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Ba0.9Co0.7Fe0.2Nb0.1O3 − δ as cathode material for intermediate temperature solid oxide fuel cells
Electrochemistry Communications 13 (2011) 882–885
Contents lists available at ScienceDirect
Electrochemistry Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / e l e c o m
Ba0.9Co0.7Fe0.2Nb0.1O3 − δ as cathode material for intermediate temperature solid oxide fuel cells Zhibin Yang a, b, Chenghao Yang b, Chao Jin b, Minfang Han a,⁎, Fanglin Chen b,⁎ a b
Union Research Center of Fuel Cell, School of Chemical & Environment Engineering, China University of Mining & Technology, Beijing, 100083, China Department of Mechanical Engineering, University of South Carolina, 300 Main Street, Columbia, SC 29208, USA
a r t i c l e
i n f o
Article history: Received 20 April 2011 Received in revised form 20 May 2011 Accepted 24 May 2011 Available online 1 June 2011 Keywords: Ba0.9Co0.7Fe0.2Nb0.1O3 − δ La0.8Sr0.2Ga0.83Mg0.17O3 − δ Solid oxide fuel cells Cathode
a b s t r a c t Ba0.9Co0.7Fe0.2Nb0.1O3 − δ (BCFN) perovskite material was synthesized and evaluated as cathode for La0.8Sr0.2Ga0.83Mg0.17O3 − δ (LSGM) electrolyte supported intermediate temperature solid oxide fuel cells (IT-SOFCs). X-ray diffraction results showed that BCFN was chemically compatible with the LSGM electrolyte. Maximum power densities of 0.36, 0.57, 0.80 and 1.1 W/cm2 were obtained for LSGM electrolyte supported cells with BCFN as cathode and Ni-GDC as anode operated at 650, 700, 750 and 800 °C, respectively. Further, the cell performance was stable under a constant current of 0.6 A/cm 2 for over 204 h at 750 °C. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Intermediate temperature solid oxide fuel cells (IT-SOFCs) have been considered to be promising energy conversion technology since they possess the merits of both high and low temperature fuel cells, such as low emissions, fuel flexibility, mitigated materials issues and low system cost [1]. Unfortunately, at reduced operating temperatures, cell resistance increases rapidly and is often dominated by interfacial polarization resistance between the cathode and the electrolyte [2]. Consequently, developing cathode with low cathode interfacial resistance is vital for successful development of IT-SOFCs. SrCoO3 − δ and BaCoO3 − δ-based perovskite mixed ionic and electronic conductors (MIECs) have been extensively investigated as cathode for IT-SOFCs because they can potentially extend the oxygen reduction reaction from the cathode/electrolyte physical interface region to the entire electrode surface [3,4]. Ba0.5Sr0.5Co0.8Fe0.2O3 − δ (BSCF) has been reported to be an excellent cathode material for IT-SOFCs in the last decade [5]. However, cobalt containing perovskite materials typically lack chemically stability and compatibility; for example, cobalt-based cathode tends to react with zirconia-based electrolyte, SrCo0.8Fe0.2O3 − δ can be easily transformed from perovskite to brownmillerite phase at low oxygen partial pressure [6], and BSCF can be easily disintegrated, especially under CO2 rich atmosphere at elevated temperatures [7,8]. Recently, it has been reported that introduction of Nb in the B-site of cobalt-based perovskites can significantly improve chemical stability and enhance oxygen permeability [9]. It has been demonstrated that barium cobalt
⁎ Corresponding authors. E-mail addresses: [email protected] (M. Han), [email protected] (F. Chen). 1388-2481/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2011.05.029
