Potentiometric NOx sensor based on stabilized zirconia and NiCr2O4 sensing electrode operating at high temperatures

The two types of electrochemical sensors using stabilized zirconia and the oxide sensing electrode (SE) were developed for NOx detection at high temperatures. For the mixed-potential-type sensor, NiCr2O4 was found to give fairly excellent NOx sensing characteristics in air among several spinel-type oxides tested. This NOx sensor provided a linear correlation between EMF and the logarithm of NO or NO2 concentration in the range 25¯436 ppm and in the temperature range 550¯650°C. With fixed bias voltage being applied between the SE (oxide) and the counter (Pt) electrode (CE), the EMF between SE and the reference (Pt) electrode (RE) was measured as a sensing signal. The NiCr2O4-attached tubular device was found to provide selective response to NO over NO2 if SE was polarized at +175 mV versus RE. It was also found that this device gave selective response to NO2 over NO, if SE was polarized at -250 mV versus CE. The new design of the planar device was proposed to avoid the crosssensitivities to the others gases usually coexisting in car exhausts.