Patent Publication Number: US-11655549-B2

Title: Carbon dioxide treatment apparatus, carbon dioxide treatment method, and method for producing carbon compound

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority from Japanese Patent Application No. 2021-048648, filed on Mar. 23, 2021, the content of which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to a carbon dioxide treatment apparatus, a carbon dioxide treatment method, and a method for producing a carbon compound. 
     Description of Related Art 
     A technology for recovering carbon dioxide in exhaust gas and the atmosphere and electrochemically reducing the recovered carbon dioxide to obtain valuable resources is a promising technology that has the potential to achieve carbon neutrality, but economic efficiency is the biggest issue. In order to improve the economic efficiency, it is important to improve energy efficiency and to reduce loss of carbon dioxide in the recovery and reduction of carbon dioxide. 
     As a technology for recovering carbon dioxide, a technology in which carbon dioxide in a gas is physically or chemically adsorbed in a solid or liquid adsorbent and then desorbed by energy such as heat is known. Further, as a technology for electrochemically reducing carbon dioxide, a technology in which a carbon dioxide gas is supplied from the side opposite to a catalyst layer of a gas diffusion layer to a cathode having a catalyst layer formed on the side in contact with an electrolytic solution of the gas diffusion layer using a carbon dioxide reduction catalyst and is then electrochemically reduced is known (WO 2018/232515 A1 (Patent Document 1)). 
     SUMMARY OF THE INVENTION 
     Carbon dioxide reduction is a promising technology that has the potential to achieve carbon neutrality, but the economic efficiency is the biggest issue. In order to improve the economic efficiency, it is important to recover carbon dioxide efficiently and to convert the carbon dioxide without loss. 
     One of causes of energy loss in carbon dioxide electrolysis is generation of hydrogen by water electrolysis which is a side reaction that does not involve a desired carbon dioxide reduction reaction. According to a deterioration state of a catalyst in each of the cathode and the anode, the generation of hydrogen may not be able to be suppressed simply by controlling a voltage. 
     An object of the present invention is to provide a carbon dioxide treatment apparatus, a carbon dioxide treatment method, and a method for producing a carbon compound that have high energy efficiency in recovery and reduction of carbon dioxide and are highly effective in reducing loss of carbon dioxide. 
     The present invention has adopted the following aspects. 
     (1) A carbon dioxide treatment apparatus according to one aspect of the present invention (for example, a carbon dioxide treatment apparatus ( 100 ) of an embodiment) includes a recovery device (for example, a recovery device ( 1 ) of the embodiment) configured to recover carbon dioxide, an electrochemical reaction device (for example, an electrochemical reaction device ( 2 ) of the embodiment) configured to electrochemically reduce carbon dioxide, and a pH adjuster (for example, a pH adjuster ( 52 ) of the embodiment), wherein the recovery device includes an absorption part (for example, an absorption part ( 12 ) of the embodiment) in which an anode side electrolytic solution composed of a strong alkaline aqueous solution and carbon dioxide gas are brought into contact with each other so that the carbon dioxide is dissolved and absorbed in the anode side electrolytic solution, and a concentration part (for example, a concentration part ( 11 ,  13 ) of the embodiment) that concentrates carbon dioxide, 
     the electrochemical reaction device includes an anode (for example, an anode ( 22 ) of the embodiment), a cathode (for example, a cathode ( 21 ) of the embodiment), an anion exchange membrane (for example, an anion exchange membrane ( 23 ) of the embodiment) provided between the anode and the cathode, a liquid flow path (for example, a liquid flow path ( 29   a ) of the embodiment) provided between the anode and the anion exchange membrane and through which the anode side electrolytic solution that has absorbed the carbon dioxide in the absorption part flows, and a liquid flow path (for example, a liquid flow path ( 28   a ) of the embodiment) provided between the cathode and the anion exchange membrane and through which a cathode side electrolytic solution composed of a strong alkaline aqueous solution of which a pH has been adjusted by the pH adjuster flows, the pH of the cathode side electrolytic solution being higher than that of the anode side electrolytic solution, and the carbon dioxide gas is supplied from the concentration part to a gas flow path (for example, a gas flow path ( 24   a ) of the embodiment) on a side of the cathode opposite to the anode, and the carbon dioxide gas is reduced at the cathode. 
     (2) The carbon dioxide treatment apparatus according to the aspect of the present invention may further include a power storage device (for example, a power storage device ( 3 ) of the embodiment) configured to supply electric power to the electrochemical reaction device, and the power storage device may include a conversion part (for example, a conversion part ( 31 ) of the embodiment) that converts renewable energy into electrical energy, and a storage part (for example, a storage part ( 32 ) of the embodiment) that stores the electrical energy converted by the conversion part. 
     (3) The storage part may be a nickel metal hydride battery, the nickel metal hydride battery may include a positive electrode (for example, a positive electrode ( 33 ) of the embodiment), a negative electrode (for example, a negative electrode ( 34 ) of the embodiment), a separator (for example, a separator ( 37 ) of the embodiment) provided between the positive electrode and the negative electrode, a positive electrode side flow path (for example, a positive electrode side flow path ( 36 ) of the embodiment) provided between the positive electrode and the separator, and a negative electrode side flow path (for example, a negative electrode side flow path ( 35 ) of the embodiment) provided between the negative electrode and the separator, when the nickel metal hydride battery is discharged, the anode side electrolytic solution may be circulated in an order of the absorption part, the negative electrode side flow path, the electrochemical reaction device, and the absorption part, and when the nickel metal hydride battery is charged, the anode side electrolytic solution may be circulated in an order of the absorption part, the negative electrode side flow path, the electrochemical reaction device, the positive electrode side flow path, and the absorption part. 
     (4) The pH adjuster may bring the cathode side electrolytic solution into contact with the carbon dioxide gas. 
     (5) The carbon dioxide treatment apparatus according to the aspect of the present invention may further include a carbon increase reaction device (for example, a carbon increase reaction device ( 4 ) of the embodiment) that increases an amount of ethylene generated by reducing carbon dioxide in the electrochemical reaction device and increases the number of carbon atoms. 
