, the first position being closer to an inlet of the cathode space than an outlet of the cathode space, the second position being opposite the first position with the diaphragm therebetween, the third position being closer to the outlet than the inlet, and the fourth position being opposite the third position with the diaphragm therebetween.

(Clause 2)

The method according to clause 1, wherein

• • the electrolytic unit is controlled during each of the startup process, the operation process, and the shutdown process so that the first time-averaged pressure is higher than the second time-averaged pressure and the third time-averaged pressure is higher than the fourth time-averaged pressure.

(Clause 3)

The method according to clause 1 or clause 2, wherein:

• • the operation process is performed next to the startup process;
  • the shutdown process is performed next to the operation process; and
  • the storage process is performed next to the shutdown process.

(Clause 4)

The method according to clause 3, wherein

• • the startup process is performed next to the storage process again; and
  • a sequence of the startup process, the operation process, the shutdown process, and the storage process is repeated multiple times.

(Clause 5)

The method according to any one of clause 1 to clause 4, wherein

• • the diaphragm is a porous membrane.

(Clause 6)

The method according to any one of clause 1 to clause 5, wherein

• • a reduction reaction of carbon dioxide occurs on the cathode.

(Clause 7)

The method according to any one of clause 1 to clause 6, wherein

• • in the startup process, a humidified gas is supplied as the first fluid to the cathode space.

(Clause 8)

The method according to any one of clause 1 to clause 7, wherein

• • in the startup process, the electrolytic unit is preheated.

(Clause 9)

The method according to any one of clause 1 to clause 8, wherein

• • in the shutdown process, a purge operation in the anode space and the cathode space is performed.

(Clause 10)

The method according to clause 9, wherein

• • in the purge operation, a fluid in the cathode space is replaced with a humidified gas.

(Clause 11)

The method according to any one of clause 1 to clause 10, wherein:

• • during the operation process,
  • a flow rate of the reducible material to be supplied to the cathode space is constantly equal to or more than a theoretical amount of the reducible material calculated from a current flowing through the electrolytic unit; and
  • a flow rate of the oxidizable material to be supplied to the anode space is constantly equal to or more than a theoretical amount of the oxidizable material calculated from the current.

(Clause 12)

The method according to any one of clause 1 to clause 10, wherein:

• • during the startup process and the shutdown process,
  • a flow rate of the reducible material to be supplied to the cathode space is constantly equal to or more than a theoretical amount of the reducible material calculated from a current flowing through the electrolytic unit; and
  • a flow rate of the oxidizable material to be supplied to the anode space is constantly equal to or more than a theoretical amount of the oxidizable material calculated from the current.

(Clause 13)

The method according to any one of clause 1 to clause 12, wherein

• • during the startup process, the operation process, the shutdown process, and the storage process,
  • a current density in the electrolytic unit is constantly-5 mA/cm² or higher with a direction of flowing from the cathode through an external circuit including a power supply into the anode set to be positive.

(Clause 14)

The method according to any one of clause 1 to clause 12, wherein

• • at least one of the startup process and the shutdown process includes a period during which a current density in the electrolytic unit is lower than zero with a direction of flowing from the cathode through an external circuit including a power supply into the anode set to be positive.

(Clause 15)

The method according to any one of clause 1 to clause 14, wherein

• • during the startup process, the operation process, the shutdown process, and the storage process,
  • an electric potential of the cathode based on a standard hydrogen electrode reference is constantly +1.5 V or less.

(Clause 16)

The method according to any one of clause 1 to clause 15, wherein

• • during the storage process, a current density in the electrolytic unit is zero.

(Clause 17)

The method according to any one of clause 1 to clause 15, wherein

• • during the storage process, a current density in the electrolytic unit is higher than zero.

(Clause 18)

The method according to any one of clause 1 to clause 17, wherein

• • the electrolytic unit is controlled during the storage process so that the first time-averaged pressure is equal to or higher than the second time-averaged pressure and the third time-averaged pressure is equal to or higher than the fourth time-averaged pressure.

(Clause 19)

The method according to any one of clause 1 to clause 17, wherein

• • the electrolytic unit is controlled during the storage process so that the first time-averaged pressure is higher than the second time-averaged pressure, and the third time-averaged pressure is higher than the fourth time-averaged pressure.

(Clause 20)

The method according to any one of clause 1 to clause 19, wherein

• • regarding a first time-averaged inter-electrode differential pressure between a first pressure at the first position and a second pressure at the second position and a second time-averaged inter-electrode differential pressure between a third pressure at the third position and a fourth pressure at the fourth position, the electrolytic unit is controlled so that the first and second time-averaged inter-electrode differential pressures in the storage process are equal to or lower than the first and second time-averaged inter-electrode differential pressures in the operation process respectively.

(Clause 21)

The method according to any one of clause 1 to clause 20, wherein

• • during the startup process,
  • a maximum increase rate of a current supplied to the electrolytic unit is 1 mA/cm² per second or less.

(Clause 22)

An electrochemical reaction device operable by the method according to any one of clause 1 to clause 21,

• • the device including:
  • the electrolytic unit;
  • a power supply configured to supply current or voltage to the electrolytic unit;
  • a temperature regulator configured to regulate a temperature in the electrolytic unit;
  • a pressure regulator configured to regulate a pressure in the electrolytic unit; and
  • a controller configured to control the electrolytic unit, the power supply, the temperature regulator, and the pressure regulator to control an operation of operating the device according to the method.