Method and apparatus for producing formic acid using conductive diamond electrodes

Abstract

To provide an efficient method and apparatus for producing formic acid.SOLUTION: The provided method and apparatus for producing formic acid comprises electro-reducing carbon dioxide using an activated conductive diamond electrode.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The present disclosure relates to a method and apparatus for producing formic acid by electrolytic reduction of carbon dioxide using a conductive diamond electrode. [Background technology]

[0002] Formic acid is an important industrial raw material and has been produced by methods such as the acid decomposition of sodium formate, the oxidation of hydrocarbons, and the direct hydrolysis of methyl formate. In recent years, from the perspectives of preventing global warming, conserving petroleum resources, and realizing a sustainable society, the production of formic acid using carbon dioxide as a raw material has attracted attention.

[0003] Patent Document 1 describes an apparatus for electrolytic reduction of carbon dioxide. In the method of Patent Document 1, carbon dioxide is saturated in a highly concentrated aqueous potassium carbonate solution of about 7M, and electrolytic reduction is carried out. The products are described as acetic acid and formic acid.

[0004] Patent Document 2 describes an electrochemical reduction device using a diamond electrode. In the method of Patent Document 2, carbon dioxide is saturated in a methanol solution under high pressure and electrolytic reduction is carried out.

[0005] Patent Document 3 describes a method and an apparatus for producing formic acid from carbon dioxide with high selectivity using a diamond electrode. Patent Document 3 also discloses a method and an apparatus using potassium chloride, rubidium chloride, or cesium chloride at a relatively high current density.

[0006] Non-Patent Document 1 describes a method and an apparatus for producing formic acid from carbon dioxide with high selectivity using a diamond electrode.

[0007] There is a need for an efficient method for producing formic acid. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] JP 2011-174139 A (Patent No. 5368340 A) [Patent Document 2] JP 2014-167151 A (Patent No. 6042749 A) [Patent Document 3] Japanese Patent Application Laid-Open No. 2018-141220 [Non-patent literature]

[0009] [Non-Patent Document 1] S. Kaneko, R. Iwao, K. IIba, K. Ohta and T. Mizuno, Energy, 23 (1998) No. 12, pp. 1107-1112 Summary of the Invention [Problem to be solved by the invention]

[0010] In the method for producing formic acid by electrolytic reduction of carbon dioxide using a diamond electrode described in Patent Document 3, the potential of the working electrode must be greater than -2.2 V relative to the reference electrode to obtain a practical amount of formic acid, and improvements were needed from the perspective of power consumption. Furthermore, the formic acid obtained by electrolytic reduction of carbon dioxide using a diamond electrode described in Patent Document 3 contains halides derived from the electrolyte, such as potassium chloride. The formic acid obtained by these methods has the problem that it cannot be used in applications that require contact with a metal catalyst, such as fuel cells or hydrogen storage, due to the inactivation of the metal catalyst by the halides.

[0011] The present disclosure provides a method and apparatus for producing formic acid that at least partially solves the problems of the prior art. In certain embodiments, the present disclosure provides an apparatus and method for producing formic acid using carbon dioxide as a raw material with low power consumption. In certain embodiments, the present disclosure also provides a method for producing formic acid using a halide-free electrolyte. [Means for solving the problem]

[0012] In certain embodiments, the present disclosure provides an apparatus and method for producing formic acid with low power consumption in the electrolytic reduction of carbon dioxide by activating a diamond electrode. Also, in certain embodiments, the present disclosure provides a method for producing formic acid using a halide-free electrolyte in the electrolytic reduction of carbon dioxide by activating a diamond electrode.

[0013] That is, the present disclosure encompasses the following embodiments: [1] A method for producing formic acid by electrolytic reduction of carbon dioxide using an electrolytic cell equipped with an anode and a cathode, the cathode is a conductive diamond electrode, In an electrolyte solution containing carbon dioxide and a supporting electrolyte, a cathodic potential was applied in the range of -2.0 V to -3.0 V relative to the reference electrode, and a current of -0.5 mA / cm 2 an activation step including activating the conductive diamond electrode by electrolysis at a current density of at least 1000 kJ / cm; a formic acid production step, which includes performing electrolytic reduction using the activated conductive diamond electrode as a cathode in an electrolyte solution containing carbon dioxide and a supporting electrolyte at a cathode potential of −1.5 V to −2.2 V relative to a reference electrode; A method for producing formic acid, comprising: [2] The method for producing formic acid according to embodiment 1, wherein both the supporting electrolyte used in the activation step and the supporting electrolyte used in the formic acid production step are halide-containing supporting electrolytes. [3] The method for producing formic acid according to embodiment 2, wherein the halide-containing supporting electrolyte is potassium chloride. [4] A method for producing formic acid by electrolytic reduction of carbon dioxide using an electrolytic cell equipped with an anode and a cathode, the cathode is a conductive diamond electrode, In an electrolyte solution containing carbon dioxide and a halide-containing supporting electrolyte, a cathodic potential was applied in the range of -2.0 V to -3.0 V relative to the reference electrode, and a voltage of -0.5 mA / cm was applied. 2an activation step including activating the conductive diamond electrode by electrolysis at a current density of at least 1000 kJ / cm; a solution exchanging step of replacing the electrolyte solution containing the halide-containing supporting electrolyte with an electrolyte solution containing a halide-free supporting electrolyte; a formic acid production step, which includes electrolytic reduction using an electrolyte solution containing the halide-free supporting electrolyte and carbon dioxide dissolved therein; A method for producing formic acid, comprising: [5] The method for producing formic acid according to embodiment 4, wherein the halide-containing supporting electrolyte is potassium chloride and the halide-free supporting electrolyte is potassium sulfate. [6] The method according to any one of embodiments 1 to 5, wherein the activation step is carried out for 30 minutes or more. [7] The current density of the cathode potential applied during the activation process is -0.5 mA / cm 2 More than ~-5.0mA / cm 2 7. The method according to any one of embodiments 1 to 6, wherein: [8] A formic acid production apparatus that produces formic acid by reducing carbon dioxide at a cathode, an electrolytic cell having an anode and a cathode; the cathode is a conductive diamond electrode, The conductive diamond electrode is placed in an electrolyte solution containing carbon dioxide and a supporting electrolyte, and a cathodic potential in the range of -2.0 V to -3.0 V is applied to the reference electrode. 2 It is activated by electrolyzing carbon dioxide at a current density of The formic acid manufacturing apparatus. [9] The conductive diamond electrode is placed in an electrolyte solution containing carbon dioxide and a supporting electrolyte, and a cathodic potential in the range of -2.0 V to -3.0 V is applied to the reference electrode, and a potential of -0.5 mA / cm 2 9. The device of embodiment 8, wherein the device has been activated by electrolyzing carbon dioxide at a current density of at least 1000 kJ / s for at least 30 minutes.

