Journal of Power Sources, 14 (1985) 295 - 303 295

OXYGEN ION CONDUCTION IN PURE AND YTTRIA-DOPED BARIUM CERATE

A N VIRKAR and H S MAITI

Materzak Science Centre, Zndaan Znstltute of Technology, Kharagpur - 721 302 (Zndla)

(Received March 1, 1984, accepted m revised form September 22, 1984)

summary

Electrical conductivity of pure and Y,03-doped BaCeOs has been mea- sured as a function of oxygen partial pressure (down to lo-l4 atm) m the temperature range 600 - 1000 “C In the high partial pressure region the specimens behave as mixed conductors with sigmficant electron-hole con- duction However, below about 10m6 atm they become predommantly ionic conductors The observed conductivity variation has been explamed on the basis of anmhllation of existmg oxygen vacancies with excess oxygen. The ionic transference number has been determmed as a function of oxygen partial pressure

1. Introduction

Oxygen ion conductors have received considerable attention due to their use as electrolytes m oxygen sensors, oxygen pumps, fuel cells, etc To improve the performance of these devices, it is necessary to seek new and better electrolytes The electrolytes most widely studied for this purpose are those with the fluorite structure They include calcia and yttria-doped ZrOz [l, 21, HfOz [3], ThO? [4 - 71 and CeO,! [8,9] In recent years mixed oxides havmg the perovskite structure (A*+ B4+ 0,) have also been reported to exhibit srgmficant oxygen ion conduction [ 10 - 121

Usually ionic conduction is not favoured m these oxides because of their close-packed structure The oxygen ion conduction m perovskites, how- ever, can be strongly enhanced by partially substitutmg cations of lower valency for the A or B cation so as to introduce oxygen vacancies m the structure [13, 141 Therefore, a distmct possibihty of their application as oxide electrolytes exists. The conductivities of some of the oxygen deficient perovskites such as CaTlo ,Al,s02 ss, CaTi,, gsMg, sSOz s5 are comparable with those of stabilized zlrcoma [15]

The compounds BaZr, ai, i02 95, BaCe,-,_&,102.95 and BaTh, gsLa,os - 0 z 975 have also been reported to show high ionic conductivity [ 161 Ste- phenson and Flanegan [17] have reported evidence of oxygen ion conduc-

037%1753/85/$3 30 0 Elsevler Sequola/Prmted m The Netherlands

296

bon from e.m f. measurements m PbZr ,, 53Ti0 4,0s (PZT). The electrical con- ductivity of SrCe03 is also reported to be due to the mobility of oxygen ions [ 181 A prehmmary mvestigation by Longo et al [ 181 mdicated the possibil- ity of oxygen ion conduction m pure BaCe03 The dependence of the elec- trical conductivity of Y,O,-doped BaCeOs on dopant concentration has been reported by Vvkar et al [ 191 In this paper, we report a more detailed mves- tigation mto the ionic conductivity of pure and yttna-doped BaCeOs

2. Expenmental

The samples studied, z e , pure and Y,O,-doped BaCeO, with dopant concentrations of 1, 2 and 10 mole % (expressed as mole percent. of CeOz), were prepared by a solid state smtermg process using ‘AnalaR’ grade BaC03, Y *O, and CeOz. Detals are given elsewhere [ 191. The formation of the com- pound and the sohd solubllity were confirmed by X-ray diffraction measure- ments (Cu Ka radiation)

For the electrical measurements, platinum paste (Engelhard No 6082) electrodes were applied on the flat surfaces of the specimens Platmum foils attached to lead wires were pressed agamst the specimen surfaces by a sprmg loadmg arrangement Altematmg current (1 kHz) electrical conductivity measurements were carried out with an LCR bridge The temperature of the specimen was controlled withm +l “C and the desired oxygen partial pres- sure around the sample was generated usmg an electrochemical oxygen pump, fabricated m this laboratory using a stabihsed znoma tube The con- struction is similar to that described by Agarwal et al [20] The oxygen partial pressure of the flowing gas was varied by manual control of the current through the electrochemical oxygen cell The gas, with controlled oxygen partial pressure, was transferred to the specimen holder through a copper tube The exact oxygen partial pressure of the gas around the spec- imen holder was determined using a stabilized zircoma oxygen sensor, sited close to the specimen. Dunng measurement, sufficient time was allowed to establish equihbnum

