JPH0745718B2 - Liquid junction type semiconductor electrode and its use - Google Patents

Description

TECHNICAL FIELD The present invention relates to a stable liquid junction type semiconductor electrode, an electrolysis method using the liquid junction type semiconductor electrode, a photovoltaic cell, and an energy storage device.

[Background of the Invention]

A semiconductor in which a semiconductor is immersed in an electrolyte solution to cause a Schottky junction near the interface is called a liquid junction semiconductor, and it is possible to cause various chemical reactions by irradiating the surface of this semiconductor with light. Are known.

For example, in a liquid junction type n-type semiconductor, the valence band and the electron level of a conductor bend in a positive direction from the surface toward the inside.
When this surface is irradiated with light having an energy higher than the band width, electrons in the valence band are excited and rise to the conductor, while holes remain in the valence band. Electrons move toward the inside, and holes in the valence band move toward the surface, so that so-called photocharge separation occurs. If the electrons and holes separated in this way are given to an appropriate compound to cause a reduction and an oxidation reaction, it means that a chemical reaction can be induced by light. A typical example of this is so-called photolysis of water in which water is reduced and oxidized by light to obtain hydrogen and oxygen. For example, by dipping an n-type titanium oxide (TiO ₂ ) semiconductor electrode in an aqueous electrolyte solution,
When platinum is used as the counter electrode and the surface of the semiconductor is irradiated with ultraviolet light, the electrons that have risen to the conduction band move to the counter electrode through the inside of the semiconductor, where the protons are reduced to generate hydrogen (Equation 1). The holes remaining in the band move to the surface of TiO ₂ and oxidize water there to generate oxygen (Equation 2), resulting in the decomposition of water (Equation 3).

_{^{4H + + 4e - → 2H 2}} (1) 2H 2 O + 4h + → O 2 + 4H + (2) 2H 2 O → 2H 2 + O 2 (3) light current is generated in the same time external conductors this time.

When a p-type semiconductor is immersed in an electrolyte solution, the band bending near the surface is opposite to that of the n-type, so that the electrons generated in the semiconductor by light irradiation move toward the semiconductor surface,
The holes move toward the inside.

Using such a liquid junction type semiconductor, various photochemical reactions such as an organic synthetic reaction by light can be performed not only by photolysis by utilizing oxidation and reduction reaction by light.

[Problems to be Solved by the Invention]

However, conventional semiconductors that can be used stably have a bandwidth
It is limited to materials that can only use ultraviolet light of 3 eV or more, and semiconductors that can absorb visible light have been left as a major problem because they cannot be used because they are inactivated under light irradiation.

[Means for Solving the Problems] Therefore, the present inventor uses a compound or complex of a metal dispersed in a coated polymer film in order to stabilize and use such an unstable semiconductor in the visible region. In view of the above, the present invention has been completed.

That is, the present invention relates to a liquid junction type semiconductor electrode characterized by being coated with a film of a polymer or clay in which at least one redox agent which can serve as a water oxidation catalyst is dispersed.

The present invention also relates to the above liquid-bonded semiconductor electrode, characterized in that it is coated with a polymer or clay film in which a manganese complex that can serve as an oxidation catalyst for water is dispersed.

The present invention also relates to the above liquid junction type semiconductor electrode, which is characterized in that it is coated with a polyanionic polymer film or a clay film in which a ruthenium complex which can serve as an oxidation catalyst for water is dispersed.

Further, the present invention is to immerse any of the liquid junction type semiconductor electrodes described above together with a counter electrode in an electrolyte solution, and apply a voltage in the range of −0.7 to +1.0 V (vs. NHE) to the liquid junction type semiconductor electrodes. The present invention relates to an electrolysis method of an electrolyte solution, which is characterized by irradiating visible light.

Further, the present invention is to connect a plurality of electrolytic cells formed by dipping any of the liquid junction type semiconductor electrodes described above together with a counter electrode in an electrolyte solution in series, and irradiate the liquid junction type semiconductor electrodes of each electrolytic cell with visible light. The present invention relates to an electrolytic solution electrolysis method characterized by the above.

Further, the present invention, a plurality of electrolytic cells formed by dipping any of the above liquid junction type semiconductor electrodes in an electrolyte solution together with a counter electrode are connected in series, and the liquid junction type semiconductor electrodes are exposed so that light irradiation is possible. The present invention relates to a photovoltaic cell.

The present invention also relates to an energy storage device including an electrolytic cell formed by immersing any of the liquid junction type semiconductor electrodes described above together with a counter electrode in an electrolyte solution and a tank for storing gas generated by electrolysis.

Hereinafter, the present invention will be described in more detail.

i) Principle of the present invention Inactivation of a liquid junction type optical element using a semiconductor having a small band gap is a problem particularly when an n-type semiconductor is used. This inactivation is caused by the holes generated by light irradiation reaching the surface of the semiconductor, which oxidatively dissolves the semiconductor itself, or forms an inactive oxide film.

