JP2016098419A - Hydrogen generation method, hydrogen generating apparatus and anode electrode for hydrogen generation - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method and apparatus for highly efficiently generating hydrogen by reducing water.SOLUTION: A hydrogen generation method uses a hydrogen generating apparatus 300 comprising a cathode tank 302, an anode tank 305, a proton permeable membrane 306, a cathode electrode 301 including platinum or a platinum compound, and an anode electrode 304. In the hydrogen generating apparatus 300, light is irradiated to the anode electrode 304, and hydrogen is generated on the cathode electrode 301. The anode electrode 304 includes a first semiconductor layer formed by a nitride semiconductor having a laminated structure comprising an InGaN layer (0<x≤0.40) and a GaN layer, and a second semiconductor layer formed by a semiconductor having a pn junction structure. The cathode electrode 301 and the anode electrode 304 are electrically connected to each other not through an external power source.SELECTED DRAWING: Figure 3

Description

  The present disclosure relates to a hydrogen generation method, a hydrogen generation apparatus, and an anode electrode for hydrogen generation.

  Conventionally, as a method of using a semiconductor material that functions as a photocatalyst, it is known that hydrogen is generated by decomposing water by irradiating the semiconductor material with light.

  Patent Documents 1 and 2 disclose a method for generating hydrogen using a nitride semiconductor as an anode electrode (photochemical electrode).

JP 2003-24764 A JP 2013-49891 A

  However, further efficiency in hydrogen production has been desired.

  An object of the present disclosure is to provide a more efficient method for generating hydrogen, a hydrogen generator, and an anode electrode for hydrogen generation.

A method for generating hydrogen according to the present disclosure includes a cathode tank holding a first electrolyte, an anode tank holding a second electrolyte, a proton permeable membrane sandwiched between the cathode tank and the anode tank, A step (a) of preparing a hydrogen generator comprising a cathode electrode having platinum or a platinum compound in contact with the first electrolyte solution and an anode electrode in contact with the second electrolyte solution; and irradiating the anode electrode with light And (b) generating hydrogen by reducing water contained in the first electrolyte solution on the cathode electrode. In the hydrogen generation device, the anode electrode includes a first semiconductor layer formed of a nitride semiconductor having a stacked structure of an In _x Ga _1-x N layer (0 <x ≦ 0.40) and a GaN layer; A second semiconductor layer composed of a semiconductor having a junction structure, and an In _x Ga _1-x N layer of the first semiconductor layer from the side in contact with the second electrolytic solution in the anode electrode, A GaN layer of one semiconductor layer, a p-type layer of the second semiconductor layer, and an n-type layer of the second semiconductor layer are arranged in this order, and the cathode electrode and the anode electrode are through an external power source There is no electrical connection between them.

  In the method for generating hydrogen according to the present disclosure, hydrogen can be generated with high efficiency.

It is sectional drawing which shows typically an example of the anode electrode (photochemical electrode) which concerns on this indication. It is sectional drawing which shows typically another example of the anode electrode (photochemical electrode) which concerns on this indication. It is sectional drawing which shows typically another example of the anode electrode (photochemical electrode) which concerns on this indication. It is sectional drawing which shows typically another example of the anode electrode (photochemical electrode) which concerns on this indication. It is a schematic diagram showing an example of a hydrogen generator concerning this indication.

  Patent Documents 1 and 2 disclose that hydrogen is generated by performing a redox treatment of water using carriers (electrons and holes) generated by light irradiation on a photochemical electrode. The amount of hydrogen produced in the hydrogen production reaction using such light energy depends on the amount of carriers produced by photoexcitation and the value of the photovoltaic force generated at the photochemical electrode. However, in the conventional method, the amount of carriers generated and the amount of reaction current obtained by photoexcitation are not sufficient, and it has been desired to further increase the amount of generation and reaction current to increase the amount of hydrogen generated.

The present inventors, as an anode electrode (photochemical electrode), a first semiconductor layer composed of a nitride semiconductor having a stacked structure of an In _x Ga _1-x N layer (0 <x ≦ 0.40) and a GaN layer And an anode electrode having a second semiconductor layer composed of a semiconductor having a pn junction structure, it has been found that hydrogen generation efficiency is increased. The method for generating hydrogen according to the present disclosure has been made based on such knowledge.

  Based on the above findings, the inventors have come up with the following aspects of the invention.

The method for generating hydrogen according to the first aspect of the present disclosure includes a cathode tank holding a first electrolyte, an anode tank holding a second electrolyte, and a proton sandwiched between the cathode tank and the anode tank. A step (a) of preparing a hydrogen generator comprising: a permeable membrane; a cathode electrode having platinum or a platinum compound in contact with the first electrolyte; and an anode electrode in contact with the second electrolyte; and And (b) generating hydrogen by irradiating light to reduce water contained in the first electrolytic solution on the cathode electrode. In the hydrogen generation device, the anode electrode includes a first semiconductor layer formed of a nitride semiconductor having a stacked structure of an In _x Ga _1-x N layer (0 <x ≦ 0.40) and a GaN layer; A second semiconductor layer composed of a semiconductor having a junction structure, and an In _x Ga _1-x N layer of the first semiconductor layer from the side in contact with the second electrolytic solution in the anode electrode, A GaN layer of one semiconductor layer, a p-type layer of the second semiconductor layer, and an n-type layer of the second semiconductor layer are arranged in this order, and the cathode electrode and the anode electrode are through an external power source There is no electrical connection between them.

