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Electrospinning La0.8Sr0.2Co0.2Fe0.8O3−δ tubes impregnated with Ce0.8Gd0.2O1.9 nanoparticles for an intermediate temperature solid oxide fuel cell cathode
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Electrospinning La0.8Sr0.2Co0.2Fe0.8O3Ld tubes impregnated with Ce0.8Gd0.2O1.9 nanoparticles for an intermediate temperature solid oxide fuel cell cathode Erqing Zhao a, Chao Ma b,c,d, Wei Yang b,c,d, Yueping Xiong a,*, Jianqi Li b,c,d, Chunwen Sun b,c,d,** a
School of Chemical Engineering and Technology, Harbin Institute of Technology, Harbin 150001, PR China Beijing National Laboratory for Condensed Mater Physics, Institute of Physics, Chinese Academy of Science, Beijing 100190, PR China c Key Laboratory for Renewable Energy, Chinese Academy of Sciences, Beijing 100190, PR China d Beijing Key Laboratory for New Energy Materials and Devices, Beijing 100190, PR China b
article info
abstract
Article history:
In order to reduce the polarization resistance of the cathode, we have developed one-
Received 23 December 2012
dimensional (1D) nanostructured La0.8Sr0.2Co0.2Fe0.8O3d (LSCF) tubes/Ce0.8Gd0.2O1.9 (GDC)
Received in revised form
nanoparticles composite cathodes for solid oxide fuel cell. Uniform LSCF/PVP composite
1 March 2013
nanofibers have been firstly synthesized by a single-nozzle electrospinning technique,
Accepted 21 March 2013
followed by firing at 800 C for 2 h to form one-dimensional LSCF tubes. Subsequently, the
Available online 21 April 2013
GDC phases were introduced into tube structured LSCF scaffold pre-sintered on a GDC pellet by a multi-impregnation process. Electrochemical Impedance spectra reveal that
Keywords:
nanostructured LSCF tubes/GDC nanoparticles composite cathodes have a better electro-
La0.8Sr0.2Co0.2Fe0.8O3d
chemical performance, achieving area-specific resistances of 4.70, 1.12, 0.27 and 0.07 U cm2
Ce0.8Gd0.2O1.9
at 500, 550, 600 and 650 C for the composite of GDC and LSCF in a weight ratio of 0.52:1. The
Composite cathode
low ASR values are mainly related to its optimal microstructure with larger triple-phase
Electrospinning
boundaries and higher porosity. These results suggest that LSCF tube/GDC nanoparticle
Impregnation
composite can be an alternative cathode material for intermediate temperature solid oxide
Intermediate temperature solid
fuel cell (IT-SOFC).
oxide fuel cell
Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
In recent years, great efforts have been devoted to developing low or intermediate temperature solid oxide fuel cells (SOFCs) operating at 500e800 C [1e3]. Lowering the operating temperature can suppress degradation of components and extend the range of acceptable material selection, this
also serves to improve cell durability and reduce the system cost. However, reducing the operating temperature decreases the electrode kinetics and thus results in large interfacial polarization resistances. This effect is most pronounced for the oxygen reduction at the cathode. In order to lower the polarization resistance of the cathode, a favorable electronic and ionic conductivity as well as high
* Corresponding author. Mailbox 1247, School of Chemical Engineering and Technology, Harbin Institute of Technology, 92 West Dazhi Street, Nan Gang District, Harbin 150001, PR China. Tel.: þ86 451 86413721; fax: þ86 451 86418616. ** Corresponding author. Beijing National Laboratory for Condensed Mater Physics, Institute of Physics, Chinese Academy of Science, South 3rd Street, Zhongguancun, Beijing 100190, PR China. Tel.: þ86 10 82649901; fax: þ86 10 82649046. E-mail addresses: [email protected] (Y. Xiong), [email protected] (C. Sun). 0360-3199/$ e see front matter Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2013.03.111
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catalytic activity for the oxygen reduction must be maintained. Currently, tailoring the electrode microstructure has become an important approach to the fabrication of high performance cathodes. Furthermore, nanostructure cathodes have been believed as the optimal electrode structure due to their high electrocatalytic activity and large TPB length. So far, one-dimensional (1D) nanostructured materials with significantly higher surface areas, including nanowires, nanorods and nanotubes, being used as SOFC cathodes, have been demonstrated superior electrochemical properties. The La0.6Sr0.4CoO3 (LSCO) nanotubes [4] have been synthesized by the pore wetting technique and made into the nanotube structured LSCO cathodes, which exhibited lower polarization resistance compared with those of conventional LSCO cathodes. Recently, a 3D fibrous Sm0.5Sr0.5CoO3/Ce0.8Sm0.2O1.9 cathode has been fabricated, which possesses high porosity and interconnectivity, and low polarization resistance [5]. La0.8Sr0.2MnO3d/Zr0.92Y0.08O2 composite nanotubes [6] were cosynthesized by a pore wetting technique as a cathode material for SOFC. The area-specific resistance (ASR) values of these nanotube structured composite cathode reach 0.17, 0.25, 0.39 and 0.52 U cm2 at 850, 800, 750 and 700 C, respectively, which are attributed to the high specific surface area and multi-scale porosity. Similarly, nanostructured Pr0.6Sr0.4Fe0.8Co0.2O3 (PSFC)/ Ce0.8Sm0.2O2 (SDC) composite cathodes, made of PSFC/SDC composite nanotubes, have been investigated, exhibiting a smaller ASR value than that for the conventional sample [7]. All of the above-mentioned nanostructured electrode materials have been synthesized by a pore wetting technique. In addition to this method, the electrospinning technique has been exploited to prepare nanostructured electrode materials for SOFC. Electrospinning is a simple, versatile and convenient technique to synthesize nanofibers from polymeric solutions [8]. In an electrospinning process, a high voltage is applied to the surface of polymeric solutions followed by ejection of a liquid jet through the spinneret. Due to bending instability, the jet is converted to continuous and ultrathin fibers. The electrospun nanofibers can be calcined to