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Fabrication of GdBaCo2O5+δ cathode using electrospun composite nanofibers and its improved electrochemical performance
Journal of Alloys and Compounds 557 (2013) 184–189
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Fabrication of GdBaCo2O5+d cathode using electrospun composite nanofibers and its improved electrochemical performance Xuening Jiang a,⇑, Hongxia Xu a, Qian Wang a, Lei Jiang b, Xiangnan Li a, Qiuli Xu a, Yuchao Shi a, Qingyu Zhang a a b
Key Laboratory of Materials Modification by Laser, Ion, and Electron Beams, Dalian University of Technology, Ministry of Education, Dalian 116024, China Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China
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Article history: Received 15 November 2012 Received in revised form 4 January 2013 Accepted 5 January 2013 Available online 17 January 2013 Keywords: SOFC Cathode Electrospun nanofibers Microstructure Electrochemical performance
a b s t r a c t GdBaCo2O5+d (GBCO)/poly (vinyl pyrrolidone) (PVP) composite nanofibers were prepared by electrospinning. Structural and morphological evolution of the GBCO/PVP fibers under calcinations at various temperatures was studied. Using 600 °C pre-calcined GBCO/PVP composite nanofibers, pure phase GBCO cathode with homogeneous network structure was easily fabricated on Ce0.9Gd0.1O1.95 (GDC) electrolyte. The as-prepared GBCO cathode was characterized by electrochemical impedance spectra (EIS) measurements based on a GBCO/GDC/GBCO symmetric cell. Its electrochemical performance was compared with the cathode prepared with the GBCO powders synthesized with sol–gel method. The GBCO cathode fabricated using GBCO/PVP composite fibers had area specific resistances (ASRs) of 0.53 X cm2 at 600 °C, 0.22 X cm2 at 650 °C, 0.10 X cm2 at 700 °C and 0.043 X cm2 at 750 °C respectively, which are lower than ASR results of the GBCO cathode prepared with powders. The results have demonstrated that the GBCO cathode fabricated using electrospun composite nanofibers has significantly enhanced electrochemical activities for oxygen reduction reaction, and it can serve as a promising cathode material for intermediate-temperature solid oxide fuel cell. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Solid oxide fuel cells (SOFCs) are chemical–electrical energy conversion devices with the advantages of high working efficiency, low emissions and excellent fuel flexibility. The present research focus in the SOFC field is lowering the cell working temperature to the intermediate-temperature (IT) range of 600–800 °C, in order to extend the choice of component materials, lengthen the cell duration time and realize cost reduction [1,2]. At the lowered temperatures, polarization resistance of the cathode becomes the key factor in determining the overall performance of the IT-SOFCs due to high activation energy for oxygen reduction reaction (ORR) occurring over the cathode [2,3]. Thus, great attention has been paid on studies of novel cathode materials for IT-SOFCs in recent years [1,3–9]. It is known that two main factors determine electrochemical catalytic activities of the cathode materials. One is the intrinsic properties including oxygen ionic conductivity and oxygen transport kinetics, which depend on compositions and crystal structure of the material [10]. To improve these intrinsic properties, A-site or B-site cationic doping as well as adjustment of crystal structures ⇑ Corresponding author. Tel.: +86 41184708380x8204; fax: +86 41184708389. E-mail address: [email protected] (X. Jiang). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.01.015
have been adopted for the perovskite-typed cathode materials [5,7,8,11–15]. Besides that, the cathode performance is also closely related to the microstructures such as porosity, particle sizes and particle connectivity [16,17], which vary with the cathode fabrication methods and conditions. In this aspect, nanostructured electrodes are most attractive since the microstructures in nanometer scale make available large three-phase boundaries for the electrocatalytic process of oxygen reduction reaction, and therefore enhance the electrocatalytic activity and cathode reaction [4,18,19]. The commonly used cathode fabrication methods include high-temperature solid-state reaction [13] and sol–gel synthesis methods [15]. These methods, however, are time-consuming and usually produce non-homogeneous microstructures. In recent years, nanostructured cathode materials have been synthesized by a variety of methods such as laser ablation, chemical vapor deposition, and aerosol deposition [20,21], but all these methods have disadvantages of being complicated and high cost and requiring large and expensive equipments. Electrospinning proves to be an easy, versatile and effective way to synthesize nanoscale fibers of ceramics [22] and it has been used to fabricate nanostructured electrodes of SOFCs [23–26]. For example, a high performance SOFC cathode was obtained by using YSZ electrospun nanofiber scaffold with infiltrated La1 xSrxMnO3 material [24]; La0.58Sr0.4Co0.2Fe0.8O3 and BaBi0.05Sc0.1Co0.85O3 d nanofibers were
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synthesized by electrospinning and the cathodes prepared using the electrospun nanofibers exhibited fast oxygen transport and high electrochemical catalytic reactivity at intermediate temperatures [25,26]. In this work, we choose the double-layered perovskite oxide GdBaCo2O5+d (GBCO) as the target of nanofiber-electrospinning because it has shown promising application as cathode material of IT-SOFCs [27–31]. In literature, various methods have been applied for synthesis of GBCO powders, such as solid state reaction [27–30] and sol–gel [31] methods; however, no work is available for preparation of GBCO nanofibers or nanostructures using electrospinning technique for usage as cathodes of IT-SOFCs. Here, we present the synthesis and characterizations of GBCO nanofibers and the nanofibers were used to fabricate the GBCO cathode layer on Ce0.9Gd0.1O1.95 (GDC) electrolyte pellet in an easy way. The asfabricated GBCO cathode has exhibited significantly improved electrochemical performance as compared to the GBCO cathode fabricated under the same conditions but using the GBCO powders synthesized with sol–gel method.
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formances, the GBCO cathode layer in cell-2 was also screen-printed on the GDC pellet and calcined at 1000 °C for 2 h in air as the case in cell-1. Both the GBCO/ GDC/GBCO symmetric cells (cell-1 and cell-2) were characterized with electrochemical impedance spectra (EIS) measurements. The EIS data were collected by a Solartron 1260 Frequency Response Analyzer combined with a Solartron 1287 potentiostat under open circuit voltage (OCV) condition as a function of temperature (600–750 °C) in flowing air. Microstructures of the post-measured GBCO cathodes were characterized by SEM measurement.
