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United States Patent Application 20170241026
Kind Code A1
ONO; Akihiko ;   et al. August 24, 2017

ELECTROCHEMICAL REACTION DEVICE

Abstract

An electrochemical reaction device includes: a first electrolytic solution tank including first and second storage parts storing first and second electrolytic solutions containing carbon dioxide and water respectively; reduction and oxidation electrodes immersed in the first and second electrolytic solutions respectively; a generator connected to the reduction and oxidation electrodes; a second electrolytic solution tank including a third storage part storing a third electrolytic solution containing carbon dioxide; and a flow path connecting the first and third storage parts. The third electrolytic solution is lower in temperature than the first electrolytic solution.


Inventors: ONO; Akihiko; (Kita, JP) ; MIKOSHIBA; Satoshi; (Yamato, JP) ; KUDO; Yuki; (Yokohama, JP) ; KITAGAWA; Ryota; (Setagaya, JP) ; TAMURA; Jun; (Chuo, JP) ; SUGANO; Yoshitsune; (Kawasaki, JP) ; TSUTSUMI; Eishi; (Kawasaki, JP) ; YAMAGIWA; Masakazu; (Yokohama, JP)
Applicant:
Name City State Country Type

KABUSHIKI KAISHA TOSHIBA

Minato-ku

JP
Assignee: KABUSHIKI KAISHA TOSHIBA
Minato-ku
JP

Family ID: 1000002184692
Appl. No.: 15/261095
Filed: September 9, 2016


Current U.S. Class: 1/1
Current CPC Class: C25B 9/10 20130101; C25B 15/08 20130101; C25B 1/003 20130101; C25B 1/10 20130101
International Class: C25B 9/10 20060101 C25B009/10; C25B 1/00 20060101 C25B001/00; C25B 15/08 20060101 C25B015/08; C25B 1/10 20060101 C25B001/10

Foreign Application Data

DateCodeApplication Number
Feb 23, 2016JP2016-032480

Claims



1. An electrochemical reaction device comprising: a first electrolytic solution tank including a first storage part storing a first electrolytic solution containing carbon dioxide and a second storage part storing a second electrolytic solution containing water; a reduction electrode immersed in the first electrolytic solution; an oxidation electrode immersed in the second electrolytic solution; a generator connected to the reduction electrode and the oxidation electrode; a second electrolytic solution tank including a third storage part storing a third electrolytic solution containing carbon dioxide; and a flow path connecting the first storage part and the third storage part, wherein a temperature of the third electrolytic solution is lower than a temperature of the first electrolytic solution.

2. The device of claim 1, further comprising: a first separation tank including a fourth storage part storing a fourth electrolytic solution containing carbon dioxide and a first gas-liquid separation membrane dividing the fourth storage part into a plurality of regions; a second separation tank including a fifth storage part storing a fifth electrolytic solution containing water and a second gas-liquid separation membrane dividing the fifth storage part into a plurality of regions; a second flow path connecting the first storage part and the fourth storage part; a third flow path connecting the third storage part and the fourth storage part; and a fourth flow path connecting the second storage part and the fifth storage part.

3. The device of claim 2, further comprising: a carbon dioxide generation source containing carbon dioxide having a higher temperature than the temperature of the first electrolytic solution; a reduction reaction device reducing a product produced by a reduction reaction of the carbon dioxide; a distiller disposed on the third storage part; a fifth flow path connecting the first storage part and the carbon dioxide generation source; and a sixth flow path connecting the third storage part and the reduction reaction device.

4. The device of claim 3, further comprising: a first cooler disposed at the third storage part; a first heater disposed at the second flow path; a second cooler disposed at the fourth flow path; and a second heater disposed at the fifth flow path.

5. The device of claim 3, wherein the device pertains at least one of heat exchange between the second electrolytic solution tank and the second separation tank, heat exchange between the first separation tank and the second separation tank, heat exchange between the reduction reaction device and the second electrolytic solution tank, heat exchange between the carbon dioxide generation source and the distiller, or heat exchange between the reduction reaction device and the distiller.

6. The electrochemical reaction device of claim 3, further comprising at least one of a heat transfer member connecting between the second electrolytic solution tank and the second separation tank, a heat transfer member connecting between the first separation tank and the second separation tank, a heat transfer member connecting between the reduction reaction device and the second electrolytic solution tank, a heat transfer member connecting between the carbon dioxide generation source and the distiller, or a heat transfer member connecting between the reduction reaction device and the distiller.

7. The device of claim 1, wherein the generator includes a photoelectric conversion body having a first face connected to the reduction electrode and a second face connected to the oxidation electrode.

8. The electrochemical reaction device of claim 1, further comprising an ion exchange membrane disposed between the first storage part and the second storage part.

9. The device of claim 1, wherein a pressure applied to the third electrolytic solution is higher than a pressure applied to the first electrolytic solution.

10. An electrochemical reaction device comprising: a first electrolytic solution tank including a first storage part storing a first electrolytic solution containing carbon dioxide and a second storage part storing a second electrolytic solution containing water; a reduction electrode immersed in the first electrolytic solution; an oxidation electrode immersed in the second electrolytic solution; a generator connected to the reduction electrode and the oxidation electrode; a second electrolytic solution tank including a third storage part storing a third electrolytic solution containing carbon dioxide; and a flow path connecting the first storage part and the third storage part, wherein a pressure applied to the third electrolytic solution is higher than a pressure applied to the first electrolytic solution.

11. The device of claim 10, further comprising: a first separation tank including a fourth storage part storing a fourth electrolytic solution containing carbon dioxide and a first gas-liquid separation membrane dividing the fourth storage part into a plurality of regions; a second separation tank including a fifth storage part storing a fifth electrolytic solution containing water and a second gas-liquid separation membrane dividing the fifth storage part into a plurality of regions; a second flow path connecting the first storage part and the fourth storage part; a third flow path connecting the third storage part and the fourth storage part; and a fourth flow path connecting the second storage part and the fifth storage part.

12. The device of claim 11, further comprising: a carbon dioxide generation source containing carbon dioxide having a higher temperature than the temperature of the first electrolytic solution; a reduction reaction device reducing a product produced by a reduction reaction of the carbon dioxide; a distiller disposed on the third storage part; a fifth flow path connecting the first storage part and the carbon dioxide generation source; and a sixth flow path connecting the third storage part and the reduction reaction device.

13. The device of claim 10, wherein the generator includes a photoelectric conversion body having a first face connected to the reduction electrode and a second face connected to the oxidation electrode.

14. The electrochemical reaction device of claim 10, further comprising an ion exchange membrane disposed between the first storage part and the second storage part.
Description



CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2016-032480, filed on Feb. 23, 2016; the entire contents of which are incorporated herein by reference.

FIELD

[0002] Embodiments described herein relate generally to an electrochemical reaction device.

BACKGROUND

[0003] Artificial photosynthesis technology of electrochemically converting sunlight into a chemical substance in imitation of photosynthesis of plants is under development from viewpoints of energy problem and environmental problem. This is because, for example, this technology makes it possible to obtain sufficient energy even if a chemical substance produced by the conversion from sunlight in a land which is of low utility value and not used for the production of plants, such as, for example, a desert is transported to a distant place. Converting sunlight to a chemical substance to store it in a cylinder or a tank is advantageous in that it costs lower for energy storage and has a less storage loss than converting sunlight to electricity to store it in storage batteries.

[0004] As a photoelectrochemical reaction device that electrochemically converts sunlight to a chemical substance, there has been known, for example, a two-electrode type device that includes an electrode having a reduction catalyst for reducing carbon dioxide (CO.sub.2) and an electrode having an oxidation catalyst for oxidizing water (H.sub.2O), these electrodes being immersed in water in which carbon dioxide is dissolved. In this case, the electrodes are electrically connected to each other via an electric wire or the like. The electrode having the oxidation catalyst oxidizes H.sub.2O using light energy to produce oxygen (1/2O.sub.2) and obtains a potential. The electrode having the reduction catalyst obtains the potential from the electrode that causes the oxidation reaction, thereby reducing the carbon dioxide to produce formic acid (HCOOH) or the like. Such two-stage excitation for obtaining the reduction potential of the carbon dioxide makes the two-electrode type device low in conversion efficiency from the sunlight to the chemical energy.

BRIEF DESCRIPTION OF THE DRAWINGS

[0005] FIG. 1 is a schematic view illustrating a structure example of an electrochemical reaction device.

[0006] FIG. 2 is a schematic view illustrating another structure example of the electrochemical reaction device.

[0007] FIG. 3 is a schematic view illustrating a structure example of a photoelectric conversion cell.

