DE2711984A1 - ELECTROCHEMICAL DISPLAY DEVICES AND METHODS - Google Patents

Description

T.EDTKE - BüHLING - KlNNE

Dipl.-Chem. Bühling Dipl.-Ing. Chins

_{97 1} IQO / Dipl.-Ing. Group

* · '' ^{| GOS} Bavariaring 4, P.O. Box 20 24

8000 Munich 2

7. Tel .: (0 89) 53 96 53-56

'^' Telex: 5 24 845tipat

cable. Germaniapatent Munich

March 18, 1977 B 8041 / case Q.28627

IMPERIAL CHEMICAL INDUSTRIES LIMITED London, United Kingdom

Electrochemical display devices and methods

The invention relates to electrochemical devices with working and counter electrodes in contact with an electrolyte that contains an organic material capable of changing color.

Electrochemical devices and in particular display devices based on electrochemical ones Phenomena are known and include a working electrode, an electrolyte medium with an organic active material »whose oxidation state is reversibly changed by passing an electric current through it can be, resulting in a detectable or observable change in the vicinity of the working electrode and a counter electrode, which is also in contact with the active material. For display purposes, the Working electrode radiation transmission or reflection

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properties and radiation is usually visible radiation and the counter electrode is related to the working electrode is arranged in such a way that changes occurring at the working electrode are apparent to an observer are not covered by reactions occurring at the counter electrode. The electrodes and the active one Material are contained in a suitable housing which has means (such as a clear window) to through which the working electrode can be observed.

Such display devices are for example in GB-PS 1,314,049 (corresponding to US-PS 3,712,709), 1,302,000 and 1,407,133 (corresponding to U.S. Patent 3,930,717) and U.S. Patent 3,806,229.

The active materials in such electrochemical devices are able to accept electrons or to give off with conversion to radical ions with a high extinction value, usually in the visible part of the spectrum. In general, active materials which are in an oxidation state are preferred for display devices show little or no color, and even if only a small current is passed through, with a suitable one EMF change into a colored oxidation state, so that a high contrast can be achieved. It is also It is necessary that the active material is relatively at least in the oxidation state that offers the display effect is immobile in the medium and for visual display purposes in appearance in the various reduced and differentiated from oxidized states. The relative immobility is achieved when the active material is in the suitable oxidation state is relatively insoluble and is deposited, for example, on an electrode surface. Such active materials are known and are described, for example, in the above-mentioned patents

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the content of which is specifically referred to here.

It is often preferred that the electrolyte medium also have an auxiliary redox system, in particular one capable of undergoing oxidation or reduction at a potential that is lower than the one in which the active material undergoes the same change and that after oxidation or reduction at a potential in close proximity to its standard reduction or oxidation potential can be brought back to its original or basic state and that in any (reduced or oxidized) state is able to interact with the active material (in the colored state) to produce a alternative body to respond. The use of such auxiliary redox systems has also already been described.

The apparatus also optionally includes electrical connection and control means with which selected Electrodes can be activated (i.e. powered) or deactivated as required.

According to the invention, the operation of electrochemical and in particular electrochromic devices will now be carried out improved that contain an active material.

It is believed that, for example, in a display device as described above, using an aqueous electrolyte medium with an active material such as CPQ sulfate (ie CPQ ⁺ " ¹ " and SO ^) and as an auxiliary redox system such as ferric ammonium sulfate (ie Fe ⁺⁺ , Fe ⁺⁺⁺ , NH ^ ⁺ and SO ^), the ions are evenly distributed over the medium and the electrodes are free of any deposits when the device is freshly manufactured

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or has not been in operation for a while. The appearance of the working electrode can now be determined by the passage of a special negative current for a special period of time (for the sake of simplicity referred to as "color pulse") between working and counter electrode can be changed by the electrolyte medium.

Two reactions then occur on or near the surface of the working electrode, namely: 10

Fe ⁺⁺⁺ L ^ Fe ⁺⁺ (1)

CPQ ^{++ +} e CPQ ⁺ (2)

Since ferric ions have a higher electron affinity than CPQ ⁺⁺ , essentially all ferric ions in the vicinity of the electrode are reduced by reaction (1) before reaction (2) can take place. CPQ ⁺ , which is insoluble and colored green, is then generated and a green precipitate forms on or near the electrode.

Simultaneously with the above reaction, the following reaction takes place on the counter electrode:

Fe ⁺⁺ " ^e ^ Fe ⁺⁺⁺

Subsequent removal of the CPQ ⁺ precipitate from the working electrode can be accomplished by passing an appropriate positive current, which for convenience will be referred to as the "bleaching pulse", thereby reversing the above reactions.

