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Interlayer-free electrodes for IT-SOFCs by applying Co3O4 assintering aid

Dengjie Chen, Fucun Wang, Zongping Shao*

State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry & Chemical Engineering,

Nanjing University of Technology, No. 5 Xin Mofan Road, Nanjing 210009, PR China

a r t i c l e i n f o

Article history:

Received 11 April 2012

Received in revised form

10 May 2012

Accepted 12 May 2012

Available online 8 June 2012

Keywords:

Solid oxide fuel cells

Stabilized zirconia electrolyte

Cobaltite electrodes

Interlayer-free

Sintering aid

* Corresponding author. Tel.: þ86 25 8317225E-mail address: shaozp@njut.edu.cn (Z. S

0360-3199/$ e see front matter Copyright ªdoi:10.1016/j.ijhydene.2012.05.053

a b s t r a c t

The influence of Co3O4 as a sintering aid for a series of cobalt-containing perovskite oxides

on the microstructure and electrical properties have been investigated. X-ray diffraction

and scanning electron microscopic results showed that well connected electrode particles

with firm adhesion to the 8 mol% yttria-stabilized zirconia (YSZ) electrolyte surface were

realized at a temperature free from interfacial phase reaction. Both ohmic and polarization

resistances of symmetric cells by adopting YSZ electrolyte, measured by electrochemical

impedance spectroscopy, were much lower than that without adding Co3O4. The peak

power density of 1176 mW cm�2 at 750 �C was achieved when La0.6Sr0.4Co0.2Fe0.8O3�d

þ Co3O4 was selected as a representative cobaltite cathode, which is much higher than

a similar fuel cell with the cathode fabricated by a conventional way. Fabrication of

interlayer-free electrodes by applying Co3O4 as a sintering aid is very simple and general,

applicable for a wide range of cobalt-containing electrode materials.

Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

1. Introduction Lower operating temperature can accelerate the commer-

With the quickly expanding energy demand and increasing

environmental pollution created by traditional technologies

such as low energy utilization efficiency of fossil fuels based

on combustion techniques, solid oxide fuel cells (SOFCs),

which are high-efficient, environmental-friendly electro-

chemical energy conversion devices, have gained increasing

attention worldwide for efficient and clean electricity gener-

ation [1]. Conventional SOFCs are based on dense 8 mol%

yttria-stabilized zirconia (YSZ) electrolyte and porous

La0.8Sr0.2MnO3 (LSM) cathode and operate at around 1000 �C[2]. Although there are many decades of intensive efforts on

developing SOFCs into practical devices, widespread practical

application of them has still not been realized because there

are several important drawbacks associated with such high

operating temperatures. Current efforts are desired to reduce

operating temperature to intermediate range, i.e. 500e800 �C.

6; fax: þ86 25 83172242.hao).2012, Hydrogen Energy P

cialization of SOFCs because of improved cell lifetime,

reduced cell materials and operation cost, as well as increased

versatility in cell sealing [3,4]. Unfortunately, poor cathode

performance has become a new emerging problem due to the

difficulty of oxygen reduction at reduced temperatures.

Since theelectrocatalytic activityof SOFCcathodes ishighly

dependent on the materials composition, nowadays there are

considerable research efforts on new cathode materials

development. Easy thermal reduction of cobalt ion benefits the

formation of oxygen vacancy and promotes the diffusion of

oxide ions within the oxide lattice, consequently many

cobalt-based perovskite oxides have both mixed oxygen ionic

and electronic conductivity at elevated temperatures [5].

Because of the mixed conductivity, the oxygen reduction

active sites can be expanded from the typical triple phase

boundary (electrode-electrolyte-air) region for most iron- and

manganese-containing perovskite oxides with negligible ionic

ublications, LLC. Published by Elsevier Ltd. All rights reserved.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 1 1 9 4 6e1 1 9 5 4 11947

conductivity to the entire exposed electrode surface if the

oxygen ionic conductivity is sufficiently high, thus cobalt-

based perovskite oxides potentially display much better elec-

trochemical activity for oxygen reduction reaction at reduced

temperatures than the state-of-the-art LSM electrode [6e10].