     (6) The carbon dioxide treatment apparatus according to the aspect of the present invention may further include a heat exchanger (for example, a heat exchanger ( 43 ) of the embodiment) that heats the anode side electrolytic solution by exchanging heat between a heat medium heated by heat due to a reaction in the carbon increase reaction device and the anode side electrolytic solution. 
     (7) A carbon dioxide treatment method according to an aspect of the present invention includes bringing carbon dioxide gas into contact with an anode side electrolytic solution composed of a strong alkaline aqueous solution so that the carbon dioxide is dissolved and absorbed in the anode side electrolytic solution, adjusting a pH of a cathode side electrolytic solution to be higher than that of the anode side electrolytic solution, and supplying the cathode side electrolytic solution between a cathode and an anion exchange membrane, supplying the anode side electrolytic solution between an anode and the anion exchange membrane, supplying carbon dioxide gas to a side of the cathode opposite to the anode, and electrochemically reducing the carbon dioxide gas to generate a carbon compound and hydrogen. 
     (8) In adjusting the pH of the cathode side electrolytic solution, the cathode side electrolytic solution may be brought into contact with carbon dioxide, and the carbon dioxide may be dissolved in the cathode side electrolytic solution. 
     (9) A method for producing a carbon compound, wherein the carbon compound in which carbon dioxide is reduced is produced using the carbon dioxide treatment method according to the aspect of (7) or (8). 
     (10) The method for producing a carbon compound according to the aspect of the present invention may further include increasing an amount of ethylene generated by reducing the carbon dioxide. 
     According to the aspects of (1) to (10), it is possible to provide a carbon dioxide treatment apparatus, a carbon dioxide treatment method, and a method for producing a carbon compound that have high energy efficiency in recovery and reduction of carbon dioxide and are highly effective in reducing loss of carbon dioxide. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a block diagram showing a carbon dioxide treatment apparatus according to an embodiment. 
         FIG.  2    is a schematic cross-sectional view showing an example of an electrolytic cell of an electrochemical reaction device. 
         FIG.  3    is a schematic diagram showing an electrochemical reaction that occurs in the electrolytic cell. 
         FIG.  4    is a schematic cross-sectional view showing a nickel metal hydride battery as an example of a storage part. 
         FIG.  5    is a graph showing electrolysis test results according to Examples and Comparative examples. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinafter, an embodiment of the present invention will be described with reference to the drawings. The dimensions and the like in the drawings exemplified in the following description are examples, and the present invention is not necessarily limited thereto, and the present invention can be appropriately modified without changing the gist thereof. 
     [Carbon Dioxide Treatment Apparatus] 
     As shown in  FIG.  1   , a carbon dioxide treatment apparatus  100  according to one aspect of the present invention includes a recovery device  1 , an electrochemical reaction device  2 , a power storage device  3 , a carbon increase reaction device  4 , a heat exchanger  43 , and a pH adjuster  52 . The recovery device  1  includes a concentration part  11 , an absorption part  12 , and a concentration part  13 . The power storage device  3  includes a conversion part  31  and a storage part  32  electrically connected to the conversion part  31 . The carbon increase reaction device  4  includes a reactor  41  and a gas-liquid separator  42 . 
     In the carbon dioxide treatment apparatus  100 , the concentration part  11  and the absorption part  12  are connected by a gas flow path  75 . The concentration part  11  and the concentration part  13  are connected by a gas flow path  77 . The absorption part  12  and the storage part  32  are connected by a liquid flow path  62 . The electrochemical reaction device  2  and the storage part  32  are connected by a liquid flow path  65 . The electrochemical reaction device  2  and the absorption part  12  are connected by a liquid flow path  66 . The electrochemical reaction device  2  and the reactor  41  are connected by a gas flow path  74 . The reactor  41  and the gas-liquid separator  42  are connected by a gas flow path  72  and a gas flow path  73 . A circulation flow path  69  of a heat medium is provided between the reactor  41  and the heat exchanger  43 . The concentration part  11  and the concentration part  13  and the gas-liquid separator  42  are connected by a gas flow path  71 . The concentration part  13  and the electrochemical reaction device  2  are connected by a gas flow path  76 . The pH adjuster  52  and the electrochemical reaction device  2  are connected by a liquid flow path  63  and a liquid flow path  64 . 
     Each of the flow paths is not particularly limited, and known piping or the like can be appropriately used. Air supply means such as a compressor, a pressure reducing valve, a measuring device such as a pressure gauge, and the like can be appropriately installed in the gas flow paths  71  to  77 . In  FIG.  1   , a cooler  50  is installed in the gas flow path  71 . Further, liquid supply means such as a pump, a measuring device such as a flow meter, and the like can be appropriately installed in the liquid flow paths  62  to  66 . In  FIG.  1   , a pH measuring device  51  is installed in the liquid flow path  66 , and a pH measuring device  53  is installed in the liquid flow path  63 . 
     The recovery device  1  is a device that recovers carbon dioxide. 
     A gas G 1  containing carbon dioxide such as the atmosphere and exhaust gas is supplied to the concentration part  11  and the concentration part  13 . In the concentration part  11  and the concentration part  13 , carbon dioxide of the gas G 1  is concentrated. As the concentration part  11  and the concentration part  13 , a known concentrating device can be adopted as long as it can concentrate carbon dioxide, and for example, a membrane separation device that utilizes a difference in a permeation rate with respect to the membrane, or an adsorption separation device that utilizes chemical or physical adsorption and desorption can be used. Among them, adsorption using chemical adsorption, particularly temperature swing adsorption, is preferable from the viewpoint of excellent separation performance. 
     A concentrated gas G 2  in which carbon dioxide is concentrated in the concentration part  11  is sent to the absorption part  12  through the gas flow path  75 . Further, a separated gas G 3  separated from the concentrated gas G 2  is sent to the gas-liquid separator  42  through the gas flow path  71 . 
     A concentrated gas G 4  in which carbon dioxide is concentrated in the concentration part  13  is sent to the electrochemical reaction device  2  through the gas flow path  75 . Further, a separated gas separated from the concentrated gas G 4  is sent to the gas-liquid separator  42  through the gas flow path  71  together with the separated gas G 3 . 
