Impedance spectroscopy analysis of WO3 thin film electrodes during electrochromic colouring-bleaching cycling

Materials Science and Engineering, BIO ( 1991 ) 3 ! 3-320

313

Impedance spectroscopy analysis of WO3 thin film electrodes during electrochromic colouring-bleaching cycling CI. Bohnke and M. Rezrazi Laboratoire d'Electrochimie des Solides (Unit~ associ~e au CNRS 436), Unit~ de Formation et de Recherche des Sciences et des Techniques, 25030 Besan¢on (.~dex (France)

(Received June 19, 1991 )

Abstract This paper deals with the variations in the electrical components (resistances, capacitances and constant phase element) of an electrochromic electrode during colouring-bleaching cycling. The complex impedance method is used to determine these parameters. Two types of electrochromic material are investigated: tungsten trioxide thin films obtained by anodic oxidation of tungsten sheet or by thermal vacuum evaporation of WO3 onto an SnO2:F electrode. An electrical model can be associated with the electrochromic electrode. The behaviours of these electrodes during cycling are compared and explained in terms of interface modification.

1. Introduction Amorphous tungsten trioxide (WO3) exhibits a good electrochromic effect when used with the appropriate electrolyte. The reversible insertion reaction is commonly written as xM + + x e - +WO3 ~ (transparent)

MxWO3

(blue)

WO 3 thin films are obtained by several methods: vacuum evaporation [1], potentiostatic anodic oxidation [2, 3], anodic oxidation with pulsed potential or current [4] or chemical vapour deposition [5, 6]. The complex impedance analysis is a powerful method which can give useful information on the electrical properties of the electrochromic cell and particularly on the electronic conductor-WO 3 and WO3-electrolyte interfaces. The aim of this work is to determine electrical parameters by impedance data analysis when the electrodes are coloured and bleached during a long time in hydrated LiCIOa-propylene carbonate electrolyte. This electrolyte contains 1 wt.% H20 and has been currently used in previous work [1, 7]. Cyclic voltammetry and complex impedance analysis have been used in order to determine the electrical parameters characteristic of an electrochromic electrode and their variations during colouring-bleaching cycling. The 0921-5107/91/$3.50

results obtained from these two methods are compared.

2. Experimental procedure

2.1. Film preparation 2.1.1. Anodic films Anodic tungsten oxide films were obtained by the procedure described in ref. 4. The electrodes are obtained from a cylindrical tungsten rod embedded in a polymeric resin (area, 0.196 cm 2). 2.1.2. Evaporated thin films The evaporated thin films are obtained by the procedure described in ref. 1. The film thickness is about 480 nm and was measured with a Taylor-Hobson Talystep ( + 4 nm). The substrate is fluorine-doped SnO2 (SnOz:F) coated on a glass slide (Glaverbel) with a square resistance of 14 f2/tz. The current collector is a copper wire sealed with a silver-conductive seal (Epotek). The electrode is coated with a UV resin (Loctite 350) in order to form a small window in contact with the electrolyte. 2. 2. Electrochemical measurements 2.2.1. Electrochemical cell A three-electrode set-up is used in the l%H20-LiCIO4 (1 M) propylene carbonate electrolyte. The counterelectrode is a platinum © Elsevier Sequoia/Printed in The Netherlands

314

grid (area, 3 cm 2) and the reference electrode is an aqueous Ag]AgCIICI- electrode (Tacussel) in a double-junction bridge containing the hydroorganic electrolyte. The electrolyte is deoxygenated with a dehydrated N 2 stream. 2.2.2. Colouring-bleaching and cycfic voltammetry To perform coiouring-bleaching cycling, a programmable arbitrary function generator (Tektronix AFG 5101 ) drives a potentiostat (Sotelem PG Stat Zt) with the electrical signal described in Fig. 1. At each cycle the injected (or extracted) charge is about 2.7 inC. After a certain number of colouring-bleaching cycles, cyclic voltammetry is performed at a sweep rate of 38 mV s- ' on the electrochromic electrodes. The same generator and potentiostat as described above were used. The voltammetric curves are recorded on a X - Y recorder (Sefram).

at equilibrium (curve a). After a long colouring-bleaching cycling run we cannot observe a significant change in this shape (curve b). A total number of 181 520 cycles was reached with these films• Table 1 summarizes the various parameters associated with the equivalent electrical circuit shown in Fig. 3. The frequency of the minimum of the imaginary part or the inflection point of the impedance 2400

2920

/ /

T_

2. 2. 3. Impedance measurements The Nyquist plots are obtained with a Solartron Schlumberger 1174 frequency analyser and a Sotelem PG Stat ZI potentiostat with a frequency sweep from 20 kHz to 0.075 Hz. The measurements are automatized with a Apple lie microcomputer. The measurements are recorded at the equilibrium potential of the non-coloured electrode.

