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

Abstract

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 dependence on time of oxide-scale growth rate, but a significantly increased growth rate with a coating of LSCo (La 0.6Sr 0.4CoO 3− δ ) compared to those without and with the coatings of LSM (La 0.85Sr 0.15MnO 3)+LSGM (La 0.8Sr 0.2Ga 0.83Mg 0.17O 2.815) and platinum. Short-term resistance measurements in stagnant air as a function of temperature with pre-oxidized alloys indicate that the oxide scale has a semiconducting transport property. The overall activation energy includes a term from small-polaron hopping inside the oxide scale Δ H m and terms Δ H i and Δ H j from charge transfers at the electrode/oxide-scale and alloy/oxide-scale interfaces, respectively. For the LSCo electrode, long-term resistance measurements as a function of time with un-oxidized alloys reveal a secondary oxidation mechanism related to the formation of an insulating spinel phase in addition to a primary oxidation mechanism associated with the formation of Cr 2O 3. SEM observations show that oxidation of the un-oxidized alloy in the presence of an oxide electrode results in considerable interdiffusion of Cr and the electrode cations, especially Co, across the interfaces. Since the ASR values of the oxide scale measured with oxide electrodes quickly approach the permitted limit of a practical SOFC, highly recommended for prevention of a secondary electrochemical oxidation of iron-based alloy interconnects is (1) the use of an oxide coating having purely electronic conductivity and/or (2) prior-to-use conditioning of the alloys via pre-oxidation.