| Fuel cells which is an energy conversion device, directly convert the chemical energy of gaseous or liquid fuels into electrical energy by a highly efficient and clean electrochemical oxidation process. Not limited by the Carnot cycle, fuel cells offer high chemical-to-electrical conversion efficiency. In order to guarantee future energy requirements and reduce pollutant emissions precludes, the widespread development of fuel cells technology will become the most important.Among all types of fuel cells, SOFCs are very promising because they have their own advantages, for example, flexibility and anti-corrosive. However, the current state of the art SOFC still has many disadvantage related to its materials compatibility, high costs and longevity due to its high operation temperature of around 1000℃, as a consequence, significant effort has been devoted to the development of intermediate-temperature (IT,500-800℃) SOFCs. Lowering the operation temperature of SOFCs can not only extend the range of material selection, significantly reduce the cost of production and application, but also improve the stability and reliability for the SOFC system. However, a key obstacle to reduced-temperature operation of SOFCs is the poor activity of traditional cathode materials. The cathode becomes the limiting factor in determining the overall cell performance. Therefore, the development of new electrodes with high electro catalytic activity for the oxygen-reduction reaction is critical for intermediate-temperature (IT)-SOFCs. This thesis systematically studied physical and chemical properties of four kinds of new cathode materials and composite cathode materials for SmBaCoFeO5+δ, SmBaCoCuO5+δ, LnBa0.5Sr0.5Co2O5+δ(Ln=Pr, Nd) and BaCo0.7Fe0.2Nb0.1O5+δ. The aim of this paper is to develop new cathodes with high catalytical activity, good compatibility, long life-time, less thermal mechanical mismatch with the other components, suitable to practical applications of IT-SOFCs.The samples of SmBaCoFeO5+δ(SBCF) were prepared by a combined EDTA-ecitrate complexing sole-gel. The basic physical properties of these materials were investagated. The suitability of SBCF as cathode materials for IT-SOFCs was evaluated. The results show that SBCF cathodes are chemically compatible with the intermediate-temperature electrolyte GDC. No chemical reaction of the binary-mixed SBCF-GDC system is detected upon sintering at 1000℃for 3h. The average TEC values is 18.9×10-6K-1 in the temperature range of 30-850℃. The conductivity of the SBCF sample was 12-82 S cm-1. In addition, the half-cell and single-cell performances with SBCF cathode on GDC electrolyte were studied. The results show that the polarization resistances for SBCF cathode on GDC electrolyte are 0.103Ωcm2 at 800℃,0.217Ωcm2 at 750℃and 1.084Ωcm2 at 700℃, respectively. For SBCF/GDC/NiO-GDC single cell, the maximum power density is 432 mW cm-2 at 800℃. In order to further reduce the match with the electrolyte and the promotion of the electrochemical properties, we mixed the GDC electrolyte powder into SBCF. We found that the addition of GDC increased the length of three-phase interface and effectively improved the performance of the cathode. The SBCF-50GDC showed the best performance, the results showed that the polarization resistances for SBCF-50GDC cathode on GDC electrolyte are 0.036Ωcm2 at 800℃. For SBCF-50GDC/GDC/NiO-GDC single cell, the maximum power density is 691mW cm-2 at 800℃. The results showed that the TECs of the SBCF cathodes can be reduced through adding GDC electrolyte into the SBCF to form the composite cathodes. The average TEC of SBCF-50GDC was 14.2×10-6K-1.Double-perovskites cuprate material SmBaCoCuO5+δ(SBCC) were synthesized by solid-state reaction method. SBCC had an orthorhombic perovskite structure after sintering at 1000℃for 10h and have a good matching and chemical compatibility with GDC. Using impedance spectra, the electrochemical performance of SBCC cathode in the temperature range of 650-800℃on GDC electrolyte, and the effect of sintering temperature on performance of cathode were investigated. The area specific resistance (ASR) value of SBCC sintered at 950℃on GDC electrolyte is 0.086Qcm2 at 800℃, which is much lower than the 0.168Qcm2 