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Fig. 2. Nyquist plot of the impedance data for the anodic W O 3 electrode at various cycling times: data a, 70 000 cycles; data b, 181 (J00 cycles•

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3.1. Anodic tungsten oxide The shape of the impedance plots (Nyquist plane) is well defined (Fig. 2). The results are relative to the electrode in the non-coloured state

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Fig. 1. Applied signal during the cycling of the electrochromic electrodes•

Impedance analysis of anodic WO 3 thin film in the noncoloured state (area, 0.196 cm 21 Number of cycles

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R~.t (~2)

('~ (/~F)

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('b, (/~F)

0 1000 35000 38000 70120 74620 78000 181520

175 172 185 181 178 186 179 178

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177 177 157 157 19 15 25 9.98

212 399 340 357 340 354 305 416

(Hz)

315

tin oxide substrate (ITO) and WO 3 800 nm thick in the coloured state [10]. The values of the constant phase angle element (CPE) are constant for each impedance datum and they are extracted from the experimental results by the formulae

data is also listed. This variable was used by Randin [9] to determine the state of discharge of Zn-AgO button cells. 3.2. Evaporated WO¢ onto SnO2.'F The impedance diagrams in the non-coloured state are shown in Fig. 4 (0 cycles) and Fig. 5 (18000 and 2 3 0 0 0 cycles). Typically, the diagrams can be decomposed in two parts: an arc at high frequencies and a linear part at low frequencies inclined with a 10°-30 ° angle vs. the imaginary axis. This constant phase angle (CPA) was observed in previous work with a low conductive indium

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n = 1 --

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where e is the angle between the imaginary axis and the CPE straight line in the Nyquist plane and to (rad s-~) is the pulse frequency. Linear regression fitting is used to obtain the value of a. Equation (2) gives the corresponding n value (Fig. 6). The CPE behaviour cannot be modellized by a simple analogous equivalent circuit. From the impedance data we have extracted four parameters: the high frequency resistance (Rht), the radius of the semicircle obtained by extrapolation of the circular part, the intercept of the CPA straight line with the real axis and the values of A from eqn. (1). The electrodes were cycled and the number of cycles reached were 60 000, 250 000 and 300 000 respectively. The results of the last two samples were analysed and are shown in Table 2 and in Fig. 7. During the

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Fig. 3. Electrical equivalent of the W/WO~ (anodic) electrode where Rh~ is the hydro-organic electrolyte resistance, R~, and (?2 are the parallel equivalent circuit associated with the charge transfer reaction between the tungsten sustrate and WO 3 [8]; C,f is the low frequency capacitance associated with the inserted charges.

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Fig. 4. Nyquist plot of the impedance data for the SnO2:F/evaporated WO 3 electrode (number of cycles, 0; area, 2.16 cm2).

316 400

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240 360 480 S00 REAL PART (ohm) Fig. 5. Nyquist plot of the impedance data for the SnO~:F/evaporated WO 3electrode: ", 18 (100cycles; A, 23 000 cycles.

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CYCLESNUMBER~(IO-3) Fig. 6. Variation in n, the exponent associated with the CPA during the cycling runs on the SnO2:F/evaporated W O 3 electrode.

colouring-bleaching cycling experiment it is possible to observe that Rhf remains constant (Table 2) throughout the whole experiment, as previously shown with anodic WO 3 (Table 1). However, concerning the radius of the semicircle, the intercept of the CPA straight line with the real axis (Fig. 7) and the value of the CPE (Table 2),

we can observe two different behaviours. On the one hand, before 80 000 cycles these parameters vary irregularly but, on the other hand, after 80 000 cycles they remain almost constant. The same behaviour is followed by the slope of the cyclic voltammetry curve during cathodic insertion, as shown in Figs. 8 and 9. Such a behaviour

317 TABLE 2 High frequency resistance and low frequency constant phase element of SnO2:F/WO 3 evaporated thin films Number of cycles

Rhf (f2)

Sample I (area, 2.16 cm 2) 0 89 1800 72 18540 76 23625 78 39825 77 44325 82 61425 81 65925 80 83000 82 87950 80 104825 75 126425 80 149151) 78 168000 80 232815 82 254000 81 Sample 2 (area, 1.8 cm-') 0 132 3600 132 19800 134 24300 139 32200 137 36700 110 52900 150 60900 139 79900 139 82000 138 138700 154 160300 150 187300 111 230500 117 304300 120