3. Results and discussion 3.1. Phase and microstructure evolutions of GBCO/PVP composite nanofibers To determine the proper temperature for GBCO phase formation from the GBCO/PVP composite fibers, we firstly calcined the
2. Experimental 2.1. Electrospinning of GBCO/PVP composite nanofibers Gd(NO3)36H2O (AR), Ba(NO3)2 (AR), Co(NO3)26H2O (AR) and poly (vinyl pyrrolidone) (PVP, Mw = 360,000 from Alfa Aesar) were used as starting materials for preparation of electrospinning solution. In a typical procedure, 1.0 g of PVP was dissolved in 10 mL of 1:1 (v/v) ethanol/water co-solvent under stirring, making a viscous PVP solution. Then, 0.0903 g of Gd(NO3)36H2O, 0.0523 g of Ba(NO3)2, and 0.1164 g of Co(NO3)26H2O were added into the PVP solution. The mixture solution was stirred for 5 h to get a viscous and uniform precursor solution of (Gd, Ba, Co)(NO3)/PVP composites. The precursor solution was ultrasonically vibrated for 10 min to get rid of the inside gas and was then transferred to a syringe for electrospinning. A grounded iron drum, covered with an aluminum foil, served as the spunfiber collector. During the electrospinning, the applied voltage was kept at 7500 V, and the distance between the tip of syringe and the collector was 4 cm. To study the phase and microstructure evolutions of the as-spun GBCO/PVP composite fibers, the collected fibers were calcined at 600 °C, 900 °C, and 1000 °C for 5 h in air respectively with a heating/cooling rate of 2 °C/min. The as-spun and calcined samples were characterized by field emission scanning electron microscopy (SEM, Hitachi S-4800) and X-ray diffraction measurement (XRD, Rigaku D/Max 2400). Thermal gravimetric analysis (TGA) was carried out from room temperature to 1100 °C with a heating rate of 5 °C/min in air. 2.2. Synthesis of GBCO powders with sol–gel method For comparisons between performance of the GBCO cathodes prepared using the electrospun nanofibers and GBCO powders, the GBCO powders were synthesized with a combined EDTA-citrate complexing sol–gel method, similar to the procedure described previously [6]. Briefly, Gd(NO3)36H2O (AR), Ba(NO3)2 (AR) and Co(NO3)36H2O (AR) at stoichiometric proportions of GBCO were firstly dissolved into EDTA–NH3H2O solution (pH 6) to form an aqueous solution; acid–NH3H2O solution (pH 6) was then added at a mole ratio of 1:1:2 for EDTA: total metal ions: citric acid with stirring. The mixed solution was heated at 80 °C and 200 °C in sequence to obtain a dark dry foam structure. The precursor was decomposed on a hot plate, followed by calcinations at 600 °C for 8 h and 1050 °C for 6 h respectively in air to yield the desired powders. 2.3. Fabrication of GBCO/GDC/GBCO symmetric cells GBCO/GDC/GBCO symmetric cells based on Ce0.9Gd0.1O1.95 (GDC) electrolyte (from Ningbo Institute of Materials Technology and Engineering), cell-1 and cell2, were fabricated using the GBCO/PVP composite nanofibers and sol–gel synthesized GBCO powders respectively, according to the fabrication procedures illustrated in Fig. 1. For fabrication of cell-1, the as-spun GBCO/PVP composite nanofibers were peeled off from the collector of alumina foil and calcined at 600 °C for 1 h in air; the pre-calcined nanofibers were then slightly ground in a mortar and mixed with a-terpineol and ethyl cellulose to make the precursor ink, which was screen-printed onto both sides of the GDC pellet followed by calcination at 1000 °C for 2 h in air, forming the GBCO/GDC/GBCO symmetric cell (cell-1). In cell-2, the GBCO cathode layer was prepared using the GBCO powders synthesized with the sol–gel method under multiple calcinations (at 80 °C, 200 °C, 600 °C and 1050 °C in sequence). The as-synthesized GBCO powders were ball-milled for 3 h to get a fine powder for preparation of the precursor cathode ink mixed with a-terpineol and ethyl cellulose. For a reasonable comparison between the cathode per-
Fig. 1. Illustration of fabrication processes of GBCO/GDC/GBCO symmetric cells using GBCO/PVP composite nanofibers (cell-1) and GBCO powders synthesized with sol–gel method (cell-2) respectively.
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as-spun GBCO/PVP composite fibers at various temperatures followed by XRD measurements. The XRD patterns of the calcined GBCO/PVP composite fibers were shown in Fig. 2. A broad diffuse peak at 20–35° was observed in the XRD pattern of the sample calcined at 600 °C, demonstrating existence of amorphous phases likely of incompletely decomposed PVP. Meanwhile, several small diffraction peaks were also found, thus some crystalline intermediate phases were formed at this stage. The XRD pattern for 900 °C was composed of complicated X-ray diffraction peaks, which were indexed by dominant phases of GdCoO3 (JCPDS card No. 25-1057) and BaCoO3 d (JCPDS card No. 26-0144), and some minor phases of Co3O4 (JCPDS card No. 76-1802) and BaO (JCPDS card No. 65-2923). These results indicated that 600–900 °C was the temperature range where intermediate phases of metal oxides were formed. Sharp diffraction peaks were observed in the XRD pattern of 1000 °C, which were indexed by an orthorhombic Pmmm space group with lattice parameters of a = 3.908(7) Å, b = 3.885(6) Å, c = 7.517(3) Å, and cell volume of V = 114.170(1) Å3, matching well with the literature data for GBCO [31]. Thus a pure GBCO phase was formed after the as-spun GBCO/PVP composite nanofibers were calcined at 1000 °C in air. The GBCO powders synthesized with the sol–gel method have the similar XRD pattern shown in Fig. 2. Fig. 3 presents TG curve of the as-spun GBCO/PVP composite fibers measured in air. A dramatic weight loss was observed from room temperature to 430 °C, which was ascribed to decomposition and removal of organic materials (PVP, ethanol), NO3 groups of nitrates, and the other volatiles (H2O, COx, etc.) [32,33]. A magnified TG plot for the temperatures above 400 °C was inserted, from which we found that the composite underwent a slight weight drop at 600–800 °C, which was probably associated with formation of the intermediate phases of metal oxides as demonstrated by the XRD result (Fig. 2). There was a weight platform at temperatures around 1000 °C, indicating formation of the GBCO phase according to the XRD result shown in Fig. 2. Slight weight loss at 1100 °C was probably due to thermal-driven releasing of lattice oxygen in GBCO, as observed in other perovskite oxides such as PrBaCo2O5+d [34]. Morphology and microstructure of the as-spun GBCO/PVP composite fibers and the fibers calcined at various temperatures in air are presented in Fig. 4. The results show that the as-spun composite fibers were straight and smooth with diameters of 100–200 nm. After calcined at 800 °C, the composite fibers shrank with decreased diameter of 50 nm, and also became rough due to formation of polycrystalline intermediate-phase grains, as demonstrated by the XRD results (Fig. 2). At 1000 °C, the nanofibers were broken and developed as coral like aggregates, which, as clearly shown in the magnified image (Fig. 4d), were composed of bundles of rods
Fig. 2. XRD patterns of GBCO/PVP composite fibers calcined at various temperatures in air.
with length of 30 nm. This nanostructure aroused much interest to us because as a cathode material the resultant large surface areas are expected to enhance the electrocatalytic activity for the oxygen reduction reaction and can thus result in improved electrochemical performance [4,18].
