JP2014227563A - Photochemical electrode for reducing carbon dioxide, apparatus for reducing carbon dioxide, and method for reducing carbon dioxide - Google Patents

Abstract

Provided are a photochemical electrode for carbon dioxide reduction, a carbon dioxide reduction device, and a carbon dioxide reduction method capable of reducing carbon dioxide with higher efficiency. A photochemical electrode for carbon dioxide reduction includes a region where an AlxGa1-xN layer (0 <x≰0.25) 101, a GaN layer 102, a gallium oxide layer 103, and an electrode layer 104 are stacked from the light irradiation surface side. It has on the surface, or has a region where an AlxGa1-xN layer (0 <x≰0.25), a GaN layer, a gallium oxide layer, a pn junction semiconductor layer, and an electrode layer is laminated on the surface from the light irradiation surface side. Thereby, highly efficient carbon dioxide reduction is performed. The light absorbed by the AlxGa1-xN layer 101 generates carriers composed of electrons and holes. The AlxGa1-xN layer 101 functions as an anode electrode, holes move to the surface, oxidize water, The electrons are supplied to the cathode electrode via the electrode layer 104 through the GaN layer 102 and the gallium oxide layer 103. [Selection] Figure 1A

Description

  The present disclosure relates to a photochemical electrode, a carbon dioxide reduction device, and a method for reducing carbon dioxide.

  Patent Documents 1 to 9 disclose methods for reducing carbon dioxide using light energy.

  Patent Document 1 and Patent Document 2 disclose a method of reducing carbon dioxide using an oxide semiconductor such as titanium oxide as a photocatalytic material.

  Patent Literature 3 and Patent Literature 4 disclose a technique for reducing carbon dioxide using a photocatalytic material made of a predetermined metal and a semiconductor.

  Patent Documents 5 and 6 disclose a method for reducing carbon dioxide by irradiating light to a cathode electrode made of a photocatalyst containing a semiconductor and a metal complex.

  Patent Document 7 and Patent Document 8 disclose a method of irradiating an anode electrode made of a semiconductor such as titanium oxide with light to reduce carbon dioxide at the cathode electrode. Note that the methods disclosed in Patent Document 7 and Patent Document 8 require an external power source such as a solar cell or a potentiostat in addition to the cathode electrode and the anode electrode.

  Patent Document 9 discloses a method of irradiating an anode electrode formed of gallium nitride with light and reducing carbon dioxide at the cathode electrode. Note that the method disclosed in Patent Document 9 does not require an external power source such as a solar cell or a potentiostat, and reduces carbon dioxide only by light irradiation.

JP-A-55-105625 Japanese Patent No. 25526396 Japanese Patent No. 3876305 Japanese Patent No. 4158850 JP 2010-066406 A JP 2011-094194 A JP 05-311476 A JP 07-188961 A International Publication No. 2012/046374

  However, in the conventional method, the efficiency is insufficient as a photochemical electrode for reducing carbon dioxide, and the amount of reduction product obtained from carbon dioxide is small.

  An object of the present disclosure is to provide a novel photochemical electrode for carbon dioxide reduction used for carbon dioxide reduction, a carbon dioxide reduction device, and a carbon dioxide reduction method.

The photochemical electrode for carbon dioxide reduction according to the present disclosure is a photochemical electrode for carbon dioxide reduction that is used to reduce carbon dioxide with light energy, and includes an Al _x Ga _1-x N layer (0 <X ≦ 0.25), a photochemical electrode for carbon dioxide reduction having a surface on which a GaN layer, a gallium oxide layer, and an electrode layer are stacked.

  The photochemical electrode for carbon dioxide reduction according to the present disclosure can improve the reduction efficiency of carbon dioxide and increase the amount of reduction product obtained from carbon dioxide.

It is sectional drawing which shows the basic structure of the photochemical electrode for carbon dioxide reduction which concerns on this indication. It is sectional drawing which shows the conventional structure of the photochemical electrode for a carbon dioxide reduction. It is sectional drawing which shows an example of the photochemical electrode for carbon dioxide reduction which concerns on this indication. It is sectional drawing which shows an example of the photochemical electrode for carbon dioxide reduction which concerns on this indication. It is sectional drawing which shows an example of the photochemical electrode for carbon dioxide reduction which concerns on this indication. It is sectional drawing which shows an example of the photochemical electrode for carbon dioxide reduction which concerns on this indication. It is sectional drawing which shows another example of the photochemical electrode for carbon dioxide reduction which concerns on this indication. It is sectional drawing which shows another example of the photochemical electrode for carbon dioxide reduction which concerns on this indication. It is sectional drawing which shows another example of the photochemical electrode for carbon dioxide reduction which concerns on this indication. It is sectional drawing which shows another example of the photochemical electrode for carbon dioxide reduction which concerns on this indication. It is the schematic which shows the carbon dioxide reduction apparatus which concerns on this indication.

<Knowledge that was the basis for this disclosure>
Patent Documents 1 to 9 report that carbon dioxide can be reduced by light irradiation to a photocatalyst material or a photochemical electrode. That is, Patent Documents 1 to 9 disclose that carbon dioxide can be reduced and converted into an organic substance by using carriers (electrons and holes) generated by light irradiation.

  In such a carbon dioxide reduction reaction using light energy, the amount of reaction product obtained from carbon dioxide depends on the amount of carriers produced by photoexcitation and the value of the photovoltaic force generated at the photochemical electrode. Therefore, in order to increase the carbon dioxide reduction reaction efficiency, the recombination of excited carriers is suppressed, the loss due to the series resistance component contained in the photochemical electrode, etc. is reduced, and the photovoltaic value applied to the photochemical electrode is reduced. It was necessary to enlarge it.

  However, in the previous reports, there was a problem that the amount of reaction current was rate-determined because a structure for reducing the loss of excited carriers in the photochemical electrode or improving the photovoltaic value was not incorporated. .

On the other hand, the present inventors configured a photochemical electrode (anode electrode) for carbon dioxide reduction with a nitride semiconductor layer and a gallium oxide layer made of aluminum gallium nitride (Al _x Ga _1-x N) and gallium nitride (GaN). As a result, it was found that the photovoltaic value generated in the photochemical electrode was improved, and the reduction reaction efficiency of carbon dioxide using light energy was increased. The photochemical electrode for carbon dioxide reduction according to the present disclosure has been made based on such knowledge.