[10] The conductive diamond electrode is placed in an electrolyte solution containing carbon dioxide and a supporting electrolyte, and a cathode potential in the range of -2.0 V to -3.0 V is applied to the reference electrode, and a potential of -0.5 mA / cm 2 More than ~-5.0mA / cm 2 10. The device of embodiment 8 or 9, wherein the device has been activated by electrolyzing carbon dioxide at a current density of [Effects of the Invention]

[0014] According to the present disclosure, an apparatus and method for producing formic acid with low power consumption by activating a conductive diamond electrode can be provided, and a method for producing halide-free formic acid by activating a conductive diamond electrode can be provided. [Brief explanation of the drawings]

[0015] [Figure 1] An example of an apparatus of the present disclosure is shown in FIG. [Figure 2-1] FIG. 10 is a diagram showing changes in current density and potential during the activation process. [Figure 2-2] FIG. 10 is a diagram showing changes in current density in the formic acid production step. [Figure 3] The faradaic efficiency of formic acid production in the formic acid production step is shown in FIG. [Figure 4] This figure shows the faradaic efficiency of formic acid production in the formic acid production process for an activated electrode (b) and an unactivated electrode (a) after solution exchange. The electrolyte solution after solution exchange was 0.25 M K2SO4. [Figure 5] The graph shows the change in current density when formic acid is generated by constant-potential electrolysis using conductive diamond electrodes with (bottom) and without (top) activation and applying a cathode potential of -1.8 V. [Figure 6] The graph shows the change in current density when formic acid is generated by constant-potential electrolysis using conductive diamond electrodes with (bottom) and without (top) activation and applying a cathode potential of -1.9 V. [Figure 7]The graph shows the change in current density when formic acid is generated by constant-potential electrolysis using conductive diamond electrodes with (bottom) and without (top) activation and applying a cathode potential of -2.2 V. DETAILED DESCRIPTION OF THE INVENTION

[0016] Formic Acid Manufacturing Equipment In one embodiment, the present disclosure provides an apparatus for producing formic acid by reducing carbon dioxide at a cathode to produce formic acid. The apparatus for producing formic acid of this embodiment has an electrolytic cell equipped with an anode and a cathode, and the cathode is a conductive diamond electrode. In a specific embodiment, the conductive diamond electrode is an activated conductive diamond electrode. In one embodiment, the activated conductive diamond electrode is in an electrolyte solution in which carbon dioxide and a supporting electrolyte are dissolved, and a cathode potential is applied in the range of -2.0 V to -3.0 V, for example, in the range of -2.2 V to -3.0 V, for example, in the range of -2.2 V to -2.8 V, relative to a reference electrode, and a current of -0.5 mA / cm is applied. 2 or more, e.g., -1.0 mA / cm 2 Carbon dioxide is activated by electrolysis at the above current density.

[0017] A conductive diamond electrode is defined as an electrode formed by imparting electrical conductivity to thin-film or bulk diamond. There are no particular limitations on the method for imparting electrical conductivity to diamond, but diamond can be doped with trace amounts of impurities. Examples of impurities include boron (B), sulfur (S), nitrogen (N), oxygen (O), and silicon (Si). For example, thin-film or bulk diamond can be obtained by vapor-phase synthesis. To dope boron, diborane, trimethoxyborane, or boron oxide can be added to a source gas containing a carbon source. Sulfur oxide or hydrogen sulfide can be added to dope sulfur, oxygen or carbon dioxide can be added to dope oxygen, ammonia or nitrogen can be added to dope nitrogen, and silane can be added to dope silicon. Boron-doped conductive diamond electrodes are particularly advantageous because they have a wide potential window and a small background current compared to other electrode materials. Hereinafter, conductive diamond electrodes may be referred to simply as diamond electrodes, and boron-doped diamond electrodes may be referred to as boron-doped diamond electrodes or BDD electrodes.

[0018] In some embodiments, conductive diamond electrode can be a thin film that is formed on substrate.When conductive diamond is formed on substrate, the substrate often becomes hot, so the substrate can be made of semiconductors such as silicon or high-melting-point metals such as tungsten, niobium, molybdenum, titanium.The method of forming the thin film of conductive diamond electrode on substrate can be known method, for example, can be made of chemical vapor deposition (CVD) method, as described in the following examples.

[0019] In some embodiments, the anode may be a metal electrode such as platinum or nickel, a common carbon electrode such as glassy carbon or graphite, or a conductive diamond electrode. In some embodiments, the electrolytic cell may take various forms, such as a batch cell type or a flow cell type. For example, in some embodiments, the electrolytic cell may be a flow cell type electrolytic cell that can continuously produce formic acid.

[0020] In the present disclosure, the configuration of the electrolytic cell is not particularly limited and may have various configurations. In some embodiments, the electrolytic cell may have a reference electrode, which allows for accurate control of the potential. Examples of the reference electrode include known electrodes such as a standard hydrogen electrode, a silver-silver chloride electrode, and a saturated calomel electrode. Although the electrode potential varies depending on the type of reference electrode used in the electrolytic cell, in this specification, unless otherwise specified, the electrode potential refers to the potential of the cathode relative to a silver-silver chloride electrode (vs. Ag / AgCl).

[0021] In some embodiments, the electrolytic cell may be a two-compartment electrolytic cell having a diaphragm. By separating the electrolytic cell into an anode chamber and a cathode chamber with a diaphragm, independent electrochemical reactions can be carried out in the anode chamber and the cathode chamber, making it easier to control the reactions. The diaphragm may be, but is not limited to, an ion exchange membrane such as a cation exchange membrane, an anion exchange membrane, or a bipolar membrane. In certain embodiments, the diaphragm may be a cation exchange membrane.

[0022] In one embodiment, the formic acid production apparatus can use an electrolyte solution in which carbon dioxide, which is a raw material for formic acid, and a supporting electrolyte are dissolved. The electrolyte solution can be an aqueous solution, which is easily available and has excellent conductivity. Carbon dioxide can be dissolved, for example, by bubbling carbon dioxide gas into the electrolyte solution. In a specific embodiment, before bubbling carbon dioxide gas, the electrolyte solution can be bubbled with an inert gas such as nitrogen or argon to remove dissolved gases, such as dissolved oxygen, contained in the electrolyte solution.

