JPS6360888B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a display element that utilizes an electrochemical redox reaction. This invention relates to an electrochromic display device (ECD) that has improved performance and can be easily enlarged.

In recent years, there has been active research into so-called electrochromic display elements in which light absorption characteristics are reversibly changed by electrochemical redox reactions and applied as display elements. Compared to other display elements such as liquid crystal display elements and light emitting diodes (LEDs), the two most important features of this electrochromic display element are (1) wide viewing angle, and (2) memory properties. Research is progressing from this perspective, and two types of electrochromic devices have been known so far.

The first method is to perform an electrochemical redox reaction on an electrochromic substance, such as a viologen derivative, dissolved in a solution, and to
Display is performed by depositing a colored reaction product on an electrode. In this case, decoloring is performed by applying a reverse voltage to return the colored substance deposited on the electrode surface to the original substance and redissolving it in the solution.

As a solution used for this first type of electrochromic display element, an aqueous potassium bromide solution in which an electrochromic substance such as heptyl viologen bromide is dissolved is known.

The second method uses a solution-insoluble transition metal oxide film such as tungsten oxide (WO ₃ ) or molybdenum oxide (MoO ₃ ) as the electrochromic material. This electrochromic display element has an electrochromic thin film on a transparent electrode provided on a transparent substrate, and utilizes changes in light absorption characteristics due to electrochemical redox reactions of this thin film compound. In this case, the color-developing substance is always fixed on the electrode surface.

These two types of electrochromic display elements have different structures, but the basic structure of an electrochromic display element is to use a display material that can be electrochemically reversibly colored and erased, and an electrolyte, and this electrolyte has a display electrode. A voltage is applied between the display electrode and the counter electrode, and a pattern is displayed on the display electrode through an electrochemical oxidation or reduction reaction at the display electrode, and a reverse reaction is caused by applying a reverse voltage to cause the display to disappear. It is something that does color. As is clear from the principle of this electrochromic display element, in an electrochromic display element, the amount of electricity that flows in the display electrode for coloring and erasing must also necessarily flow in the counter electrode. For this reason, in electrochromic display elements, the characteristics of the electrochromic material as well as the characteristics of the counter electrode are extremely important.

Conventionally, the first type of electrochromic display element uses a metal electrode as the counter electrode and utilizes the oxidation reaction of halogen ions on the metal electrode as a reversible electrochemical reaction on the counter electrode (particularly (Japanese Patent Application Laid-open No. 71380/1983), one that utilizes the reversible redox reaction of a silver/silver chloride electrode (Japanese Patent Application Laid-open No. 1983-71380),
69127) is known. These counter electrodes had the drawback that the reaction product on the electrode formed a complex with the dipyridinium compound, and the reverse reaction rate of this product was slow or lacked reversibility. In addition, as another counter electrode of the first type of electrochromic display element,
There has also been a proposal to utilize the reversible reaction of the redox pair Fe ⁽ 〓 ⁾ /Fe ⁽ 〓 ⁾ as a reaction at the counter electrode (Japanese Patent Laid-Open Publication No. 1562/1983). The reversible redox reaction of Fe ⁽ 〓 ⁾ /Fe ⁽ 〓 ⁾ occurs only when the electrolyte pH is about 2.5 or less;
When a dipyridinium compound other than dicyanophenyl dipyridinium chloride is used as an electrochromic substance, hydrogen is generated by decomposition of water before the potential at which the coloring reaction occurs, resulting in major drawbacks such as damage to the device.

Further, as the counter electrode of the second type of electrochromic display element, a type using a reduced form of WO ₃ on a conductor (Japanese Patent Application Laid-open No. 50-50893),
Or platinum (JP 53-33093), carbon (JP 52-73051), gold (JP 52-58941)
There are known methods using only metal electrodes such as Nickel (Japanese Unexamined Patent Publication No. 56-92523).
However, when a reduced form of WO ₃ is used, the stability of the reduced form is poor, so the storage performance of the electrochromic display element is poor, and when only metal electrodes such as platinum, carbon, gold, or nickel are used. Since there is no reversible electrochemical redox substance, a coloring/discoloring current flows through the display electrode, and an electrochemical decomposition reaction of the solvent or supporting electrolyte occurs on the counter electrode, significantly shortening the service life.

