electrochemical properties of bace08gd02o3 electrolyte...

J. Electrochem. Soc., Vol. 144, No. 3, March 1997 The Electrochemical Society, Inc. 1035

10. B. A. Levy M. L. Green, and P. K. Gallagher, ThisJournal, 131, 2175 (1984).

11. H. W. Piekaar, L. F Tz. Kwakman, and E. H. A.Granneman, in Proceedings of the 6th InternationalIEEE VLSI Multilevel Interconnection Conference,-1989, p. 122, IEEE, New York (1989).

12. K. P. Cheung, C. J. Case, R. J. Schutz, B. S. Wagner,L. F. Tz. Kwakman, D. Huibregtse, H. W. Piekaar,and E. H. A. Granneman, in Proceedings of the 7thInternational IEEE VLSI Multilevel InterconnectionConference,-1990, p. 303, IEEE, New York (1990).

13. W. Y-C. Lal, K. P. Cheung, C. J. Case, D. P. Favreau, C.Case, B. Liu, B. G. Murray, L. F. Tz. Kwakman, andD. Huibregtse, in Proceedings of the 8th Inter-national IEEE VLSI Multilevel InterconnectionConference-1991, p. 89, IEEE, New York (1991).

14. K. Sugai, H. Okabayashi, A. Kobayashi, T. Yako, andS. Kishida, Jpn. J. Appl. Phys., 34, L429 (1995).

15. A. Kobayashi, K. Sugai, T. Yako, A. Sekiguchi, T.Shinzawa, S. Kishida, H. Okabayashi, H. Kadokura,0. Okada, and N. Hosokawa, in Extended Abstracts55th Autumn Meeting of the Japan Society ofApplied Physics, p. 722, The Japan Society ofApplied Physics, Nagoya) (1994) [in Japanese].

16. K. Tsubouchi, K. Masu, N. Sigeeda, T Matano, Y. Hiura,N. Mikoshiba, S. Matsumoto, P Asaba, P. Marui, andT. Kajikawa, in Digest of Technical Papers, 1990Symposium on VLSI Technology, p. 5 (1990).

17. G. A. Dixit, A. Paranjpe, Q. Z. Hong, L. M. Ting, J. D.Luttmer, B. H. Haveman, D. Paul, A. Morrison, K.Littau, M. Eizenberg, and A. K. Sinha, in Abstractsof International Electron Devices Meeting-1995,p. 1001, IEEE, Piscataway, NJ (1995).

18. K. Sugai, H. Okayashi, T. Shinzawa, S. Kishida, P.Kobayashi, N. Hosokawa, T. Yako, H. Kadokura, M.Isemura, and M. Naruse, Abstract 315, p. 482, The

Electrochemical Society Meeting Abstracts, Vol. 93-1, Honolulu, HI, May 16-21, 1993.

19. K. Sugai, H. Okabayashi, T. Shinzawa, S. Kishida, T.Kobayashi, N. Hosokawa, T. Yako, H. Kadokura, M.Isemura, and M. Naruse, in Proceedings of Symposiaon Interconnects, Metallization, and MultilevelMetallization and Reliability for SemiconductorDevices, Interconnects and Thin Insulator Materials,P.O. Hemdon, H. 0. Okabayashi, N. AM, H. S. Rathom,B. A. Susko, and M. Kashiwagi, Editors, PV 93-25,pp. 290-298, The Electrochemical Society ProceedingsSeries, Pennington, NJ (1993).

20. K. Sugai, H. Okabayashi, P Shinzawa, S. Kishida, P.Kobayashi, N. Hosokawa, T. Yako, H. Kadokura, M.Isemura, and K. Kamio, in Proceedings of the 10thInternational IEEE VLSI Multilevel InterconnectionConference-1993, p. 463, IEEE, New York (1993).

21. A. Takamatsu, M. Shuto, and T. Yoshimi, Abstract 334,p. 501, The Electrochemical Society MeetingAbstracts, Vol. 86-2, San Diego, CA, Oct. 9-14, 1996.

22. K. Sugai, H. Okabayashi, P. Shinzawa, S. Kishida, A.Kobayashi, T. Yako, and H. Kadokura, J. Vac. Sci.Technol., B13, 2115 (1995).

