WO2024116235A1 - Gas phase reduction apparatus for carbon dioxide - Google Patents

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

Gas-phase carbon dioxide reduction equipment

This disclosure relates to a gas-phase carbon dioxide reduction device.

The technology of promoting the oxidation of water and the reduction of carbon dioxide by irradiating an oxidation electrode made of a photocatalyst with light is called artificial photosynthesis. The technology of promoting the oxidation of water and the reduction of carbon dioxide by applying a voltage between an oxidation electrode and a reduction electrode made of metal is called electrolytic reduction of carbon dioxide.

Artificial photosynthesis technology that uses sunlight and electrolytic reduction technology that uses electricity derived from renewable energy sources can recycle carbon dioxide into carbon monoxide, formic acid, hydrocarbons such as ethylene, and alcohols such as methanol and ethanol.

　Conventionally, as described in Non-Patent Documents 1 and 2, artificial photosynthesis technology and carbon dioxide electrolytic reduction technology have used a reaction system in which a reduction electrode is immersed in an aqueous solution and carbon dioxide dissolved in the aqueous solution is supplied to the reduction electrode for reduction. However, in this carbon dioxide reduction method, there are limitations to the concentration of carbon dioxide dissolved in the aqueous solution and the diffusion coefficient of carbon dioxide in the aqueous solution, which limits the amount of carbon dioxide supplied to the reduction electrode.

In order to address this issue and increase the amount of carbon dioxide supplied to the reduction electrode, research is being conducted into supplying gas-phase carbon dioxide to the reduction electrode. According to Non-Patent Document 3, by using a reaction device with a structure that can supply gas-phase carbon dioxide to the reduction electrode, the amount of carbon dioxide supplied to the reduction electrode is increased, and the reduction reaction of carbon dioxide is promoted.

Satoshi Yotsuhashi and six others, “CO2 Conversion with Light and Water by GaN Photoelectrode”, Japanese Journal of Applied Physics, 51, 2012, p.02BP07-1-p.02BP07-3 Yoshio Hori and two others, “Formation of Hydrocarbons in the Electrochemical Reduction of Carbon Dioxide at a Copper Electrode in Aqueous Solution”, Journal of the Chemical Society, 85(8), 1989, p.2309-p.2326 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 Letters, 47, 2018, p.436-p.439

In a conventional gas-phase carbon dioxide reduction device, the water oxidation reaction shown in formula (1) proceeds in the oxidation tank. In the reduction tank, the carbon dioxide reduction reactions shown in formulas (2) to (5) proceed in combination with the water oxidation reaction on the oxidation tank side.

_2H2O +4h ⁺ → _O2 +4H ⁺ ...(1)
CO ₂ + 2H ⁺ 2e ^- → CO + H ₂ O ... (2)
CO ₂ + 2H ⁺ + 2e ^- → HCOOH ... (3)
CO ₂ + 6H ⁺ + 6e ^- → CH ₃ OH + H ₂ O ... (4)
CO ₂ + 8H ⁺ + 8e ^- → CH ₄ + 2H ₂ O ... (5)
To realize the reduction reaction of carbon dioxide, a three-phase interface of [electrolyte - reduction electrode - carbon dioxide] is required between the oxidation cell and the reduction cell. Since protons that move from the oxidation cell to the reduction cell via the electrolyte cannot move in the gas phase in the reduction cell, the electrolyte and the reduction electrode are in contact (joined) with each other. The reduction reaction of carbon dioxide proceeds by directly supplying gas-phase carbon dioxide to the interface between the electrolyte and the reduction electrode.

In this case, if carbon dioxide is supplied to the interface between the electrolyte and the reduction electrode beforehand in order to stabilize the concentration of carbon dioxide in the reduction tank, the reduction reaction can be started with carbon dioxide adsorbed at a high concentration at the interface in the early stages of the reaction, allowing the carbon dioxide reduction reaction to proceed with high efficiency.

However, as the reduction reaction progresses, the carbon dioxide that comes into contact with the interface is gradually consumed, which reduces the contact rate (adsorption rate) of carbon dioxide at the interface and causes the hydrogen production reaction shown in formula (6) to proceed preferentially as a side reaction, resulting in a problem of reduced efficiency of the carbon dioxide reduction reaction.