The photochemical electrode for carbon dioxide reduction according to the first aspect of the present disclosure is a photochemical electrode for carbon dioxide reduction that is used for reducing carbon dioxide with light energy, and is Al _x Ga _1-x from the light irradiation surface side. The surface has a region in which an N layer (0 <x ≦ 0.25), a GaN layer, a gallium oxide layer, and an electrode layer are stacked.

  According to the first aspect, when the photochemical electrode surface (anode electrode surface) is irradiated with light, the photovoltaic value generated by the light irradiation is improved, and as a result, the reduction reaction efficiency of carbon dioxide at the cathode electrode is increased. Can do. That is, the amount of reduction product obtained from carbon dioxide can be increased.

The photochemical electrode for carbon dioxide reduction according to the second aspect of the present disclosure is a photochemical electrode for carbon dioxide reduction that is used for reducing carbon dioxide by light energy, and is Al _x Ga _1-x from the light irradiation surface side. The surface has a region where an N layer (0 <x ≦ 0.25), a GaN layer, a gallium oxide layer, a pn junction semiconductor layer, and an electrode layer are stacked.

  According to the second aspect, when the photochemical electrode surface (anode electrode surface) is irradiated with light, the photovoltaic value generated by the light irradiation is further improved, and as a result, the reduction reaction efficiency of carbon dioxide at the cathode electrode is increased. be able to. That is, the amount of reduction product obtained from carbon dioxide can be increased.

  In the photochemical electrode for carbon dioxide reduction according to the third aspect of the present disclosure, in the first aspect or the second aspect, the gallium oxide layer may be a single crystal β-type gallium oxide base material.

  According to the said 3rd aspect, since it becomes possible to form a nitride semiconductor layer on a high quality and highly conductive gallium oxide base material, the efficiency as a photochemical electrode can be improved.

In the photochemical electrode for carbon dioxide reduction according to the fourth aspect of the present disclosure, in the first aspect or the second aspect, an x value of the Al _x Ga _1-x N layer may be 0.10 or more and 0.15 or less. Good.

According to the fourth aspect, for general light source, the Al x _Ga 1-x _N for the wavelength range of absorbable light spreads in layers, it is possible to effectively utilize the illumination light.

In the photochemical electrode for carbon dioxide reduction according to the fifth aspect of the present disclosure, in the first aspect or the second aspect, the GaN layer may be n-type or n ^{+ -type} .

  According to the said 5th aspect, the electrical resistance value of the GaN layer to which the electron produced | generated by optical excitation moves becomes small, and the performance as a photochemical electrode for carbon dioxide reduction can be improved.

The photochemical electrode for carbon dioxide reduction according to a sixth aspect of the present disclosure is the metal oxide according to the first aspect or the second aspect, wherein at least a part of the surface of the Al _x Ga _1-x N layer contains nickel. May be coated.

  According to the said 6th aspect, the oxygen production | generation efficiency in a photochemical electrode can be improved by what is called a promoter action which the metal oxide containing nickel has.

  In the photochemical electrode for carbon dioxide reduction according to the seventh aspect of the present disclosure, in the first aspect or the second aspect, the metal oxide containing nickel may be in the form of fine particles.

According to the seventh aspect, it is possible to easily and with good controllability, placing the metal oxide on a part of the surface or the surface of the Al x _Ga 1-x _N layer constituting the photochemical electrode.

  In the photochemical electrode for carbon dioxide reduction according to the eighth aspect of the present disclosure, in the second aspect, the pn junction semiconductor layer may be silicon.

  According to the said 8th aspect, the light which permeate | transmitted the nitride semiconductor layer and the gallium oxide layer, and reached | attained the said pn junction semiconductor layer can be utilized effectively.

  In the photochemical electrode for carbon dioxide reduction according to a ninth aspect of the present disclosure, in the second aspect, the pn junction semiconductor layer may be gallium arsenide.

  According to the ninth aspect, light that has passed through the nitride semiconductor layer and the gallium oxide layer and reached the pn junction semiconductor layer can be used effectively.

  The photochemical electrode for carbon dioxide reduction according to a tenth aspect of the present disclosure may have a structure in which a plurality of pn junction semiconductor layers made of different materials are stacked in the pn junction semiconductor layer in the second aspect.

  According to the tenth aspect, the light that has passed through the nitride semiconductor layer and the gallium oxide layer and reached the pn junction semiconductor layer can be further effectively utilized by a combination of suitable materials, and a single pn junction can be used. Compared with the structure, the photovoltaic value generated in the photochemical electrode can be increased.

A carbon dioxide reduction device according to an eleventh aspect of the present disclosure is a carbon dioxide reduction device that reduces carbon dioxide by light energy, and includes a cathode tank for containing a first electrolyte solution containing carbon dioxide, 2 An anode tank for containing an electrolyte, a proton permeable membrane sandwiched between the cathode tank and the anode tank, and a metal or metal that is installed inside the cathode tank so as to be in contact with the first electrolyte A cathode electrode having a compound on the surface, and an anode tank disposed in contact with the second electrolyte solution, and an Al _x Ga _1-x N layer (0 <x ≦ 0.25), GaN from the light irradiation surface side An anode electrode on the surface of which a layer, a gallium oxide layer, and an electrode layer are laminated, and the cathode electrode and the anode electrode are electrically connected without an external power source. It has been continued.

  According to the eleventh aspect, by irradiating the surface of the anode electrode with light, a reduction reaction of carbon dioxide occurs at the cathode electrode and the photovoltaic value generated by the light irradiation is improved, so that carbon dioxide is efficiently reduced. can do.

A carbon dioxide reduction device according to a twelfth aspect of the present disclosure is a carbon dioxide reduction device that reduces carbon dioxide with light energy, and includes a cathode tank for containing a first electrolyte solution containing carbon dioxide, 2 An anode tank for containing an electrolyte, a proton permeable membrane sandwiched between the cathode tank and the anode tank, and a metal or metal that is installed inside the cathode tank so as to be in contact with the first electrolyte A cathode electrode having a compound on the surface, and an anode tank disposed in contact with the second electrolyte solution, and an Al _x Ga _1-x N layer (0 <x ≦ 0.25), GaN from the light irradiation surface side An anode electrode laminated in the order of a layer, a gallium oxide layer, a pn junction semiconductor layer, and an electrode layer, and the cathode electrode and the anode electrode are electrically connected without an external power source It is connected.

