JP2004315942A - Water decomposition mechanism - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a water decomposition mechanism which can electrolyze water without using a power source for applying a voltage between both electrodes of an anode and a cathode. <P>SOLUTION: The water decomposition mechanism is constituted by using a solar battery 16 formed by coupling a p type silicon layer 12 on an anode side and an n type silicon layer 13 on a cathode side, forming a titanium oxide layer 11 acting as a photocatalyst layer on the surface of the p type silicon layer 12 of the solar battery 16 by a sol-gel method, depositing a platinum layer 14 by evaporation on the surface of the n type silicon layer 13 and arranging such solar battery into an aqueous sodium sulfide solution 6 filled in a transparent container 8. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a water splitting apparatus that obtains hydrogen and oxygen, which are fuels for fuel cells and the like and raw materials for the chemical industry, by electrolysis of water.
[0002]
[Prior art]
The decomposition reaction of water can be expressed as 2H ₂ O → 2H ₂ + O ₂ , and the change in free energy is known to be 474.26 kJ per mol of oxygen. In order to supply this energy by electricity, it is understood that a voltage of 1.2288 V is required as a voltage between the anode and the cathode by using a relation of electric energy = charge × voltage.
[0003]
In a general water splitting apparatus shown in FIG. 2 for electrolyzing water, a vessel 8 is filled with an aqueous solution of sodium sulfate 6, and the aqueous solution of sodium sulfate 6 contains an anode electrode 2 and a cathode electrode 3 using a platinum wire. Are arranged at predetermined intervals, and a power source 1 is connected to the ends of the anode electrode 2 and the cathode electrode 3 protruding from the aqueous sodium sulfate solution 6. However, the + terminal of the power supply 1 is connected to the anode electrode 2, and the − terminal of the power supply 1 is connected to the cathode electrode 3.
[0004]
In such a water splitting apparatus, it is necessary to provide activation potential energy for the reaction. That is, in order to electrolyze water (here, aqueous sodium sulfate solution 6) at the platinum electrodes 2 and 3, the potential of the anode electrode 2 is more positive than +1.2 V with respect to the general silver chloride standard electrode potential. , The potential of the cathode electrode 3 must be more negative than -0.8V. Therefore, electrolysis of water does not occur unless a voltage exceeding 2 V is applied between the electrodes 2 and 3.
[0005]
This extra voltage is called overvoltage. This overvoltage is generated when the solution to be electrolyzed generates polarization energy for forming an electric double layer near the electrode, or in the case of a semiconductor electrode, when potential energy is bent inside the semiconductor. This is the potential required to move electrons beyond.
[0006]
When this overvoltage is applied to both electrodes 2 and 3 by the power supply 1, oxygen gas (bubbles) 4 is generated from the anode electrode 2, and hydrogen gas 5 is generated from the cathode electrode 3. That is, hydrogen gas and oxygen gas can be produced by electrolysis of water. However, since the overvoltage must be applied by the power supply 1, only a conversion efficiency of about 60% or less can be obtained.
[0007]
The water splitting device shown in FIG. 3 uses a semiconductor electrode 7 as the anode electrode 2 of the water splitting device shown in FIG. 2 in order to improve the conversion efficiency. The semiconductor electrode 7 is obtained by forming titanium oxide of an anatase type crystal on the surface of a metal titanium plate by a sol-gel method or the like. In such a water splitting apparatus, when +0.5 V is applied from the power supply 1 between the electrodes 7 and 3 and the semiconductor electrode 7 is irradiated with light 9 in the ultraviolet region, oxygen gas 4 from the semiconductor electrode 7 and oxygen gas 4 from the cathode electrode 3 Generation of hydrogen gas 5 is observed, and the electrolysis of water by the Honda Fujishima effect can be observed. This effect is well known as the photocatalytic effect of titanium oxide.
However, in the conventional water splitting apparatus, even in the water splitting apparatus using the photocatalytic effect shown in FIG. 3, a power source 1 for applying a voltage of 0.5 V between both the anode and cathode electrodes 7, 3 is used. Is necessary.
On the other hand, as an electrolyzer that does not require a power supply, there is, for example, one disclosed in Japanese Patent Application Laid-Open No. 5-253571. In this electrolyzer, an optical semiconductor electrode and a counter electrode are set inside a casing as a water container, and the optical semiconductor electrode is irradiated with light including ultraviolet rays to generate an electromotive force. It is electrically differentiated into alkaline water and acidic water.
[0008]
[Patent Document 1]
JP-A-5-253571
[Problems to be solved by the invention]
However, according to the electrolyzer disclosed in JP-A-5-253571, the optical semiconductor electrode and the counter electrode are arranged via an ion-exchange membrane or the like, so that the configuration becomes complicated.
[0010]
The present invention has been made in view of such a point, and an object of the present invention is to provide a water splitting device using a solar cell with a simplified configuration.
[0011]
[Means for Solving the Problems]
In order to solve the above-mentioned problems, a water splitting device of the present invention is a water splitting device for electrolyzing water, in which an anode part and a cathode part are integrated, and a photocatalyst layer is formed on the whole or a part of the light receiving surface. The solar cell is characterized by being arranged in water filled with a transparent container.
[0012]
Further, the photocatalyst layer is made of a semiconductor material.
[0013]
Further, the photocatalytic layer is made of titanium oxide or a composite material containing the same.
[0014]
Further, the photocatalyst layer is formed on the light receiving surface of the solar cell in a small island shape of 1 micron or less.
[0015]
Further, the solar cell is a solar cell based on silicon.
[0016]
Further, the solar cell is a solar cell based on a group IIIV compound semiconductor.