2H ⁺ +2e ⁻ →H ₂ ... (6)
The present disclosure has been made in consideration of the above circumstances, and an object of the present disclosure is to provide a technique capable of improving the efficiency of the reduction reaction of carbon dioxide in a gas-phase reduction device of carbon dioxide.

In one embodiment of the present disclosure, the gas-phase carbon dioxide reduction device reduces gas-phase carbon dioxide, and includes 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, in which an electrolyte membrane and a reduction electrode are joined to each other, the electrolyte membrane is disposed on the oxidation tank side, and the reduction electrode is disposed on the reduction tank side, and a switch that connects the reduction electrode to the oxidation electrode or the reference electrode, and the switch is controlled by a control device or a user to connect the reduction electrode to the oxidation electrode, to connect the reduction electrode to the reference electrode when the rate of decrease in the concentration of a product generated by the reduction reaction of carbon dioxide at the reduction electrode exceeds a first threshold, and to connect the reduction electrode to the oxidation electrode when the change in the potential difference between the reduction electrode and the reference electrode falls below a second threshold.

This disclosure provides technology that can improve the efficiency of the carbon dioxide reduction reaction in a gas-phase carbon dioxide reduction device.

FIG. 1 is a diagram showing the overall configuration of a system according to a first embodiment. FIG. 2 is a flow chart showing a method of operating the gas phase reduction device for carbon dioxide. FIG. 3 is a diagram showing the overall configuration of a system according to the second embodiment. FIG. 4 is a configuration diagram showing the overall configuration of a system according to a first comparative example. FIG. 5 is a configuration diagram showing the overall configuration of a system according to the second comparative example. FIG. 6 is a graph showing the change over time in the Faraday efficiency of the reduction reaction of carbon dioxide in Examples 1 and 2 and Comparative Examples 1 and 2.

Below, an embodiment of the present disclosure will be described with reference to the drawings. The present disclosure is not limited to the examples described in the embodiments, and can be modified within the scope of the spirit of the present disclosure. Each example can also be combined.

[Summary of the Disclosure]
The gas-phase carbon dioxide reduction device according to the present disclosure stops the carbon dioxide reduction reaction when the efficiency of the carbon dioxide reduction reaction has decreased by a certain amount, provides a time (standby time) for saturating the reaction field with carbon dioxide, and then restarts the carbon dioxide reduction reaction after the carbon dioxide is saturated.

As a result, the light energy or electrical energy provided during the redox reaction can be used efficiently for a long period of time in the carbon dioxide reduction reaction, without wasting it in the side reactions shown in formula (6). In addition, the energy not consumed in the redox reaction during standby time can be stored or used for other purposes, making it possible to make effective use of energy.

[Example 1]
1 is a configuration diagram showing an overall configuration of a system 1 according to Example 1. The system 1 includes a gas-phase carbon dioxide reduction device 10, a concentration measurement device 20, an electrochemical measurement device 30, and a control device 40.

(Configuration of the carbon dioxide gas phase reduction device 10)
The gas-phase carbon dioxide reduction device 10 according to the first embodiment is a device that performs artificial photosynthesis as described in the Background Art section. Specifically, the gas-phase carbon dioxide reduction device 10 is a device that irradiates light onto an oxidation electrode in an oxidation tank and performs a reduction reaction of gas-phase carbon dioxide at a reduction electrode in the reduction tank.

As shown in FIG. 1, the gas-phase carbon dioxide reduction device 10 according to the first embodiment includes an oxidation tank 101 and a reduction tank 102 formed by dividing the internal space of a single housing in two. The oxidation tank 101 is filled with a specific 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 adjacent to the oxidation tank 101.

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

The oxidation electrode 104 and the reduction electrode 105b are connected by a wire via the 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 reduction electrode 105b are connected by a wire via the switch 106 and a voltmeter 108. The switch 106 connects the reduction electrode 105b to the oxidation electrode 104 or the reference electrode 107. The voltmeter 108 is also connected to the control device 40, as the control device 40 controls the switch 106 using the potential difference measured by the voltmeter 108.

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

The aqueous solution 103 is, for example, an aqueous solution of potassium bicarbonate, an aqueous solution of sodium bicarbonate, an aqueous solution of potassium chloride, an aqueous solution of sodium chloride, an aqueous solution of potassium hydroxide, an aqueous solution of sodium hydroxide, an aqueous solution of rubidium hydroxide, or an aqueous solution of cesium hydroxide.

