PHOTOELECTROCHEMICAL REACTION DEVICE

A photoelectrochemical reaction device in an embodiment includes: first and second photovoltaic cells each including a first electrode, a second electrode, and a photovoltaic layer; first and second reaction electrode pairs each including a third electrode and a fourth electrode; and an electrolytic bath storing a first electrolytic solution in which the third electrodes are immersed and a second electrolytic solution in which the fourth electrodes are immersed. One of the third and fourth electrodes causes an oxidation reaction, and the other of the third and fourth electrodes causes a reduction reaction. The first photovoltaic cell is electrically connected to the first reaction electrode pair, and the second photovoltaic cell is electrically connected to the second reaction electrode pair.

TECHNICAL FIELD

Embodiments described herein generally relate to a photoelectrochemical reaction device.

BACKGROUND ART

In recent years, there has been concern about the depletion of fossil fuel such as petroleum and coal, and renewable energy that can be sustainably utilized is increasingly expected. As one of the renewable energies, a solar cell and heat power generation which use sunlight are under development. The solar cell has problems that it requires cost for storage batteries used when the generated power (electricity) is stored and a loss occurs at the time of the power storage. A technique of directly converting the sunlight to a chemical substance (chemical energy) such as hydrogen (H2), carbon monoxide (CO), methanol (CH3OH), or formic acid (HCOOH) instead of converting the sunlight to electricity has been drawing attention. Storing the chemical substance converted from the sunlight in a cylinder or a tank has advantages that it requires less cost for storing the energy and further the storage loss is smaller, as compared with storing electricity converted from the sunlight in the storage battery.

As a device that converts sunlight energy to chemical energy, there are known photoelectrochemical reaction devices in which a photovoltaic unit and an electrolytic unit are integrated together. The photoelectrochemical reaction devices are roughly classified into a cell-integrated type device in which a photovoltaic cell is not immersed in an electrolytic solution but integrally arranged on an electrolytic bath, and a cell-immersed type device in which a photovoltaic cell is immersed in an electrolytic solution. In the photoelectrochemical reaction device of the cell-integrated type, when a plurality of photovoltaic cells are used for enhancing the electromotive force, it is conceivable to connect the plurality of photovoltaic cells connected in parallel, to electrodes (anode and cathode). In this case, when part of the plural photovoltaic cells becomes shaded due to could or a failure occurs in part of the plural photovoltaic cells, not only electromotive force decreases correspondingly to the portion of the failed cell but also the conversion efficiency of the whole device decreases by the effect of the cell decreased in parallel resistance due to the failure.

DETAILED DESCRIPTION

According to one embodiment, there is provided a photoelectrochemical reaction device including: a first photovoltaic cell including a first electrode, a second electrode, and a photovoltaic layer provided between the first electrode and the second electrode; a second photovoltaic cell including a first electrode, a second electrode, and a photovoltaic layer provided between the first electrode and the second electrode; a reaction electrode pair including at least one third electrode and two divided fourth electrodes, and one of the third and fourth electrodes causing an oxidation reaction, and the other of the third and fourth electrodes causing a reduction reaction; a first connecting member electrically connecting the first electrodes of the first and second photovoltaic cells to the third electrode of the reaction electrode pair; a second connecting member electrically connecting the second electrode of the first photovoltaic cell to one of the two fourth electrodes of the reaction electrode pair; a third connecting member electrically connecting the second electrode of the second photovoltaic cell to the other of the two fourth electrodes of the reaction electrode pair; and an electrolytic bath storing a first electrolytic solution in which at least the third electrode is immersed and a second electrolytic solution in which at least the fourth electrodes are immersed.

Hereinafter, photoelectrochemical reaction devices of embodiments will be described with reference to the drawings.

First Embodiment

FIG. 1toFIG. 4are views illustrating a photoelectrochemical reaction device according to a first embodiment.FIG. 1is a plan view of the photoelectrochemical reaction device,FIG. 2is a cross-sectional view taken along a line A-A inFIG. 1,FIG. 3is a cross-sectional view taken along a line B-B inFIG. 1, andFIG. 4is a view illustrating an electrical connection state ofFIG. 1. The photoelectrochemical reaction device1of the first embodiment includes a first photovoltaic cell2A, a second photovoltaic cell2B, a first reaction electrode pair3A, a second reaction electrode pair3B, and an electrolytic bath4. Each of the first and second photovoltaic cells2A,2B includes a first electrode11, a second electrode21, and an photovoltaic layer31which is provided between the first and second electrodes11and21and performs charge separation by light energy. The first and second photovoltaic cells2A,2B are arranged outside the electrolytic bath4.

Each of the first and second reaction electrode pairs3A,3B includes a third electrode41and a fourth electrode42arranged to be opposed to the third electrode41. The first and second reaction electrode pairs3A,3B are arranged inside the electrolytic bath4. The electrolytic bath4includes a first storage part52storing a first electrolytic solution51in which the third electrodes41are immersed, a second storage part54storing a second electrolytic solution53in which the fourth electrodes42are immersed, and an ion migration layer (an ion migration layer also serving as a separation wall)55allowing ions to migrate while separating the first electrolytic solution51and the second electrolytic solution53. One of the third electrode41and the fourth electrode42causes an oxidation reaction and the other of the third electrode41and the fourth electrode42causes a reduction reaction. Specifically, a photoelectromotive force generated by radiating the sunlight or the like to the photovoltaic cells2A,2B causes the oxidation and reduction reactions by the reaction electrode pairs3A,3B.

As illustrated inFIG. 4, the first electrode11of the first photovoltaic cell2A is electrically connected to the third electrode41of the first reaction electrode pair3A via a connecting member6A, and the second electrode21of the first photovoltaic cell2A is electrically connected to the fourth electrode42of the first reaction electrode pair3A via a connecting member6B. Similarly, the first electrode11of the second photovoltaic cell2B is electrically connected to the third electrode41of the second reaction electrode pair3B via a connecting member6C, and the second electrode21of the second photovoltaic cell2B is electrically connected to the fourth electrode42of the second reaction electrode pair3B via a connecting member6D. Electrical connection of the photovoltaic cell2and the reaction electrode pair3as a set prevents a failure, even when occurring in one photovoltaic cell2, from adversely affecting the other combination of the photovoltaic cell2and the reaction electrode pair3. The conversion efficiency from light energy to chemical energy by the photoelectrochemical reaction device1can be maintained.