form crystalline oxide nanofibers. The YSZ nanofibers infiltrated with LSM nanoparticles have been used as the cathode of SOFC [9], which show high catalytic activity toward O2 reduction reaction and offer larger triple-phase boundary length. Subsequently, the La0.58Sr0.4Co0.2Fe0.8 nanofibers [10] were fabricated and applied as the cathode of IT-SOFC, the cell with LSCF nanofiber cathode shows a high power density. Li et al. [11] synthesized uniform YSZ nanofibers by electrospinning 8YSZ dispersion and applied it as a SOFC anode, the performance of the cell with the fiber-derived anode is better than that with the powder-derived anode due to the larger TPB sites in fiber structured electrode. Hieu et al. [12] obtained nanofiber-structured Ba0.5Sr0.5Co0.8Fe0.2O3d perovskite-oxide as a cathode material for low-temperature solid oxide fuel cells, the ASR of the BSCF nanofiber cathode was determined to be 0.094 U cm2 at 600 C, whereas that of the BSCF powder cathode was 0.468 U cm2 under the similar conditions. The difference of ASR was attributed to high specific surface area of the BSCF nanofibers. Shahgaldi et al. [13] prepared the cobalt-free perovskite-oxide, Ba0.5Sr0.5Fe0.8Cu0.2O3d (BSFC) nanofibers and demonstrated that the feasibility of BSFC
nanofibers served as effective cathode materials for IT-SOFCs. Very recently, the Nd1.93Sr0.07CuO4 nanofibers [14] were synthesized by electrospinning technique and used as cathode material for SOFC, and the nanofiber cathode shows better electrochemical performance than the power cathode with the same composition, which is attributed to the former’s optimal microstructure. Our group [15] also reported onedimensional LSCF/GDC nanocomposite cathode by the combination of electrospinning and infiltration methods. The above-mentioned references involved in electrospinning all studied non-hollow electrode materials for SOFC. To our knowledge, there are few reports on producing the porous and hollow LSCF materials for application in SOFC by electrospinning technique and adding GDC phases into the tube structured LSCF scaffold to form the LSCF/GDC composite cathode by an infiltration process. Herein, we reported preparation of one dimensional and porous La0.8Sr0.2Co0.2Fe0.8O3d (LSCF) tubes using a single-nozzle electrospinning technique. Subsequently, the GDC phases were added into LSCF tubes to form LSCF tubes/GDC nanoparticles composite cathodes by an impregnation process [16e22] which involves the addition of an ion conductor phase or electron conductor phase into a pre-sintered porous backbone. The electrochemical performances of GDC-impregnated LSCF nanostructured composite cathodes have been tested as well.
2.
Experimental
2.1.
Preparation of GDC electrolyte pellets
GDC nanopowders were prepared by means of the citrate complexion method [23]. Then, the as-synthesized GDC were pressed into pellets using a stainless steel mold (25 mm in diameter) under 120 MPa, followed by sintering at 1450 C for 20 h in air to obtain dense electrolytes with a thickness of 1.1 mm and a diameter of 19 mm.
2.2.
Synthesis of porous LSCF tubes
Uniform LSCF/PVP nanofibers were fabricated by electrospinning LSCF precursor solution consisting of La(NO3)3$6H2O,
4000
Intensity(a.u)
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3000
2000
1000
0
20
40
60
2Theta/( degree) Fig. 1 e XRD pattern of the LSCF/PVP nanofibers after calcination at 800 C for 2 h.
80
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by dissolving the above metal nitrates in DMF solvent in a molar ratio of La:Sr:Co:Fe ¼ 0.8:0.2:0.2:0.8 and stirred at ambient temperature. The proper quantity of PVP was added into the resulting solution, followed by magnetic stirring for 4 h to get a transparent precursor solution for electrospinning. The as-prepared LSCF precursor solution was loaded into a plastic syringe equipped with 8-gauge stainless steel needle. For the electrospinning experiment, the anode of a high voltage supply was clamped on the spinneret (stainless steel needle) and the cathode was connected to a nickel mesh as a collector placed 12 cm away from the orifice. The applied voltage was kept at 20 kV. The electrospun products were initially dried for 12 h in air to evaporate the solvent and then calcined at 800 C for 2 h, with a heating rate of 2 C min1.
2.3.
Preparation of tube structured LSCF cathodes
The LSCF tube powers were then mixed with 3 wt%-ethyl cellulose terpineol solution to form cathode pastes. The obtained LSCF cathode pastes were coated on one side of the GDC electrolyte, which followed by sintering at 904 C for 1 min with a heating rate of 15 C min1 in air to form the tube structured LSCF cathode. The active electrode area was 0.785 cm2 and the thickness was about 60 mm.
2.4. Preparation of GDC-impregnated LSCF composite cathodes
Fig. 2 e (a) SEM image of the LSCF/PVP nanofibers fabricated by electrospinning of LSCF precursor solution; (b) SEM image of as-calcined LSCF/PVP nanofibers at 800 C for 2 h.
Sr(NO3)2, Co(NO3)2$6H2O, Fe(NO3)3$9H2O, Polyvinylpyrrolidone (PVP) and N,N-dimethylformamide (DMF), followed by high temperature treatment to form porous LSCF tubes. In a typical procedure, the electrospinning solution was prepared
The GDC impregnation solution with a concentration of 0.25 mol L1 was prepared by dissolving stoichiometric Gd(NO3)3$6H2O and Ce(NO3)3$6H2O in a mixed solvent of ethanol and deionized water. The GDC-impregnated LSCF composites were obtained by infiltration of GDC nitrate solution into the LSCF scaffold pre-sintered on a GDC electrolyte pellet. For one time impregnation, 20 mL GDC precursor solution was introduced into the LSCF substrate. After that, the impregnated sample was calcined at 700 C for 0.5 h with a heating rate of 10 C min1. To further increase the GDC content, the impregnation process was repeated. Finally, the impregnated electrodes were fired at 800 C for 1 h in order to obtain the GDC phases.
Fig. 3 e (a) HAADF-STEM image of the as-calcined product; (b) energy dispersive X-ray spectroscopy of the La0.8Sr0.2Co0.2Fe0.8O3Ld nanofibers.