3.2. Fabrication process optimization and microstructure of the GBCO cathode To prepare the GBCO cathode layer using the GBCO/PVP composite nanofibers, we firstly tried to electrospin the GBCO/PVP composite nanofibers directly on the GDC electrolyte pellet followed by step-by-step calcinations at 300 °C, 600 °C and 1000 °C for 5 h respectively. Unfortunately, however, the as-prepared cathode layer easily peeled off from the GDC pellet, which was probably caused by fast burning of the as-spun GBCO/PVP composite nanofibers. As shown in the TG curve (Fig. 3), about 90% weight ratio of the as-spun GBCO/PVP composite nanofibers was burnt off at the temperature up to 400 °C; Releasing of so much burning gases could lead to poor connection between the GBCO and GDC layers and cause the cathode layer delamination. To improve the GBCO/ GDC interface connection, an updated method was adopted to fabricate the GBCO cathode based on the electrospun GBCO/PVP composite nanofibers collected on the alumina foil, as illustrated in Fig. 1a. The key point for this fabrication process is the 600 °C pre-calcination of the as-spun GBCO/PVP composite nanofibers, during which most of the organic compositions decomposed and released (Fig. 3). As a result, a good connection was obtained between the GBCO cathode and GDC electrolyte layers (Fig. 5a), thus no delamination occurred in the cell (cell-1). Significantly, as shown in Fig. 5b, a homogeneous network structure of well-connected grains with sizes smaller than 1 lm and uniformly distributed pores were formed in the prepared cathode layer, similar to the microstructure shown in Fig. 4c on the whole while the nanostructures could not be observed. In contrast, as shown in Fig. 5c, microstructure of the GBCO cathode in cell-2 fabricated using the sol–gel synthesized GBCO powders is not uniform and the particles vary in size and are not well-connected, although the used GBCO powders were ball-milled for 3 h in advance and the cathode layer was calcined under the same conditions with the GBCO cathode in cell-1. The porous network structure of the GBCO cathode prepared using the GBCO/PVP composite nanfibers has enhanced the electrode reaction as demonstrated below. Furthermore, as illustrated in Fig. 1, the cathode fabrication process using the electrospun composite nanofibers is time- and cost-saving and easy for manipulation as compared to the fabrication process using the GBCO
Fig. 3. TG curve of the as-spun GBCO/PVP composite fibers measured in air.
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Fig. 4. SEM images of as-spun GBCO/PVP composite fibers (a) and the fibers after calcination at 800 °C (b) and 1000 °C (c and d) in air.
Fig. 5. Cross-sectional and surface SEM images of the GBCO/GDC interfaces with the GBCO cathode layer prepared at 1000 °C using GBCO/PVP composite nanofibers (a and b) and GBCO powders synthesized with sol–gel method (c and d) respectively.
powders synthesized with sol–gel method in this work and solid state reaction method reported in literatures [27–30]. This is because multiple steps of high-temperature calcination were required for formation of the GBCO pure phase from the sol–gel precursor or solid reaction oxides; high-temperature calcinations are time-wasting and could cause powder aggregations, therefore, the as-synthesized powders had to be ball-milled usually for several hours before preparation of the cathode ink. In contrast, only a short period of pre-calcination at 600 °C was needed before the GBCO/PVP composite nanofibers were used for preparation of the
GBCO cathode layer on the GDC pellet, and the ball-milling step was saved because the pre-calcined GBCO/PVP nanofibers are uniform and easily broken with a hand-grinding. 3.3. Electrochemical performance of GBCO cathode Fig. 6 shows the typical EIS results for both GBCO/GDC/GBCO symmetric cells (cell-1 and cell-2). The ohmic resistance arising from GDC electrolyte and lead wires were normalized to zero for clarity. The frequency of the impedance arcs is in the range of
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1–104 Hz, which could be associated with multiple elementary steps of the cathode reaction, such as oxygen gas diffusion, surface oxygen diffusion, adsorption, dissociation, and oxygen ionic/electronic charge transfers [3,6,35]. The span of the arcs at the real axis in the spectra corresponds to polarization resistance (Rp) of the cathodes in the symmetric cell. Apparently, the polarization resistances of the GBCO cathode prepared with electrospun nanofibers (in cell-1) are much smaller than the cathode prepared with the sol–gel-synthesized powders (in cell-2). From the equation of ASR = Rp A/2, where A is geometric area of the cathode, area specific resistances (ASRs) of the GBCO cathodes for both cells were calculated, and the results are shown in Fig. 7. Fig. 7 also presents the ASR results of the GBCO cathodes reported in literatures [27,29] for comparison. The GBCO cathode prepared with the sol–gel-synthesized powders (in cell-2) has the ASR values ranging from 0.78 X cm2 at 600 °C to 0.084 X cm2 at 750 °C, in-between the reported ASR values for the GBCO cathodes [27,29]. In contrast, the ASRs of the GBCO cathode fabricated from the electrospun nanofibers (in cell-1) are 0.53 X cm2 at 600 °C, 0.22 X cm2 at 650 °C, 0.10 X cm2 at 700 °C and 0.043 X cm2 at 750 °C respectively, which are smaller than the results of the cathode prepared with the sol–gel-synthesized powders in this work. For example, the ASR value of GBCO cathode in cell-1 decreased 49% at 750 °C and 32% at 600 °C respectively, as compared to the GBCO cathode in cell-2. These results have demonstrated that the GBCO cathode prepared using the GBCO/PVP composite nanofibers has better electrochemical performance than the cathode prepared with the sol–gel-synthesized GBCO powders as expected. It’s known that in addition to increase in oxygen ionic conductivity and oxygen transport kinetics, electrochemical performance of the cathode can also be improved by adjustment of the cathode microstructures [16,17]. In this work, the two GBCO cathodes in
Fig. 7. Temperature-dependence of area specific resistances (ASRs) of the GBCO cathodes for cell-1 and cell-2 together with the ASR results of GBCO cathode reported in literatures [27,29].
cell-1 and cell-2 were fabricated under the same conditions (at 1000 °C for the same 3 h); their different microstructures shown in Fig. 5 were due to different nature of the starting materials (GBCO/PVP composite nanofibers and GBCO powders) for preparation of the cathode. In comparison, the GBCO cathode (in cell-1) prepared using the GBCO/PVP composite nanofibers had a more homogeneous and porous network structure with better-connected smaller grains (about several hundred nanometers) than the GBCO cathode (in cell-2) fabricated using the as-synthesized GBCO powders (Fig. 5). Such cathode microstructure in cell-1 could facilitate air diffusion through the cathode layer and enhance the electrode electrochemical activities with enlarged three-phase boundaries (TPBs) and more active sites for oxygen reduction reactions due to its enlarged specific area [4,18], which was believed to be the main contribution to its improved electrochemical performance.
