Electrolysis cell, and production method for compound using the same

Abstract

To provide an electrolysis cell for producing formic acid useful as an energy carrier and excellent in energy efficiency, while suppressing byproduction of hydrogen production, a production method for formic acid using the electrolysis cell, and a fuel cell using the acquired formic acid, and a production method therefor.SOLUTION: An electrolysis cell comprises a unit membrane, in which a porous anode membrane, a non-porous proton exchange membrane, a porous hydrophilic filtration membrane, and a porous and hydrophilic cathode membrane are laminated in this order. In the electrolysis cell, the hydrophilic filtration membrane and the cathode membrane communicate with each other through pores.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to an electrolytic cell and a method for producing a compound using the same. [Background technology]

[0002] From the perspectives of negative emissions and the utilization of renewable energy, direct air capture (DAC) and fuel production through the electrolytic reduction of carbon dioxide (CO2) are noteworthy technologies. CO2 electrolysis cells have been studied that include a cathode in contact with CO2 gas and the cathode solution, an anode in contact with the anode solution, and an ion exchange membrane (usually an anion exchange membrane or bipolar membrane) separating the cathode and anode (Patent Document 1). However, gas-supply CO2 electrolysis cells require high costs for CO2 gas purification and compression, and the distance between the anode and cathode results in high cell resistance and significant voltage loss. Furthermore, low CO2 utilization efficiency due to crossover to the anode and slow CO2 reduction reaction results in unreacted CO2 being emitted from the anode and cathode. DAC technology also faces the problem of significant energy loss when capturing alkaline-absorbed CO2 as gas. Therefore, the development of reactive CO2 capture (RCC) technology, which directly utilizes alkaline absorption solutions, is anticipated. In recent years, a method for producing useful compounds by CO2 electrolysis using aqueous hydrogencarbonate (bicarbonate) solutions or aqueous carbonate solutions, which are alkaline absorption solutions, as raw materials has been reported (Patent Document 2). Formic acid is the most promising compound as a hydrogen carrier for the effective use of renewable energy, and the production of formic acid by electrolysis of aqueous bicarbonate solutions has also been reported (Non-Patent Documents 1 and 2).

[0003] According to Non-Patent Document 1, by using a bicarbonate aqueous solution, which is made by reacting CO with an alkaline aqueous solution to produce bicarbonate ions, as a carbon source instead of high-purity CO gas, it is possible to eliminate processes that require a large amount of energy, such as CO recovery, purification, and compression, and to reduce energy consumption overall. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 2019-157252 [Patent Document 2] International Publication No. 2019 / 204938 [Non-patent literature]

[0005] [Non-Patent Document 1] ACS Energy Lett. 2020, 5, 2624-2630 Conversion of Bicarbonate to Formate in an Electrochemical Flow Reactor [Non-patent document 2] ACS Appl. Mater. Interfaces 2022, 14, 30760-30771 Engineering Aspects for the Design of a Bicarbonate Zero-Gap Flow Electrolyzer for the Conversion of CO2 to Formate Summary of the Invention [Problem to be solved by the invention]

[0006] However, including Non-Patent Documents 1 and 2, the current efficiency (Faraday efficiency) of formic acid production in bicarbonate electrolysis cells was at most 60%. For example, in the electrolytic cell of Non-Patent Document 1, formic acid is produced by the reactions represented by the following formulas (1) and (2). However, it is believed that the reaction represented by the following formula (3) competes with the reaction represented by the following formula (2), resulting in a decrease in the faradaic efficiency.

[0007] HCO3 - +H + →CO2+H2O (1) CO2+2H+ +2e - →HCOOH (2) 2H + +2e - →H2···(3)

[0008] Gas-supply CO2 electrolysis cells (Patent Document 1) have problems such as high CO2 gas purification costs and low carbon utilization efficiency. Bicarbonate and carbonate electrolysis cells (Patent Document 2, etc.) that produce gases such as carbon monoxide and ethylene using an alkaline absorption solution have been reported. However, gaseous products cannot be used as energy carriers. CO2 electrolysis cells (Non-Patent Documents 1 and 2) that produce liquid formic acid using an alkaline absorption solution are promising RCC technologies. However, the faradaic efficiency of formic acid production is at most 60%, and hydrogen is produced as a by-product. Furthermore, the problems of conventional gas-supply CO2 electrolysis cells, namely, high cell resistance, low carbon utilization efficiency, and instability during long-term reactions, remain unresolved.

[0009] The problem to be solved by the present invention is to provide an electrolytic cell for producing formic acid that can suppress the by-production of hydrogen, a method for producing formic acid from an alkaline CO absorption solution using the same, a fuel cell that uses the obtained formic acid, and a system equipped with the electrolytic cell and the fuel cell. [Means for solving the problem]

[0010] As a result of extensive research into solving the above problems, the inventors discovered that the above problems can be solved by rapidly transporting a bicarbonate aqueous solution into the cathode and continuously supplying bicarbonate ions to the proton exchange membrane separating the anode and cathode via a hydrophilic filtration membrane, thereby efficiently generating CO2 inside the cathode, and thus completed the present invention. The efficient neutralization reaction between bicarbonate ions and protons in the hydrophilic filtration membrane consumes protons, suppressing the hydrogen generation reaction at the cathode. By supplying a bicarbonate aqueous solution to the cathode, which includes a porous transport layer and a catalyst layer, and by placing a hydrophilic filtration membrane, a hydrophilic porous membrane, between the proton exchange membrane and the cathode, the bicarbonate aqueous solution is rapidly transported to the vicinity of the proton exchange membrane, providing highly efficient RCC technology. This technology differs from conventional prior art in that it provides highly efficient RCC technology.

[0011] That is, the present invention has the following aspects. [1] An electrolysis cell comprising a unit membrane in which a porous anode membrane, a non-porous proton exchange membrane, a porous hydrophilic filtration membrane, and a porous and hydrophilic cathode membrane are stacked in this order, The electrolysis cell, wherein the hydrophilic filtration membrane and the cathode membrane are in communication with each other via pores. [2] The anode membrane generates protons and electrons by electrolysis of an oxidizable substance, supplies the protons to the proton exchange membrane, and supplies the electrons to the cathode membrane; The hydrophilic filtration membrane reacts the protons supplied from the proton exchange membrane with bicarbonate ions to generate CO2 and water, The electrolysis cell according to [1], wherein the cathode membrane is configured to react CO2 produced in the hydrophilic filtration membrane with electrons supplied from the anode membrane to produce formic acid. [3] The bicarbonate ions are supplied to the hydrophilic filtration membrane from the cathode membrane side as an alkaline absorption solution for CO2, The electrolytic cell according to [2], wherein the concentration of the bicarbonate ions is 3 mmol / L to 3 mol / L. [4] The electrolytic cell according to [1], wherein the thickness of the hydrophilic filtration membrane is more than 0.1 μm and not more than 5 mm. [5] The electrolysis cell according to [1], further comprising a cathode-side flow plate having a flow path for flowing an aqueous bicarbonate solution on the side of the cathode membrane opposite to the hydrophilic filtration membrane side. [6] The electrolytic cell according to [5], wherein the flow path is a serpentine flow path, a lattice groove flow path, or a parallel flow path. [7] A method for producing formic acid, using the electrolytic cell according to any one of [1] to [6]. [8] The method for producing formic acid according to [7], which comprises supplying an alkaline absorption solution for CO2 into the unit membrane and recovering the produced aqueous formic acid solution. [9] The method for producing formic acid according to [7], which is a circulating method in which an alkaline absorption solution for CO2 is circulated to produce formic acid, or a continuous method in which an alkaline absorption solution is continuously supplied without circulating it.

