Abstract
Composite NOx sensors were fabricated by combining partially and fully stabilized yttria-doped zirconia with alumina forming a composite electrolyte, Y2O3-ZrO2-Al2O3, and strontium-doped lanthanum manganese oxide mixed with gold to form the composite sensing electrode, La0.8 Sr0.2MnO3-Au. A surface chemistry analysis of the composite sensor was conducted to interpret defects and the structural phases present at the Y2O3-ZrO2-Al2O3 electrolyte, as well as the charge conduction mechanism at the LaSrMnO3-Au electrode surface. Based on the surface chemistry analysis, ionic and electronic transport properties, and microstructural features of sensor components, the working principle was described for NOx sensing at the composite sensor. The role of the composite materials on the NOx sensing response, cross-sensitivity to O2, H2O, CO, CO2, and CH4, and the response/recovery rates relative to sensor accuracy were characterized by operating the composite sensors via the impedimetric method. The composite sensors were operated at temperatures ranging from 575 to 675 °C in dry and humidified gas environments with NO and NO2 concentrations varying from 0 to 100 ppm, where the balance gas was N2. It was found that the microstructure of the composite NOx sensor electrolyte and sensing electrode had a significant effect on interfacial reactions at the triple phase boundary, as well as the density of active sites for oxygen reactions. Overall, the composite NOx sensor microstructure enabled a high NOx sensing response, along with low cross-sensitivity to O2, CO, CO2, and CH4, and promoted NO detection down to 2 ppm.
Highlights
As emission laws become more stringent, modern diesel engines are becoming more environmentally friendly, while offering greater torque, increased durability, higher fuel economy, and lower CO2 emissions than their predecessors
The present study explores the influence of composite electrode and composite electrolyte materials on the impedimetric response of NOx sensors to further NOx gas-sensing capabilities in the single ppm detection range
The scanning electron microscopy (SEM) images of the FSZ composite electrolyte cross-section are shown in Figure 2c–f, along with corresponding elemental mapping by energydispersive X-ray spectroscopy (EDS)
Summary
As emission laws become more stringent, modern diesel engines are becoming more environmentally friendly, while offering greater torque, increased durability, higher fuel economy, and lower CO2 emissions than their predecessors. Key factors that restrict higher-accuracy NOx sensing include limited sensitivity to the analyte gas, cross-sensitivity to other exhaust gases, and sluggish sensor response and recovery rates. The sensing behavior of various metals and metal oxides has been studied in exhaust gas environments for potential composite electrode materials for NOx sensors [6,7,8,9,10,11,12,13,14]. The overall electrical conductivity enhances the overall NOx sensitivity [9,14] Some of these studies have found that combining a noble metal with a metal oxide can enhance the sensing response to the analyte gas. The authors selected these materials, as prior studies suggested that composite electrolytes contributed to greater NOx sensing capabilities, along with reduced water cross-sensitivity [22,24].
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