23. K. Sugai, P. Shinzawa, S. Kishida, and H. Okabayashi,in Proceedings of 1991-4th MicroProcess Conference,p. 206, Business Center for Academic Societies Japan,Tokyo (1991).

24. K. Sugai, H. Okabayashi, S. Kishida, and P. Shinzawa,Thin Solid Films, 280, 142 (1996).

25. M. M. IslamRaja, M. A. Cappelli, J. P. McVittie, andK. C. Saraswat, J. Appl. Phys., 70, 7137 (1991).

26. A. Kobayashi, K. Sugai, A. Sekiguchi, S. Kishida, H.Okabayashi, P. Yako, P. Shinzawa, 0. Okada, N.Hosokawa, and H. Kadodura, Electron. Commun.Jpn., Part 2, 78, No. 12, 50 (1995).

Electrochemical Properties of BaCe08Gd02O3 Electrolyte Films

Deposited on Ni-BaCe08Gd02O3 Substrates

Vishcil Agarwal* ond Meilin Uu**

School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 3 0332-0245, USA

ABSTRACT

Solid oxide fuel cells (SOFCs) based on Ba Ce4 4Gd4 203 (BCG) electrolyte films are constructed and tested at interme-diate temperatures (700 to 800°C) using hydrogen as fuel and air as oxidant. The ionic and electronic conductivitiesaswell as the interfacial properties of the BCG electrolyte films, as deposited on Ni-BCG substrates, are also determinedfrom impedance and open-cell voltage measurements. Results indicate that the performance of the fuel cells isvery sen-sitive to materials selection and to processing. Diffusion of Ni from substrates into the BCG electrolyte filmsduring pro-cessing increases not only the bulk and interfacial resistances but also the electronic transference numbers of the elec-trolyte films, resulting in reduced open-cell voltages and poor power output. The performance of solid oxide fuel cellsbased on BCG electrolyte films may be substantially improved, however, by avoiding Ni diffusion into BCGelectrolytethrough proper selection of materials and modifications in processing.

IntroductionBarium cerate-based materials exhibit relatively high

ionic conductivities at intermediate temperatures (600 to800°C) and are thus an attractive choice as electrolytesfor intermediate-temperature solid oxide fuel cells.Gd-doped barium cerate with a composition of BaCe04Gd403 exhibits the highest conductivity among the bari-um cerate-based electrolytes studied.2 Further, the long-term stability of the BCG electrolyte has been demon-strated under hydrogen-air fuel cell conditions4'5 and inH30-containing conditions.6 The performance of SOFCs,however, can be further enhanced by reducing the thick-ness of the BCG electrolyte.4

For successful construction of an SOFC based on a BCGelectrolyte film, several technical challenges must be over-come. The electrolyte film must be dense and crack-free to

° Electrochemical Society Student Member.* Electrochemical Society Active Member,

avoid short-circuiting and fuel leakage; the electrodesmust be electrically conductive, catalytically active, andporous (to facilitate gas transport); and the seals must beelectrically insulating and gastight (to prevent leakage). Inour earlier studies, a colloidal7 and a modified Pechini°process were investigated to prepare BCG electrolyte filmson various porous substrates. Electrically conductive andporous Ni-BCG disks were used as substrates for BCGfilms to facilitate the electrical characterization of thesefilms. In addition, the use of Ni-BCG reduces the thermalexpansion mismatch between the electrolyte film and thesubstrate.

In this study, single cells based on BCG electrolyte films,prepared on porous Ni-BCG substrates, were constructedand tested. Open-cell voltage (OCV), impedance spectra(IS), and current-voltage characteristics of each cell wereacquired under various testing conditions to characterizethe electrochemical properties of each cell component aswell as the overall performance of each cell.

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1036 J. Electrochem. Soc., Vol. 144, No. 3, March 1997 The Electrochemical Society, Inc.

