Journal o f Electronic Materials, Iiol. 8, No. 3, 1979
SPEED CONSIDERATIONS FOR ELECTROCHROMIC DISPLAYS
D J Barclay, C L Bird, D H Martin IBM United Kingdom Laboratories Limited Hursley Park Winchester, England
(Received April 25, 1978) The electrochromic effect based on the reversible electrodeposition of a viologen radical cation has been applied to information display systems. Problems of multiplexing can be overcome by the use of an auxiliary switch, such as a thinfilm transistor. The question of speed is the subject of this paper, which discusses the factors influencing the contrast ratio of an electrochromic display. The contrast ratio is determined by electrochemical and optical considerations, the former governing the amount of material deposited and the latter the visual effect of the deposit. Electrochemically, the deposition may be influenced by a variety of mechanisms, including diffusion, migration, electrode kinetics and deposit resistance. Multiple pulse driving gives increased speed in diffusion-limited cases. Optically, the perceived contrast is maximised by increasing the absorbance of the deposited material and by optimising the diffuse reflectance of the display electrode. Key words: electrochromic, display, viologen
311 0361-5235/79/0500-0311503.00/1 9 1 9 7 9 A I M E
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Barclay, Bird, and Martin
Introduction The electrochromic effect of the type based on the electrodeposition of a highly coloured organic material has sufficiently interesting characteristics to encourage consideration of its application to information display systems. A major breakthrough in organic electrochromics was the discovery by C. J. Schoot et al.(1) of the reversible electrochemical precipitation of the violet bromide salt of 1,1'-diheptyl-4,4'-bipyridinium radical cation by electroreduction of the colourless di-cation. A display based on this effect has the properties of passivity, in the sense that the display modulates rather than emits light; has memory inasmuch as once written a display does not require power to sustain it; operates at a low voltage; comsumes low average power as a consequense of its memory and low voltage; and, unlike liquid crystals, does not have a viewing angle problem.
These f a c t o r s have engendered a great deal o f i n t e r e s t in the a p p l i c a t i o n o f e l e c t r o c h r o m i c s to small d i s p l a y s , such as watches, o r c a l c u l a t o r s , but two o u t s t a n d i n g problems have mediated a g a i n s t t h e i r a p p l i c a b i l i t y to l a r g e r area d i s p l a y s . These problems are d i f f i c u l t i e s in m u l t i - p l e x i n g and speed. A conventional, direct, matrix addressing scheme involves, for example, the deposition of two orthogonal sets of conductor lines with the electrolyte sandwiched between them. By applying +V/2 and -V/2 volts to selected rows and columns respectively a voltage of +V is generated accross the electrolyte in the position where a picture element (PEL) is to be written, and of +/-V/2 volts in positions not to be written. This method of driving requires an electrolyte with a non-linear writing characteristic, an appreciable voltage threshold in the transition from writing to erasing, and an electrolyte resistance which is high so that any reaction is localised at the desired PEL. For electrochromics the low electrolyte resistance and lack of write-erase threshold make this method of driving impractical for large matrices. These addressing problems can be overcome by isolating each PEL with a switch, such as a thin film transistor (TFT), organised for example as indicated in Fig. I. The display can be written row-by-row by activating all the switches of one row and writing selected PELs with a constant current pulse in the corresponding column. Electrochromics have advantages over liquid crystal and electroluminescent displays addressed via
Speed Considerations for Electrochromic Displays
J.__
313
_L
_L
_L
J_
J_
J_
- ~
-s
J.__ J_
/
FIGURE I.
_L
_L
/
Possible Arrangement of a TFT Electrochromic Display Having One TFT Per Picture Element
314
Barclay, Bird, and Martin
TFTs (2, 3) inasmuch as the memory property reduces the requirement to one active device per PEL and eliminates the need for rapid screen refresh. The speed limitations of electrochromics are a consequence of the contrast ratio of the display being determined, for a given system, by the amount of material deposited. The minimum time in which this can occur is determined by the rate at which the electroactive species can reach and react at the electrode. A variety of mechanisms such as diffusion, migration and electrode kinetics can control this process. If we consider the electrochromic system based on the reduction of l,l'-diheptyl-4,4'-bipyridinium dibromide, also known as heptyl viologen dibromide (HVBr2) the electrochemical reaction during write is simply
HV §
+ Br" §
HVBr|
(1)
Qualitatively, the contrast ratio, CR, which defines the appearance of the display may be expressed as
OR or E . Q f .
