16530 Phys. Chem. Chem. Phys., 2011, 13, 16530–16533 This journal is c the Owner Societies 2011

Cite this: Phys. Chem. Chem. Phys., 2011, 13, 16530–16533

Fast oxygen exchange kinetics of pore-free Bi1�xSrxFeO3�d thin filmsw

Anja Wedig,* Rotraut Merkle, Benjamin Stuhlhofer, Hanns-Ulrich Habermeier,

Joachim Maier and Eugene Heifets

Received 24th May 2011, Accepted 8th August 2011

DOI: 10.1039/c1cp21684h

The oxygen incorporation/extraction kinetics of the potential

solid oxide fuel cell (SOFC) cathode material Bi1�xSrxFeO3�d

with x = 0.5 and 0.8 was studied by electrochemical impedance

spectroscopy on geometrically well-defined pore-free thin film

electrodes. The oxygen exchange rate was found to be higher than

that of La1�xSrxFeO3�d and—among cobalt-free perovskites—

only surpassed by Ba1�xSrxFeO3�d which is however known to

be unstable in a SOFC environment.

Mixed-conducting oxides have emerged as preferential candidates

for solid oxide fuel cell cathode materials, since oxygen

reduction can proceed on the whole electrode surface (‘‘bulk

path’’)1 instead of being limited to the electrode–electrolyte–air

triple phase boundary. Especially oxygen-deficient perovskite-

type oxides such as La1�xSrxMnO3�d or A1�xSrxCo1�yFeyO3�d(A = La or Ba) have been the subject of intense studies focusing

not only on application-related aspects, but also on a funda-

mental understanding of the mechanism of surface oxygen

incorporation. The cathode reaction often determines the

overall cell efficiency, particularly at low temperatures, since

its activation energy is typically higher than that of electrolyte

conductivity and anode reaction.2–13 Up to date, the investigations

of the mechanisms involved indicated that not only a high concen-

tration of oxygen vacancies,14 but also a high vacancy mobility

are highly beneficial for fast oxygen incorporation kinetics.13

The perovskite with the highest oxygen exchange rate,

Ba0.5Sr0.5Co0.8Fe0.2O3�d, has not made its way into applications

due to significant drawbacks such as a high reactivity towards

common SOFC electrolytes15–17 and CO218 as well as the

detrimental structural transformation into a hexagonal

perovskite phase between 850 1C and 900 1C.19,20 We expect

that the substitution of Ba by Bi might be a promising

approach towards an alternative high-performance SOFC

cathode material. Owing to a lower basicity compared to

Ba2+, Bi3+ is assumed to increase the chemical stability. In

addition, the high polarizability of Bi3+ attributable to its 6s

lone pair can be expected to provide the required high mobility

of oxygen vacancies. Based on these considerations and further

encouraged by first tentative results, we proposed Bi-containing

perovskites as SOFC cathode materials.21,22 High ionic

conductivities at 800 1C of 0.016 S cm�1 for Bi0.5Sr0.5FeO3�d23

and 0.005–0.022 S cm�1 for Bi0.4Sr0.6FeO3�d and

Bi0.7Sr0.3FeO3�d24 were extracted from permeation experiments.

Recently, area specific resistances (ASR) as low as

0.14–0.49 O cm2 at 700 1C have been measured on porous thick

film electrodes of Bi1�xSrxFeO3�d (x=0.3, 0.5 and 0.8).23,25,26

However, a reliable comparison with ASR values of related

perovskites or any specific conclusions about the oxygen

incorporation mechanism are not possible because of the

complex and hardly reproducible morphology of the porous

electrode films investigated. For this reason, we studied the

oxygen incorporation kinetics on geometrically well-defined

pore-free thin film electrodes of Bi1�xSrxFeO3�d with x = 0.5

and 0.8 in two different measuring arrangements.

Before dealing with the electrocatalytic activity for oxygen

incorporation, let us briefly describe the Bi1�xSrxFeO3�d bulk

properties. For 0.2 r x r 0.8 the materials crystallize in the

cubic perovskite structure with the lattice constant decreasing

from 3.95 A to 3.91 A upon increasing the Sr content.24,27,28

Thermogravimetry in combination with cerimetric titration to

determine a reference point yields the oxygen stoichiometry

(Fig. 1a). While for Bi0.5Sr0.5FeO3�d iron is essentially Fe3+,

Bi0.2Sr0.8FeO3�d contains up to 50% Fe4+ depending on the

applied conditions, and the oxygen vacancy concentration is

approximately p (p(O2))�0.05. As a consequence, the electronic

conductivity (Btotal conductivity) is rather low for

Bi0.5Sr0.5FeO3�d (Fig. 1b). Local lattice distortions as evidenced

by extended X-ray adsorption fine structure (EXAFS)

measurements29 may also contribute to this.

