PHOTOELECTROCHEMICAL REACTION SYSTEM

A photoelectrochemical reaction system of an embodiment includes: a CO2 generation unit, a CO2 reduction unit, and a CO2 supply unit supplying gas containing CO2 generated in the CO2 generation unit into the CO2 reduction unit. The CO2 reduction unit includes: a stack 3 including an oxidization electrode layer 11 oxidizing H2O, a reduction electrode layer 21 reducing CO2, and a photovoltaic layer 31 provided between the electrode layers 11, 21; an electrolytic solution tank 2 storing a first electrolytic solution 4 in which the oxidization electrode layer 11 is immersed and a second electrolytic solution 5 in which the reduction electrode layer 21 is immersed; and an ion migration pathway 6 allowing ions to migrate between the first electrolytic solution 4 and the second electrolytic solution 5. The gas containing CO2 generated in the CO2 generation unit is supplied into the second electrolytic solution 5 by a gas supply pipe 51 of the CO2 supply unit.

FIELD

Embodiments described herein relate generally to a photoelectrochemical reaction system.

BACKGROUND

From the viewpoint of an energy problem and an environmental problem, a technology of efficiently reducing CO2using light energy like plants is required. The plants use a system, called a Z-scheme, which is excited at two stages by light energy. Namely, the plants obtain electrons from water (H2O) by light energy, and synthesize cellulose and saccharide by reducing carbon dioxide (CO2) using the electrons. In an artificial photoelectrochemical reaction, low decomposition efficiency is obtained in a technology of decomposing CO2without using a sacrificial reagent.

As an artificial photoelectrochemical reaction device, a two-electrode type device is known in which an electrode having a reduction electrode reducing carbon dioxide (CO2) and an oxidization electrode oxidizing water (H2O) are included, and these electrodes are immersed in water where CO2is dissolved. The oxidization electrode oxidizes H2O by light energy to obtain oxygen (½O2) and potential. The reduction electrode reduces CO2by receiving the potential from the oxidization electrode so as to generate a chemical substance (chemical energy) such as formic acid (HCOOH). In the two-electrode type device, a reduction potential of CO2is obtained by two-stage excitation similarly to the Z-scheme of the plants, and therefore, conversion efficiency from the sunlight to the chemical energy is very low, namely, about 0.4%.

As a photoelectrochemical reaction device splitting water (H2O) by light energy to obtain oxygen (O2) and hydrogen (H2), use of a stack (silicon solar cell or the like) in which a photovoltaic layer is sandwiched between a pair of electrodes is under consideration. For example, an electrode on a light irradiation side oxidizes water (2H2O) by light energy to obtain oxygen (O2) and hydrogen ions (4H+) The electrode on the opposite side obtains hydrogen (2H2) as a chemical substance using the hydrogen ions (4H+) generated by the electrode on the light irradiation side and the potential (e−) generated in the photovoltaic layer. The conversion efficiency from the sunlight to the chemical energy (O2and H2) is as high as about 2.5%.

However, CO2decomposition with high efficiency by light energy has not been realized in the conventional photoelectrochemical reaction device. In order to enhance the efficiency of the reduction reaction of CO2, it is necessary to promote migration of the hydrogen ions or the like generated by the oxidation reaction of H2O to the opposite electrode, which is not into consideration in the conventional device. In order to enhance the practicality of the photoelectrochemical reaction device decomposing CO2, the transfer efficiency of gas containing CO2from a device exhausting CO2to the photoelectrochemical reaction device needs to be considered but is not taken into consideration in the conventional device. If transfer of the gas containing CO2requires energy, the energy efficiency as a photoelectrochemical reaction system decreases.

DETAILED DESCRIPTION

According to one embodiment, there is provided a photoelectrochemical reaction system including a CO2generation unit generating gas containing carbon dioxide, a CO2reduction unit, and a CO2supply unit. The CO2reduction unit includes: a stack including an oxidization electrode layer oxidizing water, a reduction electrode layer reducing carbon dioxide, and a photovoltaic layer provided between the oxidization electrode layer and the reduction electrode layer and performing a charge separation by light energy; an electrolytic solution tank storing a first electrolytic solution in which the oxidization electrode layer is immersed and a second electrolytic solution in which the reduction electrode layer is immersed; and an ion migration pathway allowing ions to migrate between the first electrolytic solution and the second electrolytic solution. The CO2supply unit includes a gas supply pipe supplying the gas containing carbon dioxide generated in the CO2generation unit into the second electrolytic solution.

Hereinafter, a photoelectrochemical reaction system of an embodiment will be described referring to the drawings.

First Embodiment

FIG. 1is a configuration diagram of a photoelectrochemical reaction system according to a first embodiment. A photoelectrochemical reaction system100of the first embodiment includes a CO2generation unit101, an impurity removal unit102, a CO2supply unit103, a CO2reduction unit104, and a product collection unit105. As a representative example of the CO2generation unit101, a power plant can be exemplified. However, the CO2generation unit101is not limited to this but may be an iron factory, a chemical factory, a disposal center or the like.

