GB1580027A - Devices - Google Patents

Description

(54) DEVICES (71) We, IMPERIAL CHEMICAL INDUSTRIES LIMITED, Imperial Chemical House, Millbank, London SW1P 3JF, a British company do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to electrochemical devices.

Electrochemical devices, and particularly display devices based on electrochemical phenomena are known which comprise a working electrode; an electrolyte medium comprising an organic active material the oxidation state of which may be reversibly changed by passage of an electric current through it resulting in a detectable change in the vicinity of the working electrode; and a counter electrode also in contact with the active material. For display purposes the working electrode will have radiation transmission or reflection properties and radiation will usually be visible radiation, and the counter electrode will be so disposed with respect to the working electrode that changes occurrring at the working electrode are not obscured from an observer thereof by reactions occurring at the counter electrode.The said electrodes and active material will be contained within a suitable housing comprising means, for example a transparent window, through which the working electrode may be observed.

Such display devices have been described for example in UK Patent No. 1,314,049 (equivalent to US Patent No. 3,712,709) UK Patent No. 1,302,000 in US Patent No.

3,806,229 and UK Patent No. 1,407,133 (US Patent No. 3,930,717).

The active materials in such electrochemical devices are capable of accepting or donating electrons and being thereby converted into radical ions which have a high extinction value, usually in the visible part of the spectrum. In general for display devices the active materials are preferably such that they have in one oxidation state no, or hardly any, colour, and that they may be converted, even by the passage of a small current at an appropriate EMF, to a coloured oxidation state so that a high contrast image may be obtained. It is also necessary that at least in the oxidation state which provides the display effect the active material is relatively immobile in the medium and for visual display purposes differs in visual appearance in its different reduced or oxidised states.The relative immobility is attained if the active material in the appropriate oxidation state is relatively insoluble and for example, deposits out upon an electrode surface. Such active materials are known and are described for example in the aforementioned patents.

It is often preferred that the electrolyte medium comprises also an auxiliary redox system capable of undergoing oxidation or reduction at a lower potential than that at which the active material undergoes the same change, and which after oxidation or reduction can be reconverted to its original state at a potential close to its standard reduction or oxidation potential, and which in either the reduced or oxidised state is capable of reacting with the coloured state of the active material to give the alternate species. Again, the use of such auxiliary redox systems has been described in the prior art.

The device also optionally comprises electrical connection and control means whereby selected electrodes may be activated (i.e. a current applied thereto) or deactivated as required.

The present invention provides an improvement in the operation of electrochemical, and particularly electrochromie devices comprising an active material.

It is conjectured that for example in a display device as described above, and comprising an aqueous electrolyte medium containing as an active material, say, CPQ (see below) sulphate (i.e. CPQ++ and SO4--) and as auxiliary redox system, say, ferrous ammonium sulphate (i.e.

Fe++, Fe+++, NH4+ and SO4--), when the device is fresh or has not been driven for some time the ions are evenly distributed throughout the medium and the electrodes are free from any deposit. The visual appearance of the working electrode may now be changed by passing a specified negative current for a specified period, called for convenience a 'colour pulse', between the working and counter electrode through the electrolyte medium.

Two reactions then occur at or near the surface of the working electrode, i.e.: Fe$++eFe++ .........

CPQ+A,CPO+ 2.

As ferric ions have a higher electron affinity than does CPQ++, substantially 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 green in colour is then produced and forms a green deposit at or near the electrode.

Simultaneously with the above reaction FE+±e,FE+++ ..... ................ 3.

is taking place at the counter electrode.

Subsequent removal of the CPQ+ deposit from the working electrode may be brought about by passing an appropriate positive current, which for convenience we may describe as a 'bleach pulse', whereby the above reactions are reversed.

Migration of charged ions occurs between the electrodes during the period between the colour and bleach pulscs, however, and we find that there is a tendency for Fe+++ ions to migrate towards the working electrode as their concentration in the vicinity thereof is reduced as a consequence of reaction 1, so that a gradual accumulation of FE+++ ions tends to occur, with increasing difficulty in colouration in subsequent cycles as more Fe+++ has to be reduced before reaction 2 can commence.

This explanation applics specifically to systems comprising CPQ+ and ferrous ammonium sulphate, but it will be appreciated that the principle applies equally to other systems, and the invention is not limited in its application only to the aforemcntioned system. We have now found it advantageous thercfore to apply to the system an "equilibration pulse" the purpose of which is to reduce at Icast a portion of the oxidiscd ions tending to accumulate in the region of the cathode.

