Electrochemical system and method for producing valuable material

Abstract

An electrochemical system (100) is provided with: an electrochemical cell (10) provided with a positive electrode (11) and a negative electrode (12) having a hole through which a fluid can pass; an energization control unit (20) that energizes between the positive electrode (11) and the negative electrode (12); an introduction control unit (30) that introduces a liquid electrolyte and carbon dioxide together into the electrochemical cell (10); and a control device (40) that controls the reduction rate of carbon dioxide at the negative electrode (12) by controlling the energization control unit (20), and controls the introduction rate of carbon dioxide into the electrochemical cell (10) by controlling the introduction control unit (30). An electrochemical cell (10) is provided with: an introduction port (15) that introduces a liquid electrolyte and carbon dioxide into an electrochemical cell (10); and an outflow port (17) that is disposed on the opposite side of the negative electrode (12) from the introduction port (15), and causes a product generated by permeation of the negative electrode (12) with carbon dioxide to flow out from the electrochemical cell (10) together with the liquid electrolyte.

Description

Electrochemical system and method for producing valuable materials

[0001] The present disclosure relates to an electrochemical system and a method for producing valuable materials.

[0002] In order to mitigate the recent rapid climate change, there is an urgent need to reduce emissions of greenhouse gases, such as carbon dioxide, which causes global warming, or to develop technologies for their utilization (consumption) and storage. As an electrolytic method for electrochemically reducing carbon dioxide, a system that can not only consume carbon dioxide but also convert it into valuable materials such as ethylene and ethanol is expected. Because such a system can be powered by renewable energy, it is expected to be used as a system for sustainably generating valuable materials.

[0003] Patent Document 1 describes an electrochemical reaction device comprising: "an electrochemical reaction cell including a first storage section for storing carbon dioxide, a second storage section for storing a water-containing electrolytic solution or water vapor, a diaphragm provided between the first storage section and the second storage section, a reduction electrode disposed in the first storage section, and an oxidation electrode disposed in the second storage section; a detection section for detecting the reaction amount of the electrochemical reaction cell; an adjustment section for adjusting the amount of carbon dioxide supplied to the first storage section; and a control section for controlling the adjustment section based on a detection signal from the detection section."

[0004] Patent Document 2 describes a method including "(a) a step of introducing an anolyte into an anolyte compartment of an electrochemical reactor, the anolyte compartment at least partially containing an anode; and (b) a step of introducing carbon dioxide gas into a catholyte compartment of the electrochemical reactor, the catholyte being separated from the anolyte compartment by a membrane, the catholyte compartment at least partially containing a cathode, the cathode including a cathode catalyst, the cathode catalyst electrochemically reducing carbon dioxide to formate," and further describes a method including "(a) introducing an anolyte into an anolyte compartment of an electrochemical reactor, the anolyte being separated from the anolyte compartment by a membrane, the catholyte compartment at least partially containing a cathode, the cathode including a cathode catalyst electrochemically reducing carbon dioxide to formate," the method further describes a method ...b) introducing an anolyte into an anolyte compartment of an electrochemical reactor, the catholyte being separated from the anolyte compartment by a membrane, the catholyte compartment at least partially containing a cathode, the cathode including a cathode catalyst electrochemically reducing carbon dioxide to formate," the method further describes a method "(c) introducing an anolyte into an anolyte compartment ２ The gas 30 is dispersed (indicated by arrows 40) under different pressures across the porous cathode 16a and flows through the porous cathode 16a into the catholyte mixture 26."

[0005] Japanese Patent Application Laid-Open No. 2021-46574 Special Publication No. 2013-54495

[0006] In the technology described in Patent Document 1, a fluid containing carbon dioxide diffuses after being introduced into a reaction vessel, and a portion of the fluid flows, for example, along the surface of a flat reduction electrode. As will be described in detail later, the inventors of the present invention have found that the utilization rate of carbon dioxide is poor. Therefore, it is preferable to configure a system in which unused carbon dioxide is separated and reintroduced, and to repeat the separation and introduction process multiple times until the carbon dioxide is reduced.

[0007] Furthermore, the technology described in Patent Document 2 neither describes nor suggests an operation of simultaneously controlling the supply of carbon dioxide and controlling the current. Therefore, it is difficult to reduce the difference between the supply rate of carbon dioxide and the reduction rate and improve the carbon utilization rate. The problem to be solved by the present disclosure is to provide an electrochemical system having a high utilization rate of a substance to be reduced, such as carbon dioxide, and a method for producing valuable materials.

[0008] The electrochemical system of the present disclosure includes an electrochemical cell including an anode, a cathode, and a reaction tank containing the anode and the cathode, a current control unit that applies current between the anode and the cathode, an introduction control unit that introduces an electrolyte and a substance to be reduced into the electrochemical cell, either independently or in combination, and a control device that controls the current control unit to control the reduction rate of the substance to be reduced at the cathode and the introduction control unit to control the introduction rate of the substance to be reduced into the electrochemical cell. Other solutions will be described later in the description of the preferred embodiments of the present invention.

[0009] According to the present disclosure, it is possible to provide an electrochemical system having a high utilization rate of a substance to be reduced, such as carbon dioxide, and a method for producing valuable materials.

[0010] 8 is a schematic diagram of an electrochemical system according to the present disclosure. FIG. 9 is a schematic diagram of a cathode provided in the electrochemical system shown in FIG. 1. FIG. 10 is a simulation result showing the relationship between the pore size of the cathode and the utilization rate of carbon dioxide. FIG. 11 is a block diagram showing a specific hardware configuration of a control device. FIG. 12 is a control flow diagram of the reduction rate and introduction rate when the value of X is brought closer to 0. FIG. 13 is a control flow diagram of the reduction rate and introduction rate when the value of Y is brought closer to 0. FIG. 14 is a schematic diagram of an electrochemical system according to another embodiment. FIG. 15 is a schematic diagram of a cathode provided in the electrochemical system shown in FIG. 7. FIG. 16 is a schematic diagram of a cathode provided in the electrochemical system shown in FIG. 7, showing an embodiment different from FIG. 10. FIG. 11 is a flowchart showing a method for producing valuable resources according to the present disclosure.

[0011] Hereinafter, modes for carrying out the present disclosure (hereinafter referred to as "embodiments") will be described with reference to the drawings. In the following description of one embodiment, other embodiments applicable to the one embodiment will also be described as appropriate. The present disclosure is not limited to the one embodiment described below, and different embodiments can be combined with each other or modified as desired without significantly impairing the effects of the present disclosure. Furthermore, the same components will be given the same reference numerals, and duplicate descriptions will be omitted. Furthermore, components having the same functions will be given the same names. The contents shown are merely schematic, and for convenience of illustration, changes may be made from the actual configuration within the scope of not significantly impairing the effects of the present disclosure, and some components may be omitted or modified between drawings. Furthermore, the same embodiment does not necessarily have to include all of the configurations.

