Impedancemetric gas sensor based on zirconia solid electrolyte and oxide sensing electrode for detecting total NOx at high temperature

A solid-state electrochemical NOx sensor was fabricated by using a closed-one-end yttria-stabilized zirconia (YSZ) tube, an oxide sensing electrode (SE) and a Pt counter electrode (CE). The complex impedance of the device using each of several oxide SEs was measured in the frequency and the temperature ranges of 0.1 Hz to 100 kHz and 600–700 °C, respectively. In most cases, a large semicircular arc was observed in complex impedance spectra (Nyquist plots) in the lower frequency range examined in dry-air flow. Only in the case of the device using ZnCr2O3 SE, the semi-arc shrank to some extent upon exposure to NOx gas. The resistance value (Z′) at the intersection of the semi-arc with the real axis at lower frequencies (around 0.1 Hz) varied with concentration of both NO and NO2 in the sample gas. 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. The 90% response and 90% recovery times were as short as less than few seconds at 700 °C. Furthermore, it is noted that the sensitivity of NO is almost equal to that of NO2. This indicates that the present device can detect the total NOx at higher temperatures.