JPS626311B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a reversible light-rechargeable photochemical cell, and more particularly, it is capable of chemically storing light energy and converting it into electrical energy; The present invention relates to a photochemical cell having the property that, upon energy conversion, both of the two half cells that make up the cell return to their original state before storage.

In recent years, research into converting solar energy into electrical energy has become active.
Among these, so-called photochemical cells, which utilize photochemical reactions and extract the resulting chemical changes in the form of electrical energy, are one of the most anticipated for practical use because they can be manufactured at low cost.

In order to create a photochemical cell with good performance, the photochemical reaction used must occur endothermically with a high quantum yield (at least a few tens of percent or more), and the substances produced by the photochemical reaction must later be converted into electrical energy. It must meet requirements such as being able to be stored in a form that can be extracted as electrical energy, and being able to return to the stored state using light after extracting electrical energy through discharge (i.e., being able to be photoregenerated). . Furthermore, it is advantageous to store energy as compactly as possible, and it is desirable to store it in a solid phase.

A photochemical cell that has attracted attention in view of these requirements is the photogalvanic cell, which consists of a combination of silver chloride and a redox system, and is described in the Journal of the Electrochemical Society, Vol. 42, p. 108 (1961). There's a battery. This photochemical cell covers a metal electrode with silver chloride, irradiates the silver chloride with light in an aqueous solution containing Fe ²⁺ ions, uses the produced silver as a negative electrode, and uses a platinum electrode as a positive electrode to discharge and extract electrical energy. be. In this photochemical cell, it is possible to restore it to a storage state by repeated light irradiation after discharge, and storage is performed by silver trapped in AgCl lattice defects, making it possible to form a compact battery. However, self-discharge occurs during storage, and the quantum yield of photolysis is low (approximately 2%).
The disadvantages were that the amount of photodecomposed silver itself was small and the amount of storage (charging) was small. U.S. Patent No. 3,114,658 discloses that AgCl is used in the diaphragm separating the positive and negative electrodes to eliminate self-discharge of the photochemical cell.
However, this did not solve the problems other than self-discharge.

The applicant first immersed an optical semiconductor in an electrode solution containing metal ions, irradiated it with light to deposit metal on the semiconductor, and connected the deposited metal to a separately prepared redox half cell. He applied for a patent for the invention of an electrode-light regenerating photorechargeable photochemical cell that extracts electrical energy (Japanese Patent Application No. 1983-62939). This photochemical cell has the advantage that it has a higher quantum efficiency in storing light energy and a larger storage amount than conventional ones. However, since the oxidant in the redox half cell serving as the counter electrode is consumed during discharge, it is necessary to replenish the oxidant during repeated use. Furthermore, since the structure was such that metal was deposited on the surface of the optical semiconductor, light incidence on the optical semiconductor was blocked by the deposited metal, and there was a limit to the amount that could be stored.

Therefore, it is an object of the present invention to be able to store light energy in the form of deposited metal, to store a large amount of light energy, and to have a structure in which, upon discharging, a reaction that is completely opposite to that during storage occurs within the battery, returning the battery to its initial state. An object of the present invention is to provide a reversible light-rechargeable photochemical cell that does not require replenishment other than light energy during use.

The present invention that achieves these objects has the following configuration. That is, the present invention includes (a) a first electrode made of a photosemiconductor for charging and a second electrode for discharging, and (b) as an electrode liquid.
(b) A half-cell using a solution containing a redox system having a standard electrode potential nobler than that of a half-cell; a half-cell using a solution of metal ions that can be deposited on the surface of the metal electrode; (c) a connecting part that allows the transfer of ions between the two half-cells; and (d) charging the metal electrode. The present invention is a reversible light-rechargeable photochemical cell characterized by comprising a switch section for connecting to the first electrode at times and to the second electrode at the time of discharging.

Hereinafter, the present invention will be explained based on the drawings.
The figure shows the basic configuration of the photochemical cell of the present invention, (a)
is a half-cell constructed by immersing a first electrode 1 made of an optical semiconductor for charging and a second electrode 2 for discharging in an electrode solution 3. Lead wires 4 and 5 are electrically attached to each electrode 1 and 2, respectively, and the other end of each lead wire 4 and 5 reaches a switch portion d. The second electrode 2 in this half-cell a is placed at a position where it does not interfere with the incidence of light into the first electrode 1, or it is lifted from the electrode solution 3 during charging.

