Carbon dioxide gas phase reduction device

Abstract

A gas phase reduction apparatus 1 for carbon dioxide reduces carbon dioxide in a gas phase, and comprises: an oxidation tank 101 including an oxidation electrode and a reference electrode; a reduction tank 102 to which carbon dioxide is supplied; a composite 105 which is disposed between the oxidation tank and the reduction tank, and in which an electrolyte film and a reduction electrode are joined to each other, and the electrolyte film is disposed on the oxidation tank side and the reduction electrode is disposed on the reduction tank side; and a switch 106 that connects the reduction electrode to the oxidation electrode or to the reference electrode. Under control of a control device or a user, the switch connects the reduction electrode to the oxidation electrode, connects the reduction electrode to the reference electrode if the decrease rate of the concentration of a product generated by a reduction reaction of carbon dioxide at the reduction electrode becomes higher than a first threshold, and connects the reduction electrode to the oxidation electrode if the change level of the electric potential difference between the reduction electrode and the reference electrode becomes lower than a second threshold.

Description

[Technical Field]

[0001] This disclosure relates to a gas-phase reduction apparatus for carbon dioxide. [Background technology]

[0002] Artificial photosynthesis is a technique that uses light irradiation on an oxidation electrode made of a photocatalyst to promote the oxidation of water and the reduction of carbon dioxide. Similarly, electrolytic reduction of carbon dioxide is a technique that uses voltage applied between an oxidation electrode and a reduction electrode made of metal to promote the oxidation of water and the reduction of carbon dioxide.

[0003] Artificial photosynthesis technology using sunlight and electrolytic reduction technology using electricity derived from renewable energy can recycle carbon dioxide into hydrocarbons such as carbon monoxide, formic acid, and ethylene, as well as alcohols such as methanol and ethanol.

[0004] Conventionally, as described in Non-Patent Documents 1 and 2, artificial photosynthesis technology and electrolytic reduction technology for carbon dioxide have used reaction systems in which a reduction electrode is immersed in an aqueous solution, and carbon dioxide dissolved in the solution is supplied to the reduction electrode for reduction. However, this method of reducing carbon dioxide has limitations in the solubility of carbon dioxide in the aqueous solution and the diffusion coefficient of carbon dioxide in the aqueous solution, which limits the amount of carbon dioxide that can be supplied to the reduction electrode.

[0005] To address this problem, research is underway to increase the supply of carbon dioxide to the reduction electrode by supplying gaseous carbon dioxide to the reduction electrode. According to Non-Patent Literature 3, by using a reactor with a structure that can supply gaseous carbon dioxide to the reduction electrode, the supply of carbon dioxide to the reduction electrode is increased, and the reduction reaction of carbon dioxide is promoted. [Prior art documents] [Non-patent literature]

[0006] [Non-Patent Document 1] Satoshi Yotsuhashi and 6 others, “CO2Conversion with Light and Water by GaN Photoelectrode”, Japanese Journal of Applied Physics, 51, 2012, p.02BP07-1-p.02BP07-3 [Non-Patent Document 2] Yoshio Hori and others, “Formation of Hydrocarbons in the Electrochemical Reduction of Carbone Dioxide at a Copper Electrode in Aqueous Solution”, Journal of the Chemical Society, 85(8), 1989, p.2309-p.2326 [Non-Patent Document 3] Qingxin Jia and 2 others, “Direct Gas-phase CO2 Reduction for Solar Methane Generation Using a Gas Diffusion Electrode with a BiVO4:Mo and a Cu-In-Se Photoanode”, Chemistry Letter, 47, 2018, p.436-p.439 [Overview of the project] [Problems that the invention aims to solve]

[0007] In a conventional gas-phase carbon dioxide reduction apparatus, the oxidation reaction of water shown in equation (1) proceeds in the oxidation tank. In the reduction tank, the reduction reaction of carbon dioxide shown in equations (2) to (5) proceeds in combination with the oxidation reaction of water in the oxidation tank.