As to the electrolyte, YSZ is still the most applied in SOFCs

due to its high mechanical strength, favorable oxygen ionic

conductivity, negligible electronic conductivity, relatively low

price, and high chemical stability over awide oxygen potential

window and temperatures. However, the direct application of

cobaltite perovskite oxides on YSZ electrolyte is still not very

successful up to now, because of serious interfacial reaction

between them during the high-temperature fabrication

[11,12], which is typically conducted at 1000e1150 �C. Suchphase reaction may create an insulating interfacial layer

between the electrode and the electrolyte layers to block oxide

ion conduction, thus significantly increasing the charge

transfer resistance. Thereby, the direct application of cobaltite

perovskite oxides onto YSZ electrolyte often results in poor

electrode performance.

Some researchers have tried advanced synthetic tech-

niques to prepare ultrafine electrode powders with high

surface area to reduce firing temperature of the electrode layer

to a range that the interfacial phase reaction can be effectively

suppressed [13e15]. Although it is somewhat successful, the

complexity in powder synthesis makes it economically less

attractive and also difficult for scale up. Another way often

applied is to adopt a buffer layer, typically doped ceria, to avoid

the direct contact between cobaltite electrodes and stabilized

zirconia electrolyte [16e20]. Significant improvement in elec-

trode performance at the intermediate temperature range has

been successfully demonstrated [21], however, the way by

applyinga buffer layernot only increases the complexity in cell

fabrication but also introduces new potential interfacial reac-

tion between the buffer layer and the electrolyte if the fabri-

cation process was not properly managed [22].

In literature, the application of a sintering aid is often used

to reduce sintering temperature of ceramics, which otherwise

require a firing temperature, several hundred degrees higher,

for achieving total densification [23e26]. If a sintering aid can

bedeveloped for the cobalt-containingelectrodes, allowing the

formation of well connected microstructure at a temperature

lower than 850 �C, the interfacial reaction between cobalt-

containing electrodes and electrolyte may be effectively avoi-

ded, consequently an improvement in electrode performance

is expected. It iswell knownsome transitionmetal oxides such

as CO3O4, ZnO and NiO can perform as sintering aid for

ceramics [27e30]. For example, Kleinlogel et al. reported that

the introduction of 1mol% Co3O4 into Ce0.8Gd0.2O2�x (CGO) can

effectivelyachieveasinteringdensityof99%at900 �C,while for

a pure CGO, a sintering temperature of 1300 �C is required to

reach a density of higher than 98%.

Previously we have demonstrated Co2O3 can perform as

a sintering aid for Ba0.5Sr0.5Co0.8Fe0.2O3�d (BSCF) to promote its

sintering at lower temperature, in that study, we tried to

reduce the sintering temperature of BSCF as ceramic

membrane for oxygen separation [31]. More recently, we have

found that the introduction of a small amount of Co3O4 into

BSCF can also improve the electric conductivity and electro-

chemical activity for oxygen reduction over samaria-doped

ceria electrolyte [32]. It suggests a synergistic effect between

BSCF and Co3O4 was likely created.

In the present study, we reported the investigation of Co3O4

as a sintering aid for a series of cobalt-containing perovskite

oxides as oxygen reduction electrodes of SOFCs. Our main

purpose is to reduce the sintering temperature of such elec-

trodes onto the YSZ electrolyte directly at a temperature lower

than the appearance of interfacial reaction, thus improving

the electrode performance. The five most investigated cobalt-

containing electrodes including, La0.6Sr0.4Co0.2Fe0.8O3�d (LSCF),

BSCF, Sm0.5Sr0.5CoO3�d (SSC), SrSc0.2Co0.8O3�d (SScC) and

PrBaCo2O5þd (PBC) were selected and investigated. Some

interesting results were obtained.

2. Experimental

Perovskite powders (LSCF, BSCF, SSC, SScC and PBC) were

synthesized by a complexing process with EDTA and citric

acid as the complexing agents and metal nitrates as the raw

materials [10]. Co3O4 was a commercial product with a specific

surface area of 20 m2 g�1 and the Co3O4 content in the

composite electrodes throughout the paper was fixed at

5 wt.%. The fabrication process is fairly simple, conventional

micrometer-sized electrode materials were first mixed well

with Co3O4 by ball milling for 30 min, which was then

deposited onto electrolyte by spray deposition and finally fired

at 800 �C for 5 h. For comparison, micrometer-sized electrode

materials without Co3O4 sintering aid was also fabricated onto

YSZ electrolyte and fired at 800 or 1000 �C for 5 h.

