The non-Nernstian behavior of zirconia-based electrochemical NOx and CO sensors with various semiconducting oxides as sensing electrodes was extensively studied in the temperature range 450-700°C. Both pellets and tape-cast layers of yttria-stabilized zirconia were used for the fabrication of the sensors. It was found that the gas-sensing mechanism of the sensors cannot be explained always by the mixed potential theory. When semiconducting oxides are used as sensing electrodes, the semiconductor sensing mechanism and the adsorption-desorption behavior of the different oxides dominate the sensor response if both metal and oxide are exposed to the same environment. © 2004 The Electrochemical Society. All rights reserved.

Electrochemical sensors based on yttria-stabilized zirconia ͑YSZ͒ with WO 3 as a sensing electrode were fabricated using either Pt or Au electrodes. The sensors were studied in the temperature range 500-700°C in the presence of different concentrations ͑300-1000 ppm͒ of NO 2 and CO in air. The response to NO 2 was very stable with fast response time ͑within 20 s͒. The best sensitivity was observed at 600 and 650°C using Pt and Au electrodes, respectively. At all temperatures investigated a cross sensitivity to CO gas was also noticed. The response to CO was decreased using Au electrodes. The role played by WO 3 as a sensing electrode was investigated.The environmental concerns have challenged scientists to develop suitable and reliable sensors to detect pollutants such as NO x , CO, HCs, etc. Strict norms on pollution control are being enforced worldwide, especially for emissions of vehicles. 1 The automotive industry urgently needs rugged and reliable sensors to monitor and decrease the level of pollutants (NO x and CO/HC s ) in the exhaust emissions at elevated temperatures. Solid-state electrochemical sensors with metal oxide auxiliary phase seem to be the most suitable in such environments. 2-7 Several reports are available on solidelectrolytes-based sensors combined with metal ͑Pt, Au, etc.͒ and oxide electrodes for NO x , 7-14 CO/HCs 15,16 detection. Miura and coworkers have been working for a long time on electrochemical sensors for the detection of NO x . 7-12,14 In a recent paper 11 they reported WO 3 as a suitable auxiliary oxide for selective detection of NO x in the range 500-700°C. Some of the authors of this paper 17 have used LaFeO 3 coupled with yttria-stabilized zirconia ͑YSZ͒ and sodium silicon conductor ͑NASICON͒ electrolytes and obtained stable and fast responses to NO 2 at 400 and 450°C. Brosha et al. 18 have studied LaCoO 3 and La 0.8 Sr 0.2 CoO 3Ϫ␦ perovskite oxide electrodes in zirconia-based sensors for the detection of CO/HCs at 600-700°C. Recently, Ménil et al. 19 have reviewed the actual trends of these kinds of sensors. They claimed that the major issues concerning the selectivity and the long-term stability are yet to be overcome for high-temperature sensors. This fact drives scientists to search for new materials, to improve the device fabrication techniques, and to investigate the sensing mechanism.According to many authors 12,15,16,20 the sensing mechanism of electrochemical sensors based on coupling a solid electrolyte with semiconducting oxides, can be explained using a mixed potential theory. The mixed potential mechanism was claimed both for NO x 7,12,14 and CO/HC s 15,16,18 gas sensors based on similar electrochemical cells. However, a different explanation of the NO x sensing mechanism has been proposed and named the different electrode equilibria. 21 Different electrode equilibria is a more general concept to explain the NO x sensitivity that is due not only to electrochemical reactions, but also to different electrocatalytic activity and/or sorption-desorption behavior of ...

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Electrochemical NO x sensors based on coupling different ionic conductors with perovskite-type LaFeO 3 as auxiliary phase were prepared. The ionic conductors used were yttria-stabilized zirconia that is an oxygen ion conductor and sodium superionic conductor whose charge carriers are sodium ions. Thick films of LaFeO 3 powders were deposited using a screen printing oil on one side of the solid electrolyte pellets, using powders having different grain sizes. The obtained sensors were wholly exposed to the same atmosphere. Electromotive force ͑emf͒, polarization curves, and amperometric measurements were performed in air and at different concentrations of NO 2 in air, at selected temperatures. Quite stable emf values and fast response times were obtained. The NO 2 response was much larger when powders with nanosized grains were used for the preparation of the electrodes. From polarization curves, all sensors showed a nonlinear behavior. The role played by the electrolyte/electrode interface was determined by electrochemical impedance spectroscopy measurements; exposure to NO 2 did not affect the bulk but only the electrolyte/ electrode interface.

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