4. Conclusions GdBaCo2O5+d (GBCO)/poly (vinyl pyrrolidone) (PVP) composite nanofibers were prepared by electrospinning. A pure orthorhombic GBCO phase with coral like nano-structured aggregates was formed after the GBCO/PVP composite fibers were calcined at 1000 °C. Using the GBCO/PVP composite nanofibers pre-calcined at 600 °C, the pure phase GBCO cathode layer was easily fabricated on the GDC pellet, forming the GBCO/GDC/GBCO symmetric cell. This fabrication process is time- and cost-saving and easy for manipulation as compared with the fabrication process using the as-synthesized GBCO powders with sol–gel method. More significantly, the GBCO cathode prepared with the GBCO/PVP composite nanofibers has exhibited improved electrochemical performance than the cathode prepared with the GBCO powders, characterized by its lower area specific resistance values of 0.53 X cm2 at 600 °C, 0.22 X cm2 at 650 °C, 0.10 X cm2 at 700 °C and 0.043 X cm2 at 750 °C respectively. The homogenous network structure of the GBCO cathode prepared with the GBCO/PVP composite nanofibers was believed to enhance the cathode electrochemical activities and realize the improved performance. Acknowledgements
Fig. 6. Typical EIS plots for GBCO/GDC/GBCO symmetric cells (cell-1 and cell-2) measured at different temperatures.
This work was financially supported by the ‘‘Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry’’ and ‘‘the Fundamental Research Funds for the Central Universities (DUT12LAB02)’’.
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References [1] W. Zhou, F.L. Liang, Z.P. Shao, J.L. Chen, Z.H. Zhu, Sci. Rep. 1 (2011) 155–160. [2] D. Han, X.J. Liu, F.R. Zeng, J.Q. Qian, T.Z. Wu, Z.L. Zhan, Sci. Rep. 2 (2012) 462– 466. [3] S.B. Adler, Chem. Rev. 104 (2004) 4791–4844. [4] C. Ding, T. Hashida, Energy Environ. Sci. 3 (2010) 1729–1731. [5] Z.B. Yang, C.H. Yang, C. Jin, M.F. Han, F.L. Chen, Electrochem. Commun. 13 (2011) 882–885. [6] S.L. Pang, X.N. Jiang, X.N. Li, Q. Wang, Z.X. Su, J. Power Sources 204 (2012) 53– 59. [7] Y. Li, J.G. Cheng, J. Song, J.A. Alonso, M.T. Fernández-Díaz, J.B. Goodenough, Chem. Mater. 24 (2012) 4114–4122. [8] S.G. Huang, G.J. Wang, X.H. Sun, C.M. Lei, T. Li, C.C. Wang, J. Alloys Compd. 543 (2012) 26–30. [9] S.Y. Park, H. Ji, H.R. Kim, K.J. Yoon, J.W. Son, B.K. Kim, H.J. Je, H.W. Lee, J.H. Lee, J. Power Sources 228 (2013) 97–103. [10] L. Malavasi, C. Tealdi, C. Ritter, Angew. Chem. Int. Ed. 48 (2009) 8539–8542. [11] Z.L. Ding, Z.J. Yang, D.M. Zhao, X.L. Deng, G.L. Ma, J. Alloys Compd. 550 (2013) 204–208. [12] S.L. Pang, X.N. Jiang, X.N. Li, Q. Wang, Q.Y. Zhang, Mater. Chem. Phys. 131 (2012) 642–646. [13] S. Lü, G. Long, Y. Ji, X. Meng, H. Zhao, C. Sun, J. Alloys Compd. 509 (2011) 2824– 2828. [14] H.P. Ding, X.J. Xue, J. Alloys Compd. 496 (2010) 683–686. [15] X.N. Li, X.N. Jiang, S.L. Pang, Q. Wang, Z.X. Su, Q.Y. Zhang, Int. J. Hydrogen Energy 36 (2011) 3850–3857. [16] J.C. Ruiz-Morales, D. Marrero-López, M. Gálvez-Sánchez, J. Canales-Vázquez, C. Savaniu, S.N. Savvin, Energy Environ. Sci. 3 (2010) 1670–1681. [17] Y. Zhang, S. Zha, M. Liu, Adv. Mater. 17 (2005) 487–491.
189
[18] F. Zhao, Z. Wang, M. Liu, L. Zhang, C. Xia, F. Chen, J. Power Sources 185 (2008) 13–18. [19] J.J. Choi, J.H. Choi, J.H. Ryu, B.D. Hahn, J.W. Kim, C.W. Ahn, W.H. Yoon, D.S. Park, J. Alloys Compd. 545 (2012) 186–189. [20] S. Wang, J. Yoon, G. Kim, D. Huang, H. Wang, A.J. Jacobson, Chem. Mater. 22 (2010) 776–782. [21] J.J. Choia, S.H. Oh, H.S. Noh, H.R. Kim, J.W. Son, D.S. Park, J.H. Choi, J. Ryu, B.D. Hahn, W.H. Yoon, H.W. Lee, J. Alloys Compd. 509 (2011) 2627–2630. [22] V. Thavasi, G. Singh, S. Ramakrishna, Energy Environ. Sci. 1 (2008) 205–221. [23] S. Shahgaldi, Z. Yaakob, D.J. Khadem, M. Ahmadrezaei, W.R. Wan Daud, J. Alloys Compd. 509 (2011) 9005–9009. [24] M.J. Zhi, N. Mariani, R. Gemmen, K. Gerdes, N.Q. Wu, Energy Environ. Sci. 4 (2011) 417–420. [25] M.J. Zhi, S.W. Lee, N. Miller, N.H. Menzler, N.Q. Wu, Energy Environ. Sci. 5 (2012) 7066–7071. [26] J. Sunarso, S. Liu, Y.S. Lin, J.C. Diniz da Costa, Energy Environ. Sci. 4 (2011) 2516–2519. [27] Li. N, Z. Lü, B. Wei, X. Huang, K. Chen, Y. Zhang, W. Su, J. Alloys Compd. 454 (2008) 274–279. [28] A. Tarancón, S.J. Skinner, R.J. Chater, F. Hernández-Ramírez, J.A. Kilner, J. Mater. Chem. 17 (2007) 3175–3181. [29] A. Tarancón, J. Peña-Martínez, D. Marrero-López, A. Morata, J.C. Ruiz-Morales, P. Núñez, Solid State Ionics 179 (2008) 2372–2378. [30] J.H. Kim, A. Manthiram, J. Electrochem. Soc. 155 (4) (2008) B385–390. [31] K. Zhang, L. Ge, R. Ran, Z. Shao, S. Liu, Acta Mater. 56 (2008) 4876–4889. [32] Y. Zhang, J. Yang, Q. Li, X. Cao, J. Cryst. Growth 308 (2007) 180–184. [33] C. Shao, N. Yua, Y. Liu, R. Mu, J. Phys. Chem. Solids 67 (2006) 423–426. [34] C. Frontera, A. Caneiro, A.E. Carrillo, J. Oro-Sole, J.L. Garcıa-Munoz, J. Mater. Chem. 17 (2005) 5439–5445. [35] M.J. Escudero, A. Aguadero, J.A. Alonso, L. Daza, J. Electroanal. Chem. 611 (2007) 107–116.
