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In this communication we elucidate a microstructural picture of proton conduction in nano-crystalline yttria-stabilized zirconia at low temperatures (Kim et al. Adv. Mater., 2008, 20, 556). Based on careful analysis of electrical impedance spectra obtained from samples with grain sizes of approximately 13 and approximately 100 nm under both wet and dry atmospheres over a wide range of temperatures (room temperature-500 degrees C), we were able to identify the pathway for proton conduction in this material. It was found that the grain boundaries in nano-crystalline yttria-stabilized zirconia are highly selective for ion transport, being conductive for proton transport but resistive for oxygen-ion transport.

The conductivity of dense ceramics of nanocrystalline yttria-stabilized zirconia (nano-YSZ), with average grain sizes ranging from 13 nm to 100 nm, was measured in wet and dry air as a function of temperature between 30 C and 500 C. Under wet conditions (p H 2 O ¼ 2.3 Â 10 À2 atm) the measured conductivity at low temperatures (<150 C) was found to increase strongly with decreasing grain size, displaying a highly non-linear dependence on grain size. This is interpreted as evidence of the protonic conductivity of grain boundaries increasing with decreasing grain size.

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The effect of crystallite size on cation coordination environments and oxygen vacancy ordering has been investigated in micro-and nanocrystalline Y-and Sc-doped ZrO 2 and CeO 2 by using high-resolution 89 Y and 45 Sc magic-angle-spinning nuclear magnetic resonance (MAS NMR) spectroscopy. Our results indicate that irrespective of crystallite size the vacancies are preferentially associated with the host cation (i.e., Zr) in Y-doped ZrO 2 while they display a preference for the dopant cation (i.e., Sc) in Sc-doped ZrO 2 . On the other hand, vacancies prefer to be associated with the dopant cation in both Y-and Sc-doped CeO 2 . However, the reduction of crystallite size to a few nanometers shows an unexpected and remarkable effect of increasing randomness in the vacancy distribution in all materials. Such an effect is hypothesized to result from a higher degree of short-range structural disorder in the cation coordination environments in nanocrystals compared to that in their microcrystalline counterparts that controls the energetics of vacancy ordering via a complex balance between electrostatic and strain energy terms. Finally, a clear connection is established between vacancy ordering, oxygen ion transport, and electrical conductivity in microcrystalline Y-doped CeO 2 and its possible implications on ionic transport in nanocrystalline materials are discussed.

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We report a correlation between oxygen ionic conductivity and the enthalpy of formation of trivalent-doped ceria from the component binary oxides observed at relatively low temperatures (150-275 degrees C). The bulk conductivities of La-doped ceria samples identical to those previously examined by thermochemical studies were measured as a function of La content for a direct comparison. The conductivity showed a maximum at a La concentration of 5 mol%, implying that the number of freely mobile oxygen vacancies reaches a maximum near that doping level in the temperature range of interest. The formation enthalpies previously reported by Chen and Navrotsky also show a maximum, indicating destabilization near that composition. Additional measurements show that this maximum is very pronounced and sharply peaked near that composition. These enthalpies suggest that the energetically favorable long-range interactions between the charged defects that trap the oxygen vacancies become dominant above 5 mol% doping in the CeO2-LaO1.5 solid solution. In addition, the conductivities measured from independently prepared Gd-doped ceria samples show a maximum at around 10 mol% doping below 450 degrees C as anticipated from a pronounced maximum in the formation enthalpies of the CeO2-GdO1.5 solid solution. These empirical findings confirm that the ionic conductivity of trivalent-doped ceria is strongly enough correlated with its formation enthalpy at relatively low temperatures so that information about the critical dopant concentration associated with the conductivity maximum may be gained from the formation enthalpies of the solid solutions, and vice versa. We have no direct information about this correlation at higher temperatures; both thermodynamics and conductivity maximum might change if the defect clusters dissociate to any significant extent.