Though the photoelectrochemical reaction device1having a combination of the first photovoltaic cell2A and the first reaction electrode pair3A and a combination of the second photovoltaic cell2B and the second reaction electrode pair3B is illustrated inFIG. 1toFIG. 4, the number of combinations of the photovoltaic cell2and the reaction electrode pair3is not limited to this. The photoelectrochemical reaction device1may have three sets or more of the combination of the photovoltaic cell2and the reaction electrode pair3. Also in this case, one photovoltaic cell2and one reaction electrode pair3are combined as a set and electrically connected.FIG. 5illustrates a photoelectrochemical reaction device1including three sets of the combination of the photovoltaic cell2and the reaction electrode pair3. The photoelectrochemical reaction device1illustrated inFIG. 5further includes a combination of a third photovoltaic cell2C and a third reaction electrode pair3C. Also in the combination of the third photovoltaic cell2C and the third reaction electrode pair3C, a first electrode11is electrically connected to a third electrode41and a second electrode21is electrically connected to a fourth electrode42, via connecting members6E,6F respectively.

The configuration of the photoelectrochemical reaction device1of the first embodiment will be described in detail. The photovoltaic cell2has a flat plate shape spreading in a first direction and a second direction perpendicular to the first direction, and is composed of, for example, the second electrode21as a substrate, and the photovoltaic layer31and the first electrode11which are formed in order on the second electrode21. Here, a description will be given on assumption that a light irradiated side is a front surface (upper surface) and a side opposite the light irradiated side is a rear surface (lower surface). Concrete structural examples of the photovoltaic cell2will be described with reference toFIG. 6andFIG. 7.FIG. 6illustrates a photovoltaic cell (photoelectrochemical cell)201which uses a silicon-based solar cell as a photovoltaic layer311.FIG. 7illustrates a photovoltaic cell (photoelectrochemical cell)202which uses a compound semiconductor-based solar cell as a photovoltaic layer312.

In the photovoltaic cell201illustrated inFIG. 6, a second electrode21has electrical conductivity. As a formation material of the second electrode21, metal such as Cu, Al, Ti, Ni, Fe, or Ag, an alloy such as SUS containing at least one of these metals, conductive resin, a semiconductor such as Si or Ge, or the like is used. The second electrode21is formed on a substrate22having electrical conductivity, so that mechanical strength of the photovoltaic cell201is maintained. The second electrode21itself may have a function as a support substrate. In such a case, as the second electrode21, a metal plate, an alloy plate, a resin plate, a semiconductor substrate, or the like is used. The second electrode21may be composed of an ion exchange membrane.

The photovoltaic layer311is formed on the second electrode21. The photovoltaic layer311is composed of a reflective layer32, a first photovoltaic layer33, a second photovoltaic layer34, and a third photovoltaic layer35. The reflective layer32is formed on the second electrode21and has a first reflective layer32aand a second reflective layer32bwhich are formed in order from a lower side. As the first reflective layer32a, metal such as Ag, Au, Al, or Cu, an alloy containing at least one of these metals, or the like that has light reflectivity and electrical conductivity is used. The second reflective layer32bis provided in order to adjust an optical distance to enhance light reflectivity. The second reflective layer32bis joined to a later-described n-type semiconductor layer of the photovoltaic layer31and therefore is preferably formed of a material having a light transmitting property and capable of coming into ohmic contact with the n-type semiconductor layer. As the second reflective layer32b, a transparent conductive oxide such as ITO (indium tin oxide), zinc oxide (ZnO), FTO (fluorine-doped tin oxide), AZO (aluminum-doped zinc oxide), or ATO (antimony-doped tin oxide) is used.

The first photovoltaic layer33, the second photovoltaic layer34, and the third photovoltaic layer35are each a solar cell using a pin junction semiconductor and their light absorption wavelengths are different. Stacking them in a planar manner makes it possible for the photovoltaic layer311to absorb light in a wide range of wavelength of sunlight, which makes it possible to more efficiently utilize energy of the sunlight. Since the photovoltaic layers33,34,35are connected in series, it is possible to obtain a high open-circuit voltage.

The first photovoltaic layer33is formed on the reflective 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) layer33cin order from a lower side. The a-SiGe layer33bis a layer that absorbs light in a long wavelength range of about 700 nm. In the first photovoltaic layer33, charge separation is caused by energy of the light in the long wavelength range.

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 layer34cwhich are formed in order from a lower side. The a-SiGe layer34bis a layer that absorbs light in an intermediate wavelength range of about 600 nm. In the second photovoltaic layer34, charge separation is caused by energy of the light in the intermediate wavelength range.

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 layer35cwhich are formed in order from a lower side. The a-Si layer35bis a layer that absorbs light in a short wavelength range of about 400 nm. In the third photovoltaic layer35, charge separation is caused by energy of the light in the short wavelength range. In the photovoltaic layer311, the charge separations are caused by the lights in the respective wavelength ranges. Specifically, holes are separated to a first electrode (anode)11side (front surface side) and electrons are separated to a second electrode (cathode)21side (rear surface side), so that an electromotive force is generated in the photovoltaic layer311.

The first electrode11is formed on the p-type semiconductor layer (p-type me-Si layer35c) of the photovoltaic layer311. The first electrode11is preferably formed of a material capable of coming into ohmic contact with the p-type semiconductor layer. As the first electrode11, metal such as Ag, Au, Al, or Cu, an alloy containing at least one of these metals, a transparent conductive oxide such as ITO, ZnO, FTO, AZO, or ATO, or the like is used. The first electrode11may have, for example, a structure in which the metal and the transparent conductive oxide are stacked, a structure in which the metal and other conductive material are compounded, a structure in which the transparent conductive oxide and other conductive material are compounded, or the like.

In the photovoltaic cell (the photoelectrochemical cell using the silicon-based solar cell)201illustrated inFIG. 6, irradiating light passes through the first electrode11to reach the photovoltaic layer311. The first electrode11disposed on a light irradiated side (upper side inFIG. 6) has a light transmitting property for the irradiating light. The light transmitting property of the first electrode11on the light irradiated side is preferably 10% or more of an irradiation amount of the irradiating light, and more preferably 30% or more thereof. The first electrode11may have an aperture through which the light is transmitted. An open area ratio in this case is preferably 10% or more, and more preferably 30% or more.

In order to enhance electrical conductivity while maintaining the light transmitting property, a collector electrode made of metal such as Ag, Au, or Cu, or an alloy containing at least one of these metals may be provided on at least part of the first electrode11on the light irradiated side. The collector electrode has a shape transmitting the light, and examples of its concrete shape are a liner shape, a lattice shape, a honeycomb shape, and so on. In order to maintain the light transmitting property, an area of the collector electrode is preferably 30% or less of an area of the first electrode11, and more preferably 10% or less thereof.

