Inhibition of diffusion between metallic substrates and Ni-YSZ anodes during sintering

Abstract

The BMW Group is engaged in the development of a SOFC system for mobile applications (auxiliary power unit - APU). Since this application demands low weight and high durability, metal supported-cells are preferred instead of sophisticated SOFCs using a thick anode support. Therefore tape-cast and pre-fired ferritic steel foils are coated with a Ni/YSZ-anode layer and finally are fired under conditions compatible to anode and substrate requirements. Since the nickel-containing anode is in direct contact with the high-chromium ferritic steel "Crofer 22 APU", diffusion of nickel, iron and chromium takes place under standard conditions, coupled with degradation of the anode. To reduce or even avoid diffusion of these elements during production of the SOFC, reduction of the sintering temperature and experiments with different diffusion barriers appear to be reasonable. Basic experiments were carried out, to study diffusion processes during production of a metal-substrate supported Ni/YSZ-anode, which was done in two different ways. One Crofer 22 APU sheet was screen-printed with metallic nickel, another one with NiO. To investigate if oxide scales act as effective diffusion-barriers, pre-oxidized Crofer 22 APU sheets were coated with nickel and NiO respectively. Samples with a metallic nickel layer were sintered at 1100°C for 3 h in a purified argon atmosphere, samples with a NiO layer were sintered at 950°C for 3 h in air. It could be demonstrated that pre-oxidation is an easy way to reduce interaction between the substrate and the anode layer.