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Synthesis and electrocatalytic performance of La0.3Ce0.1Sr0.5Ba0.1TiO3 anode catalyst for solid oxide fuel cells
Electrochemistry Communications 43 (2014) 79–82
Contents lists available at ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Short communication
Synthesis and electrocatalytic performance of La0.3Ce0.1Sr0.5Ba0.1TiO3 anode catalyst for solid oxide fuel cells Xin-Wen Zhou a,b, Yi-Fei Sun a, Guang-Ya Wang a, Tong Gao a, Kart T. Chuang a, Jing-Li Luo a,⁎, Min Chen c, Viola I. Birss c a b c
Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta T6G 2V4, Canada College of Biological and Pharmaceutical Science, China Three Gorges University, Yichang 443002, China Department of Chemistry, University of Calgary, Alberta T2N 1N4, Canada
a r t i c l e
i n f o
Article history: Received 13 January 2014 Received in revised form 21 March 2014 Accepted 21 March 2014 Available online 29 March 2014 Keywords: Solid oxide fuel cells Anode catalyst Ce-doped La0.4Sr0.5Ba0.1TiO3–δ Rheological phase reaction
a b s t r a c t Ce-doped La0.4Sr0.5Ba0.1TiO3–δ (LCSBT) perovskite anode catalysts in solid oxide fuel cells were successfully synthesized by a modified rheological phase reaction for the first time. Pure LCSBT could be obtained under a reducing atmosphere and nano-CeO2 particles could be exsoluted from LCSBT after being sintered in air. The catalytic activity and electrochemical performance of LCSBT anodes for the H2 oxidation were obviously improved comparing with the pure La0.4Sr0.5Ba0.1TiO3–δ (LSBT) and LSBT&CeO2 admixture anodes. The improved performance could be attributed to the nanostructure of LCSBT and the exsoluted nano-CeO2 particles. © 2014 Elsevier B.V. All rights reserved.
1. Introduction La-substituted SrTiO3 (LST) perovskite-based oxide is a potential candidate anode material because of its high electronic conductivity in reducing atmospheres, its high chemical stability and excellent sulfur and coking tolerance [1,2]. Low ionic conductivity and poor electrocatalytic activity are the main disadvantages of LST-based anode. Presently, the main methods to improve these properties include (1) doping LST with other metal ions [3,4], (2) introducing an active second phase to LST to form composite anodes [5,6], and (3) synthesizing nano-structured LST anodes [7]. Among the doped-LST anodes, Ba-doped LST has shown strong doping effectiveness in increasing its ionic conductivity and improving the electrochemical performance [8,9]. CeO2 is an active catalyst to enhance the electrochemical performance of fuel cells. Therefore, some Ce-containing compounds have been studied in recent years, e.g., Ce-doped LST [10] and CeO2based LST composite anodes including CeO2 [2,11], Gd-doped CeO2 (GDC) [12], Y-doped CeO2 (YDC) [13] and Sm-doped CeO2 (SDC) [14] etc. Then, the comparison of the performance of Ce-doped LST and CeO2&LST composite anodes is attractive. Rheological phase reaction (RPR) is an effective soft chemical way to prepare nanomaterials because of the efficient utilization of the solid particle surface, the close and uniform contact between solid particles
⁎ Corresponding author. Tel.: +1 780 492 2232; fax: +1 780 492 2881. E-mail address: [email protected] (J.-L. Luo).
http://dx.doi.org/10.1016/j.elecom.2014.03.019 1388-2481/© 2014 Elsevier B.V. All rights reserved.
