Electrochromic display apparatus

In a container which contains an electrochromic solution, a pair of electrodes, namely an indication electrode and a counter-electrode are disposed and an electric circuit is constituted in a manner such that a D.C. pulse is impressed across the electrodes and thereafter the electrodes are shortcircuited to each other. By means of the shortcircuiting, a color indicating substance formed on the indication electrode by the electrochemical reduction reaction or electrochemical oxidation reaction is reversably dissolved from the indication electrode into the solution so as to erase the indication, and therefore contamination of the indication electrode is less and hence a longer life is achieved in comparison with conventional apparatus where such shortcircuiting is not made and simply an inverse pulse is impressed across the electrodes for erasing.

BACKGROUND OF THE INVENTION 
This invention relates to electrochromic display apparatuses and 
particularly concerns electrochromic display apparatus employing an 
electrochemical reaction. 
The electrochromic display apparatuses are expected to have a bright future 
since the color of indication is brilliant, necessary voltage and current 
are small and there is no restriction in the observation angle. However 
hitherto, the life of the electrochemical display apparatus have not been 
satisfactorily long. 
The electrochromic display apparatuses are divided into two groups, namely, 
physical type electrochromic display apparatuses and electrochemical type 
electrochromic display apparatuses. Each of the groups are further divided 
into an inorganic type and an organic type. Electrochromic substances used 
in the abovementioned various types of apparatus are shown in the Table 1 
below. 
TABLE 1 
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types electrochromic substance 
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Physical type 
inorganic 
WO.sub.3, MoO.sub.3, TiO.sub.2 
electrochromic 
organic metanitroanilins, merocyanine dyes 
apparatuses such as indophenol blue. 
Electro- inorganic 
ions of metal (Ag.sup.+, Na.sup.+), ions of 
chemical type non-metal (I.sup.-), polytungsten- 
electro- anions. 
chromic organic viologen compounds such as hexyl 
apparatuses viologens, heptyl viologens, octyl 
viologens, benzyl viologens; 
redox dyes such as leucodyes and 
tetraalkyl-p-arylenediamines; 
PH indicators such as phenol- 
phthalene, phenol red, cresol red. 
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Among the abovementioned listed electrochromic display apparatuses, the 
apparatuses of the electrochemical type make use of the change of light 
absorption or light reflection of the substance formed on an indication 
electrode by the electrochemical reaction of oxidation or reduction of the 
electrochromic substance. 
A typical example of the prior art devices of the electrochromic display 
apparatus of the elctrochemical type is elucidated referring to 
FIG. 1, which is a circuit diagram of the apparatus of the prior art, and 
FIG. 2 which shows a voltage wave form of pulses to be impressed across the 
electrodes 1 and 2. 
As shown in FIG. 1, in a container 4, for example, of glass, which contains 
electrochromic substance 3 or solution in the liquid phase, gel phase or 
solid phase, a pair of chemically stable electrode, namely, an indication 
electrode 1 and a counter-electrode 2 are disposed with a specified gap 
inbetween. Therefore, the gap between the electrodes 1 and 2 is filled 
with the electrochromic solution 3. The electrodes 1 and 2 are connected 
to a D.C. power source 5 through a polarity inversing switch 6. By 
impressing a D.C. voltage 7 shown in FIG. 2 of a polarity selected by the 
switch 6, for a specified time periode on the electrodes 1 and 2, cations 
A.sup.+ and anions B.sup.-, respectively, which have been produced by a 
dissociation shown by the belowmentioned reaction formula (1) of a 
electrochromic substance AB, there results a reduction reaction of the 
belowmentioned reaction formula (2) at the cathode and an oxidation 
reaction of the belowmentioned reaction formula (3) at the anode, 
respectively. 
EQU AB.fwdarw.A.sup.+ +B.sup.- ( 1) 
EQU A.sup.+ +e.sup.- .revreaction.A (2) 
EQU B.sup.- -e.sup.- .revreaction.B (3). 
Accordingly, reduction product A onto the cathode and oxidation product B 
onto the anode, respectively are formed. 
The polarity of the voltage impressed on the electrodes are selected in a 
manner that, when the reduction product A is used for the indication, then 
the indication electrode is connected as the cathode and, when the 
oxidation product B is used for the indication, then the indication 
electrode is connected as the anode. By opening the polarity inversing 
switch 6 after the preset time, the redox reaction stops and the reduction 
product A and the oxidation product B are retained on the abovementioned 
respective electrodes. Then, by inversely connecting the inversing switch 
6 after a preset time period, an inverse polarity voltage 8 shown in FIG. 
2 is impressed on the electrode, and the reverse reaction takes place, 
thereby forming A.sup.+ cations from the reduction product A and B.sup.- 
anions from the oxidation product B and dissolving them from the 
electrodes, and resultantly, the indication is erased. 
