Carbon dioxide electrolytic reduction method

Abstract

The present invention provides a carbon dioxide electrolytic reduction method with which it is possible to: suppress salt precipitation at the cathode of a carbon dioxide electrolytic reduction cell and achieve long-term operation of the carbon dioxide electrolytic reduction cell; and improve the electrolytic performance in the early operation start stage.　Provided is a carbon dioxide electrolytic reduction method which is for electrolyzing carbon dioxide by using a solid electrolyte-type electrolytic device comprising a cathode, an ion exchange membrane, an anode, and an anodic electrolytic solution tank, said method including a step for electrolyzing carbon dioxide by supplying, to the cathode, a gas containing cations which are one or more of alkali metal ions and alkaline earth metal ions.

Description

Carbon dioxide electrolysis reduction method

[0001] This invention relates to a method for electrolytic reduction of carbon dioxide.

[0002] Research on carbon dioxide reduction using electrical energy is being conducted widely worldwide. Among these, the solid electrolyte type electrolytic device (also called a polymer electrolyte type electrolytic cell or carbon dioxide electrolytic reduction cell) equipped with an ion exchange membrane-electrode assembly (MEA) with the structure described in Patent Document 1 (Figure 1) has advantages in that it can directly reduce gaseous carbon dioxide and reduces ion transport resistance by using a thin film polymer electrolyte.

[0003] The cathode for carbon dioxide reduction in an ion exchange membrane-electrode junction generally comprises a catalyst layer containing a mixture of catalyst nanoparticles, a conductive carrier, and a binder. The binder is used to improve the adhesion of the catalyst nanoparticles to the electrode surface. Ion exchange resins can also be used as the binder, in which case the binder also functions to transport ions consumed or generated by the reduction reaction. These ion exchange resins are generally referred to as ionomers.

[0004] In an ion exchange membrane-electrode junction, the ion exchange membrane, due to its structure, contains ions (e.g., H) that contribute to the electrolytic reaction. ＋ , OH － ) In addition to the electrolytes contained in the electrolyte solution (for example, K ＋ It has the property of permeating even ions. As a result, the electrolyte supplied to the anode often penetrates the ion exchange membrane and precipitates as salt inside the cathode. This salt precipitation phenomenon hinders the supply of carbon dioxide to the cathode catalyst and causes a decrease in electrolytic performance, such as CO generation current density and CO selectivity. The adverse effects of this salt precipitation become more pronounced as the electrolyte concentration of the anode electrolyte increases.

[0005] On the other hand, it has been reported that when pure water without dissolved electrolyte is used as the electrolyte for the anode in an ion-exchange membrane-electrode junction, the rate and selectivity of the carbon dioxide reduction reaction decrease significantly (see, for example, Non-Patent Documents 1 and 2). Therefore, the presence of an electrolyte (especially alkali metal ions as cationic species) is necessary to drive the carbon dioxide reduction reaction with high selectivity. This is due, firstly, to the effect of the electrolyte (alkali metal ions) stabilizing the intermediate by temporarily bonding with it in the carbon dioxide reduction reaction, and secondly, to the effect of reducing the resistance of the electrolyte.

[0006] From the above, there are conflicting requirements: one to reduce the electrolyte concentration in the anodic electrolyte to suppress salt deposition at the cathode, and another to increase the electrolyte concentration to improve the selectivity of the carbon dioxide reduction reaction.

[0007] Several methods have been proposed to resolve these conflicting technical challenges. For example, Non-Patent Document 3 reports a method in which an ion exchange membrane is pre-substituted with alkali metal ions. However, this method cannot supply sufficient alkali metal ions near the cathode catalyst, resulting in a selectivity of less than 1% for the carbon dioxide reduction reaction.

[0008] International Publication No. 2024 / 101045

[0009] MCO Monteiro, F. Dattila, B. Hagedoorn, R. Garcia-Muelas, N. Lopez, MTM Koper, Nature catalysis, 2021, 4, 654-662.JA Rebstock, Q. Zhu, LR Baker, Chemical Science, 2022, 13, 7634-7643.W. Liao, D. Tsai, W. Hong, Y. Huang, L. Lin, Y. Pan, Chemical Engineering Journal, 2022, 434, 134765.

[0010] As described above, a major challenge in conventional carbon dioxide electrolytic reduction cells has been to appropriately control the alkali metal ion concentration near the cathode while simultaneously reducing the concentration of the anodic electrolyte to prevent salt deposition.

[0011] Furthermore, as the operating time of the carbon dioxide electrolytic reduction cell increases, the amount of salt precipitated increases, further degrading the electrolytic performance. Therefore, establishing a carbon dioxide electrolytic reduction technology that can operate for extended periods is a crucial challenge.

[0012] Furthermore, there is a need for carbon dioxide electrolysis reduction technology that can improve the electrolysis performance of carbon dioxide electrolysis reduction cells during the initial stages of operation.

[0013] Therefore, the object of the present invention is to provide a carbon dioxide electrolytic reduction method that suppresses salt deposition at the cathode of a carbon dioxide electrolytic reduction cell, enables long-term operation of the carbon dioxide reduction cell, and improves electrolytic performance in the initial stages of operation.

[0014] To achieve the above objective, the present invention provides the following method for electrolytic reduction of carbon dioxide.