InFIG. 6, the photovoltaic layer311having the stacked structure of the three photovoltaic layers is described as an example, but the photovoltaic layer31is not limited to this. The photovoltaic layer31may have a stacked structure of two, or four or more photovoltaic layers. In place of the photovoltaic layer31having the stacked structure, a single 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. A semiconductor layer may be made of a compound semiconductor such as, for example, GaAs, GaInP, AlGaInP, CdTe, or CuInGaSe, not limited to Si or Ge. As the semiconductor layer, any of various forms such as monocrystalline, polycrystalline, and amorphous forms is applicable. The first electrode11and the second electrode21may be provided on the whole surface of the photovoltaic layer31or may be provided on part thereof.

Next, the photovoltaic cell (the photoelectrochemical cell using the compound semiconductor-based solar cell as the photovoltaic layer)202illustrated inFIG. 7will be described. The photovoltaic cell202illustrated inFIG. 7is composed of a first electrode11, a photovoltaic layer312, and a second electrode21. The photovoltaic layer312in the photovoltaic cell202is composed of a first photovoltaic layer321, a buffer layer322, a tunnel layer323, a second photovoltaic layer324, a tunnel layer325, and a third photovoltaic layer326.

The first photovoltaic layer321is formed on the second electrode21and has a p-type Ge layer321aand an n-type Ge layer321bwhich are formed in order from a lower side. On the first photovoltaic layer321(Ge layer321b), the buffer layer322containing GaInAs and the tunnel layer323are formed for the purpose of lattice matching and electrical joining with GaInAs used in the second photovoltaic layer324.

The second photovoltaic layer324is formed on the tunnel layer323and has a p-type GaInAs layer324aand an n-type GaInAs layer324bwhich are formed in order from a lower side. On the second photovoltaic layer324(GaInAs layer324b), the tunnel layer325containing GaInP is formed for the purpose of lattice matching and electrical joining with GaInP used in the third photovoltaic layer326. The third photovoltaic layer326is formed on the tunnel layer325and has a p-type GaInP layer326aand an n-type GaInP layer326bwhich are formed in order from a lower side.

The photovoltaic layer312in the photovoltaic cell (the photoelectrochemical cell using the compound semiconductor-based solar cell as the photovoltaic layer)202illustrated inFIG. 7is different in a stacking direction of the p-type and the n-type from the photovoltaic layer311in the photovoltaic cell (the photoelectrochemical cell using the silicon semiconductor-based solar cell)201illustrated inFIG. 6, and therefore polarities of their electromotive forces are different. Specifically, when the charge separation is caused in the photovoltaic layer312by the irradiated light, electrons are separated to the first electrode (cathode)11side (front surface side), and holes are separated to the second electrode (anode)21side (rear surface side).

The first and second photovoltaic cells2A,2B are arranged on the electrolytic bath4. The photovoltaic cells2A,2B are in close contact with the electrolytic bath4. The photovoltaic cells2A,2B may be in close contact with the electrolytic bath4via an insulating member. The first photovoltaic cell2A and the second photovoltaic cell2B are preferably arranged to be as short as possible in connection distances to the first reaction electrode pair3A and the second reaction electrode pair3B respectively, namely, in length of connecting members6A to6D. The first photovoltaic cell2A is preferably arranged to be located above the first reaction electrode pair3A electrically connected thereto. The second photovoltaic cell2BA is preferably arranged to be located above the second reaction electrode pair3B electrically connected thereto.

The reaction electrode pair3has the third electrode41immersed in the first electrolytic solution51, and the fourth electrode42immersed in the second electrolytic solution53. The electrodes41,42are formed of a material having electrical conductivity. As each of the electrodes41,42, a metal plate of Cu, Al, Au, Ti, Ni, Fe, Co, Ag, Pt, Pd, Zn, In or the like, an alloy plate containing at least one of these metals, a conductive resin plate, a semiconductor substrate of Si or Ge, or the like is used. The third electrode41and the fourth electrode42are preferably arranged to be opposed to each other for ions to rapidly migrate. The fourth electrode42is preferable arranged as close as possible to the third electrode41. The distance between the electrodes41and42is preferably 500 mm or less, and more preferably 100 mm or less. To arrange the ion migration layer55, the distance between the electrodes41and42is preferably 100 micrometer or more.

The ion migration layer55arranged in the electrolytic bath4is composed of an ion exchange membrane or the like which allows ions to migrate between the third electrode41and the fourth electrode42and can separate the first electrolytic solution51and the second electrolytic solution53. As the ion exchange membrane, a cation exchange membrane such as Nafion or Flemion or an anion exchange membrane such as Neosepta or Selemion can be used. Materials other than the above are applicable as the ion migration layer55, as long as they are materials allowing the ions to migrate between the third electrode41and the fourth electrode42.

The third and fourth electrodes41,42may have fine pores or slits for allowing ions to migrate. The fine pores or slits are provided to cause ions to migrate while maintaining the mechanical strength of the third and fourth electrodes41,42. The fine pores or slits only need to have a size enabling the ions to migrate. For example, a lower limit value of a diameter (circle-equivalent diameter) of the fine pores is preferably 0.3 nm or more. The circle-equivalent diameter is defined as ((4×area)/{pi})1/2. The shape of the fine pores is not limited to a circle and may be an ellipse, a triangle, a square, or the like. The fine pores are arranged in a square lattice form, a triangular lattice form, a random form, or the like. The fine pores or slits may be filled with an ion exchange membrane. The fine pores or slits may be filled with a glass filter or agar.

Though the state in which the third electrode41and the fourth electrode42are individually arranged in the electrolytic bath4is illustrated inFIG. 1toFIG. 3, but the configuration of the reaction electrode pair3is not limited to this. The reaction electrode pair3may be a stack in which the third electrode41and the fourth electrode42are stacked via an ion migration layer. In this case, the ion migration layer is compose of an electrolytic solution filled in a glass filter, agar or the like or an ion exchange membrane. A concrete example of the ion exchange membrane is as described above.

As illustrated inFIG. 8andFIG. 9, the third electrode41may have a first catalyst layer43, and the fourth electrode41may have a second catalyst layer44. Each of the catalyst layers43,44may be provided on both faces of each of the electrodes41,42as illustrated inFIG. 8andFIG. 9or one face thereof. When the photovoltaic cell201illustrated inFIG. 6is used, holes are separated to the first electrode11side and electrons are separated to the second electrode21side. Accordingly, the oxidation reaction is caused near the third electrode41, and the reduction reaction is caused near the fourth electrode42. A catalyst promoting the oxidation reaction is used for the first catalyst layer43, and a catalyst promoting the reduction reaction is used for the second catalyst layer44.

