Preparation And Performances Of Electrode Materials For Intermediate-temperature Solid Oxide Fuel Cells

Solid oxide fuel cell （SOFC） has attracted much attention due to its highefficiency, environmentally friendly and fuel adaptability. However, its highoperating temperature limits the practical use of SOFC. In recent years, significantefforts have been devoted to decrease the operation temperature of SOFC to reducethe manufacturing costs and to prolong the lifetime of SOFC. One of the importantways is to develop new electrode materials, because the high electrode polarizationresistance at reduced temperature greatly decreases cell performance.Recently, RBaCo_4-xM_xO_7+δ（R=Y, Ca, In, Tb, Eu and M=Zn, Fe, Al） oxideshave drawn much attention in use as cathode materials for SOFC since they exhibitgood oxygen absorption/desorption properties and electrochemical properties.These cathode materials not only have the advantages of the traditional Co-basedcathode materials, but also have low thermal expansion coefficient （TEC） comparedwith other cobalt containing oxides. It has shown that substituting Y by Ca inYBaCo₄O₇materials is beneficial to the diffusion of oxygen ion in lattice andsubstituting Co by Zn improves the long-term phase stability at a high temperature.In this dissertation, nanosized Y_0.5Ca_0.5BaCo_4-xZnxO₇powders were prepared byEDTA–citric complexing method. The electrochemical property and the thermalexpansion coefficient of the sintered Y_0.5Ca_0.5BaCo_4-xZnxO₇materials wereinvestigated. X-ray diffraction analysis shows that nanosizedY_0.5Ca_0.5BaCo_4-xZnxO₇powders with RBaCo4O7-type structure can be obtained bycalcined the precursors at1000oC for2h. The electrical conductivity of theY_0.5Ca_0.5BaCo_4-xZnxO₇samples decreases with Zn content increases, but theactivation energy shows reverse change. The sample with x=1.0has an electricalconductivity of20.87at800oC and an activation energy of0.29eV at800oC. In therange of200-800oC, the Y_0.5Ca_0.5BaCo_4-xZnxO₇samples have TEC values of9.32×10-6K-1-11.56×10-6K-1. The polarization resistance of Y_0.5Ca_0.5BaCo_4-xZnxO₇cathode increase with Zn content increase, and the sample with x=1.0has apolarization resistance of0.629cm². At a same temperature, as the SDC contentincreases, the polarization resistance of Y_0.5Ca_0.5BaCo_4-xZnxO₇-ySDC composite cathode decreases and Y_0.5Ca_0.5BaCo₃ZnO₇-50SDC material has the lowestpolarization resistance （0.106cm²）.In recent years, researchers have paid much attention on doped ceria（DCO）materials due to their high oxygen ionic conductivity at intermediate temperature.Consequently, Ni/DCO anode materials have also been developed. It have beenindicated that the characteristics of the raw powders have an important effect on theperformance of the anode materials. In this work, NiO/Sm_0.2Ce_0.8O_1.9（NiO/SDC）powders were prepared by mechanically mixing method and gelcasting processwith NiO content50wt%and60wt.%, respectively. Ni/SDC anodes were obtainedby reducing the NiO/SDC powders in H2at800oC. Phase constitution andmorphology of the powders were characterized by X-ray diffraction and TEM.Microstructure of the Ni/SDC anode was observed by SEM. Compared withpowders prepared by the mechanically mixing method, powders synthesized by thegelcasting process have smaller particle size and uniform distribution. Properties ofthe Ni/SDC cermets change with sintering temperature of the NiO/SDC samples.Relative density of the NiO/SDC samples from four powders increases as thesintering temperature increases, but the porosity shows reverse change. For thesame method, NiO/SDC samples with60wt.%NiO have higher porosity than thesample with50wt.%NiO. Conductivity of the Ni/SDC cermets increases with thetesting temperature decreases. Electrical conductivity of the Ni/SDC cermets fromNiO/SDC powders prepared by mechanically mixing method is lower than Ni/SDCcermets from NiO/SDC powders prepared by gelcasting process. The sampleprepared by gelcasting process with60wt.%NiO reaches the highest value（432.83S cm-1） at600oC.To further investigate the feasibility of using Y_0.5Ca_0.5BaCo_4-xZnxO₇-SDCnovel composite cathode and the effects of preparation method of anode on theproperties of cell, single cells based on different Ni/SDC anode, SDCelectrolyte（⁴0μm） and Y_0.5Ca_0.5BaCo_4-xZnxO₇-SDC（³0μm） composite cathodewere constructed. Power density and microstructure of the cells were alsocharacterized. With increasing temperature, the power density increases and opencircuit voltage decreases. At700oC, the maximum power density and open circuitvoltage of the cell prepared from NiO/SDC powder by gelcasting process is320mWcm^-2and0.79V, which is higher than those of cell prepared from NiO/SDC powderby mechanically mixing method.