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Wet powder spraying fabrication and performance optimization of IT-SOFCs with thin-film ScSZ electrolyte
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Wet powder spraying fabrication and performance optimization of IT-SOFCs with thin-film ScSZ electrolyte Huangang Shi, Ran Ran, Zongping Shao* State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry and Chemical Engineering, Nanjing University of Technology, No. 5 Xin Mofan Road, Nanjing 210009, PR China
article info
abstract
Article history:
We have successfully fabricated a dense-type Scandia-stabilized zirconia (ScSZ) thin-film
Received 15 November 2010
electrolyte with thickness down to w4 mm for intermediate temperature solid oxide fuel
Received in revised form
cells (IT-SOFCs; 600e800 C) by a wet powder spraying technique. With an anode-supported
18 January 2011
design and a La0.8Sr0.2Sc0.1Mn0.9O3 (LSScM) oxide cathode, a peak power density of
Accepted 14 February 2011
788 mW cm2 was achieved at 800 C for a cell operating on hydrogen fuel. The impedance
Available online 13 March 2011
results demonstrated that the electrode polarization resistance is the main source of electrochemical performance loss for the single cell. Ba0.5Sr0.5Co0.8Fe0.2O3d þ Sm0.2Ce0.8O1.9
Keywords:
(BSCF þ SDC) composite cathode was then applied to improve the cell power output. An
Scandia-stabilized zirconia
SDC interlayer was adopted to avoid the interfacial phase reaction between ScSZ and BSCF,
Solid oxide fuel cell
which was also fabricated by the wet powder spraying. The effect of interlayer processing
Samaria-doped ceria
on the morphology and cell performance was studied by SEM-EDX, IeV polarization and
Interlayer
EIS. Pre-sintering of the ScSZ layer was found to be superior to the co-sintering of SDC interlayer and ScSZ electrolyte layer. An anode-supported cell with SDC interlayer fabricated on pre-sintered ScSZ electrolyte film and a BSCF þ SDC cathode delivered a peak power density as high as 1760 mW cm2 at 800 C. Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
As one of the high-temperature electrochemical energy conversion devices, solid oxide fuel cells (SOFCs) have received considerable attention recently because of their high energy conversion efficiency, low emissions and fuel flexibility [1,2]. Conventional SOFCs are based on thick yttriastabilized zirconia (YSZ) electrolyte and operate at around 1000 C. It is now generally accepted that the reduction of operation temperature to the intermediate temperature range of 500e800 C is the key for the successful implementation of SOFCs technology in the near future. One challenge is the sharp decrease in ionic conductivity of electrolyte with the drop of operation temperature, as a result, an obvious
increase in ohmic resistance and consequently a sharp decrease in cell power output is observed for a cell with thick YSZ electrolyte. In order to maintain high power output at intermediate temperature, a reduction of the electrolyte membrane thickness and the development of new electrolyte materials with high ionic conductivity become necessary [3,4]. Stabilized zirconia materials are the state-of-the-art electrolyte materials of sensors [5], high-temperature SOFCs [6] and solid oxide electrolysis cells [7], which possess pure and acceptable oxygen-ionic conductivity at high temperature, high mechanical strength and good thermal and chemical stability. Scandia-stabilized zirconia (ScSZ) shows the highest oxygen-ion conductivity in the cubic-fluorite zirconia phase because of the similar ionic radii of Sc3þ to Zr4þ [8], which is
* Corresponding author. Tel./fax: þ86 25 83172256. E-mail address: [email protected] (Z. Shao). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.02.077
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more promising than YSZ for reduced temperature application. If the electrolyte membrane thickness is reduced to tens of micron, SOFCs with ScSZ electrolyte could be operated at temperature down to 650 C with allowable ohmic resistance. A number of advanced techniques have been developed to fabricate thin-film electrolytes on porous substrates for SOFCs applications [9e14]. Recently wet powder spraying, sometimes called as suspension spraying, has been applied for the facile fabrication of thin-film electrolytes on porous anode substrate [15e19]. As a non-contact technique, it is suitable for flat substrates, corrugated sheets, tubes and a variety of other substrates and easy to scale up from laboratory to industrial fabrication. Previously we have successfully applied this technique for the fabrication of single layer of dense-type thin-film YSZ and samaria-doped ceria (SDC) electrolytes with the membrane thickness less than 10 mm [20,21]. With thin-film electrolyte structure, large electrode polarization resistance becomes the main obstacle for achieving high cell output at reduced temperature. The improvement of cathode performance becomes the key point to realize the reduced temperature operation of SOFCs [22e25]. Among the various cathode materials, some cobalt-containing perovskite-type composite oxides show mixed electronic and oxygen-ionic conductivities, and also high catalytic activity for oxygen electrochemical reduction at elevated temperatures. In particular Ba0.5Sr0.5Co0.8Fe0.2O3d (BSCF) is a novel cathode material for intermediate temperature solid oxide fuel cells (IT-SOFCs) with operation temperature down to 600 C. However, serious interfacial reaction between BSCF and stabilized zirconia was happened, which could result in a poor cell performance [26]. A doped ceria interlayer has been proposed to avoid the direct contact of BSCF with stabilized zirconia electrolyte since BSCF has good chemical compatibility with doped ceria electrolyte [26]. High cell performance at intermediate temperature was reported for a fuel cell based on stabilized zirconia electrolyte with BSCF cathode and a doped ceria interlayer [26,27]. In present work, the wet powder spraying technique was applied to fabricate thin-film ScSZ electrolyte on porous anode substrate for reduced temperature operation. The fuel cell performance was first measured by adopting a La0.8Sr0.2Sc0.1Mn0.9O3d (LSScM) cathode. To further improve the cell performance, BSCF was also investigated as a potential cathode in connection with an SDC interlayer, which was prepared by the same wet powder spraying technique. The effect of interlayer processing on the electrochemical performances of the single cells with thin-film ScSZ electrolyte and BSCF cathode was studied in detail.
2.
Experimental
2.1.
Powder synthesis
All the materials for powder synthesis are in AR grade. The (ZrO2)0.9(Sc2O3)0.1 (ScSZ) powder was synthesized by a combined ethylene diamine tetraacetic acid-citrate (EDTACA) complexing solegel process using Sc2O3 and Zr(NO3)4$5H2O as the staring materials. For a typical synthesis,
Sc2O3 was first dissolved in a nitric acid solution, then Zr(NO3)4$5H2O, EDTA and citric acid were added in sequence to the solution with the mole ratio of EDTA and citric acid to the total metal ions of 1:2:1, the pH value of the solution was adjusted to w6 with the addition of NH4$H2O, the solution was heated to evaporate the water under magnetic stirring until a transparent gel was obtained, which was then fired in oven at 250 C to form a solid precursor and further fired at 900 C for 5 h under an air atmosphere to result in an ScSZ powder for anode substrate tape casting. The SDC powder with the composition of Sm0.2Ce0.8O1.9 for the interlayer was synthesized by a hydrothermal treatment method. Solegel precursor for hydrothermal synthesis was prepared by co-precipitation of the aqueous solution of Sm(NO3)3$6H2O and Ce(NO3)3$6H2O with NH4$H2O at pH ¼ 10. The precursor was transferred to a Teflon vessel and hydrothermally treated at 180 C for 24 h. The product from the vessel was washed with water and ethanol repeatedly. Dried powder was subsequently calcinated at 800 C in air for 5 h to yield the SDC powder for the interlayer fabrication. La0.8Sr0.2Sc0.1Mn0.9O3 (LSScM) and Ba0.5Sr0.5Co0.8Fe0.2O3d (BSCF) cathode powders were also synthesized by the combined EDTA-citrate solegel process [28,29].
2.2.