     In the absorption part  12 , a carbon dioxide gas in the concentrated gas G 2  supplied from the concentration part  11  comes into contact with an anode side electrolytic solution A, and carbon dioxide is dissolved and absorbed in the anode side electrolytic solution A. A method of bringing the carbon dioxide gas into contact with the anode side electrolytic solution A is not particularly limited, and for example, a method in which the concentrated gas G 2  is blown into the anode side electrolytic solution A and bubbling the solution is performed can be exemplified. 
     In the absorption part  12 , the anode side electrolytic solution A made of a strong alkaline aqueous solution is used as an absorption solution for absorbing carbon dioxide. In the carbon dioxide, a carbon atom is positively charged (6+) because oxygen atoms strongly attract electrons. Therefore, in a strong alkaline aqueous solution in which a large amount of hydroxide ions are present, a dissolution reaction of carbon dioxide easily proceeds from a hydrated state to CO 3   2−  via HCO 3− , and an equilibrium state in which an abundance ratio of CO 3   2−  is high is reached. For this reason, carbon dioxide is more easily dissolved in a strong alkaline aqueous solution than other gases such as nitrogen, hydrogen, and oxygen, and in the absorption part  12 , the carbon dioxide in the concentrated gas G 2  is selectively absorbed in the anode side electrolytic solution A. In this way, the concentration of carbon dioxide can be assisted using the anode side electrolytic solution A in the absorption part  12 . Therefore, it is not necessary to concentrate carbon dioxide to a high concentration in the concentration part  11 , and energy required for the concentration in the concentration part  11  can be reduced. 
     An anode side electrolytic solution B in which carbon dioxide is absorbed in the absorption part  12  is sent to the electrochemical reaction device  2  through the liquid flow path  62 , the storage part  32 , and the liquid flow path  65 . Further, the anode side electrolytic solution A flowing out of the electrochemical reaction device  2  is sent to the absorption part  12  through the liquid flow path  66 . In this way, in the carbon dioxide treatment apparatus  100 , the anode side electrolytic solution is circulated and shared between the absorption part  12 , the storage part  32 , and the electrochemical reaction device  2 . 
     Examples of the strong alkaline aqueous solution used for the anode side electrolytic solution A include a potassium hydroxide aqueous solution and a sodium hydroxide aqueous solution. Among them, the potassium hydroxide aqueous solution is preferable, because it has excellent solubility of carbon dioxide in the absorption part  12  and the reduction of carbon dioxide in the electrochemical reaction device  2  is promoted. 
     The electrochemical reaction device  2  is a device that electrochemically reduces carbon dioxide. As shown in  FIG.  2   , the electrochemical reaction device  2  includes a cathode  21 , an anode  22 , an anion exchange membrane  23 , a liquid flow path structure  28  for forming a liquid flow path  28   a , a liquid flow path structure  29  for forming a liquid flow path  29   a , a gas flow path structure  24  in which the gas flow path  24   a  is formed, a gas flow path structure  25  in which a gas flow path  25   a  is formed, a power supply body  26 , and a power supply body  27 . 
     In the electrochemical reaction device  2 , the power supply body  26 , the gas flow path structure  24 , the cathode  21 , the liquid flow path structure  28 , the anion exchange membrane  23 , the liquid flow path structure  29 , the anode  22 , the gas flow path structure  25 , and the power supply body  27  are laminated in this order. A slit is formed in the liquid flow path structures  28  and  29 , and regions surrounded by the cathode  21 , the anode  22 , and the liquid flow path structures  28  and  29  in the slit become the liquid flow paths  28   a  and  29   a , respectively. A groove is formed on the cathode  21  side of the gas flow path structure  24 , and a portion of the groove surrounded by the gas flow path structure  24  and the cathode  21  becomes the gas flow path  24   a . A groove is formed on the anode  22  side of the gas flow path structure  25 , and a portion of the groove surrounded by the gas flow path structure  25  and the anode  22  becomes the gas flow path  25   a.    
     As described above, in the electrochemical reaction device  2 , the liquid flow path  28   a  is formed between the cathode  21  and the anion exchange membrane  23 , the liquid flow path  29   a  is formed between the anode  22  and the anion exchange membrane  23 , the gas flow path  24   a  is formed between the cathode  21  and the power supply body  26 , and the gas flow path  25   a  is formed between the anode  22  and the power supply body  27 . The power supply body  26  and the power supply body  27  are electrically connected to the storage part  32  of the power storage device  3 . Further, the gas flow path structure  24  and the gas flow path structure  25  are conductors, and a voltage can be applied between the cathode  21  and the anode  22  by electric power supplied from the storage part  32 . 
     The cathode  21  is an electrode that reduces carbon dioxide to produce a carbon compound and reduces water to produce hydrogen. The cathode  21  may be any one as long as it can electrochemically reduce carbon dioxide and the generated gaseous carbon compound and hydrogen permeate to the gas flow path  24   a , and for example, an electrode in which a cathode catalyst layer is formed on the liquid flow path  23   a  side of a gas diffusion layer can be exemplified. A part of the cathode catalyst layer may enter the gas diffusion layer. A porous layer that is denser than the gas diffusion layer may be disposed between the gas diffusion layer and the cathode catalyst layer. 
     As a cathode catalyst that forms the cathode catalyst layer, a known catalyst that promotes the reduction of carbon dioxide can be used. Specific examples of the cathode catalyst include metals such as gold, silver, copper, platinum, palladium, nickel, cobalt, iron, manganese, titanium, cadmium, zinc, indium, gallium, lead, and tin, alloys thereof, intermetallic compounds thereof, and metal complexes such as ruthenium complexes and rhenium complexes. Among them, copper and silver are preferable because the reduction of carbon dioxide is promoted, and copper is more preferable. As the cathode catalyst, one type may be used alone, or two or more types may be used in combination. 
     As the cathode catalyst, a supported catalyst in which metal particles are supported on a carbon material (carbon particles, carbon nanotubes, graphene, or the like) may be used. 
     The gas diffusion layer of the cathode  21  is not particularly limited, and examples thereof include carbon paper and carbon cloth. 