[0008] FIG. 4 is a schematic view illustrating another structure example of the electrochemical reaction device.

[0009] FIG. 5 is a schematic view illustrating another structure example of the electrochemical reaction device.

[0010] FIG. 6 is a schematic view illustrating another structure example of the electrochemical reaction device.

[0011] FIG. 7 is a schematic view illustrating another structure example of the electrochemical reaction device.

DETAILED DESCRIPTION

[0012] An electrochemical reaction device of an embodiment includes: a first electrolytic solution tank including a first storage part storing a first electrolytic solution containing carbon dioxide and a second storage part storing a second electrolytic solution containing water; a reduction electrode immersed in the first electrolytic solution; an oxidation electrode immersed in the second electrolytic solution; a generator connected to the reduction electrode and the oxidation electrode; a second electrolytic solution tank including a third storage part storing a third electrolytic solution containing carbon dioxide; and a flow path connecting the first storage part and the third storage part. A temperature of the third electrolytic solution is lower than a temperature of the first electrolytic solution.

[0013] Embodiments will be hereinafter described with reference to the drawings. The drawings are schematic, and for example, the sizes such as the thickness and width of each constituent element may differ from the actual sizes of the constituent element. In the embodiments, substantially the same constituent elements are denoted by the same reference signs and a description thereof will be omitted in some case. In this specification, the term "connect" not only means "directly connect" but also may include the meaning of "indirectly connect".

[0014] FIG. 1 is a schematic view illustrating a structure example of an electrochemical reaction device. As illustrated in FIG. 1, the electrochemical reaction device includes an electrolytic solution tank 11, an electrolytic solution tank 12, a reduction electrode 31, an oxidation electrode 32, a photoelectric conversion body 33, an ion exchange membrane 4, a flow path 51, and a flow path 52.

[0015] The electrolytic solution tank 11 has a storage part 111 and a storage part 112. The electrolytic solution tank 11 is not limited to have a particular shape and may have any three-dimensional shape having a cavity serving as the storage part.

[0016] The storage part 111 stores an electrolytic solution 21 containing a substance to be reduced. The substance to be reduced is a substance that undergoes a reduction reaction to be reduced. The substance to be reduced contains, for example, carbon dioxide. Further, the substance to be reduced may contain hydrogen ions. Changing an amount of water contained in the electrolytic solution 21 or changing electrolytic solution components can change reactivity to change selectivity of the substance to be reduced and a ratio of a produced chemical substance.

[0017] The storage part 112 stores an electrolytic solution 22 containing a substance to be oxidized. The substance to be oxidized is a substance that undergoes an oxidation reaction to be oxidized. The substance to be oxidized is, for example, water, or an organic matter such as alcohol or amine, or an inorganic oxide such as iron oxide. The electrolytic solution 22 may contain the same substance as that contained in the electrolytic solution 21. In this case, the electrolytic solution 21 and the electrolytic solution 22 may be regarded as one electrolytic solution.

[0018] The electrolytic solution 22 preferably has higher pH than pH of the electrolytic solution 21. This facilitates the migration of hydrogen ions, hydroxide ions, and the like. Further, al quid junction potential due to the difference in pH enables effective progress of an oxidation-reduction reaction.

[0019] The electrolytic solution tank 12 has a storage part 113 storing an electrolytic solution 23. The electrolytic solution 23 contains carbon dioxide, for instance. The electrolytic solution tank 12 has a function as a reduction catalyst absorber. The temperature of the electrolytic solution 23 is lower than the temperature of the electrolytic solution 21.

[0020] The reduction electrode 31 is immersed in the electrolytic solution 21. The reduction electrode 31 contains a reduction catalyst for the substance to be reduced, for instance. A compound produced by the reduction reaction differs depending on, for example, the kind of the reduction catalyst. For example, the compound produced by the reduction reaction is: a carbon compound such as carbon monoxide (CO), formic acid (HCOOH), methane (CH.sub.4), methanol (CH.sub.3OH), ethane (C.sub.2H.sub.6), ethylene (C.sub.2H.sub.4), ethanol (C.sub.2H.sub.5OH), formaldehyde (HCHO), or ethylene glycol; or hydrogen. The compound produced by the reduction reaction may be recovered through a product flow path, for instance. In this case, the product flow path is connected to, for example, the storage part 111. The compound produced by the reduction reaction may be recovered through another flow path.

[0021] The reduction electrode 31 may have a structure in a thin film form, a lattice form, a granular form or a wire form, for instance. The reduction electrode 31 does not necessarily contain the reduction catalyst. A reduction catalyst provided separately from the reduction electrode 31 may be electrically connected to the reduction electrode 31.

[0022] The oxidation electrode 32 is immersed in the electrolytic solution 22. The oxidation electrode 32 contains an oxidation catalyst for the substance to be oxidized, for instance. A compound produced by the oxidation reaction differs depending on, for example, the kind of the oxidation catalyst. Examples of the compound produced by the oxidation reaction include hydrogen ions. The compound produced by the oxidation reaction may be recovered through a product flow path, for instance. In this case, the product flow path is connected to, for example, the storage part 112. The compound produced by the oxidation reaction may be recovered through another flow path.

[0023] The oxidation electrode 32 may have a structure in a thin film form, a lattice form, a granular form, or a wire form, for instance. The oxidation electrode 32 does not necessarily contain the oxidation catalyst. An oxidation catalyst provided separately from the oxidation electrode 32 may be electrically connected to the oxidation electrode 32.

[0024] In a case where the oxidation electrode 32 is stacked and immersed in the electrolytic solution 22, and where light is radiated to the photoelectric conversion body 33 through the oxidation electrode 32 to cause the oxidation-reduction reaction, the oxidation electrode 32 needs to have a light transmitting property. Light transmittance of the oxidation electrode 32 is preferably, for example, at least 10% or more, more preferably 30% or more of an irradiation amount of the light irradiating the oxidation electrode 32.

[0025] This is not restrictive, and the photoelectric conversion body 33 may be irradiated with the light through the reduction electrode 31, for instance.

[0026] The photoelectric conversion body 33 has a face 331 electrically connected to the reduction electrode 31 and a face 332 electrically connected to the oxidation electrode 32. In FIG. 1, the face 331 and the reduction electrode 31, and the face 332 and the oxidation electrode 32 are connected by heat transfer members such as wiring lines having a heat transfer property, for instance. Connecting the photoelectric conversion body to the reduction electrode or the oxidation electrode by the wiring line or the like is advantageous as a system, since constituent elements are separated according to the function. The photoelectric conversion body 33 may be disposed outside the electrolytic solution tank 11. Incidentally, the photoelectric conversion body 33 does not necessarily have to be provided. Another generator may be connected to the oxidation electrode 32 and the reduction electrode 31. The generator is not limited to the photoelectric conversion element having the photoelectric conversion body. Examples of the generator include a system power supply, a storage battery, or the renewable energy such as the wind power, water power, and the geothermal power.

[0027] The photoelectric conversion body 33 has a function of separating electric charges when given energy of the irradiating light such as sunlight. Electrons and holes generated by the charge separation migrate to the reduction electrode side and the oxidation electrode side respectively. Consequently, the photoelectric conversion body 33 can generate an electromotive force. As the photoelectric conversion body 33, a pn-junction or pin-junction photoelectric conversion body is usable, for instance. The photoelectric conversion body 33 may be fixed to the electrolytic solution tank 11, for instance. Incidentally, the photoelectric conversion body 33 may be composed of a stack of a plurality of photoelectric conversion layers.

[0028] The reduction electrode 31, the oxidation electrode 32, and the photoelectric conversion body 33 may be different in size.

[0029] The ion exchange membrane 4 is disposed so as to separate the storage part 111 and the storage part 112. Examples of the ion exchange membrane 4 include Neosepta (registered trademark) manufactured by ASTOM Corporation, Selemion (registered trademark) and Aciplex (registered trademark) manufactured by Asahi Glass Co. Ltd., fumasep (registered trademark) and fumapem (registered trademark) manufactured by Fumatech GmbH, Nafion (registered trademark), which is a fluorocarbon resin produced through polymerization of sulfonated tetrafluoroethylene, manufactured by Du Pont, Lewabrane (registered trademark) manufactured by LANXESS, IONSEP (registered trademark) manufactured by IONTECH, Mustang (registered trademark) manufactured by Pall Corporation, ralex (registered trademark) manufactured by MEGA a.s., and Gore-Tex (registered trademark) manufactured by W. L. Gore & Associates. The ion exchange membrane 4 may be formed of a film having a hydrocarbon basic skeleton or for anion exchange, may be formed of a film having an amine group. Incidentally, the ion exchange membrane 4 does not necessarily have to be provided.

