The Fabrication Of Dual Phase BaCe_0.5fe_0.5O_3-δ Cathode For Intermediate Temperature Solid Oxide Fuel Cells

Solid Oxide Fuel Cell（SOFC）is considered to be one of the most promising energy conversion devices due to its low environmental pollution,high efficiency and fuel flexibility.The operating temperature（800-1000°C）required for typical SOFC limits the choice of materials.To achieve operating at intermediate temperature（600-800°C）for SOFC,reducing its operating temperature to 600°C or even lower is an important prerequisite for the industrialization of SOFC.While lowering the temperature will also decreases the electrode reaction process kinetics,and resulting in a sharp increase of polarization resistance and ohmic resistance.Therefore,the development of novel cathode materials with high electrocatalytics activity for the oxygen reduction reaction（ORR）at intermediate temperature and electrolyte thin films are the main research directions of SOFC.The work of this dissertation is based on the background of intermediate temperature SOFC,and investigates a novel oxygen ion conducting SOFC cathode material with high performance at intermediate temperature.Most of the polarization resistance of SOFC is generated at the cathode.Among the cathode materials of SOFC,a perovskite oxide as mixed oxygen ionic and electronic conductor（MIEC）can expand the active site of ORR to the entire electrode surface,therefore improve the electrocatalytic activity significantly.In the second chapter of the dissertation,Ba Ce_0.5Fe_0.5O_3-δ（BCF）cathode powders were synthesized by EDTA-citric acid method using Ba（NO₃）₂,Ce（NO₃）₃·6H₂O,Fe（NO₃）₃·9H₂O as raw materials,and sintered samples were prepared.Phase composition,electrode microstructure and oxygen reduction process of the BCF cathode were investigated.Symmetrical cells and single cells based on the BCF cathode were assembled with Sm_0.2Ce_0.8O_2-δ（SDC）as electrolyte and Ni O-SDC as anode,and the related properties were investigated.The results show that the BCF powders can in situ decompose into Ba Ce_0.15Fe_0.85O_3-δand Ba Ce_0.85Fe_0.15O_3-δphases.The K_chem values of BCF samples are 7.25×10^-4 cms^-1,3×10^-4cms^-1,and 3.8×10^-5 cms^-1 at 800,700 and 600°C,respectively.Area specific resistance（ASR）of symmetrical cell is 1.04Ωcm² at 600°C.After the long-term test of 400 h at600°C,ASR value remains stable at about 1.65Ωcm².Maximum power density of Ni O-SDC||SDC||BCF single cell reaches 228 m Wcm^-2 at 650°C with the 0.79V open circuit voltage,and the corresponding ohmic resistance and polarization resistance are0.12 and 0.20Ωcm²,respectively.In order to further reduce the polarization resistance of BCF cathode.In the third chapter of the dissertation,Co-doped Ba Ce_0.5Fe_0.4Co_0.1O_3-δ（BCFC）cathode powders were prepared by the EDTA-citric acid method,and its composition and properties were investigated.The results show that the BCFC powder consisted of a mixture of Ba Ce_0.15Fe_0.75Co_0.1O_3-δand Ba Ce_0.85Fe_0.05Co_0.1O_3-δ.The lattice expansion of BCFC increased as induced by Co doping.The relaxation equilibrium time of BCFC at 800°C is only 195s,which is about 22%faster than that of BCF.Surface oxygen exchange coefficient of BCFC are 1.2×10^-3 scm^-1,1.04×10^-3 scm^-1 and 3.8×10^-4 scm^-1 at 800°C,750°C and 700°C,respectively,showing better catalytic activity for ORR.ASR of BCFC/SDC/BCFC symmetric cell are 0.088,0.269,and 0.819Ωcm²at 700,650,and600°C,which are 26.4%,20.4%,and 21.2%lower than BCF,respectively.And after400 h long-term testing and 10 times thermal cycle experiments,the ASR remains stable at about 1.60Ωcm².The cathode morphology remained unchanged before and after the long-term test.Maximum power densities of Ni O-SDC||SDC||BCFC cells were 290 and204 m Wcm^-2at 700°C and 650°C,respectively.The open circuit voltages are 0.80V and 0.82V,and the polarization resistances are 0.15 and 0.37Ωcm².The preliminary results indicate that BCFC show good electrochemical performance and stability as cathode materials for intermediate temperature SOFC.In the fourth chapter of the dissertation,La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ（LSCF）cathode was prepared by solid-state method.SDC electrolyte precursor slurry was prepared with Sm₂O₃ and Ce O₂ as raw materials,which was coated on the anode support of Ni O-SDC by dip-coating method,and the single cell Ni O-SDC||SDC||LSCF was obtained by one-step sintering.The thickness of the electrolyte layer is 15-18μm,which is about30%lower than that of co-press method.The results show that Ni O-SDC anode is well combined with the SDC electrolyte.With hydrogen as fuel,maximum power densities of Ni O-SDC||SDC||LSCF cells are 376 and 295 m Wcm^-2at 700,650°C,and the corresponding polarization resistances are 0.11,0.15Ωcm²,with the ohmic resistances of 0.11,0.13Ωcm².The results show that the single cell with excellent electrochemical performance was successfully prepared by dip-coating method.