When a solution (aqueous solution) containing H2O is used as the first electrolytic solution51, the third electrode41oxidizes H2O to generate O2and H+. Therefore, the first catalyst layer43is made of a material which reduces activation energy for oxidizing H2O. The first catalyst layer43is made of a material which lowers an overvoltage when H2O is oxidized to generate O2and H+. Examples of such a material are binary 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), and ruthenium oxide (Ru—O), ternary metal oxides such as Ni—Co—O, Ni—Fe—O, La—Co—O, Ni—La—O, and Sr—Fe—O, quaternary metal oxides such as Pb—Ru—Ir—O and La—Sr—Co—O, or metal complexes such as a Ru complex and a Fe complex. A shape of the first catalyst layer43is not limited to a thin film shape, and may be an island, a lattice, a granular, or a wire.

When an aqueous solution containing CO2is used as the second electrolytic solution53, the fourth electrode42reduces CO2to generate a carbon compound (CO, HCOOH, CH4, CH3OH, C2H5OH, C2H4or the like). Therefore, the second catalyst layer44is made of a material which reduces activation energy for reducing CO2. The second catalyst layer44is made of a material which lowers an overvoltage when CO2is reduced to generate the carbon compound. Examples of such a material are metals such as Au, Ag, Cu, Pt, Pd, Ni, and Zn, an alloy containing at least one of these metals, carbon materials such as carbon (C), graphene, CNT (carbon nanotube), fullerene, and ketjen black, and metal complexes such as a Ru complex and a Re complex.

When a solution containing H2O is used as the second electrolytic solution53, H2is sometimes generated by reducing H2O. In this case, the second catalyst layer44is made of a material which reduces activation energy for reducing H2O. The second catalyst layer44is made of a material which lowers an overvoltage when H2O is reduced to generate H2. Examples of such a material are metals such as Ni, Fe, Pt, Ti, Au, Ag, Zn, Pd, Ga, Mn, and Cd, an alloy containing at least one of these metals, and carbon materials such as carbon (C), graphene, CNT (carbon nanotube), fullerene, and ketjen black. A shape of the second catalyst layer44is not limited to a thin film shape, and may be an island shape, a lattice shape, a granular shape, or a wire shape.

When the photovoltaic cell202illustrated inFIG. 7is used, electrons are separated to the first electrode11side, and holes are separated to the second electrode21side. Accordingly, an oxidation reaction is caused near the fourth electrode42, and a reduction reaction is caused near the third electrode41. The first catalyst layer43is made of a material which promotes the reduction reaction, and the second catalyst layer44is made of a material which promotes the oxidation reaction. In other words, the material of the first catalyst layer43and the material of the second catalyst layer44are counterchanged as compared with the case where the photovoltaic cell201illustrated inFIG. 6is used. Thus, the polarity of the photovoltaic layer31and the materials of the first catalyst layer43and the second catalyst layer44are arbitrary. The oxidation and reduction reactions by the first catalyst layer43and the second catalyst layer44are decided by the polarity of the photovoltaic layer31, and the materials are selected according to the oxidation and reduction reactions.

As a formation method of the first catalyst layer43and the second catalyst layer44, a thin-film forming method such as a sputtering method or a vapor deposition method, a coating method using a solution in which the catalyst material is dispersed, an electrodeposition method, a catalyst forming method by heat treatment or electrochemical treatment of the third electrode41or the fourth electrode42itself, or the like is usable. Only one of the first catalyst layer43and the second catalyst layer44may be formed. The catalyst layers43,44are arbitrarily formed and are formed according to desired oxidation and reduction reactions.

The electrolytic bath4includes the first storage part52storing the first electrolytic solution51and the second storage part54storing the second electrolytic solution53. The third electrode41is arranged in the first storage part52storing the first electrolytic solution51. The fourth electrode42is arranged in the second storage part54storing the second electrolytic solution53. Of the first and second electrolytic solutions51,53, one is a solution containing, for example, H2O and the other is a solution containing, for example, CO2. In place of the solution containing CO2, a solution containing H2O may be used. When the photovoltaic cell201illustrated inFIG. 6is employed, the solution containing H2O is used as the first electrolytic solution51, and the solution containing CO2is used as the second electrolytic solution53. When the photovoltaic cell202illustrated inFIG. 7is employed, the solution containing CO2is used as the first electrolytic solution51, and the solution containing H2O is used as the second electrolytic solution53.

As the solution containing H2O, an aqueous solution containing an arbitrary electrolyte is used. The solution is preferably an aqueous solution that promotes the oxidation reaction of H2O. Examples of the aqueous solution containing the electrolyte are aqueous solutions containing phosphoric acid ions (PO42−), boric acid 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−), carbonate ions (CO32−), and so on.

The solution containing CO2is preferably a solution having a high CO2absorptance. Examples of the solution containing H2O are LiHCO3, NaHCO3, KHCO3, and CsHCO3as aqueous solutions. As the solution containing CO2, alcohol such as methanol, ethanol, or acetone may be used. The solution containing H2O and the solution containing CO2may be the same solution. Since the solution containing CO2is preferably high in a CO2absorption amount, a different solution from the solution containing H2O may be used as the solution containing CO2. The solution containing CO2is desirably an electrolytic solution that reduces a reduction potential of CO2, has a high ion conductivity, and contains a CO2absorbent which absorbs CO2.