In such a conventional electrochemical electrochromic display apparatus, as 
described in the above, the pulses impressed on the electrodes 1 and 2 
have the positive writing pulse 7 and the negative erasing pulse 8. Since 
the voltage of each electrode is inversed at every writing-in and 
erasing-off, the reduction product A and the oxidation product B are 
alternately produced and dissolved on the same indication electrode 1. 
Since some small amounts of the reaction products are remaing on the 
indication electrode, both remaining products may coexist on the 
electrode. Such a reduction product A and the oxidation product B are 
generally liable to chemical reaction with each other as shown by the 
belowmentioned irreversible reaction formula (4), and therefore, such a 
series of the impressing of the alternate polarity of pulses is likely to 
form an undesirable irreversible reaction product C deposited on the 
indication electrode. 
EQU A+B.fwdarw.C (4). 
Such a reaction product C coats and contaminates the surface of the 
indication electrode thereby causing an increase in the resistance of the 
apparatus. Therefore, the reactions of reduction and oxidation at the 
electrode surface are hindered thereby adversely effecting normal 
electrochemical electrochromic operation of the apparatus. 
SUMMARY OF THE INVENTION 
The object of the present invention is to eliminate the abovementioned 
undesirable depositing of irreversible reaction product coating the 
electrodes, thereby considerably decreaseing contamination of the 
electrode surface and increase the life of the apparatus.

DETAILED DESCRIPTION OF THE PRESENT INVENTION 
The present invention is elucidated referring to the Figures of FIG. 3 and 
thereafter. 
FIG. 3 shows one example of the state of the voltage of the electrodes 1 
and 2. 
FIG. 4 shows an example of the electrochromic display apparatus. In FIG. 4, 
a container 4, for example, of glass contains electrochromic substance 3 
of solution in the liquid phase, gel phase or solid phase. A pair of 
chemically stable electrodes, namely, an indication electrode 1 and a 
counter-electrode 2 are disposed with a specified gap inbetween. Namely 
the gap between the indication electrode 1 and the counter-electrode is 
filled with the electrochromic substance 3. The electrodes 1 and 2 are 
connected to a D.C. power source 5 through a switching circuit 11. A 
shortcircuiting circuit 12 is connected across the electrodes 1 and 2. The 
switching circuit 11 and the shortcircuiting circuit 12 have input 
terminals 111 and 121 and are made "ON" when control signals 14 and 15 
from a controlling circuit 19 are impressed on the terminals 11 and 12, 
respectively. 
The feature of the present invention is that, after an impressing of a D.C. 
pulse across the electrodes of the electrochemical electrochromic 
apparatus for writing-in of the indication, the erasing of the indication 
can be made by shortcircuiting the electrodes to each other. 
Namely, a positive pulse 7 for writing is impressed on one of the 
electrode, for example, the indication electrode 1, with respect to the 
other electrode. The impressing of the positive pulse 7 is made by closing 
a switch 11 which is connected in series with a D.C. power source 5 for a 
specified preset time period. By means of this impression of the pulse 7, 
an indication is written in to change the color of the indication 
electrode. Then, the written-in indication is erased by shortcircuiting 
the electrodes for a preset time period as indicated by a short line 9 on 
the zero volt line in FIG. 3. Such shortcircuiting for an erasing is the 
important feature of the present invention. 
Theory: As a result of the products of the reduction reaction and the 
oxidation reaction, these products are deposited on the surface of the 
electrodes. By deposition of different substances on the first and the 
second electrodes, the cell consisting of the pair of electrodes and the 
electrochromic solution form a battery, wherein an electromotive force is 
necessarily generated across the electrode. Therefore, during the 
indication being made, such an electromotive force exists. Accordingly, 
when both electrodes are shortcircuited with each other, a discharging 
takes place by means of the electromotive force, and the discharging makes 
the deposited reduction product A and the oxidation product B disolve into 
the electrochromic solution returning into cations and anions A.sup.+ and 
B.sup.-, respectively, by means of reverse reactions (from right side to 
left side) of the reaction formulae (2) and (3), respectively. Therefore, 
the written indication is erased without impressing an inverse voltage 
pulse which has been hitherto used. 
By the abovementioned shortcircuiting, reduction product A or the oxidation 
product B deposited during the indication is entirely removed. Therefore, 
in the conventional electrochromic apparatus, there is no undesirable 
hitherto observed reaction that a part of the reduction product A or the 
oxidation product B, deposited for the indication and remaining thereafter 
on the indication electrode, reacts with opposite reaction product, 
namely, the oxidation product B or the reduction product A deposited by an 
impression of opposite voltage for erasing. 
