Synthesis, structure and electrochemical properties of In-doped BaCeO3

Solid State Ionics 107 (1998) 221–229

Synthesis, structure and electrochemical properties of In-doped BaCeO 3 *, H.-J. Lang, A. Maiwald, G. Tomandl ¨ K. Kunstler Freiberg University of Mining and Technology, Institute of Ceramic Materials, D-09596 Freiberg, Germany Received 16 June 1997; accepted 22 October 1997

Abstract Samples in the system BaCe 12x In x O 32 a were prepared in a concentration range 0.05 mole # x # 0.25 mole InO 1.5 and the physical–chemical properties determined. The maximum doping level is 0.20 mole InO 1.5 in BaCeO 3 for which almost single phase products were obtained. The doped barium cerates correspond with the structure of BaCeO 3 with a small amount of CeO 2 . The compounds are stable in reducing and oxidizing atmospheres and have a high solubility of oxygen. The In-doped BaCeO 3 shows a point defect behaviour in the oxygen pressure range from 1 bar to 10 26 bar. The highest total electrical conductivity was observed for a content of 0.20 mole InO 1.5 in BaCeO 3 . It is influenced by the water vapour partial pressure in reducing and inert gases in the temperature range from 7008C to 10008C. The electrical conductivity of In-doped BaCeO 3 does not exceed the conductivity of yttria-stabilized zirconia and not even that of the brownmilleritestructure oxides Ba 2 In 22x Ce x O 5 . The mean ionic transport number is ¯t ion 5 1 in hydrogen between 6308C and about 8008C whereas above 8008C a deviation from 1 is detected. In argon larger deviations from 1 are observed in the whole temperature range.  1998 Elsevier Science B.V. All rights reserved. Keywords: In-doped BaCeO 3 ; Preparation; Perovskite-type oxide; Electrical conductivity; Mean ionic transport number; Oxygen exchange

1. Introduction Solid electrolytes (SE) are used in sensors, solid oxide fuel cells or as oxygen pumps. For using oxide-ionic conductive ceramics it is necessary to have an electrical conductivity greater than 10 mS cm 21 at operating temperature, an almost pure oxygen ionic conductivity and a long-term stability of the SE under operating conditions. Yttria-stabi*Corresponding author. Present address: TU Bergakademie ¨ Keramische Werkstoffe, Gustav-Zeuner-Str. Freiberg, Institut fur 3, Freiberg, Germany. Tel.: (03731) 39-2807 / 2549; fax: (03731) 39-3662.

lized zirconia is commonly used as the oxygen ionic conducting ceramic electrolyte material. The application of this material is limited by a too low oxideionic conductivity below 8008C. Therefore the development of other solid electrolyte materials is required with a higher oxygen ion conductivity. Compounds with perovskite type structure can be considered as a prospective SE in electrochemical cells. These oxides have advantages in the variation of elements in the crystal lattice and of the simplicity in doping aliovalent cations for the formation of oxygen ion vacancies. The doped BaCeO 3 -system was selected for our investigations [1–7]. The investigations were started by testing rare earth doped

0167-2738 / 98 / $19.00  1998 Elsevier Science B.V. All rights reserved. PII S0167-2738( 97 )00542-0

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BaCeO 3 for its suitability as SE in sensors and solid oxide fuel cells [8,9]. Furthermore BaCeO 3 was doped with InO 1.5 in the concentration range 0.05 mole # x # 0.25 mole. The ceramic and electrical properties of these compounds were ascertained. The electrical conductivity values are compared with the values for Zr 0.83 Y 0.17 O 1.91 and Ba 2 In 22x Ce x O 51x . The electrical conductivities of these compounds are higher than those of In-doped BaCeO 3 . Furthermore our investigations have shown that the conductivities for In-doped BaCeO 3 are influenced by humid gases in the temperature range of 7008C to 10008C. This dependence is only predominant at low oxygen partial pressures. The rare earth doped BaCeO 3 prepared by us does not indicate any dependence of the electrical conductivity on the humidity of the gases in the temperature range of 7008C to 10008C [8,9].