Cell fabrication
The anode substrates were prepared by a tape casting process. The slurry for tape casting process was prepared by two-step ball milling. Commercial nickel oxide (Chengdu Shudu Nanomaterials Technology Development Co. Ltd.), ScSZ (prepared by combined EDTA-citrate solegel process, calcinated at 900 C) and starch was ball milled together with organic solvent in an agate jar for 24 h. Triethanolamine was added as surfactant for the dispersion of oxide powder in organic solvent in the first step. In the second step, polyvinyl butyral (PVB) as binder, polyethylene glycol (PEG) and dibutyl o-phthalate (DOP) as plasticizer were added to the slurry and then ball milled again for another 24 h. The slurry was vacuum pumped under 200 mbar (absolute pressure) to remove air and then cast onto the polymer carrier on a tape casting machine. The slurry was dried in air for 24 h and then detached from the tape. Anode substrates for single cells were drilled from the NiO þ ScSZ tape with a diameter of 16 mm. The disks were then sintered at elevated temperature for subsequent electrolyte deposition. The ScSZ electrolyte layer and SDC interlayer were prepared by a wet powder spraying technique. The ScSZ and SDC powders were prepared into colloidal suspensions with solid content of about 5%. The colloidal suspension was then sprayed under the drive of 1 atm nitrogen carrier gas onto the anode substrate (or ScSZ) using a modified spraying gun (BD128, Fenghua Bida Machinery Manufacture Co. Ltd., China) with a nozzle size of 0.35 mm (pore diameter). The spray gun was aligned above the heated substrate (250 C on a hot plate) at a distance of 10 mm. The effective deposition speed calculated was about 0.005e0.008 g cm2 min1 ScSZ/SDC. The cells were then sintered at 1400 C for 5 h in air. LSScM or BSCF þ SDC (7:3 in mol ratio) was applied as the cathode, which was painted on the central surface of the electrolyte and fired at 1100 and 1000 C respectively in air for 2 h to allow
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the firm attachment of the cathode layer onto the electrolyte (interlayer) surface. The coin-shape cathode had an effective area of 0.48 cm2.
2.3.
Electrochemical test
Electrochemical performance test of the single cell was carried out in an in-lab constructed fuel cell test station. The fuel cell was first sealed onto a quartz tube using silver paste to form an anode chamber, which was then heated to 800 C at a rate of 2 C min1 and dwelled for 5 h. During the process, the organic in the paste was slowly burned out and the silver was softened
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to make an effective sealing. Dry hydrogen at a flow rate of 20 ml min1 [STP] was then introduced into the anode chamber to initiate the in-situ reduction of anodic NiO to metallic nickel. After the in-situ reduction for about 5 h, the hydrogen flow rate was increased to 80 ml min1 [STP] and bubbled through a water bottle at 25 C for IeV cell polarization test. During the test, ambient air was applied as the cathode atmosphere. IeV polarization curves were collected at 50 C intervals over a temperature range of 600e800 C using a digital source meter (model 2420, Keithley, USA) with a four-probe configuration. Electrochemical impedance spectroscopy (EIS) of the cell under open circuit voltage (OCV) conditions was measured with an electrochemical workstation comprised of a Solartron 1260 A frequency response analyzer with a Solartron 1287 Potentiostat. EIS was recorded in a frequency range of 105 Hze0.1 Hz with signal amplitude of 10 mV.
2.4.
Other characterization
The fuel cell morphologies were observed using an environmental scanning electron microscope (ESEM, Model QUANTA-200, FEI Company, Hillsboro, OR). Energy disperse spectroscopy (EDS) analysis were performed to analyze the reaction between SSZ electrolyte and SDC interlayer.
3.
Results and discussion
3.1.
Thin-film ScSZ electrolyte fuel cell without interlayer
The successful fabrication of crack-free dense ScSZ thin-film on porous anode substrate by wet powder spraying technique requires the preparation of high-quality starting powder with ultrafine particles, homogenous particle size distribution and no large amount of aggregates. EDTAeCA complexing solegel process can result in fine powder with high purity and high surface activity; however, a lot of soft aggregates are also presented, while the high energy ball milling can effectively break such soft aggregates [20]. Therefore, in this study the assynthesized ScSZ powder from the EDTAeCA complexing process was subjected for ball milling for 30 min in an
Fig. 1 e SEM images of the surface morphologies of: (a) precalcinated anode substrate, (b) sintered ScSZ electrolyte, and (c) cross-sectional view of the single cell with single layer ScSZ electrolyte and LSScM cathode.
Fig. 2 e IeV and IeP curves for a typical SOFC with single layer ScSZ electrolyte and LSScM cathode.
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Fig. 3 e Electrochemical impedance spectra under open circuit conditions at different temperatures for the single cell with single layer ScSZ electrolyte and LSScM cathode.
ethylene glycol liquid medium before applying for thin-film electrolyte fabrication. The green electrolyte layer from the wet powder spraying was built from the loose packing of the ScSZ particles, thus a following sintering process is needed for the densification of
the electrolyte and also the firm adhesion of the electrolyte layer to the anode substrate. Since the gas evolved during the organic burning may lead to the formation of crack in the electrolyte layer, the anode substrate with the organic additives is preferred to be pre-fired to get rid of the organic substances before the deposition of the electrolyte layer. A proper firing temperature is critical to allow sufficient mechanical strength and matched sintering shrinkage to the electrolyte layer during the following co-sintering process. In this study, a firing temperature of 1150 C was found to be optimal, and the shrinkage of SDC and ScSZ layers for the later sintering was found to match each other pretty well. Shown in Fig. 1 are the SEM images of the surface morphologies of the anode substrate and the sintered ScSZ electrolyte. The surface of the anode substrate fabricated by the tape casting method was relatively rough after the firing at 1150 C. A proper surface roughness for the anode substrate is beneficial for the well adhesion of the electrolyte layer to the substrate because it will increase the contact area between them. As shown in Fig. 1b, there are no observable pores and cracks on the surface of ScSZ electrolyte after the sintering at 1400 C, suggesting the well densification of the electrolyte layer. LSScM was selected as the cathode, which was deposited onto the electrolyte surface by spray deposition and calcinated at 1150 C for 5 h in air. As shown in Fig. 1c is SEM image of a single cell with unreduced anode from crosssectional view. The electrolyte layer has a thickness of around 4 mm, which is well adhered to the anode substrate.
Fig. 4 e SEM images from cross-sectional view and the surface morphologies of the SOFC with SDC interlayer, a and c: Cell 1 and, b and d: Cell 2.
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Fig. 5 e IeV and IeP curves of the fuel cells with SDC interlayer between 600 and 800 C, with humidified H2 as fuel and ambient air as oxidant, (a) Cell 1, and (b) Cell 2.
The fuel cell was then subjected for electrochemical performance test. During the measurement, humidified hydrogen (3% H2O) was the introduced to the anode while the cathode was exposed to the ambient atmosphere. Under open circuit conditions, the cell voltages reached 1.08, 1.09 and 1.10 V at 800, 750 and 700 C respectively, as shown in Fig. 2. The high voltages further demonstrate the electrolyte layer prepared by the wet powder spraying technique is well densified. The peak power densities reached 788, 607 and 375 mW cm2 at 800, 750 and 700 C, respectively. Fig. 3 shows the EIS of the cell under OCV conditions at different temperatures. The low-frequency intercept gives the total resistance, which includes the ohmic resistance of the cell, the concentration polarization (mass-transfer or gas-diffusion polarization) resistance and the effective interfacial polarization resistances associated with the electrochemical reactions at both electrodeeelectrolyte interfaces (anode-electrolyte and cathode-electrolyte). The ohmic resistance is 0.23, 0.26 and 0.31 U cm2 respectively at 800, 750 and 700 C. The ratio of ohmic resistance to total cell resistance is about 12% at 800 C, which further reduces to only 9% at 700 C. It indicates that the electrode polarization resistance plays a dominant role in the electrochemical performance loss at these operating temperatures for the current anode-supported thin-film ScSZ electrolyte fuel cell with LSScM cathode. It suggests the cathode
Fig. 6 e EIS plots of the SOFC with SDC interlayer at different temperatures under open circuit conditions, (a) Cell 1, and (b) Cell 2.
optimization is critical to further improve the cell performance especially at the lower temperature range.