[0023] The supporting electrolyte for the activation step includes, but is not limited to, an electrolyte containing a halide, such as potassium chloride, rubidium chloride, cesium chloride, etc. By using an electrolyte containing a halide, electrons are actively transferred from the conductive diamond electrode to carbon dioxide, resulting in a high activation effect.

[0024] In one embodiment, the conductive diamond electrode of the present disclosure is in an electrolyte solution containing dissolved carbon dioxide and a supporting electrolyte, and is subjected to a cathodic potential in the range of −2.0 V to −3.0 V, for example, in the range of −2.2 V to −3.0 V, for example, in the range of −2.2 V to −2.8 V, relative to a reference electrode, and exhibits a resistance of −0.5 mA / cm 2 or more, e.g., -1.0 mA / cm 2 The conductive diamond electrode may be activated by electrolysis at a current density of 1000 kJ / cm or more. Activation involves electrolyzing carbon dioxide at a relatively high potential in an electrolyte solution containing carbon dioxide, i.e., by actively inducing electron transfer from the diamond electrode (cathode) to carbon dioxide dissolved in the electrolyte solution, thereby activating the electrode itself. The present inventors, while conducting various attempts at electrolytic reduction of carbon dioxide using a conductive diamond electrode, unexpectedly discovered that electrolysis proceeds more actively when an activated electrode is used, compared to when electrolysis is performed using an unactivated electrode. Without being bound by theory, the present inventors believe that one of the factors that activates the conductive diamond electrode is that a bond is formed between the carbon dioxide carbon atom and the carbon atom present on the surface of the conductive diamond electrode, generating a carboxyl group derived from carbon dioxide on the surface of the conductive diamond electrode, thereby improving the affinity between the electrode surface and carbon dioxide molecules. However, the present disclosure is not limited to such a mechanism.

[0025] The present inventors have attempted to identify activated conductive diamond electrodes based on the types of adsorbed molecules and functional groups on their surfaces. The inventors' current attempts are described below. It is impossible to simply identify the types of adsorbed molecules and functional groups near the surface, within a few micrometers of the surface, given that the structure and associated characteristics vary from one conductive diamond electrode to another. Furthermore, it is impossible or nearly impossible to identify the structure or characteristics of activated conductive diamond electrodes through measurement-based analysis using analytical techniques available at the time of filing this application. While scanning electron microscopes (SEMs) and X-ray photoelectron spectroscopy (XPSs) are used to measure the state of existence of materials in detail, these measurement devices require the measurement object to be observed in an ultra-high vacuum chamber. In such an environment, adsorbed molecules on the surface are desorbed, preventing the conductive diamond electrode from maintaining its activated state. Furthermore, measurement devices that do not require vacuum conditions, such as Fourier transform infrared spectroscopy (FT-IR) and Raman spectroscopy, lack the spatial resolution required to identify the surface structure, making it impossible to identify the surface structure of activated conductive diamond electrodes.

[0026] In an embodiment, the potential applied to the cathode when activating the diamond electrode is in the range of −2.0V to −3.0V, for example, in the range of −2.1V to −3.0V, for example, in the range of −2.2V to −3.0V, for example, in the range of −2.0V to −2.9V, for example, in the range of −2.1V to −2.9V, for example, in the range of −2.2V to −2.9V, for example, in the range of −2.0V to −2.8V, for example, in the range of −2.1V to −2.8V, For example, it may be in the range of -2.2V to -2.8V, for example, -2.0V to -2.7V, for example, -2.1V to -2.7V, for example, -2.2V to -2.7V, for example, -2.0V to -2.6V, for example, -2.1V to -2.6V, for example, -2.2V to -2.6V, for example, -2.0V to -2.5V, for example, -2.1V to -2.5V, for example, -2.2V to -2.5V. In certain embodiments, the cathode potential for activating the conductive diamond electrode is greater than -2.0V and less than -2.5V. If the cathode potential is too small, electron transfer from the diamond electrode to carbon dioxide may not occur, resulting in insufficient activation. Furthermore, if the cathode potential is too large, electron transfer from the diamond electrode to water may be prioritized, resulting in hydrogen generation. In this case, electron transfer from the electrode to carbon dioxide may not occur, resulting in insufficient activation.

[0027] In one embodiment, the current density for activating the diamond electrode is −0.5 mA / cm 2 More than -0.6 mA / cm 2 More than -0.7 mA / cm 2 More than -0.8 mA / cm 2 or more, -0.9 mA / cm 2 or more, e.g., -1.0 mA / cm 2It can be more than that. If the current density is too small, electron transfer from the diamond electrode to carbon dioxide does not occur, and sufficient activation effect may not be obtained. In addition, although there is no upper limit for the current density, as the current density increases, the cathode potential increases. Therefore, it is preferable to appropriately adjust the current density within the range not exceeding the above-mentioned cathode potential range. In a specific embodiment, the current density when activating the diamond electrode is -5.0 mA / cm 2 Below -4.5 mA / cm 2 Less than -4.0 mA / cm 2 Below, -3.5 mA / cm 2 Less than -3.0 mA / cm 2 Below -2.5 mA / cm 2 Less than -2.0 mA / cm 2 Below -1.5 mA / cm 2 Below, for example, -0.5 mA / cm 2 ~-5.0 mA / cm 2 , -0.6 mA / cm 2 ~-4.0 mA / cm 2 , -0.7 mA / cm 2 ~-3.0 mA / cm 2 , -0.8 mA / cm 2 ~-2.0 mA / cm 2 , e.g., -0.9 mA / cm 2 ~-1.5 mA / cm 2 It could be.

[0028] In one embodiment, during the diamond electrode activation process, a voltage can be applied between the cathode and anode using an external power supply. The external power supply can be, but is not limited to, a potentio-galvanostat. The potentio-galvanostat controls the electrochemical reaction of the working electrode in the electrolyte and measures the resulting potential and current. To achieve the cathode potential and current density relative to the reference electrode within the aforementioned ranges using a potentio-galvanostat, the potential can be controlled to obtain the desired current density using constant-potential electrolysis, or the current can be controlled to obtain the desired current value using constant-current electrolysis. A potentiostat or galvanostat can be used as the external power supply.

[0029] In some embodiments, the time required for the activation process of diamond electrode can be appropriately set.In some embodiments, the activation process can be 30 seconds or more, 1 minute or more, 5 minutes or more, 10 minutes or more, 15 minutes or more, for example, 30 minutes or more, 90 minutes or less, for example, 60 minutes or less, for example, 1 to 90 minutes, 10 to 60 minutes, for example, but not limited thereto.If the activation time is too short, it may not be possible to obtain sufficient activation effect, and if it is too long, it may be disadvantageous in terms of productivity and power consumption of the formic acid production device.

