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Characterization and evaluation of La0.8 Sr0.2Co0.8

Ni0.2O3-d prepared by a polymer-assisted combustionsynthesis as a cathode material for intermediatetemperature solid oxide fuel cells

Jing Chena, Fengli Lianga, Lina Liua, San Ping Jiangb,*, Li Jiana,**aSchool of Materials Science and Engineering, State Key Laboratory of Material Processing and Die & Mould Technology,

Huazhong University of Science and Technology, Wuhan, Hubei 430074, PR ChinabSchool of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore 639798

a r t i c l e i n f o

Article history:

Received 6 January 2009

Received in revised form

18 March 2009

Accepted 29 May 2009

Available online 24 June 2009

Keywords:

La0.8Sr0.2Co0.8Ni0.2O3-d

LSCN

Polymer-assisted combustion

synthesis

Nano-structured

Cathode

SOFC

* Corresponding author.** Corresponding author. Tel.: þ86 027 875578

E-mail addresses: mspjiang@ntu.edu.sg (0360-3199/$ – see front matter ª 2009 Interndoi:10.1016/j.ijhydene.2009.05.124

a b s t r a c t

A modified polymer-assisted combustion synthesis method is developed for preparation of

La0.8Sr0.2Co0.8Ni0.2O3-d (LSCN) nano-sized cathode particles by using organic additives

(glucose and acrylamide) and metal nitrates. The effect of the organic additives, pH value of

starting solution and calcination temperature on the formation of the LSCN perovskite

phase and microstructure of the powders is investigated. Chemical compatibility between

the LSCN and Y2O3 stabilized ZrO2 (YSZ) and Gd2O3 doped CeO2 (GDC) is evaluated and

electrochemical activity of LSCN cathode is evaluated. The prepared LSCN is chemically

compatible with the YSZ only at temperatures below 850 �C. The electrode area specific

resistance (ASR) is 0.30 and 0.10 U cm2 at 700 and 750 �C, respectively. These results suggest

that such prepared LSCN is a promising alternative cathode material for intermediate

temperature SOFCs.

ª 2009 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights

reserved.

1. Introduction reduced operating temperature are the increase in electrolyte

Solid oxide fuel cells (SOFCs) are considered as one of the most

promising energy conversion devices that exhibit advantages

of high efficiency, fuel flexibility and low environmental

pollution. Recently, significant progress has been achieved in

reducing the operation temperature of SOFCs from traditional

1000 �C to intermediate temperature range between 600 and

800 �C [1,2]. However, several major issues associated with the

49.S.P. Jiang), plumarrow@12ational Association for H

and electrode resistivities and the polarization losses of elec-

trode reactions, particularly the oxygen reduction reaction in

the cathode. In order to compensate for the ohmic losses at

lower temperatures, electrolytes with higher ionic conduc-

tivities and thin film electrolyte/electrode assemblies have

been developed [3,4]; and alternative cathode materials with

a high mixed ionic-electronic conductivity (MIEC) have been

employed. Compared to traditional La1�xSrxMnO3 (LSM)

6.com (L. Jian).ydrogen Energy. 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 e n e r g y 3 4 ( 2 0 0 9 ) 6 8 4 5 – 6 8 5 16846

perovskite cathode material, the MIECs can effectively

decrease the cathode polarization at reduced temperatures by

extending the reactive zones from the electrode–electrolyte

interface region to the whole body of the electrode [5,6].

Cathode material performance is very dependent on temper-

ature, grain size, microstructure and the formation or depo-

sition process [7], and the microstructure which is closely

related to the morphological characteristics of the starting

powder materials and the firing temperatures to fix the elec-

trode material on the electrolyte. The morphology of the

powder is affected by the synthesis techniques.

