Improved thermal expansion and electrochemical performance of La0.4Sr0.6Co0.9Sb0.1O3-δ-Ce0.8Sm0.2O1.9 composite cathode for IT-SOFCs

Improved thermal expansion and electrochemical performance of La0.4Sr0.6Co0.9Sb0.1O3-δ-Ce0.8Sm0.2O1.9 composite cathode for IT-SOFCs

Solid State Sciences 91 (2019) 126–132 Contents lists available at ScienceDirect Solid State Sciences journal homepage: www.elsevier.com/locate/sssc...

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Solid State Sciences 91 (2019) 126–132

Contents lists available at ScienceDirect

Solid State Sciences journal homepage: www.elsevier.com/locate/ssscie

Improved thermal expansion and electrochemical performance of La0.4Sr0.6Co0.9Sb0.1O3-δ-Ce0.8Sm0.2O1.9 composite cathode for IT-SOFCs

T

Lei Zhanga,b, Xiaowu Lia,∗, Leilei Zhangb,∗∗, Hongdong caib, Jingsheng Xub, Li Wangb, Wen Longb a Department of Materials Physics and Chemistry, School of Materials Science and Engineering, And Key Laboratory for Anisotropy and Texture of Materials, Ministry of Education, Northeastern University, Shenyang, 110819, PR China b College of Sciences, Liaoning Shihua University, Fushun, 113001, PR China

A R T I C LE I N FO

A B S T R A C T

Keywords: Solid oxide fuel cells Composite cathodes Chemical compatibility Thermal expansion Electrochemical performance

La0.4Sr0.6Co0.9Sb0.1O3-δ (LSCSb)-xCe0.8Sm0.2O1.9 (LSCSb-xSDC, x = 0, 20, 30, 40 and 50 wt%) composite materials have been studied systemically as cathodes for intermediate-temperature solid oxide fuel cells (IT-SOFCs). XRD result shows an excellent chemical compatibility between LSCSb cathode and SDC/LSGM electrolytes. Although electrical conductivity is reduced with the introduction of SDC content, the electrical conductivities of all the composite cathodes are still above 100 S cm−1 at 300–850 °C. Thermal expansion result shows that the increase in SDC content greatly reduces thermal expansion coefficients (TECs) and the TEC for the LSCSb-50SDC cathode attains as low as 13.1 × 10−6 K−1 at 30–900 °C. Polarization resistance (Rp) decreases monotonously with increasing SDC content, and the LSCSb-50SDC cathode shows the lowest polarization with the Rp of 0.086 Ω cm2 at 700 °C. For a 300 μm thick La0.9Sr0.1Ga0.8Mg0.2O3 (LSGM) electrolyte-supported single cell, the maximum power density (Pmax) of the cell with LSCSb-50SDC cathode achieves 432 mW cm−2 at 700 °C. Moreover, the single cell with the LSCSb-50SDC cathode also shows a good cell stability at operating temperature. Above results indicate that LSCSb-50SDC composite material is a potential cathode for IT-SOFCs.

1. Introduction In recent years, SOFCs have drawn wide attentions in virtue of their advantages of all solid state structure, high energy utilization ratio, fuel diversity and low letting off [1,2]. However, the higher working temperature (800–1000 °C) limits the application of traditional SOFCs with (La,Sr)MnO3 (LSM) cathode. Studies indicated that the electrocatalytic activity of the LSM cathode significantly degrades with the decrease in temperature [3,4]. Then the development of IT-SOFCs (600–800 °C) with high catalytic active cathodes have been regarded as an effective way to promote the practical application and commercialization [5–7]. Many studies indicate that Co-based perovskite oxides, such as Ba0.6Sr0.4Co0.9Nb0.1O3-δ [8]and Sm0.5Sr0.5CoO3-δ [9], generally possess excellent mixed ionic and electronic conductivity and extremely good electrocatalytic activity for oxygen reduction at intermediate temperatures. Thus, these perovskite oxides have attracted much attention due to their potential application in the cathodes of IT-SOFCs. In our previous study, the La0.4Sr0.6Co0.9Sb0.1O3-δ (LSCSb) cathode presents the highest electrical conductivity among LaxSr1-xCo0.9Sb0.1O3δ system [10]. For example, the conductivity values of LSCSb are among



