- Email: [email protected]
Preparation and characterization of Nd2−xSrxCoO4+δ cathodes for intermediate-temperature solid oxide fuel cell
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 5 ( 2 0 1 0 ) 5 5 9 4 e5 6 0 0
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Preparation and characterization of Nd2LxSrxCoO4Dd cathodes for intermediate-temperature solid oxide fuel cell Yue Cao, Haitao Gu, Han Chen, Yifeng Zheng, Ming Zhou, Lucun Guo* College of Materials Science and Engineering, Nanjing University of Technology, 5 Xinmofan Road, Nanjing 210009, People’s Republic of China
article info
abstract
Article history:
K2NiF4-type structural Nd2xSrxCoO4þd (x ¼ 0.8, 1.0, 1.2) was synthesized and evaluated as
Received 29 October 2009
cathodes for intermediate-temperature solid oxide fuel cell (IT-SOFC). The crystal struc-
Received in revised form
ture, thermal expansion, electrical conductivity and electrochemical properties were
6 March 2010
investigated by X-ray diffraction, dilatometry, DC four-probe method, AC impedance and
Accepted 11 March 2010
polarization techniques. It is found that the electrochemical properties were remarkably
Available online 18 April 2010
improved with the increasing of Sr in the experiment range. Nd0.8Sr1.2CoO4þd showed the highest electrical conductivity of 212 S cm1 at 800 C, the lowest polarization resis-
Keywords:
tance and cathodic overpotential, 0.40 Ucm2 at 700 C and 35.6 mV at a current density of
Solid oxide fuel cell
0.1 A cm2 at 700 C, respectively. The chemical compatibility experiment revealed that
Cathode
Nd0.8Sr1.2CoO4þd cathode was chemically stable with the SDC electrolyte. The thermal
Thermal expansion
expansion coefficient also increased with the Sr content.
Electrical conductivity
ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.
Electrochemical properties
1.
Introduction
Solid oxide fuel cell (SOFC) is a device that can convert chemical energy of fuels directly to electricity in a highly efficient and environmental way and draws a wide attention in recent years [1e4]. Nowadays, one of the most important research goals is to develop intermediate-temperature solid oxide fuel cell (IT-SOFC). IT-SOFC will solve various problems associated with the high operating temperature (900e1000 C), such as sealing and thermal degradation. Decreasing the operating temperature leads to improved long term stability of the system and the cost can be reduced by using less costly metal alloys as interconnects [5e7]. However, reducing the operating temperature results in a significant decreasing of the electrochemical activity of the cathode, thus a dramatically lowering of the output power density of the cells. For example, the conventional cathode
prepared with strontium doped lanthanum manganite (LSM) poorly performs at intermediate temperatures [8,9]. Therefore, the development of new cathode materials with high electrocatalytic activity for the oxygen reduction reaction is critical for IT-SOFC. Considerable investigations have been conducted on various perovskite-type oxides to improve the cathode performance at intermediate temperatures [10e14]. Among these oxides, the cobalt-based materials were received extensive attention because of their relatively high conductivity and excellent electrochemical properties. However, some cobaltites have low chemical stability and unacceptably high thermal expansion coefficients, causing the mismatch with other components, such as the electrolyte [15]. Recently, many investigations on mixed ioniceelectronic conductors (MIECs) with K2NiF4 structure were reported. These oxides are usually formulated as A2BO4, which can be regarded as a staking of ABO3 perovskite layers alternating
* Corresponding author. Tel.: þ86 25 83587261; fax: þ86 25 83306152. E-mail address: [email protected] (L. Guo). 0360-3199/$ e see front matter ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.03.046
5595
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 5 ( 2 0 1 0 ) 5 5 9 4 e5 6 0 0
14
a
K2NiF4 phase
Nd1.2Sr0.8CoO4+δ NdSrCoO4+δ Nd0.8Sr1.2CoO4+δ
12
Nd0.8Sr1.2CoO4+
Fitting results
-1
Scm K
Intensity (a.u.)
10
ln σT
NdSrCoO4+
8
6
4
Nd1.2Sr0.8CoO4+
2 20
30
40
50
60
70
0.8
80
1.0
1.2
1.4
1000/T K
2θ(°)
b
Intensity (a.u.)
Nd0.8Sr1.2CoO4+ -SDC
SDC
Nd0.8Sr1.2CoO4+
30
40
50
60
70
80
2θ(°)
Fig. 1 e XRD patterns of Nd2LxSrxCoO4Dd (x [ 0.8, 1.0, 1.2) calcined at 1200 C for 4 h in air (a) and Nd0.8Sr1.2CoO4DdSDC mixture calcined at 1100 C for 4 h in air (b).
1.4
6 -1 TEC ×10 K
dL/L 0 (%)
1.2
1.8
2.0
2.2
-1
Fig. 3 e Arrhenius plots of ln(sT ) vs. 1000/T for Nd2LxSrxCoO4Dd (x [ 0.8, 1.0, 1.2) in air.
Nd0.8Sr1.2CoO4+ SDC
20
1.6
with AO rock-salt layers along the c-direction [16]. These oxides have been demonstrated to be able to accommodate a significant oxygen nonstoichiometry. Previous studies have shown that A2BO4 oxides have better thermochemical stability and lower thermal expansion coefficient compared with perovskite ABO3 oxides [17,18]. It was also found that these compounds exhibit relatively high oxygen diffusion and surface exchange coefficients, which are two vital factors for cathode performance [19,20]. These favorable properties mentioned above make A2BO4 based oxide a promising candidate of cathode material for IT-SOFC. It was reported that the formation of K2NiF4-type structure of A2BO4 composition is partially related to the oxidation state of cations at B site [18]. James et al. [21] reported relatively high amount dopant of the Sr at A site apparently favors the formation of K2NiF4-type structure for Co at B site. They found that singlephase K2NiF4-type structure phases of Ln2xSrxCoO4þd were obtained over the composition range of 0.75 x 1.50 for Ln ¼ La and Nd. For system of Nd2xSrxCoO4þd, extensive studies have been performed on its catalytic and magnetic properties [22,23]. In the present study, Nd2xSrxCoO4þd (x ¼ 0.8, 1.0, 1.2) was investigated to evaluate their potential as cathode materials for IT-SOFC.
Nd1.2Sr0.8CoO4+
13.8
1.0
NdSrCoO4+
15.0
2.
Experimental
0.8
Nd0.8Sr1.2CoO4+
15.8
2.1.
Powder synthesis
0.6
Powder of Nd2xSrxCoO4þd (x ¼ 0.8, 1.0, 1.2) and Sm0.2Ce0.8O1.9 (SDC) was synthesized by solid state reaction method. Nd2O3
0.4 0.2
Table 1 e The electrical conductivity at 800 C and the activation energy in air.
0.0 0
100
200
300
400
Temperautre
500 o
600
700
800
C
Fig. 2 e Thermal expansion curves of Nd2LxSrxCoO4Dd (x [ 0.8, 1.0, 1.2) from 25 C to 800 C in air.