2. Preparation and ceramic properties The samples were prepared starting with analytical reagent grade BaCO 3 , CeO 2 and In 2 O 3 . The amounts of dopants were calculated for different values of x in the formula BaCe 12x In x O 32x / 2 assuming the dopant cation replaces the cerium ions in the BaCeO 3 lattice. The samples were prepared by the mixed oxide method in a concentration range 0.05 mole # x # 0.25 mole InO 1.5 . The powders were mixed for 2 h in turbulastirrer, calcined at 12008C for 5 h, milled in a planetary ball mill with isopropanol for 4 h and dried at 1208C. The X-ray diffraction analysis indicated that the solid state reaction BaCO 3 1 (1 2 x)CeO 2 1 (x / 2)In 2 O 3 → BaCe 12x In x O 32 a 1 CO 2 is finished under the applied calcination conditions (where a incorporated the number of oxygen ion vacancies per perovskite-type unit cell, 0.05 mole# x#0.25 mole InO 1.5 ). The calcined powders were ground once again and pressed at a pressure of 500 MPa into pellets and rods without pressing aids. The green densities of the pressed samples were between 70% and 75% of the theoretical density. The pellets

and rods were subsequently sintered at 17008C for 5 h in air. The ceramic samples were gas-tight. The pressure had to be increased up to 884 MPa with increasing doping of the BaCeO 3 in order to obtain a sufficiently high green density. The BaCeO 3 samples with 0.05 mole InO 1.5 achieved 97.3%, with 0.15 mole InO 1.5 91.5% and with 0.25 mole InO 1.5 90.1% of the theoretical density. Microstructure investigations showed with increasing doping content a growth of pores. The pores are evenly distributed and mostly closed. For the lower doped samples the microanalyses indicated a homogeneous distribution of the elements Ba, Ce and In. At the higher doped samples with 0.25 mole InO 1.5 fluctuations appeared in the distribution of the elements. The measurement of sintering shrinkage of the sample BaCe 0.85 In 0.15 O 32 a showed a two-stage sintering. The dilatometer curve showed an initial shrinkage at 8508C. It is caused by shrinkage of the small pores. The highest shrinkage rate occurred between 13008C and 15008C. It is caused by the compaction of the agglomerates. The thermal expansion coefficient of the sample with 0.15 mole InO 1.5 was ascertained to be 1.24310 25 K 21 in the temperature range between 1008C and 10508C.

3. Structural and chemical characterization

3.1. Structure The SE-samples of the composition BaCe 12x In x O 32 a with 0.05 mole#x#0.25 mole InO 1.5 were submitted to extensive X-ray diffraction analyses. The crystal structure of BaCeO 3 is reported to be either orthorhombic or tetragonal [10,11]. In this study BaCeO 3 was found to be orthorhombic accompanied by a small amount of CeO 2 (Fig. 1). In the X-ray diffraction pattern the 011-line (d-value: 0.441 nm) and the 331-line (d-value: 0.163 nm) of orthorhombic BaCeO 3 are always present. The orthorhombic perovskite structure of the BaCeO 3 is also observed for the In-doped BaCeO 3 . In the samples, the fraction of CeO 2 is not connected with the doping content. In the sintered samples the CeO 2 content is always higher than in the calcined. The X-ray densities for the system are presented in

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223

Fig. 1. Typical X-ray diffraction pattern of In-doped BaCeO 3 .

Table 1. The unit cell of the BaCeO 3 was taken as a basis for the determination of the densities of the In-doped samples. The lattice is slightly compressed by the addition of InO 1.5 . This causes a small distortion of the BaCeO 3 lattice. The X-ray density Table 1 X-ray densities of the In-doped BaCeO 3 System

X-ray densities g cm 23

BaCeO 3 (Literature) BaCeO 3 15 mole% InO 1.5 BaCeO 3 110 mole% InO 1.5 BaCeO 3 115 mole% InO 1.5 BaCeO 3 120 mole% InO 1.5 BaCeO 3 125 mole% InO 1.5

6.36 (calc.)

6.29 (exp.) 6.41 6.38 6.35 6.33 6.35

of the BaCeO 3 was calculated to be 6.36 g cm 23 and experimentally determined to be 6.29 g cm 23 [11].