ExperimentalPorous Ni-BCG composites were used as anode and as

supporting substrates for preparation of BCG electrolytefilms. The BCG powders used in the Ni-BCG compositewere prepared by conventional ceramic processing.9 Toensure that the substrate is electrically conductive, thecomposition of the composite was so chosen that the vol-ume fraction of Ni metal would be 40% after reduction ofNiO to Ni metal. Disks of NiO-BCG (1.7 cm diam and 1.5 mmthick) were prefired at 1000°C and BCG electrolyte filmswere subsequently deposited as described elsewhere.7Unfritted silver paste (Heraeus: C4400UF) was screen-printed on the top of each electrolyte film and fired at800°C for 10 mm to form a porous silver electrode. Thesamples were then attached to an alumina tube using aglass seal. A hermetic seal is crucial to the reliable testingof SOFCs. The sealing glass composition was chosen suchthat its thermal expansion coefficient lies between alumi-na and BCG and its softening point was above 800°C toform a hard seal at or below 800°C. The sealing glass con-sists of three glasses, glass A, glass V-960, and glass V805-B, with a weight ratio of 1:2:4. Glass A was prepared in ourlab with the following composition: 49.3 weight percent(w/o) Si02, 17.7 w/o Na,O, 13.7 w/o B203, 7.6 w/o CaO,6.8 w/o A1203, 3.7 w/o K20, 0.66 w/o Li20, 0.3 w/o MnO,,and 0.24 w/o Co20.).'° Glasses V-960 and V805-B wereobtained from Vitrifunction. The glasses were fired in situand the heating and cooling rates were carefully con-trolled to avoid cracking of the glass seal. The furnacewas heated at 7°C/mm to 840°C, held at the temperaturefor 5 mm to soften the glass, and then cooled down at1 .5°C/mm to the desired temperature for testing.

A scheme of the cell test apparatus is shown in Fig. 1.After the glass seals were fired in situ, argon was suppliedto the anode while the cathode was exposed to ambient air.Approximately 30 mm after the cell reached the operatingtemperature (700 to 800°C), the anode gas was switched tohydrogen (water content about 1 volume percent (yb)) toreduce NiO in the porous substrate to Ni metal. At 800°C,NiO was completely reduced to Ni in about 1 h.

Impedance spectra of the cells were obtained at temper-atures ranging from 700 to 800°C in the frequency rangefrom mHz to MHz using a computerized impedance analy-sis system consisting of a frequency response analyzer(Solatron 1255) and an electrochemical interface (Solatron1286). The OCVs and current-voltage characteristics were

also recorded under identical conditions for each imped-ance measurement.

A Hitachi S-800 scanning electron microscope (SEM)was used to characterize the microstructures of cellsbefore and after cell operation. A statistical "point count-ing" method in quantitative stereology" was used to esti-mate the porosities of substrates and electrodes.

Results and DiscussionMicrostructure.—Figure 2a shows a cross-sectional view

of a BCG film (sintered at 1200°C for 2 h) on a NiO-BCG

—Furnace

—Glass seal—. anode

ElectmchemicalTestEquipment

Fig. 2. Cross-sectional views of (a) a BCG film on the top of aNiO-BCG substrate and (b) a single cell after 150 h of operation at750°C. The lower layer was a porous Ni-BCG anode, the middle

Fig. 1. A schematic showing the experimental arrangement for layer was a nonporous BCG electrolyte film, and the top layer wasfuel cell testing. a porous Ag cathode.




substrate. The micrograph indicates that the film wasdense and the grain size ranged from 1 to 3 p.m. The thick-ness of the film was about 8 to 9 p.m, which was derivedfrom two consecutive coatings. The porosity of the NiO-BCG substrate was estimated to be about 10% beforereduction. Figure 2b shows a cross-sectional view of a fuelcell after 150 h of operation at 750°C. The top layer was aporous Ag cathode, the middle layer was a nonporous elec-trolyte film, and the bottom layer was a porous Ni-BCGsubstrate. There were no observable changes inmicrostructure of the electrolyte film after testing, imply-ing that the microstructure of the electrolyte film is stable.The estimated porosity of the Ni-BCG anode, however;increased to 25% due to the additional porosity created bythe reduction of NiO to Ni metal. The porosity of the Agcathode was estimated to be about 22%. The highly porousstructure of the anode and the cathode is desirable for gastransport during cell operation.