[10] Use of a formic acid-containing composition obtained by the method for producing formic acid according to [7] as fuel for a fuel cell.

[11] A fuel cell that uses formic acid obtained by the method for producing formic acid according to [7] as fuel.

[12] A system comprising the electrolysis cell according to [1] and the fuel cell according to

[11] . [Effects of the Invention]

[0012] According to the present invention, it is possible to provide an electrolytic cell for producing formic acid that can suppress the by-production of hydrogen, a method for producing formic acid from an alkaline absorption solution of CO using the electrolytic cell, a fuel cell that uses the obtained formic acid, and a system including the electrolytic cell and the fuel cell. [Brief explanation of the drawings]

[0013] [Figure 1] FIG. 1 is a front view showing an example of an electrolysis cell of the present invention. [Figure 2] FIG. 1 is an exploded view showing an example of an electrolysis cell of the present invention. [Figure 3] FIG. 1(a) is a front view showing an example of a continuous electrolytic cell of the present invention, and FIG. 1(b) is a front view showing an example of a circulation electrolytic cell of the present invention. [Figure 4] FIG. 1 is a conceptual diagram showing an example of a direct formic acid fuel cell that uses formic acid produced in the electrolysis cell of the present invention as fuel. [Figure 5] FIG. 1 is a conceptual diagram of a bicarbonate ion circulation power generation system including an electrolysis cell of the present invention and a direct formic acid fuel cell that uses formic acid as fuel. [Figure 6]1 is a graph showing the state of CO2 in an aqueous solution depending on pH. [Figure 7] 1 is a graph comparing the faradaic efficiency of formic acid when a hydrophobic cathode membrane is used and when a hydrophilic cathode membrane is used. [Figure 8] 1 is a graph comparing the faradaic efficiency of formic acid when a hydrophilic cathode membrane is used, with and without a hydrophilic filtration membrane. [Figure 9A] FIG. 2(a) is a perspective view showing an example of a serpentine flow channel as a cathode-side flow plate, and FIG. 2(b) is a perspective view showing an example of a grid groove flow channel as a cathode-side flow plate. [Figure 9B] (c) is a graph comparing the faradaic efficiency of formic acid when a serpentine channel is used and when a lattice groove channel is used, and (d) is a graph comparing the molar concentration ratio of the product when a serpentine channel is used and when a lattice groove channel is used. [Figure 10] (a) is a graph comparing the faradaic efficiency of formic acid when a serpentine flow channel is used as the cathode-side flow plate and the flow rate of the bicarbonate solution is changed; (b) is a graph comparing the faradaic efficiency of formic acid when a lattice groove flow channel is used as the cathode-side flow plate and the flow rate of the bicarbonate solution is changed. [Figure 11] 1 is a graph showing the faradaic efficiency of formic acid when the current density is changed. [Figure 12] 1 is a graph comparing the faradaic efficiency of formic acid between the electrolysis cell of the present invention and a conventional gas-fed CO electrolysis cell having a gas diffusion electrode. [Figure 13] 1 is a graph showing the faradaic efficiency of formic acid and the cell voltage when a 3 mol / L aqueous KHCO3 solution is used and the reaction is carried out at a current density of 100 mA / cm2 for 30 hours. [Figure 14] 1 is a graph showing the molar concentration and weight fraction of formic acid in the aqueous formic acid solution produced when a 3 mol / L aqueous KHCO3 solution is reacted at a current density of 100 mA / cm2 for 30 hours. [Figure 15]1 is a table comparing the type of catalyst, the faradaic efficiency of formic acid, and the voltage value required at a current density of 100 mA / cm 2 between the electrolytic cell of the present invention and a conventional electrolytic cell. DETAILED DESCRIPTION OF THE INVENTION

[0014] The present invention will be described in detail below. Note that the present invention is not limited to the following embodiments, and can be practiced in various modifications within the scope of the invention.

[0015] <Electrolytic cell> The electrolysis cell of the present invention is an electrolysis cell comprising a unit membrane in which a porous anode membrane, a non-porous proton exchange membrane, a porous hydrophilic filtration membrane, and a porous and hydrophilic cathode membrane are stacked in this order, and the hydrophilic filtration membrane and the cathode membrane are in communication with each other via pores. The anode membrane generates protons and electrons by electrolysis of water, supplies the protons to the proton exchange membrane, and supplies the electrons to the cathode membrane. The hydrophilic filtration membrane reacts the protons supplied from the proton exchange membrane with bicarbonate ions to generate CO2 and water. The cathode membrane reacts the CO2 generated by the hydrophilic filtration membrane with the electrons supplied from the anode membrane to generate formic acid. The porous membrane has pores that communicate with adjacent porous membranes and do not prevent gas or liquid from passing through. A non-porous membrane does not have pores that communicate with adjacent membranes.

[0016] The electrolysis cell 1 shown in Figure 1 has a unit membrane that integrates an anode membrane 3, a proton exchange membrane 4, a hydrophilic filtration membrane 5, and a cathode membrane 6. All of the unit membranes except for the proton exchange membrane are made of porous materials, and the interior is connected so that liquids and gases can be introduced from the cathode membrane 6 to the hydrophilic filtration membrane 5. An alkaline CO2 absorption solution is introduced from the outside of the cathode membrane 6. An oxidizable substance is supplied from the outside of the anode membrane 3.

[0017] First, an oxidation reaction that releases electrons occurs in the anode film 3. Specifically, when pure water is supplied, an oxygen generation reaction occurs due to the electrolysis of water, as expressed by the following formula (4). 2H2O→O2+4H + +4e - ···(4) The generated protons are supplied to the proton exchange membrane 4 adjacent to the anode membrane 3, and the electrons are supplied to the cathode membrane 6 via a lead wire and a power source. Meanwhile, in the hydrophilic filtration membrane 5, a neutralization reaction between bicarbonate ions contained in the alkaline CO2 absorption solution supplied from the anode membrane 3 side and protons supplied from the proton exchange membrane 4 proceeds, producing CO2 and water. Specifically, the neutralization reaction represented by the following formula (1) occurs. HCO3 - +H + →CO2+H2O (1) The generated CO2 is hardly soluble in neutral or alkaline aqueous solvents and is retained as a gas within the porous hydrophilic filtration membrane 5, which then travels in the form of bubbles through the flow of the supply liquid and is supplied to the adjacent porous cathode membrane 6. Finally, a reduction reaction occurs in the cathode film 6 to convert the produced CO2 into formic acid. Specifically, a reaction to produce formic acid occurs through the reduction of CO2, as shown in formula (2) below. CO2+2H + +2e - →HCOOH (2) The produced formic acid is the strongest acid among monovalent aliphatic carboxylic acids, and is in equilibrium with formate ions in aqueous solution. When the pH of the supplied bicarbonate aqueous solution is near neutral, most of the converted formic acid exists as formate ions in the aqueous solution. Therefore, formula (2) can also be expressed as the following formulas (5) and (6). CO2+H + +2e - →HCOO - ···(5) CO2+H2O+2e - →HCOO - +OH - ···(6)

[0018] As shown in Fig. 2, the electrolysis cell 100 of the present invention has flow plates 7 and 8 on the outside of the anode membrane 3 and the cathode membrane 6, respectively, current collectors 9 and 10 on the outside of the flow plates 7 and 8, respectively, and may have end plates 11 and 12 on the outside of the current collectors 9 and 10, respectively. The anode-side end plate 11 has a supply port 13A for supplying an oxidizable substance and a discharge port 13B for discharging an oxidation product. The cathode-side end plate has a supply port 14A for supplying an alkaline absorption solution for CO2 and a discharge port 14B for discharging the generated formic acid.