[0015] [1] A carbon dioxide electrolytic reduction method using a solid electrolyte electrolytic apparatus equipped with a cathode, an ion exchange membrane, an anode, and an anode electrolyte tank, comprising the step of supplying a gas containing one or more alkali metal ions and alkaline earth metal ions to the cathode to electrolyze carbon dioxide. [2] The carbon dioxide electrolytic reduction method according to [1], wherein the gas is a mixed gas of the carbon dioxide to be electrolyzed and water vapor. [3] The carbon dioxide electrolytic reduction method according to [1], wherein the gas is water vapor. [4] The carbon dioxide electrolytic reduction method according to any one of [1] to [3], comprising the step of passing the gas through a processing solution containing one or more alkali metal ions and alkaline earth metal ions before supplying the gas to the cathode, thereby incorporating one or more alkali metal ions and alkaline earth metal ions into the gas. [5] The carbon dioxide electrolytic reduction method according to [4], wherein the processing solution is a solution containing one or more alkali metal ions and alkaline earth metal ions in a total concentration of 0.01 to 5 mol / L. [6] The carbon dioxide electrolytic reduction method according to any one of [1] to [5], wherein the cathode has been pretreated by a process including holding a solution containing one or more alkali metal ions and alkaline earth metal ions on the cathode. [7] The carbon dioxide electrolytic reduction method according to [6], wherein the process includes impregnating the cathode with a treatment solution containing one or more alkali metal ions and alkaline earth metal ions. [8] The carbon dioxide electrolytic reduction method according to [7], wherein the impregnation process is carried out under a reduced pressure environment of 0 to 90 kPa. [9] The carbon dioxide electrolytic reduction method according to [7] or [8], wherein the treatment solution is a solution containing one or more alkali metal ions and alkaline earth metal ions in a total concentration of 0.01 to 5 mol / L.

[10] The carbon dioxide electrolytic reduction method according to [8], wherein the impregnation process includes immersing the cathode in the treatment solution for 10 to 120 minutes.

[11] The carbon dioxide electrolytic reduction method according to [6], wherein the processing step includes applying a voltage to a solid electrolyte electrolytic apparatus equipped with a cathode, an ion exchange membrane, an anode, and an anode electrolyte tank, so that one or more alkali metal ions and alkaline earth metal ions in a processing solution containing one or more alkali metal ions and alkaline earth metal ions placed in the anode electrolyte tank permeate the ion exchange membrane, and the solution containing the permeated cations is held in the cathode.

[12] The carbon dioxide electrolytic reduction method according to

[11] , wherein the processing solution is a solution containing one or more alkali metal ions and alkaline earth metal ions in a total concentration of 0.03 to 2 mol / L.

[13] The carbon dioxide electrolytic reduction method according to

[11] or

[12] , wherein the voltage application time is 0.1 to 24 hours.

[14] The method for electrolytic reduction of carbon dioxide according to any one of

[11] to

[13] , wherein the voltage is such that the applied potential of the cathode to a silver / silver chloride reference electrode (having a saturated potassium chloride solution in its internal solution) is -1.1 to -1.8 V.

[15] The processing step is such that the total amount of one or more cations, either alkali metal ions or alkaline earth metal ions, held in the cathode is 0.3 to 170 μmol / cm. ２ A carbon dioxide electrolytic reduction method according to any one of [6] to

[14] , comprising a step carried out to such effect.

[16] A carbon dioxide electrolytic reduction method according to any one of [1] to

[15] , wherein in the step of electrolyzing carbon dioxide, the electrolyte in the anodic electrolyte tank is a solution containing one or more cations from alkali metal ions and alkaline earth metal ions in a total concentration of 0.01 to 0.5 mol / L.

[17] A carbon dioxide electrolytic reduction method according to any one of [1] to

[16] , wherein the alkali metal ions are one or more from lithium ions, sodium ions, potassium ions, cesium ions, and rubidium ions.

[18] A carbon dioxide electrolytic reduction method according to any one of [1] to

[17] , wherein the alkaline earth metal ions are one or more from beryllium ions, magnesium ions, calcium ions, strontium ions, and barium ions.

[0016] According to the present invention, it is possible to provide a carbon dioxide electrolytic reduction method that suppresses salt deposition at the cathode of a carbon dioxide electrolytic reduction cell, enables long-term operation of the carbon dioxide reduction cell, and improves electrolytic performance in the initial stages of operation.

[0017] The reason why the present invention achieves the above effects is presumed to be as follows. In the production of carbon monoxide using a carbon dioxide electrolytic reduction cell, the cationic species in the cathode are presumed to promote the carbon dioxide electrolytic reduction reaction by exhibiting the effect of lowering the ohmic resistance as an electrolyte, locally increasing the carbon dioxide concentration on the electrode surface by adsorbing carbon dioxide, and stabilizing the intermediate by temporarily binding with it as a Lewis acid. Furthermore, if the cationic species in the cathode are present in a solution state, it is presumed that under negative voltage application conditions, the effects of the cationic species are further promoted as the cationic species accumulate on the electrode surface. However, it is presumed that trace amounts of cationic species in the cathode are constantly emitted from the system along with the generated gas, and during long-term operation, it is presumed that the balancing of the amount of cations in the carbon dioxide electrolytic reduction cell is disrupted, leading to a deficiency of cationic species in the cathode. Herein, the carbon dioxide electrolytic reduction method according to the embodiment of this disclosure is presumed to be a method for producing carbon monoxide with high electrolytic performance from the initial stage of operation by using a specific cationic species in a solution state that is pre-held in the cathode, thereby exhibiting the above effects and promoting the carbon dioxide electrolytic reduction reaction. Furthermore, due to the aforementioned effects, it is possible to reduce the concentration of the anodic electrolyte, which is presumed to suppress the phenomenon of electrolytes in the anodic electrolyte permeating the ion exchange membrane and causing salt to precipitate inside the cathode. Moreover, the carbon dioxide electrolytic reduction method according to this disclosure allows for the supply of carbon dioxide, which is the supply gas, by passing it through a treatment solution containing specific cation species and supplying it to the carbon dioxide electrolytic reduction cell. This makes it possible to supply clustered cation species together with water vapor into the cathode, which is presumed to prevent cation species depletion during long-term operation and enable long-term operation of the carbon dioxide reduction cell.In addition, by maintaining a humid state inside the carbon dioxide electrolytic reduction cell through the supply of water vapor, it is possible to prevent the treatment solution held inside the cathode from evaporating to dryness, which is presumed to promote the aforementioned salt precipitation suppression effect and long-term operation capability.

[0018] 1 is a schematic diagram of an apparatus for treating a supply gas. FIG. 2 is a schematic diagram of an apparatus for pretreating a cathode by reduced pressure impregnation. FIG. 3 shows XRF spectra of respective electrodes after electrolysis (Comparative Examples 1 to 4). FIG. 4 shows XRF spectra of respective electrodes after electrolysis (Examples 1 to 6). FIG. 5 shows XRF spectra of respective electrodes after electrolysis (Examples 7 to 10). FIG. 6 shows a GC chart of Example 1. FIG. 7 shows a GC chart of Example 2. FIG. 8 shows a GC chart of Example 3. FIG. 9 shows a GC chart of Example 4. FIG. 10 shows a GC chart of Example 5. FIG. 11 shows a GC chart of Example 6.