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Fabrication of GdBaCo2O5+d cathode using electrospun composite nanofibers and its improved electrochemical performance Xuening Jiang a,⇑, Hongxia Xu a, Qian Wang a, Lei Jiang b, Xiangnan Li a, Qiuli Xu a, Yuchao Shi a, Qingyu Zhang a a b
Key Laboratory of Materials Modification by Laser, Ion, and Electron Beams, Dalian University of Technology, Ministry of Education, Dalian 116024, China Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China
a r t i c l e
i n f o
Article history: Received 15 November 2012 Received in revised form 4 January 2013 Accepted 5 January 2013 Available online 17 January 2013 Keywords: SOFC Cathode Electrospun nanofibers Microstructure Electrochemical performance
a b s t r a c t GdBaCo2O5+d (GBCO)/poly (vinyl pyrrolidone) (PVP) composite nanofibers were prepared by electrospinning. Structural and morphological evolution of the GBCO/PVP fibers under calcinations at various temperatures was studied. Using 600 °C pre-calcined GBCO/PVP composite nanofibers, pure phase GBCO cathode with homogeneous network structure was easily fabricated on Ce0.9Gd0.1O1.95 (GDC) electrolyte. The as-prepared GBCO cathode was characterized by electrochemical impedance spectra (EIS) measurements based on a GBCO/GDC/GBCO symmetric cell. Its electrochemical performance was compared with the cathode prepared with the GBCO powders synthesized with sol–gel method. The GBCO cathode fabricated using GBCO/PVP composite fibers had area specific resistances (ASRs) of 0.53 X cm2 at 600 °C, 0.22 X cm2 at 650 °C, 0.10 X cm2 at 700 °C and 0.043 X cm2 at 750 °C respectively, which are lower than ASR results of the GBCO cathode prepared with powders. The results have demonstrated that the GBCO cathode fabricated using electrospun composite nanofibers has significantly enhanced electrochemical activities for oxygen reduction reaction, and it can serve as a promising cathode material for intermediate-temperature solid oxide fuel cell. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Solid oxide fuel cells (SOFCs) are chemical–electrical energy conversion devices with the advantages of high working efficiency, low emissions and excellent fuel flexibility. The present research focus in the SOFC field is lowering the cell working temperature to the intermediate-temperature (IT) range of 600–800 °C, in order to extend the choice of component materials, lengthen the cell duration time and realize cost reduction [1,2]. At the lowered temperatures, polarization resistance of the cathode becomes the key factor in determining the overall performance of the IT-SOFCs due to high activation energy for oxygen reduction reaction (ORR) occurring over the cathode [2,3]. Thus, great attention has been paid on studies of novel cathode materials for IT-SOFCs in recent years [1,3–9]. It is known that two main factors determine electrochemical catalytic activities of the cathode materials. One is the intrinsic properties including oxygen ionic conductivity and oxygen transport kinetics, which depend on compositions and crystal structure of the material [10]. To improve these intrinsic properties, A-site or B-site cationic doping as well as adjustment of crystal structures ⇑ Corresponding author. Tel.: +86 41184708380x8204; fax: +86 41184708389. E-mail address: [email protected] (X. Jiang). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.01.015
have been adopted for the perovskite-typed cathode materials [5,7,8,11–15]. Besides that, the cathode performance is also closely related to the microstructures such as porosity, particle sizes and particle connectivity [16,17], which vary with the cathode fabrication methods and conditions. In this aspect, nanostructured electrodes are most attractive since the microstructures in nanometer scale make available large three-phase boundaries for the electrocatalytic process of oxygen reduction reaction, and therefore enhance the electrocatalytic activity and cathode reaction [4,18,19]. The commonly used cathode fabrication methods include high-temperature solid-state reaction [13] and sol–gel synthesis methods [15]. These methods, however, are time-consuming and usually produce non-homogeneous microstructures. In recent years, nanostructured cathode materials have been synthesized by a variety of methods such as laser ablation, chemical vapor deposition, and aerosol deposition [20,21], but all these methods have disadvantages of being complicated and high cost and requiring large and expensive equipments. Electrospinning proves to be an easy, versatile and effective way to synthesize nanoscale fibers of ceramics [22] and it has been used to fabricate nanostructured electrodes of SOFCs [23–26]. For example, a high performance SOFC cathode was obtained by using YSZ electrospun nanofiber scaffold with infiltrated La1 xSrxMnO3 material [24]; La0.58Sr0.4Co0.2Fe0.8O3 and BaBi0.05Sc0.1Co0.85O3 d nanofibers were
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synthesized by electrospinning and the cathodes prepared using the electrospun nanofibers exhibited fast oxygen transport and high electrochemical catalytic reactivity at intermediate temperatures [25,26]. In this work, we choose the double-layered perovskite oxide GdBaCo2O5+d (GBCO) as the target of nanofiber-electrospinning because it has shown promising application as cathode material of IT-SOFCs [27–31]. In literature, various methods have been applied for synthesis of GBCO powders, such as solid state reaction [27–30] and sol–gel [31] methods; however, no work is available for preparation of GBCO nanofibers or nanostructures using electrospinning technique for usage as cathodes of IT-SOFCs. Here, we present the synthesis and characterizations of GBCO nanofibers and the nanofibers were used to fabricate the GBCO cathode layer on Ce0.9Gd0.1O1.95 (GDC) electrolyte pellet in an easy way. The asfabricated GBCO cathode has exhibited significantly improved electrochemical performance as compared to the GBCO cathode fabricated under the same conditions but using the GBCO powders synthesized with sol–gel method.
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formances, the GBCO cathode layer in cell-2 was also screen-printed on the GDC pellet and calcined at 1000 °C for 2 h in air as the case in cell-1. Both the GBCO/ GDC/GBCO symmetric cells (cell-1 and cell-2) were characterized with electrochemical impedance spectra (EIS) measurements. The EIS data were collected by a Solartron 1260 Frequency Response Analyzer combined with a Solartron 1287 potentiostat under open circuit voltage (OCV) condition as a function of temperature (600–750 °C) in flowing air. Microstructures of the post-measured GBCO cathodes were characterized by SEM measurement.