900
Composition Nd1.2Sr0.8CoO4þd NdSrCoO4þd Nd0.8Sr1.2CoO4þd
s800 (S cm1)
Ea (kJ mol1)
89 128 212
62 53 50
5596
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 5 ( 2 0 1 0 ) 5 5 9 4 e5 6 0 0
Nd2xSrxCoO4þd was calcined at 1200 C for 4 h in air, but Sm0.2Ce0.8O1.9 at 1200 C for 2 h in air.
1.5 o
1000 C o
1100 C o
1200 C Fitting results
-Z" (ohm)
1.0
2.2. 103Hz
104Hz 0.5
102Hz 10Hz 1Hz
0.0
-0.5 0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
Z' (ohm)
Fig. 4 e Impedance spectra of Nd0.8Sr1.2CoO4Dd cathodes sintered at 1000e1200 C for 2 h under open circuit potential at 700 C in air.
(99.9%), SrCO3 (99%), Co2O3 (99%), Sm2O3 (99.9%), CeO2 (99.95%) were used as starting materials. The raw materials with appropriate stoichiometric ratio were mixed for 8 h in distilled water with zirconia grinding media. The mixed powder
Sample preparation
The Nd2xSrxCoO4þd powder was pressed uniaxially at 50 MPa to form a bar and then sintered at 1300 C for 2 h. Obtained specimens with dimensions of 3 mm 5 mm 50 mm were used to measure the thermal expansion and electrical conductivity. Pressed pellets of SDC (18 mm in diameter, 1 mm in thickness) were sintered at 1550 C in air for 2 h. The Nd2xSrxCoO4þd powder was mixed with ethyl cellulose and terpinol, and the mixture was subsequently painted onto both side of the SDC pellets by screen painting, and then sintered at 1000e1200 C for 2 h. The Ag electrodes as the current collector and the reference electrode were painted onto the electrode and electrolyte surface, and then baked for 0.5 h at 600 C in air. Nd0.8Sr1.2CoO4þd powder and SDC powder were mixed at a weight ratio of 1:1 and then sintered at 1100 C for 4 h to check the chemical stability between them.
2.3.
Characterization
The crystal structure at room temperature was determined by XRD using the ARL X’TRA diffractometer with Cu Ka radiation
Fig. 5 e SEM images of Nd0.8Sr1.2CoO4Dd cathodes sintered at 1000 C (a); 1100 C (b); 1200 C (c) and the cross-section image of the test cell (d).
5597
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 5 ( 2 0 1 0 ) 5 5 9 4 e5 6 0 0
h ¼ E I Rohm
(1)
a
3.1.
Powder characterization
Fitting results
0.6
0.4
102Hz
103Hz
10Hz
0.2
1Hz
104Hz 0.0
-0.2 5.0
5.2
5.4
5.6
5.8
6.0
6.2
6.4
6.6
6.8
Z' (ohm)
b
0.6
Nd1.2Sr0.8CoO4+
o
750 C,air
NdSrCoO4+ Nd0.8Sr1.2CoO4+
0.4
-Z" (ohm)
Results and discussion
Nd1.2Sr0.8CoO4+ NdSrCoO4+ Nd0.8Sr1.2CoO4+
o
700 C,air
where E, I and Rohm indicate the applied cathode voltage, current and ohmic resistance measured by the impedance spectra, respectively.
3.
1.0
0.8
-Z" (ohm)
operating at 40 kV, 35 mA; l ¼ 0.15418 nm. The diffraction angle was 20 e80 with a step size of 0.02 . The thermal expansion of the specimens was measured in air from room temperature to 800 C, using a dilatometer (RPZ-01, Luoyang, China) with a heating rate of 5 C/min. The electrical conductivity was measured by the four-probe DC technique, using Ag paste as electrodes. AC impedance spectroscopy and polarization were performed using PARSTAT2273 electrochemical workstation. The impedance measurements were carried out in the frequency range of 0.1 MHze0.1 Hz at temperature between 500 C and 800 C. The applied voltages to the cathodes were changed between 0 and 1 V dc bias vs. the reference electrode at 5 mV/s. The overpotential (h) of the cathodes under cathodic polarization was estimated by the following equation:
Fitting results
103Hz
0.2
102Hz
10Hz 1Hz
104Hz 0.0
3.2.
Thermal expansion
Fig. 2 shows the thermal expansion curves of Nd2xSrxCoO4þd (x ¼ 0.8, 1.0, 1.2) in the temperature range 25e800 C. The average values of thermal expansion coefficients are listed in Fig. 2. The thermal expansion coefficients increased with increasing of the Sr content. This may be caused by the loss of lattice oxygen and the formation of oxygen vacancies, a process which is enhanced as the Sr content increased [20]. The thermal expansion compatibility with solid electrolyte could promote by reducing the thickness of the cathode film. It should be noted that the thermal expansion curves of the present study are not linear for the system, and that the deviation of the curves become more obvious with the increase of Sr content. However, the underlining mechanism for this unusual phenomenon is not clear so far.
3.3.
Electrical conductivity
The logarithm of electrical conductivity as a function of reciprocal temperature for Nd2xSrxCoO4þd (x ¼ 0.8, 1.0, 1.2) is
-0.2 3.6
3.8
4.0
4.2
4.4
4.6
Z' (ohm)
c
0.3
Nd1.2Sr0.8CoO4+
o
800 C,air
NdSrCoO4+ Nd0.8Sr1.2CoO4+
0.2
Fitting results -Z" (ohm)
Fig. 1(a) shows the XRD patterns of Nd2xSrxCoO4þd (x ¼ 0.8, 1.0, 1.2) at room temperature. It can be seen that only a single K2NiF4-structural phase was identified for all samples. Fig. 1(b) shows the XRD patterns of mixture of Nd0.8Sr1.2CoO4þd and SDC (1:1 weight ratio) after heat treatment at 1100 C for 4 h. It was observed the Nd0.8Sr1.2CoO4þd and SDC remained their structures unchanged. There were no secondary phases identified, indicating that Nd0.8Sr1.2CoO4þd has good chemical compatibility with the SDC electrolyte in the experiment range.
103Hz
0.1
102Hz 10Hz 1Hz
0.0
104Hz
-0.1 2.4
2.6
2.8
3.0
3.2
Z' (ohm)
Fig. 6 e Impedance spectra of Nd2LxSrxCoO4Dd cathodes ((a) at 700 C, (b) at 750 C, (c) at 800 C) under open circuit potential in air.
