Modeling And Diagnosis Of AC Impedance For Metal-Supported SOFCs

Metal-supported solid oxide fuel cells (SOFCs) are recognized to have a high potential for use in mobile application, such as vehicle and naval vessel. However, in order to make a breakthrough in metal-supported SOFC technologies, two major challenges, high cost and low reliability, have to be overcome in the path towards commercialization. In the continuing effort to address these challenges, fuel cell testing for performance optimization and failure mode diagnosis is the necessary step. Among the major fuel cell diagnostic tools, AC impedance, also called electrochmical impedance spectroscopy (EIS), has been widely used for performance evaluation and degradation diagnosis. At present work, AC impedance and its model-based diagnosis method, combined with current-voltage-power curve, energy-dispersive X-ray spectroscopy (EDX), scanning electron microscope (SEM), X-ray Diffractometer (XRD), and fast thermal cycle techniques, was applied to investigate polarization characteristics, to diagonize the degradation mechanism, and to evaluate the dynamic properties of metal-supported SOFCs with SDC as electrolyte. These research contents are summarized as follows:The studying objects SS430/ NiO-SDC/ScSz/SDC/SSCo-SDC and Hastelloy X/NiO-SDC/SDC/SSCo-SDC, fabricated by pluse laser deposition and suspension plasm spray respectively, were detailedly stated firstly. The discussions concentrate on cell structure, key materials, fabrication procedures, and characterization methods adopted at present work. The nature of AC impedance diagnosis was addressed as well. In fuel cell field, the complete theoretical framework for AC impedance diagnosis was firstly presented from the viewpoint of pattern recognition and system diagnosis, and the framework mainly consists of six states and five actions. In addition, SOFC impedance models were analyzed and compared completely and systematically in terms of the application properties.Based on AC impedance equivalent circuit model for SOFC, polarization characteristics of metal-supported SOFCs were investigated in deep. The two metal-supported SOFC cells with single-layer electrolyte of SDC and double-layer electrolyte of SDC-ScSz, fabricated by suspension plasma spray and pulse laser deposition respectively, were compared and analyzed from the viewpoint of electrochemistry, focusing on cathode exchange current density, polarization loss, and maximum power density over the temperature range of 400～600℃. Results from experiments and simulation indicate that fabrication processes and operation temperatures play an important role in the electrochemical mechanism for the linear polarization characteristics of metal-supported SOFCs. Consequently, it is necessary to optimize the deposition processes related to interfacial morphology in order to reduce the cell polarization loss.In order to diagnosize the degradation mechanism of the metal-supported SOFC, consisted of Hastelloy X/NiO-SDC/SDC/SSCo-SDC, an equivalent circuit model considering mixed ionic-electronic conducters was presented in present work. In metal-supported SOFC field, the presented equivalent circuit model firstly correlates the necessary parameters for the degradation diagnosis; moreover the model exhibits mathematical tractability. The diagnosis results based on the presented equivalent circuit model indicate that the high contact resistance is a prominent factor impeding the performance of metal-supported SOFCs at 450～600℃. The observed oxide scale at the interface between metallic substrate and anode, and, the weak bonding between the electrolyte and the cathode may be responsible for the high contact resistances. These observations also validate the presented equivalent circuit model. In addition, based on the improved equivalent circuit model, internal shorting current of metal-supported SOFCs due to electronic conduction was evaluated quantitatively.Furthermore, in order to evalue the dynamic properties of metal-supported SOFC aiming at mobile application, thermal cycle test was conducted as well. Throughout the thermal cycles, the open circuit voltage values retained relatively constant; however, impedance measurement indicated the cell performance deteriorated obviously. The relatively constant values of open circuit voltage suggest the electrolyte layer withstands thermal shock due to high thermal conductivity and excellent ductibility of the metal substrate. The observed degradation phenomina are most likely due to the thermal expansion coefficience mismatch and metal oxidation. These conclusions are consistent with the degradation mechanism discussed above by means of equivalent circuit model. This consistence also verifies the validation of the presented equivalent circuit model.To sum up, at present work the complete theoretical framework and feasiable diagnosis method were built for SOFC impedance diagnosis. Furthermore, the diagnosis results at present work offer precondition and theory foundation for performance optimization and reliability improvement of metal-supported SOFCs, which subsequently speeds up the commercialization process of metal-supported SOFCs.