Gadolinia-doped ceria and yttria stabilized zirconia interfaces: regarding their application for SOFC technology

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 Ce_0.43Zr_0.43Gd_0.10Y_0.04O_1.93 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.

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 La_0.85Sr_0.15MnO₃ (LSM) cathode material [1], [2] and lower polarization 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 CeO₂-based 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 the YSZ–GDC interface after sintering at 1400°C for 4 h in air, conditions usually used for the sintering of the electrolyte in SOFC technology. The atomic distributions of Zr, Y, Ce and Gd across the interface [Fig. 1(b)] suggest the involvement of enhanced solid state reaction and interdiffusion phenomena, resulting in the formation of an interaction zone between the materials, which is most extensive in the zirconia part of the sample. According to the results of previous work [8], [9], the composition of the reaction zone formed at the interface is governed by the attainment of a chemical equilibrium, which imposes the formation of a reaction product enriched in Gd with a nominal composition of Ce_0.37Zr_0.38Gd_0.18Y_0.07O_1.87, exhibiting at 800°C ionic conductivity lower by two orders of magnitudes than YSZ. Emerging porosity can also be recognized on the ceria side, resulting from the differences in diffusivity of the counter-diffusing cations (Kirkendahl effect).

The aim of the present work is to examine how solid state reaction and interdiffusion phenomena at the interfaces of YSZ in contact with both gadolinia doped ceria (GDC) and a ceria-doped YSZ–Ni cermet can be suppressed or even avoided. For the YSZ–GDC interface the contribution of a composition-graded microstructure is studied. The interaction between YSZ and the ceria-doped anode cermet is studied as a function of ceria content in the anode cermet for a Ni content of 40% vol. Microstructure and elemental distribution across the interfaces under consideration are examined with the aid of scanning electron microscopy and electronic probe microanalysis.