Impedancemetric NO[sub x] Sensing Using YSZ Electrolyte and YSZ∕Cr[sub 2]O[sub 3] Composite Electrodes

An impedancemetric method for N O x sensing using an yttria-stabilized zirconia (YSZ)-based electrochemical cell is described. The sensor cell consists of a planar YSZ electrolyte and two identical YSZ ∕ Cr 2 O 3 composite electrodes exposed to the test gas. The sensor response to a sinusoidal ac signal applied between the two electrodes is measured via two parameters calculated from the complex impedance, the modulus ∣ Z ∣ and phase angle Θ. While either of these parameters can be correlated to the N O x concentration in the test gas, Θ was found to provide a more robust metric than ∣ Z ∣ . At frequencies below approximately 100 Hz , Θ is sensitive to both the N O x and O 2 concentrations. At higher frequencies, Θ is predominantly affected by the O 2 concentration. A dual frequency measurement is demonstrated to compensate for changes in the O 2 background between 2 and 18.9%. Excellent sensor performance is obtained for N O x concentrations in the range of 8 – 50 ppm in background. An equivalent-circuit model was used to extract fitting parameters from the impedance spectra for a preliminary analysis of N O x -sensing mechanisms.