Measurement and modelling of the defect chemistry and transport properties of ceramic oxide mixed ionic and electronic conductors

The mixed ionic and electronic conducting fluorite and perovskite materials examined in this thesis are all oxide ion conducting materials. The defect chemistry and transport properties of a number of these materials are measured using: 1) a measurement technique using an oxygen pump and an electrolyte probe; 2) coulometric titration; 3) standard techniques such as thermogravimetry, conductivity relaxation and X-ray Absorption Near Edge Structure (XANES). Models of the defect chemistry and oxygen permeation phenomena are used to interpret the measurements. The models are further used to predict the performance of oxygen separation membranes and fuel cells with mixed conducting materials. Electrolyte probe measurements revealed that the surface reaction rate constants of oxygen leaving a membrane (reduction) and oxygen entering the membrane (oxidation) of the investigated MIECs are the same. Both the oxide site reduced diffusion coefficient, analogous to the mobility of a single ion vacancy, and the oxide site vacancy concentration were dependent on the chemical potential of oxygen and the temperature. Consequently, the dependencies of both parameters must be measured to predict the bulk transport. Slow transients in the MIEC samples altered the transport parameters of the mixed conductors over the course of days or weeks, especially when the membranes were partially reduced. According to model studies gadolinia doped ceria membranes are candidates for syngas production membranes. In many conditions gadolinia doped ceria furthermore performs better than conventional electrolyte membranes based on zirconia, especially when high power densities are required.