Oxygen Reduction Reaction on Ruddlesden-Popper Phases Studied by Impedance Spectroscopy

The oxygen reduction mechanism of Ruddlesden-Popper phases Sr3FeMO6+delta (M = Fe, Co, Ni) has been investigated by impedance spectroscopy at 500, 600, and 700 degrees C under oxygen partial pressure pO(2) between 10(-5) and 1 atm using both He and Ar as gas carriers. Thick porous electrodes were sprayed on dense Ce0.9Gd0.1O2-x and impedance spectra data were collected on symmetrical cells. An equivalent circuit was proposed considering the electrolyte resistances R-el, a Warburg element W-HF, and two parallel elements RCpe (RCpe(IF) and RCpe(LF)). For the three compounds, W-HF has been assigned to the oxygen vacancies diffusion in the bulk, the intermediate component, RCpe(IF), to oxygen dissociative adsorption in the electrode surface, and the low frequency element, RCpe(LF), to oxygen diffusion in the gas phases. In the case of the Sr3Fe2O6+delta and Sr3FeCoO6+delta compounds, the pO(2) dependence of Warburg high frequency component suggests a complex process involving both oxygen bulk diffusion and charge transfer. The results of Sr3(F)eMO(6+delta) ( M = Fe, Co, Ni) compared with those of La0.6Sr0.4Co0.8Fe0.2O3-delta perovskite electrodes, allowing us to discuss the effect of the crystal structure on the electrochemical behavior of these layered compounds. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3511770] All rights reserved.