Impedance analysis of electrochemical NO, sensor using an Au/Yttria-stabilized zirconia (YSZ)/Au cell

An electrochemical cell employing a YSZ electrolyte and two Au electrodes was utiized as a model system for investigating., the mechanisms responsible for impedancemetric NOx (NO and NO2) sensing. The cell consists of two dense Au electrodes on top of a porous/dense YSZ bilayer structure (with the additional porous layer present Only under the An electrodes). Both electrodes were co-located on the same side of the cell, resulting., in an in-plane geometry for the current path. The porous YSZ appears to extend the triple phase boundary and allows for enhanced NO,, sensing, performance, although the (exact role of the porous layer is not completely understood. Impedance darn were obtained over the frequency ran-c of 0.1 Hz to 1 MHz. and over a ran-c of oxygen (2 to 18.9%) and NOx, (10 to 100 ppm) concentrations, and temperatures (600 to 700 degrees C). Data were fitted with an equivalent circuit, and the values of the circuit elements were obtained for different concentrations and temperatures. changes in a single low-frequency are. were found to correlate with concentration changes, and to be temperature dependent. In the absence of the effect of O-2 Oil the low-frequency resistance could be described by a power law, NOx and the temperature, dependence described by a sin.-le apparent activation energy at all O-2 concentrations. When both O-2 and NOx 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.