Perovskite cathode materials were prepared previously by

numerous methods, including EDTA-citric complex method

[8], sol–gel method [9], glycine–nitrate method [10,11], Pechini

method [12,13] and freeze–drying method [14]. In the present

study, a modified polymer-assisted combustion method is

introduced for the preparation of La0.8Sr0.2Co0.8Ni0.2O3-

d (LSCN). The thermal decomposition behavior of the gelled

precursor, the phase formation of the oxide and the

morphology of the powder were examined. The chemical

compatibility and electrocatalytic activity for the oxygen

reduction reaction were evaluated.

2. Experimental

2.1. Materials synthesis

The LSCN perovskite oxide powder was synthesized using

a modified polymer-assisted combustion synthesis method

with glucose and acrylamide as the fuel and dispersing

agent, respectively. In this method, stoichiometric amounts

of La(NO3)3$6H2O, Sr(NO3)2, Co(NO3)2$6H2O, Ni(NO3)2$6H2O,

C6H12O6$H2O and acrylamide (99.9%, Sinopharm Chemical

Reagent Co. Ltd) in 2:3 molar ratio were first dissolved in

distilled water under magnetic agitation. The water content in

the nitrates was confirmed by thermogravimetric analysis

(TG/DTA, PerkinElmer Instruments Co. Ltd.) at a heating rate

of 5 �C/min in flowing air. For most of the experiments (unless

otherwise stated), 5:1 molar ratio of the organic additives

(glucose and acrylamide) to the metal nitrates was used. In

order to study the effect of the organic additives on the phase

formationof the LSCN,themolar ratio wasvariedfrom2:1 to7:1.

Ammonia solution (25 wt%) was added into the solution drop-

wise under stirring. The pH value of the solution was controlled

withinthe range of 6 to10. Suchprepared solution was heatedto

180 �C in an oven for 10 h, forming a viscous gel and then

changing quickly to a porous and black-colored xerogel. Finally,

the xerogel was calcined at various temperatures between

450 �C and 800 �C for 2 h in air to form the oxide powders.

2.2. Materials characterization

The behavior of the xerogel during temperature increase was

analyzed by thermogravimetric analysis and differential

thermal analysis (TG/DTA, PerkinElmer Instruments Co. Ltd.).

The formed phase in calcined powders was identified by X-ray

diffraction (XRD) using a Phillips X’Pert Pro diffractometer

with Cu Ka radiation. The diffraction patterns were registered

over a 2q range between 20 and 80� and the lattice parameters

were calculated using the Jade-5 software (Material Data, Inc.).

The specific surface area of the powder calcined at 800 �C was

determined as 16.12 m2 g�1 by the BET method (Micromeritics

Instrument Co. Ltd.). The chemical compatibility between the

prepared LSCN powder and Y2O3 stabilized ZrO2 (TZ8Y, Tosoh,

Japan) and in-house Ce0.8Gd0.2O1.9 (GDC) electrolytes was

studied by firing LSCN/YSZ and LSCN/GDC mixed powders at

1:1 weight ratio at temperatures 800, 850, 900, 1000 and 1100 �C

in air for 10 h, followed by XRD phase analysis. A Sirion 200

scanning electron microscope (SEM) and a Tecnai G2 20

transmission electron microscopy (TEM) were employed to

examine the morphology of the LSCN powders.

Using a specimen sintered at 1450 �C for 2 h with a dimen-

sion of 5� 5� 20 mm, the coefficient of thermal expansion

(CTE) of the prepared LSCN was measured in air in the

temperature range of 30–1000 �C by a DIL402C thermal

mechanical analyzer (NETZSCH Ltd.); and the electrical

conductivity of the prepared LSCN was measured by a DC

four-point method at temperatures ranging from 50 �C to

1000 �C in flowing air.