3.2. Thin-film ScSZ electrolyte fuel cell with SDC interlayer BSCF has been found to show excellent electrochemical activity for oxygen reduction on doped ceria electrolyte at intermediate temperature [30]. However, BSCF cannot be directly applied to the stabilized zirocnia electrolyte because serious interfacial reaction happens between BSCF and stabilized zirconia at elevated temperature during the fuel cell fabrication or operation [27], which leads to a significant increase in interfacial polarization resistance. The application of a doped ceria as an interlayer can effectively avoid the direct contact between BSCF and stabilized zirconia
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Fig. 7 e Ohmic resistance (Ro), polarization resistance (Rp) and total resistance (Rt) at different temperatures for, (a) Cell 1, and (b) Cell 2.
electrolyte; as a result, a significant improvement in cell performance was observed [26]. In this study, SDC was adopted as an interlayer to improve the cell performance by using BSCF as the cathode layer, a wet powder spraying technique was also applied for fabricating the interlayer. Although the introduction of SDC interlayer can effectively avoid the direct contact of BSCF with ScSZ electrolyte, it introduces the new possibility of interfacial reaction between SDC and ScSZ, which could lead to an increase in ohmic resistance of the fuel cell. Optimizing the fabrication process of the interlayer is crucial to achieve low ohmic resistance of the cell. In this study, two different ways were tried for the fabrication of the interlayer. For Cell 1, single layer ScSZ electrolyte was first deposited on anode substrate and fired at 1400 C in air for 5 h for the sintering of the electrolyte layer, and then additional SDC layer was sprayed on the sintered ScSZ electrolyte as interlayer and further sintered at 1400 C for 5 h in air. For Cell 2, the SDC interlayer was deposited on the green ScSZ electrolyte, and then the ScSZ electrolyte layer and the SDC layer were co-sintered at 1400 C for 5 h in air. Fig. 4 shows the SEM images of Cell 1 and Cell 2 from crosssectional view. For both cells, the ScSZ layers are well
Fig. 8 e EDS line-scan results for the SOFC with SDC interlayer with the scan lines shown in Fig. 4, (a) Cell 1, and (b) Cell 2.
densified and free of crack, which have a membrane thickness of w4 mm and adhere to anode substrate pretty well. Obvious sintering of the SDC interlayer was observed for Cell 1 and Cell 2, but the SDC layer in Cell 2 is much better sintered than that in Cell 1. Furthermore, the connection between SDC interlayer and ScSZ electrolyte layer is slightly better for Cell 1. For Cell 1, the ScSZ layer is pre-sintered before the deposition of the SDC interlayer, the sintering of the SDC layer was constricted since the ScSZ layer experienced little shrinking during the sintering process. For Cell 2, the co-sintering of SDC and ScSZ layers then allows much better sintering of the SDC interlayer. Fig. 4c and d shows the surface morphologies of the SDC interlayers in Cell 1 and Cell 2, respectively. Many cracks were formed over the SDC interlayer of Cell 1, while much fewer cracks were detected in the SDC interlayer of Cell 2. Such difference can be attributed to the larger mismatch in shrinkage between SDC layer and ScSZ layer in Cell 1 than Cell 2 during the sintering. The shrinkage of the ScSZ in Cell 1 during the sintering is w 0% because it was already sintered during the first sintering process, while it reached around 18% for Cell 2. For both cells, rough SDC surfaces are observed,
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beneficial for increasing the contacting surface between BSCF cathode and the SDC interlayer. Fig. 5 gives the cell performance at different temperatures of Cell 1 and Cell 2 with BSCF þ SDC cathode, operating on hydrogen fuel. The peak power densities are 1760, 1450, 1090, 713 and 418 mW cm2 respectively at 800, 750, 700, 650 and 600 C for Cell 1, while the corresponding values are 960, 733, 510, 287 and 154 mW cm2 for Cell 2. As compared to the cell without the SDC interlayer (Fig. 2), the cell performance was obviously improved for both cells, especially at lower temperature. As a comparison of the power outputs of Cell 1 and Cell 2, a significant effect of the processing of the interlayer on the cell performance was demonstrated. It is very interesting that although Cell 2 has much better densified SDC interlayer than Cell 1, however, much higher cell performance was observed for Cell 1. To get more information to interpret such phenomena, the electrochemical impedances of two cells under OCV conditions were measured with the results shown in Fig. 6. The total polarization resistance of Cell 2 is larger than Cell 1 at all corresponding temperatures. All the spectra can be separated into a high frequency arc and a low frequency arc. The impedance spectra can be fitted to an equivalent circuit containing parallel resistor/constant phase element components and an inductor along with the series resistor, as shown in Fig. 6c. The series resistance is contributed from the ohmic resistance of the cell, which is somewhat smaller than the high frequency intercept of the impedance spectra. The low-frequency intercept gives the total resistance, which includes the ohmic resistance of the cell, the concentration polarization (mass-transfer or gas-diffusion polarization) resistance and the effective interfacial polarization resistances associated with the electrochemical reactions at both electrodeeelectrolyte interfaces (anode-electrolyte and cathode-electrolyte). Fig. 7 gives the results of the ohmic resistance (Ro), polarization resistance (Rp) and total resistance (Rt) at different temperatures for both cells. The ohmic resistances for Cell 2 are 0.22, 0.29, 0.39, 0.61 and 1.12 U cm2 at 800, 750, 700, 650 and 600 C, respectively. The corresponding values for Cell 1 are 0.22, 0.23, 0.26, 0.33 and 0.47 U cm2. The ohmic resistances for Cell 1 and Cell 2 are contributed from both the ScSZ electrolyte layer and the SDC interlayer. Although the SDC layer in Cell 2 is much more densified than that in cell 1, it is very interesting that the ohmic resistances of Cell 2 are higher than cell 1. Furthermore, the electrode polarization resistances of cell 1 are also somewhat smaller than Cell 2. As a result, much higher cell performance was observed for Cell 1 than Cell 2, as shown in Fig. 5. As mentioned previously, the application of an SDC interlayer can effectively avoid the direct contact between BSCF cathode and ScSZ electrolyte which is beneficial to reduce the interfacial polarization. Indeed, the interfacial polarization resistance of BSCF cathode deposited directly on YSZ electrolyte reached as large as w12 U cm2 at 700 C (a value from the symmetrical cell under ambient air), while it is only 0.42 U cm2 for Cell 1 with an SDC interlayer. However, SDC interlayer also introduces a new SDCeScSZ interface. The cation inter-diffusion between SDC and ScSZ layers could be happened at elevated temperature. The diffusion of Sc from ScSZ to SDC layer and the diffusion of Sm and Ce from SDC to YSZ could result in a decrease in the ionic conductivity of SDC
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and ScSZ. Fig. 8 shows the EDS profiles by line-scanning through cross-section of Cell 1 and Cell 2 (scan from the top of SDC layer to the ScSZ electrolyte, with scan lines shown in Fig. 4). For Cell 2, the reaction thickness reaches w2.1 mm, which is deeper than Cell 1 with a thickness of w1.3 mm. This is well understood because the ScSZ in Cell 2 had higher reactivity and diffusivity than that in Cell 1 since the ScSZ layer was not sintered before the deposition of the SDC interlayer in Cell 2. It then well explained the better cell performance of Cell 1 than Cell 2.
4.