[0030] In addition, with regard to the potential and current density that are applied to the cathode in the process of activating the diamond electrode of the present disclosure, the formic acid manufacturing method of the prior art may teach the applied potential and current density that are greater than the applied potential and current density.In such a case, the formic acid manufacturing method of the prior art may momentarily cross the cathode applied potential and current density in the process of activating the diamond electrode of the present disclosure.However, it is considered that the cathode applied potential and current density that are momentarily crossed in the process of activating the diamond electrode of the present disclosure alone cannot obtain sufficient activation effect.Therefore, such momentary cathode applied potential and current density in the conventional formic acid manufacturing method are not considered to be the potential and current density that are applied to the cathode in the process of activating the diamond electrode of the present disclosure.

[0031] Method for producing formic acid In one embodiment, the present disclosure provides a method for producing formic acid by electrolytically reducing carbon dioxide using an electrolytic cell equipped with an anode and a cathode. The method for producing formic acid of this embodiment includes an activation step and a formic acid production step. In a specific embodiment, the activation step involves applying a cathode potential to the conductive diamond electrode, which is the cathode, in the range of -2.0 V to -3.0 V, for example, in the range of -2.2 V to -3.0 V, for example, in the range of -2.2 V to -2.8 V, relative to a reference electrode, and activating the conductive diamond electrode at a current of -0.5 mA / cm. 2 or more, e.g., -1.0 mA / cm 2In a specific embodiment, the formic acid production step is a step of performing electrolytic reduction in an electrolyte solution containing carbon dioxide and a supporting electrolyte at a cathode potential relative to a reference electrode of −1.5 V to −2.2 V, −1.5 V to −2.0 V, for example, −1.8 V to −2.0 V. The activation step is as described above.

[0032] In one embodiment, the present disclosure provides a method for activating a conductive diamond electrode for producing formic acid by electrolytic reduction of carbon dioxide, the method comprising applying a cathodic potential to the conductive diamond electrode in a range of −2.0 V to −3.0 V relative to a reference electrode in an electrolyte solution containing dissolved carbon dioxide and a supporting electrolyte, and applying a cathodic potential of −0.5 mA / cm 2 and activating the conductive diamond electrode by electrolyzing carbon dioxide at a current density of at least 1000 kJ / cm2 or more. The conductive diamond electrode activated by this method can be used for producing formic acid.

[0033] In the formic acid generation process of this embodiment, the conductive diamond electrode activated by the activation process is used.When the electrolytic reduction of carbon dioxide is carried out using an electrolytic cell equipped with an activated conductive diamond electrode as a cathode, electron transfer to carbon dioxide occurs at a lower cathode potential than when using an unactivated conductive diamond electrode.Therefore, formic acid can be produced at a low cathode potential, for example, in the range of -1.5V to -2.0V with respect to the reference electrode, and the power consumption required for electrolytic reduction can be reduced.

[0034] In one embodiment, the cathode potential in the formic acid production step can be, for example, -1.5 V or more, -1.6 V or more, -1.7 V or more, or -1.8 V or more, for example, -2.2 V or less, -2.1 V or less, or -2.0 V or less, for example, -1.5 V to -2.2 V, -1.6 V to -2.1 V, -1.7 V to -2.0 V, or -1.8 V to -2.0 V. If the electrode potential is too low, the amount of formic acid produced by the electrolytic reduction of carbon dioxide may decrease. If the electrode potential is too high, the amount of formic acid produced may increase, but the power consumption required for electrolysis may increase.

[0035] The current density in the formic acid production step is not particularly limited, but if it is too small or too large, it may fall outside the above-mentioned range of the cathode potential, which is undesirable. From the viewpoint of the amount of formic acid produced, it is preferable to adjust the current density so as to maximize the above-mentioned range of the cathode potential.

[0036] In addition, in certain embodiments, the formic acid production method using the activated diamond electrode of the present disclosure can reduce the cathode potential in the formic acid production process compared with the formic acid production method of the prior art.That is, with regard to the potential applied to the cathode in the formic acid production process, the formic acid production method of the prior art may teach a higher applied potential than the applied potential of the present disclosure.In such cases, in the formic acid production method of the prior art, when the potential is applied, at the start or end, especially at the end of the operation of the device, the cathode applied potential in the formic acid production process of the present disclosure may be instantaneously crossed.However, simply crossing the cathode applied potential in the formic acid production process of the present disclosure in the present disclosure does not reduce the power consumption required for formic acid production, and remains the same as conventional power consumption.Therefore, such instantaneous crossing of the cathode applied potential in the formic acid production process of the prior art formic acid production does not correspond to the potential applied to the cathode in the formic acid production process of the present disclosure.

[0037] The supporting electrolyte used in the formic acid generation process of this embodiment includes, but is not limited to, electrolytes containing halides such as potassium chloride, rubidium chloride, and cesium chloride. Electrolytes containing halides can actively transfer electrons from the conductive diamond electrode to carbon dioxide, so they can also be used in the activation process of the diamond electrode. If an electrolyte containing a halide is used in the formic acid generation process after the activation process, formic acid can be produced at a low cathode potential. In certain embodiments, the process can be simplified by continuing to use the same electrolyte solution used in the activation process in the subsequent formic acid generation process.

[0038] In the formic acid generation process of this embodiment, as in the diamond electrode activation process, a voltage can be applied between the cathode and anode using an external power supply mechanism. The external power supply mechanism is not particularly limited, but a potentio-galvanostat can be used. To obtain the desired potential using a potentio-galvanostat, the potential can be directly controlled using a constant potential method, or the current value can be controlled using a constant current method to provide the desired potential. A potentiostat or a galvanostat can be used as the external power supply mechanism.

[0039] In this embodiment, the time for carrying out the formic acid production step can be set appropriately. In certain embodiments, the time for carrying out the formic acid production step can be, but is not limited to, 10 minutes or more, 30 minutes or more, 60 minutes or more, 2 hours or more, 3 hours or more, 6 hours or more, 12 hours or more, 24 hours or more, 48 hours or more, or, for example, 72 hours or more. The amount of formic acid produced can be increased by extending the time for carrying out the formic acid production step as long as possible without reducing the supply of carbon dioxide as a raw material or the production rate of formic acid produced. In certain embodiments, the time for carrying out the formic acid production step can be, but is not limited to, 144 hours or less, 72 hours or less, 48 ​​hours or less, or 24 hours or less.

