Novel CeO₂ Based Electrolyte For Low And Intermediate Temperature Solid Oxide Fuel Cells

Low and intermediate temperature solid oxide fuel cells (LITSOFCs) that operate at 400-800℃have received much interest in recent years in terms of greatly reduction of material and fabrication problems. This thesis focuses on a novel H⁺/O^2- co-conducting CeO₂ based electrolyte for LITSOFCs. The preparation, structure, electrical properties and possible mechanism are studied in this thesis.The nano SDC powders were prepared by co-precipitation and combustion method, respectively. The SDC powder made by co-precipitation is an agglomerate of spherical particles, while the SDC powder prepared by nitrate-citric combustion is an aggregation of reticulation. The low conductivity and reduction of Ce4+ reduces both OCV and maximum power density of the fuel cells based on SDC.The SDC-carbonate electrolytes composite SDC with (Li/Na)₂CO₃, (Li/K)₂CO₃ and (K/Na)₂CO₃, respectively. The carbonate is amorphous and coated on the surface of SDC particles to form a stable composite. The electrical impedance measurements were measured in both gaseous reducing and oxidizing environments and at different humidities as well as carbon dioxide, oxygen and hydrogen partial pressures. The results show that the conductivity of composite electrolytes is greatly improved by composition and sharply increased at the temperature close to melting point of carbonate. The conductance below melting point attributes to composite enhancement effect while the conductivity above melting point is due to the moving ability of ions in molten carbonate and superionic phase transition in the boundary phase between the SDC and carbonate.The composite electrolyte single fuel cells were fabricated and evaluated. All the cells with different composite electrolytes illustrate excellent performance. The cell based on electrolyte with 20% carbonate content shows the highest OCV and maximum power density which is 580 mW cm^-2 at 650℃.The ionic transport numbers under humidified oxidising atmosphere were measured by gas concentration cells. The result indicates that the oxygen ion predominant the conduction. The O^2- conduction in oxygen and H+ conduction in hydrogen was measured by dc polarization method. It shows that the oxygen conduction is lower than proton conduction. The oxygen ions transport in bulks and interfaces while protons transport through the interfaces by combining with CO₃^2- to form HCO₃^-. The conductivity is related to the continius phases of different materials and characters of oxides and carbonates.