J.R. Frade

Ceria-based materials for solid oxide fuel cells

This paper is focused on the comparative analysis of data on electronic and ionic conduction in gadolinia-doped ceria (CGO) ceramics as well as on the electrochemical properties of various oxide electrodes in contact with ceria-based solid electrolytes. Properties of electrode materials, having thermal expansion compatible with that of doped ceria, are briefly reviewed. At temperatures below 1000 K, Ce0.90Gd0.10O2−δ (CGO10) was found to possess a better stability at reduced oxygen pressures than Ce0.80Gd0.20O2−δ (CGO20). Incorporation of small amounts of praseodymium oxide into Ce0.80Gd0.20O2−δ leads to a slight improvement of the stability of CGO20 at intermediate temperatures, but the difference between electrolytic domain boundaries of the Pr-doped material and CGO10 is insignificant. Since interaction of ceria-based ceramis with electrode materials, such as lanthanum-strontium manganites, may result in the formation of low-conductive layers at the electrode/electrolyte interface, optimization of electrode fabrication conditions is needed. A good electrochemical activity in contact with CGO20 electrolyte was pointed out for electrodes of perovskite-type La0.8Sr0.2Fe0.8Co0.2O3−δ and LaFe0.5Ni0.5O3−δ, and LaCoO3−δ/La2Zr2O7 composites; surface modification of the electrode layers with praseodymium oxide results in considerable decrease of cathodic overpotentials. Using highly-dispersed ceria for the activation of SOFC anodes significantly improves the fuel cell performance.

A combustion synthesis method to obtain alternative cermet materials for SOFC anodes

Abstract Homogeneous mixtures of nanocrystalline powders of Ni–YSZ, (Ni,Co)–YSZ, (Ni,Fe)–YSZ, and (Ni,Cu)–YSZ were obtained by combustion synthesis from mixtures of molten nitrates and urea. Electrolyte/anode bilayers were then prepared by pressing a cermet layer onto a previously pressed yttria-stabilised zirconia (YSZ) electrolyte layer, at room temperatures, and then co-firing; this yields a porous cermet anode, and a dense YSZ layer with good adhesion. The anode layer remains sufficiently porous even after firing at temperatures up to 1450°C. This technique is thus suitable to obtain porous cermets for SOFC anodes with homogeneous distribution of the metallic and ceramic components. The effects of different parameters of this chemical preparation route, such as the furnace temperature, the amount of urea and the cooling rate, have been studied, together with the preparation of different compositions of cermets. These results have shown that the composition and the volume percentage of the metallic phases can be predicted. Easy control of the composition and a good reproducibility have been obtained. In terms of physical properties, nanometric particles with high specific surface area have been produced. The resulting electrodes co-pressed on the electrolyte have exhibited fine grains and porous microstructures, as well as a very good adherence to the electrolyte, even at high sintering temperature.

The effect of cobalt oxide sintering aid on electronic transport in Ce0.80Gd0.20O2−δ electrolyte

Abstract Additions of 2 mol% CoO 1.333 into gadolinia-doped ceria (CGO) solid electrolyte considerably improve sinterability and make it possible to obtain Ce 0.8 Gd 0.2 O 2− δ ceramics with 95–99% density at 1173–1373 K. The effect of cobalt oxide on the total electrical conductivity in air is negligible if the sintering is performed at 1173 K, although p-type electronic conduction measured at 900–1200 K increases with doping by 10–30 times. When increasing the sintering temperature up to 1773 K, grain growth in Co-containing CGO ceramics is accompanied with a decrease in both ionic and electron-hole transport. The oxygen ion transference numbers under oxygen/air gradient vary in the range 0.89–0.99. The n-type conductivity measured by the ion-blocking technique is lower for Co-containing materials than for undoped CGO, suggesting that the electrolytic domain can, to some extent, be enlarged by cobalt oxide additions. The relative role of both p- and n-type electronic contributions to the total conductivity of CGO increases with increasing temperature. The results show that Co-doped materials can still be used as solid electrolyte for intermediate-temperature electrochemical applications, when the operation temperature is 770–970 K.

n‐Type Conductivity in Gadolinia‐Doped Ceria

The ionic and n-type conductivities of ceria-based electrolytes doped with gadolinium (10 and 20 cation %) or gadolinium and praseodymium (2% Pr and 18% Gd) were studied by impedance spectroscopy in air, between 573 and 1,273 K, and by constant frequency (10 kHz) conductivity measurements as a function of the oxygen partial pressure (p{sub O{sub 2}}) formed between 1,073 and 1,273 K with air at the reversible electrode. From data obtained with different experimental techniques it was concluded that estimates for the electronic conductivity of all compositions were consistent and that all materials behaved in a rather similar manner. Estimated n-type conductivities suggest that Pr-doped materials have a slightly lower electronic conductivity and a larger electrolytic (and ionic) domain, but overall differences are quite small. The model behavior used in analyzing experimental results is discussed based on the existing knowledge of the defect chemistry of ceria-based electrolytes, and a range of working conditions is identified where the ionic conductivity can be assumed constant and the electronic conductivity proportional to p{sub O{sub 2}}{sup {minus}1/4}.