  According to the said 1st aspect, hydrogen can be produced | generated more efficiently by irradiating light to an anode electrode (photochemical electrode).

  The method for generating hydrogen according to the second aspect of the present disclosure includes, in addition to the characteristics of the first aspect, the light applied to the anode electrode has a wavelength that can be absorbed by the first semiconductor layer, and the second semiconductor. A wavelength that can be absorbed by the layer.

  According to the second aspect, it is possible to effectively use light having various wavelengths.

In addition to the features of the second aspect, the method for generating hydrogen according to the third aspect of the present disclosure includes the first semiconductor layer when the value of x of the In _x Ga _1-x N layer is 0.4. The wavelength that can be absorbed is 625 nm or less, and the wavelength that can be absorbed by the second semiconductor layer is 625 nm or more.

The method for generating hydrogen according to the fourth aspect of the present disclosure includes, in addition to the characteristics of the second aspect, when the value of x of the In _x Ga _1-x N layer is 0.1, The wavelength that can be absorbed is 415 nm or less, and the wavelength that can be absorbed by the second semiconductor layer is 415 nm or more.

In addition to the features of any one of the first to fourth aspects, the method for generating hydrogen according to the fifth aspect of the present disclosure is such that the value of x of the In _x Ga _1-x N layer is 0.05 ≦ x ≦ 0.15.

  In the method for generating hydrogen according to the sixth aspect of the present disclosure, in addition to the characteristics of any one of the first to fifth aspects, the GaN layer is n-type.

  According to the sixth aspect, the electrical resistance of the GaN layer to which carriers generated by photoexcitation move is reduced and the ohmic loss is reduced, so that the performance of the anode electrode as a photochemical electrode can be enhanced.

  According to a seventh aspect of the present disclosure, in addition to the features of any one of the first to sixth aspects, the second semiconductor layer is made of at least one of Si, GaAs, GaP, and Ge. Composed.

  According to the said 7th aspect, the light which permeate | transmitted the said 1st semiconductor layer and arrived at the said 2nd semiconductor layer can be utilized effectively.

  In the hydrogen generation method according to the eighth aspect of the present disclosure, in addition to the features of any one of the first to seventh aspects, the p-type layer and the n-type layer of the second semiconductor layer are different from each other. It is composed of a semiconductor.

  According to the eighth aspect, the light transmitted through the first semiconductor layer and reaching the second semiconductor layer can be used more effectively. In addition, the photovoltaic value generated in the anode electrode can be increased.

The method for generating hydrogen according to the ninth aspect of the present disclosure includes the surface of the In _x Ga _1-x N layer on the side in contact with the second electrolytic solution, in addition to the characteristics of the first to eighth aspects. In addition, NiO _y (0 <y ≦ 1) is arranged.

According to the ninth aspect, the oxygen production efficiency of the anode electrode can be increased by the promoter action of nickel oxide NiO _y .

  According to a tenth aspect of the present disclosure, in addition to the characteristics of any one of the first to ninth aspects, the anode electrode includes a plurality of the second semiconductor layers, and the anode electrode includes There is a pair of adjacent second semiconductor layers in which the n-type layer of one of the second semiconductor layers is electrically connected to the p-type layer of the other second semiconductor layer.

  According to the tenth aspect, the light that has passed through the first semiconductor layer and reached the second semiconductor layer can be further effectively used. In addition, the photovoltaic value generated in the anode electrode can be increased.

  According to an eleventh aspect of the present disclosure, in addition to the characteristics of any one of the first to tenth aspects, the first electrolytic solution includes potassium hydrogen carbonate, sodium hydrogen carbonate, potassium chloride, sodium chloride. , An aqueous solution containing at least one of potassium sulfate and potassium phosphate.

  According to the eleventh aspect, these electrolytic solutions are suitable as the electrolytic solution accommodated in the cathode chamber.

  In the hydrogen production method according to the twelfth aspect of the present disclosure, in addition to the characteristics of any one of the first to eleventh aspects, the second electrolytic solution contains at least one of sodium hydroxide and potassium hydroxide. It is an aqueous solution containing.

  According to the twelfth aspect, this electrolytic solution is suitable as an electrolytic solution accommodated in the anode tank.

  In addition to the features of any one of the first to twelfth aspects, the method for generating hydrogen according to the thirteenth aspect of the present disclosure includes the hydrogen generation apparatus installed at room temperature and atmospheric pressure in the step (b). The

  According to the thirteenth aspect, hydrogen is generated by light energy without being installed in a special environment.