DR
(2)
where ~ is a materials property of the display, Qf is the Faradaic charge/m 2 passed through the solution and DR is the diffuse reflectance of the display electrode. The structure of the display is based on that of a conventional electrochemical cell; this is illustrated in Figure 2. The Faradaic charge is the total charge injected into the system less that required (Qal) to raise the potential of the electrode to a value at ~fiich reaction (1) occurs. Generally, Qdl is small compared with Qf and to a first approximatior can be ignored. If the display is written at constant current, Qf is simply I x t, where I is the current density and t is the deposition time. Thus
CR
E,I.t.
DR
(3)
For optimum d i s p l a y w r i t e time to a given r e q u i r e d c o n t r ast t has to be minimised by maximising I, ~ and DR.
Speed Considerations for Electrochromic Displays
Current
315
source
Electrochromic solution
Display electrode
FIGURE 2.
Counter electrode
Representation of an electrochromic cell
316
Barclay, Bird, and Martin
Electrochemical Considerations As indicated above, a variety of mechanisms can limit the write speed of an electrochromic reaction of the viologen type. Previous authors (4, 5) have considered the case where diffusion limitations dominate but have not dealt in any detail with effects such as migration, electrode kinetics, or deposit resistance. Convection has been shown to be negligible at practical write speeds (7). This paper treats these effects in a general sense and gives practical examples where possible. Much of the analysis is based on computer simulation of equivalent circuits (8,10). The case of diffusion limitation is expanded to indicate that line-by-line addressing of electrochromic displays can result in display speeds sufficiently fast for practical application. Deposit Resistance A fundamental aspect of the electrochromic reactions of the viologen family is the site of the electron transfer reaction. The possibilities are transfer at the electrode-dep0sit interface or deposit-solution interface. For the latter, electronic conduction in the electrochromic deposit is required. If the deposit is an insulator, then at the one-electron step thick deposits can only be grown if the viologen di-cation can be transported through the growing film. Analysis of systems which result in films with negligible electronic resistance and no ionic conductivity is straightforward and can be treated in a similar fashion to electrodeposition of metals. If the resistance is not negligible and ionic conductivity plays a role the situation is complex and has to be analysed in terms of the specific electrical characteristics of the film. No quantitative data on the latter has appeared in the literature. Films with various characteristics have been reported for the n-heptyl viologen di-cation (6), the variation being _ achieved by selection of the anion. When the anion is Br experimental evidence suggests that the deposit has some electronic conductivity. Electrochemical transients can be analysed in terms of diffusion control of solution species, even for thick films (7). The dihydrogenphosphate anion results in non-classical electrochemical behaviour in the sense that the response of a heptyl viologen system containing
Speed Considerations for Eiectrochromic Displays
317
that anion to a current or potential step cannot be explained in terms of diffusion, migration or kinetic control. Jasinsky has suggested that in this case, the deposit is an insulator (9). While the mechanism of film growth has not been worked out, the practical consequence is that the deposit resistance drastically limits response times. The subsequent sections of this paper will assume that the electrochromic deposit is a good conductor. A further publication will treat instances of poorly conducting or insulating films. Mi~iration In an electrochemical (or e l e c t r o c h r o m i c ) c e l l e l e c t r o a c t i v e ions are transported to the e l e c t r o d e surface under the influence of a concentration g r a d i e n t ( d i f f u s i o n ) or an e l e c t r i c field (migration). The l a t t e r is o f t e n minimised by having present in the e l e c t r o l y t e a large excess of i n e r t ions which carry the c u r r e n t through the bulk of the s o l u t i o n . In p r a c t ical e]ectrochromic c e l l s , however, the t r a n s p o r t number of the e l e c t r o a c t i v e species is often comparable with t h a t of the i n e r t ions and i t is e s s e n t i a l to understand whether or not migration c o n t r i b u t e s s i g n i f i c a n t l y to d i s p l a y response.
The data in Fig. 3 represents the current response of a 0.1M viologen dibromide solution to an applied potential 200mY negative of the reversible potential for one-electron reduction. This was obtained by computer simulation of an equivalent c i r c u i t model (10) which includes the effects of d i f f u s i o n , migration, electrode k i n e t i c s , and the e l e c t r o l y t e IR drop between the reaction s i t e and the reference electrode. In Fig. 3 (a) the reaction is assumed to be d i f f u s i o n c o n t r o l led (see below) and in Fig. 3 (b) the influence of migration is included. In the l a t t e r case a less than 10% change in the simulated current r e s u l t s , t h i s change being beneficial to the fast d r i v i n g of a display.