The phase structure of oxideswas characterized by powder

X-ray diffraction (XRD) with diffractometer (D8 Advance,

Bruker) using Cu-Ka radiation (l ¼ 1.5418 A) at 40 kV and

40mA. Oxygen temperature programmed desorption (O2-TPD)

was measured by a mass spectrometer (QIC-20, Hiden) and

a temperature controller was used to control the heat velocity

of the furnace at a ramp rate of 10 �Cmin�1 from 100 to 930 �C.The microstructure was determined by using scanning elec-

tron microscope (QUANTA-200, FEI). Electrochemical imped-

ance spectra (EIS) were collected using a symmetric cell

configuration under open circuit conditions, using an elec-

trochemical workstation Solartron (1260 A and 1287, Solartron

Analytical). IeV and IeP polarization tests of a single cell

(Ni þ YSZ/YSZ (20 mm)/electrode, NiO:YSZ ¼ 60:40 by weight)

were measured using a digital source meter (2420, Keithley).

3% H2O humidified H2 was fed into the anode chamber at

a flow rate of 80mlmin�1 as anode fuel gas and ambient air as

cathode atmosphere during the measurement. Electrical

conductivity was tested by a four-terminal direct current

technique with sintered bars and the current and the voltage

were detected by a source meter (2420, Keithley) over

a temperature range of 300e900 �C.

3. Results and discussion

It is well known that the interfacial reaction between two solid

oxides is thermally excited. To determine the proper fabrica-

tion temperature to effectively avoid or suppress the interfa-

cial reaction between cobaltite cathodes and YSZ electrolyte,

Fig. 2 e O2-TPD curves of the mixture powders (LSCF and

YSZ) after firing at 1100 �C (a), 1000 �C (b), 900 �C (c), 850 �C(d), 800 �C (e) and at room temperature (f) for 5 h.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 1 1 9 4 6e1 1 9 5 411948

LSCF was selected as a representative cobaltite electrode

material and its reaction with YSZ electrolyte at different

temperatures was first investigated by powder reaction. LSCF

and YSZ were mixed at a weight ratio of 50 to 50 in powder

formand then fired at various temperatures in air for 5 h. Fig. 1

presents XRD patterns of the mixture powder after the firing.

The diffraction patterns for the powder mixture fired at 800 �Ccan be indexed well based on a physical mixture of LSCF

perovskite and YSZ fluorite, suggesting the negligible phase

reaction between YSZ and LSCF at this temperature. Impurity

phases of La2Zr2O7 and SrZrO3, which are insulators for oxide

ion, started to appear at a calcination temperature of 850 �Cand became obvious at temperatures higher than 900 �C.

To further exploit the potential phase reaction between

LSCF and YSZ, the mixtures after the firing at various

temperatures were also subjected for O2-TPD analysis.

Thermal reduction of cobalt ion would accompany with the

release of oxygen from oxide lattice to surrounding atmo-

sphere, which can be detected by an on-line mass spectrom-

eter. The formation of molecular oxygen is closely related to

not only the reducibility of cations but also the oxygen

mobility in the oxide lattice, while the oxygen mobility is

dependent on the composition as well as the lattice structure

of the material. As shown in Fig. 2, the fresh sample displayed

a big oxygen desorption peak starting from around 750 �C,suggesting the oxide ion in LSCF lattice had good mobility at

elevated temperature. The oxygen desorption peak of the

sample fired at 800 �C did not change obviously as compared

to the fresh sample, implying the phase reaction between

LSCF and YSZ was not significant at 800 �C. The intensity of

oxygen desorption peak reducedwhen the samplewas fired at

850 �C, implying the phase reaction started to occur. With the

further increase of firing temperature to 900 �C or higher, the

oxygen desorption peak became very weak or even totally

disappeared, it suggests serious phase reaction between LSCF

and YSZ was likely happened and consequently greatly

inhibited the oxygen transportation. Above conclusion is in

well agreement with that from XRD.