[0017]
Further, the solar cell has a pn junction.
[0018]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
[0019]
(Embodiment)
The water splitting apparatus according to the present embodiment shown in FIG. 1 uses a solar cell 16 in which a p-type silicon layer 12 on the anode side and an n-type silicon layer 13 on the cathode side are combined. A titanium oxide layer 11 serving as a photocatalyst layer is formed on the surface of the layer 12 by a sol-gel method, and a platinum layer 14 is deposited on a surface of the n-type silicon layer 13. It is arranged inside. The titanium oxide layer 11 serves as an anode electrode, and the platinum layer 14 serves as a cathode electrode. The titanium oxide layer 11 is formed on the entire surface or a part of the p-type silicon layer 12. For example, the titanium oxide layer 11 is formed on the surface of the p-type silicon layer 12 in a small island shape of 1 μm or less. An end face protective film 21 was formed on end faces of the titanium oxide layer 11, the p-type silicon layer 12, the n-type silicon layer 13, and the platinum layer 14.
[0020]
In the water splitting apparatus having such a configuration, when sunlight 10 is irradiated from the outside of the container 8, electrons in the valence band are excited in the solar cell 16 to generate a pair of an electron and a hole. In the layer 13, holes move to the p-type silicon layer 12, and a voltage is generated between the n-type and the p-type 13 and 12.
[0021]
Normally, in the solar cell 16, the ultraviolet rays of the sunlight 10 recombine on the surface of the solar cell 16 and hardly contribute to the current. However, in the present water splitting apparatus, since the titanium oxide layer 11 is formed on the surface of the p-type silicon layer 12, when the titanium oxide layer 11 is irradiated with sunlight 10, the electrons of the titanium oxide emit light energy. The absorbed and activated electrons move to the solar cell 16. Thereby, the solar cell 16 can also utilize ultraviolet rays, so that a higher voltage can be generated between the n-type and the p-type 13 and 12. Accordingly, more oxygen gas 4 and hydrogen gas 5 can be generated.
[0022]
Therefore, the electrolysis of water can be performed without using a power supply for applying a voltage between the anode 11 and the cathode 11. More specifically, the present water splitting device can efficiently produce hydrogen and oxygen gas from light energy such as sunlight, and is an energy saving and environmentally friendly energy source.
[0023]
Next, as a prototype device of the present water splitting device, a platinum layer 14 was deposited to a thickness of 100 nm and irradiated with sunlight 10 having an intensity of 100 mW / cm ² from the outside of the container 8. The hydrogen gas 5 and the oxygen gas 4 could be generated without applying a bias voltage unlike the apparatus. In this prototype water splitting apparatus, 10 cc / min of hydrogen gas 5 and 5 cc of oxygen gas 4 could be produced with the titanium oxide layer 11 having an area of 100 cm ² .
[0024]
In addition, the solar cell may be not only a single crystal material of silicon but also a polycrystalline or amorphous material. When a solar cell using GaAs of a group IIIV compound semiconductor other than silicon is used, the efficiency is further improved. Of course, the same effect can be obtained even in a mixed crystal system mixed with Al or the like.
[0025]
The same effect can be obtained by using another semiconductor material such as a wide-gap oxide as the photocatalyst layer. Further, a sol-gel method is used as a method for forming a photocatalyst on a solar cell, but a method such as a CVD method or a thermal oxidation method may be used.
[0026]
【The invention's effect】
As described above, according to the present invention, the anode part and the cathode part are integrated, a photocatalyst layer is formed on the light receiving surface of the solar cell, and the photocatalyst layer is irradiated with light, so that the electron of the photocatalyst layer is irradiated. Absorbs light energy and transfers the activated electrons to the solar cell, which allows the solar cell to also utilize ultraviolet light and generate higher voltages, thus complicating the configuration. More oxygen and hydrogen can be generated without causing oxidation. Therefore, electrolysis of water can be performed without using a power supply for applying a voltage between the anode and the cathode.
[Brief description of the drawings]
FIG. 1 is a diagram showing a configuration of a water splitting device according to an embodiment of the present invention.
FIG. 2 is a diagram showing a configuration of a conventional water splitting apparatus.
FIG. 3 is a diagram showing a configuration of a conventional water splitting apparatus using a photocatalyst.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Power supply 2 Anode electrode 3 Cathode electrode 4 Oxygen gas 5 Hydrogen gas 6 Sodium sulfate aqueous solution 7 Semiconductor electrode 8 Container 9 Ultraviolet light 10 Sunlight 11 Titanium oxide layer (anode electrode)
12 p-type silicon layer 13 n-type silicon layer 14 platinum layer (cathode electrode)
16 Solar cell 21 Edge protection film

Claims (7)

In a water splitter that performs electrolysis of water,
A water splitting device comprising a solar cell having an anode part and a cathode part integrated and a photocatalyst layer formed on the entire surface or a part of a light receiving surface, and arranged in water filled in a transparent container. . The said photocatalyst layer is a semiconductor material, The water splitting apparatus of Claim 1 characterized by the above-mentioned. The water splitting apparatus according to claim 1, wherein the photocatalyst layer is made of titanium oxide or a composite material containing the same. The water splitting apparatus according to claim 1, wherein the photocatalyst layer is formed on the light receiving surface of the solar cell in a small island shape of 1 micron or less. The water splitting device according to claim 1, wherein the solar cell is a silicon-based solar cell. The water splitting device according to claim 1, wherein the solar cell is a solar cell based on a group IIIV compound semiconductor. The water splitting device according to claim 1, wherein the solar cell has a pn junction.

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