Impedance Characterization of a Model Au/Yttria-Stabilized Zirconia (YSZ)/Au Electrochemical Cell in Varying Oxygen and NOx Concentrations

An electrochemical cell (Au/YSZ/Au) serves as a model system to investigate the effect of O{sub 2} and NO{sub x}. Possible mechanisms responsible for the response are presented. Two dense Au electrodes are co-located on the same side of a dense YSZ electrolyte and are separated from the electrolyte by a porous YSZ layer, present only under the electrodes. While not completely understood, the porous layer appears to result in enhanced NO{sub x} response. Impedance data were obtained over a range of frequencies (0.1 Hz to 1 MHz), temperatures (600 to 700 C), and oxygen (2 to 18.9%) and NO{sub x} (10 to 100 ppm) concentrations. Spectra were fit with an equivalent circuit, and values of the circuit elements were evaluated. In the absence of NO{sub x}, the effect of O{sub 2} on the low-frequency arc resistance could be described by a power law, and the temperature dependence by a single apparent activation energy at all O{sub 2} concentrations. When both O{sub 2} and NO{sub x} were present, however, the power-law exponent varied as a function of both temperature and concentration, and the apparent activation energy also showed dual dependence. Adsorption mechanisms are discussed as possibilities for the rate-limiting steps. Implications for impedance metric NO{sub x} sensing are also discussed