[0040] In one embodiment, the present disclosure provides a method for producing formic acid by electrolytic reduction of carbon dioxide using an electrolytic cell equipped with an anode and a cathode. The method for producing formic acid in this embodiment includes an activation step, a liquid exchange step, and a formic acid production step. In the activation step, a cathode potential in the range of −2.0 V to −3.0 V, for example, in the range of −2.2 V to −3.0 V, for example, in the range of −2.2 V to −2.8 V, is applied relative to a reference electrode in an electrolyte solution in which a supporting electrolyte containing carbon dioxide and a halide is dissolved, and a current of −0.5 mA / cm is applied. 2 or more, e.g., -1.0 mA / cm 2 By electrolysis at the above current density, the conductive diamond electrode can be activated. In the liquid exchange process, the electrolyte solution in which the supporting electrolyte containing halide ions is dissolved can be exchanged for the electrolyte solution in which the supporting electrolyte not containing halide ions is dissolved. In the formic acid generation process, electrolytic reduction can be carried out with the electrolyte solution in which the supporting electrolyte not containing halide ions and carbon dioxide are dissolved. The activation process is as described above.

[0041] The liquid exchange process of this embodiment is a process of exchanging the electrolyte solution that contains the halide-containing supporting electrolyte used in the activation process with the electrolyte solution that does not contain the halide-containing supporting electrolyte used in the formic acid generation process.For the conductive diamond electrode that has been activated by the activation process, in order to exchange the electrolyte solution while maintaining the activated state, it is preferable to exchange the electrolyte solution without drying the surface of the activated conductive diamond electrode.This is because if the electrode surface is dried, the activation effect of the surface that has been generated by the activation process may be lost.

[0042] Any method can be selected for replacing the electrolyte solution without drying the electrode surface. In certain embodiments, for example, to prevent the residual electrolyte solution containing a halide-containing supporting electrolyte used in the activation step from remaining, the halide-containing supporting electrolyte can be replaced with pure water, and then the pure water can be replaced with an electrolyte solution containing a halide-free supporting electrolyte used in the formic acid production step. For example, when using a flow-cell electrolytic cell, the electrolyte solution used in the activation step can be removed from the electrolyte solution tank using a liquid transfer pump, followed by filling the electrolyte solution tank with pure water. The pure water can then be circulated through the electrolytic cell to clean the electrolytic cell and the electrolyte solution tank. This procedure allows the electrolyte solution containing a halide-containing supporting electrolyte used in the activation step to be replaced with pure water without drying the electrode surface. Furthermore, using a similar procedure, the pure water in the electrolyte solution tank can be replaced with an electrolyte solution containing a halide-free supporting electrolyte used in the subsequent formic acid production step. In certain embodiments, the liquid transfer pump can be a pump that performs intermittent liquid transfer, such as a peristaltic pump. This can also be used in the formic acid production step. In some embodiments, the electrolyte solution may be exchanged without drying out the electrode surface, while in other embodiments, all or part of the electrode surface may be temporarily dried out when the electrolyte solution is exchanged.

[0043] In the formic acid production step after the solution exchange, electrolytic reduction can be performed using an electrolyte solution containing a halide-free supporting electrolyte and carbon dioxide. The halide-free supporting electrolyte is not particularly limited, and examples thereof include, but are not limited to, potassium sulfate, potassium carbonate, potassium hydroxide, potassium bicarbonate, tetrabutylammonium tetrafluoroborate (TBABF4), and potassium perchlorate.

[0044] In the formic acid generation process of this embodiment, as in the diamond electrode activation process, a voltage can be applied between the cathode and anode using an external power supply mechanism. The external power supply mechanism is not particularly limited, but a potentio-galvanostat can be used. To obtain the desired potential using a potentio-galvanostat, the potential can be directly controlled using a constant potential method, or the current value can be controlled using a constant current method to provide the desired potential. A potentiostat or galvanostat can be used as the external power supply mechanism.

[0045] In this embodiment, the time for carrying out the formic acid production step can be set appropriately. In certain embodiments, the time for carrying out the formic acid production step can be, but is not limited to, 10 minutes or more, 30 minutes or more, 60 minutes or more, 2 hours or more, 3 hours or more, 6 hours or more, 12 hours or more, 24 hours or more, 48 hours or more, or, for example, 72 hours or more. The amount of formic acid produced can be increased by extending the time for carrying out the formic acid production step as long as possible without reducing the supply of carbon dioxide as a raw material or the production rate of formic acid produced. In certain embodiments, the time for carrying out the formic acid production step can be, but is not limited to, 144 hours or less, 72 hours or less, 48 ​​hours or less, or 24 hours or less.

[0046] In the present disclosure, the electrolytic reduction may be performed in a potentiostatic manner, in which the cathode potential relative to the reference electrode is kept constant, a constant voltage manner, in which the voltage between the cathode and the anode is kept constant, or a constant current manner, in which the cathode current density is kept constant. In some embodiments, the current may be direct current.

[0047] When the constant potential system is adopted in the present disclosure, the formic acid production apparatus can have a three-electrode configuration having a cathode, an anode, and a reference electrode. Furthermore, when the constant voltage system or constant current system is adopted, the formic acid production apparatus can have a three-electrode configuration having a cathode, an anode, and a reference electrode, or a two-electrode configuration having a cathode and an anode but no reference electrode. When the three-electrode configuration is adopted, the present disclosure also includes the case where the potential of the reference electrode is within the range described herein. Furthermore, when the two-electrode configuration is adopted, the cathode potential relative to the reference electrode is not measured, but if measured, the potential of the reference electrode is within the range described herein. In other words, with regard to the method for producing formic acid of the present disclosure, for example, applying a cathode potential in the range of −2.0 V to −3.0 V relative to a reference electrode does not necessarily stipulate that the cathode potential must be measured relative to the reference electrode in a three-electrode configuration. Rather, when a voltage is applied between the cathode and anode in a two-electrode configuration, if the cathode potential is measured relative to the reference electrode and the potential is ultimately within the range described in the present disclosure, the applied voltage will essentially correspond to applying a cathode potential in the range of −2.0 V to −3.0 V relative to the reference electrode in the present disclosure.

[0048] In one embodiment, the current of the conductive diamond electrode in the formic acid production reaction can be −0.1 mA to −50 mA, −0.5 mA to −40 mA, −1 mA to −30 mA, or −2 mA to −20 mA, for example, −15 mA, −10 mA, −5 mA, or −2 mA.