iron niobium oxide, BCFN can be a very promising cathode material for IT-SOFCs with ceria-based electrolyte, showing similar electrochemical activity to that of BSCF [10–12]. However, there has been no report on the cell performance stability of BCFN cathode [1]. On the other hand, La0.8Sr0.2Ga0.83Mg0.17O3 − δ (LSGM) has been considered as a promising electrolyte material for IT-SOFCs due to it higher ionic conductivity, negligible electronic conductivity and high chemical stability over a broad range of oxygen partial pressures [13]. Therefore, it will be valuable to assess the performance and stability of BCFN cathode for LSGM-electrolyte supported IT-SOFCs. It has been demonstrated that using BSCF as cathode for SOFCs, A-site deficiency in BSCF can create additional oxygen vacancies, facilitate oxygen reduction reaction and promote oxygen ion diffusion within the cathode material [5]. Consequently, in this work, A-site deficient BCFN, Ba0.9Co0.7Fe0.2Nb0.1O3 − δ has been systematically evaluated as cathode for LSGM-electrolyte supported IT-SOFCs. 2. Experimental Ba0.9Co0.7Fe0.2Nb0.1O3 − δ (BCFN) and LSGM powders were prepared through the solid-state reaction method [12]. La0.4Ce0.6O2 − δ (LDC) and Ce0.9Gd0.1O2 − δ (GDC) powders were prepared by a sol–gel process. LSGM powders were uniaxially pressed into pellets at 500 MPa and then sintered at 1450 °C for 10 h to obtain dense LSGM electrolyte. LDC ink was then screen-printed on the surface of the LSGM electrolyte and co-sintered at 1400 °C for 4 h to get a LDC buffer layer. BCFN/LSGM/LDC/Ni-GDC IT-SOFCs were finally prepared by screen-printing on the LSGM electrolytes with BCFN and Ni-GDC ink, respectively, and then co-fired at 1000 °C. The obtained button cells have 1.3 cm in diameter with an active electrode area of 0.33 cm2. Pt
Z. Yang et al. / Electrochemistry Communications 13 (2011) 882–885
883
0.15
paste and Pt wires were used as current collection and lead wire for both electrodes. Button cells were sealed on the anode side to one end of an alumina tube with a ceramic paste (Aremco-552). Ambient air was used as the oxidant in the cathode side. The phase structures of BCFN, LSGM and BCFN-LSGM powders were characterized using a Philips X-ray diffractometer (Model PW1830). The cell current voltage characteristics as well as the impedance spectra were measured with a four probe method using a multi-channel VersaSTAT (Princeton Applied Research). Scanning electron microscopy (SEM, FEI Quanta 200) operating at 30 kV was used to characterize the microstructure of BCFN/LSGM/LDC/Ni-GDC single cells after the cell performance test.
650 oC 700 oC 750 oC
-Z,
cm2
0.10
800 oC
0.05
0.00 0.00
3. Results and discussion
0.05
0.10
Fig. 1(a) shows the XRD patterns of LSGM, BCFN as well as LSGMBCFN mixed powders after being fired at 1000 °C for 10 h in air. No impurity phase has been detected for the heat-treated LSGM-BCFN mixed powders, indicating that BCFN is chemically compatible with the LSGM electrolyte. Fig. 1(b, c and d) show the cross-sectional SEM images of the BCFN/LSGM/LDC/Ni-GDC electrolyte supported single cell, the BCFN/LSGM as well as the LSGM/LDC/Ni-GDC interfaces. The thickness of the Ni-GDC anode is ~50 μm, the LDC membrane interlayer ~8 μm, the LSGM electrolyte ~300 μm and the BCFN cathode ~30 μm. The LSGM electrolyte is crack-free and gas-tight. There is a good contact between the porous BCFN cathode and the dense LSGM electrolyte after ~204 h short-term test. As can be seen from Fig. 2(b and d), the thin LDC interlayer adheres well both to the LSGM electrolyte and the Ni-GDC
a
0.15
0.20
cm2
Z,
Fig. 2. Impedance spectra of BCFN/LSGM/BCFN symmetrical cell tested in air at 650, 700, 750 and 800 °C, respectively.
anode, sufficiently dense to effectively prevent the solid-state reaction between Ni and LSGM. The BCFN electrode polarization resistance was evaluated at 650, 700, 750 and 800 °C in air, as shown in Fig. 2. The polarization resistance decreased with the increase in operating temperature from 0.18 Ωcm2 at 650 °C to 0.07, 0.04 and 0.02 Ωcm 2 at 700, 750 and 800 °C, respectively. The cathode interfacial resistance of BCFN is much smaller than that of LSCF cathode using Ce0.9Gd0.1O1.95 as the electrolyte
b
LSGM
Intensity (a.u.)
BCFN LSGM + BCFN
20
30
40
50
60
70
80
10 m
2θ (degree)
c
d
BCFN
LSGM
10 m
LSGM
LDC
Ni-GDC
10 m
Fig. 1. (a) X-ray diffraction patterns of BCFN and LSGM powder as well as BCFN-LSGM powder mixture after being fired at 1000 °C in air for 10 h, and SEM images of the tested cell: (b) cross-section of BCFN/LSGM/LCD/Ni-GDC electrolyte supported cell, (c) BCFN/LSGM interface and (d) LSGM/LDC/Ni-GDC interface.