     A method for manufacturing the cathode  21  is not particularly limited, and for example, a method in which a liquid composition containing a cathode catalyst is applied to a surface of the gas diffusion layer on the liquid flow path  23   a  side and is then dried can be exemplified. 
     The anode  22  is an electrode that oxidizes hydroxide ions to generate oxygen. The anode  22  may be any one as long as it can electrochemically oxidize hydroxide ions and the generated oxygen permeates to the gas flow path  25   a , and for example, an electrode in which an anode catalyst layer is formed on the liquid flow path  23   a  side of the gas diffusion layer can be exemplified. 
     An anode catalyst that forms the anode catalyst layer is not particularly limited, and a known anode catalyst can be used. Specifically, examples thereof include metals such as platinum, palladium, and nickel, alloys thereof, intermetallic compounds thereof, metal oxides such as manganese oxide, iridium oxide, nickel oxide, cobalt oxide, iron oxide, tin oxide, indium oxide, ruthenium oxide, lithium oxide and lanthanum oxide, and metal complexes such as ruthenium complex and rhenium complex. As the anode catalyst, one type may be used alone, or two or more types may be used in combination. 
     Examples of the gas diffusion layer of the anode  22  include carbon paper and carbon cloth. Further, as the gas diffusion layer, a porous body such as a mesh material, a punching material, a porous material, and a metal fiber sintered body may be used. Examples of a material of the porous body include metals such as titanium, nickel, and iron, and alloys thereof (for example, SUS). 
     As a material of the liquid flow path structures  28  and  29 , for example, a fluororesin such as polytetrafluoroethylene can be exemplified. 
     Examples of a material of the gas flow path structures  24  and  25  include metals such as titanium and SUS, and carbon. 
     Examples of a material of the power supply bodies  26  and  27  include metals such as copper, gold, titanium, and SUS, and carbon. As the power supply bodies  26  and  27 , those having a surface of a copper base material plated with gold or the like may be used. 
     The anion exchange membrane  23  is not particularly limited, and a conventionally known anion exchange membrane can be used. 
     The electrochemical reaction device  2  is a flow cell in which the anode side electrolytic solution B supplied from the absorption part  12  flows through the liquid flow path  29   a , a cathode side electrolytic solution D supplied from the pH adjuster  52  flows through the liquid flow path  28   a , and the concentrated gas G 4  supplied from the concentration part  13  flows through the gas flow path  24   a . Then, when a voltage is applied to the cathode  21  and the anode  22 , carbon dioxide in the concentrated gas G 4  flowing through the gas flow path  24   a  is reduced, and a carbon compound and hydrogen are generated.  FIG.  3    shows an electrochemical reaction in an electrochemical cell of the electrochemical reaction device  2 . Since CO 3   2−  is consumed at the cathode, a pH of the cathode side electrolytic solution is higher on the outlet side than on the inlet side of the liquid flow path  28   a . The anode consumes hydroxide ions, but since an equal amount of hydroxide ions are supplied from the cathode side, the pH of the anode side electrolytic solution does not change between the inlet side and the outlet side of the liquid flow path  29   a . The pH adjuster  52  adjusts the pH of the cathode side electrolytic solution C to generate the cathode side electrolytic solution D, but in the adjustment of the pH, an alkali such as KOH or an alkaline aqueous solution can be used to raise the pH, or carbon dioxide gas can be used to lower the pH. As the carbon dioxide supplied to the pH adjuster  52 , for example, carbon dioxide in the concentrated gas generated by the concentration part  11  or the concentration part  13  can be used. 
     Examples of the carbon compound generated by reducing carbon dioxide at the cathode  21  include carbon monoxide, ethylene, ethanol, and the like. For example, as shown in  FIG.  3   , carbon monoxide and ethylene are generated as gaseous products by the following reaction. Hydrogen is also generated at the cathode  21  by the following reaction. The generated gaseous carbon compound and hydrogen pass through the gas diffusion layer of the cathode  21  and flow out of the gas flow path  24   a.  
 
CO 2 +H 2 O→CO+2OH − 
 
2CO+8H 2 O→C 2 H 4 +8OH − +2H 2 O
 
2H 2 O→H 2 +2OH − 
 
     Further, the hydroxide ions generated at the cathode  21  move to the anode  22  in the anode side electrolytic solution B and are oxidized by the following reaction to generate oxygen. The generated oxygen passes through the gas diffusion layer of the anode  22  and is discharged from the gas flow path  25   a.  
 
4OH − →H 2 +2H 2 O
 
     At the cathode  21 , H 2  generation due to water electrolysis which is a side reaction that does not involve the desired CO 2  reduction reaction contributes to energy loss in CO 2  electrolysis. 
     In order to suppress the generation of H 2  at the cathode in the embodiment, a balance in catalytic activity of the anode and the cathode is very important. For example, when the anode has high activity and the cathode has low activity, O 2 generation at the anode must occur actively, and a reaction with the same number of electrons must also occur at the cathode, but when a CO 2  electrolysis reaction rate at the cathode is not sufficiently obtained, a water electrolysis H 2  generation reaction which is a side reaction occurs. And, in that event, a solution of an optimum reaction rate management changes according to the level and balance of deterioration of the catalyst in each of two electrodes during an operation of the carbon dioxide treatment apparatus. Therefore, it is useful when there is a means for flexibly managing the reaction rate at both electrodes. In the embodiment, the pH of the electrolytic solution is used as the means for flexibly managing the reaction rate. More specifically, the pH of the anode side electrolytic solution is made lower than the pH of the cathode side electrolytic solution. Examples of a means for adjusting the pH of the electrolytic solution include a means for adding an alkali such as KOH or an alkaline aqueous solution such as a KOH aqueous solution to the electrolytic solution (pH increase), a means for dissolving carbon dioxide in the electrolytic solution which is an alkaline aqueous solution (pH decrease), and the like. Normally, since the pH is lowered by blowing carbon dioxide and dissolving the carbon dioxide in an electrolytic solution which is a strong alkaline aqueous solution, the pH can be adjusted by controlling an amount of carbon dioxide dissolved in the electrolytic solution which is a strong alkaline aqueous solution. Further, a product thereof is usually a gas, and a generation rate of the target carbon compound and by-product H 2  can be quantified by sensing a gas flow rate, a H 2  concentration, and a target product concentration at the outlet of the carbon dioxide treatment apparatus  100 . From the quantification results, an optimum reaction rate at that time can be obtained under any catalyst deterioration condition by feeding-back [an amount of CO 2  dissolved in cathode side electrolytic solution] and [an amount of CO 2  dissolved in the anode side electrolytic solution] with [maximization of the generation rate of the target product] and [minimization of the generation rate of the by-product H 2 ] as objective variables. 
