26 include metals such as titanium and SUS and carbon.

Examples of the material of the feed conductors 27 and 28 include metals such as copper, gold titanium and SUS and carbon. As the feed conductors 27 and 28, a material obtained by performing plating treatment such as gold plating on the surface of a copper base material may be used.

The electrolytic cell 2a of the electrochemical reaction unit 2 is a flow cell in which the electrolytic solution B supplied from the CO₂absorption unit 12 and sent via the electric energy storage unit 32 and the heat exchanger 5 flows into the cathode-side liquid flow path 24a. Then, a voltage is applied to the cathode 21 and the anode 22, and thus the dissolved carbon dioxide in the electrolytic solution B flowing through the cathode-side liquid flow path 24a is electrochemically reduced in the cathode 21, with the result that a carbon compound and hydrogen are generated. The electrolytic solution B at the inlet of the cathode-side liquid flow path 24a is in a weak alkaline state with a high abundance of CO₃²⁻ because carbon dioxide is dissolved therein. On the other hand, as the electrolytic solution flows through the cathode-side liquid flow path 24a and the reduction proceeds, the amount of dissolved carbon dioxide, that is, the amount of CO₃²⁻ in the electrolytic solution is lowered, with the result that the electrolytic solution is changed into the electrolytic solution A in a strong alkaline state at the outlet of the cathode-side liquid flow path 24a.

Examples of the carbon compound generated by reducing carbon dioxide in the cathode 21 include carbon monoxide, ethylene and the like. For example, the following reactions proceed, and thus carbon monoxide and ethylene are generated as gaseous products. In the cathode 21, hydrogen is also generated by the following reaction. The gaseous carbon compound and hydrogen generated flow out from the outlet of the cathode-side liquid flow path 24a.

CO₂+H₂O→CO+2OH⁻

2CO+8H₂O→C₂H₄+8OH⁻+2H₂O

2H₂O→H₂+2OH⁻

The hydroxide ions generated in the cathode 21 permeate the anion exchange membrane 23 to move to the anode 22, and are oxidized by the following reaction, with the result that oxygen is generated. The generated oxygen permeates the gas diffusion layer of the anode 22, flows into the anode-side liquid flow path 26a and flows out from the outlet of the anode-side liquid flow path 26a.

4OH⁻→O₂+2H₂O

As described above, in the carbon dioxide treatment apparatus 100, the electrolytic solution used in the electrochemical reaction unit 2 is also used as the absorption solution for the CO₂absorption unit 12, and carbon dioxide is supplied to the electrochemical reaction unit 2 while being dissolved in the electrolytic solution B and is electrochemically reduced. In this way, for example, as compared with a case where carbon dioxide is adsorbed on an adsorbent and is desorbed by heating so as to be reduced, energy necessary for desorption of carbon dioxide is reduced, with the result that energy efficiency can be increased.

Here, in the reduction reactions of carbon dioxide which proceeds in the cathode 21, by-products are generated in addition to carbon compounds such as desired ethylene. Specifically, by-products such as methanol, ethanol, acetic acid and formic acid are generated, and these by-products are dissolved in the electrolytic solution and are difficult to separate. Hence, a loss of carbon dioxide occurs, and thus it is desirable to reduce the loss.

Specifically, in the cathode 21, the reduction reactions of carbon dioxide as described below proceed, and thus methanol, ethanol, acetic acid and formic acid are generated. Hence, the electrolytic solution A which has flowed through the cathode-side liquid flow path 24a includes the by-products such as methanol, ethanol, acetic acid and formic acid.

