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.

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We report on dispersive conductivity measurements in the oxygen ion conductor Gd 2 ͑Ti 2−y Zr y ͒O 7 . Increasing Zr content leads to higher concentration of oxygen vacancies and results in higher activation energies for long-range ion transport, whilst the microscopic energy barrier for single ion hopping remains constant. We find evidence that, besides oxygen binding energy, enhanced cooperativity in oxygen ion dynamics determines the activation energy for long-range diffusion. DOI: 10.1103/PhysRevB.71.132301 PACS number͑s͒: 66.30.Ϫh Oxygen ion conductors of fluorite structure, such as yttria stabilized zirconia ͑YSZ͒, are used commercially as electrolytes in solid oxide fuel cells. 1,2 Temperatures for applications of oxide conductors 3,4 are typically higher than 500°C due to low conductivity values at lower temperatures. A major challenge is to increase oxygen conductivity to allow room temperature applications. Long-range migration of oxygen ions takes place by thermally activated hopping to adjacent oxygen vacancies, which yields a dc conductivity of the form dc = ͑ ϱ / T͒exp͑−E dc / kT͒. Increasing conductivity, thus, would require increasing the prefactor ϱ and/or decreasing the activation energy E dc . However, increasing the number of charge carriers to increase ϱ leads to an undesired increase in E dc and eventually to lower conductivity values, 1 thus limiting the strategies to obtain high oxygen conductivity at lower temperatures. The origin of this behavior has remained not well understood. [1][2][3][4] Here we show that the increase of the activation energy E dc is determined by cooperative effects in oxygen dynamics.Among oxide-ion conductors, those of pyrochlore structure A 2 B 2 O 7 have been shown to be promising candidates to substitute materials currently used in fuel cells. [5][6][7] Gd 2 Ti 2−y Zr y O 7 pyrochlores are particularly interesting since the concentration of mobile oxygen vacancies can be increased by substitution of Zr for Ti, and oxygen ion conductivity shows the highest value found among materials with pyrochlore structure. For y Ϸ 1.8 the conductivity is comparable to that of YSZ ͑10 −2 S / cm at 700°C͒. 5 It has been recently suggested from molecular dynamics 8,9 and static lattice energy minimization simulations 10 that oxygen diffusion in Gd 2 Ti 2−y Zr y O 7 occurs by hopping from 48f to 48f sites. This result has been later confirmed by XPS measurements. 7The oxygen occupancy of 48f sites is 1 ͑or very close to 1͒ for Zr contents below y = 0.6, but decreases progressively as Zr content is further increased. 11 These vacancies in 48f sites are responsible for oxygen hopping and diffusion, and explain the increase of more than two orders of magnitude in dc conductivity at 600°C observed when increasing Zr content from Gd 2 Ti 1.4 Zr 0.6 O 7 to Gd 2 Zr 2 O 7 .5 On the other hand, the energy barrier for oxygen hopping from 48f to 48f sites has been previously calculated and shown to be much smaller than that observed in experimental conductivity data. 10,12 These fact...

In this work we evaluate the effect of cation size on the dc activation energy needed for oxygen ion migration, E dc , in highly disordered pyrochlore-type ionic conductors A 2 B 2 O 7 . Twenty six compositions with the general formula, Ln 2 Zr 2−y Ti y O 7 , Ln 1.7 Mg 0.3 Zr 2 O 7 ͑Ln= Y, Dy, and Gd͒, and Gd 2−y La y Zr 2 O 7 , were prepared by mechanical milling, and their electrical properties were measured by using impedance spectroscopy as a function of frequency and temperature. By using the coupling model we also examine the effect of cation radii R A and R B on the microscopic potential-energy barrier, E a , which oxygen ions encounter when hopping into neighboring vacant sites. We find that, for a fixed B-site-cation radius R B , both activation energies decrease with increasing A-site-cation size, R A , as a consequence of the increase in the unit-cell volume. In contrast, for a given R A size, the E dc of the Ln 2 Zr 2−y Ti y O 7 series increases when the average R B size increases. This behavior is associated with enhanced interactions among mobile oxygen ions as the structural disorder increases with R B .

We report on the influence of oxygen order in the oxygen-ion dynamics in the ionic conductor Gd 2 ͑Ti 0.65 Zr 0.35 ͒ 2 O 7 . The metastable Gd 2 ͑Ti 0.65 Zr 0.35 ͒ 2 O 7 powders prepared by mechanical milling present an anion-deficient fluorite type of structure, stable up to about 800°C. Thermal treatments at higher temperatures facilitate the gradual rearrangement of the cation and anion substructures and the relaxation of mechanochemically induced defects. Interestingly, metastable pyrochlores showing a very unusual cation distribution were observed during the thermally induced defect-recovery process. We have found that the ionic conductivity due to mobile oxygen ions increases significantly with increasing sintering temperature from 800 to 1500°C 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 stretched exponentials of the form ⌽͑t͒ = exp͓−͑t / ͒ 1−n ͔, and the fractional exponent n decreases systematically from n = 0.51 to 0.18 with increasing sintering temperature. These results are explained in terms of weaker ion-ion interactions in the increasingly ordered structure of the samples sintered at higher temperatures, and point to the importance of structural disorder in determining the dynamics of mobile oxygen ions.