Fig. 1 e XRD patterns of the mixture powders (LSCF and

YSZ) after firing at 1100 �C (a), 1000 �C (b), 900 �C (c), 850 �C(d), 800 �C (e) and at room temperature (f) for 5h.

In connection with the results as shown in Figs. 1 and 2,

a firing temperature of less than 850 �C is preferred in order to

get an interlayer-free LSCF electrode on YSZ electrolyte.

Similar conclusion was also drawn for the other cobaltite-

based electrodes under test. Unfortunately, it was difficult to

fabricate the electrode onto electrolyte surface at such low

temperature (<850 �C) by applying conventional coarse

cathode materials.

We first examined the promotion effect of Co3O4 for the

sintering of LSCF electrode in real fuel cells. A coarse LSCF

powder mixed with Co3O4 was directly deposited onto thin-

film YSZ electrolyte surface by spray deposition and then

fired at 800 �C for 5 h, for comparison, LSCF without Co3O4

sintering aidwas also fabricated onto YSZ electrolyte and fired

at 800 or 1000 �C for 5 h. Fig. 3 presents the typical SEM images

from cross-sectional view of the cells with the LSCF electrode

fired at 800 �C (a) and 1000 �C (c), and also the cells with

LSCF þ Co3O4 electrode sintered at 800 �C (b). All three elec-

trodes showed high porosity, which is important for free gas

diffusion within the electrode layer, thus minimizing

concentration polarization resistance. According to the SEM

images, the electrode adhesion to the electrolyte was sound

for all three electrodes too. However, the Co3O4-free LSCF

electrode showed relatively poor sintering after the firing at

800 �C, indicated by smaller particle size than the other elec-

trodes and also irregular grain shape. The grain shape of the

other two electrodes is more spheric. Anyway, there seems no

very significant difference in electrodemorphology among the

three electrodes. However, the Co3O4-free LSCF electrode fired

at 800 �C showed poor mechanical strength, which was loose

and easily detached from the electrolyte.

To interpret the improved mechanical strength of LSCF

electrode by adopting Co3O4 sintering aid, both LSCF and

LSCF þ Co3O4 electrode materials were fabricated into pellets

and sintered at 1000 �C for 5 h with the typical SEM photos

shown in Fig. 4. The grains in the LSCF pellet with Co3O4 sin-

tering aid showed obvious fusion to form well connected

network, while the grains in the pure phase LSCF were still

Fig. 5 e Typical electrochemical impedance spectroscopy of

the various symmetric cells tested in air with LSCF cathode

fired at 800 �C and 1000 �C and LSCF D CO3O4 fired at

800 �C.Fig. 3 e SEM photos from cross-sectional a view of the cells

with the LSCF electrode sintered at 800 �C (a), LSCFD Co3O4

electrode sintered at 800 �C (b) and LSCF electrode sintered

at 1000 �C (c).

Table 1 e Electrode polarization resistances and ohmicresistances of the various symmetric cells with differentelectrodes.a

Polarization resistance/Ohmicresistance (U cm2)

750(�C)

700(�C)

650(�C)

LSCF-800 0.029/1.552 0.129/2.418 0.389/4.204

LSCF-1000 33.012/1.594 97.619/2.563 286.999/4.548

LSCF þ Co3O4 � 800 0.015/1.511 0.071/2.356 0.324/4.099

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 1 1 9 4 6e1 1 9 5 4 11949

well separated. It indicates that Co3O4 promoted the connec-

tion of particles, which is important to reduce contact resis-

tance and increasemechanical strength. The poormechanical

strength implies the connection between particles in the

Co3O4-free LSCF electrode after the sintering at 800 �C was

likely still weak, while it was much improved by adopting

Co3O4 sintering additive in the electrode.