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

The electrolyte membrane 105a is, for example, Nafion (registered trademark), Forblue, or Aquivion, which are electrolyte membranes having a carbon-fluorine skeleton. The electrolyte membrane 105a may also be Selemion or Neocepta, which are electrolyte membranes having a hydrocarbon skeleton.

The reduction electrode 105b is, for example, copper, platinum, gold, silver, indium, palladium, gallium, nickel, tin, or cadmium. The reduction electrode 105b may be a porous body of an alloy of these. The reduction electrode 105b may be a porous body of silver oxide, copper oxide, copper(II) oxide, nickel oxide, indium oxide, tin oxide, tungsten oxide, tungsten(VI) oxide, copper oxide, or the like. The reduction electrode 105b may be a porous metal complex having a metal ion and an anionic ligand.

The switch 106 is, for example, a switch circuit that switches the connection destination to be selectable, or an on-off circuit that turns the electrical or physical connection state on and off. The switch 106 may be controlled by the control device 40 or by the user. The switch 106 may be built into the control device 40.

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.

The light source 109 is, for example, a xenon lamp, a pseudo-sun light source, a halogen lamp, a mercury lamp, or sunlight. The light source 109 may be a combination of these.

(Functions of concentration measuring device 20)
The concentration measuring device 20 is connected to a gas output port 112 formed in the upper part of the reduction tank 102 by a gas pipe (not shown). The concentration measuring device 20 is a device that measures the concentration of a product generated by a reduction reaction in the reduction tank 102. The concentration measuring device 20 is, for example, a gas chromatograph, a gas chromatograph mass spectrometer, a liquid chromatograph, a semiconductor gas concentration sensor, or a gas concentration detector tube using a chemical reaction. The concentration measuring device 20 may be configured by combining these.

(Functions of the electrochemical measurement device 30)
The electrochemical measurement device 30 is connected to a conductor that is connected to the oxidation electrode 104. The electrochemical measurement device 30 measures a current value during the reduction reaction (while the connection destination of 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 a current value described later in "Modification 2 of Example 1".

(Functions of the control device 40)
The control device 40 is a device that 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 measurement device 30. The control device 40 is, for example, a computer device equipped with a CPU and a memory.

(Method of Producing Reduction Electrode/Electrolyte Membrane Composite 105)
Next, a method for producing the reduction electrode/electrolyte membrane composite 105 will be described. In Example 1, a metal porous body having a thickness of 0.2 mm and a porosity of 80% was used as the reduction electrode 105b. Furthermore, Nafion, which is a cation exchange membrane, was used as the electrolyte membrane 105a.

Nafion was layered on top of the metal porous body and placed between two copper plates. This sample was then placed between a thermocompression device (hot press) and left for three minutes with a constant pressure applied vertically to the porous electrode surface of the metal porous body at a heating temperature of 150°C. The sample was then quickly cooled and removed from the thermocompression device to obtain a reduction electrode/electrolyte membrane composite 105.

(Method for reducing carbon dioxide gas)
Next, a method for reducing carbon dioxide in the gas phase will be described.

The oxidation tank 101 is filled with a specific aqueous solution 103. A 1.0 mol/L aqueous solution of potassium hydroxide is used as the aqueous solution 103.

Gallium nitride (GaN) and aluminum gallium nitride (AlGaN), which are n-type semiconductors, were epitaxially grown (thin film formed) in that order on a sapphire substrate, and nickel (Ni) was vacuum-deposited on top of the GaN and then heat-treated to form a thin film of nickel oxide (NiO) as a promoter. The substrate thus formed was used as the oxidation electrode 104, and the oxidation electrode 104 was placed in the oxidation tank 101 so that it was immersed in the aqueous solution 103.

A 300 W high-pressure xenon lamp (cutting wavelengths of 450 nm or more, illuminance 2.2 mW/cm ² ) was used as the light source 109. The light source 109 was fixed so that the surface of the oxidation electrode 104 on which the oxidation promoter was formed was the irradiated surface. The light irradiated area of the oxidation electrode 104 was 2.3 cm ² .

