JPS6124694B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an improvement in an image display element that performs decoloring and coloring by electrochemical redox.

Conventionally, metal oxides such as tungsten trioxide WO ₃ , molybdenum trioxide MoO ₃ , and titanium dioxide TiO ₂ that can be decolorized and colored by electrochemical redox are deposited on a transparent electrode of tin oxide or indium oxide.
So-called electrochromic image display elements are known in which an inert metal such as platinum, gold, or carbon or the like is used as a counter electrode.

As the electrolyte for this type of element, a sulfuric acid aqueous solution or a sulfuric acid glycerin solution containing sulfuric acid as a main component is generally used because the applied voltage is relatively low and the response speed is fast.

In the above element, when WO ₃ is used as a metal oxide capable of decoloring and developing color, a blue color develops when the transparent electrode is set at a negative potential (reduction) with respect to the counter electrode.
Then, by reversing the polarity of the applied voltage, the WO ₃ film returns to its original state. Although the mechanism of color development and decolorization has not yet been fully elucidated, it is thought to be due to the following electrochemical redox reaction that occurs with an aqueous sulfuric acid solution.

That is, the color development is due to the formation of tungsten bronze due to the injection of H ⁺ ions from the sulfuric acid solution into WO ₃ and the injection of electrons from the electrodes. According to this mechanism, oxygen abstraction requires proton donation from sulfuric acid, and the electrolyte must generally be a proton donor, ie, an acid.

The present invention provides, as a proton conductive solid electrolyte,
Relatively low resistance value (10 ^-2 ~ 10 ^-3 Ω ^-1・Cm ^-1 20
By using a liquid electrolyte in which compounds collectively known as heteropolyacids, which are known to have a The aim is to improve lifespan.

As a heteropolyacid, phosphomolybdic acid _H3
[PMo ₁₂ O ₄₀ ]・30H ₂ O, phosphotungstic acid H ₃
[PW ₁₂ O ₄₀ ]・29H ₂ O is common, and the giant anions are [PMo ₁₂ O ₄₀ ] ^3- and [PW ₁₂ O ₄₀ ] ^3- , respectively.
It is an ionic crystal composed of a water molecule cluster that incorporates H ⁺ ions into its center through hydrogen bonds, and H ⁺ ions move within this water molecule cluster.

In the above example, the heteroatom P exists at the center of the giant anion, but other possible central heteroatoms include B, Al, Si, S,
There are various metals such as As, Cr, Ce, Bi, and Ti, and metals that have the ability to create heteropolyacids include Mo,
In addition to W, V, Nb, and Ta are also known. The ratio of the central heteroatom to the adjacent metal atom in the giant anion is 1:11, 1:10, 1: in addition to the 1:12 in the above example.
9, 1:6, 2:18, 2:17, 2:13, 2:5,
4:12 is known. Further, the amount of water molecules in the crystal varies depending on the crystal production conditions, storage conditions, etc.

As mentioned above, various heteropolyacids are known, but from the viewpoint of ease of production and stability, the central heteroatom is B, P, Si, Ge, and the ratio of the central heteroatom to the adjacent metal atom is 1: 12 is most preferred.

Hereinafter, the present invention will be described using WO ₃ as a color-decoloring substance and silicotungstic acid H ₄ [SiW ₁₂ O ₄₀ ] as an electrolyte.
This will be explained by an example using 30H ₂ O dissolved in γ-butyrolactone or 1,2-dimethoxyethane.

Figure 1 shows an example of the configuration of an image display element.
is a transparent electrode in which a tin oxide SnO ₂ film 3 is attached to a glass substrate 2, and a 1 μm thick circular slightly bluish transparent film 4 is attached to its surface by electron beam evaporation of WO ₃ . ing. 5 is a counter electrode made of a platinum disk, and 6 is an electrolyte. Reference numeral 7 denotes a ring-shaped spacer made of an insulating material, which maintains the distance between both electrodes at 1 mm. The size of the working surface of both electrodes is circular with a diameter of 10 mm.

Next, in the above configuration, element A uses γ-butyrolatone in which tungstic acid is dissolved in the electrolyte at a rate of 4 g/10 c.c.; - Comparison results of characteristics of element B using 2-dimethoxyethane and element C using 7NH ₂ SO ₄ are shown.

To color WO ₃ , apply a constant voltage rectangular wave (1.5V, 1 second) so that the transparent electrode side is negative, and to decolor, apply the same rectangular wave with the polarity reversed. . FIGS. 2, 3, and 4 show changes over time in the current flowing through the elements A, B, and C, respectively, when coloring and decoloring were performed under the above conditions at 20°C.