measured at 800℃for SBCC sintered at 900℃Its value is a little lower than that of SBCC sintered at 1000℃, which is 0.127Ωcm2. The SEM results showed that SBCC cathode material shows a smaller grain size and reasonable porosity to ensure gas diffusion. Also it appears good contact with the dense electrolyte pellet after sintered 950℃. Considering these conclusions, we think 950℃is the appropriate sintering temperature for SBCC cathode. For SBCC/GDC/NiO-GDC single cell, the maximum power density is 517 mW cm-2 at 800℃. In the 30-850℃temperature range, the average thermal expansion was 15.5×10-6 K-1. In order to improve the properties of the SBCC cathode material and reduce the TECs, the composite cathode of the SBCC-GDC was prepared. The results showed that the TECs of SBCC-GDC composite cathode reduce through adding 10wt%, 30wt%.50wt%,60wt% GDC to SBCC, respectively. The TECs of the SBCC-GDC composite cathodes were 15.2×10-6 K-1,14.0×10-6 K-1,13.1×10-6 K-1 and 12.9 x 10"6 K"1 in the temperature range of 30-850×, respectively. In addition, the polarization resistance of the composite cathodes is also improved. The optimum electrochemical performance of composite cathodes can achieve through adding 50wt% GDC into the SBCC cathode. The polarization resistance is 0.042Qcm2 for SBCC-50GDC.The LnBa0.5Sr0.5Co2O5+δ(LnBSC)(Ln=Pr, Nd)cathode materials was prepared by solid state reaction method. The XRD results showed that LnBSC cathodes crystallize in a single phase double-perovskite after sintering at 1100℃for 12h. which were chemically compatible with the intermediate-temperature electrolyte material La0.9Sr0.1Ga0.8Mg0.2O2.85 (LSGM). No chemical reaction of the binary-mixed LnBSC-LSGM systems is detected upon sintering at 1000℃for 10h. In the operating temperature range, the conductivity of PBSC and NBSC were more 300 S cm-1 which meet the needs of the cathode materials. The average thermal expansion coefficients of PBSC and NBSC between 30 to 850℃are about 22.9×10-6K-1 and 22.1×10-6K-1, respectively. The polarization resistances for PBSC and NBSC materials on LSGM electrolyte are 0.027Ωcm2 and 0.039Qcm2 at 800℃, respectively. The polarization resistances of all the samples are all lower than 0.15Ωcm2 at 700℃for the LnBSC cathodes-on LSGM. For LnBSC/LSGM/GDC/Ni-GDC single cell, the maximum power density are 1021 mW cm-2 and 774 mW cm-2 at 800℃, respectively. LnBSC-GDC composite cathodes were also prepared and characterized. The results show that the TECs of the LnBSC cathodes can be reduced through adding GDC electrolyte into the LnBSCO to form the composite cathodes. However, it is found that the addition of GDC into LnBSC results in a slight inscrease of the polarization resistance compared to the LnBSCcathodes. The polarization resistance is still lower than 0.15Ωcm2 at 700℃. The power density of cell with composite cathode is slightly lower than that of LnBSCO cathodes. The power density of the cell with PBCO and NBCO as cathodes attains 645 mW cm-2 and 382 mW cm-2at 800℃, respectively.The BaCo0.7Fe0.2Nb0.1O5+δ(BCFN) perovskite, formerly, as oxygen permeation membrane, was investigated as a novel cathode for SOFCs. The XRD results show that BCFN cathode is chemically compatible with the intermediate-temperature electrolyte materials such as GDC and LSGM. No chemical reaction of the binary-mixed BCFN-GDC and BCFN-LSGM systems is detected upon sintering at 1000℃for 5h. The conductivity of the BCFN cathode was about 13.89 Scm-1 at 850℃. It shows best lowest ASR for BCFN sintered at 950℃. The ASR values of BCFN based on the GDC and LSGM electrolytes are 0.014Ωcm2 and 0.051Ωcm2 at 800℃, respectively. At 800℃, the overpotential for the BCFN based on the GDC and LSGM electrolytes is 124 mA cm-2 and 46mA cm-2. Using BCFN as the cathode, the GDC electrolyte-supp-orted single cell exhibited peak power intensitie of 618 mW cm-2 at 800℃In this paper, four kinds of cathode materials were investigated. And based on composite cathode materials design idea, we explored effective way of enhancing ion conductivity in cathode. The properties of these mixed conducting cathodes were systemically studied, which exhibited attractive performances for IT-SOFCs. |
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