Examples of the aforesaid electrolytic solution are an ionic liquid which is made of salts of cations such as imidazolium ions or pyridinium ions and anions such as BF4−or PF6−and which is in a liquid state in a wide temperature range, or aqueous solutions thereof. Other examples of the electrolytic solution are amine solutions of ethanolamine, imidazole, or pyridine, or aqueous solutions thereof. Amine may be any of primary amine, secondary amine, and tertiary amine. Examples of the primary amine are methylamine, ethylamine, propylamine, butylamine, pentylamine, hexylamine, and the like. Hydrocarbons of the amine may be substituted by alcohol, halogen, or the like. Examples of the amine whose hydrocarbons are substituted are methanolamine, ethanolamine, chloromethyl amine, and so on. Further, an unsaturated bond may exist. These hydrocarbons are the same in the secondary amine and the tertiary amine. Examples of the secondary amine are dimethylamine, diethylamine, dipropylamine, dibutylamine, dipentylamine, dihexylamine, dimethanolamine, diethanolamine, dipropanolamine, and so on. The substituted hydrocarbons may be different. This also applies to the tertiary amine. Examples in which the hydrocarbons are different are methylethylamine, methylpropylamine, and so on. Examples of the tertiary amine are trimethylamine, triethylamine, tripropylamine, tributylamine, trihexylamine, trimethanolamine, triethanolamine, tripropanolamine, tributanolamine, triexanolamine, methyldiethylamine, methyldipropylamine, and so on. Examples of the cations of the ionic liquid are 1-ethyl-3-methylimidazolium ions, 1-methyl-3-propylimidazolium ions, 1-butyl-3-methylimidazole ions, 1-methyl-3-pentylimidazolium ions, 1-hexyl-3-methylimidazolium ions, and so on. A second place of imidazolium ions may be substituted. Examples in which the second place of the imidazolium ions is substituted are 1-ethyl-2,3-dimethylimidazolium ions, 1-2-dimethyl-3-propylimidazolium ions, 1-butyl-2,3-dimethylimidazolium ions, 1,2-dimethyl-3-pentylimidazolium ions, 1-hexyl-2,3-dimethylimidazolium ions, and so on. Examples of pyridinium ions are methylpyridinium, ethylpyridinium, propylpyridinium, butylpyridinium, pentylpyridinium, hexylpyridinium, and so on. In both of the imidazolium ions and the pyridinium ions, an alkyl group may be substituted, or an unsaturated bond may exist. Examples of the anions are fluoride ions, chloride ions, bromide ions, iodide ions, BF4−, PF6−, CF3COO−, CF3SO3−, NO3−, SCN−, (CF3SO2)3C−, bis(trifluoromethoxysulfonyl)imide, bis(perfluoroethylsulfonyl)imide, and so on. Dipolar ions in which the cations and the anions of the ionic liquid are coupled by hydrocarbons may be used.

Next, an operation principle of the photoelectrochemical reaction device1will be described with reference to an electrical connection diagram inFIG. 4. Here, the operation will be described, taking, as an example, the polarity when the photovoltaic cell (the photoelectrochemical cell using the silicon semiconductor-based solar cell as the photovoltaic layer)201illustrated inFIG. 6is used. A case where an absorbing liquid in which CO2is absorbed is used as the second electrolytic solution53in which the fourth electrode42is immersed will be described. Incidentally, when the photovoltaic cell (the photoelectrochemical cell using the compound semiconductor-based solar cell as the photovoltaic layer)202illustrated inFIG. 7is used, the polarity is reversed and therefore, the absorbing liquid in which CO2is absorbed is used as the first electrolytic solution51.

When the light is irradiated from above the first and second photovoltaic cell2A,2B, the irradiating light passes through the first electrode11to reach the photovoltaic layer31. When absorbing the light, the photovoltaic layer31generates electrons and holes which make pairs with the electrons, and separates them. Specifically, in the first photovoltaic layer33, the second photovoltaic layer34, and the third photovoltaic layer35which constitute the photovoltaic layer31, the electrons migrate to the n-type semiconductor layer side (second electrode21side) due to a built-in potential, and the holes generated as the pairs with the electrons migrate to the p-type semiconductor layer side (first electrode11side), to thereby cause charge separation. Such charge separation generates the electromotive force in the photovoltaic layer31.

The holes generated in the photovoltaic layer31in each of the first and second photovoltaic cells2A,2B migrate to the first electrode11. The holes combine with electrons which are generated by the oxidation reaction caused near the third electrode41, via the connecting member6A,6B and the third electrode41. The electrons which have migrated to the second electrode21are used in the reduction reaction caused near the fourth electrode42, via the connecting member6B,6D and the fourth electrode42. Concretely, near the third electrode41in contact with the first electrolytic solution51, a reaction of the following formula (1) occurs. Near the second electrode42in contact with the second electrolytic solution53, a reaction of the following formula (2) occurs.

Near the third electrode41, H2O contained in the first electrolytic solution51is oxidized (loses electrons), so that O2and H+are generated, as expressed by the formula (1). H+generated on the third electrode41side migrates to the fourth electrode42side via the ion migration layer55. Near the fourth electrode42, CO2contained in the second electrolytic solution53is reduced (obtains electrons) as expressed by the formula (2). Concretely, CO2contained in the second electrolytic solution53, H+which has migrated to the fourth electrode42from the third electrode41, and the electrons which have migrated to the fourth electrode42react with one another, so that CO and H2O are generated, for instance.

In this event, the photovoltaic layer31needs to have an open-circuit voltage equal to or larger than a potential difference between a standard oxidation-reduction potential of the oxidation reaction occurring near the third electrode41and a standard oxidation-reduction potential of the reduction reaction occurring near the fourth electrode42. For example, the standard oxidation-reduction potential of the oxidation reaction in the formula (1) is 1.23 V, and the standard oxidation-reduction potential of the reduction reaction in the formula (2) is −0.1 V. Therefore, the open-circuit voltage of the photovoltaic layer31needs to be 1.33 V or more. The open-circuit voltage of the photovoltaic layer31is preferably equal to or more than the potential difference inclusive of overvoltages. Concretely, when 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 desirably 1.73 V or more.

Near the fourth electrode42, it is possible to cause not only the reduction reaction from CO2to CO expressed by the formula (2) but also a reduction reaction from CO2to formic acid (HCOOH), methane (CH4), ethylene (C2H4), methanol (CH3OH), ethanol (C2H5OH), acetic acid (CH3COOH) or the like. It is also possible to cause the reduction reaction of H2O used in the second electrolytic solution53to generate H2. By varying an amount of moisture (H2O) in the second electrolytic solution53, it is possible to change a generated reduced substance of CO2. For example, it is possible to change a generation ratio of CO, HCCOH, CH4, C2H4, CH3OH, C2H5OH, CH3COOH, H2, and the like which are generated by the reduction reaction of CO2.

When generating H2near the fourth electrode42, the reaction of the formula (1) occurs near the third electrode41, and the reaction of the following formula (3) occurs near the second electrode42.

Near the third electrode41, H2O contained in the first electrolytic solution51is oxidized (loses electrons), so that O2and H+are generated. H+generated on the third electrode41side migrates to the fourth electrode42side via the ion migration layer55. Near the fourth electrode42, H2is reduced (obtains electrons) to generate a H2gas as expressed by the formula (3).

In the photoelectrochemical reaction device1of the first embodiment, electrical connection of the photovoltaic cell2and the reaction electrode pair3as a set prevents a failure, for example, even when occurring in the first photovoltaic cell2A, from adversely affecting the combination of the second photovoltaic cell2B and the second reaction electrode pair3B. Accordingly, the conversion efficiency from light energy to chemical energy by the second photovoltaic cell2B and the second reaction electrode pair3B can be maintained. As a concrete example of the conversion efficiency from light energy to chemical energy, currents flowing through the first reaction electrode pair3A and the second reaction electrode pair3B are listed in Table 1. Table 1 lists the currents flowing through the reaction electrode pairs3A,3B regarding the case where the first and second photovoltaic cells2A,2B normally operate (case 1), the case where the first photovoltaic cell2A does not generate power (case 2), and the case that the first photovoltaic cell2A does not generate power and leakage occurs (case 3).