3.2. Chemical composition The quantitative analysis was carried out on sintered samples which were ground to a grain size smaller than 63 mm and then dissolved in concentrated phosphoric acid. The ICP-OES-analysis was applied for the determination of the elements Ba, Ce and In (Table 2). The sintered samples always had a higher content of Ce than the mixed powders. BaO and In 2 O 3 are evaporated during sintering. It was attempted to determine the total oxygen content in the ceramic samples using hot extraction analysis. By this analytical method the oxides are

Table 2 Chemical composition of the ceramic samples of the system BaCe 12x Inx O 32 a System

BaCe 0.95 In 0.05 O 2.975 BaCe 0.90 In 0.10 O 2.950 BaCe 0.85 In 0.15 O 2.925 BaCe 0.80 In 0.20 O 2.900 BaCe 0.75 In 0.25 O 2.875

Ba (wt%)

Ce (wt%)

In (wt%)

Exp.

Theor.

Exp.

Theor.

Exp.

Theor.

40.02 40.67 40.35 41.49 39.86

42.41 42.74 42.85 43.08 43.31

41.23 39.42 37.24 36.14 34.23

41.10 39.15 36.43 35.43 33.14

1.56 2.66 4.34 6.04 7.12

1.77 3.56 5.37 7.20 9.05

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reduced in a graphite crucible at 19008C and the released CO is measured by an infrared detector. Up to now no satisfying results could be obtained. It is necessary to carry out further investigations to use the hot extraction analysis for the determination of the total oxygen content in the ceramic samples. In the empirical formula besides BaCeO 3 (In 2 O 3 ), CeO 2 and CeO 2 (In 2 O 3 ) can be assumed. This assumption is confirmed by X-ray powder diffraction measurements.

3.3. Redox behaviour In the temperature range from room temperature to 10008C under inert, oxidizing and reducing conditions the system BaCe 12x In x O 32 a in the concentration range 0.05 mole#x#0.25 mole InO 1.5 was investigated by differential thermal and thermogravimetric (DTA / TG-analysis). The DTA-curves of the doped BaCeO 3 indicated no peculiarities. The TGanalysis showed a small weight loss of 0.1%. The system is stable in reducing and oxidizing atmospheres.

3.4. Oxygen exchange The oxygen exchange behaviour is an important physical property for the characterization of function ceramics. Under equilibrium conditions, the oxygen content of an oxide ceramics is fixed by the oxygen pressure of the ambient phase and the operation temperature. In the thermodynamic equilibrium a definite oxygen amount is dissolved by the oxide lattice at a constant temperature, provided the oxygen pressure of the ambient atmosphere is increased. Vice versa, the dissolved oxygen is released by the solid oxide when the oxygen pressure is decreased again. The exchanged oxygen amount is an essential parameter for the characterisation of the crystal lattice of an oxide functional ceramics. For the determination of the exchanged oxygen amount the pulverized material was annealed between 1008C and 9008C in a gas stream with a pH 2 O /pH 2 5 15 and quenched afterwards. The amount of oxygen, exchanged with the sample, is determined by the oxygen partial pressure change in a nitrogen carrier gas with 2 vpm oxygen by means of amperometric titration with potentiometric end point indication. For

the measurements, the solid electrolyte device OXYLYT was used [12]. In Fig. 2, the absorption of oxygen of the BaCe 0.80 In 0.20 O 32 a -sample is presented as dependant on the temperature. The exchanged oxygen amounts increase with the temperature and obey the equation log y 5 2 (4.09 6 0.09) 2 (1170 6 60) /T. The numerical value of y is the exchanged oxygen amount in O mole per one gram weighed sample of BaCe 0.80 In 0.20 O 32 a . The minimum temperature for the absorption of oxygen into the crystal lattice is about 2008C. This behaviour was observed for all InO 1.5 -doped BaCeO 3 -samples. The higher the doping the greater were the exchanged oxygen amounts.

