Development of a NOx gas sensor for exhaust

Communication présentée dans la session "Automative Exhaust". Jean-Paul Viricelle préside la session "Automative Exhaust"et participe à ce congrès dans la catégorie "Conférence Invitée".International audienceA high-sensitive sensor for NOx detection based on selective electrochemical reduction of NO2 is proposed. Electrochemical cell consisting of three metallic electrodes and YSZ electrolyte operates under electric polarization and allows selective NO2 detection in exhaust gases at 400-550°C. Electrical polarization applied between working and counter electrodes accelerates partial electrochemical reaction: oxygen reduction on cathode and oxygen-ion oxidation on anode. Variation of working electrode potential, where NO2 reduction is possible, is proportional to NO2 concentration. Cross-sensitivity of sensor to hydrocarbons, NO and CO was not observed. Combination with a catalytic filter allows NO measurement and distinction between NO and NO2, given that oxygen concentration is known and ammonia concentration remains negligible.Motivation The interest for reliable electrochemical gas sensors for automotive exhausts application to on-board control the emissions of nitrogen oxides is still challenging with the strengthening of international legislations. Electrochemical NOx sensors based on yttria-stabilized zirconia (YSZ) solid electrolyte have been and are still extensively studied, focusing either on materials approach or on running mode (polarization). In a past project, a planar potentiometric YSZ based sensor with platinum and gold electrodes associated with a catalytic filter covering the sensing element was shown to be fully selective to NOx but unable to distinguish between NO and NO2. Without a catalytic filter, the Au/YSZ/Pt sensor is sensitive to reducing gas (CO, hydrocarbons, NO…) with a positive response (&#916V= VPt-VAu) while opposite behavior is observed for oxidant gas NO2. A catalytic filter removes interferences from CO and hydrocarbons and leads to NO/NO2 thermodynamic equilibrium so that a similar response is obtained either for NO or NO2. Hence, the objective is to improve such sensor to overcome this inconvenient; a strategy based on electrode polarization was developed