In other words, in the apparatus of FIG. 4, when the substance used for the 
indication is the reduction product A, the operation of the indication 
electrode is always as a cathode and that of the counter-electrode is 
always as an anode; when the substance used for the indication is the 
oxidation product B, the operation of the indication electrode is always 
as an anode and that of the counter-electrode is always a cathode. 
Therefore, each electrode has deposition of either one of the reaction 
products A or B only for writing the indication in, and hence, there is no 
possibility of forming undesirable deposition on the indication electrode 
by means of the abovementioned irreversible reaction of the formula (4). 
As the counter-electrode a polarizable electrode material such as gold or 
platinum is used. 
It is of course necessary that the conditions of the writing-in pulse, 
namely, voltage, polarity and duty time (i.e., pulse width) should be 
selected according to known procedures so as to meet the reduction and 
oxidation potentials of the electrochromic substance used. Such selection 
is for achieving procedures the desired optical density of the indicated 
color and to prevent generation of an adverse side reaction such as 
producing a gas. The pulse shape is not limited to the square wave, but a 
triangular wave, a saw tooth wave, a half cycle of sine wave, etc., can be 
used. 
The shortcircuiting for the erasing can be made directly across the 
electrodes to each other, or indirectly through the grounding of both 
electrodes. The necessary time period i.e., time width of the short 
circuiting time, varies depending on the optical density of the indicated 
color to be erased and circuit resistance of the circuit including the 
cell and the switching means. The higher the optical density and the 
circuit resistance are, the longer the necessary shortcircuiting time 
period is. If an excessively long time period for erasing is necessary, 
the apparatus has an unsatisfactory response characteristic for erasing. 
Therefore, it is recommended to use an electrochromic cell having a 
characteristic such that a perfect erasing is made with shortcircuiting of 
a time period within 20 times of that for the writing pulses. 
According to the operation including shortcircuiting in accordance with the 
present invention, the deposition of an undesirable contaminating 
substance to the electrodes due to the simultaneous existence of the 
reduction product and the oxidation product is satisfactorily prevented. 
However, after many cycles, for example, 5,000 to 100,000 cycles, of 
writing-in and erasing-off of the indication, a very small amount of 
deposition of the reduction product or the oxidation product may take 
place. In order to remove such a small amount of the deposited substance, 
it is useful to impress a pulse of the inverse polarity after the series 
of 5,000 to 100,000 cycles of writing-in and erasing-off. 
Construction of the apparatus: FIG. 4 shows an example of the mechanical 
and electrical construction of the electrochromic display apparatus in 
accordance with the present invention. The cell comprises a transparent or 
translucent container 41, an electrochromic solution 3 contained in the 
container, an indication electrode 1 and a counter-electrode 2. The 
indication electrode 1 and the counter electrode 2 are connected to the 
negative terminal and the positive terminal of a D.C. power source, 
through a switching circuit 11, which is made "ON" during receiving a 
writing-in signal 14 at its input terminal 111. A pair of output terminals 
of a shortcircuiting circuit 12, which shortcircuits the output terminal 
during receiving of an erasing-off signal 15 at its input terminal 11, is 
connected across the indication electrode 1 and the counter-electrode 2. 
For the switching circuit 11 and the shortcircuiting circuit 12, any of 
mechanical relays, electronic relay circuits such as transistor switching 
circuit, mercury relays, etc., can be used. 
The writing-in of the indication in the apparatus of FIG. 4 is made by 
impressing the writing-in signal 14 to the input terminal 111, thereby 
impressing a writing-in pulse 7 of FIG. 3 to the indication electrode 1. 
The erasing-off is made by impressing the erasing-off signal 15 to the 
input terminal 12 input terminal 121, thereby shortcircuiting the 
electrodes 1 and 2 to each other as shown by the line 9 of FIG. 3. The 
writing-in signal 14 and the erasing-off signal 15 are generated by a 
controlling signal circuit 19. 
FIG. 5 shows an actual example of the circuit of the electrochromic display 
apparatus shown in FIG. 4. The switching circuit consists of an n-p-n 
transistor 11 and the shortcircuiting circuit consists of another n-p-n 
transistor 12. The transistors 11 and 12 are made "ON" during receiving 
input signals 14 and 15 at their base and writes in and erases off the 
indication, respectively. 
For the electrochromic substance of the apparatus in accordance with the 
present invention, one of the metal ions, non-metal ions, polytungsten 
anions, viologen compounds, or redox dyes are usable. Especially, among 
the abovementioned substances, hexyl viologens, heptyl viologens, octyl 
viologens and benzyl viologens are satisfactory for practical use, since 
these viologens have the characteristic that, from -10.degree. C. to 
+70.degree. C., the abovementioned shortcircuiting time period necessary 
for erasing the indication is less than 10 times of the writing-in time 
period. For example, the erasing time period is less than 1 second against 
100 ms (milli-second) of the writing-in time period. 
