August 24, 2011 6:46 S1793-6047 S1793604711001737 fa2

Functional Materials LettersVol. 4, No. 2 (2011) 157–160© World Scientific Publishing CompanyDOI: 10.1142/S1793604711001737

INVESTIGATION OF GdBaCo2−xFexO5.5−δ AS A CATHODE MATERIAL FORINTERMEDIATE TEMPERATURE SOLID OXIDE FUEL CELLS

ANDRZEJ KULKA∗,§, YANG HU‡, GUILHEM DEZANNEAU‡

and JANINA MOLENDA†

∗Faculty of Materials Science and Ceramics†Faculty of Energy and Fuels

AGH University of Science and Technology, al. Mickiewicza 3030-059 Kraków, Poland

‡Laboratoire SPMS, Ecole Centrale Paris (UMR CNRS 8580)Grande voie des vignes, 92295 Chatenay-Malabry Cedex, France

§akulka1986@o2.pl

Received 31 December 2010; Revised 8 April 2011

In this work, we present an evaluation of the layered perovskite materials with chemical composition GdBaCo2−x Fex O5.5−δ(x = 0.0, 0.3, 0.6) as a cathode materials for intermediate temperature solid oxide fuel cell (IT-SOFC). We first present resultsconcerning their crystal structure and oxygen nonstoichiometry, and then give results concerning their electrical conductivity andperformance in button-type SOFCs.

Keywords: Layered perovskites; cathode material; SOFC.

Recently, lowering of the operational temperature of solidoxide fuel cells (SOFC) down to the 600–800C temperaturerange has become one of the main goals in SOFC technology.It is motivated by advantages such as better materials compati-bility, better long-term stability and reduced cost. However, inthis temperature range the cathodic overpotential constitutesa big part of overall potential losses in SOFC cell. Thus newcathode materials with satisfactory electrochemical propertiesmust be found, in order to obtain good cells performances.

Perovskite type materials with general formulaLn1−xAx BO3−δ (were Ln — lanthanide element, A — Sr,Ba and B — a transition metal) are already known as poten-tial cathode materials for SOFC applications. It is due totheir mixed ionic-electronic conductivity, which can enlargeeffective reaction zone over the entire cathode-gas interfa-cial area, resulting in better electrochemical performance.1

A more recent promising subgroup among this family are lay-ered cobaltites ReBaCo2O5.5−δ (Re — rare earth element),which have been extensively studied in order to explain a num-ber of interesting phenomena such as spin-state transitions,2

charge ordering3 and giant magnetoresistance4 occurring inthese materials. It was noticed that these compounds areformed by [CoO2][BaO][CoO2][ReOδ] plane sequence along

the c direction. In addition if Re = Gd and in case of δ < 1the oxygen vacancies were found to be located in gadolin-ium planes at (0, 0, 1/2) positions. Furthermore, at RT thecoexistence of two types of cobalt coordination environmentswith different spin state was observed: pyramidal CoO5 (Co3+intermediate spin state, IS, t52ge1

g) and octahedral CoO6 (Co3+

low spin state, LS, t62ge0g).5 Particularly, GdBaCo2O5.5−δ has

been reported to exhibit very good transport properties i.e.high surface exchange coefficient and oxygen diffusivity.6,7

Moreover this compound has been reported to undergo metal–insulator (MI) transition, which was connected with the spinstate switch in the Co3+ ions located at the octahedra (fromlow spin t62ge0

g to high spin t42ge2g state) and resulted in

very high electric conductivity.5,6 These results indicated thatGdBaCo2O5.5−δ can be a potential cathode material for SOFCapplications.8–10

Despite good electrochemical performance, the use ofthis specific material as an electrode in SOFC technol-ogy is limited by its thermal expansion coefficient (TEC)and chemical reactivity with zirconia-based electrolytes. ForGdBaCo2O5.5−δ, the TEC is in the range of 16.4 × 10−6 K−1

−20.1 × 10−6 K−1 11,12 and using it with Ce0.80Gd0.2O2−δ

electrolyte (TEC ∼ 12.5 × 10−6 K−1) can result in the

157

Func

t. M

ater

. Let

t. 20

11.0

4:15

7-16

0. D

ownl

oade

d fr

om w

ww

.wor

ldsc

ient

ific

.com

by M

ON

ASH

UN

IVE

RSI

TY

on

12/0

4/14

. For

per

sona

l use

onl

y.