3. Results and discussion

The effect of the oxygen partial pressure (PO,) on the conductivity (a) of these specimens, measured at fixed temperatures of 600, 700 and 800 “C, is shown m Fig 1 It will be seen that, m each case, the conductivity drops mitially with decreasing PO, At still lower Pot, however, it becomes mde- pendent of partial pressure and continues so to be down to a value of lo-l4 atm. As expected, the conductivity at each PO, increases with temperature and Y,03 content [19]. In the high PO, region the slope of the curve varies slightly (between l/7 and l/9) with temperature as well as with Y?Os addi- tion [4] The slope tends to increase with temperature but to decrease with

297

--a. __ l -

‘i”5 0 -2 -4 -6 -8 -10

log Pop (otm)

-12 -14

Fig 1 Electmal conductwty as a function of log PO, at 600, 700 and 800 “C for ‘pure’ and YzOs-doped BaCeOa

addition of Y,Os It is difficult to identify the exact knee of the curves; however, for all the specimens it lies m the PO2 range of lo-’ - lo-’ atm.

These results mdicate that both pure and Y,O,doped BaCeO, spec- imens are not fully ionic conductors throughout the PO2 range of measure- ment. Above about 1r6 atm the material behaves as a mixed conductor with sigmficant electron-hole conductivity, while below this pressure it behaves as a predommantly iomc conductor mdicated by the Po2-mdependent con- ductivity. This is very similar to that observed m Th02-base electrolytes [ 41.

Partial hole conduction m the high partial pressure range may be explamed on the basis of two probable defect reactions. One of these alter-

298

natives is that the oxygen from the gas phase IS mtroduced m the lattice as doubly charged mterstitials Accordmgly, the defect reaction and the corre- spondmg neutrality condition are written as (using conventional notations)

;w +00:‘+2h

2[0:‘] = [h] (2)

These lead to a l/6 dependence of log u on log PO, [21] The other possible defect reaction is that the existing oxygen vacancies

are anmhllated by the excess oxygen according to the defect reaction

(3)

which leads to a l/4 dependence of log o on log PO, [ 211. The theoretical slopes of either l/6 or l/4 may, however, be observed only when the mate- rial is prepared m the extremely pure state and the measurements are carried out m the high PO_ range sufficiently far from the knee region It is unreal- istic to assume that even ‘pure’ material does not contam traces of ahovalent impurities The lower slopes observed are due to the fact that the PO, range of measurement is close to the knee region and there is appreciable iomc trans- port along with hole conduction [ 41.

It may be mentioned that reaction (3) is more likely to take place m these materials than reaction (1) since the creation of oxygen mterstitu-& is quite difficult m the closely packed perovskite lattice

When BaCeOs is doped with Y,Os it is expected that Y 3+ ions go preferably to the Ce4+ sites, creating negatively charged defects such as Y& It may be assumed that the charge compensation takes place by the forma- tion of doubly charged oxygen vacancies Therefore the defect reaction is written as

Y*Oa - 2Y& + v,- + 300 (4)

leadmg to the formation of a high concentration of oxygen vacancies, re- sponsible for enhanced ionic conduction In the high PO2 regron, on the other hand, reaction (3) is still valid and contributes to the partial pressure depen- dence of the conductivity. The mcreased ionic contribution is, however, responsible for the observed lower slope for the higher dopant concentra- tion (Fig. 1).