     Specific examples of the pH include the pH of the anode side electrolytic solution set to 14 or less, for example, in a range of 8 to 14, and the pH of the cathode side electrolytic solution set to more than 14. 
     In the carbon dioxide treatment apparatus  100 , the hydrogen generation at the cathode  21  is suppressed by making the pH of the cathode side electrolytic solution used in the electrochemical reaction device  2  higher than the pH of the anode side electrolytic solution. Thus, for example, as compared with a case in which carbon dioxide is adsorbed on an adsorbent and desorbed by heating to be reduced, the energy required for desorption of carbon dioxide can be reduced, energy efficiency can be increased, and the loss of carbon dioxide can also be reduced. 
     The power storage device  3  is a device that supplies electric power to the electrochemical reaction device  2 . 
     In the conversion part  31 , renewable energy is converted into electrical energy. The conversion part  31  is not particularly limited, and examples thereof include a wind power generator, a solar power generator, and a geothermal power generator. The number of conversion parts  31  included in the power storage device  3  may be one, or two or more. 
     In the storage part  32 , electrical energy converted by the conversion part  31  is stored. Electric power can be stably supplied to the electrochemical reaction device  2  even during a time period when the conversion part does not generate power by storing the converted electrical energy in the storage part  32 . Further, when renewable energy is used, voltage fluctuations tend to be large in general, but once the renewable energy is stored in the storage part  32 , electric power can be supplied to the electrochemical reaction device  2  at a stable voltage. 
     The storage part  32  in this example is a nickel metal hydride battery. The storage part  32  may be any one as long as it can be charged and discharged, and may be, for example, a lithium ion secondary battery or the like. 
     As shown in  FIG.  4 A , the storage part  32  is a nickel metal hydride battery including a positive electrode  33 , a negative electrode  34 , a separator  37  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  37 , and a negative electrode side flow path  35  formed between the negative electrode  34  and the separator  37 . The positive electrode side flow path  36  and the negative electrode side flow path  35  can be formed using, for example, the same liquid flow path structure as that of the liquid flow path  28   a  ( 29   a ) of the electrochemical reaction device  2 . 
     As the positive electrode  33 , for example, one in which a positive electrode active material is applied on the positive electrode side flow path  36  side of a positive electrode current collector can be exemplified. 
     The positive electrode current collector is not particularly limited, and examples thereof can include a nickel foil and a nickel-plated metal foil. 
     The positive electrode active material is not particularly limited, and examples thereof can include nickel hydroxide and nickel oxyhydroxide. 
     As the negative electrode  34 , for example, a negative electrode active material applied on the negative electrode side flow path  35  side of a negative electrode current collector can be exemplified. 
     The negative electrode current collector is not particularly limited, and examples thereof can include nickel meshes. 
     The negative electrode active material is not particularly limited, and examples thereof can include known hydrogen storage alloys. 
     The separator  37  is not particularly limited, and examples thereof can include ion exchange membranes. 
     The nickel metal hydride battery of the storage part  32  is a flow cell in which the electrolytic solution flows in each of the positive electrode side flow path  36  on the positive electrode  33  side of the separator  37  and the negative electrode side flow path  35  on the negative electrode side  34  side of the separator  37 . In the carbon dioxide treatment apparatus  100 , the anode side electrolytic solution B supplied from the absorption part  12  through the liquid flow path  62 , and the anode side electrolytic solution A supplied from the electrochemical reaction device  2  through the liquid flow path  66   a  flow through the negative electrode side flow path  35  and the positive electrode side flow path  36 , respectively. Further, the connection of the liquid flow paths  62  and  65  to the storage part  32  can be switched between a state in which each of the liquid flow paths  62  and  65  is connected to the negative electrode side flow path  35  and a state in which each of the liquid flow paths  62  and  65  is connected to the positive electrode side flow path  36 . Similarly, the connection of the liquid flow paths  66   a  and  66   b  to the storage part  32  can be switched between a state in which each of the liquid flow paths  66   a  and  66   b  is connected to the positive electrode side flow path  36  and a state in which each of the liquid flow paths  66   a  and  66   b  is connected to the negative electrode side flow path  35 . 
     When the nickel metal hydrogen battery is discharged, hydroxide ions are generated from water molecules at the positive electrode, and the hydroxide ions transferred to the negative electrode receive hydrogen ions from the hydrogen storage alloy to generate water molecules. Therefore, from the viewpoint of discharge efficiency, it is advantageous that the electrolytic solution flowing through the positive electrode side flow path  36  is in a strong alkaline state, and it is advantageous that the electrolytic solution flowing through the negative electrode side flow path  35  is in a weak alkaline state. Therefore, at the time of discharge, as shown in  FIG.  4 A , it is preferable that the liquid flow paths  62  and  65  be connected to the negative electrode side flow path  35 , the liquid flow paths  66   a  and  66   b  be connected to the positive electrode side flow path  36 , the anode side electrolytic solution B (weak alkali) supplied from the absorption part  12  flow through the negative electrode side flow path  35 , and the anode side electrolytic solution A (strong alkali) supplied from the electrochemical reaction device  2  flows through the positive electrode side flow path  36 . That is, at the time of discharge, it is preferable that the electrolytic solution be circulated in the order of the absorption part  12 , the negative electrode side flow path  35  of the storage part  32 , the electrochemical reaction device  2 , the positive electrode side flow path  36  of the storage part  32 , and the absorption part  12 . 