In addition, as another counter electrode that can be commonly used in both types of electrochromic display elements, a hot-press molded body of reversible redox material powder, carbon, and binder powder has been disclosed (Japanese Patent Laid-Open No. 55-69127). Public bulletin). However, with this configuration, the electrical conductivity of the counter electrode deteriorates, and the redox reaction of the reversible redox material becomes difficult to occur, making it impossible to function as a counter electrode. Therefore, the binder accounts for about 1/5 to 1/10 of the total weight. In addition, reducing the amount of binder can increase the true surface area of the counter electrode, thus suppressing the potential change of the counter electrode during the color development/decolorization reaction, and making the counter electrode suitable as a reference electrode. It is preferable because it can provide functions. However, if the amount of binder is small, the counter electrode formed under pressure by hot pressing becomes extremely brittle in terms of strength. For this reason, after the cell of an electrochromic display element is constructed, carbon falls off and becomes liberated due to the shock received when injecting the electrolyte or the external shock received during actual use, contaminating the display electrode and shortening its lifespan. Furthermore, it is difficult to manufacture a large-sized counter electrode, making it impossible to produce a large-sized electrochromic display element. .

SUMMARY OF THE INVENTION An object of the present invention is to provide an electrochromic display element that is stable, has improved shock resistance, and can be easily enlarged, eliminating the drawbacks associated with conventional counter electrodes as described above.

Therefore, the electrochromic display element of the present invention is characterized in that a conductive material on which a Prussian blue film is formed as a reversible redox material is used as a counter electrode.

The counter electrode used in the present invention has a Prussian blue film formed on the outer surface of a conductive material. Prussian blue is an extremely stable compound that has been used in large quantities as a blue pigment in inks and paints, and KFe ⁽ 〓 ⁾ [Fe ⁽ 〓 ⁾ (CN) ₆ ] or Fe ₄ ⁽ 〓 ⁾ [Fe ⁽ 〓 ⁾ (CN) ₆ ] It is an iron cyano complex represented by the chemical formula ₃ , and is generally potassium ferricyanide, potassium ferrocyanide, or potassium ferrocyanide. Immediately as a precipitate by mixing the derivative with a solution containing other divalent or trivalent iron ions,
Alternatively, it is obtained by post-processing the precipitate produced by mixing, and it is usually difficult to obtain a thin film having a shape that covers the counter electrode as shown in FIG. 3a. Further, although Prussian blue decomposes in alkalinity, it is insoluble in other water or organic solvents, so it is impossible to form a Prussian blue thin film on the electrode surface by a spinner method, a dip coating method, or the like. Furthermore, since Prussian blue decomposes even at high temperatures, a Prussian blue thin film cannot be formed on the electrode surface by vapor deposition or the like. For this reason, it has been extremely difficult to use Prussian blue as a film in the prior art. However, when a mixed solution of potassium ferricyanide and a trivalent iron salt such as ferric chloride or ferric sulfate is used,
It has been found that a Prussian blue film can be easily formed on an electrode display by the following method.

(1) When an electrode is placed in the above mixed solution and the mixed solution is reduced, a Prussian blue film is formed on the electrode surface.

(2) Ionic species or complexes in the mixed solution are reduced.Metals with an anode dissolution potential less than about 0.6 (relative to SCE), such as Ni, Fe, and Cu, become electroless when immersed in the mixed solution. However, the ionic species in the solution are reduced and a Prussian blue film is deposited on the metal electrode.

(3) When a thin layer of the above mixed solution is applied to the electrode display by a spinner method or dip coating method and dried, both compounds in the solution react and a Prussian blue derivative is formed on the electrode surface.

In these methods, sodium ferricyanide or the like may be used instead of potassium ferricyanide.