A hydrogen generation device according to a fourteenth aspect of the present disclosure is sandwiched between a cathode tank capable of holding a first electrolyte, an anode tank capable of holding a second electrolyte, and the cathode tank and the anode tank. A proton permeable membrane; a cathode electrode having platinum or a platinum compound disposed inside the cathode chamber so as to be in contact with the first electrolyte during hydrogen generation; and the cathode so as to be in contact with the second electrolyte during hydrogen generation. And an anode electrode installed inside the anode tank. The anode electrode includes a first semiconductor layer made of a nitride semiconductor having a stacked structure of an In _x Ga _1-x N layer (0 <x ≦ 0.40) and a GaN layer, and a semiconductor having a pn junction structure In the anode electrode, the In _x Ga _1-x N layer of the first semiconductor layer, from the side in contact with the second electrolyte at the time of hydrogen generation, the first semiconductor The GaN layer of the layer, the p-type layer of the second semiconductor layer, and the n-type layer of the second semiconductor layer are arranged in this order, and the cathode electrode and the anode electrode do not pass through an external power source, They are electrically connected to each other.

An anode electrode for hydrogen generation according to a fifteenth aspect of the present disclosure is the anode electrode used in the hydrogen generation apparatus according to the fourteenth aspect. More specifically, the semiconductor device includes a first semiconductor layer made of a nitride semiconductor having a stacked structure of an In _x Ga _1-x N layer (0 <x ≦ 0.40) and a GaN layer, and a pn junction structure. A second semiconductor layer composed of a semiconductor, an In _x Ga _1-x N layer as the first semiconductor layer, a GaN layer as the first semiconductor layer, a p-type layer as the second semiconductor layer, and An n-type layer of the second semiconductor layer is an anode electrode for hydrogen generation, which is arranged in this order.

  Hereinafter, a hydrogen generation method and a hydrogen generation apparatus according to embodiments of the present disclosure will be described with reference to the drawings. The present invention is not limited to the embodiments described below.

(Embodiment)
FIG. 1A is a schematic diagram illustrating an example of the structure of an anode electrode (photochemical electrode) 10A used in the hydrogen generation process according to the present disclosure. An anode electrode 10A shown in FIG. 1A includes a first semiconductor layer 11 made of a nitride semiconductor material and a second semiconductor layer 12 having a pn junction structure from the side irradiated with light (light irradiation surface). Have a laminated structure. The first semiconductor layer 11 is an indium gallium nitride layer (compositional formula In _x Ga _1-x N; the x value representing the indium (In) composition amount satisfies 0 <x ≦ 0.40; hereinafter also referred to as an InGaN layer) 13 and a gallium nitride layer (compositional GaN; hereinafter also referred to as GaN layer) 14. The second semiconductor layer 12 is a semiconductor layer having a pn junction structure. The GaN layer 14 of the first semiconductor layer 11 and the p-type layer of the second semiconductor layer 12 are electrically connected. As shown in FIG. 1A, the first and second semiconductor layers 11 and 12 are connected to the GaN layer 14 side of the first semiconductor layer 11 and the p of the second semiconductor layer 12 via the conductive base material 15 and the electrode portion 16. The shape layer is electrically connected.

As a method for manufacturing such an anode electrode, first, a second semiconductor layer 12 having a pn junction structure is first formed on a base material, and the first semiconductor layer 11 is continuously formed of a nitride semiconductor material. There is a method of forming (Method A). Apart from this, there is a method (method B) in which the first semiconductor layer and the second semiconductor layer are individually formed and then the respective electrode regions are joined. Method B is simpler. The anode electrode 10A shown in FIG. 1A has a structure manufactured based on the method B. Specifically, the first semiconductor layer 11 made of a nitride semiconductor material is formed on the conductive substrate 15. It has a structure in which the second semiconductor layer 12 formed and joined separately is joined by the electrode portion 16. The terminal electrode portion 17 is a connection terminal of the anode electrode 10A, and is connected to the cathode electrode through a conducting wire. The terminal electrode portion 17 is made of, for example, a transparent conductive material or metal. The transparent conductive material is, for example, ZnO, ITO (Indium Tin Oxide), or SnO ₂ . The metal is, for example, aluminum, copper, nickel or silver.

  Hereinafter, the production method of the anode electrode 10A by the method B will be described more specifically.

The first semiconductor layer 11 is formed, for example, by forming a nitride semiconductor layer (InGaN layer 13 and GaN layer 14) as a thin film on the conductive substrate 15. For this purpose, a method for forming a thin film of a nitride semiconductor on a substrate can be adopted without any particular limitation. This method is, for example, a metal organic vapor phase epitaxy method. A specific material used for the conductive base material 15 is, for example, a low-resistance single crystal gallium nitride (GaN) substrate, a gallium oxide (Ga ₂ O ₃ ) substrate, a silicon carbide (SiC) substrate, or the like.

  A method for forming the electrode portion 16 disposed on the conductive substrate 15 is not particularly limited, but a vacuum vapor deposition method (resistance heating vapor deposition, electron beam vapor deposition, etc.) used as a normal metal thin film forming method is suitable. . The electrode portion 16 shown in FIG. 1A is disposed on the surface of the conductive substrate 15 opposite to the surface on the first semiconductor layer 11 side.

  The basic function of the first semiconductor layer 11 is to absorb light in the region composed of the InGaN layer 13 and contribute to the redox reaction by the action of carriers (electrons and holes) generated by the photoexcitation. Specifically, holes generated by photoexcitation in the InGaN layer 13 move to the surface of the anode electrode 10A (the surface on the InGaN layer 13 side), and oxidize water in contact with the anode electrode 10A to generate oxygen. To do. That is, the anode electrode 10A itself functions as an oxygen generation electrode.

