Impedance simulations of SOFC LSM/YSZ cathodes with distributed porosity

The cathode represents the main source of energy loss in hydrogen fed solid oxide fuel cells (SOFCs). In order to reduce the polarization resistance, porous composite cathodes, which consist of sintered random structures of electron-conducting (e.g., strontium-doped lanthanum manganite, LSM) and ion-conducting (e.g., yttria-stabilized zirconia, YSZ) particles, are often used. The optimization of the electrode performance requires the understanding of all the phenomena involved (e.g., electrochemical reaction, charge and gas phase mass transport) and how they interplay with the geometric and microstructural electrode features. Both mathematical models and impedance measurements are usually used to get this goal. In this study, a mechanistic model for composite LSM/YSZ cathodes is presented. The model is based on mass and charge balances in transient conditions and accounts for the variation of porosity along the electrode thickness as experimentally observed on scanning electron microscope images. The continuum approach is used, which describes the composite structure as a continuum phase characterized by effective properties, related to morphology and material properties by percolation theory. The model is used to simulate impedance spectra. Simulations allow a physically-based interpretation of experimental impedance spectra. The impedance simulations are performed by applying a sinusoidal overpotential with a specified frequency and solving the system of equations in time domain. The current density as a function of time is obtained as solution of the model and it is integrated in order to get the real and imaginary components of the impedance. The procedure is repeated for several frequencies. In this way, the modeled procedure reproduces the experimental method used to get the impedance spectra. Simulated results are compared with experimental spectra for different electrode thicknesses (5-85um) and temperatures (650-850°C). The comparison allows the evaluation of a macroscopic capacitance of the double layer at each interface LSM-YSZ, which is constant with electrode thickness. It is found that the low frequency arc (from 3.5 to 250Hz for temperatures respectively from 650°C to 850°C) is due to the double layer capacitance. However, there is not a clear relationship between the latter and the temperature, suggesting that the macroscopic capacitance gathers in itself several phenomena which have different behaviors with temperature.