[0012] FIG. 1 is a schematic diagram of an electrochemical system 100 according to the present disclosure. In the example shown in FIG. 1, the electrochemical system 100 is a system that reduces a reduction target, such as carbon dioxide, to produce a carbon compound (product) containing, for example, 1 to 3 carbon atoms. The reduction target may be a gas, liquid, or solid at room temperature (e.g., 25°C). However, the reduction target is preferably a component that can be dissolved in a solvent. Examples of carbon compounds containing 1 to 3 carbon atoms include methanol, ethanol, and ethylene. However, examples of the product include carbon compounds with 4 or more carbon atoms.

[0013] The produced carbon compounds (e.g., ethanol, ethylene, etc.) are components that can be traded on the market, and therefore valuable materials can be produced by the electrochemical system 100. In the following examples, unless otherwise specified, the object to be reduced is carbon dioxide, and the product that has permeated the cathode 12 and is produced is a carbon compound containing 1 to 3 carbon atoms. However, the object to be reduced and the product are not limited to these.

[0014] The electrochemical system 100 includes an electrochemical cell 10 , a current control unit 20 , an introduction control unit 30 , and a control device 40 .

[0015] The electrochemical cell 10 is a structure for reducing carbon dioxide by an electrochemical method. The electrochemical cell 10 includes an anode 11, a cathode 12, and a reaction vessel 14 containing the anode 11 and the cathode 12. The electrochemical cell 10 is preferably configured as a cell having a three-electrode structure equipped with a reference electrode for controlling the reduction reaction with good reproducibility, specifically, for example, a potentiostat.

[0016] The reaction vessel 14 is filled with an electrolyte, which is an example of a fluid. The anode 11 is placed in the electrolyte that does not contain carbon dioxide. Any electrolyte that can cause an oxidation-reduction reaction at the anode 11 and the cathode 12 can be used. At the anode 11, oxygen is produced, for example, by an oxidation reaction of water, and the anode 11 is, for example, a conductive metal plate. On the other hand, the cathode 12 is placed in an electrolyte in which carbon dioxide is dissolved (or an electrolyte in which carbon dioxide bubbles are dispersed). In the electrolyte in which carbon dioxide is dissolved, the carbon dioxide exists as, for example, carbonate ions. At the cathode 12, a reduction reaction of the carbon dioxide produces, for example, ethanol.

[0017] FIG. 2 is a schematic diagram of the cathode 12 included in the electrochemical system 100 shown in FIG. 1 . The cathode 12 has pores 121a that allow fluid (e.g., liquid) to pass through. The fluid here is an electrolyte solution containing dissolved carbon dioxide, as described above. In the example shown in FIG. 2 , the cathode 12 is, for example, a porous body having numerous pores 121a, or a mesh body in which pores 121a are formed between adjacent metal wires, for example. However, the mesh body can also be referred to as a porous body having numerous pores 121a that allow fluid to pass through. The mesh body can be formed by forming a rod member, such as a conductor 121 (e.g., a metal wire), into a mesh shape. The conductor 121 can be made of a metal, such as gold, silver, copper, aluminum, nickel, iron, or platinum.

[0018] Electricity is passed through the conductor 121. Therefore, carbon dioxide is reduced on the surface of the conductor 121. The porous or mesh body allows the fluid to pass through to the cathode 12, and as the fluid passes through, carbon dioxide diffuses to the surface of the cathode 12, allowing the carbon dioxide in the fluid to be reduced.

[0019] The shape of the hole 121a, which is an opening formed by adjacent conductors 121, is, for example, rectangular (square or oblong), but may also be, for example, triangular or polygonal with pentagons or more. The shape of the opening may also be circular or elliptical. In the illustrated example, the distance between adjacent conductors 121 is d, and the diameter of the hole 121a is also d. There are no particular limitations on the method for measuring the diameter, but the length of the narrowest (shortest) part of the hole 121a can be used as the diameter of the hole 121a.

[0020] The shorter the distance from the cathode 12, the easier it is for carbon dioxide to diffuse to the cathode, and as a result, the easier it is to be reduced. Generally, carbon dioxide used in the reduction reaction is present within a distance of about 50 μm from the cathode 12.

[0021] Figure 3 shows the results of a simulation showing the relationship between the pore size of the cathode 12 (diameter d of the pores 121a) and the carbon dioxide utilization rate. The graph shown in Figure 3 shows the results of a simulation of the relationship between the pore size of the cathode 12 and the carbon dioxide utilization rate, assuming that 50% of the carbon dioxide or carbonate ions present within an effective distance of 50 µm from the catalyst are reduced. The solid line shows the results of the simulation, and the dashed dotted line shows the results of a simulation based on the method described in Patent Document 1. In the technology described in Patent Document 1, the cathode chamber width (the width in the direction perpendicular to the longitudinal direction of the reduction electrode) was assumed to be 5 mm, and the catalyst was applied to both sides of a flat plate.

[0022] The carbon dioxide utilization rate (unit: %) was calculated according to the following method: Carbon dioxide utilization rate of the present disclosure = 100 × (reduction rate: 50%) × (effective distance: horizontal axis) / (pore diameter) Carbon dioxide utilization rate of Patent Document 1 = 100 × (reduction rate: 50%) × 2 (both sides) × (effective distance: horizontal axis) / (cathode width: 5 mm)

[0023] As shown in the figure, the cathode 12 of the present disclosure exhibited a higher utilization rate than the technology of Patent Document 1 when the pore diameter was 2000 μm or less. A higher utilization rate corresponds to a larger amount of product relative to the amount of carbon dioxide introduced. Furthermore, judging from the shape of the graph, it is believed that the present disclosure exhibits a higher utilization rate than the technology of Patent Document 1 even when the pore diameter exceeds 2000 μm. The cathode 12 has pores 121 a that are permeable to fluid, but if the diameter d of the pores 121 a is too long, it is no longer considered a pore and is not practical. Therefore, the diameter d of the pores 121 a in the cathode 12 is, for example, 4000 μm or less, preferably 3000 μm or less, more preferably 1000 μm or less, even more preferably 500 μm or less, and even more preferably 100 μm or less. The cathode 12 is not limited to a single layer, and multiple layers can be stacked.

[0024] Returning to FIG. 1 , a catalyst for reducing carbon dioxide is supported on the surface of the conductor 121. The catalyst may be appropriately determined depending on the type of material to be reduced. When the material to be reduced is carbon dioxide, the catalyst is preferably a material that is conductive and electrochemically reactive with the electrolyte and carbon dioxide. For example, C, Al, Si, Mg, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Sr, Y, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Cd, In, Sn, Sb, Te, Ba, Hf, Ta, W, Re, Ir, Pt, Au, Pb, Bi, La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, or Yb may be used, or an alloy or compound thereof may be used. Examples of compounds include TiO ２ , SrTiO ３ , SrTiO ２ N, BaTaO ２ N, NaTaO ３ , W.O. ３ , ZnO, Fe ２ O ３ , CuO, Ta ２ O ５ , Bi ２ O ３ , SnO ２ , BiVO ４ , Y ２ Ti ２ O ５ S ２ , IrO ２ , MnO, CoO, etc. The catalyst may be used alone or in combination of two or more.

