Impedancemetric Technique for NO x Sensing Using a YSZ-Based Electrochemical Cell
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0972-AA12-04
Impedancemetric Technique for NOx Sensing Using a YSZ-Based Electrochemical Cell L. Peter Martin, Leta Y. Woo, and Robert S. Glass Lawrence Livermore National Laboratory, Livermore, CA, 94551
ABSTRACT An impedancemetric technique for NOx sensing using a yttria-stabilized zirconia (YSZ) electrochemical cell is reported. The cell consists of a dense YSZ substrate disk with two YSZ/metal-oxide electrodes deposited on the same side. The cell is completely exposed to the test gas (no air reference). The NOx and O2 response of the cell were evaluated during constantfrequency operation at frequencies in the range from 1 to 1000 Hz. At 10 Hz, the NOx response (as measured by phase angle shift) is shown to be linear with concentration over the range from 8-50 ppm, with comparable response to both NO and NO2. A method of operation is described which enables compensation for the O2 response at oxygen concentrations greater than approximately 4%. This mode of operation allows the sensor to provide sub-10 ppm detection of NOx irrespective of the O2 concentration. The sensor exhibits good stability during continuous operation for more than 150 hr. It was observed that the O2 response of the cell may be too slow to be of practical use, taking several minutes to equilibrate after changing the concentration by a few percent. However, data will be presented which demonstrate that this response is related to the metal oxide used for the electrode, and that more rapid response times can be achieved by modification of the electrode material. INTRODUCTION 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, CO, and NOx. Sensors using yttriastabilized zirconia (YSZ) as an oxygen ion conducting electrolyte have been widely reported for both amperometric and potentiometric modes of operation.1 Amperometric sensors can be configured as NO-selective or total-NOx sensors, and significant recent efforts have gone into the investigation of various metal-oxide electrodes to optimize the sensing response.2,3 These sensors are generally quite sensitive to the O2 concentration unless multiple-cell configurations are used to control the O2 concentration. Potentiometric sensors can exhibit high sensitivity to low concentrations of various species including NOx, CO, HC, and H2.4-7 However, the response to NO2 is generally opposite (in sign) to, and larger than, the NO response, which presents certain complications for sensor operation in ‘real-world’ applications. Recently, an ac impedance-based technique has been reported for sensing NOx, CO, HC, and humidity using a YSZ electrolyte and various electrode materials.8-10 In that technique, the modulus of the impedance, |Z|, is measured at 1 Hz as the sensing signal at 600-700oC. Total NOx sensing, i.e. equal response to NO and NO2, was demonstrated using a tubular YSZ electrolyte, ZnCr2O4 sensing ele
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