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Enhanced performance of solid oxide fuel cells by introducing a transition layer between nanostructured cathode and electrolyte
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 0 ( 2 0 1 5 ) 5 0 1 e5 0 8
Available online at www.sciencedirect.com
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Enhanced performance of solid oxide fuel cells by introducing a transition layer between nanostructured cathode and electrolyte Kaiyue Zhu a,b, Huanying Liu a, Xuefeng Zhu a,*, Yan Liu a,b, Weishen Yang a,* a
State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China b University of Chinese Academy of Sciences, Beijing 100039, China
article info
abstract
Article history:
A transition layer between Ce0.8Sm0.2O2d (SDC) electrolyte and Sr0.8Co0.8Fe0.2O3d (SCF0.8)
Received 13 June 2014
nano cathode was introduced to improve electrochemical performances of solid oxide fuel
Accepted 27 July 2014
cells (SOFCs). A calcining e stripping method was used to introduce the transition layer on
Available online 29 November 2014
the SDC electrolyte. Then a nano porous cathode was coated on the transition layer. The microstructure of the nano cathode was optimized by adjusting its calcining temperature.
Keywords:
It was found that the transition layer played a vital role in improving the electrochemical
Solid oxide fuel cell
performances of the nanostructured cathode by enhancing the interface connection be-
Electrochemical performance
tween cathode and electrolyte. Good electrochemical performance was obtained as the
Nanostructured cathode
nano cathode calcined at 700 C after introducing the transition layer. The polarization
Transition layer
resistance of the anode-supported SOFC at 700
Perovskite
0.20 U cm2 to 0.05 U cm2, while the power density of the single cell was increased from 116
C was significantly reduced from
to 444 mW cm2. Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Solid oxide fuel cells (SOFCs) have attracted much attention because of their high efficiency, good environmental benefits and high flexibility to various fuels [1e3]. However, the hightemperature (800e1000 C) operation causes many key technical issues, such as higher systems costs, performance degradation and slow start-up cycles, and these technical difficulties have limited the commercialization of this
transformative technology [4,5]. These drawbacks are easy to be overcome by lowering the operation temperature of SOFCs because the problems related to materials costs of interconnectors, sealing, durability of all the components are no longer difficult to be solved in the intermediate-low (IL) temperature range (600e800 C) [6e9]. However, a loss of power density is inevitable at lower operation temperatures due to a considerable decrease of ionic conductivity of the electrolyte and catalytic activity of the electrodes. The ohm losses can be
* Corresponding authors. Tel.: þ86 411 84379073; fax: þ86 411 84694447. E-mail addresses: [email protected] (X. Zhu), [email protected] (W. Yang). http://dx.doi.org/10.1016/j.ijhydene.2014.07.184 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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minimized by reducing the thickness of the electrolyte or/and adopting electrolytes with high ionic conductivity [10,11]. While now the overall performance loss of anode-supported SOFCs at lower operation temperatures is mainly dominated by the activation polarization of the sluggish electrode reactions, i.e., oxygen reduction reactions on the cathode [12,13]. Oxygen reduction reactions take place at the triple phase boundaries (TPBs) of traditional composite cathodes like yttria-stabilized zirconia (YSZ)/La0.8Sr0.2MnO3d, or on the whole surface of mixed ionic-electronic conducting (MIEC) cathodes like Sm0.5Sr0.5CoO3d [14,15]. To improve SOFCs performances at IL temperature, it is necessary and feasible to reduce the polarization loss of the cathode by developing cathode materials with high catalytic activity or decreasing the grain size of cathode materials down to nano-scale to increase the active area. Nanoparticles with large surface area have long TPBs and more catalytic active sites, and will show high oxygen reduction rate at the gasesolid interfaces. In the recent studies, many efforts are focused on the nanostructured cathode to improve the electrochemical performance of SOFCs at IL temperature [16e22]. Usually, SOFCs cathodes are prepared via a calcining step at high temperatures (>1000 C) to enhance the connection between the cathode layer and the electrolyte layer. The connection between cathode and electrolyte has strongly influence on the performance of the cells. However, nanoparticles cannot sustain their sizes after calcined at such high temperatures. One feasible approach is to impregnate cathode precursors into porous layer of ionic conductor oxides, for instance SDC, GDC, YSZ, etc, to form nano composite cathodes after heat treated at lower temperature [12]. For example, performance was remarkable enhancement by impregnating a solution into the pre-sinterted porous YSZ structure to form a nano-structrured La0.8Sr0.2Co0.5 Fe0.5O3 þ YSZ composite cathode [23]. Additionally, electrostatic spray deposition method was used to prepare a double layer nano/micro porous cathode, which was efficient to expand the triple phase boundary and improve the catalytic activity of the cathode [15]. Thus, optimization of the cathode microstructure with high porosity, large TPBs and good adhesion between cathode and electrolyte is useful to enhance the electrochemical performance of SOFCs. The previous investigation on oxygen absorption/desorption kinetics indicated that Sr0.8Co0.8Fe0.2O3d (SCF0.8) has high catalytic activity toward oxygen reduction reactions in the temperature range of 450e600 C [24]. Therefore, we chose SCF0.8 as the cathode material. Here, we report a new method
for the preparation of nanostrucutred cathodes to enhance the electrochemical performances of anode-supported SOFCs. The nanoparticles of SCF0.8 with size around 238 nm were prepared by high-speed ball milling. A SCF0.8 transition layer was introduced on the SDC electrolyte by high-temperature sintering, followed by stripping porous SCF0.8 from the SDC electrolyte. Performance of the anode-supported SOFCs with nano cathode was effectively improved through the introduction of transition layer. The results demonstrated the vital role of transition layer for the connection of nano cathode and electrolyte.
