| Cerium oxide, CeO2, has been considered for use as an electrolyte in solid oxide fuel cells (SOFC). Ceria demonstrates high ionic conductivity at moderate temperatures. Cells operating at moderate temperature could employ a wider selection of materials within the cell, and allow more convenient and efficient operation. Under reducing conditions, however, cerium ions change valence from 4+ to 3+. This leads to "electron hopping," (i.e. polaron conduction) and electronic current within the electrolyte degrades ionic conduction.; It has been shown previously that mixing insulating phases with ionic conductors may increase the conduction. This investigation studies effects of alumina-based phases on electronic and ionic conduction in ceria-based materials. Interfacial effects within such composites create "electron traps" at the interface of the ceria and insulating phases, which limits electronic conduction.; Three classes of ceria-based materials were studied: undoped ceria, sol-gel-derived ceria of composition Ca0.12Ce0.88O 2-x, and "rare earth oxide" containing 60 mol% Ce and 35 mol% La cations. Each class of ceria was mixed with various quantities of alumina, to explore electron trapping effects. Undoped ceria demonstrates the clearest relation between insulator content and changes in electronic and ionic conduction. Impedance tests showed that at 1 MHz, a composite including 40 wt% alumina demonstrated a factor of 18 higher resistivity than ceria without insulating phase. DC conductivity for the same materials showed resistivity of the composite to be 6-17 times as high as for ceria. Under methane, resistivity of 20 wt% alumina composite was 4 times higher at 800°C-900°C than for ceria, and 40 wt% composite demonstrated an increase in resistivity by a factor of 12-13. XRD and EDS analysis demonstrated that the phases did not react in these sintered samples; thus changes in conductivity are not attributable to newly formed phases. Observed increase in electronic resistivity may be linked to electron traps at material interfaces. Ionic transference number of the composite increases under low oxygen partial pressure as compared to ceria without insulating phase added. Composites of ceria and alumina showed improved ionic conduction characteristics under reducing conditions at moderate temperatures (500-600°C). |