Gas containing CO2generated in the CO2generation unit101, for example, exhaust gas exhausted from the power plant, iron factory, chemical factory, disposal center or the like is sent to the impurity removal unit102. In the impurity removal unit102, a CO2gas is separated, for example, by removing impurities such as sulfur oxide and the like from, for example, the gas (exhaust gas) containing CO2. As the impurity removal unit102, various dry-type or wet-type gas processing apparatus (sulfur oxide absorption apparatus or the like) is employed. Depending on the kind of the CO2generation unit101, conditions or the like, the generated gas containing CO2is sent directly to the CO2supply unit103without passing through the impurity removal unit102in some cases.

The CO2gas from which the impurities have been removed in the impurity removal unit102is sent by the CO2supply unit103to the CO2reduction unit104. The CO2supply unit103has, as will be described later, a gas supply pipe that supplies the CO2gas into an electrolytic solution in the CO2reduction unit104. The CO2reduction unit104includes a photoelectrochemical module1illustrated, for example, inFIG. 2toFIG. 4. FIG.2is a sectional view illustrating a first example of the photoelectrochemical module1.FIG. 3Ais a sectional view illustrating a second example of the photoelectrochemical module1, andFIG. 3Bis a plan view illustrating a photovoltaic cell used in the photoelectrochemical module1in the second example.FIG. 4is a sectional view illustrating a third example of the photoelectrochemical module1.

The photoelectrochemical module1illustrated inFIG. 2includes a stack3arranged in an electrolytic solution tank2. The stack3includes a first electrode layer11, a second electrode layer21, a photovoltaic layer31provided between the electrode layers11,21, a first catalyst layer12provided on the first electrode layer11, and a second catalyst layer22provided on the second electrode layer21. The constitutional layers of the stack3will be described later. The electrolytic solution tank2is divided into two chambers by the stack3. The electrolytic solution tank2is divided into a first liquid chamber2A where the first electrode layer11and the first catalyst layer12are arranged, and a second liquid chamber2B where the second electrode layer21and the second catalyst layer22are arranged. A first electrolytic solution4is filled in the first liquid chamber2A, and a second electrolytic solution5is filled in the second liquid chamber2B. The electrolytic solution tank2is provided with a not-illustrated window member having a light-transmission property to apply light from the outside to the stack3.

The first liquid chamber2A and the second liquid chamber2B are connected to each other via an electrolytic solution flow path6provided lateral to the electrolytic solution tank2as an ion migration pathway. In a part of the inside of the electrolytic solution flow path6, an ion exchange membrane7is filled. The electrolytic solution flow path6equipped with the ion exchange membrane7allows specific ions (for example, H+) to migrate between the first electrolytic solution4and the second electrolytic solution5while separating the first electrolytic solution4filled in the first liquid chamber2A and the second electrolytic solution5filled in the second liquid chamber2B. As the ion exchange membrane7, for example, a cation exchange membrane such as Nafion or Flemion or an anion exchange membrane such as Neocepter or SELEMION is used. In the electrolytic solution flow path6, a glass filter, agar or the like may be filled. When the first electrolytic solution4and the second electrolytic solution5are the same solution, the ion exchange membrane7does not have to be provided. To efficiently migrate the ions, a plurality of (two or more) electrolytic solution flow paths6may be provided in the electrolytic solution tank2. The dimension of each member of the photoelectrochemical module illustrated inFIG. 2does not indicate its actual size. To facilitate the movement of the ions, the cross-sectional area of the electrolytic solution flow path6may be larger than that of the stack3.

The ion migration pathway is not limited to the electrolytic solution flow path6provided lateral to the electrolytic solution tank2. The ion migration pathway between the first electrolytic solution4and the second electrolytic solution5may be composed of a plurality of pores (through holes)8provided in the stack3. The pore8only needs to have a size through which the ions can move. For example, the lower limit of the diameter (circle-equivalent diameter) of the pore8is preferably 0.3 nm or more. The circle equivalent diameter is defined by ((4×area)/{pi})1/2. The shape of the pore8is not limited to a circle but may be an ellipse, a triangle, or a square. The arrangement of the pores8is not limited to a square lattice shape but may be a triangle lattice shape, random or the like. The ion migration pathway is not limited to the pores8but may be a long hole, or a slit.

In the photoelectrochemical module illustrated inFIG. 3, a not-illustrated ion exchange membrane is filled in the pores8in order to separate the first electrolytic solution4filled in the first liquid chamber2A from the second electrolytic solution5filled in the second liquid chamber2B. Concrete examples of the ion exchange membrane7are as described above. In the pores8, a glass filter, agar or the like may be filled in place of the ion exchange membrane7. When the first electrolytic solution4and the second electrolytic solution5are the same solution, the ion exchange membrane does not have to be provided. The shape and the formation pitch of the pores8as the ion migration pathway are preferably set in consideration of the migratory property of ions and the area of the electrode layer (and the catalyst layer) reduced due to the provision of the pores8. Concretely, the ratio of the area of the pores8to the area of the electrode layer is preferably 40% or less, and more preferably, 10% or less.

