ICMCTF2012 Session A3-1/F8-1: Coatings for Fuel Cells & Batteries

Solid oxide fuel cells (SOFCs) are promising power generating system to achieve high energy efficiency and less CO₂ emission. Operating temperature has been lowered from 1273 K to 1023 K and the candidate materials for interconnects changes from ceramics to ferritic Cr₂O₃-forming alloys.

This paper focus on high temperature oxidation of metal interconnects and the effects of atmosphere and electric current on oxidation behavior of metal interconnects are discussed. Moreover, the improvement of electrical performance by the coating of La_0.85Sr_0.15CoO₃ will be also mentioned in this paper.

Interconnects are exposed to both fuel and air sides at operating temperature and electric current passes through interconnect during the operation. The oxidation of Fe-16Cr ferritic alloy was carried out at 1073 K in the single atmosphere.¹⁾ Cr₂O₃ scale formed on the alloy and Mn-Cr spinel was located on the top of the Cr₂O₃ scale. The oxidation in dual atmosphere without electric current exhibited the similar morphology and growth rate to those in the single atmosphere.²⁾ Parabolic rate constants in both sides were almost the same because the n-type defect structure may be predominant in both cases. The oxidation of Fe-25Cr ferritic alloy at 1073 K in dual atmosphere with electric current prevailed that the growth of Cr₂O₃ scale was accelerated at the anode side and suppressed at the cathode side.³⁾ The kinetic equation describing the growth rate of Cr₂O₃ scale was derived in terms of electric current, which enables to estimate both the scale thickness and areal specific resistivity (ASR). The control of oxidation kinetics at the anode side is essential to improve electrical performance of interconnect during the long-term operation.

The coating of La_0.85Sr_0.15CoO₃ onto alloy surface is effective way of improving electrical conductivity of interconnect in oxidizing atmosphere at high temperatures.^4,5) Solid state reaction between the coating and Cr₂O₃ scale gives the conductive oxide phases in the Sr-Cr-O ternary system. ⁶⁾

1) T. Brylewski, M. Nanko, T. Maruyama and K. Przybylski, Solid State Ionics, 143, 131-150(2001).

2) H. Kurokawa, K. Kawamura and T. Maruyama, Solid State Ionics, 168, 13-21(2004).

3) K. Kawamura, T. Nitobe, H. Kurokawa, M. Ueda and T. Maruyama, unpublished data.

4) T. Kadowaki, T. Shiomitsu, E. Matsuda, H. Nakagawa, H. Tsuneizumi and T. Maruyama, Solid State Ionics, 67, 65-69(1993).

5) T. Shiomitsu, T. Kadowaki, T. Ogawa and T. Maruyama, Proceeding of The Electrochemical Society, PV 95-1, 850-857(1995).

6) T. Maruyama, T. Inoue and K. Nagata, Proceeding of The Electrochemical Society, PV 95-1, 889-894(1995).

For the solid oxide fuel cell (SOFC) design, both ceramic and metallic materials are considered as construction materials for the interconnectors. Compared to the ceramics, metallic materials are easier to fabricate, and therefore cheaper, they are less brittle, easier to machine and they possess higher electrical and thermal conductivity than most ceramics. Based on the requirements such as oxidation resistance, low thermal expansion coefficient and electrical conductivity of surface oxide scales, high-Cr ferritic steels seem to be promising metallic interconnector materials. However, there are some problems that have to be overcome. The main problem for application of metallic interconnector materials in SOFCs is their reactivity with the anode and cathode side environments at the high operating temperatures. The resulting high temperature corrosion leads to e.g. dimensional changes, deterioration of mechanical properties and formation of oxide scales on the surface that often leads to low electrical conductivity. Another related problem is the chromium evaporation from the steel resulting in poisoning of the electrode surfaces in the fuel cell. One way to overcome these problems is to coat the steel with a thin (in the nm range) coating that improves the relevant properties of the steel. In this work Co- and reactive element (RE)-coatings have been deposited on FeCr steel (Sanergy HT). Furnace exposures are used to evaluate the effect of the coatings. The exposures are made in cathode-like environment (air containing 3% H₂O) during 168 h at 850°C.