3. Results and discussion 3.1. Phase and microstructure evolutions of GBCO/PVP composite nanofibers To determine the proper temperature for GBCO phase formation from the GBCO/PVP composite fibers, we firstly calcined the
2. Experimental 2.1. Electrospinning of GBCO/PVP composite nanofibers Gd(NO3)36H2O (AR), Ba(NO3)2 (AR), Co(NO3)26H2O (AR) and poly (vinyl pyrrolidone) (PVP, Mw = 360,000 from Alfa Aesar) were used as starting materials for preparation of electrospinning solution. In a typical procedure, 1.0 g of PVP was dissolved in 10 mL of 1:1 (v/v) ethanol/water co-solvent under stirring, making a viscous PVP solution. Then, 0.0903 g of Gd(NO3)36H2O, 0.0523 g of Ba(NO3)2, and 0.1164 g of Co(NO3)26H2O were added into the PVP solution. The mixture solution was stirred for 5 h to get a viscous and uniform precursor solution of (Gd, Ba, Co)(NO3)/PVP composites. The precursor solution was ultrasonically vibrated for 10 min to get rid of the inside gas and was then transferred to a syringe for electrospinning. A grounded iron drum, covered with an aluminum foil, served as the spunfiber collector. During the electrospinning, the applied voltage was kept at 7500 V, and the distance between the tip of syringe and the collector was 4 cm. To study the phase and microstructure evolutions of the as-spun GBCO/PVP composite fibers, the collected fibers were calcined at 600 °C, 900 °C, and 1000 °C for 5 h in air respectively with a heating/cooling rate of 2 °C/min. The as-spun and calcined samples were characterized by field emission scanning electron microscopy (SEM, Hitachi S-4800) and X-ray diffraction measurement (XRD, Rigaku D/Max 2400). Thermal gravimetric analysis (TGA) was carried out from room temperature to 1100 °C with a heating rate of 5 °C/min in air. 2.2. Synthesis of GBCO powders with sol–gel method For comparisons between performance of the GBCO cathodes prepared using the electrospun nanofibers and GBCO powders, the GBCO powders were synthesized with a combined EDTA-citrate complexing sol–gel method, similar to the procedure described previously [6]. Briefly, Gd(NO3)36H2O (AR), Ba(NO3)2 (AR) and Co(NO3)36H2O (AR) at stoichiometric proportions of GBCO were firstly dissolved into EDTA–NH3H2O solution (pH 6) to form an aqueous solution; acid–NH3H2O solution (pH 6) was then added at a mole ratio of 1:1:2 for EDTA: total metal ions: citric acid with stirring. The mixed solution was heated at 80 °C and 200 °C in sequence to obtain a dark dry foam structure. The precursor was decomposed on a hot plate, followed by calcinations at 600 °C for 8 h and 1050 °C for 6 h respectively in air to yield the desired powders. 2.3. Fabrication of GBCO/GDC/GBCO symmetric cells GBCO/GDC/GBCO symmetric cells based on Ce0.9Gd0.1O1.95 (GDC) electrolyte (from Ningbo Institute of Materials Technology and Engineering), cell-1 and cell2, were fabricated using the GBCO/PVP composite nanofibers and sol–gel synthesized GBCO powders respectively, according to the fabrication procedures illustrated in Fig. 1. For fabrication of cell-1, the as-spun GBCO/PVP composite nanofibers were peeled off from the collector of alumina foil and calcined at 600 °C for 1 h in air; the pre-calcined nanofibers were then slightly ground in a mortar and mixed with a-terpineol and ethyl cellulose to make the precursor ink, which was screen-printed onto both sides of the GDC pellet followed by calcination at 1000 °C for 2 h in air, forming the GBCO/GDC/GBCO symmetric cell (cell-1). In cell-2, the GBCO cathode layer was prepared using the GBCO powders synthesized with the sol–gel method under multiple calcinations (at 80 °C, 200 °C, 600 °C and 1050 °C in sequence). The as-synthesized GBCO powders were ball-milled for 3 h to get a fine powder for preparation of the precursor cathode ink mixed with a-terpineol and ethyl cellulose. For a reasonable comparison between the cathode per-
Fig. 1. Illustration of fabrication processes of GBCO/GDC/GBCO symmetric cells using GBCO/PVP composite nanofibers (cell-1) and GBCO powders synthesized with sol–gel method (cell-2) respectively.
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as-spun GBCO/PVP composite fibers at various temperatures followed by XRD measurements. The XRD patterns of the calcined GBCO/PVP composite fibers were shown in Fig. 2. A broad diffuse peak at 20–35° was observed in the XRD pattern of the sample calcined at 600 °C, demonstrating existence of amorphous phases likely of incompletely decomposed PVP. Meanwhile, several small diffraction peaks were also found, thus some crystalline intermediate phases were formed at this stage. The XRD pattern for 900 °C was composed of complicated X-ray diffraction peaks, which were indexed by dominant phases of GdCoO3 (JCPDS card No. 25-1057) and BaCoO3 d (JCPDS card No. 26-0144), and some minor phases of Co3O4 (JCPDS card No. 76-1802) and BaO (JCPDS card No. 65-2923). These results indicated that 600–900 °C was the temperature range where intermediate phases of metal oxides were formed. Sharp diffraction peaks were observed in the XRD pattern of 1000 °C, which were indexed by an orthorhombic Pmmm space group with lattice parameters of a = 3.908(7) Å, b = 3.885(6) Å, c = 7.517(3) Å, and cell volume of V = 114.170(1) Å3, matching well with the literature data for GBCO [31]. Thus a pure GBCO phase was formed after the as-spun GBCO/PVP composite nanofibers were calcined at 1000 °C in air. The GBCO powders synthesized with the sol–gel method have the similar XRD pattern shown in Fig. 2. Fig. 3 presents TG curve of the as-spun GBCO/PVP composite fibers measured in air. A dramatic weight loss was observed from room temperature to 430 °C, which was ascribed to decomposition and removal of organic materials (PVP, ethanol), NO3 groups of nitrates, and the other volatiles (H2O, COx, etc.) [32,33]. A magnified TG plot for the temperatures above 400 °C was inserted, from which we found that the composite underwent a slight weight drop at 600–800 °C, which was probably associated with formation of the intermediate phases of metal oxides as demonstrated by the XRD result (Fig. 2). There was a weight platform at temperatures around 1000 °C, indicating formation of the GBCO phase according to the XRD result shown in Fig. 2. Slight weight loss at 1100 °C was probably due to thermal-driven releasing of lattice oxygen in GBCO, as observed in other perovskite oxides such as PrBaCo2O5+d [34]. Morphology and microstructure of the as-spun GBCO/PVP composite fibers and the fibers calcined at various temperatures in air are presented in Fig. 4. The results show that the as-spun composite fibers were straight and smooth with diameters of 100–200 nm. After calcined at 800 °C, the composite fibers shrank with decreased diameter of 50 nm, and also became rough due to formation of polycrystalline intermediate-phase grains, as demonstrated by the XRD results (Fig. 2). At 1000 °C, the nanofibers were broken and developed as coral like aggregates, which, as clearly shown in the magnified image (Fig. 4d), were composed of bundles of rods
Fig. 2. XRD patterns of GBCO/PVP composite fibers calcined at various temperatures in air.
with length of 30 nm. This nanostructure aroused much interest to us because as a cathode material the resultant large surface areas are expected to enhance the electrocatalytic activity for the oxygen reduction reaction and can thus result in improved electrochemical performance [4,18].