673–1637 S cm−1 at 300–850 °C. However, in addition to good electrical conductivity, the cathode material should also possess high electrocatalytic activity for the oxygen reduction and matching thermal expansion behavior with the electrolyte. Unfortunately, the LSCSb cathode shows poor electrocatalytic activity for oxygen reduction with the polarization resistance (Rp) of 0.191 Ω cm2 at 700 °C, which is much larger than a generally accepted criterion (0.15 Ω cm2) for the Rp of the SOFC cathode [11]. Moreover, like other cobalt-based perovskite oxides [10], the TEC of LSCSb cathode is as high as 22.5 × 10−6 K−1 from 30 to 900 °C, which is clearly mismatched with the common intermediatetemperature electrolyte LSGM [12,13]. This mismatch does not facilitate the long-term thermal stability of SOFC. Adding electrolyte materials to the cathode has been demonstrated to be an effective way to improve the electrocatalytic activity and reduce the TEC. For example, adding electrolyte to cathode increases the length of the triple phase boundary (TPB), which could improve electrochemical performance obviously; meanwhile the TECs of the cathodes could be obviously reduced by the addition of electrolyte materials [14–17]. Therefore, the LSCSb-xSDC composite cathodes obtained by physical mixing are expected to possess better performances than the

Corresponding author. Corresponding author. E-mail addresses: [email protected] (X. Li), [email protected], [email protected] (L. Zhang).

∗∗

https://doi.org/10.1016/j.solidstatesciences.2019.03.023 Received 2 November 2018; Received in revised form 20 March 2019; Accepted 29 March 2019 Available online 06 April 2019 1293-2558/ © 2019 Elsevier Masson SAS. All rights reserved.

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pure LSCSb cathode. In this study, we give a systematic investigation on the performances of LSCSb-xSDC composite cathodes for IT-SOFCs. Chemical compatibility, conductivity properties, thermal matching and electrochemical properties of LSCSb-xSDC composite materials were assessed and discussed. In the end, the stability of the cell with LSCSb50SDC cathode was detected. 2. Experimental procedures 2.1. Samples preparation Solid state reaction method was used to synthesize La0.4Sr0.6Co0.9Sb0.1O3-δ (LSCSb) sample. Stoichiometric amounts of original powders SrCO3 (99%), La2O3 (99%), Sb2O3 (99%) and Co3O4 (99%) were mixed and ground with alcohol in a mortar using an agate pestle. The mixed precursors were calcined in air at 1000 °C for 10 h. The obtained powder was then pressed into thin disks and calcined in air at 1050 °C for 20 h, and then calcined at 1200 °C in air repeatedly after grinding. LSGM, SDC and NiO powders were synthesized with the glycine-nitrate process described elsewhere [18]. The composite cathodes were obtained by mixing LSCSb powder with SDC powder at weigh ratios of 80:20, 70:30, 60:40 and 50:50, respectively, marked as LSCSb-xSDC (x = 20, 30, 40, 50). Then the composite powders were ground and pressed into thin disks with a diameter of 13 mm for electrical conductivity test, and cylinders with a diameter of 6 mm for linear thermal expansion test. Here, the calcination temperature for both the disks and cylinders was set to be at 1050 °C in air for 10 h. LSGM electrolyte disks used for assembling cells were sintered at 1450 °C in air for 10 h.

