Research On A New Type NOx Sensor Based On Electrode Selectivity

With the popularity of the automotive, the environmental pollution caused by NOxgas from automotive exhaust become more and more serious. And coupled withincreasingly stringent emission regulations, research and development on NOx sensor isparticularly urgent. However, the potential response to NO₂is opposite to that of NO,which makes the structure of NOx sensors complex and it is a major obstacle to achievecommercial application. In order to simplify the structure, this paper come out with aconcept of dual-sensitive electrode NOx sensor.In this article, NOx sensors were fabricated with Cr₂O₃, LSFNiO, NiO, LSFMnO,LSFCO, LSNiO, and then their sintering process and the relationship between responseand operating temperature were examined. Cr₂O₃-LSFNiO、 NiO-LSFMnO andLSFCO-LSNiO dual-sensitive electrode NOx sensors were prepared and their potentialresponse to different NO/NO₂concentration ratio was studied.Sintering process of sensitive electrode materials was studied. The volatile amount ofsensitive electrode increased with the increase of sintering temperature. Sensitiveelectrodes could hardly sinter at1100℃; sintered at1300℃or above resulting in somedefects such as micro-cracks, particle harden, and lead to a smaller three-phase interfacearea. When sintering at1300℃or above， LSFCO could react with YSZ to formLa_0.5Zr_0.5O_1.75or (La_0.1Zr_0.9)O_1.95, and it’s characteristics peak slightly offset to the left.Relationship between potential response and operating temperature were examined.With increase of operating temperature, the response of sensitive electrode increased firstand then decreased. The response of Cr₂O₃, NiO, LSFCO, LSNiO obtained maximum at550℃and that of LSFNiO and LSFMnO at600℃. Sensitivity of sensitive electrodesexcept LSFNiO under550℃were higher than of600℃. With the elevation of operatingtemperature, the adsorption capacity to NOx molecules of sensitive electrodes diminished,so the recovery rate, but the response rate rarely increased.Potential response of dual-sensitive electrode NOx sensor to different NO/NO₂concentration ratio was analyzed. Results showed that there was a good linear relationship between the response of the sensitive electrodes and the logarithm of NOx concentration，and increased with the increase of NO₂while decreased with the increase of NO. And thenempirical formulas of sensitive electrodes were obtained. The NO sensitivity and NO₂sensitivity were in opposite directions.The electrochemical impedance spectroscopies under operating temperature differentand NO₂concentration were discussed. As the temperature increase, the interfaceresistance between sensitive electrode and YSZ，the bulk resistance of sensitive electrodeand YSZ decreased. With the increase of NO₂concentration, the amount of NO₂absorbedon sensitive electrode surface increased, then more NO₂was involved in theelectrochemical reaction, and so accelerated the rate of charge transfer at YSZ/sensitiveelectrode interface, resulting in the decrease of interfacial resistance. The fitting resultsshowed that the interface resistance of sensitive electrode and YSZ decreased linearly withthe increase of NO₂concentration.