[0019] Hereinafter, an embodiment of the present invention (hereinafter referred to as "the present embodiment") will be described in detail. However, the present invention is not limited thereto, and various modifications can be made without departing from the gist of the present invention.

[0020] In this specification, unless otherwise specified, the description of a numerical range "X (lower limit) to Y (upper limit)" means "X or more and Y or less". In addition, when a plurality of numerical ranges are described, the lower limit values and upper limit values of the respective numerical ranges can be arbitrarily combined.

[0021] [Carbon dioxide electrolytic reduction method] The carbon dioxide electrolytic reduction method according to the present embodiment is a carbon dioxide electrolytic reduction method for electrolyzing carbon dioxide using a solid electrolyte-type electrolytic device including a cathode, an ion exchange membrane, an anode, and an anode electrolyte tank, the method including a step of electrolyzing carbon dioxide by supplying a gas containing one or more cations selected from the group consisting of alkali metal ions and alkaline earth metal ions to the cathode.

[0022] The alkali metal ion in the present embodiment is preferably one or more selected from the group consisting of lithium ion, sodium ion, potassium ion, cesium ion, and rubidium ion, more preferably one or more selected from the group consisting of sodium ion, potassium ion, and cesium ion, and still more preferably potassium ion.

[0023] In the present embodiment, the alkaline earth metal ion is preferably any one or more of beryllium ion, magnesium ion, calcium ion, strontium ion and barium ion, more preferably any one or more of magnesium ion, calcium ion and strontium ion, and further preferably magnesium ion.

[0024] In one preferred embodiment (hereinafter referred to as Embodiment 1), the gas is a mixed gas of carbon dioxide to be electrolyzed and water vapor. Any one or more cations selected from alkali metal ions and alkaline earth metal ions are contained in the water vapor.

[0025] In another preferred embodiment (hereinafter referred to as Embodiment 2), the gas is water vapor. Any one or more cations selected from alkali metal ions and alkaline earth metal ions are contained in the water vapor. The water vapor is preferably supplied to the cathode together with carbon dioxide to be electrolyzed.

[0026] [Treatment of Supply Gas] In Embodiment 1, a mixed gas of carbon dioxide and water vapor that has been treated to contain any one or more cations selected from alkali metal ions and alkaline earth metal ions is used as the supply gas to the cathode.

[0027] Referring to FIG. 1, the method for treating the supply gas in Embodiment 1 will be described. A treatment apparatus 100 comprising a container 101 accommodating a treatment liquid 10 containing any one or more cations selected from alkali metal ions and alkaline earth metal ions is prepared. In addition to the container 101, the treatment apparatus 100 includes a pipe 103 having one end serving as an inlet for carbon dioxide gas, a pipe 104 having one end serving as an outlet for the mixed gas, and a stopper 102 having holes for inserting these pipes. The pipe 103 has a length such that the other end is immersed in the treatment liquid 10, and the pipe 104 has a length such that the other end is not immersed in the treatment liquid 10. Each of the pipe 103 and the pipe 104 may be formed by connecting two or more pipes.

[0028] By heating the container 101 to a predetermined temperature, preferably 5 to 80°C, more preferably 20 to 70°C, and even more preferably 40 to 60°C, the processing liquid 10 is also heated to the same temperature. By passing carbon dioxide gas supplied from the inlet of the pipe 103 through the heated processing liquid 10, a mixed gas of carbon dioxide gas and water vapor containing one or more cations from alkali metal ions and alkaline earth metal ions can be obtained. The carbon dioxide gas supply rate is preferably 20 to 150 cc / min, more preferably 40 to 120 cc / min, and even more preferably 60 to 100 cc / min.

[0029] In Embodiment 2, electrolyzed carbon dioxide and treated water vapor containing one or more alkali metal ions and alkaline earth metal ions are used as the supply gas to the cathode.

[0030] The method for treating the supply gas in Embodiment 2 is the same as the method for treating the supply gas in Embodiment 1, except that the gas supplied from the inlet of pipe 103 is changed from carbon dioxide gas to air. The water vapor discharged from the outlet of pipe 104 is supplied to the cathode together with the carbon dioxide gas.

[0031] The above-mentioned treatment solution 10 is preferably a solution containing one or more cations, either alkali metal ions or alkaline earth metal ions, in a total concentration of 0.01 to 5 mol / L. From the viewpoint of supplying a sufficient amount of cations to balance the amount of cations in the carbon dioxide electrolytic reduction cell, the lower limit of the total concentration is more preferably 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, or 0.09 mol / L, even more preferably 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, or 0.9 mol / L, and most preferably 1.0, 1.1, 1.2, 1.3, 1.4, or 1.5 mol / L. Furthermore, from the viewpoint of suppressing salt precipitation in the treatment solution due to exceeding the saturation concentration, the upper limit of the total concentration is more preferably 4.9, 4.8, 4.7, 4.6, 4.5, 4.4, 4.3, 4.2, or 4.1 mol / L, even more preferably 4.0, 3.9, 3.8, 3.7, or 3.6 mol / L, and most preferably 3.5 mol / L. The total concentration of one or more alkali metal ions and alkaline earth metal ions in the mixed gas and water vapor supplied to the cathode is lower than the total concentration of one or more alkali metal ions and alkaline earth metal ions in the treatment solution 10 used.

[0032] The carbon dioxide electrolytic reduction method according to the above embodiment preferably includes a step of passing the gas through a treatment solution containing one or more alkali metal ions and alkaline earth metal ions before supplying the gas to the cathode, thereby incorporating one or more alkali metal ions and alkaline earth metal ions into the gas (hereinafter referred to as the cathode pretreatment step used for carbon dioxide electrolysis).

[0033] [Pretreatment step for cathode used in carbon dioxide electrolysis] The pretreatment step for the cathode used in carbon dioxide electrolysis according to this embodiment includes a treatment step of holding a solution containing one or more cations, either alkali metal ions or alkaline earth metal ions, on the cathode. Holding the solution on the cathode means keeping the solution at the cathode.