Fig. 1. XRD patterns of LSGM sintered at 1450 °C for 10 h, SDC powder sintered at 800 °C for 2 h, LSCSb powders calcined at 1200 °C for 10 h, LSCSb-50LSGM mixtures and LSCSb-50SDC mixtures both calcined at 1050 °C for 10 h.

convenient observation, the XRD patterns of the pure LSCSb, SDC and LSGM powders were also given in this figure. It can be seen that the LSCSb sample crystallizes in a tetragonal perovskite structure with space group of P4/mmm. In addition, there is neither additional diffraction peak nor peak shift that could be observed, indicating that LSCSb cathode is compatible well with SDC/LSGM electrolytes. Above result implies the feasibility of using LSCSb-xSDC composites as cathodes for IT-SOFCs.

2.2. Characterization X-ray diffractometer (XRD, Rigaku-D-Max γA) was used to determine the crystal structure and analyze the chemical compatibility between LSCSb and SDC/LSGM. SDC/LSGM and LSCSb mixed powders with weight ratio of 1:1 used for chemical compatibility test was calcined at 1050 °C in air for 6 h. The electrical conductivities of LSCSbxSDC composites were tested utilizing the four probe DC technique between 300 and 850 °C in air. The TECs of LSCSb-xSDC composites were tested between 30 and 900 °C, using a dilatometer (Netzsch DIL 402C) with an air flow rate of 60 mL min−1. The LSCSb-xSDC/LSGM/LSCSb-xSDC (x = 0, 20, 30, 40, 50) symmetrical cells were fabricated to measure the electrochemical impedance spectra (EIS). Both sides of the dense electrolyte LSGM disks were coated with LSCSb-xSDC pastes, and then calcined at 950 °C in air for 2 h. The EIS with an AC signal of 10 mV in the frequency range of 10−2-106 Hz were tested under open-circuit conditions by an electrochemical instrument (CHI604D, Chenhua) in the range of 600–850 °C. The NiO-SDC/SDC/LSGM/LSCSb-xSDC (x = 0, 20, 30, 40, 50) single cells supported by LSGM electrolyte were fabricated for cell power output. The SDC interlayer was screen-printed onto one side of dense LSGM disk (0.3 mm thick) and calcined at 1300 °C for 1 h. The NiO-SDC anodes printed onto the interlayer were calcined at 1250 °C for 4 h, and then the LSCSb-xSDC cathodes were painted onto the other side of LSGM electrolyte and calcined at 950 °C for 2 h. The same electrochemical workstation (CHI604D, Chenhua) was used to detect the power output and electrochemical stability of the cells.

3.2. Electrical conductivity test Fig. 2 presented the temperature dependence of electrical conductivities for the LSCSb-xSDC samples. It can be observed that electrical conductivity values of all samples decrease as the temperature increases, which indicates all the samples show a metal-like conducting behavior. The electrical conductivity value of the LSCSb-20SDC is 407 S cm−1 at 300 °C, and then drops rapidly down to 170 S cm−1 at 850 °C. A large number of oxygen vacancies formed during thermal reduction process of Co4+ to Co3+ with increasing temperature have been demonstrated to be the main reason for the rapid drop of conductivity [17]. Then the decreased concentration of charge carriers, i.e., Co4+ ions, with temperature leads to the decrease in electrical

3. Results and discussion 3.1. Chemical compatibility To evaluate the chemical compatibility between LSCSb cathode and common electrolytes (e.g., LSGM and SDC), the LSCSb-50LSGM and LSCSb-50SDC mixtures were both calcined at 1050 °C in air for 10 h. The XRD patterns of these mixtures were presented in Fig. 1. For

Fig. 2. Temperature dependence of electrical conductivity for the LSCSb-xSDC (x = 0–50) samples in air. 127

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Fig. 3. Thermal expansion curves of LSCSb-50SDC (x = 0–50) samples between 30 and 900 °C in air.

