Electrochemical NOx Sensor for Monitoring Diesel Emissions [electronic resource].
- Washington, D.C. : United States. Dept. of Energy, 2008. and Oak Ridge, Tenn. : Distributed by the Office of Scientific and Technical Information, U.S. Dept. of Energy.
- Physical Description:
- PDF-file: 12 pages; size: 1.6 Mbytes
- Additional Creators:
- Lawrence Berkeley National Laboratory, United States. Department of Energy, and United States. Department of Energy. Office of Scientific and Technical Information
- Restrictions on Access:
- Free-to-read Unrestricted online access
- Increasingly stringent emissions regulations will require the development of advanced gas sensors for a variety of applications. For example, compact, inexpensive sensors are needed for detection of regulated pollutants, including hydrocarbons (HCs), CO, and NOₓ, in automotive exhaust. Of particular importance will be a sensor for NOₓ to ensure the proper operation of the catalyst system in the next generation of diesel (CIDI) automobiles. Because many emerging applications, particularly monitoring of automotive exhaust, involve operation in harsh, high-temperature environments, robust ceramic-oxide-based electrochemical sensors are a promising technology. Sensors using yttria-stabilized zirconia (YSZ) as an oxygen-ion-conducting electrolyte have been widely reported for both amperometric and potentiometric modes of operation. These include the well-known exhaust gas oxygen (EGO) sensor. More recently, ac impedance-based (i.e., impedance-metric) sensing techniques using YSZ have been reported for sensing water vapor, hydrocarbons, CO, and NOₓ. Typically small-amplitude alternating signal is applied, and the sensor response is measured at a specified frequency. Most impedance-metric techniques have used the modulus (or magnitude) at low frequencies (< 1 Hz) as the sensing signal and attribute the measured response to interfacial phenomena. Work by our group has also investigated using phase angle as the sensing signal at somewhat higher frequencies (10 Hz). The higher frequency measurements would potentially allow for reduced sampling times during sensor operation. Another potential advantage of impedance-metric NOₓ sensing is the similarity in response to NO and NO₂ (i.e., total-NOₓ sensing). Potentiometric NOₓ sensors typically show higher sensitivity to NO2 than NO, and responses that are opposite in sign. However, NO is more stable than NO₂ at temperatures > 600 C, and thermodynamic calculations predict ≈90% NO, balance NO₂. Since automotive exhaust sensors will probably be required to operate at temperatures > 600 C, NO is the dominant component in thermodynamic equilibrium and the target NOx species. Also, the use of upstream catalysts could further promote the conversion of NOₓ species to NO. Therefore, the focus of current work is to investigate the response to NO. Nevertheless, minimizing the sensitivity to a variety of competing species is important in order to obtain the accuracy necessary for achieving the emission limits. Mitigating the effect of interfering gases (e.g., O₂, water vapor, HCs, etc.) is an area of current study. For impedance metric NOₓ sensors, our previous work has demonstrated that the cross-sensitivity to O₂ may be accounted for by comparing measurements at multiple frequencies. Other strategies for compensation are also being explored, including calibration using data from existing sensors located nearby. Our current work has made significant advances in terms of developing prototype sensors more suitable for commercialization. Also, dynamometer testing has provided real-world sensor performance data that will be useful in approaching potential suppliers to whom we can transfer the technology for commercialization. The advances are a direct result of understanding the sensing mechanisms responsible for impedance-based NOₓ sensing and the effect of materials choice and sensor design/geometry.
- Published through SciTech Connect., 11/14/2008., "llnl-tr-408750", and Glass, R S; Woo, L Y.
- Funding Information:
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