JP6610442B2 - Artificial photosynthesis device - Google Patents

Description

  The present invention relates to an artificial photosynthesis apparatus capable of artificially performing photosynthesis.

  2. Description of the Related Art Conventionally, artificial photosynthesis systems that artificially perform photosynthesis that generates oxygen from water and carbon dioxide (such as methanol) together with hydrogen and carbon from water and carbon dioxide have been studied.

  For example, Patent Document 1 discloses that carbon dioxide is reduced to formic acid using a metal complex that exhibits a reducing action of a semiconductor and carbon dioxide. Patent Document 2 discloses that a carbon compound or hydrogen is generated using a SiC / Si based semiconductor. Non-Patent Document 1 discloses generating hydrogen using a GaAs-based semiconductor.

JP 2011-94194 A JP 2016-34611 A

Turner et al., Science 17 April 1998 vol.280, Issue 5362, pp.425-427

  However, the above-described conventional artificial photosynthesis has a problem that the production efficiency of reaction products such as hydrogen and carbon compounds is low.

  In view of the above points, an object of the present invention is to improve the production efficiency of reaction products in an artificial photosynthesis apparatus.

In order to achieve the above object, in the invention described in claim 1, a semiconductor layer (12 a) that generates an internal photoelectric effect when irradiated with light, and a reduction electrode that generates a carbon compound by a reduction reaction of carbon dioxide ( 12b) and a semiconductor photocatalyst (12c) formed by integrating an oxidation electrode (12c) that generates oxygen and hydrogen ions by an oxidation reaction of water, and collects light irradiated to the semiconductor photocatalyst at a predetermined magnification. The semiconductor layer generates heat when irradiated with light, and heat generated in the semiconductor layer is transmitted to the reduction electrode and the oxidation electrode to adjust the light collection by the condenser lens. Thus, the temperature of the semiconductor layer can be controlled .

According to the present invention, a semiconductor photocatalyst in which a semiconductor layer, a reduction electrode, and an oxidation electrode are integrated is condensed by a condensing lens, so that the temperature of the electrode is increased by thermal energy generated in the semiconductor layer by light irradiation It is possible to increase the reaction rate of the electrode reaction. As a result, the thermal energy generated in the semiconductor layer can be actively used for the electrode reaction on the electrode surface.
In the invention according to claim 4, a semiconductor layer (12a) that generates an internal photoelectric effect when irradiated with light, a reduction electrode (12b) that generates a carbon compound by a reduction reaction of carbon dioxide, water, A semiconductor photocatalyst (12) configured integrally with an oxidation electrode (12c) that generates oxygen and hydrogen ions by an oxidation reaction of the above, and a condensing lens that condenses light irradiated on the semiconductor photocatalyst at a predetermined magnification ( 14) and a container (10) containing an electrolytic solution (11) dipped in a semiconductor photocatalyst, the semiconductor layer generates heat when irradiated with light, and the heat generated in the semiconductor layer is reduced and oxidized. The reduction electrode is formed on one side of the semiconductor layer, the oxidation electrode is formed on the other side of the semiconductor layer, and the reduction electrode is provided so as to extend from the surface of the one side of the semiconductor layer, Semiconductor layer The light is irradiated from one side, a portion is the reduction electrode not provided on the surface of the one side of the semiconductor layer is equal to or exposed to the atmosphere.

  In addition, the code | symbol in the bracket | parenthesis of each said means shows the correspondence with the specific means as described in embodiment mentioned later.

It is explanatory drawing which shows the whole structure of the artificial photosynthesis apparatus of 1st Embodiment. It is explanatory drawing which shows the energy band of a semiconductor layer. It is a graph which shows the conversion efficiency of the semiconductor photocatalyst at the time of temperature change, the reaction rate of an electrode reaction, and the production | generation efficiency of a reaction product. It is a graph which shows the conversion efficiency of the semiconductor photocatalyst at the time of temperature change, the reaction rate of an electrode reaction, and the production | generation efficiency of a reaction product. It is explanatory drawing which shows the whole structure of the artificial photosynthesis apparatus of 2nd Embodiment.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. In the following embodiments, parts that are the same or equivalent to each other will be described with the same reference numerals.