Experimental Preparation of nanoparticles Sr0.8Co0.8Fe0.2O3d (SCF0.8) powder was prepared by a combined Ethylene Diamine Tetra-acetic Acid (EDTA)ecitric acid method [25]. Stoichiometric amounts of Sr(NO3)2(99.5%), Co(NO3)2$6H2O(99.0%), Fe(NO3)3$9H2O(98.5%) were dissolved in deionized water to form an aqueous solution. Then citric acid and EDTA acid were added in the above solution with a mole ratio of citric acid, EDTA and total metal ions of 1.5:1:1. pH value of the solution was adjusted to about 7 by concentrated ammonia. After water was evaporated on a hot plate, the resultant purple gel was combusted on an electric oven. Then, the ash was calcined in air for 5 h at 950 C to remove the carbon residues. To obtain the nano particles, the SCF0.8 powder was repeatedly ballmilled in ethyl alcohol, and then the resultant suspension was centrifuged at 3000 rpm for 3 min to remove the big particles and obtain the nano-size particles (denoted as SCF0.8nano).
Preparation of single cells The electrochemical performances of the SCF0.8nano as cathodes of SOFCs were characterized on anode-supported Ce0.8Sm0.2O2d (SDC) electrolyte. A highly fluffy SDC powder was used to prepare the electrolyte membranes. The powder was synthesized via the combined EDTAecitric acid complexing route and calcined in air at 800 C for 5 h. The following shows the preparation procedure of an anode-supported electrolyte disc (see Fig. 1), (1) a mixed powder containing NiO, SDC and graphite with weight ratio of 45:45:10 was firstly pressed at ~60 MPa to produce a substrate; (2) the fluffy SDC powder was then uniformly added on the substrate followed by co-pressing at ~210 MPa for several minutes; (3) the shaped green discs were sintered at 1490 C for 200 min. The resultant anode-supported electrolyte discs were about 18 mm in diameter and 540 mm in
Fig. 1 e Schematic figure for the preparation of anode-supported solid oxide fuel cells.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 4 0 ( 2 0 1 5 ) 5 0 1 e5 0 8
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Testing of single cells Electrochemical characterizations were performed from 550 to 700 C under ambient pressure. Humidified hydrogen (bubbled through water) (100 mL min1 at STP) was fed to the anode as fuel gas and air as the oxidant (300 mL min1 at STP) to the cathode. An electrochemical workstation was used to monitor the currentevoltage characteristics of the cells based on a Solartron 1287 electrochemical interface and a Solartron 1260 frequency response analyzer. Impedence spectra were measured in the frequency range from 102 Hze105 Hz at five points per frequency decade with amplitude of 10 mV. It usually takes about 30 min for the cells to reach steady states, so all the tests of single cells shown in this paper were performed after the cells running at a condition for more than 30 min. Fig. 2 e Powder X-ray diffraction patterns of (a) SCF0.8 and (b) SCF0.8nano.
thickness. A paste made of the as-prepared SCF0.8 powder and terpineol was then coated onto the SDC electrolyte surface by a doctor blade processing with a thickness about 30 mm. The painted paste was dried and then sintered at 1050 C for 2 h in air. A very thin SCF0.8 transition layer was formed by stripping the SCF0.8 layer from the SDC electrolyte surface. Subsequently, a slurry made of the as-prepared SCF0.8nano powder and terpineol was then coated onto the transition layer in the same way as mentioned above. The painted pastes were dried and then calcined at target temperatures for 2 h in air with heating and cooling rates of 1 C/min. The abbreviation like tlSCF0.8nano(700) for this type of cathodes means that the cathode consists of a SCF0.8 transition layer and a SCF0.8nano cathode calcined at 700 C. For comparison purpose, the nanostrucutred cathode without SCF0.8 transition layer was prepared by coating SCF0.8nano slurry directly onto the anode-supported electrolyte disc, and followed by drying and calcining at 700 C for 2 h. The final effective areas of all the cathodes were about 0.39 cm2.
