Study of electrode kinetics at the interface between high-Tc superconductors and solid electrolytes

15 April 1994

CHEMICAL PHYSICS LETTERS ELSEVIER

Chemical Physics Letters 221 ( 1994) 23-26

Study of electrode kinetics at the interface between high-T, superconductors and solid electrolytes Hala Saad Zaghloul, M.H. Zayan, T.M. Nazmy, M.A. Abdel Raouf

’

Physics Department, Faculty of Science, Ain Shams University Abbassia, Cairo, Egypt

Received 14 December 1993; in final form 22 February 1994

Four different contact methods have been utilized to study the electrode kinetics of high-T, superconductor (HTSC)/solid electrolyte (SE) electrochemical systems. By using a transient technique, cell currents were measured versus time at a constant potential. Investigations with different electrochemical systems were implemented to calculate the potential-dependent charge transfer resistance of the electrochemical phase boundary reaction at the used solid electrolyte, the contact resistance and ohmic resistance of the solid electrolyte in contact with the HTSC. From the dc limits of the transients, a small increase in the electrode kinetic current is observed around r, which must be correlated to the superconducting state of the HTSC. T’he results are interpreted as an enhancement of the electrode kinetics according to the values of the parameters mentioned before.

1. Introduction Electrochemical experiments on different interfaces, formed by condensed matter, have been carried out extensively, to study the transfer of charge carriers across different interfaces. Electrochemical studies on ceramic high-temperature superconductor (HTSC) and ionic conductor (IC) interfaces in a wide range of temperatures are of great interest for both kinds of ionic conductors. Electrochemical charge transfer processes can be studied with either liquid or solid electrolytes. Liquid electrolytes (LE) have the advantage of making contact with the substrate perfectly and so exhibit a well-studied electrochemical double-layer structure. Unfortunately, the choice of inorganic or organic solvent mixtures, being liquid at actual T, and dissolving a supporting electrolyte to maintain a sufficiently high ionic conduc’ Present address: Solid State Division, Oak Ridge NL, Oak Ridge, TN 37831-6032, USA. 0009-2614/94/$07.00

tivity, is restricted to only a few systems [ 11. On the other hand, the obvious disadvantages of solid electrolytes include contact problems with the HTSC, the largely unknown structure of the electrochemical double layer and low ionic conductivity at low temperatures. However, it is easy to prepare thin electrolyte layers in order to reduce the ohmic resistance. Electrochemical experiments on HTSC/LE interfaces were mainly carried out at elevated temperatures, i.e. T>> T,, in order to study surface stability, ageing effects, phase boundary reactions such as corrosion, metal deposition and redox reactions, photoelectrochemical effects, etc. [ 2-51. The mechanism of the charge-transfer process across a HTSC/LE interface can only be studied at low temperatures T<< T,, in order to get information about the electrode

0 1994 Elsevier Science B.V. All rights reserved

SSDI0009-2614(94)00237-K

kinetic

current,

what

happens

to this current

the system Ag/HTSC/SE/Ag is in the superconducting state and what is the effect of the contact resistance. This Letter deals with explaining the above queswhen

24

H.S. Zaghloul et al. /Chemical

tions. Results on four different contact methods with electrodes are given. Investigations of the charge carriers transfer phenomenon from the point of view of the current electrode kinetics are presented.

2. Experimental apparatus The electrochemical measurements of the electrochemical cell Ag/HTSC/SE/Ag were performed with press contact, i.e. mechanically, between a polycrystalline HTSC as a working electrode and RbAg& as a counter and reference electrode by four different types of contacts: by sputtering silver (20 nm thickness) on the back of the SE, silver foil (0.1 mm in thickness), silver powder and special silver epoxy resin for low temperatures (polytec). All external contacts to the HTSC as well as to the counter electrode and reference electrodes were made using the silver epoxy resin as a contact material for metallic silver fibers. The methods used are the four-probe technique for the determination of the bulk resistance of the HTSC and the SE contact and interfacial resistance. In order to work in an appropriate temperature range, a cryostat based on a liquid-helium cooling system (Cryophysics LTS-2ZDRC 9 1C) was used to adjust the temperature in the range 12 K< T-c 298 K under vacuum conditions with an accuracy of 1 K. Two different techniques were applied potentiostatically at the rest potential AE. The first was the electrochemical impedance spectroscopy (EIS ) technique in the frequency domain followed by a transfer function analysis. The second was a special transient technique in the time domain using a large signal system perturbation with a periodic rectangular function of the potential. Both techniques gave identical results; the EIS measurements were used to confirm the results from the transient technique.

