High-temperature operating characteristics of mixed-potential-type NO2 sensor based on stabilized-zirconia tube and NiO sensing electrode

This paper focuses on the gas sensing properties of the mixed-potential-type yttria stabilized zirconia (YSZ)-based NO2 sensor aiming at monitoring high-temperature car emission. The sensor device was fabricated by using an YSZ tube and a NiO sensing electrode (SE), and then its NO2 sensing properties were investigated in the operating temperature range of 800–900 °C. The X-ray diffraction (XRD) patterns confirmed that the SE film containing the face-centered cubic NiO phase was thermally stable after the 1400 °C sintering. The scanning electron microscope (SEM) observations showed that the SE film sintered at 1400 °C was smooth and uniform with 3–5 μm sized NiO grains. The thickness of the SE film was about 33 μm. The emf output of the sensor increased linearly with an increase in NO2 concentration on a logarithmic scale at each temperature examined. Among the various single-oxide SEs tested here and reported to date, NiO provided the highest sensitivity to NO2 even at 850 °C. The present NO2 sensor using NiO-SE showed faster recovery rate in the wet sample gas containing 5 vol.% water vapor than that in a dry sample gas. The Δemf value to 400 ppm NO2 under the wet condition was as high as 75 mV at 850 °C, while the value under dry condition was 60 mV. The improvement effect of water vapor on the sensing performances was examined by means of impedance and polarization techniques.