The first electrode 1 can be made by cutting out a single crystal or ribbon crystal of an optical semiconductor, by pressing and shaping semiconductor powder and then sintering it, or by dispersing optical semiconductor powder in a binder and forming a film. It is produced by various known methods, such as burning the surface of a metal plate with city gas or the like to oxidize the surface. The size and thickness of the electrode may be changed as appropriate depending on the intended use of the photochemical cell of the present invention.
In the present invention, an optical semiconductor refers to a substance that can be excited by light irradiation and generate photoelectrons, such as TiO ₂ ,
ZnO, _SnO2 , _V2O5 , _{Fe2O3 , SrTiO3} , _{KTaO3 ,}
CaTiO ₃ , CdS, SiC, GaP, GaAs, CdSe or
Examples include CdTe. Among these, n-type oxide semiconductors are particularly preferred.

On the other hand, for the second electrode 2, a material that does not or does not easily dissolve into the electrode solution 3 is selected from materials known as good electrical conductors. For example, Pt,
In addition to metals such as Au, Ag, Cu, Pb, and Sn, carbon can also be used.

The electrode solution 3 of half cell a contains a redox system having a standard electrode potential nobler than the standard electrode potential of half cell b, which will be described later. For example, Ce ³⁺ /Ce ⁴⁺ , Co ² +/
Co ³⁺ , MnO ₂ /MnO ₄ ^- (acidic), Cr ³⁺ /Cr ₂ O ^2- ₇
(acidic), H ₂ O/O ₂ (acidic), and other redox systems according to the above selection criteria. The concentrations of these redox systems may be determined arbitrarily.

The electrode solution 3 basically consists of the above-mentioned redox system and water, but preferably contains a supporting electrolyte, such as a metal salt, hydrochloric acid, or sulfuric acid, which has a standard electrode potential lower than that of half cell b, which will be described later. Strong acids such as (NH ₄ ) ₂ SO ₄ or ammonium salts are usually used at concentrations of 0.1 to 10N. Electrode solution 3
A suitable buffer may also be added therein.
For example, Clark-Labs buffer, Korthoff buffer, Walpol buffer, Märzen buffer, Matskin-Bain buffer, Michaelis buffer, or Britton-Robinson buffer can be used.

When a dye is added to the solution of half cell a, the sensitive wavelength range can be expanded to the visible region. In this case, it is desirable to bring the electrode 1 closer to the wall of the vessel on the light irradiation side to reduce light absorption by the dye in the liquid phase and to increase the amount of light that reaches the electrode. Any dye that can be used may be used as long as it is easily adsorbed to the semiconductor and the excited level of the adsorbed dye is higher (base) than the bottom level of the conduction band of the semiconductor. Although it varies depending on the type of semiconductor, for example, uranine, rhodamine B, rose bengal, erythrosin, eosin, fluorescein, auramine, crystal violet, cryptocyanin, pinacyanol, methylene blue, etc. are useful.

In FIG. 1, b is a half-cell constructed by immersing a metal electrode 6 in an electrode solution 7. A lead wire 8 is attached to the metal electrode 6, and the other end of the lead wire 8 reaches the switch portion d.

Materials for forming the metal electrode 6 include a wide variety of metals. For example, Pr, Au, Ag,
Cu, Pb, Sn, Ni, Co, In, Cd, Fe, Ga, Cr,
Examples include Zn, Mn, Zr, Ti, Al, and C. The metal electrode 6 is preferably used in a shape that allows reduction deposits of metal ions, which will be described later, to easily adhere to it (for example, a comb shape with narrow tooth spacing, a fine mesh shape, a porous shape, etc.). . Alternatively, a metal electrode may be provided at least at the bottom of the inner surface of the vessel wall containing the electrode solution 7.

