Characterization of iron-based alloy interconnects for reduced temperaturesolid oxide fuel cells

The oxidation kinetics and electrical properties of oxide scales thermally grown on the surface of a commercial ferritic alloy have been investigated on the un-oxidized and pre-oxidized alloys as functions of temperature and time under oxidizing atmospheres with four different electrodes. Oxidation kinetic studies with the un-oxidized alloys show a nearly parabolic depende nce on time of oxide-scale growth rate, but a significantly increased growt h rate with a coating of LSCo (La0.6Sr0.4CoO3-delta) compared to those with out and with the coatings of LSM (La0.85Sr0.15MnO3) + LSGM (La0.8Sr0.2Ga0.8 3Mg0.17O2.815) and platinum. Short-term resistance measurements in stagnant air as a function of temperature with pre-oxidized alloys indicate that th e oxide scale has a semiconducting transport property. The overall activati on energy includes a term from small-polaron hopping inside the oxide scale Delta H-m and terms Delta H-i and Delta H-j from charge transfers at the e lectrode/oxide-scale and alloy/oxide-scale interfaces, respectively. For th e LSCo electrode, long-term resistance measurements as a function of time w ith un-oxidized alloys reveal a secondary oxidation mechanism related to th e formation of an insulating spinel phase in addition to a primary oxidatio n mechanism associated with the formation of Cr2O3. SEM observations show t hat oxidation of the un-oxidized alloy in the presence of an oxide electrod e results in considerable interdiffusion of Cr and the electrode cations, e specially Co, across the interfaces. Since the ASR values of the oxide scal e measured with oxide electrodes quickly approach the permitted limit of a practical SOFC, highly recommended for prevention of a secondary electroche mical oxidation of iron-based alloy interconnects is (1) the use of an oxid e coating having purely electronic conductivity and/or (2) prior-to-use con ditioning of the alloys via pre-oxidation. (C) 2000 Published by Elsevier S cience B.V. All rights reserved.