Conclusions
In this study, an anode-supported thin-film ScSZ electrolyte with membrane thickness down to 4 mm was successfully fabricated by a wet powder spraying technique. Without the SDC interlayer, peak power densities of 788 and 379 mW cm2 were successfully achieved respectively at 800 and 700 C for the cell with a LSScM cathode. The electrode polarization resistance was demonstrated the main source of the electrochemical performance loss. By adopting BSCF þ SDC composite cathode with an SDC interlayer, the different ways of SDC interlayer processing was found to have significant effect on the cell performance. Pre-sintering of the ScSZ electrolyte layer before the deposition of SDC interlayer is preferred to the cosintering of SDC and ScSZ layers for achieving a higher power output. It effectively suppressed the cation diffusion, which could result in an increase in ohmic resistance of the electrolyte layer and interlayer. Cell with a BSCF þ SDC composite cathode and SDC interlayer deposited on dense ScSZ electrolyte exhibited peak power densities as high as 1760 mW cm2 at 800 C and 418 mW cm2 at 600 C. It highly promises the wet powder spraying technique for fabrication of dual layer electrolyte SOFCs with high power output.
Acknowledgments This work was supported by the National 863 Program under contract No. 2007AA05Z133, by the National Basic Research Program of China under contract No. 2007CB209704, by “Outstanding Young Scholar Grant at Jiangsu Province” under contract No. 2008023, and by program for New Century Excellent Talents (2008), and by Fok Ying Tung Education Foundation under contract No. 111073.
references
[1] Steele BCH, Heinzel A. Materials for fuel-cell technologies. Nature 2001;414:345e52. [2] Gorte RJ. Recent developments towards commercialization of solid oxide fuel cells. AIChE J 2005;51:2377e81. [3] Su PC, Chao C, Shim JH, Fasching R, Prinz FB. Solid oxide fuel cell with corrugated thin film electrolyte. Nano Lett 2008;8:2289e92. [4] Huang JB, Mao ZQ, Liu ZX, Wang C. Development of novel low-temperature SOFCs with co-ionic conducting SDCcarbonate composite electrolytes. Electrochem Commun 2007;9:2601e5.
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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 7 ( 2 0 1 2 ) 1 1 2 5 e1 1 3 2
[5] Sato T, Plashnitsa VV, Utiyama M, Miura N. Potentiometric YSZ-based sensor using NiO sensing electrode aiming at detection of volatile organic compounds (VOCs) in air environment. Electrochem Commun 2010;12:524e6. [6] Wen TL, Wang D, Chen M, Tu H, Lu Z, Zhang Z, et al. Material research for planar SOFC stack. Solid State Ionics 2002;148: 513e9. [7] Ebbesen SD, Mogensen M. Electrolysis of carbon dioxide in solid oxide electrolysis cells. J Power Sources 2009;193: 349e58. [8] Politova TI, Irvine JTS. Investigation of scandiaeyttriaezirconia system as an electrolyte material for intermediate temperature fuel cellsdinfluence of yttria content in system (Y2O3)x(Sc2O3)(11x)(ZrO2)89. Solid State Ionics 2004;168:153e65. [9] Rousseau F, Awamat S, Nikravech M, Morvan D, Amouroux J. Deposit of dense YSZ electrolyte and porous NiOeYSZ anode for SOFC device by a low pressure plasma process. Surf Coat Technol 2007;202:1226e30. [10] Talebi T, Haji M, Raissi B, Maghsoudipour A. YSZ electrolyte coating on NiOeYSZ composite by electrophoretic deposition for solid oxide fuel cells (SOFCs). Int J Hydrogen Energy 2010; 35:9455e99. [11] Brahim C, Ringuede´ A, Gourba E, Cassir M, Billard A, Briois P. Electrical properties of thin bilayered YSZ/GDC SOFC electrolyte elaborated by sputtering. J Power Sources 2006;56: 45e9. [12] Chen YY, Wei CWJ. Processing and characterization of ultrathin yttria-stabilized zirconia (YSZ) electrolytic films for SOFC. Solid State Ionics 2006;177:351e7. [13] Jung HY, Hong KS, Kim H, Park JK, Son JW, Kim J, et al. Characterization of thin-film YSZ deposited via EB-PVD technique in anode-supported SOFCs. J Electrochem Soc 2006;153:A961e6. [14] Ge XD, Huang XQ, Zhang YH, Lu Z, Xu JH, Chen KF, et al. Screen-printed thin YSZ films used as electrolytes for solid oxide fuel cells. J Power Sources 2006;159:1048e50. [15] Souza S, Visco SJ, Jonghe LC. Thin-film solid oxide fuel cell with high performance at low temperature. Solid State Ionics 1997;98:57e61. [16] Zhang YH, Liu J, Huang XQ, Lu Z, Su WH. Performance evaluation of thin membranes solid oxide fuel cell prepared by pressure-assisted slurry-casting. Int J Hydrogen Energy 2008;33:775e80. [17] Schu¨ller E, Vaßen R, Sto¨ver D. Thin electrolyte layers for SOFC via wet powder spraying (WPS). Adv Eng Mater 2002;4: 659e62.
[18] Leng YJ, Chan SH, Khor KA, Jiang SP. Performance evaluation of anode-supported solid oxide fuel cells with thin film YSZ electrolyte. Int J Hydrogen Energy 2004;29:1025e33. [19] Liu QL, Chan SH, Fu CJ, Pasciak G. Fabrication and characterization of large-size electrolyte/anode bilayer structures for low-temperature solid oxide fuel cell stack based on gadolinia-doped ceria electrolyte. Electrochem Commun 2009;11:871e4. [20] Zhou W, Shi HG, Ran R, Cai R, Shao ZP, Jin WQ. Fabrication of an anode-supported yttria-stabilized zirconia thin film for solid-oxide fuel cells via wet powder spraying. J Power Sources 2008;184:229e37. [21] Shi HG, Zhou W, Ran R, Shao ZP. Comparative study of doped ceria thin-film electrolytes prepared by wet powder spraying with powder synthesized via two techniques. J Power Sources 2010;195:393e401. [22] Imanishi N, Ohno R, Murata K, Hirano A, Takeda Y, Yamamoto O, et al. LSM-YSZ cathode with infiltrated cobalt oxide and cerium oxide nanoparticles. Fuel Cells 2009;3:215e21. [23] Vohs JM, Gorte RJ. High-performance SOFC cathodes prepared by infiltration. Adv Mater 2009;21:943e56. [24] Chen J, Liang FL, Liu LN, Jiang SP, Li J. Characterization and evaluation of La0.8Sr0.2Co0.8Ni0.2O3d prepared by a polymerassisted combustion synthesis as a cathode material for intermediate temperature solid oxide fuel cells. Int J Hydrogen Energy 2009;34:6845e51. [25] Gao Z, Mao ZQ, Wang C, Liu ZX. Preparation and characterization of La1xSrxNiyFe1yO3d cathodes for lowtemperature solid oxide fuel cells. Int J Hydrogen Energy 2010;35:12905e10. [26] Duan ZS, Yang M, Yan AY, Hou ZF, Dong YL, Chong Y, et al. Ba0.5Sr0.5Co0.8Fe0.2O3d as a cathode for IT-SOFCs with a GDC interlayer. J Power Sources 2006;160:57e64. [27] Kao WX, Lee MC, Lin TN, Wang CH, Chang YC. Fabrication and characterization of a Ba0.5Sr0.5Co0.8Fe0.2O3d-Gadoliniadoped ceria cathode for an anode-supported solid-oxide fuel cell. J Power Sources 2010;195:2220e3. [28] Gu HX, Zheng Y, Ran R, Shao ZP, Jin WQ, Xu NP, et al. Synthesis and assessment of La0.8Sr0.2ScyMn1yO3d as cathodes for solid-oxide fuel cells on scandium-stabilized zirconia electrolyte. J Power Sources 2008;183:471e8. [29] Shao ZP, Yang WS, Cong Y, Dong H, Tong JH, Xiong GX. Investigation of the permeation behavior and stability of a Ba0.5Sr0.5Co0.8Fe0.2O3d oxygen membrane. J Membr Sci 2000;172:177e88. [30] Shao ZP, Haile SM. A high-performance cathode for the next generation of solid-oxide fuel cells. Nature 2004;439:170e3.
