The Effect of a Protective Overcoat on Mixed-Potential Sensor Response

Nitrogen oxide (NO_x) and ammonia (NH₃) sensing technology is being developed for application in vehicle emissions monitoring of diesel and lean-burn gasoline engines.  In lean-burn engines, Selective Catalyst Reduction (SCR) utilizes NH₃ to reduce NO_x. The purpose of the NH₃ sensor is to ensure that the NH₃ is completely consumed in the NO_x reduction reaction, and it is not emitted from the tailpipe. Both the NO_x and NH₃ sensors described herein are mixed-potential sensors that measure the non-nernstian potential created at an electrode/electrolyte interface exposed to a mixture of reducing/oxidizing gases. The NO_x sensor is composed of La_0.8Sr_0.2CrO₃ (lanthanum strontium chromite) and Pt electrodes with a YSZ electrolyte and a protective porous ceramic overcoat. The NH₃ sensor is composed of Au/Pd and Pt electrodes with a YSZ electrolyte and protective overcoat. The purpose of the overcoat is to protect the electrodes from contaminants such as heavy metals and water present in harsh exhaust environments. We examine the effect of the protective overcoat on the sensor performance. Preliminary data on the NO_x sensors indicates that the overcoat serves as a thermal blanket; decreasing the temperature difference observed between the heater and the sensing element in bare sensors. Sensing characteristics of response time, selectivity and sensitivity will be compared and discussed.