     Further, when the nickel metal hydrogen battery is charged, water molecules are generated from hydroxide ions at the positive electrode, water molecules are decomposed into hydrogen atoms and hydroxide ions at the negative electrode, and the hydrogen atoms are stored in the hydrogen storage alloy. Therefore, from the viewpoint of charging efficiency, it is advantageous that the electrolytic solution flowing through the positive electrode side flow path  36  is in a weak alkaline state, and it is advantageous that the electrolytic solution flowing through the negative electrode side flow path  35  is in a strong alkaline state. Therefore, at the time of charging, as shown in  FIG.  4 B , it is preferable that the liquid flow paths  62  and  65  be connected to the positive electrode side flow path  36 , the liquid flow paths  66   a  and  66   b  be connected to the negative electrode side flow path  35 , the anode side electrolytic solution B (weak alkali) supplied from the absorption part  12  flows through the positive electrode side flow path  36 , and the anode side electrolytic solution A (strong alkali) supplied from the electrochemical reaction device  2  flow through the negative electrode side flow path  35 . That is, at the time of charging, it is preferable that the electrolytic solution be circulated in the order of the absorption part  12 , the positive electrode side flow path  36  of the storage part  32 , the electrochemical reaction device  2 , the negative electrode side flow path  35  of the storage part  32 , and the absorption part  12 . 
     In general, when a secondary battery is incorporated into the apparatus, the overall energy efficiency tends to decrease by an amount of charge and discharge efficiency. However, as described above, the charge and discharge efficiency according to an amount of “concentration overvoltage” of an electrode reaction represented by the Nernst equation can be improved by appropriately replacing the electrolytic solutions flowing through the positive electrode side flow path  36  and the negative electrode side flow path  35  of the storage part  32  using pH gradients of the anode side electrolytic solution A and the anode side electrolytic solution B before and after the electrochemical reaction device  2 . 
     The carbon increase reaction device  4  is a device for increasing an amount of ethylene generated by reducing carbon dioxide in the electrochemical reaction device  2  to increase the number of carbon atoms. 
     Ethylene gas E generated by the reduction at the cathode  21  of the electrochemical reaction device  2  is sent to the reactor  41  through the gas flow path  74 . In the reactor  41 , an ethylene multimerization reaction is carried out in the presence of an olefin multimerization catalyst. Thus, for example, olefins, in which the number of carbon atoms is increased, such as 1-butene, 1-hexene, and 1-octene can be produced. 
     The olefin multimerization catalyst is not particularly limited, and a known catalyst used for a multimerization reaction can be used, and examples thereof can include a solid acid catalyst using silica alumina or zeolite as a carrier and a transition metal complex compound. 
     In the carbon increase reaction device  4  of this example, a generated gas F after the multimerization reaction flowing out of the reactor  41  is sent to the gas-liquid separator  42  through the gas flow path  72 . An olefin having 6 or more carbon atoms is a liquid at room temperature. Therefore, for example, when the olefin having 6 or more carbon atoms is set as the target carbon compound, the olefin having 6 or more carbon atoms (an olefin liquid J 1 ) and the olefin having less than 6 carbon atoms (an olefin gas J 2 ) can be easily separated into gas and liquid by setting a temperature of the gas-liquid separator  42  to about 30° C. Further, the number of carbon atoms in the obtained olefin liquid J 1  can be considerably increased by raising the temperature of the gas-liquid separator  42 . 
     When the gas G 1  supplied to the concentration part  11  of the recovery device  1  is the atmosphere, the separated gas G 3  sent from the concentration part  11  through the gas flow path  71  may be used for cooling a generated gas D in the gas-liquid separator  42 . For example, using the gas-liquid separator  42  equipped with a cooling pipe, the separated gas G 3  is passed through the cooling pipe, a generated gas F is passed outside the cooling pipe and is aggregated on a surface of the cooling pipe to obtain the olefin liquid J 1 . Further, since the olefin gas J 2  separated in the gas-liquid separator  42  contains unreacted components such as ethylene and an olefin having a smaller number of carbon atoms than that in the target olefin, the olefin gas J 2  can be returned to the reactor  41  through the gas flow path  73  and reused for the multimerization reaction. 
     The ethylene multimerization reaction in the reactor  41  is an exothermic reaction in which a supplied substance has a higher enthalpy than a generated substance and reaction enthalpy is negative. In the carbon dioxide treatment apparatus  100 , a heat medium K may be heated using reaction heat generated in the reactor  41  of the carbon increase reaction device  4 , the heat medium K may be circulated to the heat exchanger  43  through the circulation flow path  69 , and heat may be exchanged between the heat medium K and the anode side electrolytic solution B in the heat exchanger  43 . In this case, the anode side electrolytic solution B supplied to the electrochemical reaction device  2  is heated. In the anode side electrolytic solution B using a strong alkaline aqueous solution, since dissolved carbon dioxide is less likely to separate as a gas even when the temperature is raised, and a temperature of the anode side electrolytic solution B rises, a reaction rate of oxidation and reduction in the electrochemical reaction device  2  improves. 
     The carbon increase reaction device  4  may further include a reactor in which a hydrogenation reaction of an olefin obtained by increasing the amount of ethylene is performed, or a reactor in which an isomerization reaction of an olefin or paraffin is performed using the hydrogen generated in the electrochemical reaction device  2 . 
     [Carbon Dioxide Treatment Method] 
     A carbon dioxide treatment method according to one aspect of the present invention is a method including the following Steps (a) and (b). The carbon dioxide treatment method of the present invention can be used in a method for producing a carbon compound. That is, a carbon compound obtained by reducing carbon dioxide or a carbon compound obtained using a carbon compound obtained by reducing carbon dioxide as a raw material can be produced using the carbon dioxide treatment method of the present invention. 
     Step (a): The carbon dioxide gas is brought into contact with the anode side electrolytic solution composed of a strong alkaline aqueous solution, and carbon dioxide is dissolved and absorbed in the anode side electrolytic solution. 
     Step (b): The pH of the cathode side electrolytic solution is adjusted to be higher than the pH of the anode side electrolytic solution. 
     Step (c): The cathode side electrolytic solution is supplied between the cathode and the anion exchange membrane, the anode side electrolytic solution is supplied between the anode and the anion exchange membrane, the carbon dioxide gas is supplied to the side of the cathode opposite to the anode, and the carbon dioxide gas is electrochemically reduced to generate a carbon compound and hydrogen. 
