CO/HC sensors based on thin films of LaCoO3 and La0.8Sr0.2CoO3-δ metal oxides

We have demonstrated a new type of mixed potential, zirconia-based sensor that utilizes dense, thin films of either La-Sr-Co-O or La-Co-O perovskite transition metal oxide vs. a Au counter electrode to generate an EMF that is proportional to the oxidizable gas species (carbon monoxide (CO), C3H6, and C3H8) concentration in a gas stream containing oxygen. The devices reported in this work were tested at 600 degrees C and 700 degrees C and in gas mixtures containing 0.1% to 20% O-2 concentrations. The metal-oxide-based sensors exhibited an improvement in operating temperature and level stability at elevated temperatures compared to Au-zirconia-Pt mixed potential devices already reported in the literature. However, as with Au-zirconia-Pt devices previously reported, the response behavior reproducibility from device to device was dependent on the Au morphology, which could vary significantly between samples under identical thermal histories. The changing Au morphology on both the Au counter electrode and the Au current collector on the metal oxide electrode were responsible for sensor aging and changes in device response over time. No change in the crystal structure of the perovskite thin film could be seen from XRD. A significant hysteresis in sensor response was found as the background oxygen concentration was cycled through stoichiometry, and this may be attributed to a change in the oxidation state of the cobaltate-based metal oxide electrode. In an effort to mitigate device aging, we replaced the Au counter electrode with a second metal oxide thin film, doped LaMnO3, and demonstrated the operation of a mixed potential sensor based on dual metal oxide electrodes. (C) 2000 Elsevier Science S.A. All rights reserved.