EXAMPLE 1 
Electrochromic aqueous solution contains: 
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heptyl viologen bromide 0.1 mol/l, 
potassium bromide as supporting electrolyte 
0.3 mol/l. 
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Indication electrode 1 and counter-electrode 2: a pair of platinum 
electrodes disposed with a 2 mm gap inbetween. 
Writing-in pulse across the electrodes: 
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pulse width 100 milli seconds, 
voltage 2 volts. 
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Erasing-off: by shortcircuiting for 500 milli seconds. 
Life test run of writing-in and erasing-off: at the rate of 1 cycle per 
second. 
In the abovementioned life test run, no contamination on the indication 
electrode is observed even after 5,000 cycles of the writing-in and the 
erasing-off; while under the conventional way of erasing by means of 
impressing 2 volts inverse voltage, the indication electrode becomes 
noticeably contaminated by the irreversible product, so that normal 
indications are not made after only 5,000 cycles of writing-in and 
erasing-off. 
EXAMPLE 2 
Electrochromic aqueous solution: same as of the Example 1. 
Indication electrode 1 and counter-electrode 2: a pair of gold electrodes 
disposed with a 2 mm gap inbetween. 
Writing-in pulse across the electrodes: 
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pulse width 100 milli seconds, 
voltage 2 volts. 
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Erasing-off: by shortcircuiting for 500 milli seconds. 
Life test run of writing-in and erasing-off: at the rate of 1 cycle per 2.5 
second (0.4 cycle per second). 
In the above mentioned life test run, no contamination on the indication 
electrode is observed and the apparatus works still satisfactory even 
after 1,000,000 cycles of the writing-in and the erasing-off: while under 
the conventional way of erasing by means of impressing 2 volts 
inverse-voltage, the indication electrode becomes noticeably contaminated 
by the irreversible product so that normal indications are not made after 
only 5,000 cycles of writing-in and erasing-off. 
EXAMPLE 3 
Electrochromic aqueous solution: same as of the Example 1. 
Indication electrode 1: a platinum electrode. 
Counter-electrode 2: a gold electrode disposed with a 2 mm gap from the 
indication electrode 1. 
Writing-in pulse across the electrodes: 
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pulse width 200 milli seconds, 
voltage 2 volts. 
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Erasing-off: by shortcircuiting for 300 milli seconds. 
Life test run of writing-in and erasing-off: at the rate of 1 cycle per 
second. 
In the abovementioned life test run, no contamination on the indication 
electrode is observed even after 1,000,000 cycles of the writing-in and 
the erasing-off; while under the conventional way of erasing by means 
impressing 2 volts inverse voltage, the indication electrode becomes 
noticeably contaminated by the irreversible product so that normal 
indication are not made after only 5,000 cycles of writing-in and 
erasing-off. 
EXAMPLE 4 
Electrochromic aqueous solution: same as of the Example 1. 
Indication electrode 1 and counter-electrode 2: same as of the Example 3. 
Writing-in pulse across the electrodes: same as of the Example 3. 
Erasing-off: by shortcircuiting for 100 milli second. 
This example was intentionally made shorter than that of Ex. 3 in order to 
realize incomplete erasing and slight accumulation of the reduction 
product on the indication electrode. 
Life test run of writing-in and erasing-off: at the rate of 1 cycle per 
second, but including impressing of inverse-voltage, pulse of 2 volts 300 
milli seconds across the electrode after every 5,000 cycles of writing-in 
and erasing-off. 
In the abovementioned test run, reduction product accumulated due to 
insufficient erasing on the surface of the indication electrode 1 is 
entirely oxidized and cleaned out at every impression of the 
abovementioned inverse voltage pulse. No contamination on the indication 
electrode is observed at all even after 3,000,000 cycles in total of the 
writing-in and the erasing-off according to the test run of this example. 
As elucidated in detail with respect to the foregoing examples, in the 
apparatus of the present invention, there is no sequence of impressing 
pulses of opposite polarity. The erasing of the written indication is made 
by shortcircuiting the electrodes, which may be followed by an impression 
of an inverse voltage pulse. Or in the other example, a shortcircuiting 
between the electrodes anticipates each impressing of the pulses. Namely, 
the surface of the indication electrode is cleaned before each impressing 
of the pulse across the electrodes. Accordingly, there is no undesirable 
production of irreversible product on the indication electrode, and hence 
the indication electrode can be kept clean for longer life time, for 
example in some cases more than 100 times longer than those of the prior 
art devices. 
In the case more than two electrochromic substances, which do not 
chemically react with each other, are contained in the solution, the 
electrochromic indication on the indication electrode becomes a mixed 
indication of these two electrochromic substances. Therefore, if desired a 
mixed color of these substances is obtainable.