Characterizations The crystalline structures of the as-synthesized powders were examined by powder X-ray diffraction (XRD, Rigaku D/Max e 2500 diffractometer with a Cu Ka radiation source, l ¼ 0.15418 nm, 40 kV, 200 mA) at room temperature. Microscopic morphologies of the powders and anode-supported single cells were observed on a scanning electron microscope (SEM, FEI Quanta 200F).
Results and discussion Crystalline structures of the SCF0.8 and SCF0.8nano powders were analyzed by X-ray diffraction, as shown in Fig. 2. It can be seen from the diffraction patterns that both powders have a cubic perovskite oxide as the major phase and a cubic spinel oxide and a brownmillerite oxide as the minor phases. Previous investigation discloses that SrCo0.8Fe0.2O3d with A-site deficient shows faster oxygen adsorption/desorption and higher oxygen activation rate than the stoichiometric one due to the appearance of spinel phase [26]. The broader diffraction peaks of the ball milled SCF0.8 powders than the as-prepared
Fig. 3 e SEM images of the as-prepared powders, (a) SCF0.8 and (b) SCF0.8nano. Insert of (b) is the size distribution of SCF0.8nano particles.
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one reveals the significant decrease of crystallite size. The SEM images shown in Fig. 3 clearly reveal that particle size is decreased significantly after ball milling. In addition, particle size analysis shows the particles size of SCF0.8nano powder is centered at 238 nm, as shown in the inset of Fig. 3. Fig. 4 shows the surface and cross-section morphologies of three different cathodes on the corresponding anodesupported single cells after electrochemical measurement. The thin transition layer consisting of SCF0.8 grains with size of 2e5 mm covers the surface of the SDC electrolyte, as shown in Fig. 4c. Cobalt-based perovskite-type cathodes have larger thermal expansion coefficients (TECs) of ~20 106 K1 than ceria-based electrolyte of ~10 106 K1; therefore, it is
difficult to achieve a good adhesion between perovskite cathode and ceria electrolyte. Delamination of perovskite cathode from the electrolyte is frequently found after the cathode calcined at high temperature (>1000 C). Here, the SCF0.8 transition layer just likes the residual perovskite on the electrolyte. The residual layer made of perovskite particles has a thickness of 2e5 mm and shows strong adhesion with electrolyte. The SCF0.8nano(700) and tl-SCF0.8nano(700) cathode show very similar microstructure features, as shown in Fig. 4c and e, that are compact structures composed of a large amount of nano-sized particles with many small pores. The interfaces between cathode and electrolyte are well adhered to each
Fig. 4 e SEM images of the surfaces and cross-sections of the electrolyte and cathodes, (a) SDC, (b) tl, (c) and (d) SCF0.8nano(700), (e) and (f) tl-SCF0.8nano(700).
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other without noticeable cracking or delamination, as shown in Fig. 4d and f. The ohmic resistances and polarization resistances of the cells are investigated via the typical AC impedance spectroscopy on single cells. In this arrangement, AC impedance spectroscopy reveals the total polarization resistance of the cell comprising both anodic and cathodic responses. Here, the single cells were made of the same anode-supported electrolytes and various cathodes. Therefore, any significant differences in impedance spectroscopy are related to the different cathodes. In this work, the area specific resistance (ASR) is used to describe all resistance terms. From the results of impedance spectra presented in Fig. 5, the ohm resistances of cells with cathodes of SCF0.8nano(700), tl and tl-SCF0.8nano(700) at 700 C are 1.08, 0.85 and 0.22 U cm2, respectively, and the polarization resistances are 0.20, 0.39 and 0.05 U cm2, respectively. The significant decrease in both ohm and polarization resistances by introducing the tl indicates that tl improves the adhesion between electrolyte and nano cathode. Fig. 6 shows the currentevoltage characteristics and power densities of single cells with different cathodes at 700 C. For the nano cathode prepared by coating SCF0.8 nano particles directly on electrolyte calcined at 700 C, i.e. SCF0.8nano(700), the peak power density is only 116 mW cm2. The low power density is attributed to the weak adhesion of the nano cathode to the SDC electrolyte as revealed by the high ohm resistances. Usually, the calcining temperature of cathodes is high up to 1000e1200 C; however, to keep the nano size the calcining temperature was decreased to 700 C. If the cathode only consists of the SCF0.8 transitional layer, i.e. tl, the peak power density is only 98 mW cm2. However, after a nano SCF0.8 layer is coated onto the tl and then calcined at 700 C, the peak power density increases to 444 mW cm2. The significant enhanced performance of the cell with tl-SCF0.8nano(700) cathode indicates the importance of SCF0.8 transition layer. The lower peak power density of tl-SCF0.8nano(700) cell compared with other SDC-based SOFCs is related to the thick electrolyte membrane (~40 mm) used in our experiments. Its ohm resistance is 4 times higher than the polarization
Fig. 5 e Impedance spectra under open circuit conditions at 700 C for single cells with SCF0.8nano(700), tl and tlSCF0.8nano(700) cathodes.