Since the amount of products by the above-described oxidation and reduction reactions, and the conversion efficiency from sunlight to chemical energy are proportional to the current flowing through the reaction electrode pair3, a larger current flowing through the reaction electrode pair3is more preferable. As listed in Table 1, the currents flowing through the first and second reaction electrode pairs3A,3B are at the same level. As listed as the case 2, even in the case where the first photovoltaic cell2A does not generate power due to cloud or the like, the current flowing through the second reaction electrode pair3B does not change and is thus not adversely affected by the failure of the first photovoltaic cell2A. Further, as listed as the case 3, even in the case where leakages occurs in the first photovoltaic cell2A, the current through the second reaction electrode pair3B does not change and is thus not adversely affected by the failure of the first photovoltaic cell2A.

Table 2 lists, as comparative examples of the photoelectrochemical reaction device in the embodiment, currents flowing through the reaction electrode pairs in the case 1, the case 2, and the case 3, as in Table 1, regarding a photoelectrochemical reaction device in which an oxidation reaction electrode pair composed of the third electrode and the fourth electrode is not provided for every photovoltaic cell but two photovoltaic cells connected in parallel are connected to one reaction electrode pair. As indicated in the case 1 in Table 2, when the two photovoltaic cells normally operate, the current flowing through the reaction electrode pair is the same as that in the case 1 of the embodiment. In contrast, as indicated in the case 2 and the case 3 in Table 2, when a failure occurs in one photovoltaic cell, the current flowing through the reaction electrode pair decreases as compared with that of the embodiment even though the other photovoltaic cell normally operates. In particular, when leakage occurs in one photovoltaic cell, the current flowing through the reaction electrode pair greatly decreases.

TABLE 2CURRENTTHROUGHREACTIONELECTRODEPAIR [mA/cm2](CASE 1) WHERE FIRST AND SECOND6.72PHOTOVOLATIC CELLS NORMALLYOPERATE(CASE 2) WHERE FIRST PHOTOVOLATIC3.10CELL DOES NOT GENERATE POWER(CASE 3) WHERE FIRST PHOTOVOLATIC0.67CELL DOES NOT GENERATE POWER ANDLEAKAGE OCCURS

As described above, in the case where a plurality of photovoltaic cells are provided but the reaction electrode pair is not provided for each of the photovoltaic cells, a failed photovoltaic cell affects the other photovoltaic cell, even normally operating, and therefore decreases the current which contribute to oxidation and reduction reactions. Regarding this point, in the photoelectrochemical reaction device1in the embodiment, even if a failure occurs in the photovoltaic cell (2A) being a part thereof, its effect is limited only to the operation of the reaction electrode pair (3A) connected to the failed photovoltaic cell (2A) but not to the operations of the other photovoltaic cell (2B) and the reaction electrode pair (3B). Accordingly, an excellent conversion efficiency from light energy to chemical energy can be maintained.

Second Embodiment

A photoelectrochemical reaction device according to a second embodiment will be described with reference toFIG. 10toFIG. 12.FIG. 10is a plan view illustrating the photoelectrochemical reaction device of the second embodiment.FIG. 11is a view illustrating an electrical connection state of a plurality of photovoltaic cells in a photovoltaic module of the photoelectrochemical reaction device illustrated inFIG. 10.FIG. 12is a view illustrating an electrical connection state between the plurality of photovoltaic modules in the photoelectrochemical reaction device illustrated inFIG. 10and reaction electrode pairs. Note that the same parts as those of the photoelectrochemical reaction device of the first embodiment will be denoted by the same reference signs, and a description of part thereof will be sometimes omitted.

A photoelectrochemical reaction device1X illustrated inFIG. 10includes a first photovoltaic module7A, a second photovoltaic module7B, a first reaction electrode pair3A, a second reaction electrode pair3B, and an electrolytic bath4. Each of the first and second photovoltaic modules7A,7B has a plurality of photovoltaic cells2. In the photoelectrochemical reaction device1X of the second embodiment, in place of the combinations of the photovoltaic cells2and the reaction electrode pairs3of the first embodiment, the first photovoltaic module7A having the plurality of photovoltaic cells2and the first reaction electrode pair3A are combined and are electrically connected, and the second photovoltaic module7B having the plurality of photovoltaic cells2and the second reaction electrode pair3B are combined and are electrically connected. The other configurations are the same as those in the first embodiment.

Each of the first and second photovoltaic modules7A,7B has six photovoltaic cells2A to2F. In the six photovoltaic cells2A to2F, first electrodes11are connected to be three in series and two in parallel, and second electrodes21are also connected to be three in series and two in parallel. As illustrated inFIG. 12, the first electrodes11connected to be three in series and two in parallel of the first photovoltaic module7A are electrically connected to the third electrode41of the first reaction electrode pair3A. The second electrodes21connected to be three in series and two in parallel are electrically connected to the fourth electrode42of the first reaction electrode pair3A. Similarly, the first electrodes11connected to be three in series and two in parallel of the second photovoltaic module7B are electrically connected to the third electrode41of the second reaction electrode pair3B. The second electrodes21connected to be three in series and two in parallel are electrically connected to the fourth electrode42of the second reaction electrode pair3B.

As described above, also in the case where the plurality of photovoltaic modules7A,7B are applied, electrical connection of the photovoltaic module7and the reaction electrode pair3as one set prevents a failed photovoltaic module7from adversely affecting the other photovoltaic module7. Even if a failure occurs in the photovoltaic module (7A) being a part, its effect is limited only to the operation of the reaction electrode pair (3A) connected to the failed photovoltaic module (7A) but not to the operations of the photovoltaic module (7B) and the reaction electrode pair (3B). Accordingly, an excellent conversion efficiency from light energy to chemical energy can be maintained.

Third Embodiment

A photoelectrochemical reaction device according to a third embodiment will be described with reference toFIG. 13toFIG. 15.FIG. 13is a plan view illustrating the photoelectrochemical reaction device of the third embodiment,FIG. 14is a cross-sectional view taken along a line A-A inFIG. 13, andFIG. 15is a view illustrating an electrical connection state ofFIG. 13. Note that the same parts as those of the photoelectrochemical reaction device of the first embodiment will be denoted by the same reference signs, and a description of part thereof will be sometimes omitted. A photoelectrochemical reaction device1Y of the third embodiment includes a first photovoltaic cell2A, a second photovoltaic cell2B, a reaction electrode pair3, and an electrolytic bath4.

