Date of Award

Spring 5-25-2019

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Molecular Science and Nanotechnology

First Advisor

Erica P. Murray

Abstract

The nitrogen oxides (NOx) sensors available on the market are unable to detect minute concentrations of NOx (<10 ppm) and, as a result, they are unable to meet the new US standards, i.e. Tier 3 standards for the period of 2017-2025. As per these standards sensors should be highly sensitive and capable of determining NOx on the order of single parts per million. As compared to gasoline engines, diesel engines have greater efficiency per unit quantity of fuel consumed, but they emit more smog causing NOx.

The three-way catalyst systems in modern diesel engines are ineffective in chemically reducing nitrogen oxides in the presence of excess air. Even though a number of NOx abatement systems have been developed over the years, the after- treatment systems in vehicles require a highly sensitive NOx sensor capable of monitoring low concentrations of NOx in the exhaust gases. The solid state electrochemical sensors, having porous yttria-stabilized zirconia (YSZ), along with dense electrodes, exhibit enhanced NOx sensitivity. Porous electrolytes support gas diffusion, while the dense electrodes limit heterogeneous catalysis reactions that interfere with accurate sensing of NOx.

The primary aim of this study was to investigate porous electrolytes composed of fully-stabilized YSZ (FSZ) and partially-stabilized YSZ (PSZ) incorporated with α- alumina (Al2O3) as well as gold (Au) wires as electrodes. The impedancemetric study of porous PSZ based sensors added with different weight percentages (2, 3.8, 5 and 10) of Al2O3 was conducted at varying operating conditions. The addition of 2 wt% Al2O3 to PSZ resulted in an increase in the NOx sensor impedance and grain boundary conductivity. However, above 2 wt% Al2O3 in the PSZ, the resistance of the electrolyte bulk and grain boundaries was increased. The PSZ based sensor with 2 wt% Al2O3 resulted in greater sensitivity at concentrations as low as 5 ppm NOx.

Composite electrolytes consisting of PSZ, FSZ, and PSZ–FSZ were studied by using impedance spectroscopy for the electrochemical responses under dry and humidified gas conditions. The response of 50 PSZ–50 FSZ based sensors indicated contribution of PSZ to lower water cross-sensitivity, while FSZ promoted NOx sensitivity. The microstructure of each electrolyte influenced sensor sensitivity, but there was no impact on water cross-sensitivity.

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