Oxidation Kinetics, Electrical Conductivity And Surface Modification Of Metallic Interconnect Materials For Solid Oxide Fuel Cells

In order to apply metallic interconnect for solid oxide fuel cell (SOFC), the oxidation behaviors of Haynes 230 alloy and SUS 430 alloy at high temperature (650℃, 750℃and 850℃) for up to 500 hours have been investigated, simulating the environment of SOFC cathode atmosphere (air) and anode atmosphere (fuel gases: H₂+H₂O) respectively. The oxidation kinetics of the two alloys in different atmosphere and temperature has been investigated by the weight gain of oxidation. The oxidation behaviors of the samples such as phase constitution, phase evolvement, microstructure and thickness, and elements of the oxide scales which formed on the surface of the alloys were analyzed by the thin film X-ray diffraction (XRD), scanning electron microscope (SEM) and energy dissipation spectroscopy (EDS) attachment. Further more, a LaCoO3 conducting coating synthesized by Sol-Gel method was applied on SUS 430 alloy, whose effect on the SUS 430 alloy as metallic interconnect in SOFC was investigated. The structure and surface morphology of coating and oxide film were characterized by XRD and SEM. The experimental results show that multi-stages oxidation kinetics was observed about the two alloys at various oxidation conditions, and each stage follows Wagner's parabolic law with different parabolic oxidation rate constants. And a duplex oxide scale was found formed on the surface of alloys, corresponding to the various oxidation kinetics stages. Formations of oxides were mainly controlled by the diffusion of ions. Cr2O3 and MnCr2O4 spinel were the main oxidation products in all oxide scales. The oxide scale cracking and spallation from the substrate were observed when SUS 430 alloy oxidized in air at 850℃. FeO was also the main oxidation product with some dissolved Fe in the oxide scale when SUS 430 alloy oxidized in anode atmosphere at 850℃for up to 500 hours. The oxidation rate of SUS 430 metallic interconnect oxidized in air at 850℃between 200h and 500h by applied LaCoO3 conducting coating have been reduced approximately a magnitude. The LaCoO3 conducting coating was effective in protecting the metallic interconnect from oxidation. The area-specific resistance (ASR) of the oxide scales was measured by"four-probe"method. The results show a typical of semiconductor behavior, the resistance decrease gradually along with the rise of temperature. Haynes 230 have higher electric capability, but the ASR of SUS 430 alloy and Haynes 230 alloy oxidized in anode atmosphere are both higher than the ASR of the two alloys oxidized in air, especially oxidized in high temperature. By evaluating the resistance of the oxide which was applied the coating, showing that the LaCoO3 conducting coating was effective in improving the high temperature electric capability of SUS 430 alloy.