Reaction Mechanisms of La2NiO4+delta Oxygen Electrodes Operated in Electrolysis and Fuel Cell Mode

The reaction mechanisms governing the electrochemical behavior of La2NiO4+delta (LNO) oxygen electrodes for Solid Oxide Cells have been investigated through a coupled experimental and modeling approach. In this frame, a set of experiments was performed on a symmetrical cell using a three-electrode setup. A micro-scale electrode model considering two reaction pathways, i.e. bulk and surface paths, has been developed to describe the experimental results. The microstructural parameters of the electrode were obtained by FIB-SEM tomography. The model was calibrated using the experimental polarization curves measured at different temperatures, and it was validated using electrochemical impedance diagrams recorded at open circuit potential (OCP) and under polarization for different oxygen partial pressures. It has been evidenced that the LNO reaction mechanism depends on both the temperature and the polarization. At OCP, the reaction mechanism is controlled by the bulk path at 650 degrees C and by the surface path at higher temperatures. A transition from the bulk path towards the surface path was observed under cathodic polarizations. These results have been interpreted by considering the evolution of the LNO over stoichiometry with the electrode polarization. The evolution of the electrode polarization resistance with the oxygen partial pressure has been also investigated.