(First embodiment)
An artificial photosynthesis device 1 according to a first embodiment of the present invention will be described. The artificial photosynthesis apparatus 1 of the present embodiment is used to perform oxygen generation, hydrogen generation, and carbon compound (methanol etc.) generation by irradiating sunlight.

  As shown in FIG. 1, the artificial photosynthesis device 1 includes a container 10. The container 10 contains an electrolyte solution 11. The electrolytic solution 11 is not particularly limited, but in this embodiment, an aqueous potassium hydrogen carbonate solution is used. The concentration of the aqueous potassium hydrogen carbonate solution is 0.1 mol / L.

  The container 10 is provided with a semiconductor photocatalyst 12. The semiconductor photocatalyst 12 is immersed in the electrolytic solution 11. The semiconductor photocatalyst 12 has the same configuration as that of a solar cell, and includes a semiconductor layer 12a, a reduction electrode 12b, and an oxidation electrode 12c.

Semiconductor layer reduction electrode 12b on one side on the surface of 12a is formed, the oxidation electrode 12 a is formed on the other side surface of the semiconductor layer 12a. In FIG. 1, a reduction electrode 12b is formed on the upper side of the semiconductor layer 12a, and an oxidation electrode 12c is formed on the lower side of the semiconductor layer 12a. That is, the semiconductor photocatalyst 12 is configured by integrating the semiconductor layer 12a, the reduction electrode 12b, and the oxidation electrode 12c.

In the semiconductor photocatalyst 12, when sunlight is irradiated, electron / hole pairs are generated by the internal photoelectric effect (photovoltaic effect). As a result, electrons move to the reduction electrode 12b, holes move to the oxidation electrode 12c, and a potential difference is generated between the electrodes 12b and 12c.

  In the present embodiment, a III-V semiconductor that is a direct transition semiconductor is used as the semiconductor layer 12a. The III-V semiconductor of this embodiment has a two-junction or three-junction tandem structure and is a tunnel junction. Examples of such III-V semiconductors include GaInP / GaAs / Ge, GaInP / GaAs / GaNAs, GaInP / GaAs, AlGaInP / GaAs / Ge, AlGaInP / GaAs / GaNAs, and AlGaInP / GaAs. A PN junction is formed, and the substrate is a Ge substrate or a GaAs substrate. Two PN junctions are two junctions and three PN junctions are three junctions. Further, a Si semiconductor may be used as the substrate of the III-V group semiconductor.

  The reduction electrode 12b is formed on the surface of the semiconductor layer 12a. By forming a transparent electrode such as ITO or FTO on the surface of the semiconductor layer 12a, the reduction electrode 12b can be formed. On the surface of the reduction electrode 12b, the following reduction reaction of carbon dioxide is performed, and a carbon compound (such as methanol) and hydrogen are generated.

CO ₂ + 6H ⁺ + 6e ⁻ → CH ₃ OH + H ₂ O
2H ^{+ +} 2e ^- → H ₂
The oxidation electrode 12c is formed on the surface of the semiconductor layer 12a opposite to the reduction electrode 12b. An oxidation electrode 12c can be formed by forming an IrO ₂ electrode or a RuO ₂ electrode on the surface of the semiconductor layer 12a. On the surface of the oxidation electrode 12c, the following water oxidation reaction is performed, and hydrogen ions and oxygen are generated.

2H ₂ O + 4h ⁺ → 4H ⁺ + O ₂
The hydrogen ions generated on the oxidation electrode 12c side move to the reduction electrode 12b side and are used for the carbon dioxide reduction reaction described above.

  The container 10 is provided with an electrolyte membrane 13. The electrolyte membrane 13 is disposed between the reduction electrode 12b and the oxidation electrode 12c. The electrolyte membrane 13 separates the electrolyte solution 11 on the reduction electrode 12b side and the oxidation electrode 12c side while allowing movement of hydrogen ions. For example, Nafion (registered trademark of DuPont) can be used as the electrolyte membrane 13.

