Enhanced SOFC cathode performance by infiltrating Ba0.5Sr0.5Co0.8Fe0.2O3 − δ nanoparticles for intermediate temperature solid oxide fuel cells

FUPROC-04249; No of Pages 6 Fuel Processing Technology xxx (2014) xxx–xxx

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Enhanced SOFC cathode performance by infiltrating Ba0.5Sr0.5Co0.8Fe0.2O3 − δ nanoparticles for intermediate temperature solid oxide fuel cells Xifeng Ding a,⁎, Wenliang Zhu a, Xiaojia Gao b, Guixiang Hua b, Jianfei Li a a b

School of Materials Science and Engineering, Nanjing University of Science and Technology, Nanjing, Jiangsu 210094, PR China Microelectronics Ministry, North General Electronics Group Co. Ltd., Suzhou, Jiangsu 215163, PR China

a r t i c l e

i n f o

Available online xxxx Keywords: Solid oxide fuel cells Composite cathode Infiltration Surface modification

a b s t r a c t Solid oxide fuel cells (SOFCs) as alternatives for energy conversion have exhibited some unique advantages such as wide fuel flexibility, high energy efficiency and exhaust heat. An innovative composite cathode for SOFCs consisting of a high conductive La0.7Sr0.3CuO3 − δ (LSCu) electrode backbone infiltrated by nano and Ba0.5Sr0.5Co0.8Fe0.2O3 − δ (BSCF) particles with high electro-catalytic activity has been successfully fabricated by a simple solution infiltration process. The effect of BSCF nano catalyst on the electrochemical activity and stability of LSCu cathode was investigated and the inherent oxygen reduction reaction (ORR) mechanism is discussed. The polarization resistance is 0.043 Ω cm2 for 15 wt.% BSCF infiltrated LSCu cathode, and is reduced ~92.6% to that pure LSCu (0.58 Ω cm2) at 700 °C. The short term stability demonstrates that the electrochemical activity can be further improved in the first period about 40 h at 700 °C. After operated for 70 h, the electrochemical performance of the composite cathode became stable. A conductive scaffold (e.g. LSCu) modified with a highly catalytic nano particles (e.g. BSCF) is an attractive approach to fabricate cathodes with high electrochemical performance and stability. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Electrochemical energy conversions have received special attention due to their high energy conversion efficiency and low reduction of pollutant emission [1,2]. Solid oxide fuel cells (SOFCs) exhibit some unique advantages such as wide fuel flexibility, high energy efficiency and exhaust heat [3]. Current developments for SOFCs focus on reducing the operation temperature to intermediate-to-low temperature (400–800 °C), which can prolong the lifetime, solve the problems with sealing and directly operate on hydrocarbon fuels without carbon deposition [4,5]. However, lowering the operation temperature sacrifices fuel cell power density, since the conducting species encounter higher activation barriers at lower temperature and the oxygen reduction reaction (ORR) kinetics occurred at cathode side has become the main obstacle for reducing the operation temperature of SOFCs [6,7]. Development of novel cathode materials and microstructures with superior electrocatalytic activity and long term stability is critically important to achieve high power density at reduced temperature [8,9]. In recent years, numerous cobalt-based perovskite materials like GdBa 0.5 Sr0.5 Co2 O 5 + δ [10], PrBa 0.5 Sr0.5 Co2 − x Fex O 5 + δ [6], Sr1 − xCexCoO3 − δ [7], Ba0.5Sr0.5Co0.8Fe0.2O3 − δ (BSCF) [11,12], YBaCo2O5+ δ [13], and SmxSr1 − xCoO3 − δ [14] mixed ionic-electronic conductors (MIEC) have been paid great attention because they usually exhibited ⁎ Corresponding author. Tel.: +86 25 84313349. E-mail address: [email protected] (X. Ding).

