6442 Phys. Chem. Chem. Phys., 2011, 13, 6442–6451 This journal is c the Owner Societies 2011 Cite this: Phys. Chem. Chem. Phys., 2011, 13, 6442–6451 Unusual decrease in conductivity upon hydration in acceptor doped, microcrystalline ceria William C. Chueh, a Chih-Kai Yang, a Carol M. Garland, a Wei Lai b and Sossina M. Haile* a Received 19th October 2010, Accepted 27th January 2011 DOI: 10.1039/c0cp02198a The impact of hydration on the transport properties of microcrystalline Sm 0.15 Ce 0.85 O 1.925 has been examined. Dense, polycrystalline samples were obtained by conventional ceramic processing and the grain boundary regions were found, by high resolution transmission electron microscopy, to be free of impurity phases. Impedance spectroscopy measurements were performed over the temperature range 250 to 650 1C under dry, H 2 O-saturated, and D 2 O-saturated synthetic air; and over the temperature range 575 to 650 1C under H 2 –H 2 O atmospheres. Under oxidizing conditions humidification by either H 2 O or D 2 O caused a substantial increase in the grain boundary resistivity, while leaving the bulk (or grain interior) properties unchanged. This unusual behavior, which was found to be both reversible and reproducible, is interpreted in terms of the space-charge model, which adequately explains all the features of the measured data. It is found that the space-charge potential increases by 5–7 mV under humidification, in turn, exacerbating oxygen vacancy depletion in the space-charge regions and leading to the observed reduction in grain boundary conductivity. It is proposed that the heightened space-charge potential reflects a change in the relative energetics of vacancy creation in the bulk and at the grain boundary interfaces as a result of water uptake into the grain boundary core. Negligible bulk water uptake is detected under both oxidizing and reducing conditions. Introduction Ceria-based oxides have been investigated extensively for application in a range of energy technologies. These include heterogeneous catalysis, where they serve as active supports, 1 fuel cells, where they can serve as either electrolyte (due to fast oxygen-ion conduction) 2,3 or anode electrocatalyst (due to mixed conduction), 4–6 and, more recently, as oxygen storage materials for solar thermochemical dissociation of water 7,8 and carbon dioxide 7 to fuels. In all of these processes, water vapor is present at significant pressures and hence may plausibly impact overall device operation. Proton solubility in pure and doped ceria and zirconia has been investigated by secondary ion mass spectrometry 9,10 and shown to be higher in polycrystalline samples than single crystals. 9 In addition, water uptake in the grain boundary regions of nanostructured doped ceria 11–15 and zirconia 14,16,17 has been shown to dominate ionic transport at temperatures below B150 1C. This intriguing result motivates a quantitative exploration of the limits of the influence of water on the properties, particularly grain boundary properties, of doped ceria with dopant concentrations and microstructure relevant to applications as solid electrolytes and electrodes. Furthermore, because changes in water vapor pressure are often employed to vary gas-phase oxygen chemical activity in the study of electrolyte and mixed conducting materials, it is essential to establish whether observed changes in properties are due solely to induced variations in oxygen partial pressure or possibly reflect bulk-phase interactions with water vapor. Background In acceptor-doped ceria examined under dry conditions ranging from oxidizing (i.e. air) to moderately reducing (i.e. p O 2 B 10 25 atm at 650 1C), the majority carriers are oxygen vacancies, generated as a consequence of the doping and requirements of charge neutrality in the bulk. Specifically, the bulk vacancy concentration is given by 2½V  O  dry 1 ¼½Sm 0 Ce  1 ð1Þ where Kro¨ ger-Vink notation has been employed, square brackets indicate concentration, the subscript N indicates the bulk (located far from any interface), and the superscript ‘dry’ indicates a system without an appreciable proton concentration. a Materials Science, California Institute of Technology, 1200 E. California Blvd. Pasadena, California 91125. E-mail: smhaile@caltech.edu b Department of Chemical Engineering and Materials Science, Michigan State University, 2527 Engineering Building, East Lansing, Michigan 48824 PCCP Dynamic Article Links www.rsc.org/pccp PAPER Downloaded by California Institute of Technology on 11 April 2011 Published on 08 March 2011 on http://pubs.rsc.org | doi:10.1039/C0CP02198A View Online