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Layered PrBaCo2O5+δ perovskite as a cathode for proton-conducting solid oxide fuel cells
Journal of Alloys and Compounds 494 (2010) 359–361
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Layered PrBaCo2 O5+ı perovskite as a cathode for proton-conducting solid oxide fuel cells Meifang Jin a,∗ , Xiuling Zhang b , Yu’e Qiu b , Jianmin Sheng c a b c
Department of Chemical and Environmental Engineering, Anyang Institute of Technology, Anyang 455000, China Department of Chemistry, Dezhou University, Dezhou 253011, China Medical Department of Dezhou University, West University Road, No. 566, Dezhou, 253011, China
a r t i c l e
i n f o
Article history: Received 1 December 2009 Received in revised form 7 January 2010 Accepted 9 January 2010 Available online 18 January 2010 Keywords: Solid oxide fuel cells BaCe0.5 Zr0.3 Y0.16 Zn0.04 O3−ı Layered perovskite Cathode
a b s t r a c t The layered PrBaCo2 O5+ı (PBCO) perovskite oxides were synthesized by modified Pechini method and investigated as a cathode material for solid oxide fuel cells (SOFCs) based on a stable and easily sintered perovskite oxide BaCe0.5 Zr0.3 Y0.16 Zn0.04 O3−ı (BCZYZ) as electrolyte. The fabricated single cell of NiO-BCZYZ/BCZYZ (∼20 m)/PBCO was operated from 550 to 700 ◦ C with humidified hydrogen (∼5% H2 O) as fuel and the static air as oxidant. The BCZYZ perovskite electrolyte was completely dense after sintered at 1250 ◦ C for 5 h, lower than that without zinc dopant about 150 ◦ C. A high open-circuit potential of 1.007 V, a peak power density of 361 mW cm−2 , and a low polarization resistance of the electrodes of 0.12 cm2 was achieved at 700 ◦ C. The ratio of polarization resistance to total cell resistance decreased with the increase of operating temperature, from 54.2% at 550 ◦ C to 17.9% at 700 ◦ C, respectively. The experimental results indicated that PBCO is a promising cathode material for proton-conducting SOFCs. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Solid oxide fuel cells (SOFCs) are attracting more and more attention, as a new electric power generation system with high energy conversion efficiency, all solid state components, less emission of pollutant and excellent fuel flexibility [1,2]. At present, intermediate temperature SOFCs (IT-SOFCs) represent the research mainstream in this realm. Typical SOFCs are based on oxygen-ionconducting electrolyte [3–5]. Recently, there is now considerable interests in proton-conducting oxide electrolytes for SOFCs at reduced-temperature range, which exhibit more advantages than that based on oxygen-ion-conducting electrolyte, such as low activation energy [6] and water produced in cathode side which will not dilute the fuel concentration in anode side [7]. Many perovskitetype (ABO3 ) oxides show high proton conductivity in reducing atmosphere. One of the major challenges for this type of proton conductor is a proper compromise between conductivity and chemical stability. Generally, doped-barium cerates exhibit the highest proton conductivity in humid atmosphere. However, the samples easily degrade when exposed to practical operating conditions due to the formation of insulating barium carbonate (BaCO3 ) and cerium oxide (CeO2 ) [8–10]. The stability of doped-barium cer-
∗ Corresponding author. Tel.: +86 372 3221970. E-mail address: [email protected] (M. Jin). 0925-8388/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2010.01.040
ates can be improved by the dopant of Zr element at B site in order to achieve high proton conductivity and sufficient chemical stability in practical operation conditions. However, barium zirconates must be sintered at a high temperature to be dense due to its low sintering activity. Some research groups [11,12] introduced Zn into Zr doped BaCeO3 for lowering sintering temperature. The results show that it is an effective way to obtain a stable proton-conducting electrolyte BaCe0.5 Zr0.3 Y0.16 Zn0.04 O3−ı (BCZYZ) with lower sintering temperature and good chemical stability by co-doping of Zr and Zn in the lattice. In this work, 20 m dense BCZYZ electrolyte was fabricated on porous anode substrates by simple dry-pressing method. An issue of significant importance for the development of ITSOFCs is to select a proper cathode materials. At present, a key obstacle to reduced-temperature operation of SOFCs is the relative poor activity of traditional cathode materials for electrochemical reduction of oxygen in the lower temperature range. Many simple perovskite-type mixed ionic-electronic conductors (MIEC) such as doped Sm0.5 Sr0.5 CoO3 [13,14], LaFeO3 [15] or BaCoO3 [16,17] have been extensively studied as possible cathode materials, however not much attention has been paid to the perovskite related structures such as the layered perovskite. In general, layered perovskite oxides exhibit lower activity energy for oxygen-ion mobility compared with simple perovskite-type conductors. Recently, Kim et al. [18] and Zhang et al. [19] reported that PrBaCo2 O5+ı (PBCO) has high bulk diffusion coefficient and surface exchange coefficient,
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M. Jin et al. / Journal of Alloys and Compounds 494 (2010) 359–361
Fig. 1. XRD diffraction patterns for (a) the layered PBCO perovskite powders and (b) BCZYZ powders. Fig. 3. Performance of the single cell with humidified hydrogen measured at 550, 600, 650 and 700 ◦ C.