excellent electrochemical performance at intermediate-to-low temperature SOFCs. But the large thermal expansion coefficient, chemical instability with electrolyte materials and high temperature volatility prevent them from practical application [15]. Thus several cobalt-free oxides have been investigated as candidate cathode materials, such as LnBaFe2O5 + δ [16,17], La0.6Sr0.4Fe1 − xNbxO3 − δ [18], Sr0.7Y0.3CuO2+ δ [19], Bi0.5Sr0.5FeO3 − δ [20], SmBa0.5Sr0.5Cu2O5+ δ [9], and Ba0.5Sr0.5Fe0.9Ni0.1O3 − δ-Sm0.2Ce0.8O1.9 (BSFN-SDC) [21]. Compared with cobalt-based cathode materials, these cobalt-free cathode materials show close thermal expansion with SOFC electrolytes and long-term stability sacrificing partly electrochemical activity [22,23]. As an excellent MIEC material, the thermal expansion coefficient of La1 − xSrxCuO3 − δ (LSCu) is about 14.3–16.8 × 10−6/°C at 20–800 °C, the electrical conductivity is 2400–833 S cm−1 at 300–800 °C and the polarization resistance is 0.2 Ω cm2 at 800 °C [24–26]. Although exhibited high electrical conductivity, the electro-catalytic performance is desired to further improve. Several methods have been exploited to increase the reduced temperature performance for oxygen reduction electrode such as surface modification of electrode by infiltration of nano catalysts [8,27,28], 3D architectures to increase the effective electrode surface area [29,30] and a combination of these two methods [31]. Reducing the size of catalysts can accelerate oxygen reduction kinetics by providing enlarged number of active sites for surface oxygen exchange [28,29,32–34]. A cost-effective approach for obtaining nano-scale cathodes is to incorporate nano particles by infiltrating appropriate metal salt solutions with subsequent calcinations [8,29]. Ba0.5Sr0.5Co0.8Fe0.2O3 − δ (BSCF) is a

http://dx.doi.org/10.1016/j.fuproc.2014.09.030 0378-3820/© 2014 Elsevier B.V. All rights reserved.

Please cite this article as: X. Ding, et al., Enhanced SOFC cathode performance by infiltrating Ba0.5Sr0.5Co0.8Fe0.2O3 − δ nanoparticles for intermediate temperature solid oxide fuel cells, Fuel Processing Technology (2014), http://dx.doi.org/10.1016/j.fuproc.2014.09.030

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very promising electro-catalyst for oxygen reduction reaction, showing very low polarization resistance at low temperature [35]. Here we report an innovative composite cathode consisting of a high conductive LSCu electrode backbone infiltrated by nano BSCF particles with high electro-catalytic activity. Through the electrode surface modification, the effect of BSCF nano catalyst on the electrochemical activity and stability of LSCu cathode was investigated and the inherent ORR mechanism was discussed. 2. Experimental 2.1. Powder synthesis La0.7Sr0.3CuO3 − δ (LSCu) and Ba0.5Sr0.5Co0.8Fe0.2O3 − δ (BSCF) powder was synthesized via glycine-nitrate combustion method, where glycine was used as both complexing agents and fuels during the combustion reaction. Stoichiometric amount of La(NO3)2 (99.5%), Sr(NO3)3 (99.5%), and Cu(NO3)2·3H2O (99.5%) were dissolved in deionized water according to the formula of La0.7Sr0.3CuO3 − δ. The molar ratio of glycine to metal ions in the final solution was 3:1. Then, the solution was heated on an electro-thermal furnace until it spontaneously combusted. The precursor was then pre-calcined at 800 °C for 2 h. The powers of Ba0.5Sr0.5Co0.8Fe0.2O3 − δ were synthesized by the same method above using Ba(NO3)3 (99.5%), Sr(NO3)3 (99.5%), Co(NO3)2·6H2O (99.5%) and Fe(NO3)2·9H2O (99.5%) as raw materials. The phase composition of the powders was evaluated by X-ray diffraction (XRD) using a Rigaku D/max-III equipped with a Cu Kα radiation source. The LSCu and BSCF powders were mixed in a weight ratio of 1:1 and calcined at 950 °C for 10 h, and the chemical compatibility between them was also examined by XRD.

stability of infiltrated cathode, the symmetrical cell was operated at 700 °C for 150 h, and EIS test was conducted every 2 h.

3. Results and discussion 3.1. XRD characterization Fig. 1 shows XRD of BSCF powders calcined at 800 and 900 °C for 2 h and chemical compatibility of the mixture of LSCF and BSCF calcined at 950 °C for 10 h. The perovskite phase was formed at 800 °C for BSCF although the crystallinity is not as good as that calcined at 900 °C. After fired at 950 °C for 10 h, no second phases were detected according to XRD, implying good chemical compatibility between them.