     When a carbon dioxide treatment apparatus including the carbon increase reaction device as in the carbon dioxide treatment apparatus  100  is used, the carbon dioxide treatment method further includes the following Step (d) in addition to Steps (a) to (c). Hereinafter, as an example of the carbon dioxide treatment method, a case in which the above-described carbon dioxide treatment apparatus  100  is used will be described. 
     Step (d): The amount of ethylene generated by the reduction of carbon dioxide is increased. 
     In the carbon dioxide treatment method using the carbon dioxide treatment apparatus  100 , first, exhaust gas, the atmosphere, and the like are supplied to the concentration part  11  as the gas G 1 , and the carbon dioxide is concentrated to obtain the concentrated gas G 2 . As described above, since the absorption of carbon dioxide by the anode side electrolytic solution A in the absorption part  12  assists the concentration, it is not necessary to concentrate carbon dioxide to a high concentration in the concentration part  11 . The concentration of carbon dioxide in the concentrated gas G 2  can be appropriately set and can be, for example, 25 to 85% by volume. 
     In Step (a), the concentrated gas G 2  is supplied from the concentration part  11  to the absorption part  12 , the concentrated gas G 2  is brought into contact with the anode side electrolytic solution A, and carbon dioxide in the concentrated gas G 2  is dissolved and absorbed in the anode side electrolytic solution A. The anode side electrolytic solution B in which carbon dioxide is dissolved is in the weak alkaline state. Further, the anode side electrolytic solution B may be supplied from the absorption part  12  to the heat exchanger  43  via the storage part  32 , and the anode side electrolytic solution B heated by heat exchange with the heat medium K may be supplied to the electrochemical reaction device  2 . The temperature of the anode side electrolytic solution B supplied to the electrochemical reaction device  2  can be appropriately set and can be, for example, 65 to 105° C. 
     In Step (b), the pH of the cathode side electrolytic solution is adjusted to be higher than the pH of the anode side electrolytic solution. The pH of the cathode side electrolytic solution can be adjusted by adding an alkali or an alkaline aqueous solution (pH increase), coming into contact with carbon dioxide (pH decrease), or the like. For example, the pH of the cathode side electrolytic solution may be more than 14, and the pH of the anode side electrolytic solution may be 14 or less, specifically in a range of 8 to 14. 
     In Step (c), the anode side electrolytic solution B flows through the liquid flow path  29   a  of the electrochemical reaction device  2 , the cathode side electrolytic solution D flows through the liquid flow path  28   a , the concentrated gas G 4  generated by the concentration part  13  flows through the gas flow path  24   a , and power is supplied from the power storage device  3  to the electrochemical reaction device  2  so that a voltage is applied between the cathode  21  and the anode  22 . The carbon dioxide gas contained in the concentrated gas G 4  is electrochemically reduced to generate a carbon compound, and water is reduced to generate hydrogen. At this time, at the anode  22 , the hydroxide ions in the anode side electrolytic solution B are oxidized to generate oxygen. An amount of dissolved carbon dioxide in the anode side electrolytic solution B decreases as the reduction progresses, and the anode side electrolytic solution A in the strong alkaline state flows out of an outlet of the liquid flow path  29   a . The gaseous carbon compound and hydrogen generated by the reduction of carbon dioxide permeate the gas diffusion layer of the cathode  21 , flow out of the electrochemical reaction device  2  through the gas flow path  24   a , and are sent to the carbon increase reaction device  4 . 
     In Step (d), ethylene gas E generated by the reduction of carbon dioxide is sent to the reactor  41  and is brought into gas phase contact with the olefin multimerization catalyst in the reactor  41  to increase the amount of ethylene. Thus, an olefin in which the amount of ethylene is increased can be obtained. For example, when an olefin having 6 or more carbon atoms is set as the target carbon compound, the generated gas F from the reactor  41  is sent to the gas-liquid separator  42  and is cooled to about 30° C. Then, since the target olefin having 6 or more carbon atoms (for example, 1-hexene) is liquefied, and the olefin having less than 6 carbon atoms remains as a gas, the olefin liquid J 1  (the target carbon compound) can be easily separated from the olefin gas J 2 . The number of carbon atoms in each of the olefin liquid J 1  and the olefin gas J 2  to be separated into a gas and a liquid can be adjusted according to a temperature in the gas-liquid separation. 
     The olefin gas J 2  after the gas-liquid separation can be returned to the reactor  41  and can be reused for the multimerization reaction. In this way, when the olefin having a smaller number of carbon atoms than that of the target olefin is circulated between the reactor  41  and the gas-liquid separator  42 , in the reactor  41 , it is preferable to adjust a contact time between a raw material gas (a mixed gas of the ethylene gas E and the olefin gas J 2 ) and the catalyst and to perform the control under a condition in which each molecule causes averagely one multimerization reaction. Thus, since an unintentional increase of the number of carbon atoms in the olefin generated in the reactor  41  is suppressed, the olefin having the desired number of carbon atoms (the olefin liquid J 1 ) can be selectively separated in the gas-liquid separator  42 . 
     According to such a method, valuable resources can be efficiently obtained from a renewable carbon source with high selectivity. Therefore, a large-scale refining facility such as a distillation column required in conventional petrochemistry using a Fisher-Tropsch (FT) synthesis method or a MtG method is not required, and an economic advantage is provided overall. 
     A reaction temperature of the multimerization reaction is preferably 200 to 350° C. 
     A reaction time of the multimerization reaction, that is, the contact time between the raw material gas and the olefin multimerization catalyst is preferably 10 to 250 g·min/mol in terms of W/F from the viewpoint of suppressing an excessive multimerization reaction and improving selectivity of the target carbon compound. 
     An olefin having a smaller number of carbon atoms than that in the target olefin may be circulated between the reactor  41  and the gas-liquid separator  42  to adjust the contact time between the raw material gas and the catalyst, and thus the selectivity of the produced carbon compound may be improved. 
     Further, the olefin obtained by increasing the amount of ethylene may be hydrogenated to obtain paraffin, or the isomerization may be further performed. 