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Fig. 4 e Representive cross-sectional SEM images of the LSCF cathode and GDC-infiltrated LSCF composite cathodes: (a) blank LSCF cathode; (b)e(f) the infiltrated cathodes with GDC/LSCF weight ratio of 0.09:1, 0.17:1, 0.35:1, 0.52:1 and 0.67:1, respectively; (g) the infiltrated cathodes with GDC/LSCF weight ratio of 0.52:1 under a constant current density of 200 mA cmL2 at 750 C for 85 h.
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Fig. 4 e (continued).
2.5.
Characterization
The surface morphologies of electrospun and calcined LSCF/ PVP nanofibers were observed by scanning electron microscope (SEM, FEI Quanta 200, Holland) and transmission electron microscopy (FEI Tecnai-F20 TEM). The chemical compositions were analyzed by EDX measurements in TEM. X-ray diffraction (XRD, Rigaku D/max-UB, Cu Ka radiation) was applied to identify the phase of calcined products. The element compositions of infiltrated LSCF tubes were also confirmed by EDX in TEM. The electrochemical performances of the cathodes were evaluated by electrochemical impedance spectra. Threeelectrode system was used with the cathodes as the working electrodes. Pt pastes were applied to the rim of the GDC
electrolyte pellet as the reference electrode and to one side of the GDC electrolyte as the counter electrode. Two Pt meshes were attached to the working electrode (a pure LSCF cathode or a GDC-infiltrated LSCF cathode) and to the counter electrode as current collectors for the electrochemical measurement. Electrochemical impedance spectra of LSCF electrodes were measured with an electrochemical work station (Chi604D) in a frequency range of 0.1 Hze100 kHz and an amplitude of 5 mV in the temperature range of 500 C and 650 C. The area-specific resistances were derived from the difference between the low and high-frequency intercepts at the real impedance axis. And, the LSCF tube-based cathode was treated under a constant current density of 200 mA cm2 at 750 C for 85 h with the aim of evaluating the adhesion characteristic between the cathode and electrolyte. The
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microstructures of the LSCF cathode and GDC-infiltrated LSCF cathodes after the impedance testing and the electrode/electrolyte interface after the long-term operation were detected by scanning electron microscope.
3.
Results and discussion
The phase purity and crystal structure of the as-calcined products of LSCF/PVP composite fibers were examined by X-ray powder diffraction (XRD). Fig. 1 shows that the XRD pattern of the calcined products at 800 C can be indexed as a pure perovskite-type phase of LSCF (JCPDS No. 48-0125). The sharp peaks indicate that the as-calcined LSCF nanofibers have high crystalline characteristics. Fig. 2 presents the scanning electron microscopy (SEM) images of LSCF/PVP composite nanofibers and calcined LSCF nanofibers. From Fig. 2(a), it can be found that the electrospun LSCF/PVP composite nanofibers are smooth and uniform, with diameters in the range of 300e400 nm. Fig. 2(b) shows that the calcined product is consisting of one-dimensional nanofibers with a diameter in the range of 200e300 nm. It can be clearly seen that these nanofibers have limited length, smooth surfaces, and open terminals after calcination at 800 C for 2 h. The morphology of the LSCF nanofibers was further characterized by transmission electron microscopy (TEM). Fig. 3(a) is a typical high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) image of the as-calcined product. It can be seen that the nanofibers form a tube structure after calcination at 800 C. To further clarify the chemical composition of the calcined tube, we performed energy dispersive X-ray spectroscopy, as shown in Fig. 3(b). It reveals the calcined tube is composed of La, Sr,
Co, Fe and O elements. Such tube structured LSCF materials as a SOFC cathode are expected to reduce the cathode polarization resistance due to their high porosity and hollow structure which facilitate the gas diffusion through the cathode. Fig. 4 shows typical cross-sectional SEM images of the LSCF cathode and GDC-infiltrated LSCF composite cathodes. From Fig. 4(a), it can be found that the blank LSCF cathode still retain excellent one-dimensional tube structure and the size of LSCF tubes almost keep unchanged, which attributed to the faster firing and the shorter dwell time. As can be seen from the upper image in Fig. 4(a), there is good adhesion and no delamination between the cathode and the electrolyte, suggesting the tube structured LSCF cathode was sintered well. Also, this tube structured LSCF cathode has higher porosity compared to the nanoparticle structured LSCF cathode [15]. After the GDC phases were added into LSCF cathode, onedimensional LSCF-GDC composite cathodes can be still observed from Fig. 4(b)e(f). Moreover, with increasing the amount of GDC loading, the surface of LSCF substrate became coarser, implying more and more GDC phases appeared on the LSCF scaffold. Fig. 4(g) displays the sectional SEM image of the infiltrated LSCF cathode with GDC/LSCF weight ratio of 0.52:1 under a constant current density of 200 mA cm2 at 750 C for 85 h. From Fig. 4(g), delamination can not be observed at the electrode/electrolyte interface, which indicates the compact adhesion between the electrode and electrolyte after the longterm operation. The GDC phases can be defined by aid of the element analysis of the infiltrated LSCF cathode. Fig. 5 displays the TEM image and EDX spectra of the infiltrated LSCF tube. From Fig. 5, it can be obtained that the Gd and Ce elements appeared on the infiltrated LSCF tubes, while not on the LSCF tubes before the infiltration (shown in Fig. 3(b)). This result demonstrates
Fig. 5 e The TEM image and EDX spectra of the infiltrated LSCF tubes in the LSCF cathode infiltrated with GDC solution.