The stack3arranged in the electrolytic solution tank2has a flat plate shape spreading in a first direction and a second direction perpendicular thereto. The stack3is constituted, for example, by forming the photovoltaic layer31and the first electrode layer11on the second electrode layer21as a base member. Here, the stack3will be described with a light irradiation side regarded as a front surface (upper surface) and an opposite side to the light irradiation side regarded as a rear surface (lower surface). A concrete configuration example of the stack3will be described referring toFIG. 5andFIG. 6.FIG. 5illustrates a photovoltaic cell3A using a silicon-based solar cell as the photovoltaic layer31A.FIG. 6illustrates a photovoltaic cell3B using a compound semiconductor-based solar cell as the photovoltaic layer31B. In each of the photovoltaic cells3A,3B illustrated inFIG. 5andFIG. 6, the first electrode layer11side is the light irradiation side.

The stack (photovoltaic cell using the silicon-based solar cell)3A illustrated inFIG. 5will be described. The photovoltaic cell3A illustrated inFIG. 5is composed of the first catalyst layer12, the first electrode layer11, the photovoltaic layer31A, the second electrode layer21, and the second catalyst layer22. The second electrode layer21has a conductive property. As the forming material of the second electrode layer21, a metal such as Cu, Al, Ti, Ni, Fe, Ag or the like, an alloy containing at least one of the metals, a conductive resin, a semiconductor such as Si, Ge or the like is used. The second electrode layer21also has a function as a support base member and thus maintains the mechanical strength of the photovoltaic cell3A. The second electrode layer21is composed of a metal plate, an alloy plate, a resin plate, and a semiconductor substrate which are made of the above-described material. The second electrode layer21may be composed of an ion exchange membrane.

The photovoltaic layer31A is formed on the front surface (upper surface) of the second electrode layer21. The photovoltaic layer31A is composed of a reflection layer32, a first photovoltaic layer33, a second photovoltaic layer34, and a third photovoltaic layer35. The reflection layer32is formed on the second electrode layer21and has a first reflection layer32aand a second reflection layer32bformed in order from the lower side. As the first reflection layer32a,a metal such as Ag, Au, Al, Cu or the like having a light-reflection property and a conductive property, an alloy containing at least one of the metals or the like is used. The second reflection layer32bis provided to enhance the light-reflection property by adjusting an optical distance. The second reflection layer32bis to be joined with a later-described n-type semiconductor layer of the photovoltaic layer31and is thus preferably formed of a material having light-transmission property and capable of ohmic contact with the n-type semiconductor layer. As the second reflection layer32b,a transparent conductive oxide such as ITO (indium tin oxide), zinc oxide (ZnO), FTO (fluorine-doped tin oxide), AZO (aluminum-doped tin oxide), ATO (antimony-doped tin oxide) or the like is used.

Each of the first photovoltaic layer33, the second photovoltaic layer34, and the third photovoltaic layer35is a solar cell using a pin-junction semiconductor. The photovoltaic layers33,34,35are different in absorption wavelength of light. Stacking them in a plane state makes it possible to absorb light in a wide range of wavelength of sunlight by the photovoltaic layer31A and efficiently utilize the energy of sunlight. The photovoltaic layers33,34,35are connected in series, and can obtain a high open-circuit voltage.

The first photovoltaic layer33is formed on the reflection layer32and has an n-type amorphous-silicon (a-Si) layer33a,an intrinsic amorphous silicon germanium (a-SiGe) layer33b,and a p-type microcrystalline silicon (mc-Si) layer33cformed in order from the lower side. The a-SiGe layer33babsorbs light in a long wavelength region of about 700 nm. In the first photovoltaic layer33, charge separation is caused by the light energy in the long wavelength region.

The second photovoltaic layer34is formed on the first photovoltaic layer33and has an n-type a-Si layer34a,an intrinsic a-SiGe layer34b,and a p-type mc-Si layer34cformed in order from the lower side. The a-SiGe layer34babsorbs light in an intermediate wavelength region of about 600 nm. In the second photovoltaic layer34, charge separation is caused by the light energy in the intermediate wavelength region.

The third photovoltaic layer35is formed on the second photovoltaic layer34and has an n-type a-Si layer35a,an intrinsic a-Si layer35b,and a p-type mc-Si layer35cformed in order from the lower side. The a-Si layer35babsorbs light in a short wavelength region of about 400 nm. In the third photovoltaic layer35, charge separation is caused by the light energy in the short wavelength region.

The first electrode layer11is formed on the p-type semiconductor (p-type mc-Si layer35c) of the photovoltaic layer31. The first electrode layer11is preferably formed of a material capable of ohmic contact with the p-type semiconductor layer. As the first electrode layer11, a metal such as Ag, Au, Al, Cu or the like, an alloy containing at least one of the metals, a transparent conductive oxide such as ITO, ZnO, FTO, AZO, ATO or the like is used. The first electrode layer11may have, for example, a structure in which the metal and the transparent conductive oxide are layered, a structure in which the metal and another conductive material are combined, a structure in which the transparent conductive oxide and another conductive material are combined or the like.

In the photovoltaic cell3A illustrated inFIG. 5, irradiation light passes through the first electrode layer11and reaches the photovoltaic layer31A. The first electrode layer11arranged on the light irradiation side (the upper side inFIG. 5) has light-transmission property with respect to the irradiation light. The light-transmission property of the first electrode layer11on the light irradiation side is preferably 10% or more of the irradiation amount of the irradiation light, and more preferably 30% or more. The first electrode layer11may have an aperture through which the light is transmitted. The aperture ratio in this case is preferably 10% or more, and more preferably 30% or more. Further, to enhance the conductive property while maintaining the light-transmission property, a collector electrode in a linear shape, a lattice shape, a honeycomb shape or the like may be provided on at least a part of the first electrode layer11on the light irradiation side.

