Preparation And Performance Research On Cathode And Sealing Materials For Intermediate Temperature SOFC

Solid Oxide Fuel Cells (SOFCs) have attracted worldwide attention due to their major advantages such as high efficiency and very low pollutant emissions. Cathode materials and sealant materials are two of the most important components for SOFC. In this paper, cathode materials and sealant materials were investigated in detail. Cathode materials included traditional La_1-xSr_xMnO₃-бmaterials and new type Pr_1-xSr_xFeO₃ cathode materials. Sealant materials included the BaO-CaO-Al₂O₃-B₂O₃-SiO2-La2O3(BCAS) system for electrolyte-supported SOFC and SrO-Al₂O₃-B₂O₃-SiO2-La2O3(SAS) system for anode-supported SOFC.Compared with other methods, the powders prepared by co-precipitation-azeotropic distillation method possessed smaller particle size and did not agglomerate after sintering at high temperature. It possesed better catalytic activity for oxygen reduction reaction. The La_0.8Sr_0.2MnO₃(LSM20) with 20 mol% of Sr content showed the highest conductivity, the best electrochemical activity and good thermal expansion compatibility with other cell components. At 800℃, the conductivity of LSM20 was 202.98 S·cm^-1 and polarization resistance was 0.396Ω·cm2. The optimized process parameters for screen-printing cathode film were as fallows: using ethyl cellulose as binder, using 120 mesh screen-printing and sintering at 1200℃for 2 h.Compared with mono-layer cathode, multilayer composite cathode showed lower polarization over-potential. At 800℃, the polarization resisitance of mono-layer composite cathode with 20 mass% YSZ was 0.329Ω·cm2. The polarization resistance of double-layer composite cathode was 0.243Ω·cm2 and it was 0.182Ω·cm2 for triple-layer composite cathode. Ohm resistance decreased from 0.87Ω·cm2 for mono-layer cathode to 0.57Ω·cm2 for triple-layer. The triple-layer composite cathode possessed optimal performance. The better layers components ratio (mass): the first layer was LSM20, the second layer was 80%LSM20+20%YSZ, the third layer was 60%LSM20+40%YSZ. Optimizing the function layer in composite cathode can improve the performance of cathode. The power density of the cell was enhanced 205 mW·cm-2 after mixing the YSZ particles and LSM particles with both large size and small size, respectively. After adding 12 mass% starch in function layer as pore-former, the power density of cell increased about 119 mW·cm-2.In this paper, the crystal structure and electrochemical performance of Pr_1-xSr_xFeO₃ cathode were investigated. All the samples were single phase perovskite when x increased from 0.1 to 0.5. The structure of the compounds with x≤0.3 was the orthorhombic GdFeO3-type structure. The structure change to cubic structure when x was 0.4 and 0.5. With the increasing of Sr content, the cell volume Pr_1-xSr_xFeO₃ decreased and the conductibity, thermal expansion coefficient (TEC) increasd. The conductivities were all higher than 100 S·cm^-1 when x=0.3⁰.5. The conductivity of Pr0.8Sr0.2FeO3 was 78 S·cm^-1 at 800℃. The TEC of Pr_1-xSr_xFeO₃ for x=0.1⁰.3 matched well with YSZ electrolyte.Compared with the traditional La_0.8Sr_0.2MnO₃ cathode, the Pr0.8Sr0.2FeO3 showed higher polarization current density and possessed lower polarization resistance. The polarization resistance was 0.2038Ω·cm2 at 800℃. There are superior chemical compatibility between Pr0.8Sr0.2FeO3 and YSZ electrolyte at high temperature.The thermal expansion coefficient of BCAS system increased with the decrease of CaO content and the increase of BaO content. Glass transition temperature and crystallization temperature increase greatly with the increase of CaO content. The thermal expansion coefficient of the sealant with Ba2+ content of 25.4 mol%(BCAS551) was 10.8×10-6 K-1. The glass transition temperature was 643℃and crystallization temperature was 786℃. The glass-ceramic sealant was well bonded to the other components. No weight loss for BCAS551 sealant was found after being heat-treated at 800℃for 100 h both in reducing and oxidizing atmospheres. There was no more than 10μm diffusion layer between sealant and other components except Fe-Cr alloy connector, which was about 30μm. It did not influence the performance of connector.In SAS system, the sealant with 19.85 mol% SrCO3(SAS3#) was determined to be suitable for anode-supported SOFC. The thermal expansion coefficient of the SAS3# sealant was 12.5×10-6 K-1. The thermal expansion mismatch rate between the SAS3# sealant and Ni/YSZ anode was only 3.2%. After being sintered at 800℃for 120 h, it showed no weight. After being sintered for 140 h, its weight loss was 0.378%. The open circuit voltage(OCV) of the sealed single cell was 1.03 V.