August 24, 2011 6:46 S1793-6047 S1793604711001737 fa2

158 A. Kulka et al.

decrease of the SOFC performances caused by thermo-mechanical incompatibility.

In this work we compare GdBaCo2O5.5−δ,GdBaCo1.7Fe0.3O5.5−δ and GdBaCo1.4Fe0.6O5.5−δ in termsof crystal structure, oxygen nonstoichiometry, electricalconductivity and single button-type SOFC cell measure-ments. Powders of GdBaCo2O5.5−δ , GdBaCo1.7Fe0.3O5.5−δ

and GdBaCo1.4Fe0.6O5.5−δ were fabricated viasoft chemistry method using Gd(NO3)3 ·6H2O, Ba(NO3)2,Co(NO3)2 ·6H2O, Fe(NO3)3 ·9H2O and citric acid as sub-strates. The aqueous solution of nitrates, in appropriate ratios,was first rapidly evaporated. Subsequently obtained precur-sors were ground thoroughly in agate mortar and heated at850C for 4 h. Obtained powders were once again groundand fired at 1050C for 5 h in air. The phase characteriza-tion was performed using a Rigaku RU-300 18 kW Bragg–Brentano diffractometer coupled to a rotating anode X-raysource (Cu Kα source) in the 20–90 2-theta range. Theobtained diffractograms were analyzed using Rietveld method(GSAS/EXPGUI software13,14). With the aim to investigatethe mass change of the samples as a function of tempera-ture, the thermogravimetric measurements were carried outon STA-SDT 2960 TA Instruments apparatus in RT-800Crange with 1/min heating/cooling rate in synthetic air atmo-sphere. The initial oxygen contents were obtained from reduc-tion measurements in 5% H2 in Ar atmosphere.

Transport properties were characterized by high tem-perature electrical conductivity, measured by four-probe DCmethod and Seebeck coefficient measurements, which werecarried out using dynamical method with temperature gra-dient equal 2–3C. In order to investigate the influence ofsubstitution of Co by Fe on the SOFC performance, elec-trolyte supported button-type SOFC cells with different cath-ode materials were fabricated. Ce0.80Gd0.2O2−δ electrolytepowder was prepared by a coprecipitation method as describedelsewhere15 and then pressed into pellets and heated at 1400Cfor 4 h in order to obtain gas-tight (relative density ≥ 95%)samples. Composite cathodes were fabricated by mixing cath-ode powders with Ce0.80Gd0.2O2−δ powders in a 1 to 1 volumeratio and grinding the mixture in an agate mortar. An organicbinder was used to ensure appropriate viscosity for the paste.Single SOFC cells were fabricated in subsequent stages: sep-arate firings of an anode (1400C for 4 h), composite cathodeand pure cathode deposition by screen-printing and treat-ment (both at 1050C for 4 h). Button-type cell performanceswere examined using home-made SOFC test unit equippedwith Solatron SI 1287 electrochemical interface and Solatron1252A frequency response analyzer in a 600–800C tempera-ture range. To the anode chamber the humidified hydrogen(3% H2O) was provided, whereas to the cathode chamberair was provided with flow rates around 40 cm3 min−1 and500 cm3 min−1, respectively.

20 40 60 80

1 GdBaCo2O5.5-δ

2 GdBaCo1.7Fe0.3O5.5-δ

3 GdBaCo1.4Fe0.6O5.5-δ

Inte

nsiv

ity [a

.u]

2θ [deg]

1

2

3

Fig. 1. XRD pattern for GdBaCo2O5.5−δ , GdBaCo1.7Fe0.3O5.5−δ andGdBaCo1.4Fe0.6O5.5−δ .