A migration of charged ions takes place in time

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span between the coloring and bleaching pulses takes place between the electrodes. However, it has been found that the Fe ⁺⁺⁺ ions tend to migrate toward the working electrode because their concentration in the vicinity thereof is decreased as a result of reaction (1) so that there is a gradual accumulation of Fe ⁺⁺⁺ ions with increasing difficulty in color formation in subsequent cycles as more Fe ⁺⁺⁺ must be reduced before reaction (2) can begin.

This explanation relates in particular to systems with CPQ ⁺ and ferric ammonium sulfate, but it is clear that the principle also applies in the same way to other systems and the invention is not restricted solely to the system described above. It has now been found that it is advantageous to apply a "compensation pulse" to the system, which serves ^{to reduce or increase at least some of the oxidized ions (eg Fe +++} ) that tend to accumulate in the area of the cathode Reduce.

The balancing pulse (so named because of its tendency to counter_act the uneven accumulation of ions of the auxiliary redox system within the cell) can be ^{impressed during each bleach / color cycle 1} or at any suitable evenly or unevenly distributed time, as appropriate. For example, in an embodiment in which an electrochromic device is subjected to a regular repetitive cycle between a state in which the working electrode is colored and a state in which the color is "erased", a balance pulse is preferably used between each bleach and color pulse inserted. However, the possibility of impressing a compensation pulse with a frequency that is lower than with the cyclical change of state and for example is not switched off

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may be one equalizing pulse per 5 or 10 cycles, or any other suitable number. the desirable frequency of the compensation pulses is easily determined by a simple experiment, and it is for example by the migration speed of the ions through the electrolyte and the dimensions of the Affects the device and the arrangement of the electrodes therein.

The frequency of the equalization pulses can vary and even an uneven frequency is possible, a compensation pulse is only then impressed if the ion accumulation appears to be desirable lets what can be determined by suitable sensors or detection means, e.g. by application a suitable reference electrode.

The compensation pulse preferably does not impair the excretion of active material.

The voltage applied for the compensation pulse can vary within wide limits, with only care should be taken that it has a tendency to eliminate the accumulation of oxidized ions near the cathode or to counteract this and that it does not cause any coloring of the working electrode. Preferably goes the impulse immediately precedes a color impulse.

The compensation pulse should preferably also be of sufficient length or duration to allow back diffusion of the majority of the excess oxidized ions generated during the bleaching pulse to the working electrode. Typically the compensation pulse be of low source impedance (e.g. less than 500OiI and usually less than 200 A-

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for an electrode of 0.1 cm) with a voltage close to zero (e.g. up to 100 mV). One a suitable pulse duration is 300 ms, although of course any other suitable and effective duration (with appropriate voltage) can be applied.

The individual working electrodes are expediently driven during the coloring and bleaching pulse with a sufficiently high source impedance to prevent color variation between the working electrodes. Typically, this impedance is of the order of 100 times that of the electrolyte (measured between an electrode and the counter electrode).

The invention is described below on the basis of a series of comparative tests, which have the advantage show the application of a compensation pulse according to the invention.

The experiments were carried out using a cell with display or working electrodes arranged in a conventional manner to form a 7-prime digit, with a counter electrode being provided on the circumference of the cell. The cell was built up on an aluminum oxide base of 25 x 75 x 1 mm as follows: A uniform transparent glaze layer (79T5 in medium 65/101 from Blythe Colors, Stoke-on-Trent) of 10 μm thickness was printed on the aluminum oxide and carried out for 30 minutes Baked in at 95O ⁰ C for a long time.

Electrodes, leads and contact extensions were applied through a stencil or a screen pattern on the glaze using gold resinate 9425 (Engelhard) by screen printing and baked at 850 ⁰ C while achieving a thickness of components

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0.3 µ.

Thereafter, a dielectric glaze was printed, which covered the entire substrate except the contact electrodes and fired extensions (11 glaze H34 63/2 in the medium from Blythe Colors), and at 800 ⁰ C.

A UV light curable resin was then screen printed around the perimeter of the substrate and a glass ring 1 mm thick and sealed therewith. The cell was sealed by sealing the top of the glass ring completed with a transparent glass cover.

The cavity of the cell was filled with an aqueous solution containing the following components:

N, N'-bis (p-cyanophenyl) -4,4'-

dipyridylium (CPQ) sulfate 0.02 M.

Lithium sulfate 0.5M

Iron ammonium sulfate 0.05 M

Sulfuric acid for pH 2.7

example 1

The cell was connected to the individual working electrodes with separate series resistors of 5.6 kü. operated, the other end of which was connected to the output side of separate electronic CMOS switches, while the input sides were connected together and connected to a variable voltage, the common source of which was connected to the counter electrode, while the on / off switches were individually through an electronic Logic system have been operated.

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• Λή.

The cell was cycled for 1 hour using a series of colored pulse voltages of -3.75 V, pulse times of 400 ms, a storage period of 1 minute and a bleaching pulse voltage of +3.75 V with a pulse duration of 300 ms. This attempt was repeated (using different but identical cells) with bleaching pulse durations of 320 ms, 340 ms, 360 ms, 380 ms, 400 ms, 420 ms and 440 ms.