shown in Fig. 3. It can be seen that there is a linear relationship between ln(sT) and 1/T for all samples, indicating a small polaron hopping mechanism, which follows the Arrhenius formula: s¼
A Ea exp T kT
(2)
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 5 ( 2 0 1 0 ) 5 5 9 4 e5 6 0 0
Electrochemical performance
Fig. 4 shows the impedance spectra of Nd0.8Sr1.2CoO4þd cathodes sintered at various temperatures under open circuit potential at 700 C in air. The impedance spectra was fitted by the equivalent circuit LRohm(R1Q1)(R2Q2), where L is the inductance, which is given by the measuring system. The intercept value of the impedance arcs with the real axis at high-frequency side corresponds to the overall ohmic resistance (Rohm) including the electrolyte, electrodes, current collectors and lead wires. For the sake of clear comparison in evaluating the catalytic activity of cathodes sintered at different temperatures, the ohmic resistance of bulk electrolyte was subtracted. Two impedance arcs are observed at low and high frequencies, which indicates that there are at least two electrode processes limiting the oxygen reduction reaction. The resistance at high frequency corresponds to the charge transfer resistance (R1), which is contributed by the electrochemical reaction at electrode/electrolyte interface, while the one at the low-frequency arc is attributed to the oxygen adsorption or desorption process on the electrode surface and the diffusion of oxygen ions (R2) [24,25]. The total cathodic polarization resistance (Rp) is the sum of R1 and R2. Q1 and Q2 are the corresponding constant phase elements [26]. It was found that the sintering temperature had a remarkable effect on the electrode performance. From the impedance spectra, it was observed that the electrode polarization was relatively high when sintered at 1000 C. Rp reduced to the lowest value at 1100 C. And at the same time, Rp increased again as for that sintered at 1200 C. Fig. 5 shows the SEM images of Nd0.8Sr1.2CoO4þd cathode sintered at three temperatures. It can be observed the sample sintered at 1100 C has a moderate porosity. When sintered at the lower temperature of 1000 C, however, the contacts among particles seem to be rather poor. On the other hand, for the sample sintered at 1200 C has a larger grains size and a lower porosity than that at 1100 C. Fig. 5(d) shows the microstructure around the interface between Nd0.8Sr1.2CoO4þd cathode (denoted by NSC) and SDC electrolyte. The good adhesion between electrode and electrolyte was observed. The thickness of the cathode is about 25 mm. According to the above results, the sintering condition of 1100 C for 2 h was chosen as a fixed parameter for the following study. The impedance spectra under open circuit potential at 700e800 C in air is shown in Fig. 6. It was found that the polarization resistance decreased with the increasing of Sr. It may be attributed to the loss of lattice oxygen and the
T( C)
Nd1.2Sr0.8CoO4þd NdSrCoO4þd Nd0.8Sr1.2CoO4þd 700 7
L(H) 10 R1 (Ucm2) R2 (Ucm2) Rp (Ucm2)
750
800 700 750 800
700
750
800
8.19 10.21 12.74 6.73 8.92 12.16 3.65 0.55 0.27 0.16 0.44 0.22 0.13 0.37 0.08 0.07 0.06 0.05 0.04 0.05 0.03 0.63 0.34 0.22 0.49 0.26 0.18 0.40
6.39 0.17 0.05 0.22
7.69 0.07 0.06 0.13
formation of oxygen vacancies [27], resulting in the improving of the oxygen reduction reaction. The polarization resistances at various temperatures are calculated and summarized in Table 2. The high-frequency resistance (R1) is larger than that of low-frequency (R2), implying that the oxygen reduction reaction is limited primarily by the charge transfer process. R1 decreased significantly with increasing temperature, indicating that the charge transfer process was enhanced by increasing temperature. Nd0.8Sr1.2CoO4þd showed the lowest polarization resistance of 0.40 Ucm2 at 700 C in this experiment. This value is lower than that of Nd1.6Sr0.4NiO4 cathode material [28]. The relaxation frequency of the high-frequency semicircle in Figs. 4 and 6 is in the order of 103 Hz. This high frequency contribution may not solely be ascribed to the electrode mechanism but also to a blocking phenomenon at the interface between electrode material and electrolyte. This blocking effect can be related to the microstructure of the interface and not to the electrode process. It might be interesting to investigate the effect of the relaxation frequency and this will be investigated in future work. The polarization resistances as a function of temperature of the Nd2xSrxCoO4þd cathodes are shown in Fig. 7. The value of apparent activation energy (Ea) calculated from the slope of the ln(Rp) versus 1000/T plots was also included in Fig. 7. The Ea of Nd1.2Sr0.8CoO4þd, NdSrCoO4þd and Nd0.8Sr1.2CoO4þd was 1.43 eV, 1.41 eV, 1.40 eV, respectively, similar to that of Nd1.8Ce0.2CoO4d electrode material reported by Masood Soorie et al. [29]. 2.0
Ea/eV 1.5 1.0 0.5
Nd1.2Sr0.8CoO4+
1.43
NdSrCoO4+
1.41
Nd0.8Sr1.2CoO4+
1.40
Fitting results
2
3.4.
Table 2 e The polarization resistance of Nd2LxSrxCoO4Dd cathodes in the temperature range 700e800 C.
cm
where A the pre-exponential factor, Ea the activation energy, k the Boltzmann constant, and T the absolute temperature. The values of Ea calculated from the plots are given in Table 1. As shown in Fig. 3 and Table 1, the electrical conductivity increased as the Sr content increased and the incorporation of Sr in Nd2xSrxCoO4þd leads to a decrease of the activation energy. This may be attributed to Sr doping in Nd2xSrxCoO4þd which results in generating holes and thus increasing the conductivity. The conductivity of all specimens at 800 C was also listed in Table 1. The conductivity of both NdSrCoO4þd and Nd0.8Sr1.2CoO4þd was higher than 100 S cm1 at 800 C, a sufficient value for the cathode providing low ohmic losses.
lnR p (
5598
0.0 -0.5 -1.0 -1.5 -2.0 0.95
1.00
1.05
1000/T K
1.10
1.15
-1
Fig. 7 e Temperature dependence of polarization resistances (Rp) for Nd2LxSrxCoO4Dd (x [ 0.8, 1.0, 1.2) cathodes 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 5 ( 2 0 1 0 ) 5 5 9 4 e5 6 0 0
0.00
-0.02
Overpotential (V)
-0.04
-0.06
Nd1.2Sr0.8CoO4+δ
-0.08
NdSrCoO4+δ Nd0.8Sr1.2CoO4+δ
-0.10
-0.12 0.00
0.05
0.10
0.15
0.20
-2
Current density Acm
Fig. 8 e Cathode polarization curves of Nd2LxSrxCoO4Dd (x [ 0.8, 1.0, 1.2) at 700 C in air.
The cathode overpotential of Nd2xSrxCoO4þd as a function of current density at 700 C is shown in Fig. 8. It can be seen that the cathode overpotential was also affected by Sr content. The increase of Sr content resulted in a significant reduction of the cathode overpotential in the experiment range. When the current density reached 0.1 A cm2, the cathode overpotential of Nd0.8Sr1.2CoO4þd was about 35.6 mV.
4.
Conclusions
All the samples obtained as a single K2NiF4-structural phase by solid state reaction method. Nd0.8Sr1.2CoO4þd had good chemical compatibility with SDC electrolyte. Thermal expansion coefficients of the specimens increased with increasing of the Sr content. The electrical conductivity of the samples showed a semiconducting electrical behavior and the conductivity of Nd0.8Sr1.2CoO4þd was 212 S cm1 at 800 C. AC impedance spectroscopy and polarization measurements showed that the electrochemical properties improved with increasing the Sr content. The polarization resistance of Nd0.8Sr1.2CoO4þd was 0.40 Ucm2 at 700 C, and the lowest overpotential was 35.6 mV at a current density of 0.1 A cm2 at 700 C in air, exhibiting a potential cathode material for IT-SOFC.
