Impedance Analysis of Electrochemical NO x Sensor Using a Au/Yttria-Stabilized Zirconia (YSZ)/Au cell
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0972-AA12-02
Impedance Analysis of Electrochemical NOx Sensor Using a Au/Yttria-Stabilized Zirconia (YSZ)/Au cell Leta Y. Woo1,2, L. Peter Martin1, Robert S. Glass1, and Raymond J. Gorte2 1 Energy and Environment Directorate, Lawrence Livermore National Laboratory, Livermore, CA, 94550 2 Chemical and Biomolecular Engineering, University of Pennsylvania, Philadelphia, PA, 19104 ABSTRACT An electrochemical cell employing a YSZ electrolyte and two Au electrodes was utilized 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 Au 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 NOx sensing performance, although the exact role of the porous layer is not completely understood. Impedance data were obtained over the frequency range of 0.1 Hz to 1 MHz, and over a range of oxygen (2 to 18.9%) and NOx (10 to 100 ppm) concentrations, and temperatures (600 to 700◦ C). Data were fit with an equivalent circuit, and the values of the circuit elements were obtained for different concentrations and temperatures. Changes in a single low-frequency arc were found to correlate with concentration changes, and to be temperature dependent. In the absence of NOx , the effect of O2 on the low-frequency resistance could be described by a power law, and the temperature dependence described by a single apparent activation energy at all O2 concentrations. When both O2 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. INTRODUCTION NOx sensor development is motivated by environmental concerns and poses numerous challenges including cost, sensitivity, stability, and response time. Yttria-stabilized zirconia (YSZ) is currently used for automotive O2 sensors and is particularly suited to meet the harsh, high-temperature operation requirements. Development of YSZ-based NOx sensors has focused on amperometric and potentiometric types, usually relying on various metal-oxide electrodes to optimize the response [1-3]. Two major drawbacks have been device stability and the need for complicated structures to account for interfering gases, such as O2 . This paper presents an impedancemetric type YSZ-based model system, which has the possibility of overcoming problems associated with other types of sensors [4]. Previous work reported an impedancemetric NOx sensor using Au, porous YSZ/Cr2 O3 composite electrodes, and a YSZ electrolyte in an in-plane geometry [4]. Understanding the sensing mechanisms is necessary to optimize sensor operation. The present work uses a model electrochemical cell t
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