2.3. Electrocatalytic activity evaluation

Electrolyte substrates were prepared by die pressing 8% mol

Y2O3–ZrO2 powder (YSZ, Tosoh, Japan), followed by sintering

at 1500 �C for 4hrs in air. The substrate disks were 21 mm in

diameter and 1.2 mm in thickness. For preparing LSCN elec-

trodes onto YSZ electrolyte disks with a thin GDC buffer layer

of approximately 8 mm in between, the GDC buffer layer was

firstly applied to the YSZ electrolyte disk by paste screen

printing, followed by sintering at 1250 �C for 2 h in air. The

LSCN paste was then screen printed on the buffer layer and

sintered at 1000 �C for 2 h in air to form the cathode with

a thickness of 8 to 10 mm and an active area of 0.5 cm2. 5 wt%

cellulose binder was used for preparing the pastes. For elec-

trochemical impedance measurement of the cathode, Pt paste

was painted on top of the cathode and was fired at 850 �C for

2 h as the current collector and on the opposite side of the

electrolyte disk as the counter and reference electrodes. The

counter electrode was positioned symmetrically opposite to

the working electrode and the reference electrode was a ring

at the edge of the electrolyte substrate. Electrochemical

impedance spectra of the above prepared cells were obtained

in a frequency range of 0.1 Hz to 100 kHz with signal ampli-

tude of 10 mV at temperatures between 600 and 750 �C using

an impedance/gain phase analyzer (Solartron 1260) and an

electrochemical interface analyzer (Solartron 1287) at open

circuit. The electrode interface (polarization) resistance (RE)

was derived from the difference between the low- and high-

frequency intercepts at the real impedance axis. The cross-

sectioned morphology of the specimen assembly was

observed by the SEM mentioned above.

3. Results and discussion

3.1. Thermal analysis of precursor powders

Fig. 1 is the TG and DTA results of the xerogel, represented by

the LSCN with 5:1 molar ratio of the organic additives to

Fig. 1 – TG-DTA curves of LSCN xerogel after the heat

treatment at 180 8C. Fig. 3 – SEM micrographs of LSCN powder sintered at 800 8C

for 2hrs.

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 e n e r g y 3 4 ( 2 0 0 9 ) 6 8 4 5 – 6 8 5 1 6847

nitrates. With temperature increase, the weight of the xerogel

sample decreases as expected. Three weight loss steps are

observed in the TG curve between 30–224, 224–396, and 396–

424 �C, corresponding the exothermic peaks at around 311 �C

and near 414 �C in the DTA curve. They are caused by residual

water removal, decomposition and oxidation of organic

additives [9,15] and burning out of nitrate radicals, respec-

tively. With temperature increase above 424 �C, the weight

loss is not significant, and formation of the LSCN perovskite

phase is initialized.

3.2. Phase identification and morphological examination

Fig. 2 shows the XRD patterns of the LSCN xerogels calcined at

various temperatures above 450 �C in air for 2 h. The rudiment

of perovskite phase starts to appear at 500 �C with poor crys-

tallinity indicated by the tiny peaks. Diffraction peaks of the

Fig. 2 – XRD patterns of LSCN powders calcined at different

temperatures for 2hrs in air.

calcined xerogel become noticeable when the calcination

temperature was increased to 600 �C; a well-crystallized LSCN

with a rhombohedral perovskite structure is presented. The

diffraction peaks turn out to be sharper with further

increasing the calcination temperature and the corresponding

grain size is increased in the range between 20 to 30 nm esti-

mated by the Scherrer equation incorporated in the Jade-5. For

all solution formulas, single-phased LSCN were synthesized

by the modified combustion method at calcination tempera-

ture as low as 600 �C. Fig. 3 is the morphology of the LSCN

powder. The particles exhibit a narrow size distribution in the

range of 20–50 nm with slight agglomeration. The morphology

of the LSCN powders is similar to that of La0.6Sr0.4CoO3-x,

La0.6Sr0.4Fe0.8Co0.2O3-x and LSCN powders synthesized by the

glycine–nitrate process [16,17] and the Pechini method [12].

Shown in Fig. 4 is the XRD patterns of the LSCN powders

prepared with different pH values and calcined at 800 �C for

Fig. 4 – The XRD patterns of the LSCN powders prepared

with different pH values and calcined at 800 8C for 2 h in

air.

Fig. 5 – TEM micrographs of LSCN powders synthesized with various pH values starting solutions: (a) pH [ 6; (b) pH [ 8; (c)

pH [ 10. The powders were calcined at 800 8C for 2hrs in air.