Fig. S1 (ESIw) shows the XRD patterns of 150 nm thin

Bi1�xSrxFeO3�d (x = 0.5 and 0.8) films grown by pulsed laser

deposition (PLD) on Y2O3-doped ZrO2 (YSZ) single crystals

(for experimental details, see ESIw). PLD targets were prepared

from the phase-pure stoichiometric powders obtained by solid

state reaction. Both compositions were found to form polycrystalline

films with a cubic perovskite structure, and no secondary

phases could be detected in the X-ray diffraction patterns.

The pore-free film growth was confirmed by SEM (Fig. S2 and

S3, ESIw). Quantitative analysis by means of ICP-OES

indicated the actual compositions (Bi0.48Sr0.52)1.01(1)FeO3�dand (Bi0.18Sr0.82)1.00(2)FeO3�d (average values derived from

two samples produced in the same PLD run).

Max Planck Institute for Solid State Research, Heisenbergstrasse 1,70569 Stuttgart, Germany. E-mail: a.wedig@fkf.mpg.de,s.weiglein@fkf.mpg.de; Fax: +49 711 689 1722;Tel: +49 711 689 1771w Electronic supplementary information (ESI) available: Experimentalprocedures, definition of TEC, XRD/SEM of thin films, EIS ofBi0.2Sr0.8FeO3�d, equivalent circuit, temperature dependence of Cchem.See DOI: 10.1039/c1cp21684h

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Electrochemical impedance spectroscopy (EIS) was performed

on YSZ crystals coated on both sides with a 150 nm thin film

of Bi1�xSrxFeO3�d (‘‘macroscopic samples’’) as well as on

arrays of circular microelectrodes with 20–100 mm diameter

and 150 nm thickness (obtained by photolithography and Ar

ion beam etching) on YSZ with an extended Ag paste counter

electrode on the backside (for details see e.g. ref. 9). A fine Au

mesh (1000 wires inch�1, Precision Eforming, USA) was

attached as a current collector to both large faces of the

macroscopic samples. For the microelectrode samples, one

of the microelectrodes and the Ag counter electrode were

contacted by Pt/Ir probe needles (tip radius: 2.5 mm, Moser

Company, USA). Impedance spectra were recorded using an

Alpha High Resolution Dielectric Analyser (Novocontrol,

Germany) with an AC amplitude of 10 mV. XRD analysis

of the thin film samples after EIS measurements showed no

reaction between the electrode film and the electrolyte YSZ, in

agreement with XRD investigations of powder mixtures of

Bi1�xSrxFeO3�d (x= 0.5 or 0.8) and YSZ annealed for 12 h at

800 1C which did not reveal any reaction products.

The impedance spectra are typically dominated by one

semicircle at low frequencies (Fig. 2 and Fig. S4, ESIw). ForBi0.5Sr0.5FeO3�d, an additional pronounced arc at intermediate

frequencies could be detected in microelectrode measurements

(Fig. 2b), which is most probably caused by sheet resistance

resulting from the low electronic conductivity and the electrode’s

extreme aspect ratio (150 nm thickness but 60 mm diameter).z30The equivalent circuit used for analysis (Fig. S5, ESIw) has

been derived from a general model for mixed-conducting

electrodes31 assuming that oxygen incorporation mainly occurs

via the bulk path and is limited by the surface reaction rather

than by oxygen transport through the electrode bulk.32 The

resistance Rs corresponding to the diameter of the low-

frequency semicircle is attributed to the oxygen exchange at

the electrode surface, and the associated large capacitance is

interpreted as ‘‘chemical capacitance’’ Cchem originating from

oxygen stoichiometry changes within the electrode. The

appearance of the large Cchem combined with a high ionic

conductivity (410�3 S cm�1 at 800 1C for Bi1�xSrxFeO3�d,

0.3 r x r 0.6)23,24 and the pronounced p(O2) dependence of

Rs (see below) strongly support this interpretation. From the

good agreement between (high-temperature) Rs from macro-

scopic and microscopic measurements, respectively, it can be

concluded that neither the Au mesh nor the Pt/Ir probe needle

exhibits any catalytic effect on the oxygen incorporation.

From the impedance spectra of both macroscopic and

microscopic samples, Rs was determined to be 4.8 O cm2 for

Bi0.5Sr0.5FeO3�d and 2.8 O cm2 for Bi0.2Sr0.8FeO3�d at 750 1C

and p(O2) = 0.2 bar. Note that these values refer to the actual

electrode surface in contrast to the ASR of porous films

typically based on the ‘‘footprint’’ area of the electrode. The

estimated real surface area of a porous Bi0.5Sr0.5FeO3�d film

(annealed at 1000 1C) is about a factor of 30 larger,y thus thereported ASR of 0.09 O cm2 at 750 1C26 is in good agreement

with our results for pore-free PLD thin films.