  A condenser lens 14 is provided outside the container 10. The condensing lens 14 has a function of concentrating sunlight on the reduction electrode 12b. As the condenser lens 14, for example, a spherical lens or a Fresnel lens can be used.

  By condensing sunlight with the condensing lens 14, the conversion efficiency from the light energy to the electrical energy in the semiconductor layer 12a can be improved. In particular, the group III-V semiconductor used as the semiconductor layer 12a in the present embodiment has a large effect of improving the conversion efficiency by light collection.

  In the present embodiment, the magnification of the condenser lens 14 is 5 to 1000 times. The magnification of the condenser lens 14 can be adjusted by the refractive index of the condenser lens 14 or the like. As will be described later, the semiconductor layer 12a generates heat by absorbing sunlight. By changing the magnification of the condenser lens 14, the temperature of the semiconductor layer 12a can be controlled. Specifically, when the magnification of the condenser lens 14 is increased, the temperature of the semiconductor layer 12a is increased, and when the magnification of the condenser lens 14 is decreased, the temperature of the semiconductor layer 12a is decreased.

A CO ₂ supply pipe 15 is inserted into the container 10. Carbon dioxide is supplied from the CO ₂ supply pipe 15 to the reduction electrode 12 b side in the electrolyte solution 11.

  FIG. 2 shows the potential measured with reference to a standard hydrogen electrode, the oxidation potential of water, the reduction potential of carbon dioxide, and the energy band diagram of the semiconductor layer 12a. As shown in FIG. 2, in this embodiment, a two-junction composed of a GaInP layer and a GaAs layer is employed as the semiconductor layer 12a. The GaInP layer and the GaAs layer are heterojunction. Sunlight enters from the GaInP layer side of the semiconductor layer 12a.

  As shown in FIG. 2, the lower end potential of the conduction band on the GaInP side is more negative than the reduction potential of carbon dioxide, and the upper end potential of the valence band on the GaAs side is more positive than the oxidation potentials of oxygen and water. I have to. Specifically, the lower end potential of the conduction band on the GaInP layer side and the upper end potential of the valence band on the GaAs layer side are set by adjusting the conductivity type and impurity concentration of the GaAs layer and GaInP layer.

  Furthermore, in order to continuously connect electrons / holes excited on the GaAs side and electrons / holes excited on the GaInP side, the energy level difference between the conduction band of the GaAs layer and the valence band of the GaInP layer is small. In addition, the vicinity of the heterojunction in the GaAs side N-type region and the vicinity of the heterojunction in the GaInP side P-type region are made to be highly doped semiconductors. As described above, the tunneling phenomenon is caused by reducing the energy level difference between the lower end of the conduction band of the GaAs layer and the upper end of the valence band of the GaInP layer and making the transition region of the heterojunction thin and high in concentration. A tunnel current flows.

  Further, the band is bent by the PN junction so that the electrons of the electron / hole pair generated in the GaAs layer move to the heterojunction side and the holes move to the surface side opposite to the heterojunction. That is, the band on the conduction band side of the GaAs layer is bent so as to incline downward toward the heterojunction side, and the band on the valence band side is also bent so as to incline downward toward the heterojunction side. It is done.

  Similarly, the band is bent by the PN junction so that the electrons of the electron / hole pair generated in the GaInP layer move to the back side opposite to the heterojunction, and the holes move to the heterojunction side. That is, the band on the conduction band side of the GaInP layer is bent so as to incline upward toward the heterojunction side, and the band on the valence band side is also bent so as to incline upward toward the heterojunction side.

  In this way, electrons are more likely to move to the surface side of the GaInP layer on the conduction band side, and holes are more likely to move to the surface side of the GaAs layer on the valence band side. The hole can be kept away. Accordingly, carriers can be separated and recombination can be suppressed. Further, electrons on the GaAs side and holes on the GaInP side gather at the heterojunction, and tunneling due to recombination is likely to occur.