Electrochemical properties of BCG films—Open-cellvoltage—Figure 3 shows the OCVs of fuel cells based ondifferent electrolytes as constructed in Fig. 1. Clearly, theOCVs of the cells based on BCG films were lower than thatof the cells based on BCG pellets.12 Three factors mayinfluence the observed OCV across a BCG film on a Ni-BCG substrate: (i) the presence of Ni in the film,7 (ii)reduced thickness of the BCG film, and (iii) gas leakagethrough the glass seals. The readings of the yttria-stabi-lized zirconia (YSZ) oxygen sensor indicated that therewas very little gas leakage through the glass seals; thus,the presence of Ni in the BCG films may reduce the OCVvalues by decreasing the ionic transference number of theelectrolyte. In order to determine if Ni doping has anyeffect on observed OCVs, the transport properties of theBCG films were characterized.

Bulk resistance.—Typical impedance spectra of singlecells based on BCG films are shown in Fig. 4. The inter-cepts of the impedance locus with the real axis at high fre-quencies correspond to the bulk resistances (Rb) of theelectrolyte film, while the intercepts at low frequenciescorrespond to the total resistances (RT) of the cell. Plottedin Fig. 5 are the bulk conductivities of BCG films, whichwere calculated from the bulk resistances as determinedfrom the impedance spectra. For comparison, the data for

0.8 I

650 700

Temperature ( °C)

Fig. 3. Nernslian potential (EN) and open-cell voltages of fuel cellsbased on different electrolytes: (a) YSZ, (b) BCG electrolyte pelletwithout Ni, and (c) BCG film deposited on Ni-BCG substrates. Thecells were exposed to air on one side and to hydrogen (1 yb H20)on the other side.

2

3.2x1(litz 800°C

,'0.05 Hz.2.sxlcfHz •e•••s•.••

•S%.qI. S.

S.U S I I I

13 14 15 16 17 IS

,I.

7S0C1.9xlOHz

1I6xllfHz a

I L

L L1 0.04Hza

aI I

17 19 21 23 25 2

3.8x ló'Hz 70O3.lxldHz ''

0.05 H

I. . U

UU— U

Re {Z, }Fig. 4. Impedance spectra of fuel cells based on BCG electrolyte

films measured at different temperatures (electrode area 0.1 cm2,electrolyte thickness 12 p.m).

a BCG pellet with and without Ni'3 are also plotted inFig. 5. It can be seen that the conductivities of the BCGfilms on Ni-BCG substrates are very close to (slightlyhigher than) those of the Ni-doped BCG pellets but nearlyone order of magnitude lower than those of BCG pellets(without Ni).'3 Thus, it is reasonable to state that Ni sig-nificantly reduces the conductivity of BCG electrolyte,even though the exact amount of Ni in the film isunknown.

0.95 1 1.05

iooorr (K)Fig. 5. Temperature dependance of the total conductivities of dif-

ferent electrolytes tested under H2-air fuel cell conditions: (a) BCGpellet, (b) BCG film (with a small amount of Ni), and (c) Ni-dopedBCG pellet (BaCe08Gd01Ni01O3).

a'NE

2

4

3

2

21 24 27 30 33

1.2

10

0.9C

Temperature800 70

(a) YSZ sensor

(b)BCG pellet

- -. - - - - - (c)BCG film.----.--.--

(tb)70

1E-1

IE-2

1E-3

0

0

000F-750 800

aCG pelletfilm

(c) Ni-doped BCG pellet850

0.9 1.1




0

Temperature ( C)800 750 700

Rb

0685 C

Rp

Interfacial resistance—When the electrolyte of a cell hasconsiderable electronic conduction, the polarizationresistance ((1) of the interfaces (R5) can be calculated fromthe following equation14

p = RT-Rbp l [1]

EN RTI\ EN

where RT is the total resistance of the cell, Rb is the resist-ance of the bulk electrolyte, V0 is the open-cell voltage,and EN is the Nernstian potential applied to the cell.

Shown in Fig. 6 are the specific interfacial resistances(H cm2) of a fuel cell based on a BCG film on a Ni-BCGsubstrate with a configuration of air; Ag/BCG/Ni-BCG,H5(1 yb H20). For rough comparison, the specificinterfacial resistances of two symmetrical cells based onBCG pellets (without Ni) are also plotted in the same fig-ure, one cell using pure Ag electrodes and the other usingAg-BCG composite electrodes (with composition of 62 ybAg and 38 yb BCG).15 The symmetrical cells were tested inair in a four-electrode configuration.'5 The specific inter-facial resistances of the cell based on a BCG film on a Ni-BCG substrate, Ni-BCG/BCG/Ag, are about twice those ofthe symmetrical cell of Ag/B CG(pellet)/Ag, and abouttwenty times those of the symmetrical cell of Ag-BCG/BCG(pellet)/Ag-BCG. These data indicate that inter-facial resistances are very sensitive to materials selectionand to processing. Interfacial resistances can be orders ofmagnitude higher if the electrodes are improperly chosenor processed. It is crucial to have an electrode that is notonly physically and chemically compatible with the elec-trolyte, but also electrically conductive, catalyticallyactive, and can be made porous. The use of Ag-BCG com-posite cathodes for the BCG films may result in lowerinterfacial resistance and, hence, improved performance.