[0034] As the cathode to which the above-mentioned pretreatment process is carried out, a known cathode used in carbon dioxide electrolysis can be used. The known cathode has at least a catalyst layer and a gas diffusion layer. As the gas diffusion layer, for example, carbon paper, nonwoven fabric, metal mesh, etc., can be used. Examples of these materials include graphite carbon, glassy carbon, titanium, SUS steel, etc.

[0035] The pretreatment step of holding a solution containing one or more alkali metal ions and alkaline earth metal ions on the cathode is not particularly limited and can be carried out by various methods. For example, preferred embodiments include the "pretreatment by impregnation" and the "pretreatment by voltage application" described below.

[0036] [Pretreatment by impregnation] As a specific example of the above treatment steps in this embodiment, a step of impregnating the cathode with a treatment solution containing one or more cations from alkali metal ions and alkaline earth metal ions and holding it therein can be cited. Impregnating the cathode with the treatment solution and holding it therein means keeping the treatment solution in the cathode. In other words, in this embodiment, after impregnation with the treatment solution, the cathode contains the treatment solution. If the cathode is water-repellent, even if the treatment solution is impregnated into the cathode, the treatment solution will be released from inside the cathode to outside the cathode over time, making it difficult to keep the treatment solution inside the cathode. In this embodiment, there are no particular restrictions on the method of impregnating the cathode with the treatment solution and holding it therein. When a water-repellent cathode is used, for example, after impregnating the cathode with the treatment solution, the cathode and the ion exchange membrane may be joined in a short time to confine and hold the treatment solution inside the cathode. The time from impregnating the cathode with the treatment solution to joining the cathode with the ion exchange membrane is not particularly limited as long as the treatment solution can be retained on the cathode. For example, it may be 0.5 seconds to 10 hours, 0.7 seconds to 1 hour, 1.0 second to 20 minutes, or 1.0 second to 5 minutes. When a water-repellent cathode is used, the time from impregnating the cathode with the treatment solution to joining the cathode with the ion exchange membrane is preferably 0.5 seconds to 60 minutes, preferably 0.7 seconds to 40 minutes, more preferably 1.0 second to 20 minutes, and even more preferably 1.0 second to 5 minutes. Here, a water-repellent cathode is a cathode with a static contact angle of 90° or more on the electrode surface, including superhydrophobic (150° to 180°) cathodes. In this embodiment, a water-repellent cathode with a static contact angle in the range of, for example, 100-175°, 105-170°, 110-165°, 115-165°, 120-170°, 125-165°, 130-160°, 135-155°, or 140-150° can be used. The static contact angle can be measured, for example, by the static droplet method (also called the liquid droplet method).

[0037] The impregnation step described above is preferably carried out under reduced pressure. The reduced pressure is preferably 0 to 90 kPa (absolute pressure), more preferably 0.1 to 50 kPa (absolute pressure), even more preferably 0.5 to 40 kPa (absolute pressure), and particularly preferably 1 to 20 kPa (absolute pressure), from the viewpoint of sufficiently degassing the air inside the cathode and encapsulating the treatment solution. This embodiment will be described with reference to Figure 2. A container 2 containing a treatment solution 4 containing one or more cations of alkali metal ions and alkaline earth metal ions, and a cathode 3 immersed in the treatment solution 4, is placed in a reduced pressure desiccator 1, and then the desiccator is evacuated to create a reduced pressure state. From the viewpoint of sufficiently degassing the air inside the cathode and preventing physical damage to the cathode under reduced pressure conditions, this immersion state is maintained for a predetermined time, preferably 10 to 120 minutes, more preferably 15 to 90 minutes, and even more preferably 30 to 60 minutes, to impregnate the cathode with the treatment solution.

[0038] The above-mentioned treatment solution 4 is preferably a solution containing one or more cations, either alkali metal ions or alkaline earth metal ions, in a total concentration of 0.01 to 5 mol / L. From the viewpoint of supplying a sufficient amount of cations to exhibit the reaction-promoting effect of cations in the carbon dioxide electrolytic reduction reaction, particularly the stabilizing effect of the reaction intermediate, the lower limit of the total concentration is more preferably 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, or 0.09 mol / L, even more preferably 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, or 0.9 mol / L, and most preferably 1.0, 1.1, 1.2, 1.3, 1.4, or 1.5 mol / L. Furthermore, from the viewpoint of suppressing salt precipitation due to exceeding the saturation concentration, the upper limit of the total concentration is more preferably 4.9, 4.8, 4.7, 4.6, 4.5, 4.4, 4.3, 4.2, or 4.1 mol / L, even more preferably 4.0, 3.9, 3.8, 3.7, or 3.6 mol / L, and most preferably 3.5 mol / L.

[0039] If the above-mentioned processing solution 4 contains potassium ions, it is preferable to use a solution containing the same alkaline cations as the electrolyte in the anodic electrolyte tank of the above-mentioned solid electrolyte electrolytic device, from the viewpoint of the ion membrane permeation rate.

[0040] To prevent the discharge of the treatment solution, it is preferable to keep the cathode pretreated by the above pretreatment method immersed in the same solution as the treatment solution until the ion exchange membrane-electrode assembly is manufactured.

[0041] [Pretreatment by voltage application] As another specific example of the processing steps in this embodiment, a voltage is applied to a solid electrolyte electrolytic device equipped with a cathode, an ion exchange membrane, an anode, and an anode electrolyte tank, thereby causing one or more alkali metal ions and alkaline earth metal ions in a processing solution containing one or more alkali metal ions and alkaline earth metal ions placed in the anode electrolyte tank to permeate the ion exchange membrane, and the solution containing the permeated cations is held in the cathode.

[0042] The above-mentioned treatment solution is preferably a solution containing one or more alkali metal ions and alkaline earth metal ions in a total concentration of 0.03 to 2.0 mol / L, more preferably 0.1 to 1.0 mol / L, and even more preferably 0.2 to 0.5 mol / L, from the viewpoint of supplying a sufficient amount of cations to exhibit the reaction-promoting effect of cations in the electrolytic reduction reaction of carbon dioxide, while suppressing salt deposition due to the supply of excess ions.

