Preparation And Properties Research Of WO₃-based Sensing Electrode Used In Mixed-potential NO_x Sensors Of Vehicles

Nowadays, the pollution of NOx gas from vehicles becomes more and moreserious.Strict emission standards have been issued by many countries all over theworld.The using of SCR technology and lean burn engines reduce the emission of NOx inthe exhaust,but it needs NOx sensor with high sensitivity, perfect selectivity and reliableperformance to detect the NOx concentration in the exhaust.Scientists have thereforelaunched studies on NOx gas sensors.Mixed-potential type NOx sensor has become thefocus of the researches due to the excellent performance.This thesis focused on the sensing electrode material of WO3used inMixed-potential type NOx sensors,and mainly studied the NOx sensing performance ofthe electrode samples made by doping WO3with TiO2and CuO,and found out two kindsof electrodes based on WO3which selective to NO2and NO respectively.Adual-electrodesample with these two sensing electrodes screen-printed on each side of theYSZ electrolyte substraterespectively was then made,and the NOx sensing performance ofthis sample to NO2, NO and the mixture of NO2and NO was studied.Our specific worksare as follows:TiO2-WO3mixed oxides were made by the method of precipitation, and thenscreen-printed on YSZ substrate to make electrode samples. The sensing performance wasthen studied and the results showed that the electrodes were made up by WO3inmonoclinic phase and TiO2in tetragonal phase which had an average particle size of180nm. The doping of TiO2improved the WO3sensing performance to NO2at lowtemperature, but depressed the sensing performance to NO2at high temperature as well asthe sensing performance to NO. By changing the TiO2contents in the electrode, we can obtain an electrode of20wt.%TiO2-WO3which selective to NO2from NO at500-700℃.Sensing electrodes of CuO doped WO3were made and screen-printed on a YSZsubstrate to make electrode samples. The sensing performance was then studied and theresults showed that the electrodes were made up by WO3in monoclinic phase and CuWO4intriclinic phase which had large particle sizes. The distribution of CuWO4particlesaffected the gas sensing performance heavily. By changing the CuO contents in theelectrode, we can obtain an electrode of10wt.%CuO-WO3which selective to NO fromNO2at500-700℃.The20wt.%TiO2-WO3and10wt.%CuO-WO3sensing electrodes were thenscreen-printed on each side of a YSZ substrate respectively. The NOx sensingperformance of this dual-electrode sample was then studied and the results showed that thesample had potential response to NO, NO2and the mixture gas of NO and NO2.Thisresponse was attributed to the difference in electrochemical reaction kinetics between thetwo electrodes. There was a good linear relationship between the potential response andthe logarithm of the concentration of NO and NO2.The potential response to the fixed totalconcentration mixture of NO and NO2increased as the proportion of NO2in the mixtureincreased, and this phenomenon can be qualitatively explained by superimpose thepotential response of the dual-electrode sample to NO and NO2.