[0030] The flow path 51 and the flow path 52 have a function as electrolytic solution flow paths to distribute the electrolytic solutions. Their function is not limited to this, and the electrolytic solutions and the products by the oxidation-reduction reaction may be distributed through the flow path 51 and the flow path 52. For the electrolytic solution tanks 11, 12 and the flow paths 51, 52, materials that transmit light may be used, for instance.

[0031] The flow path 51 connects the storage part 111 and the storage part 113. The ions and other substances contained in the electrolytic solution 21 can move to the electrolytic solution tank 12 through the flow path 51.

[0032] The flow path 52 connects the storage part 111 and the storage part 113. Ions and other substances contained in the electrolytic solution 23 can move to the electrolytic solution tank 11 through the flow path 52.

[0033] The shape of the flow path 51 and the flow path 52 is not limited to a particular shape, provided that they have a shape having a cavity allowing the electrolytic solutions to flow therethrough, such as a pipe shape. The electrolytic solution of at least one of the flow path 51 and the flow path 52 may be circulated by a circulation pump. At least part of the electrolytic solution 21 moves to the storage part 113 through the flow path 51, for instance. At least part of the electrolytic solution 23 moves to the storage part 111 through the flow path 52, for instance. The arrows illustrated in FIG. 1 indicate circulation directions of the electrolytic solutions.

[0034] Next, an operation example of the electrochemical reaction device illustrated in FIG. 1 will be described. When light is incident on the photoelectric conversion body 33, the photoelectric conversion body 33 generates photoexcited electrons and holes. At this time, the photoexcited electrons gather to the reduction electrode 31 and the holes gather to the oxidation electrode 32. Consequently, the electromotive force is generated in the photoelectric conversion body 33. As the light, sunlight is preferable, but light of a light emitting diode, an organic EL, or the like may be incident on the photoelectric conversion body 33.

[0035] The following describes a case where electrolytic solutions containing water and carbon dioxide are used as the electrolytic solution 21 and the electrolytic solution 22 and carbon monoxide is produced. Around the oxidation electrode 32, as expressed by the following formula (1), the water undergoes an oxidation reaction and loses electrons, so that oxygen and hydrogen ions are produced. At least one of the produced hydrogen ions migrates to the storage part 111 through the ion exchange membrane 4.

2H.sub.2O.fwdarw.4H.sup.++O.sub.2+4e.sup.- (1)

[0036] Around the reduction electrode 31, as expressed by the following formula (2), the carbon dioxide undergoes a reduction reaction and the hydrogen ions react with the carbon dioxide while receiving the electrons, so that carbon monoxide and water are produced. Further, in addition to the carbon monoxide, hydrogen is produced by the hydrogen ions receiving the electrons as expressed by the following formula (3). At this time, the hydrogen may be produced simultaneously with the carbon monoxide.

CO.sub.2+2H.sup.++2e.sup.-.fwdarw.CO+H.sub.2O (2)

2H.sup.++2e.sup.-.fwdarw.H.sub.2 (3)

[0037] The photoelectric conversion body 33 needs to have an open-circuit voltage equal to or more than a potential difference between a standard oxidation-reduction potential of the oxidation reaction and a standard oxidation-reduction potential of the reduction reaction. For example, the standard oxidation-reduction potential of the oxidation reaction in the formula (1) is 1.23 [V]. The standard oxidation-reduction potential of the reduction reaction in the formula (2) is -0.03 [V]. The standard oxidation-reduction potential of the reaction in the formula (3) is 0 V. In this case, the open-circuit voltage needs to be 1.26 [V] or more in the reactions of the formula (1) and the formula (2).

[0038] The open-circuit voltage of the photoelectric conversion body 33 is preferably higher than the potential difference between the standard oxidation-reduction potential of the oxidation reaction and the standard oxidation-reduction potential of the reduction reaction by a value of overvoltages or more. For example, the overvoltages of the oxidation reaction in the formula (1) and the reduction reaction in the formula (2) are both 0.2 [V]. The open-circuit voltage is preferably 1.66 [V] or more in the reactions of the formula (1) and the formula (2). Similarly, the open-circuit voltage is preferably 1.63 [V] or more in the reactions of the formula (1) and the formula (3).

[0039] The reduction reactions of hydrogen ions and carbon dioxide are reactions consuming hydrogen ions. This means that a small amount of the hydrogen ions results in low efficiency of the reduction reaction. So, the electrolytic solution 21 and the electrolytic solution 22 preferably have different hydrogen ion concentrations so that the concentration difference facilitates the migration of the hydrogen ions. The concentration of anions (for example, hydroxide ions) may be made different between the electrolytic solution 21 and the electrolytic solution 22.

[0040] Reaction efficiency of the formula (2) varies depending on the concentration of the carbon dioxide dissolved in the electrolytic solution. The higher the concentration of the carbon dioxide, the higher the reaction efficiency, and as the former is lower, the latter is lower. Since solubility of the carbon dioxide is low, it is difficult to increase the concentration of the carbon dioxide in the electrolytic solution. The reaction efficiency of the formula (2) also varies depending on the concentration of hydrogen carbonate ions or carbonate ions. However, the concentration of hydrogen carbonate ions or the concentration of carbonate ions can be adjusted by an increase of the electrolytic solution concentration or the adjustment of pH and thus is more easily adjusted than the carbon dioxide concentration. Incidentally, even if the ion exchange membrane is provided between the oxidation electrode and the reduction electrode, carbon dioxide gas, carbonate ions, hydrogen carbonate ions, and so on pass through the ion exchange membrane 4 and thus it is difficult to completely prevent performance deterioration.

[0041] A possible method to increase the carbon dioxide concentration may be, for example, a method of blowing the carbon dioxide directly to the electrolytic solution tank 11. However, in a case where the reduction product is gaseous carbon monoxide or the like, the carbon dioxide gas and the carbon monoxide gas need to be separated. This results in a cost increase due to the complication of the device, and an energy loss due to the need for energy for the separation.

[0042] The electrochemical reaction device of this embodiment includes the first electrolytic solution tank used for the oxidation-reduction reaction and the second electrolytic solution tank connected to the first electrolytic solution tank. The temperature of the electrolytic solution stored in the storage part of the second electrolytic solution tank is lower than the temperature of the electrolytic solution stored in the storage part of the first electrolytic solution tank. For example, cooling the storage part in the second electrolytic solution tank can make the temperature of the electrolytic solution stored in the second electrolytic solution tank lower than the temperature of the electrolytic solution stored in the storage part of the first electrolytic solution tank. Solubility of the carbon dioxide in the second electrolytic solution tank is higher than solubility of the carbon dioxide in the first electrolytic solution tank.

[0043] It is possible to increase the carbon dioxide concentration in the second electrolytic solution tank also by making a pressure applied to the electrolytic solution 23 higher than a pressure applied to the electrolytic solution 21. In this case, the pressure of the storage part of the second electrolytic solution tank may be set higher than the pressure of the storage part of the first electrolytic solution tank. Further, a pressure regulator may be provided in the flow path 52.

[0044] By supplying the first electrolytic solution tank with the electrolytic solution whose carbon dioxide concentration has been adjusted high in the second electrolytic solution tank, it is possible to increase the carbon dioxide concentration of the electrolytic solution stored in the first electrolytic solution tank. This can improve efficiency of the reduction reaction.

[0045] If the storage parts of the first electrolytic solution tank are cooled, the reactions by the catalysts deteriorate and accordingly reaction efficiency tends to lower. If the pressure is applied to the storage parts of the first electrolytic solution tank, pressure resistance of the electrolytic solution tank needs to be increased, leading to an increased cost and a complicated structure. Further, the increase of the pressure resistance worsens maintainability, for example, making the change of the electrodes troublesome.

[0046] For a reduction of a supply amount of the carbon dioxide and efficient absorption of the carbon dioxide in the electrolytic solution, an interval between bubbles of the carbon dioxide passing through the electrolytic solution needs to be wide. However, the interval of the bubbles becomes short when the carbon dioxide concentration is increased, allowing the downsizing of the electrolytic solution. A cooling temperature is preferably equal to or lower than the temperature of the electrolytic solution in the first electrolytic solution tank, for instance. If the temperature of the electrolytic solution is increased by the oxidation-reduction reaction, the cooling temperature is preferably not lower than the room temperature nor higher than the temperature of the electrolytic solution of the first electrolytic solution tank. The cooling temperature is more preferably not lower than a temperature at which the electrolytic solution freezes nor more than the temperature of the electrolytic solution.

