Date of Award

Winter 2016

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Micro and Nanoscale Systems

First Advisor

Erica Murray

Abstract

Yttria-stabilized zirconia has been utilized as an electrolyte of SOFC (Solid Oxide Fuel Cell) studies for years. It is also preferred as the electrolyte for solid state electrochemical sensor which is able to measure nitrogen oxides in the exhaust system. Electrochemical Impedance Spectroscopy method yields the change in impedance which is caused by an electron exchange of NO x gases at the sensing electrodes. From the EIS data, the fractional change in the phase angle is calculated for detecting sensor sensitivity.

Considering the fabrication of a sensor, several variables have been investigated which include the choice of electrolyte processing, sensing electrode dimension, varying sensor configurations, and materials selection for the sensing electrode. These factors directly influence the sensor response, which are able to change the sensor selectivity, stability and sensitivity.

In this study, porous electrolyte (YSZ) with dense electrode (Au) sensor was demonstrated, which is outside the scope of the current work by others. The difference of this work is that NOx sensor has porous YSZ contact with a dense electrode which makes the interfacial reaction mechanism change.

The role of porosity on impedancemetric NOx sensing was studied for Au/YSZ/Au sensor. NOx sensors were fired at various temperatures to establish different porosity. The impedance of the sensors demonstrated a strong dependence on porosity as if decreases in porosity of about 4% with resulted in nearly a 50% decrease in the impedance. Analysis of the impedance data indicated NOx sensitivity increased as the YSZ electrolyte porosity decreased. The responses to NOx concentrations ≤ 10 ppm were distinguishable at operating frequencies as high as 40 Hz enabling rapid sensing. Activation energies calculated based on data from the impedance measurements increased in magnitude (97.4 – 104.9 kJ/mol for 100 ppm NO) with respect to increasing YSZ porosity.

Sensors with two types of electrode (Au) configuration were tested in 1-18% 02 with 0-100 ppm NO at 600-700°C temperature. The temperature dependence of the NO sensors studied was independent of the electrode configuration. Analysis of the impedance data, along with equivalent circuit modeling which is (RHFCPEHF)-(RLFCPELF) circuit, indicated the electrode configuration of the sensor affected gas and ionic transport pathways, capacitance behavior, and NO sensitivity.

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