Shown in Fig. 7 are the bulk (Rb) and interfacial (R)resistance of a cell based on a BCG film deposited on a Ni-BCG substrate. The interfacial resistance increased rapid-ly as temperature was reduced because of large activationenergies for interfacial processes.15 The "crossover temper-ature" is about 685°C, below which the interfacial resist-

Temperature (°C)800 750

790

0.95 i 1.05

10

I

0.9

100011' (1(1)1.1

Fig. 7. Temperature dependance of the bulk and interfacial resis-tances of a fuel cell based on a BCG electrolyte film. The cell wastested under H2-air fuel cell conditions (electrode area was 0.1 cm2and electrolyte thickness wos 12 rm).

ance is greater than the bulk resistance. Accordingly, if theoperating temperature is above the crossover temperature,efforts should focus on reducing the bulk resistance toimprove the overall performance of the cell effectively.Below the crossover temperature, however, attempts toreduce the interfacial resistance would be more effectivein improving the overall performance of the cell. In gener-al, the interfacial resistance plays a dominant role indetermining the performance of a device based on a thin-film electrolyte and operated at low temperatures. Oneway to reduce the interfacial resistance is to engineer themicrostructure of the interfaces. The use of Ag-BCG com-posite electrodes, for instance, is proven to be chemicallyand physically compatible with BCG electrolyte andshows improved performance over silver electrodes.15

Transference number—From this discussion it is clearthat the presence of Ni in the electrolyte increased bothbulk and interfacial resistances of the electrolyte. Further;it is more instructive to know the effect of Ni on the ionic(R1) and electronic (Re) resistance of the BCG electrolytefilm using the following equations14

if = [2]

Re =1

[3]

where the OCV measurements and the impedance meas-urements must be performed under identical conditions.14

The estimated ionic and electronic conductivities areplotted in Fig. 8a. It can be seen that the electrolyte filmsexhibit considerable electronic conduction (10 to 15%).

1.1 The ionic transference number (ti) as defined by

R=R,

[41

can be further calculated from R1 and Re. Figure 8b showsthe ionic transference numbers and VIC/EN ratios for BCGpellets (without Ni)12 and for BCG films deposited on Ni-BCG substrates as measured at different temperatures. It

I11

I.)Ca

(a) Ag/SCU (filrn)fNi-BCG

(b) AgIBCG (pdllet)/Ag

0.01 I I I

0.9 0.95 1 1.05

iooorr (K4)

Fig. 6. Temperature dependance of the specific interfacial resis-tances of (a) a fuel cell based on a BCG film on a Ni-BCG substrate(i.e., H2,Ni-BCG I BCG(film) I Ag,air) and two symmetrical cells basedon BCG pellets with different electrodes: (b) air, Ag I BCG I Ag, air,and (c) air, Ag-BCG I BCG I Ag-BCG, air.




Temperature (°C)800 750 700

I

..4I I

(a)

CGfl

(a)

IE-3

IE-4

£..

,—. 0.8

4)

0.6

= 0.4VU

0.2

.£

.A

0.9

£800 C

£

0.95 1 1.05

700C 7•50°C

1.1

£

0 100 200 300 400 5 '0

0,95 }(BcopcItet)Voc/EN V

90

A

800CA

::0.8

.VOc/EN

I(Gmm) 4)

'I

I

301.1 1.15

•

''

•700C

75k

0.7 I I0.9 0.95 1 1.05

100011' ( K')

Fig. 8. Temperature dependance of (a) the ionic and electronicconductivities and (b) the cakulated ionic transference numbers andVOC/EN ratios of BCG electrolyte films tested under Hfair fuel cellconditions.

