D. Pérez-Coll

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.

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.

Improved Conductivity of Ce1 − x Sm x O2 − δ Ceramics with Submicrometer Grain Sizes

Freeze-drying was used to prepare nanocrystalline powders of Ce 1-x Sm x O 2-δ , x = 0.05-0.3, with high-purity and crystallite sizes in the range 10-15 nm. These powders were used to obtain ceramic samples with 4-7 μm in average diameter by firing at 1873 K, without sintering additive, and < 1 μm for samples sintered at 1423 K, with previous addition of cobalt nitrate solution to the freeze-dried powders. Impedance spectroscopy was used to deconvolute components of the spectra related to grain interiors and grain boundaries. For samples with grain sizes in the range 4-7 μm, the bulk conductivity increases with decreasing content of Sm up to 10%, whereas the grain boundary conductivity shows the opposite trend. However, samples with relatively low content of Sm and submicrometer grain sizes, obtained with addition of Co, retain the highest conductivity for both the bulk and grain boundary conductivities. The differences in grain boundary conductivities between samples with small and large grain sizes were about 1 order of magnitude for 20% Sm and 2 orders of magnitude for 5% Sm and 10% Sm. Differences in grain boundary behavior were interpreted on the basis of a space-charge layer adjacent to the boundary core. The estimated values of grain boundary thickness were in the order of 1 nm for samples prepared with and without addition of cobalt nitrate.

On the use of multichannel data acquisition of impedance spectra

Impedance spectroscopy is a powerful technique for electrical and electrochemical characterisation of ionic conductors and other electroceramics. Thus, one might tempted to use multichannel data acquisition to allow the utilisation of expensive LCR meters for simultaneous measurements. This work shows results for YSZ and strontium titanate ceramics, obtained with a multi-channel data acquisition system, to demonstrate the limitations of this approach and the applicability of some corrections. Impedance spectra are affected under multi-channel conditions, mainly in the high frequency contributions, and this is approximately described by a stray capacitance. The bulk contribution of the spectra is most affected. Contributions of internal interfaces (e.g. grain boundaries) are relatively well characterised, mainly after the proposed corrections. The characterisation of electrode processes is not affected.

Structure and Electrical-Transport Relations in Ba(Zr,Pr)O3−δ Perovskites

Members of the perovskite solid solution BaZr1-xPrxO3-δ (0.2 ≤ x ≤ 0.8) with potential high-temperature electrochemical applications were synthesized via mechanical activation and high-temperature annealing at 1250 °C. Structural properties were examined by Rietveld analysis of neutron powder diffraction and Raman spectroscopy at room temperature, indicating rhombohedral symmetry (space group R3̅c) for members x = 0.2 and 0.4 and orthorhombic symmetry (Imma) for x = 0.6 and 0.8. The sequence of phase transitions for the complete solid solution from BaZrO3 to BaPrO3 is Pm3̅m → R3̅c → Imma → Pnma. The structural data indicate that Pr principally exists as Pr4+ on the B site and that oxygen content increases with higher Pr content. Electrical-conductivity measurements in the temperature range of 250-900 °C in dry and humidified (pH2O ≈ 0.03 atm) N2 and O2 atmospheres revealed an increase of total conductivity by over 2 orders of magnitude in dry conditions from x = 0.2 to x = 0.8 (σ ≈ 0.08 S cm-1 at 920 °C in dry O2 for x = 0.8). The conductivity for Pr contents x > 0.2 is attributable to positively charged electronic carriers, whereas for x = 0.2 transport in dry conditions is n-type. The change in conduction mechanism with composition is proposed to arise from the compensation regime for minor amounts of BaO loss changing from predominantly partitioning of Pr on the A site to vacancy formation with increasing Pr content. Conductivity is lower in wet conditions for x > 0.2 indicating that the positive defects are, to a large extent, charge compensated by less mobile protonic species. In contrast, the transport mechanism of the Zr-rich composition (x = 0.2), with much lower electronic conductivity, is essentially independent of moisture content.

Reducibility of Ceria-Based Materials Exposed to Fuels and under Fuel/Air Gradients

Ceria-based materials have been extensively studied in recent years for their potential use as solid electrolytes for alternative solid oxide fuel cells (SOFC) concepts, with emphasis on intermediate temperature SOFCs (Steele, 2000), single chamber SOFC (Yano et al., 2007), etc. However, the decrease in the oxygen partial pressure exerted by the presence of the fuel increases the non-stoichiometry accompanied by the reduction of Ce4+ to Ce3+. In these cases, reducibility of ceria-based solid electrolytes is a critical limitation, mainly because this implies onset of electronic conductivity (Blumenthal & Hofmaier, 1974; Blumenthal & Sharma, 1975; Tuller & Nowick, 1977; Navarro et al., 1997) and corresponding risks of internal short circuiting and decrease in cell voltage. The reducibility of cerias is affected by trivalent additives and their contents (Wang et al. 1997; Wang et al. 1998; Kobayashi et al., 1999), altering the mixed transport properties under low values of oxygen pressure according to the defect chemistry models. However some discrepancies are found in literature regarding the level of oxygen losses as well as the electronic conductivity (Mogensen et al., 2000; Zachau-Christiansen et al., 1996). On the other hand, ceria and related materials are also promising catalysts, including use as SOFC anode components for hydrocarbon conversion (McIntosh & Gorte, 2004; Tsipis et al., 2004). Performance in these prospective applications is likely to be promoted by the redox behaviour of cerias. Thus, one has studied the onset of electronic conductivity of the most promising ceria-based materials Ce1-xGdxO2-0.5x-Δδ (CGO) and Ce1-xSmxO2-0.5x-Δδ (CSO), including its dependence on temperature and composition (Perez-Coll et al., 2004; Abrantes et al., 2003). The impact of low temperature sintering with suitable additives on the onset of n-type contribution under reducing conditions has also been studied (Fagg et al., 2003). In this chapter the dependence on oxygen partial is revised in detail, and corresponding effects imposed by fuels such as hydrogen or methane are examined by taking into account thermodynamic correlations between oxygen partial pressure and fuel conditions. This is extended for fuel/electrolyte/air gradients, as expected for SOFC operation.