[0019] As shown in FIGS. 3(a) and (b), the electrolysis cell 100a of the present invention may be of a continuous type or a circulating type. As shown in FIG. 3(a), the electrolysis cell 100a of the present invention may be configured so that an aqueous bicarbonate solution of a predetermined concentration is continuously supplied to a supply port 14A provided in the end plate 12 on the cathode membrane side, and the generated aqueous formic acid solution is collected from a discharge port 14B provided in the end plate 12 on the cathode membrane side. By carrying out the reaction continuously, a large amount of aqueous formic acid solution can be produced with high selectivity. Formic acid can be concentrated to a desired concentration from an aqueous formic acid solution by conventional means, such as distillation, extraction, or a known separation method such as a separation membrane equipped with a reverse osmosis membrane. Formate ions may be protonated. Alternatively, formic acid can be recovered as a solid formate salt by concentration and drying.

[0020] 3(b), the electrolysis cell 100b of the present invention may be provided with a tank 15 for storing an alkaline absorption solution for CO2 outside the cathode membrane 6. The electrolysis cell 100b may be configured so that an aqueous solution containing bicarbonate ions stored in the tank is supplied to the cathode membrane 6, the generated formic acid aqueous solution is discharged into the tank, and an aqueous bicarbonate solution containing the generated formic acid aqueous solution is supplied from the tank 15 to the cathode membrane 6, and so that the aqueous bicarbonate solution stored in the tank 15 is circulated. By circulating the solution, an aqueous solution of formic acid having a high concentration of formic acid can be produced.

[0021] <Anode film> The anode membrane used in the present invention is preferably a catalytic electrode containing a porous conductive support and an anode active material.

[0022] The conductive support is not particularly limited, but examples thereof include nonwoven fabric made of metal fibers, porous sintered bodies of metal particles, and porous conductive carbon materials, such as carbon paper and carbon cloth.

[0023] In the present invention, an oxidation reaction that releases electrons occurs in the anode film. Specifically, when water is supplied as an oxidizable substance, an oxygen generating reaction occurs due to the electrolysis of water, as shown in the following formula (4). 2H2O→O2+4H + +4e - ···(4) The anode membrane contains an anode active material to promote the water oxidation reaction. The anode active material for promoting the reaction of formula (4) may be any known catalyst for converting water into protons, oxygen, and electrons. It may also be any known catalyst for converting hydroxide ions into oxygen and electrons. For example, a metal selected from Groups 4 to 12 of the periodic table or a metal compound thereof can be used. Examples of metals or metal compounds include ruthenium, iridium, nickel, manganese, iron, platinum, and palladium, as well as metal compounds containing these metals. Among these, ruthenium oxide and iridium oxide are more preferred. A catalytic electrode in which an anode active material is attached to a conductive carbon material or a conductive support may also be used. Preferred examples of conductive carbon materials include carbon black, carbon fiber, graphene, carbon whiskers, and carbon nanotubes. Examples of conductive supports include titanium mesh and sintered titanium fiber.

[0024] In the present invention, it is sufficient that an oxidation reaction that releases electrons occurs in the anode film, and an electrolytic cell driven by electric energy may be used by using an anode film that functions as a catalytic electrode, or a photoanode film that functions as a semiconductor electrode may be used as the anode film by using a light energy-driven electrolytic cell. Solar energy may be used as the light energy. Examples of the photoanode film for promoting the oxidation reaction of the above formula (4) include oxides such as titanium oxide, strontium titanate, tungsten oxide, bismuth vanadate, and iron oxide, and n-type semiconductors such as oxynitrides and oxysulfides, which are attached to a porous conductive support.

[0025] <Proton exchange membrane> The proton exchange membrane of the present invention is a nonporous polymer electrolyte membrane and a proton conductor without electronic conductivity. As long as a solid electrolyte membrane is used, it may be a cation exchange membrane, an anion exchange membrane, or a bipolar membrane, but a proton exchange membrane is preferred. The material of the proton exchange membrane is not particularly limited, but examples include fluorine-based polymer membranes having sulfonic acid groups, such as perfluorosulfonic acid polymers. Specific examples include Nafion (registered trademark) and Aquivion (registered trademark). Because the proton exchange membrane of the present invention is nonporous, it can prevent products such as oxygen generated in the anode membrane from easily reaching the cathode membrane (crossover). Furthermore, the proton exchange membrane of the present invention has hydrophilic and hydrophobic moieties. It is preferable that the proton exchange membrane be cationic to prevent crossover of bicarbonate ions supplied from the hydrophilic filtration membrane to the anode membrane. Since the use of a bipolar membrane increases cell resistance, it is preferable to use an ion exchange membrane with low membrane resistance.

[0026] <Hydrophilic filtration membrane> The hydrophilic filtration membrane of the present invention is a hydrophilic porous membrane made of an insulating material. The hydrophilic filtration membrane of the present invention is preferably an insulator that does not have electronic conductivity, but may also be a semiconductor or an electric conductor. It may also be an ion conductor. Furthermore, the hydrophilic filtration membrane of the present invention is preferably porous because it needs to have substance diffusibility. In the hydrophilic filtration membrane, a neutralization reaction of bicarbonate ions by protons occurs, as shown in the following formula (1), generating CO2. HCO3 - +H + →CO2+H2O (1) Bicarbonate ions may be generated by a neutralization reaction of carbonate ions represented by the following formula (7), and CO2 may be generated by the neutralization reaction of these bicarbonate ions. CO3 2- +H + →HCO3 - ···(7) Although the material of the hydrophilic filtration membrane does not contribute to the neutralization reaction, the hydrophilic filtration membrane, which is a porous and hydrophilic membrane, provides a reaction field with a large surface area for the neutralization reaction, allowing the protons supplied from the proton exchange membrane to react quickly with bicarbonate ions. However, since the cathode has a catalyst that promotes the reduction reaction, when protons are supplied to the cathode, they compete with the CO2 reduction reaction of the above formula (2), and the reaction of the above formula (3), i.e., 2H + +2e - →The proton reduction reaction of H2 proceeds. The progression of the reaction (3) above is thought to be the reason why the faradaic efficiency of conventional electrolysis cells is 60% or less. Therefore, by placing a hydrophilic filtration membrane without a reduction catalyst between the proton exchange membrane and the cathode, the hydrophilic filtration membrane consumes protons in a reaction with bicarbonate ions, preventing the protons from reaching the cathode. This enables formic acid to be produced with high selectivity. Bicarbonate ions are ions generated by the ionization of potassium bicarbonate and other compounds. They are in equilibrium with carbonate ions and CO2 in aqueous solutions. Bicarbonate ions themselves are stable in aqueous solutions and do not readily react with electrons. When protons are supplied from the proton exchange membrane, the local proton concentration increases, facilitating the hydrogen generation reaction via the reaction of protons with electrons. Therefore, by placing a hydrophilic filtration membrane between the proton exchange membrane and the cathode, protons supplied from the proton exchange membrane can react quickly with bicarbonate ions before being reduced at the cathode to produce hydrogen as a by-product. This prevents the hydrogen by-production caused by the reduction of protons supplied to the cathode. Furthermore, CO2 generated by the neutralization reaction has low solubility in aqueous solvents and is therefore retained in a gaseous state by the porous hydrophilic filtration membrane and the adjacent cathode membrane. By supplying a high concentration of CO2 to the cathode and reducing it, formic acid can be produced with high selectivity.