[0047] The temperature of the electrolytic solution in the first electrolytic solution tank is preferably higher than the freezing point. For example, in a case where the electrolytic solution contains ions such as potassium ions or sodium ions for the purpose of increasing an absorption amount of carbon dioxide, increasing the concentrations of carbon dioxide ions and HCO.sub.3 ions, and increasing solution resistance of the electrolytic solution, the electrolytic solution does not freeze at .degree. C. However, extreme cooling requires a large cooler, leading to a cost increase and an energy loss, and thus the temperature of the electrolytic solution is preferably 0.degree. C. or higher in some case. Further, 5.degree. C. or higher or 10.degree. C. or higher is preferable in some case because of a concern about an energy loss of the whole electrochemical reaction device and reaction deterioration due to the extreme cooling of the electrolytic solution.

[0048] Temperature regulators may be provided in the electrolytic solution tanks 11 12 or the flow paths 51, 52 to impede the deterioration of reaction efficiency due to a temperature decrease of the electrolytic solution. Adjusting the temperature by the temperature regulator improves the reaction efficiency. For example, a cooler may be provided in the flow path 51 and a heater may be provided in the flow path 52 Further since even a temperature difference of several .degree. C. can produce the effect, irradiating the electrolytic solution flow path between the first electrolytic solution tank and the second electrolytic solution tank or irradiating the electrolytic solution tank with sunlight to heat it is efficient owing to the use of natural energy. Further, in a later-described case where the primary reaction is caused by electric energy generated by the conversion from sunlight, heat energy and light energy of the sunlight can be efficiently used, resulting in further improvement of efficiency.

[0049] Structure examples of the constituent elements in the electrochemical reaction device will be further described. As a water-containing electrolytic solution usable as the electrolytic solution, an aqueous solution containing a desired electrolyte is usable, for instance. This solution is preferably an aqueous solution that promotes the oxidation reaction of water. Examples of the aqueous solution containing the electrolyte include aqueous solutions containing phosphoric acid ions (PO.sub.4.sup.2-), boric acid ions (BO.sub.3.sup.3-), sodium ions (Na.sup.+), potassium ions (K.sup.+), calcium ions (Ca.sup.2+), lithium ions (Li.sup.+), cesium ions (Cs.sup.+), magnesium ions (Mg.sup.2+), chloride ions (Cl.sup.-), or hydrogen carbonate ions (HCO.sub.3.sup.-).

[0050] Examples of an electrolytic solution containing carbon dioxide usable as the electrolytic solution include aqueous solutions containing LiHCO.sub.3, NaHCO.sub.3, KHCO.sub.3, CsHCO.sub.3, phosphoric acid, or boric acid. The electrolytic solution containing carbon dioxide may contain alcohol such as methanol, ethanol, or acetone. The electrolytic solution containing water may be the same as the electrolytic solution containing carbon dioxide. However, an absorption amount of carbon dioxide in the electrolytic solution containing carbon dioxide is preferably high. So, as the electrolytic solution containing carbon dioxide, a solution different from the electrolytic solution containing water may be used. The electrolytic solution containing carbon dioxide is preferably an electrolytic solution that lowers a reduction potential of carbon dioxide, has high ion conductivity, and contains a carbon dioxide absorbent that absorbs carbon dioxide.

[0051] As the aforesaid electrolytic solution, an ionic liquid that contains salts of cations such as imidazolium ions or pyridinium ions and anions such as BF.sub.4.sup.- or PF.sub.6.sup.- and is in a liquid state in a wide temperature range, or its aqueous solution is usable, for instance. Other examples of the electrolytic solution include solutions of amine such as ethanolamine, imidazole, and pyridine, and aqueous solutions thereof. Examples of the amine include primary amine, secondary amine, and tertiary amine. These electrolytic solutions may be high in ion conductivity, have a property of absorbing carbon dioxide, and have a property of lowering reduction energy.

[0052] Examples of the primary amine include methylamine, ethylamine, propylamine, butylamine, pentylamine, and hexylamine. Hydrocarbons of the amine may be substituted by alcohol, halogen, or the like. Examples of the amine whose hydrocarbons are substituted include methanolamine, ethanolamine, and chloromethyl amine. Further, an unsaturated bond may exist. The same thing can be said for hydrocarbons of the secondary amine and the tertiary amine.

[0053] Examples of the secondary amine include dimethylamine, diethylamine, dipropylamine, dibutylamine, dipentylamine, dihexylamine, dimethanolamine, diethanolamine, and dipropanolamine. The substituted hydrocarbons may be different. This also applies to the tertiary amine. Examples of the amine having different hydrocarbons include methylethylamine and methylpropylamine.

[0054] Examples of the tertiary amine include trimethylamine, triethylamine, tripropylamine, tributylamine, trihexylamine, trimethanolamine, triethanolamine, tripropanolamine, tributanolamine, tripropanolamine, triexanolamine, methyldiethylamine, and methyldipropylamine.

[0055] Examples of the cations of the ionic liquid include [0056] 1-ethyl-3-methylimidazolium ions, 1-methyl-3-propylimidazolium ions, 1-butyl-3-methylimidazole ions, 1-methyl-3-pentylimidazolium ions, and 1-hexyl-3-methylimidazolium ions.

[0057] A second place of imidazolium ions may be substituted. Examples of the cations in which the second place of the imidazolium ions is substituted include [0058] 1-ethyl-2,3-dimethylimidazolium ions, 1-2-dimethyl-3-propylimidazolium ions, 1-butyl-2,3-dimethylimidazolium ions, 1,2-dimethyl-3-pentylimidazolium ions, and 1-hexyl-2,3-dimethylimidazolium ions.

[0059] Examples of pyridinium ions include methylpyridinium, ethylpyridinium, propylpyridinium, butylpyridinium, pentylpyridinium, and hexylpyridinium. In both of the imidazolium ions and the pyridinium ions, an alkyl group may be substituted, or an unsaturated bond may exist.

[0060] Examples of the anions include fluoride ions, chloride ions, bromide ions, iodide ions, BF.sub.4.sup.-, PF.sub.6.sup.-, CF.sub.3COO.sup.-, CF.sub.3SO.sub.3.sup.-, NO.sub.3.sup.-, SCN.sup.-, (CF.sub.3SO.sub.2).sub.3C.sup.-, bis(trifluoromethoxysulfonyl)imide, bis(trifluoromethoxysulfonyl)imide, and bis(perfluoroethylsulfonyl)imide. Dipolar ions in which the cations and the anions of the ionic liquid are coupled by hydrocarbons may be used. Incidentally, a buffer solution such as a potassium phosphate solution may be supplied to the storage parts 111, 112.

[0061] FIG. 2 is a view illustrating another example of the electrochemical reaction device. The electrochemical reaction device illustrated in FIG. 2 is different from the electrochemical reaction device illustrated in FIG. 1 in that the reduction electrode 31, the oxidation electrode 32, and the photoelectric conversion body 33 are stacked. The reduction electrode 31 is in contact with the face 331 and the oxidation electrode 32 is in contact with the face 332. In this case, a stack including the reduction electrode 31, the oxidation electrode 32, and the photoelectric conversion body 33 is also called a photoelectric conversion cell. The photoelectric conversion cell penetrates through the ion exchange membrane 4 and is immersed in the electrolytic solution 21 and the electrolytic solution 22.

[0062] FIG. 3 is a schematic cross-sectional view illustrating a structure example of the photoelectric conversion cell. The photoelectric conversion cell illustrated in FIG. 3 includes a conductive substrate 30, the reduction electrode 31, the oxidation electrode 32, the photoelectric conversion body 33, a light reflective body 34, a metal oxide body 35, and a metal oxide body 36.

[0063] The conductive substrate 30 is in contact with the reduction electrode 31. The conductive substrate 30 may be regarded as part of the reduction electrode. Examples of the conductive substrate 30 include a substrate containing at least one or more of Cu, Al, Ti, Ni, Fe, and Ag. For example, a stainless steel substrate containing stainless steel such as SUS may be used. The conductive substrate 30 is not limited to the above and may be formed of a conductive resin. Alternatively, the conductive substrate 30 may be constituted by a substrate of a semiconductor such as Si or Ge. Further, a resin film or the like may be used as the conductive substrate 30. For example, the film usable as the ion exchange membrane 4 may be used as the conductive substrate 30.

[0064] The conductive substrate 30 has a function as a support. The conductive substrate 30 may be disposed so as to separate the storage part 111 and the storage part 112. The presence of the conductive substrate 30 can improve mechanical strength of the photoelectric conversion cell. Further, the conductive substrate 30 may be regarded as part of the reduction electrode 31. Further, the conductive substrate 30 does not necessarily have to be provided.