[0049] In one embodiment, the current density of the conductive diamond electrode in the formic acid production reaction is −0.5 mA / cm 2 ~-5mA / cm 2 , -0.6mA / cm 2 ~-4.5mA / cm 2 , -0.7mA / cm 2 ~-4.0mA / cm 2 , -0.8mA / cm 2 ~-3.5mA / cm 2 , -0.9mA / cm 2 ~-3.0mA / cm2 , -1.0mA / cm 2 ~-2.5mA / cm 2 , -1.5mA / cm 2 ~-2.0mA / cm 2 , e.g., -0.5mA / cm 2 , -1.0mA / cm 2 , -2.0mA / cm 2 , -3mA / cm 2 , -4mA / cm 2 , or -5mA / cm 2 It can be said that:

[0050] In one embodiment, the BDD electrode has a diamond layer formed by vapor-depositing diamond containing 0.01 to 8% w / w boron raw material on the substrate surface. The substrate can be a Si substrate, a glass substrate such as SiO2, a quartz substrate, a ceramic substrate such as Al2O3, silicon carbide, or silicon nitride, or a metal such as tungsten, molybdenum, niobium, or titanium. All or part of the substrate surface can be made into a diamond layer. In another embodiment, the electrode portion of the BDD electrode can have bulk diamond.

[0051] The size of the electrode part of the conductive diamond electrode is not particularly limited, but is preferably 1 cm 2 More than 5cm 2 More than 10cm 2 More than 50cm 2 The area can be set to the above. All or part of the diamond layer can be brought into contact with a solution containing carbon dioxide and used in the formic acid production reaction. The area and shape of the electrode part can be appropriately determined depending on the configuration of the device.

[0052] In one embodiment, the BDD electrode has a diamond layer formed by depositing diamond with a high boron content (0.01 to 8% w / w boron content) on the surface of a Si substrate. The boron content is, for example, 0.01 to 5% w / w, 0.02 to 4% w / w, 0.03 to 3% w / w, 0.04 to 2% w / w, or 0.05 to 1% w / w, e.g., about 0.1 to 1.0% w / w.

[0053] The deposition process of diamond mixed with boron raw material onto a substrate can be carried out, for example, at 700 to 900°C for 2 to 12 hours. Conductive diamond thin films can be produced by chemical vapor deposition (CVD), for example, microwave plasma chemical vapor deposition (MPCVD). For example, a substrate such as a silicon single crystal (100) is placed in a film-forming apparatus, and a film-forming gas containing high-purity hydrogen gas as a carrier gas is flowed. The film-forming gas contains gaseous components including carbon and boron. When a microwave is applied to the film-forming apparatus through which the film-forming gas is flowing to cause a plasma discharge, carbon radicals are generated from the carbon source in the film-forming gas, and sp 3 The diamond thin film is formed by deposition while maintaining the structure and incorporating boron. Unless otherwise specified, the conductive diamond of the present disclosure, particularly the boron-doped conductive diamond, is sp 3 In certain embodiments, the conductive diamond of the present disclosure, particularly the boron-doped conductive diamond, has an sp 2 It has no structure.

[0054] The thickness of the diamond thin film can be controlled by adjusting the film formation time, and can be, for example, 100 nm to 1 mm, 1 μm to 0.1 mm, 1 μm to 10 μm, or 2 μm to 20 μm.

[0055] The conditions for the deposition process of boron-doped diamond on the substrate surface can be determined according to the substrate material.For example, the plasma power can be 500W-7000W, for example, 3kW-5kW, and preferably 5kW.If the plasma power is within this range, synthesis can proceed efficiently, and a high-quality conductive diamond thin film can be formed with few by-products.

[0056] Any known method can be used to manufacture a BDD electrode, and in addition to CVD methods (including those using the hot filament method), methods such as vacuum deposition, ion plating, and ion implantation can also be used.

[0057] In some embodiments, the BDD electrode may be hydrogen-terminated or cathodically reduced. In some embodiments, the BDD electrode may be oxygen-terminated or anodized. Specific methods for hydrogen termination include annealing (heating) or hydrogen plasma treatment of the conductive diamond electrode in a hydrogen atmosphere. Specific methods for cathodic reduction include, for example, applying a potential of -3 V in a 0.1 M sodium perchlorate solution for 5 to 10 minutes to continuously generate hydrogen. Specific methods for oxygen termination include annealing (heating) or oxygen plasma treatment of the conductive diamond electrode in an oxygen atmosphere (in air). Specific methods for anodic oxidation include, for example, applying a potential of +3 V in a 0.1 M sodium perchlorate solution for 5 to 10 minutes to continuously generate oxygen.

[0058] The above electrodes are disclosed in JP 2006-98281 A, JP 2007-139725 A, JP 2011-152324 A, JP 2015-172401 A, JP 2018-141220 A, etc., and can be prepared according to the descriptions in these publications.

[0059] The conductive diamond electrode of the present disclosure has high thermal conductivity, high hardness, chemical inertness, a wide potential window, low background current, and excellent electrochemical stability.

[0060] An example of the device of the present disclosure is shown in FIG. 1. The device includes a cathode (cathode electrode), an anode (anode electrode), a cathode tank (cathode tank), an anode tank (anode tank), a solid electrolyte membrane, an external power supply, a carbon dioxide supply unit, a liquid pump, a first reservoir, and a second reservoir. In FIG. 1, the cathode is a BDD electrode. The anode is a metal electrode, and may be made of silver, gold, platinum, carbon, stainless steel, iridium, palladium, osmium, rhodium, ruthenium, or the like. In FIG. 1, the anode is a platinum electrode. Carbon dioxide is supplied to the first reservoir from the carbon dioxide supply unit. Optionally, a sampling bag for collecting gas components may be connected to the first reservoir. The sampling bag may be made of, but is not limited to, aluminum, fluororesin, polyvinyl fluoride, polyester, or the like. The solid electrolyte membrane is sandwiched between the cathode and anode tanks. The solid electrolyte membrane may be, but is not limited to, a fluoropolymer membrane with sulfonic acid groups, such as a Nafion® membrane (THE CHEMOURS COMPANY FC LLC), a sulfonic acid-containing ion-exchange resin membrane, a Flemion™ ion-exchange membrane, or an Aciplex™ ion-exchange membrane. Because the solid electrolyte membrane separates the cathode and anode chambers, formic acid generated at the cathode is not oxidized at the anode. The cathode chamber contains a first electrolyte solution, which may contain carbon dioxide. The first electrolyte solution is supplied from a first reservoir by a liquid supply pump. In FIG. 1, a reference electrode is disposed in the cathode chamber. The anode chamber contains a second electrolyte solution, which is supplied from a second reservoir by a liquid supply pump. The BDD electrode is disposed in the cathode chamber so as to contact the first electrolyte solution. The anode is disposed in the anode chamber so as to contact the second electrolyte solution. The first and second electrolyte solutions may be the same or different.

