Sofc

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  • Topic: Electrochemistry, Partial pressure, Yttria-stabilized zirconia
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Acta mater. 48 (2000) 4709–4714 www.elsevier.com/locate/actamat

GADOLINIA-DOPED CERIA AND YTTRIA STABILIZED ZIRCONIA INTERFACES: REGARDING THEIR APPLICATION FOR SOFC TECHNOLOGY A. TSOGA1*, A. GUPTA1, A. NAOUMIDIS1 and P. NIKOLOPOULOS2
Institut fur Werkstoffe und Verfahren der Energietechnik (IWV1), Forschungszentrum Julich, D-52425 ¨ ¨ Julich, Germany and 2Chemical Engineering Department, University of Patras, GR 265 00 Patras, Greece ¨ 1

Abstract—For solid oxide fuel cells (SOFCs) operating at intermediate temperatures the adjacency of the state-of-the-art yttria-stabilized zirconia (YSZ) electrolyte with ceria-based materials to both anodic and cathodic sides is regarded as crucial for the effectiveness of the cell. Solid-state reaction, however, and interdiffusion phenomena between YSZ and ceria-based materials can cause degradation of the electrolyte. When a gadolinia-doped-ceria (GDC) layer is used to protect YSZ against interaction with Co-containing cathodes, an unfavorable solid state reaction at the YSZ–GDC interface can be efficiently suppressed when a thin ( 1 µm thick) interlayer with nominal composition of Ce0.43Zr0.43Gd0.10Y0.04O1.93 is incorporated at the interface. When ceria is to be employed at the electrolyte–anode interface to reduce polarization losses, use of a ceria–40% vol Ni cermet is recommended, since suppression of the reactivity between YSZ and ceria can also be achieved in the presence of Ni. © 2000 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved. Keywords: Solid oxide fuel cells; Interface; Diffusion; Microstructure

1. INTRODUCTION

Reduction of the operation temperature of solid oxide fuel cells (SOFCs) from 900–1000°C to 700–800°C is of great importance because it means both a prolonged stack lifetime and a cost reduction, since the use of low-cost metallic components as separator materials is then possible. However, for the operation of SOFCs at intermediate temperature to be technically feasible two parameters should be considered: the development of high-performance electrodes, because the electrode reaction rates decrease at such temperatures, and minimization of cell resistance. The latter means minimization of both the ohmic losses inside the electrolyte and the polarization losses at the electrolyte–electrode interfaces. It is known that La(Sr)CoO3-based perovskites (LSC), when sputtered on the yttrium-stabilized zirconia (YSZ) electrolyte, exhibit higher cathodic performance than state-of-the-art La0.85Sr0.15MnO3 (LSM) cathode material [1, 2] and lower polarization

* To whom all correspondence should be addressed at: CERECO, 72nd km of the national road, Athens-Lamia, PO Box 146, 34 100 Chalkida, Greece. Tel.: 30-262-71226; Fax: 30-262-71461. E-mail address: a.tsoga@chemeng.upatras.gr (A. Tsoga)

values also at intermediate temperatures. However, LSC tends to react with YSZ, forming isolating reaction products such as La2Zr2O7 or SrZrO3 [3, 4]. The only materials chemically compatible with LSC are those based on CeO2 [5] which, although they possess a higher ionic conductivity than YSZ, cannot be used as electrolytes because under a fuel gas atmosphere they are prone to develop electronic conductivity, resulting from the reduction of Ce4 to Ce3 . As a solution to this problem, consideration is being given to the use of a CeO2-based interlayer between a thin YSZ electrolyte and LSC electrode. From the anodic side, when CeO2-based materials are employed at the electrolyte–anode interface they significantly decrease polarization losses and enhance the performance of the cell [6, 7]. From these findings it is obvious how beneficial the presence of CeO2based materials is on both sides of the YSZ electrolyte. However, the chemical compatibility between YSZ and CeO2-based materials is not without problems, since the two materials react and diffuse into each other during the sintering process at 1200°C [8, 9]. Figure 1(a) shows the microstructure of...
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