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Z. Yang et al. / Electrochemistry Communications 13 (2011) 882–885
material (~0.1 Ωcm2 at 800 °C) [14] and BSCF cathode using LSGM as the electrolyte material (~0.1 Ωcm2 at 800 °C) [15], but comparable to that of LSCF-LSGM composite cathode using LSGM as the electrolyte material (0.19–0.24 Ωcm2 at 650 °C) [16]. The low cathode polarization resistance of BCFN observed in this work indicates that BCFN can be a potentially very promising cathode for IT-SOFCs. Fig. 3(a) shows the impedance spectra of BCFN/LSGM/LDC/Ni-GDC electrolyte supported single cell for different H2 flow rates from 30 to 70
a
0.15
(i) H2 flow rate
Rtotal
30 sccm 50 sccm 70 sccm
0.10
Rohmic RHF
0.387 0.381 0.375
0.216 0.215 0.213
0.088 0.087 0.086
RLF 0.083 0.080 0.076
0.05
H2- 50 sccm
H 2- 70 sccm
-Z ,
cm2
o
750 C H 2- 30 sccm
0.00 Rohmic
(ii) -0.05 0.20
RHF
RLF
CPEHF
CPELF
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0.30
b
0.35
0.40
cm2
Z,
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(iii) Temperature 650 oC 700 oC 750 oC 800 oC
Rohmic RHF
RLF
0.849 0.533 0.375 0.287
0.495 0.308 0.213 0.160
0.087 0.081 0.076 0.074
0.267 0.144 0.086 0.053
-Z ,
cm2
0.4
Rtotal
o
0.2
o
650 C 700 C o o 750 C 800 C H2-70 sccm
0.0 0.2
0.4
c
0.8
0.6
Z,
cm2
1.2
standard cubic centimeters per minute (sccm) at 750 °C. It can be seen that total cell resistance (Rtotal) decreased with the increase in H2 flow rate. Shown in the insets in Fig. 3(a) are the proposed equivalent circuit (inset ii) used to fit the experiment data (using the ZView software) and the fitting results (inset i), where CPEHF and CPELF refer to the highfrequency and low-frequency constant phase element, respectively. The impedance spectra were characterized by a small arc at high frequencies and a small arc at low frequencies. Ohmic resistance (Rohmic) was dominating in the Rtotal due to the electrolyte supported cell configuration tested in the work and the thickness of the LSGM electrolyte is ~300 μm. The polarization resistance corresponding to the high-frequency arc (RHF) seemed to be insensitive to the H2 flow rate, indicating that the RHF may be related to the cathode process. However, the polarization resistance corresponding to the low-frequency arc (RLF) decreased with the increase in H2 flow rate, indicating that RLF may be related to the anode of the cell and the gas diffusion process in porous Ni-GDC anode side is enhanced by increasing H2 flow rate. This is consistent to the phenomenon reported in the literature that the higherfrequency arc is typically associated with the O2−transfer while the lower-frequency arc is normally associated with gas diffusion in the electrode [15]. Fig. 3(b) shows the impedance spectra of the cell tested at different temperatures from 650 to 800 °C, with 70 sccm H2 flow rate. It can be seen that Rtotal and Rohmic decreased with the increasing temperature. However, RHF decreased significantly with the increase in temperature while RLF kept nearly the same. A careful examination of the impedance spectra data (in the inset iii) with the equivalent fitting circuit (in the inset ii) revealed that RHF associated with O 2−charge transfer in the electrode dominated the overall electrode kinetic at lower temperature, however, it decreased substantially with the increasing temperature. At 800 °C, RHF decreased to 0.053 Ωcm2 and was much smaller than RLF, which is associated with the gas-diffusion process. Cell maximum output power density reached 1.1 W/cm2 at 800 °C as shown in Fig.3(c), which is much higher than that of cells of Ni-SDC/LDC/LSGM/LSGMLSCF with the thickness of the LSGM electrolyte of ~30 μm (0.9 W/cm 2 at 800 °C) [17] but similar to that of cells of Ni-GDC/LDC/LSGM/LSC-LDC (LSC: La0.9Sr0.1CoO3) (1.1 W/cm2 at 800 °C) with the thickness of the LSGM electrolyte of ~60 μm [18], although the thickness of the LSGM electrolyte is ~300 μm in the present work. Much higher cell output power density is expected if thinner LSGM electrolyte is used in this work. Consequently, BCFN possesses better cathode performance than LSCF and LSC and is a very promising cathode for IT-SOFCs. The stability of LSGM electrolyte supported IT-SOFC with BCFN as cathode has been further examined in a short-term cell test. Fig. 4 shows the cell voltage as a function of operation time at a constant
1.2
0.6
0.6 0.4
Voltage, V
Voltage, V
0.8
0.8
Power density, W/cm2
o
650 C o 700 C o 750 C o 800 C H 2-70 sccm
Measured cell voltage 0.6 A/cm2, 750 oC
1.0
1.0 1.0
0.8
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0.2 0.4 0.0 0.0
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Current density, A/cm2
0.4 0
50
100
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Time, h Fig. 3. (a) Impedance spectra of BCFN/LSGM/LDC/Ni-GDC tested at 750 °C with different H2 flow rate; (b and c) impedance spectra and V–I curves of BCFN/LSGM/LDC/Ni-GDC tested from 650 °C to 800 °C with 70 sccm H2 flow rate.
