Cerium-terbium mixed oxides as potential materials for anodes in solid oxide fuel cells

Highly homogeneous (Ce,Tb) oxides are prepared by a microemulsion technique, and their structural and electronic state after high temperature calcination is examined with X-ray diffraction, high resolution transmission electron microscopy, X-ray photoelectron and absorption (XANES) spectroscopies and impedance spectroscopy measurements. Addition of Tb stabilizes significantly (in comparison to pure ceria) specific surface area and small particles sizes during high temperature calcination (up to 1100 °C); phase decomposition at these high temperatures, similar to that occurring when stabilization of ceria is carried out with Zr, does not occur, and the mixed oxide remains homogeneous throughout. Tb addition to ceria may thus be beneficial when used as a component of SOFC anodes. TEM data indicate reshaping of oxide particles and provide evidence of crystal superstructures after high temperature treatments, while XPS and XANES reveal an increase in the Tb4+/Tb3+ ratio (for a given pretreatment) with the Tb/Ce ratio; Ce seems to be less reducible to Ce3+ in the presence of Tb. Total electrical conductivity of CT samples under H2 is mediated by electron transport (involving probably only Ce) and is lower than in gadolinia-doped ceria (GCO); in air conductivity is higher than for GCO, particularly at low temperatures, and it is probable that a p-type transport mechanism predominates in this case.