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Composite cathodes based on Sm0.5Sr0.5CoO3−δ with porous Gd-doped ceria barrier layers for solid oxide fuel cells
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 5 3 2 0 e1 5 3 3 3
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Composite cathodes based on Sm0.5Sr0.5CoO3Ld with porous Gd-doped ceria barrier layers for solid oxide fuel cells Young Min Park a, Haekyoung Kim b,* a b
Fuel Cell Project, Research Institute of Industrial Science and Technology, Pohang 790-330, Republic of Korea School of Materials Science & Engineering, Yeungnam University, Gyeongsan 712-749, Republic of Korea
article info
abstract
Article history:
Cobaltite-based perovskites based on Sm0.5Sr0.5CoO3d (SSC) are attractive as a cathode
Received 21 June 2012
material with a barrier layer for solid oxide fuel cells (SOFC) due to their high electro-
Received in revised form
chemical activity and electrical conductivity. SSC, synthesized by a complex method, is
24 July 2012
used as a cathode material in a composite cathode with Gd-doped ceria (GDC). A porous
Accepted 27 July 2012
GDC layer is fabricated as a barrier to resist reactions of SSC with yttria-stabilized zirconia
Available online 15 August 2012
(YSZ). The effects of the ratio of SSC on GDC in composite cathodes and the thickness of the GDC barrier are characterized in this study. An SOFC with an SSC7eGDC3 composite
Keywords:
cathode on a 4 mm GDC layer at 0.8 V yields the highest fuel cell performance: 1.24 W cm2
Solid oxide fuel cell
and 0.61 W cm2 at 780 C and 680 C, respectively. Impedance analysis indicates that the
Composite cathode
ohmic resistances are more dependent upon the GDC barrier thickness than the cathode
Anode supported cell
composition. The polarization resistances at 780 C and 730 C exhibit similar values, but
Barrier layer
with decreasing temperature, the polarization resistances change dramatically according
Sm0.5Sr0.5CoO3d
to the composition and barrier thickness. The ohmic and polarization resistances show different trends in different temperature ranges, due to the different charge transfer mechanisms of SSC and GDC within those temperature ranges. To obtain higher fuel cell performance, the addition of GDC into the porous SSC is effective, and the compositions of the composite cathode as well as the thickness of the barrier layer need to be optimized. Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
The solid oxide fuel cell (SOFC) is considered to be the most promising future component of energy generation systems for power plants and distributed power systems. High operating temperature limits the selection of materials and the longterm stability of materials and components, which
motivates reducing the operation temperature of SOFCs. However, the ohmic and polarization resistances increase when the operation temperature decreases, which decreases the fuel cell performance. The La1xSrxMnO3 (LSM)-based cathode is a commonly used cathode material for high temperature SOFCs, but it performs poorly when the operating temperature of the SOFC is below 800 C, because of
* Corresponding author. Tel.: þ82 53 810 2536; fax: þ82 53 810 4628. E-mail address: [email protected] (H. Kim). 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2012.07.118
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Fig. 1 e Cross-sectional images of SOFCs.
cathodes and YSZ electrolyte, dense or porous interlayers such as GDC and SDC has been used [32e35]. The performance of a cathode is related to the sintering temperature, microstructure (such as the grain size and porosity) and composition [3,18,36e42]. Adding GDC to the porous SSC cathode is believed to be beneficial because it reduces the difference between thermal expansion coefficients of the electrolyte and cathode. Furthermore, the electrochemical performance of SSC porous cathodes can be improved by adding GDC, which suppresses coarsening of the SSC particles, and thereby maintains porosity and the triple phase boundary (TPB)
*SSC +GDC +
+
*
+
900 C
+
*
*
*
Intensity(a.u.)
sluggish oxygen reduction reaction (ORR) kinetics. Hence, considerable efforts are underway to develop a new class of perovskite-based cathode materials for SOFCs with mixed ionic and electronic conductivity and high electro-catalytic activity for oxygen reduction, at a lower operating temperature (700e800 C) [1e11]. Cobaltite-based perovskites such as (LSC), Sm0.5Sr0.5CoO3d (SSC), La0.5Sr0.5CoO3d La0.5Sr0.5Co0.2Fe0.8O3d (LSCF), and Ba0.5Sr0.5Co0.8Fe0.2O3d (BSCF) are attractive due to their high electrochemical activity and electrical conductivity [12e19]. BSCF cathodes exhibit very low polarization resistance due to their high oxygen diffusion coefficient, and yield high power density at a low temperature [20]. However, their large thermal expansion coefficient and low electrical conductivity result in mismatch with the electrolyte, and this limits the cell performance [21]. LSC possesses high electronic and oxygen ion conductivity over a wide temperature range, but it also has large mismatch in thermal expansion coefficient [22]. Although its thermal expansion coefficient can be reduced by doping Co-sites with Fe3þ (LSCF), the Fe3þ doping decreases the ionic conductivity of LSC [18,23e26]. Sm1xSrxCoO3d compounds show higher ionic conductivity than La1xSrxCoO3d [13,27,28], and are good catalysts for oxygen reduction [29]. Sm1xSrxCoO3d composite exhibits better electronic conductivity than pure BSCF [30]. Ishihara et al. [31] and Fukunaga et al. [27] also reported that the overpotential of dense SSC film is lower than that of dense LSC film under the same conditions, and that the oxygen adsorptionedesorption rate of SSC is one order of magnitude higher than that of LSC. The disadvantages of SSC are its large thermal expansion coefficient and its high reactivity with YSZ electrolyte above 900 C [28]. To reduce the mismatch and prevent reactions between porous SSC
+
(111)
(200)
(220)
+
+*
+
+
(300) (222)
+
+
*
(400)
+
+ +
+
*
(331) (420)
+ + GDC +
*
(002)
*
20
(022)
*
(231)
*
40
(422)
*
60
SSC
(611)
*
*
80
2 Theta
Fig. 2 e X-ray diffraction patterns of SSC and GDC mixture.