[0008] 2H2O+4h + →O2+4H + ...(1) CO2 + 2H + +2e - →CO+H2O ···(2) CO2 + 2H + +2e- →HCOOH ···(3) CO2 + 6H + + 6e - →CH3OH + H2O ···(4) CO2 + 8H + + 8e - →CH4 + 2H2O ···(5) In order to realize the above reduction reaction of carbon dioxide, a three-phase interface of [electrolyte - reduction electrode - carbon dioxide] is required between the oxidation tank and the reduction tank. Since protons moving from the oxidation tank to the reduction tank through the electrolyte cannot move in the gas phase in the reduction tank, the electrolyte and the reduction electrode are brought into contact (bonded) with each other. By directly supplying gaseous carbon dioxide to the interface between this electrolyte and the reduction electrode, the reduction reaction of carbon dioxide proceeds.

[0009] At this time, in order to stabilize the concentration of carbon dioxide in the reduction tank, if the reduction reaction is started in a state where carbon dioxide is previously supplied to the interface between the electrolyte and the reduction electrode, carbon dioxide is adsorbed at a high concentration on the interface at the initial stage of the reaction, and the reduction reaction of carbon dioxide can proceed with high efficiency.

[0010] However, as the reduction reaction progresses, the carbon dioxide in contact with the interface is gradually consumed, so the contact rate (adsorption rate) of carbon dioxide at the interface decreases, and the hydrogen generation reaction shown in formula (6) preferentially proceeds as a side reaction, resulting in the problem that the reduction reaction efficiency of carbon dioxide decreases.

[0011] 2H + + 2e - →H2···(6) The present disclosure has been made in view of the above circumstances, and the object of the present disclosure is to provide a technology capable of improving the reduction reaction efficiency of carbon dioxide in a gas-phase reduction device for carbon dioxide.

Means for Solving the Problems

[0012] A gas-phase reduction apparatus for carbon dioxide according to one aspect of the present disclosure is a gas-phase reduction apparatus for carbon dioxide that reduces gaseous carbon dioxide, comprising: an oxidation tank including an oxidation electrode and a reference electrode; a reduction tank to which carbon dioxide is supplied; a composite disposed between the oxidation tank and the reduction tank, wherein an electrolyte membrane and a reduction electrode are joined to each other, the electrolyte membrane is located on the oxidation tank side and the reduction electrode is located on the reduction tank side; and a switch for connecting the reduction electrode to the oxidation electrode or the reference electrode, wherein the switch, by a control device or a user, connects the reduction electrode to the oxidation electrode, connects the reduction electrode to the reference electrode when the rate of decrease in the concentration of the product produced by the reduction reaction of carbon dioxide at the reduction electrode exceeds a first threshold, and connects the reduction electrode to the oxidation electrode when the amount of change in the potential difference between the reduction electrode and the reference electrode falls below a second threshold. [Effects of the Invention]

[0013] According to this disclosure, a technology is available that can improve the efficiency of the carbon dioxide reduction reaction in a gas-phase carbon dioxide reduction device. [Brief explanation of the drawing]

[0014] [Figure 1] Figure 1 is a configuration diagram showing the overall system configuration according to Example 1. [Figure 2] Figure 2 is a flowchart illustrating the operation method of a gas-phase carbon dioxide reduction device. [Figure 3] Figure 3 is a configuration diagram showing the overall system configuration according to Example 2. [Figure 4] Figure 4 is a diagram showing the overall configuration of the system related to Comparative Example 1. [Figure 5] Figure 5 is a diagram showing the overall configuration of the system related to Comparative Example 2. [Figure 6] Figure 6 shows the time evolution of the Faraday efficiency of the carbon dioxide reduction reaction in Examples 1 and 2 and Comparative Examples 1 and 2. [Modes for carrying out the invention]

[0015] The embodiments of this disclosure will be described below with reference to the drawings. This disclosure is not limited to the embodiments described herein and can be modified without departing from the spirit of this disclosure. It is also possible to combine the embodiments.

[0016] [Summary of this disclosure] The carbon dioxide gas phase reduction apparatus described herein stops the carbon dioxide reduction reaction when the carbon dioxide reduction reaction efficiency drops to a certain level, provides a waiting period to saturate the reaction field with carbon dioxide, and then restarts the carbon dioxide reduction reaction after the carbon dioxide has saturated the field, repeating this process.