When a bleach pulse duration of 380 ms or less was used, the working electrodes were permanently covered with CPQ ⁺ prior to the completion of the one-hour test.

At 400 ms, none of the working electrodes were permanently colored, but rather there was a considerable variation the color between the separate working electrodes and between different parts of the same.

At 420 msec and above, the CPQ ⁺ on the working electrode brightened or faded noticeably during the storage period before the one hour test was completed.

Example 2

The experiment of Example 1 was repeated, except that a compensation pulse of -20 mV and 2 s was the color pulse preceded. Bleaching pulses of 380 ms, 400 ms, 420 ms, 440 ms, 460 ms, 480 ms, 500 ms and 520 ms were used.

At 380 msec, excess CPQ ^{+ built up} on some of the working electrodes by the end of the test. With bleaching pulse settings of 400 ms to 440 ms, the CPQ ⁺ excretion remained visually evenly colored over the working electrode area, while at 460 ras and above a certain just noticeable bleaching

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of the colored electrode occurred during the 1 minute storage period.

Example 3
5

The sequence of operations of Example 2 was again applied except that bleaching pulses of 520 msec were applied The coloring and bleaching voltages were set to -0.75 V and +0.75 V; -1.5 V and +1.5 V; -2.25 V and +2.25 V; -3.0 V and +3.0 V; -3.75 V and +3.75 V; as well as -4.5 V and +4.5 V.

At 2.25 V and below, the working electrodes were not evenly colored; at 2.25 V the unevenness was only just noticeable. At all other voltages, the CPQ ⁺ excretion gave visually identical colorations over the surface of the electrodes.

Example 4
20th

The same operating sequence as in Example 2 was used, except that the compensation pulse was in a direct Connection of the working electrodes to the counter electrode by means of a CMOS switch (10OiI), switching off the 5.6 k-fL series resistors existed.

With a compensation pulse of 200 ms there was no visual difference between the color of the electrodes and that obtained in Example 2 was observed. 30th

Using the above-described electrolyte composition and cell, the following became optimal Cycle conditions found:

2 2

Color pulse current density 4 mA / cm to 8 mA / cm

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2

Color pulse charge density 1.6 mC / cm to 3.2 mC / cm

Compensation pulse duration> 150 ms

Ratio of bleach to
Color pulse charge from 1: 1 to 1.4: 1

Example 5

Using a cell with a 0.25 mm deep Cell cavity and operation as in Example 2, but with different numbers of permanently colored Working electrodes could be determined that the color precipitation on the cyclically operated electrodes was influenced by the number of permanently colored electrodes and the voltage between working and Counter electrodes varied by as much as 400 mV at the end of the test period (apparently due to a current χ Resistance-voltage drop through the electrolyte).

Example 6

The experiments of Example 5 were repeated using a cell depth of 1 mm, and it was found that the voltage variations dropped to 100 mV with no noticeable change in the color of the electrodes was.

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Claims (7)

B 8041 claims 1. Electrochemical device with (at least) a working electrode and a counter electrode in contact with an electrolyte medium containing an organic active compound (with a color change when the oxidation state changes) comprises, with electrodes and electrolyte medium being housed in a suitable housing and electrical connection and control means by which the electrodes are alternately controlled by a color pulse and a bleaching pulse can be activated, characterized in that the control means for the application of a compensation pulse of the same folarity as the color pulse and of a lower potential than the redox potential of the organic active material are adapted to the electrodes. 2. Apparatus according to claim 1, characterized by an auxiliary redox system. 3. Apparatus according to claim 1 or 2, characterized in that the Kontrollraittel a regular Imposition of compensation pulses, preferably with a frequency of one compensation pulse per color / fading cycle are adapted. 4. Device according to one of the preceding claims, characterized in that the control means of the imprint a compensation pulse (via a low source impedance) that is equal to zero or comes close to zero, are adapted. 5. Device according to one of the preceding claims, 709839/0936 ORIGINAL INSPECTED <. B 8041 t characterized in that it is characterized by an electrochromic Display system is formed. 6. Device according to one of the preceding claims, characterized in that the compensating pulse — actuator respond to a certain concentration of ions on or near a working electrode. 7. Procedure for operating an electrochemical plant direction with a working electrode and a counter electrode in contact with an electrolyte medium in which a organic active compound (with different oxidation states of different color) and an auxiliary redox system is contained within a suitable Housing with devices through which the working electrode can be observed and electrical connection and control means for the alternative actuation of the electrodes by means of a color pulse and a Bleaching pulse, characterized in that the ion concentrations in the vicinity of the electrodes are kept essentially constant by a compensation pulse of the same polarity as the color pulse and with a potential lower than the redox potential of the organic active material to which electrodes are applied. 709839/0936