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EE11.3.1

Electrochemical NOx Sensors for Automotive Diesel Exhaust Louis P. Martin, Ai-Q. Pham, Robert S. Glass Lawrence Livermore National Laboratory P.O. Box 808, L-353 Livermore, CA, 94551-0808, U.S.A.

ABSTRACT New emissions regulations will increase the need for compact, inexpensive sensors for monitoring and control of automotive exhaust gas pollutants. Species of interest include hydrocarbons, carbon monoxide, and oxides of nitrogen (NOx). The current work is directed towards the development of fast, high sensitivity electrochemical NOx sensors for automotive diesel applications. We have investigated potentiometric NO sensors with good sensitivity and fast response when operated in 10% O2. The sensors consist of yttria-stabilized zirconia substrates attached with NiCr2O4 sensing electrodes and Pt reference electrodes. A composite NiCr2O4:Rh sensing electrode is shown to give significantly faster response than NiCr2O4 alone. The exact role of the Rh in enhancing the response speed is not clear at present. However, the Rh appears to accumulate at the contacts between the NiCr2O4 particles and may enhance the inter-particle electronic conduction. Ongoing testing of these sensors is being performed to elucidate the sensing mechanisms and to quantify cross sensitivity to, for example, NO2.

INTRODUCTION Increasingly stringent emissions regulations will introduce a need for compact, inexpensive sensors for monitoring and control of regulated exhaust gas pollutants including hydrocarbons, carbon monoxide, and oxides of nitrogen (NOx). Significant progress as been made towards the development of electrochemical sensors using ionically conducting ceramic electrolytes, usually yttria-stabilized zirconia (YSZ), and catalytically active metal oxide sensing electrodes [1-3]. The suitability of numerous single- and mixed-metal oxides for use as NOx sensor electrodes, usually in air, has been explored in the literature [4, 5]. However, reliable, cost effective sensors suitable for diesel exhaust gas applications have not yet emerged. Improvements are still needed in sensitivity, response time, reliability, and cross-sensitivity. The current work is directed towards the development of electrochemical NOx sensors for automotive diesel exhaust monitoring. The high oxygen content (5-15%) of the diesel exhaust suggests the use of a potentiometric sensor using the response of catalytic metal or metal-oxide electrodes on a solid ionic-conducting electrolyte. This response is most often described as a ‘mixed potential’ response, however alternate sensing mechanisms have been proposed in the literature [6, 7]. At present, the exact mechanism of NO response in this type of sensor does not appear to be fully understood, and there is some doubt as to the general applicability of the ‘mixed potential’ mechanism. The current investigation was performed by applying catalytic electrodes of NiCr2O4 or NiCr2O4:Rh to a planar O2 sensor substrate. Mixed potential response to NO in 5-15% O2 was monitored as a function of gas and substrate temp