Nitrogen (N) was flowed through the tube 110 into the aqueous solution 103 in the oxidation tank 101 at a flow rate of 30 ml/min. Carbon dioxide (CO ₂ ) was also flowed through the gas inlet 111 into the reduction tank 102 at the same flow rate. Then, the oxidation tank 101 was sufficiently replaced with nitrogen for 15 hours or more, and the reduction tank 102 was sufficiently replaced with carbon dioxide for the same period or more, after which light was uniformly irradiated from the light source 109 onto the oxidation electrode 104.

In this state, the switch 106 is controlled according to the flowchart shown in FIG. 2. Here, a case where the control device 40 controls the switch 106 will be described.

Step S1:
The control device 40 selects the connection terminal T1 on the oxidation electrode side as the connection destination of the switch 106, and connects the oxidation electrode 104 and the reduction electrode 105b. This causes the reduction reaction of carbon dioxide to proceed in the reduction electrode 105b.

Step S2:
Next, the concentration of the product generated by the reduction reaction of carbon dioxide is measured by 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, and only carbon monoxide (CO) was detected as the product. In addition, the current value between the oxidation electrode 104 and the reduction electrode 105b was measured by the electrochemical measurement device 30.

Step S3:
Next, the control device 40 calculates the rate of decrease in the concentration of the product (CO concentration) being measured/detected, and determines whether or not the rate of decrease in the concentration has exceeded a first threshold value X. The first threshold value X is the rate of decrease (%) of the concentration at which the concentration of the product can be considered to have started to decrease/fall. Specific examples will be described below.

In the first embodiment, the rate of decrease of the CO concentration from the initial value was calculated by the formula (7). Q ₀ is the initial value of the CO concentration. Q is the CO concentration at an arbitrary time.

Decrease rate of CO concentration from initial value=(Q ₀ −Q )×100/Q ₀ (7)
In Example 1, the slope of the change in concentration over time, E _Q, was used to determine whether the CO concentration had started to decrease or decline. E _Q was calculated from equation (8) using Q _t as the CO concentration at any time t and Q _t+Δt as the CO concentration at time (t+Δt).

_EQ = (Qt _{+ Δt} - _Qt ) × 100 / Δt ... (8)
From the time change of the CO concentration measured in advance, the _EQ when the CO concentration can be considered to have turned to a decreasing trend was -7.3 (ppm/h). Therefore, in Example 1, the first threshold value X was determined to be 10 (%) as a value at which this _EQ can be detected.

The method of determining the first threshold value X is not limited to the above method. The first threshold value X may be any value that can detect a decrease or decline in the CO concentration. For example, the first threshold value X may be determined based on the amount of change from the average CO concentration, by fitting the change in the CO concentration over time, or based on the user's experience.

If the rate of decrease in density does not exceed the first threshold value X, the process returns to step S2.

Step S4:
Next, when the rate of decrease in the concentration of the product (CO concentration) being measured/detected exceeds the first threshold value X, the control device 40 cuts off the connection to the connection terminal T1 on the oxidation electrode side of the switch 106 at that point, selects the connection terminal T2 on the reference electrode side, and connects the reduction electrode 105 b to the reference electrode 107.

If the reduction reaction at the reduction electrode 105b is stopped and the supply of carbon dioxide is continued by operating the switch 106, the reaction field will be saturated with carbon dioxide, and the chemical potential of the electrode will change. Therefore, from this point on, this change will be measured as the potential difference from the reference electrode 107. As the reaction field becomes saturated with carbon dioxide, the potential difference will become smaller, and this point will be utilized.

Step S5:
Next, the potential difference between the reference electrode 107 and the reduction electrode 105b is measured by a voltmeter 108. The reference electrode 107 is a silver-silver chloride electrode.

Step S6:
Next, the control device 40 calculates the amount of change in the potential difference between the reference electrode 107 and the reduction electrode 105b, and determines whether or not the amount of change in the potential difference has fallen below a second threshold value Y. The second threshold value Y is the amount of change in the potential difference (V) at which the potential difference between the reference electrode 107 and the reduction electrode 105b can be considered to be stable. Specific examples will be described below.

In Example 1, the stability of the absolute value |ΔV| of the potential difference was determined using the slope _FV of the time change of the potential difference. _FV was calculated from formula (9) by assuming that the potential difference at any time t is _Vt and the potential difference at time (t+Δt) is Vt _+Δt .

F _V = ( V _{t + Δt} - V _t ) × 100 / Δt ... (9)
From the time change of the potential difference measured in advance, _FV when the amount of change of the potential difference was deemed stable was 0.01 (V/s). Therefore, in Example 1, 0.01 (V) was determined as the second threshold value Y.