  As shown in FIG. 2, in the GaInP layer and the GaAs layer constituting the semiconductor layer 12a, electrons are excited by the internal photoelectric effect when irradiated with sunlight, but a part of the incident light energy is released as heat. Is done. For this reason, the semiconductor layer 12a generates heat when irradiated with sunlight.

  In this embodiment, since the semiconductor layer 12a, the reduction electrode 12b, and the oxidation electrode 12c are integrated, the heat generated in the semiconductor layer 12a is directly transmitted to the reduction electrode 12b and the oxidation electrode 12c. Thereby, the temperature of the semiconductor layer 12a, the reduction electrode 12b, and the oxidation electrode 12c rises.

  3 and 4 show the conversion efficiency of the semiconductor layer 12a from sunlight to electricity and the reaction rate ratio of the electrode reaction on the surfaces of the electrodes 12b and 12c. 3 and 4, the conversion efficiency from sunlight to electricity at 20 ° C. is 1, and the reaction rate ratio at 100 ° C. is 1. FIG. 3 shows a case where the activation energy is 10 kJ / mol, and FIG. 4 shows a case where the activation energy is 30 kJ / mol. The activation energy is energy necessary for the electrode reaction described above to proceed at the reduction electrode 12b and the oxidation electrode 12c.

  As shown in FIGS. 3 and 4, the conversion efficiency from sunlight to electricity of the semiconductor layer 12a decreases as the temperature rises. On the other hand, the reaction rate of the electrode reaction increases with increasing temperature.

  The production efficiency of the reaction product in the electrode reaction can be obtained by multiplying the conversion efficiency of the semiconductor layer 12a from sunlight into electricity and the reaction rate ratio of the electrode reaction. As shown in FIG. 3, when the activation energy is 10 kJ / mol, the generation efficiency of the reaction product is maximized around 80 ° C. As shown in FIG. 4, when the activation energy is 30 kJ / mol, the production efficiency of the reaction product is maximized around 100 ° C. At a temperature exceeding 100 ° C., the temperature of the semiconductor photocatalyst 12 is desirably 100 ° C. or less because the electrodes 12b and 12c are deteriorated and the electrolytic solution 11 is boiled.

  In the present embodiment, the temperature of the semiconductor photocatalyst 12 is controlled aiming at a temperature at which the generation efficiency of the reaction product is maximized. Thereby, the thermal energy which generate | occur | produces in the semiconductor photocatalyst 12 by condensing sunlight can be positively utilized for the electrode reaction which advances on the surface of the electrodes 12b and 12c.

  By adjusting the condensing by the condensing lens 14, the temperature of the semiconductor layer 12a can be controlled. Specifically, by adjusting the magnification, focus, position, or direction of the condensing lens 14, the condensing by the condensing lens 14 can be adjusted, and the temperature of the semiconductor layer 12a can be controlled. For example, the magnification of the condenser lens 14 may be set in advance so that the semiconductor layer 12a has a desired temperature.

  With the configuration as described above, the artificial photosynthesis device 1 of the present embodiment is configured. Next, operation | movement of the artificial photosynthesis apparatus 1 of this embodiment is demonstrated.

As described above, the semiconductor photocatalyst 12 is used in a state of being immersed in the electrolytic solution 11 accommodated in the container 10 in the artificial photosynthesis apparatus 1, and artificial photosynthesis is performed by irradiating sunlight from the reduction electrode 12b side. Done.

  When light energy is absorbed by the semiconductor layer 12a by the irradiation of sunlight, the electron / hole pair is excited by the absorbed light on the GaAs layer side, and the bending of the band causes the electrons to be on the heterojunction side and the holes to be the oxidation electrode 12c. Move to the side (opposite the heterojunction). Similarly, on the GaInP layer side, the electron / hole pair is excited by the absorbed light, and the holes move to the heterojunction side and the electrons move to the reduction electrode 12b side (the opposite side of the heterojunction).

  Then, electrons collected at the heterojunction on the GaAs layer side and holes collected at the heterojunction on the GaInP layer side are recombined by a tunnel phenomenon, and two-stage excitation between the GaAs layer side and the GaInP layer side is continuously connected. .