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that the GDC phases have been deposited onto the LSCF cathode. Especially, it is noted that the LSCF tubes gradually disappeared when the GDC infiltration loading increased, which indicates that some GDC phases may be formed in the interior of LSCF substrates. The electrochemical performance was evaluated by electrochemical impedance spectroscopy (EIS) on a half cell consisting of a dense GDC electrolyte pellet, LSCF tube-based cathode and porous Pt counter electrode over the temperature range of 500e650 C in air. The impedance spectrum of the blank tube structured LSCF cathode at 650 C is shown in Fig. 6(a), and the typical impedance spectra obtained at 650 C for the infiltrated LSCF cathodes with different GDC/LSCF weight ratio are exhibited in Fig. 6(b). The impedance response for oxygen reduction on the LSCF tubes-based cathodes is characterized as a depressed arc. The high-frequency intercept of the electrode impedance on the real axis is the total resistance of the electrolyte. The difference between the lowfrequency and the high-frequency intercepts on the real axis corresponds to the area-specific resistance (ASR) of the two interfaces. The ASR is the overall resistance related to the oxygen reduction, oxygen surface/bulk diffusion, and the gasphase oxygen diffusion. In Fig. 6, all electrolyte ohmic resistances were removed from the impedance spectra for clearly showing the difference. For the blank LSCF cathode, its
ASR value is 1.56 U cm2 at 650 C. After GDC infiltration, the ASR value of LSCF cathode reduces significantly with the increasing of the GDC amount. The ASR values of GDCinfiltrated tube structured LSCF cathodes are reduced to 0.53, 0.31, 0.12 and 0.07 U cm2 at 650 C, respectively, corresponding to the weight ratio of GDC/LSCF of 0.09:1, 0.17:1, 0.35:1, 0.52:1. When the GDC/LSCF weight ratio increased to 0.67:1, the ASR of the infiltrated cathode became higher, which may be interpreted by its microstructure (shown in Fig. 4(f)). At GDC/LSCF weight ratio of 0.67:1, the infiltrated cathode appears to be much less porous, which blocks the gas diffusion, decreases the TPB length, and thus results in a higher ASR. However, the lowest ASR of 0.07 U cm2 obtained in this work at 650 C is much lower than those of other LSCF composite cathodes (Table 1) reported in the literature [24e29], as well as LSCF nanorod/GDC nanoparticle composite that we investigated recently [15]. The typical impedance spectra obtained over the temperature range of 500e600 C for the infiltrated LSCF cathode with GDC:LSCF weight ratio of 0.52:1 are shown in Fig. 6(c). At 600, 550, and 500 C, the ASR values are 0.27, 1.12 and 4.70 U cm2, respectively. The excellent performance of tube structured LSCF cathode impregnated with GDC nanoparticles can be ascribed to its optimized electrode microstructure with high porosity and extended triple-phase boundaries [30e33] where the
1.6
(a)
0.48
blank LSCF cathode
(b)
0.09:1 0.17:1 0.35:1 0.52:1 0.67:1
1.2 0.8
-Z''/Ω cm2
-Z''/Ω cm2
0.36
0.4
0.24 0.12
0.0 0.00
-0.4 -0.4
0.0
0.4 0.8 Z'/Ω cm2
1.2
1.6
0.00
0.12
0.24 0.36 Z'/Ω cm2
0.48
5
(c)
500ºC 550ºC 600ºC
4
-Z''/Ω cm2
3 2 1 0 0
1
2 3 Z'/Ω cm2
4
5
Fig. 6 e (a) The impedance spectrum of the blank tube structured LSCF cathode at 650 C; (b) the typical impedance spectra of the infiltrated LSCF cathodes with different GDC/LSCF weight ratio measured at 650 C; (c) the EIS spectra of the infiltrated LSCF cathode with GDC/LSCF weight ratio of 0.52:1 measured at different temperatures.
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Table 1 e Comparison of the ASR for LSCF composite cathodes, measured at 650 C. Cathode LSCF tube/GDC LSCF rod/GDC LSCF/GDC LSCF/SDC LSCF/GDC LSCF/SSC LSCF/SDC LSCF/YSZ
Fabrication method
ASR (U cm2)
References
Infiltration Infiltration Mechanical mixed Infiltration Infiltration Infiltration Polymerizable complex Infiltration
0.07 0.10 0.52 0.40 0.45 0.12 0.265
Present work Zhao et al [15] Fu et al. [24] Nie et al. [25] Chen et al. [26] Lou et al. [27] Lee et al. [28]
0.218
Chen et al. [29]
oxygen reduction reaction takes place. By adding the GDC phases into tube structured LSCF scaffold, the TPB region changes from a two-dimensional interface between the cathode and the electrolyte to an entire three-dimensional cathode. It is well known that the enlargement of TPB region can promote the charge transfer ability and increase the number of the active reaction sites, which results in the reduction of the cathode ASR. Due to the hollow structure of tube structured LSCF cathode, the TPB can be formed on the outer and inner surfaces of LSCF tubes, while just on the outer surface for LSCF nanorods [15] when the GDC phases are introduced into the LSCF cathode by the impregnation method. Therefore, the LSCF tube/GDC nanoparticle composite cathode has lower ASR values than LSCF nanorod/GDC nanoparticle composite cathode. The related mechanism can be illustrated by a schematic diagram, as shown in Fig. 7. In addition, the LSCF tube/GDC nanoparticle composite cathode retained higher porosity compared to our previous GDCimpregnated nanoparticle structured LSCF cathode, which facilitates the gas-phase diffusion process and contributes to a lower polarization resistance. Moreover, the electrochemical performance of LSCF tube/GDC nanoparticle composites might be further improved by optimizing the microstructures.
Fig. 7 e Schematic of the microstructure of the GDCimpregnated tube structured LSCF cathode and GDCimpregnated rod structured LSCF cathode.
4.
Conclusions
In summary, we have synthesized LSCF tubes/GDC nanoparticle composite using a single-nozzle electrospinning technique combined with solution impregnation process. These composite materials exhibit excellent characteristics in terms of the area-specific resistance. For the infiltrated LSCF cathodes with GDC/LSCF weight ratio of 0.09:1, 0.17:1, 0.35:1 and 0.52:1, the area-specific resistances are 0.53, 0.31, 0.12 and 0.07 U cm2 at 650 C, respectively. The excellent electrochemical performance of the LSCF tube/GDC nanoparticle composite cathode is ascribed to its large TPB length and high porosity. These results show that LSCF tube/GDC nanoparticle composite is a promising cathode material for intermediate temperature solid oxide fuel cells.
