Reaction mechanisms of La2NiO4+δ oxygen electrodes operated in electrolysis and fuel cell mode
Résumé
The reaction mechanisms governing the electrochemical behavior of porous La2NiO4+δ (LNO) nickelate used as oxygen electrode in SOCs (Solid Oxide Cells) have been investigated through a coupled experimental and modeling approach. In this frame, a set of experiments has been performed on a symmetrical cell using a three-electrode setup. A micro-scale electrode model taking into account two reaction pathways, i.e. bulk and surface paths, has been developed to describe the experimental results. The microstructural parameters of the LNO electrode have been obtained by FIB-SEM tomography, thus reducing the number of unknown model parameters. For the determination of the missing parameters, the model has been calibrated using the experimental polarization curves measured at different temperatures. The model was then validated using electrochemical impedance diagrams recorded at open circuit potential (OCP) and under polarization for different oxygen partial pressures.
It has been here evidenced that the LNO reaction mechanism depends on both the temperature and the dc polarization. Under air and at OCP, the reaction mechanism was found to be controlled by the bulk path at 650 °C and by the surface path at higher temperatures. A transition from the bulk path towards the surface path was observed when increasing the cathodic dc current. These results have been interpreted by considering the evolution of the LNO over stoichiometry as a function of the polarization. Better electrochemical performances have been detected under anodic polarization. In addition, the evolution of the electrode polarization resistance with the oxygen partial pressure has been also investigated.