[0061] In certain embodiments, the device of the present disclosure may further include a means for controlling the current at a constant level (also referred to as a galvanostat or amperostat). A galvanostat or the like can be used to control the current at a constant level during the formic acid production reaction. In certain embodiments, the device of the present invention may further include a reference electrode. An example of a reference electrode is a silver-silver chloride electrode. In this case, the device of the present invention may further include a potentiostat.

[0062] In one embodiment, the electrolytic reduction of carbon dioxide can be carried out by the following procedure: (1) The working electrode is a BDD electrode, and the BDD electrode and a counter electrode are installed in the electrolysis cell. Also, a reference electrode is installed if necessary. (2) Inject an electrolyte solution into the reactor. The electrolyte solution for the anode and the electrolyte solution for the cathode may be the same or different. (3) Optionally, nitrogen is bubbled through the aqueous solution to remove oxygen. (4) Bubbling carbon dioxide into the aqueous solution. (5) Optionally, adjust the pH of the electrolyte solution to a pH suitable for the formic acid production reaction. (6) Activating the BDD electrode (activation step). (7) Carbon dioxide is electrolytically reduced (formic acid production process).

[0063] In certain embodiments, a liquid exchange step can be performed between steps (6) and (7). The electrolytic reduction can be performed at room temperature or at a low temperature. The electrolytic reduction can be performed at atmospheric pressure or at a high pressure.

[0064] In some embodiments, the apparatus of the present disclosure may include instructions for use. The instructions may include instructions specifying that the activation conditions (e.g., applied potential, current density, etc.) for activating the conductive diamond electrode are the conditions described herein. The apparatus of the present disclosure may include a program or software implementing the program that controls the activation of the conductive diamond electrode and the formic acid production reaction under such conditions. That is, in some embodiments, the present disclosure provides a control program for activating the conductive diamond electrode and the formic acid production reaction, or software implementing the program. In some embodiments, the present disclosure also provides a formic acid production apparatus equipped with a control program for activating the conductive diamond electrode and the formic acid production reaction, or software implementing the program. In some embodiments, the program or software may be stored on a recording medium. In some embodiments, the program or software may be a cloud application. In other embodiments, the program or software may be stored on the cloud and controlled by an appropriate implementation means. In these embodiments, the program or software may be temporarily stored in the hardware, for example, memory, of the formic acid production apparatus of the present disclosure, and control the formic acid production apparatus of the present disclosure. Therefore, these embodiments also apply to the case where the formic acid producing apparatus of the present disclosure is equipped with the program or software.

[0065] The method or apparatus of the present disclosure can produce formic acid with a high faradaic efficiency. The faradaic efficiency is the ratio (percentage) of the charge used to produce the reaction product to the total charge of the reaction: Faraday efficiency (%) = 100 × (amount of charge used to generate reaction product) / (total reaction charge) In this specification, when the production efficiency of formic acid is mentioned, it refers to the faradaic efficiency of the formic acid produced, unless otherwise specified.

[0066] The surface of a conductive diamond electrode can be activated by electrolytic reduction in an electrolyte solution containing carbon dioxide and potassium chloride. This activation process enables sufficient production of formic acid even at low electrode potentials of -2.2 V or less. This activation process also significantly improves the faradaic efficiency of formic acid production in electrolytic reduction in an electrolyte solution containing carbon dioxide and potassium sulfate but not potassium chloride. [Example]

[0067] The present invention will be described in more detail below using examples, but the technical scope of the present invention is not limited to these examples in any way.

[0068] [Example 1] Boron-doped diamond electrodes were fabricated using a microwave plasma CVD apparatus (Model AX5400, manufactured by Cornes Technology). Specifically, the Si(100) surface of a silicon substrate was pretreated with diamond powder, and then a film was deposited on the substrate using 50 ml of acetone and 0.4 ml of trimethoxyborate (0.1% boron concentration) as carbon sources at a plasma power of 5000 W for 6 hours at a pressure of 115 Torr. The electrolytic cell used was a two-chamber flow cell electrolytic cell as shown in Figure 1.

[0069] Activation process For the activation process, a conductive diamond electrode (electrode area 9.62 cm) was used as the cathode. 2 ), and a platinum electrode (electrode area 9.62 cm) as the anode. 2An electrolytic cell was used, equipped with a silver-silver chloride electrode as the reference electrode, and a Nafion membrane, a cation exchange membrane, as the diaphragm. The electrolytic reduction of carbon dioxide was carried out using 50 ml of a 0.5 M potassium chloride aqueous solution saturated with carbon dioxide by bubbling nitrogen for 30 minutes followed by carbon dioxide for 60 minutes as the catholyte, and 50 ml of a 1.0 M potassium hydroxide aqueous solution as the anolyte. During the electrolytic reduction, the electrolytes of the catholyte and anolyte were circulated using a liquid feed pump. The electrolytic reduction was carried out at a current density of -1.0 mA / cm while continuing to bubble carbon dioxide into the 0.5 M potassium chloride aqueous solution, which was the catholyte. 2 Constant current electrolysis was carried out for 60 minutes at 100°C. The 0.5 M potassium chloride aqueous solution used as the catholyte after the activation step was analyzed by high performance liquid chromatography to determine the amount of formic acid produced.

[0070] Formic acid production process In the formic acid production step, constant current electrolysis was carried out for 3 hours by applying a catholyte potential of -2.0 V to the silver-silver chloride reference electrode while continuing to bubble carbon dioxide into the catholyte (0.5 M potassium chloride aqueous solution). The 0.5 M potassium chloride aqueous solution used as the catholyte after the formic acid production step was analyzed by high-performance liquid chromatography, and the amount of formic acid produced was calculated from the difference with the amount after the activation step.

[0071] [Example 2] The current density during the activation process was set to -2.0 mA / cm 2 Formic acid was produced by electrolytic reduction of carbon dioxide in the same manner as in Example 1, except that:

[0072] [Comparative Example 1] Formic acid was produced by electrolytic reduction of carbon dioxide in the same manner as in Example 1, except that the activation step was omitted and the conductive diamond electrode was not activated.

[0073] Comparative Example 2 The current density during the activation process was set to -0.1 mA / cm 2 Formic acid was produced by electrolytic reduction of carbon dioxide in the same manner as in Example 1, except that:

[0074] The results are shown in Figures 2-1 and 2-2. Since electrolysis was performed using a constant potential method, the greater the increase in current density in the figure, the more formic acid is produced. Compared to Comparative Example 1, where the conductive diamond electrode was not activated, it can be seen that the current density increased due to the activation process as shown in Examples 1 and 2. Although Comparative Example 2 performed the activation process, the increase in current density was small, so it can be said that the activation effect was not necessarily sufficient.