D i f f u s i o n Limited Currents I f a constant c u r r e n t is applied to an e l e c t r o d e in a s o l u t i o n o f an e l e c t r o a c t i v e species, X and the r a p i d e l e c trode reaction
318
Barclay, Bird, and Martin
X*§+
!
Y
+
e - ---- XY~
occurs, the p o t e n t i a l - t i m e
E = EZ
RT zF
relationship
In(T.~-t-~)
(4) is given by
(5)
where E ~ is c l o s e l y r e l a t e d to Eo, the r e v e r s i b l e e l e c t r o chemical p o t e n t i a l o f the system, and '~ the t r a n s i t i o n time is the time taken f o r the a p p l i e d c u r r e n t to exceed the f l u x o f species X to the e l e c t r o d e . The symbols R, T, Z and F have th_eir usual meaning. I t is assumed t h a t the c o n c e n t r a t i o n o f Y at the e l e c t r o d e s u r f a c e i s always h i g h , t h a t XY is a good e l e c t r o n i c conductor and t h a t m i g r a t i o n is not s i g nificant. The t r a n s i t i o n time is given by
"l~ =
ztF=A=~D
c=
=
k
(6)
I
D being the diffusion coefficient of X++, and C its concentration. The electrode area is A. A current I cannot be passed for times larger than ~ w i t h o u t some other, probably undesirable, electrode reaction occurring (see footnote). Referring to equation (2) and noting that for a given electrochromic system a charge, QD, is required to give the desired contrast, the minimum time to deposit QD coulombs is then
O 2
t
min
=
k
(7)
N.B. In principle the occurrence of another electrochemical reaction at a more negative potential is not necessarily destructive. For example if the two electrode reactions:
and
X++ A+
+ +
Y- + e---,- XY~ e- --~ A ~
El E2
occur and E2 is more negative than El, then the chemical reaction: A~
+
X"I+ +
Y- ~- XY~+ A+
can occur with a free energy of -F(IE2-EI I), and if this is sufficiently negative the net result can be the precipitation of XY, the desired display reaction.
Speed Considerations for Electrochromic Displays
319
Interface Current Density (Am -2)
8OO
600
400
(a)
200
20
40
60
80
lO0
Time (ms) Figure 3.
Simulated Current Response of O.Im Viologen Dibromide Solution to Potential Step. migration (a) excluded, (b) included
320
Barclay, Bird, and Martin
If a n-line display is written line-by-line then the fastest addressing time is ~ [ ~ . U s i n g values of C=1.5xi02 mole m -3 and D=2.15 x 10 -10 m2s -I for viologen dibromide a 1000 line display based on that system may be written in 11.4 secs. If it were feasibleI~o write the display potentiostatically the charge-time relationship for maximum diffusion limitation would apply. This gives
O = 2zFA(-
-
t
= k( t
(8)
Hence, the minimum time for deposition of QD t
rn in
~
,~
(9)
and
k(,) -
16.k
(lO)
so that under potentiostatic write the display would be driven in approximately half the time compared with constant current conditions. However, potentiostatic driving is impractical for large PEL (picture element) content displays owing to IR losses down conductor lines and through the electrolyte. In general, it is apparent that rapid response times are achieved by high solubility and large diffusion coefficients for the display precursor.
Multipulsin~ The single-pulse per line techniques described above (in the section on Diffusion Limited Currents) do not result in fast (
Speed Considerations for Electrochromic Displays
321
The effect of using the multipulse technique is shown in Table I. It is clear that pulsing is useful only when I> K/Q but I can be very large. For example a lO00 line heptyl viologen display is addressable in l sec if the pulsed current density is 2 x lO4 amp/m 2. This corresponds to 800~A per pel for a display at 5 PELs/mm. Fig.4 is a practical illustration of the effect of multipulsing. Fig. 4(a) shows the potential-time transient for a 7 millisecond pulse at 2 x 103Am -2 applied to a platinum eletrode in 0.1M HVBr 2 with 0.25 KBr added. The potential is rising to levels associated with undesirable further reactions. Fig. 4(b) shows a succession of l millisecond pulses at the same current delivered every lO0 milliseconds to the same electrode and solution. The rise in potential is slight by comparison with Fig 4(a). In both cases the solution IR contribution is not negligible for the current density used.