  Further, holes having energy capable of oxidizing water gather at the oxidation electrode 12c, and oxidation of water, that is, oxygen generation and hydrogen ion generation are performed. In the reduction electrode 12b, hydrogen ions generated on the oxidation electrode 12c side are reduced to generate hydrogen. When carbon dioxide is flowed, carbon dioxide is reduced at the reduction electrode 12c, and a carbon compound (methanol or the like) is synthesized. By repeating such a reaction, oxygen generation and carbon compound generation are continuously performed.

  In the present embodiment described above, sunlight is collected by the condenser lens 14 with respect to the semiconductor photocatalyst 12 in which the semiconductor layer 12a, the reduction electrode 12b, and the oxidation electrode 12c are integrated. Thereby, the thermal energy generated in the semiconductor layer 12a can be actively used for the electrode reaction on the surfaces of the electrodes 12b and 12c.

  In the present embodiment, the temperature of the semiconductor photocatalyst 12 is controlled by optimizing the condensing lens 14 so that the generation efficiency of the reaction product is maximized. As a result, the semiconductor layer 12a balances the decrease in the conversion efficiency of the semiconductor layer 12a from sunlight into electricity accompanying the temperature rise and the increase in the reaction rate of the electrode reaction accompanying the temperature rise. It is possible to make the most effective use of the generated heat energy.

(Second Embodiment)
Next, a second embodiment of the present invention will be described. The present embodiment is different from the first embodiment in the configuration of the reduction electrode 12b, and is otherwise the same as the first embodiment. Only the parts different from the first embodiment will be described below.

  As shown in FIG. 5, in the second embodiment, the reduction electrode 12b is provided so as to extend from one surface side (the upper side in FIG. 5) of the semiconductor layer 12a. The reduction electrode 12b has a substantially conical surface shape or a slope shape, and the diameter increases as the distance from the semiconductor layer 12a increases.

In the second embodiment, a metal electrode such as Cu, In, Sn, or a conductive material having conductivity can be used as the reduction electrode 12b. Further, Pt, NiCo ₃ O ₄ or the like may be provided as a catalyst electrode on the surface of the reduction electrode 12b.

The electrolytic solution 11 exists below the reduction electrode 12b, but does not exist above the reduction electrode 12b. Therefore, one side reduction electrode 12b is al provided in the semiconductor layer 12a is exposed to the atmosphere. Therefore, sunlight is directly applied to the surface on the one surface side of the semiconductor layer 12a.

  According to the second embodiment described above, as in the first embodiment, the heat energy generated in the semiconductor layer 12a by condensing sunlight by the condenser lens 14 is transferred to the surfaces of the electrodes 12b and 12c. It can be used effectively for electrode reactions.

In the second embodiment, the electrode reaction proceeds on the surface of the reduction electrode 12b extending from the semiconductor layer 12a. For this reason, the electrode reaction does not proceed on the surface on the one surface side (upper side in FIG. 5) where the reduction electrode 12b of the semiconductor layer 12a is provided, and the deterioration of the one surface surface of the semiconductor layer 12a can be suppressed.

  In the second embodiment, sunlight is directly incident on one surface side of the semiconductor layer 12a exposed to the atmosphere. Thereby, the conversion efficiency from sunlight to electricity in the semiconductor layer 12a can be improved.

(Other embodiments)
The present invention is not limited to the above-described embodiment, and can be variously modified as follows without departing from the spirit of the present invention. Further, the means disclosed in each of the above embodiments may be appropriately combined within a practicable range.

  (1) For example, a lens adjustment mechanism that can change the magnification, focus, position, or direction of the condenser lens 14 may be provided. By adjusting at least one of the magnification, focal point, position, and direction of the condenser lens 14 with such a lens adjustment mechanism, it is possible to adjust the temperature when the semiconductor photocatalyst 12 absorbs sunlight.

  In addition to the lens adjustment mechanism, a temperature sensor that detects the temperature of the semiconductor photocatalyst 12 and a control device that controls the lens adjustment mechanism based on the temperature of the semiconductor photocatalyst 12 detected by the temperature sensor may be provided. Thereby, the condensing lens 14 can be adjusted based on the temperature of the semiconductor photocatalyst 12, and the temperature of the semiconductor photocatalyst 12 can be controlled more appropriately.