showing the potential as cathode material for proton-conducting SOFCs. In this work, layered PBCO perovskite was examined as a new cathode for SOFCs based on proton-conducting BZCYZ electrolyte. 2. Experimental Both BCZYZ and PBCO powders were synthesized by the modified Pechini method with citrate and ethylenediamine tetraacetic acid (EDTA) as parallel complexing agents. The synthesis of BZCYZ, as an example, is described as follows. Y2 O3 and ZnO were dissolved in nitric acid first, and stoichiometric amounts of Ba(NO3 )2 ·9H2 O (99.9%), Ce(NO3 )3 ·6H2 O (99.9%), Zr(NO3 )4 ·4H2 O (99.9%) were dissolved in EDTA-NH3 aqueous solution. After agitation for a certain time, a proper amount of citric acid was introduced with the molar ratio of EDTA: citric acid: total of metal cations controlled at 1:1.5:1. The original powders were afterwards calcined in air at 1100 ◦ C for 5 h. The anode-supported bi-layer was prepared by a dry-pressing method. NiO, BCZYZ and starch mixture (65%:35%:20% in weight) was pre-pressed at 250 MPa as substrate. The BCZYZ powders synthesized above were uniformly distributed onto the anode substrate, then co-pressed at 250 MPa to form bi-layer cell and sintered subsequently at 1250 ◦ C for 5 h. The PBCO cathode slurry was then painted on BCZYZ electrolyte films, and sintered at 950 ◦ C for 5 h to form single cells of NiO-BCZYZ/BCZYZ/PBCO. The final geometry of cell pellets is approximately 11 mm in diameter and active area of cathode is about 0.5 cm2 . The crystal structure of prepared BCZYZ electrolyte and PBCO cathode powders were studied with the powder X-ray diffraction by Cu-K␣ radiation. After being sealed, the single cells were tested in an electrical furnace with humidified hydrogen (∼5% H2 O) as fuel and the static air as oxidant, respectively. The power output performance and ac impedance spectroscopy under open-circuit condition were obtained from 550 to 700 ◦ C. The frequency range was 0.01–105 Hz with the signal amplitude of 10 mV. The morphology of the single cell after the electrochemical tests was characterized by a scanning electron microscope (SEM).
3. Results and discussion As shown in Fig. 1(a), the PBCO powder exhibits a layered perovskite phase without peaks attributable to other impurities [18]. Fig. 1(b) presents the XRD spectra of BCZYZ electrolyte calcined at 1100 ◦ C for 5 h. The diffraction peaks were identical with those of the barium cerate standard (JCPDS Card No. 82-2425), which were also identical with those of the BCZYZ in other literature [12]. Fig. 2(a) shows the SEM image of surface morphology of BCZYZ electrolyte on the porous anode support after testing. It can be seen that the electrolyte membrane is completely dense after sintering at 1250 ◦ C. There is no pores and cracks on the surface. The result indicates that sintering temperature is lowered significantly by doping zinc at B site. The introduction of zinc into the lattices of barium zirconates and cerates decreases the primitive perovskite cell volume by about 4%, which is the indication of solid-solution formation [12]. From the cross-section view of single cell (Fig. 2(b)), it is found that the BCZYZ film is only about 20 m-thick, and adheres very well to the anode substrate and the cathode layer. The effective contact indicates that PBCO is compatible with electrolyte in TEC even after several thermal cycles. Fig. 3 presents the I–V and I–P characteristics of NiBCZYZ/BCZYZ/PBCO cell using H2 as the fuel and static air as the oxidant in the temperature range of 550–700 ◦ C. In H2 –O2 system, the exchange current density in anode is 105 times than that of
Fig. 2. SEM micrographs of cell after testing: (a) the surface of electrolyte and (b) the cross-section of cell with a 20 m-thick BCZYZ membrane.