Kinetic Complications The section on Diffusion Limited Currents assumes that the rate of electron transfer of reaction (4) is very fast. However, all reactions of this nature proceed at a finite rate and it is important to understand the consequence of this. The rigorous relationship between current and potential is
c
]
(,2)
where I i s the exchange c u r r e n t d e n s i t y o f r e a c t i o n ( 4 ) , q i s th~ o v e r p o t e n t i a l (E - Eo, where E i s the a c t u a l p o t e n t i a l and Eo is the r e v e r s i b l e p o t e n t i a l ) , C is the s u r f a c e c o n c e n t r a t i o n o f X, Cb the b u l k c o n c e n t r a t i o n . el, is the t r a n s f e r c o - e f f i c i e n t ( g e n e r a l l y = 0 . 5 ) , and O i s the coverage o f the e l e c t r o d e by XY. This equation takes into account kinetic and diffusion limitations. The influence of kinetics is easily demonstrated by calculating the overpotential or current at t = 0 from the simple equation
I = Ioexp
F. RT q
(13)
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Barclay, Bird, and Martin
Table I. Multipulsed Display Pulses
16
Minimum
Current
Pel
Time (sec)
Density (A/m 2)
Current (~A)
1.14
1.8 x 103
113
0.57
3.6 x 103
225
0.29
7.2 x 103
450
0.14
1.4 x 104
900
0.07
2.9 x 104
1800
No of lines
100
PEL Density
4 PELs/mm
Q(viologen)
20 C/m2
Speed Considerations for Electrochromic Displays
323
I 0.2 volt
Ims
(a) Single 7ms pulse
l 0.2 vol t
100ms (b) Ims pulses repeated every 100ms
FIGURE 4.
Potential-time Transients for 2 x 103Am -2 Pulses Applied to a 0.1M HVBr 2 Solution.
324
Barclay, Bird, and Martin
If the overpotential is limited to n L volts to prevent alternative electrochemical reactions occurring then the constraint on current is
I <
I o e x p - ~ET -z, F n L
(14)
For example, f o r heptyi v i o l o g e n dibromide I has been measured as about 102A/m z (10) and n L should not ~xceed 400my to ensure t h a t the second e l e c t r o n t r a n s f e r r e a c t i o n o f v i o l o g e does not occur. Thus I must be less than 2.4 x 105/m 2. In the case o f heptyl v i o l o g e n dihydrogenphosphate in phosphate b u f f e r , pH = 5.6, nL should be less than 100my to prevent hydro gen e v o l u t i o n at an e l e c t r o d e w i t h a high c u r r e n t d e n s i t y f o r hydrogen e v o l u t i o n . Thus I should be less than 0.7A/m 2. The maximum current density for the bromide system allows an estimate of the fastest possible display based on viologen. 1000 lines may be addressed in 1000 x Q/24 = 18.3 ms using 53 pulses of 1.6 ~s duration. For the dihydrogenphosphate system, on for example platinum, pulsing is of no use as the kinetic current limitation is smaller than K/Q. This reasoning ignores the further complication in these systems caused by deposit resistance. Equation (13) is only valid at t = 0; for complicated situations involving mixed kinetic-diffusion limitations computer simulation is generally used. Other Rate-Limitin~ Steps Durin~ Writin~ A variety of other effects can limit the rate of writing an electrochromic display of the type given in equation (4). These include following or preceding chemical reactions and crystallisation or nucleation overpotential. These effects are not amenable to the general discussion in this paper.
Erasure L i m i t a t i o n s The total addressing time of an electrochromic display has to include the erase time as well as the write time. With bulk erasure, the contribution of the erase time is minimised so it is generally sufficient to optimise conditions for writing. However, a line select is desirable to permit interaction with the display so the erasure time must not be too long - probably less than 50ms.
