Electrochemical analysis of Pr0.3Sr0.7CoxB(1 −x)O3 − δ (B = Fe, Mn; x = 0, 0.3, 0.5, 0.7, and 1) as cathode materials for intermediate temperature SOFCs
Introduction
A solid oxide fuel cell (SOFC) is an efficient and environmentally friendly method for combined generation of heat and electricity [1], [2]. However, problems associated with fabrication and reliability occur over long-term operation at high temperatures (> 900 °C) [2], [3]. Therefore, the operating temperature must be decreased to an intermediate temperature (600 °C–700 °C) [1], [2], [3], [4]; however, these lower temperatures decrease the catalytic activity of the cathode, reducing the rate of the oxygen reduction reaction (ORR) [5]. Mixed ionic/electronic oxide conductors (MIECs) are promising materials for use as cathode materials that may increase the ORR rate within the desired temperature range (500 °C–700 °C). Perovskite-type oxides are most commonly used as cathode materials for SOFCs due to their high electronic/ionic conductivity and chemical stability [6]. Perovskite-type structures (ABO3) with the composition (Ln/A)MTO3 are commonly used in this application and consist of lanthanides (Ln: primarily La) and alkaline earth metals (A: Sr, Ca) as dopants on the A-site and transition metals (MT: Cr, Mn, Fe, Co, Ni) on the B-site [7], [8], [9], [10]. Many research groups have studied other materials and compositions of perovskite-type to increase the ORR rate and to decrease area-specific resistance (ASR, RN) [2], [3], [4], [5], [6], [7]. Praseodymium (Pr) is a candidate material among the various materials available for use on an A-site. Its electron distribution and structural characterization are similar to other light rare-earth elements such as lanthanum (La) and cerium (Ce) [4]. Several Sr-doped rare earth manganites that incorporated Pr3 + ions at the A-site have exhibited the highest electrical conductivity and maintained the lowest overpotential values [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18]. The Sr-doped Pr cobaltites have also displayed the highest ionic and electronic conductivities and have achieved the best performance [2], [9]. Pr0.3Sr0.7CoO3 − δ (PSC37) has exhibited low ASR values (0.18 Ω·cm2 at 700 °C on Ce0.9Gd0.1O2 (CGO91) electrolyte) in a previous study by Kim et al. [2]. Ceria-based oxides, such as CGO91 and samarium-doped ceria (SDC), have been used as electrolytes instead of yttria-stabilized zirconia (Y0.08Zr0.92O2, 8-YSZ) because these oxides can maintain higher conductivities at low temperatures. However, due to the large thermal expansion coefficient (TEC) for Sr-doped Pr cobaltites, the cathode may cause deterioration during thermal cyclic operation and long-term operation. This problem can be reduced by doping with materials such as ferrites and manganites in the B-site (i.e., replacing cobalt); doping in the B-site can enhance the ORR [3], [5], [6]. These effects have been confirmed by many research groups [4], [7], [10], [11], [12].
This paper will discuss Pr0.3Sr0.7CoxB(1 −x)O3 − δ (PSCB'37X(1-X)), where B = Fe, Mn; and x = 0, 0.3, 0.5, 0.7, and 1; how the material's TEC can be lowered; and how to obtain higher reaction activity for the ORR. Powders were synthesized and characterized by x-ray diffraction (XRD), dilatometry, scanning electron microscope (SEM), and electrochemical impedance spectroscopy (EIS) on electrodes using a CGO91-pellet as the electrolyte.
Section snippets
Experiment
Pr0.3Sr0.7CoxB(1 −x)O3 − δ powders were synthesized using the glycine-nitrate process (GNP) [3]. The precursors used included Pr(NO3)3-6H2O, Sr(NO3)2, Co(NO3)2-6H2O, Fe(NO3)3-9H2O, and Mn(NO3)2-6H2O with purities of > 99.9%. Aqueous solutions were prepared by mixing the corresponding nitrates according to the molar ratio of Pr:Sr/Co:B = 3:7/X:1-X (x = 0, 0.3, 0.5, 0.7, and 1). The appropriate amount of glycine was also added. The solution was combusted, and the combusted powder was calcined at 1250 °C
Results and discussion
The XRD patterns of the Pr0.3Sr0.7CoxB(1 −x)O3 − δ (B = Fe, Mn; x = 0, 0.3, 0.5, 0.7, 1) samples are shown in Fig. 2. Most powders were synthesized properly, as shown by the XRD results, and did not show any splitting of the diffraction peaks. As the amount of Mn or Fe increased, the peak shifted towards a lower angle; this was likely due to the larger ion radius of the dopants compared to that of Co. This trend is similar to that reported previously by Lee et al., who studied Nd0.6Sr0.4CoxMn(1 −x)O3 − δ
Conclusion
The complex perovskite structure of Pr0.3Sr0.7CoxB(1 −x)O3 − δ (B = Fe, Mn; x = 0, 0.3, 0.5, 0.7, and 1) was examined as an IT-SOFC cathode material. Single phase perovskites were obtained for B = Fe, Mn. The ASRs of Pr0.3Sr0.7Co0.3Fe0.7O3 − δ (PSCF3737) and Pr0.3Sr0.7Co0.7Mn0.3O3 − δ (PSCM3773) were 0.134 Ω·cm2 and 0.174 Ω·cm2 at 700 °C, respectively. The good electrochemical performances of these cathode materials were likely a result of the combination of their high electronic conductivity and ionic
Acknowledgments
This work was supported by the Global Frontier R&D Program on Center for Multiscale Energy System funded by the National Research Foundation of Korea under the Ministry of Science, ICT & Future, Korea (2011-0031569). The authors are grateful for the support from the new and renewable energy (Diesel desulfurization for 300 kW MCFCs) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea Government Ministry of Knowledge and Economy (MKE) and Basic
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