[0017] This allows the light or electrical energy supplied during the oxidation-reduction reaction to be efficiently used for the reduction of carbon dioxide for an extended period without being wasted on the side reactions shown in equation (6). Furthermore, energy not consumed in the oxidation-reduction reaction during waiting times can be stored or used for other purposes, enabling efficient energy utilization.

[0018] [Example 1] Figure 1 is a configuration diagram showing the overall configuration of System 1 according to Example 1. System 1 comprises a carbon dioxide gas phase reduction device 10, a concentration measuring device 20, an electrochemical measuring device 30, and a control device 40.

[0019] (Configuration of the carbon dioxide gas phase reduction device 10) The carbon dioxide gas-phase reduction apparatus 10 according to Example 1 is a device that performs artificial photosynthesis as described in the background art section. Specifically, the carbon dioxide gas-phase reduction apparatus 10 is a device that irradiates an oxidation electrode in an oxidation tank with light and performs a reduction reaction of gaseous carbon dioxide at a reduction electrode in a reduction tank.

[0020] The carbon dioxide gas-phase reduction apparatus 10 according to Example 1 comprises an oxidation tank 101 and a reduction tank 102, which are formed by dividing the internal space of a single housing into two, as shown in Figure 1. The oxidation tank 101 is filled with a predetermined aqueous solution 103. An oxidation electrode 104 is inserted into the aqueous solution 103. Carbon dioxide or a gas containing carbon dioxide is supplied to the reduction tank 102, which is adjacent to the oxidation tank 101.

[0021] A reduction electrode / electrolyte membrane composite (composite) 105 is positioned between the oxidation tank 101 and the reduction tank 102, with an electrolyte membrane 105a and a reduction electrode 105b in contact (joined) with each other. The electrolyte membrane 105a is positioned on the oxidation tank 101 side, and the reduction electrode 105b is positioned on the reduction tank 102 side.

[0022] The oxidizing electrode 104 and the reducing electrode 105b are connected by a wire via a switch 106. A reference electrode 107 is also inserted into the aqueous solution 103 in the oxidation tank 101. The reference electrode 107 and the reducing electrode 105b are connected by a wire via a switch 106 and a voltmeter 108. The switch 106 connects the reducing electrode 105b to either the oxidizing electrode 104 or the reference electrode 107. The voltmeter 108 is also connected to the control device 40 so that the control device 40 can control the switch 106 using the potential difference measured by the voltmeter 108.

[0023] To operate the gas-phase carbon dioxide reduction device 10, a light source 109 is positioned opposite the oxidation electrode 104.

[0024] Aqueous solution 103 is, for example, a potassium bicarbonate aqueous solution, a sodium bicarbonate aqueous solution, a potassium chloride aqueous solution, a sodium chloride aqueous solution, a potassium hydroxide aqueous solution, a sodium hydroxide aqueous solution, a rubidium hydroxide aqueous solution, or a cesium hydroxide aqueous solution.

[0025] The oxide electrode 104 is, for example, a nitride semiconductor, titanium oxide, or amorphous silicon. The oxide electrode 104 may also be a compound that exhibits photoactivity or redox activity, such as a ruthenium complex or a rhenium complex.

[0026] The electrolyte membrane 105a may be, for example, Nafion (trademark registered), Foablue, or Aquivion, which are electrolyte membranes having a carbon-fluorine skeleton. The electrolyte membrane 105a may also be Celemion or Neosepta, which are electrolyte membranes having a hydrocarbon skeleton.

[0027] The reducing electrode 105b may be, for example, copper, platinum, gold, silver, indium, palladium, gallium, nickel, tin, or cadmium. The reducing electrode 105b may also be a porous material of an alloy thereof. The reducing electrode 105b may also be a porous material of silver oxide, copper oxide, copper(II) oxide, nickel oxide, indium oxide, tin oxide, tungsten oxide, tungsten(VI) oxide, or copper oxide. The reducing electrode 105b may also be a porous metal complex having a metal ion and an anionic ligand.

[0028] Switch 106 is, for example, a switch circuit that selects and switches the connection destination, or an on / off circuit that turns an electrical or physical connection state on or off. Switch 106 may be controlled by the control device 40 or by the user. Switch 106 may have the control device 40 built into it.