Accordingly the present invention provides An electrochemical device comprising a working electrode and a counter electrode in contact with an electrolyte medium comprising an organic active compound as hereinbefore defined, said electrodes and electrolyte medium being contained with a suitable housing comprising means through which the working electrode may be observed, and comprising electrical connection and control means whereby the electrodes may be activated alternately by a colour pulse and a bleach pulse characterised in that the control means is adapted to apply an equilibration pulse of the same polarity but lower amplitude than, the potential difference required to produce colouration at the working electrode.

In a further embodiment there is provided an electrochemical device comprising a working electrode and a counter electrode in contact with an electrolyte medium comprising an organic active compound as hereinbefore defined and an auxiliary redox system as hereinbefore defined, said electrodes and electrolyte medium being contained within a suitable housing comprising means through which the working electrode may be observed, and electrical connection and control means whereby the electrodes may be activated alternately by a colour pulse and a bleach pulse. a method of maintaining the ionic concentrations in the vicinity of the electrodes substantially constant by applying an equilibration pulse having the same polarity as, but lower amplitude than. the potential difference required to produce colouration at the working electrode.

The equilibration pulse. so callcd because it tends to counteract the unevcn accumulation of positive oxidised ions within the cell. may be applied during every bleach/colour cycle or at any convenient time regularly or irregularly as may be appropriate. For example, in an embodiment whcre an electrochromic device is regularly and frequently cycled between a state in which the working electrode is coloured and a state in which the colour is discharged, we prefer to interpose an equilibration pulse between every bleach and colour pulse.

However, we do not exclude the possibility of applying an equilibration pulse at a frequency of less than every cycle; say one equilibration pulse per 5 to 10 cycles or any other convenient number. The desirable frequency of the equilibration pulse is quite easily determined by simple experiment and will be influenced for example by the speed of migration of ions through the electrolyte and the dimensions of the device and the disposition of the electrodes within it.

The frequency of the equilibration pulse may vary, and it may even br irregular, being applied only where ionic accumulation makes it desirable, which may be determined by appropriate sensing or detecting means, e.g. using a suitable reference electrode.

The equilibration pulse preferably does not adversely affect the deposition of active material.

The voltage employed for the equilibration pulse may vary within wide limits, the only requirement being that it tends ot negate the accumulation of oxidised ions in the vicinity of the cathode and that it does not produce colouration of the working electrode. Preferably the equilibration pulse should also be of sufficient duration to permit most of the excess oxidised ions produced during the bleach pulse to diffuse back to the surface of the working electrode.

Typically the equilibration pulse will be from a low source impedance (e.g. less than 5000Q and usually less than 200 Q for an electrode of 0.1 cm 2) and at a voltage approaching zero (e.g.

up to 100 mv). A convenient pulse duration is 300 ms although obviously any other convenient and effective duration (with corresponding voltage) may be employed.

Conveniently the drive to each working electrode during the colour and bleach pulse will be from a sufficiently high source impedance to prevent variation in colour between the working electrodes. Typically this impedance will be of the order of 100 times that of the electrolyte as measured between an electrode and the counter electrode.

The invention is illustrated by the following description of a series of comparative experiments which show the advantage of employing an equilibration pulse according to the invention.

The experiments were carried out using a cell containing display (working) electrodes disposed in the conventional manner to form a 7 bar digit, the periphery of the cell being provided with a counter electrode. The cell was built up upon an alumina substrate 25 x 75 x 1 mm as follows: A uniform transparent glaze (79T5 in medium 65/101 ex Blythe Colours, Stoke-on-Trent) coat 10,u thick was printed upon the alumina and fired at 950"C for 30 minutes.

Electrodes, leads and contact pads were screen printed upon the glaze through a figured screen using gold 9425 resinate (ex Engelhard) and fired at 8500C to give a component thickness of 0.3,u.

A dielectric glaze was next screen printed covering the entire substrate except the electrodes and contact pads (glaze 11 H34 in 63/2 medium ex Blythe Colours) and fired at 800"C.

A UV light curable resin was then screen printed around the periphery of the substrate and a glass gasket 1 mm thick located and sealed to it. The cell was completed by sealing to the other surface of the gasket a transparent glass cover.