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length. Performance can also be improved by optimizing the conductivity and size distribution of particles of each solid phase to yield a larger TPB contact area that is accessible for oxygen reduction. In this study, we synthesized a cathode material of Sm0.5Sr0.5CoO3d (SSC) using a complex method, and
fabricated the composite cathode with SSC and GDC. Porous GDC barrier layers of various thicknesses were applied to prevent reactions between SSC and YSZ. The fuel cell performances and impedances are studied and characterized according to the ratio of SSC to GDC, and the thickness of the barrier layer.
Fig. 3 e Fuel cell performances of SOFCs with compositions at 780 C with GDC layer thickness of (a) 4 mm, (b) 7 mm, and (c) 11 mm.
Fig. 4 e Fuel cell performances of SOFCs with compositions at 680 C with GDC layer thickness of (a) 4 mm, (b) 7 mm, and (c) 11 mm.
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 5 3 2 0 e1 5 3 3 3
2.
Experimental
Materials used included Sm(NO3)3$6H2O (4.440 g, 0.01 mol), Sr(NO3)2 (2.116 g, 0.01 mol) and Co(NO3)2$6H2O (5.821 g, 0.02 mol), all in analytical grades. They were dissolved in 100 ml of deionized water to prepare a homogeneous nitrate solution. Ethylenediaminetetraacetic acid (EDTA, 5.845 g, 0.04 mol) and citric acid (7.645 g, 0.02 mol) powders, each with a purity level higher than 99.5%, were added for chelating into a homogeneous nitrate solution. The solution was heated in a water bath at 70 C. After obtaining a clear solution, and the mixture was continuously heated to evaporate the water until the magnetic bar stopped rotating. The remaining mixture was dried at 80 C in a vacuum oven overnight to remove residual water. The dried powders were crushed and calcined at 700 C to form a perovskite phase, removing the organic compounds.
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To characterize the fuel cell performance, anodesupported cells (ASCs), which consist of Ni-YSZ of e 900 mm as a support layer, Ni-YSZ of e 20 mm as an anode functional layer (AFL), and YSZ of e 10 mm as an electrolyte, were fabricated using a tape casting and co-firing method at 1370 C. ASCs were cut into circles with diameters of 2.6 cm from a 25 cm 25 cm plate. To fabricate a GDC barrier layer, GDC powder was prepared by a complex method, using 0.1 mol Gd(NO3)3$6H2O, 0.9 mol Ce(NO3)3$6H2O, and 1 mol of citric acid. GDC powder was calcinated at 800 C for 3 h, and then mixed with organic binder solution of ethyl cellulose, di-ethylene glycol butyl ether, and a-terpinol. To enhance the sinterability, 1 mol % of Co3O4 was added. GDC pastes were screen-printed onto the Ni-YSZ/YSZ ASC and then sintered at 1100 C for 3 h. The thicknesses of the GDC barrier layers were 4, 7, and 10 mm, which were controlled by screen printing time. Composite cathodes were prepared from synthesized SSC powders and GDC
Fig. 5 e Summary of fuel cell performances at 0.8 V with (a) The cathode compositions and (b) The barrier thicknesses.
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Fig. 6 e Impedance spectra of SOFCs with GDC layer thickness of 4 mm.
Fig. 7 e Impedance spectra of SOFCs with GDC layer thickness of 7 mm.
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Fig. 8 e Impedance spectra of SOFCs with 11 mm GDC layer.
powders with organic binder solution and screen-printed on the Ni-YSZ/YSZ/GDC anode-supported cell. The composite cathodes were coated and coded as SSCXeGDCY (X and Y are respective weight ratios of SSC and GDC), and were sintered at 800 C for 3 h. The active cathode area was 0.785 cm2. The fabricated cells were assembled and sealed with Ceramabond 571 from AREMCO, to measure currentevoltage characteristics and impedances. Pt paste and mesh were used for current collecting. The SOFCs were heated, and the anode reduction was performed with 300 cc min1 of 97% H2 e 3% H2O for 3 h. The fuel cell performance was measured at each temperature with 300 cc min1 of 97% H2 e 3% H2O and 1000 cc min1 of air. The impedances were measured with the WEIS system from Wonatech. The impedance spectra were obtained in the frequency range of 100 kHz to 0.1 Hz with an
applied AC voltage amplitude of 100 mV at each temperature and open circuit voltage. After electrochemical measurements, the microstructures of the SOFCs were characterized via scanning electron microscopy using JSM-6480LV equipment.
3.
Results and discussions
The cross-sectional images of the SOFCs based on the SSCeGDC composite cathodes are shown in Fig. 1. The porous GDC barrier layers of 4, 7, and 11 mm are shown in the SOFCs. SOFCs have 20e30 mm cathode layers. The cathode needs to be chemically stable with an electrolyte. SSC is reactive with YSZ, and the 2nd resistive phase peaks, such as SrZrO3 and Sm2Zr2O7, appeared at 800 C [28]. A barrier layer
Fig. 9 e Equivalent circuit.
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such as GDC is needed to prevent reactions between SSC and YSZ [27,28]. The stability of SSC with GDC is shown in the Xray diffraction spectra of Fig. 2. SSC is stable with GDC below 900 C, which is a temperature high enough for SOFC
operation. Thus, the stability of SSC with GDC is proven in this study, and GDC is proposed as a barrier layer for SSC. The fuel cell performances of SOFCs are shown and summarized in Figs. 3e5. The electrochemical performances
Fig. 10 e Fitting results of impedance spectra in SOFCs at open circuit voltages at (a) 780 C with 4 mm, (b) 7 mm, and (c) 11 mm.
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Fig. 11 e Fitting results of impedance spectra in SOFCs at open circuit voltages at 680 C with (a) 4 mm, (b) 7 mm, and (c) 11 mm.
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Table 1 e Summary of impedance values from the fitting model.