Acknowledgement This work was financially supported by Jiangsu Planned Projects for Postdoctoral Research Funds (0801017B). We also acknowledge the support of Jiangsu Provincial Key Laboratory of Inorganic and Composite Materials.
references
[1] Minh Nguyen Q. Ceramic fuel cells. J Am Ceram Soc 1993;76: 563e88.
5599
[2] Weber Andre´, Ivers-Tiffe´e Ellen. Materials and concepts for solid oxide fuel cells (SOFCs) in stationary and mobile applications. J Power Sources 2004;127:273e83. [3] Singhal SC. Advances in solid oxide fuel cell technology. Solid State Ionics 2000;135:305e13. [4] Liu Bangwu, Zhang Yue, Zhang Limin. Oxygen reduction mechanism at Ba0.5Sr0.5Co0.8Fe0.2O3d cathode for solid oxide fuel cell. Int J Hydrogen Energy 2009;34:1008e14. [5] Ivers-Tiffe´e Ellen, Weber Andre´, Herbstritt Dirk. Materials and technologies for SOFC-components. J Eur Ceram Soc 2001;21:1805e11. [6] Leng Yongjun, Chan Siew Hwa, Liu Qinglin. Development of LSCFeGDC composite cathodes for low-temperature solid oxide fuel cells with thin film GDC electrolyte. Int J Hydrogen Energy 2008;33:3808e17. [7] Wei Zhou, Wanqin Jin, Zhonghua Zhu, Zongping Shao. Structural, electrical and electrochemical characterizations of SrNb0.1Co0.9O3d as a cathode of solid oxide fuel cells operating below 600 C. [8] Dutta Atanu, Mukhopadhyay Jayanta, Basu RN. Combustion synthesis and characterization of LSCF-based materials as cathode of intermediate temperature solid oxide fuel cells. J Eur Ceram Soc 2009;29:2003e11. [9] Parka Kwangjin, Lee Changbo, Bae Joongmyeon, Yoo Youngsung. Structural and electrochemical properties of Pr0.3Sr0.7Co0.3Fe0.7O3d cathode for IT-SOFC. Int J Hydrogen Energy 2009;34:6852e60. [10] Skinner Stephen J. Recent advances in perovskite-type materials for SOFC cathodes. Fuel Cells Bull 2001;33:6e12. [11] Kostogloudis GCh, Ftikos Ch. Characterization of Nd1xSrxMnO3d SOFC cathode materials. J Eur Ceram Soc 1999;19:497e505. [12] Gao Jianfeng, Liu Xingqin, Peng Dingkun, Meng Guangyao. Electrochemical behavior of Ln0.6Sr0.4Co0.2Fe0.8O3d (Ln ¼ Ce, Gd, Sm, Dy) materials used as cathode of IT-SOFC. Catal Today 2003;82:207e11. [13] Xia Changrong, Rauch William, Chen Fanglin, Liu Meilin. Sm0.5Sr0.5CoO3 cathodes for low-temperature SOFCs. Solid State Ionics 2002;149:11e9. [14] Zhou Wei, Ran Ran, Shao Zongping. Progress in understanding and development of Ba0.5Sr0.5Co0.8Fe0.2O3dbased cathodes for intermediate-temperature solid-oxide fuel cells: a review. J Power Sources 2009;192:231e46. [15] Petric Anthony, Huang Peng, Tietz Frank. Evaluation of LaeSreCoeFeeO perovskites for solid oxide fuel cells and gas separation membranes. Solid State Ionics 2000;135: 719e25. [16] Le Flem Gilles, Demazeau Ge´rard, Hagenmuller Paul. Relations between structure and physical properties in K2NiF4-type oxides. J Solid State Chem 1982;44:82e8. [17] Al Daroukha M, Vashook VV, Ullmann H, Tietz F, Arul Raj I. Oxides of the AMO3 and A2MO4-type: structural stability, electrical conductivity and thermal expansion. Solid State Ionics 2003;158:141e50. [18] Nie HW, Wen TL, Wang SR, Wang YS, Guth U, Vashook V. Preparation, thermal expansion, chemical compatibility, electrical conductivity and polarization of A2aA0 aMO4 (A ¼ Pr, Sm; A ¼ Sr; M ¼ Mn, Ni; a ¼ 0.3, 0.6) as a new cathode for SOFC. Solid State Ionics 2006;177:1929e32. [19] Skinner SJ, Kilner JA. Oxygen diffusion and surface exchange in La2xSrxNiO4þd. Solid State Ionics 2000;135:709e12. [20] Boehm E, Bassat JM, Steil MC, Dordor P, Mauvy F, Grenier J-C. Oxygen transport properties of La2Ni1xCuxO4þd mixed conducting oxides. Solid State Sci 2003;5:973e81. [21] James M, Tedesco A, Cassidy D, Colella M, Smythe PJ. The phase diagram and crystal chemistry of strontium-doped rare earth cobaltates: Ln2xSrxCoO4þd (Ln ¼ LaeDy). J Alloys Compd 2006;419:201e7.
5600
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 5 ( 2 0 1 0 ) 5 5 9 4 e5 6 0 0
[22] Luo Laitao, Hua Zhong, Mao Yangxiao. Syntheses of cocontaining mixed oxides with K2NiF4 structure and their catalytic activities in oxidation. Chin J Appl Chem 2004;21 (11):1150e4. [23] Sa´nchez-Andu´jar M, Castro-Couceiro A, Rivas-Murias B, arı´s-Rodrı´guez MA. Influence of the Mira J, Rivas J, Sen dimensionality of the structure and the nature of the rare earth on the magneto-transport properties of Nd1xSr1þxCoO4 (0 x 0.30). J Alloys Compd 2007;437: 64e70. [24] Gu Haitao, Chen Han, Gao Ling, Zheng Yifeng, Zhu Xiaofang, Guo Lucun. Oxygen reduction mechanism of NdBaCo2O5þd cathode for intermediate-temperature solid oxide fuel cells under cathodic polarization. Int J Hydrogen Energy 2009;34: 2416e20. [25] Li Shuyan, Lu¨ Zhe, Huang Xiqiang, Su Wenhui. Thermal, electrical, and electrochemical properties of Nd-doped Ba0.
[26]
[27]
[28]
[29]
5Sr0.5Co0.8Fe0.2O3d as a cathode material for SOFC. Solid State Ionics 2008;178:1853e8. Ding Xifeng, Cui Chong, Guo Lucun. Thermal expansion and electrochemical performance of La0.7Sr0.3CuO3d-Sm0.2Ce0.8 O2d composite cathode for IT-SOFCs. J Alloys Compd 2009; 481:845e50. Tai LW, Nasrallah MM, Anderson HU, Sparlin DM, Sehlin SR. Structure and electrical properties of La1xSrxCo1yFeyO3. Part 1. The system La0.8Sr0.2Co1yFeyO3. Solid State Ionics 1995;76:259e71. Sun Liping, Li Qiang, Zhao Hui, Huo Lihua, Grenier Jean Claude. Preparation and electrochemical properties of Sr-doped Nd2NiO4 cathode materials for intermediate-temperature solid oxide fuel cells. J Power Sources 2008;183:43e8. Soorie Masood, Skinner Stephen J. Ce substituted Nd2CuO4 as a possible fuel cell cathode material. Solid State Ionics 2006; 177:2081e6.
