High-temperature potentiometric NO₂ and CO sensors based on stabilized zirconia with oxide sensing electrodes

This paper reports the efforts made in our laboratory to develop electrochemical sensors that might detect NO₂ and CO at high temperatures for On Board Diagnostic (OBD) application. The non-Nernstian behaviour of zirconia-based electrochemical NO₂ sensors with various oxides as sensing electrodes was studied in the temperature range 450-700°C. Both pellets and tape-casted layers (150 μm of thickness) of yttria-stabilized zirconia (YSZ) were used for fabrication of the sensors. Pt electrodes were painted on both sides of the pellets or as two parallel fingers on one face of the layers. One of the Pt electrodes was covered with a thick-film oxide electrode. Various oxides were tested as sensing electrodes, either p- or n-type semiconductors, including WO₃ and LaFeO₃. The role of ionic conductivity of the oxide electrodes was investigated using Sr-doped perovskite-type oxides, such as La_xSr_1-xFeO₃, a mixed ionic-electronic conductor. The sensors were tested as potentiometric and amperometric devices. The performance of these devices was promising: fast and stable responses to different NO₂ concentrations (20-1000 ppm in synthetic air) were observed at high temperatures. The role of the metallic electrodes is also studied. The sensing mechanism of the sensors is discussed.