The effective surface exchange rate constant kq

kq ¼ kBT

4e2RscOð1Þ

(kB: Boltzmann constant, T: temperature, e: elementary

Fig. 1 Bulk properties of Bi1�xSrxFeO3�d. (a) Temperature and

p(O2) dependence of the oxygen deficiency d (powder samples).

(b) Temperature dependence of the electrical conductivity at

p(O2) = 0.2 bar (sintered pellets).

Fig. 2 Impedance spectra of pore-free 150 nm thin Bi0.5Sr0.5FeO3�dfilms measured on (a) a macroscopic sample (YSZ single crystal coated

on both sides) and (b) a microelectrode with 60 mm diameter on a YSZ

single crystal.

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16532 Phys. Chem. Chem. Phys., 2011, 13, 16530–16533 This journal is c the Owner Societies 2011

charge, Rs: area specific surface resistance, cO: concentration

of occupied oxygen lattice sites)33 amounts to 6.4 � 10�7 cm s�1

and 1.1 � 10�6 cm s�1 for Bi0.5Sr0.5FeO3�d and

Bi0.2Sr0.8FeO3�d at 750 1C and p(O2) = 0.2 bar. In the

literature, an effective surface exchange rate constant kd of

5.4 � 10�3 cm s�1 at 750 1C was obtained for a ceramic

Bi0.5Sr0.5FeO3�d sample by conductivity relaxation (sample

density not indicated).23 The relation between kq determined

from electrochemical measurements and kd received via a

chemical experiment can be expressed by the thermodynamic

factor oO

kd ¼ oOkq ¼ cO

RT

@mO@cO

kq ð2Þ

(R: gas constant, mO: chemical potential of oxygen).34 From

d(p(O2)) (Fig. 1a), kd is calculated to be 0.6 � 10�3 cm s�1 for

Bi0.5Sr0.5FeO3�d at 750 1C and p(O2) = 0.2 bar.

Activation energies for Rs of 1.1 eV for Bi0.5Sr0.5FeO3�d and

1.2 eV for Bi0.2Sr0.8FeO3�d were measured on the macroscopic

samples (Fig. 3a). The activation energies obtained on micro-

electrodes increase upon decreasing temperature. The reason

for this observation is not yet clear, but there seems to be a

general tendency towards overestimation of the activation

energy of Rs by microelectrode measurements also reported

for Ba- or La-containing compounds.9,13 The p(O2) dependence

of Rs follows Rs p (p(O2))�n with average n values of 0.56 for

Bi0.5Sr0.5FeO3�d and 0.63 for Bi0.2Sr0.8FeO3�d (Fig. 3b). The

temperature and p(O2) dependence of the high-frequency axis

intercept detected on macroscopic and microelectrode samples is

largely consistent with that of the ionic conductivity of YSZ.35

The chemical capacitances Cchem

Cchem ¼4F2

Vm

@cO@mO

ð3Þ

(F: Faraday constant, Vm: molar volume at the considered

temperature)4 of Bi1�xSrxFeO3�d obtained from impedance

spectra are lower than those of related (Ba, Sr, La)(Fe, Co)O3�dperovskites (Table 1) and depend only weakly on temperature

or p(O2) (Fig. S6, ESIw). They may be compared to Cchem

calculated using d(p(O2)) from Fig. 1a which yields

0.4 kF cm�3 and 2.5 kF cm�3 for Bi0.5Sr0.5FeO3�d and

Bi0.2Sr0.8FeO3�d at 750 1C and p(O2) = 0.2 bar. Cchem of

the bulk materials is therefore by a factor of 3–4 larger than

that of thin films. A similar difference has been reported

previously.4

A low Cchem is expected to result in a low thermochemical

expansion coefficient (TEC), since the expansion of the

perovskite lattice upon heating is largely a consequence of

oxygen vacancy formation. A low TEC is favourable for

SOFC applications as it reduces the thermochemical expansion

mismatch to the electrolyte and thus increases the durability of

the cell. Our measurements (Fig. 4) show that the mean TEC

of Bi0.5Sr0.5FeO3�d, TECw, is comparable to that of

La0.6Sr0.4FeO3�d (Table 1), whereas Bi0.2Sr0.8FeO3�d is subject

to stronger thermochemical expansion, in particular at high

p(O2).

