Impedance-based total-NOx sensor using stabilized zirconia and ZnCr2O4 sensing electrode operating at high temperature

We report here a new-type zirconia-based sensor that can detect total NOx content at high temperatures such as 700 degreesC. A closed-one-end yttria-stabilized zirconia (YSZ) tube was used as a base sensor material. An oxide sensing electrode (SE) and a Pt counter electrode (CE) were formed on the outer and inner surfaces of the YSZ tube. respectively. The complex impedance of the device using a ZnCr2O4-sensing electrode was measured with an impedance analyzer in the frequency and the temperature ranges 0.1 Hz 100 kHz and 600-700 degreesC, respectively. A large semicircular arc was observed in complex impedance plots (Cole-Cole plots) in the lower frequency range examined and it seemed to correspond to the electrolyte/electrode interface. The impedance value at 1 Hz of the present device was found to vary almost linearly with the concentration of NO (or NO2) from 50 to 400 ppm in the sample gas at 600-700 degreesC. Furthermore, it is noted that the sensitivity of NO is almost equal to that of NO2. This means that the present device can detect the total NOx at higher temperatures. (C) 2002 Elsevier Science B.V. All rights reserved.