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Fig. 6 e Cell voltages and power densities as a function of current density at 700 C for cells with SCF0.8nano(700), tl and tl-SCF0.8nano(700) cathodes.
resistance, as shown in Fig. 5. Combination of the above analysis, one can infer that the transition layer enhances the interface adhesion without calcining the nano SCF0.8 at elevated temperatures. Thus, we can conclude that the transition layer plays a vital role in improving the electrochemical performance of the single cell with a nano cathode. Based on the introduction of the thin SCF0.8 transitional layer, the influence of calcined temperature of the nano SCF0.8 on the cell performance was also investigated. Fig. 7 shows the surface morphologies and cross-section microstructures of the SCF0.8nano cathodes calcined at 800 and 900 C, respectively. It can be seen from Fig. 4e and f and Fig. 7 that uniform pores were formed through a necked connection between particles at the surface of SCF0.8nano cathode calcined at 700 C. The particle size and porosity gradually become larger and lower, respectively, when the calcining temperature increased from 700 to 900 C. Although the increase of calcining temperature of cathodes can strengthen the adhesion of the interface between cathode and transition layer, it causes a dramatic reduction of porosity and limits gases diffusion to and from the reaction sites. Fig. 8 shows the impedance spectra obtained at 700 C of tlSCF0.8nano cathodes calcined at 700, 800 and 900 C, respectively. The dependence of cell voltages and power densities of cells tested at 700 C with tl-SCF0.8nano cathodes calcined at different temperature are shown in Fig. 9. It is very clear that at 700 C, tl-SCF0.8nano(700) has the best electrochemical performances, i.e. the highest power density and the smallest ohm and polarization resistances. The maximum power densities (Pmax) at 700 C of the single cells with tl-SCF0.8nano cathodes calcined at 700, 800 and 900 C are 444, 183 and 251 mW cm2, respectively. Accordingly, the ohm resistances at 700 C of the cells with tl-SCF0.8nano cathodes calcined at 700, 800 and 900 C are 0.22, 0.79 and 0.47 U cm2, respectively. However, the polarization resistances are very close to each other, 0.05, 0.07 and 0.07 U cm2, respectively. Compared with polarization resistances, ohm resistances are strongly affected by the calcining temperature. Thus, on the basis of introducing the thin SCF0.8 transition layer, the adhesion between cathode and electrolyte is relatively good and cannot
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Fig. 7 e SEM images of the cathode surfaces and cross-sections of cells with tl-SCF0.8nano cathodes calcined at 800 C (a and b) and 900 C (c and d).
be improved by increasing the calcining temperature from 700 to 900 C. As a consequence, it is reasonable to believe that the main factor affecting the resistances is related to the porosity of the SCF0.8nano cathodes for the cells with the transition layer. The currentevoltage characteristics and corresponding power densities are shown in Fig. 10 for the optimal single cell
we obtained by introducing tl-SCF0.8nano cathode. Maximum power densities were 444, 414, 280 and 172 mW cm2 at 700, 650, 600 and 550 C, respectively. Furthermore, it seems that the power densities of the cell at 650 and 700 C can be enhanced by decreasing the thickness of the nanostructured cathode, because concentration polarization becomes the limitation step as reflected by the slopes of the VeI relationship with the increase of current densities.
Fig. 8 e Impedance spectra under open circuit conditions at 700 C for single cells with tl-SCF0.8nano cathodes calcined at different temperature.
Fig. 9 e Cell voltages and power densities as a function of current density at 700 C of single cells with tl-SCF0.8 nano cathodes calcined at different temperatures.
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Fig. 10 e Cell voltage and power density as a function of current density of the tl-SCF0.8nano(700) cathode at different operation temperatures.
Conclusions Sr0.8Co0.8Fe0.2O3d was synthesized by a combined EDTAecitric acid complexing method. The nano SCF0.8 cathode materials with diameter of about 238 nm were obtained by ball milling and centrifugation separation. The ohm and polarization resistances of the cells were effectively reduced by introducing SCF0.8 transition layer between cathode and electrolyte; as a result the peak power density was significantly improved. The results demonstrate that the SCF0.8 transition layer plays a vital role in improving the electrochemical performance. Furthermore, the calcining temperature of the cathode was optimized based on the SCF0.8 transition layer. It was found that the slightly sintered SCF0.8nano cathode at the calcining temperature of 700 C shows a good electrochemical performance. All the results show that the introduction of a transition layer through the simple calcining e stripping method is feasible and promising to strengthen the adhesion between nanostructured cathode and electrolyte, and enhance the electrochemical performance of SOFCs at intermediate low temperatures. Although the power density of the cell is not high in this work, it can be significantly improved by decreasing the thickness of electrolyte and optimizing the thickness and porous structure of the nano cathode.