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Preparation and characterization of Nd2LxSrxCoO4Dd cathodes for intermediate-temperature solid oxide fuel cell Yue Cao, Haitao Gu, Han Chen, Yifeng Zheng, Ming Zhou, Lucun Guo* College of Materials Science and Engineering, Nanjing University of Technology, 5 Xinmofan Road, Nanjing 210009, People’s Republic of China
article info
abstract
Article history:
K2NiF4-type structural Nd2xSrxCoO4þd (x ¼ 0.8, 1.0, 1.2) was synthesized and evaluated as
Received 29 October 2009
cathodes for intermediate-temperature solid oxide fuel cell (IT-SOFC). The crystal struc-
Received in revised form
ture, thermal expansion, electrical conductivity and electrochemical properties were
6 March 2010
investigated by X-ray diffraction, dilatometry, DC four-probe method, AC impedance and
Accepted 11 March 2010
polarization techniques. It is found that the electrochemical properties were remarkably
Available online 18 April 2010
improved with the increasing of Sr in the experiment range. Nd0.8Sr1.2CoO4þd showed the highest electrical conductivity of 212 S cm1 at 800 C, the lowest polarization resis-
Keywords:
tance and cathodic overpotential, 0.40 Ucm2 at 700 C and 35.6 mV at a current density of
Solid oxide fuel cell
0.1 A cm2 at 700 C, respectively. The chemical compatibility experiment revealed that
Cathode
Nd0.8Sr1.2CoO4þd cathode was chemically stable with the SDC electrolyte. The thermal
Thermal expansion
expansion coefficient also increased with the Sr content.
Electrical conductivity
ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.
Electrochemical properties
1.
Introduction
Solid oxide fuel cell (SOFC) is a device that can convert chemical energy of fuels directly to electricity in a highly efficient and environmental way and draws a wide attention in recent years [1e4]. Nowadays, one of the most important research goals is to develop intermediate-temperature solid oxide fuel cell (IT-SOFC). IT-SOFC will solve various problems associated with the high operating temperature (900e1000 C), such as sealing and thermal degradation. Decreasing the operating temperature leads to improved long term stability of the system and the cost can be reduced by using less costly metal alloys as interconnects [5e7]. However, reducing the operating temperature results in a significant decreasing of the electrochemical activity of the cathode, thus a dramatically lowering of the output power density of the cells. For example, the conventional cathode
prepared with strontium doped lanthanum manganite (LSM) poorly performs at intermediate temperatures [8,9]. Therefore, the development of new cathode materials with high electrocatalytic activity for the oxygen reduction reaction is critical for IT-SOFC. Considerable investigations have been conducted on various perovskite-type oxides to improve the cathode performance at intermediate temperatures [10e14]. Among these oxides, the cobalt-based materials were received extensive attention because of their relatively high conductivity and excellent electrochemical properties. However, some cobaltites have low chemical stability and unacceptably high thermal expansion coefficients, causing the mismatch with other components, such as the electrolyte [15]. Recently, many investigations on mixed ioniceelectronic conductors (MIECs) with K2NiF4 structure were reported. These oxides are usually formulated as A2BO4, which can be regarded as a staking of ABO3 perovskite layers alternating
* Corresponding author. Tel.: þ86 25 83587261; fax: þ86 25 83306152. E-mail address: [email protected] (L. Guo). 0360-3199/$ e see front matter ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.03.046
5595
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 5 ( 2 0 1 0 ) 5 5 9 4 e5 6 0 0
14
a
K2NiF4 phase
Nd1.2Sr0.8CoO4+δ NdSrCoO4+δ Nd0.8Sr1.2CoO4+δ
12
Nd0.8Sr1.2CoO4+
Fitting results
-1
Scm K
Intensity (a.u.)
10
ln σT
NdSrCoO4+
8
6
4
Nd1.2Sr0.8CoO4+
2 20
30
40
50
60
70
0.8
80
1.0
1.2
1.4
1000/T K
2θ(°)
b
Intensity (a.u.)
Nd0.8Sr1.2CoO4+ -SDC
SDC
Nd0.8Sr1.2CoO4+
30
40
50
60
70
80
2θ(°)
Fig. 1 e XRD patterns of Nd2LxSrxCoO4Dd (x [ 0.8, 1.0, 1.2) calcined at 1200 C for 4 h in air (a) and Nd0.8Sr1.2CoO4DdSDC mixture calcined at 1100 C for 4 h in air (b).
1.4
6 -1 TEC ×10 K
dL/L 0 (%)
1.2
1.8
2.0
2.2
-1
Fig. 3 e Arrhenius plots of ln(sT ) vs. 1000/T for Nd2LxSrxCoO4Dd (x [ 0.8, 1.0, 1.2) in air.
Nd0.8Sr1.2CoO4+ SDC
20
1.6
with AO rock-salt layers along the c-direction [16]. These oxides have been demonstrated to be able to accommodate a significant oxygen nonstoichiometry. Previous studies have shown that A2BO4 oxides have better thermochemical stability and lower thermal expansion coefficient compared with perovskite ABO3 oxides [17,18]. It was also found that these compounds exhibit relatively high oxygen diffusion and surface exchange coefficients, which are two vital factors for cathode performance [19,20]. These favorable properties mentioned above make A2BO4 based oxide a promising candidate of cathode material for IT-SOFC. It was reported that the formation of K2NiF4-type structure of A2BO4 composition is partially related to the oxidation state of cations at B site [18]. James et al. [21] reported relatively high amount dopant of the Sr at A site apparently favors the formation of K2NiF4-type structure for Co at B site. They found that singlephase K2NiF4-type structure phases of Ln2xSrxCoO4þd were obtained over the composition range of 0.75 x 1.50 for Ln ¼ La and Nd. For system of Nd2xSrxCoO4þd, extensive studies have been performed on its catalytic and magnetic properties [22,23]. In the present study, Nd2xSrxCoO4þd (x ¼ 0.8, 1.0, 1.2) was investigated to evaluate their potential as cathode materials for IT-SOFC.
Nd1.2Sr0.8CoO4+
13.8
1.0
NdSrCoO4+
15.0
2.
Experimental
0.8
Nd0.8Sr1.2CoO4+
15.8
2.1.
Powder synthesis
0.6
Powder of Nd2xSrxCoO4þd (x ¼ 0.8, 1.0, 1.2) and Sm0.2Ce0.8O1.9 (SDC) was synthesized by solid state reaction method. Nd2O3
0.4 0.2
Table 1 e The electrical conductivity at 800 C and the activation energy in air.
0.0 0
100
200
300
400
Temperautre
500 o
600
700
800
C
Fig. 2 e Thermal expansion curves of Nd2LxSrxCoO4Dd (x [ 0.8, 1.0, 1.2) from 25 C to 800 C in air.
900
Composition Nd1.2Sr0.8CoO4þd NdSrCoO4þd Nd0.8Sr1.2CoO4þd
s800 (S cm1)
Ea (kJ mol1)
89 128 212
62 53 50
5596
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 5 ( 2 0 1 0 ) 5 5 9 4 e5 6 0 0
Nd2xSrxCoO4þd was calcined at 1200 C for 4 h in air, but Sm0.2Ce0.8O1.9 at 1200 C for 2 h in air.