X-ray diffraction indicate that both GdBaCo2O5.5−δ andGdBaCo1.7Fe0.3O5.5−δ samples exhibit orthorhombic Pmmmspace group symmetry with doubled b and c cell parameters(ap × 2ap × 2ap where ap is the basic cubic perovskite cellparameter), while GdBaCo1.4Fe0.6O5.5−δ was found to exhibittetragonal P4/mmm space group symmetry, with only c cellparameter doubled (ap ×ap×2ap). In Fig. 1 the XRD patternsat RT for all samples are presented.

The Fe content is thus shown to alter the structural prop-erties by leading to smaller distortion of the orthorhombicstructure and to tetragonal symmetry for the high Fe content.Cell parameters are listed in Table 1. Those values show thatafter substitution of Co by Fe, the volume of the cell tendsto increase. It was shown through Mössbauer spectroscopy,that for TbBaCo2−xFexO5.5−δ system Fe3+ ions adopt a HSstate and replace the Co3+ ions (at the IS state) at the pyrami-dal sites.16 The radius of the HS Fe3+ ions is 0.645 Å and isslightly higher compared to the radius of the IS Co3+ (0.56 Å)at the pyramidal site17 which can explain observed increase ofthe volume of the cell. Below it will be shown that this mightalso be due to the Co reduction associated with Fe substitutionas observed from thermogravimetry.

Microstructure of the obtained powders was investi-gated using scanning electron microscopy. From Fig. 2, onecan observe slight differences between the different sam-ples. Average grain size was found to lie around 5µm forGdBaCo2O5.5−δ and 10µm for both GdBaCo1.7Fe0.3O5.5−δ

and GdBaCo1.4Fe0.6O5.5−δ powders.

Table 1. Cell parameters for GdBaCo2−x Fex O5.5−δ .

Composition x a [Å] b [Å] c [Å] Vp (Vp = a3p) [Å3]

0.0 3.8770(3) 7.8211(5) 7.5346(7) 57.11(8)0.3 3.8758(9) 7.8243(8) 7.5660(2) 57.36(3)0.6 3.8944(1) 3.8944(1) 7.5944(4) 57.59(1)

Func

t. M

ater

. Let

t. 20

11.0

4:15

7-16

0. D

ownl

oade

d fr

om w

ww

.wor

ldsc

ient

ific

.com

by M

ON

ASH

UN

IVE

RSI

TY

on

12/0

4/14

. For

per

sona

l use

onl

y.

August 24, 2011 6:46 S1793-6047 S1793604711001737 fa2

Investigation of GdBaCo2−x Fex O5.5−δ as a Cathode Material 159

Fig. 2. SEM microphotographs of GdBaCo2O5.5−δ (left) andGdBaCo1.4Fe0.6O5.5−δ (right).

In order to investigate structural properties ofGdBaCo2−xFex O5.5−δ as a function of temperature, thermo-gravimetric (TG) and differential thermal analysis (DTA) mea-surements were carried out (Fig. 3). From TG results, mass losswith the increase of temperature was observed. Using value ofoxygen content at RT, taken from reduction studies (Table 2)and assuming that the observed weight change can be ascribedonly to the formation of oxygen vacancies, the actual value ofthe oxygen content can be calculated as a function of temper-ature (upper part of Fig. 3).

Comparing results obtained for samples with dif-ferent Fe content, it seems that its introduction leadsto a partial reduction of Co3+ to Co2+ and signif-icant oxygen loss. Same situation was observed for

2 0 0 4 0 0 6 0 0 8 0 0

0.0

0.5

1.0

1.5

4.6

4.8

5.0

5.2

5.6

5.4

46 0 4 8 0 5 0 0 5 2 0

0 .30

0 .35

0 .40

0 .45

475°C

t [oC ]

Tem

per

atu

re D

iffer

en

ce [

o Cm

g-1

]

Tem

pera

ture

diff

eren

ce[o C

mg-1

]O

xyge

n co

nten

t (5.

5-δ)

GdBaCo2O5.5-δ

475°CT

∆∆

∆∆

t

T∆

∆

∆

t

t [oC ]

1 GdBaCo2O5.5-δ

2 GdBaCo1.7Fe0.3O5.5-δ

3

Fig. 3. (Color online) Oxygen content as a function of temperature forGdBaCo1.7Fe0.3O5.5−δ in air and DTA curve for GdBaCo2O5.5−δ withmarked phase transition temperature (Tt ).