[0025] Preferably, these catalysts are also provided in the anode 11. In order to promote the oxidation reaction in the anode 11, the surface of the conductor 121 may be appropriately modified with a material that can act as a promoter (promoter material). Typical promoter materials include, for example, Pt, Ru, Rh, and Au.

[0026] The catalyst can be supported by, for example, adhering or applying it to the surface of the conductor 121 by a chemical or physical method. In addition, if the catalyst is conductive, the cathode 12 may be formed by molding the catalyst itself into a mesh body.

[0027] The surface area of ​​the anode 11 and the surface area of ​​the cathode 12 are preferably large enough to prevent the catalytic reaction area from becoming rate-limiting with respect to the amount of current applied. The area ratio of the anode 11 to the cathode 12 is preferably adjusted depending on the type of catalyst.

[0028] The reaction tank 14 is a structure that accommodates a fluid therein so that it can flow through. The reaction tank 14 includes the anode 11 and the cathode 12 as described above, as well as an ion exchange membrane 13. The ion exchange membrane 13 is provided in the electrochemical cell 10 and is a structure that separates the reaction tank 14 into spaces 192 and 193 in which the cathode 12 is disposed and the space 191 in which the anode 11 is disposed. The spaces 191, 192, and 193 are all spaces formed inside the reaction tank 14. The ion exchange membrane 13 separates the space 191 from the space 192. The space 192 and the space 193 are separated by the cathode 12. The provision of the ion exchange membrane 13 allows only specific ions to pass through, allowing a continuous reaction to proceed, while suppressing unexpected chemical reactions other than the expected oxidation-reduction reaction.

[0029] The electrochemical cell 10 has inlets 15, 16 and outlets 17, 18. The inlets 15, 16 and the outlets 17, 18 are provided in the reaction vessel 14. The inlet 15 is an opening for introducing an electrolytic solution and carbon dioxide into the electrochemical cell 10. The inlet 16 is an opening for introducing an electrolytic solution into the electrochemical cell 10. The electrolytic solution introduced from the inlet 16 may or may not contain carbon dioxide.

[0030] The outlet 17 is an opening through which a product produced by the permeation of carbon dioxide through the cathode 12 flows out of the electrochemical cell 10 together with the electrolyte. The outlet 17 is disposed on the opposite side of the cathode from the inlet 15. The outlet 18 is an opening through which a product produced by the oxidation reaction at the anode 11 flows out of the electrochemical cell 10 together with the electrolyte.

[0031] The inlet 16 and the outlet 18 are connected to a space 191. The electrolytic solution that flows in through the inlet 16 flows into the space 191. In the space 191, for example, water in the electrolytic solution is oxidized to generate oxygen. The generated oxygen and the electrolytic solution flow out of the space 191 through the outlet 18. Meanwhile, the inlet 15 is connected to a space 192, and the outlet 17 is connected to a space 193. The electrolytic solution that flows into the space 192 from the inlet 15 permeates the cathode 12. As it permeates the cathode 12, carbon dioxide in the electrolytic solution is reduced at the cathode 12 to generate products such as ethanol and ethylene. The electrolytic solution and the products flow out of the space 193 through the outlet 17. Unreacted carbon dioxide may also flow out from the outlet 17.

[0032] As described above, the electrochemical cell 10 includes a space 192 (first space) and a space 193 (second space) separated by the cathode 12, an inlet 15, and an outlet 17. The inlet 15 is an opening for introducing a substance to be reduced (e.g., carbon dioxide) into the space 192 of the electrochemical cell 10. The outlet 17 is an opening for allowing a product generated after permeating the cathode 12 to flow out from the space 193 of the electrochemical cell 10.

[0033] Furthermore, as described above, in the present disclosure, a first fluid, which is at least one of an electrolyte solution having carbon dioxide dissolved therein and an electrolyte solution having carbon dioxide bubbles dispersed therein, is introduced into the electrochemical cell 10 through the inlet 15. The first fluid is preferably an electrolyte solution having carbon dioxide dissolved therein, and more preferably an electrolyte solution having carbon dioxide dissolved therein in an amount that results in a saturated concentration at the reduction reaction temperature inside the electrochemical cell 10. The first fluid is introduced into the space 192 through the inlet 15. That is, the carbon dioxide and the electrolyte solution are supplied together to the space 192 within the same system.

[0034] The first fluid (containing the electrolyte and carbon dioxide) introduced into the space 192 passes through the cathode 12. By controlling the flow rate of the first fluid in which carbon dioxide has been previously dissolved or dispersed, the amount of carbon dioxide introduced into the electrochemical cell 10 can be controlled, making it easier to control the reduction reaction and the amount of product.

[0035] The cathode 12 is a member that separates the space 192 in which the inlet 15 is provided from the space 193 in which the outlet 17 is provided inside the reaction vessel 14. In this way, the carbon dioxide introduced from the inlet 15 can undergo a reduction reaction at the cathode 12 while passing through the cathode 12 and heading toward the outlet 17. This allows the utilization rate of carbon dioxide to be improved.

[0036] The current control unit 20 is a structure that applies current between the anode 11 and the cathode 12. The current control unit 20 includes, for example, a power supply, and applies current from the power supply to the anode 11 and the cathode 12, causing a current to flow between the anode 11 and the cathode 12. The current control by the current control unit 20 is performed by a control device 40, which will be described later.

[0037] The introduction control unit 30 is a structure that introduces the electrolyte solution and carbon dioxide, either independently or in combination, into the electrochemical cell 10. In the example of FIG. 1 , the introduction control unit 30 introduces a fluid in which carbon dioxide and the electrolyte solution coexist into the electrochemical cell 10. The introduction control unit 30 includes, for example, a liquid delivery pump that can control the flow rate, pressure, etc. of the delivered fluid. The introduction control unit 30 introduces a first fluid containing carbon dioxide and the electrolyte solution into the space 192 of the electrochemical cell 10 through the introduction port 15. Meanwhile, the introduction control unit 30 introduces the electrolyte solution into the space 191 of the electrochemical cell 10 through the introduction port 16. The electrolyte solution introduced into the space 191 may or may not contain carbon dioxide.

[0038] The control device 40 is a device that controls the rate of reduction of carbon dioxide (an example of a substance to be reduced) at the cathode 12 by controlling the current control unit 20. The reduction of carbon dioxide is carried out using electrons supplied from the current control unit 20. Therefore, the rate of reduction of carbon dioxide can be controlled by controlling the amount of electrons supplied to the cathode 12 (applied current value, voltage value, etc.).

[0039] The reduction rate can be calculated based on, for example, the value of the current passed between the anode 11 and the cathode 12 and the electrical conversion efficiency (Faraday efficiency) determined from the catalytic performance.

[0040] In addition, the control device 40 is a device that controls the introduction control unit 30 to control the rate at which carbon dioxide is introduced into the electrochemical cell 10. The control device 40 controls, for example, the rotation speed of the liquid feed pump. This makes it possible to control the flow rate of the electrolytic solution containing carbon dioxide and the amount of carbon dioxide dissolved or dispersed in the electrolytic solution, thereby controlling the introduction rate of carbon dioxide into the electrochemical cell 10.

