Mixed ionic-electronic conduction in a number of fluorite and pyrochlore compounds

Abstract

While all compounds exhibit mixed ionic and electronic conduction to some degree, the relative ratio of these partial conductivities becomes particularly important when considering solids for use in solid-state electrochemical devices as electrolytes or electrodes. In this review, we focus on several oxide systems with fluorite and pyrochlore structures which hold particular promise in solid oxide fuel cell systems. Solid electrolytes should exhibit high ionic and low electronic conductivities to minimize internal cell resistance and short-circuiting of the cell EMF, respectively. The oxygen ion conductivity is most commonly optimized by doping as in stabilized zirconia. More recently, interest has focused on systems which exhibit high intrisic disorder. Current work on the Gd 2(Zr x Ti 1− x ) 2O 7 (GZT) and related systems, which allow control of the relative degree of intrinsic and extrinsic disorder by control of composition, is summarized. Mixed conducting electrodes are believed to catalyze the redox reactions at the gas-solid interface most effectively. Methods for introducing enhanced electronic conductivity into fluorite and pyrochlore oxides including solid-solution formation with variable valent ions, and/or shallow dopants are discussed. Examples are drawn from work on U: CeO 2, Ce:ThO 2 and GZT. Defects models, which possess improved predictive abilities regarding the doping temperature, and atmosphere dependence of the ionic and electronic transference numbers, are presented.