In order to prevent this inactivation, it is sufficient that holes are quickly moved from the semiconductor surface to cause a necessary oxidation reaction. For this reason, in the present invention, the semiconductor is coated with a polymer or clay film in which a redox agent such as a ruthenium complex or a manganese complex is dispersed to stabilize the semiconductor.

ii) Material and Manufacturing Method of Liquid Junction Type Semiconductor Electrode Next, materials and manufacturing method used for manufacturing the liquid bond type optical element of the present invention will be described.

A) Semiconductor The object of the present invention is to prevent inactivation, which is a problem particularly in the case of an n-type semiconductor, but a p-type semiconductor may be used.

Further, the semiconductor used in the present invention must be one capable of absorbing visible light, and therefore a semiconductor having a band width of 3 eV or less is used. For example, cadmium sulfide, indium phosphide, cadmium selenide, gallium arsenide, silicon, gallium phosphide and the like can be mentioned.

(B) Redox agent The redox agent to be dispersed in the film is selected according to the purpose of the reaction. As an example, to decompose water into visible light, Ru
Metal oxides such as O ₂ and MnO ₂ , ruthenium red (((NH
_{3) 4 Ru · O-Ru} (NH 3) 4 -O-Ru (NH 3) 5 ] ^6+), penta ammine ruthenium complex ([Ru (NH _{3) 5]} ^3+),
Tetrakis (bipyridine) -μ-oxo diruthenium complex Tetrakis (bipyridine) di-μ-oxo-dimanganese complex Etc. are dispersed in the film as a catalyst for oxidizing water to coat the semiconductor.

The method of dispersing these redox agents in the polymer film is to absorb the redox agent by coating the semiconductor film coated with the polymer film in an aqueous solution of the redox agent, or to introduce a redox agent by a covalent bond in advance. After synthesizing, there is a method of forming a film, or forming a mixed dispersion solution of a redox agent and a polymer to form a film from this.

In order to disperse the redox agent in the clay film, adsorption or mixed casting method can be used.

C) Coating film As the material of the polymer film used in the present invention, Nafion,
Sodium polystyrene sulfonate, polyacrylic acid,
Examples thereof include synthetic or natural polymers such as polymethacrylic acid, polymethylmethacrylic acid, polyvinyl alcohol, polyvinylpyridine, polyvinylpyridinium, collagen, cellulose and silk. The synthetic polymer can be used as a homopolymer or a copolymer, and can be used after being crosslinked with an appropriate crosslinking agent in order to suppress dissolution and peeling. These are formed into a film on the semiconductor by a casting method, a spin coating method, or the like.

A clay film is used as the coating film, and examples thereof include kaolin and montmorillonite. These can be formed into a film by placing fine particles as a water suspension on a semiconductor and drying. The appropriate thickness of the coating film is 0.1 μm to 500 μm.

iii) Electrolyte solution As the electrolyte solution used in the present invention, hydrogen and oxygen can be obtained by electrolysis when only water is used, and hydrogen and halogen molecules are obtained when an aqueous solution of hydrogen halide is used. Can be done. The use of these is particularly preferable because it converts light energy into chemical energy and accumulates it, but in addition to this, an aqueous solution of a salt of an alkali metal or an alkaline earth metal can be used as an electrolyte solution in the present invention.

iv) Electrolysis method Various electrolyte solutions can be electrolyzed using the electrolysis method of the present invention. For example, water is decomposed to obtain hydrogen and oxygen, or hydrogen halide is decomposed to generate halogen and hydrogen. Can be obtained.

When the liquid junction type semiconductor electrode of the present invention is used alone and immersed in an electrolyte solution together with a counter electrode, the liquid junction type semiconductor electrode is -0.7 to +1.0 V (vs. NHE: NHE is a standard hydrogen electrode). It is necessary to perform light irradiation while applying the voltage of 1).

The reason is that at −0.7 V or less, hydrogen generation by electrolytic reduction of water occurs, and at +1.0 V or more, oxygen generation by electrolytic oxidation of water occurs.

However, if a plurality of the above electrolytic cells are connected in series, electrolysis can be performed without applying a voltage. In this case, the number of electrolytic cells connected in series is preferably 2 to 5. This is because if 5 or more are connected, an excessive voltage is generated and the electrodes are damaged.

v) Photocell A photocell can be made using a plurality of the electrolytic cells described above. Also in this case, the number of electrolytic cells connected in series is preferably 2 to 5 for the same reason.

vi) Energy storage device By providing a storage tank for gas generated by electrolysis in the above electrolytic cell or photovoltaic cell, it can be used as an energy storage device. For example, water can be decomposed and accumulated into hydrogen and oxygen, hydrogen gas can be burned to extract energy, and then regenerated water can be reused as a raw material. In a closed space such as a spacecraft, it is effective to use it as an energy storage device that also serves as a photocell.