The method of determining the second threshold value Y is not limited to the above method. The second threshold value Y may be any value that can detect that the amount of change in the potential difference has stabilized. For example, the second threshold value Y may be determined by fitting the change in the voltage value over time, or based on the user's experience.

After that, if the result of the judgment in step S6 is that the change in the potential difference between the reference electrode 107 and the reduction electrode 105b is below the second threshold Y, the process returns to step S1. In step S1, the control device 40 at that point in time cuts off the connection to the connection end T2 on the reference electrode side of the switch 106, selects the connection end T1 on the oxidation electrode side, and connects the oxidation electrode 104 and the reduction electrode 105b. This causes the reduction reaction of carbon dioxide to proceed again. On the other hand, if the change in the potential difference is not below the second threshold Y, the process returns to step S5.

Then, the reaction was stopped when the total reaction time due to the flow of electrons between the oxidation electrode 104 and the reduction electrode 105b reached 100 hours.

(Modification 1 of the first embodiment)
The switch 106 may be operated by a person who visually checks the product concentration and voltage value and makes the switching decision, or the concentration and voltage value may be automatically transmitted to a computer and automatic control may be performed using a control circuit or control device.

(Modification 2 of Example 1)
In Example 1, since the change in the current value between the oxidation electrode 104 and the reduction electrode 105b was within ±2%, the current value was not used, and the product concentration reduction rate was simply used. However, when the input energy changes, such as in sunlight, and the current value changes significantly, it is preferable to use the normalized product concentration reduction rate normalized by dividing the product concentration by the current value. In this case, the electrochemical measurement device 30 and the control device 40 are connected to each other, and the control device 40 executes a calculation process of dividing the product concentration measured by the concentration measurement device 20 by the current value measured by the electrochemical measurement device 30. By normalizing the product concentration by the current value, the reduction in the Faraday efficiency can be accurately detected.

[Example 2]
3 is a diagram showing the overall configuration of a system 1 according to a second embodiment. The system 1 has the same configuration as that of the first embodiment.

(overall structure)
The gas-phase carbon dioxide reduction device 10 according to the second embodiment is the carbon dioxide electrolytic reduction device described in the Background Art section. Specifically, the gas-phase carbon dioxide reduction device 10 is a device that applies a voltage between an oxidation electrode and a reduction electrode, and performs a reduction reaction of gas-phase carbon dioxide at the reduction electrode in a reduction tank.

The carbon dioxide gas-phase reduction device 10 according to the second embodiment includes a power source 113 connected between the oxidation electrode 104 and the reduction electrode 105b as shown in FIG. 3, instead of the light source 109 shown in FIG. 1. The oxidation electrode 104 according to the second embodiment is, for example, platinum, gold, silver, copper, indium, nickel, zinc, tin, or lead. The oxidation electrode 104 may be an oxide of these metals. The rest of the configuration is the same as that of the first embodiment.

(Method for reducing carbon dioxide gas)
The oxidation tank 101 is filled with a predetermined aqueous solution 103. A 1.0 mol/L aqueous solution of potassium hydroxide is used as the aqueous solution 103. The oxidation electrode 104 is placed in the oxidation tank 101 so that about 0.4 ^cm2 of its surface area is submerged in the aqueous solution 103. The oxidation electrode 104 is made of platinum.

Helium (He) was flowed through the tube 110 into the aqueous solution 103 in the oxidation tank 101 at a flow rate of 30 ml/min. Carbon dioxide (CO ₂ ) was also flowed through the gas inlet 111 into the reduction tank 102 at the same flow rate. The oxidation tank 101 was then thoroughly replaced with helium for 15 hours or more, and the reduction tank 102 was thoroughly replaced with carbon dioxide for the same period or more.

In this state, the switch 106 is controlled according to the flowchart shown in FIG. 2. The specific control method is the same as in Example 1. A simple explanation will be given here.

First, the switch 106 is connected to the connection end T1 on the oxidation electrode side to connect the oxidation electrode 104 and the reduction electrode 105b. Then, the power supply 113 is operated at a constant current of 3.0 mA to allow the reduction reaction of carbon dioxide to proceed at the reduction electrode 105b. The concentration of the product in the reduction tank 102 was measured every 30 minutes by 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 a product.