Fig. 4. Cell voltage as a function of time of BCFN/LSGM/LDC/Ni-GDC tested at 750 °C under constant current of 0.6 A/cm2.
Z. Yang et al. / Electrochemistry Communications 13 (2011) 882–885
current density of 0.6 A/cm 2 at 750 °C with 60 sccm H2 flow rate. The cell showed very stable performance with no significant cell voltage degradation in the 204 h testing period. 4. Conclusions A-site deficient BCFN perovskite material has been synthesized and evaluated as cathode in IT-SOFCs with the configuration of BCFN/LSGM/ LDC/Ni-GDC. The BCFN cathode polarization resistances are 0.18, 0.07, 0.04 and 0.02 Ωcm2 at 650, 700, 750 and 800 °C, respectively. High cell power output densities of 0.36, 0.57, 0.80 and 1.1 W/cm 2 have been achieved at 550, 650, 750 and 800 °C, respectively, with the thickness of the electrolyte ~300 μm. The cell shows very stable performance under a constant current density of 0.6 A/cm 2 for 204 h at 750 °C.
885
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
Acknowledgments
[14] [15]
Financial supports from the US NSF (1000068), the NSF of China (50730004), the Ministry of Science and Technology of China (2009 DFA61360) and the 111 project contract no. B08010 are greatly appreciated.
[16] [17] [18]
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Contents lists available at ScienceDirect
Electrochemistry Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / e l e c o m
Ba0.9Co0.7Fe0.2Nb0.1O3 − δ as cathode material for intermediate temperature solid oxide fuel cells Zhibin Yang a, b, Chenghao Yang b, Chao Jin b, Minfang Han a,⁎, Fanglin Chen b,⁎ a b
Union Research Center of Fuel Cell, School of Chemical & Environment Engineering, China University of Mining & Technology, Beijing, 100083, China Department of Mechanical Engineering, University of South Carolina, 300 Main Street, Columbia, SC 29208, USA
a r t i c l e
i n f o
Article history: Received 20 April 2011 Received in revised form 20 May 2011 Accepted 24 May 2011 Available online 1 June 2011 Keywords: Ba0.9Co0.7Fe0.2Nb0.1O3 − δ La0.8Sr0.2Ga0.83Mg0.17O3 − δ Solid oxide fuel cells Cathode
a b s t r a c t Ba0.9Co0.7Fe0.2Nb0.1O3 − δ (BCFN) perovskite material was synthesized and evaluated as cathode for La0.8Sr0.2Ga0.83Mg0.17O3 − δ (LSGM) electrolyte supported intermediate temperature solid oxide fuel cells (IT-SOFCs). X-ray diffraction results showed that BCFN was chemically compatible with the LSGM electrolyte. Maximum power densities of 0.36, 0.57, 0.80 and 1.1 W/cm2 were obtained for LSGM electrolyte supported cells with BCFN as cathode and Ni-GDC as anode operated at 650, 700, 750 and 800 °C, respectively. Further, the cell performance was stable under a constant current of 0.6 A/cm 2 for over 204 h at 750 °C. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Intermediate temperature solid oxide fuel cells (IT-SOFCs) have been considered to be promising energy conversion technology since they possess the merits of both high and low temperature fuel cells, such as low emissions, fuel flexibility, mitigated materials issues and low system cost [1]. Unfortunately, at reduced operating temperatures, cell resistance increases rapidly and is often dominated by interfacial polarization resistance between the cathode and the electrolyte [2]. Consequently, developing cathode with low cathode interfacial resistance is vital for successful development of IT-SOFCs. SrCoO3 − δ and BaCoO3 − δ-based perovskite mixed ionic and electronic conductors (MIECs) have been extensively investigated as cathode for IT-SOFCs because they can potentially extend the oxygen reduction reaction from the cathode/electrolyte