[0029] The reference electrode 107 is, for example, a silver-silver chloride electrode, a silver-silver ion electrode, a standard hydrogen electrode, a reversible hydrogen electrode, a calomel electrode, a mercury-mercury oxide electrode, or a mercury-mercury sulfide electrode.

[0030] The light source 109 may be, for example, a xenon lamp, a pseudo-sunlight source, a halogen lamp, a mercury lamp, or sunlight. The light source 109 may also be composed of a combination of these.

[0031] (Functions of the concentration measuring device 20) The concentration measuring device 20 is connected to a gas output port 112 formed at the top of the reduction tank 102 by gas piping (not shown). The concentration measuring device 20 is a device for measuring the concentration of products generated by the reduction reaction in the reduction tank 102. The concentration measuring device 20 can be, for example, a gas chromatograph, a gas chromatograph-mass spectrometer, a liquid chromatograph, a semiconductor gas concentration sensor, or a gas concentration detection tube that utilizes a chemical reaction. The concentration measuring device 20 may also be configured by combining these components.

[0032] (Functions of the electrochemical measuring device 30) The electrochemical measuring device 30 is connected to a conductor connected to the oxidation electrode 104. The electrochemical measuring device 30 measures the current value during the reduction reaction (when the switch 106 is connected to the connection end T1 on the oxidation electrode side). The measured current value is used to calculate the Faraday efficiency of the reduction reaction and as the current value explained in "Modification 2 of Example 1" below.

[0033] (Functions of the control device 40) The control device 40 controls the switch 106 based on the concentration of the product measured by the concentration measuring device 20 and the current value measured by the electrochemical measuring device 30. The control device 40 is, for example, a computer device equipped with a CPU and memory.

[0034] (Method for fabricating the reduction electrode / electrolyte membrane composite 105) Next, the method for fabricating the reduction electrode / electrolyte membrane composite 105 will be described. In Example 1, a porous metal material with a thickness of 0.2 mm and a porosity of 80% was used as the reduction electrode 105b. In addition, Nafion, a cation exchange membrane, was used as the electrolyte membrane 105a.

[0035] Nafion was layered on top of a porous metal material and placed between two copper plates. This sample was then placed between thermocompression presses and a constant pressure was applied perpendicular to the porous electrode surface of the porous metal material at a heating temperature of 150°C for 3 minutes. After that, the sample was rapidly cooled and removed from the thermocompression press to obtain the reduction electrode / electrolyte membrane composite 105.

[0036] (Method for reducing carbon dioxide in the gas phase) Next, we will explain the method for reducing carbon dioxide in the gas phase.

[0037] The oxidation tank 101 was filled with a predetermined aqueous solution 103. A 1.0 mol / L potassium hydroxide aqueous solution was used as the aqueous solution 103.

[0038] Gallium nitride (GaN) and aluminum gallium nitride (AlGaN), both n-type semiconductors, were epitaxially grown (thin film formation) in that order on a sapphire substrate. Nickel (Ni) was then vacuum deposited on top of these films and heat-treated to form a nickel oxide (NiO) co-catalyst thin film. The resulting substrate was used as the oxidation electrode 104, and this oxidation electrode 104 was placed in an oxidation tank 101 so as to be immersed in an aqueous solution 103.

[0039] Light source 109 is a 300W high-pressure xenon lamp (cuts wavelengths above 450nm, illuminance 2.2mW / cm²). 2 A light source 109 was used. The light source 109 was fixed so that the oxidation co-catalyst formation surface of the oxidation electrode 104 became the irradiation surface. The light irradiation area of ​​the oxidation electrode 104 was 2.3 cm². 2 That's what I decided.

[0040] Nitrogen (N) was flowed through tube 110 at a flow rate of 30 ml / min into the aqueous solution 103 in oxidation tank 101. At the same rate, carbon dioxide (CO2) was flowed through gas inlet 111 into reduction tank 102. Then, oxidation tank 101 was thoroughly replaced with nitrogen for more than 15 hours, and reduction tank 102 was thoroughly replaced with carbon dioxide for more than the same amount of time. After that, light from light source 109 was uniformly irradiated onto oxidation electrode 104.

[0041] In this state, switch 106 is controlled according to the flowchart shown in Figure 2. Here, we will explain the case where the control device 40 controls switch 106.