[0027] Examples of hydrophilic filtration membranes include cellulose filters, nylon filters, hydrophilic polytetrafluoroethylene filters, and nonwoven fabrics made of glass fibers or chemical fibers, each having fine pores on the membrane surface. In the present invention, a membrane filter made of a cellulose mixed ester is preferably used. Nanofibers produced by electrospinning or sintered particles may also be used.

[0028] <Cathode film> The cathode membrane used in the present invention is preferably hydrophilic and is a catalytic electrode containing a porous conductive support and a cathode active material. Examples of the porous conductive support include those similar to those described above for the anode membrane. The cathode may have, for example, a multilayer structure having a catalyst layer and a porous conductive support, in which the catalyst layer has a cathode active material, or may have a single-layer structure in which the cathode active material as a catalyst is supported on a porous conductive support.

[0029] The cathode active material may be any known catalyst that promotes the reduction of CO. A reduction reaction is a reaction in which a substance loses oxygen or combines with hydrogen, or more generally, any reaction in which electrons are gained. In the present invention, a CO2 reduction reaction represented by the following formula (2) occurs in the cathode membrane to produce formic acid. CO2+2H + +2e - →HCOOH (2) Products of the CO2 reduction reaction include formic acid, carbon monoxide, methanol, ethylene, ethanol, propanol, acetic acid, ethane, and methane.

[0030] Catalysts for the CO2 reduction reaction include metals selected from Groups 4 to 15 of the periodic table, metal oxides, hydroxides, alloys or intermetallic compounds containing at least one of these metals, and composite metal oxides. Nitrides, sulfides, selenides, etc. can also be used. Catalysts containing copper, silver, gold, zinc, palladium, cadmium, bismuth, indium, tin, lead, mercury, etc. are preferred. Catalysts containing bismuth, indium, tin, lead, etc. are preferred for producing formic acid.

[0031] The reduction catalyst can also be used by immobilizing it on a porous conductive support. The support is not particularly limited, but examples include porous conductive carbon materials, nonwoven fabrics made of metal fibers, and porous sintered bodies of metal particles. Examples of porous conductive carbon materials include carbon paper and carbon cloth. The reduction catalyst and the conductive support may be subjected to hydrophilic and hydrophobic treatments as needed. Second and third components may be added to the reduction catalyst to improve catalytic performance, such as catalytic activity, reaction selectivity, and catalyst life. The reduction catalyst itself may also be made porous before use.

[0032] Examples of methods for supporting the reduction catalyst on a support include a method in which a solvent containing the reduction catalyst is dropped onto the support and then dried to support the catalyst, a method in which a solvent containing the reduction catalyst is impregnated into the support and then dried to support the catalyst, and a method in which a solvent containing the reduction catalyst is sprayed onto the support and then dried to support the catalyst, etc. The reduction catalyst may also be immobilized on a conductive support by methods such as electrolytic deposition, vacuum deposition, and sputtering.

[0033] As another catalyst, a mixture of a metal complex coordinated with a nitrogen-containing organic compound and a conductive carbon material is preferably heat-treated. Examples of conductive carbon materials include carbon black, carbon fiber, graphene, carbon whiskers, and carbon nanotubes. The heat treatment can be carried out in oxygen, air, or an inert gas, but is preferably carried out in an inert gas such as nitrogen, helium, or argon. The heat treatment temperature is preferably 100 to 1000°C, more preferably 300 to 900°C, and even more preferably 500 to 800°C. Examples of methods for supporting a catalyst on a support include a method in which a solvent containing the catalyst is dropped and dried to support the catalyst, a method in which a solvent containing the catalyst is impregnated into a support and then dried to support the catalyst, and a method in which a solvent containing the catalyst is sprayed onto a support and then dried to support the catalyst.

[0034] In the present invention, the cathode membrane may function as a catalytic electrode to form an electrolytic cell driven by electric energy, or may use a photocathode membrane that functions as a semiconductor electrode to form an electrolytic cell driven by light energy. Solar energy may be used as the light energy. The photocathode film for promoting the CO2 reduction reaction includes p-type semiconductors such as oxides and sulfides deposited on a porous conductive support.

[0035] In the present invention, the anode membrane, proton exchange membrane, hydrophilic filtration membrane, and cathode membrane are pressure-bonded together to form a unit membrane in the form of a sheet, which is then placed in the electrolysis cell of the present invention. Heat at 80 to 120°C may be applied during pressure bonding. The anode membrane, proton exchange membrane, hydrophilic filtration membrane, and cathode membrane can each be formed by a method commonly used in fuel cells and electrolysis cells, such as a coating method or a drop-casting method.

[0036] The thickness of the anode film is preferably 0.001 to 2.0 mm, more preferably 0.01 to 1.0 mm, and even more preferably 0.1 to 0.5 mm. The thickness of the proton exchange membrane is preferably 10 to 300 μm, more preferably 20 to 200 μm, and even more preferably 30 to 70 μm. The thickness of the hydrophilic filtration membrane is preferably more than 0.1 μm and 5.0 mm, more preferably 10 to 500 μm, and even more preferably 50 to 150 μm. The thickness of the cathode membrane is preferably 0.001 to 2.0 mm, more preferably 0.01 to 1.0 mm, and even more preferably 0.1 to 0.5 mm. The thickness of the unit membrane is preferably 0.003 to 10 mm, more preferably 0.03 to 3.0 mm, and even more preferably 0.3 to 1.5 mm.

[0037] When the electrolytic cell of the present invention has a flow plate as shown in Figure 2, the flow plate is not particularly limited as long as it is a conductor, but from the viewpoint of cost, a flow plate made of carbon is preferred on the cathode side. The shape of the flow path is not particularly limited, but preferred are a serpentine flow path, which is a serpentine flow path, a grid groove flow path, in which the grooves that form the flow path are in a grid pattern, and a parallel flow path, in which the grooves are parallel. A flow plate may or may not be provided, but is preferably provided at least on the cathode side. A serpentine flow path or a grid groove flow path is more preferred because it does not cause a large pressure loss and can quickly supply bicarbonate ions to the vicinity of the proton exchange membrane.

[0038] The width of the grooves in the flow plate having serpentine flow channels is preferably 5 mm, more preferably 2 mm, and even more preferably 1 mm. The depth of the grooves in the flow plate having serpentine channels is preferably 5 mm, more preferably 2 mm, and even more preferably 1 mm.

[0039] The width of the grooves of the flow plate having the grid groove flow channels is preferably 5 mm, more preferably 2 mm, and even more preferably 1 mm. The depth of the grooves in the flow plate having the grid groove flow channels is preferably 5 mm, more preferably 2 mm, and even more preferably 1 mm.

[0040] When the electrolytic cell of the present invention has a current collector as shown in FIG. 2 , the current collector is not particularly limited as long as it is a conductor, but from the viewpoints of preventing metal corrosion and improving electrical conductivity, it is preferable that the current collector be made of a gold-plated copper plate.

[0041] When the electrolytic cell of the present invention has end plates as shown in FIG. 2, the end plates may be either conductive or insulating, but are preferably made of stainless steel from the viewpoint of durability. Each supply port and discharge port may be made of either a conductor or an insulator, but is preferably made of stainless steel from the viewpoint of durability.