physical interface region to the entire electrode surface [3,4]. Ba0.5Sr0.5Co0.8Fe0.2O3 − δ (BSCF) has been reported to be an excellent cathode material for IT-SOFCs in the last decade [5]. However, cobalt containing perovskite materials typically lack chemically stability and compatibility; for example, cobalt-based cathode tends to react with zirconia-based electrolyte, SrCo0.8Fe0.2O3 − δ can be easily transformed from perovskite to brownmillerite phase at low oxygen partial pressure [6], and BSCF can be easily disintegrated, especially under CO2 rich atmosphere at elevated temperatures [7,8]. Recently, it has been reported that introduction of Nb in the B-site of cobalt-based perovskites can significantly improve chemical stability and enhance oxygen permeability [9]. It has been demonstrated that barium cobalt
⁎ Corresponding authors. E-mail addresses: [email protected] (M. Han), [email protected] (F. Chen). 1388-2481/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2011.05.029
iron niobium oxide, BCFN can be a very promising cathode material for IT-SOFCs with ceria-based electrolyte, showing similar electrochemical activity to that of BSCF [10–12]. However, there has been no report on the cell performance stability of BCFN cathode [1]. On the other hand, La0.8Sr0.2Ga0.83Mg0.17O3 − δ (LSGM) has been considered as a promising electrolyte material for IT-SOFCs due to it higher ionic conductivity, negligible electronic conductivity and high chemical stability over a broad range of oxygen partial pressures [13]. Therefore, it will be valuable to assess the performance and stability of BCFN cathode for LSGM-electrolyte supported IT-SOFCs. It has been demonstrated that using BSCF as cathode for SOFCs, A-site deficiency in BSCF can create additional oxygen vacancies, facilitate oxygen reduction reaction and promote oxygen ion diffusion within the cathode material [5]. Consequently, in this work, A-site deficient BCFN, Ba0.9Co0.7Fe0.2Nb0.1O3 − δ has been systematically evaluated as cathode for LSGM-electrolyte supported IT-SOFCs. 2. Experimental Ba0.9Co0.7Fe0.2Nb0.1O3 − δ (BCFN) and LSGM powders were prepared through the solid-state reaction method [12]. La0.4Ce0.6O2 − δ (LDC) and Ce0.9Gd0.1O2 − δ (GDC) powders were prepared by a sol–gel process. LSGM powders were uniaxially pressed into pellets at 500 MPa and then sintered at 1450 °C for 10 h to obtain dense LSGM electrolyte. LDC ink was then screen-printed on the surface of the LSGM electrolyte and co-sintered at 1400 °C for 4 h to get a LDC buffer layer. BCFN/LSGM/LDC/Ni-GDC IT-SOFCs were finally prepared by screen-printing on the LSGM electrolytes with BCFN and Ni-GDC ink, respectively, and then co-fired at 1000 °C. The obtained button cells have 1.3 cm in diameter with an active electrode area of 0.33 cm2. Pt
Z. Yang et al. / Electrochemistry Communications 13 (2011) 882–885
883
0.15
paste and Pt wires were used as current collection and lead wire for both electrodes. Button cells were sealed on the anode side to one end of an alumina tube with a ceramic paste (Aremco-552). Ambient air was used as the oxidant in the cathode side. The phase structures of BCFN, LSGM and BCFN-LSGM powders were characterized using a Philips X-ray diffractometer (Model PW1830). The cell current voltage characteristics as well as the impedance spectra were measured with a four probe method using a multi-channel VersaSTAT (Princeton Applied Research). Scanning electron microscopy (SEM, FEI Quanta 200) operating at 30 kV was used to characterize the microstructure of BCFN/LSGM/LDC/Ni-GDC single cells after the cell performance test.