In the photovoltaic layer31A of the photovoltaic cell3A illustrated inFIG. 5, charge separation is caused by the light energy in each wavelength region of the irradiation light (sunlight or the like). In the photovoltaic cell3A using the silicon-based solar cell as the photovoltaic layer31A, holes are separated to the first electrode layer (anode)11side (front surface side) and electrons are separated to the second electrode layer (cathode)21side (rear surface side) to cause electromotive force in the photovoltaic layer31A. As will be described later in detail, an oxidation reaction of water (H2O) is caused near the first electrode layer11to which the holes migrate, and a reduction reaction of carbon dioxide (CO2)k is caused near the second electrode layer21to which the electrons migrate. In the photovoltaic cell3A using the silicon-based solar cell, the first electrode layer11is an oxidation electrode and the second electrode layer21is a reduction electrode.

The first catalyst layer12formed on the first electrode layer11is provided to enhance the chemical reactivity (oxidation reactivity inFIG. 5) near the first electrode layer11. The second catalyst layer22provided on the second electrode layer21is provided to enhance the chemical reactivity (reduction reactivity inFIG. 5) near the second electrode layer21. Utilizing the accelerative effects of the oxidation and reduction reactions by the catalyst layers12,22makes it possible to reduce the overvoltage of the oxidation and reduction reactions. Accordingly, the electromotive force generated in the photovoltaic layer31A can be more effectively utilized.

In the photovoltaic cell3A using the silicon semiconductor-based solar cell, a catalyst accelerating the oxidation reaction is used as the first catalyst layer12. Near the first electrode layer11, H2O is oxidized to generate O2and H+. Therefore, the first catalyst layer12is composed of a material that decreases the activation energy for oxidizing H2O. In other words, the first catalyst layer12is composed of a material that decreases the overvoltage when H2O is oxidized to generate O2and H+. Examples of the material include binary system metal oxides such as 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), ruthenium oxide (Ru—O) and the like, ternary system metal oxides such as Ni—Co—O, Ni—Fe—O, La—Co—O, Ni—La—O, Sr—Fe—O and the like, quaternary system metal oxides such as Pb—Ru—Ir—O, La—Sr—Co—O and the like, and metal complexes such as Ru complex, Fe complex and the like. The shape of the first catalyst layer12is not limited to a thin film shape but may be an island shape, a lattice shape, a grain shape, or a wire shape.

A material accelerating the reduction reaction is used as the second catalyst layer22. Near the second electrode layer21, CO2is reduced to produce a carbon compound (for example, CO, HCOOH, CH4, CH3OH, C2H5OH, C2H4or the like). The second catalyst layer22is composed of a material that decreases the activation energy for reducing CO2. In other words, the second catalyst layer22is composed of a material that decreases the overvoltage when CO2is reduced to produce the carbon compound. Examples of the material include metals such as Au, Ag, Cu, Pt, Pd, Ni, Zn and the like, an alloy containing at least one of the metals, carbon materials such as C, graphene, CNT (carbon nanotube), fullerene, Ketjen black and the like, and metal complexes such as Ru complex, Re complex and the like. The shape of the second catalyst layer22is not limited to a thin film shape but may be an island shape, a lattice shape, a grain shape, or a wire shape.

As a manufacturing method of the first catalyst layer12and the second catalyst layer22, a thin film forming method such as a sputtering method, a vapor deposition method or the like, a coating method using a solution in which a catalyst material is dispersed, an electrodeposition method, a catalyst forming method by thermal processing or electrochemical processing of the first electrode layer11or the second electrode layer21itself can be used. The formation of the first catalyst layer12and the second catalyst layer22is optional, and therefore they may be formed when necessary. The photovoltaic cell3A may have both or only one of the first catalyst layer12and the second catalyst layer22.

The photovoltaic layer31has been described using the photovoltaic layer31A having the stack structure of the three photovoltaic layers as an example inFIG. 5, but is not limited to this. The photovoltaic layer31may have a stack structure of two or four or more photovoltaic layers. In place of the photovoltaic layer31in the stack structure, one photovoltaic layer31may be used. The photovoltaic layer31is not limited to the solar cell using the pin-junction semiconductor but may be a solar cell using a pn-junction semiconductor. The semiconductor layer is not limited to Si or Ge, but may be composed of a compound semiconductor such as GaAs, GaInP, AlGaInP, CdTe, CuInGaSe, GaP, GaN or the like. For the semiconductor layer, various forms such as single crystal, polycrystal, amorphous and the like can be used. The first electrode layer11and the second electrode layer21may be provided entirely or partially on the photovoltaic layer31.

The stack (photovoltaic cell using the compound semiconductor-based solar cell)3B illustrated inFIG. 6will be described. The photovoltaic cell3B illustrated inFIG. 6is composed of the first catalyst layer12, the first electrode layer11, the photovoltaic layer31B, the second electrode layer21, and the second catalyst layer22. The photovoltaic layer31B in the photovoltaic cell3B is composed of a first photovoltaic layer36, a buffer layer37, a tunnel layer38, a second photovoltaic layer39, a tunnel layer40, and a third photovoltaic layer41.

