Oxygen reduction on porous Ln2NiO4+δ electrodes

Ln2NiO4+delta based materials (Ln=La, Nd or Pr), show very good electrocatalytic performances as SOFC cathode: the oxygen diffusion coefficient D(*)and the surface exchange coefficient k measured by isotopic exchange are several orders of magnitude higher than that of the standard LSM cathode material. They are good mixed ionic and electronic conductors (MIEC) due to the mixed valence of the transition metal cation M and to the presence of mobile additional oxygen atoms. Therefore, the O2 reduction is not limited by a charge transfer process occurring usually at the one--dimensional ''three--phase boundary'' interface between gas, electrode and electrolyte characteristic of metallic cathodes. This study aims to characterise the reaction kinetics at O2(g), Ln2NiO4+delta/zirconia porous electrodes in the temperature range 500--800 deg C, under air. In order to identify interfaces and electrode processes, ac electrochemical impedance spectroscopy was used under zero bias conditions with symmetrical cells. Using the Schouler method, the electrode/electrolyte interface impedance has been clearly identified as the limiting step. Furthermore, electrode properties have also been measured under non--zero dc conditions with a three--electrode cell. The polarisation curves allow to confirm that Ln2NiO4+delta oxides are promising materials for SOFC cathode. The observed overpotentials are lower than those observed for LSM under the same current and temperature conditions. Nevertheless, the interface between Ln2NiO4+delta and zirconia should be optimised by a better shaping because the interfacial resistance appears to be the most important contribution to the total impedance.