[0042] The aqueous solution supplied to the cathode membrane in the present invention is not particularly limited as long as it is an alkaline solution for absorbing CO2, but an aqueous carbonate solution or an aqueous bicarbonate solution is preferably used. An aqueous potassium carbonate solution or an aqueous potassium bicarbonate solution, which have high solubility in water, is more preferred. The concentration of potassium carbonate is preferably equal to or lower than the saturated concentration, and is preferably 1 mmol / L to 8 mol / L, more preferably 0.1 to 8 mol / L, and even more preferably 1 to 8 mol / L.The concentration of potassium bicarbonate is preferably equal to or lower than the saturated concentration, and is preferably 1 mmol / L to 3 mol / L, more preferably 0.1 to 3 mol / L, and even more preferably 1 to 3 mol / L. The pH of the alkaline CO2 absorption solution is preferably 5-12, more preferably 6-11, and even more preferably 7-10.

[0043] In the present invention, various conditions that change with the scale of the electrolytic cell (e.g., the flow rate of the aqueous bicarbonate solution, the current, etc.) can be appropriately selected according to the scale of the electrolytic cell. For example, the flow rate of the aqueous bicarbonate solution can be appropriately selected according to the scale of the electrolytic cell, and is preferably 1 to 10,000 mL / min, more preferably 10 to 1,000 mL / min.

[0044] In the present invention, the current density flowing between the anode film and the cathode film is preferably 1.0 to 3000 mA / cm 2 , more preferably 10 to 1000 mA / cm 2 , and more preferably 100 to 300 mA / cm 2 It can be said that: In the present invention, the reaction can be accelerated by applying a voltage between the anode film and the cathode film. When a voltage is applied in combination with an anode for the oxygen generation reaction, the voltage can be preferably 5 V or less, more preferably 2.5 V or less. In the case of a light energy-driven type, the reaction can occur without applying a voltage, but a voltage may be applied to accelerate the reaction. The reaction temperature is preferably selected from the range of 0 to 80°C, more preferably 5 to 30°C. The reaction time may be appropriately selected by determining the substantial target values ​​of the selectivity and yield of the reaction product, and is not particularly limited, but is preferably several hours to several tens of hours.

[0045] The size of the electrolysis cell is not particularly limited, but for example, the volume of the cathode chamber is about 0.1 cm 3 Approximately 1m from 3 The sizes of the anode membrane, proton exchange membrane, hydrophilic filtration membrane, and cathode membrane, as well as the unit membranes composed of these, can be adjusted accordingly.

[0046] <Method for producing formic acid> The method for producing formic acid of the present invention uses the electrolytic cell of the present invention. The method for producing formic acid of the present invention preferably includes the steps of supplying an aqueous bicarbonate solution into the unit membrane of the electrolysis cell of the present invention, and recovering the aqueous formic acid solution produced. The reaction temperature is preferably 0 to 80° C., more preferably 5 to 30° C. When the reaction temperature is within the above range, the amount of energy consumed for heating and cooling can be reduced. The reaction time is not particularly limited, but the durability of the electrolytic cell can be maintained for at least 30 hours or more, and formic acid can be produced with high selectivity while maintaining high faradaic efficiency.

[0047] <How to use formic acid> The formic acid-containing composition containing formic acid obtained by the method for producing formic acid of the present invention can be used as a fuel for fuel cells. The formic acid-containing composition can contain water, an organic solvent, etc. in addition to formic acid.

[0048] ≪Fuel cell≫ The fuel cell of the present invention uses, as fuel, formate ions obtained by the method for producing formic acid of the present invention. FIG. 4 is a conceptual diagram showing an example of a direct formate fuel cell that uses formate ions produced in the electrolysis cell of the present invention as fuel. 4 includes an anode 21 having a first diffusion layer 23 and an oxidation catalyst 24, and a cathode 29 having an electrolyte membrane 25, a reduction catalyst 26, and a second diffusion layer 27, stacked in this order. A first current collector 22 and a second current collector 28 are provided on the outside of the anode 21 and the cathode 29, respectively, and are electrically connected via lead wires and electrical equipment such as a load device. At the anode 21, the reaction represented by the following formula (8) proceeds, and at the cathode 29, the reaction represented by the following formula (9) proceeds. HCOO - +3OH - →CO3 2- +2H2O+2e - ···(8) O2+2H2O+4e - →4OH -···(9) Formate ions are supplied to the anode 21 as fuel, and air is supplied to the cathode 27 as an oxygen source. At the anode 21, the oxidation catalyst 24 generates carbonate ions, water, and electrons from the formate ions supplied from the first diffusion layer and the hydroxide ions generated at the cathode 27. The generated carbonate ions and water pass through the first diffusion layer 23 and are discharged to the outside of the system. At the cathode 27, the reduction catalyst 26 causes oxygen supplied from the second diffusion layer 27 to react with the electrons generated at the anode 21 to generate hydroxide ions. The hydroxide ions generated at the cathode 27 pass through the electrolyte membrane 25 and are supplied to the anode 21. The formate ions supplied to the fuel cell may be those produced by the electrolytic cell of the present invention, and the fuel cell may be directly connected to the electrolytic cell of the present invention, or may be a fuel cell independent of the electrolytic cell.

[0049] <System> The system of the present invention comprises the electrolysis cell of the present invention and the fuel cell of the present invention. The system of the present invention supplies formic acid produced in the electrolytic cell of the present invention directly as fuel for a fuel cell via transportation such as piping and transportation. FIG. 5 shows an example of a bicarbonate ion circulation power generation system including the electrolysis cell of the present invention and a direct formate fuel cell that uses formic acid as fuel. In the system 30 of FIG. 5, the formate ions produced in the electrolysis cell 1 are supplied to the anode of the fuel cell 20 as is without being protonated. In the anode membrane of the electrolysis cell 1, the following reaction (4) proceeds, in the hydrophilic filtration membrane the following reaction (1) proceeds, and in the cathode membrane the following reaction (6) proceeds. 2H2O→O2+4H + +4e - ···(4) HCO3 - +H + →CO2+H2O (1) CO2+H2O+2e - →HCOO - +OH - ···(6) The following reaction (8) occurs at the anode of the fuel cell 20, and the following reaction (9) occurs at the cathode. HCOO - +3OH - →CO3 2- +2H2O+2e - ···(8) O2+2H2O+4e - →4OH - ···(9)

[0050] In the fuel cell 20, formate ions and hydroxide ions react with each other at the anode to produce carbonate ions, water, and electrons. At the cathode, the electrons and water produced at the anode react with oxygen to produce hydroxide ions. Humidified air is supplied to the cathode as an oxygen source. High-concentration oxygen may be supplied instead of air, or hydrogen peroxide may be supplied as the reducible substance. When hydrogen peroxide is supplied, the following reaction (10) occurs: H2O2+2e - →2OH - ···(10) FIG. 6 is a graph showing the state of CO2 in an aqueous solution as a function of pH. As shown in Figure 6, carbonate ions change into bicarbonate ions depending on the pH. The carbonate solution produced at the anode absorbs acidic CO2, which causes the pH to become close to neutral, converting it into a bicarbonate solution. By supplying this to the cathode side of the electrolysis cell, CO2 in the air can be reduced. As described above, according to the system of the present invention, formic acid produced in the electrolytic cell of the present invention is supplied to a direct formate fuel cell that uses formate ions as fuel to generate electricity, and carbonate ions contained in the waste liquid discharged from the direct formate fuel cell are converted to bicarbonate ions and reused in the electrolytic cell, thereby enabling effective use of energy without emitting CO2.