A (mF s" i)

AA

2.08 13 7.09 3.99 15.8 3.5 4.52 6.04 8.62 9.11 8.33 9.14 9.56 8.97 6.35 6.3

0.03 0.3 0.3 0.03 0.2 1.2 0.22 0.16 0.05 0.1 0.15 0.15 0.3 0,3 0.05 0.4

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0.05

2.3 4.4 15 3.4 3.6 2.2 2.8 1.9

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conductivity is close to 5 × 10- 3 Q - a cm- ' and the high frequency resistance values are in agreement with the values obtained using the Newman [ 11 ] formula for an electrode with a small area:

Re,

is characteristic of WO~ obtained by vacuum thermal evaporation onto an SnO2:F electrode and has not been observed with anodic WO 3. 4. Discussion

Figure 2 clearly shows that the anodic films obtained by pulsed current are stable over the colouring-bleaching cycle run. On the contrary, the as-prepared vacuum-evaporated thin films are much less stable. Figures 7 and 9 show that they need to be cycled up to 8 0 0 0 0 colouring-bleaching cycles to have constant electrical parameters. The cycling process seems to stabilize these vacuum-evaporated thin films.

4.1. Anodic films Undoubtedly, Rnf is the hydro-organic electrolyte resistance because the measured electrolyte

(3)

where p (ff~ cm) is the electrolyte resistivity and a (cm) is the radius of the circular electrode. The second reason is the fact that some experiments made in H2SO 4 ( 1 M) give the value of the aqueous electrolyte with both films: anodic oxide and WO 3 evaporated on tungsten sheet [8]. The cycling gives an enhancement of the Rot resistance of the charge transfer mechanism between tungsten and WO3. The frequency of the minimum of the imaginary part decreases with the increasing number of cycles, and the low frequency capacitance is constant (350_+ 50 /~F) during the cycling and becomes greater at the end. The measurement of the frequency of the minimum of imaginary part can be used to control the electrode lifetime.

4.2. Evaporated thin films The high frequency resistance gives the order of the magnitude and the SnO2:F substrate resistance. The highly conductive SnO 2 used in this work ( 10 ~ / D ) gives a very low value of the latter parameter because the Rhf values are very close to the theoretical values obtained using the formula

R =p-

l

(4)

s

where p(ff2 cm) is the electrolyte resistivity, l (cm) is the distance between the working electrode and the reference electrode and s (cm 2) is the working electrode area. The experimental results clearly show two important features presented by the films obtained by vacuum thermal evaporation of tungsten oxide onto SnO2:F transparent electrodes: we can observe first a different behaviour of the film before and after 80 000 colouring-bleaching cycles (Figs. 6-9) and second the existence of a CPA (Figs. 4 and 5). Such features have not been observed with anodic tungsten oxide (Fig. 2). According to Jacquelin [12, 13], this CPE behaviour appears frequently when the electrical current penetrates a material or an interracial zone in

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CYCLES NUMBER ~ ( 1 0 - 3 )

Fig. 7. Circle radius values of the charge transfer circle of the SnO2:F/evaporated WO~ electrode.

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(v)

Fig. 8. Voltammetric curve of the SnO2:F/evaporated WO 3 electrode and definition of the i = f ( v ) curve slope.

which various dissipative and storage phenomena coexist. Jonscher [14] explains the same behaviour in a humid dielectric in the particular case of a two-dimensional flow of charge in the planes of the interface. Wang and Bates [15] make the same observation in the case of horn-shaped pores at the interface of a solid electrolyte and a blocking metal electrode. The fractal approach of Le Mehaute and Crepy [16] and Nyikos and Pajkossy [17] may also be applicable to describe the interfaces. In a previous paper [9] we have already shown the presence of a CPA in the case of WO3, either coloured or bleached, obtained by vacuum thermal evaporation onto an ITO electrode of low conductivity. We had also shown that the low frequency CPE A (mF s"-') was dependent on the substrate conductivity. Moreover we determined a fractional exponent n close to 0.7, leading to the conclusion that the surface was porous. As suggested by Le Mehaute and Crepy [16], the determination of the fractional exponent n may give an idea of the electrode interface roughness. In this paper, we found that the value of n varies with the colouring-bleaching cycling (Fig. 6). A value between 0.6 and 0.8 was found before 80 000 cycles. For more than 80 000 cycles, a value very close to 0.7 was determined. The inter-

319

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Fig. 9. Variation in the i = f ( v ) curve slope during the cycling run on the SnO2:F/evaporated WO3 electrode.