  According to the twelfth aspect, by irradiating the surface of the anode electrode with light, a reduction reaction of carbon dioxide occurs at the cathode electrode and the photovoltaic value generated by the light irradiation is further improved. Can be reduced.

  In the carbon dioxide reduction device according to the thirteenth aspect of the present disclosure, in the eleventh aspect or the twelfth aspect, the metal on the surface of the cathode electrode may be copper, gold, silver, indium, or an alloy thereof. Good.

  According to the thirteenth aspect, as reduction products of carbon dioxide, hydrocarbons such as methane and ethylene, alcohols such as ethanol, formic acid, and the like can be efficiently generated.

  In the carbon dioxide reduction device according to a fourteenth aspect of the present disclosure, in the eleventh aspect or the twelfth aspect, the first electrolytic solution is a potassium hydrogen carbonate aqueous solution, a sodium hydrogen carbonate aqueous solution, a potassium chloride aqueous solution, or a sodium chloride aqueous solution. There may be.

  According to the fourteenth aspect, the electrolyte solution accommodated in the cathode chamber is suitable as an electrolyte solution for reducing carbon dioxide simply and with light energy.

  In the carbon dioxide reduction device according to the fifteenth aspect of the present disclosure, in the eleventh aspect or the twelfth aspect, the second electrolytic solution may be an aqueous sodium hydroxide solution.

  According to the fifteenth aspect, the electrolytic solution accommodated in the anode tank is suitable as an electrolytic solution for reducing carbon dioxide simply and by light energy.

A carbon dioxide reduction method according to a sixteenth aspect of the present disclosure is a carbon dioxide reduction method using a carbon dioxide reduction device, and includes the following steps: a step of preparing a carbon dioxide reduction device including the following: (A) a cathode tank, an anode tank, a proton permeable membrane, a cathode electrode, and an anode electrode, wherein the cathode electrode has a metal or a metal compound on its surface, and the anode electrode is from the light irradiation surface side. A region in which an Al _x Ga _1-x N layer (0 <x ≦ 0.25), a GaN layer, a gallium oxide layer, and an electrode layer are stacked in this order is provided on the surface. A second electrolyte is held in the anode tank, the cathode electrode is in contact with the first electrolyte, the anode is in contact with the second electrolyte, Proton permeability The permeation is sandwiched between the cathode tank and the anode tank, the first electrolyte contains the carbon dioxide, and the cathode electrode is electrically connected to the anode electrode, Irradiating the anode electrode with light having a wavelength that can be absorbed by the Al _x Ga _1-x N layer, and reducing the carbon dioxide contained in the first electrolyte solution by the cathode electrode (b).

  According to the sixteenth aspect, a novel method for efficiently reducing carbon dioxide by irradiating light to the anode electrode is provided.

A carbon dioxide reduction method according to a seventeenth aspect of the present disclosure is a carbon dioxide reduction method using a carbon dioxide reduction device, and includes the following steps: a step of preparing a carbon dioxide reduction device including the following: (A) a cathode tank, an anode tank, a proton permeable membrane, a cathode electrode, and an anode electrode, wherein the cathode electrode has a metal or a metal compound on its surface, and the anode electrode is from the light irradiation surface side. A region in which an Al _x Ga _1-x N layer (0 <x ≦ 0.25), a GaN layer, a gallium oxide layer, a pn junction semiconductor layer, and an electrode layer are sequentially stacked is provided on the surface. The first electrolytic solution is retained, the second electrolytic solution is retained in the anode tank, the cathode electrode is in contact with the first electrolytic solution, and the anode electrode is in contact with the second electrolytic solution. Contact The proton permeable membrane is sandwiched between the cathode tank and the anode tank, the first electrolyte contains the carbon dioxide, and the cathode electrode is electrically connected to the anode electrode. And irradiating the anode electrode with light having a wavelength that can be absorbed by the Al _x Ga _1-x N layer and light having a wavelength that can be absorbed by the pn junction semiconductor layer. (B) reducing carbon dioxide contained in the cathode electrode.

  According to the seventeenth aspect, a novel method for more efficiently reducing carbon dioxide by irradiating light to the anode electrode is provided.

  In the carbon dioxide reduction method according to the eighteenth aspect of the present disclosure, in the sixteenth aspect or the seventeenth aspect, in the step (b), the carbon dioxide reduction device may be placed at room temperature and atmospheric pressure. .

  According to the sixteenth aspect, carbon dioxide is reduced by light energy without being installed in a special environment.

  The method for reducing carbon dioxide according to a nineteenth aspect of the present disclosure is the method according to the sixteenth aspect or the seventeenth aspect, wherein carbon dioxide is converted into methane, ethylene, methanol, ethanol by the step (b) of irradiating the anode electrode with light. , Isopropanol, allyl alcohol, acetaldehyde, propionaldehyde, formic acid, and carbon monoxide may be obtained.

  According to the nineteenth aspect, carbon dioxide is immobilized by a reduction reaction of carbon dioxide with light energy, and various useful substances can be obtained.

  Hereinafter, a photochemical electrode for carbon dioxide reduction, a carbon dioxide reduction device, and a carbon dioxide reduction method according to embodiments of the present disclosure will be described with reference to the drawings.

(Embodiment)
(Photochemical electrode for carbon dioxide reduction (anode electrode))
FIG. 1A is a cross-sectional view showing the basic structure of a photochemical electrode for carbon dioxide reduction (hereinafter also referred to as “anode electrode”). The anode electrode 100a includes an Al _x Ga _1-x N layer 101 (0 <x ≦ 0.25), a GaN layer 102, a gallium oxide layer 103, and an electrode layer 104 from the light irradiation surface side.

The anode 100a is generally produced by forming a nitride semiconductor layer as a thin film on the gallium oxide layer 103. Here, the nitride semiconductor layer is configured by stacking the GaN layer 102 and the Al _x Ga _1-x N layer 101.