[0041] 4 is a block diagram showing the hardware configuration of the control device 40. The control device 40 is configured to include, for example, a CPU (Central Processing Unit) 1001, a RAM (Random Access Memory) 1002, a ROM (Read Only Memory) 1003, an I / F (Interface) 1004, and a bus 1005. The CPU 1001, RAM 1002, ROM 1003, and I / F 1004 are connected via, for example, the bus 1005. The control device 40 is realized by loading a predetermined control program (for example, a method for producing a valuable resource according to the present disclosure) stored in the ROM 1003 into the RAM 1002 and executing the program by the CPU 1001. In terms of hardware, signals and information are exchanged between the control device 40 and various devices (such as servers), external networks, and the like, via an I / F 1004 .

[0042] Returning to Fig. 1, the control device 40 controls at least one of the energization control unit 20 and the introduction control unit 30 to increase the carbon utilization rate (= reduction rate / introduction rate), which is the ratio between the carbon dioxide reduction rate and the carbon dioxide introduction rate, compared to when this control is not performed. By using this control, the ratio between the carbon dioxide reduction rate and the introduction rate can be increased compared to when control is not performed, thereby improving the carbon dioxide utilization rate. The control is, for example, feedback control.

[0043] The ratio of the reduction rate to the introduction rate (reduction rate / introduction rate; the value obtained by dividing the reduction rate by the introduction rate) can be calculated as follows. The reduction rate can be calculated in units such as mol / sec based on, for example, the current value, voltage value, Faraday efficiency, or the production of products (components produced by the reduction of carbon dioxide; the carbon compounds described above). For example, the reduction rate can be calculated directly by measuring the amount of product produced at regular time intervals. Alternatively, the Faraday efficiency of the product corresponding to a certain current value and voltage value can be obtained in advance as data. The Faraday efficiency can then be estimated by reading the current and voltage values, and the production rate of the product can be calculated. The Faraday efficiency is expressed as the percentage of the current used to produce a specific product out of the total current applied. The number of electrons required to produce 1 mol of a product is determined depending on the substance, and the production rate of a specific product can be calculated from the number of electrons and the current value used to produce the product (= total current × Faraday efficiency). When calculating the reduction rate, the reduction rate of carbon dioxide can be obtained by multiplying the production rates of all products by the number of carbon atoms (for example, 1 for carbon monoxide, 2 for ethylene, and 3 for n-propanol) and adding them up.

[0044] On the other hand, the introduction rate can also be calculated, for example, based on the liquid delivery rate of the liquid delivery pump and the dissolved concentration in the electrolyte, and the amount of carbon dioxide introduced per second (units: mL, mol, for example). The carbon dioxide introduced here also includes the form of carbonate ions dissolved in the liquid. The ratio between the reduction rate and the introduction rate can be calculated by using the same units for the reduction rate and the introduction rate and calculating the ratio between them.

[0045] The control device 40 controls the reduction rate and introduction rate of carbon dioxide so that the carbon utilization rate (= reduction rate / introduction rate) is, for example, 40% or more, preferably 50% or more, more preferably 60% or more, and particularly preferably 70% or more. That is, the control device 40 controls the reduction rate so that the reduction rate is, for example, 40% or more of the introduction rate. This improves the carbon utilization rate. By improving the carbon utilization rate, the amount of unreacted carbon dioxide flowing out from the electrochemical cell 10 can be reduced, eliminating the need to transport unreacted carbon dioxide back to the reaction site, thereby simplifying and reducing the cost of the electrochemical system 100. Alternatively, the frequency of transporting carbon dioxide back to the reaction site can be significantly reduced, reducing circulation costs and allowing the electrochemical system 100 to operate with a smaller amount of carbon dioxide introduced.

[0046] The control is performed, for example, in accordance with the ratio between the reduction rate and the introduction rate, so as to increase the ratio. For example, in the technology described in Patent Document 1, a flat electrode is used, and the carbon utilization rate is controlled to a constant value by controlling only the introduction rate of carbon dioxide with respect to a fluctuating reduction current (reduction rate). When the aim is to improve the carbon utilization rate by similar control, in the technology described in Patent Document 1, the reduction rate is generally maintained at a reduction current value of at most 30 mA / cm. ２ 0.1 μmol / cm ２ The reduction rate is limited to about 1 / 2 second or less. In the case of the technology described in Patent Document 1, gaseous carbon dioxide is bubbled and dissolved in the electrolyte, and then transported by diffusion to the electrode surface, which is the reaction site.

[0047] To improve carbon utilization, the carbon dioxide introduction rate should ideally be 0.1 μmol / cm ２ However, carbon dioxide is easily dissolved only in the region near the cathode 12 where the carbon dioxide concentration is low, and in other regions, the carbon dioxide concentration is almost saturated. Therefore, most of the introduced carbon dioxide does not dissolve in the electrolyte and is discharged as unreacted carbon dioxide. In fact, ２ To dissolve carbon dioxide at a rate of 1 / 2 sec, an introduction rate several tens of times that may be required.

[0048] Furthermore, if the carbon dioxide concentration in the electrolyte is reduced to increase the amount of carbon dioxide dissolved in the electrolyte relative to the introduction rate, the carbon dioxide concentration in the vicinity of the cathode 12 decreases, and the reduction rate drops significantly. Therefore, the effect of improving the carbon utilization rate by the control method in the technology described in Patent Document 1 is extremely limited.

[0049] Therefore, if carbon dioxide dissolved in the electrolyte is supplied to the porous or mesh cathode 12 so as to permeate the electrolyte, the reduction rate can be increased to 3000 A / cm ２ This corresponds to 11 μmol / cm ２ ・It can be controlled to about 2000 s. In addition, for example, the flow rate of the electrolyte is 500 ml / min, and the cathode area is 100 cm ２ , mesh diameter 50-100 μm, 1 atmosphere, room temperature, saturated solubility of carbon dioxide 330 μmol / cm ３ Assuming that the feed rate is 28 μmol / cm ２ It is possible to fully control the reaction time to within 1 / 2 second. In principle, it is difficult to utilize all of the carbon dioxide dissolved in the electrolyte for the reaction. However, since the carbon dioxide is directly transported to the vicinity of the reaction site on the surface of the cathode 12, the supplied carbon dioxide can be utilized to the maximum extent, and the carbon utilization rate can be significantly improved compared to conventional methods.

[0050] As a specific control method, the control device 40 varies the reduction rate by controlling the current density or potential using the current control unit 20, and varies the introduction rate by controlling the flow rate of carbon dioxide gas or the electrolyte solution containing dissolved carbon dioxide using the introduction control unit 30. Specifically, the control device 40 controls the reduction rate and the introduction rate so that X = (change in carbon utilization rate) / (change in reduction rate) and Y = (change in carbon utilization rate) / (change in introduction rate) are 0 or approaching 0. This maximizes the carbon utilization rate. Note that X is the value obtained by dividing the change in carbon utilization rate by the change in reduction rate, and Y is the value obtained by dividing the change in carbon utilization rate by the change in introduction rate.