[0075] Furthermore, the faradaic efficiency of formic acid production in the formic acid production step is shown in Figure 3. The faradaic efficiency indicates how much of the current flowing in Examples 1 and 2 and Comparative Examples 1 and 2 was used for formic acid production, and it can be said that the higher the faradaic efficiency, the higher the selectivity of formic acid production.

[0076] From the results of Figure 3, it can be seen that, compared to the case where the conductive diamond electrode of Comparative Example 1 was not activated, the current density increased and the faradaic efficiency also improved as shown in Examples 1 and 2 by the activation process. This shows that the amount of formic acid produced is greatly increased by the activation process. In addition, although Comparative Example 2 was subjected to the activation process, the increase in current density and the improvement in faradaic efficiency were small, so it was determined that the activation effect was not necessarily sufficient.

[0077] [Example 3] Next, the case where different electrolyte solutions were used in the activation step and the formic acid production step was investigated. A conductive diamond electrode was prepared in the same manner as in Example 1, and the same electrolytic bath as in Example 1 was used.

[0078] Activation process For the activation process, a conductive diamond electrode (electrode area 9.62 cm) was used as the cathode. 2 ), and a platinum electrode (electrode area 9.62 cm) as the anode. 2An electrolytic cell was used, equipped with a silver-silver chloride electrode as the reference electrode, and a Nafion membrane, a cation exchange membrane, as the diaphragm. The electrolytic reduction of carbon dioxide was carried out using 50 ml of 0.5 M potassium chloride aqueous solution as the catholyte, saturated with carbon dioxide by bubbling nitrogen for 30 minutes followed by carbon dioxide for 30 minutes, and 50 ml of 0.5 M potassium hydroxide aqueous solution as the anolyte. The electrolytic reduction was carried out at a current density of -2.0 mA / cm while continuing to bubble carbon dioxide into the 0.5 M potassium chloride aqueous solution, which was the catholyte. 2 Constant current electrolysis was carried out at 40°C for 60 minutes.

[0079] Liquid exchange process In the solution exchange process, the 0.5M potassium chloride aqueous solution in the catholyte tank was replaced with 50ml of pure water using a liquid pump, while being careful not to dry out the conductive diamond electrode (the cathode), and the pure water was circulated through the electrolytic cell. After repeating this operation three times, the pure water in the catholyte tank was replaced with 50ml of 0.25M potassium sulfate aqueous solution using a liquid pump, and the 0.25M potassium sulfate aqueous solution was circulated through the electrolytic cell. After repeating this operation twice, the 50ml of 0.25M potassium sulfate aqueous solution in the catholyte tank was saturated with carbon dioxide by bubbling nitrogen for 30 minutes and carbon dioxide for 30 minutes.

[0080] Formic acid production process In the formic acid generation process, carbon dioxide was continuously bubbled into 50 ml of 0.25 M potassium sulfate aqueous solution as the catholyte, and a current density of -2.0 mA / cm was applied to the silver-silver chloride reference electrode. 2 Constant-current electrolysis was carried out for 4 hours at 1000 kJ / h. Catholyte samples were taken every hour and analyzed by high-performance liquid chromatography to determine the amount of formic acid produced.

[0081] Comparative Example 3 Formic acid was produced by electrolytic reduction of carbon dioxide in the same manner as in Example 3, except that the activation step was omitted and the conductive diamond electrode was not activated.

[0082] The results are shown in Figure 4. It can be said that the higher the faradaic efficiency, the higher the selectivity for formic acid production, and since a constant current system was used in Example 3 and Comparative Example 3, it can be said that the higher the faradaic efficiency, the higher the amount of formic acid produced.

[0083] In the case of Comparative Example 3, when the conductive diamond electrode is not activated, the Faraday efficiency of formic acid is only 10% after 1 hour of electrolysis. Although the Faraday efficiency gradually increases over time, it is still about 20% even after 4 hours of electrolysis. As shown in Example 3, by activating the conductive diamond electrode, the Faraday efficiency of formic acid after 1 hour is 95%, and the amount of formic acid produced is dramatically increased.

[0084] [Example 4] Next, the applied potential in the formic acid production step was investigated. While the cathode potential in the formic acid production step was -2.0 V in Example 1, in this example, constant current electrolysis was performed for 3 hours at -1.8 V, -1.9 V, and -2.2 V. Other conditions were the same as in Example 1.

[0085] Figure 5 shows the results when a cathode potential of -1.8 V was applied, Figure 6 shows the results when a cathode potential of -1.9 V was applied, and Figure 7 shows the results when a cathode potential of -2.2 V was applied. Under all application conditions, formic acid was efficiently produced when the activated electrode was used. Furthermore, based on these results, it is believed that formic acid is produced with similarly high faradaic efficiency when generated at -1.5 V. [Industrial Applicability]

[0086] The formic acid generating device and method of the present invention enable efficient generation of formic acid. [Explanation of symbols]

[0087] 1 cathode 2 Anode 3 Reference electrode 4 Solid electrolyte membrane 5 Cathode bath 6 Anode bath 7 Carbon dioxide supply unit 8 Liquid transfer pump 9 1st storage tank 10 Second storage tank 11 External power supply

Claims

1. A method for producing formic acid by electrolytic reduction of carbon dioxide using an electrolytic cell equipped with an anode and a cathode, the cathode is a conductive diamond electrode, In an electrolyte solution containing carbon dioxide and a halide-containing supporting electrolyte, a cathodic potential was applied in the range of -2.0 V to -3.0 V relative to the reference electrode, and a voltage of -0.5 mA / cm was applied. 2 an activation step including activating the conductive diamond electrode by electrolysis at a current density of at least 1000 kJ / cm; a solution exchanging step of replacing the electrolyte solution containing the halide-containing supporting electrolyte with an electrolyte solution containing a halide-free supporting electrolyte; a formic acid production step, which includes electrolytic reduction using an electrolyte solution containing the halide-free supporting electrolyte and carbon dioxide dissolved therein; A method for producing formic acid, comprising:

2. 2. The method for producing formic acid according to claim 1, wherein the halide-containing supporting electrolyte is potassium chloride, and the halide-free supporting electrolyte is potassium sulfate.

3. 3. The method according to claim 1, wherein the activation step is carried out for 30 minutes or more.

4. The current density of the cathode potential applied in the activation step is -0.5 mA / cm 2 More than -5.0mA / cm 2 The method according to any one of claims 1 to 3, wherein:

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