3.2. Fabrication process optimization and microstructure of the GBCO cathode To prepare the GBCO cathode layer using the GBCO/PVP composite nanofibers, we firstly tried to electrospin the GBCO/PVP composite nanofibers directly on the GDC electrolyte pellet followed by step-by-step calcinations at 300 °C, 600 °C and 1000 °C for 5 h respectively. Unfortunately, however, the as-prepared cathode layer easily peeled off from the GDC pellet, which was probably caused by fast burning of the as-spun GBCO/PVP composite nanofibers. As shown in the TG curve (Fig. 3), about 90% weight ratio of the as-spun GBCO/PVP composite nanofibers was burnt off at the temperature up to 400 °C; Releasing of so much burning gases could lead to poor connection between the GBCO and GDC layers and cause the cathode layer delamination. To improve the GBCO/ GDC interface connection, an updated method was adopted to fabricate the GBCO cathode based on the electrospun GBCO/PVP composite nanofibers collected on the alumina foil, as illustrated in Fig. 1a. The key point for this fabrication process is the 600 °C pre-calcination of the as-spun GBCO/PVP composite nanofibers, during which most of the organic compositions decomposed and released (Fig. 3). As a result, a good connection was obtained between the GBCO cathode and GDC electrolyte layers (Fig. 5a), thus no delamination occurred in the cell (cell-1). Significantly, as shown in Fig. 5b, a homogeneous network structure of well-connected grains with sizes smaller than 1 lm and uniformly distributed pores were formed in the prepared cathode layer, similar to the microstructure shown in Fig. 4c on the whole while the nanostructures could not be observed. In contrast, as shown in Fig. 5c, microstructure of the GBCO cathode in cell-2 fabricated using the sol–gel synthesized GBCO powders is not uniform and the particles vary in size and are not well-connected, although the used GBCO powders were ball-milled for 3 h in advance and the cathode layer was calcined under the same conditions with the GBCO cathode in cell-1. The porous network structure of the GBCO cathode prepared using the GBCO/PVP composite nanfibers has enhanced the electrode reaction as demonstrated below. Furthermore, as illustrated in Fig. 1, the cathode fabrication process using the electrospun composite nanofibers is time- and cost-saving and easy for manipulation as compared to the fabrication process using the GBCO
Fig. 3. TG curve of the as-spun GBCO/PVP composite fibers measured in air.
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Fig. 4. SEM images of as-spun GBCO/PVP composite fibers (a) and the fibers after calcination at 800 °C (b) and 1000 °C (c and d) in air.
Fig. 5. Cross-sectional and surface SEM images of the GBCO/GDC interfaces with the GBCO cathode layer prepared at 1000 °C using GBCO/PVP composite nanofibers (a and b) and GBCO powders synthesized with sol–gel method (c and d) respectively.
powders synthesized with sol–gel method in this work and solid state reaction method reported in literatures [27–30]. This is because multiple steps of high-temperature calcination were required for formation of the GBCO pure phase from the sol–gel precursor or solid reaction oxides; high-temperature calcinations are time-wasting and could cause powder aggregations, therefore, the as-synthesized powders had to be ball-milled usually for several hours before preparation of the cathode ink. In contrast, only a short period of pre-calcination at 600 °C was needed before the GBCO/PVP composite nanofibers were used for preparation of the
GBCO cathode layer on the GDC pellet, and the ball-milling step was saved because the pre-calcined GBCO/PVP nanofibers are uniform and easily broken with a hand-grinding. 3.3. Electrochemical performance of GBCO cathode Fig. 6 shows the typical EIS results for both GBCO/GDC/GBCO symmetric cells (cell-1 and cell-2). The ohmic resistance arising from GDC electrolyte and lead wires were normalized to zero for clarity. The frequency of the impedance arcs is in the range of
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1–104 Hz, which could be associated with multiple elementary steps of the cathode reaction, such as oxygen gas diffusion, surface oxygen diffusion, adsorption, dissociation, and oxygen ionic/electronic charge transfers [3,6,35]. The span of the arcs at the real axis in the spectra corresponds to polarization resistance (Rp) of the cathodes in the symmetric cell. Apparently, the polarization resistances of the GBCO cathode prepared with electrospun nanofibers (in cell-1) are much smaller than the cathode prepared with the sol–gel-synthesized powders (in cell-2). From the equation of ASR = Rp A/2, where A is geometric area of the cathode, area specific resistances (ASRs) of the GBCO cathodes for both cells were calculated, and the results are shown in Fig. 7. Fig. 7 also presents the ASR results of the GBCO cathodes reported in literatures [27,29] for comparison. The GBCO cathode prepared with the sol–gel-synthesized powders (in cell-2) has the ASR values ranging from 0.78 X cm2 at 600 °C to 0.084 X cm2 at 750 °C, in-between the reported ASR values for the GBCO cathodes [27,29]. In contrast, the ASRs of the GBCO cathode fabricated from the electrospun nanofibers (in cell-1) are 0.53 X cm2 at 600 °C, 0.22 X cm2 at 650 °C, 0.10 X cm2 at 700 °C and 0.043 X cm2 at 750 °C respectively, which are smaller than the results of the cathode prepared with the sol–gel-synthesized powders in this work. For example, the ASR value of GBCO cathode in cell-1 decreased 49% at 750 °C and 32% at 600 °C respectively, as compared to the GBCO cathode in cell-2. These results have demonstrated that the GBCO cathode prepared using the GBCO/PVP composite nanofibers has better electrochemical performance than the cathode prepared with the sol–gel-synthesized GBCO powders as expected. It’s known that in addition to increase in oxygen ionic conductivity and oxygen transport kinetics, electrochemical performance of the cathode can also be improved by adjustment of the cathode microstructures [16,17]. In this work, the two GBCO cathodes in
Fig. 7. Temperature-dependence of area specific resistances (ASRs) of the GBCO cathodes for cell-1 and cell-2 together with the ASR results of GBCO cathode reported in literatures [27,29].
cell-1 and cell-2 were fabricated under the same conditions (at 1000 °C for the same 3 h); their different microstructures shown in Fig. 5 were due to different nature of the starting materials (GBCO/PVP composite nanofibers and GBCO powders) for preparation of the cathode. In comparison, the GBCO cathode (in cell-1) prepared using the GBCO/PVP composite nanofibers had a more homogeneous and porous network structure with better-connected smaller grains (about several hundred nanometers) than the GBCO cathode (in cell-2) fabricated using the as-synthesized GBCO powders (Fig. 5). Such cathode microstructure in cell-1 could facilitate air diffusion through the cathode layer and enhance the electrode electrochemical activities with enlarged three-phase boundaries (TPBs) and more active sites for oxygen reduction reactions due to its enlarged specific area [4,18], which was believed to be the main contribution to its improved electrochemical performance.