Acknowledgment The authors gratefully acknowledge the National Science Fund of China (21271169 and 212111089) for the financial support.
references
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Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Enhanced performance of solid oxide fuel cells by introducing a transition layer between nanostructured cathode and electrolyte Kaiyue Zhu a,b, Huanying Liu a, Xuefeng Zhu a,*, Yan Liu a,b, Weishen Yang a,* a
State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China b University of Chinese Academy of Sciences, Beijing 100039, China
article info
abstract
Article history:
A transition layer between Ce0.8Sm0.2O2d (SDC) electrolyte and Sr0.8Co0.8Fe0.2O3d (SCF0.8)
Received 13 June 2014
nano cathode was introduced to improve electrochemical performances of solid oxide fuel
Accepted 27 July 2014
cells (SOFCs). A calcining e stripping method was used to introduce the transition layer on
Available online 29 November 2014
the SDC electrolyte. Then a nano porous cathode was coated on the transition layer. The microstructure of the nano cathode was optimized by adjusting its calcining temperature.
Keywords:
It was found that the transition layer played a vital role in improving the electrochemical
Solid oxide fuel cell
performances of the nanostructured cathode by enhancing the interface connection be-
Electrochemical performance
tween cathode and electrolyte. Good electrochemical performance was obtained as the
Nanostructured cathode
nano cathode calcined at 700 C after introducing the transition layer. The polarization
Transition layer
resistance of the anode-supported SOFC at 700
Perovskite
0.20 U cm2 to 0.05 U cm2, while the power density of the single cell was increased from 116
C was significantly reduced from
to 444 mW cm2. Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Solid oxide fuel cells (SOFCs) have attracted much attention because of their high efficiency, good environmental benefits and high flexibility to various fuels [1e3]. However, the hightemperature (800e1000 C) operation causes many key technical issues, such as higher systems costs, performance degradation and slow start-up cycles, and these technical difficulties have limited the commercialization of this
transformative technology [4,5]. These drawbacks are easy to be overcome by lowering the operation temperature of SOFCs because the problems related to materials costs of interconnectors, sealing, durability of all the components are no longer difficult to be solved in the intermediate-low (IL) temperature range (600e800 C) [6e9]. However, a loss of power density is inevitable at lower operation temperatures due to a considerable decrease of ionic conductivity of the electrolyte and catalytic activity of the electrodes. The ohm losses can be
* Corresponding authors. Tel.: þ86 411 84379073; fax: þ86 411 84694447. E-mail addresses: [email protected] (X. Zhu), [email protected] (W. Yang). http://dx.doi.org/10.1016/j.ijhydene.2014.07.184 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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minimized by reducing the thickness of the electrolyte or/and adopting electrolytes with high ionic conductivity [10,11]. While now the overall performance loss of anode-supported SOFCs at lower operation temperatures is mainly dominated by the activation polarization of the sluggish electrode reactions, i.e., oxygen reduction reactions on the cathode [12,13]. Oxygen reduction reactions take place at the triple phase boundaries (TPBs) of traditional composite cathodes like yttria-stabilized zirconia (YSZ)/La0.8Sr0.2MnO3d, or on the whole surface of mixed ionic-electronic conducting (MIEC) cathodes like Sm0.5Sr0.5CoO3d [14,15]. To improve SOFCs performances at IL temperature, it is necessary and feasible to reduce the polarization loss of the cathode by developing cathode materials with high catalytic activity or decreasing the grain size of cathode materials down to nano-scale to increase the active area. Nanoparticles with large surface area have long TPBs and more catalytic active sites, and will show high oxygen reduction rate at the gasesolid interfaces. In the recent studies, many efforts are focused on the nanostructured cathode to improve the electrochemical performance of SOFCs at IL temperature [16e22]. Usually, SOFCs cathodes are prepared via a calcining step at high temperatures (>1000 C) to enhance the connection between the cathode layer and the electrolyte layer. The connection between cathode and electrolyte has strongly influence on the performance of the cells. However, nanoparticles cannot sustain their sizes after calcined at such high temperatures. One feasible approach is to impregnate cathode precursors into porous layer of ionic conductor oxides, for instance SDC, GDC, YSZ, etc, to form nano composite cathodes after heat treated at lower temperature [12]. For example, performance was remarkable enhancement by impregnating a solution into the pre-sinterted porous YSZ structure to form a nano-structrured La0.8Sr0.2Co0.5 Fe0.5O3 þ YSZ composite cathode [23]. Additionally, electrostatic spray deposition method was used to prepare a double layer nano/micro porous cathode, which was efficient to expand the triple phase boundary and improve the catalytic activity of the cathode [15]. Thus, optimization of the cathode microstructure with high porosity, large TPBs and good adhesion between cathode and electrolyte is useful to enhance the electrochemical performance of SOFCs. The previous investigation on oxygen absorption/desorption kinetics indicated that Sr0.8Co0.8Fe0.2O3d (SCF0.8) has high catalytic activity toward oxygen reduction reactions in the temperature range of 450e600 C [24]. Therefore, we chose SCF0.8 as the cathode material. Here, we report a new method
for the preparation of nanostrucutred cathodes to enhance the electrochemical performances of anode-supported SOFCs. The nanoparticles of SCF0.8 with size around 238 nm were prepared by high-speed ball milling. A SCF0.8 transition layer was introduced on the SDC electrolyte by high-temperature sintering, followed by stripping porous SCF0.8 from the SDC electrolyte. Performance of the anode-supported SOFCs with nano cathode was effectively improved through the introduction of transition layer. The results demonstrated the vital role of transition layer for the connection of nano cathode and electrolyte.