Acknowledgments Financial support from the National Natural Science Foundation (No. 51072040), National Program on Key Basic Research Project (973 Program No. 2012CB215400) and National Science Foundation (Nos. 51172275 and 11004229) of China.
references
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Electrospinning La0.8Sr0.2Co0.2Fe0.8O3Ld tubes impregnated with Ce0.8Gd0.2O1.9 nanoparticles for an intermediate temperature solid oxide fuel cell cathode Erqing Zhao a, Chao Ma b,c,d, Wei Yang b,c,d, Yueping Xiong a,*, Jianqi Li b,c,d, Chunwen Sun b,c,d,** a
School of Chemical Engineering and Technology, Harbin Institute of Technology, Harbin 150001, PR China Beijing National Laboratory for Condensed Mater Physics, Institute of Physics, Chinese Academy of Science, Beijing 100190, PR China c Key Laboratory for Renewable Energy, Chinese Academy of Sciences, Beijing 100190, PR China d Beijing Key Laboratory for New Energy Materials and Devices, Beijing 100190, PR China b
article info
abstract
Article history:
In order to reduce the polarization resistance of the cathode, we have developed one-
Received 23 December 2012
dimensional (1D) nanostructured La0.8Sr0.2Co0.2Fe0.8O3d (LSCF) tubes/Ce0.8Gd0.2O1.9 (GDC)
Received in revised form
nanoparticles composite cathodes for solid oxide fuel cell. Uniform LSCF/PVP composite
1 March 2013
nanofibers have been firstly synthesized by a single-nozzle electrospinning technique,
Accepted 21 March 2013
followed by firing at 800 C for 2 h to form one-dimensional LSCF tubes. Subsequently, the
Available online 21 April 2013
GDC phases were introduced into tube structured LSCF scaffold pre-sintered on a GDC pellet by a multi-impregnation process. Electrochemical Impedance spectra reveal that
Keywords:
nanostructured LSCF tubes/GDC nanoparticles composite cathodes have a better electro-
La0.8Sr0.2Co0.2Fe0.8O3d
chemical performance, achieving area-specific resistances of 4.70, 1.12, 0.27 and 0.07 U cm2
Ce0.8Gd0.2O1.9
at 500, 550, 600 and 650 C for the composite of GDC and LSCF in a weight ratio of 0.52:1. The
Composite cathode
low ASR values are mainly related to its optimal microstructure with larger triple-phase
Electrospinning
boundaries and higher porosity. These results suggest that LSCF tube/GDC nanoparticle
Impregnation
composite can be an alternative cathode material for intermediate temperature solid oxide
Intermediate temperature solid
fuel cell (IT-SOFC).
oxide fuel cell
Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
In recent years, great efforts have been devoted to developing low or intermediate temperature solid oxide fuel cells (SOFCs) operating at 500e800 C [1e3]. Lowering the operating temperature can suppress degradation of components and extend the range of acceptable material selection, this
also serves to improve cell durability and reduce the system cost. However, reducing the operating temperature decreases the electrode kinetics and thus results in large interfacial polarization resistances. This effect is most pronounced for the oxygen reduction at the cathode. In order to lower the polarization resistance of the cathode, a favorable electronic and ionic conductivity as well as high
* Corresponding author. Mailbox 1247, School of Chemical Engineering and Technology, Harbin Institute of Technology, 92 West Dazhi Street, Nan Gang District, Harbin 150001, PR China. Tel.: þ86 451 86413721; fax: þ86 451 86418616. ** Corresponding author. Beijing National Laboratory for Condensed Mater Physics, Institute of Physics, Chinese Academy of Science, South 3rd Street, Zhongguancun, Beijing 100190, PR China. Tel.: þ86 10 82649901; fax: þ86 10 82649046. E-mail addresses: [email protected] (Y. Xiong), [email protected] (C. Sun). 0360-3199/$ e see front matter Copyright ª 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2013.03.111
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catalytic activity for the oxygen reduction must be maintained. Currently, tailoring the electrode microstructure has become an important approach to the fabrication of high performance cathodes. Furthermore, nanostructure cathodes have been believed as the optimal electrode structure due to their high electrocatalytic activity and large TPB length. So far, one-dimensional (1D) nanostructured materials with significantly higher surface areas, including nanowires, nanorods and nanotubes, being used as SOFC cathodes, have been demonstrated superior electrochemical properties. The La0.6Sr0.4CoO3 (LSCO) nanotubes [4] have been synthesized by the pore wetting technique and made into the nanotube structured LSCO cathodes, which exhibited lower polarization resistance compared with those of conventional LSCO cathodes. Recently, a 3D fibrous Sm0.5Sr0.5CoO3/Ce0.8Sm0.2O1.9 cathode has been fabricated, which possesses high porosity and interconnectivity, and low polarization resistance [5]. La0.8Sr0.2MnO3d/Zr0.92Y0.08O2 composite nanotubes [6] were cosynthesized by a pore wetting technique as a cathode material for SOFC. The area-specific resistance (ASR) values of these nanotube structured composite cathode reach 0.17, 0.25, 0.39 and 0.52 U cm2 at 850, 800, 750 and 700 C, respectively, which are attributed to the high specific surface area and multi-scale porosity. Similarly, nanostructured Pr0.6Sr0.4Fe0.8Co0.2O3 (PSFC)/ Ce0.8Sm0.2O2 (SDC) composite cathodes, made of PSFC/SDC composite nanotubes, have been investigated, exhibiting a smaller ASR value than that for the conventional sample [7]. All of the above-mentioned nanostructured electrode materials have been synthesized by a pore wetting technique. In addition to this method, the electrospinning technique has been exploited to prepare nanostructured electrode materials for SOFC. Electrospinning is a simple, versatile and convenient technique to synthesize nanofibers from polymeric solutions [8]. In an electrospinning process, a high voltage is applied to the surface of polymeric solutions followed by ejection of a liquid jet through the spinneret. Due to bending instability, the jet is converted to continuous and ultrathin fibers. The electrospun