[0051] When attempting to control only the reduction rate, if the current density is made too small by the current control unit 20 for a certain introduction rate, the reduction rate relative to the introduction rate will be small. This will increase the amount of carbon dioxide that is discharged without being reduced, and the carbon utilization rate will decrease. If the current density is made too large by the current control unit 20, the supply of carbon dioxide will not keep up, promoting hydrogen generation and suppressing the reduction of carbon dioxide.

[0052] Furthermore, when attempting to control only the introduction rate, if the introduction rate is set too low for a certain reduction rate, the carbon dioxide supply will not keep up, promoting hydrogen generation and suppressing carbon dioxide reduction. If the introduction rate is set too high, the introduction rate will exceed the reduction rate, and an increase in the amount of carbon dioxide that is discharged without being reduced will occur. This will result in a decrease in carbon utilization. Therefore, by simultaneously controlling the reduction rate and the introduction rate, it is possible to maximize the carbon utilization rate while suppressing the side reaction of hydrogen generation.

[0053] Furthermore, the electrochemical cell 10 has a cell structure that transports the introduced carbon dioxide directly to the vicinity of the reaction field on the surface of the cathode 12. Therefore, the carbon utilization rate responds quickly to changes in the reduction rate and introduction rate, and is easy to control.

[0054] For example, at the start of startup of the electrochemical system 100, the control device 40 changes the reduction rate using the current control unit 20 while maintaining a certain introduction rate, and sets the reduction rate at which the carbon utilization rate is maximized. Thereafter, the control device 40 varies the introduction rate using the introduction control unit 30 to maximize the carbon utilization rate. If necessary, the carbon utilization rate may be further improved by repeatedly adjusting the above X and Y so that they approach 0. In this case, the change in the carbon utilization rate is the difference between the numerical values ​​before and after control of the reduction rate or introduction rate. The control device 40 can operate the electrochemical system 100 in a steady state with the carbon utilization rate maximized in this way.

[0055] For example, while maintaining a predetermined introduction rate, the control device 40 controls the reduction rate by the current control unit 20 so that X=0 or approaches 0, and then controls the reduction rate by the introduction control unit 30 so that Y=0 or approaches 0 at least once. In this way, the carbon utilization efficiency can be improved.

[0056] Alternatively, to produce the target product with high energy efficiency, steady-state operation may be performed as follows: First, a current value is set at which the faradaic efficiency of the target product is maximized, and then the introduction rate is increased by the introduction control unit 30 until Y=0 so that the carbon utilization rate is maximized in accordance with the reduction rate at that current value. This allows steady-state operation.

[0057] During steady-state operation, it is desirable to further actively control the reduction rate and introduction rate depending on the conditions of the processes before and after carbon dioxide reduction. Specific control flows are shown in Figures 5 and 6.

[0058] Fig. 5 is a control flow diagram of the reduction rate and introduction rate when the value of X is brought closer to 0. Before explaining Fig. 5, the concept of the flow in Fig. 5 will first be explained. For example, it is conceivable that, depending on the carbon dioxide production status at the carbon dioxide supply source, a situation may arise in which an increase or decrease in the supply amount (introduction amount) of carbon dioxide is required during steady operation of the electrochemical system 100. For example, if the carbon dioxide supply source is a facility such as a factory or power plant, and carbon dioxide is a by-product associated with the operation of these facilities, the supply amount of carbon dioxide may increase or decrease depending on the operating status of the facility.

[0059] Here, an increase or decrease in the amount of carbon dioxide supplied can be determined based on the magnitude of the carbon dioxide pressure before (preceding) the introduction control unit 30. In other words, if the pressure before the introduction control unit 30 is higher than atmospheric pressure by a certain amount or more, control is performed to increase the amount of carbon dioxide introduced. On the other hand, if the pressure is lower than the certain amount or more, control is performed to decrease the amount of carbon dioxide introduced.

[0060] For example, when the amount of carbon dioxide introduced is increased, the introduction control unit 30 controls to increase the introduction rate in accordance with the required amount of carbon dioxide introduced. Thereafter, the energization control unit 20 controls to increase the reduction rate so that the carbon utilization rate is maximized, i.e., so that X approaches 0. On the other hand, when the amount of carbon dioxide introduced is decreased, the introduction control unit 30 controls to decrease the introduction rate in accordance with the required amount of carbon dioxide introduced. Thereafter, the energization control unit 20 controls to decrease the reduction rate so that the carbon utilization rate is maximized, i.e., so that X approaches 0.

[0061] The above concept will be explained with reference to Fig. 5. The control device 40 determines whether the carbon dioxide pressure is within a reference value (within an allowable range) before the introduction control unit 30 (step S11). If it is within the reference value (Yes), the control device 40 ends the flow shown in Fig. 5. On the other hand, if it is outside the reference value (outside the allowable range) (No), the control device 40 determines whether the carbon dioxide pressure is greater than the reference value (greater than the upper limit of the allowable range) or less than the reference value (less than the lower limit of the allowable range) (step S12).

[0062] If it is greater, the control device 40 increases the introduction rate (step S13). Next, the control device 40 also increases the reduction rate (step S14). As a result of the increase, the value of X approaches 0, and the control device 40 determines whether the value of X is 0 (step S15). If the result of the determination is not 0 (NO), the control device 40 performs step S14 again. On the other hand, if the value is 0 (YES), the control device 40 performs step S11 again.

[0063] If the carbon dioxide pressure is lower than the reference value in step S12, the control device 40 reduces the introduction rate (step S16). Next, the control device 40 also reduces the reduction rate (step S17). As a result of the reduction, the value of X approaches 0, and the control device 40 determines whether the value of X is 0 (step S18). If the result of the determination is not 0 (NO), the control device 40 performs step S17 again. On the other hand, if the value is 0 (YES), the control device 40 performs step S11 again.

[0064] As described above, the control device 40 controls the introduction rate so that the required amount of carbon dioxide is introduced based on the amount of carbon dioxide (substance to be reduced) supplied to the introduction control unit 30, and then controls the current supply control unit 20 so that the reduction rate is X = 0 or close to 0. This makes it possible to respond to fluctuations in the amount of carbon dioxide supplied.

[0065] FIG. 6 is a control flow diagram for the reduction rate and introduction rate when the value of Y approaches 0. Before explaining FIG. 6 , the concept of the flow in FIG. 6 will be explained. Depending on the status of the utilization process of the target product, it is expected that the production rate of the product may be required to be increased or decreased during steady-state operation of the electrochemical system 100. For example, when synthesizing a new substance using the product, it is expected that the demand for the supply amount of the product may increase or decrease in order to control the reaction. Here, the increase or decrease in the production rate of the product can be determined by the amount of product stored. Alternatively, it may be determined by the pressure of the container or piping in which the product is stored. In other words, if a certain amount of product is stored or more, the reduction rate is decreased, and if the amount of product stored is below a certain level, the reduction rate is increased.