3.2. SEM image Shown in Fig. 2(a) is the SEM images of as-prepared LSCu scaffold. The grain surface of LSCu was smooth before infiltration and the structure was very porous. The grain size is about 0.3–1.0 μm. After infiltration of ~ 20 wt.% BSCF into LSCu scaffold and fired at 800 °C for 1 h shown in Fig. 2(b), it is clearly observed that nano BSCF particles with the size ~30 nm modified the surface of LSCu. From the previous XRD pattern, the precursor of BSCF was successfully transformed to perovskite oxide even after calcinations at 800 °C. Combined with the SEM images, it is found that nano BSCF particles were successfully deposited on LSCu scaffold.

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Electrolyte-supported symmetric cells for impedance studies were prepared by screen printing. The SDC electrolyte powder was synthesized by carbonate co-precipitation method [36]. The powder was ground in mortar with 5 wt.% PVA, pressed into pellets (Φ15 mm) under 200 MPa and then sintered at 1200 °C for 6 h. The cathode slurry of La0.7Sr0.3CuO3 − δ powder mixed with ethyl cellulose and terpineol was printed onto both sides of SDC disks, followed by calcination at 950 °C for 2 h under stagnant air to form porous LSCu scaffold. The weight ratio of cathode powder, ethyl cellulose and terpineol is 1:0.06:3. Nano BSCF catalyst was prepared by a facile technique combined with solution infiltration and thermal treatment. Appropriate nitrate salts of Ba, Sr, Co and Fe according to chemical formula Ba0.5Sr0.5Co0.8Fe0.2O3 − δ were dissolved in deionized water. Glycine was added into the solution for complexing the metal cations at a mole ratio of glycine to total metal cations of 3:1. The solution was diluted to 0.25 mol/L, and then 1.5 wt.% PVP was added as a surfactant. BSCF solution was infiltrated into the porous LSCu scaffold using a micro-liter syringe. The loading of nominal BSCF was controlled at ~ 5%, 10%, 15% and 20 wt.% according to the equation of BSCF % = mBSCF /mBSCF + mLSCu × 100%, in which mBSCF and mLSCu were based on the amounts weighed with an electronic balance. After the infiltration of appropriate amount of solution, the electrodes were fired at 800 °C for 1 h.

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Ag paste was printed on electrode surfaces as current collector and sintered at 700 °C for 0.5 h. The electrochemical performance of the infiltrated composite cathode was characterized by electrochemical impedance spectroscopy (EIS) using an electrochemical workstation CHI604D. The applied frequency was in the range of 0.1 Hz to 100 kHz with the signal amplitude of 10 mV. The EIS was measured at 500– 800 °C with an increment of 50 °C in air. To investigate the short term

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2Theta(°) Fig. 1. XRD of (a) BSCF powders calcined at 800 and 900 °C for 2 h and (b) mixture of LSCF and BSCF, pure LSCu and BSCF powders calcined at 950 °C for 10 h.

Please cite this article as: X. Ding, et al., Enhanced SOFC cathode performance by infiltrating Ba0.5Sr0.5Co0.8Fe0.2O3 − δ nanoparticles for intermediate temperature solid oxide fuel cells, Fuel Processing Technology (2014), http://dx.doi.org/10.1016/j.fuproc.2014.09.030

X. Ding et al. / Fuel Processing Technology xxx (2014) xxx–xxx

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Fig. 2. Cross-sectional SEM images of (a) as-prepared LSCu scaffold and (b) BSCF infiltrated LSCu composite cathode after sintering at 800 °C for 1 h.

3.3. Electrochemical performance Electrochemical activity for ORR of various infiltrated electrodes was investigated by EIS in a symmetric cell configuration, and a typical Nyquist impedance spectra of BSCF infiltrated LSCu electrode (BSCF loading ~ 16.2 wt.%) was demonstrated in Fig. 3(a). The impedance spectra are evaluated by fitting impedance data with the equivalent circuits inserted in the figure, where L is the inductance, and Rs is the ohmic resistance including the electrolyte, electrode ohmic resistance and lead resistance. R1 is interpreted to oxygen ion transferring from

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Ωcm2 Fig. 3. (a) Typical Nyquist impedance spectra of BSCF infiltrated LSCu electrode (BSCF loading ~16.2 wt.%) at 500–700 °C. A equivalent circuit LR(QR)(QR) was inserted in the figure. And (b) Nyquist plots of LSCu, BSCF infiltrated LSCu and SDC infiltrated LSCu cathodes at 700 °C, and a magnified plot showing BSCF infiltrated LSCu cathode was inserted in the figure.

