| Solid oxide fuel cells (SOFCs) are energy conversion devices with high working efficiency, low emissions and excellent fuel flexibility. Lowering the operating temperature from～1000 ℃ high temperature to intermediate-temperature (IT) range of 500-800 ℃ can reduce fabrication and working cost, improve structure and performance stabilities and realize long lifetime of SOFCs, which, however, is also accompanied with performance degradation of the component materials of cathode, electrolyte and anode. Particularly, due to relatively high activation energy of oxygen reduction reaction (ORR) occurring over the cathode, polarization resistance of the cathode increases fast with the lowering temperature, and the cathode resistance becomes the main contribution to the performance degradation of the IT-SOFCs. Therefore, it is of great significance to obtain cathode materials with excellent performance for development and practical applications of IT-SOFCs.Cobalt-based perovskite oxides are the important candidate cathode materials of IT-SOFCs due to their distinctive advantages of high electronic-ionic mixed conductities and high ORR catalytic activities. However, this type of materials commonly has the problem of high TEC values that don’t match with TEC of the commonly used electrolyte materials of Gd0.1Ce0.9O1.95 (GDC), Sm0.1Ce0.9O1.95 (SDC) and La0.8Sr0.2Ga0.83Mg0.17O3-δ (LSGM), which can cause structural crack and performance degradation during high-teperarure calcination and working process of SOFCs. Therefore, it is crucial to decrease the TEC values for matching with the electrolyte materials and enhance the structure and performance stabilities of the cobalt-based perovskite cathodes. It’s known that the high TEC values of the cobalt-based perovskite oxides are caused by reduction of B-site Con+(n=2,3,4) ions and/or their low spin/high spin transition. One way to decrease the TEC values is partial substitution of Con+ ions with other valence stable transition metal ions. The other way is to make composite cathode materials by adding certain amount of other component material with small TEC. Modification of electrical and electrochemical performance can also be realized in both ways.In this work, two cobalt-based perovskite oxides, La0.5Ba0.5CoO3-δ with cubic perovskite structure and PrBaCo2O6-δ with double-layered perovskite structure, have been chosen as the study targets of property modification by B-site doping and preparation of composite materials. The main contents and experimental results are as below:1. Cubic perovskite oxides La0.5Ba0.5Co1-yFey03-δ (LBCF-y) with B-site Fen+(n=3,4) doping contents of y=0.1-0.9 were synthesized by sol-gel method and studied with respect to oxygen content, defect chemistry, thermal expansion behavior, and electrical and electrochemical properties. The XRD results have indicated that the LBCF-y oxides are pure phases of cubic pervoskite structure at Fen+ doping contents of y=0.1-0.7 while some impurity phase appears at y> 0.7. Besides, structural expansion occurs in LBCF-y with the increasing Fen+ doping content. Iodomatric titration measurements at room temperature have demonstrated that oxygen content and average valences of the B-site cations increase gradually with the higher Fen+ doping content; The B-site cations are Fe4+, Co4+ and Co3+ at y=0.1-0.3 and Fe4+, Fe3+ and Co+ ions at y=0.5-0.7 respectively. Thermal expansion behaviors of LBCF-y were measured in air at 20-1000 C, and the results have shown that the TEC values firstly increase to a maximum at y=0.3 then decrease gradually with the bigger y and the lowest TEC values are obtained at y=0.7. Electrical conductivites of LBCF-y were measured in air at 100-850 C using DC four-electrode method. For each sample, the conductivities firstly increase then decrease with the higher temperatures. At the same tempature, the conductivities decrease with the higher Fen+ doping content in LBCF-y. Effects of Fen+ doping on the TEC and electrical conductivites of LBCF-y are closely associated with the changes in the chemical defects in the oxide. Electrochemical performance of LBCF-y cathodes on the Gd0.1Ce0.901.95 (GDC) electrolyte was characterized by AC impedance spectra measurements in air at 650-800 C and in the atmospheres with various oxygen partial pressures. The results have demonstrated that polarization resistances of LBCF-y slightly increase with the higher Fen+ doping content because that the Fen+doping has caused decrease in concentration of oxygen vacancy, which inhibits oxygen ionic diffusion process at the high-frequency range. However, all the LBCF-y (y= 0.1-0.7) oxides show low ASR values of< 0.1 Ω·cm2 at 650 C, demonstrating high ORR catalytic activities of the cathodes.2. B-site scandium-doped PrBaCo2-xScxO6-δ (PBCS-x, x=0.00-1.00) oxides have been synthesized and evaluated as cathode materials of IT-SOFCs with respect to phase structure, oxygen content and chemical defects, thermal expansion behavior as well as electrical and electrochemical properties. The XRD results have demonstrated a phase transition in PBCS-x with the increasing Sc3+ doping content:tetragonal double-layered perovskite structure at x= 0.00-0.20, bi-phase mixtures at x=0.30-0.40, cubic perovskite structure at x=0.50-0.90 and a minor impurty appears at x=1.0. The iodomatric titration measurement results indicate that with the higher Sc3+ doping content in PBCS-x, oxygen contents (6-8) and average valences of cobalt ions decrease while concentration of oxygen vacancy increases. Such changes in the chemical defects have caused the changes in structure and properties of PBCS-x. Sc3+ doping has also led to decreased TECs of PBCS-x, which improves the TECs compatibility between the electrolyte materials. The AC impedance spectra measurement results of PBCS-x/GDC/PBCS-x symmetric cells have demonstrated that partial substitution of B-site cobalt ions with Sc3+ has greatly enhanced the ORR catalytic activities of PBCS-x cathodes characterized by decreasing ASR values with the higher Sc3+ doping contents. Among the studied samples, the PBCS-0.50 oxide with Sc3+-doping content of x=0.50 exhibits the best electrochemical performance on Ce0.9Gd0.1O1.95 electrolyte. Its ASR values range from 0.123 Ω·cm2 at 600 ℃ to 0.022 Ω·m2 at 750 ℃, which are much lower than the related cathode materials. These results have demonstrated that the PBCS-0.50 oxide is a promising cathode material for IT-SOFCs.3. Pr0.83BaCo1.33Sc0.506-δ-0.17PrCo03 (PBCS-0.17PCO) nano-composite material has been synthesized by a combined EDTA-citrate complexing sol-gel method and characterized as cathode material of IT-SOFCs with respect to phase structure, themal expansion coefficients as well as electrical and electrochemical properties. The results have also been compared with the results of single-phase cathode material of PrBaCo1.5Sco.5O6-δ (PBCS-0). The XRD results have shown that the composite material is composed of two cubic perovskite phases, Pro.83BaCo1.33Sc0.5O6-δ(83 mol%) and PrCOO3 (17 mol%), with nano-scaled grain sizes (< 25 nm). TEC of the composite material was measured to be 18.4×10-6/℃ at temperatures of 30-900 ℃, which is lower than the TEC of PBCS-0. Electrical conductivities of the composite cathodes were measured in air at 50-850℃ by DC four-electrode method, and the conductivity values increase monotonically with the higher temperatures from 50 ℃ up to 850 C, different from the conductive behavior of the PBCS-0 single phase cathode. Electrochemical performance of the PBCS-0.17PCO composite materials was studied by AC impedance spectra measurements using GDC-based symmetric cell. Low ASR values ranging from 0.127 Ω·cm2 at 600 ℃,0.069 Ω·m2 at 650 ℃,0.039 Ω·m2 at 700 ℃ to 0.026 Ω·m2 at 750 ℃ were achieved for the composite cathode, demonstrating its promising application as cathode material of IT-SOFCs. |
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