M. Jin et al. / Journal of Alloys and Compounds 494 (2010) 359–361
Fig. 4. Impedance spectra of the single cell measured at 550, 600, 650 and 700 ◦ C under open-circuit condition.
361
and interfacial polarization resistance (Rp ) as determined from the impedance spectra at different temperatures are shown in Fig. 5. The high frequency intercept is corresponded to Ro which includes ionic resistance of the electrolyte, electronic resistance of the electrodes and some contact resistances associated with interfaces [16]. The low frequency intercept corresponds to Rt . As expected, the increase of operating temperature resulted in a significant reduction of Rp , typically from 1.15 cm2 at 550 ◦ C to 0.12 cm2 at 700 ◦ C. Furthermore, Fig. 5 showed that Rt is mainly dominated by Rp . The ratio of Rp to Rt increases with a decrease in the operating temperature, from 17.9% at 700 ◦ C to 54.2% at 550 ◦ C, implying that Rt is dominated by the cathode–electrolyte interface at low temperature range since the resistance of the anode–electrolyte is negligible for excellent catalytic activity of Ni in anode. At 550 ◦ C, the polarization resistance of the electrodes is 1.15 cm2 whereas the resistance of the electrolyte is only 0.97 cm2 . It is worthwhile to reduce cathode–electrolyte interfacial resistance for cell performance improvement in our future studies. 4. Conclusions In this work, layered perovskite PBCO oxide was investigated as a cathode for proton-conducting SOFCs with an easily sintered BCZYZ electrolyte. The tri-layer single cells were fabricated by a simple dry-pressing/co-firing process with the structure of NiO-BCZYZ/BCZYZ/PBCO and tested from 550 to 700 ◦ C fed with humidified H2 (∼5% H2 O). The cell shows a maximal power density of 361, 266, 194 and 125 mW cm−2 at 700, 650, 600 and 550 ◦ C, respectively. The polarization resistance of the electrodes was as low as 0.12 cm2 at 700 ◦ C. These results indicated that the PBCO cathode is a good candidate in lower temperature range due to the high activation for oxygen reduction. References
Fig. 5. (a) Rp , Ro , and Rt determined from the impedance spectra of the cell measured under open-circuit condition at different temperatures. The ratio of Rp /Rt is shown in (b).
cathode. Therefore the activation polarization of cathode affects the cell performance significantly and that of anode is negligible. The almost linear I–V behavior is mainly caused by the high ohmic resistance from electrolyte. At high current density region, the absence of I–V curve slope change also implies that the concentration polarization is not present in practical operation. The open-circuit voltages (OCV) was 1.007, 1.021, 1.042 and 1.059 V at 700, 650, 600 and 550 ◦ C, respectively which was close to the theoretical values 1.141, 1.147, 1.152 and 1.159 V at 700, 650, 600 and 550 ◦ C which were calculated by Nernst equation, indicating that the electrolyte membrane was sufficiently gastight. Since the electrolyte is not a pure proton-conducting oxide, current leakage might take place. The OCV value is also related with denseness of electrolyte resulting in fuel-oxidant cross flow. The maximal power densities were 361, 266, 194 and 125 mW cm−2 at 700, 650, 600 and 550 ◦ C, respectively. The impedance spectra of the as-prepared cells are obtained under open-circuit conditions at different temperatures, and are shown in Fig. 4. The total cell resistance (Rt ), ohmic resistance (Ro ),
[1] N.Q. Minh, J. Am. Ceram. Soc. 76 (3) (1993) 563. [2] Q. Ma, R. Peng, L. Tian, G. Meng, Electrochem. Commun. 8 (2006) 1791. [3] C. Zhou, X. Liu, C. Yi, L. Pei, D. Yan, J. Niu, D. Wang, W. Su, Electrochem. Commun. 11 (2009) 958. [4] F. Zhao, Z. Wang, M. Liu, L. Zhang, C. Xia, F. Chen, J. Power Sources 185 (2008) 13. [5] M. Liu, R. Peng, D. Dong, J. Gao, X. Liu, G. Meng, J. Power Sources 185 (2008) 188. [6] A.F. Sammells, R.L. Cook, J.H. White, J.J. Osborne, R.C. MacDuff, Solid State Ionics 52 (1992) 111. [7] L. Yang, C. Zuo, S. Wang, Z. Cheng, M. Liu, Adv. Mater. 