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

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.