The amount of light absorption in the InGaN layer 13 is determined by the band gap value (forbidden body width), which can be decreased by increasing the In composition. Therefore, the InGaN layer 13 can be adjusted to absorb light having a wavelength of about 365 nm or more by controlling the band gap value. However, when the light absorption of the long wavelength by the InGaN layer 13 increases excessively, the light absorption in the second semiconductor layer 12 decreases. As a result, the current is limited by the second semiconductor layer 12, and the entire anode electrode As a result, the photovoltaic power value is reduced. Therefore, from the viewpoint of effective use of light, the In composition of the InGaN layer 13 of the anode electrode 10A is in the range of 0 <x ≦ 0.40 in terms of the value of x in the composition formula In _x Ga _1-x N. In this case, the InGaN layer 13 absorbs light having a wavelength of 625 nm or less. The In composition of the InGaN layer 13 is preferably 0.05 ≦ x ≦ 0.15 in terms of the value of x.

  When the surface of the InGaN layer 13 is irradiated with light having a component in the above wavelength range, the absorption region is approximately 100 nm from the light irradiation surface, although depending on the value of the band gap of the InGaN layer. For this reason, the thickness of the InGaN layer 13 is, for example, not less than 70 nm and not more than 300 nm, and more preferably not less than 80 nm and not more than 150 nm.

Electrons generated in the InGaN layer 13 by photoexcitation are supplied to the p-type layer side of the second semiconductor layer 12 via the GaN layer 14 of the first semiconductor layer 11. The GaN layer 14 is preferably n-type (configured with n-type GaN). When the GaN layer 14 is n-type, the electrical resistance component of the GaN layer 14 is reduced, and therefore ohmic loss associated with carrier transport can be reduced. As the n-type GaN layer 14, it is desirable to apply a low resistance gallium nitride layer (n ^{+ -type} GaN layer) to which an impurity element (for example, silicon) is added. In this embodiment, this configuration is adopted. The carrier concentration of the n ^{+ -type} GaN layer 14 to which silicon is added is preferably 1 × 10 ¹⁸ / cm ³ or more, and more preferably about 2 × 10 ¹⁸ to 8 × 10 ¹⁸ / cm ³ .

  The second semiconductor layer 12 having a pn junction structure includes a p-type layer composed of a material (semiconductor material) exhibiting p-type characteristics and an n-type layer composed of a material (semiconductor material) exhibiting n-type characteristics. It has a junction structure. A material exhibiting i-type characteristics may be included between the p-type layer and the n-type layer. That is, the pn junction structure included in the second semiconductor layer 12 includes a pin junction structure. Similarly, the pn junction structure of the second semiconductor layer 12 includes a structure including a buffer layer introduced into a junction interface such as a pi layer or an in layer.

  In general, a material exhibiting p-type characteristics and a material exhibiting n-type characteristics are made of the same material, but a pn junction structure may be formed of different materials. That is, the p-type layer and the n-type layer of the second semiconductor layer 12 may be composed of different semiconductors. The second semiconductor layer 12 is made of at least one of Si, GaAs, GaP, and Ge, for example.

  In the second semiconductor layer 12 having a pn junction structure, an absorbable light component contained in the transmitted light of the first semiconductor layer 11 is absorbed to generate excited carriers and a photovoltaic power is generated. As a result, holes excited by light irradiation recombine with carriers supplied from the first semiconductor layer 11 side, but electrons excited by the second semiconductor layer 12 are terminals arranged on the anode electrode 10A. Collected in the electrode part 17 and supplied to the cathode electrode side for reducing water through the electrically connected wiring (conductive wire). The potential applied to the cathode electrode is the sum of the photovoltaic power generated in the first semiconductor layer 11 and the photovoltaic power generated in the second semiconductor layer 12. That is, by using the anode electrode 10A in which the first semiconductor layer 11 and the second semiconductor layer 12 are stacked, recombination of carriers as a whole of the anode electrode is suppressed. Further, it is possible to improve the value of the photovoltaic power generated by light irradiation, and it is possible to increase the amount of water reduction treatment at the cathode electrode.

  The anode electrode may have a plurality of second semiconductor layers 12. In this case, the anode electrode includes a pair of adjacent second semiconductor layers 12 in which the n-type layer of one second semiconductor layer 12 is electrically connected to the p-type layer of the other second semiconductor layer. Preferably, all the second semiconductor layers 12 included in the anode electrode are electrically connected to the n-type layer (or p-type layer) and the p-type layer (or n-type layer) of the adjacent second semiconductor layer 12. It is further desirable to be connected. In order to achieve electrical connection, the n-type layer of one second semiconductor layer 12 and the p-type layer of the other second semiconductor layer 12 are not necessarily in direct contact with each other. For example, the n-type layer of one second semiconductor layer 12 and the p-type layer of the other second semiconductor layer 12 may be electrically connected via a conductive layer (with sandwiched therebetween). The conductive layer is, for example, a transparent conductive layer 19 or an intermediate reflection layer. The intermediate reflection layer is made of, for example, a low refractive index material, and the low refractive index material is, for example, ZnO or SiO. An example of a pair of the second semiconductor layers 12 electrically connected via the intermediate reflection layer is “amorphous silicon layer having a pn junction structure (or pin junction structure) / intermediate reflection layer / pn junction structure (or pin A microcrystalline silicon layer having a junction structure).