  Note that a method for manufacturing the anode electrode 100 a is not particularly limited as long as the method can form a nitride semiconductor thin film on the gallium oxide layer 103. For example, a metal organic vapor phase epitaxy method can be mentioned. The electrode layer 104 is a metal thin film, and is formed by resistance heating vapor deposition, electron beam vapor deposition, or the like, which is used as a normal metal thin film formation method.

The basic function of the anode electrode 100a will be described. When the surface of the anode electrode is irradiated with light, the light is absorbed by the Al _x Ga _1-x N layer 101 and photoexcitation occurs. Carriers composed of electrons and holes generated by photoexcitation contribute to a reduction reaction and an oxidation reaction, respectively. Specifically, holes generated in the Al _x Ga _1-x N layer 101 by photoexcitation move to the surface of the anode electrode 100a, and oxidize water in contact with the anode electrode 100a to generate oxygen. That is, the anode electrode 100a functions as an oxygen generation electrode.

Since the band gap value (forbidden body width) of the Al _x Ga _1-x N layer 101 is 3.4 eV or more, in order to use the Al _x Ga _1-x N layer 101 as a photochemical electrode using light, It is necessary to irradiate the Al _x Ga _1-x N layer 101 with light having a wavelength that can be absorbed by the Al _x Ga _1-x N layer 101, specifically, light having a wavelength of 360 nm or less. Therefore, from the viewpoint of effective use of light, the x value representing the composition ratio of Al contained in the Al _x Ga _1-x N layer 101 is preferably in the range of 0 <x ≦ 0.25. More preferably, it is in the range of 0.10 ≦ x ≦ 0.15.

Moreover, _(distance from Al x _{Ga 1-x} N layer 101 surface) thickness of a region where the light having the wavelength range described above is absorbed in _Al x _{Ga 1-x} N layer _101, Al x _{Ga 1-} It is approximately 100 nm from the _xN surface. This absorption region thickness can also depend on the band gap value of the Al _x Ga _1-x N layer 101. Therefore, the Al _x Ga _1-x N layer 101 preferably has a thickness of 70 nm to 1000 nm. More preferably, it has a thickness of 80 nm to 200 nm.

In order to efficiently collect electrons generated by light irradiation in the electrode layer 104, the Al _x Ga _1-x N layer 101 is stacked on the GaN layer 102. By forming the Al _x Ga _1-x N layer 101 on the GaN layer 102, an internal electric field is induced in the Al _x Ga _1-x N layer 101, and recombination of excited electrons can be suppressed. The GaN layer 102 is preferably n-type or n ^{+ -type} . The n-type GaN layer 102 can be realized by adding an appropriate amount of impurities such as silicon. When the GaN layer 102 is n-type or n ^{+ -type} , the electrical resistance value of the GaN layer 102 becomes small, so that loss due to carrier transport can be reduced.

  As described above, electrons generated in the nitride semiconductor layer by photoexcitation are not consumed in the anode electrode 100a but are supplied to the cathode electrode side through the gallium oxide layer 103 and the electrode layer 104. That is, if the carrier supply capability from the anode electrode 100a is improved, the reduction efficiency of carbon dioxide can be increased. As a specific method thereof, the value of photovoltaic force applied to the cathode electrode can be improved by further reducing the internal resistance component contained in the anode electrode and eliminating the resistance loss.

FIG. 1B is a cross-sectional view (comparative example) showing a basic structure of a conventional anode electrode. Similarly to the anode electrode 100a of the present disclosure, the anode electrode 100b having a conventional structure also includes an Al _x Ga _1-x N layer 101 (0 <x ≦ 0.25), a GaN layer 102, and an electrode layer 104. The function of each layer is the same as that of the anode electrode 100a. However, the base material 105 used for the anode electrode 100b having the conventional structure is generally an insulating material such as a sapphire base material. As a result, the electrode layer 104 is arranged at a GaN layer as shown in FIG. 1B. 102 is limited to a part of the area. Therefore, in the conventional structure, the internal resistance of the path for supplying the electrons generated in the Al _x Ga _1-x N layer 101 to the cathode electrode is high, and the loss is large.

On the other hand, since the gallium oxide layer used for the anode electrode 100a of the present disclosure can form a nitride semiconductor and exhibits good conductivity, the electrode layer 104 can be formed on the entire back surface of the substrate as shown in FIG. 1A. As a result, the movement of electrons generated in the Al _x Ga _1-x N layer 101 is facilitated, so that the resistance loss can be reduced. That is, the photovoltaic value obtained by light irradiation can be improved and the carrier supply capability to the cathode electrode is improved, so that the amount of reaction current, that is, the reduction amount of carbon dioxide can be increased.

2A to 2D are diagrams illustrating a configuration example of a photochemical electrode for carbon dioxide reduction according to the present disclosure. The anode electrode 200a of FIG. 2A has the above-described basic structure. However, in order to increase the efficiency of oxygen generation generated on the surface of the anode electrode, the surface is formed on the surface of the Al _x Ga _1-x N layer 201 as shown in FIG. 2B. A covering layer 206 may be disposed. The surface coating layer is a layer having translucency. The surface coating layer 206 is a layer containing a metal oxide, and the main metal species contained in the surface coating layer is nickel. The surface coating layer 206 preferably has a thickness of 10 nm or less so as not to block light irradiated toward the Al _x Ga _1-x N layer 201. In addition, as shown in FIG. 2C, it is also preferable to dispose the surface coating layer 206 in an island shape so that a part of the surface of the Al _x Ga _1-x N layer 201 is exposed. In this case, the shape of each surface coating layer does not need to be uniform. A plurality of surface coating layer regions having various shapes and sizes can be randomly distributed on the surface of the Al _x Ga _1-x N layer 201. Furthermore, as shown in FIG. 1D, it is also preferable to disperse and arrange a large number of fine-particle-shaped metal oxides 207 on the surface of the Al _x Ga _1-x N layer 201. The specification of US patent application 13/453669 filed by the inventors is hereby incorporated by reference.

