Pt

Sohd State lomcs 36 (1989) 205-208 North-Holland, Amsterdam

IMPEDANCE STUDIES OF T H E CELL Pt/Ag + C O N D U C T I N G GLASS/Pt H DURAKPASA, P L I N H A R D T and M W BREITER lnstttut fur Techmsche Elektrochemw, TU Wwn, 9 Getretdemarkt, A- I060 Vtenna, Austrta

Received ! 8 July i 988, accepted for publication 1 February 1989

Impedance measurements were camed out on the cell Pt/Ag + conductmgglass/Pt at constant temperature between 25 and 320°C or 25 and 450°C at frequencies between 10-2 and 10s Hz Reproducibleresults were obtained m the first temperature range by measurements, startmg at 320°C, gomgdown to 25°C and then up again At low temperatures the Pt electrode was of the blocking type Largechanges occurred m the mteffacmltmpedance and the bulk reststance when heatmg the cell above the glass transmon temperature in the secondtemperature range

1. Introduction The symmetrical cell Pt/glass/Pt conststs of two electrodes whtch should be of the blocking type and an amorphous sohd electrolyte The behavtor of thts cell was studted to obtam answers to the following questtons (a) Do the Pt electrodes dtsplay the properttes of blockmg electrodes m the range of room temperature to glass transmon temperature9 (b) How do the cell propertxes change when the temperature ~s mcreased above the glass transmon temperature9 Impedance spectroscopy covenng a wtde frequency range was used as the techmque of mvesUgatton

2. Experimental The silver ton conductmg glass consisted of 34 mol% AgI, 26 mol% Ag20 and 40 mol% B203 It possessed a relatively hxgh glass transmon temperature of 340°C The glass surfaces were pohshed by hand before sputtenng on the Pt electrodes at room temperature of the substrate Contact to the leads of the measunng system was made by stlver pamt The cell was mstde a glass vessel under N2. The IM 5E automated Impedance meter, produced and sold by Zahner-elektnk ( F R G ) was used The data had to

be transferred to a separate PC for further processmg wRh subsequent plottmg However, smgle Bode plots are dtrectly obtainable from the automated set-up and the soft ware package supphed by the manufacturer

3. Results Data, representattve for measurements when the temperature remams below the glass transmon temperature, are gwen m a Bode plot for three dtfferent temperatures m fig 1 The logarithm of the absolute value of the cell xmpedance is plotted at the left side versus the logarithm of the frequency The phase angle ts gwen at the same temperatures as a funcUon of the logarithm of the frequency on the right side. The measurements were started at the htghest temperature They were continued down to room tern. perature and then up to the htghest temperature again The data m fig. 2 correspond to the second temperature range. Here the temperature was increased from room temperature to 450°C by setting the temperature controller of the furnace appropriately. Then the measurements were made at decreasing temperature at first and subsequently at increasing temperature.

0 167-2738/89/$ 03 50 © Elsevier Science Pubhshers B V (North-Holland Physics Pubhshmg Division )

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Fig 2 Impedancedata m Bode plot for measurementsup to 450°C 4. Discussion At high frequencies the phase angle becomes small m fig 1 Simultaneously the absolute value of the impedance approaches that of the cell resistance, computed from the geometric dimensions of the cell

and the specific conducttv]ty of the glass The latter was °dependently deter°meal by a four-probe tcch. tuque [ 1 ] The said agreement xs demonstrated by the two upper curves which are reasonably close m fig 3 The products ofconducttv~ty and temperature are plotted versus 1000/T there The conductivity

H Durakpasa et al I Cell Pt/Ag + conductmg glass/Pt

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A comparison of the frequency of the phase angle at constant temperature in figs 1 and 2 shows a marked change above 100°C The interracial impedance Pt/glass has also been affected by heating above the glass transition temperature Since the cell impedance is controlled by the lnterfaclal behavior at low frequencies, it follows from the data In fig 1 and fig 2 that the relative contribution of the interfacial impedance to the cell impedance became smaller at temperatures above 100°C for the measurements in the second temperature range The platinum electrodes do not behave as blocking electrodes in this temperature range any longer Additional processes, probably of a Faradalc nature, occur at the interface Finally it should be mentioned that the changes in the impedance behavior due to heating above the glass transition temperature remain reproducible in subsequent measurements if a temperature of about 150°C is not surpassed The cell is symmetrical Therefore Zcell