Table 2. Oxygen content at RT for GdBaCo2−x Fex O5.5−δ .

Composition x 0.0 0.3 0.6Oxygen content 5.520 5.356 5.063

TbBaCo2−xFexO5.5−δ layered perovskite, which has struc-ture similar to GdBaCo2−xFexO5.5−δ .17

For GdBaCo2O5.5−δ and GdBaCo1.7Fe0.3O5.5−δ

samples, phase transition was observed (Fig. 3 — marked Tt

temperature). Additionally, with increasing Fe content tem-perature of phase transition (Tt ) decreases (475C for x = 0and 441C for x = 0.3). For the x = 0.6, the RT phase isalready tetragonal and thus no phase transition is observed.Analyzing the data shown in Fig. 3 one can distinguish forsamples with Fe content x = 0.0 and 0.3, two regimes withdifferent oxygen uptake/release rate corresponding to high andlow temperature phases.

Electrical conductivity and thermoelectric power in airfor all samples was measured using DC 4-probe method anddynamical method with temperature gradient equal 2–3Crespectively (Fig. 4).

An increase in conductivity observed at lower temper-atures, T < Ts (Ts — temperature of spin state transition)(Fig. 4) is typical for semiconductor-type behavior. Activation

10

100

1000

∆

∆

σΩ

[-1

cm-1

]

GdBaCo2O5.5-δGdBaCo1.7Fe0.3O5.5-δGdBaCo1.4Fe0.6O5.5-δ

GdBaCo2O5.5-δGdBaCo1.7Fe0.3O5.5-δGdBaCo1.4Fe0.6O5.5-δ

Ea= 0.42 eV

Ea = 0.20 eV

Ea = 0 .13 eV

∆

Ts

Tt

800 600 400 200t [oC ]

1.0 1.5 2.0 2.5 3.0 3.5

-150

-120

-90

-60

-30

0

1000/T [K-1]

α [µ

V/K

]

Ts

Fig. 4. (Color online) Electrical conductivity (σ) and thermoelectric power(α) of GdBaCo2−x Fex O5.5−δ in air as a function of temperature. Ts —temperature of spin state transition and Tt — temperature of phase transition.

Func

t. M

ater

. Let

t. 20

11.0

4:15

7-16

0. D

ownl

oade

d fr

om w

ww

.wor

ldsc

ient

ific

.com

by M

ON

ASH

UN

IVE

RSI

TY

on

12/0

4/14

. For

per

sona

l use

onl

y.

August 24, 2011 6:46 S1793-6047 S1793604711001737 fa2

160 A. Kulka et al.

energies decrease for GdBaCo2−xFexO5.5−δ with Fe substi-tution and which were estimated to be 0.42 eV; 0.20 eV and0.13 eV for x = 0.0, 0.3 and 0.6, respectively. Furthermoreat specific temperature Ts , change of electrical behavior wasobserved and for GdBaCo2O5.5−δ it has been attributed to themetal–insulator transition caused by change of spin state ofcobalt ions in octahedral environment.5 It has been noticedthat with increasing Fe content the temperature of this transi-tion shifts to the higher values (100C, 180C and 300C forx = 0.0, 0.3, 0.6, respectively). At temperatures T > Tt

decrease of the electrical conductivity was observed. Tak-ing into account results of TG measurements (Fig. 3), forsamples with Fe content x = 0.0 and 0.3 it correspondswell with increase of concentration of the oxygen vacancieswithin greater oxygen uptake/release rate regime. Electricalconductivity for all samples, even for GdBaCo1.4Fe0.6O5.5−δ

(max ∼ 170 Scm−1) which exhibit the lowest values, is highenough for applications. The temperature dependence of ther-moelectric power is correlated with that of the electrical con-ductivity (Fig. 4). From obtained results (Fig. 4) one cansee that in whole temperature range sign of thermoelectricpower is negative, indicating that electrons are major chargecarriers. At temperatures T < Ts its linear dependence vs.1/T confirms semiconductor-type behavior. At temperature Ts

semiconductor–metal transition takes place (above Ts absolutevalues of thermoelectric power increase with temperature).