     As the hydrogenation reaction of the olefin, a known method can be adopted, and for example, a method in which the hydrogenation reaction is performed using a solid acid catalyst such as silica alumina or zeolite can be exemplified. 
     As the isomerization reaction, a known method can be adopted, and for example, a method in which the isomerization reaction is performed using a solid acid catalyst such as silica alumina or zeolite can be exemplified. 
     A reaction temperature in the reactor  84  is preferably 200 to 350° C. 
     As described above, in an aspect of the present invention, an electrolytic solution composed of a strong alkaline aqueous solution is used, and the electrolytic solution in which carbon dioxide is dissolved is supplied between the cathode and the anode by the recovery device, and the dissolved carbon dioxide in the electrolytic solution is electrochemically reduced. Therefore, the energy efficiency in the carbon dioxide recovery and the reduction is high, and loss of carbon dioxide is also reduced. 
     The present invention is not limited to the above-described aspect. 
     Further, the carbon dioxide treatment apparatus of the embodiment may be a carbon dioxide treatment apparatus that includes none of the carbon increase reaction device, the heat exchanger, and the pH adjuster. For example, ethylene may be produced using a carbon dioxide treatment method in which the carbon dioxide treatment apparatus is adopted. 
     Further, in the carbon dioxide treatment apparatus of the embodiment, the electrochemical reaction device and the power storage device may not share the electrolytic solution, and the electrolytic solution may be circulated only between the absorption part of the recovery device and the electrochemical reaction device. 
     In addition, it is appropriately possible to replace the constituent elements in the above-described embodiment with well-known constituent elements without departing from the spirit of the present invention, and the above-described modified examples may be appropriately combined. 
     EXAMPLES 
     In the carbon dioxide treatment apparatus  100  shown in  FIG.  1   , a CO 2  electrolysis test was conducted by changing the combination of potassium hydroxide concentration (molar concentration) of each of the cathode side electrolytic solution and the anode side electrolytic solution. The results of the CO 2  electrolysis test (Faraday efficiency (%) of ethylene, carbon monoxide, methane and hydrogen) are shown in the graph of  FIG.  5   . 
     Example 1 
     A KOH concentration of the cathode side electrolytic solution was 7 M, and a KOH concentration of the anode side electrolytic solution was 1 M. 
     Comparative Example 1 
     The KOH concentration of both the cathode side electrolytic solution and the anode side electrolytic solution was set to 1 M. 
     Comparative Example 2 
     The KOH concentration of both the cathode side electrolytic solution and the anode side electrolytic solution was set to 7 M. 
     Comparative Example 3 
     The KOH concentration of both the cathode side electrolytic solution and the anode side electrolytic solution was set to 10 M. 
     &lt;Results&gt; 
     Example 1 showed the highest ethylene Faraday efficiency. From this, it was found that the CO 2  electrolysis efficiency can be improved by making the pH of the electrolytic solution higher than the pH of the anode side electrolytic solution, that is, creating a hydrogen ion concentration gradient. 
     In Example 1, it is considered that, when the KOH concentration of the anode side electrolytic solution is set to 1 M, the oxygen generation became milder than in a case in which the KOH concentration was 7 M, and the reaction rate balance between the two electrodes was improved. As a result, charge compensation can be achieved without any problem even at the cathode, and it is considered that CO 2  electrolysis became the main reaction. 
     In Comparative example 2, the oxygen generation becomes advantageous, and the cathode becomes rate-determining Therefore, it is thought that an overvoltage is locally applied in a region in which electrons are likely to be supplied when trying to perform the charge compensation at the cathode forcibly, and thus a potential region in which hydrogen generation is advantageous is also shifted. 
     While preferred embodiments of the invention have been described and illustrated above, it should be understood that these are exemplary of the invention and are not to be considered as limiting. Additions, omissions, substitutions, and other modifications can be made without departing from the spirit or scope of the present invention. Accordingly, the invention is not to be considered as being limited by the foregoing description, and is only limited by the scope of the appended claims. 
     EXPLANATION OF REFERENCES 
     
         
         
           
               1  Recovery device 
               2  Electrochemical reaction device 
               3  Power storage device 
               4  Carbon increase reaction device 
               6  Number of carbon atoms 
               11  Concentration part 
               12  Absorption part 
               13  Concentration part 
               21  Cathode 
               22  Anode 
               23  Anion exchange membrane 
               23   a  Liquid flow path 
               24  Gas flow path structure 
               24   a  Gas flow path 
               25  Gas flow path structure 
               25   a  Gas flow path 
               26  Power supply body 
               27  Power supply body 
               28  Liquid flow path structure 
               28   a  Liquid flow path 
               29  Liquid flow path structure 
               29   a  Liquid flow path 
               31  Conversion part 
               32  Storage part 
               33  Positive electrode 
               34  Negative electrode 
               35  Negative electrode side flow path 
               36  Positive electrode side flow path 
               37  Separator 
               41  Reactor 
               42  Gas-liquid separator 
               43  Heat exchanger 
               50  Cooler 
               51  pH measuring device 
               52  pH adjuster 
               53  pH measuring device 
               62  Liquid flow path 
               63  Liquid flow path 
               64  Liquid flow path 
               65  Liquid flow path 
               66  Liquid flow path 
               66   a  Liquid flow path 
               66   b  Liquid flow path 
               69  Circulation flow path 
               70  Gas flow path 
               71  Gas flow path 
               72  Gas flow path 
               73  Gas flow path 
               74  Gas flow path 
               75  Gas flow path 
               76  Gas flow path 
               77  Gas flow path 
               84  Reactor 
               100  Carbon dioxide treatment apparatus 
             A Anode side electrolytic solution 
             B Anode side electrolytic solution 
             C Cathode side electrolytic solution 
             D Cathode side electrolytic solution 
             E Ethylene gas 
             F Generated gas 
             G 1  Gas 
             G 2  Concentrated gas 
             G 3  Separated gas 
             G 4  Concentrated gas 
             G 5  Concentrated gas 
             J 1  Olefin liquid 
             J 2  Olefin gas 
             K Heat medium