Physically-based impedance simulation to decouple convoluted transport and reaction phenomena in SOFC cathodes

A mechanistic model, based on mass and charge conservation equations [1], is presented for the physically-based simulation of impedance spectra in composite solid oxide fuel cell cathodes, taking into account the complex interaction between transport and reaction phenomena. The impedance simulation, which reproduces the same procedure used in laboratory frequency response analyzers, allows the de-convolution of distinct elementary processes and the identification of a specific double layer chemical capacitance, describing the possible accumulation of adsorbed species and reaction intermediates at the interface between electron-conducting and ion-conducting particles. The satisfactory agreement of simulated spectra with experimental data for different operating conditions and electrode thicknesses reveals that the model is capable to reproduce the transient behavior of composite electrodes by relying on only one fitted parameter. Model simulations show that mass-transfer processes within the electrode produce a resistive contribution in the impedance spectra related to the effect of the local oxygen partial pressure on the reaction kinetics. In addition, the pores act as a buffer for molecular oxygen, leading to a capacitive contribution in the frequency range 10^2-10^4Hz, more pronounced at high current densities.