Invited Presentation: Chemo-Mechanical Stress and Its Impact on the Stability and Reliability of Solid Oxide Fuel Cells

Abstract

As a device of ceramic multilayers, solid oxide fuel cell has a risk of having mechanical failures when material selection is not appropriate. It may cause a fatal damage of a cell and the whole stack. Thermo-mechanical properties, i.e. thermal expansion mismatch between the materials or temperature gradient inside a cell is the primary cause of a mechanical failure. In addition, chemical strain can also lead to a serious problem if a nonstoichiometric compound is used as a major component of the cell stack, which is the case of most SOFCs under development. The chemical strain is a function of local temperature and oxygen potential which is determined by mass, charge, and heat transport in the materials and at the interfaces. Thus, the chemo-mechanical or electro-chemo-mechanical aspects of the materials should be carefully taken into consideration in designing and operating the cells. Impact of the chemical strain on the reliability is highest when electrolyte itself shows oxygen nonstoichiometry, and is used in an electrolyte-support type configuration. Doped ceria is an example as pointed out by several researchers. Transition metal doped lanthanum gallate also shows significant chemical stress when used as an electrolyte. The valence state of cobalt changes from 4+ to 2+ depending the local oxygen potential, the oxygen vacancy concentration changes significantly. The stress distribution was calculated under open circuit, operation, and short-circuited conditions for the flat- constrained and partially constrained situations. Large tensile stress is anticipated on the cathode side. Temperature distribution also enhances the development of the stress. Similarly as the electrolyte material, oxide interconnects such as lanthanum chromite based materials can also causes chemical stress in the cell. On the other hand, electrode material such as lanthanum cobaltite or cobalt ferrite has less effect of chemical expansion under normal operation conditions even though chemical strain is large when oxygen partial pressure decreases. Although overpotential at the electrode locally reduces the cathode material, the thickness of the electrochemically active layer is around or less than 10 micrometer in air. Together with the high porosity of the electrode, those materials are safely used in actual cell stacks. However, under accidental conditions such as stoppage of air flow, the large chemical expansion of the mixed conductor cathode causes fatal failure in a cell. In contrast to the significance of chemical to mechanical coupling in SOFC materials, effects of mechanical stress on the chemical properties are not well understood, and expected to be negligible in a practical SOFCs. Direct effect of stress on the oxygen nonstoichiometry was found negligible by using electromotive force measurements.