Karla J. Moreno

Tailoring Disorder and Dimensionality: Strategies for Improved Solid Oxide Fuel Cell Electrolytes

Reducing the operation temperature of solid oxide fuel cells is a major challenge towards their widespread use for power generation. This has triggered an intense materials research effort involving the search for novel electrolytes with higher ionic conductivity near room temperature. Two main directions are being currently followed: the use of doping strategies for the synthesis of new bulk materials and the implementation of nanotechnology routes for the fabrication of artificial nanostructures with improved properties. In this paper, we review our recent work on solid oxide fuel cell electrolyte materials in these two directions, with special emphasis on the importance of disorder and reduced dimensionality in determining ion conductivity. Substitution of Ti for Zr in the A(2)Zr(2-) (y)Ti(y)O(7) (A = Y, Dy, and Gd) series, directly related to yttria stabilized zirconia (a common fuel cell electrolyte), allows controlling ion mobility over wide ranges. In the second scenario we describe the strong enhancement of the conductivity occurring at the interfaces of superlattices made by alternating strontium titanate and yttria stabilized zirconia ultrathin films. We conclude that cooperative effects in oxygen dynamics play a primary role in determining ion mobility of bulk and artificially nanolayered materials and should be considered in the design of new electrolytes with enhanced conductivity.

Dynamics of Mobile Oxygen Ions in Disordered Pyrochlore-Type Oxide-Ion Conductors

We investigate the effect of cation size in the dc activation energy needed for oxygen ion conductivity, Edc, in highly disordered pyrochlore-type ionic conductors A2B2O7. Twenty compositions of general formula Ln2Zr2-yTiyO7 (Ln = Y, Dy and Gd) and Gd2-yLayZr2O7, were prepared by mechanical milling and their electrical properties measured by using impedance spectroscopy at different temperatures. We also evaluate, by using Ngai’s Coupling Model, the effect of cation radii RA and RB, on the microscopic potential-energy barrier, Ea, that oxygen ions encounter when jumping into neighboring vacant sites. We find that for a fixed B-site cation radius RB, both activation energies decrease with increasing A-site cation size, RA, as a consequence of the increment in the unit cell volume. In contrast, and for a given RA size, the dc activation energy Edc of the Ln2Zr2-yTiyO7 series increases when the average RB size increases. The latter behavior is explained in terms of the enhanced interactions among mobile oxygen ions as the structural disorder increases when RB approaches RA.

Room-Temperature Synthesis and Electrical Properties of La, Nd and Gd Apatite-Type Silicates

We show in this paper the possibility of using mechanical milling to prepare apatitetype La, Nd and Gd silicates starting from stoichiometric mixtures of the constituent oxides. XRD patterns collected after grinding the starting mixtures for 9 hours contain only the characteristic reflections of the target materials with no other phase apparently present. Electrical conductivity data were successfully fitted to a Jonscher-type empirical expression with fractional exponent n included in the 0.35-0.75 range. Activation energies for oxygen migration were found to decrease as the size of the rare-earth cation increases. Therefore, the highest conductivity values were found for the apatite-type lanthanum silicate.

Structural Characterization and Ionic Conductivity of Metastable Gd2(Ti0.65Zr0.35)2O7 Powders Prepared by Mechanical Milling

We analyze in this work the influence of ordering on the oxygen ion dynamics in the ionic conductor Gd~(Tio.65Zr0.35)207, prepared by mechanical milling. As-prepared powder phase presents a metastable anion deficient fluorite -type of structure below 800°C becoming a disordered pyrochlore above this temperature. Such phase transformation implies a significant increase in the ionic conductivity of this material as a result of a systematic decrease in the activation energy for the dc conductivity, from 1.23 to 0.78 eV. Electrical conductivity relaxation is well described by the Kohlrausch-Williams-Watts stretched exponential function with the fractional exponent n decreasing with increasing sintering temperature (increasing ordering) as a result of decreasing ion-ion interactions in better ordered samples.