To prove the beneficial effect of Co3O4 sintering aid in

improving electrode performance, we first measured EIS of

the various LSCF electrodes based on the symmetric cell

configuration. Fig. 5 shows the typical EIS at 700 �C tested in

air. Although the pure LSCF electrode fired at 1000 �C showed

good particle connection in the electrode and well adhesion of

the electrode to the electrolyte layer as demonstrated in Fig. 3,

large electrode polarization resistance and ohmic resistance

of 97.619 and 2.563 U cm2 respectively were obtained. The cell

ohmic resistance was contributed from the electrolyte, the

electrode, possible interfacial layer between the electrolyte

and the electrode, the contact resistance between electrode

and electrolyte and between electrode particles as well as the

connection leads, while the electrode polarization resistance

Fig. 4 e SEM photos of LSCFD Co3O4 (a) and LSCF (b) pellets

sintered at 1000 �C.

was contributed from charge transfer polarization resistance

at both the electrodeeair interface and electrodeeelectrolyte

interface as well as the surface diffusion polarization resis-

tance. In connection with XRD and O2-TPD results, the

formation of insulating phases of La2Zr2O7 and SrZrO3 likely

introduced large resistance for oxygen-ion transfer across the

electrodeeelectrolyte interface and thus led to the large

electrode polarization resistance. The cell with LSCF electrode

fired at 800 �C showed electrode polarization resistance and

ohmic resistance of 0.129 and 2.418 U cm2 respectively at

700 �C. The electrode polarization resistance was much

BSCF-800 0.034/1.559 0.123/2.450 0.426/4.192

BSCF-1000 6.105/1.626 16.687/2.448 47.902/3.986

BSCF þ Co3O4 � 800 0.007/1.515 0.066/2.383 0.298/4.069

SSC-800 0.043/1.581 0.178/2.411 0.609/3.883

SSC-1000 36.291/1.590 110.692/2.504 365.149/4.343

SSC þ Co3O4 � 800 0.020/1.534 0.172 /2.306 0.591/3.837

SScC-800 0.011/1.553 0.073/2.466 0.314/4.206

SScC-1000 2.014/1.599 5.159/2.487 14.428/4.296

SScC þ Co3O4 � 800 0.006/1.448 0.042/2.284 0.218/3.892

PBC-800 0.142/1.587 0.400/2.534 1.230/4.331

PBC-1000 108.462 /1.969 371.784/3.732 1236.660/6.392

PBC þ Co3O4�800 0.081/1.508 0.228/2.340 0.642/4.081

a Electrode-number means sintering temperature of the electrode.

Table 2 e Peak power densities and shorted currentdensities of the various cells with cobaltite cathodes firedat different temperatures.a

PPD (mW cm�2)/SCD (mA cm�2)

750 (�C) 700 (�C) 650 (�C) 600 (�C)

LSCF-800 881/3705 647/2992 440/2059 262/1218

LSCF-1000 110/423 54/225 16/62 8/34

LSCF þ Co3O4 � 800 1176/4590 876/3820 571/2790 336/1725

BSCF-800 962/4270 641/3100 363/1920 192/1013

BSCF-1000 224/775 116/410 61/212 31/112

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reduced as compared to that of LSCF electrode sintered at

1000 �C, while the ohmic resistance reduced just slightly. The

sharp decrease in electrode polarization resistance was due to

the avoidance of interfacial phase reaction between LSCF and

YSZ layers. As to the cell with Co3O4 sintering aid in the

electrodes, the electrode polarization resistance and ohmic

resistance were 0.071 and 2.356 U cm2 at 700 �C respectively.

As compared to the Co3O4-free LSCF electrode fired at the

same 800 �C, the polarization resistance of the electrode with

Co3O4 sintering aid was further reduced to only 55% of the

former, while their cell ohmic resistance was almost the

Fig. 6 e Typical IeV and IeP curves of the cells with pure

LSCF cathode sintered at 800 �C (a), the LSCF cathode with

Co3O4 sintering aid fired at 800 �C (b), and pure LSCF

cathode sintered at 1000 �C (c).

BSCF þ Co3O4 � 800 1227/4450 832/3530 650/2990 442/2095

SSC-800 751/3446 512/2325 312/1372 174/752

SSC-1000 75/259 44/154 27/87 18/57

SSC þ Co3O4 � 800 1043/4010 806/3291 465/2085 223/1042

SScC-800 749/2830 430/1750 245/1020 137/570

SScC-1000 725/2965 427/1995 249/1125 131/650

SScC þ Co3O4 � 800 1622/5410 1027/4600 561/3035 284/1575

PBC-800 834/3238 560/2342 284/1476 163/928

PBC-1000 53/188 30/115 18/69 10/40

PBC þ Co3O4 � 800 970/3768 663/2950 420/1986 222/1192

a Cathode-number means firing temperature of the cathode.