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Wet powder spraying fabrication and performance optimization of IT-SOFCs with thin-film ScSZ electrolyte Huangang Shi, Ran Ran, Zongping Shao* State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry and Chemical Engineering, Nanjing University of Technology, No. 5 Xin Mofan Road, Nanjing 210009, PR China
article info
abstract
Article history:
We have successfully fabricated a dense-type Scandia-stabilized zirconia (ScSZ) thin-film
Received 15 November 2010
electrolyte with thickness down to w4 mm for intermediate temperature solid oxide fuel
Received in revised form
cells (IT-SOFCs; 600e800 C) by a wet powder spraying technique. With an anode-supported
18 January 2011
design and a La0.8Sr0.2Sc0.1Mn0.9O3 (LSScM) oxide cathode, a peak power density of
Accepted 14 February 2011
788 mW cm2 was achieved at 800 C for a cell operating on hydrogen fuel. The impedance
Available online 13 March 2011
results demonstrated that the electrode polarization resistance is the main source of electrochemical performance loss for the single cell. Ba0.5Sr0.5Co0.8Fe0.2O3d þ Sm0.2Ce0.8O1.9
Keywords:
(BSCF þ SDC) composite cathode was then applied to improve the cell power output. An
Scandia-stabilized zirconia
SDC interlayer was adopted to avoid the interfacial phase reaction between ScSZ and BSCF,
Solid oxide fuel cell
which was also fabricated by the wet powder spraying. The effect of interlayer processing
Samaria-doped ceria
on the morphology and cell performance was studied by SEM-EDX, IeV polarization and
Interlayer
EIS. Pre-sintering of the ScSZ layer was found to be superior to the co-sintering of SDC interlayer and ScSZ electrolyte layer. An anode-supported cell with SDC interlayer fabricated on pre-sintered ScSZ electrolyte film and a BSCF þ SDC cathode delivered a peak power density as high as 1760 mW cm2 at 800 C. Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
As one of the high-temperature electrochemical energy conversion devices, solid oxide fuel cells (SOFCs) have received considerable attention recently because of their high energy conversion efficiency, low emissions and fuel flexibility [1,2]. Conventional SOFCs are based on thick yttriastabilized zirconia (YSZ) electrolyte and operate at around 1000 C. It is now generally accepted that the reduction of operation temperature to the intermediate temperature range of 500e800 C is the key for the successful implementation of SOFCs technology in the near future. One challenge is the sharp decrease in ionic conductivity of electrolyte with the drop of operation temperature, as a result, an obvious
increase in ohmic resistance and consequently a sharp decrease in cell power output is observed for a cell with thick YSZ electrolyte. In order to maintain high power output at intermediate temperature, a reduction of the electrolyte membrane thickness and the development of new electrolyte materials with high ionic conductivity become necessary [3,4]. Stabilized zirconia materials are the state-of-the-art electrolyte materials of sensors [5], high-temperature SOFCs [6] and solid oxide electrolysis cells [7], which possess pure and acceptable oxygen-ionic conductivity at high temperature, high mechanical strength and good thermal and chemical stability. Scandia-stabilized zirconia (ScSZ) shows the highest oxygen-ion conductivity in the cubic-fluorite zirconia phase because of the similar ionic radii of Sc3þ to Zr4þ [8], which is
* Corresponding author. Tel./fax: þ86 25 83172256. E-mail address: [email protected] (Z. Shao). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.02.077
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more promising than YSZ for reduced temperature application. If the electrolyte membrane thickness is reduced to tens of micron, SOFCs with ScSZ electrolyte could be operated at temperature down to 650 C with allowable ohmic resistance. A number of advanced techniques have been developed to fabricate thin-film electrolytes on porous substrates for SOFCs applications [9e14]. Recently wet powder spraying, sometimes called as suspension spraying, has been applied for the facile fabrication of thin-film electrolytes on porous anode substrate [15e19]. As a non-contact technique, it is suitable for flat substrates, corrugated sheets, tubes and a variety of other substrates and easy to scale up from laboratory to industrial fabrication. Previously we have successfully applied this technique for the fabrication of single layer of dense-type thin-film YSZ and samaria-doped ceria (SDC) electrolytes with the membrane thickness less than 10 mm [20,21]. With thin-film electrolyte structure, large electrode polarization resistance becomes the main obstacle for achieving high cell output at reduced temperature. The improvement of cathode performance becomes the key point to realize the reduced temperature operation of SOFCs [22e25]. Among the various cathode materials, some cobalt-containing perovskite-type composite oxides show mixed electronic and oxygen-ionic conductivities, and also high catalytic activity for oxygen electrochemical reduction at elevated temperatures. In particular Ba0.5Sr0.5Co0.8Fe0.2O3d (BSCF) is a novel cathode material for intermediate temperature solid oxide fuel cells (IT-SOFCs) with operation temperature down to 600 C. However, serious interfacial reaction between BSCF and stabilized zirconia was happened, which could result in a poor cell performance [26]. A doped ceria interlayer has been proposed to avoid the direct contact of BSCF with stabilized zirconia electrolyte since BSCF has good chemical compatibility with doped ceria electrolyte [26]. High cell performance at intermediate temperature was reported for a fuel cell based on stabilized zirconia electrolyte with BSCF cathode and a doped ceria interlayer [26,27]. In present work, the wet powder spraying technique was applied to fabricate thin-film ScSZ electrolyte on porous anode substrate for reduced temperature operation. The fuel cell performance was first measured by adopting a La0.8Sr0.2Sc0.1Mn0.9O3d (LSScM) cathode. To further improve the cell performance, BSCF was also investigated as a potential cathode in connection with an SDC interlayer, which was prepared by the same wet powder spraying technique. The effect of interlayer processing on the electrochemical performances of the single cells with thin-film ScSZ electrolyte and BSCF cathode was studied in detail.
2.
Experimental
2.1.
Powder synthesis
All the materials for powder synthesis are in AR grade. The (ZrO2)0.9(Sc2O3)0.1 (ScSZ) powder was synthesized by a combined ethylene diamine tetraacetic acid-citrate (EDTACA) complexing solegel process using Sc2O3 and Zr(NO3)4$5H2O as the staring materials. For a typical synthesis,
Sc2O3 was first dissolved in a nitric acid solution, then Zr(NO3)4$5H2O, EDTA and citric acid were added in sequence to the solution with the mole ratio of EDTA and citric acid to the total metal ions of 1:2:1, the pH value of the solution was adjusted to w6 with the addition of NH4$H2O, the solution was heated to evaporate the water under magnetic stirring until a transparent gel was obtained, which was then fired in oven at 250 C to form a solid precursor and further fired at 900 C for 5 h under an air atmosphere to result in an ScSZ powder for anode substrate tape casting. The SDC powder with the composition of Sm0.2Ce0.8O1.9 for the interlayer was synthesized by a hydrothermal treatment method. Solegel precursor for hydrothermal synthesis was prepared by co-precipitation of the aqueous solution of Sm(NO3)3$6H2O and Ce(NO3)3$6H2O with NH4$H2O at pH ¼ 10. The precursor was transferred to a Teflon vessel and hydrothermally treated at 180 C for 24 h. The product from the vessel was washed with water and ethanol repeatedly. Dried powder was subsequently calcinated at 800 C in air for 5 h to yield the SDC powder for the interlayer fabrication. La0.8Sr0.2Sc0.1Mn0.9O3 (LSScM) and Ba0.5Sr0.5Co0.8Fe0.2O3d (BSCF) cathode powders were also synthesized by the combined EDTA-citrate solegel process [28,29].
2.2.