〔The invention's effect〕

The liquid-junction type semiconductor electrode of the present invention is stable, and therefore, the liquid-junction type semiconductor electrode of the present invention can be used to manufacture an electrolysis device, a photovoltaic cell, an energy storage device and the like having a long life.

Example 1 A Nafion solution (5 wt%) was cast at a rate of ² μ / mm ^{2 on} the surface of an n-type cadmium sulfide semiconductor electrode and dried to obtain a Nafion coating film. This is dried with warm air and then immersed in a 1 mM aqueous solution of ruthenium red for 1 hour to adsorb the complex into the Nafion membrane. When the optical characteristics of this modified semiconductor were examined by a cyclic voltammogram (CV) in an electrolyte solution, it was found that n-CdS was stabilized because a stable CV was given and a constant photocurrent was given at a constant potential. I understand.

Example 2 Using n-CdS stabilized in Example 1 as a working electrode,
Immersing the platinum counter electrode and silver reference electrode in an aqueous solution of 10ml containing 0.MKNO ₃ and 10mMHNO _3, the cell provided a silicone stopper for introducing gas and gas sampling was sealed. After blowing argon gas into the water for 2 hours, the potential of -0.3V (vs, Ag) was changed to n-.
It was applied to CdS and irradiated with visible light of 500 nm on n-CdS.
Photocurrent was generated with irradiation, and bubbles were observed on CdS and the counter electrode.

After irradiation with light for 8 hours, sampling from the gas phase and examination by gas chromatography revealed that hydrogen and oxygen were generated at a ratio of 2: 1, and it was clear that water was decomposed by visible light. The water splitting efficiency was about 15% per irradiation light.

Example 3 In Example 1, n-CdS was stabilized in the same manner as in Example 1 except that tetrakis (bipyridine) -diμoxo-dimanganese complex was used in place of ruthenium red, and water was added in the same manner as in Example 2. Was subjected to visible light decomposition to generate photocurrent and obtain hydrogen and oxygen at an efficiency of 9% per irradiation light.

Example 4 In Example 2, 0.1 M HCl aqueous solution was used as the electrolyte aqueous solution, and 400 to 8 from a 500 W xenon lamp was used as irradiation light.
When a photoreaction was carried out in the same manner as in Example 2 except that light of 00 nm was used, a photocurrent was generated and at a high rate of 3%.
HCl was decomposed into hydrogen and chlorine.

Example 5 An aqueous suspension of kaolin clay was placed on an n-CdS electrode and dried to form a clay film having a thickness of about 100 μm. This clay film-coated n-CdS was treated with tris (2,2'-bipyridine)
Both complexes were adsorbed by immersing in a mixed aqueous solution containing 10 mM of ruthenium (II) complex and (Ru (NH ₃ ) ₅ ] ^{3 +} 1 mM for 30 minutes.
When the optical characteristics of this modified semiconductor were examined as in Example 1, it was found that n-CdS was stabilized.

Example 6 Three modified electrolytic n-CdS electrodes, which were stabilized in Example 1, were immersed in an aqueous solution containing 1 MHBr together with a platinum counter electrode and connected in series, and the modified n-CdS was irradiated with visible light. However, with the generation of photocurrent, 4
HBr was decomposed to produce hydrogen and bromine with an efficiency of%.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification number Office reference number FI technical display location H01M 14/00 P // H01L 29/872

Claims (7)

[Claims] 1. A liquid junction type semiconductor electrode characterized by being coated with a film of polymer or clay in which at least one redox agent which can serve as an oxidation catalyst of water is dispersed. 2. A liquid junction type semiconductor electrode according to claim 1, which is coated with a film of a polymer or clay in which a complex of manganese which can serve as an oxidation catalyst of water is dispersed. 3. The liquid-junction type semiconductor electrode according to claim 1, which is coated with a polyanion polymer film or a clay film in which a ruthenium complex that can serve as an oxidation catalyst for water is dispersed. 4. The liquid junction type semiconductor electrode according to any one of claims (1) to (3) is immersed in an electrolyte solution together with a reference electrode, and the liquid junction type semiconductor electrode has a -0.7 to +1.0 V ( vs. NH
An electrolysis method for an electrolytic solution, which comprises irradiating visible light while applying a voltage in the range E). 5. A plurality of electrolytic cells formed by immersing the liquid-junction type semiconductor electrode according to claim 1 in an electrolyte solution together with a counter electrode are connected in series, and An electrolysis method of an electrolytic solution, which comprises irradiating a liquid junction type semiconductor electrode with visible light. 6. A plurality of electrolytic cells, which are obtained by immersing the liquid-junction type semiconductor electrode according to any one of claims 1 to 3 in an electrolyte solution together with a counter electrode, are connected in series, and the liquid-bonding is performed. Photovoltaic cell, characterized in that the type semiconductor electrode is exposed for light irradiation. 7. An electrolytic cell formed by immersing the liquid junction type semiconductor electrode according to any one of claims 1 to 3 in an electrolyte solution together with a counter electrode, and a tank for storing gas generated by electrolysis. An energy storage device comprising: