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High performance Ni–Fe alloy supported SOFCs fabricated by low cost tape casting-screen printing-cofiring process
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 9 ( 2 0 1 4 ) 1 9 7 4 7 e1 9 7 5 2
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High performance NieFe alloy supported SOFCs fabricated by low cost tape casting-screen printing-cofiring process Kai Li, Xin Wang, Lichao Jia, Dong Yan*, Jian Pu, Bo Chi, Li Jian Center for Fuel Cell Innovation, State Key Laboratory for Coal Combustion, School of Materials Science and Engineering, Huazhong University of Science & Technology, Wuhan 430074, China
article info
abstract
Article history:
Novel NieFe alloy supported solid oxide fuel cells, with Ni cermet as functional anode,
Received 24 June 2014
La0.8Sr0.2MnO3 coated Ba0.5Sr0.5Co0.2Fe0.8O3 as cathode and Gd-doped Ce2O3 as electrolyte,
Received in revised form
are successfully fabricated by the cost effective method of tape casting-screen printing-
8 September 2014
cofiring. The NieFe porous substrate is obtained by reduction (in H2 at 650 C for 2 h) of
Accepted 26 September 2014
sintered NiO-10 wt% Fe2O3 consisting of NiO and NiFe2O4. The cell is subjected to evalua-
Available online 14 October 2014
tion in the aspects of electrochemical performance and redox capability at temperatures between 500 and 650 C. The result shows a peak power density of 1.04 W cm2 at 650 C.
Keywords:
Furthermore, the metal support cell exhibits excellent tolerance to redox cycles. Five redox
Solid oxide fuel cell
recycles for cells are operated at 600 C, which shows no significant degradation in open
Metal-support
circuit voltage and power density.
NieFe alloy
Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Redox
Introduction Different from conventional electrolyte- and electrodesupported solid oxide fuel cells (SOFCs), metal-supported SOFCs proposed firstly by Villarreal et al. [1] are built on a porous metal substrate, which possesses high thermal conductivity and ductility that improve cell temperature gradient and thermal shock resistance [2] as well as capability withstanding vibration and mechanical loading [3,4]. Ferritic stainless steels, such as SUS 430, 409, 410 and 441 [3], have been employed as the support material due to their low cost and thermal expansion coefficient (TEC) matching with other cell components. Perforated metal sheets [5e8] or porous metal [9] were used as supports on which cells were built by
vacuum plasma spraying (VPS) [10,11], pulsed laser deposition (PLD) [7,12], electrostatic spray assisted vapor deposition [13], or and co-sintering in reduced atmospheres [14]. Porous Ni has also been considered as the support material [15,16]; however, the TEC (16.5 106 k1) of Ni is not well matched with those of other cell components (10e13 106 k1), which hinders its utilization for this purpose. Adding Fe into Ni decreases the TEC of Ni [17]; therefore NieFe alloy has been selected as another kind of alternative support materials [17e20]. Porous NieFe alloy support is obtained by reducing porous NiOeFe2O3 formed by either screen printing [17] or die-pressing [18e20] and sintering; other cell components are applied on to the substrate by sputtering [17], slurry coating [18] or PLD [19,20]. In the present study, a different approach, tape castingscreen printing-cofiring, was employed to fabricate NieFe
* Corresponding author. Tel.: þ86 27 87557849; fax: þ86 27 87558142. E-mail address: [email protected] (D. Yan). http://dx.doi.org/10.1016/j.ijhydene.2014.09.146 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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alloy supported SOFCs. Mixed NiO and Fe2O3 powders were used as the materials for NiOeFe2O3 substrate made by tape casting. Compared with Y2O3 stabilized ZrO2 (YSZ), Gd-doped CeO2 (GDC) possesses higher ionic conductivity at lower temperatures and suitable for metal-supported cells [21e23]. Therefore NiO-GDC (Ce0.9Gd0.1O1.95) functional anode and GDC electrolyte were applied onto the substrate by screen-printing. The half cell (substrate/functional anode/electrolyte) was cofired; and functional cathode was then built on the electrolyte by screen-printing and sintering before reduction in H2. This fabrication method is suitable for mass production of large cells at low cost. The performance and redox stability of such prepared cells were evaluated at temperatures between 500 and 650 C.
Experimental Substrate preparation For preparation of the NiOeFe2O3 (10 wt%) substrate by tape casting, NiO (average particle size 1 mm, Haite Advanced Materials) and Fe2O3 (average particle size 0.5 mm, Sinopharm Chemical Reagent) were ball-milled at room temperature for 24 h in an organic mixed solvent consisting of xylene and ethanol with Menhaden fish oil (Richard E. Mistler, Inc.) dispersant and 5 wt% of corn starch as poor former. Organic binder and plasticizer (Solutia Inc.) were then added, followed by ball-milling for another 24 h. Such prepared slurry was degassed and tape cast to form a flexible green tape with a thickness of ~1.2 mm.
Cell fabrication The green tape was die-cut into discs with a diameter of 25 mm as the cell support, on which functional anode (50 wt% NiO-50 wt% GDC) and GDC (Ningbo Institute of Materials Technology and Engineering) electrolyte were screen-printed in sequence to form a green half cell. Dried in air at room temperature for 2 h, the green half cell was sintered at 1450 C in air for 5 h. A layer of novel composite cathode, La0.8Sr0.2MnO3-coated Ba0.5Sr0.5Co0.8Fe0.2O3 (LSM-BSCF) [24], was screen-printed on the sintered GDC electrolyte, followed by sintering in air at 1050 C for 3 h. The LSM-BSCF contained LSM and BSCF at a weight ratio of 50 to 50, possessing the merits of high electronic and ionic conductivity for the need of oxygen reduction reaction. The sintered cell was reduced in H2 at 650 C for 2 h to form porous NieFe alloy supported SOFC cell with an active area of 0.5 cm2. The porosity of sintered and reduced substrates was obtained by the Archimedes method.
composite sealant (Aremco Product, Inc.). Ni foam and Pt mesh were used as anode and cathode current collector, respectively. Impedance analysis of the cell was carried out using the four-probe method with Solartron 1260A under the conditions of open circuit and 10 mV excitation in the frequency range from 100 KHz to 0.01 Hz at temperatures between 450 C and 600 C. The cell voltage as a function of applied current density was measured by Solartron 1260A at temperatures from 500 to 650 C. In addition, 5 redox cycles were performed at 600 C to examine the redox capability of the cell.
Results and discussion Cell characterization Fig. 1 shows XRD patterns of as-sintered and reduced substrate. The sintered substrate was consisted of two phases of NiO and NiFe2O4; and reduced in H2 at 650 C for 2 h the substrate was converted to a single solid solution of NieFe alloy. Since Fe dissolved in Ni reduced the d-spacing of Ni lattice, the diffraction peaks of the NieFe alloy were left-side shifted compared to those of pure Ni, as shown by the insert in Fig. 1. Even though the reduction caused 9.4% cell shrinkage, the integrity of the reduced cell was well maintained, which indicates that the NieFe alloy supported cell is capable of redox cycles. In the sintered substrate, the pores were uniformly distributed with a pore size ranging from 5 to 10 mm (Fig. 2a); and the porosity was around 40%. After reduction, the substrate contained bimodal pores (Fig. 2b) with a much higher porosity of 62%. The bigger pores were inherited from those formed between the sintered oxide particles; and the smaller ones were formed due to the reduction of the oxide scaffold. Such highly porous microstructure of the reduced substrate is expected to be beneficial for fuel transport to
Cell characterization and testing Scanning electron microscope (SEM, FEI sirion 200) and X-ray diffraction (XRD, X'Pert Pro) were employed for examining microstructure/composition and identifying phases of the cell, respectively. The performance of prepared cells was evaluated with H2 as the fuel and air as the oxidant at a flow rate of 0.1 L min1. The disc-shaped cell (3.8 cm2) was hermetically sealed in the cell holder with a ceramic-glass
Fig. 1 e XRD patterns of as-sintered and reduced NiO-10 wt % Fe2O3 support. The reduction was performed at 650 C in H2 for 2 h.
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Fig. 2 e Cross-sectional microstructure of NiO-10 wt% Fe2O3 support: (a) sintered at 1450 C in air for 5 h; (b) reduced at 650 C for 2 h.
Fig. 3 e Cross-sectional microstructure of reduced NieFe alloy supported cell (a) and surface microstructure of sintered GDC electrolyte (b).
and steam removal from the anode, resulting in low concentration polarization [25] and in turn high cell power output. Fig. 3 shows the microstructure of cross-sectioned cell (Fig. 3a) and the surface of GDC electrolyte (Fig. 3b). The thickness of the metallic support was around 1 mm, and that of the functional anode, electrolyte and cathode was 15, 10 and 7 mm, respectively. Each layer in the cell was well adhered to each other after reduction. The GDC electrolyte was fully dense with a grain size between 2 and 20 mm. It is expected that high cell performance can be achieved due to the low ohmic resistance associated with the thin thickness of the cell components and the intimate contact between the layers.
at 500 C to 1.04 WW cm2 at 650 C, which is higher than that previously reported for metal-supported cells with GDC electrolyte at 650 C [27]. Fig. 5 shows the measured electrochemical impedance spectra (Fig. 5a) and derived resistances (Fig. 5b, RT: total resistance, RU: ohmic resistance, RP: polarization resistance) of the metallic support cell at various temperatures. The RU is the sum of ohmic resistances contributed
Cell performance Fig. 4 shows the currentevoltage (IeV) curves of the NieFe alloy supported cell at temperatures range from 500 to 650 C. Open circuit voltage (OCV) of the cell decreased with temperature from 0.88 for 500 C to 0.78 V for 650 C. This tendency is predictable according to the Nernst equation. And the lower OCV values than the Nernst potential (~1.2 V) is caused by the electronic conducting behavior of GDC electrolyte in low oxygen partial pressure atmosphere [26,27]. Even though the OCV value was relatively low, high cell performance was still achieved. The peak power density increased from 0.47 W cm2
Fig. 4 e Electrochemical performance of NieFe alloy supported cell at temperatures between 500 C and 650 C with H2 as the fuel and air as the oxidant.
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Fig. 6 e Open circuit voltage of NieFe alloy supported cell after each redox cycle at 600 C.
Fig. 5 e Total (RT), ohmic (RU) and polarization (RP) resistances of the cell as a function of testing temperature obtained from measured electrochemical impedance spectra at open circuit voltage and various temperatures between 500 and 650 C.
by the cell support, electrolyte, electrodes and contact between the layers; and the RP characterizes the electrocatalytic activity of electrodes. The RU and RP decreased from 0.12 and 0.33 U cm2 at 500 C to 0.06 and 0.04 U cm2 650 C, respectively, resulting in a low value of total resistance between 0.10 and 0.45 U cm2 in the tested temperature range. The low polarization resistance suggests that the selected electrode materials, especially the novel LSM-BSCF cathode material, were extremely active for electrochemical reactions occurred in the electrodes. And the low ohmic resistance implies that the cell was well prepared by the process of tape cast-screen printingcofiring, with robust contact between cell components. It was the low cell total resistance that assured the high cell performance. The most vulnerable aspect of metal-supported cells is the redox instability, since the metallic support is subjected to oxidation, similar to the state-of-the-art Ni cermet [28], while it is exposed to oxidizing atmosphere. Cyclic oxidation and reduction of the metallic substrate, accompanying with volume change, may causes cell cracking, delamination or more
seriously disintegration. As a result, the OCV of the cell will decrease dramatically as a response. Thus the prepared cell was also evaluated by OCV measurement to confirm its tolerance to redox cycles at 600 C. Fig. 6 demonstrates the OCV after each 4-h redox cycle for up to 5 cycles. It remained essentially unchanged at slightly above 0.780 V, indicating perfect redox stability of the NieFe alloy supported cell. This is attributed to the excellent oxidation resistance of the NieFe alloy, as shown in Fig. 7. Different from what previously reported [29], a relatively dense oxide scale (~2e3 mm) was formed on the inner surface of the NieFe scaffold after oxidation in air at 600 C for 2 h, preventing further inward oxidation of the stem of the porous structure. The oxide scale was Fe-rich, possibly NiFe2O4 spinel according to XRD (Fig. 1); and the inside remained virtually metallic. Fig. 8 shows the cell performance before and after 5 redox cycles at 600 C; only insignificant degradation at high current densities was observed possibly caused by slight densification of the substrate during the test. This result also confirms that the NieFe
Fig. 7 e Cross-sectional microstructure of NieFe alloy support oxidized at 600 C in air for 2 h.
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references
Fig. 8 e Electrochemical performance of NieFe alloy supported cell before and after 5 redox cycles at 600 C.
alloy supported cell with 10 mm thick electrolyte is capable of redox cycles.
Conclusions NieFe alloy supported SOFC was fabricated by tape castingscreen printing-cofiring process, and evaluated in the aspects of electrochemical performance and redox capability at temperatures ranging from 500 to 650 C. From the obtained results, the following conclusions are made: 1) Tape casting-screen printing-cofiring is a low cost process that can be utilized for massive production of NieFe alloy supported SOFC cells. Porous NieFe alloy substrate can be obtained by reduction of the sintered NiO-10 wt% Fe2O3, which is consisted of NiO and NiFe2O4, in H2 at 650 C for 2 h. 2) The NieFe alloy supported cell can demonstrate significantly high electrochemical performance. With H2 as the fuel and air as the oxidant this cell can reach a peak power density as high as 1.04 W cm2 at 650 C due to its low ohmic and polarization resistances benefited from intimate contact between cell components and highly active LSM-BSCF cathode material. 3) The NieFe alloy supported cell is capable of redox cycles, since a relatively dense Fe-rich oxide scale, primarily NiFe2O4 spinel, is formed on the inner surface of the NieFe alloy scaffold, which prevents further inward oxidation of the stem of the porous structure.
Acknowledgments This research was financially supported by the National “863”project (2011AA050702) and Natural Science Foundation of China (U1134001 and 51271083). The SEM and XRD characterizations were assisted by the Analytical and Testing Center of Huazhong University of Science and Technology.
[1] Villarreal I, Jacobson C, Leming A, Matus Y, Visco S, De Jonghe L. Metal-supported solid oxide fuel cells. Electrochem Solid-State Lett 2003;6:A178. [2] Blennow P, Hjelm J, Klemenso T, Ramousse S, Kromp A, Leonide A, et al. Manufacturing and characterization of metal-supported solid oxide fuel cells. J Power Sources 2011;196:7117e25. [3] Tucker MC. Progress in metal-supported solid oxide fuel cells: a review. J Power Sources 2010;195:4570e82. [4] Blennow P, Klemenso T, Persson AH, Brodersen K, Srivastava AK, Sudireddy BR, et al. Metal-supported SOFC with ceramicbased anode. In: Singhal SC, Eguchi K, editors. Solid oxide fuel cells 122011. pp. 683e692. [5] Lee CB, Bae JM. Fabrication and characterization of metalsupported solid oxide fuel cells. J Power Sources 2008;176:62e9. [6] Leah R, Bone A, Selcuk A, Corcoran D, Lankin M, DehaneySteven Z, et al. Development of highly robust, volumemanufacturable metal-supported SOFCs for operation below 600 C. In: Singhal SC, Eguchi K, editors. Solid oxide fuel cells 122011. pp. 351e367. [7] Chen X, Wu NJ, Smith L, Ignatiev A. Thin-film heterostructure solid oxide fuel cells. Appl Phys Lett 2004;84:2700. [8] Oishi N, Yoo Y. Fabrication of cerium oxide based SOFC having a porous stainless steel support. 2009. p. 739e44. [9] Molin S, Kusz B, Gazda M, Jasinski P. Evaluation of porous 430L stainless steel for SOFC operation at intermediate temperatures. J Power Sources 2008;181:31e7. [10] Henne R. Solid oxide fuel cells: a challenge for plasma deposition processes. J Therm Spray Technol 2007;16:381e403. [11] Schiller G, Ansar A, Lang M, Patz O. High temperature water electrolysis using metal supported solid oxide electrolyser cells (SOEC). J Appl Electrochem 2008;39:293e301. s-Petit C, Qu W, et al. [12] Hui S, Yang D, Wang Z, Yick S, Dece Metal-supported solid oxide fuel cell operated at 400e600 C. J Power Sources 2007;167:336e9. [13] Yan JW, Hou XH, Choy KL. The electrochemical properties of LSM-based cathodes fabricated by electrostatic spray assisted vapour deposition. J Power Sources 2008;180:373e9. [14] Matus Y, Dejonghe L, Jacobson C, Visco S. Metal-supported solid oxide fuel cell membranes for rapid thermal cycling. Solid State Ionics 2005;176:443e9. [15] Mineshige A, Fukushima K, Okada S, Kikuchi T, Kobune M, Yazawa T, et al. Porous metal tubular support for solid oxide fuel cell design. Electrochem Solid State Lett 2006;9:A427e9. [16] Kong Y, Hua B, Pu J, Chi B, Jian L. A cost-effective process for fabrication of metal-supported solid oxide fuel cells. Int J Hydrogen Energy 2010;35:4592e6. [17] YLaGM Choi. Ceria film supported on NieFe metal film. ECS Trans 2009;25:727e30. [18] Park HC, Virkar AV. Bimetallic (NieFe) anode-supported solid oxide fuel cells with gadolinia-doped ceria electrolyte. J Power Sources 2009;186:133e7. [19] Ishihara T, Yan J, Enoki M, Okada S, Matsumoto H. NieFe alloy-supported intermediate temperature SOFCs using LaGaO3 electrolyte film for quick startup. J Fuel Cell Sci Technol 2008;5. 031205. [20] Ishihara T, Yan J, Shinagawa M, Matsumoto H. NieFe bimetallic anode as an active anode for intermediate temperature SOFC using LaGaO3 based electrolyte film. Electrochim Acta 2006;52:1645e50. [21] Fu C, Chan SH, Liu Q, Ge X, Pasciak G. Fabrication and evaluation of Ni-GDC composite anode prepared by aqueous-
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based tape casting method for low-temperature solid oxide fuel cell. Int J Hydrogen Energy 2010;35:301e7. Fu CJ, Liu QL, Chan SH, Ge XM, Pasciak G. Effects of transition metal oxides on the densification of thin-film GDC electrolyte and on the performance of intermediatetemperature SOFC. Int J Hydrogen Energy 2010;35:11200e7. Ge L, Li R, He S, Chen H, Guo L. Effect of titania concentration on the grain boundary conductivity of Ce0.8Gd0.2O1.9 electrolyte. Int J Hydrogen Energy 2012;37:16123e9. Meng L, Wang FZ, Wang A, Pu J, Chi B, Li J. High performance La0.8Sr0.2MnO3-coated Ba0.5Sr0.5Co0.8Fe0.2O3 cathode prepared by a novel solid-solution method for intermediate temperature solid oxide fuel cells. Chin J Catal 2014;35:38e42. Lee KT, Vito NJ, Yoon HS, Wachsman ED. Effect of NieGd0.1Ce0.9O1.95 anode functional layer composition on
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Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
High performance NieFe alloy supported SOFCs fabricated by low cost tape casting-screen printing-cofiring process Kai Li, Xin Wang, Lichao Jia, Dong Yan*, Jian Pu, Bo Chi, Li Jian Center for Fuel Cell Innovation, State Key Laboratory for Coal Combustion, School of Materials Science and Engineering, Huazhong University of Science & Technology, Wuhan 430074, China
article info
abstract
Article history:
Novel NieFe alloy supported solid oxide fuel cells, with Ni cermet as functional anode,
Received 24 June 2014
La0.8Sr0.2MnO3 coated Ba0.5Sr0.5Co0.2Fe0.8O3 as cathode and Gd-doped Ce2O3 as electrolyte,
Received in revised form
are successfully fabricated by the cost effective method of tape casting-screen printing-
8 September 2014
cofiring. The NieFe porous substrate is obtained by reduction (in H2 at 650 C for 2 h) of
Accepted 26 September 2014
sintered NiO-10 wt% Fe2O3 consisting of NiO and NiFe2O4. The cell is subjected to evalua-
Available online 14 October 2014
tion in the aspects of electrochemical performance and redox capability at temperatures between 500 and 650 C. The result shows a peak power density of 1.04 W cm2 at 650 C.
Keywords:
Furthermore, the metal support cell exhibits excellent tolerance to redox cycles. Five redox
Solid oxide fuel cell
recycles for cells are operated at 600 C, which shows no significant degradation in open
Metal-support
circuit voltage and power density.
NieFe alloy
Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Redox
Introduction Different from conventional electrolyte- and electrodesupported solid oxide fuel cells (SOFCs), metal-supported SOFCs proposed firstly by Villarreal et al. [1] are built on a porous metal substrate, which possesses high thermal conductivity and ductility that improve cell temperature gradient and thermal shock resistance [2] as well as capability withstanding vibration and mechanical loading [3,4]. Ferritic stainless steels, such as SUS 430, 409, 410 and 441 [3], have been employed as the support material due to their low cost and thermal expansion coefficient (TEC) matching with other cell components. Perforated metal sheets [5e8] or porous metal [9] were used as supports on which cells were built by
vacuum plasma spraying (VPS) [10,11], pulsed laser deposition (PLD) [7,12], electrostatic spray assisted vapor deposition [13], or and co-sintering in reduced atmospheres [14]. Porous Ni has also been considered as the support material [15,16]; however, the TEC (16.5 106 k1) of Ni is not well matched with those of other cell components (10e13 106 k1), which hinders its utilization for this purpose. Adding Fe into Ni decreases the TEC of Ni [17]; therefore NieFe alloy has been selected as another kind of alternative support materials [17e20]. Porous NieFe alloy support is obtained by reducing porous NiOeFe2O3 formed by either screen printing [17] or die-pressing [18e20] and sintering; other cell components are applied on to the substrate by sputtering [17], slurry coating [18] or PLD [19,20]. In the present study, a different approach, tape castingscreen printing-cofiring, was employed to fabricate NieFe
* Corresponding author. Tel.: þ86 27 87557849; fax: þ86 27 87558142. E-mail address: [email protected] (D. Yan). http://dx.doi.org/10.1016/j.ijhydene.2014.09.146 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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alloy supported SOFCs. Mixed NiO and Fe2O3 powders were used as the materials for NiOeFe2O3 substrate made by tape casting. Compared with Y2O3 stabilized ZrO2 (YSZ), Gd-doped CeO2 (GDC) possesses higher ionic conductivity at lower temperatures and suitable for metal-supported cells [21e23]. Therefore NiO-GDC (Ce0.9Gd0.1O1.95) functional anode and GDC electrolyte were applied onto the substrate by screen-printing. The half cell (substrate/functional anode/electrolyte) was cofired; and functional cathode was then built on the electrolyte by screen-printing and sintering before reduction in H2. This fabrication method is suitable for mass production of large cells at low cost. The performance and redox stability of such prepared cells were evaluated at temperatures between 500 and 650 C.
Experimental Substrate preparation For preparation of the NiOeFe2O3 (10 wt%) substrate by tape casting, NiO (average particle size 1 mm, Haite Advanced Materials) and Fe2O3 (average particle size 0.5 mm, Sinopharm Chemical Reagent) were ball-milled at room temperature for 24 h in an organic mixed solvent consisting of xylene and ethanol with Menhaden fish oil (Richard E. Mistler, Inc.) dispersant and 5 wt% of corn starch as poor former. Organic binder and plasticizer (Solutia Inc.) were then added, followed by ball-milling for another 24 h. Such prepared slurry was degassed and tape cast to form a flexible green tape with a thickness of ~1.2 mm.
Cell fabrication The green tape was die-cut into discs with a diameter of 25 mm as the cell support, on which functional anode (50 wt% NiO-50 wt% GDC) and GDC (Ningbo Institute of Materials Technology and Engineering) electrolyte were screen-printed in sequence to form a green half cell. Dried in air at room temperature for 2 h, the green half cell was sintered at 1450 C in air for 5 h. A layer of novel composite cathode, La0.8Sr0.2MnO3-coated Ba0.5Sr0.5Co0.8Fe0.2O3 (LSM-BSCF) [24], was screen-printed on the sintered GDC electrolyte, followed by sintering in air at 1050 C for 3 h. The LSM-BSCF contained LSM and BSCF at a weight ratio of 50 to 50, possessing the merits of high electronic and ionic conductivity for the need of oxygen reduction reaction. The sintered cell was reduced in H2 at 650 C for 2 h to form porous NieFe alloy supported SOFC cell with an active area of 0.5 cm2. The porosity of sintered and reduced substrates was obtained by the Archimedes method.
composite sealant (Aremco Product, Inc.). Ni foam and Pt mesh were used as anode and cathode current collector, respectively. Impedance analysis of the cell was carried out using the four-probe method with Solartron 1260A under the conditions of open circuit and 10 mV excitation in the frequency range from 100 KHz to 0.01 Hz at temperatures between 450 C and 600 C. The cell voltage as a function of applied current density was measured by Solartron 1260A at temperatures from 500 to 650 C. In addition, 5 redox cycles were performed at 600 C to examine the redox capability of the cell.
Results and discussion Cell characterization Fig. 1 shows XRD patterns of as-sintered and reduced substrate. The sintered substrate was consisted of two phases of NiO and NiFe2O4; and reduced in H2 at 650 C for 2 h the substrate was converted to a single solid solution of NieFe alloy. Since Fe dissolved in Ni reduced the d-spacing of Ni lattice, the diffraction peaks of the NieFe alloy were left-side shifted compared to those of pure Ni, as shown by the insert in Fig. 1. Even though the reduction caused 9.4% cell shrinkage, the integrity of the reduced cell was well maintained, which indicates that the NieFe alloy supported cell is capable of redox cycles. In the sintered substrate, the pores were uniformly distributed with a pore size ranging from 5 to 10 mm (Fig. 2a); and the porosity was around 40%. After reduction, the substrate contained bimodal pores (Fig. 2b) with a much higher porosity of 62%. The bigger pores were inherited from those formed between the sintered oxide particles; and the smaller ones were formed due to the reduction of the oxide scaffold. Such highly porous microstructure of the reduced substrate is expected to be beneficial for fuel transport to
Cell characterization and testing Scanning electron microscope (SEM, FEI sirion 200) and X-ray diffraction (XRD, X'Pert Pro) were employed for examining microstructure/composition and identifying phases of the cell, respectively. The performance of prepared cells was evaluated with H2 as the fuel and air as the oxidant at a flow rate of 0.1 L min1. The disc-shaped cell (3.8 cm2) was hermetically sealed in the cell holder with a ceramic-glass
Fig. 1 e XRD patterns of as-sintered and reduced NiO-10 wt % Fe2O3 support. The reduction was performed at 650 C in H2 for 2 h.
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Fig. 2 e Cross-sectional microstructure of NiO-10 wt% Fe2O3 support: (a) sintered at 1450 C in air for 5 h; (b) reduced at 650 C for 2 h.
Fig. 3 e Cross-sectional microstructure of reduced NieFe alloy supported cell (a) and surface microstructure of sintered GDC electrolyte (b).
and steam removal from the anode, resulting in low concentration polarization [25] and in turn high cell power output. Fig. 3 shows the microstructure of cross-sectioned cell (Fig. 3a) and the surface of GDC electrolyte (Fig. 3b). The thickness of the metallic support was around 1 mm, and that of the functional anode, electrolyte and cathode was 15, 10 and 7 mm, respectively. Each layer in the cell was well adhered to each other after reduction. The GDC electrolyte was fully dense with a grain size between 2 and 20 mm. It is expected that high cell performance can be achieved due to the low ohmic resistance associated with the thin thickness of the cell components and the intimate contact between the layers.
at 500 C to 1.04 WW cm2 at 650 C, which is higher than that previously reported for metal-supported cells with GDC electrolyte at 650 C [27]. Fig. 5 shows the measured electrochemical impedance spectra (Fig. 5a) and derived resistances (Fig. 5b, RT: total resistance, RU: ohmic resistance, RP: polarization resistance) of the metallic support cell at various temperatures. The RU is the sum of ohmic resistances contributed
Cell performance Fig. 4 shows the currentevoltage (IeV) curves of the NieFe alloy supported cell at temperatures range from 500 to 650 C. Open circuit voltage (OCV) of the cell decreased with temperature from 0.88 for 500 C to 0.78 V for 650 C. This tendency is predictable according to the Nernst equation. And the lower OCV values than the Nernst potential (~1.2 V) is caused by the electronic conducting behavior of GDC electrolyte in low oxygen partial pressure atmosphere [26,27]. Even though the OCV value was relatively low, high cell performance was still achieved. The peak power density increased from 0.47 W cm2
Fig. 4 e Electrochemical performance of NieFe alloy supported cell at temperatures between 500 C and 650 C with H2 as the fuel and air as the oxidant.
