Impedancemetric NOx Sensing Using Yttria-Stabilized Zirconia (YSZ) Electrolyte and YSZ/Cr2O3 Composite Electrodes

An impedancemetric method for NO{sub 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{sub 2}O{sub 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 {Theta}. While either of these parameters can be correlated to the NO{sub x} concentration in the test gas, {Theta} was found to provide a more robust metric than |Z|. At frequencies below approximately 100 Hz, {Theta} is sensitive to both the NO{sub x} and O{sub 2} concentrations. At higher frequencies, {Theta} is predominantly affected by the O{sub 2} concentration. A dual frequency measurement is demonstrated to compensate for changes in the O{sub 2} background between 2 and 18.9%. Excellent sensor performance is obtained for NO{sub 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 NO{sub x} sensing mechanisms.