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 e n e r g y 3 4 ( 2 0 0 9 ) 6 8 4 5 – 6 8 5 16848

2 h in air. All the samples display identical diffraction

patterns, indicating the formation of a rhombohedral perov-

skite phase and the same crystalline size of w30 nm according

to the Scherrer formula. Therefore, it can be concluded that

variation of pH value between 6 and 10 has no effect on LSCN

phase formation and particle size of the powders, even though

the viscosity of the prepared solution was changed. Fig. 5

shows the TEM micrographs of LSCN powders calcined at

800 �C for 2 h in air with various solution pH values. The small

particles are agglomerated, and the average particle size is in

the neighborhood of 30 nm.

3.3. Effect of the ratio of organic additive to metal nitrate

Another important parameter affecting LSCN phase forma-

tion is the molar ratio of the organic additives to the metal

nitrates. Fig. 6 shows the XRD patterns of the LSCN powders

synthesized with variation of the ratio. It is noticed no

impurity phase was formed with the 5:1 molar ratio; and in all

other cases, an impurity phase La4Ni3O10 was appeared. The

Fig. 6 – XRD patterns of LSCN powders synthesized with

various molar ratios of the organic additives to the metal

nitrates. The powders were calcined at 800 8C for 2 h in air.

diffraction peaks from the impurity phase near 25� are turned

out to be more obvious at the lower ratios owing to the lack of

blocking effect of organic additives on suppressing impurity

formation [18]; however, excessive content of the organic

additives is also prone to the formation of the impurity

possibly due to excess of fuel and the high combustion

temperature [19,20]. Fig. 7 shows the TEM micrograph of the

LSCN powders prepared at 2:1 ratio for comparison with that

shown in Fig. 5c, the particle size of the LSCN with 2:1 ratio is

significantly larger than that with 5:1 ratio. The organic

additives of glucose and acrylamide act not only as the fuel,

but also as the chelate agent. Glucose may degrade in alkaline

solution at about 150 �C into lactic acid, formate, glycolic acid,

and acetate [21], forming complexes with metal cations. On

the other hand, acrylamide itself is an effective complex

Fig. 7 – TEM micrograph of LSCN powders synthesized with

2:1 molar ratio of the organic additives to the metal nitrates

and a pH of 10. The powders were calcined at 800 8C for

2hrs in air.

Fig. 9 – SEM micrographs of fractured cross-section of LSCN

electrode.

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 e n e r g y 3 4 ( 2 0 0 9 ) 6 8 4 5 – 6 8 5 1 6849

forming agent with metal cations due to the amine group [20].

A proper ratio of the organic additives to nitrates is beneficial

for the metal ions to be completely engaged into the

complexes, leading to simultaneous complete combustion

and phase formation. Less than that, metallic cations cannot

be fully complexed, resulting in incomplete combustion and

phase separation. The ratio of 5:1 seems to be in the vicinity of

the critical value.

3.4. Chemical and thermal compatibility

The chemical compatibility of the LSCN with the YSZ and GDC

electrolyte was investigated by firing the mixed powders at

800, 850, 900, 1000, 1100 �C for 10 h. Fig. 8 shows the XRD

patterns of the sintered LSCN/YSZ and LSCN/GDC mixture.

The results show that the LSCN reacts readily with the YSZ to

form La2Zr2O7 and SrZrO3 phases at temperatures above

800 �C; however, the GDC is chemically inert to the LSCN at all

sintering temperatures up to 1100 �C. Thus, in order to use the

LSCN cathode with the YSZ electrolyte, a GDC interlayer is

conventionally required to prevent the formation of resistive

La2Zr2O7 and SrZrO3 phases during cell preparation.

Fig. 8 – XRD patterns of (a) LSCN/YSZ and (b) LSCN/GDC

mixtures sintered at 800, 850, 900, 1000, 1100 8C in air for

10 h.

