High-Temperature Potentiometric NO 2 and CO Sensors Based on Stabilized Zirconia with Oxide Sensing Electrodes
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High-Temperature Potentiometric NO2 and CO Sensors Based on Stabilized Zirconia with Oxide Sensing Electrodes E. Di Bartolomeo, M. L. Grilli, N. Kaabbuathong, and E. Traversa Department of Chemical Science and Technology, University of Rome Tor Vergata, Via della Ricerca Scientifica, 00133 Rome, Italy ABSTRACT This paper reports the efforts made in our laboratory to develop electrochemical sensors that might detect NO2 and CO at high temperatures for On Board Diagnostic (OBD) application. The non-Nernstian behaviour of zirconia-based electrochemical NO2 sensors with various oxides as sensing electrodes was studied in the temperature range 450-700°C. Both pellets and tapecasted layers (150 µm of thickness) of yttria-stabilized zirconia (YSZ) were used for fabrication of the sensors. Pt electrodes were painted on both sides of the pellets or as two parallel fingers on one face of the layers. One of the Pt electrodes was covered with a thick-film oxide electrode. Various oxides were tested as sensing electrodes, either p- or n-type semiconductors, including WO3 and LaFeO3. The role of ionic conductivity of the oxide electrodes was investigated using Sr-doped perovskite-type oxides, such as LaxSr1-xFeO3, a mixed ionic-electronic conductor. The sensors were tested as potentiometric and amperometric devices. The performance of these devices was promising: fast and stable responses to different NO2 concentrations (20-1000 ppm in synthetic air) were observed at high temperatures. The role of the metallic electrodes is also studied. The sensing mechanism of the sensors is discussed. INTRODUCTION Strict norms on pollution control are being enforced worldwide, especially for what concerns emissions of vehicles [1]. To satisfy the new standards on automobile emissions, the On-Board Diagnostic (OBD) system has been introduced inside the vehicles to control the main pollutants, such as NOx, HCs and CO. The OBD is a quite complex, closed-loop system to continuously monitor the pollutant concentrations in the exhausts. At present, it consists of two solid state oxygen sensors (lambda sensors): one placed upstream the three way catalytic converter (to control the fuel/air ratio), the second one located downstream in the exhaust to control, through an electronic unit, the efficiency of the catalytic converter. The availability of new, reliable and fast NOx, HCs and CO solid state sensors will allow a direct and precise analysis of the pollutants easily integrable in the OBD system. Moroever, NOx sensors are required to control de-NOx systems for fuel-lean applications in gasoline engines. Solid-state electrochemical sensors with metal oxide auxiliary phase seem to be the most suitable for applications at high temperatures and in harsh environments. Several reports are available on solid electrolytes based sensors combined with metal (Pt, Au, etc.) and oxide electrodes for NOx [2-16] and CO/HCs [17-19] detection. Different types of solid electrolytes have been tested in the electrochemical sensors such as: NASICON [2, 3, 6,
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