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A novel one step synthesized Co-free perovskite/brownmilleritenanocomposite for solid oxide fuel cells

Nagore Ortiz-Vitoriano,a Idoia Ruiz de Larramendi,a Izaskun Gil de Muro,a Aitor Larra~naga,b

Jose Ignacio Ruiz de Larramendia and Teofilo Rojo*ac

Received 2nd March 2011, Accepted 13th April 2011

DOI: 10.1039/c1jm10911a

Cobalt-free perovskite oxides Pr1�xCaxFe0.8Ni0.2O3 (PCFN) were investigated as novel cathodes for

intermediate temperature solid oxide fuel cells (IT-SOFCs). Ca and Ni substitution in the PrFeO3

material shows that a wide range of perovskites Pr1�xCaxFe0.8Ni0.2O3 (0 < x < 0.9) can be prepared by

sintering in air at 600 �C. Perovskites with 0 < x < 0.4 exhibit orthorhombic single phases (Pnma space

group), whereas 0.4 < x < 0.9 show a coexistence of the perovskite and the brownmillerite-type

structure (Ca2Fe2O5). The structure of the polycrystalline powders was analyzed by X-ray powder

diffraction, Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM).

The analysis on the synthesized powders shows the presence of clusters formed by 30–100 nm

nanoparticles. High-Resolution TEM (HRTEM) studies were carried out to confirm the existence of

Ca2Fe2O5. The dc four-probe measurement exhibits a total electrical conductivity, over 100 S cm�1 at

T $ 600 �C when 0 < x < 0.6, pointing out that strontium can be substituted for calcium without

modifying the electrochemical properties. The Pr0.4Ca0.6Fe0.8Ni0.2O3/Ca2Fe2O5 composite cathode

presents the better performance in electrochemical measurements, showing an area specific resistance

value of 0.09 ohm cm2 at 850 �C.

Introduction

Solid oxide fuel cells (SOFCs) convert chemical energy to elec-

trical energy exhibiting main advantages such as high efficiency

and low environmental impact.1 SOFC exploits the high mobility

of oxygen ions, O2�, in certain oxides at temperatures higher than

600 �C. So, they are good candidates for stationary power

installations and auxiliary power units in trucks.2,3 The tradi-

tional SOFCs with yttria-stabilized zirconia (YSZ) as electrolyte

and strontium-doped lanthanum manganite (LSM) as cathode

need to operate at high temperature (800–1000 �C), thus limiting

the commercial use due to the cost of the materials and the

maintenance of the cell.4 One effective way to overcome this

problem is to reduce the SOFCs working temperature from

1000 �C to an intermediate temperature range between 600 and

800 �C giving plenty of benefits such as versatile cell materials,

prolonged lifetime and reduced fabrication cost.5,6

A good cathode material requires a mixed ionic–electronic

conductor (MIEC) exhibiting a thermal expansion coefficient

similar to that of the electrolyte.7 The search for new materials

aDepartamento de Qu�ımica Inorg�anica, Facultad de Ciencia y Tecnolog�ıa,Universidad del Pa�ıs Vasco UPV/EHU, Apdo.644, 48080 Bilbao, Spain.E-mail: teo.rojo@ehu.es; Fax: +34 94 601 3500; Tel: +34 94 601 2458bGeneral Research Services (SGIker), Universidad del Pa�ıs Vasco UPV/EHU, Apdo.644, 48080 Bilbao, SpaincCIC energiGUNE, Parque Tecnol�ogico de �Alava, Albert Einstein 46-ED.E7, 01510 Mi~nano, �Alava, Spain

9682 | J. Mater. Chem., 2011, 21, 9682–9691

has been concentrated on LaFeO3, LaCoO3 and/or LaNiO3

perovskites doped with Sr or Ca. The best of these MIEC cath-

odes has been La1�xSrxFe1�yCoyO3.8 These cobalt-based cath-

odes have exhibited higher electrocatalytic performance than

those of the conventional LSM cathodes. Unfortunately, these

cathodes have high thermal expansion coefficient (TEC), high

cost of cobalt element and easy evaporation and reduction of

cobalt.9

A-Site doped rare earth orthoferrite compounds have been

studied as candidates to replace the LSM cathode due to its high

catalytic activity and mixed ionic and electronic conductivities at

reduced temperature (stotal $ 100 S cm�1 at 600–800 �C).10,11 Thechoice of using Pr instead of La as the rare earth element is due to

its peculiarity to show 3+ and 4+ oxidation states, which might

induce interesting electrical properties.12 Fe-based perovskite

oxides have attracted much attention as possible alternatives to

cobaltites due to their interesting transport properties. In this way

when the Fe fraction is higher than 0.5, the materials exhibit a high

electronic conductivity as was observed for the LaNi0.2Fe0.8O3

phase in which a conductivity of 135 S cm�1, at 800 �C, wasobtained.13 Therefore, a combination of Pr for A-site and Fe and

