Title

Durability study and multi-physics based life prediction method investigation of a solid oxide fuel cell

Date of Completion

January 2006

Keywords

Engineering, Mechanical

Degree

Ph.D.

Abstract

The durability of solid oxide fuel cell (SOFC) is generally quite good, but the science and engineering associated with the mechanisms that control that durability is not well developed. In order to obtain long cell-life performance, it is critical to explore the major causes of performance deterioration and to develop precautions against them. Generally, SOFC performance degradation is a combined result of different time-dependent changes in the characteristics of cell materials such as microstructures and material properties, element migration, impurity or compound formation, etc. ^ This research is designed to understand degradation mechanisms and correlate them to the measured performance. 10 mol.% scandia stabilized zirconia (10ScSZ) electrolyte pellets are fabricated and aged for 1500 hours. The material properties of as sintered and as-aged samples are characterized by XRD, FESEM, EBSD, AFM, AC/DC electrochemical impedance spectroscopy, nanoindentation, and microindentation, ageing test results demonstrate the ionic conductivity degradation with the ageing time. Although both bulk and grain boundary conductivites decrease due to ageing, the ionic conductivity is governed by grain boundary conductivity. ^ Lanthanum strontium ferrite cathode (LSF40) and 6 mol% scandia stabilized zirconia (6ScSZ) symmetric cathode half cell shows three major resistances from series resistance (RS), interfacial ionic transfer resistance (Ri), and cathode surface oxygen exchange reaction resistance (R C); Cathode surface oxygen exchange reaction resistance accounts for over 70% of total resistance at all measured temperature ranges. Electrochemical ageing tests illustrate that series resistance (RS) and cathode surface oxygen exchange reaction resistance (RC) have experienced severe degradation in performance. Interfacial ionic transfer resistance (R i) has relatively much smaller absolute value and changes due to ageing, which indicated the prevented interfacial reaction between 6ScSZ electrolyte and LSF cathode. The observed in situ electrolyte sintering causes grain growth, accumulative pore volume decrease, and improved the mechanical properties (Young's modulus, hardness, fracture toughness, and interfacial energy release rate). ^ LSCo-La0.6Sr0.4CoO3/LSGM/Ni electrolyte-supported tubular SOFC under study, cathode morphology change is observed that can be the cause for cell degradation. Cathode/electrolyte interface interaction can also be a cause. Received LSCF/6ScSZ/Ni full-cell exhibit degradation under 850°C with 0.7 V loads. AC impedance data shows electrolyte and cathode polarization are 90% of the total resistance. They are also responsible for cell degradation. There is an additional diffusion layer formed at the cathode/electrolyte interface for received LSCF/6ScSZ/Ni full-cell that can be the reason for increased cathode polarization due to ageing. ^ Finally, a 2-D axi-symmetric multi-physics model is built to study the relationship between time-dependent property changes and cell performance in order to obtain insight into the main causes for the increasing/decreasing pattern of the performance of the solid oxide fuel cell. Real material microstructure from SEM is also analyzed for size dependent and geometry restrains on electrolyte ionic conductivity. ^ Suggestions are given for future research activities in cathode/electrolyte ageing characterization, cathode microstructure optimization from nano-scale manufacturing, and related systematic SOFC life prediction methodologies. ^