Fig. 4. (a)–(e) Typical impedance spectra of LSCSb-xSDC (x = 0–50) cathodes on LSGM electrolyte measured at 700 °C and 750 °C; (f) Polarization resistance values (Rp) measured at 700 °C and 750 °C for LSCSb-xSDC (x = 0–50) cathodes.

conductivity. In addition, the formed oxygen vacancies can interfere with the O-Co-O periodic potential and then result in the localization of carriers [19], which is not beneficial for the transfer of charge carriers. It can also be seen that the SDC addition results in a decrease in electrical conductivity. Similar result has been reported in the previous studies by Zhou et al. [15,16]. The reason is that the SDC is a good oxygen ionic conductor but a poor electronic conductor, whose conductivity values are much lower than those of the electrode materials [15]. Although the electrical conductivities of the LSCSb-xSDC system show a monotonous decrease in electrical conductivity with increasing temperature, their values are still much larger than the usually required value of 100 S cm−1 for an SOFC cathode [20]. Therefore, the electrical conductivities of the LSCSb-xSDC composites can entirely fulfill the requirement for cathode materials of IT-SOFCs.

in the slops of the thermal expansion curves among the 300–400 °C range. These slope changes are generally considered to be associated with chemical expansion, which is caused by the thermal reduction of Co4+ to Co3+ and/or transition of Co3+ from low-to high-spin state [22,23]. However, with the addition of SDC, this abrupt change in the slop is clearly weakened. Thus, adding SDC electrolyte to LSCSb cathode could greatly ameliorate thermal expansion matching between the electrolyte and cathode components.

3.4. AC impedance analysis Some important information of cathodes such as oxygen diffusion, surface adsorption/dissociation and charge transfer can been obtained from impedance spectra [24] of the LSCSb-xSDC/LSGM/LSCSb-xSDC symmetrical cells. The impedance spectra were tested under opencurrent condition in the range of 700–800 °C. Fig. 4(a)-(e) show the impedance spectra of LSCSb-xSDC composite cathodes on LSGM electrolytes at different temperatures. It can be seen that at least two obvious arcs at low- and high-frequency ranges appear in the spectra, indicating two or more polarization processes during oxygen reduction. The intercepts on the real axis at low frequency and high frequency represent the total resistance and ohm resistance of the half symmetrical cell, respectively. The polarization resistance values (Rp) are then calculated by the difference between the two intercepts. Fig. 4(f) shows the SDC content dependence of the Rp measured at 700 °C and 750 °C for LSCSb-xSDC composite cathodes. The Rp values of the LSCSb-xSDC composite cathodes with x = 0, 20, 30, 40 and 50 are 0.191, 0.185, 0.171, 0.115, and 0.086 Ω cm2 at 700 °C, respectively, and are 0.099, 0.085, 0.082, 0.055, and 0.045 Ω cm2 at 750 °C, respectively. As can be seen in Fig. 4(f), the Rp values decrease with increasing SDC content. It is the excellent oxygen ionic conductivity of SDC electrolyte [25] that improves the oxygen ion transport ability of the composite cathodes and enlarges their area of the TPB zone. In addition, the improved thermal expansion matching between the cathode and electrolyte caused by SDC introduction may also contribute to the decrease in Rp. Therefore the introduction of SDC into LSCSb cathode could significantly enhance its electrochemical performance. Furthermore, the electrochemical performance of the LSCSb-50SDC composite cathode is clearly better than those of some other composite cathodes, such as the LaBaCuFeO5+δ-10Ce0.8Sm0.2O1.9, Pr2CuO4-40Ce0.9Gd0.1O1.95 and La2NiO4+δ-Ce0.55La0.45O2-δ [15,26,27]. To describe the polarization processes more intuitively, the