and fluids, and the proper heat exchange [15]. LiMn2O4 [15], LiFePO4 [16,17], Li4Ti5O12 [18] and KMn8O16 [19] electrode materials have been synthesized by the RPR method. BaTiO3 perovskite oxide having proved to be a promising sulfur and coking tolerant anode material [20], was also synthesized successfully by the RPR method [21]. In this study, we focus on the synthesis of Ce-doped La0.4Sr0.5Ba0.1TiO3–δ (LCSBT) perovskite anode catalysts by a modified RPR method. The electrochemical performances of LCSBT and LSBT&CeO2 anodes are also compared in details. 2. Experimental sections 2.1. Preparation and characterization of anode catalysts La0.3Ce0.1Sr0.5Ba0.1TiO3–δ (LCSBT) and La0.4Sr0.5Ba0.1TiO3–δ (LSBT) perovskite nanopowders were prepared using a modified RPR method [18]. The stoichiometric amounts of La(NO3)3 ⋅ 6H2O, Ce(NO3)3 ⋅ 6H2O, Sr(NO3)2 ⋅ 6H2O, Ba(NO3)2 ⋅ 6H2O, Ti(OC3H7)4 and citric acid (C6H8O7) were sufficiently ground at room temperature. During the grinding process, Ti(OC3H7)4 started to hydrolyze, accompanied by the evaporation of parts of propyl alcohol and nitrogen oxides. A proper amount of deionized water was added to accelerate the hydrolysis of Ti(OC3H7)4 and accommodate the conglutination resistance. The obtained rheological phase material was then transferred into a container that was subsequently sealed in a stainless autoclave at 120 °C for 12 h. The precursor was calcined in 10% H2–N2 (Praxair) at 1300 °C for 10 h. Finally, the black product was calcined in air at 400 °C to dispose of the
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remaining carbon. LSCBT powders sintered in 10% H2–N2 and air were denoted as LCSBT-H and LCSBT-A, respectively. LSBT and CeO2 (molar ratio: 10:1) was ball milled for 10 h to obtain the LSBT&CeO2 admixture. All the chemicals were purchased from Fisher Scientific and used as received without further purification. The phase structures and morphologies of the anode materials were characterized using a Rigaku Rotaflex X-ray diffractometer (XRD) with Co Kα radiation and a JEOL 6301F field emission scanning electron microscope (SEM), respectively. 2.2. Fuel cell fabrication and test
LSBT in air [10]. When the pure LCSBT was sintered in air at 1200 °C for 2 h, the peaks of CeO2 appeared (Fig. 1c), indicating that Ce could be separated from LCSBT to form a nano-CeO2 phase. The exsoluted CeO2 could not be re-compounded into the LSBT structure completely when sintering in reducing atmosphere again, ensuring the presence of the catalytically active nano-CeO2 phase [10]. In Fig. 1d, the peaks of LSBT and CeO2 were clearly observed in LSBT&CeO2. At the same time, the peak located near 28° in Fig. 1c was broadened compared with
a
The fuel cell testing system was fabricated and installed as described previously [3,8,9]. The fuel cells were fabricated using commercial YSZ disks (300 μm in thickness and 25 mm in diameter) as the electrolyte, strontium doped lanthanum manganite (LSM) powders as the cathode and LCSBT-H as the anode. The obtained electrode ink was screen printed onto the face of YSZ to form a membrane electrode assembly (MEA) with a circular area of 0.75 cm2, and then sintered in air at 1200 °C for 2 h. An Au anode current collector has been proven to be sulfur-tolerant, which is favorable for our next H2S-containing fuel application. For comparison, Au was used as the anode current collector and Pt was the most used cathode current collector because of its high stability and high electronic conductivity. The anodes were reduced in-situ at 950 °C in H2 for 10 h before the tests. The anode and cathode were fed with pure H2 and oxygen that were dry and had a flow rate of 100 mL min−1 for both. Fuel cell testing was performed with standard DC and AC electrochemical techniques using a Solartron instrument (SI1287 EI). The polarization resistance of the cell was measured using electrochemical impedance spectroscopy (EIS) from 1.0 MHz to 0.1 Hz at open circuit voltage (OCV). Each sample was tested 3 times under the same conditions and the error range between each measurement is controlled in 5%.
b
3. Results and discussion Fig. 1 shows the XRD of different LCSBT powders obtained under different conditions. In Fig. 1a, it can be seen that the pure cubic perovskite structure of the SrTiO3 type was synthesized under a reducing atmosphere [4,8,10]. The substitution for La by Ce led to the displacement of all the XRD peaks to higher angles and a slight contraction of lattice parameter (a = 5.5430 Å, b = 5.5279 Å, c = 7.8076 Å) compared with the literature [10]. In contrast, LCSBT obtained in air showed evidence of impurity peaks, which was attributed to cubic CeO2 (Fig. 1b). The results indicated that Ce could not be totally doped into
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Fig. 2. (a) Surface and (b) fracture cross-sectional SEM images of a typical cell before a fuel cell test, and (c) SEM images of LCSBT powders after sintering in air at 1200 °C for 2 h.