FIG. 3 is a schematic view showing another structural example of the photochemical electrode for carbon dioxide reduction (anode electrode) according to the present invention. The anode electrode 300a illustrated in FIG. 3A has a structure in which an Al _x Ga _1-x N layer 301, a GaN layer 302, a gallium oxide layer 303, a pn junction semiconductor layer 309, and an electrode layer 304 are stacked from the light irradiation surface side. . The functions of the nitride semiconductor layer 308 and the gallium oxide layer 303 constituting the anode electrode 303a are the same as described above. Also in the anode electrode 300a, a metal oxide 307 having a fine particle shape or the like may be dispersed on the surface of the Al _x Ga _1-x N layer 301 in order to increase oxygen generation efficiency. Further, in order to electrically connect the gallium oxide layer 303 and the pn junction semiconductor layer 309, a connection layer 310 made of a metal thin film may be inserted as necessary.

  The pn junction semiconductor layer 309 has a pn junction structure formed of silicon, gallium arsenide, or the like, and is electrically connected to the gallium oxide layer 303 side through a p-type layer. As a method for manufacturing such an anode electrode 300a, a method in which a nitride semiconductor layer 308 is manufactured on a gallium oxide base material by the same method as described above, and a separately manufactured pn junction semiconductor layer is prepared and bonded to each other. Is simple.

In the configuration of the anode electrode 303 a illustrated in FIG. 3A, a pn junction semiconductor layer 309 is bonded to the back surface side of the gallium oxide layer 303. Therefore, electrons generated in the Al _x Ga _1-x N layer 301 by photoexcitation are supplied to the p-type layer side of the pn junction semiconductor layer 309 via the GaN layer 302 and the gallium oxide layer 303. The pn junction semiconductor layer 309 has a junction structure of a material exhibiting p-type characteristics and a material exhibiting n-type characteristics, and includes a material exhibiting i-type characteristics between the p-type layer and the n-type layer. You can leave. That is, the pn junction structure includes a pin junction structure. In general, a material exhibiting p-type characteristics and a material exhibiting n-type characteristics are formed of the same material, but a pn junction structure may be formed of different materials.

The pn junction semiconductor layer 309 is irradiated with light transmitted through the nitride semiconductor layer 308 and the gallium oxide layer 303. Therefore, the pn junction semiconductor layer absorbs an absorbable light component included in the wavelength component of transmitted light and generates excited carriers. As a result, the holes excited in the pn junction semiconductor layer 309 by light irradiation recombine with carriers supplied from the gallium oxide layer 303 side, and the excited electrons are disposed in the electrode layer 304 disposed on the anode electrode 300a. The carbon dioxide is supplied to the cathode electrode side through which the carbon dioxide is reduced. The potential applied to the cathode electrode is the sum of the photovoltaic power generated in the nitride semiconductor layer 308 and the photovoltaic power generated in the pn junction semiconductor layer 309. That is, by using the anode electrode 300a in which the Al _x Ga _1-x N layer, the GaN layer, the gallium oxide layer, the pn junction semiconductor layer, and the electrode layer are stacked, the photovoltaic value generated by light irradiation can be improved. Thus, it becomes possible to increase the reduction treatment amount of carbon dioxide at the cathode electrode.

  FIG. 3B shows a configuration example in which the gallium oxide layer 303 and the pn junction semiconductor layer 309 are bonded via a transparent conductive layer 311. As described above, if the gallium oxide layer on which the nitride semiconductor layer is formed and the pn junction semiconductor layer are electrically connected and the pn junction semiconductor layer is irradiated with transmitted light from the nitride semiconductor layer, The configuration of the connection layer is not particularly limited.

  FIG. 3C and FIG. 3D are schematic views of a configuration including a plurality of pn junction layer semiconductor layers in another structural example of the photochemical electrode for carbon dioxide reduction according to the present invention. As a specific combination of pn junction structures constituting the pn junction semiconductor layer 309, a material having a band gap capable of effectively absorbing light transmitted through a nitride semiconductor layer, such as gallium arsenide and silicon, or amorphous silicon and crystalline silicon. If it is a combination of these, it will not specifically limit.

(CO2 reduction device)
FIG. 4 is a schematic diagram showing the carbon dioxide reduction device 400. The carbon dioxide reduction device 400 includes a cathode tank 402, an anode tank 405, and a proton permeable membrane 406.

  A first electrolytic solution 407 is held inside the cathode chamber 402, and a cathode electrode 401 is provided in the cathode chamber 402. The cathode electrode 401 is in contact with the first electrolyte solution 407. Specifically, the cathode electrode 401 is immersed in the first electrolyte solution 407.

  The first electrolytic solution 407 is, for example, a potassium hydrogen carbonate aqueous solution, a sodium hydrogen carbonate aqueous solution, a potassium chloride aqueous solution, or a sodium chloride aqueous solution. The concentration of the first electrolytic solution is preferably 1 mol / L or more in any aqueous solution. More preferably, the concentration is 3 mol / L or more.

  The first electrolyte solution 407 contains carbon dioxide. The concentration of carbon dioxide is not limited. The first electrolytic solution 407 is preferably acidic in a state where carbon dioxide is dissolved in the first electrolytic solution 407.

  A material constituting the cathode electrode 401 in which carbon dioxide is reduced is a metal or a metal compound. The metal that is a material constituting the cathode electrode 401 is, for example, copper, gold, silver, indium, or an alloy thereof. The surface of the metal material may contain a metal compound such as an oxide or chloride of the metal material. Furthermore, as a metal compound suitable for carbon dioxide reduction, for example, tantalum carbide or tantalum nitride can also be applied.

  The metal which is a material constituting the cathode electrode 401 is particularly preferably one containing either copper or indium. By adopting copper, hydrocarbons and alcohols can be obtained as reduction products of carbon dioxide. Further, by adopting indium, formic acid can be selectively produced as a reduction product of carbon dioxide.

  The cathode electrode 401 can be composed of only a metal or a metal compound, but may be composed of a base material that holds the metal or metal compound. For example, the cathode electrode 401 may be formed by forming a predetermined metal or metal compound into a thin film on a substrate such as glass or glassy carbon (registered trademark). The cathode electrode 401 may be formed by dispersing a plurality of fine particles made of a metal or a metal compound on a conductive substrate. As shown in FIG. 4, at least a part of the cathode electrode 401 can be immersed in the first electrolyte solution 407.