Speed Considerations for Electrochromic Displays
325
A variety of types of erasure behaviour is exhibited by different electrochromic materials within the viologen class. For example the para-cyanophenyl viologen dication (11) is readily reduced to the radical cation, but is not electrochemically reoxidisable. The oxidation of the radical cation from the heptyl viologen dihydrogenphosphate is complex and slow and very temperature dependent. Neither of these effects is clearly understood. The erasure of the heptyl viologen bromide radical cation is well behaved as long as the deposit does not dwell on the electrode surface ( under the latter conditions it apparently recrystallises). (12). The erase time should, by consideration of Eq. 12 be very fast as there are apparently no diffusion limitations and I is relatively high. Experimentation has suggested however? that erasure speed may be limited by a mechanism which prevents over-saturation of the electrolyte with HVBr 2 in the vicinity of the electrode (10). To maximise erase speed the bulk concentration of the dication should therefore be low. This however conflicts with write optimisation and the preferred electrolyte composition has to be a compromise between write and erase requirements. Optical Considerations
Referring to equation 3, display write time is optimised by maximising the parameters ~ and DR. ~ is frequently treated as the extinction coefficient of the display deposit; a more general treatment is given below. For both ~ and diffuse reflectance (DR), the opportunities for optimisation are discussed.
Extinction Coefficient To achieve maximum contrast, the absorption in the visible spectrum of both the display deposit and its precursor - such as the heptyl viologen di-cation - are important: the precursor should be transparent in the visible region, the deposit should have a broad band with a large extinction coefficient.
326
Barclay, Bird, and Martin
Although the extinction coefficient is a convenient measure of the intensity of an absorption band, a more useful parameter is the oscillator strength, f. This is a function of the area under a band when the absorption is recorded as a function of frequency. For a reasonably symmetrical band, the formal expression may be simplified to
f
4-6 x IO -i~ r rnax.
where l~i~is the band-width (in m -l) at half the maximum extinction ~ m a x (13). Table II shows some values for methyl and heptyl viologen cation-radicals and for the quinone diimine species derived from two-electron oxidation of o-tolidine. Contrast is increased if the amount of light reaching the human eye is decreased as a result of the absorption process. The eye itself is differently sensitive to the wavelengths comprising the visible spectrum. Peak sensitivity occurs around 550nm. To account for this, the absorption of the deposit should be integrated with respect to the eye sensitivity curve. Experimentally, a filter calibrated to the eye response may be used. Multiple absorption peaks may also contribute significantly to the perceived contrast. Published experimental data can be used to gain some insight into feasible speeds for electrochromic materials. For example, o-tolidine can undergo a pH-dependent oxidation to a cation radical which appears to form a dimer having absorptions at ~ m a x = 365nm ( ~ max = 4.8 x I03m 2 mole -l ) and ~ m a x = 630nm ( ~ m a x = 2.8 x 103m2mole -I) (17). The twoelectron oxidation product, which is formed exclusively below pH3, h a s ' m a x = 437nm and { m a x = 6.1 x 103m2mole -1 (15,17,18). The estimated half-width ( ~ ) of 3.2 x I05m -I for this latter peak yields a value of 0.90 for f. It is apparent that the opportunity exists to improve electrochromic speed through the discovery of systems with higher absorption.
D i f f u s e Reflectance A high d i f f u s e r e f l e c t a n c e w i ] / increase the c o n t r a s t between the o f f and on s t a t e s of the d i s p l a y . To achieve t h i s , the r e f l e c t e d l i g h t should be of high i n t e n s i t y but w i t h a l l d i r e c t i o n s o f r e f l e c t i o n having equal p r o b a b i l i t y . This i m p l i e s
Speed Considerations for Electrochromic Displays
327
that reflectivity and scattering have both to be optimised. Optimal scattering additionally eliminates any viewing angle problems.
Table II Typical absorption values. Species
(m2mole - ~ m a x i)
~ (m-l)
f
Ref.
Methyl viologen cation radical
1.05x103
4.1xi05
0.20
14
1.26x103
4.1xi05 (a)
0.24
15
Heptyl viologen Solution cation Radical Deposit
1.2x103 (b)
3.5xi05
0.19
16
4.2xi0 2 (c)
4.5xi05
0.09 (d)
16
Quinone diimine from o-tolidine
6.1xi03
3.2xi05 (e)
0.90
17
(pH<3)
(a)
Spectrum not given in Ref. 15. Value assumed from Ref. 14.
(b)
Value taken from two methyl viologen values.
(c)
Estimated from relative heights of peaks in Fig. 3 of Ref. 16.
(d)
Aggregate formation usually results in loss of absorption intensity (19).
(e)
Estimated from Fig. 6 of Ref. 17.