Ni for B-site is expected to give rise to better properties as

a cathode material at low temperature. In complex metal oxides,

the transition metal elements exhibiting anomalously high-valence

states work as catalytic active sites as in the case of LaFe1�xNixO3

where the Ni3+ and Fe4+ cations act as active sites for the oxygen

reduction.14

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On the other hand, calcium is another effective doping-element

for the A-site of ABO3, with low cost. The Pr1�xMxMnO3 (M ¼Ca, Sr) phases have been previously studied for SOFC applica-

tions, showing that the Ca-doped PrMnO3 phases exhibit better

characteristics than those of the Sr doped PrMnO3. In this

manner, the Ca-doped phases show higher electrical conduc-

tivity, lower cathodic overpotential, non-reactivity with YSZ and

more similar thermal expansion coefficient.15,16

As far as we are aware, the Ca and Ni-doped praseodymium

orthoferrites have not been studied for intermediate temperature

solid oxide fuel cells (IT-SOFCs) and especially the electrode

morphology behaviour and the electrical properties. Therefore,

the development of Co-free cathodes that combine electronic and

ionic conductivities was our goal in this work.

A growing interest of the use of nanostructured materials as

SOFC electrode has appeared in the last few years. These new

morphologies could provide some advantages with respect to the

physico-chemical properties due to the increase in the specific

area. This fact is closely related to the creation of double and

triple phase boundary (DPB and TPB) which has a great influ-

ence in the catalytic activity of the electrodes.17

In this paper we report on a new method to prepare nano-

structured composites in one step, which has permitted the

attainment of mixed conductor electrodes in a quick and easy

way. Nanosized Pr1�xCaxFe0.8Ni0.2O3�d (0 < x < 0.9) powders of

30–40 nm were obtained. The compounds were structurally and

morphological characterized by powder X-ray diffraction,

Scanning Electron Microscopy (SEM) and Transmission Elec-

tron Microscopy (TEM). Dc four-probe measurements on sin-

tered bars and electrochemical performance of PCFN electrodes

are also described.

Experimental section

Sample preparation

Samples of nominal composition Pr1�xCaxFe0.8Ni0.2O3 (PCFN)

from x ¼ 0 to 0.9 were prepared by the liquid mix process as

described elsewhere.18 Stoichiometric amounts of the nitrate salts

[Pr(NO3)3$5H2O, Ca(NO3)2$H2O, Fe(NO3)3$9H2O, Ni

(NO3)2$6H2O] and citric acid were dissolved in distilled water

and later a suitable volume of ethylene glycol was added. The

resulting solution was agitated and heated in a heating plate until

the formation of a gel. After that the gel, which was already

treated in a sand bath, was calcined at 600 �C in an oven for 12

hours with a 1� min�1 rate.

Structural and morphological characterization

X-Ray diffraction (XRD) measurements were carried out on

Philips PW1710 and Philips X’Pert-MPD (Bragg–Brentano

geometry) diffractometers, with CuKa radiation. The diffraction

data were refined by the Rietveld method.19 Morphological

studies were based on extended SEM observations on a JEOL

JSM-6400 microscope at 20 kV accelerating voltage and TEM

observations on a Philips CM200 microscope. High-Resolution

Transmission Electron Microscopy (HRTEM) studies were

performed using a Philips CM200 microscope. The thermal

expansion coefficient has been measured with a LINSEIS vertical

dilatometer (L75 Platinum Series). Dense sintered samples

This journal is ª The Royal Society of Chemistry 2011

exhibiting coplanar surfaces were thermal cycled in the temper-

ature range between 200 and 1000 �C in air.

Electrochemical measurements

The electrical conductivity of sintered bars (sintering temperature

of 1200 �C) of approximate 1 � 3 � 7 mm dimensions was

measured in air from 600 to 850 �C at 50 �C intervals by the Van

der Pauw’s four probe technique. Electrical contacts were made

using Pt wires and Pt paste placed over whole end faces ensuring

ahomogeneous current flow.The conductivity (s)was determined

froma set ofV�I values by takings¼ 1/r¼L/A� dI/dV, whereL

is the distance between voltage contacts andA is the sample cross-

section. A current load of 5–100 mA was applied with a Keithley

6221DC andAC current source. The corresponding voltage drop

was recorded with a 2182A nanovoltmeter.

The study of the conducting properties was carried out using

a two-electrode configuration, being necessary to obtain the

impedance spectra for symmetrical cells. In this way, the cells were

made of electrolyte pellets onto which symmetrical electrodes

were deposited. A commercial electrolyte (ceria doped samaria,

SDC) and platinum as current collector have been used. The SDC

pellets were made using NexTech materials as they were received.

