Effect Of Aliovalent Doping On Hydration Behavior And Electrochemical Properties Of La₂Ce₂O₇ Fluorite-Type Oxide

As one of the most efficient and environmentally friendly energy conversion devices,solid oxide fuel cells(SOFCs)have received a lot of attention in recent years.The greatest problem in the commercialization of SOFC is its high operating temperature.Lowering the operating temperature of SOFCs will improve their stability and thereby promoting the commercialization of SOFCs.SOFCs use ceramic electrolytes that are oxide ion conductors or proton conductors.The use of proton conductor electrolyte is an effective means of lowering the operating temperature of SOFCs.50%La-doped CeO2(La2Ce2O7)has been shown to exhibit pure ionic conductivity with the contribution of proton conductivity at low temperatures.The alio valent doping of these materials enhances the concentration of oxygen vacancies and protons upon hydration.In this thesis,we chose La2Ce2O7 as an electrolyte material and investigated the effects of various doping strategies on its conduction behavior.Firstly,the effect of doping by partial substitution of La3-with divalent alkaline earth cations A2+(A=Ca,Sr,Ba)on the hydration behavior and electrical conductivity of La2Ce2O7 was studied.A single cubic fluorite structure was obtained for La2Ce2O7,(La1-xCax)2Ce2O7-δ(x=0-0.3),(La1-xSrx)2Ce2O7-δ(x=00.2),and(La1-xBax)2Ce2O7-δ(x=0-0.05).Substitution by A2+ enhanced the hydration ability,attributed to the increased concentration of hydratable oxygen vacancies and enhanced basicity of lattice oxide ions.The doping increased ionic conductivity slightly,while under reducing atmospheres the n-type electronic conductivity increased greatly.XPS analyses revealed that the binding energy of Ce 3d electrons decreased as the Ca doping level increased,indicating increased mobility of electrons.Secondly,Ce4+ is partially substituted with trivalent rare earth cations R3+(R=Y,Ho,Er,Tm,Yb).The effect of doping and partial pressure of water vapor on the conductivity of La2Ce2O7 was investigated.A single fluorite phase was obtained for La2(Ce1xRx)2O7-δ with the doping level(x)not higher than 0.2,regardless of the dopants.Doping rare earth elements R3+ increased the hydration ability and conductivity.The total conductivity increased with the increasing pH2O,attributed to the contribution of protons.Y3-with the larger ionic radii than other dopants imparted the sample with relatively higher proton concentration and proton conductivity which may be due to larger lattice space induced by larger dopants,facilitating the transport of ionic species.In summary,this thesis provides a systematic study and profound understanding of the doping behavior of alkaline earth and rare earth elements into La2Ce2O7,and the influence of such doping on the hydration behavior and electrochemical properties of La2Ce2O7,leading to the profound understanding of the conduction behavior of protons in the fluorite-type oxides.