Also in the present invention, a Prussian blue film can be easily formed on the outer surface of a strong conductive material used as a counter electrode using the above method.

Further, in the present invention, as the conductive material
Transparent electrodes such as SnO ₂ , In ₂ O ₃ , platinum, gold, nickel, iron, stainless steel, aluminum, tantalum,
It is possible to use metal plates such as titanium, nets, sintered bodies, vapor deposited films, foams, etc., or conductive carbon nonwoven fabrics, fibers, etc. Among them, metal nets, sintered bodies, foams, carbon non-woven fabrics, and fibers can increase the electrode surface area per apparent area, so counter electrodes made of these materials are better than other conductive materials. Compared to the case where a material is used, it is preferable to pass the same amount of charge because it is possible to suppress the change in potential of the opposing electrode to a further small value, and it can also function as a reference electrode.

The counter electrode of the present invention, which is made of a conductive material with Prussian blue fixed on its surface and constructed by the above-mentioned method, does not have the brittleness seen in graphite molded bodies, so it can be destroyed by electrolyte injection or shock during use. Never. Further, according to the above-described method, it is clear that by using a large-area conductive material, a large-area counter electrode can be easily created.

However, when the Prussian blue film formed on the surface of the conductive material becomes thick, it may fall from the electrode surface, so the thickness needs to be 10 μm or less.

In addition, this Prussian blue, as mentioned above,
It is traditionally used as a blue pigment,
Its stability is obvious. Furthermore, the reduced form of Prussian blue is extremely stable in neutral to acidic aqueous solutions and organic solvents such as propylene carbonate, acetonitrile, and N,N-dimethylformamide.

Therefore, the electrochromic display element of the present invention utilizes Prussian blue and its reduced form as a reversible redox material for the counter electrode, and uses a counter electrode made of a conductive material coated with a Prussian blue film, thereby achieving stability. The shock resistance has been improved, and it has become possible to increase the size, so it has extremely high practical value.

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

FIG. 1 and FIG. 2 each show an electrochromic display element according to an example of the present invention.

In FIG. 1, reference numeral 1 denotes a transparent substrate made of glass or the like, on which a transparent conductive electrode 2 made of SnO ₂ or the like is provided. This transparent electrode generally has a low resistance of 50 [Ω/□] or less and a high light transmittance of 80 [%] or more. An insulating film 3 made of SiO, SiO ₂ , MgF ₂ , polyethylene, etc. is provided in addition to the pattern portion and the electrode lead connection portion on the transparent electrode. On the other hand, the rear board 4
A counter electrode 5 is provided on top, and the display electrode (transparent electrode pattern portion) and the counter electrode are held in parallel with each other via a spacer 7 as shown in the figure. Display electrode and counter electrode 5
A light scattering plate 6 made of porous alumina etc.
will be established. Next, an electrolyte solution 9 in which an electrochromic substance is dissolved is injected into the cell, and the injection port is sealed with an epoxy resin 8.

In the electrochromic display element constructed in this way, when an appropriate voltage is applied between the display electrode and the counter electrode, the electrochromic substance in the electrolyte solution develops a color, and conversely, when a reverse voltage is applied between the two electrodes, a display is displayed. The color of the image disappears and it returns to its transparent state.

FIG. 2 shows another example of an electrochromic display element which differs from the electrochromic display element of FIG. 1 in that a display electrode made of an electrochromic material is provided on a transparent substrate.

Example 1 A nickel (Ni) plate with a length of 50 mm and a width of 30 mm was used as the conductive material 12 for the counter electrode, and ferric chloride and potassium ferricyanide were each 0.05 [M/
] Immerse in the dissolved solution for 15 seconds and apply 350 Å to the surface.
Form a Prussian blue film 10 with a thickness of
A counter electrode was prepared by electrolytic reduction in a 1 [M/] aqueous solution. FIGS. 3a and 3b show a cross-sectional view and a plan view of the counter electrode thus prepared. In addition, numeral 11 in the drawing is a lead wire for a counter electrode.