[0043] The voltage application time may be 0.01 to 36 hours, 0.1 to 24 hours, 0.1 to 12 hours, 0.1 to 6.0 hours, preferably 0.1 to 3.0 hours, more preferably 0.2 to 2.0 hours, and even more preferably 0.5 to 1.0 hours, from the viewpoint of supplying a sufficient amount of cations to exhibit the reaction-promoting effect of cations in the electrolytic reduction reaction of carbon dioxide, while suppressing salt deposition due to the supply of excess ions.

[0044] From the viewpoint of supplying a sufficient amount of cations by ion transport to exhibit the reaction-promoting effect of cations in the electrolytic reduction reaction of carbon dioxide, while suppressing salt deposition due to excess ion transport, the voltage applied to the cathode relative to the silver / silver chloride reference electrode (which has a saturated potassium chloride solution in its internal solution) is preferably -1.1 to -1.8 V, more preferably -1.3 to -1.8 V, and even more preferably -1.5 to -1.8 V.

[0045] It is preferable that the processing solution used is a solution containing the same alkaline cations as the electrolyte in the anodic electrolyte tank of the solid electrolyte electrolytic apparatus described above.

[0046] The cathode pretreated by the above pretreatment method is preferably stored in the solid electrolyte electrolytic apparatus until carbon dioxide electrolysis is performed using the solid electrolyte electrolytic apparatus, in order to prevent the discharge of the treatment solution. Alternatively, the cathode may be removed from the electrolytic apparatus and immersed in the same solution as the treatment solution.

[0047] The above-mentioned "pretreatment by impregnation" and "pretreatment by voltage application" may be performed individually or in combination.

[0048] In the above pretreatment step, the treatment step of holding a solution containing one or more alkali metal ions and alkaline earth metal ions on the cathode is performed such that the total amount of one or more alkali metal ions and alkaline earth metal ions held on the cathode is 0.3 to 175 μmol / cm³. ２ It is preferable that the process be carried out in such a manner. From the viewpoint of supplying a sufficient amount of cations to exhibit the reaction-promoting effect of cations in the carbon dioxide electrolytic reduction reaction, particularly the stabilizing effect of the reaction intermediate, the lower limit of the total retention amount is 0.5 to 1.9 μmol / cm³. ２ It is more preferably any value within the range of 2 to 19 μmol / cm³. ２ It is even more preferable that the value be any number within the range of 20 to 30 μmol / cm³. ２ It is most preferable that the value be any value within the specified range. Furthermore, from the viewpoint of suppressing salt precipitation due to exceeding the saturation concentration, the upper limit of the total retention amount should be 110 μmol / cm³.２ more than 175 μmol / cm ２ any numerical value within the range below (for example, 170, 165, 160, 155, 150, 145, 140, 135, 130, 125, 120, 115, 110, etc.) is more preferred, and 94 to 109 μmol / cm ２ any numerical value within this range (for example, 105, 100, 95, etc.) is even more preferred, and 93 μmol / cm ２ is the most preferred.

[0049] The carbon dioxide electrolytic reduction method according to the present embodiment includes a step of electrolyzing carbon dioxide using a solid electrolyte-type electrolysis device comprising a cathode for a known solid electrolyte-type electrolysis device or a cathode pretreated by the aforementioned pretreatment step, an ion exchange membrane, an anode, and an anode electrolyte tank.

[0050] In the step of electrolyzing the aforementioned carbon dioxide, from the viewpoint of sufficiently lowering the ohmic resistance of the anode electrolyte and preventing excessive ion migration and salt precipitation caused by ion transport that occurs when a negative voltage is applied, the electrolyte in the anode electrolyte tank is preferably a solution containing one or more of potassium ions, cesium ions, and rubidium ions at a total concentration of 0.01 to 0.5 mol / L, more preferably a solution containing the same at 0.01 to 0.3 mol / L, and even more preferably a solution containing the same at 0.01 to 0.2 mol / L.

[0051] The present embodiment can be implemented by known techniques, except for treating the supply gas according to the present embodiment, using the cathode pretreated by the aforementioned pretreatment step, and using the aforementioned electrolyte.

[0052] [Cathode for Solid Electrolyte Type Electrolyzer] The cathode for the solid electrolyte type electrolyzer used in this embodiment preferably holds a solution containing one or more potassium ions, cesium ions, and rubidium ions in a total concentration of 0.01 to 5 mol / L. From the viewpoint of supplying a sufficient amount of cations to exhibit the reaction-promoting effect of cations in the carbon dioxide electrolytic reduction reaction, particularly the stabilizing effect of the reaction intermediate, the lower limit of the total concentration is more preferably 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, or 0.09 mol / L, even more preferably 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, or 0.9 mol / L, and most preferably 1.0, 1.1, 1.2, 1.3, 1.4, or 1.5 mol / L. Furthermore, from the viewpoint of suppressing salt precipitation due to exceeding the saturation concentration, the upper limit of the total concentration is more preferably 4.9, 4.8, 4.7, 4.6, 4.5, 4.4, 4.3, 4.2, or 4.1 mol / L, even more preferably 4.0, 3.9, 3.8, 3.7, or 3.6 mol / L, and most preferably 3.5 mol / L.

[0053] The above-mentioned cathode can be manufactured, for example, by pre-treating a known cathode for a solid electrolyte electrolytic device using the above-mentioned pre-treatment step.

[0054] [Method for Manufacturing an Ion Exchange Membrane-Electrode Assembly] The method for manufacturing an ion exchange membrane-electrode assembly used in this embodiment preferably includes a step of providing a cathode that has been pre-treated by the above pre-treatment step. The ion exchange membrane may be a commercially available product such as a cation exchange membrane or an anion exchange membrane. It is preferable that the ion exchange membrane includes an anion exchange membrane.

[0055] From the viewpoint of reducing ion resistance, the average thickness of the ion exchange membrane in a dry state is preferably 60 μm or less. Furthermore, from the viewpoint of preventing pinhole formation, the average thickness in a dry state is preferably 10 μm or more. From the viewpoint of balancing ion resistance and pinhole prevention, the average thickness of the ion exchange membrane in a dry state is more preferably 15 to 50 μm, and even more preferably 20 to 40 μm.