nanofibers can be calcined to form crystalline oxide nanofibers. The YSZ nanofibers infiltrated with LSM nanoparticles have been used as the cathode of SOFC [9], which show high catalytic activity toward O2 reduction reaction and offer larger triple-phase boundary length. Subsequently, the La0.58Sr0.4Co0.2Fe0.8 nanofibers [10] were fabricated and applied as the cathode of IT-SOFC, the cell with LSCF nanofiber cathode shows a high power density. Li et al. [11] synthesized uniform YSZ nanofibers by electrospinning 8YSZ dispersion and applied it as a SOFC anode, the performance of the cell with the fiber-derived anode is better than that with the powder-derived anode due to the larger TPB sites in fiber structured electrode. Hieu et al. [12] obtained nanofiber-structured Ba0.5Sr0.5Co0.8Fe0.2O3d perovskite-oxide as a cathode material for low-temperature solid oxide fuel cells, the ASR of the BSCF nanofiber cathode was determined to be 0.094 U cm2 at 600 C, whereas that of the BSCF powder cathode was 0.468 U cm2 under the similar conditions. The difference of ASR was attributed to high specific surface area of the BSCF nanofibers. Shahgaldi et al. [13] prepared the cobalt-free perovskite-oxide, Ba0.5Sr0.5Fe0.8Cu0.2O3d (BSFC) nanofibers and demonstrated that the feasibility of BSFC
nanofibers served as effective cathode materials for IT-SOFCs. Very recently, the Nd1.93Sr0.07CuO4 nanofibers [14] were synthesized by electrospinning technique and used as cathode material for SOFC, and the nanofiber cathode shows better electrochemical performance than the power cathode with the same composition, which is attributed to the former’s optimal microstructure. Our group [15] also reported onedimensional LSCF/GDC nanocomposite cathode by the combination of electrospinning and infiltration methods. The above-mentioned references involved in electrospinning all studied non-hollow electrode materials for SOFC. To our knowledge, there are few reports on producing the porous and hollow LSCF materials for application in SOFC by electrospinning technique and adding GDC phases into the tube structured LSCF scaffold to form the LSCF/GDC composite cathode by an infiltration process. Herein, we reported preparation of one dimensional and porous La0.8Sr0.2Co0.2Fe0.8O3d (LSCF) tubes using a single-nozzle electrospinning technique. Subsequently, the GDC phases were added into LSCF tubes to form LSCF tubes/GDC nanoparticles composite cathodes by an impregnation process [16e22] which involves the addition of an ion conductor phase or electron conductor phase into a pre-sintered porous backbone. The electrochemical performances of GDC-impregnated LSCF nanostructured composite cathodes have been tested as well.
2.
Experimental
2.1.
Preparation of GDC electrolyte pellets
GDC nanopowders were prepared by means of the citrate complexion method [23]. Then, the as-synthesized GDC were pressed into pellets using a stainless steel mold (25 mm in diameter) under 120 MPa, followed by sintering at 1450 C for 20 h in air to obtain dense electrolytes with a thickness of 1.1 mm and a diameter of 19 mm.
2.2.
Synthesis of porous LSCF tubes
Uniform LSCF/PVP nanofibers were fabricated by electrospinning LSCF precursor solution consisting of La(NO3)3$6H2O,
4000
Intensity(a.u)
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3000
2000
1000
0
20
40
60
2Theta/( degree) Fig. 1 e XRD pattern of the LSCF/PVP nanofibers after calcination at 800 C for 2 h.
80
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by dissolving the above metal nitrates in DMF solvent in a molar ratio of La:Sr:Co:Fe ¼ 0.8:0.2:0.2:0.8 and stirred at ambient temperature. The proper quantity of PVP was added into the resulting solution, followed by magnetic stirring for 4 h to get a transparent precursor solution for electrospinning. The as-prepared LSCF precursor solution was loaded into a plastic syringe equipped with 8-gauge stainless steel needle. For the electrospinning experiment, the anode of a high voltage supply was clamped on the spinneret (stainless steel needle) and the cathode was connected to a nickel mesh as a collector placed 12 cm away from the orifice. The applied voltage was kept at 20 kV. The electrospun products were initially dried for 12 h in air to evaporate the solvent and then calcined at 800 C for 2 h, with a heating rate of 2 C min1.
2.3.
Preparation of tube structured LSCF cathodes
The LSCF tube powers were then mixed with 3 wt%-ethyl cellulose terpineol solution to form cathode pastes. The obtained LSCF cathode pastes were coated on one side of the GDC electrolyte, which followed by sintering at 904 C for 1 min with a heating rate of 15 C min1 in air to form the tube structured LSCF cathode. The active electrode area was 0.785 cm2 and the thickness was about 60 mm.
2.4. Preparation of GDC-impregnated LSCF composite cathodes
Fig. 2 e (a) SEM image of the LSCF/PVP nanofibers fabricated by electrospinning of LSCF precursor solution; (b) SEM image of as-calcined LSCF/PVP nanofibers at 800 C for 2 h.
Sr(NO3)2, Co(NO3)2$6H2O, Fe(NO3)3$9H2O, Polyvinylpyrrolidone (PVP) and N,N-dimethylformamide (DMF), followed by high temperature treatment to form porous LSCF tubes. In a typical procedure, the electrospinning solution was prepared
The GDC impregnation solution with a concentration of 0.25 mol L1 was prepared by dissolving stoichiometric Gd(NO3)3$6H2O and Ce(NO3)3$6H2O in a mixed solvent of ethanol and deionized water. The GDC-impregnated LSCF composites were obtained by infiltration of GDC nitrate solution into the LSCF scaffold pre-sintered on a GDC electrolyte pellet. For one time impregnation, 20 mL GDC precursor solution was introduced into the LSCF substrate. After that, the impregnated sample was calcined at 700 C for 0.5 h with a heating rate of 10 C min1. To further increase the GDC content, the impregnation process was repeated. Finally, the impregnated electrodes were fired at 800 C for 1 h in order to obtain the GDC phases.
Fig. 3 e (a) HAADF-STEM image of the as-calcined product; (b) energy dispersive X-ray spectroscopy of the La0.8Sr0.2Co0.2Fe0.8O3Ld nanofibers.