4. Electrical properties

4.1. Electrical conductivity The total electrical conductivities of various Indoped BaCeO 3 were measured by the dc four probe method in oxygen, air, argon and N 2 / H 2 (with 2 vol.% H 2 ) in the temperature range between 7008C and 10008C. Each of these gases was conducted over the samples. Three different moisture levels of the gases were used: dried with P2 O 5 , as supplied in the steel cylinder, artificially humidified (Table 3). In Fig. 3 the electrical conductivities are plotted as a function of doping content and the temperature in dried N 2 / H 2 . In the BaCeO 3 -system with 0.20 mole InO 1.5 , the highest values of the electrical conductivities were found. The conductivity decreases in the higher doped samples of BaCeO 3 a little more than in the less doped ones. The electrical conductivity increases with the increase of oxygen partial pressure in the applied gas (Fig. 4). This behaviour was observed by all doping contents in the BaCeO 3 . According to the theory of disorder in solids the dependence of the concentration of electron holes h ? on the oxygen partial pressure pO 2 is obtained from the following equation 1 ] 2

O 2 (g) 1 V ?? → O o 1 2h ?

which describes the incorporation of oxygen in the lattice (O o means oxygen on lattice place). Using the

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225

Fig. 2. Dependence of oxygen absorption in BaCe 0.80 In 0.20 O 32 a on temperature.

Table 3 Gases used for the determination of the electrical conductivities Gases

pH 2 O /pH 2

pH 2 O (bar)

pO 2 (bar)

Dry As supplied Humidified

2310 27 5310 26 2.8310 22

,3310 26

Synthetic air

Dry As supplied Humidified

2310 27 8310 23 2.8310 22

0.21 0.21 0.21

Sauerstoff

Dry As supplied Humidified

2310 27 5310 26 2.8310 22

1 1 1

98 vol % N 2 2 vol % H 2

Dry As supplied Humidified

Argon

0.012 0.025 0.33

law of mass action and when the concentration of oxygen ion vacancies [V ?? ] is constant we obtain [h ? ] | p O1 /24 . The pure ion conductivity is only observed in reducing gaseous atmosphere in the temperature range from about 7008C to about 8008C (Table 4). For the determination of the constituent of the hole conduction the values of the conductivities were

calculated for the various gases and from them the values in N 2 / H 2 subtracted. The plot of the logarithm of the obtained values of the conductivity in dependence on log pO 2 at the different temperatures showed straight lines with a slope of about 0.25. This result shows that there exists a contribution of hole conduction in oxygen ion conductors. Further investigations have shown that the conductivities of In-doped BaCeO 3 are influenced by the partial pressure of water in the tested temperature range (Fig. 5). Under dry and humid conditions the conductivities again agree at a temperature from 10008C. This means that the influence of water decreases with increasing temperature. Only under reducing and inert gases the electrical conductivities are decisively influenced by the content on water vapour in the gases. In Table 5 this behaviour is illustrated by the comparison of the activation enthalpies and the pre-exponential factors for the different gases. This behaviour is observed with all doped samples. In our earlier investigations of the BaCeO 3 -system with rare earth dopings no dependence could be found on the water vapour partial pressure in the gas in the temperature range between 7008C and 10008C [8,9]. In Fig. 6 the electrical conductivities of different

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Fig. 3. Dependence of total electrical conductivity of BaCeO 3 on doping content and temperature under dried N 2 / H 2 .

Fig. 4. Dependence of total electrical conductivity of BaCe 0.80 In 0.20 O 32 a on temperature and oxygen partial pressure (dried).

SE-materials are presented. It is that the compounds BaCe 0.80 In 0.20 O 32 a in air and in hydrogen, Ba 2 In 1.75 Ce 0.25 O 5.215 under a pO 2 510 26 bar as well as the Zr 0.83 Y 0.17 O 1.91 prepared by us. The

In-doped BaCeO 3 which crystallizes in the perovskite structure does not attain the electrical conductivity of the brownmillerite-structure oxide (Ba 2 In 1.75 Ce 0.25 O 5.215 ). The electrical conductivity

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227

Table 4 Mean ionic transport numbers of In-doped BaCeO 3 in hydrogen and in argon Temperature (8C)

940 865 785 694 665 630

BaCeO 3 with InO 1.5 (mole) 0.05 H2

0.05 Ar

0.10 H2

0.10 Ar

0.15 H2

0.15 Ar

0.20 H2

0.20 Ar

0.64 0.74 0.92 1.02 1.04 1.05

0.50 0.88 0.90 0.87 0.83 0.65

0.82 0.80 0.93 0.98 0.98 0.95

0.62 0.88 0.81 0.76 0.75 0.71

0.89 0.89 0.98 0.99 0.99 0.99

0.80 0.79 0.81 0.83 0.77 0.75

0.66 0.73 0.92 0.90 0.95 0.96

0.66 0.80 0.81 0.83 0.77 0.77

Fig. 5. Dependence of total electrical conductivity of BaCe 0.80 In 0.20 O 32 a on doping content and temperature under humidified N 2 / H 2 .