X.-W. Zhou et al. / Electrochemistry Communications 43 (2014) 79–82
respectively. The maximum power densities were 74 and 57 mW cm−2 using LSCBT and LSBT&CeO2 admixtures as anodes at 850 °C, respectively. The corresponding current densities were 153 and 108 mA cm−2. The Rohm of LCSBT and LSBT&CeO2 anodes were similar because of the same YSZ electrolyte and larger than expected for a 300 μm electrolyte. But the cause was not definitively identified [22]. The Rp of LCSBT to LSBT&CeO2 anodes was increased from 4.12 Ω cm−2 to 9.35 Ω cm− 2 which indicated that the enhancement of Ce-doping effect is better than that of CeO2 adding directly. These results suggest that the performance of LCSBT anode catalyst is better than that of LSBT&CeO2 admixture for H2 fuel cells. The improved electrochemical performance of the LCSBT anode might be attributable to the following factors. The first one is that the LCSBT anode catalysts obtained by the RHR method were smaller and much more uniform than those obtained by conventional solid state reaction [8]. These could increase the surface and the triple-phase boundary (TPB) lengths of the LCSBT anode, resulting in the improved cell performance. The second factor is related to the Ce-doping effect. The doped Ce in LSBT body could increase the electrical conductivity and the exsoluted active nano-CeO2 could enhance the cell performance. The exsoluted CeO2 was in nanoscale and the contact with LSBT was much more compact than that between the LSBT and the additive CeO2, which could increase the TPB effectively.
Fig. 1b and d, indicating that the exsoluted CeO2 had a smaller grain size according to the Scherrer formula. Fig. 2a and b shows the surface and fracture cross-sectional SEM images of a typical cell of LSCBT anode before a fuel cell test, respectively. In Fig. 2a, it can be seen that the LSCBT and YSZ were uniform and wellproportionally mixed together. The thickness of the LSCBT anode was about 13.0 μm. The contact between the LSCBT anode and the dense YSZ was compact and clear in Fig. 2b. The exsoluted CeO2 with an average diameter of about 100 nm could be observed clearly when the LCSBT was sintered in air in Fig. 2c. The distances among these nano-CeO2 particles were broad enough to prevent the aggregation at high temperatures. The voltage–current and power density curves of the single fuel cell operated from 800 °C to 900 °C using pure H2 as fuel are presented in Fig. 3a. The OCV was about 1.05 V. At 800 °C, the maximum power density of the fuel cell was 46 mW cm− 2 at a corresponding current density of 99 mA cm−2. The maximum power density and corresponding current density increased to 110 mW cm−2 and 225 mA cm−2 when the operating temperature was increased to 900 °C. These values are much higher than that of pure LSBT anode materials reported in our earlier work [8]. The power density of LSCBT is still low because of the limited amount of active CeO2. In order to improve the electrocatalytic performance, the work about the nano-CeO2 and Nano-Ni seperated from LST-based anode materials at the same time is undergoing. Fig. 3b shows the electrochemical impedance spectra (EIS) of a LCSBTbased fuel cell using H2 as fuel under OCV at different temperatures. The high frequency intercept in the X-axis corresponds to the ohmic resistance of the electrolyte (Rohm) and the diameter of the semi-circle at low frequency is assigned to the charge transfer polarization resistances (Rp) [22,23]. Both Rohm and Rp increased apparently as the operating temperature decreased from 900 to 800 °C. The Rohm and Rp were 0.93 Ω cm−2 and 2.54 Ω cm−2 at 900 °C and later increased to 1.52 Ω cm−2 and 7.13 Ω cm−2 at 800 °C. Fig. 3c and d compares the performance and EIS of LSCBT and LSBT&CeO2 admixture anodes at 850 °C,
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In summary, pure Ce-doped La0.4Sr0.5Ba0.1TiO3 − δ (LCSBT) perovskite anode catalysts were successfully synthesized by a modified rheological phase reaction under a reducing atmosphere. The doped Ce in LCSBT could be separated to form nano-CeO2 after sintering the LCSBT in air. The catalytic performance of LCSBT for the oxidation of H2 was improved in comparison with the LSBT and LSBT&CeO2 admixture anodes. The nanostructures of LCSBT and the exsoluted nano-CeO2 might play key roles in contributing to the improvement.
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Fig. 3. Current density–voltage and power density curves of (a) LCSBT anode at different temperatures and (c) LCSBT and LSBT&CeO2 at 850 °C in pure H2. Electrochemical impedance spectra (EIS) of (b) LCSBT at different temperatures and (d) LCSBT and LSBT&CeO2 at 850 °C in pure H2.