Cell fabrication
The anode substrates were prepared by a tape casting process. The slurry for tape casting process was prepared by two-step ball milling. Commercial nickel oxide (Chengdu Shudu Nanomaterials Technology Development Co. Ltd.), ScSZ (prepared by combined EDTA-citrate solegel process, calcinated at 900 C) and starch was ball milled together with organic solvent in an agate jar for 24 h. Triethanolamine was added as surfactant for the dispersion of oxide powder in organic solvent in the first step. In the second step, polyvinyl butyral (PVB) as binder, polyethylene glycol (PEG) and dibutyl o-phthalate (DOP) as plasticizer were added to the slurry and then ball milled again for another 24 h. The slurry was vacuum pumped under 200 mbar (absolute pressure) to remove air and then cast onto the polymer carrier on a tape casting machine. The slurry was dried in air for 24 h and then detached from the tape. Anode substrates for single cells were drilled from the NiO þ ScSZ tape with a diameter of 16 mm. The disks were then sintered at elevated temperature for subsequent electrolyte deposition. The ScSZ electrolyte layer and SDC interlayer were prepared by a wet powder spraying technique. The ScSZ and SDC powders were prepared into colloidal suspensions with solid content of about 5%. The colloidal suspension was then sprayed under the drive of 1 atm nitrogen carrier gas onto the anode substrate (or ScSZ) using a modified spraying gun (BD128, Fenghua Bida Machinery Manufacture Co. Ltd., China) with a nozzle size of 0.35 mm (pore diameter). The spray gun was aligned above the heated substrate (250 C on a hot plate) at a distance of 10 mm. The effective deposition speed calculated was about 0.005e0.008 g cm2 min1 ScSZ/SDC. The cells were then sintered at 1400 C for 5 h in air. LSScM or BSCF þ SDC (7:3 in mol ratio) was applied as the cathode, which was painted on the central surface of the electrolyte and fired at 1100 and 1000 C respectively in air for 2 h to allow
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the firm attachment of the cathode layer onto the electrolyte (interlayer) surface. The coin-shape cathode had an effective area of 0.48 cm2.
2.3.
Electrochemical test
Electrochemical performance test of the single cell was carried out in an in-lab constructed fuel cell test station. The fuel cell was first sealed onto a quartz tube using silver paste to form an anode chamber, which was then heated to 800 C at a rate of 2 C min1 and dwelled for 5 h. During the process, the organic in the paste was slowly burned out and the silver was softened
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to make an effective sealing. Dry hydrogen at a flow rate of 20 ml min1 [STP] was then introduced into the anode chamber to initiate the in-situ reduction of anodic NiO to metallic nickel. After the in-situ reduction for about 5 h, the hydrogen flow rate was increased to 80 ml min1 [STP] and bubbled through a water bottle at 25 C for IeV cell polarization test. During the test, ambient air was applied as the cathode atmosphere. IeV polarization curves were collected at 50 C intervals over a temperature range of 600e800 C using a digital source meter (model 2420, Keithley, USA) with a four-probe configuration. Electrochemical impedance spectroscopy (EIS) of the cell under open circuit voltage (OCV) conditions was measured with an electrochemical workstation comprised of a Solartron 1260 A frequency response analyzer with a Solartron 1287 Potentiostat. EIS was recorded in a frequency range of 105 Hze0.1 Hz with signal amplitude of 10 mV.
2.4.
Other characterization
The fuel cell morphologies were observed using an environmental scanning electron microscope (ESEM, Model QUANTA-200, FEI Company, Hillsboro, OR). Energy disperse spectroscopy (EDS) analysis were performed to analyze the reaction between SSZ electrolyte and SDC interlayer.
3.
Results and discussion
3.1.
Thin-film ScSZ electrolyte fuel cell without interlayer
The successful fabrication of crack-free dense ScSZ thin-film on porous anode substrate by wet powder spraying technique requires the preparation of high-quality starting powder with ultrafine particles, homogenous particle size distribution and no large amount of aggregates. EDTAeCA complexing solegel process can result in fine powder with high purity and high surface activity; however, a lot of soft aggregates are also presented, while the high energy ball milling can effectively break such soft aggregates [20]. Therefore, in this study the assynthesized ScSZ powder from the EDTAeCA complexing process was subjected for ball milling for 30 min in an
Fig. 1 e SEM images of the surface morphologies of: (a) precalcinated anode substrate, (b) sintered ScSZ electrolyte, and (c) cross-sectional view of the single cell with single layer ScSZ electrolyte and LSScM cathode.
Fig. 2 e IeV and IeP curves for a typical SOFC with single layer ScSZ electrolyte and LSScM cathode.
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Fig. 3 e Electrochemical impedance spectra under open circuit conditions at different temperatures for the single cell with single layer ScSZ electrolyte and LSScM cathode.
ethylene glycol liquid medium before applying for thin-film electrolyte fabrication. The green electrolyte layer from the wet powder spraying was built from the loose packing of the ScSZ particles, thus a following sintering process is needed for the densification of
the electrolyte and also the firm adhesion of the electrolyte layer to the anode substrate. Since the gas evolved during the organic burning may lead to the formation of crack in the electrolyte layer, the anode substrate with the organic additives is preferred to be pre-fired to get rid of the organic substances before the deposition of the electrolyte layer. A proper firing temperature is critical to allow sufficient mechanical strength and matched sintering shrinkage to the electrolyte layer during the following co-sintering process. In this study, a firing temperature of 1150 C was found to be optimal, and the shrinkage of SDC and ScSZ layers for the later sintering was found to match each other pretty well. Shown in Fig. 1 are the SEM images of the surface morphologies of the anode substrate and the sintered ScSZ electrolyte. The surface of the anode substrate fabricated by the tape casting method was relatively rough after the firing at 1150 C. A proper surface roughness for the anode substrate is beneficial for the well adhesion of the electrolyte layer to the substrate because it will increase the contact area between them. As shown in Fig. 1b, there are no observable pores and cracks on the surface of ScSZ electrolyte after the sintering at 1400 C, suggesting the well densification of the electrolyte layer. LSScM was selected as the cathode, which was deposited onto the electrolyte surface by spray deposition and calcinated at 1150 C for 5 h in air. As shown in Fig. 1c is SEM image of a single cell with unreduced anode from crosssectional view. The electrolyte layer has a thickness of around 4 mm, which is well adhered to the anode substrate.
Fig. 4 e SEM images from cross-sectional view and the surface morphologies of the SOFC with SDC interlayer, a and c: Cell 1 and, b and d: Cell 2.
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Fig. 5 e IeV and IeP curves of the fuel cells with SDC interlayer between 600 and 800 C, with humidified H2 as fuel and ambient air as oxidant, (a) Cell 1, and (b) Cell 2.
The fuel cell was then subjected for electrochemical performance test. During the measurement, humidified hydrogen (3% H2O) was the introduced to the anode while the cathode was exposed to the ambient atmosphere. Under open circuit conditions, the cell voltages reached 1.08, 1.09 and 1.10 V at 800, 750 and 700 C respectively, as shown in Fig. 2. The high voltages further demonstrate the electrolyte layer prepared by the wet powder spraying technique is well densified. The peak power densities reached 788, 607 and 375 mW cm2 at 800, 750 and 700 C, respectively. Fig. 3 shows the EIS of the cell under OCV conditions at different temperatures. The low-frequency intercept gives the total resistance, which includes the ohmic resistance of the cell, the concentration polarization (mass-transfer or gas-diffusion polarization) resistance and the effective interfacial polarization resistances associated with the electrochemical reactions at both electrodeeelectrolyte interfaces (anode-electrolyte and cathode-electrolyte). The ohmic resistance is 0.23, 0.26 and 0.31 U cm2 respectively at 800, 750 and 700 C. The ratio of ohmic resistance to total cell resistance is about 12% at 800 C, which further reduces to only 9% at 700 C. It indicates that the electrode polarization resistance plays a dominant role in the electrochemical performance loss at these operating temperatures for the current anode-supported thin-film ScSZ electrolyte fuel cell with LSScM cathode. It suggests the cathode
Fig. 6 e EIS plots of the SOFC with SDC interlayer at different temperatures under open circuit conditions, (a) Cell 1, and (b) Cell 2.
optimization is critical to further improve the cell performance especially at the lower temperature range.