The electrode solution 7 of the half cell b is formed on the metal electrode 6 when the first electrode 1 of the half cell a and the metal electrode 6 of the half cell b are connected at the switch part d and the first electrode 1 is irradiated with light. Contains metal ions that can be deposited in Various metal ions are selected depending on the type of optical semiconductor constituting the first electrode 1.
Examples of metal ions preferable for use in the optical semiconductors specifically exemplified above include Ag ⁺ ,
Examples include Fe ³⁺ , Fe ²⁺ , Cu ²⁺ , Cu ⁺ , Hg ⁺ , and Au ⁺ . These metal ions are usually introduced into the electrode solution 7 by adding their salts. Therefore, it is desirable that the metal salt used has high solubility. In general
A salt having a solubility in water at 20° C. of 0.03 mol/min or more is used. For example, AgNO ₃ , AgClO ₄ ,
FeCl ₃ , Fe(NO ₃ ) ₂ , Cu(ClO ₃ ) ₂ , Hg ₂ (ClO ₄ ) ₂
There are metal salts such as Although the amount of metal salt added to the electrode solution 7 has no critical meaning for the present invention,
Usually set in the range of 0.03N to 3N.

The electrode solution 7 is basically made from the above-mentioned metal ions and water, but it is preferable to add the above-mentioned supporting electrolyte, and the above-mentioned buffer solution may be added as appropriate.

In FIG. 1, c is a connection part that enables the movement of ions between half-cells a and b, and usually uses a salt bridge, unglazed chip, glass frit, ion exchange membrane, etc. Ru.

The switch part d of the photochemical cell of the present invention constructed in this way is connected so that the first electrode 1 of the half cell a and the metal electrode 6 of the half cell b are connected, and the switch part d is provided on the wall of the half cell a. Light corresponding to the excitation wavelength of the optical semiconductor constituting the first electrode 1 is transmitted through the window 9 to the first electrode 1.
When the electrode 1 is irradiated, holes and electrons are generated on the surface of the first electrode. The generated holes move to the interface due to the bending of the band formed at the interface of the first electrode 1 when the first electrode 1 and the electrode solution 3 come into contact, and the redox system contained in the electrode solution 3 moves to the interface. oxidizes the reduced form of On the other hand, the electrons move to the bulk, travel through the lead wires 4 to 8, reach the metal electrode 6, reduce the metal ions contained in the electrode solution 7, and zero-valent metal is deposited on the surface of the metal electrode 6. Precipitate. This process can be expressed as follows.

Half-cell a: Red.+h ⁺ →Oxd. () Half-cell b: M ⁿ⁺ +ne ^- →M ⁰ () (Here, Red. and Oxd. respectively represent the reduced form and oxidized form of the redox system, and M ⁽ⁿ⁺ represents a positive n-valent metal ion, and ^M0 represents a deposited metal.) This process is the charging step in the present invention.

Next, when switch part d is switched to connect the second electrode 2 of half cell a and the metal electrode 6 of half cell b, the reverse reaction of the above reaction formulas () and () occurs in each half cell, An electromotive force is generated between the two poles and current flows. This process is expressed as follows.

Half-cell a: Oxd.+e ^- →Red. () Half-cell b: M ⁰ →M ⁿ⁺ +ne ^- () This process is the discharge step in the present invention.
As is clear from the reaction equations () and (), in the discharging step, each half-cell system reversibly returns to the state before charging.

As described above, according to the photochemical cell of the present invention, since light energy is stored in a solid phase called precipitated metal, the entire device during storage can be made compact, and also because it is deposited on the metal electrode that is the opposite electrode of the photosemiconductor electrode. , unlike when a structure is adopted in which the optical semiconductor is directly deposited, the absorption of light by the optical semiconductor is not hindered by the deposited metal, so there is no limit to the amount of deposition, and the amount of optical energy stored can be increased. . Furthermore, according to the present invention, each half-cell system that changes during charging returns to the state before charging by discharging, so that only light energy is consumed, and the electrode solution is replenished during battery operation. It is not necessary and is very economical.

The present invention will be explained in more detail with reference to Examples below.

Example In FIG. 1, a metal titanium plate is baked with city gas as the first electrode 1 of half cell a (1300 to 1350
℃, 5 minutes) with an oxide film formed on the surface, and platinum black was used as the second electrode 2 of half cell a.
0.05N Ce ₂ (SO ₄ ) ₃ as the electrode solution 3 of half cell a.
Using a solution containing 0.1N Ce(SO ₄ ) ₂ and 1N HNO ₃ ,
An anion exchange membrane (ACT-45T,
(manufactured by Tokuyama Soda Co., Ltd.), platinum black was used as the electrode 6 of half cell b, and 0.1N was used as the electrode liquid of half cell b.
Using a solution containing AgNO ₃ and 1N KNO ₃ , the first electrode 1 of half cell a and the electrode 6 of half cell b were connected as shown in Figure 1 to assemble the battery shown below (however, The connection of one electrode 1 was made at the metal titanium part).