was either obtained by the four-probe techmque or computed with the aid of the cell dimensions from the absolute value of the impedance at high frequencies As it will be discussed later, the Pt electrode behaves like a blocking type electrode at low temperatures Additional effects the nature of which is not known with certainty become noticeable in the low frequency range at higher temperatures If the temperature is increased above 340 ° C, the impedance behavior in fig 2 is observed The absolute value of the impedance approaches a constant value at high frequencies and the phase angle becomes small This implies that the cell resistance is measured The comparison of the conductIVitles, computed from the the absolute value of the impedance at high frequencies, with the conductlvities of the measurements in the first temperature range In fig 3 demonstrates that the cell resistance has increased considerably for the measurements in fig 2 Since the cell dimensions remained the same, the Increase of the cell resistance is due to a decrease of the specific conductivity of the glass Partial crystalhzation of the glass during heating above the glass transition temperature is responsible for the latter effect

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Here Zce., Zg~assand Zmter designate the impedances of the cell, the glass and the interface respectively The behavior of Zeal] at high frequencies (compare figs 1 and 2) suggests that Zs]as~ can be replaced by Rs~as~ in the frequency range studied In this paper Zglass = Rglass + 2Z, nter

(2)

The data in the first temperature range were taken to test the applicability ofeq. (2), using the analogue circuit in the upper left corner of fig. 4 There a constant phase element [2] which describes,the propertles of the blocking interface is represented by the capacitor symbol with inclined lines between the plates The Interfacial impedance is represented by a constant phase element with a resistor (charge transfer resistance) in parallel A second constant phase element describes the bulk impedance Good agreement was found between the theoretical curves and the experimental data at temperatures up to about 100°C This is demonstrated by the results at 25°C In fig 4 where the fitted curve is given by the solid line The optimal parameters for the fit were obtained by a computer program As to be expected the constant phase element on the left of the analog circmt turned out to be predominantly ohmic In contrast, the constant phase element parallel to the ohmic

208

H Durakpasa et al /Cell Pt/Ag+ conductmg glass/Pt

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Ftg 4 Comparison of experimental data (symbols) and theoretical curve (sohd hne). computed w~th optimal values efthe parameters for the analogue orcmt m the upper left corner of fig 4 for the measurements at 25°C m the first temperature region

resistor was p r e d o m i n a n t l y capacttive The charge transfer resistance was relatively large. T h e p l a t i n u m electrodes possess the p r o p e r t i e s o f blocking electrodes m the said t e m p e r a t u r e range. M o r e c o m p l i c a t e d o r c u i t s h a d to be e m p l o y e d at temperatures above 100 °C to obtain a reasonable fit In spite o f the m o r e c o m p h c a t e d circuits the agreem e n t between e x p e r i m e n t a l d a t a a n d theoretical curve b e c o m e worse with increasing temperature. Therefore an m t e r p r e t a t i o n o f the d a t a a b o v e 100 ° C on the basis o f the s i m u l a t i o n results was not att e m p t e d However, it cart be stated m a qualitative fashion that the blocking b e h a v i o r o f the p l a t i n u m electrodes is still reflected at higher a n d m i d d l e frequencies while the influence o f a F a r a d a l c reaction becoraes m o r e p r o n o u n c e d at lower frequencies with mcreasmg temperature T h e simulation a p p r o a c h taken in this m a n u s c r i p t

for the first t e m p e r a t u r e range has not yet been app h e d to the d a t a in the second t e m p e r a t u r e range

Acknowledgement T h e p r e p a r a t i o n o f the glass by the group o f P r o f B D u n n , U C L A , U S A is gratefully acknowledged

References [ 1] P Llnhardt, M Maly-Schrelber and M W Brelter, m Proc 6th Intern Syrup Metallurgy and Materials Soence, eds F W Poulsen, N Hessel Anderson, K. Clau~n. S. Skaarup and O Tort Serensen (Rise National Laboratory, Roskdde, 1985 ) p 474 [2] J Brumlnk, J Eieetroanal Chem 51 (1974) 141