The first photovoltaic layer36is formed on the second electrode layer21and has a p-type Ge layer36aand an n-type Ge layer36b formed in order from the lower side. On the first photovoltaic layer36, the buffer layer37and the tunnel layer38containing GaInAs are formed for lattice matching and electrical connection with GaInAs used for the second photovoltaic layer39. The second photovoltaic layer39is formed on the tunnel layer38and has a p-type GaInAs layer39aand an n-type GaInAs layer39bformed in order from the lower side. On the second photovoltaic layer39, the tunnel layer40containing GaInP is formed for lattice matching and electrical connection with GaInP used for the third photovoltaic layer41. The third photovoltaic layer41is formed on the tunnel layer40and has a p-type GaInP layer41aand an n-type GaInP layer41bformed in order from the lower side.

The photovoltaic layer31B in the photovoltaic cell3B illustrated inFIG. 6is opposite in direction of stacking the p-type and n-type layers to the photovoltaic layer31A in the photovoltaic cell3A illustrated inFIG. 5and is thus different in polarity of electromotive force thereto. When charge separation is caused in the photovoltaic layer31B by the irradiation light, electrons are separated to the first electrode layer (cathode)11side (front surface side) and holes are separated to the second electrode layer (anode)21side (rear surface side). A reduction reaction of CO2is caused near the first electrode layer11to which the electrons migrate. An H2O oxidation reaction is caused near the second electrode layer21to which the holes migrate. Accordingly, in the photovoltaic cell3B using the compound semiconductor-based solar cell, the first electrode layer11is a reduction electrode and the second electrode layer21is an oxidation electrode.

The photovoltaic cell3B illustrated inFIG. 6is opposite in polarity of electromotive force and the oxidation and reduction reactions to the photovoltaic cell3A illustrated inFIG. 5. Therefore, the first catalyst layer12is composed of a material accelerating the reduction reaction and the second catalyst layer22is composed of a material accelerating the oxidation reaction. With respect to the case of using the photovoltaic cell3A illustrated inFIG. 5, the material of the first catalyst layer12and the material of the second catalyst layer22are changed with each other in the photovoltaic cell3B. The polarity of the photovoltaic layer31and the materials of the first catalyst layer12and the second catalyst layer22are arbitrary. Since the oxidation and reduction reactions of the first catalyst layer12and the second catalyst layer22are decided depending on the polarity of the photovoltaic layer31, the materials are selected according to the oxidation and reduction reactions.

One of the first and second electrolytic solutions4,5is a solution containing H2O and the other is a solution containing CO2. In the case of employing the photovoltaic cell3A illustrated inFIG. 5, the solution containing H2O is used as the first electrolytic solution4and the solution containing CO2is used as the second electrolytic solution5. In the case of employing the photovoltaic cell3B illustrated inFIG. 6, the solution containing CO2is used as the first electrolytic solution4and the solution containing H2O is used as the second electrolytic solution5.

As the solution containing H2O, a solution containing an arbitrary electrolyte is used. This solution is preferably a solution accelerating the oxidation reaction of H2O. Examples of the solution containing an electrolyte include solutions containing phosphate ions (PO42), borate ions (BO33−), sodium ions (Na−), potassium ions (K+), calcium ions (Ca2+), lithium ions (Li+), cesium ions (Cs+), magnesium ions (Mg2−), chloride ions (Cl−), hydrogen carbonate ions (HCO3−) and the like.100471The solution containing CO2is preferably a solution high in CO2absorption rate. Examples of the solution containing CO2include solutions such as LiHCO3, NaHCO3, KHCO3, CsHCO3and the like as a solution containing H2O. For the solution containing CO2, alcohols such as methanol, ethanol, acetone and the like may be used. The solution containing H2O and the solution containing CO2may be the same solution. Since the solution containing CO2is preferably high in CO2absorption amount, a solution different from the solution containing H2O may be used. The solution containing CO2is desirably an electrolytic solution containing a CO2absorbent that decreases a reduction potential of CO2, is high in ion conductivity, and absorbs CO2.