3.3. Thermal expansion behavior The internal stress induced by the large difference in TECs between electrolyte and electrode materials is apt to destroy the stability of SOFC system. Thus it is necessary for the thermal expansion behaviors of LSCSb-xSDC samples to be investigated systematically. Fig. 3 presents the thermal expansion curves of the LSCSb-xSDC cathodes. It is expected that the TECs of the LSCSb-xSDC samples drop obviously along with introducing SDC. For example, the average TEC of the pure LSCSb is as high as 22.5 × 10−6 K−1 but only 13.1 × 10−6 K−1 for LSCSb-50SDC at 30–900 °C. As reported by Pikalova et al. [21], the TEC of SDC is only 12.3 × 10−6 K−1 at 350–900 °C in air, which is clearly much smaller than that of pure LSCSb reported in our previous work [10]. Therefore, adding SDC powder into LSCSb cathode contributes to the decrease in TECs. For ease of observation for the relationship between TECs and temperature, the average TEC values corresponding to different temperature ranges were listed in Table 1. It can be seen that the TEC values from 30 °C to 300 °C are obviously lower than those from 300 °C to 900 °C. As expected, there are sudden changes occurring Table 1 TECs ( × 10−6 K−1) for the LSCSb–xSDC system in the different temperature ranges. LSCSb-xSDC

TEC30-300°C

TEC

x=0 x = 20 x = 30 x = 40 x = 50

16.9 12.3 11.2 9.9 9.6

25.0 21.3 19.9 17.7 14.7

300-900 °C

TEC

30-900 °C

22.5 18.5 17.2 15.3 13.1

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Fig. 4. (continued)

Fig. 4. (continued)

Fig. 4. (continued)

Fig. 4. (continued)

Fig. 4. (continued)

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Fig. 6. (a)–(e) Cell voltage and power density as a function of current density for NiO-SDC/SDC/LSGM/LSCSb-xSDC (x = 0–50) cell measured at 600–850 °C using dry H2 as fuel and ambient air as oxidant. (f) Electrochemical stability of the NiO-SDC/SDC/LSGM/LSCSb-50SDC single cell at 700 °C in H2.

Fig. 5. Arrhenius plots of polarization resistance (Rp) for LSCSb-xSDC (x = 0–50) cathodes.

frequency (f) values for the solid symbol are denoted by 4, 3, 2, 1, 0 and −1, which correspond to 104, 103, 102, 101, 100 and 10−1 Hz, respectively. In general, the low frequency arcs (f < 1 Hz) are associated with oxygen adsorption/desorption and oxygen diffusion processes, which are non-charge transfer processes, while the medium-high frequency arcs are charge transfer processes [28–30]. In Fig. 4(a)-(e), the polarization related to the medium-high frequency arcs (f > 10 Hz) accounts for a large proportion in total polarization, indicating that the charge transfer of electrons and ions is the main rate-limiting step during the oxygen reduction processes [10]. Clearly, the SDC addition in LSCSb-xSDC system gradually reduces the polarization resistance induced by charge transfer process. This can be attributed to the increased area of TPB induced by SDC addition, which increases the active sites for oxygen reduction reaction. The Arrhenius plots of ln(1/Rp) vs. 1000/T. for LSCSb-xSDC composite cathodes are shown in Fig. 5. The calculated activation energy values (Ea) from the slops of the Arrhenius curves are 138.2, 131.3, 126.0, 113.7 and 97.5 kJ mol−1, respectively. It is known that the smaller activation energy means the better oxygen permeability [31]. The decreased activation energy with SDC addition suggests that the introduction of electrolyte powder makes the oxygen reduction reaction easier and faster. This result can be attributed to the high oxygen ionic conductivity of SDC, which improves the oxygen permeability of the cathode.

that the current density reduced slightly after a 20 h test, indicating that the LSCSb-50SDC cathode has a good electrochemical stability. In conclusion, the LSCSb-50SDC composite material is a very potential and promising cathode material for IT-SOFCs. 4. Conclusions In this study, LSCSb-xSDC composite materials are investigated as potential cathodes for IT-SOFCs. XRD result demonstrated that the LSCSb cathode is chemical compatible with the LSGM/SDC electrolyte. Electrical conductivities of the LSCSb-xSDC decrease with increasing content of SDC, which can be explained by the poor electrical conductivity of the SDC electrolyte. Thermal expansion test indicates that the introduction of SDC into LSCSb reduces the TEC, which faciliates the thermal expansion matching between the cathode and LSGM electrolyte. Impedance spectra indicate that the Rp values decrease with SDC introduction and the LSCSb-50SDC cathode show the lowest Rp value, which can be ascribed to the introduction of SDC which improves the oxygen ionic conducting ability of the cathode and enlarges the area of the TPB zone. The maximum power density of LSGM electrolyte supported single cell with the LSCSb-50SDC cathode reached 432 mW cm−2 at 700 °C. Furthermore, the long-term stability test demonstrated that the LSCSb-50SDC cathode possesses good electrochemical stability performance. All the experimental results indicate that LSCSb-50SDC could be used as a candidate cathode material for ITSOFC.