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Conflicts of interest The authors declared that they have no conflicts of interest to this work. Acknowledgments This research was supported through funding to the Solid Oxide Fuel Cell Canada Stategic Research Network from the Natural Science and Engineering Research Council. References [1] P.I. Cowin, C.T.G. Petit, R. Lan, J.T.S. Irvine, S. Tao, Adv. Energy Mater. 1 (2011) 314–332. [2] X.W. Zhou, N. Yan, J.L. Luo, K.T. Chuang, RSC Adv. 4 (2014) 118–131. [3] S.H. Cui, J.H. Li, X.W. Zhou, G.Y. Wang, J.L. Luo, K.T. Chuang, Y. Bai, L.J. Qiao, J. Mater. Chem. A 1 (2013) 9689–9696. [4] A.D. Aljaberi, J.T.S. Irvine, J. Mater. Chem. A 1 (2013) 5868–5874. [5] C.D. Savaniu, D.N. Miller, J.T.S. Irvine, J. Am. Ceram. Soc. 96 (2013) 1718–1723. [6] D. Neagu, G. Tsekouras, D.N. Miller, H. Ménard, J.T.S. Irvine, Nat. Chem. 5 (2013) 916–923. [7] S.P. Jiang, Int. J. Hydrogen Energy 37 (2012) 449–470. [8] A. Vincent, J.L. Luo, K.T. Chuang, A.R. Sanger, J. Power Sources 195 (2010) 769–774.
[9] A. Vincent, A.R. Hanifi, J.L. Luo, K.T. Chuang, A.R. Sanger, T.H. Etsell, P. Sarkar, J. Power Sources 215 (2011) 301–306. [10] C. Périllat-Merceroz, G. Gauthier, P. Roussel, M. Huvé, P. Gélin, R.N. Vannier, Chem. Mater. 13 (2011) 1539–1550. [11] S. Lee, G. Kim, J.M. Vohs, R.J. Gorte, J. Electrochem. Soc. 155 (2008) B1179–B1183. [12] K.B. Yoo, G.M. Choi, Solid State Ionics 192 (2011) 515–518. [13] M. Roushanafshar, J.L. Luo, A.L. Vincent, K.T. Chuang, A.R. Sanger, Int. J. Hydrogen Energy 37 (2012) 7762–7770. [14] M.R. Pillai, I. Kim, D.M. Bierschenk, S.A. Barnett, J. Power Sources 185 (2008) 1086–1093. [15] H. Tang, C.Q. Feng, Q. Fan, T.M. Lei, J.T. Sun, L.J. Yuan, K.L. Zhang, Chem. Lett. 31 (2002) 822–823. [16] X.W. Zhou, D. Zhan, L.N. Wang, Q.Y. Liu, H.X. Zong, K.L. Zhang, Wuhan Univ. J. Nat. Sci. 10 (2005) 909–912. [17] Y.J. Zhong, J.T. Li, Z.G. Wu, X.D. Guo, B.H. Zhong, S.G. Sun, J. Power Sources 234 (2013) 217–222. [18] S.Y. Yin, L. Song, X.Y. Wang, M.F. Zhang, Y.X. Zhang, Electrochim. Acta 54 (2009) 5629–5633. [19] H. Zheng, C. Feng, S.J. Kim, S. Yin, H. Wu, S. Wang, S. Li, Electrochim. Acta 88 (2013) 225–230. [20] J.H. Li, X.Z. Fu, J.L. Luo, K.T. Chuang, A.R. Sanger, J. Power Sources 213 (2012) 69–77. [21] Y. Zhang, S.T. Luo, Y. Fu, K.L. Zhang, J. Mater. Sci. 41 (2006) 3179–3182. [22] F.X. Zhu, J.L. Luo, A.R. Sanger, Z.R. Xu, K.T. Chuang, Electrochim. Acta 55 (2010) 1145–1149. [23] A.M. Hussain, J.V.T. HøGH, W. Zhang, P. Blennow, N. Bonanos, B.A. Boukamp, Electrochim. Acta 113 (2013) 635–643.