[0056] When an anion exchange membrane is used as the ion exchange membrane, the base point density is 0.5 to 5.0 mmol / cm³ in a dry state. ３ Preferably, the concentration is 2.5 mmol / cm ３ Above, 4.5 mmol / cm ３ It is more preferable to be less than 2.9 mmol / cm³. ３ Above, 4.5 mmol / cm ３ It is even more preferable that it be less than [amount].

[0057] Examples of cation exchange membranes that can be used include strongly acidic cation exchange membranes such as Nafion 117, Nafion 115, Nafion 212, and Nafion 350 (manufactured by Chemours), which are formed by introducing sulfone groups into a fluororesin matrix, and Neosepta CSE (manufactured by Astom), which are formed by introducing sulfone groups into a styrene-divinylbenzene copolymer matrix.

[0058] Examples of anion exchange membranes include those having one or more ion exchange groups selected from the group consisting of quaternary ammonium groups, primary amino groups, secondary amino groups, and tertiary amino groups. Specifically, examples include Neosepta® ASE, AHA, ACS, AFX (manufactured by Astom Co., Ltd.), and Celemion® AMVN, DSVN, AAV, ASVN, AHO (manufactured by Asahi Glass Co., Ltd.).

[0059] The ion exchange membrane-electrode assembly used in this embodiment can be manufactured according to known manufacturing methods, except for the use of the cathode according to this embodiment.

[0060] [Method for Manufacturing a Solid Electrolyte Type Electrolyte] The method for manufacturing a solid electrolyte type electrolyte used in this embodiment preferably includes a step of providing a cathode that has been pre-treated by the above pre-treatment step. Another method for manufacturing a solid electrolyte type electrolyte according to this embodiment preferably includes a step of providing an ion exchange membrane-electrode assembly according to this embodiment.

[0061] The electrolytic apparatus used in this embodiment can be manufactured according to known manufacturing methods, except for the use of the cathode or ion exchange membrane-electrode assembly according to this embodiment.

[0062] The present invention will now be described with reference to examples, but the present invention is not limited to these examples.

[0063] [Examples 1-21, Comparative Examples 1-7] [Synthesis Process of Ni Monoatom Catalyst] In a beaker, 8.0 mmol of pentaethylenehexamine and 1.3 mmol of nickel(II) chloride hexahydrate were mixed in ethanol solvent to form a nickel-pentaethylenehexamine complex. Next, 3.0 g of carbon black support (conductive support: VULCAN® XC-72 manufactured by Cabot) was mixed in, and the resulting ethanol dispersion was irradiated with ultrasound for 10 minutes. After that, the ethanol was evaporated by heating and drying the ethanol dispersion, and the resulting mixture was calcined in a calcination furnace in an inert gas at 890-900°C for 10 seconds or more. After that, the product was washed with an aqueous sulfuric acid solution, the solid was recovered using a suction filter, and the solid was dried under reduced pressure at 60°C overnight to obtain a catalyst powder on which the nickel complex was supported.

[0064] [Cathode (Reduction Electrode) Fabrication Process] The obtained catalyst powder was dispersed in ethanol, and a cation exchange resin (Nafion® dispersion solution DE521, manufactured by Chemours) was mixed with the dispersion as a binder. After mixing, sonication was performed for 10 minutes, and the mixture was exposed for 10 minutes in a desiccator under a reduced pressure of 10 kPa (absolute pressure). Using a spray coater, the mixture was loaded onto microporous carbon paper (SIGRACET® 39BB, manufactured by SGL Carbon) with a dry load of 1.5-2 mg / cm². ２ The material was coated in this manner to obtain a cathode (reduction electrode). The static contact angle of the obtained cathode was measured and found to be within the range of 140 to 150°. The static contact angle was measured using a contact angle meter with the droplet method. 2 to 4 μL of pure water was dropped onto the catalyst layer surface of the cathode, and the change in contact angle using the θ / 2 method was measured for 5 seconds, and the average value over 5 seconds was calculated. Furthermore, the measurement was repeated 5 times per sample while changing the measurement location within the catalyst layer surface, and the average value was taken as the static contact angle.

[0065] [Treatment of supply gas] <Treatment method of supply gas in Examples 3-6 and 11-17> A potassium bicarbonate aqueous solution (2 mol / L) in a container heated to 40°C is mixed with pure CO2. ２ By passing gas through, CO ２ Gas and K ＋ A mixed gas (mixed gas A) with ion-containing water vapor was used as the supply gas to the cathode in the electrolysis process described later. ２ The gas supply rate was 60 cc / min. <Method for treating the supply gas in Examples 7-8 and 18-19> Pure CO2 was added to an aqueous sodium bicarbonate solution (1 mol / L) in a container heated to 40°C. ２ By passing gas through, CO ２ Gas and Na ＋ A mixed gas (mixed gas B) containing ionized water vapor was used, and this was used as the supply gas to the cathode in the electrolysis process described later. ２ The gas supply rate was 60 cc / min. <Method for treating the supply gas in Examples 9-10 and 20-21> Pure CO2 was added to a magnesium sulfate aqueous solution (2 mol / L) in a container heated to 40°C. ２ By passing gas through, CO ２ Gas and Mg ２＋ A mixed gas (mixed gas C) with ion-containing water vapor was used as the supply gas to the cathode in the electrolysis process described later. ２ The gas supply rate was 60 cc / min.

[0066] [Cathode Pretreatment by Vacuum Impregnation] In Examples 1-2, 11-12, and the Comparative Example, the obtained cathodes were used as is without pretreatment. On the other hand, in Examples 3-6 and 13-17, the obtained cathodes were immersed in a 2 mol / L potassium bicarbonate aqueous solution. In Examples 7-8 and 18-19, the obtained cathodes were immersed in a 1 mol / L sodium bicarbonate aqueous solution. In Examples 9-10 and 20-21, the obtained cathodes were immersed in a 0.2 mol / L magnesium sulfate aqueous solution. With the electrodes immersed, they were left standing in a desiccator under a reduced pressure of 2 kPa (absolute pressure) for 30 minutes to impregnate the electrode voids of the cathode with the potassium bicarbonate aqueous solution. The pretreated cathodes remained immersed in the aqueous solution until the film-electrode assembly was manufactured.

