No abstract

One of the most challenging problems in SOFC is the thermal compatibility of materials. Mechanical failure, or cathode delamination induced performance degradation, is related to local heat generations. An accurate measurement of spatial temperature distribution with correlated electric current provides good information for fuel cell performance and thermal management. Because insufficient ability of measuring electric current introduced heat generation, infrared thermography, instead of thermocouple, was used to measure the instantaneous cathode surface temperature response to the electric current in an operating electrolyte supported planar solid oxide fuel cell (LSCF-6ScSZ-NiO). The numerical model was built to study the coupled current and temperature relation by incorporating the temperature dependent material properties, i.e., Ohmic resistance and activation resistance, as a global function in the model. The thermal and electric fields were solved simultaneously. The measured and the predicted results agreed to each other well. The cathode polarization overpotential tended to increase with the current at the low current densities, but the simulated polarization-current curve exhibited a decreased slope under higher current densities that is ascribed to the local temperature increases due to the current.

Time dependent properties and performance of tubular solid oxide fuel cells were studied numerically and experimentally. The numerical model incorporated local characteristics such as porosity, tortuosity, grain size, and conductivity and was used to evaluate the specific and relative changes in performance caused by the effect of time-dependent material changes of those characteristics. A 500 hour experimental study was conducted at 800°C in 97%H2∕3%H2O on an extruded LSCo-La0.6Sr0.4CoO3∕LSGM∕Ni electrolyte-supported tubular SOFC made in our laboratory. Changes in current density with time (at constant voltage) formed a curve with initial convex (upward) curvature, becoming monotonic decreasing. The microstructure of the constituent layers was examined by scanning electron microscopy. Comparisons between model predictions and experimental observations were made. For the situation modeled and tested, the porosity and ionic conductivity were found to be most influential on performance. More importantly, the effect of porosity is a trade-off between the influence on gas transport and the mixed conductor influence on the electrochemical reactions at the electrode.

The current interest in power generation industry with more efficient and clean, and more environmentally friendly ways has attracted the research and development in solid oxide fuel cell (SOFC). In SOFC, the chemical energy is converted into electric energy and heat as the by-product. In order to make a thermally self-sustained fuel cell stack, the understanding and management of the heat generation and electric power is a critical issue. Infrared thermography provides a non-destructive way to measure surface temperature. It was used to measure the instantaneous cathode surface temperature response to the current in an operating electrolyte supported planar solid oxide fuel cell (LSCF-6ScSZ-NiO). A numerical model was built to study the coupled electric current and temperature relation by incorporating the temperature dependent material properties, i.e. ohmic resistance and activation resistance, as global functions in the model. The thermal and electric fields were solved simultaneously. The measured and the predicted results agree to each other reasonably well. The cathode polarization overpotential tends to increase with the current at low current densities, but the simulated polarization-current curve exhibited a decreased slope under higher current densities that is ascribed to the local temperature increases due to the high current energy losses.

Ceramic fuel cell, such as solid oxide fuel cell (SOFC), usually has three functional layers with one dense electrolyte in the middle and two porous electrodes on each side of it, which operates around 1000°C. Recent research activities in SOFC tend to lower the operation temperature to the range of 700°C-800°C due to improvement in mechanical properties, and reduction in costs. However, the state-of-the-art electrolyte yttria-stabilized zirconia (YSZ) under this reduced temperature produces relatively poor ionic conductivity. Ceria-based electrolyte is an excellent candidate in electrical properties under intermediate temperature range, even though it shows a lattice expansion by cerium reduction at the very low oxygen partial pressure occurring at the anode side. Hence, a bilayer yttria doped ceria (YDC) with thin YSZ protection at anode side is designed to maximize the ionic conductivity. However, this lattice expansion of cerium results in an internal stress under this SOFC consideration. In this paper, oxygen partial pressure dependent creep behavior of an edge crack at the bi-material interface (YSZ:YDC) is studied numerically. The steady state C* path independent integral is obtained from ABAQUS code. Bi-material and homogeneous cases are discussed under extensive creep. Finally, fracture analysis of an edge crack at the bilayer electrolyte is also investigated for homogeneous bilayer materials.