[0065] The reduction electrode 31 preferably contains a reduction catalyst. The reduction electrode 31 may contain both a conductive material and the reduction catalyst. Examples of the reduction catalyst include a material that reduces activation energy for reducing hydrogen ions or carbon dioxide. In other words, a material that lowers the overvoltages when hydrogen and a carbon compound are produced by the reduction reactions of hydrogen ions and carbon dioxide is usable. For example, a metal material or a carbon material is usable. For example, in the production of hydrogen, a metal such as platinum or nickel, or an alloy containing this metal is usable as the metal material. In the reduction reaction of carbon dioxide, a metal such as gold, aluminum, copper, silver, platinum, palladium, or nickel, or an alloy containing this metal is usable. As the carbon material, graphene, carbon nanotube (CNT), fullerene, or ketjen black is usable, for instance. The reduction catalyst is not limited to these, and may be, for example, a metal complex such as a Ru complex or a Re complex, or an organic molecule having an imidazole skeleton or a pyridine skeleton, or may be a mixture of a plurality of materials.

[0066] The oxidation electrode 32 preferably contains an oxidation catalyst. The oxidation electrode 32 may contain both a conductive material and the oxidation catalyst. Examples of the oxidation catalyst include a material that reduces activation energy for oxidizing water. In other words, a material that lowers the overvoltage when oxygen and hydrogen ions are produced by the oxidation reaction of water is usable. Examples thereof include iridium, platinum, cobalt, and manganese. Further, as the oxidation catalyst, a binary metal oxide, a ternary metal oxide, or a quaternary metal oxide is usable, for instance. Examples of the binary metal oxide include manganese oxide (Mn--O), iridium oxide (Ir--O), nickel oxide (Ni--O), cobalt oxide (Co--O), iron oxide (Fe--O), tin oxide (Sn--O), indium oxide (In--O), and ruthenium oxide (Ru--O). Examples of the ternary metal oxide include Ni--Co--O, La--Co--O, Ni--La--O, and Sr--Fe--O. Examples of the quaternary metal oxide include Pb--Ru--Ir--O and La--Sr--Co--O. The oxidation catalyst is not limited to these, and may be a metal complex such as a Ru complex or a Fe complex, or a mixture of a plurality of materials.

[0067] At least one of the reduction electrode 31 and the oxidation electrode 32 may have a porous structure. Examples of a material usable for the electrode having the porous structure include, in addition to the above-listed materials, carbon black such as ketjen black and VULCAN XC-72, activated carbon, and metal fine powder. The porous structure can increase the area of an active surface contributing to the oxidation-reduction reaction and thus can increase conversion efficiency.

[0068] In a case where an electrode reaction with a low current density is caused using relatively low irradiation energy of light, the catalyst material can be selected from a wide range of options. Accordingly, it is easy to cause the reaction using, for example, a ubiquitous metal, and it is also relatively easy to obtain selectivity of the reaction. On the other hand, in a case where the photoelectric conversion body 33 is not disposed in the electrolytic solution tank 11 and is electrically connected to at least one of the reduction electrode 31 and the oxidation electrode 32 by, for example, a wiring line, the electrode area is usually decreased due to a reason such as the downsizing of the electrolytic solution tank, and the reaction is sometimes caused with a high current density. In this case, a noble metal is preferably used as the catalyst.

[0069] The photoelectric conversion body 33 has a stacked structure of a photoelectric conversion layer 33x, a photoelectric conversion layer 33y, and a photoelectric conversion layer 33z. The number of the stacked photoelectric conversion layers is not limited to that in FIG. 3.

[0070] The photoelectric conversion layer 33x has, for example, an n-type semiconductor layer 331n containing n-type amorphous silicon, an i-type semiconductor layer 331i containing intrinsic amorphous silicon germanium, and a p-type semiconductor layer 331p containing p-type microcrystalline silicon. The i-type semiconductor layer 331i is a layer that absorbs light in a short wavelength range including 400 nm, for instance. Accordingly, in the photoelectric conversion layer 33x, charge separation is caused by energy of light in the short wavelength range.

[0071] The photoelectric conversion layer 33y has, for example, an n-type semiconductor layer 332n containing n-type amorphous silicon, an i-type semiconductor layer 332i containing intrinsic amorphous silicon germanium, and a p-type semiconductor layer 332p containing p-type microcrystalline silicon. The i-type semiconductor layer 332i is a layer that absorbs light in an intermediate wavelength range including 600 nm, for instance. Accordingly, in the photoelectric conversion layer 33y, charge separation is caused by energy of light in the intermediate wavelength range.

[0072] The photoelectric conversion layer 33z has, for example, an n-type semiconductor layer 333n containing n-type amorphous silicon, an i-type semiconductor layer 333i containing intrinsic amorphous silicon, and a p-type semiconductor layer 333p containing p-type microcrystalline silicon. The i-type semiconductor layer 333i is a layer that absorbs light in a long wavelength range including 700 nm, for instance. Accordingly, in the photoelectric conversion layer 33z, charge separation is caused by energy of light in the long wavelength range.

[0073] The p-type semiconductor layers or the n-type semiconductor layers each can be formed of, for example, a semiconductor material to which an element that is to be a donor or an acceptor is added. Incidentally, in the photoelectric conversion layer, as the semiconductor layers, the semiconductor layers containing silicon, germanium, or the like are used, but the semiconductor layers are not limited to these, and maybe compound semiconductor layers, for instance. As the compound semiconductor layers, semiconductor layers containing, for example, GaAs, GaInP, AlGaInP, CdTe, or CuInGaSe are usable, for instance. Further, layers containing a material such as TiO.sub.2 or WO.sub.3 may be used, provided that photoelectric conversion is possible. Further, the semiconductor layers each may be monocrystalline, polycrystalline, or amorphous. Further, the photoelectric conversion layer may include a zinc oxide layer.

[0074] The light reflective body 34 is between the conductive substrate 30 and the photoelectric conversion body 33. Examples of the light reflective body 34 include a distributed Bragg reflection layer composed of a stack of metal layers or semiconductor layers, for instance. Owing to the presence of the light reflective body 34, light that cannot be absorbed by the photoelectric conversion body 33 can be reflected to enter one of the photoelectric conversion layer 33x to the photoelectric conversion layer 33z, enabling to enhance conversion efficiency from light to a chemical substance. As the light reflective body 34, a layer of a meal such as Ag, Au, Al, or Cu or an alloy containing at least one of these metals is usable, for instance.

[0075] The metal oxide body 35 is between the light reflective body 34 and the photoelectric conversion body 33. The metal oxide body 35 has a function of enhancing light reflectivity by adjusting an optical distance, for instance. For the metal oxide body 35, a material capable of ohmic contact with the n-type semiconductor layer 331n is preferable used. As the metal oxide body 35, a layer of a light-transmissive metal oxide such as, for example, indium tin oxide (ITO), zinc oxide (ZnO), fluorine-doped tin oxide (FTO), aluminum-doped zinc oxide (AZO), or antimony-doped tin oxide (ATO) is usable.

[0076] The metal oxide body 36 is between the oxidation electrode 32 and the photoelectric conversion body 33. The metal oxide body 36 may be disposed on a surface of the photoelectric conversion body 33. The metal oxide body 36 has a function as a protective layer preventing the photoelectric conversion cell from being broken by the oxidation reaction. The presence of the metal oxide body 36 can prevent the corrosion of the photoelectric conversion body 33 to extend the life of the photoelectric conversion cell. Incidentally, the metal oxide body 36 does not necessarily have to be provided.

[0077] As the metal oxide body 36, a dielectric thin film of TiO.sub.2, ZrO.sub.2, Al.sub.2O.sub.3, SiO.sub.2, or HfO.sub.2 is usable, for instance. The metal oxide body 36 preferably has a thickness of 10 nm or less, further 5 nm or less. This is intended to obtain electrical conductivity by a tunnel effect. As the metal oxide body 36, a layer of a light transmissive metal oxide such as, for example, indium tin oxide (ITO), zinc oxide (ZnO), fluorine-doped tin oxide (FTO), aluminum-doped zinc oxide (AZO), or antimony-doped tin oxide (ATO) may be used.

[0078] The metal oxide body 36 may have, for example, a stacked structure of a metal and a transparent conductive oxide, a composite structure of a metal and another conductive material, or a composite structure of a transparent conductive oxide and another conductive material. The above structure can decrease the number of parts, decrease the weight, and facilitate the manufacture, enabling cost reduction. The metal oxide body 36 may have functions as a protective layer, a conductive layer, and a catalyst layer.

