Thermodynamic Stability of Gadolinia-Doped Ceria Thin Film Electrolytes for Micro-Solid Oxide Fuel Cells Jennifer L.M. Rupp, w Anna Infortuna, and Ludwig J. Gauckler Nonmetallic Inorganic Materials, Department of Materials, Swiss Federal Institute of Technology, Zurich, Switzerland Next-generation micro-solid oxide fuel cells for portable devices require nanocrystalline thin-film electrolytes in order to allow fuel cell fabrication on chips at a low operation temperature and with high power outputs. In this study, nanocrystalline gadolin- ia-doped ceria (Ce 0.8 Gd 0.2 O 1.9x ) thin-film electrolytes are fab- ricated and their electrical conductivity and thermodynamic stability are evaluated with respect to microstructure. Nano- crystalline gadolinia-doped ceria thin-film material (Ce 0.8 Gd 0.2 O 1.9x ) exhibits a larger amount of defects due to strain in the film than state-of-the-art microcrystalline bulk material. This strain in the film decreases the ionic conductivity of this ionic O 2 conductor. The thermodynamic stability of a nanocrystal- line ceria solid solution with 65 nm grain size is reduced com- pared with microcrystalline material with 3–5 lm grain size. Nanocrystalline spray-pyrolyzed and PLD Ce 0.8 Gd 0.2 O 1.9x thin films with average grain sizes larger than 70 nm show predom- inantly ionic conductivity for temperatures lower than 7001C, which is high enough to be potentially used as electrolytes in low to intermediate-temperature micro-solid oxide fuel cells. I. Introduction G ADOLINIA-DOPED ceria thin films have attracted considerable interest as electrolyte materials for solid oxide fuel cells (SOFCs) operating at intermediate temperatures. 1,2 The major advantages of SOFC with Ce 0.8 Gd 0.2 O 1.9x (CGO) electrolyte thin films are (i) the four to five times higher ionic conductivity at intermediate operating temperatures compared with state-of- the-art yttria-stabilized zirconia (YSZ), 2–4 (ii) the possibility to combine low-cost ceramic thin film methods like spray pyrolysis with traditional silicon micro-machining technologies for the production of micro-solid oxide fuel cells, and (iii) the reduced ohmic losses resulting in higher fuel cell power outputs. 5 One of the major disadvantages of microcrystalline CGO compared with YSZ is its mixed ionic-electronic conductivity, which be- comes dominant especially for high fuel cell operating tempera- tures between 8001 and 10001C. In this temperature regime, electronic leakage through the fuel cell electrolyte leads to short circuiting and decreased power performance. However, if nano- crystalline CGO thin films are used, the ohmic resistance of the electrolyte, which scales linearly with its thickness, is drastically reduced and SOFC operation temperatures can be lowered to 5001–6001C. However, it is unclear whether these nanocrystal- line thin films are as thermodynamically stable as standard mi- crocrystalline-sintered CGO ceramics. Spray pyrolysis 5,6 and pulsed laser deposition (PLD) 7 thin- film synthesis techniques allow the production of nanocrystal- line SOFC electrolyte thin films with average grain sizes below 100 nm and film thicknesses between 100 and 500 nm. CGO spray-pyrolyzed and PLD thin films have dense microstructures after annealing for a short time at temperatures higher than 5001C and no further sintering steps are required. 8,9 It was re- ported in this recent study that the thin films exhibit a high amount of microstrain in the crystalline grains for average grain sizes below 100 nm. In electrical conductivity experiments meas- ured in air, nanocrystalline CGO thin films prepared by spin coating, spray pyrolysis, and PLD exhibited a reduced oxygen vacancy mobility and a higher migration enthalpy with decreas- ing grain size and increasing microstrain. 10,11 The latter studies give the first indications that the electrical properties and ther- modynamic stability might change in materials with high strain concentrations compared with strain-poor materials with micro- meter grain size. In the present investigation, we want to study the relation between ionic and electronic conductivity of CGO ceramics as a function of grain size, oxygen partial pressure, and temperature. The thermodynamic stability of CGO spray pyrolysis and PLD thin films with nano-sized grains is compared with that of a sintered bulk sample with grain sizes in the micrometer range. II. Electric Conductivity of Gadolinia-Doped Ceria Ceria-based materials are mixed ionic-electronic conductors transporting electrons via n-type small polaron hopping and oxygen ions via oxygen vacancies. 12,13 It depends on the doping level, temperature, and oxygen partial pressure of the surround- ing gas phase whether gadolinia-doped ceria ceramics are mainly ionic or electronic conductors. At low oxygen partial pressures and elevated temperatures, the ceria becomes partially reduced, whereby oxygen vacancies and electrons are formed in order to maintain charge neutrality 2Ce x Ce þ O x O $2Ce 0 Ce þ V  O þ 1 2 O 2 (1) Under these reducing conditions, ceria-based ceramics are pre- dominantly electronic conductors. Additions of small gadolinia contents below 30 mol% 14,15 create additional oxygen vacan- cies, which can increase the overall ionic conductivity Gd 2 O 3 $2Gd 0 Ce þ V  O þ 3O x O (2) In oxygen partial pressure and temperature-dependent elec- trical conductivity measurements, information on the dominant conductivity type, ionic (s i ) or electronic (s e ), and the thermo- dynamic stability is provided. In general, the total conductivity (s tot ) of gadolinia-doped ceria can be expressed by s tot ¼ s i þ s e (3) The ionic conductivity is given as s i ðT Þ¼ð2qÞ V  O   v i T ð Þ ¼ð2qÞ V  O   v 0i T exp  DH m k B T   (4) T. Gur—contributing editor Funding from BBW under contract number 03.0170-7 within the EU REAL–SOFC project is gratefully acknowledged w Author to whom correspondence should be addressed. e-mail: jennifer.rupp@ mat.ethz.ch Manuscript No. 22093. Received August 15, 2006; approved November 22, 2006. J ournal J. Am. Ceram. Soc., 90 [6] 1792–1797 (2007) DOI: 10.1111/j.1551-2916.2007.01531.x r 2007 The American Ceramic Society 1792