7 mm
630 C
680 C
730 C
780 C
630 C
Ro R1 R2 R3 R4 R5
0.586 0.2256 0.3109 0.3391 0.1798 0.0679
0.3664 0.0914 0.1441 0.1866 0.1452 0.0664
0.2287 0.0466 0.095 0.0633 0.1193 0.0855
0.162 0.0247 0.0593 0.0402 0.1221 0.0893
0.5233 0.1149 0.24 0.3379 0.1836 0.0534
Ro R1 R2 R3 R4 R5
0.3543 0.1292 0.2245 0.2619 0.1011 0.1076
0.2129 0.0632 0.1152 0.1462 0.1083 0.0847
0.138 0.0311 0.0798 0.0875 0.1056 0.0912
0.1064 0.0096 0.0517 0.0618 0.112 0.0965
0.4283 0.1104 0.1629 0.2058 0.0861 0.0934
Ro R1 R2 R3 R4 R5
0.3219 0.0848 0.2179 0.2821 0.1011 0.1164
0.1919 0.0527 0.1197 0.1557 0.0785 0.1109
0.1224 0.0338 0.1009 0.0929 0.1067 0.0714
0.0837 0.0155 0.0723 0.0525 0.1081 0.0768
0.4776 0.0695 0.161 0.2708 0.1181 0.0961
Ro R1 R2 R3 R4 R5
0.6277 0.3455 0.3829 0.5452 0.3444 0.1062
0.3916 0.0846 0.1268 0.3797 0.0514 0.2086
0.2308 0.0434 0.0843 0.167 0.1741 0.0607
0.1573 0.0232 0.0731 0.0773 0.1682 0.0636
0.6507 0.231 0.3694 0.5496 0.2056 0.0683
680 C SSC5 GDC5 0.3177 0.0818 0.1708 0.1083 0.059 0.1618 SSC6 GDC4 0.2563 0.069 0.0972 0.0835 0.0784 0.0982 SSC7 GDC3 0.271 0.0533 0.1289 0.0995 0.0972 0.0977 SSC10 0.4027 0.0791 0.1288 0.2786 0.1689 0.0568
11 mm
730 C
780 C
630 C
680 C
730 C
780 C
0.1974 0.0428 0.1002 0.0599 0.1524 0.0634
0.1398 0.0269 0.0666 0.031 0.1359 0.0727
0.4511 0.0817 0.1998 0.5679 0.2175 0.0768
0.2833 0.0448 0.096 0.3268 0.1539 0.0569
0.1915 0.0363 0.0739 0.1951 0.109 0.0593
0.1437 0.0224 0.0804 0.085 0.0905 0.0625
0.1639 0.0253 0.0862 0.061 0.0872 0.0874
0.1176 0.018 0.0764 0.0479 0.0986 0.0803
0.4987 0.1004 0.1775 0.2273 0.1048 0.0764
0.2945 0.0449 0.119 0.113 0.0989 0.0803
0.1848 0.0345 0.0988 0.0416 0.082 0.0829
0.1329 0.0383 0.0852 0.0459 0.113 0.0746
0.1716 0.0377 0.0995 0.0435 0.0815 0.1
0.1077 0.0207 0.0602 0.0436 0.0911 0.1052
0.5849 0.0717 0.219 0.4113 0.1343 0.0918
0.3331 0.0816 0.1421 0.1566 0.136 0.0633
0.2014 0.0491 0.1361 0.0653 0.1237 0.0691
0.1293 0.0264 0.0568 0.0333 0.0718 0.0861
0.2451 0.0418 0.1082 0.0889 0.1582 0.0546
0.1755 0.0305 0.0584 0.0429 0.1485 0.0635
0.495 0.1594 0.3848 0.6656 0.1629 0.0573
0.295 0.0591 0.0512 0.1852 0.2979 0.1099
0.2023 0.0275 0.0852 0.183 0.0994 0.0693
0.1552 0.0283 0.0681 0.0639 0.0908 0.0712
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4 mm
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of SSC porous cathodes are improved by adding GDC as shown in Fig. 5(a), which results from resisting the coarsening of SSC particles, thereby maintaining the porosity and the TPB length. SOFCs with SSC7eGDC3 show higher values than those with SSC6eGDC4, SSC5eGDC5, or SSC10. The effects of the barrier thickness may be discerned from Fig. 5(b), which shows the current densities at 0.8 V with the temperature and the GDC barrier thickness. The fuel cell performances of SSC7eGDC3 and SSC6eGDC4 increase with decreasing GDC barrier thickness. A thicker GDC barrier might increase the ohmic resistance, resulting in lower fuel cell performances. However, SOFCs with SSC10 and SSC5eGDC5 exhibit similar performances, even with various GDC barrier thicknesses. The effect of a thinner GDC barrier, the reactivity of SSC with YSZ through porous GDC layer, and the reaction sites might be compensated, resulting in similar fuel cell performances. In this work, SOFC at 0.8 V with SSC7eGDC3 composite cathode on a 4 mm GDC layer yielded the highest fuel cell performances: 1.24 W cm2 and 0.61 W cm2 at 780 C and 680 C, respectively. The fuel cell performance here is a function of the SSC reaction sites, the ohmic resistance of barrier, and the reactivity of SSC
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through porous GDC, which could be compensated. Higher fuel cell performance might be observed due to optimization of the conductivity, a larger TPB contact area of SSC, reactivity of SSC and YSZ, and GDC in composite cathode. To study the effects of barrier and cathode compositions, the impedance spectra at open circuit voltage (OCV) are shown with SSC and GDC composition in Figs. 6e8. The equivalent circuits for analysis are presented in Fig. 9. Leonide et al. show an equivalent circuit of the anode-supported SOFC, which has five impedance elements connected in series by distribution of relaxation time [43,44]. Based on their circuit, which contains generalized finite Warburg elements and Gerischer elements, the impedances are modeled by five RQ elements. The fitting results of the composite cathodes at OCV are presented in Figs. 10 and 11, and the fitting results show good matching with experimental data. R0 is from the ohmic resistance of the YSZ electrolyte, anode, composite cathode, GDC barrier, and the connection wires; L is the inductance, which is attributed to the platinum currentevoltage probes or the heating elements of the furnace used to heat the sample; R1Q1 and R3Q3
Fig. 12 e Plot of ohmic resistance with (a) The cathode compositions and (b) The barrier thickness.