A second electrolytic solution 408 is held inside the anode tank 405, and the anode tank 405 includes an anode electrode 404. In the present application, since the anode electrode 404 is irradiated with light, the anode electrode 404 is a photochemical electrode. The anode electrode 404 includes a region where an Al _x Ga _1-x N layer, a GaN layer, a gallium oxide layer, an electrode layer, and the like are stacked. An example of the anode electrode configuration is the anode electrode 200d, for example. The anode electrode 404 is in contact with the second electrolytic solution 408. Specifically, the anode electrode 404 is immersed in the second electrolyte solution 408.

  The second electrolytic solution 408 is, for example, a sodium hydroxide aqueous solution. The concentration of the second electrolytic solution is preferably 1 mol / L or more in any aqueous solution. More preferably, the concentration is about 5 mol / L. The second electrolytic solution 408 is preferably basic.

As will be described later, the anode electrode 404 immersed in the second electrolyte solution 408 is irradiated with light having a wavelength that can be absorbed by the Al _x Ga _1-x N layer from the light source 403. In the structure having a pn junction semiconductor layer as shown in FIG. 3, light having a wavelength that can be absorbed by the Al _x Ga _1-x N layer and light having a wavelength that can be absorbed by the pn junction semiconductor layer are emitted. Irradiated from light source 403. As a specific example of the light source 403, a xenon lamp, a mercury lamp, a halogen lamp, or the like can be used alone or in combination, or a pseudo solar light source or sunlight can be used.

  In order to separate the first electrolytic solution 407 from the second electrolytic solution 408, a proton permeable membrane 406 is sandwiched between the cathode tank 402 and the anode tank 405. In other words, in the carbon dioxide reduction device according to the embodiment of the present disclosure, the first electrolytic solution 407 and the second electrolytic solution 408 are not mixed with each other.

  The proton permeable membrane 406 is not particularly limited as long as it substantially passes only protons and other substances cannot pass through the proton permeable membrane 406. The proton permeable membrane 406 is, for example, Nafion (registered trademark).

  The cathode electrode 401 and the anode electrode 404 include an electrode terminal 310 and an electrode terminal 311, respectively. These electrode terminals 310 and 311 are electrically and directly connected to each other by a conducting wire 312. An external power source such as a battery or a potentiostat is not electrically sandwiched between the electrode terminals 310 and 311.

(Method of reducing carbon dioxide)
Next, a method for reducing carbon dioxide using a carbon dioxide reducing device will be described.

  The carbon dioxide reduction device 400 can be placed at room temperature and atmospheric pressure.

  As shown in FIG. 4, light is irradiated from the light source 403 to the anode electrode 404. The light source 403 is, for example, a xenon lamp. The light emitted from the light source 403 has a wavelength of 360 nm or less. This light preferably has a wavelength of 250 nm or more and 325 nm or less. In addition, the light emitted from the light source 403 can include light having a wavelength of a visible light component.

  As shown in FIG. 4, the carbon dioxide reduction device preferably includes a gas introduction pipe 409. It is preferable that carbon dioxide contained in the first electrolyte solution 407 is reduced while carbon dioxide is supplied to the first electrolyte solution 407 through the gas introduction pipe 409. One end of the gas introduction pipe 409 is immersed in the first electrolyte solution 407. It is also preferable that carbon dioxide is supplied to the first electrolyte solution 407 through the gas introduction pipe 409 and a sufficient amount of carbon dioxide is dissolved in the first electrolyte solution 407 before starting the reduction of carbon dioxide.

  Carbon dioxide contained in the first electrolytic solution 407 is reduced on the cathode electrode 401, and at least of formic acid, carbon monoxide, hydrocarbons such as methane and ethylene, alcohols such as ethanol, and aldehydes such as acetaldehyde. Generate one species.

(Example)
With reference to the following examples, the photochemical electrode for carbon dioxide reduction, the carbon dioxide reduction device, and the carbon dioxide reduction method according to the embodiment of the present disclosure will be described in more detail.

Example 1
A low resistance single crystal β-type gallium oxide base material (thickness: about 0.68 mm) was used for the gallium oxide layer constituting the photochemical electrode for carbon dioxide reduction. As the GaN layer, an n ^{+ type} low-resistance GaN layer doped with silicon (thickness: 3.0 μm, silicon doping amount: 4.0 × 10 ¹⁸ / cm ³ ) was used. The GaN layer was grown on the gallium oxide substrate by metal organic vapor phase epitaxy. For the Al _x Ga _1-x N layer, a film having a thickness of 100 nm and x = 0.10 was used. Similarly, the Al _x Ga _1-x N layer was grown on the GaN layer using the metal organic vapor phase epitaxy method. Further, in order to further increase the oxygen generation efficiency, nickel oxide fine particles (fine particle size: several tens of nm to several μm) were dispersedly arranged on the Al _x Ga _1-x N layer. Furthermore, an electrode layer (thickness: about 500 nm) made of titanium / gold was formed on the back side of the gallium oxide substrate (the side on which the nitride semiconductor layer was not formed). In this way, an anode electrode 200d as shown in FIG. 2D was obtained.

A copper plate (thickness: 0.5 mm) was used for the cathode electrode. The area of the copper plate immersed in the first electrolytic solution was about 4 cm ² .

  Using the photochemical electrode (anode electrode) and cathode electrode as described above, the carbon dioxide reduction apparatus shown in FIG. 4 was produced. The distance between the anode electrode and the cathode electrode was about 8 cm. The configuration of other carbon dioxide reduction devices is as follows.

First electrolyte solution: potassium chloride aqueous solution having a concentration of 3.0 mol / L Second electrolyte solution: sodium hydroxide aqueous solution having a concentration of 5.0 mol / L Proton permeable membrane: Nafion membrane (manufactured by DuPont, Nafion 117)
Light source: xenon lamp (output: 300 W, light irradiation area: about 4 cm ² )
Carbon dioxide gas was supplied to the first electrolyte solution through the gas introduction pipe 409 for 30 minutes. The anode tank was equipped with a light irradiation window (not shown). The light emitted from the light source 403 was irradiated to the anode electrode surface for a certain period of time through the light irradiation window. This light had a wavelength of 360 nm or less and had a broad spectrum.

(Comparative Example 1)
Except for an anode electrode in which an Al _x Ga _1-x N layer and a GaN layer are deposited on an insulating sapphire substrate under the same growth conditions, and an electrode layer is formed in a partial region on the n ^{+ -type} GaN layer, An experiment similar to Example 1 was performed.

