Ceria electrolytes preparation

Most of the literature focuses on the aspects of sinterability and microstructure, but limited data on the electrical properties is available. Tok [152] reported a conductivity of 18.3 x 10-3 Scm-1 at 600°C for Gd0 jCeo.gOj 95, and we measured a high conductivity of 22 x 10-3 scm-1 for Sm0 2Cc08O 9 at the same temperature. Their activation energies are relatively low—less than 0.7 eV. Although conductivity data reported for doped ceria prepared with carbonate precipitation is varied from different authors [153-155], the conductivity is generally high and the activation energy is usually low for ceria electrolytes fabricated with this method. 

Mizutania Y, Matsudaa H, Ishijia T, Furuya N, Takahashi K (2005) Improvement of electrochemical NOj sensor by use of carbon-fluorocaibon gas permeable electrode. Sens Actuators B 108 815-819 Mukundem E, Brosha E, Brown D, Garzon F (1999) Ceria-electrolyte-based mixed potential sensors for the detection of hydroceubons and ceubon monoxide. Electrochem Sohd State Lett 2 412-414 Neikayama S, Sadaoka Y (1994) Preparation of Na3ZrjSijPOjj-sodium aluminosilicate composite and its application as a solid-state electrochemiceil COj gas sensor. J Mater Chem 4(5) 663-668... 

Liu, Q.L., Khor, KA., Chan, SXI., and Chen, X.J. (2006) Anode-supported solid oxide fuel cell with yttria-stabilized zirconia/gadolinia-doped ceria bilayer electrolyte prepared by wet ceramic cosintering process. J. Power Sources,... 

Nguyen TL, Kobayashi K, Honda T, Iimura Y, Kato K, Neghisi, A et al. Preparation and evaluation of doped ceria interlayer on supported stabilized zirconia electrolyte SOFCs by wet ceramic processes. Solid State Ionics 2004 174 163-174. 

The redox properties of ceria-zirconia mixed oxides are interesting, because these materials find applications as electrolytes for solid oxide fuel cells, supports for catalysts for H2 production, and components in three-way automobile exhaust conversion catalysts. The group of Kaspar and Fornasiero (Montini et al., 2004, 2005) used TPR/TPO-Raman spectroscopy to identify the structural features of more easily reducible zirconia-ceria oxides and the best method for their preparation by suitable treatments. TPR/TPO experiments and Raman spectra recorded during redox cycles demonstrated that a pyrochlore-type cation ordering in Ce2Zr2Og facilitates low temperature reduction. 

Rare-earth nanomaterials find numerous applications as phosphors, catalysts, permanent magnets, fuel cell electrodes and electrolytes, hard alloys, and superconductors. Yan and coauthors focus on inorganic non-metallic rare-earth nanomaterials prepared using chemical synthesis routes, more specifically, prepared via various solution-based routes. Recent discoveries in s)mthesis and characterization of properties of rare-earth nanomaterials are systematically reviewed. The authors begin with ceria and other rare-earth oxides, and then move to oxysalts, halides, sulfides, and oxysulfides. In addition to comprehensive description of s)mthesis routes that lead to a variety of nanoforms of these interesting materials, the authors pay special attention to summarizing most important properties and their relationships to peculiar structural features of nanomaterials s)mthesized over the last 10-15 years. 

The most commonly used electrolyte materials in SOFCs are based on zirconia and ceria doped with a suitable cation, normally a rare earth (see Chapter 9). The properties that make these two materials attractive for use in fuel cells are discussed in Section 4.4.4, and it is sufficient to note that the most important feature is that they are good oxygen ion conductors. We will focus here on some recent investigations of these materials, with emphasis placed on their methods of preparation. 

It is considered that the bulk area specific resistance i o must be lower than l o = k/<7 = 0.15 Qcm, where L is the electrolyte thickness and a is its total conductivity, predominantly ionic [39]. At present, fabrication technology allows the preparation of reliable supported structures with film thicknesses in the range 10-15 pm consequently, the electrolyte ionic conductivity must be higher than 10 Scm. As shown in Figure 12.9, a few electrolytes (ceria-based oxides, stabihzed zirconias, and doped gallates) exceed this minimum ionic conductivity above 500 °C. 

An electrochemical cell system with ceria-based solid electrolyte coated wdth YSZ prepared by the spin coating method showed higher selectivity to acrylsildehyde than that with ceria-based solid electrol5rte alone. This may be due to the fact that a film of YSZ on the ceria-based solid electroljde to suppress the complete oxidation of propene. When the YSZ SDC disk was used as an electrolyte membrane, selectivity of the oxidation products did not depend on the thickness of YSZ. This indicates that the selective oxidation of propene occurred at the Au-YSZ-gas triple phase boundary by the oxygen species pumped electrochemicaUy through the ceria-based solid electrolyte and the YSZ. 

Lanthanum gallate is doped with divalent metal ions, for instance, strontium and magnesium Lai jSrjMgyGai j,03 usually, x and y have values between 1 and 0. Unlike the ceria-based electrolytes, this electrolyte can also be used at low oxygen partial pressures without the menace of developing an electronic conductivity. When in contact with cathodes of the LSM- or LSC type, some diffusion of manganese and cobalt from the cathode into the electrolyte is possible, but this has little effect on the fuel cell s performance. A certain defect of this electrolyte material is its complicated preparation procedure. 

As with zirconia, ohmic losses can be significantly reduced by decreasing the thickness of the electrolyte. Burninskas and co-workers used Raman spectroscopy to characterise GDC electrolytes formed from solid state annealing of sputtered Gd203/Ce02 multilayers [71], Solid solutions of Gd doped ceria were observed to form when multilayer structures were annealed at a temperature of 900 °C. Films had to be annealed for more than one hour and prepared with a bilayer period of less than 70 mn in order to produce a well-ordered and uniformly doped phase. 

---