[0042] Step S1; The control device 40 selects the connection terminal T1 on the oxidation electrode side as the connection destination for switch 106, and connects the oxidation electrode 104 and the reduction electrode 105b. This allows the reduction reaction of carbon dioxide to proceed at the reduction electrode 105b.

[0043] Step S2; Next, the concentration of the product generated by the reduction reaction of carbon dioxide is measured using the concentration measuring device 20. In Example 1, a gas chromatograph was used as the concentration measuring device 20, and the concentration of the product was measured every 30 minutes, detecting only carbon monoxide (CO) as the product. In addition, the current value between the oxidation electrode 104 and the reduction electrode 105b is measured using the electrochemical measuring device 30.

[0044] Step S3; Next, the control device 40 calculates the rate of decrease in the concentration of the product (CO concentration) being measured and detected, and determines whether the rate of decrease in that concentration exceeds the first threshold X. The first threshold X is the rate of decrease (%) of the concentration at which the product concentration can be considered to have started to decrease or decline. Specific examples will be explained below.

[0045] In Example 1, the rate of decrease from the initial value of the CO concentration was calculated using equation (7). Q0 is the initial value of the CO concentration. Q is the CO concentration at any given time.

[0046] The rate of decrease in CO concentration from its initial value = (Q0 - Q) × 100 / Q0 ... (7) Furthermore, in Example 1, the decrease in CO concentration was indicated by the slope E of the concentration change over time. Q The determination was made using Q, where Q is the CO concentration at any given time t. t Q is the CO concentration at time (t+Δt). t+Δt Therefore, from equation (8), E Q The result was calculated.

[0047] E Q =(Q t+Δt -Q t ) × 100 / Δt ... (8) E when the CO concentration is considered to have shifted to a decreasing trend based on the time change of the CO concentration measured in advance. Q The value was -7.3 (ppm / h). From this, this E Q In Example 1, 10% was determined as the first threshold X to be a value that could detect the condition.

[0048] The method for determining the first threshold X is not limited to the method described above. The first threshold X only needs to be a value that can detect when the CO concentration begins to decrease or decline. For example, it may be determined based on the change from the average value of the CO concentration, by fitting the change in CO concentration over time, or by using the user's experience.

[0049] If the rate of decrease in concentration does not exceed the first threshold X, the process returns to step S2.

[0050] Step S4; Next, if the rate of decrease in the concentration of the product (CO concentration) being measured and detected exceeds the first threshold X, the control device 40 disconnects the switch 106 from the oxidizing electrode side connection terminal T1, selects the reference electrode side connection terminal T2, and connects the reduction electrode 105b and the reference electrode 107.

[0051] Here, by operating switch 106, the reduction reaction at the reduction electrode 105b is temporarily stopped and the supply of carbon dioxide is continued. As a result, the reaction field becomes saturated with carbon dioxide, and the chemical potential of the electrode changes. Therefore, from here on, this change is measured as the potential difference from the reference electrode 107. This potential difference decreases as the reaction field becomes saturated with carbon dioxide, and this point is utilized.

[0052] Step S5; Next, the potential difference between the reference electrode 107 and the reduction electrode 105b is measured using the voltmeter 108. A silver-silver chloride electrode was used as the reference electrode 107.

[0053] Step S6; Next, the control device 40 calculates the change in potential difference between the reference electrode 107 and the reduction electrode 105b, and determines whether the change in potential difference falls below a second threshold Y. The second threshold Y is the change in potential difference (V) at which the potential difference between the reference electrode 107 and the reduction electrode 105b can be considered stable. A specific example will be explained below.

[0054] In Example 1, the stability of the absolute value of the potential difference |ΔV| was determined by the slope F of the time change of the potential difference. V The determination was made using the following: The potential difference at any time t is V t The potential difference at time (t+Δt) is V t+Δt F V This was calculated using equation (9).

[0055] F V =(V t+Δt -V t ) × 100 / Δt ... (9) Based on the time change of the potential difference measured in advance, F is the value at which the change in potential difference can be considered stable. V The value was 0.01 (V / s). Therefore, in Example 1, 0.01 (V) was determined as the second threshold Y.

[0056] The method for determining the second threshold Y is not limited to the method described above. The second threshold Y can be any value that allows for the detection of a stable change in the potential difference, and may be determined by fitting the time change of the voltage value, or by determining it based on the user's experience.