3.2. Thin-film ScSZ electrolyte fuel cell with SDC interlayer BSCF has been found to show excellent electrochemical activity for oxygen reduction on doped ceria electrolyte at intermediate temperature [30]. However, BSCF cannot be directly applied to the stabilized zirocnia electrolyte because serious interfacial reaction happens between BSCF and stabilized zirconia at elevated temperature during the fuel cell fabrication or operation [27], which leads to a significant increase in interfacial polarization resistance. The application of a doped ceria as an interlayer can effectively avoid the direct contact between BSCF and stabilized zirconia
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Fig. 7 e Ohmic resistance (Ro), polarization resistance (Rp) and total resistance (Rt) at different temperatures for, (a) Cell 1, and (b) Cell 2.
electrolyte; as a result, a significant improvement in cell performance was observed [26]. In this study, SDC was adopted as an interlayer to improve the cell performance by using BSCF as the cathode layer, a wet powder spraying technique was also applied for fabricating the interlayer. Although the introduction of SDC interlayer can effectively avoid the direct contact of BSCF with ScSZ electrolyte, it introduces the new possibility of interfacial reaction between SDC and ScSZ, which could lead to an increase in ohmic resistance of the fuel cell. Optimizing the fabrication process of the interlayer is crucial to achieve low ohmic resistance of the cell. In this study, two different ways were tried for the fabrication of the interlayer. For Cell 1, single layer ScSZ electrolyte was first deposited on anode substrate and fired at 1400 C in air for 5 h for the sintering of the electrolyte layer, and then additional SDC layer was sprayed on the sintered ScSZ electrolyte as interlayer and further sintered at 1400 C for 5 h in air. For Cell 2, the SDC interlayer was deposited on the green ScSZ electrolyte, and then the ScSZ electrolyte layer and the SDC layer were co-sintered at 1400 C for 5 h in air. Fig. 4 shows the SEM images of Cell 1 and Cell 2 from crosssectional view. For both cells, the ScSZ layers are well
Fig. 8 e EDS line-scan results for the SOFC with SDC interlayer with the scan lines shown in Fig. 4, (a) Cell 1, and (b) Cell 2.
densified and free of crack, which have a membrane thickness of w4 mm and adhere to anode substrate pretty well. Obvious sintering of the SDC interlayer was observed for Cell 1 and Cell 2, but the SDC layer in Cell 2 is much better sintered than that in Cell 1. Furthermore, the connection between SDC interlayer and ScSZ electrolyte layer is slightly better for Cell 1. For Cell 1, the ScSZ layer is pre-sintered before the deposition of the SDC interlayer, the sintering of the SDC layer was constricted since the ScSZ layer experienced little shrinking during the sintering process. For Cell 2, the co-sintering of SDC and ScSZ layers then allows much better sintering of the SDC interlayer. Fig. 4c and d shows the surface morphologies of the SDC interlayers in Cell 1 and Cell 2, respectively. Many cracks were formed over the SDC interlayer of Cell 1, while much fewer cracks were detected in the SDC interlayer of Cell 2. Such difference can be attributed to the larger mismatch in shrinkage between SDC layer and ScSZ layer in Cell 1 than Cell 2 during the sintering. The shrinkage of the ScSZ in Cell 1 during the sintering is w 0% because it was already sintered during the first sintering process, while it reached around 18% for Cell 2. For both cells, rough SDC surfaces are observed,
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beneficial for increasing the contacting surface between BSCF cathode and the SDC interlayer. Fig. 5 gives the cell performance at different temperatures of Cell 1 and Cell 2 with BSCF þ SDC cathode, operating on hydrogen fuel. The peak power densities are 1760, 1450, 1090, 713 and 418 mW cm2 respectively at 800, 750, 700, 650 and 600 C for Cell 1, while the corresponding values are 960, 733, 510, 287 and 154 mW cm2 for Cell 2. As compared to the cell without the SDC interlayer (Fig. 2), the cell performance was obviously improved for both cells, especially at lower temperature. As a comparison of the power outputs of Cell 1 and Cell 2, a significant effect of the processing of the interlayer on the cell performance was demonstrated. It is very interesting that although Cell 2 has much better densified SDC interlayer than Cell 1, however, much higher cell performance was observed for Cell 1. To get more information to interpret such phenomena, the electrochemical impedances of two cells under OCV conditions were measured with the results shown in Fig. 6. The total polarization resistance of Cell 2 is larger than Cell 1 at all corresponding temperatures. All the spectra can be separated into a high frequency arc and a low frequency arc. The impedance spectra can be fitted to an equivalent circuit containing parallel resistor/constant phase element components and an inductor along with the series resistor, as shown in Fig. 6c. The series resistance is contributed from the ohmic resistance of the cell, which is somewhat smaller than the high frequency intercept of the impedance spectra. The low-frequency intercept gives the total resistance, which includes the ohmic resistance of the cell, the concentration polarization (mass-transfer or gas-diffusion polarization) resistance and the effective interfacial polarization resistances associated with the electrochemical reactions at both electrodeeelectrolyte interfaces (anode-electrolyte and cathode-electrolyte). Fig. 7 gives the results of the ohmic resistance (Ro), polarization resistance (Rp) and total resistance (Rt) at different temperatures for both cells. The ohmic resistances for Cell 2 are 0.22, 0.29, 0.39, 0.61 and 1.12 U cm2 at 800, 750, 700, 650 and 600 C, respectively. The corresponding values for Cell 1 are 0.22, 0.23, 0.26, 0.33 and 0.47 U cm2. The ohmic resistances for Cell 1 and Cell 2 are contributed from both the ScSZ electrolyte layer and the SDC interlayer. Although the SDC layer in Cell 2 is much more densified than that in cell 1, it is very interesting that the ohmic resistances of Cell 2 are higher than cell 1. Furthermore, the electrode polarization resistances of cell 1 are also somewhat smaller than Cell 2. As a result, much higher cell performance was observed for Cell 1 than Cell 2, as shown in Fig. 5. As mentioned previously, the application of an SDC interlayer can effectively avoid the direct contact between BSCF cathode and ScSZ electrolyte which is beneficial to reduce the interfacial polarization. Indeed, the interfacial polarization resistance of BSCF cathode deposited directly on YSZ electrolyte reached as large as w12 U cm2 at 700 C (a value from the symmetrical cell under ambient air), while it is only 0.42 U cm2 for Cell 1 with an SDC interlayer. However, SDC interlayer also introduces a new SDCeScSZ interface. The cation inter-diffusion between SDC and ScSZ layers could be happened at elevated temperature. The diffusion of Sc from ScSZ to SDC layer and the diffusion of Sm and Ce from SDC to YSZ could result in a decrease in the ionic conductivity of SDC
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and ScSZ. Fig. 8 shows the EDS profiles by line-scanning through cross-section of Cell 1 and Cell 2 (scan from the top of SDC layer to the ScSZ electrolyte, with scan lines shown in Fig. 4). For Cell 2, the reaction thickness reaches w2.1 mm, which is deeper than Cell 1 with a thickness of w1.3 mm. This is well understood because the ScSZ in Cell 2 had higher reactivity and diffusivity than that in Cell 1 since the ScSZ layer was not sintered before the deposition of the SDC interlayer in Cell 2. It then well explained the better cell performance of Cell 1 than Cell 2.
4.