2CO₃²⁻+12H₂O+12e⁻→2CH₃OH+16OH⁻

2CO₃²⁻+11H₂O+12e⁻→C₂H₅OH+16OH⁻

2CO₃²⁻+8H₂O+8e⁻→CH₃COOH+12OH⁻

2CO₃²⁻+6H₂O+4e⁻→2HCOOH+8OH⁻

By contrast, the electrolytic cell 2a of the electrochemical reaction unit 2 in the present embodiment includes a first liquid supply path 20 which supplies, to the anode-side liquid flow path 26a, the electrolytic solution A which has flowed through the cathode-side liquid flow path 24a. The first liquid supply path 20 supplies, from the inlet of the anode-side liquid flow path 26a into the anode-side liquid flow path 26a, the electrolytic solution A which flows out from the outlet of the cathode-side liquid flow path 24a and includes the by-products such as methanol, ethanol, acetic acid and formic acid. As shown in FIG. 2, the electrolytic solution flowing through the anode-side liquid flow path 26a consists only of the electrolytic solution A that has passed through the cathode-side liquid flow path 24a and been supplied via the first liquid supply path 20. In other words, the anode-side liquid flow path 26a does not receive any electrolytic solution that has not passed through the cathode-side liquid flow path 24a. In this way, the by-products such as methanol, ethanol, acetic acid and formic acid are oxidized by oxidation reactions which proceed in the anode 22, and thus carbon dioxide is captured in the form of carbon dioxide (CO32-) and electrons (e-).

Specifically, in the anode 22, the oxidation reactions of the by-products such as methanol, ethanol, acetic acid and formic acid as described below proceed, and thus these by-products are converted into the form of carbon dioxide (CO₃²⁻) and electrons (e⁻). The electrolytic solution A which flows through the anode-side liquid flow path 26a and in which the by-products are converted into the form of carbon dioxide (CO₃²⁻) and electrons (e⁻) is supplied by the second liquid supply path 65 to a nickel-hydride battery which forms the electric energy storage unit 32 to be described later. As described above, in the electrolytic cell 2a of the electrochemical reaction unit 2 in the present embodiment, carbon dioxide can be captured and recycled, and thus it is possible to reduce a loss of carbon dioxide and enhance energy efficiency.

2CH₃OH+16OH⁻→2CO₃²⁻+12H₂O+12e⁻

C₂H₅OH+16OH⁻→2CO₃²⁻+11H₂O+12e⁻

CH₃COOH+12OH⁻→2CO₃²⁻+8H₂O+8e⁻

2HCOOH+8OH⁻→2CO₃²⁻+6H₂O+4e⁻

With reference back to FIG. 1, the electric energy storage device 3 is a device which supplies power to the electrochemical reaction unit 2. In the conversion unit 31, renewable energy is converted into electric energy. The conversion unit 31 is not particularly limited, and examples thereof include a wind power generator, a solar power generator, a geothermal power generator and the like. One or a plurality of conversion units 31 may be included in the electric energy storage device 3.

The electric energy storage unit 32 is electrically connected to the conversion unit 31. In the electric energy storage unit 32, the electric energy converted by the conversion unit 31 is stored. The converted electric energy is stored in the electric energy storage unit 32, and thus it is possible to stably supply power to the electrochemical reaction unit 2 even when the conversion unit 31 does not generate power. When renewable energy is utilized, though in general, large voltage fluctuations easily occur, the electric energy is temporarily stored in the electric energy storage unit 32, and thus it is possible to stably supply power to the electrochemical reaction unit 2.

The electric energy storage unit 32 in the present embodiment includes a nickel-hydride battery. However, as long as the electric energy storage unit 32 can perform charging and discharging, the electric energy storage unit 32 may include, for example, a lithium-ion secondary battery or the like.

Here, FIG. 3A is a diagram showing the nickel-hydride battery in the electric energy storage unit 32 during discharge. FIG. 3B is a diagram showing the nickel-hydride battery in the electric energy storage unit 32 during charge. As shown in FIGS. 3A and 3B, the electric energy storage unit 32 is the nickel-hydride battery which includes a positive electrode 33, a negative electrode 34, a separator 35 provided between the positive electrode 33 and the negative electrode 34, a positive electrode side flow path 36 formed between the positive electrode 33 and the separator 35 and a negative electrode side flow path 37 formed between the negative electrode 34 and the separator 35. The positive electrode side flow path 36 and the negative electrode side flow path 37 can be formed using, for example, the same liquid flow path structures as the cathode-side liquid flow path 24a and the anode-side liquid flow path 26a in the electrochemical reaction unit 2.

