Digital Voltage-Current Time Differential Method for Operating Zirconia-Based NOx Gas Sensors

Tuesday, October 13, 2015: 10:00
106-C (Phoenix Convention Center)
L. Woo, F. Bell, M. Boettcher, J. Chee, J. Fitzpatrick, B. Henderson, L. Sorensen (EmiSense Technologies, a CoorsTek Sensors Company), V. Wang (EmiSense Technologies, a CoorsTek Sensors Company), R. Novak (Ford Motor Company), and J. Visser (Ford Motor Company)
Gas sensors are important for monitoring and controlling combustion technologies. In particular, NOx compounds (NO and NO2) that are present in diesel exhaust cause poor air quality and act as both pollutants and greenhouse gases. Commercially available NOx sensors for vehicle applications use high-temperature solid-state ceramic electrochemical technology similar to the ubiquitous automotive oxygen sensor; however, commercial NOx sensors are much more expensive than their oxygen sensor counterparts due to their complicated design with multiple cells (as opposed to the single-cell oxygen sensor) and their expensive electronics for measuring low-current signals (as opposed to the voltage signal from oxygen sensors). Therefore, a NOx sensor alternative that meets stringent operational requirements at a reduced cost is desired for automotive and other combustion applications.

Conventional solid-state electrochemical sensors operate with direct current (dc) methods that are either current-based/amperometric or voltage-based/potentiometric. In this work, we use alternating current (ac) impedance-based/impedancemetric operation of zirconia-electrolyte-based sensors for NOx detection. The solid-state electrochemical cells are comprised of two electrodes separated by the zirconia electrolyte, where both electrodes are exposed to the test gas. The impedance-based response of the simple single-cell sensor relies primarily on multiple concurrent non-equilibrium steady-state interfacial redox reactions. Impedance spectroscopy was used to investigate sensing mechanisms, and sensor operation was performed at pre-determined frequencies to demonstrate stable, reproducible sensor behavior.

Since laboratory impedance spectroscopy evaluation usually employs expensive analytical equipment, a novel, low-cost, portable signal processing method was developed using a digital voltage-current time differential method. The applied signal was an alternating current electrical waveform, and the response of the sensor was digitally measured directly in the time domain; laboratory impedance evaluation usually measures in the frequency domain. Results of the low-cost digital signal processing indicated the potential for increased NOx sensitivity and improved sensor performance compared to the much more expensive frequency-domain impedancemetric method.