Examples of the electrolytic solution include ionic liquids composed of salt of cations such as imidazolium ion, pyridinium ion and the like and anions such as BF4−, PF6−and the like and are in a liquid state in a wide temperature range, and their solutions. Other examples of the electrolytic solution include amine solutions such as ethanolamine, imidazole, pyridine and the like and their solutions. Amine may be any of primary amine, secondary amine, and tertiary amine. Examples of the primary amine include methylamine, ethylamine, propylamine, butylamine, pentylamine, hexylamine and the like. The hydrocarbon of the amine may be replace with alcohol, halogen or the like. Examples of the amine whose hydrocarbon is replaced include methanolamine, ethanolamine, chloromethylamine and the like. Besides, an unsaturated bond may exist. Those hydrocarbons also apply to secondary amine and tertiary amine. Examples of the secondary amine include dimethylamine, diethylamine, dipropylamine, dibutylamine, dipentylamine, dihexylamine, dimethanolamine, diethanolamine, dipropanolamine and the like. The replaced hydrocarbons may be different. This also applies to tertiary amine. Examples of the amine with different hydrocarbon include methylethylamine, methylpropylamine and the like. Examples of the tertiary amine include trimethylamine, trihexylamine, tripropylamine, tributylamine, trihexylamine, trimethanolamine, triethanolamine, tripropanolamine, tributanolamine, trihexanolamine, methyldiethylamine, methyldipropylamine and the like. Examples of cation in the ionic liquid include 1-ethyl-3-methylimidazolium ion, 1-methyl-3-propylimidazolium ion, 1-butyl-3-methylimidazolium ion, 1-methyl-3-pentylimidazolium ion, 1-hexyl-3-methylimidazolium ion and the like. The position2of imidazolium ion may be replaced. Examples of the imidazolium ion whose position2is replaced include 1-ethyl-2,3-dimethylimidazolium ion, 1, 2-dimethyl-3-propylimidazolium ion, 1-butyl-2,3-dimethylimidazolium ion, 1, 2-dimethyl-3-pentylimidazolium ion, 1-hexyl-2,3-dimethylimidazolium ion and the like. Examples of pyridinium ion include methylpyridinium, ethylpyridinium, propylpyridinium, butylpyridinium, pentylpyridinium, hexylpyridinium and the like. In both of imidazolium ion and pyridinium ion, an alkyl group may be replaced and an unsaturated bond may exist. Examples of anion include fluoride ion, chloride ion, bromide ion, chloride ion, BF4−, PF6−, CF3COO−, CF3SO3−, NO3−, SCN−, (CF3SO2)3C−, bis(trifluoromethoxysulfonyl)imide, bis(perfluoroethylsulfonyl)imide and the like. Dipolar ion made by bonding the cation and the anion in the ionic liquid by hydrocarbon may be adoptable.

As illustrated inFIG. 2, in the second liquid chamber2B of the electrolytic solution tank2in which the second electrolytic solution5is stored, a gas supply pipe51constituting the CO2supply unit103is provided. The gas supply pipe51is arranged to be immersed in the second electrolytic solution5.FIG. 2illustrates the configuration of the photoelectrochemical module1based on the polarity of the electromotive force of the photovoltaic cell3A illustrated inFIG. 5. The gas supply pipe51is arranged in the second electrolytic solution5in which the second electrode layer21that is the reduction electrode is immersed. In the photoelectrochemical module1configured based on the polarity of the electromotive force of the photovoltaic cell3B illustrated inFIG. 6, the gas supply pipe51is arranged in the first electrolytic solution4in which the first electrode layer11that is the reduction electrode is immersed. Hereafter, the configuration of the photoelectrochemical module1based on the polarity of the electromotive force of the photovoltaic cell3A will be mainly described unless otherwise noted.

The CO2gas separated by removing the impurities such as sulfur oxide and so on in the impurity removal unit102is introduced into the gas supply pipe51of the CO2supply unit103. The gas supply pipe51has a plurality of gas supply holes (through holes)52. The CO2gas introduced into the gas supply pipe51is released into the second electrolytic solution5from the gas supply holes52. Since the second electrolytic solution5is composed of the solution high in CO2absorption amount as described above, the CO2gas released into second electrolytic solution5from the gas supply holes52is absorbed by the second electrolytic solution5. The CO2absorbed by the second electrolytic solution5is reduced by the oxidation and reduction reactions which will be described hereafter in detail.

A principle of operation of the photoelectrochemical module1will be described referring toFIG. 7. Here, the operation will be described using, as an example, the polarity in the case of using the stack illustrated inFIG. 5, that is, the photovoltaic cell3A using the silicon semiconductor-based solar cell as the photovoltaic layer31A. The case where an absorbing liquid absorbing CO2is used as the second electrolytic solution5in which the second electrode layer21and the second catalyst layer22are to be immersed will be described. In the case of using the stack illustrated inFIG. 6, that is, the photovoltaic cell3B using the compound semiconductor-based solar cell as the photovoltaic layer31B, the polarity is reversed and therefore an absorbing liquid absorbing CO2is used as the first electrolytic solution4.

As illustrated inFIG. 7, light irradiated from above (the first electrode layer11side of) the photoelectrochemical module1passes through the first catalyst layer12and the first electrode layer11and reaches the photovoltaic layer31. Upon absorption of the light, the photovoltaic layer31generates electrons and holes paired therewith and separate them. In the photovoltaic layer31, the electrons migrate to the n-type semiconductor layer side (the second electrode layer21side) and the holes generated as companions to the electrons migrate to the p-type semiconductor layer side (the first electrode layer11side). This charge separation causes electromotive force in the photovoltaic layer31.

The holes generated in the photovoltaic layer31migrate to the first electrode layer11and combine with the electrons generated by the oxidation reaction caused near the first electrode layer11and the first catalyst layer12. The electrons generated in the photovoltaic layer31migrate to the second electrode layer21and are used for the reduction reaction caused near the second electrode layer21and the second catalyst layer22. Concretely, near the first electrode layer11and the first catalyst layer12in contact with the first electrolytic solution4, the reaction of the following Expression (1) is caused. Near the second electrode layer21and the second catalyst layer22in contact with the second electrolytic solution5, the reaction of the following Expression (2) is caused.