[0057] Subsequently, if the result of the determination in step S6 indicates that the change in potential difference between the reference electrode 107 and the reduction electrode 105b falls below the second threshold Y, the process returns to step S1. In step S1, the control device 40 disconnects the connection of switch 106 to the reference electrode side connection terminal T2, selects the oxidation electrode side connection terminal T1, and connects the oxidation electrode 104 and the reduction electrode 105b. This restarts the reduction reaction of carbon dioxide. On the other hand, if the change in potential difference does not fall below the second threshold Y, the process returns to step S5.

[0058] The reaction was stopped when the total reaction time due to the flow of electrons between the oxidizing electrode 104 and the reducing electrode 105b reached 100 hours.

[0059] (Modification 1 of Example 1) Switch 106 can be operated by a person visually checking the product concentration and voltage value and making a judgment to switch it, or the concentration and voltage value can be automatically transmitted to a computer and controlled automatically using a control circuit or control device.

[0060] (Modification 2 of Example 1) In Example 1, since the change in current value between the oxidation electrode 104 and the reduction electrode 105b was within ±2%, the decrease rate of the product concentration was simply used instead of using the current value. However, when the input energy changes, such as in sunlight, and there is a large change in the current value, it is preferable to use the decrease rate of the normalized product concentration, which is obtained by dividing the product concentration by the current value. In this case, the electrochemical measuring device 30 and the control device 40 are connected to each other, and the control device 40 performs a calculation by dividing the product concentration measured by the concentration measuring device 20 by the current value measured by the electrochemical measuring device 30. By normalizing the product concentration with the current value, the decrease in Faraday efficiency can be accurately detected.

[0061] [Example 2] Figure 3 is a configuration diagram showing the overall configuration of System 1 according to Example 2. System 1 has the same configuration as in Example 1.

[0062] (Overall structure) The carbon dioxide gas-phase reduction apparatus 10 according to Example 2 is an electrolytic carbon dioxide reduction apparatus as described in the background art section. Specifically, the carbon dioxide gas-phase reduction apparatus 10 is an apparatus that applies a voltage between an oxidation electrode and a reduction electrode, and carries out the reduction reaction of gaseous carbon dioxide at the reduction electrode in the reduction tank.

[0063] The gas-phase carbon dioxide reduction apparatus 10 according to Example 2 includes a power supply 113 connected between the oxidation electrode 104 and the reduction electrode 105b, as shown in Figure 3, instead of the light source 109 shown in Figure 1. The oxidation electrode 104 according to Example 2 is, for example, platinum, gold, silver, copper, indium, nickel, zinc, tin, or lead. The oxidation electrode 104 may also be an oxide of these materials. The other components are the same as in Example 1.

[0064] (Method for reducing carbon dioxide in the gas phase) The oxidation tank 101 was filled with a predetermined aqueous solution 103. A 1.0 mol / L potassium hydroxide aqueous solution was used for the aqueous solution 103. The oxidation electrode 104 was placed with a surface area of ​​approximately 0.4 cm². 2 The device was placed inside the oxidation tank 101 so that it was immersed in the aqueous solution 103. Platinum was used for the oxidation electrode 104.

[0065] Helium (He) was flowed through tube 110 at a flow rate of 30 ml / min into the aqueous solution 103 in oxidation tank 101. At the same time, carbon dioxide (CO2) was flowed through gas inlet 111 into reduction tank 102 at the same flow rate. Then, oxidation tank 101 was thoroughly replaced with helium for 15 hours or more, and reduction tank 102 was thoroughly replaced with carbon dioxide for the same amount of time or more.

[0066] In this state, switch 106 is controlled according to the flowchart shown in Figure 2. The specific control method is the same as in Example 1. A brief explanation is provided here.

[0067] First, switch 106 is connected to the connection terminal T1 on the oxidation electrode side, and the oxidation electrode 104 and reduction electrode 105b are connected. Then, the system is operated at a constant current of 3.0 mA using power supply 113, and the reduction reaction of carbon dioxide proceeds at reduction electrode 105b. The concentration of the product in the reduction tank 102 was measured every 30 minutes using the concentration measuring device 20. In Example 2, a gas chromatograph was also used as the concentration measuring device 20, and only carbon monoxide (CO) was detected as the product.