Experimental Preparation of nanoparticles Sr0.8Co0.8Fe0.2O3d (SCF0.8) powder was prepared by a combined Ethylene Diamine Tetra-acetic Acid (EDTA)ecitric acid method [25]. Stoichiometric amounts of Sr(NO3)2(99.5%), Co(NO3)2$6H2O(99.0%), Fe(NO3)3$9H2O(98.5%) were dissolved in deionized water to form an aqueous solution. Then citric acid and EDTA acid were added in the above solution with a mole ratio of citric acid, EDTA and total metal ions of 1.5:1:1. pH value of the solution was adjusted to about 7 by concentrated ammonia. After water was evaporated on a hot plate, the resultant purple gel was combusted on an electric oven. Then, the ash was calcined in air for 5 h at 950 C to remove the carbon residues. To obtain the nano particles, the SCF0.8 powder was repeatedly ballmilled in ethyl alcohol, and then the resultant suspension was centrifuged at 3000 rpm for 3 min to remove the big particles and obtain the nano-size particles (denoted as SCF0.8nano).
Preparation of single cells The electrochemical performances of the SCF0.8nano as cathodes of SOFCs were characterized on anode-supported Ce0.8Sm0.2O2d (SDC) electrolyte. A highly fluffy SDC powder was used to prepare the electrolyte membranes. The powder was synthesized via the combined EDTAecitric acid complexing route and calcined in air at 800 C for 5 h. The following shows the preparation procedure of an anode-supported electrolyte disc (see Fig. 1), (1) a mixed powder containing NiO, SDC and graphite with weight ratio of 45:45:10 was firstly pressed at ~60 MPa to produce a substrate; (2) the fluffy SDC powder was then uniformly added on the substrate followed by co-pressing at ~210 MPa for several minutes; (3) the shaped green discs were sintered at 1490 C for 200 min. The resultant anode-supported electrolyte discs were about 18 mm in diameter and 540 mm in
Fig. 1 e Schematic figure for the preparation of anode-supported solid oxide fuel cells.
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Testing of single cells Electrochemical characterizations were performed from 550 to 700 C under ambient pressure. Humidified hydrogen (bubbled through water) (100 mL min1 at STP) was fed to the anode as fuel gas and air as the oxidant (300 mL min1 at STP) to the cathode. An electrochemical workstation was used to monitor the currentevoltage characteristics of the cells based on a Solartron 1287 electrochemical interface and a Solartron 1260 frequency response analyzer. Impedence spectra were measured in the frequency range from 102 Hze105 Hz at five points per frequency decade with amplitude of 10 mV. It usually takes about 30 min for the cells to reach steady states, so all the tests of single cells shown in this paper were performed after the cells running at a condition for more than 30 min. Fig. 2 e Powder X-ray diffraction patterns of (a) SCF0.8 and (b) SCF0.8nano.
thickness. A paste made of the as-prepared SCF0.8 powder and terpineol was then coated onto the SDC electrolyte surface by a doctor blade processing with a thickness about 30 mm. The painted paste was dried and then sintered at 1050 C for 2 h in air. A very thin SCF0.8 transition layer was formed by stripping the SCF0.8 layer from the SDC electrolyte surface. Subsequently, a slurry made of the as-prepared SCF0.8nano powder and terpineol was then coated onto the transition layer in the same way as mentioned above. The painted pastes were dried and then calcined at target temperatures for 2 h in air with heating and cooling rates of 1 C/min. The abbreviation like tlSCF0.8nano(700) for this type of cathodes means that the cathode consists of a SCF0.8 transition layer and a SCF0.8nano cathode calcined at 700 C. For comparison purpose, the nanostrucutred cathode without SCF0.8 transition layer was prepared by coating SCF0.8nano slurry directly onto the anode-supported electrolyte disc, and followed by drying and calcining at 700 C for 2 h. The final effective areas of all the cathodes were about 0.39 cm2.
