Morphology and electrochemical activity of SOFC composite cathodes: II. Mathematical modelling

This paper presents a mathematical model of mass and charge transport and electrochemical reaction in porous composite cathodes for application in solid oxide fuel cells. The model describes a porous composite cathode as a continuum, and characterises charge and mass transfer and electrochemical kinetics using effective parameters (i.e. conductivity, diffusivity, exchange current) related to mor- phology and material properties by percolation theory. The model accounts for the distribution of morphological prop- erties (i.e. porosity, tortuosity, density of contacts among particles) along cathode thickness, as experimentally observed on scanning electron microscope images of LSM/ YSZ cathodes of varying thickness. This feature allows the model to reproduce the dependence of polarisation resistance on thickness, as determined by impedance spectroscopy on LSM/YSZ cathodes of varying thickness. Polarisation resis- tance in these cathodes is almost constant for thin cathodes (up to 10 lm thickness), it sharply decreases for intermediate thickness, to reach a minimum value for about 50 lm thickness, then it slightly increases in thicker cathodes