The above is the main configuration of the anode electrode used in the hydrogen generation process according to the present embodiment. The InGaN layer 13 is configured to increase the oxygen generation efficiency, which is a function of the anode electrode, and to increase the durability of the anode electrode. It is desirable that a large number of nickel oxides (NiO _y : 0 <y ≦ 1) 18 are dispersedly arranged on the surface (the surface on the light irradiation surface side and in contact with the second electrolytic solution) or a part of the surface. . In FIG. 1A, fine nickel oxide 18 is disposed, but instead of this, the surface of the InGaN layer 13 or a surface of the thin film containing nickel oxide within a range in which the light irradiation to the InGaN layer 13 is not hindered. It is also possible to arrange it on a part of the surface. In this case, the thickness of the thin film is desirably thin enough to prevent light irradiation, for example, 10 nm or less. The shape of the nickel oxide layer is not necessarily uniform, and various shapes or sizes may be randomly distributed on the surface of the InGaN layer 13. The present inventors have confirmed that these structures have an effect of increasing the oxygen generation efficiency in the anode electrode by the promoter action of nickel oxide.

FIG. 1B is an example of an anode electrode having a configuration in which a first semiconductor layer 11 made of a nitride semiconductor and a second semiconductor layer 12 having a pn junction structure are joined via a transparent conductive layer 19. As described above, if the first semiconductor layer 11 and the second semiconductor layer 12 are electrically connected and the second semiconductor layer 12 is irradiated with the transmitted light of the first semiconductor layer 11, the semiconductor layer 11. , 12 are not particularly limited in configuration. The transparent conductive layer 19 is made of a transparent conductive material. The transparent conductive material is, for example, ZnO, ITO, SnO ₂ or the like.

  2A and 2B show an example (anode electrodes 20A and 20B) that is an anode electrode used in the hydrogen generation process according to the present disclosure and includes a plurality of second semiconductor layers 12 having a pn junction structure. 2A and 2B, the two second semiconductor layers 12 are stacked on each other with the transparent conductive layer 19 interposed therebetween.

  Specific combinations of pn junction structures of the second semiconductor layer 12 include gallium arsenide (GaAs) and silicon (Si), amorphous silicon (a-Si) and crystalline silicon (c-Si), and microcrystalline silicon (μc). -Si), gallium phosphide (GaP), germanium (Ge), or the like.

  The pn junction of the second semiconductor layer 12 is not particularly limited as long as it is a combination of materials having a band gap that can effectively absorb the transmitted light of the first semiconductor layer 11. In the embodiment, a layer in which a pin junction structure made mainly of amorphous silicon and a pin junction structure made of microcrystalline silicon are stacked is used as the second semiconductor layer 12.

(Device for generating and processing hydrogen by light irradiation)
FIG. 3 is a schematic view showing an example of an apparatus for generating and processing hydrogen by light irradiation. The apparatus 300 includes a cathode chamber 302, an anode chamber 305, and a proton permeable membrane 306. A first electrolytic solution 307 is held inside the cathode chamber 302, and the cathode chamber 302 includes a cathode electrode 301. The cathode electrode 301 is in contact with the first electrolytic solution 307. Specifically, the cathode electrode 301 is immersed in the first electrolytic solution 307.

As the first electrolytic solution 307 held in the cathode tank, a general electrolytic solution can be used, and in particular, at least one of potassium hydrogen carbonate, sodium hydrogen carbonate, potassium chloride, sodium chloride, potassium sulfate, and potassium phosphate. An aqueous solution containing one species is desirable. The first electrolytic solution 307 includes a potassium hydrogen carbonate aqueous solution (KHCO ₃ aqueous solution), a sodium hydrogen carbonate aqueous solution (NaHCO ₃ aqueous solution), a potassium chloride aqueous solution (KCl aqueous solution), a sodium chloride aqueous solution (NaCl aqueous solution), and potassium sulfate (K ₂ SO _4). Aqueous solution) or an aqueous potassium phosphate solution (K ₃ PO ₄ aqueous solution). In any case, the concentration of the first electrolytic solution is preferably 0.5 mol / L or more, and more preferably 3 mol / L or more. The first electrolyte solution 307 is preferably acidic.

  The cathode electrode 301 has a catalyst layer made of a material mainly composed of platinum on the surface. Further, the cathode electrode 301 may be formed only of the material constituting the catalyst layer, but may have a laminated structure of the catalyst layer and a base material that holds the catalyst layer. Examples of the latter cathode electrode 301 are an electrode in which a catalyst layer is formed in a thin film on a substrate such as glass or glassy carbon (registered trademark), and an electrode in which a large number of particulate catalyst layers are supported on a conductive substrate. . The configuration is not limited as long as the cathode electrode 301 has the ability to reduce water. As long as the material is in contact with the first electrolytic solution 307, only a part of the cathode electrode 301 can be immersed in the first electrolytic solution 307.

