Gadolinium-doped ceria has demonstrated a high-ionic conductivity at moderate temperatures and is a potential candidate as electrolyte in solid oxide fuel cell (SOFC) devices. However, Ce ions can undergo a valency change from +IV to +III under reducing conditions. That valency change then leads to electronic (polaron) conduction and thus, degrades the ionic conduction. It has been demonstrated that the incorporation of electrically insulating particles will reduce the electronic conduction by an 'electron-trapping' mechanism (ideally) without affecting the ionic conductivity. This design is principally similar to the grain boundary design in zinc-oxide varistors.
In order to make this design effective, the insulating (electron trapping) particles have to be spaced close to each other. The spacing between 50 and 100 nm is assumed to be necessary for optimum performance. In order to not overload the entire composition with insulating particles (and thus reducing the ionic conductivity substantially due to volumetric dilution) the insulating grains have to be small (nanometer sized) and uniformly distributed throughout the matrix (cerium oxide). Moreover, the insulating grains should not dissolve or otherwise alter the cerium oxide matrix.
The present study now focuses on the precipitation of nanometer-sized alumina particles and coating these 'seed' particles with a 50 nm layer of gadolinium-doped cerium oxide. Small sizes for the alumina particles will prevent the overall composition from being overloaded with nonconducting particles and the coating process will enhance a very uniform distribution of the alumina particles in the cerium oxide matrix. Afterwards, the powders were calcined, compacted and (microwave) sintered. Characterization by SEM, TEM, XRD, density, and conductivity measurements are presented to evaluate properties of the proposed nano-composite electrolyte.