[0051] The electrolytic cell of the present invention is an electrolytic cell that can produce useful compounds under mild conditions with high selectivity, efficiently, and economically, and overcomes the problems associated with conventional catalytic processes. Furthermore, the electrolytic cell of the present invention can be used to produce formic acid under mild conditions with high selectivity, efficiently, and economically. In particular, the use of the electrolytic cell of the present invention to produce formic acid from water and bicarbonate ions can solve problems associated with conventional production methods, such as high energy consumption.

[0052] Furthermore, since the electrolytic cell of the present invention is provided with a hydrophilic filtration membrane, it is possible to suppress the by-production of hydrogen and reduce electrical energy consumption without excessively increasing the cell resistance. According to the present invention, as shown in the examples below, by using the electrolytic cell of the present invention, formic acid can be produced with high selectivity from water and bicarbonate ions. Furthermore, the electrolytic cell of the present invention solves the problems of conventional gas supply type CO electrolytic cells, namely, high cell resistance, low carbon utilization efficiency, and instability during long-term reactions, and can achieve highly selective production of formic acid. [Example]

[0053] The present invention will be described in more detail below with reference to examples and comparative examples, but the present invention is not limited thereto. Those skilled in the art will be able to implement the present invention by making various modifications in addition to the examples shown below, and such modifications are also encompassed within the scope of the present claims.

[0054] [Manufacturing Example 1] In Production Example 1, formic acid was produced using the electrolytic cell shown in Figures 1 and 2. A unit membrane was fabricated for the electrolytic cell, integrating an anode membrane 3, a proton exchange membrane 4, a hydrophilic filtration membrane 5, and a cathode membrane 6. A flow plate 7 with serpentine channels was attached to the outside of the anode membrane 3, and a flow plate 8 with lattice grooves was attached to the outside of the cathode membrane 6. Gold-plated copper current collectors 9 and 10 were attached to the outside of these, and stainless steel end plates 11 and 12 were attached to the outside of these. The end plates 11 and 12 were secured in place with bolts. The bolts were tightened with a torque of 2.0 N m. The anode-side end plate 11 had an inlet 13A for supplying water and an outlet 13B for discharging oxygen. The cathode-side end plate 12 had an inlet 14A for supplying a bicarbonate aqueous solution and an outlet 14B for discharging the resulting formic acid aqueous solution. Lead wires were connected to the current collectors 9 and 10, and the anode and cathode were connected to a power source via the flow plate, the current collectors, and the lead wires.

[0055] The anode membrane 3, the proton exchange membrane 4, the hydrophilic filtration membrane 5, and the cathode membrane 6 were fabricated as follows.

[0056] The anode film 3 was prepared by the following procedure. Carbon paper Sigracet GDL39BC (SGL Carbon Japan Co., Ltd., thickness 0.32 mm) was used as the porous support. First, 0.2 g of IrO2 powder (Tanaka Kikinzoku Kogyo, Ir75%) was placed in 8 mL of ethanol and ground using a ball mill at 500 rpm for 3 hours. Next, 1.1 mg of the obtained powder, 1.2 mL of 1-propanol, and 7.2 mL of 2-propanol were added and ultrasonically stirred. 300 μL of 5 wt% Nafion dispersion (Fujifilm Wako Pure Chemical Industries, Ltd.) was added to this mixture and ultrasonically stirred again to prepare a catalyst ink. The obtained catalyst ink was prepared with a catalyst loading of 1 mg / cm. 2 The anode was prepared by spray coating onto the microporous layer surface of the carbon paper so that the anode was in the range of 0.1 to 1.0 μm. The anode was dried overnight.

[0057] The proton exchange membrane 4 used was Nafion NR212 (Chemours, thickness 50 μm), which is a perfluorosulfonic acid (PFSA) polymer membrane.

[0058] The hydrophilic filtration membrane used was a cellulose mixed ester membrane filter (Merck Millipore, thickness 0.14 mm, porosity 84%, pore size 8 μm) cut into a 2 cm square.

[0059] The cathode film 6 was prepared in the following manner. Carbon paper Sigracet GDL39AA (SGL Carbon Japan Co., Ltd., thickness 0.28 mm, porosity 80%) was used as the porous support. First, 0.80 g of bismuth nitrate pentahydrate (Fujifilm Wako Pure Chemical Industries, Ltd., purity 99.9%) was weighed out, and 0.5 mol / L nitric acid was added to make 50 mL. The electrolyte was then prepared by ultrasonic stirring for 10 minutes. Next, the carbon paper cut into a length of 4.0 cm and a width of 2.0 cm was used as the working electrode. The bottom 2.0 cm of the cut carbon paper was immersed in the electrolyte, and a current density of -32 mA (-8 mA / cm) was applied. 2 A constant current of 1000 kJ / s was applied for 5 minutes to electrolytically deposit a metal bismuth catalyst onto the surface of the carbon paper. The membrane was then washed with ion-exchanged water and dried at 80°C for 10 minutes to obtain a hydrophilic cathode membrane 6. 80 μL of PTFE dispersion (FUELCELLStore) was then drop-cast onto the membrane and heated in a dryer at 250°C for 30 minutes to obtain a hydrophobic cathode membrane 6.

[0060] The reaction was carried out using an electrolysis cell consisting of the anode membrane 3, proton exchange membrane 4, hydrophilic filtration membrane 5, and cathode membrane 6 stacked together to form a unit membrane, as follows. A bicarbonate aqueous solution was supplied to the cathode membrane 6 at a predetermined flow rate under normal pressure. Pure water was supplied to the anode membrane 3 at a predetermined flow rate under normal pressure, and the current density was controlled to a predetermined value using an electrochemical measurement device (Ivium, Vertex 1A). The reaction was carried out at room temperature (approximately 25°C). 1 mL of the aqueous solution circulating through the electrolysis cell was sampled every 30 minutes, and the concentration of the generated formate ions was quantified using an ion chromatograph (Metrohm, EcoIC). Gas components were quantified using a gas chromatograph (Shimadzu, GC8A). During this time, the voltage applied externally between the anode film 3 and the cathode film 6 was measured using an electrochemical measurement device. Furthermore, the Faraday efficiency of formic acid production was calculated based on the current value that flowed, using the following formula. Faraday efficiency (%) = amount of formic acid produced (mol) x 2 x 96485 (C / mol) x 100 / amount of electricity passed (C) The quantity of electricity (C) in the formula is the current value (A) multiplied by the time (s). The current value is expressed as the current density (mA / cm 2 ) to the geometric area of ​​the cathode membrane (cm 2 ) is the value obtained by multiplying Similarly, the faradaic efficiencies of the by-products hydrogen and carbon monoxide were calculated.

[0061] [Example 1] The faradaic efficiency of formic acid was compared between the case where a hydrophobic cathode membrane was used and the case where a hydrophilic cathode membrane was used. A grid groove flow plate was used, and the flow rate of a 3 mol / L KHCO3 aqueous solution was set to 32 mL / min, and the current density was set to 100 mA / cm. 2 The test was carried out with the reaction time set to 1 hour. FIG. 7 is a graph comparing the faradaic efficiency of formic acid when a hydrophobic cathode membrane is used and when a hydrophilic cathode membrane is used. Typically, hydrophobic cathode membranes are used in CO2 electrolysis cells. However, when a hydrophobic cathode membrane was used, the faradaic efficiency of formic acid was less than 10%, as shown in the graph on the left side of Figure 7. On the other hand, when a hydrophilic cathode membrane was used, the faradaic efficiency of formic acid was over 90%, as shown in the graph on the right side of Figure 7. In electrolysis cells using conventional technology, the faradaic efficiency of formic acid was a maximum of 60%, but it was found that the faradaic efficiency of formic acid could be significantly improved by using a hydrophilic cathode membrane. This is thought to be because the use of a hydrophilic cathode makes it easier to supply bicarbonate ions to the hydrophilic filtration membrane.