[0079] In the photoelectric conversion cell illustrated in FIG. 3, a face of the n-type semiconductor layer 331n opposite to its contact surface with the i-type semiconductor layer 331i is a first face of the photoelectric conversion body 33, and a face of the p-type semiconductor layer 333p opposite to its contact surface with the i-type semiconductor layer 333i is a second face. The photoelectric conversion cell illustrated in FIG. 3 has the stacked structure of the photoelectric conversion layer 33x to the photoelectric conversion layer 33z as described above and thus is capable of absorbing lights in a wide wavelength range of sunlight, enabling more efficient use of energy of sunlight. In this case, a high voltage can be obtained owing to the series connection of the photoelectric conversion bodies.

[0080] In FIG. 3, electrons and holes having undergone the charge separation can be used as they are in the oxidation-reduction reaction, since the electrodes are stacked on the photoelectric conversion body 33. Further, the photoelectric conversion body 33 and the electrodes need not be electrically connected by wiring lines or the like. This enables a high-efficiency oxidation-reduction reaction.

[0081] The plural photoelectric conversion bodies may be electrically connected in parallel. A dual junction or single-layer photoelectric conversion body may be used. A stack of two photoelectric conversion bodies, or four photoelectric conversion bodies or more may be used. A single-layer photoelectric conversion layer may be used instead of the stack of the plural photoelectric conversion layers.

[0082] The electrochemical reaction device of this embodiment is a simplified system with a reduced number of parts owing to the integration of the reduction electrode, the oxidation electrode, and the photoelectric conversion body. This facilitates at least one of, for example, manufacture, installation, and maintenance. Further, this structure eliminates a need for wiring lines connecting the photoelectric conversion body to the reduction electrode and the oxidation electrode, achieving an increased light transmittance and an increased light-receiving area.

[0083] The photoelectric conversion body 33 is in contact with the electrolytic solution, which may lead to its corrosion and the dissolving of corrosive products in the electrolytic solution to deteriorate the electrolytic solution. A possible measure to prevent the corrosion may be to provide a protective layer. However, components of the protective layer may dissolve in the electrolytic solution. Here, providing a filter such as a metal ion filter in the flow path or the electrolytic solution tank hinders the deterioration of the electrolytic solution.

[0084] The electrochemical reaction device of this embodiment is an art suitable as a measure for surplus power and is required to make good use of solar energy. In a case where illuminance of sunlight is high, when there is no surplus power, energy is obtained as much as possible, and when there is surplus energy, the energy is consumed by being used for circulating the electrolytic solution. This enables efficient energy mix to increase the total energy utilization ratio. In a case where a buffer solution is used as the electrolytic solution, a small reaction amount also results in a small pH change caused by the reaction. So, during a non-reaction period, the electrolytic solution is circulated to keep the electrolytic solution components uniform, and during the reaction, the supply of the electrolytic solution is restricted or stopped. This can prevent a decrease of total efficiency and reduce the cost. For example, preferably, the electrolytic solution is circulated using nighttime wind power or low-cost surplus power, and in the daytime, the oxidation-reaction reaction is caused, with the circulation of the electrolytic solution being stopped or with the minimum supply amount of the electrolytic solution.

[0085] A structure example of the electrochemical reaction device is not limited to that in FIG. 1. FIG. 4 is a schematic view illustrating another example of the electrochemical reaction device. The electrochemical reaction device illustrated in FIG. 4 is different from the electrochemical reaction device illustrated in FIG. 1 at least in that it further includes a separation tank 13, a separation tank 14, a flow path 53 to a flow path 55.

[0086] The separation tank 13 has a storage part 114a storing an electrolytic solution 24 and a gas-liquid separation membrane 114b dividing the storage part 114a into a plurality of regions. The gas-liquid separation membrane 114b includes, for example, a hollow fiber membrane and so on. The hollow fiber membrane contains, for example, a silicone resin, a fluorine-based resin (perfluoroalkoxyalkane (PFA), a perfluoroethylene propene copolymer (F E P), polytetrafluoroethylene (PTFE), an ethylene-tetrafluoroethylene copolymer (ETFE), polyvinylidene fluoride (PVDF), polychlorotrifluoroethylene (PCTFE), an ethylene-chlorotrifluoroethylene copolymer (ECTFE)), or the like.

[0087] In the electrochemical reaction device illustrated in FIG. 4, part of the reduction product in the electrolytic solution tank 11 is extracted in the separation tank 13. An outer side of the gas-liquid separation membrane 114b (its surface side opposite to its contact surface with the electrolytic solution 24) is pressure-reduced and the electrolytic solution 24 containing a gaseous product passes through the gas-liquid separation membrane 114b, enabling the efficient separation of the gaseous product and carbon dioxide. In a case where the product is, for example, carbon monoxide, only carbon monoxide gas can be separated by the gas-liquid separation in the separation tank 13.

[0088] The flow path 51 is connected to the storage part 114a. The flow path 53 connects the storage part 113 and the storage part 114a. The separation tank 14 has a storage part 115a storing an electrolytic solution 25 and a gas-liquid separation membrane 115b dividing the storage part 115a into a plurality of regions. The flow path 54 connects the storage part 112 and the storage part 115a. The flow path 55 connects the storage part 112 and the storage part 115a. At least part of the electrolytic solution 22 is supplied to the storage part 115a through the flow path 54. At least part of the electrolytic solution 25 is supplied to the storage part 112 through the flow path 55. Circulation pumps or the like may be provided in the flow path 54 and the flow path 55.

[0089] An outer side of the gas-liquid separation membrane 115b (its surface side opposite to its contact surface with the electrolytic solution 25) is pressure-reduced and the electrolytic solution containing a gaseous product passes through the gas-liquid separation membrane 115b, so that oxygen gas and dissolved oxygen can be separated similarly to the carbon dioxide. It can be conceived to directly recover and use the oxygen gas generated in the electrolytic solution tank 11, but since the oxygen gas is dissolved in the electrolytic solution 22, it is difficult to completely recover the oxygen gas. Since the dissolved oxygen deteriorates performance of the oxide electrode, the dissolved oxygen is desirably recovered in the form of gas. Unlike the gas separation in the electrolytic solution tank 11, it is possible to recover gases generated in a plurality of cells at a time. Accordingly, the total flow path length for the gas recovery is shortened, enabling a simplified system. In this case, by providing temperature regulators in the separation tank 14 or the flow path 54 and the flow path 55 as in the electrolytic solution tank 12 in order to efficiently recover the oxygen gas, it is possible to efficiently separate oxygen from the electrolytic solution.

[0090] By providing a temperature regulator in the separation tank 13 or the flow path 51, it is possible to enhance separation efficiency of the product. For the complete gas separation, the dissolved gas in the electrolytic solution is preferably removed as much as possible. An agitator is preferably provided in the separation tank 13 to enhance efficiency of removing the dissolved gas by temperature distribution or the like.

[0091] A difference between the temperature of the electrolytic solution 24 in the separation tank 13 and the temperature of the electrolytic solution 21 in the electrolytic solution tank 11 may be not less than -10.degree. C. nor more than 10.degree. C. Too high a temperature of the electrolytic solution 24 in the separation tank 13 is likely to decrease the gas concentration of the product due to the vaporization of carbon dioxide dissolved in the electrolytic solution 24. Excessive heating leads to efficiency deterioration because of a large energy loss by the heating.

[0092] In a case where the product is a water-soluble liquid substance such as methanol or ethanol, a separation method in the separation tank 13 may be distillation or membrane separation, for instance. In this case, a temperature regulator is desirably provided to improve separation efficiency. The separation membrane may be zeolite, for instance. Heat especially on an upstream side is large and thus is likely to deteriorate the total efficiency. To cope with this, providing a heat insulator in the separation tank 13 can prevent the efficiency deterioration.

[0093] In a case where an ion exchange membrane or a flow path is provided between the oxidation electrode and the reduction electrode in the electrolytic solution tank, the electrolytic solution in contact with the oxidation electrode may be different from the electrolytic solution in contact with the reduction electrode. By the above structure, it is possible to easily separate and extract oxygen being the reaction product in the oxidation side.

[0094] A suitable electrolytic solution differs depending on each catalyst, and by making the electrolytic solutions in contact with the catalyst layers different, it is possible to improve efficiency. Furthermore, making pH on the oxidation side larger than that on the reduction side is advantageous in that a liquid junction potential caused by the pH difference can compensate for an insufficient potential of the reaction.

[0095] An electrochemical reaction device illustrated in FIG. 5 includes the structure of the electrochemical reaction device illustrated in FIG. 4, a flow path 56, a cooler 61a, a cooler 61b, a heater 62a, a heater 62b, a pump 71, and a pressure valve 72.