can be seen that the ionic transference numbers of theBCG films (with a small amount of Ni) are lower thanthose of the BCG pellets. This strongly suggests that thepresence of Ni decreases ionic conductivity and increasesthe electronic transference number of the electrolyte,which reduces OCVs and is detrimental to the perfor-mance of fuel cells based on the electrolyte. Another inter-esting observation is that the deviations of ionic transfer-ence numbers from VOC/EN ratios are larger for BCG filmsthan for the BCG pellet. This is due to the effect of inter-facial resistance and electrolyte thickness, as explainedelsewhere. 14

Fuel cell performance—Figure 9 shows the current-voltage characteristics of a cell in the temperature rangefrom 700 to 800°C. As seen in the plot, a current density ofabout 280 mA/cm2 was obtained at 800°C when the cellvoltage was 0.4 V. At the same cell voltage, the currentdensity was about 200 mA/cm1 at 750°C and 160 mA/cm2at 700°C. The estimated total resistances of the cell (Rr)from the I-V plots in Fig. 9a agree well with the RT valueestimated from the impedance spectra in Fig. 4. The corre-sponding power densities of the cell are shown in Fig. 9b.The peak power output was approximately 113 mW/cm2 at800°C. The observed low current and power densities canbe attributed to the observed degradation in bulk andinterfacial properties of the electrolyte film due to the dif-fusion of Ni into the BCG electrolyte. Thus, significantimprovements in cell performances may be possible if thediffusion of Ni into BCG electrolyte can be prevented dur-

0 100 200 300 400 500

Current Density (mA/cm2)

Fig. 9. Typical (a) current-voltage characteristics and (b) powerdensities of a fuel cell based on a BCG electrolyte film tested inH2-air fuel cell conditions.

ing processing. For instance, the power density would beexpected to increase by one order of magnitude just bypreventing Ni from diffusing into the BCG electrolyte andby the use of an Ag-BCG composite electrode.

ConclusionsSOFCs based on BaCe08Gd02O3 electrolyte films deposit-

ed on Ni-BCG substrates were constructed and testedunder hydrogen-air fuel cell conditions. The diffusion ofNi from the substrate into the electrolyte film not onlyincreased the bulk and interfacial resistances of the elec-trolyte but also reduced the ionic transference number ofthe electrolyte film, resulting in reduced OCV. The ionictransference numbers deviate from the VOC/EN ratios due tothe effect of interfacial resistances. Low conductivities ofthe electrolyte film and high resistances of the interfacesresulted in poor cell performance. While the feasibility ofconstruction and operation of SOFCs based on BCG elec-trolyte films on porous substrates has been demonstrated,many challenges still remain to improve the performanceof the cells through modifications in processing or materi-als selection.

--C.-...-This work was supported by Electric Power Research

Institute (EPRI) under Contract No. RP 1676-19 and byNational Science Foundation (NSF) under Award No.




DMR-9357520, and their financial support of this researchis gratefully acknowledged.

Manuscript submitted Sept. 6, 1996; revised manuscriptreceived Nov. 22, 1996.

REFERENCES1. N. Bonanos, B. Ellis, K. S. Knight, and M. N.

Mahmood, Solid State lonics, 35, 179 (1989).2. N. Taniguchi, K. Hatoh, J. Niikura, and T. Gamo, ibid.,

53-56, 998 (1992).3. H;Iwahara, Solid State lonics, 52, 111 (1992).4. N. Taniguchi, E. Yasumoto, and T. Gamo, This Journal,

143, 1886 (1996).5. N. Bonanos, B. Ellis, and M. N. Mahmood, Solid State

Iortics, 44, 305 (1991).6. Z. L. Wu and M. Liu, This Journal, Submitted.7. V. Agarwal and M. Liu, ibid., 143, 3239 (1996).

8. V. Agarwal and M. Liu, J. Mater. Sci., 32, 619 (1997).9. W. Rauch and M. Liu, in Role of Ceramics in Advanced

Electrochemical Systems, P N. Kumta, G. S. Rohrerand U. Balachandran, Editors, Ceramic Transactions,Vol. 65, p. 73, The American Ceramic Society, Wester-yule, OH (1996).