20 (2008) 3280. [8] N. Bonanos, K.S. Knight, B. Ellis, Solid State Ionics 79 (1995) 161. [9] S. Gopalan, A.V. Virkar, J. Electrochem. Soc. 140 (1993) 1060. [10] U. Balachandran, Proton-conducting Membranes, Annual Report for FY2001, Argonne National Laboratory, 2001. [11] P. Babilo, S.M. Haile, J. Am. Ceram. Soc. 88 (2005) 2362. [12] S.W. Tao, J.T.S. Irvine, Adv. Mater. 18 (2006) 1581. [13] S. McIntosh, J.F. Vente, W.G. Haije, D.H.A. Blank, H.J.M. Bouwmeester, Chem. Mater. 18 (2006) 2187. [14] S. McIntosh, J.F. Vente, W.G. Haije, D.H.A. Blank, H.J.M. Bouwmeester, Solid State Ionics 177 (2006) 1737. [15] H. Ding, B. Lin, X. Liu, G. Meng, Electrochem. Commun. 10 (2008) 1389. [16] K. Yoon, W. Huang, G. Ye, S. Gopalan, U. Pal, D.A. Seccombe, J. Electrochem. Soc. 154 (2007) B389. [17] L. Zhang, S. Jiang, W. Wang, Y. Zhang, J. Power Sources 170 (2007) 55. [18] G. Kim, S. Wang, A. Jacobson, L. Reimus, P. Brodersen, C. Mims, J. Mater. Chem. 17 (2007) 2500. [19] K. Zhang, L. Ge, R. Ran, Z.P. Shao, S.M. Liu, Acta Mater. 56 (2008) 4876.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Layered PrBaCo2 O5+ı perovskite as a cathode for proton-conducting solid oxide fuel cells Meifang Jin a,∗ , Xiuling Zhang b , Yu’e Qiu b , Jianmin Sheng c a b c
Department of Chemical and Environmental Engineering, Anyang Institute of Technology, Anyang 455000, China Department of Chemistry, Dezhou University, Dezhou 253011, China Medical Department of Dezhou University, West University Road, No. 566, Dezhou, 253011, China
a r t i c l e
i n f o
Article history: Received 1 December 2009 Received in revised form 7 January 2010 Accepted 9 January 2010 Available online 18 January 2010 Keywords: Solid oxide fuel cells BaCe0.5 Zr0.3 Y0.16 Zn0.04 O3−ı Layered perovskite Cathode
a b s t r a c t The layered PrBaCo2 O5+ı (PBCO) perovskite oxides were synthesized by modified Pechini method and investigated as a cathode material for solid oxide fuel cells (SOFCs) based on a stable and easily sintered perovskite oxide BaCe0.5 Zr0.3 Y0.16 Zn0.04 O3−ı (BCZYZ) as electrolyte. The fabricated single cell of NiO-BCZYZ/BCZYZ (∼20 m)/PBCO was operated from 550 to 700 ◦ C with humidified hydrogen (∼5% H2 O) as fuel and the static air as oxidant. The BCZYZ perovskite electrolyte was completely dense after sintered at 1250 ◦ C for 5 h, lower than that without zinc dopant about 150 ◦ C. A high open-circuit potential of 1.007 V, a peak power density of 361 mW cm−2 , and a low polarization resistance of the electrodes of 0.12 cm2 was achieved at 700 ◦ C. The ratio of polarization resistance to total cell resistance decreased with the increase of operating temperature, from 54.2% at 550 ◦ C to 17.9% at 700 ◦ C, respectively. The experimental results indicated that PBCO is a promising cathode material for proton-conducting SOFCs. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Solid oxide fuel cells (SOFCs) are attracting more and more attention, as a new electric power generation system with high energy conversion efficiency, all solid state components, less emission of pollutant and excellent fuel flexibility [1,2]. At present, intermediate temperature SOFCs (IT-SOFCs) represent the research mainstream in this realm. Typical SOFCs are based on oxygen-ionconducting electrolyte [3–5]. Recently, there is now considerable interests in proton-conducting oxide electrolytes for SOFCs at reduced-temperature range, which exhibit more advantages than that based on oxygen-ion-conducting electrolyte, such as low activation energy [6] and water produced in cathode side which will not dilute the fuel concentration in anode side [7]. Many perovskitetype (ABO3 ) oxides show high proton conductivity in reducing atmosphere. One of the major challenges for this type of proton conductor is a proper compromise between conductivity and chemical stability. Generally, doped-barium cerates exhibit the highest proton conductivity in humid atmosphere. However, the samples easily degrade when exposed to practical operating conditions due to the formation of insulating barium carbonate (BaCO3 ) and cerium oxide (CeO2 ) [8–10]. The stability of doped-barium cer-