[0067] [Manufacturing of Solid Electrolyte Type Electrolyte Device] An anion exchange membrane with a thickness of approximately 30 μm (ion exchange capacity 1.5 meq / g), the prepared cathode (reduction electrode) (Examples 1-2, 11-12, Comparative Example) or the cathode (reduction electrode) obtained by the above pretreatment by vacuum impregnation (Examples 3-6, 13-21), and an oxide electrode (commercial product) prepared by supporting iridium oxide on a titanium mesh (Taiyo Kanami Co., Ltd., aperture ratio 56%) were bonded together within 60 seconds after the above pretreatment to form a membrane-electrode assembly. The anode (oxidation electrode) was prepared using an electrolyte (KHCO3 at concentrations listed in Tables 1-2). ３ The structure was designed to be in contact with the aqueous solution tank.

[0068] [Implementation and evaluation of the electrolysis process] Using the manufactured solid electrolyte electrolytic device, CO ２ Gas and K ＋ (Na ＋ , or Mg ２＋ ) The above mixed gases A to C with ion-containing water vapor (examples), or pure CO ２ Under conditions where gas (comparative example) is supplied to the cathode and the cell is heated to 40°C, the applied potential of the cathode is set to -1.8V relative to a silver / silver chloride reference electrode (containing saturated potassium chloride solution in its internal solution), CO ２ The current density for CO production when electrolyzing to produce CO [mA / cm²] ２The CO selectivity [%] was measured. The electrolysis time in each example and comparative example is shown in Tables 1 and 2. The carbon dioxide electrolysis reduction cell had an effective area of ​​2.25 cm². ２ A serpentine channel shape was used, and the cathode gas supply rate was set to 60 mL / min. The electrolysis test was performed using an electrochemical measurement system (SI1287; Solartron Analytical), and the cathode-side generated gas during electrolysis was analyzed using a gas chromatograph (Nexis GC-2030; Shimadzu Corporation) (Figures 6-11). In addition, the presence or absence of salt deposition at the cathode at the end of electrolysis was confirmed by visual inspection or optical microscope observation. Furthermore, the fluorescence X-ray spectrum at the Kα-ray energy position (approximately 3.31 keV) of K (potassium) at the cathode at the end of electrolysis was measured using fluorescence X-ray analysis (NEX DE; Rigaku). The results are shown in Table 1 and Figures 3-5. In Figures 3-5, the region with a spectral intensity of 0.12 or higher is shown in gray, with the fluorescence X-ray spectrum of Comparative Example 2, in which salt deposition was observed, as the reference. The various indicators of electrolysis performance are explained below. The CO selectivity (Faraday efficiency of carbon monoxide) of the cathode-side generated gas is the percentage of the amount of carbon monoxide actually observed relative to the amount of carbon monoxide produced per unit time if all the current applied to the electrolytic cell were used for carbon monoxide generation. The current applied to the electrolytic cell was measured using an electrochemical measuring device, and the amount of carbon monoxide actually produced was calculated by measuring the gas discharged from the cathode-side gas discharge channel using a gas chromatograph. The current density was calculated by measuring the current applied to the electrolytic cell using an electrochemical measuring device and dividing the current value by the electrode area. The CO generation current density (CO partial current density) was calculated by multiplying the current density by the CO selectivity.

[0069] The potassium ion content of the cathode after pretreatment in Examples 1-6 and 11-17, the sodium ion content of the cathode after pretreatment in Examples 7, 8, 18, and 19, and the magnesium ion content of the cathode after pretreatment in Examples 9, 10, 20, and 21 were calculated using the following calculation method and are shown in Tables 1-2. In Tables 1-2, the content of each ion (μmol / cm³) is shown in the "Cation Content" column.２ ) was shown.

[0070] <Method for calculating potassium ion content in Examples 1-6 and 11-17 (Pretreatment: Vacuum impregnation)> The reducing electrode was impregnated with electrolyte (2.0 mol / L KHCO3 ３ The electrodes were immersed in an aqueous solution. With the electrodes immersed, they were left standing in a desiccator under a reduced pressure of 2 kPa (absolute pressure) for 30 minutes, and KHCO3 was applied to the electrode gap. ３ The electrode was impregnated with an aqueous solution. The weight of the electrode before and after impregnation was used to determine the amount of KHCO impregnated into the electrode void. ３ The weight of the aqueous solution was calculated, and the potassium ion content was determined from the electrolyte concentration and weight.

[0071] <Method for calculating sodium ion content in Examples 7, 8, 18, and 19 (Pretreatment: Vacuum impregnation)> The reducing electrode was placed in the electrolyte (1.0 mol / L NaHCO3 ３ The electrodes were immersed in an aqueous solution. With the electrodes immersed, they were left standing in a desiccator under a reduced pressure of 2 kPa (absolute pressure) for 30 minutes, and NaHCO3 was applied to the electrode gap. ３ The electrode was impregnated with an aqueous solution. The weight of the electrode before and after impregnation was used to determine the amount of NaHCO impregnated into the electrode void. ３ The weight of the aqueous solution was calculated, and the sodium ion content was determined from the electrolyte concentration and weight.

[0072] <Method for calculating magnesium ion content in Examples 9, 10, 20, and 21 (Pretreatment: Vacuum impregnation)> The reducing electrode is placed in the electrolyte (0.2 mol / L MgSO ４ The electrodes were immersed in an aqueous solution. With the electrodes immersed, they were left standing for 30 minutes in a desiccator under a reduced pressure of 2 kPa (absolute pressure), and MgSO4 was placed in the electrode gap. ４ The electrode was impregnated with an aqueous solution. The amount of MgSO impregnated into the electrode void was determined from the electrode weight before and after impregnation. ４ The weight of the aqueous solution was calculated, and the magnesium ion content was determined from the electrolyte concentration and weight.