[0062] [Example 2] When a hydrophilic cathode membrane was used, the faradaic efficiency of formic acid was compared between the case where a hydrophilic filtration membrane was not provided and the case where a hydrophilic filtration membrane was provided. A grid groove flow plate was used, and the flow rate of a 3 mol / L KHCO3 aqueous solution was set to 32 mL / min, and the current density was set to 100 mA / cm. 2 The test was carried out with the reaction time set to 1 hour. FIG. 8 is a graph comparing the faradaic efficiency of formic acid when a hydrophilic cathode membrane is used, with and without a hydrophilic filtration membrane. When the proton exchange membrane and anode membrane were closely attached without a hydrophilic filtration membrane, the faradaic efficiency of formic acid was approximately 45%, as shown in the graph on the left side of Figure 8. On the other hand, when a hydrophilic filtration membrane was placed between the proton exchange membrane and anode membrane, the faradaic efficiency of formic acid was over 90%, as shown in the graph on the right side of Figure 8. These results demonstrate that the faradaic efficiency of formic acid can be improved by placing a hydrophilic filtration membrane between the proton exchange membrane and anode membrane. This is thought to be because the use of a hydrophilic filtration membrane reduces the proton concentration at the cathode, making the reaction between protons and bicarbonate ions more likely to proceed faster than the reaction between protons and electrons.

[0063] [Example 3] The faradaic efficiency and production rate of formic acid and other products were compared between a serpentine flow plate and a lattice groove flow plate. Using each flow plate, the flow rate of a 3 mol / L KHCO3 aqueous solution was set to 4 mL / min, and the current density was set to 100 mA / cm. 2 The test was carried out with the reaction time set to 1 hour. Figure 9A is a perspective view showing an example of a serpentine flow plate as a cathode-side flow plate (a), a perspective view showing an example of a grid groove flow plate as a cathode-side flow plate (b), a graph showing a comparison of the Faraday efficiency of formic acid between a serpentine flow plate and a grid groove flow plate (c), and a graph showing a comparison of the product production rate between a serpentine flow plate and a grid groove flow plate (d). When a serpentine flow channel was used as the cathode flow plate, the faradaic efficiency of formic acid was approximately 70%, as shown in the graph on the left side of Figure 9B(c). When a lattice groove flow channel was used, the faradaic efficiency of formic acid was approximately 80%, as shown in the graph on the right side of Figure 9B(c). This is thought to be because the use of a lattice groove flow channel enabled rapid supply of bicarbonate ions to the proton exchange membrane, promoting the reaction between protons and bicarbonate ions in the proton exchange membrane. Comparing the production rates of each product when using a serpentine flow plate and a grid groove flow plate as the cathode flow plate, the hydrogen production rate was lower and the formic acid production rate was higher when using the grid groove flow plate (see the graph on the left in Figure 9B(d)). While a large amount of unreacted CO2 was emitted when using a conventional gas supply-type CO2 electrolysis cell, CO2 emissions were low when using either flow plate in this test. These results indicate that most (approximately 80%) of the CO2 generated inside the electrolysis cell (CO2 generated by the reaction of Equation (1) above) was used as a raw material for formic acid production, demonstrating excellent carbon utilization efficiency.

[0064] [Example 4] The faradaic efficiencies of formic acid and other products were compared when a serpentine flow channel and a lattice groove flow channel were used as the flow plate and the flow rate of the bicarbonate aqueous solution was changed. Using each flow plate, the flow rates of a 3 mol / L KHCO3 aqueous solution were set to 4, 16, and 32 mL / min, and the current density was set to 100 mA / cm. 2 The test was carried out with the reaction time set to 1 hour. Figure 10 is a graph comparing (a) the Faradaic efficiency of formic acid and other products when the flow rate of the bicarbonate aqueous solution is changed when a serpentine flow channel is used as the cathode-side flow plate, and (b) the Faradaic efficiency of formic acid and other products when the flow rate of the bicarbonate aqueous solution is changed when a lattice groove flow channel is used as the cathode-side flow plate. As shown in Figure 10(a), when a serpentine flow channel was used as the cathode flow plate, the faradaic efficiency of formic acid improved as the flow rate of the bicarbonate solution increased. Similarly, as shown in Figure 10(b), when a grid groove flow channel was used as the cathode flow plate, the faradaic efficiency of formic acid also improved as the flow rate of the bicarbonate solution increased. This is thought to be because a sufficient amount of bicarbonate ions was quickly supplied to the hydrophilic filtration membrane to react with the protons supplied from the proton exchange membrane, which efficiently generated CO2 inside the electrolysis cell and promoted the production of formic acid through the reduction reaction of CO2. 10(a) and (b), it was found that a higher flow rate of the bicarbonate aqueous solution suppressed the by-production of hydrogen. This is thought to be because the protons supplied from the proton exchange membrane reacted quickly with a sufficient amount of bicarbonate ions supplied to the hydrophilic filtration membrane, suppressing the supply of protons to the cathode, thereby suppressing the side reaction of hydrogen production caused by the reaction of protons with electrons at the cathode.

[0065] [Example 5] The faradaic efficiency of formic acid and other products was compared when the current density was changed. A grid-groove flow plate was used, and the flow rate of a 3 mol / L KHCO3 aqueous solution was set to 32 mL / min. The current densities were set to 50, 100, 200, and 300 mA / cm. 2 The test was carried out with the reaction time set to 1 hour. FIG. 11 is a graph showing the faradaic efficiency of formic acid and other products when the current density is changed. Current density 50mA / cm 2 , and 100mA / cm 2 In the case of 200mA / cm, the faradaic efficiency of formic acid was over 90%. 2 , and 300mA / cm 2 Even in this case, the faradaic efficiency of formic acid was maintained at 80% or more. These results demonstrate that formic acid can be produced with higher selectivity than conventional methods, even when the current density is changed.

[0066] [Example 6] The faradaic efficiency of formic acid was compared between a conventional gas-supply type CO2 electrolysis cell having a gas diffusion electrode and the electrolysis cell of the present invention. The conventional gas supply type CO2 electrolysis cell used a three-chamber cell consisting of a gas flow chamber, a cathode chamber, and an anode chamber. The flow rate of CO2 gas in the gas flow chamber was 50 mL / min, and the flow rate of 3 mol / L KHCO3 aqueous solution in the cathode chamber was 4 mL / min. In the electrolysis cell of the present invention, the flow rate of the 3 mol / L KHCO3 aqueous solution was 32 mL / min. In both cases, the current density was 100 mA / cm. 2 The test was carried out with the reaction time set to 1 hour. FIG. 12 is a graph comparing the faradaic efficiency of formic acid and other products between a conventional gas-fed CO electrolysis cell having a gas diffusion electrode and the electrolysis cell of the present invention. A comparison of the faradaic efficiencies of a conventional gas supply-type CO electrolysis cell having a gas diffusion electrode and the electrolysis cell of the present invention in this test showed that, as shown in the graph on the right side of Figure 12, the faradaic efficiencies of the conventional gas supply-type CO electrolysis cell having a gas diffusion electrode and the electrolysis cell of the present invention in this test were almost equivalent, indicating that formic acid could be produced with equivalent selectivity.