Physics Letters 221(1994)

23-26

electrochemical studies on the interfacial behavior of the HTSC/SE cell at temperatures above and below T,. Accordingly, the increase in the contact resistance, R,, increases the electrochemical measurements Z(s) so utilizing different contact methods gives the possibility to select one of the optimum results. The temperature dependence of the four contact resistances is shown in Fig. 1. It is seen from the figure that the contact resistance R, (Ag/SE) sharply increases with decreasing temperature. Also, results show that the contact using silver sputtering and silver foil are of lower contact resistance than using silver powder and silver paint. Indeed, this difference in R, values with different contact methods was related to the effective area of the deposited silver on the contact surface. The transfer process of the charge carriers across the interface HTSC/SE was studied via the transient technique by employing an alternating potential with amplitude 1 V between the working and the counter or reference electrodes. The current transients were recorded versus time. The dc limits of the cathodic and anodic current transients were found to be nearly symmetrical with respect to i=O. Alternating pulse polarization was chosen in order to avoid irreversible polarization effects within the SE. The difference of the anodic and cathodic, quasi-dc current 1Ai 1, measured after t= 20 s, is logarithmically plotted versus T-’ in Fig. 2 for the polycrystalline high-T, superconductor T1,BazCaCu,Os measured by the twoprobe technique. Two principal features of this result are apparent; the first is the strong decrease of 1Ail with decreasing temperature which is observed at Ts T, corresponding to the increase in the resistivity. Another important feature of the results is that

14)

I

3. Results and discussion Electrochemical measurements were performed under exact potential and temperature control to get the ohmic resistance Rn of the superionic conductor. The values of R were found to be rather high at low temperatures. However, they were low enough for

2

82 9’6 temperature/K

lb8

120

Fig. 1. Four different contact resistances of the system Ag/RbAgJJAg. (0 ) Ag sputtering; ( l ) Ag foil; ( + ) Ag powder, ( * ) Ag paint.

25

H.S. Zaghloul et al. / Chemical Physics Letters 221(1994) 23-26

Therefore, the extraordinary exchange in the current density would correspond to either a decrease in the charge transfer resistance R, (kinetic effect) or a decrease in the ohmic resistance RQ (proximity effect ) [ 9 1. To distinguish between the above two proposals, electrochemical experiments on the system Ag/HTSC/Ag, Ag/SE/Ag and Ag/HTSC/SE/Ag are considered. The experimental overall impedance of the latter system consists of a series combination of the following components: z=

ZHTSC

+ z,,HTSC

+ ZSE

+&E/&

+ ZHTSC,SE

. (1)

T-‘*103(K-‘) Fig. 2. Temperature dependence of the current density measured by the two-probe transient technique in the system T12Ba2CaCu20,/Ag+ glass.

around T,, an increase in the current is observed which coincides with the transition or critical temperature of the superconductor. The electrode kinetic current increases by 1.4 times around the transition temperature. However, the increase is not maintained and when the temperature has dropped approximately another 10 K the current density resumes the behavior it would have had if superconductivity had not set in. It is easy to give a hypothesis as to the increase in the current density in the superconducting state. Thus, here, the Cooper pairs which easily perform transitions through the crystal come to the interface since the coherence length of Cooper pairs in HTSCs is about 0.6 + 0.1 nm, and their properties of minimizing the interaction of the surroundings are maintained so that the ease of electron transfer across the double layer is increased. However, it would have been expected that this enhanced current would have continued as the temperature fell but this is not the case. Similar results have been observed by Bockris and Wass [ 61 for the hyevolution drogen reaction in the system YBa,Cu30,_Jfrozen HC104.5Hz0. Pinkowski et al. [ 7 ] observed a hump in the exchange current density in some redox reactions examined in silver l3”-alumina. Murray [ 8 ] also reported a non-monotonic temperature dependence of the double layer capacitance around T, in the system T12Ba2Ca2Cu20LO/liquid

organic

temperature.

electrolyte

mixture

with

low

melting

The first four components were determined using two- and four-probe techniques of the first two systems as shown in Figs. 3 and 4. The results show a

ll-

7

80

85

90

1M)

95

lb5

110

115

Temperature/K

Fig. 3. Temperature dependence of RbAg& impedance: (a) bulk resistance measured by the four-probe technique, (b) overall impedance measured by the two-probe technique and (c) contact resistance calculated from (a) and (b). 0.71

I

,

I

I

T/K Fig. 4. Temperature dependence of the HTSC resistance: (a) bulk resistance measured by the four-probe technique, (b) overall impedance measured by the two-probe technique and (c) contact resistance calculated from (a) and (b).