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correspond to the gas diffusion coupled with the charge transfer reaction and ionic transport; R2Q2 corresponds to the oxygen surface exchange kinetics and oxygen ion diffusivity at the cathode; while R4Q4 and R5Q5 correspond to the gas diffusion reaction [44e47]. The impedance values from the fitting model are summarized in Table 1. The ohmic resistances and polarization resistances of the SOFCs are plotted in Figs. 12 and 13. Fig. 12 shows that the ohmic resistances of the SOFCs with SSC7eGDC3 cathodes are the lowest. By adding GDC to the porous SSC cathode, the ohmic resistance decreases. With decreasing barrier layer, SOFCs exhibit lowered ohmic resistances in SSC7eGDC3 and SSC6eGDC4. However, in SSC10 and SSC5eGDC5, the ohmic resistance increases with decreasing barrier thickness. In this study, the ohmic resistance differences are expected from the interfacial resistance induced by the barrier layer and the conductivity by the cathode compositions. In SSC7eGDC3 and SSC6eGDC4, the increment of interfacial-resistance-induced barrier layer might be more dominant than the conductivity of the composite cathode. In SSC10 and SSC5eGDC5, the conductivity of the cathode might be more dominant than the resistance increment of the barrier layer. The polarization resistances are also slightly lowered by adding GDC to the porous SSC as shown in Fig. 13. The effects of adding GDC are dramatically increased with decreasing temperature. The
barrier thicknesses of 7 mm are optimized in the polarization resistance. The polarization resistances at 780 C exhibit similar values, but with decreasing temperature, the resistances change significantly according to the composition and GDC thickness. The polarization resistances of SSC7eGDC3 and SSC6eGDC4 show lower values than those of SSC10 and SSC5eGDC5. From the impedances analysis, the ohmic and polarization resistances are lowered with GDC added to the porous SSC. The increments of the fuel cell performances result from the lowered ohmic resistance concerned with the higher conductivity and lower interfacial resistance. The effects of polarization resistances with the cathode compositions and the barrier thickness are dominant at temperatures lower than 680 C. However, at temperatures higher than 730 C, the polarization effects are not very significant. When the ohmic resistances are plotted with temperature in Fig. 14, the slopes of the SOFCs are similar. The effects of cathode composition and GDC barrier thickness might be compensated, which results in similar activation energy in the ohmic resistance. However, the polarization resistances at different temperatures exhibit different slopes, due to the charge transfer and gas-coupled charge transfer. Furthermore, the polarization resistance has two slopes in different temperature ranges, which can be induced by the different charge transfer mechanisms.
Fig. 13 e Plot of polarization resistance with (a) The cathode compositions and (b) The barrier thickness.
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Fig. 14 e Plot of (a) Ohmic resistances and (b) Polarization resistance with temperature.
4.
Conclusions
SSCs were synthesized using a complex method with citric acid and EDTA. SSCeGDC composite cathodes were fabricated with different ratios of SSC and GDC on ASC, consisting of an Ni-YSZ anode support, an Ni-YSZ anode functional layer, YSZ electrolyte, and a porous GDC barrier layer. The electrochemical performances of SSC porous cathodes were improved by adding GDC, which resulted from helping to resist the coarsening of SSC particles, thereby maintaining the porosity and the TPB length. In this work, the SOFC with the SSC7eGDC3 composite cathode on a 4 mm GDC layer operating at 0.8 V yielded the highest fuel cell performances: 1.24 W cm2 and 0.61 W cm2 at 780 C and 680 C, respectively. Higher fuel cell performance might be observed due to optimization of the conductivity, a larger TPB contact area of SSC, reactivity of SSC and YSZ, and GDC in the composite cathode. From the impedances analysis, the ohmic and polarization resistances are lowered after adding GDC to the porous SSC. The increments of the fuel cell performances
result from the lowered ohmic resistance concerned with the higher conductivity and lower interfacial resistance. The effects of polarization resistances with the cathode compositions and the barrier thickness are dominant at temperatures lower than 680 C.
Acknowledgments This work was supported by the National Research Foundation (NRF) of Korea Grant funded by the Korean Government (MEST) (NRF-2012-0004203).
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[22] Adler SB. Limitations of charge-transfer models for mixedconducting oxygen electrodes. Solid State Ionics 2000;135: 603e12. [23] Yamamoto O, Takeda Y, Kanno R, Noda M. Perovskite-type oxides as oxygen electrodes for high temperature oxide fuel cells. Solid State Ionics 1987;22:241e6. [24] Jiang SP, Wang W. Novel structured mixed ionic and electronic conducting cathodes of solid oxide fuel cells. Solid State Ionics 2005;176:1351e7. [25] Haanappel VAC, Mai A, Mertens J. Electrode activation of anode-supported SOFCs with LSM- or LSCF-type cathodes. Solid State Ionics 2006;177:2033e7. [26] Petric A, Tietz PHF. Evaluation of LaeSreCoeFeeO perovskites for solid oxide fuel cells and gas separation membranes. Solid State Ionics 2000;135:719e25. [27] Fukunaga H, Koyama M, Takahashi N, Wen C, Yamada K. Reaction model of dense Sm0.5Sr0.5CoO3 as SOFC cathode. Solid State Ionics 2000;132:279e85. [28] Tu HY, Takeda Y, Imanishi N, Yamamoto O. Ln1xSrxCoO3(Ln ¼ Sm, Dy) for the electrode of solid oxide fuel cells. Solid State Ionics 1997;100:283e8. [29] Baumann FS, Maier J, Fleig J. The polarization resistance of mixed conducting SOFC cathodes: a comparative study using thin film model electrodes. Solid State Ionics 2008;179: 1198e204. [30] Zhu W, Lu Z, Li S, Wei