650 oC 700 oC 750 oC
-Z,
cm2
0.10
800 oC
0.05
0.00 0.00
3. Results and discussion
0.05
0.10
Fig. 1(a) shows the XRD patterns of LSGM, BCFN as well as LSGMBCFN mixed powders after being fired at 1000 °C for 10 h in air. No impurity phase has been detected for the heat-treated LSGM-BCFN mixed powders, indicating that BCFN is chemically compatible with the LSGM electrolyte. Fig. 1(b, c and d) show the cross-sectional SEM images of the BCFN/LSGM/LDC/Ni-GDC electrolyte supported single cell, the BCFN/LSGM as well as the LSGM/LDC/Ni-GDC interfaces. The thickness of the Ni-GDC anode is ~50 μm, the LDC membrane interlayer ~8 μm, the LSGM electrolyte ~300 μm and the BCFN cathode ~30 μm. The LSGM electrolyte is crack-free and gas-tight. There is a good contact between the porous BCFN cathode and the dense LSGM electrolyte after ~204 h short-term test. As can be seen from Fig. 2(b and d), the thin LDC interlayer adheres well both to the LSGM electrolyte and the Ni-GDC
a
0.15
0.20
cm2
Z,
Fig. 2. Impedance spectra of BCFN/LSGM/BCFN symmetrical cell tested in air at 650, 700, 750 and 800 °C, respectively.
anode, sufficiently dense to effectively prevent the solid-state reaction between Ni and LSGM. The BCFN electrode polarization resistance was evaluated at 650, 700, 750 and 800 °C in air, as shown in Fig. 2. The polarization resistance decreased with the increase in operating temperature from 0.18 Ωcm2 at 650 °C to 0.07, 0.04 and 0.02 Ωcm 2 at 700, 750 and 800 °C, respectively. The cathode interfacial resistance of BCFN is much smaller than that of LSCF cathode using Ce0.9Gd0.1O1.95 as the electrolyte
b
LSGM
Intensity (a.u.)
BCFN LSGM + BCFN
20
30
40
50
60
70
80
10 m
2θ (degree)
c
d
BCFN
LSGM
10 m
LSGM
LDC
Ni-GDC
10 m
Fig. 1. (a) X-ray diffraction patterns of BCFN and LSGM powder as well as BCFN-LSGM powder mixture after being fired at 1000 °C in air for 10 h, and SEM images of the tested cell: (b) cross-section of BCFN/LSGM/LCD/Ni-GDC electrolyte supported cell, (c) BCFN/LSGM interface and (d) LSGM/LDC/Ni-GDC interface.
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Z. Yang et al. / Electrochemistry Communications 13 (2011) 882–885
material (~0.1 Ωcm2 at 800 °C) [14] and BSCF cathode using LSGM as the electrolyte material (~0.1 Ωcm2 at 800 °C) [15], but comparable to that of LSCF-LSGM composite cathode using LSGM as the electrolyte material (0.19–0.24 Ωcm2 at 650 °C) [16]. The low cathode polarization resistance of BCFN observed in this work indicates that BCFN can be a potentially very promising cathode for IT-SOFCs. Fig. 3(a) shows the impedance spectra of BCFN/LSGM/LDC/Ni-GDC electrolyte supported single cell for different H2 flow rates from 30 to 70
a
0.15
(i) H2 flow rate
Rtotal
30 sccm 50 sccm 70 sccm
0.10
Rohmic RHF
0.387 0.381 0.375
0.216 0.215 0.213
0.088 0.087 0.086
RLF 0.083 0.080 0.076
0.05
H2- 50 sccm
H 2- 70 sccm
-Z ,
cm2
o
750 C H 2- 30 sccm
0.00 Rohmic
(ii) -0.05 0.20
RHF
RLF
CPEHF
CPELF
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0.30
b
0.35
0.40
cm2
Z,
0.6
(iii) Temperature 650 oC 700 oC 750 oC 800 oC
Rohmic RHF
RLF
0.849 0.533 0.375 0.287
0.495 0.308 0.213 0.160
0.087 0.081 0.076 0.074
0.267 0.144 0.086 0.053
-Z ,
cm2
0.4
Rtotal
o
0.2
o
650 C 700 C o o 750 C 800 C H2-70 sccm
0.0 0.2
0.4
c
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Z,
cm2
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standard cubic centimeters per minute (sccm) at 750 °C. It can be seen that total cell resistance (Rtotal) decreased with the increase in H2 flow rate. Shown in the insets in Fig. 3(a) are the proposed equivalent circuit (inset ii) used to fit the experiment data (using the ZView software) and the fitting results (inset i), where CPEHF and CPELF refer to the highfrequency and low-frequency constant phase element, respectively. The impedance spectra were characterized by a small arc at high frequencies and a small arc at low frequencies. Ohmic resistance (Rohmic) was dominating in the Rtotal due to the electrolyte supported cell