The reaction electrode pair3includes a third electrode41as a common electrode and two fourth electrodes42A,42B as individual electrodes. In a first storage part52of the electrolytic bath4, the third electrode41common to the first and second photovoltaic cells,2A,2B is arranged. In a second storage part54of the electrolytic bath4, the fourth electrode42A corresponding to the first photovoltaic cell2A and the fourth electrode42B corresponding to the second photovoltaic cell2B are arranged. The reaction electrode pair3includes the third electrode41common to the first and second photovoltaic cells2A,2B, and the fourth electrode42A and the fourth electrode42B individually corresponding to the first and second photovoltaic cells2A,2B. The other configurations are the same as those in the first embodiment.

As illustrated inFIG. 15, a first electrode11of the first photovoltaic cell2A and a first electrode11of the second photovoltaic cell2B are electrically connected to the third electrode41of the reaction electrode pair3via a connecting member6A. A second electrode21of the first photovoltaic cell2A is electrically to the fourth electrode42A of the reaction electrode pair3via a connecting member6C. Similarly, a second electrode21of the second photovoltaic cell2B is electrically connected to the fourth electrode42B of the reaction electrode pair3via a connecting member6D. Here, the third electrode41of the reaction electrode pair3is the common electrode and the fourth electrode42is the individual electrode, but the common electrode and the individual electrode may be reversed. It is only necessary that one of electrodes of the reaction electrode pair3is the common electrode and the other of the electrodes is the individual electrode. The number of combinations of the photovoltaic cell2and the individual electrode is not limited to two but may be three or more.

As described above, also in the case where the electrode42that is one of electrodes of the reaction electrode pair3is an individual electrode, electrically connecting the individual electrode42and the photovoltaic cell2as a set makes it possible to decrease the effect of a failed photovoltaic cell2on the other photovoltaic cell2. Table 3 lists, as in Table 1, currents flowing through the reaction electrode pair in the case 1, the case 2, and the case 3, regarding the third embodiment. It is found that the current flowing through the reaction electrode pair in the third embodiment is larger than that in the comparative example, in any of the case 2 and the case 3. Accordingly, an excellent conversion efficiency from light energy to chemical energy can be maintained. Note that in the case where a plurality of third electrodes41and fourth electrodes42are provided as illustrated inFIG. 16, when only either of the third electrodes41and fourth electrodes42are electrically connected in parallel, the same effect as that in the third embodiment can be obtained.

TABLE 3CURRENTTHROUGHREACTIONELECTRODEPAIR [mA/cm2](CASE 1) WHERE FIRST AND SECOND6.72PHOTOVOLATIC CELLS NORMALLYOPERATE(CASE 2) WHERE FIRST PHOTOVOLATIC3.83CELL DOES NOT GENERATE POWER(CASE 3) WHERE FIRST PHOTOVOLATIC3.44CELL DOES NOT GENERATE POWER ANDLEAKAGE OCCURS

Fourth Embodiment

A photoelectrochemical reaction device according to a fourth embodiment will be described with reference toFIG. 17toFIG. 20.FIG. 17is a top perspective view illustrating the photoelectrochemical reaction device of the fourth embodiment,FIG. 18is a cross-sectional view taken along a line A-A inFIG. 17,FIG. 19is a cross-sectional view taken along a line B-B inFIG. 17, andFIG. 20is a view illustrating an electrical connection state ofFIG. 17. Note that the same parts as those of the photoelectrochemical reaction device of the above-described embodiments will be denoted by the same reference signs, and a description of part thereof will be sometimes omitted.

A photoelectrochemical reaction device100according to the fourth embodiment includes a photovoltaic cell2, a reaction electrode101, and an electrolytic bath4. The photovoltaic cell2includes two divided first electrodes11A,11B, one second electrode21, a first photovoltaic layer31A which is provided between one first electrode11A and the second electrode21, and a second photovoltaic layer31B which is provided between the other first electrode11B and the second electrode21. Note that concrete configurations and so on of the first electrode11, the photovoltaic layer31, the second electrode21, the electrolytic bath4including the electrolytic solutions51,53, and the reaction electrode101corresponding to the fourth electrode are the same as those in the first embodiment, and their description will be omitted here.

The photovoltaic cell2of the fourth embodiment includes the second electrode21serving as a common electrode, a first stack unit102A having the photovoltaic layer31A and the first electrode11A stacked in order on the second electrode21, and a second stack unit102B having the photovoltaic layer31B and the first electrode11B similarly stacked in order on the second electrode21. The photovoltaic cell2is arranged in the electrolytic bath4. The electrolytic bath4includes a first storage part52storing the first electrolytic solution51in which the photovoltaic cell2is immersed, a second storage part54storing the second electrolytic solution53in which the reaction electrode (corresponding to the fourth electrode)101is immersed, and an ion migration layer (an ion migration layer also serving as a separation wall)55allowing ions to migrate while separating the first electrolytic solution51and the second electrolytic solution53. The concrete configuration of the ion migration layer55is as described above.

As illustrated inFIG. 20, the second electrode21of the photovoltaic cell2is electrically connected to the reaction electrode101immersed in the second electrolytic solution53via a connecting member6. The second electrode21does not contribute to oxidation and reduction reactions and therefore may be coated with an insulating member. The second electrode21serves as the common electrode with respect to the first stack unit102A and the second stack unit102B and is therefore equivalent to being connected in parallel. The first stack unit102A and the second stack unit102B are geometrically separated. The thicknesses of the first electrode11and the photovoltaic layer31are as thin as about 1 micrometer to 10 micrometer, and the solution resistance between the first stack unit102A and the second stack unit102B is high. Accordingly, the first stack unit102A and the second stack unit102B are equivalent to being electrically insulated. The first electrode11A of the first stack unit102A and the first electrode11B of the second stack unit102B are equivalent to being not electrically connected. The number of the stack units102having the photovoltaic layer31and the first electrode11is not limited to two but may be three or more.

In the photoelectrochemical reaction device100of the fourth embodiment, one of the first electrode11A,11B and the reaction electrode101causes an oxidation reaction and the other of the first electrode11A,11B and the reaction electrode101causes a reduction reaction. As in the first embodiment, the first electrodes11A,11B and the reaction electrode101may have a catalyst layer which promotes the oxidation reaction or the reduction reaction. When light is irradiated to the photovoltaic cell2, H2O is oxidized so that O2and H+are generated (the formula (1)), for example, near the first electrodes11A,11B in contact with the first electrolytic solution51. H+generated on the first electrodes11A,11B side migrates to the reaction electrode101side via the ion migration layer55. Near the reaction electrode101in contact with the second electrolytic solution53, for example, CO2is reduced so that CO and H2O are generated (the formula (2)).