B, Miao J, Huang X, et al. Study on Ba0.5Sr0.5Co0.8Fe0.2O3de Sm0.5Sr0.5CoO3d composite cathode materials for IT-SOFCs. J Alloy Comp 2008;465:274e9. [31] Ishihara T, Honda M, Shibayama T, Minami H, Nishiguchi H, Takita V. Intermediate temperature solid oxide fuel cells using a new LaGaO3 based oxide ion conductor. J Electrochem Soc 1998;145:3177e83. [32] Jung H, Sun Y, Jung H, Park J, Kim H, Kim G, et al. Investigation of anode-supported SOFC with cobaltcontaining cathode and GDC interlayer. Solid State Ionics 2008;179:1535e9. [33] Yang D, Zhang X, Nikumb S, Deces-Petit C, Hui R, Maric R, et al. Low temperature solid oxide fuel cells with pulsed laser deposited bi-layer electrolyte. J Power Sources 2007;164: 182e8. [34] Zhang X, Robertson M, Deces-Petit C, Xie Y, Hui R, Qu W, et al. Solid oxide fuel cells with bi-layered electrolyte structure. J Power Sources 2008;175:800e5. [35] Xia CR, Rauch W, Chen FL, Liu ML. Sm0.5Sr0.5CoO3 cathodes for low-temperature SOFCs. Solid State Ionics 2002;149: 11e9. [36] Gunasekaran N, Saddawi S, Carberry JJ. Effect of surface area on the oxidation of methane over solid oxide solution catalyst La0.8Sr0.2MnO3. J Catal 1996;159:107e11. [37] Liu Y, Zheng HT, Liu JR, Zhang T. Preparation of high surface area La1xAxMnO3 (A¼Ba, Sr or Ca) ultra-fine particles used for CH4 oxidation. Chem Eng J 2002;89: 213e21. [38] Dutta A, Mukhopadhyay J, Basu RN. Combustion synthesis and characterization of LSCF-based materials as cathode of intermediate temperature solid oxide fuel cells. J Eur Ceram Soc 2009;29:2003e11. [39] Shukla S, Seal S, Vij R, Bandyopadhyay S. Reduced activation energy for grain growth in nanocrystalline yttria-stabilized zirconia. Nano Lett 2003;3:397e401. [40] Leng YJ, Chan SH, Liu Q. Development of LSCF-GDC composite cathodes for low-temperature solid oxide fuel cells with thin film GDC electrolyte. Int J Hydrogen Energy 2008;33:3808e17. [41] Qiang F, Sun K, Zhang N, Zhua X, Le S, Zhou D. Characterization of electrical properties of GDC doped A-site deficient LSCF based composite cathode using impedance spectroscopy. J Power Sources 2007;168:338e45.
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Composite cathodes based on Sm0.5Sr0.5CoO3Ld with porous Gd-doped ceria barrier layers for solid oxide fuel cells Young Min Park a, Haekyoung Kim b,* a b
Fuel Cell Project, Research Institute of Industrial Science and Technology, Pohang 790-330, Republic of Korea School of Materials Science & Engineering, Yeungnam University, Gyeongsan 712-749, Republic of Korea
article info
abstract
Article history:
Cobaltite-based perovskites based on Sm0.5Sr0.5CoO3d (SSC) are attractive as a cathode
Received 21 June 2012
material with a barrier layer for solid oxide fuel cells (SOFC) due to their high electro-
Received in revised form
chemical activity and electrical conductivity. SSC, synthesized by a complex method, is
24 July 2012
used as a cathode material in a composite cathode with Gd-doped ceria (GDC). A porous
Accepted 27 July 2012
GDC layer is fabricated as a barrier to resist reactions of SSC with yttria-stabilized zirconia
Available online 15 August 2012
(YSZ). The effects of the ratio of SSC on GDC in composite cathodes and the thickness of the GDC barrier are characterized in this study. An SOFC with an SSC7eGDC3 composite
Keywords:
cathode on a 4 mm GDC layer at 0.8 V yields the highest fuel cell performance: 1.24 W cm2
Solid oxide fuel cell
and 0.61 W cm2 at 780 C and 680 C, respectively. Impedance analysis indicates that the
Composite cathode
ohmic resistances are more dependent upon the GDC barrier thickness than the cathode
Anode supported cell
composition. The polarization resistances at 780 C and 730 C exhibit similar values, but
Barrier layer
with decreasing temperature, the polarization resistances change dramatically according
Sm0.5Sr0.5CoO3d
to the composition and barrier thickness. The ohmic and polarization resistances show different trends in different temperature ranges, due to the different charge transfer mechanisms of SSC and GDC within those temperature ranges. To obtain higher fuel cell performance, the addition of GDC into the porous SSC is effective, and the compositions of the composite cathode as well as the thickness of the barrier layer need to be optimized. Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
The solid oxide fuel cell (SOFC) is considered to be the most promising future component of energy generation systems for power plants and distributed power systems. High operating temperature limits the selection of materials and the longterm stability of materials and components, which
motivates reducing the operation temperature of SOFCs. However, the ohmic and polarization resistances increase when the operation temperature decreases, which decreases the fuel cell performance. The La1xSrxMnO3 (LSM)-based cathode is a commonly used cathode material for high temperature SOFCs, but it performs poorly when the operating temperature of the SOFC is below 800 C, because of
* Corresponding author. Tel.: þ82 53 810 2536; fax: þ82 53 810 4628. E-mail address: [email protected] (H. Kim). 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2012.07.118
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Fig. 1 e Cross-sectional images of SOFCs.
cathodes and YSZ electrolyte, dense or porous interlayers such as GDC and SDC has been used [32e35]. The performance of a cathode is related to the sintering temperature, microstructure (such as the grain size and porosity) and composition [3,18,36e42]. Adding GDC to the porous SSC cathode is believed to be beneficial because it reduces the difference between thermal expansion coefficients of the electrolyte and cathode. Furthermore, the electrochemical performance of SSC porous cathodes can be improved by adding GDC, which suppresses coarsening of the SSC particles, and thereby maintains porosity and the triple phase boundary (TPB)
*SSC +GDC +
+
*
+
900 C
+
*
*
*
Intensity(a.u.)