Conclusions
In this study, an anode-supported thin-film ScSZ electrolyte with membrane thickness down to 4 mm was successfully fabricated by a wet powder spraying technique. Without the SDC interlayer, peak power densities of 788 and 379 mW cm2 were successfully achieved respectively at 800 and 700 C for the cell with a LSScM cathode. The electrode polarization resistance was demonstrated the main source of the electrochemical performance loss. By adopting BSCF þ SDC composite cathode with an SDC interlayer, the different ways of SDC interlayer processing was found to have significant effect on the cell performance. Pre-sintering of the ScSZ electrolyte layer before the deposition of SDC interlayer is preferred to the cosintering of SDC and ScSZ layers for achieving a higher power output. It effectively suppressed the cation diffusion, which could result in an increase in ohmic resistance of the electrolyte layer and interlayer. Cell with a BSCF þ SDC composite cathode and SDC interlayer deposited on dense ScSZ electrolyte exhibited peak power densities as high as 1760 mW cm2 at 800 C and 418 mW cm2 at 600 C. It highly promises the wet powder spraying technique for fabrication of dual layer electrolyte SOFCs with high power output.
Acknowledgments This work was supported by the National 863 Program under contract No. 2007AA05Z133, by the National Basic Research Program of China under contract No. 2007CB209704, by “Outstanding Young Scholar Grant at Jiangsu Province” under contract No. 2008023, and by program for New Century Excellent Talents (2008), and by Fok Ying Tung Education Foundation under contract No. 111073.
references
[1] Steele BCH, Heinzel A. Materials for fuel-cell technologies. Nature 2001;414:345e52. [2] Gorte RJ. Recent developments towards commercialization of solid oxide fuel cells. AIChE J 2005;51:2377e81. [3] Su PC, Chao C, Shim JH, Fasching R, Prinz FB. Solid oxide fuel cell with corrugated thin film electrolyte. Nano Lett 2008;8:2289e92. [4] Huang JB, Mao ZQ, Liu ZX, Wang C. Development of novel low-temperature SOFCs with co-ionic conducting SDCcarbonate composite electrolytes. Electrochem Commun 2007;9:2601e5.
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[5] Sato T, Plashnitsa VV, Utiyama M, Miura N. Potentiometric YSZ-based sensor using NiO sensing electrode aiming at detection of volatile organic compounds (VOCs) in air environment. Electrochem Commun 2010;12:524e6. [6] Wen TL, Wang D, Chen M, Tu H, Lu Z, Zhang Z, et al. Material research for planar SOFC stack. Solid State Ionics 2002;148: 513e9. [7] Ebbesen SD, Mogensen M. Electrolysis of carbon dioxide in solid oxide electrolysis cells. J Power Sources 2009;193: 349e58. [8] Politova TI, Irvine JTS. Investigation of scandiaeyttriaezirconia system as an electrolyte material for intermediate temperature fuel cellsdinfluence of yttria content in system (Y2O3)x(Sc2O3)(11x)(ZrO2)89. Solid State Ionics 2004;168:153e65. [9] Rousseau F, Awamat S, Nikravech M, Morvan D, Amouroux J. Deposit of dense YSZ electrolyte and porous NiOeYSZ anode for SOFC device by a low pressure plasma process. Surf Coat Technol 2007;202:1226e30. [10] Talebi T, Haji M, Raissi B, Maghsoudipour A. YSZ electrolyte coating on NiOeYSZ composite by electrophoretic deposition for solid oxide fuel cells (SOFCs). Int J Hydrogen Energy 2010; 35:9455e99. [11] Brahim C, Ringuede´ A, Gourba E, Cassir M, Billard A, Briois P. Electrical properties of thin bilayered YSZ/GDC SOFC electrolyte elaborated by sputtering. J Power Sources 2006;56: 45e9. [12] Chen YY, Wei CWJ. Processing and characterization of ultrathin yttria-stabilized zirconia (YSZ) electrolytic films for SOFC. Solid State Ionics 2006;177:351e7. [13] Jung HY, Hong KS, Kim H, Park JK, Son JW, Kim J, et al. Characterization of thin-film YSZ deposited via EB-PVD technique in anode-supported SOFCs. J Electrochem Soc 2006;153:A961e6. [14] Ge XD, Huang XQ, Zhang YH, Lu Z, Xu JH, Chen KF, et al. Screen-printed thin YSZ films used as electrolytes for solid oxide fuel cells. J Power Sources 2006;159:1048e50. [15] Souza S, Visco SJ, Jonghe LC. Thin-film solid oxide fuel cell with high performance at low temperature. Solid State Ionics 1997;98:57e61. [16] Zhang YH, Liu J, Huang XQ, Lu Z, Su WH. Performance evaluation of thin membranes solid oxide fuel cell prepared by pressure-assisted slurry-casting. Int J Hydrogen Energy 2008;33:775e80. [17] Schu¨ller E, Vaßen R, Sto¨ver D. Thin electrolyte layers for SOFC via wet powder spraying (WPS). Adv Eng Mater 2002;4: 659e62.
[18] Leng YJ, Chan SH, Khor KA, Jiang SP. Performance evaluation of anode-supported solid oxide fuel cells with thin film YSZ electrolyte. Int J Hydrogen Energy 2004;29:1025e33. [19] Liu QL, Chan SH, Fu CJ, Pasciak G. Fabrication and characterization of large-size electrolyte/anode bilayer structures for low-temperature solid oxide fuel cell stack based on gadolinia-doped ceria electrolyte. Electrochem Commun 2009;11:871e4. [20] Zhou W, Shi HG, Ran R, Cai R, Shao ZP, Jin WQ. Fabrication of an anode-supported yttria-stabilized zirconia thin film for solid-oxide fuel cells via wet powder spraying. J Power Sources 2008;184:229e37. [21] Shi HG, Zhou W, Ran R, Shao ZP. Comparative study of doped ceria thin-film electrolytes prepared by wet powder spraying with powder synthesized via two techniques. J Power Sources 2010;195:393e401. [22] Imanishi N, Ohno R, Murata K, Hirano A, Takeda Y, Yamamoto O, et al. LSM-YSZ cathode with infiltrated cobalt oxide and cerium oxide nanoparticles. Fuel Cells 2009;3:215e21. [23] Vohs JM, Gorte RJ. High-performance SOFC cathodes prepared by infiltration. Adv Mater 2009;21:943e56. [24] Chen J, Liang FL, Liu LN, Jiang SP, Li J. Characterization and evaluation of La0.8Sr0.2Co0.8Ni0.2O3d prepared by a polymerassisted combustion synthesis as a cathode material for intermediate temperature solid oxide fuel cells. Int J Hydrogen Energy 2009;34:6845e51. [25] Gao Z, Mao ZQ, Wang C, Liu ZX. Preparation and characterization of La1xSrxNiyFe1yO3d cathodes for lowtemperature solid oxide fuel cells. Int J Hydrogen Energy 2010;35:12905e10. [26] Duan ZS, Yang M, Yan AY, Hou ZF, Dong YL, Chong Y, et al. Ba0.5Sr0.5Co0.8Fe0.2O3d as a cathode for IT-SOFCs with a GDC interlayer. J Power Sources 2006;160:57e64. [27] Kao WX, Lee MC, Lin TN, Wang CH, Chang YC. Fabrication and characterization of a Ba0.5Sr0.5Co0.8Fe0.2O3d-Gadoliniadoped ceria cathode for an anode-supported solid-oxide fuel cell. J Power Sources 2010;195:2220e3. [28] Gu HX, Zheng Y, Ran R, Shao ZP, Jin WQ, Xu NP, et al. Synthesis and assessment of La0.8Sr0.2ScyMn1yO3d as cathodes for solid-oxide fuel cells on scandium-stabilized zirconia electrolyte. J Power Sources 2008;183:471e8. [29] Shao ZP, Yang WS, Cong Y, Dong H, Tong JH, Xiong GX. Investigation of the permeation behavior and stability of a Ba0.5Sr0.5Co0.8Fe0.2O3d oxygen membrane. J Membr Sci 2000;172:177e88. [30] Shao ZP, Haile SM. A high-performance cathode for the next generation of solid-oxide fuel cells. Nature 2004;439:170e3.