Next, when the rate of decrease in the detected CO concentration exceeds the first threshold value X, the connection end T1 of the switch 106 on the oxidation electrode side is turned off and connected to the connection end T2 on the reference electrode side, connecting the reference electrode 107 and the reduction electrode 105b. The potential difference between the reference electrode 107 and the reduction electrode 105b is then measured by the voltmeter 108. A silver-silver chloride electrode was used for the reference electrode 107.

The rate of decrease from the initial value of the CO concentration was calculated by the above formula (7). From the time change of the CO concentration measured in advance, the _EQ when the CO concentration was deemed to have turned to a decreasing trend was -290 (ppm/h). For this reason, in Example 2, the first threshold value X was determined to be 10 (%) as a value at which this _EQ can be detected.

After that, when the change in the potential difference between the reference electrode 107 and the reduction electrode 105b falls below the second threshold Y, the connection end T2 of the switch 106 on the reference electrode side is turned off and connected to the connection end T1 on the oxidation electrode side, connecting the oxidation electrode 104 and the reference electrode 107. This allows the carbon dioxide reduction reaction to proceed again. The second threshold Y was set to 0.01 (V) for the same reasons as in Example 1.

Then, the reaction was stopped when the total reaction time due to the flow of electrons between the oxidation electrode 104 and the reduction electrode 105b reached 20 hours.

[Comparative Example 1]
For comparison with Example 1, the gas-phase carbon dioxide reduction device 10 shown in Fig. 4 was used. Comparative Example 1 differs from Example 1 shown in Fig. 1 in that it does not have the switch 106, the reference electrode 107, or the voltmeter 108. Also, the oxidation electrode 104 and the reduction electrode 105b are connected by a conductor. The rest of the configuration is the same as in Example 1. The gas-phase carbon dioxide reduction device 10 according to Comparative Example 1 was operated to continuously carry out the oxidation-reduction reaction, and was stopped when the total reaction time reached 100 hours.

[Comparative Example 2]
For comparison with Example 2, the gas-phase carbon dioxide reduction device 10 shown in Fig. 5 was used. Unlike Example 2 shown in Fig. 3, Comparative Example 2 does not have the switch 106, the reference electrode 107, or the voltmeter 108. In addition, the oxidation electrode 104 and the reduction electrode 105b are connected by a conductor via the power source 113. The rest of the configuration is the same as that of Example 2. The gas-phase carbon dioxide reduction device 10 according to Comparative Example 2 was operated to continuously carry out the oxidation-reduction reaction, and was stopped when the total reaction time reached 20 hours.

[Effects of Examples 1 and 2]
6 is a diagram showing the change over time in the Faraday efficiency of the reduction reaction of carbon dioxide in Examples 1 and 2 and Comparative Examples 1 and 2. The Faraday efficiency, as shown in formula (10), indicates the ratio of the total charge amount used in the reduction reaction to the total charge amount flowing between the oxidation electrode 104 and the reduction electrode 105b during light irradiation or application of a power supply voltage.

Faraday efficiency of reduction reaction=(total charge consumed in reduction reaction)/(total charge flowing between oxidation electrode and reduction electrode) (10)
The "total charge consumed in the reduction reaction" (C) can be calculated by converting the measured amount of reduction product produced into the number of electrons required for the production reaction. It was calculated using formula (11) where A (ppm) is the concentration of the reduction reaction product, B (L/sec) is the flow rate of the carrier gas, Z (mol) is the number of electrons required for the reduction reaction, F (C/mol) is the Faraday constant, V _m (L/mol) is the molar mass of gas, and t (sec) is the reaction time.

Total charge consumed in reduction reaction=(A×B×Z×F×t×10 ⁻⁶ )/V _m (11)
A higher faradaic efficiency of the carbon dioxide reduction reaction indicates that the electrons flowing between the oxidation electrode 104 and the reduction electrode 105b can be efficiently consumed in the carbon dioxide reduction reaction.

As can be seen from Figure 6, in Comparative Examples 1 and 2, the Faraday efficiency of carbon dioxide decreases at the beginning of the reaction and then settles at a constant low value. On the other hand, in Examples 1 and 2, when the rate of decrease in the Faraday efficiency exceeds a threshold value, a waiting time is provided to saturate the reaction field with carbon dioxide, and the Faraday efficiency recovers to its initial value, and by repeating this process, high efficiency can be maintained.