∗ Corresponding author. Tel.: +86 372 3221970. E-mail address: [email protected] (M. Jin). 0925-8388/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2010.01.040
ates can be improved by the dopant of Zr element at B site in order to achieve high proton conductivity and sufficient chemical stability in practical operation conditions. However, barium zirconates must be sintered at a high temperature to be dense due to its low sintering activity. Some research groups [11,12] introduced Zn into Zr doped BaCeO3 for lowering sintering temperature. The results show that it is an effective way to obtain a stable proton-conducting electrolyte BaCe0.5 Zr0.3 Y0.16 Zn0.04 O3−ı (BCZYZ) with lower sintering temperature and good chemical stability by co-doping of Zr and Zn in the lattice. In this work, 20 m dense BCZYZ electrolyte was fabricated on porous anode substrates by simple dry-pressing method. An issue of significant importance for the development of ITSOFCs is to select a proper cathode materials. At present, a key obstacle to reduced-temperature operation of SOFCs is the relative poor activity of traditional cathode materials for electrochemical reduction of oxygen in the lower temperature range. Many simple perovskite-type mixed ionic-electronic conductors (MIEC) such as doped Sm0.5 Sr0.5 CoO3 [13,14], LaFeO3 [15] or BaCoO3 [16,17] have been extensively studied as possible cathode materials, however not much attention has been paid to the perovskite related structures such as the layered perovskite. In general, layered perovskite oxides exhibit lower activity energy for oxygen-ion mobility compared with simple perovskite-type conductors. Recently, Kim et al. [18] and Zhang et al. [19] reported that PrBaCo2 O5+ı (PBCO) has high bulk diffusion coefficient and surface exchange coefficient,
360
M. Jin et al. / Journal of Alloys and Compounds 494 (2010) 359–361
Fig. 1. XRD diffraction patterns for (a) the layered PBCO perovskite powders and (b) BCZYZ powders. Fig. 3. Performance of the single cell with humidified hydrogen measured at 550, 600, 650 and 700 ◦ C.
showing the potential as cathode material for proton-conducting SOFCs. In this work, layered PBCO perovskite was examined as a new cathode for SOFCs based on proton-conducting BZCYZ electrolyte. 2. Experimental Both BCZYZ and PBCO powders were synthesized by the modified Pechini method with citrate and ethylenediamine tetraacetic acid (EDTA) as parallel complexing agents. The synthesis of BZCYZ, as an example, is described as follows. Y2 O3 and ZnO were dissolved in nitric acid first, and stoichiometric amounts of Ba(NO3 )2 ·9H2 O (99.9%), Ce(NO3 )3 ·6H2 O (99.9%), Zr(NO3 )4 ·4H2 O (99.9%) were dissolved in EDTA-NH3 aqueous solution. After agitation for a certain time, a proper amount of citric acid was introduced with the molar ratio of EDTA: citric acid: total of metal cations controlled at 1:1.5:1. The original powders were afterwards calcined in air at 1100 ◦ C for 5 h. The anode-supported bi-layer was prepared by a dry-pressing method. NiO, BCZYZ and starch mixture (65%:35%:20% in weight) was pre-pressed at 250 MPa as substrate. The BCZYZ powders synthesized above were uniformly distributed onto the anode substrate, then co-pressed at 250 MPa to form bi-layer cell and sintered subsequently at 1250 ◦ C for 5 h. The PBCO cathode slurry was then painted on BCZYZ electrolyte films, and sintered at 950 ◦ C for 5 h to form single cells of NiO-BCZYZ/BCZYZ/PBCO. The final geometry of cell pellets is approximately 11 mm in diameter and active area of cathode is about 0.5 cm2 . The crystal structure of prepared BCZYZ electrolyte and PBCO cathode powders were studied with the powder X-ray diffraction by Cu-K␣ radiation. After being sealed, the single cells were tested in an electrical furnace with humidified hydrogen (∼5% H2 O) as fuel and the static air as oxidant, respectively. The power output performance and ac impedance spectroscopy under open-circuit condition were obtained from 550 to 700 ◦ C. The frequency range was 0.01–105 Hz with the signal amplitude of 10 mV. The morphology of the single cell after the electrochemical tests was characterized by a scanning electron microscope (SEM).