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Fig. 4 e Representive cross-sectional SEM images of the LSCF cathode and GDC-infiltrated LSCF composite cathodes: (a) blank LSCF cathode; (b)e(f) the infiltrated cathodes with GDC/LSCF weight ratio of 0.09:1, 0.17:1, 0.35:1, 0.52:1 and 0.67:1, respectively; (g) the infiltrated cathodes with GDC/LSCF weight ratio of 0.52:1 under a constant current density of 200 mA cmL2 at 750 C for 85 h.
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Fig. 4 e (continued).
2.5.
Characterization
The surface morphologies of electrospun and calcined LSCF/ PVP nanofibers were observed by scanning electron microscope (SEM, FEI Quanta 200, Holland) and transmission electron microscopy (FEI Tecnai-F20 TEM). The chemical compositions were analyzed by EDX measurements in TEM. X-ray diffraction (XRD, Rigaku D/max-UB, Cu Ka radiation) was applied to identify the phase of calcined products. The element compositions of infiltrated LSCF tubes were also confirmed by EDX in TEM. The electrochemical performances of the cathodes were evaluated by electrochemical impedance spectra. Threeelectrode system was used with the cathodes as the working electrodes. Pt pastes were applied to the rim of the GDC
electrolyte pellet as the reference electrode and to one side of the GDC electrolyte as the counter electrode. Two Pt meshes were attached to the working electrode (a pure LSCF cathode or a GDC-infiltrated LSCF cathode) and to the counter electrode as current collectors for the electrochemical measurement. Electrochemical impedance spectra of LSCF electrodes were measured with an electrochemical work station (Chi604D) in a frequency range of 0.1 Hze100 kHz and an amplitude of 5 mV in the temperature range of 500 C and 650 C. The area-specific resistances were derived from the difference between the low and high-frequency intercepts at the real impedance axis. And, the LSCF tube-based cathode was treated under a constant current density of 200 mA cm2 at 750 C for 85 h with the aim of evaluating the adhesion characteristic between the cathode and electrolyte. The
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microstructures of the LSCF cathode and GDC-infiltrated LSCF cathodes after the impedance testing and the electrode/electrolyte interface after the long-term operation were detected by scanning electron microscope.
3.
Results and discussion
The phase purity and crystal structure of the as-calcined products of LSCF/PVP composite fibers were examined by X-ray powder diffraction (XRD). Fig. 1 shows that the XRD pattern of the calcined products at 800 C can be indexed as a pure perovskite-type phase of LSCF (JCPDS No. 48-0125). The sharp peaks indicate that the as-calcined LSCF nanofibers have high crystalline characteristics. Fig. 2 presents the scanning electron microscopy (SEM) images of LSCF/PVP composite nanofibers and calcined LSCF nanofibers. From Fig. 2(a), it can be found that the electrospun LSCF/PVP composite nanofibers are smooth and uniform, with diameters in the range of 300e400 nm. Fig. 2(b) shows that the calcined product is consisting of one-dimensional nanofibers with a diameter in the range of 200e300 nm. It can be clearly seen that these nanofibers have limited length, smooth surfaces, and open terminals after calcination at 800 C for 2 h. The morphology of the LSCF nanofibers was further characterized by transmission electron microscopy (TEM). Fig. 3(a) is a typical high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) image of the as-calcined product. It can be seen that the nanofibers form a tube structure after calcination at 800 C. To further clarify the chemical composition of the calcined tube, we performed energy dispersive X-ray spectroscopy, as shown in Fig. 3(b). It reveals the calcined tube is composed of La, Sr,
Co, Fe and O elements. Such tube structured LSCF materials as a SOFC cathode are expected to reduce the cathode polarization resistance due to their high porosity and hollow structure which facilitate the gas diffusion through the cathode. Fig. 4 shows typical cross-sectional SEM images of the LSCF cathode and GDC-infiltrated LSCF composite cathodes. From Fig. 4(a), it can be found that the blank LSCF cathode still retain excellent one-dimensional tube structure and the size of LSCF tubes almost keep unchanged, which attributed to the faster firing and the shorter dwell time. As can be seen from the upper image in Fig. 4(a), there is good adhesion and no delamination between the cathode and the electrolyte, suggesting the tube structured LSCF cathode was sintered well. Also, this tube structured LSCF cathode has higher porosity compared to the nanoparticle structured LSCF cathode [15]. After the GDC phases were added into LSCF cathode, onedimensional LSCF-GDC composite cathodes can be still observed from Fig. 4(b)e(f). Moreover, with increasing the amount of GDC loading, the surface of LSCF substrate became coarser, implying more and more GDC phases appeared on the LSCF scaffold. Fig. 4(g) displays the sectional SEM image of the infiltrated LSCF cathode with GDC/LSCF weight ratio of 0.52:1 under a constant current density of 200 mA cm2 at 750 C for 85 h. From Fig. 4(g), delamination can not be observed at the electrode/electrolyte interface, which indicates the compact adhesion between the electrode and electrolyte after the longterm operation. The GDC phases can be defined by aid of the element analysis of the infiltrated LSCF cathode. Fig. 5 displays the TEM image and EDX spectra of the infiltrated LSCF tube. From Fig. 5, it can be obtained that the Gd and Ce elements appeared on the infiltrated LSCF tubes, while not on the LSCF tubes before the infiltration (shown in Fig. 3(b)). This result demonstrates
Fig. 5 e The TEM image and EDX spectra of the infiltrated LSCF tubes in the LSCF cathode infiltrated with GDC solution.
