Investigations of Fe-Cr Ferritic Steels as Sofc Interconnect Material
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ABSTRACT Pure Fe-Cr alloys with a Cr-content between zero and 60 wt% have been investigated with respect to their thermal expansion from room temperature to 1100°C and their corrosion resistance in air and humidified hydrogen up to 1300'C. The thermal expansion coefficient of the ferritic Fe-Cr steels is close to that of the supports used in the manufacture of thin-electrolyte cells. The corrosion resistance in air and humidified hydrogen appears to be best for a Cr content around 20 wt%. In addition, a ceria coating is found to reduce significantly the scale growth. Preliminary results for a Fe78 Cr 22 foil indicates that an electrical contact resistance less than 3 mQcm 2 can be obtained between the alloy and a Ni/YSZ anode. INTRODUCTION The purpose of the interconnect in SOFC stacks is two-fold, namely to separate the fuel gas on the anode side and the oxidant (air) on the cathode side and to act as an electrical interconnector between cells [1]. There are several general demands to the interconnector in SOFC stacks [2] of which matching of the thermal expansion coefficient (TEC), low reactivity with other components, low diffusion rate of oxygen and hydrogen, and high electrical and thermal conductivity are among the most important. In the case of traditional self-supported yttria-stabilized zirconia (YSZ) electrolytes the thermal expansion coefficient of the interconnect is governed by the YSZ electrolyte, i.e. the interconnector should have an approximate TEC of 11.10.6 K-' (RT-10000 C). This puts a strong requirement on possible interconnect candidates [2]. Ceramic lanthanum-chromite (LC) based perovskites may fulfil most of the requirements to the interconnect material [1]. However, bending of the LC interconnects when exposed to a P0 2 gradient has caused problems in stack tests [3]. The dimensional instability is sought solved by use of various dopants, e.g. Zr, so far with an expense on the electrical conductivity and the thermal expansion coefficient [4]. Metallic alloys have been investigated for some years due to their better workability, higher mechanical strength, electrical and thermal conductivity, and potentially lower cost compare to LC materials [2,5]. For use with thick YSZ electrolytes, Cr-rich alloys have proven to be the best candidates [2]. The problems arising with metallic interconnects are especially the oxidation of the alloy and the evaporation of Cr-O-OH species [6]. The best candidate, at present, is probably the Cr-5Fe-1Y 20 3 alloy from Metallwerk Plansee [2]. However, the cost of this alloy is high due to a laborious fabrication technique [7]. An LC coating is being applied [8] for reducing the evaporation of Cr at high operating temperatures (above approximately 900'C). The apparent need for reduction of stack and system costs for commercialisation of SOFC points towards the use of ferritic steel. The mechanical and corrosion properties of ferritic steels will probably demand a lowering of the operation temperature. This has invoked the search for the development of thin electro
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