configuration tested in the work and the thickness of the LSGM electrolyte is ~300 μm. The polarization resistance corresponding to the high-frequency arc (RHF) seemed to be insensitive to the H2 flow rate, indicating that the RHF may be related to the cathode process. However, the polarization resistance corresponding to the low-frequency arc (RLF) decreased with the increase in H2 flow rate, indicating that RLF may be related to the anode of the cell and the gas diffusion process in porous Ni-GDC anode side is enhanced by increasing H2 flow rate. This is consistent to the phenomenon reported in the literature that the higherfrequency arc is typically associated with the O2−transfer while the lower-frequency arc is normally associated with gas diffusion in the electrode [15]. Fig. 3(b) shows the impedance spectra of the cell tested at different temperatures from 650 to 800 °C, with 70 sccm H2 flow rate. It can be seen that Rtotal and Rohmic decreased with the increasing temperature. However, RHF decreased significantly with the increase in temperature while RLF kept nearly the same. A careful examination of the impedance spectra data (in the inset iii) with the equivalent fitting circuit (in the inset ii) revealed that RHF associated with O 2−charge transfer in the electrode dominated the overall electrode kinetic at lower temperature, however, it decreased substantially with the increasing temperature. At 800 °C, RHF decreased to 0.053 Ωcm2 and was much smaller than RLF, which is associated with the gas-diffusion process. Cell maximum output power density reached 1.1 W/cm2 at 800 °C as shown in Fig.3(c), which is much higher than that of cells of Ni-SDC/LDC/LSGM/LSGMLSCF with the thickness of the LSGM electrolyte of ~30 μm (0.9 W/cm 2 at 800 °C) [17] but similar to that of cells of Ni-GDC/LDC/LSGM/LSC-LDC (LSC: La0.9Sr0.1CoO3) (1.1 W/cm2 at 800 °C) with the thickness of the LSGM electrolyte of ~60 μm [18], although the thickness of the LSGM electrolyte is ~300 μm in the present work. Much higher cell output power density is expected if thinner LSGM electrolyte is used in this work. Consequently, BCFN possesses better cathode performance than LSCF and LSC and is a very promising cathode for IT-SOFCs. The stability of LSGM electrolyte supported IT-SOFC with BCFN as cathode has been further examined in a short-term cell test. Fig. 4 shows the cell voltage as a function of operation time at a constant
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Time, h Fig. 3. (a) Impedance spectra of BCFN/LSGM/LDC/Ni-GDC tested at 750 °C with different H2 flow rate; (b and c) impedance spectra and V–I curves of BCFN/LSGM/LDC/Ni-GDC tested from 650 °C to 800 °C with 70 sccm H2 flow rate.
Fig. 4. Cell voltage as a function of time of BCFN/LSGM/LDC/Ni-GDC tested at 750 °C under constant current of 0.6 A/cm2.
Z. Yang et al. / Electrochemistry Communications 13 (2011) 882–885
current density of 0.6 A/cm 2 at 750 °C with 60 sccm H2 flow rate. The cell showed very stable performance with no significant cell voltage degradation in the 204 h testing period. 4. Conclusions A-site deficient BCFN perovskite material has been synthesized and evaluated as cathode in IT-SOFCs with the configuration of BCFN/LSGM/ LDC/Ni-GDC. The BCFN cathode polarization resistances are 0.18, 0.07, 0.04 and 0.02 Ωcm2 at 650, 700, 750 and 800 °C, respectively. High cell power output densities of 0.36, 0.57, 0.80 and 1.1 W/cm 2 have been achieved at 550, 650, 750 and 800 °C, respectively, with the thickness of the electrolyte ~300 μm. The cell shows very stable performance under a constant current density of 0.6 A/cm 2 for 204 h at 750 °C.
885
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
Acknowledgments
[14] [15]
Financial supports from the US NSF (1000068), the NSF of China (50730004), the Ministry of Science and Technology of China (2009 DFA61360) and the 111 project contract no. B08010 are greatly appreciated.
[16] [17] [18]
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