The CTE of the prepared LSCN is 16.67� 10�6 K�1 and quite

comparable to those reported in references [12] (16.5� 10�6 K�1)

and [14] (15.6� 10�6 K�1). For the most commonly used YSZ

electrolyte, its CTE is in the range of 10.3–10.5� 10�6 K�1 in the

temperature range from 50 to 1000 �C in air atmosphere; and for

the GDC electrolyte, its CTE is between 11.8� 10�6 K�1 and

13.4� 10�6 K�1 depending on the content of doping [21,22].

Therefore, the LSCN is more thermally compatible with the

GDC electrolyte than the YSZ.

3.5. Electrical conductivity and electrocatalytic activity

The total electrical conductivity of the prepared LSCN

measured by the DC four-point method is 1243 S cm�1 at

800 �C and 2379 S cm�1 at room temperature, which is close to

the values reported in reference [11] and exhibits the charac-

teristic of temperature dependence of metallic materials. This

total electrical conductivity includes the electronic and ionic

contribution; however, the ionic contribution is almost

negligible compared to the electronic contribution as the

difference between them is several orders of magnitude. Such

a high electrical conductivity can meet the requirement of

a SOFC cathode.

Fig. 9 is the SEM micrograph of the cross-sectioned spec-

imen assembly for electrochemical impedance measurement,

showing a layered structure of Pt current collector, cathode,

the GDC buffer and the YSZ electrolyte substrate and

adequate adherence between them. With this half cell, elec-

trochemical impedance spectra of the LSCN electrode at

temperatures between 600 and 750 �C in air were obtained, as

shown in Fig. 10. The O2 reduction reaction on the LSCN

electrode is characterized by a depressed impedance arc over

the temperature range studied and there was no clear sepa-

ration between low- and high- frequency arcs. The cathode

polarization resistance (RE) at open circuit condition for the O2

reduction reaction in air, obtained from the distance between

the high- and low-frequency intercepts of the arc on the real

axis, is 2.69, 0.65, 0.30, and 0.10 U cm2 at 600, 650, 700, and

750 �C, respectively. The corresponding activation energy is

Fig. 10 – Impedance spectra of LSCN electrode measured at

different temperatures in air.

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 e n e r g y 3 4 ( 2 0 0 9 ) 6 8 4 5 – 6 8 5 16850

1.64� 0.03 eV. These results are in the vicinity of that reported

with La0.8Sr0.2Co0.98Ni0.02O3-d [10], (La0.8Sr0.2)0.99Co0.8Ni0.2O3-

d [11] and LSCF [22,23] cathodes, which suggests that such

prepared LSCN with high electrocatalytic activity and sub-

micron sized microstructure be a promising alternative

cathode material for intermediate temperature SOFCs.

4. Conclusions

Based on the present study, the following conclusions can be

made:

1) The modified polymer-assisted combustion process is

suitable for the synthesis of the LSCN nano-sized powders.

The rhombohedral LSCN perovskite structure forms at

calcination temperature above 600 �C.

2) The pH value of the starting solution between 6 and 10 has

no significant effect on the formation and morphology of

LSCN phase; however, the ratio of the organic additives

(glucose and acrylamide) to the metal nitrates affects the

formation of pure LSCN phase. 5:1 ratio seems to be an

appropriate choice for forming single LSCN phase without

impurity of La4Ni3O10.

3) The prepared LSCN is chemically compatible with the GDC

and YSZ electrolytes at temperatures below 1100 �C and

850 �C, respectively, and thermally matched to the GDC

better than the YSZ.

4) Such prepared LSCN with polarization resistances of 0.30

and 0.10 U cm2 at 700 and 750 �C, respectively, demon-

strates a comparable polarization resistance to the state of

the art cathode LSCF, suggesting the potential of being

a promising alternate cathode material for intermediate

temperature SOFCs.

Acknowledgements

This research was financially supported by the NSFC project

50571038 and the national ‘‘863’’ project 2006AA05Z148. SEM

and XRD characterizations were assisted by the Analytical and

Testing Center of Huazhong University of Science and

Technology.

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