Contents lists available at ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Short communication
Synthesis and electrocatalytic performance of La0.3Ce0.1Sr0.5Ba0.1TiO3 anode catalyst for solid oxide fuel cells Xin-Wen Zhou a,b, Yi-Fei Sun a, Guang-Ya Wang a, Tong Gao a, Kart T. Chuang a, Jing-Li Luo a,⁎, Min Chen c, Viola I. Birss c a b c
Department of Chemical and Materials Engineering, University of Alberta, Edmonton, Alberta T6G 2V4, Canada College of Biological and Pharmaceutical Science, China Three Gorges University, Yichang 443002, China Department of Chemistry, University of Calgary, Alberta T2N 1N4, Canada
a r t i c l e
i n f o
Article history: Received 13 January 2014 Received in revised form 21 March 2014 Accepted 21 March 2014 Available online 29 March 2014 Keywords: Solid oxide fuel cells Anode catalyst Ce-doped La0.4Sr0.5Ba0.1TiO3–δ Rheological phase reaction
a b s t r a c t Ce-doped La0.4Sr0.5Ba0.1TiO3–δ (LCSBT) perovskite anode catalysts in solid oxide fuel cells were successfully synthesized by a modified rheological phase reaction for the first time. Pure LCSBT could be obtained under a reducing atmosphere and nano-CeO2 particles could be exsoluted from LCSBT after being sintered in air. The catalytic activity and electrochemical performance of LCSBT anodes for the H2 oxidation were obviously improved comparing with the pure La0.4Sr0.5Ba0.1TiO3–δ (LSBT) and LSBT&CeO2 admixture anodes. The improved performance could be attributed to the nanostructure of LCSBT and the exsoluted nano-CeO2 particles. © 2014 Elsevier B.V. All rights reserved.
1. Introduction La-substituted SrTiO3 (LST) perovskite-based oxide is a potential candidate anode material because of its high electronic conductivity in reducing atmospheres, its high chemical stability and excellent sulfur and coking tolerance [1,2]. Low ionic conductivity and poor electrocatalytic activity are the main disadvantages of LST-based anode. Presently, the main methods to improve these properties include (1) doping LST with other metal ions [3,4], (2) introducing an active second phase to LST to form composite anodes [5,6], and (3) synthesizing nano-structured LST anodes [7]. Among the doped-LST anodes, Ba-doped LST has shown strong doping effectiveness in increasing its ionic conductivity and improving the electrochemical performance [8,9]. CeO2 is an active catalyst to enhance the electrochemical performance of fuel cells. Therefore, some Ce-containing compounds have been studied in recent years, e.g., Ce-doped LST [10] and CeO2based LST composite anodes including CeO2 [2,11], Gd-doped CeO2 (GDC) [12], Y-doped CeO2 (YDC) [13] and Sm-doped CeO2 (SDC) [14] etc. Then, the comparison of the performance of Ce-doped LST and CeO2&LST composite anodes is attractive. Rheological phase reaction (RPR) is an effective soft chemical way to prepare nanomaterials because of the efficient utilization of the solid particle surface, the close and uniform contact between solid particles
⁎ Corresponding author. Tel.: +1 780 492 2232; fax: +1 780 492 2881. E-mail address: [email protected] (J.-L. Luo).
http://dx.doi.org/10.1016/j.elecom.2014.03.019 1388-2481/© 2014 Elsevier B.V. All rights reserved.