From the experimental results, the variation of hole conductivity (oh) with PO may be obtained by subtracting the PO independent ionic con- ducti& from the measured total conductivity m the mixed conductron region, assummg that the magnitude of iomc conductivity remams constant, even m this range Such plots of the difference between total and iomc conductivltles (err - u,) agamst log PO2 for different specimens are shown m Fig 2 It may be noted that the slopes of these plots compare quite well with the predicted value of l/4 (eqn (3)), particularly for the ‘pure’ sample

299

.6

- ‘Pure’ BaCeOg, --- I ml0 Y203,

- - IOmlo Y24,

-0 6OO.C

& 700~C

*eoo’c

0 2 4 6 8

-log Po2 (atml

Fig 2 Plot of (uT- u& us log P4 for ‘pure’ and doped BaCeOJ at different temper- atures

and that with lower dopant concentration. A small devration m the slope for the highly doped specimen is due to the larger contribution of the ionic conductron [ 41.

It is observed that both pure and doped specimens of BaCe03 become predommantly ionic conductors only at lower partial pressures. The values of ioruc conductivity m the lower PO, range are lower compared with those measured m au (z e , III high Pol).

The variation of iomc conductivity with temperature and Y *Os content is shown m Fig. 3 As m the case of air, the conductivity increases with tem- perature; however, with a lower activation energy, except for the 10 mole 7%

0 -‘Pure* BaCeO3, E 0 0 25 ev

A- I m/o Y24, E = 0220~

l - 2ml0 Y203, E =023ov

A - lomlo ‘2O3, E - 0 58 ev

YSi!+ ZrO2 -9mlo Y2O3 (22 1

YDTd ThO2 - lSm/o Y2O3 (23 1

I I I I I

3 09 IO I I I 2 I 3

I/T x Iti3 ( Ki’

Fig. 3 Iomc conductwty us l/T for ‘pure’ and YzOs-doped BaCeOB (1, 2 and 10 mole %)

Y,Os specimen for which the value is shghtly higher than that observed m an [ 191. The ionic conduction actlvatlon energy, 1 e , measured m the low PO, rmon, 1s lower m the case of 1 and 2 mole % Y,03doped BaCe03 than m those obtained for conductivity measured m an, z e , m the high PO, regron [ 191. This might be due to the predommant lomc conduction at low PO1 m doped BaCeOs. However, the activation energy (0.57 eV) of highly doped BaCeO, IS high as compared with that calculated for lower dopant concen- tration (~0.22 eV) which might be the result of ordermg of the oxygen vacancies Ordering occurs readily at higher dopant concentration.

For comparison, the lomc conductivltles of two commercial solid electrolytes, yttria stabrhzed zvconia (YSZ) [22] and yttrla doped thorra

301

IOU Concentration of Y203 (added (m/o1

IO'

Fig 4 Conductnnty as a function of Yz03 content at fixed temperatures

(YDT) [23] are also plotted m the Figure It may be noted that even though the pure and low-Y,Os-contammg specimens have lower conductivities, the value for the 10 mole 7% Y,Os specimen compares quite well with those of YDT and YSZ Due to its lower activation energy the conductivity even becomes higher than that of YSZ below about 500 “C

It follows from reaction (4) that the iomc conductivity is proportional to the Y,Os content, so there should be a lmear dependence between log o1 (ionic) and the logarithm of concentration. This is observed m Fig 4, m which both the ionic and the total conductivity are plotted against concen- tration for a fixed temperature. It is evident that the ionic conductivity is l/4 to l/5 of the total conductivity Beyond 10 mole % however, the con- ductivity becomes almost independent of concentration, probably due to the clustermg of defects

From the data m Fig. 3 a rough estimate of the ionic transport number is made by using the relation t, = uJ,/uT at different oxygen partial pressures. The values are plotted m Fig. 5 It is observed that the trarsference number, as expected, mcreases with mcreasmg dopant concentration and becomes umty below the oxygen partial pressure of about 10F8 atm