1.5 o
1000 C o
1100 C o
1200 C Fitting results
-Z" (ohm)
1.0
2.2. 103Hz
104Hz 0.5
102Hz 10Hz 1Hz
0.0
-0.5 0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
Z' (ohm)
Fig. 4 e Impedance spectra of Nd0.8Sr1.2CoO4Dd cathodes sintered at 1000e1200 C for 2 h under open circuit potential at 700 C in air.
(99.9%), SrCO3 (99%), Co2O3 (99%), Sm2O3 (99.9%), CeO2 (99.95%) were used as starting materials. The raw materials with appropriate stoichiometric ratio were mixed for 8 h in distilled water with zirconia grinding media. The mixed powder
Sample preparation
The Nd2xSrxCoO4þd powder was pressed uniaxially at 50 MPa to form a bar and then sintered at 1300 C for 2 h. Obtained specimens with dimensions of 3 mm 5 mm 50 mm were used to measure the thermal expansion and electrical conductivity. Pressed pellets of SDC (18 mm in diameter, 1 mm in thickness) were sintered at 1550 C in air for 2 h. The Nd2xSrxCoO4þd powder was mixed with ethyl cellulose and terpinol, and the mixture was subsequently painted onto both side of the SDC pellets by screen painting, and then sintered at 1000e1200 C for 2 h. The Ag electrodes as the current collector and the reference electrode were painted onto the electrode and electrolyte surface, and then baked for 0.5 h at 600 C in air. Nd0.8Sr1.2CoO4þd powder and SDC powder were mixed at a weight ratio of 1:1 and then sintered at 1100 C for 4 h to check the chemical stability between them.
2.3.
Characterization
The crystal structure at room temperature was determined by XRD using the ARL X’TRA diffractometer with Cu Ka radiation
Fig. 5 e SEM images of Nd0.8Sr1.2CoO4Dd cathodes sintered at 1000 C (a); 1100 C (b); 1200 C (c) and the cross-section image of the test cell (d).
5597
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 5 ( 2 0 1 0 ) 5 5 9 4 e5 6 0 0
h ¼ E I Rohm
(1)
a
3.1.
Powder characterization
Fitting results
0.6
0.4
102Hz
103Hz
10Hz
0.2
1Hz
104Hz 0.0
-0.2 5.0
5.2
5.4
5.6
5.8
6.0
6.2
6.4
6.6
6.8
Z' (ohm)
b
0.6
Nd1.2Sr0.8CoO4+
o
750 C,air
NdSrCoO4+ Nd0.8Sr1.2CoO4+
0.4
-Z" (ohm)
Results and discussion
Nd1.2Sr0.8CoO4+ NdSrCoO4+ Nd0.8Sr1.2CoO4+
o
700 C,air
where E, I and Rohm indicate the applied cathode voltage, current and ohmic resistance measured by the impedance spectra, respectively.
3.
1.0
0.8
-Z" (ohm)
operating at 40 kV, 35 mA; l ¼ 0.15418 nm. The diffraction angle was 20 e80 with a step size of 0.02 . The thermal expansion of the specimens was measured in air from room temperature to 800 C, using a dilatometer (RPZ-01, Luoyang, China) with a heating rate of 5 C/min. The electrical conductivity was measured by the four-probe DC technique, using Ag paste as electrodes. AC impedance spectroscopy and polarization were performed using PARSTAT2273 electrochemical workstation. The impedance measurements were carried out in the frequency range of 0.1 MHze0.1 Hz at temperature between 500 C and 800 C. The applied voltages to the cathodes were changed between 0 and 1 V dc bias vs. the reference electrode at 5 mV/s. The overpotential (h) of the cathodes under cathodic polarization was estimated by the following equation:
Fitting results
103Hz
0.2
102Hz
10Hz 1Hz
104Hz 0.0
3.2.
Thermal expansion
Fig. 2 shows the thermal expansion curves of Nd2xSrxCoO4þd (x ¼ 0.8, 1.0, 1.2) in the temperature range 25e800 C. The average values of thermal expansion coefficients are listed in Fig. 2. The thermal expansion coefficients increased with increasing of the Sr content. This may be caused by the loss of lattice oxygen and the formation of oxygen vacancies, a process which is enhanced as the Sr content increased [20]. The thermal expansion compatibility with solid electrolyte could promote by reducing the thickness of the cathode film. It should be noted that the thermal expansion curves of the present study are not linear for the system, and that the deviation of the curves become more obvious with the increase of Sr content. However, the underlining mechanism for this unusual phenomenon is not clear so far.
3.3.
Electrical conductivity
The logarithm of electrical conductivity as a function of reciprocal temperature for Nd2xSrxCoO4þd (x ¼ 0.8, 1.0, 1.2) is
-0.2 3.6
3.8
4.0
4.2
4.4
4.6
Z' (ohm)
c
0.3
Nd1.2Sr0.8CoO4+
o
800 C,air
NdSrCoO4+ Nd0.8Sr1.2CoO4+
0.2
Fitting results -Z" (ohm)
Fig. 1(a) shows the XRD patterns of Nd2xSrxCoO4þd (x ¼ 0.8, 1.0, 1.2) at room temperature. It can be seen that only a single K2NiF4-structural phase was identified for all samples. Fig. 1(b) shows the XRD patterns of mixture of Nd0.8Sr1.2CoO4þd and SDC (1:1 weight ratio) after heat treatment at 1100 C for 4 h. It was observed the Nd0.8Sr1.2CoO4þd and SDC remained their structures unchanged. There were no secondary phases identified, indicating that Nd0.8Sr1.2CoO4þd has good chemical compatibility with the SDC electrolyte in the experiment range.
103Hz
0.1
102Hz 10Hz 1Hz
0.0
104Hz
-0.1 2.4
2.6
2.8
3.0
3.2
Z' (ohm)
Fig. 6 e Impedance spectra of Nd2LxSrxCoO4Dd cathodes ((a) at 700 C, (b) at 750 C, (c) at 800 C) under open circuit potential in air.
