The Ionic Conduction In Doped Ceria-Carbonate Composite Electrolytes And The Performance Of Related Fuel Cells

Lowering the operating temperature is a key requirement for thecommercialization of the solid oxide fuel cell. A promising electrolyte material, oxide-inorganic salt composite, which has a high ionic conductivity in the temperature rangeof500-650oC, has been investigated extensively in recent years. This work focuseson the conducting mechanisms of O^2-and H⁺in the samarium doped ceria （SDC）-carbonate composite electrolytes with different components, and the effect factors ofthe fuel cell performance.In chapter2, the SDC powders are synthesized by the oxalate coprecipitationmethod and the carbonate coprecipitation method respectively, and mixed withNa₂CO₃or (Li_0.52Na_0.48)₂CO₃to form the composite electrolytes. The effects ofpreparing method, composition and mixing process on the structure and property ofthe composite electrolytes are examined by XRD, SEM and TG-DTA. The SDCretains its intrinsical characters in the composite, and possesses a high chemicalstability with the carbonate. The distribution of the two phases and the ACconductivity of the composite are highly influenced by the carbonate content and themixing method in the preparation process.In chapter3, the conductions of H⁺and O^2-in the SDC-(Li_0.52Na_0.48)₂CO₃composite electrolyte made by the oxalate coprecipitation route are investigated bythe H2and O₂electrochemical pumping methods, which eliminate the errors causedby the gas leakage through composite electrolyte membranes effectively. Thecomposite materials show efficient conductivities of both of the two ions at650oC.Both of the conductivities of H⁺and O^2-improved with the increase of the carbonatecontent, implying the molten carbonate phase provided a path for the conduction ofboth of the two ions. The effect of the operating temperature on the conductivities ofthe two ions is also studied.In chapter4, the direct-current four-probe method is employed to investigate theeffects of species and content of the carbonate and the distribution of the two phasesin the composite electrolyte on the conducitvities of O^2-and H⁺. Meanwhile, theconductivities of O^2-and H⁺in the SDC-(Li_0.52Na_0.48)₂CO₃made by the oxalatecoprecipitation method are calculated using the effective medium percolation theory,and then compared with experimental values. O^2-conducts through the SDC andcarbonate bulk phases, and higher migration energy is required at the interface. H⁺ conducts through the carbonate bulk phase and the interface between the two phases.The conductivity of H⁺at the interface is influenced by the ceramic phase. In thecomposite electrolyte made by solid-mixed method, which has a small interfacialregion, the conductivities of O^2-and H⁺could be estimated by the effective mediumpercolation theory.In chapter5, composite electrolyte-based single cells are fabricated by co-pressing method with NiO as the anode and LiNiO₂as the cathode. The performancesof the cells are measured and products during the cell operation are quantitativelyanalyzed. The carbonate phase suppresses the electronic conductivity of the compositeand reduces the densification of the electrolyte layer simultaneously. The ionicconductivity of the electrolyte is the major factor that influences the current densityand power density of the single cell. Meanwhile, the performance of the cell is alsoaffected by the thickness of the electrolyte layer and the reacting atmosphere. Theconduction of CO₃^2-dominates when CO₂is added in the oxidant, while theconductivities of O^2-and H⁺decrease at the same time, implying the interation ofthese ions.