  (2) Further, a mediator may be included on the reducing electrode 12b side of the electrolytic solution 11. A mediator is a compound that mediates electron transfer through a carbon dioxide reduction reaction. A nitrogen-containing aromatic compound can be used as the mediator. The aromatic compound is a planar ring having a delocalized π electron system containing 4n + 2 (n is an integer) π electrons. Aromatic rings can be formed by 5, 6, 7, 8, 9, or 10 or more atoms. Aromatic compounds include monocyclic and fused ring polycyclic.

  The nitrogen-containing aromatic compound is a heteroaromatic compound in which one or more constituent atoms of the aromatic ring are N atoms. In the nitrogen-containing aromatic compound, one or more hydrogen atoms bonded to the constituent atoms of the aromatic ring may be substituted with a linear or branched lower alkyl group, a hydroxy group, an amino group, or a pyridyl group. Examples of nitrogen-containing aromatic compounds constituting such mediators include imidazole, methylimidazole, dimethylimidazole, triazole, pyridine, and dimethylaminopyridine.

1 Artificial Photosynthesis Device 11 Electrolytic Solution 12 Semiconductor Photocatalyst 12a Semiconductor Layer 12b Reduction Electrode 12c Oxidation Electrode 14 Condensing Lens

Claims (5)

A semiconductor layer (12a) that generates an internal photoelectric effect when irradiated with light, a reduction electrode (12b) that generates a carbon compound by a reduction reaction of carbon dioxide, and oxygen and hydrogen ions that are generated by an oxidation reaction of water A semiconductor photocatalyst (12) configured integrally with the oxidation electrode (12c);
A condenser lens (14) for condensing the light irradiated to the semiconductor photocatalyst at a predetermined magnification;
The semiconductor layer generates heat when irradiated with light, and heat generated in the semiconductor layer is transmitted to the reduction electrode and the oxidation electrode ,
An artificial light synthesizer capable of controlling the temperature of the semiconductor layer by adjusting light collection by the condenser lens . The so that the temperature of the semiconductor layer is 100 ° C. or less, artificial photosynthetic apparatus of claim 1, the condenser is adjusted by the condenser lens. A container (10) containing an electrolytic solution (11) immersed in the semiconductor photocatalyst;
The reduction electrode is formed on one side of the semiconductor layer, and the oxidation electrode is formed on the other side of the semiconductor layer;
The reduction electrode is provided so as to extend from the surface on one side of the semiconductor layer,
The semiconductor layer is irradiated with light from the one surface side,
The artificial photosynthesis apparatus according to claim 1 or 2, wherein a portion of the one side of the semiconductor layer where the reduction electrode is not provided is exposed to the atmosphere. A semiconductor layer (12a) that generates an internal photoelectric effect when irradiated with light, a reduction electrode (12b) that generates a carbon compound by a reduction reaction of carbon dioxide, and oxygen and hydrogen ions that are generated by an oxidation reaction of water A semiconductor photocatalyst (12) configured integrally with the oxidation electrode (12c);
A condenser lens (14) for condensing the light irradiated to the semiconductor photocatalyst at a predetermined magnification;
A container (10) containing an electrolytic solution (11) immersed in the semiconductor photocatalyst,
The semiconductor layer generates heat when irradiated with light, and heat generated in the semiconductor layer is transmitted to the reduction electrode and the oxidation electrode,
The reduction electrode is formed on one side of the semiconductor layer, and the oxidation electrode is formed on the other side of the semiconductor layer;
The reduction electrode is provided so as to extend from the surface on one side of the semiconductor layer,
The semiconductor layer is irradiated with light from the one surface side,
An artificial photosynthesis device in which a portion of the semiconductor layer on which one surface of the semiconductor layer is not provided is exposed to the atmosphere. The artificial photosynthesis apparatus according to any one of claims 1 to 4 , wherein the semiconductor layer is formed of a group III-V semiconductor.

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