  When light was applied to the surface of the anode electrode, it was observed that a reaction current flowed through the conductor in both cases of Example 1 and Comparative Example 1. At that time, the amount of reaction current obtained in Example 1 was about 1.5 times that of Comparative Example 1. On the other hand, when light irradiation was interrupted, it was observed that no reaction current flowed through the conductor. This means that some reaction occurs at the anode electrode and the cathode electrode by light irradiation.

  The present inventors investigated the state of carbon dioxide reduction in detail as follows. Specifically, each anode electrode was irradiated with light in a state where the cathode chamber was sealed, that is, in a state where carbon dioxide was sealed. This light irradiation reduces carbon dioxide in the cathode chamber. The type and amount of reaction products produced by the carbon dioxide reduction were measured by gas chromatography and liquid chromatography, respectively.

  As a result, it was found that carbon monoxide, formic acid and the like were produced in the cathode tank in both cases of Example 1 and Comparative Example 1. The amount of each reaction product increased in proportion to the light irradiation time. From the above, it has been found that the catalytic reaction in which carbon dioxide is reduced at the cathode electrode is caused by the light irradiation to the anode electrode.

  Furthermore, as a result of comparing the reaction product amounts of Example 1 and Comparative Example 1, Example 1 was about 1.5 times that of Comparative Example 1. That is, by using an anode electrode produced using a gallium oxide substrate, the amount of reaction current is about 1.5 times that of an anode electrode produced using a sapphire substrate, and a reaction is generated. It was found that the amount of carbon dioxide was also about 1.5 times, and carbon dioxide was efficiently reduced.

(Example 2)
An experiment similar to that of Example 1 was performed, except that fine particle-shaped nickel oxide was not disposed on the surface of the Al _x Ga _1-x N layer.

  As a result, it was observed that a reaction current flows through the conductive wire by irradiating the anode electrode with light. The observed current amount was almost the same as in Example 1. Moreover, it was confirmed that the reaction product similar to Example 1 was obtained as the reduction product of carbon dioxide.

Example 3
The same experiment as in Example 1 was performed except that an electrode in which copper fine particles were dispersed and arranged on a glassy carbon (registered trademark) base material was used as the cathode electrode.

  As a result, it was observed that a reaction current flows through the conductive wire by irradiating the anode electrode with light. The observed current amount was almost the same as in Example 1. Moreover, it was confirmed that the reaction product similar to Example 1 was obtained as the reduction product of carbon dioxide.

  In addition, when an electrode is prepared with copper nickel alloy fine particles containing a small amount of nickel component instead of copper fine particles, the reaction current amount and the reduction product of carbon dioxide are almost the same as the case of only copper fine particles. Results were obtained.

Example 4
The same experiment as in Example 1 was performed except that an indium plate was used as the cathode electrode.

  As a result, it was observed that a reaction current flows through the conductive wire by irradiating the anode electrode with light. Further, the observed amount of reaction current was almost equal to that of Example 1. On the other hand, it was confirmed that most of the reaction product obtained by carbon dioxide reduction was formic acid. That is, it was confirmed that formic acid was selectively generated by using indium as the cathode electrode.

(Example 5)
The same experiment as in Example 1 was performed except that the first electrolytic solution was changed to a potassium hydrogen carbonate aqueous solution and a sodium chloride aqueous solution.

  As a result, it was observed that a reaction current flows through the conductive wire by irradiating the anode electrode with light. Further, the observed reaction current amount was almost the same as that of Example 1.

(Example 6)
An experiment similar to that of Example 1 was performed except that the x value of the Al _x Ga _1-x N layer was changed from x = 0.10 to x = 0.05, 0.15, 0.20.

As a result, even when the amount of Al contained in the Al _x Ga _1-x N layer was changed, it was confirmed that the same reaction product as that of Example 1 was obtained as the reduction product of carbon dioxide.

(Example 7)
Bonding the photochemical electrode described in Example 1 and a pn junction semiconductor layer produced by introducing an impurity exhibiting p-type conduction into the surface of a single crystal n-type silicon substrate via a connection layer made of a metal thin film. Thus, the anode electrode 300a shown in FIG. 3A was produced.

  The anode electrode was irradiated with light having a broad spectrum consisting of ultraviolet light having a wavelength of 360 nm or less and visible light from the light source 403.

  As a result, the main reaction products in Example 1 were formic acid and carbon monoxide, but in the configuration of Example 7, hydrocarbons such as methane and ethylene, alcohols such as ethanol, acetaldehyde and the like Aldehydes were also obtained.

  That is, by using an anode electrode in which a nitride semiconductor layer and a pn junction semiconductor layer formed on a gallium oxide base material are stacked, an organic substance component having a more complicated structure than formic acid and carbon monoxide by only light irradiation. Has been found to be derived from carbon dioxide.

As described above, the structure of the photochemical electrode for carbon dioxide reduction (anode electrode) includes a region in which an Al _x Ga _1-x N layer, a GaN layer, a gallium oxide layer, and an electrode layer are stacked, thereby forming an anode electrode. It was found that the amount of reaction current due to light irradiation increased. It was also confirmed that carbon dioxide was efficiently reduced by light energy at the cathode electrode. Furthermore, it was shown that various kinds of organic substances can be generated from carbon dioxide by combining pn junction semiconductor layers.

  The present disclosure provides a photochemical electrode for carbon dioxide reduction, a carbon dioxide reduction device, and a carbon dioxide reduction method used for reducing carbon dioxide with light energy.