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Fig. 6 e Open circuit voltage of NieFe alloy supported cell after each redox cycle at 600 C.
Fig. 5 e Total (RT), ohmic (RU) and polarization (RP) resistances of the cell as a function of testing temperature obtained from measured electrochemical impedance spectra at open circuit voltage and various temperatures between 500 and 650 C.
by the cell support, electrolyte, electrodes and contact between the layers; and the RP characterizes the electrocatalytic activity of electrodes. The RU and RP decreased from 0.12 and 0.33 U cm2 at 500 C to 0.06 and 0.04 U cm2 650 C, respectively, resulting in a low value of total resistance between 0.10 and 0.45 U cm2 in the tested temperature range. The low polarization resistance suggests that the selected electrode materials, especially the novel LSM-BSCF cathode material, were extremely active for electrochemical reactions occurred in the electrodes. And the low ohmic resistance implies that the cell was well prepared by the process of tape cast-screen printingcofiring, with robust contact between cell components. It was the low cell total resistance that assured the high cell performance. The most vulnerable aspect of metal-supported cells is the redox instability, since the metallic support is subjected to oxidation, similar to the state-of-the-art Ni cermet [28], while it is exposed to oxidizing atmosphere. Cyclic oxidation and reduction of the metallic substrate, accompanying with volume change, may causes cell cracking, delamination or more
seriously disintegration. As a result, the OCV of the cell will decrease dramatically as a response. Thus the prepared cell was also evaluated by OCV measurement to confirm its tolerance to redox cycles at 600 C. Fig. 6 demonstrates the OCV after each 4-h redox cycle for up to 5 cycles. It remained essentially unchanged at slightly above 0.780 V, indicating perfect redox stability of the NieFe alloy supported cell. This is attributed to the excellent oxidation resistance of the NieFe alloy, as shown in Fig. 7. Different from what previously reported [29], a relatively dense oxide scale (~2e3 mm) was formed on the inner surface of the NieFe scaffold after oxidation in air at 600 C for 2 h, preventing further inward oxidation of the stem of the porous structure. The oxide scale was Fe-rich, possibly NiFe2O4 spinel according to XRD (Fig. 1); and the inside remained virtually metallic. Fig. 8 shows the cell performance before and after 5 redox cycles at 600 C; only insignificant degradation at high current densities was observed possibly caused by slight densification of the substrate during the test. This result also confirms that the NieFe
Fig. 7 e Cross-sectional microstructure of NieFe alloy support oxidized at 600 C in air for 2 h.
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Fig. 8 e Electrochemical performance of NieFe alloy supported cell before and after 5 redox cycles at 600 C.
alloy supported cell with 10 mm thick electrolyte is capable of redox cycles.
Conclusions NieFe alloy supported SOFC was fabricated by tape castingscreen printing-cofiring process, and evaluated in the aspects of electrochemical performance and redox capability at temperatures ranging from 500 to 650 C. From the obtained results, the following conclusions are made: 1) Tape casting-screen printing-cofiring is a low cost process that can be utilized for massive production of NieFe alloy supported SOFC cells. Porous NieFe alloy substrate can be obtained by reduction of the sintered NiO-10 wt% Fe2O3, which is consisted of NiO and NiFe2O4, in H2 at 650 C for 2 h. 2) The NieFe alloy supported cell can demonstrate significantly high electrochemical performance. With H2 as the fuel and air as the oxidant this cell can reach a peak power density as high as 1.04 W cm2 at 650 C due to its low ohmic and polarization resistances benefited from intimate contact between cell components and highly active LSM-BSCF cathode material. 3) The NieFe alloy supported cell is capable of redox cycles, since a relatively dense Fe-rich oxide scale, primarily NiFe2O4 spinel, is formed on the inner surface of the NieFe alloy scaffold, which prevents further inward oxidation of the stem of the porous structure.
Acknowledgments This research was financially supported by the National “863”project (2011AA050702) and Natural Science Foundation of China (U1134001 and 51271083). The SEM and XRD characterizations were assisted by the Analytical and Testing Center of Huazhong University of Science and Technology.
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