The powder was pressed under 10 T uniaxial forces to form green

pellets. The pellets were sintered at 1050 �C (4 h) and subsequently

at 1500 �C (4 h). The density of the obtained pellets is higher than

93% opposite to the theoretical value. The surface of the pellets

was polished with grit paper and then cleaned with ethanol and

acetone solutions. In order to prepare the electrodes, the obtained

powders were dispersed in a vehicle ink in a 1 : 1 weight propor-

tion forming a paste. This paste was painted with a paintbrush in

both faces of the pellets forming symmetrical cells which were

sintered at 1050 �C for four hours to form porous electrodes well

adhered to the surface of the electrolytes.

Electrochemical Impedance Spectroscopy (EIS) measurements

of PCFN/SDC/PCFN test cells were conducted using a Solartron

1260 Impedance Analyzer. The frequency range was 10�2 to 106

Hz with a signal amplitude of 50 mV. All these electrochemical

experiments were performed at equilibrium from 850 �C to room

temperature, under zero dc current intensity and under air over

a cycle of heating and cooling. Impedance diagrams were ana-

lysed and fitted using the Zview software.

Results and discussions

Room temperature XRD patterns of Pr1�xCaxFe0.8Ni0.2O3 (x ¼0–0.9) exhibit perovskite phases (Fig. 1). A secondary Ca2Fe2O5

phase appears from x ¼ 0.5 to x ¼ 0.9 which was increased in

amount with the Ca content (inset in Fig. 1). This phase presents

brownmillerite structure, which is derived from the strong

reduction of the perovskite-type structure with the oxygen

vacancies ordered along alternate rows giving rise to a value of

the c-parameter double than that of the starting structure.20 The

crystal structure of brownmillerite exhibits a suitable matrix to

accommodate the Jahn–Teller cations favoring a distorted

octahedral environment.21 The brownmillerite structure can be

considered as a perovskite one with one sixth of the oxide-ions

removed. At room temperature, the oxide-ion vacancies are

usually ordered to generate a structure with alternating layers of

J. Mater. Chem., 2011, 21, 9682–9691 | 9683

Fig. 1 X-Ray diffraction patterns of the Pr1�xCaxFe0.8Ni0.2O3 (0 < x <

0.9) samples. Inset: detailed portion of the patterns showing the peaks

corresponding to the new phase.

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BO6 octahedra and BO4 tetrahedra. At high temperatures the

oxide-ion vacancies can be disordered, in a similar way to some

pyrochlores. The oxygen vacancies in brownmillerite structure

were assumed to contribute to ionic transport, forming one-

dimensional pathway for oxygen ion migration in the tetrahedral

layers.22 This structure that appears in the Pr1�xCaxFe0.8Ni0.2O3

(0.5 < x < 0.9) phases enhanced the electrochemical properties

giving rise to lower resistance values. The crystal structure of the

pure perovskite phases could be defined from the XRD data in

a PrFeO3 structural type (space group Pnma).23,24

The results of the Rietveld refinement for Pr0.6Ca0.4Fe0.8Ni0.2O3

are shown in Fig. 2. Changes in the crystal structure properties as

a result of Ca doping are listed in Table 1. These values are in good

agreement with those reported in the literature.14,25 As can be seen

in this table, the ‘b’ cell parameter decreases from x ¼ 0 to x ¼ 0.4

but, on the contrary, an increase in the ‘a’ and ‘c’ parameters is

observed. A systematic increase in volume with increasing doping

level is also observed. These changes can be attributed to the

different ionic radii and volumes of the Pr3+ (1.179 �A) and Ca2+

(1.34�A) cations in the A-site. A similar behavior in the variation of

Fig. 2 Fitted diffraction profile of Pr0.6Ca0.4Fe0.8Ni0.2O3 showing

observed (crosses), calculated (lines) and difference (lower) profiles.

9684 | J. Mater. Chem., 2011, 21, 9682–9691

the lattice parameters with the Ca amount was observed by Vidal

et al. in the Ln1�xMxFeO3�d (Ln ¼ La, Nd, Pr; M ¼ Sr, Ca)

system.26

Rietveld refinement of the structural data showed that the

substitution of Pr3+ by Ca2+ is accompanied by a local distortion of

the Pr1�xCaxFe0.8Ni0.2O3 structure (see Fig. 3). The bond lengths

and angles are also affected byCa doping as can be seen in Table 2.

A decrease in the average distances Fe–O can be also observed

which can be attributed to the size reduction in the position of iron,

from Fe 3+ to Fe4+, to ensure the electroneutrality of the samples.

The increase in the Fe–O1–Fe bond angle and the decrease in Fe–

O2–Fe values are directly related to the distortion of the BO6

octahedra as a consequence of the Fe4+ Jahn–Teller effect.