sluggish oxygen reduction reaction (ORR) kinetics. Hence, considerable efforts are underway to develop a new class of perovskite-based cathode materials for SOFCs with mixed ionic and electronic conductivity and high electro-catalytic activity for oxygen reduction, at a lower operating temperature (700e800 C) [1e11]. Cobaltite-based perovskites such as (LSC), Sm0.5Sr0.5CoO3d (SSC), La0.5Sr0.5CoO3d La0.5Sr0.5Co0.2Fe0.8O3d (LSCF), and Ba0.5Sr0.5Co0.8Fe0.2O3d (BSCF) are attractive due to their high electrochemical activity and electrical conductivity [12e19]. BSCF cathodes exhibit very low polarization resistance due to their high oxygen diffusion coefficient, and yield high power density at a low temperature [20]. However, their large thermal expansion coefficient and low electrical conductivity result in mismatch with the electrolyte, and this limits the cell performance [21]. LSC possesses high electronic and oxygen ion conductivity over a wide temperature range, but it also has large mismatch in thermal expansion coefficient [22]. Although its thermal expansion coefficient can be reduced by doping Co-sites with Fe3þ (LSCF), the Fe3þ doping decreases the ionic conductivity of LSC [18,23e26]. Sm1xSrxCoO3d compounds show higher ionic conductivity than La1xSrxCoO3d [13,27,28], and are good catalysts for oxygen reduction [29]. Sm1xSrxCoO3d composite exhibits better electronic conductivity than pure BSCF [30]. Ishihara et al. [31] and Fukunaga et al. [27] also reported that the overpotential of dense SSC film is lower than that of dense LSC film under the same conditions, and that the oxygen adsorptionedesorption rate of SSC is one order of magnitude higher than that of LSC. The disadvantages of SSC are its large thermal expansion coefficient and its high reactivity with YSZ electrolyte above 900 C [28]. To reduce the mismatch and prevent reactions between porous SSC
+
(111)
(200)
(220)
+
+*
+
+
(300) (222)
+
+
*
(400)
+
+ +
+
*
(331) (420)
+ + GDC +
*
(002)
*
20
(022)
*
(231)
*
40
(422)
*
60
SSC
(611)
*
*
80
2 Theta
Fig. 2 e X-ray diffraction patterns of SSC and GDC mixture.
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length. Performance can also be improved by optimizing the conductivity and size distribution of particles of each solid phase to yield a larger TPB contact area that is accessible for oxygen reduction. In this study, we synthesized a cathode material of Sm0.5Sr0.5CoO3d (SSC) using a complex method, and
fabricated the composite cathode with SSC and GDC. Porous GDC barrier layers of various thicknesses were applied to prevent reactions between SSC and YSZ. The fuel cell performances and impedances are studied and characterized according to the ratio of SSC to GDC, and the thickness of the barrier layer.
Fig. 3 e Fuel cell performances of SOFCs with compositions at 780 C with GDC layer thickness of (a) 4 mm, (b) 7 mm, and (c) 11 mm.
Fig. 4 e Fuel cell performances of SOFCs with compositions at 680 C with GDC layer thickness of (a) 4 mm, (b) 7 mm, and (c) 11 mm.
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 5 3 2 0 e1 5 3 3 3
2.
Experimental
Materials used included Sm(NO3)3$6H2O (4.440 g, 0.01 mol), Sr(NO3)2 (2.116 g, 0.01 mol) and Co(NO3)2$6H2O (5.821 g, 0.02 mol), all in analytical grades. They were dissolved in 100 ml of deionized water to prepare a homogeneous nitrate solution. Ethylenediaminetetraacetic acid (EDTA, 5.845 g, 0.04 mol) and citric acid (7.645 g, 0.02 mol) powders, each with a purity level higher than 99.5%, were added for chelating into a homogeneous nitrate solution. The solution was heated in a water bath at 70 C. After obtaining a clear solution, and the mixture was continuously heated to evaporate the water until the magnetic bar stopped rotating. The remaining mixture was dried at 80 C in a vacuum oven overnight to remove residual water. The dried powders were crushed and calcined at 700 C to form a perovskite phase, removing the organic compounds.
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To characterize the fuel cell performance, anodesupported cells (ASCs), which consist of Ni-YSZ of e 900 mm as a support layer, Ni-YSZ of e 20 mm as an anode functional layer (AFL), and YSZ of e 10 mm as an electrolyte, were fabricated using a tape casting and co-firing method at 1370 C. ASCs were cut into circles with diameters of 2.6 cm from a 25 cm 25 cm plate. To fabricate a GDC barrier layer, GDC powder was prepared by a complex method, using 0.1 mol Gd(NO3)3$6H2O, 0.9 mol Ce(NO3)3$6H2O, and 1 mol of citric acid. GDC powder was calcinated at 800 C for 3 h, and then mixed with organic binder solution of ethyl cellulose, di-ethylene glycol butyl ether, and a-terpinol. To enhance the sinterability, 1 mol % of Co3O4 was added. GDC pastes were screen-printed onto the Ni-YSZ/YSZ ASC and then sintered at 1100 C for 3 h. The thicknesses of the GDC barrier layers were 4, 7, and 10 mm, which were controlled by screen printing time. Composite cathodes were prepared from synthesized SSC powders and GDC
Fig. 5 e Summary of fuel cell performances at 0.8 V with (a) The cathode compositions and (b) The barrier thicknesses.
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Fig. 6 e Impedance spectra of SOFCs with GDC layer thickness of 4 mm.
Fig. 7 e Impedance spectra of SOFCs with GDC layer thickness of 7 mm.
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Fig. 8 e Impedance spectra of SOFCs with 11 mm GDC layer.
powders with organic binder solution and screen-printed on the Ni-YSZ/YSZ/GDC anode-supported cell. The composite cathodes were coated and coded as SSCXeGDCY (X and Y are respective weight ratios of SSC and GDC), and were sintered at 800 C for 3 h. The active cathode area was 0.785 cm2. The fabricated cells were assembled and sealed with Ceramabond 571 from AREMCO, to measure currentevoltage characteristics and impedances. Pt paste and mesh were used for current collecting. The SOFCs were heated, and the anode reduction was performed with 300 cc min1 of 97% H2 e 3% H2O for 3 h. The fuel cell performance was measured at each temperature with 300 cc min1 of 97% H2 e 3% H2O and 1000 cc min1 of air. The impedances were measured with the WEIS system from Wonatech. The impedance spectra were obtained in the frequency range of 100 kHz to 0.1 Hz with an
applied AC voltage amplitude of 100 mV at each temperature and open circuit voltage. After electrochemical measurements, the microstructures of the SOFCs were characterized via scanning electron microscopy using JSM-6480LV equipment.