[0066] When increasing the production amount (production rate), the energization control unit 20 controls to increase the current value so that the required production amount is achieved. The introduction control unit 30 controls to increase the introduction rate so that Y approaches 0 so that the carbon utilization rate is maximized for the reduction rate at that time. On the other hand, when decreasing the production amount, the energization control unit 20 controls to decrease the current value so that the required production amount is achieved. The introduction control unit 30 controls to decrease the introduction rate so that Y approaches 0 so that the carbon utilization rate is maximized for the reduction rate at that time.

[0067] The above concept will be explained with reference to Fig. 6. The control device 40 determines whether the storage amount of the product is within a reference value (within an expected range) (step S21). If it is within the reference value (YES), the control device 40 ends the flow in Fig. 6. If it is outside the reference value (NO), the control device 40 determines whether the storage amount of the product is greater than the reference value (greater than the upper limit of the expected range) or smaller than the reference value (smaller than the lower limit of the expected range) (step S22).

[0068] If it is smaller, the control device 40 increases the reduction rate (step S23). Next, the control device 40 also increases the introduction rate (step S24). As a result of the increase, the value of Y approaches 0, and the control device 40 determines whether the value of Y is 0 (step S25). If the result of the determination is not 0 (NO), the control device 40 performs step S24 again. On the other hand, if the value is 0 (YES), the control device 40 performs step S21 again.

[0069] If the amount of stored product is greater than the reference value in step S22, the control device 40 reduces the reduction rate (step S26). Next, the control device 40 also reduces the introduction rate (step S27). As a result of the reduction, the value of Y approaches 0, and the control device 40 determines whether the value of Y is 0 (step S28). If the result of the determination is not 0 (NO), the control device 40 performs step S27 again. On the other hand, if the value is 0 (YES), the control device 40 performs step S21 again.

[0070] As described above, the control device 40 controls the reduction rate by the current control unit 20 so as to achieve the required production amount based on the production amount of the product that has permeated the cathode 12, and then controls the introduction rate by the introduction control unit 30 so that Y=0 or approaches 0. This makes it possible to prevent the amount of product from becoming excessive or insufficient.

[0071] In this way, the carbon utilization rate can be improved while maintaining a high target product ratio by controlling the reduction rate and introduction rate using the current control unit 20 and the introduction control unit 30. Furthermore, the carbon utilization rate can be maximized by controlling the reduction rate of carbon dioxide to an arbitrary value according to the status of the carbon dioxide production process.

[0072] However, in each control, X and Y do not necessarily have to be exactly 0, and it is preferable to make them as close to 0 as possible within a controllable range. For example, the amount of change in the carbon utilization rate is preferably 6% or less, preferably 5% or less, more preferably 4% or less (i.e., about 5% or less) compared to the carbon utilization rate before control. More preferably, it is 1% or less.

[0073] The target product can be any reduction product of the introduced substance.

[0074] 1 , in order to improve the efficiency of producing the reduction product, the electrolyte introduced into the space 192 (cathode chamber) containing the cathode 12 may be alkaline. Also, an acidic electrolyte may be used in order to partially reduce carbon dioxide that is dissolved in the electrolyte and not used in the reduction reaction.

[0075] The separation mechanism 50 separates the fluid flowing out of the space 193 through the outlet 17 into gas and liquid. The separation mechanism 50 includes, for example, a hydrophobic membrane that selectively allows gas to pass through. By including the separation mechanism 50, the components, production amounts, etc. of the liquid product and the gas product can be evaluated independently. The separation mechanism 50 may be, for example, a storage tank. By storing an electrolyte solution in which the product is dissolved, dispersed, etc., in the storage tank, the gas phase and liquid phase are separated inside the storage tank. Then, the gas phase can be removed to separate the gas product. At this time, if necessary, the product dissolved in the liquid phase may be further separated from the liquid phase using, for example, a separation membrane. Furthermore, when separating the liquid product, the separation mechanism 50 may include, for example, a distillation device. Furthermore, the separation mechanism 50 may be installed within the space 193.

[0076] If the electrolyte contains unreacted carbon dioxide, the separation mechanism 50 can also separate the unreacted carbon dioxide from the electrolyte. If the amount of separated carbon dioxide is large (for example, if the carbon dioxide concentration is greater than a predetermined threshold), the separated carbon dioxide can be returned to the introduction control unit 30. On the other hand, if the amount of separated carbon dioxide is small (for example, if the carbon dioxide concentration is equal to or less than a predetermined threshold), the separated carbon dioxide can be released, for example, into the atmosphere.

[0077] 7 is a schematic diagram of an electrochemical system 100 according to another embodiment. In the embodiment shown in FIG. 7 , the introduction control unit 30 introduces the electrolyte and carbon dioxide independently into the electrochemical cell 10. Therefore, the inlet 19 includes an inlet 195 (first inlet) through which the electrolyte (electrolytic solution without carbon dioxide; only the electrolyte) is introduced into the electrochemical cell 10, and an inlet 196 (second inlet) through which gaseous carbon dioxide (only the carbon dioxide; no electrolyte) is introduced into the electrochemical cell 10.

[0078] The introduction control unit 30 includes an introduction control unit 301 that introduces the electrolyte into the electrochemical cell 10 and an introduction control unit 302 that introduces carbon dioxide into the electrochemical cell 10. The introduction control unit 301 introduces the electrolyte into the space 192 through the introduction port 195. The introduced electrolyte does not need to contain carbon dioxide. The introduction control unit 301 includes, for example, a liquid feed pump. The introduction control unit 302 introduces gaseous carbon dioxide into the space 194 through the introduction port 196. The introduction control unit 302 includes, for example, an air feed pump, a mass flow controller, etc. The carbon dioxide in the space 194 permeates the cathode 12 and reaches the space 192 where the electrolyte is present. Therefore, in the embodiment of FIG. 7 , the electrolyte is present only in the spaces 191 and 192, and no electrolyte is present in the space 194.

[0079] One of the fluids, the electrolytic solution introduced from inlet 195 (first inlet) and heading toward outlet 17, or the carbon dioxide introduced from inlet 196 (second inlet) and heading toward outlet 17, is joined with the other fluid that has permeated the cathode 12. In the example of FIG. 7 , the electrolytic solution (one fluid) introduced from inlet 195 and heading toward outlet 17 is joined with carbon dioxide (the other fluid) that has permeated the cathode 12. With this configuration, by introducing carbon dioxide in the form of gas, the amount of carbon dioxide supplied to the catalyst section can be significantly increased, and the carbon dioxide reduction rate can also be increased.

