Date of Award

2014

Document Type

Open Access Thesis

Department

Mechanical Engineering

First Advisor

Xingjian Xue

Abstract

Solid oxide fuel cell (SOFC) has been well demonstrated as a promising clean energy conversion technology. For practical applications, the SOFC systems should have both good electrochemical performance and high reliability. The SOFCs are usually operated under very aggressive conditions, e.g., high temperatures (600-1000oC) and extremely low oxygen partial pressures (anode electrode). These aggressive operating conditions could lead to a variety of material system degradations, imposing great challenges on meeting lifetime requirement of SOFC commercial applications. It is therefore essential to increase the understanding of fundamental SOFC degradation mechanisms.

The basic structure of SOFCs is a positive electrode-electrolyte-negative electrode (PEN) tri-layer assembly, in which the dense electrolyte is sandwiched by porous electrode on either side. Because the material is different from one layer to another in PEN structure, and different materials have different thermal expansion coefficients, thermal stress occurs in PEN structure at elevated temperatures. The thermal stress effects on SOFC structures have been investigated extensively, however, the chemical stress effects are rarely studied, particularly their effects on the delamination at the cathode/electrolyte interface. The study of such chemical stress is very difficult or even impossible for present experimental techniques, but could be potentially feasible for modeling techniques.

The defect transport process in conducting ceramics and non-stoichiometric conditions are closely related to the multi-physicochemical processes in SOFC devices, so the multi-physicochemical modeling is developed in the first section in both SOFC and SOEC mode. The models are validated with experimental V-I curves and utilized to investigate the performance degradation resulted from oxygen electrode/electrolyte interface delamination in chapter 3. Results indicate that delaminations significantly influence local charge current density distributions since the charge transport path is cutoff. In both parallel flow and counter flow settings, electrolysis performance is more sensitive to the delamination occurred at the center of the cell than those occurred at the edges of the cell.

To better understand the mechanism governing the delamination phenomenon, the chemical stress generated due to the non-uniform oxygen vacancy distribution at the interface is analyzed at Micro scale. The micro model considers the complicated interactions between structural mechanics and ionic transport process through conductive defects. While both the chemical and thermal stresses are complicated at the interface, the chemical stresses show different distribution patterns from the thermal stresses. The results of combined thermal and chemical stresses show that these two kinds of stresses can be partially canceled out with each other, leading to the reduced overall stresses at the cathode/electrolyte interface. The distributions of oxygen partial pressure and thus the oxygen vacancy concentration on the cathode particle surface have significant effects on chemical stress distribution and consequently on the principal stresses at the cathode/electrolyte interface.

For practical SOFC, the defect transport process is closely related to the multi-physicochemical processes, to predict the chemical stress generated in the cell under operating condition, a mathematical model is developed to study oxygen ionic transport induced chemical stress in a cell level in chapter 5. Comprehensive simulations are performed to investigate chemical stress distribution in the PEN assembly under different operating conditions and design parameters as well as mechanical constraints. Principal stress analysis is employed to identify the weakest zones in the cell. The Weibull approach is utilized to analyze failure probability of each components and the elastic energy stored in the cathode layer is employed to evaluate potential delamination failure at cathode/electrolyte interface. For the first time we build a chemical-mechanical coupling model at a cell level and is an important module complementary to the state-of-the-art electrochemical-thermal-mechanical model of SOFCs.

Rights

© 2014, Xinfang Jin

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