Compact electrochemical bifunctional NOx/O2 sensor with metal/metal oxide internal reference electrode for high temperature applications

Simultaneous measurement of total NO_x and O₂ using two electrochemical methods are demonstrated using metal/metal oxide internal oxygen reference electrode-based sensors at high temperatures. The Pd/PdO-containing reference chamber was sealed within a stabilized zirconia superstructure by a high pressure/temperature plastic deformation bonding method exploiting grain boundary sliding between the ceramic components. Amperometric and potentiometric NO_x sensing devices were assembled on the outside of the sensor. Pt-loaded zeolite Y was used to obtain total NO_x capability. Both the amperometric and potentiometric type sensors showed total NO_x response, with the potentiometric device showing better NO_x/O₂ signal stability and lower NO_x–O₂ cross-interference. Since these sensors do not require plumbing for reference air, there is more flexibility in the placement of such sensors in a combustion stream.