10. W. Rauch, H. Hu, V. Agarwal, Z. Wu, and M. Liu,Unpublished work.

11. E. E. Underwood, in Quantitative Microscopy, p. 94,Addison Wesley Publ. Co., Reading, MA (1968).

12. H. Hu and M. Liu, Unpublished work.13. W. Rauch and M. Liu, in Ceramic Membranes I, H. U.

Anderson, A. C. Khandkar, and M. Liu, Editors,PV 95-24, p. 146, The Electrochemical Society Proceed-ings Series, Pennington, NJ (1997).

14. M. Liu and H. Hu, This Journal, 143, L109 (1996).15. H. Hu and M. Liu, ibid., 143, 859 (1996).

Ionic Transport Properties of Mixed Conductors: Applicationof AC and DC Methods to Silver Telluride

R. Andreaus' and W. Sitte*,b

°Institut für Chemische Technologie Anorganischer Stoffe, and blnstitut fUr Physikalische und Theoretische Chemie,Technische Universität Graz, A-801 0 Graz, Austria

ABSTRACT

The chemical diffusion coefficient and the ionic conductivity of the fast mixed conductor a'-Ag2+0Te have been meas-ured with high stoichiometric resolution at 160 and 200°C within the range of homogeneity by applying both ac and dcmethods. All measurements were performed on the same symmetric electrochemical cell. The stoichiometry of a'-Ag2Tewas varied in situ by coulometric titration with AgI as a solid ionic conductor. Chemical diffusion coefficients and silverionic conductivities are available from the transient voltage response of the ionic probes during polarization/depolariza-tion experiments as well as from an analysis of the impedance data using an equivalent circuit model. The ac and dcresults regarding the composition dependence of the chemical diffusion coefficient and the ionic conductivity of a'-Ag23Te are in full agreement with each other within the limits of error.

InfroductionDue to their high cationic and electronic conductivities

as well as their high chemical diffusion coefficients, themedium temperature phases of the silver chalcogenides (a'phases) may be regarded as model substances for mixedconducting materials. The a' phase represents one of twostructurally cationic disordered, mixed conducting phasesof Ag2Te.1 The 3—*a' phase transition occurs at about 132 to145°C.'

Transport properties of mixed conductors in varioussolid-state electrochemical cells have been determined bydc and ac techniques, and reviews are available for dc(transient)3'4 as well as ac techniques.'' Maier' gives areview on various solid-state electrochemical dc and actechniques, taking into account internal defect chemicalreactions. Honders et al.'°' measured chemical diffusioncoefficients and thermodynamic factors of solid solutionelectrode materials by ac and dc methods. Recently, thetheory of galvanostatic processes in mixed conductorsemployed in asymmetric cells has been extended to thegeneral case of arbitrary eletronic transport numbers ofthe mixed conductor.'3

In this study we report on the agreement between theresults of ac and dc measurements regarding the determi-nation of the chemical diffusion coefficient and the ionicconductivity of the mixed conductor a'-Ag,,Te using arecently developed solid-state electrochemical cell.'4 Bothchemical diffusion coefficients and ionic conductivities areobtained at 160 and 200°C as a function of composition,the latter being varied by coulometric titration in the solidstate. The advantage of our cell is that the ionic probes are

* Electrochemical Society Active Member. °'ion

located at the opposite ends of the mixed conducting sam-ple; therefore, problems with the exact determination oftheir geometrical position (due to, e.g., "soft" electrolyteedges) can be avoided. Some preliminary results obtainedwith this cell are given in Ref. 15.

TheoryElectrochemical cell—All ac and dc experiments were

performed on the same symmetric solid-state cellequipped with two ionic electrodes and probes at theopposite ends of the sample

Ag -- a' — Ag,, Te Ag [I]Ag,, Pt

An additional platinum contact on the sample allows elec-tromagnetic force (EMF) measurements and the in situvariation of the silver content of a'-Ag,+0Te by means ofcoulometric titration within the whole range of composition.

DC method—The chemical diffusion coefficient, D, andthe partial silver ionic conductivity o can be calculatedfrom the dc voltage response U(t) of the silver referenceelectrodes (Ag). For the polarization process (afterswitching on a constant current at t = 0 and for times t �/2) we obtain the following voltage response'4

U(t) — U(t = c°) = 82Lte exp [1]

with

U(t = ') = --- [2]



electrochemical properties of bace08gd02o3 electrolyte...

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