Fig. 7 e Time dependence of ASRs of LSCF D Co3O4 (a) and

LSCF (b) electrodes fired at 800 �C.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 1 1 9 4 6e1 1 9 5 4 11951

same. This is in well accordance with the sound adhesion of

both electrodes to the YSZ electrolyte (thus similar ohmic

resistance) while much improved particles connection in the

electrode by introducing the Co3O4 sintering aid (thus further

decreased polarization resistance).

The effect of Co3O4 sintering aid on the performance of

other electrodes was also investigated by EIS. Table 1 shows

the ohmic resistances and electrode polarization resistances

of the various symmetric cells with different electrodes

prepared with or without the Co3O4 sintering aid at selected

temperatures. Much lower electrode polarization resistances

were observed for the cells with the electrodes containing

Co3O4 sintering aid. The lowest ohmic resistance was also

observed for the cell with Co3O4 sintering aid in the electrodes.

It implies that Co3O4 is a general purpose sintering aid, which

is suitable for a wide range of electrode materials.

Fig. 8 e Electrical conductivities of the selected coba

The performance of the various electrodes with and

without Co3O4 sintering aid was further investigated in single

cells. Fig. 6 shows the typical IeV and IeP curves of the cells

with pure LSCF cathodes (sintered at 800 or 1000 �C), and the

LSCF cathode with Co3O4 sintering aid fired at 800 �C, by

applying 3% humidified H2 at a flow of 80 ml min�1 [STP] as

anode fuel gas and ambient air as cathode atmosphere during

the measurement. For all cells, no concentration polarization

was appeared, in good agreement with the high porosity as

observed by SEM. For the cell with pure LSCF cathode fired at

800 �C, peak power densities (PPDs) of 881, 647, 440 and

262 mW cm�2 were achieved respectively at operating

temperatures of 750, 700, 650 and 600 �C. The performance is

very low for the cell with LSCF electrode fired at 1000 �C with

PPDs of only 110, 54, 16 and 6 mW cm�2 respectively at 750,

700, 650 and 600 �C, clearly due to the serious interfacial

ltite oxides and cobaltite D Co3O4 composites.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 1 1 9 4 6e1 1 9 5 411952

reaction between LSCF and YSZ during the cell fabrication. As

expected, much higher cell performance was observed for the

cell with LSCF þ Co3O4 cathode fired at 800 �C, which reached

PPDs of 1176, 876, 571 and 336 mW cm�2 respectively at 750,

700, 650 and 600 �C. The electrochemical performance of the

cell with LSCF þ Co3O4 cathode fired at 800 �C in this work are

comparable to those of cells with LSCF cathode prepared by

advanced synthetic techniques [15] and BSCF þ SDC cathode

prepared by adopting a buffer layer at corresponding

temperatures with YSZ electrolyte [17].

Table 2 lists the detail PPDs and shorted current densities

(SCDs) of the various cells with cobaltite cathodes fired at 800

Fig. 9 e EIS of the various electrodes at 650 �C with and withou

Arrhenius plots of electrodes at different test temperatures. The

while the electrodes with Co3O4 were sintered at 850 �C.

or 1000 �C without the Co3O4 sintering aid, and with

cobaltite þ Co3O4 cathodes fired at 800 �C. All fuel cells were

fabricated from the same batch to ensure the best reliability.

By applying Co3O4 sintering aid in the cathodes, all fuel cells

showed promising PPDs higher than 970 mW cm�2 at 750 �Cwith the maximum PPD obtained by applying SScC cathode,

while they are much lower for the cells with cobaltite cath-

odes free from sintering aid, in particular for the cells with

cobaltite cathodes fired at 1000 �C.Operational stability is an important issue for fuel cells.

Fig. 7 shows the time dependence of ASRs of LSCF and

LSCF þ Co3O4 electrodes fired at 800 �C, measured by

t the Co3O4 sintering aid on SDC electrolyte. Insets are the

electrodes without Co3O4 sintering aid were fired at 1000 �C

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 7 ( 2 0 1 2 ) 1 1 9 4 6e1 1 9 5 4 11953

symmetric cells, and the EIS at selected times are also pre-

sented as inserts. For the LSCF electrode fired at 800 �C, anobvious increase in ASR with operating time was demon-

strated, while the LSCFþ Co3O4 electrode showedmuch better

stability during the investigated period of more than 200 h. It

can be explained by the fact that theparticles in LSCF electrode

fired at 800 �C is loosely contacted, with the time on operation,

the electrode became slowly powderized and resulted in the

worsened connection between electrode particles and contact

between the electrode and the electrolyte layer.