In an electrochromic device, the amount of electricity that flows and the coloring density are generally proportional, so the more current that flows in the shortest possible time, the more intense coloring will occur in a shorter time. In other words, the response speed is fast. The same thing can be said in the case of decolorization. Comparing FIGS. 2 to 4, it can be seen that the response characteristics of elements A and B are improved compared to element C. It is interesting to note that in an aqueous solution of silicotungstic acid (concentration 4 g/10 c.c.), under the same conditions with the same structure,
No color development of WO ₃ was observed. more voltage
When the voltage was set to 2V, water electrolysis was clearly observed, and the generation of hydrogen gas and oxygen gas was observed. On the other hand, in the case of elements A and B, the solvent did not decompose even when the voltage was increased to 2.0 V, and the response speed for coloring and decoloring could be further increased. In the case of element C
At 2.0V, decomposition of the solvent water still occurred.

As mentioned above, when a non-aqueous solvent is used, the response speed can be further increased by further increasing the driving voltage. Furthermore, compared to when a heteropolyacid is used alone, that is, as a solid electrolyte, by dissolving it in a non-aqueous solvent, it has the advantage that the operating characteristics of an electrochromic image display element can be particularly improved at low temperatures.

Figure 5 shows a device at -20°C using pellets with a diameter of 10 mm and a thickness of 1 mm formed by molding solid tungstic acid crystals, which are one of the heteropolyacids, at a pressure of 1 ton/cm ² as the electrolyte. Indicates response characteristics. Further, FIGS. 6 to 8 respectively show the elements A, B,
The response characteristics of C at -20°C are shown. As can be seen from these figures, when silicotungstic acid is used in solid form as a solid electrolyte, the characteristics deteriorate significantly at -20°C. Element C using an aqueous electrolyte also suffers from significant deterioration. On the other hand, element A,
It can be seen that B has improved low temperature response characteristics compared to the solid electrolyte type and the sulfuric acid aqueous solution type.

Note that when comparing elements A and B, element B has better low-temperature characteristics. This is γ-butyrolactone,
Freezing point of 1,2-dimethoxyethane (respectively -
43.5℃, -71℃). In other words, this is thought to be due to the fact that a solvent with a lower freezing point increases less in viscosity at low temperatures and is less likely to impede the movement of H ⁺ ions.

Next, it seems that the same effect can be produced by dissolving sulfuric acid in the above-mentioned non-aqueous solvent, but in this case, the reactivity of sulfuric acid with the solvent is large and it cannot be used as a practical device. On the other hand, heteropolyacids are considered to be relatively inert to various solvents and have the ability to sufficiently liberate H ⁺ ions in the solvent. The results of investigating the solubility of silicotungstic acid in various non-aqueous solvents are shown below.

(1) All showed a solubility of 8 g/10 c.c. or higher in ethers such as tetrahydrofuran, dioxane, diethyl ether, 1,2-dimethoxyethane, diethylene glycol, and dimethyl ether.

(2) All esters such as γ-butyrolactone, propylene carbonate, ethylene carbonate, methyl formate, methyl acetate, and ethyl acetate showed a solubility of 7 g/10 c.c. or more.

(3) It showed a solubility of 5 g/10 c.c. or more in nitriles such as acetonitrile, propionitrile, valeronitrile, and isobutyrone nitrile.

(4) It showed a solubility of 8 g/10 c.c. or higher in amides such as dimethylformamide and dimethylacetamide.

(5) It showed a solubility of 7 g/10 c.c. or more in alcohols such as methanol, ethanol, and isopropanol.

(6) 0.1 for hydrocarbons such as cyclopentane, cyclohexane, hexane, heptane, etc.
g/10 c.c. could not be dissolved.

(7) It showed a solubility of 6 g/10 c.c. or more in sulfur-containing solvents such as sulfolane, dimethyl sulfate, and dimethyl sulfite.

From the above, suitable electrolyte solvents for silicotungstic acid include oxygen atoms, nitrogen atoms,
It can be said that it contains sulfur atoms on its constituent atoms. The hydrocarbons of No. 6 containing no hetyl atoms hardly dissolved tungstic silicoic acid. Similar trends were observed in the dissolving ability of these solvents for other heteropolyacids such as phosphomolybdic acid and phosphotungstic acid.

As mentioned above, there are many types of heteropolyacids, and they are not limited to the above examples, and it is also possible to mix two or more types of heteropolyacids or to mix two or more types of solvents. . In addition to WO ₃ , MoO ₃ ,
Metal oxides such as TiO ₂ or organic compounds that change color upon reaction with hydrogen ions, such as PH indicators, can also be used. In addition to platinum, palladium, gold, stainless steel, titanium, tantalum, carbon, etc. can be used as the counter electrode.

[Brief explanation of the drawing]

FIG. 1 is a vertical cross-sectional view showing an example of the structure of an image display element of the present invention, and FIGS. 2 to 4 are views of elements using various electrolytes.
Figures 5 to 8 show the response characteristics at 20°C.
The response characteristics at °C are shown. 1... Electrode, 4... Film of material capable of decoloring and developing color,
5... Counter electrode, 6... Electrolyte.

Claims (1)

[Claims] 1 A proton conductive device comprising an electrode having a substance capable of decoloring and developing color by electrochemical redox, a counter electrode, and an electrolyte interposed between both electrodes, the electrolyte being collectively referred to as at least one heteropolyacid. An image display element comprising a non-aqueous solvent in which a compound is dissolved.