Experimental validation of a La0.6Sr0.4Co0.2Fe0.8O3-δ electrode model operated in electrolysis mode: Understanding the reaction pathway under anodic polarization

In order to validate the reaction mechanism of porous LSCF oxygen electrodes, a set of experiments has been conducted on two types of symmetrical cells exhibiting different microstructures. In both cases, the polarization curves exhibit a dissymmetry with a transition at low anodic overpotential associated to a modification in the shape of the electrochemical impedance spectra. To interpret the experimental results, a micro-scale electrode model including two reaction pathways has been used. The model considers an oxidation/reduction at TPBs (surface path) in parallel to an oxygen transfer at the gas/LSCF interface (bulk path). Thanks to a 3D electrode reconstruction, the simulations have been performed with a reduced number of unknown parameters. It has been found that the simulated data are in good agreement with the experimental polarization curves and impedance spectra at OCP as well as under anodic polarization. Once validated, the model has been used to unravel the complex electrode operating mechanisms in electrolysis mode. The simulations have shown that the transition detected at low anodic polarization is due to a change in the dominant reaction mechanism passing from the bulk to the surface path. Moreover, the relative contribution of the two pathways has been investigated as a function of temperature.