A fast regression model for the interpretation of electrochemical impedance spectra of Intermediate Temperature Solid Oxide Fuel Cells

A frequency domain transformation method is applied to a physically consistent model of an IT-SOFC with MIEC electrolyte. Fast computation of the impedance spectra is achieved, which allows to apply the model to kinetic studies using traditional regressive techniques. The model includes conservation equations of mass and charge, gas diffusive transport effects and leakage current effects due to the semiconductor electrolyte. Global rate equations for the electrocatalytic semi-reactions guarantee economy of calibrated parameters. Two sets of impedance experiments collected on electrolyte-supported Cu-Pd-CZ80-SDC/SDC/LSCF-GDC cells are analyzed. The description of the H2 oxidation process is improved based on experiments with H2/N2 mixtures at fixed temperature and increasing H2 molar fraction (700 °C, 30-100% H2). The introduction of constant phase elements proves essential to closely predict Nyquist and Bode plots: strong inhomogeneity of the anode structure emerges, while the cathodic determining process is better identified via refinement of the double layer capacitance. Novel impedance experiments, performed with a CO-rich mixture (97% CO, 3% CO2) between 600 °C and 700 °C, allow to estimate the kinetic constant for CO electro-oxidation. The variation of the CPE parameters suggests that carbon deposition during exposure to CO simultaneously reduces the anodic active area and double layer capacitance.