302

01 I I I I I 0 2 4 6 8 IO

-log P% lotml

Fig 5 Ionic transference numbers for Y20rEiaC!e03 solid solution as a function of partial pressure of oxygen at 800 “C

4. Conclusions

BaCeO,, which has a perovsklte structure, shows slgmflcant oxygen ion conduction below an oxygen partial pressure of around 10m6 atm In the high partial pressure range it behaves as a mured conductor with appreciable electron-hole conduction On the addition of Y 203, Ce4+ ions are substituted by Y3+ ions with a sigmflcant mcrease m the vacancy concentration and a consequent mcrease m electrical conductlvlty and lomc transport number From the point of view of electrical conductlvlty, Y,03-doped BaCe03 may act as a good substitute for Y,O,doped ThO, as an oxide solid electrolyte

Acknowledgements

The authors thank Mr S K Sundaram and Mr B K Das for their help m specimen preparation and X-ray dlffractlon work Fmanclal support by the DST, Govt. of India (ProJect No. HCS/DST/393/76) 1s gratefully ac- knowledged

References

1 D K Hohnke, m P D Vashlshta, J N Mundy and G K Shenoy (eds ), Fast Zon Trnnsport m Sohds, Elsevler, Amsterdam, 1979

303

2 W D Kmgery, J Pappas, M E Dotty and D C Hdl, J Am Ceram Sot, 42 (1959) 393

3 N M Tallan, W C Trlpp and R W Vest, J Am Ceram Sot , 50 (1967) 279 4 H S Malt1 and E C Subbarao, J Electrochem Sot, 123 (1976) 1713 5 K Kmkola and C Wagner, J Electrochem Sot , 104 (1957) 379 6 B C H Steele and C B Alcock, Trans Met Sot AIME, 233 (1965) 1359 7 J M Wmner, L R Bldwell and N M Tallan, J Am Ceram Sot , 50 (4) (1967) 198 8 M D Hurley and D K Hohnke, J Phys Chem Sohds, 41 (1980) 1349 9 H L Tuller and A $ Nowlck, J Electrochem Sot, I22 (1975) 255

10 Y Matsuo and H Sasakl, Jpn J Appl Phys, 3 (1964) 799 11 D B Schwarz and H U Anderson, J Electrochem Sot, 12 (1975) 707 12 C V Stephenson and C E Flanagan, J Chem Phys, 34 (1961) 2203 13 T Takahashl and H Iwahara, Denkz Kagaku, 35 (1967) 433 14 T Takahashl, H Iwahara and T Imalchl, Denkz Kagaku, 37 (1969) 857 15 K W Browall, 0 Muller and R H Doremus, Mater Res Bull, 11 (1976) 1475 16 T Takahashl and H Iwahara, Energy Cowers, 11 (1971) 105 17 C V Stephenson and C E Flanagan, J Chem Phys, 34 (1961) 2203 18 V Longo, F Rlcclard Iellow and 0 Stabzer, Energy and Ceramics, m P Vmcertgm

(ed ), Proc 4th Int Meetzng on Modern Ceramzc Technologzes, San-Vzncent, Italy, May, 1979, Elsevler, Amsterdam, 1980

19 A N Vlrkar, B K Das, S K Sundaram and H S Malti, Trans Znd Ceram Sac , 41 (3) (1982) 63

20 Y K Agrawal, D W Short, R Guenke and R A Rapp, J Eiectrochem Sot , 121 (3) (1974) 354

21 F A Kroger and H J Vmk, m F Se&z and D Turnbull (eds ), Solzd State Physzcs, Vol 3, Academic Press, New York, 1956, p 307

22 J M Dixon, L D LaGrange, V Merten, C F Miller and J T Porter, J Electrochem Sot, 110 (3) (1963) 276

23 M F Lasker and R A Rapp, 2 Phys Chem N F, 49 (1966) 198