In Table 1 data for several Fe-based perovskite cathode

materials are compiled. The ASR of Bi0.5Sr0.5FeO3�d and

Bi0.2Sr0.8FeO3�d is higher than that of Ba0.5Sr0.5FeO3�d, but

comparable to SrFeO3�d and lower than for La0.6Sr0.4FeO3�d.

While the low electronic conductivity (particularly for

Bi0.5Sr0.5FeO3�d) may require an additional current collecting

layer, other problems (carbonate formation in

Ba0.5Sr0.5FeO3�d similar to Ba0.5Sr0.5Co0.8Fe0.2O3�d, phase

transformations in SrFeO3�d40) are reduced in the Bi-based

perovskites. Table 1 further indicates that as a general trend

the oxygen exchange rate increases with increasing ionic

(oxygen vacancy) conductivity, well in line with results obtained

for Ba1�xSrxCoyFe1�yO3�d perovskites.13

To summarize, this study on morphologically well-defined

pore-free thin film electrodes shows that a partial occupation

of the perovskite A site by the highly polarizable Bi3+

Fig. 3 Surface oxygen incorporation resistance Rs of

Bi1�xSrxFeO3�d with x = 0.5 and 0.8 from impedance spectra.

(a) Temperature dependence at p(O2) = 0.2 bar. (b) Oxygen partial

pressure dependence at 750 1C. Solid symbols: macroscopic samples,

open symbols: microelectrode samples.

Table 1 Comparison of Rs and Cchem from (microcontact) impedancemeasurements on pore-free PLD thin films, sion (wwfrom tracer diffu-sion experiments, zzfrom permeation experiments) and TEC at 750 1Cand p(O2) = 0.2–0.5 bar for perovskite-type SOFC cathode materials

Rs/O cm2Cchem/kF cm�3

sion/S cm�1 TEC=10�6 K�1

Ba0.5Sr0.5FeO3�d 1.013 1.821 0.15ww,13 B2236

SrFeO3�d 3.613 2.121 0.03ww,13 B2137

Bi0.2Sr0.8FeO3�d 2.8 0.8 16a–21b

Bi0.5Sr0.5FeO3�d 4.8 0.1 0.006zz,23 14a,b

La0.6Sr0.4FeO3�d 8.29 1.39 1338

La0.5Sr0.5FeO3�d 0.005zz,33

La0.75Sr0.25FeO3�d B0.001ww,39

a measured at p(O2) = 10�3 bar. b measured at p(O2) = 1 bar.

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This journal is c the Owner Societies 2011 Phys. Chem. Chem. Phys., 2011, 13, 16530–16533 16533

improves the surface oxygen exchange kinetics over the respective

La1�xSrxFeO3�d perovskites despite a decrease in the materials’

electronic conductivity. In contrast to Ba substitution, this can

be achieved without increasing the danger of detrimental

surface carbonate formation. Further experimental and quantum-

chemical studies on the detailed oxygen incorporation reaction

mechanism are underway.

Acknowledgements

We thank G. Christiani and S. Schmid for assistance with

PLD and lithography/ion beam etching, G. Gotz for XRD,

B. Fenk and T. Reindl for SEM/EDX, P. Lupetin and

K. Adepalli for SPS (all MPI for Solid State Research,

Stuttgart), and A. Meyer for ICP-OES (MPI for Intelligent

Systems, Stuttgart). This study was in part supported by GIF

under contract No. 1025-5.10/2009.

Notes and references

z The alternative assignment of the intermediate-frequency contribu-tion in the Bi0.5Sr0.5FeO3�d microelectrode spectra to processes at theelectrode–electrolyte interface is questionable, since the correspondingcapacitance of 5.2 � 10�4 F cm�2 is more than one order of magnitudehigher than a typical interfacial capacitance.13,32 This indicates thatthe intermediate-frequency feature rather arises from sheet resistancedue to the low electrical conductivity of Bi0.5Sr0.5FeO3�d. A roughestimate gives a resistance contribution of 29 kO from the electronicconductivity (Fig. 1a) of a 150 nm thick strip with 30 mm in length andwidth which is in good agreement with the diameter of theintermediate-frequency semicircle in Fig. 2b. The intermediate-frequency resistance exhibits an activation energy of 0.28 eV and isproportional to (p(O2))

�0.20, in fair agreement with the electricalresistivity being proportional to (p(O2))

�0.23 and having an activationenergy of 0.08 eV between 550 1C and 750 1C. Detailed numericalsimulations are underway.41

y The actual surface of a porous electrode was estimated from the totalparticle perimeter in an area cross-section derived from SEM imagesusing an image processing program (ImageJ) multiplied by theelectrode thickness.

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Fig. 4 Temperature and p(O2) dependence of the mean thermochemical

expansion coefficient of Bi1�xSrxFeO3�d with x = 0.5 and 0.8.

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