[0096] The flow path 56 is connected to the storage part of the electrolytic solution tank 12. For example, the flow path 56 is connected to a carbon dioxide generation source 80.

[0097] The cooler 61a has a function of cooling the electrolytic solution flowing in the flow path 56. The cooler 61a may be disposed inside or outside the flow path 56, for instance.

[0098] The cooler 61b has a function of cooling the electrolytic solution 23. The cooler 61b may be disposed inside or outside the storage part 113, for instance.

[0099] The heater 62a has a function of heating the electrolytic solution 25. The heater 62a may be disposed inside or outside the storage part 115a, for instance.

[0100] The heater 62b has a function of heating the electrolytic solution flowing in the flow path 54. The heater 62b may be disposed inside or outside the flow path 54, for instance.

[0101] The pump 71 has a function of promoting the supply of the electrolytic solution from the storage part 114a to the storage part 113. The pump 71 is disposed inside or outside the flow path 53, for instance. The pump 71 does not necessarily have to be provided.

[0102] The pressure valve 72 has a function of promoting the supply of the electrolytic solution from the storage part 113 to the storage part 111. The pressure valve 72 is disposed inside or outside the flow path 52, for instance. Examples of the pressure valve 72 include an orifice valve and a pulse valve. The pressure valve 72 does not necessarily have to be provided.

[0103] Heat exchange between the separation tank 13 and the separation tank 14 may be performed. The heat exchange is possible by providing a heat transfer member 91 connecting, for example, the separation tank 13 and the separation tank 14. The heat transfer member 91 may be provided so as to connect the storage part 114a and the storage part 115a, for instance. Alternatively, a heat exchanger or the like may be separately connected.

[0104] An electrochemical reaction device illustrated in FIG. 6 further includes a cooler 61c in addition to the structure illustrated in FIG. 5, and does not include the separation tank 13.

[0105] The flow path 53 connects the storage part 111 and the storage part 113. The flow path 56 is connected to the storage part 111. The flow path 56 connects, for example, the storage part 111 and the carbon dioxide generation source 80. The carbon dioxide generation source 80 may be disposed inside or outside the electrochemical reaction device.

[0106] Heat exchange between the electrolytic solution tank 12 and the separation tank 14 may be performed. The heat exchange is possible by providing a heat transfer member 92 connecting, for example, the electrolytic solution tank 12 and the separation tank 14.

[0107] The heat transfer member 92 may be provided so as to connect the flow path 53 and the flow path 54, for instance. Alternatively, a heat exchanger or the like may be separately connected.

[0108] The cooler 61c has a function of cooling the electrolytic solution flowing in the flow path 53. The cooler 61c is disposed inside or outside the flow path 53, for instance.

[0109] The pump 71 has a function of promoting the supply of the electrolytic solution from the storage part 113 to the storage part 111. The pump 71 is disposed in the flow path 52, for instance.

[0110] The pressure valve 72 has a function of promoting the supply of the electrolytic solution from the storage part 111 to the storage part 113. The pressure valve 72 is disposed inside or outside the flow path 52, for instance. Examples of the pressure valve 72 include an orifice valve and a pulse valve. Incidentally, the pressure valve 72 does not necessarily have to be provided.

[0111] In the electrochemical reaction devices illustrated in FIG. 5 and FIG. 6, the use of the coolers can facilitate lowing the temperature of the electrolytic solution on the reduction side. Further, the use of the heaters can facilitate raising the temperature of the electrolytic solution on the oxidation side. This can enhance reaction efficiency.

[0112] High-temperature carbon dioxide is generated in power plants, incinerators, and the like. The direct supply of the high-temperature carbon dioxide to the electrolytic solution tank 11 causes a temperature increase. The temperature increase is preferably reduced by providing the cooler in the flow path 56 between the carbon dioxide generation source 80 and the electrolytic solution tank 11. A cooler which cools the flow path by, for example, the atmospheric air, seawater, river water, or the like can also produce a sufficient effect.

[0113] It is possible to reduce an energy loss by supplying carbon dioxide pressurized in the carbon dioxide generation source 80 such as the power plant or the incinerator to the electrolytic solution tank 11 or the electrolytic solution tank 12 through the flow path without using a pump or the like. A pressure regulator may be provided for pressure stabilization. Owing to the pressure regulator, carbon dioxide with a stable pressure can be absorbed in the electrolytic solution. This can enhance stability of the whole device. Further, by improving efficiency by performing voltage control across the reduction electrode and the oxidation electrode and temperature control and pressure control of the electrochemical reaction device according to a supply amount and the temperature of carbon dioxide from the electrolytic solution tank 11 and an operation signal of a carbon dioxide supply device, it is possible to make the best use of performance of the device to improve the efficiency.

[0114] In a case where the separation tank 13 is heated, the use of heat of the carbon dioxide generation source or the like for the heating reduces an energy loss to improve efficiency. On the other hand, the use of heat of the high-temperature carbon dioxide gas supplied from the carbon dioxide generation source lowers the temperature of the carbon dioxide gas supplied to the electrolytic solution tank 12 to improve efficiency.

[0115] An electrochemical reaction device illustrated in FIG. 7 further includes, in addition to the structure of the electrochemical reaction device illustrated in FIG. 6, a distiller 81a, a reduction reaction device 81b, and a flow path 57 connecting the storage part 113 and the reduction reaction device 81b. The electrochemical reaction device further includes a cooler 61d instead of the cooler 61c. Incidentally, it may include both the cooler 61c and the cooler 61d.

[0116] The cooler 61d has a function of cooling the electrolytic solution flowing in the flow path 52. The cooler 61d is disposed inside or outside the flow path 52, for instance.

[0117] The distiller 81a has a function of distilling the product in the storage part 113. The distiller 81a is connected to the storage part 113. The distiller 81a is disposed on the electrolytic solution tank 12, for instance. In the electrochemical reaction device illustrated in FIG. 7, efficiency can be improved since heat deprived of by the distillation in the distiller 81a and the high-temperature carbon dioxide gas from the carbon dioxide generation source can be efficiently used. However, since an efficient heat exchanger leads to a cost increase, a simple heat exchange method such as connecting pipes or the like by a heat transfer member can also produce the effect. It is also possible to exchange the heat of the high-temperature carbon dioxide gas supplied from the carbon dioxide generation source 80 between the carbon dioxide generation source 80 and the separation tank 13.

[0118] The reduction reaction device 81b has a function of reducing the product in the storage part 113. In the reduction reaction device 81b, a catalyst in which Al.sub.2O.sub.3 or the like carries a metal such as an oxide of copper, palladium, or silver, or Cu--ZnO, Pd--ZnO, or Cu--Zn--Cr is used, for instance, and methanol can be mainly manufactured when hydrogen and CO gas which are raw materials are made to flow at, for example, 150 to 300.degree. C. under pressurization. Methanol can also be produced by a liquid phase method that passes the hydrogen and the CO gas in a slurry of the aforesaid catalyst under pressurization. The reduction reaction device 81b includes a heat exchanger for removing heat generated by the reaction, for instance. Further, the reduction reaction device 81b may be a device that produces ethanol or nickel by using rhodium or the like, or produces methane by using ruthenium.

[0119] Examples of the product by the reduction reaction in the reduction reaction device 81b include hydrocarbons such as methane, methanol, ethanol, acetic acid, dimethyl ether, wax, olefin, naphtha, and light oil. A heat source is not only the carbon dioxide from the carbon dioxide generation source but also may include at least part of the heat of the reaction between the reduction product of carbon dioxide and hydrogen, for instance. For example, the mutual heat utilization of using part of the reaction heat obtained when methanol is produced by the reaction of carbon monoxide and hydrogen in the reduction reaction device 81b improves efficiency.

[0120] Heat exchange may take place between the carbon dioxide generation source 80 and the electrolytic solution tank 12. The heat exchange is possible by providing a heat transfer member 93 connecting, for example, the carbon dioxide generation source 80 and the electrolytic solution tank 12. The heat transfer member 93 may be provided so as to connect the flow path 56 and the distiller 81a, for instance. Alternatively, a heat exchanger or the like may be separately connected.

[0121] Heat exchange may take place between the reduction reaction device 81b and the distiller 81a. The heat exchange is possible by providing a heat transfer member 94 connecting, for example, the reduction reaction device 81b and the distiller 81a. Further, a heat exchanger or the like may be separately connected.