of both compounds does not exceed the conductivity of the yttria-stabilized zirconia with cubic-fluorite structure.

calculated using the relation between the change of the EMF of the measuring cell and the change of the EMF of the reference cell (with ZrO 2 (Y 2 O 3 ) as SE) at different oxygen partial pressures:

4.2. EMF behaviour In solid electrolytes with an electronic contribution to the conductivity the ionic transport number of the oxygen ions is dependent on the oxygen partial pressure which can be determined experimentally from EMF measurements. The mean ionic transport numbers for the system BaCe 12x In x O 32 a were determined as a function of the oxygen pressure in the gas while simultaneously measuring the EMF changes. The mean ionic transport number ¯t ion is

¯tion 5 DEmeasuring cell /DEreference cell . The oxygen pressure in argon was changed between 2.0310 25 bar and 4.2310 23 bar in the temperature range between 6308C and 9408C. The change of the oxygen partial pressure under argon took place with addition of synthetic air with a water content of 5310 24 vol%. For the measurements under N 2 / H 2 , the oxygen was pumped into the gas using a solid

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228

Table 5 Conductivity parameters for the BaCe 0.80 In 0.20 O 32 a in the temperature range 7008C to 10008C under dried, as supplied and humidified gases Gases

Conditions

DH kJ mole 21

log s0 SK cm 21

Nitrogen with 2 vol% hydrogen Argon

dried as supplied humidified dried as supplied humidified dried as supplied humidified dried regular moistened

97 93 73 91 88 73 86 83 80 86 85 81

5.2 5.1 4.2 5.0 4.9 4.3 5.2 5.0 4.9 5.3 5.2 5.0

Air

Oxygen

electrolyte system in the gas flow. The oxygen pressure in N 2 / H 2 was changed between 10 220 bar and 10 226 bar. In Table 4 the obtained mean ionic transport numbers are summarized for the doped BaCeO 3 . The almost single phase products with 0.05 mole#x# 0.20 mole InO 1.5 in BaCeO 3 indicated a pure ionic conductivity under hydrogen in the temperature range between 6308C and about 8008C. At higher

temperatures the mean ionic transport numbers are between 0.6 and 0.9. It is to be supposed that another contribution arises in addition to the ionic conductivity above 8008C. Furthermore the ionic transport numbers drop down with an increase of the oxygen partial pressure in the gas. In the SE-materials the hole conduction rises as it has already been shown by the measurements of electrical conductivity. The EMF measurements with the two-phase BaCeO 3 -samples with 0.25 mole InO 1.5 were not reproducible.

5. Conclusions The In-doped BaCeO 3 ceramics have been prepared as almost single phase perovskite compounds with a InO 1.5 content up to 20 mole %, a doping level for which the maximal electrical conductivity was observed. There exists three temperature ranges of the electrical conductivity. The first temperature range is below 7008C, the second between 7008C and 9508C and the third above 9508C. The second temperature range of electrical conductivity was investigated in this study. The In-doped BaCeO 3 is

Fig. 6. Electrical conductivity of BaCe 0.80 In 0.20 O 32 a in comparison with other solid electrolyte materials.

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less conducting than the BaCeO 3 with rare earth dopant and than Zr 0.83 Y 0.17 O 1.91 . The conductivity is distinctly influenced by the water vapour partial pressure in reducing inert gases in the measured temperature range from 7008C to 10008C. Further investigations will be continued to optimize the solid electrolyte ceramics. These investigations should show the influence of the deviations from the ideal stoichiometry of the compound BaCe 12x In x O 32 a on the electrical conductivity and the behaviour of these electrolyte materials against water vapour. They will be carried out in comparison to measurements on rare earth-doped BaCeO 3 .

Acknowledgements This work was supported by the Bundesminis¨ Bildung und Forschung (BMBF, FRG). terium fur

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