Diesel engine dynamometer testing of impedancemetric NO sensors

Abstract

Prototype solid-state electrochemical sensors using a dense gold sensing electrode, porous yttria-stabilized zirconia (YSZ) electrolyte, and a platinum counter electrode (Au/YSZ/Pt) were evaluated for measuring NO x (NO and NO 2 ) in diesel exhaust. Both electrodes were exposed to the test gas (i.e., there was no reference gas for the counter electrode). An impedancemetric method was used for NO x measurements, where the phase angle was used as the response signal. A portion of the tailpipe exhaust from the dynamometer test stand was extracted and fed into a furnace containing the experimental sensor. The prototype sensor was tested along with a commercially available NO x sensor. Simultaneous measurements for NO x , O 2 , CO 2 , H 2 O, CO, and CH 4 in a separate feed stream were made using Fourier transform infrared (FTIR) spectroscopy and an oxygen paramagnetic analyzer. The experimental sensor showed very good measurement capability for NO in the range of 25-250 ppm, with a response paralleling that of the FTIR and commercial sensor. The prototype sensor showed better sensitivity to NO x at the lower concentration ranges. O 2 is an interferent for the experimental sensor, resulting in decreased sensitivity for measurement of NO x . Methods to overcome this interference are discussed.

Managing H2O Cross‐Sensitivity Using Composite Electrolyte NOx Sensors

“…Zirconiabased NO x sensors are favored for their stability and electrochemical performance under stringent exhaust conditions. Various research studies have achieved substantial NO x sensitivity at sensors utilizing novel porous electrolytes, as an alternative to the conventional dense electrolyte microstructure [1][2][3]. NO x sensors based on the conventional architecture commonly have porous precious metal electrodes to support exhaust gas transport to the electrode/electrolyte interface where NO x sensing reactions take place.…”

FY2011 Progress Report: Agreement 8697 - NOx Sensor Development

“…12 The prototype used a dense alumina substrate (99.6% Al 2 O 3 , 10 mm x 10 mm x 0.5 mm, Valley Design Corp.). The counter electrode was formed using platinum ink (Engelhard 6082) that was applied to the substrate and fired for two hours at 1200°C.…”

“…The approach is described in detail in previous reports and several publications. [8][9][10][11][12] Impedancemetric operation has shown the potential to overcome the drawbacks of other approaches, including higher sensitivity towards NO x , better long-term stability, potential for subtracting out background interferences, total NO x measurement, and lower cost materials and operation. [8][9][10][11] Past LLNL research and development efforts have focused on characterizing different sensor materials and understanding complex sensing mechanisms.…”

“…[8][9][10][11] Past LLNL research and development efforts have focused on characterizing different sensor materials and understanding complex sensing mechanisms. [8][9][10][11][12] Continued effort has led to improved prototypes with better performance, including increased sensitivity (to less than 5 ppm) and long-term stability, with more appropriate designs for mass fabrication, including incorporation of an alumina substrate with an imbedded heater and a protective sensor housing (see Fig. 1).…”

“…The phase angle has been found to provide a more stable response at higher operating frequencies and we prefer it for the sensor signal. [8][9][10][11][12] In previous work, impedancemetric sensing using either gold or strontium-doped lanthanum manganite (LSM) electrodes, the latter being an electronically conducting metal oxide, was investigated in laboratory and engine testing. Preliminary results indicated that gold electrodes have good stability and the potential for low water crosssensitivity, but also have a higher thermal expansion coefficient and lower melting temperature than the YSZ electrolyte, which limit processing flexibility.…”