Ti―TiO ₂ ｜Ce ₂ (SO ₄ ) ₃ (0.05N), Ce (SO ₄ ) ₂
(0.1N), HNO ₃ (1N) | Anion exchange membrane |
AgNO ₃ (0.1N), KNO ₃ (1N) | Pt-Pt A Hg lamp is used for the first electrode 1 of this battery.
The photovoltaic force and photocurrent when irradiated with 365 nm monochromatic light were measured, and the relationship between them is shown in Figure 2. As is clear from Figure 2, the open circuit photovoltaic force is 370mV,
The short circuit photocurrent was 0.2mA.

Separately, when a 1Ω resistor is placed between the two electrodes of the above battery and irradiated with 365 nm monochromatic light for 198 minutes using an Hg lamp, metallic silver is deposited on the electrode 6 of half cell b, and the light energy is transferred to the form of metallic silver. storage (charging) in
I found out that it would happen. After 198 minutes of light irradiation,
Turn switch d to select electrode 6 (Pt-Ag) of half-cell b.
It was found that when the electrode) was connected to the second electrode of half-cell a, a discharge occurred, and the photo-charged energy could be taken out as a current. The relationship between the discharge current and the output voltage at this time was measured by changing the resistance 10, and is shown as a curve 31 in FIG. For reference, FIG. 3 shows a curve 32 showing the relationship between discharge current and output voltage when metallic silver having the same area as the Pt--Ag electrode is used as the electrode of half-cell b. curve 3
2, the open circuit output voltage of this reference cell is 780 mV, but the open circuit output voltage of the photochemical cell of the present invention is 770 mV, and the photocharged Pt
- It was found that the Ag electrode has almost the same output as the metallic silver electrode.

Moreover, the second electrode of the photochemical cell of the present invention and
A load of 1KΩ was applied between the Pt-Ag electrodes to cause discharge, and the time dependence of the discharge current was investigated, as shown in Figure 4. As is clear from FIG. 4, the discharge current is kept constant for the first 35 minutes, then decreases with time, and the discharge almost ends after about 60 minutes.

Turn the switch d of the photochemical cell of the present invention, which has finished discharging in this way, to connect the electrode 6 of the half cell b to the first electrode of the half cell a, and repeat the process as described above.
The relationship between the discharge current and the output voltage is shown as a curve 33 in FIG. 3 when the electrode 6 was photoregenerated by exposure to a Hg lamp for 198 minutes and then discharged.

The characteristics shown by curve 33 are very similar to curve 31, and it is clear that the photochemical cell of the present invention can perform photocharging and discharging reversibly.

[Brief explanation of the drawing]

FIG. 1 shows the basic configuration of the photochemical cell of the present invention. In the figure, 1 is a first electrode for charging, 2 is a second electrode for discharging, 3 is an electrode solution containing a redox system, and a is a half cell made of these. Further, 6 indicates a metal electrode, 7 indicates an electrode solution containing metal ions,
b shows a half cell consisting of these. Further, 4, 5 and 8 indicate lead wires, 9 indicates a window provided in the wall of the half cell a, and 10 indicates a load. c designates a connection part that allows the movement of ions between half cells a and b, and d designates a switch part. FIG. 2 is a graph showing the relationship between the photovoltaic force and photocurrent of one embodiment of the photochemical cell of the present invention, and FIG. 3 is a graph showing the relationship between the output voltage and discharge current of the photochemical cell. , FIG. 4 is a graph showing the time dependence of the discharge current of the photochemical cell.

Claims (1)

[Claims] 1a A redox compound comprising a first electrode made of a photosemiconductor for charging and a second electrode for discharging, and having a standard electrode potential more noble than the half cell of (b) as an electrode liquid. (b) a half-cell using a solution containing a metal electrode; (b) a solution of metal ions that is equipped with a metal electrode and that can be deposited on the surface of the metal electrode by excited electrons generated when the first electrode is irradiated with light; (c) a connection allowing the transfer of ions between said half-cells; and (d) connecting said metal electrode to said first electrode during charging and to said second electrode during discharging. A reversible light-rechargeable photochemical cell characterized by comprising a switch section for connection to an electrode.