High-temperature ceramic fuel cells such as the solid-oxide fuel cell (SOFC) are among the highest types of fuel cells in terms of volumetric power density with the ability to use a variety of fuels. However, poor mechanical robustness and thermal cycling stability are the major disadvantages of these fuel cells as the operating temperature is very high, leading to high thermal stresses between layers. A new concept of a micro-monolithic ceramic cell suitable for portable applications was previously introduced that had extremely high power-densities (&gt;10 W/cm3) as well as high mechanical robustness and thermal cycling stability. The cell was characterized in terms of power density and bending strength and was found to be highly fragile in the longitudinal direction. In this study, a 3D comprehensive theoretical model is presented for ceramic fuel cells, which includes equations for species, charge, momentum, and energy transport. Moreover, the bilinear elastoplastic material model is used to estimate thermal stresses in ceramics at high temperatures. This model is then used to study the details of thermal stresses occurring in different layers, as well as to apply the concept of functionally graded material composition to the electrodes in order to lower the values of stresses and create a more robust design suitable for portable applications. A study for the operating conditions and channel geometry is also investigated in order to locate the optimum working conditions of such a cell, which corresponds to the highest fuel conversion efficiency. Results indicate that circular anode channels are best in terms of electrochemical performance and thermal stresses. Furthermore, moderate flow rates of fuel must be kept in order to maximize the fuel conversion efficiency, as very low flow rates create severe mass transport losses, and large flow rates significantly reduce the fuel conversion efficiency. Optimum operating conditions for the cell which correspond to a fuel conversion efficiency of 59% are with a hydrogen flow rate of 30 ml/min at a temperature of 800 ºC and a voltage of 0.35 V. Moreover, grading the electrodes with a grading parameter of m = 1 yielded the lowest equivalent stress values, which is 30% lower than in the conventional ungraded electrodes. A temperature as high as 1200 ºC can be achieved safely, which yielded a maximum power density of 20 W/cm3 at 0.35 V. The results of this study create an opportunity to obtain ultra-high power density fuel cells with higher mechanical robustness suitable to portable applications.

Mass Transport and Thermal Stresses Evaluation of Micro Solid-Oxide Fuel Cells: A Numerical Study

Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2021/volume/85543

ASME 2021 International Mechanical Engineering Congress and Exposition (IMECE2021)

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