Near the first electrode layer11and the first catalyst layer12, H2O contained in the first electrolytic solution4is oxidized (lose electrons) to generate O2and H+as expressed in Expression (1). H+generated on the first electrode layer11side migrates to the second electrode layer21side via the electrolytic solution flow path6(FIG. 2) provided in the electrolytic solution tank2as the ion migration pathway or the pores8(FIG. 3) provided in the stack3. Near the second electrode layer21and the second catalyst layer22, CO2supplied into the second electrolytic solution5from the gas supply pipe51is reduced (gains electrons) as expressed in Expression (2). Concretely, CO2in the second electrolytic solution5, H+migrated to the second electrode layer21side via the ion migration pathway and the electrons migrated to the second electrode layer21react to generate, for example, CO and H2O.

The photovoltaic layer31needs to have an open-circuit voltage equal to or higher than a potential difference between a standard oxidation-reduction potential of the oxidation reaction caused near the first electrode layer11and a standard oxidation-reduction potential of the reduction reaction caused near the second electrode layer21. For example, the standard oxidation-reduction potential of the oxidation reaction in Expression (1) is 1.23 V, and the standard oxidation-reduction potential of the reduction reaction in Expression (2) is −0.1 V. Therefore, the open-circuit voltage of the photovoltaic layer31needs to be 1.33 V or higher. The open-circuit voltage of the photovoltaic layer31is preferably equal to or higher than a potential difference including the overvoltage. Concretely, when each of the overvoltage of the oxidation reaction in Expression (1) and the reduction reaction in Expression (2) is 0.2 V, the open-circuit voltage is desirably 1.73 V or higher.

Near the second electrode layer21, not only the reduction reaction from CO2to CO expressed in Expression (2) but also a reduction reaction from CO2to fonnic acid (HCOOH), methane (CH4), ethylene (C2H4), methanol (CH3OH), ethanol (C2H5OH) or the like can also be caused. A reduction reaction of H2O used in the second electrolytic solution5can be further caused to generate H2. By changing the moisture (H2O) amount in the second electrolytic solution5, a reducing substance of CO2to be produced can be changed. For example, it is possible to change a generation ratio of CO, HCOOH, CH4, C2H4, CH3OH, C2H5OH, H2and the like.

The photoelectrochemical module1in the photoelectrochemical reaction system100of the embodiment includes the ion migration pathway allowing ions to migrate between the first electrolytic solution4and the second electrolytic solution5. The hydrogen ions (H+) generated on the first electrode layer11are sent to the second electrode layer21side via electrolytic solution flow path6or the pores8as the ion migration pathway. Efficiently sending the hydrogen ions (H+) generated on the first electrode layer11side to the second electrode layer21side accelerates the reduction reaction of CO2near the second electrode layer21and the second catalyst layer22. The reduction efficiency of CO2by light can be enhanced. In other words, the photoelectrochemical reaction system100of this embodiment can efficiently decompose CO2by light energy, thereby making it possible to improve the conversion efficiency, for example, from sunlight to chemical energy.

The CO2supply unit103in the photoelectrochemical reaction system100of this embodiment utilizes the pressure (exhaust pressure) of the gas containing CO2(exhaust gas or the like) exhausted from the CO2generation unit101to supply the CO2gas into the second electrolytic solution5via the gas supply holes52of the gas supply pipe51. For example, in the case of sending CO2to the electrolytic solution tank after being absorbed by the CO2absorbent, energy to send the CO2absorbent (absorbing liquid) to the electrolytic solution tank is required. Considering sending of the CO2absorbent absorbed CO2by a pump, energy to operate the pump is required. This decreases the energy efficiency as the whole photoelectrochemical system. In contrast, utilizing the exhaust pressure of the gas in the CO2generation unit101makes it possible to supply the CO2gas into the second electrolytic solution5without consuming energy for transfer.

Further, a gaseous product such as a carbon compound (for example, CO, CH4, C2H4or the like) and H2produced by reducing CO2and H2O are sent from the electrolytic solution tank2of the CO2reduction unit104to the product collection unit105utilizing the pressure (exhaust pressure) of the CO2gas released from the gas supply pipe51into the second electrolytic solution5. Therefore, the gaseous product can be accumulated in the product collection unit105without separately generating a transfer means for the gaseous product, that is, airflow or the like required for transfer of the gaseous product. These can enhance the energy efficiency as the photoelectrochemical reaction system100. Consequently, it becomes possible to provide the photoelectrochemical reaction system100high in CO2decomposition efficiency and excellent in energy efficiency as the whole system.

In the photoelectrochemical reaction system100of the embodiment, the ion migration pathway allowing ions to move between the first electrolytic solution4and the second electrolytic solution5is not limited to the electrolytic solution flow path6provided in the electrolytic solution tank2and the pores8provided in the photovoltaic cell (stack)3. For example, an ion migration pathway may be provided in the base plate (second electrode layer21) that substantially divides the electrolytic solution tank2into two chambers, or the photovoltaic cell3may be divided into a plurality portions and an ion migration pathway may be provided between them. The structure of the photoelectrochemical module1is not limited to the structures illustrated inFIG. 2andFIG. 3. For example, a photoelectrochemical module lA having a structure in which a photovoltaic cell3formed in a tubular shape and a tubular electrolytic solution tank2are arranged in order around a gas supply pipe51as illustrated inFIG. 4, may be employed.