[0122] In the electrochemical reaction device illustrated in FIG. 7, the heat exchange between the flow path 56 and the distiller 81a and the heat exchange between the distiller 81a and the reduction reaction device 81b make it possible to efficiently use and remove the heat of the heat source.

[0123] Incidentally, the electrochemical reaction device illustrated in FIG. 7 may include the separation tank 14, the flow path 54, and the flow path 55 illustrated in FIG. 4 and so on. Further, an agitator may be provided in an oxygen gas separator to enhance efficiency of separating dissolved gas by temperature distribution or the like. In this case, the use of the carbon dioxide generation source 80, the high-temperature carbon dioxide gas obtained from the carbon dioxide generation source 80, the heat generated in the reduction reaction device 81b, or the like as the heat source can improve efficiency. The combination of these heats may be any, and an operation method for the heat exchange with any of them can improve efficiency. Further, connecting the flow paths or the like by the heat transfer member in order to mutually use these heats can improve efficiency. The storage part 114a may be connected to at least one of the storage part 112 and the storage part 115a via a heat transfer member, for instance.

EXAMPLE

Example 1

[0124] An electrochemical reaction device having a structure was fabricated. The structure includes a three-junction photoelectric conversion body with a 500 nm thickness, a 300 nm thick ZnO layer provided on a first face of the three-junction photoelectric conversion body, a 200 nm thick Ag layer provided on the ZnO layer, a 1.5 mm thick SUS substrate provided on the Ag layer, and a 100 nm thick ITO layer provided on a second face of the three-junction photoelectric conversion body.

[0125] The three-junction photoelectric conversion body has a first photoelectric conversion layer that absorbs light in a short wavelength range, a second photoelectric conversion layer that absorbs light in an intermediate wavelength range, and a third photoelectric conversion layer that absorbs light in a long wavelength range. The first photoelectric conversion layer has a p-type microcrystalline silicon layer, an i-type amorphous silicon layer, and an n-type amorphous silicon layer. The second photoelectric conversion layer has a p-type microcrystalline silicon layer, an i-type amorphous silicon germanium layer, and an n-type amorphous silicon layer. The third photoelectric conversion layer has a p-type microcrystalline silicon layer, an i-type amorphous silicon layer, and an n-type amorphous silicon layer.

[0126] An open-circuit voltage when the structure was irradiated with light using a solar simulator (AM1.5, 1000 W/cm.sup.2) was measured. The open-circuit voltage was 2.1 V.

[0127] A Ni(OH).sub.2 layer with a 200 nm thickness was formed as an oxidation catalyst on the ITO layer on the structure of the three-junction photoelectric conversion body by an electrodeposition method using nickel nitrate. A 500 nm thick gold nanoparticle layer carried by carbon was formed as a reduction catalyst on the SUS substrate.

[0128] The above structure was cut into a square shape and its edge portions were sealed with a thermosetting epoxy resin. The periphery of the structure was surrounded by an ion exchange membrane (Nafion (registered trademark)), whereby a single sheet-shaped structure was formed. A 10 cm square unit was fabricated from the combination of the ion exchange membrane and a plurality of cells, and ten pieces of the units were arranged in each of the vertical and lateral directions to fabricate a 100 cm square photoelectrochemical reaction unit. The sheet-shaped structure may be formed by, for example, embedding photoelectric conversion cells in a plurality of holes of one ion exchange membrane having the plural holes. The sheet-shaped structure may be formed by arranging a plurality of structures in each of which a photoelectric conversion cell is embedded in a hole of an ion exchange membrane having one hole. Ion exchange membranes may be embedded in holes of photoelectric conversion cells each having a hole.

[0129] This sheet-shaped photoelectrochemical reaction unit is sandwiched by a pair of 3 cm thick frames each having a hollow portion with 100 cm length.times.100 cm width, and a silicone resin layer was formed between the pair of frames. A window formed of non-reflective glass for solar cell was fabricated to cover the hollow portion of one of the pair of frames. An acrylic resin plate was formed to cover the hollow portion of the other of the pair of frames. Consequently, a sealed body encapsulating the photoelectrochemical reaction unit was fabricated. Flow paths were provided on the Ni(OH).sub.2 layer side and the gold nanoparticle layer side of the photoelectrochemical reaction unit respectively. As an electrolytic solution, a 0.5 M aqueous potassium hydrogen phosphate solution containing saturated carbon dioxide gas was used. A gas recovery flow path for capturing produced gas was provided in part of an electrolytic solution tank. Through the above, a photoelectrochemical reaction module was fabricated. An acrylic vessel with an internal volume of 30 cm.times.3 cm.times.3 cm was connected as a mixing tank to the gold nanoparticle layer side of the module.

[0130] This module was immersed in an electrolytic solution tank which was a cylindrical glass vessel with a 30 cc volume, and 50 cc/min CO.sub.2 gas was blown to the electrolytic solution tank to be dissolved in the electrolytic solution. This electrolytic solution was supplied to the reduction electrode side of the module at a 0.1 cc/min flow rate to be circulated. Further, a potassium borate buffer solution on the oxidation electrode side was circulated at a 0.1 cc/min flow rate via a buffer tank, which was a cylindrical vessel with a 30 cc volume, without blowing CO.sub.2.

[0131] In the module of the example 1, when A.M.1.5 pseudo sunlight was radiated from the oxidation electrode side to cause a 0.5 hour reaction, a current value was approximately 1 mA/cm.sup.2 at an initial stage, but decreased to 0.4 mA/cm.sup.2.

[0132] In the module of the example 1, when the electrolytic solution tank was put in ice water to be cooled after the 0.5 hour reaction was caused by the radiation of the A.M.1.5 pseudo sunlight from the oxidation electrode side, the current value recovered to approximately 0.7 mA/cm.sup.2. From this, it is seen that cooling the electrolytic solution containing carbon dioxide can improve reaction efficiency.

[0133] In the module of the example 1, when the A.M.1.5 pseudo sunlight was radiated from the oxidation electrode side and the flow rate was set to 0.2 cc/min, it was possible to make the current decrease time about 1.7 times. From this, it is seen that increasing the circulation flow rate can impede the decrease of the current.

Example 2

[0134] A composite substrate (4 cm square) having a 1.5 mm thick SUS substrate connected to a generator via a lead and a gold-carrying carbon film provided on the SUS substrate and carrying 0.25 mg/cm.sup.2 gold, and a platinum foil (4 cm square) were prepared. The generator is a simulation device of a solar cell. A flow path and a gas flow path were formed on each of an oxidation electrode side and a reduction electrode side of a 5 cm square acrylic frame with a 1 cm thickness. The composite substrate and the platinum foil were enclosed in the frame, an ion exchange membrane (Nafion 117, 6 cm square) was provided between the composite substrate and the platinum foil, and a silicon rubber sheet and an acrylic plate (7 cm length.times.7 cm width.times.3 mm thickness) were provided on each of an outer side of the composite substrate and an outer side of the platinum foil, whereby a module sandwiched by these was fabricated. A potassium phosphate buffer solution with pH7 was supplied into the module. The composite substrate was used as a reduction electrode, the platinum foil was used as an oxidation electrode, and a silver-silver chloride electrode was used as a reference electrode. Carbon dioxide was decomposed by passing a current under a 37 mA: 2.3 mA/cm.sup.2 condition using a galvanostat. This module was immersed in an electrolytic solution tank which was a cylindrical glass vessel with a 30 cc volume, and CO.sub.2 gas at 50 cc/min was blown into the electrolytic solution tank to be dissolved in the electrolytic solution. This electrolytic solution was supplied to the reduction electrode side of the module at a 0.1 cc/min flow rate to be circulated. A potassium borate buffer solution on the oxidation electrode side was circulated at a 0.1 cc/min flow rate via a buffer tank, which was a 30 cc cylindrical vessel, without blowing CO.sub.2.

[0135] In the module of the example 2, when a 0.5 hour reaction was caused under a 37 mA current and a 0.1 cc/min circulation flow rate, a potential was approximately -1 V at an initial stage, but decreased to -1.4 V.

[0136] In the module of the example 2, when the electrolytic solution tank was put in ice water to be cooled after the 0.5 hour reaction was caused under the 37 mA current and the 0.1 cc/min circulation flow rate, the potential recovered to approximately -0.8 V. From this, it is seen that cooling the electrolytic solution containing carbon dioxide can improve reaction efficiency.

[0137] In the module of the example 2, when the flow rate was changed to 0.2 cc/min after the 0.5 hour reaction was caused under the 37 mA current and the 0.1 cc/min circulation flow rate, it was possible to make the potential decrease time about twice. From this, it is seen that increasing the circulation flow rate can impede the decrease of the potential

[0138] While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the inventions.

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