Examples of the positive electrode 33 include a positive electrode in which a positive electrode active material is applied to the surface of a positive electrode current collector on the side of the positive electrode side flow path 36. The positive electrode current collector is not particularly limited, and examples thereof include nickel foil and nickel plated metal foil. The positive electrode active material is not particularly limited, and examples thereof include nickel hydroxide and nickel oxyhydroxide.

Examples of the negative electrode 34 include a negative electrode in which a negative electrode active material is applied to the surface of a negative electrode current collector on the side of the negative electrode side flow path 37. The negative electrode current collector is not particularly limited, and examples thereof include nickel mesh. The negative electrode active material is not particularly limited, and examples thereof include a known hydrogen storage alloy.

The separator 35 is not particularly limited, and examples thereof include an ion exchange membrane.

The nickel-hydride battery of the electric energy storage unit 32 is a flow cell in which the electrolytic solution flows through each of the positive electrode side flow path 36 on the side of the positive electrode 33 with respect to the separator 35 and the negative electrode side flow path 37 on the side of the negative electrode 34 with respect to the separator 35. In the carbon dioxide treatment apparatus 100 of the present embodiment, the electrolytic solution B supplied from the CO₂absorption unit 12 via the liquid flow path 62 and the electrolytic solution A supplied from the electrochemical reaction unit 2 via the second liquid supply path 65 are respectively supplied to the positive electrode side flow path 36 and the negative electrode side flow path 37.

Each of the connections of the liquid flow path 62 and the liquid flow path 63 to the electric energy storage unit 32 is switched by, for example, a switching valve between a state where the liquid flow path is connected to the positive electrode side flow path 36 and a state where the liquid flow path is connected to the negative electrode side flow path 37. Likewise, each of the connections of the second liquid supply path 65 and the liquid flow path 66 to the electric energy storage unit 32 is switched by, for example, a switching valve between a state where the path is connected to the positive electrode side flow path 36 and a state where the path is connected to the negative electrode side flow path 37.

When the nickel-hydride battery is discharged, hydroxide ions are generated from water molecules in the positive electrode 33, the hydroxide ions which have moved to the negative electrode 34 receive hydrogen ions from a hydrogen storage alloy to generate water molecules. Hence, in terms of discharge efficiency, the electrolytic solution flowing through the positive electrode side flow path 36 is advantageous to be in a weak alkaline state, and the electrolytic solution flowing through the negative electrode side flow path 37 is advantageous to be in a strong alkaline state. Hence, preferably, during discharge, as shown in FIG. 3A, the liquid flow paths 62 and 63 are connected to the positive electrode side flow path 36, the second liquid supply path 65 and the liquid flow path 66 are connected to the negative electrode side flow path 37 such that the electrolytic solution B in a weak alkaline state supplied from the CO₂absorption unit 12 flows through the positive electrode side flow path 36 and the electrolytic solution A in a strong alkaline state supplied from the electrochemical reaction unit 2 flows through the negative electrode side flow path 37. In other words, preferably, during discharge, the electrolytic solution is circulated from the CO₂absorption unit 12, to the positive electrode side flow path 36 of the electric energy storage unit 32, to the electrochemical reaction unit 2, to the negative electrode side flow path 37 of the electric energy storage unit 32 and back to the CO₂absorption unit 12.

When the nickel-hydride battery is charged, water molecules are generated from hydroxide ions in the positive electrode 33, the water molecules are decomposed into hydrogen atoms and hydroxide ions in the negative electrode 34 and the hydrogen atoms are stored in the hydrogen storage alloy. Hence, in terms of charge efficiency, the electrolytic solution flowing through the positive electrode side flow path