328
Barclay, Bird, and Martin
The scattering properties of a surface are a function of its topography. The latter should be chosen to provide efficient scattering without an excessive number of reflections within the surface. The increased absorption from these would result in a grey appearance. The bulk optical constants of the display electrode metallurgy should be such as to maximise the reflectivity. Table Ill shows calculated reflectances for silver and gold (20). It is apparent that silver metal offers the desired high reflectivity and indeed may be electrop]ated to give a diffuse reflective appearance approaching that of white paper.
TABLE III Calculated Reflectances for Silver and Gold (20)
~(nm) Ag (evaporated)
450
500
550
600
650
0.968
0.979
0.982
0.984
0.985
0.397
0.504
0.815
0.919
0.955
Au (evaporated)
Speed Considerations for Electrochromic Displays
329
Conclusions The minimum write time of viologen based electrochromic systems is determined by a combination of mechanisms, the most important of which are diffusion, electrode kinetics, and electricalproperties of the electrochromic deposit. In systems where the latter are not complicating, the diffusion limitation may be overcome by multiple-pulsing techniques (if line-by-line addressing is used) so that the ultimate limitation becomes the rate of electron transfer between the electrode and the display precursor. Convection effects are not significant (7) in viologen electrochromic systems so far studied, whereas migration has a small, but beneficial, contribution.
Erase response times are not so c l e a r l y understood and t h i s , along w i t h e l e c t r i c a l p r o p e r t i e s o f the d e p o s i t , are fruitful areas f o r f u r t h e r research aimed at u n d e r s t a n d i n g and improving e l e c t r o c h r o m i c d i s p l a y s . The comments on o s c i l l a t o r s t r e n g t h i n d i c a t e t h a t t h e r e i s s u b s t a n t i a l area f o r improvement here, but the o t h e r requirements o f d e p o s i t i n s o l u b i l i t y and electrochemical reversibility mediate a g a i n s t the l i k e l i h o o d o f a l a r g e number o f e l e c t r o c h r o m i c systems s u p e r i o r in speed to the v i o l o g e n s e r i e s .
330
Barclay, Bird, and Martin
Refe rences l.
C. J. Schoot,J. J. Ponjee, H. T. Van Dam, R. A. Van Doorn, and P. T. Bolwijn, Appl. Phys. Lett. 23, 64 (1973).
21
A. G. Fischer, Microelectronics ~, 5 (1976).
3.
T. P. Brody,
IEEE Trans. Consumer Electronics CE-21, 260
(1975). .
I. F. Chang, B. L. Gilbert and T. I. Sun, J. Electrochem. Soc. 122, 955 (1975).
.
I. F. Chang and W. E. Howard, ED-22, 749 (1975).
.
R. J. Jasinski, J. Electrochem.
7. .
IEEE Trans. Electron Devices
Soc. 124, 637, (1977).
J. Bruinink and P. Van Zouten, J. Electrochem Soc. 124, 1232, (1977). M. Ichise, Y. Nagayanagi and T. Kojima, J. Electroanal. Chem. Interfacial Electrochem. 3_~3, 253 (1971).
9.
R. J. Jasinski, U.S. Patent 3,961,842.
10.
D. J. Barclay, C. L. Bird and D. H. Martin, unpublished data.
11.
British Patent 1314049.
12.
J. Bruinink, C. G. A. Kregting and J. J. Ponjee, J. Electrochem. Soc. 12/4, 1854 (1977).
13.
A. B. P. Lever, "Inorganic Electronic Spectroscopy", Elsevier, Amsterdam (1968).
14.
W. M. Schwarz Jr., Ph.D. Thesis, University of Wisconsin (1961).
15.
T. Kuwana and N. Winograd, Electroanalytical I (1974).
16.
H. T. van Dam and J. J. Ponjee, ]555 (1974).
Chemistry ~,
J. Electrochem.
Soc. 12l
Speed Considerations for Electrochromic Displays
331
17.
T. Kuwana and J. W. Strojek, Disc. Farad. Soc. 45,134 (1968).
18.
J. W. Strojek and T. Kuwana, J. s facial Electrochem. I_6.6,471 (1968).
19.
S. F. Mason, in "The Chemistry of Synthetic Dyes", Vol Ill, ed. K.Venkataraman, Academic Press. New York (1970).
20.
D. E. Gray (Ed), 4'American Inst. of Physics Handbook" 3rd edition, McGraw-Hill, New York (1972).
Chem. Inter-