3.5. Cell performance Fig. 6(a)-(e) present the voltage/power density as a function of current density for LSCSb-xSDC/LSGM/SDC/NiO-SDC (x = 0, 20, 30, 40 and 50) single cells at 600–850 °C. The hydrogen was used as the fuel gas and the ambient air was used as the oxidant. It can be seen that the maximum power densities (Pmax) increase with increasing SDC content. The Pmax values are 297, 329, 359, 365 and 432 mW cm−2 at 700 °C for x = 0, 20, 30, 40 and 50, respectively. The maximum power output increases monotonously with SDC introduction. This result of the cell performances reported here is consistent with that of the impedance spectra. Then it is reasonable to consider that the best power output for the cell with LSCSb-50SDC cathode may be associated with its lowest Rp. Furthermore, the reduced TEC value of the LSCSb-50SDC cathode, which greatly improves the thermal expansion matching between electrolyte and cathode components, may also contribute to the cell power output. Fig. 6(f) shows the long-term stability of the LSCSb-50SDC/LSGM/ SDC/NiO-SDC single cell. The stability measurement was carried out at constant voltages of 0.2 and 0.6 V at 700 °C for 20 h. The result shows

Prime novelty statement In this work, the La0.4Sr0.6Co0.9Sb0.1O3-δ-Ce0.8Sm0.2O1.9 (LSCSbxSDC) samples were evaluated as potential cathodes for intermediate–temperature solid oxide fuel cells based on La0.9Sr0.1Ga0.8Mg0.2O3–δ (LSGM) electrolyte. To the best of our knowledge, this is a first study on the performances of the LSCSb-xSDC composite cathodes based on oxygen–ion conducting electrolyte LSGM. In this research, the prime novelties were concluded as follow: ● Electrical conductivities of LSCSb-xSDC cathodes were systematically investigated and their values are generally above 100 S cm−1. ● Thermal expansion behaviors of the LSCSb-xSDC cathodes were systematically investigated and, with SDC introduction, the thermal expansion matching between the cathode and electrolyte components was improved. 130

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Fig. 6. (continued)

● The effect of SDC introduction on the electrochemical performance of the LSCSb cathode on LSGM electrolyte were systematically investigated. The results indicate that, among the LSCSb-xSDC system, the LSCSb-50SDC cathode shows the optimal performance. In addition, the single cell with LSCSb-50SDC cathode shows good cell stability at operating temperature (700 °C).

candidate for use in the cathode of IT–SOFC. Acknowledgements This work was supported by the National Natural Science Foundation of China (Nos. 21403101), the Foundation of Education Department of Liaoning Province (Nos. L2012135 and L2016013), the Foundation of the Science and Technology Department of Liaoning

All results indicate that the LSCSb-50SDC is a very promising

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Fig. 6. (continued)

Fig. 6. (continued)

Province (Nos. 2013020151 and 201602475), the Program for the development of Science and Technology of Fushun city (Nos. 20153310 and 20141117), and the Doctoral Scientific Reaearch Foundation of Liaoning Province (Nos. 20170520381). References [1] B. Zhu, X.R. Liu, Z.G. Zhu, R. Ljungberg, Solid oxide fuel cell (SOFC) using industrial grade mixed rare-earth oxide electrolytes, Int. J. Hydrogen Energy 33

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