Development of NOx sensing devices based on YSZ and oxide electrode aiming for monitoring car exhausts

Recent progress in the development of zirconia-based sensing devices using zinc-family oxide electrodes for both mixed-potential and complex impedance-based types has been focused on the detection of NOx in the temperature range of 600–700 °C. The sensing characteristics of these sensors were investigated in different gaseous environments. The mixed-potential-type NOx sensor using ZnFe2O4 sensing electrode (SE) have shown the highest sensitivity to NOx among other spinel-type oxides tested as SEs in the temperature range of 600–700 °C. It was also shown that the complex-impedance-based NOx sensor attached with ZnCr2O4-SE gave rather good sensing characteristics to NOx at 700 °C. Furthermore, it was discovered that the sensitivity to NO is almost equal to the sensitivity to NO2 from 0 to 200 ppm for this sensor at 700 °C. The observed NOx sensitivity and the measured NOx concentration had almost linear correlation even in the presence of 8 vol.% H2O and 15 vol.% CO2. Based on the results obtained, we proposed laminated-type sensor structure that is the best for the present complex impedance-based NOx sensor.