[0080] 7, the distance between the cathode 12 and the ion exchange membrane 13 and the distance between the anode 11 and the ion exchange membrane 13 may be set as close to zero as possible (not shown). In this case, the anode 11 is preferably positioned at the left end of the space 191 in FIG. 7. The width of the space 192 is also preferably designed to be close to zero so that the cathode 12 can be placed near the ion exchange membrane 13. For example, the distance between the cathode 12 and the ion exchange membrane 13 is preferably 100 μm or less, more preferably 50 μm or less. In this case, it is not necessary to introduce the electrolyte from the inlet 195. Instead, produced water that permeates the ion exchange membrane 13 from the space 191 side can be used as the electrolyte.

[0081] 8 is a schematic diagram of the cathode 12 provided in the electrochemical system 100 shown in FIG. 7 . The cathode 12 is a member that separates a space 192 in which an electrolyte flow is formed and a space 194 in which an inlet 196 is formed inside the reaction vessel 14. The cathode 12 includes a substrate layer 122, a substrate layer 123, and a catalyst layer 124. The substrate layer 122 is exposed to the space 194, and the catalyst layer 124 is exposed to the space 192. The substrate layer 123 is disposed between the substrate layer 122 and the catalyst layer 124. At least the space 192 side of the cathode 12 preferably has hydrophobic properties; for example, it is preferable that at least one layer out of the substrate layer 122, the substrate layer 123, and the catalyst layer 124 has hydrophobic properties.

[0082] The substrate layer 122 is, for example, a porous layer that is at least permeable to gas. The substrate layer 122 is, for example, made of an aggregate of carbon fibers having a predetermined length. The substrate layer 123 is, for example, a porous layer that is at least permeable to gas and has hydrophobic properties. The substrate layer 123 is, for example, made of an aggregate of carbon-based nanoparticles.

[0083] The catalyst layer 124 is a porous layer that is at least permeable to gas. The catalyst layer 124 is composed of, for example, an aggregate of catalyst particles (e.g., copper-based particles). The catalyst layer 124 can be formed, for example, by chemically or physically adsorbing a catalyst onto the surface of the substrate layer 123 (the surface opposite to the side where the substrate layer 122 is disposed), or by dripping and drying a slurry in which the catalyst particles are dispersed in an arbitrary dispersion medium. Note that by dispersing the catalyst particles in at least one of the substrate layer 122 or the substrate layer 123, at least one of the substrate layer 122 or the substrate layer 123 containing the catalyst particles may also serve as the catalyst layer 124.

[0084] As described above, the base material layer 122, the base material layer 123, and the catalyst layer 124 are all porous layers, and the cathode 12 is a porous body as a whole. Therefore, although not shown in Fig. 8 (and Fig. 9 described later), fluid can pass through the pores present in the base material layer 122, the base material layer 123, and the catalyst layer 124.

[0085] FIG. 9 is a schematic diagram of the cathode 12 provided in the electrochemical system 100 shown in FIG. 7 , showing an embodiment different from that shown in FIG. 8 . In the embodiment shown in FIG. 9 , the catalyst layer 124 is disposed between the substrate layer 122 and the substrate layer 123. In the embodiment of FIG. 9 , the substrate layer 122 is made of a porous material such as a fluorine-based resin. It is desirable that at least one of the substrate layer 122 and the substrate layer 123 is conductive. Even in this manner, the cathode 12 having pores 121 a that are permeable to a fluid can be formed.

[0086] 7 , carbon dioxide gas in space 194 is released (e.g., diffused) through the surface of cathode 12 (the surface facing space 192) relative to the flow of the electrolyte in space 192. In this manner, products are produced when the gaseous carbon dioxide permeates cathode 12, and the products and unreacted carbon dioxide are released into the electrolyte. Then, the products and the like can be discharged through outlet 17.

[0087] It is preferable to control the flow of carbon dioxide so that the carbon dioxide in space 194 merges with the electrolyte in space 192 while preventing the electrolyte in space 192 from flowing back into space 194. Backflow of the electrolyte into space 194 reduces the amount of carbon dioxide that can reach the catalyst surface in gaseous form, thereby reducing the carbon dioxide supply and the carbon dioxide reduction efficiency (Faraday efficiency). As a specific countermeasure, for example, the pressure of carbon dioxide in space 194 may be made higher than the water pressure of the electrolyte in space 192. This allows efficient transport of carbon dioxide in gaseous form to the catalyst surface and maintains the supply rate. Furthermore, during steady-state operation of the electrochemical system 100, intermittent operation may be repeated for a certain period of time, in which at least one of the carbon dioxide supply rate or reduction rate is reduced or stopped. This allows the electrolyte that has penetrated into the vicinity of the catalyst or into space 194 to be partially discharged to space 192, enabling stable and efficient carbon dioxide reduction (high Faraday efficiency) for a long period of time.

[0088] In the above-described embodiments, the substance to be reduced is carbon dioxide, and the product generated by the reduction reaction is a carbon compound containing mainly carbon atoms having a carbon number of 1 to 3. However, the above-described embodiments can be similarly applied to an electrochemical system 100 in which the substance to be reduced is nitrogen or nitrogen oxide and the product generated after permeation through the cathode 12 is ammonia, by using a catalyst compatible with nitrogen or nitrogen oxide as the catalyst.

[0089] 10 is a flowchart showing a method for producing a valuable material according to the present disclosure (hereinafter referred to as the production method of the present disclosure). The production method of the present disclosure can be executed by, for example, a control device 40. As described above, the products produced in the electrochemical system 100 are, for example, carbon compounds containing 1 to 3 carbon atoms, ammonia, and the like, which are valuable materials that have value in the market. Therefore, according to the production method of the present disclosure, a valuable material can be produced using, for example, the electrochemical system 100. Hereinafter, the production method of the present disclosure will be described using, as an example, a carbon compound containing 1 to 3 carbon atoms as the valuable material; however, the valuable material may also be ammonia, etc., produced by the reduction of nitrogen or nitrogen oxides.

[0090] The manufacturing method of the present disclosure includes an introduction step S1, a first control step S2, a reduction step S3, a second control step S4, and an outflow step S5. The introduction step S1, the first control step S2, the reduction step S3, the second control step S4, and the outflow step S5 do not necessarily have to be performed in this order, and at least two steps may be performed simultaneously.

[0091] The introduction step S1 is a step of introducing the electrolyte solution and carbon dioxide, either independently or together, into an electrochemical cell 10 including an anode 11, a cathode 12 having fluid-permeable holes 121a, and a reaction vessel 14 having the anode 11 and the cathode 12 therein. When the electrolyte solution and carbon dioxide are introduced independently into the electrochemical cell 10, for example, the electrochemical cell 10 shown in FIG. 7 above can be used. When the electrolyte solution and carbon dioxide are introduced together into the electrochemical cell 10, for example, the electrochemical cell 10 shown in FIG. 1 above can be used. The introduction of the carbon dioxide and the electrolyte solution can be performed by, for example, an introduction control unit 30.

[0092] The first control step S2 is a step of controlling the introduction rate of carbon dioxide (substance to be reduced) in the introduction step S1. The first control step S2 is usually performed simultaneously with the introduction step S1. Therefore, carbon dioxide is introduced into the electrochemical cell 10 while controlling the introduction rate of carbon dioxide. However, in the present disclosure, for convenience of explanation and illustration, the introduction step S1 and the first control step S2 are described separately. The control of the introduction rate can be performed by, for example, the introduction control unit 30.