The Pr1�xCaxFe0.8Ni0.2O3 phases exhibit a solid solution

behavior in the range from x¼ 0 to 0.5. Essays to introduce higher

Ca content (x $ 0.5) gave rise to the segregation of Ca2Fe2O5 as

a secondary phase which increases when the Ca content is

increased (Table 2).27The results of theRietveld refinement for the

Pr0.2Ca0.8Fe0.8Ni0.2O3 sample are shown in Fig. 4. The analysis of

these data revealed that the sample exhibits two orthorhombic

phases, one perovskite structure (space group Pnma; a ¼ 5.5308

(6) �A, b ¼ 15.4258 (8) �A and c ¼ 5.4491 (8) �A), similar to that of

PrFeO3, and other one with brownmillerite structure (Ca2Fe2O5,

space group Pnma; a ¼ 5.2739 (6) �A, b ¼ 15.3163 (2) �A and c ¼5.4664 (7) �A). These results are in good agreement with those

reported in the literature.21,28 However, in the present study we

confirm the formation of a perovskite phase from x¼ 0 to x¼ 0.4

appearing a new phase, Ca2Fe2O5, at higher amounts of calcium.

XRD measurements as a function of temperature were carried

out. The 3D plot of the recorded diffraction patterns for the

Pr0.9Ca0.1Fe0.8Ni0.2O3 material is shown in Fig. 5. From the

analysis of these data cannot be assured a structural change in

brownmillerite phase, although an increase in the crystallinity of

the phase from 660 �C is observed. The results indicate the

possibility of brownmillerite transformation from perovskite

phase decreases with the La content as a result of increased

oxygen vacancy concentration. These materials require two

additional thermal treatments before electrochemical tests, in

order to improve the adhesion of the cathode to the electrolyte

and also of the platinum ink, which acts as current collector.

These thermal treatments need temperatures around 1050 �C,this means that the crystal structure of the samples can change

and chemical reactions between perovskite and brownmillerite

can take place. For this reason, different thermodiffractograms

have been recorded using similar temperature conditions. After

thermal treatment during 4 hours at 1050 �C, the percentages ofperovskite/brownmillerite phases remain unchanged and no

crystal structure modification or reactions have been observed.

The conductive properties of the samples are influenced not

only by the structural aspects but also by the particle size. In this

way, it is very important to get phases with smaller particle size in

order to obtain a better behavior of the material as cathode. The

results of the Scanning Electron Micrographs (SEM) of the

PCFN samples are shown in Fig. 6. As can be seen, the particles

exhibit a wide size distribution with agglomeration and

a morphology similar to that of La0.8Sr0.2Fe0.8Ni0.2O3.6 The

porosity of the samples increases with increasing the Ca content

up to 40% Ca and then a lowering of the porosity with the

formation of agglomerates is observed.

This journal is ª The Royal Society of Chemistry 2011

Table 1 Crystallography parameters of the Pr1�xCaxFe0.8Ni0.2O3 (0 < x < 0.4) series obtained from XRD Rietveld refinement

x ¼ 0 x ¼ 0.1 x ¼ 0.2 x ¼ 0.3 x ¼ 0.4

Lattice parametersa/�A 5.4814(5) 5.4809(8) 5.4907(8) 5.4921(3) 5.4976(9)b/�A 5.5565(1) 5.5487(7) 5.5484(7) 5.5497(7) 5.5421(3)c/�A 7.7723(1) 7.7727(9) 7.7809(6) 7.7842(1) 7.7871(6)V/�A3 236.726(3) 236.391(6) 237.050(5) 237.265(7) 237.266(6)Pr/Cax 0.9863(1) 0.9863(1) 0.9886(5) 0.9914(4) 0.9908(5)y 0.0429(9) 0.041481) 0.0404(8) 0.0364(2) 0.0349(7)z 0.25 0.25 0.25 0.25 0.25Fe/Nix 0 0 0 0 0y 0.5 0.5 0.5 0.5 0.5z 0 0 0 0 0O1

x 0.077(6) 0.059 (5) 0.060(8) 0.054(1) 0.059(5)y 0.477(5) 0.482 (3) 0.487(5) 0.487(5) 0.479(2)z 0.25 0.25 0.25 0.25 0.25O2

x 0716(8) 0.699 (3) 0.694(1) 0.701(3) 0.699(4)y 0.304(6) 0.305 (3) 0.303(8) 0.302(9) 0.303(3)z 0.041(3) 0.04480 0.040(4) 0.042(4) 0.046(7)OccPr 0.5 0.45 0.4 0.35 0.3Ca 0 0.05 0.1 0.15 0.2Fe 0.4 0.4 0.4 0.4 0.4Ni 0.1 0.1 0.1 0.1 0.1O1 0.5 0.5 0.5 0.5 0.5O2 1 1 1 1 1R-Factors (%)Rwp 13.9 13.0 12.0 11.6 11.8Rexp 8.46 8.71 8.24 9.58 9.82RBragg 5.54 6.74 5.95 5.45 5.72Bond lengths/�AFe1–O1 1.993(1) 1.972 (5) 1.974(9) 1.970(3) 1.977(5)Fe1–O2

