Influence of Microstructure on the Sensing Behavior of NO_xExhaust Gas Sensors

Abstract

The role of porosity on impedancemetric NO x sensing was investigated for sensors composed of a porous Y-stabilized ZrO 2 (YSZ) electrolyte and Au wire electrodes. NO x sensors were fabricated at firing temperatures of 950 • C, 1000 • C, and 1050 • C to establish different porous microstructures. Porosity calculations were determined from scanning electron microscopy images of the porous electrolytes using a three-dimensional statistical method. The mean porosity of sensors fired at 950 • C was 50.2%, and the mean porosity value decreased to 46.7% for sensors fired at 1050 • C. Impedance spectroscopy was used to measure the electrical response of the sensors while operating at 650 • C, in gas atmospheres of 0-100 ppm NO x and O 2 concentrations ranging from 1-18% in a balance of N 2 . The impedance of the sensors demonstrated a strong dependence on porosity as a decrease in porosity of about 4% resulted in nearly a 50% decrease in the impedance. Analysis of the impedance data indicted NO x sensitivity increased as the YSZ electrolyte porosity decreased. The response to NO x concentrations ≤ 10 ppm were distinguishable at operating frequencies as high as 40 Hz enabling rapid sensing.

Managing H2O Cross‐Sensitivity Using Composite Electrolyte NOx Sensors

“…This trend was observed for each of the various gas compositions measured and likely relates to the electrolyte microstructure. Other studies have reported that the activation energy in porous electrolyte NO x sensors tends to increase with increasing electrolyte porosity [3]. Table 1 shows the porosity of the electrolytes increased as the FSZ content in the sensor electrolyte increased.…”

“…Studies have found the porosity of the electrolyte effects gas transport to the triple phase boundary (TPB) [3,7]. The TPB is the location where the electrolyte, electrode, and gas phase are in contact.…”

“…Zirconiabased NO x sensors are favored for their stability and electrochemical performance under stringent exhaust conditions. Various research studies have achieved substantial NO x sensitivity at sensors utilizing novel porous electrolytes, as an alternative to the conventional dense electrolyte microstructure [1][2][3]. NO x sensors based on the conventional architecture commonly have porous precious metal electrodes to support exhaust gas transport to the electrode/electrolyte interface where NO x sensing reactions take place.…”

Electrochemical deposition of gold on indium zirconate (InZrOx with In/Zr atomic ratio 1.0) for high temperature automobile exhaust gas sensors

“…In a recent critical review of the chemoresistive gas sensors, Sadek et al [13] believe that dependence of sensor response on operating temperature originates from varying adsorption and desorption kinetics for oxygen ions on the oxide surfaces. The investigated material in this publication has a similar structure to the so called lambda sensor based on YZrOx, for which oxygen ion diffusion through vacancies in the material and detection of the potential gradient over the material, constitutes the detection mechanism [14][15][16]. Therefore, also oxygen diffusion through vacancies in the Au/InZrOx may form at least part of the sensor detection mechanism.…”

“…Zirconia is one of the most commonly used O2 and NOx sensing material [14][15][16], whereas indium oxide is a semiconductor oxide that is recently being used as active material in exhaust gas sensors [17][18][19].…”