8 00 oC

7 50 oC

7 00 oC

6 50 oC

6 00 oC

0 .0 5

0 .1 0

0 .1 5

0 .2 0

P [W

/cm

2 ]

U [V

]

N i/Y SZ-G D C20-G dBaCo2O 5.5-δ

0.0 0.1 0.2 0.3 0.4 0.5 0.60.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Ni/YSZ-GDC20-GdBaCo1.7

Fe0.3

O5.5-δ

H2 40ml/min + 2.3% H2O

H2 40ml/min + 2.3% H2O

I [A/cm 2]

0.00

0.05

0.10

0.15

800oC

750oC

700oC

650oC

600oC

Fig. 5. (Color online) Voltage and power density as a function of currentdensity characteristics for selected cathode materials.

Table 3. Power density [W] for GdBaCo2−x Fex O5.5−δ is afunction of temperature.

Composition x 0.0 0.3 0.6

At 600C 0.022 0.029 0.021At 750C 0.132 0.112 0.109

With the aim of investigating the influence of Fe dopingon electrode performance under working conditions measure-ments of single SOFC cell were carried out. In Fig. 5 voltageand power density as a function of current density character-istics are shown.

As can be seen, voltage drop with increasing current isalmost linear, therefore indicating that major part in the wholeelectrical losses can be ascribed to the ohmic resistivity ofthe electrolyte. It is caused by the specific construction of thesingle SOFC cells where thick (∼800µm) Ce0.8Gd0.2O2−δ

electrolyte was used. The obtained values of maximum powerdensity of a single SOFC cell are shown in Table 3. From theseresults, it can be seen that at high temperatures, the maximumpower density decreases with Fe content, while at lower tem-perature, a slightly improved performance can be observed forcomposition with x = 0.3.

In conclusion, all materials exhibit layered perovskitestructure with space group symmetry dependent on Fe content.For composition with x = 0.0, 0.3 structural phase transitionoccurs. High electrical conductivity and high concentration ofthe oxygen vacancies suggest that these materials are potentialcathode materials in SOFC application.

References1. H. Uchida et al., J. Electrochem. Soc. 149, A13 (2002).2. Y. Moritomo et al., Phys. Rev. B 61, R13325 (2000).3. T. Voght et al., Phys. Rev. Lett. 84, 2969 (2000).4. M. Respaud et al., Phys. Rev. B 64, 214401 (2001).5. C. Frontera et al., Phys. Rev. B 65, 180405(R) (2002).6. A. A. Taskin et al., Phys. Rev. B 71, 134414 (2005).7. A. A. Taskin et al., Appl. Phys. Lett. 86, 91910 (2005).8. A. Chang et al., Solid State Ionics 177, 2009 (2006).9. A. Tarancón et al., J. Power Sources 171, 349 (2007).

10. A. Tarancón et al., J. Mater. Chem. 17, 3175 (2007).11. A. Tarancón et al., Solid State Ionics 179, 611 (2008).12. N. Li et al., J. Alloy. Compd. 454, 274 (2008).13. A. C. Larson et al., Los Alamos Natl. Lab. Rep. – LAUR 86-748

(2004).14. B. H. Toby, J. Appl. Cryst. 34, 210 (2001).15. W. Zaj et al., J. Power Sources 194, 2 (2009).16. M. Kopcewicz et al., J. Appl. Phys. 93, 479 (2003).17. R. D. Shannon, Acta Crysallogr. Sect. A: Cryst. Phys., Diffr.,

Theor. Gen. Crystallogr. A32, 751 (1976).

Func

t. M

ater

. Let

t. 20

11.0

4:15

7-16

0. D

ownl

oade

d fr

om w

ww

.wor

ldsc

ient

ific

.com

by M

ON

ASH

UN

IVE

RSI

TY

on

12/0

4/14

. For

per

sona

l use

onl

y.