Reduction and re-oxidation of anodes for solid oxide fuel cells

doi:10.1016/j.ssi.2005.06.014

Solid State Ionics

Volume 176, Issues 29–30, September 2005, Pages 2201-2203

https://doi.org/10.1016/j.ssi.2005.06.014 Get rights and content

Abstract

The influence of reduction and re-oxidation cycles on microstructure and residual stress of anode supported planar solid oxide fuel cells (SOFCs) has been studied. Shrinkage caused by the reduction of NiO to Ni is not reversible. Microstructural scanning electron microscope observations reveal that after re-oxidation, the NiO has a higher porosity and requires a larger volume than in the initial, oxidised state. As a consequence of the anode expansion, tensile stresses develop in the electrolyte, ultimately causing fracture.

Introduction

In the layered composite of a planar solid oxide fuel cells (SOFC), the materials are rigidly bonded. Thus, differences in materials properties result in residual stresses [1]. In addition, stresses emanate from the constrained fixation of the cells in a SOFC stack [1]. Hence, mechanical integrity is an important aspect for reliable operation and safe thermal cycling of the components. The mechanical properties of SOFC materials, cells and interfaces within cells and stacks have been characterised in the past [1], [2]. However, the behaviour of the cells in the case of re-oxidation at high temperature that might occur due to a lack of fuel gas, e.g., by sealant damage, is an aspect of concern which has only received limited attention to date [3], [4], [5]. It has been suggested that under these circumstances fracture occurs due to the rapid oxidation and volume expansion of the Ni particles in the YSZ matrix [3]. Furthermore, it has been reported that reduction and oxidation cycles resulted in an increase of the polarisation resistance which was attributed to the formation of cracks within the anode [6]. Macroscopically, the volume expansion during re-oxidation causes a volume increase of an anode as compared to the initial oxidised situation [6]. In this report, results are given on the microstructure and related stress changes of anode and electrolyte to gain further insight into the reduction and re-oxidation of planar SOFCs.

Section snippets

Experimental

The re-oxidation tests were carried out with anode supported SOFC half-cells supplied by Research Centre Juelich (FZJ). The half-cells (total thickness ∼ 1.5 mm) had a porous NiO–YSZ–Cermet as anode current collector (ACC) which supported an anode functional layer (AFL) and a YSZ electrolyte film (of ∼5 μm and ∼10 μm thickness, respectively) [7], [8]. The weight ratio of NiO to YSZ in the ACC and AFL is 55:45. This results in 40% volume of Ni in the reduced state. Starting from as-received larger

Results and discussion

Micrographs were taken from the very same location of a cell in the oxidised, reduced and re-oxidised state. Fig. 1a–c shows an example. Fig. 1 permits a clear distinction of Ni/NiO particles (dark in Fig. 1a and c) and YSZ matrix. The reduction results in material shrinkage when the NiO particles are converted to Ni (Fig. 1a and b). After re-oxidation, the morphology of the NiO has changed compared to the initial oxidised microstructure (Fig. 1a and c and d). Before reduction the porosity of

Conclusions

It has been shown that the re-oxidation is accompanied by an expansion of the NiO as compared to the initial NiO before reduction. The expansion strain in the anode results in a tensile strain in the electrolyte and a respective stress that in our investigation led to fracture. To obtain re-oxidation stable cells, fracture of the electrolyte has to be inhibited. From the thermoelastic point of view, an increase in the porosity of the anode should be beneficial since the strain during the

Acknowledgements

The work was financially supported by the European Community within the project “Component Reliability in Solid Oxide Fuel Cell Systems for Commercial Operation (CORE - SOFC)”. The scientific support of L. Singheiser is kindly acknowledged.

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