3.
Results and discussions
The cross-sectional images of the SOFCs based on the SSCeGDC composite cathodes are shown in Fig. 1. The porous GDC barrier layers of 4, 7, and 11 mm are shown in the SOFCs. SOFCs have 20e30 mm cathode layers. The cathode needs to be chemically stable with an electrolyte. SSC is reactive with YSZ, and the 2nd resistive phase peaks, such as SrZrO3 and Sm2Zr2O7, appeared at 800 C [28]. A barrier layer
Fig. 9 e Equivalent circuit.
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such as GDC is needed to prevent reactions between SSC and YSZ [27,28]. The stability of SSC with GDC is shown in the Xray diffraction spectra of Fig. 2. SSC is stable with GDC below 900 C, which is a temperature high enough for SOFC
operation. Thus, the stability of SSC with GDC is proven in this study, and GDC is proposed as a barrier layer for SSC. The fuel cell performances of SOFCs are shown and summarized in Figs. 3e5. The electrochemical performances
Fig. 10 e Fitting results of impedance spectra in SOFCs at open circuit voltages at (a) 780 C with 4 mm, (b) 7 mm, and (c) 11 mm.
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 5 3 2 0 e1 5 3 3 3
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Fig. 11 e Fitting results of impedance spectra in SOFCs at open circuit voltages at 680 C with (a) 4 mm, (b) 7 mm, and (c) 11 mm.
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Table 1 e Summary of impedance values from the fitting model.
7 mm
630 C
680 C
730 C
780 C
630 C
Ro R1 R2 R3 R4 R5
0.586 0.2256 0.3109 0.3391 0.1798 0.0679
0.3664 0.0914 0.1441 0.1866 0.1452 0.0664
0.2287 0.0466 0.095 0.0633 0.1193 0.0855
0.162 0.0247 0.0593 0.0402 0.1221 0.0893
0.5233 0.1149 0.24 0.3379 0.1836 0.0534
Ro R1 R2 R3 R4 R5
0.3543 0.1292 0.2245 0.2619 0.1011 0.1076
0.2129 0.0632 0.1152 0.1462 0.1083 0.0847
0.138 0.0311 0.0798 0.0875 0.1056 0.0912
0.1064 0.0096 0.0517 0.0618 0.112 0.0965
0.4283 0.1104 0.1629 0.2058 0.0861 0.0934
Ro R1 R2 R3 R4 R5
0.3219 0.0848 0.2179 0.2821 0.1011 0.1164
0.1919 0.0527 0.1197 0.1557 0.0785 0.1109
0.1224 0.0338 0.1009 0.0929 0.1067 0.0714
0.0837 0.0155 0.0723 0.0525 0.1081 0.0768
0.4776 0.0695 0.161 0.2708 0.1181 0.0961
Ro R1 R2 R3 R4 R5
0.6277 0.3455 0.3829 0.5452 0.3444 0.1062
0.3916 0.0846 0.1268 0.3797 0.0514 0.2086
0.2308 0.0434 0.0843 0.167 0.1741 0.0607
0.1573 0.0232 0.0731 0.0773 0.1682 0.0636
0.6507 0.231 0.3694 0.5496 0.2056 0.0683
680 C SSC5 GDC5 0.3177 0.0818 0.1708 0.1083 0.059 0.1618 SSC6 GDC4 0.2563 0.069 0.0972 0.0835 0.0784 0.0982 SSC7 GDC3 0.271 0.0533 0.1289 0.0995 0.0972 0.0977 SSC10 0.4027 0.0791 0.1288 0.2786 0.1689 0.0568
11 mm
730 C
780 C
630 C
680 C
730 C
780 C
0.1974 0.0428 0.1002 0.0599 0.1524 0.0634
0.1398 0.0269 0.0666 0.031 0.1359 0.0727
0.4511 0.0817 0.1998 0.5679 0.2175 0.0768
0.2833 0.0448 0.096 0.3268 0.1539 0.0569
0.1915 0.0363 0.0739 0.1951 0.109 0.0593
0.1437 0.0224 0.0804 0.085 0.0905 0.0625
0.1639 0.0253 0.0862 0.061 0.0872 0.0874
0.1176 0.018 0.0764 0.0479 0.0986 0.0803
0.4987 0.1004 0.1775 0.2273 0.1048 0.0764
0.2945 0.0449 0.119 0.113 0.0989 0.0803
0.1848 0.0345 0.0988 0.0416 0.082 0.0829
0.1329 0.0383 0.0852 0.0459 0.113 0.0746
0.1716 0.0377 0.0995 0.0435 0.0815 0.1
0.1077 0.0207 0.0602 0.0436 0.0911 0.1052
0.5849 0.0717 0.219 0.4113 0.1343 0.0918
0.3331 0.0816 0.1421 0.1566 0.136 0.0633
0.2014 0.0491 0.1361 0.0653 0.1237 0.0691
0.1293 0.0264 0.0568 0.0333 0.0718 0.0861
0.2451 0.0418 0.1082 0.0889 0.1582 0.0546
0.1755 0.0305 0.0584 0.0429 0.1485 0.0635
0.495 0.1594 0.3848 0.6656 0.1629 0.0573
0.295 0.0591 0.0512 0.1852 0.2979 0.1099
0.2023 0.0275 0.0852 0.183 0.0994 0.0693
0.1552 0.0283 0.0681 0.0639 0.0908 0.0712
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 5 3 2 0 e1 5 3 3 3
4 mm
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 5 3 2 0 e1 5 3 3 3
of SSC porous cathodes are improved by adding GDC as shown in Fig. 5(a), which results from resisting the coarsening of SSC particles, thereby maintaining the porosity and the TPB length. SOFCs with SSC7eGDC3 show higher values than those with SSC6eGDC4, SSC5eGDC5, or SSC10. The effects of the barrier thickness may be discerned from Fig. 5(b), which shows the current densities at 0.8 V with the temperature and the GDC barrier thickness. The fuel cell performances of SSC7eGDC3 and SSC6eGDC4 increase with decreasing GDC barrier thickness. A thicker GDC barrier might increase the ohmic resistance, resulting in lower fuel cell performances. However, SOFCs with SSC10 and SSC5eGDC5 exhibit similar performances, even with various GDC barrier thicknesses. The effect of a thinner GDC barrier, the reactivity of SSC with YSZ through porous GDC layer, and the reaction sites might be compensated, resulting in similar fuel cell performances. In this work, SOFC at 0.8 V with SSC7eGDC3 composite cathode on a 4 mm GDC layer yielded the highest fuel cell performances: 1.24 W cm2 and 0.61 W cm2 at 780 C and 680 C, respectively. The fuel cell performance here is a function of the SSC reaction sites, the ohmic resistance of barrier, and the reactivity of SSC