The photoelectrochemical module1A illustrated inFIG. 4has a structure in which the gas supply pipe51, the photovoltaic cell3formed in a tubular shape, and the tubular electrolytic solution tank2are concentrically arranged for instance. The tubular electrolytic solution tank2is composed of a material having a light-transmission property so as to allow light to reach the photovoltaic cell3arranged therein. The tubular photovoltaic cell3has a structure in which layers are stacked to have a circular cross-sectional shape such that the first electrode layer11that is on the light irradiation side is located on an outer side. A plurality of electrolytic solution flow paths6are provided and their shape is not limited a circle but may be an ellipse, a triangle, a square, a slit shape or the like. Between the tubular photovoltaic cell3and the tubular electrolytic solution tank2, the first liquid chamber2A in which the first electrolytic solution4is filled is formed. Between the gas supply pipe51and the tubular photovoltaic cell3, the second liquid chamber2B in which the second electrolytic solution5is filled is formed. The outside diameters and inside diameters of the gas supply pipe51, the tubular photovoltaic cell3, and the electrolytic solution tank2are adjusted so that the first liquid chamber2A and the second liquid chamber2B are formed.

In the photoelectrochemical module lA illustrated inFIG. 4, the tubular photovoltaic cell3is arranged around the gas supply pipe51via the second electrolytic solution5. Therefore, feeding the CO2gas through gas supply pipe51makes it possible to efficiently release the CO2gas from the gas supply holes52into the second electrolytic solution5. Further, it is also possible to allow the gaseous product such as the carbon compound (for example, CO, CH4, C2H4or the like) and H2produced by reducing CO2and H2O to flow along the direction of a tube axis of the tubular photovoltaic cell3utilizing the exhaust pressure of the CO2gas. Accordingly, transfer of the gaseous product is facilitated. It is also possible to allow O2generated by the oxidation reaction in the first liquid chamber2A to flow along the direction of a tube axis of the electrolytic solution tank2, thus also facilitating transfer of O2.

In the photoelectrochemical reaction system100illustrated inFIG. 1, the carbon compound produced by the reduction reaction in the CO2reduction unit104is collected to a tank or the like as the product collection unit105. The carbon compound produced in the CO2reduction unit104may be supplied as a carbon fuel to a combustion furnace of the CO2generation unit101of for example, a power plant, iron factory, chemical factory, disposal center or the like. O2generated by the oxidation reaction in the CO2reduction unit104may be similarly collected to a tank or the like, or may be supplied to the combustion furnace as a combustion improver. In addition to the above, O2can be utilized for various uses such as supply to a breeding pond so as to promote growth of living things, supply to a sewage disposal plant for improvement in processing efficiency by bacteria, supply to an air purification system, water clarification system and the like.

Second Embodiment

FIG. 8is a configuration diagram of a photoelectrochemical reaction system according to a second embodiment. A photoelectrochemical reaction system110of the second embodiment includes a CO2generation unit101, an impurity removal unit102, a CO2supply unit103, a CO2reduction unit104, a CO2separation unit106, and a product collection unit105. The constitutional units101,102,103,104,105other than the CO2separation unit106have the same configurations as those in the photoelectrochemical reaction system100of the first embodiment.

In the photoelectrochemical module1constituting the CO2reduction unit104, the carbon compound and hydrogen produced by the reduction reaction of CO2and H2O are collected to a tank or the like as the product collection unit105. There is a possibility that CO2which has not been decomposed is mixed in the produced carbon compound and hydrogen. In the photoelectrochemical reaction system110of the second embodiment, the CO2separation unit106is provided between the CO2reduction unit104and the product collection unit105. To the CO2separation unit106, for example, a molecular sieve using a polymeric film, zeolite, a carbon film, CO2absorbent using amine, KOH or NaOH solution, and the like, is applicable. Separation of CO2from the produced carbon product enables enhancement of the utility value of the product. The CO2gas separated from the product may be returned to the CO2reduction unit104or may be sent to a CO2absorption unit as illustrated in the third embodiment.

Third Embodiment

FIG. 9is a configuration diagram of a photoelectrochemical reaction system according to a third embodiment. A photoelectrochemical reaction system120of the third embodiment includes a CO2generation unit101, an impurity removal unit102, a CO2supply unit103, a CO2reduction unit104, a CO2separation unit106, a product collection unit105, and a CO2absorption unit107. The constitutional units101,102,103,104,106,105other than the CO2absorption unit107have the same configurations as those in the photoelectrochemical reaction systems100,110of the first and second embodiments.

The CO2absorption unit107is, for example, a CCS (Carbon Dioxide Capture and Storage). In the CO2absorption unit107, a part of CO2separated in the impurity removal unit102and/or CO2separated from the product in the CO2separation unit106is absorbed by a CO2absorbent. Concrete examples of the CO2absorbent are as described above. By heating the CO2absorbent absorbed CO2, CO2is separated. The separated CO2is stored underground or the like. By using both the CO2reduction unit104(CCU: Carbon dioxide Capture and Utilization) and the CO2absorption unit107(CCS: Carbon dioxide Capture and Storage), the CO2gas generated in the CO2generation unit101can be decomposed or stored without being released into the atmosphere.