[0073]

[0074]

[0075] As shown in Tables 1 and 2, the examples using the carbon dioxide electrolytic reduction method of this embodiment were able to suppress salt deposition at the cathode of the carbon dioxide electrolytic reduction cell. Furthermore, Examples 1 to 10, with electrolysis times of 5 to 90 hours, were able to suppress salt deposition and exhibit high electrolysis performance, allowing the carbon dioxide reduction cell to be operated for a long period of time. In addition, Examples 11 to 18, 20 and 21, with an electrolysis time of 1 hour, showed superior CO generation current density compared to Comparative Examples 5 to 7, which also had an electrolysis time of 1 hour. Example 19, with an anode electrolyte concentration of 0.01 mol / L, had inferior electrolysis performance compared to the other examples, but showed improved CO generation current density compared to Comparative Example 7, which also had an anode electrolyte concentration of 0.01 mol / L. Therefore, Examples 11 to 21 showed improved electrolysis performance in the initial stages of operation. On the other hand, in comparative examples using a method that does not include the step of supplying a predetermined gas to the cathode to electrolyze carbon dioxide, Comparative Examples 1 and 2, with an electrolysis time of 20 hours, were unable to suppress salt deposition. Furthermore, Comparative Example 3, in which the anode electrolyte concentration was 0.02 mol / L, exhibited inferior electrolysis performance compared to Example 2, in which the anode electrolyte concentration was 0.02 mol / L, and the carbon dioxide reduction cell could not be operated for an extended period. Comparative Example 4, in which the anode electrolyte concentration was 0.01 mol / L, exhibited inferior electrolysis performance compared to Examples 5, 6, and 8, in which the anode electrolyte concentration was 0.01 mol / L, and the carbon dioxide reduction cell could not be operated for an extended period.

[0076] 100: Processing device, 101: Container, 102: Stopper, 103: Inlet tube, 104: Outlet tube, 10: Processing liquid 1: Vacuum desiccator, 2: Container, 3: Cathode, 4: Processing liquid

Claims

1. A method for electrolytic reduction of carbon dioxide using a solid electrolyte electrolytic apparatus equipped with a cathode, an ion exchange membrane, an anode, and an anode electrolyte cell, comprising the step of supplying a gas containing one or more cations that are either alkali metal ions or alkaline earth metal ions to the cathode to electrolyze carbon dioxide.

2. The carbon dioxide electrolytic reduction method according to claim 1, wherein the gas is a mixed gas of the carbon dioxide and water vapor to be electrolyzed.

3. The carbon dioxide electrolytic reduction method according to claim 1, wherein the gas is water vapor.

4. The carbon dioxide electrolytic reduction method according to any one of claims 1 to 3, comprising the step of passing the gas through a processing solution containing one or more alkali metal ions and alkaline earth metal ions before supplying the gas to the cathode, thereby incorporating one or more alkali metal ions and alkaline earth metal ions into the gas.

5. The carbon dioxide electrolytic reduction method according to claim 4, wherein the treatment solution is a solution containing one or more cations, either alkali metal ions or alkaline earth metal ions, in a total concentration of 0.01 to 5 mol / L.

6. The method for electrolytic reduction of carbon dioxide according to any one of claims 1 to 5, wherein the cathode has been pretreated, the process of holding a solution containing one or more alkali metal ions and alkaline earth metal ions at the cathode.

7. The carbon dioxide electrolytic reduction method according to claim 6, wherein the processing step includes impregnating the cathode with a processing solution containing one or more cations that are alkali metal ions and alkaline earth metal ions.

8. The carbon dioxide electrolytic reduction method according to claim 7, wherein the impregnation step is carried out under a reduced pressure environment of 0 to 90 kPa.

9. The carbon dioxide electrolytic reduction method according to claim 7 or 8, wherein the processing solution is a solution containing one or more cations, either alkali metal ions or alkaline earth metal ions, in a total concentration of 0.01 to 5 mol / L.

10. The carbon dioxide electrolytic reduction method according to claim 8, wherein the impregnation step includes immersing the cathode in the processing solution for 10 to 120 minutes.

11. The carbon dioxide electrolytic reduction method according to claim 6, wherein the processing step includes applying a voltage to a solid electrolyte electrolytic apparatus equipped with a cathode, an ion exchange membrane, an anode, and an anode electrolyte tank, so that one or more alkali metal ions and alkaline earth metal ions in a processing solution containing one or more alkali metal ions and alkaline earth metal ions placed in the anode electrolyte tank permeate the ion exchange membrane, and the solution containing the permeated cations is held at the cathode.

12. The carbon dioxide electrolytic reduction method according to claim 11, wherein the processing solution is a solution containing one or more cations, either alkali metal ions or alkaline earth metal ions, at a total concentration of 0.03 to 2 mol / L.

13. The method for electrolytic reduction of carbon dioxide according to claim 11 or 12, wherein the application time of the voltage is 0.1 to 24 hours.

14. The method for electrolytic reduction of carbon dioxide according to any one of claims 11 to 13, wherein the voltage is such that the applied potential of the cathode to a silver / silver chloride reference electrode (having a saturated potassium chloride solution in its internal solution) is -1.1 to -1.8 V.

15. The processing step ensures that the total amount of one or more cations, either alkali metal ions or alkaline earth metal ions, held in the cathode is 0.3 to 170 μmol / cm³. ２ A method for electrolytic reduction of carbon dioxide according to any one of claims 6 to 14, comprising a step performed to such an extent.

16. The carbon dioxide electrolytic reduction method according to any one of claims 1 to 15, wherein in the step of electrolyzing carbon dioxide, the electrolyte in the anodic electrolyte tank is a solution containing one or more cations that are alkali metal ions and alkaline earth metal ions in a total concentration of 0.01 to 0.5 mol / L.

17. The carbon dioxide electrolytic reduction method according to any one of claims 1 to 16, wherein the alkali metal ion is one or more of lithium ions, sodium ions, potassium ions, cesium ions, and rubidium ions.

18. The carbon dioxide electrolytic reduction method according to any one of claims 1 to 17, wherein the alkaline earth metal ion is one or more of beryllium ions, magnesium ions, calcium ions, strontium ions, and barium ions.

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