The Faraday efficiency of the carbon dioxide reduction reaction for Examples 1 and 2 and Comparative Examples 1 and 2 is shown in Table 1.

　表１より、実施例１、２は、比較例１、２よりも二酸化炭素還元のファラデー効率が高いことが分かる。これは、比較例１、２では、反応開始とともに徐々に反応場の二酸化炭素量が低下したのに対して、実施例１、２では、効率が閾値よりも低下した時点で二酸化炭素を飽和させる待機時間を設けたことで、常に反応場に二酸化炭素を十分に吸着させることができ、高いファラデー効率を維持したまま運転できたことが要因と考えられる。これにより、与えた光エネルギーや電気エネルギーを副反応に無駄遣いすることなく、二酸化炭素の還元反応に効率的に長時間使い続けることができる。

From Table 1, it can be seen that Examples 1 and 2 have a higher Faraday efficiency for carbon dioxide reduction than Comparative Examples 1 and 2. This is thought to be 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 time was provided to saturate the carbon dioxide at the point when the efficiency fell below a threshold, so that carbon dioxide could always be sufficiently adsorbed in the reaction field and operation could be performed while maintaining a high Faraday efficiency. As a result, the applied light energy and electrical energy can be used efficiently for a long period of time in the carbon dioxide reduction reaction without being wasted on side reactions.

[Effects of the present disclosure]
The switch 106 in Examples 1 and 2 connects the reduction electrode 105b to the oxidation electrode 104, connects the reduction electrode 105b to the reference electrode 107 when the rate of decrease in the concentration of a product produced by the reduction reaction of carbon dioxide at the reduction electrode 105b exceeds a first threshold value X, and connects the reduction electrode 105b to the oxidation electrode 104 when the amount of change in the potential difference between the reduction electrode 105b and the reference electrode 107 falls below a second threshold value Y, by the control device 40 or a user.

As a result, the light energy and electrical energy provided during the redox reaction can be used efficiently for a long period of time in the carbon dioxide reduction reaction, without wasting it in side reactions. In addition, the energy not consumed in the redox reaction during standby time can be stored or used for other purposes, making it possible to make effective use of energy.

In particular, artificial photosynthesis technology that utilizes solar energy can be effectively used when the reaction time and standby time cycles described in Examples 1 and 2 are shorter than the sunlight cycle of the land where the carbon dioxide gas phase reduction device 10 is used.

1: System 10: Carbon dioxide gas phase reduction device 20: Concentration measurement device 30: Electrochemical measurement device 40: Control device 101: Oxidation cell 102: Reduction cell 103: Aqueous solution 104: Oxidation electrode 105: Reduction electrode/electrolyte membrane composite 105a: Electrolyte membrane 105b: Reduction electrode 106: Switch 107: Reference electrode 108: Voltmeter 109: Light source 110: Tube 111: Gas input port 112: Gas output port 113: Power source

Claims (3)

In a gas phase carbon dioxide reduction apparatus for reducing gas phase carbon dioxide,
an oxidation chamber including an oxidation electrode and a reference electrode;
a reduction tank to which carbon dioxide is supplied;
a composite disposed between the oxidation chamber and the reduction chamber, in which an electrolyte membrane and a reduction electrode are joined to each other, the electrolyte membrane being disposed on the oxidation chamber side and the reduction electrode being disposed on the reduction chamber side;
a switch for connecting the reduction electrode to the oxidation electrode or the reference electrode;
The switch is
A gas phase carbon dioxide reduction device in which, by a control device or a user, the reduction electrode is connected to the oxidation electrode, the reduction electrode is connected to the reference electrode when a rate of decrease in concentration of a product produced by a carbon dioxide reduction reaction at the reduction electrode exceeds a first threshold, and the reduction electrode is connected to the oxidation electrode when an amount of change in potential difference between the reduction electrode and the reference electrode falls below a second threshold. The first threshold value is
2. The gas phase reduction device for carbon dioxide according to claim 1, wherein the concentration of the product produced by the reduction reaction of carbon dioxide at the reduction electrode is a value that can be regarded as starting to decrease. The second threshold value is
2. The gas phase carbon dioxide reduction device according to claim 1, wherein the potential difference between the reduction electrode and the reference electrode is a value that can be regarded as stable.

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