the three phase boundary (TPB) to the electrolyte, and R2 is related to dissociative absorption of O2 and the diffusion of neutral oxygen on surface. Q1 and Q2 are the corresponding constant phase elements [37]. The polarization resistance (Rp) is the sum of R1 and R2. It is found that Rp decreased with the elevated temperature. For the infiltrated composite cathode, the oxygen reduction reaction was occurred not only at TPB, but also at prolonged cathode surface. As the temperature increased, the lattice oxygen was released and the oxygen vacancy concentration was improved. On the other hand, the thermal kinetics and diffusion rate were enhanced with increasing temperature. For comparison, ionic conducting SDC with the same loading as that of BSCF was infiltrated into LSCu backbone shown in Fig. 3(b). Only a small decrease of Rp was found for SDC-infiltrated LSCu cathode. The Rp values are 0.58 Ω cm2, 0.45 Ω cm2 and 0.043 Ω cm2 for pure LSCu, SDCinfiltrated LSCu and BSCF-infiltrated LSCu cathodes at 700 °C, respectively. It was reported that the ionic conductivity of BSCF was pretty high about 0.006 S cm−1 at 600 °C) [38], which was very close to that of SDC about 0.005–0.012 S cm−1 at 600 °C) [39]. Moreover, as a superior cathode material, the electronic conductivity of BSCF was about 20–50 S cm−1 [40], much higher than that of SDC. For SDC-infiltrated LSCu cathode, the promoted activity sites for ORR lie in O2/SDC/LSCu three phase boundary (TPB), while for BSCF-infiltrated LSCu cathode, the ORR occurred not only at the TPB of O2/BSCF/LSCu, but at the surface of BSCF. Since the BSCF was fired at relative low temperature, the specific surface was very high, which greatly promoted oxygen adsorption and dissociation process on the cathode surface. The polarization resistance (Rp) of the LSCu cathode infiltrated with different BSCF loading calculated from EIS in Nyquist plots under open circuit conduction at 500–800 °C is shown in Fig. 4(a). The Rp of blank LSCu is 0.18 Ω cm2 at 800 °C, which is close to that about 0.2 Ω cm2 reported by Yu et al. [24]. For the infiltrated cathodes, Rp is 0.028, 0.023, 0.087 and 0.049 Ω cm2 at 800 °C for that with 5.2, 9.4, 16.2 and 20.9 wt.% BSCF loading, respectively, indicating a superior improvement on electro-catalytic activity for LSCu cathode after a surface modification with nano BSCF catalyst. Moreover, it is found that Rp reached the minimum when infiltrated with 16.2 wt.% BSCF, and it increased again with more loading. The activation energies of the composite cathodes are calculated according to the Arrhenius plots in Fig. 4(b) and the values are 110–116 kJ mol− 1, very close to that of blank LSCu and BSCF reported in literature [35]. Based on the previous results, schematic diagrams of the nano particle decorated electrode with different BSCF loading were proposed in Fig. 5. For the blank LSCu cathode (Fig. 5(a)), the ORR occurred mainly at the TPB of O2/SDC electrolyte/LSCu cathode and the area near TPB because LSCu is also a mixed ionic and electronic conductor, but the electro-catalytic activity was not satisfactory since the oxygen ionic conductivity of LSCu is low at 600–800 °C. For the cathode infiltrated with a small BSCF loading (Fig. 5(b)), nano particles randomly distributed on

Please cite this article as: X. Ding, et al., Enhanced SOFC cathode performance by infiltrating Ba0.5Sr0.5Co0.8Fe0.2O3 − δ nanoparticles for intermediate temperature solid oxide fuel cells, Fuel Processing Technology (2014), http://dx.doi.org/10.1016/j.fuproc.2014.09.030

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were greatly improved. However, while the infiltration loading was further increased (Fig. 5(d)), the cathode porosity as well as the TPB sites was decreased with overloaded BSCF particles which increased the concentration polarization resistance. And the overloaded nano particles may even sinter and grow to bigger grains after a long time operation at high temperature.