and fluids, and the proper heat exchange [15]. LiMn2O4 [15], LiFePO4 [16,17], Li4Ti5O12 [18] and KMn8O16 [19] electrode materials have been synthesized by the RPR method. BaTiO3 perovskite oxide having proved to be a promising sulfur and coking tolerant anode material [20], was also synthesized successfully by the RPR method [21]. In this study, we focus on the synthesis of Ce-doped La0.4Sr0.5Ba0.1TiO3–δ (LCSBT) perovskite anode catalysts by a modified RPR method. The electrochemical performances of LCSBT and LSBT&CeO2 anodes are also compared in details. 2. Experimental sections 2.1. Preparation and characterization of anode catalysts La0.3Ce0.1Sr0.5Ba0.1TiO3–δ (LCSBT) and La0.4Sr0.5Ba0.1TiO3–δ (LSBT) perovskite nanopowders were prepared using a modified RPR method [18]. The stoichiometric amounts of La(NO3)3 ⋅ 6H2O, Ce(NO3)3 ⋅ 6H2O, Sr(NO3)2 ⋅ 6H2O, Ba(NO3)2 ⋅ 6H2O, Ti(OC3H7)4 and citric acid (C6H8O7) were sufficiently ground at room temperature. During the grinding process, Ti(OC3H7)4 started to hydrolyze, accompanied by the evaporation of parts of propyl alcohol and nitrogen oxides. A proper amount of deionized water was added to accelerate the hydrolysis of Ti(OC3H7)4 and accommodate the conglutination resistance. The obtained rheological phase material was then transferred into a container that was subsequently sealed in a stainless autoclave at 120 °C for 12 h. The precursor was calcined in 10% H2–N2 (Praxair) at 1300 °C for 10 h. Finally, the black product was calcined in air at 400 °C to dispose of the
80
X.-W. Zhou et al. / Electrochemistry Communications 43 (2014) 79–82
remaining carbon. LSCBT powders sintered in 10% H2–N2 and air were denoted as LCSBT-H and LCSBT-A, respectively. LSBT and CeO2 (molar ratio: 10:1) was ball milled for 10 h to obtain the LSBT&CeO2 admixture. All the chemicals were purchased from Fisher Scientific and used as received without further purification. The phase structures and morphologies of the anode materials were characterized using a Rigaku Rotaflex X-ray diffractometer (XRD) with Co Kα radiation and a JEOL 6301F field emission scanning electron microscope (SEM), respectively. 2.2. Fuel cell fabrication and test
LSBT in air [10]. When the pure LCSBT was sintered in air at 1200 °C for 2 h, the peaks of CeO2 appeared (Fig. 1c), indicating that Ce could be separated from LCSBT to form a nano-CeO2 phase. The exsoluted CeO2 could not be re-compounded into the LSBT structure completely when sintering in reducing atmosphere again, ensuring the presence of the catalytically active nano-CeO2 phase [10]. In Fig. 1d, the peaks of LSBT and CeO2 were clearly observed in LSBT&CeO2. At the same time, the peak located near 28° in Fig. 1c was broadened compared with
a
The fuel cell testing system was fabricated and installed as described previously [3,8,9]. The fuel cells were fabricated using commercial YSZ disks (300 μm in thickness and 25 mm in diameter) as the electrolyte, strontium doped lanthanum manganite (LSM) powders as the cathode and LCSBT-H as the anode. The obtained electrode ink was screen printed onto the face of YSZ to form a membrane electrode assembly (MEA) with a circular area of 0.75 cm2, and then sintered in air at 1200 °C for 2 h. An Au anode current collector has been proven to be sulfur-tolerant, which is favorable for our next H2S-containing fuel application. For comparison, Au was used as the anode current collector and Pt was the most used cathode current collector because of its high stability and high electronic conductivity. The anodes were reduced in-situ at 950 °C in H2 for 10 h before the tests. The anode and cathode were fed with pure H2 and oxygen that were dry and had a flow rate of 100 mL min−1 for both. Fuel cell testing was performed with standard DC and AC electrochemical techniques using a Solartron instrument (SI1287 EI). The polarization resistance of the cell was measured using electrochemical impedance spectroscopy (EIS) from 1.0 MHz to 0.1 Hz at open circuit voltage (OCV). Each sample was tested 3 times under the same conditions and the error range between each measurement is controlled in 5%.
b
3. Results and discussion Fig. 1 shows the XRD of different LCSBT powders obtained under different conditions. In Fig. 1a, it can be seen that the pure cubic perovskite structure of the SrTiO3 type was synthesized under a reducing atmosphere [4,8,10]. The substitution for La by Ce led to the displacement of all the XRD peaks to higher angles and a slight contraction of lattice parameter (a = 5.5430 Å, b = 5.5279 Å, c = 7.8076 Å) compared with the literature [10]. In contrast, LCSBT obtained in air showed evidence of impurity peaks, which was attributed to cubic CeO2 (Fig. 1b). The results indicated that Ce could not be totally doped into
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2-Theta / o Fig. 1. XRD patterns of (a) LSCBT-H, (b) LSCBT-A, and (c) LSCBT-H after sintering in air, and (d) LSBT&CeO2.
Fig. 2. (a) Surface and (b) fracture cross-sectional SEM images of a typical cell before a fuel cell test, and (c) SEM images of LCSBT powders after sintering in air at 1200 °C for 2 h.
