Stationary 2D FEM Model Framework for SOFC Stack Performance Prediction

High performing anode-supported cells (ASCs) undergo a significant power reduction when contacted by a metallic interconnector (MIC) and operated in a stack layer [1,2]. The performance limiting factors can be identified by a stationary 2D FEM model, which calculates current/voltage (C/V) characteristics for specific SOFC stack layer setups. The required parameter set which describes (i) electrical/ionic conduction, (ii) electrochemical charge transfer and (iii) porous media gas transport is obtained via electrochemical impedance spectroscopy (EIS), 4-point DC conductivity measurements and 3D microstructures extracted from FIB-tomography [3]. The applicability of our stationary 2D FEM model was verified by comparing simulated and measured C/V characteristics over a broad range of operation conditions, and published in [3,4].

For state-of-the-art LSCF cathodes as thin as 45 µm, the simulation results demonstrate significant performance loss due to O₂starvation underneath the MIC contact ribs and/or electrical cross conduction limitations in the cathode material underneath the gas channel area in each individual stack layer [3,4]. Following up these results, a systematic variation of the cathode parameters thickness, porosity, tortuosity, mean pore diameter, and electrical conductivity as well as of the MIC flowfield design has been carried out numerically. In this way, the sensitivity of each parameter on stack layer performance became transparent.

It will be shown in this work, that (i) choosing an optimized cathode/MIC design combination can increase current density at 0.7V by more than 25% in an SOFC stack assembly (see figure 1), that (ii) the magnitude of possible loss minimization requires the ability to precisely control the system parameters, and that (iii) an optimized MIC design avoids microstructural changes occurring in the cathode/CCL material because of its inherent chemical stability limitations.

[1] L. Blum, W. A. Meulenberg, H. Nabielek, R. Steinberger-Wilckens, Worldwide SOFC Technology Overview and Benchmark, International Journal of Applied Ceramic Technology 2, pp. 482-492 (2005).

[2] M. Kornely, A. Leonide, A. Weber, E. Ivers-Tiffée, Journal of Power Sources, Volume 196, Issue (17), pp. 7209-7216 (2011).

[3] H. Geisler, A. Kromp, A. Weber, E. Ivers-Tiffée, Journal of the Electrochemical Society, 161 (6) F778-F788 (2014).

[4] H. Geisler, A. Kromp, A. Weber, E. Ivers-Tiffée, Proceedings of the 11th European Solid Oxide Fuel Cell Forum, A0904 (2014).