  A second electrolytic solution 308 is held inside the anode tank 305, and the anode tank 305 includes an anode electrode 304. The anode electrode 304 is an anode electrode of the present disclosure, for example, the anode electrodes 10A to 20B described above. The anode electrode 304 is in contact with the second electrolytic solution 308. Specifically, the anode electrode 304 is immersed in the second electrolyte solution 308.

  The example of the 2nd electrolyte solution 308 hold | maintained at the anode tank 305 is the aqueous solution containing at least 1 sort (s) among sodium hydroxide and potassium hydroxide. The second electrolytic solution may be a sodium hydroxide aqueous solution (NaOH aqueous solution) or a potassium hydroxide aqueous solution (KOH aqueous solution). The concentration of the second electrolytic solution is preferably 1 mol / L or more, and particularly preferably about 5 mol / L. The second electrolytic solution 308 is desirably basic.

  As will be described later, at least light having a wavelength that can be absorbed by the first semiconductor layer 11 and light that can be absorbed by the second semiconductor layer 12 are formed in the region of the anode electrode 304 that is immersed in the second electrolyte solution 308. The included light is emitted from the light source 303. Specific examples of the light source 303 are a xenon lamp, a mercury lamp, or a halogen lamp, which can be used alone or in combination. Also, a so-called pseudo solar light source or sunlight can be used as the light source 303.

In order to separate the first electrolyte solution 307 and the second electrolyte solution 308, a proton permeable membrane 306 is sandwiched between the cathode chamber 302 and the anode chamber 305. That is, in this apparatus, the first electrolytic solution 307 and the second electrolytic solution 308 are not mixed with each other. The proton permeable membrane 306 is not particularly limited as long as it transmits protons (H ⁺ ) and suppresses the passage of other substances. A specific example of the proton permeable membrane 306 is a Nafion membrane (registered trademark).

  The cathode electrode 301 and the anode electrode 304 have electrode terminals 310 and 311, respectively. These electrode terminals 310 and 311 are electrically connected by a conducting wire 312 without going through an external power source such as a potentiostat. That is, the cathode electrode 301 is electrically connected to the anode electrode 304 through the conducting wire 312 without going through an external power source.

(Water reduction treatment method by light irradiation)
Next, a method for generating and processing hydrogen using the above-described apparatus will be described.

The hydrogen generator 300 can be placed at room temperature and atmospheric pressure. As shown in FIG. 3, light is irradiated from the light source 303 to the anode electrode 304. More specifically, the InGaN layer 13 included in the anode electrode 304 is irradiated with light. An example of the light source 303 is a xenon lamp, and light from the light source 303 includes light that can be absorbed by the first semiconductor layer 11 and light that can be absorbed by the second semiconductor layer 12. The light that can be absorbed by the second semiconductor layer is preferably light transmitted through the first semiconductor layer (the second semiconductor layer preferably has a configuration capable of absorbing light transmitted through the first semiconductor layer). Thereby, the light irradiated to the InGaN layer 13 can be used effectively. Specifically, when the value of x of the In _x Ga _1-x N layer 13 is 0.4, the first semiconductor layer absorbs light having a wavelength of 625 nm or less, and thus the second semiconductor layer has a wavelength of 625 nm or more. It is desirable to have a configuration that can absorb light of a wavelength. Further, when the value of x of the In _x Ga _1-x N layer 13 is 0.1, the first semiconductor layer absorbs light having a wavelength of 415 nm or less, and thus the second semiconductor layer has light having a wavelength of 415 nm or more. It is desirable to have a configuration that can absorb

  When the cathode electrode 301 includes a material having a catalyst layer made of a material mainly composed of platinum, the water contained in the first electrolytic solution 307 is reduced by light irradiation to the anode electrode 304. As a result, hydrogen can be generated at the cathode electrode 301.

(Example)
Hereinafter, the hydrogen generation method and the hydrogen generation apparatus of the present disclosure will be described in more detail with reference to examples.

Example 1
In this example, the anode electrode 20A shown in FIG. 2A was used. The anode electrode 20A was produced as follows.

A low-resistance single crystal gallium nitride substrate (thickness: about 0.4 mm, size: about 50 mm in diameter) was prepared as the conductive base material 15 constituting the anode electrode (photochemical electrode). Next, on the single crystal gallium nitride substrate, an n ^{+ -type} low-resistance GaN layer 14 doped with silicon (thickness: 3.0 μm, silicon doping amount: 4.0 × 10 ¹⁸ / cm ³ ) and InGaN layer 13 (Composition formula: In _0.05 Ga _0.95 N, thickness: 70 nm) was grown by metal organic vapor phase epitaxy. Next, in order to increase the oxygen generation efficiency in the anode electrode, a large number of nickel oxide fine particles 18 (fine particle size: several tens of nanometers to several μm) are dispersedly arranged on the surface of the InGaN layer using a solution reaction. Furthermore, an electrode layer 16 (thickness: about 500 nm) made of titanium (Ti) / aluminum (Al) / gold (Au) is formed on the back side of the gallium nitride substrate (the side on which the nitride semiconductor layer is not formed). Formed.

