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lyte. Consequently the mobility of ionic species involved in the EC reaction of. Ni(OQ2 ... /6/ R. E. CARBONIO, V. A. MACAGNO, M.C. GIORDANO, J.R. VILCHE,.
Short Notes

K197

phys. stat. sol. (a) 84, K197 (1984) Subject classification: 14.3.3 and 14.3.4; 14.4; 22.8.1 Institute of Solid State Physics, Latvian State University, Riga 1) Modelling of the Solid State Electrochromic System

W03/HSb03 -2H20/Ni(OH) BY J . L . LAGZDONS, G. E. B A J U S , and A . R . LUSIS Thin film electrochromic (EC) m a t e r i a l s (ECM) and s y s t e m s (ECS) present a b a s i s for displays, light modulators, and other electro-optic devices. The ECS can be presumed as a system E1/M1/I/M2/E2,

containing a n ECM (M1), a n

ion conducting material (r) , and the so-called counter-electrode (Ma) between two electronic conductors (El and E2) serving as ohmic contacts. If M1 is a n ECM which colours during the cathodic p r o c e s s /1/ when electrons and small cations (H+, Li',

Na'.,

and others) are being injected, then M1 must be r e v e r s -

ible to ions involved in the EC reaction. However, the most promising ECS a r e those containing two ECM/2/,

i. e. colouring cathodically

/$/(M1)

and colour-

ing anodically /3/ (M2). T h i s leads to a n i n c r e a s e in the optical efficiency of the ECS. A s the colouration mechanism of the anodically colouring ECM is still questionable /3, 4/, the most important problem in the development of promising EC devices is the compatibility of M1 and M2 with the solid electrolyte (I). The problem of compatibility and the selection of solid electrolytes can be solved successfully by m e a n s of modelling - the construction and investigation of s y s t e m s involving massive m a t e r i a l s instead of thin films. The present note d e a l s with the investigation of model ECS containing cathodically colouring ECM (W03), anodically colouring ECM (Ni(0H)2), and a solid proton electrolyte (HSb03- 2 H20), as well as a n ECS containing HxW03

as counter-electrode M2. The solid electrolyte HSb03- 2 H 2 0 h a s been obtained by hydrolysis of The protonic conductivity of the electrolyte is equal to 0.26 S m - l -1 a t 293 K, the activation energy - 15.5 kJ mol . Tablets of polycrystalline

SbC15 /5/.

initial m a t e r i a l s were pressed between platinum electrodes in a n acrylic plastic cell. This enables visual observation of the colouration-bleaching processes. 1) Kengaraga S t r . 8, 226063 Riga, USSR.

physica status solidi (a) 84

K198

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Fig. 1. a) Colouration scheme and b) cyclic voltammogram of the electrochromic system W03/HSb03 2 H20/HxW03 between two platinum electrodes

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Fig. 2. a) Colouration scheme and b) cyclic voltammogram of the electrochromic system WO3/HSW3' 2 H20/Ni(OH) between two platinum electrodes

The symmetr$ of cyclic voltammogram of W 0 3 - and HxW03-containing ECS (Fig. 1) and i t s preservation under cyclic regime shows that the EC reaction involving proton insertion

wo3 + XH+ + xe- s?:H,WO~

( 1)

is completely reversible in the system under discussion. With increase in voltage up to 5 V the current goes down to zero, i. e. no decomposition of the solid

electrolyte takes place. The colouration of W 0 3 as well as the bleaching of HxWQj form a front (Fig. la) starting at the solid electrolyte/ECM interface and spreading through the bulk of tablets towards the ohmic contacts. The formation of such a front shows that electrons and protons taking part in the EC reaction (1) have different mobility. Thus, the colouration process of W03 is limited by diffusion of H+ through the growing layer of HxW03, whereas the bleaching process of

Short Notes HxW03

- by H+ migration through W03.

K199 The ratio. between electronic and ionic

conductivities of ECM determines the appearance and advance of the front. Direct experimental observations show that there are two reactions going on in the ECS containing W 0 3 and Ni(0H)

a t the same time (see Fig. Za) : the

reduction of W03 with the formation of HxW03 (1) and the oxidation of Ni(OQ2 with the formation of NiOOH (2) The cyclic voltammogram of the given system at positive voltage exhibits a linear dependence, a t reverse polarity a current maximum appears associated with the oxidation of HxW03. In case of Ni(OW2 it is characteristic that the black coloured front according to the formation of NiOHH, appears at the platinum electrode and moves towards the solid electrolyte. Consequently the mobility of ionic species involved in the EC reaction of Ni(OQ2, is much higher than the mobility of the electrons. W e know /5/ that Ni(0H) has reasonable protonic, conductivity and low electronic conductivity. The possibility of colouration of N4(OQ2 in conjunction with the solid proton electrolyte leads to the conclusion that the following reaction of the formation of NiOOH is much more reliable: N i ( O H ) 2 z NiOOH

+ H+ + e- ,

i, e. the reaction with the proton extraction contrasted to the proposed OH in-

jection / 6 / , Consequently, both the cathodic (W03) and anodic (Ni(0H).J ECM a r e compatible with the solid proton electrolyte and effective electrochromic devices can be worked out on their basis. The existence of the colouration front connected I

with the different mobility of the particles involved in the electrochromic reaction allows u s to make theoretical calculations of kinetic characteristics of the thin film electrochromic devices. References

/I/ W.C. DAUTREMONT-SMITH, Displays 3 , 3 (1982). /2/ A.R. LUSIS, in :Oksidniye elektrokhromniye materialy, Riga 1981 (p. 13).

/3/ W.C. DAUTREMONT-SMITH, Displays 3, 67 (1982). /4/ J.D.E. MCINTYRE, S. BASU, W.F. PECK, W.L. BROWN, and W.M.

-, 7242 (1982). AUGUSTYNIAK, Phys. Rev. B 25 /5/ R. BARNARD, C. F. RANDELL, and F.L. TYE, J . appl. Eleckrochem. 10,

K200

physica status solidi (a) 84

109 (1980). /6/ R. E. CARBONIO, V. A. MACAGNO, M.C. GIORDANO, J.R. VILCHE, and A. J. ARVIO, J. Electrochem. SOC. -129, 983 (1982)

.

(Received May 9, 1984)