Characterizations The crystalline structures of the as-synthesized powders were examined by powder X-ray diffraction (XRD, Rigaku D/Max e 2500 diffractometer with a Cu Ka radiation source, l ¼ 0.15418 nm, 40 kV, 200 mA) at room temperature. Microscopic morphologies of the powders and anode-supported single cells were observed on a scanning electron microscope (SEM, FEI Quanta 200F).
Results and discussion Crystalline structures of the SCF0.8 and SCF0.8nano powders were analyzed by X-ray diffraction, as shown in Fig. 2. It can be seen from the diffraction patterns that both powders have a cubic perovskite oxide as the major phase and a cubic spinel oxide and a brownmillerite oxide as the minor phases. Previous investigation discloses that SrCo0.8Fe0.2O3d with A-site deficient shows faster oxygen adsorption/desorption and higher oxygen activation rate than the stoichiometric one due to the appearance of spinel phase [26]. The broader diffraction peaks of the ball milled SCF0.8 powders than the as-prepared
Fig. 3 e SEM images of the as-prepared powders, (a) SCF0.8 and (b) SCF0.8nano. Insert of (b) is the size distribution of SCF0.8nano particles.
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one reveals the significant decrease of crystallite size. The SEM images shown in Fig. 3 clearly reveal that particle size is decreased significantly after ball milling. In addition, particle size analysis shows the particles size of SCF0.8nano powder is centered at 238 nm, as shown in the inset of Fig. 3. Fig. 4 shows the surface and cross-section morphologies of three different cathodes on the corresponding anodesupported single cells after electrochemical measurement. The thin transition layer consisting of SCF0.8 grains with size of 2e5 mm covers the surface of the SDC electrolyte, as shown in Fig. 4c. Cobalt-based perovskite-type cathodes have larger thermal expansion coefficients (TECs) of ~20 106 K1 than ceria-based electrolyte of ~10 106 K1; therefore, it is
difficult to achieve a good adhesion between perovskite cathode and ceria electrolyte. Delamination of perovskite cathode from the electrolyte is frequently found after the cathode calcined at high temperature (>1000 C). Here, the SCF0.8 transition layer just likes the residual perovskite on the electrolyte. The residual layer made of perovskite particles has a thickness of 2e5 mm and shows strong adhesion with electrolyte. The SCF0.8nano(700) and tl-SCF0.8nano(700) cathode show very similar microstructure features, as shown in Fig. 4c and e, that are compact structures composed of a large amount of nano-sized particles with many small pores. The interfaces between cathode and electrolyte are well adhered to each
Fig. 4 e SEM images of the surfaces and cross-sections of the electrolyte and cathodes, (a) SDC, (b) tl, (c) and (d) SCF0.8nano(700), (e) and (f) tl-SCF0.8nano(700).
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other without noticeable cracking or delamination, as shown in Fig. 4d and f. The ohmic resistances and polarization resistances of the cells are investigated via the typical AC impedance spectroscopy on single cells. In this arrangement, AC impedance spectroscopy reveals the total polarization resistance of the cell comprising both anodic and cathodic responses. Here, the single cells were made of the same anode-supported electrolytes and various cathodes. Therefore, any significant differences in impedance spectroscopy are related to the different cathodes. In this work, the area specific resistance (ASR) is used to describe all resistance terms. From the results of impedance spectra presented in Fig. 5, the ohm resistances of cells with cathodes of SCF0.8nano(700), tl and tl-SCF0.8nano(700) at 700 C are 1.08, 0.85 and 0.22 U cm2, respectively, and the polarization resistances are 0.20, 0.39 and 0.05 U cm2, respectively. The significant decrease in both ohm and polarization resistances by introducing the tl indicates that tl improves the adhesion between electrolyte and nano cathode. Fig. 6 shows the currentevoltage characteristics and power densities of single cells with different cathodes at 700 C. For the nano cathode prepared by coating SCF0.8 nano particles directly on electrolyte calcined at 700 C, i.e. SCF0.8nano(700), the peak power density is only 116 mW cm2. The low power density is attributed to the weak adhesion of the nano cathode to the SDC electrolyte as revealed by the high ohm resistances. Usually, the calcining temperature of cathodes is high up to 1000e1200 C; however, to keep the nano size the calcining temperature was decreased to 700 C. If the cathode only consists of the SCF0.8 transitional layer, i.e. tl, the peak power density is only 98 mW cm2. However, after a nano SCF0.8 layer is coated onto the tl and then calcined at 700 C, the peak power density increases to 444 mW cm2. The significant enhanced performance of the cell with tl-SCF0.8nano(700) cathode indicates the importance of SCF0.8 transition layer. The lower peak power density of tl-SCF0.8nano(700) cell compared with other SDC-based SOFCs is related to the thick electrolyte membrane (~40 mm) used in our experiments. Its ohm resistance is 4 times higher than the polarization
Fig. 5 e Impedance spectra under open circuit conditions at 700 C for single cells with SCF0.8nano(700), tl and tlSCF0.8nano(700) cathodes.