X.-W. Zhou et al. / Electrochemistry Communications 43 (2014) 79–82
respectively. The maximum power densities were 74 and 57 mW cm−2 using LSCBT and LSBT&CeO2 admixtures as anodes at 850 °C, respectively. The corresponding current densities were 153 and 108 mA cm−2. The Rohm of LCSBT and LSBT&CeO2 anodes were similar because of the same YSZ electrolyte and larger than expected for a 300 μm electrolyte. But the cause was not definitively identified [22]. The Rp of LCSBT to LSBT&CeO2 anodes was increased from 4.12 Ω cm−2 to 9.35 Ω cm− 2 which indicated that the enhancement of Ce-doping effect is better than that of CeO2 adding directly. These results suggest that the performance of LCSBT anode catalyst is better than that of LSBT&CeO2 admixture for H2 fuel cells. The improved electrochemical performance of the LCSBT anode might be attributable to the following factors. The first one is that the LCSBT anode catalysts obtained by the RHR method were smaller and much more uniform than those obtained by conventional solid state reaction [8]. These could increase the surface and the triple-phase boundary (TPB) lengths of the LCSBT anode, resulting in the improved cell performance. The second factor is related to the Ce-doping effect. The doped Ce in LSBT body could increase the electrical conductivity and the exsoluted active nano-CeO2 could enhance the cell performance. The exsoluted CeO2 was in nanoscale and the contact with LSBT was much more compact than that between the LSBT and the additive CeO2, which could increase the TPB effectively.
Fig. 1b and d, indicating that the exsoluted CeO2 had a smaller grain size according to the Scherrer formula. Fig. 2a and b shows the surface and fracture cross-sectional SEM images of a typical cell of LSCBT anode before a fuel cell test, respectively. In Fig. 2a, it can be seen that the LSCBT and YSZ were uniform and wellproportionally mixed together. The thickness of the LSCBT anode was about 13.0 μm. The contact between the LSCBT anode and the dense YSZ was compact and clear in Fig. 2b. The exsoluted CeO2 with an average diameter of about 100 nm could be observed clearly when the LCSBT was sintered in air in Fig. 2c. The distances among these nano-CeO2 particles were broad enough to prevent the aggregation at high temperatures. The voltage–current and power density curves of the single fuel cell operated from 800 °C to 900 °C using pure H2 as fuel are presented in Fig. 3a. The OCV was about 1.05 V. At 800 °C, the maximum power density of the fuel cell was 46 mW cm− 2 at a corresponding current density of 99 mA cm−2. The maximum power density and corresponding current density increased to 110 mW cm−2 and 225 mA cm−2 when the operating temperature was increased to 900 °C. These values are much higher than that of pure LSBT anode materials reported in our earlier work [8]. The power density of LSCBT is still low because of the limited amount of active CeO2. In order to improve the electrocatalytic performance, the work about the nano-CeO2 and Nano-Ni seperated from LST-based anode materials at the same time is undergoing. Fig. 3b shows the electrochemical impedance spectra (EIS) of a LCSBTbased fuel cell using H2 as fuel under OCV at different temperatures. The high frequency intercept in the X-axis corresponds to the ohmic resistance of the electrolyte (Rohm) and the diameter of the semi-circle at low frequency is assigned to the charge transfer polarization resistances (Rp) [22,23]. Both Rohm and Rp increased apparently as the operating temperature decreased from 900 to 800 °C. The Rohm and Rp were 0.93 Ω cm−2 and 2.54 Ω cm−2 at 900 °C and later increased to 1.52 Ω cm−2 and 7.13 Ω cm−2 at 800 °C. Fig. 3c and d compares the performance and EIS of LSCBT and LSBT&CeO2 admixture anodes at 850 °C,
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In summary, pure Ce-doped La0.4Sr0.5Ba0.1TiO3 − δ (LCSBT) perovskite anode catalysts were successfully synthesized by a modified rheological phase reaction under a reducing atmosphere. The doped Ce in LCSBT could be separated to form nano-CeO2 after sintering the LCSBT in air. The catalytic performance of LCSBT for the oxidation of H2 was improved in comparison with the LSBT and LSBT&CeO2 admixture anodes. The nanostructures of LCSBT and the exsoluted nano-CeO2 might play key roles in contributing to the improvement.
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Fig. 3. Current density–voltage and power density curves of (a) LCSBT anode at different temperatures and (c) LCSBT and LSBT&CeO2 at 850 °C in pure H2. Electrochemical impedance spectra (EIS) of (b) LCSBT at different temperatures and (d) LCSBT and LSBT&CeO2 at 850 °C in pure H2.
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