shown in Fig. 3. It can be seen that there is a linear relationship between ln(sT) and 1/T for all samples, indicating a small polaron hopping mechanism, which follows the Arrhenius formula: s¼
A Ea exp T kT
(2)
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 5 ( 2 0 1 0 ) 5 5 9 4 e5 6 0 0
Electrochemical performance
Fig. 4 shows the impedance spectra of Nd0.8Sr1.2CoO4þd cathodes sintered at various temperatures under open circuit potential at 700 C in air. The impedance spectra was fitted by the equivalent circuit LRohm(R1Q1)(R2Q2), where L is the inductance, which is given by the measuring system. The intercept value of the impedance arcs with the real axis at high-frequency side corresponds to the overall ohmic resistance (Rohm) including the electrolyte, electrodes, current collectors and lead wires. For the sake of clear comparison in evaluating the catalytic activity of cathodes sintered at different temperatures, the ohmic resistance of bulk electrolyte was subtracted. Two impedance arcs are observed at low and high frequencies, which indicates that there are at least two electrode processes limiting the oxygen reduction reaction. The resistance at high frequency corresponds to the charge transfer resistance (R1), which is contributed by the electrochemical reaction at electrode/electrolyte interface, while the one at the low-frequency arc is attributed to the oxygen adsorption or desorption process on the electrode surface and the diffusion of oxygen ions (R2) [24,25]. The total cathodic polarization resistance (Rp) is the sum of R1 and R2. Q1 and Q2 are the corresponding constant phase elements [26]. It was found that the sintering temperature had a remarkable effect on the electrode performance. From the impedance spectra, it was observed that the electrode polarization was relatively high when sintered at 1000 C. Rp reduced to the lowest value at 1100 C. And at the same time, Rp increased again as for that sintered at 1200 C. Fig. 5 shows the SEM images of Nd0.8Sr1.2CoO4þd cathode sintered at three temperatures. It can be observed the sample sintered at 1100 C has a moderate porosity. When sintered at the lower temperature of 1000 C, however, the contacts among particles seem to be rather poor. On the other hand, for the sample sintered at 1200 C has a larger grains size and a lower porosity than that at 1100 C. Fig. 5(d) shows the microstructure around the interface between Nd0.8Sr1.2CoO4þd cathode (denoted by NSC) and SDC electrolyte. The good adhesion between electrode and electrolyte was observed. The thickness of the cathode is about 25 mm. According to the above results, the sintering condition of 1100 C for 2 h was chosen as a fixed parameter for the following study. The impedance spectra under open circuit potential at 700e800 C in air is shown in Fig. 6. It was found that the polarization resistance decreased with the increasing of Sr. It may be attributed to the loss of lattice oxygen and the
T( C)
Nd1.2Sr0.8CoO4þd NdSrCoO4þd Nd0.8Sr1.2CoO4þd 700 7
L(H) 10 R1 (Ucm2) R2 (Ucm2) Rp (Ucm2)
750
800 700 750 800
700
750
800
8.19 10.21 12.74 6.73 8.92 12.16 3.65 0.55 0.27 0.16 0.44 0.22 0.13 0.37 0.08 0.07 0.06 0.05 0.04 0.05 0.03 0.63 0.34 0.22 0.49 0.26 0.18 0.40
6.39 0.17 0.05 0.22
7.69 0.07 0.06 0.13
formation of oxygen vacancies [27], resulting in the improving of the oxygen reduction reaction. The polarization resistances at various temperatures are calculated and summarized in Table 2. The high-frequency resistance (R1) is larger than that of low-frequency (R2), implying that the oxygen reduction reaction is limited primarily by the charge transfer process. R1 decreased significantly with increasing temperature, indicating that the charge transfer process was enhanced by increasing temperature. Nd0.8Sr1.2CoO4þd showed the lowest polarization resistance of 0.40 Ucm2 at 700 C in this experiment. This value is lower than that of Nd1.6Sr0.4NiO4 cathode material [28]. The relaxation frequency of the high-frequency semicircle in Figs. 4 and 6 is in the order of 103 Hz. This high frequency contribution may not solely be ascribed to the electrode mechanism but also to a blocking phenomenon at the interface between electrode material and electrolyte. This blocking effect can be related to the microstructure of the interface and not to the electrode process. It might be interesting to investigate the effect of the relaxation frequency and this will be investigated in future work. The polarization resistances as a function of temperature of the Nd2xSrxCoO4þd cathodes are shown in Fig. 7. The value of apparent activation energy (Ea) calculated from the slope of the ln(Rp) versus 1000/T plots was also included in Fig. 7. The Ea of Nd1.2Sr0.8CoO4þd, NdSrCoO4þd and Nd0.8Sr1.2CoO4þd was 1.43 eV, 1.41 eV, 1.40 eV, respectively, similar to that of Nd1.8Ce0.2CoO4d electrode material reported by Masood Soorie et al. [29]. 2.0
Ea/eV 1.5 1.0 0.5
Nd1.2Sr0.8CoO4+
1.43
NdSrCoO4+
1.41
Nd0.8Sr1.2CoO4+
1.40
Fitting results
2
3.4.
Table 2 e The polarization resistance of Nd2LxSrxCoO4Dd cathodes in the temperature range 700e800 C.
cm
where A the pre-exponential factor, Ea the activation energy, k the Boltzmann constant, and T the absolute temperature. The values of Ea calculated from the plots are given in Table 1. As shown in Fig. 3 and Table 1, the electrical conductivity increased as the Sr content increased and the incorporation of Sr in Nd2xSrxCoO4þd leads to a decrease of the activation energy. This may be attributed to Sr doping in Nd2xSrxCoO4þd which results in generating holes and thus increasing the conductivity. The conductivity of all specimens at 800 C was also listed in Table 1. The conductivity of both NdSrCoO4þd and Nd0.8Sr1.2CoO4þd was higher than 100 S cm1 at 800 C, a sufficient value for the cathode providing low ohmic losses.
lnR p (
5598
0.0 -0.5 -1.0 -1.5 -2.0 0.95
1.00
1.05
1000/T K
1.10
1.15
-1
Fig. 7 e Temperature dependence of polarization resistances (Rp) for Nd2LxSrxCoO4Dd (x [ 0.8, 1.0, 1.2) cathodes 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 5 ( 2 0 1 0 ) 5 5 9 4 e5 6 0 0
0.00
-0.02
Overpotential (V)
-0.04
-0.06
Nd1.2Sr0.8CoO4+δ
-0.08
NdSrCoO4+δ Nd0.8Sr1.2CoO4+δ
-0.10
-0.12 0.00
0.05
0.10
0.15
0.20
-2
Current density Acm
Fig. 8 e Cathode polarization curves of Nd2LxSrxCoO4Dd (x [ 0.8, 1.0, 1.2) at 700 C in air.
The cathode overpotential of Nd2xSrxCoO4þd as a function of current density at 700 C is shown in Fig. 8. It can be seen that the cathode overpotential was also affected by Sr content. The increase of Sr content resulted in a significant reduction of the cathode overpotential in the experiment range. When the current density reached 0.1 A cm2, the cathode overpotential of Nd0.8Sr1.2CoO4þd was about 35.6 mV.
4.
Conclusions
All the samples obtained as a single K2NiF4-structural phase by solid state reaction method. Nd0.8Sr1.2CoO4þd had good chemical compatibility with SDC electrolyte. Thermal expansion coefficients of the specimens increased with increasing of the Sr content. The electrical conductivity of the samples showed a semiconducting electrical behavior and the conductivity of Nd0.8Sr1.2CoO4þd was 212 S cm1 at 800 C. AC impedance spectroscopy and polarization measurements showed that the electrochemical properties improved with increasing the Sr content. The polarization resistance of Nd0.8Sr1.2CoO4þd was 0.40 Ucm2 at 700 C, and the lowest overpotential was 35.6 mV at a current density of 0.1 A cm2 at 700 C in air, exhibiting a potential cathode material for IT-SOFC.