The cavity of the cell was filled with an aqueous solution of: N,N'-di(p-cyanophenyl) 4,4'-bipyridylium (CPQ) sulphate 0.02M lithium sulphate 0.5M ferrous ammonium sulphate 0.05M Sulphuric acid to pH 2.7 Example 1 The cell was driven by connecting separate resistors of 5.6kit to each of the working electrodes, the other ends of the resistors being connected to the output side of separate electronic CMOS switches, the input sides being connected together and to a variable voltage the common. source of which was connected to the counter electrode, while the on-off switches were individually operated by an electronic logic system.

The cell was cycled for 1 hour using a sequence of colour pulse voltage of -3.75 v and duration 400 ms, a memory period of 1 minute, and a bleach pulse voltage of +3.75 v and duration 300 ms. This experiment was repeated (using different but identical cells) with bleach pulse durations of 320 ms, 340 ms, 360 ms, 380 ms, 400 ms, 420 ms and 440 ms.

Using a bleach pulse duration of 380 ms or less the working electrodes became permanently coated with CPQ+ before one hour test period was completed.

At 400 ms none of the working electrodes became permanently coloured but considerable variation in colour occured between the separate working electrodes and between different parts of the same electrode.

At 420 ms and above the CPQ on the working electrode faded noticeably during the memory period before the one hour test period was completed.

Example 2 The experiment of Example 1 was repeated except that an equilibration pulse of -20 mv for 2 seconds preceded the colour pulse. Bleach pulses of 380 ms, 400 ms, 420 ms, 440 ms, 460 ms, 480 ms, 500 ms and 520 ms were used.

At 380 ms an excess of CPQ+ built up on some of the working electrodes by the end of the test. With bleach pulse settings from 400 ms to 440 ms the CPQt deposit remained visually equally coloured over the working electrode surfaces, while at 460 ms and above there was some just noticeable fading of the coloured electrode during the one minute memory period.

Example 3 The drive sequence of Example 2 was again employed, except that the bleach pulse used was 520 ms. The colour and bleach voltages were set respectively 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 + 3.0v; -3.75 v and +3.75 v; and -4.5 v and +4.5v.

At 2.25 and below the working electrodes were not evenly coloured; at 2.25 v unevenness was only just apparent. At all other voltages the CPQ+ deposit gave visually equal colouration over the surface of the electrodes.

Example 4 The same drive sequence was used as in Example 2 except that the equilibration pulse consisted of connecting the working electrodes directly by CMOS switches (100or) to the counter electrode, thus eliminating the 5.6or resistors.

With an equilibration pulse of 200 ms there was no visual difference between the colour of the electrodes and those achieved in Example 2.

Using the electrolyte composition and cell described above it was found that optimum cycle conditions were as follows: Colour pulse current density 4 mA/cm2 to 8 mA/cm2 Colour pulse charge density 1.6 mc/cm2 to 3.2 mc/cm2 Equilibration pulse duration > 150 ms Ratio of bleach to colour impulse charge from 1:1 1.4:1 Example 5 Using a cell with .25 mm deep cavity and driving the cell as in Example 2 but with different numbers of the working electrodes permanently coloured, it was noticeable that the deposit of colour upon the cycled electrodes was influenced by the number of permanently coloured electrodes and the voltage between working and counter electrodes at the end of the test period varied by as much as 400 mv apparently due to current x resistance voltage drop through the electrolyte.

Example 6 The experiments of Example 5 were repeated using a cell depth of 1 mm and the voltage variations were found to have dropped to 100 mv with no noticeable variation in the colour of the electrodes.

WHAT WE CLAIM IS: 1. An electrochemical device comprising a working electrode and a counter electrode in contact with an electrolyte medium comprising an organic active compound as hereinbefore defined, said electrodes and electrolyte medium being contained within a suitable housing comprising means through which the working electrode may be observed, and comprising electrical connection and control means whereby the electrodes may be activated alternately by a colour pulse and a bleach pulse respectively for producing colouration and bleaching of the working electrode characterised in that the control means is adapted to apply an equilibration pulse of the same polarity but lower amplitude than, the potential difference required to produce colouration at the working electrode.

2. A device according to claim 1 comprising an auxiliary redox system.

3. A device according to claim 1 or 2 wherein the control means is adapted to apply regularly the equilibration pulse.

4. A device according to any one of the preceding claims wherein the control means is adapted to apply one equilibration pulse per colour/bleach cycle.

5. A device according to any one of the preceding claims wherein the control means is adapted to apply through a low source impedance an equilibration pulse which is or approaches zero.

6. A device according to any one of the preceding claims which is an electrochromic display device.

7. A device according to any one of the preceding claims comprising equilibration pulse actuating means responsive to a predetermined ionic concentration at or in the vicinity of a working electrode.