An electrochromic display element shown in FIG. 1 was produced using the counter electrode. In addition, electrolyte solution 9
As, N,N'-diheptyl-4,4'-dipyridinium chloride is 0.05 [M/], KCl is 1
[M/] A dissolved aqueous solution was used. When this electrolyte was injected into the display cell, the counter electrode did not break. −0.6 to the display electrode and to the counter electrode
When a voltage of [V] was applied, the display electrode developed color, and when a positive voltage was applied, the color rapidly disappeared. The voltage and application time for color development were -0.6 [V] (0.5 seconds),
A repeated life test was performed using an erasing voltage and application time of +0.3V [1 second], and the result was 5×10 ⁵
No change was observed in the display and erasing characteristics even after the test was repeated several times. Furthermore, this display element was actually dropped to examine its impact resistance, and although the glass of the transparent substrate was broken, no damage to the counter electrode was observed.

Example 2 As the conductive material for the counter electrode used in Example 1, carbon fiber (manufactured by Nippon Carbon) was used instead of the nickel plate of Example 1. Its surface area was 23 times the apparent surface area. 0.01 [M/] each
carbon fibers are soaked in a solution containing ferric sulfate and potassium ferricyanide, and
Electrolytic reduction was performed at a current density of 2.3 mA/cm ² for 40 seconds to form a Prussian blue thin film on the surface, which was used as a counter electrode. The rest of the structure was the same as in Example 1, and an electrochromic display element was produced. The characteristics of this display element were examined under the same conditions as in Example 1, and as a result, it showed almost the same characteristics as in Example 1.

Example 3 A carbon nonwoven fabric (manufactured by Nippon Carbon) was used as the conductive material for the counter electrode. The surface area of this nonwoven fabric was about 4.5 times the apparent surface area. This nonwoven fabric was immersed in an aqueous solution containing 0.01 [M/] of ferric chloride and potassium ferricyanide,
Electrolysis was performed as a cathode at 0.9 mA/cm ² (apparent surface area) for 30 seconds to form a Prussian blue film on the surface. The counter electrode prepared in this way and lithium perchlorate as an electrolyte were
], using a solution dissolved in propylene carbonate, an electrochromic display element as shown in FIG. 2 was prepared having a thin film 13 of WO ₃ as an electrochromic material on a transparent electrode 2 as a display electrode.

The electrochromic display element thus constructed has a display voltage of -1.0V (1 second) and an erase voltage of +
As a result of a repeated life test by applying a 1.0 V (1 second) square wave to the display electrode, no change was observed in the display characteristics even after 5×10 ⁵ cycles or more. Further, the counter electrode did not break due to shocks received during electrolyte injection and use.

In the examples, N, N′-
Although only electrochromic display elements using diheptyl-4,4'-dipyridinium chloride and WO ₃ thin film are shown, other display materials,
For example, the present invention is applicable to electrochromic display elements using a series of electrochromic substances such as di-p-cyanophenyl-4,4'-dipyridinium chloride, molybdenum oxide, and acridine derivatives. The principle of the element is also self-evident.

[Brief explanation of drawings]

FIGS. 1 and 2 are cross-sectional views of an electrochromic display element according to an example of the present invention, and FIG.
is a cross-sectional view of the counter electrode used in the present invention;
Figure b is a plan view of the counter electrode of Figure 3a. 1... Transparent electrode, 2... Conductive transparent electrode, 3...
... Insulating film, 4 ... Back substrate, 5 ... Counter electrode, 6 ... Light scattering plate, 7 ... Spacer, 8 ... Sealing resin, 9 ... Electrolyte solution, 10 ... Prussian blue film, 11...Lead wire for counter electrode, 1
2... Conductive material for counter electrode, 13... WO ₃ thin film.

Claims (1)

[Scope of Claims] 1. An electrochromic display element using an electrochemical redox reaction, characterized in that a conductive material on which a Prussian blue film is formed is used as a counter electrode. 2. The electrochromic display element according to claim 1, wherein the Prussian blue film has a thickness of 10 μm or less.