400 Carbon dioxide reduction device 401 Cathode electrode

Claims (19)

A photochemical electrode for carbon dioxide reduction used for reducing carbon dioxide by light energy,
A photochemical electrode for carbon dioxide reduction having, on the surface, a region in which an Al _x Ga _1-x N layer (0 <x ≦ 0.25), a GaN layer, a gallium oxide layer, and an electrode layer are stacked from the light irradiation surface side. A photochemical electrode for carbon dioxide reduction used for reducing carbon dioxide by light energy,
Carbon dioxide reduction having on the surface a region in which an Al _x Ga _1-x N layer (0 <x ≦ 0.25), GaN layer, gallium oxide layer, pn junction semiconductor layer, and electrode layer is laminated from the light irradiation surface side Photochemical electrode. The photochemical electrode for carbon dioxide reduction according to claim 1 or 2, wherein the gallium oxide layer is a single crystal β-type gallium oxide base material. The photochemical electrode for carbon dioxide reduction according to claim 1 or 2, wherein an x value of the Al _x Ga _1-x N layer is 0.10 or more and 0.15 or less. The photochemical electrode for carbon dioxide reduction according to claim 1 or 2, wherein the GaN layer is n-type or n ^{+ -type} . The photochemical electrode for carbon dioxide reduction according to claim 1 or 2, wherein at least a part of the surface of the Al _x Ga _1-x N layer is coated with a metal oxide containing nickel. The photochemical electrode for carbon dioxide reduction according to claim 6, wherein the nickel-containing metal oxide has a fine particle shape. The photochemical electrode for carbon dioxide reduction according to claim 2, wherein the pn junction semiconductor layer is silicon. The photochemical electrode for carbon dioxide reduction according to claim 2, wherein the pn junction semiconductor layer is gallium arsenide. The photochemical electrode for carbon dioxide reduction according to claim 2, wherein the pn junction semiconductor layer has a structure in which a plurality of pn junction semiconductor layers made of different materials are stacked. A carbon dioxide reduction device for reducing carbon dioxide with light energy,
A cathode tank for containing a first electrolyte containing carbon dioxide;
An anode tank for containing a second electrolyte solution;
A proton permeable membrane sandwiched between the cathode cell and the anode cell;
A cathode electrode installed inside the cathode chamber so as to be in contact with the first electrolyte solution and having a metal or a metal compound on the surface;
It is installed inside the anode tank so as to be in contact with the second electrolyte solution, and an Al _x Ga _1-x N layer (0 <x ≦ 0.25), a GaN layer, a gallium oxide layer, and an electrode layer are provided from the light irradiation surface side. An anode electrode having a stacked region on the surface,
A carbon dioxide reduction device in which the cathode electrode and the anode electrode are electrically connected without an external power source. A carbon dioxide reduction device for reducing carbon dioxide with light energy,
A cathode tank for containing a first electrolyte containing carbon dioxide;
An anode tank for containing a second electrolyte solution;
A proton permeable membrane sandwiched between the cathode cell and the anode cell;
A cathode electrode installed inside the cathode chamber so as to be in contact with the first electrolyte solution and having a metal or a metal compound on the surface;
It is installed inside the anode tank so as to be in contact with the second electrolytic solution, and from the light irradiation surface side, an Al _x Ga _1-x N layer (0 <x ≦ 0.25), a GaN layer, a gallium oxide layer, and a pn junction semiconductor And an anode electrode having a region on which the electrode layer is laminated,
A carbon dioxide reduction device in which the cathode electrode and the anode electrode are electrically connected without an external power source. The carbon dioxide reduction device according to claim 11 or 12, wherein the metal on the surface of the cathode electrode is copper, gold, silver, indium, or an alloy thereof. The carbon dioxide reduction device according to claim 11 or 12, wherein the first electrolytic solution is a potassium hydrogen carbonate aqueous solution, a sodium hydrogen carbonate aqueous solution, a potassium chloride aqueous solution, or a sodium chloride aqueous solution. The carbon dioxide reduction device according to claim 11 or 12, wherein the second electrolytic solution is an aqueous sodium hydroxide solution. A method for reducing carbon dioxide using a carbon dioxide reduction device comprising the following steps:
A step (a) of preparing a carbon dioxide reduction device comprising:
Cathode chamber,
Anode tank,
Proton permeable membrane,
A cathode electrode, and
Anode electrode, where
The cathode electrode comprises a metal or a metal compound on the surface,
The anode electrode has a surface on which the Al _x Ga _1-x N layer (0 <x ≦ 0.25), the GaN layer, the gallium oxide layer, and the electrode layer are stacked from the light irradiation surface side,
A first electrolyte is held inside the cathode chamber,
A second electrolyte is held inside the anode tank,
The cathode electrode is in contact with the first electrolyte;
The anode electrode is in contact with the second electrolytic solution;
The proton permeable membrane is sandwiched between the cathode chamber and the anode chamber;
The first electrolyte contains the carbon dioxide; and
The cathode electrode is electrically connected to the anode electrode;
Irradiating the anode electrode with light having a wavelength that can be absorbed by the Al _x Ga _1-x N layer to reduce carbon dioxide contained in the first electrolyte solution at the cathode electrode (b). A method for reducing carbon dioxide using a carbon dioxide reduction device comprising the following steps:
A step (a) of preparing a carbon dioxide reduction device comprising:
Cathode chamber,
Anode tank,
Proton permeable membrane,
A cathode electrode, and
Anode electrode, where
The cathode electrode comprises a metal or a metal compound on the surface,
The anode electrode has, on the surface, a region where an Al _x Ga _1-x N layer (0 <x ≦ 0.25), a GaN layer, a gallium oxide layer, a pn junction semiconductor layer, and an electrode layer are stacked from the light irradiation surface side. Equipped,
A first electrolyte is held inside the cathode chamber,
A second electrolyte is held inside the anode tank,
The cathode electrode is in contact with the first electrolyte;
The anode electrode is in contact with the second electrolytic solution;
The proton permeable membrane is sandwiched between the cathode chamber and the anode chamber;
The first electrolyte contains the carbon dioxide; and
The cathode electrode is electrically connected to the anode electrode;
The anode electrode is irradiated with light having a wavelength that can be absorbed by the Al _x Ga _1-x N layer and light having a wavelength that can be absorbed by the pn junction semiconductor layer, and is contained in the first electrolyte solution. A step (b) of reducing carbon dioxide at the cathode electrode. The method for reducing carbon dioxide according to claim 16 or 17, wherein, in the step (b), the carbon dioxide reducing device is placed at room temperature and atmospheric pressure. The step (b) of irradiating the anode electrode with light obtains at least one of methane, ethylene, methanol, ethanol, isopropanol, allyl alcohol, acetaldehyde, propionaldehyde, formic acid, and carbon monoxide from carbon dioxide. Item 18. The method for reducing carbon dioxide according to Item 16 or Item 17.

Images