3. Results and discussion As shown in Fig. 1(a), the PBCO powder exhibits a layered perovskite phase without peaks attributable to other impurities [18]. Fig. 1(b) presents the XRD spectra of BCZYZ electrolyte calcined at 1100 ◦ C for 5 h. The diffraction peaks were identical with those of the barium cerate standard (JCPDS Card No. 82-2425), which were also identical with those of the BCZYZ in other literature [12]. Fig. 2(a) shows the SEM image of surface morphology of BCZYZ electrolyte on the porous anode support after testing. It can be seen that the electrolyte membrane is completely dense after sintering at 1250 ◦ C. There is no pores and cracks on the surface. The result indicates that sintering temperature is lowered significantly by doping zinc at B site. The introduction of zinc into the lattices of barium zirconates and cerates decreases the primitive perovskite cell volume by about 4%, which is the indication of solid-solution formation [12]. From the cross-section view of single cell (Fig. 2(b)), it is found that the BCZYZ film is only about 20 m-thick, and adheres very well to the anode substrate and the cathode layer. The effective contact indicates that PBCO is compatible with electrolyte in TEC even after several thermal cycles. Fig. 3 presents the I–V and I–P characteristics of NiBCZYZ/BCZYZ/PBCO cell using H2 as the fuel and static air as the oxidant in the temperature range of 550–700 ◦ C. In H2 –O2 system, the exchange current density in anode is 105 times than that of
Fig. 2. SEM micrographs of cell after testing: (a) the surface of electrolyte and (b) the cross-section of cell with a 20 m-thick BCZYZ membrane.
M. Jin et al. / Journal of Alloys and Compounds 494 (2010) 359–361
Fig. 4. Impedance spectra of the single cell measured at 550, 600, 650 and 700 ◦ C under open-circuit condition.
361
and interfacial polarization resistance (Rp ) as determined from the impedance spectra at different temperatures are shown in Fig. 5. The high frequency intercept is corresponded to Ro which includes ionic resistance of the electrolyte, electronic resistance of the electrodes and some contact resistances associated with interfaces [16]. The low frequency intercept corresponds to Rt . As expected, the increase of operating temperature resulted in a significant reduction of Rp , typically from 1.15 cm2 at 550 ◦ C to 0.12 cm2 at 700 ◦ C. Furthermore, Fig. 5 showed that Rt is mainly dominated by Rp . The ratio of Rp to Rt increases with a decrease in the operating temperature, from 17.9% at 700 ◦ C to 54.2% at 550 ◦ C, implying that Rt is dominated by the cathode–electrolyte interface at low temperature range since the resistance of the anode–electrolyte is negligible for excellent catalytic activity of Ni in anode. At 550 ◦ C, the polarization resistance of the electrodes is 1.15 cm2 whereas the resistance of the electrolyte is only 0.97 cm2 . It is worthwhile to reduce cathode–electrolyte interfacial resistance for cell performance improvement in our future studies. 4. Conclusions In this work, layered perovskite PBCO oxide was investigated as a cathode for proton-conducting SOFCs with an easily sintered BCZYZ electrolyte. The tri-layer single cells were fabricated by a simple dry-pressing/co-firing process with the structure of NiO-BCZYZ/BCZYZ/PBCO and tested from 550 to 700 ◦ C fed with humidified H2 (∼5% H2 O). The cell shows a maximal power density of 361, 266, 194 and 125 mW cm−2 at 700, 650, 600 and 550 ◦ C, respectively. The polarization resistance of the electrodes was as low as 0.12 cm2 at 700 ◦ C. These results indicated that the PBCO cathode is a good candidate in lower temperature range due to the high activation for oxygen reduction. References
Fig. 5. (a) Rp , Ro , and Rt determined from the impedance spectra of the cell measured under open-circuit condition at different temperatures. The ratio of Rp /Rt is shown in (b).
