CO/HC sensors based on thin films of LaCoO3 and La0.8Sr0.2CoO3-delta metaloxides

We have demonstrated a new type of mixed potential, zirconia-based sensor t hat utilizes dense, thin films of either La-Sr-Co-O or La-Co-O perovskite t ransition metal oxide vs. a Au counter electrode to generate an EMF that is proportional to the oxidizable gas species (carbon monoxide (CO), C3H6, an d C3H8) concentration in a gas stream containing oxygen. The devices report ed in this work were tested at 600 degrees C and 700 degrees C and in gas m ixtures containing 0.1% to 20% O-2 concentrations. The metal-oxide-based se nsors exhibited an improvement in operating temperature and level stability at elevated temperatures compared to Au-zirconia-Pt mixed potential device s already reported in the literature. However, as with Au-zirconia-Pt devic es previously reported, the response behavior reproducibility from device t o device was dependent on the Au morphology, which could vary significantly between samples under identical thermal histories. The changing Au morphol ogy on both the Au counter electrode and the Au current collector on the me tal oxide electrode were responsible for sensor aging and changes in device response over time. No change in the crystal structure of the perovskite t hin film could be seen from XRD. A significant hysteresis in sensor respons e was found as the background oxygen concentration was cycled through stoic hiometry, 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 sen sor based on dual metal oxide electrodes. (C) 2000 Elsevier Science S.A. Al l rights reserved.