H.S. Zaghloulet al. / Chemical PhysicsLetters221(1994) 23-26

26

relation between the resistances of the HTSC and SE as a function of temperature. In both figures, curve (a) gives the bulk resistance of the material Rn using four-probe measurements, lim[Z(s)]=R,(HTSCorSE).

(2)

Curve (b) represents the two-probe measurements which include the contact resistance term given by lim [Z(s) ] =2R,(Ag/HTSC +Rn(HTSC

or SE)

or SE) .

(3)

Curve (c) was calculated from (a) and (b) to get the contact resistance of both HTSC and SE with silver. The dc currents of ( Ai 1HTSCand 1Ai I Agat a constant potential AE between the working and reference electrodes are given approximately by IAilj=AE/(Ri,+Ri+R&),

RzT:Tsc

<< R

FTsc

z

RF=.”

,

(6)

was not seen in our measurements. Therefore, the extraordinary exchange in the current density is interpreted as a real quantum electrochemical kinetic phenomenon [ 91 caused by the transfer of the charge carriers across the interface HTSC/SE. which

4. Conclusion The present investigations emphasize that the change in the number of charge carriers transferring across the interface HTSC/SE around T, affects the electrode kinetics causing an extraordinary enhancement in the current density. Using different contacts confirm that the non-monotonic behavior of the current density can be interpreted as a real quantum electrochemical kinetic phenomenon.

(4) Acknowledgement

where R’,, R{ and RJA denote the contact resistance between j=HTSC or Ag and the solid electrolyte SE, the charge transfer resistance at the interface and the ohmic resistance of SE in contact with j, respectively. According to the afore-mentioned first proposal, RpTsC decreases sharply at T-e T,; I Ail HTSCin Eq. (4) is determined by the values of RgTsc directly only under the condition

The experimental work reported here was carried out at the Institute of Physical Chemistry and Electrochemistry, Karlsruhe University, Germany. One of us (HSZ) would like to thank Professor Dr. W.J. Lorenz for the use of the facilities of his Institute and for many stimulating and valuable discussions.

R FTso << R FTsc x R gTso ,

References (5)

which is observed in the electrochemical impedance measurements shown in Fig. 5. On the other hand, the second interpretation presumes that RzTSC drops at T-c T, due to a proximity effect. This leads to an increase in I Ai I HTsc under the condition 1

r

Fig. 5. Electrochemical impedance measurements versus temperature on the system TlzBa2CaCu,0s/RbAg&.

[ 1] A. Pinkowski, J. Doneit, K. Jiittner, W.J. Lorenz, G. Saemann-Ischenko, T. Zetterer and M.W. Breiter, Electrochim. Acta 34 ( 1989) 1113. [2] M. Bachtler, W.J. Lorenz, W. Schindler and Cl. SaemannIschenko, Modem Phys. Letters B 2 ( 1988) 8 19. [3] M. Hampel, E.W. Grabner, M. Bachtler and W.J. Lorenz, Modem Phys. Letters B 3 ( 1989) 303. [4] J.M. Rosamilia and B. Miller, J. Electroanal. Chem. 249 (1988) 205. [ 51 S. Rochani, D.B. Hibbert, S.X. Dou, A.J. Bourdillon, H.K. Liu, J.P. Zhou and C.C. Sorell, J. Electroanal. Chem. 248 (1988) 461. [6] J. O’M. Bockris and J. Wass, J. Electroanal. Chem. 267 (1990) 329. [ 71 A. Pinkowski, J. Doneit, K. Jtlttner, W.J. Lorenz, G. Saemann-Ischenko and M.W. Breiter, Europhys. Letters 9 (1989) 269. [S] R.W. Murray, unpublished lectures at the University of Karlsruhe ( 1990). [9] H.S. Zaghloul, M.H. Zayan and M.A. Abdel-Raouf, Proceedings of the International Symposium on High-T, Superconductivity and Its Applications, Cairo, Egypt ( 1993).