  Separately, a pair of second semiconductor layers 12 having a laminated structure of an amorphous silicon pin layer and a microcrystalline silicon pin layer (that is, having two pn junction structures) is formed, and the electrode layer 16 is interposed through The first semiconductor layer and the p-type layer of the second semiconductor layer were electrically connected. In this way, an anode electrode 20A as shown in FIG. 2A was obtained.

On the other hand, a platinum plate (thickness: 0.1 mm) was used for the cathode electrode. The area immersed in the 1st electrolyte solution in the platinum plate was about 13 cm < ² >.

  Using these anode and cathode electrodes, a hydrogen generator as shown in FIG. 3 was produced. The distance between the anode electrode and the cathode electrode was about 8 cm. Table 1 shows other configurations of the produced hydrogen generator.

  Through a light irradiation window (not shown) installed in the anode tank, the surface of the anode electrode is irradiated with light having an ultraviolet light component and a visible light component from the light source 303 for a predetermined time, and the charge amount reaches 10 C (Coulomb). Water reduction treatment was performed. In this embodiment, a light source 303 that irradiates light over a wavelength range of 290 nm to 840 nm is used.

(Comparative Example 1)
An experiment similar to that of Example 1 was performed except that an anode electrode having no second semiconductor layer was used.

  When light from the xenon lamp is irradiated on the surface of the anode electrode (InGaN surface), it is observed that in both cases of Example 1 and Comparative Example 1, a reaction current flows through the conducting wire 312 connecting the anode electrode and the cathode electrode. It was. On the other hand, when light irradiation was interrupted, it was observed that no reaction current flowed through the conducting wire 312 in both cases of Example 1 and Comparative Example 1. This means that some reaction occurs at the anode electrode and the cathode electrode by light irradiation. When the reaction current amounts were compared, as shown in Table 2, Example 1 was about 37 times larger than Comparative Example 1. This result shows that the reaction amount of Example 1 is larger than that of Comparative Example 1.

  From this result, it is clear that in order to increase the amount of reaction occurring at each electrode, a second semiconductor layer made of a semiconductor having a pn junction structure is required in addition to the first semiconductor layer made of a nitride semiconductor. It became.

  Next, the present inventors investigated in detail the reaction caused by light irradiation, that is, the state of the redox reaction of water as follows. Specifically, in the state where the cathode tank was sealed, each anode electrode of Example 1 and Comparative Example 1 was irradiated with light for the same time. This light irradiation causes a reduction reaction of water in the cathode chamber, and the amount of hydrogen produced was measured by gas chromatography. The results are shown in Table 3.

As shown in Table 3, the amount of hydrogen produced in Example 1 (converted to solar energy 100 mW / cm ² ) was 145.9 μmol / h. On the other hand, the hydrogen production amount of Comparative Example 1 was about 3.0 μmol / h. That is, it was found that the amount of hydrogen generated was greatly improved by the configuration of Example 1. That is, it was found that the structure of Example 1 can efficiently generate hydrogen from water in a short time.

(Example 2)
Example 1 except that the value of x representing the In composition amount of the InGaN layer (In _x Ga _1-x N layer) constituting the first semiconductor layer was increased from x = 0.05 to x = 0.15. The same experiment was conducted.

  As a result, it was confirmed that hydrogen was efficiently generated at the cathode electrode as in Example 1.

(Example 3)
Example 1 except that the value of x representing the In composition amount of the InGaN layer (In _x Ga _1-x N layer) constituting the first semiconductor layer was increased from x = 0.05 to x = 0.40. The same experiment was conducted.

  As a result, it was confirmed that hydrogen was efficiently generated at the cathode electrode as in Example 1.

(Comparative Example 2)
An experiment was performed with the same configuration as in Example 1 except that a gallium nitride layer (composition formula: GaN, film thickness: 100 nm) was used instead of the In _0.05 Ga _0.95 N layer of the first semiconductor layer.

  As a result, the amount of reaction current in Comparative Example 2 was lower than that in Example 1. Moreover, the hydrogen production amount of Comparative Example 2 was 105.1 μmol / h.

(Comparative Example 3)
The experiment was performed with the same configuration as in Example 1 except that an aluminum gallium nitride layer (composition formula: Al _0.10 Ga _0.90 N, film thickness: 100 nm) was used instead of the In _0.05 Ga _0.95 N layer of the first semiconductor layer. went.

  As a result, the amount of reaction current in Comparative Example 3 was lower than that in Example 1. Moreover, the hydrogen production amount of Comparative Example 3 was 34.9 μmol / h.

  The present invention provides a method for producing hydrogen with high efficiency.

