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Research Papers

Anode Side Diffusion Barrier Coating for Solid Oxide Fuel Cells Interconnects

[+] Author and Article Information
J. Froitzheim

 Forschungszentrum Jülich, Institut für Energieforschung (IEF-2), 52425 Jülich, Germanyj.froitzheim@fz-juelich.de

L. Niewolak

 Forschungszentrum Jülich, Institut für Energieforschung (IEF-2), 52425 Jülich, Germanyl.niewolak@fz-juelich.de

M. Brandner

 Plansee SE, Innovation Services, 6600 Reutte, Austriamarco.brandner@plansee.com

L. Singheiser

 Forschungszentrum Jülich, Institut für Energieforschung (IEF-2), 52425 Jülich, Germanyl.singheiser@fz-juelich.de

W. J. Quadakkers

 Forschungszentrum Jülich, Institut für Energieforschung (IEF-2), 52425 Jülich, Germanyj.quadakkers@fz-juelich.de

J. Fuel Cell Sci. Technol 7(3), 031020 (Mar 17, 2010) (7 pages) doi:10.1115/1.3182731 History: Received May 20, 2008; Revised November 12, 2008; Published March 17, 2010; Online March 17, 2010

During the operation of solid oxide fuel cells (SOFCs) the Ni base anode and/or Ni-mesh is in direct contact with the ferritic steel interconnect or the metallic substrate. For assuring long-term stack operation a diffusion barrier layer with high electronic conductivity may be needed to impede interdiffusion between the various components. A pre-oxidation layer on the ferritic steel turned out to be not viable as a barrier layer since a Ni-layer tends to dissociate the oxide scale. Therefore the potential of ceria as a diffusion barrier layer for the anode side of the SOFC was estimated. The barrier properties of a ceria coating between the Ni and the ferritic steel Crofer 22 APU were tested for 1000 h in Ar4H22H2O at 800°C. Conductivity experiments were performed in the same atmosphere at different temperatures. After long-term exposures no indication of interdiffusion between Ni and ferritic steel could be detected, however, sputtered coatings on ferritic steel substrates showed significantly lower conductivities than bulk ceria samples because of void formation between the ceria and the oxide on the steel surface. The latter could be prevented by an intermediate copper layer, which resulted in overall area specific resistance values lower than 20mΩcm2 after 100 h exposure at 800°C.

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Copyright © 2010 by American Society of Mechanical Engineers
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Figures

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Figure 1

Electrical conductivity data at 800°C reported by different authors as a function of oxygen partial pressure

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Figure 2

The Ni-mesh in direct contact with the interconnect plate of an SOFC stack, which operated for 3000 h at 800°C. An austenite layer of approximately 70 μm width can be found in places where Ni and Crofer 22 APU were in direct contact. Significant amounts of Cr can be found in the Ni-mesh, which becomes oxidized at the surface and the grain boundaries (31).

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Figure 3

The cross section of a sample with a Ni-coating on bare Crofer 22 APU after 1000 h exposure at 800°C in Ar–4H2–2H2O. It can be seen that Cr and Mn can easily diffuse through the Ni-coating and form an oxide scale at its surface. Vice versa, Ni diffuses up to a depth of approximately 30 μm into the steel, resulting in an austenite formation.

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Figure 4

The cross section of a sample with a Ni-coating on pre-oxidized Crofer 22 APU. The sample was pre-oxidized for 100 h in Ar–4H2–2H2O at 800°C. After the Ni-coating was applied, the sample was exposed for 1000 h under the same conditions.

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Figure 5

The cross section of a sample exposed for 1000 h at 800°C to Ar–4H2–2H2O. A ceria coating of about 4 μm thickness was sputtered onto a Crofer 22 APU substrate. Subsequently a Ni-coating was applied to simulate the anode material. Beneath the ceria coating an oxide scale of approximately 1 μm thickness can be found beneath, which an internal oxidation zone with small spherical TiO2 particles is visible. The ceria coating itself shows a porous structure, which is columnar in the upper part. This type of microstructure is commonly observed for sputtered films (28).

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Figure 6

The linescan from the substrate (left) through the ceria coating into the Ni-coating (right) of the sample shown in Fig. 5

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Figure 7

The cross section of a pre-oxidized Crofer 22 APU substrate with a multilayer coating. On the pre-oxidized substrate a Cu layer was sputtered, which was followed by a CeO2 layer, and subsequently by a Ni layer. After 1000 h of exposure in Ar–4H2–2H2O at 800°C the internal oxidation zone of Ti-oxides and the two-layered oxide scale are clearly visible.

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Figure 8

The electrical conductivity (σ) for CeO2 plotted as ln(σT) as a function of temperature in an Ar–4H2–2H2O atmosphere. The open symbols show values reported by various authors (18,20,27). The filled symbols correspond to their own experimental data. Samples A and B are bulk ceria samples, whereas “sputter coating” refers to a 2.6 μm thick ceria coating on an Al2O3 substrate.

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Figure 9

The area specific resistance versus the inverse temperature of different ceria based coating systems after 100 h of measurement at 800°C. Each data point is based on four samples (five in the case of coating on bare Crofer 22 APU). For comparison the calculated resistance of a 3 μm thick ceria layer is included in the graph. The resistance was derived from the conductivity data of the sputter coating on an alumina substrate shown in Fig. 8.

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Figure 10

The cross section of a sample after conductivity measurement. A 3 μm ceria coating was applied on bare Crofer 22 APU. During the measurement at 800°C for 100 h an oxide scale is formed at the steel/coating interface. Numerous voids and pores can be seen above the oxide scale.

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Figure 11

The cross section of a sample (pre-oxidized Crofer 22 APU coated with CeO2) after the conductivity measurement. Below the ceria coating the oxide scale is present, the interface seems to have less voids than in Fig. 1. However, the coating itself shows high porosity, which is related to the manufacturing process.

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Figure 12

The cross section after conductivity measurement (100 h). Between the pre-oxidized (pre-oxidation 100 h) substrate and the CeO2 coating an approximately 3 μm thick Cu layer is present. The layer shows some oxide nodules and is in good contact with the ceria coating.

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