Title page for ETD etd-12292003-101214


Type of Document Dissertation
Author An, Ke
Author's Email Address kean@vt.edu
URN etd-12292003-101214
Title Mechanical Properties and Electrochemical Durability of Solid Oxide Fuel Cells
Degree PhD
Department Engineering Science and Mechanics
Advisory Committee
Advisor Name Title
Reifsnider, Kenneth L. Committee Chair
Case, Scott W. Committee Member
Hendricks, Scott L. Committee Member
Lesko, John J. Committee Member
Prather, Carl L. Committee Member
Keywords
  • Durability
  • Impedance
  • Composite Cathode
  • SOFC
  • Fracture Strength
  • Aging
  • Mechanical Properties
  • Hardness
Date of Defense 2003-12-19
Availability unrestricted
Abstract
The mechanical properties of unaged and aged constituent materials for solid oxide fuel cells were evaluated using microindentation, plate tensile, four-point bend, ball on ring and pressure on ring tests. The Vickers hardness of the anode, interconnect and electrolyte was determined before and after 1000 hours aging at 1000 oC in air. The fracture toughness KIC was found for the electrolyte materials. Finite element analysis (FEA) was validated and used to calculate the stress distribution and peak stress for the biaxial strength test. A Weibull analysis was carried out on the test/FEA-predicted peak stresses, and Weibull strength, modulus and material scale parameters were found for each test methodology. The methodologies were evaluated based on the results of the Weibull analysis and the pressure on ring test is preferred one for brittle thin film fracture strength testing.

Half cell SOFCs with composite cathode (Pr0.7Sr0.3)MnO3±δ /8YSZ on the 8YSZ electrolyte were aged 1000 hours at 1000 oC in air with/without polarization and investigated using Electrochemical Impedance Spectroscopy (EIS), Scanning Electron Microscopy (SEM), Brunauer-Emmett-Teller (B.E.T.) method and X-ray Diffraction (XRD). The performance of the half cell SOFCs degraded after aging with/without polarization compared to the initial state, which was ascribed to the decrease of the electrolyte conductivity. The current load was shown to have impact on the performance by slowing down the decreasing rate of the polarization resistance of the SOFCs. After aging, the microstructural properties - pore size and pore volume changed, and growth of grains was found on the (Pr0.7Sr0.3)MnO3 phases, which may have contributed to the decrease of the activation polarization by decreasing the capacitance and increasing the number of active sites. After aging the high frequency EIS arcs/peaks shifted to a lower frequency range, and the low frequency arcs/peaks became unapparent compared to before aging.

A 3-D multiphysics finite element model was used to simulate the performance of the half cell SOFC. The effective exchange current density and the effective ionic conductivity of the cathodes showed much influence on the performance of the SOFC. Predicted and observed performance was compared.

Suggestions were given for the further experiments on the composite cathode.

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