[0068] Next, when the rate of decrease in the detected CO concentration exceeds the first threshold X, the connection terminal T1 on the oxidation electrode side of switch 106 is switched off and connected to the connection terminal T2 on the reference electrode side, thereby connecting the reference electrode 107 and the reduction electrode 105b. Then, the potential difference between the reference electrode 107 and the reduction electrode 105b is measured using the voltmeter 108. A silver-silver chloride electrode was used for the reference electrode 107.

[0069] The rate of decrease from the initial value of the CO concentration was calculated using the above formula (7). E is calculated when the CO concentration can be considered to have shifted to a decreasing or declining trend based on the time change of the CO concentration measured in advance. Q It was -290 (ppm / h). From this, this E Q In Example 2, 10% was determined as the first threshold X to be a value that could detect the condition.

[0070] Subsequently, when the change in potential difference between the reference electrode 107 and the reduction electrode 105b falls below the second threshold Y, the connection terminal T2 on the reference electrode side of switch 106 is switched off and connected to the connection terminal T1 on the oxidation electrode side, thereby connecting the oxidation electrode 104 and the reference electrode 107. This restarts the carbon dioxide reduction reaction. The second threshold Y was set to 0.01 (V) for the same reasons as in Example 1.

[0071] The reaction was stopped when the total reaction time due to the flow of electrons between the oxidizing electrode 104 and the reducing electrode 105b reached 20 hours.

[0072] [Comparative Example 1] For comparison with Example 1, the gas-phase carbon dioxide reduction apparatus 10 shown in Figure 4 was used. Comparative Example 1 differs from Example 1 shown in Figure 1 in that it lacks the switch 106, the reference electrode 107, and the voltmeter 108. Also, the oxidation electrode 104 and the reduction electrode 105b are connected by a wire. The other configurations are the same as in Example 1. The gas-phase carbon dioxide reduction apparatus 10 according to Comparative Example 1 was operated to allow the oxidation-reduction reaction to proceed continuously, and was stopped when the total reaction time reached 100 hours.

[0073] [Comparative Example 2] For comparison with Example 2, the gas-phase carbon dioxide reduction apparatus 10 shown in Figure 5 was used. Unlike Example 2 shown in Figure 3, Comparative Example 2 lacks the switch 106, reference electrode 107, and voltmeter 108. Also, the oxidation electrode 104 and reduction electrode 105b are connected by a wire via the power supply 113. The other configurations are the same as in Example 2. The gas-phase carbon dioxide reduction apparatus 10 according to Comparative Example 2 was operated to allow the oxidation-reduction reaction to proceed continuously, and was stopped when the total reaction time reached 20 hours.

[0074] [Effects of Examples 1 and 2] Figure 6 shows the time evolution of the Faraday efficiency of the carbon dioxide reduction reaction in Examples 1 and 2 and Comparative Examples 1 and 2. Faraday efficiency, as shown in equation (10), represents the ratio of the total charge used in the reduction reaction to the total charge that flowed between the oxidation electrode 104 and the reduction electrode 105b during light irradiation or power supply voltage application.

[0075] Faraday efficiency of a reduction reaction = (Total charge consumed in the reduction reaction) / (Total charge flowing between the oxidizing electrode and the reducing electrode) ... (10) The "total charge consumed in the reduction reaction" (C) can be determined by converting the measured amount of reduction product produced into the number of electrons required for the reaction. Let A (ppm) be the concentration of the reduction reaction product, B (L / sec) be the flow rate of the carrier gas, Z (mol) be the number of electrons required for the reduction reaction, F (C / mol) be the Faraday constant, and V be the number of moles of the gas. m The reaction time was given as (L / mol) and t (sec), and the result was calculated using equation (11).

[0076] Total charge consumed in the reduction reaction = (A × B × Z × F × t × 10 -6 ) / V m ...(11) The higher the Faraday efficiency of this carbon dioxide reduction reaction, the more efficiently the electrons flowing between the oxidizing electrode 104 and the reducing electrode 105b are consumed in the carbon dioxide reduction reaction.