0a 2.093(1) 2.04 (1) 2.025(8) 1.999(1) 2.040(1)Fe1–O2

00b 1.921(3) 2.00 (1) 2.019(5) 2.038(9) 2.012(5)Bond angles/�Fe–O1–Fe 154.30(5) 160.24(1) 160.12(1) 162.21(2) 159.78(1)Fe–O2–Fe 152.87(8) 149.0 (7) 149.53(1) 150.38(8) 148.79(1)

a Symmetry operators: x + 1/2, �y + 1/2, �z; �x + 1/2, y + 1/2, z. b x, y, z; �x, �y, �z.

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In order to get better measurements of the morphology of the

samples Transmission Electron Microscopy (TEM) was carried

out. TEMmicrographs of the PCFN samples are shown in Fig. 7.

All powders showed globular shapes but considering the

different doping level a different particle size distribution is

obtained. The sample without Ca exhibits particles with a grain

size of about 40–100 nm showing tendency for agglomeration

and low porosity. Samples with a Ca amount from 20 to 40%

exhibit smaller particle size (30–40 nm) and greater porosity. The

sample with 80% of Ca shows many particles that after sintering

they have tendency to form conglomerates. In previous studies,

we had demonstrated that after electrochemical test, the grains of

the samples synthesized by the liquid mix method had grown, but

particles still showed nanosize (75–100 nm).18

The High-Resolution Transmission Electron Microscopy

(HRTEM) measurements of the Pr0.2Ca0.8Fe0.8Ni0.2O3 sample

corroborate the existence of the brownmillerite (Ca2Fe2O5)

phase. Similar results were observed for the phases with Ca

content higher than 50%. The Fourier transform of the HRTEM

image (Fig. 8) shows the electron diffraction pattern of one

crystallite, indicating that this particle is oriented along the [342]

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zone axis. The measured lattice spacings can be related to the

(0�12), (�211) and (�22�1) planes of the brownmillerite-like structure,

corroborating the presence of this phase as was suggested from

the XRD data.

The thermal expansion coefficients (TECs) of the cathodic

materials were determined by dilatometry. The values for

cathode and electrolyte phases must be as close as possible to

avoid the thermal stresses that can cause delamination and

cracks, leading to a poorer adhesion of the components. The

thermal expansion coefficients of PCFN samples are reported in

Fig. 9. From x ¼ 0 to x ¼ 0.5 TEC curves exhibit a linear

behaviour, but with higher Ca content (x $ 0.5) it is possible to

distinguish a change in curvature in 650–750 �C range. This

change becomes more important in samples with higher

brownmillerite content (x > 0.7) (see Fig. 9a). This anomalous

behaviour observed for the samples with x > 0.5 can be attrib-

uted either to the presence of a phase transition where the cell

volume decreases in the brownmillerite phase (Pnma–Imma) or

changes in the oxidation states of Fe for compensating the charge

neutrality of Ca2Fe2O5 by the formation of oxygen vacancies.

A similar behavior was observed by Li et al.29 As can be observed

J. Mater. Chem., 2011, 21, 9682–9691 | 9685

Fig. 3 Crystal structure distortion of (a) PrFe0.8Ni0.2O3 and (b)

Pr0.6Ca0.4Fe0.8Ni0.2O3.

Table 2 Percentages of perovskite and brownmillerite structures (0.5 < x< 0.9) obtained by XRD Rietveld refinement

x ¼ 0.5 x ¼ 0.6 x ¼ 0.7 x ¼ 0.8 x ¼ 0.9

%Perovskite 77.40 68.55 68.37 51.92 47.21Brownmillerite 22.60 31.45 31.63 48.08 52.79

Fig. 4 Fitted diffraction profile of Pr0.2Ca0.8Fe0.8Ni0.2O3 showing

observed (crosses), calculated (lines) and difference (lower) profiles.

Fig. 5 X-Ray thermodiffractogram for the Pr0.9Ca0.1Fe0.8Ni0.2O3

sample measured from room temperature to 1050 �C. Inset: detail cor-responding to the contour map of the three-dimensional representation

of the high intensity diffraction peak.

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in Fig. 9b, the TEC value strongly depends on the calcium

content. With increasing the amount of Ca in the samples, the

TEC value also increases up to x¼ 0.3. From x¼ 0.4, TEC value

decreases which coincides with the appearance of brownmillerite

phase that presents a TEC value of 11.3 � 10�6 K�1 in the

temperature range 700–1000 �C.30 As mentioned, thermal

expansion compatibility between cathode and electrolyte is

crucial in order to avoid mechanical problems due to thermal

stress. In this case, PCFN samples have been compared with that

of SDC, pointing out that the phase without Ca and those with x

¼ 0.6 and 0.7 are the most thermo-mechanical compatible

materials with this electrolyte.