[0093] The reduction step S3 is a step of reducing carbon dioxide at the cathode 12 by permeating at least carbon dioxide through the cathode 12. As a method of permeating carbon dioxide to the cathode 12, for example, when carbon dioxide is permeated alone, the electrochemical cell 10 shown in FIG. 7 above can be used. For example, when carbon dioxide and an electrolyte are permeated together (for example, dissolved in the electrolyte), the electrochemical cell 10 shown in FIG. 1 above can be used. The reduction of carbon dioxide can be performed by the current control unit 20.

[0094] The second control step S4 is a step for controlling the rate of reduction of carbon dioxide in the reduction step S3. The second control step S4 is usually performed simultaneously with the reduction step S3. Therefore, the reduction of carbon dioxide is performed while controlling the reduction rate. However, in this disclosure, for convenience of explanation and illustration, the reduction step S3 and the second control step S4 are described separately. The control of the reduction rate can be performed by, for example, the energization control unit 20.

[0095] The outflow step S5 is a step of causing the product produced in the reduction step S3 to flow out of the electrochemical cell 10 together with the electrolyte. The outflow is performed by the introduction control unit 30. For example, when the substance to be reduced is carbon dioxide, the product is a carbon compound containing 1 to 3 carbon atoms. Also, for example, when the substance to be reduced is nitrogen or nitrogen oxide, the product is ammonia or the like. Carbon compounds containing 1 to 3 carbon atoms, ammonia, and the like are all valuable materials. Therefore, valuable materials can be produced according to the production method disclosed above.

[0096] Control indices in the first control step S2 and the second control step S4 include carbon utilization rate, Faraday efficiency, energy efficiency, etc. The carbon utilization rate is expressed as "(amount of reduced carbon dioxide) / (amount of introduced carbon dioxide)" (i.e., the value obtained by dividing the amount of reduced carbon dioxide by the amount of introduced carbon dioxide).

[0097] REFERENCE SIGNS LIST 10 Electrochemical cell 100 Electrochemical system 11 Anode 12 Cathode 121 Conductor 122 Substrate layer 123 Substrate layer 124 Catalyst layer 13 Ion exchange membrane 14 Reaction vessel 15 Inlet 16 Inlet 17 Outlet 18 Outlet 19 Inlet 191 Space 192 Space 193 Space 194 Space 195 Inlet 196 Inlet 20 Current control unit 30 Introduction control unit 301 Introduction control unit 302 Introduction control unit 40 Control device 50 Separation mechanism 913 Space S1 Introduction step S2 First control step S3 Reduction step S4 Second control step S5 Outlet step

Claims

1. An electrochemical system comprising: an electrochemical cell including an anode, a cathode, and a reaction vessel containing the anode and the cathode; an energization control unit that energizes between the anode and the cathode; an introduction control unit that introduces an electrolyte and a substance to be reduced into the electrochemical cell, either independently or in combination; and a control device that controls the energization control unit to control the reduction rate of the substance to be reduced at the cathode, and controls the introduction control unit to control the introduction rate of the substance to be reduced into the electrochemical cell.

2. An electrochemical system according to claim 1, wherein the cathode has holes that allow liquid to pass through.

3. An electrochemical system according to claim 2, wherein the electrochemical cell comprises: a first space and a second space separated by the cathode; an inlet for introducing the substance to be reduced into the first space of the electrochemical cell; and an outlet for allowing the product generated by permeating the cathode to flow out of the second space of the electrochemical cell.

4. An electrochemical system according to claim 1, wherein the control device controls the reduction rate and the introduction rate so that X = (amount of change in carbon utilization rate) / (amount of change in reduction rate) and Y = (amount of change in carbon utilization rate) / (amount of change in introduction rate) become 0 or approach 0.

5. An electrochemical system according to claim 4, wherein the control device performs the following at least once, while maintaining the predetermined introduction rate, by using the current control unit to control the reduction rate so that X = 0 or approaches 0, and then by using the introduction control unit to control the reduction rate so that Y = 0 or approaches 0.

6. An electrochemical system according to claim 4, wherein the control device controls the introduction rate by the introduction control unit based on the amount of the object to be reduced supplied to the introduction control unit so that the required amount of the object to be reduced is introduced, and then controls the reduction rate by the current control unit so that X = 0 or approaches 0.

7. An electrochemical system according to claim 4, wherein the control device controls the reduction rate by the current control unit based on the amount of product generated through the cathode to achieve the required amount of product, and then controls the introduction rate by the introduction control unit to Y=0 or approaching 0.

8. An electrochemical system according to claim 1, characterized in that the control device controls at least one of the current control unit and the introduction control unit to control the ratio of the reduction rate of the substance to be reduced to the introduction rate of the substance to be reduced (reduction rate / introduction rate) to be larger than when the control is not performed.

9. An electrochemical system according to claim 8, characterized in that the control device controls the reduction rate so that it is 40% or more of the introduction rate.

10. The electrochemical system according to claim 8, wherein the cathode is a porous body or a mesh body.

11. An electrochemical system as claimed in claim 3, wherein the inlets include a first inlet for introducing the electrolyte into the electrochemical cell and a second inlet for introducing the gaseous target substance to be reduced into the electrochemical cell, and one of the fluids of the electrolyte introduced from the first inlet and heading towards the outlet, or the target substance to be reduced introduced from the second inlet and heading towards the outlet, is joined by the other fluid that has permeated the cathode.

12. An electrochemical system as claimed in claim 11, wherein the cathode is a member that separates the space inside the reaction vessel in which the flow of the electrolyte is formed from the space in which the second inlet is formed, and the gas of the substance to be reduced is released through the surface of the cathode in response to the flow of the electrolyte.

13. An electrochemical system according to claim 1, wherein the electrochemical cell is provided with an ion exchange membrane that separates the space in which the cathode is disposed from the space in which the anode is disposed inside the reaction vessel.

14. An electrochemical system according to claim 1, wherein the substance to be reduced is carbon dioxide, and the product generated by permeating the cathode is a carbon compound containing 1 to 3 carbon atoms.

15. An electrochemical system according to claim 10, characterized in that the diameter of the pores in the cathode is 2000 μm or less.

16. The electrochemical system according to claim 1, wherein the substance to be reduced is nitrogen or nitrogen oxide, and the product generated by permeation through the cathode is ammonia.

17. A method for producing valuable resources, comprising: an introduction step of introducing an electrolyte and a material to be reduced, either independently or together, into an electrochemical cell having an anode and a cathode having pores that allow fluid to pass through; a first control step of controlling the introduction rate of the material to be reduced in the introduction step; a reduction step of reducing the material to be reduced at the cathode by allowing at least the material to be reduced to pass through the cathode; a second control step of controlling the reduction rate of the material to be reduced in the reduction step; and a discharge step of discharging a product produced in the reduction step from the electrochemical cell together with the electrolyte.

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