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through porous GDC, which could be compensated. Higher fuel cell performance might be observed due to optimization of the conductivity, a larger TPB contact area of SSC, reactivity of SSC and YSZ, and GDC in composite cathode. To study the effects of barrier and cathode compositions, the impedance spectra at open circuit voltage (OCV) are shown with SSC and GDC composition in Figs. 6e8. The equivalent circuits for analysis are presented in Fig. 9. Leonide et al. show an equivalent circuit of the anode-supported SOFC, which has five impedance elements connected in series by distribution of relaxation time [43,44]. Based on their circuit, which contains generalized finite Warburg elements and Gerischer elements, the impedances are modeled by five RQ elements. The fitting results of the composite cathodes at OCV are presented in Figs. 10 and 11, and the fitting results show good matching with experimental data. R0 is from the ohmic resistance of the YSZ electrolyte, anode, composite cathode, GDC barrier, and the connection wires; L is the inductance, which is attributed to the platinum currentevoltage probes or the heating elements of the furnace used to heat the sample; R1Q1 and R3Q3
Fig. 12 e Plot of ohmic resistance with (a) The cathode compositions and (b) The barrier thickness.
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correspond to the gas diffusion coupled with the charge transfer reaction and ionic transport; R2Q2 corresponds to the oxygen surface exchange kinetics and oxygen ion diffusivity at the cathode; while R4Q4 and R5Q5 correspond to the gas diffusion reaction [44e47]. The impedance values from the fitting model are summarized in Table 1. The ohmic resistances and polarization resistances of the SOFCs are plotted in Figs. 12 and 13. Fig. 12 shows that the ohmic resistances of the SOFCs with SSC7eGDC3 cathodes are the lowest. By adding GDC to the porous SSC cathode, the ohmic resistance decreases. With decreasing barrier layer, SOFCs exhibit lowered ohmic resistances in SSC7eGDC3 and SSC6eGDC4. However, in SSC10 and SSC5eGDC5, the ohmic resistance increases with decreasing barrier thickness. In this study, the ohmic resistance differences are expected from the interfacial resistance induced by the barrier layer and the conductivity by the cathode compositions. In SSC7eGDC3 and SSC6eGDC4, the increment of interfacial-resistance-induced barrier layer might be more dominant than the conductivity of the composite cathode. In SSC10 and SSC5eGDC5, the conductivity of the cathode might be more dominant than the resistance increment of the barrier layer. The polarization resistances are also slightly lowered by adding GDC to the porous SSC as shown in Fig. 13. The effects of adding GDC are dramatically increased with decreasing temperature. The
barrier thicknesses of 7 mm are optimized in the polarization resistance. The polarization resistances at 780 C exhibit similar values, but with decreasing temperature, the resistances change significantly according to the composition and GDC thickness. The polarization resistances of SSC7eGDC3 and SSC6eGDC4 show lower values than those of SSC10 and SSC5eGDC5. From the impedances analysis, the ohmic and polarization resistances are lowered with GDC added to the porous SSC. The increments of the fuel cell performances result from the lowered ohmic resistance concerned with the higher conductivity and lower interfacial resistance. The effects of polarization resistances with the cathode compositions and the barrier thickness are dominant at temperatures lower than 680 C. However, at temperatures higher than 730 C, the polarization effects are not very significant. When the ohmic resistances are plotted with temperature in Fig. 14, the slopes of the SOFCs are similar. The effects of cathode composition and GDC barrier thickness might be compensated, which results in similar activation energy in the ohmic resistance. However, the polarization resistances at different temperatures exhibit different slopes, due to the charge transfer and gas-coupled charge transfer. Furthermore, the polarization resistance has two slopes in different temperature ranges, which can be induced by the different charge transfer mechanisms.
Fig. 13 e Plot of polarization resistance with (a) The cathode compositions and (b) The barrier thickness.
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Fig. 14 e Plot of (a) Ohmic resistances and (b) Polarization resistance with temperature.
4.
Conclusions
SSCs were synthesized using a complex method with citric acid and EDTA. SSCeGDC composite cathodes were fabricated with different ratios of SSC and GDC on ASC, consisting of an Ni-YSZ anode support, an Ni-YSZ anode functional layer, YSZ electrolyte, and a porous GDC barrier layer. The electrochemical performances of SSC porous cathodes were improved by adding GDC, which resulted from helping to resist the coarsening of SSC particles, thereby maintaining the porosity and the TPB length. In this work, the SOFC with the SSC7eGDC3 composite cathode on a 4 mm GDC layer operating at 0.8 V yielded the highest fuel cell performances: 1.24 W cm2 and 0.61 W cm2 at 780 C and 680 C, respectively. Higher fuel cell performance might be observed due to optimization of the conductivity, a larger TPB contact area of SSC, reactivity of SSC and YSZ, and GDC in the composite cathode. From the impedances analysis, the ohmic and polarization resistances are lowered after adding GDC to the porous SSC. The increments of the fuel cell performances
result from the lowered ohmic resistance concerned with the higher conductivity and lower interfacial resistance. The effects of polarization resistances with the cathode compositions and the barrier thickness are dominant at temperatures lower than 680 C.
Acknowledgments This work was supported by the National Research Foundation (NRF) of Korea Grant funded by the Korean Government (MEST) (NRF-2012-0004203).
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