9686 | J. Mater. Chem., 2011, 21, 9682–9691

The electrical conductivity (s) for the Pr1�xCaxFe0.8Ni0.2O3

phases measured at different temperatures is shown in Fig. 10. The

expected behavior for this system was to observe an increasing

conductivity as Ca content increases due to the formation of

anionic vacancies and the change in the oxidation state of Fe

cations from trivalent to tetravalent, showing mixed oxygen ionic

and electrical conductivities. On the other hand, in this case, s

shows a continuous increment up to values of x¼ 0.4 with a sharp

drop for x > 0.5 and a strong decrease for x ¼ 0.9.31 As can be

observed in Fig. 10a, until x ¼ 0.3 the electrical conductivity is

higher at 800 �C but from x ¼ 0.4 to x ¼ 0.9 the trend is reversed,

being the higher electrical conductivity obtained at 600 �C. Thisfact may be related to the presence of a secondary phase that could

strongly vary the electronic conduction.32 When the influence of

the temperature on the electrical conductivity is studied, similar

behaviour is observed (Fig. 10b). The temperature dependence of

the electronic conductivity is relatively strong, as quantitatively

expressed by the activation energy of about 1.35 eV for x # 0.3,

while this value increases to 1.98 eV for higher Ca doping.

When brownmillerite phase appears (x $ 0.4), a change in the

conduction mechanism is produced, and conductivity decreased

when temperature is increased. For Ca content below 40%,

a small-polaron semiconducting behaviour is found, while for the

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Fig. 6 Scanning Electron Microscopy (SEM) images obtained at 20 000

and 100 000 times magnification for (a and b) PrFe0.8Ni0.2O3, (c and d)

Pr0.8Ca0.2Fe0.8Ni0.2O3, (e and f) Pr0.6Ca0.4Fe0.8Ni0.2O, (g and h)

Pr0.4Ca0.6Fe0.8Ni0.2O3 and (k and l) Pr0.2Ca0.8Fe0.8Ni0.2O3.

Fig. 7 Transmission Electron Microscopy (TEM) micrographs of PCFN

samples: (a and b) PrFe0.8Ni0.2O3, (c and d) Pr0.8Ca0.2Fe0.8Ni0.2O3, (e and f)

Pr0.6Ca0.4Fe0.8Ni0.2O3 and (g and h) Pr0.2Ca0.8Fe0.8Ni0.2O3.

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samples with x $ 0.4, the decrease in conductivity when temper-

ature increases could be attributed to the loss of oxygen from the

lattice at high temperatures, consistent with the findings of Ase-

nath-Smith et al.33 An increase in ionic conductivity in Ca2Fe2O5

with increasing temperature is expected, giving rise to a decrease in

electronic (hole) conduction.

The higher electrical conductivity is achieved at 600 �C for an

amount of 40% Ca, which can be related to the appearance of

Ca2Fe2O5 (not observable by XRD in Pr0.6Ca0.4Fe0.8Ni0.2O3)

showing an activated mobility, consistent with polaron conduc-

tion mechanism of hole transport. At low temperature hole-type

conduction due to the existence of oxygen interstitials in

Ca2Fe2O5 appears. At temperatures higher than 677 �C a phase

transition (Pnma–Imma) takes place and the new phase shows

a lower electrical conduction.33,34 Thus, in perovskite/brown-

millerite composites have been found an increased electronic

This journal is ª The Royal Society of Chemistry 2011

conduction at 600 �C than at higher temperatures. The cause of

this behavior is due to the next fact: when T < 650 �C Ca2Fe2O5

presents electronic conduction and for T > 650� C it becomes

mainly ionic conductor due to the generation of oxygen vacan-

cies in its new high-temperature structure.

This kind of material has attracted much attention due to its

ability of accommodation of large amounts of oxygen vacancies

in the lattice, resulting in a fast oxygen ionic conductivity in the

material.29,35 This study implies that the partial substitution of

Ca2+ ion in the A-site and Ni2+ in the B-site for PrFeO3 has an

advantageous effect on its electrical conductivity up to 60% Ca.

This value is over 100 S cm�1 at temperatures higher than 600 �Csatisfying the general requirement for electrode materials in

intermediate temperature SOFC.

J. Mater. Chem., 2011, 21, 9682–9691 | 9687

Fig. 8 Transmission Electron Microscopy (HRTEM) image of

Pr0.2Ca0.8Fe0.8Ni0.2O3. The inset shows the indexed Fourier Transform

pattern.