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In addition to electrochemical activity, operation stability is also an important concern on SOFC cathode material development [29]. The impedance spectra of a symmetrical cell infiltrated with 16.2 wt.% BSCF calcined at 700 °C after operation for 0, 20, 40, 60, 80, 120 h and 150 h are shown in Fig. 6(a) and the polarization resistances as a function of operation time are plotted in Fig. 6(b). The change of Rp can be summarized to three stages. Before operated for 40 h, it is found Rp decreased with operation time. Considering the poor crystallinity of BSCF fired at 800 °C from the previous XRD results, the improved electrochemical performance (i.e., decreased Rp) could be attributed to the improved phase purity and crystallinity. At the second period of 40–70 h, Rp increased a little and then remained constant after operated for 70 h. The SEM images of infiltrated cathode after operation at 700 °C for 150 h are shown in Fig. 6(c) and (d), and it is found that small particles about 300–500 nm were deposited on the surface of LSCu scaffold. During the long time operation at 700 °C, the BSCF nano gains grew and thus the specific surface area of BSCF reduced, which deteriorated a little the electrochemical activity during 40–70 h. After operated for 70 h, the electrochemical performance became stable, suggesting the gain grew stopped. Therefore, the long-term stability was expected for BSCF infiltrated LSCu cathode operated at intermediate temperature.

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the LSCu surface, and the surface of BSCF became active sites for oxygen reduction reaction which could be associated with reduced polarization value (0.14 Ω cm2 for 5.2 wt.% BSCF infiltrated LSCu versus 0.58 Ω cm2 for pure LSCu at 700 °C). When the electrode surface were uniformly modified by appropriate amount of BSCF particles (Fig. 5(c)), the oxygen adsorption, dissociation and transform processes were greatly enhanced since BSCF was also a nice ionic conductor. The ionic conductivity of BSCF was even better than the excellent oxygen ionic conductor SDC. Thus the oxygen absorption and transfer processes

4. Conclusions An innovative composite cathode consisting of a high conductive LSCu electrode backbone infiltrated by nano BSCF particles with high electro-catalytic activity has been successfully fabricated by a one-step solution infiltration process. The as-prepared composite cathode shows high chemical compatibility between them, thus producing excellent electrochemical activity. The Rp is 0.043 Ω cm2 at 700 °C for 15 wt.% BSCF infiltrated LSCu cathode, about 7.4% of that pure LSCu. The nano BSCF particles can increase the active sites and enhance the surface activity for oxygen reduction reaction on the LSCu cathode. The short term stability demonstrates that the electrochemical activity can be further improved in the first period about 40 h at 700 °C. After operated for 70 h, the electrochemical performance of the composite cathode became stable (70–150 h with no degradation at 700 °C). The electrode electrochemical performance improvement can be realized by a simple infiltration technique combined with thermal treatment, which may accelerate the commercialization of SOFC technology at low cost.

Fig. 5. Schematic diagrams of the nano particle decorated electrode with different BSCF loading (a) LSCu blank, (b) a small amount of BSCF nano particles, (c) an appropriate loading and (d) overloading.

Please cite this article as: X. Ding, et al., Enhanced SOFC cathode performance by infiltrating Ba0.5Sr0.5Co0.8Fe0.2O3 − δ nanoparticles for intermediate temperature solid oxide fuel cells, Fuel Processing Technology (2014), http://dx.doi.org/10.1016/j.fuproc.2014.09.030

X. Ding et al. / Fuel Processing Technology xxx (2014) xxx–xxx

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Fig. 6. (a) Impedance spectra of a symmetrical cell infiltrated with 16.2 wt.% BSCF calcined at 700 °C under OCV conditions, (b) polarization resistance as a function of operation temperature, (c) SEM of BSCF infiltrated LSCu cathode after operation at 700 °C for 150 h and (d) SEM in a magnification.

Acknowledgments The authors gratefully acknowledge the financial supports provided by the Natural Science Foundation of China (No. 50902069), the Natural Science Foundation of Jiangsu Province (No. BK2012806) and the Fundamental Research Funds for the Central Universities (No. 30920130111022).

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Please cite this article as: X. Ding, et al., Enhanced SOFC cathode performance by infiltrating Ba0.5Sr0.5Co0.8Fe0.2O3 − δ nanoparticles for intermediate temperature solid oxide fuel cells, Fuel Processing Technology (2014), http://dx.doi.org/10.1016/j.fuproc.2014.09.030

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Please cite this article as: X. Ding, et al., Enhanced SOFC cathode performance by infiltrating Ba0.5Sr0.5Co0.8Fe0.2O3 − δ nanoparticles for intermediate temperature solid oxide fuel cells, Fuel Processing Technology (2014), http://dx.doi.org/10.1016/j.fuproc.2014.09.030