To enhance the insulating property between the first stack unit102A and the second stack unit102B, an insulating member103may be arranged between them as illustrated inFIG. 21. The insulating member103may be provided to cover the peripheries of the first stack unit102A and the second stack unit102B. To cause H+ions and the like generated near the first electrodes11A,11B to rapidly migrate to the reaction electrode101side, the photovoltaic cell2may have an ion migration unit104such as fine pores or slits as illustrated inFIG. 22andFIG. 23. The fine pores or slits only need to have a size enabling the ions to migrate. The concrete size is as described above. A shape of the fine pores is not limited to a circle and may be an ellipse, a triangle, a square, or the like. The fine pores are arranged in a square lattice form, a triangular lattice form, a random form, or the like. The fine pores or slits may be filled with an ion exchange membrane. The fine pores or slits may be filled with a glass filter or agar.

The photoelectrochemical reaction device100of the fourth embodiment can be recognized as including a first photovoltaic cell based on the first stack unit102A and a second photovoltaic cell based on the second stack unit102B, because the second electrode21serves as a common electrode. Additionally, the first electrode11A of the first stack unit102A and the first electrode11B of the second stack unit102B are electrically insulated. Accordingly, as in the third embodiment, a failure, even when occurring in one photovoltaic cell (stack102), never adversely affects the combination of the other photovoltaic cell (stack102) and the reaction electrode101. The conversion efficiency from light energy to chemical energy by the photoelectrochemical reaction device100can be maintained.

Fifth Embodiment

A photoelectrochemical reaction device according to a fifth embodiment will be described with reference toFIG. 24toFIG. 26.FIG. 24is a top perspective view illustrating the photoelectrochemical reaction device of the fifth embodiment,FIG. 25is a cross-sectional view taken along a line A-A inFIG. 24, andFIG. 26is a cross-sectional view taken along a line B-B inFIG. 24. Note that the same parts as those of the photoelectrochemical reaction devices of the above-described embodiments will be denoted by the same reference signs, and a description of part thereof will be sometimes omitted. A photoelectrochemical reaction device110of the fifth embodiment includes a photovoltaic cell2and an electrolytic bath4. The photovoltaic cell2includes two divided first electrodes11A,11B, one second electrode (common electrode)21, a first photovoltaic layer31A provided between one first electrode11A and the second electrode21, and a second photovoltaic layer31B provided between the other first electrode11B and the second electrode21.

The photovoltaic cell2of the fifth embodiment includes, as in the fourth embodiment, a first stack unit102A having the photovoltaic layer31A and the first electrode11A stacked in order on the second electrode21, and a second stack unit102B having the photovoltaic layer31B and the first electrode11B stacked in order on the second electrode21. The photovoltaic cell2is arranged in the electrolytic bath4. The electrolytic bath4includes a first storage part52storing a first electrolytic solution51, a second storage part54storing a second electrolytic solution53, and an ion migration layer (an ion migration layer also serving as a separation wall)55allowing ions to migrate while separating the first electrolytic solution51and the second electrolytic solution53.

The photovoltaic cell2is arranged in the first storage part52of the electrolytic bath4so that the second electrode21is located on the ion migration layer55. The ion migration layer55has an opening55afor exposing the rear surface of the second electrode21. The photovoltaic cell2is arranged in the first storage part52, so that the first electrode11A of the first stack unit102A and the first electrode11B of the second stack unit102B are in contact with the first electrolytic solution51. The second electrode21is in contact with the second electrolytic solution53via the opening55aprovided in the ion migration layer55.

The second electrode21serves as the common electrode with respect to the first stack unit102A and the second stack unit102B and is thus equivalent to being connected in parallel. The first stack unit102A and the second stack unit102B are geometrically separated. The thicknesses of the first electrode11and the photovoltaic layer31are as thin as about 1 micrometer to 10 micrometer, and the solution resistance between the first stack unit102A and the second stack unit102B is high. Accordingly, the first stack unit102A and the second stack unit102B are equivalent to being electrically insulated. The number of the stack units102having the photovoltaic layer31and the first electrode11is not limited to two but may be three or more.

In the photoelectrochemical reaction device100of the fifth embodiment, one of the first electrode11A,11B and the second electrode21causes an oxidation reaction and the other of the first electrode11A,11B and the second electrode21causes a reduction reaction. As in the first embodiment, the first electrodes11A,11B and the second electrode21may have a catalyst layer which promotes the oxidation reaction or the reduction reaction. When light is irradiated to the photovoltaic cell2, H2O is oxidized so that O2and H+are generated, for example, near the first electrodes11A,11B in contact with the first electrolytic solution51as in the fourth embodiment. H+generated on the first electrodes11A,11B side migrates to the second electrode21side via the ion migration layer55or a later-described ion migration unit104. Near the second electrode21in contact with the second electrolytic solution53, CO2is reduced so that CO and H2O are generated.

To enhance the insulating property between the first stack unit102A and the second stack unit102B, an insulating member103may be arranged between them as illustrated inFIG. 27. The insulating member103may be provided to cover the peripheries of the first stack unit102A and the second stack unit102B. To cause H+ions generated near the first electrodes11A,11B to rapidly migrate to the second electrode21side, the ion migration unit104such as fine pores or slits may be provided at a portion of the second electrode21located between the first stack unit102A and the second stack unit102B. The ion migration unit104may be provided to penetrate the first stack unit102A and the second stack unit102B. The fine pores or slits only need to have a size enabling the ions to migrate. The concrete size and shape are as described above. The fine pores or slits may be filled with an ion exchange membrane, or may be filled with a glass filter or agar.

The photoelectrochemical reaction device110of the fifth embodiment can be recognized as including a first photovoltaic cell based on the first stack unit102A and a second photovoltaic cell based on the second stack unit102B, because the second electrode21serves as a common electrode. Additionally, the first electrode11A of the first stack unit102A and the first electrode11B of the second stack unit102B are electrically insulated. As in the fourth embodiment, a failure, even when occurring in one photovoltaic cell (stack102), never adversely affects the combination of the other photovoltaic cell (stack102) and the second electrode21. Accordingly, the conversion efficiency from light energy to chemical energy by the photoelectrochemical reaction device110can be maintained.

Note that the configurations of the first to fifth embodiments can be applied in combination. Further, parts thereof can be substituted. 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 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.