Fig. 9 Thermal expansion coefficients (TECs) measured in air for

Pr1�xCaxFe0.8Ni0.2O3 in the temperature range between 700 and 1000 �C(dashed region: TEC value for the Sm doped ceria SDC electrolyte).

Fig. 10 (a) Effect of the Ca2+ amount on the electrical conductivity (s) at

different temperatures and (b) variation of the electrical conductivity

with temperature.

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Recently studies have shown that the oxides with brownmil-

lerite structure have potential applications in separation of

oxygen from air and partial oxidation of light hydrocarbons, due

to their high oxygen permeability.36,37 These results indicate that

perovskite–brownmillerite composites are good candidates to be

a new generation of cathode materials for IT-SOFC.

Several strategies have provided promising results in the

performance of these devices, but the knowledge of the processes

taking part in SOFC electrodes is partially understood, which

makes more difficult the optimization of the materials that

present degradation problems at short and long term. Its oper-

ation varies strongly with many variables that are still unknown

and their understanding is limited as to how material properties

and microstructure are related to the operation and long-term

9688 | J. Mater. Chem., 2011, 21, 9682–9691

stability. Electrochemical impedance spectroscopy is a useful

technique to clarify the different processes that take place in these

devices.

The impedance spectra of Pr0.8Ca0.2Fe0.8Ni0.2O3 and

Pr0.4Ca0.6Fe0.8Ni0.2O3 at 800�C are shown in Fig. 11. To carry

out the study of the different processes that occur at the elec-

trode, it is important to take into account the different semi-

circles involved. At high frequencies, a smaller semicircle

attributed to charge transfer followed by the ion incorporation

into the electrolyte is observed. This contribution is related to the

interface and not to the electrode surface. The charge transfer

models describe interfacial resistance as a function of the elec-

tronic and ionic transport within the two components and the

morphology of these components.38 Other models have been

used to describe the interfacial resistance primarily in terms of

the internal porosity and the kinetics of oxygen diffusion and

surface exchange in the composite material.39 Both models

provide insights into the parameters that influence the perfor-

mance of composite electrodes. At lower frequencies, a higher

semicircle attributed to the oxygen diffusion through the elec-

trode is observed. As can be seen, this semicircle is larger and

provides a greater contribution to the total resistance of the

electrode. It is possible to compare the obtained impedance

spectra for the Pr0.8Ca0.2Fe0.8Ni0.2O3 sample (only perovskite)

This journal is ª The Royal Society of Chemistry 2011

Fig. 11 Impedance plot of Pr0.8Ca0.2Fe0.8Ni0.2O3 and Pr0.4Ca0.6Fe0.8Ni0.2O3 on SDC electrolyte at 800 �C.

Fig. 12 Arrhenius plots of area specific resistance ASR (a) PCFN

electrodes at 850 �C and (b) Pr0.4Ca0.6Fe0.8Ni0.2O3 electrode at different

temperatures with SDC electrolyte.

Fig. 13 Temperature dependence of area specific resistance (ASR) for

the Pr1�xCaxFe0.8Ni0.2O3 compounds.

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composites and the Pr0.4Ca0.6Fe0.8Ni0.2O3 sample (perovskite/

brownmillerite) as shown in Fig. 11. This result confirms that the

diffusion of oxide ions generated in the material surface occurs in

an easier way when Ca2Fe2O5 appears as an ionic conductor

mixed with perovskite phase.

Another way to confirm these processes is associating capac-

itance values to each semicircle. Conducting this study has been

observed that semicircles at high frequencies present capacitance

values of 10�6 F while those observed at low frequencies present

capacitance values of 10�3 F. These results are in good agreement

with those described in the literature.40

The cathodic Area Specific Resistance (ASR) for PCFN-SDC

electrodes has been deduced from the relation: ASR¼Relectrode�surface/2. The results at 850 �C as a function of Ca content in

Pr1�xCaxFe0.8Ni0.2O3 are shown inFig. 12 and 13.As can be seen,

with increasing SDC content, a drop of the ASR value up to 50–

60%Ca is observed and then, the ASR values increase again up to

90%. The samples with 50–60%Ca containingCa2Fe2O5 show the

lower resistance values due to the increase in the ionic conductivity

of this phase which is added to the electronic conductivity

provided by PCFN. Furthermore, the microstructure of these

samples shows a grain size of about 40–100 nm. These results are

important because the microstructure and polarization resistance

(Rp) of the electrode are found to be correlated being the high

porosity and small particles which exhibit low Rp values.41,42

The main processes which can take place in the oxygen

reduction mechanism on porous MIEC (mixed ionic and elec-

tronic conductors) electrodes are the following: (i) diffusion of

O2 molecules in the gas phase to the electrode, (ii) oxygen

dissociative adsorption on cathode surface, (iii) surface diffusion

of oxygen on the cathode, (iv) incorporation of oxygen into

electrolyte via the triple phase boundary (TPB) in the electrode/

electrolyte interface, (v) oxide ion incorporation in the bulk of

cathode (if mixed conducting), (vi) bulk or surface transport of

oxide ion from cathode to electrolyte, and (vii) electrochemical

charge transfer across the electrode/electrolyte interface.43–45

Pure perovskite structure materials exhibit a polarization resis-

tance much greater, suggesting negligible oxide ion conduction.