cathode. Therefore the activation polarization of cathode affects the cell performance significantly and that of anode is negligible. The almost linear I–V behavior is mainly caused by the high ohmic resistance from electrolyte. At high current density region, the absence of I–V curve slope change also implies that the concentration polarization is not present in practical operation. The open-circuit voltages (OCV) was 1.007, 1.021, 1.042 and 1.059 V at 700, 650, 600 and 550 ◦ C, respectively which was close to the theoretical values 1.141, 1.147, 1.152 and 1.159 V at 700, 650, 600 and 550 ◦ C which were calculated by Nernst equation, indicating that the electrolyte membrane was sufficiently gastight. Since the electrolyte is not a pure proton-conducting oxide, current leakage might take place. The OCV value is also related with denseness of electrolyte resulting in fuel-oxidant cross flow. The maximal power densities were 361, 266, 194 and 125 mW cm−2 at 700, 650, 600 and 550 ◦ C, respectively. The impedance spectra of the as-prepared cells are obtained under open-circuit conditions at different temperatures, and are shown in Fig. 4. The total cell resistance (Rt ), ohmic resistance (Ro ),
[1] N.Q. Minh, J. Am. Ceram. Soc. 76 (3) (1993) 563. [2] Q. Ma, R. Peng, L. Tian, G. Meng, Electrochem. Commun. 8 (2006) 1791. [3] C. Zhou, X. Liu, C. Yi, L. Pei, D. Yan, J. Niu, D. Wang, W. Su, Electrochem. Commun. 11 (2009) 958. [4] F. Zhao, Z. Wang, M. Liu, L. Zhang, C. Xia, F. Chen, J. Power Sources 185 (2008) 13. [5] M. Liu, R. Peng, D. Dong, J. Gao, X. Liu, G. Meng, J. Power Sources 185 (2008) 188. [6] A.F. Sammells, R.L. Cook, J.H. White, J.J. Osborne, R.C. MacDuff, Solid State Ionics 52 (1992) 111. [7] L. Yang, C. Zuo, S. Wang, Z. Cheng, M. Liu, Adv. Mater. 20 (2008) 3280. [8] N. Bonanos, K.S. Knight, B. Ellis, Solid State Ionics 79 (1995) 161. [9] S. Gopalan, A.V. Virkar, J. Electrochem. Soc. 140 (1993) 1060. [10] U. Balachandran, Proton-conducting Membranes, Annual Report for FY2001, Argonne National Laboratory, 2001. [11] P. Babilo, S.M. Haile, J. Am. Ceram. Soc. 88 (2005) 2362. [12] S.W. Tao, J.T.S. Irvine, Adv. Mater. 18 (2006) 1581. [13] S. McIntosh, J.F. Vente, W.G. Haije, D.H.A. Blank, H.J.M. Bouwmeester, Chem. Mater. 18 (2006) 2187. [14] S. McIntosh, J.F. Vente, W.G. Haije, D.H.A. Blank, H.J.M. Bouwmeester, Solid State Ionics 177 (2006) 1737. [15] H. Ding, B. Lin, X. Liu, G. Meng, Electrochem. Commun. 10 (2008) 1389. [16] K. Yoon, W. Huang, G. Ye, S. Gopalan, U. Pal, D.A. Seccombe, J. Electrochem. Soc. 154 (2007) B389. [17] L. Zhang, S. Jiang, W. Wang, Y. Zhang, J. Power Sources 170 (2007) 55. [18] G. Kim, S. Wang, A. Jacobson, L. Reimus, P. Brodersen, C. Mims, J. Mater. Chem. 17 (2007) 2500. [19] K. Zhang, L. Ge, R. Ran, Z.P. Shao, S.M. Liu, Acta Mater. 56 (2008) 4876.