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that the GDC phases have been deposited onto the LSCF cathode. Especially, it is noted that the LSCF tubes gradually disappeared when the GDC infiltration loading increased, which indicates that some GDC phases may be formed in the interior of LSCF substrates. The electrochemical performance was evaluated by electrochemical impedance spectroscopy (EIS) on a half cell consisting of a dense GDC electrolyte pellet, LSCF tube-based cathode and porous Pt counter electrode over the temperature range of 500e650 C in air. The impedance spectrum of the blank tube structured LSCF cathode at 650 C is shown in Fig. 6(a), and the typical impedance spectra obtained at 650 C for the infiltrated LSCF cathodes with different GDC/LSCF weight ratio are exhibited in Fig. 6(b). The impedance response for oxygen reduction on the LSCF tubes-based cathodes is characterized as a depressed arc. The high-frequency intercept of the electrode impedance on the real axis is the total resistance of the electrolyte. The difference between the lowfrequency and the high-frequency intercepts on the real axis corresponds to the area-specific resistance (ASR) of the two interfaces. The ASR is the overall resistance related to the oxygen reduction, oxygen surface/bulk diffusion, and the gasphase oxygen diffusion. In Fig. 6, all electrolyte ohmic resistances were removed from the impedance spectra for clearly showing the difference. For the blank LSCF cathode, its
ASR value is 1.56 U cm2 at 650 C. After GDC infiltration, the ASR value of LSCF cathode reduces significantly with the increasing of the GDC amount. The ASR values of GDCinfiltrated tube structured LSCF cathodes are reduced to 0.53, 0.31, 0.12 and 0.07 U cm2 at 650 C, respectively, corresponding to the weight ratio of GDC/LSCF of 0.09:1, 0.17:1, 0.35:1, 0.52:1. When the GDC/LSCF weight ratio increased to 0.67:1, the ASR of the infiltrated cathode became higher, which may be interpreted by its microstructure (shown in Fig. 4(f)). At GDC/LSCF weight ratio of 0.67:1, the infiltrated cathode appears to be much less porous, which blocks the gas diffusion, decreases the TPB length, and thus results in a higher ASR. However, the lowest ASR of 0.07 U cm2 obtained in this work at 650 C is much lower than those of other LSCF composite cathodes (Table 1) reported in the literature [24e29], as well as LSCF nanorod/GDC nanoparticle composite that we investigated recently [15]. The typical impedance spectra obtained over the temperature range of 500e600 C for the infiltrated LSCF cathode with GDC:LSCF weight ratio of 0.52:1 are shown in Fig. 6(c). At 600, 550, and 500 C, the ASR values are 0.27, 1.12 and 4.70 U cm2, respectively. The excellent performance of tube structured LSCF cathode impregnated with GDC nanoparticles can be ascribed to its optimized electrode microstructure with high porosity and extended triple-phase boundaries [30e33] where the
1.6
(a)
0.48
blank LSCF cathode
(b)
0.09:1 0.17:1 0.35:1 0.52:1 0.67:1
1.2 0.8
-Z''/Ω cm2
-Z''/Ω cm2
0.36
0.4
0.24 0.12
0.0 0.00
-0.4 -0.4
0.0
0.4 0.8 Z'/Ω cm2
1.2
1.6
0.00
0.12
0.24 0.36 Z'/Ω cm2
0.48
5
(c)
500ºC 550ºC 600ºC
4
-Z''/Ω cm2
3 2 1 0 0
1
2 3 Z'/Ω cm2
4
5
Fig. 6 e (a) The impedance spectrum of the blank tube structured LSCF cathode at 650 C; (b) the typical impedance spectra of the infiltrated LSCF cathodes with different GDC/LSCF weight ratio measured at 650 C; (c) the EIS spectra of the infiltrated LSCF cathode with GDC/LSCF weight ratio of 0.52:1 measured at different temperatures.
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Table 1 e Comparison of the ASR for LSCF composite cathodes, measured at 650 C. Cathode LSCF tube/GDC LSCF rod/GDC LSCF/GDC LSCF/SDC LSCF/GDC LSCF/SSC LSCF/SDC LSCF/YSZ
Fabrication method
ASR (U cm2)
References
Infiltration Infiltration Mechanical mixed Infiltration Infiltration Infiltration Polymerizable complex Infiltration
0.07 0.10 0.52 0.40 0.45 0.12 0.265
Present work Zhao et al [15] Fu et al. [24] Nie et al. [25] Chen et al. [26] Lou et al. [27] Lee et al. [28]
0.218
Chen et al. [29]
oxygen reduction reaction takes place. By adding the GDC phases into tube structured LSCF scaffold, the TPB region changes from a two-dimensional interface between the cathode and the electrolyte to an entire three-dimensional cathode. It is well known that the enlargement of TPB region can promote the charge transfer ability and increase the number of the active reaction sites, which results in the reduction of the cathode ASR. Due to the hollow structure of tube structured LSCF cathode, the TPB can be formed on the outer and inner surfaces of LSCF tubes, while just on the outer surface for LSCF nanorods [15] when the GDC phases are introduced into the LSCF cathode by the impregnation method. Therefore, the LSCF tube/GDC nanoparticle composite cathode has lower ASR values than LSCF nanorod/GDC nanoparticle composite cathode. The related mechanism can be illustrated by a schematic diagram, as shown in Fig. 7. In addition, the LSCF tube/GDC nanoparticle composite cathode retained higher porosity compared to our previous GDCimpregnated nanoparticle structured LSCF cathode, which facilitates the gas-phase diffusion process and contributes to a lower polarization resistance. Moreover, the electrochemical performance of LSCF tube/GDC nanoparticle composites might be further improved by optimizing the microstructures.
Fig. 7 e Schematic of the microstructure of the GDCimpregnated tube structured LSCF cathode and GDCimpregnated rod structured LSCF cathode.
4.
Conclusions
In summary, we have synthesized LSCF tubes/GDC nanoparticle composite using a single-nozzle electrospinning technique combined with solution impregnation process. These composite materials exhibit excellent characteristics in terms of the area-specific resistance. For the infiltrated LSCF cathodes with GDC/LSCF weight ratio of 0.09:1, 0.17:1, 0.35:1 and 0.52:1, the area-specific resistances are 0.53, 0.31, 0.12 and 0.07 U cm2 at 650 C, respectively. The excellent electrochemical performance of the LSCF tube/GDC nanoparticle composite cathode is ascribed to its large TPB length and high porosity. These results show that LSCF tube/GDC nanoparticle composite is a promising cathode material for intermediate temperature solid oxide fuel cells.
Acknowledgments Financial support from the National Natural Science Foundation (No. 51072040), National Program on Key Basic Research Project (973 Program No. 2012CB215400) and National Science Foundation (Nos. 51172275 and 11004229) of China.
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