This conduction must occur through oxygen vacancies, although

in this case by refining the diffraction data it has been estimated

that they are virtually non-existent. Thus, the oxide ion forma-

tion and the transfer to the electrolyte must occur at TPB regions.

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However, when the brownmillerite phase appears the response of

the material is very different. In this case, the composite is able to

conduct oxygen ions, so the incorporation of oxygen into the

electrolyte can take place at double phase boundary (DPB)

through cathode/electrolyte interface. Likewise, oxide ion diffu-

sion in the bulk of cathode and oxide ion transport from the

cathode to the electrolyte are favored by the presence of

brownmillerite, thereby decreasing the polarization resistance.

Although the electronic conductivity is higher at 600 �C than

at higher temperatures, there are other factors that influence

polarization resistance and ASR, as the ionic conductivity or the

active surface area. In this case, the limiting process is the ionic

conductivity that is thermally activated. In this manner, when

temperature is increased, the conduction gets facilitated, giving

rise to a lower polarization resistance.

From 60% the resistance value begins to increase, decreasing

the electronic conductivity from 243 to 37 S cm�1, approximately.

At this point, the contact between the gas and the electrode

surface is limited, which difficulties the reactions of adsorption

and dissociation of oxygen. Therefore, oxide ions are formed in

smaller quantities. Increasing the amount of brownmillerite the

regions of 2 PB (gas/electrode) are reduced, this will be the

limiting factor of conductivity, thus increasing the ASR values.

The Pr0.4Ca0.6Fe0.8Ni0.2O3 cathode presents the better

performance in electrochemical measurements, showing an ASR

value of 0.09 ohm cm2 at 850 �C. This remarkable behavior is due

to the presence of a primarily electronic conductor material

(perovskite) with an ionic conductor (brownmillerite) giving rise

to a composite. It is clear the interest of this type of composites as

cathodes for IT-SOFC. In this case, there is no need to carry out

a mixing of both compounds with consecutive heat treatments,

because in the same synthesis both nanoscale phases are

obtained. The fact of obtaining a nanostructured composite will

lead to a more specific area on the electrode, as well as promote

greater contact between the two phases, which translates into

better performance.

Conclusions

A high-performance nanostructured perovskite/brownmillerite

composite electrode was successfully prepared by a novel one

step synthesis route using the liquid mixed method. These cobalt-

free Pr1�xCaxFe0.8Ni0.2O3 (PCFN) oxides show a nanosize

distribution, exhibiting the smallest particle size and greater

porosity for the samples with 20 and 40% of Ca content

(30–40 nm).

XRD analysis suggests that the PCFN samples (0 < x < 0.4)

have orthorhombic symmetry corresponding to a perovskite

structure, whereas 0.4 < x < 0.9 samples show a coexistence of

the perovskite and the brownmillerite-type structure. This

statement has been confirmed by means of high resolution

transmission microscopy. The electrical conductivity of the

PCFN oxides goes through a maximum at x ¼ 0.4, the same

composition at which brownmillerite phase appears. The

decreasing electrical conductivity is associated to the appearance

of the ionic phase (Ca2Fe2O5). The electrical conductivity at

T$ 600 �C is over 100 S cm�1 (0 < x < 0.6), indicating the interest

of these materials to be considered as electrode in intermediate

temperature SOFC devices.

9690 | J. Mater. Chem., 2011, 21, 9682–9691

The best electrochemical performance is for Pr0.4Ca0.6Fe0.8Ni0.2O3, exhibiting an ASR value of 0.09 ohm cm2 at 850 �C.Taking into account the ASR value and the electrical conduc-

tivity of this cathode (at temperatures above 600 �C is over

100 S cm�1), this material is a promising cathode for intermediate

temperature-operation solid oxide fuel cell (IT-SOFC)

applications.

Acknowledgements

This work has been partially financed by the Spanish CiCyT

under projects MAT2007-66737-C02-01 and MAT2010-19442

and by the Government of the Basque Country under project IT-

312-07 and SAIOTEK S-PE09UN61. N. Ortiz-Vitoriano thanks

the Eusko Jaurlaritza/Gobierno Vasco for her predoctoral

fellowship. I. Ruiz de Larramendi thanks the Government of the

Basque Country for funding her research activities as postdoc

within the Project GIC07/126-IT-312-07.

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