[0077] As shown in Figure 6, in Comparative Examples 1 and 2, the Faraday efficiency of carbon dioxide decreases in the initial stages of the reaction and then settles at a low value. On the other hand, in Examples 1 and 2, when the rate of decrease in Faraday efficiency exceeds a threshold, a waiting period is provided to saturate the reaction field with carbon dioxide, which allows the Faraday efficiency to recover to its initial value. By repeating this process, a high efficiency can be maintained.

[0078] Table 1 shows the Faraday efficiencies of the carbon dioxide reduction reaction in Examples 1 and 2 and Comparative Examples 1 and 2.

[0079] [Table 1] Table 1 shows that Examples 1 and 2 have higher Faraday efficiency for carbon dioxide reduction than Comparative Examples 1 and 2. This is likely because, in Comparative Examples 1 and 2, the amount of carbon dioxide in the reaction field gradually decreased as the reaction started, whereas in Examples 1 and 2, a waiting period was provided to saturate the carbon dioxide when the efficiency fell below a threshold. This allowed for sufficient carbon dioxide to be adsorbed in the reaction field at all times, enabling operation while maintaining high Faraday efficiency. As a result, the supplied light energy and electrical energy can be efficiently used for the carbon dioxide reduction reaction for a long period of time without being wasted on side reactions.

[0080] [Effects of this disclosure] In the first and second examples, the switch 106 is configured by the control device 40 or the user to connect the reduction electrode 105b to the oxidation electrode 104, connect the reduction electrode 105b to the reference electrode 107 when the rate of decrease in the concentration of the product generated by the reduction reaction of carbon dioxide at the reduction electrode 105b exceeds a first threshold X, and connect the reduction electrode 105b to the oxidation electrode 104 when the change in the potential difference between the reduction electrode 105b and the reference electrode 107 falls below a second threshold Y.

[0081] This allows the light and electrical energy supplied during the oxidation-reduction reaction to be efficiently used for the reduction of carbon dioxide for extended periods without being wasted on side reactions. Furthermore, energy not consumed during the oxidation-reduction reaction can be stored or used for other purposes during waiting times, enabling more efficient energy utilization.

[0082] In particular, in artificial photosynthesis technology utilizing solar energy, this technology can be effectively used when the reaction time and waiting time cycles described in Examples 1 and 2 are shorter than the solar radiation cycle of the land where the carbon dioxide gas phase reduction device 10 is used. [Explanation of symbols]

[0083] 1: System 10: Gas-phase reduction device for carbon dioxide 20: Concentration measuring device 30: Electrochemical measuring device 40: Control device 101: Oxidation tank 102: Reduction tank 103:Aqueous solution 104: Oxidation electrode 105: Reducing electrode / electrolyte membrane complex 105a: Electrolyte membrane 105b: Reduction electrode 106: Switch 107:Reference electrode 108: Voltmeter 109: Light source 110: Tube 111: Gas Inlet 112: Gas output port 113: Power supply

Claims

1. In a gas-phase carbon dioxide reduction apparatus that reduces gaseous carbon dioxide, An oxidation tank including an oxidation electrode and a reference electrode, A reduction tank to which carbon dioxide is supplied, A composite structure is placed between the oxidation tank and the reduction tank, with an electrolyte membrane and a reduction electrode joined to each other, the electrolyte membrane being located on the oxidation tank side and the reduction electrode being located on the reduction tank side. The system includes a switch for connecting the reduction electrode to the oxidation electrode or the reference electrode, The aforementioned switch is A gas-phase carbon dioxide reduction apparatus in which a control device or user connects the reduction electrode to the oxidation electrode, connects the reduction electrode to the reference electrode when the rate of decrease in the concentration of the product generated by the reduction reaction of carbon dioxide at the reduction electrode exceeds a first threshold, and connects the reduction electrode to the oxidation electrode when the amount of change in the potential difference between the reduction electrode and the reference electrode falls below a second threshold.

2. The first threshold is, The gas-phase carbon dioxide reduction apparatus according to claim 1, wherein the concentration of the product generated by the reduction reaction of carbon dioxide at the reduction electrode is considered to have begun to decrease.

3. The second threshold is, The gas phase reduction apparatus for carbon dioxide according to claim 1, wherein the potential difference between the reduction electrode and the reference electrode is a value that can be considered stable.

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