As an electrode of SOFCs for practical application, suffi-

ciently high electrical conductivity is also an important

requirement. Since Co3O4 is a semiconductorwithmuch lower

electronic conductivity than the cobaltite perovskite oxides,

the introduction of Co3O4 into cobaltites may decrease the

electronic conductivity of the later. Very interestingly,wehave

demonstrated previously that the formation of composite

with Co3O4 significantly increased the electrical conductivity

of BSCF at elevated temperature instead [32]. To determine the

effect of Co3O4 on electrical conductivity of the various

cobaltites, the electrodeswith andwithout the Co3O4 sintering

aid were fabricated into dense pellets and their conductivity

was measured by 4-probe DC conductivity. As shown in Fig. 8,

the electrical conductivity of the cobaltite oxides with Co3O4

sintering aid is comparable to the single phase oxides, as

a whole. It implies that Co3O4 did not cause significant effect

on the electrical conductivity of the oxides, which may due to

the low Co3O4 content in the composites. Similar to BSCF, an

increased electrical conductivity was observed for SScC by

applying Co3O4 sintering aid at intermediate temperature. It

suggests the Co3O4 had certain type of interaction with the

parent oxide, which not only promoted the sintering of the

parent oxide but also enhanced the electron charge transfer.

It is well known that Co3O4 or its hybrid is a good catalyst

for oxygen reduction reaction [33e35]. Zhang et al. considered

that the electrochemical performance enhancement of the

SSC cathode after adding 30wt.% Co3O4 was due to the

improved oxygen surface exchange coefficient of the

SSC þ Co3O4 composites. To investigate the effect of Co3O4 on

the intrinsic activity of the various electrodes for oxygen

reduction reaction, symmetric cells were prepared with

Sm0.2Ce0.8O1.9 (SDC) electrolyte since negligible interfacial

reaction between cobaltite and SDCwas happened at elevated

temperature. Shown in Fig. 9 are EIS of the various electrodes

with and without the Co3O4 sintering aid. To minimize the

morphologic effect on electrode performance, the electrodes

without Co3O4 sintering aid were fired at 1000 �C while the

electrodes with Co3O4 were sintered at 850 �C to ensure the

similar electrode morphology between the two electrodes. For

all electrodes, the corresponding electrodes with and without

the Co3O4 sintering aid had comparable ASRs at correspond-

ing temperatures, demonstrating that the introduction of

Co3O4 did not cause a significant effect on the intrinsic elec-

trochemical activity of the electrode for the oxygen reduction

reaction. Interestingly, the cobaltite þ Co3O4 composite elec-

trode showed even higher electrochemical activity for oxygen

reduction than pure electrode, this is due to a synergistic

effect occurred between cobalt-based perovskite oxides and

Co3O4 and an improved oxygen surface exchange coefficient

of composites [32,35].

4. Conclusions

In summary, we have reported here that, by simply applying

Co3O4 as a sintering aid, cobaltite electrodes can be directly

fabricated onto stabilized zirconia electrolyte surface at 800 �Cwith well connected electrode particles, firm adhesion and

free from interfacial reaction between the electrode and the

electrolyte layers. Both ohmic and polarization resistances of

as-fabricated symmetric cells with YSZ electrolyte weremuch

lower than that without adding Co3O4 in cobaltite electrodes.

We also found that the as-fabricated fuel cells show much

improved peak power density as compared to similar fuel

cells with cathodes fabricated by a conventional way. For

example, a peak power density of 1176 mW cm�2 at 750 �Cwas achieved when La0.6Sr0.4Co0.2Fe0.8O3�d þ Co3O4 was

selected as a representative cobaltite cathode. It thus intro-

duces a new way for the development of high performance

electrodes for SOFCs.

Acknowledgments

This work was supported by the National Science Foundation

for Distinguished Young Scholars of China under contract No.

51025209.

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