8. A device according to claim 1 and substantially as hereinbefore described.

9. In the use of an electrochemical device comprising a working electrode and a counter electrode in contact with an electrolyte medium comprising an organic active compound as hereinbefore defined and an auxiliary redox system as hereinbefore defined, said electrodes and electrolyte medium being contained within a suitable housing comprising means through which the working electrode may be observed, and electrical connection and control means whereby the electrodes may be activated alternately by a colour pulse and a bleach pulse, a method of maintaining the ionic concentrations in the vicinity of the electrodes substantially constant by applying an equilibration pulse having the same polarity but lower amplitude

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (10)

**WARNING** start of CLMS field may overlap end of DESC **. equally coloured over the working electrode surfaces, while at 460 ms and above there was some just noticeable fading of the coloured electrode during the one minute memory period. Example 3 The drive sequence of Example 2 was again employed, except that the bleach pulse used was 520 ms. The colour and bleach voltages were set respectively 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 + 3.0v; -3.75 v and +3.75 v; and -4.5 v and +4.5v. At 2.25 and below the working electrodes were not evenly coloured; at 2.25 v unevenness was only just apparent. At all other voltages the CPQ+ deposit gave visually equal colouration over the surface of the electrodes. Example 4 The same drive sequence was used as in Example 2 except that the equilibration pulse consisted of connecting the working electrodes directly by CMOS switches (100or) to the counter electrode, thus eliminating the 5.6or resistors. With an equilibration pulse of 200 ms there was no visual difference between the colour of the electrodes and those achieved in Example 2. Using the electrolyte composition and cell described above it was found that optimum cycle conditions were as follows: Colour pulse current density 4 mA/cm2 to 8 mA/cm2 Colour pulse charge density 1.6 mc/cm2 to 3.2 mc/cm2 Equilibration pulse duration > 150 ms Ratio of bleach to colour impulse charge from 1:1 1.4:1 Example 5 Using a cell with .25 mm deep cavity and driving the cell as in Example 2 but with different numbers of the working electrodes permanently coloured, it was noticeable that the deposit of colour upon the cycled electrodes was influenced by the number of permanently coloured electrodes and the voltage between working and counter electrodes at the end of the test period varied by as much as 400 mv apparently due to current x resistance voltage drop through the electrolyte. Example 6 The experiments of Example 5 were repeated using a cell depth of 1 mm and the voltage variations were found to have dropped to 100 mv with no noticeable variation in the colour of the electrodes. WHAT WE CLAIM IS:

1. An electrochemical device comprising a working electrode and a counter electrode in contact with an electrolyte medium comprising an organic active compound as hereinbefore defined, said electrodes and electrolyte medium being contained within a suitable housing comprising means through which the working electrode may be observed, and comprising electrical connection and control means whereby the electrodes may be activated alternately by a colour pulse and a bleach pulse respectively for producing colouration and bleaching of the working electrode characterised in that the control means is adapted to apply an equilibration pulse of the same polarity but lower amplitude than, the potential difference required to produce colouration at the working electrode.

2. A device according to claim 1 comprising an auxiliary redox system.

3. A device according to claim 1 or 2 wherein the control means is adapted to apply regularly the equilibration pulse.

4. A device according to any one of the preceding claims wherein the control means is adapted to apply one equilibration pulse per colour/bleach cycle.

5. A device according to any one of the preceding claims wherein the control means is adapted to apply through a low source impedance an equilibration pulse which is or approaches zero.

6. A device according to any one of the preceding claims which is an electrochromic display device.

7. A device according to any one of the preceding claims comprising equilibration pulse actuating means responsive to a predetermined ionic concentration at or in the vicinity of a working electrode.

8. A device according to claim 1 and substantially as hereinbefore described.

9. In the use of an electrochemical device comprising a working electrode and a counter electrode in contact with an electrolyte medium comprising an organic active compound as hereinbefore defined and an auxiliary redox system as hereinbefore defined, said electrodes and electrolyte medium being contained within a suitable housing comprising means through which the working electrode may be observed, and electrical connection and control means whereby the electrodes may be activated alternately by a colour pulse and a bleach pulse, a method of maintaining the ionic concentrations in the vicinity of the electrodes substantially constant by applying an equilibration pulse having the same polarity but lower amplitude

than, the potential difference required to produce colouration at the working electrode.

10. A method according to claim 9 and substantially as hereinbefore described.