4. Conclusions GdBaCo2O5+d (GBCO)/poly (vinyl pyrrolidone) (PVP) composite nanofibers were prepared by electrospinning. A pure orthorhombic GBCO phase with coral like nano-structured aggregates was formed after the GBCO/PVP composite fibers were calcined at 1000 °C. Using the GBCO/PVP composite nanofibers pre-calcined at 600 °C, the pure phase GBCO cathode layer was easily fabricated on the GDC pellet, forming the GBCO/GDC/GBCO symmetric cell. This fabrication process is time- and cost-saving and easy for manipulation as compared with the fabrication process using the as-synthesized GBCO powders with sol–gel method. More significantly, the GBCO cathode prepared with the GBCO/PVP composite nanofibers has exhibited improved electrochemical performance than the cathode prepared with the GBCO powders, characterized by its lower area specific resistance values of 0.53 X cm2 at 600 °C, 0.22 X cm2 at 650 °C, 0.10 X cm2 at 700 °C and 0.043 X cm2 at 750 °C respectively. The homogenous network structure of the GBCO cathode prepared with the GBCO/PVP composite nanofibers was believed to enhance the cathode electrochemical activities and realize the improved performance. Acknowledgements
Fig. 6. Typical EIS plots for GBCO/GDC/GBCO symmetric cells (cell-1 and cell-2) measured at different temperatures.
This work was financially supported by the ‘‘Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry’’ and ‘‘the Fundamental Research Funds for the Central Universities (DUT12LAB02)’’.
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References [1] W. Zhou, F.L. Liang, Z.P. Shao, J.L. Chen, Z.H. Zhu, Sci. Rep. 1 (2011) 155–160. [2] D. Han, X.J. Liu, F.R. Zeng, J.Q. Qian, T.Z. Wu, Z.L. Zhan, Sci. Rep. 2 (2012) 462– 466. [3] S.B. Adler, Chem. Rev. 104 (2004) 4791–4844. [4] C. Ding, T. Hashida, Energy Environ. Sci. 3 (2010) 1729–1731. [5] Z.B. Yang, C.H. Yang, C. Jin, M.F. Han, F.L. Chen, Electrochem. Commun. 13 (2011) 882–885. [6] S.L. Pang, X.N. Jiang, X.N. Li, Q. Wang, Z.X. Su, J. Power Sources 204 (2012) 53– 59. [7] Y. Li, J.G. Cheng, J. Song, J.A. Alonso, M.T. Fernández-Díaz, J.B. Goodenough, Chem. Mater. 24 (2012) 4114–4122. [8] S.G. Huang, G.J. Wang, X.H. Sun, C.M. Lei, T. Li, C.C. Wang, J. Alloys Compd. 543 (2012) 26–30. [9] S.Y. Park, H. Ji, H.R. Kim, K.J. Yoon, J.W. Son, B.K. Kim, H.J. Je, H.W. Lee, J.H. Lee, J. Power Sources 228 (2013) 97–103. [10] L. Malavasi, C. Tealdi, C. Ritter, Angew. Chem. Int. Ed. 48 (2009) 8539–8542. [11] Z.L. Ding, Z.J. Yang, D.M. Zhao, X.L. Deng, G.L. Ma, J. Alloys Compd. 550 (2013) 204–208. [12] S.L. Pang, X.N. Jiang, X.N. Li, Q. Wang, Q.Y. Zhang, Mater. Chem. Phys. 131 (2012) 642–646. [13] S. Lü, G. Long, Y. Ji, X. Meng, H. Zhao, C. Sun, J. Alloys Compd. 509 (2011) 2824– 2828. [14] H.P. Ding, X.J. Xue, J. Alloys Compd. 496 (2010) 683–686. [15] X.N. Li, X.N. Jiang, S.L. Pang, Q. Wang, Z.X. Su, Q.Y. Zhang, Int. J. Hydrogen Energy 36 (2011) 3850–3857. [16] J.C. Ruiz-Morales, D. Marrero-López, M. Gálvez-Sánchez, J. Canales-Vázquez, C. Savaniu, S.N. Savvin, Energy Environ. Sci. 3 (2010) 1670–1681. [17] Y. Zhang, S. Zha, M. Liu, Adv. Mater. 17 (2005) 487–491.
189
[18] F. Zhao, Z. Wang, M. Liu, L. Zhang, C. Xia, F. Chen, J. Power Sources 185 (2008) 13–18. [19] J.J. Choi, J.H. Choi, J.H. Ryu, B.D. Hahn, J.W. Kim, C.W. Ahn, W.H. Yoon, D.S. Park, J. Alloys Compd. 545 (2012) 186–189. [20] S. Wang, J. Yoon, G. Kim, D. Huang, H. Wang, A.J. Jacobson, Chem. Mater. 22 (2010) 776–782. [21] J.J. Choia, S.H. Oh, H.S. Noh, H.R. Kim, J.W. Son, D.S. Park, J.H. Choi, J. Ryu, B.D. Hahn, W.H. Yoon, H.W. Lee, J. Alloys Compd. 509 (2011) 2627–2630. [22] V. Thavasi, G. Singh, S. Ramakrishna, Energy Environ. Sci. 1 (2008) 205–221. [23] S. Shahgaldi, Z. Yaakob, D.J. Khadem, M. Ahmadrezaei, W.R. Wan Daud, J. Alloys Compd. 509 (2011) 9005–9009. [24] M.J. Zhi, N. Mariani, R. Gemmen, K. Gerdes, N.Q. Wu, Energy Environ. Sci. 4 (2011) 417–420. [25] M.J. Zhi, S.W. Lee, N. Miller, N.H. Menzler, N.Q. Wu, Energy Environ. Sci. 5 (2012) 7066–7071. [26] J. Sunarso, S. Liu, Y.S. Lin, J.C. Diniz da Costa, Energy Environ. Sci. 4 (2011) 2516–2519. [27] Li. N, Z. Lü, B. Wei, X. Huang, K. Chen, Y. Zhang, W. Su, J. Alloys Compd. 454 (2008) 274–279. [28] A. Tarancón, S.J. Skinner, R.J. Chater, F. Hernández-Ramírez, J.A. Kilner, J. Mater. Chem. 17 (2007) 3175–3181. [29] A. Tarancón, J. Peña-Martínez, D. Marrero-López, A. Morata, J.C. Ruiz-Morales, P. Núñez, Solid State Ionics 179 (2008) 2372–2378. [30] J.H. Kim, A. Manthiram, J. Electrochem. Soc. 155 (4) (2008) B385–390. [31] K. Zhang, L. Ge, R. Ran, Z. Shao, S. Liu, Acta Mater. 56 (2008) 4876–4889. [32] Y. Zhang, J. Yang, Q. Li, X. Cao, J. Cryst. Growth 308 (2007) 180–184. [33] C. Shao, N. Yua, Y. Liu, R. Mu, J. Phys. Chem. Solids 67 (2006) 423–426. [34] C. Frontera, A. Caneiro, A.E. Carrillo, J. Oro-Sole, J.L. Garcıa-Munoz, J. Mater. Chem. 17 (2005) 5439–5445. [35] M.J. Escudero, A. Aguadero, J.A. Alonso, L. Daza, J. Electroanal. Chem. 611 (2007) 107–116.