[0067] [Example 7] A 3 mol / L KHCO3 aqueous solution was used, and the current was 100 mA / cm 2 The durability of the electrolytic cell was confirmed when the reaction was carried out for 30 hours at a current density of 1000 kJ / s. A grid-grooved flow plate was used, and iridium oxide was used as the oxygen-evolving catalyst for the anode membrane. Pure water was supplied to the anode membrane. The flow rate of the electrolyte supplied from the cathode membrane side was 32 mL / min, and the bicarbonate ion concentration was 3 mol / L. The electrolyte was circulated rather than continuously supplied with fresh 3 mol / L bicarbonate solution. Specifically, as shown in Figure 3(b), a tank containing a 3 mol / L KHCO3 solution was installed on the cathode side. The solution in the tank was supplied to the electrolysis cell, the generated formic acid solution was discharged into the tank, and the bicarbonate solution containing the formic acid solution in the tank was again supplied to the electrolysis cell. The system was designed so that the amount of formic acid produced could be determined by measuring the formic acid concentration in the tank after a certain period of time. The current density was 100 mA / cm. 2 The test was carried out with the reaction time set to 30 hours. Figure 13 shows the results of a 3 mol / L KHCO3 aqueous solution at 100 mA / cm 2 1 is a graph showing the faradaic efficiency of formic acid and the cell voltage when reacted for 30 hours at a current density of 1000 kJ / s. As shown in Figure 13, formic acid could be produced continuously for 30 hours at a constant cell voltage (3.1 V). Conventional gas-supply CO2 electrolysis cells face challenges in stably running CO2 reduction reactions over long periods of time due to factors such as a flooding phenomenon, in which electrolyte in the cathode chamber penetrates the gas diffusion electrode due to a decrease in the hydrophobicity of the gas diffusion electrode, and the precipitation of carbonates on the flow plate, which inhibits the supply of CO2 gas. The electrolysis cell of the present invention does not encounter these problems and therefore exhibits high stability even over long periods of time. The faradaic efficiency also remained above 80%. Although the faradaic efficiency appears to have dropped from the 90% range to the 80% range after approximately 10 hours, this is presumably due to the use of a circulating 3 mol / L KHCO3 aqueous solution, which reduces the amount of bicarbonate ions supplied to the proton exchange membrane as bicarbonate ions are used to produce formic acid, resulting in a drop in the faradaic efficiency from 90% to 80%. It is estimated that this is because, by using a continuous system that constantly supplies a 3 mol / L potassium bicarbonate aqueous solution rather than a circulating system, the device has the durability to maintain a faradaic efficiency of 90%.

[0068] Figure 14 shows the results of a 3 mol / L KHCO3 aqueous solution at 100 mA / cm 2 1 is a graph showing the molar concentration and weight fraction of formic acid in the aqueous formic acid solution produced when the reaction was carried out for 30 hours at a current density of 1000 kJ / cm 2 . In this test, as shown in Figure 14, a 2 mol / L formic acid solution was produced from a 3 mol / L KHCO3 aqueous solution after 30 hours. The bicarbonate ion conversion rate was approximately 70%, indicating that most of the bicarbonate ions, which are the carbon source, were converted into formic acid. The weight fraction of formic acid in the resulting aqueous solution was 9 wt%, indicating that a high concentration of formic acid can be accumulated by using a circulating system.

[0069] [Example 8] The electrolysis cell of the present invention and the prior art electrolysis cell are compared in terms of catalyst type, faradaic efficiency of formic acid, and 100 mA / cm 2 The cell voltages at the current densities were compared. The type of catalyst, the faradaic efficiency of formic acid, and the cell voltage of the prior art electrolytic cell were determined from the values ​​described in the literature. The electrolytic cell of the present invention was one that produced formic acid under the same conditions as in Example 7. FIG. 15 shows the relationship between the type of catalyst, the faradaic efficiency of formic acid, and the 100 mA / cm electrolysis efficiency of the electrolysis cell of the present invention and the bicarbonate electrolysis cell of the prior art. 2 1 is a table comparing cell voltages in When comparing the electrolytic cell of the present invention with a bicarbonate electrolytic cell of the prior art, the electrolytic cell of the present invention can produce formic acid with a selectivity much higher than that of the prior art, which is 100 mA / cm 2 The cell voltage, which previously required about 4V, is now 100mA / cm 2 It was found that the cell voltage could be suppressed to about 3 V. While the conventional technology uses a bipolar membrane, the present invention uses a proton exchange membrane, which significantly reduces the cell voltage. [Industrial Applicability]

[0070] According to the present invention, it is possible to provide an electrolytic cell for producing formic acid that suppresses the by-production of hydrogen, is useful as an energy carrier, and has excellent energy efficiency, a method for producing formic acid using the same, a fuel cell that uses the obtained formic acid, and a method for producing the same. [Explanation of symbols]

[0071] 1 unit membrane 2 power supply 3. Anode film 4. Proton Exchange Membrane 5 Hydrophilic filtration membrane 6. Cathode membrane 7, 8 Flow plates 9, 10 Current collector 11, 12 End plates 13A Anode supply port 13B Anode outlet 14A cathode supply port 15B Cathode outlet 20 Fuel Cell 21 Anode 22 Diffusion layer 23 Electrolyte membrane 24 Catalyst 25 cathode 30 systems

Claims

1. An electrolysis cell comprising a unit membrane in which a porous anode membrane, a non-porous proton exchange membrane, a porous hydrophilic filtration membrane, and a porous and hydrophilic cathode membrane are stacked in this order, The electrolysis cell, wherein the hydrophilic filtration membrane and the cathode membrane are in communication with each other via pores.

2. The anode membrane generates protons and electrons by electrolysis of an oxidizable substance, supplies the protons to the proton exchange membrane, and supplies the electrons to the cathode membrane; The hydrophilic filtration membrane reacts the protons supplied from the proton exchange membrane with bicarbonate ions to produce CO ２ and water, The cathode membrane is a membrane that absorbs CO generated by the hydrophilic filtration membrane. ２ with electrons supplied from the anode membrane to produce formic acid.

3. The bicarbonate ion is CO ２ is supplied to the hydrophilic filtration membrane from the cathode membrane side as an alkaline absorption solution, 3. The electrolytic cell of claim 2, wherein the concentration of the bicarbonate ions is 3 mmol / L to 3 mol / L.

4. 2. The electrolysis cell of claim 1, wherein the thickness of the hydrophilic filtration membrane is greater than 0.1 μm and not greater than 5 mm.

5. 2. The electrolysis cell according to claim 1, further comprising: a cathode-side flow plate having a flow path for flowing an aqueous bicarbonate solution on the side of the cathode membrane opposite to the hydrophilic filtration membrane side.

6. 6. The electrolysis cell of claim 5, wherein the flow channels are serpentine channels, grid groove channels, or parallel channels.

7. A method for producing formic acid using the electrolytic cell according to any one of claims 1 to 6.

8. CO ２ and recovering the produced aqueous formic acid solution.

9. CO ２ 8. The method for producing formic acid according to claim 7, wherein the method is a circulation method in which the alkaline absorption liquid is circulated to produce formic acid, or a continuous method in which the alkaline absorption liquid is continuously supplied without being circulated.

10. 8. Use of a formic acid-containing composition obtained by the method for producing formic acid according to claim 7 as a fuel for a fuel cell.

11. A fuel cell using, as fuel, formic acid obtained by the method for producing formic acid according to claim 7.

12. A system comprising the electrolysis cell of claim 1 and the fuel cell of claim 11.

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