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Fig. 6 e Cell voltages and power densities as a function of current density at 700 C for cells with SCF0.8nano(700), tl and tl-SCF0.8nano(700) cathodes.
resistance, as shown in Fig. 5. Combination of the above analysis, one can infer that the transition layer enhances the interface adhesion without calcining the nano SCF0.8 at elevated temperatures. Thus, we can conclude that the transition layer plays a vital role in improving the electrochemical performance of the single cell with a nano cathode. Based on the introduction of the thin SCF0.8 transitional layer, the influence of calcined temperature of the nano SCF0.8 on the cell performance was also investigated. Fig. 7 shows the surface morphologies and cross-section microstructures of the SCF0.8nano cathodes calcined at 800 and 900 C, respectively. It can be seen from Fig. 4e and f and Fig. 7 that uniform pores were formed through a necked connection between particles at the surface of SCF0.8nano cathode calcined at 700 C. The particle size and porosity gradually become larger and lower, respectively, when the calcining temperature increased from 700 to 900 C. Although the increase of calcining temperature of cathodes can strengthen the adhesion of the interface between cathode and transition layer, it causes a dramatic reduction of porosity and limits gases diffusion to and from the reaction sites. Fig. 8 shows the impedance spectra obtained at 700 C of tlSCF0.8nano cathodes calcined at 700, 800 and 900 C, respectively. The dependence of cell voltages and power densities of cells tested at 700 C with tl-SCF0.8nano cathodes calcined at different temperature are shown in Fig. 9. It is very clear that at 700 C, tl-SCF0.8nano(700) has the best electrochemical performances, i.e. the highest power density and the smallest ohm and polarization resistances. The maximum power densities (Pmax) at 700 C of the single cells with tl-SCF0.8nano cathodes calcined at 700, 800 and 900 C are 444, 183 and 251 mW cm2, respectively. Accordingly, the ohm resistances at 700 C of the cells with tl-SCF0.8nano cathodes calcined at 700, 800 and 900 C are 0.22, 0.79 and 0.47 U cm2, respectively. However, the polarization resistances are very close to each other, 0.05, 0.07 and 0.07 U cm2, respectively. Compared with polarization resistances, ohm resistances are strongly affected by the calcining temperature. Thus, on the basis of introducing the thin SCF0.8 transition layer, the adhesion between cathode and electrolyte is relatively good and cannot
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Fig. 7 e SEM images of the cathode surfaces and cross-sections of cells with tl-SCF0.8nano cathodes calcined at 800 C (a and b) and 900 C (c and d).
be improved by increasing the calcining temperature from 700 to 900 C. As a consequence, it is reasonable to believe that the main factor affecting the resistances is related to the porosity of the SCF0.8nano cathodes for the cells with the transition layer. The currentevoltage characteristics and corresponding power densities are shown in Fig. 10 for the optimal single cell
we obtained by introducing tl-SCF0.8nano cathode. Maximum power densities were 444, 414, 280 and 172 mW cm2 at 700, 650, 600 and 550 C, respectively. Furthermore, it seems that the power densities of the cell at 650 and 700 C can be enhanced by decreasing the thickness of the nanostructured cathode, because concentration polarization becomes the limitation step as reflected by the slopes of the VeI relationship with the increase of current densities.
Fig. 8 e Impedance spectra under open circuit conditions at 700 C for single cells with tl-SCF0.8nano cathodes calcined at different temperature.
Fig. 9 e Cell voltages and power densities as a function of current density at 700 C of single cells with tl-SCF0.8 nano cathodes calcined at different temperatures.
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Fig. 10 e Cell voltage and power density as a function of current density of the tl-SCF0.8nano(700) cathode at different operation temperatures.
Conclusions Sr0.8Co0.8Fe0.2O3d was synthesized by a combined EDTAecitric acid complexing method. The nano SCF0.8 cathode materials with diameter of about 238 nm were obtained by ball milling and centrifugation separation. The ohm and polarization resistances of the cells were effectively reduced by introducing SCF0.8 transition layer between cathode and electrolyte; as a result the peak power density was significantly improved. The results demonstrate that the SCF0.8 transition layer plays a vital role in improving the electrochemical performance. Furthermore, the calcining temperature of the cathode was optimized based on the SCF0.8 transition layer. It was found that the slightly sintered SCF0.8nano cathode at the calcining temperature of 700 C shows a good electrochemical performance. All the results show that the introduction of a transition layer through the simple calcining e stripping method is feasible and promising to strengthen the adhesion between nanostructured cathode and electrolyte, and enhance the electrochemical performance of SOFCs at intermediate low temperatures. Although the power density of the cell is not high in this work, it can be significantly improved by decreasing the thickness of electrolyte and optimizing the thickness and porous structure of the nano cathode.
Acknowledgment The authors gratefully acknowledge the National Science Fund of China (21271169 and 212111089) for the financial support.
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