The stability and mixed conductivity in La and Fe doped SrTiO3 in the search for potential SOFC anode materials

Both physical properties and the level of mixed conduction obtained in La and Fe doped SrTiO3 are widely influenced by composition. In contrast to La free compositions, La containing compositions show high stability against reaction with yttria stabilised zirconia (YSZ) and a closely matching thermal expansion coefficient (∼1×10−5 K−1). Faradaic efficiency measurements for Sr0.97Ti0.6Fe0.4O3–δ and La0.4Sr0.5Ti0.6Fe0.4O3–δ show ionic transference numbers in air between 5 × 10−3 to 4 × 10−2, and 2 × 10−4 to 6 × 10−4 respectively, decreasing with decreasing temperature. The substitution of La for Sr is observed to deplete the level of both ionic and total conductivity obtained in air.

La2Zr2O7 formed at ceramic electrode/YSZ contacts

Strontium-doped LaCoO3 or LaMnO3 materials have been studied for use as cathodes for solid oxide fuel cells (SOFCs). This choice relies on the required properties and competitive cost. However, formation of reaction products under typical electrode-firing conditions may affect the performance of SOFCs. La2Zr2O7 was detected at YSZ/electrode interfaces. This reaction product was synthesized from powders and characterized to obtain a better understanding of its effects on cell performance. Its structural, thermal, and electrical properties are reported.

Applicability of the brick layer model to describe the grain boundary properties of strontium titanate ceramics

AbstractImpedance spectroscopy was used to evaluate the bulk and grain boundary resistances (R B and R gb ) of undoped strontium titanateceramicsinairandinN 2 .Thehighfrequencycontributionofthespectradoesnotdependsignificantlyontheaveragegrainsizeorgrainsize distributions, and was thus ascribed to the bulk behaviour. The grain boundary results obtained for several samples vary with theaverage grain size, and these e•ects nearly agree with predictions by a simple brick layer model. The deviations from this model mayincrease with decreasing temperature, and are somewhat greater in N 2 than in air. # 2000 Elsevier Science Ltd. All rights reserved. Keywords: Electrical resistance; Grain boundaries; Impedance spectroscopy; SrTiO 3 1. IntroductionThe barrier character of grain boundaries of strontiumtitanate ceramics play an important role on low tempera-ture applications such as multilayer capacitors. 1 Thisinterest thus prompted investigations to separate the bulkand grain boundary contributions by comparing impe-dance spectra obtained for single crystals, bicrystals, andceramic samples,

Combustion synthesis of iron-substituted strontium titanate perovskites

Perovskites in the system SrOTiO2Fe2O3 can be good candidates for electrochemical applications, and the properties of sintered, fully dense bodies depend on the Fe to Ti stoichiometry and the prevailing temperature and oxygen partial pressure. Conductivity studies normally require that the oxides with the desired stoichiometry and structure be synthesized. Solid state synthesis of perovskites requires reasonably high temperatures and full conversion is not guaranteed. Moreover, with this method, it is difficult to obtain a compositionally homogeneous product. The present work discusses a straightforward combustion synthesis technique to prepare submicron iron-substituted strontium titanate, the Sr(Fe0.5Ti0.5)O3 perovskite, using the corresponding metal salts-urea mixtures, at low temperature and short reaction times. The effect of the ratio oxidizerfuel in the redox mixture was investigated, namely to find out how that ratio affected the phase formation and the characteristics, such as morphology and grain size, of the powder produced.

An alternative representation of impedance spectra of ceramics

Abstract Suitable approximations are derived to extract the relevant parameters of the bulk, grain boundary, and electrode contributions of impedance spectra from Z′ vs. Z″/f plots. Corrections are proposed for cases where the difference between bulk and grain boundary relaxation frequencies and/or the difference between grain boundary and electrode relaxation frequencies is relatively small. In these cases, the Nyquist plot (Z″ vs. Z′) shows overlapped arcs, but inspection of alternative Z′ vs. Z″/f representations may reveal contributions with relatively small differences in relaxation frequencies. These plots and the corresponding derivative dZ′/d(Z″/f) may be useful to interpret cases showing a significant distribution of grain boundary relaxation frequencies.

Conductivity of CGO and CSO ceramics obtained from freeze-dried precursors

Ce0.8Gd0.2O1.9 (CGO) and Ce0.8Sm0.2O1.9 (CSO) have been prepared as polycrystalline materials using a freeze-dried precursor. This method yields amorphous nanometric powders. Crystallization of the fluorite phase occurred on heating at 600 8C or higher temperatures. The grain size of freeze-dried powders increases to about 100 nm after calcination at 800 8C, or about 200 nm after firing at 1000 8C. Freeze-dried powders were used to prepare dense ceramic disks by sintering at 1400 8C. Some disks were sintered at 1000 8C by adding small amounts of cobalt nitrate solution to assist the densification. The electrical conductivity results obtained for these gadolinia-doped ceria and samaria-doped ceria ceramics are similar to those obtained for CGO pellets obtained from commercial nanopowders (Rhodia). Though the bulk conductivity of CSO is probably higher than that of CGO, its grain boundary conductivity is inferior, and tends to control the overall behaviour, at least at relatively low temperatures. # 2003 Elsevier Science Ltd. All rights reserved.