face roughness varies with the number of cycles and finally stabilizes. The problem now is to determine whether the interface concerned is either the oxide-electrolyte interface as suggested in our previous paper [9] or the transparent conductor-oxide interface. Since such a CPE behaviour is not observed with anodic WO 3, we lean towards the latter assumption. Moreover, it has been shown by Falaras [2] that the anodic oxide presents a rough surface as the vacuum-evaporated film. The technology used to prepare the electrochromic films may then greatly influence the properties of the substrate-oxide interface. Because of this method used, the W-anodic oxide interface is much less rough than the substrate-evaporated film interface which may present some aggregates on it. In this latter case, the WO 3 thin film may not be considered as a simple coverage of the conductive substrate but as a porous media. The CPA behaviour could be explained by a dielectric dispersion (low frequency dispersion) and could be associated with the porosity of the aggregates of the film with an amorphous structure. The modification of this interface as cycling proceeds may explain the instability of the fractional exponent in Fig. 6 and of the values of the CPE real axis intersection in Fig. 7. It may also explain the high transfer resistance obtained with evaporated films, i.e.

300-500 £2 for a 2 c m 2 electrode, and its variation with cycling (Figs. 4 and 5). For comparison, this transfer resistance is of the order of 200-300 £2 for a 0.2 c m 2 electrode obtained by anodic oxidation (Table 1). Another study made on the kinetics behaviour of the electrochromism phenomenon also shows that the resistance of this interface plays an important role in the colouration kinetics. This result is shown in Fig. 9 which displays the slope of the voltammetric curve as a function of the number of cycles. 5. Conclusion

The colouring-bleaching cycling of WO 3 electrochromic thin film electrodes obtained by two methods, namely anodic oxidation of tungsten with a pulsed current and vacuum evaporation in a hydro-organic electrolyte, can be carried out up to about 80000 cycles and 3000000 cycles respectively. The analysis of the impedance data and the voltammetric curve equilibrium potential in the non-coloured state give evidence of the two different behaviours for the two preparation methods. The anodic WO3 is more stable in time and gives constant electrical parameters during cycling. The measurement of a particular frequency can be used in order to determine the lifetime of

320

electrodes. The evaporated WO 3 thin films are less stable and need cycling up to 80 000 colouring-bleaching cycles to obtain constant electrical parameters. The thin film preparation induces a CPA behaviour of the SnO2:F/WO3 electrode. A combination of optical density measurements, impedance data and the other electrochemical parameters will be the next investigation to be undertaken and the impedance data on coloured MxWO3/SnO2:F and M,WO~/W during cycling will be analysed. The comparison with the recent results of Koksbang et al. [18] on a similar material (Li,V60~3) used in lithium batteries will be very useful.

References 1 0 . Bohnke, Thesis, University of Besangon, 1984. 2 P. Falaras, Thesis, University of Paris VI, 1986. 3 P. Delichere, P. Falaras and A. Hugot-Le Goft', Thin Solid Films, 161 1988)47.

4 M. Rezrazi, O. Bohnke and J. Pagetti, Displays, 7(1987) 119. 50. Bohnke, CI. Bohnke, A Donnadieu and D. Davazoglou, J. Appl. Electroehem., 18 (1988) 447. 6 0 . Bohnke and CI. Bohnke, Dgplays, 10 (1988) 199. 7 0 . Bohnke, CI. Bohnke, G. Robert and B. Carquille, Solid State Ion., 6 (1982) 121. 8 CI. Bohnke, Thesis, University of Besanqon, 1986. 9 J. P. Randin, J. Appl. Electroehem., 15 (1985) 365. 10 CI. Bohnke and O. Bohnke, Solid State Ion., 39 (1990) 195. 11 J. Newman, J. Electroehem. Sot., 113 (1966) 501. 12 J. Jacquelin, in C. Gabrielli (ed.), Actes du 3 ° Forum sur les impedances l~lectrochirniques, Montrouge, 1988, p. 91. 13 J. Jaequelin, in C. Gabrielli (ed.), 1st Electrochemical Impedance Spectroscopy Symp., Bombannes, 1989, Extended Abstract C.26. 14 A. K. Jonscher, Electrochim. Acta, 35 (1990) 1595. 15 J. C. Wang and J. B. Bates, Solid State Ion., 18-19 (1986) 224. 16 A. Le Mehaute and G. Crepy, Solid State Ion., 9-10 (1983) 17. 17 L. Nyikos and T. Pajkossy, Eleetrochim. Acta, 30 (1985) 1533. 18 R. Koksbang, 1. T. Olsen, P. E. Tonder, N. Knudsen and D. Fauteux, J. Appl. Eleetrochem., 21 ( 1991 ) 3(11.