Acknowledgement This work was financially supported by Jiangsu Planned Projects for Postdoctoral Research Funds (0801017B). We also acknowledge the support of Jiangsu Provincial Key Laboratory of Inorganic and Composite Materials.
references
[1] Minh Nguyen Q. Ceramic fuel cells. J Am Ceram Soc 1993;76: 563e88.
5599
[2] Weber Andre´, Ivers-Tiffe´e Ellen. Materials and concepts for solid oxide fuel cells (SOFCs) in stationary and mobile applications. J Power Sources 2004;127:273e83. [3] Singhal SC. Advances in solid oxide fuel cell technology. Solid State Ionics 2000;135:305e13. [4] Liu Bangwu, Zhang Yue, Zhang Limin. Oxygen reduction mechanism at Ba0.5Sr0.5Co0.8Fe0.2O3d cathode for solid oxide fuel cell. Int J Hydrogen Energy 2009;34:1008e14. [5] Ivers-Tiffe´e Ellen, Weber Andre´, Herbstritt Dirk. Materials and technologies for SOFC-components. J Eur Ceram Soc 2001;21:1805e11. [6] Leng Yongjun, Chan Siew Hwa, Liu Qinglin. Development of LSCFeGDC composite cathodes for low-temperature solid oxide fuel cells with thin film GDC electrolyte. Int J Hydrogen Energy 2008;33:3808e17. [7] Wei Zhou, Wanqin Jin, Zhonghua Zhu, Zongping Shao. Structural, electrical and electrochemical characterizations of SrNb0.1Co0.9O3d as a cathode of solid oxide fuel cells operating below 600 C. [8] Dutta Atanu, Mukhopadhyay Jayanta, Basu RN. Combustion synthesis and characterization of LSCF-based materials as cathode of intermediate temperature solid oxide fuel cells. J Eur Ceram Soc 2009;29:2003e11. [9] Parka Kwangjin, Lee Changbo, Bae Joongmyeon, Yoo Youngsung. Structural and electrochemical properties of Pr0.3Sr0.7Co0.3Fe0.7O3d cathode for IT-SOFC. Int J Hydrogen Energy 2009;34:6852e60. [10] Skinner Stephen J. Recent advances in perovskite-type materials for SOFC cathodes. Fuel Cells Bull 2001;33:6e12. [11] Kostogloudis GCh, Ftikos Ch. Characterization of Nd1xSrxMnO3d SOFC cathode materials. J Eur Ceram Soc 1999;19:497e505. [12] Gao Jianfeng, Liu Xingqin, Peng Dingkun, Meng Guangyao. Electrochemical behavior of Ln0.6Sr0.4Co0.2Fe0.8O3d (Ln ¼ Ce, Gd, Sm, Dy) materials used as cathode of IT-SOFC. Catal Today 2003;82:207e11. [13] Xia Changrong, Rauch William, Chen Fanglin, Liu Meilin. Sm0.5Sr0.5CoO3 cathodes for low-temperature SOFCs. Solid State Ionics 2002;149:11e9. [14] Zhou Wei, Ran Ran, Shao Zongping. Progress in understanding and development of Ba0.5Sr0.5Co0.8Fe0.2O3dbased cathodes for intermediate-temperature solid-oxide fuel cells: a review. J Power Sources 2009;192:231e46. [15] Petric Anthony, Huang Peng, Tietz Frank. Evaluation of LaeSreCoeFeeO perovskites for solid oxide fuel cells and gas separation membranes. Solid State Ionics 2000;135: 719e25. [16] Le Flem Gilles, Demazeau Ge´rard, Hagenmuller Paul. Relations between structure and physical properties in K2NiF4-type oxides. J Solid State Chem 1982;44:82e8. [17] Al Daroukha M, Vashook VV, Ullmann H, Tietz F, Arul Raj I. Oxides of the AMO3 and A2MO4-type: structural stability, electrical conductivity and thermal expansion. Solid State Ionics 2003;158:141e50. [18] Nie HW, Wen TL, Wang SR, Wang YS, Guth U, Vashook V. Preparation, thermal expansion, chemical compatibility, electrical conductivity and polarization of A2aA0 aMO4 (A ¼ Pr, Sm; A ¼ Sr; M ¼ Mn, Ni; a ¼ 0.3, 0.6) as a new cathode for SOFC. Solid State Ionics 2006;177:1929e32. [19] Skinner SJ, Kilner JA. Oxygen diffusion and surface exchange in La2xSrxNiO4þd. Solid State Ionics 2000;135:709e12. [20] Boehm E, Bassat JM, Steil MC, Dordor P, Mauvy F, Grenier J-C. Oxygen transport properties of La2Ni1xCuxO4þd mixed conducting oxides. Solid State Sci 2003;5:973e81. [21] James M, Tedesco A, Cassidy D, Colella M, Smythe PJ. The phase diagram and crystal chemistry of strontium-doped rare earth cobaltates: Ln2xSrxCoO4þd (Ln ¼ LaeDy). J Alloys Compd 2006;419:201e7.
5600
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 5 ( 2 0 1 0 ) 5 5 9 4 e5 6 0 0
[22] Luo Laitao, Hua Zhong, Mao Yangxiao. Syntheses of cocontaining mixed oxides with K2NiF4 structure and their catalytic activities in oxidation. Chin J Appl Chem 2004;21 (11):1150e4. [23] Sa´nchez-Andu´jar M, Castro-Couceiro A, Rivas-Murias B, arı´s-Rodrı´guez MA. Influence of the Mira J, Rivas J, Sen dimensionality of the structure and the nature of the rare earth on the magneto-transport properties of Nd1xSr1þxCoO4 (0 x 0.30). J Alloys Compd 2007;437: 64e70. [24] Gu Haitao, Chen Han, Gao Ling, Zheng Yifeng, Zhu Xiaofang, Guo Lucun. Oxygen reduction mechanism of NdBaCo2O5þd cathode for intermediate-temperature solid oxide fuel cells under cathodic polarization. Int J Hydrogen Energy 2009;34: 2416e20. [25] Li Shuyan, Lu¨ Zhe, Huang Xiqiang, Su Wenhui. Thermal, electrical, and electrochemical properties of Nd-doped Ba0.
[26]
[27]
[28]
[29]
5Sr0.5Co0.8Fe0.2O3d as a cathode material for SOFC. Solid State Ionics 2008;178:1853e8. Ding Xifeng, Cui Chong, Guo Lucun. Thermal expansion and electrochemical performance of La0.7Sr0.3CuO3d-Sm0.2Ce0.8 O2d composite cathode for IT-SOFCs. J Alloys Compd 2009; 481:845e50. Tai LW, Nasrallah MM, Anderson HU, Sparlin DM, Sehlin SR. Structure and electrical properties of La1xSrxCo1yFeyO3. Part 1. The system La0.8Sr0.2Co1yFeyO3. Solid State Ionics 1995;76:259e71. Sun Liping, Li Qiang, Zhao Hui, Huo Lihua, Grenier Jean Claude. Preparation and electrochemical properties of Sr-doped Nd2NiO4 cathode materials for intermediate-temperature solid oxide fuel cells. J Power Sources 2008;183:43e8. Soorie Masood, Skinner Stephen J. Ce substituted Nd2CuO4 as a possible fuel cell cathode material. Solid State Ionics 2006; 177:2081e6.