10A, 10B, 20A, 20B Anode electrode 11 First semiconductor layer 12 Second semiconductor layer 13 Indium gallium nitride (InGaN) layer 14 Gallium nitride (GaN) layer 15 Conductive substrate 16 Electrode portion 17 Terminal electrode portion 18 Nickel oxide ( NiO _y )
19 Transparent conductive layer 300 Hydrogen generator 301 Cathode electrode 302 Cathode tank 303 Light source 304 Anode electrode 305 Anode tank 306 Proton conductive membrane 307 First electrolyte 308 Second electrolyte 310, 311 Terminal 312 Conductor

Claims (15)

A method for producing hydrogen, comprising:
A cathode chamber holding a first electrolyte solution;
An anode tank holding a second electrolyte solution;
A proton permeable membrane sandwiched between the cathode cell and the anode cell;
A cathode electrode in contact with the first electrolyte and having platinum or a platinum compound;
An anode electrode in contact with the second electrolytic solution;
Preparing a hydrogen generator comprising: (a);
(B) generating hydrogen by irradiating the anode electrode with light and reducing water contained in the first electrolyte solution on the cathode electrode;
Including
In the hydrogen generator,
The anode electrode includes a first semiconductor layer made of a nitride semiconductor having a stacked structure of an In _x Ga _1-x N layer (0 <x ≦ 0.40) and a GaN layer, and a semiconductor having a pn junction structure A second semiconductor layer configured by:
In the anode electrode, from the side in contact with the second electrolyte, the In _x Ga _1-x N layer of the first semiconductor layer, the GaN layer of the first semiconductor layer, the p-type layer of the second semiconductor layer, and N-type layers of the second semiconductor layer are arranged in this order;
The cathode electrode and the anode electrode are electrically connected to each other without an external power source,
How to generate hydrogen. The light applied to the anode electrode has a wavelength that can be absorbed by the first semiconductor layer and a wavelength that can be absorbed by the second semiconductor layer.
The method for producing hydrogen according to claim 1. When the value of x of the In _x Ga _1-x N layer is 0.4, the wavelength that can be absorbed by the first semiconductor layer is 625 nm or less, and the wavelength that can be absorbed by the second semiconductor layer Is a wavelength of 625 nm or more,
The method for producing hydrogen according to claim 2. When the value of x of the In _x Ga _1-x N layer is 0.1, the wavelength that can be absorbed by the first semiconductor layer is 415 nm or less, and the wavelength that can be absorbed by the second semiconductor layer Is a wavelength of 415 nm or more,
The method for producing hydrogen according to claim 2. X value of the In _x Ga _1-x N layer is 0.05 ≦ x ≦ 0.15,
The method for producing hydrogen according to any one of claims 1 to 4. The GaN layer is n-type;
The method for producing hydrogen according to any one of claims 1 to 5. The second semiconductor layer is composed of at least one of Si, GaAs, GaP and Ge;
The method for producing hydrogen according to any one of claims 1 to 6. The p-type layer and the n-type layer of the second semiconductor layer are composed of different semiconductors;
The method for producing hydrogen according to any one of claims 1 to 7. NiO _y (0 <y ≦ 1) is disposed on the surface of the In _x Ga _1-x N layer on the side in contact with the second electrolyte solution.
The method for producing hydrogen according to any one of claims 1 to 8. The anode electrode has a plurality of the second semiconductor layers,
The anode electrode includes a pair of adjacent second semiconductor layers in which an n-type layer of one of the second semiconductor layers is electrically connected to a p-type layer of the other second semiconductor layer,
The method for producing hydrogen according to any one of claims 1 to 9. The first electrolytic solution is an aqueous solution containing at least one of potassium hydrogen carbonate, sodium hydrogen carbonate, potassium chloride, sodium chloride, potassium sulfate and potassium phosphate.
The method for producing hydrogen according to any one of claims 1 to 10. The second electrolytic solution is an aqueous solution containing at least one of sodium hydroxide and potassium hydroxide.
The method for producing hydrogen according to any one of claims 1 to 11. In the step (b), the hydrogen generator is installed at room temperature and atmospheric pressure.
The method for producing hydrogen according to any one of claims 1 to 12. A hydrogen generator,
A cathode cell capable of holding a first electrolyte solution;
An anode tank capable of holding a second electrolyte solution;
A proton permeable membrane sandwiched between the cathode cell and the anode cell;
A cathode electrode having platinum or a platinum compound disposed inside the cathode chamber so as to be in contact with the first electrolyte during hydrogen generation;
An anode electrode installed inside the anode tank so as to be in contact with the second electrolyte during hydrogen generation;
With
The anode electrode includes a first semiconductor layer made of a nitride semiconductor having a stacked structure of an In _x Ga _1-x N layer (0 <x ≦ 0.40) and a GaN layer, and a semiconductor having a pn junction structure A second semiconductor layer configured by:
In the anode electrode, from the side in contact with the second electrolyte during hydrogen generation, the In _x Ga _1-x N layer of the first semiconductor layer, the GaN layer of the first semiconductor layer, and the p-type of the second semiconductor layer And the n-type layer of the second semiconductor layer are arranged in this order,
The cathode electrode and the anode electrode are electrically connected to each other without an external power source,
Hydrogen generator. A first semiconductor layer made of a nitride semiconductor having a stacked structure of an In _x Ga _1-x N layer (0 <x ≦ 0.40) and a GaN layer, and a first semiconductor layer made of a semiconductor having a pn junction structure Two semiconductor layers,
The In _x Ga _1-x N layer of the first semiconductor layer, the GaN layer of the first semiconductor layer, the p-type layer of the second semiconductor layer, and the n-type layer of the second semiconductor layer are arranged in this order. An anode electrode for hydrogen generation.

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